Green Manufacturing Process of Chromium Compounds

Transcription

1 Green Manufacturing Process of Chromium Compounds Yi Zhang, Zuo-Hu Li, Tao Qi, Shi-Li Zheng, Hui-Quan Li, and Hong-Bin Xu Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing , China Published online 19 November 2004 in Wiley InterScience ( DOI /ep Chromium compounds are important basic chemicals essential to many industries. In the traditional process for manufacturing these compounds, the utilization efficiency of resources and energy is quite low. Large amounts of chromium-containing toxic solid wastes and exhaust gas are discharged, resulting in serious pollution problems. A green manufacturing process of chromium compounds has been developed by the Institute of Process Engineering, Chinese Academy of Sciences in Beijing, China. With the design objective of eliminating pollution at the source, this green process achieves higher resource utilization efficiency and zero emissions of chromium-containing waste residue by alteration of process chemistry, change of reactor and operation, regeneration and recycle of reaction media, and comprehensive use of resources. By use of the green process, a demonstration plant with an annual production capability of 10,000 tons has been built and operated in Henan Province, China. The green process exhibits a promising prospect for the industrial production of chromium compounds American Institute of Chemical Engineers Environ Prog, 24: 44 50, 2005 Keywords: chromite ore, chromium compounds, green manufacturing process, submolten salt medium, pollution prevention, zero emission 2004 American Institute of Chemical Engineers INTRODUCTION Chromium compounds are essential to many industries, but their manufacturing process is usually a major source of pollution [1 4]. In the traditional manufacturing process that uses oxidation roasting at a high temperature (1200 C), water leaching and multistage evaporation crystallization [3, 4], the utilization efficiency of resources and energy is quite low. Chromate plants discharge large amounts of chromium-containing residues, dusts, and waste gases. Chromium-containing residues create serious pollution problems that threaten groundwater, rivers, and marine areas [2]. Although the less calcium [5] or noncalcium [6, 7] roasting technologies, as well as many other new technologies [8 12], of the chromium compounds production were developed, the pollution problem has not been thoroughly resolved [4]. Recently, a green manufacturing process for chromium compounds has been developed by the Institute of Process Engineering, Chinese Academy of Sciences [13 16]. With the design objective of eliminating pollution at the source [17 21], the green process includes: continuous oxidization of chromite ore in a submolten 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 submolten salt medium at 300 C in a multiphase reactor. BRIEF DESCRIPTION OF THE TRADITIONAL PROCESS As shown in Figure 1, the traditional manufacturing process for chromium compounds involves a considerable number of unit operations, and produces useful products of sodium dichromate (Na 2 Cr 2 O 7 ) and chromium anhydride (CrO 3 ), as well as several kinds of toxic residues, including aluminum-enriched residue, chromium-containing residue, and chromium-containing Glauber s salt (Na 2 SO 4 10H 2 O) and sodium bisulfate (NaHSO 4 ). The major chromium-bearing mineral can be ideally represented as FeO Cr 2 O 3, but it rarely exists as pure iron protoxide (FeO) and chromic oxide (Cr 2 O 3 ). Chromic oxide (Cr 2 O 3 ) and iron protoxide (FeO) are isomorphously displaced in part by aluminum oxide (Al 2 O 3 ) and magnesium oxide (MgO), respectively. The general formula is described as (Mg x Fe 1 x )O (Al y Cr 1 y ) 2 O 3. Silicon is mixed as gangue in chromite ores. 44 April 2005 Environmental Progress (Vol.24, No.1)

2 Figure 1. Illustrative flow sheet of the traditional process for manufacturing chromium compounds. The main chemical reactions in the oxidation roasting of chromite ore with sodium carbonate (Na 2 CO 3 ) and oxygen (O 2 ) are listed as follows: FeO Cr 2 O 3 2Na 2 CO 3 7/4O 2 3 2Na 2 CrO 4 1/2Fe 2 O 3 CO 2 (1) MgO Cr 2 O 3 2Na 2 CO 3 3/2O 2 3 2Na 2 CrO 4 MgO CO 2 (2) Cr 2 O 3 2Na 2 CO 3 3/2O 2 3 2Na 2 CrO 4 CO 2 (3) Cr series products The chemical reaction in the acidification of sodium chromate (Na 2 CrO 4 ) alkaline liquor is 2Na 2 CrO 4 H 2 SO 4 3 Na 2 Cr 2 O 7 Na 2 SO 4 H 2 O (4) For the fusing of sodium dichromate (Na 2 Cr 2 O 7 ) with concentrated sulfuric acid (H 2 SO 4 ), the chemical reaction is Na 2 Cr 2 O 7 2H 2 SO 4 3 2CrO 3 2NaHSO 4 H 2 O (5) In the traditional process, three problems are highly distinguishable. The first is the environmental pollution. During production of one ton of chromium anhydride (CrO 3 ) 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 (CaCrO 4 ) 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 Glauber s salt (Na 2 SO 4 10H 2 O) and sodium bisulfate (NaHSO 4 ) produced are of little use and constitute a pollution Environmental Progress (Vol.24, No.1) April

3 Figure 2. Schematic idea of the proposed green manufacturing process for chromium compounds. Figure 3. Illustrative flow sheet of the green manufacturing process for chromium compounds. source. Consequently, the total atom utilization efficiency [22] of the traditional process is quite low [23]. GREEN MANUFACTURING PROCESS The design of the green process is on the basis of the principle of the 3Rs: reduce, recycle, reuse. 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 2 illustrates the main design idea of the proposed new green process. Accordingly, the green manufacturing process, as shown in Figure 3, achieves the zero emissions of chromium-containing residues and eliminates the pollution problem at the source. ANALYSIS OF THE GREEN PROCESS FOR MANUFACTURING CHROMIUM COMPOUNDS High Efficiency of Chemical Reaction in the Fluid Salt Medium Compared with the gas solid heterogeneous reactions of chromite ore with sodium carbonate (Na 2 CO 3 ) 46 April 2005 Environmental Progress (Vol.24, No.1)

4 Figure 4. Change of standard Gibbs free energy vs. temperature for Reactions 6 through 8, respectively. Figure 5. Effect of temperature on the total conversion of chromium for Reactions 6 through 8. and oxygen (O 2 ) at 1200 C, the liquid-phase oxidation of chromite ore in submolten fluid more easily and quantitatively controls the reaction and separation because the oxygen anion (O 2 ) exchange in the salt medium is similar to that in the solvent and the heat and mass transfer is enhanced, so that the efficiency and selectivity of chromite ore oxidation are significantly improved. The reaction temperature is decreased from 1200 to 300 C, thus substantially decreasing the energy consumption. Thermodynamic Analysis of the Main Reactions Reacting with oxygen (O 2 ) in potassium hydroxide (KOH) fluid medium, chromite ore would undertake the following main chemical reactions: 1/2FeO Cr 2 O 3 2KOH 7/8O 2 3 1/4Fe 2 O 3 H 2 O K 2 CrO 4 (6) 1/2Cr 2 O 3 2KOH 3/4O 2 3 H 2 O K 2 CrO 4 (7) 1/2MgO Cr 2 O 3 2KOH 3/4O 2 3 1/2MgO H 2 O K 2 CrO 4 (8) By thermodynamic calculation, the relationship between the change of standard Gibbs free energy ( r G ) and temperature for the main Reactions 6 8 is shown in Figure 4. The standard Gibbs free energy data of the related substances were found in the literature [24]. As shown in Figure 4, the change of standard Gibbs free energy, r G, of Reaction 6 is negative so that trivalent chromium is easily oxidized to hexavalent chromium. Under the same reaction conditions, the reaction tendency is FeO Cr 2 O 3 Cr 2 O 3 MgO Cr 2 O 3. The thermodynamic tendency of the reaction for FeO Cr 2 O 3 is greater than that of traditional roasting of chromite ore with sodium carbonate (Na 2 CO 3 ), considering that the change of standard Gibbs free energy, r G, of Reaction 6 is kj/mol K 2 CrO 4 at 300 C, which is approximately twice as much as that of Reaction 1 in the traditional process at 1200 C [25]. The main reaction of liquid-phase oxidation of chromite ore in potassium hydroxide (KOH) medium at 327 C is an exothermic reaction and the reaction heat of kj/mol K 2 CrO 4 is favored to lower the energy consumption [25]. Kinetic Analysis of the Main Reactions The reaction kinetics of liquid-phase oxidation of chromite ore in the submolten salt medium were investigated to discover approaches to reactor scale-up and project development. The effect of reaction temperature on the conversion of chromium is shown in Figure 5. Kinetic analysis shows that the best fit of the chromium extraction data was obtained by using the unreacted shrinking core model controlled by chemical reaction with the experimental observations. As shown in Figures 6 and 7, the reaction rate increases with the reaction time and the relationship can be expressed as 1 (1 x) 1/3 kt, where x is the extraction yield of chromium, t is the reaction time, and k is the reaction rate constant. The specific activation energy of the main reactions is calculated through the Arrhenius diagram, as shown in Figure 7, to be 52.5 kj/mol. Compared to the traditional process, the reaction and mass transfer rates are significantly accelerated in the proposed new green process. It can be hypothesized that the chromite particles can be dispersed almost evenly in potassium hydroxide (KOH) fluid medium, and the mass transfer resistance in particle surface film is reduced. In the green process, the reaction and separation are intensified by the reaction system substitution of liquid-phase oxidation of chromite ore in the potassium hydroxide (KOH) fluid medium at 300 C, as contrasted to oxidation roasting at 1200 C. The liquid-phase oxidation reaction and the crystallization separation of the semifinished product are integrated and, thus, the reaction and transport are significantly improved [26]. The chemical conversion rate of chromium approaches 100%. Compared to the traditional process, the recovery of chromium is increased Environmental Progress (Vol.24, No.1) April

5 Figure 6. Relationship between 1 (1 x) 1/3 and reaction time for Reactions 6 through 8. Figure 7. Relationship between ln k and 1/T for Reactions 6 through 8. by more than 20%, but the reaction temperature decreases by 900 C and, consequently, the energy consumption is decreased by 20% [27]. Furthermore, the amount of chromium-containing residue is decreased to 500 kg/ton of product from 2.5 tons per ton of product in the traditional process. Additionally, the chromium content in the residue is decreased to 1% of that in the traditional process. All reactions and separation operations are carried out in liquid media so that pollution arising from toxic dust is prevented. As noted above, the green process achieves higher resource utilization efficiency than that of the traditional process. Use of CO 2 as the Acidulant Carbon dioxide (CO 2 ) exhaust gas from a nearby coal gas plant is used in the green process as a substitute for sulfuric acid (H 2 SO 4 ) in the process of converting semifinished product to potassium dichromate (K 2 Cr 2 O 7 ), as shown in Reaction 9. Compare this process reaction to Reaction 4. The transition between alkaline chromate liquor and acidic dichromate liquor makes it possible to recover and recycle the potassium cation, which is used as the reaction medium and as a carrier of chromium in the green process. As a result, the pollution resulting from chromium-containing Glauber s salt (Na 2 SO 4 10H 2 O) produced in the traditional process is eliminated. 2K 2 CrO 4 H 2 O 2CO 2 3 K 2 Cr 2 O 7 2KHCO 3 (9) Zero Emissions of Chromium-Containing Residue In developed countries, the general method of disposing of chromium-containing residue is landfilling or stacking until the hexavalent chromium is detoxified [1 4]. Although there are a number of reports and patents for disposing of chromium-containing residue [1, 2, 28], few processes are really economical. With the green process, a new practical, economical integrated technology has been developed. It was originally developed to dispose of chromium-containing residue. The process consists of carbonate decomposition of chromium-containing residue, extraction of magnesium, Cr 6 detoxification, and preparation of magnetic ferrite. In the process of carbonate leaching, the chromium-containing residue after magnesium extraction can be used as iron ore or raw material in the cement industry [29]. In addition, aluminum in chromite ore can be extracted and recovered in the green process as aluminum hydroxide [Al(OH) 3 ]. Thus, the comprehensive use of all valuable elements (Cr Al Mg Fe) in chromite ore and the zero emission of chromium-containing toxic waste are achieved in the green process. Production of Chromic Oxide In the green process, chromic oxide (Cr 2 O 3 )ischosen as one of the final products and manufactured by the roasting reduction of potassium dichromate (K 2 Cr 2 O 7 ) with carbon black (C) or sucrose, as shown in Reaction 10. The reaction conditions for Reaction 10 become much milder than those for Reaction 5 in the traditional process that yields chromic anhydride (CrO 3 ) as the final product. The chromic oxide (Cr 2 O 3 ) product is suitable for both metallurgical and pigmental uses. The by-product, potassium carbonate (K 2 CO 3 ), can be converted inside the process into potassium hydroxide (KOH) [30], which is one of the raw materials in the green process. 2K 2 Cr 2 O 7 2C 3 Cr 2 O 3 K 2 CO 3 CO 2 (10) ADVANTAGES OF THE GREEN PROCESS Chromite ore and air are the only raw materials needed to manufacture chromium series products and other valuable by-products. The general reaction equation for the green process is as follows: recyclable reaction media FeCr 2 O 4 O 2 O Fe2 O 3 Cr series products (11) 48 April 2005 Environmental Progress (Vol.24, No.1)

6 Table 1. Comparison of the green process with the traditional process [3, 4, 15, 16, 23]. Item Green process Traditional process Cr recovery yield, % Amount of chromium-containing residue, tons per ton of product 0.5* 2.5 Cr content in residue, % w/w Cr 6 content in residue, % w/w Below Cr-containing waste gases and dusts Almost eliminated Significant problem Atom economy, % *The composition of the chromium-containing residue is eligible for the raw material in cement industry [29]. Figure 9. Illustrative diagram of the demonstration EIP constructed in Henan Province, China. Figure 8. Photograph of the chromium production demonstration plant constructed in Henan Province, China. [Color figure can be viewed in the online issue, which is available at In brief, the green manufacturing process of chromium compounds not only yields a higher amount of chromium but also comprehensively uses the accompanying elements so as to improve the atom utilization [22] of chromite ore resources and elimination of process waste. The main results of laboratory- and smallscale pilot-plant studies are given in Table 1. By use of the green process described above, a demonstration project with an annual production capability of 10,000 tons of potassium dichromate has been built in Henan Province, China (Figure 8). The technology has exhibited promising results for the industrial production of chromium compounds. By linking the chromate manufacturing plant with the upstream coal gas plant and the downstream cement production plant, a demonstration Eco-Industrial Park (EIP) has also been constructed, as shown in Figure 9. CONCLUSION In summary, a novel and original green process has been developed and commercialized. Thermodynamic analysis and reaction kinetic analysis of liquid-phase oxidation of chromite ore in potassium hydroxide (KOH) fluid media has been carried out. The analysis shows that the change of free energy of liquid-phase oxidation of chromite ore is approximately twice as great as that of the traditional roasting process. Analysis also shows that the reaction kinetics of liquid-phase oxidation of chromite ore is controlled by the rate of chemical reactions and the chromium yield is apparently improved at higher reaction temperatures. The main reaction is exothermic and is beneficial in lowering the energy consumption. The pilot-plant tests demonstrate that the chromium yield is greater than 99.5% and the amount of discharged chromium-containing residue is only one-fifth that in the traditional process. A novel process with zero emission of chromium-containing residue has been proposed. Magnesium oxide (MgO) and other valuable by-products are prepared from toxic chromium-containing residue. The green chemical system and technology for manufacturing chromium compounds present a good example of the feasibility of minimizing chemical losses through the comprehensive use of natural resources and elimination of pollution at the source. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant No ) and the Major Program Project of the Knowledge Innovation Program of the Environmental Progress (Vol.24, No.1) April

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