Gold Smelting

Many people confuse gold smelting with melting. Gold melting is simply the process of heating gold to turn it into a liquid, in order to form gold bars, coins or other shapes. Gold smelting is a more complex method used to purify gold. The gold smelting process removes impurities from gold ore. These could be the original impurities found in gold deposits in the earth, or impurities added to gold in manufacturing. To remove these impurities, extremely high temperatures, pressure, and a number of chemicals are used.

Gold Smelting vs Gold Refining

The terms smelting and refining are sometimes used interchangeably, but they describe related yet distinct stages of the process. It is thought that gold has been smelted since 6000 BC; first done by the Babylonians, in current day Syria. Today, it is a highly sophisticated treatment process, but the fundamentals for smelting gold remain the same.

Smelting typically refers to the initial melting and separation of gold from ore or scrap material using heat. The result is often a crude gold alloy that still contains impurities. Refining takes this material further, removing those impurities to achieve a precise purity level, for example, the 999.9 fine gold (also expressed as 24 carat) used in the highest-grade investment bullion.

How Is Gold Smelted? The Key Stages

1. Ore Preparation and Concentration

Gold is rarely found in large, pure deposits. Most commercially mined gold occurs in very low concentrations within rock ore, often alongside other minerals and metals. Before smelting can begin, the ore must be crushed, ground and processed to increase the concentration of gold.

Common methods include gravity separation, which exploits gold's density to separate it from lighter minerals, and froth flotation, which uses chemical reagents to make gold particles attach to air bubbles that rise to the surface of a liquid, collecting the gold-rich froth.

2. Leaching

Many modern gold operations use a chemical leaching process (most commonly cyanide leaching) to dissolve gold from concentrated ore. The gold binds with the cyanide solution to form a gold-cyanide complex, which is then separated from the remaining solid material. The gold is subsequently recovered from the solution through a process called carbon adsorption or by adding zinc.

While effective, leaching produces a gold product that still requires further processing to reach investment-grade purity.

3. Smelting

The concentrated gold material is mixed with flux materials such as borax, silica and soda ash. These fluxes bind with impurities and slag, separating them from the molten gold when the mixture is heated.

The mixture is loaded into a furnace and heated to temperatures above gold's melting point of 1,064°C (1,947°F). At these temperatures, the gold melts and the lighter flux-bound impurities rise to the surface as slag, which is skimmed off. The molten gold is then poured into moulds to produce what is known as a gold doré bar, a semi-pure gold alloy, typically containing gold, silver and traces of other metals.

4. Refining to Investment Grade

Doré bars are transported to specialist refineries, where they undergo further processing to achieve the purity levels required for investment bullion. Two primary refining methods are used at this stage:

• Miller process: Chlorine gas is bubbled through the molten gold, causing silver and base metal impurities to form chloride compounds that separate from the gold. The Miller process can achieve purities of around 99.5%, and is relatively fast and cost-effective.
• Wohlwill process: An electrolytic refining method in which the impure gold is used as an anode in an electrolytic cell containing a gold chloride solution. Pure gold is deposited on the cathode at very high purity levels — typically 999.9 fine, or 99.99% pure. This is the standard used for the highest-grade investment bullion products.

Gold Purity Standards in Investment Bullion

The purity of refined gold is expressed either as a fineness (parts per thousand) or as a carat rating. Investment-grade gold bullion is held to strict purity standards, and understanding these standards is useful when comparing products.

• 999.9 fine (24 carat): The standard for the modern investment gold bar, representing 99.99% pure gold. This is also the purity level of many gold bullion coins such as the Gold Britannia (from 2013 onwards).
• 999 fine (24 carat): 99.9% pure gold, still considered investment grade and used in a wide range of bullion bars and coins worldwide.
• 916.7 fine (22 carat): The purity of the Gold Sovereign, which contains 91.67% gold with the remainder being copper.

The gold bullion price quoted on markets and by dealers such as BullionByPost is based on 24 carat fine gold. Products of lower purity are priced accordingly, reflecting their actual gold content.

Gold Recycling

Gold recycling is a large part of the market, and contributes around 25% to the market each year. The majority of this (90%) comes from jewellery and scrap gold. These gold items are purchased, then sent to refiners who smelt the jewellery back into pure gold.

BullionByPost offer market-leading rates for scrap gold ready to be smelted back into pure gold. For a quote, call 0121 634 8060 and our team will be happy to help.