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Show For a specific particle size, the washability analysis represents the theoretical maximum amount of coal that can be recovered at a given quality by gravity separation. It is often used as a method for evaluating the performance of coal cleaning processes. A perfonnance curve in tenns of Btu recovery and pyritic sulfur rejection for Pittsburgh #8 coal based on the washability analysis of 325M coal and the 12-inch diameter CAER column pilot test results are ploned in Figure 8. The advanced flotation column perfonnance falls below what is predicted by the washability curve. eo o o -- WASHABILITY ANALYStS t;:,. - - - - RELEASE ANALYSIS o -- 12- COLUMN RESUL T5 20 ooL ----2-0 L------40 -~-----10 -~-----eo -~----~ PERCENT PYRmc SULFUR R£JE~ Figure 8. Washability AnalysIs, Flotation Release Analysis, and Performance Test Results Column Performance' ersus Release Analysis As described earlier, washability analysis is based on gravitational separation. Froth flotation is based not only on gravitational separation, but also on the differences in the surface properties of the material being treated. The flotation release analysis technique developed by Dell in the early 1950s and improved in 1964 is a more appropriate method to evaluate froth flotation perfonnance than washabilityanalysis(ll ). In release analysis, a sample of coal is initially floated in a conventional batch flotation cell using every means possible (i.e., high pulp level, high aeration rate, extra frother, etc.) to produce a high recovery. The concentrate from the cell is cleaned an additional three or four times under the same conditions to remove entrained clays and fine mineral matter. The procedure results in a separation of the truly non-floatable material from the floatable material. The floatable material is placed back in the cell and a final separation is made between the highly floatable material and the progressively less floatable material. For this final separation, the air is initially turned off and the impeller speed is reduced to the point where froth formation and flotation ceases. Both air and speed are than increased very slowly until flotation is just di scernible. This froth is scraped for as long as it keeps appearing. The collection basin is then changed and the aeration rate and impeller speed are increased slightly to collect slightly less floatable material. This procedure i 10 continued until all floatable material has been recovered. Any remaining tailings are combined with the original non-floatable materials as part of the overall tailings sample. Each fraction of material is weighed and analyzed to detennine cumulative yield, Btu recovery, ash, sulfur, etc. The data are plotted as cumulative Btu recovery versus pyritic sulfur rejection. The use of the rejection term nonnalizes the effect of changes in feed sulfur from one sample to the next so that all samples can be compared on the same basis. The release analysis curve of Btu recovery versus pyritic sulfur rejection represents the optimum result that can possibly be obtained by any flotation process for that particular sample and size distribution. It primarily reflects cleaning down to the pyrite liberation limit of the sample. Extensive test work using various frothers and frother dosages shows no effect on the release curve. In addition , the dosage of kerosene or fuel oil seems to have liuJe effect on the release curve. The only parameters which are found to change the shape of the release curve are liberation as reflected by the feed size distribution, and reagents, such as pyrite depressants, which change the relative flotation rates of the various components in the sample. With the above procedure in mind, release analysis tests were conducted on a sample of Pittsburgh #8 coal by VPI. This analysis for pyritic sulfur rejection and Btu recovery is shown in Figure 8. The difference between washability (based on gravity separation) and release analysis (based on differences in surface properties) is clearly indicated. Release analysis appears to be a more appropriate indicator of ultrafine flotation perfonnance than washability analysis. The CAER column shows improved perfonnance when compared to the release analysis as opposed to the washability analysis. However, the results indicate that pyriti sulfur rejection and Btu recovery of the CAER advanced flotation column are lower than can be theoretically e - peeted for Pittsburgh #8 coal and could be further optimized. Column Scale-Up Most of the current scientific literature on column flotation recognizes that, when floating coal, the solids removal rate is more influential on throughput rate than any of the other capacity-limiting factors. The tenn for product solids removal is called the carrying capacity of the column and is typically expressed as tph/fl2. Carrying capacity is directly related to particle size, specific gravity of solids, and air flow rate, and is inversely related to the diameter of the bubble. Researchers have shown that there is a theoretical limit to the carrying capacity, regardless if the material to be floated is coal or any other mineral. For the most economical size of column for any given throughput, the carrying capacity should be at. or near, this theoretical limit. |