Contact angle measurements of water on coal

Experiments on the removal of water and wetting liquids from a bed of high‐rank coal using a development of Bartell's method showed that the receding contact angle was nearly zero. Observations of drops of water on coal surfaces confirmed that on polished plates the receding contact angle was low: for high‐ and medium‐rank coals it was about 10°, for a low‐rank coal near to zero. For cleaved plates or rough surfaces the receding angle was indistinguishable from zero. The advancing contact angle, on the other hand, was between 60° and 85° on polished plates, depending on coal rank. Fuel oil on the coal surface increased the receding contact angle considerably. Potassium permanganate treatment gave a zero receding angle but a finite advancing angle. The implications of contact hysteresis for dewatering and froth flotation are discussed. It is concluded that knowledge of both advancing and receding angles is necessary to understand these processes.