Water and carbon footprints of mining and producing Cu, Mg and Zn : a comparative study of primary and secondary sources

Sammanfattning: Finding sustainable alternatives to the current degrading activities associated with metal mining and production is essential in order to meet increased resource demand and stricter environmental regulations. One option that recently has gained interest is to recover metals from fly ash, which is produced when incinerating municipal solid waste. As of today, metal resources in fly ash are not utilized in Sweden or in most other parts of the world, but are put on landfills where they are made unavailable for future use. Incentives in terms of economic and environmental gains of recovering metals from fly ashes are needed in order to realize the required installments for such recovery. A few studies have been dedicated to examine the economic (and to a smaller extent environmental) benefits of up-scaling successful laboratory attempts of these practices in the past. However, these studies are often compared to the current practices at a certain waste-to-energy plant, or for a specific metal. In general data on environmental parameters, especially water use, for implementing techniques for metal recovery from fly ash is scarce. This study compared the environmental burden in terms of water and carbon footprint of recovering Cu, Mg and Zn from fly ash, compared to conventional mining and production of these metals. To enable such a comparison, a literature study of water use and greenhouse gas emissions for the respective metal recovery methods has been made. Both the current practices in use today, as well as a few potential future alternatives are discussed. The overall findings are that both the carbon- and the water footprint is much larger for mining and producing the studied metals using present practices, i.e. primary metal recovery, compared to recovering them from fly ash. The investigated sources indicate that greenhouse gas emissions for magnesium and zinc recovery could be reduced by a factor of 2 and 15 respectively, if secondary sources are used instead. With currently available techniques, the water footprint can be considered negligible when recovering metals from fly ash. A general lack of data on water use and large inconsistencies in ways of reporting water footprint were affecting the results of this study markedly. Still, it could firmly be concluded that if Cu, Mg and Zn were recovered from fly ash, large savings of CO2e emissions and water could be made compared to only producing these metals from primary sources.