Preparation of Aluminum Hydroxide from Bauxite with Alkaline Additives.
Byline: SHAKILA BEGUM, MAQSOOM H. SHAH AND MOHAMMAD NAUMANSummary: The goal of the present investigation was to study the leach conditions of a low - grade bauxite in the area of Khushab district, Pakistan for local production of Al (OH)3. The chemical composition was 56.04 % Al2O3, 15.77 % Fe2O3, 19.59 % SiO2 and 3.94 % TiO2. Emphasis was placed on the effect of (CaO/Na2CO3) additives, pH and precalcination temperature on the percent recovery of Al (OH)3 and the extraction of Al2O3 from bauxite. Bauxite with the alkaline additives was fired at 550, 750 and 950 Co.
The extraction efficiency of Al2O3 increased noticeably (96.58 %, 97.00 % and 98.09 % ) at 550, 750 and 950 Co by leaching sodium aluminate in soda - based liquor in an open system and precipitating Al (OH)3 with CO2 gas than in a reflux condensed system (73.82 %, 78.94 % and 84.28 %) and neutralizing the leach solution with acid.
The Al2O3 was 90.57 %, 92.00 % and 94.16 % by precipitating Al (OH)3 from water - based liquor with CO2 gas over similar temperatures and leaching period of 25 minutes.
The feasible conditions of the process were the addition of alkaline in 60 % by weight, 750 to 950 Co, precalcination temperature, 2 hours of calcination time, 25 minutes of leaching time and precipitation of Al (OH)3 from water - based liquor by carbonation.
Introduction
The development of new methods for preparation of Al (OH)3 has always been important, because Al (OH)3 is of great practical significance.
Bauxite has received considerable attention especially as potential raw material for the production of Al (OH)3. In Pakistan large deposits of bauxite are found in Khushab district. It is evident that these ores will gain worth for the production of aluminum hydroxide, alumina and eventually, if viable, for metallic aluminum in future. Pakistan is inadequate in these commercial grade chemicals and the country largely depends upon the import of these materials. The choice of process technology and the market of production are mainly influenced by the quality of bauxite with regard to its impurities mainly iron.
Bauxite deposits from Khushab have very high iron and silica contents. Production of Al2O3 from bauxite has been the subject of interest of several researchers [1 - 14]. Some of the investigators have recovered Al2O3 by acid leaching [15 - 18]. However, the present beneficiation processes are undertaken to remove low-grade materials from bauxite and to add value to bauxite as an industrial raw material for the production of Al (OH)3 and Al2O3.
Bauxite is the most important aluminum ore. It consists largely of the minerals gibbsite Al (OH)3, boehmite g-AlO(OH) and diaspore a-AlO(OH), together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase TiO2. It was named after the village Les Baux in Southern France, where it was first discovered in 1821 by the geologist Pierre Berthier [19, 20].
Bauxite is strip mined because it is found at the surface, with little or no overburden. Approximately 95% of the world's bauxite production is processed into aluminum. Bauxites are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical and refractory.
The two types of bauxites that are used commercially as aluminum ores are laterite and karst. Lateritic bauxites (silicate bauxites) are distinguished from karst bauxites (carbonate bauxites) [21].
Nanoparticles of Al2O3 have antibacterial properties [22, 23]. In this study, bauxite from Khushab has been selected for evaluation largely because of its bulk availability.
Results and Discussion
Effect of Alkaline Additives
Increasing the content of soda increases the dissolution of bauxite. When only soda is added to the bauxite, it facilitates the occurrence of sodium aluminum silicates (Na2O. Al2O3. 2SiO2) or camegites and it decreases Al2O3 extraction. Thus the dissolution rate of only SiO2 increases. When CaO is added, SiO2 shows lower efficiency of dissolution as it results into the formation of calcium silicates (Fig. 1). So, the extraction of Al2O3 increases. The addition of soda and calcium oxide convert iron, silica and titania impurities into insoluble sodium and calcium ferrates, silicates as well as sodium titanate. CaO was selected as an additive because with lime stone bauxite first forms alumino - silicate. Thus with the addition of lime stone the dissolution rate of SiO2 increases and then alumino - silicates convert to sodium aluminate at high temperature, which is soluble in alkaline solution and to dicalcium silicate which is insoluble in the same solution.
The effect of alkaline additives on Al2O3 extraction from bauxite is mentioned in detail in the recent literature [24].
For the calcium aluminates, e.i. CaO. Al2O3 formed in the lime soda sintering, the leach solution must contain high concentration of Na2CO3 for maximum Al2O3 extraction. Therefore, Na2CO3 was used as a lixiviant (a and b). However, procedure (c) demonstrates that Al2O3 dissolution in Na2CO3 and H2O are comparable. These results reveal that when bauxite is calcined with the chosen alkaline additives at high temperature, the liquor is moderately clear and does not require further purification. The results show that equal percent amounts of soda and calcium oxide are needed to increase the dissolution of bauxite and to improve the brightness of the product.
Effect of Calcination Temperature
As shown in Table-1, the extraction of Al2O3 from bauxite increases with increasing precalcination temperature. The degree of extraction at 750 and 950 Co is higher than the lower temperature range of the calcination. Sodium and Calcium aluminates occur highly proficiently in the solid phase during precalcination at 950 Co as seen in Fig. 1. Table-2 also indicates that the production of Al (OH)3 increases with temperature.
Table-1: Effect of Leaching Solutions on Extraction of Al2O3 at Different Precalcination Temperatures###
Temperature###Alkaline###% Al2O3###% Al2O3###% Al2O3
Co###Additives in###in recovered###in recovered###in recovered
###wt %###Al(OH)3###Al(OH)3###Al(OH)3
###(a)###(b)###(c)
CaO Na2CO3
950###60###60###84. 28###98. 09###94. 16
750###60###60###78. 94###97. 00###92.00
550###60###60###73. 82###96. 58###90. 57
Conditions: (a): Homogenous mixture 110 g containing bauxite: 50 g: Na2CO3: 2.5 M, leached for two hours in a reflux condensed system (b): 20 gm of homogenous mixtures containing 9. 0 gm of bauxite, stirred for 25 minutes in boiling solution of 2. 5 M Na2CO3 in an open system (c): same amount as in (b), agitated for 25 minutes in boiling H2O.
Concentration of Al determined by atomic absorption spectrophotometer in the recovered Al(OH)3, presenting percent recovery from bauxite
Effect of Calcination Time
After the attainment of precalcination temperature, the homogenous mixtures of bauxite with alkaline additives were calcined for two hours after which little enhancement occurs over longer period of time.
Effect of Leaching Time:
(a): The treated homogenous mixtures were leached for two hours in Na2CO3 solution after boiling. (b): The homogenous mixtures were stirred for 25 minutes in hot Na2CO3 solution while in procedure (c) the mixtures were agitated in hot H2O for 25 minutes. In procedure (b) almost all of the extractable Al2O3 goes into solution because of the high dissolving power of Na2CO3. The percent recovery in (a) is less than (b and c) because in accordance with the procedures (b and c), in an open system, a substantial amount of organic contaminants and humic substances are vented out from the liquor, which are extracted into the liquor from bauxite during digestion, thereby improving the productivity of the process.
Effect of Solution Concentration:
As the alkaline aluminates are formed by solid states reactions in the solid phase (Fig. 1), the treated material dissolves easily in the 2.5 M Na2CO3 solution as well as in H2O.
Effect of Particle Size
Various particle size distributions do not influence the dissolution of SiO2 from bauxite into the leaching solution as no detectable silica was observed in the recovered Al (OH)3. Similarly, the extraction of Al2O3 from bauxite is also not affected by particle size as it increases consistently with temperature.
Effect of pH:
(a): The liquor containing sodium aluminates hydrolyses with the addition of HCl at room temperature as shown in equation 1
NaAlO2 + HCl + H 2O - Al (OH) 3 + NaCl [1]
and aluminum hydroxide results into white precipitate at pH 6. 0 +- 0.1. The relationship of aluminum hydroxide with alumina is presented in equation (2):
2 Al (OH ) 3 - Al O 2 3 + 3H O 2 [2]
(b): In this procedure alumina extracts as sodium aluminate into the basic liquour.
Al O 2 3+ Na CO 2 3 - 2 NaAlO 2 + CO 2 [3]
Table-2: Characterization of the Produced Al (OH)3, (a), (b) and (c)
Temperature###Al(OH)3###Al(OH)3###Al(OH)3###% Fe2O3###% Chlorides###% Calcium###% Sodium
Co###wt %###wt %###wt %###in product###in product###in product###in product
###(a)###(b)###(c)###Al(OH)3###Al(OH)3###Al(OH)3###Al(OH)3
950###72.60###84. 50###81. 12###0. 01###0. 00###0.00###9. 02
750###68.00###83. 90###79. 90###0. 01###0. 00###0.00###10. 00
550###63.61###83. 20###78.03###0. 01###0. 00###0.00###11. 19
Conditions: (a): precipitated by HCl , (b) and (c): precipitated by carbonation
Carbonation neutralizes the basicity and precipitates gibbsite.
2 NaAlO 2 + CO 2 + 3H 2 O - 2 Al (OH ) 3 + Na 2 CO3 [4]
(c): The dissolution of the sodium aluminate is believed to proceed as follows.
NaAlO2 + 2 H O - Al (OH ) [?] + Na + [5]
Al (OH) [?] - Al (OH ) 3 + OH [?] [6]
2(OH) [?] + CO 2 - CO3 [?] [?] + H O 2 [7]
Characterization of Aluminum Hydroxide:
Table-2 shows that the impurities in the recovered Al (OH)3 are minimal at 950 Co. The production aptitude of Al (OH)3 is also elevated at high temperatures.
Experimental
Experiments were performed with representative samples of bauxite from Khushab district, Pakistan. The ore was ground to particle size ranging from less than 300 um and was analyzed chemically by standard methods of chemical analysis [25-27]. The data of the Chemical Analysis are given in Table-3.
Table-3: Chemical Analysis of Bauxite.
Compound###%###Compound###%
Loss on Ignition###01.77###
Al2O3###56.04###CaO###1.47
Fe2O3###15.77###SO3###0.48
SiO2###19. 59###MgO###0.54
TiO2###3.94###K2O###0.21
Na2O###0.13###CO2###No trac
Alkaline Additives
The equivalent weight percents of alkaline compounds (CaO and Na2CO3) were blended with 150 g bauxite by Siebtechnik grinding machine in order to form aluminate, silicate, ferrate and titanate (Table-1).
Calcination Temperature
The homogenous mixtures were fired at 550, 750 and 950 Co in a graphite crucible for two hours after the attainment of temperature in a tilting temperature controlled furnace having natural gas as a source of heat. The fired samples were allowed to cool. The homogenous sintered mixtures were again ground with the same machine and a X-ray diffraction pattern was recorded for treated bauxite fired at 950 Co (Fig. 1).
A great deal of effort has been expended in intensifying gibbsite precipitation from sodium aluminate liquor and progress has been achieved.
The sintered material was processed and gibbsite was precipitated by three different procedures to obtain the maximum extraction of Al2O3 and formation of Al (OH)3.
(a): Leaching Time:
At each temperature 110 g calcined material containing calculated amount of 50 g bauxite was leached for two hours at the boiling temperature to determine the optimum leaching time.
Solution Concentration
The calcined samples above were leached in 500 mL of 2. 5 M Na2CO3 solution. The other investigators [24] leached it in NaOH containing Na2O. The leaching experiments were carried out in a pyrex flask equipped with a reflux condenser placed over a hot plate with a magnetic stirrer. The experimental results indicated that an amount of 60 % by weight of each alkaline additive (Na2CO3 and CaO) to the bauxite sample lead to a significantly clear sodium aluminate liquor.
(b): 20 gm of homogenous calcined mixtures containing 9. 0 gm of bauxite fired at 550, 750 and 950 C were stirred for 25 minutes in boiling solution of 2. 5 M Na2CO3 in an open system.
c): The same amount of mixtures at the given temperatures in process (b) was agitated for 25 minutes in boiling H20.
Separation of Mixtures:
After leaching in process (a) the mixtures were cooled and then rapidly centrifuged while in process (b) and (c) the mixtures were filtered to separate the clear liquor from the residual bauxite or red mud.
Particle Size
Bauxite ore was ground to less than 300 um. Each sample with various particle size distributions was calcined together with CaO and Na2CO3 at different temperatures and leached for two hours in 2. 5 M Na2CO3 solutions (a), stirred in 2.5 M hot Na2CO3 solution (b) and hot H2O (c). Precipitation of Al (OH) 3
(a) After separation the pH of the clear liquor obtained was approximately 13.0. For the percent recovery known amount of liquor was titrated using 6M HCl as a titrant. Small amount of titrant was added and the solution was constantly stirred with a magnetic stirrer. Al (OH)3 was precipitated from the undiluted leach solutions by neutralizing with hydrochloric acid to pH 6. 0 +- 0.1.
The minor adjustment of pH was done with 1M HCl. The recovered Al (OH)3 was washed several times to remove any traces of chlorides and other impurities.
(b) and (c)
After filtration the precipitation in the clear liquor obtained in processes (b) and (c), the modified procedures, was done by bubbling CO2 gas through the liquor.
The precipitated Al (OH)3 obtained from each process was kept overnight to settle down. It was filtered through whatman 42 filter paper and dried in an oven at 100C0.
Characterization of Recovered Aluminum Hydroxide
In (a) 1.0001, 1.0157 and 1.0304 g of Al (OH)3 recovered at 550, 750 and 950 Co were dissolved in aqua regia while in processes (b) and (c) 0. 2 g Al (OH)3 was dissolved and the volumes were made to 250 mL. The solutions were further diluted and then analyzed by atomic absorption spectrophotometer in order to calculate percentages of Al2O3 (Table-1), percentages of Al(OH)3 and presence of iron in the product (Table-2). Calcium and sodium were determined by flame photometer. No detectable SiO2 by gravimetric methods was found. For chlorides the Al (OH)3 at different temperatures was dissolved in nitric acid. Chlorides were determined in the solutions by AgNO3 titration method. No chlorides were found (Table-2).
Conclusion
The maximum extraction efficiency of Al2O3 obtained in process (a) was 84.28 % and in (b and c) were 98.09 % and 94.16 % with precalcination at 950 Co. The low grade bauxite containing high content of SiO2 can be easily evaluated by this process because with the addition of CaO, SiO2 shows lowest efficiency of dissolution. Though procedure (b) provides the highest recovery, however, procedure (c) is preferred most feasible for industrial purposes.
References
1. Q. Wang, J. Deng, X. Liu, Q. Zhang, S. Sun, C. Jiang and F. Zhou, Journal of Asian Earth Sciences, 39, 6, 701 (2010).
2. T. Salmi, D. Y. Murzin and J. A. Mensah, Hydrometallurgy, 102, 22 (2010).
3. Q. Wang, J. Deng, Q. Zhang, S. Sun and J. Meng, Journal of Geochemical Exploration, 105, 137 (2010).
4. L. Bao and A. V. Nguyen, Hydrometallurgy, 104, 86 (2010).
5. X. Li, S. Gu, Z. Yin, G. Wu and Y. Zhai, Hydrometallurgy, 104, 313 (2010).
6. G.M. Brodie, G. Power and C.F. Vernon, Hydrometallurgy, 104, 278 (2010).
7. H. Zamanian and E. Hejazi, Journal of Geochemical Exploration, 107, 77 (2010).
8. H. Grenman, T. Salmi, D. Y. Murzin and J. A. Mensah, Hydrometallurgy, 102, 22 (2010).
9. Y. Li, Y. Zhang, C. Yang and Y. Zhang, Hydrometallurgy, 98, 52 (2009).
10. S. h. M. Z. Wen, J. N. Chen and S. l. Zheng, Minerals Engineering, 22, 793 (2009).
11. S. O. Lee, K. H. Jung, C. J. Oh, Y. H. Lee, T. Tran and M. J. Kim, Hydrometallurgy, 98, 156 (2009).
12. Y. Zhang, S. Zheng, H. Du, H. Xu, S. Wang and Y. Zhang, Hydrometallurgy, 98, 38 (2009).
13. S. Cao, Y. Zhang and Y. Zhang, Hydrometallurgy, 98, 298 (2009).
14. W. Haipeng, S. Peter, I. Don, Xu, Bingan, Zhang, Jian, Zhang, Yao, Newcombe, Bianca, Wang, Xiaodong, Weng, Wubiao, T. Yuan, Wang, Kai and N. Yung, Australia Chemeca, 36, 1678 (2008).
15. C. Bazin, K. El-Ouassiti and V. Ouelle, Hydrometallurgy, 88, 196 (2007).
16. B. R. Reddy, S. K. Mishra and G. N. Banerjee, Hydrometallurgy, 51, 131 (1999).
17. D. Ivan, C. Zivkovi, D Nada and S. trbac, Hydrometallurgy, 36, 247 (1994).
18. G. Patermarakis and Y. Paspaliaris, Hydrometallurgy, 23, 77 (1989).
19. Eastern Industries (2007), http://www.eastern.net.cn/Bauxite.html
20. http://en.wikipedia.org/wiki/List_of_countries_b y_bauxite_production
21. http://en.wikipedia.org/wiki/Bauxite_ore
22. W. Jiang, H. Mashayekhi, B. Xing, Environmental Pollution, 157, 1619 (2009).
23. I. M. Sadiq, B. Chowdhury, N. Chandrasekaran and A. Mukherjee, Journal of Nanomedicine, Nanotechnology, Biology and Medicine, 5, 3, 282 (2009).
24. A. Alp and A. O. Aydin, Canadian Metallurgical Quarterly, 51, 1, 41 (2002).
25. W. W. Scott and N. H. Furman, Sixth Edition, Vol(1) - The Elements, D. Van Mostrand Company, Inc. Princeton, New Jersey, p. 68 (1962).
26. Inam-Ul-Haque and M. Tariq, Journal of the Chemical Society of Pakistan, 33, 529 (2011).
27. S. Perveen, I. Ihsanullah, Z. Shah, W. Nazif, S. S. Shah and H. Shah, Journal of the Chemical Society of Pakistan, 33, 220 (2011).
SHAKILA BEGUM, MAQSOOM H. SHAH AND MOHAMMAD NAUMAN
Pakistan Council of Scientific and Industrial Research Laboratories, Jamrud Road, Peshawar, Pakistan., *To whom all correspondence should be addressed.
 Printer friendly
 Cite/link
 Email
 Feedback
| |
| Author: | Begum, Shakila; Shah, Maqsoom H.; Nauman, Mohammad |
|---|---|
| Publication: | Journal of the Chemical Society of Pakistan |
| Article Type: | Report |
| Geographic Code: | 9PAKI |
| Date: | Feb 29, 2012 |
| Words: | 2977 |
| Previous Article: | Preparation of SO42-/ Fe2O3-TiO2-Nd2O3 Solid Acid and its Activity for the Synthesis of Menthyl Acetate. |
| Next Article: | Synthesis, Analytical and Antibacterial Studies of N-[4-(phenyliminomethyl)phenyl]acetamide 0.67-hydrate and its Complexes with Manganese (II),... |
| Topics: | |