Since a mining company extracts gold-rich ore from the earth until a coin is minted or an ingot or jewel is made with this metal, time passes and many industrial processes have evolved for centuries. It is the process known as refining, which converts a raw mineral, with numerous impurities, into a metal with a purity greater than 900 thousandths.
First of all, it must be clarified that what is extracted from the mines is not gold: it is tons of earth with a concentration of just a few grams of metal per ton.
That land is taken to a special plant for processing, which is in charge of separating the gold from the rest of the materials. This plant is usually close to the mining operation, although it is not uncommon for several mines to share the same processing plant, for reasons of cost or organization of the company.
Once this material that has been extracted from the mine has been processed, the result is a series of bars (they are too coarse to call them ingots) of a material called doré, which is actually an alloy of various metals, mainly gold and silver.
The composition of this doré can vary depending on the company that produces it and the mine from which it was extracted. For example, the doré bars received by the Western Australian Mint , the Perth Mint , for refining are typically composed of 70-80% gold and 10-15% silver .
These bars are sent to the refineries, which are the plants where sophisticated physical and chemical processes are carried out to eliminate impurities from the metal until it is left in an almost pure state.
The main European refineries are concentrated in Switzerland and, specifically, in the Italian-speaking canton of Ticino . In fact, Switzerland typically refines approximately 70% of the world’s gold each year .
The London Bullion Market Association (LBMA) Accredited Refinery Good Delivery List brings together the world’s leading refiners. The six most important on this list account for around 90% of the gold refined worldwide annually.
Most of the gold circulating in Europe, in any of its forms, comes from these four large refineries.
Once the refinery receives the doré bars sent by the mining company, they are weighed and melted to ensure that the metal is homogeneous, that is, that there are no points of greater or lesser metal purity inside.
When this amount is determined, a document called an ‘outturn’ is sent to the mining company containing a statement of the weight of the doré bar, the percentage of gold and silver it contains and, based on that data, the amount of gold and pure silver that can be mined.
The mining company, once the document is received, then decides whether to sell the gold and silver to the refinery or carry out a ‘crazy swap’ with it, which consists of an exchange of the precious metals that are in different places, without the need for move them.
The doré first goes through a chlorine refining process, known as the ‘Miller process’ . Named after the chemist who devised it, Francis Bowyer Miller , this process is used to refine gold to a high degree of purity.
It consists of blowing a stream of pure chlorine gas over and through a crucible filled with molten gold containing impurities. These impurities, as well as silver and other metals that may be in the doré alloy, react with chlorine to form silver chloride and other compounds, which are deposited on the surface.
The result of this process is gold with a purity of 99.5%, which is later melted in molds to form banked ingots of 400 troy ounces (12.44 kilos) that are used in international banking transactions.
For its part, the silver chloride obtained is subjected to a leaching process to eliminate the rest of the metals and, later, to electrolysis, which allows pure silver to be obtained.
Returning to gold, sometimes the market demands that this metal have an even higher purity. In these cases, the 99.5% pure gold obtained by the Miller process is put through a new process, called the ‘Wohlwillprocess’ , named after the German electrochemical engineer Emil Wohlwill .
It consists of using lower purity gold as an anode and subjecting it to an electrochemical reaction in which the cathode is made up of 24-carat gold sheets or stainless steel.
Once the current is applied, the anode dissolves in the chlorine solution and pure gold ends up coating the cathode.
This is melted down to obtain gold granules of 99.999% purity . These granules are of different sizes, in order to achieve the exact weight needed when melting them into ingots.
The pellets are combined to fill the smaller ingot molds, ranging in weight from one kilo (32.15 ounces) to half an ounce (15.55 grams) . These ingots go through a process similar to coin minting (‘minting’) that gives them their final rectangular shape, with the characteristic marks or designs of each refinery.