Depression of pyrite flotation by yeast and bacteria

Microorganisms, such as the bacteria Thiobacillus ferrooxidans, have been r eported to depress pyrite flotation (Elzeky and Attia, 1987; Atkins, 1990). However, the dependence of the depression on the type of organism or on su spension pH is still being determined. In this study, the relative effectiv eness of various microorganisms (chemolithotrophic bacteria, chemoorganotro phic bacteria and yeast) over the pH range of 2 to 12 was studied. Screenin g tests using micro flotation showed that every microorganism tested was ca pable of depressing naturally hydrophobic pyrite at acidic pH. Larger-scale experiments with both mineral pyrite and coal pyrite using Thiobacillus fe rrooxidans and Saccharomyces cerevisiae as depressants, showed that these m icroorganisms ale very effective depressants for mineral pyrite at acid pH, but they are largely ineffective at neutral and alkaline pH, where the min eral pyrite surface is not naturally hydrophobic. The flotation response of the coal pyrite was completely different from the mineral pyrite. The coal pyrite was most floatable near neutral pH, with the floatability decreasin g in acidic or alkaline solutions. Depression of the coal pyrite by yeast w as not selective between the pyrite and the associated coal under the exper imental conditions.