Introduction

Vertical roller mill is widely used in cement raw material, fuel and cement grinding system, and is equipped with feeding and discharging system. However, the feeding and discharging system often has an unsatisfactory effect. For this reason, we analyze and deal with the common problems of the system and solve the problems.

01 Process flow

The feed discharge system of the vertical roller mill is composed of feeding belt conveyor, metal detector and three-way valve (see Figure 1), which is used to remove iron and other metal substances in the grinding material, so as to protect the grinding disc, roller and stabilize the production of the system:

Fig. 1 flow chart of feed discharge system of vertical roller mill

For the uncleaned iron and other metal substances, the metal detector will detect and send a signal to the three-way valve. The three-way valve will discharge the materials containing iron or other metal substances on time (do not feed into the vertical mill).

02 Common Questions and Causes

Table 1 shows the common problems and causes of feed discharge system of vertical roller mill.

Table 1 Common problems and causes of feed discharge system of vertical roller mill

Common problem	Cause analysis
Metal material cannot be detected, the sensitivity of the external metal detector is too low or the interference of the three-way guide valve does not operate.	The signal strength of metal detector is weak due to high sensitivity or interference
Frequent alarm and discharge	Metal detectors are too sensitive or disturbed
The material discharged is not material containing metal	The three-way valve does not work on time

03 Solutions

In view of the above problems, our solution is mainly from two aspects: one is the correct equipment selection; Second, reasonable arrangement and correct installation of metal detector.

3.1 Correct equipment selection

(1) Selection of three-way valve. In order to ensure the rapid response of the system, the three-way valve should be pneumatic, and the switching time should be controlled in 3 ~ 5s. Therefore, the three-way valve should be equipped with a large air inlet pneumatic solenoid valve, so that the compressed air into the cylinder time is shorter. For large valves, double cylinders (both ends of the valve) should be provided.

(2) The selection of metal detector. The metal detector with appropriate sensitivity is selected according to the maximum metal block that the mill can bear. Too low sensitivity is easy to lead to large metal material into the mill and cause damage to the grinding wheel roller; too high sensitivity is easy to lead to false alarm too high and frequent discharge, resulting in frequent changes in the amount of material in the mill, material layer instability.

Metal detectors generally have three limits of sensitivity, respectively, the metal detector can detect the diameter of the metal ball is 10%, 5% and 2% of the height of the detection coil hole. For example, the belt width is 800mm, the detector coil hole height is 250 ~ 300mm, the limit sensitivity is 2%, it is expected to be able to detect the minimum diameter of 5 ~ 6mm of the metal ball.

(3) The selection of feeding belt machine into the mill. The feeding belt machine into the mill should choose low speed (usually <1.0m/s), frequency conversion belt machine, because the metal detector should take into account the detection of materials from the batching station and to stand on its own grinding slag material. Due to this limitation, the metal detector can only be arranged near the discharging end of the human mill feeding belt conveyor. Considering the operation time of the three-way valve, the belt speed of the belt conveyor must be limited. For example, when the metal detector can only be arranged in the distance from the grinding feeding belt machine discharging end 5.5m, the pneumatic three way valve action time is 5s, then it must be required to feed the grinding belt machine with speed <1.0m/s, to ensure that the three way valve on time, to ensure that the material discharged by the metal detector is the material with metal.

3.2 Reasonable layout and correct installation of metal detector

The metal detector coil support structure and coil must be properly installed to ensure the detector to work normally. Since the metal objects near the coil are easy to lead to signal strength weakening, in order to avoid signal weakening, the following equipment layout and installation conditions should be met:

(1) there should be no metal objects within the scope of 1.2m near the coil of the metal detector, such as metal beam, cover plate, skirt plate, return roller, etc. (except conveying roller and longitudinal steel structure beam of conveyor).

(2) The spacing between the rollers in the coil area of the metal detector shall not be less than 1200mm. If it cannot be satisfied for some reason, the rubber buffer roller can be used to replace the roller in the original position.

(3) The control device of the metal detector is installed in a non-vibrating position within 3m from the coil of the metal detector.

(4) Other considerations include: cover plates or skirt plates on either side of the metal detector coil support structure must be removed or replaced with non-metallic materials. The feeding chute must be at least 2.4m away from the coil metal detector support structure. Electromagnet equipment (except iron, high-power motor, frequency converter, etc.) must be at least 5.5m away from the metal detector.

04 conclusions

To sum up, in order to ensure the stable operation of the feeding and discharging system of the vertical roller mill and the accurate action of the three-way valve, the correct selection of the feeding belt conveyor, metal detector and three-way valve in the feeding and discharging system of the vertical roller mill should be carried out, and the reasonable arrangement and correct installation of the metal detector should be carried out.

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