Often dewatering of coal tailings is inhibited by the presence of clays,
making it difficult to obtain a high concentration, handleable coal
tailings. The net result can be a low concentration, chocolate mousse-like
tailings which poses significant sedimentation problems in a tailings dam
and represents a significant environmental problem. This paper examines
the coal tailings from a Hunter Valley colliery where the tailings
properties are influenced significantly by the presence of montmorillonite
clay. The influence of the calcium ion on the rheology and
sedimentation/compression behaviour of coal tailings . containing
montmorillonite already dispersed in water was investigated and revealed
that very little improvement in the properties is achieved through the
coagulation. The preliminary investigations also indicated that the
properties of the tailings were dominated by the presence of the
montmorillonite clay. Thus investigations were carried out on the clay
alone in water. Investigations into the effect of dispersing samples of the clay alone in
water and then coagulating with calcium compared with dispersing the
clay directly into calcium ion solutions at the same calcium concentration
showed huge improvements in the rheology, compressibility,
sedimentation rates and sediment solids concentrations for the second
dispersion method, which can be called controlled dispersion. The
sediment volume fraction solids improved by more than a factor of two
by controlling the dispersion of the clay. The reason for the improved
properties lies in the suppression of the initial swelling of the clay, which
is initially in the sodium form, thus maintaining the parallel orientation of
the platelets that is present before wetting. While swelling is being
suppressed, ion exchange occurs with the calcium which results in
permanent swelling suppression. In the uncontrolled dispersion case
where swelling is uninhibited, the clay is allowed to break up and despite
the powerful coagulating nature of the calcium ion, the random
orientation of the high aspect ratio clay platelets inhibits effective
coagulation. Results indicate that the initial electrolyte concentration of
the washwater in a coal washery is the critical factor controlling the
dewaterability of clay tailings and the handleability of the dewatered
product rather than the actual act of coagulating with the calcium ion. The results presented are of clear practical significance and indicate
how coal tailings problems where clay dominates the rheology can be
improved considerably by knowledge of the surface chemistry.