Experimental Study on Dense Settlement of Full-Tail Mortar under Mechanical Vibration

: There are some problems in the application of slurry preparation technology, such as wide ﬂuctuation range of underﬂow concentration, long settling time and low efﬁciency of solid– liquid separation. This is an important basis for researching the thick settling law of tailings slurry under the action of mechanical vibration and its inﬂuencing factors to solve these problems. To this end, a small vibration thickening testing machine and vibrating rod were designed and developed. Physical simulation experiments were conducted to analyze the settling characteristics of tailings slurry under different vibration duration, start time, vibration frequency, and vibration inertia single factors. The results show that: (1) Mechanical vibration can effectively accelerate the settling speed of tailings particles, but the relationship between them is a non-positive correlation, and mechanical vibration time control with in 5 mins is the best. With the delay of starting the vibration time, the ﬁnal mass concentration ﬁrst increases and then decreases. (2) As the vibration frequency increases, the ﬁnal mass concentration of tailings settlement ﬁrst increases and then decreases. When the eccentric vibrator speed is 6000 r/min, the best slurry settlement effect is achieved. (3) When the vibration inertia of the eccentric oscillator is 0.158 g · cm 2 and the ﬁnal mass concentration reaches 70.1%, the settling time only takes 210 min. (4) The lower the slurry concentration, the faster the settling speed. As the initial concentration increases, the ﬁnal thickening time is also gradually prolonged. The research results provide some insights for the rapid thickening technology of rake-free paste thickeners.


Introduction
Due to the characteristics of no segregation, no stratification and no bleeding, pastefilling technology has been widely used in underground mines at home and abroad [1]. In 1992, Canada first used paste-filling technology at the Creighton mine in the Sudbury area. Paste filling has many advantages, such as a high tailings utilization rate, good mechanical properties, and low overall cost [2]. Scholars have defined a paste as a structural slurry with a sufficient number of fine particles to ensure that the viscous force in the flow field is greater than the inertial force and reaches saturation [3]. For paste filling in underground metal mines, the paste is composed of 20 µm structural slurry, which consists of fine particles with a particle content greater than 15%, a slump between 18-25 cm, a yield stress of 100-200 Pa, and a bleeding rate of 1.5~5%.
Full tailings thickening dehydration is a key link in the paste-filling process technology, and is the core technology for the preparation of high concentration paste filling slurry [4,5]. The quality of tailings slurry produced at the end of the beneficiation process in metal mines is generally between 10% and 35%. Low concentration tailings slurry mainly uses sand silos, deep cone thickeners, unpowered paste thickeners, and various thickening systems for dehydration and concentration [6,7]. In recent years, the rake-free paste thickener using the principle of gravity compression has attracted industry attention, and its thickening, dehydration, and concentration mechanism and efficient methods have been studied [8,9].
In the state of flocculation settlement, when the polymer concentration is low, the long chains of the polymer are simultaneously adsorbed on the surface of multiple particles, connecting them together through a "bridging" method leading to flocculation [10]. Before the addition of polymer flocculant, due to the electrostatic action the particles will have a small range of self-adsorption to form part of a small floc. After adding flocculant, although a large floc was formed, it could not eliminate the large amount of water enclosed inside the floc. The special internal structure of flocculants determines the different flocs that are formed [11]. Mpofu et al. [12] found that APAM was mainly adsorbed to the surface of montmorillonite particles through electron sharing by analyzing the changes of diffuse reflectance infrared spectrum. Glover et al. [13] obtained the size and density of the flocs after flocculation using static light scattering and sedimentation image methods. Blanco et al. and Kou et al. [14,15] used infrared technology to identify the characteristics of acidic adsorption and desorption on the surface of clay minerals. Bürgera et al. [16] conducted a study on the flocculation settling performance using five settling devices with different cross-sectional shapes, and obtained a mathematical model for continuous settling and concentration of solid particles. During the paste mixing process, solid particles tend to aggregate into small spheres due to the surface tension of the fluid, which are surrounded by the fluid medium known as the aggregation phenomenon of microscopic components [17]. The shear action destroys the shear stress, surface tension and cohesion of the agglomerated particles, realizes the destruction of the agglomerated particle sphere itself, realizes the uniform distribution of the mixed material on a micro level, and further improves the thickening effect of the paste [18].
The mud layer compression theory is an important basis for tailings thickening. Buscall et al. [19] takes the formation concentration of continuous network structure, compression yield stress and interference sedimentation coefficient as the basic parameters to characterize the dewatering capacity of flocs. Controlling the height of the mud layer within an appropriate range is not only a necessary guarantee for avoiding excessive suspended solids in overflow water, but also an important prerequisite for obtaining appropriate bottom flow concentration. The mud layer is too thin and easily damaged. The mud layer is too thick and the flow rate is too small, making it difficult to achieve the desired concentration adjustment effect. The height of the mud layer refers to the distance from the free settlement and interference settlement interface to the bottom of the container when the tailings particles or flocs settle [20]. The bottom flow concentration is mainly affected by the pressure formed by the height of the slurry, and the bottom flow concentration varies depending on the pressure on the slurry. Buscall et al. [21] found in their study of the thickening and compression process of mud layers that the compression dehydration of the network flocs formed during the thickening process is closely related to pressure. Farrow et al. [22] showed that the concentration of the slurry increases vertically along the height of the mud layer as it descends. Rudman et al. [23] focused on exploring the effects of slurry yield stress and target speed on torque, but there is relatively little research on the impact of mud layer height on tailings settlement and concentration patterns. Gladman et al. [24] believe that increasing mechanical stirring during the settling process of tailings can to some extent increase the dehydration rate and dehydration area.
Mechanical vibration is considered an effective method to promote rapid settling of tailings [25]. When mechanical vibration occurs, the friction between tailings particles decreases sharply, and the shearing stress of the mixed slurry is overcome to liquefy, thus promoting the slurry to accelerate settlement and compaction. In the process of the propagation of tailings slurry, ultrasonic makes the tailings particles vibrate alternatingly, which promotes the process of particle collision condensation settlement. Ultrasonic enhancement of tailings particle settlement has become a research focus. Omran et al. [26] used ultrasound to process magnesite flotation, effectively increasing foam stability, reducing foam  [27] accelerated the deposition of sulfates in wastewater by applying 24 kHz ultrasonic waves. In addition, the vibration method has also been widely used in settlement reinforcement. Green et al. [28] built a prediction model of soil dissipation capacity around the vibrator, based on the assumption that soil failure is caused by a vertical shear wave. Massarsch et al. [29] divided the dense region near the vibrator into three parts: elastic region, elastoplastic region and plastic region according to the soil shear strain level γ. Wang et al. [30] improved the vibro-punching method to solve the problem of a weak silt interlayer, and proposed methods of vibro-punching hole encryption and adding coarse coral sand into vibro-punching holes, respectively.
This research was conducted to reveal the thickening and settling law of tailings slurry under the effect of mechanical vibration, and to lay the foundation for the research and development of a rake-less thickening machine. To this end, a small vibration densification testing machine was developed, vibration rods were developed, vibration settlement testing devices were designed, and sedimentation process tests of slurry were conducted under different vibration frequencies, inertia, opening times, and initial concentrations. The influence of vibration on the densification settlement of tailings slurry was studied, providing a theoretical basis for studying vibration densification technology.

Mechanism of Vibration Thickening of Tailings Slurry
The high concentration tail mortar conforms to the Bingham model [31], and the rheological properties of the slurry are characterized based on yield stress and plastic viscosity, expressed by the following equation: In the equation: τ 0 -yield stress, Pa µ p -plastic viscosity, Pa·s ∂γ/∂t-velocity gradient or speed index, 1/s. When τ − τ 0 ≤ 0, the slurry remains almost unchanged and appears as a solid state. When τ − τ 0 > 0, the structure of the slurry is sheared and destroyed, leading to liquefaction and flow. At this time, the flow characteristics of the slurry are affected by plastic viscosity. The higher the plastic viscosity, the more difficult it is for the slurry to liquefy and flow, and on the contrary, it is easier to liquefy. It can be seen that the main factors affecting the rheological properties of the slurry are yield stress and plastic viscosity [32].
As shown in Figure 1, mechanical vibration will generate a shear stress near the rod that is higher than the yield stress of the tailings slurry, and it monotonically decreases with the increase of the distance from the axis of the vibrating rod. At a certain distance, the shear stress is lower than the yield stress. Therefore, during the vibration process, a cylindrical liquefaction area will be formed around the vibrating rod, and the tailings slurry within this area is a liquid in a flowing state.
In the liquefaction area, the shear stress at the contact interface between the vibrating rod and the tailings slurry is the highest, and the shear stress at the activation interface of the tailings slurry is equal to the yield stress.
In the equation: r ls -the radius of action of the vibrating rod, m/s τ ω -maximum shear stress at the contact surface between the vibrating rod and the tail mortar body, Pa τ 0 -yield stress, Pa r i -the radius of the shape of the vibrating rod, m. τ0-yield stress, Pa ri-the radius of the shape of the vibrating rod, m From the above equation, it can be seen that the higher the shear stress generated near the vibrating rod, the lower the yield stress of the slurry, the larger the radius of the vibrating rod shape, and the larger the action radius.
The yield stress and plastic viscosity have opposite effects on the action radius. The lower the yield stress and the higher the plastic viscosity, the larger the action radius. The characteristics of low yield stress, high plastic viscosity and self-compacting concrete are the same. In addition, the higher the plastic viscosity, the slower the exhaust rate of the bubble, and the longer the vibration time is required to achieve the goal of compaction [10].
At the same vibration frequency, the radius of action of the vibrating rod increases with the increase of amplitude. The commonly used amplitude of a vibrating rod is around 1 mm. At the same amplitude, there is a maximum effect of vibration frequency on the radius of action. When the vibration frequency is less than the optimal frequency, the radius of action increases as the frequency increases; when it is greater than the optimal frequency, the radius of action decreases as the frequency increases. As the amplitude increases, the optimal frequency decreases slightly. The optimal frequency of vibration of the vibrating rod is close to the natural vibration frequency of the tailings slurry, which is conducive to generating resonance. As the vibration time of the vibrating rod increases, the radius of action increases.

Materials
The sample of this experiment is full-grain tailings. The tailings sample was taken from the tailings aggregate tank of a certain mining beneficiation plant. To ensure representativeness of the sampling, the method of equal amount sampling was adopted at equal intervals. The samples were bagged and transported to the laboratory and then dried at a temperature of 110 °C. The samples were dried to constant weight for future use. The basic physical properties of the measured tailings samples are shown in Table 1, and the chemical composition and content of the tailings measured using the X-ray fluorescence qualitative (semi-quantitative) analysis method are shown in Table 2. From the above equation, it can be seen that the higher the shear stress generated near the vibrating rod, the lower the yield stress of the slurry, the larger the radius of the vibrating rod shape, and the larger the action radius.
The yield stress and plastic viscosity have opposite effects on the action radius. The lower the yield stress and the higher the plastic viscosity, the larger the action radius. The characteristics of low yield stress, high plastic viscosity and self-compacting concrete are the same. In addition, the higher the plastic viscosity, the slower the exhaust rate of the bubble, and the longer the vibration time is required to achieve the goal of compaction [10].
At the same vibration frequency, the radius of action of the vibrating rod increases with the increase of amplitude. The commonly used amplitude of a vibrating rod is around 1 mm. At the same amplitude, there is a maximum effect of vibration frequency on the radius of action. When the vibration frequency is less than the optimal frequency, the radius of action increases as the frequency increases; when it is greater than the optimal frequency, the radius of action decreases as the frequency increases. As the amplitude increases, the optimal frequency decreases slightly. The optimal frequency of vibration of the vibrating rod is close to the natural vibration frequency of the tailings slurry, which is conducive to generating resonance. As the vibration time of the vibrating rod increases, the radius of action increases.

Materials
The sample of this experiment is full-grain tailings. The tailings sample was taken from the tailings aggregate tank of a certain mining beneficiation plant. To ensure representativeness of the sampling, the method of equal amount sampling was adopted at equal intervals. The samples were bagged and transported to the laboratory and then dried at a temperature of 110 • C. The samples were dried to constant weight for future use. The basic physical properties of the measured tailings samples are shown in Table 1, and the chemical composition and content of the tailings measured using the X-ray fluorescence qualitative (semi-quantitative) analysis method are shown in Table 2. The sample tailings were tested using an X-ray fluorescence qualitative (semi-quantitative) analysis method, and the results are shown in Table 2. The main components of the tailings in this sample are SiO 2 , MgO, Fe 2 O 3 , CaO, and Al 2 O 3 , with a main component content of 88.69%.
The particle size distribution of tailings directly affects its packing compactness and the compactness of the consolidated filling body. The particle size distribution of tailings is measured by the Malvern MS3000 laser diffraction particle size analyzer, as shown in Figure 2. The average particle size of tailings is 210.50 µm. The specific surface area is 602.5 m 2 /kg, the non-uniformity coefficient of tailings particles Cu = 42.9, and the curvature coefficient Cc = 1.55. The composition of tailings particle size is uneven and the grading is good. The sample tailings were tested using an X-ray fluorescence qualitative (semi-quantitative) analysis method, and the results are shown in Table 2. The main components of the tailings in this sample are SiO2, MgO, Fe2O3, CaO, and Al2O3, with a main component content of 88.69%. The particle size distribution of tailings directly affects its packing compactness and the compactness of the consolidated filling body. The particle size distribution of tailings is measured by the Malvern MS3000 laser diffraction particle size analyzer, as shown in Figure 2. The average particle size of tailings is 210.50 μm. The specific surface area is 602.5 m 2 /kg, the non-uniformity coefficient of tailings particles Cu = 42.9, and the curvature coefficient Cc=1.55. The composition of tailings particle size is uneven and the grading is good.

Test Device
The test device is a self-developed full tailings vibration thickening test system, as shown in Figure 3, which mainly consists of a small bionic thickening test machine, a vibration rod and its control system, and an iron frame.

Test Device
The test device is a self-developed full tailings vibration thickening test system, as shown in Figure 3, which mainly consists of a small bionic thickening test machine, a vibration rod and its control system, and an iron frame. Minerals 2023, 13, 1072 6 of 16 The small biomimetic density testing machine is an Alec settling bucket with a height of 500 mm and an inner diameter of 100 mm, with an accuracy of 1 mm.
The vibrating rod is made of a transparent acrylic tube, with an outer diameter of 22 mm, an inner diameter of 16 mm, and a length of 600 mm. It is equipped with a DC-BL1634 series hollow cup motor and can be adjusted for speed through a computer. The vibration rod is excited using an eccentric oscillator, which is fixed to the motor bearing. The eccentric vibrator is fan-shaped and made of steel plates with different thicknesses. The vibration inertia of the eccentric vibrator is related to its sector size, mass, thickness, and circumferential motion speed. In order to obtain different vibration inertia under the same rotational speed and test the effect of different vibration inertia on the settlement of tailings sand particles, five types of vibrators were designed. The different oscillator parameters are shown in Table 3.  The small biomimetic density testing machine is an Alec settling bucket with a height of 500 mm and an inner diameter of 100 mm, with an accuracy of 1 mm.
The vibrating rod is made of a transparent acrylic tube, with an outer diameter of 22 mm, an inner diameter of 16 mm, and a length of 600 mm. It is equipped with a DC-BL1634 series hollow cup motor and can be adjusted for speed through a computer. The vibration rod is excited using an eccentric oscillator, which is fixed to the motor bearing. The eccentric vibrator is fan-shaped and made of steel plates with different thicknesses. The vibration inertia of the eccentric vibrator is related to its sector size, mass, thickness, and circumferential motion speed. In order to obtain different vibration inertia under the same rotational speed and test the effect of different vibration inertia on the settlement of tailings sand particles, five types of vibrators were designed. The different oscillator parameters are shown in Table 3. During the experiment, the vibrating rod is inserted into the thickener, and the upper part is suspended at the proper position of the suspension bracket above the Retort stand with a flexible rope, so that the vibrating rod is located at the center of the thickener and 1 cm away from the bottom. The vibration rod generates vibration by driving the eccentric rotor to rotate through a built-in motor.

Test Measurement Indicators
(1) Final thickening settling time The settling speed of tailings varies at different stages, and the height of the settling interface of the tailings at different times is recorded until the height of the settling interface no longer changes. This time is the final time of dense settlement.
(2) Final settling mass concentration When the settlement interface no longer changes, above the settlement interface is the clarification zone, and below the settlement interface is the dense tailings slurry. Calculate the maximum settlement mass concentration using Equation (1): In the formula, C f inal is the maximum settling mass concentration, G 0 , G 1 , G 2 are the total mass of slurry in the settling bucket, the mass of clarified water, and the mass of dense mortar, respectively. R is the radius of the settling bucket, 50 mm, and r is the radius of the vibrating rod taken as 11 mm.

Experimental Procedure
(1) Preparation of tailings slurry: First, pour 2.0 kg of water into the dense bucket, then use an electronic scale to weigh 2.5 kg of tailings, pour it into the dense bucket for homogenization using a mixing rod, and then add water until the liquid level in the dense bucket reaches 450 mm. Record the second amount of water added. After multiple experiments, it can be concluded that the total amount of water added is 2.56 kg. At this time, the calculated mass concentration of the prepared tailings slurry is 49.4%.
(2) Use a stirring rod to stir again evenly, keeping the vibrating rod centered during the mixing process, and keep stirring for 1 min.
(3) First, carry out the scientific control of natural settlement of tailings without vibration, record the settlement interface height at different time nodes, and collect the final settlement time and settlement interface height.
(4) Fix the vibrator and rotational speed, conduct density tests at different times when vibration starts, and study the influence of vibration start time on the settling law of tailings slurry.
(5) Fix the vibrator, set different motor speeds, and conduct motor speed tests of 2000 rpm, 4000 rpm, 6000 rpm, 8000 rpm, 10,000 rpm, and 12,000 rpm to study the influence of vibration frequency on the settling law of tailings slurry.
(6) At a fixed speed, density tests were conducted on oscillators 1, 2, 3, 4, and 5 to investigate the influence of different vibration inertia or amplitudes on the settling law of tailings slurry.
(7) After the completion of the above experiment, different tailings weights were set, including 1 kg, 1.5 kg, 2 kg, 2.5 kg, and 3 kg (to obtain different mud layer heights), and different slurry concentrations were configured, with fixed oscillators and rotational speeds. The effect of different slurry concentrations on the thickening law of tailings slurry under vibration was studied. The experimental process is shown in Figure 4.

Impact of Vibration Time on the Dense Settlement of Tailings
From Figure 5, it can be seen that before the vibration effect, the settling speed of the entire tail mortar is basically the same, and the difference in liquid level height after 5 min is not significant. After applying vibration, the liquid level of the entire tail mortar drops faster than without vibration, and the settlement height is the lowest at 253 mm at 300 min under 5 min of vibration. As the vibration time increases, the settling liquid level of the full tail mortar actually shows an upward trend, indicating that the vibration effect accelerates the settling speed of the tail sand particles, but it is not better to control the vibration time at 5 min in the experiment.

Impact of Vibration Time on the Dense Settlement of Tailings
From Figure 5, it can be seen that before the vibration effect, the settling speed of the entire tail mortar is basically the same, and the difference in liquid level height after 5 min is not significant. After applying vibration, the liquid level of the entire tail mortar drops faster than without vibration, and the settlement height is the lowest at 253 mm at 300 min under 5 min of vibration. As the vibration time increases, the settling liquid level of the full tail mortar actually shows an upward trend, indicating that the vibration effect accelerates the settling speed of the tail sand particles, but it is not better to control the vibration time at 5 min in the experiment. bration, and the best concentration effect is achieved after 5 min of vibration. As the vibration time increases, the mass concentration shows a decreasing trend. During the vibration process, a liquefied area of a cylinder will be formed around the vibrating rod, and the plastic viscosity in this area will increase due to vibration, achieving the effect of concentration. As the vibration time of the vibrating rod increases, the radius of the activation zone increases and further affects the flow characteristics of the tailings slurry, leading to a continuous increase in shear stress. After 5 min, the thickening effect gradually decreases. Start the vibrating rod at 0, 5, 10, 20, 30, 45, 60, and 90 min, respectively, to observe the dense settlement law of the full tail mortar, and draw the relevant relationship curve as shown in Figure 4. It can be seen that the final height of the slurry liquid level after applying vibration is slightly lower than that without vibration. In particular, the slurry liquid level with a starting vibration time of 45 min has the lowest final height, and the difference between the final height of the slurry liquid level without vibration is the largest, reaching 14 mm. The application of vibration has varying degrees of influence on the settling and thickening stages of the tailings slurry, and the activation of vibration at different times has different effects on the final settling and thickening time and final settling height of the tailings slurry.
Under the action of vibration, the final mass concentration of the tailings slurry has been improved, and as the starting vibration time increases, the mass concentration first increases and then decreases. The effect of starting vibration is most significant when the natural settlement is 45 min, at which time the mass concentration increases by 3.6% compared to the situation without vibration, reaching 72.5%. The final thickening time of The mass concentration of tailings slurry has been improved under the action of vibration, and the best concentration effect is achieved after 5 min of vibration. As the vibration time increases, the mass concentration shows a decreasing trend. During the vibration process, a liquefied area of a cylinder will be formed around the vibrating rod, and the plastic viscosity in this area will increase due to vibration, achieving the effect of concentration. As the vibration time of the vibrating rod increases, the radius of the activation zone increases and further affects the flow characteristics of the tailings slurry, leading to a continuous increase in shear stress. After 5 min, the thickening effect gradually decreases.
Start the vibrating rod at 0, 5, 10, 20, 30, 45, 60, and 90 min, respectively, to observe the dense settlement law of the full tail mortar, and draw the relevant relationship curve as shown in Figure 4. It can be seen that the final height of the slurry liquid level after applying vibration is slightly lower than that without vibration. In particular, the slurry liquid level with a starting vibration time of 45 min has the lowest final height, and the difference between the final height of the slurry liquid level without vibration is the largest, reaching 14 mm. The application of vibration has varying degrees of influence on the settling and thickening stages of the tailings slurry, and the activation of vibration at different times has different effects on the final settling and thickening time and final settling height of the tailings slurry.
Under the action of vibration, the final mass concentration of the tailings slurry has been improved, and as the starting vibration time increases, the mass concentration first increases and then decreases. The effect of starting vibration is most significant when the natural settlement is 45 min, at which time the mass concentration increases by 3.6% compared to the situation without vibration, reaching 72.5%. The final thickening time of starting vibration before 45 min showed a significant downward trend, and the final settling thickening time was the shortest at 180 min. After exceeding 45 min, the final thickening time actually increases, and with the increase of the opening vibration time, the final thickening time remains basically unchanged and tends to stabilize at approximately 240 min.
Under the action of vibration, the tailings particles undergo strong mechanical vibration in the medium around the vibrating rod, creating a liquefied area of the cylinder. Under the action of vibration energy, the probability of collision between the tailings particles increases, and the particles maintain a high-intensity motion state and undergo condensation.
On the one hand, the loose structure balance of the tailings particles is disrupted and loses stability, and the tailings particles are rearranged under the action of gravity and the gravity of the overlying particles. Micro particles with small particle sizes continue to move and fall into the pores of coarse particles, making the slurry structure denser. On the other hand, under the action of vibration-especially within the range of vibration excitation-the continuity of the water channel is significantly improved. The plastic viscosity of the tailings slurry decreases, the permeability increases, and the water between pores is accelerated and squeezed out, rising to the upper part of the tailings slurry, accelerating the settling speed and achieving the effect of density and concentration. However, when the settlement of tailings reaches a certain degree of compaction, the transmission and attenuation of vibration energy of the vibrating rod intensify and the effect of vibration concentration decreases instead.

Effect of Vibration Frequency on the Dense Settlement of Tailings
According to the operation steps in Section 3.4, prepare the tail mortar body, pour it into the test thickener, use a No. 2 vibrator, insert the vibrating rod, and use a mixing rod to stir for 1 min. Start the vibration and ensure the vibration time is 5 min. Conduct density settling tests at six different motor speeds (vibration frequencies) of 2000 r/min, 4000 r/min, 6000 r/min, 8000 r/min, 10,000 r/min, and 12,000 r/min. Figure 6 shows the settlement law of the full tail mortar under different vibration frequencies. Under different vibration frequencies, the settlement rate of the tail sand particles shows different rates. There is a sudden change in the settling rate of tailings. At speeds of 2000, 4000, 8000, 10,000, and 12,000 r/min, the settling rate of the full tailings mortar suddenly slows down at 240 min, while at speeds of 6000 r/min, this sudden change is advanced by 30 min, and the settling of tailings is basically completed. Under the vibration action of the first 5 min, the liquid level height decreases the fastest at 6000 r/min, followed by 10,000 r/min. The difference in the decrease rate among the other three different frequencies is not significant. From the perspective of final mass concentration, with the increase of vibration frequency, the final mass concentration of tailings settlement first increases and then decreases. It reaches its maximum at 6000 r/min, reaching 71.5%. Therefore, under different vibration frequencies, the final mass concentration of the full-tail mortar does not differ significantly. When the vibration frequency is 6000 r/min, the final thickening time is the shortest and the final mass concentration is the highest.

Effect of Vibration Inertia on the Dense Settlement of Tailings
Pour the prepared tailings slurry into the experimental thickener and mix evenly. At Therefore, under different vibration frequencies, the final mass concentration of the full-tail mortar does not differ significantly. When the vibration frequency is 6000 r/min, the final thickening time is the shortest and the final mass concentration is the highest.

Effect of Vibration Inertia on the Dense Settlement of Tailings
Pour the prepared tailings slurry into the experimental thickener and mix evenly. At a fixed frequency of 6000 r/min, the final settling concentration and settling time were tested using five types of oscillators, as shown in Table 1. In order to facilitate the observation of the vibration inertia on the law of settlement of tailings particles, as far as possible, exclude the interference of the initial state. After testing, the test conditions of 60 min of resting and then 5 min of turning on the vibration were selected. Figure 7 shows the density pattern of tailings under different vibration inertia effects. From Figure 7, it can be seen that the influence of different vibration inertia on the settlement of tailings shows obvious regularity. As the vibration inertia decreases, the settling rate of the slurry first increases and then decreases. The curve of the liquid level height of the No. 2 oscillator with time is located at the bottom, indicating that the settling rate of tailings is the fastest at this time. From the perspective of the final settlement time, the second oscillator takes significantly less time than other oscillators, i.e., 0.158 g·cm 2 . Under the vibration inertia, it only takes 210 min for the tailings to reach the final settlement. The final mass concentration of tailings slurry settlement is obtained through calculation. As the vibration inertia decreases, the final mass concentration first increases and then decreases, reaching 0.158 g·cm 2 . Under the action of vibration inertia, the final mass concentration reaches its maximum, reaching 70.1%.
It can be found that the final thickening time is the shortest and the final mass concentration is highest under the action of the vibration inertia of the second oscillator. It can be found that the final thickening time is the shortest and the final mass concentration is highest under the action of the vibration inertia of the second oscillator.

Settlement Law of Tailings with Different Initial Concentrations under Vibration
Fixed vibration frequency (6000 r/min) and vibration inertia (No. 2 oscillator) were used to conduct vibration densification tests under different mud layer thicknesses. During the experiment, weigh 1.0 kg, 1.5 kg, 2.0 kg, 2.5 kg, and 3.0 kg of tailings, pour them into a thickener, and then add diluted water to the mark of 450 mm. Record the mass of water added to obtain the mass concentration of the slurry (see Table 4). After stirring evenly, natural settlement is carried out first. After 60 min of natural settlement, vibration is turned on to observe the height of liquid surface settlement and draw the vibration concentration law of the slurry under different mud layer thicknesses, as shown in Figure 8.

Settlement Law of Tailings with Different Initial Concentrations under Vibration
Fixed vibration frequency (6000 r/min) and vibration inertia (No. 2 oscillator) were used to conduct vibration densification tests under different mud layer thicknesses. During the experiment, weigh 1.0 kg, 1.5 kg, 2.0 kg, 2.5 kg, and 3.0 kg of tailings, pour them into a thickener, and then add diluted water to the mark of 450 mm. Record the mass of water added to obtain the mass concentration of the slurry (see Table 4). After stirring evenly, natural settlement is carried out first. After 60 min of natural settlement, vibration is turned on to observe the height of liquid surface settlement and draw the vibration concentration law of the slurry under different mud layer thicknesses, as shown in Figure  8.  From Figure 8, it can be seen that the lower the slurry concentration, the faster the settling speed. The settling speed at 1 kg of tailings is much higher than other concentrations, and the settlement is basically completed in about 150 min, and complete settlement is achieved at 180 min. Under natural settling conditions, the slurry with an initial concentration of 24.6% had the fastest settling speed, settling 233 mm within 60 min, while the slurry with an initial concentration of 55.3% only settled 41 mm. After the vibration is activated, the settling speed of the tailings is less affected. Only the slurry with a high initial concentration is weakly affected, indicating that the vibration has a smaller impact on the settling of the tailings. This may also be related to the fact that the vibration is only From Figure 8, it can be seen that the lower the slurry concentration, the faster the settling speed. The settling speed at 1 kg of tailings is much higher than other concentrations, and the settlement is basically completed in about 150 min, and complete settlement is achieved at 180 min. Under natural settling conditions, the slurry with an initial concentration of 24.6% had the fastest settling speed, settling 233 mm within 60 min, while the slurry with an initial concentration of 55.3% only settled 41 mm. After the vibration is activated, the settling speed of the tailings is less affected. Only the slurry with a high initial concentration is weakly affected, indicating that the vibration has a smaller impact on the settling of the tailings. This may also be related to the fact that the vibration is only activated for 5 min. As the initial concentration increases, the final thickening time also gradually extends, and the complete settling time for 1 kg of tailings is about 240 min. However, due to the lack of comparative experiments on the presence or absence of vibration at different initial concentrations, this experiment only demonstrates the settling patterns at different initial concentrations. Figure 9 shows the relationship between different initial concentrations of slurry and final mass concentration. As the initial concentration increases, the final mass concentration gradually decreases. The slurry with tailings content of 1.0, 1.5, 2.0, 2.5, and 3.0 kg settles completely, and the mass concentration increases by 69.8%, 42.8%, 29.6%, 20.0%, and 12.3%, respectively. As the initial concentration increases, the thickening effect actually decreases. This indicates that when using a rake-free paste thickener, for low concentration tailings, it has a better thickening effect.
Minerals 2023, 13, 1072 14 of 16 activated for 5 min. As the initial concentration increases, the final thickening time also gradually extends, and the complete settling time for 1 kg of tailings is about 240 min. However, due to the lack of comparative experiments on the presence or absence of vibration at different initial concentrations, this experiment only demonstrates the settling patterns at different initial concentrations. Figure 9 shows the relationship between different initial concentrations of slurry and final mass concentration. As the initial concentration increases, the final mass concentration gradually decreases. The slurry with tailings content of 1.0, 1.5, 2.0, 2.5, and 3.0 kg settles completely, and the mass concentration increases by 69.8%, 42.8%, 29.6%, 20.0%, and 12.3%, respectively. As the initial concentration increases, the thickening effect actually decreases. This indicates that when using a rake-free paste thickener, for low concentration tailings, it has a better thickening effect.

Conclusions
This research independently designed and developed a small vibration densification testing machine, which used the final mass concentration and final densification time of tail mortar settlement densification as experimental investigation indicators. Experiments were conducted on the densification settling characteristics of tail sand slurry under different single factors such as vibration duration, start time, vibration frequency, and vibration inertia. The following main conclusions were obtained: (1) Mechanical vibration can effectively accelerate the settling rate of tailings particles, but this effect is nonlinear. In this experiment, the best effect was achieved when the action time of mechanical vibration was 5 min. With the delay of vibration starting time, the final mass concentration first increases and then decreases, and the effect of starting vibration is most significant when the natural settlement is 45 min.

Conclusions
This research independently designed and developed a small vibration densification testing machine, which used the final mass concentration and final densification time of tail mortar settlement densification as experimental investigation indicators. Experiments were conducted on the densification settling characteristics of tail sand slurry under different single factors such as vibration duration, start time, vibration frequency, and vibration inertia. The following main conclusions were obtained: (1) Mechanical vibration can effectively accelerate the settling rate of tailings particles, but this effect is nonlinear. In this experiment, the best effect was achieved when the action time of mechanical vibration was 5 min. With the delay of vibration starting time, the final mass concentration first increases and then decreases, and the effect of starting vibration is most significant when the natural settlement is 45 min. (4) The lower the slurry concentration, the faster the settling speed. Under natural settling conditions, the slurry with an initial concentration of 24.6% has the fastest settling speed. As the initial concentration increases, the final thickening time is also gradually prolonged.
Author Contributions: Conceptualization, W.L. and K.Z.; methodology, F.G.; Experimental design and data curation, F.G. and Z.P.; writing, review and editing, Z.P.; funding acquisition, W.L.; investigation, X.G. All authors have read and agreed to the published version of the manuscript. Data Availability Statement: All the data can be found in this article.