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من طرف **Admin** الثلاثاء يناير 27, 2009 8:28 am

How Gold is Produced
This chart illustrates the general steps in open-pit gold mining. The specifics of the process vary from mine to mine.Geologists use the latest technology such as satellite surveys and geochemistry to locate an ore deposit.Computers are used to design the mine which requires precise and accurate measurement of the ore deposit. Construction begins following the lengthy process of receiving permits.As holes are drilled for blasting, samples of ore are examined to determine grade and metallurgical characteristics. The broken rock is marked by type for efficient processing.Based on its metallurgical makeup, a dispatcher directs truck operators to deliver the ore to the correct processing location. Low Grade Ore is roughly broken into small chunks and placed on carefully lined pads where a dilute cyanide solution is distributed over the surface of the heap. The solution percolates through the heap and the cyanide dissolves the gold. This solution containing dissolved gold is then collected.

High Grade Ore is delivered to a grinding mill where the ore is pulverized to a powder. Depending on its metallurgical characteristics the ore may be treated in one of three recovery circuits.

Refractory ore containing carbon is roasted to over 1,000 degrees Fahrenheit, burning off the sulfide and carbon. The product of this process is an oxide ore, which is routed to the leaching circuit.Oxide ore is sent directly to the leaching circuit where cyanide dissolves the gold.

Sulfide refractory ore without carbon is oxidized in an autoclave to liberate the gold from sulfide minerals, then it is sent to the leaching circuit. Treated, high-grade ore is leached with cyanide.The gold is absorbed (collected) out of solution onto activated carbon. The remaining cyanide solution is recycled.The gold loaded carbon is moved into a vessel where the gold is chemically stripped from the carbon which is then recycled.Gold is precipitated from the solution electrolytically or by chemical substitution.The pure gold is then melted into dore' bars containing up to 90 percent gold. Dore' bars are then sent to an external refinery to be refined to bars of 999.9 parts per thousand pure gold.Reclamation is a long-term investment made by every gold mining company, and can cost anywhere from $2,000 to $10,000 per acre. It is the cornerstone of every mine plan and is considered the first and last step of the mining process. (For more information on reclamation, see the Reclamation section.)