Tutorials on Refining Precious Metals

Metals contain a number of impurities and, hence, need to be purified. Impurities may include other metals, non-metals, oxides and sulfides of the metals, slags and dissolved gases. The method used for the purification of a particular metal depends on the nature of the metal to be refined, the impurities to be removed and the purpose for which the metal is to be used.

Amalgamation

• The amalgamation process was first used in Mexico in the 1500s. It is a refining process to extract silver from ore. When silver is allowed to react with mercury, an amalgam (an alloy of mercury with silver) is formed. Mercury is more volatile than silver, so when the amalgam is heated in an iron retort (a vessel in which substances are distilled and decomposed by heat), mercury is distilled off and silver is left behind.

Extraction of Silver from Silver Glance

• Silver glance typically is crushed in stamp mills. The ground ore is mixed with a cuprous chloride solution and mercury and spread in a 1- or 2-foot-thick layer in an enclosure. After it has been soaked in the sunlight for a couple of weeks, a complex reaction leads to the formation of an amalgam with mercury. The amalgam is washed off and allowed to distill in an iron retort, while mercury, being more volatile, is distilled off. The result is the refined silver being left behind.

Extraction of Gold from Auriferous Quartz Rocks

• Auriferous (gold-bearing) quartz rocks are broken into smaller pieces by means of rock crushers. The smaller pieces obtained are finely powdered in big stamp mills. This is called pulverization of the quartz. The pulverized ore is carried off by a current of water through a screen at the other end of the mill, and a slurry (i.e. mixture of finer particles of quartz and water) is obtained. The coarse particles, if any, are retained and the ore is again pulverized.

  The slurry mass is allowed to flow over copper plates coated with mercury and are placed in an inclined position. The particles of gold metal present in the slurry mass form an Au-Hg amalgam with mercury, which is retained on the copper plates. The amalgam is removed from the plates and distilled in iron retorts, and the gold metal is left behind. Nearly 75 to 80 percent of the gold originally present in the quartz rock is thus purified, and the remaining 20 to 25 percent remains associated with rock particles. These rock particles are called "tailings," from which the gold is further refined through a separate purification process called the cyanide process or the Macarthur-Forrest process.