Unexpectedly, some of this arsenic appeared in the precipitator, but much reported instead to the calcine product.
The air flow rate to the first roaster is set within narrow bounds to ensure reducing conditions prevail but some latitude is available because the sulphur content of the calcine leaving the first roaster bed may vary between 3 and 4%.
Mathematical correlations from 13 sets of roaster variables found that gold dissolution obtainable from the second-stage bed overflow rose with the increase in ferric-iron content of the calcine, increased with decrease in the temperature of the first roaster bed, and increased with decrease in the gold content of the calcine.
Gold dissolution obtainable by cyaniding the calcine collected in the electrostatic dust precipitator increased with gold grade, increased with decrease in temperature of the first-stage roaster bed, and decreased with increase in the roaster solids feed rate.
The above data indicate that carbon is concentrated in the calcine dust collected by the EDP.
However, the challenge remains for the development of an economic process for the recovery of the considerable amounts of gold that are still locked in the calcine after fine milling and cyanidation.
PHOTO : At the New Consort gold mine, left to right, roaster house, calcine precipitator