Treatment of Spent Pot Lining by Chemical Leaching Using Nitric Acid for Enrichment of Its Fuel Value and Optimzation of The Process Parameters

Abstract

Over the years numerous methods have been suggested for treatment of Spent Pot Lining (SPL)- a waste produced from aluminum industries that has been declared as hazardous by the US Environmental Protection Agency. In this project chemical leaching treatment method was adopted and several process parameters and their dependency on each other were studied. SPL was subjected to caustic leaching followed by HNO3 leaching at different temperature and liquid to solid ratio combinations. Again an optimized set of values was obtained via Taguchi method. The main objective of the optimization methodology was to determine the effect of acid concentration, alkali concentration, temperature and L/S ratio on the leaching percentage in terms of percentage contribution. Alkali concentration contributed 51.80% in the leaching process, whereas acid concentration contributed only 2.61 %. The maximum leaching percentage was found out to be 50.62% by Taguchi optimization. It was concluded that 2.5 M NaOH, 5 M HNO3 concentration, 4.5 L/S ratio and temperature 75 ±1°C was the best combination for the optimum leaching percentage for final treated SPL. The carbon percentage of SPL was increased from 33.27% to 87.31% as confirmed from the elemental composition obtained by Enegry Dispersive X-ray spectroscopy. Again with the help of existing data, an extensive comparison was made to find out the best suitable acid out of HCl, HNO3, H2SO4 and HClO4 for leaching process keeping L/S ratio constant. It was observed that at normal temperatures H2SO4 gave maximum leaching percentage for all values of alkali concentrations. But as temperature was increased from 25°C to 100°C, the leaching percentage obtained by all acids converged slowly to a constant range.