In saline water, however, a strong attractive force at a separation of 2.5 nm was still observed, and there was a slight decrease
in the overall attraction in the presence of lignosulfonate. Obviously, a small amount of lignosulfonate failed to generate enough
repulsive forces to offset van der Waals forces in saline water. This
result is consistent with the depressed coal flotation in the presence of a small amount of lignosulfonate in process water. As discussed earlier, the adsorbed lignosulfonate could not generate
strong electrostatic repulsion between coal and kaolinite particles
because of the screened electrical double layer in the presence of
salts. Herb and Ross [37] also pointed out that the saline water
with high ionic strength could suppress the range of electrostatic
repulsion, and although electrostatic repulsion still existed and
took part in the interactions, the main cause of stability came from
the steric hindrance of the adsorbed lignosulfonate dispersant.
However, as found in the adsorption isotherms section, a small
amount of lignosulfonate displayed a low adsorption density on
both coal and clay surfaces, and therefore could not produce sufficient steric repulsion to separate clay and coal particles. On the
other hand, the adsorbed lignosulfonate on the clay coated coal
surface may make it more hydrophilic and therefore decrease the
flotation of the problematic coal in process water.