In this study a new method was created for the treatment of waste pickling liquor originating from the hot-dip galvanizing process. This master’s project was initiated due to a local galvanizer contacting Associate Professor Aaron Marshall and asking him to help solve their expensive waste acid problem. Hence, this project was designed to provide a commercial solution to the galvanizing industry from its onset.

By far the most valuable material with the waste is the zinc ($4000/tonne). For a recycling process to be economically viable, zinc must be recovered in a high value form. Directly electrowinning zinc from the waste is ineffective due to the large amount of iron also dissolved in the waste acid. This is because iron both reduces zinc purity and provides sites that the undesirable hydrogen evolution reaction to occur. Thus, the selective separation of iron and zinc is an essential component of the technology.

Sonmez had already shown that iron could be selectively precipitated from zinc in sulphuric acid by first oxidising the majority of Fe²⁺ to Fe³⁺ and then raising the pH to approximately 2. However, it was found that the H₂O₂ oxidation method used by Sonmez was very inefficient. Other oxidation methods were also analysed. The electrochemical method was by-far the most effective method tested. Precipitation experiments showed that Fe³⁺ could be selectively precipitated from zinc within HCl solutions. An electrochemical cell was designed where iron oxidation and zinc electrowinning are performed at the anode and cathode respectively. The issue of iron transportation across the membrane and contaminating the zinc electrowining was solved by the use of the anion exchange membrane FAP-375-PP. Surprisingly, for the tested reactions the anion exchange membrane had comparable performance to Nafion membranes whilst being only a fraction of the cost. The most cost effective cell potential over a ten year period was calculated to be 2.5 V.

Within this master project a novel technology has been developed to recycle and treat galvanizer’s waste acid. An equally large amount of effort was conducted to validate the market and generate a strong business case. The result of this work is that Kiwinet and the University of Canterbury have awarded $480,000 to build a pilot plant and commercialize the technology resulting from this Master’s project.