The amount of process sand ranges from < 1% to as high as 50% in different digestion residues, but its quantity is normally about 5%. In several cases the process sand is separated before the clarification and is transferred to washing in a separate system (Bánvölgyi and Huan, 2010).
Source
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Bánvölgyi, Gy., Huan, T.M., 2010. De-watering, disposal and utilization of red mud: state of the art and emerging technologies. TRAVAUX 35 (39), 431–443. Available from: https://www.yumpu.com/en/document/view/9042102/de-watering-disposal-and-utilization-of-red-mud-state-redmudorg Accessed June 7, 2018.