Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating

Article pubs.acs.org/IECR

Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating Fluidized Bed Combustion Fly Ash with Hydrochloric Acid Qing Luo, Guilan Chen, Yuzhu Sun, Yinmei Ye,* Xiuchen Qiao, and Jianguo Yu National Engineering Research Center for Integrated Utilization of Salt Lake Resources, School of Resources and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ABSTRACT: The dissolution of aluminum, calcium, and iron from calcined fly ash with hydrochloric acid was investigated using a batch reactor at temperatures from 315 to 345 K and over the acid concentration range of 1.4−6.0 M. Calcium was preferentially released compared to aluminum though these two elements in the calcined fly ash were mainly incorporated into the same mineralogical phase of anorthite. The dissolved fractions of aluminum, calcium, and iron increased with leaching time, but it decreased for silicon, which indicated the continuous precipitation of silicon during the leaching process. The dissolution kinetic data were successfully examined according to a semiempirical Avrami-type equation. The activation energies for dissolution of aluminum, calcium, and iron from the calcined fly ash with hydrochloric acid are 32, 28, and 19 kJ/mol, respectively.

1. INTRODUCTION With the rapid development of circulating fluidized bed combustion (CFBC) technology in thermal power plants, more than 50 million tons of CFBC fly ash is discharged annually in China. The huge quantities of fly ash not only lead to land occupation but also bring pollution risks to the environment, which is becoming the major bottleneck for the further development of thermal power plants in China. On the other hand, as the content of Al2O3 in fly ash is typically as high as 20−50 wt %, the fly ash is considered to be an economical source of aluminum.1,2 Recovering aluminum from this massive waste slag could not only offer a substitute aluminum source for the diminishing bauxite resources but also reduce the environmental contamination of coal fly ash. The process for aluminum extraction from coal fly ash could be mainly classified into acidic3−5 and alkali6−8 methods. Compared to the alkali methods, one major advantage of the acidic method is the simultaneous dissolution of aluminum and other valuable elements such as Fe, Ca, Ge, and Ga in fly ash with depressing the silica dissolution and thus reducing the slag amount.9−11 The acidic method includes a direct acid leaching process with low leaching rate of aluminum12 and calcined-acid leaching process with high leaching rate of aluminum by activation of fly ash before acid leaching.1,5 The acid leachants commonly used in the acid leaching process are sulfuric acid, hydrochloric acid, and hydrofluoric acid. Though the hydrofluoric acid can effectively decompose mullite of coal fly ash, it is limited in the industrial application ascribed to its environmental hazard. Seidel et al.13,14 studied the mechanism and kinetics of aluminum and iron leaching from the pulverized coal combustion (PC) fly ash using sulfuric acid, and they found that the formation of calcium sulfate was a major selfinhibition for releasing aluminum from fly ash. Due to the addition of sulfur-fixing agent, the content of calcium in the CFBC fly ash is much higher than the PC fly ash. To overcome the self-inhibition dissolution of calcium and obtain the maximum dissolution rate of valuable elements, hydrochloric acid is preferred as the leachant for CFBC fly ash in this work. © 2013 American Chemical Society

Another advantage of using hydrochloric acid as the leachant is that it could not only recover aluminum but also produce compound flocculants containing chlorides of aluminum, calcium, and iron, and the solid residues could be used for refractory bricks. Furthermore, as the leachant, hydrochloric acid could be taken from a waste HCl solution of chemical plants. Therefore, research using waste HCl solution to extract valuable inorganic elements from waste fly ash could benefit environmental protection and bring us economic profits as well. Although the acidic method for recovering elements from coal fly ash has been intensively studied, most work was focused on the leaching process optimization and few researchers investigated the kinetics of different element dissolution from fly ash.13 The objective of this study is to obtain the dissolution kinetics of aluminum, calcium, and iron from the calcined CFBC fly ash with hydrochloric acid and to understand the dissolution mechanism, which was not systematically reported to our knowledge. A comprehensive understanding of the dissolution behavior of fly ash during leaching would greatly contribute to the design of an applicable industrial process for recovering valuable elements from CFBC fly ash, and this study could also offer reference for recovering elements from other solid waste slags.

2. EXPERIMENTAL SECTION 2.1. Material Pretreatment and Characterization. The CFBC fly ash used in this work was from the Shanghai Coking & Chemicals Company. It was crushed, ground, well mixed with the addition agents of CaCl2, and calcined at 1223 K for about 1 h to obtain the calcined fly ash. The chemical composition of calcined CFBC fly ash was determined by X-ray fluorescence (XRF) analysis using a Thermo ARL ADVANT’X Received: Revised: Accepted: Published: 18184

August 16, 2013 November 21, 2013 December 2, 2013 December 2, 2013 dx.doi.org/10.1021/ie4026902 | Ind. Eng. Chem. Res. 2013, 52, 18184−18191

Industrial & Engineering Chemistry Research

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2290 spectrometer, and the results were summarized in Table 1. The crystalline phase of the calcined CFBC fly ash before Table 1. Chemical Composition of Calcined CFB Fly Ash component

SiO2

Al2O3

CaO

Fe2O3

SO3

TiO2

K2O

content (wt %)

48.28

28.74

15.64

4.48

0.39

0.75

0.44

and after leaching was investigated with X-ray diffraction (XRD) spectra on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation in the range of 10−80° 2θ. The particle size distribution was measured by using a MasterSizer Malven 2000 Laser Diffraction Particle Size Analyzer. Brunauer− Emmett−Teller (BET) specific surface area measurements were performed by Quantachrome Instruments (USA). Scanning electron microscopy (SEM) images of samples were taken on a Nova 450 SEM equipped with an energy-dispersive X-ray spectroscopy (EDS). 2.2. Leaching Experiments. Leaching experiments were performed in a glass batch of 250 mL heated in a temperaturecontrolled oil bath. The reactor with a stirrer motor for mixing was equipped with a condenser to prevent the solution loss by evaporation. Hydrochloric acid was used as the leaching agent and the concentrations varied from 1.4 to 6.0 M. As the particle size of the calcined fly ash was usually smaller than 50 μm after calcination, the effect of particle size on the dissolution rate was not studied in this work. Preliminary experiments showed that the rate of dissolution was independent of the stirring speeds in the range of 200−400 rpm, which means the solids were well suspended in the liquid above 200 rpm and the external diffusion was not the rate determining. The stirring speed was thus fixed at 300 rpm for all further leaching experiments. To observe the influence of solid-to-liquid ratio (S/L), the ratio of fly ash to HCl solution was varied from 1/10 to 1/100 g/cm3. The reaction temperature used in this work was typically in the range of 315−345 K. Once the temperature of hydrochloric acid in the reactor reached thermal equilibrium, a given amount of calcined CFBC fly ash was quickly added into the solution with stirring and a timer was started. The leaching samples was filtered and washed at a given time. The species of aluminum and calcium in the filtrate were analyzed by chemical titration method with ethylenediamine tetra-acetic acid (EDTA) and ethylene glycol tetra-acetic acid (EGTA), respectively. By using a 721 UV−vis spectrophotometer, the content of iron in the filtrate was determined colorimetrically at 570 nm after adding tiron reagent and silicon concentration was measured at 620 nm with adding molybate solution. More details about the concentration measurement of aluminum, calcium, iron, and silicon could be found in GB/T 1574−2007 (China). The fraction of dissolution x was calculated as follows: x(%) =

Figure 1. Dissolution behavior of cations of calcined fly ash using 6 M hydrochloric acid at 335 K and S/L of 1/20 g/cm3 with stirring speed of 300 rpm.

aluminum and calcium reached a plateau (maximal values), e.g., 71.6% for aluminum and 86.9% for calcium at 600 min. Actually, dissolution fractions of aluminum and calcium had already reached values of 69.9% and 83.1% at 92 min, respectively, which are very close to their maximal values. In addition, iron could be completely dissolved as far as the reaction time was long enough. It should be mentioned that the maximal dissolution fractions were further confirmed by conducting the leaching experiment at 378 K with 6 M HCl solution for 800 min, and the fractions dissolved were 71.8% for aluminum, 87.2% for calcium and 95.9% for iron, respectively. Compared to iron, the maximal dissolution fractions of calcium and aluminum did not approach 100%, which might be ascribed to the inert phases of aluminum/calcium in the calcined fly ash and the secondary precipitation of amorphous alumino-silicate during leaching. Furthermore, the dissolution curves of calcium and aluminum were similar and they were different from that of iron, which was likely because that calcium and aluminum existed mainly together in the form of anorthite but iron was in the isolated mineralogical phase of hematite (Figure 2).

mole numbers of cation in the filtrate after leaching mole numbers of cation in the fly ash before leaching × 100

Figure 2. Evolution of XRD patterns as a function of leaching time. (1)

Figure 2 shows the evolution of X-ray patterns of the CFBC fly ash with the leaching time. It can be observed that the anorthite and quartz are the major mineral phases, and hematite is the minor phase in the calcined CFBC fly ash before leaching (0 min). In addition, from the XRF results (Table 1), the molar ratio of aluminum and calcium of the calcined CFBC fly ash is 2.05, which is also very close to that of anorthite (CaAl2Si2O8). With reaction progress, the anorthite phase gradually disappeared and finally the main crystalline phase was quartz

All data presented in this work were obtained by the average value of two repeated analysis of samples.

3. RESULT AND DISCUSSION 3.1. Dissolution Progress of Elements. As illustrated in Figure 1, the dissolution rates of aluminum, calcium, and iron were fast at the initial stage of leaching, and then, it became slower with time. Moreover, the dissolution fractions of 18185

dx.doi.org/10.1021/ie4026902 | Ind. Eng. Chem. Res. 2013, 52, 18184−18191


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Figure 3. SEM images of CFBC fly ash particles at leaching times of (a) 0, (b) 5, (c) 15, (d) 30, and (e) 120 min.

with a minority of hematite and anorthite, and trace of mullite phase could be detected as well. Though the molar ratio of aluminum and calcium in the fly ash was 2.05, and they mainly existed together in the phase of anorthite, the relative dissolution fraction of aluminum was lower than that of calcium, as observed in Figure 1. That means nonstoichiometric dissolution of aluminum and calcium

occurred, which might be attributed to the impurity phase or zones in the fly ash such as trace of inert mullite phase and amorphous alumina, to the secondary precipitation phase such as amorphous alumino-silicate, or to the preferential leaching of Ca element from the mineral surface.15−17 It should be stressed that the higher dissolution fraction of calcium than aluminum was not ascribed to preferentially dissolving the ultrafine 18186



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fly ash after leaching was increased from 4 m2/g before the leaching to 157 m2/g after the leaching of 120 min though the particle size had no obvious change from the particle size distribution measurement. Therefore, it could be concluded that the ash layer covering the unreacted layer was predominated by amorphous substances. The corresponding EDS results were concluded in Table 2. Compared to the fresh sample (0 min), the contents of calcium

particles or other impurity phase of calcium in fly ash by conducting a stepwise leaching experiment. In such an experiment, the fresh fly ash was first dissolved in the 6 M hydrochloric acid solution for 30 min and filtrated with washing, and then the residual fly ash was dissolved again at the same conditions for 30 min. It was found that the dissolution fractions of calcium were higher than that of aluminum in both leaching steps. The nonstoichiometric release of aluminum and calcium was also observed by Zhou et al.18 in the anorthite dissolution process promoted by bacterial adhesion. Casey et al.19 reported that the exchange of calcium by hydrogen ions produced a Cadepleted surface layer of up to 1000 Å in depth for dissolution of labradorite feldspar at acidic condition, which indicated calcium was preferentially dissolved. The rapid exchange reaction of H+ with the large intrastructural Ca17 is one reason for the higher dissolution fractions of calcium than aluminum, as shown in Figure 1. Since aluminum and calcium exhibited a nonstoichiometric dissolution behavior and iron was a substantive phase being not incorporated in the same mineralogical phase of aluminum and calcium, the dissolution kinetics of fly ash were analyzed in terms of dissolution rates of aluminum, calcium, and iron, respectively. As observed in Figure 1, in contrast to aluminum, calcium, and iron, the dissolution fraction of silicon decreased with the leaching time, indicating the precipitation of silicon during the leaching process. According to the mineral phases of fly ash and the dissolution behavior of aluminum, calcium, iron, and silicon, the overall reactions of CFBC fly ash at acidic conditions could be represented as follows: CaAl 2Si 2O8 + 8H+ → 2Al3 + + Ca 2 + + 2H4SiO4

(2)

Fe2O3 + 6H+ → 2Fe3 + + 3H 2O

(3)

H4SiO4 → SiO2 ·x H 2O + (2 − x)H 2O

(4)

Table 2. Evolution of Surface Element Composition (wt %) element time (min)

Al

Ca

Fe

Si

O

0 5 15 30 120

18.63 22.87 18.64 10.89 5.82

15.49 3.26 2.83 1.87 1.21

1.21 0.47 0.29 0.22 0.16

22.98 24.60 28.35 34.12 49.01

40.91 45.83 46.74 49.71 42.35

and iron on the sample surface were dramatically depleted at the initial period of leaching (5 min); whereas, the relative percentage of aluminum and silicon increased. The relatively higher content of aluminum and silicon at the initial reaction stage confirmed that the calcium was dissolved preferentially compared to aluminum and silicon. With further leaching, the relative content of aluminum decreased, while that of silicon clearly increased, which is consistent with the dissolution behavior of Si as concluded in Figure 1. Since the silicon mainly existed in the solid residue after leaching, the kinetics study of fly ash with hydrochloric acid was focused on the dissolution of aluminum, calcium, and iron in the following work. 3.2. Effect of Processing Parameters. 3.2.1. Effect of Solid/Liquid Ratio. The effect of the solid to liquid ratios on the dissolution rate was studied at temperature of 335 K and HCl concentration of 2.9 M. According to eqs 2 and 3 as well as the contents of elements as listed in Table 1, with 2.9 M HCl solution the stoichiometric S/L ratio for completely dissolving all aluminum, calcium, and iron elements should be 1/8.5 g/ cm3. Herein the S/L ratios varied from 1/10 to 1/100 g/cm3 to obtain a fair comparison, which means the amount of acid is enough to dissolve all aluminum, calcium, and iron elements in theory. As exhibited in Figure 4, the influence of S/L ratio on the dissolution fraction of aluminum was not obvious as S/L ≤ 1/10 g/cm3. For calcium and iron, the dissolution fractions were slightly lower at S/L = 1/10 g/cm3, and then, they had no obvious change with S/L ratios at S/L ≤ 1/20 g/cm3. Therefore, the solid is well mixed with the HCl solution at S/L ≤ 1/20 g/cm3. The similar results of S/L ratio influence were obtained with 4.4 and 6.0 M HCl. Unless otherwise specified, all further batch leaching experiments were performed at the S/L ratio of 1/20 g/cm3. 3.2.2. Effect of Acid Concentration. Figure 5a−c show the dissolution fractions of aluminum, calcium, and iron at 325 K by varying concentrations of hydrochloric acid. The dissolution of aluminum, calcium, and iron was increased with increasing initial acid concentration from 1.4 to 6.0 M. In an acidic environment, feldspar dissolves primarily by the protonattacking mechanism,17 and thus, the rate of dissolution is accelerated by increasing proton concentration. Moreover, the similar dependence of dissolution rate on the initial acid concentrations was observed at 315 and 335 K as well. 3.2.3. Effect of Reaction Temperature. The effect of temperature on the dissolution rates of aluminum, calcium,

Once silicon is dissolved, it could further form a precipitate under the investigated conditions with extending reaction time, as illustrated in eq 4. It was found that the liquid became viscous and the filtration became difficult with leaching time, likely indicating the formation of silicon gel. Fogler et al.20,21 also observed the Si concentration in the liquid decreased with time in the dissolution of mineral analcime with 2−8 M HCl solution and found the formation of 5 nm primary silica particles being followed by particle flocculation. The SEM images of fly ash are shown in Figure 3. The fresh calcined fly ash were multilayered platelike shape with a minority of small spherical particles before leaching (Figure 3a), and its surface looked smooth and tight. After leaching of 5 min using 6 M HCl solution at 335 K, the porous solid residues with etch pits could be easily observed (Figure 3b), suggesting that dissolution occurred preferentially at the active sites. With leaching progress, the sample became fragmented (Figure 3c) and lots of small particles covered the surface of the unreacted particles and blocked the pores (Figure 3d). Finally, the sample surface was fluffy and loose and the pores became invisible (Figure 3e), which was due to pores that were initially formed by acid attack on the surface of particles being blocked by the silica precipitation and the fragmented small particles. Therefore, both internal diffusion of H+ into the unreacted active sites and the detachment of products from the active sites to the liquid were hindered by the leached layer, which could partly reduce the dissolution rate. Moreover, the BET surface area of 18187



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Figure 4. Effect of solid to liquid ratio on the dissolution of (a) aluminum, (b) calcium, and (c) iron at 335 K and 6 M hydrochloric acid with stirring speed of 300 rpm.

Figure 5. Effect of initial acid concentration on the dissolution of (a) aluminum, (b) calcium, and (c) iron at 325 K and S/L of 1/20 g/cm3 with stirring speed of 300 rpm.

and iron was investigated at 1.4 M HCl by varying temperature from 315 to 345 K. As presented in Figure 6a−c, the dissolution of aluminum, calcium, and iron was favored at higher temperature. The trends of reaction temperature effect were similar with 2.9 and 6.0 M HCl, which are not plotted here. 3.3. Dissolution Kinetics. The leaching of CFBC fly ash with HCl solution is a heterogeneous noncatalytic solid−liquid reaction. For describing the dissolution kinetics of such case of solids in liquids, the shrinking core model (SCM)22 is commonly adopted. Moreover, the semiempirical Avrami equation x = 1 − exp( −kt m)

neous reactions by Christian.28 Kabai29 used this model to correlate data for the batch dissolution of over 50 substances consisting of metal oxide and metal oxide hydrate, and he concluded that the rate constant k is dependent on the reaction temperature and the initial acid concentration as well. Various SCM equations were adopted to fit the experimental data in this work, and no satisfactory fit was obtained. However, the experimental data could be fitted successfully with the modified semiempirical Avrami equation: x /x∞ = 1 − exp( −kt m)

(6)

in which x∞ is the dissolution fraction of elements as t → ∝. By linearizing eq 6, it can be rewritten as

(5)

has been successful to explain the kinetic data of solid−liquid reaction for dissolution of minerals and drugs,23−26 where m is a phase-specific constant, k is the rate constant, t is the reaction time, and x is the dissolution fraction of solid. This equation was originally derived by Avrami27 for the kinetic modeling of a new phase nucleation and was later developed for heteroge-

ln[− ln(1 − x /x∞)] = m ln t + ln k

(7)

By applying eq 7 to the experimental data presented in Figures 5 and 6, dissolution rate parameters m and k were obtained. The results as well as the correlation coefficients are summarized in Tables 3 and 4, which show that each set of 18188



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Table 4. Dependence of Dissolution Rate Parameters on Temperature at the Initial Acid Concentration of 1.4 M elements

temperature (K)

m

k (min−m)

R2

aluminum

315 325 335 345 315 325 335 345 315 325 335 345

0.455 0.458 0.455 0.460 0.375 0.384 0.369 0.377 0.237 0.240 0.238 0.233

0.202 0.287 0.427 0.586 0.261 0.347 0.490 0.653 0.135 0.172 0.210 0.252

0.987 0.997 0.995 0.982 0.993 0.996 0.989 0.992 0.991 0.994 0.996 0.993

calcium

iron

not affected by operation parameters, which is in line with Kabai’s conclusion.29 Kabai pointed out that the parameter m was independent of temperature and initial acid concentration but was a function of substance being dissolved. The average value of parameter m for release aluminum, calcium, and iron elements from the calcined CFBC fly ash is 0.460, 0.378, and 0.234, respectively, and the derivation of m was ±5% about the average value for all sets of experimental data in this work. On the basis of the data in Table 3, the dependence of reaction rate constant k on the initial acid concentration CA,0 is plotted (Figure 7). The linear relationship between ln k and

Figure 6. Effect of reaction temperature on the dissolution of (a) aluminum, (b) calcium, and (c) iron using 1.4 M hydrochloric acid and S/L of 1/20 g/cm3 with stirring speed of 300 rpm. Figure 7. Dependence of reaction rate constant on the initial acid concentration.

Table 3. Dependence of Dissolution Rate Parameters on Initial Acid Concentration at a Temperature of 325 K elements

concentration (M)

m

k (min−m)

R2

aluminum

1.4 2.9 4.4 6.0 1.4 2.9 4.4 6.0 1.4 2.9 4.4 6.0

0.458 0.461 0.467 0.468 0.384 0.376 0.380 0.379 0.240 0.231 0.228 0.228

0.287 0.334 0.358 0.397 0.347 0.399 0.439 0.462 0.172 0.197 0.221 0.239

0.997 0.998 0.988 0.990 0.996 0.994 0.993 0.996 0.994 0.988 0.995 0.992

calcium

iron

ln(CA,0) indicates the reaction rate is directly related to the pH values of liquids. The slopes for aluminum, calcium, and iron were 0.214, 0.199, and 0.226, respectively, which were obtained by the linear regression analysis of curves in Figure 7. The activation energies were calculated from Arrhenius plots (Figure 8), and they are approximately 32, 28, and 19 kJ/mol for the dissolution of aluminum, calcium, and iron elements from fly ash, respectively. The apparent activation energy obtained here for dissolution of aluminum is close to the value of 33 kJ/mol reported for anorthite dissolution at pH = 2 by Fleer.30 The activation energy of iron dissolution are obviously different from those of aluminum and calcium, which again confirms that iron in the fly ash was not incorporated with aluminum and calcium into the same mineralogical phase. Moreover, it is difficult to selectively leach aluminum and calcium as they have very similar dissolution behavior, but it could selectively depress the iron dissolution fraction by reducing the leaching time and the initial acid concentration.

data can be mathematically represented by the semiempirical Avrami equation. As seen, the value of m remains almost constant and it is only related to the elements dissolved but is 18189



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ing to aluminum though they were mainly incorporated into the same mineralogical phase of anorthite. The dissolution fractions of aluminum, calcium, and iron increased, but that of silicon decreased with time, indicating the continuous silica precipitation during dissolution. SEM images in this study revealed that the dissolution of fly ash occurs primarily at active sites and not by uniform attack over the entire surface of the solids. It is believed that the internal diffusion of H+ into the unreacted active sites and the detachment of products from the active sites to the liquid were hindered by the leached ash layer, which may partly reduce the dissolution rate. The dissolution rate of aluminum, calcium, and iron ions from the calcined CFBC fly ash with hydrochloric acid were mainly influenced by the initial acid concentration and the reaction temperature. The dissolution kinetic data can be successfully explained by the semiempirical Avrami-type equation. The activation energies for dissolution of aluminum, calcium, and iron from fly ash with hydrochloric acid are 32, 28, and 19 kJ/mol, respectively. The obtained kinetic equations could predict dissolution rates of the CFBC fly ash with hydrochloric acid well.

Figure 8. Arrhenius plots of aluminum, calcium, and iron dissolution from fly ash by hydrochloric acid.

On the basis of the fitting results with eq 6, the dissolution rates of aluminum, calcium, and iron from the calcined fly ash by hydrochloric acid under the investigated conditions could be concluded as the following kinetic expressions: For Al xAl = xAl, ∞[1 − exp( −45722.8e−3906.9/ T CA,0 0.21t 0.46)]

■

(8)

For Ca

*E-mail: [email protected].

xCa = xCa, ∞[1 − exp(− 10731.3e−3363.3/ T CA,0 0.20t 0.38)]

Notes

The authors declare no competing financial interest.

(9)

■

For Fe x Fe = x Fe, ∞[1 − exp( −167.1e−2264.4/ T CA,0 0.23t 0.23)]

AUTHOR INFORMATION

Corresponding Author

ACKNOWLEDGMENTS This research is supported by the Fundamental Research Funds for the Central Universities of China (WB1213008) and the National High Technology Research and Development (863 Program 2011AA06A102).

(10)

where xAl,∞, xCa,∞, and xFe,∞ are the dissolution fractions of aluminum, calcium, and iron at t → ∝, and they are accordingly 72%, 88%, and 100% in this study. According to eqs 8−10, the fractions of aluminum, calcium, and iron dissolved in hydrochloric acid were calculated and compared to 180 sets of experimental data obtained over acid concentrations of 1.4−6 M, temperatures of 313−375 K, and solid/liquid ratios of 1/4−1/100 g/cm3 with stirring speed of 300 rpm. It was found that the calculated values according to the kinetic equations do not deviate by more than ±5.8% from the experimental results, indicating that the above kinetic equations could be used to predict batch dissolution behavior of aluminum, calcium, and iron from the CFBC fly ash with hydrochloric acid. Hulbert and Huff24 pointed out that the dissolution data should fit a nucleation equation, e.g., Avramitype equation, if the reaction process involves the following: (1) diffusion of hydronium ions to active sites, (2) the nucleation of products at active sites, and (3) hydration of products and diffusion of products into solution. SEM images in this study revealed that the dissolution of fly ash occurs primarily at active sites and not by uniform attack over the entire surface of the solids. It was commonly considered by previous investigators17,31 that the surface precursor complex was formed at the active sites with adsorbed hydrogen ions during dissolution of alumino-silicate minerals. The formation of precursor complex and the in situ silica precipitation at the active sites might be the main reason why the Avrami equation could successfully model the dissolution data in this work.

■

REFERENCES

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4. CONCLUSION In a batch leaching of the calcined CFBC fly ash with hydrochloric acid, calcium was preferentially released compar18190



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Dissolution Kinetics of Aluminum, Calcium, and Iron from Circulating

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