Application of frequency converter in centrifugal slurry pump in mine

1. Introduction

A mining unit is a nonferrous metal mining unit under the State-owned Geological Bureau, which provides one-stop services from ore mining to ore dressing and processing. The mined lead and zinc ores are first crushed by a jaw crusher, and then evenly sent to a ball mill through an elevator and a feeder after being crushed to a reasonable fineness. The ball mill crushes and grinds the ore. The fine ore ground by the ball mill enters the classification process. The spiral classifier uses the principle that the solid particles have different specific gravities and different precipitation speeds in the liquid to wash and classify the ore mixture. When the washed and classified mineral mixture passes through the magnetic separator, the magnetic substances in the mixture are separated by magnetic and mechanical forces due to the different specific magnetic susceptibility coefficients of various minerals. The mineral particles that have been initially separated by the magnetic separator are sent to the flotation machine, and different drugs are added according to the different mineral characteristics to separate the desired minerals from other substances. Taking environmental protection factors into consideration, the waste liquid containing slag after sorting is sent to the slag slurry pool, and then sent to the tailings pond by a slurry pump. After the slag is separated from the liquid, the liquid is recycled.

At present, most of the slurry pumps used in the beneficiation plants of major mining areas are centrifugal slurry pumps. Their structure is basically the same as that of centrifugal water pumps, but they are very different from ordinary water pipe networks in terms of pipelines. Because the liquid flowing in the pipe network contains the waste slag after the ore is ground by the ball mill, the wear on the pump body and the pipe wall is large, which requires the pump body and the pipe network to have a high wear resistance, so the inner wall of the pump body and the pipe network wall of the slurry pump must be processed by special processes. Generally, the terrain of the tailings pond is slightly higher than the installation position of the slurry pump. Therefore, there is a check valve at the output end of the pipe network to prevent the slurry from flowing back when the pump stops. In terms of design, the flow rate of the slurry pump should be greater than the amount of waste liquid discharged into the slurry pool. If the slurry pump is running all the time, the slurry in the slurry pool may not be pumped enough to cause the pump to run idle; but the slurry pump should not be allowed to run intermittently, which will cause the slag to precipitate. Therefore, there is a valve at the output end of the pump to adjust the amount of slag discharge, and the flow rate of the slurry pump can be adjusted according to production needs. The opening and closing degree of the valve needs to be adjusted by the operator according to the amount of slurry generated during the production process. Similarly, when the valve is opened too wide, the slurry in the slurry pool may not be pumped out enough, causing the pump to run idle; if the valve is opened too small, the slurry pool will be filled and overflow. Regardless of the flow rate of the slurry pump, the motor runs at full speed. Although it is simple to use a valve for throttling adjustment, it increases the damping of the pipeline and wastes electrical energy.

2. Frequency conversion speed regulation scheme

In order to save energy, the inverter is used to control the speed of the slurry pump. It only needs to open the valve to the maximum and the damping of the pipe network is the minimum. The liquid level signal of the slurry pool is used for automatic control to adjust the frequency of the inverter. At present, there are two types of sensors commonly used for liquid level control. One is to use a pressure sensor to convert the liquid depth signal of the slurry pool into a 4-20mA analog liquid level signal through a transmitter; the other is to use a liquid level relay. The liquid level relay has three contacts of high, medium and low, corresponding to the high, medium and low liquid levels in the slurry pool. The former is more expensive and can accurately control the liquid level at a certain height; the latter is cheaper and can only control the liquid level in the slurry pool to change between high, medium and low. In order to save costs, the liquid level relay is selected as the liquid level sensor, which can fully meet the production process requirements.

The liquid level in the slurry pool of the ore dressing plant is detected by using a liquid level relay. The high, medium and low nodes of the liquid level relay are connected to the input contacts X1, X2 and X3 of the frequency converter, and the control process is as follows: when the liquid level is below the "low level", the frequency converter operates at a frequency of 25Hz; if the flow rate of the slurry pump is less than the input amount of the waste liquid, the liquid level in the slurry pool gradually rises, and when the liquid level rises to the "low level", the frequency converter operates at a frequency of 38Hz; if the flow rate of the slurry pump is still less than the input amount of the waste liquid, the liquid level in the slurry pool continues to rise, and when the liquid level rises to the "middle level", the frequency converter operates at a frequency of 50Hz, and the liquid level in the slurry pool generally does not exceed the "high level". At this time, if the flow of the slurry pump is greater than the input of the waste liquid, the liquid level of the slurry pool will gradually decrease. When the liquid level drops to the "middle position", the inverter will operate at a frequency of 38Hz; if the flow of the slurry pump is still greater than the input of the waste liquid, the liquid level of the slurry pool will continue to decrease. When the liquid level drops to the "low position", the inverter will operate at a frequency of 25Hz. It can be seen from the above process that the switching frequencies are different when the liquid level rises and falls and passes through the high, middle and low levels, which avoids short-term repeated switching at the switching point due to the fluctuation of the liquid level, that is, oscillation.

When selecting a frequency converter, the load characteristics of the slurry pump must be taken into consideration. That is, the slurry waste liquid contains a large number of impurities with a specific gravity greater than that of water. The frequency converter cannot use a fan or water pump type special frequency converter. A general frequency converter with constant torque mechanical characteristics should be selected. Therefore,

Considering that the efficiency of the frequency converter is 95%, the energy saving rate of the slurry pump after frequency conversion speed regulation is about 41%. After the transformation is completed, the energy saving rate is measured by active power meter, which is 40%, which is not much different from the calculation. The calculation shows that the frequency conversion system after the transformation saves 40% of energy compared with the original power frequency system, and the investment can be recovered within a one-year operation cycle.

4. Other effects after transformation

1) Since frequency conversion can achieve soft starting of the motor, the impact on the power grid and machinery when the motor starts is eliminated or reduced, thus extending the life of the equipment.

2) After using frequency conversion speed regulation, the speed of the slurry pump slows down most of the time, reducing the wear of the waste liquid medium on the pump, valves and pipelines. Before the frequency converter was changed, the outlet valve of the pipeline network had to be replaced every 3-5 months. After using frequency conversion control, it was only replaced once a year, and the maintenance cost was greatly reduced.

3) Previously, there were people specifically responsible for supervising and operating this equipment. After switching to frequency conversion control, it achieved automatic operation and unattended operation, saving labor costs.

V. Conclusion

The mining centrifugal slurry pump control system is composed of Senlan SB70 inverter, liquid level relay and auxiliary circuit. It can realize automatic operation and unattended operation of the system. It has the advantages of simple operation, convenient maintenance, small impact on the power grid and mechanical system, safe and reliable operation, and obvious energy-saving effect. It can be used in slurry pump equipment in major mines and mineral processing plants, providing another example for energy saving and consumption reduction of slurry pumps.