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Show 6" x 0 1/4" X 0 RAW FEED COARSE HYDROCYCLONE TOTAL REFUSE Figure 2. Schematic of Pittsburgh 18 Sfim Cosl Pr~Ss lot-scale wet stirred ball mill operated in an open circuit was used to conduct the continuous microgrinding tests. Stirred ball mill media size, media volume, rpm, coal feed percent solids, and coal feed rate were varied to control the viscosity and particle size distribution (PSD) of the product. Advanced Froth Flotation Microbubble flotation is a nablral extension of conventional flotation processes for cleaning fine coal and concentrating valuable minerals in mineral processing applications. A generally accepted flotation theory is that fine particle flotation selectivity will be enhanced if the bubble size can be sufficiently reduced. By using very small bubbles, flotation can effectively separate the fine coal and pyrite. Several alternative microbubble flotation techniques have been proposed and evaluated in a number of bench-scale research programs().'). These various alternatives differ primarily in the procedure used to generate the microbubbles, the device for injecting the fine bubbles into the flotation cell, and the physical design of the flotation cell. Column flotation was selected for the advanced flotation process because it is designed to recover fine particles that conventional cells have difficulty recovering. Figure 3 shows a simple schematic of a column flotation cell. The column cell minimizes the non-coal particle entrainment problems associated with conventional cells. The J ADVANCED COLUMN FLOTATION CLEAN COAL essential features of column flotation are a countercurrent feeding arrangement, wash water addition, and generation and use of microbubbles. There are three operating wnes in the flotation column: the recovery zone where the feed entering the column descends against a rising stream of bubbles generated by the diffuser; a cleaning zone where a bubble bed is washed concurrently with a spray of water; and a conventional froth zone to transpon particles to the launder. The tailings are withdrawn at the bottom of the column at a slightly higher rate than the feed flow rate. This feed! withdrawal differential is called a positive bias. The wash water, which is added below the conventional froth wne, is sufficient to prevent coalescence of rising air bubbles and allows free drainage of any entrained ash particles back into the column. The Quiescent operating conditions of the column allow for beneficiation of very fine coal particl es (7) • PROCESS ISSUES Several key process issues relating to ulttafme grinding and advanced column flotation were identified by the project team for evaluation. Additional experimental data was needed to resolve the engineering uncertainties and aid in the design of a semi-works plant These issues were the focus of the pilot microgrinding and advanced flotation testing in the project. |