US2872280A - Process of bleaching cellulose pulps by absorbing alkali in the pulp and then applying hydrogen peroxide - Google Patents

Description

PROCESS OF BLEACHING (IELLULOSE PULPS BY ABSORBING ALKALI IN THE PULP AND THEN APPLiiING HYDROGEN PEROXIDE Robert R. Kinda-on. -Malden, Mass, assignor, by mesne assignments, to Food Machinery and Chemical Corporation, San Jose, 'Calif., a corporation of Delaware No Drawing. Application May 27, 1953 Serial No. 357,889

1 Claim. (Cl. 8-104) This invention pertains to a method of bleaching cellulosic materials and more particularly to a method of bleaching self-sustaining sheets, webs or other structures composed of felted cellulosic fibers by employment of alkali and peroxide.

Sheets, webs or other felted and compacted fibrous structures composed of cellulosic fibers, hereinafter referred to as sheets, are commonly bleached by applying to one or both of their surfaces relatively restricted amounts of bleach solution and subsequently effecting the bleach by submitting the impregnated sheets to relatively long term room temperature storage or to relatively short term storage at elevated temperature. Such bleaching methods are disclosed, e. g. by Heritage 2,125,634, Kauff- .mann and Wolfe 2,150,926, McEwen 2,513,344, McEwen and Sheldon 2,510,595 and McEwen 2,613,579.

In the above cited as well as in related methods, the bleach solution used commonly comprises two essential ingredients, namely hydrogen peroxide or a hydrogen peroxide forming compound as the bleaching agent and one or more alkaline compounds as bleach bath modifiers. As in all hydrogen peroxide bleaching procedures, these two essential ingredients are necessary prerequisites and work in conjunction with each other. It is also necessaryfor satisfactory bleaching results that these two ingredients be present not only in the bleach bath but also in and on the material to be bleached in predetermined amounts and in predetermined ratio to each other as well as to the material to be bleached. If, for example, a bleach bath containing 1.5% H 50%, 5.0% sodium silicate 41 B. and 1% NaOH gives satisfactory bleach results under certain conditions of use, then a deviation from this composition will obviously produce diiferent and usually less satisfactory results. Such a deviation may be caused even with a correctly composed bleach bath, if under conditions of use, factors operate which prevent the essential bleach bath ingredients from reacting conjointly and simultaneously with the material to be bleached, or to react in proportions different from those known to be optimum proportions.

When operating in accordance with the bleaching methods disclosed in the above cited patents or in accordance with similar methods, the essential bleach bath ingredients are first combined in aqueous solution by dissolving in water the proper amounts of ingredients in their proper ratio and the resulting bleach bath is then applied in a suitable manner to the material to be bleached. In doing so the assumption is made that the essential bleach bath ingredients will be and will remain in the same percentage combination both in the bulk of the bleach bath and in that fraction of the bleach bath which is actually taken up by the material to be bleached. However, practical experience has shown that this assumption is not generally valid.

A damp sheet composed of felted cellulosic fibers represents a complicated physical system exhibiting strong adsorptive power, so that if a bleach bath or other solution be applied to such a sheet, wetting and penetration of the 2. sheet by the solution will take place. This is of course true if plain water be applied to the sheet and also true if an aqueous solution of chemicals, such as a bleach bath be applied. It is, however, well known, that cellulosic fibers have the property of selectively adsorbing solutes from a solution. This means that if a solution containing two or more solutes is applied to such a sheet, the solutes will be absorbed by the sheet at different rates and to different extents. It also means that if a bleach bath solution contains solutes in certain amounts and proportions, it cannot be taken for granted that these solutes will be present in the same proportions after the solution has been applied to and adsorbed by the sheets. in consequence, this shift from optimum proportions of ingredients in the bath to different proportions of ingredients after a fraction of the bath has been adsorbed by the sheet, can, and very often will, result in bleaching results which are below the standard bleaching results to be expected on the basis of bath composition.

Practice has confirmed that such a shift in proportions does take place. It has been found that when applying a bleach bath comprising as essential ingredients hydrogen peroxide and one or more alkalies by means of a padder, sprayer or other device, or even by immersion, to a sheet, the hydrogen peroxide will penetrate or diffuse into the sheet so as to reach the center of the sheet thickness. The alkali or alkalies however, behave in a difierent manner. Whether these alkalies be sodium silicate, sodium hydroxide, phosphates such as sodium pyrophosphate, trisodium phosphate, sodium tripolyphosphate, sodium hexametaphosphate, or other alkalies, used singly or in combination, they will not penetrate or diffuse into the sheet when applied to its surface, but will become selectively adsorbed in a relatively shallow surface layer of the sheet, where their concentration will therefore be relatively greater than in the bulk of bleach bath solution. In consequence, the center portions of the sheet will receive little or no alkali, although they will receive their full share of hydrogen peroxide, and in this way a very pronounced shift from optimum proportions of ingredients in the bath to less than optimum proportions in the sheet will take place and result in bleaching results of a much lower order than expected on the basis of bath composition.

These unsatisfactory bleach results are due to the fact that the bleaching agent, hydrogen peroxide, is evenly distributed throughout the sheet thickness, the modifier, that is the alkali, is however unevenly distributed. In the center of the sheet hydrogen perox-. ide activation is insufficient due to lack of alkali, resulting in sub-standard bleach. In the surface layers there is an alkali excess, resulting in wasteful hydrogen peroxide decomposition, the two essential ingredients being incorrectly proportioned. Moreover, the high alkali concentration in the sheet surface, under the influence of the surrounding air, causes a well-known browning reaction to set in, which interferes with proper bleaching in the sheet surface. The overall result is then a sheet improperly bleached in the center because of hydrogen peroxide excess and improperly bleached in the surface layers because of alkali excess. In the case of a sheet of pulp intended for use in a paper furnish, the resulting bleach or brightness of the sheet after breaking up and forming a uniform crumb mass is actually even lower than the arithmetic mean of the brightness of inner layers and outer layers, as explained by the Kubelka-Munk equations for analyzing mixed paper furnish brightness.

In other words, selective adsorption of bleach bath ingredients by the sheet makes it impossible to obtain the brightness or bleach theoretically obtainable with a given bleach bath formulation, unless steps are taken to counteract selective adsorption phenomena. This is usually accomplished by applying a large excess of bleach bath solution to the sheet and also by forcing the solution into and through the sheet by mechanical means, such as press rolls. With a suitably compounded bleach bath solution retention by the sheet must be of the order of about -20% by weight based on the weight of the dry cellulose fibers forming the sheet, if commercially acceptable bleach results are to be obtained. Because uniform sheet impregnation, even with the help of mechanical means e. g. press rolls, demands the application of bleach solution in amounts of at least about 100% by weight on the basis of dry cellulose weight, as much as 80% and more of the bleach solution volume goes to waste as run-off from the press rolls, making such a procedure highly uneconomical. If roll pressure is reduced for the purpose of reducing run-off and retaining a higher percentage of the bleach solution in the sheet, even impregnation will result, but at the same time bleaching efiiciency will drop because of lowered sheet density, i. e. larger amounts of dilute bleach solution will be present in relation to the solids content of the sheet, a condition known to adversely aifect bleaching efficiency. Moreover, if the sheet carrying increased amounts of bleach solution is to be bleached by means of a high temeprature bleaching method, for example by passing it through a drying oven, increased steam consumption resuts because of the increased amounts of water to be driven olf. Obviously, all these factors militate against bleach economy.

It is an object of this invention to provide means to obtain uniform distribution of essential bleach bath ingredients throughout the thickness of a sheet composed of cellulosic fibers.

It is a further object of this invention to provide means to reduce Waste of bleach baths used in the bleaching of such sheets.

It is also an object of this invention to provide means to obtain maximum utilization of essential bleach baths ingredients and thereby optimum brightness results in the bleached sheet.

It is still another object of this invention to reduce steam consumption in any drying step used in carrying out a high temperature bleach of bleach bath impregnated sheets.

I have now found that uniform sheet impregnation with all its concomitant advantages results, if the essential bleach bath ingredients, hydrogen peroxide and alkali, are applied to the sheet in two separate operations.

In accordance with the method of this invention alkali is first applied to the sheet and then only hydrogen peroxide is applied. Even and uniform impregnation of the sheet with alkali is achieved by employing the principle of excess solution. As explained above, application of a substantial excess of solution to a sheet will result in uniform impregnation throughout the whole thickness of the sheet. 0nce there exists a sheet evenly and uni formly impregnated with the correct amount of alkali, impregnation of that sheet with the correct amount of bleaching agent, that is hydrogen peroxide, may easily be carried out in a second operation. In that second operation hydrogen peroxide solution is applied to the sheet in relatively restricted amounts. Because hydrogen peroxide is not selectively adsorbed by cellulosic fibers, application of excess solution is not necessary and even restricted amounts of hydrogen peroxide solution will penetrate and diffuse into the sheet quite uniformly. As a consequence, hydrogen peroxide application does not then involve excess solution application and thus avoids loss and waste of expensive bleaching solution due to run-otf of excess solution.

Furthermore because any excess of alkali solution may be removed from the alkali impregnated sheet prior to the application of relatively minor amounts of hydrogen peroxide solution, the solids content of the sheet after impregnation with both alkali and hydrogen peroxide remains relatively high. This high solids content or h1gh density of the sheet contributes to bleaching efficiency. Thus, optimum utilization of essential bleach bath ingredents is assured therefor by first adding alkali in excess to the sheet, pressing out the excess and thereafter adding the peroxide to the sheet.

Also, heat requirements for carrying out a bleach at elevated temperatures are reduced because the relatively small amounts of bleach solution present in the sheet after impregnation in accordance with this invention re quire the removal of correspondingly small amounts of water during high temperature bleaching or in any subsequent drying step. This fact also helps to reduce over-all bleaching costs.

In accordance with my invention the alkali solution may be applied to the material to be bleachedun any manner suitable to assure complete and uniform impregnation. The alkali solution may be added to the cellulose fiber mass prior to sheet formation, that is to say the alkali solution may be added in an aqueous slurry of: cellulosic fibers, from which a sheet is then formed in conventional manner on vacuum cylinder machines as used in the paper industry, or on Fourdrinier-type paper ma: chines, or other sheet forming equipment. The alkali solution may also be added to the formed sheet, for example by spraying the solution on the sheet by means of spray nozzles or other applicators, advantageously wh1le passing the sheet over suction devices which help to pull solution into and through the sheet. A very suitable mode of application consists in applying the alkali solution to the sheet by means of spray nozzles or other applicators immediately prior to passing the sheet through press rolls With any one of these or equivalent methods of alkali application there will result a sheet uniform mpregnated with the correct and desired amount of alkall.

To the properly alkalized sheet is then applied the second essential bleach solution ingredient, hydrogen peroxide. This may be applied to the sheet by means of spray nozzles, transfer rolls, or other suitable appicators. The sheet is thus evenly and uniformly impregnated with both the alkali and the hydrogen peroxide; that is, each of the essential bleach solution ingredients, and holds the minimum of solution or water consistent with proper bleaching operations.

The amount of alkali solution applied in the first operation will ordinarily be from about 10% to about 30% by weight based on dry fiber weight and the amount of hydrogen peroxide solution applied in the second operation will ordinarily be from about 2% to 20% by weight based on dry fiber weight. Total solution retention, that is total weight of alkali solution and hydrogen peroxide solution in the fully impregnated sheet, will ordinarily be from about 5% to about 20% by weight based on dry fiber weight. The amounts of alkali and peroxide based upon dry fiber weight are conventional for the bleach method used.

The bleaching agent used in the method of my invention may be hydrogen peroxide, or a compound libera ting or forming hydrogen peroxide upon dissolution in water.

The alkalies which I prefer to use are sodium silicate and sodium hydroxide, singly or combined, but other alkalies such as phosphates, carbonates, borates or combinations of alkalies may be used all as conventionally known.

The alkali solution used may also contain as optional ingredients sequestering agents, wetting agents, and the like.

Although the following examples describe the principle of my invention as applied to the bleaching of pulp sheets as used in the manufacture of paper, my invention is applicable to other sheets, webs or generally structures composed of cellulosic fibers and limited in its scope only by the appended claims.

Example I (a) A pulp sheet containing 100% conventionally bleached sulfate fiber was super-bleached by applying to the sheet 20.5% on a dry weight basis of a solution containing 0.186 1b. H 50% and 0.254 lb. sodium silicate 41 Baum per gallon. Thus, on a dry weight basis, 0.46% H 0 50% and 0.62% sodium silicate 41 Baum were applied to the sheet. The impregnated sheet was then dried and stored. After 30 days, sheet brightness was found to be 77.6 G. E. units.

(b) Another pulp sheet containing 100% conventionally bleached sulfate fiber and of the same stock as above was super-bleached by first applying to the sheet 10.9% on a dry weight basis of a solution containing 0.667 lb. sodium silicate 41 Baum per gallon and then expressing the excess by passing the sheet through press rolls, thereby reducing silicate content of the sheet to 0.61% on dry weight basis with accompanying loss of excess silicate solution. To the alkalized sheet after leaving the press rolls, was then further applied 18.75% on a dry weight basis of a solution containing only 0.182 lb. H 0 50% per gallon. Thus the completely impregnated sheet contained on a dry Weight basis 0.41% H 0 50% and 0.61% sodium silicate 41 Baum. The impregnated sheet was then dried and stored. After 30 days sheet brightness was found to be 84.5 G. E. units.

Example II (a) A pulp sheet containing 100% unbleached sulfite fiber was bleached by applying to the sheet 40% on a dry weight basis of a solution containing 0.313 lb. H 0 50%, 1.04 lbs. sodium silicate 41 Baum and 0.21 lb. of sodium hydroxide per gallon. Thus, on a dry weight basis, 1.5% H 0 50%, 5.0% sodium silicate 41 Baum and 1.0% sodium hydroxide were applied to the sheet. The impregnated sheet was then stored at room temperature without prior drying. After 32 days sheet brightness was found to be 75.7 G. E. units.

(11) Another pulp sheet of the same stock as Example II(a) containing 100% unbleached sulfite fiber was bleached by applying to the sheet 13% on a dry weight basis of a solution containing 3.21 lbs. sodium silicate 41 Baum and 0.64 lb. sodium hydroxide per gallon. To the alkalized sheet was then further applied 9% on a dry weight basis of a solution containing only 1.39% H 0 50% per gallon. Thus the completely impregnated sheet contained on a dry weight basis 1.5 H 0 50%, 5.0% sodium silicate 41 Baum and 1.0% sodium hydroxide. The impregnated sheet was then stored at room temperature without prior drying. After 32 days sheet brightness was found to be 78.6 G. E. units.

Example III (a) A pulp sheet containing 100% unbleached sulfite fiber was bleached by applying to the sheet 25% on a dry weight basis of a solution containing 0.334 lb. H 0 50%, 1.00 lb. sodium silicate 41 Baum and 0.167 lb. sodium hydroxide per gallon. Thus, on a dry weight basis, 1.0% H 0 50%, 3.0% sodium silicate 41 Baum and 0.5% sodium hydroxide were applied to the sheet. The impregnated sheet was then dried and stored. After 7 days sheet brightness was found to be 64.4 G. E. units.

(b) Another pulp sheet containing 100% unbleached sulfite fiber of the same stock as Example III(a) was bleached by applying to the sheet 15% on a dry weight basis of a solution containing 1.67 lbs. sodium silicate 41 Baum and 0.278 lb. sodium hydroxide per gallon. To the alkalized sheet was then further applied on a dry weight basis of a solution containing 0.834

lb. H 0 50% per gallon. Thus the completely impregnated sheet contained on a dry weight basis 1.0% H 0; 50%, 3.0% sodium silicate 41 Baum and 0.5% sodium hydroxide. The impregnated sheet was then dried and stored. After 7 days sheet brightness was found to be 69.4 G. E. units.

Example IV (a) A pulp sheet containing conventionally bleached sulfate fiber was super-bleached by applying to the sheet 25 on a dry weight basis of a solution containing 0.167 lb. H 0 50% and 0.333 lb. sodium silicate 41 Baum per gallon. Thus, on a dry weight basis, 0.5% H 0 50% and 1.0% sodium silicate 41 Baum were applied to the sheet. The impregnated sheet was then dried and stored. After 7 days sheet brightness was found to be 80.1 G. E. units.

(12) Another pulp sheet containing 100% conventionally bleached sulfate fiber of the same stock as Example IV(a) was super-bleached by applying to the sheet 12.5% on a dry weight basis of a solution containing 0.666 lb. sodium silicate 41 Baum per gallon. To the alkalized sheet was then further applied 12.5% on a dry weight basis of a solution containing 0.333 lb. H 0 50% per gallon. Thus the completely impregnated sheet contained on a dry weight basis 0.5% H 0 50% and 1.0% sodium silicate 41 Baum. The im pregnated sheet was then dried and stored. After 7 days sheet brightness was found to be 84.3 G. E. units.

The experimental results obtained and described in Examples 1 to 4 are summarized in the following table. In that table simultaneous application of hydrogen peroxide and alkali is termed unit application and application of alkali in a first operation and application of hydrogen peroxide in a second application is termed split application.

It is evident from the data given in Examples 1 to 4 and summarized in the table, that the same percentage of chemicalsapplied to a sheet will result in a substantially greater end brightness after bleaching if the chemicals are applied in two separate operations than if the same chemicals are applied to the sheet in a single operation.

The improvement of the present invention is suitable for use with all methods of pulp bleaching effected at high pulp densities, i. e. densities of greater than 20%. At pulp densities greater than 20% and where the pulp is still damp, water is no longer the continuous phase as is true of the usual pulp slurries. The fibers are merely soaked with the treating liquid and the treating liquid is carried into-the fibers themselves to produce better bleaching thereof. The improvement may be employed with all types of pulp fiber amenable to peroxide bleaching i. e. ground wood pulp, semi-chemical pulp and chemical pulp and the improvement may be employed with either high temperature or low temperature bleaching of such pulps provided only that the fiber be bleached at high pulp density. In all of such operations the alkali equivalent to NaOH shall be from 0.05% to 3% NaOH and the peroxide from 0.1% to 1% H 0 (100%) calculated on an oven dry fiber basis.

What' is claimed is:

In the bleaching of cellulose -pulps'with alkaline peroxide, the improvement which comprises adding to the pulp a solution consisting-of water and an alkali, until the pulp has absorbed an amount of alkali equivalent to 0.05% to 3% by weight ofsodium hydroxide, said amount being calculated as percent by weight of the pulp on an oven dry basis, and then after the pulp has absorbed the alkali, introducing into the so impregnated pulp a solution in water of from 0.1' to 1% by weight, on an oven dry pulp basis, of hydrogen peroxide, and then permitting the so treated pulp to bleach.

References Cited in the file of this patent UNITED STATES PATENTS 2,037,119 Comey Apr. 14, 1936 8 Kaufimann et' a1. Mar. 21, Kauffmann Feb. 6, Kaufimann et al. Dec. 30, McEwen Mar. 29, McEwen et a1. June 6, McEwen July 4, McEwen et a1. May 27, McEwen Oct. 14-,

OTHER REFERENCES See, p. 186.

Jr Textile Inst., December 1944, p. A-S 18.