1.0 INTRODUCTION:

Mining of bauxite, followed by refining of bauxite to alumina by the Bayer process (invented by Karl Bayer in 1887) and finally smelting of alumina to aluminium (Hall –Heroult process) are the three stages in the primary aluminium production process. Production of alumina is a process of separating alumina from undesired components like oxides of iron, titanium, silicon, calcium, vanadium, manganese etc. in bauxite. The Bayer process of extraction of alumina from bauxite remains the most economical process till date. In the Bayer process, the insoluble product generated after bauxite digestion with sodium hydroxide at elevated temperature and pressure to produce alumina is known as ‘red mud’ or ‘bauxite residue’. The waste product derives its color and name from its iron oxide content. Red mud is a mixture of compounds originally present in the parent mineral, bauxite and of compounds formed during the Bayer process. As the bauxite has been subjected to sodium hydroxide treatment, the red mud is highly caustic with a pH in the range of 10.5-12.5.

Considerable research and development work for the storage, disposal and utilization of red mud is being carried out all over the world. An enormous quantity of red mud is generated worldwide every year posing a major environmental problem. The worldwide alumina production is around 58 million tonnes in which India counts for 2.7 million tonnes [1]. Depending on the raw material processed, 1- 2.5 tons of red mud is generated per ton of alumina produced [2]. One of the methods for making red mud safe for disposal is its treatment with seawater. Research have been carried out to utilize seawater treated red mud and thermally activated seawater treated red mud for removal of arsenate, vanadate, and molybdate.

High concentrations of fluoride occurring naturally in groundwater cause a serious bone disease, fluorosis among local populations. One of the methods of treating fluoride containing water is through adsorption on activated alumina, activated charcoal and ion exchange resins. This water can then be made suitable for use. Seawater neutralized red mud contains precipitated hydroxide, carbonate and aluminate ions along with magnesium and calcium. In the paper, a study has been made for utilizing seawater neutralized red mud for fluoride adsorption. Adsorption of fluoride has been facilitated by heat activating the seawater neutralized red mud.

2.0 PRODUCTION AND MAIN CHARACTERISTICS OF RED MUD/BAUXITE RESIDUES:

The sodium aluminate liquor is separated from the undigested bauxite or red mud and is disposed off in red mud ponds. Chemical analysis shows that red mud contains oxides of silicon, aluminium, iron, calcium, titanium, sodium as well as an array of minor elements namely K, Cr, V, Ba, Cu, Mn, Pb, Zn, P, F, S, As, and etc. The variation in chemical composition between red mud worldwide is high. Typical composition of red mud is given in Table 1.

Table 1: Typical composition of red mud

Composition	Percentage
Fe₂O₃	30-60%
Al₂O₃	10-20%
SiO₂	3-50%
Na₂O	2-10%
CaO	2-8%
TiO₂	trace-25%

(Source: Red mud Project. http://www.redmud.org/Characteristics.html [3]

Mineralogically, red mud has a very high number of compounds present. These are: Hematite (Fe₂O₃), goethite Fe_(1-x)Al_xOOH (x = 0.33), gibbsite Al(OH)₃, boehmite AlO(OH), diaspore AlO(OH), calcite(CaCO₃), calcium aluminium hydrate (x^.CaO^.yAl₂O₃^.zH₂O), quartz (SiO₂), rutile (TiO₂), anatase (TiO₂), CaTiO₃, Na₂TiO₃, kaolinite Al₂O₃^.2SiO₂^.2H₂O, sodalites, aluminum silicates, cancrinite (NaAlSiO₄)6CaCO₃, hydroxycancrinite (NaAlSiO₄) 6NaOH^.H₂O, chantalite CaO^.Al₂O₃^.SiO₂^.2H₂O, hydrogarnet Ca₃Al₂(SiO₄)n(OH)_12-4n [3].

3.0 ENVIRONMENTAL CONCERNS:

Red mud is disposed as dry or semi dry material in red mud pond or abandoned bauxite mines. It is also disposed as slurry having a high solid concentration of 30-60% and with a high ionic strength. Very large quantity of red mud generated and its causticity are the two main environmental concerns. Research and development work for the utilization, storage and disposal is being carried out all over the world. Problems associated with the disposal of red mud waste include its high pH, alkali seepage into underground water, safety in storage, and alkaline air borne dust emissions and the vast area of land required for disposal. Up to 2 tons of liquor with a significant alkalinity of 5- 20 g/L caustic (as Na₂CO₃) accompanies every ton of dry mud. Red mud is a very fine material in terms of particle size distribution. Average particle size of red mud is less than 10 microns. The specific surface area (BET) of red mud is between 10 and 30 m²/g depending upon the grinding of bauxite.

4.0 STORAGE AND DISPOSAL OF RED MUD:

4.1. Red Mud Disposal

Seawater discharge, lagooning, dry stacking and dry disposal are the methods currently in use for the disposal of bauxite residue. In seawater discharge, after washing and thickening process of red mud, the slurry is disposed directly via a pipeline into the deep sea. This process reduces environmental impact of land disposal but may release toxic metals to the marine environment and increase the turbidity of the sea due to the fine mud and the formation of colloidal magnesium and aluminium compounds. Nevertheless, French and Japanese practices have favored disposal at sea as the best option on economic and environmental grounds. Lagooning is the conventional disposal method in which the residue slurry is directly pumped into land- based ponds. The residue is filtered to a dry cake (>65% solids) and the material is washed on the filter with water or steam to recover soda and minimize the alkalinity of residue in dry disposal method. Without further treatment, the dry residue is carried by truck or conveyor to the disposal site. This reduces the storage area but requires installation and operation of filtration plant. Solids contents of greater than 75% have been achieved with Bokela hyperbaric filtration technology at the Stade plant in Germany [4]. Even with the excellent washing performance offered by hyperbaric steam filtration, significant alkalinity remains associated with the solids because of the complex nature of red mud. Hence these hazards associated with alkalinity may be further reduced by employing suitable methods for neutralizing the red mud slurry.

4.2. Red Mud Neutralization

This large quantity of waste material is required to be disposed economically and safely to the environment and can be achieved by residue neutralization which will convert the highly caustic state of red mud to a state which is no longer highly caustic and is less hazardous. Neutralization will also open opportunities for re-use of the residue which to date have been prevented because of the high pH. Amongst the several pH- reduction steps to be incorporated to ameliorate the red mud, one of the methods for neutralization is seawater neutralization. Instead of direct discharge of red mud into the seawater, red mud can be mixed with seawater to reduce the pH of red mud slurry. Mixing red mud with seawater involves precipitation of hydroxide, carbonate and aluminate ions with magnesium and calcium. The red mud slurry after the treatment becomes non-hazardous and environmentally benign.

4.3 Red Mud Utilization as an adsorbent

In environmental field, red mud can be utilized in pollution control by using as an adsorbent for cleaning of industrial gases, as synthetic coagulants in waste water treatment and as a catalyst especially for coal hydrogenation. After adequate neutralization, red mud can be utilized for remediation of contaminated sites and treatment of contaminated liquid waste.

Red mud presents a promising application in water treatment for removal of toxic heavy metal and metalloid ions, inorganic anions such as nitrate, fluoride, and phosphate, as well as organics including dyes, phenolic compounds and bacteria [5]. The researchers have used acid and acid-thermal treated raw red mud to develop effective adsorbents to remove phosphate from aqueous solution. Study on the use of red mud for removal of dyes from textile effluents has also been conducted. Efforts have been made to use red mud for the removal of chlorophenols from wastewater [6]. Neutralized red mud in batch adsorption technique was used for the removal of phenol from aqueous solution [7]. Removal of boron from aqueous solution has also been studied by using neutralized red mud [8]. Red mud has been converted into an inexpensive and efficient adsorbent to remove cadmium, zinc, lead and chromium from aqueous solutions [9, 10]. Removal of arsenate from aqueous solutions using activated CO₂ neutralized red mud has been studied by Sahu et al, 2010 [11].

Hofstede et al. [12] have made use of bauxite refining residue to reduce the mobility of heavy metals in municipal waste compost. A US Patent Application 20090234174 [13] shows that a neutralized and activated red mud is suitable for heavy metals remediation in soil and water. Entrapped metals are not easily exchangeable and removable. However, more investigation would be needed to further understand the metal trapping mechanisms of red mud.

5.0 SEAWATER NEUTRALIZED RED MUD AND ITS UTILIZATION FOR ADSORPTION:

In neutralization of red mud with seawater, when seawater is added to caustic red mud, the pH of the mixture is reduced causing hydroxide, carbonate or hydroxycarbonate minerals to be precipitated [14]. Average seawater contains 965 gm of water and 35 gm of salts (i.e. 3.5% salinity). The concentration of various salt ions in seawater is 55% Chlorine (Cl^-), 30.6% sodium (Na⁺), 7.7% sulphate (SO₄^-2), 3.65% magnesium (Mg²⁺), 1.17% calcium (Ca²⁺), 1.13% potassium (K⁺) and 0.7% others [15]. Seawater neutralization does not eliminate hydroxide from the system but converts the readily soluble, strongly caustic wastes into less soluble, weakly alkaline solids. Brunori et al. [16] studied the possibility of reusing treated red mud (through the technology patented by Virotec International, consisting of a seawater treatment for pH neutralization) in the Eurallumina SpA bauxite refinery, located in Sardinia (Italy) for treating contaminated waters and soils. Researchers have investigated the effectiveness of using thermally activated seawater neutralized red mud for the removal of arsenate, vanadate, and molybdate in individual and mixed solutions [17, 14]. They found that thermally activated seawater neutralized red mud removes at least twice the concentration of anionic species than thermally activated red mud alone, due to the formation of 40–60% hydrotalcite during the neutralization process in seawater neutralized red mud. Hydrotalcite structure in the seawater neutralized red mud has been determined to consist of magnesium and aluminium with a ratio between 3.5:1 and 4:1 [14]. Removal of arsenate from aqueous solutions has also been studied by other researchers [18]. Fuhrman et al [19] studied arsenic removal from water using 4 sorbents namely seawater-neutralized red mud (Bauxsol), acid treated Bauxsol (ATB), activated Bauxsol (AB), Bauxsol coated sand (BCS), and activated Bauxsol coated sand (ABCS). The affinity of the developed sorbents towards arsenic in a decreasing order is AB > ATB >ABCS > BCS > Bauxsol, and sorptive capacity of all tested sorbents compares well with conventional sorbents such as activated alumina and ferric oxides. The removal of arsenate using seawater neutralized red mud is sensitive to several parameters such as pH, ionic strength, adsorbent dosage, initial arsenate concentration and the source water composition. Arsenate adsorption is favoured by slightly alkaline pH values with maximum adsorption recorded at pH 8.5.

Seawater neutralized red mud indicate the formation of brucite (magnesium oxide, Mg(OH)₂ magnesium aluminium oxide (MgAl₂O₄7.9H₂O). The other phases are of hematite (Fe₂O₃), magnetite (Fe₃O₄), gibbsite (Al(OH)₃), sodalite, halite, calcite and ileminite. Feasibility of using seawater neutralization has been studied in detail by Rai et al, 2012 [20]. Characterization of seawater neutralized bauxite residue using XRD has been studied in detail by researchers (Palmer and Ray, 2009) [17]. Neutralized red mud show large agglomerates of size of about 80-150 micron formed after neutralization which improves the physical characteristic of red mud. This is due to the fact that when unneutralized red mud comes in contact with seawater, the fine mineral particles flocculate into large agglomerates with the multivalent exchange cations, Ca and Mg, forming electrostatic bridges (Mcbride, 1994) [21]. These sites then act as nucleation sites for the precipitation of magnesium and calcium hydroxides, taking hydroxide ions from solution and lowering the pH (McConchie et al. 2000) [22]. Due to these characteristics of seawater neutralized red mud studies were carried out for fluoride adsorption on neutralized red mud.

6.0 FLUORIDE ADSORPTION ON RED MUD:

The removal of fluoride from aqueous solution using the original and HCl‐activated red mud forms has been studied by Cengeloglu et al. (2002) [23]. The fluoride adsorption capacity of the activated form was found to be higher than that of the original form and the required time for adsorption equilibrium of fluoride ions was 2 h. The removal of fluoride ion using red mud was explained on the basis of the chemical nature and specific interaction with metal oxide surfaces, and the results were interpreted in terms of pH variations. Tor et al., 2009 [24], also reported the feasibility of granular red mud for the removal of fluoride from water. The experiments showed that maximum fluoride removal (0.644 mg/g) was obtained at pH 4.7 and it took 6 h to attain equilibrium. Red mud after modification with AlC1₃ and by heat activation was tested for the removal of fluoride from water by Wei et al, 2009 [25]. The results showed that the adsorption capacities of these samples were 68.07 and 91.28 mg/g, respectively, which were much higher than that of red mud 13.46 mg/g. The solution pH values affected the removal efficiency significantly, and the highest removal efficiency was achieved at pH 7–8.

7.0 METHODOLOGY, RESULTS AND DISCUSSION:

Seawater neutralized red mud, 4 ppm fluoride solution, distilled water, buffer solution (TISAB II solution), 0.1 N KNO₃, 0.1 N HCl, 0.1 N NaOH were taken for study. Seawater neutralized red mud was calcined to temperatures: 200, 300, 400 and 500°C. BET surface area of seawater neutralized red mud and heat activated seawater neutralized red mud at 300 and 500°C mud were analyzed using Micromeritics USA, ASAP 2020, Surface Area and Porosity Analyzer. The surface area of as it is seawater treated red mud was observed to be 20.46 m²/g while the heat activated red mud at 300 and 500°C was 42.62 and 46.32 m²/g respectively.

0.2 g of each of the samples of heat activated red mud at 200, 300, 400 and 500°C was taken in conical flask containing 50 ml of 4 ppm fluoride solution (according to 4 g/L of dose). The conical flasks were placed in Orbital Shaking Incubator (REMI make) at 30°C, 150 rpm and 60 min residence time. Each of the solution was then taken out and filtered using Whatman 42 filter paper. The filtrate was analyzed for fluoride adsorption using Fluoride Ion Selective Electrode (HI3222 pH/ORP/ISE Meter of Hanna Instruments, USA. Point of zero discharge was determined using KNO₃, HCl and HNO₃ solutions.

In neutralization of red mud with seawater, the calcium and magnesium carbonates of seawater form their hydroxides and get precipitated in red mud. As seawater neutralized red mud and activated seawater neutralized red mud have been used for adsorption of arsenate, molybdate and vanadate, fluoride adsorption on these red mud were studied. It is seen that with an increase in temperature of activation of red mud, quantity of fluoride adsorption on the surface increases. About 8-10% fluoride adsorption takes place on the surface of seawater neutralized red mud activated to 500°C. Point of zero charge was found to be at about pH 9.0 and this indicated that the adsorption of fluoride can be increased if the pH of the slurry is between 6.0- 7.0. Hence the adsorption can be enhanced if seawater neutralized red mud is acid treated. Acid treated red mud would increase the adsorption due to neutralization of hydroxide ions and the change of surface negative charge.

8.0 CONCLUSION:

As it is apparent red mud is a highly complex material that differs due to the different bauxites used and the different process parameters. It is highly alkaline, mainly composed of oxides of aluminium, iron, silicon, titanium and sodium. It has relatively high specific surface area and a fine particle size distribution. Utilization of seawater neutralized red mud by heat activating it and using it for adsorption of fluoride has been studied. Up to 10 % fluoride adsorption has been observed on activated seawater red mud. This can be increased if activated seawater red mud is acid treated. Treatment of this red mud with acid will increase the active sites of adsorption on its surface.

Until now several applications of red mud have been studied. In general all these applications concern the use of red mud in relatively small amounts while the current need is safe disposal of red mud and its bulk utilization.

9.0 REFERENCES:

1. Indian Aluminium Industry “Indian Primary Aluminium Market” (2009). www.scribd.com/doc/.../19149792-Indian-Aluminium-Industry

2. Paramguru RK, Rath PC, Misra VN (2005). Trends in red mud utilization- A Review. Mineral Processing and Extractive Metall. Rev. 26: 1-29.

3. Red mud Project. http://www.redmud.org/Characteristics.html

4. Bott R, Langeloh T, Hahn J (2002). 6^th International Alumina Quality Workshop, Chandrashekar S (Ed). AQW Inc., Brisbane. Dry bauxite residue by hi-bar steam pressure filtration: 24-32.

5. Huang W, Shaobin W, Zhonghua Z, Li L, Xiangdong Y, Victor R et al. (2008). Phosphate removal from wastewater using red mud. Journal of Hazardous Materials 158 (1):35- 42.

6. Gupta V K, Ali I, Saini VK (2004). Removal of chlorophenols from wastewater using red mud: an aluminum industry waste. Environmental Science and Technology 38 (14):4012-4018.

7. Tor A, Cengeloglu Y, Mehmet EA, Mustafa E (2006). Removal of phenol from aqueous phase by using neutralized red mud. Journal of Colloid and Interface Science 300 (2):498- 503.

8. Cengeloglu Y, Tor A, Gulsin A, Mustafa E, Sait G (2007). Removal of boron from aqueous solution by using neutralized red mud. Journal of Hazardous Materials 142 (1-2): 412-417.

9. Gupta VK, Gupta M, Sharma S (2001). Process development for the removal of lead and chromium from aqueous solutions using red mud--an aluminium industry waste. Water Research 35 (5):1125-1134.

10. Gupta VK, Sharma S (2002). Removal of cadmium and zinc from aqueous solutions using red mud. Environmental Science and Technology 36 (16):3612-3617.

11. Sahu RC, Patel R, Ray BC, Utilization of activated CO2 neutralized red mud for removal of arsenate from aqueous solutions, J Hazard Mater 2010 Jul 15;179(1-3):1007-13.

12. Hofstede HT (1994). Use of bauxite refining residue to reduce the mobility of heavy metals in municipal waste compost. PhD thesis, School of Biological and Environmental Sciences. Murdoch University Digital Theses Program.

13. Westman AL, Rouse JV, Jonas JP, JR, Bardach NM (2009). Solid-phase activation of bauxite refinery residue for heavy metals remediation. US Patent 20090234174.

14. Palmer Sara J, Nothling M, Bakon K, Frost R (2010). Thermally activated seawater neutralised red mud used for the removal of arsenate, vanadate and molybdate from aqueous solutions. Journal of Colloid and Interface Science 342 (1):147-154.

15. Oceanplasma. Chemistry of Seawater. http://oceanplasma.org/documents/chemictry.html#concentrations

16. Brunori C, Cremisini C, Massanisso P, Pinto V, Torricelli L (2005). Reuse of a treated red mud bauxite waste: studies on environmental compatibility. Journal of Hazardous Materials 117 (1): 55–63

17. Palmer SJ, Ray LF (2009). Characterisation of bauxite and seawater neutralized bauxite residue using XRD and vibrational spectroscopic techniques. Journal of Material Science 44 (1):55-63.

18. Shuwu Z, Changjun L, Zhaokun L, Xianjia P, Haijing R, Jun W (2008). Arsenate removal from aqueous solutions using modified red mud. Journal of Hazardous Materials 152 (2):486 - 492.

19. Fuhrman GH, Jens CT, McConchie D (2004). Adsorption of arsenic from water using activated neutralized red mud. Environmental Science and Technology 38 (8):2428 – 2434.

20. Rai ,SB, Wasewar, KL_,Lataye, DH, Mukhopadhyay, J. (2012). Feasibility of red mud neutralization with Seawater using Taguchi’s Methodology, in International Journal of Environmental Science and Technology, Available online 30/10/12.

21. Mcbride, M., (1994): Environmental Chemistry of Soils. New York; Oxford University Press.

22. McConchie, D.; Clark, M.; Hanahan, C., (2000). In 3^rd Queensland Environmental Conference on Sustainable Solutions for Industry and Government, Brisbane, QLD; Australia. The use of seawater neutralized bauxite refinery residues in the management of acid sulphate soils, sulphidic mine tailings and acid mine drainage. 201-208.

23. Cengeloglu, Y., Kir, E. and Ersoz, M. (2002). Removal of fluoride from aqueous solution by using red mud. Sep. Purif. Techol., 28: 81-86.

24. Tor A, Danaoglu N, Cengeloglu Y (2009). Removal of fluoride from water by using granular red mud: Batch and column studies. Journal of Hazardous Materials 164 (1): 271-278.

25. Wei, N., Luan, ZK., Wang, J., Shi, L., Zhao, Y. and Wu, JW. (2009). Preparation of modified red mud with aluminum and its adsorption characteristics on fluoride removal. Chin. J. Inorg. Chem., 25: 849–854.

Received on 30.01.2013 Accepted on 14.03.2013

Research J. Engineering and Tech. 4(2): April-June, 2013 page 57-61