Flotation separation of phosphate from dolomite using bacteria as depressants

Three bacterial strains, Bacillus subtilis Marburg, Bacillus licheniformis JF-2, and Mycobacterium phlei, were investigated for their uses as modifiers in a mineral flotation system. Minerals studied were apatite, dolomite, and quartz. The flotation of actual Florida phosphate rock ores was also performed using the bacteria as flotation modifiers. The characteristics of the bacteria and minerals studied were investigated by electrokinetic, contact angle, and surface tension measurements. Experimental work on the binding of calcium and magnesium ions to the surfaces of bacteria, and the adhesion of bacteria to surfaces of minerals were conducted. Micro-flotation of pure apatite and dolomite, and flotation of Florida dolomitic phosphate pebbles were also conducted using bacteria as collectors and as depressants in anionic collector flotation. Results of the experimental investigations indicated that the outer surface of cell walls of the three bacteria includes very active biosurfactants. These substances can remarkably reduce the surface tension of water. In addition, through accompanying carboxylate groups, they are responsible for the highly negative surface charges of the bacteria in aqueous solutions, for adsorption of alkaline earth metal ions on bacteria, and for adhesion of bacteria to minerals. M. phlei has the highest hydrophobicity on its surface, and can increase the contact angle on apatite and dolomite. Magnesium ions bind more readily on B. subtilis than do calcium ions. As a consequence, the bacterium adheres more readily to dolomite than to apatite. Over the neutral and acidic pH regions M. phlei also tends to more readily adhere to dolomite than to apatite. All bacteria have less ability to adhere onto the surfaces of quartz than onto the surfaces of dolomite and apatite. The kinetic study of bacterial adhesion revealed that adhesion of both B. subtilis and M. phlei to dolomite, apatite, and quartz follows a reversible adhesion process. The adhesion data fit the Langmuir isotherm. Both adhesion and de-adhesion processes are controlled by diffusion. M. phlei functions as a collector for fine particle flotation of both dolomite and apatite. Both B. subtilis and M. phlei can be used as depressants for anionic flotation of apatite from dolomite. When diphosphonic acid was used as a collector and M. phlei as a depressant in flotation of Florida dolomitic phosphate pebbles containing a relatively low content of MgO, a concentrate with a MgO grade less than 1% and a {dollar}rm Psb2Osb5{dollar} recovery between 60 and 65% can be obtained.