The Bayer Process for the production of alumina from Bauxite ore is characterized by low energy efficiency and it results in the production of significant amounts of dust-like, high alkalinity bauxite residues known as red mud. Red mud or red sludge is a solid waste product of the Bayer process, the principal industrial means of refining bauxite in order to provide alumina as raw material for the electrolysis of aluminium by the Hall–Héroult process.
Red mud is composed of a mixture of solid and metallic oxide-bearing impurities, and presents one of the aluminium industry's most important disposal problems. The red colour is caused by the oxidised iron present, which can make up to 60% of the mass of the red mud. The other dominant particles include silica, unleached residual aluminium, and titanium oxide.
The red mud is washed several times with water to recover caustic soda. Sea water is then added to neutralise any remaining caustic soda, before being pumped to the Residual Disposal Area (RDA).
Two Process for production of Red Mud
Bayer Process
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide). Bauxite, the most important ore of aluminium, contains only 30–54% aluminium oxide, (alumina), AlβOβ, they are mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide is purified and refined to aluminium metal.
Hall–Héroult process
The Hall–Héroult process is the major industrial process for the production of elemental aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell.
Red Mud- How it is treated
Red mud from the washing stage in is alkaline at pH 14. Seawater, at pH 8, is added to the mud to neutralise residual caustic, reducing the pH of the mud to 8. The mud is then thickened and deposited in the dam as slurry from multiple points around the dam perimeter to form drying beaches. The water that drains from the red mud is of similar pH to sea water and is discharged continuously from the RDA into the estuary.
Application areas for Red Mud
Building materials
Red Mud is utilized for building materials production such as cement, bricks, roofing tiles and glassceramics.
Use as a coagulant, adsorbent and catalyst
Red Mud can be employed as catalysts for hydrogenation, hydrodechlorination and hydrocarbon oxidation. It is a support in catalytic wet oxidation of organic substances present in industrial wastewaters.
Recovery of metals
Iron is the major constituent of red mud. Iron is obtained as value added product and alumina and soda is recycled in the process. Some red mud contains titania in substantial amount, if successfully recovered, has the most potential value.
Soil remediation
After adequate neutralization, red mud can be used for the remediation of contaminated sites and treatment of contaminated liquid waste. A novel process for making radiation shielding materials utilizing red mud is developed by adopting ceramic chemical processing route using phosphate bonding.
Red mud is utilised for developing plasma spray coatings (ceramic and cermet) on metal substrates, stainless steel, mild steel, Cu & Al. As red mud consists of metal oxides of iron, titanium, silicon, aluminium, it is felt that red mud can be spray coated.
Red Mud in India
More than 4 million tons of red mud is generated annually in India. It is stored or dumped on land, or in the oceans near alumina refineries. India is amongst the major producers of alumina in the world. There are some differences in mineralogical composition between the residues from India and other countries due to the difference in the ore type in its production processes.
Vedanta Aluminium Ltd (VAL) commissioned a red mud powder producing unit at Lanjigarh refinery in Odisha, the first of its kind in alumina industry tackling major environmental hazards.
Building Material and Technology Promotion Council of India (BMPTC) produced composite from red mud, polymer and natural fibres, called Red Mud Jute Fibre Polymer composite (RFPC), to replace wood in the wood based panel products in the building industry.
