Method for brightening mechanical pulps

Abstract

The present invention is directed to an improved method for brightening mechanical pulp under neutral or alkaline papermaking conditions. The improvement comprises the steps of: (a) separating neutral or alkaline pulp dilution water into a high-solids stream and a neutral or alkaline low-solids stream; and (b) reusing the neutral or alkaline low-solids stream for pulp dilution purposes prior to a bleaching process.

Description

[0001] This invention relates generally to a method for brightening mechanical pulp.[0002] Paper pulp typically is subjected to a brightening process prior to paper making. The presence of transition metal ions in paper pulp is known to be detrimental to the brightening process. Chelation techniques, also known as Q stage techniques, have been used to remove transition metal ions from pulp, thereby enhancing brightness levels. Y. Ni et al., Pulp & Paper Canada, vol. 98, T285 (1998). Treatment of pulp with sodium hydrosulfite prior to chelation, known as the Q.sub.y stage technique, is believed to improve chelation of metals, thereby further enhancing pulp brightness. However, this technique is less effective at higher pH values, such as those encountered when precipitated calcium carbonate ("PCC") is used as a filler in a pulp and paper mill.[0003] The problem addressed by this invention is to find a more effective method for brightening mechanical pulp at high pH values.STATEMENT O...

AI Technical Summary

Benefits of technology

[0011] Dilution water (i.e. cloudy white water) typically is added to mechanical pulp prior to a bleaching step. In an integrated pulp and paper mill, the dilution water is a recycled stream from the paper making operations. In acid-based mills, the dilution water typically is at a pH from 4 to 5, and contains impurities such as pulp fines, suspended and dissolved solids, fillers and transition metal ions. In a neutral to alkaline paper making environment, i.e., one that utilizes PCC as a filler, the dilution water is at a pH from 6 to 8, and contains impurities such as pulp fines, suspended calcium carbonate, and transition metal ions. Of these impurities, transition metals can be especially troublesome as they can catalyze the decomposition of bleaching chemicals resulting in reduced bleaching efficiency and lower brightness levels. They also tend to increase brightness reversion and thus further contribute to lowering the brightness of bleached pulp. Both reductive and oxidative bleaching chemicals are affected by transition metals. The most commonly used oxidative bleaching chemical is hydrogen peroxide. Reductive bleaching chemicals typically are aqueous reducing agents, including, e.g., dithionite anion, also known as hydrosulfite, borohydrides and bisulfites, and formamidine sulfinic acid.

[0013] According to this invention, the use of the low solids stream under neutral or alkaline conditions (pH 6 to 8) minimizes the adverse effects on the brightness of the resulting pulps. The Examples demonstrate that the reduction of the amount of solids in the cloudy white water significantly lowers the levels of transition metals and other impurities in the low solids stream and thereby improves the efficiency of bleaching. Preferably, the pH of the dilution water is from 6.5 to 7.5.

[0015] Chemical treatment of the high solids stream recovered from cloudy dilution water from neutral or alkaline processes according to the method of this invention allows recycling of the solids without adverse effects on brightness of the resulting paper and pulps. Without being bound by theory, it is believed that addition of a reducing agent to the solids recovered from the pulp dilution water reduces the valences of the transition metal ions. The reduced valences in turn result in better chelation of transition metals, and treated solids that typically have reduced levels of transition metals, and thus can be introduced into the bleaching and paper making process without adversely affecting pulp brightness.

Problems solved by technology

The presence of transition metal ions in paper pulp is known to be detrimental to the brightening process.

However, this technique is less effective at higher pH values, such as those encountered when precipitated calcium carbonate ("PCC") is used as a filler in a pulp and paper mill.

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