Justification for mining overburden without the use of conveyor transport at the Pivnichnyi open pit of the Pokrovskyi mining and processing enterprise

The current state of mining operations in the open pits of the Pokrovsky mining and processing enterprise has been analyzed, where the shortcomings of existing technological schemes have been identified. Technical and organizational criteria have been identified that must be met by technological schemes for overburden and mining operations for the open-pits of the Pokrovsky mining and processing enterprise, where conveyor transport is used. Five technological schemes have been proposed for the Pivnichnyi open-pit, which provide for the reallocation of the height of overburden benches and the movement of overburden only to internal dumps without the use of conveyors. The dependences of the height of the internal dump and its resulting angle of slope on the re-excavation factor for the proposed technological schemes were obtained. The indicators have been calculated at which the most operating efficiency of mining and transport equipment for the Pivnichnyi open-pit of the Pokrovsky mining and processing enterprise (MPE) will be achieved and complete extraction of overburden rocks with cross movement to the internal dump without the use of conveyor transport will be ensured, and the production capacity of the quarry is also given according to the demand for ore.


1.Introduction
The search for solutions aimed at improving the technological schemes for developing open-pits at the Pokrovsky mining and processing enterprise (MPE) is one of the directions for increasing the competitiveness of its products, especially during the period of war and after martial law in Ukraine.
The demand for manganese ore is determined by production volumes in metallurgy.Crisis phenomena in the country's economy in recent years have caused a significant decrease in industrial production in Ukraine, exports of ore raw materials, engineering products, and metallurgy.Under such economic conditions, it is not practical to extract manganese ore in the same volumes.The dynamics [1] of manganese ore production are presented in Figure 1.
Analysis of the above indicators, as well as the performance indicators of the Pokrovsky MPE (Figure 2) shows a decrease in the productivity of metallurgical and mining enterprises over the past 15-20 years and their achievement of capacities of 30-50% of their design [2].
A decrease in manganese ore production due to a semi-fixed part of costs leads to an increase in production costs, which reduces its competitiveness.Therefore, the purpose of this study is to search for new organizational and technological solutions aimed at reducing production costs in the extraction of manganese ore.

2.Methods
Achieving efficient operation of mining enterprises can be achieved in different ways, but most of them are very dependent on the technology of mining operations in open-pits, the choice of mining and transport equipment, technical indicators, etc.
Work [3] considers the issues of transportation of overburden and backfilling productivity considering different depths and stripping ratios during open-pit mining at great depths.
Article [4] describes that bringing the stripping ratio to the balance and stability of production leads to economic benefits for the mining enterprise.And in works [5][6], the stripping ratio, mineral extraction, overburden transportation distance and lifting height were compared and analyzed considering technical and economic indicators.At the Pivnichnyi and Chkalovskyi open-pit №2 of the Pokrovskyi MPE, technological schemes for the development of overburden rocks in three benches according to transport, transport-dump and stripping schemes are used [7].The thickness of the overburden rocks in these open-pits is about 70 m.The overburden bench with a height of 17 ± 2 m is mined with two ESh-15/90 draglines.The same draglines are used to extract manganese ore, temporarily store it on a dump, and then load it into dump trucks using a single-bucket straight shovel type excavator and deliver it to the surface.After ore is extracted in isolated areas along the work front, the mined-out space is filled with waste rock.
The overlying bench with a height of 27 ± 3 m is developed with a bucket wheel complex consisting of a bucket wheel excavator SRs-2400, a loader BRs-2250/60 and a spreader A2Rs-8800/150.The width of the overburden stope is 70 m, the angle of repose of the bench is 40 0 .
The dumper forms the second tier of the dump with a height along the ridge of 43 ± 1 m with a resulting angle of slope of 15 0 .
The forward bench with a height of 25 ± 5 m is developed with a bucket wheel complex consisting of a bucket wheel excavator SRs-2400, a system of face, end and dump conveyors and a spreader A2Rs -8800/110.The width of the overburden stope is 70 m, and the angle of slope of the bench is 40 0 .
The dumper forms the third tier of the dump with a height of about 70 m with a resulting angle of repose of 14 0 .The average annual productivity of the bucket wheel complex is 10 million m 3 .
The estimated annual volumes of ore production with a bench height of the bucket wheel complex equal to 27, 30 and 24 m, respectively, will be 1346.9;1210.9;1514.9 thousand m 3 with an average value of 1375.6 thousand m 3 .
The entire overburden in the Pivnichnyi open-pit is mined by two SRs-2400 bucket wheel complexes (the average productivity of each of them is 10 million m 3 per year) and one dragline ESh-15/90 with an annual productivity of 3.8 million m 3 per year.
One ESh-15/90 excavator is used for re-excavation of overburden rocks.Taking into account the given performance indicators of mining transport equipment, the re-excavation factor is Kr =0.16.
Based on the current situation at open pits of the Pokrovskyi MPE and the loss of their production capacity due to a decrease in demand for raw materials, the following shortcomings have been identified: 1.The ore is mined using an overburden dragline with its temporary storage on a dump, which does not ensure high-quality and complete extraction of manganese ore from the rock mass.2. The use of conveyor transport increases the distance of transportation of overburden to the dump, and, consequently, operating costs.3. The total productivity of stripping equipment is significantly greater than that required to open ore reserves in volumes corresponding to its possible sale.Taking into account the above, it is necessary to improve the technological scheme for the development of the Pivnichnyi open-pit of the Pokrovskyi MPE, which must meet the following requirements.
1. Mining operations at the open-pit are carried out on a separate bench using a straight shovel type excavator, followed by subsequent transportation of ore to the side of the open-pit by dump trucks.2. The scheme should ensure the cross movement of overburden into the dump (i.e.without the use of conveyor transport).3.For stripping operations, equipment is used that is available at the Pokrovskyi MPE. 4. The scheme should ensure the production of manganese ore in volumes corresponding to the possible volume of its sales.The technological schemes of the Shevchenkivskyi open-pit and the Chkalovskyi open-pit №2 of the Pokrovsky MPE [8] have similar disadvantages to the scheme at the Pivnichnyi open-pit: 1.The need to purchase an expensive conveyor belt for the upper ledge.
2. The high cost of transporting overburden by belt conveyors.
3. Increased dilution of ore due to the difficulty of removing it from under the overburden and storing it in a pre-dump.4. A large amount of used equipment, which is idle when the demand for ore decreases.

3.Results and discussion
This state of development of open pits at the Pokrovskyi mining and processing enterprise requires the development of new technological schemes.For the conditions of the Pivnichnyi open-pit, several variants of technological schemes were considered, which exclude the use of a bucket wheel complex at the leading edge using a system of belt conveyors.At the same time, it is necessary to form an additional capacity in the internal dump to accommodate the increased volume of overburden.A special feature of the first proposed scheme is that the leading bench with a height of 42 m is mined with a bucket wheel complex consisting of a bucket wheel excavator, a loader and spreader.At the same time, using the ESр-5/45 dragline, the height of the forward ledge is reduced to 40 m and a trench is formed along the final contour of the stope.This ensures that a 42-meter-high bench can be mined with a bucket wheel excavator.At the dump, an ESр-15/90 excavator is used to re-excavate the rocks and create an additional capacity for dumping rocks with a cantilever dumper.
In scheme 2, a forward bench 60 m high is mined by two sub-benches, each of which is 30 m high.The rocks of the upper sub-bench are loaded onto the working platform of the lower sub-bench and then excavated into the internal dump.At the dump, two ESh-15/90 excavators re-excavate the rocks and create an additional capacity to accommodate overburden, which is transferred to the dump by the dumper from the lower bench.
In scheme 3, the leading bench is worked off with two sub-benches.The upper sub-bench, which has 20 m high, is mined with an ESh-15/90 excavator and the overburden is transferred to the working platform of the lower sub-bench.The lower sub-bench, which has 40 m high, is mined by a bucket wheel excavator, which transfers the rocks of the upper and lower sub-bench into the internal dump using a loader and a spreader.The rocks dumped from the lower sub-bench into the internal dump by the spreader are re-excavated by a second bucket wheel excavator, which is installed on the 2nd tier of the dump and, with the help of the spreader, places them in the upper tier of the dump.
In Schemes 4 and 5, the development of the forward bench is carried out similarly to Scheme 3. In Scheme 4, on the dump using an ESh-20/90 excavator, the rocks are re-excavated and an additional capacity is created to accommodate overburden, which is reloaded into the dump by a spreader.And in scheme 5, on the upper bench of the dump, instead of the ESh-20/90 excavator, the ESh-15/90 and ESh-5/45 excavators are installed.
For the Pivnichnyi open-pit of the Pokrovskyi MPE, five technological schemes for developing overburden with two benches using non-transport and transport-dump systems are proposed.These schemes are assessed based on stripping ratios, the cost of mining of one ton of ore, and capital costs for the purchase of stripping excavators [9][10][11].A comparison was made of the proposed schemes with the scheme actually used in the open-pit for the development of overburden with three benches.Capital costs were compared to each other both in terms of the residual cost of the equipment and the replacement cost if the bucket wheel excavators and draglines were purchased at the present time.To calculate the cost of ore mining, the actual costs of stripping operations for the equipment used were taken.exclude the development transport system (conveyor transport) on the upper bench [12][13][14][15].Such dependencies show a strict correlation between the angle of repose of the blade and the re-excavation factor.However, the interdependence of the height of the internal dump on the re-excavation factor is quite low (R²=0.343),which indicates the influence of other factors [16][17][18][19], such as the height of the upper bench, the excavation equipment used, and the bench development system.which is inappropriate as practice has shown in the conditions of the Pivnichnyi open-pit, since the specific pressure of the walking stroke on the ground exceeds the permissible limit.Therefore, scheme 5 is rational for the conditions of the Pivnichnyi open-pit (Figure 4).The required equipment productivity is calculated by comparing the cross-sectional areas of the working equipment and the cross-sectional area of the workings of the SRs-2400 bucket wheel excavator.The re-excavation factor is defined as the ratio of the sum of the areas of the stopes according to re-excavation Sr to the sum of the cross-sections of the areas of the stopes along the pillar Smass in a loosened state according to the formula: where n and m are the number of excavation stops for re-excavation of rocks and for the pillar, respectively.Thus, the re-excavation factor is equal to: • on the over-ore bench -Kr =1.13 • on the upper bench -Kr =0.83 • overall according to the scheme -Kr =0.92 The recommended scheme ensures mining of the excavated material in the Pivnichnyi open-pit with transverse movement of the rock to the dump and eliminates the use of conveyor transport.The scheme ensures that mining operations are carried out using a separate bench using a straight shovel type excavator, loading ore into dump trucks and transporting it on board of the open pit.The annual volume of ore production according to the proposed scheme is Q=682 tons, which will amount to 50% of its design production capacity and corresponds to the current state of mining volumes at the Pivnichnyi open-pit.

4.Conclusions
The technical and organizational state in the open pits of the Pokrovsky mining and processing enterprise is analyzed, which shows the discrepancy between the current productivity of mining and transport equipment relative to the production capacity of the open-pits.
Using the example of the Pivnichnyi open-pit, research was carried out to change the technological scheme for extracting and moving overburden rocks to dumps without the use of conveyor transport.
Five technological schemes have been proposed that provide for the movement of overburden only to internal dumps.It is proposed to redistribute the height of the mined overburden benches with transverse movement of the overburden and the creation of additional receiving tanks to accommodate an increased volume of overburden rocks in the internal dump.
The dependences of the height of the internal dump and its resulting angle of repose on the reexcavation factor for the proposed five development schemes of the Pivnichnyi open-pit were obtained, which show a strict relationship (R²=0.8939) between the resulting angle of repose of the dump and the re-excavation factor.At the same time, the dependence of the height of the internal dump on the reexcavation factor is R²=0.343, which indicates the influence of other factors, such as the height of the upper bench, the excavation equipment used, the ledge development system, etc.
Based on the results of the study, the recommended scheme for the development of the Pivnichnyi open-pit (Scheme 5) ensures complete mining of overburden rocks and their movement to an internal dump without the use of conveyor transport, as well as the necessary productivity for ore mining (Q=682 thousand tons) at the level of modern demand for the open-pits of Pokrovskyi MPE.

5 .
Rigid interconnection of equipment complexes with each other.6.The presence of complex work on moving face and dump conveyors.

Figure 3 .
Figure 3. Graphs of the dependence of the height of the internal dump (a) and the resulting slope angle of the dump (b) on the re-excavation factor.The graphs (figure3) show the dependences of the height of the internal dump and its resulting angle on the re-excavation factor for the proposed development schemes for the Pivnichnyi open-pit, which

Figure 4 .
Figure 4. Recommended technological scheme for the Pivnichnyi open-pit According to the criteria of «re-excavation factor» and « power of overburden produced», schemes 4 and 5 are acceptable.However, scheme 4 provides for work on the dump of an ESh-20/90 excavator,

Table 1 .
Characteristics of technological schemes of Pivnichnyi open pit.