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Catalytic oxidation and kinetics of oxidation of coal-derived pyrite by electrolysis
Fuel Processing Technology 82 (2003) 75 – 85 www.elsevier.com/locate/fuproc
Catalytic oxidation and kinetics of oxidation of coal-derived pyrite by electrolysis Li Dengxin a,*, Gao Jinsheng b, Yue Guangxi a a
b
Department of Thermal Engineering, Tsinghua University, Tsinghua 100084, China East China University of Science and Technology, 130, Meilong Road, Shanghai 200237, China
Abstract The catalytic oxidation of coal-derived pyrite (CDP) into SO42 and Fe3 + and kinetics of oxidation of coal-derived pyrite by electrolysis were studied in a batch reactor. In this paper, the influence of electrolysis conditions on the pyrite conversion rate, such as particle size, electrolysis potential, reaction time, etc. was reported. It is obvious that the rate of conversion of pyrite (Cr) increases with the increase of the electrolysis potential, reaction time, temperature, acidity and the concentration of catalyst (MnSO4), but decreases with the increase of concentration of pyrite. The experimental data show that the reaction is marching with shrinking core model. The reactioncontrolling step was found to be the reaction between Mn3 + and pyrite. The active energy for oxidation of CDP into SO42 and Fe3 + by electrolysis is 29.6 kJ/mol. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Coal-derived pyrite; Electrolysis; Kinetic
1. Introduction The resources of coal-derived pyrite (CDP) are abundant and can be found in many areas in China, such as Shandong, Shichuan, Shanxi and Heibei provinces. The proven reservation of CDP is about 1.6 billion tons and the year’s production of it is over 1 million tons. Most of CDP is used for the production of sulfur. But the conversion rate to sulfur is lower and the SO2 emission of the process is serious. Consequently, this process is already prohibited by the Chinese government. The CDP is also not suitable for the production of sulfuric acid by fluidized bed combustion because of high carbon content in CDP [1]. * Corresponding author. Tel.: +86-010-62781559-808. E-mail address: [email protected] (L. Dengxin). 0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3820(03)00040-7
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A number of environmental friendly chemical processes have been suggested for the utilization of CDP. In general, there are three chemical oxidation pyrite techniques: (1) oxidation of pyrite to form soluble sulfates; (2) conversion of sulfur to sulfur; (3) reaction with hydrogen to form gaseous hydrogen sulfide [2 –4]. Various agents are employed for these processes such as nitric acid, hydrogen peroxide, ozone, oxygen, chlorine, potassium dichromate, ferric salts and cupric salts. The active oxygen or high valence metal ion from electrolytic anode is very effective oxidants. The electrolysis oxidation of pyrite is a simple process by active oxygen from anodic surface in the electrolytic cell. Besides, the high-purity hydrogen is the by-product for the process. Therefore, the cost of electrolysis of CDP is lower [5]. So far, there is not enough investigation on the detail of the electrolysis CDP process. The present work is aiming to analyze the electrolysis conditions and their influence on the conversion rate of pyrite (Cr) besides the kinetics of pyrite oxidation.
2. Experimental 2.1. Experimental apparatus The schematic of electrolysis apparatus for the experiment is shown in Fig. 1. The electrode is made by Pt foils with surface area of 0.5 cm2. There is no membrane in the electrobath. The electrolysis potential was controlled by a potentiometer. The electrobath is inside of a temperature control box that can maintain constant temperature of the reaction.
Fig.1. Schematic diagram of electrolysis apparatus. Note: 1—constant temperature installation; 2—cell; 3—cathode; 4—magnetic stirrer; 5—anode; 6—reference electrode; 7—potentiometer.
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2.2. Electrolysis of pyrite Certain amount of CDP powder with certain size (samples from Wennan mine, Shandong province of China, FeS2, 86.27 wt.%) was put in a liquor of H2SO4 (10%) and MnSO4 (2%) and well mixed by magnetic stirrer. Then, pyrite was electrolyzed in the electrobath for a certain time at preset temperature. The reacted residue was then filtered and washed by water and 1 wt.% HCl solution, respectively, for the later analysis. 2.3. Analysis of sample Sulfur content in the residue of reaction was analyzed by elemental analysis meter (made in Germany, Vario EL). Ferrous ion in the electrolysis solutions was analyzed by 72-1 spectrophotometers. The different kinds of sulfur compounds in electrolysis solutions can be distinguished by the conventional chemical methods shown in Ref. [6]. Calculation of sulfur conversion ratio Cr: Cr ¼
W 1 S1 W 2 S2 100% W 1 S1
ð1Þ
note: W1—weight of coal-derived pyrite; S1—sulfur in coal-derived pyrite; W2—weight of reacted pyrite; S2—sulfur in reacted pyrite.
3. Result and discussion 3.1. Influence of particle size on Cr The effect of particle size on Cr at two different pyrite concentrations and two temperatures is given in Fig. 2. As can be seen from these figures, Cr of pyrite at any time increased with decreasing particle size due to the increase in surface area. This implies that the reaction takes places at the surfaces of the particles. This conclusion is in agreement with the microscopic structure of pyrite, which is generally nonporous. For the case of the continuous reaction model, there will be no effect of particle size [7,8]. 3.2. Influence of anodic potential on Cr When pyrite is electrolyzed, Mn2 + is oxidized into Mn3 + at surface of anode at first. Because the Mn3 + is a strong oxidizing agent, pyrite is easily oxidized into Fe3 + and SO42 , while Mn3 + is reduced into Mn2 +. The reduced Mn2 + can be oxidized into Mn3 + at the anodic surface again. Therefore, oxidizing reaction of pyrite is carried on in a sustainable way. So, Mn ion is recycling without consumption in the reaction (Mn2 + – Mn3 + –Mn2 +). When anodic potential is high, the concentration of Mn3 + from Mn2 + is
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Fig. 2. Effect of particle size on the rate of pyrite oxidation. 2.0% pyrite, 298 K, 2% MnSO4, 2.7 V; 1.0% pyrite, 298 K, 2% MnSO4, 2.7 V 3488 K, 2.7 V; 2% pyrite; 2% MnSO4; 10% H2SO4 348 K, 3.0 V; 1% pyrite; 2% MnSO4; 10% H2SO4.
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Fig. 3. Effect of potential on conversion of pyrite. Average diameter 50 Am, 298 K, 2% MnSO4, 1% pyrite, 10% H2SO4.
high. So, the higher concentration of Mn3 + results in higher speed of oxidization of CDP. The influence of anodic potential on Cr is shown in Fig. 3. Cr increases rapidly as electrolysis potential increases. When pyrite was electrolyzed at 3.0 V for about 16 h, Cr is 90%. It is only 6% at 2.0 V. 3.3. Influence of concentration of acid on Cr The reaction between Mn3 + and pyrite is affected by the acidity of electrolysis solutions because the acidity of electrolysis solutions is related to the form of Mn2 + when it was electrolyzed at the surface of anode. If the acidity of electrolysis solutions is lower, Mn2 + is easily oxidized into MnO2. Because MnO2 is easily absorbed on the surface of anode, the rate of reaction about oxidation of Mn2 + into Mn3 + is low. Fig. 4
Fig. 4. Effect of concentration of acid on conversion of pyrite. Average diameter 50 Am, 298 K; 1% pyrite; 3.0 V; 2% MnSO4.
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Fig. 5. Effect of amount of MnSO4 on conversion of pyrite. Average diameter 50 Am, 298 K, 2.7 V, 10% H2SO4, 1% pyrite.
shows influence of concentration of acid on Cr. The higher the concentration of acid, the greater the Cr, as shown in Fig. 4. 3.4. Influence of the concentration of MnSO4 on Cr Because Mn2 + is electrolyzed into Mn3 + at the surface of anode, the higher concentration of MnSO4 can yield higher concentration of Mn3 + which promotes more oxidation of pyrite. Fig. 5 shows the influence of the concentration of MnSO4 on Cr. Cr increases with the raise of amount of MnSO4. 3.5. Influence of amount of pyrite on Cr Fig. 6 shows the influence of concentration of pyrite on Cr. Cr decreases with the raise of concentration of pyrite. When the concentration of pyrite is 0.5%, Cr is high up to 100%
Fig. 6. Effect of concentration of pyrite on conversion. Average diameter 50 Am, 298 K, 8 h, 2% MnSO4, 2.7 V.
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Fig. 7. Effect of temperature on conversion. Average diameter 50 Am, 3.0 V; 1% pyrite; 2% MnSO4; 10% H2SO4.
for a 6-h reaction. While for a 2.0% pyrite concentration and a 12-h reaction, Cr. is only 47%. 3.6. Influence of electrolysis temperature on Cr Because the high temperature can accelerate the speed of oxidizing reaction of pyrite, Cr is certain to be high when pyrite is electrolyzed at high temperature. As shown in Fig. 7, Cr increases with the raise of temperature. When pyrite is electrolyzed at 348 K for about 12 h, Cr is high, 96%, but at 298 K and also at the same time, Cr is only 73%.
4. Process of conversion of pyrite by electrolysis When the electrolysis solutions were analyzed, Fe3 + was found at a higher anodic potential; Fe2 + and Fe3 + exist in the electrolysis solutions at the same time at a lower potential ( < 2.4 V). For Sulfur conversion, high potential generates SO42 . Sulfur and SO42 are found at lower potential and lower temperature. In the electrolysis process of pyrite, Mn3 + exists at any selected electrolysis conditions. We can detect the specific color of Mn3 + (purplish red) at all conditions. When pyrite is electrolyzed, Mn2 + is oxidized into Mn3 + at surface of anode at first. Because the Mn3 + is a strong oxidizing agent, pyrite is easily oxidized into Fe3 + and SO42 while Mn3 + is reduced into Mn2 +. The reduced Mn2 + can be oxidized into Mn3 + at the anodic surface again. Therefore, oxidizing reaction of pyrite is carried on in a sustainable way. So the process of pyrite conversion is actually manganese ion recycle (Mn2 + – Mn3 + – Mn2 +). So, the conversion of pyrite can be described as follows: At the anode: Mn2þ ! Mn3þ þ e
ð2Þ
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In the electrolysis solutions and high potential: 2þ 8H2 O þ 15Mn3þ þ FeS2 ! 16Hþ þ Fe3þ þ 2SO2 4 þ 15Mn
ð3Þ
4.1. Reaction kinetics In order to evaluate the kinetic parameters and rate-controlling step, the experimental data were analyzed based on the shrinking core model [9] because pyrite particles are generally non-porous. The reaction occurs on the surface of pyrite particles [9,10]. According to this model, the reaction between a fluid and a solid may be represented by [7]: AðfluidÞ þ BðsolidÞ ! CðproductsÞ
ð4Þ
where A and B represent Mn3 + and pyrite particles, respectively. The reaction rate may be controlled by one of the following steps: diffusion through boundary layer, diffusion through the ash (converted solids or inert materials) and the chemical reaction at the surface of the core of the unreacted materials [7,9]. If there is no residue layer formed on the surface of particles, there should be only two controlling steps: diffusion in boundary layer and chemical reaction. Because the pyrite sample used in this work is relatively pure, there is no solid product (see Eq. (3)), the rate-controlling step is either the diffusion of Mn3 + from liquid phase to the pyrite surface or the chemical reaction between Mn3 + and pyrite on shrinking surface. When overall rate is controlled by the chemical reaction on the surface, the relationship between time and the rate of conversion of pyrite is [7]: t ¼ 1 ð1 Cr Þ1=2 * tf t *f ¼
qB R1 Kp CMn3þ
where : qB ¼ Cp
ð5Þ
ð6Þ ð7Þ
When the anodic potential is certain, the concentration of Mn3 + can be expressed as follows: CMn3þ ¼ K1 CMn2þ In Eq. (8), K1 is the reactive velocity constant of electrolysis reaction: Mn2þ ! Mn3þ þ e:
ð8Þ
L. Dengxin et al. / Fuel Processing Technology 82 (2003) 75–85
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Fig. 8. Relationship between l (l Cr)1/2 and time at different temperatures.
K1 is expressed as follows: K1 ¼ e
FðE 0 EÞ RT
ð9Þ
where F = Faraday’s constant = 9.6500 C/mol; E0 = standard potential of the Mn2 +/Mn3 + redox couple; E = anodic potential. Therefore: t *f ¼ Kp e
Cp R1
FðE 0 EÞ RT
ð10Þ CMn2þ
To determine the rate-controlling step in the overall reaction, the experimental results were plotted on the basis of Eq. (5) which is shown in Fig. 8. It is found that the experimental results are better correlated by the reaction-controlled mechanism (Eq. (5)). From Fig. 8, the value of t*f can be calculated from slope of these straight lines. Then, t*f will be used in calculation of the first-order rate constant for surface reaction (Kp) according to Eq. (10). The value of Kp in different temperatures is shown in Table 1. From Table 1, rate constants increase with raise of temperature. Arrhenius plot of ln(Kp) versus
Table 1 Reactive velocity constants of pyrite at different temperature Temperature (K)
KP (m s 1 10 5)
303 313 338 348
0.56 0.96 2.21 2.58
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Fig. 9. Relationship between (-lnK) and 1/T.
1/T are shown in Fig. 9. From the slope coefficient of straight line, the activation energy is found (by regress analysis) to be 29.6 kJ/mol.
5. Conclusion The CDP electrolysis was experimentally investigated. The rate of conversion of pyrite (Cr) increases with the raise of potential, time, temperature, acidity and the concentration of additive agent, but decreases with the raise of concentration of pyrite. According to shrinking core model, the rate-controlling step was found to be the chemical reaction between Mn3 + and pyrite. The reactive activity energy for oxidation of coal-derived pyrite into SO42 and Fe3 + by electrolysis is 29.6 kJ/mol.
Acknowledgements The former National Coal Department of China financially supported this research.
Appendix A t*f Kp K1 t Cp Cr CMn2+
time required for complete conversion (s) first-order rate constant for surface reaction (m s 1) the reactive velocity constants of electrolysis reaction: Mn2 + ! Mn3 + + e. reaction time (s) concentration of pyrite (mol m 3) rate of conversion of pyrite (%) concentration of MnSO4 (mol m 3)
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CMn3+ concentration of Mn3+ (mol m 3) R1 mean radii of original particle of pyrite (m) r correlation coefficient T temperature (K) qB molar density of pyrite (mol m 3) F = Faraday’s constant = 9.6500 C/mol; E0 = Standard potential of the Mn2+/Mn3+ redox couple E = anodic potential of the Mn2+/Mn3+ redox couple
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
L. Zushe, Coal Utilization (in Chinese) 13 (1) (1996) 21. S. Damste, Fuel Progress and Technology 30 (1) (1992) 109. Y. Kadiogive, Fuel Progress and Technology 41 (3) (1997) 273. Y. Kadiogive, Fuel 71 (8) (1992) 929. L. Dengxin, G. Jinsheng, Journal of East China University of Science and Technology (in Chinese) 24 (6) (1998) 664. W. Zilrong, L. Xiangzhen, Inorganic Chemical Test (in Chinese), Beijing: Tsing Hua University Press, 1987, pp. 71 – 75, pp. 123 – 126. D. Slagle, Y.T. Shah, J.B. Joshi, Kinetics of oxydesulfurization of upper freeport coal, Industrial & Engineering Chemistry Process Design and Development 19 (1980) 294 – 300. T. Bronikowski, Model selection for aqueous slurry coal desulfurization, Fuel 63 (1984) 116 – 120. O. Levenspiel, Chemical Reaction Engineering, 2nd ed., Wiley, New York, NY, 1972, p. 357. C.C. Hwang, R.K. Streeter, Y.T. Shah, Kinetics of the ozonation of pyrite in aqueous suspension, Fuel 66 (1987) 1575.
Catalytic oxidation and kinetics of oxidation of coal-derived pyrite by electrolysis Li Dengxin a,*, Gao Jinsheng b, Yue Guangxi a a
b
Department of Thermal Engineering, Tsinghua University, Tsinghua 100084, China East China University of Science and Technology, 130, Meilong Road, Shanghai 200237, China
Abstract The catalytic oxidation of coal-derived pyrite (CDP) into SO42 and Fe3 + and kinetics of oxidation of coal-derived pyrite by electrolysis were studied in a batch reactor. In this paper, the influence of electrolysis conditions on the pyrite conversion rate, such as particle size, electrolysis potential, reaction time, etc. was reported. It is obvious that the rate of conversion of pyrite (Cr) increases with the increase of the electrolysis potential, reaction time, temperature, acidity and the concentration of catalyst (MnSO4), but decreases with the increase of concentration of pyrite. The experimental data show that the reaction is marching with shrinking core model. The reactioncontrolling step was found to be the reaction between Mn3 + and pyrite. The active energy for oxidation of CDP into SO42 and Fe3 + by electrolysis is 29.6 kJ/mol. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Coal-derived pyrite; Electrolysis; Kinetic
1. Introduction The resources of coal-derived pyrite (CDP) are abundant and can be found in many areas in China, such as Shandong, Shichuan, Shanxi and Heibei provinces. The proven reservation of CDP is about 1.6 billion tons and the year’s production of it is over 1 million tons. Most of CDP is used for the production of sulfur. But the conversion rate to sulfur is lower and the SO2 emission of the process is serious. Consequently, this process is already prohibited by the Chinese government. The CDP is also not suitable for the production of sulfuric acid by fluidized bed combustion because of high carbon content in CDP [1]. * Corresponding author. Tel.: +86-010-62781559-808. E-mail address: [email protected] (L. Dengxin). 0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3820(03)00040-7
76
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A number of environmental friendly chemical processes have been suggested for the utilization of CDP. In general, there are three chemical oxidation pyrite techniques: (1) oxidation of pyrite to form soluble sulfates; (2) conversion of sulfur to sulfur; (3) reaction with hydrogen to form gaseous hydrogen sulfide [2 –4]. Various agents are employed for these processes such as nitric acid, hydrogen peroxide, ozone, oxygen, chlorine, potassium dichromate, ferric salts and cupric salts. The active oxygen or high valence metal ion from electrolytic anode is very effective oxidants. The electrolysis oxidation of pyrite is a simple process by active oxygen from anodic surface in the electrolytic cell. Besides, the high-purity hydrogen is the by-product for the process. Therefore, the cost of electrolysis of CDP is lower [5]. So far, there is not enough investigation on the detail of the electrolysis CDP process. The present work is aiming to analyze the electrolysis conditions and their influence on the conversion rate of pyrite (Cr) besides the kinetics of pyrite oxidation.
2. Experimental 2.1. Experimental apparatus The schematic of electrolysis apparatus for the experiment is shown in Fig. 1. The electrode is made by Pt foils with surface area of 0.5 cm2. There is no membrane in the electrobath. The electrolysis potential was controlled by a potentiometer. The electrobath is inside of a temperature control box that can maintain constant temperature of the reaction.
Fig.1. Schematic diagram of electrolysis apparatus. Note: 1—constant temperature installation; 2—cell; 3—cathode; 4—magnetic stirrer; 5—anode; 6—reference electrode; 7—potentiometer.
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2.2. Electrolysis of pyrite Certain amount of CDP powder with certain size (samples from Wennan mine, Shandong province of China, FeS2, 86.27 wt.%) was put in a liquor of H2SO4 (10%) and MnSO4 (2%) and well mixed by magnetic stirrer. Then, pyrite was electrolyzed in the electrobath for a certain time at preset temperature. The reacted residue was then filtered and washed by water and 1 wt.% HCl solution, respectively, for the later analysis. 2.3. Analysis of sample Sulfur content in the residue of reaction was analyzed by elemental analysis meter (made in Germany, Vario EL). Ferrous ion in the electrolysis solutions was analyzed by 72-1 spectrophotometers. The different kinds of sulfur compounds in electrolysis solutions can be distinguished by the conventional chemical methods shown in Ref. [6]. Calculation of sulfur conversion ratio Cr: Cr ¼
W 1 S1 W 2 S2 100% W 1 S1
ð1Þ
note: W1—weight of coal-derived pyrite; S1—sulfur in coal-derived pyrite; W2—weight of reacted pyrite; S2—sulfur in reacted pyrite.
3. Result and discussion 3.1. Influence of particle size on Cr The effect of particle size on Cr at two different pyrite concentrations and two temperatures is given in Fig. 2. As can be seen from these figures, Cr of pyrite at any time increased with decreasing particle size due to the increase in surface area. This implies that the reaction takes places at the surfaces of the particles. This conclusion is in agreement with the microscopic structure of pyrite, which is generally nonporous. For the case of the continuous reaction model, there will be no effect of particle size [7,8]. 3.2. Influence of anodic potential on Cr When pyrite is electrolyzed, Mn2 + is oxidized into Mn3 + at surface of anode at first. Because the Mn3 + is a strong oxidizing agent, pyrite is easily oxidized into Fe3 + and SO42 , while Mn3 + is reduced into Mn2 +. The reduced Mn2 + can be oxidized into Mn3 + at the anodic surface again. Therefore, oxidizing reaction of pyrite is carried on in a sustainable way. So, Mn ion is recycling without consumption in the reaction (Mn2 + – Mn3 + –Mn2 +). When anodic potential is high, the concentration of Mn3 + from Mn2 + is
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Fig. 2. Effect of particle size on the rate of pyrite oxidation. 2.0% pyrite, 298 K, 2% MnSO4, 2.7 V; 1.0% pyrite, 298 K, 2% MnSO4, 2.7 V 3488 K, 2.7 V; 2% pyrite; 2% MnSO4; 10% H2SO4 348 K, 3.0 V; 1% pyrite; 2% MnSO4; 10% H2SO4.
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Fig. 3. Effect of potential on conversion of pyrite. Average diameter 50 Am, 298 K, 2% MnSO4, 1% pyrite, 10% H2SO4.
high. So, the higher concentration of Mn3 + results in higher speed of oxidization of CDP. The influence of anodic potential on Cr is shown in Fig. 3. Cr increases rapidly as electrolysis potential increases. When pyrite was electrolyzed at 3.0 V for about 16 h, Cr is 90%. It is only 6% at 2.0 V. 3.3. Influence of concentration of acid on Cr The reaction between Mn3 + and pyrite is affected by the acidity of electrolysis solutions because the acidity of electrolysis solutions is related to the form of Mn2 + when it was electrolyzed at the surface of anode. If the acidity of electrolysis solutions is lower, Mn2 + is easily oxidized into MnO2. Because MnO2 is easily absorbed on the surface of anode, the rate of reaction about oxidation of Mn2 + into Mn3 + is low. Fig. 4
Fig. 4. Effect of concentration of acid on conversion of pyrite. Average diameter 50 Am, 298 K; 1% pyrite; 3.0 V; 2% MnSO4.
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Fig. 5. Effect of amount of MnSO4 on conversion of pyrite. Average diameter 50 Am, 298 K, 2.7 V, 10% H2SO4, 1% pyrite.
shows influence of concentration of acid on Cr. The higher the concentration of acid, the greater the Cr, as shown in Fig. 4. 3.4. Influence of the concentration of MnSO4 on Cr Because Mn2 + is electrolyzed into Mn3 + at the surface of anode, the higher concentration of MnSO4 can yield higher concentration of Mn3 + which promotes more oxidation of pyrite. Fig. 5 shows the influence of the concentration of MnSO4 on Cr. Cr increases with the raise of amount of MnSO4. 3.5. Influence of amount of pyrite on Cr Fig. 6 shows the influence of concentration of pyrite on Cr. Cr decreases with the raise of concentration of pyrite. When the concentration of pyrite is 0.5%, Cr is high up to 100%
Fig. 6. Effect of concentration of pyrite on conversion. Average diameter 50 Am, 298 K, 8 h, 2% MnSO4, 2.7 V.
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Fig. 7. Effect of temperature on conversion. Average diameter 50 Am, 3.0 V; 1% pyrite; 2% MnSO4; 10% H2SO4.
for a 6-h reaction. While for a 2.0% pyrite concentration and a 12-h reaction, Cr. is only 47%. 3.6. Influence of electrolysis temperature on Cr Because the high temperature can accelerate the speed of oxidizing reaction of pyrite, Cr is certain to be high when pyrite is electrolyzed at high temperature. As shown in Fig. 7, Cr increases with the raise of temperature. When pyrite is electrolyzed at 348 K for about 12 h, Cr is high, 96%, but at 298 K and also at the same time, Cr is only 73%.
4. Process of conversion of pyrite by electrolysis When the electrolysis solutions were analyzed, Fe3 + was found at a higher anodic potential; Fe2 + and Fe3 + exist in the electrolysis solutions at the same time at a lower potential ( < 2.4 V). For Sulfur conversion, high potential generates SO42 . Sulfur and SO42 are found at lower potential and lower temperature. In the electrolysis process of pyrite, Mn3 + exists at any selected electrolysis conditions. We can detect the specific color of Mn3 + (purplish red) at all conditions. When pyrite is electrolyzed, Mn2 + is oxidized into Mn3 + at surface of anode at first. Because the Mn3 + is a strong oxidizing agent, pyrite is easily oxidized into Fe3 + and SO42 while Mn3 + is reduced into Mn2 +. The reduced Mn2 + can be oxidized into Mn3 + at the anodic surface again. Therefore, oxidizing reaction of pyrite is carried on in a sustainable way. So the process of pyrite conversion is actually manganese ion recycle (Mn2 + – Mn3 + – Mn2 +). So, the conversion of pyrite can be described as follows: At the anode: Mn2þ ! Mn3þ þ e
ð2Þ
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In the electrolysis solutions and high potential: 2þ 8H2 O þ 15Mn3þ þ FeS2 ! 16Hþ þ Fe3þ þ 2SO2 4 þ 15Mn
ð3Þ
4.1. Reaction kinetics In order to evaluate the kinetic parameters and rate-controlling step, the experimental data were analyzed based on the shrinking core model [9] because pyrite particles are generally non-porous. The reaction occurs on the surface of pyrite particles [9,10]. According to this model, the reaction between a fluid and a solid may be represented by [7]: AðfluidÞ þ BðsolidÞ ! CðproductsÞ
ð4Þ
where A and B represent Mn3 + and pyrite particles, respectively. The reaction rate may be controlled by one of the following steps: diffusion through boundary layer, diffusion through the ash (converted solids or inert materials) and the chemical reaction at the surface of the core of the unreacted materials [7,9]. If there is no residue layer formed on the surface of particles, there should be only two controlling steps: diffusion in boundary layer and chemical reaction. Because the pyrite sample used in this work is relatively pure, there is no solid product (see Eq. (3)), the rate-controlling step is either the diffusion of Mn3 + from liquid phase to the pyrite surface or the chemical reaction between Mn3 + and pyrite on shrinking surface. When overall rate is controlled by the chemical reaction on the surface, the relationship between time and the rate of conversion of pyrite is [7]: t ¼ 1 ð1 Cr Þ1=2 * tf t *f ¼
qB R1 Kp CMn3þ
where : qB ¼ Cp
ð5Þ
ð6Þ ð7Þ
When the anodic potential is certain, the concentration of Mn3 + can be expressed as follows: CMn3þ ¼ K1 CMn2þ In Eq. (8), K1 is the reactive velocity constant of electrolysis reaction: Mn2þ ! Mn3þ þ e:
ð8Þ
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Fig. 8. Relationship between l (l Cr)1/2 and time at different temperatures.
K1 is expressed as follows: K1 ¼ e
FðE 0 EÞ RT
ð9Þ
where F = Faraday’s constant = 9.6500 C/mol; E0 = standard potential of the Mn2 +/Mn3 + redox couple; E = anodic potential. Therefore: t *f ¼ Kp e
Cp R1
FðE 0 EÞ RT
ð10Þ CMn2þ
To determine the rate-controlling step in the overall reaction, the experimental results were plotted on the basis of Eq. (5) which is shown in Fig. 8. It is found that the experimental results are better correlated by the reaction-controlled mechanism (Eq. (5)). From Fig. 8, the value of t*f can be calculated from slope of these straight lines. Then, t*f will be used in calculation of the first-order rate constant for surface reaction (Kp) according to Eq. (10). The value of Kp in different temperatures is shown in Table 1. From Table 1, rate constants increase with raise of temperature. Arrhenius plot of ln(Kp) versus
Table 1 Reactive velocity constants of pyrite at different temperature Temperature (K)
KP (m s 1 10 5)
303 313 338 348
0.56 0.96 2.21 2.58
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Fig. 9. Relationship between (-lnK) and 1/T.
1/T are shown in Fig. 9. From the slope coefficient of straight line, the activation energy is found (by regress analysis) to be 29.6 kJ/mol.
5. Conclusion The CDP electrolysis was experimentally investigated. The rate of conversion of pyrite (Cr) increases with the raise of potential, time, temperature, acidity and the concentration of additive agent, but decreases with the raise of concentration of pyrite. According to shrinking core model, the rate-controlling step was found to be the chemical reaction between Mn3 + and pyrite. The reactive activity energy for oxidation of coal-derived pyrite into SO42 and Fe3 + by electrolysis is 29.6 kJ/mol.
Acknowledgements The former National Coal Department of China financially supported this research.
Appendix A t*f Kp K1 t Cp Cr CMn2+
time required for complete conversion (s) first-order rate constant for surface reaction (m s 1) the reactive velocity constants of electrolysis reaction: Mn2 + ! Mn3 + + e. reaction time (s) concentration of pyrite (mol m 3) rate of conversion of pyrite (%) concentration of MnSO4 (mol m 3)
L. Dengxin et al. / Fuel Processing Technology 82 (2003) 75–85
85
CMn3+ concentration of Mn3+ (mol m 3) R1 mean radii of original particle of pyrite (m) r correlation coefficient T temperature (K) qB molar density of pyrite (mol m 3) F = Faraday’s constant = 9.6500 C/mol; E0 = Standard potential of the Mn2+/Mn3+ redox couple E = anodic potential of the Mn2+/Mn3+ redox couple
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