The use of bioconditioning prior to a flotation process to help remove pyrite from coal is described and the effect of bacterial adaptation on the activity of the microorganisms demonstrated. Because coal is largely hydrophobic, and is made more so by additives such as fuel oil or kerosene, it is possible to recover more coal through the froth, while the ash-forming minerals, because they are hydrophilic, remain behind. This process is not practiced to remove pyrite, which has a moderate hydrophobicity. As a result, a significant proportion of pyrite, typically about 50%, normally floats with the coal when the pyrite is totally liberated.
Very often, pyrite is in the same matrix as coal and to some extent in the same matrix as shale (the main ash-forming minerals).
An objective is to use chemical or biological additives to selectively suppress the floatability of pyrite so it stays behind with the remainder of the ash-minerals in the flotation cell. There are a number of substances (some commercial chemicals and others new) that can do this. There is also some evidence that suggests that biological agents such as thiophilic bacteria have a selective tendency to coat and/or oxidize pyrite but not coal. Only small modifications of the surface, for example, one or two molecular layers, are needed to change the surface characteristics significantly. The possible mechanisms for the selective bacterial modification of pyrite floatability are listed in Table 1.
The bacteria could compete with the adsorption of the flotation collector. Normally, even though coal is largely hydrophobic, it is still not enough to float all of it, so oil collector has to be added to induce greater hydrophobicity on coal. An alcohol frother is also added to stabilize the air bubbles to form a stable froth. What is typically used as a collector is not selective to coal, but it also adsorbs on pyrite. That is another reason why pyrite floats with coal.
Thus if the microorganisms adsorb on the pyrite surface, there will be less surface available for adsorption of the collector. At the standard flotation pH (8-9), the adsorbed microbial cells often carry electrically negative charges, which increase the electrical potential of the already negatively charged pyrite