Gold Refining Process

A gold refining process is the sequence of chemical and electrochemical operations that takes gold which is impure but concentrated — a doré bar, a batch of 22K scrap, a filtered gold sponge, or the gold fraction of an anode slime — and raises it to a fineness that can be traded as investment-grade metal. In plant terms it is the refining half of a gold refinery plant: dissolution and reduction vessels, gold electrolysis equipment, the gold refining machine units that melt, granulate and cast the product, and the scrubber train that keeps the whole circuit legal. Specified as one supply, those units make up a single gold refining system engineered around one feed material and one required daily output. It is not smelting, and it is not melting. Smelting separates gold from ore and from gangue at temperatures above 1,000 °C and produces doré at roughly 90–95% gold; melting simply liquefies metal without changing its composition. Refining is the controlled part: gold is either dissolved into solution and chemically brought back as metal, or dissolved anodically and re-deposited cathodically in an electrolytic cell. The target is an internationally recognised fineness — for investment and central-bank trade this is 99.99% (Au99.99), and for premium or high-reliability industrial applications 99.999%.

The line described on this page is the complete refining circuit of a gold works, arranged in the order the chemistry demands. Feed arrives and is assayed and granulated, because a refining reaction runs at the speed of its surface area: a melt that has been poured into water to form shot dissolves in a few hours where the same weight of bar would take days. High-silver feed is then pre-treated with nitric acid, which removes silver and base metals ahead of the refining step instead of letting silver chloride form inside the dissolving vessel or the electrolytic cell. The prepared feed is attacked with aqua regia — one part concentrated nitric acid to three parts concentrated hydrochloric acid — which converts gold to chloroauric acid (HAuCl₄); the reaction is run hot, the vapour is condensed and returned, and the solution is cooled, filtered and clarified. Silver and any insoluble fraction stay behind as a chloride residue and are filtered off as a by-product. Gold is then brought back out of solution as metal by selective reduction with oxalic acid, sodium metabisulfite or ferrous sulfate at controlled temperature, pH and dosing rate, producing sponge gold or a fine gold powder that is washed, acid-scrubbed, washed again to neutral and dried.

Sponge gold is not yet 99.99%. The step that reaches investment grade is electrolysis. The dried gold is melted and cast into anode plates, which are suspended in an electrolyte of gold chloride in free hydrochloric acid at 60–70 °C against cathode starter sheets of pure titanium or fine gold. Under current, gold dissolves from the anode as Au³⁺ and plates out on the cathode as coarse crystalline gold of very high purity, while base metals stay in solution and insoluble impurities drop as anode slime into the anode bag. The two process problems that decide whether a gold electrolytic refinery performs are silver and copper. Silver above roughly 6% in the anode forms a silver chloride film that passivates the plate and stops the reaction; the line answers this with a DC supply with superimposed AC, which is an asymmetric pulsed current that breaks the silver chloride layer off the anode as it forms, so the plate dissolves evenly and completely and the silver is recovered from the anode bag. Copper above roughly 2% in the anode co-deposits at the cathode and contaminates the product, which is why copper is removed in pre-treatment and why the electrolyte is monitored, polished and periodically bled. The cathode gold is harvested, washed to remove occluded electrolyte, dried and melted and cast into ingots — under vacuum and inert gas where the customer requires porosity-free, ripple-free bars to an LBMA-approved trading specification.

The reason a gold refinery is bought as an engineered line rather than assembled from vessels is that dissolving gold generates the most aggressive off-gas in precious-metal processing. Aqua regia evolution, electrolysis at 60–70 °C and anode melting all release acid mist, chlorine, nitrosyl chloride and nitrogen oxides, and every one of those streams carries metal. Vapour above a dissolving reactor is therefore condensed and returned to the reactor, not vented; the enclosed hood keeps the operator out of the plume; the residual NOₓ is absorbed in a multi-stage train — two-stage bubble absorption, four-stage jet absorption and a final two-stage alkaline spray — at 99.5% or better absorption, which takes an inlet of 20,000 ppm down below 150 ppm and has been measured in service at 80 mg/m³ against a 100 mg/m³ regulatory limit; and the acid effluent is neutralised in an agitated PPH neutralisation tank before discharge or recycling, with washing water cross-reused inside the circuit so that the plant approaches closed-loop operation rather than merely compliant disposal.

What the operator ends up with is a compact, cyanide-free refining circuit — aqua regia, chloride, oxalic acid, sodium metabisulfite, hydrochloric acid and nitrite-free reduction chemistry, with no cyanide anywhere in the refining route — that runs on a 10–20 hour electrolysis cycle, locks up roughly 0.15 kg of gold per litre of electrolyte, consumes about 150 litres of hydrochloric acid per tonne of gold in the electrolytic step, and returns chemistry the operator can verify: Au99.99 (Grade I) on the Shanghai Gold Exchange standard, or 99.999% by a second pass through the same cell. Wetted parts are selected against the duty from high-performance polypropylene with anti-ageing additives, PPH structural framing, titanium, glass and titanium-clad copper; every vessel, filter, tank and duct can be built to the layout of an existing refinery building or supplied as a containerised line for a greenfield site.

A gold refining process line is specified wherever gold arrives in a form that is rich but not yet tradable, and where the operator wants to own the final step — the step that converts metal at a discount into metal at the market price — instead of paying a third-party refiner’s deduction. The following are the situations in which this line is installed.

• In-house refineries for jewellery manufacturers and goldsmiths — refine workshop scrap, sprues, filings, polishing dust, sweeps and old gold taken in part-exchange, and put the metal straight back into production as certified fine gold. Output: Au99.99 ingots, granules or granulate for alloying.
• Karat gold and old-jewellery processors — take 18K, 22K, dental gold, watch cases and mixed karat scrap through pre-treatment, dissolution and electrolysis in one line rather than in separate chemical batches. Output: Au99.99 with the silver fraction recovered as silver chloride.
• Precious-metal recyclers and urban-mining operators — finish the gold fraction produced by e-waste, PCB and plating-sludge recovery circuits into saleable fine gold on the same site. Output: Au99.99 ingots plus gold granules.
• Gold mines, doré smelters and mine-site refineries — refine gravity concentrate, doré bullion and electrowinning cathode from cyanidation or gravity circuits into market metal, shortening the chain from mine to bank. Output: Au99.99 bars to a trading specification.
• Copper and nickel electrorefineries — treat the gold fraction of anode slime, where gold arrives as a rich but contaminated residue after silver removal. Output: fine gold ingots with the silver and PGM fractions routed onward.
• E-waste and PCB recycling plants — refine the gold-bearing concentrate, gold powder or gold alloy produced by mechanical and hydrometallurgical front ends. Output: Au99.99 in a cyanide-free refining circuit.
• Mints, assay houses and trading houses — convert returned, off-spec, contaminated or mixed gold material back to assay-certified metal with full batch traceability. Output: certified ingots with a Certificate of Analysis per batch.
• Banks, pawn networks and bullion dealers — bring melting and refining in-house so that bought-in scrap is settled against a documented assay rather than an external refinery’s deduction. Output: standard-weight fine gold bars.
• Industrial and electronics manufacturers — recover gold from rejected plating lines, sputtering targets, bond wire, contact scrap and process residue, and close the loop back into the plating bath. Output: fine gold for anode or target manufacture.
• Dental laboratories and medical device makers — refine crown, bridge and alloy scrap, crucible residue and bench sweeps in-house. Output: high-fineness gold for alloy make-up.
• Hydrometallurgical plants and precious-metal works — add a dedicated gold finishing circuit behind a leaching, solvent-extraction or ion-exchange train. Output: fine gold plus a bleed stream routed to the PGM circuit.
• Greenfield refinery projects and national precious-metal programmes — build domestic refining capacity with a single supplier for dissolution, electrolysis, casting and emissions control. Output: a standalone gold refinery with auditable metal balance.
• Pilot, laboratory and teaching facilities — small-batch refining and process development on the same flow sheet as a production refinery. Output: analytical-grade fine gold for development work.

Q1. What is the gold refining process in simple terms?
The gold refining process turns impure but concentrated gold — doré bullion, karat scrap, gold sponge or anode slime — into tradeable fine gold by dissolving the gold and bringing it back as metal. Feed is assayed and granulated, silver and base metals are removed with nitric acid, the gold is dissolved in aqua regia to form chloroauric acid, a selective reducing agent brings it back as sponge gold, the sponge is melted and cast into anode plates, and electrolysis in gold chloride electrolyte deposits 99.99% gold on the cathode, which is then washed, dried and cast into ingots.

Q2. What is the difference between the Miller process, the Wohlwill process and aqua regia refining?
The Miller process blows chlorine gas through molten gold; impurities and silver form chlorides that float as slag, giving roughly 99.5% gold quickly and simply. The Wohlwill process is electrolytic: an impure gold anode dissolves in gold chloride and hydrochloric acid and pure gold plates onto the cathode, reaching 99.99% or better. Aqua regia refining dissolves gold chemically and reduces it back as powder, which is the practical route for low-grade, mixed or high-silver material that cannot be cast into a stable anode. This line combines aqua regia refining and Wohlwill-type electrolysis in one flow sheet, so it accepts the full range of feed.

Q3. Why does the process need electrolysis if chemical reduction already reaches 99.9%?
Chemical reduction reliably reaches around 99.9% and is limited by the purity of the reducing agent, the completeness of the wash and the behaviour of the liquor. It also struggles to deliver a consistent five-nines product. Electrolysis separates gold from the remaining impurities electrochemically: base metals stay in solution and insoluble material drops as anode slime, so the deposit on the cathode is purer than anything the chemical step can produce, and repeatability is set by current, temperature and electrolyte concentration rather than by operator judgement.

Q4. Can the line refine karat gold and jewellery scrap, or only doré bars?
It refines both. Karat gold and jewellery scrap go through granulation, nitric acid pre-treatment to strip the alloying silver and copper, aqua regia dissolution and reduction, and then to anodes and electrolysis. Because karat scrap already carries a high gold content, it does not need enrichment before refining — it needs the alloying metals taken out, which is exactly what pre-treatment does.

Q5. How much gold is locked up inside the equipment at any time?
The electrolytic circuit holds approximately 0.15 kg of gold per litre of electrolyte, so a 12-litre cell locks up about 1.8 kg of gold in solution, against roughly 4 kg or more for a conventional gold electrolytic circuit of the same duty. Anode plates in the cell add the working load, and sponge, powder and solution in the chemical half of the line add their own inventory. The figure is calculated for each project from the cell volume and the batch size, and is stated in the technical proposal because it is a direct working-capital number at today’s gold price.

Q6. What is the refining loss, and how is it verified?
Refining loss on this line is below 0.02% of the gold in the batch. It is verified by closing a metal balance for every lot: feed weight and assay in, product weight and assay out, plus the weighed and assayed residues — anode slime, filter cake, bag contents, bleed-liquor recovered gold and clean-up material. The difference is the reported loss. Nothing is estimated, and the batch record is available to the customer.

Q7. How is the purity of the finished gold confirmed?
Purity is confirmed by fire assay with inductively coupled plasma optical emission spectrometry confirmation on a sample taken from the finished ingot or granulation lot. Fire assay is the reference method for gold because it is not a surface measurement. X-ray fluorescence should not be relied on as the release method for a bar, because it reads only the top few microns and its error on gold alloys can reach a few percent; a bar must be sampled from the melted, homogenised metal to be assayed properly.

Q8. What fineness can the line guarantee, and against which standard?
The standard product is 99.99% gold, cast to the Shanghai Gold Exchange Au99.99 (Grade I) standard gold ingot specification, and cast bars can be produced to any LBMA-approved trading specification the buyer nominates. Where a five-nines product is required, the cathode gold is recast and passed a second time through the same electrolytic cell to reach 99.999%.

Q9. What silver and copper content can the feed contain?
For stable direct electrolysis, silver in the anode should stay below about 6% and copper below about 2%. Silver above that level forms silver chloride that passivates the anode plate, and copper above that level co-deposits at the cathode and reduces product purity. Feed that exceeds either limit is not rejected — it is routed through nitric acid pre-treatment, which removes silver and base metals first, so high-silver and high-copper material is handled rather than turned away.

Q10. How long does a batch take, and how many batches per day?
The electrolytic step runs 10–20 hours per batch, so a cell completes one batch per working day or overnight. The dissolution and reduction stages ahead of it run in hours; a granulated batch dissolves far faster than a solid bar of the same weight because the reaction is surface-controlled. The number of batches per day is set by the number of cells, which is why capacity is configured to the required daily output rather than fixed by a single model.

Q11. Why does the line use a DC supply with superimposed AC?
Because silver in the anode forms a silver chloride film that coats the plate and stops it dissolving. A DC supply with an AC component superimposed creates an asymmetric pulsed current that breaks the film off continuously as it forms. The anode therefore dissolves evenly and completely, unreacted plates do not have to be returned for remelting, and the silver reports to the anode bag as recoverable silver chloride instead of blocking the cell.

Q12. Do I need cyanide to refine gold with this line?
No. The refining circuit uses aqua regia, gold chloride electrolyte, hydrochloric acid, nitric acid, oxalic acid and metabisulfite, and alkaline scrubber liquor. Cyanide has no role in refining this feed range; it belongs to ore leaching, not to refining. Removing cyanide from the refining circuit simplifies permitting, removes a cyanide-bearing effluent stream and reduces the hazard the operator is exposed to.

Q13. What does the plant do with the acid mist and nitrogen oxides it produces?
Dissolution and electrolysis are vented at source into a multi-stage absorption train — two-stage bubble absorption, four-stage jet absorption and a final two-stage alkaline spray — with automatic chemical dosing and a demister. Absorption efficiency is 99.5% or better, taking an inlet above 20,000 ppm down below 150 ppm, and the same train has been measured in service at 80 mg/m³ against a local limit of 100 mg/m³. Scrubber liquor, filter wash water and cathode washing water are cross-reused in the circuit, and the acid effluent balance is neutralised to a controlled pH in an agitated PPH neutralisation tank.

Q14. What space, power and utilities does the line need?
Footprint, clear height, installed power and utility loads are all configured to the feed assay, the required daily output and the customer’s building, and are stated in the technical proposal rather than fixed here. The process parameters are not configured — current density up to 1,000 A/m², electrolyte at 60–70 °C and 120–150 g/L gold, a 10–20 hour cycle and 99.5% or better absorption are the values the line is guaranteed to hold. The line is built as function-matched units and can be laid out into an existing refinery or containerised for a new or remote site.

Q15. What is included with the plant, and what is not?
Included: the reaction and dissolution vessel, condenser and gas–liquid separator, reduction and precipitation unit, filtration and washing unit, storage and dosing tanks, electrolysis cell with hood, anode bags and cathode starter sheets, rectifier with superimposed AC supply, electrolyte circulation and temperature control, PLC control system with touchscreen, melting furnace, granulation machine, anode and ingot moulds, vacuum ingot casting machine, multi-stage scrubber train, effluent neutralisation tank, monitoring and interlocks, and the documentation, training and commissioning that go with them. Not included unless separately agreed: site civil works and foundations, building and extraction ductwork outside the equipment battery limit, chemical storage buildings and bunding, electrical supply to the plant terminal, reagents, and local permits and approvals.

Q16. How long does delivery and commissioning take, and what does the warranty cover?
Delivery time and commissioning duration are quoted per project because they depend on the number of cells, the degree of containerisation and the destination. The service process is fixed: laboratory testing of the feed, flowsheet and layout design, manufacture with a factory acceptance test, shipment and site installation, commissioning against a performance test on fineness, recovery, cycle time and emission, operator training and handover. The warranty covers defects in design, materials and workmanship of the supplied equipment, runs from successful commissioning for a period fixed in the contract, excludes consumable and wearing items and damage from operation outside the agreed parameters, and is supported by a recommended spare-parts list and remote diagnostics on the batch data.