Chemical pulping methods include the soda process, the sulfite process, and the solvent process. The soda process is further divided into the lime process, the caustic soda process, the sulfate process, and the sodium polysulfide process. Among these, the sulfate process is the absolute mainstream method in the pulp industry, especially for wood-based raw materials. Pulp produced by the sulfate process accounts for more than 80% of the pulp market. The fluctuating pulp prices in the market mainly refer to the prices of sulfate pulp. When we talk about the fluctuating pulp prices, it basically refers to the prices of sulfate pulp.
Why do we need to make pulp?
The purpose of making pulp is to convert wood chips or wood shavings into individual fibers. The main components of wood are cellulose and lignin. Cellulose is the main component of fibers and is the substance we need. The reason why wood chips can form compact block or sheet structures is that wood contains lignin. Lignin binds fibers together and provides binding strength. However, lignin cannot form a linear structure of fibers and is dark in color and inflexible, making it unsuitable for papermaking. Therefore, when producing fiber raw materials for papermaking (i.e., pulp), it should be separated from cellulose and removed. However, in its natural state, lignin is bound to fibers and is insoluble. Therefore, liquid agents must be added and high-temperature and high-pressure conditions must be applied to dissolve the lignin. These liquid agents are called "cooking liquor". We will discuss cooking liquor in more detail later. At the same time, during the process of dissolving lignin, it is necessary to retain the cellulose components as much as possible and not dissolve them - cellulose constitutes fibers, and if cellulose is dissolved, the fibers will be destroyed. This process of dissolving lignin is called "pulping". Since the pulping process mainly involves dissolving lignin and leaving the wood fibers, the study of lignin structure becomes a crucial step. Research on lignin began nearly 200 years ago. French botanist and chemist Anselme Payen first proposed in 1830 that wood is composed of cellulose and another substance. After more than a hundred years of research by generations of scientists, the structure of lignin has been basically clarified.
The basic units of lignin are three types:
These three structures respectively polymerize into the corresponding lignins. It has been determined that the lignin of gymnosperms (coniferous wood) is mainly guaiacyl lignin. The lignin of dicotyledons (broad-leaved wood) is mainly guaiacyl-syringyl lignin. Monocotyledons (herbaceous plants, such as rice straw, wheat straw, reed, carex, bamboo, etc.) are mainly guaiacyl-syringyl-p-hydroxyphenyl lignin. The following figure shows a small fragment of the molecular structure of lignin in herbaceous plants. For a long time, the research focus of foreign scientists has been on lignin in woody plants, and there has been relatively little research on lignin in herbaceous plants. However, China is short of wood resources, but has abundant herbaceous plant resources. Therefore, how to use herbaceous plants for pulping has become an urgent problem to be solved. After the founding of the People's Republic of China, generations of scientific and technological workers overcame numerous difficulties and finally, in the mid-1980s, thoroughly clarified the structure of lignin in herbaceous plants related to pulping, as well as the chemical reactions and dissolution mechanisms during the pulping process. These studies have established China's leading position in the field of herbaceous plant pulping in the academic world, filled an important gap in plant fiber chemistry and pulping and papermaking technology, and written a brilliant chapter. We must give a thumbs up to our scientific and technological workers!
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