Desulfurization of coal by microbial flotation in a semicontinuous system

A microbiological flotation system that facilitates the removal of pyritic sulfur from coal is reported. The chemoautotrophic bacterium Thiobacillus f errooxidans is utilized as a pyrite-selective biological surfactant that au gments the flotation-mediated rejection of pyritic sulfur. The bacteria wer e produced hv continuous cultivation in 50-L tanks, where it was possible t o produce 3.8 x 10(12) cells/day when the dilution rate was optimal and the pH was 1.6, a condition that prevented ferric precipitation. Prior to flot ation, the bacteria were mired with coal, which allowed cells To selectivel y adhere to the pyrite. The adherence of bacteria made the pyrite surfaces more hydrophilic, which facilitated separation by increasing the tendency o f pyrite to sink, while the hydrophobic coal floated. When a synthetic coal /pyrite mixture (10.1% sulfur) was subjected to microbial flotation, the le vel of sulfur rejection was stable during the entire period of operation (i .e., one hour). In the absence of bacteria, floatation reduced the sulfur c ontent of a coal/pyrite mixture to within the range of 6.7% to 7.4%, with t he combustible recoveries ranging from 93.9% to 99.0%. In the presence of b acteria, the sulfur content of cleaned coal was in the range of 1.3% to 1.4 %, with the combustible recoveries ranging from 80.8% to 86.3%. Bacterial a ctivity accounted for 57% of the total sulfur rejected through flotation. T he desulfurization capacity of the system was further confirmed by testing it with run-of-mine coal. The pyritic sulfur content was reduced from 2.9% in the feed coal to around 1.2% in the froth product, while 80% of the comb ustible recovery was retained. Microbial flotation removed 60% of the pyrit ic sulfur from the feed. These findings demonstrated that microbial flotati on could be utilized for the removal of pyritic sulfur from coal in a semic ontinuous system.