The utilization of forward osmosis for coal tailings dewatering

• Dewatering of coal tailings slurry can be accomplished using an osmotic gradient.
• A solids content of greater than seventy percent can be achieved at ambient pressure.
• The dewatering rate was increased by increased osmotic gradient, bed mixing, and larger slurry particle size.

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.