Abstract: This paper introduces the application of frequency converter in high-pressure leaching feed pump. Keywords: frequency converter; PID; automatic control. Pressure leaching is the main process in the Bayer process for producing alumina, and the production of alumina by the Bayer process accounts for 70% of the alumina output of our plant. Therefore, the pressure leaching process plays a very important role in the alumina production process and is a key link in our plant's alumina production. We know that when the leaching time is constant, the faster the leaching rate, the higher the alumina leaching rate and the greater the production capacity. This not only yields a leaching solution with a lower caustic ratio but also relatively reduces steam consumption. Therefore, the ratio of various materials in the leaching reaction must be within a certain range. To ensure a high alumina leaching rate, the relative balance of various materials must be ensured. Under the condition of relative changes in other conditions, the feed rate of the slurry pump must also be adjusted accordingly. For the transmission system, this is achieved by adjusting the speed of the motor to adjust the piston stroke of the feed pump, thereby realizing the reasonable adjustment of the feed rate. 1 Problems in the original system Our plant's pressure leaching feed pump system has six units, all of which are wound-rotor asynchronous motors. Based on the characteristic of adjusting the speed using series resistance in the rotor circuit of a wound-rotor asynchronous motor, this speed regulation method essentially changes the slip rate by altering the slip power consumed in the external series resistance of the rotor, thus achieving speed regulation. This is an energy-intensive speed regulation method. Furthermore, it suffers from a high system failure rate, requires significant maintenance and repair work, and incurs high maintenance costs, impacting equipment uptime and the continuity of production. Given the shortcomings of this speed regulation system, to optimize the production process, we designed a simple and practical automatic control system. The composition of the control system is shown in Figure 1. It includes a frequency converter, pipeline pressure sensor, slurry flow sensor, and peripheral control circuits. The frequency converter is the key equipment in the automatic feeding control system. It drives the motor and uses its own PID control function to achieve automatic control of the material quantity. We selected the Mitsubishi FR-F540L series frequency converter. It features high output, low noise, multi-functional control (PID control), high-performance speed and torque control, superior dynamic and static performance indicators, and comprehensive protection functions such as overvoltage, overcurrent, overload, and grounding protection. 2. System Composition and Control Method 2.1 System Composition The pressure sensor converts the pump outlet pipeline pressure into a standard 4-20 mA signal, which is then sent to the secondary display instrument, allowing operators to constantly monitor the pipeline's working pressure. To ensure safe pipeline operation, a pressure indicator with live contacts is also installed on the pipeline to immediately shut down the system in case of overpressure. The pressure transmitter is the E+H PMC133, the display instrument is the SMZ series digital display instrument, and the local pressure gauge is the Y150 series Bourdon tube pressure gauge. The slurry flow sensor is a key detection element of the control system. It converts the detected slurry flow rate into a standard 4-20 mA signal, which is then sent to the secondary instrument for display and participates in the control. The flow meter used is the Japanese Ganghe AE2 series electromagnetic flow meter for flow detection. 2.2 System Working Principle Based on the slurry flow rate requirements of the production process, the frequency converter is started, and the pump unit begins operation. The electromagnetic flowmeter converts the detected slurry flow rate into a 4-20mA standard signal, which is sent to the digital display for display and control. When the pipeline flow rate is greater than the given value, the frequency converter's output frequency is automatically reduced through the PID control, thus decreasing the pipeline flow rate. When the pipeline flow rate is less than the given flow rate, the frequency converter's output frequency is automatically increased through the PID control, thus automatically increasing the pipeline flow rate. This adjustment process continues until the pipeline flow rate and the given flow rate are consistent. The dynamic control principle is shown in Figure 2. Based on the system's dynamic and static performance indicators, PI control, PD control, or PID control can be flexibly selected. When the deviation x (given value - feedback value) is positive, the execution quantity (output frequency) is increased; when the deviation is negative, the execution quantity is decreased. The execution process is shown in Figure 3. 2.3 System Protection To ensure the safe and reliable operation of the system, in addition to the conventional electrical protection of the equipment, we have also installed detection, action, alarm, and operating systems to ensure the safe operation of the system. When the pipeline pressure exceeds the limit, an audible and visual alarm is issued, and the operator can decide to stop the system operation according to the situation. At the same time, we have also installed a Y150 local pressure gauge with electrical contacts on the feeding pipeline. When the pipeline pressure exceeds the allowable pressure, the electrical contacts open, the frequency converter is de-energized, the system stops operating, and the audible and visual alarm system issues an audible and visual alarm. 3 Conclusion Since its commissioning two months ago, the high-pressure leaching automatic control system has operated reliably and stably, with flexible and convenient control, high accuracy, and high precision. It has fully ensured the accurate feeding of the feed pump, greatly optimized production indicators, increased the alumina leaching rate in the slurry and the hourly capacity of the equipment, and achieved significant economic benefits. Furthermore, due to the advanced system settings, high level of automation, and high system reliability, the equipment's uptime has been improved, providing equipment and control guarantees for increased alumina production. The application of variable frequency speed control technology ensures smooth and continuous speed regulation, achieving energy saving; the improvement and optimization of equipment technology level has greatly reduced equipment maintenance work and maintenance costs, reduced the cost of unit products, and generated good economic benefits.