Investigation and optimization of main materials consumption when mining iron ores at deep levels of the Underground Mine Group of the PJSC “ArcelorMittal Kryvyi Rih”

. The work considers conditions of deep levels of the Underground Mine Group for underground ore mining (as underground mines) of the Mining Department of the PJSC “ArcelorMittal Kryvyi Rih” (the PJSC “ArcelorMittal Kryvyi Rih”). The research aims to improve indicators of mined ore mass extraction when mining rich iron ores through studying and optimizing consumption of explosives, enhancing mining technology to provide fulfilment of the underground iron ore mining program. During the research, there are analyzed mining geological and technical conditions of the deposit mining as well as current technologies of iron ore mining at the Underground Mine Group of the PJSC “ArcelorMittal Kryvyi Rih”. The work analyzes the achieved indices and consumption of explosives for drilling and blasting at the Underground Mine Group. The mining geological and technical conditions of the deposit mining as well as current technologies of mining, parameters of preparatory operations, the nomenclature and qualitative characteristics of many types of explosives are determined to have changed. This complicates planning consumption of explosives and making their estimates for work sites. However, this is a reason for selecting highly efficient technology and machinery in deteriorating mining and geological conditions of operating at over 1200 m depths. The work determines dependencies of a stress value on a mining depth and physical properties of rocks, as well as parameters of drilling and blasting operations considering the stress-strain state of the massif under high rock pressure at deep levels of the Mining Group of the PJSC “ArcelorMittal Kryvyi Rih”.


Introduction
Rich iron ores (with the useful component content 58 -65% in the massif) of the Underground Mine Group (as underground mines) of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih" (the PJSC "ArcelorMittal Kryvyi Rih") are mined applying underground methods at the depths of over 1100 m [1 -3]. Deepening of mining operations causes deterioration of mining-geological conditions of the iron ore deposit. Decrease of the deposit thickness and iron content in ores is one of the major deterioration indicators. Iron content decrease results in loss of iron ore consumers as current leading iron ore producers offer ores with iron content of not less than 62% [4 -6].
Deterioration of mining-geological conditions caused by increased depths of mining operations leads to greater expenses on mining useful minerals [7 -9, 13, 26, 27], increased costs of opening new levels [10 -12], stricter requirements to applied underground mining systems [12 -16] and improvement of main and auxiliary underground mining operations [24, 29, 31 -37] that impact efficiency and safety of the applied technologies.
The first step is improvement of ore breaking in stopes [6,10,20]. The basic requirement is consideration of the stress-strain state (SSS) of the massif [2,8,11,12]. Deepening of mining operations results in increased rock pressure [26,28,30]. Fulfilment of these requirements leads to additional costs for efficient drilling and blasting operations (DBO) [23,28].
When drawing and transporting ore, it is necessary to consider the most efficient flowsheets of extracting ore from the massif which are used or may be used in underground operations [1,5,13]. Deepening requires additional expenses to maintain safe conditions of draw workings as well as for transporting ore to the shaft and winding [3,7].
Increased depths of mining require greater attention to controlling the state of the massif as disturbed stability of preparatory and development workings and stopes need additional expenses for maintaining their safe condition [4,8,19].
Costs for application of various concentrating methods in underground or on-surface conditions rise as well [38 -42]. Some authors suggest enhancing the mined ore concentrating processes through their automation [43 -47, 49 -52]. For instance, at Kiruna mine, ore is sorted underground [51 -53].
The technology like that enables obtaining the final product of the appropriate quality of the useful component without concentration, clean ore extraction reaching 90 -92% [54,55]. Application of similar technologies at Kryvbas will lead to 2 -5-fold increase of operating costs.
Thus, investigation and optimization of main materials consumption when mining iron ores at deep levels of the Underground Mine Group (as underground mines) of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih" (the PJSC "ArcelorMittal Kryvyi Rih") are proved to be necessary.

Methods
To achieve the set objective, the authors apply the complex method of solving the following tasks: -Analysis of mining geological and technical conditions of mining the deposit as well as the state of mining operations at deep levels of the Underground Mine Group (as underground mines) of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih"; -Analysis of the technology of mining iron ores applied at the Underground Mine Group of the PJSC "ArcelorMittal Kryvyi Rih"; -Basic statements on investigation and optimization of main materials consumption for drilling and blasting operations when mining iron ores at deep levels of the Underground Mine Group; -Investigation and optimization of main materials consumption for drilling; -Research and optimization of main materials consumption for blasting; -Calculation of DBO parameters considering the SSS of the massif at deep levels of at the Underground Mine Group of the PJSC "ArcelorMittal Kryvyi Rih".

Results and discussion
The minefield of the deposit is located in the southern part of Saksahanska band which is the eastern Kryvorizka syncline limb.
The deposit contains six schist and six ferruginous levels.
The fifth horizon is an economic one and occurs directly on the rocks of the 5 th schist level excluding the anticlinal part where the 5 th ferruginous level rests on the 4 th ferruginous and schist level rocks due to several complicated tectonic shifts.
This level is the richest in ore and its thickness is 45 -120 m. The lithographic composition of the level is characterized by dominance of martite hornfels and jaspilites over hydrohematite-martite hornfels and jaspilites occurring in the form of small benches in the hanging and footwalls of the thickness and above iron-mica-martite hornfels and jaspilites.
The largest deposits of martite and hydrohematite-martite ores are assigned to the 5 th ferruginous level. The ore body at the main levels is opened by haulage crosscuts. Preparation of a haulage level is fulfilled by the ort-ring pattern where field haulage drifts in the footwall and access crosscuts from the western drift are driven.
An interval between levels is 90 m and a level consists of three sublevels. Deposit opening at intermediate levels is performed by crosscuts from auxiliary sub-verticals of shafts to fringedrifts and then by orts driven from these drifts. At the Underground Mine Group, level block preparation of the ore body is applied. For this, the ore body within the level is divided on strike into 100 m long blocks as wide as the horizontal thickness of the ore body. The block is divided vertically into three sublevels of 30 m in height each. A sublevel consists of extraction panels mined onto two scraping orts with the following transportation of the ore along the storage drift to the orepass raise where the ore is passed to the main haulage level for loading into electromotive cars.
Due to low hardness of ore and country rock, a relatively low value of mined iron ores at the Underground Mine Group, the sublevel caving system is only used for mining. In addition, the system of drilling the ore massif with a bunch of deep raising holes from drilling cutouts of the level onto the vertical slot (compensation area) has been applied for a long time.
Figs. 2 and 3 present the mining layout on the combined level of ore drilling, drawing and transporting and vertical projections by slots (vertical compensation rooms) and drilling cutouts respectively. In blocks, caved ore is only transported by scrapers 30LS-2S in scraping orts to the storage drifts and then winches 55LS-2S take the ore to the orepass. The applied technology is characterized by the following technical and economic features: -The required specific amount of preparatory-development workings -3.5 -5.5, m/thou t; -Labor productivity of: a worker at driving -0.6 m/manshift; a deep-drilling machine operator -350 -390 t/manshift; an ore transportation worker -120 -125 t/manshift; -Average monthly productivity of a face -20.0 thou t; -Ore output from 1 m of a hole: when forming a slot -9.5 -10.0, t/m; when bulkcaving -11.5 -12.0, t/m; -Ore losses -17 -19%, ore dilution -16 -18%; -Specific consumption of main materials: explosives for primary ore breaking -0.5 -0.6 kg/t; explosives for secondary fragmentation -0.05 kg/t; steel timbering -0.9 -1.0 kg/t; timber -1.5 -2 m 3 /1000 t; power -1.5 -2.0 kWh/t.
Main advantages of applying the technology consist in its constructive simplicity, low metreage of development workings and high safety of operations due to combination of drilling and transportation on a single level.
Disadvantages include some decrease of stability of haulage workings on sublevels caused by increased massif disturbance of the block foundation (especially at cutouttransportation working junction), and a low degree of mechanization (average 50 -60%) resulted from application of obsolete equipment and almost complete absence of self- propelled machinery. As a result, labor productivity of separate processes and the entire mining system is rather low. To optimize material consumption for mining iron ores at deep levels of the Underground Mine Group of the PJSC "ArcelorMittal Kryvyi Rih", the authors have developed an Enterprise Standard (ES). The document contains consumption standards for main materials for mining operations at the Underground Mine Group for underground ore mining (as underground mines) of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih".
Standardizing materials consumption for mining iron ores at deep levels of the Underground Mine Group includes solving the following tasks: analysis of production conditions of materials consumption and data from leading national and foreign mining enterprises; materials consumption rate setting based on scientifically substantiated standards; implementation of materials consumption rates; control of materials consumption standards in production, planning, material consumption accounting and procurement of production; implementation of technical and organizational arrangements to provide more rational and efficient use of materials; regular revision of materials consumption standards to decrease specific material consumption through implementation of innovative technologies and best practices, enhancement of production processes, etc.
The document is based on regulatory and engineering documents, technical literature and production data of the Underground Mine Group of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih" and other mining enterprises of Kryvbas.
The document offers more accurate dependencies and correction factors for perforator efficiency depending on: compressed air pressure (Table 1); the borehole diameter (Table 2); the borehole depth (Table 3). There are improved regularities of specific consumption of explosives in drifting faces depending on the number of exposures. Thus, specific consumption of explosives in drifting faces with two and three exposure planes is determined with the correction factors 0.85 and 0.65 respectively.
When breaking ore onto the "compressed" environment, specific consumption of explosives is increased by the value of the compression factor Fc. The factor values (depending of the value of the ore layer shift when breaking ΔВ, m) are given in Table 4. Use of the explosion energy is ensured by the greatest degree of the massif resistance to explosive attacks along both the line of least resistance (LLR) and the hole. Excessive increase of the LLR value and decreased spacing of holes result in greater probabilities for the massif separating along the hole than before breaking. This causes shorter blasting effects on rocks and, consequently, additional decrease of breaking intensity. Excessive spacing of holes with corresponding decrease of the LLR value may lead to incomplete massif fragmentation along the hole and a wavy separation surface.
The research shows that the coefficient of column charge interaction which is a relation of optimal LLR values at companion and single blasting varies from 1.0 -1.4 if charge spacing decreases. The dependence of the coefficient of column charge interaction on the distance between charges at companion and single blasting is approximated by the expression: Test blasting shows that increase of charge spacing enhances quality of massif breaking. Critical hole spacing equals the single column charge effects radius at the unlimited plane of the exposure: which is practically proved by drilling and blasting operations at Kryvbas underground mines. However, to decrease substandard yield, closer borehole spacing is sometimes applied as compared with maximum parameters. In this case, the burden-to-spacing ratio will not remain fixed as the optimal distance between holes is not proportionally dependent on the LLR value change.
On replacing W s = W cci /K cci in the expression and taking the optimal value K cci = 1.15, the authors obtain the formula for determining the optimal distance between column charges at their simultaneous firing for any LLR values:  Also, this technique enables considering effects of the stress-strain state of the massif on DBO parameters that may considerably impact ore breaking quality. As for massif breaking, the value of stresses and their distribution are conditioned mainly by the mining depth, sizes and orientation of compensation rooms or exposures onto which ore is broken.
When designing DBO, the stress-strain state of the massif is considered through the coefficient of energy intensity of breaking Kσ. The coefficient equals the ratio of specific consumption of explosives which is determined considering the impact of mining depth and stope sizes q σ to specific consumption of explosives determined considering hardness of ore to break q: The numerical value of the coefficient is calculated by the expressions: -when breaking ore onto the horizontal compensation area: -when breaking ore onto vertical slots or exposures: where B is the width of the stoping area, m; S h , S v are the area of the horizontal or vertical stope respectively, m 2 ; H is the mining depth, m; μ is the Poisson ratio.
Considering the stress-strain state of the massif, blastability indices of specific consumption of explosives for breaking, the LLR value is determined by the formulas: It is advisable to use TNT-free explosives as TNT is dangerous due to its explosibility, and its toxicity causes up to thirty human diseases. Wider application of non-electric blasting supplies is also expedient.

Conclusions
The authors have: analyzed mining geological and technical conditions of the deposit development as well as the current state of mining operations at deep levels of the Underground Mine Group for underground ore mining (as underground mines) of the Mining Department of the PJSC "ArcelorMittal Kryvyi Rih"; revealed major disadvantages of the iron ore mining technology applied at the Underground Mine Group of the PJSC "ArcelorMittal Kryvyi Rih"; designed main directions of investigation and optimization of drilling and blasting costs when mining iron ore at deep levels of the Underground Mine Group; investigated and optimized material consumption for drilling; specified dependencies and suggested correction factors for drilling the rock massif with drilling equipment; determined dependencies of the stress value on the mining depth and physical properties of rocks; determined parameters of DBO considering the stress-strain state of the massif; optimized explosives consumption for mining iron ores under high rock pressure at deep levels of Underground Mine Group of the PJSC "ArcelorMittal Kryvyi Rih".