Grinding and classifying automatic control
Grinding and classifying automatic control

This article describes the latest beneficiation automatic control technology that sinonine has mastered.

Part2:Grinding and classifing automatic control

Grinding process in a mineral processing plant is generally divided into two categories: self grinding (semi self grinding) and conventional grinding (rod mill, ball mill). Each type of process includes different grinding processes. Figure 1 is a semi autogenous grinding mill + ball mill+hard stone crushing process. Figure 2 is one conventional closed circuit ball mill process

semi autogenous grinding mill + ball mill+hard stone crushing process.

Fig1. semi autogenous grinding mill + ball mill+hard stone crushing process

one closed circuit ball mill process

Fig2. one closed circuit ball mill process

1.Grinding mill feeding control

No matter it is a semi self mill or a ball mill, a smooth, continuous and uniform feeding condition is needed to make the energy transfer in the grinding process even, achieving a better grinding effect and reducing the wear of the mill liners. Therefore, the grinding control of the mill is the most important and basic control. The grinding process of the mill has the following characteristics:

(1) The grinding machine is instantaneously given the amount of ore entry, and it is measured in real time through an electronic belt scale.

(2) The semi self grinding mill usually selects heavy apron feeder, which is arranged in parallel with multiple blanking ports along the ore yard. Generally, the ball mill generally adopts vibrating feeder, which is symmetrically arranged at the bottom of one or more fine material hoppers.

3) The electronic belt weigher is installed near the inlet of the mill. The feeding equipment is installed at the bottom of the ore pile or the powder mine. It is far from the entrance of the mill. Therefore, the control of the mill has the characteristic of large lag.

(4) The dischaging speed is positively related to the motor speed of the feeding equipment, but each device will change the correlation between the feed volume and the motor speed due to the height of ore accumulation, the loose degree of ore and the particle size of the ore.

The ball mill feeding control principle shown in Figure 3, the electronic belt scale mill feed belt will measure instantaneous ore value transmitted to the distributed control system (DCS), and to mine the set value, the controller according to the calculated set frequency vibration feeder, up to a frequency conversion controller; a feed opening corresponding to the position less than the lower limit, the control system must close the corresponding feed feeding equipment, and other high level selection of feeding equipment.

ball mill feeding control principle

Fig3. ball mill feeding control principle

Semi self mill feed from the crushed ore and rock is composed of two parts, stone crushing ore quantity change is one of the main interference factors of the volume of ore mill. The control principle shown in Figure 4 is the total amount of ore into the mill semiwhite control as outer loop control loop to ore feed control, stone crusher feeding equipment as the auxiliary control and adjustment measures.

Semi self mill feeding control principle

Fig4. Semi self mill feeding control principle

2. Grinding water supply control

There are two water adding operations in the grinding process, one is to add water to the mill and also called water supply, the other is to add water to the pump pool, also called the water supply after the grinding. The effect of adding water to the mill is to form a reasonable grinding concentration, and urge the qualified granularity products to flow out of the mill conveniently, leaving enough space for the grinding ore. The function of filling water in the pump pool is to adjust the concentration of the hydrocyclone, and to ensure the reasonable distribution of the overflow granularity of the cyclone. The electromagnetic flowmeter and regulating valve of the feed water supply water supply pipe are composed of a closed loop control loop, as shown in Figure 5.

The difficulty of grinding water supply control is the choice of setting value. Usually, mill feed water is used to calculate the set value of mill water supply by the set value of mill feed rate and the proportion coefficient method, so it is called proportional feed water control. The water supply of the pump generally adopts the artificial setting method.

Grinding water supply control

Fig5. Grinding water supply control

3.Ball adding of ball mill control

Steel ball is an important grinding medium. If the amount of steel ball is too large, it will occupy much more effective space of ball mill, resulting in a decrease in ore handling capacity. If the steel ball is too small and the grinding degree of the ore is not enough, the cycle load will increase and the energy consumption will increase. Therefore, the control of adding ball is an important part of grinding control. Generally, the amount of ball added in accordance with the amount of ore is controlled according to a certain proportion, that is, proportional plus ball control, as shown in Figure 6. The actuator that controls the amount of the ball is an automatic ball - adding machine. It is an automatic device that controls the amount of steel ball added by the speed control device.

Steel ball adding control

Fig6. Steel ball adding control 

4. Level control of the mill pump pool

The ball mill pump pool is the intermediate buffer of grinding and classifing, and the effect is to reduce the influence of the fluctuation of the grinding process on the classifing. Monitoring the pump pool level of the ball mill can dynamically reflect the full performance of the current buffer action of the pump pool. The level is too high, that the buffer capacity of pump tank was excessive overdrafts, once beyond the upper limit will result in pulp running failure and production safety; level is too low, that the buffer capacity of pump pools have not been fully played out, grinding fluctuation will affect the classification stability of frequent production, resulting in the grinding process is not stable.

The main factors that affect the level of the pump pool are the amount of the slurry in the ball mill, the discharge of the slurry pump, and the water supply of the pump pool. There is no on-line measurement for the concentration and concentration of the pulp in the ball mill, and it is uncontrollable for the classification process. In order to stabilize the concentration of the hydrocyclone, it is not allowed to regularly adjust the water supply of the pump pool. Therefore, the main means of adjusting the level of the pump pool is to adjust the slurry pump to control the discharge of the pulp. The principle of pump pool level control is shown in Figure 7. The sump level control can be used to control the interval: first determine the interval sump level permitted fluctuation limit, when the liquid level reaches the upper limit or lower limit, the slurry pump need to adjust quickly, will be back to the middle position level adjustment range as soon as possible; when the level in the middle position after a period of time, even if the level fluctuations, as long as do not exceed the limit, do not adjust the slurry pump speed.

principle of pump pool level control

Fig7. principle of pump pool level control

5.Feeding Concentration control of Hydrocyclone

The feed concentration of hydrocyclone for work efficiency has great influence, therefore the production by adjusting the pump pool of make-up water to achieve the cyclone gives the ore concentration adjustment, which adopts the cascade control scheme, the outer ring is concentration control, inner control loop pump pool water addition, as shown in figure 8.

Feeding Concentration control of Hydrocyclone

Fig8. Feeding Concentration control of Hydrocyclone

6. Classification pressure control of hydrocyclone

Hydrocyclone is a separation device which uses inertia centrifugal force and particle size difference. The stable cyclone pressure is the key to ensure the classification effect. The hydrocyclone works mostly in the form of the swirler group, and each swirler is controlled by the switch and valve to enter the grading work. The hydrocyclone group itself is an important regulating device. By switching the number of swirler and changing the working load of each cyclone, it can adjust the working pressure of the hydrocyclone. However, because of the inability to achieve continuous regulation, the swirler switching control is more applied to the larger fluctuation of production load and beyond the working capacity of a single cyclone.

The feeding slurry pump of hydrocyclone is the most important regulating device for the working pressure of the cyclone. It has good real-time performance, sensitive reaction and continuous adjustment. The pressure transmitter is installed at the top of the hydrocyclone to detect the working pressure of the cyclone and adjust the speed of the slurry pump to control the working pressure, as shown in Figure 9.

Classification pressure control of hydrocyclone

Fig9. Classification pressure control of hydrocyclone

From Figure 9, it can be seen that the contradiction between the working pressure control of the cyclone and the level control of the pump pool. In order to buffer the fluctuation of pulp discharge volume, the pump pool level is bound to change the speed of slurry pump to maintain the pulp level. According to the need of steady working state, we hope that the speed of slurry pump is best and stable. The coordination of this contradiction is the main task of the hierarchical control.

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