Straw Pulp Making Process

CNBM displacement digester system is a good choice for your straw pulp production. Get a quote now!

wheat straw for paper pulp making
Wheat straw for straw pulp making

Rice straw and wheat straw are traditional raw materials of paper pulp making. Since ancient times, they have been widely used in pulp production all over the world.

Straw belongs to Gramineae and is an annual herb. The average length and width of rice straw fiber are about 1mm and 8μm respectively. The content of miscellaneous cells in rice straw is high, which accounts for about half of the total area of rice straw cells, and the most abundant cells are parenchyma cells, which appear in an aggregated state and are not easy to separate. Compared with wood, straw contains less cellulose and lignin, more hemicelluloses and ash, and the structure of rice straw fiber is loose.

Straw Pulp Paper Features

The stiffness of straw pulp is weak, only about one-fourth of that of softwood kraft pulp. The weak stiffness leads to poor papermaking performance. However, the wheat straw pulp has a higher tensile strength than most other pulp even without beating.

The density and air permeability of paper are important indexes reflecting the internal structure of paper. Due to the existence of a large number of fine fibers in straw pulp, the density and air permeability of straw pulp paper are lower than those of wood pulp paper. The water absorption ability of straw pulp paper is also worse than that of wood pulp paper. However, because straw pulp has a better distribution of fiber length, straw pulp paper is smooth and has very good evenness.

straw pulp paper
Printing paper made from straw pulp and wood pulp

The straw pulp is fit to manufacture printing paper and writing paper without mechanical wood pulp. The advantages of straw pulp like good evenness and high smoothness can be brought into full play in these products, while the main disadvantage of low strength can be avoided.

Displacement Digester System for Straw Pulp Making

The main pulping methods of straw pulp are caustic soda pulping and Kraft pulping (sulfate pulping). The CNBM displacement digester system is ideal for the cooking process of straw pulp.

Displacement digester system for chemical paper pulping process.
Displacement digester system

Displacement Digester System Working Principle

Displacement digesting is a process in which the digesting liquid or washing water is used to replace the waste liquid and heat by using the principle of replacing circulating black liquor and diffusion washing in the batch digester.

Displacement digesting is to reuse the black liquor discharged from the last digesting process, which can remove part of the lignin, hemicellulose and extractives from the digester system at the end of delignification stage, and add digesting liquor in the supplementary delignification stage. This operation reuses the heat and chemicals in the black liquor, and makes the digesting conditions of the whole pulp digestion process tend to be balanced, which is conducive to improving paper pulp quality and production efficiency.

Steps of Straw Pulp Displacement Digesting

Displacement digester system for straw pulp making
Displacement Digester System Process

1) Loading: put the slices, cold black liquor, and some cold white liquor into the digester, and ensure the pH≥12. The cold white liquor is used for neutralizing acid on the surface of material slices.

2) Warm black liquor filling: fill the digester with warm black liquor and cold white liquid, and the cold black liquor added in the loading process is replaced to start preheating and delignification.

3) Hot black liquor filling: fill the digester with hot black liquor and hot white liquid, and replace the warm black liquor added in the previous warm black liquor filling operation.

4) Digestion: steam the digester to the required temperature and keep the temperature for a proper time. During this period, according to the temperature of the upper, middle, and lower parts of the digester, it is decided whether to continue circulating the digestion liquid.

5) Displacement: the hot digestion liquid is replaced by the filtrate of washing pulp, in the meantime, the material liquid in the digester is cooled. The displaced hot black liquor is stored in the tank area and reused in the next digesting operation. After replacement, the materials in the digester are cooled to a flash temperature below atmospheric pressure (100 ℃).

6) Tank blow: pump out the material which was cooled to below the atmospheric flash temperature out of the digester and spray it cold. The rest of the process is the same as conventional batch cooking.

Advantages of Displacement Digester System

1) Compared with the conventional digesting method, the black liquor of replacement digested pulp has a higher residual alkali, less silica content, and lower viscosity, which is conducive to the alkali recovery of waste liquor.

2) The displacement digester system brings full automation to pulp production. It also reduces the coarse slag rate and the drug consumption in the bleaching process.

3) The displacement digester system is attached with a fully closed waste heat and waste steam recovery system, which helps save more energy. Due to its obvious replacement effect, there are more thermal energy recovered from the pulping process, and the utilization of energy and chemicals is high.

4) The displacement digester system improves the yield of pulp. Because a large amount of delignification has been achieved during the filling process of warm black liquor and the amount of alkali used in the filling process of hot black liquor is reduced, the digesting reaction is more complete, and the coarse slag rate is greatly reduced and the pulp yield is greatly improved.

5) High temperature washing in the digester can improve the washing efficiency, reduce the residual alkali in the pulp after washing, and reduce the dilution factor.

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