Neutralization Reactor

— Neutralization Reactor: Acid Load, pH Control and Salt Balance

A neutralization reactor is a vessel in which an acidic stream and an alkaline stream are mixed until the mixture reaches a neutral, dischargeable pH — published practice quotes 6.0 to 8.5, rising to 9, and the site’s own permit governs. Its design comes down to four elements: a vessel in a corrosion-resistant material — thermoplastic, glass-reinforced plastic or stainless steel; agitation by a top-entry agitator, a high-shear mixer or a circulation pump, against local hot spots and unreacted pockets; an inline pH probe signalling pumps that dose caustic soda, milk of lime or dilute sulphuric acid; and a retention volume sized for 15 to 60 minutes of residence time. The same unit serves industrial effluent before discharge, surfactant and chemical manufacture where a sulphonic acid is neutralized with caustic to SLES (sodium lauryl ether sulphate), and edible-oil refining where free fatty acids are neutralized in crude vegetable oil.

Product Overview

A neutralization reactor is an agitated vessel in which an acid-bearing or alkali-bearing process liquor is brought into a discharge pH band by the metered addition of the opposite reagent, and in which four duties are performed in a fixed order. The reagent is first dispersed into the bulk liquid so that it cannot react locally and create a pocket of extreme pH; the neutralization reaction then runs, which it does quickly once the reagents are in contact; the mixture is held under agitation long enough for the measurement at the probe to represent the whole vessel rather than the region around the injection point; and the treated liquor is then released against a proved figure. The mechanical description is short and the vessel is not the difficult part. What decides whether an installation works is the accounting of the acid or alkali load the feed carries, because that load sets the reagent, the dosing pumps, the heat released, the salt made and the vapour given off, while the volume only sets the residence time in which the loop has to act.

That distinction between load and volume is the reason this page is written around a titration rather than around a tank size. Two liquors at the same measured pH can carry two different acid loads, because a buffered liquor holds its pH down while the load rises; the load is therefore established by titrating a representative sample to the target band of the site’s own permit and expressing the result as moles of hydrogen ion per litre, or as kilograms of calcium carbonate equivalent per cubic metre. From that figure the reagent equivalent follows directly — one mole of sodium hydroxide neutralizes one mole of nitric acid, one mole of calcium hydroxide neutralizes two — and the buffer measured on the same sample is added as an allowance rather than absorbed silently into a safety factor. The result is a dose range, a reagent consumption per kilogram of acid neutralized, and a heat load that can be checked before a shell is selected instead of after a discharge has failed.

The mode of operation follows from the variability of the feed rather than from a preference. A batch reactor collects the liquor, meters reagent into it against the probe, holds the batch until the band is reached and then releases it, which suits effluent that arrives in slugs, effluent from a plant that runs campaigns, and any duty where the discharge has to be proved one batch at a time. A continuous stirred vessel meters reagent into a stream that is flowing through it, which suits a steady high-volume discharge and a feed whose composition changes slowly. Published guidance for this class of equipment states the trade rather than a rule: continuous flow systems suit a flow that is stable in volume, pH and chemistry, and become unsuitable where the outflow must be held inside a narrow band, because a disturbance in the feed passes through the vessel and reaches the discharge point; batch systems cost less but are focused on lower volumes, and suit highly acidic or highly alkaline waste because the neutralization can be adjusted to the composition of the batch and an exact pH can be attained. Where the feed cannot be made steady, the structural answer recorded in the same literature is to arrange two or more vessels in series, so that a slug is absorbed as it passes rather than arriving at the discharge intact.

What is delivered is a package rather than a vessel. The vessel itself, with the head arrangement that the duty requires and a freeboard sized for the foam and level swell the liquor produces; the agitator with its impeller, shaft, seal and drive, baffled to prevent the vortex that would starve the lower part of the vessel; the reagent storage with its secondary containment and its own material of construction, together with the metering pumps, the dilution tie and the dosing point placed in the impeller discharge stream; the measurement chain, comprising the pH probe, where the hazard requires it an oxidation-reduction potential probe, a temperature measurement, a level measurement and a flow measurement, each with its tapping position and its calibration arrangement; the closed vent with its defined destination; the sludge and precipitate draw; and the control panel with the sequence, the interlock schedule, the dosing curve, the alarm list and the data record. The scope boundaries between this reactor and the units either side of it are drawn in the same package: what arrives at the reactor inlet flange, what leaves at the discharge flange, what leaves through the vent, and what leaves as sludge.

The reactor is equally explicit about what it is not. It is not a metal recovery unit, and where a liquor still carries recoverable metal above the site’s economic threshold the metal is won first and the stripped liquor is neutralized afterwards, because precipitation of the metal as a hydroxide converts its value into a disposal cost and removes the solution from which it could have been recovered. It is not a settlement or holding basin, so the precipitation and the settling of a hydroxide sludge are designed as a separate step downstream of the reaction rather than bought from the residence time in the reactor. It is not a gas treatment device, so a liquor that gives off nitrogen oxides, hydrogen sulphide or ammonia is routed to a vent designed for those species rather than sealed into the vessel head. And it is not a solids handling plant, so a feed carrying appreciable undissolved solids is screened or settled upstream, because the agitator that is correct for blending a liquor is the wrong agitator for keeping a heavy slurry in suspension.

The two things that decide whether the installation meets its discharge figure are therefore both process items rather than mechanical ones. The first is mixing, because the reaction is fast and the dispersion of the reagent is not, and published practice for neutralization mixing states the requirement in a form that can be checked: the mixer must achieve at least sixteen complete tank turnovers within the tank residence time, so a vessel holding sixteen minutes of flow needs a mixer whose pumping capacity equals the whole tank volume every minute. The second is control, because the response of pH to reagent addition is logarithmic, and published guidance for pH control records the consequence directly — on and off control, or a single linear proportional-integral-derivative loop, is far too simple to account for the ever changing control gain encountered with the logarithmic reaction of pH to control input. A reactor that satisfies both, and that was sized from a titration of the customer’s own liquor, discharges inside its band on the first campaign instead of on the fifth.

Product Core Selling Points

The reactor is sized from a titration of the customer’s own liquor rather than from a volume quoted on an enquiry. A representative sample is titrated to the site’s own discharge band, the result is expressed as an acid load in moles of hydrogen ion per litre, and the working volume, the number of stages, the reagent equivalent and the dosing range all follow from that measurement. The quotation therefore arrives with the number it was built on, and a customer who changes the feed changes the load and can see what changes with it, instead of discovering the difference at the acceptance test.
The reagent is quoted as an equivalent and a consumption, not as a tank size. Every reagent recommendation carries the stoichiometry it is based on — one mole of sodium hydroxide per mole of nitric acid, one mole of calcium hydroxide per two moles — together with the buffer allowance measured on the sample, the delivered strength the reagent arrives at, and the consumption expressed per kilogram of acid neutralized. That last figure is what a customer can compare against a competitor’s, and it is also what turns a reagent choice into a decision about operating cost rather than about the price of a drum.
Batch and continuous modes are both built from the same shell, and the mode is chosen from the feed rather than from preference. A batch reactor collects the liquor, meters reagent into it against the probe, holds the mixture until the band is reached and then releases it, which suits effluent that arrives in slugs and effluent that has to be proved batch by batch. A continuous stirred vessel meters reagent into a flowing stream against a continuous measurement, which suits a steady high-volume discharge. Published guidance for neutralization describes the trade exactly and is used here rather than paraphrased: continuous flow systems suit a flow that is stable in volume, pH and chemistry, and become unsuitable where the outflow has to be held inside a narrow band because a disturbance passes straight through, while batch systems suit low volumes, highly concentrated waste and any duty where an exact pH has to be attained.
The pH loop is programmed to the site’s own titration curve, with a coarse stage and a trim stage running independently. The load is taken down by the first stage and held by the second, and the tuning of each stage is set against the measured curve rather than against a fixed gain, because the gain of a pH loop changes by orders of magnitude between the flat and the steep part of the curve. The sensor position, its calibration interval, the dosing valve and pump characteristic and a manual override are all part of the delivered loop, and the first campaign is used to replace the assumed reagent consumption with the measured one.
The agitator is selected for blend time and verified by a turnover count rather than by a motor rating. Published practice for neutralization mixing requires at least sixteen complete tank turnovers within the residence time, and the impeller, its diameter, its speed and the baffling are chosen to deliver that, with the injection point placed in the discharge stream of the impeller so that the reagent is dispersed before it can react locally. A reactor whose mixer was chosen for a suspended solids duty rather than for a blending duty will pass its mechanical test and still fail its pH test.
The heat of neutralization is computed from the load before the material is chosen. At about 55.83 kilojoules per mole of hydrogen ion neutralized, a concentrated feed releases enough heat to carry the liquor past the working temperature of an ordinary thermoplastic shell, and published guidance for concentrated acid service records temperature rises large enough to melt thermoplastic piping. The computed rise, the shell’s temperature limit and the quench or cooling arrangement are therefore presented together, and where the feed cannot be diluted below the safe rate the reactor is specified with a rate limit rather than with a promise.
The vapour route is designed as part of the reactor rather than as part of the building. Every reactor is delivered with a closed vent and a defined destination: an ordinary acid or alkali duty vents through the plant’s scrubber, while a nitric-bearing duty vents to the nitrogen oxides train because its decomposition liberates toxic nitrous oxide gases and its outgassing above about five per cent strength is significant. The same drawing fixes the vapour-space corrosion allowance, the vent sizing and the nozzle that carries the vapour out of the vessel head.
The hazard sequence for dangerous liquors is published rather than implied. A cyanide-bearing liquor is held above a pH of about 10.5 and oxidized in two stages, so that hydrogen cyanide is never formed in the vessel head; a sulphide-bearing liquor is kept on the alkaline side so that hydrogen sulphide is not released by acidification; an ammonia-bearing liquor is neutralized with an acid and its ammonium salt is routed to a defined destination; and a liquor carrying residual solvent-extraction organics is checked for its reaction with any oxidant before an oxidant is specified. Each of these sequences is written into the interlock schedule as well as into the procedure.
The salt balance is closed, and the three destinations are separated rather than merged. The soluble sodium salts of the common acid duties leave with the discharge inside a stated concentration limit; the sparingly soluble calcium salts of sulphate and fluoride are held by a scaling control and a bleed take-off rather than being allowed to deposit on the wetted surfaces; and the hydroxide sludge produced where a metal-bearing liquor is neutralized is routed to a sludge handling arrangement with its own volume and its own destination. A reactor that promises neutralization without saying where the salt goes has not finished its design.
The material envelope is delivered as an exclusion list, not as a preference. Thermoplastic construction for the ordinary acid and alkali duties inside its temperature band, glass-reinforced plastic where the shell is large or the service temperature is high, stainless steel where the service is oxidizing or sanitary, and no glass-containing material wherever fluoride is present. The exclusions are stated in the specification table so that a customer with a fluoride-bearing liquor receives the thermoplastic vessel that suits it and is not sold the enamel-lined one that does not.
The consumables and the calibration interval are part of the delivered package. The wetted parts of the dosing system, the reagent filters, the probe with its spare and its buffer solutions, the gaskets for the nozzle set, the impeller and its shaft seal are all listed with their replacement intervals, and the probe calibration interval is fixed from the fouling tendency of the liquor rather than left to the site to discover. The first year’s consumable campaign is issued with the acceptance test record so that the operating cost of the reagent and the instruments can be compared against the quotation.
The boundary is written down before the quotation, and the honest alternative is offered where it exists. A liquor that still carries recoverable metal above the site’s economic threshold is directed to recovery before neutralization, because raising its pH destroys the solution from which the metal could have been won and converts the value into a hydroxide sludge; a feed whose peak load has never been titrated is not sized on an estimate; and a cyanide, sulphide or ammonia-bearing liquor without a designed hazard sequence is not quoted as an ordinary acid duty. Refusing those three cases is part of the product, because each of them ends in a delivered reactor that cannot hold the band it was bought to hold.

Application Scenarios

A neutralization reactor is applied wherever a liquid has to be brought to a neutral pH, and it is applied for two different reasons: to make a stream dischargeable, and to make a product neutral. The scenarios below are the duties this reactor is built for, and each one is described by what the liquor is and by what has to be settled before the vessel can be specified. The list is deliberately accompanied by the cases that are routed elsewhere, because the most expensive mistake in this equipment class is a reactor bought for a duty that belonged to a recovery unit.

Barren liquor from a precious-metal hydrometallurgical circuit. After gold, platinum, palladium, rhodium and silver have been taken out of a chloride, nitrate or sulphate circuit, the filtrate that remains is an acidic brine carrying base metals, residual oxidant and dissolved salts at a pH far below any discharge band, and it frequently arrives hot and still evolving traces of the reagent it was treated with. This is the reactor’s principal duty, and its design case is the combination of streams that can be discharged at the same time rather than the sum of the individual ratings. The stream itself is produced by the circuit described under precious metal hydrometallurgical refining plant, and the reactor’s place is at the end of that circuit rather than inside it.
Nitric and aqua regia parting effluent, where the vapour route matters as much as the vessel. A parting or inquartation circuit produces a nitrate-bearing filtrate that is conventionally neutralized with any inorganic base, because nitric acid is fully dissociated and its neutralization reaction is unremarkable. What is not unremarkable is its behaviour as an oxidant: published technical guidance records that above about five per cent concentration the outgassing of nitric acid is significant and requires good ventilation, that its decomposition liberates toxic nitrous oxide gases, and that its reactions with organic materials can be violent because of its oxidizing power. The vessel head is therefore closed and its vent is routed to the NOx treatment equipment, and the organic loading in the feed is controlled before the reactor is specified. The upstream circuit that produces the liquor is the one described under gold refining process.
Silver electrorefining and copper tankhouse bleed. Electrolyte purification in a silver or copper circuit produces a bleed stream that has to leave the circuit to hold its impurities down, and that stream is strongly acidic and carries dissolved metal. Its neutralization is a duty with two stages rather than one, because the metal is usually worth recovering before the acid is killed, and the reactor’s design case is set from the bleed volume and its rate rather than from the cellhouse volume. The cell-side duty belongs to the silver electrorefining cells and to the copper electrolytic cell, and the liquor-side duty belongs here.
Cyanide-bearing barren solution from a leaching or stripping circuit. Free cyanide is in equilibrium with hydrogen cyanide, so a cyanide liquor is never admitted to the acid side of a reactor and is never acidified in the vessel. Published practice in this field holds cyanide-bearing solutions above a pH of about 10.5 so that the cyanide remains in solution as the cyanide ion and hydrogen cyanide gas is not formed, and describes cyanide destruction as a one or two stage chemical treatment in which the first stage raises the pH and oxidizes the cyanide to the far less toxic cyanate and the second stage oxidizes the cyanate to its end products, with the oxidant dosed against an oxidation-reduction potential setpoint that is held under mixing until the consumption stops. The reactor for this duty is therefore specified as an alkaline-hold vessel first and a reactor second, and its interlock schedule is what makes it safe.
Metal-bearing rinse water, where the metal is recovered before the acid is killed. A rinse or wash water that carries copper, nickel, zinc or a precious metal above the site’s economic threshold is a dilute ore rather than a waste, and neutralizing it converts that value into a hydroxide sludge and destroys the solution from which it could have been won. The published route is to concentrate and recover first — by precious metal electrowinning where the concentration permits it, by cementation or by selective precipitation where it does not — and to neutralize the stripped liquor afterwards. Where a customer prefers to combine the two, this page states the consequence rather than the choice, and the quotation says so as well.
Pickling and plating rinse water. A surface-finishing line produces a continuous acidic rinse whose load is low per cubic metre and large in total, and whose pH swings as the line’s tanks are dumped. This is the classic continuous-flow duty, and it is the case for which published guidance is most explicit: continuous neutralization suits a flow that is stable in volume, pH and chemistry, a batch vessel suits the slug, and two vessels in series absorb the surge that neither alone can. The tank-side agitation duty belongs to the pickling agitation tank, and the rinse-water duty belongs here.
Wash water from spent battery and photovoltaic recycling. Leaching and washing circuits in spent lithium battery processing and in photovoltaic panel processing produce a wash water whose load comes from the leaching agent rather than from a mineral acid — sulphate, carbonate, chloride and, in the photovoltaic case, fluoride. That last species is what makes this scenario different, because fluoride disqualifies any glass-containing material and confirms a thermoplastic vessel. The upstream circuits producing these streams — the battery black mass and printed circuit board fractions these wash waters come from — are described under non-ferrous metal extraction equipment, whose stripping and etching effluent arrives with the same class of load and the same requirement for a vent that is designed rather than improvised, and the reactor treats what that line rejects.
Blowdown and bleed from the gas treatment train. A wet scrubber accumulates the salt it has just made and holds it down with a bleed stream, and that bleed is itself a liquor with a pH outside the discharge band and a salt concentration near the limit of the receiving route. Neutralizing a scrubber bleed is a small-volume duty with a very high salt load, which makes it a batch duty by nature, and it is the point at which this reactor and the gas treatment package meet. The scrubber that produces the bleed is described under acid and alkaline gas treatment equipment, and the reactor’s design case is the bleed’s own volume and strength.
Pilot, laboratory and campaign-scale duties. A pilot plant, a laboratory waste line or a plant that runs a neutralization campaign a few weeks a year needs the same chemistry in a smaller vessel, and it needs the same vapour discipline: the vent of a small reactor handling a nitric or chloride duty is a genuine gas treatment duty rather than a length of duct to the window. These duties are built as scaled-down versions of the same package, using the vent and instrumentation arrangements described under laboratory waste gas treatment equipment, with the control loop and the hazard sequence unchanged.
Surfactant and chemical manufacture, where the neutralization is the process rather than the clean-up. An anionic surfactant is made by neutralizing a sulphonic acid with a caustic solution, and the reactor is then the vessel the reaction is carried out in rather than the vessel an effluent is disposed of from. Three properties of this duty differ from every effluent duty on this page: the reaction is strongly exothermic and is run under heat removal rather than under a rate limit; the viscosity climbs steeply as the salt forms, so the agitator is selected from a viscosity the reaction itself creates rather than from the viscosity of the feed; and the product is judged on its active matter and its pH together, which makes the measurement chain a quality instrument as well as a control instrument. Where the product viscosity puts a top-entry agitator beyond its blending duty, a high-shear mixer or a circulation pump is used instead. The vessel is specified for atmospheric service with the heat removed, so a duty that genuinely requires pressure belongs to a pressure-rated unit rather than to this one. Output: a neutralized surfactant at its target active matter and pH, with the reaction heat removed and the dosing curve programmed to the titration of the acid intermediate.
Edible-oil refining, where the free fatty acids in a crude oil are neutralized. A crude vegetable oil carries free fatty acids whose concentration sets the refining loss, and chemical refining neutralizes them with a caustic solution so that they leave as a soapstock rather than remaining in the oil. The duty has two halves. The caustic must be dispersed finely enough to reach the acid rather than to saponify the neutral oil surrounding it, which makes the agitator a dispersion device at a controlled intensity rather than a machine run at the maximum blend the vessel can deliver; and the soapstock that forms has to be separated, which is a settling or centrifuging duty downstream of the reactor rather than a duty the reactor’s own volume is asked to perform. The caustic strength and the dose are fixed against the measured free fatty acid figure rather than against a nominal one. Output: a deacidified oil at a stated free fatty acid figure, and a soapstock drawn to the separation step that follows the reactor.
Where this reactor is not applied, and what is offered instead. A liquor whose peak acid load has never been titrated is not sized from an estimate, because a reactor bought against an unmeasured load holds its band only until the first concentrated slug arrives. A liquor still carrying recoverable metal above the site’s economic threshold is routed to recovery before neutralization, because the reactor destroys the solution rather than the value. A feed carrying appreciable undissolved solids is screened or settled upstream, because a blending agitator is the wrong agitator for a heavy slurry, and the vessel suited to that duty is a double-impeller leaching agitator tank rather than this reactor. A liquor that cannot be made steady and cannot be batched is a candidate for two vessels in series rather than one larger vessel, and a site whose problem is the gas rather than the liquid is served by the gas treatment pages instead.

Applicable Raw Materials

The feed to this reactor is a liquid, and specifying a liquid properly means stating more than its pH. Every stream below is described as it arrives at the reactor inlet — its acid or alkali load, its buffer content, its temperature, its salt concentration, its oxidant content, its organic content and its shape in time — because those seven properties, and not the pH reading alone, decide the reagent, the vessel, the agitator, the vent and the material. A stream is also assigned a hazard class at this point, because a cyanide, sulphide or oxidant-bearing liquor cannot be handled by the ordinary sequence no matter how ordinary its pH looks. Output in each case is the condition in which the liquor leaves the reactor, and the arrangement of the whole list is that the load is neutralized, the heat is carried away, the salt is accounted for and the vapour is routed.

Spent aqua regia and nitric parting liquor. The filtrate left after gold has been reduced out of a chloride or nitrate circuit, still carrying free nitric and hydrochloric acid, dissolved base metals, residual oxidant and a very high chloride content. It arrives hot, it outgasses above about five per cent nitric strength, and it cannot be vented to the room. Output: a neutralized chloride-and-nitrate brine inside the discharge band, a precipitated base-metal hydroxide where the load is high, and a nitrogen oxide bearing vapour stream routed to the gas treatment train.
Chloride barren liquor from precious-metal refining. The acidic mother liquor from a chloride circuit after platinum, palladium and rhodium have been separated and precipitated, carrying ammonium salts, residual reductant and traces of the metals that were not recovered. The circuit that produced it is described under precious metal extraction equipment, and this reactor handles what that circuit rejects. Output: a neutralized ammonium-chloride brine whose residual metal has already been assessed against the recovery threshold, and whose precipitates are drawn to the sludge route.
Sulphate tankhouse bleed from silver and copper electrorefining. A purified electrolyte bleed taken to hold iron, arsenic or antimony down, strongly acidic, warm and carrying dissolved metal at concentrations that make recovery worth examining before neutralization. Output: a neutralized sulphate brine, a hydroxide sludge where the metal was not recovered upstream, and a salt concentration held below the receiving route’s limit by the discharge rate.
Cyanide-bearing barren solution and stripping liquor. The liquor left after gold has been recovered from a cyanide leach, together with the strip liquor from a carbon or resin circuit. This feed never sees the acid side of a reactor. Published practice holds such solutions above a pH of about 10.5 so that the cyanide remains in solution as the cyanide ion and hydrogen cyanide gas is not formed, and treats cyanide destruction as a one or two stage chemical treatment — first the pH is raised and the cyanide is oxidized to cyanate, then the cyanate is oxidized to its end products — with the oxidant dosed against an oxidation-reduction potential setpoint held under mixing until the reaction stops consuming it. Output: a neutralized, oxidized liquor whose free cyanide has been destroyed, with the alkaline hold maintained throughout and the oxidant consumption recorded.
Thiosulphate, thiourea and glycine leach liquors. The non-cyanide alternatives to a cyanide circuit carry their own reagents, and their spent liquors are alkaline or mildly acidic and heavily buffered, which means the pH reading understates the load by a wide margin. Output: a neutralized liquor whose buffer has been titrated rather than assumed, and whose reagent demand is established from the sample instead of from a pH reading.
Alkaline strip liquor and eluate from a desorption circuit. The hot alkaline eluate produced when gold is stripped from loaded carbon or resin, which arrives warm, alkaline and carrying cyanide or hydroxide depending on the circuit. Its neutralization is an acid-side dose and its hazard is the same as the cyanide feed above. The circuit that produces it is the elution and electrowinning circuit described under gold electrowinning equipment. Output: a neutralized eluate whose acid dose was set from the titrated alkali load, and whose heat was carried away without cooling the vessel below the working range of its shell.
Ammonia- and ammonium-bearing liquor. A liquor carrying free ammonia or an ammonium salt, produced where ammonium chloride, ammonium hydroxide or ammoniacal leaching is used. Its neutralization is done on the acid side, and its product is an ammonium sulphate or ammonium chloride solution that has a destination of its own rather than being a discharge stream. Output: an ammonium salt solution at a defined strength, together with a vent sized for the free ammonia the liquor can release.
Raffinate from solvent extraction. The aqueous raffinate leaving a solvent-extraction circuit after the metal has been extracted, and the stream that carries the plant’s acid load most consistently because the extractants in this class of plant work at low pH. Two properties of this feed are decisive. The first is organic carryover: entrained extractant such as P204, P507 or C272 is a dilute solvent in an aqueous stream, and published guidance for concentrated acid service records that reactions between nitric acid and organic materials can be violent because of the acid’s oxidizing power, so the organic content is measured and the oxidant specification is settled before the reactor is built. The second is fouling: the same organic films over a pH probe and blinds it, so the measurement arrangement for this feed includes a cleaning provision rather than a bare probe. Output: a neutralized raffinate with its organic content controlled, and a probe whose calibration interval was set from the measured fouling rate rather than from a default.
Pickling and plating rinse water. A low-load, high-volume, intermittent stream that swings in pH as the line’s tanks are dumped, and the feed for which the continuous and batch distinction matters most. Output: a neutralized rinse water inside the band, produced either batch by batch where the line runs campaigns or continuously where the flow is steady, and arranged as two vessels in series where the surges cannot be made steady.
Fluoride-bearing etch and photovoltaic wash water. Etch waste from glass and semiconductor processing, and the wash water from photovoltaic panel treatment, both of which carry fluoride in addition to their acid load. Fluoride is the species that decides the material rather than the chemistry: published guidance for glass-lined equipment records that a glass lining resists most acids but not hydrofluoric acid, so a fluoride-bearing feed selects a thermoplastic vessel and simultaneously disqualifies a glass-lined one. The same guidance makes the enamel-lined vessel the correct choice for the aggressive dissolution duties its lining was designed for, which is the boundary described under enamel reactor. The circuit that produces the photovoltaic wash water is a photovoltaic panel recycling line, and its wash water carries the same fluoride load. Output: a neutralized fluoride-bearing liquor in a thermoplastic vessel, with the calcium fluoride or sodium fluoride it precipitates accounted for in the scaling control and the bleed.
Battery leaching and wash water. The wash water from a spent battery leaching circuit, whose load comes from the leaching agent and from the residual acid of the leach rather than from a mineral acid added by the operator, and which routinely carries dissolved lithium, nickel, cobalt and manganese at low concentration. Output: a neutralized wash water with the metal hydroxides it precipitates drawn to the sludge route, and with the soluble salt held below the receiving route’s limit.
Scrubber blowdown and tower bleed. The bleed taken from a wet scrubber’s liquor circuit to hold its salt concentration down, which is a small volume with a very high dissolved salt content and a pH outside the band. Output: a neutralized bleed whose salt has been diluted or held below the receiving route’s acceptance limit, and whose own discharge rate was fixed as part of the water balance rather than by a level switch.
Rare-metal and rare-earth leach raffinate. The acidic raffinate from a rare-metal or rare-earth leach and separation circuit, which combines a high acid load with a very high salt concentration and often with fluoride or sulphate that precipitates on neutralization. Output: a neutralized raffinate whose sparingly soluble salts have been managed by a scaling control and a bleed rather than allowed to deposit on the wetted surfaces.
Filter-cake wash water and hydroxide slurry liquor. The liquor that drains from a precipitated hydroxide or a filter cake during washing, which carries a low but persistent alkali load and a fine solid that the agitator must not be asked to keep in suspension. Output: a neutralized wash liquor with its suspended solid settled or filtered upstream, and with the vessel specified for blending rather than for solids suspension.
Sulphonic acid intermediates from surfactant manufacture. The acidic intermediate that is neutralized with a caustic solution to make an anionic surfactant such as SLES, arriving as a viscous acidic liquid whose neutralization releases a heat load comparable to that of a concentrated mineral acid and whose viscosity climbs by a wide margin as the salt forms. Output: a neutralized surfactant at its target active matter and pH, with the reaction heat removed and the agitator selected against the viscosity the product reaches rather than the viscosity of the feed.
Crude vegetable oil carrying free fatty acids. A triglyceride stream whose free fatty acid content sets the refining loss, neutralized with a caustic solution so that the acids leave as a soapstock rather than remaining in the oil. Its two governing properties are the free fatty acid figure, which sets the dose, and the readiness of the neutral oil to saponify if the caustic is over-dispersed, which sets the agitation intensity. Output: a deacidified oil at a stated free fatty acid figure, and a soapstock drawn to a separation step downstream of the reactor.
Feeds this reactor refuses, and the reason for each refusal. A feed whose peak acid load has not been titrated is refused, because a vessel sized on an estimate holds its band only until the first concentrated slug and the failure appears as a discharge excursion rather than as a mechanical fault. A feed still carrying recoverable metal above the site’s economic threshold is refused without a recovery step ahead of it, because raising the pH precipitates the value into a sludge and destroys the solution it came from. A feed carrying more than a trace of undissolved solids is refused, because the blending agitator this reactor needs is the wrong agitator for a heavy slurry. A feed containing a complexing agent that holds metals in solution is refused where the intent was to precipitate those metals, because the precipitation depends on the complex’s own stability rather than on the pH. A cyanide, sulphide, ammonia or oxidant-bearing feed is refused without its hazard sequence designed, because the ordinary sequence would liberate hydrogen cyanide, hydrogen sulphide or free ammonia in the vessel head. And a feed whose concentration of a species capable of a violent reaction with nitric acid has not been measured is refused until it has been, because published guidance records that those reactions can be violent or explosive.

Finished Products

The neutralization reaction converts an acid and a base into a salt and water, and everything that leaves the reactor is one of four things: a neutralized liquid, a salt dissolved in it, a solid precipitated out of it, or a vapour carried away from it. The list below is written in those terms, because the products of a neutralization reactor are obligations as much as they are outputs — each one has a destination, a limit and a recording requirement, and a reactor that reaches its pH band while leaving one of the four unaccounted for has not finished its job.

Neutralized effluent inside the discharge pH band. The primary product is a liquid whose pH sits inside the band the site is permitted to discharge, at the temperature and the salt concentration the receiving route allows. Published practice for this equipment class states the band in ordinary terms — a typical target of 6 to 9 is considered safe for disposal in a sewer system, with the acceptable level varying by location and by the destination of the discharge — and the site’s own permit governs over the general figure. Output: a discharge stream proved against the standard’s own basis and conditions, not against a remembered number.
A dissolved salt brine at a stated concentration. The reaction between an acid and a base makes a salt, and published guidance for neutralization states the behaviour that decides where that salt goes: when the cation is sodium and the reactants are relatively dilute, the products of neutralization remain soluble and can simply be discharged, whereas a calcium cation or elevated concentrations of both reactants produce precipitation products that drop out of solution. Output: a chloride, sulphate, nitrate or fluoride brine whose strength is held below the receiving route’s limit by the reactor’s discharge rate rather than by dilution at the drain.
A precipitated hydroxide sludge, where the feed carried metal. Where the feed carries dissolved metal above its hydroxide solubility, raising the pH precipitates it, and this solid rather than the liquid becomes the stream with the disposal cost. Output: a hydroxide sludge at a stated dry solids content, drawn to a thickening, filtration or dewatering step downstream of the reactor, with its volume, its metal content and its destination stated before the reactor is quoted.
Ammonium sulphate or ammonium chloride solution where the alkaline side is neutralized. An ammonia-bearing or ammonium-bearing liquor is neutralized with an acid rather than with an alkali, and its product is an ammonium salt solution that has a use or a disposal route of its own. Output: an ammonium salt solution at a defined strength, whose concentration is held by the acid dose and whose recovery or consignment route is settled as part of the specification rather than after commissioning.
A vapour stream with a defined destination. Every neutralization releases some water vapour and, depending on the feed, something more than water. A nitric-bearing liquor gives off nitric acid vapour and, from its decomposition, nitrogen oxides; an ammonia-bearing liquor can release free ammonia; a sulphide-bearing liquor acidified by mistake releases hydrogen sulphide. Output: a closed vent whose destination is a named treatment device, with the vapour-space material, the vent sizing and the nozzle arrangement all part of the reactor’s own drawing rather than the building’s.
Heat, carried away by the reaction’s own water or by a cooling arrangement. The neutralization of a strong acid by a strong base releases about 55 to 57 kilojoules per mole of water formed, and the working figure used in published concentrated-acid guidance is 55.83 kilojoules per mole of hydrogen ion. A dilute feed dissipates that heat through the water it already contains; a concentrated feed does not, and the reactor’s products then include the duty it passes to a cooler, a quench tie or a rate limit. Output: a vessel whose contents stay inside the working temperature of their own material, and a computed heat load that was checked before the shell was selected.
Calcium sulphate and calcium fluoride held in suspension rather than deposited. Where a lime route is used, the salts formed are only sparingly soluble, and the product that matters is the one the plant does not want: scale on the wetted surfaces, the agitator and the instruments. Output: a scaling control and a bleed take-off specified with the lime route, with the supersaturation created at the dosing point managed by the dilution tie and the discharge rate.
Water fit to be returned to the washing circuit. Where the neutralized liquor is a wash water or a rinse whose dissolved solids are low enough, the product is not a discharge but a reusable stream, and returning it reduces the site’s make-up demand and its discharge volume at the same time. Output: a recovered water stream whose dissolved solids and residual reagent are inside the acceptance limit of the circuit it returns to, such as the washing arrangement of a multi-stage washing concentrator.
A proved batch or a proved period, on a record. In a batch reactor the product is a batch that was proved before release, and in a continuous reactor it is a period whose measurement chain held its band without an excursion longer than the permit allows. Published practice for neutralization systems arranges this as continuous pH measurement at the discharge with recording, flow totalisation to track the treated volume and data logging to document compliance, and the record is a delivered item rather than an optional one. Output: a retained record carrying the batch or period identity, the pH and volume figures, the reagent consumed, the calibration state of the instruments and the release decision.
A neutralized surfactant at its target active matter and pH. Where the duty is a process duty rather than a clean-up duty, the product is the neutralized surfactant itself, and it is judged on two figures at once: the active matter the reaction has produced and the pH it has settled at. Output: a surfactant at a stated active matter and a neutral pH, with the reaction heat removed and the dosing curve programmed to the titration of the acid intermediate.
A deacidified oil and the soapstock it separates. Where free fatty acids are neutralized in a crude oil, the reactor produces two streams rather than one: the oil, whose free fatty acid content is the figure it is sold against, and the soapstock, which carries the acids out and has a value or a disposal route of its own. Output: a deacidified oil at a stated free fatty acid figure and a soapstock at a stated strength, with the separation of the two designed downstream of the reactor rather than bought from its residence time.
What the reactor specifically does not produce. It does not produce recovered metal, because the precipitate it makes is a sludge rather than a product and the solution it came from is destroyed by the precipitation; metal belongs to a recovery step ahead of the reactor. It does not produce a clean effluent, because neutralization controls pH and not dissolved solids, chemical oxygen demand or the metals still in solution below their hydroxide solubility. And it does not produce a settled sludge, because the settling, thickening and dewatering of a hydroxide precipitate are separate operations with their own residence times, and a reactor asked to do both is a reactor that does neither well.

Neutralization Reactor Process Flow

The nine stages below are the working sequence of a neutralization reactor, written at the level of the decisions the chemistry forces rather than at the level of the vessel’s dimensions. It is not commissioned by filling it and reading the probe, but by proving that the load was titrated rather than estimated, that the heat released stays inside the shell’s working temperature, and that the discharge figure was proved on the standard’s own basis.

Neutralization reactor working sequence — nine stages from the effluent survey that produces a titrated acid load rather than a pH reading, through the reagent equivalent and the vessel envelope it fixes, the agitation the control loop requires, the heat and vapour the reaction produces, and the salt and sludge destinations that close the design.

Steps 1 — Effluent Survey and Acid Load Titration. The design starts with a list of every liquid stream the reactor will see, and the first field is not the pH but the load: the titrated acid or alkali content in moles of hydrogen ion per litre or in kilograms of calcium carbonate equivalent per cubic metre, with the buffer content that explains why a pH reading understates it. That load is titrated on a representative sample against the target band of the site’s own permit, because the band decides where the equivalence point of the design lies. The survey names the governing simultaneous combination — the streams discharged at the same time, not the sum of the ratings — and records each stream’s hazard class, because those classes change the sequence that follows. It refuses a load inferred from a pH meter, and refuses to proceed without an answer to whether recoverable metal is still present above the site’s economic threshold. Setting and utility input: the effluent list with volumes and temperatures, the operating schedule that fixes which streams coincide, and a sample of each stream. Output: a titrated load per stream, the buffer quantified, the hazard class recorded, and the recoverable-metal question answered.

Steps 2 — Reagent Selection and Equivalent Dose. The reagent is chosen from what the reaction makes and where that product has to go, and only then from what it costs. On an acid-side duty the reagents in ordinary use are sodium hydroxide and hydrated lime; on an alkaline-side duty, a dilute mineral acid. The choice is decided by the salt it creates: sodium makes a salt that stays in solution and leaves with the discharge, while calcium makes a salt that is only sparingly soluble in the sulphate and fluoride cases and deposits on the wetted surfaces unless a scaling control and a bleed are specified to match. The equivalent dose follows from the titration — one mole of sodium hydroxide neutralizes one mole of a monobasic acid, one mole of calcium hydroxide neutralizes two — with the buffer added as an explicit allowance, so what is fixed here is a dose range and a consumption per kilogram of acid neutralized. Setting and utility input: the titrated loads from Step 1, the permit’s limit on the salt the reagent would make, and the reagents available with their price. Output: a reagent specification and dose range, a consumption per kilogram of acid neutralized, and a bleed rate for a calcium reagent.

Steps 3 — Vessel Sizing and Material Envelope. The working volume follows from two figures, not a catalogue number: the batch size where the duty is a batch, or the residence time the control loop needs where the duty is continuous. Published practice for neutralization tanks states a residence time of fifteen to sixty minutes, longer for waste that is difficult to neutralize, sizes the operating capacity for 110 to 150 per cent of the maximum flow, and describes one, two and three stage arrangements that absorb a slug rather than pass it on. The material envelope is set here too: thermoplastic for the ordinary acid and alkali duties inside its temperature band, glass-reinforced plastic where a larger shell or a higher service temperature is required, and stainless steel where the service is oxidizing or sanitary. One exclusion is absolute: glass-lined equipment resists most acids but not hydrofluoric acid, so a fluoride-bearing feed confirms the thermoplastic vessel. Setting and utility input: the batch size or flow and residence time from Steps 1 and 2, the peak-to-average ratio, and the liquor’s fluoride content. Output: a working volume and stage count, a freeboard for the swell, and a material envelope with its temperature limit and exclusions.

Steps 4 — Agitation, Turnover and Blend Time. Neutralization is a mixing duty before it is a reaction duty: the reaction is fast while the dispersion of the reagent into the bulk is not, and an undersized agitator shows up as a probe reading the band while the vessel does not. Published guidance for neutralization mixing states the requirement as something to check rather than argue about: at least sixteen complete tank turnovers within the residence time, so a vessel holding sixteen minutes of flow needs a mixer whose pumping capacity equals the vessel volume every minute. The impeller is therefore selected for pumping and blending rather than shear, and the injection point belongs in the impeller’s discharge stream rather than at the surface or the wall, so the reagent is dispersed into moving bulk liquid instead of reacting locally in a pocket at an extreme pH. Where the liquor carries solids, the duties are separated: a heavy slurry needs a suspension duty this reactor is not specified for, and the feed is settled or filtered upstream instead. Setting and utility input: the working volume and geometry from Step 3, the liquor’s viscosity, density and solids content, and the injection rate from Step 2. Output: an impeller with its diameter, speed and power, a baffle arrangement, a turn-over count against the residence time, and the solids duty reassigned.

Steps 5 — Heat Load and Vapour Route Design. The heat of neutralization of a strong acid by a strong base is about 55 to 57 kilojoules per mole of water formed; published concentrated-acid guidance works from 55.83 kilojoules per mole of hydrogen ion. Multiplied by the titrated load from Step 1 that figure gives the heat released, and the same guidance supplies the rule that decides whether it matters: the acid concentration is usually too low for the rise to concern anyone, but above about one per cent the rise is measurable, and a ten per cent and a seventy per cent nitric solution are named as cases whose rise can melt thermoplastic piping and risk steam explosions. Where the computed rise would carry the liquor past the shell’s working temperature, the design answers with continuous temperature monitoring, control of the reaction rate and the addition of quench water, and specifies the rate limit or the cooler rather than promising a temperature. The vapour route follows from the liquor rather than the building: above about five per cent the outgassing of nitric acid is significant and requires good ventilation, its decomposition liberates toxic nitrous oxide gases, and it reacts violently with organic materials. Setting and utility input: the titrated load from Step 1 and the shell’s material and working temperature from Step 3. Output: a computed heat load, a quench or rate-limit arrangement where the margin is thin, and a closed vent with its destination.

Steps 6 — pH Loop, Sensor and Control Law. The loop is built around the shape of the response rather than a controller’s default, because pH responds logarithmically and the gain therefore changes by orders of magnitude between the ends of the titration curve and its middle. Published guidance for pH control is blunt: most controllers on the market are not truly pH controllers because they use simple on and off control or the linear control of a proportional-integral-derivative algorithm, which is far too simple to account for the ever changing control gain of a logarithmic response, and a true pH controller must control to a curve such as the site’s own titration. The design therefore has a coarse stage that takes the bulk of the load into the band and a trim stage that holds it, each with its own dosing device and tuning, as published practice describes in one to three stages. The sensor is specified with the three things that decide whether its reading can be trusted: its position in bulk liquid rather than near the injection point, its cleaning arrangement where the liquor fouls, and its calibration interval. Setting and utility input: the titration curve measured on the customer’s own sample, the dose range from Step 2, and the volume and blend time from Steps 3 and 4. Output: a sensor specification with its position, cleaning and calibration interval, a two-stage coarse and trim loop, a tuning curve programmed to the measured titration, and a manual override with a rate limit.

Steps 7 — Hazard Sequence and Interlock Chain. Some liquors are more dangerous when their pH is changed than when it is left alone, and this stage writes the order for each. A cyanide-bearing liquor never sees the acid side: published practice holds it above a pH of about 10.5 so that cyanide stays in solution and hydrogen cyanide gas is not formed, and describes destruction as a one or two stage treatment that first oxidizes the cyanide to the far less toxic cyanate and then the cyanate to its end products, with the oxidant dosed to an oxidation-reduction potential setpoint. A sulphide-bearing liquor is kept alkaline so that hydrogen sulphide is not released; an ammonia-bearing liquor is neutralized on the acid side and vented; and a concentrated feed is introduced at a rate that holds the computed temperature rise. The interlock chain is then defined against those sequences — what the plant does when the pH leaves its band, when the oxidation-reduction potential setpoint is missed, when the reagent strength fails, or when the discharge reading exceeds the permit — and which condition alarms, which holds the batch, which ramps the feed down and which trips it. Setting and utility input: the hazard class and titration of each stream from Step 1, the reagent and oxidant specification from Step 2, and the plant’s alarm philosophy. Output: a written sequence for each hazard class, an interlock schedule with a defined action for each condition, and a rate limit written as a number.

Steps 8 — Salt Balance and Sludge Handling. The reaction converts the pollutant into a dissolved salt, and this stage decides where that salt and any precipitate go before discharge is promised. Published guidance gives the rule: with sodium as the cation and dilute reactants the salt stays soluble and can be discharged, while a calcium cation or elevated concentrations of both produce precipitation products that drop out. The balance is written stream by stream: the soluble salt’s concentration against the receiving route’s limit, the bleed rate that holds it below that limit rather than the dilution needed at the drain, and the water the reactor adds or removes. Where the feed carried dissolved metal, the hydroxide it precipitates becomes a sludge with a volume, a dry solids content and a destination, and its settling, thickening and dewatering are a separate step with its own residence time. Where a calcium reagent is used, the sparingly soluble sulphate and fluoride salts are held by the scaling control and the bleed specified in Step 2. Setting and utility input: the titrated load and reagent stoichiometry from Steps 1 and 2, the salt solubilities involved, and the receiving limits. Output: a discharge specification with its salt concentration and volume, a bleed rate sized against the receiving limit, a sludge specification with its dry solids, and a water balance.

Steps 9 — Batch Acceptance and Record Handover. The outlet figure is a measurement with a species, a basis, an instrument and a record, and this stage builds all four before the reactor is released to the drain. The measurement chain is completed: continuous pH at the discharge with recording, flow totalisation so the treated volume is known rather than assumed, reagent consumption logged against the batch, and each instrument’s calibration state recorded. The acceptance test is then run under the plant’s own duty rather than on water: a batch or flow equal to the design case from Step 1’s governing combination, a titration of the feed taken at the same time, a dose range check, a turn-over verification from the agitator’s measured performance, and a discharge figure proved on the standard’s own basis. Where a batch fails, the failure is recorded as a load or a mixing finding rather than an instrument error, because the two have different remedies. What is handed over is the operating envelope the vessel was proved in, the reagent consumption measured, and the calibration interval the probe needs on this liquor. Setting and utility input: the permit requirements for measurement, reporting and retention, the acceptance-test load, and the calibration standards. Output: a monitoring arrangement with sampling points, instruments and calibration intervals, a commissioning record, and an operating envelope with the loads it was proved against.

Product Structure (Main Equipment)

The package is described below in the order a buyer meets it on the drawing: the vessel and its containment first, then the agitator that makes the reaction possible, then the reagent system that feeds it, then the measurement chain that decides when to stop, then the vent that carries away what the liquor gives off, and finally the arrangement that takes the product out. Each group is presented with the reason it exists rather than as a list of parts, because every item in it was fixed by one of the nine stages above and a part specified without its reason is a part that gets substituted at the first cost review.

Vessel, head and containment

The vessel is a cylindrical shell with a flat, dished or conical bottom chosen from what the product does rather than from what is cheapest to roll: a conical bottom wherever a hydroxide sludge or a sparingly soluble precipitate has to be drawn out completely, a flat bottom where the duty is a clean liquor and the draw is by pump, and a dished bottom where the vessel is the larger of a multi-stage arrangement. Published practice for this class of vessel places the ordinary service at atmospheric pressure down to a small vacuum rather than at pressure, and the shell’s thickness and its reinforcement are therefore set by its own hydrostatic load, by the vacuum it may see and by the wind and seismic load where it stands outdoors, not by an internal pressure. The head carries the agitator’s mounting flange, the reagent injection nozzle, the instrument tappings, the vent and the manway, and its arrangement is fixed by the ones that need to point somewhere: the vent must leave the highest point of the vapour space so that it does not become a liquid trap, and the instrument tappings must reach bulk liquid at the lowest level the vessel is allowed to operate at. Secondary containment is part of the vessel rather than part of the building, which means a bund or a double wall with a leak indication capable of holding the largest single volume the vessel can release, and published practice for batch neutralization systems with demanding chemical duty describes double containment with leak detection as the standard arrangement for exactly this reason. The level range between the normal low level and the normal high level is stated on the drawing, because the working volume is the volume between those two marks and not the volume of the shell.

Agitator, baffling and the injection point

The agitator is a top-entry machine in the ordinary case, with a shaft supported by the drive’s bearings and by a steady bearing at the bottom where the shaft’s length and the vessel’s geometry require one, and it is selected from the pumping and blending duty established in Step 4 rather than from a shear or dispersion duty. The impeller’s diameter and its position are the two variables that decide whether the lower part of the vessel is mixed or merely stirred: an impeller set too high and too small leaves a stagnant zone beneath it, and the probe mounted in bulk liquid will then report a band that the lower zone has not reached. Baffles are fitted to break the rotating flow that would otherwise let the vessel’s contents turn with the impeller as a solid body, and their number and width are part of the agitator’s selection rather than an accessory, because the turn-over count the mixing requirement is stated in cannot be delivered in an unbaffled vessel. The wetted materials of the shaft, the impeller, the fasteners and the steady bearing are chosen from the same material envelope as the shell, with the steady bearing named separately because it is the part most often specified in a different material for wear reasons and most often forgotten at the ordering stage. The injection point is a nozzle rather than a pipe end, and it is placed so that the reagent enters the impeller’s discharge stream, which disperses it into moving bulk liquid in the shortest possible distance and prevents the local excursion that a reagent dropped onto the surface or sprayed against a wall will always produce.

Reagent storage, dosing and dilution

The reagent side of the package is storage, transfer, metering and dilution, and each has its own material and its own containment obligation. Storage is a vessel in the same material family as the reactor where the reagent is dilute, and in a material chosen for the concentrated reagent where it is not, with its own bund and, where the reagent is an oxidant, its segregation from anything it can oxidize — published guidance for reagent handling in this service treats the acid, alkali and oxidant store as three separate arrangements rather than as one chemical area. The metering device is a positive-displacement pump or a control valve with a characterised trim, chosen from the dose range of Step 2 and from the turndown the loop of Step 6 needs, and it is arranged with a calibration means, a strainer, a non-return valve and a spare installed or on the shelf, because the reagent line is the part of a neutralization plant that stops it most often. Where the reagent is a solid or a slurry rather than a solution, the dosing device is a dry feeder of the same class as a disc-type zinc powder feeder, with its own hopper, its own level indication and its own accuracy statement, because a solid reagent metered by a screw without a stated accuracy is a loop that cannot be closed. Where the reagent arrives concentrated and the reaction’s heat or the material’s temperature limit requires it to be diluted before it is added, the dilution tie is part of the dosing skid and its water is accounted for in the balance of Step 8.

Measurement and the control loop

The measurement chain is smaller than it looks and more decisive than it looks. It comprises a pH probe mounted where it sees bulk liquid at a representative temperature, where the hazard sequence requires it an oxidation-reduction potential probe for the oxidant dose, a temperature measurement for the heat calculation and the interlock, a level measurement with its high and high-high alarm, and a flow measurement on the feed and on the reagent. The probe’s position is the part of the chain that is most often wrong: a probe placed in the reagent’s path reads the local reaction rather than the vessel, and a probe placed in a quiescent corner reads a mixture that has been standing for a while rather than the mixture the agitator just made, and either error appears as a loop that cannot be tuned rather than as an instrument fault. The probe’s cleaning and calibration arrangement is specified with the fouling tendency of the liquor, which is the reason a raffinate carrying residual extractant such as P204, P507 or C272 is given a washable or withdrawable mounting and a shorter calibration interval than a clean rinse: a film of organic on the glass is a reading that drifts slowly and then fails a discharge without ever looking like a fault. The controller is the plant’s own platform wherever it has one, programmed with the two-stage structure and the tuning curve of Step 6, with the dosing device’s characteristics entered rather than assumed, and with a manual override that carries a rate limit so that a manual correction cannot reproduce the excursion the automatic loop was built to avoid.

Vent, vapour route and vapour-space material

The vent is a designed item rather than a nozzle. Its sizing follows the vapour the liquor can actually release — water vapour from a warm feed in every case, nitric acid vapour and nitrogen oxides from a nitrate-bearing feed, free ammonia from an ammoniacal feed, hydrogen sulphide from a sulphide-bearing feed that has been acidified by error — and its destination is a named device rather than a length of duct: a scrubber or absorption tower of the class described under flue gas absorption tower for an ordinary acid or alkali duty, and a nitrogen oxides train for the nitrate-bearing one. The vapour-space material, the vent’s own material and the condensate drain are fixed by the same species, which is why a reactor handling a chloride duty and a reactor handling a nitrate duty are not the same vessel even when their loads match. A demister or a droplet separator is fitted where the vent is short enough for entrained liquid to reach the downstream device, and the vapour space is given a wash provision wherever the vapour can deposit a salt on the shell above the liquid line. The vapour route is also the reason the reactor’s boundary is drawn at a flange and not at the vessel wall: what leaves through the vent has a destination, a material and a limit of its own, and it belongs to the package in the same way the discharge does.

Sludge draw, discharge and level control

The outlet side of the package is the part that decides whether the reactor is emptied cleanly. One draw is provided for the treated liquor, taken at a level above the settled solids where a precipitate is produced, and a second is provided for the sludge or the precipitate, taken at the lowest point of a conical bottom with a valve sized for a thickened slurry rather than for a clear liquid. Where the duty produces a hydroxide sludge, the draw is arranged so that it can be taken while the agitator is running or after a defined settling period, and the choice is written into the operating procedure rather than left to the operator, because the two give different sludge concentrations and different discharge qualities. The discharge arrangement carries the flow measurement and the sampling point of Step 9 and, where the permit requires it, the pH measurement is taken after the last point at which the liquor can still be recirculated rather than at a point where a correction is still possible, so that the recorded figure is the figure that was actually discharged. Level control is arranged with a normal high and low range, an independent high-high alarm, and an overflow that is directed to containment rather than to the yard, and the interlock schedule of Step 7 is what ties the level, the temperature, the agitation and the reagent together so that a single failure holds the batch instead of releasing it.

Product Features

Sized from a titrated acid load rather than from a pH reading or a tank volume. The load is measured on the customer’s own sample against the band of the customer’s own permit, and it is the figure the working volume, the stage count, the reagent equivalent, the dosing range and the heat computation are all derived from. A quotation that names a volume and a flow has left the two variables the reactor is actually sized on unstated, and the difference between the two approaches appears at the first concentrated slug rather than at the drawing review.
Batch, continuous and multi-stage arrangements are built from one engineering team and arbitrated on the feed. A batch vessel where the effluent arrives in slugs or a campaign has to be proved one batch at a time, a continuous stirred vessel where the flow is steady in volume, pH and chemistry, and two or more vessels in series where a surge cannot be made steady and has to be absorbed as it passes. The arbitration is written down with the rejected arrangement and the reason, so that the mode follows from the plant rather than from the product line.
The pH loop is programmed to the customer’s own titration curve, not to a controller’s default. A coarse stage takes the bulk of the load into the band and a trim stage holds it, each with independent tuning set against the measured curve, because the gain of a pH loop changes by orders of magnitude between the flat and the steep part of the curve and a linear tuning that behaves well in one region overshoots violently in the other.
The agitator is specified for the blend time the loop needs, and the turn-over count is delivered as a figure. Published practice for neutralization mixing requires at least sixteen complete vessel turnovers within the residence time, and the impeller’s diameter, its speed, its immersion and the baffling are chosen to deliver that figure, with the reagent injected into the impeller’s discharge stream so that it is dispersed before it can react locally.
The heat of neutralization is computed from the load before a material is selected. At about 55.83 kilojoules per mole of hydrogen ion, a concentrated feed releases enough heat to carry a thermoplastic shell past its working temperature, and published guidance for concentrated acid service records temperature rises large enough to melt thermoplastic piping. The computed rise, the shell’s limit and the quench, cooling or rate-limit arrangement are presented together, and where the feed cannot be introduced slowly enough for the limit the reactor is specified with a rate figure rather than with a promise.
The vent is part of the reactor rather than part of the building, and it has a named destination. An ordinary acid or alkali duty vents through the plant’s scrubber; a nitrate-bearing duty is given a closed head and a route to the nitrogen oxides train, because published guidance records that its outgassing above about five per cent strength is significant, that its decomposition liberates toxic nitrous oxide gases and that its reactions with organic materials can be violent. The vapour-space material and the condensate drain follow the same species list.
The hazard sequence is written down rather than implied, and it is interlocked as well as documented. A cyanide-bearing liquor is held above a pH of about 10.5 before any oxidant is admitted, so that hydrogen cyanide is never formed in the vessel head, and its destruction runs in two stages with the oxidant dosed against an oxidation-reduction potential setpoint held under mixing. A sulphide-bearing liquor is kept alkaline, an ammonia-bearing liquor is vented for free ammonia, and a liquor carrying residual organic from a solvent-extraction circuit is measured for that organic content before an oxidant is specified.
The material envelope is delivered as a list of exclusions rather than as a preference. Thermoplastic construction for ordinary acid and alkali service inside its temperature band and with no lining to fail; glass-reinforced plastic where the shell is large, the temperature is high or the installation is outdoors; stainless steel where the service is oxidizing or sanitary; and no glass-containing material wherever fluoride is present, which is a published limitation of glass linings rather than a commercial preference.
The salt balance and the sludge route are closed in figures rather than described in principle. The soluble salt’s concentration in the discharge, the rate that holds it below the receiving route’s limit, the water the reactor adds through reagent dilution and quench, the volume of any hydroxide sludge and its dry solids content, and the scaling control and bleed that a calcium reagent requires — each stated as a number, because a salt balance without a concentration is not a balance.
The measurement chain is delivered with its cleaning and its calibration rather than as a bare probe. The probe’s position where it sees bulk liquid, its cleaning provision on a liquor that fouls, its buffer solutions, its spare and its calibration interval are all part of the package, and the interval is set from the measured fouling rate of the actual liquor rather than from a default, because a film of organic or scale on the glass is the failure mode that drifts slowly and then fails a discharge without ever looking like an instrument fault.
The consumables and the operating envelope are handed over with the acceptance record. The wetted parts of the dosing line, the reagent filters, the probe, its buffers, the gaskets and the shaft seal are listed with their intervals, and the acceptance test is run on the plant’s own duty rather than on water so that the reagent consumption is measured rather than assumed and the operating envelope is issued with the loads it was proved against.
The boundary is published with the quotation and the honest alternative is offered with it. A liquor still carrying recoverable metal above the site’s economic threshold is directed to recovery before neutralization; a feed whose peak load has never been titrated is not sized from an estimate; a feed carrying appreciable undissolved solids is routed to a suspension vessel rather than to a blending one; and a cyanide, sulphide or ammonia-bearing liquor without a designed hazard sequence is not quoted as an ordinary acid duty. Each refusal is accompanied by the route that does fit, because a refusal without an alternative is a lost customer and a refusal with one is a specification.

Technical Parameters And Specifications

The tables below separate three things that are usually run together: what is specified for a site, what is measured on it, and what the published literature establishes independently of either. Values that belong to a site — load, flow, temperature, space and services — are given as engineered quantities together with the basis on which they are calculated, because a reactor quoted with a fixed volume and a fixed flow is a reactor quoted for a plant that does not exist. Values that belong to the chemistry — the heat of neutralization, the titration behaviour of pH, the published residence and turn-over requirements, the solubility behaviour of the salts formed and the material limitations — are given with their published basis so that they can be checked independently of this page.

Table 1 — Unit and system specification

Parameter	Specification
Equipment type	Neutralization reactor package: vessel with agitation and baffling, reagent storage and dosing, pH and oxidation-reduction potential measurement, closed vent with a named destination, sludge and discharge arrangement, control panel with sequence and interlocks
Duties covered	Acid-side neutralization of an acidic liquor with an alkaline reagent; alkaline-side neutralization of an alkaline liquor with a dilute acid; two-stage destruction of a cyanide-bearing liquor; alkaline hold of a cyanide, sulphide or ammonia-bearing feed before any other step
Liquors handled	Refining barren liquor, aqua regia and nitric parting effluent, tankhouse bleed, spent leach and strip liquor, rinse and wash water, solvent-extraction raffinate, scrubber blowdown, filter-cake wash water
Load basis	Titrated acid or alkali load in moles of hydrogen ion per litre, or in kilograms of calcium carbonate equivalent per cubic metre, measured on a representative sample against the band of the site’s own permit
Buffer allowance	Measured on the same sample and stated separately, because a buffered liquor holds its pH down while its load rises and a pH reading therefore understates the reagent demand
Operating mode	Batch where the effluent arrives in slugs, where a plant runs campaigns or where each batch has to be proved; continuous stirred where the flow is steady in volume, pH and chemistry; two or more vessels in series where a surge cannot be made steady
Stage count	One, two or three stages, with the coarse stage taking the bulk of the load and the trim stage holding the band; published practice for this class of equipment describes one, two and three stage arrangements
Residence time	Published practice for neutralization service: 15 to 60 minutes, with longer times for waste that is difficult to neutralize or whose pH varies widely
Operating capacity basis	Published practice for neutralization service: sized for the peak flow with a margin, typically designed for 110 to 150 per cent of the maximum flow
Turn-over requirement	Published practice for neutralization mixing: at least 16 complete vessel turnovers within the residence time, so that the mixer’s pumping capacity equals the working volume once per residence-time interval
Blend time	Delivered as a computed figure from the impeller’s pumping capacity and the working volume, and reconciled with the control loop’s tolerable dead time
Heat of neutralization	Published figure for a strong acid and a strong base: about 55 to 57 kJ per mole of water formed, taken as 55.83 kJ per mole of hydrogen ion in published concentrated-acid guidance
Temperature-rise threshold	Published guidance: below about 1 per cent acid concentration the temperature rise is generally not a concern, and above it the rise is measurable and can be of concern
Working temperature	Set by the material envelope and by the computed heat load, with a stated margin and with a quench, cooling or rate-limit arrangement where that margin is thin
Reagent, acid side	Sodium hydroxide solution as the common alkaline reagent, or hydrated lime where the cost balance favours it and a scaling-control means and a bleed are provided; sodium carbonate or carbon dioxide where a milder alkali is wanted
Reagent, alkaline side	Dilute mineral acid, selected against the salt it produces and the receiving route’s acceptance limits
Oxidant, where required	Hypochlorite or another oxidant for a cyanide-bearing liquor, dosed against an oxidation-reduction potential setpoint held under mixing until the consumption stops
Measurement chain	pH at a bulk-liquid position, oxidation-reduction potential where an oxidant is used, temperature, level with an independent high-high alarm, feed flow and reagent flow
Control structure	Coarse stage and trim stage with independent tuning, programmed to the site’s own measured titration curve rather than to a linear gain, with a manual override carrying a rate limit
Vent arrangement	Closed head with a sized vent and a named destination; vapour-space material fixed by the species the liquor can release; demister or droplet separator where the run is short; wash provision where the vapour can deposit salt
Product outlets	Treated liquor, sludge or precipitate, ammonium salt solution where the alkaline side is neutralized, and a routed vapour stream
Discharge proof	Continuous pH at the discharge with recording, flow totalisation of the treated volume, reagent consumption logged per batch or per period, instrument calibration state recorded with its date
Construction	Thermoplastic for ordinary acid and alkali service inside its temperature band; glass-reinforced plastic for a larger shell, a higher temperature or an outdoor installation; stainless steel for oxidizing or sanitary service; no glass-containing material where fluoride is present
Capacity	Engineered per project against the titrated acid or alkali load of the governing simultaneous combination, the peak-to-average ratio of the flow, the residence time the control loop requires and the batch size the plant discharges, with the working volume, the number of stages and the shell diameter all fixed from those figures; no fixed treatment volume is quoted in advance
Utilities	Electrical supply for the agitator, the dosing pumps and the control system; process water for reagent dilution, quench and probe cleaning; drainage for the treated liquor and the sludge; steam or cooling water where the heat balance requires one; compressed air where a valve or an instrument needs it
Footprint	Engineered against the shell diameter and height, the reagent and dosing area with its containment, the sludge holding volume, the vent route and the access the internals, the manway and the instrument tappings require
Boundary	Inlet flange for the feed liquor, discharge flange for the treated liquor, a flange for the sludge draw, a flange for the vent, and the reagent receiving connection; the settling, thickening and dewatering of a sludge, the recovery of metal and the treatment of any gas are outside this battery limit

Table 2 — Acid load, titration and reagent equivalents

Quantity	Basis and value
Load expression	Moles of hydrogen ion per litre, or kilograms of calcium carbonate equivalent per cubic metre, from a titration of a representative sample to the site’s own target pH
Why the titration is required	pH is logarithmic and a buffered liquor holds its pH down while its load rises, so two liquors at the same pH can carry loads differing by an order of magnitude
Sodium hydroxide equivalence	One mole of NaOH neutralizes one mole of a monobasic acid: NaOH + HNO₃ → NaNO₃ + H₂O
Calcium hydroxide equivalence	One mole of Ca(OH)₂ neutralizes two moles of a monobasic acid: Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O
Nitric acid behaviour	Published technical guidance: HNO₃ is a very strong acid that fully dissociates at all concentrations, is a one-normal solution, and reacts with an inorganic base conventionally
Nitric acid outgassing	Published technical guidance: above about 5 per cent concentration the outgassing of HNO₃ can be quite significant and good ventilation is required
Nitric acid as an oxidant	Published technical guidance: its decomposition liberates toxic nitrous oxide gases, and its reactions with organic materials can be violent or explosive because of its oxidizing power
Soluble salt case	Published guidance: where the cation is sodium and the reactants are relatively dilute, the products of neutralization remain soluble and can be discharged
Precipitating salt case	Published guidance: where the cation is calcium, or where the concentration of both reactants is elevated, precipitation products form as insoluble salts
Fluoride service	Published guidance for glass-lined equipment: the glass lining resists most acids but not hydrofluoric acid, so fluoride selects a thermoplastic vessel and disqualifies a glass-containing one
Cyanide-bearing feed	Published practice: hold above about pH 10.5 so that cyanide remains in solution as the cyanide ion and hydrogen cyanide gas is not formed; then oxidize in two stages, cyanide to cyanate and cyanate to its end products
Oxidant endpoint	Published practice: the oxidant is dosed against an oxidation-reduction potential setpoint of about +300 mV and held there under mixing until the consumption stops
Reagent consumption	Delivered as kilograms of reagent per kilogram of acid neutralized, measured on the first campaign and compared with the quotation
Reaction rate versus dispersion	The neutralization itself is fast; the rate that governs the vessel’s size is the dispersion of the reagent into the bulk, which is why the mixing requirement is stated in turn-overs rather than in minutes

Table 3 — Vessel, geometry and material envelope

Parameter	Value or basis
Shell form	Vertical cylindrical shell as the default; flat, dished or conical bottom selected from the product — conical wherever a precipitated sludge has to be drawn out completely
Working volume	The volume between the normal low-level and normal high-level marks, not the volume of the shell; derived from the batch size or from the flow and the residence time
Freeboard	Sized for the foam and the level swell the liquor produces at the dosing rate and the agitation level specified, and stated on the drawing
Pressure basis	Published practice for this class of vessel: atmospheric service down to a small vacuum rather than pressure, with the shell’s thickness set by hydrostatic, vacuum, wind and seismic load
Temperature basis	Set by the material envelope and by the computed heat load of Table 1, with the working range of the chosen material stated explicitly
Agitation	Top-entry machine with an impeller selected for pumping and blending, baffles to prevent the rotating flow that would otherwise starve the lower vessel, and a steady bearing where the shaft’s length requires one
Injection point	A nozzle placed in the impeller’s discharge stream, so that the reagent is dispersed into moving bulk liquid before it can react locally
Level range	Normal high and low, an independent high-high alarm, and an overflow directed to containment rather than to the yard
Secondary containment	Bund or double wall with leak indication capable of holding the largest single volume the vessel can release; published practice for demanding chemical duty describes double containment with leak detection
Thermoplastic envelope	Polypropylene and polypropylene homopolymer for ordinary acid and alkali service inside their temperature band; the standard range in this class of vessel runs from 0.05 to 5 cubic metres, with larger duties arranged as multiple vessels
Glass-reinforced plastic	Where a larger shell, a higher service temperature or an outdoor installation is required
Stainless steel	Where the service is oxidizing or sanitary, with the grade selected against the actual halide and oxidant content rather than against a generic corrosion table
Excluded materials	Glass-containing materials wherever fluoride is present, and any material whose temperature limit the computed heat load would exceed at the specified dosing rate
Sludge draw	Lowest point of a conical bottom with a valve sized for a thickened slurry, arranged so that it can be taken while the agitator runs or after a defined settling period, with the choice written into the procedure

Table 4 — pH, control and measurement bands

Parameter	Value or basis
Discharge band	The site’s own permit governs; published practice for this equipment class states a typical target of pH 6 to 9 as safe for disposal to a sewer, with the acceptable level varying by location and destination
Control loop shape	Non-linear: published guidance for pH control states that pH response is not linear and cannot be controlled as such, and that simple on-off control or a linear PID algorithm is far too simple to account for the ever changing control gain
Control law	A tuning curve programmed to the site’s own measured titration curve, with the acid and reagent devices of each stage tuned independently; published practice for neutralization systems describes this explicitly
Coarse stage	Takes the bulk of the load into the band, with a dosing device sized for the titration’s steep middle section
Trim stage	Holds the band against drift and disturbance, with a dosing device sized for the small remaining load and a correspondingly finer resolution
Probe position	In bulk liquid at a representative temperature, away from the reagent’s injection path and away from a quiescent corner, so that the reading represents the vessel rather than the region around the nozzle
Probe fouling provision	Washable or withdrawable mounting and a shortened calibration interval wherever the liquor carries residual organic — for example entrained P204, P507 or C272 from a solvent-extraction raffinate — or a scale-forming salt
Calibration	Two-point calibration with fresh buffer solutions, a recorded date and value, and a stated interval set from the measured fouling rate of the actual liquor rather than from a default
Oxidation-reduction potential	Required wherever an oxidant is dosed, with a setpoint of about +300 mV in published practice for cyanide destruction and the measurement held under mixing
Temperature	Measured in the vessel and interlocked against the material’s limit at the working concentration, with the heat load of Table 1 as its reference
Level	Measured with a normal range and an independent high-high alarm, interlocked to hold the feed rather than to release the batch
Flow	Measured on the feed, so that the treated volume is known and logged rather than assumed, and on the reagent, so that consumption is measured per batch or per period
Recording	Continuous pH at the discharge with recording, flow totalisation and data logging, which published practice for neutralization systems arranges as standard equipment rather than as an option

Table 5 — Hazardous liquor sequencing

Feed class	Required sequence and the reason for it
Cyanide-bearing	Raise the pH above about 10.5 first and hold it there, because free cyanide is in equilibrium with hydrogen cyanide and the equilibrium is set by pH; then oxidize in two stages, cyanide to cyanate and cyanate to its end products, with the oxidant dosed against an oxidation-reduction potential setpoint held under mixing. Never admit this feed to the acid side and never acidify it in the vessel
Sulphide-bearing	Hold on the alkaline side and never acidify, because acidification releases hydrogen sulphide; vent the vessel head for it and interlock the reagent against an unintended acid dose
Ammonia- or ammonium-bearing	Neutralize on the acid side with the product routed as an ammonium salt solution, and vent the head for the free ammonia the liquor can release; keep any oxidant segregated from the ammonium liquor
Nitrate-bearing	Conventional neutralization with any inorganic base, but with a closed head and a vent to the nitrogen oxides train, because above about 5 per cent strength the outgassing of nitric acid is significant and its decomposition liberates toxic nitrous oxide gases; exclude or control organic material, because reactions between nitric acid and organic materials can be violent
Organic-bearing	Measure the organic content before specifying any oxidant, and provide a probe cleaning provision, because the same organic films the glass and blinds the measurement as well as reacting with an oxidant
Concentrated acid	Introduce the reagent into the bulk at a rate that holds the computed temperature rise, because published guidance records rises large enough to melt thermoplastic piping; use continuous temperature monitoring, reaction-rate control and quench water where the rise requires it
Metal-bearing	Settle whether the metal is recovered first or whether the site has chosen to pay for sludge disposal, because raising the pH destroys the solution the metal came from and converts the value into a hydroxide precipitate
Complexed metal	Check the complexing agent’s own stability before promising a precipitate, because precipitation depends on the complex’s stability rather than on the pH alone, and a chelated metal will stay in solution through the band

Table 6 — Materials of construction by liquid service

Service condition	Material and the reason
Hydrochloric, sulphuric and dilute nitric acid within the temperature band	Polypropylene and polypropylene homopolymer: inert to the liquor, no lining to fail, and the ordinary choice for this class of vessel
Caustic and alkaline liquor within the temperature band	Polypropylene and polypropylene homopolymer, with the reagent’s own crystallisation behaviour accounted for in the storage and transfer arrangement
Fluoride-bearing liquor	Thermoplastic only; published guidance for glass-lined equipment records that a glass lining resists most acids but not hydrofluoric acid, so glass-containing materials are excluded outright
Larger shell, higher temperature or outdoor installation	Glass-reinforced plastic, which carries the size and the weather that a thermoplastic shell does not
Oxidizing or sanitary service	Stainless steel with the grade selected against the actual halide and oxidant content, because a halide-bearing oxidant service attacks grades that a generic corrosion table lists as acceptable
Concentrated reagent storage	A material chosen for the concentrated reagent rather than for the working strength, kept in its own containment, and segregated from anything it can oxidize
Sparingly soluble salt service (calcium route)	The same vessel material as the duty, with a scaling control and a bleed specified for the deposit rather than the material changed for it
Aggressive dissolution duties outside this envelope	Not this reactor; the enamel-lined vessel described elsewhere in this series is the correct unit for those duties, and the boundary is drawn at the feed rather than at the material

Table 7 — Acceptance tests and verification criteria

Test	Criterion
Feed titration at the test	A titration of the feed taken at the same time as the test load, compared with the load the reactor was sized on, recorded as a load rather than as a pH
Load test	A batch or a flow equal to the design case drawn from the governing simultaneous combination, run for long enough for the loop’s dead time to be visible in the record
Dose-range check	The delivered reagent dose range checked against the range the loop was designed for, with the consumption logged per batch or per period
Blend-time verification	Turn-over count and blend time taken from the agitator’s measured performance rather than from its nameplate, against the published requirement of at least 16 complete turnovers within the residence time
Working-temperature check	The vessel’s contents held inside the working temperature of the chosen material at the specified dosing rate, with the computed heat load as the reference figure
Vent check	Vent flow and pressure verified, and the vapour route confirmed to be carrying the species the liquor can release to the destination named on the drawing
Discharge figure	Proved per batch or per period on the standard’s own basis and with its own reference conditions, against the site’s permitted band rather than against a remembered number
Salt concentration check	The discharged salt concentration measured and compared with the receiving route’s acceptance limit, with the discharge or bleed rate that holds it confirmed
Sludge check, where a precipitate is produced	Sludge volume and dry solids content measured at the design load, against the figures the sludge handling arrangement was specified for
Instrument state	Probe, oxidation-reduction potential electrode, temperature, level and flow instruments calibrated and recorded with their dates and their buffer or standard values
Failure recording	A failed batch recorded as a load or a mixing finding or as an instrument finding, because the two have different remedies and the record has to state which one applied
Handover documentation	Operating envelope with the loads it was proved against, measured reagent consumption, calibration interval for the actual liquor, consumable and spares schedule, interlock schedule and the retained data set with its retention period

Technical Advantages

The reactor is specified from the acid load, which is the only place the design can honestly start. A pH reading is a logarithm and a volume is a geometry, and neither of them is the quantity the reagent, the dosing pumps, the heat and the salt are computed from. The load is titrated on the customer’s own sample against the customer’s own permit band, and it is the first deliverable rather than an assumption buried in a safety factor, so that a customer who changes the feed can see what changes with it instead of discovering the difference at the acceptance test.
The titration curve is treated as the design object rather than as an inconvenience. Published guidance for pH control states plainly that most controllers on the market are not truly pH controllers because they use simple on-off control or the linear control provided by a proportional-integral-derivative algorithm, which is far too simple to account for the ever changing control gain encountered with the logarithmic reaction of pH to control input. The loop is therefore built as a coarse stage and a trim stage with independent tuning against the customer’s measured curve, and where the feed cannot be made steady enough for a loop the answer is a second vessel in series rather than a controller tuned harder.
The mixing duty is stated in a figure that can be checked. Published practice for neutralization mixing requires at least 16 complete vessel turnovers within the residence time, which converts directly into a pumping capacity of one working volume per residence-time interval, and that number is delivered rather than the motor’s rating. The reagent is injected into the impeller’s discharge stream so that it is dispersed into moving bulk liquid before it can react locally, and the turn-over count and blend time are handed over as measured figures instead of as design intentions.
The heat the reaction releases is computed before a material is chosen. At about 55.83 kilojoules per mole of hydrogen ion neutralized, a concentrated feed releases enough heat to carry a thermoplastic shell past its working temperature, and published guidance records rises large enough to melt thermoplastic piping. The computed heat load, the shell’s working temperature and the quench, cooling or rate-limit arrangement are presented together, which turns a temperature that would otherwise be discovered on the first campaign into a figure the customer can review at the quotation stage.
The vapour route is designed as part of the reactor. A neutralization reactor handling a nitrate-bearing liquor produces nitrogen oxides from the decomposition of nitric acid, an ammoniacal liquor can release free ammonia, and a sulphide-bearing liquor acidified by error releases hydrogen sulphide. Each of those has a destination on the drawing rather than a hope that the room’s ventilation is sufficient, and the vapour-space material and the condensate drain follow the same species list, which is what makes a chloride duty and a nitrate duty genuinely different vessels at the same load.
The hazard sequence runs in the right order, and the interlock schedule enforces it. A cyanide-bearing liquor is held above a pH of about 10.5 before any oxidant is admitted and is oxidized in two stages with the oxidant dosed against an oxidation-reduction potential setpoint held under mixing; a sulphide-bearing liquor is kept alkaline; an ammonia-bearing liquor is vented for free ammonia; and a liquor carrying residual solvent-extraction organic is measured for that organic content before an oxidant is specified. The sequence is a written procedure and an interlocked state, because a procedure alone is not a control.
The salt balance is closed in figures, and the three destinations are kept separate. The soluble salt’s concentration in the discharge and the rate that holds it below the receiving route’s limit; the sparingly soluble sulphate and fluoride salts of a calcium route held by a scaling control and a bleed; and the hydroxide sludge of a metal-bearing feed drawn to a handling arrangement with its own volume and dry solids content. A reactor that promises a discharge without stating a salt concentration has not finished its balance, and this page states it in the specification table instead of in a footnote.
The material envelope is delivered as an exclusion list, and the crucial exclusion is published rather than commercial. Thermoplastic for ordinary acid and alkali service inside its temperature band, glass-reinforced plastic for the larger or hotter shell, stainless steel for oxidizing or sanitary service, and no glass-containing material wherever fluoride is present — which is a documented limitation of glass linings rather than a preference between suppliers. The exclusions are in the specification table so that a customer with a fluoride-bearing liquor is sold the vessel that suits it and is not sold the one that does not.
The measurement chain is delivered complete, including the parts that decide whether it can be trusted. The probe’s position in bulk liquid, its cleaning provision on a fouling liquor, its buffer solutions, its spare and its calibration interval are part of the package, and the interval is set from the measured fouling rate of the actual liquor. Published practice for neutralization systems arranges continuous pH measurement with recording, flow totalisation and data logging as standard, and this page follows that arrangement rather than treating the record as an optional extra, because the record is what a discharge figure is proved by.
The boundary and the alternative are offered together. A liquor still carrying recoverable metal above the site’s economic threshold is directed to recovery ahead of neutralization, because raising the pH converts the value into a sludge and destroys the solution it came from; a feed carrying appreciable undissolved solids is routed to a suspension vessel rather than to a blending one; a feed whose peak load has never been titrated is not sized from an estimate; and a cyanide, sulphide or ammonia-bearing liquor without a designed hazard sequence is not quoted as an ordinary acid duty. Each refusal is accompanied by the route that does fit, which turns a lost sale into a specification the customer can act on.

Service Process

The service process is built around one fact: a neutralization reactor cannot be specified from a catalogue, so the first work is a titration and the last work is a measured acceptance test under the design load. The ten steps below run from an enquiry to a commissioned reactor with its own record, and every step ends by naming what it hands to the next one.

1. Enquiry review and boundary check. The enquiry is read for the four things that decide whether a reactor is the right unit at all: the streams that produce a load and whether they are acid-side or alkaline-side, the process that produces each of them, the site’s permitted discharge band and its limits on the salt the reaction will make, and whether recoverable metal is still present above the site’s economic threshold. Enquiries whose liquor still carries a value are separated from enquiries whose liquor is a genuine waste, because those two paths diverge before any vessel is discussed. What leaves this step: a written read-back of the duty, a statement of which hazard class each stream falls into, and a decision on whether to proceed to sampling with the reason stated either way.

2. Sampling plan and safe sample dispatch. A representative sample is what the whole design rests on, so it is taken properly: the sample point is chosen where the liquor is well mixed rather than where it is convenient, the sample is taken at the timing that catches the peak load rather than the average, the container is chosen for the chemistry and the hazard class, and the packaging and labelling follow the consignment rules that apply to the species present — a cyanide, sulphide or nitrate-bearing sample is not shipped as an ordinary liquid. What leaves this step: a sample with a documented point, timing, temperature and container, and a chain-of-custody record.

3. Laboratory titration and load report. The sample is titrated twice against the site’s own target band, with a buffer determination and a solids determination run alongside it, and the results are reported as an acid or alkali load in moles of hydrogen ion per litre with the buffer stated separately rather than merged into a single figure. Where the first result disagrees with the plant’s own expectation, the titration is repeated on a second sample before the report is issued, because a load report that is wrong by a factor passes every drawing review and fails the acceptance test. What leaves this step: a laboratory report containing the titrated load, the buffer, the solids, the temperature behaviour on neutralization and a statement of what the load implies for the reagent and the heat.

4. Flowsheet, mode and stage selection. The mode is chosen from the shape of the feed in time, and the stage count from the variability that the mode cannot absorb: a batch vessel where the liquor arrives in slugs or a plant runs campaigns, a continuous stirred vessel where the flow is steady, two or more vessels in series where a surge has to be absorbed as it passes. The reagent is chosen from the salt it makes and from the receiving route’s limit on that salt, and the heat and vapour consequences are written into the flowsheet at the same time as the vessel. What leaves this step: a process flowsheet with the mode, the stage count, the reagent, the dose range, the heat load, the vapour route and the salt destinations all stated, together with the arrangements that were rejected and the reason for each.

5. Equipment specification and quotation. The vessel, the agitator, the reagent system, the measurement chain, the vent and the sludge arrangement are specified against the flowsheet rather than against a product list, and the quotation carries the figures it was built on: the titrated load, the reagent consumption per kilogram of acid neutralized, the working volume, the residence time, the turn-over count, the heat load, the discharge salt concentration and the calibration interval. Where a constraint cannot be met with the liquor as it stands, the quotation says so and states the change that would make it meetable rather than absorbing the problem into a larger vessel. What leaves this step: a specification with its basis stated, a quotation with the boundary written into it, and a deliverables list that matches what will actually be handed over.

6. Design review and drawing approval. The general arrangement, the piping and instrumentation diagram, the nozzle schedule, the material schedule and the interlock schedule are reviewed with the customer before anything is cut, and the review covers the items most often discovered too late: the routing of the vent to the device that will actually take it, the position of the probe relative to the agitator’s flow, the access required to withdraw the probe and to change the impeller seal, the space for the reagent area with its containment, and the load the vessel imposes on the floor it will stand on. What leaves this step: an approved drawing set with a revision that the acceptance test will be run against, and a list of the site services the customer is responsible for bringing to the battery limit.

7. Fabrication, assembly and internal tests. The vessel is fabricated and its wetted surfaces examined, the agitator is assembled and run on its own mounting, the dosing skid is assembled and its pumps are calibrated against a measured volume, the instrument chain is loop-checked with the actual transmitters, and the control sequence and the interlocks are simulated before power is applied to anything that moves. What leaves this step: an internal test record for each assembly, a calibrated dosing range, and a simulated interlock schedule with each defined action demonstrated.

8. Factory acceptance test under the design load. The package is run on a liquor whose titre matches the sample rather than on water, so that the delivered dose range, the blend time, the working temperature, the salt concentration and the control response are all demonstrated against the load the reactor was sized on. A test run on water proves that the agitator turns and that the probe reads; it does not prove that the loop can hold the band against the load, and the two are not interchangeable. What leaves this step: a factory acceptance record carrying the titre used, the dose range delivered, the blend time measured, the temperature held, and the control response at the design load, with any deviation noted rather than smoothed over.

9. Site installation, commissioning and operator training. The package is installed to the approved drawing, the services are connected and proved at the battery limit, the instruments are calibrated on site against fresh buffers and standards, and the first batches are run with the customer’s operators under supervision until the loop’s behaviour on the real liquor is understood. Because the first campaign exists to replace the assumed reagent consumption with the measured one, the commissioning period is treated as a measurement exercise rather than as a formality. What leaves this step: a site commissioning record, a probe calibration interval set from the observed fouling rate, a measured reagent consumption, and operators trained on the sequence including the hazard sequences and the alarm responses.

10. Handover, record and after-sales entry. The operating envelope with the loads it was proved against, the measured consumption, the calibration schedule, the consumable and spares list, the interlock schedule and the retained data set are handed over together, and the reactor is entered into the after-sales arrangement for its warranty period and its planned support. What leaves this step: a signed handover with the record set, and an open support line with the duty as it was actually proved rather than as it was originally quoted.

What the customer is asked to bring to the battery limit. A representative sample or a documented sampling point for each stream; the applicable discharge permit with its pH band and its salt limits; the volumes, temperatures and timings of the streams, including the simultaneous case; the services available at the installation point — electrical supply, process water, drainage and the sludge route; the space in plan and in height with the floor’s load capacity; and a nominated operator who will be trained and who will run the reactor after handover. A specification written without the permit and the simultaneous case is a specification written against an assumption, and the assumption is the item most often wrong.

Warranty And After-Sales Service

A neutralization reactor is a chemical unit and it is judged over campaigns rather than over days, so the warranty is written against what the equipment is expected to hold and against what the site is expected to do. The sections that follow set out the cover, the exclusions, the spares position and the support that continues once the acceptance test has been signed.

What the warranty covers

The vessel, its agitator and drive, the reagent dosing skid, the instrument chain, the control panel and the vent arrangement are warranted against defects in material and workmanship for the period stated in the contract, counted from the date of the signed acceptance test rather than from the date of shipment. The cover extends to the figures the acceptance test was run against: where the reactor does not hold the discharge band at the load it was proved on, the cause is investigated at our cost and the package is corrected, whether the correction is a nozzle position, an impeller, a dosing device, a probe mounting or a control parameter. The operating envelope issued at handover is the reference for that comparison, which is why it is issued with the loads it was proved against rather than as a general statement of capability.

What is excluded

The warranty does not cover wear parts — the pH probe and the oxidation-reduction potential electrode, their buffer and standard solutions, the gaskets, the dosing pump diaphragms and valves, the filters and the shaft seal — which are consumables with stated intervals rather than defects. It does not cover damage from operating outside the envelope that was handed over: a load materially above the titrated design load, a temperature above the material’s stated limit, an unneutralized reagent addition, or a hazard sequence that was bypassed. It does not cover a feed whose hazard class was not declared at the enquiry stage, and the reason for that exclusion is stated plainly rather than buried: a cyanide, sulphide, ammonia, oxidant or organic-bearing feed changes the sequence, the material and the vent, and a reactor built as an ordinary acid duty was never specified for it. It does not cover corrosion or scaling caused by a change in the feed’s chemistry after handover without the design being revisited, and it does not cover the consequences of a discharge excursion where the site’s own permit limits differ from the ones supplied at the enquiry stage.

Spare parts

A first-year spares set is recommended with every package and its composition follows the duty rather than a standard list: one pH probe and its buffer solutions, one oxidation-reduction potential electrode where an oxidant is dosed, one set of dosing pump diaphragms and valves, one set of reagent-line filters, one gasket set for the nozzle and manway arrangement, one shaft seal, and one impeller key and fastener set. Consumable items are quoted with their intervals rather than as a list, because a spares list without intervals is a list the customer buys once and then substitutes from a local supplier. Long-lead items — the impeller where it is a moulded part, the probe where it is a specific mounting, the dosing pump head — are identified at handover so that a replacement can be ordered before it is needed rather than after a batch has failed.

Remote support

The control panel is provided with the communications arrangement the site’s platform allows, so that the sequence, the tuning parameters, the alarm history and the logged record can be read remotely. The first months of operation are supported from that record rather than from a description of the symptom, which is the difference between a loop that is corrected in one exchange and a loop that is discussed for a month: the recorded pH trace at the design load shows whether the problem is a load the reactor was not sized for, a mixing shortfall, a probe that has fouled or a control curve that was programmed against the wrong titration.

Field service

Field service is provided for commissioning, for operator training and for the diagnosis of a discharge excursion, and the attendance is scheduled against the site’s own operating pattern so that the engineer is present for a real batch rather than for an idle vessel. Where a fault is found to be a defect under the warranty, the attendance is at our cost; where it is found to be a change in the feed or a departure from the procedure, the attendance is charged and the finding is documented with the remedy, because a finding recorded is a repeat failure avoided.

Maintenance and operating support

A maintenance schedule is issued with the handover and it is built from the duty rather than from a generic interval: the probe’s calibration interval from the measured fouling rate of the actual liquor, the dosing pump’s diaphragms and valves from the reagent’s own attack and the delivered strength, the filter cleaning from the solids the reagent and the feed carry, the seal’s inspection from the agitator’s duty, and the sludge draw’s operation from the precipitate the reaction produces. The schedule is accompanied by the operating procedure, including the hazard sequences, the alarm responses and the batch release rule, so that the operator who runs the reactor at three in the morning has the same instructions as the engineer who commissioned it.

Upgrade and re-configuration

Because the vessel, the agitator, the dosing skid and the control system are specified as modules, a change in the duty is answered by re-specifying the module that changed rather than by replacing the plant. Where a load rises, the first step is to re-titrate and to re-examine the mixing and the control curve before any hardware is added, because a loop that was tuned against a lower load will often hold a higher one once its curve is reprogrammed; where the load genuinely exceeds the envelope, the usual answers are an additional stage with its own dosing device, a higher-capacity reagent pump, a larger impeller, a probe relocated to a better mixing position, a second vessel in series, or a cooler where the heat load is what binds. Where the feed changes class — for example where a cyanide-bearing stream is added to a plant that previously neutralized only acid rinse water — the hazard sequence, the material envelope and the vent are re-specified before the new stream is admitted.

Frequently Asked Questions

What is a neutralization reactor?

A neutralization reactor is an agitated vessel in which an acid-bearing or alkali-bearing process liquor is brought into a discharge pH band by the metered addition of the opposite reagent, with the dose controlled against a continuous measurement of pH. Its four duties are to disperse the reagent so that it cannot react locally, to let the neutralization reaction run, to hold the mixture long enough for the measurement to represent the whole vessel, and to release the treated liquor against a proved figure. The vessel is the simple part; what decides whether the installation works is the accounting of the acid or alkali load the feed carries, because that load sets the reagent, the dosing pumps, the heat released and the salt produced, while the volume only sets the residence time the control loop has to work in.

Why can two liquors at the same pH need different reactors?

Because pH is a logarithm and it does not measure how much acid is there. A hydrochloric rinse at pH 2 and a buffered plating bath at pH 2 describe two different quantities of acid, since a buffer holds the pH down while the load rises behind it. The load is what the reagent dose, the vessel, the dosing pumps and the heat are computed from, so the two liquors need different reactor specifications even when their pH readings are identical. This is why the load is established by titrating a sample to the target band and expressing it in moles of hydrogen ion per litre, rather than by reading a meter.

How is the acid load measured, and why not use the pH reading?

The load is measured by titrating a representative sample with a standard alkali or acid to the target pH of the site’s own permit band, and expressing the result as moles of hydrogen ion per litre or as kilograms of calcium carbonate equivalent per cubic metre. A pH reading cannot serve for two reasons: the scale is logarithmic, so equal pH intervals are not equal quantities of acid, and a buffer absorbs reagent without letting the pH move. The titration is run against the site’s own band rather than a generic figure because the band decides where the equivalence point of the design lies, and it is repeated on a second sample whenever the first result disagrees with the plant’s own expectation.

Why does a pH control loop oscillate, and how is it corrected?

Because the gain between reagent flow and measured pH rises by orders of magnitude as the mixture approaches the equivalence point. Published guidance for pH control states that most controllers on the market are not truly pH controllers, since they use simple on and off control or the linear control provided by a proportional-integral-derivative algorithm, which is far too simple to account for the ever changing control gain encountered with the logarithmic reaction of pH to control input. The correction is structural rather than a matter of tuning harder: a coarse stage that takes the bulk of the load into the band and a trim stage that holds it, each with independent tuning programmed to the site’s own measured titration curve.

When should a neutralization reactor be batch and when continuous?

A batch reactor suits effluent that arrives in slugs, a plant that runs campaigns, and any duty where each batch has to be proved before release. A continuous stirred reactor suits a flow that is stable in volume, pH and chemistry, because a disturbance in the feed passes through the vessel to the discharge point. Published guidance for this equipment states the trade in exactly those terms, and adds the structural answer for the case in between: where a surge cannot be made steady, two or more vessels are arranged in series so that the slug is absorbed as it passes rather than arriving at the discharge intact.

How long should the liquor stay in the reactor?

Published practice for neutralization service gives a residence time of 15 to 60 minutes, with longer times for waste that is difficult to neutralize or whose pH varies widely. Within that range the figure is set by the control loop rather than by the reaction, because the neutralization itself is fast while the dispersion of the reagent into the bulk and the loop’s own response are not. The vessel’s volume is then sized for the peak flow with a margin rather than for the average, with published practice describing a design for 110 to 150 per cent of the maximum flow so that the vessel absorbs variability instead of passing it to the discharge.

How much mixing does a neutralization reactor need?

It needs enough to deliver blend time, and published practice for neutralization mixing states the requirement as at least 16 complete vessel turnovers within the residence time. That converts into a usable figure: a vessel holding 16 minutes of flow needs a mixer whose pumping capacity equals the whole working volume once per minute. The impeller is therefore selected for pumping and blending rather than for shear, the vessel is baffled so that the contents cannot rotate as a solid body with the impeller, and the turn-over count and blend time are handed over as measured figures rather than as an intention.

Where should the reagent be injected?

Into the impeller’s discharge stream, through a nozzle rather than at the end of a pipe. An impeller produces a high-velocity stream in a defined direction, and a reagent introduced into that stream is dispersed into moving bulk liquid within the shortest possible distance. A reagent dropped onto the liquid surface or sprayed against a wall disperses slowly and produces a local region at an extreme pH, which the probe may or may not see depending on where it sits, and the same reactor then reads a band it has not actually reached throughout its volume.

How much heat does neutralization release, and when does it matter?

A strong acid and a strong base release about 55 to 57 kilojoules per mole of water formed, because the reaction that matters is the combination of hydrogen and hydroxide ions, and published concentrated-acid guidance uses 55.83 kilojoules per mole of hydrogen ion as its working figure. Multiplied by the titrated load this gives the heat the reactor must carry away. Published guidance adds the threshold: below about one per cent acid concentration the rise is generally not a concern, but above it the rise is measurable and can be of concern, with rises large enough to melt thermoplastic piping recorded for concentrated solutions.

Why does a nitric-bearing liquor need a different vent?

Because nitric acid is not only an acid but an oxidant and a decomposable species. Published technical guidance records that above about five per cent concentration the outgassing of nitric acid is significant and requires good ventilation, that its decomposition liberates toxic nitrous oxide gases, and that its reactions with organic materials can be violent or explosive because of its oxidizing power. A reactor handling aqua regia residues, nitric parting liquors or nitrate-bearing rinse water is therefore given a closed head and a vent routed to a nitrogen oxides treatment train, and the organic content of the feed is measured and controlled before the reactor is specified.

How is a cyanide-bearing liquor neutralized safely?

By raising the pH before anything else happens. Free cyanide is in equilibrium with hydrogen cyanide and the position of that equilibrium is set by pH, so published practice holds cyanide-bearing solutions above a pH of about 10.5 to keep the cyanide in solution as the cyanide ion and prevent hydrogen cyanide gas from forming. Cyanide destruction follows as a one or two stage treatment in which the first stage raises the pH and oxidizes cyanide to the far less toxic cyanate and the second oxidizes the cyanate to its end products, with the oxidant dosed against an oxidation-reduction potential setpoint held under mixing until the consumption stops. A cyanide liquor is never admitted to the acid side of a reactor.

What happens to the salt the reaction makes?

It either leaves in the discharge or it deposits in the vessel, and which one happens is decided by the reagent. Published guidance for neutralization states the behaviour directly: where the cation is sodium and the reactants are relatively dilute the products remain soluble and can be discharged, while a calcium cation or elevated concentrations of both reactants produce precipitation products that drop out of solution. A sodium route therefore needs a salt concentration and a discharge rate that hold it below the receiving route’s limit, and a calcium route needs a scaling control and a bleed to match its sparingly soluble sulphate and fluoride salts.

Does neutralization recover metal?

No, and it destroys the conditions under which the metal could have been recovered. Raising the pH of a metal-bearing liquor converts free acid into a dissolved salt and, once the solubility product of the corresponding hydroxide is exceeded, precipitates the metal as a hydroxide sludge that is a disposal cost rather than a product. The liquor it came from is no longer a solution from which the metal can be won by electrowinning, cementation or selective precipitation. Where a liquor still carries recoverable metal above the site’s economic threshold, the metal is recovered first and the stripped liquor is neutralized afterwards.

Which material should the reactor be made of, and when is glass excluded?

Thermoplastic construction covers the ordinary acid and alkali duties inside its temperature band and has the advantage that there is no lining to fail; glass-reinforced plastic is used where a larger shell, a higher temperature or an outdoor installation is required; stainless steel is used where the service is oxidizing or sanitary, with the grade selected against the actual halide and oxidant content. One exclusion is absolute: published guidance for glass-lined equipment records that a glass lining resists most acids but not hydrofluoric acid, so a fluoride-bearing liquor selects a thermoplastic vessel and disqualifies any glass-containing material outright.

Why does the pH probe drift or fail, and what can be done about it?

Because the probe measures through a surface that can be coated. A film of organic, scale or precipitate on the glass produces a reading that drifts slowly and then fails a discharge without ever looking like an instrument fault. The answer is designed rather than diagnosed: a mounting that can be withdrawn or washed, a position in bulk liquid rather than in a quiescent corner, fresh buffer solutions with a recorded calibration date and value, a spare electrode, and a calibration interval set from the measured fouling rate of the actual liquor. A solvent-extraction raffinate carrying residual extractant is the case that forces a short interval.

Can a neutralization reactor treat a liquor that also carries solids?

Not as its primary duty. The agitator specified for blending a liquor is the wrong agitator for keeping a heavy slurry in suspension, and a vessel asked to do both is a vessel that does neither well. A feed carrying more than a trace of undissolved solids is therefore screened, settled or filtered upstream, and a feed whose solids content is genuinely part of the duty belongs to a suspension vessel instead. The exception that is designed rather than refused is the precipitate the reaction itself makes, which is drawn from a conical bottom with a valve sized for a thickened slurry and routed to its own settling, thickening or dewatering step.

What happens when the feed surges or the composition changes?

A surge that passes through a single continuous vessel reaches the discharge point, which is why a narrow permitted band and a variable feed are a poor combination for a one-stage continuous reactor. The answers are structural and are settled at the design stage: a batch vessel where the surge can be collected and proved before release, two or more vessels in series where the slug has to be absorbed as it passes, a reagent dose rate that follows the load rather than a fixed setpoint, and a level range with an independent high-high alarm that holds the feed instead of releasing the batch. A change of composition is a load change, and it is answered by re-titrating rather than by re-tuning.

How is the reactor proved before it is released to the drain?

By an acceptance test run on the plant’s own duty rather than on water, so that the delivered dose range, the blend time, the working temperature, the salt concentration and the control response are all demonstrated against the load the reactor was sized on, with a titration of the feed taken at the same time for comparison. The discharge figure is then proved per batch or per period on the standard’s own basis and with its own reference conditions, and the record carries the batch or period identity, the pH and volume figures, the reagent consumed and the calibration state of the instruments. A failed batch is recorded as a load or a mixing finding rather than as an instrument error.

What capacity should be quoted, and why is no figure given in advance?

The capacity is engineered per project from the titrated acid or alkali load of the governing simultaneous combination of streams, the peak-to-average ratio of the flow, the residence time the control loop requires and the batch size the plant discharges, and the working volume, the stage count and the shell diameter all follow from those figures. No fixed treatment volume is quoted in advance because a reactor quoted against a volume and a flow has left the two variables it is actually sized on unstated, and the difference appears at the first concentrated slug rather than at the drawing review. The standard thermoplastic range in this class of vessel runs from 0.05 to 5 cubic metres, with larger duties arranged as multiple vessels.

What must the customer provide for a firm quotation?

A representative sample or a documented sampling point for each stream, with its volume, temperature and timing including the case in which several streams are discharged together; the applicable discharge permit with its pH band and its limits on the salt the reaction will make; the services available at the installation point, meaning the electrical supply, the process water, the drainage and the sludge route; the space available in plan and in height, and the floor’s load capacity; and an answer to the question of whether recoverable metal is still present above the site’s economic threshold. A specification written without the permit and the simultaneous discharge case is written against an assumption, and the assumption is the item most often wrong.