Dynamic factors of formation of mining development strategy in the conditions of incomplete certainty of the raw material market

The article deals with the issue of planning the development of mining operations in the context of a changing demand for iron ore products. One of the problems regarding iron ore deposit mining at the current stage of open mining development presented in this work. It includes the following: mines operate with a constant operational capacity while iron ore products price and demand significant variation is observed in mineral markets. In these conditions, there is a possibility of a significant increase in economic efficiency of mining due to the implementation of a flexible change in the main parameters of the open-pit, which is achieved by a justified correction of the development of mining operations by changing the dimensions, parameters and design of the active part of the working area in an open-pit. A technique for planning the development of mining operations in an open-pit has been developed, which differs from the known ones by the possibility of regulating the main parameters of the open-pit under conditions of a changing demand for iron ore products, and also takes into account the interrelationship of mining method parameters ensuring a basic ore stock ready for excavation in the open-pit.


Introduction
Modern approaches to planning the development of mining operations are based on the assumption that the parameters of the external and internal environment will be relatively constant and, therefore, do not provide for changes in the main parameters of the quarry in the long term [1][2][3][4].Therefore, when designing, engineers always tried to adopt a stable calendar schedule for a fairly long development period, basing it on investments that would help solve long-term problems in the development of the work area [5][6][7][8].
However, the practice of open-pits shows that during periods of crisis there is a sharp drop in the volume of stripping work [9,10].This reduction reflects the intention of enterprises to limit the increase in production costs by forming temporarily non-working open-pit faces, as well as working faces with increased angles of inclination.This is largely due to the inability of mining enterprises to provide a flexible response to changes in internal and external demand for products without a significant increase in development costs, since deposit development projects do not provide for changes in the main parameters of the open-pit over long periods.Under these conditions, the actual development schedule cannot remain stable for a long period of time and differs sharply from the design one, which additionally leads to a delay in stripping work, untimely transfer of opening 2 workings and, in general, to a general deterioration in mining and technical development conditions.Therefore, ensuring operational regulation of the main parameters of the open-pit becomes the most relevant for mining enterprises.
The main parameters of the open-pit must be adjusted in accordance with changing external conditions throughout the entire period of its operation [11,12].It is possible to significantly increase the economic efficiency of development by implementing flexible changes in the main parameters of the quarry when the demand for extracted products changes [13].
The calendar distribution of overburden and ore volumes significantly depends on the adopted mining development option.From a design point of view, it is important to know not only the possible value of the open-pit's productivity in terms of ore or rock mass, but also how the development of the open-pit space should be planned [14,15].Therefore, timely regulation of the main parameters of mining operations in accordance with changes in demand for iron ore products can be carried out by managing the development of mining operations [16,17].
Management of the mining development is understood as a set of sequential operations to assess the state of mining operations, change the size, parameters and design of the active part of the working area of the open-pit, determine the direction of development of mining operations and control specified technological parameters in order to maintain the required volume of production and create conditions for safe and economical open-pit work.
Therefore, choosing a rational procedure for mining deposit reserves is one of the main and complex tasks in open-pit design [18,19].This task becomes even more complicated when the demand for iron ore products and, accordingly, the ore productivity of the open-pit changes [20].
In this regard, the goal was set: to develop a methodology for planning the mining development, which will allow for flexible regulation of the main parameters of the open-pit in conditions of changing demand for iron ore raw materials.

Methods
Working in a market environment makes mining enterprises dependent on the global market conditions for mineral raw materials, and also requires a new, more flexible approach to the development of deposits.In this regard, issues of ensuring competitiveness, which depends on the adopted mining development strategy, have become of paramount importance.In these conditions, the strategy must first of all assume the possibility of obtaining benefits from changes.Special requirements are imposed on decisions that determine the volume and quality of mined ore, the costs and profits of a mining enterprise, which directly depend on the main parameters of open-pits, as well as the direction of development of mining operations [21].
Many scientists have dealt with the issue of planning mining operations and designing the main parameters [22][23][24][25], but the current scientific and methodological base does not allow for a flexible response to fluctuations in the demand for extracted raw materials.
In this regard, a methodology was developed for planning the development of mining operations in an open-pit when the demand for iron ore products changes.The methodology for planning the development of mining operations is based on the idea of concentrating mining operations on individual sections of the open-pit field (the active part of the working zone), the length of which is determined by the optimal values of the width of the working site and the length of the active front of mining operations, ensuring the standard volume of reserves ready for extraction for a given open-pit productivity by ore.It is proposed to regulate the productivity of the open-pit by changing the face angle of the working area of the open-pit in the areas of mining operations concentration, as well as the extent and number of these sections.At the same time, due to the change in the order of working out the mining areas where works are concentrated, it is possible to provide the open-pit works with a minimum current stripping ratio.

Results and discussion
Planning the development of mining operations must be carried out taking into account the following requirements: -the size of the working area of the open-pit in any period of its operation must be minimal, while ensuring the implementation of ore production program; -when determining the optimal parameters for the mining system, it is necessary to take into account the influence of the concentration of mining on the cost of marketable mineral products; -when determining the productivity for the ore, it is necessary to take into account the relationship between the width of the work site and the length of the active front of mining operations, ensuring the specified ore reserve in the open-pit ready for excavation; -when increasing the ore mining productivity in the open-pit, it is necessary to take into account the change in the regime of mining operations due to the increase in the width of the work benches; -at any planning stage, it is necessary to be able to achieve the highest possible ore mining productivity with minimum stripping ratio.
The application of the proposed methodology was considered using the example of a typical openpit developing a steeply dipping deposit (figure 1).

Figure 1. Consolidated plan of mining operations upon reaching the open-pit bottom at -30 m
The general sequence of calculations assumes the following order: 1.For a given design productivity of an open-pit for ore, using a nomogram, we determine the optimal values of the width of the working benches and the length of the active front of mining In the case of allocating areas with concentration of mining in the open-pit, when determining the parameters for the mining system, it is necessary to take into account not only a reduction in the length of the benches involved in working out, but also a reduction in the maximum possible length of the active front of mining operations with an increase in the width of the work site.
Number of sites involved in the work: Da -is the active area of the working zone, %.The total number of sites of concentration of mining operations: Ore productivity of the site: where Apthe productivity of the open-pit ore, t/year.2. We divide the open-pit field into equal areas of concentration of mining operations, limited by vertical planes (figure 1).
3. For each section, with deepening of mining operations, in with the accepted direction of deepening the bottom of the open-pit (the direction of deepening the bottom of the open-pit must be determined with slope angles of the working faces, which correspond to work with the maximum possible productivity of the open-pit for ore), we determine the maximum possible productivity and stripping ratios, taking into account their relationships.We build a graph of changes in current stripping ratios as mining operations decrease in each section of the deposit (figure 3).According to the calendar plan for the development of mining operations, we compile a summary table of the increasing volumes of overburden and ore excavation as the open-pit deepens.In accordance with the table data, we construct graphs V=f(P).We average the graphs V=f(P) and determine the operational stripping ratios for the periods of open-pit operation.-with uneven development of the open-pit space (curve II), when the development of mining operations is carried out according to the developed methodology.
As can be seen from figure 5, the curve reflecting the planning of the development of mining operations according to the proposed methodology (curve II) provides a better mode of mining operations than with their uniform development.-the open-pit operation with minimum current stripping ratios at the maximum degree of mining operations concentration; -mining with a specified ore productivity.

Figure. 2 .
Figure. 2. Change in the length of the front of mining operations, depending on the increase in the specified width of the work site for different ore productivity options.

Figure. 3 .
Figure. 3. The graph of the change in the current stripping ratios and the value of horizontal advance of the mining front at various sections of the working zone at pit deepening 4. Determine the rational sequence of mining the quarry field.The deepening of mining operations in different areas of the deposit is accompanied by different stripping ratios.Obviously, in order to operate the entire open-pit with the minimum current stripping ratio, it is necessary to begin lowering the mines in the area with the minimum stripping ratio, and until the stripping ratio is comparable to the closest stripping ratio of any other area.The main condition for the involvement of any section of the open-pit in the development is the equality of the current stripping ratio of this section (nj) and the current stripping ratio in the whole open-pit (nк): nк = nI.

Figure 4 .
Figure 4. Nomogram for determining the duration of mining of open-pit sections at different intensity of mining operations Using a nomogram, you can determine and draw the position of mining operations in an open-pit for any calculation year with the sequence of mining of various sections of the deposit, ensuring the operation of the entire open-pit with minimal current stripping ratios for the entire or long period of time.Using a nomogram, we build a linear calendar plan for the development of mining operations at each section of the open-pit.According to the calendar plan for the development of mining operations, we compile a summary table of the increasing volumes of overburden and ore excavation as the open-pit deepens.In accordance with the table data, we construct graphs V=f(P).We average the graphs V=f(P) and determine the operational stripping ratios for the periods of open-pit operation.Figure 5 illustrates a comparison of the mining mode for the open-pit deepening option: -with uniform development of the working area of the open-pit (curve I), when the deepening of the open-pit is carried out in all areas simultaneously.-withuneven development of the open-pit space (curve II), when the development of mining operations is carried out according to the developed methodology.As can be seen from figure5, the curve reflecting the planning of the development of mining operations according to the proposed methodology (curve II) provides a better mode of mining operations than with their uniform development.

Figure 5
illustrates a comparison of the mining mode for the open-pit deepening option: -with uniform development of the working area of the open-pit (curve I), when the deepening of the open-pit is carried out in all areas simultaneously.

7 Figure 5 .
Figure 5. Change in current stripping ratios with mining operations deepening 4. Conclusions A technique for planning the development of mining operations in an open-pit has been developed, which differs from the known ones by the possibility of regulating the main parameters of the openpit under conditions of a changing demand for iron ore products, and also takes into account the interrelationship of mining method parameters ensuring a basic ore stock ready for excavation in the open-pit.The developed methodology for planning the mining operations development in an open-pit provides:-the open-pit operation with minimum current stripping ratios at the maximum degree of mining operations concentration;-mining with a specified ore productivity.