Red Mud uses from Alumina Production

 

Review on utilization of Red Mud

Red mud is solid waste which is produced as residue during the manufacturing of alumina. Alumina is produce from bauxite ore. About 35-40% of bauxite is converted into waste as alkaline red mud during alumina production which contains 15-40% solid. A large amount of red mud is produced every year during alumina production. Red mud is discharged on to the ground as slurry near production plant, in ponds or nearby sea. It contains several metal oxides like Fe₂O₃, Na₂O, SiO₂, CaO, and Al₂O_3. Red mud has alkaline pH greater than 10. Due to presence of heavy metals and alkaline pH, the disposal of huge amount of red mud lead to serious environmental problems like soil contamination, ground water pollution and particle suspension in sea. Also disposal of red mud cost very high. Therefore it is necessary to find alternative solution for red mud solid waste so that it cannot cause harmful effects to environment.

 Alumina is produced by two methods; Bayer’s process and Sintering process. Red mud is produce by both methods in large quantity. Stockpiling of red mud is done by two methods; wet stocking and dry stocking. In wet stocking, waste is transported into yard as slurry and then stocked after precipitation. While in dry stocking it is transported to yard and then waste is either air or solar dried. Wet stocking has more requirements and cost high as compared to dry stocking. Wet stocking is used for stocking of red mud obtained by sintering process and dry stocking is used for red mud obtained by Bayer’s process. Later on, an intermediate method for stocking of red mud is developed known as mixed stocking. It utilizes red mud obtained from sintering as well as Bayer’s process.

Research is carried out to recover valuable metals present in red mud. Several techniques are developed for the recovery of components present in waste. Red mud contains high concentration of iron oxide. The recycling process of iron from mud include; roasting magnetic recovery, the reducing smelting method, the direct magnetic separation method and the leaching extraction method. Researchers from the University of Central South have made steel directly with iron recovered from red mud. Aluminum and sodium are also present in higher content in red mud. Al₂O₃ and Na₂O are recovered through sub molten salt method by Zhong et al with recovery rate of Al₂O₃ up to 86%. Red mud is treated with NaOH solution to recover Na₂O from residue.

Red mud is used in construction material production such as cement, bricks, roofing tiles and glass ceramics. The production of construction material in large amount from red mud can reduce the disposal problem. When red mud is treated with alkali metal silicate solution under alkaline conditions geopolymers are formed. Geopolymers are synthetic aluminosilicate materials that can be used in replacement of Portland cement and high tech composites and ceramic applications. Clay based products are prepared from red mud. These products have advantges of high compressive strength, decreased hydraulic conductivity and swelling percentage as compared to ordinary clay products. A mixture of red mud and cement red mud is used for stabilization of clay liners in geotechnical products.

Red mud is also used in production of cement along with fly ash, lime and gypsum as raw material. It increases early strength of cement and resistance to sulfate attack. Red mud helps in crystallization of cement clinkers. The use of red mud in brick production reduced its cost and resulted in environmental friendly production of bricks. Non steam cured and non-fired bricks are obtained from red mud and fly ash using cement and lime as binder. Red mud and chromium slag along with other materials are used for manufacturing of black glass decorative materials. These materials possess good mechanical strength, optical properties and chemical stability. Red mud is also used as raw material in concrete block making.

Red mud is used as filling material. Red mud obtained from sintering process is used as base material in construction of roads. First experiment in this respect was performed in China which revealed that red mud meets the strength requirements and it can be used as a base material to build highways. Red mud is also used in mining. Red mud is mixed with fly ash, lime and water and is pumped into the mine to prevent subsidence during bauxite mining. This method not only utilize red mud in large amount but also reduce filling cost and increase benefits of mining. Red mud is used in plastic making like PVC. PVC obtained from red mud is used as filler and thermal stabilizer which provide anti-aging property to filled PVC products. The fluidity of red mud is better than other fillers which provide plastic with good processing properties.  

Red mud is used in pollution control. It is used in wastewater treatment to remove toxic heavy metals, inorganic ions, organic compounds and bacteria. Wastewater treatment mostly utilized neutralized red mud for boron, phenol removal or seawater neutralized red mud for arsenate removal. Red mud is also used to neutralize the acid forming gasses produced during coal combustion. Red mud and cement kiln can be used to limit acid mining drainage.

Red mud can absorb heavy metal ions such as Cu²⁺, Ni²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Cr⁶⁺, Mn⁴⁺, Co³⁺ and Hg²⁺ in the soil. There is precipitation reaction between carbonates in red mud and heavy metals ions that reduces the reactivity of heavy metals ions in soil, microbial activity and plant growth is promoted.  Red mud is used to absorb acidic SO₂ exhaust gas produced by roasting process by treating it directly with alkaline red mud generated in alumina production process.

Red mud also finds its applications in catalysis. It is used as catalyst in hydrogenation, hydro-chlorination and hydro-oxidation. Red mud based catalyst have advantages over commercial catalysts due to its properties such as high surface area, sintering resistance, iron content in form of ferric oxide and low cost. It is also used as adsorbent and coagulant for water and gas treatment.

All these technologies used a small amount of red mud as compared to annual production of red mud. Therefore the comprehensive utilization of red mud generated during alumina production is still a challenging problem for world. Some technologies are in practice and researchers are working to develop new methods for utilization of red mud solid waste in order to recover valuable metals present in residue and reduce environmental effects caused by red mud.