US2741536A - Available chlorine multi-stage bleaching of sulfite pulp - Google Patents

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AVAILABLE CHLORINE MULTl-STAGE BLEACHING F SULFITE PULP William A. Stone, New Orleans, La., assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Application March 26, 1952, Serial No. 278,747

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

In the conventional methods of multi-stage bleaching of sulfite cellulose pulps, chlorine has been acommonly 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 sulfite pulp, the tendency being particularly marked in cases where a high degree of bleaching is required. The strong oxidizing power of chlorine and the hypochlorites is apparently a chief cause of this undesirable degradation, although direct reaction of the chlorine and sulfite pulp may be partially responsible. For example, although sulfite pulp has the advantage over sulfate pulp in that it is more readily bleached to a high whiteness and is thus more suitable for many grades of paper and other cellulose products, the high whiteness is frequently obtained at the expense of a considerable loss of strength. For this reason sulfate pulp is sometimes blended with sulfite pulp in spite of the sacrifice in color in order to obtain adequate strength.

In the bleaching of sulfite pulp many variations in procedure are possible. However, the minimum operation ordinarily consists of three stages, the first of which is a chlorination stage in which the pulp is treated with chlorine water for a period of about one hour at an acid pH. 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, the pH normally being about 1.8 to 2.0, which restricts the formation of hypochlorous acid, the oxidizing agent in the system. Furthermore, since the chlorination reaction is relatively rapid compared to the oxidation reaction, limiting the time of contact also assists in avoiding oxidation. In this preliminary or chlorination stage it is customary to supply about 50 to 60 per cent of the chlorine demand of the pulp as determined by the permanganate number. The pulp consistency is about 1.5 to 3 per cent and the preliminary bleach is carried out at the temperature of the available water. The chlorinated pulp is extracted with caustic, for example with 2 per cent caustic, at 120 to 140 F. using a 10 per cent pulp consistency. After washing, the bleach is completed by treatment with a hypochlorite under alkaline conditions. The resulting washed pulp has a brightness of about 80 to 82.

While chlorine dioxide and chlorites have been widely used in bleaching sulfite 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 bleaching stage where the principal advantages of the chlorine dioxide and chlorites are utilized to the maximum degree.

2 ,741,536 Patented Apr. 10, 1956 I have now found that a combination chlorine dioxidechlorine bleach in the first or preliminary stage in bleaching a sulfite pulp permits the realization of substantial advantages for the bleaching operation, particularly in chlorine savings. For example, I have found that the combination chlorine dioxide-chlorine 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, equivalent or higher brightnesses, greater strength retention, easier and faster bleaching and less color reversion are obtained. Moreover, the reduction in acidity and lower temperature in the preliminary chlorination stage avoids damage to the pulp by the acid and saves corrosion of the equipment besides making possible the elimination of the caustic extraction stage.

According to my multi-stage process a sulfite cellulose pulp in aqueous medium is partially bleached in a preliminary stage with sufiicient chlorine dioxide and chlorine to supply about 50 to 60 per cent of the chlorine requirement of the pulp as determined by the permanganate number and then bleached in a subsequent stage with an aqueous solution of a hypochlorite. The amount of the chlorine requirement of the pulp supplied to the preliminary partial bleaching stage by the chlorine dioxide is about 50 to per cent on an equivalent available chlorine basis. Advantageously, the pulp in aqueous medium is partially bleached in the preliminary stage with a chlorite and chlorine at a pH of about 2.5 to 2.7 at approximately room temperature.

In general, I have found that best results at the most economical level are obtained when the amount of the chlorine requirement of the pulp supplied to the preliminary partial bleaching stage by the chlorine dioxide is about 50 to 75 per cent on an equivalent available chlorine basis. Particularly if a chlorite is used in the preliminary partial bleaching stage, in order to realize the advantages of my invention, it is important that more than sufficient chlorine be supplied than is required to activate the chlorite.

Although the following examples deal with the multistage bleaching of a sulfite spruce pulp, it is to be understood that the examples are merely illustrative of my invention. Preliminary conventional multi-stage bleachings of this sulfite spruce pulp were performed using 30 pounds of chlorine per ton of pulp in the preliminary or chlorination stage, 0 to 20 pounds of caustic per ton of pulp in the second or caustic extraction stage, and 30 to 34 pounds of available chlorine ar calcium hypochlorite per ton of pulp in the third or hypochlorite bleaching stage. The resulting pulp had a brightness of from 78.0 to 83.0 depending on the amounts of caustic and hypochlorite used in the second and third stages respectively. For a brightness of 7879, no caustic and about 30 to 34 pounds of hypochlorite per ton of pulp were used. Twenty pounds of caustic and 34 pounds of hypochlorite per ton of pulp were used when a brightness of 82-83 was desired. Strength retention of test sheets of 8283 brightness was 94.8 per cent based on per cent Mullen values and 100.5 per cent based on per cent tear values.

Example I Instead of the 30 pounds of chlorine conventionally supplied to partially bleach this pulp in the preliminary chlorination stage, 10 pounds of sodium chlorite together with 15 pounds of chlorine, 3 of which were necessary to activate the chlorite, were used. After the treatment had continued for one-half hour the pulp was filtered and washed. The caustic extraction was omitted and the pulp at a consistency of 12 per cent was treated with 28 pounds of available chlorine as calcium hypochlorite at a temperature of F. for two hours. test sheets showed a GE brightness of 80.7. The strength After washing,

retention after completion of the treatment was 112 per cent based on the per cent Mullen values which were 182 for the unbleached stock and 204 for the final pulp. Based on the per cent tear values for the unbleached stock of 171 and on the final pulp of 179 the strength retention was 104.6 per cent. By this operation a saving of 21 pounds of chlorine per ton of pulp or 32.8 per cent of the chlorine normally required was effected.

Example II The same pulp at a consistency of 2.5 per cent was treated with 20 pounds of available chlorine as sodium chlorite and pounds of chlorine per ton, 4 of which were utilized to activate the chlorite. After treatment in the preliminary chlorination stage for one-half hour the pulp was filtered, washed and reslurried to a consistency of 10 per cent with pounds of caustic per ton. The caustic extraction was continued at a temperature of 140 F. for one hour. After washing the pulp and reslurrying to a consistency of 12 per cent, it was treated with 9 pounds of available chlorine as calcium hypochlorite at a temperature of 95 F. for two hours. The finished pulp gave a test sheet having a brightness of 83.9. The strength retention was 92.8 per cent based on per cent Mullen values of 184 for the unbleached stock and 171 for the final pulp. Based on per cent tear values the strength retention was 111.5 per cent. The chlorine requirement in the first stage was reduced by 70 per cent of that when no chlorite was used and the total available chlorine requirement was reduced by per cent.

Example 111 to per cent of the chlorine requirement of the pulp as determined by the permanganate number is supplied by chlorine followed by a later stage of bleaching with an aqueous solution of hypochlorite, the improvement which comprises partially bleaching the pulp in the preliminary stage in aqueous medium and under acid conditions with chlorine dioxide and chlorine, the amount of the chlorine requirement supplied to the preliminary partial bleaching stage by the chlorine dioxide being about 50 to per cent on an equivalent available chlorine basis.

2. In the multi-stage process of bleaching sulfite cellulose pulps in which the pulp to be bleached is subjected in a preliminary stage to a partial bleaching in aqueous medium in which about 50 to 60 per cent of the chlorine requirement of the pulp as determined by the permanganate number is supplied by chlorine followed by a later stage of bleaching with an aqueous solution of hypochlorite, the improvement which comprises partially bleaching the pulp in aqueous medium at a pH of about 2.5 to 2.7 at approximately room temperature with a chlorite and chlorine, the amount of the chlorine requirement supplied to the preliminary partial bleaching stage by the chlorite being about 50 to 75 per cent on an equivalent available chlorine basis.

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

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

Claims (1)

1. IN THE MULTI-STAGE PROCESS OF BLEACHING SULFITE CELLULOSE PULPS IN WHICH THE PULP TO BE BLEACHED IS SUBJECTED IN A PRELIMINARY STAGE TO A PARTIAL BLEACHING IN AQUEOUS MEDIUM AND UNDER ACID CONDITIONS IN WHICH ABOUT 50 TO 60 PER CENT OF THE CHLORINE REQUIREMENT OF THE PULP AS DETERMINED BY THE PERMANGANATE NUMBER IS SUPPLIED BY CHLORINE FOLLOWED BY A LATER STAGE OF BLEACHING WITH AN AQUEOUS SOLUTION OF HYPOCHLORITE, THE IMPROVEMENT WHICH COMPRISES PARTIALLY BLEACHING THE PULP IN THE PRELIMINARY STAGE IN AQUEOUS MEDIUM AND UNDER ACID CONDITIONS WITH CHLORINE DIOXIDE AND CHLORINE, THE AMOUNT OF THE CHLORINE REQUIREMENT SUPPLIED TO THE PRELIMINARY PARTIAL BLEACHING STAGE BY THE CHLORINE DIOXIDE BEING ABOUT 50 TO 75 PER CENT ON AN EQUIVALENT AVAILABLE CHLORINE BASIS.