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Feed Dilution-Based Design of a Thickener for Refuse Slurry of a Coal Preparation Plant

Thickening is the most widely applied dewatering technique in mineral processing. Thickeners are used to increase the concentration of suspensions by sedimentation, accompanied by the release of a clear liquid. As the particles get finer the thickening process encounters difficulty due to a significant change in the particles settling behavior. The batch settling tests of coal refuse of a coal washing plant that contained 91% particles smaller than 38 µm and 0.6% coarser than 75 µm showed that the optimum feed percent solids that provided highest flux (solids handling capacity) was 4%. The flux of the pulp with the plant solids concentration (i.e., 10% by weight) was 60% lower than that of the pulp with 4% solids. A thickener with a diameter of 22 m based on the dilution of feed from solids concentration of 10% to 4% was designed. Monitoring of the thickener performance for a period of one month in the plant indicated that an average feed rate of 25t/h (dry solids) with solids concentration of 10% could be thickened to an underflow concentration of 26.5% with a clear water overflow. It was found that the key component of the successful operation of the thickener is the dilution of the feed, without dilution the overflow loses its clarity and the system ceases to operate under predetermined conditions. Based on the results of established CFD (Computational Fluid Dynamics) studies, a feeding system that efficiently dissipated the energy of the incoming flow and a staged flocculant addition regime were utilized in the design and operation of the thickener.