Methods for Extraction of Precious Metals for Analysis
Precious metals are naturally occurring metallic elements such as gold, platinum and silver, which are less reactive than other forms of metals. These metals are rare and have high economic value. Some of them, such as gold and silver were used to make coins but now they are commonly used for jewelry or saved as investments. There are various methods that are used to extract these precious metals for analysis before large-scale mining can be undertaken.
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Leaching
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A common method of precious metal extraction from a base ore is leaching. Leaching involves dissolving the ore sample into a liquid. The precious metal "leaches" out of the ore substrate and combines with the liquid to form a complex which is easier to separate and process. The ore remains suspended and unreactive to the liquid solution so there is no contamination of the separated precious metal. Cyanide is the leaching agent used in the leaching of gold and silver. However, it is a dangerous undertaking because cyanide is toxic. Acidic levels should be regularly monitored to prevent the formation of hydrogen cyanide gas which is the toxic form of cyanide.
Cupellation via Lead Collectors
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Cupellation, which is also called fire assaying, requires the use of a fusion mixture to draw out the precious metal. The fusion mixture is made from lead oxide and fluxes, including sodium carbonate, borax, silica, potassium nitrate and flour. The fusion mixture reacts with the raw ore and the precious metals are oxidized by lead and combine to form molten lead droplets which collect the precious metals. The fluxes are reduced to a borosilicate slag that contains all the other impurities. Lead is then separated from the precious metals by heating the mixture in a furnace.
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Cupellation via Nickel Collectors
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As an alternative to lead, nickel can be used to react with the precious metals in the raw mixture to form the metal-nickel mixture called a "button." The same process as in cupellation via lead is observed except that separating the nickel from the precious metals requires a different approach. Instead of subjecting it to high temperatures inside a furnace, a hydrochloric acid solution is used to dissolve the nickel leaving behind the precious metals for analysis.
X-Ray Fluorescence (XRF) Analysis Methods
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The X-ray method is a modern method of precious metal extraction. The process, which is called X-ray fluorescence, relies on the ability of different metals to give off different signatures when exposed to X-rays. With this method, separating the precious metals from the ore fully is not required. You only need to prepare a composite sample of the metal and ore, and then subject the whole sample to X-ray radiation which then gives off X-ray fluorescence signatures indicating the quality and composition of metals in the ore.
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References
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very rich information. thanks
