METHOD FOR INHIBITING THE FORMATION AND
DEPOSITION OF SILICA SCALE IN AQUEOUS SYSTEMS
TECHNICAL FIELD
This invention relates generally to silica scale inhibitors. More specifically, this invention
relates to a method for inhibiting the formation and deposition of silica and silicate compounds in
water systems with low molecular weight anionic polymers.
BACKGROUND OF THE INVENTION
In many parts of the world, amorphous silica scales cause significant fouling problems
when industrial waters contain high quantities of silica. For the most part, high quantities of
silica means that the industrial waters contain at least 5 ppm and up to about 500 ppm dissolved
silica and may contain higher quantities of silica either in dissolved, dispersed or colloidal forms.
The solubility of silica adversely limits the efficient use of water in industrial
applications, such as cooling, boiler, geothermal, reverse osmosis, and papermaking.
Specifically, water treatment operations are limited because the solubility of silica at about 150
ppm can be exceeded when minerals are concentrated during processing. This excess can result
in the precipitation and deposition of amorphous silica and silicates with consequential loss of
equipment efficiency. Moreover, the accumulation of silica on internal surfaces of water
treatment equipment, such as boilers, cooling, and purification systems, reduces heat transfer and
fluid flow through heat exchange tubes and membranes.
Once the silica scale forms on water treatment equipment, the removal of such scale is
very difficult and costly. With high silica water, therefore, cooling and reverse osmosis systems
typically operate at low water-use efficiency to assure that the solubility of silica is not exceeded.
Under these conditions, however, reverse osmosis systems must limit their pure water recovery
rate and cooling systems must limit water recycling. In both cases, water discharge volumes are
large.
Various additives have been employed over the years to inhibit silica deposition. The
current technologies for silica scale control in industrial cooling systems involve the use of either
colloidal silica dispersants or silica polymerization inhibitors, Dispersant technologies have
shown little activity, being able to stabilize only slight increases of total silica in a tower. For
instance, by feeding a dispersant, silica levels may increase from 150-200 to 180-220 ppm, which
is often an undetectable increase in silica cycles.
On the other hand, silica polymerization inhibitors have shown to be more effective
against silica scale deposition. For example, U.S. Patent No. 4,532,047 to Dubin relates to the
use of a water-soluble low molecular weight polypolar organic compound for inhibiting
amorphous silica scale formation on surfaces in contact with industrial waters. Likewise, U.S.
Patent No. 5,658,465 to Nicholas et al. relates to the use of polyoxazoline as a silica scale
inhibition technology. These polymerization inhibitors have allowed for increases In soluble
silica to greater than 300 ppm without scale formation.
There thus exists an industrial need for scale control agents having increased performance
over those currently known in the art.
SUMMARY OF THE INVENTION
Accordingly, the present disclosure provides a method for inhibiting the formation and
deposition of silica and silicate compounds in a water system. The inventors have discovered
that certain low molecular weight polymers are effective inhibitors of soluble silica
polymerization and scale deposition in water systems. In an embodiment of the invention, the
method includes adding to the water in the water system an effective inhibiting amount of one or
more relatively low molecular weight anionic polymers. The polymer is preferably selected from
a group comprised of homopolymers of acrylic acid, copolymers of methacrylic acid, and
polyethylene glycol ally] ether, homopolymers of methacrylic acid, copolymers of acrylic acid
and polyethylene glycol allyl ether and copolymers of acrylic acid and 1-allyloxy-2-hydroxy
propane sulfonic acid, homopolymers of maleic anhydride, copolymers of maleic anhydride and
polyethylene glycol allyl ether and combinations thereof. Such polymers are disclosed in, for
example, in JP2138319 (A), "Allyl Ether-Maleic Anhydride Copolymer," to Yasukochi Tom et
al.
It is an advantage of the invention to provide a >50% increase in the dispersency of both
polymeric and monomelic silica over the current art.
It is another advantage of the invention that the disclosed chemistry works in a manner to
slow the self-polymerization of silica, maintaining a portion of the silica in monomeric form.
It is a further advantage of the invention to allow for the softening and easier removal of
existing silica scale.
It is yet another advantage of the invention that the chemistry is thermally stable at
temperatures in excess o 300°C for greater than Shours.
It is another advantage of the invention that it can be coupled with tracing capabilities,
making it compatible with fluorescent tracing technology such TRASAR® technology (available
from Nalco® Company, Naperville, Illinois, USA).
The foregoing has outlined rather broadly the features and technical advantages of the
present invention in order that the detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention will be described hereinafter
that form the subject of the claims of the invention. It should be appreciated by those skilled in
the art that the conception and the specific embodiments disclosed may be readily utilized as a
basis for modifying or designing other embodiments for carrying out the same purposes of the
present invention. It should also be realized by those skilled in the art that such equivalent
embodiments do not depart from the spirit and scope of the invention as set forth in the appended
claims.
DETAILED DESCRIPTION OF THE INVENTION
Polymers for use in the disclosed invention are relatively low molecular weight polymers
and preferably have an average molecular weight ranging from about 5,000 to 200,000 as
described in more detail below. The organic polymers of the present invention are preferably
those polymers or copolymers which have acrylic acid or methacrylic acid functionality.
Exemplary polymers include: homopolymers of acrylic acid having an average molecular weight
from about 5,000 to about 200,000; copolymers of methacrylic acid and polyethylene glycol allyl
ether having average molecular weights of from about 5,000 to about 7,000; homopolymers of
methacrylic acid having an average molecular weight of about 15,000; copolymers of acrylic acid
and polyethylene glycol allyl ether having an average molecular weight of from about 5,000 to
about 7,000; copolymers of acrylic acid and 1-allyloxy-2-hydroxypropane sulfonic acid having
an average molecular weight of about 32,000; and combinations thereof.
In an embodiment, the polymers of the invention are water soluble homo-polymers and
co-polymers utilizing carboxylated and alkoxylated monomers. Representative alkoxy groups
include propoxy (propylene oxide), ethoxy (ethylene oxide), and hydroxyl substituted alkyl
chains and combinations therein. In an embodiment, the polymers of the invention are water
soluble homo-polymers and co-polymers utilizing carboxylated and alkoxylated monomers.
More specifically the alkoxylated monomer can be derivatized from either ethylene oxide,
propylene oxide, or any combination thereof. Average substitution ranges from about 4 to about
20 mole percent.
The weight average molecular weight (MW) of the polymers preferably ranges from
about 5,000 Da to about 200,000 Da, with the polymer actives typically between about 25%-
100%. The dosage ranges for the invention are about 1-100 ppm.
Representative polymers of the invention include water soluble co-polymers of either
acrylic acid, methacrylic acid, or malcic anhydride and an ethoxylated monomer where that
monomelic head is either an acrylate, methacrylate, or allylic moiety and the ethoxylate
appendage generally between 5-15 units. The monomer units in these co-polymers can be ratioed
from 90:10 to 10:90, respectively. Preferably, the ratio of the monomer units are between 40:60
and 60:40, respectively. Polymer actives typically exist at upwards of 100% but due to viscosity
limitations for certain applications (e.g., pumpability far transport or dosing) most exemplary
polymer actives levels have been positioned between 35% and 50%. Typical working MW
ranges are between about 10,000 Da and about 100,000 Da based on manufacturing conditions
and supplier quality, but targeted MW are between about 20,000 Da and about 35,000.
In an embodiment, the polymer is 2-Propenoic acid, polymer with a-2-propen-1-yl-w-
hydroxypoly(oxy-1,2-ethanediyl), sodium salt, peroxydisulfuric acid ([(HO)S(O)2]202) sodium
salt (1:2)-initiated (CAS No. 137898-98-7).
In an embodiment, the polymer is 2-Propenoic acid, polymer with a-2-propen-1-y1-w-
hydroxypoly(oxy-1,2-ethanediyl) block copolymer (CAS No. 1010818-79-7).
In an embodiment, the polymer is 2-Propenoic acid, polymer with a-2-propen-1-y1-w-
hydroxypoly(oxy-1,2-ethanediyl) graft copolymer (CAS No. 185506-87-0).
In embodiments, the polymer of the invention exists in various salt forms having a
counterfoil, such as sodium, potassium, and ammonium.
This invention provides methods for inhibiting the formation and deposition of silica and
silicate compounds in water systems. The methods include adding to the water in a water system
an effective amount inhibiting amount of a polymer according to mis invention.
The precise effective dosages at which the polymers can be employed will vary
depending upon the makeup of the water being treated. For example, an effective dosage (based
on total polymer) for treating cooling water will usually be in the range of about 0.5 to about 500
ppm. In alternative embodiments dosage ranges of about 1 to about 100 ppm or about 5 to about
60 ppm may be used. In embodiments, dosages between about 5 ppm and 50 ppm may also be
used. Typical dosages for treating industrial system water can range from about 10,000 to about
100,000 ppra. In embodiments, the polymers may be added directly into the water system being
treated as an aqueous solution intermittently or continuously.
The industrial waters that require treatment with the polymers of this invention are
generally waters that contain silica in a dissolved, suspended or colloidal form. The silica is
present as dissolved, silicic species, silicates, or their complex ions and may also be present as
colloidal silica or suspended silica. The total silica concentration in these industrial waters is
normally low. When it exceeds about 120-ISO ppm in total concentration; amorphous silica
scale formation then becomes a problem. However, in the presence of common cations, such as
Ca, Mg, Zn, Al, Se, etc, present in the water, much lower level of silica can cause
scaling/deposition problems. Obviously, the higher the concentration of total silica from all
sources in these waters, the more difficult is the problem created by amorphous silica scale
formation.
The industrial waters may be cooling waters, geothermal waters, salt water for
desalinization purposes, industrial waters being prepared for boiler treatment and steam
generation, downhole waters for petroleum crude recovery, pulp and paper mill waters, mining
and mineral processing waters and the like. The problem of amorphous silica scale formation on
the surfaces in contact with these industrial waters is particularly noted when the industrial
waters are alkaline, having a pH of at least S.O or above, and contain at least 5 ppm total silica as
SiO2. The effective use of the polymers of this invention are preferably at pH's of at least S.O and
above and may be at temperatures ranging between ambient temperatures to temperatures in
excess of 500 °F. However, as one skilled in the art of water treatment would appreciate, the
polymers of this invention should also be effective in waters having a pH lower than S.O.
Of particular importance is the treatment of alkaline industrial waters being used as
cooling waters, either on a once-through basis or particularly in a recirculating cooling water
system. When these alkaline cooling waters contain sufficient total silica, the problem of
amorphous silica scale formation on surfaces in contact with these cooling waters is exaggerated.
As the alkalinity increases, the problem of amorphous silica scale formation also increases.
Therefore, the effectiveness of the polymers used in this invention must also be demonstrated at
pH in excess of about 8.0.
Although not required to implement this invention, it is contemplated that the scaleinhibiting
polymers of the invention may be combined with one or more corrosion inhibitors, one
or more other scale inhibitors, one or more fluorescent tracers, one or more water treatment
polymers, one or more polyalkoxy compounds, or any other suitable adjunct or additional
component. Any such adjuncts may be part of an existing program to which the invention
becomes an additional component or program. In alternative embodiments, such adjuncts may
be added simultaneously or sequentially with the polymers of the invention.
It should be appreciated that the method, in certain embodiments, may be combined with
other utilities known in the industry. Representative utilities include sensors for measuring the
content of various additives in the system; dissolved or particulate contaminant sensors: other
sensors based upon resistance, capacitance, spectroscopic absorbance or transmittance,
colorimetric measurements, and fluorescence; and mathematical tools for analyzing
sensor/controller results (e.g., multivariate analysis, chemometrics, on/off dosage control, PID
dosage control, the like, and combinations thereof).
In another embodiment, an inert fluorescent tracer is included in the synergistic blend to
provide a means of determining the dosage level. A known proportion of me fluorescent tracer is
added either simultaneously or sequentially with the blend. Effective inert fluorescent tracers
include those substances that are chemically non-reactive with other components in the system
and that do not significantly degrade with time. Such tracers should also be completely (or
essentially completely) soluble in the blend at all relevant levels of concentration and preferably
the fluorescence intensity should be substantially proportional to its concentration and not
significantly quenched or otherwise diminished by other components in the system.
Furthermore, the inert fluorescent tracer should not be appreciably or significantly affected by
any other chemistry in the system. The statement, "not appreciably or significantly affected,"
means that an inert fluorescent compound generally has no more than about a 10% change in its
fluorescent signal, under conditions normally encountered in fuel ethanol.
Desired characteristics for an inert fluorescent tracer preferably include: fluorescence
excitation/emission wavelengths that do not have significant overlap with light absorbing
substances present in the water of the system, other additives, contaminants, etc.; high solubility;
excellent chemical stability; suitable fluorescence properties at manageable wavelengths (e.g.,
other components in the system should not interfere with the fluorescence properties at those
wavelengths) and excitation/emission wavelengths that are separate from other fluorescent
components mat may be present in the system to prevent interference; and avoiding negative
impacts on the properties of the system.
Representative inert fluorescent tracers include fluorescein or fluorescein derivatives;
rhodamine or rhodamine derivatives; naphthalene sulfonic acids (mono-, di-, tri-, etc.); p rene
sulfonic cids (mono di tri tetr etc ); stilbene deriv tives cont ining sulfonic acids
(including optical brighteners); biphenyl sulfonic acids; phenylalanine; tryptophan; tyrosine;
vitamin B2 (riboflavin); vitamin B6 (pyridoxin); vitamin E (a-tocopherols); ethoxyquin; caffeine;
vanillin; naphthalene sulfonic acid formaldehyde condensation polymers; phenyl sulfonic acid
formaldehyde condensates; lignin sulfonic acids; polycyclic aromatic hydrocarbons; aromatic
(poly)cyclic hydrocarbons containing amine, phenol, sulfonic acid, carboxylic acid
functionalities in any combination; (poly)heterocyclic aromatic hydrocarbons having N, O, or S;
a polymer containing at least one of the following moieties: naphthalene sulfonic acids, pyrene
sulfonic acids, biphenyl sulfonic acids, or stilbene sulfonic acids. Additional examples of such
inert fluorescent tracers may be found in U.S. Patent Nos. 6,966,213 B2, entitled "Rapid Method
for Detecting Leaks of Hydraulic Fluids in Production Plants" and 7,169,236 B2, entitled
"Method of Monitoring Membrane Cleaning Process." These inert fluorescent tracers are either
commercially available, for example, under the tradename TRASAR from Nalco Company or
may be synthesized using techniques known to persons of ordinary skill in the art of organic
chemistry.
Finally, the polymers of this invention may be combined with other water treating agents.
For example, the polymers may be used with water treatments, such as those used to inhibit
corrosion and those treatments used to disperse or prevent scale formation of other types.
Representative scale inhibitors include, but are not limited to, inorganic and organic
polyphosphate, phosphonates, and polycarboxylates. These inhibitors help inhibit or disperse
other scales such as calcium carbonate, calcium sulfate, calcium phosphate, calcium fluoride,
barium sulfate, calcium oxalate, and the like. Inhibition of these scales helps the polymer reach
its full potential for inhibiting silica/ silicate deposit.
Inorganic polyphosphates include compounds composed of phosphate units linked by
phosphoanhydride bonds as shown in the following formula;
Organic polyphosphates (polymeric organic phosphate) include esters of polyphosphates
as shown in the following formula:
where R is substituted or unsubstituted alkyl or aryl and n = 2-20. Representative inorganic and
organic polyphosphates include sodium tripolyphosphate, sodium hexametaphosphates, anionic
silicone phosphate ester, alkyl phosphate esters, and the like.
Phosphonates include compounds containing the structural moiety
where R is H or substituted or unsubstituted alkyl, or aryl. Representative phosphonates include
commercially available products including HEDP (1-hydroxy ethylidene 1,1-diphosphonic acid
and its salts), AMP (amino tri(methylene phosphonic acid) and its salts), PAPEMP (polyamino
polyether methylene phosphonic acid and its salts), and the like.
Polycarboxylates comprise polymers composed of monomers containing carboxylic acid
functional group or salts thereof including, for example, acrylic acid, methacrylic acid, a-
haloacrylic acid, maleic acid or anhydride, vinylacetic acid, allylacetic acid, fumaric acid, and b-
carboxylethylacrylate , and the like. Representative polycarboxylates include low molecular
weight commercially available water soluble polyacrylic acid, polymaleic acid, acrylic acid-AMP
copolymers, and the like.
Polyphosphate, phosphonates and polycarboxylates and their use for inhibiting scale is
known in the art See, for example, U.S. Patents 4,874,527, 4,933,090 and 5,078,879.
The foregoing can be better understood by reference to the following examples, which are
presented for purposes of illustration and are not intended to limit the scope of the invention.
Example 1: Stagnant Flask Study
This test was conducted using a 300 ppm test solution comprised of sodium silicate as
SiO2, 80 ppm as Mg from magnesium sulfate, 100 ppm as total alkalinity from sodium
bicarbonate, and 200 ppm as calcium from calcium chloride. The pH of the test solution was
adjusted between 8.5- 9.0. These test solutions were dosed with varying amounts of silica
inhibitor and a constant amount (2 ppm) of 2-phosphonobutane-l, 2, 4-tricarboxylic acid
(generally referred to as PBTC) as calcium carbonate inhibitor. Theses samples were
thermostated at 60°C. Samples were withdrawn at various times and filtered through 2.5 micron
filter prior to being analyzed for, silica, at pH 7-7.5. Sample 1 was 50/50 acrylic acid (AA)/
polyethoxy methacrylate (HEMA) at 40% actives. Sample 2 was 40/60 AA/ hydroxypolyethoxy
allyl other AAE at 40% actives. Sample 3 was 50/50 AA/AAE at 40% actives. Sample 4 was
60/40 AA/AAE at 40% actives.
Example 2: Pilot Cooling Tower Study
The following water was used in this study: Calcium (Ca) 12 mg/L; Magnesium (Mg) 4.0
mg/L; Potassium (K) 2.5 mg/L 2.6 mg/L; Silica (SiO2) 89 mg/L; Sodium (Na) 13 mg/L; Chloride
(CI) 43 mg/L Nitrate (NC¾) 1.6 mg/L; Sulfate (SO4) 2.4 mg/L; Chloride (CaCO3) 6.1 mg/L;
Total Alkalinity (CaCO3) 7 1 mg/L; Conductivity at 25°C 150 uS/cm; and pH @ 25°C 8.3 pH
Units.
The water was dosed with 20 ppra of the silica inhibitor of the invention and 20 ppm of
the calcium carbonate inhibitor (PBTC). The water was recirculated in several heat exchanger
loops and heat rejected through a cooling tower. In this process, the water was concentrated
between 3-3 .5 times (concentration factor) from the original water chemistry. The pH in the
recirculating loop was recorded at -8.9. The delta temperature between the heat exchanger inlet
and exit was 10°F. The water chemistry was monitored in the recirculating loop and the heat
exchangers were monitored for any fouling.
Results found that there was no observable deposition found on the heat exchangers and
there was 97 % recovery for all the ions in the concentration loop as a result of the silica
inhibitor.
All of the compositions and methods disclosed and claimed herein can be made and
executed without undue experimentation in light of the present disclosure. While this invention
may be embodied in many different forms, there are described in detail herein specific preferred
embodiments of the invention. The present disclosure is an exemplification of the principles of
the invention and is not intended to limit the invention to the particular embodiments illustrated.
In addition, unless expressly stated to the contrary, use of the term "a" is intended to include "at
least one" or "one or more." For example, "a device" is intended to include "at least one device"
or "one or more devices."
Any ranges given either in absolute terms or in approximate terms are intended to
encompass both, and any definitions used herein are intended to be clarifying and not limiting.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the
invention are approximations, the numerical values set forth in the specific examples are reported
as precisely as possible. Any numerical value, however, inherently contains certain errors
necessarily resulting from the standard deviation found in their respective testing measurements.
Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges
(including all fractional and whole values) subsumed therein.
Furthermore, the invention encompasses any and all possible combinations of some or all
of the various embodiments described herein. Any and all patents, patent applications, scientific
papers, and other references cited in this application, as well as any references cited therein, are
hereby incorporated by reference in their entirety. It should also be understood that various
changes and modifications to the presently preferred embodiments described herein will be
apparent to those skilled in the art Such changes and modifications can be made without
departing from the spirit and scope of the invention and without diminishing its intended
advantages. It is therefore intended that such changes and modifications be covered by the
appended claims.

CLAIMS
The claimed invention is:
1. A method for inhibiting the formation and deposition of silica and silicate
compounds in a water system, the method comprising adding to the water in the water system an
effective inhibiting amount of one or more low molecular weight anionic polymers selected from
the group consisting of: homopolymers of acrylic acid; copolymers of methacrylic acid and
polyethylene glycol allyl ether, homopolymers of methacrylic acid; copolymers of acrylic acid
and polyethylene glycol allyl ether; copolymers of acrylic acid and 1-allyloxy-2-hydroxypropane
sulfonic acid; homopolymers of maleic anhydride; copolymers of maleic anhydride and
polyethylene glycol allyl ether; and combinations thereof,
2. The method of Claim 1, wherein the one or more low molecular weight polymers
have a weight average molecular weight of about 5,000 Da to about 200,000 Da.
3. The method of Claim 1, wherein the one or more low molecular weight polymers are
homopolymers of acrylic acid having an average molecular weight from about 5,000 to about
200,000 Da.
4. The method of Claim 1, wherein the one or more low molecular weight polymers are
copolymers of methacrylic acid and polyethylene glycol allyl ether having average molecular
weights of from about 5,000 to about 7,000 Da.
5. The method of Claim 1, wherein the one or more low molecular weight polymers are
homopolymers of methacrylic acid having an average molecular weight of about 15,000 Da.
6. The method of Claim 1, wherein the one or more low molecular weight polymers are
copolymers of acrylic acid and polyethylene glycol allyl ether having an average molecular
weight of from about 5,000 to about 7,000 Da.
7. The method of Claim 1, wherein the one or more low molecular weight polymers are
copolymers of acrylic acid and 1-allyJoxy-2-hydroxypropane sulfonic acid having an average
molecular weight of about 32,000 Da.
8. The method of Claim 1, wherein the one or more low molecular weight anionic
polymers comprise water soluble homo-polymers and co-polymers utilizing carboxylated and
alkoxylated monomers.
9. The method of Claim 8, wherein the one or more low molecular weight polymers
comprise propoxy, ethoxy, hydroxy substituted alkyl chains, and combinations thereof.
10. The method of Claim 9, wherein the one or more low molecular weight polymers
comprise an average substitution ranges from about 4 to about 20 mole percent
11. The method of Claim 1, wherein the water system is selected from cooling water
systems, geotherrnal water systems, salt water desalinization systems, boiler water systems,
downhole water systems for petroleum crude recovery, pulp and paper mill water systems and
mining and mineral processing water systems.
12. The method of Claim 1, wherein the water system is a cooling water system.
13. The method of Claim 1, further comprising adding one or more corrosion inhibitors,
scale inhibitors, or dispersants to the water system.
14. The method of Claim 13, wherein the scale inhibitors or dispersants are selected
from the group consisting of: inorganic and organic polyphosphates, phosphonates,
polycarboxylates, and combinations thereof.