Research Note pubs.acs.org/IECR
Novel Process for Alumina Extraction via the Coupling Treatment of Coal Gangue and Bauxite Red Mud Yanxia Guo,† Qian Zhao,† Kezhou Yan,† Fangqin Cheng,*,† and Helen H. Lou‡ †
Institute of Resources and Environment Engineering, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, Taiyuan, Shanxi 030006, China ‡ Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710, United States ABSTRACT: A novel process for the coupling treatment of coal gangue and red mud is reported. Red mud containing 23.4% Al2O3, 19.1% SiO2, and 9.4% Na2O by weight was used as the blended ingredients of coal gangue for alumina extraction. Calcination of coal gangue with the addition of Na2CO3 at 850 °C could obtain more than 90% of Al2O3 extraction. Nevertheless, the high consumption of Na2CO3 is equivalent to the same weight amount of coal gangue. Both nepheline and sodium aluminum silicate formed during the process have the molecular structure of NaAlSiO4. The 1:1:1 of the theory molar ratio of Na, Al, and Si suggested that the minimum consumption of Na2CO3 might be obtained at the optimal Al/Si molar ratio of 1:1. Red mud with an Al/Si molar ratio of 1.44 mixed with coal gangue with a ratio of 0.64 was used to extract alumina. The results showed that red mud could adjust the Al/Si molar ratio of coal gangue to an appropriate value and Na2O in red mud was also used to partly substitute for Na2CO3 required for coal gangue activation, which consequently decreased the consumption of Na2CO3 from 100% to 12.1−20.5%.
1. INTRODUCTION Coal gangue produced from coal mining and coal processing is a kind of industrial solid waste. It is one of the largest quantities of industrial solid wastes in China.1,2 More than 500 million tons of coal gangue was produced and about 200 million tons were piled up annually in China.3 Coal gangue contains 15− 40% of Al2O3. Extraction of alumina as one of the most important value-added utilization ways has attracted many researchers’ attention.4−6 The most common method of alumina extraction is direct calcination−acid leaching method. The direct calcination at 650−850 °C could make the inert kaolinite in coal gangue transform into active metakaolinite by removing its crystal water. The Al2O3 dissolution in acid, however, is as low as ∼70% at the optimal calcination temperature of 700 °C.5 Our previous work found that coal gangue activation by calcining at 800−900 °C with the addition of Na2CO3 obtained a >90% of Al2O3 extraction. In the process, aluminosilicates in coal gangue was transformed into nepheline (NaAlSiO4) and became easier to be dissolved in acid. The consumption of Na2CO3, however, is almost the same weight amount as that of coal gangue. Because the molar ratio of Na, Al, and Si in NaAlSiO4 are 1:1:1, the optimal atom ratio in the reactants should be of the same value. However, the common molar ratio of Al/Si in coal gangue is 0.4−0.8, resulting in the increase of the consumption of Na2CO3 due to the superfluous Si. The weight ratio of Na2CO3 to coal gangue was about 1:1, in which the molar ratio of Na:Al reached about 4:1. An important way to decrease the consumption of Na2CO3 is to adjust the Al/Si molar ratio in the raw materials. Bauxite red mud is another kind of industrial waste derived from alumina production. As the second largest aluminaproducing country, China generated more than 30 million tons of bauxite red mud annually.7,8 Red mud is highly alkaline, with a pH ranging from 10 to 13.9,10 Its high alkalinity limits the © 2014 American Chemical Society
utilization of red mud. Less than 5% of red mud was utilized in China.7,8 Almost all of the red mud is stored in impoundment. In general, red mud still contains 15−25% of Al2O3 after alumina extraction.9,11 The impoundment not only results in the waste of resources but also is of major concern of environmental pollution.12 Great efforts have been devoted to the study of alkali removal13 or neutralization treatment14 for better utilization of the red mud. Red mud contains 15−25% of Al2O3, 10−20% of SiO2, and 8−12% of Na2O, in which the Al/Si molar ratio generally reaches 1.5. We hypothesized that red mud might be used as the blended ingredients for alumina extraction from coal gangue, and it might be able to not only adjust the Al/Si molar ratio of coal gangue (0.4−0.8) to an appropriate value (1:1) but also provide some Na2O sources for coal gangue activation. Three benefits might be gained by the process: (1) decreasing the consumption of Na2CO3 for coal gangue activation because the appropriate Al/Si molar ratio and the indispensable Na2O could be obtained, (2) simplifying the complicated process for the treatment of red mud as a result of Na2O recovery or removal is nonessential, and (3) a novel process for cotreatment of red mud and coal gangue would be established. In this work, the effect of red mud on the consumption of Na 2 CO 3 and the alumina extraction was investigated tentatively. This novel process would provide a new concept for the treatment of red mud. Received: Revised: Accepted: Published: 4518
January 21, 2014 February 16, 2014 March 5, 2014 March 5, 2014 dx.doi.org/10.1021/ie500295t | Ind. Eng. Chem. Res. 2014, 53, 4518−4521
Industrial & Engineering Chemistry Research
Research Note
Table 1. Chemical Compositions of Coal Gangue and Red Mud contents (wt %) component
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
TiO2
loss on ignition
coal gangue red mud
41.1 19.1
22.9 23.4
2.5 15.7
2.6 13.3
0.34 1.3
9.4
0.86 4.4
33.2 5.9
2. EXPERIMENTAL SECTION 2.1. Raw Materials. The as-received coal gangue (CG) and red mud (RM) were taken from Changcun Coal Mine in Lu’an Coal Mining Industry (Shanxi, China) and Zhaofeng Alumina Industry Co. Ltd. in Yangquan, respectively. Their chemical and mineral compositions are shown in Table 1 and Figure 1, respectively.
3. RESULTS AND DISCUSSION The Al2O3 dissolution (weight percentage) of the as-received coal gangue in HCl is only about ∼10%.16 After activation by direct calcination at 650−850 °C, the dissolution percentage only could reach ∼70%.16 Figure 2 shows the Al2O3 dissolution
Figure 2. Effect of the weight ratio of Na2CO3 to coal gangue on the dissolution of Al2O3 at 850 °C. Figure 1. XRD spectra of as-received coal gangue (CG) and red mud (RM). 1, kaolinite (Al2O3·2SiO2·2H2O); 2, quartz (SiO2); 3, nephenline (NaAlSiO4); 4, albite (NaAlSi3O8); 5, perovskite (CaTiO3); 6, hematite (Fe2O3).
of coal gangue calcined at 850 °C with the addition of various amount of Na2CO3. As can be seen, the Al2O3 dissolution increased with the increases of Na2CO3 to coal gangue weight ratios in the range of 0−1. The Al2O3 dissolution increased up to 90% at the ratio exceeding 0.8. Figure 3 shows the XRD patterns of the as-received coal gangue (CG), the calcined coal gangue with the addition of Na2CO3 at 850 °C (Na2CO3-850), and the acid leached residue of sample Na2CO3-850 (R-Na2CO3-850). The weight ratio of Na2CO3 to coal gangue is 1:1. It is clearly seen that the diffractions corresponding to kaolinite (1) and quartz (2) present in CG spectra. It is generally accepted that the presence of the inert kaolinite and quartz causes the low activity of coal gangue, which resulted in the Al2O3 dissolution of as-received being as low as ∼10%.3,4 After calcination with the addition of Na2CO3 at 850 °C (Na2CO3-850), the diffractions of kaolinite and quartz disappeared. The new strong diffractions corresponding to nepheline (3), sodium aluminum silicate (4), and Na2SiO3 (5) were observed. In the XRD spectra of the acidleached residue of sample Na2CO3-850 (R-Na2CO3-850), however, the diffractions of 3, 4, and 5 were invisible. Combined with the high alumina dissolution obtained from HCl leaching of sample Na2CO3-850, it is deduced that the formation of nepheline or sodium aluminum silicate improved the alumina dissolution of coal gangue. However, the consumption of Na2CO3 reached almost the same weight amount as that of coal gangue, which would increase the utilization cost and consequently hinder the industrial application of alumina extraction from coal gangue.
2.2. Methods and Processes. The as-received coal gangue and red mud were sieved to 60−100 meshes (0.25−0.15 mm), respectively. When using red mud as the blended ingredient, coal gangue, red mud, and Na2CO3 were mixed completely depending on the Al/Si/Na molar ratio. The mixture is then calcined at the desired temperature. For comparison, the thermal activation of coal gangue only with the addition of Na2CO3 was carried out. The calcination operations were performed in a muffle furnace (SX2-12-10) at the desired temperature for 2 h. Alumina extraction process of the activated samples by hydrochloric acid leaching was as follows. A 20 wt % hydrochloric acid solution was mixed with the activated samples in a four-mouth flask with a solid to liquid weight ratio of 1:4 at 100 °C for 2 h. After that, the mixture stood and was cooled to room temperature and then separated by filtration. The dissolution efficiency of Al3+ in hydrochloric acid by weight percentage was served as the index of evaluating the extraction of Al2O3 from coal gangue. The content of Al3+ in the filtrate was determined using copper salts back-titration.15 The phase analysis of solid phases was performed by using a Bruker D2 Advance X-ray diffract meter with a Cu Kα source at 40 kV and 40 mA. The 2θ scans were performed from 10° to 80° at a 6°/min speed. 4519
dx.doi.org/10.1021/ie500295t | Ind. Eng. Chem. Res. 2014, 53, 4518−4521
Industrial & Engineering Chemistry Research
Research Note
coal gangue was blended with 175 g of red mud. With the addition of Na2CO3 in the mixture of coal gangue and red mud, the mixed systems with various molar ratios of Na/Al/Si could be controlled. Figure 4 shows the comprehensive Al2O3 dissolution of the mixed samples containing coal gangue, red mud, and Na2CO3
Figure 3. XRD spectra of as-received coal gangue (CG), coal gangue calcination with the same weight ratio of Na2CO3 at 850 °C (Na2CO3850), and its acid-leached residue (R-Na2CO3-850). 1, kaolinite (Al2O3·2SiO2·2H2O); 2, quartz (SiO2); 3, nephenline (NaAlSiO4); 7, sodium aluminum silicate (NaAlSiO4); 8, sodium silicate (Na2SiO3). Figure 4. Al2O3 dissolution of the mixed samples by coal gangue, red mud, and Na2CO3 at various molar ratios of Na:Al:Si.
When the molecular structures of nepheline and sodium aluminum silicate in sample Na2CO3-850 are NaAlSiO4 with a Na, Al, and Si atom molar ratio of 1:1:1 are considered, the minimum consumption of Na2CO3 could be obtained at the optimal Al/Si molar ratio of 1:1. Therefore, the theoretical consumption of Na2CO3 based on the composition of Al2O3 in coal gangue shown in Table 1 should only be 23.8% of the weight amount of coal gangue used, which is much lower than the actual consumption (∼100%). The extra consumption of Na2CO3 might be attributed to the redundant SiO2, which formed Na2SiO3 during the calcination process (diffraction 5 of Na2CO3-850 in Figure 3). According to the data in Table 1, the molar ratio of Al/Si is 0.66 for the as-received coal gangue. Thus, the excessive SiO2 reaches 34% based on the molar ratio of Na, Al, and Si in NaAlSiO4. It would increase 105% of the extra Na2CO3 consumption. One important way to reduce the consumption of Na2CO3 is to adjust the Al/Si molar ratio of the reactants. As shown in Table 1, the red mud contains 23.4% of Al2O3 and 19.1% of SiO2, with an Al/Si molar ratio of 1.44. The red mud could be used to adjust the Al/Si molar ratio of coal gangue to an appropriate value (1:1). Furthermore, Na2O in red mud could also be used to substitute for the needed Na2CO3 during the activation of coal gangue, which might make the consumption of Na2CO3 even less. Table 2 shows the weight fraction of coal gangue, red mud, and Na2CO3 at various molar ratios of Na/Al/Si. It shows that an Al/Si molar ratio of 1:1 could be obtained when 100 g of
at various molar ratio of Na/Al/Si. Obviously, the overall Al2O3 dissolution increases with the increase of the weight fraction of Na2CO3. The comprehensive Al2O3 dissolution reaches ∼78% at the weight fraction of Na2CO3 of 0, with a Na/Al/Si molar ratio of 0.4:1:1. It suggests that Na2O in red mud played the important role in replacing Na2CO3. The Al2O3 dissolution is higher than 90% when the molar ratio of Na/Al/Si is more than 1:1:1. Nevertheless, the practical consumption of Na2CO3 is only 12.1−20.5%, which is almost one-fifth of that used in coal gangue. The low Na2CO3 consumption might be attributed to the appropriate Al/Si molar ratio adjusted by blending with red mud as well as the partial substitution for Na2CO3 by Na2O in red mud. As shown in Figure 1, kaolinite (Al2O3·2SiO2·2H2O) as the major Al-containing mineral in coal gangue (CG) was transformed into nephenline, which is easily soluble in HCl under the action of Na2O. Most of the Al-containing minerals, such as nephenline (NaAlSiO4), albite (NaAlSi3O8) as shown in Figure 1, and any other amorphous substances undetected by XRD, were transformed into the acid-soluble substances under these conditions based on more than 90% of the comprehensive alumina dissolution (Figure 4). The results above showed that the blends of red mud decreased the consumption of Na2CO3 for alumina extraction from coal gangue. This work presents a new concept and a novel process for the coupling red mud and coal gangue treatment.
Table 2. Weight and Weight Fraction of Coal Gangue, Red Mud, and Na2CO3 Used at Various Molar Ratio of Na:Al:Si molar ratio of Na:Al:Si 0.4:1:1
0.8:1:1
1:1:1
1.2:1:1
1.5:1:1
component
weight (g)
weight fraction (wt %)
weight (g)
weight fraction (wt %)
weight (g)
weight fraction (wt %)
weight (g)
weight fraction (wt %)
weight (g)
weight fraction (wt %)
coal gangue red mud Na2CO3
100 175 0
36.4 63.6 0.0
100 175 25
33.3 58.3 8.3
100 175 38
31.9 55.9 12.1
100 175 51
30.7 53.7 15.6
100 175 71
28.9 50.6 20.5
4520
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Industrial & Engineering Chemistry Research
Research Note
(6) Qiao, X.; Si, P.; Yu, J. A Systematic Investigation into the Extraction of Aluminum from Coal Spoil through Kaolinite. Environ. Sci. Technol. 2008, 42, 8541. (7) Zhang, R.; Zheng, S.; Ma, S.; Zhang, Y. Recovery of alumina and alkali in Bayer red mud by the formation of andradite-grossular hydrogarnet in hydrothermal process. J. Hazard. Mater. 2011, 189, 827. (8) The Twelfth Five Year Plan for the comprehensive utilization of the bulk of industrial solid waste. Ministry of industry and information technology of the people’s Republic of China, No. 600; Ministry of Industry and Information Technology: Beijing, China, 2011; http:// www.miit.gov.cn/n11293472/n11293832/n11293907/n11368223/ 14416612.html. (9) Sushil, S.; Batra, V. S. Catalytic applications of red mud, an aluminium industry waste: A review. Appl. Catal., B 2008, 81, 64. (10) Zhang, N.; Liu, X.; Sun, H.; Li, L. Evaluation of blends bauxitecalcination-method red mud with other industrial wastes as a cementitious material: Properties and hydration characteristics. J. Hazard. Mater. 2011, 185, 329. (11) Palmer, S. J.; Reddy, B. J.; Frost, R. L. Characterisation of red mud by UV−vis NIR spectroscopy. Spectrochim. Acta, Part A 2009, 71, 1814. (12) Jones, H.; Boger, D. V. Sustainability and waste management in the resource industries. Ind. Eng. Chem. Res. 2012, 51, 10057. (13) Couperthwaite, S. J.; Johnstone, D. W; Millar, G. J.; Frost, R. L. Neutralization of Acid Sulfate Solutions Using Bauxite Refinery Residues and Its Derivatives. Ind. Eng. Chem. Res. 2013, 52, 1388. (14) Cengeloglu, Y.; Tor, A.; Arslan, G.; Ersoz, M.; Gezgin, S. Removal of boron from aqueous solution by using neutralized red mud. J. Hazard. Mater. 2007, 142, 412. (15) Guo, Y.; Li, Y.; Cheng, F.; Wang, M.; Wang, X. Role of additives in improved thermal activation of coal fly ash for alumina extraction. Fuel Process. Technol. 2013, 110, 114. (16) Cui H. Research on the key technology in the process of the preparation of crystalline aluminum chloride from coal gangue. M.S. Thesis, Shanxi University, 2013.
It is understood that the mineralogical phases in red mud are complicated. For example, it contains hematite (Fe2O3), goethite (FeO(OH), sodalite (NaAlSi4O12Cl), cancrinite (3NaAlSiO4·NaOH), katoite (Ca3Al2SiO2(OH)12), and rutile (TiO2) etc.9 The reactions among these minerals, coal gangue, and Na2CO3 are still unclear. Moreover, the effect of other impurities, such as Fe, Ca, Mg, Ti, and so forth, on the transformation of aluminosilicates and alumina extraction is needed to be investigated. These studies facilitate the optimization of the process technology further.
4. CONCLUSIONS Calcination of coal gangue with the addition of Na2CO3 at 850 °C could obtain more than 90% of Al2O3 extraction at the Na2CO3 to coal gangue at weight ratios of 0.8−1. The XRD patterns showed that the aluminosilicates in coal gangue were transformed into nepheline and sodium aluminum silicate, both of which have the molecular structure of NaAlSiO4, during the calcination process with the addition of Na 2CO3. An appropriate Al/Si molar ratio of 1:1 could be obtained by blending 100 g of coal gangue with 175 g of red mud. Na/Al/Si molar ratios from 0.4:1:1 to 1.5:1:1 in blended materials could be reached at the amount of Na2CO3 added being 0−71 g. When Na/Al/Si molar ratios were 1:1:1−1.5:1:1, more than 90% of the alumina extraction could be achieved, in which the consumption of Na2CO3 only account for 12.1−20.5% of the total blends’ amount by weight. The results showed that red mud could be used to adjust the Al/Si molar ratio of coal gangue to the appropriate value, and Na2O in red mud was also used to partially substitute for Na2CO3 required for coal gangue activation, which consequently decreased the consumption of Na2CO3 from 100% to 12.1−20.5%.
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AUTHOR INFORMATION
Corresponding Author
*F. Cheng. Tel.: +86 351 7016893. Fax: +86 351 7016893. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (21306109) and National Hi-Tech Research and Development Program of China (863 Program, 2011AA06A103).
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REFERENCES
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