IMPROVED CAUSTIC COAL DESULFURIZATION BY A FLOAT SINK LEACHING PROCESS EMPLOYING 50-PERCENT AQUEOUS NAOH SOLUTIONS

The lowest residual sulfur levels ever attained by caustic leaching of Illinois No. 6 coal samples have resulted from the application of a c ombined float-sink/leaching (FL) process in which 50% aqueous NaOH sol utions serve as the heavy medium during the float-sink step and as the reagent during subsequent atmospheric pressure leaching. In addition to yielding coal with extremely low residual sulfur levels, this proce ss also requires significantly less energy and less caustic for cleani ng coal than molten caustic leaching (MCL) procedures from which it wa s derived. In one variation of the FL process, coal samples are pretre ated by a float-sink procedure using 50% aqueous NaOH as the heavy med ium. The floated coal plus a portion of the 50% aqueous caustic used d uring float-sink are then heated at 390-degrees-C at atmospheric press ure for 30 minutes. The resulting coal-caustic cake is then washed to remove solubilized materials, and a clean coal product is recovered. W hen applied to Illinois No. 6 coal samples initially containing 3.8% s ulfur and 11% ash, this variation of the FL process led to recovery of a cleaned coal containing only 0.2% sulfur and 0.3% ash. In other var iations of the FL procedure, the amount of caustic, the leaching tempe rature, and leaching durations have been reduced. Even under these mil der conditions, use of the FL process resulted in cleaned coals at lea st as low in sulfur as can be attained by MCL. An important advantage of the FL process compared with MCL is the reuse of recovered caustic solutions without need for drying. It is estimated that 90% of the ene rgy consumed during MCL is required for evaporation of water from rege nerated caustic process streams to produce a dry caustic for reuse. Wi th the FL process, unreacted caustic is recovered as a 60% solution th at has been reused without drying and without loss of cleaning effecti veness. The effectiveness of the FL process may result from the remova l of a sulfur-containing species during the float-sink step that is un reactive with caustic during the subsequent leaching step.