TECHNOLOGY ALTERNATIVES FOR ALUMINIUM INDUSTRY IN INDIA
 
Aluminium Industry

The manufacture of aluminium involves two important stages (i) refining bauxite to aluminium oxide (alumina) by the Bayer process and (ii) reducing alumina, in Hall-Heroult electrolytic cells to the aluminium metal.

In Bayer process, the bauxite material is crushed and digested in a heated caustic solution. The solution is then filtered to remove insoluble residue (red mud) and the pregnant liquor is cooled so that precipitation of aluminium oxide occurs. The precipitated material is removed from the process stream by filtration and calcined to produce aluminium oxide.

In the Hall-Heroult process, aluminium oxide is dissolved in an electrolytic bath composed mainly of cryolite, sodium aluminium fluoride and aluminium fluoride. The electrolytic cells consist of open steel vessels, that are lined with carbon. The carbon lining serves as the cathode for electrical conductance. Consumable carbon anodes (Soderberg or prebaked type) are used to provide a carbon source to react with the oxygen liberated in the electrolysis.

The major pollutants/wastes generated are (i) red mud (solid waste) from refining of bauxite: (ii) fluoride emission (air pollutant) and spent pot lining (solid waste) from electrolytic stage. The red mud is highly alkaline and hence needs special precaution while disposing to avoid pollution of surface as well ground water resources. The specific generation of red mud in our country is 1.16- 1.4 T/T of alumina production. The total generation of red mud in India is about 2 million TPA.

The specific generation rate for spent pot lining in our country varies from 43-62 Kg per tonne of aluminium produced. The spent pot lining carbon portion contains 4-8% leachable fluoride as well 0.01 to 0.025% leachable cyanide, requiring handling and management as per Hazardous Wastes (Management & Handling) Rules, 1989.

The existing pollution control/waste management systems and the requirements tabulated below:
 

Existing Management System
Requirements
Red Mud :
(a) Wet disposal : In this method, washed red mud slurry containing 10-30% solids is pumped to the pond.
(b) Dry disposal : In this method, the red mud disposed contains 30-50% moisture and also known as thickened tailing disposal		 Dry disposal in secured land fill, as dry disposal requires much less space (1/3 to 1/5 of Wet disposal) and less seepage is expected.
Spent Pot Lining :
(a) Flouride recovery followed by use as fuel.
(b) Disposal in secured landfill to avoid leakage of fluoride and cyanide.		 (a) Fluoride recovery followed by reuse for carbon portion (Impact of such reuses need to be investigated).
(b) Disposal in secured landfill for refractory portion.
Fluoride emission :
(a) Dry scrubbing (using alumina): By this most of the fluoride is recycled into the system.
(b) Wet scrubbing. This causes water pollution. The treatment of water pollutants results in generation of solid waste (calcium fluoride).		 Dry scrubbing, as this helps in recycling fluoride and also there is no water pollution.

SOURCE: CENTRAL POLLUTION CONTROL BOARD
DATE : 28TH NOVEMBER, 2000
 
KEY WORDS :

TECHNOLOGY ALTERNATIVES, ALTERNATIVE TECHNOLOGIES, BEST PRACTICES, INDIAN INDUSTRY, TECHNOLOGY TIPS, POLLUTION CONTROL, CENTRAL POLLUTION CONTROL BOARD, CPCB, TECHNOLOGY OPTIONS, ALUMINIUM INDUSTRY, ALUMINIUM