Cyanidation in Heap Leaching of Gold & Silver

A New Tool for Small Mining
This chapter was written with the intention of giving a vision of the developments reached in the lixiviation in heap leaching of gold-silver minerals, and the recuperation of these precious metals from the strong solutions through techniques of adsorption with activated carbon and electro-obtaining, including a procedure of separation of the silver with sulphur which reduces the inventory of carbon required.

The mineralogical conditions described and the necessary studies for determining the feasibility of treatment of a mineral, some factors of design and techniques used in commercial operations, besides the aspects of security and of environmental protection related with the handling and discarding of the cyanide. Finally, data related to the economy of the process are also given, together with an estimate of costs for a size of operation representative of the reality of the nation.

The lixiviation in heap leaching is a lixiviation by percolation of the stocked up mineral over an impermeable surface, prepared for collecting the sollutions. Even though this concept is pretty antique, being extensively applied for lixiviating minerals of copper and of uranium, has found a comparatively recent use in the cyanidation of the minerals of gold and of silver.

Even though this concept has been conceived especially for exploiting large auriferous deposits of very low law, the cyanidation in heap leaching, due to its minor costs of capital and of operation, is also attractive for the development of small deposits. Its great operational flexibility allows it to include short treatments (weeks) with the fine crushed mineral, or very mush prolongued (months, even up to years) with thick material, to the size produced in the mine.

In general lines, the fractured, or crushed mineral is placed on top of an impermeable floor forming a heap leaching of a determined height, over which one spreads out the diluted sollution of cyanide which percolates through the bed dissolving the precious metals finely disseminated. The sollution of the lixiviation, enriched in gold and silver, is collected on top of the impermeable floor which, disposed in a slightly inclined form, makes it flow towards a well of storage. From this well, the sollution is alimented to a series of reservoirs or collumns with activated carbon where these elements are adsorbed, returning the sterile effluent to another well for its adjusting and posterior recirculation to the heap leaching of the mineral. Alternatively, the strong sollution can also be treated by the Merril-Crowe method of precipitation.