Density of the suspension is determined according to the requirements of the fine coal ash indicators. However, due to the influence of fluid motion in the sorter, the density of the suspension is different from the actual sorting density. For the lump coal heavy media sorting machine with rising medium flow, the suspension density is generally lower than the actual sorting density by 0.03 -0.1g/cm3; if a heavy medium cyclone is used, as mentioned above, the suspension density is more than the actual fraction. The density is chosen to be 0.2 ~ 0.4 g/cm3. The magnitude of the required density difference, in addition to the viscosity, stability, raw coal particle size composition and density composition of the suspension, for the lump coal heavy medium sorter, the density difference and the horizontal medium and the rising or falling medium flow The size is related. When the descending medium flow is large, the actual sorting density may be slightly lower than the density of the suspension. For heavy medium cyclones, the difference in density is related to the feed pressure, liquid-solid ratio (ratio of the amount of suspension to the amount of material selected), and the structural parameters of the cyclone.
In daily production, controlling the density of the suspension is an important part of achieving good process results. There are two methods for checking the density of the suspension. One is manual inspection. The physical density of the suspension is determined by weighing a certain volume of the suspension, that is, it is measured by a concentration pot, and can also be measured by a densitometer. However, this method of inspection is laborious and difficult to adjust in time after inspection. Another method is automatic detection by instruments. The devices used are dual-tube differential pressure densitometer, water column equilibrium densitometer and radioactive density meter. The primary signal obtained by these devices is converted into an electrical signal by an electronic device, transmitted to an actuator, and the density of the suspension is maintained in a stable state by adding water or supplementing the weight.
(1) Double pipe differential pressure densitometer
Figure 4-4-59 is a schematic diagram of a dual tube differential pressure meter. In the suspended liquid 1 to be tested, two pressure measuring tubes 2 inserted in different depths are placed. The compressed air having a pressure of 0.015 MPa enters the two pressure measuring tubes through the throttle valve 7, and the compressed air is filled in the double tubes, and the pressure at the tube end enters the suspension. Due to the action of the throttle valve, the pressure in the tube will be balanced with the pressure of the heavy medium at the end of the tube. The lateral side hose at the upper end of the long tube is connected to the sealed water tank 4, and the lower portion of the water tank communicates with the bottoms of the indicator tube 5 and the electrode 6. The lateral side hose 3 of the short tube is connected to the indicator tube 5 and the upper portion of the electrode 6. Therefore, the long and short tubes constitute a closed circuit. Due to the different depths of the piezometer insertion, the static pressure of the suspension at the nozzle will also differ. For example, h1 and h2 are set. The insertion depth of the long tube and the short tube, respectively, the pressure at the end of the long tube 
, the pressure at the end of the short pipe p2= . Since the pressure p1 at the end of the long pipe is greater than the pressure p2 at the end of the short pipe, the water level in the water tank is pushed down, indicating that the water column in the pipe rises by H height. When the pressure reaches equilibrium, there is
In the formula, [h1h1 - h1h2] is a fixed value, so that the water column height H in the pipe can be used to indicate the density of the suspension. Obviously, the greater the difference between h1 and h2, the higher the sensitivity at the time of measurement. If the device is used in a lump coal heavy medium sorting machine, the pressure measuring tube can be directly inserted into the sorting tank, and the average density of the suspensions in each layer of the tank can be determined at will. In the normal production process, the pressure measuring tube is placed in the sorting zone.
If a heavy medium cyclone is used for coal preparation, a part of the qualified medium can be taken out and the density of the suspension can be measured in a special measuring box.
The double tube differential pressure suspension density automatic measuring device is composed of three parts: the first part is the measuring part. It detects the change in density at any time, and when the suspension deviates from the specified density value, it will sound an audible and visual signal. The liquid level in the electrode tube rises or falls in parallel with the water column in the indicator tube, so the resistance between the two electrodes changes with the rise and fall of the liquid surface; the second part is to send the rise and fall of the resistance between the electrodes to the potential amplifier 8 and pass the amplifier identification control. The adjustment system; the third part is the actuator, which is indicated by the amplifier to add water or increase the weight according to the change of the density of the suspension.
The device is used to control the density fluctuation range of the suspension, which does not exceed ±0.015 g/cm3.
(2) Water column balance density meter
Figure 4-4-60 Water column balance density meter device schematic
1-Medium tank; 2-media tube; 3 - Confluence tank; 4-nozzle; 5-pressure water tank; 6-water pipe; 7-mechanical tank overflow pipe; 8-electrode;
Figure 4-4-60 is a schematic diagram of a water column equilibrium densitometer. The densitometer also uses the differential pressure principle to determine the density of the suspension. The heavy medium to be tested is removed from the large-sized material through the fixed sieve of the medium tank 1 and then enters the medium pipe 2, and then flows out from the nozzle 4. The clean water of the constant pressure water tank 5 also flows through the water pipe 6 through the nozzle 4. 3 is the confluence tank of the suspension and the clean water. In order to maintain the suspension in the medium tank at a certain height h, excess medium entering the tank flows out of the overflow pipe 7. The mixture of water and suspension flowing from the nozzle 4 is removed from the dilute medium drum.
According to the principle of liquid column balance, when both water and suspension flow out from the nozzle at the same time, the pressure of the water column and the pressure of the suspension must be flat.
Visible when the density of water When the water column height h is fixed, H will change with the change of the density of the suspension, which is proportional to the relationship. Therefore, the level of the water surface reflected on the scale 9 indicates the density of the suspension.
Suspension density changes of water from a measured equilibrium density meter, the electrode 8 by an electric signal converted by the sense amplifier enable transistor electromagnet in the magnetic amplifier starts operation, the barrel of the suspension density of the medium is adjusted (or supplemented Clean water or add magnetite powder] to keep the suspension density at the specified value. The control system of the water column balance density meter is shown in Figure 4-4-61.
(3) Radioactive density measuring instrument
The radioactivity density meter is also called an isotope density meter. The principle is to indicate the density of the suspension based on the degree of absorption of the gamma rays as they pass through the suspension.
The source is placed on the side of the tube containing the suspension stream, and the receiver that measures the intensity of the gamma ray is placed on the other side of the tube. ^ After the radiation passes through the suspension in the pipeline, a portion of the radiation will be absorbed by the suspension, and the amount of absorption is proportional to the density of the suspension. At this time, the density of the suspension during the work can be determined according to the total radiation intensity measured by the receiver.
[IV] Suspended Wave Density Automatic Control System
Figure 4-4-62 is a commonly used automatic suspension density control system.
After the densitometer 1 measures the density of the controlled suspension, the signal is fed to the automatic control box 2. If the measured density does not match the required density, the difference in density forms a signal that is amplified and sent to the actuator. When the density difference is very weak, the actuator will fine-tune after obtaining the signal. If the difference in density is large, for example, the density of the tested suspension is low, that is, the acceptable medium is unqualified due to the lower density. At this time, the signal passes through the control box, and the variable flow box 3 is instructed to increase the flow rate, and more media that have passed the qualified and unqualified are sent to the thin medium barrel 6 to participate in the concentration and purification. Return to the qualified media bucket 5 again. If the suspension density has not increased, the partial flow rate is further increased until the suspension density reaches a qualified value. At this time, the traffic is returned to normal. If the density of the suspension fed into the magnetic separator is high, the signal enters the control box, and the water valve 4 is commanded to be added to the qualified medium tank. The density of the suspension in the qualified medium barrel begins to gradually decrease. At this time, the signal is again Through the control box, the water valve is instructed to reduce the amount of fresh water added, and the water valve stops supplying water until the density of the suspension in the qualified medium tank reaches the specified value and is truly qualified.
The liquid level automatic measuring instrument is arranged in the medium barrel and is considered together with the automatic density control. During the production process, the liquid level in the dilute medium tank should be kept at a medium position. In other words, there should be a maximum and a minimum limit for the liquid level in the tank.
When the density of the suspension reaches the specified value, the level of the qualified medium tank generally depends on the total amount of magnetite powder in the system. The liquid level is too low, indicating that the total amount of magnetite powder is too small, and new magnetite powder should be added; if the liquid level is too high, the density of the qualified suspension becomes small and becomes unqualified, and the partial flow should be increased to concentrate. At this time, the action of density and liquid level automatic control is consistent.
In short, the function of the automatic control system is: once the density of the suspension in the qualified medium tank is increased, the fresh water is immediately added; once the density is lowered, the partial flow rate is increased and concentrated. The liquid level is low, and new magnetite powder is added; the liquid level is high, and the flow rate is increased. In the normal production process, the liquid level is generally stable, and the fluctuation of the suspension density is small, so the liquid level change is difficult to reflect. Density and liquid level automatic control systems should have the ability to quickly adjust the density when the density of the suspension in the acceptable media barrel varies widely.
For dual-density systems, all magnetically selected concentrates from both density systems can be fed to high-density media barrels and some of the high-density suspension can be split into low-density barrels to adjust the density of low-density suspensions.
The quality of the density automatic control system works not only related to the working effect of the measuring and control device, but also related to the process flow, the size of magnetite powder loss and other factors. For example, when the raw coal is too much water or drowning in a qualified medium drum, it can be found that the liquid level of the suspension in the qualified medium barrel rises too high, the density of the suspension drops greatly, and the density fluctuation is excessive, the magnetite powder is lost. increase. At this time, if the automatic control system is used alone, the problem cannot be solved. For example, when the loss of magnetite powder is too much, the automatic control system is very frequent, but the medium is difficult to stabilize, and the liquid level often drops. This should take other measures to fundamentally eliminate the excessive loss of magnetite powder. The hidden dangers are difficult to achieve by relying solely on automatic control systems.
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