Recovery of carbon rich material:-Recycling of spent pot lining: A review

Spent pot lining (SPL) is a carbon rich material generated during primary aluminium smelting process. The major constituents of SPL are carbon (40–50%), fluoride (10–18%), aluminium (10–13%), cyanide (700–4500 ppm) and sodium (12–16%) along with other metals like calcium, iron, lithium, titanium and magnesium in trace amounts. SPL ends up getting landfilled but the hazardous composition of the waste (presence of fluoride and cyanide), coupled with toxicity and reactivity with water, harm the environment. Gases like methane, ammonia and hydrogen cyanide evolve when SPL is exposed to water. Water and soil resources are hampered due to the SPL landfilling. This review focuses on the recovery of carbon and fluoride for its utilization as fuel in cement industry and as a carbonaceous material in iron making industry. The article describes hydrometallurgical extraction processes from different grades of SPL in which cyanide is extracted in water washing stage whereas fluoride is recovered as CaF2. The acid leaching (by sulphuric acid, perchloric acid, hydrofluoric acid and hydrofluorosilicic acid) removes the rest of the fluorides, silicates, cryolite, alumina and aluminium fluoride. Final treated SPL is used as reducing agent in iron making industry. SPL has high mechanical strength against breakage, abrasion and compression. Due to these features it can be used as granulated material in reduction reactors. This review article discusses the composition of different grades of SPL, the environmental impacts, methods for recovery of carbon, corundum, cyanides and fluorides and its applications in iron making and cement industry in economical and environment friendly manner.