GOLD ORE PROCESSING

Gold has been recovered from its ores in many ways throughout the centuries. These ways range from the rocker or long tom of the California Forty-Niner and the noisy stamp mill of the 19th century to modern methods of leaching with cyanide.
Any method of treating ores of gold must take advantage of the natural characteristics of the metal. Cyanide solution which is not like the majority of other liquids is able to dissolve gold, and this way, is used in the processing of gold ore. Cyanide slowly attacks fine particles of gold and ultimately dissolves them when in solution and in the presence of oxygen. It is strange, but fortunate due to the fact that cyanide is extremely toxic, that a weak cyanide solution attacks the gold particles faster than a strong solution.
It is necessary that the gold first be freed from the gangue rock that is worthless which surrounds it for the cyanide to attack the gold particles because cyanide will not attack or dissolve most other minerals.
In general, the process of cyanide is very efficient. A gold ore that contains less than one gram of gold per tonne can in some cases (and depending on the price of gold), be treated profitably. A modern cyanide mill extracts or recovers 95% to 98% of the gold which is in the ore.
Lime and cyanide are added to the ore pulp in the grinding circuit of a cyanide mill. The lime has a variety of functions: it protects the cyanide from being destroyed by chemicals that occur naturally called cyanicides and improves the settlement rate of the pulp in the thickening stage.
The actual dissolution of the gold (cyanidation) begins in the grinding step. Cyanide and lime solutions are introduced here, where newly liberated gold particles are being constantly polished by the grinding action and the friction is what heats the solutions. From 30% to 70% of the gold may be dissolved during the grinding process depending on the ore and the fineness of the grind.

Additional time is required to place the balance of the liberated gold into solution. This is done by pumping the pulp that bears gold to a large amount of mixing tanks, which are known as agitators. Here the pulp is aerated either by compressed air or mechanically, or by a combination of both, for a predetermined period of time. This varies anywhere from 24 to 48 hours.
The 1980s saw a quick expansion in gold production from low-grade oxide deposits around the world. That expansion would not have been able to occur without the development of a new, low-cost method of recovering the gold. That process is called heap leaching.
Heap leaching avoids most of the previous steps, and does not even require that a mill be built, making it a very inexpensive method of processing ore. Here, broken ore is heaped onto a thick polyethylene sheet, which is called a liner, and then dilute cyanide solution is sprinkled on top of the heap. The gold is dissolved as the solution trickles down through the ore. Before the heap is constructed, the polyethylene liner is laid down in a certain way that the cyanide solution will drain to a central point. From here the gold-laden solution is channeled into a pond which is made by man.