Mining

The hunt for gold: above and below ground

Gold mining is one of the oldest activities known to mankind. The Turin Papyrus Map – generally considered the world’s earliest surviving map – shows the location of an Ancient Egyptian gold mine in c1150 BC. However, this is not the world’s oldest mine. Archaeologists have found evidence of gold mining as early as the third millennium BC, near the Georgian capital Tblisi.

Modern mines

There are two types of large scale gold mine: open pit and underground. The type of mine is determined by the nature of the gold deposits and whether they are close to ground level or deep beneath the surface.

Open pit mines

Open pits are suitable for mining large deposits near the surface, so all the excavation is “top down” from ground level. Advances in mining techniques have made open pit mines much more common in recent years, because larger, more efficient equipment can move rock more cheaply. For example, 20 years ago mining trucks typically carried about 70 tonnes; many of today’s trucks can carry 250 tonnes. This makes it cost effective to mine lower-grade ore near the surface, which previously would not have been profitable.

An open-pit mining team will usually lower the floor of the pit in “benches” of about 30 feet or 10 metres. This gives a mature open pit mine its distinctive stepped appearance. To loosen the rock for removal, the team drills holes and sets off explosive charges. After blasting, geologists mark the broken rock as being either ore or waste, and the ore is taken away to be milled.

Underground mines

By definition, underground mines are suitable for deposits that lie deep underground. Their depth varies greatly, but some of the world’s deepest gold mining takes place more than two miles or five kilometres below ground level in South Africa.

Underground mines vary more than open pit mines because rock formations and conditions are so diverse. However, all rely on shafts dug deep into the ground. The approach taken is determined by the configuration of the ore zones and the strength of the ore and the surrounding rock.

Long hole and cut and fill are two main underground mining methods.

Long hole stoping

This method is suitable for large blocks of ore contained in surrounding rock that is reasonably strong; the result is not unlike an underground quarry. The miners dig tunnels to provide access to the top and bottom of the ore block, and create a vertical hole (slot raise) within the ore from the top of the block to the bottom. Long holes are drilled to blast vertical slabs off the ore block. Normally a remote-controlled loader will pick up the broken ore from the lower tunnel and take it away to an ore pass.

Cut and fill

In this method, top and bottom access is also created, but the ore is taken out in horizontal slices (or lifts) from the bottom. After a slice is blasted and the broken ore is carried away, the void is backfilled to provide a platform for the equipment to stand and to support the side walls. This process is repeated until the block of ore is extracted.

Cut and fill is a more expensive method of mining. However, it can be cost effective where the ore is of a higher grade and the surrounding rock is too weak to use long hole stopping, or the sides of the ore zone are irregular.

Milling

Milling crushes the ore and prepares it for physical or chemical treatment to extract the gold. In this process, large chunks of ore are crushed into much finer particles like gravel.

Cyanidation

Cyanide is responsible for the majority of gold extracted today. The process was introduced towards the end of the 19th century and replaced older methods of extracting gold from ore, such as mercury amalgamation.

In cyanidation, water is added to the milled ore to create a slurry, which is mixed with cyanide to form a solution. Once in solution, the green ‘pregnant liquor’ that forms is separated from the remaining ‘rock flour’. Gold is normally precipitated from the liquor by adding zinc powder. While cyanide bonds naturally with gold, it bonds even more readily with zinc, enabling the gold to be filtered out of the solution. Cyanide is a highly toxic substance, so responsible gold processing plants observe the International Cyanide Code and have the cyanide antidote available for possible accidents.

Floatation

Because cyanide also has an affinity for copper, copper’s presence in ore can prevent or complicate the use of cyanide to extract gold. The solution to the presence of copper in mixed sulphide and oxide ores is a process called floatation. Here, after the addition of special chemicals, small sulphide particles can be made to attach to air bubbles. The ground ore is put in floatation cells where air is injected. More sulphide particles then attach to bubbles and float on the surface as a froth, where they are collected, dried and shipped to a smelter for gold recovery.

Smelting

The gold extracted from ore is further purified by smelting. A chemical flux is mixed with the metal, which is then melted in a furnace at over 1,000 C, causing the gold to separate from most of the impurities – such as zinc, copper and iron. The molten gold is then poured into a mould and allowed to cool. The resulting bar is known as “doré”, which is up to 90% pure gold.