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- Feb 25, 2007
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As Chris alluded, cyanide, used very dilute, becomes very selective. It's one of the reason's it has been used with success. Consumption tends to be only on the values involved, so a ton of ore can often be treated with an ounce of cyanide. All depends on the volume of values contained within the ore. It won't react with the gangue under normal circumstances, so there are few losses.
When ores are processed, if the level is kept down around .02% or less, no copper will be dissolved. It's a little different with plated objects, where base metal tends to interfere with recovery, but there are buffering reagents that limit the problems. I used a commercially prepared buffered compound with great success after I closed the doors on my refining business. I had bought and stored about 180 pounds of very lightly plated wire wrap pins and related materials years earlier, and had placed them in storage because my time was better spent working on customer materials. I recovered nearly six ounces of gold from the components. All action ceased when the nickel barrier was exposed, where, by sharp contrast, some heavily plated copper alloy items I had tried stripping years earlier, using just cyanide, were very troublesome once the gold perforated. Base metal dissolved instead of the plated gold, so the process was abandoned.
It is recommended that free oxygen be removed from cyanide solutions before introducing zinc, to minimize zinc loss, but I didn't worry about that. At the time, a pail of zinc cost so little that it wasn't worth the time to mess with the solution. I simply introduced a greater amount of zinc than I expected to recover in values, then decanted the solution immediately upon settling. As I recall, an ounce of zinc will precipitate an ounce of gold. I then precipitated a second time to insure that no values were left behind. A small amount of free cyanide should be in solution, which accelerates the conversion. When conditions are proper, it happens almost instantly. My practice was to stir the solution well, getting it in motion, then I'd sprinkle the required amount of zinc in the solution and continue to stir. I used a 20 gallon Nalgene container and stirred with an aluminum rod. The zinc almost immediately turned dark brown, a sure sign it was precipitating the gold. There was a minimal reaction with the aluminum rod, but it was exposed to the pregnant solution only briefly.
Regards destroying cyanide for disposal, I'm at a loss to recall the compound used, but it liberates a huge amount of chlorine, which kills the cyanide. It can then be safely discharged to a sewer. Perhaps Chris, or one of the others can provide the common name of the compound. I believe Lou has made mention in one of his posts some time ago.
Harold
When ores are processed, if the level is kept down around .02% or less, no copper will be dissolved. It's a little different with plated objects, where base metal tends to interfere with recovery, but there are buffering reagents that limit the problems. I used a commercially prepared buffered compound with great success after I closed the doors on my refining business. I had bought and stored about 180 pounds of very lightly plated wire wrap pins and related materials years earlier, and had placed them in storage because my time was better spent working on customer materials. I recovered nearly six ounces of gold from the components. All action ceased when the nickel barrier was exposed, where, by sharp contrast, some heavily plated copper alloy items I had tried stripping years earlier, using just cyanide, were very troublesome once the gold perforated. Base metal dissolved instead of the plated gold, so the process was abandoned.
It is recommended that free oxygen be removed from cyanide solutions before introducing zinc, to minimize zinc loss, but I didn't worry about that. At the time, a pail of zinc cost so little that it wasn't worth the time to mess with the solution. I simply introduced a greater amount of zinc than I expected to recover in values, then decanted the solution immediately upon settling. As I recall, an ounce of zinc will precipitate an ounce of gold. I then precipitated a second time to insure that no values were left behind. A small amount of free cyanide should be in solution, which accelerates the conversion. When conditions are proper, it happens almost instantly. My practice was to stir the solution well, getting it in motion, then I'd sprinkle the required amount of zinc in the solution and continue to stir. I used a 20 gallon Nalgene container and stirred with an aluminum rod. The zinc almost immediately turned dark brown, a sure sign it was precipitating the gold. There was a minimal reaction with the aluminum rod, but it was exposed to the pregnant solution only briefly.
Regards destroying cyanide for disposal, I'm at a loss to recall the compound used, but it liberates a huge amount of chlorine, which kills the cyanide. It can then be safely discharged to a sewer. Perhaps Chris, or one of the others can provide the common name of the compound. I believe Lou has made mention in one of his posts some time ago.
Harold