Zinc ores, cadmium from

Cadmium is usually found in zinc ores and is extracted from them along with zinc (p. 416) it may be separated from the zinc by distillation (cadmium is more volatile than zinc. Table 14.2) or by electrolytic deposition. 

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. 

Cadmium production is dependent on the processing of zinc ores, which often contain 0.2 to 0.4% cadmium. As can be seen from Table 2, U.S. demand for cadmium normally exceeds the domestic supply and the United States is dependent on imports. 

The isolation of zinc, over 90% of which is from sulfide ores, depends on conventional physical concentration of the ore by sedimentation or flotation techniques. This is followed by roasting to produce the oxides the SO2 which is generated is used to produce sulfuric acid. The ZnO is then either treated electrolytically or smelted with coke. In the former case the zinc is leached from the crude ZnO with dil H2SO4, at which point cadmium is precipitated by the addition of zinc dust. The ZnS04 solution is then electrolysed and the metal deposited — in a state of 99.95% purity — on to aluminium cathodes. 

Name from cadmia (Latin = calamine), a zinc ore in which cadmium occurs... 

Cadmium also may be recovered from zinc ores and separated from other metals present as impurities by fractional distillation. Alternatively, the cadmium dust obtained from the roasting of zinc ore is mixed with sulfuric acid. Zinc dust is added in small quantities to precipitate out copper and other impurities. The metal impurities are removed by filtration. An excess amount of zinc dust is added to the solution. A spongy cadmium-rich precipitate is formed which may he oxidized and dissolved in dilute sulfuric acid. Cadmium sulfate solution is then electrolyzed using aluminum cathodes and lead anodes. The metal is deposited at the cathode, stripped out regularly, washed and melted in an iron retort in the presence of caustic soda, and drawn into desired shapes. More than half of the world s production of cadmium is obtained by elecrolytic processes. 

Dr. Stromeyer detected cadmium in tutty and other kinds of zinc oxide, in metallic zinc, in Silesian zinc ores, and in several blendes, especially one from Przibram, Bohemia, which contained 2 or 3 per cent of it (10, 24). Thus it is evident that cadmium was first discovered in substances of which it is merely a non-essential constituent. 

In a letter to the Annals of Philosophy, dated Cambridge, February 18, 1820, Edward Daniel Clarke wrote as follows Some varieties of radiated blende from Przibram in Bohemia are described by Stromeyer as containing two or three per cent of cadmium. At a sale. .. in London, I procured specimens of the particular mineral thus alluded to, which were sold under the name of splendent fibrous blende from Przibram, pronounced Pritzbram. I found afterwards that they had been brought to England by Mr. J. Sowerby of Lisle-street, a dealer in minerals.. . . Upon my return to Cambridge, I endeavoured to obtain cadmium from this ore, and succeeded. . . (133). Clarke also found this element in die zinc silicate from Derbyshire, England, and his results were soon confirmed by W. H. Wollaston and J. G. Children. In 1822 Clarke published a paper on the presence of cadmium in commercial sheet zinc (134). 

In 1863 Reich began a search for thallium in some Freiberg zinc ores from the Himmelsfurst mine consisting mainly of arsenical pyrites, blende, lead glance, silica, manganese, copper, and small amounts of tin and cadmium (19, 43). After roasting the blende to remove most of the sulfur and arsenic, he decomposed it with hydrochloric acid (47). When Clemens Winkler, who was then a metallurgist in the Saxon smalt works, visited Professor Reich in 1863, the latter showed him a straw-yellow precipitate and said, This is the sulfide of a new element (52). Because of his colorblindness, however, Reich entrusted the spectroscopic examination to his assistant, Richter. 

Zinc.—The chief ore of zinc is the sulphide. To convert it into the oxide, it is roasted on a flat hearth in a current of air 2ZnS + 302= 2ZnO + 2S02. The oxide is mixed with small coal (slack) and placed in cylindrical retorts of fireclay. These retorts have pipes of rolled sheet-iron luted to the open ends with fireclay they are packed into a furnace in tiers, and the temperature is raised to bright redness. The coal distils first, giving off coal-gas, which expels air from the retorts. When the temperature exceeds 1000% the zinc distils and condenses in the iron pipes. It happens that almost all zinc ores contain cadmium sulphide, which, like zinc sulphide, is converted into oxide by roasting and on distillation, the cadmium, which is the more volatile metal, distils over first and condenses in the outer portion of the tubes. These are untwisted and the metal removed with a chisel. 

Along with mercury and lead, cadmium (Cd) is one of the big three heavy metal poisons. Cadmium occurs as a constituent of lead and zinc ores, from which it can be extracted as a byproduct. Cadmium is used to electroplate metals to prevent corrosion, as a pigment, as a constituent of alkali storage batteries, and in the manufacture of some plastics. 

The major ore of zinc (though it occurs in numerous other minerals) is sphalerite, a form of ZnS usually found to also contain iron and commonly associated with galena (PbS). Cadmium is mostly found isomorphously replacing zinc in zinc minerals. To obtain metallic zinc from its ore, the ore is first calcined to ZnO and this is then roasted at ca. 1000°C with charcoal in the absence of air, whence the metal distills out. Cadmium can be separated from sulfate solutions of both zinc and cadmium by addition of zinc dust ... 

Cadmium (2 x 10- % of the earth s crust) is derived exclusively from zinc ores the mineral greenockite, CdS, is without economic importance. Its lower b.p. enables cadmium metal, which may amount to 0.5% of crude zinc, to be separated by distillation. It is used in electroplating, in bearing metals, in fusible alloys, and in atomic reactors to absorb neutrons. 

Derivation (1) Dust or fume from roasting zinc ores is collected, mixed with coal or coke and sodium or zinc chloride, and sintered. The cadmium fume is collected in an electrostatic precipitator, leached, fractionally precipitated, and distilled. (2) By direct distillation from cadmium-bearing zinc. (3) By recovery from electrolytic zinc process (approximately 40%). 

Smoke from the factories was a constant element of the community s life. The aged zinc plant belched out prodigious amounts of fume and dust full of sulfur, zinc, lead, cadmium, and arsenic. Control technology existed—these emissions could be trapped in baghouses and electrostatic precipitators—but U.S. Steel, like its competitors, only installed these devices where it was profitable to do so. Thus the plant used the sulfur from its roasters to make acid and collected metallic fume from ore sintering. The sale of these by-products was vital to the economics of zinc making, a business whose personnel,... 

Jinzu Valley, Japan. One of the most infamous cases of contaminated land and health occurred in Japan and the effects were most prominent immediately after the Second World War. Around the end of the 19th century, soils in the Jinzu River basin, part of the Toyama prefecture, became contaminated with cadmium as a result of activities upstream at the Kamioka mines. The main activity at this mine was the mining and processing of zinc (cadmium is often associated with zinc ores) with the result that wastewater rich in heavy metals was discharged into the Jinzu River. Contaminants from this industry moved down-stream and caused contamination of soils in paddy fields as a result of abstraction of river water into fields in order to cultivate the local rice crop. Under favourable conditions, cadmium can be a fairly mobile heavy metal, particularly in soils with low pH, and increases in soil cadmium can often result in an increase in the uptake of cadmium by plants. This in turn results in an increase in dietary exposure and the consumption of contaminated agricultural crops can be a major pathway of human exposure. 

In the 50 s, electrochemical and hydrometallurgical processes were tested and used on a small scale for the production of recovered cadmium. The principle whereby the cadmium was extracted by distillation is described and used in the production of primary cadmium (coming from zinc ores). 

The abundance of zinc in the Earth s crust averages 70 g ton . Zinc is a chalcophUic element like copper and lead, and a trace constituent in most rocks. Zinc rarely occurs naturally in its metallic state, but many minerals contain zinc as a major component from which the metal may be economically recovered. The mean zinc levels in soils and rocks usually increase in the order sand (10-30 mg kg ), granitic rock (50 mg kg ), clay (95 mg kg ) and basalt (100 mg kg ) (Adriano 1986, Malle 1992). Sphalerite (zinc blende, cubic ZnS) is the most important ore mineral and the principal source for zinc production. Smithsonite (ZnGOj) and hemimorphite (Zri4(Si207) (0H)2 X H2O) were mined extensively before the development of the froth-flotation process (Jolly 1989, Graf 2002). The main impurities in zinc ores are iron (1-14%), cadmium (0.1-0.6%), and lead (0.1-2%), depending on the location of the deposit (ATSDR 1994). 

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