Talk about Low Concentration Hydrogen Peroxide Bleaching Process Control

In recent years, with the needs of production development, our factory has used low-concentration hydrogen peroxide bleaching in chemical pulps and mechanical pulps, using existing production conditions, and achieved relatively satisfactory results. In connection with production practices, we will talk about low-concentration hydrogen peroxide bleaching. The role of hydrogen peroxide in pulp bleaching is mainly to change the structure of the colored groups in the lignin, make it a colorless gene, and to some extent make it dissolve. Therefore, hydrogen peroxide is widely used for the bleaching of mechanical pulps. For bleaching chemical pulps that require the dissolution of lignin as much as possible, hydrogen peroxide is mostly used in the final stage to finally change the structure of various gene-generating genes to stabilize the whiteness of the pulp. . Hydrogen peroxide is a non-volatile water-like liquid that acts as a bleaching agent, mainly an oxidation of hydrogen peroxide anion (OOH-). Therefore, in hydrogen peroxide bleaching, the concentration of OOH should be increased as much as possible to reduce the ineffective decomposition of hydrogen peroxide. The control of process parameters is particularly important. 1. Changes in Pulp Concentration Because China's current level of pulp and paper production is relatively low, the use of high-concentration hydrogen peroxide bleaching is not much, whether it can be performed in the hydrogen peroxide bleaching in the pulp after the ordinary concentrator, so that the pulp quality on the grade, We tried this. After the multi-disk concentrator, we performed hydrogen peroxide bleaching on ordinary milled wood pulp. The bleaching concentration was about 5% to 6%, and other process parameters were well controlled. The amount of hydrogen peroxide used was 1% to 2%. The whiteness of the machine has been increased by about 10% to meet the production requirements for the production of high brightness newsprint. In the chemical pulp, we conducted the HHP bleaching experiment of the natural bamboo pulp in the bleaching machine, and the bleaching concentration was 3%. In the case of 4%, whiteness of the pulp reached more than 80%. Therefore, as long as the bleaching process parameters are well controlled, low-concentration hydrogen peroxide bleaching can also achieve satisfactory results. 2. Pretreatment of Pulp before Bleaching Because Fe+, Mn, Cu, Fe and other metal ions will accelerate the inefficient decomposition of hydrogen peroxide, in the low-concentration hydrogen peroxide bleaching, the white water content in the pulp is high, and the metal ion content is also expected to be higher. It is very important to pretreat pulp with retanning agents, especially mechanical pulp bleaching. The selection of lanthanum mixture should be based on the type and content of metal ions in the slurry of each plant. It is generally believed that DTPA has a good chelating effect on Mn, Fe, and Cu, while sodium tripolyphosphate has a good effect on the coordination of Fe, 2%-5% CaCl2 has a good effect on Mn, and EDTA does not work as well as DTPA on Mn and Cu. The effect of lanthanum mixture is best in the alkali-free system. Only adding 0.05% EDTA can avoid the decomposition of hydrogen peroxide produced by various metal ions. Therefore, the addition of the chelating agent should be advanced, and it should be based on the different parts of the addition. The type of chelating agent should also be different. 3, the order of the preparation of liquid hydrogen peroxide bleaching generally used in the order: first alkali and magnesium sulfate and other auxiliary drugs into a mixture of liquid, mixed with hydrogen peroxide before entering the process. Into the pulp. Through practice, we first added sodium silicate to a dilute solution of magnesium sulfate to form a colloidal suspension of magnesium silicate, and then added alkali to form a mixture of the mixture and the slurry. Finally, a concentrated hydrogen peroxide solution was added. Hydrogen peroxide has a good reaction environment at the beginning of bleaching so that the bleaching efficiency is high. 4. pH Control The pH of hydrogen peroxide is between 9 and 13. The higher the pH, the greater the ionization of OOH. However, when the pH is greater than 11, the hydrogen peroxide decomposes and enhances. Experiments have shown that when the pH value is greater than 12, hydrogen peroxide is almost completely decomposed in 4O minutes. Therefore, in actual production, the bleaching pH value must be strictly controlled, and the pH at the beginning of bleaching should be controlled at IO. Between 5-11, the end point of the bleaching pH is controlled at 9. O ~ 9.5 is good. 5. Control of bleaching end point Low-concentration hydrogen peroxide bleaching should adopt high-temperature and rapid bleaching mode. The determination of the bleaching end point should be based on the content of residual hydrogen peroxide and the pH level. The time should not be too long. Even if the hydrogen peroxide bleaching process conditions are well controlled, a small amount of hydrogen peroxide will be decomposed during the bleaching process, and the oxygen generated will cause the secondary chromic groups in the pulp to be produced. We have adopted a post-bleaching wash, which is beneficial to the stability of whiteness. For the mechanical pulp, we have restored the SO2 treatment system. We use the SO2 to neutralize the residual hydrogen peroxide at the bleaching end point, and the pH is controlled at 4.5-5. O, it has achieved very good results. As sulfur dioxide is dissolved in water, sulfurous acid is generated, which can also increase the whiteness of the pulp by 2% to 3%.