Note: Descriptions are shown in the official language in which they were submitted.
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Description
This invention relates to the decolorization of
wood pulp effluents, particularly unbleached kraft wood
pulp effluents. More particularly, the invention relates
to the decolorization of such effluents by utilizing a
macroreticular aliphatic, preferably acrylic, weak anion ~
exchange resin. -
Today's pulp industry provides for a process of
chemically breaking down wood chips into brown pulp and
further washing and processing such pulp to produce light
brown unbleached paper. These washing and processing streams ~,
result in large quantitites of highly colored effluent in
which the color bodies are in the form of lignin and '!
lignin by-products. Such lignin products cannot be effectively
removed or broken down by biological treatment. They are
of concern to the industry since the lignin and lignin by- - ,
products when discharged into a receiving water inhibit
the penetration of sunlight and consequently the photo- `,
synthesis of aquatic biota. It disrupts normal behavior of
aquatic life and creates an aesthetically unpleasant,
environment. Due to such detrimental features government
regulations have become more strict to protect the environment
from such hazardous streams being discharged into waters
serving as municipal or industrial or recreational water -
sources.
The present invention provides for an effective
method of removing at an economic advantage and to a deg~ee
not heretofore possible the color bodies from spent pulping
liquors or unbleached effluents. By unbleached effluent ;
is meant any pulpmill stream not containing the effluent
from an oxidizing step. For example, in bleach pulpmills
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the pulp is frequently washed prior to the chlorine bleaching
step. This wash water despite being present in a bleach pulp-
mill is an unbleached effluent as defined hereinbefore. Typical
unbleached effluents are caustic extraction, washing and
screening waters, decker effluent, black liquor spills, dilute
spent pulping liquor and unbleached paper machine water. In
accordance with the known techniques the separated color bodies -
are removed from the resin, and ultimately burned in a re-
covery furnace which produces by-products such as carbon
dioxide and H20. The process may involve filtering the effluent
to decrease the suspended solids level and subsequently acid~
ifying the effluent to a pH suitable for ion exchange. The
effluent is then passed through a bed of macroreticular
acrylic weak base anion exchange resin in the free base form
whereby the color bodies are removed by ion exchange and
chemisorption. By free base form is meant any resin re-
generated with caustic that is not pretreated or activated
with an acid wash. Although the resin contains substantial
amounts of free base sites, it should be understood that ;-;
~ other sites may have been protonated. The resin form is ;~
therefore defined as the form of a resin that has not been ;
preactivated with a mineral acid wash. A preferred embodiment
would provide for the regeneration of the resin and the
chemical elution of the color bodies from the resin by using
one of the mill's own caustic process streams known as weak
wash or white liquor. Such an embodiment would further pro~
vide that the eluate caustic stream with the desorbed color
bodies is then returned for normal use in the mill's pulping
operation. The color bodies further find their way to the
spent pulping liquor called weak black liquor which liquor is
subsequently evaporated to increase the solids level and then
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burned in the mill's caustic recovery furnace to recover the
solids that are re-used to make pulping liquor. Alternatively,
the eluate caustic stream could be returned directly to the ~
weak black liquor recovery system. ;;
The process of the present invention may provide
specific advantages since when the resin is used without
pretreatment with mineral acid it avoids precipitation of -~
color bodies in the resin column which frequently may occur
if the resin is used inthe salt form such as the HSO4 form.
Additionally it provides for the use of an on-site regenerant
such as the mill caustic stream containing Na2S which, due
to the form of the resin, excludes the creation of H2S. In
addition to removing lignin and lignin by-products the process
of the invention lowers biological oxygen demand (BOD) and
chemical oxygen demand- (COD) and further removes toxic
organic impurities such as natural resin acids and fatt~ acids.
Prior art processes available to decolorize
general pulp mill effluents may be classified into three
distinct categories. One process is commonly referred to
as the mini-lime process. Another process utilizes alum
precipitation and the third is an ion exchange process
designed specifically for bleached effluents.
The mini-lime process involves the addition of
1000 - 2000 ppm of calcium hydroxide to the effluent to form 1;
a calcium organic precipitate. This sludge, containing the
color bodies, is de-watered and then burned in the mill's ~ '
lime kiln to recover calcium oxide and to incinerate the
color bodies. The alum precipitation process is similar to
the mini-lime process in that the alum forms an aluminum
organic precipitate which is again de-watered and burned
to dispose of the color bodies. Such processes suffer the
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disadvantage of the requirement of large quantities of ex-
pensive chemicals, and additionally are severely hampered
by sludge handling problems. The third process available
provides for an ion exchange system. This process, described
in published Swedish Patent Application No. 11726/71, pro-
vides for the use of a weak base anion exchange resin in the
salt form, preferably HS04. This resin must be regenerated
with purchased sulfur (Na2S) free caustic to prevent the -
creation of toxic hydrogen sulfide. It has been discovered
that the use of such a resin in the bisulfate form when
applied to unbleached effluents will cause color bodies to ~ -
precipitate in the resin bed leading to excessive pressure
drop across the bed which eventually will severely decrease
the possible flow rate.
It has surprisingly been discovered that should
the resins be used in the free base form for unbleached
effluents the disadvantages of the prior art ion exchange
approach disappear. ~ ;
It should be noted that there is a related fourth
prior art process which is also specifically designed for
the decolorization of bleached kraft pulp mill effluents, '
and which is described in U.S. Patent 3,652,407. This pro- ~
cess utilizing a polymeric adsorbent possessing no ionic ~ -
functionality treats highly colored bleached effluents. Due
to the nature of these effluents the streams possess a pEI
of less than 3 and therefore ideally use a non ionic
adsorbent~ ~ills providing for unbleached effluents do not
normally have sources of acid at hand and therefore dis-
charge an effluent which is alkaline in nature.
It is therefore an object of the present invention
to provide for an effective process for decolorizing un-
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bleached effluents. It is a specific object of the present
invention to provide for a process which obviates the
necessity of handling sludge. It is a further specific
objective of the present invention to provide for an economical
process which utilizes only on-site materials. It is a
further objective of the present invention to avoid the
creation of excessive quantities of salt cake Na2SO4. It
is a further objective of the present invention to provide
for a process which is characterized by relatively low cost
operation and low cost installation.
The present invention, then, resides in a process
for purifying, including decolorizing, a pulp mill stream
which comprises passing said effluent as a pH of 6 to 8.5
through a mass or bed of macroreticular aliphatic weak anion
exchange resin in the free base form~ thereby removing the
color ~odies and impurities on to the resin.
The resins useful in the process of the present
invention comprise the macroreticular acrylic weak base ion
exchange resins. The resins are further characterized as
possessing high pKb. Such resins are derived from suitable ~ ,
monoethyleneically unsaturated monomers which monomers in- -
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clude or may be selected from such monomers as methylacrylate, ;
ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl
acrylate, tertbutyl acrylate, ethylhexyl acrylate, cyclohexyl
acrylate, isobornyl acrylate, benzyl acrylate, phenyl
acrylate, alkylphenyl acrylate, ethoxymethyl acrylate,
ethoxyethylacrylate, ethoxypropyl acrylate, propoxymethyl
acrylate, propoxyethyl acrylate, propoxypropyl acrylate,
ethoxyphenyl acrylate, ethoxybenzyl acrylate, ethoxycyclo-
hexyl acrylate, and the corresponding esters of methacrylic
acid, ethylene, propylene, isobutylene, diisobutylene,
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styrene, vinyltoluene, vinyl chloride, vinyl acetate, vinyli-
dene chloride, vinyl acetate, vinylidene chloride and acrylo-
nitrile. Polyethylenically unsaturated monomers which
contain only one polymerizable ethylenically unsaturated
group, such as isoprene, butadiene, and chloroprene, are
also to be regarded as alling within the category of mone-thy-
lenically unsaturated monomexs.
The above mentioned monomers preferably are copoly-
merized withicomonomers normally referred to as crosslinkers
in an amount between 4 - 50% by weight of monomer or monomer
mixture.
Such polyethylenically unsaturated crosslinking
monomers may be of the type of polyfunctional methacrylates,
for example, ethylene glycol dimethacrylate or trimethylol-
propane dimethacrylate. Other crosslinking monomers may ;~
include divinylbenzene, trivinylbenzene, alkyl divinylbenzene ~,.
having from 1 to 4 alkyl groups of 1 to 2 carbon atoms sub- ;
stituted in the benzene nucleus, and alkyltrivinylbenzenes
having 1 to 3 alkyl groups of 1 to 2 carbon atoms substituted
in the benzene nucleus, which are described in U.S. Patent
3,531,463. Besides the homopolymers and copolymers of these
polyfunctional monomers, one or more of them may be copoly~
merized with up to 50~ (by weight of the total monomer
mixture) of (1) monoethylenically unsaturated monomers, or
(2) polyethylenically unsaturated monomers other than the ~ .
polyfunctional mers just defined or (3) a mixture of (1) or (2). ~ `
Examples of the alkyl-substituted di- and tri- .
vinyl-benzenes are the various divinyltoluenes, the divinyl :
xylenes, divinylethylbenzene, 1,4-divinyl-2,3,5,6-tetra- ;i
methylbenzene, 1,3,3-trivinyl-2,4,6-trimethylbenzene, 1,4-
divinyl, 2,3,6-triethylbenzene, 1,2,4-trivinyl-3,5-diethyl-
benzene, 1,3,5-trivinyl-2-methylbenzene.
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~xamples of other non-ionogenic, polyethylenically
unsaturated compounds, which can comprise up to 50 weight
percent of the polymer, include:~divinylnaphthalenes, diallyl
phthalate, ethylene glycol diacrylate, ethylene glycol
dimethacrylate, divinylsulfone, polyvinyl or polyallyl ethers
of glycol, of glycerol, of pentaerythritol, of monothio or
dithio-derivatives of glycols, and of resorcinol, divinyl-
ketone, divinylsulfide, allyl acrylate, diallyl maleate,
diallyl fumaratei diallyl succinate,diallyl carbonate,
diallyl malonate, diallyl oxalate, diallyl adipate, diallyl
sebacate, divinyl sebacate, diallyl tartrate, diallyl
silicate, triallyl tricarballylate, triallyl aconitate,
triallyl citrate, triallyl phosphate, N,N'-methylenediacryl-
amide, N,N'-methylenedimethacrylamide, N,N'-ethylenediacryl- -
amide, trivinylnaphthalenes, and polyvinylanthracenes.
As heretofore mentioned, it is preferred to use a
suspension polymerization technique. The polymerization is
carried out between about 40C and 120C, and at atmospheric,
subatmospheric or superatmospheric pressure, thereby ~-
producing polymer beads or granules. A variety of suspending
agents may be used to aid in the suspension of the monomer `~`
solvent mixture in an aqueous medium as particles of the `
desired size. Typical of these materials, a great number of
which are known in the art, are water soluble polymeric
materials such as polyvinyl alcohol, hydroxyethyl cellulose,
methyl cellulose, starch and modified starches! hydrolyzed
ethylene, maleic anhydride polymers, hydrolyzed styrene-
maleic and anhydride copolymers, acrylamide-sodium acrylate
copolymers, sodium acrylate copolymers, polyvinyl imidazoline
polymers and salts thereof and like.
Other well-known suspending agents are finely
divided solids such as magnesium silicate wax and finely
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divided silica sold under the trademark CAB-O-SIL, finely
divided clays and the like. Also sometimes useful are the
conventional surface active materials such as octyl phenoxy
polyethoxy ethanol, sodium lauryl sulfate, sodium stearate
and others. Suitable catalysts, usually in the range of
.01~ to 3% by weight with reference to the weight of the
monomer or monomer mixture, may be used to provide free ~
radical initiation in the polymerization reaction. Examples `
include benzoyl peroxide, lauroyl peroxide, t-butyl hydro-
peroxide, t-butyl perbenzoate cumene peroxide and azo
catalysts such as azo-diisobutyronitrile, or azo-diisobutyr-
amide. Suitable phase-extending or phase-separating solvents
include for example, methyl isobutyl carbinol, methyl ~-
isobutyl ketone, diisobutyl ketone, N-butyl acetate, xylene,
toluene, iso-octane, and chlorobenzene, as well as others
known in the art. `
The preferred proportion of the polyethylenically
unsaturated crosslinking monomer or mixtures of said
monomers is from 5 - 30 and more preferably between 7 and 20
by weight of the total monomer mixture from which the resin
is prepared. Suspension polymerization usually produces
the resin in the form of granules or beads having an overall
bead size in the range of about .1 to about 3 millimeters
average diameter. The macroreticular resin used in the
process of the invention may range from 12 - 100 mesh
(U.S. Standard Screen Series) in particle size. The resin
particles preferably range from 20 - 60 mesh for the balance
of adequate decolorization with practical column pressure
drops.
Macroreticular resins are characterized by the
presence throughout the polymeric matrix of a network of
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"extra gellular" microchannels or pores. While these micro-
channels are very small, they are large in comparison with
the pores in conventional homogenous crosslinked gells.
Macroreticular resins suitable for use in the invention may
have specific surface areas of at least 5 sq. meters per
gram to 500 sq. meters per gram.
As said herebefore, the resins are further charact-
erized by a high pKb. The PXb f these resins may range
from 5 - 10, preferably between 6 - 9.5.
The decolorization process may be carried out in '-
the fixed bed operation or continuously. For example, the '
resin may be supported in suitable cells or vessels which '
in most practical operations take the form of a tower or
column suitably packed with the resin particles which may
be of appropriate size or mesh. The liquid mixture or ^
waste stream influent is passed through the resin mass at a ~''
suitable rate such as from top to bottom, or vice versa, so '~
that one of the components is contained on a resin surface.
Alternatively, the resin particles may pass in counter-current
to the wash water influent. For example, the particles may
be continuously fed to the top of a column or tower into the
bottom of which the'liquid is fed continuously, the particles
being removed from the bottom for subsequent treatment to -~`
remove or separate the substances sorbed. The process may
be operated at flow rates of approximately 4 - 35 bed ~'-
volum~s per hour and it is an advantage of the process that
a range of 15 - 25 bed volumes/hour is well within the "
capabilities of the process.
The optical density of the bed influent is measured
at an adjusted pH of 7.6 using a spectrophotometer set at
465 m~ and results are expressed in APHA color units (or
as ppm on the platinum cobalt scale). It should be noted
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however that in accordance with the technical bulletin for
measuring mill effluents and receiving water colors published
by the National Council of Paper Industries for Air and
Stream Improvement the samples after pH adjustment are ;~
pretreated with .8 micron millipore filter to remove non
color turbidity causing impurities.
The fo~lowing examples, in which all parts are by
weight, serve to further illustrate the process of this
invention and the products thereof.
Resin A which has been chosen to illustrate the
process of the invention is a macroreticular weak base resin
(methyl acrylate) and i9 available as "Amberlite XE-275"*
from the Rohm and Haas Company. Other macroreticular weak --
base acrylic resins are commer~ially available under various
commercial designations. Reference is also made to U.S.
Patent 2,675,359 which described acrylic weak base resins.
~xample I
Decolorization of Unbleached Kraft Pulp Effluent
, ,
A sample of unbleached kraft pulp effluent is
decolorized by passing it through a fixed bed of resin. The -;
sample is filtered by a sand filter and the pH is adjusted
to 8.0 using H2SO4. The sample is then filtered through a
0.8 ~millipore filter. The resin bed consists of 20 cm.3
of ~30, +40 mesh Resin A in a 1 cm. diameter, jacketed
burette column heated to 60C. The unbleached effluent is
treated at 20 bed volumes/hour. The sample color prior to
treatment is 1835 APHA color units (pH 7.6, 465 nm). The
color of the treated sample and the cumulative percent
decolorization are listed in Table 1.
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* Trademark
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Table 1
Decolorization of Unbleached Kraft Pulp Effluent with Resin A
.,
Elapsed Time of Treated Cumulative
Experiment (hr.) Col~r (~PH~) ~ Decolorization
1 134 92.7
2 145 92.4
3 217 91.0 ,
4 196 90.6
Example II
10Desorption of Resin
:;
Regeneration of Resin A (-30, +40 mesh) bed is first
used in a 1 cm. diameter burette column to treat a simulated
unbleached kraft pulp effluent composed of weak black liquor
diluted in water to obtain a color of 2471 APHA. The pH
is adjusted to 8.0 and the sample is treated at 45C. at a
flow rate of 16 bed volumes/hour. The cumulative percent
decolorization at the end of a 3 hr. treatment is 84.8%.
The resin bed is then regenerated using simulated weak wash,
a paper mill caustic process stream, consistlng of 1% NaOH.
The resin bed is regenerated at 45C. at 4 bed volumes/hr.
for 1 hour. The spectrophotometer absorbance of each 0.25
bed volume sample is presented in Table 2.
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Table 2
Regeneration of Loaded Resin A Using NaOH ;
.,
Absorbance
Bed Volume (465 nm, pH not adjusted to 7.6)
. .
0.25 0.109
0.5 0.171
0.75 9.3 (measured as 1:50 dilution)
1.0 38.6 "
1.25 60.95 "
1.5 55.9 "
1.75 13.71 (measured as 1:10 dilution)
2.0 2.178
2.25 0.794 ~ -
2.5 0.611
2.75 0.451
3.0 0.375
3.25 0.267
3.5 0.215 - `
3.75 0.148
4.0 0.117
It is observed that the color is chemically eluted with
1~ NaOH in a very concentrated form, with the bulk of the ~
color removed with 2 bed volumes. -
Example III
Comparison of Resins of the Invention with "Duolite"* Resins
for Unbleached_Kraft Pulp Effluent Decolorization
Twenty cm.3 each of Resin A (-30, +40 mesh) and
three "Duolite"* resins, A-4F, A-7, and S-30, are placed in
1 cm. diameter burette columns and treated with 4~ NaOH
caustic, and then water washed. The HSO4 form of the
"Duolite" resins is not used for reasons stated hereinbefore.
* Trademark for a series of synthetic ion-exchange resins
manufactured by the Diamond Alkali Company
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Each resin is used to decolorize unbleached effluent that is
filtered with "Super-Cel"~ and then pH adjusted to 8Ø A ::
flow rate of 20 BV/hour is used at 60C. with an influent
color of 3482 APHA. The decolorization results are listed
in Table 3.
Table 3
Decolorization Using Resin A and Comparative Phenolic Resins
in the Free Base Form ~,
Cumulative ~ Decolorization
,.,~
Resin1st Hour 2nd Hour 3rd Hour :~
Resin A 92.2 90.3 88.3
"Duolite" A-4F 85.0 77.4 72.3
"Duolite" A-761.4 50.6 44.4
"Duolite" S-30 31.0 28.8 27.8
*Trademark for a brand of diatomaceous silica used as a ~-
filter aid and clarifying agent.
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