Advances in Materials Science and Engineering
Volume 2016 (2016), Article ID 9817969, 9 pages
http://dx.doi.org/10.1155/2016/9817969

Research Article

Sulfur Fixation by Chemically Modified Red Mud Samples Containing Inorganic Additives: A Parametric Study

Yang Liu, Yang Li, Feng-shan Zhou, Ying-mo Hu, and Yi-he Zhang

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China

Received 23 January 2016; Revised 8 June 2016; Accepted 17 July 2016

Academic Editor: Santiago Garcia-Granda

Copyright © 2016 Yang Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Sulfur retention ability of Bayer red mud from alumina plant was investigated. Bayer red mud modified by fusel salt and waste mother liquor of sodium ferrocyanide as the main sulfur fixation agent and the calcium based natural mineral materials as servicing additives; the experimental results showed the following: (1) Through 10 wt% waste mother liquor of sodium ferrocyanide modifying Bayer red mud, sulfur fixation rate can increase by 13 wt%. (2) Magnesium oxide can obviously improve the sulfur fixation performance of Bayer red mud and up to a maximum sulfur fixation rate of 47 wt% at adding 1 wt% magnesium oxide. (3) Dolomite enhanced the sulfur fixation performances with the sulfur fixation rate of 68 wt% in optimized condition. (4) Vermiculite dust reduced sulfur dioxide during the fixed-sulfur process of modified Bayer red mud, and the desulphurization ration could reach up to a maximum 76 wt% at 950°C. (5) An advanced three-component sulfur fixation agent was investigated, in which the optimized mass ratio of modified Bayer red mud, dolomite, and vermiculite dust was 70 : 28 : 2 in order, and its sulfur fixation efficiency has reached to a maximum 87 wt% under its 20 wt% dosage in the coal.

1. Introduction

Since the 1980s, with the rapid development of China’s economy, the annual consumption of coal has been increasing by a large margin and China has become the largest producer and consumer of coal [1]. The main pollution of China is derived from emissions of SO₂ and dust caused by coal combustion. Therefore, the control of SO₂ emitted from industrial coal-fired boilers has great significance for air pollution control in our country [2]. Desulfurization technology is the main method to reduce SO₂ pollution [3]. At present, the commonly used desulphurization technology includes desulfurization before combustion, sulfur fixation during combustion, and flue gas desulfurization [4]. Developing a new type of sulfur fixing agents has become the most valuable industrial desulfurization technical way in coal combustion process [5]. At present, the mainly used sulfur-fixing agents at home and abroad include calcium-based, magnesium-based, sodium-based, and potassium-based sulfur-fixing agents [6–9]. Traditional calcium-based sulfur-fixing agent (limestone, dolomite, etc.) generally showed the disadvantages of low calcium utilization rate and low sulfur fixation rate at high temperature [10].

In recent years, many domestic and foreign researchers found that the sulfur-fixing efficiency of coal combustion could be improved by modifying the fixing agents using sodium chloride [11], ethanol, and acetic acid [12] or adding Fe₂O₃, SiO₂, Al₂O₃, and Na₂CO₃ [13–18] to the sulfur-fixing agents.

In addition to the researches of sulfur-fixing additive, some industrial solid waste is also used as desulfurization agents such as Carbide slag, black liquor of papermaking, and red mud [19–26]. Bayer red mud is an industrial solid waste with high alkalinity and environmental impact, which is discharged during the process of Al₂O₃ extraction from bauxite in alumina plant. Disposal of Bayer red mud occupies large areas of productive land, and it is harmful to the environment due to its high alkalinity, heavy metals, and sometimes radioactivity [27–29]. Red mud is mainly composed of fine particles including Al₂O₃, Fe₂O₃, CaO, MgO, and Na₂O [19, 20]. Owing to its high alkalinity and chemical properties, Bayer red mud could be used as a kind of sulfur-fixing agent. This not only opens up a new way for the utilization of red mud but also finds a cheap and efficient sulfur-fixing agent for the coal industry.

In recent years, many domestic and foreign scholars focused their research on red mud slurry for flue gas desulfurization [21, 22]. But the red mud slurry is easy to harden and not conducive for long time use [23]. By adding natural minerals to Bayer red mud, a high performance synergistic sulfur-fixing agent was developed for sulfur fixation during coal combustion in the present work. According to the sulfur-fixing product of sulfur-fixing agent, we will continue to study the preparation of oilfield waste drilling fluid treatment agent using the sulfur-fixing product, which will truly achieve high value-added and clean utilization of red mud.

2. Materials and Methods

2.1. Materials

Raw Coal. Raw coal came from Zouping Weimian Corporation (Shandong, China). The total sulfur content in the coal sample was determined by Eschka method [30]. 0.2 g coal (200 mesh) was mixed with Eschka reagent and burned. The sulfur in the coal reacted with Eschka reagent to produce sulfate. Then, sulfate ions () reacted with barium ions (Ba²⁺) to produce barium sulfate (BaSO₄). The total sulfur content was calculated according to the produced weight of BaSO₄. Experimental results showed that the total sulfur content of the raw coal was 1.685 wt%. According to the classification standard of sulfur coal [31], the raw coal belongs to the middle sulfur coal.

Bayer Red Mud. The red mud in the present work in the form of dried clay was obtained from Zhengzhou Great Wall Alumina Corporation (Henan Province, China). Table 1 shows the component contents of the red mud. The XRD pattern and its particle size distribution of the red mud sample are shown in Figures 1 and 2. The particle size of the red mud was in the range of 0.1–500 mm (generally finer than 200 mesh after drying and grinding). The mineral compositions of the Bayer red mud were very complicated [32], but the content of Al₂O₃ and SiO₂ was relatively high with 24.35 wt% and 18.12 wt%.

Table 1: Chemical composition of Bayer red mud in the present work.

Components
Al₂O₃
CaO
SiO₂
Fe₂O₃
Na₂O
MgO
Water
Loss

Content/wt%
24.35
19.96
18.12
7.79
5.54
1.47
0.87
14.25

Figure 1: XRD of Bayer red mud in the present work.

Figure 2: Particle size distribution of Bayer red mud in the present work.

Modification Agents. Modification agents were the byproduct of yellow blood sodium (sodium ferrocyanide) solution from Hebei Chengxin Co., Ltd. (Hebei, China), and fusel alcohol salts from Changchun Dacheng Industry Company (Jilin, China). The chemical composition was shown in Tables 2 and 3.

Table 2: Chemical composition of waste mother liquor of sodium ferrocyanide.

Components
Na₂Fe(CN)₆
HCOONa
Na₂CO₃
Water

Content/wt%
5.29
39.83
5.27
49.31

Table 3: Chemical composition of the fusel salt.

Components
Ethylene glycol, propylene glycol, butyl glycol, glycerin, and so forth
Salt mix (sodium formate, sodium acetate and sodium lactate, etc.)
Polyether polyols (low polymerization degree)
Water

Content/wt%
35~45
20~30
25~35
<1

Sulfur Fixation Additives. Magnesium oxide (MgO, industrial grade, purity more than 95 wt%) was obtained from Beijing Chemical Plant. The natural mineral materials brucite and dolomite were obtained from Dandong Yilong Gaoke New Material Co., Ltd. (Liaoning, China); both of their particle sizes were in the range of 800–1000 mesh.

Sulfur Fixation Promoter. Byproduct dust of vermiculite obtained from Hebei Chenxiang Mineral Materials Research Institute (Hebei, China) has been designated as promoter; its particle size was in the range of 60–200 mesh.

2.2. Laboratory Instruments

The pH of red mud suspension in the reaction process was measured by pH500 meter. The chemical analytical methods of Al₂O₃, CaO, and MgO were Karl Fischer titration, SiO₂ by gravimetry, and Fe₂O₃ by colorimetry and K₂O/Na₂O was determined by FP-640 Flame Photometer. The XRD patterns of the samples were determined by X-ray diffractometer with a Cu Kα (0.15418 nm) radiation source in a 2θ range of 10°–70° at a scanning rate of 2° min⁻¹. The morphologies of red mud were analyzed by Hitachi S-4800 high resolution ice emission scanning electron microscopy (SEM) [33].

Both of the SK-G10123K Tube furnace and the SK-G18123 Muffle furnace were purchased from Zhonghuan Experiment Electric Furnace Co., Ltd. (Tianjin, China). Heating rate of the SK-G10123K Tube furnace was 5°C/min at below 500°C, 10°C/min at 500–800°C, 5°C/min at 800–1000°C, and less than 2°C/min at 1000–1200°C. The length of its heating zone was 420 mm, and its constant temperature zone was 200 mm. Muffle furnace SK-G18123 Model with resistance wire heating was used; its heating process was 100°C to the maximum 1200°C in 40 min.

2.3. Experimental Methods

2.3.1. Modification Experiment of Bayer Red Mud

The Bayer red mud was immersed in modification agents with certain concentration for 24 h. Then, it was taken out and dried in an oven at constant temperature of 80°C for 12 h. Finally, it was taken out from the oven, milled, and screened as a reserve. Particle size distribution of original and modified Bayer red mud is shown in Figure 2.

2.3.2. Sulfur Fixation Experiment

Sulfur fixation for combustion coal was carried out in the SK-G10123K Tube furnace. The furnace was heated to 950°C according to its heating rate procedure firstly; then the specified amount of coal sample mixed sulfur fixation agents beforehand was loaded in a combustion boat, the coal sample was pushed slowly to the furnace inside, and finally air was pumped into the furnace to make a full combustion of the coal sample.

2.3.3. The Calculation of Sulfur Fixation Rate and the Utilization Rate of Calcium

The sulfur content was determined by using chemical analysis method. The sulfur fixation efficiency (see (1)) and the utilization rate of calcium (see (2)) [26, 34] were determined by testing the total sulfur content in coal ash and the coal sample:

where is sulfur fixation efficiency; is experimental coal sample mass, g; is total mass of ash after combustion, g; is the sulfur content of ash, wt%; and is total sulfur content of experimental coal sample, wt%. Consider

where is utilization rate of calcium, wt%; is total mass of ash after combustion, g; is the sulfur content of ash, wt%; is the mass of adding sulfur fixation agent in the experiment, g; is percentage content of CaO in the sulfur fixation agent, wt%.

3. Results and Discussion

3.1. Influence of Ca/S Mole Ratio on the Sulfur Fixation Effect

The Ca/S mole ratio is one of the main factors that affect the sulfur fixation effect. Using Bayer red mud as the sulfur fixation agent, the influence of Ca/S mole ratio on the sulfur fixation efficiency was studied. The results were shown in Figure 3.

Figure 3: Influence of Ca/S mole ratio on the sulfur fixation effect.

Figure 3 indicated that the sulfur fixation rate increased with the increase of the Ca/S mole ratio when Bayer red mud was used as sulfur fixation agent. However, the increase of the Ca/S mole ratio would greatly increase the amount of sulfur fixation agent and reduce the heating rate of the coal. So, the doping content of Bayer red mud in the following experiment was optimized to be 10 wt% (the mole ratio of Ca/S was 0.7).

3.2. The Influence of Modification on the Sulfur Fixation Effect

3.2.1. The Influence of Modification on the Sulfur Fixation Efficiency

Experimental results indicated that the maximum sulfur fixation efficiency of original Bayer red mud was only 42.83 wt% when the mass dosage of red mud as sulfur fixation agent was 20 wt% coal sample. Modification activation method was used to improve the sulfur fixation efficiency of red mud. Fusel salt and waste mother liquor of sodium ferrocyanide were used as modification agents. The influence of different amount wt% of fusel salt and waste mother liquor of sodium ferrocyanide on the sulfur fixation efficiency of Bayer red mud was studied.

The results of Tables 4 and 5 and Figure 4 indicated that the sulfur fixation efficiency could increase by 7 percentage points after the Bayer red mud was modified by 50 wt% fusel solution. Meanwhile, the utilization efficiency also increased from 22.02 wt% to 31.61 wt%. But the red mud after fusel modification was not easy to be dried and crushed and the mixing was not uniform. For the 10 wt% sodium ferrocyanide modified Bayer red mud, the sulfur fixation efficiency could increase by 13 percentage points and the utilization efficiency increased to 40.10 wt%.

Table 4: Effect of the fusel salt on sulfur fixation of modified Bayer red mud.

Amount/wt%
0
10
30
50

Sulfur content in the residue/wt%
0.667
0.877
1.171
1.684

Utilization rate of Ca/wt%
10.20
16.20
21.95
31.61

Sulfur fixation rate/wt%
8.333
12.06
17.87
23.54

Table 5: Effect of the waste mother liquor of sodium ferrocyanide on sulfur fixation of modified Bayer red mud.

Amount/wt%
0
10
20
30

Sulfur content in the residue/wt%
0.667
1.876
1.209
1.224

Utilization rate of Ca/wt%
10.20
40.10
26.43
23.64

Sulfur fixation rate/wt%
8.333
29.85
20.58
18.48

(a) Influence on the sulfur fixation rate

(b) Influence on the utilization rate of Ca

Figure 4: Test results for modified Bayer red mud with different solutions.

3.2.2. The Influence of Modification on the Surface Morphology of Bayer Red Mud

The results of Scanning Electronic Micrography (SEM) indicated that the morphology of the Bayer red mud modified by fusel salt and sodium ferrocyanide changed after calcination at 950°C (Figure 5). Nonsignificant structural change was observed in the three samples in Figure 5, but the sodium ferrocyanide modified sample showed a relatively more change in the formation of porous structure. Agglomeration was still noticeable in unmodified and modified red mud. The particles of unmodified Bayer red mud agglomerated more seriously, which was not conducive to the diffusion of SO₂ and the sulfur fixation reaction. In contrast, the secondary pore structure in the modified sample was beneficial to the diffusion of the gases and promoted the sulfur fixation reaction. It was possible that the specific surface area (SSA) and porosity increased the fixation degree. Figure 6 shows the secondary porous structure after sulfur fixation reaction of modified red mud; the SSA has indeed increased (Table 6). As predecessors had proposed that the influence of pore structure characteristics on their sulfur fixation performance was very important, little pores made a significant contribution to alkaline substance utilization in the initial stage of reaction and at lower temperature, while bigger pores took effect in the later stage and at higher temperature. The more the pore in range of effective aperture, the better the sulfur fixation performance [35].

Table 6: Specific surface area (SSA) of modified Bayer red mud before and after sulfur fixation reaction.

Sample
Specific surface area
with BET method (m²/g)

Original Bayer red mud
14.100

Bayer red mud modified by 50 wt% fusel solution
99.086

Bayer red mud modified by 10 wt% sodium ferrocyanide solution
142.300

Sulfur fixation ashes formed at 950°C after the sulfur fixation reaction of three-component sulfur fixation agent SFG-2
176.179

(a) Unmodified

(b) Modified by 50 wt% fusel solution

(c) Modified by 10 wt% sodium ferrocyanide solution

Figure 5: SEM photographs of modified Bayer red mud with different solutions.

(a)μm scale

(b) nm scale

Figure 6: SEM photographs of secondary porous structure of sulfur fixation ashes formed after the sulfur fixation reaction of three-component sulfur fixation agent SFG-2.

3.3. The Influence of Additives on the Sulfur Fixation Efficiency

Because single modified Bayer red mud showed lower sulfur fixation efficiency, it was necessary to add some additives to improve sulfur fixation efficiency. In this paper, the effects of additives of magnesium containing were mainly studied.

3.3.1. The Influence of MgO on the Sulfur Fixation Efficiency

MgO additives with doping content of 1 wt%, 2 wt%, and 3 wt% were added together with Bayer red mud to the coal samples. Experimental results of Figures 7 and 8 indicated that MgO significantly improved the sulfur fixation efficiency of the combustion coal. With the increase of MgO doping content, the sulfur fixation efficiency showed a decreasing trend after the first increase. The sulfur fixation efficiency was optimized to be 46.56 wt% when the doping content of MgO was 1 wt%.

Figure 7: Effect of MgO on the utilization rate of Ca.

Figure 8: Effect of MgO on the sulfur fixation rate.

During the sulfur fixation process, SO₂ transformed into sulfate by reacting with MgO. Meanwhile, MgO showed catalytic effects on sulfur fixation reaction for combustion of coal [36]. So the addition of MgO increased the sulfur fixation efficiency and improved the utilization rate of calcium (the highest utilization rate could reach 56.35 wt%).

3.3.2. The Influence of Natural Mineral Materials Additives on the Sulfur Fixation Efficiency

Both of dolomite and brucite with doping content of 1 wt%, 2 wt%, 3 wt%, and 4 wt% were added together with Bayer red mud to the coal samples. The influence of dolomite and brucite on the sulfur fixation efficiency of the combustion coal was studied, respectively. The sulfur fixation efficiency increased with the addition of the two natural mineral materials (Figure 9). As an effective sulfur fixation mineral material, dolomite was better than brucite. The sulfur fixation efficiency was optimized to be 67.70 wt% when the doping content of dolomite was 4 wt%. Meanwhile, the paper researched the effect of the mass ratio of brucite and dolomite (4 : 0, 3 : 1, 2 : 2, 1 : 3, and 0 : 4) on the sulfur fixation efficiency. According to Figure 10, the results showed that the efficiency of sulfur fixation was different under the same temperature but different dolomite/brucite ratio; with the dolomite increasing or brucite decreasing, the sulfur fixation increased, and the sulfur fixation efficiency was maximum 67.70 wt%; with the only doping content of dolomite was 4 wt%. According to above results, the paper selected dolomite as the additives of sulfur fixation agents and developed a new kind of sulfur-fixed agent named SFG-1 which is composed of modified Bayer red mud and dolomite with the mass ratio of 10 : 4 in order. At 950°C, CaO and MgO derived from the decomposition of dolomite and brucite could react with SO₂, showed a certain catalytic effect on sulfur fixation reaction, and improved the sulfur fixation efficiency. Besides, adding dolomite could improve pore structure of the red mud after high temperature calcination, accelerate the forming of high temperature stability of sulfide minerals, and restrain the decomposing of CaSO₄ [19], which made the sulfur fixation efficiency improved. And the existence of dolomite can improve the pore structure of red mud in high temperature, which also enhanced the own desulfurization efficiency of Bayer red mud.

Figure 9: Effect of natural minerals on sulfur fixation rate.

Figure 10: Effect of dolomite/brucite ratio on sulfur fixation rate.

3.4. The Influence of Promoter on the Sulfur Fixation Efficiency

In order to further improve the sulfur fixation effect of combustion, the promoter added again by experimental design. Although the layered structure and dilatability of vermiculite promoted the inner oxidation extent of the coal combustion and enhanced the fixed-sulfur efficiency [37], vermiculite was difficult to crush because of its good toughness, so as to increase the cost of crushing. However, the dust from vermiculite industry was mainly composed of fine particles (160–200 mesh) and had high specific surface area. Using the vermiculite dust replacing normal industrial vermiculite as coal-burning sulfur-fixed promoter, the effect of different doping content of vermiculite dust (0.1 wt%, 0.2 wt%, 0.3 wt%, and 0.4 wt%) on fixed-sulfur efficiency was researched.

According to the experimental results (Figure 11), vermiculite dust could promote the fixed-sulfur efficiency. Experimental results showed that vermiculite dust played an important role in reducing SO₂ during the fixed-sulfur process of modified Bayer red mud or SFG-1, and the desulphurization ration could reach up to maximum 76 wt% at 950°C. The main reasons were that further inflating of vermiculite dust at 950°C could make the inner looser and promote the inner oxidation extent of the coal combustion, which was advantageous to forming CaSO₄ and restraining the decomposing of CaSO₄ [38]. So, an advanced complex sulfur fixation agent named SFG-2 was made successfully by the experimental studies in which Bayer red mud was the main desulphurization composition, dolomite served as additives, vermiculite dust played the inflation role, and the optimized mass ratio of them in order was 70 : 28 : 2.

Figure 11: Effect of vermiculite dust on sulfur fixation rate under 4 wt% dosage.

3.5. The Influence of SFG-2 on the Sulfur Fixation Efficiency

The influence of SFG-2 content ranging in the coal from 10 wt% up to 25 wt% on the sulfur fixation efficiency was studied. Experimental results (Figure 12) showed that SFG-2 achieved desulphurization ration maximum 87 wt% when the doping content of SFG-2 in the coal was up to 20 wt% (i.e., Ca/S mole ratio was 1.3), and it may be inferred that the tricomponent composite SFG-2 was indeed an advanced desulphurization material.

Figure 12: Effect of tricomponent composite SFG-2 on sulfur fixation rate under 20 wt% dosage.

3.6. XRD of Desulphurization Produced Ashes

The structural characteristic of desulphurization produced ashes was investigated by X-ray diffraction (XRD), and the analysis results of the produced ashes used SFG-2 as sulfur fixation agent at 950°C were shown in Figure 13. The diffraction structural peaks of CaSO₄, Fe₂O₃, Ca₂Al₂SiO₇, and (Na,Ca)Al(Si,Al)₃O₈ could be clearly found, especially CaSO₄, which showed that CaSO₄ was the main component of sulfur fixation produced ashes at 950°C and the content is more. Besides, the existence of Ca₂Al₂SiO₇ and (Na,Ca)Al(Si,Al)₃O₈ also improved the sulfur fixation efficiency [32].

Figure 13: XRD of sulfur fixation produced ashes.

4. Conclusion

(1)The sulfur fixation rate of Bayer red mud could be improved by using fusel salt and waste mother liquor of sodium ferrocyanide as the modifying agents. When 50 wt% fusel salt solution was used as modifying agent, the sulfur fixation rate increased by 7 wt% and the utilization rate of calcium increased from 22.02 wt% to 31.61 wt%. When modified by waste mother liquor of sodium ferrocyanide with the concentration of 10 wt%, sulfur fixation rate could increase by 13 wt%, the highest of which could reach 29.85 wt% and the utilization rate of calcium reached 40.10 wt%.(2)Magnesium oxide (MgO) could obviously improve the sulfur fixation performance of Bayer red mud and up to a maximum sulfur fixation rate of 46.56 wt% with the 1 wt% MgO adding.(3)The sulfur fixation efficiency increased with the addition of the two natural mineral materials. Dolomite showed better effect than brucite. And the efficiency of sulfur fixation was different at 950°C with different dolomite/brucite ratio. With the dolomite increasing and brucite decreasing, the sulfur fixation increased. The sulfur fixation efficiency was 67.70 wt% when the sulfur fixation agent consisted of 10 wt% Bayer red mud and 4 wt% dolomite.(4)Vermiculite dust could promote the fixed-sulfur efficiency. The main reasons were that further inflating of vermiculite dust at 950°C could make the inner looser and promote the inner oxidation extent of the coal combustion, which was advantageous to forming CaSO₄ and restraining the decomposing of CaSO₄.(5)A new kind of tricomponent complex sulfur fixation agent SFG-2 was made successfully, in which modified Bayer red mud was the main desulphurization composition, dolomite served as additives, vermiculite dust played the inflation role, and the optimized mass ratio in order was 70 : 28 : 2. The SFG-2 achieved desulphurization ration maximum 87 wt% when the doping content of SFG-2 in the coal was 20 wt% (i.e., Ca/S mole ratio was 1.3), which indeed proved the advanced desulphurization characteristics of SFG-2.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Authors’ Contributions

Yang Liu and Yang Li contributed equally to this work.

Acknowledgments

This research was supported by the National High Technology Research and Development Program of China (863 Program 2012AA06A109).

References

J. Wang and H. M. Zhang, “Air pollution by sulfur dioxide and its control of coal fired industrial boilers in China,” Electric Power Environmental Protection, vol. 20, pp. 43–46, 2004.

H. M. Zhang and J. Wang, “Control of sulfur dioxide pollution from industrial coal fired boiler,” Electric Power Environmental Protection, vol. 20, pp. 38–42, 2004.

N. N. Zhang, Y. Jiang, and D. P. Xu, “The research status on sulfur solidifying technique during coal high-temperature combustion,” Coal Quality Technology, vol. 24, no. 1, pp. 8–10, 2010.

C. L. Chou, “Geologic characteristics of sulfur in coals and development of the clean coal technologies,” Earth Science Frontiers, no. 1, pp. 23–27, 1999.

B. Lin, Y. Z. Ge, and X. L. Zhang, “Introduction of fixed-sulphur technology on calcium-based sorbents,” Coal Chemical Industry, vol. 74, pp. 48–51, 1996.

J. H. Li and A. H. Lu, “Effect of Ca-based mineral sulfur-fixing agent on sulfur retention of different kinds of coals,” Techniques and Equipment for Environmental Pollution Control, vol. 7, pp. 36–40, 2006.

Z.-S. Yuan, D.-Y. Wu, and S.-D. Wang, “Study on sulfur retention of integrated additive during coal combustion,” Journal of Fuel Chemistry and Technology, vol. 30, no. 1, pp. 36–40, 2002.

S.-L. Wang, “Influence of CaO/MgO ratios on efficiency of sulfur fixation,” Coal Geology & Exploration, vol. 31, no. 2, pp. 4–7, 2003.

N. Li, W. P. Liu, J. H. Zhou, and K. F. Cen, “Sulfur capture mechanism and application of novel barium-based sorbents at high temperature,” Journal of Chemical Industry and Engineering (China), vol. 53, pp. 1198–1201, 2002.

Z. H. Zhang and X. D. Ma, “Research status of sulfur-fixing agent,” China Resources Comprehensive Utilization, vol. 27, pp. 13–14, 2009.

P. Davini, G. DeMichele, and P. Ghetti, “An investigation of the influence of sodium chloride on the desulphurization properties of limestone,” Fuel, vol. 71, no. 7, pp. 831–834, 1992.

Y. Fu, G. Z. Lin, and Y. H. Zhuang, “On the role of sodium additives in desulfurization during coal briquette combustion,” Environmental Chemistry, vol. 13, pp. 492–493, 1994.

L. R. Chen, L. M. Ge, and A. N. Zhou, “Study on modified calcium hydroxide with sodium alkali used as briquette capturing sulfur agent,” Clean Coal Technology, vol. 11, pp. 73–75, 2005.

K.-H. Han, J.-L. Zhao, B. Zheng, C.-M. Lu, and G.-J. Zhao, “Sulfur retention performance and modification of alkaline wastes used in coal combustion,” Journal of the China Coal Society, vol. 34, no. 12, pp. 1698–1702, 2009.

C. H. Dai and W. L. Zheng, “Catalytic sulfur retention of the transition-metal oxides under fluidized-bed combustion conditions,” Coal Chemical Industry, vol. 38, no. 1, pp. 31–34, 2011.

Z. H. Wu, P. Kou, Y. X. Shi, J. P. Zhang, and J. Wang, “Effects of alkali carbonates on the CaO desulfurization,” Coal Conversion, vol. 23, pp. 59–62, 2000.

X. Z. Gong, Z. C. Guo, and Z. Wang, “Reactivity of pulverized coals during combustion catalyzed by CeO₂ and Fe₂O₃,” Combustion and Flame, vol. 157, no. 2, pp. 351–356, 2010.

Z. Z. Li and B. Q. Xiao, “Study of a newly high-temperatured compound sulfur fixztion agent for burning-coal,” Science Technology and Engineering, vol. 7, pp. 3896–3902, 2007.

G.-J. Zhao, C.-M. Lu, Y. Tian, Z.-Y. Fan, J.-L. Zhao, and Z.-Y. Zhang, “Desulfurization characteristic and mechanism of red mud from alumina plant,” Journal of Fuel Chemistry and Technology, vol. 36, no. 3, pp. 365–370, 2008.

S. Wang, H. M. Ang, and M. O. Tadé, “Novel applications of red mud as coagulant, adsorbent and catalyst for environmentally benign processes,” Chemosphere, vol. 72, no. 11, pp. 1621–1635, 2008.

S. Z. Yu and R. L. Man, “Application of red mud in flue gas desulfurization from thermal power plant,” Mining and Metallurgical Engneering, vol. 25, pp. 63–65, 2005.

Y. N. Chen and J. X. Nie, “Adsorption of SO₂ from flue gas with wastewater in red mud,” Nonferrous Metals, vol. 59, pp. 153–155, 2007.

X. Wang, X. H. Bao, and A. X. Zheng, “Study on the sulfur fixation of coal burning by red mud,” Fly Ash Comprehensive Utilization, no. 6, pp. 23–25, 2010.

H. T. Liu, K. H. Han, H. Li et al., “Sulfur retention performance of limestone modified by red mud for CFB boiler,” Thermal Power Generation, vol. 41, no. 9, pp. 7–11, 2012.

G. X. Yin, C. Ning, and X. F. Fu, “Development of sulfur fixation and combustion-supporting agent for loose coal using red mud and black liquor of papermaking,” Environmental Engineering, vol. 22, no. 2, pp. 19–22, 2004.

C. S. Lv, J. W. Wang, and C. Y. Lu, “The summary of the desulfurization process with Bayer red mud,” Guangzhou Chemical Industry, vol. 40, pp. 20–22, 2012.

R. K. Paramguru, P. C. Rath, and V. N. Misra, “Trends in red mud utilization—a review,” Mineral Processing and Extractive Metallurgy Review, vol. 26, no. 1, pp. 1–29, 2005.

Y.-C. Huang, N. Wang, J. Wan et al., “Comprehensive utilization of red mud and control techniques of radioactive issues,” Bulletin of Mineralogy Petrology and Geochemistry, vol. 28, no. 2, pp. 128–130, 2009.

H. N. Gu, N. Wang, and N. C. Zhang, “Study on radioactivity level of red mud and radiological constraints of usability as building materials,” Light Metals, no. 5, pp. 19–21, 2011.

“Methods for analysis and testing of coal and coke—ultimate analysis of coal and coke—determination of total sulfur content—eschka method,” Tech. Rep. BS 1016-106.4.1-1993, British Standards Institution, 1993.

China Coal Industry Association & China National Standardization Management Committee, “Classification of quality of coal—part 2: sulfur content,” Tech. Rep. GB/T 15224.2, China Standard Press, Beijing, China, 2010.

K. Q. Wang and S. H. Li, “A study on occurrence mode of rare earth metals in red mud from Bayer process,” Light Metals, no. 10, pp. 13–16, 2012.

X. K. Wang, Y. H. Zhang, F. Z. Lv et al., “Removal of alkali in the red mud by SO₂ and simulated flue gas under mild conditions,” Environmental Progress & Sustainable Energy, vol. 34, no. 1, pp. 81–87, 2015.

H. Y. Qi, C. F. You, and A. J. Wang, “Influence of temperature on enhancing calcium conversion rate of sorbent and effects of steam reactivation,” Journal of Engineering Thermophysics, vol. 24, pp. 717–719, 2003.

G. J. Zhao, F. J. Yin, J. Li, and X. Y. Li, “Study on pore structure and sulfur fixation characteristics of alkali industrial waste,” Advanced Materials Research, vol. 194–196, pp. 716–721, 2011.

Y. Tan, Experiment research of additives on sulfur capture during coal combustion and analysis of mechanism on promoting effect [M.S. thesis], Hunan University, Hunan, China, 2005.

M. Geng, L. Shi, and W. D. Xu, “Experimental research on using vermiculite as coal-burning sulfur-fixed additive,” Non-Metallic Mines, vol. 29, pp. 28–30, 2006.

J. H. Li, A. H. Lu, and C. X. Chen, “The application of vermiculite to the desulphurization and dusts clearing during coal combustion,” Acta Petrologica et Mineralogica, vol. 20, pp. 515–524, 2001.