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) combustionashes by acid leaching and solvent extraction. Leaching experiments were performed by pH stattitration for bottom and fly ash from a Swedish grate fired combustor firing sorted MSW. Nitricacid 0.6-3M was used for the ash leaching. The solvent extraction of Zn(II) from acid leachateswas carried out using three extraction ligands that have been reported in the literature to beeffective 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 (CYANEX921) and Tributyl phosphate (TBP) dissolved in kerosene and the extractions were carried out atinitial 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 leachateand may thus be a good starting point for the solvent extraction step since all tested ligands bindFe(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 30vol% in kerosene was needed and for the bottom ash leachate the corresponding concentrationwas 20 vol%. The other two ligands gave significantly lower extraction results. Literature dataindicate that their performance can be much better in a chloride rich media. Thus, they can beinteresting to test if ash is leached with hydro chloric acid instead of nitric acid. Based on theresults CYANEX 272 is indicated as a suitable extractant for the recovery of zinc from MSWIplant ashes.