Cyanide leaching

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WARNING!

Cyanide is one of the more dangerous processes in refining. One mistake can be your last.

Dangers of cyanide

Cyanide is a process to leach low grade ores or gold plated base metals effectively.

Contents

Safety

Read this thread on GRF for some good advice.

Recovering precious metal

The cyanide complex is quite strong but there are a few tried methods that are used.

pH range

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.

See also

Patents

Method of desorbing gold from activated carbon US3920403

References

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