Red mud or red sludge is a waste product generated in the industrial production of aluminium. With about 77 million tons of this hazardous material produced annually, red mud is one of the most important disposal problems in the mining industry.
Production and composition
Red mud is a side-product of the Bayer process, the principal means of refining bauxite en route to alumina. The resulting alumina is the raw material for producing aluminium by the Hall–Héroult process. A typical bauxite plant produces one to two times as much red mud as alumina. This ratio is dependent on the type of bauxite used in the refining process and the extraction conditions.
Red mud is composed of a mixture of solid and metallic oxides. The red colour arises from iron oxides, which comprise up to 60% of the mass of the red mud. The mud is highly basic with a pH ranging from 10 to 13. In addition to iron, the other dominant components include silica, unleached residual alumina, and titanium oxide.
Red mud cannot be disposed of easily. In most countries where red mud is produced, it is pumped into holding ponds. Red mud presents a problem as it takes up land area and can neither be built on nor farmed, even when the mud is dry. One challenge is that drying the mud requires much energy (latent heat).
Research is being performed to find uses for red mud and some 2 to 3 million tonnes are used annually in the production of cement, road construction and as a source for iron. Potential applications include the production of low cost concrete. Vedanta Aluminium Ltd has commissioned a red mud powder-producing unit at Lanjigarh refinery in Odisha, India, describing it as first of its kind in alumina industry tackling major environmental hazards.
Discharge of red mud is hazardous environmentally because of its alkalinity.
In October 2010, approximately one million cubic meters of red mud from an alumina plant near Kolontár in Hungary was accidentally released into the surrounding countryside in the Ajka alumina plant accident, killing ten people and contaminating a large area. All life in the Marcal river was said to have been "extinguished" by the red mud, and within days the mud had reached the Danube.
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