Theoretical Foundations of Selective Oil Aggregation in Coal: Coalescence Mechanism of Coal-Oil Structure Formation

Abstract

The article deals with the theoretical foundations of selective oil aggregation (SOA) of coal. The emphasis is laid on the technologically most significant coalescence mechanism of coal-oil structures formation. At the same time, the introduction presents a comparative review of the hypotheses of the process of selective oil aggregation of coal. A phenomenological model of the process of selective oil aggregation of coal by the coalescence mechanism is offered and investigated, which includes the following subprocesses: “1. Elementary act of meeting and contact of coal grains and oil-binder droplets → 2. Adhesive fixation of the binder on hydrophobic areas of the coal surface → 3. Autohesion contacts of oiled coal grains. The emergence of primary coal-oil structures → 4. Formation and compaction of coal-oil aggregates”. The main factors that determine the course and outcome of these subprocesses are identified and characterized. In particular, it has been established that the elementary act of meeting and contact of droplets of the binder reagent and coal grains during oil aggregation is determined by the effects of close and long-range hydrodynamic interaction (CHI and LHI). For the conditions of coal SOA, the gradient and inertial mechanisms of the meeting of aggregate-forming objects are of the largest importance. The higher the rate of turbulent pulsations during agitation of the water-coal-oil suspension, the greater the proportion of "gradient" meetings, and the larger the diameter of the binder droplets, the greater the proportion of "inertial" meetings. Increasing the Re number up to 2000...3000 (developed turbulence) is a means of neutralizing the negative impact of LHI on the aggregation of fine coal particles r ≤ 1...6 μm. At Re ≥ 2000...3000, it can be considered that each meeting of coal particles calculated on the basis of rectilinear trajectories leads to their actual meeting and subsequent probable adhesion. For the conditions of selective oil agglomeration of fine coal, turbulent diffusion is the most important. At the same time, the gradient mechanism of the meeting takes place mainly in the near-wall water layer of the granulator working chamber. It is shown that the adhesive fixation of the binder on the coal surface is due, in addition to physical adhesion, to specific high-energy bonds of a chemical nature, in particular, hydrogen bonding, which promotes to the process of oil aggregation of coal. It is established that autohesive contacts of oiled coal grains and the emergence of primary coal-oil structures for SOA conditions determine the main aggregate-forming forces - cohesive and capillary. In this case, the capillary component prevails. It is shown that the subprocess of formation and compaction of coal-oil aggregates is characterized by a step curve of the dependence of the diameter of coal-oil structures on the time of agitation of the water-coal-oil mixture.