The Process of Toasting

The process of toasting and the elimination of the as – sb of the concentrates of pyrrite & arsenopyrrites

The elimination of contaminants through toasting is based on the fact that the high temperature volatilizes selectively some of the elements or compounds, separating these from the system through the gasses.

These elements can be the sulphur, the mercury, arsenic, antimonious, bismute, lead, tellure,  talio, germanium, selenium, etc. And their compounds such as sulphurs, arseniures and oxydes.

For the case of the concentrates of pyrrites and arsenopyrrites and other complex sulphurs, results to be especially conventient for the elimination of the As, Sb, etc, under controlled conditions of reductory atmosphere and optimal temperature, in which the sulphur complex minerals become unstable and the thermic decomposition occurs, successively and selectively, according to its level of internal energies, giving place to the formation of stable sulphurs and the volatization of the sulphurs of arsenic, antimonious and elemental sulphurs.

The main chemical reactions for reduction atmosphere and temperature between 500°C to 700°C, are: (1)

• 8 FeAsS – A s4S4(g) +  FeAs + 4 FeS    (1)

In the presence of the free sulphur, the iron arsenic (Fe, As ) is decomposed in:

• 4 FeAs + 4 S2 (g) – As 4 S 4 (g) + 4 FeS    (2)
• in the same manner, the decomposition of the pyrrite and the presence of the S2 (g) is due to 2 Fe S2 – 2 Fe S + S2 (g)     (3).

Such relations result from the proper handling of 4 fundamental parameters:

1. temperature of toasting.
2. reductive atmosphere.
3. time of residence, and
4. velocity of heating.

The adequate reactor through which it is possible to control such parameters come to be the Edwards oven developed in Australia and the oven of Multiple Heat NESA in Europe and in the Untied States.