Dominating method of rock destruction at open-cast mining is explosive rupture, which has an explosion efficiency coefficient not exceeding several percents. This is almost the only existing method of destroying large volumes of rocks, blasting occupies up to 30 % of mineral resources production costs. Taking into consideration that shattering explosion efficiency coefficient doesn’t exceed a few percents, then development of applied blasting technologies (considering development of blasting science and technology) appear to be obvious.
It is well-known, that charge shape is crucial in terms of environmental influence and its final results. There exist practical researches into concentrated charge (with spherical symmetry) and slab charge (with noncircular cross-section). During sand-cement blocks blasting was revealed, that while changing from the circle form of the well to rectangular and flat the output of crushed fines and oversize fines is decreasing. There are well-known production tests of slot-like (flat) wells, that are formed by extension of cylindrical wells using burner-expander without thermodrill rotation, by parallel-coupled and parallel pulled-together wells drilling. Charge form changing has provided rock output increasing up to 25% while specific consumption of explosives has decreased up to 25 %. Middle size of spalls decreasing can be seen on the hard-wax, red and fire brick blocks after square and triangular wells blasting.
The results of such experiments verify that wells’ form changing provides changing of the blasted rock qualitative composition. It is arguable that in wells with polyangular cross-section such situation is caused by crack initiation and mainly in the angles intersection of wells sides.
Therefore, the main goal of researches is the development and validation of drilling tools usage, which provides specific consumption of explosives decreasing with the purpose of energy saving and ecological environment pollution decreasing.
For practical usage of wells with noncircular cross-section is required the bit, which should be technically as good as the ones well-known in the world practice. Patent information analysis has shown that for stress-raiser well drilling is essential production of the new type of drilling machines and that drilling methods can’t be effectively applied at the open-cast mining.
Shattering effect prevalence upon the bottomhole will be reasonable during drilling of medium-grained, fine-grained sandstone, conglomerates, hard limestone (with range of strength from 6 to 18 according to the Protodyakonov Rock Hardness Scale). For these conditions the rolling cutter bits will be sufficient (for medium-hard rock with layers hardness up to 9) and roller bits (for rocks with hardness from 6 to 18). The most common in practice are the roller bits, which are used for drilling of 80% wells at the open-cast mines. Therefore usage of existing design of roller-bit drilling rig, combination of drilling and stress riser forming is a rational decision.
It might be implemented by means of rectangular, triangular wells drilling. Geometry analysis and kinematic analysis have revealed that drilling rollers can be used for rectangular and triangular wells drilling.
Predrilling of blocks with sand:cement ratio — 4:1 was carried out. It has shown that rollers may rotate synchronously (pic. 1).
Pic. 1. Well view
Rackbar of the bottomhole positively influences over noncircular cross-section well forming, because rollers should roll skidlessly and synchronously. For this purpose rollers should have a perfect cone (when tops of roller cone surface are allocated inside the well). The rollers with a profile which is close to perfect cone are used for the harder rocks, because in this situation the bottomhole equipment sliding with prevalence of shattering effect is minimal.
Therefore, in three-roller bits for square cross-section well drilling might be used features of rolling bits K, TK and partly T type. Noncircular cross-section form of the well is determined by a modified geometric shape of bit gauge rollers, that might be toothed, insert and insert-toothed, which means that they capture almost all rocks’ level of hardness for different types of bits. Synchronous movement of rollers provides decreasing of slipping and deterioration of equipment during hard and abrasive rock drilling. At the manufacturing plant in such bits may be used all significant achievements on the roller bit production sphere (oil-filled bearings, slide bearings, advanced materials).
Effectiveness of rolling cutter bit is caused by the following. Separate processing outline of the bottomhole might be used in the roller cutting bits . Peripheral part is processed by the rollers and the central one – by the cutting unit. Rock bit teeth, which are allocated at the bit crown, close to the top are less effective in destroying the rock because of the low peripheral speed and, therefore, lowest rock hit energy of the teeth. At the same time cutters, that are under peak radius, have the biggest sliding distance, which causes its deterioration densification. In such construction the leading part of the well is drilled by the cutting unit and rollers are expanding it up to the necessary diameter. In construction of rolling cutter rock bit for square cross-section well drilling might be used the previously mentioned advantages of such bits. Peripherical part of the bottomhole is destroyed by the rollers, that form noncircular profile, and the central part of the bottomhole is destroyed by the cutting unit, which has a lower deterioration. As cutting unit deteriorates, it might be replaced, because it is the cheapest part of the tool. Therefore, drilling tool is highly technically competitive with predeveloped and examined rolling cutter rock bit. Such bits are the alternative to type СТ and type С roller bits.
Introduction of such drilling tool would provide blasting effect for wells with noncircular cross-section, that (as it was previously mentioned) sometimes decreases specific consumption of explosives up to 25%. In this case the volumes of drilling are also decreasing. At the graphic log Berezovsckiy the rate of output per shift is 400 metres or 134 wells with average depth of 12 m. For example, if specific consumption of explosives is decreased up to 12,5 % during usage of wells with square cross-section, than on an equal footing will be required 117 wells, which is 12,5% less. Tool interchange is carried out after 3000 metres of drilling.
In this case, according to the economical analysis, the cost-cutting on explosives will be 17,4 million rubbles per year, even taking onto consideration the drilling tool rise in price and drilling rigs’ decline in productivity.
Therefore, expenses on drilling using wells with square cross-section are lower in comparison to the wells with circle cross-section. Suggested tool construction may develop the conditions of drilling and blasting operations and would lead to expenses decreasing.
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