Gold metallurgy

(metallurgy) The science and technology of gold recovery and refining.

Extracting gold from ores, refining it, and preparing it for use. Total world resources of gold are estimated at about 83,000 tons (75,000 metric tons). South Africa has about half of these resources, and Brazil, Russia, and the United States have about 12% each. The United States produces about 330 tons (300 metric tons) per year. In the United States, gold is used for jewelry and arts (70%), industry and electronics (23%), and dentistry (7%). There are only a few dozen placer mines in the United States, nearly all in Alaska. In such mines, gold is processed with the modern equivalent of gold panning—sluicing, tabling, and jigging. In addition, by-product gold from copper mining is only about 10% (historically, this source used to be much larger). There are several hundred lode mines in the United States, where the ore is mined from solid rock. This gold is often difficult to recover because it is associated with sulfide or carbonaceous minerals. As technology has improved, possibilities for processing the more difficult-to-treat (refractory) ores have expanded. A particular ore is more or less refractory depending on its combination of chemical compounds and minerals. See also Gold; Placer mining.

In the cyanide leaching process, ore is first crushed dry in a gyratory crusher and ground wet in a semiautogenous grinding mill. During the wet grinding process, cyanide and lime are added. The ore leaves the grinding mill as a slurry of muddy water. The gold gradually leaches out of each tiny ore particle (during its 20–30-h residence time) and dissolves into solution.

Sometimes lower-grade ore (<0.05 oz/ton or 1.57 g/metric ton of gold) is simply crushed and placed in heaps where it is slowly leached with cyanide solutions. Even though heap leaching gives lower gold recovery (sometimes lower than 60%), the cost is lower. Heap leaching is a form of solution mining that can also be applied to old tailings piles and mined overburden dumps. See also Solution mining.