THE ROLE OF SURFACE THERMODYNAMIC PROPERTIES IN THE AGGLOMERATION OF COALS

The efficiency with which Australian coals ranging from brown to low-volatile bituminous undergo the process of agglomeration was related to the surface properties of the individual coals. Surface hydrophobicity was assessed by measuring the angle of contact between light gas oil in water with discs pressed from the coal. The effectiveness of agglomeration was measured by the weighted average size of the agglomerates resulting from successful agglomeration of a < 100-mu-m sample. Coals of all ranks, when immersed in water, exhibited a finite contact angle for oil advancing over the solid surface. The effects of both the organic and inorganic surface components of coal on the surface wetting characteristics were analysed separately in terms of the C:O ratios and mineral matter contents. Extrapolation to zero mineral mater provided an intrinsic contact angle characteristic of the polar organic components (dominantly oxygen) of the coal surface. This contact angle decreased with increasing coal rank and provided a critical minimum spreading pressure which must be overcome for oil to spread spontaneously on the coal surface. This minimum spreading pressure rapidly increases with decreasing coal rank to give a lower limit below which wetting, oil film formation and agglomeration will not occur.