graphite cannot be observed any more as shown in Fig. 8. Instead,
strong repulsive forces were produced in both deionised and saline
water. As can be seen, repulsive forces produced in deionised water
were stronger than that in synthetic saline water. While the repulsive interaction was observed to start at a separation distance of
11 nm in deionised water, the repulsive interaction happened only
within a separation distance of 5 nm in saline water. As shown earlier, the adsorption amount of lignosulfonate on coal and clay surfaces increased with increasing the lignosulfonate concentration
(lower than 15 mg/L). As a result, the strong repulsion produced
in deionised water was caused by the increased electrostatic and
some steric repulsion. The weaker repulsive forces generated by
15 mg/L lignosulfonate in saline water are mainly due to the
increased steric repulsion. Indeed, a higher amount of lignosulfonate is required in saline water than in deionised water to achieve
satisfactory separation of clay particles from coal surfaces with the
minimised electrostatic repulsion. Pawlik, et al. [38] showed that
at higher ionic strength, the initial CMC concentration at which
the re-stabilisation of serpentine minerals began was much higher.
On the other hand, although a large amount of lignosulfonate could
generate strong repulsion to mitigate clay coatings in both deionised and saline water, the coal surface was made strongly hydrophilic resulting in the depression in flotation no matter whether
clay coatings were removed from the surface