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Heavy Metals
Published in Abhik Gupta, Heavy Metal and Metalloid Contamination of Surface and Underground Water, 2020
Silver (Ag), a precious metal, has an atomic number of 47, an atomic weight of 107.868, and a density of 10.50 g cm–3. Many lead, zinc, and copper ores are argentiferous, that is, these contain argentite or silver sulfide (Ag2S). Silver is extracted as a by-product from the mining of these three metals. However, being a precious metal, even small amounts of silver fetch a lucrative price. Silver is also an essential constituent of calaverite, which is a gold telluride [(AuAg)Te2]. Silver, alloyed with copper to make it hard, is used for making cutlery, utensils, coins, ornaments, and jewelry. Among the other uses of silver, silver vats are used during the production of acetic acid, vinegar, and cider, because of the resistant nature of silver to acetic acid. It is also used in silver solders, dental amalgams, batteries, ceramic paints, and other items. Silver also acts as a catalyst in the synthesis of formaldehyde and acetaldehyde. The property of silver halides of reacting to light instantaneously makes them invaluable in the production of films, plates, and photographic printing paper. Silver nitrate (AgNO3) is an important laboratory reagent in analytical chemistry, and is used for making mirrors, photography, silver plating, dyeing, and numerous other uses ranging from indelible inks and water disinfectant to antiseptic ointments and astringents (Encyclopaedia of Occupational Health and Safety 2012). Silver has very strong antimicrobial properties, and nanosilver compounds are expected to revolutionize the use of silver in antimicrobial formulations.
Gold Ore Processing and Environmental Impacts: An Introduction
Published in Sadia Ilyas, Jae-chun Lee, Gold Metallurgy and the Environment, 2018
Most of the gold occurs as local metal, and most of the minerals are naturally connected to it. Silver often coexists with gold; when it exists with >20% silver it is named as the “electrum.” Other gold amalgams are uncommon and for the most part limited to particular metals; for instance, the two gold copper composites: Cu3Au (auricupride) and [AuCu]4 (tetra-auricupride) are considered high gold-review porphyry copper minerals (Kemess, BC, Canada). Some important ones are FeS2 (pyrite), PbS (galena), ZnS (zincblende), FeAsS (Arsenopyrite), Sb2S3 (stibnite), FeS (pyrrhotite), and CuFeS2 (chalcopyrite). Different Se minerals and Fe3O4 (magnetite) may likewise be available. In Witwatersrand, South Africa, UO2 uraninite, and to a lesser degree, thucholite (a variable blend of hydrocarbons, uraninite, and sulphides) are related to the gold bearing mineral. Uranium was recuperated as a result of gold exploitation. Carbonaceous matter always relates to certain gold minerals. Gold has partiality for Te with two known minerals, the calaverite, AuTe2 and sylvanite (Ag,Au)Te2. It occurs with Pd as porpezite, and with Rd as resinrhodite. In placer stores, it might be available as small grains or huge pieces. In specific metals knowing headstrong minerals, gold is related with sulfidic minerals to a great degree of a finely separated state. Petrovskaya (1987) worked on revealing the tendency of elements to associate with gold in minerals and ore bodies; this information is presented in Figure 1.2. Notably, the lode (vein) and placer deposits are mainly recognized auriferous deposits; however, the quartz-pebble deposits supplementing ~50% of worldwide gold production are classified as modified paleo-placer (Yannopoulos, 1991).
Oxidative Decomposition of Silver Telluride (Ag2Te) Using Hypochlorite in Different Acid Environments
Published in Mineral Processing and Extractive Metallurgy Review, 2022
V.M. Rodríguez-Chávez, J.C. Fuentes-Aceituno, F. Nava-Alonso
The depletion of easily leachable gold and silver deposits has forced the mining industry to explore new processing alternatives to recover the precious metals from different ores. Among the most important silver phases, it is possible to find pyrargyrite, proustite, stephanite and silver telluride (Paterson 1990). The most common mineral phase of silver telluride is hessite (Ag2Te), and it is frequently associated with some gold tellurides, e.g. calaverite (AuTe2), petzite (Ag3AuTe2), krennerite (AuTe2), montbrayite (Au2Te3) and kostovite (CuAuTe4) (Adams 2016). However, according to Celep et al. (2014), these important silver mineral phases present a very low leaching kinetics in cyanide solutions. Furthermore, Wang and Forssberg (1990) reported that gold and silver telluride and selenide species are stable in the presence of cyanide, which suggests that direct leaching of gold and silver with cyanide is probably ineffective. The reported results demonstrate that the leaching kinetics is very slow compared to native gold and electrum (Cornwall and Hisshion 1976; Henley, Clarke and Sauter 2001; Jayasekera, Ritchie and Avraamides 1991; Johnston 1933; Marsden and House 2006; Padmanaban and Lawson 1991). As can be seen, the refractoriness of these type of minerals becomes an important challenge for the scientific and engineering viewpoint. Jha (1987) and Zhang et al. (2010) mentioned that refractoriness of some minerals including the telluride species can be usually solved by modifying the cyanidation conditions or providing an oxidation pretreatment. Roasting prior to cyanidation was the almost universal practice in old days; however, severe environmental regulations related to the toxic emissions have motivated the researchers to develop alternative leaching systems (Hiskey and Atluri 1988). In the case of gold telluride cyanidation, Haque (2007), reported the possibility to accelerate its dissolution using pre-treatments such as: thermal oxidation, roasting, chemical oxidation, (such as acid leaching, alkaline or acid pressure leaching or biological oxidation). In the case of the chemical oxidation of refractory ores, various types of oxidants have been studied e.g. ozone, hydrogen peroxide, permanganate, chlorine, bromine cyanide, Care`s acid, perchlorate, hypochlorite, ferric ion in acid media and oxygen (Canning and Woodcock 1982).