Paper-Recycling System: Part 6

Chemi-mechanical re-pulping for higher-quality products is a process where ink pigments and some other chemicals are separated from the paper and removed using either washing or foaming methods. In some cases the pulp is also bleached to meet the brightness demand of the paper manufacture. The mechanical treatement steps of the de-inked pulp are similar to the uncleaned pulp steps. Normally, the process constitutes the following steps:

1. Waste paper feed and pulping
2. Pre-cleaning (mechanical)
3. Refining
4. Pigment removal (chemi-mechanical)
5. Final cleaning (mechanical)
6. Thickening, bleaching (optional), and storing
7. Drying and packaging (only for market pulp)

Mechanical reuse pulp tends to darken when treated in alkaline conditions. Darkening is prevented by adding highly oxidizing agents, such as hydrogen peroxide or sodium hypochlorite, in pulpers. Water glass is used as the main de-inking chemical. In addition, different chemicals are used depending on the method for removing pigments. During the bleaching step, pulp is heated and mixed with bleaching chemicals for from 20 minutes to 2 hours. Ultimately, to make the brightness of the fiber last longer, the pulp is stabilized with sulfuric acid.