Calcine from the bed of the first, reducing roaster is fed to the second, oxidizing, roaster together with coarse particles from the exit gas cyclone.
Temperatures below 650 [degrees] C are avoided because the conversion of S[O.sub.2] to S[O.sub.3] is favored at lower temperatures, and the S[O.sub.3] so formed combines with calcine to form sulphates, which are sticky and tend to hang-up in the calcine overflow.
Calcine overflows from the first roaster into a standpipe from which it is air-lifted into the second roaster to join material arriving in the underflow of the first roaster cyclone.
Mass loss on roasting is around 25% so 3,000 kg/hr calcine is produced.
An intensive investigation revealed that appreciable quantities of gold were appearing in the calcine quench water, water that had been run to waste when the calcine was thickened prior to cyanidation.
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%.
