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Gold/Mining/Energy : ECHARTERS

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To: Al Cern who wrote (2120)	1/4/1998 1:40:00 PM
From: E. Charters	Read Replies (1) of 3744

CIL stands for carbon-in-leach. It is just a cyanide recovery process dressed up with a different recovery process of the solution gold from the cyanide. Its advantage, so the advertising goes, it that it can use less cyanide and recover gold from more refractory ores that tend to make metal cyanates other than gold before the gold gets a chance to dissolve. It cannot get at gold which is not releases from its matrix or exposed to solution. Only an oxidizing process that deals with the elmental sulphide by destroying it can help you there. Such processes are vat leaching, bacterial leaching and autoclave. I am not that big a fan of CIP and CIL processes. I think the mills tend to allow too little explosure time and send too much out to tails. The average mill does not know even approximately what is really in the tails and tries to cover up evidence of loss. Thoughput is their god. If they don't recover anything they tend to blame the underground, grade mixing and dilution. The average retention time of the Holt Mcdermott mill in Kirkland Lake-Matheson, which is CIL, was 16 hours. Most ores require 24 to 30 hours to get all the gold. This might explain why they are shut down now. I believe their recovery did not exceed 75%. If you take a pan out to the tailing pond I think you are going to see gold and auriferous pyrite grains. Most of the mills in Canada overstated their recovery. Some by a wide margin. The Toburn, in Kirkland Lake, ran at a head grade of .50 ounces per ton. Their stated recovery was .96%. That means that there should be .02 ounces per ton in the tailings. In fact the tailings run from .10 ounces per ton to perhaps .06 at the lowest. A company called Eastmaque ran a successful recovery operation on the Toburn tailings for a few years and that is what they recovered and assayed by bulk assay. So between 80% and 85% was the better figure for the Toburn. I think you will find that this is unbiquitous across the industry and recovery percentages are not to be trusted, then or now. Certainly the mills that are trying to recover refractory gold will not get higher than the old mills using the prefectly acceptable Merril Crowe cyanide leach process on free milling ores. echarter@vianet.on.ca The Canadian Mining Newsletter

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