Gold Ore Processing
Throughout the centuries, gold has been recovered from its ores in many ways. These 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 gold ores must take advantage of the natural characteristics of the metal. Cyanide solution, unlike most other liquids, is able to dissolve gold, and thus, is used in the processing of gold ore. When in solution (and in the presence of oxygen), cyanide slowly attacks fine particles of gold and ultimately dissolves them. It is strange, but fortunate (because cyanide is extremely toxic), that a weak cyanide solution attacks the gold particles faster than a strong solutions.
For the cyanide to attack the gold particles, it is necessary that the gold first be liberated from the worthless gangue rock which surrounds it because cyanide will not attack or dissolve most other minerals.
Overall, the cyanide process is very efficient. A gold ore containing less than one gram of gold per ton can, in some cases (and depending on the gold price), be profitably treated. A modern cyanide mill recovers or extracts 95% to 98% of the gold in the ore.
In a cyanide mill, lime and cyanide are added to the ore pulp in the grinding circuit. The lime has several functions: it protects the cyanide from being destroyed by naturally occurring chemicals called cyanicides and improves the settlement rate of the pulp in the thickening stage.
Cyanidation (the actual dissolution of the gold) begins in the grinding step. Cyanide and lime solutions are introduced here, where newly liberated gold particles are constantly being polished by the grinding action and the solutions are heated by the friction. Depending on the ore and fineness of grind, from 30% to 70% of the gold may be dissolved during the grinding process.
Additional time is required to place the balance of the liberated gold into solution. This is done by pumping the gold-bearing pulp to a number of mixing tanks, known as agitators. Here the pulp is aerated either mechanically or by compressed air, or by a combination of both, for a predetermined period of time. This varies any where from 24 to 48 hours.
The 1980s saw a rapid expansion in gold production from low-grade oxide deposits around the world. That expansion could not have occurred 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 above steps, and does not even require that a mill be built, making it a very low-cost method of processing ore. Here, broken ore is heaped onto a thick polyethylene sheet, called a liner, and then dilute cyanide solution is sprinkled on top of the heap. As the solution trickles down through the ore, the gold is dissolved. Before the heap is constructed, the polyethylene liner is laid down in such a way that the cyanide solution will drain to a central point. From here the gold-laden solution is channeled into a man-made pond.
One downside of heap leaching is lower recovery — just 65% to 85% of the gold in the ore ends up in the gold bars a heap-leach mine produces.
Extracting Gold Out of Solution: Traditionally, recovering the gold from the cyanide solution was achieved by separating the gold-laden, or pregnant, solution from the barren solids present and then precipitating the gold.
The traditional approach is called the Merrill-Crowe method. The first step is to move the pulp from the agitators to one or more thickeners — large, shallow tanks. The solution flows over the top of the tank and is collected in a launder around the tank’s perimeter, while the worthless rock particle sink to the bottom and are slowly raked to the center by mechanical arms which operate continuously. The material is discharged through a pipe at the bottom of the tank but it contains too much valuable material to be discarded, so is filtered to recover additional gold.