Â Â Â I. Overview
Jigging ore dressing is one of the main methods of gravity beneficiation and belongs to the deep tank sorting operation. The process feature is that the selected ore is continuously fed to the sieve plate of the jig chamber to form a thick material layer called a bed. The rising water flow is periodically inflated through the sieve plate, so that the bed rises loosely, and then the water flow drops (or stops rising). In this process, particles of different density are relatively transferred, heavy minerals enter the lower layer, and light minerals are transferred to the upper layer. After the discharge, the concentrate and tailings are obtained.
Most of the medium used for jigging is water, and air is used in special cases. The sieve plate in the jig is generally fixed, and the water flow passes through the sieve plate to move up and down. A few of the slide valves only intermittently feed the rising water. Another type of jig screen frame is used for reciprocating up and down, called a moving sieve jig. The device that pushes the water flow movement in the fixed sieve jig is mainly a diaphragm in the mine jig, and the piston is used in the early stage. Compressed air is used in large coal jigs.
The speed and direction of movement of the water in the jig is cyclical, and the time taken to complete each cycle is called the jigging cycle. A curve indicating the velocity of water flow over time in one cycle is called a jigging cycle curve. The maximum distance that the water flows up and down in the jigging chamber is called the water flow stroke, which is proportional to the mechanical stroke of the diaphragm or piston motion. The number of movements of water per minute is called the stroke. The thickness of the bed, the form of the cycle curve, the stroke, and the stroke are important parameters that affect the jigging.
The jigging beneficiation can process almost all kinds of mineral raw materials except for very fine materials. The process is simple, the equipment processing capacity is large, and some kind of final product can be obtained in one sorting. Therefore, it is widely used in production. The use of the jigging method to treat raw coal accounts for about 40% of the total coal. For the metal ore, it is processed coarse iron ore particles mainly method, manganese ore and chromium ore. It is also used to select the coarser fraction of tungsten or tin ore that is unevenly embedded. The use of the jigging method to treat gold ore, yttrium , yttrium, titanium , zirconium- containing primary ore and sand ore is also widely used. It is also the main method of selecting diamonds . The greater the density difference of the mineral to be separated in the ore, the wider the range of the selected particle size. For example, for gold-bearing sand ore, when the feed size is less than 25 mm, it may be selected without classification, and the lower limit of the recovery particle size may be 0.05 mm. However, the classification of general metal ore can effectively improve the sorting index and improve the processing capacity of the equipment.
Second, the theoretical basis of jigging beneficiation
Although the theory of jigging stratification has been studied since the middle of the last century, a unified jigging theory has not yet been formed. The reason is that the stratification process is carried out in an unsteady flow, and the factors are complex. There have been various theoretical insights from different angles, which can be roughly divided into two categories: one is to explore the stratification reasons from the differences in the movement of individual particles, which can be described as the dynamic system theory (also known as the fixed factor model); One is to find the stratification basis from some unbalanced factors in the bed, which is regarded as the static system theory (also known as the population statistical model). They all have a certain layering principle, but they are not perfect. They can only be quoted according to the actual situation (see the principle of re-election). [next]
From a dynamic point of view, the particles in the bed generally have three forms of motion: (1) acceleration motion; (2) interference settlement motion; and (3) clearance movement of fine particles through the bed gap. These movements have different developments at different stages of the cycle.
(1) Gravity acceleration movement of particles When the bed rises loosely, the velocity of the water flow weakens and the relative velocity is zero, the particles lose their hydrodynamic support, and they settle with their own gravitational acceleration in the medium. The value g 0 is
The acceleration increases as the particle density increases, regardless of the particle size, so it is advantageous to stratify by density. However, in the jigging cycle where the water velocity changes greatly, the effective time is very short. Extending the relative velocity of the water flow to the bed will help the light and heavy minerals to accumulate in the differential layer.
According to Gaudeng's theory of differential layer of initial acceleration, the essence is consistent with the static view of particles under their own gravity pressure stratification.
(II) Interference Settlement of Particles Most of the time during the loose period of the bed is relative movement with the water flow, and thus belongs to the category of interference settlement. However, this interference settling velocity is unstable, and is affected by the bed looseness and the water flow velocity. But in general, the bed is not loose, so the equal drop ratio will be greatly increased. Most of the heavy minerals will settle to the lower layer faster, and only those fine-grained minerals will remain in the upper layer mixed with light minerals. The more the particles and water flow are in relative motion, the more obvious the mixing. Screening the raw materials in advance and limiting the inclusion of the particle size range will help to reduce delamination.
(3) Inhalation During the period of water flow decline, the first thing that loses activity is the coarse particles, followed by the medium particles, and the fine particles can still pass through the gap of the bed to enter the lower layer when the bed is tight. For inhalation. Appropriate control of the inhalation intensity of the water stream can be carried out only by transferring the fine-grained heavy minerals to the bottom layer and supplementing them by density.
RH Richards (1909) has proposed that the granules in the ascending water flow are jigging according to the interference settlement movement and stratification by the inhalation in the descending water flow. Practice has shown that inhalation is meaningful for sorting wide grade materials.
The layered transfer process of particles in one cycle in the above-described form of motion can be schematically represented in FIG. [next]
The theory of static system of jigging stratification mainly includes the stratification theory of heavy medium action, the differential layer theory of suspension density and the theory of potential energy. They were proposed by Nathorst (1924), AA Hirst (1937), and EW Meyer (1947). The common feature is that the influence of fluid dynamics on the movement of individual particles is not considered, but the buoyancy effect indicated by the heavy mineral layer, the static pressure of the light and heavy mineral suspensions, and the spontaneous decrease of the bed center of gravity. As a layered basis.
The theory of heavy medium action and the theory of the effect of suspended matter density show that under dense conditions of the bed, there is a static extrusion action between the particles (in the form of a suspension) like two mutually incompatible fluids. As a result, light minerals are forced into the upper layer and heavy minerals enter the lower layer. The theory of potential energy explains the inevitability of stratification from the law that spontaneous energy has a spontaneous reduction. According to their static theory, the stratification results should be perfect and not affected by particle size. This gives us the inspiration that the bed should not be too loose during the jigging process. It is necessary to maintain its integrity so that the static force between the particles works. The hydrodynamic action of the water flow is a factor that destroys the static conditions and should therefore be minimized. This is consistent with the conclusion that the kinetic system theory requires sorting under conditions of minimum relative velocity. However, the dynamics of the fluid are indispensable for the loose bed, so it is impossible to achieve the ideal static stratification. Therefore, it is necessary to appropriately limit the particle size range of the selected raw materials, and the effect of reducing the hydrodynamic influence can also be obtained by improving the form of the jigging cycle curve.
HH Vinogradov (BÐ¸Ð½Ð¾Ð³Ñ€Ð°Ð´Ð¾Ð²) et al. also proposed a model using a probabilistic-statistical method to study the stratification process. This study no longer considers the mechanism of stratification, but treats stratified motion as the center of particle distribution with the same nature. The process of remigrating. The transfer of particles conforms to the law of diffusion of the element, and finally the formula can be used to express the relationship between the stratification time and the nature of the ore and the operating conditions, and thus has more practical value. However, the model establishment process is more complicated and is still in the research improvement stage. [next]
Third, the jigging cycle curve In the alternating water flow jig, the cycle curve can have three characteristic segments: water flow rise, relative static and water flow drop. They can be composed of different periodic curves with different duration ratios. The basic requirement is to adapt to the ore properties and equipment conditions. Figure 2 shows four typical jigging cycle curves.
(1) Sinusoidal periodic curve The sinusoidal periodic curve is shown in Fig. 2a. The water flow has equal rising and falling speed and time, which is expressed by the formula:
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