1.0 INTRODUCTION:

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 (containing 40-60% Al₂O₃) digestion with sodium hydroxide at elevated temperature and pressure to produce alumina is known as ‘red mud’ or ‘bauxite residue’. The waste product gets its name as red mud due to the oxidized iron content present in it. Red mud is a mixture of compounds originally present in the parent mineral, bauxite and of compounds formed during the Bayer cycle. As the bauxite has been subjected to sodium hydroxide treatment, red mud is highly caustic with a pH in the range of 10.5-12.5. The main reaction that occurs in the Bayer process (the conversion of bauxite to sodium aluminate) can be schematized as follows:

2NaOH + Al₂O₃.3H₂O → Na₂O.Al₂O₃ + 4H₂O + Red Mud (1)

About 1 tonne of alumina is produced from 3 tonnes of bauxite and about 1 tonne Aluminium is produced from 2 tonne of alumina [1]. The worldwide alumina production is around 58 million tonnes in which India counts for 2.7 million tonnes [2]. Depending on the raw material processed, 1- 2.5 tons of red mud is generated per ton of alumina produced [3]. Thus an enormous quantity of red mud is generated worldwide every year (70 million tones) posing a very serious and alarming environmental problem and hence voluminous research and development work for the storage, disposal and utilization of red mud is being carried out all over the world.

Red mud is disposed as dry or semi dry material in red mud pond or abandoned bauxite mines and as slurry having a high solid concentration of 30-60% and with a high ionic strength. Problems associated with the disposal of red mud waste include: its high pH, alkali seepage into underground water, safety in storage, alkaline air borne dust emissions and vast disposal area. Up to 2 tons of liquor with a significant alkalinity of 5- 20 g l^-1 caustic (as Na₂CO₃) accompany every ton of dry mud. Red mud mainly consists of Fe₂O₃ (35-55 wt %), Al₂O₃(16-22 wt %), SiO₂(4-16 wt %), Na₂O (3-6 wt %), TiO₂ (2-19 wt %), CaO (0.8-4.5 wt %) and other oxides in trace amount depending upon the mineralogy and process parameters used. Red mud is a very fine material in terms of particle size distribution. Average particle size of red mud is less than 10 microns. A few particles of greater than 20 microns are also available. The specific surface area (BET) of red mud is between 10 and 30 m²/g depending upon the grinding of bauxite.

Safe treatment and storage of red mud pose unique waste management challenges. Though red mud appears to be consolidated, it has a muddy consistency because of the fineness of the material involved and their colloidal nature due to the caustic soda present in it. Treatment of red mud will help to reduce its environmental impact and reuse options of red mud will open. Efforts to ameliorate red mud are being carried out by possibly incorporating a pH-reduction processing step. In some of the methods, caustic soda is neutralized to make red mud environmentally benign while method such as in sintering, red mud is mixed with materials such as fly ash and the product may be used as a building and construction material. The sintering of residue is carried out to fix all leachable soda and the mechanism can be made use in making bricks and blocks from red mud.

Several studies have been carried in utilization of red mud in the field of pollution control (in wastewater treatment, absorption and purification of acid waste gases), metal recovery (iron, titanium, aluminium, alkali, rare earths), coagulant, adsorbent, catalyst and in soil remediation. Researchers have also investigated its application in the field of building (geo-polymers, clay material, cements, ceramics, fired and non-fired building materials, concrete) industry. Construction industry being one of the fastest growing sectors in India, is facing a short fall of traditional building materials due to growing demand of houses. Red mud can be an alternative to conventional brick materials such as clay which will reduce the cost of raw materials and also promote bulk utilization of red mud. A considerable research has been done on the utilization of red mud as a raw material for production of a range of building products. Red mud can be used as a constructional / building material in bricks, blocks, light weight aggregates, roofing tiles, glass ceramics, in cement industry as cements and special cements and in concrete industry. The paper provides an important solution in red mud as an alternative to clay material for its utilization in making fired and non- fired bricks.

2.0 Utilization of red mud as a brick material:

Vast usage of red mud can be made in preparation of building materials such as fired and non-fired bricks and blocks. This will reduce the cost of raw material for making bricks and introduction of red mud in these processes would be highly beneficial. The bulk production of building materials could eliminate the disposal problem.

2.1 Fired building materials:

United States Patent 3886244 [4] claims a process for manufacturing fired bricks wherein 50-90 wt % of red mud can be used along clay and a water fixing agent. The raw bricks are dried with heated gases at a temperature below 70°C, and subsequently fired at a temperature between 900°-1,100°C. Efforts have been made at Central building Research Institute, CBRI, India [5] to produce burnt clay bricks by partially replacing the clay with red mud (from the Indian Aluminium Company), lime and fly-ash. Researchers [6] have used red mud along with plastic clay and talc to make bricks/blocks having a very high compressive strength. These bricks were sundried before firing at 950-1000^oC.

2.2 Non-fired building materials:

Red mud has been treated with hydrochloric acid to dissolve only the sodium content [7] and the treated material is mixed with kaolinitic clay and formed into construction bricks. Efforts have also been made at CBRI [5] to incorporate a small percentage of lime in red mud and compress the mix at optimum moisture content in the form of bricks with the purpose of examining their strength and stability to the erosive action of water. A maximum wet compressive strength of 3.75 MPa with 5% lime and 4.22 MPa with 8% lime has been obtained after 28 days of casting and humid curing of these bricks in the month of August. Red mud and fly ash were used in making non-steamed cured and non-fired bricks by Xing (1993)[8] and Yang (1996) [9].

Studies were carried out at Jamaica Bauxite Institute and the University of Toronto [10] using red mud to make bricks for inexpensive housing. The red mud was pressed into bricks using a standard brick press, immersed in sodium silicate followed by drying in the sun. Non-fired bricks by mixing red mud, Portland cement and river sand were also made by the researchers at the institute. Un-sintered bricks have been developed from red mud disposed from Chinese sintering alumina process cured at ambient conditions. The optimal proportions of red mud brick are suggested as the following: 25–40% red mud, 18–28% fly ash, 30–35% sand, 8–10% lime, 1–3% gypsum and about 1% Portland cement [11]. Liu et al [12] studied the recovery of iron from Bayer red mud with direct reduction roasting process followed by magnetic separation, and then building materials were prepared from aluminosilicate residues. The brick specimens were prepared with aluminosilicate residues and hydrated lime and the mean compressive strength of specimens was 24.10 MPa. It was indicated that main mineral phase nepheline (NaAlSiO₄) in aluminosilicate residues transformed into gehlenite (Ca₂Al₂SiO₇) in brick specimens as demonstrated by X-ray diffraction (XRD) technology. Combining the recovery of iron with the reuse of aluminosilicate residues, it can realize zero-discharge of red mud from Bayer process. The potential use of red mud for synthesis of inorganic polymeric materials through a geopolymerization process was studied to use it in the construction sector as artificial structural elements such as massive bricks [13].

3.0 DISCUSSION:

Red mud is a highly complex material that differs due to different bauxites used and different process parameters. As the high pH is highly lethal to natural ecosystems, disposal of red mud can unquestionably be made safer by treating and utilizing it. Most significant hazard associated with the bauxite residue can thus be removed. Fired and non-fired bricks can be manufactured from it by using red mud in place of clay as a raw material certain waste materials such as fly ash and sintering it. About 50-70% of red mud can be mixed with other wastes and materials to manufacture fired bricks. However, in fired bricks, a fairly high temperature is required for sintering to achieve high compressive strength. Non-fired bricks and blocks can also be made by utilizing red mud along with materials such as lime. In this way bulk utilization of red mud can be realized by using it in building and construction industry. Application of red mud in geo-polymers requires minimum heat treatment in which artificial structural elements such as bricks can be made.

4.0 CONCLUSION:

One of the economically viable and environmentally acceptable solutions for the utilization of large volumes of red mud appears to be in building and construction sector. Alkalinity of red mud which is the most important barrier for its reuse and disposal can be reduced by mixing red mud with certain waste materials such as fly ash and sintering it. Other materials such as lime and gypsum can also be used in the process. In nearly for all of the other applications of red mud, its use in building materials makes bulk utilization of red mud possible which simplifies its problem of disposal and residue management.

5.0 REFERENCES:

[1] Annual report, Chap V/ Department of Ministry of Mines, India 1999-2000, http://mines.nic.in/archp5.html http://www.portal.gsi. gov.in/ gsiDoc/pub/DID_Bauxite_WM.pdf

[2] Indian Aluminium Industry “Indian Primary Aluminium Market”, 2009. www.scribd.com/doc/.../19149792-Indian-Aluminium-Industry

[3] Paramguru K, Rath PC and Misra VN. Trends in red mud utilisation- A review. Mineral processing and Extractive Metallurgy Review, 2005: 26: 1-29.

[4] Garhard B, Method for producing bricks from red mud. US Patent 3886244,(1975).

[5] Dass A, Malhotra SK Lime-stabilized red mud bricks. Materials and Structures, 1990, 23: 252-255.

[6] Satpathy BK, Chaddha MJ, Sharma RJ, Rai SB, Roy A. Preparation and Characterization of bricks using bauxite residue- A Innovative Approach. ICSOBA -2011, International Committee for Study of Bauxite, Alumina and Aluminium at Goa, India , 17-19th Oct. 2011. International Seminar on Bauxite Residue.

[7] Iwu. Gregory Onyemauwa, Method of treating bauxite waste red mud with acid and making construction bricks from the treated material, United States Patent 3985567, (1976).

[8] Xing, G, Jiao, ZZ. The development of non-autoclaved brick made of red mud and fly ash. Rare Metals Cemented Carbides 1993, 6, 154-163.

[9] Yang. AP. The development of brick made of red mud and fly ash. Light Metals 1996, 12, 17-18.

[10] Peter N, IDRC Reports. Making Bricks with red mud in Jamaica, No. 21(2). International Development Research Centre, Ottawa, Canada (1997).

[11] Jiakuan Yang and Bo Xiao . Development of unsintered construction materials from red mud wastes produced in the sintering alumina process. Construction and Building Materials, 2008, 22 (12): 2299-2307.

[12] Liu W, Yang J,Xiao B. Application of Bayer red mud for iron recovery and building material production from alumosilicate residues. Journal of hazardous materials, 2009, 61 (1): 474-478.

[13] Dimas DD, Ioanna P, Panias D. Utilization of alumina red mud for synthesis of inorganic polymeric materials. Mineral processing and Extractive Metallurgy Review, 2009, 30 (3): 211 –239.

Received on 11.01.2013 Accepted on 20.01.2013

Research J. Engineering and Tech. 4(1): Jan.-Mar. 2013 page 12-14