In the case of many complex gold and silver ores roasting before cyanidation is essential if satisfactory extraction of the precious metals is to be obtained. In such cases no practical amount of grinding or prolonged contacting of the raw ore with cyanide solution will affect more than a certain low extraction; in other cases, while the extractions may be acceptable, the consumption of cyanide is prohibitive, and roasting or, occasionally, pyritic smelting of the material is the only alternative. Roasting, which is today almost always carried out following concentration of the values into a small bulk, must be used if it becomes necessary to decompose the minerals with which the gold is associated in order to expose it to solvent action. Many of the cyanicides in the raw ore are broken up and rendered harmless by roasting, but the calcine may contain new compounds that must be removed by water or acid washing before cyanide treatment can be successfully carried out.
There were instances, where at one time the whole of the ore was roasted before cyanidation, but this practice has been for the most part discontinued on account of the large plant and high overall costs involved. With ….Read more