Purification of combustion fly ashes using the SERVO process

The SERVO process involves the extraction of heavy metals using a volatile chelating extraction reagent that is selective for chosen contaminating metals. The extractant is heated and the vapour passed through a heated bed of fly ash where reaction occurs with the contaminant metals to produce volatile metal-reagent complexes at relatively low temperatures and normal pressure. These can be condensed from the vapour phase or, in suitable circumstances, reduced to the element and the reagent recycled. Previous studies demonstrated the extraction of nickel from low-grade laterite ores using beta-diketones and Schiff bases. Current research is now directed towards the selective extraction of metals such as zinc, lead, cadmium, molybdenum and vanadium from combustion ashes. New extractants and their metal complexes have been synthesised to determine their thermal stability and volatility. Of those prepared, complexes of tetra-iso-propyldithiophosphoramide are the most thermally stable. Using thermogravimetric analysis the kinetics of the process have been studied to optimise the operating conditions. The SERVO process has been used on a laboratory scale to study the extraction of metals from coal combustion fly ash, waste incinerator fly ash and Orimulsion ash. Extraction has been found to depend on metal speciation and the paper presents results of the metal removal with three different extractants correlated with the speciation of the metals in the feed. (C) 2002 Society of Chemical Industry.