Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes

Definitions

• 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 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• Wood Science & Technology (AREA)
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Citations (9)

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• 1953
  • 1953-05-27 US US357889A patent/US2872280A/en not_active Expired - Lifetime
• 1954
  • 1954-05-14 GB GB14158/54A patent/GB753769A/en not_active Expired
  • 1954-05-15 DE DEB31037A patent/DE1046467B/de active Pending
  • 1954-05-26 CH CH321535D patent/CH321535A/fr unknown
  • 1954-05-26 FR FR1105941D patent/FR1105941A/fr not_active Expired

Patent Citations (9)

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