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operating ideas continued flotation circuit. The combined circuit was able to produce combustible recoveries in the range of 80%-90% while reducing the total ash content into the 10%-11% range. Moreover, the combined circuit also attained the best reductions in sulfur. In this particular case, sulfurs under 4% were obtained while keeping combustible recoveries in the 85%-90% range. The improvement in performance achieved by the combined circuit is best illustrated by plotting the product ash and sulfur values obtained from the flotation, spiral and combined test runs on a single graph. The format clearly illustrates the inherent capabilities of the different processes in dealing with ash- and sulfur-forming minerals and the synergistic effect of the twostages combined in maximizing the purity of the final clean coal product. Industrial Demonstration In light of the promising data obtained from test programs, a new plant flowsheet was designed by engineers at Taggart Global. A simplified schematic of the upgraded flowsheet is provided in Figure 2. The new circuitry continued to use the existing water-only cyclones and clean coal classify- July 2013 ing cyclones for their primary functions of cleaning and sizing coal. In addition, the sulfur partitioning capabilities of these units were exploited to ensure that nearly all of the high density pyritic sulfur was captured in the cyclone underflows. The overflow from the clean coal cyclones, which was now depleted of pyritic sulfur, was then passed to a bank of deslime cylones where high-ash ultrafines were rejected as overflow. The low-sulfur underflow was then upgraded by column flotation cells, which used froth washing to minimize the recovery of ultrafine mineral impurities, such as clays, misplaced in the water reporting to the underflow of the deslime cyclones. The pyrite-enriched underflow from the clean coal classifying cyclones was permitted to flow to the fine wire sieves and pass as an underflow effluent into a newly installed bank of fine desulfurization spirals. The high-sulfur reject from the spirals was allowed to pass to the thickener for disposal, while the pyrite-depleted fines passed to column flotation along with the overflow from the clean coal classifying cyclones. One additional upgrade to the plant was the installation of a screenbowl centrifuge to maximize the amount of mois- ture removed from the fine clean coal products. Simulations indicated that the improved flowsheet would provide an additional 10 tons of clean coal from the flotation circuit at ash and sulfur levels below 10% and 3%, respectively. Plant sampling campaigns conducted by Taggart Global shortly after plant commissioning indicated the new flowsheet did meet expectations. The new fine coal flotation column provided approximately 9 tph of additional clean coal at ash and sulfur contents of 10.8% and 3.1%, respectively. A much more extensive sampling of the new circuitry was undertaken now that the plant operations have been fully optimized; however, the laboratory analysis of the samples was not completed at the time this article was written. The results obtained to date indicate that the twostage circuit has performed extremely well and that all projected targets have been successfully met in terms of both technical performance and financial returns. This article was adapted from a paper the authors presented at Coal Prep 2013, which took place during late April in Lexington, Ky. To see the original work, visit: www.coalprepshow.com. www.coalage.com 55