Using a mineralogical approach to recover metals efficiently from wastewater

Significant amounts of heavy metal-loaded wastewater are produced during the processing and manufacturing of metals. These wastewaters are challenging to treat owing to their chemically complex properties and are considered hazardous, as they are toxic to humans and the environment. The conventional purification method binds the heavy metals into a hydroxide complex, which causes large amounts of voluminous sludge, usually disposed of in landfills. Hence, valuable metals are lost due to dissipation. Recently, waste has received greater attention as a potential metal resource for economic and political reasons. Accordingly, research is carried out to improve wastewater treatment methods to extract metals from wastewater efficiently.

In our work, we apply insights from mineralogy to (1) recover heavy metals from wastewater so they can be recycled and (2) purify the wastewater. We explore the precipitation conditions of metal oxide conditions experimentally and study the effects of parameters such as reaction temperature, alkalization conditions, kinetic effects, and alteration conditions on the metal phases. We find that transformation reactions play a crucial role, and kinetics can, directly and indirectly, control the mineralogy of the precipitated minerals. Based on our findings, we formulated treatment recipes for individual wastewater and constructed an automated pilot facility that enables the treatment. Heavy metal-enriched wastewater from different industries, such as electroplating and chemical catalyst production, is used as sample material. Three elemental wastewater systems (Zn, Au, and Cu) were tested. However, the Cu-enriched electroplating wastewater is multi-element wastewater showing lower concentrations of Zn, Pb, Ni, Cr, and Mn.

Zn could be recovered as ZnO, and Cu-recovery is possible as Cu-ferrite (CuFe₂O₄), delafossite (CuFeO₂), or CuO depending on the addition of Fe to the system. With a two-step process, Au is retrieved as zero-valent Au during the first process step and Fe as magnetite during the second. In the Cu-system a variation of stirring speed resulted in either the formation of brochantite, a Cu-sulfate mineral, or CuO. Alteration of the suspension generally leads to reduced metal concentrations, and limit values for discharge are met for Zn, Pb, Ni, and Cr.