Note: Descriptions are shown in the official language in which they were submitted.
~1705~3
TREAT~IENT OF FILTRATES IN PEROl~lLJE
BLEACHING OF PULP
BAC~GROUND AND SUMMARY OF T~ INVENTION
In continuung efforts to reduce the environmental impact
associated with the production of kraft pulp, a number of techniques
- have been developed to "close" a bleaching system of a kraft pulping
10 facility so that there is no or very little discharge of washing filtrates
- into the environment These bleaching systems essentially elimin~te
chlorine and use in place of chlorine more enviromnentally friendly -
bleaching agents such as ozone and peroxide. For example co-
penr~inE aplication serial no. 07/861,387 filed March 31, 1992, and
co-pen~ing application serial no. 08/031,432 filed March 15, 1993,
teach various me~h~ni~m.~ for handling filtrates in bleaching celiulose
pulp. According to the FIGURE 3 embo(1im~nt of these applications
filtrate used for washing the pulp in a washing stage prior to an
ozone bleaching stage is oxidized as by purifying the off gas from the
20 ozone bleachillg stage so that essentially only the carrier gas
(typically o~ygen, but also potentially air) is used in a reactor, with
a~ali, to remove cont~min~nts from the filtrate prior to the w~hin~
stage.
It has now been found according to the present in~ention that
25 there are other situations in a pulp mill bleach plant which can
benefit from oxidizing the filtrate. For exa~ple it has been
recognized according to the present invention that pulp brightness
may be adversely affected when peroxide stage filtrate (from a
w~hing stage after a peroxide bleaching stage) is routed back to a
30 washer ahead of the P stage to be used as wash liquid in the earlier
217055~
w~hing stage. This filtrate may have a yellowish color, which
eventually carries through with the ble~t-hed pulp, having
cont~min~nts, particularly organics, therein which adversely affect
the pulp brightnes.s.
According to the present invention the pulp brightness problem
recognized above is solved by o~ ng the filtrate prior to its being
used as a wash liquid before a P-stage. This oxidizing is particularly
effective when the filtrate is treated with the off gas from an ozone
bleach stage which contains the carrier gas (typically oxygen) and a
0 residual amount of ozone, typically about one percent or less, if the
oxidizing gas is brought into intimate contact with the filtrate. This
gas is also useful in other processes, such as to treat condensates to
remove odors, and to treat off-gases from a pulp mill chemical
recovery systems to remoYe odors. An alternative source of oxidizing
15 agent is a low concentration, low pressure ozone gas that can be
obtained from a commercially available, low cost (low ozone
concentration capability, e.g. 4% or less) ozone generator.
According to the present invention a method of bleaching
cellulose pulp is provided comprising the steps of continuously: (a)
20 W~hin~ kraft cellulose pulp in a first w~.ching stage using a first
wash liquid. (b) Passing the pulp from the first w~hing stage to a
peroxide ble~ching stage. (c) After the peroxide bleaching stage,
w~hing the pulp in a second washing stage using a second wash
liquid, and producing a filtrate. (d) Bringing the filtrate from the
~5 second w~hing stage into intimate contact with an oxidizing gas to
oxidize cont~min~nts therein to produce a purified filtrate. And, (e)
after step (d), using the purified filtrate as the first wash liquid in
step (a). The w~shing steps (a) and (c) may also include pressing to
increase consistency.
2170553
According to a preferred embodiment of the invention there is
the further step (f) of before the first w~.~hin~ stage, or after the
second washing stage, bleaching the pulp in an ozone stage using
ozone cont~ininF gas, to produce an off gas cont~ining residual
5 amounts of ozone therein; and step (d) is preferably practiced using
the off gas from step (f) as at least part of the oxidizing gas. The
method may be practiced n~ inF a vertical t~nk having a gas
sparger at the bottom thereof, in which case step (d) is practiced by
introducing the oxidizing gas into the vertical tank through the gas
10 sparger. The tank may be maintained at substantially atmospheric
pressure, or at superatmospheric pressure (e.g. about 2 bar or above).
Alternatively instead of a tank with gas sparger a conventional gas
scrubber, or any suitable conventional liquid-gas contacting device,
may be utilized.
In the practice of the method there is typically the further step
of subjecting the pulp, either before or after the peroxide stage, to at
least one other ble~hing stage. Typically the steps (a) through (e)
are practiced with the cellulose pulp at a consistency throughout of
about 6-18%, and the ozone ble~- hing is also preferably carried out at
20 this medil~m consistency. The peroxide ble~ching stage may be of
superatmospheric pressure peroxide bleaching stage, the pressure
being at least 2 bar.
According to another aspect of the present invention a cellulose
pulp treating apparatus is provided. The apparatus comprises the
25 following components: A first wash stage including a first wash
liquid inlet. A peroxide bleaching stage connected to the first wash
stage, washed pulp passing from the first wash stage to the peroxide
bleaching stage. A second wash stage including a second wash liquid
inlet, and a filtrate outlet, the peroxide ble~-hinF stage connected to
30 the second wash stage so that pulp passing from the peroxide
217~3553
bleaching stage passes to the second wash stage. A vertical tank
having a gas sparger adjacent the bottom thereof, a liquid inlet, and
a liquid outlet. A source of oxidizing gas connected to the gas
sparger. The filtrate outlet connected to the tank liquid inlet. And,
5 the tank liquid outlet connected to the first wash stage wash liquid
inlet.
The apparatus preferably further comprises an ozone ble~ching
stage including an off-gas outlet for carrying carrier gas (typically
oxygen) with a residual amount of ozone gas (typically about one
o percent or less) away from the ozone ble~hin~ stage. The off-gas
outlet of the ozone ble~hing stage preferably is the source of the
oxidizing gas connected to the gas sparger. If the ozone-cont~ining
off-gases from an ozone ble~hing stage are not sufficient to oxidize
the filtrate, the off-gases may be supplemented by other sourc~s of
15 oxidizing agents. For example, ozone from a low cost ozone generator
may be used.
According to another aspect of the present invention there is
provided a method of treating a fluid of a pulp mill to remove color or
odor therefrom, the mill including at least one ozone bleaching stage
20 having off-gases therefrom. This method con,l";ses the steps of: (a)
removing the off-gases from the at least one ozone bleaching stage,
the of~ gases having residual ozone therein at a concentration of 4%
or less (e.g. 1% or less) in a carrying gas including oxygen; and (b)
without intermediate treatment which destroys the residual ozone
25 bringing the off-gases from step (a) into intimate contact with a pulp
mill fluid having adverse odor or color or both so that the ozone is
completely consumed, and so that the odor or color or both are
reduced.
Step (b) may be practiced to bring the off-gases into contact
30 with a pulp mill liquid condensate having undesirable odor, or to
2l 70~i~3
bring the off-gases into contact with a gaseous discharge from a piece
of equipment associated with the chemical recovery loop (e. g. a
recovery boiler, smelt dissolver, or lime kiln), or as described in detail
above, by bringing the off-gases into contact with a filtrate from a
5 washer after a peroxide bleaching stage, using a tank with gas
sparger or a gas scrubber.
It is the primary object of the present invention to effectively
treat cellulose pulp ntili7inF peroxide as one of the bleaching stages,
while properly re~lti~ ng the filtrates from various wash stages yet
10 not adversely affecting the bleaching of the pulp because of recycled
filtrate use, and to treat fluids from a pulp mill with off gas from an
ozone ble~chin~ stage to remove color and/or odor while
simultaneously using up the residual ozone. This and other objects of
the invention will become clear from an inspection of the detailed
description of the invention, and from the appended cl~im.~.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a srhem~tic view of an exemplary bleaching
20 methcd for cellulose pulp acco~ g to the present invention;
FIGURE 2 is a schematic view showing the particulars of one
embodiment of the filtrate treating apparatus of the system of
FIGURE 1;
FIGURE 3 is a view like that of FIGURE 2 of an alternative
construction of the filtrate treating apparatus according to the
invention; and
'~17~3
FIGURES 4 and 5 are schem~tic views of other systems
according to the invention for using the off-gases from a Z stage, with
residual ozone, to treat a pulp mill fluid having undesirable odor
and/or color.
DETATT-F',n DESCRIPTION OF THE DRAWINGS
}
FIGURE 1 s~hem~tically illustrates a system utilized for
practicing the method of treatinglbleaching cellulose pulp according
10 to the present invention. The pulp is typically kraft pulp, and is
provided in line 10 at a consistency that is typically about 6-18~o.
The pulp may optionally first be treated in one or more first
bleaching stages 11 which typically use non-chlorine bleaching
chemicals, and the pulp is subjected to washing in a fi~st w~hin~
stage 12 in which a first wash liquid is introduced through inlet 13,
and the filtrate exits the wash stage 12 in line 14. The filtrate in
line 14 may be used in earlier w~.chin~ stages, or it may be treated,
used as make-up liquid at other parts of the bleach plant or pulp
mill, or h~n~led in other manners.
After the first washing stage 12 th~ pulp passes substantially
immediately to a peroxide ble~hing stage 15. The stage 15 may
either be an atmospheric peroxide stage or a superatmospheric stage,
and the peroxide typically used is hydrogeIl peroxide, and it is used
at known or conventional temperatures, and charges. The pulp is
25 typically also at medinm consistency when bleached in the stage 15.
After the stage 15 the pulp passes substantially immediately to a
second wash stage 16 which has a wash liquid inlet 17 and a filtrate
outlet 18. The wash liquid added in line 17 may be fresh water, or
from a downstream ble~çhin~ stage. Stages 12 and 16 may include
30 pressing to increase consistency, and/or may be practiced by any
2170553
suitable techniques including displacement washing, using drums,
using pressing and dilution, etc.
Any number of downstream ble~chinF stages may also be
provided after the second wash stage 16. In a preferred embodiment
5 of the invention an ozone stage 19 is provided after the wash stage
16, or before the wash stage 12 (shown downstream of the wash stage
16 in FIGURE 1). In the ozone stage 19 ozone con~ininF gas (e.g.
oxygen cont~inin~ ozone at a concentration of at least about 8%) is
introduced in line 20, and typically is intimately mixed with the pulp,
o and produces an off-gas in line 21. The off-gas is typically at low
pressure (e.g. about 2 bar or below), and the amount of residual ozone
is low, 4% or less, typically about one percent or less (normally
significantly less than one percent).
According to the present invention it has been recognized that
when the filtrate in line 18 is introduced as the wash liquid in illlet
13 for the first washing stage 12 that the affect on bleaching is
adverse. The filtrate in line 1~ may typically have a yellowish color,
and that carries downstream after the peroxide stage 15. Normally
the off-gas from the ozone stage in line 21 is catalytically reacted to
20 remove the ozone since it is nn~lesirable to ~ h~rge the residual
ozone to the environment~ or the gas in line 21 is otherwise purified
before being discharged to the enviromnent.
According to the present invention it has been recognized that
the brightness of the pulp may be ~ignific~ntly enh~n~.ed in a
23 bleachi~g system sucln as illustrated in FIGU~E 1 by treating the
filtrate in line 18, as by using an apparatus shown generally by
reference numeral 23 in FIGURE 1. In the apparatus 23 oxidizing
gas is brought into intimate contact with the filtrate in line 18,
oxidizing cont~min~nts (typically organics causing the yellowish
2170'j53
_
color, but also other cont~min~nts) therein so that the liquid
introduced as a wash liquid in inlet 13 is a relatively clean liquid.
A preferred embodiment of the apparatus 23 is illustrated
schematically, but in more detail, in FIGUR~ 2. The apparatus 23
5 includes a vertical tank 25 having a gas sparger 26 at the bottom
thereof. The filtrate is introduced into the tan~ 25, e.g. adjacent the
bottom thereof, from line 18 as illustrated in FIGURE 2, while the
purified filtrate exits from the top of the tank 25, to pass to the inlet
13 as also illustrated in FIGURE 2. The tank 25 may be at
0 substantially atmospheric pressure, or may be held under
superatmospheric pressure (e.g. about 2 bar or above). The oxidizing
gas from source 27 is introduced into the gas sparger 26 to move into
intimate contact with the filtrate in the tank 25. The oxidizing gas
source 27 -- as illustrated schematically in FIGU~E 1 -- pre~erably
5 comprises the off-gases in line 21 from the ozone stage 19, which
contains a residual amount of ozone. If the ~mount of residual ozone
is insnffi~ient for complete oxidation it may be enh~nced by ozone
from a conventional low cost ozone generator 22 (see FIGURE 1); the
low cost generator 22 may not be able to produce an ozone
20 concentration of more than 4% and thus is ~ot suit,able for producing
ozone for stage 19, but that is entirely sllffirient. for purposes of the
invention. The ozone is a very effective oxidizing agent, and since
there is a si~nific~nt residence time in tank 25 (e.g. at least about 30
seconds, typically signific~ntly more, such as several minutes), all of
2~ the ozone is consumed, therefore elimin~ting the need for further
treatment. At the top of the tank 25 rem~ining gases may be
exhausted as indicated by reference numeral 29, or the purified
filtrate may pass through a degasser before being introduced into the
inlet 13.
217~
_
If desired, a heat e~rch~nger can be provided between the
various flows [as illustrated in FIGURE 3 of co-pending application
serial no. 07/861,387 filed March 31, 1992]. Also if it is necessary to
adjust the pH of the wash liquid, this can also be done by adding acid
5 or alkali to the ta~k 25, or to the filtrate in line 18 just before the
tank 25, good mixing taking place in the tank 25.
FIGURE 3 illustrates another embodiment that the apparatus
23 can take, in FIGURE 3 the apparatus being shown generally by
reference numeral 23'. In this embodiment a conventional gas
0 scrubber 32 is utilized, oxidizing gas from source 27 (which, again,
may be the line 21 from ozone stage 19) being scrubbed with the
filtrate introduced in line 18, as is conventional. A gas outlet 33 may
also be provided. Any conventional scrubber 32 can be used, one in
which the gas to be scrubbed is introduced at the top, or at the
5 bottom (as illustrated in FIGURE 3). Alternatively, other suitable
conventional liquid-gas contacting devices can be used instead of
structures 25, 32.
While in the preferred embodiment the pulp is at medium
consistency (about 6-18%) throughout the entire treatment seen in
20 FIGURE 1, where particular stag~s may be practiced better at
different consistencies press~g, t~ kening, or diluting apparatus
may be utilized.
FIGURE 4 illustrates another m~nner in which the off-gases
from the ozone (Z) stage 19 may be used advantageously in a
25 cellulose pulp mill. The mill includes an evaporator 40, or a like
piece of equipment (e. g. condenser, or condensing part of other
equipment) which produces a condensate in line 41. The liquid
condensate in line 41 is brought into intimate contact with the off-
gases cont~ining residual ozone [of 4% or less, typically about 1% or
30 less, in an oxygen cont~ining carrier gas, such as pure oxygen or air]
2170553
in line 21 (the gases in line 21 not being treated to remove the
residual ozone), as in contactor 42. The contactor 42 may be a tank
with gas sparger like the apparatus 23, or a gas scrubber like the
apparatus 23', or a simple mixer, or a wide variety of other
5 conven~ion~l structures. The ozone, and perhaps other oxidizing
elements of the gas in line 21, is consumed, removing at least some of
the odor from the condensate in line 41, producing a substantially
odor-free condensate in line 43. Rem~ining gas may be exhausted in
line 44, or may be removed from the condensate in line 43 by a
10 degasser, or the like.
FMURE 5 shows a system .~imil~r to that of FIGURE 4 for
treating odor-cont~ining off-gases from a piece of equipment 48 from
the pulp mill chemical recovery plant, such as a recovery boiler, lime
kiln, or smelt dissolver. The odoriferous gas in line 49 is mixed with
the off-gas (cont~;ning residual ozone) in line 21, e. g. in a mixer 50
or other contactor, the ozone and perhaps other oxidizing elements of
the gas in line 21 removing at least some of the odor from the gas in
line 49, producing relatively odor-free gas in line 51, which may be
further treated, or discharged to the environment, or used as a
20 combustion gas in the recovery boiler or the li~e.
It will thus be seen that according to the present invention a
method and apparatus have been provided which enh~nce the
brightness of pulp produced ~lt~ ing a peroxide bleaching stage,
while effectively reusing filtrates so as to minimize the amount of
25 lln(lesirable discharges to the envirGnment. The invention also
effectively utilize the off-gases from an ozone bleaching stage so that
those off-gases need not be further specially treated in order to
discharge them into the environment.
While the invention has been herein shown and described in -
30 what is presently conceived to be the most practical and preferred
2~70~53
embodiment thereof it will be apparent to those of ordinary skill inthe art that m~ny modificai~ions may be made thereof within the
scope of the invention, which scope is to be accorded the broadest
interpretation of the appended claims so as to encompass all
5 equivalent methods and apparatus.
