Improved caustic coal desulfurization by a float-sink/leaching process employing 50% aqueous NaOH solutions

Abstract

The lowest residual sulfur levels ever attained by caustic leaching of Illinois No. 6 coal samples have resulted from the application of a combined float-sink/leaching (FL) process in which 50% aqueous NaOH solutions 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 process also requires significantly less energy and less caustic for cleaning coal than molten caustic leaching (MCL) procedures from which it was derived. In one variation of the FL process, coal samples are pretreated by a float-sink procedure using 50% aqueous NaOH as the heavy medium. The floated coal plus a portion of the 50% aqueous caustic used during float-sink are then heated at 390°C at atmospheric pressure for 30 minutes. The resulting coal-caustic cake is then washed to remove solubilized materials, and a clean coal product is recovered. When applied to Illinois No. 6 coal samples initially containing 3.8% sulfur 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 variations of the FL procedure, the amount of caustic, the leaching temperature, and leaching durations have been reduced. Even under these milder conditions, use of the FL process resulted in cleaned coals at least 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 energy consumed during MCL is required for evaporation of water from regenerated caustic process streams to produce a dry caustic for reuse. With the FL process, unreacted caustic is recovered as a 50% solution that has been reused without drying and without loss of cleaning effectiveness. The effectiveness of the FL process may result from the removal of a sulfur-containing species during the float-sink step that is unreactive with caustic during the subsequent leaching step.