How to Refine Precious Metals
Gold and silver refining has been around since antiquity. The ultimate goal of refining precious metals is to obtain the utmost purity from gold and silver. Before refining, precious metals are mined from the earth as rock ore. The ore must be crushed before leaching and precipitation of the metals, which are then poured into molds to be further refined. High prices for precious metals make it economically feasible to use sophisticated refining techniques that allow for the recovery of fine particles of gold and silver from large rock ore.
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Things You'll Need
- Ore
- Crusher
- Cyanide solution
- Mercury
- Metal cathode
- Heat source
- Filters
- Molds
- Nitric acid
- Cupel
- Lead foil
- Crucible
Instructions
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Prepare the gold and silver rock ore. The metal ore extracted and smelted is impure. Several steps must take place before the metals can be refined to acceptable purity levels: Rock ore is extracted from the earth and transported to a mill, where it is crushed into a fine sand. Acids and alkalies are added to the sand to extract the precious metals; the most common additive is a cyanide solution, which leaches and dissolves the metal away from the sand particles. The metal-bearing solution is then collected and further processed.
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Add positive electric current to the solution for electroplating of metals. The positively charged metals will be drawn to a negatively charged metal plate for easy collection. The resulting material will be a metal-infused sludge, roughly 70 to 90 percent recoverable gold or silver, which can be sloughed off into a precipitation tank.
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Remove impurities from the sludge and collect in a filter. Different processes are used to remove specific impurities from the sludge. For example, to remove mercury, retorting is the acceptable process; the sludge is distilled and the mercury dissipates in the form of mercury vapors, which must be monitored for health and safety.
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Melt-refine to remove other impurities. Heat the sludge to a high temperature, but below 1250 degrees Celsius. At this temperature range, and in conjunction with atmospheric oxygen, gold and silver will result, but other impurities will vaporize and can then be collected in stack scrubbers and filters. Melt-refining is suitable to remove iron, zinc, calcium, magnesium, sodium, alumina and silica from sludge. Heat the metal again and pour the molten material into molds.
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Test the purity of gold with fire assay. Drill through a poured bar of metal and collect a sample. Weigh the sample precisely. Combine it with a specified amount of silver and wrap with lead foil. Place it in a cupel, a porous pot within a crucible that is able to withstand high temperatures. Heat it in a furnace; at this point, all nonprecious metals will be absorbed by the hot cupel and the precious metals will form a button. Remove the button and roll it flat. Add nitric acid to remove the silver. The result is a 99.999 percent pure sample of gold. Divide the original sample's weight into the weight of this sample; the result is a value of the gold purity.
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References
- Photo Credit gold mill site machinery image by Greg Pickens from Fotolia.com
