US2741537A - Available chlorine single-stage bleaching of sulfate pulp - Google Patents

Description

AVAILABLE CHLORINE SINGLE-STAGE BLEACHING OF SULFATE PULP William A. Stone, New Orleans, La., and John George Matarese, Evanston, 111., assignors to Olin Mathlesou Chemical Corporation, a corporation of Virginia N Drawing. Application March 26, 1952, Serial No. 278,748

2 Claims. (C1. 8-105) Our invention relates to improvements in the multistage bleaching of sulfate cellulose pulps and more particularly to a bleaching operation employing a combination of chlorine dioxide, particularly in the form of a water soluble chlorite, and chlorine in the first or preliminary bleaching stage.

In the conventional methods of multi-stage bleaching of sulfate cellulose pulps, chlorine has been a commonly used reagent. While this reagent, used most frequently in conjunction with hypochlorites, will produce a satisfactory color, it tends to impair the strength of the sulfate pulp, the tendency being particularly marked in cases where a high degree of bleaching is required. The strong oxidizing power of chlorine and hypochlorite is apparently a chief cause of this undesirable degradation, although direct reaction of the chlorine and sulfate pulp may be partially responsible.

In the bleaching of sulfate pulp many variations in procedure are possible. However, the minimum operation ordinarily consists of at least three stages, the first of which is a chlorination stage where the pulp is treated with chlorine at an acid pH for from about five to ten minutes. Under these conditions the lignin is largely chlorinated due to the presence in the acid solution of most of the chlorine as dissolved elemental chlorine. Oxidation is avoided by the acidity of the solution which restricts the formation of hypochlorous acid, the oxidizing agent in the system. Furthermore, since the chlorination reaction is relatively rapid as compared to the oxidation reaction, limiting the time of contact also assists in avoiding oxidation.

In a typical operation, pump consistency is about 1.5 to 3 per cent and the preliminary chlorine bleach is carried out at about 70 F. The chlorinated pulp is extracted in a second stage with caustic, for example with 2 per cent caustic, at 120 to 140 F. using a 10 per cent pulp consistency. The residual from the caustic extraction stage is maintained at about 0.9 gram per liter of available chlorine and is passed to a tile tank and held for about one hour at a temperature of about 60 F. The residual from the hold tank with about 0.2 gram per liter of available chlorine is then passed to a first washer, repulped with hot water and a hypochlorite and passed to a third stage which comprises the beating stage. Additional hypochlorite is added to the beaters and the pulp beaten for about an hour at 90 F. prior to being passed to a second washer. Brightnesses of the bleached pulp average between 68 and 70 per cent but strengths are poor.

While chlorine dioxide and chlorites have been widely used in bleaching sulfate pulps to obtain high brightness without decrease in strength, tearing or fold resistance, their use has ordinarily been restricted by the relatively high cost of chlorites as compared with chlorine to stages subsequent to the first or early chlorine bleach stage where the principal advantage of the chlorine dioxide and chlorites are utilized to the maximum degree.

nited States Patent 0 2,741,537 Patented Apr. 10, 1956 We have now found that when chlorine dioxide and chlorine are combined in particular proportions in the first or preliminary stage in bleaching a sulfate pulp, substantial advantages for the bleaching operation, particularly in chlorine savings, may be realized. For example, we have found that the combination chlorine dioxidechlorine bleach unexpectedly reduces the total available chlorine requirement of the pulp to a substantial extent, e. g. by 20 to 50 per cent. In addition, substantial savings may be realized by using less caustic for neutralization, by elimination of bleaching in and operation of the beaters, by elimination of a second stage washer, and by reduction in the amounts of pigments used for brightness and opacity. Equivalent or higher brightnesses, greater strength retention, easier and faster bleaching and less color reversion are also obtained.

Our invention resides in an improvement in the process of bleaching sulfate cellulose pulps in which the pulp to be bleached is contacted in aqueous medium with chlorine, neutralized, and washed. In the process of our invention the pulp to be bleached is contacted in aqueous medium with chlorine and chlorine dioxide, the mole ratio of chlorine to chlorine dioxide being within the range of about 2.5 to 3.5: l. Advantageously the pulp in aqueous medium is contacted with chlorine and a water soluble chlorite at a pH of about 2.5 to 2.7, the mole ratio of chlorine to chlorite being within the range of about 3.5 to 4.5. Of the water soluble chlorites, we prefer to use sodium chlorite.

The operation and advantages of the processes of our invention will be further illustrated by the following examples.

Example I Tests were conducted using 20 pounds of sodium chlorite and pounds of chlorine per ton of sulfate pulp in the preliminary or chlorination stage. After neutralization the pH was maintained at around 9 to 9.5 in order to keep the pulp slightly alkaline at the end of 1 hour in the hold tank. Brightness of pulp off the first washer averaged around 68 per cent. Five pounds of sodium chlorite per ton of pulp were added to the heaters for one hour at F. Brightness of pulp off the second washer averaged 74 per cent with good strength. However, the chlorine dioxide odor from the beaters and the second washer was objectionable to the operators.

Example II e Bleached Stock Brown Stool. (1 Stage only) Beating Time Mullen Tear Mullen Tear Percent Percent 0 minutes l7. 1 1. G0 24. 8 2. 44 20 minutes 63. 9 l. 85 89. 4 2. S6 40 minutes 87. 2 3. 13 105. 6 2. 45 60 minutes 09. 6 2. 75 127. 2 2. 34

Fadometer tests indicated about the same reversion (8%) for the one stage bleached pulp as for the two-stage bleached pulp on oiled paper when original brightness was 72% for both. Titanox added to the heaters was reduced from 39 pounds per ton for the two-stage bleached pulp to 10 pounds per ton for the one-stage bleached pulp.

We claim:

1. A single stage bleaching operation for sulfate cellulose pulps in which the only bleaching operation necessary to afiord products of acceptable color consists essentially of simultaneously contacting the pulp in aqueous medium under bleaching conditions with chlorine and chlorine dioxide, the molar ratio of chlorine to chlorine dioxide being within the range of about 2.5 to about 3.5:1.

2. A single stage bleaching operation for sulfate cellulose pulps in which the only bleaching operation necessary to afford products of acceptable color consists essentially of simultaneously contacting the pulp in aqueous medium under bleaching conditions with chlorine and a water soluble chlorite, the molar ratio of chlorine to the water soluble chlorite being within the range of about 3.5 to about 4.5:1.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,719 Vincent Sept. 13, 1938 2,166,330 Vincent July 18, 1939 2,235,837 Logan Mar. 25, 1941 FOREIGN PATENTS 191,357 Great Britain Mar. 22, 1923 446,444 Belgium July 15, 1942 OTHER REFERENCES Staff Report, Chem. and Eng. News, March 17, 1947, pg. 766.

Woodside et al.: Condensation of Round Table Discussion on Pulp Bleaching. Paper Trade Journal, December 9, 1948, pgs. 21-25, especially at page 24, col. 2.

Sen Gupta: Progressive Delignification of Jute Fibre With Chlorine Dioxide. J. Tect. Inst. (Trans), September 1951, pgs. T375-384, especially first page.

Claims (1)

1. A SINGLE STAGE BLEACHING OPERATION FOR SULFATE CELLULOSE PULPS IN WHICH THE ONLY BLEACHING OPERATION NECESSARY TO AFFORD PRODUCTS OF ACCEPTABLE COLOR CONSISTS ESSENTIALLY OF SIMULTANEOUSLY CONTACTING THE PULP IN AQUEOUS MEDIUM UNDER BLEACHING CONDITIONS WITH CHLORINE AND CHLORINE DIOXIDE, THE MOLAR RATIO OF CHLORINE TO CHLORINE DIOXIDE BEING WITHIN THE RANGE OF ABOUT 2.5 TO ABOUT 3.5:1.