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Show grinding is employed to maximize the liberation of pyritic sulfur. Thus, an appropriately designed precleaning circuit prior to advanced flotation minimizes the amount of coal which must undergo costly microgrinding and simultaneously maximizes overall circuit Btu recovery. In addition, the capability to blend coarser clean coal with the ultrafine clean coal product will improve product dewatering. Based on evaluation of the advanced flotation conceptual design, the project team identified and ranked critical areas in the process design where uncertainties existed and further engineering analysis or process testing was needed. The process areas with major design deficiencies included advanced froth flotation, microgrinding, and dewatering. As part of the project, B& W was responsible for benchand pilot-scale process testing in two of these areasmicrogrinding and advanced froth flotation. These tests generated the necessary engineering information for scaleup to a 2 - 3 ton-per-hour (tph) proof-of-concept (POC) plant and preliminary design of a 20-tph semi-works plant. PRIMARY CRUSHING TO 1/4" • COARSE & FINE HYDROCYCLONE PRE-CLEANING • ROUGHER FLOTATION PRE-CLEANING • ULTRAFINE GRINDING • ADVANCED COLUMN FLOTATION • PRODUCT DEWATERING Figure t. Advanced flotation Process FlowshBet 2 PROCESS DESCRIPTION Overall Circuit The efficiency of the fine coal grinding and flotation operations can be significantly enhanced by precleaning the relatively coarse coal. Figure 2 illustrates the overall advanced flotation process that was used in preparing the coal for the microgrinding and advanced flotation testing. This process flow sheet incorporates staged size reduction and precleaning of the coarse coal with hydrocyclones and conventional flotation prior to microgrinding and advanced flotation. The process is summarized as follows: The 6-inch x 0 raw Pittsburgh #8 seam coal from Belmont County, Ohio, was crushed to less than 1/4 inch and then washed in a coarse hydrocyclone to remove the coarse ash (refuse) material. The refuse was discarded. The clean coal product was screened at 48 mesh (48M) (300 microns). The screen oversize, 1/4 inch x 48M, was crushed to 48M x 0 and recombined with the screen through-product The total recombined 48M x 0 product was washed in a fine hydrocyclone to remove the coarse, free pyrite. The refuse was again discarded. The fme hydrocyclone clean coal product was classified and further cleaned using conventional froth flotation cells. The purpose of the flotation in the overall circuit was to further remove the fine clay and mineral matter and minimize the amount of material that had to be processed by fine grinding. The conventional flotation products were first deaerated, and then classified at 325 mesh (325M) (44 microns) in a cyclone. The flotation refuse was discarded. The material coarser than 325M underwent microgrinding in a stirred ball mill to liberate additional mineral and pyrite panicles. The stirred ball mill product was then combined with the 325M x 0 material from the classifying cyclone and used as the feed to the advanced froth flotation cell. Microgrinding To liberate finely disseminated pyrite particles from the coal matrix, it is necessary to grind the coal to an ultrafme size. The performance of ultrafine coal in advanced flota tion depends not only on the flotation conditions, but also on coal size and coal surface properties. In order to improve the Btu recovery, pyrite rejection. and ash rejection, both the panicle size and surface properties must be addressed. It has been reported that the optimum panicle size for conventional flotation is in the range of 10 to 200 microns( 2). If the particles are larger or smaller than this range, the flotation recovery will decrease. For advanced froth flotation utilizing a microbubble technique, the optimum bubble size for flotation will shift to a lower particle size range. Typically, wet grinding using a stirred ball mill is employed to produce ultrafine coal of less than 325M. A pi- |