Recovery of Zn from municipal solid waste incineration bottom and fly ash by acid leaching and solvent extraction with the ligands CYANEX 272, CYANEX 921 and TBP

Abstract

This work focuses on methods to recover zinc from Municipal Solid Waste (MSW) combustion ashes by acid leaching and solvent extraction. Leaching experiments were performed by pH stat titration for bottom and fly ash from a Swedish grate fired combustor firing sorted MSW. Nitric acid 0.6-3M was used for the ash leaching. The solvent extraction of Zn(II) from acid leachates was carried out using three extraction ligands that have been reported in the literature to be effective in separation of Zn from other metal ions in acid solutions. The ligands used were di- (2,4,4- tri methylpentyl) phosphinic acid (CYANEX 272), tri octyl phosphine oxide (CYANEX 921) and Tributyl phosphate (TBP) dissolved in kerosene and the extractions were carried out at initial pH 2, 3, 4 and 5. A special problem in these extractions is the separation between Zn(II) and Fe(III) from ash leachates. Leaching at pH 4 gives the lowest levels of iron in the leachate and may thus be a good starting point for the solvent extraction step since all tested ligands bind Fe(III) as well as Zn(II). CYANEX 272 gave the best results extracting about 90% of the Zn(II) in the ash leachates to the organic phase. For the fly ash leachate a ligand concentration of 30 vol% in kerosene was needed and for the bottom ash leachate the corresponding concentration was 20 vol%. The other two ligands gave significantly lower extraction results. Literature data indicate that their performance can be much better in a chloride rich media. Thus, they can be interesting to test if ash is leached with hydro chloric acid instead of nitric acid. Based on the results CYANEX 272 is indicated as a suitable extractant for the recovery of zinc from MSWI plant ashes.