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Sunday, July 25, 2010

Development of gold recovery techniques

 Development of gold recovery techniques


Because the gold in the California gravel beds was so richly concentrated, the early forty-niners simply panned for gold in California's rivers and streams, a form of placer mining. However, panning cannot be done on a large scale, and industrious miners and groups of miners graduated to placer mining "cradles" and "rockers" or "long-toms" to process larger volumes of gravel. In the most complex placer mining, groups of prospectors would divert the water from an entire river into a sluice alongside the river, and then dig for gold in the newly exposed river bottom. Modern estimates by the U.S. Geological Survey are that some 12 million ounces (370 t) of gold were removed in the first five years of the Gold Rush (worth approximately US$7 billion at November 2006 prices).



Gold miners excavate a gold-bearing bluff with jets of water at a placer mine in Dutch Flat, California sometime between 1857 and 1870

In the next stage, by 1853, hydraulic mining was used on ancient gold-bearing gravel beds that were on hillsides and bluffs in the gold fields. In a modern style of hydraulic mining first developed in California, a high-pressure hose directed a powerful stream or jet of water at gold-bearing gravel beds. The loosened gravel and gold would then pass over sluices, with the gold settling to the bottom where it was collected. By the mid-1880s, it is estimated that 11 million ounces (340 t) of gold (worth approximately US$6.6 billion at November 2006 prices) had been recovered by "hydraulic process." This style of hydraulic mining later spread around the world. An alternative to "hydraulic process" was "coyoteing. This method involved digging a shaft 6 to 13 meters (20 to 40 feet) deep into bedrock along the shore of a stream. Tunnels were then dug in all directions to reach the richest veins of pay dirt.

A byproduct of these extraction methods was that large amounts of gravel, silt, heavy metals, and other pollutants went into streams and rivers. Many areas still bear the scars of hydraulic mining since the resulting exposed earth and downstream gravel deposits are unable to support plant life.



Quartz Stamp Mill in Grass Valley crushes the quartz before the gold is washed out

After the Gold Rush had concluded, gold recovery operations continued. The final stage to recover loose gold was to prospect for gold that had slowly washed down into the flat river bottoms and sandbars of California's Central Valley and other gold-bearing areas of California (such as Scott Valley in Siskiyou County). By the late 1890s, dredging technology (which was also invented in California) had become economical, and it is estimated that more than 20 million ounces (620 t) were recovered by dredging (worth approximately US$12 billion at November 2006 prices).

Both during the Gold Rush and in the decades that followed, gold-seekers also engaged in "hard-rock" mining, that is, extracting the gold directly from the rock that contained it (typically quartz), usually by digging and blasting to follow and remove veins of the gold-bearing quartz. Once the gold-bearing rocks were brought to the surface, the rocks were crushed, and the gold was separated out (using moving water), or leached out, typically by using arsenic or mercury (another source of environmental contamination).Eventually, hard-rock mining wound up being the single largest source of gold produced in the Gold Country.

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