Automatic control system of hydraulic classifier

In the high purity sand washing plant, the hydraulic classifier is mainly used to remove the particle size of -0.1 mm, so as to ensure the particle size composition of fine sand. Generally speaking, the particle size of -0.1 mm should not exceed 5% (Wt) in high-quality silica sand. In order to achieve this goal, hydraulic classifier must be adopted. Because the chemical composition of silica sand is closely related to the grain size, practice has proved that only by controlling the grain size can the chemical composition and its fluctuation index be controlled. Therefore, hydraulic classification is not only a means to control the grain size composition, but also a means to control the chemical composition. In this sense, hydraulic classification is one of the most important links in the production of high quality silica sand.

   
For quartz sand, which is a light mineral, hydraulic classifier can also be used for coarse-grained classification, such as the size classification above 0.71 mm. In this case, the water consumption will be doubled, the pulp concentration in the classification area will be greatly increased, and the effect of interference settlement will be more obvious.
1.Technical route and scheme choice of hydraulic classifier
When the hydraulic classifier works, the sand in the tank will float or sink slowly under the action of hydraulic pressure, that is, the flowing sand. The density value of the slurry reflects the amount of sand in it. If the value is too large or too small, it will affect the grading effect. Therefore, the stability of the sand slurry density is the key to the production of qualified products by the hydraulic classifier（1) Because the density of the mortar in the hydraulic classifier is unevenly distributed up and down, in order to collect the density value of the key working points and improve the sensitivity, the differential pressure sampling method with small distance is adopted. (2) In order to reduce the influence of temperature, two consistent pressure sensors are used to reduce the output signal phase deceleration, so as to improve the detection accuracy. In the control, the fuzzy plus PID control mode is selected, which takes into account the dynamic response characteristics and the stability of the steady-state work, so that the electric actuator has the fast response ability and the minimum number of actions.
2.Working principle of hydraulic classifier
The settling velocity of particles with different particle size and density in fluid is different. The settling velocity of particles with small particle size and density is slow, while that of particles with large particle size and density is fast. There are two different forms of particle settlement: (1) Free settlement means that a single particle settles independently in a relatively wide space. At this time, particles are not affected by other factors except gravity, buoyancy and resistance. When the distance between particles is large and the cross-section occupied by particles is less than 3% in the total cross-section area, the interference between particles becomes very small, which can be regarded as free settlement（2) The interference settlement is the settlement of individual particles in the particle group, the scattered suspension composed of groups of particles and medium, the collision between particles and the increase of the average density of suspension reduce the settlement velocity of individual particles. In addition to the free settlement, the interference settlement also has the additional effect caused by the existence of the surrounding particles, which is mainly manifested in two aspects: one is the increase of the flow velocity of the medium. When the particle group settles down, it forces the medium to surge up along the gap between the particles and between the particles and the vessel wall to form an upward flow, which increases the flow resistance of the medium; The other is that the physical density of the suspension composed of solid particles and medium will be greater than that of the dispersion medium, and each particle will be subjected to greater buoyancy than that of the dispersion medium. Under the influence of these two additional actions, the settlement velocity of interference is much lower than that of free settlement. In upwelling flow, particles with different particle sizes and densities can be classified by controlling the upwelling velocity. When the upwelling velocity is constant, the pulp density is the main factor to determine the grading particle size and grading effect.
The mixture of sand and water is fed from the upper part through the feeding trough and moves downward in the tank under the action of gravity. In the lower part of the tank, there is a porous water spray plate, the new water will rise evenly, the rising water will interact with the falling solid particles to form a fluidized bed layer gradually, and the particles with different particle sizes will fall down or move up at the interference settling velocity of the volume concentration. Therefore, it is very important to maintain the stability of pulp density. But in the actual production, the factors such as ore feeding amount, particle size composition of ore feeding, water feeding amount and pressure, and ore discharging speed are changing with time, so it is difficult to keep stable by manual operation. With the help of the electronic pressure sensor, the pressure of the fluidized bed can be measured and the opening of the discharge valve can be automatically adjusted, so as to ensure the accurate classification according to the required separation particle size and make the product stable and uniform.
The hydraulic classifier is mainly composed of feeding trough, tank, spray plate, overflow weir, pressure sensor, electric discharge valve, controller and other components. Two pressure sensors installed on the key working point of hydraulic classifier collect two-point pressure values, send them to the computer to calculate and get the density value, send them to the digital display and compare with the set density value to complete the fuzzy plus PID, that is, intelligent PID control algorithm, send the adjustment amount to the electric actuator, control the opening amount of rubber valve and stabilize the slurry density. The output of silica sand can be controlled according to the current density and valve opening size.

3.Automatic control system of hydraulic classifier
The automatic control system of hydraulic classifier is designed according to the unstable system with large pulp change and serious random interference in the classification process, and uses ASIC and digital display technology. Compared with the conventional PID control instrument, it fully embodies the characteristics of simple structure, reasonable, intuitive display, excellent performance, low price, practical, trouble free and long service cycle. And because of its unique design of measuring probe, it is stable, reliable and easy to install. Because the static and dynamic characteristics of the automatic control system are better. It can be allowed to run in the fully automatic working state and enter the normal operation state soon. The density, valve opening and cumulative production are all digital display; Output cycle accumulation, special key view, digital cycle display is 9999.999t, one meter multi-purpose. It can not only adapt to the rapid change of large lag system, but also continuously adjust the displacement, which is conducive to the stability of the product. The unique control software can open or close the valve in tens of seconds according to the large range mutation of density. The control action frequency is very low in steady state, which extends the service life of electric actuator and valve. The changes of various process parameters are mainly reflected in the pulp concentration, so how to measure and control the concentration is the core of the problem. Laser corrected diffused silicon pressure sensor is used in the automatic control system of hydraulic classifier, which has the characteristics of good consistency, sensitivity and reliability. The two sensors sample at different heights at the same time, which can compensate the influence of environmental conditions and process fluctuations, and can be flexibly adjusted to obtain the best sampling point and the density value of the best height range, which can reflect the key role of the key position. The automatic control instrument is the heart of the automatic control system, which takes the high-performance single chip microcomputer as the core to complete the functions of signal processing, digital display and intelligent PID control. It has the characteristics of small volume, strong function, low power consumption, flexible and convenient parameter change and simple operation. The actuator adopts the matching method of electric actuator and electric hose valve, which is simple, reliable and easy to replace.

The whole automatic control system of hydraulic classifier is composed of sensor, controller, operator and electric actuator. The system diagram is shown in the figure. When the pulp concentration in the classifier exceeds the set range, the electric actuator will start to act and adjust the opening degree of the valve: when the concentration is too high, the opening degree of the valve will increase, which will speed up the discharge speed; When the concentration is too low, the opening of the valve will decrease, which will slow down the discharge speed. Therefore, the pulp concentration in the classifier can be kept stable and the classification effect can be guaranteed.

The detection unit adopts laser corrected diffused silicon pressure sensor, which has good consistency, sensitivity and reliability. Two sensors sample at different heights at the same time, which can compensate the influence of environmental conditions and process fluctuations, and can be flexibly adjusted to obtain the best sampling point and density value within the best height range.
Technical index of automatic control system of hydraulic classifier

Examples of application effect of automatic control system of hydraulic classifier
The results show that the fine powder content of quartz sand products is obviously reduced and the stability of quartz sand is improved significantly after the hydraulic classifier is equipped with automatic control system. The average content of -0.105mm in the feed of hydraulic classifier is 34.99%, and after the classification treatment of hydraulic classifier, the average subdivision reaches 8.56%, and the average decrease is 4.97% after adding the automatic control system. Mean square deviation also decreased from 2.82% to 1.32%
In addition, the classification efficiency has been improved. The average classification efficiency of hydraulic classifier without automatic control is 73%, and 80.5% after adopting automatic control system.
At present, most of the hydraulic graders in sand washing plants adopt the automatic control system, which has achieved good results.