The vacuum drum washer is a type of pulp washer which is widely applied in the pulp washing process of wood pulp and straw pulp. Its main function is to separate the pulp from the black liquor and wash out the residual substances such as alkali lignin produced during the pulp cooking process to purify the pulp for screening and bleaching.
Vacuum Drum Washer Working Principle
The vacuum drum washer can either be used alone or in series group. The main components of a vacuum drum washer include a rotary drum, a filter tank, a flat face valve, washing showers, a strip device, drive gears, a screw conveyor, and so on.
The whole drum surface of the washer drum is divided into several compartments. Each compartment is connected with the corresponding cell on the end face of the valve seat through the filtrate channel. The valve core divides the end face into vacuum area, pulp stripping area, and exhaust area. The drum rotates in the tank containing the pulp, and each outlet passes through the vacuum filtration area, pulp stripping area, and exhaust area one after another. Due to the vacuum suction, the liquid in the pulp is extracted from the filtrate flow channel and distribution valve, and the pulp is adsorbed on the surface of the drum and peeled off by a scraper to realize the process of suction filtration, pulp stripping, and exhaust.
The drum body of the vacuum washer is divided into several chambers. When the chambers rotate down clockwise into the paper pulp, they are connected with the natural filtration area with natural air pressure. At this time, the static pressure of the pulp makes part of the filtrate into the chamber, and forms a pulp layer on the mesh surface. The chamber continues to rotate, goes deep below the liquid level, and is connected with the vacuum filtration area. It is forced to filter under high pressure difference to increase the thickness of the pulp layer. After turning out of the liquid level, the pulp layer on the mesh surface continues to be sucked dry to complete the dilution and dehydration process. The chamber continues to rotate and is connected with the vacuum suction filtration area to suck the washing liquor on the surface of the pulp layer into the drum to complete replacement washing. The chamber continues to rotate and is connected with the stripping area where the vacuum degree in the chamber disappears, which is convenient for stripping off the pulp.
Operation of the Vacuum Drum Washer
As one of the key equipment in the paper pulp production process, the stability of the vacuum drum washer affects the quality and production efficiency of product pulp. In order to ensure the continuous and stable operation of the washer, the staff must pay attention to the following aspects in operation:
1. Blow Tank
In the blow tank, the pulp level and concentration should be kept stable, which is conducive to the continuous and stable pulp washing process.
2. Liquid Level in Black Liquor Level
If the liquid level of the black liquor tank is too low, the spray liquid flow on the washing drum will be reduced, the vacuum degree will be damaged, and the washing effect will deteriorate. And if the liquid level is too high, the bubbles produced by black liquor will not have enough time to defoaming automatically, causing bubbles to overflow outside the tank, resulting in environmental pollution and alkali loss.
3. Washing Drum Speed & Pulp Concentration
The relationship between the washing drum speed and the pulp concentration can be adjusted according to the filtration performance of the pulp. Generally, if the pulp is easy to dehydrate, the pulp concentration should be reduced and the washing drum speed should be increased. This is beneficial to improve the pulp washing quantity and quality.
4. Pulp Level
The filtration and washing of pulp in the vacuum drum washer do not start until the drum turns out of the pulp. Therefore, reducing the pulp level is equal to increasing the filtration area, thus improving the washing effect. But the pulp level can not be lowered too low, otherwise, the pulp layer thickness and pulp yield will decrease. The best plan is to adjust the pulp level according to the actual production conditions to achieve the best washing effect.
5. The Temperature of Washing Water
The temperature of the washing water should be controlled at about 70 ℃, and the washing effect is the best at this time. If the temperature of washing water continues to rise, the temperature of black liquor will also rise, resulting in higher steam pressure, which is not conducive to the vacuum washing.
6. Vacuum Degree
When the thickness of pulp layer is fixed, increasing the vacuum degree within a certain range can reduce the residual alkali in the pulp. However, the high vacuum degree will cause the filtrate bubble, which should be avoided. Generally speaking, the vacuum degree should not exeed 46kpa.
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