MINING GOLD: FROM MINES TO RETAIL
By: Evan Lucyk
Gold mines have been prolific and economically stimulating for Canada, with 183 tonnes of gold produced in 2018. Gold’s $9.6 billion production value makes it Canada’s most profitable and economically stimulating mineral, with Ontario and Quebec producing more than 75% of gold mined in 2018.
More than half of the global demand for gold is for one type of product: jewelry. Gold is also commonly used in technology—most often as a piece within microcircuitry in various electronic products. Some gold is sold to investors in wafers, bars, and coins because of its believed protection against inflation.
But the process of getting gold ore to a pure form takes lots of work, chemistry, and engineering, and a lot of this work happens within Canada in a landscape rich with unionized workforces. Today we share how this precious mineral goes from being buried deep in the earth to becoming a beautiful gold necklace and pendant or a circuit in your phone.
SEPARATING GOLD AND ORE
When mined, gold and ore must be separated before gold can be turned into products. There are three main processes used to refine gold: floatation, cyanidation, and the carbon-in-pulp method, which can be used independently or in conjunction with one another.
Floatation uses a mix of chemicals, including a frothing agent that causes the water to foam. A collecting agent bonds onto the gold and leaves an oily film that sticks to air bubbles. Several organic chemicals keep other contaminants from bonding to the bubbles. When the solution is aerated and the gold attaches to the bubbles, the gold can be skimmed off as the bubbles float to the top.
In cyanidation, zinc is added to a weak solution of cyanide with the ore submerged. The chemical reaction separates the gold and ore. Next, the solution is run through a filter press, which collects the gold.
Amalgamation uses a similar process to cyanidation, but with different chemicals. In amalgamation, a solution carries the mined ore over plates with mercury. The gold is attracted to the mercury on the plates and forms an alloy called an amalgam. When this alloy is heated, the mercury boils off as gas—collected and recycled in the same process—and gold is left behind.
The carbon-in-pulp method also uses a cyanide solution but relies on carbon to precipitate the gold rather than zinc, which is used in the cyanidation method. After mixing the ground ore with water to form a pulp, cyanide is added to lift the gold from the ore, and carbon is added to bond with the gold. The carbon and gold are separated using a hot caustic carbon solution.
Smelting is a process used when gold is not pure enough after going through these separation processes. A chemical substance called flux is heated with the gold. The flux floats on top of the melted gold bringing contaminants to the surface. The gold is then cooled and left to harden in molds, and the resulting flux and contaminant mixture called slag is disposed of.
REFINING GOLD
There are two major methods of refining gold. In pursuit of pure gold, the Miller process and Wohlwill process are implemented to remove impurities. In the Miller process, gaseous chlorine causes impurities to separate onto the surface of gold that has been heated to a liquid state. It’s a fast and simple process and produces gold that is around 99.5% purity. However, the Wohlwill process can produce gold with about 99.99% purity.
Gold ore is submerged in an electrolyte solution of hydrochloric acid and gold chloride, and an electrical current causes the gold to lift from the ore and move to a cathode where it is restored to a highly pure state.
Because gold is a relatively soft metal, it is often alloyed with other metals such as silver, copper, platinum, or nickel. The alloys are measured in karats, where one unit is equal to 1/24 part of pure gold in an alloy. This is why when you see that something is 24 karat gold, it is pure gold, while 18 karat gold is 18 parts pure gold to 6 parts of other metals.
After the refining process, the gold moves along on its production ride, where it is made into consumer goods from beautiful earrings, necklaces, and watches, to helping keep us connected by powering circuits within our smartphones. Transforming gold ore from the mine into a refined precious metal takes a lot of work but is necessary to get gold products to market.