Baltic oil shale has long been recognized as a raw material for the chemical and gas industries and as an energy fuel. In this regard, the issues of exploitation of shale deposits and the enrichment of extracted shale are becoming important. The beneficiation of shale, which began simultaneously with their extraction, from the very beginning to the present day is carried out in all mines by manual selection of rock (limestone). The use of mechanical (non-manual) enrichment of oil shale is unknown. The question of mechanical enrichment of oil shale arose by analogy with coal and in connection with the sharply increasing scale of its production. Of the mechanical enrichment processes, hydrogravity is the most studied. For a number of reasons, aerogravity and flotation are less studied. The materials obtained as a result of all the studies make it possible to evaluate the Baltic oil shale as an object of enrichment and to identify approximate indicators of their enrichment. This work aims to summarize the materials accumulated over the past 20-25 years on the beneficiation of oil shale in the Baltic states and outline a number of conclusions that follow from them.
Free fall is theoretically defined as the fall of an isolated grain in an unlimited space of a distributing medium (water or air). In practice, it is considered as the fall of an aggregate of grains at such a concentration that the movement of any grain is not significantly disrupted directly or through the distributing medium by accompanying grains. Free fall has an exceptional value in the process of accurate classification, when the movement of grains is considered as uniform, occurring at a constant final speed. Falling in the environment, grains displace the environment from those places that they themselves successively occupy. Their shapes and the distribution of masses within them, strictly speaking, do not remain constant during this movement, but these changes are so small that they are certainly neglected. In all cases of interaction of a grain with a liquid medium, we obtain the same phenomenon - the flow of a medium around the grain. This flow around, due to the impermeability of the space occupied by the grain to the liquid medium, causes a change in the movement of the medium flow incident on the grain. The movement is accompanied by friction at the boundary between the grain and the medium. The question of the amount of resistance experienced by a moving body is one of the most ancient problems of dynamics, but to date its final theoretical solution has not been achieved, which led to attempts to directly experimentally determine resistance and gave rise to the successful development of experimental hydro- and aerodynamics.
