Interaction between coal particles and oil droplets suspended in an aqueous electrolyte solution can result in collision (defined). Two energy barriers may hinder such interaction : (1) is the result of the interpenetration of the electrical double layers surrounding the surfaces when they have similar sign (2) which prevents spreading of the oil over the wet coal surface and leads to a difference between the advancing and receding contact angles. With an experimental system such that the kinetic energy available to overcome these energy barriers is controlled at a known value, it is shown that three kinds of behaviour can occur : (a) when the kinetic energy is less than the repulsion energy, collision between coal particles and oil droplets does not occur in such a way as to permit permanent attachment of oil to coal; (b) if the kinetic energy is much larger than the repulsion energy, collision occurs but does not necessarily produce coal surfaces which permit flotation of the coal. This is interpreted as meaning that if flotation is impossible the oil has not spread over the coal surface (c) in case (b) if a suitable surface-active reagent is already adsorbed at the coal surface the oil spreads spontaneously over the surface and air can displace water at the air-water-solid interface. This behaviour is held to confirm the model of conditioning and flotation set out.
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