A gold plant makes a concentrate or a pregnant solution. The gold refining process starts when those intermediate products leave the carbon-in-leach circuit. You'll notice a hard boundary in the flowsheet: mining, milling, leaching, and recovery are plant work; refining is a separate chemical and thermal discipline. Most mines don't refine to 99.99% on site because the capital and permitting burden rarely make sense at mine scale.
Where the Plant Stops and the Refinery Starts
In a typical CIP or CIL plant, gold dissolves into a cyanide solution and then adsorbs onto activated carbon. That carbon is stripped in an elution column to produce a concentrated gold solution. The next step, electrowinning, plates the gold onto steel wool cathodes. But the cathode deposit isn't bullion—it's a loose, impure metallic mud. This is the plant's final product boundary for many operations. Loaded carbon at that point carries 1,000 to 10,000 grams of gold per tonne, and the electrowon cathode sludge is 50% to 90% gold by mass. After this point, you're in the gold refining process, distinct from the gold milling process. A CIP plant design usually sizes the elution and electrowinning circuit to deliver a cathode product that can be smelted on site into a saleable dore bar. If your ore is refractory, you'll need pre-treatment ahead of leaching; see gold recovery methods for how that changes the front end.
Elution and Electrowinning: Producing the First Metallic Gold
Elution is a high-temperature, high-pH stripping process. Loaded carbon, typically carrying 1,000 to 10,000 grams of gold per tonne, is contacted with a hot caustic cyanide solution. The gold desorbs from the carbon pores. The pregnant eluate then flows to electrowinning cells. Stainless steel or steel wool cathodes collect gold by electrolysis at 3 to 5 volts. The result is a cathode sludge that's 50% to 90% gold by mass, depending on the feed. This is not yet dore. You'll often see the term electrowinning used loosely, but electrolytic refining later is different—electrowinning deposits gold from a leach solution, while Wohlwill electrolysis refines already metallic gold.
Electrowinning efficiency drops if the eluate contains too much copper or silver. Those base metals co-deposit and later complicate smelting. Good elution practice keeps oxygen out and temperature above 120°C for AARL or 150°C for Zadra systems. On suitable ores, Xinhai's gold CIL/CIP recovery can reach 99%; refinery purity is a separate metric and is not a plant recovery guarantee. Don't confuse the two.
Smelting to Dore Bars
Smelting is different from refining. It's a melting and fluxing step that turns cathode sludge into a solid bar called dore. The smelter loads cathode mud, silica, borax, and sometimes niter into a crucible or induction furnace. Temperatures reach 1,100°C to 1,300°C. Fluxes react with base metals and gangue to form a slag, while gold and silver sink into the metal phase. The poured bar is called dore, from the French word for 'gilded.' Dore typically contains 50% to 95% gold, with silver as the main alloying element. In practice, most cyanide-plant dore runs 75% to 90% gold.
Dore bars are porous if cooled too quickly, which traps slag. A clean melt, poured slowly and cooled under controlled conditions, yields a bar that a refiner will accept without penalty. Some mines upgrade dore on site by remelting with more flux, but that's still not chemical refining.
Miller Chlorination: Fast Pyrometallurgical Refining
Miller chlorination is the workhorse of large gold refineries. It's a pyrometallurgical process. Chlorine gas is injected into molten dore at about 1,100°C. Base metals—iron, copper, zinc, lead—form volatile chlorides and fume off. Silver also forms silver chloride, which floats as a molten slag. The gold remains behind because gold chloride is unstable at that temperature. The result is gold of 99.5% fineness, often written as 995 fine in the trade.
The process is fast: a 400 kg bar can be chlorinated in a few hours. But it can't economically reach 99.99%. Those last few thousand parts per million of silver and platinum-group metals require a different mechanism. Miller chlorination is therefore a bulk purification step before final electrolytic refining.
Wohlwill Electrolysis: Reaching 99.99% Fineness
Wohlwill electrolysis refines gold by dissolving an impure gold anode in a hot hydrochloric acid and gold chloride electrolyte. Direct current passes through the cell. Pure gold plates onto a thin gold cathode, while silver and platinum-group metals either stay in the anode slimes or remain in solution. Operating conditions are typically 65°C to 75°C and a cell voltage of 0.8 to 1.2 volts. The resulting cathode gold can exceed 99.99% fineness—the standard for London Good Delivery bars.
This aqueous electrochemical refining process is distinct from electrowinning, which recovers gold from a leach solution. The feedstock for Wohlwill cells is usually Miller-refined gold of 99.5% purity. A refinery can also feed dore directly to Wohlwill cells, but the electrolyte fouls quickly with silver chloride, so Miller chlorination upstream reduces operating cost. According to the LBMA Good Delivery Rules, a good delivery gold bar must contain 350 to 430 troy ounces of gold with a minimum fineness of 995.0 parts per thousand.
Fineness Notation: 99.5% vs 99.99%
Fineness is parts per thousand by mass. 99.5% gold is 995 fine, meaning 995 parts gold and 5 parts other metals per thousand. 99.99% gold is 999.9 fine, often called 'four nines.' The difference sounds small, but it matters in settlement. A 1,000 oz lot of 995 fine gold contains 995 oz of payable gold. At 999.9 fine, it contains 999.9 oz. That's 4.9 oz more gold—worth thousands of dollars at current prices. So a refinery's ability to move from 99.5% to 99.99% is a commercial decision, not just a technical one.
Most gold trading uses 995 fine as the minimum for good delivery bars, but exchange-traded products and many central banks demand 999.9 fine. Dore is rarely better than 950 fine, so it's sold at a discount to spot that reflects the cost and recovery of refining.
What a Mine Actually Sells
A gold mine sells three possible products: dore bars, 995 fine gold, or 999.9 fine bullion. Small and remote mines often pour dore and ship it to a third-party refinery under a tolling agreement. The refiner assays the bar, deducts treatment and refining charges, and pays for the gold and silver content. Larger operations may install a Miller chlorination unit to upgrade dore to 995 fine, cutting freight and refining charges. Only the largest mines with multi-decade reserves justify a full Wohlwill cell house.
You'll see the phrase 'dore refining' in contracts. It refers to the entire downstream chain: smelting, Miller chlorination, Wohlwill electrolysis, and final assay. When you design a gold plant, don't overbuild the refinery. The extraction side has more impact on project economics than the final fineness. Xinhai reports 2,500+ mines served and 600+ EPC+M+O projects, according to the company's published figures; most of those projects stop at dore or concentrate. The USGS Mineral Commodity Summaries notes that mine production and scrap are the two main sources of newly refined gold.
Matching Refining Choices to Your Plant
If you're designing a 500 t/d gravity and CIL plant, don't plan for a Wohlwill cell house. Your dore production is small—often a few hundred kilograms per month—so a toll refining contract is cheaper and simpler. If your plant is 3,000 t/d or larger, on-site Miller chlorination can cut shipping weight and improve cash flow. A full electrolytic refinery only becomes rational above roughly 10,000 oz of dore per month; below that, the acid storage, precious metal security, and skilled labour costs outweigh the margin gain.
Look at your feed grade and silver content before you choose. High-silver dore complicates Miller chlorination because silver chloride slag drags gold losses. In that case, a smaller plant might prefer to sell dore directly and let the refiner handle silver separation. For an overview of how leaching affects what ends up in dore, see CIL vs CIP. And if you're still at flowsheet selection, the gold CIL plant solution page shows a typical layout from crushing to elution.
Gold Refining Sequence in Seven Steps
The full gold refining process from cathode sludge to investment-grade bullion can be summarised as an ordered sequence. 1. Elute loaded carbon at 120–150°C to strip gold into a pregnant solution. 2. Electrowin the eluate at 3–5 volts to deposit cathode sludge that is 50–90% gold by mass. 3. Smelt the cathode sludge with silica and borax flux at 1,100–1,300°C; pour dore bars that assay 50–95% gold, typically 75–90%. 4. In Miller chlorination, inject chlorine gas into molten dore at about 1,100°C; base metals fume off as volatile chlorides, silver forms a chloride slag, and gold remains at 99.5% fineness. 5. Cast the 995 fine gold into anodes and place them in a Wohlwill cell. 6. Run electrolysis at 65–75°C and 0.8–1.2 volts; pure gold plates onto the cathode, while silver and platinum-group metals stay in the slimes. 7. Harvest the cathode gold, wash, dry, and melt it into final bars that exceed 99.99% fineness, meeting the LBMA Good Delivery standard of at least 995.0 parts per thousand. Each step removes a specific class of impurity: elution and electrowinning separate gold from the leach solution, smelting removes gangue and excess base metals, Miller chlorination removes most silver and remaining base metals, and Wohlwill electrolysis removes the last traces to reach four nines.
Frequently asked questions
What is the difference between dore and refined gold?
Dore is an impure gold-silver alloy from smelting, typically 50–95% gold. Refined gold is 99.5% (995 fine) or 99.99% (999.9 fine) after Miller chlorination and Wohlwill electrolysis. The refinery removes silver and base metals that a mine's smelter can't.
How does Miller chlorination work?
Chlorine gas is bubbled through molten dore at about 1,100°C. Base metals form volatile chlorides and fume off; silver forms a molten chloride slag. The remaining gold is about 99.5% pure. It's a bulk purification step, not a final step.
Why do most mines sell dore instead of pure gold?
Refining to 99.99% requires a Wohlwill cell house with acid handling, precious metal inventory, and strict permitting. The capital and operating cost only makes sense at large scale. Small mines reduce freight and charges by pouring dore and paying a toll refiner.
What does 99.99% fineness mean?
It means 999.9 parts gold per 1,000 parts by mass, or 99.99% gold. This is the standard for London Good Delivery bars and most investment products. The last 0.49% removal from 99.5% to 99.99% requires electrolytic refining rather than chlorination.