FI68683B - SAETT ATT BEHANDLA LIGNOCELLULOSAHALTIGT MATERIAL - Google Patents

Description

f ^ SSF * l. Public notice of publication

jjgöjft l ''11) UTLÄGG NINGSSKRIFT 68683 (45) (51) Kv.lk.4 / lnt.CI.4 D 21 C 9/1 FINLAND - FINLAND (21) Patent application - Patentansökning 773710 (22) Application date - Ansökningsdag 08.12. 77 (23) Start date - Glltlghetsdag 08.12.77 (41) Become Public - Blivit offentllg IQ g £ ηβ

National Board of Patents and Registration Date of publication and hearing publication, ο o6 ος

Patent and registration authorities in the United States and other Member States · · 5 (32) (33) (31) Privilege requested — Begärd prloritet 09.12.76 Sweden i-Sverige (SE) 7613827-0 (71) SCA Development Aktiebolag, 851 88 Sundsvall, Sweden-Sverige (SE) (72) Göran Erik Annergren, Sundsvall, Tjell-Äke Hägglund, Sörberge,

This invention relates to cellulosic material. This invention relates to cellulosic material. The present invention relates to cellulosic material. The present invention relates to cellulosic material. This invention relates to cellulosic material. bleaching method, more specifically the method of using chlorine dioxide for such bleaching.

The use of chlorine dioxide, for example, to bleach sulphate pulp is based on two factors. In part, chlorine dioxide has become so cheap in the main pulp-producing countries that it is also able to compete with alternative bleaching chemicals, such as hypochlorite, from an economic point of view. In part, chlorine dioxide has a very unique selectivity, which makes it possible to produce fully bleached sulphate pulps without substantially degrading the fibrous material.

The chemical relationship of chlorine dioxide in bleaching can best be characterized as follows: 1. Carbohydrates are protected from oxidation and cleavage by receiving radicals, such as chlorine radicals.

2. Lignin is selectively oxidized so that the aromatic structure is lost.

2 68683 Perhaps the most important is the first point, as it is suitable for both pre- and final pulping. By pre-bleaching is meant here the first steps of a multi-stage bleaching sequence in which lignin is mainly dissolved. Pre-bleaching usually involves the first two or three bleaching steps. Final bleaching refers to the following bleaching steps in a multi-step sequence in which the pulp is essentially white. The second point, on the other hand, mainly concerns pre-bleaching due to the special ability of chlorine dioxide to react with phenols and etherified phenols, i. with structures approximately corresponding to sulfate pulp or sulfite pulp lignin.

In the past, when the price of chlorine dioxide was higher than that of other chemicals, it was used only in final bleaching, where its advantages strongly favored such an expensive chemical. The final bleaching technique has not really evolved much in recent years, but the reaction conditions have naturally been refined. Pre-bleaching with chlorine dioxide has instead been used to an increasing extent, e.g. due to the closure of bleaching plants and increased environmental protection requirements.

The use of chlorine dioxide as the sole bleaching chemical in the first stage has been reviewed by Rapso in "The Bleaching of Pulp", pages 132-134, Pin Monograph series - 27, (1963). The bleaching rate in the first stage then remains lower than with chlorine and the bleaching conditions must therefore be adjusted so that all chlorine dioxide is used. It is also reported that more chlorine dioxide is calculated as calculated as active chlorine, despite the better selectivity of chlorine dioxide mentioned earlier. Therefore, it is reported that the use of chlorine dioxide in the first stage usually costs so much more than the corresponding use of chlorine that the benefits such as less carbohydrate degradation and better color stability can no longer normally motivate additional costs. But for pulps with a low lignin content and having, for example, severe resin problems, it has been profitable to use chlorine dioxide as the only chemical in the first stage, and this application is known in several factories.

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Recent environmental protection requirements to reduce the impact of bleaching plant effluents have, of course, greatly increased the interest in using chlorine dioxide alone in the first phase, as this can drastically reduce the amount of chlorinated organic matter in the effluent to a negligible proportion. However, the difficulty has been the change in the reaction rate, which is why it has not been possible to carry out chlorine dioxide bleaching in ordinary chlorine towers, even though these chlorine towers have been suitable for chlorine dioxide in terms of their material. In addition to this, of course, the negative experiences of using chlorine dioxide not mixed with chlorine have had a deterrent effect. Until now, it has not been possible to explain the chemical background to the fact that chlorine dioxide, despite its expected selectivity, which is otherwise highly mixed or co-occurring with chlorine, has not achieved as good a performance as pure chlorine in pre-bleaching. reduction per defined dose of active chlorine. Kappa number means that which is defined by a standard drawn up by Scandinavian Pulp, Paper and Board and referred to as SCAN-Cl: 59, September 1959.

Optimization of reaction conditions such as pH, reaction time, temperature and charge has been done mainly for chlorine dioxide in final bleaching. In The Bleaching of Pulp, pages 154-157, Tappi Monograph series - 27 (1963), Rapson reports the optimum final pH for the first chlorine dioxide stage in final bleaching 3.5-4.5 and for the second chlorine dioxide stage the final pH 4.5- 4.5. 5.5, while Virkola in Svensk Papperstidning 65 (1962) 632 reports an optimum pH for buffered bleaching liquids of 4 to 5.

With regard to other bleaching conditions in final bleaching with chlorine dioxide, it is now generally known from several optimization studies that the chlorine dioxide charge must be applied according to the desired degree of brightness of each pulp and that the time and temperature must be applied according to the level of charge used, although some caution is required. The balance between the two chlorine dioxide stages of final bleaching should be such that about 60-70% of the chlorine dioxide charge is used in the first and 30-40% in the second final bleaching stage. Small deviations are not critical.

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Similar data are not available for bleaching in which chlorine dioxide is used as the sole bleaching chemical in the first step of the bleaching sequence.

In contrast, the use of chlorine in the first bleaching step is well optimized, and the most important points are that the chlorine charge must be applied partly according to the lignin content of the unbleached pulp, partly according to the bleaching sequence used in each case, i. according to the combination of bleaching steps, which in turn must be applied to the bleaching requirement of the pulp in question.

It has resulted from the above that for most pulp types and bleaching sequences the optimal chlorine input is a multiple of about 1.2-1.5 chlorine calculated as a percentage of active chlorine in the pulp (the range of variation depends on the different pulp types and bleaching sequences).

Therefore, it has been natural that when chlorine dioxide is used as the sole bleaching chemical in the first bleaching step of bleaching, the degree of loading expressed as active chlorine is taken from conventional chlorine bleaching. Since both cases involve the leaching of lignin and, moreover, it is known that chlorination with a small charge is negative, no one has so far made a serious attempt to use low chlorine dioxide charges in the first stage. With regard to pH values, experience has been gained primarily with the use of chlorine dioxide in final bleaching, i. final pH 3.5-4.5, although there are exceptions that are mainly due to the absence of any control or pH regulation.

Thus, J.V. In Hatton's Pulp and Paper Mag Can 68, No. 4: T 181-190, T 204 (April 1967) presented a study using a low final pH in a first bleaching step using only chlorine dioxide as a bleaching chemical. However, this low final pH is the result of experimental conditions rather than a conscious, controlled reaction condition. The chlorine dioxide charge used has been too high, partly for the selected conditions, resulting in too high a residual chlorine, which confuses the comparison, partly with a suitable balance between pre- and final bleaching when using chlorine dioxide in the first stage, as shown below 68683. In addition, comparative bleaches with chlorine have been performed using an extremely low pH, which can have various effects, which also confuses the comparison. For these reasons, he has therefore used a test design in which he has not been able to obtain optimal reaction conditions for chlorine dioxide bleaching in the first stage and therefore has not developed the correct kinetics in this case, and therefore the comparison has turned to chlorine dioxide bleaching loss only in step 1.

However, in our own bleaching studies, we have found that in the first stage, chlorine dioxide reacts sharply to a different pH than in the final bleaching. This is probably based on the fact that pre-bleaching involves pure lignin leaching, while final bleaching mainly involves the decolourisation of highly modified lignin fragments.

Studies on the effect of pH on chlorine dioxide have shown, somewhat surprisingly, that pH not only significantly increases the reaction rate of chlorine dioxide, so that the reaction rate is no longer a problem, but also that lower pH makes the chlorine dioxide reaction more efficient, possibly because pH itself facilitates the dissolution of lignin, for example through acid hydrolysis of chlorine dioxide-treated lignin. However, this latter reaction mechanism is not known and can only be considered as an extrapolation of previously known facts.

In all cases, we have succeeded in keeping the pH substantially lower than normal in chlorine dioxide bleaching to improve chlorine dioxide reactions to the extent that chlorine dioxide becomes clearly competitive with chlorine in lignin decomposition, while at the same time the previously mentioned environmental benefits of chlorine dioxide are very strong. Of particular interest is the fact that this new method was able to cope with unusually low chemical inputs in the first stage and yet achieved good lignin degradation which makes final bleaching acceptable.

This latter effect is of great importance for the economic aspect of bleaching 6 68683, and makes it possible, when carried out optimally, to compete with economically conventional methods without losing the aforementioned advantages. In the first stage, the optimal chlorine dioxide charge is a factor of 0.6-1.0 calculated as the percentage of chlorine in the mass of active chlorine, i.e. the difference is considerable when compared to the optimal chlorine charge.

The invention is illustrated by the following examples:

Diagrams 1-3 show the bleaching of birch sulphate pulp in the laboratory using chlorine dioxide as the sole bleaching chemical. The pulp used had a chlorine number of 2.7, a kappa number of 19.6 and a viscosity of 1025 dm / kg. Bleaching was performed in step 1 at a pulp concentration of 3.5% and a temperature of 25 ° C. The diagram shows the residual chlorine dioxide as a function of reaction time at different final pH values. The bleaching conditions have been applied so that a conventional chlorine tower could be used. The figures clearly show the great importance of pH for the reaction rate. Similarly, it appears that the need for pH regulation increases with lower chlorine dioxide inputs, namely closer to the input optimum. High chlorine dioxide charges reduce the need to increase acidity due to the acidic reaction products formed, but unfortunately the optimum pH is not reached until the charge is suboptimal and the bleaching sequence is unbalanced, resulting in high chemical costs and unnecessarily high carbohydrate cleavage.

Table 1 shows the results of factory tests in which birch sulphate pulp was bleached using chlorine dioxide as the sole bleaching chemical in step 1. The brightness reported in the table is defined according to Scandinavian Pulp, Paper and Board standard SCAN-P3: 75, September 1975. Brightness is expressed as ISO%.

The following prices have been used to calculate chemical costs

Chlorine 0.50 FIM / kg

Chlorine dioxide 1.15 mck / kg

Hypochlorite FIM 1.50 / kg

Sodium hydroxide 0.55 mc / kg

Sulfuric acid 0.20 FIM / kg 7 68683

Comparing the experiments with pH adjustment and the experiments without pH adjustment in the chlorine dioxide bleaching step 1, it is found that test 1c, in which the pH was adjusted, is far superior in terms of pulp quality and economy than test 1b, without pH control. Also, compared to the comparative test, 1a, which was bleached with a mixture of chlorine and chlorine dioxide, the pH-controlled test is better not only in terms of pulp quality but also in terms of cost. These advantages were obtained despite the fact that the bleaching losses of the unbleached pulp and thus the buffer capacity were somewhat higher than calculated, so that a little too little acid was invested in pH control to achieve an optimal pH, about 2.5, in Experiment 1c.

Diagrams 4-6 show the laboratory bleaching of softwood sulphate pulp using chlorine dioxide as the sole bleaching chemical in step 1. The pulp used had a chlorine number of 4.4 and a viscosity of 1060 dm / kg. The pulp concentration was 12% and the temperature 23oc. The chlorine dioxide charge was 41.8 kg of active chlorine per tonne of pulp. Diagram 2a shows the residual chlorine dioxide as a function of reaction time at different final pH values. In the experiments on which diagram 2b is based, chlorine dioxide bleaching is performed with a constant amount of active chlorine used, 41.8 kg per tonne of pulp, and one diagram shows the number of pieces and the other the viscosity after standard extraction depending on the final pH in step 1. Figure 2a shows with Figure 1 the significance of pH for the reaction rate. Figure 2b shows that the low pH gives a much more efficient lignin leaching, which is reported here in kappa number, which is considered to correlate with the residual lignin content, after the first two steps in the bleaching sequence. It should be noted that the bleaches are carried out here with the same amount of active chlorine used, so that the effect of different reaction rates is eliminated. In addition, it can be stated that low pH provides a certain viscosity advantage, except at extremely low pH values.

Table 2 shows the laboratory bleaching of softwood sulphate pulps using chlorine dioxide as the sole bleaching chemical in step 1 compared to the optimized conventional chlorination. The table shows BOD = Biochemical Oxygen Demand, determined according to the Scandinavian Pulp, Paper and Board standard, labeled 8 68683 SCAN-W 5:71, from February 1971. Chemical costs have been calculated according to the above prices.

The importance of low pH in the first step of chlorine dioxide bleaching is clear when comparing experiments 2a and 2b. Low pH is by far more advantageous both in terms of pulp quality (viscosity) and chemical costs. When bleached with chlorine in the usual way, in most cases the plus effects of lowering the chlorine number, test 2d, compared to test 2e, cannot be exploited because of the quality, whereas instead it is very excellent in the case of chlorine dioxide in the first stage, test 2a. the viscosity of the bleached softwood sulphate pulp must be more than 900 dm3 / kg in order to take advantage of the potential strength properties of the fibrous material. Thus, if chlorine dioxide is optimized as the sole bleaching chemical in the first bleaching step by adjusting the pH to a suitably low value by adding a suitable acid and applying a low level which is optimal in this case, taking advantage of the unique selectivity of chlorine dioxide and resulting in a high-quality pulp and a significant reduction in environmental effluents, and bleaching is, moreover, as economically competitive as possible.

In connection with the above, the chlorine number (ISO) has been determined according to a standard prepared by Scandinavian Pulp, Paper and Board under the designation SCAN-C 29:72.

The viscosity has been determined according to a standard developed by Scandinavian Pulp, Paper and Board under the designation SCAN-C 15:62, October 1962, and the species definitions have been amended to conform to the SI system.

COD, Chemical Oxygen Demand is determined according to the method published in "Standard Methods for the Examination of Water and Waste Water", 14th Edition 508, pages 550-554, 1975.

The invention is not limited to the described embodiments, but can be varied within the scope of the idea of the invention.

II

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Table 1. Factory bleaching of birch sulphate pulp, chlorine number about 3.0. Averages from the long-term experiment

Koe n: o la Ib le

Reference period Test period 1 without Test period pH-weather-pH control with test step _ in step 1_

Bleaching sequence _ (D + C) -E-H-D-E-D D-E-H-D-E-D-_D-E-H-D-E-D -___

Final pH step 1 2.7 5.0 3.1

Step 1 Chlorine dioxide active chlorine, multiple 0.52 0.85 0.72 "kg / ton 15.6 25.5 21.6

Chlorine active chlorine, multiple 0.43 0 0 ”kg / ton 12.9 0 0

Mass concentration% 4 4 4 Temperature ° C 30 36 36

Time min 40 40 40 Residual chlorine, active chlorine kg / ton 2.3 10.5 3.9

Step 2

NaOH kg / ton 31.4 27.5 26.0

Final pH 11.0 11.4 11.2

Step 3

Na-hypochlorite active chlorine, kg / ton 9.8 10.3 9.3 Residual chlorine active chlorine, kg / ton 3.9 2.0 2.1

Final pH 9.9 9.9 10.3

Step 4 Chlorine dioxide active chlorine, kg / ton 11.9 17.2 13.4 Residual chlorine active chlorine, kg / ton 0.3 1.1 0.8

End pH 3.1 3.9 4.3

Step 5

NaOH kg / ton 8.2 8.4 8.0

Na-hypochlorite activated carbon, kg / ton 6.3 6.5 4.1

Final pH 11.1 11.0 10.9

Step 6 Chlorine dioxide active chlorine kg / ton 8.6 12.0 9.0 Residual chlorine active chlorine, kg / ton 0.8 1.1 0.0

Final pH 3.9 3.5 3.6 10 68683

Table 1 (continued) Factory bleaching of birch sulphate pulp, chlorine number approx. 3.0. Averages from the long-term experiment

Koe n: o la Ib le ·

Reference period Test period 1 without Test period pH-adjustment pH adjustment with test or- _______stage_through 1

Valence sequence (D + C) -E-H-D-E-D D-E-H-D-E-D- D-E-H-D-E-D-

Final pH step 1 2.7 5.0 3.1

Bleached pulp

Brightness,% ISO 88.8 86.0 88.6

Viscosity chn / kg 860 780 960

Chemical.

costs irik / ton 93:90 107: 85 89:40 11 686 8 3 «

Table 2. Laboratory bleaching of two softwood sulphate pulps with chlorine number 3.7 and 5.5 in D-E-D-E-D and C-E-D-E-D sequences

Bleaching sequence D-E-D-E-D- C-E-D-E-D-

Unbleached pulp

Chlorine number 3.7 5.5 3.7 5.5

Chapter number 25 35 25 35

Viscosity dm ^ Ag IHO 1170 1110 1170

Final pH step 1 2.0 4.0 2.0 2.0 1.8

Step 1 Chlorine dioxide active chlorine, coefficients 0.9 0.9 0.9 "kg / ton 33 33 49.5

Chlorine active Cl factor - - - 1.45 1.45 kg / ton - - 53.5 79.5

Mass concentration% 12 12 12 3.5 3.5 Temperature ° C 25 25 25 25 25

Time min 60 60 60 30 30 Residual chlorine active chlorine kg / ton traces 3.2 traces traces traces

Step 2

NaOH kg / ton 22 20 29 22 31

Final pH 11.5 11.5 11.5 11.5 11.5

Step 3 Chlorine dioxide active chlorine, kg / ton 15.6 20.2 14.0 15.7 13.8 Residual chlorine active chlorine kg / ton traces traces traces traces traces

Final pH 4.0 4.0 4.0 4.0 4.0

Step 4

NaCH kg / ton 8.0 9.0 10.0 8.0 10.0

Final pH 11.9 11.9 11.9 11..8 11.9

Step 5 Chlorine dioxide active chlorine kg / ton 10.4 13.5 9.3 10.5 9.2 Residual chlorine active chlorine kg / ton traces traces traces traces traces

Final pH 4.0 4.0 4.0 4.0 4.0

Bleached pulp

Brightness% ISO 89.5 89.5 89.5 89.5 89.5

Viscosity dm ^ / kg 955 850 1050 750 900 Waste liquor analyzes Color, kg Pt / ton 59.5 58.5 88.5 120 177.5 OQD kg / ton 54 55 78.5 60 86 BCD, _kg / ton_13.3 13 , 8 16.5 17.7_21.7

Chemical costsx) FIM / ton_85: 15 92:65 105: 80 73:40 88:75

Test No. 2a 2b 2c 2d 2e