At lower temperatures, roasting is slow and there is a danger of losing autogeneous roast conditions if, for example, the sulphur content of the feed drops; while at temperatures over 700 [degrees] C a less porous calcine is produced.
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.
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.
