Abstract: The research on the jigging theory of coal preparation technology has been more than 140 years old, but some problems are still not systematically solved, although a hypothesis touches the core of the jigging theory from different aspects. However, the jigging theory is a very complicated subject in the coal preparation technology. This article uses the latest coal preparation technology to systematically analyze and study the coal preparation technology on the basis of summarizing the long-term production practice.

1 Introduction

From 1830 to 1840, a mechanical piston jig appeared in the Harz mining area in Germany. In 1892, a pistonless jig was invented. The jig has been used as a kind of machinery for beneficiation for more than 100 years. In 1851, France's Bilollit published "the study of mineral machining, establishing a theoretical system of current operational systems and possible operating systems", which explored the principle of jigging sorting from the perspective of the motion dynamics of individual particles.

From this paper, the study of jigging theory has been more than 140 years old, but some problems have not been solved. Although some hypotheses have touched the core part of the problem from different sides, they have not been able to uncover the jump. The essence of the mechanism. The reason is that fluid mechanics is a very complex science, and the effect of fluids on solids is more complicated. It is difficult or even impossible to describe them with precise mathematical expressions. It can only be described by empirical equations through a large number of experiments; The various hypotheses do not explain the phenomenon of wide or no classification. Looking back at the history of the development of jigging theory, it is not difficult to see that it is basically a process of gradually increasing the "equal ratio".

The jigging theory has lagged far behind production practice. In the long-term production practice, people have already reached some consensus on the phenomenon of jigging. It is these consensus that has made the jigging technology develop. These consensuses can be summarized as follows: 1 The jigging layer is basically layered by density, and is also affected by the granularity. After stratification, the material distribution layer with the same particle size density is relatively lower, and the material distribution layer with the same density and large particle size is also relatively lower. 2 The jigging process is inseparable from the bed, and the bed is the medium in which the ore particles are layered by density. 3 The jigging process is a cyclical pulsation process to ensure a certain range of looseness. The upper limit of the looseness is too large or too small, which is unfavorable for stratification of minerals by density.

After long-term practice to obtain these consensus, the jig also evolved from piston mechanical pulsation to air pulsation, and then evolved from a periodic motion mechanical damper to a CNC pneumatic damper jig to a regenerative jig. The pulsating wind source used is getting more and more wind pressure, and the result is that the initial acceleration of the water flow increases.

2 quasi-heavy liquid hypothesis

From the above two points, it can be concluded that the jigging bed is very similar to the heavy liquid with uneven density distribution but basically low upper and lower high and low inlet and low outlet. The different density of the ore can be retained in the layer with similar density. On the top, if there is a difference in density, it will float or sink until it is close to the density.

It is well known that the buoyancy of a solid in a liquid or gas is equal to the resultant force of the static pressure of the fluid to the solid in the vertical direction. The jigging layer is carried out in water. It is impossible for the water to float at a density greater than that of water. Since, the buoyancy of the ore particles in static water is much smaller than the gravity of the ore particles. Therefore, we have reason to say that the ore particles obtained an additional buoyancy under the action of the pulsating jigging bed and the water flow, so that the ore particles can be layered by density in the jigging bed. If we can find the cause of this additional buoyancy during the jigging process, the problem of jigging layering mechanism will be solved.

2.1 Hypothesis model basis

Let us look at a simple check valve. As shown in Figure 1, there is a small ball in the lower tube body. The water is supplied from the lower end of the tube. When the water pressure reaches a certain value, the water flows out from the upper part, and the small ball hangs up. The water pressure increases, the height of the ball suspension increases, the water flow rate of the water supply pipe increases, and the flow rate of the gap water between the ball and the wall also increases.

The ball hangs up to the front of the cone, and the gap between the wall and the sphere is small. At this time, the sphere can be regarded as a piston, and the dynamic pressure of the water flow is almost completely converted into static pressure. Here, the conversion of the energy of the fluid to the pressure energy is affected by the surrounding gap. When the gap is zero, the conversion rate is 100%. When the ball hangs to a certain gap at the bottom of the cone, the energy of the water is partially converted into static pressure. When the suspension is suspended to the upper part of the cone, the large conversion rate is close to zero. The conversion rate is a monotonically decreasing function of the gap. When the fluid flows through the sphere, the sphere is subjected to the buoyancy of static buoyancy and static pressure of the fluid, in addition to Affected by the flow resistance of the gap. which is

G=F+f+S

Where G is the gravity of the ore;

F——static buoyancy;

F——additional buoyancy;

S - flow resistance.

When the gap is large enough, the additional buoyancy f of the solid disappears. At this time, the sphere exhibits free sedimentation in liquid and gas. When the gap is small enough, the S term disappears, the weight of the solid is all supported by static pressure, and the solid floats on solid and liquid. Or solid and gas interface. This is two special cases. More of the situation is the movement between the two, the so-called interference settlement.

On the basis of a large number of experiments using quartz sand and gravel, the former Soviet scholar Mingzi has mathematically derived that “the particles can float when the gap flow rate is less than the free settling velocity, and the volume concentration of the particles is higher. Large, the smaller the gap flow rate required, the conclusion.

2.2 Jigging Hypothesis

The above model shows that in this case, the solid particles can obtain an additional buoyancy, that is, there are other objects around them, and together hinder the fluid flow, so that when the fluid motion is abrupt, the relative flow resistance is between the particles and the particles. The size of the gap is related. In the jigging sorting process, people use the looseness of the mineral group to express the gap rate. Because the looseness of the material is low during the whole jigging process, the stratification process is mainly the function of hydrostatic pressure, and is also affected by the flow. The impact of resistance.

When the fluid flows through the ore particles, an additional pressure gradient is formed around the ore particles. Just as the heavy liquid of the density layering acts on the ore particles, the buoyancy of the ore particles is increased. According to the principle of action and reaction, it can be assumed that the effect of the ore particles on the surrounding fluid is equivalent to increasing the density of the fluid. When the ore is lifted, the density of the fluid is equal to the ratio of the buoyancy of the ore to the volume of the ore, ie δ = (GS) / V.

It is lower than the density of the ore particles, because the fluid flow resistance offsets part of the gravity. We regard the ore particles together with the surrounding fluid as heavy liquid. For the sake of distinction, it is called quasi-heavy liquid, and its density is called quasi-heavy liquid density. When the under-water stream lifts the material, the ore particles are stratified at the quasi-heavy liquid density. If there is no influence of flow resistance, then the ore should be stratified strictly according to density, and the particle size and shape have no effect on the movement.

The density of the quasi-heavy liquid is a function of the density of the ore and the resistance of the flow. The flow resistance of the fluid to the ore is proportional to the density of the medium, the square of the particle size d, the square of the flow velocity u, and the volume and particle size of the ore. In proportion, the quasi-heavy liquid density can be expressed as:

δ=δ0-f(ρ,u2/d)

In the jigging bed, u is proportional to the degree of looseness. Therefore, the greater the looseness, the more difficult it is to stratify by density. The smaller the particle size, the smaller the relative buoyancy, so the relative position is higher. However, as the looseness decreases, the difference also decreases, and the effect of the size of the particle size is irreversible because its determinant is the looseness.

The fluid always flows from a high pressure to a low pressure. This diffusion of the pressure inside the fluid causes the internal pressure distribution to be continuous, ie, the pressure is not high at a certain point, but at a certain point close to each other. disappear suddenly. Therefore, the solid particles accumulated under the action of pulsating water flow, the final equilibrium result is that the density and the particle size are similarly accumulated in the uniform equal pressure layer.

The jigging bed is a solid-liquid two-phase flow body which has been basically balanced by the pulsating water flow and the particles and the water flow are basically balanced.

When new materials are fed in, they move to the respective equilibrium level under the action of pulsating water flow, realize stratification by density, and at the same time replenish the bed and push the bed to move to the discharge end. This is the jigging. The essence.

2.3 jigging machine layer pressure distribution

The pressure is always transmitted in the direction of reduced pressure, but the pressure in the jigging bed is not likely to be the same as the fluid in all directions. Because it is only a temporary, periodic performance with fluid heavy liquids, the transfer of pressure is also intermittent and periodic. In this case, its pressure line cannot be straight, only to ensure that the pressure does not change at a certain point, and its pressure line is a continuous but not smooth curve. Due to the uneven accumulation of materials caused by feeding and discharging, it can only be slowed down during the jigging process. Near the feed port, the ore particles have not been layered, and their average quasi-heavy liquid density is larger than that of the upper layer, thus forming a feed zone, so that the lower layer of the layered material forms pits. The pressure distribution in the jig is basically a fan-shaped distribution from bottom to top and gradually decreasing from the feed to the discharge end.

3 The main contradiction in the jigging process

3.1 Contradiction between density stratification and looseness

According to the above hypothesis, the smaller the looseness, the better the stratification by density, and the looseness is the necessary condition for the stratification of the particles. Therefore, how to control the looseness and stratify the ore particles as much as possible is the solution to the jigging process. The main contradiction.

3.2 Loose form of integral lifting and CNC damper

It is required that the materials be stratified by density, the looseness between the particles should be as small as possible, and the position should be changed. The only way is to form a space around the lower part of the particle group, so that the particles with relative displacement can be squeezed out. When the particles move, when they are loose, the heavy particles sink and the light particles float. It can be inferred that the looseness of some areas in the bed is smaller than that of natural accumulation. That is, it is necessary to form a loose form of the scattered bed. To achieve the sag, a higher initial velocity of water flow, a higher pressure pulsating air and a higher speed switching damper, ie a pneumatic numerical control damper, are required.

3.3 The contradiction between high initial acceleration and looseness

The high initial acceleration means that there is a high initial hydraulic head, and the long-term action will cause the looseness to be too large, thereby destroying the purpose of stratification by density, and the short acting time will affect the stratification speed, so the initial acceleration is limited. However, without a high initial acceleration, the bottom bed cannot be lifted as a whole. The solution to this contradiction is generally to use a shorter intake period and a longer expansion period. However, the time to extend the stratification by natural expansion of the air is limited. The best method is to provide higher pressure at the beginning, once the bed is overcome by static friction, immediately reduce the pressure to maintain a certain degree of looseness for a long time.

3.4 Wide or non-classified

From the point of view of stacking, wide or non-graded pores can be complementary, and the stacked pores are smaller than the narrow one, so the effect of layering by density is better. The narrow particle size range is not a single particle size and shape, and the effect of particle size and shape is greater than when the particle size is mixed into a wide grade. Due to the dissociation effect, in general, the ash content of the same-density small-grain grade is lower than that of the large-grain grade, so the sorting density of the small-grain grade is higher, which coincides with the jigging layer. Therefore, in most cases, wide or non-classified selection can also achieve better results.

4 The essence of reinvigorated jigging

According to the requirements of 3.3, one of the methods can be realized by two wind sources with different wind pressure and different pulsation frequencies. This is the re-vibration jigging. The essence of the re-vibration jigging is to meet the different pressure requirements of each period of the jigging process. Maximize the stratification relative time.

4.1 The vibration recovery jig has good stratification effect and increased processing capacity.

The peak value of the vibration jigging looseness is smaller than that of the ordinary jigging, so it tends to be stratified by density. Moreover, since the loosening time is prolonged, the horizontal moving speed of the bed layer, especially the bottom layer, is improved, so the layering effect is good, and the processing amount is also larger than that of the ordinary jig.

4.2 Reverberation jig wind and water saving mechanism

Re-vibration jigging minimizes the gap flow, allowing the water to move forward under the bed, thereby increasing the accuracy of stratification by density, so the bottom water penetrates the bed to become overflowing compared to the ordinary jig The amount of flow is much less. At the same time, the regenerative jig has a relatively long intake period, and the bottom water replenishment amount is relatively small, and the water is extruded from the air chamber by the air expansion, and the air is discharged after the expansion, so the water saving is also saved.

5 Conclusion

This paper is only a point system analysis of the mechanism of coal preparation technology jigging layered quasi-heavy liquid. Many coal preparation technical problems still need long-term verification, and many problems need further discussion. However, this paper has certain guidance for the coal preparation technology jigging layer quasi-heavy liquid machine.

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