Ball mill load refers to the sum of ball load, material load and water volume in the mill. It is an important parameter of the grinding process and directly affects the effect of grinding. In the actual production process, due to the fluctuation of ore properties and the interference of a series of external factors and the difference in operating level, the load of the ball mill is difficult to maintain at the optimal level, and the ball mill can not be fully utilized. Therefore, the grinding process is automatic. In the control, the detection and control of the ball mill load is the most important content of the automatic control of the ball mill. The ability to accurately detect the load of the ball mill (including the ball load, material load and the amount of water) is the key to the success of the entire ball mill optimization control.
First, the direct detection method of the ball mill load
At the 10th International Mineral Processing Conference in 1973, the Czech Republic Institute of Mines B, Lanstiak reported that there is a functional relationship between the slurry surface of the ball mill and the output of the ball mill. The slurry surface has a close relationship with the load rate of the ball mill. When measuring the slurry surface in the ball mill, the electrode is used as a detector, and the height of the slurry surface in the ball mill can be judged according to the output of the electrode.
Although the direct detection method can obtain satisfactory results, it has the following disadvantages: (1) only the slurry surface in the ball mill can be detected, and the medium filling rate, the material ball ratio and the grinding concentration cannot be detected; (2) the electrode The installation position needs to be solved: (3) The electrode needs to be protected from mechanical damage of the falling ball, and therefore, the electrode is not easily removed. It brings difficulties for maintenance and replacement; (4) The method of detecting data transmission needs to be solved. It is precisely because of these shortcomings that this method has not been promoted and applied.
Second, the ball mill load detection method
Indirect detection of ball mill load is a common method currently used in industrial production.
(1) Single factor detection
(2) Sound method
1. The practice of industrial production shows that the sound of the ball mill has a certain relationship with the material load. The essence of this method is to determine the material load in the ball mill according to the difference between the noise generated by the ball mill. When the material load in the ball mill increases, the ball mill noise intensity decreases. Conversely, the ball mill noise intensity increases, that is, there is a relationship between the ball mill noise intensity and the material load in the ball mill. Therefore, the noise intensity of the ball mill can be utilized to evaluate the material load in the ball mill.
Acoustic detection of ball mill load can be used for both wet grinding and dry grinding, which can improve the grinding capacity and the fineness of the product, and save steel consumption and energy consumption: compared to other methods, measurement The signal is more sensitive. The main problem encountered by this method is that the interference generated by other mills and the position of the sensor must be selected correctly to avoid such interference. In addition, the sound method can only determine the ratio of the material to the ball, but can not determine the filling rate of the medium, nor can it determine the grinding concentration.
2, active power method
The operating characteristics of a wet ball mill can be directly described by the useful power consumed. When the mill and its rotation speed are both fixed, according to theoretical calculations and actual test results, there is a certain relationship between the active power of the ball mill motor and the load of the ball mill. As the load of the ball mill increases, the active power of the ball mill motor increases significantly at the beginning, reaches a certain extreme value, and as the load of the ball mill increases, the active power decreases. This extreme value can be considered as the best point of the ball mill load. Therefore, the useful power is directly related to the ball mill load.
The active power method detects that the signal is less affected by the surrounding environment, and the detection result is more accurate. However, the sensitivity of the detection signal is not high, affecting the control effect, and there are many factors affecting the active power. The active power signal of the ball mill alone can not accurately determine the ball mill. The level of media fill rate. It is also impossible to determine the level of the ball-to-batch ratio: in addition, since the ball mill has a maximum power of active power. Therefore, when the power is lowered, it is difficult to judge whether the ball mill load is increased or decreased based on the detection signal.
3. Vibration method
When the ball mill is running, the grinding body and material are biased to one side of the mill, and continuously roll and fall, impact grinding materials, and the rotating part of the mill is in a serious imbalance, resulting in unbalanced centrifugal force, making the mill system vibration. When the rotating speed of the mill is constant, the vibration intensity is related to the material to be ground. The vibration intensity of the mill can detect the material load in the mill.
The vibration detection signal has high sensitivity, can display the change of material in the ball mill at any time, and is not affected by the surrounding noise source. The sensor is well sealed and can be effectively waterproof and dustproof, so it can adapt to the harsh working environment of the concentrator. However, when the grid voltage, especially the grid frequency, changes, the slight fluctuation of the mill speed, or the loss of the grinding body in the mill, will cause the material load control point to drift, affecting the quality of the detection and control, and the grinding cannot be detected. The size of the concentration and the media filling rate.
4, pressure sensor method
(1) The loader is installed on the ore supply end of the ball mill and the bearing of the discharge end to continuously weigh the mill, thereby determining the total amount of the ball medium, ore and water in the ball mill, and determining the load of the mill. The advantage is that the simplicity and disadvantage are that the detection accuracy is not high, and the respective amounts of the ball medium, the ore and the water in the ball mill cannot be accurately detected. In addition, the wear of the ball mill liner, the change in ore properties, and the change in weight before and after the addition of the steel ball will have a relatively large impact on the test results.
(2) A research institute in South Africa used a method of installing a pressure sensor in a mill to determine the load inside the mill, and provided a new method for determining the load of the mill. The method was installed on the lining of the mill. A sensor capable of measuring radial pressure and tangential pressure, the information measured by the pressure sensor is introduced into the microcomputer through conduction, and the microcomputer can process the signal to obtain a spectrogram, and the ratio of the ball load to the material load can be known through the analysis. The transmission of the pressure sensor and the state of operation of a large number of mills. The data transfer of the detection system is a big problem.
(3) J. KOLACZ uses a piezoelectric stress sensor to measure the change in the stress of the mill housing, and the sensor is installed in the middle of the maximum change in the stress of the mill housing. When the sensor is at the top of the mill barrel, the pressure is measured, while at the bottom of the barrel, the tension is measured, and the change in total pressure is calculated based on the change in reading, which is proportional to the mill load.
5. Differential electromagnetic sensor method
The differential electromagnetic sensor method has been researched and patented by the Central Institute of Integrated Automation in Bulgaria. It only measures the useful signal portion of the mill housing vibration and eliminates the general interference signal. The sensor can be mounted 71 to 80 mm from the ball mill housing, where the output signal is almost independent of the change in distance. The reliability and sensitivity of the sensor are high and the signal transmission delay is small. However, this detection system cannot detect the specific values ​​of the media filling rate, the ball-to-ball ratio, and the grinding concentration of the ball mill.
6, Hall Power Transmitter Method
In the closed-circuit grinding system consisting of a double-helical classifier of a ball mill, the ball mill load is indirectly reflected by the method of measuring the amount of sand returned by the classifier. In the grinding production process, when the load of the ball mill exceeds the normal value, the coarse fraction of the ball mill discharge increases, which will increase the amount of sand return: otherwise the amount of sand returned decreases. Therefore, it is considered feasible to indirectly return the ball mill load by measuring the amount of sand return. For the measurement of the amount of sand return, the electric power of the classifier is measured by a Hall power transmitter. At the same time, this measurement does not require the absolute value of the measured power. Therefore, only the single-phase power variation value of the three-phase motor of the double-helical classifier is measured to reflect the relative change of the amount of sand returning, and the automatic control of the ball mill load is realized.
7. Tracer atom method
Studies have been carried out abroad, the main principle is to use the tracer atom to determine the residence time of the material in the ball mill, so as to calculate the amount of material retained in the ball mill, but this method is difficult due to the low accuracy It is applied in practice.
(two) two-factor detection
1. Power double-signal detection
Yang Xiaosheng, a master student at Northeastern University, proposed the use of a power-sound double factor combination to detect and control the ball mill load. The specific methods are as follows: 1. Due to the actual rate of rotation (ψ=60%-85%). Regardless of the media fill rate, the ball to ball ratio increases. The sound intensity is reduced. Therefore, the range value of the output ball ratio can be judged based on the falling value of the sound intensity. 2. According to the judged ball ratio φm and the active power value detected, the filling rate value of the medium in the mill is obtained according to the relationship of N=F(ψ, φm).
In this way, according to the active power-sound double signal, the medium filling rate and the ratio of the material in the ball mill production can be relatively accurately determined, and then the amount of the medium added and the amount of sand returned or the new feeding amount of the mill can be adjusted as needed to make the ball mill The most efficient.
The detection and control system has the following characteristics: 1. It can accurately control the amount of material and the filling rate of the medium in the ball mill.
However, the influence of the grinding concentration is not considered, that is, the grinding concentration cannot be determined. Therefore, the use of power-sound dual-signal automatic control does not allow the mill to operate at the optimum grinding point. It should also add the slurry concentration (or viscosity) and the ball ratio signal. 2. Compared with a single signal, dual signal detection and control is more timely and accurate.
2, sound-vibration double signal detection
The vibration signal from mechanical grinding can be detected by some instruments or meters. The change of the vibration signal depends on the change of the grinding state. Zeng Yigen (Chinese student in Sweden) conducted a preliminary study on the vibration signal of the industrial ball mill. The mechanical vibration of the ball mill was measured by an accelerometer. The change in sound pressure is measured with a microphone (the microphone is mounted on the side where the sphere falls). The author's research results show that the vibration signal changes are closely related to the changes of grinding operation parameters. The power consumption and product granularity of the ball mill are mainly related to the mechanical vibration signal, while other factors are mainly related to the sound vibration signal. The authors point out that the measurement of the vibrational signal can be used to monitor changes in the operating parameters of the grinding.
In addition, there are methods such as power-vibration double signal detection of the ball mill load. In general, however, the dual-signal detection of the ball mill load still does not determine the internal state of the ball mill.
In addition, there are methods such as power-vibration double signal detection of ball mill load. In general, however, the dual-signal detection of the ball mill load still does not determine the internal state of the ball mill.
In summary, the accurate detection of the ball mill load can not be achieved by single factor, and it is impossible to judge the media filling rate, the material ball ratio and the grinding concentration of the ball mill. Although the two-factor detection is one step ahead of the single factor detection, it is still impossible to judge the ball mill. Grinding concentration. Therefore, both single factor detection and two-factor detection cannot accurately determine the internal working state of the ball mill (ie, medium filling rate, material ball ratio, and grinding concentration), so it is difficult to achieve optimal control of the ball mill. The development trend of ball mill load detection should be multi-factor joint detection (at least three factors).
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