Cause Analysis and Countermeasure of Unsatisfactory Separation Effect of Centrifuge

Cause Analysis and Countermeasure of Unsatisfactory Separation Effect of Centrifuge
There are two centrifuges in the recycling workshop of a company's acrylic fiber plant, all of which are horizontal screw discharge sedimentation centrifuges. One of them was imported from Japan with the complete set of equipment when it was started in 1998, and the other was imported from Japan in May 2001. Imports are produced by TANABE Japan, and the main technology relies on the production technology of West Germany's FIoffwegwerk company.

The main function of the recycling workshop in the entire acrylic fiber plant is to remove and concentrate the sodium thiocyanate system. Finally, a concentration of 56% sodium thiocyanate solution is sent to the polymerization plant for the preparation of the spinning dope. In the material circulation of the entire acrylic device, sodium sulfate and sodium thiocyanate are mixed together, and nearly 90% of sodium sulfate is separated from sodium thiocyanate in the step of crystallization separation. The balance of sodium sulfate depends on the centrifuge. The separation is achieved, so in the entire recovery device, the centrifuge plays a vital role, just like the human kidney, to eliminate impurities, to ensure the clean flow of blood, the separation effect of the centrifuge is good, and directly affects Smooth production and long-cycle operation in the entire workshop and even in the entire plant.

The principle of separation of sodium sulfate in the crystallization process is as follows: since sodium sulfate has a low solubility in sodium thiocyanate solution, crystals are easily formed, so that the concentrated sodium thiocyanate solution is settled by the sedimentation tank, so that The sodium sulfate grains gradually grow at the bottom of the settling tank (sodium sulfate content is about 13% to 15%), and the centrifuge is used to separate the sodium sulfate crystals from the bottom solution (only a small part of which is transported to the evaporation plant). As a seed crystal, the separated solid phase is dissolved in a dissolution tank, sent to a sewage treatment process, replaced by a resin, and adsorbed sodium thiocyanate therein, and the waste liquid is discharged. If the separation effect of the centrifuge is not good, the sodium thiocyanate content in the solid phase separation is too high, exceeding the adsorption capacity of the sewage resin, and the environmental protection requirement that the sodium thiocyanate content in the discharged sewage should not exceed 200 ppm, the workshop has to include Sodium thiocyanate with high concentration of sodium sulphate is poured into the system to re-concentrate and recover. Over time, the sodium sulphate content will accumulate in the system, which will eventually lead to fluctuations in the production of evaporation and crystallization, thus affecting the normal production of acrylic fiber devices. run.

According to the above separation principle, in the recovery device, the separation effect of the centrifuge is focused on the dryness of the solid phase, that is, the less the sodium thiocyanate contained in the separated solid phase, the better the separation effect, and below, from the centrifugation The working principle of the machine begins, and the reasons for the unsatisfactory separation of the centrifuge are analyzed and the corresponding solutions are found.

Working principle: Horizontal spiral discharge centrifuge is used in the recovery unit for solid phase concentration of suspension. It is a column-cone combination type consisting of two independent rotating rotors, one is a cylindrical-conical non-porous rotating drum, one For a spiral with a helical blade, the two are coaxially nested together. The drum of the centrifuge rotates counterclockwise, and the drum rotates concentrically with the spiral inside, and the drum speed is slightly faster than the spiral. When the suspension is introduced into the spiral cavity from the feeding tube, it enters the sedimentation zone (cylindrical part) of the drum through the distribution port. Under the centrifugal force, the solid phase with a larger specific gravity is deposited on the inner wall of the drum, and is pushed out by the spiral blade. The zone is discharged from the small end of the drum through the drying zone, and the clarified separation liquid flows along the spiral blade passage to the large end of the drum and is discharged from the overflow hole, thereby separating the solid and liquid of the suspension. See the figure below for details.


What is carried out in the drum is sedimentation separation. The function of the spiral is to distribute the suspension introduced by the feed pipe, and to send the solid phase accumulated on the inner wall of the drum to the solid discharge port of the small end of the drum. The spiral barrel is provided with a suspension distribution chamber, four suspension distribution holes, and power is obtained from the gear box through the spline shaft. According to the above figure, the reasons for the centrifuge separation not achieving the expected results are analyzed and corresponding countermeasures are found:
1. Rotating speed: Generally speaking, the higher the rotating speed of the drum, the better the dewatering effect, because the material in the drum and the spiral rotates at high speed, and the sedimentation force acting on the solid particles can reach thousands of times of its gravity. The solid can be completely separated from the suspension. The drum of the centrifuge rotates at a constant speed around its axis. The radius of the drum is R and the speed is n (r/m). The relationship between the angular velocity ω of the drum and n is:
ω=π n/30; 1/s. (1)
The circumferential speed of the drum v = R ω; m / s. (2)
The centripetal acceleration is an = -R ω; m/s2. (3)
If the material of mass m is contained in the drum, a centrifugal force F will be generated when rotating:
F =- man = mR ω2; N (4)
The ratio of the centrifugal force to the gravity of the separated material in the centrifugal force field is called the separation factor F r :
F = F / G = mR ω2 / mg = R ω2 / g N ; (5)

The separation factor is the main indicator indicating the separation ability of the centrifuge. The larger the Fr is, the larger the sedimentation force of the material is, and the better the separation effect is. Therefore, for the solid particles, the viscosity of the liquid is large, and the suspension or emulsion which is difficult to separate, Use a centrifuge with a large separation factor (high speed or large diameter). At the same time, we can see from equation (5) that the method of increasing the rotational speed is more direct and effective than the method of increasing the diameter of the drum (modifying the diameter of the drum to modify a series of accessories related to the size); and because the separation factor is improved Limitally, the limit of Fr depends on the strength and density of the drum material, so the method of increasing the speed is relatively easier to operate in practical applications. However, increasing the rotational speed also has its disadvantages. If the centrifugal speed is too high, the solid phase discharge will be too hard to block the spiral, affecting the operation of the centrifuge, and thus having to stop the centrifuge for washing to remove the agglomerated sulfuric acid. sodium. The speed of the drum of the centrifuge can be realized by adjusting the size of the pulley. In the actual operation, the characteristics of the liquid and the diameter and material of the drum should be carefully considered, and the rotation speed of the drum and correspondingly be accurately calculated. The pulley size is only adjustable for the speed.

2. Discharge time: The discharge time is actually to adjust the depth of the pool by adjusting the height of the weir. As can be seen from the figure, when the depth of the liquid pool becomes larger, the clear liquid stays longer in the spiral, and the clear liquid will be clearer, but the clear liquid will also enter the solid phase to increase the mother liquid content in the solid phase; Conversely, if the depth of the liquid pool is small, reducing the depth of the liquid pool is equivalent to the liquid phase overflowing easily from the drum, and the solid phase is settled in the drum for a prolonged period of time, the solid phase discharge time is prolonged, It is beneficial to reduce the content of mother liquor in the solid phase discharge. In the TANABE centrifuge, the overflow radius of the clarified liquid is adjustable, and the structural design of the overflow baffle takes into consideration a reasonable adjustment range, which is convenient for adjustment and disassembly. The adjustment of the depth of the liquid pool is achieved by the overflow opening on the big end cap of the drum, and the overflow plate which is positioned by the set screw can be moved radially to change the depth of the liquid pool. The centrifuge needs to be shut down.

If, in some cases, the rate of change has no significant effect on the solid phase discharge, it may be considered to adjust the depth of the tank to change the state of the discharge. Typically, this method is more efficient and simple to operate. It is important to note that all overflow baffles must be adjusted in the same orientation or they will cause an imbalance.

3, the speed difference: As shown in the schematic diagram, the spiral and the drum rotate in the same direction, but there is a speed difference between the two. If nb is the absolute rotation speed of the drum, ns is the absolute rotation speed of the spiral, and Δ n is the difference speed between the two.
Then â–³n=ns-nb
We already know that the rotation speed of the drum is faster than the spiral, that is, ns> nb, which is a negative speed, and the difference of the speed is the ratio of the difference speed to the drum speed:
Slip rate a=(△ n/nb)× 100%
The crystallization separation of the recovery unit pursues the dryness of the solid phase, and the use of such a negative slip rate facilitates the transport of the sediment and can reduce the power transmitted by the reducer. The negative slip can be realized by changing the difference speed between the drum and the spiral, that is, changing the size of the pulley on the rotor to change the slip ratio. This method is also more operative in production practice.
In addition, belt aging or belt tightness unevenness may also cause changes in the rotational speed difference to cause changes in solid phase discharge, which is also a point to be aware of when analyzing centrifuge solid phase discharge.

4. Screw conveyor: The conveyor is a main component. Its structure, materials and parameters are directly related to the separation effect, production capacity and service life of the centrifuge. The screw conveyor is mainly composed of a spiral blade, a spiral cylinder and a feeding chamber.
1 Spiral blade: The spiral blade of the recovery device is a continuous integral spiral blade, the spiral winding direction is right-handed, the blade is perpendicular to the drum bus bar, and the pitch is arranged. Since the spiral blade is directly in contact with the solid phase sediment, the solid phase sediment and the blade have a pair of movement. If the blade wears in daily use, the ability to transport the sediment after abrasion decreases, and the moisture content of the sediment increases. In the daily inspection and maintenance work, it is necessary to pay attention to strengthen the inspection of the spiral blades. The maintenance personnel only pay attention to the inspection of each rotating component and ignore the situation of the spiral blade. It also happens occasionally if the centrifuge spiral has open welding, cracks or severe wear. When repairing, the repair welding should be carried out in time. The surface of the screw-feeding surface should be smoothed and the balance of the spiral should be corrected. When the repairing area is large, the stress-relieving heat treatment is required after welding, which is difficult to operate. It is even more important to focus on strengthening the protection of the spiral during daily use.
2 spiral cylinder: The spiral cylinder is also a cone-column combination type, the cylinder inner cavity is the feeding cavity, and there are four feeding distribution holes in the feeding cavity.
3 Feeding position: In production, the closer the feeding point is to the solution outlet, the less the small particles have the opportunity to be deposited and discharged with the centrate. The sodium sulfate crystals in the solution are sometimes finely divided. The manufacturer pays attention to this in the production of the centrifuge. The feeding point set in the small end of the spiral is precisely calculated and set in the middle and the rear of the spiral cylinder. It is equivalent to extending the drying zone, and the time for centrifugation of the solid phase liquid is also extended, which ensures the dryness of the solid phase discharge. The position of the discharge hole of the centrifuge in our workshop is not adjustable, so the solid phase dryness cannot be improved by adjusting the discharge position or the feeding tube.
4 Process conditions: The production capacity of the centrifuge depends on the difference between solid and liquid phase density and the length of the settling zone. The closer the density difference between the solid and liquid phases is, that is, the greater the viscosity of the feed slurry, the more difficult it is to separate the sedimentation. In actual production, there are three main factors affecting the separation efficiency of the centrifuge: feed temperature, feed rate, and abnormal process conditions.
a. Feeding temperature: The temperature of the slurry can directly affect the viscosity of the mother liquor. Generally, the higher the temperature of the solution, the lower the viscosity, the more liquid film on the solid phase, the easier the fine particles settle, and the pores contain The less the liquid, the better the separation will be for centrifuges that seek solid phase dryness.
b. Feed rate: Sometimes too large a feed will result in poor separation, mainly due to insufficient settling time of the particles in the drum. The premise of achieving the separation conditions of the centrifuge design is that the time for the solid phase particles to settle onto the drum wall must be less than the residence time of the particles in the drum, that is, the effective residence time of the slurry to be separated in the drum must be ensured. The solid phase particles are allowed to settle for a sufficient time. In our actual experience, the same material, when the feed amount is 1m3 / h, the separation effect is not good, but when the feed amount is 0.5m3 / h, the separation effect is very good, the reason is here.
c. Abnormal process conditions: mainly refers to the lack of crystal content in the feed slurry or the crystals are not crystallized and flocculated, which is very difficult for the centrifuge to obtain the desired separation effect. The recovery unit has experienced such an event. The sodium thiocyanate system accumulates more sodium formate due to external reasons, causing the sodium sulfate to produce crystal grains in the sodium thiocyanate solution. In any case, any adjustment of the parameters of the centrifuge is incapable of this situation, and the physical properties of the material added to the centrifuge are directly related to the separation effect.
For materials that are difficult to separate, a good method is to clean the centrifuge frequently. Flush the centrifuge with hot water or condensate higher than the temperature of the liquid. It can replace the more viscous mother liquor and can also block the spiral. The harder solid phase discharge is replaced. Normally, the centrifuge is cleaned twice a day. However, when the production is abnormal, especially if the feed liquid is abnormal and the centrifuge cannot be stopped, it is a good treatment method to clean it as needed according to the centrifuge.

It can be seen from the above that the centrifuge is a very important equipment for the recovery device. In daily use and maintenance, some experiences need to be further summarized. It is believed that for diligent and intelligent workers, the centrifuge The separation effect should be able to achieve control and easy adjustment, maximize the technical advantages of the equipment itself, and better serve the production.

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