Chromite leaching

Das, G. K. Acharya, S. Anand, S. Das, R. P. Acid pressure leaching of nickel-containing chromite over burden in the presence of additives. Hydrometallurgy 1995, 39, 117-128. 

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. 

Less than 15% of the ore is transformed into chromium compounds, principally chromates, dichromates, chromium(VI) oxide, chromium(III) oxide, and so on. Alkaline oxidative roasting of chromite in rotary kilns yields sodium chromate (see equation 1), which is leached out with water and typically converted into sodium dichromate with sulfimc acid (equation 2) or carbon dioxide (equations). Fiuther treatment of sodium dichromate with sulfuric acid yields chromium(VI) oxide ( chromic acid ), while its reduction (with carbon, sulfur, or anuuo-nium salts) produces chromium(III) oxide. Finally, basic chromium(III) salts, for example Cr(0H)S04, which are used as tanning agents for animal hides, also result from reduction of sodium dichromate. Heterogeneous chromium catalysts are used for the polymerization of ethylene. 

Cohen and Ng (C21) described a process of improving the chrome-iron ratio of reduced pelletized chromite concentrate by acid leaching of iron without significant loss of chromium. A 5% sulfuric acid solution was used to quench and leach the fired pellets from 400 C to room temperature. 

Derivation Roasting powdered chromite with potash and limestone, treating the cinder with hot potassium sulfate solution, and leaching. 

One environmental concern is that around the world there are landfill areas where over the years chromite ore processing residues (COPR) have been dumped, and chromium from these landfills is being leached into the ground water. The environmental impact of chromium in soils and sediments is dependent on specia-tion and on the response of the matrix to biological and physico-chemical conditions. These factors control the mobilization of chromium from the solid into the aquatic phase and uptake and transfer into living systems (Hursthouse 2001). 

If calcium chromate be treated with a soln. of potassium sulphate, the calcium chromate is converted into calcium sulphate, which is precipitated, and potassium chromate, which remains in soln. Instead of leaching the calcium chromate with a soln. of potassium sulphate, W. J. Chrystal showed that if ammonium sulphate is used, a soln. of ammonium chromate is produced, and J. J. Hood found that if the soln. of potassium salt be treated with sodium hydrosulphate, potassium sulphate crystallizes from the soln., while sodium dichromate remains in soln. According to F. M. and D. D. Spence and co-workers, if a mixtiu-e of ammonia and carbon dioxide be passed into the aq. extract of the calcium chromate. Calcium carbonate is precipitated while ammonium and alkali chromate remain in soln. If the liquid be boiled, ammonia is given off, and sodium dichromate remains in soln. S. Pontius used water and carbon dioxide under press, for the leaching ptocess. J. Brock and W. A. Rowell purified alkali chromite by treating the soln. with strontium hydroxide, and digesting the washed precipitate with a soln. of alkali sulphate or carbonate W. J. A. Donald used calcium hydroxide or barium chloride as precipitant. A mixture of chromite with calcium carbonate and potassium carbonate was formerly much employed. Modifications of the process were described by W. J. A. Donald, A. R. Lindblad, C. J. Head, 8. G. Thomas, W. Gow, J. Stevenson and T. Carlile, L. I. Popoff,G. Bessa, P.Weise, P. N. Lukianoff,... 

Iron chromate (as the mineral chromite) has to be powdered extremely finely and then fused in a nickel crucible with sodium peroxide. First melt about 1 g of solid Na202 in a nickel crucible by very gentle heating, allow to cool and add 10-50 mg of the mineral. Heat the mixture gently until the fusion is complete. Leach the cooled metal with hot water until the mixture dissolves. Remove the crucible and carefully acidify the solution with 6m hydrochloric acid. The metal hydroxides should dissolve completely the solution is green because of the presence of nickel ions, originating from the walls of the crucible. Iron(III) and chromium can be tested in the solution (cf. Sections 3.22 and 3.24). Examine the crucible and discard it if the walls are worn thin. 

Weng CH, Hnang CP, Allen HE, Cheng AHD, Sanders PR (1994) Chromium leaching behavior in soil derived from chromite ore processing residne. Science of the Total Environment 154 71-86. 

Leaching tests showed that the leaching of metals is generally low and that the leaching of chromiiun caused by the presence of chromite sand is negligible [169, Orkas, 2001],... 

To produce the desired hexavalent, water-soluble chromate chemicals, the ore is bail-milled to less than 100 mesh, mixed with soda (Na2C03) and lime, and roasted in rotary kilns at 1100-1150°C in air. While the mixture does not fuse, the molten soda ash reacts with the chromite to form water-soluble sodium chromate. The lime reacts with any aluminum impurities in the ore to form water-soluble compounds. The resulting yellow mixture is then leached with water and filtered the aluminum impurities are precipitated as aluminum hydroxide the solution is then acidified and concentrated to yield sodium chromate. From the sodium chromate, chromium can be obtained by electrolytic reduction or reduction with carbon. 

Geochemical speciation Batch experiments ORCHESTRA Leaching of Cr(VI) from chromite ore processing residue [4]... 

The roasted clinker is then leached out with hot water and the resulting liquor filtered to remove insoluble silica, along with aluminum and iron oxides present as impurities in the original chromite ore. Pure sodium dichromate is then crystallized and dried. The anhydrous sodium chromate can then be reduced to chromium trioxide by simple carbothermic reduction. Note that the sodium chromate produced can be converted into the dichromate by dissolving it in sulfuric acid and is the basis for the manufacture of all industrially important chromium chemicals. Once produced, chromium trioxide is mixed with aluminum powder and lime and placed inside a refractory-lined steel vessel. The exothermic reaction is started by igniting a pyrotechnic mixture made of barium peroxide and aluminum powder or potassium chlorate aluminum powder ... 

High-temperature calcination of chromite ore and soda ash leads to oxidation of chromium to sodium chromate, which is leached out by water. After acidifying, sodium dichromate, the basic material for chromium chemicals, is formed. Chromium trioxide CrOj, chromic add, prepared from sodium dichromate, is used for preparing the baths for chromium plating. Hexavalent chromium chemicals are also still used to some extent in pigments, for corrosion protection and for timber treatment Chromic add is also used for video/audio tapes although iron-based tapes have taken over more than 90% of this market. 

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