Taking the raw coal screening system of a coal preparation plant in Shanxi Province as the engineering background, the working principle of flip flow screen and banana screen selected in the system were expounded, and the screening effects of the two screening devices were compared under the same conditions.  The results show that, under the same conditions, the flip flow screen has higher processing capacity and better screening rate, and the overall screening process performance of flip flow screen is higher than that of banana screen.  Therefore, the coal preparation plant should give priority to the selection of flip flow screen when screening raw coal.
1 Overview
The coal preparation plant of a mine in Shanxi Province has achieved the annual washing capacity of 2.4 million tons after many technical reforms. In 2014, the banana screen technical reform of flotation system was completed, and the coal preparation process of jump-flotation was realized.  The specific coal washing process is: after the raw coal vibration jump screen, the size of the raw coal is less than 13mm is sold directly without flotation, the size of the raw coal is greater than the next level of screening, screening the size of the 38-80mm coal, the size of the 18-38mm small coal, the size of the 13-18mm coal washing particles and the size of the final coal powder within 13mm.
Sales prices creeping up in recent years, coal washing, in order to improve the economic benefit of coal preparation plant, nearly some adjustments of raw coal washing, will be at the end of the fine powder limit reduced from 13 mm to 8 mm, would reduce the lower limit of coal washing grains from 13 mm to 8 mm, ceiling unchanged, the raw coal washing the goods after small coal particle size range of lump coal, lump coal remains the same,  The particle size range of the washed coal is 8~18mm, and the particle size range of the final cleaned coal is within 8mm.

The adjusted co-washing process of coal preparation plant is shown in Figure 1.
As can be seen from FIG. 1, the key to improving the production rate of coal washing particles lies in the screening of fine coal with A particle size greater than 8mm in raw coal. In order to improve the screening and washing effect, the coal preparation plant replaced the screening device in the two washing systems of A and B, and replaced device A with A tensile screen (model ABFS24100). Unit B banana screen (model ABS2194) is currently in synchronous operation with two screening devices.
2. Analysis of working principle of flip flow screen and banana screen
2.1 Flip flow screen is through the double body sub-resonance to realize the vibration of the screen body, and the use of elliptical vibration belt to drive other floating bed flexicles, in order to make the vibration amplitude of the floating bed is much larger than the traditional vibrating screening equipment, at the same time, different material groups in the screening of raw coal have their own motion rules. The separation and screening of different material layers are realized by using the excitation force generated by the shaker motion.

2.2 Banana screen
Banana screen is based on the optimization design of equal thickness screening to realize the screening of raw coal.  Banana sieve motion using box shaker, and through the coupling and drive shaft will transfer vibration power.  In the normal working state, the shock device drives the motion of the screen box, so that the material on the layered cast screen can realize the oscillation forward, from the screen body into the outlet to the outlet, the material in accordance with different particle size and attributes in order to screen arrangement.  There are screen surfaces with different inclination angles on the screen frame of banana sieve, and the inclination value of each screen surface is 4°. There are 7 screen surfaces in total, which are installed in descending order.  Under the same oscillation frequency, the materials on different screen surface will have different movement speed and throwing intensity, and from the inlet to the outlet direction decreases in turn, so that the thickness of the material from the inlet to the outlet basically remains unchanged, to achieve equal thickness screening of raw coal.
3 Comparison of the screening effect between the flip flow screen and the banana screen
3.1 Technical parameters of flip flow screen and banana screen
The specific technical parameters of flip flow screen and banana screen used by coal preparation plant are shown in Table 1.
3.2 flip flow screen and banana screen
screen effect in A, B two sets of screening system, the raw coal, screen after screen of the content and filter screen on the upper part of the screen content under the granularity of the requirements for screening experiment according to the regulation, compared to two kinds of different filter sieve sieve effect, after the screening data, remittent sieve screening effect as shown in table 2,  The screening effect of banana sieve is shown in Table 3.
It can be seen from Table 2 and Table 3 that with the increase of raw coal input, the screening rate of the two kinds of sieve bodies showed a trend of decrease, but the screening rate of banana sieve decreased more obviously.  At the same time, when the screen body is under low load, the screening rate of the two kinds of screening equipment can reach more than 60%, indicating that the two kinds of screening equipment have a good application effect when the low processing capacity.  With the increase of raw coal input, when the relaxed screen reaches 60% of its rated capacity, its screening efficiency can still reach more than 60%. When the raw coal input continues to increase, its screening efficiency edge agglomeration decreases.  When the raw coal input of banana screen reaches 35 of its rated capacity, the screening efficiency can still approach 60%. When the raw coal input continues to increase, the screening rate of banana screen decreases dramatically.  If the screening rate is 60% as the limit, the processing capacity of the banana screen is lower than that of the relaxed screen, but the processing capacity of the relaxed screen is only 65% of the rated processing capacity.  With the increase of raw coal input, the difference of screening rate between the two screening equipment becomes more and more obvious.  Although the rated processing capacity of the banana sieve is higher than that of the relaxed sieve, the processing capacity of the relaxed sieve is higher than that of the banana sieve under the premise of ensuring the screening rate.
Therefore, it can be concluded that in the same working environment, the screening rate of the slack screen is higher than that of the banana screen, and the overall performance is also better than that of the banana screen.
4 Conclusion
The coal market has higher and higher requirements on the washing of coal preparation plant. In order to better use the market needs, the washing requirements of fine particles in coal preparation plant are also increasing day by day.  By comparing the screening effect of relaxation sieve and banana sieve under the same environment, it can be seen that under the same conditions, the screening rate of relaxation sieve is significantly higher than that of banana sieve, and the processing capacity is also more obvious.  Therefore, in the actual production process of coal preparation plant should give priority to the selection of slack screen to screen raw coal, in order to improve the economic benefits of coal preparation plant and the effect of raw coal washing.  At the same time, from the application effect of flip flow screen in coal preparation plant, there is still a large room to improve the washing effect of the flip flow screen on fine coal, which needs continuous optimization research, so as to better meet the needs of fine coal washing in coal preparation plant.

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