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262 Advances in Productive, Safe, and Responsible Coal Mining

Particle retained by barrier
No applied force on particle
Feed solution Semipermeable
(C ) membrane
1

1
C <C 2
Water flow due to
activity gradient
Osmotic agent
(C )
2

Fig. 13.7 Principle of osmotic dewatering technology [21].

differences in activity of an osmotic agent and FCPW slurry to instigate self-directed
transport of water from the coal slurry to the osmotic agent. It is, in essence, a passive
dewatering technology. The principle of the technology is illustrated in Fig. 13.7,
which shows a feed solution having a relatively low concentration (c 1 ) of ionic or non-
ionic solute separated from the osmotic agent or draw solution containing an elevated
concentration (c 2 ) of the ionic or nonionic solute by a semipermeable membrane bar-
rier. Since the activity of water in the feed solution is higher than that in the osmotic
agent, water flows from the feed solution to the osmotic agent effectively dewatering
the feed solution.
This technology has two key components—the semipermeable membrane and the
osmotic agent. Development of highly selective, high-flux, semipermeable cellulosic
membranes in the 1960s led to the development of forward osmosis or extracting pota-
ble water from seawater and reverse osmosis (RO) filtration for household water sys-
tems. The primary purpose of the semipermeable membrane is to separate the material
being dewatered from the osmotic agent. This allows selective flow of water without
the mixing of the two solutions.
The choice of osmotic agent is equally important as it determines regenerability,
which is a critical characteristic if the process is to be efficient. Requirements of a
good osmotic agent include the ability to lower water activity at low concentrations;
thus, low-molecular-weight solutes are more efficient than high-molecular-weight
solutes. Common osmotic agents in use are sodium chloride (readily available at
mines with high-chloride discharges), magnesium sulfate, glycerol, and sucrose.
The osmotic agent should be inexpensive, easily removed from solution to enable
recycling, and preferably noncorrosive, nonflammable, and nontoxic.
Operational parameters of osmotic dehydration include the concentration of the
osmotic agent, which depends upon the salinity of the coal slurry. The higher the salin-
ity of the feed solution, the higher the concentration of the osmotic agent; however, the
maximum concentration of the osmotic agent solution is limited by its solubility.
The minimum concentration is fixed by the desired extent of feed solution dewatering.