Hydraulic classification equipment

Hydraulic classification devices selected from coal beneficiation and used in wide variety of hydraulic classification devices are using different mineral particle sedimentation rate in an aqueous medium, the classification process is completed in a gravitational field or a centrifugal field. It is also widely used in industrial sectors such as building materials, chemicals, and food. In the coal preparation plant, the hydraulic classification is mainly used in the treatment process of coal slurry water, including sedimentation, concentration and dehydration, which are auxiliary operations in the coal preparation process. In metal concentrator, hydraulic classification is for classifying raw materials selected to obtain a level of several narrow Takeo twisted material, respectively, to the classifying device reselection mineral or ore kernel ready for reselection.
(I) Mechanical classifier In many hydraulic classification equipment, the classifier with the lifting and transporting grit mechanism is called the mechanical classifier. Like all grading equipment, the mechanical classifier grading process is carried out by the difference in sedimentation speed of the particles in the water medium. The mechanical classifier is mainly used in conjunction with the grinding machine for pre-grading and inspection grading, and can also be used for mud-bearing ore. Washing and de-sludge and dewatering.
According to the different transportation sanding mechanisms, the mechanical classifier can be divided into a spiral classifier, a jaw classifier and a float classifier. The spiral classifier continuously discharges the grit using a rotating spiral. Compared with the latter two, it has the advantages of simple structure, convenient operation, large inclination angle of the classification groove, and the like, and is convenient for self-flow connection with the grinding machine, so the spiral classifier is generally used in production.
According to the number of spirals, the spiral classifier can be divided into a spiral classifier and a double spiral classifier. According to the level of the overflow of the classifier, it can be divided into three types: high-grade, low-lying and sunken.
1. The structure and working process of the spiral classifier The structure of the spiral classifier is shown in Figure 1. The grading groove 2 is arranged obliquely, the inclination angle is generally 12° to 18.5°, and the bottom of the groove is semicircular. The slurry is fed from the feed port 7 in the middle of the tank, and the classification zone at the lower end of the classifier is classified. The fine-grained stage is discharged from the overflow discharge port 9 with the water flow through the lower end overflow 堰8. The coarse-grained product settles in the classification zone and is then discharged by the spiral 1 to the grit discharge port 10 at the upper end of the trough. The screw is mounted on the hollow main shaft 3, and the two ends of the main shaft are supported by the bearing 4. The upper end of the shaft is provided with a transmission device 5, and the lower end of the shaft is placed inside the lifting mechanism, and the height of the spiral in the groove can be adjusted if necessary.


Figure 1 Schematic diagram of the spiral classifier
1. spiral; 2. grading groove; 3. screw shaft; 4. bearing; 5. transmission; 6. spiral lifting structure;
7. Feed port; 8. Overflow raft; 9. Overflow discharge port; 10. Grit discharge port

When the slurry is continuously fed, the overflow and the grit are continuously discharged separately. If the classifier and the grinding machine form a closed circuit, the grit of the classifier enters the grinding machine through the chute and is reground, and the grit returned to the grinding machine is called “returning sand”.
2. Factors affecting the process effect of the spiral classifier The advantages and disadvantages of the classifier process are mainly two aspects. One is the working quality of the classifier, such as the fine-grained content less than the classified particle size in the grit and the larger than the graded grit in the overflow. The coarse-grained content and the moisture level of the grit; the second is the production capacity of the classifier, including the production amount calculated according to the solid content in the overflow and the production amount calculated according to the solid content in the grit.
There are many factors affecting the process effect of the classifier. In summary, there are mainly the following aspects.
1 Ore properties - mainly refers to the density, particle size composition and mud content of the ore. The ore density affects the production capacity of the classifier by mass almost in proportion. The higher the ore density, the higher the production capacity. The effect of the particle size composition and mud content of the ore is reflected in the viscosity of the slurry, the viscosity increases, the sedimentation velocity of the ore decreases, and the accuracy of the treatment capacity and classification is reduced. Therefore, when the sludge content is high, the The method of lowering the grading concentration is adopted, but this in turn leads to a decrease in the throughput. It is not entirely unfavorable for the classification to contain the proper amount of slime in the ore. Because the viscosity of the slurry is increased by the slime, the development of the slurry turbulence caused by the spiral agitation can be suppressed.
2 Classifier structure—The liquid level of the sedimentation of the ore group in the classifier is called the graded area. The size of the graded area affects the classifier's processing capacity and determines the graded granularity. Since the grading process occurs in a horizontal slurry flow close to the surface layer, as shown in Figure 3-4-16. In this horizontal flow, the ore particles, on the one hand, settle down at their own settling velocity, and at the same time are driven by the horizontal medium flow to travel toward the overflow end at a speed close to the horizontal flow velocity u of the slurry. Therefore, its direction of motion is the vector sum of the two velocities. Two ore particles with different particle sizes (assuming the same density and shape) are in the same classification tank, the horizontal flow rate is the same, and the sedimentation speed of the two ore particles is different, so the angle between the combined velocity and the horizontal plane is also different. The β angle of the coarse particles is large, and the end of the operation to the end of the classification tank is blocked by the overflow weir, so it stays in the trough and becomes a grit. The β angle of the fine particles is small. When it runs to the end of the classification tank, it is still located above the overflow weir. The horizontal flow exits the machine from the overflow.


Figure 2 Relationship between the classification area of ​​the spiral classifier and the classification process, the size of the tank and the inclination of the tank [next]

It can be seen from the influence of the classifier area on the grading process that the classifier has a large area and the classifier has a large processing capacity, but the grading granularity is reduced. As shown in Fig. 2, if the grading liquid level is l, the grading groove width is B, the lower end of the grading groove is H, and the inclination angle is a, then the classifier area is A, that is,

It can be seen that increasing the width of the grading groove, increasing the height of the overflow weir or reducing the inclination of the trough can increase the classifier area. As the size of the classifier increases, the processing capacity of the overflow volume increases, and the classification accuracy is finer.
The speed of the spiral affects the degree of agitation of the liquid surface and the ability to transport sand return. The rotational speed is inversely proportional to the diameter of the spiral. For coarse fractionation there may be a greater degree of agitation without affecting the settling of the particles. The coarse fractionation can have a greater degree of agitation without affecting the settling of the particles. For fine particle grading, strong agitation should be avoided, and the spiral running speed is sufficient to transport the return sand along the outer tank.
3 Feeding concentration—The concentration of the ore is not only affected by the graded particle size, but also affects the processing capacity at the graded particle size. Gradation size is often controlled in production by adjusting the concentration. There is a critical value for the effect of concentration on fractional particle size and productivity. On the one hand, increasing the concentration of the slurry, the interference degree of the settlement becomes larger, and the sedimentation speed of the particles will become smaller, so that some coarser particles will be brought into the overflow by the horizontal flow in the future, thus changing the granularity of the overflow. Crude. On the other hand, when the concentration is small, in order to maintain a certain productivity in terms of solids, the overflow flow rate will increase, and the increased horizontal flow rate will bring the coarser particles into the overflow, making the overflow particle size thicker. Therefore, the optimum critical pulp concentration is applied to a certain ore. At this concentration, the solid product productivity is kept constant, and the finest graded particle size can be obtained; while maintaining a certain graded particle size, the maximum productivity can be obtained.
In order to understand the working condition of the classifier, the overflow concentration of the classifier in the production is generally measured every 20~30min to ensure the control of the classification granularity. In order to improve the classification efficiency, in recent years, some concentrators have begun to use screening equipment to replace the coarse-grained grader. In the fine-grain classification, hydrocyclones have been widely used.
(2) Hydrocyclone 1. Overview In the gravity field, since the gravitational acceleration g is fixed at a certain place, the sedimentation speed of the fine particles is limited, and the processing capacity and sorting effect of the equipment are difficult to increase. In order to strengthen the grading and sorting operations, in recent decades, the inertial centrifugal force generated by the rotary flow has been widely used to greatly increase the moving speed of the particles.
There are basically two methods for making the slurry rotary motion in mineral processing. One is that the slurry is fed into the circular sorting container along the tangential line under pressure, forcing it to make a rotary motion. Such a rotary flow has a large thickness, for example This type of cyclone belongs to this type; the other is to use the rotary drum to drive the slurry for circular motion, such as various horizontal centrifugal concentrators and horizontal centrifugal dewatering machines.
The inertial centrifugal acceleration a of the particles in the swirling flow is opposite to the direction of the centripetal acceleration of the synchronously moving fluid, and the values ​​are equal. which is

Where r is the radius of the circular sorter, m;
Ω—the angular velocity of the gyroscopic motion, rad/s;
Ur - the tangential velocity of the gyroscopic motion, m/s.
Therefore the centrifugal force strength is:

Some of the centrifugal forces used in gravity beneficiation can be more than ten times larger than gravity, thus greatly enhancing the sorting process.
The hydrocyclone is a device that uses a swirling flow for grading, and is also used for concentration, de-sludge (also sand removal), and even sorting. Because of its simple structure, easy manufacture, large processing capacity and good process effect, it has been rapidly promoted and applied after its introduction.
The structure of the cyclone is shown in Figure 3. It is mainly composed of a hollow cylinder and a cone. The diameter of the cylinder means that the size of the cyclone is from 125 to 500 mm, which is commonly used by 50 to 1000 mm. An overflow pipe is inserted in the center of the cylinder, and a feed pipe is connected along the tangential direction, and a grit (or underflow) port is left in the lower part of the cone.


Figure 3 Hydrocyclone [next]

2. Principle of classification of hydrocyclone The slurry is fed into the cyclone through a tangential feed port under a certain pressure, thus forming a swirling flow in the cyclone. At the center of the cyclone, the pulp rotation speed is maximized, and the centrifugal force generated is also maximum. As a result of the expansion of the slurry to the surroundings, a low pressure zone is formed around the central axis. At this point, air is drawn through the grit chamber and a low pressure air column is formed at the central axis.
The centrifugal force acting on the ore particles in the cyclone is proportional to the mass of the ore particles, so that when the density of the ore particles is close, the particle size can be classified according to the size (the density is different to obtain the equal-falling particles).
The slurry has both tangential and rotary motions in the cyclone, while the slurry near the center moves in the axial direction (overflow tube), and the peripheral slurry is mainly downward (sanding port). movement. So it belongs to three-dimensional space movement. In the axial direction, the slurry has a zero-speed point in one direction transition, and the points connecting the spaces form an approximately conical surface in the space, which is called the zero-speed envelope surface (see Figure 4). The fine sedimentation speed of the fine particles is small, and is pushed by the centripetal liquid flow into the zero-speed envelope surface to be discharged into the overflow product by the overflow pipe; while the coarser particles are retained by the large centrifugal force and remain outside the zero-speed envelope surface. Finally, it is discharged from the grit chamber and becomes a product of grit. The position of the zero-speed envelope surface roughly determines the hierarchical granularity.
3. Process calculation of hydrocyclone The hydrocyclone process calculation includes calculation of separation particle size and treatment volume. They have many calculation formulas, some formulas are cumbersome and inconvenient to apply; some formulas differ greatly from actual values, so I won't go into details here. The commonly used formulas are listed below.

Figure 4 Schematic diagram of the classification principle of hydrocyclone

(1) The processing capacity of the cyclone Q (L/min)

Where d C and d y — the diameter of the ore pipe and the diameter of the overflow pipe, cm; when the ore is a rectangular section, where d G =, ;b and l are the width and length of the rectangular section of the feeding pipe, cm;
P—the inlet pressure of the cyclone (gauge pressure), MPa;
G—gravitational acceleration, measured in 9.8 m/s 2 ;
K 1 — Coefficient, as determined by the table below.

Table 1   Cyclone, at various K 1 d G / D value condition
d G /D
0.10
0.15
0.20
0.25
0.30
K 1
1.83
2.46
3.10
3.85
4.93
Note: D refers to the diameter of the cyclone.
(2) Separation granularity of the cyclone
The critical particle size d F (cm) of the classification is:
Where h is the axial distance from the lower edge of the overflow pipe to the cone wall, which is 2/3 of the height of the cone;
— - medium viscosity;
Q—The volume of ore slurry. Each physical quantity unit is calculated according to the CGS unit system;
Φ X — the coefficient of change of speed, the value of which is greater than 1.
There are two commonly used Φ X calculation methods:
1Dalyan's calculation formula
Where n = 0.5 ~ 0.9, generally take n = 0.64.
2 Formula proposed by the former Soviet Union Mineral Processing Research and Design Institute
Where A G - the area of ​​the mine mouth, cm 2 ;
D—the inner diameter of the cyclone, cm 2 ;
a — cyclone cone angle, (°); a 0.3 value is shown in Table 2. [next]
Table 2   The value of a 0.3 in Equation 7
a
10°
15°
20°
60°
90°
a 0.3
2
2.25
2.46
3.40
3.85
Factors affecting the operation of the hydrocyclone include structural parameters, operating conditions and ore properties. The diameter D of the cyclone, the feed port diameter dG and the overflow port diameter dy are the main structural parameters affecting the treatment amount Q and the classification particle size dF. By converting formula (4), it can be concluded that the approximate relationship between the volumetric treatment volume of the slurry and the diameter of the cyclone is:

As the slurry flow rate increases, the centripetal flow rate of the slurry also increases, and the graded grain size becomes coarse. Converting Q, dG, and h in equation (5) into the ratio of D, then
Therefore, larger diameter cyclones are often used for coarse grading; small diameter cyclones are used for subdivision. If the latter has insufficient processing capacity, multiple units can be used in parallel.
(1) Influence of the diameter of the mine pipe dG on the operation of the cyclone The size of the ore gate has a certain influence on the processing capacity, separation granularity and classification efficiency. Its diameter is often proportional to the diameter of the cyclone, mostly dG = (0.08 ~ 0.25) D. The cross-sectional shape of the feed port is preferably a rectangle. The profile is often a tangential shape as shown in Figure 5(a). Because of this feeding mode, the slurry is affected by the local vortex when the slurry enters the cyclone and affects the classification efficiency. Therefore, an involute shape and other types of feed pipes as shown in Fig. 5(b) have appeared.


Figure 5 tangential and involute shape feed pipe
(a) tangential; (b) involute

(2) Influence of overflow pipe diameter dy on cyclone operation The overflow pipe size should be proportional to the diameter of the cyclone, generally dy=(0.2~0.4)D. The diameter of the overflow pipe is increased, the overflow flow is increased, the overflow particle size is coarsened, the fine particle size is reduced in the grit, and the concentration of the grit is increased.
(3) Influence of the diameter of the grit port on the operation of the cyclone The diameter of the grit port is often proportional to the diameter of the overflow port, and the ratio is called the pyramid ratio. Test t shows that the ratio of angle to cone is 3~4, which is an effective means to change the graded particle size. The grit port is the most wearable part in the cyclone, and often increases the discharge area due to wear and tear. The sand production increased and the grit concentration decreased. If the grit is too small, the coarse particles will accumulate at the top of the cone, which will cause the grit to block. The change in the size of the grit chamber has little effect on the cyclone processing capacity.
(4) Influence of cone angle on the operation of the cyclone The cone angle affects the resistance of the downward flow of the slurry and the height of the graded free surface. Generally speaking, the subdivision or dehydration cyclone should adopt a smaller cone angle, the minimum is 10 ° ~ 15 °; the coarse classification or concentration cyclone uses a large cone solution, up to 20 ° ~ 45 °.
The height h of the cyclone cylinder mainly affects the residence time of the material in the cyclone, and generally takes h=(0.6~1.0)D. The overflow pipe is inserted into the depth hy, which is approximately close to the height of the cylinder and is (0.7~0.8)h. If it is too long or too short, the overflow will be rough.
(5) The influence of the ore pressure p on the operation of the cyclone The ore pressure is an important parameter for the operation of the cyclone. Increasing the ore pressure and increasing the slurry flow rate can improve the classification efficiency and the concentration of the grit. By increasing the pressure Reducing the graded particle size is insignificant, while the kinetic energy consumption will increase substantially, and the wear of the cyclone, especially the grit chamber, will be more serious. Therefore, when handling coarse-grained materials, low-pressure (0.05~0.1MPa) operation should be used as far as possible; only when dealing with fine-grained and muddy materials, higher pressure (0.1~0.3MPa) should be used.
There are two main ways of feeding the discharger:
1 The pressure tank is fed to the mine - the high difference is piped to the cyclone or the sand pump is used to send the slurry to the high voltage tank and then introduce the cyclone. This ore is limited by the height difference conditions and can only be used during low pressure feeding.
2 sand pump directly to the mine - this feeding method can obtain higher ore pressure, easy to configure, less pipelines, easy to maintain, so it is widely used.
(6) The effect of ore-bearing properties on the operation of the cyclone The most important of which is the grain size composition (including mud content) and the ore concentration. The particle size composition of the ore and the particle size requirements of the product affect the selected cyclone diameter and feed pressure. When the size and pressure of the cyclone are constant, the ore concentration has an important influence on the overflow particle size and classification efficiency. The concentration of the ore is high, the graded particle size becomes coarser, and the classification efficiency will also decrease. When the graded granules are 0.074mm, the ore concentration is preferably 10%~20%; when the graded particle size is 0.019mm, the ore concentration should be 5%~10%.
The optimum working condition of the cyclone used for grading shall be that the grit is sprayed in an umbrella shape, and the center of the umbrella has a small air suction port. This allows the air to carry the fine particles in the inner layer slurry out of the overflow when flowing upward, thereby contributing to an improvement in classification efficiency. At this time, the cone angle of the umbrella should be as shown in Figure 3-4-20. If the cyclone is used for concentration, it can be taken out in a rope shape, and the concentration of grit is the highest at this time. When used for dewatering, the grit should be discharged at the maximum angle of the umbrella. At this time, the concentration of the grit is the lowest, and the overflow with the least amount of solids can be obtained accordingly.
Fig. 7 Schematic diagram of different discharge conditions of the underflow of the cyclone 1. Umbrella; 2. Rope (small outlet); 3. Large cone-shaped umbrella (large outlet)

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