Extraction of metals from contaminated land and industrial solid waste using a novel technology (servo process)

Abstract

Selective Extraction and Recovery using Volatile Organic compounds is an emerging technology developed during the 1970s. This process can achieve the extraction of heavy metal contaminants from a matrix using a volatile organic reagent which passes through the feed material and reacts selectively with the desired metal salt, producing a volatile metal complex, removed from the matrix by a carrier gas. Such complexes may be decomposed to produce a pure metal product and regenerate the organic reagent for recycle. Previous studies demonstrated the possible extraction of nickel from low grade laterite ores using ß-diketones (2,4- pentanedione (Hacac)) and Schiff bases (bis(pentan-2,4-dionato)propan-1,2-diimine (H2pnaa). The current research is directed towards the selective extraction of different metals such as zinc, lead, cadmium, molybdenum, and vanadium from contaminated sediments and industrial wastes (Orimulsion ash, Municipal Solid Waste fly ash (MSW), Pulverized Coal Combustion technology fly ash (PCC)). New extractants and their metal complexes have been synthesised to determine their thermal stability and their volatility. Of those synthesised the metal complexes of tetra-propyldithiophosphoramide (Hprps) are the most thermally stable. Using a thermogravimetric analyser the reaction kinetics of the SERVO process have been studied. Equipment to study the SERVO process on a laboratory scale has been designed and constructed. This equipment has been used to study the extraction of metals from four different matrices (sediments, Orimulsion ash, and two types of fly ash) using three different extractants, with promising results. These sources have been ranked from the best to the least applicable for the technology: Orimulsion ash > sediments > MSW fly ash > PCC fly ash. Of the three extractants studied, Hacac, H2pnaa and Hprps, the latter is the most efficient in terms of the range of metals which can be extracted, the volatilisation temperature, the extent of degradation and reaction time, but unfortunately is also the most expensive. For the fly ashes, of the three ligands studied, Hprps is the preferred extractant followed by H2pnaa. Hacac is not recommended for these sources because extraction is too low.