US3694309A - Lignin-preserving bleaching of cellulose pulp - Google Patents

Abstract

A BLEACHING PROCESS FOR CELLULOSE PULPS AND WHICH ENABLES THE LIGNIN IN THE PULP TO BE PRESERVED COMPRISES BLEACHING OF THE PULP IN THE PRESENCE OF AN ORGANIC PEROXIDE, WHICH IS PRODUCED BY CONTACTING A PEROXIDE FORMING ORGANIC COMPOUND WITH AIR OR A GAS CONTAINING FREE OXYGEN.

Description

United States Patent 3,694,309 LIGNIN-PRESERVING BLEACHING 0F CELLULOSE PULP Josef Franz Gierer, Lidingo, and Carl Torbjorn Norin, Bromma, Sweden, assignors to Svenska Traforskningsinstitutet, Stockholm, Sweden No Drawing. Filed Nov. 10, 1969, Ser. No. 875,564 Claims priority, application Sweden, Nov. 22, 1968, 15,966/68 Int. Cl. D21c US. Cl. 162-65 3 Claims ABSTRACT OF THE DISCLOSURE A bleaching process for cellulose pulps and which enables the lignin in the pulp to be preservedcomprises bleaching of the pulp in the presence of an organic peroxide, which is produced by contacting a peroxide forming organic compound with air or a gas containing free oxygen.

The present invention relates to a process for bleaching cellulose pulp while preserving the lignin therein.

There are known to the art a number of processes intended for bleaching cellulose pulps without adverse effect on the lignin therein. Reduction bleaching with dithionite and oxidizing bleaching with hydrogen peroxide or sodium peroxide are those methods which have obtained the greatest practical significance.

The reduction bleaching method with dithionite or other reduction agents, such as sulphite or boron hydride, is gentle and imparts to the pulp good properties of strength. The bleaching effect, however, is not long lasting. Pulps which have been bleached by the reduction method quickly turn yellow when coming into contact with air, particularly when subjected to light irradiation or high temperatures.

Oxidative bleaching with hydrogen peroxide or sodium peroxide produces a cellulose pulp with a high degree of brightness which is relatively stable, although at the cost of a certain deterioration in the mechanical properties of the pulps. The main disadvantages associated with peroxide bleaching, however, lie in the instability of the bleaching agent and its high price. In addition to oxidizing, and thereby removing chromophore systems in cellulose pulps, hydrogen peroxide undergoes a spontaneous decomposition which is difficult to calculate and to control. This decomposition can be reduced to a certain extent, although never entirely eliminated, by the addition of stabilizers, such as sodium silicate, magnesium sulphate, or complex builders, such as for example ethylene diamine-tetraacetic acid (EDTA). The stabilizers and complex builders are not recovered and hence create extra costs. Even when using these additives it is necessary to charge a large excess of peroxide to the system if the cellulose pulp is to obtain a high degree of brightness.

These disadvantages explain why peroxide bleaching processes have hitherto been mainly applied to cellulose pulps which respond well to bleaching per unit of weight of peroxide used i.e. in single stage bleaching of relatively bright starting pulps (semi-chemical pulps and mechanical wood pulps), in two stage bleaching in combination with a reducing bleaching stage (primarily dithionite bleaching) and in multi-stage bleaching as a terminating bleaching stage.

In accordance with the invention, cellulose pulps can be favourably bleached while preserving the lignin therein by performing the bleaching process in the presence of an organic peroxide. The organic peroxide is suitably produced, in accordance with the invention, by using air or some other oxygen bearing gas and a peroxide forming 3,694,309 Patented Sept. 26, 1972 ice organic compound under correctly adjusted conditions with regard to pH, temperature, pressure, pulp consistency, reaction rate and, optionally, irradiation.

The process of the invention is suitable as a single-stage bleaching process, but may also be used to advantage in processes embodying two or more bleaching stages. Thus, it is not restricted to types of pulp which have a relatively high degree of initial brightness, such as for example mechanical, semi-chemical and pre-bleached pulps. Particularly when using air and easily recovering organic solvents for producing the active bleaching organic peroxide compounds it is possible in accordance with the invention to use the process, to advantage, for the partial or complete bleaching also of cellulose pulps which present a low initial brightness, e.g. unbleached sulphate pulp.

Processes for bleaching cellulose pulps with oxygen gas or air are well 'known to the art. According to these processes, the cellulose pulp, whilst suspended in caustic soda solution, is treated with air or oxygen gas at high temperatures and under pressure. During this treatment the major portion of the lignin is decomposed and separated and the bleaching process does not preserve the lignin. Furthermore, this treatment causes undesirable degradation of the cellulose and hence an inferior pulp yield and impaired strength properties of the pulp.

The aim of the process of the invention is, however, to combine the economic advantages and effectiveness of, respectively, the air and the oxygen gas bleaching processes with the relative leniency of the peroxide bleaching process. This is achieved by incorporating the oxygen into organic compounds, to form organic peroxide substances which, in a lignin-preserving manner, bleach the cellulose pulps to a high and lasting degree of brightness. The process is lenient in respect of the values of the pH and temperature applied, which results therein that the yield losses caused by oxidative and/or hydrolytic decomposition are small and that the properties of strength of the resulting, bleached pulps are well comparable with the corresponding properties of pulps bleached to the same degree of brightness with hydrogen peroxide in a conventional manner.

' In accordance with the invention, the preparation of the organic peroxidic substances of suitable organic compounds by reaction with air or oxygen bearing gas and reacting the formed peroxidic components with the chromophore groups of the pulps can take place either in two separate processes or simultaneously. In the first case both the preparation of the bleaching organic peroxide compounds and the reacting thereof can be carried out under optimal conditions in respect of the aforementioned variables. The advantage with this mode of proce-. dure is primarily to be found therein that the pulp need not be subjected to high temperatures (and possibly high oxygen gas pressure) for long periods of time, whereby its strength properties are only slightly impaired. The best results have, therefore, been obtained when the step of preparing the peroxide and the step of reacting it with the cellulose pulp are carried out progressively under reaction conditions which are the most favourable one for each of these two stages.

It should also be understood, however, that the second mode of procedure, i.e. simultaneous preparation and reaction of the peroxidic compounds with the cellulose pulps, can present certain advantages. In this connection, the peroxidic compounds are formed and reacted either by introducing air or some other oxygen bearing gas continuously into a suspension of the cellulose pulp in a peroxide forming organic compound under the conditions given below, or by treating the pulp suspension with air or oxygen bearing gas under pressure in a closed .system. Since this process does at the same time involve an eifective extraction of the cellulose pulps, i.e. a removal of lipophilic and hydrophilic low molecular constituents, e.g. resins, steroids, fats, monoand oligosaccharides, there are obtained in addition to the bleaching effect, the same advantages as those when treated with solely the said compound (solvent). This combined bleaching and extraction effect is particularly advantageous in connection with lignin-preserving bleaching of mechanical, thermomechanical and semi-mechanical pulps which present a relatively high content of the extractive substances. Furthermore, the use of watersoluble, peroxide forming organic solvents has the advantage that the pulps are effectively dewatered during the bleaching operation.

In accordance with the invention, peroxide forming organic compounds are reacted with oxygen gas or oxygen bearing gas, preferably air, and the resulting peroxides are reacted with cellulose pulps. The following will discuss more closely the most suitable conditions for preparing the peroxides and their reaction with cellulose pulps.

Many peroxide forming organic compounds can be used for bleaching cellulose pulps while preserving the lignin therein. The best results, however, are obtained with organic compounds which with free oxygen give Water-soluble peroxidic compounds, particularly such compounds which readily form organic hydroperoxides and which together with water swell the cellulose pulps and thereby render them more readily accessible to attacks by the organic peroxides. Dioxan, a commercially available solvent which is produced on a large scale, has the aforementioned properties. It forms together with air or free oxygen bearing gas a water-soluble hydroperoxide (M.P. 53-56 C.) and swells cellulose pulps. It can readily be recovered by distillation and be reused for the same type of lignin preserving bleaching process. The majority of the bleaching tests in accordance with the invention have therefore been carried out with dioxan as the peroxide forming organic compound. It should be mentioned, however, that also other peroxide forming organic compounds, such as other cyclic ethers such as tetrahydrofuran, tetrahydropyran, non-cyclic ethers and alcohols, e.g. isopropanol, Z-butanol, give satisfactory bleaching results.

Preparation of the bleaching organic peroxides by a separate process prior to the cellulose bleaching process is effected either by passing air or free oxygen containing gas through the peroxide forming compound at high temperature, preferably at the boiling point of the compound, or by heating the compound with air or oxygen bearing gas at pressures in excess of atmospheric in a closed system, at temperatures between the boiling point of the compound and 50 C. thereabove, generally between 80 and 150 C. Since the rate at which the peroxide is formed depends upon the amount of oxygen gas which is dissolved in the peroxide forming organic compound, and since this quantity is proportional to the partial pressure of oxygen over the compound, it is preferred for economic reasons to prepare the compound at pressures in excess of atmospheric. Generally an air pressure of between 3-50 atmospheres above atmospheric pressure is used, corresponding to an oxygen gas pressure of between 06-10 atmospheres above atmospheric pressure, preferably -30 and 4-6 atmospheres above atmospheric pressure respectively. The use of oxygen gas instead of air shortens the reaction rate for obtaining a specific peroxide content to approximately between one third and one fifth.

During the formation of peroxide, the pH falls because of the formation of acid decomposition products from the peroxidic compounds. Since the decomposition of the organic peroxides, active during the bleaching process, is catalysed by protons, the formation of the peroxides is compensated for after a certain length of time by the decomposition, i.e. the peroxide content passes through a maximum. For the purpose of avoiding, or at least delaying, decomposition of the peroxides they can, in accordance with the invention, be prepared in the presence of stabilizers, such as sodium or magnesium salts, for instance, sodium silicate, sodium carbonate, magnesium sulphate etc. Such salts have been found essential when preparing peroxides in large quantities while using long reaction times.

If an open system is employed, the rate at which the peroxides are formed can be further increased by irradiating the system with UV-light in the presence of a sensibilizer, e.g. benzophenone.

In principal, all types of cellulose pulps can be bleached by applying the method of the present invention. The

majority of tests have heretofore been made with unbleached sulphate pulps which present a low initial brightness and with unbleached 'or pre-bleached neutral pulphite pulps and mechanical pulps having relatively high degree of initial brightness. The pulp consistency can be varied within wide limits e.g. from 1 to 25%. In practice a consistency of approximately 8% (8 g. of pulp/ ml.) has been found advantageous. Under otherwise equal conditions the bleaching result is, primarily, determined by the relationship of organic peroxide to pulp.

The reaction between the organic peroxides and the cellulose pulps is suitably effected so that the brightness is increased by the largest possible extent within the shortest possible time. The optimum bleaching conditions vary of course with different types of pulp and different organic peroxidic compounds. Normally, temperatures between 40-100 C. and reaction times between 0.5-4 hours are applied. In the case of longer reaction times an addition of stabilizers, e.g. sodium silicate, provides a favourable effect on the bleaching result.

It is evident from the aforegoing that the optimal conditions for, firstly, preparing the organic peroxides and, secondly, their reaction with cellulose pulps are not identical. If the preparation of the peroxides and their reaction with the pulps is to take place simultaneously. i.e. in one and the same process stage, the selection of the optimal bleaching conditions must therefore be based on compromises, primarily with respect to temperature, pressure, reaction time and pH. These process variables are suitably selected so that the highest possible increase in brightness is obtained with the least possible decomposition of the cellulose and of the lignin. Of course, the optimal bleaching conditions in this method of procedure are also dependent on the type of pulp to be bleached and on which peroxide forming organic compound and oxygen bearing gas is to be used.

Normally the cellulose pulps are suspended in the peroxide forming organic compound and bleaching is effected by introducing air or oxygen bearing gas through the suspension, heated to the boiling point of the compound, or by heating the suspension with air or oxygen bearing gas at pressures of atmospheres above atmospheric pressure (between 3 and 50 in respect of air, and 06-10 in respect of oxygen gas) to the boiling point of the compound or up to 50 C. above said boiling point in a closed system (autoclave tube). The reaction time is primarily determined by the degree of brightness to be obtained and the extent to which deterioration in the mechanical properties of the pulp can be tolerated. In this method of procedure reaction times of between 5 and 50 hours are normally chosen.

The invention will now be described with reference to a number of examples which illustrate the influence of different process variables on the formation of the organic peroxides, on their reaction with the different cellulose pulps and on the optical and mechanical properties of the bleached pulps.

EXAMPLE 1 Oxygen gas or air was introduced into 250 ml. of the peroxide forming organic compound during a reflux di- 6 EXAMPLE 3 Mechanical pulp from spruce (50 g.) having an initial brightness of 58.8% G.E. and an intrinsic viscosity of 153 cp. was suspended in 625 ml. water containing sodium silicate (2.5 g.) and peroxide (4 g.), prepared from TABLE l.-PEROXIDE FORMATION IN DIFFERENT ORGANIC COM- POUNDS WITH AIR OR OXYGEN GAS AT BOILING POINT Peroxide With Ml consumed forming or without .1 N organic Temper- Reaction benzo- NazSzOa compound Air or ature, time phenone solution (250 ml.) oxygen (hours) (50 mg.) per ml.

Isopropanol Air 82 68 1.60 D0 Air 82 27 1.60 2-butanol 100 24 25.50 Tetrahydrofuran. 65 48 10.00 Tetrahydropyran. 81 60 15.00 D0 81 103 60.00 Dioxan... 100 24 12.50 Do--. 100 18.10 D0.-- 100 24 4.50 Do.-. 100 48 10.00 Do... 100 24 8.10 D0 100 30 12.30

EXAMPLE 2 dioxan, was added to the suspenslon. Bleaching was In orderto show the effect of different stabilizers on the formation of peroxide the method of procedure in accordance with Example 1 was repeated with dioxan effected under the conditions disclosed in Table 3, which also shows the values of brightness, viscosity and the most important mechanical properties of some of the bleached mechanical pulps.

TABLE 3 [Bleaching of mechanical pulp (50 g.) (58.8% G.E., 153 cp.) with peroxide from dioxan (4 g.) in the presence of sodium silicate (2.5 g)] Reaction Brightness Intrinsic Beating Breaking Rupture time (percent viscosity degree length strength Tearing Double Temperature, 0. (hours) G.E.) (cp.) S.R.) (m.) (kp./cm. factor ioldings Unbleachedmechanical pulp (250 ml.) and oxygen gas at 100 C., with the addition of different inorganic salts.

-TABLE 2.-FORM.ATION OF PEROXIDE IN DIOXAN WITH OXYGEN GAS AT 100 0. IN THE PRESENCE OF INOR- oaNro sat/rs Reaction ml. consumed Nags O4 (150).-

Tests at 40 C. and a reaction time of 1 hour gave a bleached pulp having a brightness of 66.1% G.E.

EXAMPLE 4 A neutral sulphite pulp from spruce (50' g.) having an initial brightness of 48.6% G.E. and an intrinsic viscosity of 781 cp. was suspended in 625 ml. of water, optionally containing sodium silicate (2.5 g.), and was treated in the manner described in Example .3. Table 4 discloses the bleaching conditions and the brightness; intrinsic viscosity and mechanical properties of the bleached pulps.

TABLE 4 [Bleaching 01 neutral sulphlte pulp 50 g.) (48.6% G.E., 781 .53 peroxide trom dioxan (4 g.) with and without sodliim silicate With Bright- Reaction or wi ness Intrinsic Beating Breaking Rupture time out (percen viscosity degree length strength Tearing Double Temperature, 0. (hours) NazSiO; G.E.) (cp.) S.R.) (m.) (kp./cm.'-) factor toldings Unbleached neutral sulphite pulp 781 43 9, 437 7. 1 64 1, 574

Test with the bleaching of a corresponding pulp with washed with distilled water, dried in air and examined in hydrogen peroxide at the temperature of 60 C. and a respect of brightness, intrinsic viscosity and mechanical reaction time of 4 hours showed that bleaching to acproperties. The results are disclosed in Table 6.

TABLE 6 [Bleaching of different types of pulp (40 g.) with air in dioxan (21.) during refluxed digestion for 48 hours.']

With

With or without without benzo- Brightness Intrinsic Beating Breaking Rupture $11 810; g)henone (percent viscosity); degree length strength Tearing Double g. cp.

Type of pulp (40 g.) (2 4 g.) G.E.) 8.3..) (m.) (kp./cm. factor ioldings 1 Corresponding values for the starting pulps are found in Tables 3-5.

ceptable brightness can only be achieved in the presence EXAMPLE 7 of a stabilizer.

EXAMPLE Oxygen gas was forced into an autoclave tube made of stainless steel and containing a suspension of pulp (40 Unbleached sulphate pulp from pine (50 g.) having an g.) in dioxan (21.), and optionally sodium silicate (2 g.) initial brightness of 23% G.E. and an intrinsic viscosity as a stabilizer, to a pressure of 5 atmospheres above of 1128 cp. was suspended in 625 ml. of water, optionally atmospheric pressure. The sample was rotated for 6, 12 containing sodium silicate (2.5 g.), and was treated in 25 and 23 hours, respectively, in a polyglycol bath heated the manner described in Example 3. Table 5 discloses to 100 C. The obtained pulp was washed with distilled the bleaching conditions and the values of the brightness, water, dried in air and examined in respect of brightness. intrinsic viscosity and mechanical properties of the Table 7 discloses the measuring values and the bleaching bleached sulphate pulps. For comparison purposes, the conditions.

TABLE 7 [Bleaching of different types of pulp g. of each) in diolzsafi ((32 11) with oxygen gas (5 atmospheres above atmospheric pressure) With orwithout Reaction Bright- Intrinsic Beating Breaking Rupture Na-1S1O; time ness(perviscosity degree length strength Tearing Double Type of pulp (40 g.) (2 g.) (hours) cent G.E.) (cp.) 8.11.) (m.) (kpJcm factor ioldings Mechanical pulp. 23 Neutral sulphite pul 23 1 Corresponding values for the starting pulps are given in Tables 3-5. 3 Treated with air instead of oxygen.

table also discloses corresponding values for the un- What is claimed is: bleached sulphate pulp. 1. A process for bleaching cellulose pulps while pre- TABLE 5 [Bleaching of sulphate pulp g.) (23.2% G.E., 1,128 cp.) with peroxide from dioxan (4 g.) with and without sodium silicate (2.5 g.)]

With Bright- Reaction or withness Intrinsic Beating Breaking Rupture time out (percent viscosity degree length strength Tearing Double Temperature, 0. (hours) NazSiOa G.E.) (op. 8.3..) (m.) (kpJcmJ) factor foldings Unbleached sulphate pulp Tests with the bleaching of a corresponding pulp with serving the lignin therein comprising adding to unhydrogen peroxide at a temperature of C. and a rebleached cellulose pulp an organic compound selected action time of 4 hours resulted therein that bleaching to from the group consisting of alcohols and ethers, conan acceptable brightness can only be achieved in the tacting the cellulose pulp and organic compound with presence of a stabilizer. free oxygen under a pressure of from about 0.6 to about EXAMPLE 6 10 atmospheres above atmospheric pressure at a temperature of from about 80 C. to about 150 C. during Dltferent types of pulp (40 g. of each type) were a reaction time from about 5 to about 50 hours whereby suspended in dioxan (2 optionally containing sodium an organic peroxide is formed and the cellulose pulp is sihcate (2 g.) as a stabilizer and/or benzophenone (0.4 bleached thereby in one stage.

g.) as a sensibllrzer, and the suspension was refluxed 2. A process as claimed in claim 1 wherein the organic digested on the water bath during constant percolation of compound is an ether selected from the group consisting air for 48 hours. The pulp was subjected to filtration, of dioxan, tetrahydrofuran and tetrahydropyran.

9 3. A process as claimed in claim 1 wherein the 01'- 2,042,705 6/ 1936 Dreyfus 162-72 ganic compound is an alcohol selected from the group 3,514,278 5/ 1970 Brink, Jr 16278 X consisting of isopropanol and Z-butanol. OTHER REFERENCES R f s Cit d 5 Chemistry of Organic Compounds, Noller, 1951, p. UNITED STATES PATENTS 1,767,543 6/ 1930 McKee et a1. 8-111 HOWARD R. CAINE, Primary 'Examiner 3,458,394 7/1969 Yiannos et a1. 162-78 X Us Cl XR 2,939,813 6/ 1960 Wayman et a1. 162----78 2,022,664 12/1935 Groombridge et a1. 162-72 X 10 8-111; 16272, 77, 78