Method for the production of high yield chemical pulp from softwood

Abstract

The present invention relates to a novel method for the production of a high yield chemical pulp from softwood in a pulp mill by the application of a delignification catalyst in a wood impregnation step and by using alkaline pulping liquors comprising boron compounds at a white liquor sulfidity level below about 25%. The method is practiced concurrently with a controlled sulfur chemicals management in the pulp mill thereby providing a pulping process which have low emission of odorous pollutants, low rate of corrosion in mill equipment and which has a rate of delignification and selectivity sufficient to produce softwood pulp in higher yield and at a quality at least on a par with a kraft pulping alternative.

Description

[0001] The present invention relates to a method for the production of high yield chemical pulp from softwood by the addition of a delignification catalyst to the wood chips prior to pulping and using alkaline pulping liquors comprising boron compounds at white liquor sulfidity levels below about 25%. BACKGROUND TO THE INVENTION [0002] Sulfate or kraft pulp with a lignin content corresponding to a Kappa number of from about 40 to about 125 is conventionally used for the production of unbleached linerboard, kraftliner and sack paper products. Linerboard and sack pulp manufactured this way has good strength properties at relatively high yields (55-65%). Softwood pulps produced in this kappa range may also be further treated by oxygen and alkali to increase brightness or be used as an intermediate step in producing fully bleached pulps from softwood. [0003] The dry weight of washed fibers, which are recovered after pulping, is generally reported as percentage of the weight of dry ligno...

AI Technical Summary

Benefits of technology

[0017] The present invention relates to a novel method for the production of high yield chemical pulp from softwood in a pulp mill by the application of a delignification catalyst in an wood impregnation step and using alkaline pulping liquors comprising boron compounds at a white liquor sulfidity level below about 25% such method practiced along with a controlled sulfur chemicals management in the pulp mill. Specifically, the novel pulping process comprises a separate impregnation step wherein softwood lignocellulosic material is contacted with an aqueous solution comprising a quinone delignification catalyst the treatment taking place at a maximum temperature of 150 degree C for a period of 1 to 120 minutes and at a pH value below about 7. The neutral or acidic impregnation step is optionally followed by extraction of excess impregnation liquor. After impregnation the chips are cooked in accordance with common practice using batch or continuous digester...

Problems solved by technology

This percentage is termed “yield.” Any decrease in yield caused by loss of lignocellulosic materials is undesirable in papermaking.

Birch or other hardwood pulps can be mixed into the raw liner pulp, however strength properties may suffer should the proportion of hardwood exceed about 20%.

While the kraft pulping process has several advantages that account for its widespread use, there are also disadvantages, including odor emissions, sulfur chemicals inventory problems and the need for a selective sulfur chemicals purge system.

To date however, alternative-pulping technologies generally cannot match the overall economics and pulp strength of the kraft process.

While AQ is effective as a catalyst for improving the rate of delignification and lowering environmental impacts, the commercial use of AQ is still limited to pulping hardwood and nonwood due to inferior pulp strength properties of softwood pulps.

Furthermore, as certain processes in the modem softwood kraft mill traditionally uses low cost sulfur chemicals for acidulation purposes it is difficult to decrease, let alone to remove, sulfur from the chemicals recovery system in the softwood kraft mill.

The management of sulfur chemicals in the modern kraft mill has become much more complicated along with stricter environmental legislation and increased recycle of mill effluents.

While sulfur management problems are present in all modern kraft mills, specific sulfur inventory build up and problems arises in sack and kraftliner pulp mills using softwood wood raw material.

The reason for this is that the natural sulfur chemicals purge in bleached kraft mills through bleach plant effluents is not an option for production of unbleached pulp qualities.

It is therefore difficult to operate kraftliner mills at liquor sulfidity levels below 35%, let alone at 25% sulfidity or lower.

Sulfur purge through removal of recovery boiler ESP ash is not economically attractive as sodium value is lost from the mill chemicals recovery cycle.

These reduced losses have a drastic impact on the make-up sodium and sulfur requirements in the kraft pulping process, which balance these losses.

Secondly, the sulfides are excellent pulping catalysts and a decrease in mill sulfidity to levels below about 25% in the white liquor has a negative impact on key kraft pulp strength properties.

While the addition of AQ to the white liquor counteract this negative effect, particularly at lower sulfidity, prior art application of AQ cannot match the efficiency and economics of sulfides used in the manufacturing of high yield chemical pulp from softwood.