Chromite Ore Processing Residue (COPR) is an industrial waste material generated during the manufacture of chromates from chromite ore. In chromite, chromium exists in the trivalent state as chromium iron oxide (FeCrO4). In this state, the chromium is inert and is not soluble in water.
As reported by Nriagu and Nieboer (Chromium in natural and human environment; John Wiley & Sons, New York, Chapter 6, 1994), chromium chemicals are manufactured by oxidising chromium(III) to chromium(VI). This is conventionally done by pulverizing the ore in a ball mill to less than 200 mesh size, mixing the pulverized ore with soda ash and lime and roasting the mixture in rotary kiln at 1100–1150° C. to obtain a molten roast. The molten roast thus obtained is then leached to extract the water-soluble sodium chromate. The lime added during the step of roasting reacts with aluminium, which is present in the ore and prevents it from dissolving when the roast is leached. A counter-current leach process is employed. The liquor released from the leaching process is a deep yellow solution, saturated with sodium chromate. The sodium chromate is either dried by conventional drying methods or converted by acidification and crystallization into the desired product, for example, into crystalline sodium dichromate. The solid material remaining after the leaching is the Chromite Ore Processing Residue.
COPR when disposed off without adequate precaution continues to leach chromate salts for decades. The total chromium content in the COPR on the residue weight can be as high as 10% and slowly solubilizing chromate compounds are present at concentrations of 0.7–5% in the COPR. Bartlett (Environmental Health Perspectives, 92, 17, 1991) reports that the soluble chromium (VI) compounds present in the COPR are known carcinogens.
The Chromite Ore Processing Residue (COPR) is a complex mixture of chromite ore, lime and soda ash. Disposal of COPR as landfills has produced areas of contaminated land across the world. Ground water from these sites can be highly contaminated with Cr (VI). Some of the residual water-soluble chromium compounds in the residue have relatively low degree of solubility and bleed very slowly, hence cannot be readily leached to exhaustion. Nevertheless, the water-soluble chromium compounds have sufficient solubility to pollute the environment by bleeding long after the residue has been discarded. Hursthouse (Journal of Environmental Monitoring, 3, 49, 2001) reports that areas like Glasgow in Scotland, where COPR was left behind in early 1960's continue to release chromium. To stock pile these residues under conditions that they could not be wetted would be difficult and expensive.
Burke et al. (Environmental Health Perspectives, 92, 131, 1991) report that the slowly bleeding chromium compounds present in the residue obtained in the manufacture of chromium chemicals from chromite ore include calcium chromate, CaCrO4, and calcium alumino-chromate, 3CaO.Al2O3.CaCrO4,12H2O, which are very slowly soluble in water, tribasic calcium chromite, Ca3(CrO4)2 which decomposes slowly in the presence of water to produce water-soluble hexavalent chromium and insoluble trivalent chromium hydroxide; and basic ferric chromate, Fe(OH)CrO4, which hydrolyses slowly in water to release chromate ions. The use of lime encapsulates chromium with calcium sulfate thus preventing its oxidation/subsequent extraction. The waste residue also contains some trivalent chromium compounds, but these are soluble to a lesser extent only. Mineralogical studies of soils and sediments contaminated by chromite ore processing residues (COPR) indicates three distinct categories of minerals—unreacted feedstock (chromite), high temperature phases produced during chromium extraction (brownmillerite, periclase and larnite), and finally, minerals formed under ambient weathering conditions, on the disposal sites (brucite, calcite, aragonite, ettringite, hydrocalumite, hydrogamet) as reported by Meegoda et al (Practice Periodical of Hazardous, Toxic and Radioactive Waste Management, 4, 7, 2000).
Several common remediation strategies are available for the management of chromium disposed off as landfills. Gancy and Wamser (U.S. Pat. No. 3,937,785, 1976) suggested the reduction of particle size of chromite ore processing residues such that at least 20 percent of the residue passes through a 200 mesh sieve to reduce the bleeding of water soluble chromium compounds from the residue. The water soluble chromium compounds are generally present only to the extent of 10–15% of the total chromium in the residue. Weathering conditions and various compounds present in the soil would continue to enable, the leaching of chromium from these residues with time.
Kapland et al. (U.S. Pat. No. 4,504,321, 1985) suggested the stabilization of the ore residue with sludge dredged from salty or brackish water to obtain a hardened mass. It was presumed that the impermeable layer around the residue would prevent the leaching of chromium when exposed to weathering conditions. However, the potential threat of chromium leaching from dumpsites containing such residues still persist as the impermeable layer can crack under natural climatic cycles.
The management of ore residue by possible conversion of chromium (VI) to chromium (III) has been a subject matter of discussion for long. Situ et al. (U.S. Pat. No. 5,395,601 1995) suggested the admixing of the residue with blast furnace coke for reducing chromium (VI), while the use of ferrous sulfate to convert chromium (VI) to chromium (III) has been suggested by Farmer et al. (Environmental Geochemistry and Health, 21, 331, 1999).
Technologies for immobilization are generally judged by the following criteria:                a) the ability to reduce hexavalent chromium to trivalent chromium and at the same time prevent its reversal back to hexavalent chromium and        b) the ability to demonstrate long term and low toxic metal leaching characteristics of the end product which these methods may seem lacking to satisfy.        
The immobilized chromium from all the above technologies ultimately end up as landfills. There are evidences for chromium cycling in soil as demonstrated by Bartlett (Environmental Health Perspectives, 92, 17, 1991) where chromium (III) can be oxidized to chromium (VI) by other compounds present in soil and subsequently enter the food cycle.
In an effort to minimize the risk of exposure to the public, regulatory authorities are now demanding the cleanup of chromium-contaminated water and soils. Vitrification or making glass out of wastes, immobilizes the chromium in the glass. Meegoda et al. (Practice Periodical of Hazardous, Toxic, and Radioactive waste management, 4, 89, 2000) has employed cold top vitrification technology to immobilize chromium from ore residues. This technology proposes the use of the vitrified product in highway construction industry. The success of this technology would lie in the effective immobilization of chromium in a siliceous matrix and use of the product by the construction industry.
Despite many years of researching alternatives, landfill is still the world-wide industry standard today. The larger western world factories are located close to existing or historic clay mining operations normally involved in refractory brick manufacture. Such plants generate approximately 1 ton of treated mineral waste per ton of rated sodium chromate capacity employed, typically containing 8–12% Cr2O3. High-lime process plants operating today collectively generate an estimated 600 kt y−1 of mineral waste. Darrie (Environmental Geochemistry and Health, 23, 187, 2001) suggests that the challenge of successful treatment of high-lime process waste is considerably greater since it typically contains 20–40% calcium oxide equivalent together with salts of iron. The industry is more than 170 years old and the inability to landfill mineral waste has been the single most significant factor dictating factory closures.
Sreeram and Ramasami (Journal of Environmental Monitoring, 3, 526, 2001) had reported characterization of the chromite ore processing residues and attempted to recover chromium from such residues through chelative and oxidative extraction procedures. The chromium in the residue exists in various phases which can be broadly classified under exchangeable, carbonate bound, Fe—Mn oxide bound or reducible, oxidizable and residual chromium. While chromium in the exchangeable, oxidizable and carbonate bound forms are easily discharged into the immediate environment, the reducible and residual chromium are not easily discharged. The reducible chromium and residual chromium phases are formed either during the high temperature roasting of the ore or from the unreacted ore. The chromium in these phases are encapsulated within the calcium matrix.
The major limitation associated with this extraction method is that only a maximum of 70% recovery of a mixture of chromium and iron is possible by chelative extraction. Also, the process requires repetitive extraction cycles making it cumbersome. The oxidative extraction process necessitates use of sodium peroxide, a designated explosive generating large quantities of heat on contact with water, along with sodium hydroxide for an economical recovery of chromium (>70%). The associated risk of using sodium peroxide in large-scale use has obviously been a limiting factor.
Sreeram and Ramasami teaches a process wherein sodium peroxide is added along with sodium hydroxide to the COPR. In this case, sodium peroxide would react with water in the residue to generate hydrogen peroxide, an oxidant. The reaction proceeds as follows:Na2O2+2H2O→2NaOH+H2O2 
It is a well known fact that hydrogen peroxides are unstable and the disproportionation of the same is fast in the presence of alkali. The reaction proceeds as follows:H2O2→H2O+½H2O
The melting point of sodium peroxide is 600° C. and hence, for this reaction to occur, a temperature above 500° C. is essential.
Also, it should be noticed that in the presence of water, sodium peroxide results in the generation of 142 kJ/mol of heat. Sodium peroxide is corrosive on metals and cannot be used in kilns, rotary kilns etc. It is highly irritating to the eyes, skin and mucous membrane and hence its use in industrial application is limited. Further, the dissociation of peroxide to yield hydrogen peroxide which in turn disproportionates to water slows down the rate of reaction and results in the addition of larger quantities of alkali and oxidant that is required for the stoichiometric conversion of Cr(III) to Cr(IV). Further, commercial sodium peroxide contains Fe2O3 as contaminant and this would add to the residue left over after chromium extraction. Finally, to the best of the Inventor's knowledge, no other prior art is available on the recovery of chromium and iron from the COPR.