Pyrite are rather based skarn deposit copper-containing pyrrhotite as a hydrothermal ore, a large thickness variation coefficient, the thinnest 1.0m, most thick 21.37m, small angle, by a subhorizontal Up to 30°, the ore and surrounding rock are stable and difficult to mine. The current mine uses the room-pillar mining method. Generally speaking, the stability of the stope during the room-column mining is mainly determined by the stability of the pillar [1]. The stability of the pillar is very important to the stope. Once the pillar is damaged, the stope will collapse in a large area. In order to achieve safe and efficient mining of the mine, this study uses Ningdu's gently inclined thin to medium-thick pyrite as background, using theoretical calculations. The method analyzes the stability of the mine pillar and optimizes the mining sequence of the stope by numerical simulation.
1 pillar stability sensitivity analysis
1.1 pillar strength
For the strength of the pillar, a lot of experiments and researches have been carried out at home and abroad, and various theoretical calculation formulas [2-4] have been proposed. Among them, the strength formula proposed by Bieniawski is representative and widely used. The specific formula is as follows:

Where σp is the strength of the pillar, MPa; σ0 is the uniaxial compressive strength of the cubic rock sample, MPa; L is the width of the pillar, m; h is the height of the pillar, m; It is a constant whose value depends on the aspect ratio of the pillar. When h/L<5,  =1; when h/L>5, =1.4.

It can be known from formula (1) that the strength of the pillar is related to the aspect ratio. The smaller the aspect ratio is, the smaller the pillar strength is, and the larger the aspect ratio is, the stronger the pillar strength is. When the aspect ratio is greater than 5, the mine The column strength varies more with the aspect ratio.
1.2 Area bearing theory
For the calculation of the load on the pillar, the area bearing theory is the most widely used. The theory holds that the load on the pillar is the overburden gravity [5]. When the mining body is opened by the room column method, the overlying rock in the stope is mainly supported by the pillar. It is assumed that the ore body is elastic and isotropic, and the pillar failure is caused by the vertical stress of the overburden. When the height of the column is not large, the self-weight of the pillar is not considered. The load-bearing area S is the sum of the area of ​​the pillar and the cross-sectional area of ​​the pillar. The calculation formula is as follows:

Where: B———the span of the mine, m;
L———the width of the pillar, m;
b———the spacing of the pillars, m;
n———The length of the pillar, m.
According to the indoor test and the field test, the vertical load of the pillar is also related to the ratio of the length L of the ore body and the thickness H of the overburden on the stope. When L/H>1.5, the load on the pillar is γHS, as their ratio becomes smaller, the overburden load on the pillar becomes smaller. The strike length of Ningdu pyrite ore and the depth of the stope are obviously more than 1.5 times. Therefore, the overburden load of the pillar is γHS, and the stress σu of the pillar is:

1.3 Analysis of factors affecting the stability of pillars
The stability of the pillar is affected by many factors. The main factors are the strength of the pillar itself, the gravity of the overlying strata, and the size of the mine. The strength of the pillar is determined by the uniaxial compressive strength of the pillar, the width of the pillar, the length of the pillar, and the mine. The column height is determined [6-7]. Therefore, eight factors must be considered in the analysis of pillar stability: uniaxial compressive strength, pillar width, pillar length, pillar height, buried depth, overburden bulk density, mine span, and pillar spacing. Taking the ratio of the self-compressive strength of the pillar to the load of the pillar, that is, the safety factor of the pillar, as an index for evaluating the stability of the pillar, and using the orthogonal range theory to analyze the influence of various factors on the stability of the pillar. size.
Safety factor of pillar: K=σp/σu
Considering that the eight factors are simultaneously analyzed by the orthogonal range theory, the test requires more and the process is difficult to operate. Therefore, the pillar is assumed to be square, so the length and width of the pillar are equal, and the spacing between the pillars is assumed to be equal to the span of the mine, simplifying After analyzing only 6 influencing factors, it is simplified:
When the aspect ratio is less than 5,

When the aspect ratio is greater than 5,

According to the actual situation of the mine project, six influencing factors are assigned within the appropriate range, and each factor considers five levels. The specific assignment is shown in Table 1.

According to the orthogonal experimental design, the problem of 5 factors and 6 levels only needs to be tested 25 times. The experimental results are shown in Table 2.

Comparing the range of the difference in Table 2, we can see that the factors affecting the stability of the pillar are: pillar width > buried depth > mine span > rock pillar height > uniaxial compressive strength > overburden rock mass density, of which pillar width The influence of the stability of the pillar is the most obvious, and the influence of the buried depth is second. The mining span and the height of the pillar are equivalent, and other factors have little effect on the stability of the pillar.
The research object of this paper is the stope of the middle section of 130. The depth of the burial is certain. Therefore, only the width of the pillar and the span of the mine are analyzed and calculated. The relationship between the span of the mine and the width of the pillar with respect to the safety factor of the pillar is plotted, as shown in Figure 1. From the figure, we can see:

(1) As the width of the pillar increases, the safety factor of the pillar increases;
(2) As the span of the mine increases, the safety factor of the pillar gradually decreases;
(3) It is known that the safety factor of the mine pillar is not less than 1.5, so the width of the mine should not exceed 17m, and the width of the pillar should not be less than 6m.
2 recovery order optimization
2.1 Calculation model and scheme
In this paper, the model is established by numerical software ANSYS, and the model is imported into FLAC3D for calculation, and the stress and displacement of the pillar are analyzed separately [8-9]. Assuming the surface contour is high, according to the local geological data, the horizontal elevation of the surface is about 100-350 m, and the simplified model of the stope is constructed. Considering the size of the disturbed area of ​​the goaf, the length of the model is 500m along the ore body, and the width of the vertical ore body is 300m, height 300m, of which the excavation ore body thickness is 4m, the inclination angle is 15°, and the buried depth is 150m. The ore blocks are arranged along the ore body and one mine is excavated. Figure 2 shows the overall calculation model established. Figure 3 shows the stope map after excavation of No. 2 mine. According to the above conclusions and the actual site of the mine, the structural parameters of the stope are set to 14m for the mine and 6m for the point. The column spacing is 7m. According to the actual production requirements and equipment requirements of the mine, the ore mining sequence is optimized. Two schemes are set: scheme 1, from the middle to the two wings, in the order of 2→1→3; scheme 2, from the two wings To the center, mining is performed in the order of 1 → 3 → 2. In order to facilitate the analysis and calculation, this paper only performs numerical simulation analysis on one of the segments.
The model established this time includes three kinds of lithology, namely the upper sandstone , the lower limestone and the ore rock. The rock mechanics parameters are obtained according to the indoor mechanical experiments and weakened into the mechanical parameters of the rock mass, as shown in Table 3.

2.2 Analysis of simulation results
2.2.1 From the middle to the two wings
Due to space reasons, this paper only gives the stress, displacement and plastic zone cloud map of the No. 2 mine after excavation, as shown in Figure 4. It can be seen from the figure that after the excavation of No. 2 mine, the original rock stress field is destroyed and the stress is redistributed. The tensile stress appears on the roof of the stope, and stress concentration occurs on the point column. The top and bottom plates are sunken and the upper drum is displaced, and the plastic zone appears on the point column. The maximum tensile stress of the top plate is 1.9875MPa, the maximum compressive stress on the point column is 9.0778MPa, and the displacement of the roof plate is 7.676mm.

Similarly, according to the simulation results, the maximum tensile stresses of the top plate at X=100 and X=81 after excavation of No. 1 mine are 1.9429 MPa and 1.5 MPa, respectively, and the point columns are at X=90 and X=110. The maximum compressive stress is 10.496 MPa and 8.7934 MPa, and the maximum sinking displacement of the top plate is 8.6738 mm. The volume of the plastic zone on the point column is slightly larger than that of step 1. The maximum tensile stress of the roof after excavation of No. 3 mine is 1.9559MPa. Due to the symmetry of X=90 and X=110, only the maximum compressive stress of X=90 is given. The value is 12.352MPa, and the maximum subsidence of the roof. The displacement is 9.8016 mm, and the plastic zone of the point column is significantly larger than before.
According to the stope stress, displacement and plastic zone of each mining step, as the mining room is mined, the stress concentration of the point column occurs, and the compressive stress becomes larger and larger, but the change is not obvious, the tensile stress is generated in the roof plate, and the tensile stress is basically not Change, but the displacement of the roof sinking is getting larger and larger, and the volume of the plastic zone of the point column is gradually increasing. Table 4 shows the stress and displacement values ​​of different excavation steps.

2.2.2 From the two wings to the center

Figure 5 is the maximum principal stress, minimum principal stress, displacement and plastic zone distribution after excavation of No.1 mine. It can be seen from the above figure that the top plate produces tensile stress, the stress is concentrated on the column, and the top plate is pulled. The stress is 1.5MPa, the maximum compressive stress of the point column is 8.5609MPa, the maximum sinking displacement of the top plate is 5.9553mm, and the plastic zone appears in the small area of ​​the point column.

After excavation, the maximum tensile stress of the roof is 1.5MPa, the maximum compressive stress of the point column is 8.6977MPa, the maximum subsidence displacement of the roof is 6.206mm, and the plastic zone area of ​​the point column is slightly larger. The stress, displacement and plastic zone after excavation of No. 2 mine is consistent with that of Step 3 of Scheme 1, and will not be stated here.
Summarizing the above two-wing to central excavation simulation, as with the central to two-wing excavation, with the excavation of the mine, the maximum compressive stress of the point column is getting larger and larger, the maximum tensile stress value of the top plate is basically unchanged, but the roof is vertically sunken. The displacement is getting larger and larger, and the plastic area of ​​the point column is gradually larger. Table 5 shows the stress and displacement values ​​of different excavation steps.

2.3 Comparison of two schemes
For the above two different mining sequences, the mining steps are different, but the stress variation trend of the roof and the point column is the same during the mining process. The tensile stress of the roof is small with the excavation, the compressive stress of the point column becomes larger, and the top plate is pulled during the mining process. The stress is less than the ultimate tensile stress of 2.48 MPa, the point column compressive stress is much less than 92.2 MPa, and the stope is stable.
Theoretically, when the stope is mined from the two wings to the center, due to the existence of the central mine, the continuous exposed area of ​​the mining area is reduced, and the stability of the stope is better. In this paper, the stresses of the two schemes are compared and analyzed. Figure 6 shows the stress and displacement line diagrams of the different excavation steps of the two schemes. From the analysis in the figure, the stress and displacement of the scheme 2 from the two wings to the central mining are less than the scheme 1. And from the above plastic zone cloud map, the plastic point of the 2-point column is obviously smaller than that of the scheme 1, so it is considered that the mining from the two wings to the central is better than the central to two-wing mining scheme, that is, the scheme 2 is optimal.

3 conclusions

(1) Through the theory of area bearing, the evaluation model and formula of pillar stability are established, and the orthogonality difference theory is used to sort the sensitivity factors of pillar stability, and the width, depth and pillar size of the pillar are obtained. The stability of the pillar is the most important, and the relationship between the width of the pillar, the width of the mine and the safety factor of the pillar is obtained. When guaranteeing the required safety factor of the pillar, the span of the mine shall not exceed 17m, and the width of the pillar shall not be less than 6m.
(2) Under the premise of ensuring the stability of the stope, two different mining sequences commonly used in the mine are selected as alternatives. Through numerical simulation, the stress, displacement and plastic zone size changes of different schemes are compared to obtain a plan. Excellent, that is, mining from the middle to the two wings is safer.
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