Drilling and blasting operations are among main and essential processes of surface mining. Nowadays there exist many different methods of effective blast rock fragmentation improvement and moreover there are conducted researches on its development. The main goal of such researches is to decrease the explosive ratio, which provides reduction in expenses on blasting materials, as well as reduction in expenses on drilling, environmental pollution, seismic effect on the nearby environment.
Mostly energy is used for rock overgrinding nearby the blasthole, because well sides receive it in large amounts during a short period of time. There might be increased a duration of blasting materials in order to improve its effective using, as well as contacting area of blasting gas at the explosion starting moment might also be increased. However, the borehole diameter increasing will lead to borehole cross-section increasing and, as a result, needless additional drilling expenses. But if the borehole has, for example, a square or triangular (in the shape of equilateral triangle) cross-section, the area of the lateral surface of the well in the ratio to the volume of explosive materials, in comparison to the circle cross-section, will be greater by 12% and 27%. Moreover, the angles, formed by the borehole walls matching, represent the concentrator of tension stress. As a result, there might be obtained predicted quantity and initial main crack propagation direction, where gas is delivered during the explosion, providing the piston type effect.
Nowadays there exist a wide range of drill bit designs and sizes, which are used for blastholes mechanical drilling and usage of wells with circle cross-section has become a mature technology. It is determined by the requirement of simplicity and design reliability, as well as necessity of drilling tool rotation for mechanical energy transmission to the drill bits and diamond tools.
One of exceptions is the irrevocable percussion drilling method, which makes cross section of wells conformed to the tool shape. However the absence of tool rotation impede the hole-bottom region clearing out of drill cuttings, because rock cutting element has to be allocated all over the cross section of the borehole.
It is recommended to use drillbits with characteristics that conform mining and geological conditions and physico-mechanical properties of drilled rock in order to get better indices during the drilling. For this reason during practical application of wells with improved noncircular cross-section the new drillbit should be technically as good as drillbits, which are well-known in the global practice.
It is common knowledge that 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 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. The harder the rock is the bigger shattering effect prevalence and the smaller cutting effect should be. The most effective type of bit (according to the scientific research and industrial tests) applied to rocks with alternating hardness of layers will be a combined rolling cutter bit.
In this article is suggested the usage of rotative bit with conical cutters, which forms a well with noncircular cross section. Unequal slant height of a cutters’ cone (Pic.1) causes unequal borehole diameter with explicit angles.
Pic. 1 Rolling bit for square cross-section well drilling
In suggested three-roller bits for square cross-section well drilling might be used features of rolling bits of K, TK and partly T type. The square 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 bit types with range of strength from 6 to 18 according to the Protodyakonov Rock Hardness Scale.
It is common knowledge that during rolling bit application at the bottomhole appears an edging, formed by the external bit crown with the greatest teeth spacing. Rackbar of the bottomhole positively influences over noncircular cross-section well forming, because rollers should roll skidlessly and synchronously. For this purpose the rollers should have a perfect cone (when tops of roller cone surface are allocated inside the well). In order to decrease rackbar’s negative impact on the deterioration process might be used constructive solutions for fractional damage of edging.
Nowadays drilling rollers with the perfect cone are not applied, because there is required an as big as possible bearing assembly in order to increase its durability. Contact voltage in rolling-contact bearing is up to 4000-5000 MPa and in sleeve bearing — 30-40 MPa.
That is why durability of bearings in such construction depends on high technology and development of bearings with larger slide area for low heat removal compensation, caused by rubbing part of the bearing.
Bearing rollers unloading is available only if they are forming a noncircular cross-section outline of the well, for example, in the construction of rolling cutter bit. Separate processing outline of the bottomhole might be used in the roller cutting bits. Peripherical 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, the 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.
During a well drilling using the combined bit the main role of bottomhole destroying belongs to the cutting unit, which is changed after deterioration. Partly the well is drilled jointly with the borehole processing using cutting and rolling tools. There are used different tools for bottom ole destruction depending on drilling conditions, which increases working capacity of rock-destroying unit. Moreover, the drilling is carried out at lower feeding pressure in comparison to the roller bit due to the joint impact of cutting tools and rollers. These all provides combined bit rollers unloading and increases its working capacity.
In suggested construction of rolling cutter rock bit for square cross-section well drilling might be used 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 and may drill rocks of medium hardness equal to 6 according to the Protodyakonov Rock Hardness Scale with hard rock layers.
The reserves of explosive action effectiveness increasing by means of wells’ cross-section form changing are not used during blasthole drilling at the open-cut mining. Mainly it is caused by the absence of drilling tool, which could compete with the noncircular cross-section well drilling bits. There are required the researches and industrial tests in the sphere of noncircular cross-section well drilling using rollers and blasting of such wells. Suggested in this article constructions have the following advantages:
· Might be based on the high and innovative technologies of bit drilling.
· Cover wide range of drilled rocks according to their hardness at the open-cut mining.
· Use rotary system of drilling and, therefore, might be applied to the most common Russian and foreign air blowing roller rigs.
· Provides technical-economic indicators improvement of rigs due to their construction development and development of bits’ construction and, moreover, through drilling volume decreasing.
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