Note: Descriptions are shown in the official language in which they were submitted.
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O~ONE BLEAC~IING OF CELLULOSIC MATERIAL~
BACI<GROUND OF THE INVENTION
Field of lhe Invention
5Bleaching li~nocellulosic ma-terial with ozone.
BRIEF SUMMARY OF T~-IE INVEI~ITION
It has been found that, during pulp bleaching with ozone,
irradiation of the pulp wi-th ultraviole-t light enhances the efficiency
10 of the ozone and the bleachabili-ty of the pulp.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following deFinitions will be used in this application.
Pulping is the changing of wood chips or o-ther lignocellulosic
15 material to fibrous form. Chemical pulping requires cooking of such
material in solu-tion with a chemical, and includes partial removal oF
the coloring matter such as lignin associa-ted with the wood.
E leaching is the trea-tment of cellulosic fibers to remove or
alter the coloring matter associated wi-th the fibers to allow tke
20 fiber to reflect white light more truly.
The presen-t invention is concerned with -the degradation of
lignin and hence "delignifica-tion" and "bleaching" are use:l
interchangeably .
Consistency is the weight of pulp fibers in a pulp suspension
25 usually expressed as a percentage. For example, one pound oven
dry fiber in one hundred pounds o-f a mixture of water and fibers
would be a pulp suspension of 1% consistency. The consistency of
the pulp will depend upon the type of dewatering equipment used.
The following de-Finitions o-f consistency are based on the use
30 of those terms in "The Bleaching of Pulp," Third Edition, Revised,
edited by the present inventor, TAPPI Press 1979, pages 243 46.
Low consistency is typically -from 4-6~. I-t is a suspension that
is pumpable in an ordinary centri-fugal pump and is obtainable using
deckers and filters without press rolls.
.. , ,~
~ `
Medium consistency is between 10 and 15%. This consistency
can be obtained by vacuum washers and vacuum thickeners. Above
15%, press rolls are neecled for dewatering. A medium consisterlcy
slurry is pumpable by speciai machinery.
High consistency is from 20-35%. This consis-tency is ob-tained
by the use of presses. ~ligh consis-tency pulp is essen-tially
nonpumpable .
One measure of -the ef-ficacy oF a bleaching process is -the
degree o-f delignifica-tion. There are many methods of measuring -the
10 degree of delignification of -the pulp, but mos-t are variations o-f -the
permangana-te tes-t.
The permanganate test used herein provides a Kappa
number--a measure of potassium permangana-te solution consumed by
oven dry pulp under specified condi-tions. The Kappa number is
15 determined in accordance wi-th TAPPI standard -test me-thod
T 214 M42.
A fairly leng-thy recitation of prior art li-terature and patent
references describing gas phase ozone bleaching o-f lignocellulosic
materials is contained in U.S. Pa-tent 4,080,249 to Kempf et al
20 column 1, lines 31-46. These re-ferences disclose the ozone bleach-
ing of pulp at low, medium and high consistencies. Therefore,
"ozone bleaching" as used herein is not to be limi-ted to any par-
ticular consistency but may be employed at whatever consistency is
preferred, as suitable apparatus is already known for all -three
25 levels. For low and medium consistency pulp, i.e. pumpable
slurries, a pho-toreac-tor such as disclosed -for example in U.S.
Paten-t 3,637,342 to Veloz can be employed, following the teachings
of Kempf et al for entraining the ozone bearing gas into the slurry
of pulp. For high consis-tency pulp reactors, such as disclosed by
30 Fritzvold in U.S. Pa-tents 4,278,496 and 4,123,317 and by Carlsmith
in U.S. Patents 3,814,664 and 3,964,962 it is con-templated tha-t
lamps emit-ting ultraviolet light be mounted on the walls of the
reaction chamber.
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In accordance with -the present invention, ultraviolet
radia-tion increases -the effec-tiveness of the ozonation.
Withou-t wishing to be bound by -theory, -the present inven-tor
believes that the ultraviolet radiation electronically excites
the lignin in the material to be bleached. The excited lignin
may then form a high energy complex with the ozone lor
possibly singlet oxygen). It is speculated that this complex
immediately breaks apart into degradation products of lignin.
Lignin, because of its aromatic nature, absorbs strongly in
the ultraviolet region of the spectrum. The -typical lignin
absorption spectrum comprises a maximum a-t 205nm (nanometers),
a less in-tense peak at 280 nm, with ]ess significant
shoulders in the spectrum at 250, 300 and 360 nm.
An aspect of the invention is as follows:
A process for bleaching lignocellulosic pulp with
ozone wherein the improvement comprises irradiating said
pulp with ultraviolet light while mixing an ozone bearing
gas with -the pulp.
In reducing the present invention to practice, pulp
was subjected to treatment with ozone carried in oxygen while
being irradiated at the aforementioned frequencies of
ultraviolet light. A 360 watt spectrophotometer was used
as the source of ultraviolet. Samples of pulp weighing
0.1 gram each (oven dried basis) were irradiated in a closed
chamber consisting of two glass cells for a period of two
hours. The cells were partially covered with reflective
foil so as to increase the energy available to the pulp.
In the data which follows the reduction in K number
~increase in brighkness) is on the order of two, which --
given the low power of the spectrophotometer -- is considered
to be significant and having the potential to be translated
to pilot or commercial scale apparatus.
Sample K Number
U I treated pul p 18.32~
Ozonated pulp 17 389
Ozonated and irradiated at
205nm 15.0~1
250nm 15.278
280nm 15.672
300nm 15.586
360nm 15.789
10 At each of -these wave leng-ths which are charac-teris-tic of the
absorption spectrum of lignin, the ul-traviolet radiation increased the
e-FFectiveness oF -the ozonation. As may be seen, ligh-t at 205nm had
the greatest eFfect. While commercially available light sources for
pho-toreac-tors do not generally emit a single wavelength, i-t will be
15 sufficient i-f some o-f the ligh-t is at one or more of the more
effective frequencies. Moreover, i-t is desirable to choose wave-
leng-ths at which cellulose does not absorb. By irradiating at
absorption peaks charac-teris-tic to lignin and not to cellulose, the
lignin can be made more susceptible to at-tack by ozone than the
20 cellulose thus tending to cause -the ozone to react preferentially
with lignin and tending to cause less degradation of cellulose.
While the present inven-tion has been described in terms of a
laboratory reduction to prac-tice, it will be apprecia-ted by one o-f
ordinary skill in the ar-t having -the benefit oF -the teachings
25 contained herein that the invention as deFined by -the appended
claims is applicable to various types of reaction vessels already
known in the ar-t and incorporated herein by re-ference.
Accordingly, it is to be understood that various changes in the
forms of the apparatus by which the invention is practiced may be
30 resorted -to without depar-ting from the spirit and scope of the
invention as de-fined by the appended claims.
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