Reducing and bleaching composition for pulp from decoloured waste paper and its bleaching method

Abstract

A reducing and bleaching composition for decoloured waste pulp is prepared from sodium dithionite (5-15 wt.%), chelating agent (1.25-15), sodium carbonate or sodium hydroxide or their mixture (0.25-3) and water (79.5-93.5) through uniform mixing. Its bleaching method features that the stabilizer and said composition are cooperated, and the oil or solvent not dissolved in water is added to isolate air, so improving the stability of bleaching liquid, reducing consumption of sodium dithionite and improving bleaching effect. Its advantages include less damage to strength and opaqueness of paper fibres, less corrosion to pipeline, less pollution and low cost.

Description

(1) Technical field [0001] The invention relates to waste paper pulping technology, in particular to a waste paper deinked pulp reduction bleaching composition and a bleaching method thereof. (2) Background technology [0002] For recycled waste paper, it is difficult to produce deinked pulp with the same brightness as virgin fiber pulp only by purification and deinking technology. In order to obtain deinked pulp with high brightness, a post-bleaching process must be arranged after deinking. Removes all colors. Color can be caused by residual lignin in deinked pulp, such as newsprint made from high-interest rate pulp, which has a high lignin content; it can also be caused by dyes, pigments, inks and Caused by other chromogenic compounds. The technical process and conditions of waste paper bleaching are generally determined according to the waste paper raw materials to be processed, the types of paper to be copied and the requirements of environmental protection. People us...

AI Technical Summary

Problems solved by technology

Hydrogen peroxide is the most widely used bleaching agent with the best effect in pollution-free bleaching. It is mainly used for waste paper pulp (such as old newspapers) with high wood pulp content in bleaching machines, but there are also many shortcomings: the decolorization ability is relatively low. Poor; due to the use of H in the pulping process 2 o 2 , then use H 2 o 2 The value-added value of bleaching whiteness is not large, only 2-3% ISO; the cost of bleaching is relatively high; there are varying degrees of negative effects on the strength and opacity of paper fibers

The bleaching effect of FAS is better, and the wastewater is non-polluting, but the cost of bleaching is higher, and the application is limited

Sodium dithionite bleaching is easy to operate, the bleaching cost per unit of whiteness is low, and the decolorization ability is strong, and H 2 o 2 Combined use of bleaching can most effectively obtain pulp with high brightness, and bleaching with sodium dithionite can improve the brightness of pulp without any negative effect on the fiber strength and opacity of paper. Therefore, sodium dithionite is very effective Bleaching agent for secondary fiber pulp, but there are also some shortcomings: poor stability, if it encounters air or dissolved oxygen in the pulp, it will promote its ineffective decomposition, and heavy metal ions in waste paper deinking pulp and circulating white water will also catalyze the decomposition. Sodium disulfite affects the bleaching effect. The ineffective decomposition products of sodium dithionite will also corrode pipeline equipment, shorten the service life of equipment and pipelines, fail to achieve the expected increase in whiteness and yellowing of pulp after bleaching, etc.

Metal ions in deinked pulp not only catalyze the decomposition of sodium dithionite, but also cause the yellowing of bleached pulp

Some studies have tried to add chelating agent before bleaching or sodium bisulfite before storage, which has a certain effect, but not obvious