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Recovery of fly ash carbon by carbochlorination via phosgene route
Powder Technology 131 (2003) 206 – 211 www.elsevier.com/locate/powtec
Recovery of fly ash carbon by carbochlorination via phosgene route F. Yang 1, P. Pranda *, V. Hlavacek Department of Chemical Engineering, SUNY Buffalo, 218 Furnas Hall, Buffalo, NY 14260, USA Received 10 July 2002; received in revised form 13 December 2002; accepted 17 December 2002
Abstract Coal fly ash consists mainly of unburned carbon and minerals. It also contains phosphorus with a concentration much higher than that acceptable for the metallurgical coke or boiler fuel. By flotation process, a large portion of the inorganic materials can be separated. However, further treatment is necessary before the fly ash carbon can be reused as a fuel or metallurgical coke. In our study, chlorination method was attempted using phosgene as a chlorinating agent. Our thermodynamic simulation shows that, by using phosgene, the minerals can be converted completely already at the temperature 200 jC. However, the experimental results showed that the reactivity of minerals with phosgene depends strongly on the reaction temperature; the reactivity is high in the first 10 min. Within 10 min, 80% of the minerals can be extracted from the fly ash at 1050 jC. Chlorination is also effective in removing phosphorus from fly ash. At 1000 jC, the content of phosphorus can be reduced from 2100 to 24 ppm in 10 min. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Fly ash carbon; Carbochlorination; Phosgene route
1. Introduction The coal ash from the combustion of coal in utility boilers is one of the major wastes produced in industry. These materials present significant disposal problems and expense, but, on the other hand, they might be potentially valuable materials. Coal ash is divided into three components: slag, bottom ash and fly ash. The great bulk of the coal ash produced in large modern utility boilers is fly ash [1]. Fly ash consists mainly of unburned carbon and minerals. By flotation process [2 –4], a large portion of the minerals can be separated. The carbon enriched coal fly ash contains about 20% ash and phosphorus compounds with a concentration much higher than required for metallurgical coke or boiler fuel. Further treatment is necessary before the fly ash carbon can be reused. Minerals recovery from ash (inorganic part of fly ash) was investigated by several researchers [5– 10]. However, a process for the treatment of the carbon enriched coal fly ash has not been developed so far.
* Corresponding author. Tel./fax: +1-716-645-3106. E-mail address: [email protected] (P. Pranda). 1 Present address: Harper International, West Drullard Avenue, Lancaster, NY 14086-1698.
The objective of our work is to investigate the feasibility of treating the carbon enriched fly ash by a chlorination process using phosgene. This process can be described as follows. 1.1. Extraction of minerals Minerals in fly ash exist primarily in the form of metal oxides. Since the oxides in fly ash are embedded in a carbon matrix, which is a reducing agent, they react with a chlorinating agent to generate the corresponding chlorides. When using phosgene, the major chlorination reaction can be written as: Ma Ob ðsÞ þ bCðsÞ þ bCOCl2 ðgÞ ! aMCl2b=a ðgÞ þ 2bCOðgÞ Here, M represents the metals in question. The chlorides are volatile at the operating temperatures. They are carried away by the gas stream, condensed and then separated by rectification to yield high-purity products. The pure chlorides can be further processed to get pure metals or metal compounds [11,12]. 1.2. Recovery of fly ash carbon Cleaning of fly ash carbon for use as a fuel includes the extraction of minerals as described in Section 1.1 and
0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0032-5910(03)00002-0
F. Yang et al. / Powder Technology 131 (2003) 206–211
removing of phosphorus compounds. The latter, perhaps the most significant goal of the coal fly ash cleaning process, is derived from the requirement of making metallurgical coke or, alternatively, fuel for boilers. Firstly, iron and steel may be of inferior quality if the phosphorous content in the coal exceeds 200 ppm and, secondly, the phosphorus, liberated together with other inorganic constituents in the combustion process, may cause troublesome deposits in heat exchanger and thus reduce steam output [13]. During the chlorination process, the phosphorus may react with chlorinating agent to form phosphorus chlorides (PCl3 and POCl3), which are volatile and can be removed from the fly ash.
2. Thermodynamic analysis 1.3. Composition of the carbon enriched coal fly ash Fly ash sample used in this study has an average particle size of 12 Am. This sample was first dried at 100 jC for 4 h to remove the volatile matters. Burning the carbon in a furnace at 1000 jC showed that the 100 jC dried sample contains 18.4% (by weight) ash (minerals). The ash was analyzed by X-ray fluorescence. The composition of ash is as follows: 56.0% SiO2, 27.6% Al2O3, 1.0% Fe2O3, 1.1% TiO2, 3.0% P2O5, 3.7% CaO, 1.5% MgO, 0.5% K2O and 5.4% SO3. The above data can be recalculated (by including carbon) to get the total composition of the fly ash: 81.6% C, 10.3% SiO2, 5.1% Al2O3, 0.2% Fe2O3, 0.2% TiO2, 0.6% P2O5, 0.7% CaO, 0.3% MgO, 0.1% K2O and 1.0% SO3. 1.4. Thermodynamic calculations and discussions Thermodynamic calculations for the reaction system ‘‘fly ash – COCl2’’ were performed using NASA chemical equilibrium program [14]. This program is based on minimization of free energy. The reaction conditions were P = 1 atm and T = 200– 1400 jC. The inlet of reactants in weight percentage was: Phosgene: COCl2 = 100 wt.% Fly ash: C = 81.6 wt.%, SiO2 = 10.3 wt.%, Al2O3(a) = 5.1 wt.%, Fe2O3(s) = 0.2 wt.%, TiO2 = 0.2 wt.%, P2O5 = 0.6 wt.%, CaO = 0.7 wt.%, MgO = 0.3 wt.%, K2O(s) = 0.1 wt.%, SO3 = 1.0. wt.%. In the initial input of the reactants, COCl2 is in excess to simulate the real reaction system in which a sufficient amount of COCl2 flows through the bed of fly ash. The equilibrium composition obtained from the calculations is presented in Table 1. The products are either in gaseous phase, which are unmarked, or, in condensed phase, which are labeled as (s) or (L) denoting solid phase or liquid phase, respectively. Some products with mole fractions below 0.0001 were not listed.
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Table 1 Equilibrium mole fractions of fly ash + COCl2 reaction system Temperature (jC)
200
400
600
800
1000
1200
1400
AlCl3 Al2Cl6 CCl4 CO COCl2 CO2 COS CS2 CaCl2 Cl Cl2 FeCl3 Fe2Cl6 KCl MgCl2 PCl3 POCl3 SCl2 SiCl4 TiCl4 C(s) CaCl2(s) CaCl2(L) KCl(s) MgCl2(s) MgCl2(L)
0.000 0.006 0.027 0.000 0.001 0.097 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.001 0.002 0.021 0.000 0.839 0.002 0.000 0.000 0.001 0.000
0.003 0.005 0.009 0.001 0.004 0.092 0.000 0.000 0.000 0.000 0.034 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.021 0.000 0.827 0.001 0.000 0.000 0.001 0.000
0.011 0.000 0.000 0.036 0.002 0.074 0.000 0.000 0.000 0.000 0.052 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.020 0.000 0.800 0.001 0.000 0.000 0.001 0.000
0.012 0.000 0.000 0.157 0.001 0.014 0.001 0.000 0.000 0.000 0.054 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.020 0.000 0.738 0.000 0.001 0.000 0.000 0.001
0.012 0.000 0.000 0.183 0.000 0.001 0.001 0.000 0.000 0.001 0.054 0.000 0.000 0.000 0.001 0.001 0.000 0.001 0.020 0.000 0.724 0.000 0.001 0.000 0.000 0.000
0.012 0.000 0.000 0.184 0.000 0.000 0.000 0.000 0.000 0.006 0.052 0.000 0.000 0.000 0.001 0.001 0.000 0.001 0.020 0.000 0.721 0.000 0.001 0.000 0.000 0.000
0.012 0.000 0.000 0.183 0.000 0.000 0.000 0.000 0.001 0.019 0.045 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.020 0.000 0.716 0.000 0.000 0.000 0.000 0.000
Equilibrium concentrations of major chlorides are plotted in Fig. 1a and b. The results reported in Table 1, as well as in Fig. 1a and b, can be summarized as the follows: (a) A complete extraction of minerals from fly ash by COCl2 is thermodynamically feasible. (b) A low temperature is in favor of the formation of CO2 rather than CO. (c) Some chlorides, such as CaCl2, MgCl2 and KCl, are in the condensed phases because the operating temperature is below their boiling or sublimation points. Due to the high melting and boiling points (CaCl2, m.p. 782 jC, b.p. >1600 jC; MgCl2, m.p. 708 jC, b.p. 1412 jC; KCl, m.p. 776 jC, b.p. or subl. p. 1500 jC), these chlorides cannot be removed from the system as volatile matters under typically reasonable temperatures (T < 1200 jC). (d) Dependence of equilibrium conversion on temperature for the most important chlorides is shown clearly in Fig. 1a and b. Fig. 1a shows that aluminum chloride at a low temperature (e.g., at 200 jC) is in the form of dimer, Al2Cl6, but as the temperature goes up it decomposes to simple form, AlCl3. Silicon chloride and titanium chloride are in the form of tetrachlorides. The same dependence for chlorides of P and Fe is shown in Fig. 1b. Phosphorous chloride exists as POCl3 at temperatures below 400 jC. From 400 to 800 jC, both PCl3 and POCl3 are generated. When temperature is higher
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Fig. 2. Experimental results—effect of reaction temperatures on reactivity.
Fig. 1. (a) Thermodynamic simulation—equilibrium mole fractions of chlorides of Al, Si and Ti. (b) Thermodynamic simulation—equilibrium mole fractions of chlorides of P and Fe.
The flow rate of COCl2 was 60 ml/min. The fly ash sample amount was 2 g. An analytical balance (Model 200 analytic balance, Sartorius, Long Island, NY) was used for the measurement of the sample mass before and after the experiments. The ceramic boat containing known amount of sample was slowly inserted into the middle of the reactor. Reactor was sealed and argon was introduced at a flow rate of 200 ml/min for 10 min to purge the system. A reaction temperature was set and the furnace was switched on. The heating rate was 50 jC/min. Fifteen minutes after the temperature reached the set point, the argon flow was shut off and gaseous reactants (COCl2 or COCl2/CO) were fed into the reactor. After a specified time interval, gaseous reactant(s) was replaced by argon again to purge the system. The reaction temperature was maintained for five more minutes in argon stream to ensure a complete removal of the volatile chlorides pro-
than 800 jC, PCl3 becomes dominant. Chloride of Fe is primarily in the form of Fe2Cl6 at temperatures below 400 jC and becomes FeCl3 when temperature is increased to 600 jC. 1.5. Experimental set-up and procedures Chlorination of fly ash was investigated in a fixed bed reactor. It consisted of a quartz tube (O.D. 32.5 mm, wall thickness 2.5 mm and length 610 mm) placed in a Lindberg tube furnace (1100 jC TF 55030 A). The furnace was equipped with a temperature controller. The fly ash sample was placed in a ceramic boat (length = 50 mm, width = 12 mm and depth = 9 mm), which was located inside the reactor tube. Phosgene, CO and Ar were stored in cylinders and were fed into the reactor through plastic tubing. Mass flow meters and metering valves were used to control the gas flowrate. Two-stage scrubbers with 5% caustic soda solution neutralized the exit gas.
Fig. 3. Experimental results—effect of CO on reactivity.
F. Yang et al. / Powder Technology 131 (2003) 206–211
209
Fig. 6. Experimental results—effect of CO on phosphorus removal.
was discontinued and the sample was taken out and weighed. Combustion of the sample in a furnace provided information about the degree of conversion.
3. Chlorination by phosgene 1.6. Effect of the reaction temperature
Fig. 4. (a) Experimental results—conversion vs. reaction time at 1000 jC. (b) Experimental results—conversion vs. reaction time at 1050 jC.
duced. Afterwards, power was switched off and the furnace body was opened for quick cooling. When the reactor tube was cooled to room temperature, purging
The fly ash sample was chlorinated for 10 min by phosgene at various temperatures. The conversion of the minerals and the remaining unreacted minerals in the fly ash after chlorination are plotted in Fig. 2. We can observe from Fig. 2 that the conversion of minerals in fly ash depends strongly on the reaction temperature. 1.7. Effect of CO Chlorination of fly ash by a mixture of COCl2 and CO was carried out to find out if the addition of CO could enhance the reactivity. The reaction temperature was 1000 jC and reaction time was 5 min. The results are plotted in Fig. 3. It is evident from Fig. 3 that the addition of CO increases the reactivity when the mole fraction of CO in the COCl2/ CO mixture is less than 0.4 but decreases the reactivity when it is greater than 0.4. The enhancement of reactivity by CO is insignificant because a higher content of CO in the
Table 2 Chlorination of fly ash by phosgene at 1000 jC in 10 min
Fig. 5. Experimental results—phosphorus removal by chlorination at 1000 jC.
Sample no.
Bed depth (mm)
Conversion of inorganic part (%)
1 2 3
2.5 5.0 10.0
73.7 65.1 31.4
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Table 3 Chlorination of fly ash particles by phosgene at 1000 jC in 10 min Sample no.
Particle diameter de (mm)
Conversion of inorganic compounds (%)
1 2 3 4 5
11.6 9.3 7.4 6.0 < 0.2
47.3 63.1 61.5 64.6 73.7
COCl2/CO mixture corresponds to a lower concentration of COCl2, which is unfavorable to the chlorination reactions. 1.8. Time dependence The conversion of minerals in fly ash was tested at 1000 and 1050 jC for different time intervals. The data are presented in Fig. 4a and b. These figures depict that, in the first 10 min, 71.6% and 80.0% of the minerals can be extracted from the fly ash at 1000 and 1050 jC, respectively. The content of minerals remaining in the fly ash sample dropped from 18.4% to 4.8% in 10 min at 1000 jC and to 4.6% in 5 min at 1050 jC. It can be noticed that, in the first 10 min, the conversion increases with time significantly, but after 10 min this increase is slow. This phenomenon may be explained by the fact that the chlorides, such as CaCl2, MgCl2 and KCl, exist in the condensed phase and cannot be removed from the unreacted solid reactants as volatile matters. The chlorides accumulated form a coating on the surface of the solid reactant, and therefore block the access of chlorinating gas and diminish a further conversion of the minerals.
It is obvious from Tables 2 and 3 that mass transfer is the controlling step under conditions of our experiment.
4. Conclusions 1. Thermodynamic calculation reveals that all minerals in fly ash can be completely extracted by phosgene chlorination already at 200 jC. 2. Chlorination is an effective method of removing phosphorus from fly ash. At 1000 jC, the content of phosphorus can be reduced from 2100 ppm to 149 in 5 min or to 24 ppm in 10 min. In other words, the 200 ppm limit required by metallurgical coke specification can be met in 5 min by chlorination at 1000 jC. 3. The reactivity of minerals with phosgene depends strongly on the reaction temperature. In the first 10 min, 71.6% and 80.0% of the minerals can be extracted from the fly ash at 1000 and 1050 jC, respectively. The content of minerals remaining in the fly ash sample dropped from 18.4% to 4.8% in 10 min at 1000 jC and to 4.6% in 5 min at 1050 jC. 4. Reactivity is high in the first 10 min and then becomes very low. The reason may be that the condensed chlorides cover the surface of the solid reactant and therefore hinder the access of chlorine species to the reactive sites. 5. Addition of CO increases the effectiveness of phosphorus removal when concentration of CO is less than 0.7. However, CO does not enhance the reactivity substantially.
1.9. Phosphorous removal
References
The removal of phosphorus is one of the major objectives of the chlorination treatment. To obtain the content of phosphorus, one blank (untreated) sample and four samples chlorinated at 1000 jC by phosgene were analyzed by colorimetric method. Reports of the analyses are shown in Fig. 5. Fig. 5 indicates that chlorination by phosgene is an effective method of phosphorus removal. In 5 min, phosphorus content can be reduced from 2100 to 149 ppm, which is well below the concentration required for metallurgical coke (200 ppm); in 10 min, the content drops to as low as 24 ppm. The effect of CO on the phosphorus removal was also tested. Experimental data are presented in Fig. 6.
[1] J.S. Watson, Potential resources from coal fly ash, Mater. Res. Soc. Symp. Proc. 43 (1985) 151. [2] C.J. McKenny, B.W. Raymond, Coal flotation process, US Patent 5,443,158, 1995. [3] W.-W. Wen, M.L. Gray, K.J. Champagne, Method for simultaneous use of a single additive for coal flotation, dewatering, and reconstitution, US Patent 5,379,902, 1995. [4] D.J.A. McCaffrey, J.P. Sheppard, Froth flotation, US Patent 4,528, 107, 1985. [5] A.K. Mehrotra, L.A. Behie, P.R. Bishnoi, W.Y. Svrcek, High-temperature chlorination of coal ash in a fluidized bed: 1. Recovery of aluminum, Ind. Eng. Process Des. Dev. 21 (1982) 37. [6] G. Burnet, M.J. Murtha, Direct utilization—recovery of minerals from fly ash, Fossil Energy Program Technical Progress Report IS-4655, 1979. [7] D.J. Adelman, G. Burnet, Carbochlorination of metal oxides with phosgene, AIChE J. 33 (1987) 64. [8] T.M. Gilliam, R.M. Canon, Economic metal recovery from fly ash, Resour. Conserv. 9 (1982) 155. [9] R.M. Canon, T.M. Gilliam, Evaluation of potential processes for recovery of metals from coal fly ash, Electric Power Research Institute Report No. EPRI CS-1992 (1981) 1 – 2. [10] R.F. Wilder, D.M. Golden, Recovery of metal oxides from fly ash, Electric Power Research Institute Report No. EPRI CS-3544 (1984) 1 – 3.
1.10. Mass transfer limitations To verify the role of mass transfer resistance, chlorination of fly ash powder with phosgene for various bed depths was performed (see Table 2). The effect of the particle size was also tested (see Table 3).
F. Yang et al. / Powder Technology 131 (2003) 206–211 [11] F. Yang, V. Hlavacek, Carbochlorination kinetics of titanium dioxide with carbon and carbon monoxide as reductant, Metall. Mater. Trans., B 29B (6) (1998) 1297. [12] F. Yang, V. Hlavacek, Carbochlorination of tantalum and niobium oxide; thermodynamic simulation and kinetic modeling, AIChE J. 45 (3) (1999) 581.
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[13] R.J. Cosstick, H.N.S. Schafer, The determination of phosphorus in coal ash, Fuel 38 (1959) 277. [14] B.J. McBride, S. Gorden, Computer program for calculation of complex chemical equilibrium compositions and applications, NASA Reference Publication, vol. 1311, 1996.
Recovery of fly ash carbon by carbochlorination via phosgene route F. Yang 1, P. Pranda *, V. Hlavacek Department of Chemical Engineering, SUNY Buffalo, 218 Furnas Hall, Buffalo, NY 14260, USA Received 10 July 2002; received in revised form 13 December 2002; accepted 17 December 2002
Abstract Coal fly ash consists mainly of unburned carbon and minerals. It also contains phosphorus with a concentration much higher than that acceptable for the metallurgical coke or boiler fuel. By flotation process, a large portion of the inorganic materials can be separated. However, further treatment is necessary before the fly ash carbon can be reused as a fuel or metallurgical coke. In our study, chlorination method was attempted using phosgene as a chlorinating agent. Our thermodynamic simulation shows that, by using phosgene, the minerals can be converted completely already at the temperature 200 jC. However, the experimental results showed that the reactivity of minerals with phosgene depends strongly on the reaction temperature; the reactivity is high in the first 10 min. Within 10 min, 80% of the minerals can be extracted from the fly ash at 1050 jC. Chlorination is also effective in removing phosphorus from fly ash. At 1000 jC, the content of phosphorus can be reduced from 2100 to 24 ppm in 10 min. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Fly ash carbon; Carbochlorination; Phosgene route
1. Introduction The coal ash from the combustion of coal in utility boilers is one of the major wastes produced in industry. These materials present significant disposal problems and expense, but, on the other hand, they might be potentially valuable materials. Coal ash is divided into three components: slag, bottom ash and fly ash. The great bulk of the coal ash produced in large modern utility boilers is fly ash [1]. Fly ash consists mainly of unburned carbon and minerals. By flotation process [2 –4], a large portion of the minerals can be separated. The carbon enriched coal fly ash contains about 20% ash and phosphorus compounds with a concentration much higher than required for metallurgical coke or boiler fuel. Further treatment is necessary before the fly ash carbon can be reused. Minerals recovery from ash (inorganic part of fly ash) was investigated by several researchers [5– 10]. However, a process for the treatment of the carbon enriched coal fly ash has not been developed so far.
* Corresponding author. Tel./fax: +1-716-645-3106. E-mail address: [email protected] (P. Pranda). 1 Present address: Harper International, West Drullard Avenue, Lancaster, NY 14086-1698.
The objective of our work is to investigate the feasibility of treating the carbon enriched fly ash by a chlorination process using phosgene. This process can be described as follows. 1.1. Extraction of minerals Minerals in fly ash exist primarily in the form of metal oxides. Since the oxides in fly ash are embedded in a carbon matrix, which is a reducing agent, they react with a chlorinating agent to generate the corresponding chlorides. When using phosgene, the major chlorination reaction can be written as: Ma Ob ðsÞ þ bCðsÞ þ bCOCl2 ðgÞ ! aMCl2b=a ðgÞ þ 2bCOðgÞ Here, M represents the metals in question. The chlorides are volatile at the operating temperatures. They are carried away by the gas stream, condensed and then separated by rectification to yield high-purity products. The pure chlorides can be further processed to get pure metals or metal compounds [11,12]. 1.2. Recovery of fly ash carbon Cleaning of fly ash carbon for use as a fuel includes the extraction of minerals as described in Section 1.1 and
0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0032-5910(03)00002-0
F. Yang et al. / Powder Technology 131 (2003) 206–211
removing of phosphorus compounds. The latter, perhaps the most significant goal of the coal fly ash cleaning process, is derived from the requirement of making metallurgical coke or, alternatively, fuel for boilers. Firstly, iron and steel may be of inferior quality if the phosphorous content in the coal exceeds 200 ppm and, secondly, the phosphorus, liberated together with other inorganic constituents in the combustion process, may cause troublesome deposits in heat exchanger and thus reduce steam output [13]. During the chlorination process, the phosphorus may react with chlorinating agent to form phosphorus chlorides (PCl3 and POCl3), which are volatile and can be removed from the fly ash.
2. Thermodynamic analysis 1.3. Composition of the carbon enriched coal fly ash Fly ash sample used in this study has an average particle size of 12 Am. This sample was first dried at 100 jC for 4 h to remove the volatile matters. Burning the carbon in a furnace at 1000 jC showed that the 100 jC dried sample contains 18.4% (by weight) ash (minerals). The ash was analyzed by X-ray fluorescence. The composition of ash is as follows: 56.0% SiO2, 27.6% Al2O3, 1.0% Fe2O3, 1.1% TiO2, 3.0% P2O5, 3.7% CaO, 1.5% MgO, 0.5% K2O and 5.4% SO3. The above data can be recalculated (by including carbon) to get the total composition of the fly ash: 81.6% C, 10.3% SiO2, 5.1% Al2O3, 0.2% Fe2O3, 0.2% TiO2, 0.6% P2O5, 0.7% CaO, 0.3% MgO, 0.1% K2O and 1.0% SO3. 1.4. Thermodynamic calculations and discussions Thermodynamic calculations for the reaction system ‘‘fly ash – COCl2’’ were performed using NASA chemical equilibrium program [14]. This program is based on minimization of free energy. The reaction conditions were P = 1 atm and T = 200– 1400 jC. The inlet of reactants in weight percentage was: Phosgene: COCl2 = 100 wt.% Fly ash: C = 81.6 wt.%, SiO2 = 10.3 wt.%, Al2O3(a) = 5.1 wt.%, Fe2O3(s) = 0.2 wt.%, TiO2 = 0.2 wt.%, P2O5 = 0.6 wt.%, CaO = 0.7 wt.%, MgO = 0.3 wt.%, K2O(s) = 0.1 wt.%, SO3 = 1.0. wt.%. In the initial input of the reactants, COCl2 is in excess to simulate the real reaction system in which a sufficient amount of COCl2 flows through the bed of fly ash. The equilibrium composition obtained from the calculations is presented in Table 1. The products are either in gaseous phase, which are unmarked, or, in condensed phase, which are labeled as (s) or (L) denoting solid phase or liquid phase, respectively. Some products with mole fractions below 0.0001 were not listed.
207
Table 1 Equilibrium mole fractions of fly ash + COCl2 reaction system Temperature (jC)
200
400
600
800
1000
1200
1400
AlCl3 Al2Cl6 CCl4 CO COCl2 CO2 COS CS2 CaCl2 Cl Cl2 FeCl3 Fe2Cl6 KCl MgCl2 PCl3 POCl3 SCl2 SiCl4 TiCl4 C(s) CaCl2(s) CaCl2(L) KCl(s) MgCl2(s) MgCl2(L)
0.000 0.006 0.027 0.000 0.001 0.097 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.001 0.002 0.021 0.000 0.839 0.002 0.000 0.000 0.001 0.000
0.003 0.005 0.009 0.001 0.004 0.092 0.000 0.000 0.000 0.000 0.034 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.021 0.000 0.827 0.001 0.000 0.000 0.001 0.000
0.011 0.000 0.000 0.036 0.002 0.074 0.000 0.000 0.000 0.000 0.052 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.020 0.000 0.800 0.001 0.000 0.000 0.001 0.000
0.012 0.000 0.000 0.157 0.001 0.014 0.001 0.000 0.000 0.000 0.054 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.020 0.000 0.738 0.000 0.001 0.000 0.000 0.001
0.012 0.000 0.000 0.183 0.000 0.001 0.001 0.000 0.000 0.001 0.054 0.000 0.000 0.000 0.001 0.001 0.000 0.001 0.020 0.000 0.724 0.000 0.001 0.000 0.000 0.000
0.012 0.000 0.000 0.184 0.000 0.000 0.000 0.000 0.000 0.006 0.052 0.000 0.000 0.000 0.001 0.001 0.000 0.001 0.020 0.000 0.721 0.000 0.001 0.000 0.000 0.000
0.012 0.000 0.000 0.183 0.000 0.000 0.000 0.000 0.001 0.019 0.045 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.020 0.000 0.716 0.000 0.000 0.000 0.000 0.000
Equilibrium concentrations of major chlorides are plotted in Fig. 1a and b. The results reported in Table 1, as well as in Fig. 1a and b, can be summarized as the follows: (a) A complete extraction of minerals from fly ash by COCl2 is thermodynamically feasible. (b) A low temperature is in favor of the formation of CO2 rather than CO. (c) Some chlorides, such as CaCl2, MgCl2 and KCl, are in the condensed phases because the operating temperature is below their boiling or sublimation points. Due to the high melting and boiling points (CaCl2, m.p. 782 jC, b.p. >1600 jC; MgCl2, m.p. 708 jC, b.p. 1412 jC; KCl, m.p. 776 jC, b.p. or subl. p. 1500 jC), these chlorides cannot be removed from the system as volatile matters under typically reasonable temperatures (T < 1200 jC). (d) Dependence of equilibrium conversion on temperature for the most important chlorides is shown clearly in Fig. 1a and b. Fig. 1a shows that aluminum chloride at a low temperature (e.g., at 200 jC) is in the form of dimer, Al2Cl6, but as the temperature goes up it decomposes to simple form, AlCl3. Silicon chloride and titanium chloride are in the form of tetrachlorides. The same dependence for chlorides of P and Fe is shown in Fig. 1b. Phosphorous chloride exists as POCl3 at temperatures below 400 jC. From 400 to 800 jC, both PCl3 and POCl3 are generated. When temperature is higher
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Fig. 2. Experimental results—effect of reaction temperatures on reactivity.
Fig. 1. (a) Thermodynamic simulation—equilibrium mole fractions of chlorides of Al, Si and Ti. (b) Thermodynamic simulation—equilibrium mole fractions of chlorides of P and Fe.
The flow rate of COCl2 was 60 ml/min. The fly ash sample amount was 2 g. An analytical balance (Model 200 analytic balance, Sartorius, Long Island, NY) was used for the measurement of the sample mass before and after the experiments. The ceramic boat containing known amount of sample was slowly inserted into the middle of the reactor. Reactor was sealed and argon was introduced at a flow rate of 200 ml/min for 10 min to purge the system. A reaction temperature was set and the furnace was switched on. The heating rate was 50 jC/min. Fifteen minutes after the temperature reached the set point, the argon flow was shut off and gaseous reactants (COCl2 or COCl2/CO) were fed into the reactor. After a specified time interval, gaseous reactant(s) was replaced by argon again to purge the system. The reaction temperature was maintained for five more minutes in argon stream to ensure a complete removal of the volatile chlorides pro-
than 800 jC, PCl3 becomes dominant. Chloride of Fe is primarily in the form of Fe2Cl6 at temperatures below 400 jC and becomes FeCl3 when temperature is increased to 600 jC. 1.5. Experimental set-up and procedures Chlorination of fly ash was investigated in a fixed bed reactor. It consisted of a quartz tube (O.D. 32.5 mm, wall thickness 2.5 mm and length 610 mm) placed in a Lindberg tube furnace (1100 jC TF 55030 A). The furnace was equipped with a temperature controller. The fly ash sample was placed in a ceramic boat (length = 50 mm, width = 12 mm and depth = 9 mm), which was located inside the reactor tube. Phosgene, CO and Ar were stored in cylinders and were fed into the reactor through plastic tubing. Mass flow meters and metering valves were used to control the gas flowrate. Two-stage scrubbers with 5% caustic soda solution neutralized the exit gas.
Fig. 3. Experimental results—effect of CO on reactivity.
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Fig. 6. Experimental results—effect of CO on phosphorus removal.
was discontinued and the sample was taken out and weighed. Combustion of the sample in a furnace provided information about the degree of conversion.
3. Chlorination by phosgene 1.6. Effect of the reaction temperature
Fig. 4. (a) Experimental results—conversion vs. reaction time at 1000 jC. (b) Experimental results—conversion vs. reaction time at 1050 jC.
duced. Afterwards, power was switched off and the furnace body was opened for quick cooling. When the reactor tube was cooled to room temperature, purging
The fly ash sample was chlorinated for 10 min by phosgene at various temperatures. The conversion of the minerals and the remaining unreacted minerals in the fly ash after chlorination are plotted in Fig. 2. We can observe from Fig. 2 that the conversion of minerals in fly ash depends strongly on the reaction temperature. 1.7. Effect of CO Chlorination of fly ash by a mixture of COCl2 and CO was carried out to find out if the addition of CO could enhance the reactivity. The reaction temperature was 1000 jC and reaction time was 5 min. The results are plotted in Fig. 3. It is evident from Fig. 3 that the addition of CO increases the reactivity when the mole fraction of CO in the COCl2/ CO mixture is less than 0.4 but decreases the reactivity when it is greater than 0.4. The enhancement of reactivity by CO is insignificant because a higher content of CO in the
Table 2 Chlorination of fly ash by phosgene at 1000 jC in 10 min
Fig. 5. Experimental results—phosphorus removal by chlorination at 1000 jC.
Sample no.
Bed depth (mm)
Conversion of inorganic part (%)
1 2 3
2.5 5.0 10.0
73.7 65.1 31.4
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Table 3 Chlorination of fly ash particles by phosgene at 1000 jC in 10 min Sample no.
Particle diameter de (mm)
Conversion of inorganic compounds (%)
1 2 3 4 5
11.6 9.3 7.4 6.0 < 0.2
47.3 63.1 61.5 64.6 73.7
COCl2/CO mixture corresponds to a lower concentration of COCl2, which is unfavorable to the chlorination reactions. 1.8. Time dependence The conversion of minerals in fly ash was tested at 1000 and 1050 jC for different time intervals. The data are presented in Fig. 4a and b. These figures depict that, in the first 10 min, 71.6% and 80.0% of the minerals can be extracted from the fly ash at 1000 and 1050 jC, respectively. The content of minerals remaining in the fly ash sample dropped from 18.4% to 4.8% in 10 min at 1000 jC and to 4.6% in 5 min at 1050 jC. It can be noticed that, in the first 10 min, the conversion increases with time significantly, but after 10 min this increase is slow. This phenomenon may be explained by the fact that the chlorides, such as CaCl2, MgCl2 and KCl, exist in the condensed phase and cannot be removed from the unreacted solid reactants as volatile matters. The chlorides accumulated form a coating on the surface of the solid reactant, and therefore block the access of chlorinating gas and diminish a further conversion of the minerals.
It is obvious from Tables 2 and 3 that mass transfer is the controlling step under conditions of our experiment.
4. Conclusions 1. Thermodynamic calculation reveals that all minerals in fly ash can be completely extracted by phosgene chlorination already at 200 jC. 2. Chlorination is an effective method of removing phosphorus from fly ash. At 1000 jC, the content of phosphorus can be reduced from 2100 ppm to 149 in 5 min or to 24 ppm in 10 min. In other words, the 200 ppm limit required by metallurgical coke specification can be met in 5 min by chlorination at 1000 jC. 3. The reactivity of minerals with phosgene depends strongly on the reaction temperature. In the first 10 min, 71.6% and 80.0% of the minerals can be extracted from the fly ash at 1000 and 1050 jC, respectively. The content of minerals remaining in the fly ash sample dropped from 18.4% to 4.8% in 10 min at 1000 jC and to 4.6% in 5 min at 1050 jC. 4. Reactivity is high in the first 10 min and then becomes very low. The reason may be that the condensed chlorides cover the surface of the solid reactant and therefore hinder the access of chlorine species to the reactive sites. 5. Addition of CO increases the effectiveness of phosphorus removal when concentration of CO is less than 0.7. However, CO does not enhance the reactivity substantially.
1.9. Phosphorous removal
References
The removal of phosphorus is one of the major objectives of the chlorination treatment. To obtain the content of phosphorus, one blank (untreated) sample and four samples chlorinated at 1000 jC by phosgene were analyzed by colorimetric method. Reports of the analyses are shown in Fig. 5. Fig. 5 indicates that chlorination by phosgene is an effective method of phosphorus removal. In 5 min, phosphorus content can be reduced from 2100 to 149 ppm, which is well below the concentration required for metallurgical coke (200 ppm); in 10 min, the content drops to as low as 24 ppm. The effect of CO on the phosphorus removal was also tested. Experimental data are presented in Fig. 6.
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1.10. Mass transfer limitations To verify the role of mass transfer resistance, chlorination of fly ash powder with phosgene for various bed depths was performed (see Table 2). The effect of the particle size was also tested (see Table 3).
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