Basic Chemistry and ore characteristics required for heap leaching.
The bas
ic principle of cyanidation process is that cyanide ions form very stable complexes with gold, silver, and other metals. Weak alkaline cyanide solutions have a preferential dissolving action on the gold and silver contained in an ore. The ionic reaction, referred as Elsener’s equation, that is traditionally accepted as the primary pathway for gold dissolution by cyanide is:
4Au + 8CN- + O2 + H2O = 4Au(CN)2- + 4OH- (1)
More recent research on the mechanism of dissolution indicates that the reaction proceeds in two stages. Most of the gold dissolves by the reaction:
2Au + 4CN- + O2 + 2H2O = 2Au(CN)2- + H2O2 + 2OH- (2)
With a small, but significant portion of the gold dissolving via reaction (1). Gold dissolution rate is dependent on the concentration of sodium cyanide and the alkalinity of the solution, the optimum pH being 10.3.
For efficient leaching (i.e. in order for the reactions defined above to occur rapidly), the gold should be as free, fine size, clean particles in an ore that contains no cyanicides or impurities that might destroy cyanide or otherwise inhibit the dissolution reaction. An adequate supply of dissolved oxygen must be present in the cyanide solution throughout the reaction period. Only certain kinds of ore have the general characteristic required. The major ore types suitable for cyanidation and heap leaching cyanidation are:
- Oxidized disseminated ores,
- Sulfide ores where the precious metals are not intimately associated with the sulfide minerals, and
- Certain lode or placer materials which contain fine particles of gold or particles with a high surface area to weight ratio.
More specific characteristics required of ores if they are to be amenable to heap leaching process include:
- Ore with contained precious metals leachable by cyanide;
- Ore with extremely small or flattened gold particles;
- Ore occurring in porous and permeable host rock (precious metals in ores of low porosity can be liberated by fracturing and crushing);
- Ore free of carbonaceous or pre-robbing material which causes premature adsorption or precipitation of dissolve values;
- Ore relatively free of cyanicides that consumes cyanide or interfere with the dissolution reactions;
- Ore relatively free of fines or clays that impede uniform solution percolation agglomeration pretreatment is required if excessive fines or clays are contained in the feed); and
- Ore free of acid forming constituents that cause high cyanide or base consumption.
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