Extraction of Cr(III) and other metals from tannery sludge by mineral acids

A comprehensive investigation on the extraction conditions of Cr(III) and o ther metals from tannery sludge using mineral acids was performed. The effe ct of various factors (the extraction time, the type of mineral acid, the c onsumption of acid, pH, sludge solids concentration, temperature, and the t ype of sludge) on the leaching yield of metals was studied. The results ind icate that the metal extraction time for most of the metals was 2h at 25 de greesC. The most suitable acid for Cr(III) extraction was sulphuric acid. A relationship for the acid requirement to adjust different sludge pH at var ying sludge solids concentration was established. The leaching yield of chr omium varied with sludge pH and the suitable pH for Cr(III) extraction was between 2.0 to 3.0. The optimum sludge solids concentration for Cr(III) ext raction was 78.5 g(.)l(-1). The leaching yield of Cr(III) decreased with th e increased temperature. The type of sludge (wet or dry) has no effect on t he leaching yield of Cr(III). The metal adsorption and bonding on the insol uble organic colloid matter as well as the formation of metal precipitates may be two possible reasons for the decreased extraction yield of metals wi th increased solids concentration.