Coal Desulfurization in Oxidative Acid Media Using Hydrogen

Energy & Fuels 2009, 23, 3703–3710

3703

Coal Desulfurization in Oxidative Acid Media Using Hydrogen Peroxide and Ozone: A Kinetic and Statistical Approach F. R. Carrillo-Pedroza,*,† A. Da´valos Sańchez,†,‡ M. Soria-Aguilar,† and E. T. Pecina Trevinõ‡ Facultad de Metalurgia, UniVersidad Autońoma de Coahuila, Carr. 57 Km 4.5 N, C.P. 5710, MoncloVa, Coahuila, Mexico, and Centro de InVestigacioń en Materiales AVanzados S. C. (CIMAV), Miguel de CerVantes 120, C.P. 39109, Complejo Industrial Chihuahua, Chihuahua, Mexico ReceiVed March 23, 2009. ReVised Manuscript ReceiVed June 11, 2009

The removal of pyritic sulfur from a Mexican sub-bituminous coal in nitric, sulfuric, and hydrochloric acid solutions was investigated. The effect of the type and concentration of acid, in the presence of hydrogen peroxide and ozone as oxidants, in a temperature range of 20-60 °C, was studied. The relevant factors in pyrite dissolution were determined by means of the statistical analysis of variance and optimized by the response surface method. Kinetic models were also evaluated, showing that the dissolution of pyritic sulfur follows the kinetic model of the shrinking core model, with diffusion through the solid product of the reaction as the controlling stage. The results of statistical analysis indicate that the use of ozone as an oxidant improves the pyrite dissolution because, at 0.25 M HNO3 or H2SO4 at 20 °C and 0.33 g/h O3, the obtained dissolution is similar to that of 1 M H2O2 and 1 M HNO3 or H2SO4 at 40 °C.

1. Introduction Sub-bituminous coal in Mexico is found mainly in the state of Coahuila. This coal is the most important fossil fuel used for energy production as well as iron and steel manufacturing in northern Mexico.1 However, the use of this coal requires a cleaning stage to meet air pollution regulations, which severely limits the formation of SO2 and H2SO4 during the coal combustion.2 The cleaning is carried out over flue gases but is expensive and energy-intensive.3,4 Previous to the combustion, coal-cleaning techniques based in physical methods are extensively used but are less efficient to remove organic sulfur and syngenetic pyrite (FeS2).5 Syngenetic pyrite is one of the two forms of pyritic sulfur found as a very fine and highly disseminated mineral in coal, which makes it difficult to separate by conventional cleaning processes. The other sulfur pyritic form is the epigenetic, which is present as larger crystals or coarse particles and is easier to eliminate by physical methods.6-8 Many studies have been realized to explore the pyrite dissolution by chemical methods. In acid solutions, the oxidation of pyrite has been extensively studied and documented because * To whom correspondence should be addressed. Telephone/Fax: +52866-6390330. E-mail: [email protected]. † Universidad Auto ´ noma de Coahuila. ‡ Centro de Investigacio ´ n en Materiales Avanzados S. C. (CIMAV). (1) Lin, C. L.; Parga, J. R.; Drelich, J.; Miller, J. D. Coal Prep. 1999, 26, 227–245. (2) Apenzaller, T. The coal paradox. National Geographic 2006, March, 99-103. (3) Baruah, B.; Khare, P. Energy Fuels 2007, 21, 2156–2164. (4) Pysh’yevl, S.; Shevchuk, K.; Chmielarz, L.; Kustrowski, P.; PattekJanczyk, A. Energy Fuels 2007, 21, 216–221. (5) Li, W.; Cho, E. H. Energy Fuels 2005, 19, 499–507. (6) Ozbayoglu, G. Mineral Processing and the EnVironment; Gallios, G. P.; Matis, K. A., Eds.; Kluwer Academic Publishers: Norwell, MA, 1998; pp 199-221. (7) Ayha, F.; Abayak, H.; Saydut, A. Energy Fuels 2005, 19, 1003– 1007. (8) Baruah, B.; Saika, B.; Kotoky, P.; Rao, P. Energy Fuels 2006, 20, 1550–1555.

Figure 1. Eh-pH diagram of the Fe-S-O system at 20 °C. [Fe] ) 1 M, [S] ) 1 M, and pO2 ) 1 atm.

of its importance in sulfur processing.6,9-13 According to the Eh-pH diagram shown in Figure 1, the sulfide species, such as pyrite or pyrrhotite, can be oxidized to sulfate in oxidative conditions and within a pH range from 2 to 14. For this purpose, various oxidizing agents, such as oxygen, hydrogen peroxide, ferric sulfate, ferric chloride, potassium permanganate, and perchloric and nitric acids, have been used to oxidize pyrite.3,14-16 (9) Deng, T. Miner. Process. Extr. Metall. ReV. 1992, 10, 325–345. (10) Elliot, R. C. Coal Desulfurization Prior to Combustion; Data Corporation Noyes: Park Ridge, NJ, 1978. (11) Bonn, M.; Heijnen, J. J. Hydrometallurgy 2001, 62, 57–66. (12) Borah, D. Energy Fuels 2006, 20, 287–294. (13) Kawatra, S. K.; Eisele, T. C. Coal Desulfurization; Taylor and Francis: Oxford, U.K., 2001. (14) Antonijevic´, M. M.; Jankovic´, Z. D.; Dimetrijevic´, M. D. Hydrometallurgy 2003, 71, 329–334. (15) Karaca, S.; Akiurek, M.; Bayrakceken, S. Fuel Process. Technol. 2003, 80, 1–8.

10.1021/ef900253g CCC: $40.75  2009 American Chemical Society Published on Web 06/25/2009

3704

Energy & Fuels, Vol. 23, 2009

Carrillo-Pedroza et al.

In general, at low pH, pyrite oxidation proceeds through two basic steps:17,18 In the first step, the dissolution of pyrite to ferrous ions in an acid medium proceeds through the formation of an iron-deficient or a sulfur-rich layer rather than elemental sulfur. In the second step, further oxidation of this layer occurs, forming sulfides of lower iron content, and eventually are converted to elemental sulfur. In severely oxidizing conditions, the elemental sulfur could be oxidized to oxy-sulfuric species. Anodic reactions, such as pyrite and sulfur oxidations, are sustained by cathodic processes, which could involve oxygen, hydrogen peroxide, or even ozone (as is the case of this work) reduction. The importance of this analysis is based on the fact that, under certain conditions, such as pH, redox potential, temperature, etc., the product layer is protective, thus limiting pyrite oxidation.19 Despite the existing discrepancies about the exact composition of the oxidation products, the most well-known general mechanism of pyrite oxidation is described in eq 1. FeS2 ) Fe2+ + 2So + 2e-

(1)

Elemental sulfur is stable at low pH and redox potential and could be oxidized to sulfate by molecular oxygen and ferric ions at higher potentials (eq 2). FeS2 + 8H2O ) Fe3+ + 2SO42- + 16H+ + 15e-

(2)

The pyrite dissolution has been characterized in the following media: (i) in the presence of oxygen at high pressure and temperature9,20 2FeS2 + 7O2 + 2H2O ) 2FeSO4 + 2H2SO4 FeS2 + 2O2 ) FeSO4 + S

o

(3) (4)

(ii) in sulfuric acid solutions9 2FeS2 + 2H2SO4 + 3O2 ) Fe2(SO4)3 + 3So + 2H2O (5) (iii) in nitric acid solutions9,15 3FeS2 + 18HNO3 ) Fe2(SO4)3 + Fe(NO3)3 + 3H2SO4 + 15NO + 6H2O (6) 2FeS2 + 10HNO3 ) Fe2(SO4)3 + H2SO4 + 10NO + 4H2O (7) (iv) in hydrogen peroxide solutions21 FeS2 + 7.5H2O2 ) Fe3+ + 2SO42- + H+ + 7H2O

(8)

(v) in highly acidic solutions (16) Mukherjee, S.; Srisvastava, S. K. Energy Fuels 2004, 18, 1746– 1769. (17) Buckley, A. N.; Hamilton, I. C.; Woods, R. Flotation of Sulphide Minerals; Forssberg, K. S. E., Eds.; Elsevier: Amsterdam, The Netherlands, 1985; pp 41-60. (18) Chander, S.; Briceno, A.; Pang, J. Miner. Metall. Process. 1993, 21, 113–118. (19) Chander, S.; Kumar, S. Flotation: Alexander Sutulov Memorial Volume, Proceedings of the 3rd Latin American Congress on Froth Flotation; Castro, S., Alvarez, J., Eds.; University of Conception, Conception, Chile, 1994; Vol. 2, pp 29-45. (20) Papangelakis, V. G.; Demopoulus, G. P. Hydrometallurgy 1991, 26, 309–325.

FeS2 + 7.5H2O2 + H+ ) Fe3+ + 2HSO4- + 7H2O (9) On the other hand, successful examples, indicating the use of ozone as a good option for increasing the oxidation potential and the oxygen content in the oxidizing treatment of sulfide minerals, have been published.22-25 Ozone has a very high oxidation potential, which is 2.07 V. It is higher when compared to 1.77 V of hydrogen peroxide or 1.4 V of chlorine. This property has been powerfully applied to several applications.26,27 As a result, the ozone can create favorable oxidation conditions for sulfide minerals in aqueous media. According to previous work,25 the oxidation of pyritic sulfur by ozone can occur by means of the following equations: Direct oxidation FeS2 + O3 + H2O + 2O2 ) FeSO4 + H2SO4

(10)

FeS2 + 7/3O3 + H2O ) FeSO4 + H2SO4

(11)

Indirect oxidation 2/3O3 (g) ) O2 (ac)

(12)

FeS2 + 7/2O2 + H2O ) FeSO4 + H2SO4

(13)

The oxidation of sulfide ores by ozone can occur by dissolution of sulfide species and the formation sulfate ion, as suggested by Elorza et al.24 Then, the global reaction of the pyrite oxidation in the presence of ozone can be described as follows: FeS2 + 2/3O3 + 5/2O2 + H2O ) Fe2+ + 2SO42- + 2H+ (14) In this context, the present work reports a comparative kinetic study and a statistical analysis of pyritic sulfur removal of a Mexican coal using different acids (sulfuric, hydrochloric, and nitric acids) and oxidants (hydrogen peroxide and ozone), at a temperature range of 30-60 °C. 2. Experimental Section 2.1. Minerals and Reagents. The coal sample was obtained from the carboniferous region of Coahuila, Mexico. The proximate and ultimate analyses of the coal indicates that it contained 0.83% moisture, 20.8% volatile matter, 34.2% ash, 72.2% fixed carbon (dry mineral matter free), 55.6% C, 3.6% H, 0.86% N, 2.13% Fe (all as FeS2; Fe as oxide was not detected), and 2.95% S total. This coal is representative of high-sulfur coal from the Sabinas region.1 The coal sample was homogenized, and the fraction comprised of -175/+104 µm particle size was employed in the experiments. The reagents used were of analytical grade. Concentrated commercial sulfuric acid was used as well as hydrogen peroxide at 30% (v/v) concentration. Distilled water 10-6 Ω-1 cm-1 was used in all tests. (21) Antonijevic´, M. M.; Dimetrijevic´, M.; Jankovic´, Z. Hydrometallurgy 1996, 46, 71–83. (22) Antwerp, W.; Lincoln, P. Precious metal recovery using UV/ozone. U.S. Patent 4,642,134:8, 1987. (23) Salinas, E.; Rivera, R.; Carrillo, F.; Patinõ, J.; Hernańdez, J. J. Chem. Soc. Mex. 2004, 48, 225–356. (24) Elorza, E.; Nava, F.; Jara, J.; Lara, C. Min. Eng. 2006, 19, 56–61. (25) Carrillo, F.; Soria, M.; Martıńez, A.; Gonzalez, A. Ozone: Sci. Eng. 2007, 29, 307–313. (26) Liu, C.; Fuchun, X. Ozone in Wastewater Treatment and Industrial Applications; Bollyky, L. J., Ed.; International Ozone Association: Scottsdale, AZ, 1989; Vol. 1. (27) Kuo, C. H.; Yocum, F. H. Handbook of Ozone Technology and Applications; Rice, R. G., Netzer, A., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1982; Vol. 1, pp 105-142.

Coal Desulfurization in OxidatiVe Acid Media

Energy & Fuels, Vol. 23, 2009 3705

Figure 2. Effect of the acid type and concentration in the pyrite oxidation.

Figure 3. Effect of the H2O2 concentration at different aqueous media in the pyrite oxidation.

2.2. Methodology. Experiments were carried out in a flask with 500 mL of solution with a variable acid/oxidant ratio (base solution). The solids content was 5% (50 g/L). The stirring rate was fixed at 160 rpm. The experiments were divided into two series: First, experimental tests without the use of ozone, where different acids (HNO3, HCl, and H2SO4) and concentrations (0.25, 0.5, and 1 M) were employed with different H2O2 solutions (0.5, 0.75, and 1.0 M) at temperatures of 10, 20, 30, and 45 °C. Second, experimental series were realized using different ozone concentrations, at selected conditions of type and concentration of acids and H2O2 concentration. The chemical analysis was carried out by sampling 11-20 mL of the target solution (in contact with the carbon) and replacing this volume for fresh base solution. Reaction kinetics was determined by measuring total iron dissolved in the solutions as a function of time. The total Fe dissolved in the solution was analyzed by spectroscopy (GENESYS 20 spectrophotometer) using a solution of potassium thiocyanate.28 The leftover solid was chemically characterized by the gravimetric determination of S via precipitation of BaSO4.29 The residue was acid-digested, and the solution was examined through atomic absorption spectroscopy (Perkin Elmer, Optima 3000XL) to determine Fe. A primary soft digestion was used to determine Fe as oxides. Analysis by atomic absorption indicate not detected. In series 2, an air-ozone mixture was injected through an aerator (pore size of 2 µm) installed at the bottom of the reactor. This gas mixture was generated using an ozone generator (Pacific Ozone L22). The ozone concentration used in the tests was 0.1 at 0.33 g/h and was determined by the iodometric method.

Some aspects of this behavior have been identified in other systems with peroxide and sulfuric acid. Antonijevic´ et al.21 report the oxidative leaching of pyrite, using 2 M H2O2; it was concluded that concentrations higher than 0.5 M H2SO4 decrease pyrite leaching. This behavior has been related to the great stability of peroxide in the presence of H2SO4. In other systems, such as chalcopyrite-H2O2-H2SO4, the same behavior has been explained on the basis of the formation and hydrolysis of ferric species and the catalytic decomposition of hydrogen peroxide at the reaction site. The results in Figure 2 reveal that the reaction is more intense during the first half hour of the reaction time. With longer periods, a lower dissolution rate occurred, probably because of the quick reaction of pyrite particles exposed to solution. The reaction between pyrite and peroxide involves stages that start with pyrite dissolution and the release of Fe2+ ions. These ions based on a pH of 0.6 and an oxidizing potential (ca. 820 mV versus SHE) generated in the sulfuric media are largely transformed to Fe3+ ions, thus contributing to the consumption of hydrogen peroxide. 3.2. Effect of the Hydrogen Peroxide Concentration. The curves for pyrite dissolution in the acid solutions with variable concentrations of H2O2 are shown in Figure 3. The results indicate higher pyrite dissolution in proportion to the increased H2O2 concentration. Similar to the previous results, the data in Figure 3 reveal that the reaction is very intense during the first 30 min; afterward, pyrite dissolution proceeds with lower intensity. For each different acid, the highest reacted fraction was obtained when 1.0 M H2O2 was employed. Previous tests indicate that the decomposition of peroxide in the presence of pyrite is very intense. According to the literature,31 the decomposition of hydrogen peroxide to water and oxygen (eq 15) can be catalyzed by Fe3+ and Zn2+ ions, metallic surfaces, solids, carbon, etc., suggesting the occurrence of parallel reactions of oxidant consumption.

3. Results and Discussion 3.1. First Experimental Series: Effect of the Type and Concentration of Acid. Experiments were oriented toward determining the optimal dissolution of pyrite from coal. Tests were carried out in 1.0 M H2O2 solutions and in the presence of H2SO4, HCl, and HNO3 (0.25-1.0 M) at 20 °C (Figure 2). Results are presented as a function of reacted pyrite. The figure shows that, at 1.0 M H2O2, the dissolution of pyrite is strongly sensitive to the type of acid used. The highest pyrite dissolution is obtained when nitric acid is used; furthermore, the increase of the acid nitric concentration favors pyrite dissolution. However, in the case of HCl and H2SO4, the optimum concentration is 0.5 M acid; a higher concentration (1.0 M H2SO4 or HCl) decreases the lixiviation of pyrite. (28) American Public Health Association (APHA)-American Water Works Association (AWWA). Standard Methods for the Examination of Water and Wastewater, 18th ed.; APHA-AWWA: Washington, D.C., 1992. (29) American Society for Testing and Materials (ASTM). Annual Books of ASTM Standards, Section 5; ASTM: West Conshohocken, PA, 1998; Vol. 05.05.

2H2O2 ) O2 + 2H2O

(15)

The ferrous ion reacts with peroxide according to the Fenton reaction, which results in the production of Fe3+, OH-, and HO•30 (30) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 1475–1488. (31) Jones, C. W. Applications of Hydrogen Peroxide and DeriVatiVes; MPG Books Ltd.: Cornwall, U.K., 1999.

3706


Carrillo-Pedroza et al.

Fe2+ + H2O2 ) Fe3+ + OH- + HO• k ) 76.5 mol-1 s-1 (16) According to eq 15, the presence of a ferrous salt increases the oxidation capacity of H2O2. Borah et al.30 show that the HO• radical takes part in the oxidation of organic sulfur compounds with the production of sulfonic acid. In accordance with Borah et al.,30 the iron deposited in the coal is present in the divalent state; therefore, ferrous ions can catalyze H2O2 decomposition during dissolution. Equally, the oxidant reacts in successive stages of oxidation and reduction and forms HO• and HO2• free radicals based on the Fenton reaction. Thus, the following reactions take place: Fe2+ + H2O2 ) Fe3+ + HO• + OH-

(17)

Fe2+ + HO• ) Fe3+ + OH-

(18)

Fe3+ + H2O2 ) Fe2+ + HO2• + H+

(19)

for control by chemical reactions on the unreacted particle surface35 is studied as a comparative option. In the mentioned models, the acid leaching of a sulfide proceeds according to the following:

Fe3+ + HO2• ) Fe2+ + H+ + O2

(20)

Af + bBs ⇒ products

The HO• and HO2• free radicals produced by catalytic decomposition of H2O2 by the Fe2+ and Fe3+ metallic ions are vigorous oxidizing agents. Previous results (not presented) show that peroxide destruction is catalyzed by the coal matrix and the pyrite oxidation products at 60 °C. Furthermore, it establishes that hydrogen peroxide does not activate at temperatures e20 °C, which explains the small desulfurization of coal at room temperature or less. 3.3. Effect of the Temperature. The effect of the temperature on pyrite dissolution from coal was studied in 1.0 M H2O2 and 0.5 M acid solution. The results are shown in Figure 4. In general, the reaction is very intense during the first 30 min. After this time, the pyrite dissolution process shows a different behavior with the temperature; in sulfuric acid medium, the pyrite dissolution has a strong dependence, and at higher temperatures, there exists greater dissolution. However, when hydrochloric acid is used, the effect of the temperature is not significant. When nitric acid is used, the increase of temperature diminished the pyrite dissolution. Antonijevic´ et al.21 reports that peroxide destruction accelerates at 40 °C. As previously stated, one of the reasons for this behavior (t > 30 min) is that the stability of the peroxide was affected by the temperature and redox species, among other things. 3.4. Kinetics of Pyrite Dissolution from Coal. With the objective of identifying the limiting stage of the process, a kinetic analysis of the data by means of the models was carried out. Because of the complexity of the fact that the peroxide concentration is dramatically affected over 30 °C, the data taken into account would be from the 20 °C experiments. In this analysis, test with ozone were not included to simplify the solid-liquid model. In an acid-leaching process, most of the sulfides follow the kinetic models for heterogeneous solid/liquid reactions, known as shrinking core models (SCMs): the SCM controlled by chemical reaction and the SCM controlled by diffusion through the solid product layer.32-34 A third model, the stochastic model (32) Habashi, F. Kinetics of Metallurgical Processes, Metallurgy Extractive Series; University of Que´bec: Que´bec City, Canada, 1999. (33) Levenspiel, O. Chemical Reaction Engineering; John Wiley and Sons: New York, 1999; p 668. (34) Sohn, Y. H.; Wadsworth, M. E. Rate Process of ExtractiVe Metallurgy; Plenum Press: New York, 1986.

Figure 4. Effect of the temperature in the pyrite oxidation.

(21)

The fraction of iron reacted at any time t, in a regime controlled by the chemical reaction, can be predicted from eq 22 kt ) 1 - (1 - x)1/3

(22)

where x is the fraction of iron reacted and can be calculated from the following relation: x)

C C0

(23)

and k is the apparent rate constant and can be calculated from the following relation: k)

ksCA R0 F

(24)

where ks is the rate constant of the reaction, F is the density of the FeS2 ore, R0 is the radius of the unreacted particle, and CA is the reactive concentration in the solution. The above equations are applied to monosized particles; thus, the average size of a narrow fraction of particles can be used in the kinetic model. When the regime is controlled by the diffusion of the reagents or dissolved species through the layer of solid reaction products, the fraction of iron reacted at any time t can be predicted from eq 25 2 kt ) 1 - x - (1 - x)2/3 3

(25)

where k can be calculated from the following relation: k)

2DCA R02F

(26)

where D is the diffusion coefficient of the iron species. In the stochastic model, the heterogeneity of solid minerals by introduction of a stochastic distribution for the rate constant is taken into account. Then, the rate constant ks from the SCM (35) Ciminelli, V.; Osseo Assare, K. Metall. Mater. Trans. B 1995, 26B, 677–685.


Energy & Fuels, Vol. 23, 2009 3707

Table 1. Correlation Coefficients (r2) Obtained from Experimental Data Plotted According to the Kinetics Models correlation coefficients (r2) for the kinetics models aqueous SCM diffusion SCM chemical stochastic medium concentration controlled reaction controlled model HCl H2SO4 HNO3

0.25 0.5 1 0.25 0.5 1 0.25 0.5 1

0.97 0.7 0.83 0.99 0.97 0.85 0.94 0.97 0.94

0.15 0.56 0.02 0.26 0.7 0.05 0.97 0.81 0.72

0.67 0.19 0.04 0.73 0.34 0.1 0.99 0.89 0.86

is transformed into a variable that changes with time or conversion, according to following relation: ks(X) ) 2k0(1 - X)

(27)

where ks ) kmax/2. According to Ciminelli and Osseo-Assare,35 the resulting equation has the following expression: kt ) (1 - x)-2/3 - 1 k)

4ksCA FR0

(28) (29)

Table 1 shows that the SCM for product layer diffusion control, represented by eq 25, describes well the experimental data with respect to others. That previously stated about the formation of elemental sulfur and/or sulfate layers, during the pyrite dissolution in sulfuric and hydrochloric acid solutions, gains relevance, because of the agreement of the results with the diffusion through a product layer as the controlling kinetic stage. Interestingly, the pyrite dissolution in the presence of hydrogen peroxide and 0.25 M HNO3 solutions also fits the SCM for the chemical reaction (see Table 1). Then, the assumption that the extent of the chemical reaction rate at the interface is similar than that of the diffusion can be precise. For nitric acid solutions, it has been reported that the presence of hydrogen peroxide or ozone prevents the formation of the aforementioned product layer of elemental sulfur because of the high anodic potential achieved or, at least, the electrochemical conditions influence the layer texture, favoring the formation of a porous cover of product.15 The dissolution of pyrite proceeds into a coal matrix; thus, the diffusion through the porous cover in the coal could limit the flux of the reagents and products to or from the reactive layer of pyrite (Figure 5).

Figure 5. SCM for diffusion control.

Figure 6. Experimental data of pyrite oxidation in different acid medium and 1 M H2O2 (black) or 0.33 g/h O3 (white), plotted according to the SCM for diffusion control.

In addition to the diffusion, the reagents (oxidant and acid) could modify the chemical properties of coal;36,37 therefore, a consumption of oxidative reagents would occur, thus leading the process to the chemical control. The diffusion and the reagents consumption could explain the fit of the results to the diffusionand chemical-controlling stage. Figure 6 shows the data of pyrite dissolution form coal according to the kinetic model of diffusion (eq 25). In each case (1 M HCl, H2SO4, and HNO3 in the presence of 1 M H2O2 and 0.25 M HNO3 in the presence of 0.33 g/h O3), the pyrite dissolution follows the kinetic described by the SCM, with diffusion through the layer of the formed product as the controlling stage. 3.5. Second Experimental Series: Statistical Analysis. The use of ozone and its effect in different conditions were investigated. As a first approach, the current study was based on an experimental design with the following parameters and levels: type and concentration of acids (HCl, HNO3, and H2SO4: 0.25, 0.5, and 1 M, respectively), concentration of H2O2, (0.5, 0.75, and 1 M), presence and concentration of O3 (0, 0.16, and 0.33 L g/h), and temperature (20, 30, 45, and 60 °C). The main factors affecting the Fe dissolution were determined by analysis of variance (ANOVA).38,39 Table 2 shows the main effect ANOVA of the results as a function of the amount of Fe extracted at 90 min of treatment. According to the table, ANOVA shows that, under the studied conditions, the type of acid employed as aqueous medium is the most important factor of the chemical dissolution of pyrite, followed of O3 and H2O2 concentrations. Results of ANOVA also indicate that acid concentration and temperature have an unimportant effect on the amount of reacted pyrite. The response curves for the individual effects of pyrite dissolution parameters are given in Figure 7. Figure 7a confirms that the maximum pyrite dissolution is reached when nitric acid is used. Increasing the acid concentration (average of the different acids) does not clearly affect the response of these curves (Figure 7b). An increase in the levels of factors, such H2O2 (Figure 7c) and O3 (Figure 7e), results in an increase in the mean value. On the basis of this, the results of ANOVA and the response curves indicate a qualitative way to know the behavior of pyrite dissolution at (36) Kawamoto, K.; Ishimaru, K.; Imamura, Y. J. Wood Sci. 2005, 51, 66–72. (37) Valdes, H.; Sanchez-Polo, M.; Rivera-Utrilla, J.; Zaror, C. Langmuir 2001, 18, 2111–2116. (38) Burke, M. I. Introduction to Design of Experiments. The Taguchi Method. Chrysler Quality Planning Manual, 1987. (39) Montgomery, D. C. Design and Analysis of Experiments; John Wiley and Sons, Inc.: New York, 1991.

3708


Carrillo-Pedroza et al. Table 2. ANOVA for Experimental Design

a

source term

DF

sum of squares

mean square

F ratio

prob level

power (R ) 0.05)

A: Sol_ac_ B: concentration C: H2O2 D: O3 E: temperature S total (adjusted) total

3 5 3 2 3 27 43 44

1158.763 452.6039 328.6117 323.7331 143.4148 2517.723 6100.13

386.2545 90.52078 109.5372 161.8665 47.80494 93.24899

4.14 0.97 1.17 174 0.51

0.015435a 0.453184 0.337738 0.195346 0.67699

0.793538 0.291345 0.279472 0.33177 0.140448

Term significant at R ) 0.05.

Figure 7. Response curves of individual effects.

different combinations of chemicals (aqueous medium, concentration, and oxidants). 3.6. Response Surface Approach. To find a better prediction about the effect of the interactions between factors, the response surface methodology (RSM) versus linear multiple regression (LMR) was used. RSM is an effective method to solve multivariable problems and to optimize the response for a given set of experiments.40-42 The first step in RSM is to find a suitable (40) Kasiri, M. B.; Aleboyeh, H.; Aleboyeh, A. EnViron. Sci. Technol. 2008, 42, 7970–7975.

approximation of the true functional relationship between the response or dependent variable (reacted pyrite) and the set of factors or independent variables. In this study, a quadratic model was applied to the response value. Figures 8 and 9 show predicted versus experimental curves of the two regression models applied to experiments with HNO3 and H2SO4 solutions, (41) Navarrete-Bolanõs, J. L.; Jimeńez-Islas, H.; Botello-Alvarez, E.; Rico-Martıńez, R. Org. Proc. Res. DeV. 2002, 6, 841–846. (42) Vegly, F.; Volpe, M.; Trifoni, M.; Toro, L. Ind. Eng. Chem. Res. 2000, 39, 2947–2953.


Energy & Fuels, Vol. 23, 2009 3709 Table 3. Optimal Conditions for the Oxidation of FeS2 at the Level Parameters Studied in This Work

Figure 8. Predicted versus experimental curves of HNO3 solutions.

aqueous medium

H2O2 (M)

temperature (°C)

ozone (g/h)

Fe extracted (%)(RSM), [exp]

1 M HNO3 0.25 M HNO3 1 M H2SO4 0.25 M H2SO4

1 0 1 0

20 20 45 20

0 0.33 0 0.33

(49.8), [49.8] (49.6), [49.5] (48.5), [46.2] (40), [40.0]

peroxide concentrations and also by the temperature. Note that the equations are empirical relationships obtained from different conditions. Then, the combinations of the variables studied determine the best parameters for the highest pyrite dissolution. The best conditions obtained from the RSM and confirmed by experimental tests are summarized in Table 3. The results from this study, with the use of different oxidants and acids, indicate that the reaction of pyrite dissolution, which was determined indirectly by the iron in solution, proceeds at the external core of this mineral according to the SCM. The strong oxidizing condition for pyrite dissolution is generated when H2O2 and O3 are employed. In general, this favorable effect is found for all types and concentrations of acids examined in this study. According to eq 1, as the reaction takes place, a significant amount of ferrous ion (Fe2+) is formed, thus increasing the redox potential of the solution. The addition of H2O2 increases the oxidizing potential, but eventually, the H2O2 is decomposed by the free ferrous ions. The principal benefit of ozone is its continuous addition into the solution, thus promoting the oxidation of Fe2+ to Fe3+ (eq 32). 6Fe2+ + O3 + 6H+ ) 6Fe3+ + 3H2O

Figure 9. Predicted versus experimental curves of H2SO4 solutions.

respectively. Because RSM includes the interaction effects and quadratic factor, the RSM correlation coefficient is greater than the LMR coefficient. The application of RSM yields the following regression equations, which are empirical relationships of the amount of pyrite dissolved (%), as a function of the test variables: for HNO3 FeS2 dissolved (%) ) 47.07 - 86.08[HNO3] 36.69[H2O2] + 0.14T - 140.97O3 + 49.51[HNO3] + 2

39.87193[H2O2]2 - 5.40 × 10-3T2 + 585.56O32 + 35.40[HNO3][H2O2] (30) for H2SO4 FeS2 dissolved (%) ) -1.06 - 58[H2SO4] - 40.6[H2O2] + 2.41T + 70.76O3 - 6.93[H2SO4]2 + 6.66[H2O2]2 0.02T2 - 71.18O32 + 70.93[H2SO4][H2O2]

(31)

where [HNO3], [H2SO4], and [H2O2] are the concentrations (mol/ L), O3 is the mass flow rate (g/h), and T is the temperature (°C). It is clear that the FeS2 dissolution in HNO3 medium is strongly affected by acid, hydrogen peroxide, or ozone concentrations. The temperature is not significant at small acid concentrations but is important at higher acid concentrations when peroxide or ozone is used. In this case, the increase in temperature promotes the decomposition of the oxidants, thus diminishing the pyrite dissolution. On the other hand, pyrite dissolution in H2SO4 is mainly affected by the ozone and

(32)

The occurrence of reaction 32 increases the concentration of ferric ion, which in turn favors reaction 1; in consequence, more sulfur is dissolved. However, the reaction products, such as the sulfate ions, propagate through the liquid film and porous cover from the coal matrix. Consequently, the kinetic of the overall process is affected by the rate of the product diffusion, and if it is the case of a precipitation, the kinetic would be slowed and eventually stopped. 4. Conclusions The dissolution of pyrite from sub-bituminous coal in different acidic media using H2O2 was studied. The results indicate that HNO3 is the stronger oxidant to remove the pyrite and its dissolution is enhanced by the H2O2 concentration. However, its efficiency decreases by the increase of temperature, because of the quick decomposition of H2O2. The behavior of pyrite dissolution in the presence of H2SO4 is different. In this case, temperature improves the dissolution reaction, because in this medium the H2O2 decomposition is slower with respect to HNO3. In the presence of H2O2 and with the acids evaluated, the kinetics of pyrite dissolution from coal follow the SCM and the global reaction is controlled by diffusion through the solid product. However, the fact that higher oxidation conditions are present in HNO3 medium makes it possible that the sulfur layer can be oxidized and removed. However, the evidence of kinetics behavior of the experimental data indicates that the reaction can be controlled by diffusion through a layer or film conformed by the surrounding surface coal. The analysis of the results with O3 as the oxidizing agent using ANOVA and one-factor plots, multiple regression, and response surface analysis methods was applied to figure out the optimum experimental conditions to improve the pyrite

3710


dissolution yield. The multifactor response surface analysis was the best approach. In addition, our results support that diffusion of oxidant agents is the rate-controlling step of the overall process: O3 diffusion from the gas bulk to dissolution to react with pyrite and diffusion of Fe3+ through the layer formed in the boundary surface pyrite and coal particles. The results clearly showed that ozone contributed to the use of lower concentrations of HNO3 or H2SO4, with consequently economical savings.

Carrillo-Pedroza et al. Acknowledgment. The authors thank CONACYT for financial support (Project 67039). A. Da´valos Sańchez thanks the Universidad Autonoma de Coahuila (Mexico) for financial support through PROMEP for the schoolarship granted. Furthermore, the authors thank the staff at Centro de Investigacioń en Materiales Avanzados S. C. (CIMAV) for all of their help that they gave to carry out this work. The assistance of S. Miranda, M. Moreno, A. Rubio, E. Elorza, L. de la Torre, R. Torres, and K. Campos is also acknowledged. EF900253G

Coal Desulfurization in Oxidative Acid Media Using Hydrogen

Recommend Documents