Pyrometallurgy of Gold

Gold melts at 1063 ^oC and boils at 3081 ^oC under atmospheric pressure; 1800 in a vacuum. The melting and boiling point for other metals and minerals are listed in the next table.

Element Melting Point (oC) Boiling Point (oC)   Gold 1063 2808 Silver 961 2210 Platinum 1769 4530 Mercury -38.9 357 Zinc 420 907 Lead 327 1744 Copper 1083 2595 SiO2 1723 2230 Fe2O3 1565 - FeS2 1171 - Al2O3 2072 2980

At temperatures above the melting point, gold volatilizes as a red coloured vapour, with the rate of volatilization increasing with increasing temperature. For example, the volatilization rate at 3000 ^oC is sufficient to completely remove a gold film on porcelain within minutes. Even though the volatility of pure gold is negligible below approximately 1050 ^oC, and is low below 1250 ^oC. The volatility increases with the presence of impurity metals, particularly Tellurium, e.g. an alloy containing 5% Tellurium loses between 2 and 4% of the contained gold in one hour at 1245 ^oC. Alloys containing 5% Hg lose approximately 0.2% of the gold under similar conditions.

Gold does not oxidize in air or oxygen, even at red heat, but does has the ability to absorb gases; for example H₂, CO, CO₂ and N₂ can be absorbed up to concentrations of 0.48%, 0.29%. 0.16% and 0.20% respectively. Gold chloride is formed when gold is heated above approximately 140 ^oC in the presence of chlorine gas. This compound has a dark red color.

Gold forms alloys with most metals. Those most commonly encountered are mercury, silver, lead, and copper.

Gold and mercury form alloys over the complete range of proportions. This characteristic is used in the amalgamation process. Mercury can be separated from gold by distillation, leaving a gold product containing 0.1-1% Hg. A piece of gold rubbed with mercury is immediately penetrated by it and becomes some brittle. The ductility is not always restored when the mercury is removed by distillation, a crystalline structure being often induced, maybe because part of mercury is retained by the gold; a particle of gold wetted by mercury at once loses its color. A solid amalgam is formed, bur it is not readily dissolved in an excess of mercury. The amalgam recovered in mills may be regarded as a collection of little nuggets of gold, coated and partially saturated with mercury. The amalgam sinks to the bottom of the mercury. Coarse particles of gold are not saturated with mercury to their centers, although the outside layers of particles consist of a saturated solution.

The saturation of the interior, depending on the diffusion of mercury through the solid amalgam, would probably not be complete for many days. The finer particles of gold, the more nearly the interior approaches to saturation. It follows that coarse gold give rich amalgam and fine gold poor amalgam. The limits are pure gold on the one hand and a saturated solution of mercury in gold on the other, neither of which exist in practice. The practical limits are amalgams containing about 50% and 25% of gold respectively. These gold amalgams usually contain impurities in the shape of amalgams of silver and of base metals, as well as non metallic substances.

Gold and silver form alloys in all the proportions. Alloys containing more than 66% Ag can be separated by acid parting. Alloys with less silver must be inquarted with additional silver to increase the silver content to allow parting. The alloys are homogeneous, malleable, soft and ductile. They are nearly uniform in composition, but here is a slight tendency for gold to concentrate in the interior of the ingot, the greatest divergence occurring in the alloy containing 36.1% Au. The color of gold is sensibly diminished by the addition of very small quantities of silver and on increasing the proportion of the latter, the color changes to greenish-yellow (20-40% Ag) to a faint yellowish-white (50% Ag) and to white with a scarcely perceptible yellow tinge (60% Ag). The yellow color finally disappears when some proportion between 60-70% Ag is present.

Gold and lead readily form a wide range of alloy compositions. Lead has a high affinity for gold and can be used as a collector for gold in pyrometallurgical processes. The lead can be subsequently separated by volatilization.

Gold and copper are miscible in all proportions when molten and form solid alloys of sensible uniform composition. Copper can be separate from gold by fusion with lead, by electrolysis, or by smelting to produce a copper matte. Copper imparts a red color to Au-Cu alloys, property that is used in the jeweler industry.