The role of surface interaction forces and mixing in enhanced dewatering of coal preparation tailings

Industrial waste containing colloidal particulate matter such as coal and mineral processing tailings presents many environmental and economic challenges. Effective management solutions for these challenges require an improved understanding of colloidal forces between tailings particles which control their flocculation and subsequent dewatering. In this paper, the colloidal and dewatering behaviours of four coal processing tailings were investigated in the presence of four high molecular weight anionic polyelectrolytes using the AFM (atomic force microscope) colloid probe technique, flocculation and sedimentation testing, and colloid and surface characterisation. Averaged snap-off distances statistically determined from thousands of force curves measured by AFM were obtained by dividing the area between the force curve and the horizontal axis by the maximum adhesion force. This novel assessment of the AFM results showed that for each of the tailing samples and flocculants tested there was statistically a maximum snap-off distance, which on average would correspond to the optimum flocculant concentration. Under this optimum concentration condition the adhesive strength is the strongest to resist potential floc breakage caused by external hydrodynamic forces. Flocculation and sedimentation tests also showed optimum flocculant concentrations for the different tailings samples but these concentrations were much higher than those corresponding to the optimum adhesive strength found with the AFM measurements. This was due to the large surface area of particles in the concentrated suspension used in the settling tests compared with just a few particles used in the atomic force measurement and therefore the two optimum concentrations per unit surface area of particles would be similar.