Abstract
In order to improve the tailings utilization rate and realize full tailings’ filling in Huanggang iron mine in Inner Mongolia, a flocculation settlement system was added, which resulted in silting and consolidation of tailings in sand silts. Based on this, the reason for the deposition and caking of tailings was obtained through analysis, and a set of coarse and fine tailings bin filling system was proposed. The tailings of the plant were pumped to the filling station by slurry pump. Firstly, the tailings with large particle size difference were divided into coarse tailings and fine tailings by hydrocyclone so that the fine tailings entered the No. 1 sand bin and the coarse tailings entered the No. 2 sand bin. Then, flocculant was added by automatic flocculant dosing device to flocculate and settle the coarse and fine tailings, and then, it was mixed, stirred, and filled into the well. The results showed the following. (1) The particle size difference of tailings in Huanggang Iron mine increased and resulted in unstable flocs. The coarse tailings settled to the bottom of the silo before the fine tailings, and the compression time was too long, leading to the phenomenon of silting and consolidation. (2) Flocculation and settlement of coarse and fine tailings in different sand silos can avoid deposition and compaction of tailings in sand silos. (3) According to the different requirements of the concentration and ratio of the filling slurry, the proportion of coarse sand and fine sand in the filling slurry can be controlled by controlling the flow size of the coarse sand and fine sand chamber, to achieve the design of reasonable particle size composition and concentration requirements. The research results are expected to provide technical support and theoretical guidance for the construction of full tailings cemented filling system in Huanggang iron mine.
1. Introduction
With the continuous development of society and the national requirements and promotion of green mine construction, the requirements of mine safety and environmental protection are becoming higher and higher, and the control of mining surface and environment is becoming more and more strict. The filling mining method has many advantages that other traditional mining methods do not have, such as controlling surface collapse, improving mining environment, reducing ore loss rate and dilution rate, controlling stope ground pressure, improving underground operation safety conditions, and other outstanding advantages. Therefore, more and more underground mining mines choose to adopt the filling mining method [1–8].
In the field of mine filling, most metal mines or some nonmetal mines begin to use the whole tailings produced by the mine dressing plant to fill the underground goaf, which can not only improve the utilization rate of tailings and reduce the discharge pressure on the tailings reservoir but also meet the rigid requirements of the state for the surface environment of the mining area. Therefore, many scholars have conducted in-depth studies on tailings’ filling [9–12]. Based on the calculation method and results of friction loss, Fan et al. [13] studied the influence of tailing slurry mass fraction, flow rate, and pipe diameter on friction loss and carried out industrial filling transportation for goaf. Zhuen et al. [14] studied the effects of solid mass fraction, waste rock dosage, and cementing powder consumption on the collapse degree, yield stress, uniaxial compressive strength, and bleeding rate of tailings’ solid waste paste. Meidao et al. [15] used MATLAB software for data analysis and obtained the sensitivity of the influence of cement-sand ratio and slurry concentration on the strength of backfill and the prediction model of various mechanical parameters. Gezhong et al. [16] studied the change rules of equivalent diameter, fractal dimension, and sedimentation velocity of tailings flocs formed under different flocculation conditions.
It is one of the main ways of tailings filling to adopt sand silo for tailings’ settlement and filling [17–20]. Jun et al. [21] proposed the calculation method of critical working pressure and working flow of high-pressure activated medium in vertical sand silo according to the basic principle of activated slurry. Ke et al. [22] studied the influence of tailings particle size composition, surface shape, and sand feeding concentration on tailings’ settling velocity in sand silo. Weicheng et al. [23] proposed a sand discharging model of a sand silo and used fluid dynamics software to simulate and analyze the continuous sand discharging model. Liyi et al. [24] analyzed and studied the influence rule of ultrasonic wave on the final mass concentration of total tailings’ slurry in the sand silo under different conditions. Due to the different mineral processing methods and ore properties adopted by mine concentrators, the properties of tailings produced are also different. There will be large differences in the particle diameter of tailings and obvious differences in the thickness and fineness of tailings, which may lead to the deposition and agglomeration of tailings in sand bins and affect the normal production. However, the research results in this field are rarely reported, and it is urgent to put forward more effective solutions.
The tailings of Huanggang iron mine in Inner Mongolia have a large proportion and obvious difference in thickness. After flocculation settlement, silting and consolidation of sand silts occur, which greatly affect the normal filling operation. Based on this, this study takes the full tailings’ filling slurry of Huanggang iron mine as the research object, obtains the reasons for tailings silting and consolidation through analysis, and puts forward a set of coarse and fine tailings’ filling system, in order to provide technical support and theoretical guidance for the construction of the full tailings’ cemented filling system of Huanggang iron mine.
2. Test and Analysis of Physical and Chemical Properties of Total Tailings
The physical and mechanical properties of the tailings, such as specific gravity, bulk density, porosity, and particle size distribution, were measured by sampling the tailings from Huanggang iron concentrator and laboratory tests were carried out.
2.1. Gravimetric Determination of Total Tailings
Specific gravity is an object of the ratio between density of the tailings and density of water, as a dimensionless quantity. Specific gravity refers to the ratio between the density of liquid or solid at a specific temperature and pressure and the maximum density of pure water at standard atmospheric pressure. Specific gravity of laboratory samples is usually measured by the pyrogen flask method, as shown in Figure 1.
The gravity determination test for the tailings of Huanggang mining was conducted for three times. The average value of the three test results was taken to obtain the gravity of the whole tailings. The test results are presented in Table 1.
It can be seen from Table 1 that the average value of the three test results of the gravity of total tailings in Huanggang iron mine was 3.23. Compared with the gravity of tailings in [9–16], the gravity of total tailings in Huanggang iron mine was large. When the slurry is formed, the settlement performance is good, the settlement speed is fast, and the bulk density of the slurry is large.
2.2. Determination of Bulk Density of Whole Tailings
The constant volume weighing method was used for the measurement, that is, the test material was loaded into the standard funnel, a standard liter bucket was placed below, the material fell freely to avoid vibration, and the weight of the balance was scraped flat after the bucket was full. The loose bulk density of the test material was calculated after several measurements, and the measurement process is shown in Figure 2.
Three tests were conducted for the determination of the dense bulk density and loose bulk density of the whole tailings in Huanggang mining. The results of the three tests were averaged to obtain the dense bulk density and loose bulk density of the whole tailings. The test results are shown in Table 2.
It can be seen from Table 2 that the average values of the three test results of loose and dense bulk density of the whole tailings in Huanggang iron mine were 2.16 and 2.46, respectively. Compared with the bulk density of the tailings in [9–16], the bulk density of the total tailings in Huanggang iron mine was also large, and the difference between the loose bulk density and the dense bulk density was small, which can reflect from the side that the porosity of the total tailings was small.
2.3. Porosity Analysis of Total Tailings
Porosity was calculated usingwhere η is the porosity of the whole tailings, %, γis the bulk density of the whole tailings, g/cm3, and ρ0 is the true density of the whole tailings.
According to the data in Tables 1 and 2, the porosity of the whole tailings in loose and dense states is calculated by substituting it into equation (1), as shown in Table 3.
It can be seen from Table 3 that the loose porosity and dense porosity of the total tailings in Huanggang iron mine were 33.12% and 23.84%, respectively, which are smaller than those in [9–16]. The porosity of the tailings can reflect the strength and water retention of the tailings to a certain extent. The total tailings in Huanggang iron mine had high strength and poor water retention.
2.4. Total Tailings’ Particle Size Distribution
The grain size distribution of filling aggregate is also called the gradation of materials, which refers to the percentage content of different particle sizes in granular materials. The particle size distribution of filling material determines the whole process of the filling process and has a great influence on preparation, transportation, and filling body quality. The particle size composition of the whole tailings was analyzed by Malvern 3000 laser particle size tester, which is shown in Figure 3.
The results of particle size distribution of the whole tailings are presented in Table 4 and shown in Figure 4, respectively, and the uniformity coefficient Cu and curvature coefficient Cc obtained are 7.05 and 1.02, respectively.
The measurement results of physical and mechanical properties of Huanggang Mining’s tailings show that(1)From the size of filling materials, the size of Huanggang iron tailings is relatively coarse. The proportion of −200 mesh (<74 μm) particles is only 21.41%, the proportion of 38 μm is about 11%, and the median particle size is 181.57 μm. Compared with the tailings in [9–16], the tailings of Huanggang iron mine were coarser, and the gravity of coarse particle size tailings was larger, and the proportion of fine particle size tailings was smaller. The difference between coarse particle size and fine particle size tailings was obvious. It is necessary to pay attention to the blockage caused by the settlement of coarse particles. The coarse particle size is beneficial to the strength development of filling body, but the less fine particles will affect the transportability of slurry.(2)From the perspective of filling material gradation, the uneven coefficient and curvature coefficient of filling material were both outside the normal range. Compared with the tailings in [9–16], the composition and distribution of tailings in Huanggang iron mine were uneven, and the gradation was poor. The curvature coefficient was slightly larger than the optimal value, indicating that there was a slight loss of fine particles. Tailings of Huanggang iron mine had great abrasion on the pipeline and were easy to be separated, which was not conducive to long-distance pipeline transportation.
3. Technical Reform Scheme and Results of Sand Silo
The original filling process of Huanggang iron mine adopts natural settlement and concentration of tailings. Due to the coarse particle size, large ratio, and large difference in particle size, the coarser tailings can naturally settle to the bottom of the sand bin for accumulation and concentration, while the finer tailings cannot naturally settle and float in the upper overflow water, and the overflow water is not clear. Therefore, the slurry particles filled into the underground are coarse, the composition distribution is uneven, and the gradation is poor. The segregation occurs in the transportation pipeline and the underground goaf, and the pipeline is greatly worn.
In view of the problems existing in the original filling process of Huanggang iron mine, such as low filling concentration, large drainage volume, serious turbidity of overflow water, and failure of filling strength to meet the standard, based on the above test results, the technical improvement scheme of the sand silo was studied.
3.1. Technical Reform Scheme
The fine tailings cannot settle naturally; therefore, a set of flocculant settling module is added to wrap coarse and fine tailings with flocculant to form flocculant groups, which settle together to the bottom of the sand bin, so as to realize full tailings’ filling and overflow water clarification. Flocculant automatic dosing machine is shown in Figures 5 and 6.
The modified filling process is as follows: the tailings for filling are provided by the concentrator, and the tailings’ slurry with a concentration of 15.81% generated in the beneficiation process is precipitated and concentrated by the tailings’ concentration pond, and its weight concentration is increased to about 40%. Then, the tailings are sent to the tailings’ bin of the filling preparation station by the tailings’ slurry pump. High concentration bottom flow mortar was formed after the sand bin was reconstructed and treated with flocculant. Cement is measured by spiral electronic balance according to the flow and concentration of tailings. At the same time, according to the concentration requirements of filling, an appropriate amount of concentration water is added. Concentrated tail mortar, cement, and water are stirred by the original mixing system, and the evenly mixed filling slurry is prepared and transported to the goaf for filling through the filling borehole and underground pipe network. The overall filling process is shown in Figure 7.
Through the sand silo technical reform program, the filling concentration is increased to 75%–80%, and the solid content of overflow water is less than 300 ppm, which greatly improves the strength of the filling body and reduces water dewatering, thus creating economic benefits for the mine.
3.2. Technical Reform Results and Reason Analysis
After the technical reform of the sand bin, the filling concentration increased from 70%∼75% to 75%∼78% due to the increase of fine particles. The bleeding rate of the filling slurry decreased from 20% to 10%. The flow state of the filling slurry is good, and segregation and stratification no longer appear. The strength of the backfill body increases and basically reaches the strength required by the mining method. The overflow water is clear, and the full tailings’ filling and continuous filling are realized. The filling effect after technical modification is shown in Figures 8 and 9, and the change of filling concentration is shown in Figure 10.
As can be seen from Figure 10, in the first 7 days of filling time, the filling slurry concentration of 1 : 4 and 1 : 15 with different cement-sand ratios was 75% or above, with roughly the same change trend and relatively stable filling. After 7 days, the filling concentration decreased sharply from the original 75% to 46% and 33% and finally to 0.
The reason for the sharp decrease in filling concentration is that the silting and compaction degree of tailings in the sand silo is getting deeper and deeper. At the beginning, the slurry concentration is unstable and the material cannot downfeed in a short time. At the later stage, the slurry concentration is low and the material cannot downfeed for a long time, and the sand surface height is getting higher and higher until the material cannot downfeed in the end. The silting and consolidation of sand silo tailings are shown in Figure 11.
It can be seen from Figure 11 that the hardness of tailings is very high after compaction, and the height of compaction is about 2–4 m, which requires clearance treatment, consuming a lot of time, and labor cost.
After analysis and research, the reasons for silting and compaction of sand silo tailings are as follows: coarse tailing sands particle size is large, coarse tailing sands ratio is too large, −38 micron particle accounted for only 21.41%, adding flocculating agent, the formation of the floccules is not stable, in the process of settling sand storehouse, floccules damage occurs, flocculant wrapped coarse tailing sands materials and a small amount of fine in the sand at the bottom of the silo, and rapid subsidence and a large number of fine tailing sands are under the parcel of flocculant slowly settling in tailing sand materials coarse surface. As a result, the coarse and fine tailings are stratified, and the coarse tailings are accumulated and concentrated for a long time due to the fast settlement speed. Finally, the tailings are silted and consolidated, and the slurry cannot down feeding.
4. Study on the Solution of Tailings’ Silting Consolidation
In view of the causes of silting and consolidation of silo tailings, in order to reduce the cost, this study improved the original filling system of Huanggang iron mine and added a new hydrocyclone. Based on this, a set of coarse and fine silo filling system was proposed to solve the problem of silting and consolidation of silo tailings. The process flow of the filling system of coarse and fine tailings’ compartments is shown in Figure 12. The tailings of the concentrator are pumped to the filling station by a slurry pump. Firstly, the tailings with large particle size difference are divided into coarse tailings and fine tailings by hydrocyclone so that the fine tailings enter the I sand bin and the coarse tailings enter the II sand bin. Then, the flocculant is added to the flocculation settlement of the coarse and fine tailings by the flocculant automatic dosing device and then mixed and filled to the underground.
In the filling system of coarse and fine tailings’ bin shown in Figure 12, according to the concentration and ratio requirements of different filling slurry, the opening and closing degrees of sand discharge valve 1 and sand discharge valve 2 under the sand bin are adjusted, respectively, to control the flow size of fine sand bin and coarse sand bin. The flow can be displayed by electromagnetic flowmeter 1 and electromagnetic flowmeter 2, to control the proportion of coarse and fine tailings in the filling slurry and achieve the reasonable particle size composition and concentration requirements.
By flocculation, sedimentation, and concentration of coarse tailings and fine tailings with different particle sizes in different sand bins, the problems of unstable flocs, separation of coarse and fine tailings, deposition and agglomeration of tailings, the shutdown of filling station, and influence of normal filling operation can be effectively avoided after adding flocculants.
5. Conclusion
In this study, the causes of siltation and compaction of tailings in Huanggang iron mine were analyzed, and a set of coarse and fine tailings’ filling system is proposed. The following conclusions are drawn:(1)The particle size difference of tailings in Huanggang iron mine increases, and the formed flocs are unstable. The coarse tailings settle to the bottom of the sand bin before the fine tailings, and the compression time is too long, resulting in the phenomenon of deposition and agglomeration.(2)Coarse tailings and fine tailings are flocculated in different sand bins, which can avoid the phenomenon of siltation and hardening of tailings in sand bins.(3)According to the different requirements of the concentration and proportion of the filling slurry, the proportion of coarse sand and fine sand in the filling slurry can be controlled by controlling the flow rate of the coarse sand bin and the fine sand bin, to achieve the reasonable particle size composition and concentration requirements.
The research results are expected to provide technical support and theoretical guidance for the construction of cemented tailings’ filling system in Huanggang iron mine.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported and financed by National Key Research and Development Program of the 13th Five-Year Plan (no. 2018YFC0604603 and 2018YFC0604606), all of which are greatly appreciated.