Procedure for the bleaching of sulphate pulp

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: The present invention relates to a procedure for the bleaching of pulp, in which procedure an oxidating bleaching chemical containing chlorine is used. Especially pulp obtained from a sulphate pulping process is of brown colour, which is mainly due to the lignin remaining in the pulp. Lignin is removed from the pulp by bleaching, which is generally a process consisting of several stages. During this process, the pulp is treated alternately with oxidizing, lignin-degrading chemicals and chemicals dissolving the degradation products. Oxidizing agents commonly used are chlorine-containing chemicals and oxygen, whereas alkali solutions are used for eliminating the degradation products. In the reactions occurring in bleaching using chlorine-containing chemicals, lignin is converted into organic chlorine compounds, which remain in the spent bleach liquor. Spent bleach liquors are a problem in regard of environmental protection because of the possible toxic nature of the organic chlorine compounds contained in the liquors. Moreover, the chemical oxygen demand in spent bleach liquors reaches detrimental levels. The spent bleach liquors causing the worst environmental pollution load are produced in the washing stages following the first chlorination and the first alkali treatment in the bleaching process. To reduce the pollution load, various methods have been employed, including the use of chlorine dioxide instead of chlorine gas, the use of oxygen as an oxidating chemical in the first bleaching stage, and biological purification of the spent bleach liquor. However, these methods have not yielded completely satisfactory results. Although the amounts of chlorophenols and other toxic-chlorine compounds in the spent bleach liquor have been significantly reduced by using chlorine dioxide and employing oxygen bleaching, it has not been possible to achieve a sufficient reduction in the chemical oxygen demand values of the effluents. Therefore, the methods referred to have required the employment of efficient biological purification. DE-A-3 636 208 (see also WO-A-8 803 190) discloses a method based on the treatment of natural or mechanically modified pulp raw material. The process is described with one example in which straw is treated with enzymes in order to dissolve the lignin. It has to be noted that there are significant differences of the structure of straw lignin and wood lignin (Wei He Diesheng Tai and Stephen Y. Lin, 1991). In addition, the known process calls for a pretreatment, whereby straw was pretreated with 1-2 % NaOH for 24-28 hrs, and was grinded before addition of enzyme. Such pretreatment is technically difficult, and not suitable for removing of the wood lignin. Int. Cont. Biotechnol. Pulp & PaperInd. (1986), 67-69, concerns the enzyme treatment of a cellulose mass concentrating mainly on the use of the hemicellulases. By the use of hemicellulases chemical savings in the chlorine bleaching have been obtained, but the use of oxidases and ligninases is based on completely different reaction mechanisms which are difficult to rule (Lehninger). In the publication it is mentioned that the use of peroxidases was tested with birch and pine peroxyacid pulps. The pulps produced by the peroxyacid treatment differ substantially from the conventionally produced pulps. Additionally, the enzymed tested were peroxidases or ligninasees. In the publication it is also mentioned that the lignin content of the pulp dit not change substantially as a result of the treatment. This shows how difficult it was to reduce the lignin content with enzymes. Tappi, Vol 67, 1984, 31-33, the possibilities of "lignin-degrading enzymes" is discussed in general terms. The publication does not contain any concrete knowledge of the subject to be discussed. The object of the present invention is to achieve a sulphate pulp bleaching procedure that enables the toxic content and chemical oxygen demand of the spent bleach liquor to be reduced so as to reduce the need for purification of the liquor. This object is achieved by the characterizing part of claim 1. It has been observed in earlier investigations that by using enzymes degrading hemicellulose it is possible to separate lignin and/or hemicellulose from cellulose and thus make the pulp more easily bleachable in the subsequent bleaching stages. This effect has not been observed before in conjunction with the use of lignin-degrading enzymes. According to the invention, it has now been observed that a treatment with a suitable redox enzyme reduces the amount of chlorine chemicals required for bleaching, thereby also reducing the amounts of organic chlorine compounds left in the spent bleach liquor. According to the invention, the redox potential of the enzymatic reaction should be below 400 mV, preferably below 250 mV. Suitable redox enzymes are phenoloxidases, such as the laccase produced by the white-rot fungus. Except for the enzyme treatment, the bleaching of pulp by the procedure of the invention can be performed in the conventional manner by employing alternate oxidation and alkali stages and washing the pulp after each of these stages to remove the bleaching chemicals and degradation products. The enzyme treatment as taught by the invention is preferably carried out in a temperature range of 10-90 °C, the most suitable range being 40-80 °C, with pH values in the range 3.0-10.0, preferably 4.0-8.0. The enzyme used is the lignin-modifying oxidation-reduction enzyme, laccase, as mentioned above. The invention also concerns the use of a lignin-modifying enzyme, e.g. the laccase produced by the white-rot fungus, in the bleaching of sulphate pulp using an oxidating bleaching chemical containing chlorine. In the following, the invention is described in greater detail by the aid of examples of embodiments based on laboratory experiments. Example 1 A diluted enzyme mixture of laccase produced by the Trametes hirsuta white-rot fungus was added to 220 g of dry matter obtained from pine sulphate pulp (with a dry matter content of 30%) so that a mixture with a consistency of 10% and a laccase activity of 60 U/kg of pulp dry matter was obtained. The temperature in the enzyme treatment was 55 °C and the duration of treatment 2 h. After the enzyme treatment the pulp wax filtered in a Buchner funnel and the pulp cake obtained was washed with ion-exchange-treated water so that a wash ratio of 20 was obtained. For reference, a corresponding portion of pulp was treated in the same way except that no enzyme was added. Next, both the pulp quantity which had undergone enzyme treatment and the reference portion were bleached by a process consisting of five successive stages of oxidation and bleaching. In the first stage, the chemicals used were chlorine (90%) and chlorine dioxide (10%), in the second stage sodium hydroxide, in the third stage chlorine dioxide, in the fourth stage sodium hydroxide and in the fifth stage chlorine dioxide. The consumption of bleaching chemicals and the analysis results representing the quality of the bleached pulp are presented in Table 1. The experiment shows that by using a lignin-modifying enzyme, i.e. laccase, a substantial reduction in the amount of chlorine has been achieved.

Claims: Procedure for the bleaching of sulphate pulp, said procedure using an oxidating bleaching chemical containing chlorine, characterized in that, before oxidation, the pulp is subjected to an enzyme treatment in which a lignin-modifying oxidation-reduction enzyme is used, and in that the enzyme used is a lignin-modifying phenoloxidase, such as the laccase produced by the white-rot fungus. Procedure according to claim 1, characterized in that the redox potential of the enzymatic reaction is below 400 mV, preferably below 250 mV. Procedure according to any one of the preceding claims, characterized in that the pulp is treated with a phenoloxidase, e.g. laccase, and washed before the first oxidation phase. Procedure according to any one of the preceding claims, characterized in that the bleaching chemical used in the oxidation phases is chlorine dioxide or a mixture of chlorine dioxide and chlorine gas, and that between the oxidation phases the pulp is treated with an alkali. Procedure according to any one of the preceding claims, characterized in that the enzyme treatment is carried out in a temperature range of 10-90 °C, preferably 40-80 °C, with pH values in the range 3.0-10.0, preferably 4.0-8.0. Procedure according to any one of the preceding claims, characterized in that the procedure is used for the bleaching of softwood pulp, e.g. pine sulphate pulp. Use of a lignin-modifying oxidation-reduction enzyme wherein the enzyme used is a phenoloxidase, e.g. the laccase produced by the white-rot fungus in a sulphate pulp bleaching process using an oxidating bleaching chemical containing chlorine.

Inventors: MIIKKI KARI, VAHERI MARJA, MIIKKI, KARI, VAHERI, MARJA
Applicants: ENSO GUTZEIT OY, ENSO-GUTZEIT OY
Application year: 1989
Main classification: D21C9
IPC codes: D21C9, D21C5, C12S3