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

A comprehensive investigation on the extraction conditions of Cr(III) and other metals from tannery sludge using mineral acids was performed. The effect of various factors (the extraction time, the type of mineral acid, the consumption of acid, pH, sludge solids concentration, temperature, and the type of sludge) on the leaching yield of metals was studied. The results indicate that the metal extraction time for most of the metals was 2h at 25 degrees C. The most suitable acid for Cr(III) extraction was sulphuric acid. A relationship for the acid requirement to adjust different sludge pH at varying sludge solids concentration was established. The leaching yield of chromium 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) extraction was 78.5 g x l(-1). The leaching yield of Cr(III) decreased with the increased temperature. The type of sludge (wet or dry) has no effect on the leaching yield of Cr(III) The metal adsorption and bonding on the insoluble organic colloid matter as well as the formation of metal precipitates may be two possible reasons for the decreased extraction yield of metals with increased solids concentration.