FI59274B - CHLORFRI FLERSTEGSBLEKNING AV CELLULOSA - Google Patents

Description

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Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) P 2219505.7 Verified-Styrkt (71) Degussa Aktiengesellschaft, Weissfrauenstrasse 9 »6000 Frankfurt 1,

Federal Republic of Germany Förbundsrepubliken Tyskland (DE) (72) Gerhard Hebbel, Traisa, Horst Kruger, Darmstadt, Federal Republic of Germany Förbundsrepubliken Tyskland (DE) (7M Oy Kolster Ab (5 ^)

Chemically prepared celluloses obtained, for example, by the sulphite process or by the alkaline sodium or sulphate process, contain, in addition to their main constituent cellulose, small amounts of lignin, hemicelluloses and certain other constituents. Said cellulose additives, in particular lignin, cause the cellulose or its products to turn yellow or discolor. To produce high whiteness papers or other products that do not tend to turn yellow, the lignin and other interfering by-products still remaining after chemical treatment must be removed by multi-stage bleaching, usually in 3 to 8 steps. Chlorine (c), chlorine dioxide (s) and sodium or calcium hypochlorite (H) are currently used mainly as bleaching chemicals.

In addition to these, hydrogen peroxide (P) and, more recently, molecular oxygen (0g) are also used. An alkaline extract (E) is often added between the individual bleaching steps. (C.V.Bailey, C.V.Dence, Tappi, Bd 49 pp. 9-15) ·

When it is desired to prepare fully bleached cellulose (degree of whiteness <88 and MgO), a conventional bleaching series includes one or two chlorination steps containing elemental chlorine and, in most cases, in addition one or two hypochlorite steps and one or two chlorine dioxide steps. In this case, 2 59274 is formed primarily in chloroform leaching, in which case a large amount of hydrochloric acid, based on cellulose, is usually added, so that the effluent contains large amounts of hydrochloric acid or, when neutralizing, sodium chloride. To a lesser extent, this also applies to hypochlorite bleaching and chlorine dioxide bleaching.

Contamination by these inorganic substances and, above all, organic matter dissolved in cellulose bleaching means a significant load on the effluent when discharged into the sewer system. loss of strength due to oxygen treatment with additives reported in the literature.

However, no method is known in which the use of oxygen could replace the addition of chlorine-containing bleaches.

Examples of currently used bleaching methods are the phase systems C / e / H, C / e / h / E / H, C / E / H / H, C / E / H / D, C / E / d / e / D, c / e / h / d / P, C / E / H / d / e / D, C / E / H / E / D and C / E / H / d / e / H.

More recently, systems A / 02 / d / E / D, 02 / d / e / D, A / 02 / d / p / d have also been studied. (A-acid pretreatment, Og «molecular oxygen bleaching). (G.Rowlandson, Tappi 54, 962-967 (1971) No. 6).

When an alkaline end step is used, this is usually accompanied by neutralization with sulfuric acid or the like, which, however, is not generally considered to be part of the bleaching steps.

In addition, chlorine dioxide can be used in the chlorine steps, e.g. in a ratio of Cl 2 ClO 2 = 10: 1 or 20: 1.

Patents and publications disclose the use of peracetic acid as a bleaching agent in single-stage bleaching or single-stage bleaching of precursor-containing material.

All bleaching processes known to date, in which cellulose must be bleached to a whiteness of at least $ 88 MgO or more preferably at least $ 90 MgO, regularly use a bleaching step containing at least 2-3 chlorine-containing bleaches, even when a peroxide bleach is added to the multi-step system.

In contrast, a multi-stage process for the complete bleaching of cellulose has now been invented, in which case the use of chlorine or chlorine-containing substances can be completely dispensed with. In this bleaching process, which may be preceded if necessary by pretreatment of acidic cellulose, as described in the literature (P. Re-rolle, H. H. Myburgh and A. Robert, Pulp and Paper International, Juli 1969 p.

29-31, G. Rowlandson, Pin 54, 962-967 (1971) No. 6), the cellulose is bleached in the first step with peroxide, then in the second step with peracid and in the third step once more with peroxide.

When treating cellulose which is difficult to bleach or in order to save bleach, at least one further peracid and / or peroxide step can be added to the actual bleach step bleaching. Cellulose that is difficult to bleach is, for example, sulphate cellulose.

It proved particularly advantageous to allow the series to end with a peroxide step, such as in a five-step series: 1st degree hydrogen peroxide. 2nd step per-acetic acid, 3 «step hydrogen peroxide, 4« step peracetic acid, 3rd step hydrogen peroxide.

As this series has made it possible to dispense not only with chlorine but also with water glass added to peroxide bleaching, it is possible to recycle the effluents from the individual bleaching steps so that a relatively concentrated bleach removal solution is formed which can be directly evaporated and incinerated or recovered. This applies when using NaOH as a base in the peroxide steps for at least all sulphate, polysulphide, soda, sodium and sodium sulphite cellulose processes.

When other bases, such as calcium hydroxide or ammonia, are used, this also applies to other cellulose processes (calcium bisulfite process, ammonium bisulfite process and magnesium bisulfite process).

The following examples illustrate a process in which the percentages always refer to percentages by weight of unbleached oven-cellulose.

Tap water with a hardness of 3 ° dH was used in the experiments in batches containing 100 g of cellulose (dry weight). As reaction vessels, enamelled vessels were used, which were kept in thermostatically controlled ovens during the bleaching process. After each step, the cellulose was washed with tap water, after the last step, it was acidified with SO 4. The reported degrees of whiteness were determined according to the German standard method (Zellcheming-Merkblätter) with a Zeiss-Elrepho whiteness meter using a filter R 46. The density of the material is then understood as the percentage by weight of cellulose in the pulp. Number of pieces according to the Scandinavian standard method.

Example 1

Spruce-pulp pulp prepared according to the potassium bisulfite method (number of pieces 15 * 6; whiteness unbleached 30.3 l »MgO) was bleached in a five-step series under the following conditions: Step 1 1.3 ^ HgOg, 2 ^ NaOH, 12 fan density 63 ° C, 4 h Step 2 0.6 peracetic acid, 12 fan density, 75 ° C, 4 h Step 3 0.75 H 02, 1.5 96 NaOH, 12 fan density, 65 ° C, 4 h

y 2. Q

4 * step 0.4 ψ peracetic acid, 12% bulk density, 75 C, 4 h Step 5 0.5 H 2 Og, 1.0 # NaOH, 12 fan bulk density, 65 ° C, 4 h Peracetic acid step pH was: n initial value 6.0.

4,5794

Initially, 40 weight percent peracetic acid solution was used. However, references to concentrations in the second and fourth bleach levels refer to $ 100 peracetic acid.

The whiteness of the cellulose after the fifth stage was 91.5 MgO, after the third stage $ 88.9> MgO. The reaction times of the peroxide steps can be substantially shortened by using higher temperatures, for example by raising the temperature to 100 ° C, for 20 minutes. The same - although not to such an extent and at such high temperatures - also applies to the peracetic acid steps.

A three-step series using peroxide in the first step, peracetic acid in the second step and peroxide once more in the third step and an extended five-step set using peracetic acid in the fourth step and once again peroxide in the fifth step, in which hydrogen or sodium peroxide has proved particularly advantageous, have proved particularly advantageous.

It is further preferred to bleach alternately alkaline - in the peroxide steps at pH 11a to 8-15 - and acidically - with peracetic acid at pH 11a to 5 to 6.5 ttt, although peracetic acid and other paracids can in principle also be bleached in an alkaline medium. Instead of peracetic acid, other peracids can be used, such as perpropionic acid or percholic acid or their salts, however, peracetic acid is preferred. Peracetic acid itself can be added not only as an equilibrium peracetic acid but also as a pure aqueous solution or formed in its cellulose pulp from acetic anhydride or glacial acetic acid and peroxide such as hydrogen peroxide. Particularly preferred are the solutions obtained according to German patents 1,165,576 and 1,170,926.

Instead of hydrogen peroxide or another peroxide, elemental oxygen could in principle also be used at a partial pressure of about 10 at and e.g. at 100 ° C, since it is known that when oxygen is used in an alkaline aqueous medium under pressure in the presence of carbohydrates, peroxides are formed. whitening as pre-added peroxide (0, Samueleon and L.Stolpe, Svenska Pappers-tidn. 72, 662-66 (1969). No. 20).

However, the peroxide concentrations formed are relatively low, which cannot be offset by the high oxygen pressure and the high density of 25-55 required for oxygen bleaching. Even high material densities are needed in oxygen bleaching, at least when it should replace the otherwise conventional bleaching steps using elemental chlorine.

In contrast, peroxides can be bleached very well without pressure according to the new method and it is also possible to achieve good results with material densities of 10-15 # already achievable with simple equipment, in addition to which smaller amounts of peracid as a second bleaching agent are required compared to peroxide addition.

In principle, the bleaching series according to the invention, starting with the peroxide stage, could also be changed to start with the peracid stage, but it is more appropriate to start the actual bleaching series with an effective alkali treatment with peroxide to use as little peracid as possible and thus bleach as economically as possible.

If the actual bleaching process is preceded by an acid pretreatment, then in principle small amounts of peracid could already be used here, although in principle it would be more economical to use peracid in another and, if necessary, later stages in the actual bleaching process. Acidic treatment is generally referred to in the literature as pretreatment, even if it starts from sulfuric acid and thus the treatment carried out before the actual bleaching in the presence of small amounts of peracid under comparable conditions in terms of density, temperature, reaction and pH must also be regarded as pretreatment. By small amounts is meant 0.25% by weight of peracid, based on the cellulose to be bleached, in which case other acids can also be used.

Example 2

Finally, the cellulose used in Example 1 was bleached in comparison with the new bleaching method according to the conventional method in five steps: Step 1 $ 4> Cl 2, $ 3 bulk density, 20 ° C, 1 h Step 2 (extraction) $ 1> NaOH, $ 14 bulk density, 55 ° C, 2 h Step 3 1 i »active chlorine as NaOCl, bulk density of 12 #, 35 ° C, 2.5 h 4 * step 0.4 $ C102, bulk density of $ 14, 70 ° C , 4 h 5 · step 0.3 active chlorine as NaOCl, density # 12 #, 50 ° C, 2.5 h

Finally, the whiteness of the cellulose was 91 # 3 i ° MgO.

In order to obtain the most economical bleaching possible with the new chlorine-free bleaching, the bleaching must be carried out in individual steps as thick bleaches with a density of at least $ 10-15, i.e. the moist pulp should contain at least 10-15 by weight of cellulose calculated as oven dry. Material densities of up to 35 i »· n can be used.

In order to achieve a certain bleaching effect, if the bleaching agent is sufficiently water soluble, the lower the substance density, the lower the amount of bleaching agent. Also according to the new method, the pulp, as with other bleaching agents, can be diluted or washed with dilution or washing water after each bleaching stage, whereby a pulp with a density of 4 * 5 io can be obtained. The cellulose may then need to be partially dewatered for the next bleaching step until the bulk density is $ 10 or greater. The filtrate thus obtained is in turn useful in the preceding bleaching steps or in one of them as dilution or washing water. At the same time, 6,59,274 individual stages of water can still be recycled. According to this completely chlorine-free and also water-glass-free bleaching method, the flow of water can be arranged so that only after the first bleaching step the filtrate obtained by dilution and re-removal of water is sealed off. This filtrate, which now contains virtually all the organic substances dissolved in the bleaching as well as the reaction products of the bleaching chemicals, can now be directly subjected to the evaporation and incineration or reuse of the cooking liquor or recovered from the chemicals without substantially changing the process steps.

This is also the case when using NaOH in peroxide grades in at least all cellulose processes based on sodium-containing cooking liquors, such as sulphate, polysulphide, sodium, soda, sodium sulphite and sodium bisulphite processes. When Ca (OH) 2 is used instead of NaOH or Na2CO2 in the peroxide steps, this is also valid in the potassium bisulfite process and when KEL is used in all cellulose processes.

j Here we can see the substantial advantage of the new method compared to all other known bleaching methods, where this possibility of wastewater removal is in most cases not possible at all and in rare cases only a part of the wastewater is feasible.

This means that conventional bleaching methods produce effluent containing cellulose-dissolved organic matter to the extent that the water to which the effluent in many cases flows without passing through the biological clarification step is unsustainably loaded over time with respect to biological oxygen demand (BOD). In this case, it must also be borne in mind that, in addition to carbohydrates and their degradation products, bleaching effluents in conventional bleaching processes contain, above all, partially chlorinated lignin degradation products which, because of their very slow biodegradation, appear to pose less risk than they actually do.

Bleaching of cellulose, based on unbleached cellulose, involves about 5-10% by weight of organic substances and about 6-12% by weight of inorganic salts or acids in the effluent.

As described in the return of water and its recycling, after one bleaching step, the remaining amounts of unused bleaching agents may be intentionally or unintentionally introduced into one of the preceding bleaching steps. In this case, the different bleaching agents - peroxide and peracid - should not interact. Of course, more peroceids can be used in advance in the peroxide steps and more peracid in the peracid steps.

In particular, in order to prevent the decomposition of percompounds due to the presence of heavy metal ions, it may be necessary to add organic complexing agents such as ethylenediaminetetraacetic acid, diethylenetriamine-nipentaacetic acid or nitrilotriacetic acid or nitrilotriacetic acid, or salts thereof advertisements P 19 04 940 · 2, P 19 04 941 · 3 »P 19 42 556.0. The use of water glass can also be avoided.

In the case where the effluent generated by the new process is separately evaporated and dried or incinerated and thus not subjected to cooking-liquor evaporation, there is still a significant advantage over conventional bleaching methods, as there is no need to worry about chlorine corrosion and virtually free salt and silicate. ash.

The new, particularly environmentally friendly method is not limited to the complete bleaching of cellulose and other cellulosic materials to a degree of whiteness of at least 88 io MgO, but can also be used for the partial bleaching of a lower degree of whiteness of cellulose.

The method is preferably used continuously.

It is also preferred that the entire bleaching process requires 10-40 times the amount of wash and dilution water based on the cellulose to be bleached and a correspondingly smaller amount of wastewater is formed. This is achieved by circulating the wash or dilution water in the peroxide steps. Thus, the washing or dilution water can be passed from the second stage back to the first stage or from the third stage to the second or first stage. Rotation is also possible within individual steps. The same applies to peracetic acid steps or peroxide and peracetic acid steps.

Claims (8)

8 59274 Claims p A multi-stage process for chlorine-free bleaching of cellulose in an aqueous medium, characterized in that the cellulose, optionally after acid pretreatment, is bleached in a first step with an inorganic or organic peroxide or hydroperocide, then in a second step with an organic peracid and in a third step again with an inorganic or organic peroxide. Process according to Claim 1, characterized in that, when cellulose which is difficult to bleach is used, at least one peracid and / or peroxide stage is added to the three-stage process. Process according to Claims 1 and 2, characterized in that the series terminates in a peroxide or hydroperokide step. Process according to Claims 1 to 3, characterized in that hydrogen peroxide, sodium peroxide and / or t-butyl hydroperoxide are used as the peroxide or hydroperokide and peracetic acid, perpropionic acid or percholic acid or their salts are used as the peracid. Process according to Claims 1 to 4, characterized in that the amounts of peroxide or hydroperoxide are from 0.3 to 6.0 by weight and the amounts of peracid are from 0.3 to 3 * 0 by weight in each case, calculated on the basis of $ 100 in HgO cellulose. Process according to Claims 1 to 3, characterized in that the bleaching is carried out in individual steps at a temperature between 20 and 140 ° C, preferably 40 ° C, at the boiling point of the bleaching solution and at ambient pressure. Process according to Claims 1 to 6, characterized in that the peroxide steps are bleached in an alkaline medium and the peracid steps in an acidic medium. Θ. Process according to Claims 1 to 7, characterized in that only 10 to 40 times the amount of washing and dilution water is used in the entire bleaching process and correspondingly only 10 to 40 times the amount of waste water is formed, based on cellulose, the washing and dilution water being passed from the second or each subsequent bleaching step to re-diluting and washing the cellulose from one of the preceding bleaching steps and, if desired, recycled within the individual bleaching steps so that all of the bleach effluent after washing the cellulose is obtained from the first bleaching step as a filtrate by re-concentrating the cellulose buepension prior to the second bleaching step. Process according to Claims 1 to 1, characterized in that the effluent from the bleaching plant is subjected to cooking evaporation and incineration or recovery.