1. Field of the Invention
The present invention relates to a process for increasing a degree of whiteness of paper fibers produced, at least in part, from printed waste paper. The process may include producing a suspension from paper pulp and water, blending bleaching aids into the suspension, adding bleaching chemicals. The process may also include intensely mechanically treating the paper pulp with a predetermined specific work, bleaching according to predetermined conditions, and removing a major portion of the color print particles separated from the fibers.
2. Discussion of the Background Information
A process similar in general to the present invention is discussed, e.g., in EP 0 557 651 A1. In this document, the process is applied to treatment of mixed waste paper. The waste paper is dissolved using NaOH and NaOCl, and subsequently undergoes conventional cleaning steps. After further thickening the waste paper is treated at a raised temperature with molecular oxygen and peroxide to release oxygen-containing radicals. This process triggers commonly known chemical reactions, in which lignin, in particular, is released from the fiber. After expulsion from the pressure chamber, the pulp is washed. The lignin, brought into solution, is removed with cleaning water. A degree of whiteness can and should be increased through such a process, i.e., the paper produced by the process should be as white as possible, even if it was recovered from printed waste paper.
In SE 94 034 949 A, a process for bleaching secondary fibrous material is illustrated. In this process, the waste paper is subjected to either a single or multiple step de-inking process, and remaining print color particles are later broken up so efficiently in a dispergator that they do not lower the quality of the paper produced. The dispergator also is increases the chances of the bleaching agents attacking the remaining ink particles. A satisfactory whiteness can be achieved in this process only if bleaching agents having relatively aggressive natures and predetermined concentrations are used. However, this leads to high bleaching material costs and could result in lower fiber quality.
A waste paper bleaching process that utilizes elementary oxygen is mentioned in, e.g., U.S. Pat. No. 5,211,809. However, the process operates exclusively through intense chemical treatment of the pulp by using a targeted chemical reaction between the contained ink materials and oxygen. In a pure chemical attack, the degree of whiteness is supposed to be raised by ink material destruction. Thus, a high degree of alkalinity is preferred in this process.