Gold Melting and Refining Process Explained Step-by-Step: From Dore to LBMA Good-Delivery Bars

Gold straight from a mine is not pure. It comes mixed with silver, copper, iron, and trace metals. The refining process transforms raw mined material into the 99.99 percent pure investment-grade bars that fill central-bank vaults and back gold ETFs. The chemistry and engineering have been refined over centuries; the modern process is precise, repeatable, and capable of producing hundreds of tonnes of investment-grade gold per year from a single facility.

Starting point: the dore bar

Mines produce dore bars by smelting concentrate at the mine site. A typical dore bar weighs 15-30 kg and contains roughly 70-90 percent gold by weight, with the remainder being silver (often 10-20 percent), copper, iron, and trace platinum-group metals. The dore is what gets shipped to LBMA-accredited refineries for full purification.

Step 1: receipt and initial assay

When dore arrives at a refinery, it is weighed, photographed, and sampled. A fire assay determines the exact gold content. This number is critical because it determines what the miner is paid. Both sides usually take independent samples to avoid disputes.

Step 2: pre-melting and homogenization

Dore bars are melted in induction furnaces at 1,064 degrees Celsius (gold's melting point). The molten gold is stirred to ensure even distribution of the contained metals. This homogenization step makes subsequent purification more efficient and consistent.

The four major refining methods

1. Miller chlorination process

Invented by Francis Bowyer Miller in 1867 in Sydney. Chlorine gas is bubbled through the molten dore at 1,100 degrees Celsius. The chlorine combines with base metals (copper, iron, silver) to form metal chlorides that float as a slag on the surface and are skimmed off. Gold remains in the molten pool because gold chlorides are unstable at these temperatures. Final purity reaches 99.5 to 99.6 percent — high enough for many uses but not for investment-grade.

2. Wohlwill electrolysis process

Invented by Emil Wohlwill in 1874 in Hamburg. Gold from the Miller process is cast into anodes (positive electrodes) and placed in a bath of gold chloride solution. When electric current is applied, pure gold migrates to the cathode (negative electrode) and deposits as a powder of 99.99 percent purity. The process takes 24-48 hours per batch. Wohlwill is the gold standard for investment-grade refining today.

3. Aqua regia process

Aqua regia (Latin for 'royal water') is a mixture of nitric acid and hydrochloric acid in a 1:3 ratio. It is one of the few chemicals that dissolves gold. Lower-purity dore is dissolved in aqua regia, then gold is selectively precipitated out by adding a reducing agent (sulfur dioxide, sodium metabisulfite, or oxalic acid). Recovery yields 99.99+ percent pure gold powder, which is then re-cast into bars. Aqua regia handles complex material but is more energy and reagent intensive than electrolysis.

4. Ion exchange and modern processes

Modern refineries use ion-exchange resins for fine purification and for recovering trace gold from secondary sources (electronics, jewelry scrap). Ion exchange is less common as a primary method but plays a critical role in modern refinery operations alongside the classical Miller-Wohlwill combination.

Comparison of refining methods

Step 3: casting and final form

After electrolysis or chemical precipitation, the pure gold is melted again at 1,064 degrees Celsius and cast into the standard 12.5 kg (400 troy ounce) LBMA good-delivery shape. Some refiners cast directly to size; others cast large pieces and re-melt to size. The bars are cooled, surface-finished, weighed precisely, and then assayed for final fineness verification.

Step 4: hallmarking and quality control

Each finished bar receives the refiner's hallmark, unique serial number, year of casting, fineness, and weight to the troy ounce. The marks are pressed into the surface with hardened steel dies. Quality control teams verify weight (within tight tolerances), surface quality (no cavities, no bubbles, no inclusions), and stamp legibility. Bars that fail any check are re-melted and re-cast.

Recycled gold refining

Roughly 1,200 tonnes of gold per year come from recycling (mostly jewelry, some electronics). The refining process is similar to mine dore but starts from messier material. Electronics scrap may contain only 200-400 grams of gold per tonne, requiring complex chemistry to extract. Modern e-waste recyclers use hybrid pyrometallurgical and hydrometallurgical processes. PAMP, Umicore, and Tanaka are leaders in recycled-gold refining.

Energy and environmental considerations

• Energy intensity: refining one kilogram of gold consumes 3-8 kWh, mostly for melting and electrolysis.
• Chemical waste: aqua regia and chlorination produce acidic waste streams that require neutralization.
• Mercury and cyanide: legacy mining processes used mercury and cyanide; modern refineries phase these out.
• ESG audits: LBMA Responsible Gold Guidance requires audited chain-of-custody to prevent conflict gold.
• Carbon emissions: typical refining emits 5-12 kg CO2 per kg of gold processed.
• Water use: significant, but recycled in modern refineries.

The major LBMA-accredited refineries

• Valcambi (Switzerland): ~2,000 tonnes/year capacity, largest single refinery.
• PAMP (Switzerland): ~450 tonnes/year, premium investment bars and designed coins.
• Argor-Heraeus (Switzerland): ~400 tonnes/year, oldest still-operating Swiss refinery.
• Metalor (Switzerland): ~650 tonnes/year, strong in industrial gold.
• Heraeus (Germany): ~250 tonnes/year, strong in industrial precious metals.
• Rand Refinery (South Africa): ~600 tonnes/year, primary refiner for African mine production.
• Tanaka (Japan): ~250 tonnes/year, strong in electronics and industrial.
• Royal Canadian Mint, Perth Mint: government refiners, also major coin producers.

Refining costs and margins

Refining typically costs 0.10 to 0.50 percent of the gold value as a fee paid by the mine. For miners, this is a significant expense given typical mine all-in sustaining costs of $1,400-$1,500 per ounce. Refining margins on commodity dore are thin (0.1-0.2 percent). Margins on coin and small-bar minting are much higher (5-15 percent over spot). This is why most refineries also operate coin and bar minting operations as a higher-margin add-on.

From dore to bar: the complete timeline

Why this matters for investors

• Refining capacity limits short-term physical supply when demand spikes.
• Refining bottlenecks in 2020 created the largest LBMA-COMEX dislocation in history.
• LBMA-accredited refining is what makes a bar tradable at full price worldwide.
• Recycled gold refining provides a flexible supply buffer during shortages.
• Refining technology has barely changed in 50 years; the bottleneck is institutional, not technical.

Frequently asked questions

How pure is mine dore?

Typically 70-90 percent gold, with the balance being silver, copper, iron, and trace metals. Refining is required to reach investment grade.

How long does refining take?

Typically 35-70 hours from dore receipt to vault-ready LBMA bar. The Wohlwill electrolysis step is the longest, taking 24-48 hours per batch.

What is the Miller process?

The first major commercial gold refining method, invented in 1867. Chlorine gas reacts with base metals in molten dore to form chlorides that float as slag. Gold remains, reaching 99.5-99.6 percent purity.

What is the Wohlwill process?

Electrolysis-based refining invented in 1874. Gold migrates from impure anode to pure cathode through a gold-chloride solution under electric current. Produces 99.99 percent purity, the global investment-grade standard.

What is aqua regia?

A mixture of nitric and hydrochloric acid in 1:3 ratio. One of the few chemicals that dissolves gold. Used for refining lower-grade or complex feeds; gold is selectively precipitated out and re-melted.

How much does it cost to refine gold?

Typically 0.10 to 0.50 percent of gold value as a refining fee paid by the mine. Coin and bar manufacturing adds higher markup but bulk refining is a thin-margin business.

Which is the largest gold refinery?

Valcambi in Balerna, Switzerland, with approximately 2,000 tonnes per year capacity, the largest single facility in the world. Located close to the Italian border.

Can I refine gold at home?

Strongly discouraged. Refining involves dangerous chemicals (concentrated acids, chlorine gas) and high temperatures. Professional refining requires safety equipment, regulatory permits, and trained personnel.

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