Gold Extraction

General Considerations
The process of cyanidation is one of the most used for the extraction of gold starting from those minerals or concentrates. It is based in which the diluted sollutions of cyanide sodium or potassium in the middle of airing, dissolve the particles of metallic gold, of prefference to the other materials which are present in the minerals to be treated.

The mechanism of reaction is emminently electrochemical being a form of corrosion.

The cyanidation of the gold in silicose gangue, without other nocive compounds, does not demand major cares. On the meantime, with the growing search of precious metals, the treatment of complex minerals has become ever the more frequent. The compounds which are prejudicial to the process, now present are called cyanicides and influence, consuming cyanide, in parallel reactions to the dissolution of gold, not allowing the proper reaction, in the form of tranquilizing the surface of contact, stopping the dissolution. 

The forms of lixiviation generally used in the cyanidation are:

Filtering in tanks or in piles and agitation.

In the first two alternatives, care is needed with the intention of avoiding the granulometric segregation, the formation of preferential canals and the compacting of minerals. In the filtering in piles, the topography of the terrain is conditioning factor of the economic viability and the investment in the usina and something less, ideal for minerals of low tenor. In the filtering in tanks, the lasting of the operation is, in general, in three days and does not adapt to the treatment of great tonnelages, fixing the level of recuperation restricted to the maximum of 80%. (10)

The laboratory tests to verify the lixiviety of the mineral were performed by agitation. Industrially, tanks with mechanical agitators or pachuca tanks (with agitation promoted by the agitation of air) are used. (11)

In this case, the parameters of cyanidation involved are: granulometry of the material, concentration of cyanide in the solution, liquid/solid relation of pulp, aeration used, temperature, time of reaction, velocity of agitation and protective alkalinity of the environment.

This last one is of extreme importance, due to the fact that it avoids the decomposition of cyanide in cyanhydric gas, which is poisonous. Generally, the pH is corrected through the addition of CaO, performed during the grinding of the material, under the form of lime milk.

The concentration of the the cyanide of sodium in the solution is significative variable for the extraction of the silver. The concentration used for obtaining a high recuperation are of 1 to 4 grams 1-1 (8) higher than the ones normally used for gold (from 0.1 to 1.5 g. 1-1). (8) the percentage of solids in the pulp varies from 20% to 50%.

The temperature accelerates the reaction of dissolution of the metals of interest, being ideal that of 85° C (12), but resluting in losses of cyanide by decomposition or evaporation. The time of reaction generally from 10 to 72 hours is related to the temperature used.

In the cases in which the minerals of gold are of nature arseno-pyrritic a stage of ustullation previous to the lixiviation becomes necessary. The objectives to reach are: elimination of the arsenic and antimonious existant, due to the fact that the corrosive effect of the process; forming of a product with a minimum of cyanided substances; the obtaining of a porous product which facilitates the dissolution of the metals of interest.

The process is performed, in general, in an oven of fluidized bed and the main variables involved are: granulometric distribution of the material, temperature, evacuation of gas and partial pressure of the gasses in the reactor.

When the gold is found included within the grain of pyrrite and the level of liberation it is under 20 mm, it is made more interesting to ustullate the material than to grind it until it reaches the necessary level.