1. Introduction
Tailings are the solid mineral waste created by the natural dewatering of tailings slurry from beneficiation plants, where high output and low comprehensive utilization efficiency are the major characteristics [
1]. This leads to a significant loss of resources and pollution of the ecosystem. In order to reduce the environmental problems from the tailings in surface stockpiles, many academics have suggested tailings as the primary backfill material in underground mined-out areas [
2,
3,
4]. The filling body in mined-out areas can support overburden pressures, ensure that vegetation, farmland, and buildings on the surface are not destroyed, and ensure the safety of underground operators and equipment. At the same time, workers can recycle underground safety pillars, reducing the waste of mineral resources [
5]. Consequently, the use of tailings to backfill the mined-out areas is one of the directions for the development of the back-filling method [
6].
Although the back-filling method could solve many of the issues brought on by underground mining, it is unable to completely dispose of the enormous volume of tailings in surface stockpiles. In addition, comprehensive utilization of tailings is also one of the most effective ways to dispose of tailings. However, less than 40% of the tailings are utilized, especially gold tailings [
5]. The problem of surface stockpiles of significant amounts of gold tailings can be resolved through the development of innovative techniques for the comprehensive utilization of gold tailings. Researchers have investigated the use of tailings as building materials. Chen et al. [
7] investigated the effect of making bricks using clay and raw materials including iron ore tailings and fly ash. Shettima et al. [
8] pointed out that when the fraction of river sand substituted by iron ore tailings grew from 25% to 50%, 75%, and 100%, the workability of concrete decreased, but all the strength and modulus of elasticity data were higher than in traditional concrete. By separating and recovering quartz, feldspar, and gold elements from gold tailings samples, Chen et al. [
9] found that they could make high-quality glass from the quartz concentrate and ceramics from the feldspar. Wang et al. [
10] found that the ideal blending ratio for using gold ore tailings as raw siliceous material for cement manufacture is 5%, and the ideal calcination temperature is 1450 °C. Deng et al. [
11] investigated the effect of the red mud-unburned ceramsite employed as an adsorbent to remove phosphate from swine water. As a result, previous studies [
7,
8,
9,
10,
11] indicate that gold mine tailings can be used as fine sand for construction and raw materials for ceramics, brickmaking, and cement. However, due to the large amount of tailings surface stockpiles, when the coarser particles and some particles with higher silica content in the tailings are utilized, there still exists a portion of fine and ultra-fine tailings that cannot be utilized, which will cause great damage to the surface ecology. Therefore, combining the comprehensive utilization of tailings with underground mined-out areas backfill will be a good way to solve the surface stockpiling of tailings. The process flow diagram is displayed in
Figure 1.
From
Figure 1, the tailings slurry from the beneficiation plant was first pumped to the tailings comprehensive utilization base, where it will be screened twice by meshes. The first level of screening uses a 60-mesh vibrating triple screen, and the products on the screen are sold as finished fine sand in concrete. The secondary screening uses a 160-mesh vibrating triple screen, and the products on the screen are transported to the magnetic separation apparatus through a belt. The magnetic material will be removed to improve the whiteness of the tailings. The tailings with high whiteness can be used as the primary raw material for ceramics and glass. Moreover, the removed iron-containing tailings can then be used as fine iron powder raw materials. After sedimentation and pressure filtration of the tailings slurry from secondary screening, the tailings can be used as a raw material for industrial unburned ceramic granules, bricks, cement, etc. The other portion of the tailings slurry can still be delivered to the mine backfill station, and the tailings deep-cone thickener can be used to raise the underflow concentration of the ultra-fine tailings (UT). Finally, the cementitious materials, water, and UT are mixed uniformly by the mixing system, and then transported to mined-out areas by means of self-flow or pumping. So, all of the tailings in surface stockpiles can be effectively handled, and there is no need for a new tailings pond.
Tailings will become finer as beneficiation technology and comprehensive utilization of tailings technology advances [
12,
13,
14,
15]. Since ultra-fine tailings (UT) have the characteristics of slow particle settling, a unique chemical makeup, an uneven distribution of gradation, etc., they can be tough to treat [
16]. Ke et al. [
17,
18] studied the effects of tailings fineness and gradation on the properties of the cemented paste backfill (CPB) and found that well-graded tailings may improve the CPB’s capacity for consolidation. Portland cement is frequently used as the cementitious material to generate CPB. However, its production produces pollutants such as dust, SO
2, and NO
2, and the cost of buying cementitious material supplies accounts for roughly 70%–80% of the backfilling cost [
19]. Therefore, establishing sustainable mine development requires a reduction in the cost of cementitious materials. After being physically or chemically activated, a few typical industrial wastes, such as blast furnace slag (BFS) (
Appendix A), steel slag, carbide slag (CS), fly ash, etc., have potential activity and can be used as cementing materials [
20]. Sun et al. [
21] created a novel binder by using calcined quarry dust and NaOH at a mass ratio of 1:1 to activate slag, which significantly increased the uniaxial compressive strength (UCS) of the CPB. The rheological and mechanical properties of cemented backfill materials are influenced by factors such as the cement-tailings (C/T) ratio, mass concentration, curing temperature and time [
22]. Underground mines will have different requirements for backfilling parameters in ultra-fine tailings cemented backfill slurry (UTCBS) because of specific surrounding rock characteristics, tailings gradation, and cost-effectiveness limits [
23]. To meet particular requirements, some common admixtures (such as water reducers, pump agents, suspend agents, accelerators) are added to the backfilling materials [
24,
25]. Sodium silicate (SS) can be used as a gel filling in a variety of concrete applications because of its gelatinous characteristics. However, it is rarely used for backfilling underground mined-out areas [
26]. The addition of SS will boost the early strength, speed up the hydration reaction, and lower the water-cement ratio of the cemented material to mine backfill [
27]. However, when more SS is injected than a certain percentage, the strength of the cemented materials decreases [
28]. There is disagreement about the mechanism by which SS affects the hydration of cementitious materials, even if some researchers concur that SS decreases the long-term mechanical strength of cemented materials [
29]. Additionally, cemented backfill slurry (CBS) pipeline transportation is one of the vital components of underground mined-out areas backfilling. Guo et al. [
30] examined the thixotropic properties of the UTCBS-added superplasticizer as well as the dynamic and static rheological properties. They also proposed a dynamic yield stress model that took into account the water content in flocs to enrich CBS rheological theory. Therefore, the use of the cementation filling method to treat UT will result in an increase in backfill cost, followed by the low strength of the filling body. It is necessary to carry out more research to enrich the theory of UT backfill and improve the application level of UT underground cemented backfill.
Due to the large surface stockpile of tailings and the high cost of backfilling ultra-fine tailings (UT), the technical difficulties are high. Most of the previous research focused on the unclassified tailings or classification tailings cementation backfill in the mined-out areas, with fewer studies on the ultra-fine tailings cementation backfill. Therefore, the main objectives of this study are to reduce the cost of backfilling UT and to improve the mechanical properties of the ultra-fine tailings cemented paste backfill (UTCPB). Firstly, suitable cementitious materials were prepared using blast furnace slag (BFS), carbide slag (CS), sodium silicate (SS), and calcium chloride (CC), to keep the cost of cementitious materials under control. Next, a slump test was used to ensure that the ultra-fine tailings cemented backfill slurry (UTCBS) met the pipeline delivery, and then the rheological test of the backfill slurry was carried out. Thirdly, on the basis of the newly prepared cementitious materials and the determined slurry proportioning parameters, mechanical experiments on the specimens of the UTCPB were carried out. Finally, the consolidation microscopic properties of the UTCPB were investigated by SEM-EDS, XRD, and mercury intrusion porosimeter (MIP) tests. The research results of this thesis can promote the improvement of the theory and application level of UT cemented filling, and lay a good foundation for the realization of tailing-free mines.
4. Conclusions
In this paper, ultra-fine tailings (UT) with low comprehensive utilization rate were treated utilizing the underground mined-out areas cementation filling method, and to improve the cementing backfill effect of UT, the method of adding sodium silicate (SS) and calcium chloride (CC) admixtures in the ultra-fine tailings cemented backfill slurry (UTCBS) were investigated. The UTCBS was subjected to fluidity, setting time, and rheological tests, and the ultra-fine tailings cemented paste backfill (UTCPB) was subjected to uniaxial compressive strength (UCS) and microscopic performance tests. The main conclusions are as follows:
- (1)
A proper admixture dosage is beneficial to shorten the setting time of UTCBS, and the setting time of UTCBS in the blast furnace slag (BFS)-based cementitious system decreases as SS increases.
- (2)
UTCBS in the rheological experiment primarily manifests as “shear thinning” characteristics. At different shear rates, the shear stress-rate curve of UTCBS can be divided into three stages: rapid growth, slow decline, and slow growth. As the hosting time increases, the second stage becomes more pronounced.
- (3)
The BFS-based cementitious material added with SS and CC significantly improved the short-term UCS of the UTCPB. When compared to P. O42.5 cement, the UCS of UTCPB made from BFS-based cementitious materials with admixture increased by 124%, 142%, and 13% at curing ages of 3, 7, and 28 days, respectively. Moreover, compared to ordinary P. O42.5 cement, this is less expensive at 22.4%.
- (4)
The BFS vitreous dissociates due to the strongly alkaline environment of the slurry produced by carbide slag (CS), producing C-S-H, C-A-H, Ca(OH)2, a small amount of AFt, and other hydration products. O, Al, Si, and Ca are the main elements of the amorphous gel found in hydration products. With the addition of SS and CC, some calcium silicate precipitate was produced and wrapped around the UT particles, which caused the UCS of the UTCPB to significantly increase. The porosity of UTCPB without and with admixture at the curing age of 7 days was 54.25% and 48.37%, respectively.
Consequently, for better consolidation of UT, increasing the cement-tailing ratio will increase the cost of backfilling, and increasing the mass concentration will lead to transport difficulties. Although adding admixtures will somewhat increase the pipe transport resistance of UTCBS, using SS and CC in BFS-based cementitious materials for UT consolidation not only significantly reduces the cost of mine backfill, it also shortens the slurry’s setting time and increases the UCS of UTCPB. As a result, this makes it gradually possible for ultra-fine tailings to backfill the mined-out areas. Finally, there is no need to construct a new tailings reservoir, minimizing the environmental impact of mining activity.