- Extraction

Generally once metallic gold or a gold alloy has been produced it can be sold to a precious metals refinery for a price quite close to its gold content. Hence the easiest mining is for "free" gold or nuggets.

Gravity separation is the simplest technique, and was used by early miners typically on placer deposits in river beds. Many mines have some free gold and often there is a small gravity circuit that uses gold's great weight relative to the rock in which it is found to extract it early in the treatment plant. This may reduce the size and thus the cost of the rest of the plant.

Cyanide leaching is the mainstay of the gold industry. Cyanide is responsible for the majority of gold extracted today and gold treatment plants have the cyanide antidote available for any possible accidents.

To mill or not to mill

By the 1980s large scale mining equipment was driving down mining costs and lower grade ores were becoming more economical. About this time it was discovered that the heavy upfront cost of a conventional treatment plant could be avoided for certain types of oxide ores.

In oxidized ore, the gold was not trapped inside other minerals and if the ore was reasonably porous and free of other oxidized metals that could interfere with gold recovery, the ore could simply be placed on a waterproof (normally plastic) pad and a weak cyanide solution could be made to percolate though the untreated rock and extract the gold. Another development about that time was the use of activated carbon to collect gold from solution.

The bottom line was that for ores that offered rapid gold recoveries using these methods, the financial risks of developing this type of mine fell significantly.

After cyanide's effectiveness for capturing gold into solution was discovered in the late 19th century, gold extraction was revolutionized and recovery efficiencies greatly improved. Once in solution, the green colour "pregnant liquor" is separated from remaining rock flour and gold is normally precipitated from this "pregnant liquor" by adding zinc powder. While cyanide "likes" bonding with gold, it "likes" zinc more.

This is called the Merrill Crowe or MacArthur-Forrest Process, and when it was introduced in the latter half of the 19th century it replaced other methods such as the previous amalgamation process, which collected gold using mercury.

Unfortunately, the low tech mercury amalgamation process remains popular among small-scale and illegal miners, with negative health effects on those using the mercury and those impacted by spillage. More recently, activated carbon has taken on a major role in capturing gold from the cyanide solution.

Unfortunately, cyanide also has an affinity for copper, and therefore if there is copper in the ore this normally prevents, or at least greatly complicates, the use of cyanide to extract gold.

The simple solution to the presence of copper in mixed sulphide/oxide ores is to use a process called flotation, where, after the addition of special chemicals, small sulphide particles can be made to attach to air bubbles. The ground ore is put in flotation cells where air is injected, and most sulphide particles then attach to bubbles and float as a froth to the surface where they are collected, dried, and shipped to a smelter for gold recovery.

Better recoveries can sometimes be achieved with a more complicated plant layout, where the ore is first treated to extract free gold using gravity methods. Then the ore is put through flotation cells to collect sulphides mixed with gold, and finally with most of the copper cleansed from the system, a cyanide leach can be used.

Refractory ores pose extra challenges, add extra costs, and typically must offer a higher incentive in the form of better grades to be economic. The essential problem with refractory ore is that the gold is intimately mixed with waste materials, sometimes at an atomic rather than at a physical level. Metallurgists have an arsenal of methods to try on these ores. In order to expose gold to the cyanide solution, the ore would have to be ground very fine (expensive) or oxidized in an autoclave.

Unfortunately it's often not commercially feasible to grind fine enough to expose the gold.

Typically when faced with a refractory ore a metallurgist will subject bulk samples to oxidation via autoclave, and roaster, bio oxidation or perhaps ultra fine grinding, just in case the oxidation step can be avoided. An autoclave is essentially a pressure cooker for ground rock. It's a large pressurized vessel where the ore is cooked in the presence of oxygen.

The throughput capacity of the plant is often related to the sulphur content of the ore input, since the sulphur is the "fuel" that is chemically burned. This can be a complicating issue for autoclave operators because in order to have steady operations, management must manage the sulphur input, and this can sometimes affect gold throughput. The oxidized ore exits though a pressure relief valve and then passes to a conventional CIL leach plant.

A roaster is another method of breaking down refractory ores. This is a relatively simple device where sulphides are "burnt"; the downside is that the exhaust gasses must pass through extensive gas cleaning facilities to generate an environmentally acceptable exhaust stream. Of note, under current rules the gas treatment facilities can easily cost more than the roaster.

Additionally, the key gaseous component will be sulphur dioxide. This will normally be converted into sulphuric acid; however with more and more roasters producing acid this product has little value. Again the oxidized ore passes to a normal leach section to extract gold.

The use of bacteria that thrive in extreme conditions is a relatively new development in the gold sector although it has been used for longer in the copper sector and has led to the new bio-leach treatment route. The bacterium, thiobacillus ferrooxidans, breaks down iron sulphides to sustain itself. These bugs are generated and are injected into large tanks containing the ground ore. After a multi hour period of treatment the ore is normally sufficiently oxidized to pass to a leach section.

Sometimes while a gold ore is considered refractory using normal techniques, acceptable gold recovery can be achieved by using extra fine grinding methods.