Gold Refining Machine

The main chain runs vertically through feed assay → preparation and granulation → aqua regia dissolution → precipitation, filtration and washing → anode casting and electrolyte make-up → electrolytic refining → electrolyte and slime treatment → melting and casting → assay, metal balance and certificate. Side branches route reactor vapour and condensed gold back to the batch, scrubber liquor back to the process, silver chloride to the silver circuit, anode slime to gold, silver and PGM recovery, barren filtrate and electrolyte bleed to neutralisation and polishing, and crucible and filter returns back to the head of the line.

Q1. What is a gold refining machine?
 A gold refining machine is an industrial or workshop system that extracts and purifies gold from scrap jewellery, doré bars, sweeps, plating residues and gold-bearing electronic fractions, and delivers it as fine gold of certified fineness. A complete machine covers both refining routes — aqua regia dissolution, which handles mixed, low-karat and powdered feed directly, and electrolytic (Wohlwill) refining, which takes high-grade feed and holds a stable 99.99% product — plus granulation, filtration, precipitation, washing, melting, casting, off-gas scrubbing, effluent neutralisation and assay. Standard product is 99.9–99.99% fine gold, rising to 99.999% on a second electrolysis pass, at a recovery above 99%.

Q2. How does a gold refining machine work, step by step?
 Feed is first weighed, sampled and assayed, and the assay fixes the route. High-grade material of roughly 90–92% gold and above is melted, fluxed and cast into crude anodes; lower-grade, mixed, plated or dust-form material is melted and granulated instead. Aqua regia feed is dissolved in a sealed reactor at about 90 °C for 4–6 hours; silver reports as silver chloride and is filtered off; gold is precipitated from the chloroauric acid liquor with oxalic acid, sulfite or ferrous sulfate, washed with dilute hydrochloric and nitric acid and dried. Gold destined for four-nines is cast into anodes and electrolysed in a chloroauric acid electrolyte at 60–70 °C and up to 1,000 A/m², depositing pure gold on the cathode and concentrating silver and platinum-group metals in the anode slime. The gold is melted under flux, cast or granulated, assayed by fire assay with ICP-OES confirmation, and released with a Certificate of Analysis against a closed metal balance.

Q3. Which route should I choose: aqua regia or electrolytic refining?
 Choose by the assay, not by preference. The electrolytic route needs feed at roughly 22 karat and above — about 90–92% gold — with silver below about 6% and copper below about 2%, and it rewards that feed with a single-step 99.99% product, very low reagent consumption and anode slime that concentrates the silver and platinum-group metals. The aqua regia route is right for mixed-karat scrap, sweeps, polishing dust, plated and gold-filled material, filter cakes, sludges and e-waste concentrates, because it dissolves varied feed directly without an anode and reaches 99.9–99.99% — at the cost of higher reagent use and a nitrogen oxide load that must be scrubbed. Most operations run both: aqua regia to bring material up to grade, then electrolysis to make it consistent. Feed high in silver is pre-treated by inquartation or nitric parting before electrolysis.

Q4. What is the maximum silver content each gold refining route tolerates?
 On the electrolytic route, silver above roughly 6% passivates the anode — a silver chloride film forms on the surface and stops dissolution — and copper above about 2% contaminates the electrolyte, so both are ceiling values for direct anode feed. On the aqua regia route silver is far less of a problem because it simply reports as low-solubility silver chloride and is filtered off, but above roughly 8% the chloride layer begins to shield the metal from the acid and slows dissolution, so the batch takes longer and the residue load rises. Material above those levels is handled by inquartation — melting the gold with silver to bring the gold content down to about 25% so the silver can be parted with nitric acid — or by a direct nitric parting step, and the recovered silver goes to a silver circuit.

Q5. What gold content must my feed have, and what pre-treatment is needed?
 There is no hard floor for the aqua regia route — sweeps and plating residues at a few percent gold are processed routinely — but the practical economics change with grade, because reagent, reactor time and gas treatment are consumed per tonne of feed rather than per gram of gold. As a working rule, feed below roughly 10–20% gold is usually pre-concentrated by physical separation, ashing or selective leaching before dissolution, and feed above roughly 90% gold with acceptable silver and copper can go straight to anode casting and electrolysis. Pre-treatment before dissolution covers milling and screening of solid and dusty feed, ashing or roasting of organics-bearing sweeps, non-cyanide stripping of plated and gold-filled scrap, and inquartation or nitric parting of heavy-silver material.

Q6. What purity can I realistically achieve — 99.9%, 99.99% or 99.999%?
 All three, but under different conditions, and the conditions matter more than the number. The aqua regia route typically reaches 99.9–99.99% depending on how thoroughly the precipitated powder is washed — the limiting factor is occluded base metal, which repeated hot dilute acid washing removes. The electrolytic route holds 99.99% repeatably when the electrolyte is kept at 120–150 g/L gold, 60–70 °C, with current density up to 1,000 A/m², anode silver below about 6% and a treated bleed stream controlling copper and silver accumulation. 99.999% requires a second electrolysis pass with a fresh or well-purified electrolyte, and it is the point at which atmospheric and crucible contamination start to matter, so melting and casting move to inert gas or vacuum.

Q7. How is the final gold purity verified, and who issues the certificate?
 Fineness is determined by fire assay with cupellation, which remains the reference method for gold, with ICP-OES for confirmation and impurity quantitation and XRF for rapid in-process and input checking. Fire assay determines gold directly from a weighed, cupelled sample rather than by difference, which is why it is the method that banks, mints and metal exchanges accept. The Certificate of Analysis issued with each lot states weight, fineness, the impurity scan and the assay method used, and it is backed by the batch record. Customers who require an independent result can have samples assayed by a third-party laboratory of their choice as part of the acceptance procedure, and the machine can be supplied with an on-site assay laboratory — fire-assay furnace, cupellation equipment, balance, XRF and ICP-OES — so the customer is not dependent on anyone else for its own numbers.

Q8. What is the typical gold recovery rate, and how is it guaranteed?
 Recovery runs above 99% on suitable feed. It is protected mechanically rather than by promise: reactor vapour is condensed and the gold-bearing condensate returns to the batch, filter wash liquor and rinse water are returned to the circuit, anode slime is processed rather than discarded, and crucible, dross and filter returns go back to the head of the line. Tail liquor is polished to below 5 ppm gold before recirculation or discharge. Process loss typically closes under 0.02% of the gold processed. The guarantee is a written batch metal-balance method — weighed and assayed input, weighed and assayed output, assayed residues and effluent — with acceptance tolerances agreed before shipment and verified at commissioning on the customer’s own material.

Q9. How much gold is tied up in the process, and how is input reconciled against output?
 Gold is immobilised mainly in the electrolyte. The figure is about 1.8 kg of gold in roughly 12 litres of electrolyte on a 5 kg/day cell, and about 14.4 kg in roughly 96 litres on a 50 kg/day cell; the aqua regia route holds far less, because its gold exists as liquor and precipitate for hours rather than sitting in a permanent bath. Together with in-process liquor, filter cakes and slime, that is the working capital tied up at any moment, and it is the reason a refining project should be sized to the throughput actually available rather than to the largest machine affordable. Reconciliation is by batch: every lot is weighed and assayed in, weighed and assayed out, and the difference against residues, slime, effluent and crucible returns is written up as a metal balance and retained with the Certificate of Analysis.

Q10. What does it cost to run, in chemicals, power, water and labour?
 The two routes have different cost profiles. The aqua regia route is dominated by reagent and gas treatment: hydrochloric and nitric acid at roughly 3:1, oxalic acid, sulfite or ferrous sulfate for reduction, and caustic for the scrubber and neutralisation, with the acid charge of the order of a few kilograms per kilogram of gold dissolved. The electrolytic route is dominated by the gold in process rather than by consumables — roughly 150 litres of hydrochloric acid per tonne of refined gold, a low-voltage rectifier duty, and modest cooling water and ventilation. Labour is one trained operator per shift for small and medium units, with a second person wherever dual-control rules apply to vault and metal movement. Because unit costs depend on local power, acid and labour prices, the operating cost is calculated per project from the test-work mass balance and the customer’s own utility tariffs, and is issued with the quotation rather than as a generic figure.

Q11. What consumables and spare parts will I need, and how often must they be replaced?
 Recurring consumables are reagents — hydrochloric and nitric acid, oxalic acid, sulfite or ferrous sulfate, caustic and flocculant — plus the stripping reagent for plated scrap; sacrificial items are graphite or cast-iron anode moulds, crucibles, anode bags, filter cloth and cartridge filter elements, tower packing and demister elements, dosing and magnetic pump diaphragms and seals, and gaskets and liners. Replacement intervals are driven by duty rather than by the calendar: filter cloth and anode bags by the number of batches and the slime load, crucibles and moulds by melt temperature and flux attack, tower packing by the acid load and the dust carried into the scrubber, and pump consumables by hours run and acid concentration. A preventive-maintenance schedule with wear-item stock levels is issued with the handover dossier, and a recommended first-year spares kit is quoted with the machine.

Q12. What nitrogen oxide, acid fume and wastewater limits must my installation meet?
 The machine is built to meet the local limit rather than a fixed international number, and the reference values used for sizing are a reactor-mouth nitrogen oxide load of the order of 30,000 mg/m³ from aqua regia dissolution, an outlet held below 100 mg/m³, and scrubber absorption efficiency of 99.5% or better across nitrogen oxide, hydrochloric acid, chlorine and acid mist. Where nitrate parting of heavy-silver feed is used, the inlet load is materially higher and the tower is sized accordingly. On the water side, acidic effluent is neutralised under pH control in a PPH reactor, the metal hydroxide sludge is settled and dewatered in a filter press, and polish water is recirculated to the rinse loop with tail liquor held below 5 ppm gold. Because discharge limits, stack-height rules, hazardous-waste classification and permit conditions vary by country and by municipality, the final sizing is confirmed against the customer’s local standard, and we assist with the emission and effluent documentation required for permitting.

Q13. What site and utility requirements do I need before installation?
 The installation needs an acid-resistant floor with containment and bunding, a bunded or diked chemical-storage area with secondary containment for acids and caustic, headroom for the reactor hood, condenser and scrubber tower plus ductwork routing, a ducted exhaust path to atmosphere that satisfies the local stack rules, cooling water for the condenser and cell temperature control, mains power for the rectifier and the heating and control circuits, and drainage routed to the neutralisation circuit rather than to a general drain. Ventilation, gas detection, eyewash and safety shower positions and the vault or strong-room for metal and product are fixed at layout stage. Utility capacities are not standard because they follow throughput, cell count, reactor size and local climate; the utility schedule is issued with the process design package after the feed assay and volume are known, so the customer sizes the building, the power connection and the cooling supply against real figures rather than against an approximate catalogue value.

Q14. What safety equipment and operator training are required?
 On the equipment side the line is supplied with sealed reaction hoods and local extraction, acid-proof flooring and matting, secondary containment and bunding, gas detection with interlocks on the reactor and scrubber circuits, eyewash and safety shower, and the operator protective equipment needed for concentrated acid handling — face shield, chemical goggles, acid-resistant apron and gloves, and respiratory protection for incidental exposure. Training is delivered on site as part of commissioning and covers safe aqua regia working, acid and caustic handling, reagent dosing, route selection and grading, dissolution and reduction control, cell operation and electrolyte correction, melting and casting, off-gas and effluent operation, emergency and spill procedures, assay practice and dual-control metal handling. The crew is signed off against written Standard Operating Procedures before handover, and refresher and supervisor-level training is available afterwards. No special operating licence is required for the standard process, because the route is cyanide-free — which is precisely why it is the preferred configuration for workshops in mixed-use or residential-adjacent locations.

Q15. How long does one refining batch take?
 It depends on the route. Aqua regia dissolution runs typically 4–6 hours at about 90 °C, followed by cooling, filtration and precipitation, which together put a complete aqua regia cycle in the region of 8–12 hours depending on feed and particle size. An electrolytic batch runs typically 15–20 hours, but it is a low-attention operation once the cell is charged, and it can run overnight. Melting, casting and assay add a few hours at the end of a campaign. In practice the constraint is not a single batch time but the number of reactors, cells and filter stations, and how the batches are scheduled across them — which is why throughput and the number of parallel units are fixed together at design stage.

Q16. What capacity machine do I need for my volume?
 Size for the gold you actually handle, not the largest machine you can afford, because the electrolyte holds gold permanently and an oversized cell ties up working capital. As a rule of thumb, one cell’s daily capacity is the practical ceiling for the material you can feed it per day, and the aqua regia reactor should be able to prepare that feed within a single shift. A workshop producing a few kilograms of scrap a day needs a compact single-cell unit with a shared dissolution reactor; a refiner handling tens of kilograms needs multiple cells and parallel reactors; a doré or e-waste operation needs the dissolution side sized to the concentrate throughput and the electrolysis side sized to the gold content of that concentrate. Because the correct size follows gold throughput per day, feed grade and silver content, capacity is engineered per project from the test-work balance and your operating calendar.

Q17. Should I refine in-house or send my gold to an external refiner?
 In-house refining captures the full spread between an impure material and fine gold, removes third-party refining charges and settlement delays, shortens the cash cycle from weeks to days, keeps the material under your own custody and lets you control assay and product certification. It also turns previously marginal material into feed worth processing, because a batch no longer has to justify an external minimum or a shipment. The counterweights are real and should be calculated rather than assumed: the gold immobilised in the electrolyte, the reagent and gas-treatment cost, the labour and training burden, the permitting and workplace-safety obligations, and the assay discipline required to trust your own numbers. The comparison is normally settled by a test-work-based mass balance on your own material, with the refining charge you currently pay per kilogram or per ounce set against the machine’s calculated unit operating cost and the value of the metal in process — and that calculation is prepared as part of our test-work report.

Q18. What is included in delivery, and what warranty and after-sales support do I get?
 Delivery covers process design and layout, manufacture and skid assembly, factory functional testing and trial running with the client invited to the acceptance test, packing in export cases with the shipping documentation, supervised installation and electrical and ductwork tie-in, commissioning on the customer’s own feed until the contracted fineness, recovery and throughput are demonstrated, scrubber outlet verification against the local limit, on-site operator training with written Standard Operating Procedures, and a handover dossier containing O&M manuals, electrical drawings, P&ID, spare-parts catalogue and safety documentation. Warranty is twelve months from commissioning or eighteen months from shipment, whichever comes first, with defective-part replacement and free remote support, backed by a guaranteed-performance clause on fineness and recovery and an agreed batch metal-balance method. After warranty expiry, remote response within 24 hours, stocked spares and consumables, and optional upgrade and optimisation services continue for the life of the machine.

Q19. What information do you need to prepare a quotation?
 A representative sample or assay of your feed giving gold, silver, copper and any platinum-group metal content; the material type and physical form, and whether it is solid, granular, dust, powder, sludge, plated or in liquor; the volume you handle per day or per month and the batch size you want; the product form and fineness you intend to sell, and whether you need standard bar dimensions; the available floor area, headroom, power supply, cooling water, drainage and exhaust routing for your site; and your local air-emission, wastewater and chemical-storage limits. The sample and the assay are what allow the route, the achievable fineness, the recovery, the electrolyte lock-up and therefore the machine size and price to be confirmed, and test work on your own material is what turns the quotation from an estimate into a commitment.