Vol 41 Iss. 1

The study of destruction of coals and rocks by water jet was carried out in four main directions: 1) the process of destruction of rocks and some artificial materials under the application of impact and static loads and the identification of factors determining the energy intensity of such destruction; 2) the structure and parameters of free high-pressure water jets; 3) the process and energy intensity of rock destruction by water jet; 4) mathematical interpretations of laboratory research data and production observations and the development of the basics of engineering theory of rock destruction by water jet.

For hydraulic transportation of bulk materials of various characteristics over significant distances, the application of feeders in combination with high-pressure centrifugal pumps is promising.

Further growth of labor productivity in the coal industry should be ensured not only by using and improving the existing system of mechanization of coal mining, but also by introducing new technologies, including the hydraulic method of coal mining. Only in this case the multiple increase of labor productivity, reduction of production costs and increase of in the efficiency of the use of capital investments will be achieved.

In the development of the scientific basis of hydromechanization of mining operations, the theory of motion of a free unsubmerged jet plays a great role. There is quite an extensive literature on the structure of the jet leaving the nozzle, the instability developing in it, jet decay, the influence of conditions preceding its exit, the resistance of the surrounding (much less dense) medium, and the dynamic properties of the jet. Nevertheless, even in the experimental field, the existing data cannot be considered exhaustive for the creation of a hydraulic theory of the jet.

In order to study some regularities determining the energy intensity of the process of destruction of a solid body by high-pressure water jet (up to 200 atm), a set of studies was carried out in the laboratory of mine transport of the Leningrad Mining Institute.

The significant magnitude of the forces arising during impact, exceeding the weight of the striking body by hundreds and thousands of times, opens wide opportunities for simplification of modern mining machines by increasing their speed and accumulating energy through impact elements. On the other hand, the study of the impact phenomenon can facilitate the study of regularities occurring during the destruction of rocks by a high‑pressure water jet. All this indicates the relevance of studying the regularities of rock destruction and, in particular, of coal by impact.

Experimental study of the dynamics of free water jets that meet a rigid barrier on their way is very important for the accumulation of experimental data contributing to the establishment of a number of regularities necessary for the creation of the theory of hydraulic breaking of coal from the massif in hydromechanization of coal mining operations. Of primary importance in such experiments is the determination of the jet impact force on the barrier depending on the water pressure in the jet‑producing device, on the diameter of the nozzle outlet, and on the distance from the latter to the barrier.

When breaking rocks with a water jet, the flow due to the potential energy of pressure in the hydromonitor at the exit of the nozzle acquires kinetic energy and, freely interacting with air, is gradually aerated, which leads to the dispersion of the jet. Since the efficiency of the destruction process depends to a large extent on the compactness of the jet, the study of compactness and factors determining it is of practical interest.

The high-speed movie camera FP-22, used in the study of the water jet in the conditions in the laboratory of the Leningrad Mining Institute, shot at a frequency of 25,000 and 50,000 frames per second. The total length of each film was approximately 30 m with a frame width of 5 mm and a height of 3.5 mm. The water pressure in the pump was varied from 2 to 150 ati. In studying each film, the following were mainly taken into account: 1) the shooting frequency (number of frames per second); 2) the diameter of the nozzle (its shape was not changed); 3) the water pressure; and 4) the presence or absence of a stabilizer.

Considering individual elements of high-pressure jets as a free-falling body thrown into space with a certain velocity, and having the parameters of the jet trajectory, it is possible to judge the amount of energy transferred by the jet to each of the sections under consideration, as well as the losses to overcome the resistance of the medium. But the determination of the trajectory parameters of the force axis of the jet is complicated by a number of features inherent in high-pressure jets. For example, at a pressure of 200 ati, a jet with a diameter of 3 mm passes the most effective zone with a length of 6 m for 0.03 sec, while in a vacuum it would fall by 4.4 mm. Under these conditions, a method with a high degree of accuracy in determining the position in space of the force axis of the high-pressure jet is needed. Determination of the altitude coordinate by visual observation is completely unacceptable, since the halo of spray surrounding and hiding the main central part of the jet has a lower velocity, and hence a greater curvature of the trajectory. The method of jet core prints does not provide the accuracy required in these conditions and requires labor‑intensive preparation and processing of jet prints.

When designing mining and tunneling machines operating on the principle of hydraulic breaking, it is important to know the dependence of productivity and energy consumption of destruction of solid bodies by a water jet on the speed of movement of the jet axis relative to the face, on the water pressure in front of the nozzle, as well as on other parameters (radius of rotation of the nozzle, distance from the nozzle to the body to be destroyed, etc.).

The study of the process of coal destruction by water jet in order to build a rational theory of the phenomenon is a difficult theoretical and experimental task. The complexity of the study is caused by insufficiently studied process of brittle fracture, the complexity of the structure of the fractured rock, the lack of our information about the destructive agent and its action. In such a complex environment, it seems natural at the first steps of the research to refrain from full consideration of all factors acting in the process of fracture, to simplify and schematize the phenomenon. The study carried out in a simplified scheme gives only approximate dependences between the mechanical characteristics of the rock being fractured and the parameters characterizing the jet producing the fracture. However, the obtained dependencies, having been subjected to experimental verification, can be evaluated with respect to the accuracy of the results they deliver and the acceptability of their use in the practice of technical calculation. On the basis of experience, corrections can be made to them, taking into account the complexity of the actual phenomenon and bringing the simplified scheme closer to reality.

The article deals with the issues related to the calculation of the conditions of brittle rock fracture by statically applied load. The rock massif is considered as an infinite half-space under the action of a load normally applied to the boundary.

In hydraulic transportation of fine-grained materials for their delivery to the surface from depths exceeding 100 m, it is necessary to feed these materials by slurry pumps into the mine drainage pipeline through a special feeder. Such a feeder is a hollow cylinder, included in the network of the main drainage header. Special gate valves allow either disconnecting the feeder from the header at the moment of loading it with material, or directing the pressure jet created by the main drainage pumps into the feeder cylinder at the moment of washing out the material. In this way, the eroded material in the form of a slurry of low concentration (H : L up to 1 : 20) is discharged to the surface by the main dewatering header.

Hydromechanization of mining operations was first applied in the 30s of the last century during the development of gold-bearing placers in the Ural Mountains by open-pit operations. Later, since 1852, hydromechanization began to be applied in similar conditions in California (USA), where this method became especially widespread. Hydromechanization of underground works began much later. The first hydro-mine in the USSR was built in 1939 in the Donetsk basin. A few years before that, experimental work was carried out in coal mines in the Kizelov basin in the Urals and manganese mines in the Nikopol region. At present, hydromechanization is widely used in open-pit mining for various purposes and in the development of coal deposits by underground works.