Gold and Gold Compounds

Abstract

Gold is characterized by high density, high electrical and thermal conductivities, and high ductility. At least 26 unstable gold isotopes have been made. Gold is the most noble of the noble metals. Other than in the atomic state, the metal does not react with oxygen, sulfur, or selenium at any temperature. It does, however, react with tellurium at elevated temperatures to produce gold ditelluride which is also found in the naturally occurring mineral, calaverite. Gold reacts with the halogens, particularly in the presence of moisture. Gold reacts with various oxidizing agents at ambient temperatures provided a good ligand is present to lower the redox potential below that of water. Thus, gold is not attacked by most acids under ordinary conditions and is stable in basic media. Gold does, however, dissolve readily aqua regia and in alkaline cyanide solutions in the presence of air or hydrogen peroxide to form (Au(CN) 2 ) − . These reactions are important to the extraction and refining of the metal. Gold is very corrosion and tarnish resistant and imparts corrosion resistance to most of the commonly used gold alloys. Placer mining is the oldest form of gold mining. The technique is still in use in places where appropriate alluvial or marine deposits exist, such as Alaska, but requires large quantities of water. At present, most gold is obtained either by deep mining, most notably in South Africa, or by open pit mining such as in the United States. In refining precious metal scrap and some concentrates, the gold is converted to HAuCl 4 by treatment with aqua regia. After heating to remove nitrogen oxides, gold is precipitated from solution by reduction with sulfur dioxide or ferrous sulfate. Gold is widely distributed and the average content in the earth's crust is estimated to be 3.5 ppb. The gold content of ocean water varies considerably with location. The average value is of the order of 10 ppt, which is well below the concentration (3 ppm) required for economic recovery. Chrysotherapy, therapy with gold compounds, remains as one of the few treatments capable of slowing or halting the damage caused by rheumatoid arthritis. Besides its use for monetary reserves, gold is used in the private sector principally for investment and fabrication. By far, the largest commercial use is jewelry. In the electronics industry, gold is used as fine wires or thin film coatings and frequently in the form of alloys to economize on gold consumption and to impart properties such as hardness. In dentistry, gold is used for a variety of restorations. The chemistry of nonmetallic gold is predominantly that of Au(I) and Au(III) compounds and complexes. Common gold compounds include halides, cyanides, oxides and hydroxides hydroxide, Au(OH) 3 ), and sulfides. Both alkyl and aryl complexes of Au(I) and Au(III) as well as olefin and acetylene complexes have been prepared and studied. An increasing amount of interest has developed in gold‐containing bimetallic cluster compounds.

Use of Metallic Mercury in Artisanal Gold Mining by Amalgamation: A Review of Temporal and Spatial Trends and Environmental Pollution

“…Originally, gold was recovered by swirling a mixture of auriferous sand and water in a shallow pan or separated using sluice boxes fitted with transverse riffles that retain the gold. Later, probably prior to 1000 B.C., gold recoveries were improved by adding Hg 0 to the system to wet the gold particles and bind them together [10,23,58,59].…”

“…Previously, global Hg emission into the atmosphere was blamed primarily on industrial and combustion sources from Europe and North America [15,19,20], while the mining sector, an important anthropogenic source of Hg emissions [19,21,22], was not always included among the dominant Hg polluters on a global scale. Today, however, and although considered as a minor but fast-growing contributor to the global Hg cycle [23][24][25], artisanal gold mining (AGM) by Hg amalgamation techniques is recognized as one of the sources of environmental pollution with Hg, with primarily local and regional implications [26,27]. This specific source of Hg pollution appears to be a consequence of socio-economic predicaments in many developing nations in Africa, Asia, and South America, where the use of Hg 0 to mine gold has rebounded and heightened in spite of the introduction of much safer mining technologies, such as cyanidation and bacterial oxidation processes [28][29][30].…”