Chromite ore

The only commercial ore, chromite [1308-31-2], which is also called chromite ore, chrome ore, and chrome, has the ideal composition Fe0-Cr2 03, ie, 68 wt % Cr202, 32 wt % FeO, or ca 46 wt % chromium. Actually the Cr Fe ratio varies considerably and the ores are better represented as (Fe,Mg)0-(Cr,Fe,Al)203. Table 1 gives the classification of chromite ores. 

During much of the nineteenth century, the United States was the principal world producer of chromite ore (37). However in the latter twentieth century the United States has become completely dependent on imports from South Africa and Turkey (chromite) South Africa, Zimbabwe, Turkey, and Yugoslavia (ferrochromium) and the Philippines (chromite for refractory brick). 

Russia and the RepubHc of South Africa account for more than half the world s chromite ore production. Almost all of the world s known reserves of chromium are located in the southeastern region of the continent of Africa. South Africa has 84% and Zimbabwe 11% of these reserves. The United States is completely dependent on imports for all of its chromium (4). The chromite s constitution varies with the source of the ore, and this variance can be important to processing. Typical ores are from 20 to 26 wt % Cr, from 10 to 25 wt % Fe, from 5 to 15 wt % Mg, from 2 to 10 wt % Al, and between 0.5 and 5 wt % Si Other elements that may be present are Mn, Ca, Ti, Ni, and V. AH of these elements are normally reported as oxides iron is present as both Fe(II) andFe(III) (5,6). 

Compounds that have the empirical formulas MCr02 and DCr204 where M is a monovalent and D a divalent cation, are known as chromites. These are actually mixed oxides and probably are better written as M20-Cr203 and D0-Cr203, respectively. The oxides of D are largely spinels, ie, the oxygen atoms define a close-packed cubic array having the octahedral holes occupied by the Cr(III) cation and the tetrahedral holes occupied by D (54). Chromite ore is an important member of this class of oxides. 

Manufacture The primary iadustrial compounds of chromium made directly from chromite ore are sodium chromate, sodium dichromate, and chromic acid. Secondary chromium compounds produced ia quantity include potassium dichromate, potassium chromate, and ammonium dichromate. 

Recommendations by the ACGIH are classified as threshold limit values (TLV) based on 8-h TWA. Chromium metal and alloys, Cr(II) compounds and Cr(III) compounds, including chromite ore, have a TLV of 0.5 mg/m Cr in air. Water-soluble Cr(VI) compounds have a TLV of 0.05 mg/m Cr. Certain water-insoluble Cr(VI) compounds, ie, the chromates of 2inc, barium, calcium, lead, strontium, sintered chromic acid, and processing chromite ores, also have a TLV of 0.05 mg/m as well as a human carcinogen designation (145). 

Cbrome bricks are manufactured in much the same way as magnesite bricks but are made from natural chromite ore. Commercial ores always contain magnesia and alumina. Unburned hydraulically pressed chrome bricks are also available. 

The sodium chromate produced in the isolation of chromium is itself the basis for the manufacture of all industrially important chromium chemicals. World production of chromite ores approached 12 million tonnes in 1995. 

Chromium compounds of high purity can be produced from chromite ore without reduction to the free metal. The first step is the roasting of chromite ore in the presence of sodium carbonate ... 

Hexavalent chromium and metals such as zinc and nickel that are present as impurities in the chromites ore are predominant pollutants associated with the sodium dichromate plant. They are generally removed through alkaline precipitation, clarification, filtration, and settling processes. The wastewater is treated with sodium sulfide to reduce hexavalent chromium to trivalent chromium,... 

James B.R. Hexavalent chromium solubility and reduction in alkaline soils enriched with chromite ore processing residue. J Environ Qual 1994 23 227-233. 

Preparation. Oxidation of the chromite ore by air in molten alkali gives sodium chromate, Na2Cr04 that is then converted to Cr203. The oxide is further reduced with aluminium or silicon to form chromium metal. Solutions suitable for electrolytic production of chromium (for plating) can be obtained from ore by oxidative roasting in alkali or by dissolution of chromite in H2S04 and especially by dissolving ferro-chromium in sulphuric acid. 

Schoenberg et al. (2008) presented the first set of Cr isotope data for rocks and Cr(ll) rich ores. Mantle derived rocks and chromite ores from layered intrusions have a uniform Cr/ Cr isotope ratio very close to the certified Cr standard NIST SRM 979. The Cr isotope composition of hydrothermal lead chromates is substantially heavier (S Cr from 0.6 to 1.0%o) than the rocks from which the chromium was leached. 

Trivalent chromium compounds (Cr )(chromic compounds) including chromic oxide (Cr203), chromic sulfate (Ct2[S04]3), chromic chloride (CrCb), chromic potassium sulfate (KCr[S04]2), and chromite ore (FeOCdCr203). 

Chromite ore roast mixed with sheep fat implanted intrapleurally in rats produced sarcomas coexisting with squamous cell carcinomas of the lungs the same material implanted in the thigh of rats produced fibrosarcomas/ However, the lARC concluded that these studies were inadequate to fully evaluate the carcinogenicity of this compound/ Other animal studies have found no increase in the incidence of tumors with chromium metal and chromite ore/ The lARC has determined that there is inadequate evidence in humans and animals for the carcinogenicity of metallic chromium and chromium(III) compounds. 

Hueper, WC Experimental studies in metal cancerigenesis. X. Cancerigenic effects of chromite ore roast deposited in muscle tissue and pleural cavity of rats. AMA Arch Ind Health 18 284-291, 1958... 

High-carbon ferrochromium alloys are made by the reduction of chromite ore with carbon in an arc furnace. On the other hand, low-carbon ferrochromium is obtained by silicon reduction of the ore. The carbon content of ferrochromium can be reduced further by heating high-carbon alloys with ground quartzite or by oxidation in vacuum and removal of carbon monoxide formed. Ferrochromium alloys are used in the manufacture of stainless steel. 

Sevenson also reported that MAECTITE was used at the Dundalk Marine Terminal in Baltimore, Maryland. At this site, approximately 8000 tons of soil were contaminated with chromite ore residuals that had been used as fill material. The costs for the project were estimated a SnO/yd (D15712K, pp. 18-21). 

Chromic acetate (see Chromium and chromium compounds) Chromic chloride (see Chromium and chromium compounds) Chromic oxide see Chromium and chromium compounds) Chromic phosphate see Chromium and ehromium compounds) Chromite ore see Chromium and ehromium compounds) Chromium and chromium compounds see also Implants, surgical)... 

Chromite ore see Chromium and chromium compounds) Chromium and chromium compounds... 

Chromium (IV) Production of chromium compounds, paint pigments, reduction of chromite ore Lung cancer... 

Cardiovascular Effects. Information regarding cardiovascular effects in humans after inhalation exposure to chromium and its compounds is limited. In a survey of a facility engaged in chromate production in Italy, where exposure concentrations were 0.01 mg chromium(VI)/m3, electrocardiograms were recorded for 22 of the 65 workers who worked in the production of dichromate and chromium trioxide for at least 1 year. No abnormalities were found (Sassi 1956). An extensive survey to determine the health status of chromate workers in seven U.S. chromate production plants found no association between heart disease or effects on blood pressure and exposure to chromates. Various manufacturing processes in the plants resulted in exposure of workers to chromite ore (mean time-weighted concentration of 0-0.89 mg chromium(ni)/m3) water-soluble chromium(VI) compounds (0.005-0.17 mg chromium(VI)/m3) and acid-soluble/water-insoluble chromium compounds (including basic chromium sulfate), which may or may not entirely represent trivalent chromium (0-0.47 mg chromium/m3) (PHS 1953). No excess deaths were observed from cardiovascular diseases and ischemic heart disease in a cohort of 4,227 stainless steel production workers from 1968 to 1984 when compared to expected deaths based on national rates and matched for age, sex, and calender time (Moulin et al. 1993). No measurements of exposure were provided. In a cohort of 3,408 individuals who had worked in 4 facilities that produced chromium compounds from chromite ore in northern New Jersey sometime between 1937 and 1971, where the exposure durations of workers ranged from <1 to >20 years, and no increases in atherosclerotic heart disease were evident (Rosenman and Stanbury 1996). The proportionate mortality ratios for white and black men were 97 (confidence limits 88-107) and 90 (confidence limits 72-111), respectively. 

Hematological Effects. Hematological evaluations of workers occupationally exposed to chromium compounds have yielded equivocal results. Ninety-seven workers from a chromate plant were exposed to a mixture of insoluble chromite ore containing chromium(III) and soluble sodium chromate and dichromate. Hematological evaluations revealed leukocytosis in 14.4% or leukopenia in 19.6% of the workers. The leukocytosis appeared to be related primarily to monocytosis and eosinophilia, but controls had slight increases in monocytes and occasional increases in eosinophils without leukocytosis. 

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