Microwave assisted alkaline roasting-water leaching for the valorisation of goethite sludge from zinc refining process

In this paper, we report on the extraction of zinc, lead and arsenic from goethite sludge, a waste material produced by the Zn refining industry. Goethite sludge is an iron-rich waste material which is currently landfilled due to the high concentrations of heavy metals (Zn, Pb, As, etc.), which labels the waste as hazardous. Valorisation of goethite sludge as a secondary raw material is challenging since many heavy metals need to be removed form its matrix, before this matrix material could be reused (e.g. as Fe-ore or pigment material) or more safely stored.

In this work, microwave assisted alkaline roasting of the waste material was investigated in order to convert the stable franklinite (ZnFe₂O₄) spinel into more soluble ZnO, which could be leached, together with Pb and As, in a subsequent water-based leaching step. Furthermore, this study yielded insights in the intricate chemical conversions of different metal species as a function of the applied alkali source (NaOH or KOH). In particularly, the influence of Mn conversion during KOH roasting and subsequent leaching as permanganate has been shown to influence the Zn leachability. Finally, the application of microwave power instead of conventional heating has shown to lower the roasting time and overall furnace temperature, because the samples absorbed MW power and locally heated to much higher temperatures than the set furnace temperature.