Certain articles appearing in the mainstream American press in recent years will have raised a few eyebrows in metallurgical circles. Namely, references to a new process developed by US researchers for the extraction of gold from its ores. The new process? No less than cyanidation.
The cyanidation process was patented in the 1880s, just in time to save the new gold mines on the Witwatersrand, South Africa.
The misinformed reporters were half right though. What researchers at the United States Bureau of Mines (USBM) did at their Reno, Nevada, laboratory, that attracted the media, was to devise a revolutionary new application for cyanidation. The then new process made the extraction of gold from low-grade orebodies profitable. It has made Nevada one of the major gold fields of the world and the process — heap leaching — is well on the way to reviving derelict gold camps across the globe.
It is, as you know, simplicity itself. No mill is needed and ore grading as little as 0.02 oz. per ton can be treated profitably. On the other hand, not all ores are amenable and gold recoveries are low, generally in the 65% range.
At American Barrick’s Goldstrike, the ore destined for the heap-leach pad is crushed to 1.5 inches and piled in layers 50 ft. thick to a height of 200 ft.
A given leach pad will eventually contain upwards of a million tons and its foundation is designed for the specific ground conditions at the site.
Goldstrike’s foundation comprises 12 inches of compacted soil, an 80-mil synthetic, waterproof liner and a two-ft. layer of screened sand. The last is to protect the liner during loading of the pad. Leak detection and cyanide collection systems are a vital part of the design.
The lowest layer of crushed ore is cyanide-sprayed over a period of four months, the second layer likewise and so on for the four layers.
Because there is a maximum volume of solution the gold precipitation plant can handle per day, and, because freshly-arrived ore is awaiting its first spraying, some parts of the pad are at rest while others are actively being leached. The leach and rest cycle is ideal for cyanidation. Oxygen is necessary for cyanidation, and the rest’ part of the cycle gives oxygen the opportunity to penetrate the pile.
Gold-bearing cyanide solution (“preg”) seeps from the base of the pile and is directed to the precipitation plant. Here the gold is adsorbed on carbon (coconut shell charcoal) and the “barren” solution is returned to the leach pads. When the carbon is loaded, it is stripped of its gold using hot cyanide/caustic solution. The enriched, gold-bearing solution is electrolysed and the metal deposited on stainless steel cathodes. The powdery, black deposit of metallic gold is vibrated from the cathodes and furnaced into bullion.
When active leaching ceases, the rock pile may stand for a number of years before it is de-toxified. Then it will be given a series of water washes to remove the bulk of the remaining cyanide. (The wash solutions will probably carry sufficient gold to make recovery worthwhile). Remaining traces of cyanide will be destroyed by hydrogen peroxide, or sulphur dioxide. Landscaping will follow. In time, the contoured pile will merge into the landscape.
Heap leaching produced 322,000 oz. gold, or 59%, of Goldstrike’s 1991 production. Leaching will account for only a small amount of future production and the final leach pad is now under construction. The bulk of the ore mined from now on will be higher-grade sulphide.
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