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 enrichment 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 enrichment 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 surrounding medium (water or air). Practically, it is considered as the fall of a set of grains at a concentration where the movement of any grain is not significantly disturbed, either directly or through the surrounding medium, by accompanying grains. Free fall is of particular importance in the process of precise classification, where the movement of grains is considered to be uniform and occurring at a constant terminal velocity. As the grains fall through the medium, they displace the medium from the places that they successively occupy. Strictly speaking, their shapes and internal mass distributions do not remain constant during such movement, but these changes are so small that they are unconditionally neglected. In all cases of grain‑liquid interaction, we observe the same phenomenon: the flow of the medium around the grain. This flow, caused by the impermeability of the space occupied by the grain to the liquid medium, causes a change in the motion of the oncoming flow of the medium. The motion is accompanied by friction at the boundary between the grain and the medium. The question of the magnitude of the resistance experienced by a moving body is one of the oldest problems in dynamics, but to this day no final theoretical solution has been achieved, which has led to attempts to directly determine the resistance experimentally and has given rise to the successful development of experimental hydro‑ and aerodynamics.