Therefore, the moving velocity v of the thin layer of fine particles at the point of X=X v is determined and substituted into the general formula (6), that is, the theoretical formula of the total separation particle size of the fine sieve is
Whether the total separation particle size formula of the above fine sieve can meet objective reality is still to be further verified. However, it will play a certain role in researching and guiding fine screening production.
C Production Management Practice
At present, domestically applied vibrating nylon fine screens are generally produced by cams that produce a self-flowing suspension vibrating fine screen parallel to the screen surface. After the structure is determined, the most important thing is to make the fine screen fully function. The most important thing is to choose the working parameters correctly. According to the production practice, the following aspects should be noted when selecting parameters:
(1) Shock force and number of shocks. Shock is the key to preventing clogging of the screen. The weight of the hammer, the hitting height, the number of beats, and the direction of the knock (vertical or parallel to the screen surface) that directly produce the impact greatly affect the size and grade of the undersize. It is known from practice that stopping the vibration can play a role in improving the grade under the sieve in a short time, but the yield under the sieve is obviously reduced, resulting in an increase in the circulation amount on the sieve, easy to produce a vicious cycle or a decrease in the processing capacity of the ball mill . Excessive vibration and too many vibration times not only affect the service life of the entire striking mechanism and screen frame, but also destroy the steady flow state of the screen surface slurry, resulting in low grade under the screen. In production, as long as the screen hole is not blocked. Generally, the hammer weight is 1kg, the hitting height is 200-250mm, the number of tapping is 5-20 times/min (the stainless copper screen surface is 30-35 times/min), and the tapping is parallel to the screen surface.
According to the US patent, in general, when the hammer is given a torque of 1.35 to 3.40 N · m, the number of strikes is 1 to 10 times/min. For a fine sieve having a width of 457 mm and a length of 1200 mm, the weight is 0.56 kg. When the hammer falls from a height of 330 mm, it is effective to strike 2 times per minute. When the screen surface is too long, two striking devices are installed on one screen frame to improve the screening efficiency. Two or more pads are mounted on one screen frame, and sometimes the hammers can be staggered on the rotating shaft at 90 or 180 degrees.
(2) Screen size and installation angle. The width of the sieve surface only affects the number of sieves used, and the length of the sieve surface and the installation inclination angle affect the grade of the sieve under the sieve and the yield under the sieve. Usually when using a fine sieve, the length is 1500~1800mm, and the second-stage fine sieve is 1200mm when used in series. Because the two-stage fine sieve series can meet the production requirements, at this time, the installation height difference is 5~7m. Fixing in the sieve hole The angle of inclination determines the size of the separation. The dip angle is large, the yield under the sieve is small, the grade is high, and vice versa. It can be selected from 45° to 65°, and is generally around 55°.
(3) The screen surface material and the screen strip are broken and shaped. The material of the sieve bar is required to have the following components: 1) good wear resistance; 2) good dimensional stability, mainly in low water absorption, long-term contact with water (pulp), stable mesh size, and no deformation of the sieve surface;
(4) Certain mechanical strength; 4) Wide source of raw materials, which can be produced in batches. Both stainless steel and nylon 1010 meet the above requirements. Practice has proved that the nylon 1010 sieve plate is superior to the stainless steel sieve bar, and the nylon sieve plate has the advantages of wear resistance, low cost, and easy meshing of the mesh hole, and the service period is 6 to 12 months.
The cross section of the sieve strip has three kinds of rectangular, right-angled trapezoid and isosceles trapezoid. Test and production practice shows that the use of isosceles trapezoidal section is more beneficial to the improvement of fine screening efficiency. Figure 13 is the size of the domestic nylon sieve plate;
The section of the domestic nylon sieve plate from Fig. 13 is similar to the section of the stainless steel sieve, but the edge of the section is sharper than the stainless steel, which is more favorable for screening. At the same time, it should be pointed out that the cross-sectional shape of the sieve plate is very important for the fine screening operation. A is wide and the effective screening area is small; B is small, and the strength of the sieve surface is insufficient, which affects a service life. Generally, A takes about 2mm, B takes about 5mm, and C takes 7°~15°.[next]
The fine sieve structure is simple, and it is easy to operate after the structural parameters are determined. Years of production practice has shown that the following points should be noted in the operation and maintenance of fine screening:
(1) Demagnetization before screening. The sieving materials are all concentrating concentrates, so demagnetization is necessary. The demagnetization effect is not good. Not only can it be classified according to the particle size and density when sieving, but the iron strips connected to the sieve plate are easy to sense, and “magnetic clusters†are generated around the iron bars, which reduces the effective screening area and causes the sieve. Hole plugging.
In production, if the magnetic separation concentrate is pumped by a rubber pump for a long distance and after several times of ore separation, and the concentration of the slurry is about 40%, the demagnetizer may be unset. When the magnetic separator is connected to the fine screen, it is necessary to install a demagnetizer. According to the production experience, when designing the demagnetizer, the choice of magnetic field strength or demagnetization time should be slightly higher than the conventional value.
(2) Reasonable control of the on-screen yield. The yield of the sieve on the fine sieve directly affects the processing capacity of the ball mill, the final concentrate grade and the production order, so it must be reasonably controlled during the operation. Reasonable values ​​for each selected plant should be determined by experiment. Generally, the yield on the sieve is preferably 25 to 30% of the amount of the fine sieve. If the ore grade is below 30%, the grade of the sieve can reach 57-60%, and the yield on the sieve can be increased to 30-50%.
In the separate regrind process, the first and second stage fine sieves are pre-classified, and the sieve yield can be controlled at 30-50%. The three-stage fine sieve is used for control classification and re-grinding machine to form a closed circuit, and reasonable sieve production. The rate should account for about 50% of the fine sieve feed.
In order to ensure that the yield on the sieve is controlled within a certain range, the grade and particle size of the sieve must be strictly controlled. Especially in the plant where the cyclone is used, it is necessary to prevent the cyclone from being overloaded, the grain size is coarsened, and the grade of the sieve is low. The large amount causes an increase in the yield on the sieve, which is prone to a vicious cycle, which makes the whole production confusing and the indicator deteriorates.
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(3) Establish maintenance operation: 1) Screen surface assembly. The fine sieve screen surface is required to be assembled during assembly: 1 Each sieve plate should be selected before the fine sieve assembly, and the sieve plate is not in conformity with the specified sieve plate; it should not be assembled; 2 when the sieve plate is assembled, the perforated iron (copper) The rods should be vertical, the thickness should be uniform, the connection between the sieve plates should be flat, and no steps can be produced. The screws or pins at the ends of each piercing rod should be balanced and not too loose or too tight to ensure that the screen surface does not deform when used. 3 When the screen surface is assembled with the screen frame, the rubber strips should be pressed on both sides of the screen surface. Prevent the slurry from flowing out from the two measurements, affecting the screening effect; 4 The installation of the screen frame should be placed on the support point of the support frame, so that the contact surface is small, and the movement effect is good when the tapping is easy; 5 the tapping device should be perfect, and the buffer spring should have elasticity. The height difference between the hammer and the striking plate should be adapted; 6 the inclination angle of the ore-mining plate should be parallel with the screen surface to prevent the phenomenon of waterfall-like ore flow; 7 the curtain of the unloading end of the sieve should be adapted to the length of the installation. It should be replaced in time to prevent the mixing of the products under the sieve and the quality of the unloading. 2) Maintenance. The fine screen operation and maintenance requirements are as follows: 1 The ore supply of the sieve should be uniform, and no overload should be given to the mine. According to the amount of ore and the ore concentration (generally 35-50%), adjust the number of sieves to prevent vicious circle and destroy the production balance; 2 when the sieve is not used, rinse it, put down the hammer and keep a good state to be opened; 3 The grade and particle size of the sieve should meet the design requirements. If the requirements are not met, it should be contacted and adjusted in time to ensure the quality of the product; 4 often check the beat device to maintain good operation. It is necessary to perform periodic beating according to the design regulations. It is strictly forbidden to stop tapping for a long time during production to prevent the screen hole from being clogged; 5 the screen surface wear should be replaced regularly. When the sieve is used, the entire screen surface is unevenly worn, and can be used according to the situation, or the bad sieve plate can be removed for individual replacement. On the premise of ensuring the quality of screening, the use of sieve plates is saved. 3) Clearing. Figure 14 shows the relationship between the operation time of the first and second stages of the fine separation of the Dagushan Concentrator and the yield under the sieve. It can be seen from the curve that it is necessary to rinse the sieve every 10 to 15 days to ensure that the undersize yield is about 60%. If the undersize yield is only required to be around 50%, the sieve surface may not be cleaned. The factory's practice is: flush the screen surface with a water pipe every day after the succession, clean it three times a month, the product is separated from the broken sieve, and the back of the sieve is washed once when the curtain is replaced. Each selection plant can determine the cleaning cycle according to the characteristics and indicators of the ore that it processes. In the past, cleaning the screen surface required to remove the sieve one by one and rinse it with water, which is time consuming and laborious. Later, the factory specially designed a spray gun for cleaning the screen surface (Fig. 15). When cleaning, the sieve surface should be stopped to feed the mine, and the high pressure wind (about 400 kPa) and water of the spray gun can be turned on, and the sieve can be sieved in 1 or 2 minutes. The plug of the hole is rinsed clean. 4) Screen hole detection. The inspection of the sieve hole is an important task for calibrating the processing quality of the sieve plate and the degree of wear of the sieve hole. The instrument such as a polarizing microscope and a reading magnifying glass can be used to detect the wear state of the new sieve surface and the screen surface. A polarizing microscope is a viable test instrument without dedicated testing equipment. The detection method is as follows:
1Select a representative fine sieve screen 1 (10 sieve plates) and 1 to 3 new sieve plates for the sieve surface to be tested;
2 calibrate the polarizer micrometer, use the eyepiece as a 5×9 times polarizer, and observe it block by block and point by point;
3 Prepare a table of identification results and perform wear calculations.
For example, the results of the wear inspection of the screen surface of a factory: the maximum sieve hole of the new sieve plate is 0.2854mm, the minimum sieve hole is 0, 1706mm, and the average is 0.2198mm. The old sieve plate of the old sieve plate is 0.3412. Mm; the minimum mesh hole is 0.2593mm, the flat is 0.2994mm. Compared with the old and new sieve plates, the old sieve plate mesh wear (ie, large) is 0.0796mm, and the screen wear (ie, wear away) is 0.326mm.
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