Energy Fuels 2009, 23, 5916–5919 Published on Web 11/19/2009
: DOI:10.1021/ef901167t
Analysis on Leaching Characteristics of Iron in Coal Fly Ash under Ammonia-Based Wet Flue Gas Desulfurization (WFGD) Conditions Xiang Gao,*,† Hong-lei Ding,† Zu-liang Wu,‡ Zhen Du,† Zhong-yang Luo,† and Ke-fa Cen† †
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China and ‡Department of Environmental Engineering, Zhejiang Gongshang University, Hangzhou 310035, China Received June 10, 2009. Revised Manuscript Received November 4, 2009
Iron leaching from fly ash has been investigated under simulated ammonia-based wet flue gas desulfurization conditions to determine the effects of the reaction temperature, initial pH value, liquid/solid ratio, and vibration frequency. The experimental results indicated that the ferric ion concentration in solution increased with the increase of the temperature and vibration frequency and the decrease of the pH value and liquid/ solid ratio. The kinetics of iron leaching can be expressed as diffusion combined with a surface chemical reaction model. The reaction path can be described by the equation: 1 - (1 - R)1/3 = kt. The apparent activation energy was estimated to be about 20.01 kJ/mol. Such a value of the activation energy indicates that the leaching of iron under experimental conditions is controlled by both chemical reaction and diffusion.
and coals.6-8 These studies mainly focus on three aspects:9-11 the leaching kinetics, environmental influence, and reuse of leached metals. Fytianos and Tsaniklidi12 investigated the potential mobility of certain heavy metals in four types of fly ash from northern Greece. They found that the leachabilities of Ca, Pb, Zn, Cd, Cr, and Mn in fly ash samples are different. In all of the samples, Ca has the highest leachability and Mn has the lowest leachability. The concentration of metals in the leachate increases with the decrease of the leachant pH value. To observe the effect of fly ash on the environment in filling abandoned coal mines, Dutta et al.13 tested the leachability of 10 elements: Fe, Mn, Ca, Na, K, Cu, Cr, Zn, As, and Pb. They reported that the elements have a high mobility at a low pH. The risk of ground and surface water pollution may be unavoidable if fly ash is used to fill the abandoned mines where acid mine drainage is prominent. Blending lime with the acid mine drainage to enhance its alkalinity is a possible solution to this problem. There are currently many studies investigating the reuse of fly ash because of the leachability of some metal and metalloid elements in ashes and coals. With its alkalinity, fly ash can be used for FGD.5,10,14 Moreover, the oxidizing ions, such as iron and manganese ions, can be leached from fly ash in the process of wet flue gas desulfurization (WFGD).15 The
1. Introduction Coal is one of the most important energy sources in China1 and is consumed mainly in the combustion in power plants. The combustion of coal produces a large number of residues, such as fly ash, bottom ash, and boiler slag. These residues always contain some metal elements whose species and content usually depend upon the characteristics of the coal. Some elements contained in the residues, such as Cr, As, and Hg, may be toxic or harmful2,3 to the environment because they can dissolve in water or certain solutions. Other elements in the residue, such as Ca, Fe, Mn, and Zn, may be harmless to the environment but will influence subsequent processes in coal combustion, such as flue gas desulfurization (FGD).4,5 The physical and chemical characteristics of fly ash vary depending upon factors, such as the coal source and combustion process. These characteristics, along with the leaching conditions, determine the leaching characteristics of fly ash. There are many studies on the leaching characteristics of ashes *To whom correspondence should be addressed. Fax: (86)57187951335. E-mail:
[email protected]. (1) Zhao, Y.; Wang, S. X.; Duan, L.; et al. Primary air pollution emission of coal-fired power plants in China: Current status and future prediction. Atmos. Environ. 2008, 42, 8442–8452. (2) Palumbo, A. V.; Tarver, J. R.; Fagan, L. A.; et al. Comparing metal leaching and toxicity from high pH, low pH, and high ammonia fly ash. Fuel 2007, 86, 1623–1630. (3) Chakraborty, M.; Mukherjee, A. Mutagenicity and genotoxicity of coal fly ash water leachate. Ecotoxicol. Environ. Saf. 2009, 72, 838– 842. (4) Zhao, J. T.; Huang, J. J.; Zhang, J. M.; et al. Influence of fly ash on high temperature desulfurization using iron oxide sorbent. Energy Fuels 2002, 16, 1585–1590. (5) Izquierdo, M. T.; Rubio, B. Carbon-enriched coal fly ash as a precursor of activated carbons for SO2 removal. J. Hazard. Mater. 2008, 155, 199–205. (6) Cornelis, G.; Johnson, C. A.; Gerven, T. V.; et al. Leaching mechanisms of oxyanionic metalloid and metal species in alkaline soild wastes: A review. Appl. Geochem. 2008, 23, 955–976. (7) Kim, A. G.; Hesbach, P. Comparison of fly ash leaching methods. Fuel 2009, 88, 926–937. (8) Paul, M.; Seferinoglu, M.; Ayc-ık, G.; et al. Acid leaching of ash and coal: Time dependence and trace element occurrences. Int. J. Miner. Process. 2006, 79, 27–41. r 2009 American Chemical Society
(9) Singh, D. N.; Kolay, P. K. Simulation of ash-water interaction and its influence on ash characteristics. Prog. Energy Combust. Sci. 2002, 28, 267–299. (10) Srinivasan, A.; Grutzeck, M. W. The adsorption of SO2 by zeolites synthesized from fly ash. Environ. Sci. Technol. 1999, 33, 1464–1469. (11) Sundaram, H. P.; Cho, E. H.; Miller, A. SO2 removal by leaching coal pyrite. Energy Fuels 2001, 15, 470–476. (12) Fytianos, K.; Tsaniklidi, B. Leachability of heavy metals in Greek fly ash from coal combustion. Environ. Int. 1998, 24, 477–486. (13) Dutta, B. K.; Khanra, S.; Mallick, D. Leaching of elements from coal fly ash: Assessment of its potential for use in filling abandoned coal mines. Fuel 2009, 88, 1314–1323. (14) Li, L.; Fan, M. H.; Brown, R.; et al. Production of a new waster treatment coagulant from fly ash with concomitant flue gas scrubbing. J. Hazard. Mater. 2009, 162, 1430–1437. (15) Guan, B. H.; Ni, W.; Wu, Z. B.; et al. Removal of Mn(II) and Zn(II) ions from flue gas desulfurization wastewater with water-solution chitosan. Sep. Purif. Technol. 2009, 65, 269–274.
5916
pubs.acs.org/EF
Energy Fuels 2009, 23, 5916–5919
: DOI:10.1021/ef901167t
Gao et al.
Table 1. Chemical Composition of the Fly Ash component
wt %
CaO MgO SiO2 Al2O3 Fe2O3 K2O others
3.62 3.33 61.3 12.2 9.31 3.13 7.11
desulfurization solution contains a lot of ions of SO3-2 and HSO3- because of the removal of SO2 by WFGD. These ions can be oxidized catalytically to SO4-2 and HSO4-.16 Ammonia-based WFGD (Am-WFGD) has drawn increasing attention in China recently17 because of its advantages, such as high desulfurization efficiency and easy use of the byproducts. By adjusting the pH value of the desulfurization solution and the oxidation degree of sulfite, the byproducts of Am-WFGD may be the solution of (NH4)2SO3, NH4HSO3, or (NH4)2SO4. As a result, sulfite oxidation becomes an important factor in the Am-WFGD process. The fly ash will be carried to the scrubber by the flue gas, where it will be captured by the scrubbing liquid. Once captured, certain oxidizing ions are leached from the fly ash by the desulfurization solution. Our previous investigations of Am-WFGD focused on developing a process that produced concentrated ammonium bisulfite as a byproduct, which would be used to synthesize hydroxylamine sulfate.18 In this process, the presence of iron will cause the degeneration of hydroxylamine sulfate and the oxidation of sulfite. These required an investigation of the leaching characteristics of iron. Herein, this report focuses on the leachability of iron with an emphasis on the differences found under Am-WFGD conditions.
Figure 1. XRD pattern of the fly ash sample.
made fresh by absorbing SO2 with an aqueous ammonia solution and diluting it to appropriate concentrations and pH values according to the Am-WFGD process. Deionized water was used for experiments and analysis. The leaching tests were contacted according to the Chinese national standard of “Test method standard for leaching toxicity of solid wastes;Horizontal vibration extraction procedure” (GB 5086.2, 1997).19 A thermostatic vibrator was used for the leaching experiments, and the experimental temperature range was determined according to the Am-WFGD process. A group of sealed cylindrical vials, which were filled with 40 mL of leachant and a certain amount of fly ash, were put together in the vibrator before the experiment. During the run, one of these vials would be taken out of the vibrator in a certain time interval. The suspension in the vial was filtered with a 0.22 μm filter and analyzed for total leached iron by spectrophotometry (1,10phenanthroline method, λ = 510 nm) with a spectrophotometer (model UV2102, Shangfen, China) according to the Chinese national standard of “Chemical products for industrial use; General method for determination of iron content;1,10-Phenanthroline spectrophotometric method” (GB/T 3049, 2006).
2. Materials and Methods 2.1. Fly Ash. The fly ash used in this study was collected from flue gas after it had passed through the electrostatic precipitators (ESPs) of a circulating fluidized-bed boiler (CFBB) in the Zhejiang province and was obtained with a measuring apparatus for dust content (model 3012H, Laoying, China). The samples of fly ash were analyzed after being dried at 55 °C for 24 h. The chemical compositions of fly ash were determined by X-ray fluorescence (XRF) (model ZSX100e, Rigaku, Japan) analysis and were shown in Table 1. The particle size distribution was measured with a particle size analyzer (model Mastersizer 2000, Malvern, U.K.), and the sample had a d50 size of approximately 17.933 μm. The X-ray diffraction (XRD) measurement was carried out on a Rigaku D/max-rA system with Cu Ka radiation. The XRD pattern for the fly ash used in the experiments was presented in Figure 1. It can be noticed from this figure that Fe2O3 is the main occurrence form of iron in the fly ash. 2.2. Experimental Procedure. To investigate the leachability of fly ash, the effects of the reaction temperature, initial pH value, liquid/solid ratio (L/S), and vibration frequency were determined. The experimental procedure used to determine these effects was to change one of the parameters while holding the others constant. The leachants used in the experiments were
3. Results and Discussion 3.1. Influence of Leaching Parameters. When the effect of the pH value on the leaching of iron was investigated, the initial pH values of leachants were 4.03, 5.0, and 6.05 while keeping other parameters fixed at temperature, 323 K; L/S, 10:1 L/kg; and vibration frequency, 120 min-1. Figure 2 shows that the leaching concentration increased with the decrease of the pH value. In a reaction time of 60 min, the iron concentration in the leachate can increase by about 52.6% when the initial pH value in the leachant decreased from 6.06 to 4.03. This phenomenon can be explained by the leaching path of iron in the leaching solution as follows:20 6Hþ þ Fe2 O3 f 2Fe3þ þ 3H2 O
ð1Þ
H2 O þ Fe3þ þ HSO3 - f Fe2þ þ SO4 2- þ 3Hþ
ð2Þ
The low pH value in the leaching solution means a high concentration of Hþ and HSO3- in the solution, which will
(16) Karatza, D.; Prisciandaro, M.; Lancia, A.; et al. Calcium bisulfite oxidation in the flue gas desulfurization process catalyzed by iron and manganese ions. Ind. Eng. Chem. Res. 2004, 43, 4876–4882. (17) Xiao, W. D.; Li, W.; Fang, Y. J.; et al. A new flue gas desulfurization process of thermal power plant-NADS ammonia-fertilizer process. Electric Power 2001, 34, 54–58. (18) Ding, H. L; Gao, X; Shi, Z. L.; et al. Research on properties of ammonia-bisulfite WFGD in stone-coal fired power plant. J. China Coal Soc. 2009, 34, 1110–1114.
(19) Li, M.; Hu, S.; Xiang, J.; et al. Characterization of fly ashes from two Chinese municipal solid waste incinerators. Energy Fuels 2003, 17, 1487–1491. (20) Ansari, A.; Peral, J.; Domenech, X.; et al. Oxidation of HSO4- in aqueous suspensions of R-Fe2O3, R-FeOOH, β-FeOOH, and γ-FeOOH in the dark and under illumination. Environ. Pollut. 1997, 95, 283–288.
5917
Energy Fuels 2009, 23, 5916–5919
: DOI:10.1021/ef901167t
Gao et al.
Figure 4. Effect of L/S on the leaching process (pH value, 6.05; temperature, 323 K; vibration frequency, 120 min-1).
Figure 2. Effect of the pH value on the leaching process (temperature, 323 K; L/S, 10:1 L/kg; vibration frequency, 120 min-1).
Figure 5. Effect of the vibration frequency on the leaching process (pH value, 5.90; temperature, 323 K; L/S, 10:1 L/kg).
in the total amount of iron in the suspension. This leads to the increase of the leached iron concentration in the solution. Figure 5 shows the effect of the vibration frequency on the iron dissolution rate in the solution. The observed leaching rate increases with the increase of the vibration frequency, which is due to the enhancement of mass transfer caused by vibration. The increase of the vibration frequency enhances the convective mass transfer between liquid and fly ash particles, thus promoting the dissolution rate. All of the above experimental data indicated that the leaching time is the one of the most important factors in the leaching procedures. Because the mass transfer is a timedependent process, the leaching ferric concentration in the leaching solution increased with the experimental time course. 3.2. Leaching Kinetics Analysis. The leaching of iron in solution is a fluid-solid heterogeneous reaction. In the fluid-solid system, the rate of the reaction may be controlled by one of the following steps:22 diffusion through the fluid film, diffusion through the ash, or chemical reaction at the surface of the unreacted solid core. To study the leaching kinetics of iron under Am-WFG conditions, a model must be estimated to represent the dissolution of iron. For this purpose, the iron concentration at different time intervals was converted to the fraction of Fe leached (R) ð3Þ R ¼ c=c0
Figure 3. Effect of the temperature on the leaching process (pH value, 5.90; L/S, 10:1 L/kg; vibration frequency, 150 min-1).
be favorable for chemical reactions 1 and 2. According to reaction 1, increasing [Hþ] can react with more Fe2O3 coming from fly ash, producing more [Fe3þ]. The Fe3þ generated is then reduced to Fe2þ by HSO3- in reaction 2. Therefore, the high concentrations of Hþ and HSO3- in the solution aid in the dissolution of iron from fly ash. Figure 3 demonstrates the effect of the temperature on the leaching of iron. The temperatures were selected as 298, 318, 338, and 358 K. It is clear that the concentration of leached iron increased with the increasing temperature. For example, the iron concentration increased significantly from 18.6 g/m3 at 298 K to 48.6 g/m3 at 358 K in 3 h. This indicates that the increase of the temperature is helpful to the leaching of iron in the leachant. According to the Arrhenius law, the chemical reaction rate is strongly dependent upon the temperature21 and increases with temperature. The rate of diffusion can also be increased with temperature. The effect of varying L/S on the leachability of iron from 5 to 10 is shown in Figure 4. When L/S was changed from 10 to 5, the iron concentration increased from 13.6 to 17.6 g/m3 in 60 min. This is due to the increase in the amount of fly ash as L/S decreases, and increasing the fly ash signifies an increase (21) Aydo gan, A.; Erdemoglu, M.; Uc-ar, G. Kinetics of galena dissolution in nitric acid solutions with hydrogen peroxide. Hydrometallurgy 2007, 88, 52–57.
(22) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; Chemical Industry Press: Beijing, China, 2002, p 379.
5918
Energy Fuels 2009, 23, 5916–5919
: DOI:10.1021/ef901167t
Gao et al.
Figure 6. Plot of 1 - (1 - R)1/3 versus t for the temperature range of 298-358 K.
Figure 7. Arrhenius plot for the leaching of iron.
where R is the fraction of Fe leached, c0 is the initial content of Fe in the fly ash (in grams), and c is the content of iron leached in the solution (in grams), respectively. The leaching kinetics can be described by the equation of the surface chemical reaction model 1 -ð1 -RÞ1=3 ¼ kt
This value for the activation energy indicates that the iron leaching under experimental conditions is controlled both chemically and by diffusion. 4. Conclusions
ð4Þ
The investigation into iron leaching from fly ash under Am-WFGD conditions has shown that all leaching parameters influence the process. The ferric ion concentration in solution increased with the increase of the temperature and vibration frequency and the decrease of the pH value and L/S. The kinetics of iron leaching can be described by the surface reaction kinetics model of the fluid-solid heterogeneous reaction. The value of the apparent activation energy is 20.01 kJ/mol, which indicates that the leaching of iron under experimental conditions is controlled both chemically and by diffusion. This study suggests that the iron-leaching concentrations are about 0-50 g/m3 under the experimental conditions. This range of iron concentrations will influence the synthesis of hydroxylamine sulfate and the oxidation of sulfite.16
where k is the leaching rate constant and t is the leaching time. Figure 6 shows the plot of 1 - (1 - R)1/3 versus t for the data presented in Figure 3 between 298 and 358 K. The lines in Figure 6 fit the experimental data well. The slope of each line is the value of k. In Figure 6, the straight fitted lines do not pass through 0 at zero time because of dissolution taking place during the heating time. From the slope of the straight line in Figure 6, the apparent rate constant (k) was evaluated. The effect of the temperature on the rate of the chemical reaction is usually expressed by the Arrhenius empirical equation. From the Arrhenius equation, the k term is defined as k ¼ Ae-E=RT
ð5Þ
where A is the pre-exponential factor, E is the activation energy (kJ mol-1), R is the universal gas constant (kJ mol-1 K-1), and T is the leaching temperature (K). The Arrhenius plot, ln k versus 1/T, has a nearly straight line (Figure 7) for each experimental temperature. Using the plot, E was determined to be 20.01 kJ mol-1.
Acknowledgment. This work was supported by the National Key Technologies R&D Program in the 11th Five-Year Period of China (2006BAA01B04) and the Program for New Excellent Talents in University (NCET-06-0513).
5919
