Paper pulping technology is very important to the paper quality. As we all know, paper pulping process can be divided into several steps, such as pulp cooking, pulp washing, and screening, pulp bleaching, etc. Except for the paper pulp machine like digester, blow tank, drum washer, and twin roll press, there is some vital technology is applied to the paper pulping process. As a paper pulp machine manufacturer with rich experience, CNBM analyzes the paper pulping technology including pulp cooking, oxygen delignification, and pulp bleaching today.
Pulp cooking
Pulp cooking is classified into various stages, including wood chip filling, charging of cooking liquor, heat to cooking temperature, final cooking and heat recovery and pulp discharge. All of stages are called a complete digester cycle, different fiber material has different digester cycle.
- Chip filling: after materials preparation, the qualified wood chips will be sent into digester.
- Charging of liquor: sent the cooking liquor into digester, cooking liquor is made up of white liquor, black liquor and water according to the settled proportion. The charging volume is decided by he liquor rate and moisture of wood chip.
- Heat temperature: heating and raise temperature up to required value (150-170℃) of chemical cooking by direct heating or indirect heating.
- Discharge: when the pulp cooking is finished, the fiber materials will blow or be pumped into blow tank.
Before pulp cooking process, there are some factors should be considered:
- Fiber materials property. Common materials for paper pulp making include wood, bamboo, sugarcane bagasse, reed, wheat straw, cotton, etc.
- The requirement of pulp types. Pulp types include wood pulp, straw pulp, and waste paper pulp.
- Working condition of the pulp mill. Paper pulp machine (digester, blow tank, vacuum drum washer, twin roll press, etc), alkaline recovery, and other equipment.
- productivity benefit: social benefit and economic benefit
Oxygen delignification
Oxygen delignification means that use oxygen and alkali to remove the most of residual lignin. In general, Oxygen delignification will ensure higher whiteness of pulp, which makes the pulp bleaching easier. Compared to the traditional pulp bleaching (without oxygen delignification), do the treatment of oxygen delignification before bleaching will be more superior. To be specific, the dissolved materials of oxidation stage can be recovered in the process of unbleached pulp washing, which means that oxygen delignification reduce the pollution.
Oxygen delignification can apply for any type of paper pulp. As the transition section between pulp cooking and pulp bleaching, Oxygen delignification is related to cost, pulp quality and environmental effect. Middle consistence pulp oxygen delignification is so common among applications, the lignin removal rate is 30%-50%.
The organics dissolved in the oxidation stage and delignification chemicals remains in the pulp will be removed in the pulp washing process, and transported into chemicals recovery system. The main function of oxygen delignification is pollution reduction. Besides, oxygen delignification also reduces the cost of bleaching agent.
Pulp bleaching
In the stages of pulp cooking and oxygen delignification, compare the carbohydrate dissolution and carbohydrate decomposition, there is a suitable choice when delignification reaches special point. According to the delignification, it is different for range of point, due to the pulp cooking process and oxygen delignification technology, the range of point is limited in every stage.
Above development can be found in most of bleaching stages, it actually caused by multiple stages bleaching. In another words, a bleaching process should not be more than the dissolved point of lignin. In the following stage, paper pulp is better to be treated by reaction of chemicals and lignin. In some special situation, there is lignin in the following stage. For example, the extraction stage is between the first and second chlorine dioxide bleaching stage, after extraction, the pulp will be treated in a new stage that is similar to the first bleaching stage.
According to the basic principle of pulp bleaching, pulp bleaching can be divided into three stages, including primary stage, transmit stage and stagnation stage. With the lignin content decrease, the pulp bleaching will be short. Especially for softwood Kraft pulp with 10 Kapa.
