Note: Descriptions are shown in the official language in which they were submitted.
SPECI~ICATIO~
The present invention relates to an improved method
for removing impurities from a liquid, and to an improved
filter be~.
It has been found that the removal of suspended
impurities from a liquid is enhanced by passing the liquid
through a filter that has been precoated with a layer of
ion exchange resin particles in the size range of 60 to
400 mesh. Such a method is described and claimed in U. S.
Patent No. 3,250,703, which is assigned to the assignee
¦ 10 of this application. It has further been found that the
pressure drop across the bed of finely divided resin par-
ticles may be reduced, while filtration efficiency is
simultaneously increased, by employina a bed comprising
a mixture of cation and anion exchange resin particles,
this mixture comprising in the range of about 5 to 95~
cation exchange resin particles based on the weight of
the mixture. The reduced pressure drop results from
"clumping," which occurs when the particles are combined
in aqueous suspension. This method is described and
20 claimed in U. S. Patent INo. 3,250,702, which is assigned
to the assignee of this application.
As used herein, the term "bed" refers to a layer,
such as a precoat layer, which has been deposited on a
filter screen, a wound, annular filter cartridge, a film,
a deep or shallow bed, or the like. Such a bed may ad- -
vantageously be deposited on a tubular filter cartridge
such as those described in U. S. Patent ~o. 3J 279,608,
which is assigned to the assignee of this application.
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3S~
Although ion exchange resin mixtures form a highly
efficient filtration system, the ion exhange resins are expensive,
and, in many instances, ion exchange capacity is unnecessary.
That is, it is sometimes desired to employ a filter having a
reduced pressure drop and increased efficiency solely for the
removal of finely divided suspended particles ("crud") from
liquids
There are also instances where it is desirable to employ
a non-ion-exchange resin overlay over the ion exchange resin
itself. Such a method is particularly desirable when the liquid
being filtered contains suspended particles which interfere with
the ion exhange resin. For example, it has been found that iron
contaminants have a tendency to cause cracking of ion exchange
resin precoats, with a resultant loss in both filtration and
ion exchange efficiency. If an overlay can be employed over the
ion exchange resin in order to remove these contaminants from
the liquid, the run length for the ion exchange resin can be
greatly increased.
Generally, the present invention relates to a method for
removing impurities from a liquid by passing the liquid through
a filter bed which comprises a mixture of oppositely charged
particles of filter aid material. The filter aid particles that
are employed normally have a surface charge in aqueous solution,
and a portion of the particles is treated with a chemical com-
pound to produce a surface charge which is opposite to the normal
surface charge. A mixture of oppositely charged particles
(normal and reversed) is therefore produced, and the "clumping"
phenomenon is achieved without the need to employ expensive
cation and anion exchange resin particles.
Thus, the invention contemplates a method for removing
impurities from a liquid which comprises passing the liquid
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throu~h a filter bed made up of a mixture of oppositely charged
particles of filter aid materïal with the particles normally
all having the same surface charge in aqueous solution, wherein
a portion of the particles has been treated with a chemical
compound that produces a surface charge opposite to the normal
surface charge. The portion of the particles comprises from
about 5 to about 95 percent of said particles hy weight.
The invention also provides an improved filter bed which
comprises a mixture of oppositely charged particles of filter
aid material. Again, these particles normally all have the same
surface charge in aqueous solution, and a portion of the parti-
cles is treated with a chemical compound to produce a surface
charge opposite to the normal surface charge. The portion
comprises from about 5 to about 95 percent of the particles
by weight.
More specifically, it has been found that the phenomenon
of "clumping" described in U. S. Patent No. 3,250,702, together
with the advantageous reduced pressure drop and increased
filtration efficiency of this phenomenon, can be achieved by
combining particles of filter aid material which have been
treated in a manner to produce an opposite charge on a portion
of such particles. This opposite charge is produced by treating
a portion of the particles with a chemical compound that pro-
duces a surface charge opposite to the normal surface charge.
In order to produce the clumping phenomenon, it is necessary
that a portion of the particles retain a charge which is the
same as the normal surface charge. These particles may be
untreated, or, desirably, may also be treated with chemical
compounds in order to enhance the normal surface charge thereon.
It is well known in the art that many types of filter aid
material normally carry a surface charge. By the term "filter
aid material," applicant refers to those materials which are
conventionally deposited on a filter screen or the like in order
to aid in the filtration which is produced by the filter.
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Most of such material are characterized by electronegatively
charged s-urface. Such materials are well known in the art, and
include diatomaceous earth, cellulose fibers, charcoal, expanded -
perlite, asbestos fibers, etc. Cation exchange resin particles
also have a negatïve surface charge, and may be employed as
filter aid particles in accordance with the present invention.
Particularly preferred filter aid particles for use in accordance
with the invention are cellulose fibers, which are available -
commercially under the trade name "SOLKA-FLOC".
Although most filter aid materials normally have a negative
surface char~e, some have a positive charge. Such particles
include, for example, anion exhange resin particles. Such filter
aid materials may also be empolyed in accordance with the present
invention, wherein they are treated with a chemical compound in
order to produce a positive surface charge on some of the part-
icles, so that the clumping phenomenon is again achieved when
oppositely charged particles are mixed.
A wide variety of chemical compounds may be empolyed in
accordance with the present invention in order to produce a
reverse surface charge on a portion of the particles. Such
compounds must be miscible with water, and the compounds must
have a plurality of charge sites. A plurality of charge sites
is required in order that the compound form a bond with the
filter aid material, and will have charge sites remaining to
produce a surface charge that is the reverse of the normal
surface charge.
When the filter aid particles normally have a negative
surface charge, a cationic electrolyte-type compound is
employed, preferably a cationic organic polyelectrolyte.
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These cationic compounds form an electrical bond with
the surface of the negatively charged filter aid material,
producing a positive charge on the surface thereof.
Suitable non-polymeric cationic-type compounds include
l-carboxymethyl pyridinium chloride and cetyl pyridinium
chloride. Suitable cationic polyelectrolytes include
linear polyelectrolytes characterized by little, if any,
cross-linking. Many such polyelectrolytes are well known
in the art, and include polyalkylene imines, polyalkylene
polyamines, polyvinyl benzyl quaternary ammonium salts,
polyvinyl benzyl tertiary amines, vinylbenzyl sufonium
polymers, etc. Specific polymeric compounds that could
be employed include, for example, poly (l-butyl-4-vinyl
pyridinium bromide), and poly (1,2-dimethyl-5-vinyl
pyridinium methyl sulfate). A particularly suitable
cationic polyamine is one characterized by the repeating
structure:
~2t
H X
However, it should be understood that the above-mentioned
specific compounds are not the only ones that can be
utilized, as many cationic polyelectrolytes are well known
in the art.
1~73S98
In the instance where the filter aid employed has an
electropositively charged surface in aqueous solution, an
anionic-type compound is employed in order to produce a
reverse surface charge~ Again, the preferred compound is
an anionic polyelectrolyte. Suita~le anionic compounds
include polymeric acids such as polyacrylic acids, poly-
sulfonic acids, etc.
As previously stated, it is frequently desirable to
treat a filter aid material with both anionic and cationic
polyelectrolytes in order to enhance the normal surface
charge in accordance with the present invention. Thus,
an electronegatively charged filter aid material, such as
cellulose fibers, can be treated with a suitable chemical
compound in order to produce a positive surace charge,
and then with another chemical compound in order to again
reverse the charge to the normal surface charge. However,
this reversed charge is often stronger, and therefore
capable of producing clumping characteristics that are
superior to those produced when untreated material is
employed.
The application of chemical compounds to the filter
aids in order to produce a reverse surface charge is nor-
mally carried out in aqueous suspension. In carrying out
the preferred method, the filter aid material is simply
suspended in water, and an adequate amount of chemical
compound is added to produce the desired reverse surface
charge. With high molecular weight polyelectrolytes
(i.e., molecular weights in excess of 100,000), the point
at which adequate electrolyte has been added can be
determined by observing the filter aid material as the
polyelectrolyte is added.
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Initially, clumping will be produced as the surface char~e
of a portion of the particles is reversed, and this clump-
ing will then disappear when a reversal of substantially
all of the surface charge is observed. However, with
lower molecular weight electrolytes, and even with some
combinations of high molecular weight polyelectrolytes
and filter aid material, no noticeable clumping is
produced when the electrolyte is added. In those instances,
an adequate amount of electrolyte must be determined
from the results obtained when the treated particles are
mixed with particles having an opposite ~urface charge.
In general, at least about 5% of the chemical compound,
based upon the weight of the dry filter aid particles, is
required. However, much larger amounts can be employed,
as there is no detriment realized from the use of an excess
amount of the surface charge reversal-producing compound.
Of course, the amount of compound required in a particular
case depends upon many factors, including the nature of
the particles being treated and the number of positive or
negative sites that are available on the chemical compound
being added.
As will be apparent from the foregoing, one method of
carrying out the present invention when high molecular
weight polyelectrolytes are employed is simply to add a
polyelectrolyte which produces a reverse surface charge to
an a~ueous suspension of filter aid particles. This
compound is added in an amount sufficient to produce the
desired clumping effect, indicating that the surface charge
of a portion of the filter aid particles in suspension has
been reversed, and that these particles have clumped with
the remaining, untreated particles.
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~ ore desirably, howe~er, the particles are first
treated in aqueous suspension to produce a complete reversal
of surface charge, and the suspension is then dewatered
through well known techniques such as decanting, filtration,
or a combination thereof. Thes-e particles are then combined
in aqueous suspension w-ith particles having the normal
surface charge. Ordinarily, this com~ination is performed in
about a 50-50 proportion, although this proportion may be
adjusted in order to produce the desired amount of clumping.
In general, clumping is produced in aqueGus suspensions
containing anywhere from 5 to 95% of particles having the
reversed surface charge.
In the most preferred embodiment of the invention,
particles having a negative surface charge in aqueous
suspension are treated with a cationic polyelectrolyte to
produce a positive surface charge. The treated particles
are dewatered and mixed in aqueous suspension with particles
having an enhanced negative surface charge. This enhanced
negative charge is produced by treating the particles in
aqueous suspension with a cationic polyelectrolyte, dewater-
ing the suspension, resuspending the particles in water, and
treating them with an anionic polyelectrolyte. The suspension
is again dewatered, and the treated particles are mixed in
aqueous suspension with particles that have been treated
with a cationic polyelectrolyte. It has been found that this
method produces a high degree of clumping and a stable
floc of filter aid material. Again, the clumping is performed
in aqueous suspension, preferably with about a 50-50 mixture
of positively and negati~ely charged polyelectrolytes,
although the ratio may be varied anywhere
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1~73598
from 5 to 95% positively charged ~ilter aid particles.
After clumping, the mixture may be dewatered and
dried, and then resuspended in water when it is desired to
precoat a filter to form a filter bed.
The following examples are intended to illustrate the
present invention, and should not be construed as limitative,
the scope of the invention being determined by the appended
claims:
Example I
36g of cellulose fibers having an average length of 96
microns and an a~erage diameter of 17 microns was suspended
in 1000 ~1 of water in a 1.0 liter beaker equipped with a
magnetic stirrer. The stirrer was turned on to maintain
the cellulose in suspension, and a water-soluble polyethylene
polyamine was added slowly. The polyelectrolyte was added
until a maximum amount of clumping was observed, which
required about 8% polyelectrolyte, based on the dry weight
of the cellulose fibers.
The slurry prepared as above was dewatered by
filtration with a Buchner funnel and dried. This material
could be resuspended in water precoated onto a filter,
such as a tubular, nylon-wound filter element, to produce
good filtration characteristics together with a lower
pressure drop than would be produced with untreated cellulose
fibers.
Example II
A suspension of 36g of anion exchange resin particles
was suspended in 1000 ml of water, as in Example I. The
particles were in the size range of 60-400 mesh, and were
of the styrene-divinylbenzene copolymer type having
quaternary ammonium active groups. The polymer was about
8~ cross-linked.
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lQ73S9~3
A solution of polyacrylic acid ha~ing a concentration of
25% by weight and an average molecular weight of about
50,000-150,000 was added slowly to the suspension until
maximum clumping was o~served, as in Example I. About 10%
polyacrylic acid, based upon the weight of the dry resin
particles, was required.
The slurry was dewatered with a Buchner funnel and
dried. This material could be resuspended in water and
precoated onto a filter, such as a tubular, nylon-wound
filter element, to produce good filtration characteristics,
together with a lower pressure drop than would be produced
with untreated cellulose fibers.
Example III
150 gallons of water were placed in a 250-gallon tank
e~uippe~ with a mechanical stirrer. 30 pounds of cellulose
fibers of the type employed in ~xample I were added, and
the cellulose was suspended using the stirrer. 4 pounds
of a polyamide cationic polyelectrolyte having a molecular
weight in the range of 20,000 to 100,000. This polyamide
is commercially available under the trade name "Betz 1175,"
sold by the Bet2 Company, Trevose, Pennsylvania. The
suspension was then stirred for one hour to thoroughly mix
the polyelectrolyte. The cellulose was then dewatered with
a Buchner funnel.
Half of the treated cellulose was separated and
transferred to a 500-gallon tank equipped with a mechanical
stirrer and containing 150 gallons of water. The stirrer
was started to resuspend the cellulose, and 8 pounds of an -
aqueous solution of polyacrylic acid having a concentration
of 25% b~ weight, and having an average molecular weight in the
range of 50,000-150,000 was added. This polyacrylic acid
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~73598
solution is commercially available under the trade name
"ACRYSOL" A-3 from Rohm & Haas Co., Philadelphia,
Pennsylvania. The stirrer was run for 30 minutes to
thoroughly coat the treated cellulose fibers to produce
a negative surface charge. The suspension was dewatered
using a Buchner funnel, and was reslurried in 300 gallons
of water. The portion of the cellulose that was treated
only with the polyamide was added, and mixed for ten
minutes, and the mixture was then dewatered.
The foregoing mixture was suspended in water and
coated on a nylon-wound tubular filter element. The
material produced excellent iltration characteristics
with a minimum o~ pressure drop.
EXample IV
A tubular, wound nylon filter element was precoated
with a mixture of 75% cation exchange resin and 25~ anion
exchange resin in the size range of about 60 to 400 mesh,
as described in U.S. Patent No. 3,250,703. This ion
exchange resin precoat was applied in an amount of 0.2
pound per square foot of filter element. A second coat
of filter aid material prepared in accordance with Example
III was then applied to the filter over the precoat of
ion exchange resin particles in an amount of 0.06 pound per
square foot. The filter cartridge was employed to filter
liquid containing both ions and suspended particles
containing iron contaminants. The coating of cellulose
fibers over the ion exchange resin effectively removed the
bulk of the iron contaminants, preventing the cracking of
the ion exchange resin that would normally be observed.
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Very little increase in pxessure drop was produced
through the use o~ this overlay.
Obviously, many modifications and variations of
the invention as hereinbefore set forth will occur to
those skilled in the art, and it is intended to cover in
the appended claims all such modifications and variations
as fall within the true spirit and scope of the invention.
