Chapter II (1)

Phase and Microstructural characterization of chromite ore and manufacturing of sodium dichromateChapter 2 (Literature review)

Submitted by: Asif khan & Waleed Subhan(BS Physics Students)Submitted To: Dr. Yaseen Iqbal DEPARTMENT OF PHYSICS UNIVERSITY OF PESHAWAR

CHAPTER II LITRATURE REVIEW1.1Chromite ore:The chemical formula of chromite ore is FeO.Cr2O3 with Cr/Fe ratio as 2:1.Chromite ores are basically classified on the basis of Cr2O3 content and Cr to Fe ratio. Naturally occurring chromite minerals tends to have a spinel structure, which is characterized by the partial substitution of Mg2+, Ca2+ or Mn2+ for Fe2+ and Al3+ or Fe3+ for Cr3+ with the general formula (Fe2+, Mg2+, Ca2+)O.(Cr3+, Al3+,Fe3+)2O3 [1]. Highest-grade chromite have Cr/Fe ratio more than 2 containing minimum of 46 to 48 wt% Cr2O3. Chemical and refractory-grade chromite typically have Cr/Fe ratios ranging from 1.4 to 2.0. Chemical-grade (high-iron) chromite contain large amounts of iron often results in Cr/Fe ratios of close to 1, absolute amount of contained chromium ranges from 40 to 46 wt % Cr203. Low-grade chromite have low Cr/Fe ratios and contain relatively small amounts of chromium. Refractory-grade chromite contain relatively large quantities of Al203 (greater than 20 wt %) and have Cr203+ Al203 level more than 60 wt % [2].1.2Phase and microstructural analysis of chromite ore: Knowledge about particle size and distribution is essential for a wide variety of industrial processes. To recover minerals in the mineral processing industry, an ore must be ground to produce liberated grains within the size range defined. Mineral recoveries from a concentrator can often be improved by utilizing mineralogical studies that affect mineral beneficiation. In order to improve metal recovery of economic minerals in beneficiation, the necessary quantitative mineralogical data can be obtained by developed image analyzer [3], cone beam X-ray microtomography system has been used for the quantitative analysis of multiphase minerals, including grain size distribution, interfacial area, textural information [4]. The use of an image analyzer in mineral processing involves analyzing unbroken ore pieces to predict how the minerals might behave during grinding and processing, and analyzing ground ores and concentrated products to determine how the minerals respond to processing [3]. The first step in studying the unbroken ore is to identify minerals and their quantitative appraisal in the ore. The second and the most important step is size analysis for establishing size distribution. Higher value of Cr content increases the significance of ore, but due to the presence of impurities in the form of silicates, oxides, hydroxides which reduces the value of the ore. To analyze the impurities present in chromite ore was collected from Prangghar (Mohmand agency), Pakistan [5]. Table 1 shows the minerals of different sorts present in chromite ores [6].

1.3Phase analysis:The XRD data for the sample showed mainly 3 major peaks as shown in Figure 1. The EDX analysis carried out were in agreement with XRD for the major compound, but some impurity phases were also detected [5]. The pattern was matched with magnesio-chromite-ferroan ((Mg, Fe) (Cr, Al)2 O4) ICDD card # 09-0353.According to the ICDD card data the spinel under investigation has a cubic structure and almost all the iron is in form of Fe2+ [6] however, a slight difference between the d-values of the first peak and that of the database was observed. The difference could be attributed to the change in the geological location of specimen used in ICDD database, as it has been reported to be from Cribou Canada having the composition: Cr2O3= 55.51%, Al2O3= 14.03%, Fe2O3= 3.79%, FeO= 11.35%, MgO= 14.83% and minor Si, Ti, Ca and Mn related compounds.[7]

Figure 1. The XRD pattern of the Mohmand agency chromite, the data has possible match with ICDD card # 09-0353.

1.4Microstructural analysis:MMI showed four different type of grain morphologies marked with G, H, I & J showing a different phase for each. Figures 3(b,d) and 3(a,c) show the metallurgical microscopy images and SEIs of the same area respectively. MMI of samples when compared with SEM/EDX analysis revealed that both G and H represented the same phase. The difference in grain morphology (grain G being more continuous than H) might be due to over etching of one region with respect to other. EDX of grains I and J showed them as impurities but of different sort, this could also be seen in color contrast of SEIs Figure 2(a, b) as I was darker and J had whitish touch. EDX analyses of each grain in Figure 2(a, b) are given in Table 2. From XRD, but based on SEM/EDX the impurities could be mainly attributed to silicates family with one of them being uvarovite [5].

Table 2. EDX analyses in weight% of the G, H, I and J grains shown in Figure

Figure 2 (a,c) and (b,d) showing the SEI and MMI of the same regions respectively. EDX analysis suggested that the grains G and H represent chromite ore and I represent silicate impuritiesThe chemical analysis of chromite ore samples exploited from Philippines obtained from Bluestar Yima Chrome Chemical Materials PR China is shown in table 3. The sample was crushed and dry-sieved to four particle-size fractions (0.0450.063, 0.0630.075, 0.0750.090, 0.0900.150 mm), and then dried overnight at 80 C. The Cr2O3 and Fe2O3 content of each size is given in table 4. All experiments were performed with the 0.0450.063 mm. The potassium hydroxide used in this work is of analytical grade and commercial pure oxygen is used in this experiment [8].

Table 3 The chemical composition of Philippine chromite ore (%, wt).

Table 4 Cr2O3 and FeO content in different size of Philippine chromite ore.

The mineralogical analysis result of the Philippine chromite ore indicates that the sample mainly consists of (Fe, Mg)(Cr, Fe)2O4 (Fig 3). The morphology of chromite ore was investigated by scanning electron microscopy (SEM). Several important features can be observed. (1) The solid particles have a compact surface (Fig. 4) and (2) The main elements including Cr, Fe and Al are uniformly distributed in the chromite ore (Fig. 5), suggesting that chromite ore has homogeneous structure [8].

Figure 3. XRD patterns of Philippine chromite ore. Figure 4. SEM of Philippine chromite ore. Figure 5. The elemental distribution map of Figure 4 detected by EDS.

The South African chromite ore from the Transvaal region of the Bushveld complex was used for the investigation. The physical and chemical properties of the ore analyzed by various techniques are given in Table 5. The X-ray diffraction (XRD) pattern and the microstructure of chromite ore is shown in figure 6 and 7 respectively. The XRD pattern of the ore indicates that the chromium is mostly present in the (Fe, Mg)(Cr, Al)2 O4 spinel phase. The microstructure shows primarily grains of well-defined chromite spinel (bright phase) and a small quantity of siliceous gangue (dark gray phase) [9].

Table 5 The Physical and Chemical Properties of South African Chromite Ores.

Figure 6. The XRD pattern of the South African chromite ore. The intensities are in arbitrary units (AU).

Figure 7. The microstructure of the chromite ore. Light gray phases are the chromite minerals whereas the dark gray phase is siliceous gangue.

The five types of chromite ore samples obtained from the run-of-mines, obtained from Bantli which belongs to Karaburhan in Eskisehir, Turkey; Dereboyu, Kef, Lasir and Yunuskuyu which belong to Guleman in Elazig, Turkey. Chemical analysis of these chromites by XRF (X-ray fluorescence) is given in Table 6. Lump samples of chromites were made thin and polished sections and then these sections have been investigated for the texture of chromite and gang minerals. The representative samples of chromite ores were examined by XRD and the patterns are given in Figure 8, 9 10, 11 and 12 [10].

Table 6. XRF analysis of chromite ores used

Figure 8. XRD patterns of BANTLI chromite ore. (Ore type consists of serpentinized olivines as the gang mineral).

Figure 9. XRD patterns of (DEREBOYU) chromite ore. (Ore type consists of pyroxene mineral as the gang mineral)

Figure 10. XRD patterns of (KEF) chromite ore. (Ore type consists of mainly unaltered olivine minerals)

Figure 11. XRD patterns of (LASIR) chromite ore.

Figure 12. XRD patterns of (YUNUSKUYU) chromite ore. (Chromite ore has dominantly serpentinized olivine as gang mineral).

Samples of chromite ores from South Africa (SA-1), India (ICO-1), and China (CCO-1) were used for experiments. The main differences between the three types of ores are apparent from their physical and chemical properties, summarized by comparing the chemical analysis in Table 7 and the X-ray diffraction (XRD) patterns shown in Figure 13. In the South African ore, the chromite spinel is predominant and the silica content is less than 1 wt%. By comparison, the Indian ore, in addition to chromite spinel, also consists of the gibbsite and Fe-rich sesquioxide solid solutions, with silica at less than 1 wt%. The Chinese chromite, on the other hand, was found to have approximately 6 wt% SiO2 in the form of pure SiO2 and forsterite (Mg2SiO4) phases. Neither of these two phases are present in the Indian and South African ores. The scanning electron microscopic examination of ores also revealed a considerable variation in the chemical compositions of chromite grains in the Chinese ore, as compared to the average compositional variation among the grains of the South African and Indian chromite spinels. The overall chemical analysis of chromite ores was carried out using the X-ray fluorescence spectrometric technique after the loss-on-ignition (LOI) measurements at 1298 K. When the Chinese and South African ores were heated in air for LOI analysis, both samples gained weight due to the oxidation of FeO to Fe2O3, whereas a major weight loss was observed in the Indian ore due to the release of water of crystallization during the thermal decomposition of gibbsite to constituent oxides [11].

Table 7. Physical and Chemical Properties of South Africa (SA-1), India (ICO-1), and China (CCO-1) Chromite Ores

Figure 13. The XRD patterns of the South Africa (SA-1), India (ICO-1), and China (CCO-1) chromite ore samples.

1.5Manufacturing 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 [12]. 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 [13]. Consequently, the total atom utilization efficiency of the traditional process is quite low [14].The major compounds which are manufactured are the chromates and the dichromate 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 [15, 16].

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 [15].

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 [15].

5. K2Cr2O7 + H2SO4 K2SO4 +H2O +2 CrO3Partial drying of the principle products, namely potassium and sodium mono- and dichromate is carried out in filter presses and the process completed in centrifugal dryers or heated baths [15].Figure 14 represents the flow sheet 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 [16].

Figure 14. Illustrate flow sheet of traditional production process for chromic oxide (Cr2O3)1.6Green manufacturing process of chromium compounds:A green manufacturing process for chromium compounds has been developed by the Institute of Process Engineering, Chinese Academy of Sciences [17], with the design objective of eliminating pollution [18] 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 [19], and aims to achieve the 3Rs (Reduce, Recycle, Reuse) objectives Combining environmental and economic benefits, the goals of comprehensive use of resources, recycling of reaction media, and zero emissions should be achieved in the green process. Figure 15 illustrates the main design idea of the proposed new green process, according to the reactions (6,7,8,9,10) [16].

Figure 15. Schematic idea of the proposed green manufacturing process for chromium compounds.1.6.1To 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.[16]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 + CO1.6.2To 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 [16].

11. 2 KHCO3 K2CO3 + H2O + CO212. K2CO3 + Ca(OH)2 CaCO3 + 2 KOH13. CaCO3 CaO + CO214. CaO + H2O Ca(OH)21.6.3To 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) [20].In the new cleaner process, illustrated in figure 16 aluminum-bearing by products and magnesium-bearing byproducts are manufactured, in addition to the products of potassium dichromate (KCr2O7) and chromic oxide (Cr2O3) [21]. As the final obtained ferrite-enriched residues were used as raw materials in the cement industry [16].

Figure 16. Illustrative ow sheet of the new cleaner production process for chromic oxide (Cr2O3).

References:1.Shen, S.-B., X.-F. Hao, and G.-W. Yang, Kinetics of selective removal of iron from chromite by carbochlorination in the presence of sodium chloride. Journal of Alloys and Compounds, 2009. 476(1): p. 653-661.2.Nafziger, R.H., A review of the deposits and beneficiation of lower-grade chromite. J. S. Afr. Inst. Min. Metall., 1982. 82(8): p. 205-226.3.Petruk, W., Automatic image analysis for mineral beneficiation. JOM Journal of the Minerals, Metals and Materials Society, 1988. 40(4): p. 29-31.4.Lin, C. and J. Miller, Cone beam X-ray microtomography-a new facility for three-dimensional analysis of multiphase materials. Minerals and Metallurgical Processing, 2002. 19(2): p. 65-71.5.Khan, A.A. and Y. Iqbal, PHASE AND MICROSTRUCTURE OF CHROMITE: A DIFFERENT APPROACH.6.Zubakov, S. and E. Yusupova, Composition and properties of chromite ores from new deposits in Kazakhstan. Refractories, 1962. 3(9-10): p. 341-344.7.Snchez-Ramos, S., et al., Analytical and mineralogical studies of ore and impurities from a chromite mineral using X-ray analysis, electrochemical and microscopy techniques. Talanta, 2008. 74(5): p. 1592-1597.8.Chen, G., et al., A new metallurgical process for the clean utilization of chromite ore. International Journal of Mineral Processing, 2014. 131: p. 58-68.9.Tathavakar, V.D., M. Antony, and A. Jha, The physical chemistry of thermal decomposition of South African chromite minerals. Metallurgical and Materials Transactions B, 2005. 36(1): p. 75-84.10.Tademir, A., Evaluation of grain size distribution of unbroken chromites. Minerals Engineering, 2008. 21(10): p. 711-719.11.Tathavadkar, V.D., A. Jha, and M. Antony, The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores. Metallurgical and Materials transactions B, 2003. 34(5): p. 555-563.12.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. 219-226.13.Trost, B.M., The atom economy--a search for synthetic efficiency. Science, 1991. 254(5037): p. 1471-1477.14.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. 343-347.15.Buckell, M. and D. Harvey, An environmental study of the chromate industry. British journal of industrial medicine, 1951. 8(4): p. 298.16.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. 211-219.17.Zhang, Y., et al., Green chemistry of chromate cleaner production. Chinese Journal of Chemistry, 1999. 17(3): p. 258-266.18.Cano-Ruiz, J. and G. McRae, Environmentally conscious chemical process design. Annual Review of Energy and the Environment, 1998. 23(1): p. 499-536.19.Manahan, S.E., Industrial ecology: environmental chemistry and hazardous waste. 1999: CRC Press.20.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.21.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.

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