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The utilization of forward osmosis for coal tailings dewatering
Salih, H., L. Wang, V. Patel, V. Namboodiri, AND K. Rajagopalan. The utilization of forward osmosis for coal tailings dewatering. Minerals Engineering. Elsevier B.V., Amsterdam, Netherlands, 81:142-148, (2015).
As-mined coal contains non-combustible impurities, sulfur containing minerals, and hazardous trace elements that need removal prior to use. The removal of impurities – coal cleaning or preparation – is predominantly carried out by water-based processes. The resulting waste streams in coal preparation plants are categorized by particle size distribution (PSD). The larger particle size fraction can be readily dewatered and disposed of in waste piles or used for backfilling. The fine size fraction poses the greatest challenge due to the presence of clays, slimes, and other mineral impurities. This fine refuse stream typically constitutes only two to three percent solids. It is thickened to about thirty percent solids content before being discharged to impoundments (Parekh, 2009). Maintenance of impoundments is expensive and several well-publicized impoundment failures (Committee on Coal Waste Impoundments et al., 2002) have sharply raised awareness of the environmental hazards of storing such wastes. Therefore, new impoundments face greater opposition and older ones are filling up leading to concerns about waste disposal becoming a critical bottleneck in coal production.
The feasibility of dewatering coal tailings slurry by forward osmosis (FO) membrane process was investigated in this research. A prototype cell was designed and used for the dewatering tests. A cellulosic FO membrane (Hydration Technology Innovations, LLC, Albany, OR) was used for the dewatering studies due to its high fouling resistance. Representative samples of coal tailings slurry were collected from the thickener outflow at American Coal Company (Galatia, Illinois). Characterization studies were conducted to obtain particle size distribution (PSD), total dissolved solids (TDS) and the solids content of the slurry. The impact of the slurry properties such as solids weight percent, osmotic pressure, and particle size on the dewatering rates was determined. Furthermore, the impact of slurry conditioning by the addition of flocculant and gypsum on the rate and extent of dewatering was also investigated. Dewatering to a total solids content of more than seventy weight percent from an initial solids content of approximately thirty percent was achieved in all cases. The dewatering rate and extent were found to be a function of particle size, particle shape, TDS content, and mixing. The membrane material was shown to withstand repeated use over a period of thirty trials without deterioration of performance. The results obtained from this research suggest that osmotic dewatering of coal refuse slurry is feasible.