Gainful utilization of spent pot lining – A hazardous waste from aluminum industry

Abstract

Spent Pot Lining (SPL) or Spent Cathode is a solid waste produced by the aluminium industry during the manufacture of aluminum metal in electrolytic cells. After 3-7 years of
operation, the cathode liner materials deteriorate and affect the cell’s performance and need to be replaced. Due to high fluoride (20 wt. %) and cyanide (1 wt. %) content SPL was listed as hazardous waste by the US Environmental Protection Agency in the year 1988.In the present study, various approaches in the treatment of SPL have been conducted to recover the valuable carbon and fluoride values. Initially a comparative study was investigated by the chemical leaching of water washed SPL with H2SO4 and HClO4 acids and the process parameters were optimized via utilization of Multiple Level Factorial design. For H2SO4 and HClO4 treatment of water washed SPL, L/S ratio was found to be the most significant factor.The carbon content was increased from 42.19% for raw SPL to 70.83% for H2SO4 and 71.76% HClO4 treatment. An approach in chemical leaching of water washed SPL was performed initially with caustic leaching followed by Perchloric acid leaching. In this case, the Temperature was found to be the most significant factor among all the parameters, whereas the L/S ratio was the least significant among the four parameters studied.The carbon percentage of SPL was increased from 42.19 to 87.03% as confirmed from the ultimate analysis. From the proximate analysis, the fixed carbon was increased from 38.96% to 82.86% from the raw to final treated SPL. Another approach in chemical leaching of water washed SPL was conducted initially with causticleaching followed by sulfuric acid leaching. In this
case, Alkali concentration and the Temperaturewere found to be the most significant and least significant factor among the four parameters studied.The process parameters used in
above approaches were optimized by Taguchi method and Classical Method.The carbon percentage of SPL was increased from 42.19% to 81.27% as confirmed from the ultimate analysis. From the proximate analysis, the fixed carbon was increased from 38.96% to 78.68% for sulfuric acid from the raw to final treated SPL.Gross calorific values (GCV) were
determined for the all optimized samples obtained from above studies. The GCV was found to be increased from 2865.04kcal/kg to 6689.69 kcal/kg for raw SPL and optimized sample obtained from caustic followed by sulfuric acid treatment respectively.