FI71177B - BLEKNING AV MASS - Google Patents

Description

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(51) Ky.lk.4 / lnt.CI. * D 21 C 9/10 SUOMI FINLAND (21) Patent application - Patentansöknlng 791681 (22) Application date - Ansökningsdag 25.05-79 (23) Starting date - Giltighetsdag 25-05-79 (41) Made public - Blivit offentlig 27 - 1 1 - 79

National Board of Patents and Registration Date of Appearance and Monthly Glare- 1A.08.86

Patent and registration authorities V 'Ansökan utlagd och utl.skriften publicerad (86) Kv. application - Int. trap (32) (33) (31) Privilege claimed - Begärd priority 26-05-78

England-England (GB) 22901/78 (71) Monsanto Europe S.A-, Avenue de Tervuren 270-272. 1150 Brussels, Belgium

Be1 g i en (BE) (72) Bronislav Henri May, Overijse, Belgium-Belgien (BE) (7 * 0 Oy Koister Ab (5 * 0 Bleaching of pulp - Blekning av pulp

The invention relates to a process for bleaching kraft pulp, which kraft pulp is a pulp obtained by treating wood fiber with an aqueous alkaline solution of sodium sulphides. This treatment results in a significant removal of lignin from the wood, but for the production of high quality papers, further pulp treatment (bleaching) is required to improve the brightness and reduce the lignin content to a low level.

Various bleaching agents have been proposed for this purpose. Exposure of kraft pulp to oxygen in an alkaline medium under elevated temperature under pressure is known to be effective, but in the absence of certain additives, such treatment also causes significant depolymerization of the cellulose, which in turn results in an unacceptable reduction in the physical properties of the pulp.

Magnesium salts are among the additives that prevent the depolymerization of cellulose during the bleaching of kraft pulp with oxygen, and certain organic complexing agents have also been found to be effective (see, for example, Ericsson, Lindgren and Theander, Svensk Papperstid., 74 (22), 757 , (1971).

These investigators found that both nitrile tri (acetic acid) and ethylenediaminetetra (acetic acid) were less effective than magnesium sulfate heptahydrate in preventing the breakage of cellulose fibers during unbleaching of unbleached sulfate pulp.

It has now been found that certain complexing agents, which are salts of aminomethylenephosphonic acids, can be used with successful effects in preventing the breakage of cellulose fibers during oxygen bleaching of kraft pulp. Aminomethylene phosphonates are useful in a concentration range of, for example, 0.05 to 1.0%, based on the dry weight of the pulp, but their use for the optimum effect varies with concentration. For example, in the upper part of the above-mentioned concentration range, aminomethylene phosphonate can completely replace magnesium sulfate as a protective agent, whereby the quality of the obtained pulp is better than that obtained by oxygen bleaching in the presence of an equivalent amount of magnesium sulfate. At the lower end of the concentration range, aminomethylene phosphonates alone do not provide significant protection for cellulose, but they significantly enhance the protective effect of magnesium sulfate.

The process of the invention is a process for bleaching kraft pulp and comprises exposing a kraft pulp whose fibers have been decomposed in an aqueous alkaline medium containing a complexing agent to oxygen, the process being characterized in that at least one aminomethylenephosphonic acid of the formula is used as a complexing agent: η2ο3ρη2 <Γ (H2O3t »H2C) 2U — j- CnH2nN J-jp CnH2nN (CHjPOjHj) j, wherein n is 2-6 and m is 0-5, and the salt is present in a concentration of at least 0.01%, calculated of the dry weight of the pulp, provided that at a concentration of 0,1% or less, the aqueous medium also contains a magnesium salt.

3,71177

Preferred alkali metal salts for use in the process are those salts of aminomethylenephosphonic acids having n in formula 2 or 3 and m in the range 1-4, examples of such aminomethylenephosphonic acids being: diethylenetriaminepenta (methylenephosphenicethylenephronenethylenephoseniphosphenic acid) ), tetraethylenepentaamine (methylenephosphonic acid) and pentaethylenehexamino (methylenephosphonic acid).

Also preferred are aminomethylenephosphonic acids where n: 11a is 6 and m is 0-5, for example hexamethylenediamine tetra (methylenephosphonic acid) and di (hexamethylene) triamine penta (methylenephosphonic acid).

When n: 11a has a value between 3-6, the group C "nH2n can have either a straight or branched chain structure.

Mixtures of aminomethylene phosphonates can be used? commercially available aminomethylenephosphonic acids or alkali metal aminomethylenephosphonates of this nature are often mixtures of compounds of the above formula in which n is constant, but m is a set of values, one of which is the predominant value of m. Such mixtures may also contain small amounts of aminomethylenephosphonic acids or aminomethylenephosphonates in which m: 11a has a value greater than 5? and mixtures containing these preferred aminomethylene phosphonates where n is 2 or 3 and m is a value from 1 to 4 may also contain small amounts of aminomethylene phosphonate and where m is greater than 4 or where m is 0. Preferred aminomethylene phosphonates , especially when used in the absence of a magnesium salt, are the alkali metal salts of diethylenetriaminepenta- (methylenephosphonic acid) or the alkali metal salts of aminomethylenephosphonic acids having the value 2 in formula n: 11a, with diethylenetriaminepenta (methylenephosphonic acid) in the n-principal moiety.

In addition, commercially available products are often aqueous solutions of aminomethylenephosphonic acids or aminomethylenephosphonates; in such cases, the amount of solution to be used to give the required concentration of aminomethylene phosphonate to the pulp will, of course, depend on the concentration of the solution.

When aminomethylene phosphonate is used in the mainly absence of magnesium salts 4,71177, the most solid amounts are generally between 0.15 and 1.0%, in particular between 0.2 and 0.6% by weight, based on the dry weight of the pulp. When used to enhance the effect of magnesium salts, the amount of aminomethylene phosphonate is at least 0.01% by weight on a dry weight basis, while amounts up to 1.0% may be used; economically effective amounts are usually in the range of 0.02 to 0.5%, for example 0.05 to 0.2% by dry weight of the pulp.

The alkali metal aminomethylene phosphonates can be added to the pulp as such, as the corresponding acids or, for example, as ammonium salts, whereby the acids and ammonium salts are converted into alkali metal salts in the alkaline environment of the pulp. As will be shown below, the alkalinity of the aqueous medium is usually provided by sodium hydroxide, so that the alkali metal salt of aminomethylenephosphonic acid is usually the sodium salt, but other salts, for example potassium salts, could also be used.

When aminomethylene phosphonates are used to enhance the protective effect of the magnesium salt, the preferred magnesium salt is magnesium sulfate, which is usually treated in the form of its heptahydrate, but other magnesium salts, for example magnesium carbonate, may be used. The concentration of the magnesium salt in the pulp is generally in the range found to be useful in previous methods in the art when using magnesium salts as preservatives for cellulose, for example between 0.5 and 3% by weight based on the dry weight of the pulp; good results have been obtained with pulps containing about 1.0% by weight of magnesium sulfate heptahydrate.

The main reason for the desirability of using aminomethylene phosphonate in the predominant absence of magnesium salts is that it is generally possible to obtain a pulp with a better brightness than that obtained when using a magnesium salt and enhanced with aminomethylene phosphonates (although such enhanced magnesium salts sodium salts only). "Primary absence of magnesium" thus means that the aqueous medium does not contain a sufficient amount to adversely affect whiteness, but small amounts, such as those naturally present in the water used in the process, need not be removed.

The consistency (expressed as the dry weight of the fibers as a percentage of the actual weight of the pulp) of the pulp pulp treated in accordance with method 5,71177 of this invention is between 10-30%, for example 15-25%. The alkalinity of the pulp is usually achieved by the addition of sodium hydroxide, but other water-soluble temporal substances could be used to partially or completely replace sodium hydroxide, for example sodium carbonate. The amount of sodium hydroxide or other alkaline substances used may be, for example, between 1 and 6% by dry weight of the pulp and preferably between 2.5 and 3.5% of this weight.

The oxygen pressure during the treatment may be, for example, in the range of 5 to 15 atmospheres, with preferred pressures being in the range of 6 to 10 atmospheres.

The temperature at which the oxygen bleaching of the pulp is carried out may be, for example, between 75 and 150 ° C and preferably between 90 and 140 ° C.

The duration of the treatment may vary depending on, for example, the quality of the original pulp, the amount of alkali and the temperature used. The optimum can be easily determined simply by experimentation and is usually not less than 1/2 hour or more than three hours.

The bleaching process of the present invention may optionally be implemented as a single step in a multi-stage pulp bleaching process. For example, a pulp bleached by the process of this invention can be further bleached by treatment with chlorine dioxide.

The effect of the additives on oxygen bleaching was studied using kraft pulp obtained from Scandinavian resinous wood, sold under the name "Obbola" pulp.

The pulp was exposed to oxygen in a heated autoclave under the following processing conditions: pulp consistency: 20% by weight oxygen pressure: 8 bar duration of treatment: 15 minutes for heating the pulp to working temperature and 45 minutes at working temperature.

• amount of sodium hydroxide: 3% by weight based on the dry weight of the fibers.

working temperature: 120 ° C.

additives: as shown in the following table.

The following pulp characteristics were determined before and after bleaching: 6,71177

Brightness according to French standard method NFQ 03039 using an Elrepho photometer.

Unit number determining the lignin removal rate of the pulp by standard measurement of the oxidation of the pulp with potassium permanganate according to the French standard method NFT 12018.

Average degree of polymerization of cellulose according to the French standard method NFT 12005.

(Measurement of the viscosity of a cellulose solution in a copper amine complex.)

After oxygen bleaching, the pulps were ground, i.e. softened and fibrillated using a Jokro mill. Sheets of 70 g / m 2 were made from the pulp by a method in which 10 l of powdered pulp with a consistency of 0.5% was sucked through a cloth sieve. The wet nonwoven mat thus formed was separated from the fabric and dried by heating under compression. The physical properties of the sheets were determined as follows:

Breaking length: ie the length at which a strip of paper suspended at one end breaks under its own weight - standard method AFNOR NFQ 03004.

Puncture index: hydrostatic pressure (applied through the membrane) at which the paper bursts - standard method AFNOR NFQ 03014.

Tear index: work required to further tear a strip of paper with a standard cut - standard method AFNOR NFQ 03011.

The results are shown in Table 1 below.

7 71177

table 1

Additive during bleaching

Characteristic Mass before Not used- AX0.5% AX1.0% MgSO0.-7H ~ 0 bleaching 4 2 * 1.0% brightness% 25.5 48 48.5 47.3 42.9 chapter number 31.8 8, 4 9.7 9.6 9.9 average. degree of polymerization 1265 680 822 903 857 breaking length 9720 to 10280 11126 9758 burst index 7.9 to 7.5 7.2 7.1 tear index 820 to 732 810 790 Aqueous solution containing about 50% by weight of aminomethylenephosphonic acids, of which diethylenetriamine penta (methylenephosphonic acid) is the main component. The percentages refer to the weight of this solution used, calculated on the dry weight. Phosphonic acids are converted in the alkaline pulp medium to sodium phosphonates, which are thus present in amounts of about 0.25% and 0.5%, respectively.

The results show the effectiveness of additive A, which at 1% gave better performance than MgSO 4 * 7H 2 O for all the properties studied, and at 0.5%, it gave better brightness, kappa number, break length and burst index.

A similar experiment was performed in which the additive was an aqueous solution containing about 40% by weight of ammonium hexamethylenediamine tetra (methylene phosphonate). This was converted in the alkaline stock medium to the corresponding sodium salt, which was present in an amount of about 0.4% by dry weight of the pulp. A product with a brightness of 44.5%, a break length of 9,718 and a tear index of 808 was obtained, i.e. a brightness better than that obtained with magnesium sulphate and the mechanical properties were approximately the same.

The results shown in Table 2 below were obtained using various aminomethylene phosphonates, with or without magnesium sulfate, in the specific tests described above. A blind test without additives has also been included. The results show that aminomethylene phosphonates promote the effect of magnesium sulfate in protecting cellulose against depolymerization when the former are used at concentrations where, in the absence of magnesium sulfate, there is essentially no protective effect. The results also show that in the presence of aminomethylene phosphonates, the products have a better brightness compared to the product obtained in the presence of magnesium sulfate alone.

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Claims (8)

10 71177 A process for bleaching kraft pulp by subjecting kraft pulp, the fibers of which are dispersed in an aqueous alkaline medium containing a complexing agent, to oxygen, characterized in that at least one aminomethylenephosphonic acid having the formula: ¥ 2 ° 3ΡΗ2Γ (H2O3P ) 2N - CnH2nN-— CnH2nN (CH2PO3H2> 3 LJ where n is 2-6 and m is 0-5, an alkali metal salt in a concentration of at least 0,01%, calculated on the dry weight of the pulp, provided that at a concentration of 0,1% or less, the aqueous medium also contains a magnesium salt. Process according to Claim 1, characterized in that the alkali metal salt is the alkali metal salt of diethylenetriaminepenta- (methylenephosphonic acid) or the alkali metal salt of aminomethylenephosphonic acids of formula 2, wherein diethylenetriaminepenta (methylenephosphonic acid) Process according to Claim 2, characterized in that the concentration of the alkali metal salt is 0.1 to 1%, based on the dry weight of the pulp. Process according to Claim 3, characterized in that the alkali metal salt is present in an amount of 0.2 to 0.6%, based on the dry weight of the pulp, and the magnesium salts are essentially absent from the aqueous medium. Process according to Claim 1, characterized in that the alkali metal salt is an alkali metal salt of at least one aminomethylenephosphonic acid of formula n or 2 and m is 1 to 4, the alkali metal salt having a content of between 0.02 and 0.5%, calculated the dry weight of the pulp, and the aqueous medium also contains a magnesium salt. Process according to Claim 5, characterized in that the alkali metal salt content is between 0.05 and 71177 0.2% and the magnesium salt content is between 0.5 and 3%, based on the dry weight of the pulp. Process according to one of Claims 1 to 6, characterized in that the alkali metal salt is the sodium salt. Process according to one of Claims 1 to 7, characterized in that the process is carried out at a temperature of between 90 and 140 ° C. 71177 12