Recovery of copper and zinc in ashes from municipal solid waste combustion
The amount of municipal solid waste incineration (MSWI) fly ash has increased in many parts of the world in recent years. Ashes from municipal solid waste incineration (MSWI) may be a cumbersome waste to handle since containing significant amounts of leachable hazardous metal species and must be landfilled in special sites at high costs. Recovery of e.g. copper, zinc and lead from MSWI ashes may not only recover valuable metals that would otherwise be land filled, but also reduce the toxicity of the ash and thereby make it possible to use a less expensive landfill option or perhaps even to utilize the ash residue as a construction material.
The work presented here discusses the development and evaluation of a metal recycling process for MSWI fly ash based on acid leaching and solvent extraction; fly ash and bottom ash from an MSWI facility was used for studying and optimization of metal leaching using different solutions (nitric acid, hydrochloric acid and sulfuric acid) and parameters (temperature, controlled pH value, leaching time, and liquid/solid ratio). It was shown that hydrochloric acid is relatively efficient in solubilizing copper (75.3 ± 3.0%), zinc (80.8 ± 5.3%) from the fly ash in less than 24 hours at 20°C. Efficient leaching of cadmium and lead (over 92% and 90% respectively) was also achieved.
After leaching, a selective solvent extraction to recover Cu using an oxime (LIX860-I) followed by a Zn extraction with phosphine oxides (Cyanex 923) were carried out sequentially. Both extraction reagents gave good yields and stripping of Cu and Zn is also possible in a single step using sulfuric acid or nitric acid respectively. The extraction of Cu was selective but a significant amount of other metals, such as Fe, were co-extracted together with Zn. It was shown that it is possible to decrease the contamination of Fe by adjusting the extraction parameters (e.g. adjusting the concentration of nitric acid solution for stripping) or by cleaning the final solution by cementation. A metal recycling process for MSWI fly ash is suggested based on the results from this study.