Bauxite residue

Red mud or red sludge is a highly alkaline waste product composed mainly of iron oxide that is generated in the industrial production of aluminum. With about 77 million tons of this hazardous material being produced annually, red mud poses a serious disposal problem in the mining industry.

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. 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.

Many studies have been conducted to develop uses of red mud.

An estimated 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, application to sandy soils to improve phosphorus cycling, amelioration of soil acidity, landfill capping and carbon sequestration.

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.

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Wikipedia