In recent years, affected China's economic transformation, the growth rate of decline, demand for iron ore resources of growth gradually slowed down [1-2], and the international iron ore production continues to expand, the global supply of iron ore is still in the growth phase, As a result, iron ore prices fell rapidly and once fell below $50/t. Due to the high production cost of iron ore in China, the sharp drop in iron ore prices has brought great blows to China's iron ore mining enterprises.
Effectively reducing mining costs and improving competitiveness have become a top priority for iron ore mining companies in China. As an important process of open-pit iron ore mining, blasting operation has a great impact on mining cost. Improving blasting effect is an effective way to reduce the mining cost of open-pit iron ore.
1 Relationship between blasting and mining cost
The processes related to blasting parameters in the exploitation cost of open-pit iron ore in China mainly include perforation, blasting, shovel loading, transportation and crushing. According to the analysis of relevant data, the proportion of the cost of each process is: perforation and blasting account for about 20%, shovel loading accounts for about 15%, transportation accounts for about 30%, crushing accounts for about 25%, and other accounts for about 10% [3].
Open-pit blasting operations generally use loose blasting, which uses the energy generated by the explosion of explosives to loosen and break the ore, and throws the ore into the direction of the step slope to form a burst of a certain shape, which provides conditions for the subsequent shovel loading process. As an important process of open-pit mining, blasting directly affects the efficiency and cost of subsequent shovel loading, transportation, and coarse crushing. The ore blasting has a uniform blockage and no roots, which can improve the production efficiency of the shovel and transportation equipment, and at the same time avoid the cost of secondary crushing of large blocks and roots, and reduce costs.
There is a correlation between the mining cost of open-pit iron ore and the quality of blasting, and the quality of blasting can be reflected by the blasting rate. The blasting block rate is too large, the cost of crushing, shovel loading and transportation is high, and the total cost of mining is correspondingly higher. As the block rate decreases, the cost of crushing, shovel loading, transportation and total cost will decrease; Decrease, although the cost of crushing, shovel loading, and transportation will be reduced, the total cost will increase due to the significant increase in blasting costs. Therefore, there are reasonable blasting parameters in open pit iron ore mining, which minimizes mining costs.
The blasting optimization proposed by the Maanshan Mine Research Institute of Sinosteel Group refers to finding the optimal blasting parameters under the condition of satisfying some special requirements of the mine under the premise of ensuring the minimum total cost of perforation, blasting, shipping and rough crushing. If one-sided pursuit of the lowest cost of blasting may result in an increase in shovel loading costs, crushing costs, and even beneficiation costs. Therefore, reducing the cost of open-pit iron ore mining not only reduces the cost of blasting, but also considers the cost of other processes. By obtaining good blasting quality, the sum of the costs of each process is the lowest.
2 factors affecting the quality of blasting
The factors affecting the quality of open pit blasting include the geological structure of the explosion zone, the mechanical properties of the rock, the performance of the explosive, the blasting technology, the process parameters, and the blasting construction management.
(1) Geological structure of the explosion area. The structural plane in the ore will affect the distribution of the explosive energy, thus affecting the fragmentation of the rock. The joint fissures in the ore are developed. The explosive energy of the explosive will first break the joint fissure, and the explosive energy will escape through the joint fissure too early, which is easy to produce large blocks and roots.
(2) Mechanical properties of ore. The tensile and compressive strength characteristics of the ore directly affect the blasting effect. The greater the tensile strength and compressive strength of the ore, the greater the energy required for the rock to break, and the more blasting, the more likely the roots are. In addition, when blasting, the interface between hard rock and soft rock is easy to form a large block.
(3) Explosive properties. Explosive speed, brilliance, function, blasting distance, density and water resistance performance of explosives directly affect the blasting effect. Explosive speed, explosion heat, density, etc. of explosives affect the peak pressure and duration of the explosion shock wave and stress wave, which in turn affects the explosive energy utilization rate of explosives [4].
(4) Blasting process and technical parameters. The blasting parameters mainly include the front row chassis resistance line, hole distance, row spacing, unit consumption, charge structure, and detonation sequence. The pore network parameters (pore spacing, row spacing, etc.) are too large and the diameter of the blasthole is small, which will result in a small consumption of explosives, which is easy to produce large blocks and roots; the center of charge of the continuous charging structure is low, and it is easy to blast at the upper part of the step. Large blocks are produced; the more free surfaces in open-air step blasting, the higher the blasting quality, and the detonation sequence can affect the blasting quality by affecting the number of blasting free faces.

(5) Quality of blasting construction. The quality of construction, such as drilling, charging, and blasting, will also affect the quality of the blasting. Insufficient drilling accuracy, such as drilling position, unreasonable inclination angle, insufficient hole depth, etc., easily lead to the formation of large blocks and roots; poor quality of blasthole packing is likely to cause “shooting”, resulting in low energy utilization rate of explosives, large blocks. High rate issues.
3 Technical measures for improving the blasting quality of Heshangqiao Iron Mine
The Heshangqiao Iron Mine is a sag mining, with an annual mining and stripping total of 15 million tons. It is perforated with a down-the-hole drilling rig and has a hole diameter of 200 mm. It has been mined to the -60m level. The ore body is mainly magnetite. The Platt's coefficient of ore is 10-15, and the fissure is relatively developed. The surrounding rock is mainly diorite porphyrite, the Platts coefficient is 8-10, and the fissure is relatively developed. The shallow ore body and surrounding rock of the mine are relatively broken, the deep ore body and surrounding rock are hard, and the joint fissures are relatively developed. It uses bagged emulsion explosives and Orica high-precision detonator blasting. In order to improve the blasting effect of the deep water-bearing ore body and surrounding rock, the following blasting technology tests were carried out.
3.1 small resistance line blasting technology
Through the forward blasting funnel test of the cement mortar model of the Maanshan Mine Research Institute of Maanshan Iron and Steel Group Co., Ltd. (the mortar model is 80cm×80cm×80cm, the test dose is 1g), it is found that the depth of the explosive is shallow to deep. Until the best drug depth is 5.5cm, the degree of fragmentation K50 (the size of the screen when the weight of the sieve is 50%, cm) also increases with the increase of the blast volume, and reaches the maximum at the best drug depth. The value is 4.9. When the blasting volume is the largest, K50 is also the largest, which indicates that although the blasting volume is the largest, the crushing effect is poor and the bulk rate is high. Through the on-site blasting funnel test, it is determined that the reasonable fracture resistance line of the mine should be 0.6 to 0.8 times the resistance line at the maximum volume.
Through field test, it is finally determined that the mine has a row spacing of 5.5m and a hole spacing of 8.0m. The row spacing is about 0.7-0.75 times of the front row resistance line, which is consistent with the field blasting funnel test results. After the application of the technology, the blasting effect of the mine is greatly improved, the bulk rate is reduced, and the shovel loading efficiency is increased.
3.2 Hydraulic booster blasting technology
The height of the mine step is 12m, and the length of the filling is generally 6m. The upper part of the step is not filled with explosives. The explosives are all concentrated in the lower half of the step, which causes the lower part of the step to be denser, but the rate of fine ore is higher, and the step is higher. The phenomenon of more large parts in half. The hydraulic supercharged blasting technology sets a certain height of water column transmission in the upper part of the blasthole explosive and increases the detonation energy of the incoming rock mass explosive. A pre-pressure plug pack with a dose of about 3 kg is set between the water column and the upper packed rock cuttings [5] . During blasting, the advance pressure plug pack detonates 25ms ahead of time, pre-compacting the plug rock to increase the packing strength, and then the main charge begins to detonate. At this time, the explosive energy is transmitted to the upper rock of the step through the upper water column to improve the explosion energy utilization. Improve the effect of the block distribution on the upper part of the step. The blasting structure is shown in Figure 1.

The field test determined that the height of the hydraulic booster blasting water column was 1.5-2.0 m, the packing height was 5.5-6.5 m (water-column height), and the upper part was filled with 2.5-4.5 m. When water is contained in the blasthole, the water in the blasthole can be fully utilized, and a positioning baffle is set at the height of the reserved water column (the front baffle pack is placed on the positioning baffle), and the positioning rope is used to fix the hole. The application of the technology in the field shows that: 1 the length of the blasting volume is increased by 10.86%, and the rupture and breaking block is uniform; 2 by strengthening the stuffing bag, the packing strength is improved, and the filling effect is effectively ensured, and no punching phenomenon occurs; The hydraulic turbocharged blasting rate is reduced by 45% compared with the conventional blasting, which greatly improves the energy utilization rate of the explosive.
3.3 short differential hole by hole initiation technology
The technology utilizes the first blast hole to generate a new free surface and a short crack for the adjacent hole, reduces the shape of the bottom of the step, avoids the generation of the root, can better control the shape of the blast, and improve the blasting quality. Because the maximum amount of drug (ie, single-pore dose) is reduced, the damage of blasting vibration can be effectively reduced [6]. According to the comparison test between the blasting vibration and the differential time of the Maanshan Iron Mine in Maanshan Iron and Steel Group of Sinosteel Group, and the waveform analysis, the reasonable time difference is 20~30ms. Mine production blasting equipment detonator adopts Orica 25, 42ms extended high-precision detonator tube detonator, the adjacent two holes have a differential time of 25 ~ 17ms. After the short-and-difference-by-hole detonation technology is applied, the blasting effect is improved, and the impact of blasting vibration on the slope and surrounding residents is greatly reduced.
3.4 Improved measures for stable explosion of water-containing blasthole explosives
The water in the blasthole, combined with the weathering of the ore body and the surrounding rock, tends to be discontinuous in the charging process, leading to the root. To this end, a detonating cord is arranged in the hole, wherein one detonator is connected with the detonating cord, which solves the problem that the water hole charging is discontinuous and the residual medicine appears.
4 Conclusion

Aiming at the current situation of the Heshangqiao iron ore mine, the use of small resistance line blasting technology, hydraulic boosting blasting technology, short differential hole-by-hole initiation technology and improved improvement of water-containing blasthole explosives were proposed. Through on-the-spot practice, the bulk rate and roots are greatly reduced, the blasting effect is improved, the shovel loading and coarse crushing efficiency are improved, the blasting vibration is greatly reduced, the mine production cost is reduced, and good economic and social benefits are achieved.
references
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[3] Wan Delin. The effect of open pit blasting effect on shovel loading efficiency and production cost [J]. World Mining Express, 2000, 16(9): 317-319.
[4] Lai Yingde. On the energy matching between explosives and rocks [J]. Engineering Blasting, 1995 (2): 22-26.
[5] Liu Weizhou, Yuan Yingjie, Zhang Xiliang, et al. Hydraulic booster blasting test [J]. Engineering Blasting, 2014(2): 10-13.
[6] Liu Weizhou. Experimental study on blasting blasting in the mining area of ​​Nanshan Mine[J]. Metal mines, 2004 (6): 22-23.
Author: Yuan Yingjie; Magang (Group) Holding Co., Ltd. Nanshan Mining Company;
Article source: "Modern Mining"; 2016.7;
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