INTRODUCTION:

Production of alumina from bauxite, called as the Bayer process is the most economic process of obtaining alumina 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’. Red mud is a mixture of compounds originally present in the parent mineral, bauxite and of compounds formed during the Bayer cycle.

Bauxite ore mined globally amounts to be around 205 million per year¹. This indicates that an enormous quantity of red mud is generated worldwide every year posing a very serious and alarming environmental problem. The red mud generation is 1.3-2 times the alumina production depending upon the quality of bauxite used. Worldwide generation of red mud is estimated to be 75 million tones per year. Red mud is a highly complex material that differs due to the different bauxites and the process parameters used in an alumina refinery.

As the bauxite has been subjected to sodium hydroxide treatment, the red mud is highly alkaline with a pH in the range of 10.5-12.5. Mineralogically, these red muds have phases of undigested alumina, aluminosilicates, phases of iron and titania. Sodium is present as free soda in the entrained liquor in the red mud slurry which gets incorporated during digestion process and remains with red mud in spite of repeated washings. Free soda is in the form of NaOH, Na₂CO_3,NaAlO₂ etc. The pH of the red mud is due to the presence of these alkaline solids in red mud. Inclusion of caustic soda in bound form (sodium aluminosilicates) in the red mud is due to the desilication step carried out in the Bayer process for removal of kaolinitic silica from the bauxite ore.

Red mud is disposed off as slurry having a high solid concentration of 30-60% with a high ionic strength. It is disposed as dry or semi dry material in red mud pond or abandoned bauxite mines which are engineered disposal areas that eventually have to be revegetated. This is challenging because of the alkaline nature of the residue. Up to 2 tons of liquor with a significant alkalinity of 5- 20 gpl caustic (as Na₂CO₃) accompany every ton of dry mud². Due to the caustic soda present in red mud, problems encountered in the past include poor drainage and water logging during the wet season, and salt scalding and capillary rise during the dry season³. High pH, electrical conductivity (E.C.), and extractable Na levels in the residue are greater than can support vigorous plant growth.

These caustic properties must be ameliorated so that vegetation could be established on the surface of large disposal ponds. Also for the restoration of the abandoned mining open pits and for the rehabilitation of the bauxite residue disposal area, the development of a vegetation cover by proper modification of the residue properties can be done. Previous work suggests that additives such as gypsum, sewage sludge have indicated that bauxite residues when amended with these can support vegetation. Studies have been carried out to study the use of red mud and lime to remediate a heavily contaminated acid soil to allow re-vegetation⁴. Work carried out for rehabilitation of red mud pond for an alumina refinery situated at Belgaum (Karnataka, India) show that a combination of 55% red mud, 25% FYM (farmyard manure), 15% gypsum, and 5% vegetative dry dust, inoculated with both bacteria and mycorrhizae, resulted in good responses from three tree species—kikar (Acacia nilotica), karanj (Pongamia pinnata ), and vilayati babul (Prosopis juliflora)—while other two species—drek (Melia azedarach) and Israeli babul (Acacia tortilis)—did not survive in any of the treatment combinations. Among the grass species, para grass (Brachiaria mutica), signal grass (B. decumbens), and shrubby stylo grass (Stylosanthes scabra) performed well in the same treatment combination as the trees, along with sesban (Sesbania sesban), a legume species⁵.

The effectiveness of various industrial wastes and low cost chemicals such as gypsum, sewage sludge, ferrous sulfate, ammonium sulphate, ammonium nitrate and calcium phosphate as ameliorants for red mud to develop and maintain a low cost, self sustaining vegetation cover has been studied by many researchers⁶. In the present study, materials such as coal dust, superphosphate and gypsum which act as neutralizers/ amenders have been used to ameliorate the caustic properties of red mud from an alumina refinery located on the eastern coast of India. The present work includes selection of materials to be used as additives in bauxite residues and experiments to study the effectiveness of the selected mixture to support vegetation.

MATERIALS AND METHODS:

Red mud from Indian alumina plant (eastern coast); coal dust from Khaparkheda Thermal power plant, Nagpur; gypsum and superphosphate were used for the study.

Red mud: Typical Chemical composition of red mud is given in Table I:

Red mud is very fine material in terms of particle size distribution. Typical values would account for over 60% less than 10 microns. The specific surface area (BET) of red mud is about 10 m²/g.

TABLE I. Typical Chemical Composition of Red Mud.

Constituents	Percentage
Al₂O₃	16.0-18.0
Fe₂O₃	54.0-57.0
SiO₂	6.0-7.0
TiO₂	5.0-6.0
Na₂O	4.0-5.0
CaO	2.0-3.0

The selection of modifiers/ amenders was based upon their characteristics. Coal dust, superphosphate and gypsum were the amenders used to modify the caustic properties of red mud.

Coal dust: Coal is a combustible carbonaceous rock, formed from accumulated vegetable matter that has been altered by decay and various amounts of heat and pressure over millions of years. The humus mass and other materials get oxidized to acids when exposed to atmosphere. 10 gm coal dust boiled with 100 ml of distilled water shows a pH value of 9.30. Table II shows the chemical composition of coal dust.

TABLE II. Composition of coal dust

Air dry basis
Ash %	24.07
VM%	24.11
FC% Moisture Content	39.07 12.75
Derived ultimate analysis
C, %	52.80
H, %	3.0
N, %	1.2
S, %	0.60
O₂, %	7.30
CO₂, %	0.81
Eq. moist %	7.45

Superphosphate: Superphosphate is a fertilizer produced by the action of concentrated sulfuric acid on powdered phosphate rock⁷. It is highly acidic in nature as 5 gm of superphosphate when boiled in 50 ml of distilled water shows a pH value of 2.17.

Gypsum: Gypsum is listed as an inorganic fertilizer, with no nitrogen, phosphorus or potassium, but 18% sulfur and a good source of calcium. Gypsum is also useful as an additive for soils with high levels of sodium and hence can suitably be used with red mud. The pH of 10 gm of gypsum when boiled in 100 ml water is slightly acidic with a value of 5.20.

As red mud has high alkalinity, salinity and sodicity, as well as low permeability, preventing the growth of most plants, it was found that the addition of 5% or more gypsum reduced the pH, electrical conductivity and sodium and aluminum content of the soil, as well as providing a continuous supply of calcium ions, thus reducing the exchangeable sodium percentage, and was effective in treating the soil to permit revegetation by Agropyron elongatum (tall wheat grass) and Cynodon dactylon (Bermuda grass)⁸.

Method:

Red mud, coal dust, superphosphate and gypsum were ground to 100 mesh size and used for the study. Magnetic stirrer (Eltek M S 204) was used for stirring of red mud slurry mixed with different materials. The pH was measured on calibrated pH meter (Orion EA940, Thermo Electron Corporation).

Effectiveness of the materials such as coal dust, superphosphate, gypsum to amend the caustic characteristics of red mud was seen by adding them in different quantities to red mud slurry (prepared by mixing 50 gm of red mud with 100 ml distilled water) and measuring the pH value.

As the effectiveness and quantity of the amenders was determined from the above experiments, they were subsequently added to 1 kg red mud in suitable proportions to carry out the vegetation studies in earthern pots 1, 2, 3, 4 and 5. Also some quantity of soil and vermicompost were added to the mixture to provide nutrients and organic constituents as red mud is devoid of these. Soil was added based upon the amender added. Soil was added in less quantity where the amender added was more e.g. coal dust. It was added in more quantity where amenders added were less in quantity to give a proper base to the plants. Vermicompost was added in equal quantity in all the earthern pots. Plant no 1 and 5 are amended with same amount of gypsum except for the soil quantity which has been varied to see the effect of quantity of soil added. After properly mixing all the ingredients and water, shrubs (ornamental plants) were planted in all the earthern pots.

RESULTS AND DISCUSSION:

Results:

Effect of coal dust, superphosphate, gypsum as modifiers/ amenders/ neutralizers in red mud:

The pH of red mud is measured with increasing quantity of these amenders. Figure 1 show the pH of red mud slurry when coal dust is used in different quantities. Figure 2 shows the pH of the red mud slurry with superphosphate, gypsum and a combination of coal dust and superphosphate. It is observed that a large amount of coal dust (50 gm) is required to lower the pH of red mud slurry. Comparatively a lesser amount of gypsum (5-10 gm) and superphosphate (5-10 gm) can be used to reduce the pH of red mud. Nearly same results are seen when superphosphate (5 gm) and coal dust (10 gm) are used at a time. The results show that gypsum is the most effective modifier/ amender followed by superphosphate and coal dust mixture.

Fig.1: Change in pH of red mud slurry with increasing quantity of coal dust

Fig. 2: Change in pH of red mud slurry with different quantity of neutralizers

Table III: Vegetation studies on red mud (1kg) with amenders.

Plant no.	Amenders	Qty of soil, gm	Qty of compost, gm	Name of the shrub
1	Gypsum (100 gm)	700	150	Dudh Mogra
2	Coal dust ( 1000 gm)	200	150	Croton
3	Superphosphate (100 gm)	1350	150	Croton
4	Superphosphate (100 gm) +Coal dust (200 gm)	1350	150	Eksora Dork
5	Gypsum (100 gm)	1200	150	Verigated Tagar

Vegetation studies:

Table III shows the details of the vegetation study carried out. All the plants were monitored for 8 months time duration for their growth. It was seen that new and green leaves have come up in all the plants. Flowers have also been observed in flower bearing plants. Two of the plants have shown flower buds in them. Soil quantity does not have much effect as the plants have grown properly even with less quantity of soil. Fig. 3, 4, 5, 6, 7 show the growing plants in earthen pots which have plants numbered according to Table III.

Fig. 3: Vegetation with red mud amended with gypsum with less quantity of soil (Plant no. 1)

Fig. 4: Vegetation with red mud amended with coal dust (Plant no. 2)

Fig. 5: Vegetation with red mud amended with super phosphate (Plant no. 3)

Fig. 6: Vegetation with red mud amended with super phosphate and coal dust (Plant no. 4)

Fig. 7: Vegetation with red mud amended with gypsum with more quantity of soil (Plant no. 5)

DISCUSSION:

The overall study shows that the alkalinity of red mud can be reduced to a great extent when mixed with coal dust, superphosphates and gypsum. Coal dust is a low-value waste material produced by coal mining and coke production operations can be used for amending the red mud. Though the amount of coal dust required is quite large, it is very effective when used along with superphosphate as the optimum results are obtained with the combination of the two. The quantity of coal dust required is very less when used with superphosphate. Superphoshate is highly acidic in nature and a very effective fertilizer. Gypsum is also a successful neutralizer as it has a very high percentage of calcium and sulfur in it. Vegetation studies show that red mud can very well be vegetated when mixed with suitable amenders in proper proportion along with soil and compost.

CONCLUSION:

Red mud is dominated by presence of alkaline solids (hydroxides, carbonates and aluminates) and the alkalinity of red mud cannot be reduced even by several washings and hence limits its further usage. The alkalinity and pH of red mud can be reduced substantially if amended in suitable proportions with modifiers such as coal dust, superphosphate and gypsum. After modifying red mud with these materials, it certainly becomes a much less hazardous material for disposal and a better waste management can be carried out. It can be used for refilling the abandoned bauxite mining open pits and for the rehabilitation of the bauxite residue disposal area above which a proper vegetation cover can be developed.

REFERENCES:

1. U.S. Geological Survey, Mineral Commodity Summaries, January 2009, http://www.usgs.gov

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

3. Wehr BJ, Fulton I and Menzies N.W. Revegetation strategies for bauxite refinery residue: a case study of Alcan Gove in Northern Territory, Australia. Environmental management, 2006; March 37(3): 297-306.

4. Gray CW, Dunham SJ, Dennis PG, Zhao FJ and McGrath SP. Field evaluation of in situ remediation of a heavy metal contaminated soil using lime and red-mud. Environmental pollution Aug. 2006; 142(3): 530-9.

5. Suresh Chauhan and Chandra Shekhar Silori. Rehabilitation of Red Mud Bauxite Wasteland in India (Belgaum, Karnataka). Ecological Restoration, March 2010; Volume 28, 12-14. Number 1, E-ISSN: 1543-4079 Print ISSN: 1543-4060.

6. Xenidis A, Harokopou AD, Mylona E, Brofas G. Modifying Alumina Red Mud to Support a Revegetation Cover, GJOM Journal of the Minerals, Metals and Materials Society 2005; 57, 42-46.

7. McMurry, J., Fay Robert C., Chemistry (4 ed.), 841–842. 2004.

8. Wong WC, Ho GE, Use of Waste Gypsum in the Revegetation on Red Mud Deposits: A Greenhouse Study. Waste Management and Research, 1993;11, 249-256.

Received on 25.05.2011 Accepted on 09.06.2011

Research J. Engineering and Tech. 2(3): July-Sept. 2011 page 109-113