Re: What are those cistern things on 178 just s of 190?
Posted by RiverRat on March 31, 2003 at 12:08:00:
In Reply to: Re: What are those cistern things on 178 just s of 190? posted by Genie on March 29, 2003 at 11:18:14:
Just to expand a bit on the use of cyanide to extract gold:
A weak cyanide solution disolves the gold in the ore. This discovery only doubled the worlds production of gold after it was introduced in the 1890s, and resulted in the (depending how you count them) fourth gold rush in California.
The original process (the "cistern things): The gold can be microscopic and still be extracted. The ore is ground fine (salt to flour size) by some combination of jaw crushers, stamp mills, ball mills, and/or rod mills, put in vats with the cyanide solution and stirred. The vats were usually redwood or concrete, since the cyanide solution disolves metal. The cyanide solution must be kept at an alkaline pH, lime was usually used to buffer the solution. When the solution has disolved the gold, it was drawn off and the gold was precipitated by zinc shavings or "zinc moss", and then refined. This process is fouled up by too many base metals in the ore (cyanide will take all the metals into solution) and the presence of acid (even weak carbolic acid contained in rainwater will decompose the cyanide solution). Most ore was run through a gravity seperator (shaker table) to recover the native (visible) gold prior to cyanide treatment. Waste products include: waste rock ("mine dump"), tailings (the fine ground treated ore, usually lifeless because it is fine ground mineral, usually quartz, usually light colored, likes to blow around), and piles of lime that was not used. I do not know if the cyanide solution was regenerated and reused after stripping by zinc.
It surprised the hell out of me once, while exploring a mill in the desert, to run across these intact galvanized metal drums labeled "Sodium Cyanide NaCN Danger Poison Keep Dry". Danger, no shit, as I backed out the same way I came in, stepping in my same footprints on the dusty floor. Was it a problem? Quien Sabe.
The heap leach (modern) process: Ore is cracked into walnut-raisin sized pieces (excessive fines will screw up this process by plugging up the collector pipes or creating preferential flow through the heap), piled on pads with collector pipes underneath, plastic sprinklers are set up and the heaps are "irrigated" with cyanide solution for years. As the solution trickles through the heaps it disolves the gold. The solution is run through activated charcoal columns that strip the gold from solution and regenerate the cyanide solution so it can be reused. Operating conditions (pH) and problems (acidic rainwater) remain the same, compounded by open collector ponds of cyanide solution in the desert attracting birds.
Hope this helps.