At the Kuzbas current graphic log the average stripping ratio is 5,8 m3/ton, which means that for 1 ton of coal production almost 6 m3 of overburden rock has to be removed into the disposal area and, moreover, it has to be shattered before excavator loading. The main method of overburden rock destruction is blasthole drilling. Specific dust generation during blasting is changing from 0,04 kg to 0,154 kg of dust per 1 kg of blasted explosions, moreover, it leads to great volumes of toxic substances releasing – mainly carbonic oxide and nitrous oxides.
Thus, decreasing the powder factor is important not only because it will be economized. While the powder factor is decreasing, the quantity of blastholes is either decreasing. As a result, the volume of drilling is also decreasing and drilling rate increase requirements become not so strict.
Different methods are successfully applied in order to decrease the powder factor and increase the quality of rock shattering. Their main goal is to transmit the blast energy to as many rock volumes as possible. The energy is mainly expended on rocks crushing nearby the blastholes, because the great amount of energy is transmitted to the wells’ side during a short period of time. Therefore, increasing of gas (generated during the explosion) and rock contact area at the initial moment of the explosion is assumed to be reasonable.
Nowadays almost all blastholes at the open cast are bored in cylindrical form. However, if the well has, for example, a square or triangular cross section, then its lateral area per volume of blasting material in comparison to the circular one will be 12% and 27% bigger. 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.
It has been established that during explosion of sandy-cement blocks and changing circular form of the borehole to the rectangular or flat form decreases the output of fine and oversized fractions.
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. Therefore, the main goal of researches is 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.
The analysis of patent information has shown that for drilling of wells with changed cross-section form is required the new type of drilling tool and machine. However, the usage of existing design of roller-bit drilling rig, combination of drilling and well forming is a rational decision. It might be implemented by means of rectangular, triangular wells drilling, that generate a stress-riser by means of their sides’ connection. Geometry analysis and kinematic analysis have revealed that in order to achieve this goal roller bit might be used for rectangular and triangular wells drilling, as a widely spread tool for blastholes drilling at the open-cast mining.
There was produced the prototype of two-roller bit with single-cone rolling cutter (pic. 2) in order to analyze the predrilling process. Predrilling (pic.1) of blocks with sand:cement ratio — 4:1 was carried out.
Defined well cross section forming is provided by rollers synchronous rolling and constant speed increasing ratio of the bit, which is achieved in the construction of perfect cone rollers. 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 shattering effect prevalence is minimal. However, the perfect cone rollers are not practically applied because of their support block small lifetime. That is the reason why there is suggested the usage of such rollers in roller cutting tools and slider bearings in support block of rollers.
The suggested drilling tool for forming of blastholes with noncircular cross-section has some significant distinctions and provides:
1. noncircular cross-section wells drilling with tool rotation and avoiding changes in widely spread technology of drilling at the open-cast mining;
2. series roller-bit drilling rig usage (almost 80% of wells at the open-cast mining are drilled using roller-bit drilling rigs);
3. wide range of rocks drilling according to the hardness, which enables to change up to 50% of roller bit types for blastholes drilling at the open casts with circular cross-section.
Pic. 1. Prototype of square cross-section roller bit for well drilling
Pic. 2 The view of initially predrilled well
Usage of suggested drilling tool for noncircular cross section wells drilling will provide noncircular cross-sectional wells bursting effect. As it has been previously mentioned – in some cases it might cause the decrease of explosive ratio up to 25 %. Moreover, drilling volumes might be reduced as well.
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