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Sustainable coal waste disposal practices 263

For example, if water activity is 0.9 at the targeted level of dewatering for the coal
slurry, the corresponding osmotic agent solution activity must be 0.9. Another factor
that influences osmotic agent concentration is the ability to be removed from solution
using energy efficient methods such as RO. This limits osmotic agent concentrations
to a maximum of 6%–7% sodium chloride or equivalent; however, using other tech-
niques such as membrane distillation allows relaxation of these limits. The other
important parameter is membrane area. Since the amount of membrane used directly
affects costs, it is desirable to use the minimum possible membrane area. This requires
a maximum activity difference between feed solution and draw solution containing
the osmotic agent.
Osmotic dehydration offers two distinct advantages over conventional pressurized
membranes or other filtration processes. First, there are no externally applied driving
forces on slurry particles, only gravitational body forces and drag forces. This helps
ensure that particles are not forced toward the filter barrier, which lowers the potential
for the plugging of the porous media. Second, while osmotic forces create pressure-
type forces equivalent to several megapascals (hundreds of pounds per square inch),
the equipment itself is unpressurized leading to simple processes and ease of equip-
ment construction and operation. For example, components for the pilot-scale system
were fabricated using a 3D printer.
Laboratory-scale testing of an osmotic dehydration system dewatering FCPW
feeds achieved 70%–80% solid content (see Fig. 13.8). Average flux is achieved,
while dewatering as-received coal refuse slurry at 25% starting solid content to a
2
final solid content of 75% was approximately 1.25L/m /h (LMH). When processing
washed coal refuse slurry (i.e., lowering the osmotic pressure of the slurry and increas-
ing driving forces), average flux achieved for dewatering feed slurry with starting
solid content of 25% to a final solid content of 80% was approximately 3LMH.
Amazingly, this degree of dewatering was achieved without the application of any

Fig. 13.8 Laboratory-scale osmotic dehydration test cell including automated membrane
scraper [21].