Silver Electrolytic Cell

— Silver Electrolytic Cell: Cell Geometry, Internals and Cathode Area Sizing

A silver electrolytic cell is an electrorefining vessel in which a cast crude-silver anode and a titanium cathode are suspended in a silver nitrate electrolyte, and a direct current dissolves the silver at the anode and deposits it on the cathode as 99.99% crystal at a cathode current density of 750 A/m². YuSai Machinery engineers the cell itself — its type and geometry, cathode area, electrode pitch, anode basket and filter bag, busbar and insulation, electrolyte hold-up and circulation, crystal release mechanism and sealed side extraction — from your own anode assay and daily silver target, so that the cell holds its grade and its throughput across the whole campaign.
Configuration: cell count, cathode area, cell volume, rectifier rating, circulation rate, cooling duty, exhaust volume and effluent duty are engineered to your anode assay, daily silver target, product form and site conditions rather than selected from a catalogue of fixed models.

Product Overview

A silver electrolytic cell is the vessel in which silver electrorefining actually happens: a cast crude-silver anode and a cathode plate hang in a silver nitrate electrolyte, a direct current is passed between them, and the silver leaves the anode, crosses the electrolyte and comes back out of solution on the cathode as crystalline metal at 99.99% purity. Everything else in a silver refinery — the melting shop that casts the anode, the rectifier, the circulation and cooling, the washing and drying unit, the absorption tower, the induction furnace — exists to feed this vessel, keep it in its operating window and take its products away. This page is about the cell as the unit that sets the throughput, the grade and the operating cost of the whole line.

The electrochemistry inside the cell is simple to state and demanding to run. At the anode, metallic silver loses an electron and enters the electrolyte as the silver ion, Ag − e⁻ = Ag⁺. At the cathode, the silver ion gains an electron and returns to the metallic state, Ag⁺ + e⁻ = Ag. The two reactions together are a shuttle for silver and a filter for everything else. Copper and other base metals dissolve anodically alongside the silver and stay in the electrolyte as nitrates, so they accumulate in the liquor instead of following the silver to the cathode. Lead and bismuth hydrolyse in the working pH band and precipitate, which is why the free-acid window has to be held. Gold, platinum, palladium and rhodium do not dissolve in nitric acid at all and report to the anode bag as insoluble slime. The cell therefore concentrates the silver and separates the impurities into exactly two places — the electrolyte and the anode slime — and the design of the cell is, in the end, a matter of controlling those two streams and the geometry that carries them.

Cell type is a decision with consequences, because the three arrangements that dominate silver electrorefining differ in ways that cannot be corrected later. In the vertical basket cell, the anodes stand in a perforated titanium basket lined with a filter bag and the cathodes hang between them, which gives a short electrode gap, a compact footprint and high current densities, and it is the arrangement that takes granulated, cemented and powdered feed without a casting step. In the horizontal tray cell, also known as the belt-discharge or boat-type cell, the anodes lie in trays and the crystal is carried out continuously on a belt under the electrodes; it is the classic arrangement of the large copper and lead smelters, it is automated and it never has to be stopped for discharge, and it carries four costs that have to be accepted knowingly: a lower current density of 250–350 A/m² in the conventional version, a slower dissolution rate, a large inventory of electrolyte held up in the cell, and crystal that accumulates under the belt where it disturbs the discharge and can go off-grade if it lies there long enough. The third arrangement, and the one this series is built around, is the integrally formed sealed cell: a high-current-density cell body formed in one piece without welded seams, closed with a transparent cover and extracted from the side, with the electrode frame, the baskets, the busbar and the discharge arranged around a cathode area sized for 750 A/m².

Current density is the parameter that decides both the output and the quality of the crystal, and it is the number most often missing from a silver cell specification. The cell this series is built around runs at a cathode current density of 750 A/m², which is between two and three times the 250–350 A/m² of a conventional silver cell and inside the 600–800 A/m² band that high-current-density silver practice occupies, with industrial high-current-density silver electrorefining cells published as operating above 1,000 A/m² depending on the composition of the doré being processed. Raising the current density is paid for in three currencies: the resistive drop across the electrode gap, which rises with current and with the amount of silver and base metal held in the liquor; the tendency of the anode face to passivate as its silver content falls away and the residual surface becomes gold-rich; and deposit morphology, because a deposit grown too fast at a starved boundary layer turns nodular and dendritic and can bridge the gap to the anode. Boundary-layer control is therefore part of the cell rather than an accessory of it: circulation holds the silver concentration at the cathode face close to the bulk concentration, agitation is set to the design rate, and the temperature is held in band, which lowers the electrolyte resistance and improves the deposition kinetics at the same time.

The electrolyte window is narrow in both directions and the cell is instrumented to hold it. Silver is held at 60–150 g/L with 10–18 g/L of free nitric acid, and potassium nitrate is added where the conductivity and the pH band need help. If the free acid falls below the window, bismuth and other base metals hydrolyse and the precipitate contaminates the silver crystal, which is a grade problem that cannot be corrected without melting the lot again. If the free acid rises above the window, the acid re-dissolves silver that has already been deposited, which is a recovery problem and also forces the silver concentration up. Because the two failure modes sit on opposite sides of the same setpoint, the liquor is titrated and corrected on a schedule rather than adjusted after a quality excursion, and the temperature is held at a 40 °C set point by a chiller that starts and stops on the measured electrolyte temperature: warmth lowers the resistance, but evaporation, acid consumption and the volatilisation of silver-bearing electrolyte all climb with it.

The liabilities of the cell are specific and the design contains them rather than describing them. Nitric acid mist and nitrogen oxides are generated in the cell headspace, and far more intensively during electrolyte preparation, where silver reacts with nitric acid according to Ag + 2HNO₃ → AgNO₃ + NO₂↑ + H₂O. The cell is therefore closed, extracted from the side of the body, and the vapour passes a condenser that returns entrained silver to the circuit before the residual stream goes to the absorption tower. Silver crystals bridging the electrode gap are a short-circuit waiting to happen, which is why scraping is continuous rather than scheduled. Anode passivation as the plate is consumed is managed by current and voltage intervention, with cell voltage read as a diagnostic. And the electrolyte and the anode slime are assayed every campaign, because those two streams are where an unmanaged silver cell quietly loses both metal and grade.

Because anode composition, gold content, impurity profile, batch size and available floor space vary widely between one project and the next, the cell is sized from your own numbers rather than from a table of standard tanks. The cathode area is calculated from the silver equivalent of the current, the target daily silver output, the current efficiency and the design current density; the cell volume and the electrolyte hold-up follow from the cathode area and the campaign length; and the rectifier rating, the circulation rate, the cooling duty, the exhaust volume and the effluent duty are then fixed around them. Working as a silver electrolytic cell manufacturer from the anode backwards, we state those figures in the process design package, and we would rather revise the anode specification or the cell geometry than supply a cell that cannot hold the grade.

Key Facts

Cell types offered: vertical basket-type cells for compact high-density duty, horizontal tray-type cells where the site and the feed suit them, and the integrally formed sealed polypropylene cell that is the standard configuration of this series.
Cathode current density: 750 A/m² is the design point, against the 250–350 A/m² of a conventional belt-discharge cell. Industrial high-current-density silver electrorefining cells are published as operating above 1,000 A/m² depending on the composition of the silver doré being processed.
Cell voltage: 1–3 V DC per cell in the normal working window. Cell voltage is a diagnostic inside the cell, not a productivity lever, because it rises with the silver concentration of the liquor, with contact and circuit resistance, and with anode passivation.
Electrolyte: silver nitrate at 60–150 g/L silver with 10–18 g/L of free nitric acid, held inside that window deliberately. Below the window bismuth and other base metals hydrolyse into the crystal; above it the acid re-dissolves silver that has already been deposited.
Cell sizing basis: a cathode area sized from the silver equivalent of the current — 4.025 g of silver per ampere-hour — the target daily silver output, the current efficiency and the design current density, so the cell is built to carry the current rather than derated afterwards.
Electrolyte hold-up: about 5–6 litres of circulating electrolyte per kilogram of daily silver capacity, which is the figure that decides floor space, reagent inventory and the value tied up in the liquor.
Electrode materials: titanium cathode plates or a rotating titanium drum cathode, a titanium basket and high-density filter bag at the anode, titanium-clad copper conductor bars and bakelite insulation between the bars and the tank.
Crystal release: continuous scraping or agitation on both cathode faces, so the deposit cannot bridge the electrode gap, short-circuit the cell or accumulate under the cathode support and starve the discharge.
Two cell-level losses are closed by design: the sealed cell is extracted from the side and the vapour is condensed so that silver returns to the circuit, and the crystal is kept out of the anode bag so that the acid demand for slime digestion and its nitrogen oxide load both fall.
Configuration: cell count, cathode area, cell volume, rectifier rating, circulation rate, cooling duty, exhaust volume and effluent duty are engineered to your anode assay, daily silver target, product form and site conditions rather than selected from a catalogue of fixed models.

Product Core Selling Points

Cell geometry is selected before it is built. The three working arrangements — the vertical basket cell, the horizontal tray or belt-discharge cell and the integrally formed sealed cell — differ in where the anode sits, how wide the electrode gap has to be and how the crystal leaves the cell, and each one wins on a different duty. The cell type is fixed from your anode size, daily silver target and site, and the geometry follows from it rather than being adapted from a standard tank.
Cathode area is calculated, not guessed. The cathode area is derived from the silver equivalent of the current, 4.025 g per ampere-hour, the target daily silver output, the current efficiency and the design current density. That single number sets the electrode count, the cell footprint and the rectifier rating, and it is stated in the process design package rather than discovered when the first campaign under-delivers.
750 A/m² as a design premise rather than a derated promise. The electrode pitch, conductor bar section, contact pressure, circulation rate and cooling duty are all sized for high-current-density duty from the start, against the 250–350 A/m² of a conventional belt-discharge silver cell and inside the 600–800 A/m² band that high-current-density practice occupies.
Integrally formed polypropylene shell without welded seams. The cell body is produced as one piece with an anti-ageing additive compounded into the material, so the nitric acid electrolyte cannot weep through a joint, and the wall thickness and ribbing are set by the hydrostatic load of the working volume.
Parallel-flow, bottom-in top-out circulation. Electrolyte enters at the bottom of the cell and leaves at the top, flowing parallel to the electrode plates. This is the mechanism that makes the current density usable: it holds the silver-ion concentration uniform across the electrode gap and suppresses the concentration polarisation that would otherwise force the silver concentration up.
Anode basket and filter bag as a cell component. Silver that cannot be cast into a plate is run from a titanium basket, and every anode is enclosed in a high-density filter bag, so the gold and the platinum group metals are held at the anode from the first minute of the run instead of migrating to the cathode.
Sealed cell with transparent cover and side extraction. The headspace is closed and drawn from the side of the cell body rather than diluted under a large open hood, which is both a containment measure and what keeps the exhaust volume that has to be treated economically small. The cover is transparent, so the electrodes and the electrolyte stay visible for inspection.
A cell that clears itself. Scraping or agitation works both cathode faces continuously and the crystal leaves the cell mechanically, so the deposit cannot bridge the electrode gap and the interval between deep cleans stretches from roughly monthly on a belt-discharge cell to a campaign measured in months.
Cell-overhaul scope stated in writing. The deep clean is a scheduled job with its own procedure — cell drained by way of the discharge line and the solid–liquid separator, anode frames and bags lifted, slime recovered from the basket and the cell floor, gaskets and seals inspected, cathode plates re-dressed and the electrode gap re-set with a gauge before the cell is re-energised.
Materials selected for nitric acid service stream by stream. Integrally formed polypropylene with an anti-ageing additive, a polypropylene homopolymer frame, titanium-clad copper conductor bars, bakelite insulation, titanium and stainless steel cathodes, and PP and PVDF wetted parts, piping and ducting.
Single-cell isolation for small and mixed batches. Each cell runs its own anode set and discharges its own crystal, so a cell can be worked on a different anode batch without cross-contaminating the crystal from another cell, and a cell can be taken out of service for its overhaul while the rest of the house keeps running.
Cell duty engineered to your assay. Cell count, cathode area, working volume, electrolyte hold-up, rectifier rating, circulation rate and cooling duty are stated in the process design package against your anode assay, daily silver target, product form, reagent supply and site conditions.

Application Scenarios

Silver refineries treating doré from gold cyanidation and gold electrowinning. Doré bars carrying silver with gold, copper and platinum group metals are parted in the cell, with the gold held back in the anode bag and the silver leaving as 9999 fine crystal.
Desilverising ahead of gold electrolysis. Where a gold doré carries silver above the ceiling a gold cell can tolerate, the silver is taken out in the silver cell first, so that the residual gold-rich plate can be cast into a gold anode instead of passivating in a chloride electrolyte.
Copper and lead smelters processing anode slime. Silver recovered from copper and lead anode slime, and from the associated doré and cement silver, upgraded to a saleable 9999 product rather than shipped as an intermediate.
Integrated precious metal refineries. A cell house that sits between the smelting shop and the gold line, taking the silver-rich fractions out of the flowsheet and returning gold-bearing slime to the gold circuit.
High-current-density cell houses converting from belt-discharge cells. Existing houses running conventional horizontal belt cells at 250–350 A/m², where the same floor space is asked to carry more silver without a second row of tanks.
Plants fighting belt-underflow crystal. Operations where crystal accumulating under the belt disturbs the discharge, forces a monthly stop for a floor clean and produces off-grade powder from material that has lain in the cell too long.
Houses with an acid-consumption problem in the slime circuit. Plants where silver finds its way into the anode bags and has to be boiled out, driving up nitric acid demand and the nitrogen oxide load that follows it.
Jewellery, silverware and holloware manufacturers. Sterling 925 scrap, bench sweeps that have been ashed and melted, and silver-copper alloys from coin and tableware manufacture, refined in-house and returned to the workshop as fine silver rather than sold at a discount.
Coin melt and bullion operations. Old coin, medallion and bullion stock with a silver content that varies batch to batch, refined in a cell that can be worked on one batch at a time with an assay certificate per lot.
E-waste and printed circuit board recyclers. Silver recovered from contacts, silver-plated components, solder and the silver-bearing fractions of mixed electronic scrap after they have been concentrated and cast into anodes.
Photographic, X-ray and imaging processors. Silver recovered as cemented silver or as a reduced silver residue from fixer, bleach-fix and wash water, refined into saleable metal instead of being sent out as a regulated waste.
Solar photovoltaic cell recyclers. Silver recovered from the metallisation paste and the etched wafer fractions of end-of-life panels, where silicon and the other layer materials have to be separated before the silver can be cast and refined.
Electroplating and surface finishing shops. Silver plating anode stubs, spent silver plating baths and drag-out silver, taken back to fine silver in a small single cell and returned to the plating line or sold.
Contract and toll refineries. Third-party crude silver refined in campaigns, where the cell has to be quick to reconfigure between feeds and every batch must close on a weighed metal balance and a certificate of analysis.
Mints, assay offices and laboratories. Small-volume, high-value work where the feed is well characterised and the requirement is a documented purity rather than throughput, served by the same platform in a single-cell configuration.

Applicable Raw Materials

The cell accepts metallic silver feed, and every feed below enters the cell through the anode. Material that is already metallic is melted, fluxed and cast into anode plates, or granulated and loaded into a titanium basket; material that arrives as a powder, a cement or a slime is dried, fluxed and consolidated before it reaches the cell; and material carrying chloride is converted to metallic silver first, because silver chloride will not dissolve anodically in a nitrate electrolyte. Output is stated for each feed.

Silver doré bars from gold cyanidation and gravity concentrate smelting. The classic feed, carrying silver with gold, copper and platinum group metals, and cast into anode plates once the gold content has been checked against the ceiling the cell can hold. Output: 99.99% silver crystal, and gold-bearing anode slime to the gold circuit.
Crude silver ingots and bullion above 97% silver. High-grade feed that goes straight to anode casting with no intermediate chemical step, and the grade the cell geometry is normally proved against. Output: 99.99% silver crystal and a low-yield slime.
Crude silver scrap between 75% and 97% silver. Feed carrying enough copper, lead or bismuth that it is fluxed and air-blown during melting so those elements oxidise into a removable slag before the anode is cast. Output: refined anode plates, a base-metal slag returned for silver recovery, and 99.99% cathode crystal.
Sterling 925 silver scrap, jewellery and holloware. Alloyed feed with a predictable 7.5% copper content, which reports to the electrolyte and is managed by the bleed schedule rather than by pretreatment, at the cost of a shorter electrolyte life. Output: 99.99% silver crystal and a copper-bearing electrolyte bleed.
Silver-copper coin alloys, solders and brazing alloys. Feed with a copper or zinc matrix, cast into anodes with the flux and blowing conditions set from the assay. Output: 99.99% silver crystal and a base-metal stream to recovery.
Cemented silver and reduced silver residues. Fine, high-surface-area material that is dried, fluxed and loaded into a titanium basket rather than cast into a plate, which removes a melting step and the metal inventory that sits in the furnace with it. Output: 99.99% silver crystal from a basket-fed cell.
Silver powder and atomised silver feed. Loose powder that will not survive in an anode bag and cannot be cast into a coherent plate, run from a lined basket so that the residue is still retained at the anode. Output: 99.99% silver crystal and a retained anode residue.
Residual anode plates from a previous campaign. Partly dissolved plates that reached the end of their dissolution cycle, finished in a basket without being re-melted or returned to the melting shop and cast into fresh anodes. Output: recovered silver crystal and either a finished plate or a recast anode.
Silver chloride and silver salt residues. Converted to metallic silver before the cell, since chloride is not compatible with the nitrate electrolyte; the conversion runs in a closed vessel with the off-gas routed to the absorption plant. Output: metallic silver feed for anode casting, and a base-metal filtrate.
Silver recovered from copper and lead anode slime. Slime-derived silver, typically arriving as doré, cement or a reduced residue depending on the upstream route. Output: 99.99% silver crystal, with any remaining gold and PGM declared and routed onward.
Silver-rich alloy plate from the gold circuit. Silver-bearing plate returned from gold refining, where the silver has to be taken out before the residual gold can be run as a gold anode. Output: 99.99% silver crystal and a gold-rich residual plate.
Silver-bearing fractions from electronic and electrical scrap. Contacts, silver-plated components, relays, switches and solder, concentrated and melted into anodes after the copper and ferrous fractions have been separated. Output: 99.99% silver crystal and a base-metal slag.
Spent silver plating baths, anode stubs and drag-out silver. Plating-side silver recovered as metal, with the plating salts and organic load removed before the material reaches the melt, in a single-cell configuration matched to the batch size. Output: 99.99% silver crystal returned to the plating line or sold.
Silver catalysts, silver oxide and silver salt process residues. Recovered from spent catalyst and process residue after the support material has been separated. Output: 99.99% silver crystal and a separated support fraction.

Finished Products

The cell is configured around the product form the customer sells and the stage at which the customer takes delivery. Where the customer already operates a melting shop and an assay laboratory, the cell house is delivered with the harvest, washing and drying stages only; where it does not, melting, casting and assay are included with it.

99.99% silver crystal. The primary product of the cell: crystalline cathode silver at 9999 grade, washed free of entrained electrolyte, dried to a moisture content fit for clean melting and held for the melting stage or sold as crystal. Output: dried 99.99% silver crystal with a lot assay.
9999 fine silver ingots and bars. Crystal melted in an induction furnace and cast into bars or ingots of the weight the market requires, marked with weight and batch number. Output: certified 99.99% silver bar with certificate of analysis.
Vacuum-cast and continuously cast silver. Where density, surface finish and freedom from porosity and oxidation matter, vacuum ingot casting and vacuum continuous casting replace open casting. Output: high-finish silver bar or ingot for investment and industrial use.
Silver granules and grain. Where the product is sold as grain rather than as bar, the melt is granulated and the grain is dried, screened and packed. Output: packed silver grain at a declared grade.
Gold-bearing anode slime. The most valuable by-product of the cell, running at about 8% of the anode mass and typically assaying 50–70% gold with 30–40% silver; washed free of entrained electrolyte, dried, weighed and assayed, and either parted with nitric acid to raise the gold grade above 90% or cast into a second anode for the gold circuit. Output: washed and assayed gold-rich slime with the silver and PGM fractions declared.
Platinum group metal fraction from the slime. Where the feed carried platinum, palladium and rhodium, that fraction is separated from the slime and declared by metal content before it leaves for the PGM refining line. Output: PGM-bearing concentrate with declared metal content.
Silver recovered from the anode bag. Crystal that reports to the bag instead of the cathode, recovered from the bag lining during the slime stage rather than boiled out of the bag with additional acid. Output: recovered silver returned to the melt.
Recast anode plates from residual anode stock. Partly dissolved plates that reached the end of their dissolution cycle, returned to the melting shop and cast into fresh anodes rather than written off. Output: remelted anode plates returned to the cell.
Regenerated electrolyte. Spent electrolyte from which the accumulated copper, bismuth and other impurities have been removed, re-acidified to its working pH and returned to the cell. Output: electrolyte returned to service at the working specification.
Silver recovered from the electrolyte bleed. The silver held in the liquor drawn off for purification is recovered in the same step rather than neutralised to waste, which is one of the two places silver quietly disappears on an unmanaged cell. Output: recovered silver returned to the anode melt.
Condensed silver returned from the fume. Vaporised electrolyte captured by the condensation system on the cell and vessel off-gas path and returned to the circuit, so that evaporation is a handling issue rather than a metal loss. Output: silver-bearing condensate returned to the cell.
Compliant off-gas and effluent. Cell and vessel off-gas treated through condensation and nitrogen-oxide absorption before release, and process effluent neutralised, metal-polished and either recirculated or discharged at the agreed limits. Output: emission and effluent records filed with the batch.
Batch documentation. Every lot is released with the anode weight and assay, the electrolyte analysis at make-up and at draw-off, the cell campaign record, the slime weight and assay, the product assay and the metal balance for that lot. Output: certificate of analysis and batch record.

Silver Electrolytic Cell Process Flow

The nine stages below are the working cycle of one silver electrolytic cell, from the duty it has to carry through to its clean-out and re-dress. It is written at cell level rather than at plant level on purpose: this is the sequence a cell designer, an operator and a metallurgist work through when the question is how a cell is configured, why it behaves the way it does and what has to be done to it between campaigns. Cell type, cathode area, working volume, electrolyte hold-up, electrode pitch, rectifier rating, circulation rate and cooling duty are fixed during process design against your own anode assay, daily silver target and site.

Silver electrolytic cell working cycle — from the duty definition and cathode area sizing through the in-cell deposition stage to the clean-out and re-dress, with the electrolyte, the crystal and the anode slime kept inside the circuit.

Steps 1 — Cell Type Selection and Cathode Area Sizing. The cell is specified before any plate is cut, and the first decision is the arrangement: a vertical basket cell, where anodes stand in a lined titanium basket and cathodes hang between them for a short electrode gap, a compact footprint and a route for granulated feed; a horizontal tray or belt-discharge cell, where anodes lie in trays and crystal is carried out on a belt under the electrodes for continuous unattended discharge at a lower conventional current density; or the integrally formed sealed cell, which carries the high-current-density duty of this series. The cathode area then follows from arithmetic rather than from a catalogue. Silver carries 4.025 g per ampere-hour, so a cell required to produce 100 kg of silver a day has to deposit 4,167 g every hour; at a current efficiency of about 95% that needs 1,090 A, and at a design current density of 750 A/m² it needs 1.45 m² of cathode area. That single figure sets the electrode count, the electrode pitch, the internal length of the cell, the working volume, the rectifier rating and the cooling duty, and it is written into the process design package against your assay. Reagent and utility input: anode assay, target daily silver output, required product form, site drawing and available floor area, electrode and basket drawings, current-efficiency basis. Output: cell type, cathode area and electrode count, cell working volume and electrolyte hold-up, and the rectifier, circulation and cooling duties that follow from them.

Steps 2 — Cell Shell Forming and Electrode Frame Setting. The shell is formed in one piece rather than welded from sheet, with an anti-ageing additive compounded into the polypropylene so that the nitric acid electrolyte cannot weep through a joint, and the wall thickness, ribbing and base support are set by the hydrostatic load of the working volume. Level is what matters next: the cell is set on a surveyed, level support structure and checked across its length, because the electrolyte surface is the datum that the electrode immersion depth is measured from, and a cell that is out of level along its length will run a different current through each electrode set. The electrode frame then goes in over the shell: rails that carry the anode and cathode sets, stops that fix the electrode pitch, and a support arrangement rigid enough that the bars do not move when the cell is loaded and unloaded. A sloped floor section and a slime collection zone are formed at the base for the arrangement that discharges crystal mechanically. Reagent and utility input: integrally formed polypropylene shell material with anti-ageing additive, HDPE, PVC or FRP where the duty specifies it, polypropylene homopolymer frame and rail stock, levelling shims and survey equipment, gaskets and sealant line. Output: a level, leak-tight cell shell with a rigid electrode frame and a defined slime collection zone.

Steps 3 — Anode Basket, Filter Bag and Lug Fit-Out. The anode side is assembled before the anode goes anywhere near the electrolyte. Where the feed can be cast, the plate is dressed, its lugs formed and the plate weighed, because the anode weight is what the cell current is set against; where the feed cannot be cast, granulated, cemented or powdered silver is loaded into a perforated titanium basket that carries the same current and retains its own residue. Either way the anode is enclosed in a high-density filter bag, and this is a functional stage rather than a finishing detail: gold, platinum, palladium and rhodium are insoluble in the nitrate electrolyte, and if they are free to move they will migrate to the cathode and report to the product as contamination. The bag is fitted so that it holds the slime against the anode without fouling the electrode face, and the lug and contact faces are cleaned and dressed before hanging, because a resistive contact on one anode shows up later as an unexplained low current on one cathode and is hard to diagnose once the cell is closed and full. Reagent and utility input: cast anode plates or a titanium basket with lining, high-density filter bag and membrane material, anode lugs, hooks and hangers, contact cleaning and dressing tools, anode handling gear, weighing equipment. Output: bagged, hung and weighed anode assemblies with verified contact, and a basket route for feed that cannot be cast.

Steps 4 — Cathode Plate Preparation and Gap Setting. The cathode is prepared for release as much as for deposition. Titanium sheet plates are dressed and, where the process calls for it, treated so that the crystal releases cleanly on scraping, after which the plate is checked for flatness — a bowed plate changes the electrode gap along its length, which shows up as a localised burn and a current distribution that cannot be rebalanced from the rectifier. The inter-electrode gap is then set with a gauge across the whole array, because the gap fixes the resistive drop in the electrolyte and therefore the cell voltage at a given current, and it fixes how much crystal can grow before the deposit is in danger of bridging to the anode. The anode and cathode sets are interleaved to the design pitch, every lug is seated on its conductor bar, and the array is checked for squareness and for the freedom of each plate to be lifted clear at discharge. Reagent and utility input: titanium sheet cathode plates or a rotating titanium drum cathode, cathode dressing and release-treatment materials, gap gauges, spacing stops, flatness check, electrode lifting gear. Output: a squared electrode array at the design inter-electrode gap, with every plate free to be lifted and a known resistance path from lug to bar.

Steps 5 — Electrolyte Charging and Thermal Loop Commissioning. The cell is charged to its working level with silver nitrate electrolyte made up to 60–150 g/L of silver with 10–18 g/L of free nitric acid, potassium nitrate being added where the conductivity or the pH band needs help, and the cell is not energised until the liquor has been titrated for silver and free acid and the result recorded against the batch. Both bounds of the acid window are deliberate and both fail in a different direction: below the window, bismuth and other base metals hydrolyse and the precipitate contaminates the crystal as a grade loss that cannot be corrected afterwards; above it, the acid re-dissolves silver that has already been deposited, which is a recovery loss and drives the silver concentration up. The thermal loop is commissioned in the same stage: the chiller is set to a 40 °C electrolyte set point and started and stopped on the measured electrolyte temperature, and the circulation pump is brought up so that electrolyte enters at the bottom of the cell and leaves at the top, flowing parallel to the electrode plates. That flow direction is what keeps the silver-ion concentration uniform across the electrode gap and suppresses the concentration polarisation that would otherwise force the silver concentration up, and it is the reason the high current density is usable at all. Reagent and utility input: nitric acid with metered dosing, high-purity silver or cathode crystal for make-up, potassium nitrate where specified, closed preparation vessel with extraction, deionised water, circulating pump with PP or PVDF wetted parts, chiller and heat exchanger, level indication, silver and free-acid titration. Output: cell charged to working level at 60–150 g/L silver with 10–18 g/L free acid, thermal loop running on a 40 °C set point, and parallel-flow circulation established.

Steps 6 — Cell Energisation and Cathode Current Balancing. Current is brought up on a ramp rather than applied at its full value, for a reason that is visible in the product: a modest initial current produces a coherent, fine-grained seed layer on the cathode that stays coherent as it thickens, whereas a cold start at full current produces a burst of nodules, a high local current density and a deposit that sheds back into the electrolyte later in the run. Individual cathode currents are verified during the ramp, because a poor contact or a mis-set electrode found on the first day costs a shift, while the same fault found at harvest costs the campaign. Cell voltage is compared against the design figure for the design current — 1–3 V DC per cell in the normal working window — and the conductor bars and lugs are checked for a stable, low temperature before the ramp is complete, the bars being engineered to stay below 70 °C in continuous operation. Where the anode face carries a silver content that tends to passivate it, or where the liquor is unusually concentrated, an alternating component can be superimposed on the rectifier output so that the polarity reverses for part of every cycle and the anode face is kept active rather than becoming film-bound. Reagent and utility input: DC rectifier with adjustable voltage and current and, where specified, an adjustable superimposed alternating component, staged current programme, per-cathode current measurement, insulation and spacing verification, busbar and lug temperature monitoring. Output: stable current distribution across the cathode set, a coherent seed deposit established, and the corrected settings recorded against the batch.

Steps 7 — Silver Deposition at Working Current Density. This is the stage the whole cell is sized around. The cell runs at a cathode current density of 750 A/m², between two and three times the 250–350 A/m² of a conventional belt-discharge silver cell and inside the 600–800 A/m² band of high-current-density practice, with industrial silver electrorefining cells published as operating above 1,000 A/m² depending on the composition of the doré being processed. At the anode the silver dissolves as Ag − e⁻ = Ag⁺ while the copper and other base metals dissolve alongside it and stay in the liquor, and the lead and bismuth that hydrolyse in the working pH band are held out of the deposit by keeping the acid window. At the cathode the silver ion is reduced as Ag⁺ + e⁻ = Ag and deposits as crystalline metal. What holds the grade at this current density is the state of the liquor at the cathode face: with circulation running parallel to the plates and the electrolyte held at its temperature set point, the silver concentration at the cathode face stays close to the bulk concentration, the deposit stays coherent, and the cell can run at a lower and more stable silver concentration than a still cell would allow. When that balance is lost the deposit says so first — silver that starts growing as long hairy or whisker-like crystal rather than as a dense granular powder is the classic sign that the silver in the liquor is being displaced by copper and that the deposition window is closing. Reagent and utility input: rectifier at design current, parallel-flow circulation at the design rate, chiller holding the electrolyte at its set point, cell voltage and per-cathode current logging, regular titration of silver and free acid, periodic crystal sampling for the co-deposition check. Output: silver depositing as crystal on the cathodes, base metals reporting to the electrolyte, and insoluble gold and platinum group metals collecting in the anode bag as slime.

Steps 8 — Crystal Release, Slime Capture and Cell Discharge. Silver leaves the cathode as a crystalline powder, and crystal left in place grows until it bridges the electrode gap, which short-circuits the cell, distorts the current distribution across the remaining electrodes, lowers the quality of the powder still in the cell and immobilises silver inside the anode bag. The release mechanism therefore runs continuously rather than on a schedule: a scraper or an agitation arrangement works both faces of the cathode and removes the crystal as it forms, and the crystal leaves the cell mechanically — carried along the cell floor to a sloped discharge section, or lifted out on a rotating drum cathode with an internal scraper — and enters the wash-and-dry unit without being shovelled, bagged or transferred by hand. Keeping the crystal out of the anode bag is a cost decision as much as a quality one: silver that sits in the bag has to be boiled out later, and the acid demand for that step and the nitrogen oxide load it generates both fall when the bag is left to carry slime rather than metal. At the end of the campaign the cell is de-energised, the electrolyte is drawn off through the discharge line by way of the solid–liquid separator into the circulation tank, and the cell is emptied for the harvest of residual anode plates and anode slime. Reagent and utility input: continuous scraper or agitation on both cathode faces, cell-floor conveyor and sloped discharge section or a rotating drum cathode, solid–liquid separator on the discharge line, discharge valve, circulation and collection tanks, anode lifting gear, slime washing and filtration equipment. Output: harvested silver crystal transferred to the wash-and-dry stage; drained electrolyte routed to regeneration or make-up; residual anode plates and bagged slime recovered from the cell.

Steps 9 — Cell Clean-Out, Inspection and Re-Dress. The deep clean is a scheduled job with its own procedure, not an unplanned stop. The cell is drained and opened, the electrode sets are lifted out, the bagged slime is removed and the crystal that has collected under the electrode support and at the base of the cell is recovered rather than left to go off-grade — which is the failure that forces a monthly floor clean on a belt-discharge cell, where crystal accumulating under the belt disturbs the discharge and material that lies there too long fails the assay. With the cell empty, the shell is inspected for weeping seams, crazing, rib damage and base deformation; the gasket line on the cover, the discharge valve, the level indication and the extraction connection are checked and replaced as required; the conductor bars, lugs and insulation are examined for erosion at the contact faces and for stray-current tracking through the frame; and the anode and cathode plates are assessed and either returned to service or sent back to the melting shop for re-casting. The array is then re-dressed, the inter-electrode gap re-set with a gauge, fresh bags fitted and the cell recharged and re-energised on the ramp, with the clean-out recorded against the cell’s history so that the overhaul interval and the wear pattern on the wetted parts are tracked over the life of the equipment. Reagent and utility input: cell drain-down and slime recovery equipment, inspection tools, gasket, seal and valve spares, contact dressing materials and insulation spares, gap gauges, fresh filter bags, cathode dressing materials and release treatment, re-dress and re-energisation procedure. Output: a cleaned and inspected cell returned to service at the design gap with fresh bags; a cell history record; residual plates and recovered silver routed onward.

Product Structure (Main Equipment)

The cell is assembled from the three equipment groups below, all of them cell-level rather than plant-level. Materials are selected stream by stream: integrally formed polypropylene for the shell, produced without welded seams, with an anti-ageing additive where the liquor is strongly oxidising; polypropylene homopolymer for the frame and support work; titanium and stainless steel for the cathodes and the basket; titanium-clad copper for the conductor bars and lugs; bakelite between the bars and the tank body; high-density filter fabric for the anode bags; and PP and PVDF for wetted parts, piping and ducting.

Cell shell, cover and process containment

Integrally formed cell shell. A polypropylene cell body produced as one piece without welded seams, so that the nitric acid electrolyte cannot weep through a joint, with the wall thickness, ribbing and base support set by the hydrostatic load of the working volume and an anti-ageing additive compounded into the material. High-density polyethylene, PVC and FRP construction are used where the liquor or the site calls for them. Output: a corrosion-resistant cell that holds its dimensions and does not develop seam leaks in nitric acid service.
Sloped floor and slime collection zone. A sloped base section formed into the shell beneath the electrode array, with a defined collection zone for the slime that falls from the bags and for the crystal that misses the discharge path. Output: a cell that can be cleaned down to the base and a slime stream that can be recovered rather than flushed.
Cell frame and structural support. A polypropylene homopolymer support structure carrying the shell, the electrode frame and the conductor bars on a surveyed level, with the load path kept clear of the busbar insulation so that no structural member can become a current path. Output: dimensional stability at operating temperature, a level electrolyte surface and a rigid electrode array.
Transparent sealed cover. A transparent cell cover that closes the process while leaving the electrodes and the liquor visible for inspection, fitted with a gasket line and secured so that the extraction draws from under the cover rather than from the room. Output: contained exhaust, a visible process and a clear workshop floor.
Side extraction plenum and ducting. Extraction taken from the side of the cell body rather than from a hood above it, because the open hood a conventional horizontal cell needs has to be moved out of the way for loading and unloading and still captures poorly at a high air volume. Sealing the cell is what makes the extraction economic: a small, well-directed air volume removes the mist that collects under the cover. Output: captured acid mist and nitrogen oxides at a reduced exhaust volume to treat.
Cell and floor containment. Bunding, drip trays and a segregated floor drainage path under the cell and the electrolyte handling area, with the drainage directed to the effluent plant rather than to the storm system. Output: containment of spills and wash-down within the process boundary.

Electrode assemblies, baskets and current delivery

Cast silver anodes and anode bags. Anode plates cast to the cell geometry with formed lugs and a dressed face, enclosed in high-density filter bags that retain insoluble gold and platinum group metals at the anode. Anode moulds and a casting arrangement are supplied as part of the equipment scope. Output: weighed, bagged anode assemblies that hold the slime where it can be recovered.
Titanium basket anodes for granulated feed. A perforated titanium basket for feed that cannot be cast into a plate, lined so that the residue is retained, allowing granulated, cemented and powdered silver to be run without being recast. Output: a cell route for granulated and cemented silver that avoids a second melting step.
Titanium cathode plates. Titanium sheet cathodes on which the silver crystallises, selected because titanium resists the nitrate electrolyte, releases the crystal easily on scraping and does not contaminate the deposit. Stainless steel plate is available where the process calls for it. Output: a cathode surface that produces a clean, easily harvested crystal.
Rotating titanium drum cathode. A rotating drum cathode with an internal scraper, where the layout and the capacity call for continuous harvesting and a compact footprint. Output: continuous crystal removal from a cell with no plate handling.
Electrode frames, stops and hangers. Rails, stops and hangers that set the electrode pitch and hold the spacing square across the array, together with the handling gear for lifting and setting plates. Output: uniform electrode spacing and a repeatable loading position.
Titanium-clad copper conductor bars. Anode and cathode conductor bars in titanium-clad copper, with the plating graded so that the current-carrying copper is protected from the acid mist and the contact faces stay clean. Output: low-resistance current delivery with a long service life in an acid atmosphere.
Bakelite and polymer insulation. Interlocking insulation between the conductor bars and the tank body, and at every point where a busbar passes near a structural member, to prevent stray current from finding a path through the frame. Output: no stray-current corrosion of the structure or the shell.

Electrolyte loop, crystal release and cell instrumentation

Circulation pump and piping. A magnetic-drive circulation pump with PP or PVDF wetted parts, piped so that electrolyte enters at the bottom of the cell and leaves at the top, parallel to the electrode plates, with a valve arrangement that sets the in-cell level. Output: uniform silver concentration across the electrode gap and suppression of concentration polarisation.
Electrolyte cooling set. A chiller with a 40 °C set point on the electrolyte temperature, starting and stopping on the measured temperature, with a heat exchanger and the control loop that holds the cell in band. Cooling is not a comfort measure: it limits evaporation, nitric acid consumption and the volatilisation of silver-bearing electrolyte. Output: electrolyte held in band, with acid consumption and fume load reduced.
Continuous crystal release mechanism. A scraper or agitation arrangement working both faces of the cathode, timed or continuous, that removes the crystal as it forms so that the deposit cannot bridge the electrode gap. Output: no bridging, no short-circuit and no silver left to harden in the anode bag.
Cell discharge and solid–liquid separator. A discharge line carrying the cell contents by way of a solid–liquid separator, so that the electrolyte is recovered to the circulation tank rather than carried out with the product, together with the slime collection and washing arrangement. Output: clean electrolyte return, a dry harvest stream and a recoverable slime fraction.
Level, temperature and current instrumentation. In-cell level indication, electrolyte temperature measurement at the cell, per-cathode current measurement and cell voltage logging, so that the state of the cell is read from its own instruments rather than inferred from the rectifier. Output: an instrumented cell with a campaign record written against the batch.
PLC control and operator interface. PLC control with a touchscreen interface covering rectifier start and stop, current and voltage setting, circulation and scraper sequences, temperature control and alarm annunciation, with an event log that records the campaign against the batch number. Output: repeatable campaigns and a process record that stands behind the batch documentation.

Product Features

Cell type chosen from the duty rather than from a stock tank. Vertical basket, horizontal tray or belt-discharge, and the integrally formed sealed cell are offered on their own merits, and the geometry, the electrode gap and the discharge arrangement follow from the choice rather than being adapted afterwards.
Cathode area derived from the silver equivalent of the current. A cathode area calculated from 4.025 g of silver per ampere-hour, the target daily silver output, the current efficiency and the design current density, so the cell is sized to carry the current rather than derated from a nominal figure.
Integrally formed polypropylene shell without welded seams. One-piece construction with an anti-ageing additive, wall thickness and ribbing set by the hydrostatic load of the working volume, and HDPE, PVC or FRP where the duty calls for it.
High-current-density internals throughout. Electrode pitch, conductor bar section, contact pressure, circulation rate and cooling duty sized for 750 A/m² from the start, against the 250–350 A/m² of a conventional belt-discharge cell and inside the 600–800 A/m² high-current-density band.
Transparent sealed cover with side extraction. The process stays visible for inspection while the headspace is closed and drawn from the side of the body, which is both a containment measure and the reason the exhaust volume that has to be treated stays economically small.
A cell that discharges itself. Continuous scraper or agitation on both cathode faces plus mechanical crystal removal, so the deposit cannot bridge the electrode gap, short-circuit the cell or overload the operator between cleans.
Titanium cathode with a release treatment. A cathode surface that releases the crystal cleanly on scraping, available as sheet plates or as a rotating drum cathode with an internal scraper where continuous harvesting suits the layout.
Titanium basket for feed that cannot be cast. Granulated, cemented and powdered silver and residual anode plates are run from a lined basket, removing a melting step and the metal inventory that sits in the furnace with it.
Anode bagged as standard. High-density filter bags hold the gold, platinum, palladium and rhodium at the anode from the first minute of the run, so the value is recovered as slime instead of being found in the crystal assay at the end of the campaign.
Titanium-clad copper conductor bars with bakelite insulation. Current-carrying copper protected from the acid atmosphere, low-resistance contacts, and interlocking insulation that prevents stray current from tracking through the tank or the frame.
Parallel-flow electrolyte circulation. Bottom inlet and top outlet flowing parallel to the plates, which holds the silver-ion concentration uniform across the electrode gap and is the mechanism that makes the high current density usable rather than aspirational.
Chiller with a 40 °C electrolyte set point. Automatic start and stop on the measured electrolyte temperature, holding evaporation, acid consumption and silver volatilisation losses down, with conductor bars and lugs engineered to stay below 70 °C.
Cell instrumentation that reads the cell, not the rectifier. In-cell level, electrolyte temperature at the cell, per-cathode current and cell voltage logging, so that a distribution fault or a rising voltage is caught on the day it appears.
Slime and residual anode routes inside the cell scope. Bag opening, slime washing and filtration and the residual plate route are part of the cell equipment scope, and the PGM fraction is declared by metal content where the feed carries it.
Single-cell isolation. Each cell runs its own anode set and discharges its own crystal, so a cell can work a different anode batch without cross-contaminating another cell’s product, and one cell can be overhauled while the rest of the house keeps running.
Cell duty engineered to your assay. Cell count, cathode area, working volume, electrolyte hold-up, rectifier rating, circulation rate, cooling duty and exhaust volume are stated in the process design package against your anode assay, daily silver target, product form and site conditions.

Technical Parameters And Specifications

Parameter	Specification
Primary function	Electrolytic refining of crude silver and silver doré in a cell, dissolving a cast or basketed anode in a silver nitrate electrolyte and depositing 99.99% silver crystal on the cathode, with the gold and platinum group metals retained as anode slime
Cell type — vertical basket cell	Anodes standing in a lined titanium basket with cathodes hung between them; short electrode gap, compact footprint, and the standard route for granulated, cemented and powdered feed
Cell type — horizontal tray or belt-discharge cell	Anodes lying in trays with a crystal conveyor under the electrodes; continuous unattended discharge at a conventional current density of 250–350 A/m², with a large electrolyte hold-up and crystal accumulating under the belt between cleans
Cell type — integrally formed sealed cell	The standard configuration of this series: one-piece polypropylene shell, sealed transparent cover with side extraction, electrode array sized for high-current-density duty
Cell shell material	Integrally formed polypropylene with an anti-ageing additive and no welded seams; HDPE, PVC or FRP where the liquor or the site specifies it
Cell structural frame	Polypropylene homopolymer frame, rails and support work, with the load path kept clear of the busbar insulation
Cover and extraction	Transparent sealed cover on a gasket line, with extraction taken from the side of the cell body rather than from an overhead hood
Anode form	Cast silver plates with formed lugs for castable feed, or a perforated titanium basket with a lining for granulated, cemented, powdered and residual-anode feed
Anode bagging	High-density filter bags retaining insoluble gold, platinum, palladium and rhodium at the anode from the first minute of the run
Feed silver content	Above 75% Ag, and in practice above 97% for direct anode casting; lower grades are flux refined during melting before the anode is cast
Cathode material	Titanium sheet plate as standard; stainless steel plate where the process specifies it; rotating titanium drum cathode with internal scraper for continuous harvest
Cathode area	Sized from the silver equivalent of the current, the target daily silver output, the current efficiency and the design current density, and stated in the process design package
Silver equivalent of the current	4.025 g of silver per ampere-hour, at a current efficiency of about 95% in normal operation
Cathode current density	750 A/m² design point, against 250–350 A/m² for a conventional belt-discharge silver cell and inside the 600–800 A/m² high-current-density band; above 1,000 A/m² is published for industrial high-current-density silver electrorefining cells depending on the silver doré composition
Inter-electrode gap	Set with a gauge across the whole electrode array; the gap fixes the resistive drop in the electrolyte and therefore the cell voltage at a given current, and limits how much crystal can grow before the deposit risks bridging to the anode
Electrode pitch and count	Fixed by the support rails and stops and derived from the cathode area; every plate is checked for freedom to be lifted clear at discharge
Cell voltage	1–3 V DC per cell in the normal working window; rising voltage at constant current is read as a fault indicator rather than used as a control handle
Cell supply and rectifier control	Adjustable voltage and current, constant-current or constant-voltage mode as the flowsheet specifies, with an adjustable superimposed alternating component where anode passivation requires it; single-phase for compact single-cell configurations and three-phase for industrial duty, at the local voltage and frequency
Conductor bars and lugs	Titanium-clad copper bars with dressed contact faces, engineered to stay below 70 °C in continuous operation
Electrical insulation	Interlocking bakelite and polymer insulation between the conductor bars and the tank body and wherever a busbar passes a structural member
Electrolyte system	Silver nitrate in nitric acid, with potassium nitrate where the conductivity or the pH band requires it
Silver concentration in electrolyte	60–150 g/L Ag
Free nitric acid	10–18 g/L; below the window bismuth and other base metals hydrolyse into the crystal, above it re-dissolves silver already deposited
Electrolyte acid control	Titrated and corrected on schedule during the campaign, and re-acidified to the working pH during regeneration
Electrolyte temperature	40 °C set point, held by a chiller that starts and stops on the measured electrolyte temperature
Electrolyte hold-up	About 5–6 litres of circulating electrolyte per kilogram of daily silver capacity, with the working volume and the in-cell level set from the cathode area and the campaign length
Circulation pattern	Parallel flow, entering at the bottom of the cell and leaving at the top, parallel to the electrode plates
Circulation function	Holds the silver-ion concentration uniform across the electrode gap and suppresses the concentration polarisation that would otherwise force the silver concentration up
Crystal release	Continuous scraper or agitation on both cathode faces, timed or continuous, so that the deposit cannot bridge the electrode gap, short-circuit the cell or harden inside the anode bag
Cell discharge	Mechanical crystal transfer along the cell floor to a sloped discharge section, or lift-out on a rotating drum cathode, followed by a solid–liquid separator and the circulation tank
Deep clean interval	On a campaign basis rather than the roughly monthly stop of a belt-discharge cell, with continuous release running in between
Anode slime yield	About 8% of the anode mass
Anode slime composition	Typically 50–70% gold with 30–40% silver, plus the platinum group metal fraction where the feed carries one
Slime and residual anode handling	Slime washed, dried, weighed and assayed at the cell; residual plates finished in a basket without re-melting or returned to the melting shop for re-casting
Cell-level acid demand	Keeping the crystal out of the anode bag removes the boiling step that would otherwise be needed to recover it, which cuts the nitric acid demand for slime treatment and the nitrogen oxide load generated with it
Fume control	Transparent sealed cell cover with side extraction, followed by a condensation system returning silver-bearing condensate to the circuit
Nitrogen oxide abatement	Multi-stage absorption tower under negative pressure, sized for the load from electrolyte preparation and cell operation, also absorbing chlorine and sulphur dioxide, and generating no secondary waste
Materials of construction — wetted parts	PP and PVDF piping, ducting, pump internals and filter media; high-density filter fabric for the anode bags
Product purity	99.99% (9999 fine) silver at the cathode, from an anode above 75% silver; 99.999% where a second pass is specified
Product forms	Silver crystal, granules or grain, and cast ingots or bars, with vacuum ingot or continuous casting where the finish and density require it
Verification	Fire assay with cupric chloride titration or ICP-OES confirmation; each lot released against a certificate of analysis with a closed metal balance
Instrumentation and control	PLC with touchscreen operator interface, rectifier control, circulation and crystal-release sequences, in-cell level and temperature measurement, per-cathode current and cell voltage logging, alarm annunciation and an event log written against the batch
Containment	Bunding, drip trays and a segregated floor drainage path under the cell and the electrolyte handling area, draining to the effluent plant
Effluent handling	Segregated process effluent, neutralised, metal-polished and either recirculated or discharged at the agreed limits, with the analysis filed against the batch
Capacity, cell count, power, footprint and utilities	Engineered to your anode assay, daily silver volume, target product form and site conditions
Installation	Indoor, on a level floor, with the containment, bunding and segregated drainage arrangements shown on the layout drawing

Technical Advantages

Throughput comes from the current density, not from cell volume. At 750 A/m² against the 250–350 A/m² of a conventional belt-discharge cell, a cell of the same working volume carries two to three times the cathode current, and the electrode array, the basket and bag arrangement, the conductor bars, the circulation rate and the cooling duty are all engineered for that regime rather than derated from a low-density design. High-current-density silver electrorefining cells in industrial service are published above 1,000 A/m² depending on the doré composition, which is the headroom this cell is built into.
Cathode area is a calculation, so the cell can be checked before it is built. Deriving the cathode area from the silver equivalent of the current, the target daily output, the current efficiency and the design current density gives a figure that can be audited on paper against the electrode count, the cell length and the rectifier rating. A cell specified this way cannot quietly under-deliver, because the number that decides its throughput is stated in the design package.
The boundary-layer limit is addressed instead of discovered. Concentration polarisation caps cathode current density long before the rectifier does, so parallel-flow circulation, agitation, a level electrolyte surface and a squared electrode array are treated as cell geometry rather than as accessories. A cell that ignores this underperforms its own specification on the first campaign.
Crystal quality is held by voltage discipline inside the cell. Within the 1–3 V window silver deposition is the favoured reaction, and a rising cell voltage at constant current is worked through in a fixed order — contacts, electrode gap, silver concentration, free acid, temperature, anode condition — and the fault is corrected rather than the current raised.
The deposit is read before the assay is. Because the release mechanism runs continuously and the crystal is sampled during the campaign, a change in deposit habit shows up as hairy or whisker-like growth before the co-depositing metal appears in the assay, which gives the operator a window to correct the liquor instead of losing the batch.
Silver is kept out of the anode bag, and that is a cost decision. Crystal that reports to the bag has to be boiled out with acid later, and the acid demand for that step and the nitrogen oxide load it generates both fall when the bag is left to carry slime. The release mechanism, the electrode pitch and the discharge arrangement are chosen together to keep the bag a slime receptacle rather than a silver trap.
The cell that clears itself also stops being cleaned monthly. Continuous release plus mechanical discharge removes the accumulation under the electrode support that forces a belt-discharge cell to be stopped roughly monthly, so production is interrupted at campaign intervals instead, and the crystal is not left lying in the cell long enough to go off-grade.
Cell type is matched to the feed and the site, not to a house standard. A cell that has to take cemented silver should not be a tray cell that needs a cast plate; a house fighting floor space should not be adding a second row of low-density tanks; and a single-batch operation should be able to isolate one cell rather than run the whole house. The arrangement is selected against the feed and the layout before the geometry is fixed.
Materials are matched to nitric acid service stream by stream. Integrally formed polypropylene without welded seams, a polypropylene homopolymer frame, titanium-clad copper bars, bakelite insulation, titanium cathodes and basket, and PP and PVDF wetted parts, selected for the chemistry rather than for first cost.
The electrolyte and the metal balance stay inside the cell circuit. The chiller holds the acid in the liquor, the sealed cover and condenser keep silver out of the stack, the bleed is regenerated and returned with its silver recovered, and the slime is washed and assayed at the cell — so what leaves the cell house is product and declared by-product rather than dilute loss.
Compliance is engineered into the cell rather than added downstream. Sealed cover, side extraction, condensation and a negative-pressure absorption tower are part of the cell scope, and the reagent chemistry of the tower converts the nitrogen oxides to nitrogen, carbon dioxide and water so that abatement does not create a secondary waste to dispose of.
Configuration stated in writing before manufacture. Cell count, cathode area, working volume, electrolyte hold-up, rectifier rating, circulation rate, cooling duty, exhaust volume and effluent duty are engineered from your anode assay and daily silver volume, and the resulting figures are set out in the process design package rather than left to be discovered at commissioning.

Service Process

Step 1 — Feed Characterisation and Cell Consultation. Send a representative sample of your crude silver, doré, cement silver, residue or slime-derived feed with an indication of grade, the batch size and the daily silver volume you intend to process. Our metallurgical team assays it for silver and gold by fire assay with an ICP-OES cross-check, and for the impurities that govern the cell design — copper, lead, bismuth, tellurium, selenium and the platinum group metals — and identifies whether the feed can be cast into a plate or has to be run from a basket. Output: assay report and a preliminary cell proposal with the achievable purity and the expected slime yield.

Step 2 — Laboratory Test Work and Cell Design. Bench and pilot campaigns confirm the anode composition and casting or granulation parameters, the electrolyte specification and the free-acid window, the current density, the expected crystal quality and habit, the anode slime yield and composition, and the bleed and regeneration schedule. The results are converted into the cell type, the cathode area and electrode count, the working volume and electrolyte hold-up, the inter-electrode gap and electrode pitch, the shell drawing, the rectifier rating, the circulation and cooling duty, the off-gas load, a layout and a utility schedule against your throughput and site. Output: test report, cell design package and firm quotation.

Step 3 — Shell Fabrication and Module Assembly. The cell shell is formed as one piece, the frame and electrode rails are fabricated, the titanium baskets and cathode plates are produced to drawing, the conductor bars are titanium-clad and the cover, side extraction plenum, discharge line and instrumentation are assembled and piped. Output: an assembled, pre-piped cell module.

Step 4 — Factory Assembly, Testing and Acceptance. Each cell is hydrostatically checked, the electrode array is trial-fitted and the gap verified, and the cell is functionally tested — wherever possible on your own material — with the client invited to attend the factory acceptance test before shipment. Output: FAT-approved, pre-commissioned cell with its inspection record.

Step 5 — Shipping, Installation and Commissioning. We supervise the civil interface, the level survey of the cell support, erection, piping, ducting and electrical tie-in, then run an instrumented commissioning campaign on your feed until the contracted purity, current density and recovery are demonstrated and the nitrogen-oxide outlet is verified against your local emission limit. Output: a producing silver electrolytic cell at contracted performance.

Step 6 — Operator Training and Handover. On-site training covers safe nitric acid handling, anode casting and bagging, basket loading, cathode preparation and gap setting, cell loading and electrode spacing, electrolyte make-up and titration, current density and temperature control, reading cell voltage and per-cathode current, crystal release and the co-deposition check, cell discharge and clean-out, off-gas and effluent operation, emergency procedures and assay quality control, supported by standard operating procedures and a full handover dossier. Output: trained crew and complete documentation.

Step 7 — Process Optimisation and Ongoing Support. Remote troubleshooting, reagent and consumable supply, electrode, bag and filter service, periodic performance audits where the crystal assay, the electrolyte analysis and the slime yield are reviewed against the design, and cell or capacity upgrade paths keep the cell house at target over its service life. Output: sustained performance and spare-part continuity.

Warranty And After-Sales Service

Warranty period. Twelve months from commissioning or eighteen months from shipment, whichever falls first, covering design, materials and workmanship of the equipment supplied, with the terms and the excluded wear parts stated in the contract.
Wear part and consumable clarity. Anode bags, filter media, gaskets, pump seals, scraper elements and cathode release treatment are identified as consumables with their expected service intervals declared up front, so that the operating cost of the cell is visible before it is commissioned rather than after.
Spare parts availability. A recommended commissioning and first-year spares set is quoted with the cell, and spare parts remain available through the equipment’s service life with lead times stated at order.
Remote support. Telephone, email and messaging support for operating questions, alarm interpretation, crystal quality problems, cell voltage deviations, current distribution faults and process troubleshooting while your cell is running. Output: same-day response on operating issues.
On-site service. Engineer attendance for warranty faults, process problems and periodical check-ups, with response and attendance times agreed in the contract according to your location. Output: on-site intervention and a service report.
Cell clean-out and overhaul support. Assistance with the first scheduled clean-out and re-dress, including shell inspection, gasket and seal replacement, contact dressing, gap re-setting and the return to service on the ramp, so that the procedure is transferred with the equipment rather than learned at the customer’s expense. Output: a documented clean-out carried out on site and a cell history record.
Assay and metal balance verification service. Periodic verification of the product assay and the electrolyte and slime analyses against your own laboratory results, so that the performance of the cell is tracked over time rather than assumed from commissioning. Output: verification report against the batch records.
Operator retraining and refresher visits. Additional training, refresher sessions and process optimisation visits as your feed mix, anode assay or product requirement changes. Output: updated training and revised procedures.
Documentation set. Operation and maintenance manuals, cell drawing and electrode arrangement, electrical and control drawings, layout, spare parts catalogue, safety data for nitric acid and the reagents used, and the emergency response plan covering acid exposure, nitrogen oxide release and electrical isolation. Output: complete documentation handover.
Compliance support. Assistance with emission and effluent parameter declarations, absorption tower performance verification, and the handling, storage and disposal requirements for acid-bearing process liquors and for the intermediates generated at the cell. Output: technical data for your permit applications.
Lifetime technical support. The cell design remains yours, and our engineering team remains available for modification and expansion advice for the operating life of the cell. Output: long-term engineering continuity.

Frequently Asked Questions

What is a silver electrolytic cell?

A silver electrolytic cell is an electrochemical vessel in which a cast crude-silver anode and a titanium cathode are suspended in a silver nitrate electrolyte and a direct current is passed between them. The anode dissolves and releases silver into the liquor as the silver ion, and the cathode reduces it back to metal as a crystalline powder at 99.99% purity, at a design cathode current density of 750 A/m². The cell separates the impurities at the same time: copper and other base metals stay dissolved in the electrolyte, while gold, platinum, palladium and rhodium do not dissolve at all and are held back in the anode bag as slime. A silver electrolytic cell is therefore the unit that sets the throughput, the grade and the operating cost of a silver refinery, and everything around it exists to feed it, hold it in its window and take its products away.

What is the difference between a Moebius cell and a Balbach-Thum cell?

They are the two classic arrangements of silver electrorefining, and they differ chiefly in where the electrodes sit rather than in the chemistry. In the Moebius, or vertical, cell the anodes are enclosed in filter bags and suspended vertically in a basket arrangement with the cathodes hung between them, which allows a short electrode gap, a compact footprint and higher current densities, and it is the arrangement that suits granulated or cemented feed loaded in a basket. In the Balbach-Thum, or horizontal, cell the anodes lie in trays with the cathodes arranged above them across a wider electrode gap, the older form of the same idea being the belt-discharge or boat-type cell that carries crystal out on a conveyor under the electrodes. The wider gap in the horizontal arrangement raises the resistive drop and therefore the cell voltage at a given current, which is why the conventional horizontal cell occupies the 250–350 A/m² band, while the vertical arrangement is the basis of high-current-density practice at 600–800 A/m² and above.

Which cell type should be chosen for a given silver output?

The choice follows from three things: the form the feed arrives in, the batch pattern, and the floor space available. Feed that cannot be cast into a coherent plate — cemented silver, atomised powder, residual anode plates — needs a vertical basket cell, because a tray cell requires an anode that will lie flat and stay in one piece. A house running continuous high tonnage in one grade is served by the horizontal belt-discharge arrangement, whose automation is its main strength, provided it accepts the lower conventional current density, the large electrolyte inventory and the floor clean that crystal accumulating under the belt will require. Where floor space is the binding constraint, or where the same throughput is wanted without adding a second row of low-density tanks, the integrally formed sealed vertical cell at 750 A/m² produces two to three times the silver of a conventional cell of the same volume. Where batches must be kept apart, single-cell isolation matters more than cell size: each cell runs its own anode set and discharges its own crystal, so a small batch can be worked in one cell without touching another cell’s product.

How is the cathode area of a silver cell sized?

It is arithmetic rather than judgement. Silver carries 4.025 g per ampere-hour, so the target daily silver output is converted to grams per hour and divided by the silver equivalent at the working current efficiency, which gives the cell current. The cathode area is then that current divided by the design cathode current density. A cell required to produce 100 kg of silver a day has to deposit 4,167 g an hour; at a current efficiency of about 95% that needs 1,090 A; and at 750 A/m² that needs 1.45 m² of cathode area. The cathode area then fixes the number of electrode sets, the electrode pitch, the internal length of the cell, the working volume, the rectifier rating and the cooling duty. Because the calculation is explicit, the cell can be audited on paper against its own specification before it is built.

How much electrolyte does a silver cell hold?

Electrolyte hold-up scales with the cell duty at roughly 5–6 litres of circulating electrolyte per kilogram of daily silver capacity, so the figure rises with the cathode area and the campaign length you specify. It is worth treating as a design output rather than a tank dimension, because the liquor is inventory in two senses: it is the silver, acid and nitrate held in solution, and it is the working volume the circulation and cooling systems have to turn over. A conventional horizontal belt cell is the arrangement that holds the most electrolyte for a given output, which is part of why it carries a slower turnover of capital. The working volume and the in-cell level are stated in the process design package, and the cell is filled, titrated and recorded against the batch before it is ever energised.

What is the inter-electrode gap in a silver cell, and why does it matter?

The gap is the distance between the anode face and the cathode face, and it is set with a gauge across the whole array rather than left to the tolerance of the frames. It matters twice over. Electrically, the gap is the electrolyte path that the current has to cross, so it fixes the resistive drop and therefore the cell voltage at a given current — a wider gap raises the voltage and the energy per kilogram of silver without changing the chemistry. Practically, the gap sets how much crystal can grow before the deposit is in danger of bridging to the anode: silver deposits as a powder, and powder that is not removed builds until it touches the anode and short-circuits the cell. A plate that is bowed or a stop that is out of position changes the gap along its length, which shows up as a localised burn and a current imbalance that cannot be corrected from the rectifier, which is why flatness is checked and the gap is re-set at every re-dress.

What anode and cathode materials are used inside the cell?

The anode is the crude silver itself, cast into a plate with formed lugs and dressed faces, or loaded as granulated, cemented or powdered silver into a perforated titanium basket that carries the current and retains its own residue. Titanium is the basket material because it resists the nitrate electrolyte and does not contaminate the deposit. The cathode is a titanium sheet plate, chosen for the same reason and because the crystal releases from titanium cleanly on scraping; stainless steel plate is available where the process specifies it, and a rotating titanium drum cathode with an internal scraper is used where continuous harvesting and a compact footprint matter more than plate handling. The conductor bars that deliver the current are titanium-clad copper, so that the current-carrying copper is shielded from the acid mist while the contact faces stay clean, and the bars are insulated from the tank body with interlocking bakelite to stop stray current from finding a path through the frame.

Why do horizontal silver cells have to be cleaned so often?

Because crystal accumulates where the discharge does not reach it. In a belt-discharge cell the conveyor runs under the electrodes and carries the powder out of the cell, but some of it falls behind and under the belt, and that material sits in the cell instead of leaving it. Once it builds up, the discharge stops running evenly, and powder that lies in the cell long enough changes character and can fail the assay when it is finally recovered. That is what puts a conventional belt cell on a roughly monthly stop for a floor clean rather than a campaign interval. The same failure mode is what the continuous release mechanism and the sloped discharge section in this cell are arranged to avoid: crystal is removed from both cathode faces as it forms and leaves the cell mechanically, so there is no accumulation under the electrodes to shut the cell down for.

What cell voltage should a silver cell run at?

The normal working window is 1–3 V DC per cell, and the operating instruction is that voltage is read, not set. Within that window silver deposition is the favoured reaction at the cathode, and the voltage in any case moves with the silver concentration of the liquor, with the resistive drop across the electrode gap, with the contact resistance from lug to busbar, and with the condition of the anode face. A cell voltage that rises at constant current is therefore a fault indicator, and it is worked through in a fixed order — contacts, electrode gap, silver concentration, free acid, temperature, anode condition — because raising the current to recover the deposition rate would raise the voltage further and take the grade with it. Rectifier control is provided in either constant-current or constant-voltage mode according to the flowsheet, and an adjustable superimposed alternating component is available where an anode face tends to passivate.

How does parallel-flow circulation work inside the cell?

Electrolyte is pumped into the bottom of the cell and leaves at the top, so that the flow direction is parallel to the plane of the electrode plates rather than across the gap between them. That direction matters because the limiting factor on cathode current density is not the rectifier but the boundary layer: silver is consumed at the cathode face faster than it can diffuse in from the bulk liquor, so the concentration at the surface falls, and a deposit grown from a starved surface turns nodular and dendritic. Parallel flow sweeps that depleted layer away and carries fresh silver-bearing liquor into the gap, which holds the silver concentration at the cathode face close to the bulk concentration, suppresses the concentration polarisation, and lets the cell run at a lower and more stable silver concentration than a still cell could sustain. It is the reason a cell can be designed at 750 A/m² instead of being limited to the 250–350 A/m² band, and it is built as cell geometry rather than added as an accessory.

What happens inside the cell when the electrolyte silver concentration falls?

It announces itself in the shape of the deposit before it appears in the assay. As the silver in the liquor is progressively displaced by copper stripped from the anodes, the deposition window narrows and the crystal starts growing as long hairy or whisker-like growth rather than as a dense granular powder, which is the classic sign that copper is about to co-deposit with the silver. The liquor colour is a second, rougher indicator, moving from a light blue towards a deeper blue as the copper content rises, with a green cast that can also come from a palladium-rich anode or from residual nitrous species left in an electrolyte that was not boiled out during make-up. Because the deposit is the earliest signal, the cell is sampled during the campaign rather than only at discharge, and the correction is made to the liquor — by bleed, by regeneration and by restoring the silver and free-acid figures to their window — rather than by working the rectifier harder.

How is the silver crystal released from the cathode in the cell?

Mechanically, and continuously. Silver leaves the cathode as a crystalline powder, and a scraper or an agitation arrangement works both faces of the cathode so that the deposit is lifted off as it forms and falls to the cell floor, where a conveyor or a sloped discharge section carries it out of the cell into the wash-and-dry unit. On a rotating drum cathode the scraper sits inside the drum and removes the crystal as the drum turns. Continuous release is not a convenience: crystals left in place grow until they bridge the electrode gap and short-circuit the cell, which distorts the current distribution across the remaining electrodes and lowers the quality of the powder still in the cell. It is also what keeps the anode bag a slime receptacle rather than a silver trap, since powder that finds its way into the bag has to be boiled out later with additional acid.

Why is sealed side extraction better than an overhead hood on a cell?

Because capture efficiency and exhaust volume move in opposite directions. Nitric acid mist and nitrogen oxides collect in the cell headspace, and a sealed cover with extraction taken from the side of the body removes them at the point of generation with a small, well-directed air volume. An open cell under an overhead hood has to be diluted with a much greater air volume to reach the same working condition in the room, which means a larger fan, a larger scrubber and a larger operating cost for the same emission result — and on a horizontal cell the hood also has to be swung clear for loading and unloading, which is when it captures least effectively. Sealing the cell also lets the vapour pass a condenser on the way to the absorption tower, so the entrained silver-bearing electrolyte is returned to the circuit instead of being diluted into the exhaust and lost.

What are the safety requirements inside and around a silver cell?

The hazards are the liquor and the headspace, not the metal. Nitric acid causes severe burns and the cell generates nitric acid mist and nitrogen oxides, so the cell is sealed and extracted at the source, electrolyte preparation runs in a closed vessel with the off-gas ducted to the absorption plant, and the working area is ventilated with the exhaust verified rather than assumed. Personal protective equipment — acid-resistant gloves, chemical splash goggles or a face shield, and an apron over suitable clothing — is required for any work on the cell, and respiratory protection is used where the extraction is opened up or where a spill has to be handled. The other cell-specific risks are electrical and mechanical: the electrode array carries a high direct current, so the cell is isolated and proved dead before plates are handled, and the lifted electrode sets, the baskets and the conveyor are handled with the gear supplied rather than by hand. Spill containment, bunding and a segregated drainage path are part of the cell installation, together with an emergency response plan covering acid exposure, nitrogen oxide release and electrical isolation.

How does the cell handle residual anode plates?

Partly dissolved plates that have reached the end of their dissolution cycle do not have to go back to the furnace. They are finished in a titanium basket inside the cell, which carries the same current and retains the residue in a lining, so the remaining silver is recovered without a re-melt and without the metal inventory that would otherwise sit in the melting shop waiting for it. Where the works does prefer to re-cast, the plates are returned to the melting shop and cast into fresh anodes with the flux and blowing conditions set from the assay. Either route keeps the residual plate in the circuit: the objective at the end of an anode’s life is to finish it as silver in the cell, not to dispose of it as scrap, and the bag and basket arrangement is what makes that possible without the residue migrating into the product.

What is the difference between a silver electrolytic cell and a silver electrowinning cell?

The difference is what is fed to it. A silver electrolytic cell, or electrorefining cell, takes metallic feed: a cast anode plate or a basket of granulated silver, which is dissolved anodically under an applied current and re-deposited on the cathode. The anode is consumable, it dissolves as the campaign runs, and it leaves behind an anode slime carrying the insoluble gold and platinum group metals. A silver electrowinning cell takes a solution — a leach liquor, a stripping liquor, a plating drag-out or a rinse water — through an inert anode that does not dissolve, so there is no anode slime and the product forms as cathode silver or silver mud from a liquor rather than from a plate. Both are electrochemical and both produce high-purity silver, but only the electrorefining cell purifies metal, and only the electrorefining cell has an anode that is consumed, a slime to recover and an anode bag to fit. This page describes the refining cell; silver recovery from solution belongs to a separate equipment family.

How is the cell verified and documented before handover?

The cell is verified in three stages. In the factory the shell is hydrostatically checked for leaks, the electrode array is trial-fitted and the inter-electrode gap verified with a gauge, and the cell is functionally tested on the customer’s own material wherever that is possible, with the client invited to attend the factory acceptance test. On site the level survey of the cell support, the electrode spacing, the per-cathode current distribution and the thermal and circulation loops are confirmed during an instrumented commissioning campaign, which continues until the contracted purity, current density and recovery are demonstrated and the nitrogen-oxide outlet is verified against the local emission limit. Thereafter each lot is released against a certificate of analysis that carries the anode weight and assay, the electrolyte analysis, the cell campaign record, the slime weight and assay, the product assay and the metal balance for the lot. The documentation set handed over with the cell includes the cell drawing and electrode arrangement, the electrical and control drawings, the layout, the operation and maintenance manuals, the spare parts catalogue, the safety data for nitric acid and the reagents used, and the emergency response plan.

Silver Electrolytic Cell

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