Cadmium minerals

T. O. Llewellyn, Cadmium Minerals Yearbook, U.S. Department of the Interior, Bureau of Mines, Washington, D.C., 1988. 

Although ranking 57th in abundance in the earth s crust tO. 15 ppm), cadmium is not encountered alone, but is always associated with zinc. The only known cadmium minerals are greenockite (sulfide) and otavite (carbonate), both minor constituents of sphalerite (zinc oxide) and smilhsoniie (zinc carbonate), respectively See also Greenockite Smithsonite and Sphalerite Blende. 

Zinc occurs widely in a number of minerals, but the main source is sphalerite, (ZnFe)S, which commonly occurs with galena, PbS cadmium minerals are scarce but, as a result of its chemical similarity to Zn, Cd occurs by isomorphous replacement in almost all zinc ores. There are numerous... 

Most important mineral Greenockite CdS is the only cadmium mineral of any importance. 

The only common cadmium mineral, greenockite CdS, is almost always associated with the zinc ore mineral sphalerite. The average ratio between zinc and cadmium in these types of ores is 400 1, which means that the cadmium content is 0.25% with reference to zinc. Of the total output of cadmium in the world, about 80% comes as byproduct from primary zinc production and a small amount comes from lead production. The remaining part, about 20%, is obtained from cadmium scrap and other secondary sources, such as for instance dust generated by recycling of iron and steel scrap. 

Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. 

Fluorspar occurs in two distinct types of formation in the fluorspar district of southern Illinois and Kentucky in vertical fissure veins and in horizontal bedded replacement deposits. A 61-m bed of sandstone and shale serves as a cap rock for ascending fluorine-containing solutions and gases. Mineralizing solutions come up the faults and form vein ore bodies where the larger faults are plugged by shale. Bedded deposits occur under the thick sandstone and shale roofs. Other elements of value associated with fluorspar ore bodies are zinc, lead, cadmium, silver, germanium, iron, and thorium. Ore has been mined as deep as 300 m in this district. 

Some elements found in body tissues have no apparent physiological role, but have not been shown to be toxic. Examples are mbidium, strontium, titanium, niobium, germanium, and lanthanum. Other elements are toxic when found in greater than trace amounts, and sometimes in trace amounts. These latter elements include arsenic, mercury, lead, cadmium, silver, zirconium, beryUium, and thallium. Numerous other elements are used in medicine in nonnutrient roles. These include lithium, bismuth, antimony, bromine, platinum, and gold (Eig. 1). The interactions of mineral nutrients with... 

Zinc minerals tend to be associated with those of other metals the most common ate zinc—lead or lead—zinc, depending upon the dominant metal, zinc— copper or copper—zinc, and base metal such as silver. Zinc does occur alone, most often in the northeastern district, and here, as elsewhere, recoverable amounts of cadmium (up to 0.5%) are present. Other minor metals recovered from zinc ores are indium, germanium, and thallium. 

Hazards of Production. In most zinc mines, zinc is present as the sulfide and coexists with other minerals, especiaHy lead, copper, and cadmium. Therefore, the escape of zinc from mines and mills is accompanied by these other often more toxic materials. Mining and concentrating, usuaHy by flotations, does not present any unusual hazards to personnel. Atmospheric poHution is of Httle consequence at mine sites, but considerable effort is required to flocculate and settle fine ore particles, which would find their way into receiving waters. 

Cadmium occurs primarily as sulfide minerals ia ziac, lead—ziac, and copper—lead—ziac ores. Beneftciation of these minerals, usually by flotation (qv) or heavy-media separation, yields concentrates which are then processed for the recovery of the contained metal values. Cadmium follows the ziac with which it is so closely associated (see Zinc and zinc alloys see also Copper Lead). 

Naturally occurring cadmium compounds are limited to the rare minerals, greenockite [1317-58 ] CdS, and otavite (1), an oxycarbonate, but neither is an economically important source of cadmium metal or its compounds. Instead, cadmium compounds are more usually derived from metallic cadmium [7440-43-9] which is produced as a by-product of lead—2inc smelting or electrolysis (see Cadmiumand cadmium alloys). Typically, this cadmium metal is burnt as a vapor, to produce the brown-black cadmium oxide [1306-19-0], CdO, which then acts as a convenient starting material for most of the economically important compounds. 

A number of process improvements have been described, and iaclude the use of white mineral oil having a boiling range of 300—400°C (60) or the use of a mixture of cresols (61). These materials act to reduce the reaction mixture s viscosity, thus improving mixing. Higher sebacic acid yields are claimed by the use of catalysts such as barium salts (62), cadmium salts (63), lead oxide, and salts (64). 

Codeposition of silver vapor with perfluoroalkyl iodides at -196 °C provides an alternative route to nonsolvated primary perfluoroalkylsilvers [272] Phosphine complexes of trifluaromethylsilver are formed from the reaction of trimethyl-phosphme, silver acetate, and bis(trifluoromethyl)cadmium glyme [755] The per-fluoroalkylsilver compounds react with halogens [270], carbon dioxide [274], allyl halides [270, 274], mineral acids and water [275], and nitrosyl chloride [276] to give the expected products Oxidation with dioxygen gives ketones [270] or acyl halides [270] Sulfur reacts via insertion of sulfur into the carbon-silver bond [270] (equation 188)... 

Scheuhammer AM. 1990. Accumulation and toxicity of mercury, cadmium and lead in vertebrates. In Workshop to Design Baseline and Monitoring Studies for the OCS Mining Program, Norton Sound, Alaska — Workshop Proceedings, US Dept, of the Interior Minerals Management Service, OCS Study, mms90-059. 

Ghnaya, T., Nouairi, I., Slama, I., Messedi, D., Grignon, C., Abdelly, C., and Ghorbel, M.H., Cadmium effects on growth and mineral nutrition of two halophytes Sesuvium portulacastrum and Mesembry-anthemum crystallinum, Journal of Plant Physiology, 162 (10), 1133-1140, 2005. 

Other studies use soil or sediment samples for a more accurate indication of microbial activity in natural environments. In these samples, organic matter and clay particles play a role in metal toxicity.76112113 Both organic material and clay particles in soil can bind metals and reduce their bioavailability. For example, Pardue et al.87 demonstrated that much less solution-phase cadmium was required to inhibit trichloroaniline (TCA) dechlorination in a mineral-based soil than in a soil containing a higher concentration of organic matter. Other studies have shown that adding clay minerals to a medium mitigates toxicity. Clay minerals, such as kaolinite, montmorillonite, bentonite, and vermiculite, can bind to metals to decrease the amount that is bioavailable.112 115... 

The pH of a medium also impacts the formation of metal-phosphate precipitates. For example, divalent ionic cadmium (Cd2+) concentrations rapidly decline as both phosphate concentration and pH increase. Sandrin and Hoffman121 determined that when no phosphate is present in a commonly used mineral salts medium, the concentration of divalent ionic cadmium remains relatively constant until an abrupt decline above pH 8. When 15 mM inorganic phosphate is added to the medium, divalent cadmium ion concentrations rapidly decline at pH values above only 6. 

Babich, H. and Stotzky, G., Reductions in the toxicity of cadmium to microorganisms by clay minerals,... 

Kamel, Z., Toxicity of cadmium to two Streptomyces species as affected by clay minerals, Plant Soil, 93 (2), 195-203, 1986. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Minerals dominated by cadmium are rare the sulfide CdS (greenockite), especially, is very rarely found. However, cadmium is widespread in zinc ores in low concentrations (0.2-0.4%) and is separated during processing of these ores and production of zinc. 

Trace elements can be precipitated as carbonates, sulfates, phosphates and hydroxides in arid and semi-arid environments. But most carbonates are more stable in arid and semi-arid soils than other solid phases. Cadmium hydroxide (Cd(OH)2), sulfate (CdS04) and phosphates (Cd3(P04)2) are more soluble than carbonate (CdC03, octavite), therefore the former minerals are not stable in arid soils. In calcareous soils, CdC03 (octavite) is the main Cd mineral to control Cd2+ activity in soil solution. At high C02... 

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