Paper pulping is the first step in pulp and papermaking. It is a vital link to determine the quality of the paper. Only using proper pulping process can produce a paper that meets different quality requirements.
As we all know, paper pulping is the process of separating the fibers in the plant fiber raw material to obtain pulp. The pulping processes mainly include mechanical pulping, chemical pulping and chemi-mechanical pulping. Since the separation of the fibers during chemical pulping is not damaged, the obtained pulp has advantages of long fiber, good stiffness, high bursting strength, and good durability. At the same time, the harmful impurities such as lignin in the raw materials are effectively removed, and the fibers are purified. Therefore, the chemical pulping process has become the main pulping process in the pulp and paper industry. Meanwhile, chemical pulping machines are also widely applied to paper and pulp production, such as pulp digester, blow tank, twin roll press, single screw press, vacuum drum washer, and etc.
Chemical pulping treat plant fiber raw materials at a certain temperature and pressure by using an aqueous solution of chemicals, dissolving lignin and non-fibrous carbohydrates in raw materials, oils, resins, etc. retaining cellulose as much as possible and retain hemicellulose in varying degrees, finally, separate the raw fiber fibers into pulp.
There are two main types of the chemical pulping process. First, alkaline pulping, including sulfate pulping, soda pulping, lime process, alkaline-oxygen pulp and ammonia pulping, the latter three methods are less applied for paper pulp making, so alkali pulping usually refers to sulfate pulping and soda pulping. The second types is sulfite pulping, including acidic sulfite pulping, bisulfite process, neutral sulfite process, alkaline sulfite process and so on.
The sulfite pulping process is a chemical pulping process that treating plant fiber raw materials with sulfite cooking liquors of different pH values. The main feature of this pulping process is that the pH of the cooking liquor has a wide selection range, from strong acidity to strong alkalinity, which can be applied for manufacturing many different types of pulp, and compared with other chemical pulps of the same lignin content, the pulp yield is higher and the color is lighter.
The earliest sulfite pulping process is the traditional method, which is low in price and low in production cost because the main chemical raw materials are sulfur and limestone. The obtained pulp is lighter in color and can be used directly to produce a wide variety of papers without bleaching. The purity of the pulp is high and it is easy to further process and refine. These important advantages make sulfite pulping the most productive chemical pulping process between 1890 and 1937. However, this traditional acidic sulfite pulping process doesn’t have a wide range of adaptation to plant materials. This pulping process should not be used for pines and some hardwood and grass materials. The cooking waste liquid produced by the pulping process is lacked a good recycling method at that time, which caused serious environmental pollution after discharge.
The sulfite pulping has been continuously improved. Gradually eliminate the calcium sulfite cooking liquor with low solubility, and switch to the cooking liquid such as magnesium sulfite, sodium sulfite and ammonium sulfite with higher solubility, and on this basis, sting a method for recovering chemicals and heat energy from the cooking waste liquid. Thereby greatly reducing the pollution of the cooking waste liquid to the environment
In addition, the use of neutral and alkaline sulfite pulping process has been developed, which greatly expands the range of application to plant materials and the variety of pulp. However, because the recycling technology of the cooking waste liquid is not as complete and mature as the recovery technology of the alkaline pulping, and the economic benefit of the recovery is low, it is still unable to compete with the sulfate pulping in the production of chemical pulp.
Alkaline pulping is another common chemical pulping, it uses alkali cooking liquor such as caustic soda to treat plant fiber raw materials and make paper pulp. The representative alkaline pulping is Kraft pulping. The main components of the kraft cooking liquor are sodium hydroxide and sodium sulfide, and also contain a small amount of sodium carbonate, sodium sulfate (salt) and sodium sulfite, etc. Due to sodium sulfate is used as a supplemental chemical in the alkali recovery process, it is called the sulfate pulping. Currently, the Kraft pulping is the most important chemical pulping process, which has a wide range of applications and is suitable for cooking both softwood fiber and hardwood and other fiber materials.
The sulfate pulping firstly realizes the industrialization of alkali recovery of cooking waste liquid. Since then, a large amount of packaging paper was required for the bulk goods to be dispensed, and as a result, the sulfate pulping was rapidly developed. Due to the emergence of chlorine dioxide bleaching technology, the difficulty of high whiteness of Kraft pulp bleaching has been overcome, which has further promoted the development of paper pulp making.
With the continuous improvement of the chemical pulping process technology, the chemical pulping process is becoming more and more scientific, the scope of application is increasingly wide, the waste recycling and treatment technology is increasingly perfect, and the environmental protection concept is increasingly rich, the chemical pulping is characterized by advantages that other pulping process never have.