Gold

Industrial Gold Extraction Process Overview

Gold is one of the most highly valued metals today, as it has been since the dawn of human civilization. Its rich yellowish color evokes the very idea that we almost always relate to it: wealth. Gold had been used for several millennia as currency in the past, with the United States being the last country to use the gold standard. Despite the takeover of fiat currency in virtually every jurisdiction, gold remains of high economic value.

This fact is particularly evidenced by the usage of newly mined gold: 50 percent goes into jewelry, 40 percent into financial investments, and 10 percent to industrial usage (such as in electronics, dentistry, and aeronautics). Curiously enough, the applications of gold are so far reaching that it is even used as ingredient in some high-culture cuisines.

Gold Bullion

As with most precious metals, gold occurs very rarely in nature. This is because of gold’s high density which causes it to sink among other elements found in the Earth crust. Thus, almost all of the gold believed to be in Earth can be found only in the planet’s core.

This is also why practically all of the gold mined from the earth comes from gold-containing meteorites that have crashed in an earlier geological period.

Because of its rarity and high value, it is absolutely essential that gold extraction processes be as close to 100-percent efficient as they can be.

Gold extraction is the process of recovering elemental gold from gold ores.

Gold typically occurs already in metal form (i.e. it is not chemically bonded to other elements as a compound) as sizable nuggets, which can be as huge as coinage or as little as fine grains of sand. In fact, gold may even occur in microscopic amounts while embedded in rocks. Because of this much of the recovery process is actually focused on two things: increasing concentration and increasing purity from contaminants, which is referred to as “refining.”

Gravity concentration is perhaps the oldest method for increasing the concentration of naturally occurring gold. This is traditionally done by using metal pans to displace lighter materials by effect of centrifugal force, thus leaving the heavier gold nuggets in the middle of the pan. This method of gold concentration still remains in use in many small-scale mining today.

The same principle is used today in industrial settings with the use of sluices. These devices are flatbeds that are lined with troughs that act as trapping mechanisms. By passing a pulp of ore and water, gold is allowed to settle in the troughs, while lighter materials that are generally found with gold (such as silica) simply flow through the sluice and eventually get disposed as effluent.

Efficiency is achieved by maintaining a consistent speed in the flow of pulp that is slow enough to allow the gold to settle in the troughs but fast enough to not let the contaminants, referred to as “gangue,” to do so.

In cases where distinct and visible gold particles still fail to appear despite undergoing gravity concentration (i.e. in instances where original gold concentration is very low), froth flotation may be employed. Froth flotation works by selectively segregating materials in terms of their hydrophobicity (i.e. their property to repel or be repelled by water molecules). Froth flotation is generally used when a high concentration of sulfide minerals are found in the ores.

During this process, surfactants and wetting agents are added into the ore to increase the difference in hydrophobicity.

Froth flotation is usually directly followed by cyanidation.

However, in cases where cyanidation is seen as too environmentally taxing, or where the ore is naturally resistant to the process, roasting or wet-pressure oxidation may be applied before cyanidation. Roasting or wet-pressure oxidation works to remove sulfides that may have associated with gold that may prevent gold from being dissolved during cyanidation.

The following is the reaction during this process: Au2S+3 O2→2 Au2O+2 SO2

Leaching involves dissolving gold with cyanide for later precipitation, which is essential in order to ensure that even microscopic amounts of gold can still be recovered. Cyanidation, also known as the “cyanide process” or the “MacArthur–Forrest process,” is the industry standard for leeching.

It uses the following reaction: 4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH

There are several methods for precipitating gold from the cyanide solution. The most economical of which is the carbon-in-pulp process, which involves passing the leeched pulp through several tanks of activated carbon. The carbon acts as a trap for Na[Au(CN)2], which is then removed from the carbon using high temperatures and pH. Afterwards, the resulting solution is passed through electroextraction (or electrowinning) cells.

As part of the refining stage, this procedure uses electrolysis to allow gold to deposit in the cathode area.