Gold Amalgamation

    Amalgamation: The physical and/or chemical characteristics that make amalgamation work are not clearly understood to this day. However, it is known that if clean mercury is brought into contact with clean gold, the gold is wetted and "drawn into" the mercury. This result in a solution of gold in mercury or an alloy of gold and mercury called amalgam. After the mercury has gathered in the gold it can be removed by dissolving it in nitric acid or by driving it off as a vapor by heat.

    Amalgamation: The physical and/or chemical characteristics that make amalgamation work are not clearly understood to this day. However, it is known that if clean mercury is brought into contact with clean gold, the gold is wetted and "drawn into" the mercury. This result in a solution of gold in mercury or an alloy of gold and mercury called amalgam. After the mercury has gathered in the gold it can be removed by dissolving it in nitric acid or by driving it off as a vapor by heat. The gold will remain behind.

    The mill operator's problem is to get the gold and the mercury into intimate contact with each other. To do this he must: (1) liberate the gold particles from the gangue; (2) remove any coating which may be covering the gold; (3) keep the mercury clean and bright; and (4) bring the gold and mercury into intimate contact. Then he must allow the amalgam to coalesce, separate it from the pulp, and extract the gold.

    Amalgamation generally works best on relatively coarse gold that can be liberated from the gangue and abraded clean without too much difficulty. Since mercury will not penetrate into minute crevasses or pores, the ore must be ground fine enough to expose the gold at the surface. If the gold is very fine, cyanidation is more effective and, in practice, a combination of amalgamation and cyanidation is usually employed. Gravity and flotation are also frequently used in conjunction with amalgamation.

    Some of the things that tend to frustrate the mill man's attempt to get the mercury and gold together are: (1) The existence of surface coatings or encasement of the gold in the gangue. Fine grinding and abrasion will usually solve this problem. (2) The presence of oil, grease, clay or iron and base metal sulfides may result in sickening or flouring of the mercury. Grinding in lime or some other alkali will usually prevent this occurrence. (3) The presence of carbon as graphite also sickens the mercury. In some instances it can be removed by flotation prior to amalgamation. (4) The presence of sulfides of arsenic, antimony or bismuth will cause flouring and sickening of the mercury. This usually makes the recovery of gold by amalgamation impractical.

    There is some confusion about the meaning of the words flouring and sickening of the mercury. If the mercury will not wet or take up the gold or coalesce into larger globules it is said to be sick. Sickening is most likely caused by impurities in or on the surface of the mercury. The most common impurities are oil, grease, clay, manganese and iron sulfates, and base metal and iron sulfides. Flouring in the strict sense is the division of the mercury into extremely small globules. This gives it a white flour-like appearance. This is not bad in itself but the mercury seems to be more susceptible to sickening while in the finely divided state. Therefore, it does not coalesce but stays in a floured condition and is lost to tails. Any gold that it took up before becoming sickened is also lost to tails.

    The traditional use of amalgamation involved the stamp mill and amalgamation plates. Today, with the exception of a few traditionalists, the stamp mill has been replaced by the ball mill for this purpose. The mercury is fed into the ball mill with the ore and is then passed over prepared plates.