Note: Descriptions are shown in the official language in which they were submitted.
2130587
POLYACRYLAMIDE-PHOSPHONATE FLOCCULANTS AND
METHODS OF MAKING AND USE THEREOF
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates generally to water
treatment polymers and, more particularly, the
invention relates to anionic polymers useful as
flocculants and methods of preparing and using the
same.
Description of Related Technology
Considerable expenditures are made each
year for materials used in water treatment, such as
flocculants and coagulants used to clarify raw
waters for potable and industrial use. Flocculants
are used to clarify municipal and industrial
effluents to remove pollutants and to improve the
efficiency of papermaking processes by increasing
the first pass retention of fiber and filler when
paper is formed on a paper machine and for other
process clarification needs. Therefore, the demand
for more efficient, inexpensive water treatment
compositions for use as flocculants is increasing.
Anionic high molecular weight polymers are
widely used as flocculants to clarify waters for
potable and industrial use. Some commercially
available anionic polymers include pendant carboxyl
units connected to a backbone chain. These units
function as active sites for linking with solid
particles suspended in the water to be treated. The
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active carboxyl units along the chain begin to
become deprotonated at about pH 4.6 and
deprotonation increases as pH increases.
Accordingly, the deprotonated sites become anionic,
exhibiting a single negative charge per carboxyl
pendant. In the flocculant-type applications
described above, it would therefore be advantageous
to increase the negative charges per active pendant
unit to increase polymer activity and efficiency.
In water stabilization applications, the
use of phosphono-methylated amine compounds to
sequester polyvalent metal ions, especially calcium
and magnesium, has been the subject of investigation
for many years. It has been found that four
phosphono units can be attached to a diamine base
while five phosphono units may be attached to a
triamine base to provide additional sequestering
ability. Amine-based phosphonates have enjoyed
commercial success as sequesterants and chelating
agents in industrial systems needing control over
such metal ions as calcium, magnesium, iron, and
manganese.
In the past, a Mannich-type reaction has
been used to prepare phosphono-methylated amines
using phosphorous acid, formaldehyde and ammonia or
amine compounds. See Moedritzer & Irani, J. Org.
Chem., 31, 1603-1607 (1966). It has also been
disclosed that certain polymers containing
phosphonate units may be copolymerized with other
materials such as acrylamide, acrylic acid and
acrylates to produce scale and corrosion inhibitors
for water systems such as cooling, boiler and gas
scrubbing systems. See Becker, U.S. Patent No.
2130~87
4,446,028 (May 1, 1984) and Hoots et al., U.S.
Patent No. 4,904,413 (Feb. 27, 1990).
SUMMARY OF THE INVENTION
It is an object of the invention to
overcome one or more of the problems described
above.
The invention comprehends the product of
the reaction between an acrylamide polymer and
phosphorous acid. The invention also comprehends
methods of preparing water treatment compositions
comprising reacting an acrylamide polymer with
phosphorous acid and water treatment methods
utilizing such inventive compositions.
Other objects and advantages of the
invention will be apparent to those skilled in the
art and from the following detailed description,
taken in conjunction with the drawing and the
appended claims.
BRIEF DESCRIPTION OF THE DRAWING
The sole figure is a schematic view of an
apparatus suitable for carrying out a method of
preparing the inventive polymers.
DETAILED DESCRIPTION OF THE INVENTION
Anionic polymers according to the
invention, which exhibit a surprisingly high charge
density and are thus useful for flocculation, are
prepared by adding phosphorous acid to an acrylamide
polymer. This results in the creation of a
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polyacrylamide-phosphonic acid material comprising
repeat phosphono pendants which are believed to
attach to the amide groups of the polyacrylamide
backbone. A phosphono pendant is defined herein by
the following formula:
H O--I' O H
When fully deprotonated, the phosphono
pendants are transformed into phosphonate pendants,
each having two negative sites, i.e., twice the
anionic charge per pendant as carboxylated
flocculants. Therefore, the inventive flocculants
display superior activity in applications having pH
levels of about 7.7 (where double deprotonation is
believed to begin) and above. The flocculants
remarkably accelerate the sedimentation of sludge
and produce excellent water clarity. At lower pH
levels where only single deprotonation per active
site is expected, the inventive flocculants at least
match the performance of carboxylated flocculants.
The inventive polymers are preferably
prepared by a reaction in which an aqueous solution
of an acrylamide polymer is mixed with phosphorous
acid. The reaction takes place under acidic
conditions, preferably at a pH ranging between about
1 and about 2. The reaction appears to be
essentially instantaneous or occurring within a few
seconds at reaction temperatures ranging between
about 34F and about 200F. The reaction may take
place in either a batch or continuous reactor.
A reaction system for preparing the0 polymers of the invention is shown in the figure.
2130SY7
An aqueous solution of acrylamide polymer is located
in a holding tank 10 having a discharge outlet 16.
The aqueous solution of acrylamide polymer may be
prepared utilizing a reaction system described in
Sortwell et al., U.S. Patent No. 4,874,588 (October
17, 1989), the disclosure of which is incorporated
by reference herein. The polymer solution may be
removed through the outlet 16 and through a line 20
and pumped, as by a lobe pump 22, through a series
of static in-line mixers 24. Phosphorous acid
(H3PO3) may be introduced into the line 20 through a
line 26 which joins the line 20 at a T-union 30.
(The phosphorous acid is metered through a metering
pump (not shown) in the line 26.) The product is
removed at 34 for direct use or to another
receptacle for storage. Prior to use, the product
may be diluted with water.
In another embodiment, the phosphorous
acid is not introduced into the line 20 and the
dissolved acrylamide polymer is removed at 34 and
sent to a storage tank for subsequent metering to an
application area. As described more fully below,
both the dissolved acrylamide polymer and the
phosphorous acid are then metered to a in-line mixer
for reaction just prior to being applied to a system
to be flocculated.
A preferred acrylamide polymer starting
material for the flocculants of the invention is an
aqueous solution of an acrylamide homopolymer.
However, the acrylamide polymer starting material
may be another acrylamide polymer, such as an
aqueous solution of methacrylamide homopolymer or a
copolymer of acrylamide and methacrylamide, acrylate
213 0~ X7
or metal salts of acrylic acid, for example. The
acrylamide polymer may be partially hydrolyzed.
It has been found that maximum activity
from the flocculants according to the invention
results when the pH of the flocculant is adjusted to
range between about 7.7 and about 8.4 (pH range of
about 8 to about 8.2 is particularly preferred)
immediately before the flocculant is applied to the
slurry to be flocculated. It has also been found
that high concentrations of hydroxide lower the
stability of the flocculant molecule. Therefore, a
preferred flocculant according to the invention is
prepared by adjusting the pH of the water to be used
for polymer dilution to a pH high enough so that the
pH of the diluted polymer is about 8.2. This is
preferred over simply adding a strong caustic
solution to the diluted polymer (which could cause
localized high hydroxide concentrations) until the
polymer and water are thoroughly mixed. By adding
the hydroxide to the dilution water, localized high
hydroxide levels in the flocculant are avoided.
It is believed that adjusting the pH of
the dilution water prior to mixing with the
flocculant accomplishes at least two objectives.
First, the pH of approximately 8.2 assures double
deprotonation of the phosphonate pendant at the time
it is added to the aqueous system to be treated to
promote immediate flocculating action. Secondly,
the double deprotonation of the flocculant is
believed to help ensure maximum polymer molecule
linear extension. It is known that a linearly
extended polymer molecule is more effective than one
which is fully or partially coiled at the time it is
added to the system to be treated. It is believed
2130~87
that at higher pH levels the molecules of the
inventive flocculant extend because of the mutually
repellant nature of the highly anionic pendants.
A particularly preferred process according
to the invention includes storing a polyacrylamide
solution and a phosphorous acid solution in separate
tanks, metering the two solutions in ratios
appropriate for a particular flocculant application
to a highly efficient in-line mixer where a
polyacrylamide-phosphonate complex is formed, and
thereafter diluting the mixture. Prior to the
dilution step, the pH of the dilution water is
adjusted so that the diluted flocculant will have a
pH ranging from about 8.0 to about 8.4 and
preferably about 8.2. The pH-adjusted dilution
water is preferably mixed with the flocculant in a
highly efficient in-line mixer.
It has been found that a caustic solution
can be added directly to a diluted or undiluted
flocculant of the invention in order to adjust the
pH thereof and such a flocculant will outperform
carboxylated anionic polymers in terms of speed of
flocculation and clarity of the resulting
supernatant. However, such a flocculant will not
perform as well as a similar one in which the pH of
the dilution water is adjusted prior to dilution of
the polymer.
It is preferred that the inventive
flocculants be prepared shortly before use. It has
been found that an extended storage time may result
in a breakdown of the polyacrylamide-phosphonate
complex (the higher the pH the more rapid the
breakdown). Tests to date indicate excellent
performance of such flocculants compared to
~130587
carboxylate polymers for one to two days after
preparation with activity declining to that of
simple polyacrylamides over a period of about seven
days.
Preferred flocculants and coagulants
according to the invention are made with about a 0.2
to about a 50 wt.% solution (about 0.2 to about 6
wt% solution is preferred and a 2 wt.% solution is
particularly preferred) of an acrylamide homopolymer
having a molecular weight ranging from about 40,000
to about 18,000,000 (particularly preferred about
5,000,000 to about 11,000,000) as measured by an
intrinsic viscosity of about 0.1 to about 28
(particularly preferred about 12 to about 14). The
maximum polymer solids level is limited only by the
ability of the polymer to quickly obtain a
homogeneous mixture with the phosphorous acid. The
amount of phosphorous acid in the reaction mixture
preferably ranges between about 5 mole % and about
200 mole % based upon the acrylamide content of the
polymer backbone. A range of about 20-80 mole %
phosphorous acid is preferred and a particularly
preferred range is about 20-50 mole % based upon the
acrylamide polymer backbone.
The flocculants of the invention are
efficient in treating materials that have a cationic
charge, either naturally or imposed thereon by
treatment with, for example, quaternary amines,
aluminum salts or iron salts. The inventive
flocculants provide good activity in applications
having pH levels of about 5.6 and above and superior
activity in applications having pH levels of about
7.7 (where double deprotonation is believed to
2l3n~s7
begin) and above. A highly preferred pH range is
between about 8 and about 8.4.
The double negative charge per phosphonate
p~n~Ant provided by the inventive polymers gives
S especially good activity wherever calcium,
magnesium, iron, barium, manganese, zinc or other
metal ions are present, whether as a part of
inorganic particles or present on the surface of
fibers, colloids, or organic particles via an
adsorption phenomenon such as ion exchange. Where
metal ions such as calcium or magnesium are not
present on the surface of the materials to be
flocculated, they can be added upstream of the
flocculant addition point where they can adsorb onto
the surface of the materials to be removed by
clarification. For example, calcium or magnesium
chloride may be added to raw water prior to
flocculation. Seawater may be used to provide the
source of magnesium and calcium.
An example of a field of application
highly suited to the use of a high molecular weight
polymer of the invention is in the papermaking
industry where calcium carbonate is used as a filler
in the manufacture of so-called alkaline-sized
papers. The effective equilibrium pH of a calcium
carbonate saturated system appears to be in the
range of about 8.0 to about 8.2 where the polymer
would be highly deprotonated. The attraction to the
calcium sites on the filler particles and to the
adsorbed calcium ions present on the fiber and other
furnish materials is very strong because of the
double negative charge per phosphono pendant.
Therefore, the flocculants according to the
invention are highly suited for use as retention
2130587
aids in the paper machines as well as for the
clarification of effluents and the dewatering of
effluent sludges in these types of paper mills.
Among the additional uses in the paper industry is
the clarification of circuit waters where calcium is
added to cause soap-ink complexes to float.
EXAMPLES
The invention is further described and
illustrated by the following detailed examples which
are not intended to be limiting.
Example 1 (Preparation of Flocculant)
A high molecular weight polyacrylamide-
phosphonate flocculant according to the invention
was prepared as follows:
An acrylamide polymer having a molecular
weight of approximately 11,000,000 measured by an
intrinsic viscosity of approximately 14 was
dissolved in water to form a one weight percent (1
wt. %) solution.
One milliliter (ml) of the polyacrylamide
solution and 0.4 ml of a one weight percent
phosphorous acid solution were combined with 38.6 ml
water and rapidly mixed for about one minute at a
temperature of about 68 F. It is believed that
approximately 40 mole percent of the phosphorous
acid was converted to phosphono pendants on the
polyacrylamide backbone.
ExamDle 2 (Flocculant Activity)
The product of Example 1 was tested as a
flocculant by comparing it to a very high molecular
weight hydrolyzed acrylamide homopolymer having 40
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mole % carboxyl sites. Solutions of all the
polymers were made so that the final dilution
contained 0.00025 wt.% solids. Accordingly, one
milliliter contained about 0.0025 milligram (mg)
polymer solids and one drop contained 0.000125 mg
polymer solids. The pH of the final dilution water
was adjusted to approximately 8.6 prior to mixing
with the inventive polymer. The pH of the diluted
flocculant of the invention was measured at
approximately 8 immediately prior to its application
to the slurry to be flocculated.
The samples tested were a standard dirt
slurry augmented with calcium chloride, a repulped
newsprint slurry with and without added calcium
carbonate, a slurry of sodium-based coating clay to
which calcium chloride had been added and a bleached
fiber slurry containing calcium carbonate pigment as
a filler at about pH 8Ø Three ml portions of the
samples to be flocculated were added to 250 ml
beakers. Because the flocculants of the invention
are at their highest activity at pH ranges of about
7.7 and above, the samples were adjusted to
approximately 8 to 8.2 pH before the flocculants
were added.
Each flocculant was added drop-wise to a
sample until good flocculation of the solids and
good clarity of the supernatant were obtained.
The 40 mole percent high molecular weight
polymer of the invention was found to be effective
at one-third the dosage of the 40 mole percent
hydrolysed polyacrylamide.
The foregoing detailed description is
given for clearness of understanding only, and no
unnecessary limitations are to be understood
2130587
therefrom, as modifications within the scope of the
invention will be apparent to those skilled in the
art.
