Removal and Recovery of Metals from Municipal Solid Waste Incineration ashes by a Hydrometallurgical Process

University dissertation from Chalmers University of Technology

Abstract: Municipal Solid Waste Incineration (MSWI) fly ash contains significant amounts of leachable metals, and is therefore considered to be a hazardous waste. The increased amounts of fly ash generated cause environmental issues, management costs and accumulation of valuable metals in landfills. All these factors contribute to an increased interests to recognize the ash as an urban mining target of metal resources. Metals recovery, e.g. copper, zinc from fly ash, is not only beneficial for the availability of valuable metals that otherwise would be lost, but it can also decrease the amount of potentially soluble and toxic metal compounds in the ash residue,  making it possible to landfill the ash at a lower cost.

A hydrometallurgical process for recovery of copper and zinc from MSWI ash is presented in this thesis. A process consisting of leaching followed by sequential solvent extraction of metals was developed. With respect to leaching efficiency and feasibility, it was found that hydrochloric acid is the best option for recovery if copper and zinc from the fly ash, in addition, efficient leaching of cadmium and lead was also achieved.

LIX860N-I, a commercial extractant was chosen for the copper extraction. No third phase formations or phase separation problems were observed. The copper extraction gave a good extraction yield with high selectivity. The extracted copper in organic phase can be stripped in a one stage process using a selected sulfuric acidic solution (1.5M). Zinc extraction followed the copper extraction by extracting the raffinate using Cyanex 272, Cyanex 572 or Cyanex 923. Cyanex 923 showed the highest efficiency and no pH adjustment of the aqueous phase was needed. However, this extractant co-extracts iron, cadmium and lead with the zinc. Cyanex 572, a novel phosphorus-based chelating extractant is less effective, but more selective for zinc extraction compared to Cyanex 923.  Selective stripping processes were suggested for zinc extraction using Cyanex 572 and Cyanex 923 respectively.

Laboratory pilot scale experiments using mixer-settler systems were carried out based on the batch experiment results. McCabe Thiele diagrams were used to model the processes with respect to the number of stages needed for conducting the extractions in mixer-settler systems. The solvent extraction process was then demonstrated in pilot size mixer settler equipment consisting of two stages for copper extraction and three stages for zinc extraction and gave a recovery of 90% of the copper and ~100% of the zinc in the leachate.