FI88316B - FOERFARANDE FOER BLEKNING AV CELLULOSAMASSA - Google Patents

Abstract

The invention concerns a procedure for the bleaching of pulp, in which procedure an oxidating bleaching chemical containing chlorine is used. The essential feature of the invention is that the pulp is treated, either in conjunction with or before oxidation, with a lignin-modifying oxidation-reduction enzyme, such as the lactase produced by the white-rot fungus. The oxidating bleaching chemical used in the procedure may be chlorine dioxide or a mixture of chlorine dioxide and chlorine gas. The use of said enzyme reduces the amount of chlorophenols and other forms of organically bound chlorine in the spent bleach liquor, simultaneously lowering its chemical oxygen demand.

Description

8831 6

METHOD FOR BLEACHING CELLULOSE PULP

The present invention relates to a process for bleaching cellulose pulp using an oxidizing chlorine-containing bleaching chemical.

In particular, the cellulose pulp obtained by sulphate soup is brown in color, mainly due to the lignin remaining in the pulp. Lignin is removed by bleaching, which is usually a multi-stage process in which the pulp is treated alternately with oxidizing, lignin-degrading chemicals and dissolving chemicals. The most widely used oxidizing chemicals have been chlorine-containing substances and oxygen, while time solutions have been used as decomposition chemicals.

Chlorine-containing bleaching chemicals react to convert the lignin in the pulp to organic chlorine compounds, which end up in the bleaching effluent leaving the pulp. Bleaching effluents are a problem for environmental protection due to the potential toxicity of organochlorine compounds entering the water, and the values of chemical oxygen demand in effluent wastes are also harmfully high.

The effluents from the bleaching, which have the greatest impact on the environment, are generated in the washing steps following the first chlorination and the first time of bleaching. Efforts have been made to reduce the load by various methods, such as replacing chlorine gas with chlorine dioxide or using oxygen as the oxidizing chemical in the first bleaching stage, or by biological treatment of the bleaching effluent. However, these measures have not yielded completely satisfactory results. Although the use of chlorine dioxide and oxygen bleaching have significantly reduced the amount of chlorophenols and other toxic chlorine compounds in wastewater, the values of chemical oxygen demand in water have not been reduced sufficiently. Thus, effective biological purification is needed to support said methods.

The object of the present invention is to provide a solution by which cellulose pulp can be bleached in such a way that the toxicity of the bleaching effluent is reduced and at the same time the value of the chemical oxygen demand of the water is reduced so that the need for water purification is reduced. The invention is characterized in that the pulp is subjected to an enzyme treatment using a lignin-modifying oxidation-reduction enzyme during or before the oxidation.

Previous studies have found that hemisululose-degrading enzymes make it possible to release lignin and / or hemisululose from cellulose and thus make the pulp more easily bleached in subsequent bleaching steps. To date, no similar effect has been shown with lignin-modifying enzymes.

According to the invention, it has now been found that treatment with a suitable oxidation-reducing enzyme reduces the amount of chlorine chemicals required for bleaching and thus also the organochlorine compounds in the bleaching effluent.

According to the invention, the redox potential of the enzymatic reaction is preferably less than 400 mV, more preferably less than 250 mV. Suitable oxidation enzymes include e.g. phenol oxidases, exemplified by the lactate produced by the white rot fungus.

With the exception of the enzyme process, the bleaching of the pulp according to the invention can take place in a conventional manner in such a way that the oxidation and scale steps take place and after each step the pulp is washed to remove bleaching chemicals and degraded constituents.

According to the invention, the enzyme treatment preferably takes place in the temperature range from 10 to 90 ° C, preferably in the range from 40 to 80 ° C, and at a pH of 3.0 to 10.0, most preferably 4.0 to 8.0. The enzyme used is a lignin-modifying oxidation-reduction enzyme, e.g. the above-mentioned laccase.

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The invention also relates to the use of a lignin-modifying oxidation-reduction enzyme, such as, for example, laccase produced by white rot fungi, in the bleaching of cellulose using an oxidizing chlorine-containing bleaching chemical.

The invention is described in more detail below by means of exemplary embodiments based on laboratory experiments.

Example 1 To 220 g of pulp sulphate soup dry matter (pulp dry matter content 30%) was added a diluted enzyme mixture, Trametes hirsuta white rot fungus laccase, so that the mixture had a consistency of 10% and a laccase activity of 60 U / kg dry matter. . The temperature of the enzyme treatment was 55 ° C and the treatment time was 2 h. After the enzyme treatment, the stock was filtered in a Buchner funnel and the resulting mass cake was washed with deionized water to a washing ratio of 20.

For comparison, the corresponding batch of pulp was treated in the same way but without the addition of enzyme.

Next, the enzyme-treated pulp stock and the control batch were subjected to bleaching in five successive oxidation and extraction steps. Chlorine (90%) and chlorine dioxide (10%) were used as chemicals in the first stage, sodium hydroxide in the second stage, chlorine dioxide in the third stage, sodium hydroxide in the fourth stage and chlorine dioxide in the fifth stage.

The consumption of bleaching chemicals and the analytical results describing the quality of the bleached pulp are shown in Table 1.

The experiment shows that the amount of chlorine in bleaching has been substantially reduced by using a lignin-modifying enzyme, lacquer glove.

* 8316 4 TABLE 1

Enzyme treatment, end pH viscous kappa yield% pine sulphate pulp, kappa 27.4 reference batch 8.70 1020 26.4 100.84 enzyme treatment pulp 8.60 1045 26.5 100.63

The test series continues with the bleaching sequence D / C-E-D-E-D

D / C 10/90 Reference enzyme - step batch treated D / C 2 * kappa pulp D / C dose% 5.28 5.30 consumption% 5.28 5.30 pH 1.65 1.55 consistency% 3.5 3.5 temperature ° C 55 55 time. 45 45 E-phase E 0.9 * kappa E-dose% 2.38 2.39 pH 11.65 11.3 kappa 9.6 8.2 yield% 96.13 96.81 D-or they dose% 3.0 3.0 consumption% 3.0 3.0 pH 1.9 1.8 consistency% 10 10 temperature ° C 70 70 time min. 180 180 E2 step: dose X 1.0 1.0 pH 11.2 11.3 consistency Ϊ 10 10 temperature ° C 60 60 for a while. 90 90 D2 phase dose Ϊ 1.5 1.5 consumption% 1.5 1.42 pH 2.2 2.2 consistency% 10 10 temperature ° C 75 75 time. 240 240 acidification pH 4.2 3 yield% 97.96 98.18 total yield% 94.96 95.65 brightness 83.8 87.1 viscosity TC1 mg / kg 3 ^ 45 265 DCM extract%

Claims (7)

A process for bleaching softwood cellulose pulp, using a method for oxidizing chlorine-containing bleaching chemical, characterized in that the pulp of coniferous wood consists of pine sulfate pulp and that the pulp is enzymatically treated during the oxidation, the potential of the enzymatic reaction is below 400 mV. Process according to claim 1, characterized in that the redox potential of the enzymatic reaction is preferably below 250 mV. 3. A process according to claim 1 or 2, characterized in that the pulp is treated with phenol oxidase as lacquered and washed before the first oxidation stage. Process according to any of the preceding claims, characterized in that as a bleaching chemical for the oxidizing stage, chlorine oxide or the mixture of chloride oxide and chlorine gas is used and that between the oxidation stage the pulp is treated with alkali, such as sodium hydroxide. Process according to any of the preceding claims, characterized in that the enzyme treatment takes place within the temperature range 10 - 90 ° C, preferably 40 - 80 ° C, and at the pH 3.0 - 10.0, preferably 4.0 - 8. 0th Use of an oxidation-reducing enzyme that modifies lignin in the bleaching of cellulose, which uses an oxidizing chlorine-containing bleaching chemical. 7. The use of phenol oxidase, which is lacquered by the production of carrot fungus, in the bleaching of cellulose, which uses a b 8 3 ϊ 6 oxidizing chlorine bleaching chemical. j