The rate of wetting of coal dust by aqueous solutions of surface active agents (surfactants) has been studied. It has been shown that surfactant is adsorbed onto the coal surface during the wetting process. Rates of wetting by solutions of the widely used sodium salt of di(2-ethylhexyl)sulphosuccinic acid (Aerosol OT) are strongly influenced by low concentrations (I-2 mg/l) of dipositive metal ions. After wetting, electron spectroscopy for chemical analysis (ESCA) has been used to show the presence of these metal ions and the surfactant on the surface of the particles of coal dust. Satisfactory control of the dust formed during coal mining and processing operations can sometimes be achieved by the use of sprays of wate?.
Under some conditions, the addition of a small amount of surfactant to the s ray may significantly improve the suppression of the dust P . A related situation occurs in the case of the mining of pyrite', and during the crushing of a variety of ores6.
A simple, effective laboratory test for the ability of a given surfactant solution to wet coal dust, by timing the rate at which the dust passes into a solution, was devised many years ago'. More recentlys, this test has been used to measure the wetting power for coal of solutions of a wide range of surfactants. An interesting comparison can be drawn between these latter results and data obtained for the efficiency of the same surfactants in promoting the spread of agricultural sprays onto waxy leaves'.
Extensive studies have been made of the nature of adsorption of surfactants onto such materials as carbon black, graphite, graphitized carbon, and charcoal". We have observed that the wetting characteristics of coal are quite different from those of the foregoing materials.
It is our objective to study and understand the fundamental processes occurring during the wetting of coal dust by aqueous surfactant solutions. In the present paper we report the results of investigations into comparative wetting rates, along with the results of our studies of changes at the coal surface during wetting as revealed by the application of ESCA spectroscopy".
EXPERIMENTAL
Materials used
Coal was obtained from the Pocahontas seam of McDowell County, West Virginia. After crushing, dust which passed through a wire screen of 45 pm mesh size (325 US mesh) was retained for testing purposest . Scanning electron photomicrographs revealed the sample to be predominantly composed of angular fragments with an estimated 90% (by mass) of the sample consisting of particles with an equivalent diameter in the 5-30 I.cm range. The presence of frag-