Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF CONTROLLING SCALE FORMATION IN THE
PRESENCE OF METAL IONS IN AQUEOUS SYSTEMS
FIELD OF THE INVENTION
The present invention relates to the treatment of water to inhibit
the formation of scale. More particularly, the present invention relates to
a treatment for aqueous systems which controls scale deposition in the
presence of metal ions.
BACKGROUND OF THE INVENTION
Although the present invention has general applicability to any
given aqueous system where the formation and deposition of scale and
in particular barium sulfate is a potential problem, the invention will be
discussed in detail as it concerns papermaking systems. The present
invention relates to methods for inhibiting scale deposition and fouling
in aqueous systems.
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The water employed in papermaking systems often contain dis-
solved salts of barium, calcium, magnesium, etc., which can lead to scale
and sludge deposits. These materials can be present in the water at its
source or enter the system with the wood or pulp. In papermaking sys-
terns barium sulfate deposition has detrimental effects an papermaking,
affecting paper quality, screening efficacy and production rates. Inhibi-
tors are often employed to control barium sulfate scales. Currently,
products such as polyphosphates and polyacrylates as well as poly-
epoxysuccinic acid (PESA) are employed as inhibitors.
In paper machines operating in an acid environment, AI*3 is pre-
sent in concentrations of up to 20 mg AI*3 per liter of process water to-
promote paper sizing and increased paper machine drainage. The pres-
ence of AI*3 has been found to markedly reduce the efficacy of scale
control products including PESA. Other metal ions that may be present
as contaminants such as Fe*3 have a similar detrimental effect.
Scale can also cause rapid localized corrosion and subsequent
penetration of metallic surfaces through the formation of differential oxy-
gen concentration cells. The localized corrosion resulting from differen-
tial oxygen cells originating from deposit is commonly referred to as
underdeposit corrosion.
The treatment of industrial waters to inhibit scale formation with
polyepoxysuccinic acid (hereinafter PESA) is disclosed in U.S. Patent No.
5,062,962. The general formula for PESA is:
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R R
HO -~- i - i - O -~ H
O - C C - O
O O
M M
where n ranges from about 2 to 50, preferably 2 to 25, and M is hydrogen
or a water soluble ration such as Na+, NH4+ or K+ and R is hydrogen, C
1-4 alkyl or C 1-4 substituted alkyl (preferably R is hydrogen). However,
it was found that when PESA was employed in an acidic paper machine
where AI+3 was present in substantial concentrations, the scale control
efficacy of PESA was markedly reduced.
In addition to PESA, other scale control agents such as polyphos-
phates and polyacrylates were found to exhibit limitations in efficacy in
such an environment. Polyphosphates function as both scale inhibitors
and metal ion binders. A high dose of a polyphosphate may overcome
the interference of AI+3 or Fe~3, however the cost would be prohibitive.
The combination of the present invention provides a scale inhibitor which
is effective at low dosages when combined with a low cost metal ion bind-
ing agent.
The scale control efficacy of PESA was also found to be decreas-
ed by the presence of acrylic acid copolymers in aqueous systems as
described in U.S. Patent 5,248,438 filed January 28, 1992.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph of minimum dose vs. % sodium hexametaphos-
phate to provide 100% inhibition.
SUMMARY OF THE INVENTION
The present invention provides an effective method of inhibiting
scale formation in an acidic aqueous system in the presence of metal
ions such as AI+3 or Fe+3 by employing PESA in combination with a
metal ion binding material. The method of the present invention en-
hances the efficacy of PESA and avoids the problems of interference
between PESA and other scale control materials in acidic environments
which include metal ions such as AI+3 or Fe+3.
The present invention is effective at inhibiting the deposition of
scale forming materials such as barium sulfate. The method of the pre-
sent invention comprises treating an acidic aqueous system having metal
ions such as AI+3 or Fe*3 present with a combination of polyepoxysuccin-
is acid of the general formula:
R R
HO -f- C - C - O ---~ H
I ,
O - C C - O
O 0
M M
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,~_ ~ ~ ~ fir.
where n ranges from about 2 to 50, preferably 2 to 25, and M is hydrogen
or a water soluble cation such as Na+, NH4+ or K+ and R is hydrogen, C
1-4 alkyl or C 1-4 substituted alkyl (preferably R is hydrogen); and a
metal ion binding agent such as polyacrylates, citric acid, tartaric acid,
5 and sodium hexametaphosphate.
In the present invention, the polyepoxysuccinic acid is added to an
acidic aqueous system including metal ions such as AI+3 or Fe+3 at sub-
stoichiometric levels to inhibit the scale formation. The metal binding
agent is added to avoid a decreased efficacy of the PESA due to the
presence of the metal ions. The metal binding agent is added in an
amount sufficient to inhibit these adverse effects.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention pertains to a novel method of inhibiting the
formation of scale such as barium sulfate in aqueous systems. Specifi
cally, the method of the present invention comprises adding to an aque
ous system a treatment solution comprising a combination of polyepoxy
succinic acid of the general formula:
R R
a
HO-E-G-C-0~, H
O - C C - O
0 0
M M
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where n ranges from about 2 to 50, preferably 2 to 25, and M is hydrogen
or a water soluble catian such as Na'", NH4* or K+ and R is hydrogen, C
1-4 alkyl or C 1-4 substituted alkyl Cpreferably R is hydrogen); and a
metal ion binding agent such as polyacrylates, citric acid, tartaric acid,
and sodium hexametaphosphate.
Polyepoxysuccinic acids are known to provide scale inhibition and
are effective in a wide variety of water systems. U.S. Patent No. 5,062,962
outlines a method of preparing the polyepoxysuccinic acid material of the
present invention. The treatment levels of polyepoxysuccinic acid added
to an aqueous system c;an range from about 25 parts per billion up to
about 400 parts per million. The preferred treatment levels range from
about 5 parts per million up to about 100 parts per million. The concentra-
tion of polyepoxysuccinic acid necessary to provide effective scale control
will vary from system to system. The treatment level will vary, in part, with
changes in temperature and pH. However, in all cases, the concentration
of polyepoxysuccinic acid added to an acidic aqueous system in accor-
dance with the present invention is at substoichiometric concentrations.
That is, the concentration of polyepoxysuccinic acid added is much lower
than the concentration of the scale forming material in the system to be
treated.
The metal binding agents of the present invention are those known
to be effective in acidic aqueous systems for binding ions such as AI+3 or
Fe+3. Exemplary binding agents include sodium tripolyphosphate and
polyacrylic acid. Also included are binding agents such as citric acid,
tartaric acid, and sodium hexametaphosphate.
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It was found that in acidic aqueous environments where metal ions
such as AI+3 were present, the combination of polyepoxysuccinic acid
and a metal binding agent was effective at reducing scale formation and
deposition. Individually, the polyepoxysuccinic acid and metal binding
agents were significantly less efficacious at inhibiting scale formation and
deposition. See Table I. The metal binding agent can be added to the
aqueous system in concentrations ranging from about 2 up to about 50
parts per million depending upon the concentration of interfering metal
ions.
The present invention will now be described with reference to a
number of specific examples which are to be regarded solely as illustra-
tive and not as restricting the scope of the present invention.
Examales
Dynamic scale inhibiting testing was undertaken by means of an
apparatus in which aqueous solutions of Ba(N0~~2, and Na2S04 and
AIC13 were blended and pumped through a stainless steel capillary col-
umn. The aqueous solutions were prepared so as to reproduce con-
centrations, pH's and temperatures typically encountered in a paper
machine. The scaling process was monitored by measuring the pressure
required to pump the solution, at constant flow, through the column. As
scale deposits, resistance to flow increases and the pumping pressure
required to maintain a constant flow increases. A second measure of the
scaling rate was obtained by chemical analysis of barium present in the
solution exiting the column.
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The efficacy of the treatment solutions added to the circulating
aqueous system was measured based upon the slope of a pressure
increase versus time piot. Percent inhibition based upon slope was
calculated according to the equation:
°~ Inh (slope) = Slope (untreated- Slooe (treated) X 100
Slope {untreated)
Inhibition based upon measurement of barium concentration
was determined according to the equation:
°~ Inh (Ba) _ [Ba. treatedL- fBa untreated) X 100
[Ba, initial] - [Ba untreated]
Test conditions for the inhibition studies reported in Table I were:
Temperature 30°C, pH 4.3, S04 concentration 1000 ppm, Ba concen-
tration 2 ppm, AI concentration 3 ppm. Table I summarizes percent
inhibition values for a variety of treatment solutions and dosages.
TABLE I
Barium Sulfate Inhibition Values
_ Products) Dose s % inhibition~ Inhibition
(ppm) (slope) (Ba)
STPP 20 0 0
Coag 88D 10 9 0
PESA 5 fi0 32
PESA/STPP 5I 5 58 47
PESA/Coag 88D 5! 5 70 71
PESA/citric acid 5110 85 72
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STPP: Sodium tripolyphosphate
Coag 88D: Polyacrylic acid (available from Betz Laboratories of
Trevose, PA).
An additional metal ion binding agent, which is normally present in
industrial PESA solutions as a consequence of the manufacturing proc-
ess is tartaric acid. It was found that the efficacy of inhibiting BaS04
scale deposition in the presence of AI*3 ions increased with increasing
tartaric acid concentration as shown in Table ll.
TABLE II
Barium Sulfate Inhibition Values
Product % Tartaric Acid Dose % Inhibition
(ppm Active) (slope)
PESA I 4.23 5 51
PESA II 5.95 5 68
PESA I 4.23 10 84
PESA II 5.95 10 100
The above described dynamic scale apparatus was used to test
the efficacy of sodium hexametaphosphate (NaHMP) in conjunction with
PESA. The test conditions for the inhibition studies reported in Table III
were: temperature fi0°C, pH 5.5, SOd concentration 1500 ppm, Ba con-
centration 25 ppm, AI*3 concentration 3 ppm. Table III summarizes per-
cent inhibition values for a variety of minimum dosages of NaHMP and
PESA to provide 100% inhibition.
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TABLE III
Dosage to Provide 100% Inhibition of BaS04
PESA (~pm~ NaHMP~(ppml
5
25 0
15.8 27
8.1 43.2
0 60
As can be seen from Tables I, li and III the combination of PESA
and a metal ion binding agent inhibits scale deposition in acidic aqueous
environments where metal ions are present.
While the present invention has been described with respect to
particular embodiments thereof, it is apparent that numerous other forms
and modifications of the invention will be obvious to those skilled in the
art. The appended claims and this invention generally should be con-
strued to cover all such obvious forms and modifications which are within
the true scope and spirit of the present invention.
