Waste paper disintegration and ink detachment

The first process of deinking waste paper is carried out in a hydraulic pulper. The purpose of the treatment is to fully disintegrate the waste paper and to facilitate complete removal of the ink from the fibers. At the same time, it is necessary to maintain the size of other impurities having a sufficient size so that it can be effectively removed by the screening and deslagging of the next step.

The pulp concentration when the waste paper is disintegrated is 5% to 15% or more. In order to reduce the adhesion of the ink particles to the fibers, chemicals such as surfactants are added when water is added to the hydraulic pulper. Due to the influence of these chemicals and the mechanical shear force, ink particles of different diameters are released from the fibers into the liquid phase.

During the waste paper disintegration stage, the pH in the pulper is maintained at a relatively high pH, ​​usually pH=8-10. If the waste paper does not contain mechanical wood pulp, the pH can be properly increased because the waste paper pulp has less yellowing. The high pH is obtained by adding sodium hydroxide and sodium silicate. Due to the addition of sodium hydroxide, the fibers swell and help the ink to separate from the fibers. In the high pH range, certain binders in printing inks can be saponified and hydrolyzed, thus promoting the water solubility of these inks. If fatty acid soaps are used as trapping agents, it is very important to maintain a high pH.

Sodium silicate is a water-soluble polymer that binds to polyvalent ions and adsorbs to the surface of the body particles. Sodium silicate promotes the dispersion of printing inks and prevents the accumulation of ink particles on the fiber surface. Sodium silicate is also a pH buffer that keeps the pH within the required high range.

Hydrogen peroxide is added to the hydrapulper as a bleaching aid. Hydrogen peroxide is effective at higher temperatures and concentrations. However, the high temperature will soften the stickies in the waste paper, making it extremely difficult to remove these impurities at a later stage. The slurry concentration in the hydraulic pulper is determined by the structure of the hydraulic pulper. Usually, the consistency of pulp in the waste paper treatment process does not reach the higher concentration required for effective bleaching.

After the offset printing ink has dried and stored for a long time, the alkyd adhesive molecules in the ink are cross-linked, which is difficult to remove during the general deinking process, and hydrogen peroxide can cleave these cross-linked molecular bonds. Helps remove these inks from waste paper.

Some metal ions, such as iron, manganese, and copper, catalyze the decomposition of hydrogen peroxide. Therefore, organic chelating agents, such as EDTA, DTPA, and DTMPA, need to be added to react with metal ions to prevent the decomposition of hydrogen peroxide.

The table lists the old newsprint containing mechanical pulp (waste paper I), the waste paper without mechanical pulp (waste paper II), the waste paper with low ink content (waste paper III) when the hydraulic pulp is used for pulping. Typical chemical formula.