The Process of Toasting and the Recuperation of Valuable Elements

When the gold is found intimately associated to pyrritical minerals and the fine grinder of the mineral does not increase the extraction of gold in a desired range, the toasting of the mineral before the cyanidation is essential for liberating the gold from these minerals (2).

In effect, in most cases of auriferous pyrrites and arsenopyrrites, the gold is found finely disseminated or forming inclusions within the crystal structure of the mineral. This aspect represents a great difficulty in the treatment of such minerals for the conventional techniques as is the case of the cyanidation and amalgamation.

Therefore, the toasting besides allowing the elimination of nocive impurities, the ones which have adverse influences in the subsequent metallurgical processes, must conduct to increase the recuperation of the valuable elements. In this sense, one must avoid the partial synthetization or the over oxydation which conduces to the re-crystallization of the material or the forming of ferrytes and sylicates, phenomenonos which encapsulate the valuable metals in the material and which logically will overflow negatively in the future treatment.

The selective toasting contributes, besides, to the forming of porosity in the grain of the calcinate, allowing the expansion of the lixiviant solutions.

It can be that the selective toasting will not be enough for letting the gold free or it might not have the adequate porosity; therefore, it will be necessary to effectuate a second stage of toasting. This will be of the oxydant type in which the solid reactors, are the pyrrotite and pyrrite resulting from the selective toasting. The gas will be composed of O2 and SO2, having as a product according to the conditions of toasting: Fe3O4, Fe2O3, FeSO4 and Fe2 (SO4) 3.

The objective of the second stage of toasting must be the forming of hematite which is not prejudicial to the process of cyanidation and has the advantage of increasing the porosity of the grain of mortar.

The dissolution of gold of the mortars of toastings is by cyanidation, being also feasible the process of thiourea. In the cyandiation the objectives are the maximum of extraction of the gold and the minimum consumption of reactives: CaO and NaCN. For this it will be necessary to determine the optimal conditions of operation, especially:

• The acid washing of the mortar.
• The violent aeration previous to the cyanidation.
• The granulometry of the mortar.
• The concentration of NaCN in the lixiviant solution.
• The alkalinity of the system.
• The consumption of oxidant reactives.

A diagram of the sequence of operations to be performed with concentrates of pyrrites and arsenopyrrites is shown.

Finally, the recuperation of the gold and the silver of the solutions are effectuated by procedures already known such as:

• Precipitation with zinc or aluminum dust.
• Absorption with active carbon.
• Electrodeposition.