Cyanidation

Is the method of the treatment of the minerals of gold most largely used.

Focused at the beginnings of the century has had continuous bettering, both in the level of the process as with its putting into action. It is then a well established and simple method, with the capacity of working with minerals of different types, through certain specific adaptations.

General Conditions of Cyanidation
The chemical of the Cyanidation of gold has been exposed and discussed by Finkelstein and Dorr and Bosqui.

The global equation generally admitted is:

• 4Au + 8CN O2+H2O
• 4Au + (CN)2 + OH

This equation places into evidence the necessity of the presence of oxygen so that the reaction will take place. The oxygen is thickened in the solution by a simple adding of air.

The attack is conducted in an alkaline ambience (tH from 9 to 11,7) to avoid the transformation of the cyanides by hydrolysis in cyanotic acids HCN gaseous; this reaction which is produced in an acid surrounding, conduces to a loss of reactive but also to the emanation of an extremely toxic gas.

The alkalinity of the surroundings is controlled by the addition of lime.

The glasswort, most expensive, is sometimes used for minimizing the problems of incrustation due to the precipitation of the calcium sulphate.

The consumption of lime strongly varies according to the minerals. It is understood between 1/kg/t mineral treated in some kilograms.

The cyanide used nowadays is the sodium cyanide or more rarely, the one of calcium. The habitual consumptions go from less than 200 g/t to 6 kilograms/t in the more difficult cases.

The concentrate in free cyanide in the solution of attack varies from some ppm to 500 ppm Na SN.

The time of attack is dependant of the treated mineral and of the dimension of the gold nuggets. Industrially, the times of remaining are generally understood between the sixteen and forty hours, but the longer times can, according to the occasion be used.

The recuperation of the particles of gross gold by gravimetric, allows to reduce the times of remaining and the losses in metal in the residues of the Cyanidation. The attack is conducted in the environmental temperature; this parameter has no influence more than above the kinetic of extraction.

The kinetic of extraction and the consumptions in reactive are greatly influenced by the mineralogy of the treated mineral.

 In some cases, the general conditions mentioned previously, are not convenient more and the cost of some parameters of the process is not economical. In numerous cases one pre-treatment allows to restitute the mineral or to the treated pulp a behavior towards the Cyanidation.