Gold Electrowinning Equipment

— Gold Electrowinning Equipment: Eluate-to-Doré Circuit for Gold Recovery Above 99%

Gold electrowinning equipment is a direct-current recovery circuit that reduces dissolved gold out of a pregnant solution onto a cathode — aurocyanide Au(CN)₂⁻ in alkaline eluate, or a gold chloride complex in chloride liquor — and delivers it as a gold sludge that is washed, calcined and smelted to doré bullion of 50–75% gold. YuSai Machinery engineers the complete gold room circuit around your assay: solution conditioning and copper knockout, eluate preheating and flow control, cell bank and cathode selection, rectifier sizing, deposit monitoring, sludge treatment, doré smelting and the barren liquor return to leach.
Gold recovery: 99% or higher electrolysis rate from pregnant solution; the barren liquor leaves below 1 g/t Au after resin and cementation polishing.
Configuration: capacity, cathode area, footprint, power and utilities are engineered to your feed assay and target gold output rather than fixed catalogue numbers.

Product Overview

Gold electrowinning is the step that turns dissolved gold back into metal, at the end of a hydrometallurgical circuit — a carbon-in-leach or carbon-in-pulp plant, a heap leach operation or a workshop stripping line. A direct current through the pregnant solution reduces the gold onto a cathode, which is then harvested, washed and melted, so there is no zinc dust to buy, no precipitate to filter out of a large flow, and no reagent left in the barren solution to destroy before it returns to the leach. The distinction from electrorefining matters, because the two are routinely conflated. Electrorefining dissolves a cast anode of impure gold anodically and plates pure gold onto the cathode, so the anode is consumed and an anode slime circuit is unavoidable. In electrowinning the anode is inert — stainless steel, MMO-coated titanium or lead alloy — and the anodic reaction is oxygen evolution, with cyanide oxidation as a secondary reaction in alkaline cyanide circuits. Nothing dissolves at the anode, so there is no anode bag and no anode slime, the only metal entering the cell is the metal already in solution, and an electrowinning gold room has a different equipment list from an electrolytic refinery.

The circuit begins with the solution. Eluate from a carbon elution column is high in gold but not clean: it carries free cyanide and caustic, silver, and the copper, nickel and iron the loaded carbon picked up from the pulp. Copper is dealt with first, because its deposition potential in cyanide solution lies close to that of gold and it will co-deposit on the same cathode; where the assay shows copper above roughly 5 g/L, a knockout step goes ahead of the cells. The liquor is then clarified, since suspended carbon fines and mineral slime short-circuit the electrodes, and its pH is brought into the 10.5–12 band that keeps hydrogen cyanide from forming and keeps the anode passive rather than corroding.

In the cell bank, gold concentration — hundreds of milligrams per litre in a real eluate, not tens of grams — limits the deposition current by how fast gold ions reach the cathode surface. That is why the cathode is steel wool or multi-layer mesh rather than a flat plate: specific surface area does the work. It is also why gold and copper electrowinning differ so sharply in current efficiency. On dilute eluate a current efficiency of 5–10% is normal and not a fault, most of the charge going into water reduction and oxygen evolution; the figures that describe performance are the electrolysis rate and the residual gold in the barren liquor. Flow and temperature decide whether the cathode area is used well: eluate reaches the gold room hot from the elution column, in the region of 60–70 °C, and is trimmed to the 40–60 °C optimum rather than simply cooled, while circulation is held at two to four bed volumes per hour so the boundary layer at the cathode stays thin, with level make-up keeping the electrodes immersed and the liquor strength steady. The cyclone cell takes the same principle further, injecting electrolyte at the base of a concentric electrode pair so that it leaves as a swirling film which continuously renews the cathode surface and holds a workable current density on a liquor a flat-plate cell cannot treat economically.

Harvesting and finishing complete the route. The deposit is not cathode-grade metal but a gold-bearing sludge mixed with steel wool, silver and traces of copper; it is rinsed in the cell, stripped from the cathode with high-pressure water, and the wool is returned to service or dissolved off where consumed. The sludge is parted with nitric acid to dissolve base metals and silver, filtered, washed and dried, and the silver is recovered from the parting liquor as silver chloride. The dried gold is calcined between roughly 400 and 750 °C, fluxed and melted in an induction furnace at 1,150–1,300 °C, and cast into doré bars assaying 50–75% gold; where four-nines metal is needed, the bars go to the refining line instead of being sold on assay. Nothing leaves the circuit prematurely: residual gold in the barren liquor is measured and the stream is polished through ion-exchange resin and cementation until it carries less than 1 g/t, with the cementation residue returning to the smelter rather than to the tailings dam, and cyanide and caustic are made up to strength and returned to the elution column or the leach. Cell vapour is drawn from beneath closed transparent hoods into a caustic scrubbing train, and mercury abatement is fitted where the feed carries mercury — so the gold room recovers the metal the leach dissolved, produces a saleable bar and returns an unchanged reagent stream to the front of the plant.

Key Facts

Gold recovery: 99% or higher electrolysis rate from pregnant solution; the barren liquor leaves below 1 g/t Au after resin and cementation polishing.
Cathode deposit: gold sludge carrying 30–75% Au as harvested, taken off cathodes made of stainless steel wool, multi-layer stainless mesh or an expanded metal sheet.
Smelted product: doré bullion of 50–75% Au with crude silver as a by-product; four-nines gold is produced on the downstream refining line.
Cell operating window: 2.0–4.0 V DC across the cell at 8–25 A/m² cathode current density, 40–60 °C electrolyte, pH 10.5–12 and conductivity above 1.7 S/m.
Accepted feed: carbon elution liquors from Zadra, AARL and IPS circuits, heap and dump leach pregnant solution, intensive cyanidation liquor from gravity concentrate, e-waste and PCB stripping solution, spent gold plating baths and drag-out rinse, chloride and aqua regia gold liquor, and jewellery workshop drag-out.
Configuration: capacity, cathode area, footprint, power and utilities are engineered to your feed assay and target gold output rather than fixed catalogue numbers.

Product Core Selling Points

A recovery circuit, not a box of cells. Solution conditioning, cell bank, rectifier, deposit control, sludge washing, smelting and barren liquor polishing are delivered as one engineered line, so the interfaces between the elution column and the gold room are no one’s problem but the supplier’s.
The gold room interface is specified, not assumed. The line is designed against the elution route you actually run — atmospheric or pressure Zadra, AARL, or an integrated pressure stripping unit — because the eluate gold concentration, temperature and free cyanide level set the current density and the cathode area.
Two cell geometries in one scope. Flat-plate cells handle the mainstream eluate duty; the cyclone cell, with a concentric electrode pair and a removable cathode sleeve, recovers gold from dilute, impurity-heavy liquor where a flat-plate cell would run at an uneconomic current efficiency.
Electrowinning explained against electrorefining where the two are confused. In electrowinning the anode is inert and evolves oxygen, so no anode metal is consumed and no anode slime circuit is needed. In electrorefining a cast anode is dissolved. The distinction decides the whole plant, and this page states it plainly.
Copper knocked out before it reaches the cathode. Copper deposits at a potential close to that of gold, so eluate carrying more than about 5 g/L Cu is treated first. The flowsheet specifies that pre-treatment from the assay rather than leaving it to a purity argument later.
Rectifier sizing given as arithmetic, not as a catalogue. Total current, current density and per-cathode current are related by an explicit three-step calculation, with a worked example, because most of the trade literature quotes only one of the three.
Sludge handled to bullion. More than 97% of the deposit is stripped from the cathode with high-pressure water, parted with nitric acid to remove base metals and silver, dried, fluxed and melted to doré — the washed sludge is not handed back to the customer as a problem.
Closed circuit on the barren liquor. Residual gold is polished below 1 g/t by ion-exchange resin and cementation, reagents are made up to strength, and the stream returns to the leach or elution circuit instead of going to the effluent plant.
Both cyanide and non-cyanide chemistry. Alkaline cyanide eluate is the mainstream duty; the same cells accept thiosulfate, glycine and chloride liquors, with a superimposed AC-DC supply available to suppress anodic film formation in chloride service.
Emissions enclosed at the source. Cells run under closed transparent hoods with adjustable extraction, chloride and cyanide-bearing mist goes to a caustic scrubbing train, and mercury abatement is available for feed that carries mercury.
Assay-linked traceability. Every campaign carries a feed assay, a cathode weight record, a barren liquor assay and a fire assay on the doré, so recovery and fineness claims can be checked against the customer’s own analysis on the same lot.
Configuration to the feed, not to a model number. What sets the size of a gold electrowinning plant is the gold load per hour and the impurity profile of the liquor, so two plants with the same nominal cell count can have very different daily output.

Application Scenarios

Carbon-in-leach and carbon-in-pulp gold plants. Gold rooms that receive loaded carbon from CIP or CIL adsorption and need a repeatable route from elution to doré bullion.
Heap and dump leach operations. Circuits where pregnant solution is concentrated on carbon and then eluted, and where the electrowinning step has to tolerate variable flow and variable grade.
Intensive cyanidation circuits. High-grade liquor from the intensive leach of gravity gold concentrate, where the gold concentration is high enough for a compact cell bank to do the whole job.
Small and medium gold mines. Operations producing a few kilograms of gold per day that cannot justify the deaeration tower, filtration plant and reagent handling that Merrill-Crowe precipitation requires.
E-waste and PCB recycling plants. Stripping and deplating lines where gold is taken off plated contacts into cyanide or non-cyanide stripping solution and recovered electrolytically on site.
Precious metal refineries. Chloride, thiourea and aqua regia gold liquors where the plant prefers to recover gold electrolytically onto a cathode instead of reducing it chemically.
Electroplating and surface finishing shops. Spent gold plating baths, drag-out rinse water and ion-exchange strip liquors, where the gold is recovered as metal and the rinse water is returned to the line.
Jewellery, watch and pen-nib manufacturers. Workshop drag-out and pickling liquor from gold-bearing process solutions, recovered inside the plant instead of sold as a low-grade residue.
Assay, jewellery and refining laboratories. Small batch recovery from process solutions and residues where the value is high and the volume is low.
Plant upgrades and retrofits. Replacement of an ageing or corroded cell bank, or the addition of a cyclone cell stage to lift recovery from an existing dilute liquor stream.
Turnkey gold room projects. Greenfield plants where the elution column, electrowinning cells, rectifier, sludge handling and smelting scope are awarded as a single package.

Applicable Raw Materials

Each feed below enters the circuit at the point its composition requires. Liquors carrying copper above about 5 g/L are treated ahead of the cells, and liquors carrying suspended solids are clarified first, which is why the flow sheets differ even though the cell bank is common. Output is stated for each feed.

Carbon elution liquor from a Zadra circuit. Atmospheric or pressure Zadra eluate carrying gold at high concentration with free cyanide and caustic. Output: gold sludge on steel wool cathode; barren liquor returned to the elution circuit.
Carbon elution liquor from an AARL circuit. Dilute caustic eluant with lower reagent strength and a lower gold concentration than Zadra eluate, handled at a reduced current density. Output: gold sludge; barren liquor returned to elution.
Carbon elution liquor from an integrated pressure stripping unit. Eluate at 150 °C and 0.5 MPa from the stripping column, trimmed to cell temperature before it reaches the cell bank. Output: gold sludge; barren liquor returned to elution.
Heap leach pregnant solution. Solution from heap or dump leaching after carbon adsorption and elution, or a high-grade pregnant solution fed directly to the cells where the grade justifies it. Output: gold sludge; barren solution returned to the leach.
Intensive cyanidation liquor. High-grade solution from the intensive leach of gravity gold concentrate. Output: gold sludge; barren liquor returned to intensive leach.
E-waste and PCB stripping solution. Cyanide or non-cyanide deplating solution from the stripping of gold-plated contacts, connectors and edge fingers. Output: gold sludge; stripped solution returned to the stripping line.
Spent gold plating baths. Exhausted cyanide, citrate or sulphite gold electrolytes from electroplating and electronic plating lines. Output: gold sludge; spent bath discharged or regenerated.
Drag-out and rinse water. Plating rinse, ion-exchange strip liquor and workshop wash water carrying gold at milligrams per litre, recovered on the cyclone cell. Output: gold sludge; polished water below discharge limits.
Chloride and aqua regia gold liquor. Gold-bearing chloride solution from in-house dissolution, where the plant prefers electrolytic recovery over chemical reduction; the superimposed AC-DC supply suppresses anodic film formation. Output: gold sludge; chloride liquor returned to the circuit.
Thiosulfate, glycine and thiourea leach liquors. Non-cyanide leach solutions from plants that have moved away from cyanide, run at the current density established by pilot test. Output: gold sludge; barren liquor returned to leach.
Silver-bearing eluate. Eluate with substantial co-dissolved silver, co-deposited at a reduced current density and separated afterwards in the nitrate parting stage. Output: gold sludge; silver chloride by-product.
Mercury-bearing solution. Liquor from feed carrying mercury, where the cell off-gas is drawn through a scrubber and the mercury abatement system. Output: gold sludge; captured mercury stream.
Barren and return liquors. Polishing duty on process liquors and pond water that carry residual gold at low concentration. Output: polished liquor below 1 g/t Au.

Finished Products

The line is configured to deliver the product form the customer actually sells, from a gold-bearing cathode to a cast bar with a floatable assay.

Gold-loaded cathode. Stainless steel wool, multi-layer mesh or expanded metal cathode carrying the deposit, removed from the cell at the end of a campaign and logged by weight. Output: gold-loaded cathode with a deposition record.
Washed gold sludge. The deposit stripped from the cathode with high-pressure water, rinsed and dewatered, carrying 30–75% Au depending on the feed and the cathode type. Output: washed gold sludge for parting.
Parted and dried gold powder. Sludge treated with nitric acid to dissolve base metals and silver, then filtered, washed and dried, ready for calcining and fluxing. Output: dried gold powder.
Doré bullion. Fusion of the dried gold with a borax and silica flux in an induction furnace and casting into bars assaying 50–75% Au. Output: doré bars with a fire assay certificate.
High-fineness gold. Doré routed to the refining line produces gold of 99.9% and above, and 99.99% where the downstream specification calls for it. Output: refined gold ingots to standard.
Silver chloride by-product. Silver dissolved in the nitric acid parting stage is precipitated as silver chloride, then reduced and melted to crude silver, or sold as the salt. Output: silver chloride or crude silver ingot.
Recovered steel wool. Cathode wool that is not consumed is washed, returned to the cathode frame and reused, which is why the consumable cost of the cell is low. Output: wool returned to cathode service.
Barren liquor returned to circuit. Reagent-adjusted liquor with residual gold below 1 g/t, returned to the elution column or the leach instead of being discharged. Output: closed-circuit process liquor.
Polished effluent. Any bleed that leaves the circuit passes resin and cementation polishing and, where required, cyanide destruction before discharge. Output: compliant effluent.
Cementation and resin residue. Gold recovered from the polish stage as a residue that is returned to the smelter, closing the mass balance on the campaign. Output: residue returned to the furnace.

Gold Electrowinning Equipment Process Flow

The nine stages below form the standard gold electrowinning flowsheet from pregnant solution to doré bullion. Where the elution column, the carbon regeneration kiln or the stripping line is supplied as part of the same contract, those units are integrated upstream. Engineering setpoints such as cathode area, current density, cell voltage, circulation rate and parting conditions are fixed during process design against your own liquor assay.

Gold electrowinning equipment flow — pregnant solution to doré bullion, with the barren liquor returned to leach.

Steps 1 — Pregnant Solution Assay and Recovery Sizing. Incoming liquor is sampled and analysed for gold, silver, copper, nickel, iron, free cyanide, pH and conductivity, by ICP-OES and AAS on filtered and digested splits with a titration for free cyanide. The assay fixes the gold load per hour, the impurity-to-gold ratio and the liquor temperature, and those three figures set the cathode area, the number of cells and the rectifier rating for the campaign. Liquors with a copper-to-gold ratio above the tolerable band are flagged at this stage because the copper knockout stage has to be brought into the flow sheet before any current is applied. Every batch is given a lot number and a route sheet so the campaign mass balance can be closed against the doré assay at the end. Reagent and utility input: sampling and splitting equipment, ICP-OES, AAS, filtration rig, titration apparatus. Output: gold load calculation, circuit sizing sheet and batch record.

Steps 2 — Gold Solution Conditioning and Copper Knockout. The liquor is clarified and its chemistry is brought into the working window before it is allowed near an electrode. Suspended carbon fines, mineral slime and precipitated matter are removed by filtration or settling, because a conductive particle bridging anode and cathode short-circuits the cell and a non-conductive film blinds the cathode. Copper is removed where the assay calls for it, by precipitation or by a dedicated cementation or electrowinning stage, so that copper does not deposit alongside gold and inflate the impurity load of the sludge. The pH is adjusted into the 10.5–12 band with sodium hydroxide and free caustic is brought to 1–2% NaOH, which keeps hydrogen cyanide from forming at the cathode and keeps conductivity above 1.7 S/m. Where chloride is present, the chloride level is set with the anode material and the superimposed AC-DC supply in mind, because chloride service attacks stainless anodes. Reagent and utility input: clarification and filtration set, sodium hydroxide dosing, copper knockout reagents, pH and conductivity instrumentation. Output: conditioned cell feed; copper-bearing by-product stream.

Steps 3 — Eluate Preheating and Flow Trim. Solution temperature and circulation rate are set before the cell is energised. Eluate usually arrives from the elution column hot, in the region of 60–70 °C, and it is trimmed to the 40–60 °C optimum for cell operation through a shell-and-tube heat exchanger, because deposition slows below 40 °C and cyanide begins to break down above 60 °C. Circulation is held at two to four bed volumes per hour through the cell bank by the circulation pump, which keeps the boundary layer at the cathode thin enough for a useful deposition rate and keeps the liquor homogeneous in the cell rather than stratified around the electrodes. Level make-up keeps the electrodes immersed as the liquor is consumed and evaporated, and a flow meter on the cell inlet confirms the circulation rate the design assumes. Reagent and utility input: shell-and-tube heat exchanger, electric heater, circulation pump, flow meter, level make-up system. Output: stable cell feed at the design temperature and flow rate.

Steps 4 — Cathode Loading and Cell Charging. The cathode is prepared and weighed before it goes into the cell, because the cathode weight is part of the deposition record. Stainless steel wool is packed into the cathode frame or basket at the specified density, multi-layer stainless mesh is assembled on its frame, or an expanded metal sheet is fitted directly, and the loaded cathode is weighed and its tare recorded. Anodes are inspected for coating damage and set on the correct pitch so that the electrode gap and therefore the current distribution are the ones the design assumes. The cell is charged, the busbars are torqued down on the flat contact faces, the hood is closed and the connections are checked for insulation integrity before the rectifier is switched on. Reagent and utility input: cathode frames and baskets, stainless steel wool or mesh, anode plates, torque tools, insulation tester, weighing station. Output: charged cell bank with a cathode tare record.

Steps 5 — Ramped Current Start-Up and Gold Deposition. The rectifier is brought up over a controlled ramp rather than switched straight on, because an abrupt current at full setpoint produces a coarse, poorly adherent deposit that sheds into the cell. Cell voltage is set in the 2.0–4.0 V DC band and cathode current density in the 8–25 A/m² band, with 10–15 A/m² as the design centre for eluate duty and a reduction to about 8 A/m² where silver is co-deposited. The automatic programme holds three consecutive voltage and time stages, so a campaign can start gently, run at the deposition setpoint and finish without an operator watching every hour. Gold ions are reduced at the cathode and deposit as metal on the wool or mesh while the inert anode evolves oxygen, and in an alkaline cyanide circuit the secondary anodic reaction oxidises cyanide to cyanate, which is why the anode is a consumable item rather than an inert surface in practice. Reagent and utility input: rectifier cabinet with three-stage programme, busbar connections, cell voltage and current instrumentation. Output: gold deposit on the cathode surface.

Au(CN)₂⁻ + e⁻ → Au + 2CN⁻

4OH⁻ − 4e⁻ → 2H₂O + O₂

Steps 6 — Deposit Quality Monitoring and Correction. Deposit quality is read from the electrical record rather than judged by eye alone. Cell voltage and busbar temperature are logged continuously, and a rising voltage at constant current is the signature of a cathode blinding over with gold, while a falling current at constant voltage points to a short circuit from suspended carbon or a stray metal particle. The cathode surface is inspected at the scheduled interval: a bright, adherent deposit is what the setpoints are aiming for, and a blackened, poorly bonded deposit means the current density is too high and the voltage is trimmed down before the metal oxidises and falls off as fine sludge that is harder to harvest. Where the deposition cycle is running long, the barren liquor is sampled and assayed so the end of the campaign is set by a measured residual gold figure rather than by a schedule. Reagent and utility input: cell voltage and busbar temperature monitoring, deposit inspection, barren liquor sampling and assay. Output: corrected setpoints; deposition cycle closed on measured residual gold.

Steps 7 — Cathode Harvest and Gold Sludge Washing. At the end of the campaign the rectifier is shut down before the cathode is touched, and the loaded cathode is lifted out and allowed to drain back into the cell. The deposit is washed in the cell where an installed spray system is fitted, which avoids taking the electrodes out for servicing, and the cathode is then moved to the wash station where high-pressure water strips more than 97% of the deposit from the wool or mesh. The stripped wool is rinsed, inspected, returned to the cathode frame and reused, and the portions that are consumed or fouled are dissolved off in hydrochloric acid to recover the residual gold rather than being scrapped. The washed sludge is dewatered on a vacuum filter and weighed, which closes the first step of the campaign mass balance. Reagent and utility input: cathode lifting gear, in-cell spray system, high-pressure wash station, vacuum filter, hydrochloric acid for wool dissolution. Output: washed gold sludge; steel wool returned to cathode service.

Steps 8 — Sludge Acid Wash, Drying and Doré Smelting. The sludge is part-dry before it is melted, because iron from the wool, copper, and silver all have to come out first. It is treated with nitric acid, which dissolves base metals and silver while leaving the gold as a residue, then filtered, washed and dried. The dissolved silver is precipitated from the parting liquor as silver chloride with sodium chloride and recovered separately. The dried gold is calcined between roughly 400 and 750 °C to burn off residual carbon, wool fibre and organic matter, then mixed with a borax and silica flux and melted in an induction furnace at 1,150–1,300 °C. The melt is poured into ingot moulds and the bars are weighed and fire assayed, giving doré bullion of 50–75% Au with the balance substantially silver. Reagent and utility input: nitric acid parting reactor, filtration and washing set, drying oven, calcining furnace, induction melting furnace, borax and silica flux, ingot moulds. Output: doré bullion at 50–75% Au; silver chloride by-product.

3Ag + 4HNO₃ → 3AgNO₃ + NO + 2H₂O

Ag⁺ + Cl⁻ → AgCl↓

Steps 9 — Barren Liquor Polishing, Make-Up and Return. The campaign is closed on the liquor, not on the bar. Spent electrolyte from the cells is assayed for residual gold, and where the figure is above the target it passes through an ion-exchange resin train followed by zinc or iron cementation on the polish filtrate, with the cementation residue returned to the smelter instead of leaving the plant. Cyanide and caustic are analysed and made up to strength, and the liquor is returned to the elution column or the leach circuit as a closed loop, which is the operational advantage of the electrochemical route over zinc precipitation. Off-gas from the cells and the parting and melting stages is drawn through a caustic scrubbing train and a demister, mercury abatement is brought into service where the feed carries mercury, and the emission and effluent records are filed with the batch record. Reagent and utility input: AAS for residual gold, ion-exchange resin columns, cementation tank, reagent make-up station, caustic scrubbing train with demister. Output: barren liquor below 1 g/t Au returned to circuit; emission and effluent record.

Zn + 2Au(CN)₂⁻ → 2Au + Zn(CN)₄²⁻

Product Structure (Main Equipment)

The line is assembled from the following equipment groups. Materials of construction are selected per stream: polypropylene and PVC for the cell bodies, hoods and atmospheric solution lines, 316L stainless steel and titanium for electrode frames, busbars and the chloride duty, and PTFE and PVDF for seals, gaskets and instrumentation wetted parts.

Solution conditioning and feed handling

Feed storage and mixing tank. Agitated polypropylene tank that receives eluate or pregnant solution, blends the batch and gives the conditioning stage a buffer volume between the elution column and the cells.
Clarification and filtration set. Bag or cartridge filtration with a settling option ahead of the cells, sized to remove carbon fines and mineral slime that would otherwise short-circuit the electrode gap or blind the cathode.
Copper knockout reactor. Precipitation, cementation or pre-electrowinning vessel for copper removal, with reagent dosing, agitation and pH control, used where the feed carries copper above the tolerable level for direct gold deposition.
pH and conductivity control loop. In-line pH and conductivity measurement with sodium hydroxide dosing, holding the liquor in the 10.5–12 pH band and above 1.7 S/m at the cell inlet.
Conditioning circulation pump set. Transfer and circulation pumps in corrosion-resistant wetted materials, with a bypass for reagent dissolution.
Reagent make-up station. Sodium hydroxide, sodium cyanide, sodium chloride and acid dosing points with metering and level indication, arranged so that reagents are dissolved and confirmed before they enter a circulating stream.

Heating, circulation and cell feed

Shell-and-tube heat exchanger. Trims eluate from elution temperature to the 40–60 °C cell operating band, with the tube material selected against the chloride and cyanide content of the stream.
Electric heater and temperature control. Supplementary heating with temperature control and high-limit protection, so the cell feed holds its setpoint when the elution column is not running.
Circulation pump with flow meter. Delivers two to four bed volumes per hour through the cell bank, with a magnetic flow meter on the cell inlet for live confirmation of the design circulation rate.
Level make-up and level control system. Automatic make-up to hold the electrode immersion depth and the liquor strength as volume is consumed and evaporated, with level indication and alarms.
Solution storage and return tank. Receives spent electrolyte for polishing, make-up and return, sized as a buffer between the cell bank and the elution or leach circuit.

Electrowinning cell bank and electrodes

Flat-plate electrowinning cells. Integrally welded polypropylene or PVC cells rated to 80 °C, with an optional 316L stainless steel shell lined with polypropylene for electrical isolation and heat insulation, and flat copper busbars at the front for cleaning and electrode repositioning.
Cyclone electrowinning cells. Concentric cylindrical cells with a removable cathode sleeve, where electrolyte is injected at the base and leaves as a swirling film past the electrodes; used on dilute and impurity-heavy liquors where a flat-plate cell cannot hold a workable current efficiency.
Cathodes and cathode frames. Stainless steel wool of about φ0.13 mm microfilament, multi-layer 304 or 316 stainless mesh, or expanded metal sheet, mounted in stainless or plastic frames sized to the cell, loaded to the specified packing density and weighed before service.
Anodes. Perforated 304 or 316 stainless steel plate for cyanide duty, MMO-coated titanium where the service life justifies the cost, and lead alloy where chloride is present, set on the design electrode pitch.
In-cell cathode wash system. Installed spray bar or portable high-pressure sprayer that strips the deposit inside the cell, cutting electrode handling and the time the cell is out of service.
Cell hoods and exhaust plenum. Closed transparent hoods with a locking facility and an adjustable extraction connection, so that hydrogen and cyanide-bearing mist are drawn away from the working face rather than into the gold room.
Sludge and launder arrangement. Sloping launder and notched outlet weir that decant liquor while the gold sludge settles in the cell, so that sludge is removed without dismantling the anode set.
Busbars, contact strips and insulation. Flat copper busbars with titanium-clad copper contact strips and insulated supports, with busbar temperature measurement at each cell.

Rectifier, control, sludge and emissions

Rectifier cabinet. Thyristor or high-frequency switch-mode rectifier converting plant AC to controlled DC in the 0–4 V and 0–200 A to 0–6 V and 0–1,250 A range by module, with steady-current and steady-voltage modes, ripple held low and a superimposed AC-DC option for chloride service.
PLC control and touch-screen HMI. Holds the three-stage voltage and time programme, the start-up ramp and the level make-up sequence, logs cell voltage, cell current and busbar temperature, and interlocks the rectifier out on an abnormal reading or on a loss of circulation.
Cathode lifting and handling set. Lifting beam, drain station and cathode trolley that move loaded cathodes between the cell, the wash station and the filter without manual lifting over the liquor.
Cathode wash and vacuum filtration station. High-pressure water stripping station with a vacuum filter plate and diaphragm pump that dewater the washed sludge while keeping the wash volume low.
Nitric acid parting reactor and silver recovery set. Heated, agitated acid-resistant reactor for sludge parting, with filtration, washing and drying equipment and a silver chloride precipitation and reduction set for the by-product.
Calcining and induction melting furnace. Calcining furnace operating to roughly 750 °C for burning off carbon and wool residue, and an induction melting furnace at 1,150–1,300 °C with fluxing and ingot moulds for casting doré bars.
Caustic scrubbing train and demister. Alkali washing tower, absorption tower and demister for the cyanide and acid mist drawn from the cells, the parting reactor and the furnace, with the scrubber liquor monitored before discharge.
Mercury abatement unit. Dedicated abatement for feed carrying mercury, applied to the cell off-gas and the furnace off-gas where the assay requires it.
Assay laboratory equipment. Atomic absorption spectrometer for residual gold in the barren liquor, ICP-OES for the feed assay, fire assay equipment and a fume hood, so that feed, process and product figures are generated in-house on the same day as production.

Product Features

Single-supplier gold room scope. Liquor conditioning, cell bank, rectifier, control system, sludge handling, smelting and scrubbing are delivered as one contract, so the interfaces between the elution column and the bullion are held by one party.
Two cell geometries, one process. The flat-plate cell covers the mainstream eluate duty and the cyclone cell covers dilute and impurity-heavy liquors, and the flowsheet assigns each of your streams to the cell that can actually recover it.
Chemistry set by the feed, not by a template. Copper knockout, clarification, pH and free caustic are fixed from the assay, because a gold electrowinning circuit that ignores the copper in the eluate produces a dirty sludge and a purity argument at the furnace.
Deposit quality held by the electrical record. Cell voltage, current and busbar temperature are logged continuously, so blinding, short circuits and a coarsening deposit are visible as a trend instead of being discovered when the cathode is pulled.
Three-stage automatic programme. Each campaign is run as a gentle start, a steady deposition stage and a finishing stage at different voltage and time settings, which is what produces an adherent deposit and makes unattended running realistic.
End of campaign judged on an assay. The deposition cycle closes when the barren liquor assays below the target, not when a timer expires, which protects the recovery figure on a feed whose grade drifts.
Deposit removed without stripping the cell. In-cell washing or a high-pressure wash station takes more than 97% of the deposit off the cathode, and the wool that survives is returned to service rather than discarded.
Sludge taken through to bullion. Nitric acid parting, silver chloride recovery, drying, calcining, fluxing, induction melting and ingot casting are inside the scope, so the customer receives a weighed, fire-assayed doré bar.
Barren liquor returned as a reagent stream. Cyanide and caustic are made up and the liquor goes back to elution or leach, which removes the reagent destruction and the effluent load that a zinc precipitation route leaves behind.
Residual gold polished to below 1 g/t. Resin and cementation are part of the standard scope, and the polish residue returns to the furnace rather than to the tailings dam.
Enclosed emission control. Closed transparent hoods, adjustable extraction, a caustic scrubbing train with a demister and optional mercury abatement keep the working face and the stack within limits.
Non-cyanide chemistry supported. Thiosulfate, glycine, thiourea and chloride liquors are accepted, with a superimposed AC-DC supply available where anodic film formation would otherwise limit the chloride duty.
Assay-linked traceability. Feed assay, cathode tare and loaded weight, barren liquor assay and doré fire assay are held together with the campaign mass balance, which is what makes a recovery guarantee checkable.
Configuration to the feed, not to a catalogue. Cathode area, cell count, rectifier rating and footprint are engineered from the gold load per hour and the impurity profile, so the plant is sized to the metal actually available.

Technical Parameters And Specifications

Engineering setpoints below are the standard design basis and are confirmed or adjusted during process design once the liquor assay is known. Capacity, cathode area, footprint, installed power and utility consumption are engineered to your throughput and site conditions.

Gold electrowinning equipment — technical parameters and specifications.

Parameter	Specification
Primary duty	Electrolytic recovery of gold, with co-deposited silver, from gold-bearing process solution onto a cathode; not electrorefining of a cast anode
Accepted liquors	Carbon elution liquor from Zadra, AARL and integrated pressure stripping circuits; heap and dump leach pregnant solution; intensive cyanidation liquor from gravity concentrate; e-waste and PCB stripping solution; spent gold plating baths and drag-out rinse; chloride and aqua regia gold liquor; thiosulfate, glycine and thiourea liquors; jewellery workshop drag-out
Feed gold concentration	About 0.1 to 2,000 mg/L Au; the cell geometry and cathode area are selected from the assay, and dilute streams are handled on the cyclone cell
Feed silver	Co-deposited with the gold; eluate carrying substantial silver is run at a reduced current density and the silver is separated in the nitrate parting stage
Copper limit into the cells	Copper above about 5 g/L in the feed is knocked out ahead of the cells, because it deposits at a potential close to that of gold
Solution pH	10.5 to 12 maintained; below pH 10.5 hydrogen cyanide evolves at the cathode, and stainless anodes corrode below pH 12.5 in chloride service
Free caustic	1 to 2% NaOH on cyanide eluate service
Solution conductivity	Above 1.7 S/m at the cell inlet
Electrolyte temperature	40 to 60 °C operating optimum; maximum electrolyte operating temperature 90 °C; eluate is normally received at 60 to 70 °C from elution and trimmed to the cell band
Cell voltage	2.0 to 4.0 V DC across the cell; 3.5 V is the practical ceiling on cyanide circuits, because a higher voltage raises energy demand and co-deposits impurities
Cathode current density	8 to 25 A/m²; 10 to 15 A/m² design centre for eluate service, reduced to about 8 A/m² where silver is co-deposited
Single cathode current	60 to 90 A per cathode on the flat-plate cell at rated load
Cathode materials	Stainless steel wool of about φ0.13 mm microfilament in a stainless or plastic frame; multi-layer 304 or 316 stainless mesh; expanded metal sheet; removable titanium drum on the cyclone cell
Cathode gold loading	Up to about 20 times the steel wool weight; the cathode is harvested before that limit is reached
Anode materials	Perforated 304 or 316 stainless steel plate; MMO-coated titanium for long service life; lead alloy where chloride is present
Anode slime	None; electrowinning uses an inert anode that evolves oxygen, so no anode bag and no anode slime circuit are required
Cell body	Integrally welded polypropylene or PVC rated to 80 °C; optional 316L stainless steel shell with polypropylene lining for electrical isolation and heat insulation
Electrode geometry	Flat vertical plates on the flat-plate cell; concentric cylinder with a removable cathode sleeve on the cyclone cell
Busbar and contacts	Flat copper busbar with titanium-clad copper contact strips, insulated supports, continuous busbar temperature measurement
Rectifier range	0 to 4 V and 0 to 200 A, up to 0 to 6 V and 0 to 1,250 A by module; steady current or steady voltage; low ripple; superimposed AC-DC option for chloride service
Current efficiency	5 to 10% typical for gold from dilute eluate; the balance of the charge runs water reduction and oxygen evolution at that concentration
Specific energy	About 10 to 20 kWh per kilogram of gold deposited
Deposition cycle	8 to 16 h per campaign, or continuous with scheduled cathode changes; the cycle closes on a measured barren liquor assay rather than on a timer
Barren liquor target	Below 1 g/t (1 mg/L) Au after resin and cementation polishing
Circulation rate	2 to 4 bed volumes per hour through the cell bank
Elution interface, atmospheric Zadra	Elution below 100 °C at atmospheric pressure, 48 to 72 h; eluant flow 1.5 to 2.5 BV per hour
Elution interface, pressure Zadra	Elution at about 140 °C, electrowinning below 90 °C, 8 to 12 h cycle
Elution interface, integrated pressure stripping	Elution at 150 °C and 0.5 MPa, 6 to 10 h cycle, eluant flow 2 to 4 BV per hour
Elution reagents	NaCN not less than 2% with NaOH not less than 1% on the high-strength route; 3 to 5% NaOH eluant on the dilute AARL route
Cathode stripping	High-pressure water removes more than 97% of the deposit; residual wool is dissolved off in hydrochloric acid where the wool is consumed
Sludge parting	Nitric acid parting to dissolve base metals and silver, followed by filtration, washing and drying
Silver by-product	Silver chloride precipitated from the parting liquor with sodium chloride, then reduced and melted to crude silver or sold as the salt
Drying and calcination	400 to 750 °C to burn off residual carbon, wool fibre and organic matter before smelting
Smelting and casting	Induction furnace at 1,150 to 1,300 °C with borax and silica flux; doré ingots assaying 50 to 75% Au
Refined product	Doré routed to the refining line for gold of 99.9% and above, and 99.99% where the downstream specification requires it
Verification	Feed assay by ICP-OES, residual gold in the barren liquor by AAS, fire assay in duplicate on each doré lot, full campaign mass balance
Solution polishing	Ion-exchange resin train followed by zinc or iron cementation, with the cementation residue returned to the smelter
Off-gas control	Closed transparent hoods, adjustable extraction and a caustic scrubbing train with a demister; mercury abatement available for mercury-bearing feed
Cyanide handling	Any bleed leaving the circuit is treated by alkaline chlorination or hydrogen peroxide; the return stream is made up to strength and recycled rather than destructed
Instrumentation and control	PLC with touch-screen HMI, three-stage voltage and time programme, start-up ramp, level make-up, busbar temperature monitoring and rectifier interlock
Operating mode	Manual or automatic; automatic mode runs the three-stage programme unattended with alarm-only supervision
Materials of construction	Polypropylene and PVC, 316L stainless steel, titanium, PTFE and PVDF
Capacity, footprint, power, utilities	Engineered to feed assay, target gold output and site conditions

Technical Advantages

The definition is stated before the specification. Electrowinning uses an inert anode and recovers metal from solution; electrorefining dissolves a cast anode and purifies it. Getting that right removes the anode bag, the anode slime circuit and the anode scrap handling from the equipment list entirely. Result: a shorter, cheaper flowsheet than an electrolytic refinery covering the same duty.
Two cell geometries matched to two liquor classes. The flat-plate cell handles mainstream eluate at 10–15 A/m², and the cyclone cell handles the dilute and impurity-heavy streams by renewing the cathode boundary layer continuously instead of relying on diffusion at a flat surface. Result: recovery from liquors a flat-plate bank would have to write off.
Rectifier sizing reduced to three numbers. Current density, cathode area and total current are linked by one relation: total current equals current density multiplied by total cathode area, and per-cathode current equals total current divided by the number of cathodes. A plant targeting 5 kg of gold per day over 20 hours needs about 8,500 ampere-hours, which at a current efficiency of 8% — normal for eluate — requires roughly 425 A; spread over 14 cathodes of about 2.5 m² that is 35 m² of cathode area and about 12 A/m², inside the working window. Result: a rectifier ordered from arithmetic rather than from a sales assumption.
Current efficiency reported honestly. 5–10% is the normal band for gold from dilute eluate, because most of the charge goes into water reduction and oxygen evolution at that concentration. Result: no over-promise on the one number that cannot be delivered, and the operator guided to judge the circuit by electrolysis rate and barren liquor assay instead.
Copper dealt with before it reaches the cathode. Copper in cyanide solution deposits close to the gold, so the flowsheet places a knockout stage ahead of the cells where the assay calls for it. Result: a cleaner sludge, a lower parting reagent demand and a doré bar that meets its assay without a re-melt.
Deposit quality managed as a trend. Cell voltage, current and busbar temperature are logged continuously and read against each other: a rising voltage at constant current means the cathode is blinding, and a falling current at constant voltage means a short circuit. Result: corrections made during the campaign instead of after a poor harvest.
Campaign ends on an assay, not a clock. The deposition cycle closes when the barren liquor assays below target, which matters on a feed whose gold grade drifts from batch to batch. Result: the recovery figure holds across variable feed instead of falling away when the liquor gets lean.
Cathode consumables kept low. Steel wool cathodes are reused after washing, the deposit is stripped with high-pressure water, and the portion of wool that is consumed is dissolved for its residual gold. Result: a cathode cost per kilogram of gold that is dominated by stripping labour rather than by consumable purchases.
Sludge converted to a saleable bar. Parting, silver recovery, calcining, fluxing, induction melting and ingot casting are all inside the equipment scope. Result: the customer sells a fire-assayed doré bar, not a wet sludge whose value is a guess.
Silver recovered rather than lost to the parting liquor. Silver dissolved in the nitric acid stage is precipitated as silver chloride and reduced to crude silver. Result: a second revenue stream from the same feed, at no extra cell capacity.
Barren liquor returned as reagent, not discharged as waste. Cyanide and caustic are made up and the liquor returns to elution or leach, and residual gold is polished below 1 g/t by resin and cementation. Result: the closed-loop operation that distinguishes electrowinning from zinc precipitation, with the effluent load removed.
Hydrometallurgical energy profile. The circuit operates between 40 and 60 °C, with the only high-temperature duty confined to the calcining and melting furnaces at the end of the route. Result: a low thermal energy demand compared with a smelting-based recovery route.
Emissions captured where they are generated. Hydrogen from the cathode, cyanide-bearing mist and acid vapour are drawn from beneath closed hoods into a caustic scrubbing train with a demister, and mercury abatement is available where the feed carries mercury. Result: a gold room that can be worked in without relying on general ventilation.
Non-cyanide routes available on the same platform. Thiosulfate, glycine, thiourea and chloride liquors are accepted, with current density and anode material set by pilot test rather than by assuming cyanide behaviour. Result: the same equipment survives a change of lixiviant, which is where several jurisdictions are heading.
Verifiable numbers. Feed assay, cathode tare and loaded weight, barren liquor assay, parting weights and doré fire assay are held together with the campaign mass balance. Result: recovery and fineness claims that can be audited against the customer’s own sampling.

Service Process

Step 1 — Enquiry and liquor data review. You provide the liquor type, an assay or typical analysis, daily gold volume and any site constraints. We confirm whether the stream suits direct electrowinning or needs conditioning, copper knockout or concentration first. Output: preliminary process route and a clear scope statement.

Step 2 — Laboratory and pilot test work. A representative liquor sample is run through conditioning, electrowinning on a bench cell and the sludge parting route. The working current density, cathode type, expected electrolysis rate, barren liquor residual and doré assay are established on real solution. Output: test report with the recovery and assay figures for your liquor.

Step 3 — Process design and flowsheet. We issue the process flow diagram, the gold and reagent mass balance, the cell and cathode area calculation, the rectifier rating calculation, the equipment list and the utility schedule, with all engineering setpoints stated. Output: process design package and mass balance.

Step 4 — Equipment manufacture and factory acceptance test. Cells, electrode frames, rectifier cabinet, control panel, parting reactor, furnaces and scrubbers are fabricated, assembled and function-tested before shipment, with material certificates retained and the control interlocks proved on the test bench. Output: factory acceptance test record and material certificates.

Step 5 — Layout and site interface design. Equipment layout, floor loading, power supply points, water, compressed air, ventilation, gas detection and the elution column interface are defined against your building and your existing circuit. Output: layout and utility interface drawings.

Step 6 — Shipment, installation and commissioning. Equipment ships with the documentation set, and our engineers supervise installation, insulation and leak testing, electrical and control checkout, and water commissioning before any liquor is introduced. Output: mechanically complete circuit, ready for liquor commissioning.

Step 7 — Operator training and SOP handover. Operating procedures, cyanide and acid handling, preparation of a cathode, current density control, deposit judgement, cathode harvesting, sludge parting, sampling and assay practice, and routine maintenance are taught on site with the actual equipment. Output: trained operators and a signed SOP set.

Step 8 — Production trial and performance acceptance. The circuit is run on your liquor and the agreement is confirmed against measured electrolysis rate, barren liquor residual gold, doré assay and reagent consumption. Output: performance test certificate and the first doré assay certificate.

Step 9 — Ongoing technical support. Process troubleshooting, setpoint review as your feed mix changes, spare parts at cost, and reagent or cathode supply on request. Output: continuing production support.

Warranty And After-Sales Service

Warranty period. Twelve months from successful commissioning or eighteen months from shipment, whichever falls earlier, covering design, manufacture and assembly defects of the supplied equipment. Excluded: consumables such as cathode wool, filter cloths, gaskets, anode coatings, crucibles, fluxes and reagents, and damage from misuse or from liquor outside the agreed specification.
Performance commitment. Where the liquor matches the tested sample, we stand behind the agreed electrolysis rate, the barren liquor residual gold figure and the doré assay band established in the process design package, with the performance test as the reference point. Output: documented performance acceptance.
Spare parts. Wear and critical spares — cathode wool, cathode frames, anodes, filter cloths, gaskets, seals, pump parts, thermowells and rectifier components — are quoted at the time of order and held available afterwards, with a recommended on-site stock list issued at handover. Output: spare parts list and supply at cost.
Remote support. Telephone, email and messaging support for operating questions, alarm interpretation, deposit quality problems and process troubleshooting, whenever your circuit is running. Output: same-day response on operating issues.
On-site service. Engineer attendance for warranty faults, process problems and periodic check-ups, with response and attendance times agreed in the contract according to your location. Output: on-site intervention and a service report.
Operator retraining and refresher visits. Additional training, refresher sessions and setpoint optimisation visits as your liquor mix, cathode type or product specification changes. Output: updated training and procedure revisions.
Documentation set. Operation and maintenance manuals, process flow diagram, piping and instrumentation diagram, equipment layout, electrical drawings, spare parts catalogue, safety data for cyanide, caustic and nitric acid, and the emergency response plan for a cyanide release. Output: complete documentation handover.
Compliance support. Assistance with effluent and emission parameter declarations, scrubber performance verification, and the handling, storage and disposal requirements for the reagents and intermediates used in the circuit. Output: technical data for your permit applications.
Lifetime technical support. The process design remains yours, and our engineering team remains available for modification and expansion advice, including the addition of a cyclone cell stage or a larger cell bank, for the operating life of the plant. Output: long-term engineering continuity.

Frequently Asked Questions

What is gold electrowinning equipment used for?

Gold electrowinning equipment recovers gold that is already dissolved in a process solution. A direct current is passed through the liquor, gold ions are reduced at the cathode and deposit as metal, and the loaded cathode is harvested, washed and melted to a doré bar. It is used at the end of carbon-in-leach, carbon-in-pulp, heap leach and stripping circuits, where the gold has been concentrated into a small volume of solution and has to be turned back into metal without buying zinc dust or building a filtration plant.

What is the difference between gold electrowinning and gold electrorefining?

Electrowinning recovers metal from solution, electrorefining purifies metal that is already solid. In electrowinning the anode is inert — stainless steel, coated titanium or lead alloy — and the anodic reaction is oxygen evolution, so nothing dissolves at the anode and there is no anode slime circuit. In electrorefining a cast anode of impure gold is dissolved anodically and pure gold plates onto the cathode, which means the feed has to be melted and cast into anodes first and the slime has to be treated. A gold room described as electrowinning therefore has a different equipment list from an electrolytic refinery, and the two are not interchangeable.

What feed solutions can the equipment recover gold from?

The circuit accepts carbon elution liquor from Zadra, AARL and integrated pressure stripping circuits, heap and dump leach pregnant solution, intensive cyanidation liquor from gravity concentrate, e-waste and PCB stripping solution, spent gold plating baths and drag-out rinse, chloride and aqua regia gold liquor, thiosulfate, glycine and thiourea liquors, and jewellery workshop drag-out. Liquors carrying copper above about 5 g/L are treated ahead of the cells, and liquors carrying suspended solids are clarified first, so the flow sheet around the cell bank differs with the feed.

How much gold does the circuit recover, and what is left in the barren liquor?

The electrolysis rate is 99% or higher from pregnant solution, and the barren liquor leaves the circuit below 1 g/t Au after ion-exchange resin and cementation polishing. Recovery is measured across the whole circuit rather than at the cell alone: spent electrolyte is assayed for residual gold, polished on resin and cementation, and the cementation residue returns to the smelter instead of leaving the plant. The practical figure for your liquor is established in the laboratory test, because it depends on the gold concentration, the copper level and how many times the liquor passes the cell bank.

What current density and cell voltage should be used?

Cell voltage is normally held between 2.0 and 4.0 V DC, with 3.5 V as the practical ceiling on cyanide circuits because a higher voltage raises energy demand and co-deposits impurities. Cathode current density runs between 8 and 25 A/m², with 10 to 15 A/m² as the design centre for eluate service and a reduction to about 8 A/m² where silver is co-deposited. Above roughly 15 A/m² the deposit coarsens, forms dendrites and blinds the cathode; below the working band the deposition rate falls away. Both figures are confirmed on your own liquor in the laboratory test before the plant is built.

How is the rectifier sized for a given gold production target?

Size the rectifier from the gold, not from the cell. Gold deposits at 7.35 g per ampere-hour in the aurocyanide system at 100% current efficiency, so at the 8% current efficiency normal for eluate the effective rate is about 0.59 g per ampere-hour. A plant targeting 5 kg of gold per day over 20 hours therefore needs about 8,500 ampere-hours, or roughly 425 A. Dividing that by the working current density gives the cathode area: at 12 A/m² the circuit needs about 35 m², which is 14 cathodes of about 2.5 m² each. We issue this calculation, with your own figures, in the process design package.

Which cathode material should I choose, and how often must it be replaced?

Stainless steel wool is the default for gold from cyanide eluate, because its specific surface area is the highest and therefore so is the deposition rate; multi-layer 304 or 316 stainless mesh is used where mechanical handling and cleaning matter more than surface area; expanded metal sheet suits short, rapid-turnover campaigns on high-grade liquor; and a removable titanium drum is used on the cyclone cell for dilute streams. Wool takes up to about 20 times its own weight in gold and is harvested before that limit is reached. It is washed and returned to service after each campaign, so replacement is driven by physical attrition rather than by gold loading, and the portion that is consumed is dissolved in hydrochloric acid so its residual gold is recovered.

Why is the current efficiency only 5 to 10%?

Because at the gold concentrations a real eluate carries, hundreds of milligrams per litre rather than tens of grams, most of the charge goes into side reactions. Water reduction to hydrogen at the cathode and oxygen evolution at the anode consume the current that does not reach a gold ion, so a current efficiency of 5 to 10% is normal for this duty and is not a fault. The number that describes circuit performance is the electrolysis rate, 99% or higher, together with the residual gold in the barren liquor. A supplier quoting a high current efficiency on dilute eluate is either describing a different duty or measuring it differently, and the figure should be checked against the barren liquor assay.

Can the same machine treat e-waste stripping solution and plating rinse water?

Yes, and it is a natural extension of the same circuit. E-waste and PCB stripping solutions carry gold at concentrations comparable to an eluate, and they are recovered on the flat-plate cells with the same conditioning, harvesting and sludge route. Plating drag-out, ion-exchange strip liquor and workshop rinse water are much more dilute, and those streams are recovered on the cyclone cell, where the swirling electrolyte film renews the cathode surface and makes a workable deposition rate possible at low gold concentration. In every case the current density and anode material are set by pilot test on the actual solution, because cyanide, thiosulfate, glycine and chloride liquors behave differently at the electrode.

What purity does the product reach, and what happens after the cell?

The cell produces a gold sludge, not refined metal. As harvested the sludge assays 30 to 75% Au depending on the feed and cathode type, and it is washed, parted with nitric acid to remove base metals and silver, dried, calcined between 400 and 750 °C, fluxed and melted in an induction furnace at 1,150 to 1,300 °C into doré bars of 50 to 75% Au. Silver leaves the parting stage as silver chloride and is recovered as a by-product. Where four-nines gold is required, the doré goes to the refining line rather than to the market, and that route delivers 99.9% gold and above.

How is the performance of the equipment proved before shipment?

By a factory acceptance test on the assembled equipment, followed by a production trial on your own liquor after commissioning. The factory test proves the control interlocks, the three-stage voltage and time programme, the start-up ramp, the level make-up sequence and the rectifier output against its rating, and it is recorded with material certificates. The production trial then runs the circuit on your liquor and confirms the electrolysis rate, the residual gold in the barren liquor, the doré assay band and the reagent consumption against the performance figures stated in the process design package. Both tests are witnessed and signed before handover.

Is cyanide used, and how are the fumes and the effluent controlled?

Where the feed is a cyanide eluate the circuit runs on cyanide, and it is handled as an engineering problem rather than a procedural one. Cells operate under closed transparent hoods with adjustable extraction, and hydrogen from the cathode, cyanide-bearing mist and acid vapour are drawn into a caustic scrubbing train with a demister, with the scrubber liquor monitored before discharge. Mercury abatement is available where the feed carries mercury. The circuit is closed: cyanide and caustic are made up to strength and the barren liquor returns to elution or leach, so no reagent destruction plant is needed, and any bleed that does leave is treated by alkaline chlorination or hydrogen peroxide. Operators are trained on cyanide and acid handling before the circuit is started.