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operating ideas continued high-density pyrite to the underflow stream. Surprisingly, the plus 325 mesh sulfur contents ranged from 2.20%-2.45% sulfur, which were well below the plant contract specification of 2.8%-3.2% sulfur. Upgrading of Clean Coal Classifying Cyclone Overflow The feed, overflow and underflow products from the raw coal deslime cyclone were subjected to laboratory froth flotation tests. The test work included both kinetic tests and release analysis tests to evaluate the practical and ultimate cleanability of each sample. The test results from the release analysis tests showed that products containing less than 10% ash could be theoretically obtained from any of the three process streams. However, high recoveries were difficult to obtain for either the feed or overflow samples due to the very high feed ash contents of these samples. In contrast, combustible recoveries approaching 90% were attainable for the deslimed underflow. The only apparent disadvantage of processing the underflow sample was that slightly higher sulfur values were obtained, perhaps due to the comparatively high sulfur content of the underflow feed sample. Data from the kinetics tests were found to be substantially inferior to those obtained from release analysis testing. This large difference suggests that column flotation with froth washing to minimize entrainment would be the preferred option for treating any of these process streams. Based on this data, column flotation would be expected to produce a froth product containing about 7%-8% ash, while con- ventional flotation would be expected to result in a froth product containing 17%18% ash. Both methods produced clean coal products containing about, 2.5% sulfur, which as indicated previously well below the sulfur was recovered by flotation as a result of either poor liberation or pyrite floatability and not due to hydraulic entrainment in the froth water. Upgrading of Clean Coal Sieve Underflow The experimental results obtained from the initial round of classification and flotation tests indicated that deslime column flotation would be an ideal approach for upgrading the overflow from the clean coal classifying cyclones currently installed in the Prairie Eagle prep plant. Further investigations indicated that flotation performed well since most of the high-density pyritic sulfur in the minus 100 mesh-feed slurry had already been captured in the underflow of the plant's clean coal classifyng cyclones. The cyclone underflow then reports to fine wire sieves where the sulfur-enriched fines eventually passed through as effluent. Sampling and size-by-size analysis of the sieve underflow showed that the effluent contained 39.8% ash and 5.6% sulfur, with the greatest proportions of the sulfur occuring in the sizes less than 100 mesh. This created a dilemma for plant designers since the sieve effluent contained too much valuable coal to discard despite being highly enriched in both ash and sulfur. Several additional series of coal cleaning tests were conducted to determine how to best process the material present in the clean coal sieve underflow. In this Figure 2: Simplified schematic of the upgraded flowsheet layout used at the Prairie Eagle prep plant. 54 www.coalage.com round of tests, two different processes were considered, i.e., column flotation and fine spirals. In addition, a multiproperty processing circuit consisting of density-based spirals followed by surface-based flotation was also evaluated for this unique application. This circuitry was considered to be a viable option since the upstream clean coal classifying cyclones and fine wire sieves had created a manageable low-volume high-impurity stream that could be easily handled by a multiproperty processing circuit. The flotation test work was conducted using both laboratory and pilot-scale units, while the spiral tests were conducted using a full-scale single-start compound spiral rig. The spiral rig was equipped with a partitioned product launder that allowed six products to be collected across the spiral profile in addition to a primary reject. Six separate sets of fine spiral tests were conducted to identify the optimum conditions (i.e., dry solids mass rate and slurry volume flow rate) for separating the nominal 100- x 325-mesh material from the sieve underflow. The data (combustible recovery vs. ash and sulfur contents) obtained from the flotation, spiral and flotation/spiral test runs effectively demonstrates that froth flotation was much superior to the fine spirals in removing ash-forming minerals. Flotation effectively reduced the ash of the feed from about 43% to about 15%18% ash, while the fine spirals never achieved ash levels lower than about 34% ash. While much of this problem can be attributed to the inability of the spiral to deal with misplaced ultrafines (minus 325 mesh solids), size-by-size analyses of the test data showed that froth flotation was superior to the fine spirals even for the coarsest size fraction of 100- x 200-mesh present in the feed. On the other hand, fine spirals were much superior to flotation for reducing sulfur levels, often rejecting solids containing double-digit sulfur contents. The spiral unit lowered the feed sulfur from about 5.6% to about 4.2% sulfur. Flotation was not able to match this level of performance and, in fact, actually concentrated the sulfur-bearing components into the clean coal product. Consequently, most of the froth products from flotation testing had higher sulfur values than the original feed. The most interesting and most promising results were obtained from the testing of the combined fine spiral and July 2013