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
  • D21C9/1057—Multistage, with compounds cited in more than one sub-group D21C9/10, D21C9/12, D21C9/16

Definitions

• This invention relates to a method of treating partially bleached chemical Wood pulps of the sulfate and sulfite type in order to produce superbleached chemical pulp fibers which possess unimpaired or improved physical properties.
• Partially bleached pulps of the sulfate or sulfite types are generally prepared by treating the original pulps with bleaching agents in single or multiple stages.
• these partially bl ached sulfate or suliite pulps are com-- prised of long, clean, good forming, medium or strong fibers, depending on the nature of the original pulp and the bleaching treatment thereon, and by reason of these properties are desirable for use in high grade papers.
• top quality paper production often require bleachto the very degree of whiteness designated as superbleached; i. e., possessing a dif fuse reflectance as compared to magnesium oxide in the range of 30 to 90+ brightness as measured by standard brightness meters.
• sulfate or suliite pulp partially bleached in multiple stages with chlorine and hypochlorite in the normal manner maybe subjected to one or more additional chlorine -01 hypochlorite treatments with a water or caustic wash after-each treatment.
• additional multi-stage treatments of this nature result not only in fiber damage, due to the action of bleaching chemicals on the pulp, but also in mechanical loss'byreason of attendant Washing steps.
• a further resultant disadvantage arises by reason of the fact that these additional treatmentsrequire extensive equipment occupying considerable floor space.
• desirable physical characteristics of a finished paper, and the pulp therein may be judged from anobservation of the results of certain --standard physical measurements of the paper as, for instance, burst, tear, and tensile strength.
• the'viscosity of pulp in acuprammonium solution as determined by standard procedure is proportional to :the 00121- bined physical characteristics and throughout the following experiments, the cuprammonium viscosity in centipoises has been determined and used as a measure of the fiber strength and cellulose degradation of a particular pulp.
• Brightness as hereinafter expressed refers to the per cent reflectance as measured by a Hunter Multipurpose reflectometer, calibrated to a General Electric brightness meter.
• Example I As mentioned hereinabove, it is possible by means of active chlorine treatments to prepare superbleached chemical pulps of the sulfate or sulfite type but at a sacrifice of fiber strength.
• a Southern pine sulfate pulp which had been treated in the conventional manner with multiple chlorinc-hypochlorite steps and intermediate washes to reach a partially bleached brightness of 77.0 units, was found to possess a cuprammonium viscosity of 28.6 centipoises.
• This pulp was then treated with an additional 1% available chlorine as hypochlorite and then Washed to produce a pulp with a brightness of 84.8, a figure sufilciently high so that the pulp fell within the range of pulps designated as superbleached.
• the cuprammonium viscosity of this pulp had dropped to 21.8 centipoises indicating that considerable fiber damage is associated with the action of the last hypochlorite treatment.
• Example II A partially bleached softwood sulfite pulp produced by conventional means possessed a brighness of 82.6 units and a cuprammonium viscosity of 22.3 centipoises.
• the partially bleached pulp was dewatered to 50% pulp density and thereafter treated with sumcient peroxide solution to add to the pulp 1% hydrogen peroxide, 4% sodium silicate 42 B. and 1% caustic soda all based on oven dried pulp.
• the resulting pulp possessed a pulp density of 43% and was maintained at this density for several days at room temperature. A maximum brightness of 90.8 units occurred after five days standing in the damp condition.
• the cuprammonium viscosity of the pulp was found to be 29.6.
• Example III Another sample of partially bleached sulfate pulp was found to have a pulp brightness of 70.3 units and a cuprammonium viscosity of 13.2 centipoises.
• a pulp brightness of 70.3 units When super-bleached at a pulp density of 45% with 1% hydrogen peroxide, 4% silicate of soda, and 1% caustic soda, a brightness of 87.2 was obtained after 14 days at room temperature. In this case, the cuprammonium viscosity of the pulp was substantially unchanged.
• the original partially bleached sulfate pulp at 70.3 brightness was superbleached at 5% density for four hours at 35 C.
• Example IV A partially bleached hardwood sulfite pulp possessed a brightness of 75.5 units and a cuprammonium viscosity of 43.4. When superbleached at 5% density for 3 hours at 35 C. with 1% available chlorine as hypochlorite and washed, the brightness of the pulp had increased to 85.4
• cuprammonium viscosity had dropped to 24.1.
• the invention as set forth herein requires, for best results, that a specific peroxide treating procedure be employed for raising the brightness of moderately or partially bleached sulfate and sulfite pulps at 65 to brightness to the superbleached range of 80 to
• the peroxide superbleaching treatment shall be performed at a relatively high pulp density, 1. e., above about 30%.
• the bleaching operation shall be performed at low temperature, preferably at room temperature and, in no instance, at a temperature higher than 130 F.
• the bleaching should be carried out over a relatively extended period of time as, for instance, from 1 to 20 days, and the fibers preferably accumulated in order to prevent evaporation of substantial quantities of moisture from the high density pulp which may be in the form of crumbs or as a lap.
• the latter form of accumulation is preferred.
• the chemical content on a dry pulp basis for the peroxide superbleaching treatment should be preferably within the following ranges:
• hydrogen peroxide is the preferred superbleaching reagent and may be used as such or may be prepared from an inorganic peroxide, preferably sodium peroxide, although other metallic peroxides such as potassium peroxide, barium peroxide, sodium perborate and the like, can be employed since they generate hydrogen peroxide in aqueous solution.
• the peroxide solution should contain an alkali.
• the alkali is preferably caustic soda although other materials generating available alkali in aqueous solutions may be employed as, for instance, trisodium phosphate, sodium carbonate, sodium silicate, as well as sodium pyrophosphate.
• the other stabilizers are soluble calcium or magnesium salts such as magnesium sulfate. Certain of these stabilizers contribute toward the desired alkali content and, where sodium peroxide is the source of hydrogen peroxide, it need not be completely neutralized but only to the extent desired to obtain a predetermined caustic content in the hydrogen peroxide solution resulting therefrom.
• the preferred procedure is to treat the partially bleached pulps with alkaline hydrogen peroxide in accordance with the special hydrogen peroxide bleaching method described above, in order to maintain chemical degradation at a minimum.
• all methods of hydrogen peroxide bleaching of partially bleached pulps are not effective for the purposes of the present invention.
• the following examples are designed to illustrate the point in question:
• Example V A sulfite chemical pulp, partially bleached by a standard chlorine-hypochlorite treatment, had a brightness of 73.1 and a cuprammonium viscosity of 34.8 centipoises.
• Example VI A sample of partially bleached Southern sulfate, prepared by a chlorine mill treatment, consisting of two stages of chlorination, each followed by a caustic wash and a final hypochlorite bleach, possessed a brightness of 68.5.
• a sample of this pulp when treated at 12% density for one hour at 60 C., with 1.5% hydrogen peroxide, 5% silicate of soda, 42 B., and 1.5% caustic on oven dried pulp, produced a pulp having a maximum brightness of 81.6 and a cuprammonium viscosity of 10.1.
• the method of preparing a, superbleached paper fiber from chemically purified fiber selected from the class consisting of sulfate fiber and sulfite fiber which comprises preliminarily prebleaching the chemically purified fiber with a hypochlorite to a brightness greater than and then completing the bleach upon the chemically purified and hypochlorite prebleached fibers at a substantially constant pulp density not less than 30% at temperatures below F. with alkaline peroxide, the total alkalinity of which is 0.3% to 3.25% calculated as NaOH, the time for obtaining maximum brightness being between 1 and 29 days, whereby the physical properties of the so treated fibers at the higher brightness level remain substantially unchanged from those existing at the lower brightness level.
• the method of preparing a superbleached paper fiber from chemically purified fiber selected from the class consisting of sulfate fiber and sulfite fiber which comprises preliminarily prebleaching the chemically purified fiber with a hypochlorite to a brightness greater than 65 and then applying an alkaline hydrogen peroxide solution to said purified and prebleached fibers at a high pulp density and thereafter maintaining the pulp density substantially constant and greater than 30% and the temperature below 130 F.
• the amount of peroxide being between 0.l% and 1.5% H202 (100%) and the total allrali calculated as NaOH, being between 0.3% and 3.25%, each based on the weight of dry fiber, whereby the physical properties of the so treated fibers at the higher brightness level remain substantially unchanged from those existing at the lower brightness level.

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• Wood Science & Technology (AREA)
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• 1950
  • 1950-06-30 US US171548A patent/US2661261A/en not_active Expired - Lifetime
• 1951
  • 1951-05-10 DE DEB14930A patent/DE973632C/de not_active Expired
  • 1951-05-16 GB GB11415/51A patent/GB684742A/en not_active Expired
  • 1951-06-04 FR FR1043060D patent/FR1043060A/fr not_active Expired
  • 1951-06-11 CH CH297856D patent/CH297856A/fr unknown

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