Issues about alkaline pulping
Alkaline pulping is a chemical pulping method in which an alkaline solution is used to cook vegetable fiber raw materials. Including sulfate method, caustic soda method, lime method, oxygen alkali method and ammonia pulping. The latter three methods are less used, so alkaline pulping usually refers to the first two.
The active ingredients of the cooking liquor are sodium hydroxide (caustic soda) and sodium sulfide, but the sodium sulfate is used to supplement the loss of sodium sulfide in the production process, so it is called the sulfate method.
Kraft pulping process: The raw materials are first prepared (skinned, decontaminated and cut into pieces), and then cooked in a cooking liquid to form a brown pulp. The cooking temperature of the wood raw material is about 160 to 180 ° C, and the time is about 2 to 4 hours. After the lignin is removed from the lignin, it is dispersed into a fiber state.
A small amount of cellulose and a part of hemicellulose are decomposed and alkali hydrolyzed in the cooking to be dissolved and lost. The crude slurry is washed to remove the cooking waste liquid, and the impurities are removed by screening to remove the contained knots, fiber bundles and dust sand into a fine pulp. The waste liquid is made into a cooking liquid from an alkali recovery system and reused. The rosin esters and fatty acid esters contained in the wood form sodium soap during the cooking process, and a mixture of resin acids and fatty acids can be separated as a by-product of the crude sulfate soap during evaporation of the alkali recovery system, and Can be processed into Tarot oil.
The grass tissue is looser than wood, and the lignin content is less and easier to remove. The cooking of grasses, under the condition that the grass pieces are uniformly saturated with the cooking liquid, the cooking temperature is lower and the time is shorter.
The sulphate method has the following advantages: 1. The recovery technology of the chemical and thermal energy in the retort waste liquid, that is, the alkali recovery technology is perfected, so that the environmental pollutants generated in the process are effectively treated and the consumption of materials and energy is reduced; The pulp has excellent mechanical strength; 3 is suitable for almost all kinds of plant fiber raw materials, and thus has been widely used in industry.
Caustic soda pulping
The process is essentially the same as the sulphate process, but is cooked using a caustic soda solution and the alkali loss is compensated for by the use of sodium carbonate or caustic soda in the alkali recovery system. This method is commonly used to cook hardwood and non-wood plant fiber raw materials, and the quality of the obtained pulp is close to that of kraft pulp.
The recovery of chemicals and thermal energy from alkaline pulping and cooking waste liquid is an important part of alkaline pulping, which has important economic significance and control of pollution.