A new treatment has been developed for industrial wastewater, to remove the heavy metal zinc in a form that would allow it to be recycled. The zinc being targeted is used in industrial galvanising to apply a protective zinc coating to weatherproof steel and iron components — everything from nuts and bolts to bridges and building frames.
The zinc-recovery process is the result of a seven-year collaboration between the University of Melbourne’s Department of Chemical Engineering and industry partner Industrial Galvanizers. Professor Sandra Kentish, has overseen the research, which provides proof of concept with bench-scale testing and accompanying mathematical modelling of the processes involved.
Professor Kentish says the zinc-recovery process has the potential to reduce the costs for galvanisers, who must dispose of wastewater as landfill because of the heavy metal contaminants, which is becoming increasingly expensive. Removing the zinc also reduces the potential environmental impacts of the wastewater and makes more efficient use of a limited natural resource.
The zinc in industrial wastewater is in the form of zinc chloride, which has limited commercial value. However, a two-part extraction process converts the zinc chloride to zinc sulfate. In this form, there is potential to sell it to mineral-processing companies for reprocessing into pure zinc, which would help to offset the cost of the extraction process.
Professor Kentish says technical challenges included finding the right solvent to extract the zinc chloride from the wastewater and also preparing the wastewater stream for the extraction process. Larger particles of paint and metals that might interfere with the extraction process need to be filtered from the water first. The final process uses a hollow fibre membrane with an organic, kerosene-based solvent that selectively binds with the zinc chloride molecules from the wastewater. A second extraction phase converts zinc chloride to zinc sulfate, which could then be sold.