Oxygen reduction at sulphide minerals .1. A rotating ring disc electrode (RRDE) study at galena and pyrite

In the flotation of sulphide minerals oxygen reduction plays an important role as the counter reaction to xanthate oxidation. Oxygen reduction is known to proceed via hydrogen peroxide as a stable product or a short lived intermediate in the course of reaction. The formation of hydrogen peroxide is dependent on the catalytic properties of the surface. In the present work oxygen reduction was studied at pyrite and galena in the pH range 2-11, and at platinum and gold in alkaline solution, using the rotating ring-disc electrode (RRDE) technique. At gold hydrogen peroxide is formed as a stable intermediate in the potential region -500 to -900 mV vs. ssce. At the maximum of the ring current, about 75% of the hydrogen peroxide formed at the disc enters the solution phase and is detected at the ring. Since the direct 4 electron path was shown to be negligible the result show that some of the hydrogen peroxide is reduced further to water also in this potential region. At platinum most of the hydrogen peroxide formed at the disc is reduced further to water and only 7% is detected at the ring. For pyrite it was shown that between 25 and 30% of the hydrogen peroxide formed at the disc could be detected at the ring. The reaction mechanism on pyrite is similar to that on gold. However, pyrite is better catalyst for the reduction of hydrogen peroxide compared to gold. Oxygen reduction at galena takes place in parallel to the reduction of the mineral, which complicates the mechanism analysis. Only about 7% of the hydrogen peroxide formed at the disc could be detected at the ring. The results show that hydrogen peroxide is formed as an stable intermediate at pyrite. The potential range for hydrogen peroxide formation overlap with the open circuit potential of pyrite in the presence of xanthate, Thus, hydrogen peroxide formed in the course of oxygen reduction can react with xanthate. On the other hand, for galena the potential region for hydrogen peroxide formation is far away from the open circuit potential for galena in the presence of xanthate.