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Cyanide is one of the more dangerous processes in refining. One mistake can be your last.
Cyanide is a process to leach low grade ores or gold plated base metals effectively.
Recovering precious metal
The cyanide complex is quite strong but there are a few tried methods that are used.
- Electrowinning, for example this cylindrical cell.
- Cementing with zinc, called the Merrill–Crowe process
- Absorption on activated carbon, also known as Carbon in pulp (CIP).
The cyanide leach needs close controlled conditions on pH, too low and there is a risk of creating hydrogen cyanide gas. Too high and the effectiveness goes down.
At pH 9.5, about half of the cyanide is in the form of dissolved HCN gas, half as CN- ions capable of dissolve gold. Raising the pH turns more HCN into CN--ions, increasing the rate of gold dissolving. At pH 11 practically all cyanide is in the form of CN-. Raising the pH above 11 will not increase the gold dissolving speed but some metal hydroxides will be going into solution and creating complexes with the cyanide ion and that will lower the effectiveness of the cyanide leach.
Most industrial cyanide leaches is run at pH 11 to have a good margin against creating hydrogen cyanide gas while keeping a good dissolving rate of gold.
- Eco-Goldex, a variation on the cyanide process.
- THE MANAGEMENT OF CYANIDE IN GOLD EXTRACTION
- GRF : The cyanide handbook
- GRF : GSP on cyanide leaching of plated material
- GRF : 4Metals recommendation on cyanide destruction and testing
- GRF : Discussion about a real case cyanide leaching
- GRF : Deano on pH levels
- GRF : Deano demonstrates a small vat leaching system
- GRF : GSP on safety while working with cyanide
- GRF : Thread about a cylindrical electrowinning cell
- LAPPEENRANTA UNIVERSITY OF TECHNOLOGY : Comparison of Cyanid and Thiosulphate Leaching for Gold Production (pdf)
- Google books : The Chemistry of Gold Extraction, chapter 6.1, Cyanidation, page 233-271.