An optimum coal depyritization process may combine the physical removal of large liberated inclusions and the microbial degradation of the micropyrite. This was demonstrated in a 1501 aerated trough slurry reactor divided into six sections. An Illinois No. 6 coal containing 1.02% pyritic sulfur and ground to 80% -100 mesh was fed into section 2. Any solids settling out in sections 2 through 6 were transferred to section 1, which acted as a rougher section for physical separation and an inoculum generator for the microorganisms.
Slurry leaving the reactor was dewatered on a screen and part of the liquid was recycled to provide Fe 3+, bacteria and biosurfactants needed to initiate the process on the feed coal. With a five day reactor residence time, and a slurry concentration of 20%, pyritic sulfur was reduced 89% and ash by 30 to 40%, with 90% recovery of heating value. Approximately 16% of the pyrite and other mineral matter appeared in the solid waste drained from section 1, and the remainder is due to microbial pyrite oxidation and solubilization of basic minerals in the coal. Increasing slurry concentration to 35% reduced pyritic sulfur removal to 78%. A slight (5%) reduction in the heating value of the product coal remains unexplained.