The purpose of pulp washing is to separate the pulp from the black liquor, so that the residual substances such as alkali lignin produced by the reaction in the cooking process are washed out, and the pulp is purified to facilitate screening and bleaching. The pulp is thoroughly washed, and the cooking waste liquid having a higher concentration and temperature is extracted and sent to the sewage treatment and acid or alkali recovery process recover the organic or inorganic substances in the waste liquid; the pulp is suitable for the next processing.
The principle of pulp washing
For unwashed pulp, most of the waste liquid is freely present in the flow space between the fibers (about 70-80% of the total liquid volume), and a part of waste liquor is present in the inner cell of the fiber cell (about 15%-20%), only a small part is present in the pores of the fiber wall (about 5%). Therefore, pulp washing is actually a process of filtration and diffusion. Filtration and diffusion are inseparable.
Factors affecting the pulp washing
- Pulp cooking process, pulp type, pulp hardness:
Long fiber pulp is good. The order of pulp washing from easy to difficult: straw pulp → wheat straw pulp → bagasse pulp → mash pulp → bamboo pulp → cotton pulp
Pulp hardness: high hardness, low viscosity, good drainage performance, but the poor cleaning performance that is mainly caused by a part of the waste liquid inside the fiber.
- Washing liquors temperature:
The higher temperature the washing liquid has, the greater ability dissolve impurities have and the lower viscosity the liquid has, which is beneficial for diffusion and filtration.
- Pressure difference or vacuum degree:
The pressure difference or the vacuum degree is large, which has a great influence on the extrusion filtration and has little effect on the diffusion;
- pulp thickness, concentration and dryness of the pulp layer:
- Thick layer result in high filtration resistance, slow filtration speed, small contact area with washing liquid during pulp washing, unfavorable for diffusion, high residual alkali content in pulp;
- Higher dryness, less residual alkali, high extraction rate;
- Increase sizing concentration, can increase production capacity, affect washing quality and black liquor extraction rate;
- Good water filtration of pulp has a thicker layer, straw pulp usually has poor water filtration and large viscosity of the black liquor, and it is not easy to access the Internet when filtering. Therefore, the concentration of the sizing is higher than that of the wood pulp, otherwise, the slurry layer is too thin; the sizing concentration of the wood pulp is 1 to 1.5%, and the sizing concentration of the straw pulp is 1.5 to 2.5%.
- Washing water quantity and washing times:
The washing water volume is large and the washing is clean, but the steam required for evaporating the black liquor is more.
The role of multi-stage countercurrent washing
The washing liquid has multiple times reverse contact with the pulp flow to increase the concentration of the washing liquid. It is characterized by low water consumption, maximum concentration of waste liquid for countercurrent recovery or comprehensive utilization, and less wastewater, which is conducive to comprehensive utilization and reduction of environmental pollution.
Reasons for the foam and elimination measures
The hazard of foam: It causes troubles in operation and management, and also causes loss of alkali, lower extraction rate of black liquor, and increase environmental pollution.
Reasons for the occurrence:
- Organic substances dissolved in raw materials such as resins, waxes, alcohols, gums and saponified compounds are internal factors of foam formation;
- Intense mechanical agitation air during operation and changes in temperature, viscosity, and pH are external causes and conditions for foam formation;
Elimination measures:
- Try to avoid or reduce the air of the black liquor;
- Wherever black liquor needs to be sprayed, the smooth spray to avoid more foam due to impact;
- Keep the waste liquid at a high temperature as much as possible, it is difficult to form a stable foam;
- The waste liquor tank of the vacuum washer should have a large volume, so that the waste liquid remains height at around 1 /3, so that there is more room for the foam to disappear on its own;
- Defoamers should be added to the tanks that are prone to foaming;
Pulp washing equipment classification
Pulp washing must adopt a variety of operating principles, different washing equipment. Any washing equipment is mainly used for one of the functions like diffusion, filtration and extrusion, and etc.
- The washing equipment mainly used for filtration: rotary screen concentrator, gravity disc thickener, side pressure concentrator, vacuum washer, falling washer, pressure washer, washing tank, etc.;
- The washing equipment mainly used for pressing: double roll press, double barrel press, screw press, etc.;
- The washing equipment mainly used for diffusion: horizontal belt vacuum washer, continuous displacement washer, etc.
Notes for the operation of the vacuum washer
To ensure continuous and stable operation of the vacuum washer, the following points must be noted in the operation:
- The spray pot should maintain a certain liquid level and a stable concentration;
- Control the liquid level of the black liquor tank; if the liquid level is too low, it will affect the flow rate of the spray liquid on the drum, affect the washing effect and destroy the vacuum degree; if the liquid level is too high, the foam generated by the black liquor will not have enough residence time. Defoaming, the foam overflows the tank and pollutes the environment.
- Master the relationship between the rotational speed of the drum and the slurry concentration;
- Grasp the proper slurry level. The filtration and washing of the washer start after the drum has left the level of the slurry tank, so the low pulp level is equivalent to increasing the filtration area and improving the washing effect.
- Master the temperature of the wash water at 60-70 ° C;
- The speed of each section shall be adjusted according to the difficulty of production and washing;
- When the thickness of the slurry layer is constant, increasing the vacuum degree can reduce the residual alkali in the slurry.
- The process of the vacuum washer is very important;
Why is the vacuum degree of the vacuum washer not reached?
- The drain pipe is directly inserted into the black liquor tank. Generally, the liquid level in the black liquor tank is at 3m height, so the direct suction of the black liquor is reduced by 30kPa.
- The black liquor is very sticky, and the black liquor flows in the tube will generate resistance, which causes pressure loss and slows the flow. Or the pipe is not in the proper position, and the elbow is too much, causing the degree of vacuum to drop;
- Some air will be leaked into the filter layer of the pulp or in the pipeline to reduce the vacuum;
- Washing pulp with hot water, the temperature of the pulp becomes higher and the steam on the surface of the water will offset a part of the vacuum.
