Hydrophobicity of Coal Particles
Coal is intrinsically hydrophobic because of its chemical composition (surface aromatic and aliphatic groups). Several explanations have been provided for poor coal flotation. The coal surface may be less hydrophobic because of its oxidation resulting in the formation of hydrophilic carbonyl, carboxyl and ester groups or because of the presence of hydrophilic species on its surface as fine particles ‘piggy-backing’ or part of a composite particle.
The aim of this study is to predict coal flotation from a knowledge of the proportion and distribution of surface species (silicate and coal), and their contact angle (hydrophobicity). The influence of surface roughness on the contact angle and therefore the attachment of coal particles to bubbles was also investigated. X-ray photoelectron spectroscopy and scanning electron microscopy (QEM-SEM) were used to measure the surface composition and the distribution of the various mineral phases in each particle, respectively. The extent of coal oxidation was provided by infrared and X-ray photoelectron spectroscopy. The roughness and contact angle of the coal surface were measured by specular reflectance spectroscopy and nanoindentation, and the captive bubble technique, respectively. The results obtained in this study have clearly shown that the presence of clay or oxidation species on the coal surface and surface roughness reduce the hydrophobicity of coal and therefore its flotation.