The key on raosting of the ore at Syama is simple. Initially Syama was a oxide plant. Once it hit sulphides a roaster was added. At 4% gold it was thought the process would be energy neutral or positive (sulphides react with oxygen to process heat). BUT the sulphur content was too low and the process did not produce heat and thus required additional energy in the form of expensive diesel.
Randgold added some concentration to increase sulfur content and make the process energy equation better. This was still not sufficient.
In the new process, RSG will use a full float to produce a concentrate which will then be roasted. This will have a much higher sulfur content and infact produce excess heat - this is known as the company was looking to use the excess heat as power - something that ended up not being cost effective. Thus diesel will not be required at all except for normal power requirements.
An additional problem is carbon in the ore which destroys cyanide and reduces recovery - thus adding further to costs. At at sufficiently high roast temperature the carbon is burnt off which is a further positive for the concentrate situation. Carbon may also be able to be removed during the float process itself as a further backup.
Further background details can be seen in this article:
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