Xiaoying Lia,
Bo Hubc,
Nengsheng Liu
*a,
Xueqing Liubc,
Chengwei Liuc,
Xintao Hec and
Sufang Hebc
aFaculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China. E-mail: liunengshengms@163.com
bResearch Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China
cFaculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
First published on 21st November 2022
The recycling of aluminum is commonly an important step to achieve the high value-added utilization of fly ash, which is a kind of solid waste generated from coal-fired power plants. In this study, high-alumina fly ash was efficiently activated by ammonium sulfate method and the alumina was efficiently extracted. The effects of roasting temperature, roasting time, and ammonium sulfate/high-alumina fly ash mass ratio on the leaching rate of alumina were fully analyzed, and the roasting kinetics and reaction mechanism in the roasting process were discussed. The experimental results showed that the leaching rate of alumina in the roasted material achieved 93.57% with the roasting temperature of 673 K, the roasting time of 60 min, and the mass ratio of ammonium sulfate to high-alumina fly ash of 6:
1. The roasting kinetics showed that the reaction between high-alumina fly ash and ammonium sulfate was controlled by internal diffusion, the apparent activation energy was 37.40 kJ mol−1, which accorded with the reaction kinetic equation 1 − 2x/3 − (1 − x)2/3 = 2.9546
exp[−37
400/(RT)]t. The reaction mechanism showed that the aluminum and structural damage mullite in the high-alumina fly ash reacted with molten ammonium sulfate to form (NH4)3Al(SO4)3 and NH4Al(SO4)2. Finally, (NH4)3Al(SO4)3 was transformed into NH4Al(SO4)2 with the increase of temperature.
It is worth mentioning in particular that there is a special kind of fly ash, which comes from Shanxi and the middle and west of Inner Mongolia with the abundant aluminum content up to 40–60%,19 called high-alumina fly ash.20,21 Due to the aluminum content in high-alumina fly ash is comparable to a medium grade bauxite, so it can be used as a potential raw material to replace bauxite and recover alumina, alleviate the shortage of bauxite resources in China, and reduce the environmental pollution caused by fly ash accumulation.19 However, aluminum in high-alumina fly ash mainly exists in the form of stable mullite, leading to the difficulty for the direct extraction of aluminum.22 Therefore, the key to the utilization of high-alumina fly ash is to destroy the structure of mullite through activation.
There are many methods about fly ash activation, such as acid method, alkali method, thermal activation by calcination, sodium hydrogen sulfate roasting method,23 thermal activation by potassium bisulfate,24 potassium pyrosulfate calcination activation method,19 etc. The traditional and effective activation method of high-alumina fly ash is alkali fusion method, which means that high-alumina fly ash is mixed with solid alkali such as NaOH, KOH, NaCO3, Na2SO4 or K2SiO3 (ref. 5) evenly and roasted at high temperature to achieve the purpose of activation.25,26 However, the process of alkali fusion is usually high cost, high temperature, high energy consumption and low activation efficiency.27 Therefore, in terms of environment and economy, it is necessary to find a new activator to further reduce energy consumption.
Ammonium sulfate method is a newly developed activation technology in recent years,28,29 which can produce alumina products with high purity and has high commercial value. Especially, the activation process of ammonium sulfate, with low cost and recyclable tail gas, does not involve strong acid, strong alkali or strong corrosive agent and is less corrosive to the equipment, so there is a broad industrial application prospect with its unique advantages.30,31 Briefly speaking, the ammonium sulfate method involves the reaction of ammonium sulfate with high alumina fly ash in the temperature range of 300–400 °C to produce soluble aluminum sulfate, which converts mullite, an inert component of high alumina fly ash, into silica and soluble aluminum salts, thus facilitating the extraction of aluminum and silica elements and the preparation of silica materials.31 For instance, Li et al.31 conducted three times recrystallization of ammonium aluminum sulfate solutions obtained by this method to obtain high-purity alumina with a purity of more than 99.9%. Park32 extracted high concentration alumina from fly ash by ammonium sulfate low-temperature sintering method, the extraction rate of aluminum can reach 94.36%. Li33 studied the process of extracting alumina from fly ash by ammonium sulfate, and the recovery rate was 95.6%. Frederic J. Doucet et al.34 studied the feasibility of recovering aluminum from ammonium sulfate and found that 95% of aluminum could be recovered at 1 h, 500 °C, with a 2:
6 weight ratio of fly ash to ammonium sulfate. Their results indicated that ammonium sulfate method was a good activation method and the study of it was very necessary.
At present, most researches on ammonium sulfate activation of high-alumina fly ash were mainly focused on the optimization of experimental process parameters to efficiently extract alumina, the in-depth studies on roasting kinetics and reaction mechanism in the roasting process, which is important to the efficient activation and comprehensive utilization of high-alumina fly ash, have been less addressed. Therefore, in order to promote further development of alumina extraction from high-alumina fly ash via the ammonium sulfate method, this method was first used to treat the raw material high-alumina fly ash from a power plant in Inner Mongolia, China. On the basis of studying the effect of process parameters on the extraction rate of alumina from high-alumina fly ash, the thermodynamics and kinetics, as well as associated factors in roasting process were examined, and the reaction mechanism was deduced. This study aims to reveal the reaction mechanism of ammonium sulfate activation high-alumina fly ash, optimize the roasting parameters and kinetic parameters, and provide certain theoretical guidance for the efficient activation and comprehensive utilization of high-alumina fly ash.
The chemical compositions (mass fraction) of raw fly ash were measured by XRF and infrared carbon and sulfur analyzer, and the results were presented in Table 1. The main components were SiO2 and Al2O3, with the contents were 37.40% and 46.22%, respectively. Obviously, it is a kind of high-alumina fly ash. As well, there were a little amount of Fe2O3 (3.05%), CaO (5.17%), TiO2 (3.51%) and C (2.54%) in this sample.
Component | SiO2 | Al2O3 | Fe2O3 | CaO | TiO2 | C |
---|---|---|---|---|---|---|
Content/wt% | 37.40 | 46.22 | 3.05 | 5.17 | 3.51 | 2.54 |
In addition, the phase compositions of raw high-alumina fly ash were studied by XRD and the results were shown in Fig. 1(A). According to PDF#10-0173, PDF#46, PDF#15-0076 and previous literature,30,37,38 the diffraction peaks at 16.44°, 23.99°, 25.99°, 31.40°, 33.16°, 37.37°, 39.39°, 40.80°, 42.43°, 54.01°, 60.74°, 62.55°, 64.70°, 70.95°, 74.94°, 84.87° and 88.65° were assigned to mullite (Al6Si2O13); while the peaks at 19.72°, 26.60°, 36.08°, 39.39°, 45.66°, 50.10°, 54.96°, 67.12° and 81.85° were corresponded to amorphous SiO2; in addition, Al2O3 phase could be also found at peaks of 25.42°, 35.53°, 43.33°, 52.23°, and 57.48°. The particle size distribution of high alumina fly ash in Fig. 1(B) showed that the maximum particle size was 30 μm, and more than 98% of the material diameter was distributed in the range of 0–60 μm, and the material size was relatively fine. The surface morphology of high-alumina fly ash was characterized by SEM, as shown in Fig. 2. Fig. 2(A) was obtained at 8k magnification, it can be seen that high-alumina fly ash was composed of a large number of spherical, spherical particles, and rod-like particles.30 Fig. 2(B) was shot at the magnification of 150k, which further showed that the spherical particles in Fig. 2(A) were smooth and dense. It had a stable structure with a particle size of about 0.9 μm.
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Fig. 2 SEM image of raw high-alumina fly ash. (A) Low magnification image and (B) high magnification image. |
As mentioned previously, the optimum roasting conditions for using ammonium sulfate to roast and activate high-alumina fly ash were as follows: the roasting temperature was 673 K, the roasting time was 60 min, and mass ratio of ammonium sulfate to high-alumina fly ash was 6:
1. Under these conditions, the leaching rate of aluminum reached 93.57%. According to previous literature (Table 2),30,40,41 the leaching rate of aluminum in this study was higher than most of them, which evidenced that this study was essential. It can be seen from the Fig. 3 that the roasting temperature had a great influence on the extraction rate of aluminum. Next, the roasting kinetics was discussed.
Equations | Equations: g(x) = k(T)t | Reaction mechanisms |
---|---|---|
a Where x is the extraction rate of aluminum; k is the apparent rate constant; t is the reaction time. | ||
F1(x) | x = k1t | External diffusion |
F2(x) | 1 − (1 − x)1/3 = k2t | Chemical reaction |
F3(x) | 1 − 2x/3 − (1 − x)2/3 = k3t | Internal diffusion |
Time/min−1 | Temperature/K | F1(x) | F2(x) | F3(x) |
---|---|---|---|---|
0–60 | 613 | 0.95048 | 0.98879 | 0.99108 |
0–60 | 633 | 0.94293 | 0.98608 | 0.98720 |
0–60 | 653 | 0.94477 | 0.99281 | 0.99476 |
0–40 | 673 | 0.90794 | 0.98026 | 0.99109 |
The simulated data of the most probably roasting kinetics were shown in Fig. 4(D). The apparent reaction rate constant k was obtained by calculating the slope of the reaction models, the values of R2 were obtained by linear fitting, and the values of k and R2 were shown in Table 5. It can be seen from Table 5 that the apparent reaction rate constant k increased from 0.00193 to 0.00374 when the thermodynamic temperature was increased from 613 K to 673 K, indicating that the thermodynamic temperature had a great influence on apparent reaction rate constant, and the increase of the thermodynamic temperature could accelerate apparent reaction rate constant.
Temperature (K) | Apparent reaction rate constant k (s−1) | Linear correlation coefficient R2 |
---|---|---|
613 | 0.00193 | 0.99108 |
633 | 0.00243 | 0.98720 |
653 | 0.00296 | 0.99476 |
673 | 0.00374 | 0.99109 |
According to the Arrhenius formula lnk = ln
A − Ea/RT (where k is the rate constant; A is the frequency factor; Ea is the apparent activation energy, J mol−1; R is the mole gas constant (8.314 J mol−1 K−1) and T is the thermodynamic temperature), ln
k was used to regress T−1 to explore the relationship between chemical reaction rate constant and temperature, and the results were shown in Fig. 5. It was very obvious that ln
k had a better linear relationship with T−1 (the R2 is close to 1 with a value of 0.99803). Combining ln
k = ln
A − Ea/RT with y = −4.49792x + 1.08335 in Fig. 5, the values of apparent activation energy Ea and A were obtained with 37.40 kJ mol−1 and 2.9546, respectively. There were some differences compared to the literature,31 which was normal because of different raw materials, different chemical reagents and different chemical reactions. Bringing the experimental data into 1 − 2x/3 − (1 − x)2/3 = kt, the kinetic equation of high-alumina fly ash–ammonium sulfate system in the roasting activation process was obtained: 1 − 2x/3 − (1 − x)2/3 = 2.9546
exp[−37
400/(RT)]t.
The possible chemical reactions in the roasting process are listed below:48,49
6(NH4)2SO4 + Al2O3 = 2(NH4)3Al(SO4)3 + 6NH3↑ + 3H2O↑ | (1) |
18(NH4)2SO4 + Al6Si2O13 = 6(NH4)3Al(SO4)3 + 18NH3↑ + 9H2O↑ + 2SiO2↑ | (2) |
(NH4)2SO4 = NH4HSO4 + NH3↑ | (3) |
Al6Si2O13 + 12(NH4)2SO4 = 6NH4Al(SO4)2 + 2SiO2 + 18NH3↑ + 9H2O↑ | (4) |
Al2O3 + 4(NH4)2SO4 = 2NH4Al(SO4)2 + 6NH3↑ + 3H2O↑ | (5) |
Fe2O3 + 3(NH4)2SO4 = Fe2(SO4)3↑ + 6NH3↑ + 3H2O↑ | (6) |
(NH4)2SO4 = 2NH3↑ + H2O↑ + SO3↑ | (7) |
4(NH4)3Al(SO4)3 + Al2O3 = 61NH4Al(SO4)2 + 6NH3↑ + 3H2O↑ | (8) |
NH4HSO4 = NH3↑ + H2O↑ + SO3↑ | (9) |
By roasting, alumina in high-alumina fly ash can be converted to soluble (NH4)3Al(SO4)3 and NH4Al(SO4)2 by the reaction between molten ammonium sulfate and alumina and damaged mullite.
The high-alumina fly ash–ammonium sulfate mixtures were roasted and activated at different temperatures (573–693 K) with the mass ratio of ammonium sulfate to high-alumina fly ash of 6:
1, roasting time of 60 min. And the phase transformations of the mixtures were investigated by XRD. The XRD comparison charts of the high-alumina fly ash–ammonium sulfate mixtures were shown in Fig. 6, the main XRD peaks for different materials are listed in Table 6.31,38,50 Obviously, the main phases of the sample roasted at 573 K were found to be (NH4)2SO4 and (NH4)3Al(SO4)3, and there was a small amount of NH4HSO4, indicating that the molten ammonium sulfate had reacted with alumina and the damaged mullite to form ammonium aluminum sulfate, so the reactions (1)–(3) were verified.31 At the same time, the extraction rate of aluminum in the roasted material was 49%, which indicated that the reaction degree was limited. After roasted at 633 K, a new phase –NH4Al(SO4)2– were observed in the sample, so the reactions (4) and (5) began to happen under this condition.31,48,50 Simultaneously, the extraction rate of aluminum in the roasted material was 86.82%, indicating that a large number of aluminum-containing materials in high-alumina fly ash had reacted with ammonium sulfate to form soluble ammonium aluminum sulfate. In addition, the main phases were NH4HSO4 and NH4Al(SO4)2 in the sample at 673 K. Compared with 633 K, the peaks of NH4HSO4 and NH4Al(SO4)2 continued to exist and were enhanced, but the characteristic peaks of (NH4)2SO4 and (NH4)3Al(SO4)3 disappeared which due to the reactions (1)–(8) were over, respectively.48,50,51 At this time, the extraction rate of aluminum in the roasted material reached 93.57%, the reaction between high-alumina fly ash and ammonium sulfate was relatively complete, achieving high efficiently activate high-alumina fly ash. As the roasting temperature continued to rise to 693 K, the characteristic peak of NH4HSO4 disappeared, the characteristic peak of NH4Al(SO4)2 continued to exist, and the extraction rate of aluminum in the roasted material under this condition was 90.14%. Compared to 673 K, the extraction rate was decreased, which may be related to the decomposition of ammonium salt in the raw material and the relatively low mass ratio of ammonium sulfate to high-alumina fly ash in the material,3 reaction (9) was finished.48,50,51
Temperature | Main phase | 2theta (°) | ||||
---|---|---|---|---|---|---|
573 K | (NH4)2SO4 | 16.76 | 20.52 | 22.94 | 29.32 | 38.84 |
(NH4)3Al(SO4)3 | 11.78 | 26.62 | — | — | — | |
NH4HSO4 | 17.94 | 23.58 | — | — | — | |
633 K | (NH4)2SO4 | 17.86 | — | — | — | — |
(NH4)3Al(SO4)3 | 11.78 | 20.62 | 26.62 | — | — | |
NH4Al(SO4)2 | 10.64 | 24.14 | 30.52 | 34.28 | 37.88 | |
NH4HSO4 | 18.68 | — | — | — | — | |
673 K | NH4Al(SO4)2 | 10.72 | 24.22 | 30.62 | 37.92 | 43.94 |
49.46 | 55.64 | 60.64 | 63.94 | 68.46 | ||
NH4HSO4 | 18.78 | 22.84 | — | — | — | |
693 K | NH4Al(SO4)2 | 10.64 | 24.12 | 30.54 | 37.86 | 43.86 |
49.72 | 55.52 | 60.56 | 63.88 | 68.38 |
The chemical compositions of the roasted sample and the acid leached residue were measured by XRF and were shown in Table 7. Sample 1 was roasted at a roasting temperature of 673 K, roasting time of 60 min, and a mass ratio of 6:
1 of ammonium sulfate to high alumina fly ash, and the total amount of elements measured by XRF was 67.5% less than 100%, which was because nitrogen and hydrogen could not be measured. The main elements were S (31.1043%), O (27.0454%), Al (5.0143%), Si (2.3086%), with higher S, O content, which corresponded to the NH4HSO4 and NH4Al(SO4)2 phases in XRD (Fig. 6 – 673 K). Sample 2 was 0.71 g residue obtained from 10 g sample 1 after acid leaching, the total amount of elements measured by XRF was 85.7%, the main elements were Si (33.6134%), O (44.6783%), Al (6.0732%), the content of Si and O was high, which corresponded to the SiO2 phase in XRD (Fig. 7), while Al was the percentage of 0.71 g residue, so the amount of residual Al amount was very small and it was calculated that 91.4% of Al was extracted by XRF. Fig. 7 showed the XRD pattern of the residue after roasting with ammonium sulfate high alumina fly ash with a mass ratio of 6
:
1 at 673 K for 60 min and leaching with 10% volume fraction of sulfuric acid at 90 °C for 60 min at a liquid to solid ratio of 10
:
1. Only amorphous silica was detected in the residue, and other substances such as mullite and aluminum-containing compounds were not detected, showing that NH4HSO4 and NH4Al(SO4)2 generated during roasting were completely dissolved in sulfuric acid solution, and the aluminum in high-alumina fly ash was leached out by the acid, further confirming that high-alumina fly ash can be activated efficiently by ammonium sulfate.
Sample | O | Al | Si | P | S | K | Ca | Ti | Fe | Zn | Sr | Y | Zr |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | 27.0454 | 5.0143 | 2.3086 | — | 31.1043 | 0.0668 | 0.7211 | 0.5349 | 0.4616 | 0.1473 | 0.0334 | — | 0.0290 |
2 | 44.6783 | 6.0732 | 33.6134 | 0.1143 | 0.2510 | 0.1227 | 0.0887 | 0.4048 | 0.2348 | — | 0.0246 | 0.0046 | 0.0529 |
Based on the analysis of XRD patterns, the reaction path of the high-alumina fly ash–ammonium sulfate mixtures under the experimental conditions was obtained, as shown schematically in Fig. 8.
In the condition of the temperatures of 573 K and 633 K, and duration of 60 min, the SEM images of high-alumina fly ash–ammonium sulfate mixtures were shown in Fig. 9. Fig. 9(A) showed that the particle structure of the mixture was similar, and there was agglomeration between particles. Fig. 9(B) showed that the material appearance and morphology further demonstrated this melt aggregation phenomenon, however, the degree of melt aggregation of the material was low. The results confirmed that the mass transfer rate of high-alumina fly ash ammonium sulfate was accelerated and the occurrence of reactions was promoted by melt polymerization at 573 K, the reactions (1)–(3) were confirmed again. Correspondingly, the XRD patterns of the activated materials showed that the main phases of the activated materials were ammonium sulfate and ammonium aluminum sulfate. Fig. 9(C) showed that the outer surface of each particle was similar and whiskers were formed. Fig. 9(D) further confirmed the appearance of crystal morphology. Correspondingly, XRD patterns of activated materials showed that the main phases of activated materials were (NH4)2SO4, (NH4)3Al(SO4)3, NH4Al(SO4)2 and NH4HSO4, indicating that a large number of aluminum-bearing materials in high-alumina fly ash had reacted with ammonium sulfate to form soluble ammonium aluminum sulfate, which confirmed the occurrence of reactions (1)–(5) again.
The above XRD and SEM analysis were complementary and revealed the mechanism of the activation process.
Fig. 10 was the reaction mechanism diagram. It can be seen that at 298 K, the reaction did not occur because the temperature range of roasting activation reaction was not reached. With the increase of temperature, ammonium sulfate reached the melting point and became a molten state. At 573 K, the molten ammonium sulfate reacted with the structurally damaged mullite to form (NH4)3Al(SO4)3, coinciding with reactions (1) and (2), and some ammonium sulfate was decomposed into NH4HSO4, such as reaction (3). At 633 K, the reactions (1)–(3) were enhanced and a new phase NH4Al(SO4)2 was formed, coinciding with reactions (4) and (5). At 673 K, (NH4)2SO4 disappeared, indicating that the reactions (1)–(7) were ended. Moreover, (NH4)3Al(SO4)3 had completely disappeared and all transformed into NH4Al(SO4)2, reaction (8) was completed. At 693 K, it can be found that only NH4Al(SO4)2 was observed, and NH4HSO4 was completely disappeared, reaction (9) was over. At the same time, the further decomposition of ammonium salt in the raw material produced ammonia, sulfur trioxide and other gases, which made the effective mass ratio of ammonium sulfate to high-alumina fly ash in the material decrease, and resulted in the whole reaction degree and the leaching rate of aluminum were decreased.
In brief, the amorphous silicon dioxide contained in high-alumina fly ash was unchanged during the whole reaction process, indicating that the reaction of high-alumina fly ash–ammonium sulfate system was mainly the reaction between molten ammonium sulfate and structurally damaged mullite.
(1) The main components of high-alumina fly ash were mullite (Al6Si2O13) and amorphous SiO2, as well as impurity oxide Fe2O3. It was found that temperature had a great influence on the effect of high-alumina fly ash activation. The highest aluminum extraction rate of 93.57% was achieved when ammonium sulfate was roasted with high alumina fly ash at a mass ratio of 6:
1 at 673 K for 60 min.
(2) The roasting activation process of high alumina fly ash–ammonium sulfate system was a liquid–solid reaction process and controlled by internal diffusion, with an apparent activation energy of 37.40 kJ mol−1. The kinetic equation was: 1 − 2x/3 − (1 − x)2/3 = 2.9546exp[−37
400/(RT)]t.
(3) The phase transformation and morphology of high-alumina fly ash–ammonium sulfate system roasted at different temperatures were thoroughly investigated by XRD and SEM. The alumina and structurally damaged mullite in high-alumina fly ash can react with the molten ammonium sulfate. At 573 K, the ammonium sulfate salt was (NH4)3Al(SO4)3; at 633 K, the ammonium sulfate salts were mainly (NH4)3Al(SO4)3 and NH4Al(SO4)2; at 673 K and 693 K, the ammonium sulfate salt was NH4Al(SO4)2. In the reaction process, the appearance morphology of the material agglomerated, which enhanced the mass transfer process and promoted the occurrence of reactions.
(4) Based on these investigations and analysis, the reaction mechanism of high-alumina fly ash–ammonium sulfate mixtures was mainly the reaction between molten ammonium sulfate and structurally damaged mullite.
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