Precious Metal Electrowinning Equipment
— Turnkey Precious Metal Electrowinning Plant
Precious metal electrowinning equipment is a cathodic recovery circuit that passes direct current through a metal-bearing process solution and reduces the dissolved metal ions onto a cathode as a harvestable deposit. YuSai Machinery engineers the whole circuit — liquor survey and conditioning, cell and cathode selection, rectifier and busbar, boundary-layer control, in-cell cathode washing, barren-liquor polishing and off-gas treatment — around your own liquor assay, and delivers gold, silver, copper and platinum group metal sludge that goes straight to melting and assay.
Configuration: cell count, cathode area, rectifier rating, circulation, cooling, exhaust and effluent polishing are engineered to your liquor assay, monthly metal volume and target product rather than taken from a catalogue of fixed models.
Product Overview
Precious metal electrowinning is a cathodic recovery step: direct current is passed through a metal-bearing solution and the dissolved metal is reduced onto a cathode as solid metal — Au(CN)₂⁻ in a carbon-desorption eluate, Ag(CN)₂⁻ in a silver strip liquor or fixing bath, AuCl₄⁻ in a chloride or aqua regia medium, palladium and platinum chloro-complexes in a platinum group metal electrolyte, and Cu²⁺ in a spent plating bath or etchant. The distinction from electrorefining is routinely confused, so it is worth stating plainly: electrorefining dissolves a solid anode to purify a metal that is already metallic, while electrowinning has no anode to dissolve and is governed instead by how fast ions can be brought to the cathode surface. In a gold plant the combination is called a desorption electrolysis circuit, and gold electrowinning from cyanide solution is its most widely installed form.
That difference fixes the physics. The controlling constraint is mass transfer, not current: current strips the layer of liquor immediately adjacent to the cathode, and current density can rise only as fast as the bulk solution replenishes that depleted film, or the surplus current reduces water to hydrogen and the deposit turns rough, nodular, dark and poorly adherent — concentration polarization. Cathode surface area and solution movement therefore matter more than rectifier size, which is why packed steel wool and multi-layer mesh, exposing many times the geometric area of a flat plate, together with forced circulation, agitation and the swirl cell’s rotational flow field, are the design’s centre of gravity — and why a precious metal electrowinning cell treated as a tank with plates in it always underperforms its specification.
YuSai Machinery builds three cell architectures and selects between them from the liquor itself. A flat-plate cell with a packed steel wool cathode or multi-layer stainless mesh — the arrangement most often catalogued as precious metal electrolytic recovery equipment — is the workhorse for concentrated eluates such as the 0.3–0.6 g/L of gold a carbon-desorption circuit delivers. A swirl cell imposes a rotational flow field across the cathode and sustains several times the current density of a flat-plate cell of the same footprint, which is what lets it work 0.x g/L liquors and tolerate the impurity load of stripping solutions, etchants and rinse water. A compact rotating-cathode unit covers the small end, turning a plated barrel through the liquor in a drag-out or rinse tank at around 150–250 W with no reagent and no dedicated operator. Selection is made on tenor, impurity profile, volume and required product form, and recorded on a route sheet before any equipment is specified.
Whichever cell is selected, the plant is the circuit around it: the liquor is clarified, the pH is raised above 11 where cyanide is present, temperature is brought into the 40–60 °C band, and free cyanide, conductivity and tenor are confirmed before the cell is charged. Current is brought up on a ramp rather than switched on at its full value, because a deposit that begins as a coherent seed film stays coherent. At steady state the cell runs at 2.0–3.5 V, with voltage an outcome of the design rather than a control handle — above 3.5 V the surplus is spent decomposing water. When the cathode has taken up its working load — for gold on stainless steel wool, twenty times the wool’s own weight — the deposit is washed off in place, then filtered, dried, fluxed, melted, sampled and released against a certificate of analysis, and the barren liquor, holding metal at low single-digit parts per million, returns as desorption solution or to the leaching circuit after make-up.
The liabilities are specific, and the design contains them rather than describing them: cyanide forms hydrogen cyanide gas if the liquor turns acidic, which makes the pH hold a safety function rather than an efficiency measure, and it breaks down above roughly 60 °C; mercury from concentrate that has seen amalgamation is retorted ahead of the circuit; hydrogen evolution makes the cell headspace a ventilation issue as much as an efficiency one; and oxygen, acid mist and nitrogen oxides are captured by the built-in exhaust plenum. Because tenor, impurity profile, volume and required product vary between projects — and a precious metal electrowinning plant is judged on its barren liquor as much as on its product — capacity is engineered from your own assay and monthly metal volume rather than selected from a catalogue. Working as an electrowinning cell manufacturer from the liquor backwards, we would rather revise the route sheet than supply a cell that does not match the chemistry.
Key Facts
- Deposition rate: more than 99% of the metal held in the feed liquor is reduced onto the cathode under the design window; the spent electrolyte leaves at low single-digit parts per million.
- Final product: gold at 99.99% and above after melting, silver at 99.9–99.99%, copper as cathode plate or powder, and platinum group metal sludge routed to the refining line.
- Cell voltage: 2.0–3.5 V per cell is the normal working window. Above 3.5 V the extra voltage is consumed by water decomposition rather than by metal deposition, so a rising cell voltage is treated as a symptom to be diagnosed, not as a productivity setting.
- Cathode current density: 8–15 A/m² is a typical operating figure for gold recovered from a cyanide eluate in a packed flat-plate cell; base metal duty runs an order of magnitude higher; a swirl cell sustains several times the current density of a flat-plate cell of the same footprint because forced convection removes the diffusion limit.
- Liquor tenor handled: one platform covers 0.x g/L up to several hundred grams per litre of metal, so dilute rinse water and concentrated eluate are served by the same circuit family.
- Cathode media: stainless steel wool, multi-layer stainless mesh, expanded metal and rotating titanium cylinders; more than 97% of the deposited metal is released by high-pressure water washing inside the cell.
- Operating window for cyanide liquors: pH held above 11 and temperature held between 40 and 60 °C, because below pH 10 cyanide forms hydrogen cyanide gas and above about 60 °C it breaks down.
- Configuration: cell count, cathode area, rectifier rating, circulation, cooling, exhaust and effluent polishing are engineered to your liquor assay, monthly metal volume and target product rather than taken from a catalogue of fixed models.
Product Core Selling Points
- Solution-side recovery, engineered as a circuit rather than as a tank. The scope covers what happens to the liquor before it reaches the cell and after it leaves, because a cell that is correctly sized for a dirty or wrongly conditioned feed will still underperform.
- One platform from dilute rinse water to concentrated eluate. The same cell family and control philosophy serve liquors holding a few hundred milligrams per litre of metal and liquors holding several hundred grams per litre, with the cathode medium, current density and circulation set from the assay.
- Three cathode architectures, selected from the liquor. A flat-plate cell with packed stainless steel wool or multi-layer stainless mesh for concentrated eluates, a swirl cell that imposes a rotational flow field for dilute and complex liquors, and a compact rotating-cathode unit for drag-out tanks and plating rinses.
- In-cell cathode washing instead of plate handling. The deposit is washed off the cathode medium with high-pressure water while it stays in the cell, which removes the transport losses and the labour that come with lifting loaded plates to a separate wash tank, and cuts service time by up to 75%.
- Deposit quality treated as a controlled variable. Current is brought up on a ramp rather than switched on at full value, the rectifier runs a staged voltage and time programme, and the standing operating rule is that a blackened, poorly adherent cathode is corrected by lowering the voltage rather than by raising it.
- Boundary-layer control designed into the cell. Expanded cathode surface area, forced circulation, agitation and, in the swirl cell, a rotational flow field lift the mass-transfer ceiling that otherwise caps the current density long before the rectifier is at its limit.
- Cyanide contained by design rather than by procedure. The pH hold above 11 is a safety function, the cell is enclosed with a built-in exhaust plenum, and the anode reaction oxidises cyanide to cyanate and then to carbon dioxide and nitrogen, so free cyanide is destroyed in the same operation that recovers the metal.
- The barren liquor goes back to work. Spent electrolyte leaves at low single-digit parts per million. It is returned as desorption solution, returned to the leach circuit after a make-up addition, rebuilt as plating electrolyte, or polished to the discharge limit.
- Selective deposition and second-pass polishing for impure liquors. Where copper, nickel, zinc or iron are present, the deposition window is set to favour the target metal, and a polishing cell takes the residual down further before the liquor leaves the plant.
- Harvest, melt and assay are inside the scope. Sludge washing, filtration, drying, fluxing, melting, sampling and a certificate of analysis are supplied as part of the line, so the customer receives certified metal rather than a wet sludge.
- Materials selected stream by stream. Integrally formed polypropylene and PPH cell bodies with an anti-ageing additive, titanium-clad copper busbars, 304 and 316 stainless wool and mesh, coated titanium or carbon anodes, and PP and PVDF wetted parts.
- Configuration to your assay, with the numbers written down. Cell count, cathode area, rectifier rating, cooling duty, exhaust volume and effluent load are engineered from your liquor analysis and monthly metal volume, and the resulting figures are stated in the process design package.
Application Scenarios
- Carbon-in-leach and carbon-in-pulp gold plants using Zadra or AARL elution. The eluate arrives at roughly 0.3–0.6 g/L of gold in a clean, high-purity cyanide solution, which is the ideal feed for electrowinning equipment for gold and silver in a packed flat-plate cell, and the barren eluate is returned to the elution column.
- Gold operations currently using zinc cementation. Merrill-Crowe circuits that want to remove zinc dust, clarification and deaeration from the flowsheet, or that want a gold electrowinning machine whose output is a sludge rather than a cemented precipitate. Where the eluate is clean, a gold electrowinning system of this type replaces the cementation step outright rather than supplementing it.
- Gravity gold concentrators running intensive cyanidation. Small volumes at high tenor, where a compact cell recovers the gold direct and the concentrated liquor can be processed in a single shift.
- Heap, dump and vat leach operations. Pregnant solution treated after carbon adsorption, or treated directly by electrowinning where the tenor and the impurity profile allow it.
- Standalone precious metal refineries. Strip liquors, deplating solutions and process liquors recovered in-house instead of being sold as a solution, and barren filtrates and mother liquors polished before they are discharged.
- Electronics and printed circuit board recyclers. Gold and copper from gold stripping and deplating solutions, from etching and micro-etch liquors, and from rinse water generated by plating and stripping lines.
- Electroplating and surface finishing shops. Gold, silver, palladium, palladium-nickel, platinum, rhodium, iridium and copper recovered from plating baths, drag-out tanks and rinse stages, with the recovered metal returned to the plating circuit as anode material or sold. This is the most common duty for electrowinning for plating wastewater, because the rinse stages carrying the highest metal load are also the ones that most often exceed the discharge limit.
- Photographic and imaging processors. Silver recovered from fixing, bleach-fix and developer solutions and from wash water, which converts a regulated waste stream into a saleable metal. These liquors sit at the low end of the tenor range and call for silver electrowinning equipment that still works once the concentration has fallen away, which is where the swirl cell earns its place.
- Jewellery manufacturers and polishers. Gold and silver recovered from plating solutions, polishing rinses and spent electrolytes, with the recovered metal returned as fine metal or as plating salt.
- Semiconductor, connector and contact manufacturers. Gold, palladium and platinum recovered from plating rinse water and stripping solutions, which also brings the rinse stream inside the discharge limit.
- Copper electrorefining and hydrometallurgical plants. Copper recovered from bleed streams, spent electrolyte and leach solution, and precious-metal-bearing liquors taken down to a few parts per million in a polishing cell before they leave the plant.
- Operations driven primarily by compliance. Sites that must remove metal-cyanide complexes and free cyanide from effluent before discharge, where the same cell both recovers the metal and oxidises the cyanide at the anode.
- Contract refineries and service providers. Toll treatment of third-party solutions and sludges where the recovery step must be quick to reconfigure between campaigns and traceable per batch.
Secondary recovery of these metals is itself a growing industry. The Business Research Company, in its July 2026 report on the precious metals e-waste recovery market, values that market at USD 11.13 billion in 2025 rising to USD 11.85 billion in 2026, a compound annual growth rate of 6.5%. Published estimates differ in scope — some houses measure equipment, others measure recovered metal — so the absolute figures move between sources, but the direction is consistent and it is what drives the demand for recovery capacity at the point where the liquor is generated.
Applicable Raw Materials
Each feed below enters the circuit at the point its composition requires. Liquors carrying suspended solids are clarified first; liquors carrying mercury are routed through a retort before the cell; liquors that are strongly acidic, nitrate-based or chloride-based are handled with the anode, cell body and exhaust configuration matched to that chemistry. Output is stated for each feed.
- Cyanide eluate from activated carbon desorption (CIP, CIL, Zadra and AARL circuits). The standard high-purity feed: gold at 0.3–0.6 g/L with silver co-present and a low base-metal load, in an alkaline cyanide solution. Output: gold-silver sludge; barren eluate returned to the elution column as desorption solution.
- High-grade eluate from intensive cyanidation of gravity gold concentrate. Small-volume, high-tenor liquor produced from a gravity concentrate, processed in a compact cell or a single flat-plate cell in campaign mode. Output: gold sludge; barren liquor returned to the intensive leach.
- Zinc-cyanide and copper-cyanide plating solutions and drag-out liquor. Plating electrolytes and tank drag-out where the metal is present as a cyanide complex; a cell voltage of approximately 2.4 V produces favourable deposition from these liquors. Output: metal deposit returned as anode material or sold; electrolyte rebuilt for further plating.
- Gold and silver cyanide strip solutions from plated scrap and deplating lines. Alkaline cyanide stripping liquors from plated electronics, connectors and jewellery, where the metal tenor is high but the liquor also carries copper, nickel and iron from the substrate. Output: gold and silver sludge; polishing-cell treatment of the residual.
- Chloride and chloride-hypochlorite strip liquors. Aqua regia and HCl–chlorine media in which gold is present as AuCl₄⁻; these liquors demand coated titanium or carbon anodes and a cell body and exhaust system rated for acid mist and chlorine. Output: gold sludge; chloride liquor conditioned for reuse or discharge.
- Nitrate electrolytes and spent silver plating baths. Silver nitrate and silver-cyanide electroplating electrolytes at end of life, together with nitrate strip liquors from silver plating. Output: silver crystals or silver sludge; nitrate liquor routed to the effluent plant.
- Fixing, bleach-fix and photographic processing solutions. Silver present as thiosulphate complexes at a low tenor, with thiosulphate and sulphite load. Output: silver sludge; treated liquor at a substantially reduced silver concentration.
- Spent palladium, palladium-nickel, platinum and rhodium plating electrolytes. Precious metal plating baths at end of life, where the metal value justifies recovery and the liquor must be taken off the discharge route. Output: platinum group metal sludge routed to the refining line.
- Platinum group metal process liquors, wash waters and barren filtrates. Dilute palladium, platinum and rhodium liquors generated inside a refinery, recovered in a swirl or polishing cell rather than sent to the effluent plant. Output: platinum group metal concentrate; polished liquor below the agreed residual.
- Copper-bearing etchants, micro-etch liquors and spent plating baths. Acid and alkaline etching and micro-etch solutions from printed circuit board manufacture, and spent acid copper plating baths. Output: copper cathode plate or copper powder; etchant regenerated where the chemistry allows.
- Dilute rinse water and drag-out rinses. Rinse stages at 0.x g/L down to tens of milligrams per litre of metal, where the objective is both recovery and compliance. Output: recovered metal; rinse water polished to the discharge limit.
- Barren filtrates, mother liquors and neutralised effluent. Solutions that already hold metal only at the parts-per-million level, treated in a second-pass polishing cell or followed by ion exchange or cementation where the residual specification is tight. Output: recovered residual metal; compliant effluent.
Finished Products
The circuit is configured around the product form the customer sells or re-uses, from a wet sludge to certified metal. Where the customer already operates a melting shop, the line is delivered with the harvest and drying stages only; where it does not, melting, casting and assay are included.
- Gold sludge and melted fine gold. The primary output from gold-bearing liquors, dried, fluxed and melted to an ingot or a button assaying 99.99% and above. Output: certified fine gold ingot or gold button with certificate of analysis.
- Gold-silver doré. Produced where silver is co-deposited with gold from an eluate or a strip liquor and the two are melted together as a saleable intermediate. Output: doré bar with declared gold and silver content.
- Silver crystals and melted fine silver. Recovered from silver strip liquors, spent plating baths, fixing solutions and silver-bearing rinse water, melted to an ingot assaying 99.9–99.99% depending on the feed. Output: certified fine silver ingot or silver crystal.
- Copper cathode plate or copper powder. Deposition from etchants, spent plating baths and bleed streams produces either a coherent cathode sheet or, where the deposit is harvested as powder, a free-flowing copper powder. Output: copper cathode sheet or copper powder.
- Platinum group metal sludge and concentrate. Palladium, platinum and rhodium recovered from dilute refinery liquors and spent plating electrolytes, supplied as a washed, dried concentrate for the refining line. Output: PGM concentrate with declared metal content.
- Loaded cathode medium as a saleable intermediate. Where the customer prefers to run its own melting, the gold-loaded stainless steel wool or mesh is supplied ready for transfer under the loading ceiling agreed in the design. Output: gold-loaded cathode medium with declared metal content.
- Barren liquor suitable for reuse. Spent electrolyte returned as desorption solution, as leach make-up, or as rebuilt plating electrolyte after a make-up addition. Output: analysed barren liquor returned to the circuit.
- Treated effluent at the discharge limit. Liquor that has passed through the polishing cell and, where specified, through ion exchange or cementation, meeting the agreed residual metal and free cyanide limits. Output: compliant effluent with the analysis record attached.
- Cyanide destruction credit. Free cyanide oxidised at the anode to cyanate and then to carbon dioxide and nitrogen, which reduces the load on the downstream cyanide destruction circuit. Output: liquor with a lower free cyanide concentration and a destruction record.
- Batch documentation. Every batch is released with the assay of the feed liquor, the intermediate liquor, the barren liquor and the final product, together with the mass balance for that batch. Output: certificate of analysis and batch record.
Precious Metal Electrowinning Equipment Process Flow
The nine stages below form the standard precious metal electrowinning flowsheet, from the liquor survey through to certified metal and a compliant barren liquor. Step 4 is the heart of the circuit and Step 6 is the stage most often missing from competing equipment specifications: the deposit is a chemical product whose quality is set by voltage and current discipline during the run, not by the length of the run. Holding time, circulation rate, current density and stage count are fixed during process design against your own liquor assay.
Precious metal electrowinning process flow — from process solution to certified metal, with cyanide destruction and off-gas control inside the same circuit.
Steps 1 — Feed Liquor Survey and Route Selection. The liquor is sampled and characterised before anything is specified. Metal tenor is determined by atomic absorption or ICP-OES for gold, silver, platinum, palladium, rhodium and copper; free cyanide is determined by titration where cyanide is present; pH, conductivity, suspended solids and base metal content covering copper, nickel, zinc and iron are measured; and mercury is tested for wherever concentrate that has seen amalgamation may have entered the circuit. Volume, flow rate and the batch or continuous nature of the feed are recorded, together with the required product form and the customer’s melting arrangements. The survey output is a route sheet that fixes the cell architecture, the cathode medium, the target current density, the operating pH and temperature window, the anode material and the residual tenor to be achieved. Getting this stage right is what keeps the rest of the circuit from being sized against the wrong constraint. Reagent and utility input: sampling equipment, ICP-OES and atomic absorption spectrometry, cyanide titration, pH and conductivity meters, mercury test. Output: route sheet fixing cell type, cathode medium, current density target and expected residual tenor.
Steps 2 — Liquor Conditioning and Cathode Preparation. The liquor is brought into the condition the cell requires, and the cathode is built to the density the liquor will support. Suspended solids are removed by settling, filtration or both, because solids blind the cathode medium and report to the sludge as contamination. Where cyanide is present, the pH is raised above 11 with sodium hydroxide and held there: this is a safety function first, since cyanide released as hydrogen cyanide gas below pH 10 is lethal, and an efficiency function second, since the deposition reaction itself consumes cyanide and shifts the liquor. Temperature is brought into the 40–60 °C band, which lowers the electrolyte resistance and improves ion mobility, while staying clear of the point at which cyanide starts to break down. Free cyanide and conductivity are checked and adjusted, and the cell body, exhaust path and ducting are confirmed against the liquor chemistry. On the cathode side, stainless steel wool is packed to the design density inside stainless or plastic frames, multi-layer mesh is assembled into its frame, or the titanium cylinder is checked for surface condition; busbar contact faces are cleaned, because a resistive contact shows up later as an unexplained low current on one cathode. Reagent and utility input: settling and filtration equipment, sodium hydroxide dosing with pH control, heating and cooling, conductivity and free cyanide analysis, cathode frames and stainless steel wool or mesh, busbar cleaning. Output: conditioned liquor at the design pH and temperature; loaded cathode assemblies ready to charge.
Steps 3 — Cell Charging and Ramped Current Start-Up. The charged cell is filled, the electrodes are immersed to depth, the design inter-electrode spacing is verified, and the cathode and anode contact faces are clamped onto the busbar with the compression the design calls for. Current is then brought up on a ramp rather than applied at its full value, and the rectifier runs a staged programme in which each stage carries its own voltage and its own duration. The purpose of the ramp is deposit structure: a modest initial current produces a coherent, fine-grained seed film that stays coherent as the deposit thickens, whereas full current from a cold start produces a burst of nodules, high local current density and a deposit that sheds back into the liquor later in the run. During the ramp the current drawn by each cathode is verified individually, so that a poor contact or a mis-set electrode is found on the first day rather than at harvest. Reagent and utility input: rectifier with adjustable voltage and current, staged voltage and time programme, insulation and spacing gauges, individual cathode current check. Output: stable current distribution across the cathode set; coherent seed deposit established.
Steps 4 — Steady-State Electrowinning and Flow Control. This is the stage that determines the metal balance. The cell runs at a cell voltage inside the 2.0–3.5 V working window, with the rectifier set to deliver the current the cathode area and the liquor tenor can support rather than the maximum it can supply. Where the target is gold recovered from a cyanide eluate in a packed flat-plate cell, a cathode current density of 8–15 A/m² is a representative operating figure; base metal duty runs an order of magnitude higher, and a swirl cell sustains several times the current density of a flat-plate cell of the same footprint because the rotational flow field keeps replacing the depleted film at the cathode surface. Circulation, agitation and, on the swirl cell, the rotational flow field are all there to serve the same purpose: keeping the concentration of metal ions at the cathode face as close as possible to the concentration in the bulk liquor. Temperature is held in the 40–60 °C band by cooling, which also limits evaporation and reagent loss. Metal tenor in the cell is monitored as it falls, and the run continues until the deposit has taken up its design load or the tenor falls to the point where the residual current would go into water decomposition. Reagent and utility input: rectifier at design current, circulation pump, agitator or swirl flow field, temperature control loop, tenor and pH monitoring. Output: deposition of more than 99% of the metal held in the liquor; metal tenor in the cell falling towards the parts-per-million range.
Steps 5 — Anode Reaction and Cyanide Destruction. The anode reaction is not a by-product of the process; on cyanide liquors it is a compliance asset and on chloride liquors it is the main materials-selection driver. At the anode, water is oxidised to oxygen and hydroxyl ions, and free cyanide is oxidised first to cyanate and then through to carbon dioxide and nitrogen, so the circuit destroys cyanide in the same operation that recovers the metal and reduces the load on any downstream destruction plant. Because the cell is enclosed, the headspace is ducted to an exhaust plenum rather than vented into the workshop: this captures oxygen, hydrogen carried over from the cathode, acid mist on chloride and nitrate liquors, and cyanide mist on alkaline liquors, and carries them to an alkali scrubbing tower, with nitrogen oxide abatement added where chloride or nitrate chemistry generates it. Where the feed may contain mercury, a retort is connected ahead of the circuit and the cell exhaust is monitored, because mercury vaporises readily under cell conditions and is a far greater hazard in the vapour phase than in the liquor. The pH hold above 11, established in Step 2, is maintained continuously here, since hydrogen cyanide formation is a function of pH and not of time. The cathode and anode reactions that govern the circuit are set out below. The first two are the metal deposition reactions that carry the recovery; the third and fourth are the competing and the counter-electrode reactions; and the last two are the cyanide destruction sequence that runs at the anode on alkaline cyanide liquors.
Au(CN)₂⁻ + e⁻ → Au + 2CN⁻
Ag(CN)₂⁻ + e⁻ → Ag + 2CN⁻
2H₂O + 2e⁻ → H₂ + 2OH⁻
4OH⁻ − 4e⁻ → 2H₂O + O₂
CN⁻ + 2OH⁻ − 2e⁻ → CNO⁻ + H₂O
2CNO⁻ + 4OH⁻ − 6e⁻ → 2CO₂ + N₂ + 2H₂O
Reagent and utility input: coated titanium, carbon or stainless anode plates, exhaust plenum and ducting, alkali scrubbing tower, nitrogen oxide abatement where required, mercury retort where required, continuous pH control. Output: cyanide destruction credit with a destruction record; scrubbed off-gas and an emission log with the batch record.
Steps 6 — Deposit Quality Monitoring and Correction. Deposit quality is inspected during the run, not only at harvest, and each observed condition maps to a specific correction. A cathode surface that has blackened and is no longer holding its deposit has been run at too high a voltage, and the correction is to reduce the voltage so that the metal is not over-oxidised and shed back into the liquor, where it costs grade as well as recovery. A rising cell voltage at constant current indicates increased resistance, and the causes are worked through in order: contact faces, electrode spacing, liquor conductivity, temperature, and anode condition. Nodular or dendritic growth is a current-density symptom and is corrected by bringing the current down and reviewing circulation. Short circuits between anode and cathode are detected by cell voltage monitoring and prevented by agitation and by the loading discipline that keeps the deposit from bridging the gap. The deposit load itself is checked against the ceiling for the medium, which for gold on stainless steel wool is twenty times the wool’s own weight, and harvesting is scheduled before that limit is reached rather than after. Reagent and utility input: cell voltage and current monitoring, visual cathode inspection, agitation, sampling of liquor and deposit. Output: corrected cell settings recorded against the batch; a deposit that meets the agreed grade.
Steps 7 — In-Cell Cathode Washing and Sludge Harvesting. The deposit is removed inside the cell with a high-pressure water spray, either from a fixed spray header or from a portable lance, and this is one of the clearest operating advantages of the design. More than 97% of the deposited metal is released from the cathode medium by the wash, and because the cathode never leaves the cell there is no transport of loaded plates, no separate wash tank, and none of the handling losses and drip losses that come with moving a heavy, dripping, metal-bearing cathode across a workshop floor. Service time is cut by up to 75% compared with a design that requires plate removal and external washing. The wash water carries the sludge into the collection and settling tank under the cell, from which it goes to filtration. The cathode medium itself is inspected for compaction, break-up and iron loss, topped up or replaced as required, and returned to service. Reagent and utility input: high-pressure water supply and spray header or lance, sludge collection and settling tank, cathode medium inspection and top-up. Output: washed and serviceable cathode medium; sludge slurry at a known metal content.
Steps 8 — Barren Liquor Polishing and Circuit Return. The spent electrolyte leaves the main cell holding metal at low single-digit parts per million, and what happens to it next is decided by the route sheet rather than by the effluent plant. Where the liquor is a desorption eluate it is returned to the elution column as desorption solution; where it is a leach or strip liquor it is returned to the leach circuit after a make-up addition of cyanide, alkali or reagent; where it is a plating electrolyte it is rebuilt to specification and returned to the plating line. Where the residual specification is tight, the liquor passes through a second-pass polishing cell, which is normally a swirl cell because it can still work at low tenor, and then through ion exchange or cementation if the limit requires it. The barren liquor is analysed before it leaves the stage, and the result is filed with the batch. Only liquor that cannot be returned to the circuit goes to neutralisation and discharge. Reagent and utility input: polishing cell, ion exchange or cementation where specified, make-up reagent dosing, transfer pumps, residual metal analysis. Output: barren liquor returned to elution, leaching or plating; or effluent at the agreed residual metal and free cyanide limits.
Steps 9 — Sludge Melting, Assay and Certificate of Analysis. The washed sludge is filtered, dried, mixed with flux and melted. Base metals oxidise into the slag and the precious metal collects at the furnace bottom because of its density and its chemical inertness, which is the same principle whether the feed was a cyanide eluate, a strip liquor or a plating electrolyte. The melt is cast into an ingot, a button or granules according to the customer’s preference, sampled, and analysed; gold and silver are determined by fire assay with an ICP-OES or atomic absorption cross-check, and the batch is released only when the analysis confirms the specified grade. Where the route requires it, the recovered metal is instead directed to the refining line as a sludge or as a loaded cathode medium. The feed liquor assay, the intermediate assay, the barren liquor assay and the final product assay are held together with the batch mass balance. Reagent and utility input: filter press or vacuum filter, drying oven, melting furnace with flux and mould set, sampling and fire assay, ICP-OES or atomic absorption spectrometry. Output: certified fine gold, fine silver or copper product with certificate of analysis; batch record and mass balance.
Product Structure (Main Equipment)
The circuit is assembled from the four equipment groups below. Materials of construction are selected stream by stream: integrally formed polypropylene and PPH for cell bodies and structural frames, with an anti-ageing additive where the liquor is strongly oxidising; titanium-clad copper for the busbars; 304 and 316 stainless steel for cathode wool and mesh; coated titanium, carbon or stainless steel for anodes according to the chemistry; and PP and PVDF for liquor piping and ducting.
Liquor handling and conditioning
- Sampling and assay station. Sample points on the feed, the cell, the polishing stage and the discharge, with the glassware and apparatus needed for cyanide titration, pH and conductivity measurement and metals analysis by ICP-OES or atomic absorption. Output: feed and stage assays on which the route sheet and the batch record are built.
- Feed storage and surge tank. Level-controlled storage that decouples the upstream process from the cell, so that a desorption column, a stripping line or a plating line can operate on its own cycle while the cell runs on its own. Output: steady feed supply with a recorded volume.
- Clarification and filtration unit. Settling tank with a filtration stage for liquors carrying suspended solids, protecting the cathode medium from blinding and the sludge from contamination. Output: clarified liquor; filtered solids routed for assay.
- Conditioning tank with pH control. Agitated tank with sodium hydroxide dosing, pH probe, temperature control and conductivity measurement, where the pH above 11 and the 40–60 °C operating window are established and held. Output: conditioned liquor at the design pH and temperature.
- Free cyanide and tenor monitoring instruments. Titration apparatus and instrumentation for tracking free cyanide and metal tenor through the run, so that the end of the cycle is set by measurement rather than by elapsed time. Output: tenor and free cyanide trend for the batch.
- Mercury pre-treatment retort. Retort and condenser connected ahead of the circuit where the feed may carry mercury, removing it before it can vaporise under cell conditions. Output: mercury recovered separately; cell feed free of mercury.
- Transfer pumps, piping and valving. Corrosion-resistant pumps with PP or PVDF wetted parts and piping sized to the circulation duty, with isolation valves that allow a single cell to be taken out of service without stopping the circuit. Output: controlled liquor transfer between stages.
Electrowinning cells and electrode assemblies
- Flat-plate electrowinning cell. Integrally formed polypropylene or PPH tank with a gas-tight lid, sized for the cathode set and the liquor volume, with the internal geometry arranged so that liquor passes across the cathode faces rather than only through the cell. Output: primary recovery cell for concentrated eluates.
- Swirl electrowinning cell. Cell that imposes a rotational flow field across the cathode, sustaining several times the cathode current density of a flat-plate cell of the same footprint and extending the working range down to 0.x g/L of metal. Output: recovery cell for dilute, complex and polishing duty.
- Compact rotating-cathode recovery unit. Plated barrel cathode that turns through the liquor in a drag-out or rinse tank, drawing approximately 150–250 W, adding no reagent and requiring no dedicated supervision. Output: continuous recovery from drag-out and rinse stages.
- Cathode assemblies. Stainless steel wool packed to design density in stainless or plastic frames; multi-layer stainless mesh; expanded metal; and the rotating titanium cylinder, each selected against the liquor and the required product form. Output: the deposition surface and the deposit carrier.
- Anode assemblies. Perforated stainless steel plate, coated titanium in the form of a dimensionally stable anode, carbon plate, or lead alloy where the chemistry calls for it, selected for corrosion resistance and for the oxygen overpotential the liquor can tolerate. Output: the counter-electrode and the site of the cyanide oxidation reaction.
- Busbar set and contacts. Titanium-clad copper busbars with compression clamps and insulated supports, holding contact resistance low across every electrode pair so that current divides evenly across the cathode set. Output: even current distribution and a measurable base for fault detection.
- In-cell washing spray header. Fixed high-pressure spray header, with a portable lance supplied as an alternative, releasing more than 97% of the deposit from the cathode medium without removing it from the cell. Output: in-place harvesting with no plate transport.
- Electrode handling and lifting frame. Lifting and setting frame for cathode and anode assemblies and for cell lids, designed so that routine service does not depend on improvisation. Output: safe and repeatable electrode handling.
Power supply, control and instrumentation
- Rectifier and DC power supply. Adjustable, current-limited DC supply sized to the cathode area and the chemistry, with the voltage range that covers the working window and the headroom the start-up ramp requires. Output: controlled direct current to the cell.
- High-frequency pulse power option. Pulse supply used where a denser, more coherent deposit is wanted, particularly on dilute liquors and on small-diameter rotating cathodes. Output: improved deposit structure at low tenor.
- PLC and operator interface. Control panel with power, pump, agitation and mode keys, navigation keys and status indicators, running an automatic mode that is set in three stages, each with its own voltage and its own operating time, and switching to manual mode for intervention. Output: repeatable automatic cycles and reproducible batches.
- Cell voltage, current and busbar temperature monitoring. Continuous monitoring with interlocks that stop the rectifier on an abnormal reading rather than allowing a fault to run to harvest. Output: fault detection before it becomes metal loss.
- pH, temperature, level and conductivity control loop. Closed-loop control of the variables that govern deposition, with the pH loop acting as a safety interlock as well as a process control. Output: a stable operating window for the whole run.
- Data logging and batch record. Storage and retrieval of the electrical and process parameters for every batch, so that a deviation can be traced to the liquor, the setting or the equipment rather than guessed at. Output: per-batch process record filed with the assay results.
Harvesting, finishing and emission control
- Sludge collection and settling tank. Receiving tank under the cell that takes the wash water and settles the sludge, with the clarified water returned to the circuit or to the effluent plant. Output: settled sludge; clarified return water.
- Filter press and vacuum filter set. Dewatering equipment for the settled sludge, with a filter medium chosen for the particle size of the deposit. Output: filter cake at a known moisture and metal content.
- Drying oven. Controlled-temperature drying ahead of fluxing and melting, so that the melt does not carry water into the furnace. Output: dry sludge ready for fluxing.
- Melting furnace, flux and mould set. Induction or resistance furnace with the flux formulation and moulds matched to the product form, whether that is an ingot, a button or granules. Output: cast precious metal product.
- Exhaust plenum, ducting and alkali scrubbing tower. Built-in exhaust at the cell, ducting under negative pressure, and an alkali scrubbing tower with a demister for acid mist, cyanide mist and entrained aerosol. Output: scrubbed off-gas with the scrubber liquor analysed and logged.
- Nitrogen oxide abatement. Added where chloride or nitrate liquors generate nitrogen oxides, using absorption stages matched to the measured inlet concentration. Output: abated off-gas meeting the discharge limit for the site.
- Effluent neutralisation and settling. Neutralisation and settling for liquor that cannot be returned to the circuit, with the hydroxide sludge routed back for metal recovery rather than discarded. Output: compliant effluent; recovered residual metal.
- Assay laboratory equipment. Fire assay apparatus, ICP-OES or atomic absorption spectrometry and a fume hood for sample preparation, so that feed, intermediate, barren liquor and product assays are produced in-house on the day the batch is run. Output: certificate of analysis issued against the batch.
Product Features
- Circuit scope rather than cell supply. Liquor survey, conditioning, cell and electrode selection, rectifier and control, washing, melting, assay and effluent polishing are delivered as one engineered line, so the interfaces between stages belong to one supplier.
- Route selection driven by the liquor. Cell architecture, cathode medium, anode material, current density, pH and temperature window, and the residual tenor target are all fixed from the assay before equipment is specified, and recorded on a route sheet.
- Three cathode architectures in one product family. A packed flat-plate cell for concentrated eluates, a swirl cell for dilute and complex liquors, and a compact rotating-cathode unit for drag-out and rinse duty, with the same control philosophy across all three.
- Working range from 0.x g/L to several hundred grams per litre. The swirl cell extends the range downwards and the packed flat-plate cell carries the high tenor, so a single project can cover both rinse water and eluate without a second equipment family.
- Staged automatic operation. Automatic mode runs three stages, each with its own voltage and its own duration, so a cycle is defined once and repeated rather than being adjusted by hand at each batch.
- Ramped start-up as standard. Current is brought up progressively at the beginning of every cycle, which is what produces a coherent seed deposit and a deposit that survives to harvest.
- Voltage discipline with a defined correction path. Blackening and poor adhesion are corrected by reducing voltage rather than raising it, and the correction is written into the operating procedure rather than left to judgement.
- In-cell washing with a fixed spray header. The deposit is released inside the cell and the cathode medium stays in place, removing transport handling, wash-tank logistics and drip losses, and cutting service time by up to 75%.
- Cyanide destroyed in the same operation. Free cyanide is oxidised at the anode to cyanate and on to carbon dioxide and nitrogen, so the recovery step also reduces the cyanide load that the downstream destruction circuit has to handle.
- Enclosed cell with built-in exhaust. The headspace is ducted to scrubbing at the point of generation rather than being allowed into the workshop, which covers oxygen, hydrogen, acid mist and cyanide mist in one arrangement.
- Materials matched to the chemistry stream by stream. Polypropylene and PPH with an anti-ageing additive for cell bodies, titanium-clad copper for busbars, 304 and 316 stainless for cathode media, and coated titanium, carbon or stainless steel for anodes.
- Return-oriented barren liquor handling. The route sheet states where the spent electrolyte goes — elution column, leach circuit, plating line, polishing cell or effluent plant — rather than treating discharge as the default outcome.
- Two-pass recovery for tight residuals. A polishing cell, with ion exchange or cementation where specified, takes the residual metal from single-digit parts per million down to the agreed discharge limit.
- Melting and assay inside the scope. Sludge washing, dewatering, drying, fluxing, melting, sampling and a certificate of analysis are supplied, so the customer’s first certified assay does not depend on finding a third-party smelter.
- Batch traceability. Feed, intermediate, barren liquor and product assays are held with the electrical and process record for every batch, which is what makes a recovery guarantee checkable rather than rhetorical.
- Configuration to the assay, not to a catalogue. Cell count, cathode area, rectifier rating, circulation, cooling, exhaust volume and effluent duty are engineered from the liquor analysis, the monthly metal volume and the required product.
Technical Parameters And Specifications
The engineering setpoints below are the standard design basis and are confirmed or adjusted during process design once the liquor assay is known. Cell count, cathode area, rectifier rating, footprint, installed power and utility consumption are engineered to your assay, monthly metal volume and site conditions.
Precious metal electrowinning equipment — technical parameters and specifications.
| Parameter | Specification |
|---|---|
| Metal tenor accepted | From 0.x g/L up to several hundred grams per litre of metal, with the cell architecture and cathode medium selected accordingly |
| Feed liquor types accepted | Cyanide eluate, cyanide strip and deplating liquor, zinc-cyanide and copper-cyanide plating solution, chloride and chloride-hypochlorite strip liquor, nitrate and silver plating electrolyte, fixing and bleach-fix solution, platinum group metal process liquor, acid and alkaline etchant, spent plating bath, drag-out and rinse water, barren filtrate and effluent |
| Feed liquor condition on charging | Suspended solids removed by settling and filtration before the cell; mercury-bearing feeds routed through a retort ahead of the circuit |
| Operating pH for cyanide liquors | Held above 11 with sodium hydroxide, and maintained continuously; below pH 10 cyanide forms hydrogen cyanide gas and the deposition efficiency also falls away |
| Operating temperature | 40–60 °C, held by cooling; the upper bound is set by the breakdown of cyanide above approximately 60 °C |
| Cell voltage, working window | 2.0–3.5 V per cell |
| Cell voltage, symptom threshold | Above 3.5 V the surplus voltage is consumed by water decomposition rather than metal deposition; a rising cell voltage at constant current is diagnosed as a fault |
| Cell voltage, zinc-cyanide and similar complex liquors | Approximately 2.4 V produces favourable deposition |
| Cathode current density, precious metal liquor in a packed flat-plate cell | 8–15 A/m² as a representative operating figure for gold recovered from a cyanide eluate; set from tenor, cathode area and impurity load |
| Cathode current density, swirl cell | Several times the current density of a flat-plate cell of the same footprint, sustained by the rotational flow field at the cathode surface |
| Cathode current density, base metal duty | Up to the low hundreds of amperes per square metre, set by the deposit quality the downstream melting step can accept |
| Inter-electrode spacing | Fixed by the cell design at the closest spacing that avoids dendrite bridging at the operating current density; too close causes short circuits, too far increases cell resistance and lowers the deposition rate |
| Current start-up | Ramped from a fraction of the target value over a defined period, on a staged voltage and time programme |
| Cathode media | Stainless steel wool packed to design density, multi-layer stainless mesh, expanded metal plate, rotating titanium cylinder |
| Cathode wool and mesh material | Grades 304 and 316 stainless steel |
| Gold loading ceiling on stainless steel wool | Twenty times the wool’s own weight; harvesting is scheduled before the ceiling is reached |
| Deposit removal method | In-cell high-pressure water washing from a fixed spray header or portable lance; more than 97% of the deposited metal is released |
| Service interval | Set by the loading ceiling and the tenor of the feed, with in-cell washing cutting service time by up to 75% compared with external plate washing |
| Anode materials | Perforated stainless steel plate, coated titanium dimensionally stable anode, carbon plate, lead alloy where the chemistry requires it |
| Anode reaction | Water oxidation to oxygen and hydroxyl ions, with free cyanide oxidised to cyanate and then to carbon dioxide and nitrogen on cyanide liquors |
| Cyanide destruction at the anode | Free cyanide oxidised in the cell, reducing the load on the downstream destruction circuit and recorded per batch |
| Busbar material | Titanium-clad copper with compression clamps and insulated supports, holding contact resistance low across every electrode pair |
| Rectifier | Adjustable, current-limited direct current supply with a voltage range covering the working window and the start-up ramp; high-frequency pulse supply available where a denser deposit is required |
| Automatic cycle control | Three stages, each with an independent voltage and duration setting, with manual mode for intervention |
| Monitoring and interlocks | Cell voltage, cell current and busbar temperature monitored continuously, with interlocks that stop the rectifier on an abnormal reading; pH, temperature, level and conductivity under closed-loop control |
| Metal deposition rate | More than 99% of the metal held in the feed liquor reduced onto the cathode under the design window |
| Residual metal in the barren liquor | Low single-digit parts per million after the main cell, reduced further where a polishing cell and ion exchange or cementation are specified |
| Barren liquor destination | Returned as desorption solution, returned to the leach circuit after make-up, rebuilt as plating electrolyte, or polished to the effluent limit |
| Hydrogen evolution | Managed as both an efficiency loss and a ventilation requirement; the competing reaction is controlled by current density discipline and by agitation |
| Mercury handling | Pre-treatment retort ahead of the circuit where mercury-bearing feed is expected, with the cell exhaust monitored |
| Off-gas capture and treatment | Built-in exhaust plenum at the cell, ducting under negative pressure, alkali scrubbing tower with demister; nitrogen oxide abatement added for chloride and nitrate liquors |
| Cell body material | Integrally formed polypropylene or PPH with an anti-ageing additive, gas-tight lid, integrally formed to avoid liquor leakage at joints |
| Wetted piping and ducting material | Polypropylene and PVDF |
| Clarification | Settling and filtration sized for the suspended solids load of the feed liquor |
| Sludge finishing | Filter press or vacuum filter, drying oven, flux formulation matched to the feed, induction or resistance melting furnace, mould set |
| Final purity, gold | 99.99% and above after melting, certificated against a fire assay determination with an ICP-OES or atomic absorption cross-check |
| Final purity, silver | 99.9–99.99% depending on the feed, melted to ingot or supplied as crystal |
| Final purity, copper | Cathode sheet or powder at the grade the etchant or plating bath chemistry supports |
| Platinum group metal handling | Recovered as a washed and dried concentrate, declared by metal content, and routed to the refining line |
| Product forms | Fine gold ingot or button, gold-silver doré, fine silver ingot or crystal, copper cathode sheet or powder, platinum group metal concentrate, loaded cathode medium, barren liquor, treated effluent |
| Quality verification | Fire assay with ICP-OES or atomic absorption cross-check on the product; ICP-OES, atomic absorption, cyanide titration, pH and conductivity on the liquors and on the barren liquor |
| Materials of construction, summary | Polypropylene and PPH with an anti-ageing additive, titanium-clad copper, 304 and 316 stainless steel, coated titanium and carbon, PP and PVDF |
| Capacity, cell count, footprint, power and utilities | Engineered to the liquor assay, monthly metal volume, required product form and site conditions |
Technical Advantages
- The mass-transfer limit is addressed by design rather than by buying a bigger rectifier. Electrowinning fails at the cathode surface, where a depleted film of liquor forms and blocks further deposition. Packed stainless steel wool, multi-layer mesh, forced circulation, agitation and the swirl cell’s rotational flow field all serve the same purpose — keeping the metal concentration at the cathode face close to the bulk concentration. Result: current density is set by what the liquor can support rather than by what the rectifier can deliver, which is the difference between a cell that meets its design rate and one that runs at a fraction of it.
- Cutting the tails rather than the average. A circuit that recovers 99% of the metal in a 0.5 g/L eluate still leaves 5 mg/L in the barren liquor, and that residual is what determines whether the liquor can be returned to the leach circuit or must go to the effluent plant. The swirl cell was selected for polishing duty precisely because it continues to work at tenor where a flat-plate cell has stopped. Result: a barren liquor that can be returned to the circuit, and a discharge stream that meets its limit without an additional treatment plant.
- Deposit quality controlled as a chemical specification, not judged at harvest. A blackened, poorly adherent deposit is a voltage problem, and the operating procedure states the correction — reduce voltage — rather than leaving it to operator instinct. A rising cell voltage at constant current is diagnosed in a fixed order: contacts, spacing, conductivity, temperature, anode condition. Result: metal that reports to the melt instead of being shed back into the liquor as fine particles that are difficult to recover.
- Cyanide destruction delivered as a second output. On cyanide liquors the anode reaction oxidises free cyanide to cyanate and then to carbon dioxide and nitrogen. The recovery step therefore reduces the cyanide load that the downstream destruction circuit must handle, and the destruction is recorded against the batch. Result: one operation that recovers the metal and simultaneously lowers the cost and the compliance exposure of the effluent plant.
- Safety designed into the pH loop instead of written into a procedure. Hydrogen cyanide formation is a function of pH and not of time, so the pH hold above 11 is implemented as a closed-loop interlock rather than as a shift instruction. The temperature ceiling of 60 °C is enforced by the cooling system for the same reason. Result: the two conditions that create the acute risk on a cyanide circuit are controlled by equipment rather than by discipline.
- In-cell washing removes the handling stage where metal is lost. Moving a loaded cathode from a cell to a wash tank drips metal-bearing liquor across the floor, requires a second tank and its own liquor inventory, and puts a heavy wet plate in the hands of an operator several times a shift. Washing in place keeps the metal in the circuit. Result: better containment and up to 75% less service time per cycle.
- A platform that spans four orders of magnitude in tenor. Rinse water at tens of milligrams per litre and eluate at several hundred grams per litre are normally two different equipment purchases with two different control philosophies. Here they share the same cell family, the same staged automatic cycle and the same control panel. Result: a plant that can be reconfigured between feeds by changing the route sheet rather than by buying another machine.
- Impurity tolerance that comes from selectivity and from equipment layout. Copper, nickel, zinc and iron co-deposit to some extent from every real liquor, and the design response is a deposition window set to favour the target metal, a swirl cell that tolerates a heavier impurity load, and a polishing pass that recovers what the first pass could not. Result: usable recovery from strip liquors, etchants and rinse streams that a single-pass flat-plate cell would not justify.
- Cathode loading discipline instead of running to a calendar. Depositing beyond the loading ceiling does not add recovery; it degrades the deposit, risks bridging the inter-electrode gap and makes the subsequent wash less complete. The ceiling for gold on stainless steel wool — twenty times the wool’s own weight — is stated as an operating limit. Result: a defined harvest point at which recovery is still complete and the deposit is still clean.
- Voltage treated as a measurement rather than a control handle. Because 3.5 V is a symptom threshold and not a productivity setting, the operator has a defined single number against which to judge the health of the cell. Result: a fault-finding path that a normal crew can follow without a metallurgist on site.
- Corrosion strategy chosen stream by stream. Chloride liquors attack stainless steel, nitrate liquors generate nitrogen oxides, alkaline cyanide liquors attack many plastics, and an anode that is stable in one chemistry may have an untenable oxygen overpotential in another. Cell body, anode, busbar and ducting are therefore selected per stream. Result: equipment life measured in years on chemistries that destroy general-purpose hardware.
- Melting and certification inside the same scope. A recovery circuit that stops at a wet sludge leaves the customer dependent on a third party for the number that actually determines the value of the metal. Fire assay, ICP-OES cross-check and a certificate of analysis are part of the delivery. Result: the grade claim is verifiable on the customer’s own batch, not on a marketing figure.
- Batch traceability from liquor to ingot. Feed assay, intermediate assay, barren liquor assay, process record and product assay are held together for every batch, with a mass balance across the circuit. Result: a recovery figure that can be audited, which is what makes a performance commitment meaningful on a toll treatment or a contract recovery duty.
- Energy and reagent profile appropriate to the duty. Electrowinning runs at 40–60 °C and at 2.0–3.5 V per cell, against high-temperature pyrometallurgical alternatives, and the compact rotating-cathode units that serve drag-out tanks draw only around 150–250 W and add no reagent at all. Result: recovery is economic on dilute streams where the metal value alone would not justify the energy of a smelting route.
- Designed to be extended rather than replaced. Cells are modular and the busbar, rectifier and control arrangement allow additional capacity in parallel, so a plant that validates the route on a single campaign can be extended when volume grows. Result: an entry point that does not become stranded capital.
Service Process
- Step 1 — Enquiry and liquor data review. You provide the liquor type, an assay or a typical analysis, the expected volume, the impurity profile and any site constraints. We confirm whether electrowinning is the right recovery step for that liquor or whether it needs an upstream clarification, retort or pre-treatment stage first. Output: preliminary process route and a clear scope statement.
- Step 2 — Laboratory test on your liquor. A representative sample is run through conditioning and electrowinning at bench and pilot scale. The achievable deposition rate, the deposit grade, the required cell voltage and current density, the cathode medium and the expected residual tenor are established on real material. Output: test report, returned product sample for your own assay, and a recommended route sheet.
- Step 3 — Process design and cell selection. We issue the process flow diagram, the material and reagent balance, the metal balance, the cell selection with cathode medium and anode material, the liquor conditioning specification and the residual tenor target, with all engineering setpoints stated. Output: process design package and mass balance.
- Step 4 — Equipment manufacture and factory testing. Cell bodies, electrode assemblies, busbars, rectifier, control panel, washing header, scrubbing tower and sludge handling equipment are fabricated, assembled and function-tested before shipment, with material certificates retained. Output: factory acceptance test record.
- Step 5 — Layout and site interface design. Equipment layout, civil loading, power supply points, water and cooling, compressed air, ventilation interfaces, gas detection and emergency provisions are defined against your building and your existing circuit. Output: layout and utility interface drawings.
- Step 6 — Shipment, installation and commissioning. Equipment ships with the documentation set, and our engineers supervise installation, electrode alignment, busbar torque-up, exhaust and duct leak testing, electrical and control checkout, and water and liquor commissioning. Output: mechanically complete circuit, ready for production liquor.
- Step 7 — Operator training and procedure handover. Operating procedures covering pH and temperature control, cyanide safety and hydrogen cyanide prevention, the start-up ramp, the staged cycle, deposit inspection and the voltage correction rule, harvesting and washing, sampling and assay practice, and routine maintenance are taught on site with the actual equipment. Output: trained operators and a signed procedure set.
- Step 8 — Production trial and performance acceptance. The circuit is run on your liquor and the agreement is confirmed against the measured deposition rate, the product grade, the residual tenor in the barren liquor and the free cyanide reduction achieved. Output: performance test certificate and the first certificate of analysis.
- Step 9 — Ongoing technical support. Process troubleshooting, route sheet revision as your feed mix changes, spare parts at cost, cathode medium and consumable supply on request, and support on effluent and emission declarations. Output: continuing production support.
Warranty And After-Sales Service
- Warranty period. Twelve months from successful commissioning or eighteen months from shipment, whichever falls earlier, covering design, manufacture and assembly defects of the supplied equipment. Excluded: consumable items such as cathode wool and mesh, gaskets, seals, filter cloths, spray nozzles, membranes and reagents, and damage arising from misuse or from a feed liquor outside the agreed specification.
- Performance commitment. Where the liquor matches the tested sample, we stand behind the deposition rate, product grade and residual tenor established in the process design package, with the performance test as the reference point. Output: documented performance acceptance.
- Cathode medium and spares. Stainless steel wool, mesh, cathode frames, anodes, busbar contacts, pump seals, spray nozzles and instrumentation are quoted at the time of order and held available afterwards, with a recommended on-site stock list issued at handover. Where a customer is adding capacity, a complete second electrowinning cell for sale at the same factory basis is quoted from the original design package rather than being re-engineered. Output: spare parts list and supply at cost.
- Remote support. Telephone, email and messaging support for operating questions, alarm interpretation, deposit quality problems, cell voltage deviations and process troubleshooting while your circuit 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.
- Assay and residual verification service. Periodic verification of the barren liquor residual and the product grade against your own analysis, so that the performance of the circuit is tracked over time rather than assumed from commissioning. Output: verification report against the batch record.
- Operator retraining and refresher visits. Additional training, refresher sessions and process optimisation visits as your feed mix, tenor or product requirement changes. Output: updated training and revised procedures.
- Documentation set. Operation and maintenance manuals, process flow diagram, cell and electrode drawings, electrical and control drawings, layout, spare parts catalogue, safety data for reagents, and the emergency response plan covering hydrogen cyanide release, mercury vapour and chlorine. Output: complete documentation handover.
- Compliance support. Assistance with effluent and emission parameter declarations, scrubbing performance verification, and the handling, storage and disposal requirements for cyanide-bearing liquors and for the intermediates generated in the circuit. Output: technical data for your permit applications.
- Lifetime technical support. The process design remains yours, and our engineering team remains available for modification and expansion advice for the operating life of the circuit. Output: long-term engineering continuity.
Frequently Asked Questions
What feed solutions can precious metal electrowinning equipment treat?
Cyanide eluates from carbon desorption in CIP, CIL, Zadra and AARL circuits; cyanide strip and deplating liquors from plated scrap and printed circuit boards; zinc-cyanide, copper-cyanide and silver plating solutions and their drag-out; chloride and chloride-hypochlorite strip liquors carrying gold as AuCl₄⁻; nitrate electrolytes and spent silver plating baths; fixing and bleach-fix solutions; spent palladium, palladium-nickel, platinum and rhodium plating electrolytes; dilute platinum group metal process liquors and barren filtrates; acid and alkaline copper etchants and micro-etch liquors; dilute drag-out and rinse water at tens of milligrams per litre; and effluent that needs polishing before discharge. Liquors carrying suspended solids are clarified first, and mercury-bearing feeds are routed through a retort ahead of the cell.
What recovery rate does electrowinning achieve, and what metal is left in the barren solution?
Under the design window, more than 99% of the metal held in the feed liquor is reduced onto the cathode, and the spent electrolyte leaves the main cell at low single-digit parts per million. The exact residual is set by the route sheet and depends on the feed tenor, the impurity load and the deposition window, and it is confirmed on your own liquor in the laboratory test. Where the specification is tighter, a second-pass polishing cell followed by ion exchange or cementation takes the residual down further before the liquor leaves the plant.
What is the difference between electrowinning and electrorefining?
Electrorefining dissolves a solid anode in order to purify metal that is already metallic, so the process is governed by how fast the anode dissolves. Electrowinning has no anode to dissolve: the metal is already in solution and is reduced onto a cathode, so the process is governed by how fast ions can be brought to the cathode surface. That is why an electrowinning cell is designed around cathode surface area, circulation and boundary layer control, and why the measures that matter are tenor, current density and flow rather than anode preparation and anode slime handling.
Why is electrowinning chosen instead of zinc cementation?
Zinc cementation requires the solution to be clarified and deaerated, it consumes zinc dust that must be paid for and accounted for, and the cemented precipitate has to be filtered and handled before melting. Electrowinning removes those steps, works directly on clarified liquor, needs no reagent addition for the deposition reaction itself, and on cyanide liquors it also oxidises free cyanide at the anode. The trade is that electrowinning is a slower, capital-bearing step whose performance depends on cathode area and liquor movement, which is why it is normally selected where the liquor is clean and the tenor is workable, or where the compliance benefit of cyanide destruction alongside recovery justifies it.
What cell voltage and current density do you operate at?
The working window is 2.0–3.5 V per cell. For gold recovered from a cyanide eluate in a packed flat-plate cell, a cathode current density of 8–15 A/m² is a representative operating figure, set from the tenor, the cathode area and the impurity load. Complex liquors such as zinc-cyanide and copper-cyanide plating solutions deposit favourably at approximately 2.4 V. Base metal duty runs an order of magnitude higher, and a swirl cell sustains several times the current density of a flat-plate cell of the same footprint because the rotational flow field keeps replacing the depleted film at the cathode surface. The exact setpoints for your liquor are established in the laboratory test.
Why does raising the voltage above 3.5 V not recover more metal?
Because the surplus voltage is consumed decomposing water rather than reducing metal ions. The current that goes into water decomposition produces hydrogen gas and hydroxyl ions instead of deposited metal, so the extra energy is spent on gas evolution and heat, and the deposit it produces is rougher, darker and more likely to shed back into the liquor. On a cell that is already at its mass-transfer limit, higher voltage does not raise the deposition rate: it lowers the current efficiency and degrades the product. A rising cell voltage at constant current is therefore treated as a fault to be diagnosed, and the first items checked are contact resistance, electrode spacing, liquor conductivity, temperature and anode condition.
What cathode material is used, and how is the deposit removed?
The cathode medium is selected against the liquor and the required product form. Stainless steel wool packed to a design density inside stainless or plastic frames is the standard medium for gold recovered from alkaline cyanide liquors because it is alkali-resistant, does not dissolve, and presents a very large surface area relative to its footprint. Multi-layer stainless mesh, expanded metal and a rotating titanium cylinder cover the other duty ranges, and grades 304 and 316 are both suitable. The deposit is removed inside the cell with high-pressure water from a fixed spray header or a portable lance, releasing more than 97% of the deposited metal without the cathode leaving the cell.
How much gold can be loaded onto stainless steel wool before harvesting?
The working ceiling is twenty times the wool’s own weight, and harvesting is scheduled before that limit is reached rather than after. Running past the ceiling does not add recovery; it degrades the deposit, restricts the flow of liquor through the wool, risks bridging the gap to the anode, and makes the subsequent wash less complete, which costs grade as well as labour. The harvest point for your circuit is set from the feed tenor and the wool charge, and the loading is tracked against the batch record.
Why must the pH stay above 11?
Because cyanide in an acidic or even a weakly alkaline solution forms hydrogen cyanide, which is a lethal gas, and the transition begins well above the neutral point. Below pH 10 the risk of hydrogen cyanide release becomes significant and the deposition efficiency also falls away. The pH in this circuit is therefore held above 11 by closed-loop sodium hydroxide dosing and is maintained continuously through the run, because hydrogen cyanide formation is a function of pH and not of elapsed time. It is implemented as a safety interlock rather than as a shift instruction, and it is the first item in the operating procedure.
Why is the solution temperature capped at 60 °C?
Two reasons, one chemical and one economic. Cyanide begins to break down above approximately 60 °C, which destroys the complex that is holding the metal in solution and can release hydrogen cyanide if the pH is not tightly controlled. Above the same range, evaporation losses rise, reagent consumption climbs and the cooling duty grows without improving the deposition rate enough to pay for it. The circuit therefore runs in a 40–60 °C band, which is warm enough to lower the electrolyte resistance and improve ion mobility, and cool enough to keep the cyanide complex intact.
What happens to the cyanide in the solution?
Part of it is consumed by the deposition reaction itself, which frees cyanide as the metal is reduced, and part is destroyed at the anode. There, water is oxidised to oxygen and hydroxyl ions while free cyanide is oxidised first to cyanate and then through to carbon dioxide and nitrogen. The circuit therefore destroys cyanide in the same operation that recovers the metal, which reduces the load on any downstream destruction plant, and the destruction is recorded against the batch. The cell is enclosed and its headspace is ducted to an alkali scrubbing tower, so cyanide mist is captured at the point of generation.
How is mercury handled?
Mercury carried in from concentrate that has seen amalgamation vaporises readily under cell conditions, and mercury vapour is a far greater hazard than mercury in the liquor. Where that feed exists, a retort is connected ahead of the circuit to remove the mercury before it reaches the cell, the cell exhaust is monitored, and the ventilation design is set from the mercury load rather than from the liquor volume. Removing mercury upstream is always the preferred solution, and the retort is specified as part of the line wherever the feed analysis shows it is needed.
What causes a blackened cathode that does not adhere, and what do you do?
Blackening with poor adhesion is the signature of running at too high a voltage: the metal is over-oxidised at the surface, the deposit forms as a loose, non-coherent layer and part of it sheds back into the liquor, which costs both recovery and grade. The correction stated in the operating procedure is to reduce the voltage. The underlying cause is normally that the cell has been pushed past its mass-transfer limit — usually because the tenor has fallen towards the end of the cycle, or because circulation has degraded, or because the deposit has thickened to the point where flow through the cathode medium is restricted. Reducing the voltage restores a coherent deposit, and the batch record then shows where the excursion started.
Can the equipment handle very dilute solutions at a few hundred milligrams per litre?
Yes, within the range of the swirl cell, and this is where the architecture choice matters most. A conventional flat-plate cell loses its deposition rate once the tenor falls far enough that the depleted film at the cathode cannot be replenished quickly, which is the situation that makes dilute rinse water and polished barren liquor uneconomic to treat. The swirl cell imposes a rotational flow field that keeps the boundary layer refreshed at low tenor, and the compact rotating-cathode units continue to work on drag-out and rinse streams while adding no reagent and drawing only around 150–250 W. The applicable range and the expected residual for a specific dilute liquor are confirmed in the laboratory test.
What happens to the barren solution after electrowinning?
The route sheet states the destination, and discharge is the last option rather than the default. A desorption eluate is returned to the elution column as desorption solution. A leach or strip liquor is returned to the leach circuit after a make-up addition of cyanide, alkali or reagent. A plating electrolyte is rebuilt to specification and returned to the plating line. Where the residual specification is tight, the liquor passes through a polishing cell and then through ion exchange or cementation if the limit requires it. Only liquor that cannot be returned to the circuit goes to neutralisation and discharge, and every stream is analysed before it leaves the stage.
What capacity can you supply, and how is it configured?
Capacity is engineered to your liquor assay and your monthly metal volume rather than taken from a catalogue of fixed models. What matters in this circuit is the metal tenor of the feed, the impurity load and the number of passes required, so two plants with the same reported treatment volume can have very different daily metal output depending on what is dissolved in the liquor. We size the number of cells, the cathode area, the rectifier rating, circulation, cooling, exhaust volume and effluent polishing duty from your analysis and your target metal volume, and we state the resulting figures in the process design package. The layout is modular, so a circuit that validates the route on one campaign can be extended in parallel rather than replaced.
How is the recovered metal verified?
By fire assay of a sample taken from the released batch, with an ICP-OES or atomic absorption cross-check, and by determination of the residual metal and free cyanide in the barren liquor by ICP-OES, atomic absorption, titration, pH and conductivity measurement. The batch is not released until the certificate of analysis confirms the specified grade, and the feed assay, the intermediate assay, the barren liquor assay, the process record and the product assay are held together with the mass balance for the batch. That is what makes the recovery and grade figures on this equipment checkable on the customer’s own material rather than only on a specification sheet.
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