Spent Pot Lining Processing
Aluminum production is a continuous process where molten aluminium is produced within a refractory lined ‘pot’. It’s a reduction cell is made up of steel rods, with a carbon cathode lining supported by refractory bricks inside a steel shell. Since the reduction of alumina occurs in a molten bath of cryolite (Na3AlF6) at 930-1000°C, the carbon cathode lining becomes impregnated with several fluoride compounds and a small but significant quantity of cyanide (primarily from the reaction between nitrogen and carbon in the presence of sodium at high temperatures). Eventually the cell fails as the voltage increases or iron starts to be detected in the aluminum metal. At the end of the operational lifetime of the cells, the linings are removed and broken up and have to be disposed in some way and the shell has to be relined. However, the spent lining material, which is composed of carbon, refractory material from insulating refractory bricks and cryolite, including fluorine, aluminum, sodium, calcium and silicon values, along with free and complex cyanides, carbides and nitrides, is hazardous and must be treated with great caution. Also, it is leachable, and it can, in certain conditions, generate a flammable and toxic gas. SPL also contains materials that are valuable if recovered and used for specific purposes. The main components with potential value are carbon and fluorides.
The safe disposal of spent linings has for a long time presented a challenge to the industry. With the given environmental regulations of Indian government, the challenge continues. The disposal residues should have low concentrations of fluorides and cyanides. The existing management system includes disposal in secured landfill to avoid leakage of fluoride and cyanide. Central Pollution Control Board reports that the specific generation rate for spent pot lining in our country varies from 43 to 62 Kg per tonne of aluminum produced which accounts for a production on 90000 tonnes of SPL every year.
The spent pot lining carbon portion typically contains 4-8% leachable fluoride as well 0.01-0.025% leachable cyanide. The composition of SPL varies from smelter to smelter. Table 1 presents the range of composition of SPL. The amount of fluorides and cyanides are hence substantial and hence, require handling and management as per Hazardous Wastes (Management & Handling) Rules, 1989.
The Ecozen approach to the processing the by-products of the aluminium production process have a number of environmental benefits:
- No landfill or storage of waste minerals at the smelter site
- Lower process temperatures for manufacturing products such as bricks and cement, meaning lower energy consumption requirements.
- A reduction in greenhouse gas emissions due to lower energy consumption requirements