WO1997010180A1

WO1997010180A1 - Composition and process for inhibiting scale formation in aqueous systems

Info

Publication number: WO1997010180A1
Application number: PCT/US1996/014788
Authority: WO
Inventors: Everett J. Fuller
Original assignee: Ashland Inc.
Priority date: 1995-09-13
Filing date: 1996-09-13
Publication date: 1997-03-20
Also published as: AU7109996A

Abstract

This invention relates to a composition and process for controlling scale on metal surfaces exposed to aqueous systems. The composition used in the process is a synergistic mixture of (1) a first component capable of inhibiting the formation of CaSO4 scale; and (2) a second component selected from the group consisting of benzoic acid, benzene sulfonic acid, water soluble derivatives thereof, and salts thereof.

Description

COMPOSITION AND PROCESS FOR INHIBITING SCALE FORMATION IN AQUEOUS SYSTEMS

FIELD OF THE INVENTION This invention relates to a composition and process for controlling scale on metal surfaces exposed to aqueous systems. The composition used in the process is a synergistic mixture of (1) a first component capable of a chemical known to inhibit the formation of CaS0₄ scale; and (2) a non polymeric aromatic acid, derivative thereof, or salt thereof.

BACKGROUND OF THE INVENTION Industrial scale formation results from the combined effects of several variables, e.g. temperature, flow rate, mixing, concentration, ionic strength, other scale forming species, dispersion, etc. Many of these variables vary throughout a process flow and are specific to the site. Calcium ions and sulfate ions are present in most water which circulates through the cooling systems and boilers of most industrial complexes. These ions deposit on the insides of pipes, heat exchangers, boilers and furnaces where they cause CaS0₄ scale formation and other damage. CaS0 poses a particularly serious problem to the operation of a cooling system because CaS0₄ has limited solubility.

Furthermore, once CaS0₄ has formed, it is very difficult to remove.

Problems from the precipitation of CaS0₄ can also arise in mining and in the production of fertilizers. In gold recovery by leaching of ore with cyanide solution, for example, a highly alkaline pH is desirable for optimal recovery of gold and to minimize the release of toxic hydrogen cyanide gas (HCN) to the environment. Lime (CaO) is commonly added to maintain a high pH. This introduces a substantial amount of calcium ions, which can combine with sulfate ions already present in the mineral being processed to form calcium sulfate scale, which tends to plug the cyanide solution distribution lines, accumulate on screens, etc.

The wet process phosphoric acid procedure is based upon treating calcium phosphate ore with sulfuric acid, H₂S0₄. The desired products are made with by-production of calcium sulfate scale:

3H₂S0₄ + Ca₃(P0₄)₂ → 2H₃PO_< + 3CaS0₄

Such scale tends to accumulate in process equipment and cause expensive shutdowns for scale removal.

It is known to use metal sequestrants to prevent crystallization and precipitation of solids such as calcium sulfate. The disadvantage of powerful sequestrants such as ethylenediamine tetraacetic acid (EDTA) is that in general, one mole of EDTA is required to complex a mole of metal ion, and the complexing is irreversible. It also known to use organic phosphorus substances such as phosphonates and phosphinates to inhibit the formation of calcium sulfate scale on metal surfaces. Organic phosphorus substances such as hexamethylene dia ine tetra (methylene phosphonic acid) are used in water to change the crystal properties of solids which form scale, and to inhibit crystal formation so that scale removal problems are greatly reduced or eliminated.

Organic polymers of acrylic, maleic or methacrylic acids or their derivatives, as well as other monomers, for example vinyl derivatives and 2-acrylamido-2- methylpropane-sulfonic acid, are also known in the art to be effective inhibitors of calcium sulfate scale formation. Combinations of some of these substances are known to be effective as well.

SUMMARY OF THE INVENTION This invention relates to a calcium sulfate scale corrosion inhibitor comprising as a mixture:

(a) a first component capable of inhibiting the formation of calcium sulfate scale; and

(b) a non polymeric aromatic acid, derivative thereof, or salt thereof.

such that the weight ratio of (a) to (b) is from about 20:1 to 1:20, preferably 10:1 to 1:10.

Preferably (b) is selected from the group consisting of benzoic acid, benzene sulfonic acid, water soluble derivatives thereof, and salts thereof. The invention also relates to a process for controlling scale on metal surfaces exposed to aqueous systems. It is surprising that the compounds, used as the second component of the mixture, inhibit scale formation when used with conventional CaS0₄ inhibitors. For benzoic acid and its salts, when used alone, do not appreciably inhibit CaS0₄ scale formation. Thus such mixtures demonstrate efficacy which was not predicted in view of the activity of each individual component.

The invention offers several advantages, particularly when benzoic acid salts are used as the second component of the mixture. Sodium benzoate is non toxic and creates less stress to the environment. It is also cheaper than conventional inhibitors of CaS0₄ scale.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a calibration curve for a standard showing the calcium sulfate inhibiting activity of polyacrylic acid.

BEST MODES AND OTHER MODES

Any known inorganic or organic chemical or combination of chemicals which inhibits the formation of CaS0₄ scale can be used in the first component of the mixture. Examples of such conventionally known CaS0₄ scale inhibitors based upon phosphorous include polyphosphates and polyorganophosphorus compounds such as phosphonates and phosphate esters. An example of a phosphorus compound known to inhibit CaS0₄ scale is hexamethylene diamine tetra (methylene phosphonic acid) .

Other examples of conventional CaS0₄ scale inhibitors include organic polymers of acrylic, maleic or methacrylic acids, copolymers thereof, derivatives thereof, and the like. Also used are polymers of vinyl derivatives (for example, 2-acrylamido-2-methylpropane- sulfonic acid) , sulfonated polymers, polymaleic anhydride, maleic anhydride copolymers, phosphono- carboxylates, organic acids such as citric acid, chelates such as EDTA, anionic and cationic polymers, and the like. Combinations of many of these substances are known to be effective as well.

Some specific examples of chemicals which can be used as the first component are polyacrylic acid, amino- tri (methylenephosphonic acid) , 2-acrylamido-2- methylpropane sulfonic acid, hexamethylenediaminetetra- (methylenephosphonic acid) , sodium lignosulfonate, bis (hexamethylene)triamine phosphonic acid, hydroxyphosphonoacetic acid, and polymaleic acid. The second component is a non polymeric aromatic acid, derivative thereof, or salt thereof. Preferably used as the second component is a compound selected from the group consisting of benzoic acid, benzene sulfonic acid, water soluble derivatives thereof, and salts thereof. These compounds can be added directly to the water where the calcium sulfate ions are present or benzoic acid can be put into the alkaline water and benzoate salt will form by the neutralization of the acid. Also sodium xylene sulfonate, a water-soluble derivative of benzenesulfonic acid can be used. It has the advantage of remaining soluble at lower pH's where benzoic acid would precipitate from a solution of its sodium salt. The weight ratio of component one to component two is from about 20:1 to 1:20, preferably 10:1 to 1:10.

ABBREVIATIONS

BZH benzoic acid

PAA polyacrylic acid

ATMPA aminotri(methylenephosphonic acid)

AMPS 2-acrylamido-2-methylpropane sulfonic acid

HMDTMPA hexamethylenediaminetetra

(methylenephosphonic acid)

SLS sodium lignosulfonate

BHTPA bis(hexamethylene)triamine phosphonic acid

HPA hydroxyphosphonoacetic acid

PM polymaleic acid

EXAMPLES

The examples will show the effect of using the compounds of the first and second component alone and mixtures of the first component and second component as CaSC scale inhibitors. In order to test the effectiveness of these substances, a standard was created using polyacrylate according to the following test:

CaS0 Precipitation Rate Teβt

(Sequoia-Turner Model 390 Spectrophotometer) It is known that CaS0₄ is precipitated by isopropyl alcohol at a rate which is proportional to the amount of solid already present (autocatalytic conditions) . This rate has been shown to be strongly inhibited by the presence of less than 1 ppm polyacrylate in solution. Under the test conditions, two absorbance readings suffice to define the rate constant and index the polymer activity, which is compared to that of a standard polyacrylate using a standard curve.

The apparatus and reagents used in the test were:

• Model 390 spectrophotometer with 13 mm O.D. cell, set at 450 nm to read absorbance.

• Pipets to deliver 10 ml, 0.5 ml and 2 ml.

• 0.06M solutions of CaCl₂.2H₂0 and Na₂S0.

• 6 ppm (active) solution of test polymer at pH «11 (will give 0. 1463 ppm polymer in test before isopropyl alcohol is added) . • Isopropyl alcohol.

• Two ounce sample bottle with screw cap.

The procedure used is described as follows: 1. Set zero and D. I. water blank to zero.

2. To a sample bottle, add 10 ml of Ca⁺⁺ solution, 0.5 ml acrylic polymer solution and 10 ml S0₄" solution. Cap bottle and shake 4x to mix. 3. At time zero, begin addition of 2 ml isopropyl alcohol slowly so that it tends to float on the liquid in the bottle. At ten seconds, cap bottle and shake 3x. 4. Rinse the cell once with test mixture, then add *3 ml of test mixture to the cell. Put the cell into the Model 390 and close the lid. Record absorbance at two minutes and at three minutes.¹

5. Discard cell contents; wash cell, bottle and cap once with D.I. water, once with «0.3N HCl and four times with D.I. water.

6. Repeat steps 1-5 until five determinations have been made.²

The precipitation rate (μ) was determined for the system as follows:

1. Calculate K_: = In A3 for each determination where

A2 Ki is the precipitation rate constant in min^"1, A2 is absorbance at two minutes, and A3 is absorbance at three minutes.

2. Calculate K, , the average value of the five

determinations.

The reaction is sensitive to minor differences in procedure, which can produce virtually no change in absorbance for up to 5 minutes in some cases. If this happens repeat the test a few times, if no change is consistently seen, try a more dilute polymer solution.

If A₂ is less than 0.01 absorbance units, repeat the test and do not use such values in computing results. 3. For each Ki, compute (A., - K ², sum these values to

get A.

4. Compute the standard deviation S = Jf .

5. Compute the true mean³:

μ = lg+ 2.776S 2.236

6. The value of μ is graphed on the standard curve.

If μ lies inside the limits of definition for the

PAA standard at the same concentration, the polymer tested is equivalent to PAA standard by this test. The value of μ defines an equivalent PAA standard concentration.

Figure 1 is a calibration curve for a standard showing the calcium sulfate inhibiting activity of polyacrylic acid. The equation of the line is Ki =

0.8764 - 2.298 (ppm active polyacrylic acid). The coordinates of the points are as follows:

ppm active PAA Ki

0.1026 0.6508 (range of from 0.5283 to 0.7733)

This calculation is based on the average of five tests. Standard t tables [for example, "Statistics for Experimenters", by G.E.P. Box, W.G. Hunter and J.S. Hunter (John Wiley & Sons, New York, New York, 1978, p.631] will provide a number different from 2.776 in the numerator, if the number of tests is different from five. Also, the denominator is the square root of the number of tests, and S - A / lJ — l for n tests. 0.5128 0.7654 (range of from 0.6762 to 0.8546)

0.30 0.2071 (range of from 0.1455 to 0.2687)

0.30 0.3800 (range of from 0.2780 to 0.4820)

Figure 1 shows a plot of the activity of the polyacrylate which can be used to determine the activity of the other compounds tested for their inhibitory effect on the formation of CaS0₄ scale. The dotted lines on Figure 1 are the average confidence which is ± 0.0938 from the line.

After the standard was established, compositions as described in Table I, were tested and compared to the polyacrylate standard which had an activity level of 1.00. The activity of the tested compounds relative to PAA is determined by measuring K_ at a given concentration of active polymer or inhibitor. Then the activity relative to PAA is determined as follows:

ppm PAA to get measured ki ppm active substance actually used"

The activity of the chemical, which is based upon experimental results, is then compared with the predicted activity for the mixture of the conventional inhibitor with benzoic acid. The predicted activity of the mixture is the weight average of the two activities of the individual chemicals and is determined as follows

(assuming two chemicals are used) :

(Ci x A ) + (C₂ x A₂) = PAM

⁴ The amount actually used in the experiments was 0.1463. 2 where C is the fraction concentration of the chemical, A is the activity of the chemical, and PAM is the weight average of the predicted activity of the mixture. When the activity of the compounds of the second component of the mixture are essentially zero, the predicted activity of the mixture will be 0 + C x A of the other component. For example, a mixture containing 50% sodium benzoate and 50% of B which has an activity level of 0.7 will be 0.35.

If the actual activity of the mixture exceeds the calculated predicted activity, this indicates that a synergy results by using the mixture. An unexpected improvement in the activity of the mixture occurred. Actual results indicate that benzoic acid alone does not inhibit the formation of CaS0₄ scale and thus has an activity value of 0. The conventional inhibitors tested did inhibit the formation of CaS0₄ scale. However, the addition of benzoic acid in various weight percentages to the conventional CaS0₄ scale inhibitors resulted in an activity which exceeded expectations based upon what was known about the activity of the individual components of the mixture, while reducing the overall cost of the inhibitor and creating less stress to the environment.

TABLE I

CORROSION INHIBITING ACTIVITY OF INHIBITORS ALONE AND AS MIXTURES WITH BZH

CONTROL A B C D E F G H

SOLE PAA BZH BHTA AMPTA HPA PM SLS PAA/AMPS INHIBITOR

ACTIVITY 1.0 0 1.114 1.208 0.950 1.182 0.820 1.029

KZAMPIXS 1 2 3 4 5 6

(MIXTURES WITH BZH)

% WEIGHT BZH 15.1 54.5 12.0 29.0 44.0 50.0 ADDED

ACTIVITY OF 1.076 1.539 1.017 1.194 0.898 1.140 MIXTURE

PREDICTED 0.95 0.7790 1.06 0.86 0.53 0.515 ACTIVITY OF MIXTURE

The results in Table I clearly indicate that the inhibiting activity of the mixture of the conventional corrosion inhibitor and benzoic acid salt was greater than what was predicted. In fact, in all cases in Table I, except the BHTA/BZH mixture, the activity of the mixture was greater than when the conventionally known corrosion inhibitor was used, even though much less of the conventionally known corrosion inhibitor was used on a weight basis when mixed with the BZH.

Claims

CLAIMS I claim:

1. A composition for inhibiting the formation of calcium sulfate scale on metals which are in contact with aqueous systems, said composition comprising as a mixture:

(a) a first component which is a chemical known to inhibit the formation of calcium sulfate scale on metals which are in contact with an aqueous system, and

(b) a second component comprising a non polymeric aromatic acid, derivative thereof, or salt thereof,

such that the weight ratio of (a) to (b) is from 20:1 to 1:20 based upon the total weight of (a) to (b) .

2. The composition of claim 1 wherein the chemical of the first component of the composition is selected from the group consisting of polyacrylic acid, aminotrimethylenephosphonic acid) , 2-acrylamido-2- ethylpropane sulfonic acid, hexamethylenediamine tetra (methylenephosphonic acid) , sodium lignosulfonate, bis (hexamethylene) triamine phosphonic acid, hydroxyphosphonoacetic acid, polymaleic acid, and mixtures thereof.

3. The composition of claim 1 wherein the weight ratio of (a) to (b) is from 10:1 to 1:10 based upon the total weight of (a) to (b) .

4. The composition of claim 3 wherein the compound used in the second component is a compound selected from the group consisting of benzoic acid, benzene sulfonic acid, water soluble derivatives thereof, and salts thereof.

5. A process for inhibiting the formation of calcium sulfate scale on metals which are in contact with aqueous systems which comprises:

adding a composition comprising as a mixture:

6. The process of claim 5 wherein the second component is a compound selected from the group consisting of benzoic acid, benzene sulfonic acid, water soluble derivatives thereof, and salts thereof.

7. The process of claim 5 wherein the chemical of the first component of the composition is selected from the group consisting of polyacrylic acid, aminotrimethylenephosphonic acid) , 2-acrylamido-2- ethylpropane sulfonic acid, hexamethylenediamine tetra(methylenephosphonic acid) , sodium lignosulfonate, bis(hexamethylene)triamine phosphonic acid, hydroxyphosphonoacetic acid, polymaleic acid, and mixtures thereof.

8. The process of claim 5 wherein the weight ratio of

(a) to (b) of the composition is from 10:1 to 1:10 based upon the total weight of (a) to (b) .

9. The process of claim 7 wherein the compound used in the second component of the composition is sodium benzoate.