Monday, December 22, 2008

Common Gold Concentration Methods, Part 2


I covered a few of the basic gold concentration methods in Part 1 of this series of posts. Here are the remaining methods:

Flotation

This is a milling concentration process that uses bubbles generated in a specific chemical solution to "capture" tiny particles of gold in a large vat or tank. The precious metal particles become entrapped in the bubbles which "float" on the surface of the solution while any waste particles sink to the bottom of the flotation tank. Flotation is a common process in most large-scale or commercially run gold operations and can include the following steps: ore regrinding, ore roasting, bio-oxidation, and pressure oxidation.

Leaching

More commonly known as heap leaching, this gold concentration method involves the piling up or "heaping" of crushed gold ore on a pad in the open air. A cyanide solution is then sprayed onto the heap, eventually percolating down through the auriferous material and dissolving the gold into a chemical state. This chemical or "pregnant" solution is collected in a large, protectively lined holding pond where it is eventually processed to recover the gold values and capture and recycle the cyanide.

Cyanide heap leaching has been the hallmark concentration process for large, open-pit gold mining operations throughout the western United States, as well as other parts of the world. It is an extremely efficient method for extracting and concentrating gold from refractory ores or ores that were considered economically "unfeasible" for processing by other means. However, the potential environmental impacts of using cyanide solutions in large-scale mining can be significant, especially if appropriate regulatory parameters are not in place.

Refining

The final gold concentration method is, of course, refining. In this step any and all gold recovered by other methods is essentially placed inside a furnace, heated to melting, and then poured into ingots or bars ranging from 5-10 troy ounces in small operations or 100-troy ounce "whopper" ingots in large-scale mining operations. Typically the gold concentrated at this stage is nearly pure, running .999 fine.

(c)  Jim Rocha (J.R.)  2008

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