Treatment of Refractory Minerals

Presently, the treatment of refractory minerals to the cyanidation has reached great importance due to the increase of occurrence of this type of minerals, especially those constituted by inclusions of submicroscopic gold or silver in sulphurs such as pyrrite and arsenopyrrite.

Other types of refractory minerals are some telurures of gold, minerals with carbonaceous material of organic origin or with clays capable of reabsorbing the already cyanized gold.

In the case of the gold in sulphurs, the principal processes recently incorporated to the industrial application, alternatives to the more traditional process of toasted-cyanidation (which has the disadvantage of the production of SO2) are:

• Oxidation by Pressure
  Even though this idea goes back to the decade of the fifties, it is not until the eighties when it enters into operation in the first commercial plant; we are talking about the McLaughlin plant of the Homestake (16). The oxidation by pressure is very effective for liberating the gold associated to the sulphurs (the recuperation of gold increases in extractions without treatment of the order of 5-75% to extractions of 85-98%). Nevertheless, the extraction of silver is affected in an adverse form by the formation of argentojarosite, which is refractory to the cyanidation. Recently, a post-treatment of high temperature which transforms it into hematite has been developed.
• Cyanidation by Pressure
  It is also an old idea taken into practice recently. It consists in cyanidating in a reactor by pressure, which will increase the velocity seven times. The industrial development has been performed by the Lurgi firm through theuse of reactor-tubing and it is used in a concentrate of estibnite of South Africa.