We can often see some terms of paper pulping process, such as black liquor, white liquor, red liquor, and green liquor. For a beginner of paper pulp making, you may confuse about them, what’s the meaning of these terms? What’s difference between black liquor and white liquor? CNBM helps you figure it out.
In the processing of kraft pulp or soda pulp, after pulp cooking process, about 50% of the fiber raw material is dissolved in the cooking liquor, which is called black liquor because of its dark color.
Black liquor contains inorganic substances in the cooking liquid and lignin, hemicellulose and cellulose degradation products and organic acids which are dissolved from the plant fiber materials. The inorganic substances include free sodium hydroxide, sodium sulfate, sodium sulfide, sodium carbonate, and sodium, silica, and the like combined with an organic substance. Black liquor contains about 30% to 35% inorganic, and the main components are sodium hydroxide, sodium carbonate, sodium sulfide, sodium sulfate and other sodium salts combined with organic matter. There are 65%~70% organic substances, and the main components are lignin, resin, starch, and low molecular compounds.
White liquor (English name white liquor) is a common name for cooking liquor in the paper and pulp industry. It consists of an aqueous solution of sodium hydroxide and sodium sulfide. The concentration of sodium hydroxide is 1.0mol/L, the concentration of sodium sulfide is 0.2mol/L, the pH value is 13.5~14.0, and the active components of the liquid are hydroxide ions (OH-) and sulfur ions (HS- ), hydroxide ions are derived from the hydrolysis of sodium hydroxide and sodium sulfide.
White liquor is cooking liquor that used in the Kraft pulping. It is so called white liquor because it should be distinguished to other liquids in the pulping process like black liquor, red liquor or green liquor.
Working principle of white liquor
In the cooking process, a non-fibrous material such as lignin in the fiber material is reacted with the chemical solution to form a compound dissolved in the cooking liquid, thereby being removed from the fiber to dissociate the fiber. When the white liquor is used for cooking, the lignin removal is divided into three stages; the initial delignification stage, the main delignification stage, and the residual delignification stage. In the initial stage, the temperature is raised by 150 ° C, the delignification rate is 20 to 25%, and the α-aryl ether bond and the phenolic β-aryl ether bond in the lignin structure are broken. The main delignification stage is heated to 150-170 ° C and kept warm, 60% lignin is removed, and the non-phenolic β-ether bond is broken in the lignin structure. In the residual delignification stage, the de-lignin rate is 10~15% at the late stage of 170 °C heat preservation. At this stage, the carbon-carbon bond is broken, the lignin condensation, and the yield of pulp decreases.
The main goal of cooking process control is to increase the selectivity of delignification. It can be determined by factors such as cooking time, cooking temperature, alkali amount, and degree of vulcanization. The alkali concentration has a great influence on the speed of delignification, and also affects the pulp yield and strength. The same degree of sulphurization has a great influence on the cooking rate, pulp yield, and strength. Therefore, the composition and use conditions of the white liquor are one of the important factors that control the process of the kraft pulp production process.
The red liquor is the washing liquor after pulp cooking in the sulfite pulping process. It is brownish red. It mainly includes lignosulfonate, and also contains sugars, organic acids, inorganic acids and etc.
Direct discharge of red liquid can cause pollution, so it should be recycled. Red liquor can be recycled from waste liquid. The general recycling treatment method is as follows: the red liquid is evaporated and concentrated (the solid matter content is 50% to 60%) and sprayed into a combustion furnace to generate sulfur dioxide, magnesium oxide, soda ash, etc., and then absorbed to form a sulfite cooking liquid. The heat of combustion generation can be recycled.
Green liquor refers to the chemical solution formed by dissolving the melt of the alkaline recovery system in water. The main components are sodium carbonate and sodium sulfide, and it contains a small amount of sodium hydroxide and sodium sulfate. Lime can be used to recover sodium hydroxide. Because there is a small amount of iron hydroxide in the composition, it is green, so it is called green liquid.
The melt after burning the black liquor, dissolved in a white liquor or water to form green liquor. The main component is sodium carbonate or sodium carbonate and sodium sulfide (sulfate method).
CNBM is a paper pulp machine manufacturer from China, we can supply paper pulp machine including digester, blow tank, vacuum drum washer, twin roll press, and etc. We also customize turnkey paper pulp solutions for pulp mill, and our applications include wood pulp, straw pulp, bamboo pulp, bagasse pulp, reed pulp, waste paper pulp and etc.