Page 288 - Advances In Productive, Safe, and Responsible Coal Mining

264 Advances in Productive, Safe, and Responsible Coal Mining

mechanical pressure. The system used in these experiments was a batch configuration
with freestanding, handmade, cylindrical membrane bags filled with feed slurry. They
were immersed in a reservoir of draw solution causing the sides of bags to collapse as
water was removed, thereby creating a moving wall effect. While this batch of con-
figuration was effective in proving the concept, it would be challenging to scale up.
A continuous forward osmosis (FO) system using membrane disks was also tested, but
it was not as successful in achieving deep dewatering due to impermeability of cake
layer that developed on the membrane. This led to creating the automated scraping
device seen in Fig. 13.8, which was designed to mix the feed solution during
dewatering in an attempt to maintain intimate contact between water in the feed slurry
and the membrane by provisioning channels for water flow.
Scale-up testing of the laboratory devices focused on membrane robustness, which
was tested by using the same membrane coupon repeatedly for over 30 trials with no
deterioration in performance observed. Throughout these tests, the membrane was
easily cleaned with a simple water rinse. As part of Rajagopalan’s research, the con-
tinuous system FO cell was scaled by a factor of 20 in terms of membrane area, and the
amount processed was scaled by a factor of 60. The performance achieved with larger
cells was comparable with that obtained with smaller cells validating the feasibility of
scale-up of continuous dewatering configuration.
The cost of dewatering a ton of solids is dependent on starting slurry concentration,
target slurry concentration, flux (measured in LMH), membrane life, and mode of
draw solution regeneration. An estimate of the cost to dewater a ton of solids given
the following parameter values, average flux of 2LMH, membrane cost of
2
$20/m , 3-year membrane life, and a target solid content of 75%, arrived at around
$3.50. This estimate assumed the use of RO for osmotic agent regeneration. This esti-
mate compares favorably with costs for alternative technologies that generally
achieve inferior dewatering success (measured in percent product solid content), such
as deep cone thickeners (45%–50%), belt presses (25%–45%), and hydraulic filter
presses (20%–80%) [22].
In summary, osmotic dehydration of fine coal refuse and ultrafine clean coal has
been shown to be technically feasible, scalable, and potentially economic. Further
research is focusing on lowering membrane replacement costs through productivity
enhancements such as decreasing cake resistance and maximizing driving forces
(i.e., lowering the osmotic pressure of the feed solution). The latter is achieved
through management of total dissolved solids (TDS) and water washing. For exam-
ple, TDS in the feed solution used for laboratory-scale testing were 8500mg/L
(0.07lb/gal), which is more than double reported values for several Illinois Basin
coal mines. Processing slurries with lower TDS is expected to result in higher aver-
age flux. Cake resistance can be lowered by increasing particle size within slurries or
by increasing the sphericity of materials in the cake bed. A doubling of the produc-
tivity reported by Rajagopalan is considered achievable with adjustments to slurry
and cake properties and membrane material. Finally, further development of the
technology should focus on continuous process systems with integrated solids han-
dling within membrane cells.