Coal Age

JUL 2013

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operating ideas continued wall and underground mining operations. It currently produces thermal coals containing 10% ash, 2.8%-3.2% sulfur and 11,200 Btu/lb. The plant has a nameplate feed capacity of 500 tons per hour (tph) and produces approximately 2.7 million tons of clean coal annually. The coarse coal circuitry at the Prairie Eagle prep plant incorporates a dense medium cyclone to upgrade particles larger than 1 mm in diameter. The original fine coal circuitry, which is shown in Figure 1, was used to clean raw feed coals in the 1 x 0.15 mm size class using a combination of water-only cyclones (380 mm diameter) and secondary scavenging spirals. The coal products from the cleaning units are initially dewatered using clean coal classifying cyclones (380 mm diameter) and fine wire sieves (0.35 mm aperture) and then fully dewatered using a centrifugal dryer (EBW-40). Particles finer than 0.15 mm from the clean coal classifying cyclones are discarded as waste along with the underflow streams from the fine wire sieves and the refuse high-frequency screen. The plant flowsheet also incorporated two recycle streams, i.e., the effluent from the centrifugal dryer is recycled back to the clean coal classifying cyclones and the spiral middlings are recycled back to the wateronly cyclones. These recycle streams are needed to avoid excessive coal losses that would otherwise occur if these streams were discarded. For the Prairie Eagle operations to respond to increased market demands, Taggart Global was contracted to design a plant expansion that would increase capacity up to 750 tph. The plant expansion also provided an opportunity for the engineering firm to work with company management in developing more efficient circuitry for processing fine coal. Due to market demands, there was considerable interest in identifying new equipment solutions that would allow the operation to recover additional fine coal without adversely impacting the clean coal ash and sulfur levels. In particular, considerable interest was expressed in adding a froth flotation circuit to the plant flowsheet. cyclones to discard the minus 325 mesh fraction followed by column flotation to upgrade the nominal 100- x 325-mesh cyclone underflow. To examine the potential benefits of this alternative, a sample of minus 100 mesh overflow was acquired from the 15-in. diameter clean coal classifying cyclones currently operating in the plant. The sample was shipped to the laboratory and passed through a 6-in. diameter deslime cylone to determine the effects of classification on ash and sulfur partitioning. The data from the deslime cyclone test results was promising in that the overflow represented 63.4% of the feed weight and contained 71.8% ash. Very little (roughly 2.6%) material in the 100- x 325-mesh size fraction reported to the overflow. The data also indicated very little difference in sulfur contents of the overflow and underflow products for sizes larger than 325 mesh. This result was unexpected since it was assumed that sulfur levels in the underflow would be higher due to preferential partitioning of Classification Testing The use of a conventional "by-zero" froth flotation circuit was not considered to be a viable option for the Prairie Eagle prep plant because of the extreme fineness (>85% minus 325 mesh) and poor quality (>60% ash) of the minus 100 mesh feed. The best alternative was considered to be a flotation circuit incorporating deslime classifying July 2013 www.coalage.com 53

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