Mechanisms of pyrite biodepression with Acidithiobacillus ferrooxidans in seawater flotation

It has been shown that bacterium Acidithiobacillus ferrooxidans can be used to depress pyrite in seawater flotation at natural pH, which opens the possibility for its use as an alternative to lime to depress pyrite in copper sulfide flotation. In order to have a better understanding of the mechanisms involved in pyrite depression with A. ferrooxidans, different kind of experiments were carried out, including, contact angle, attachment kinetics, and streaming potential measurements. All these experiments were carried out in seawater. Biodepression of pyrite was improved by increasing the pH from 4 to 8, with a decrease in recovery from 92% to 36%. This increase in depressing capability was accompanied by an increase in attachment density of bacteria on pyrite, from bacteria/g to bacteria/g at pH 4 and 8, respectively. These results suggest that the mechanism of depression is related to the attachment of bacteria to the pyrite surface. The streaming potential measurements showed that both bacteria and pyrite were negatively charged at pH 8. This indicates that electrostatic forces are mainly repulsive, therefore other forces cause the attachment of bacteria to the mineral. The contact angle of pyrite conditioned with seawater at pH 8 was 16, which increased to 54 when collector (sodium isopropyl xanthate) was added, indicating an increase in hydrophobicity. Nevertheless, when pyrite was previous conditioned with bacteria, the contact angle increased only to 44 when collector was added. Thus, the collector has a lower influence in the hydrophobicity of pyrite when the mineral has interacted with bacteria A. ferrooxidans.