Secondary crushing

(Figure # 1) The hydraulic-explosive method for the crushing

SECONDARY CRUSHING (EN: secondary crushing; DE: Blockbehandlung; FR: broyage secondaire, concassage secondaire; ES: trituration secundaria; RU: вторичное дробление) is the destruction of the oversized pieces within the mining mass during the construction, and during the opencast or underground development of the deposits. The secondary crushing is performed: at the open-pit mines - within the excavator stope, or at the transloading place; at the underground mines - immediately within the clearing stope, and within the mine workings of the screening, scraping, or loading horizons. In case of the underground development, the secondary crushing is usually divided into the two stages: the crushing of the large pieces, and the liquidation of the hangings of the ore within the issuing raises (drawholes); the destruction of the oversized pieces, which have passed through the issuing raises. In this case, the expenditures for the secondary crushing reach up to 20-30% (the breaking using the blastholes), and up to 50-100% (using the boreholes).

According to the type of the energy, which is delivered to the object for the destruction, they distinguish the following methods for the secondary crushing: blasting, mechanical, electrical, thermal, hydraulic, acoustic, optical, radiative, chemical, and combined.

The blasting methods (the most widespread) are based on the methods of the blasthole and external charges. In case of the secondary crushing using the first method, the diameter of the blastholes is usually 36-42 millimetres. The specific expenditure of the explosive substances is 0.1-0.3 kilograms per cubic metre (at the underground ore mines, up to 0.4-0.8 kilograms per cubic metre). There assists to the improvement of the effectiveness of the secondary crushing using this method the filling of the blastholes with the water, namely, the hydraulic explosion (see the Figure # 1).

In this case, the size of the charge is chosen according to the calculation of 10-50 grams per 1 cubic metre of the volume of the oversized piece. The energy of the explosion transitions into the shock wave with the small losses. The throwing action of the explosion is weakly expressed. In case of the usage of the external charges, the specific expenditure of the explosive substances increases, as a rule, up to 1.5-3 kilograms per cubic metre, and may be decreased to 0.4-0.6 kilograms per cubic metre with the usage of the shaped charges (see the Figure # 2).

 (Figure # 2) The shaped charge of the ZKP type

The usage of the polyethylene packets with the liquid as the tamping for the external charge (see the Figure # 3) allows to increase the efficiency of the explosion on the account of the participation of the reflected shock waves within the process of the crushing.

In this case, the productivity of the labour for the fragmentation of the oversized pieces increases by 2 times in comparison with the method of the blasthole charges. The crushing of the large pieces, and the liquidation of the hangings of the ore within the issuing mine workings, are performed using the pushing charges with the mass of usually 2-10 kilograms. They also use the throwing systems with the remote control, which are delivering the charges of the explosive substances to the hanging ore; within the USSR, there have been created the DRS-130, DRS-160, and DRS-200 grenade launchers.

The major advantages of the blasting methods for the secondary crushing are the universality, and the destruction of the pieces with almost any size. The major disadvantages are the relatively high specific expenditures of the energy (up to 15•10^6 joules per cubic metre), the cost (up to 0.8-1.2 rubles per cubic metre), especially in case of the crushing of the pieces with the sizes of less than 0.8-1 metres, and the specific peculiarities of the explosive technology, which are disrupting the rhythmicity of the production.

(Figure # 3) The external charge with the hydraulic screen(Figure # 4) The electrohydraulic crushingAmong the mechanical methods for the secondary crushing, they distinguish the destruction of the pieces of the mining mass using the blow, and the gravitational destruction. The first method is realized primarily within the crushers, which are installed stationarily or on the self-propelled crushing aggregates (the most prospective type of the crushers is the rotary crushers). For the destruction of the large single rock blocks, there is expedient the usage of the hammers of different designs (pneumatic, hydraulic, and so on). They also perform the destruction, using the delivery of the compressed air into the blastholes, which have been drilled within the oversized piece. The performance of the pneumatic hammers for the rocks of the medium strength is 30-40 cubic metres per hour, the cost of the process of the destruction is 0.07-0.1 rubles per tonne. The usage of the hydraulic hammers allows to increase the performance of the process of the destruction up to 80-100 cubic metres per hour. The gravitational method for the secondary crushing is realized using the falling load (the destructors of the stone, of the excavator type and crane type), or because of the action of the own weight of the falling oversized piece. This method is distinguished with the low specific energy consumption of the process of the destruction (up to 0.4•10^6 joules per cubic metre); this method is effective enough in case of the rocks of the medium strength. For the destruction of the oversized pieces, they also use the hydraulic wedges. The advantages of the mechanical methods for the secondary crushing are the safety, the low energy consumption, the simplicity of the delivery of the energy to the object of the impact, the possibility of the automation of the process of the crushing.

The thermal method for the secondary crushing is based on the uneven expansion of the bodies during the concentrated heating. The destruction of the oversized pieces is performed, using the manual thermal borers with the fire-jet burners of the rocket type and thermites. For the oxidation of the liquid fuel within the manual thermal borers, there are used the oxygen or air. The usage of the thermal borers is limited; the process of the destruction is distinguished with the relatively high energy consumption (up to 7•10^6 joules per cubic metre). The performance of the manual thermal borer with the power of the burner of 100 kilowatts (at the open-pit mines within the Krivoy Rog basin) is 10-15 cubic metres per hour. The secondary crushing using the thermite is based on the impact onto the oversized piece, using the heat, which is obtained during the burning of the thermite composition. The process of the destruction proceeds rapidly, does not produce the dispersion of the pieces and the forming of the harmful gases (except the crushing of the oversized pieces of the sulfur-containing ores), however, it requires the additional mechanical impact for the complete destruction of the oversized pieces. The effectiveness of the secondary crushing using the thermite increases with the increasing of the content of the quartz within the rocks.

In case of the electric method (with the contact, or without the contact), there serves most often as the basis of the process for the destruction of the rocks the thermal effect, and less often the electrohydraulic effect. The essence of the latter effect is the discharge of the battery of the capacitors (the voltage is up to 100 kilovolts) into the aqueous interval (see the Figure # 4); the rock is being destructed under the action of the cavitation and shock waves of the explosive character.

(Figure # 5) The low-frequency electrothermal destruction

The electric contact method, which is used within the open-pit mines, is realized with the help of the installations (of the 2URN type), the major assembly unit of which is the single-phase transformer with the power of 100 kilowatts (the electric current of the industrial frequency). The oversized piece, which has been placed between the two electrodes (see the Figure # 5), is destructed as the result of the thermal breakdown, heating, and expansion of the current-conductive channel within the rocks.

The method is distinguished with the simplicity, and with the high safety. The average energy consumption of the destruction is 20•10^6 joules per cubic metre. The performance during the destruction of the quartzites with the help of the 2URN installations (the "Novokrivorozhsky" mining-beneficiation combined enterprise) is 14 cubic metres per hour. The cost (the open-pit mines within the Krivoy Rog basin) is 0.3-0.5 rubles per cubic metre. For the destruction of the semiconducting rocks (the ferruginous quartzites, and others), there is more effective the high-frequency contact method (of the thermal breakdown). There proceeds between the electrodes the high-frequency breakdown (see the Figure # 6), and there emerge within the rocks the thermoelastic stresses, which are leading to the destruction of the oversized piece.

For the destruction of the rocks-dielectrics (granite, basalts, and others), there is used the method of the uneven dielectric heating (the high-frequency contact method). The electric contactless method, that is the weakening of the rocks using the electromagnetic field of the capacitor or solenoid, is not used widely because of the low performance.

 (Figure # 6) The low-frequency electrothermal destruction

There are being advanced the processes of the secondary crushing, which are based on the hydraulic methods for the destruction (see the "Hydraulic destruction of the rocks" article), and on their combination with other methods. The acoustic methods for the secondary crushing are based on the destruction of the rocks using the oscillations with various frequencies, including the ultrasonic range of the frequency spectrum. The usage of these methods is effective mainly in combination with the mechanical methods for the secondary crushing. The processes of the secondary crushing, which are based on the chemical methods for the destruction, are distinguished with the low performance; the area of their usage is limited. There has emerged the tendency of the advancement of the combined methods (thermo-mechanical, acoustic, mechanical).