NOTE Effect of Gas Nuclei on Hydrophobic Coagulation
EXPERIMENTAL Experimental results on the coagulation of fine coal particles in water (a case of natural hydrophobicity) and silica in dodecylam-A Pittsburgh #8 coal was used. The coal chunk was broken with a ine solution (chemically induced hydrophobicity) are presented.
hammer, and then dry pulverized in a bench-scale hammer mill. The minus Different behavior was observed depending whether or not the 400 mesh (38 mm) fraction was obtained by dry screening, and stored in slurry was degassed. After degassing, the apparent size of both coal a freezer to minimize oxidation. The surface hydrophobicity of coal was and silica flocs became smaller, which was attributed to removal of estimated from the contact angle (measured by a goniometer). An advanc- gas nuclei and consequent reduction in the interparticle attractive ing contact angle of about 65ะ was obtained by placing a water drop on a force. The effect on settling, however, was different for both: the flat surface of a coal chunk. settling rate of silica decreased while for coal it increased. This A 15 g sample was mixed with 600 ml water (volume fraction, 1.85%) in a beaker for 15 to 20 min by magnetic stirring. (Caution was taken to was modeled in terms of the relative effect of size and density of avoid the entrainment of air and thus the formation of bubbles during coal and silica particles with and without gas nuclei. Apart from stirring.) While still mixing, slurry was transferred to fill a Plexiglas cylinder practical consequences in fine particle processing, a fundamental (i.d. 3.8 cm and 16.5 cm high) with a volume of 187 ml, and the top of one is that to evaluate true hydrophobic coagulation or to measure the cylinder was sealed by a piece of parafilm (Fig. ). After shaking the the true hydrophobic force, gas nuclei have to be eliminated. slurry by turning the cylinder end over end, the particles (aggregates) were