Lixiviation with Reactives
Different To Cyanide

The search for an alternative reactive to the cyanidation continues calling the interest of the investigators, nevertheless, the highest cost of them frequently does not compare with the advantages which are obtained by them. Normally, the advantage of a rapid cinetic is not as important as an increase of the recuperation.

In continuation, we will briefly analyze the principal alternatives:

Thiourea – CS (NH2)
the alternative of thiourea has been studied very frequently, due to its cynetic of its very fast lixiviation (an order of magnitude superior to the one of the cyanidation). The disadvantages are the minor recuperations caused by its decomposition, which also implies high consumptions of reactives and cinetic difficulties for passivity of the surface. For the estabilization there has been proposed the use of SO2, urea and other reactives. As an oxidant Fe2 (SO4)3 and the pH is adjusted between 1 and 1.5 with H2SO4 is used. Among the commercial operations known, we can mention the operation of the New England Antimony Mines in Australia and the Jamestown project of the Sonora Gold Corporation in the United States. The recuperation of gold and of silver of the rich solutions has not been very much mentioned in literature.

We know that the absorption in carbon or in strongly acidic interchangeable pitches can be used. The carried carbon can be washed with hot water and afterwards be processed through the conventional elusion with cyanide, and immediately after, the electrodepositation. Direct electrodepositation, cementation and reduction with gasses can also be used.

A successful industrial operation is that of Vielle Montagne in Viviez, France, which uses thiourea for treating residues with silver of a refinery of zinc which have been concentrated by flotation. The cementation is used with aluminum in powder.

The most recent investigations refer to a combined process of lixiviation in heap leaching using thiourea and bacteria for treating the minerals of low refractory laws. First, the bacterial lixiviation of the sulphurs has been performed, and, after, the acomplexing with thiourea in the presence of ferric sulphate. The thiourea is an excellent nutrient for the bacteria. The initial tests have been performed by Mountain States R & D Int. Inc.