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Concentration process
Concentration is the process of enriching a thinner slurry into a thicker slurry while separating a liquid that contains little or no solid material.
The concentration process of mineral processing products is divided into the following types according to the nature of the main forces on the solid particles in the slurry:
(1) Gravity sedimentation and concentration. The slurry is settled by the action of the gravity field;
(2) Centrifugal sedimentation and concentration. The slurry is settled by the centrifugal force field;
(3) Magnetic concentration. A slurry composed of a magnetic material is aggregated under a magnetic field and a part of the moisture is removed therefrom.
Basic principle of gravity settlement and settlement speed calculation
The monomer (free) sedimentation or aggregate (interference) sedimentation of the particles is not only governed by its own characteristics, such as particle shape, density, particle size composition and composition, but also by temperature, magnetic agglomeration, colloidal effect, heterogeneous flow, The effects of lateral pulsation flow rate, hydraulic sling, mechanical agitation, and drug content. Many experimental studies have confirmed that the sedimentation and concentration process involves a complex combination of physical and chemical effects. At present, the study of concentration theory is limited to the range of gravity sedimentation, that is, based on the sedimentation of solid particles suspended in a liquid.
1. Gravity settlement principle and settlement speed calculation
Initially, the behavior of spherical particles with free sedimentation in suspensions of different concentrations was investigated. There are three main forces that the particles sink in the slurry, namely gravity, buoyancy and resistance. For certain particles and a certain slurry, gravity and buoyancy are constant, while the resistance changes with the relative movement speed between the particles and the slurry. Small particles have a tendency to be dragged down by large particles that settle faster. In the process of sedimentation of uniform particles, the increase of drag force is mainly caused by the increase of velocity gradient, while the viscosity change caused by the increase of solid concentration has little effect on it.
The algebraic sum of the forces acting on the particles should be equal to the product of the particle mass and its acceleration (in accordance with Newton's second law of motion). The sedimentation process of the particles is divided into two phases, an acceleration phase and a constant velocity phase. In the isokinetic stage of sedimentation, the rate of movement of the particles relative to the slurry is referred to as the "settling velocity." Because the settling velocity is the velocity of the particles relative to the fluid at the end of the acceleration phase, it is also referred to as the end-set velocity or "terminal velocity." Since the particles treated by the industrial settlement operation tend to be small, the contact surface between the particles and the slurry is relatively large. Therefore, in the process of gravity sedimentation, the time root of the acceleration phase is short and often negligible.
The relationship between the forces received by the particles and the settling velocity during the gravity settling process is based on Newton's second law. The finishing and the time-series analysis show that the drag coefficient affecting the gravity settling velocity should be the relative motion of the particles and the fluid. The function of the Reynolds number Re. The comprehensive test can obtain the relationship between the drag coefficient of the spherical particles and the Reynolds number Re. The curve can be roughly divided into three zones according to the Re value, that is, the stagnation zone, the transition zone, and the turbulence zone. The curves in each zone are expressed by the corresponding relationship, and the corresponding Reynolds number in the constant velocity sedimentation stage is also replaced by the settlement velocity Ï… t, so that the spheres with smooth surface can be freely settled in the fluid. Settling velocity formula:
twenty four
Stagnation zone 10 -4 <1, ξ=-------
Re
d 2 (δ-Ï).g
v 1 =----------------
18μ
(1)
18.5
Transition zone 1 8, ξ =-----
Re 0.6
(2)
Turbulence area 10 3 ×10 5 , ξ = 0.45
(3)
Where υ t —— the free settling velocity of spherical particles, m/s;
d - particle diameter, m;
δ - particle density, kg / m 3 ;
Ï - fluid density, kg / m 3 ;
g - gravity acceleration, m / s 2 ;
ξ —— resistance coefficient, no dimension, related to the Reynolds number;
μ - the viscosity of the fluid, Pa s. [next]
Equations (1), (2), and (3) are called the Stokes formula, the Allen formula, and the Newton formula, respectively, in the stagnation zone, the surface caused by the viscosity of the fluid. Frictional resistance dominates. In the turbulent zone, the body resistance caused by the boundary layer separation of the fluid at the tail of the particle to form a vortex is dominant, and the viscosity μ of the fluid has no effect on the settling velocity υ t . In the transition zone, both frictional resistance and physical resistance are not negligible.
Free settling occurs when the particles in the fluid are sparse. Therefore, the above-mentioned settling velocity formula must be applied under the condition that the size of the container is much larger than the size of the particles (more than 100 times) to eliminate the significant retardation of the wall of the vessel. Secondly, the particles must not be too small to prevent Brownian motion from colliding with fluid molecules or leaking between fluid molecules to achieve a sedimentation velocity higher than the calculated value. This is why when the Re<10 -4 (the size of the ore particles reaches 0.1 to 0.5 microns), the Stokes formula is no longer applicable.
A slurry composed of fine ore is a suspension. During the sedimentation process, small particles have a tendency to be dragged downward by large particles that settle faster due to the accompanying turbulence in the fluid. The flocculation of fine ore particles also changes the effective size of the particles. Therefore, the sedimentation and dewatering of the slurry generally belongs to interference settlement, in which the large particles are disturbed more, and the sedimentation speed is slowed down; while the small particles are dragged, the sedimentation speed is relatively accelerated. However, tests have shown that for suspensions having a solid particle size that differs by no more than 6 times, all of the particles settle at substantially the same rate. When the concentrator uses the concentrator to dewater, in order to prevent the granules from being too thick and "pressing", it is generally necessary to pre-screen the +0.25-0.8 mm ore in the slurry, so the interference during the sedimentation process is not serious. Furthermore, when the ore product is dehydrated, the ore particles are all required to sink to obtain a clear liquid. Therefore, the sedimentation velocity of the slurry must be calculated according to the final sedimentation velocity of the smallest particles in the grit, and the Stokes formula [Equation (1)] suitable for the smaller Reynolds number range (stagnation zone) is generally adopted. It has a particle size range of up to Re≈1, which corresponds to the case where the ore particles having a diameter of 0.15 mm are settled in water; the finest is about 0.5 μm, which corresponds to the case before the suspension is converted into a colloidal solution. It fails when the particle size reaches 0.5 microns or less. In the specific calculation, it is generally assumed that the settlement belongs to a certain flow pattern, such as stagnation. Use the Stokes formula corresponding to the flow pattern to find υ t , and press υ t to calculate the Re value, and check whether the obtained Re t value is Within the range of 1 × 10 -4 ~ 1.0. If this range is exceeded, the flow pattern should be set separately, and the corresponding other formula should be used to find υ t until the Re t value calculated according to the obtained υ t coincides with the range of Re t values ​​that the formula fits. In addition, the settling speed can also be calculated by the frictional group method which avoids the trial difference by means of the converted curve of the ξ-Re relation curve.
Mineral processing products generally consist of non-spherical natural particles after crushing and grinding. The resistance of the particles during sedimentation is closely related to their shape, in addition to the above factors. The smaller the sphericity (the ratio of the surface area of ​​the particles to the surface area of ​​the sphere of the same volume), the larger the resistance coefficient corresponding to the same Re value, and the lower the sedimentation speed. This effect gradually increases as the value of Re increases. However, the effect of sphericity on resistance in the stagnation zone is not significant. According to the measured data of natural-shaped quartz particles, the Stokes formula is used to calculate the sedimentation velocity of fine-grained beneficiation products. It must be multiplied by the shape coefficient K of the particles (ie, the sedimentation velocity of the ore sediment and the same volume of the spherical sediment) The ratio of the end speed). The K values ​​of different shaped ore particles are roughly: rounded shape 0.78; polygonal shape 0.72; rectangular shape 0.67; flat shape 0.52.
2. Settlement process
When the suspension containing the ore particles of different sizes is precipitated, the coarser ore particles are first settled to the bottom of the container, and the fine liquid forms a turbid liquid, and the sedimentation speed is slow. In the richer pulp, or when the coagulant is used, due to the agglomeration of the ore particles, the larger ore particles drive the smaller ore particles to settle. At this time, the amount of liquid clarified in the upper layer gradually increases, and the slurry in the container gradually appears. The layer phenomenon, that is, divided into four regions from top to bottom, and the size of the ore particles and the concentration of the precipitate gradually increase from the top to the bottom, as shown in FIG. In the figure, zone A is a clarification zone, which has a low solid particle content and a small cohesive force between the particles; zone B is a settling zone, and its concentration is the same as that of the suspension before the sedimentation starts. The solid content of this section increases, and the cohesion between them is greater than the resistance of the solid particles when it settles; the C zone is the transition zone, the cohesive force between the solid particles in the zone increases, and the solid particle content increases accordingly; the D zone is compressed In the zone, the cohesion between the solids is greater, the concentration is higher, and the viscosity between the particles is also increased. As the precipitation process progresses, the D zone and the A zone gradually increase, while the B zone gradually decreases or disappears, and the C zone also disappears. At this point, the slurry is at the critical point of the sedimentation process. After the critical point, only the A zone and the D zone remain.
Figure 1 Â Concentration process diagram
In the continuous operation of the concentrator, the slurry is continuously fed and discharged, and the above four zones are always present. Therefore, the sedimentation velocity of the slurry is calculated based on the sedimentation velocity of the sedimentation zone. The final concentration of the concentrated product is determined by the time the pulp stays in the compression zone. The compression process often takes up most of the entire concentration process. When the feeding and discharging speed of the thickener is constant, the height of the compression zone of the thickener determines the concentration of the bottom stream. Practice has shown that an increase in the height of the compression zone results in an increase in the underflow concentration. However, since the slurry in the compression zone is shifted at a variable speed and the sedimentation speed is small, it is generally not necessary to increase the height of the compression zone to increase the underflow concentration. Therefore, the total height of the clarification zone and the settling zone of the actual produced concentrator is about 0.8 to 1.0 meters. The height of the compression zone shall be determined by experimentation and calculation.