Chemical pulping refers that make pulp from raw material with a chemical agent. This method uses fiber plants (mainly wood and straw) as raw materials, and removes most of the lignin by the chemical agent that can selectively react with the lignin contained in the raw materials, and makes the single fiber of material is sufficiently separated into pulp, generally, obtained pulp has a high purity of cellulose. There are two types of pulp commonly used in industrial production: alkali pulping and sulfite pulping. About 70% of the world’s pulp was produced by chemical pulping in the 1980s.
Chemical pulp classification
- According to the processing, it can be classified into alkali pulp, sulfate pulp, sulfite pulp, neutral sulfite pulp, and chlorinated pulp.
- According to the raw materials, it can be divided into chemical wood pulp, straw pulp, bamboo pulp, sugar cane bagasse pulp, cotton pulp, reed pulp and etc.
- Pulp quality and use are various because the different raw materials and pulping process. For example, sulfite wood pulp, clean and soft, easy to bleach, can be used to make high-grade writing paper and printing paper; sulfate wood pulp, fiber is slender and strong, can be used to manufacture industrial technical paper.
Chemical pulping process and paper pulp machines
Pulping process of chemical pulping consists of three basic steps including preparation, pulp cooking and purification, as well as auxiliary process.
The wood is peeled and cut into wood chips or the grass stems are cut into grass segments. And then filtered to remove impurities, so that the raw materials are relatively clean, the size and specifications are relatively uniform. Preparation is prepared for the physical and chemical reactions in the next cooking process.
Pulp cooking means that basic process of transforming raw materials into chemical pulp. Put the qualified material or section and a proper amount of chemicals into the digester, strictly control temperature, pressure, time and chemical composition, the chemical penetrates into the raw material and chemically reacts with the lignin in the raw material, thereby The individual fibers are loosely separated into pulp, and the pulp obtained by cooking is called a brown pulp. In most cases, the primary wall of the fiber cell is also destroyed. In the case of a relatively strong cooking reaction, a part of the lignin is also dissolved from the second wall of the fiber, and other components such as carbohydrates, resins, inorganic salts are also partially removed, thus the pulp produced by the chemical process, usually contain a high content of cellulose.
The digester is the basic equipment for the chemical pulping and cooking process. It is divided into two categories according to the pulping process: batch digester and continuous digester. The commonly used batch digester has a rotary digester and a fixed vertical digester. The equipment is relatively simple, the required investment is low, and the operation and maintenance are relatively easy. It is more suitable for small and medium-sized pulp mills, but the heating steam load is not balanced, the waste heat recovery is difficult, and the uniformity of the prepared pulp is poor. The continuous digester is divided into vertical type, horizontal tube type, and inclined tube type according to the arrangement of the main body of the equipment. The main parts of various digester are metal pressure vessels with a large volume, and the chemical reaction of the cooking process is carried out in this container. The continuous digester equipment is complex, with high investment and high requirements for operation and management. However, the steam load for heating is balanced, the waste heat recovery is easy, and the uniformity of the pulp produced is good, which is suitable for large-scale pulp mills. After the 1970s, the digester used in the new pulp mill around the world, for the production capacity, the batch digester and the continuous digester equally shared the market. In the 1980s, some batch digesters were able to approach or even better than continuous digester after adopting computer program control and energy-saving new technology.
After pulp cooking, the coarse pulp can be used for papermaking after proper purification. Purification of pulp generally includes pulp washing, pulp screening, pulp bleaching.
- Pulp washing separates the pulp from the cooking waste liquid. In order to wash the pulp with a minimum amount of water and obtain a higher concentration of cooking waste liquid for comprehensive utilization, three or four vacuum drum washer or pressure washer are generally used for washing.
- Pulp screening removes the coarse chips and fiber bundles from the washed pulp, using screener and various types of hydrocyclones (conical sand removal) to remove the heavier impurities such as sediment contained in the pulp.
- The role of pulp bleaching is to reduce or remove residual lignin in the pulp or the chromophores (the main colored material in the pulp) to achieve a higher whiteness of the pulp. common bleaching equipment includes disc filter, twin roll press, single screw press, etc.
Auxiliary processes include cooking liquor preparation, chemicals for pulping and cooking waste, and heat recovery processes.