Establishment of Intensive Ash and Sulfur Rejection from Coals.

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Two kinds of coal cleaning methods were applied to the Illinois No.2 and No.6 coals, which were 1. flotation as major physical cleaning method and 2. aqueous caustic leaching (ACL) and molten caustic leaching (MCL) as chemical cleaning methods. The objective of this study is to clarify the limit of flotation and the reaction mechanism of ACL and MCL.The major results obtained are summarized as follows ;1. Ash and sulfur rejection by flotationNormal flotation was applied to the Illinois No.2 coal with the addition of potassium chromate and iron oxidation bacteria and changing conditioning pH to depress iron sulfide minerals. 60-70% of ash rejection, 70-75% sulfur rejection was obtained with 40-45% recovery of coal substances. Bacteria treatment could achieve higher coal substance recovery but with relatively lower ash and sulfur rejection.Reverse flotation was applied to the Illinois No.6 coal which was very low floatability of coal substance. The cleaning results were 25% ash rejection, 38% sulfur rejection and 89% coal substance recovery. Searching relationship between the libeation of iron sulfide minerals and flotation behavior, it was demonstrated that the particles with the ratio of lower than 30% of iron sulfide minerals were difficult to be floated.2. Ash and sulfur rejection by ACL and MCLACL was applied to the Illinois No.6 coal with NaOH,to obtain the result of 88.8% ash rejection, 28.6% sulfur rejection and 90.4% coal substance recovery. 94.9% ash rejection, 91.4% sulfur rejection and 73.0% coal substance recovery was obtained by the MCL with alkali of NaOH/KOH=3/1.The recovery of each maceral group in the stage of MCL were calculated to 98% of vitrodetrinite+exinite, 67% of inertinite and 58% of telinite+collinite.