Effect of alkali consumption on hydrogen peroxide bleaching of pulp
Alkali is a very important factor in the process of pulp hydrogen peroxide bleaching. On the one hand, alkali promotes the formation of peroxide anion, which is beneficial to the bleaching process, but too high will inhibit the bleaching.
Sodium hydroxide reacts with hydrogen peroxide to form peroxide anion (HOO-):
Peroxyanion (HOO -) as an active species reacts with the chromogenic group in the pulp to bleach or whiten the pulp.
(HOO-) + chromophoric group in pulp → pulp bleaching or whitening （2）
At the same time, due to the alkaline darkening reaction, the color of the pulp will also darken, which can be simply expressed as:
Lignin + OH-→ chromophoric group (3)
According to the viewpoint of chemical reaction engineering, (1) and (3) are parallel reactions, (1) is the desired main reaction, and (3) is the undesirable side reaction.
On the other hand, the bleaching process gradually releases organic acids, so that the pH value of the subsequent reaction system decreases, which is also harmful to the reaction. Therefore, in the whole hydrogen peroxide bleaching process, there is an optimal amount of sodium hydroxide to maximize the content of peroxide anion and the maximum whiteness.
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