Higher concentration is beneficial for the Paste and Thickened Tailings (PTT) operation in metal mine. Partial paste thickeners are produced lower density underflow. Flocculated tailings are intended to form a water entrapped network structure in thickener, which is detrimental to underflow concentration. In this study, the continuous thickening experiment was carried out for ultra-fine tungsten tailings to study the influence of rake shearing on underflow. The micro pores structure and seepage flow in tailings bed before and after shearing are studied by CT and simulation approach to reveal the shearing enhancement mechanism of thickening process. The results shown that, the underflow concentration is increased from 61.4 wt% to 69.6 wt% by rake shearing in a pilot scale thickener, the porosity decreased from 46.48% to 37.46%. The entrapped water discharged from sticks structure more than sphere spaces. In items of seepage, after shearing, the seepage flow channel of tailings underflow is becoming longer, which caused the decreasing average flow rate decreases and absolute permeability. The absolute permeability is negatively correlated with tortuosity. The rake shearing can destroy the flocs structure; change the effective stress to increase the concentration. Higher underflow concentration improves the waste recycling and water recovery rate, especially for arid areas.
The metal minerals processing industry produces million tons of fluidized tailings annually [
The gel-like tailings flocs produced by polyacrylamides entrapped water inside the honeycomb network structure [
Furthermore, the flocs compaction behavior upon shear is also a key factor to improve the raking thickener performance [
The shearing lead to particles restructuring and reorganization of the flocs, the connection of pores structure can produce the channels for drainage water. The complex dewatering behavior is occurring through the flow channel [
However, there appears to be no combined study of rake shearing and water seepage processes for high solids volume fraction flocculated tailings thickening bed. The process of flow channel formation, development and annihilation need to be further investigated. Meanwhile, visualization of floc microstructure is beneficial for the explanation of the interaction between the pore structure and the flow channel. This information observed in this study can, in turn, help in the water recovery, tailings waste recycling, heavy metals ions solidification, and underground cemented paste backfill operation.
The dynamic shear test was carried out on a pilot-scale continuous thicker, as shown in
The unclassified tailings were from Xianglushan Tungsten Mine in Jiangxi Province. The tailings were neutral to alkali, and high porosity. The physical parameters are shown in
Material | Specific gravity(t/m³) | Bulk density |
Porosity |
Natural repose angle (°) |
---|---|---|---|---|
Tungsten Tailings | 2.992 | 1.392 | 34.659 | 42.997 |
The flotation tailings of tungsten ore are ultra-fine, which is difficult to be dewatered and thickened. The average particle diameter is less than 0.03 mm; more than 50% particles are finer than 0.019 mm; less than 10% particles larger than 0.074 mm is; less than 30% larger than 0.037 mm. The particle size distribution (PSD) is shown in
The scanning device is a phoenix v high-precision industrial micro-CT scanning system (
The original CT image need to be de-noised [
The Feret Diameter (FD) is used to measure the irregular pores and characterize the pore geometry [
The pore geometry parameters are calculated by the pore contour perimeter P, μm, area A, μm2, Feret Diameter (FD), etc., where FDmax is the maximum length of single pore and FDmin is the maximum width of single pore, μm. The pores shape could be quantitatively analyzed by morphological parameters such as concavity (CV), roundness (RN) and Aspect Ratio (AR) [
The CV is a measure of the topological properties of particle projection, which is the ratio of the area of particle projection to the area of convex hull polygon. The CV can represent the smoothness of pore profile, follow the
RN is a measure of the ratio of the particle’s projected area to the area of a circle with a diameter equal to the longest dimension, reflecting the extent to which the aperture is nearly circular. RN reflects the degree of pores close to the circle, follow the
The AR is the maximum ratio between the length and width of a bounding box. AR represents the overall ductility of the pore, the larger the AR value, the thinner the pore, follow the
The porosity is the ratio of pore voxel value to total value in the 3D reconstructed model [
The shearing can make tailings bed compacting, which is beneficial for reducing the porosity of underflow. Parameters CV, RN and AR can characterize the morphological of the single pore, as shown in
The bed porosity of before and after shearing is higher in the range of 0.45–0.5 and 0.95–1, the content is 32.09% and 37.51% of total pores, respectively. After shearing, the porosity reached 14.45% in the range of 0.45–0.5 and 0.95–1.0. The CV distribution indicates the pores are mainly shape of round and elliptical, the shape is sensitive to the rake shear.
The RN before and after shearing are mostly in range of 0.8–1, accounting for 45.28% and 35.85%, respectively. Shearing reduces 0.8–1 RN pores quantity by 20.82%, but the 0–0.8 RN pores shape is not sensitive to shear. The RN distribution indicates pores have acceptable roundness, approximately half circular shape. The data show that shear reduces the content of circular pores.
The AR reflects the ductility of pore geometry. The peak values of pore AR before and after shearing are 2.0 and 3.0, respectively. After reaching the peak value, the number of pores decreases rapidly with the increase of AR. When AR greater than 3.5, pores quantity under before/after shearing reaches 19.57% and 27.21%, respectively. As a result, shearing makes the pores more slender.
The PNM represents the topological structure between pores in bed, as shown in
As shown in
Experiment condition | Before shear | After shear |
---|---|---|
Model size | 1 mm × 1 mm × 1 mm | |
Spheres quantity | 238 | 202 |
Sticks quantity | 480 | 343 |
The position and geometrical parameters of spheres or sticks are keys to the quantitative description of pores microstructure. The results of sphere radius, stick radius and length, and coordination number is shown in
(1) The radius of spheres after and before shearing is the majority in the range of 20–30 μm, 28.16% and 32.15% respectively. The average radius is 27.80 μm and 32.52 μm with/without shearing, respectively, which is reduced 14.51% by shearing.
(2) The coordination number (CN) represents the connectivity of pore spaces, which is positively correlated with the connectivity. The average CN of the spheres with/without shearing is 9.38 and 10.15, respectively, reduced 7.6% by shearing.
(3) The radius of the larger-scale stick body can be reduced by shearing, resulting in a significant decrease in stick radius. The average stick radius was 18.63 μm and 15.07 μm, before and after shearing, respectively, which was reduced 19.11% by shearing. The stick radius distribution is concentrated in the range of 5–10 μm and 0–5 μm, 19.87%, 25.90% of the total, respectively.
(4) Shearing increases the length of stick space, as shown in
The shear conducts significant influence on the stick structure rather than the sphere space. The spheres and sticks quantity were decreased by 15.13% and 28.54% by shearing, the average radius was reduced by 14.51% and 19.11%, respectively. The results of morphology and topology indicated that the rake shear made the pores loose, the connectivity deteriorated, and the porosity decreased. The shearing force changes the floc network structure, a large-size pores evolve into several small pores by an external force. The porosity of tailings bed decreases and the underflow concentration increases, indicating that the water entrapped in pores is discharged upward, the source of drainage is mainly from throat (stick) spaces.
The pore network in tailing thickening bed is the storage space and flow channel of moisture. The seepage can be assigned to the PNM to predict the reverse direction seepage in the tailings bed porous media [
It is crucial to select the optimal pores sample connectivity to ensure the smooth flow simulation results. Assuming that the seepage flowing in PNM is an incompressible fluid, the inflow and outflow of the connecting pore throat should follow the law of conservation of mass, that is:
The flow formula between any of the two adjacent pores i and j can be derived based on the Hagen-Poiseuille law, as follows:
From Hagen-Poiseuille’s law, the flow formula between two adjacent pores i and j can be derived as follows:
Where, G is the seepage fluid flow rate, μm/s, L is the length of flow channel, μm, P is the seepage pressure, Pa. The pore connectivity of the full flow region is obtained by calculating the average connectivity of the whole spaces between the two pores i and j.
Based on
The pure water was setting as the flow medium, density ρ = 1000 kg/cm3, viscosity μ = 0.001 Pa∙s. The inlet pressure was 1.3 × 105 Pa; outlet pressure was 1.0 × 105 Pa. The bottom boundaries is setting as the velocity inlet, the top is the pressure outlet boundary. The side boundary and wall are regarded as the non-slip wall surface. The simulation, controlled by the combination of Darcy’s law, was conducted to obtain the absolute seepage velocity and flow channels in bed.
The water seepage flow channel was identified and found that the pores and water seepage flow channel have significant relevance. The tailings bed before shear contained a large amount of liquid (blue color in
In sheared tailings bed, the entrapped water volume is reduced obviously in
The rake shear directly affects the distribution and connectivity of the flow channel, the internal pores of tailings are zigzag, and the liquid seepage flow channels are less, the water has been effectively discharged, the pressure drop mainly occurs at both ends of the test piece; the streamline distribution is sparse, arranged intermittently, the dominant channel disappears, and the permeability is poor. This is due to the evolution of pore structure and the uniform filling of pore space by tailings particles, resulting in the compression of an effective channel. At this time, the channel’s water conductivity is weak, indicating that a large amount of pore water is discharged during the shearing disturbance.
The tailings thickening bed is a porous medium [
where: Lt is the length of the curved line, μm; L0 is the length of the straight line of the medium, μm.
From the
Sheared period/h | Before shear | After shear |
---|---|---|
Porosity/% | 46.48 | 37.46 |
Tortuosity of flow channels | 1.813 | 2.240 |
Average channel diameter/μm | 41.32 | 28.57 |
Absolute permeability/10-3μm2 | 13.702 | 9.931 |
The flow channel evolution process is containing four stages in the shearing condition. Firstly, the formation stage of the flow channel, the flocs and pores structure were changed by the rake and pickets, the isolated pores connected with surrounding flocs to form branch channel, several branch channels connected to form a major seepage channel. Secondly, the reverse direction seepage stage, at the moment of channel connection, the entrapped water flow upward along the open channel by hydrostatic pressure. Thirdly, the pore network re-organization stage, after the water discharged, the tailings particles occupied the original water spaces, which caused the re-organization of flocs network, the throat of channel tend to be narrow, the major channel evaluated back to the branch channels. At last, the channel disappears stage, the rake shear impacts the flocs structure denser, the pores and channels would be blocked by tailings, the flow channel annihilated. As a result, the tailings bed concentration is increasing, the thickening process arrived a new stage.
(1) The gravity shearing dewatering process can be explained as the flocs and pores structure changed by the rake and pickets, the isolated pores connecting to form seepage channel. The entrapped water flow upward along the open channel by hydrostatic pressure.
(2) Adding 2 rpm rake shear can increases the underflow concentration from 61.4 %wt to 69.6 %wt, decreases the porosity from 46.48% to 37.46%. In the morphological analysis, the pores are mainly shape of round and elliptical, the shape is sensitive to the shear. The microstructure of pore was identified into sphere space and stick space by PNM. The rake shear reduced the quantity of spheres and sticks by 15.13% and 28.54%, respectively, the water released from the pores of the bed is more from the stick space rather than sphere.
(3) The shearing evolution of the seepage flow channel can be divided into four stages, the formation stage of the flow channel, the reverse direction seepage stage, pore network re-organization stage, and the channel disappear stage. After that, the tailings bed is increasing to higher concentration to fulfill the Paste and Thickened Tailings requirement.