Looseness of the bed is a prerequisite for the stratification of mineral particles by density; insufficient looseness makes it difficult for particles of different properties to shift relative to each other, and even loses the possibility of stratification. It can be said that within the jig machine, the looseness of the bed not only determines the quality of the bed stratification but also affects the stratification speed. Generally, a greater bed looseness correlates with a faster stratification speed.
Loose bedding is a prerequisite for the stratification of mineral particles by density; insufficient looseness makes it difficult for particles of different properties to shift positions, and even loses the possibility of stratification. It can be said that within the jig machine, the looseness of the bed not only determines the quality of stratification but also the speed of stratification. Generally, a greater degree of bed looseness corresponds to a faster stratification speed.
Due to the fact that the stratification process can only be facilitated when the bed is in a loose condition, it is crucial to quickly loosen the bed and extend the time it remains loose during a jigging cycle. To achieve this, the process of transitioning the bed from compact to loose should be accelerated at the beginning of the cycle. Once the bed reaches an appropriate level of looseness, efforts should be made to maintain this state, slowing down the sedimentation process from loose to compact. However, after the bed becomes compact, it is essential to promptly initiate a new jigging cycle with water flow, absolutely avoiding any extended downtime between cycles. If the jigging cycle T is too long (i.e., the water flow frequency is too low), it may result in an excessively long pause between cycles or improper water flow characteristics.
The measurement of the state and operating parameters of the gravity separation process is a prerequisite for its automatic control. Since the gravity separation process is a dynamically changing two-phase fluid (liquid-solid) process, the state and operating parameters are influenced by many factors and are interdependent. Therefore, accurately obtaining the state variables and operating parameters of the gravity separation process is a significant challenge. Real-time detection of the density stratification status in the gravity separator is a key unresolved issue in the coal preparation industry, and its resolution is crucial for the realization of automatic control in gravity separators. This article mainly discusses the methods for obtaining bed stratification state information and briefly describes the measurement methods for other process parameters (including auxiliary parameters).
In the control systems of gravity separation machines used domestically and internationally, the common method is to use buoyancy sensors to detect the heavy material layer in the pulsating bed. This allows for the measurement of the thickness of the heavy product bed, which is then used to control the discharge, aiming to maintain a stable bed thickness. Buoys operate on the principle of gravity-induced buoyancy, mimicking the movement of material of the same density within the pulsating bed. By determining the position of the buoy, the thickness of the heavy product layer with a higher density can be ascertained. Therefore, buoy sensors do not provide precise whole-bed density stratification information.
In recent years, both domestically and internationally, various detection methods have been researched that can directly or indirectly reflect the state of the material bed in a jigging machine. These include the use of multiple pressure sensors placed within the material layer to indirectly reflect the stratification of the material by density; an "intelligent probe" mimicking an artificial probe, which features a force sensor mounted at a fixed end and can reciprocate at a certain speed within the bed layer, detecting the bed condition and looseness through changes in motion resistance. However, due to technical limitations, none of these devices have been successfully applied in industry.
