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Development of a new cleaner production process for producing chromic oxide from chromite ore
Journal of Cleaner Production 14 (2006) 211e219 www.elsevier.com/locate/jclepro
Development of a new cleaner production process for producing chromic oxide from chromite ore Hong-Bin Xu*, Yi Zhang, Zuo-Hu Li, Shi-Li Zheng, Zhi-Kuan Wang, Tao Qi, Hui-Quan Li Key Laboratory for Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, P. O. Box 353 sub 26, Beijing 100080, PR China Received 18 December 2003; accepted 7 September 2004 Available online 18 November 2004
Abstract Chromic oxide (Cr2O3) is an important chemical with numerous industrial applications. The traditional process used for manufacturing chromic oxide from chromite ore has low resources and energy efficiency. Moreover, large quantities of chromiumcontaining toxic solid wastes are discharged, posing serious pollution concerns. To reduce the environmental impact of this procedure, a new cleaner process was developed by the Institute of Process Engineering, Chinese Academy of Sciences in Beijing, PR China, based upon the 3Rs (Reduce, Recycle, Reuse) principles of cleaner production and industrial ecology; this new cleaner process utilizes resources more efficiently, and does not discharge emissions of chromium-containing waste residue. A demonstration plant featuring this process, as well as an Eco-Industrial Park (EIP) has been built in He’nan Province, PR China. The new cleaner process is a promising advancement for the industrial production of chromic oxide (Cr2O3) as well as other chromium compounds such as potassium chromate (K2CrO4) and potassium dichromate (K2Cr2O7). Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Cleaner production; Chromic oxide; Chromite ore; Sub-molten salt medium; Zero emission
1. Introduction Chromic oxides have a variety of applications. They serve as pigments in paints and coatings, enamels, additives in concrete and other building products, floor coverings, and in other color applications where permanence of color is paramount. Other applications include catalysts, abrasives, polishing media, and refractories where chemical composition and physical properties other than color are important. While many minerals contain chromium, chromite (FeCrO4) is the only commercial ore mineral of
* Corresponding author. Fax: C86 10 6256 1822. E-mail address: [email protected] (H.-B. Xu). 0959-6526/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2004.09.001
chromium. The general formula of chromite ore can be described as (MgxFe1x) O(AlyCr1y)2O3, since silicon is mixed as gangue in chromite ores. As the main source for manufacturing chromic oxides, chromium hexavalent compounds are known to be human carcinogens; there is sufficient evidence of carcinogenicity in humans who have been occupationally exposed in the chromate production, chromate pigment production, and chromium plating industries [1]. In the traditional manufacturing process for producing chromium hexavalent compounds from chromite ore, which utilizes oxidation roasting at 1100 C, water leaching and multi-stage evaporation crystallization [2,3], the utilization efficiency of resources and energy is generally quite low. Furthermore, chromate plants discharge large amounts of chromium-containing residues, dusts and waste gases, creating serious pollution
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problems that threaten underground waters, rivers, marine areas, and human health [4,5]. Although roasting technologies which produce minimal calcium [6e8], as well as a variety of other technologies [9e14] have since been developed, the industry’s pollution problem has not been thoroughly resolved. The People’s Republic of China is currently considered one of the largest producers of chromium compounds and its pollution problems from chromate production is increasingly spawning public concern [15]. Recently, a cleaner process for producing chromic oxide from chromite ore was developed by the Institute of Process Engineering, Chinese Academy of Sciences [16e19]. Traditional oxidation roasting of chromite ore with sodium carbonate at temperatures in the range of 1100 C in a rotary kiln has been replaced with a system featuring continuous liquid-phase oxidation of chromite ore in a sub-molten salt medium at 300 C in a gas lift loop reactor. A demonstration plant which uses this process was built in He’nan Province, PR China. By linking of the upstream coal gas plant and the downstream cement plant, the chromate production plant maximizes resource utilization and minimizes waste discharge. 2. Brief description of the traditional process Fig. 1 provides an illustrative flowsheet of the traditional process used for producing chromic oxide (Cr2O3) from chromite ore. During the initial stage, insoluble chromite ore is converted into water-soluble chromate. Roasting a mixture of chromite ore, sodium carbonate (Na2CO3), limestone, and dolomite at 1100 C for several hours produces sodium chromate (Na2CrO4). The limestone and dolomite acts as a mechanical separator, allowing oxygen to react with the chromite and sodium carbonate (Na2CO3). Reactions (1) through (3) represent the main chemical reactions in oxidation roasting. 1=2FeO Cr2 O3 CNa2 CO3 C7=8O2 /Na2 CrO4 C1=4Fe2 O3 CCO2 1=2MgO Cr2 O3 CNa2 CO3 C3=4O2 /Na2 CrO4 C1=2MgOCCO2
ð1Þ
ð2Þ
1=2Cr2 O3 CNa2 CO3 C3=4O2 /Na2 CrO4 CCO2
ð3Þ
2Na2 CrO4 CH2 SO4 /Na2 Cr2 O7 CNa2 SO4 CH2 O
ð4Þ
(H2SO4), as shown in Reaction (4). The sodium dichromate (Na2Cr2O7) obtained can be sold as commercial product. Na2 Cr2 O7 C2ðNH4 Þ2 SO4 /Cr2 O3 CNa2 SO4 CN2 C4H2 O
ð5Þ
Na2 Cr2 O7 C2H2 SO4 /2CrO3 C2NaHSO4 CH2 O
ð6Þ
2CrO3 /Cr2 O3 C3=2O2
ð7Þ
Traditionally, chromic oxide (Cr2O3) was produced using two approaches. The first involves roasting a mixture of sodium dichromate (Na2Cr2O7) and ammonium sulfate ((NH4)2SO4), as shown in Reaction (5); the second involves thermal decomposition of chromic anhydride (CrO3), another commercial product obtained by fusing sodium dichromate (Na2Cr2O7) with concentrated sulfuric acid (H2SO4), as shown in Reactions (6) and (7). Generally, the former approach is for manufacturing chromic oxide (Cr2O3) of metallurgical grade, and the latter, pigment grade. The traditional process does not extract all of the chromium from the chromite ore. The residue contains unreacted chromite ore and unextracted chromate, and usually must be disposed of on-site. The chromiumcontaining sodium sulfate (Na2SO4) formed as a byproduct has little commercial value and also must be disposed of on-site. 3. The new cleaner process flowsheet The new cleaner process is based on the principles of cleaner production and industrial ecology [20], and aims to achieve the 3Rs (Reduce, Recycle, Reuse) objectives, according to the following reactions: 1=2FeO Cr2 O3 C2KOHC7=8O2 /1=4Fe2 O3 CH2 OCK2 CrO4
ð8Þ
1=2MgO Cr2 O3 C2KOHC3=4O2 /1=2MgO CH2 OCK2 CrO4
ð9Þ
1=2Cr2 O3 C2KOHC3=4O2 /H2 OCK2 CrO4
ð10Þ
2K2 CrO4 CH2 OC2CO2 /K2 Cr2 O7 C2KHCO3
ð11Þ
K2 Cr2 O7 CC/Cr2 O3 CK2 CO3 CCO
ð12Þ
3.1. To reduce The sodium chromate (Na2CrO4) produced is extracted in water, and the residue, discarded. The sodium chromate (Na2CrO4) is then converted into sodium dichromate (Na2Cr2O7) by reaction with sulfuric acid
A new reaction path, as shown in Reactions (8)e(10), was designed and no solid additives were needed in the oxidation of chromite ore. Consequently, the amount of
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213
Fig. 1. Illustrative flowsheet of the traditional production process for chromic oxide (Cr2O3).
waste residue remaining following leaching was remarkably reduced. Furthermore, benign reactants such as carbon dioxide (CO2) (Reaction (11)) and carbon black (C) (Reaction (12)) were employed to convert the semi-finished product to the final products [21,22], thereby minimizing possible pollution sources.
medium, and most was recycled after the reaction occurred. Only a small amount was consumed during the reaction and needed to be supplemented.
2KHCO3 /K2 CO3 CH2 OCCO2
ð13Þ
3.2. To recycle
K2 CO3 CCaðOHÞ2 /CaCO3 C2KOH
ð14Þ
An excessive amount of potassium hydroxide (KOH), in sub-molten salt state, was employed as the reaction
CaCO3 /CaOCCO2
ð15Þ
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CaOCH2 O/CaðOHÞ2
ð16Þ
4. Improvements and innovations in the new cleaner process
3.3. To reuse The produced intermediates such as potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) were reused to manufacture potassium hydroxide (KOH) [21], as shown in Reactions (13) and (14). The calcium hydroxide (Ca(OH)2) can be recovered via Reactions (15) and (16). In the new cleaner process, illustrated in Fig. 2, aluminum-bearing byproducts and magnesium-bearing byproducts are manufactured, in addition to the products of potassium dichromate (K2Cr2O7) and chromic oxide (Cr2O3) [23,24]. As the final obtained ferrite-enriched residues were used as raw materials in the cement industry, the process can therefore be officially labeled a ‘‘zero emissions’’ technology.
4.1. Simpler reaction system for the oxidation of chromite ore In the traditional process, the oxidation of chromite ore occurs in the presence of sodium carbonate (Na2CO3) and other additives including limestone and dolomite. When the temperature increases as high as 1100 C, these additives will decompose and react with sodium carbonate (Na2CO3) as well as chromite ore. Therefore, dozens of subsidiary reactions accompany the main reactions shown in Reactions (1)e(3) [2,3], producing a higher amount of chromite ore processing residues. Additionally, the increase in the variety of the reaction products makes it difficult to separate them from one another. Considering that the object product
Fig. 2. Illustrative flowsheet of the new cleaner production process for chromic oxide (Cr2O3).
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of these reactions, sodium chromate (Na2CrO4), is embedded within the solid mixture, the successive leaching process becomes exceedingly challenging, requiring the application of substantial solvent (water). However, in the new cleaner process, only potassium hydroxide (KOH) features in the reaction of chromite ore with oxygen (O2). Although other components present in the chromite ore, such as aluminum oxide (Al2O3), iron protoxide (FeO), and silicon oxide (SiO2) can also react with oxygen (O2), potassium hydroxide (KOH), or one another, and be dissolved into the aqueous solution [25], they hardly have any impact on the separation of potassium chromate (K2CrO4). This is due in large part because within certain ranges of potassium hydroxide (KOH) concentrations and temperatures, the solubility of potassium chromate (K2CrO4) in aqueous potassium hydroxide (KOH) solutions becomes very small and almost all of potassium chromate (K2CrO4) is precipitated as crystals [23,26]. The simpler reaction system results in an easier separation operation and therefore, produces purer products. Through comparing the flowsheets, as shown in Figs. 1 and 2, it will be found that the new cleaner process has a much shorter path than the traditional one. 4.2. Higher efficiency of the oxidation reaction Traditional oxidation roasting of chromite ore is a gassolid heterogeneous reaction in a rotary kiln. By applying the Shrinking-core Model for Spherical Particles of Unchanging Size to the oxidation reaction, it is found that [3], diffusion through the ash layer is the controlling resistance and controls the rate of the reaction when the temperature is no lower than 1100 C; therefore, an effective measure to raise the conversion rate is to reduce the size of the solid reactants. However, in the industrial process of oxidation roasting, the agglomeration of solid reactants remains one of the most pressing problems encountered in most chromate manufacturing plants and determines the low conversion efficiency. In comparison with the oxidation roasting of chromite ore using sodium carbonate (Na2CO3) and oxygen (O2) at 1100 C in a rotary kiln, the liquid-phase oxidation of chromite ore in sub-molten fluid media at 300 C in a gas lift loop reactor is a gas-liquid-solid three-phase non-catalytic reaction. Kinetic investigation results [25] show that the rate of the reaction is controlled by chemical reaction, providing that the mass transfer is greatly enhanced through intensifying the agitation and thus improving the mixing. Furthermore, the fluidness of the system enables the easier and more quantitative control of reaction and separation. As a result, with a temperature decrease from 1100 C to 300 C, the conversion rate of trivalent chromium in
215
chromite ore is increased from 76% in the traditional process to above 99% in the new cleaner process. 4.3. Thermodynamically favorable main reactions Utilizing the standard Gibbs energy data taken from the related reference [27], the changes of standard Gibbs energy (DrGo) for Reactions (1)e(3) in the traditional process and Reactions (8)e(10) in the new cleaner process were calculated at different temperatures. Figs. 3 and 4 illustrate the relationship between the change of standard Gibbs energy (DrGo) and the corresponding temperature (T ). As shown in Figs. 3 and 4, the changes of standard Gibbs energy, DrGo, of Reactions (1)e(3) and (8)e(10) are all negative at the respective given temperature range so that the oxidation of trivalent chromium to hexavalent chromium is relatively simple. The changes of standard Gibbs energy, DrGo, of Reactions (8)e(10) at 300e800 K are at least twice as much as those of Reactions (1)e(3) at 1100e1500 K, respectively. Generally, a larger negative DrGo value means greater reaction tendency. Therefore, the liquid-phase oxidation at 300 C (527.15 K) is more favorable than the oxidation roasting at 1100 C (1373.15 K). However, under the same reaction conditions, the reaction tendency for FeOCr2O3 (FeCr2O4), MgOCr2O3 (MgCr2O4) and Cr2O3 is FeOCr2O3>Cr2O3> MgOCr2O3 [28]. The oxidation of chromite ore, in both the traditional and the new cleaner processes, is an exothermic reaction and the values of reaction heat are 76.5, 17.4, 36.0, for Reactions (1)e(3) at 1123 C (1400K) in kJ/mol Na2CrO4, and 294.5, 231.0, 250.2, for Reactions (8)e(10) at 323 C (600 K) in kJ/mol K2CrO4, respectively. The reaction heat in the new cleaner
Fig. 3. Relationship between the change of standard Gibbs energy and the temperature for Reactions (1)e(3) in the traditional process, respectively.
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erable amounts of aluminum (Al) are dissolved into the aqueous solution, as shown in Reaction (17). The potassium aluminate (KAlO2) produced can be separated by adjusting the concentration, composition and temperature of the aqueous solution [23,25] and converted into other products; thus, valuable aluminum-bearing byproducts are obtained. Considering that the chromium concentration of the aqueous solution after liquid-phase oxidation and dilution is typically below 1.0 g of CrC6 per liter of solution, the chromium carried in the aluminum-bearing byproducts is slight in concentration. Al2 O3 C2KOH/2KAlO2 CH2 O Fig. 4. Relationship between the change of standard Gibbs energy and the temperature for Reactions (8)e(10) in the new cleaner process, respectively.
process is greater in amount than that in the traditional process and therefore is more likely to lower the energy consumption [28]. 4.4. Utilization of benign reactants In the new cleaner process, benign reactants are widely used to reduce the pollution sources and improve the reaction conditions. Carbon dioxide (CO2), from another industrial process or inside the new cleaner process, is employed as a substitute for sulfuric acid (H2SO4) in converting potassium chromate (K2CrO4) to potassium dichromate (K2Cr2O7), as shown in Reaction (11) and compared with Reaction (4). The transition between alkaline chromate liquor and acidic dichromate liquor makes it possible to recover and recycle the potassium cation (KC), which is the reaction medium and carrier of chromium in the new cleaner process. As a result, the pollution resulting from chromiumcontaining Glauber’s salt (Na2SO410H2O) produced in the traditional process is radically eliminated. In the production of the final product chromic oxide (Cr2O3), Reaction (12) is chosen to substitute for Reaction (5) or Reactions (6) and (7). Consequently, the reaction conditions are greatly improved, and the side reactions for Reaction (5) [3] are effectively prevented. The byproduct, potassium carbonate (K2CO3), can be converted inside the process into potassium hydroxide (KOH) (Reaction (14)), one of the raw materials in the new cleaner process. 4.5. More commercially valuable byproducts In chromite ore, besides chromium, valuable elements include aluminum (Al), iron (Fe) and magnesium (Mg). During liquid-phase oxidation of chromite ore, consid-
ð17Þ
The magnesium in chromite ore, in the form of magnesium oxide (MgO) after the decomposition of the chromite ore (Reaction (9)), hydrolyzes into magnesium hydroxide (Mg(OH)2) and enters the residue phase, during the process of liquid-phase oxidation. Magnesium hydroxide (Mg(OH)2) can be extracted by carbonation, and thus converted into magnesium carbonate (MgCO3) [24], another valuable byproduct. The iron in chromite ore persists in the form of iron oxide (Fe2O3) after the decomposition of chromite ore (Reaction (8)) during the liquid oxidation phase. Iron oxide (Fe2O3) is inert, and thus enters the residue phase. Nevertheless, after washing chromium and extracting magnesium, the ferrite-enriched residue can be used as raw materials in other industries (see also Section 4.6). Comparatively, during the traditional oxidation roasting process, aluminum (Al), magnesium (Mg) and iron (Fe) are present in the forms of aluminum oxide (Al2O3), magnesium oxide (MgO) and iron oxide (Fe2O3), respectively. All are chemically inactive and fixed in the residue after reaction. However, there are no effective extraction approaches available. 4.6. Zero emission of chromium-containing residues In developed countries, the general method for disposal of chromium-containing residue is land filling or stacking until the hexavalent chromium is detoxified. There were a number of reports and patents for disposing of chromium-containing residues [1e3,5,29], but few were really economically applicable. Recently, a widely discussed topic is the deployment of chromiumcontaining residue as a raw material input in cement industry. Portland cement normally contains CaO, SiO2, Al2O3, Fe2O3, MgO, K2O, Na2O, TiO2, and P2O5, as well as a small amount of other components. Correspondingly, the residue produced from the chromate industry also contains most of the components in the Portland cement clinker [30], which suggests the
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possibility of using the chromium-containing residue as the raw materials in cement industry. However, among the raw materials for cement manufacturing, two components are highly sensitive and of great importance. One is magnesium oxide (MgO); the other is chromic oxide (Cr2O3). Quantities of magnesium oxide (MgO) in excess of about 2% by weight can occur as periclase, which through slow reaction with water, can cause destructive expansion of hardened concrete [31]. An excessive amount of chromium can directly lead to industrial problems [32]. In the China National Standards for Portland cement and ordinary Portland cement products, the content of MgO is limited to below 5% by weight, and for the chromium-containing raw materials for cement industry, the content of Cr2O3 is limited to below 4.5% by weight [33]. As seen in Table 1, the residue produced in the liquidphase oxidation process contains a very small amount of chromium and magnesium, after washing of chromium and extraction of magnesium. It thereby becomes eligible for raw materials in cement industry. As for the traditional process, the residue produced does not meet the requirements mentioned above due to its high content of chromium and magnesium.
4.7. Practical results of utilizing the new cleaner process The main results, including the technical and economic index, of both laboratory and small-scale pilot plant studies are provided in Table 2. The data are based upon the production of 1.0 tonne of K2Cr2O7 or Na2Cr2O7 products. In brief, the new cleaner production process for chromic oxide (Cr2O3) not only achieved a higher extraction yield of chromium but also utilized the accompanying elements comprehensively and therefore improved atom utilization [35] of chromite ore resources, and eliminated waste. A demonstration plant featuring this new process has been built in He’nan Province, PR China. The plant’s annual production capacity is 10,000 tonnes, and has
Table 2 Comparison of the new cleaner process with the traditional process in technical and economic index when producing 1.0 tonne of K2Cr2O7 or Na2Cr2O7 [2,3,18,19,34] Items
The new cleaner process
The traditional process
Cr recovery yield, % by weight Amount of chromium residue (tonnes) Cr content in residue, % by weight Cr6C in residue, % by weight Cr-containing waste gases and dusts Ore consumption (tonnes) Alkali consumption (tonnes) Energy cost (counted by nature gas; Nm3) Atom economy (%) [35]
99
76
0.5a
2.5
0.5
4.0
Below 0.1 Almost eliminated 1.07 Below 0.5 (KOH) 650
1.0 Significant problem 1.35e1.50 0.9 (Na2CO3) 850
90.5
14.7
a
Eligible as the raw materials for cement industry.
been linked with the upstream coal gas plant and the downstream cement plant, as well as a newly-built alkali manufacturing plant. A demonstration Eco-Industrial Park (EIP) has also been constructed, as shown in Fig. 5. The technology has exhibited promising results for the industrial production of chromium compounds.
5. Conclusion In summary, a novel and original cleaner production process has been developed and commercialized. Thermodynamic analysis and reaction kinetic analysis of liquid-phase oxidation of chromite ore in potassium
Table 1 Typical chemical composition of the chromium-containing residues produced in both the traditional and the new cleaner processes [24] Components
Content, % by weight The traditional process
The new cleaner process
CaO MgO SiO2 Fe2O3 Al2O3 SCr CrC6
28e35 20e29 7e12 9e13 6e9 3e7 1.0
None 3.5 7.6 64.2 16.0 0.5 Below 0.1
Fig. 5. Illustrative diagram of the demonstration EIP con constructed in He’nan Province, PR China.
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hydroxide (KOH) fluid media has been carried out. The analysis shows that, when compared with the traditional roasting process, the liquid-phase oxidation of chromite ore in sub-molten potassium hydroxide (KOH) medium is thermodynamically favorable and the main reactions of liquid-phase oxidation are exothermic, which is beneficial in lowering the energy consumption. The pilot plant tests demonstrate that, the chromium yield is greater than 99% and the amount of produced chromium-containing residue is only one-fifth that of the traditional process. In the demonstration EIP constructed, carbon dioxide (CO2) in the waste gas from a nearby coal gas plant was used as the acidulant in the new cleaner process. Valuable magnesium-bearing byproducts are prepared from toxic chromium-containing residue. Moreover, the treated residues are used as raw material in the cement plant. A novel process with zero emission of chromium-containing residues has been achieved. The new cleaner chemical system and technology for producing chromic oxide (Cr2O3) is a good example of the feasibility of minimizing chemical loses through comprehensive utilization of natural resources and elimination of pollution at the source.
Acknowledgements The authors gratefully acknowledge the financial supports from the Key Project Program of the National Natural Science Foundation of China (Grant No. 50234040) and the Major Project of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KCCX1-SW-22).
References
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Development of a new cleaner production process for producing chromic oxide from chromite ore Hong-Bin Xu*, Yi Zhang, Zuo-Hu Li, Shi-Li Zheng, Zhi-Kuan Wang, Tao Qi, Hui-Quan Li Key Laboratory for Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, P. O. Box 353 sub 26, Beijing 100080, PR China Received 18 December 2003; accepted 7 September 2004 Available online 18 November 2004
Abstract Chromic oxide (Cr2O3) is an important chemical with numerous industrial applications. The traditional process used for manufacturing chromic oxide from chromite ore has low resources and energy efficiency. Moreover, large quantities of chromiumcontaining toxic solid wastes are discharged, posing serious pollution concerns. To reduce the environmental impact of this procedure, a new cleaner process was developed by the Institute of Process Engineering, Chinese Academy of Sciences in Beijing, PR China, based upon the 3Rs (Reduce, Recycle, Reuse) principles of cleaner production and industrial ecology; this new cleaner process utilizes resources more efficiently, and does not discharge emissions of chromium-containing waste residue. A demonstration plant featuring this process, as well as an Eco-Industrial Park (EIP) has been built in He’nan Province, PR China. The new cleaner process is a promising advancement for the industrial production of chromic oxide (Cr2O3) as well as other chromium compounds such as potassium chromate (K2CrO4) and potassium dichromate (K2Cr2O7). Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Cleaner production; Chromic oxide; Chromite ore; Sub-molten salt medium; Zero emission
1. Introduction Chromic oxides have a variety of applications. They serve as pigments in paints and coatings, enamels, additives in concrete and other building products, floor coverings, and in other color applications where permanence of color is paramount. Other applications include catalysts, abrasives, polishing media, and refractories where chemical composition and physical properties other than color are important. While many minerals contain chromium, chromite (FeCrO4) is the only commercial ore mineral of
* Corresponding author. Fax: C86 10 6256 1822. E-mail address: [email protected] (H.-B. Xu). 0959-6526/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2004.09.001
chromium. The general formula of chromite ore can be described as (MgxFe1x) O(AlyCr1y)2O3, since silicon is mixed as gangue in chromite ores. As the main source for manufacturing chromic oxides, chromium hexavalent compounds are known to be human carcinogens; there is sufficient evidence of carcinogenicity in humans who have been occupationally exposed in the chromate production, chromate pigment production, and chromium plating industries [1]. In the traditional manufacturing process for producing chromium hexavalent compounds from chromite ore, which utilizes oxidation roasting at 1100 C, water leaching and multi-stage evaporation crystallization [2,3], the utilization efficiency of resources and energy is generally quite low. Furthermore, chromate plants discharge large amounts of chromium-containing residues, dusts and waste gases, creating serious pollution
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problems that threaten underground waters, rivers, marine areas, and human health [4,5]. Although roasting technologies which produce minimal calcium [6e8], as well as a variety of other technologies [9e14] have since been developed, the industry’s pollution problem has not been thoroughly resolved. The People’s Republic of China is currently considered one of the largest producers of chromium compounds and its pollution problems from chromate production is increasingly spawning public concern [15]. Recently, a cleaner process for producing chromic oxide from chromite ore was developed by the Institute of Process Engineering, Chinese Academy of Sciences [16e19]. Traditional oxidation roasting of chromite ore with sodium carbonate at temperatures in the range of 1100 C in a rotary kiln has been replaced with a system featuring continuous liquid-phase oxidation of chromite ore in a sub-molten salt medium at 300 C in a gas lift loop reactor. A demonstration plant which uses this process was built in He’nan Province, PR China. By linking of the upstream coal gas plant and the downstream cement plant, the chromate production plant maximizes resource utilization and minimizes waste discharge. 2. Brief description of the traditional process Fig. 1 provides an illustrative flowsheet of the traditional process used for producing chromic oxide (Cr2O3) from chromite ore. During the initial stage, insoluble chromite ore is converted into water-soluble chromate. Roasting a mixture of chromite ore, sodium carbonate (Na2CO3), limestone, and dolomite at 1100 C for several hours produces sodium chromate (Na2CrO4). The limestone and dolomite acts as a mechanical separator, allowing oxygen to react with the chromite and sodium carbonate (Na2CO3). Reactions (1) through (3) represent the main chemical reactions in oxidation roasting. 1=2FeO Cr2 O3 CNa2 CO3 C7=8O2 /Na2 CrO4 C1=4Fe2 O3 CCO2 1=2MgO Cr2 O3 CNa2 CO3 C3=4O2 /Na2 CrO4 C1=2MgOCCO2
ð1Þ
ð2Þ
1=2Cr2 O3 CNa2 CO3 C3=4O2 /Na2 CrO4 CCO2
ð3Þ
2Na2 CrO4 CH2 SO4 /Na2 Cr2 O7 CNa2 SO4 CH2 O
ð4Þ
(H2SO4), as shown in Reaction (4). The sodium dichromate (Na2Cr2O7) obtained can be sold as commercial product. Na2 Cr2 O7 C2ðNH4 Þ2 SO4 /Cr2 O3 CNa2 SO4 CN2 C4H2 O
ð5Þ
Na2 Cr2 O7 C2H2 SO4 /2CrO3 C2NaHSO4 CH2 O
ð6Þ
2CrO3 /Cr2 O3 C3=2O2
ð7Þ
Traditionally, chromic oxide (Cr2O3) was produced using two approaches. The first involves roasting a mixture of sodium dichromate (Na2Cr2O7) and ammonium sulfate ((NH4)2SO4), as shown in Reaction (5); the second involves thermal decomposition of chromic anhydride (CrO3), another commercial product obtained by fusing sodium dichromate (Na2Cr2O7) with concentrated sulfuric acid (H2SO4), as shown in Reactions (6) and (7). Generally, the former approach is for manufacturing chromic oxide (Cr2O3) of metallurgical grade, and the latter, pigment grade. The traditional process does not extract all of the chromium from the chromite ore. The residue contains unreacted chromite ore and unextracted chromate, and usually must be disposed of on-site. The chromiumcontaining sodium sulfate (Na2SO4) formed as a byproduct has little commercial value and also must be disposed of on-site. 3. The new cleaner process flowsheet The new cleaner process is based on the principles of cleaner production and industrial ecology [20], and aims to achieve the 3Rs (Reduce, Recycle, Reuse) objectives, according to the following reactions: 1=2FeO Cr2 O3 C2KOHC7=8O2 /1=4Fe2 O3 CH2 OCK2 CrO4
ð8Þ
1=2MgO Cr2 O3 C2KOHC3=4O2 /1=2MgO CH2 OCK2 CrO4
ð9Þ
1=2Cr2 O3 C2KOHC3=4O2 /H2 OCK2 CrO4
ð10Þ
2K2 CrO4 CH2 OC2CO2 /K2 Cr2 O7 C2KHCO3
ð11Þ
K2 Cr2 O7 CC/Cr2 O3 CK2 CO3 CCO
ð12Þ
3.1. To reduce The sodium chromate (Na2CrO4) produced is extracted in water, and the residue, discarded. The sodium chromate (Na2CrO4) is then converted into sodium dichromate (Na2Cr2O7) by reaction with sulfuric acid
A new reaction path, as shown in Reactions (8)e(10), was designed and no solid additives were needed in the oxidation of chromite ore. Consequently, the amount of
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213
Fig. 1. Illustrative flowsheet of the traditional production process for chromic oxide (Cr2O3).
waste residue remaining following leaching was remarkably reduced. Furthermore, benign reactants such as carbon dioxide (CO2) (Reaction (11)) and carbon black (C) (Reaction (12)) were employed to convert the semi-finished product to the final products [21,22], thereby minimizing possible pollution sources.
medium, and most was recycled after the reaction occurred. Only a small amount was consumed during the reaction and needed to be supplemented.
2KHCO3 /K2 CO3 CH2 OCCO2
ð13Þ
3.2. To recycle
K2 CO3 CCaðOHÞ2 /CaCO3 C2KOH
ð14Þ
An excessive amount of potassium hydroxide (KOH), in sub-molten salt state, was employed as the reaction
CaCO3 /CaOCCO2
ð15Þ
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CaOCH2 O/CaðOHÞ2
ð16Þ
4. Improvements and innovations in the new cleaner process
3.3. To reuse The produced intermediates such as potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) were reused to manufacture potassium hydroxide (KOH) [21], as shown in Reactions (13) and (14). The calcium hydroxide (Ca(OH)2) can be recovered via Reactions (15) and (16). In the new cleaner process, illustrated in Fig. 2, aluminum-bearing byproducts and magnesium-bearing byproducts are manufactured, in addition to the products of potassium dichromate (K2Cr2O7) and chromic oxide (Cr2O3) [23,24]. As the final obtained ferrite-enriched residues were used as raw materials in the cement industry, the process can therefore be officially labeled a ‘‘zero emissions’’ technology.
4.1. Simpler reaction system for the oxidation of chromite ore In the traditional process, the oxidation of chromite ore occurs in the presence of sodium carbonate (Na2CO3) and other additives including limestone and dolomite. When the temperature increases as high as 1100 C, these additives will decompose and react with sodium carbonate (Na2CO3) as well as chromite ore. Therefore, dozens of subsidiary reactions accompany the main reactions shown in Reactions (1)e(3) [2,3], producing a higher amount of chromite ore processing residues. Additionally, the increase in the variety of the reaction products makes it difficult to separate them from one another. Considering that the object product
Fig. 2. Illustrative flowsheet of the new cleaner production process for chromic oxide (Cr2O3).
H.-B. Xu et al. / Journal of Cleaner Production 14 (2006) 211e219
of these reactions, sodium chromate (Na2CrO4), is embedded within the solid mixture, the successive leaching process becomes exceedingly challenging, requiring the application of substantial solvent (water). However, in the new cleaner process, only potassium hydroxide (KOH) features in the reaction of chromite ore with oxygen (O2). Although other components present in the chromite ore, such as aluminum oxide (Al2O3), iron protoxide (FeO), and silicon oxide (SiO2) can also react with oxygen (O2), potassium hydroxide (KOH), or one another, and be dissolved into the aqueous solution [25], they hardly have any impact on the separation of potassium chromate (K2CrO4). This is due in large part because within certain ranges of potassium hydroxide (KOH) concentrations and temperatures, the solubility of potassium chromate (K2CrO4) in aqueous potassium hydroxide (KOH) solutions becomes very small and almost all of potassium chromate (K2CrO4) is precipitated as crystals [23,26]. The simpler reaction system results in an easier separation operation and therefore, produces purer products. Through comparing the flowsheets, as shown in Figs. 1 and 2, it will be found that the new cleaner process has a much shorter path than the traditional one. 4.2. Higher efficiency of the oxidation reaction Traditional oxidation roasting of chromite ore is a gassolid heterogeneous reaction in a rotary kiln. By applying the Shrinking-core Model for Spherical Particles of Unchanging Size to the oxidation reaction, it is found that [3], diffusion through the ash layer is the controlling resistance and controls the rate of the reaction when the temperature is no lower than 1100 C; therefore, an effective measure to raise the conversion rate is to reduce the size of the solid reactants. However, in the industrial process of oxidation roasting, the agglomeration of solid reactants remains one of the most pressing problems encountered in most chromate manufacturing plants and determines the low conversion efficiency. In comparison with the oxidation roasting of chromite ore using sodium carbonate (Na2CO3) and oxygen (O2) at 1100 C in a rotary kiln, the liquid-phase oxidation of chromite ore in sub-molten fluid media at 300 C in a gas lift loop reactor is a gas-liquid-solid three-phase non-catalytic reaction. Kinetic investigation results [25] show that the rate of the reaction is controlled by chemical reaction, providing that the mass transfer is greatly enhanced through intensifying the agitation and thus improving the mixing. Furthermore, the fluidness of the system enables the easier and more quantitative control of reaction and separation. As a result, with a temperature decrease from 1100 C to 300 C, the conversion rate of trivalent chromium in
215
chromite ore is increased from 76% in the traditional process to above 99% in the new cleaner process. 4.3. Thermodynamically favorable main reactions Utilizing the standard Gibbs energy data taken from the related reference [27], the changes of standard Gibbs energy (DrGo) for Reactions (1)e(3) in the traditional process and Reactions (8)e(10) in the new cleaner process were calculated at different temperatures. Figs. 3 and 4 illustrate the relationship between the change of standard Gibbs energy (DrGo) and the corresponding temperature (T ). As shown in Figs. 3 and 4, the changes of standard Gibbs energy, DrGo, of Reactions (1)e(3) and (8)e(10) are all negative at the respective given temperature range so that the oxidation of trivalent chromium to hexavalent chromium is relatively simple. The changes of standard Gibbs energy, DrGo, of Reactions (8)e(10) at 300e800 K are at least twice as much as those of Reactions (1)e(3) at 1100e1500 K, respectively. Generally, a larger negative DrGo value means greater reaction tendency. Therefore, the liquid-phase oxidation at 300 C (527.15 K) is more favorable than the oxidation roasting at 1100 C (1373.15 K). However, under the same reaction conditions, the reaction tendency for FeOCr2O3 (FeCr2O4), MgOCr2O3 (MgCr2O4) and Cr2O3 is FeOCr2O3>Cr2O3> MgOCr2O3 [28]. The oxidation of chromite ore, in both the traditional and the new cleaner processes, is an exothermic reaction and the values of reaction heat are 76.5, 17.4, 36.0, for Reactions (1)e(3) at 1123 C (1400K) in kJ/mol Na2CrO4, and 294.5, 231.0, 250.2, for Reactions (8)e(10) at 323 C (600 K) in kJ/mol K2CrO4, respectively. The reaction heat in the new cleaner
Fig. 3. Relationship between the change of standard Gibbs energy and the temperature for Reactions (1)e(3) in the traditional process, respectively.
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erable amounts of aluminum (Al) are dissolved into the aqueous solution, as shown in Reaction (17). The potassium aluminate (KAlO2) produced can be separated by adjusting the concentration, composition and temperature of the aqueous solution [23,25] and converted into other products; thus, valuable aluminum-bearing byproducts are obtained. Considering that the chromium concentration of the aqueous solution after liquid-phase oxidation and dilution is typically below 1.0 g of CrC6 per liter of solution, the chromium carried in the aluminum-bearing byproducts is slight in concentration. Al2 O3 C2KOH/2KAlO2 CH2 O Fig. 4. Relationship between the change of standard Gibbs energy and the temperature for Reactions (8)e(10) in the new cleaner process, respectively.
process is greater in amount than that in the traditional process and therefore is more likely to lower the energy consumption [28]. 4.4. Utilization of benign reactants In the new cleaner process, benign reactants are widely used to reduce the pollution sources and improve the reaction conditions. Carbon dioxide (CO2), from another industrial process or inside the new cleaner process, is employed as a substitute for sulfuric acid (H2SO4) in converting potassium chromate (K2CrO4) to potassium dichromate (K2Cr2O7), as shown in Reaction (11) and compared with Reaction (4). The transition between alkaline chromate liquor and acidic dichromate liquor makes it possible to recover and recycle the potassium cation (KC), which is the reaction medium and carrier of chromium in the new cleaner process. As a result, the pollution resulting from chromiumcontaining Glauber’s salt (Na2SO410H2O) produced in the traditional process is radically eliminated. In the production of the final product chromic oxide (Cr2O3), Reaction (12) is chosen to substitute for Reaction (5) or Reactions (6) and (7). Consequently, the reaction conditions are greatly improved, and the side reactions for Reaction (5) [3] are effectively prevented. The byproduct, potassium carbonate (K2CO3), can be converted inside the process into potassium hydroxide (KOH) (Reaction (14)), one of the raw materials in the new cleaner process. 4.5. More commercially valuable byproducts In chromite ore, besides chromium, valuable elements include aluminum (Al), iron (Fe) and magnesium (Mg). During liquid-phase oxidation of chromite ore, consid-
ð17Þ
The magnesium in chromite ore, in the form of magnesium oxide (MgO) after the decomposition of the chromite ore (Reaction (9)), hydrolyzes into magnesium hydroxide (Mg(OH)2) and enters the residue phase, during the process of liquid-phase oxidation. Magnesium hydroxide (Mg(OH)2) can be extracted by carbonation, and thus converted into magnesium carbonate (MgCO3) [24], another valuable byproduct. The iron in chromite ore persists in the form of iron oxide (Fe2O3) after the decomposition of chromite ore (Reaction (8)) during the liquid oxidation phase. Iron oxide (Fe2O3) is inert, and thus enters the residue phase. Nevertheless, after washing chromium and extracting magnesium, the ferrite-enriched residue can be used as raw materials in other industries (see also Section 4.6). Comparatively, during the traditional oxidation roasting process, aluminum (Al), magnesium (Mg) and iron (Fe) are present in the forms of aluminum oxide (Al2O3), magnesium oxide (MgO) and iron oxide (Fe2O3), respectively. All are chemically inactive and fixed in the residue after reaction. However, there are no effective extraction approaches available. 4.6. Zero emission of chromium-containing residues In developed countries, the general method for disposal of chromium-containing residue is land filling or stacking until the hexavalent chromium is detoxified. There were a number of reports and patents for disposing of chromium-containing residues [1e3,5,29], but few were really economically applicable. Recently, a widely discussed topic is the deployment of chromiumcontaining residue as a raw material input in cement industry. Portland cement normally contains CaO, SiO2, Al2O3, Fe2O3, MgO, K2O, Na2O, TiO2, and P2O5, as well as a small amount of other components. Correspondingly, the residue produced from the chromate industry also contains most of the components in the Portland cement clinker [30], which suggests the
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possibility of using the chromium-containing residue as the raw materials in cement industry. However, among the raw materials for cement manufacturing, two components are highly sensitive and of great importance. One is magnesium oxide (MgO); the other is chromic oxide (Cr2O3). Quantities of magnesium oxide (MgO) in excess of about 2% by weight can occur as periclase, which through slow reaction with water, can cause destructive expansion of hardened concrete [31]. An excessive amount of chromium can directly lead to industrial problems [32]. In the China National Standards for Portland cement and ordinary Portland cement products, the content of MgO is limited to below 5% by weight, and for the chromium-containing raw materials for cement industry, the content of Cr2O3 is limited to below 4.5% by weight [33]. As seen in Table 1, the residue produced in the liquidphase oxidation process contains a very small amount of chromium and magnesium, after washing of chromium and extraction of magnesium. It thereby becomes eligible for raw materials in cement industry. As for the traditional process, the residue produced does not meet the requirements mentioned above due to its high content of chromium and magnesium.
4.7. Practical results of utilizing the new cleaner process The main results, including the technical and economic index, of both laboratory and small-scale pilot plant studies are provided in Table 2. The data are based upon the production of 1.0 tonne of K2Cr2O7 or Na2Cr2O7 products. In brief, the new cleaner production process for chromic oxide (Cr2O3) not only achieved a higher extraction yield of chromium but also utilized the accompanying elements comprehensively and therefore improved atom utilization [35] of chromite ore resources, and eliminated waste. A demonstration plant featuring this new process has been built in He’nan Province, PR China. The plant’s annual production capacity is 10,000 tonnes, and has
Table 2 Comparison of the new cleaner process with the traditional process in technical and economic index when producing 1.0 tonne of K2Cr2O7 or Na2Cr2O7 [2,3,18,19,34] Items
The new cleaner process
The traditional process
Cr recovery yield, % by weight Amount of chromium residue (tonnes) Cr content in residue, % by weight Cr6C in residue, % by weight Cr-containing waste gases and dusts Ore consumption (tonnes) Alkali consumption (tonnes) Energy cost (counted by nature gas; Nm3) Atom economy (%) [35]
99
76
0.5a
2.5
0.5
4.0
Below 0.1 Almost eliminated 1.07 Below 0.5 (KOH) 650
1.0 Significant problem 1.35e1.50 0.9 (Na2CO3) 850
90.5
14.7
a
Eligible as the raw materials for cement industry.
been linked with the upstream coal gas plant and the downstream cement plant, as well as a newly-built alkali manufacturing plant. A demonstration Eco-Industrial Park (EIP) has also been constructed, as shown in Fig. 5. The technology has exhibited promising results for the industrial production of chromium compounds.
5. Conclusion In summary, a novel and original cleaner production process has been developed and commercialized. Thermodynamic analysis and reaction kinetic analysis of liquid-phase oxidation of chromite ore in potassium
Table 1 Typical chemical composition of the chromium-containing residues produced in both the traditional and the new cleaner processes [24] Components
Content, % by weight The traditional process
The new cleaner process
CaO MgO SiO2 Fe2O3 Al2O3 SCr CrC6
28e35 20e29 7e12 9e13 6e9 3e7 1.0
None 3.5 7.6 64.2 16.0 0.5 Below 0.1
Fig. 5. Illustrative diagram of the demonstration EIP con constructed in He’nan Province, PR China.
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hydroxide (KOH) fluid media has been carried out. The analysis shows that, when compared with the traditional roasting process, the liquid-phase oxidation of chromite ore in sub-molten potassium hydroxide (KOH) medium is thermodynamically favorable and the main reactions of liquid-phase oxidation are exothermic, which is beneficial in lowering the energy consumption. The pilot plant tests demonstrate that, the chromium yield is greater than 99% and the amount of produced chromium-containing residue is only one-fifth that of the traditional process. In the demonstration EIP constructed, carbon dioxide (CO2) in the waste gas from a nearby coal gas plant was used as the acidulant in the new cleaner process. Valuable magnesium-bearing byproducts are prepared from toxic chromium-containing residue. Moreover, the treated residues are used as raw material in the cement plant. A novel process with zero emission of chromium-containing residues has been achieved. The new cleaner chemical system and technology for producing chromic oxide (Cr2O3) is a good example of the feasibility of minimizing chemical loses through comprehensive utilization of natural resources and elimination of pollution at the source.
Acknowledgements The authors gratefully acknowledge the financial supports from the Key Project Program of the National Natural Science Foundation of China (Grant No. 50234040) and the Major Project of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KCCX1-SW-22).
References
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