How Gold Smelting Works

Gold is rarely found in its pure state. It’s usually mixed with other metals and impurities that must be removed before it can be used for jewellery, bullion, or industrial applications. This is where smelting comes in.

In this article, we explain the gold smelting process, including what it is, how it works, and what happens next (gold refining and assaying).

What is gold smelting?

Gold smelting is the process of removing impurities from gold ore to produce pure, usable gold. Mined gold often contains impurities and other metals like silver, copper, and iron, and smelting helps remove these metals to produce a purer form of gold.

Smelting involves heating gold to its melting point of -1,064°C (1,947°F) and combining it with specific chemicals that help separate out unwanted elements. This process has been refined over time with advances in science and technology, but its principle remains the same: purifying raw gold into a form that can easily be shaped into jewellery, coins, bullion, or used in industry.

History of gold smelting

Gold smelting is one of the oldest known metallurgical practices, thought to date back as far as 6,000-3,000 BC. Early gold smelting dates back to Mesopotamia and Syria, where ancient civilisations first learned how to extract pure gold from raw ores.

By 600 BC, the Egyptians had mastered gold smelting by using furnaces to separate impurities from the yellow metal. The technique spread across the Mediterranean, and in ancient Greece, the philosopher Heraclitus described the process of smelting.

Throughout time, the tools and technologies used for smelting have evolved, but the fundamental principle of gold smelting still involves applying heat and fluxes to separate pure gold from everything else.

Learn More: How is Gold Formed and Where Does it Come From?

How does gold smelting work?

Gold smelting is a multi-step process that requires extremely high heat and specialized chemicals. Here’s a look at the gold smelting process, step-by-step:

1. Pulverising the ore

The first step is to crush or pulverise the gold-bearing ore into fine particles. This increases the surface area of the material and makes it easier to separate the gold from surrounding minerals during smelting.

2. Heating gold ore in a furnace

The crushed ore is then placed into a furnace capable of reaching temperatures well above gold’s melting point of 1,064°C. Blast furnaces or induction furnaces are often used to achieve this intense heat. Once molten, the gold begins to separate from the surrounding rock and metals.

3. Adding flux

Next, chemicals are added to the molten mixture. These are called ‘fluxes’ and include borax, silica, sodium nitrate, sodium carbonate, and lead oxide. Fluxes serve several purposes in the smelting process:

• They bond with impurities like copper, zinc, or iron to help separate them from the gold
• They lower the melting point of the charge to make smelting more efficient
• They create a protective slag layer that floats to the surface and traps unwanted oxides.

Because gold is a noble metal that resists reacting with most chemicals, the impurities bond with the flux instead of the gold itself. The slag can then be skimmed off to leave behind a precious metal alloy.

4. Cooling and separating

When the molten mixture cools, the heavier gold and silver settle to the bottom of the furnace while the lighter slag remains on top. Once solidified, the slag is removed and often reprocessed to recover any remaining precious metals.

5. Refining to a higher purity

Smelting results in a product called a doré bar, which typically contains about 90-98% gold with traces of silver and other metals. To achieve the 99.99% purity required for investment-grade bullion, doré bars are sent for further refining using methods like electrolysis or chemical treatment.

What happens after gold is smelted?

As mentioned, gold smelting doesn’t produce pure gold, but gold nuggets or doré bars, which are rough, unrefined blocks that typically contain a mix of gold and silver. Gold content at this stage is usually around 90% purity, which means it needs to be further refined before it can be used in jewellery or bullion.

See: What is a Gold Ingot?

What is gold refining?

Gold refining is the next step after smelting, taking gold from around 90% pure to the high levels of purity needed for jewellery, investment, or industry applications. The purpose of refining is to remove the last traces of other metals so the final product reaches 99.5-99.999% purity.

There are several methods used to refine gold into a higher purity:

• Miller process: This involves introducing chlorine gas to molten gold, which reacts with impurities and separates them out. It’s a fast, efficient, and cost-effective method that produces gold about 99.5% purity, making it a common first step in the refining process.
• Wohlwill process: This is an electrolytic process that’s often used after Miller refining. It involves dissolving gold into an acid solution and then passing an electrical current through it. Impurities are stripped away to produce gold of up to 99.999% purity, the highest level achievable on a commercial scale.
• Merrill-Crowe process: This process is common in mining operations, relying on zinc precipitation to help recover gold from cyanide solutions.
• Aqua regia process: On a smaller scale, gold can be refined using aqua regia, a mix of nitric and hydrochloric acids. This dissolves gold and allows it to be re-precipitated and purified to achieve up to 99.9% purity.

Note that even with these advanced refining methods, it’s impossible to achieve 100% pure gold. The highest recorded gold purity was 99.9999% (six nines fine) set by the Perth Mint in 1957.

Once gold has been refined, it’s melted again and poured into molds to form bullion bars (i.e. cast bullion bars). These bars are then stored in vaults and traded on global markets. Besides cast bars, gold can also be shaped into coins, jewellery, or smaller minted bars.

See: What are the Types of Gold Bars?

Besides freshly mined bars, gold refining is also used for recycled gold, which accounts for around 25% of supply each year. Most recycled gold comes from old jewellery and scrap items that are purchased, smelted, and refined back into pure gold. This helps keep gold available while reducing the need for new mining, offering a more sustainable path for gold investments.

What is slag?

When gold is smelted, the impurities and fluxes used in the process combine to form a byproduct known as slag. Slag is a glassy, stony material that floats on top of the molten gold (or dore metal) inside the crucible. It’s then skimmed away and discarded, or in some cases reprocessed to recover any small amounts of precious metals that remain trapped inside.

Slag is important because it shows whether the smelting process was carried out correctly. When slag is well-formed, it shows that impurities like iron, copper, or zinc have bonded successfully with fluxes like silica, borax, or sodium nitrate, leaving behind the purer gold underneath.

Depending on how it cools, slag can take on a different appearance and texture:

• Slow-cooled slag is hard, compact, and stony, often cooled under normal air conditions
• Water-jacket cooled slag forms quickly with cavities from steam, creating a lighter and less dense material
• Granulated slag is quenched rapidly in water, resulting in small, sand-like glass particles.

Gold assaying

Gold assaying involves measuring gold’s purity and content before it can be sold, refined, or valued. The most reliable and widely used technique is also one that has been used for hundreds of years – the fire assay.

Read More: What are Gold Hallmarks?

Here’s how fire assay works:

1. A gold-bearing sample is placed in a clay crucible along with fluxes like silica and borax, lead oxide (litharge), and a reducing agent like flour
2. The mixture is heated to very high temperatures so that the fluxes lower the melting point of unwanted materials
3. The lead oxide is reduced to metallic lead, which collects and absorbs the precious metals (gold, silver, and platinum-group metals except osmium and ruthenium)
4. As the sample cools, the lead forms a button at the bottom of the crucible while lighter impurities form a glassy slag on top
5. The lead button is then reheated in an oxidising environment to remove the lead and leave behind a shiny bead of precious metals
6. Finally, the bead is treated with nitric acid to dissolve silver and leave behind pure gold that can be weighed for analysis.

Fire assay is destructive but also extremely accurate, which is why it’s the standard for testing gold purity both in mining and refining.

In jewellery, gold purity is expressed in karats (also spelled carats), with 100% pure gold being 24 karats. Each karat equals 4.167% gold content, so 18 karat gold is 75% pure. Fineness is another way to measure gold purity, expressed in parts per thousand. Gold that’s 99.9% pure is referred to as ‘three-nines fine’.

Read More: Gold Purity, Fineness, And Karat – What Is It And How To Check It

Buy pure, investment-grade gold

For gold to be considered investment-grade, it must meet the highest purity standards – usually 99.5% or above. At StoneX Bullion, we offer a wide range of investment-grade gold bullion bars and gold coins from the world’s most trusted mints and refineries. Whether you’re looking for the rustic look of cast gold bars or the artistic appeal of gold coins, you’ll find a high-purity item in our range.

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