Manufacturing of Chromium compounds (Traditional Process):Chromium compounds are of considerable importance to many industries, but their manufacturing is a major source of pollution. Chromate plants discharge large amounts of chromium-containing residues, dusts, and waste gases [1]. In the traditional process, three problems are highly distinguishable. The first is the environmental pollution. During production of one ton of chromium anhydride (CrO3) product, the chromate production plant has to discharge approximately 2.5 to 3.0 tons of toxic chromium-containing residues that are difficult to be detoxified and comprehensively used because of their high content of hexavalent chromium. Also, the produced calcium chromate (CaCrO4) is highly toxic and carcinogenic. Furthermore, the discharge of large amounts of chromium-containing gases and dusts creates serious pollution. The second problem is the low conversion efficiency of the main element chromium. Although the reaction temperature may be as high as 1200 C, the conversion efficiency of chromium is only 76%, which means that a considerable amount of chromium is discharged into the residue. The third problem with this process is the production of by-products that are not valuable. The chromium-containing Glaubers salt (Na2SO4.10H2O) and sodium bisulfate (NaHSO4) produced are of little use and constitute a pollution source [2]. Consequently, the total atom utilization efficiency of the traditional process is quite low [3].The major compounds which are manufactured are the chromates and the dichromates of sodium, potassium and chromic acid. The main step in manufacturing chromium compounds involves the conversion of the water insoluble ore (chromite) into a water soluble chromate. Finely grounded chromite (FeOCr2O3) is intimately mixed with sodium carbonate (Na2CO3) and crude calcium carbonate (as dolomite or limestone). This mixture is conveyed to a rotating furnace to be roasted at 1100 0C. The limestone and dolomite acts as a mechanical separator, allowing oxygen to react with the chromite and sodium carbonate (Na2CO3) A series of chemical reactions takes place which can be represented as follows [4, 5]. 1. 4 (FeOCr2O3) + 4 (Na2CO3) 4 (Na2Cr2O4) + 4 (FeO) + 4 (CO2)2. 4 (Na2Cr2O4) + 4 (Na2CO3) + 6 (O2) 8 (Na2CrO4) + 4 (CO2)3. 4 (FeO) + (O2) 2 (Fe2O3)Calcium carbonate is added to increase heat of the reaction some calcium chromate is formed during the process and is mostly decomposed by excess soda. The red hot roasted product (frit) is transferred to wash tanks where hot water is carefully added this flooding give rise to a strong exothermic reaction, as a result sodium mono chromate is leached out and the liquor run off and is treated according to special treatments. In order to obtain dichromate, the mono chromate liquor is treated with dilute sulphuric acid (H2SO4) in special tanks. The reaction proceeds as [4].4. 2 Na2CrO4 + H2SO4 Na2SO4 + Na2Cr2O7 + H2OThe sodium sulphate is less soluble then the dichromate and much of it separates out and is removed more sodium sulphate separates and is removed. The final mother liquor contains the dichromate which crystallizes out on concentration. In order to obtain potassium dichromate a concentrated solution of potassium salt (e.g potassium chloride) is added to a concentrated solution of sodium dichromate. Potassium dichromate is precipitated immediately. Another chromium compound is chromic acid (CrO3) is prepared by adding strong sulphuric acid to a concentrated solution of potassium dichromate, the reaction is given as [4].5. K2Cr2O7 + H2SO4 K2SO4 +H2O +2 CrO3Partial drying of the principle products, namely potassium and sodium mono- and dichromates is carried out in filter presses and the process completed in centrifugal dryers or heated baths [4].Figure 1 represents the flowsheet of the traditional process used for producing chromic oxide (Cr2O3) from chromite ore. The traditional process does not extract all of the chromium from the chromite ore. The residue contains unreacted chromite ore and un extracted chromate, and usually must be disposed of on-site. The chromium containing sodium sulfate (Na2SO4) formed as a byproduct has little commercial value and also must be disposed of on-site [5].

Green manufacturing process of chromium compounds:A green manufacturing process for chromium compounds has been developed by the Instituteof Process Engineering, Chinese Academy of Sciences [6], with the design objective of eliminating pollution [7], the green process includes continuous oxidization of chromite ore in a sub molten salt medium at 300 C coupling of reaction and separation, recovery, and recycling of the reaction medium and comprehensive use of the multiple components in chromite ore. The essence of the green process is that traditional oxidation roasting of chromite ore with sodium carbonate at 1200 C is replaced by continuous liquid-phase oxidation of chromite ore in the sub molten salt medium at 300 C in a multiphase reactor. at the source The new cleaner process is based on the principles of cleaner production and industrial ecology [8], and aims to achieve the 3Rs (Reduce, Recycle, Reuse) objectives, according to the reactions (6,7,8,9,10).To Reduce: A new reaction path (reaction 6, 7, 8) was designed and no solid additives were needed in the oxidation of chromite ore, the amount of waste residue remaining following leaching was remarkable reduced. The reactants such as (CO2) (reaction 9) and carbon black (C) (reaction 10) we employed to convert the semi-finished products to the final products, minimizing possible pollution sources.6. FeO.Cr2O3 + 2KOH + 7/8 O2 Fe2O3 + H2O + K2CrO47. MgO.Cr2O3 + 2 KOH + O2 MgO + H2O + K2CrO48. Cr2O3 + 2 KOH + 3/4 O2 H2O + K2CrO49. 2 K2CrO4 + H2O + 2 CO2 K2Cr2O7 + 2 KHCO310. K2Cr2O7 + C Cr2O3 + K2CO3 + COTo Recycle:An excessive amount of potassium hydroxide (KOH), in sub-molten salt state, was employed as the reaction medium, and most was recycled after the reaction occurred. Only a small amount was consumed during the reaction and needed to be supplemented.

11. 2 KHCO3 K2CO3 + H2O + CO212. K2CO3 + Ca(OH)2 CaCO3 + 2 KOH13. CaCO3 CaO + CO214. CaO + H2O Ca(OH)2

To Reuse:

The produced intermediates such as potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) were reused to manufacture potassium hydroxide (KOH) (reaction 11, 12) the calcium hydroxide (Ca (OH) 2) can be recovered (reaction 13, 14).[9]In the new cleaner process, illustrated in figure 2 aluminum-bearing byproducts and magnesium-bearing byproducts are manufactured, in addition to the products of potassium dichromate (KCr2O7) and chromic oxide (Cr2O3) [10]. As the final obtained ferrite-enriched residues were used as raw materials in the cement industry.[5]

Refrences:1.Walawska, B. and Z. Kowalski, Environmental evaluation of the effects of using chromic waste in the production of chromium compounds. Journal of Cleaner Production, 2001. 9(3): p. 219-226.2.Trost, B.M., The atom economy--a search for synthetic efficiency. Science, 1991. 254(5037): p. 1471-1477.3.Zheng, S., Y. Zhang, and Z. Li, The quantitative evaluation index of the greenization of traditional process industry. Journal of Chemical and Industrial Engineering Supplement (China), 2000. 51: p. 343-347.4.Buckell, M. and D. Harvey, An environmental study of the chromate industry. British journal of industrial medicine, 1951. 8(4): p. 298.5.Xu, H.-B., et al., Development of a new cleaner production process for producing chromic oxide from chromite ore. Journal of Cleaner Production, 2006. 14(2): p. 211-219.6.Zhang, Y., et al., Green chemistry of chromate cleaner production. Chinese Journal of Chemistry, 1999. 17(3): p. 258-266.7.Cano-Ruiz, J. and G. McRae, Environmentally conscious chemical process design. Annual Review of Energy and the Environment, 1998. 23(1): p. 499-536.8.Manahan, S.E., Industrial ecology: environmental chemistry and hazardous waste. 1999: CRC Press.9.Yang, R., The study on recovery of alkali and refinement of products in clean production of chromate. 2001, PhD Thesis, Beijing: Institute of Process Engineering, Chinese Academy of Sciences.10.Wang, W., Comprehensive utilization and zero emission of chromium containing residue in cleaner production of chromates. 2003, PhD Thesis, Beijing: Institute of Process Engineering, Chinese Academy of Sciences.