JPH08505907A - Chlorine-free bleaching method for pulp using acetic acid as acidifying agent - Google Patents

Abstract

  (57) [Summary] A method is described for bleaching pulp of cellulosic fiber materials without the use of chlorine when producing chemical pipes, in which the incoming pulp is continuously fed to a bleaching series to adjust the pH to a value of 7 or less. For this purpose it is treated with a sequestering agent in the presence of an acid and then bleached with a series of several solutions consisting of a peroxide, at least one organic peracid and a peroxide. According to the invention, it is proposed to acidify the pulp with acetic acid in the treatment with the sequestrant.

Description

DETAILED DESCRIPTION OF THE INVENTION Chlorine-free bleaching method for pulp using acetic acid as acidifying agent The present invention is a method for bleaching pulp of cellulosic fiber material without using chlorine when producing chemical pulp. The pulp is continuously fed to the bleaching series and treated with a sequestering agent in the presence of at least one acid to adjust the pH value below 7, followed by peroxide, one or more The present invention relates to a method for bleaching pulp, which is bleached with a series of multistage solutions consisting of an organic peracid and a peroxide. Due to the stringent environmental requirements for the use of chlorine-containing bleaching agents to bleach pulp, chlorine-containing bleaching agents that form chlorinated organic compounds are disadvantageous while other environmentally harmful substances are not adversely affected. Extensive research has been done in finding bleach. It is therefore proposed to bleach the pulp using ozone and peroxide in various orders following treatment with a sequestrant, for example as described in Swedish patent publication (SE-A) 9101300-3. Was done. However, the use of ozone causes a number of drawbacks. Ozone has a degradative effect on cellulose and the equipment for producing and supplying ozone in connection with the bleaching process is large and expensive. U.S. Pat. No. 3,867,246, corresponding to Swedish patent (SE-B) 405130, describes an alternative bleaching method including the process sequence of peroxide, peracetic acid and peroxide without the use of chlorine. Prior to the first peroxide step, a treatment with a sequestrant in the presence of acid is carried out in order to adjust the pH value to a low value. The acid used is generally sulfuric acid. However, like acetic acid, peracetic acid also has a strong degrading effect on cellulose. The object of the present invention is to bleach pulp without chlorine using peracetic acid in combination with peroxide in the bleaching process PPaP following treatment with a sequestrant (Q) in the presence of acid. It solves the drawbacks of the prior art and thus gives an instant bleached pulp with satisfactory viscosity and strength properties and a market demand of 85-90% ISO (International Standard) brightness. The process of the invention is characterized by acidifying the pulp with acetic acid during treatment with the sequestrant. The raw pulp is preferably produced by a steaming process that gives a low Kappa number, followed by delignification with oxygen gas so that the pulp has a Kappa number of about 12 or less and a viscosity of at least about 1000 dm ³ / kg. Delignify to some extent. The peroxide used is preferably hydrogen peroxide. Following the first peroxide stage, a second bleaching stage is carried out with performic acid, peracetic acid or perpropionic acid, preferably peracetic acid or a mixture of at least two of said peracids. According to a suitable embodiment, acetic acid is added in an amount such that the pH value is below 6, preferably below 5. The temperature during all bleaching stages is 80 ° C or higher, preferably 90 ° C. Suitably each peroxide step is carried out in an alkaline solution. The peracetic acid step is preferably carried out in acidic solution. Acetic acid and peracetic acid or other suitable peracids are preferably prepared in situ. The consistency of the pulp during the various treatment steps is about 8-16%, preferably 9-11%. The following example further illustrates the present invention and shows its unexpected results. EXAMPLE A number of experiments were carried out with softwood sulphate pulp which had been delignified with oxygen gas and washed. The delignified pulp had a Kappa number of 12.1, an ISO whiteness of 33.7% and a viscosity of 1020 dm ³ / kg. The pulp was fed into the bleaching series at a consistency of 10% and was maintained during the treatment with the sequestering agent EDTA and during all bleaching stages. In experiments 1 and 2 sulfuric acid was used as the acidifying agent in the Q-stage, while in experiments 3 and 4 according to the invention acetic acid was used as the acidifying agent. Experiment 1-4 The results show that, surprisingly, the pulp pretreated and bleached according to the invention has a significantly improved viscosity compared to the pretreated and bleached pulp according to Experiments 1 and 2, It also shows improved strength properties. In the second bleaching stage, an equivalent amount of peracetic acid (Pa), ie 20 kg in experiments 1 and 3 and 30 kg in experiments 2 and 4, was used in the Q-stage and 26 units each were obtained by using acetic acid instead of sulfuric acid as the acidifying agent. And the viscosity increased by 50 units. As mentioned above, acetic acid and peracetic acid or other suitable peracids are preferably prepared in situ. For example, acetic acid can be recovered from black liquor when producing pulp in an amount of up to 50 kg per tonne of pulp. For example, peracetic acid can be produced from recovered acetic acid in a known manner. Therefore, the method of the invention can be carried out in a very advantageous manner from an economical point of view.

─────────────────────────────────────────────────── ─── Continued front page    (81) Designated countries EP (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, M C, NL, PT, SE), OA (BF, BJ, CF, CG , CI, CM, GA, GN, ML, MR, NE, SN, TD, TG), AT, AU, BB, BG, BR, BY, CA, CH, CN, CZ, DE, DK, ES, FI, G B, HU, JP, KP, KR, KZ, LK, LU, LV , MG, MN, MW, NL, NO, NZ, PL, PT, RO, RU, SD, SE, SK, UA, US, UZ, V N ,,

Claims (1)

[Claims]   1. Cellulose fiber material without using chlorine when manufacturing chemical pulp A method of bleaching the pulp of , Metal ion sealing in the presence of at least one acid to adjust the pH to a value of 7 or less. Treatment with a chain agent, followed by peroxide, one or more organic peracids and peroxides. Treatment with a sequestering agent in a bleaching method with a series of several steps The pulp of cellulosic fiber material characterized by acidifying pulp with acetic acid Bleaching method.   2. The method according to claim 1, wherein the peroxide used is hydrogen peroxide.   3. The one or more organic peracids used are performic acid, peracetic acid or perpropionic acid. Preferably peracetic acid or a mixture of at least two of said peracids. Is the method described in 2.   4. Claim to add acetic acid in an amount such that the pH value is 6 or less, preferably 5 or less. 5. The method according to any one of ranges 1 to 3.   5. Each of the peroxide steps is carried out in an alkaline solution. The method described in.   6. The consistency of the pulp during the treatment with the sequestrant and during the continuous bleaching stage is about 6 -16%, preferably 9-11%. 6. A method according to any of claims 1-5.   7. Treating the incoming pulp with oxygen gas, A Kappa number of about 14 or less, preferably 12 or less, most preferably 10 The method according to any one of claims 1 to 6, which has a Kappa number of less than that.   8. Incoming chemical pulp is continuously supplied to the bleaching series to reach a pH value of 7 or less. Treated with a sequestrant in the presence of acetic acid to adjust, then peroxide, Bleaching in a series of steps consisting of one or more organic peracids and peroxides At the time of bleaching pulp without chlorine, the chemical pipe Use of acetic acid as an acidifying agent when treating with an ion sequestrant.