NZ227748A - Process for producing semibleached kraft pulp - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £27748 Fro/it', D SpC^nXl/ ri>'.»C<) "■•<••• !••• C.„S9. '//?'■> 7J. \\ 1111 4 8 Pv'^rV or* 1 v * • t.'t »'«»•• i ^ V i«- it . \. . , - 1 hj.y' ■ * ' \ w' - Patents Form No. 5 NEW ZEALAND PATENTS ACT 195 3 COMPLETE SPECIFICATION PROCESS FOR PRODUCING SEMIBLEACHED KRAFT PULP { J X/We, DEGUSSA AKTIENGESELLSCHAFT, Weissfrauenstrabe 9, D—6000 Frankfurt am Main, FEDERAL REPUBLIC OF GERMANY hereby declare the invention, for which ^/we pray that a patent may be granted to n^/us, and the method by which it is to be performed, to be particularly described in and by the following statement: Process for producing seraibleached kraft pulp Description: The invention relates to a process for producing semibleached kraft pulp without chlorine-containing bleaches. Semibleached or medium degrees of whiteness mean, in this connection, the same as degrees of whiteness between 60 and 80 (ISO).

According to the state of the art, bleaching of kraft pulps requires chlorine or chlorinated compounds, even if only medium degrees of whiteness are desired. High degrees of whiteness generally require the use of multi-stage bleaching sequences. Starting with a chlorination stage, bleaching for medium degrees of whiteness is usually carried out in two further stages (alkaline extraction and hypochlorite treatment). If higher or very high whitenesses are to be achieved, these three stages must be followed by a treatment with chlorine dioxide. Hydrogen peroxide is used only in small quantities, for instance for stabilizing the whiteness or for reducing the chlorine dioxide requirement. Current conventional bleaching sequences are, for example, C-E-H-(D) for semibleached pulps (with C = chlorination, E = extraction, H = hypochlorite, D = chlorinedioxide). For highly bleached pulps, bleaching sequences such as D-E-D-E-D-P, C-E-H-D-E-D or CD-EO-D-P-D have become established (P = peroxide, EO = extraction intensified by oxygen).

The delignification, nowadays conventional in Scandinavia, of unbleached pulp with oxygen allows a reduction in the chlorine requirement and hence also in the total of the chlorinated compounds in the effluent. Nevertheless, the pollution with chlorinated compounds is increasingly regarded as a problem. Chlorinated compounds are mutagenic, such as, for example, chloroacetone, or toxic, such as, for example, chlorophenols. The biodegradability of the chlorinated compounds is limited, so that the fundamental problem of persistence in the environment can also not be 22 7 7 48 solved by simple techniques such as, for example, the reaction with SO2• Processes , in which the chlorine requirement is drastically reduced by the use of oxygen for delignification, are therefore nowadays state of the art in countries where high environmental sensitivity has evolved. Bleaching sequences for high final whitenesses are then, for example, O-CD-EO-D-P or O-DC-EO-D. The use of large quantities of chlorine dioxide in sequential chlorination (D+C, DC) also markedly reduces the quantity of chlorinated compounds in the effluent. Medium whitenesses, however, still require the application of chlorine after the oxygen stage, for example in sequences such as O-CD-E-H or O-CD-E-D.

The further development of analaysis techniques . has led to the characterization of extremely small quantities of the highly toxic dioxin in effluent and in significantly greater quantities even in the pulp itself. Particularly in hygiene products, such as tissue for napkins, tampons and the like, the quantity of dioxin found is regarded as hazardous in spite of , the low level. There is therefore considerable interest in a process, in which at least semibleached pulps are accessible without the nowadays indispensable use of chlorine.

In the sulphate process or kraft process, the lignin is largely dissolved under strongly alkaline conditions. The high temperature and alkalinity here lead to extensive condensation and dark-brown discoloration of the lignin, which is very difficult to bleach. Under relatively mild conditions, only chlorine reacts very selectively with the residual lignin, whereas hydrogen peroxide leads only to a moderate increase in whiteness in the case of the highly condensed kraft lignin. The pulp can be brightened, for example, with 2% of H2O2 from a whiteness of 22 (ISO) to 28. Markedly higher peroxide quantities give hardly better whitenesses ((1) Degussa company brochure "Wasserstoffperoxid, Anwendung in der Zellstoffbleiche 22 7 7 4 8 [Hydrogen peroxide, use in pulp bleaching]", 1987).

The use of oxygen requires drastic reaction . conditions, under which the cellulose is also attacked.

It is therefore hardly possible, without noticeable losses in viscosity and strength, to remove more than 45-50% of the residual lignin, still present after the digestion, by means of oxygen.

This problem can be considerably diminished by an acid wash stage which follows the alkaline digestion and which in turn precedes the alkaline delignification with oxygen. ("The Bleaching of Pulp", Tappi Press 1979, 177-178, ISBN 0-89852-043-6).

The preceding wash also leads to an improvement in the whiteness by a few points.

If the acid wash is followed by a bleach with . hydrogen peroxide, an additional slight brightening is also detectable. Whitenesses of 30-33 are then accessible (Degussa company brochure, page 3), but these are inadequate for most of the desired uses.

Markedly higher whitenesses are also hardly accessible by combining the conventional oxygen delignification with a peroxide bleach. 45-50 ISO whiteness points are obtained as a maximum.

When this process is carried out in conjunction with the acid wash, a rise to the desired values is again not to be expected in view of the small increase in whiteness when oxygen delignification or hydrogen peroxide bleaching is combined with the acid wash.

The object of the invention is a process for producing semibleached kraft pulp without the use of chlorine or chlorinated compounds. It should also be possible here to employ pulps of relatively high residual lignin content.

The subject of the invention is a process for producing semibleached kraft pulp by delignification and bleaching, in which the kraft pulp is subjected to an acid wash after the alkaline digestion and then delignified with oxygen under alkaline conditions, and which is characterized in that this is followed by a 22 7 7 4 8 peroxide stage at 60-90°C, 8-25% pulp consistency and a residence time of 1-8 hours, using 0.5-4% of H2O2/ 0.2-2.5% of NaOH and 0.05-3% of water glass, the percentage data relating to the total weight of the pulp.

The acid wash is preferably carried out with the use of inorganic mineral acid, in particular sulphuric acid, at a pH value of < 4 and > 0.5 and at a temperature of 30-60°C within 1-60 minutes.

The treatment with oxygen takes place in the presence of 0.05-1% of Mg salts known per se, in particular magnesium sulphate, 0.5-7% of NaOH and an oxygen pressure of 0.1-0.5 MPa. In this step, the consistency of the pulp is 5-25%, and the selected residence time is 20-120 minutes. In place of water glass, 0.01-0.8% of a generally known complexing agent can also be used in the peroxide stage.

It is found that, by the process according to the invention, bleaching of kraft pulp even to medium whitenesses is possible by the use of oxygen and hydrogen peroxide, even in the case of pulps having a relatively high residual lignin content.

The process can readily be put into practice in plants with an existing oxygen stage, after installation of a wash system. The effluent from the O2 stage can be recycled in countercurrent to the evaporation unit, so that only the small quantity of polluted effluent from the peroxide stage must be passed to an effluent treatment plant.

Example 1-3: Combination of an oxygen treatment and a hydrogen peroxide bleach with upstream acid wash.

Example 1: A spruce kraft pulp having a basic whiteness of 4.1 (ISO) and a degree of digestion of Kappa 30 is subjected for 5 minutes to an acid treatment with H2S04 at pH 2.0 and 45°C. The pulp is washed and subjected at 100°C for 90 minutes to an oxygen delignification with 227748 2-5% of NaOH and 0.5% of MgS04 under 0.4 MPa pressure. This gives a pulp having a Kappa number of 14. Whiteness (ISO) 34.8 after the O2 stage.

In bleaching with hydrogen peroxide, the following whitenesses result at 12% stock consistency, 75°C and 4 hours residence time: H202 % 12 3 3 NaOH % 0.5 0.8 1.0 1.2 Water glass % 1.0 1.0 1.9 Whiteness (ISO) 66.3 70.2 75.8 75.2 The strengths of the pulp after bleaching, in spite of the more intensive delignification, at 20 SR are 8.8 km breaking length and 10.4 mN m2/g tearing resistance, as compared with the original values of 9.8 km and 11.5 mN m2/g.

Example 2: A pine kraft pulp having a whiteness of 22.8 (ISO) and a Kappa number of 46 is, after an acid wash for eliminating the heavy metals, subjected to an , intensive oxygen delignification at 115°C, 90 minutes residence time and 12% stock consistency. The oxygen pressure is 0.5 MPa, and the caustic soda content is 4% with 0.5% of MgSO^j. The lignin content is reduced to Kappa 15. The whiteness (ISO) after the 02 stage is 33.8. The peroxide bleach at 75°C, 20% stock consistency and 5 hours residence time gives the following whitenesses: H202 % 1 2 3 2 3 NaOH % 0.5 0.8 1.0 1.0 1.2 Water glass % 1.0 1.0 1.0 - - Whiteness (ISO) 65.2 69.1 74.3 69.5 74.1 The strengths of the unbleached pulp at 20 SR were 8.9 km breaking length and 11.9 mN m2/g tearing resistance. In the trial with 3% H202 anc* water glass stabilization, the breaking length changed to 8.0 Jan 22 7 7 4 8 and the tearing resistance changed to 10.1 mN m2/g (at 20 SR).

Example 3: A eucalyptus kraft pulp having a whiteness of 26.4 and a Kappa number of 16 is treated at 60°C for 5 minutes at pH 2 (adjusted with H2S04) and then washed. Delignification is then carried out with 2.0% NaOH at 100°C and initially 0.3 MPa oxygen pressure for 90 minutes in a tower open at the top. After the wash, the lignin content is Kappa 8. After the 02 stage the whiteness is 41.4 ISO. At 14% stock consistency, 80°C and a residence time of 4 hours, the peroxide bleach gives the following whitenesses: H2O2 % 1 NaOH % 0.5 Water glass % 1.0 Whiteness (ISO) 70.2 2 3 4 0.8 1.0 1.0 1.0 - 1.0 74.0 78.8 79.3 Examples 1-3 show that kraft pulp having , adequate whitenesses can be obtained by means of the process according to the invention.

Surprisingly, the combination of the known acid wash before the oxygen stage with a subsequent hydrogen peroxide bleach results in a synergistic effect. The increase in whiteness is greater them the sum of the increases in the combination, each carried out individually, of the oxygen or hydrogen peroxide stage with the wash stage.

The resulting cellulose pulps cannot contain any chlorinated organic compounds, since they are produced without the use of chlorine-containing compounds.

They are therefore particularly attractive precisely for the field of hygiene papers. The achievable whiteness is also entirely adequate for the hygiene paper sector and for many graphical papers, especially ligneous papers.

At & 227748 1'f ' V t

Claims (3)

WHAT WE CLAIM IS;

1. A process for producing semibleached kraft pulp, the process comprising, after the kraft pulp has been subjected to alkaline digestion, an acid wash and delignification with oxygen under alkaline conditions, subjecting the delignified pulp to a peroxide treatment at a temperature in the range 60-90°C and at a pulp consistency of 8 to 25% for a residence time of 1-8 hours, using 0.5-4% of HjOj# 0.2-2.5% of A-HeOff-and 0.05-3% of water glass or 0.01 to 0.8% of a complexing agent, the percentages being based on the total weight of the pulp.

2. A process according to claim 1 in which 0.01-0.8% of a complexing agent is used.

3. A process according to claim 1 and substantially as described in this specification with reference to any one of the examples. DEGUSSA AKTIENGESEllLSCHAFT