After caustic or sulphate cooking, a small part of the black liquor (about 5% of the total volume) remains in the capillary inside the cell wall, and part of the black liquor (about 15% to 20% of the total volume) is contained in the fiber. In the intercellular layer and in the cell. The remaining black liquor, which accounts for 75% to 80% of the total amount, is a free black liquor surrounding a single fiber or fiber bundle.
The most important physico-mechanical and physicochemical phenomena that occur during slurry washing include: black liquor extruding from the slurry; black liquor passing through the filtration of the slurry; black liquor dissolving substances out of the pulp fibers; due to Adsorption and desorption re-equilibrium, while the absorption of black liquor dissolved in the adsorbed part of the dissolution; black foaming [1]. Below we briefly analyze these processes and phenomena.
Extrusion: Usually the pulp press uses mechanical extrusion to discharge the black liquor from the pulp. The advantage of this method is that the undiluted black liquor can be separated. However, single-stage extrusion does not guarantee complete separation of the black liquor because the pressure generated during the extrusion process is insufficient to allow the black liquor in the capillary inside the pulp fibers to escape. As we all know, the height of the liquid rising in the capillary is determined by the following formula:
Where: δ-liquid surface tension; r-liquid density; d-capillary diameter; h-capillary column height The corresponding capillary pressure is: when extruding, the slurry discharges black liquor until the pressure in the capillary due to the pulp The compaction of the material and the increase of the diameter of the capillary are increased, and the pressure is balanced with the external pressure. For this reason, some experiments have been carried out on sulphate and sulphite pulps [2]: Static squeezing tests were carried out in a small laboratory apparatus. When the pressure reaches 15MPa, the amount of black liquor left in the slurry is 0.7kg/kg air-dried fiber, and the corresponding relative dryness is about 59%. The ultra-microscopic pores of the fiber wall have a moisture content of about 0.3 g/g air-dried fiber. Therefore, under the pressure of 15 MPa, not only the black liquor remained in the cell wall but also in the fiber cavity. Of course, in the actual operation of the pulper, a pressure of 15 MPa is impossible to reach. In addition, even if it is achieved, the slurry at such dryness is quite difficult for the subsequent dilution washing.
Filtration: In chemical technology, filtration is interpreted as the process of separating a suspension through a filter cloth or a porous membrane, retaining solid particles, and filtering out the liquid. All pulp washing methods require the passage of a liquid through the fibrous material. The speed of the liquid passing through the passage depends on the nature of the fiber, the viscosity of the liquid, the amount and size of the suspended solids in the liquid, and to a large extent also on the pulp. Concentration and force applied to the fibrous material. This process occurs when the drum of the vacuum washer is immersed in the black liquor separated in the immersion zone of the slurry suspension. The washing liquid (water or diluted black liquor) in the drum washing zone is filtered out of the concentrated slurry layer, and the thick black liquor is displaced from the slurry layer. When washing in a wash basin, cooker or diffuser, the entire process is the same as the filtration of the liquid through the pulp layer.
Diffusion: Reliance on diffusion can extract dissolved substances from pulp fibers. The diffusion rate is proportional to the diffusion area, the diffusion coefficient, and the concentration difference between the inside and outside of the fiber waste liquid. The diffusion coefficient is related to the temperature, viscosity, and pressure during washing. The result of approximate integration of the differential equations of diffusivity shows that for a single pulp fiber submerged in water, the black liquor can be separated in less than one second, but the black liquor is diffused out of a cubic fiber bundle with a volume of 1 cm2. It takes more than one hour [1].
Mc-kibbins directly extracted the black liquor from the slurries that retained the shape of the wood chips after cooking [3], and measured the diffusion coefficient of sodium ions (see Table 1). The diffusion coefficient in the transverse direction of the fiber is about 1/2 of the longitudinal direction. With a temperature increase of 10° C., the diffusion coefficient of the alkali in the fiber increases by 0.2 to 0.3 times and the activation energy is 20 to 27 kJ/mol. Therefore, to speed up the spread during washing, hot water must be used. Table 1 Sodium ion diffusion coefficient diffusion direction diffusion distance diffusion coefficient D × 105cm2 / s mm 38 °C 52 °C 65 °C 71 °C fiber longitudinal 3 0.71 1.20 1.80 1.90 6 0.84 1.40 2.10 - fiber transverse 3 0.36 0.58 0.68 - 6 0.36 0.60 0.75 -
Adsorption: As we all know, pulp fibers are substances that have a large adsorption capacity for metal ions. The carboxyl group is the adsorption center in the alkaline medium, and the adsorption capacity increases as the valence of the ion increases. The adsorption of sodium ions during washing is of practical significance. It affects the amount of alkali that is carried away by the washed pulp and the adsorption of sodium ions by the kraft pulp is reversible. Addition of other sodium salts (NaCl, Na2S) and sodium hydroxide to black liquor can enhance sodium ion adsorption. Measurements at a plant showed that [4] after diffusion washing, the amount of alkali in the form of adsorbed form per ton of kraft air-dried slurry was 0.6-2.5 kg Na2O.
Blistering of black liquor: Black liquor foam can cause great difficulties during drum washing. After cooking conifers, there is a large amount of surface active substances in black liquor, mainly foams caused by resin soaps and fatty soaps. Saponification can reduce the surface tension of water on the interface between black liquor and air, so it adsorbs on the surface layer to form a sealing film with a crystalline structure, high strength, and covered air bubbles. The number and size of the bubbles also have a certain influence on the speed of the drainage. This is all the more true when the black liquor containing suspended air is used instead of concentrated black liquor. The air bubbles have solid-state properties and are completely filtered out of the fibrous material, thus blocking the fluid passages and reducing the speed of the filtration. 2 Washing method All the paste washing has only one purpose. It is to extract the soluble organic and inorganic solids in the cooking process at the highest concentration, and use the least amount of dilution water while keeping the minimum when the pulp leaves the washing system. Loss of soluble solids. Only by using multiple stages of countercurrent washing, it is possible to fully exert the diffusion effect with a smaller dilution factor and achieve better washing results.
Different process conditions have a great influence on the washing efficiency. Grass pulp contains a large amount of silicon, a large number of heterocytic cells, short fibers, and a large viscosity of the black liquor after cooking. The mean diameter of the pores in the filter layer during washing is much smaller than that of wood fibers, so the dewatering rate is slow, and it is easy to paste the web, greatly reducing the Washing efficiency. The boiled pulp is too soft and dewatering is difficult, while too much black liquor remains in the slurry after pulp washing, which affects the extraction rate. When the pulp contains bubbles, the bubbles will block the pores between the fibers, affect dehydration, the temperature of the slurry is high, and the viscosity of the black liquor is small and the filtration of water is easy.