Polymeric dispersants for coal-water slurries

The effect of several low molecular weight polymers (MW < 100,000) on the surface properties of a medium-volatile bituminous coal in concentrated aqueous suspensions was investigated through adsorption, flotation, electroacoustic, and rheological measurements. The dispersing capabilities of the tested polymers, i.e. polystyrene sulfonate, humic acids, carboxymethyl cellulose, dextrin, hydroxyethyl cellulose, and hydroxypropyl cellulose depend not only on their ionic/nonionic character but also on the ability to increase the wettability of the coal surface. The anionic polymers (carboxymethyl cellulose, polystyrene sulfonate, and humic acids) adsorb less densely on the coal surfaces compared to the nonionic ones. Despite the lower adsorption density, the anionic polymers are much stronger coal dispersants since their action is a combination of steric and electrostatic repulsive forces. In contrast, the nonionic cellulose ethers can act only through steric effects. Good coal dispersants are also strong flotation depressants. The poor dispersing performance of hydroxypropyl cellulose correlates well with the inability of the polymer to depress the natural floatability of the bituminous coal. The results are discussed in terms of the extended Deryaguin-Landau-Verwey-Overbeek theory of colloid stability. The data demonstrate that rendering the coal particles hydrophilic is the common mode of the dispersing action of both anionic and nonionic polymers. (c) 2005 Elsevier B.V. All rights reserved.