Method and Composition for Treating Aqueous Mediums

The present invention relates to a method and composition for controlling the formation and deposition of scale forming salts, particularly calcium carbonate, calcium phosphate, and calcium sulfate, in aqueous mediums. The compositions and methods of the present invention also act as dispersants for suspended inorganic matter, such as clay and iron oxides which may be present in the aqueous medium.

Although the invention has general applicability to any given system where the formation and deposition of calcium carbonate, calcium phosphate and/or calcium sulfate is a potential problem, or where other problems due to deposition or agglomeration of suspended inorganic matter, such as iron oxide and clay, are encountered, the invention will be discussed in detail as it concerns cooling water and boiling water systems.

The term "cooling water" is applied whenever water is circulated through equipment to absorb and carry away heat. This definition includes air conditioning systems, engine jacket systems, refrigeration systems as well as the multitude of industrial heat exchange operations, such as found in oil refineries, chemical plants, steel mills, etc.

The once-through cooling system, as the name implies, is one in which the water is passed through the heat exchange equipment and the cooling water is then discharged to waste. Usually, a once-through system is employed only where water at suitably low temperature is readily available in large volume and at low cost. The normal source of once-through cooling water is from wells, rivers and lakes where the cost involved is that of pumping only. In a once-through system, no evaporation takes place and consequently the water does not concentrate. Circulating water characteristics are the same as the makeup water.

The use of a recirculating system, in which a cooling tower, spray pond, evaporative condenser and the like serve to dissipate heat, permits great economy in makeup water requirements. With dwindling supplies of fresh cold water available for industries' cooling requirements, increased use must be made of recirculating systems in which the cooling water is used over and over again.

After passage of the circulating water through the heat exchange equipment, the water is cooled when passing over the cooling tower. This cooling effect is produced by evaporation of a portion of the circulating water in passing over the tower. By virtue of the evaporation which takes place in cooling, the dissolved solids and suspended solids in the water become concentrated.

The circulating water becomes more concentrated than the makeup water due to this evaporation loss. Cycles of concentration is the term employed to indicate the degree of concentration of the circulating water as compared with the makeup. For example, two cycles of concentration indicates the circulating water is twice the concentration of the makeup water.

Deposits in lines, heat exchange equipment, etc., may originate from several causes. For example, the precipitation of, inter alia, calcium carbonate, calcium sulfate and calcium phosphate will form scale. In addition, solids foulant particles may enter the system. Through collisions with neighboring solids particles, these foulants may agglomerate to a point where they either foul a heat transfer surface or begin to accumulate in lower flow areas of the system.

On the other hand, corrosion is the electrochemical reaction of a metal with its environment. It is a destructive reaction and, simply stated, is the reversion of refined metals to their natural state. For example, iron ore is iron oxide. Iron ore is refined into steel. When steel corrodes, it also forms iron oxide.

In speaking of deposits which form in cooling water systems, it is important to bear in mind the mechanism causing the deposit, otherwise confusion may result. In general, the term "scale" applies to deposits which result from crystallization or precipitation of salts from solution. Wasting away of a metal is the result of corrosion. The agglomeration of suspended solids particles also results in deposit formation. While a deposit results in all of these cases, the mechanisms of formation are different and different corrective methods are required to prevent each type of deposit.

Some of the factors which affect scale formation are temperature, rate of heat transfer, the calcium, sulfate, magnesium, silica, phosphate, carbonate, alkalinity, dissolved solids and pH of the water.

In the past in order to minimize the formation of the scale forming salts, cooling water systems were operated at pH's where the solubility of the "hardness" or "scale forming" ions was the greatest. Because the pH's of the systems were acidic, corrosion inhibitors together with dispersants were the normal treatment. Corrosion inhibition in most instances required chromate treatment. With the advent of tight control regarding toxic pollutant discharge, operating parameters of cooling water systems had to be changed in an attempt to utilize non-chromate treatment. The development of high pH and/or non-chromate corrosion programs over the past few years has concurrently enhanced the potential for heat exchange fouling due to chemical precipitation. Presently, most non-chromate treatments include phosphate and/or phosphonic acid compounds, such as the alkali metal polyphosphates, organophosphates, e.g., phosphate esters, etc., amino-trimethylenephosphonic acid, hydroxy ethylidene diphosphonic acid, and water soluble salts thereof. However, the reversion of the polyphosphates and the organic phosphates plus the use of alkaline operating conditions leads to the formation and deposition of the highly insoluble calcium phosphate. Also since there may be phosphate in the makeup water supply, for example, tertiary sewage treatment effluent for makeup water, calcium phosphate scaling has become one of the major problems encountered. Of course, the formation of calcium sulfate in cooling water systems also results in a scale formation problem. Calcium sulate is often associated with the use of sulfuric acid for pH control, especially in connection with sidestream softening, and with the greater calcium concentrations associated with higher cycles of concentration.

Another principal scale-forming material encountered in cooling water systems is calcium carbonate formed by the decomposition of calcium bicarbonate. This compound has an inverse solubility curve (i.e., the solubility decreases as the system temperature increases), and its solubility is lower than most of the other potential scale imparting moieties that may be present in cooling systems. Calcium carbonate is soluble in acidic solutions, but since most cooling systems are operated at alkaline conditions to retard corrosion, calcium carbonate scaling remains a problem.

Although steam generating systems are somewhat different from cooling water systems, they share a common problem in regard to deposit formation.

As detailed in the Betz Handbook of Industrial Water Conditioning, 8th Edition, 1980, Betz Laboratories, Inc., Trevose, PA, Pages 85-96, the formation of scale and sludge deposits on boiler heating surfaces is a serious problem encountered in steam generation. Although current industrial steam producing systems make use of sophisticated external treatments of the boiler feedwater, e.g., coagulation, filtration, softening of water prior to its feed into the boiler system, those operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, slits and hardness-imparting ions escape the treatment, and eventually are introduced into the steam generating system. The problems which result from their introduction into the steam generating system are apparent. Since the deposit forming materials are present, they have a tendency to accumulate upon concentration of the water and to settle at points in the system where there is low flow, thus restricting water circulation. The baking of mud and/or sludge on tubes and sheets will result in overheating and failure, thereby requiring downtime for repair or replacement of the structural parts. In addition, mud, sludge and slits may become incorporated in scale deposits adding to their volume and heat insulating effect.

Accordingly, internal treatments have been necessary to maintain the mud and slits in a suspended state. These internal treatments have been generally referred to in the industry as sludge conditioning agents.

In addition to the problems caused by mud, sludge or silts, the industry has also had to contend with boiler scale. Although external treatment is utilized specifically in an attempt to remove calcium and magnesium from the feedwater, scale formation due to residual hardness, i.e., calcium and magnesium salts, is always experienced. Accordingly, internal treatment, i.e., treatment of the water fed to the system, is necessary to prevent, reduce and/or retard formation of the scale imparting compounds and their deposition. The carbonates of magnesium and calcium are not the only problem compounds as regards scale, but also waters having high contents of phosphate, sulfate or silicate ions either occurring naturally or added for other purposes cause problems since calcium and magnesium, and any iron or copper present, react with each and deposit as boiler scale. As is obvious, the deposition of scale on the structural parts of a steam generating system causes poorer circulation and lower heat transfer capacity, resulting accordingly in an overall loss in efficiency.

Although the foregoing is directed for the most part to cooling water systems and boiler water systems, or more specifically steam generating systems, the same problems occur in scrubber systems and the like. Any aqueous system having the requisite cations and anions will experience the formation and deposition of scaling salts.

Many and different type materials have been used for the treatment of water systems. Of the vast number may be mentioned alginates, lignins, lignosulfonates, tannins, carboxymethyl cellulose materials, and synthetic polymers such as polyacrylates and polymethacrylates. For instance, in U.S. Pat. No. 4,029,577 (Godlewski et al), of common assignment herewith, certain acrylic acid/hydroxylated lower alkyl acrylate copolymers are disclosed as being effective in controlling the formation and deposition of scale and/or suspended solid matter which otherwise would occur in aqueous mediums containing scale imparting ions and dispersed particles.

In U.S. Pat. No. 3,663,448 (Ralston), the formation of solid scale-forming salts in aqueous solution is inhibited by adding to the solution small amounts of certain amino phosphonate compounds, together with a water soluble polymer having a molecular weight from about 500 to about 12,000 selected from the group consisting of polyacrylic acid, copolymers of acrylic acid and up to 50% acrylamide and polyacrylamide in which at least 50% of the amide groups are hydrolyzed.

U.S. Pat. No. 4,209,398 (Ii et al) discloses yet another water treating process wherein a polymer having a structural unit derived from a monomer having an ethylenically unsaturated bond and having one or more COOH radicals is combined with inorganic phosphates, phosphonic acids, organic phosphonic acid esters, or polyvalent metal salts, to prevent scale formation and corrosion.

Water treatment compositions comprising certain telomeric phsophinocarboxylic compounds are disclosed in U.S. Pat. No. 4,239,648 (Marshall et al). The disclosure states that these compounds are prepared via reaction of acrylic acid and a hypophosphorous acid derivative. Other similar treatments are disclosed in U.S. Pat. No. 4,127,483 (Smith et al), and U.S. Pat. No. 4,159,946 (Smith et al).

Despite the efforts of the prior art, the water treatment industry is constantly searching for means for inhibiting scale formation and/or for dispersing inorganic solids particulate matter, efficiently and in a cost effective manner.

The present inventors have discovered that acrylic acid/lower alkyl hydroxy acrylate copolymers (I) in combination with a water soluble telomeric phosphinocarboxylic acid compound (II) not only effectively control the formation of certain scale forming salts, but also act as an effective dispersant for inorganic matter. Specifically, the combined treatment of the present invention comprising the copolymer (I) and telomeric phosphinocarboxylic acid compound (II) has proven effective in controlling the formation and deposition of calcium sulfate, calcium carbonate, and calcium phosphate and also effectively disperses iron oxide and clay.

The specific acrylic acid/lower alkyl hydroxy acrylate copolymers (I) utilized in accordance with the present invention are disclosed in U.S. Pat. No. 4,029,577 (Godlewski et al). The entire disclosure of this patent is accordingly incorporated by reference.

Basically, the polymers (I) which are to be utilized in conjunction with the telomeric phosphinocarboxylic acid component (II) in the combined treatment are those containing essentially moieties (a) derived from an acrylic acid compound, i.e., ##STR3## where R is hydrogen or a lower alkyl of from 1 to 3 carbon atoms and R.sub.1 =OH, OM, NH.sub.2, where M is a water soluble cation, e.g., NH.sub.4, alkali metal (K, and Na), etc.; and (b) moieties of an hydroxylated lower alkyl (C=2-6) acrylate as represented for example by the formula ##STR4## where R is H, or CH.sub.3 and R.sub.2 is a lower alkyl having from about 2 to 6 carbon atoms (the OH moiety may be attached to any of the C atoms in the alkyl group).

These polymers most advantageously have a mole ratio of moieties derived from an acrylic acid compound to hydroxy alkyl acrylate derived moieties of from about 34:1 to about 1:4, and preferably 11:1 to 1:2, and possess a molecular weight of from 500 to 1,000,000 and preferably 1,000 to 500,000. The only criteria that is of importance that applies to the molar ratios of the described monomers in the copolymer, is that it is desirable to have a copolymer which is soluble in water. It should be noted that as the proportion of hydroxylated alkyl acrylate moieties increase, the solubility of the copolymer decreases.

The polymers (I) utilized in accordance with the invention can be prepared by vinyl addition polymerization or by treatment of an acrylic acid or salt polymer. More specifically, acrylic acid or derivatives thereof or their water soluble salts, e.g., sodium, potassium, ammonium, etc. can be copolymerized with the hydroxyalkyl acrylate under standard copolymerization conditions utilizing free radicals such as benzoyl peroxide, azo bisisobutyronitrile or redox initiators such as ferrous sulfate and ammonium persulfate. The molecular weights of the resulting copolymer can be controlled utilizing standard chain control agents such as secondary alcohols (isopropanol), mercaptans, halocarbons, etc. Copolymers which may be utilized in accordance with the present invention are commercially available from National Starch Company. One preferred copolymer is sold by National Starch under the trademark "Natrol 42". This particular copolymer is an acrylic acid/2-hydroxypropyl acrylate copolymer having an acrylic acid: 2-hydroxypropyl acrylate molar ratio of 3:1 and a molecular weight of about 6000.

The hydroxyalkyl acrylate can be prepared by the addition reaction between the acrylic acid or its derivatives or water soluble salts and the oxide of the alkyl derivative desired. For example, the preferred monomer of the present invention is the propyl derivative. Accordingly, to obtain the hydroxylated monomer, acrylic acid is reacted with propylene oxide to provide the hydroxypropylacrylate monomer constituent of the copolymer utilized in accordance with the present invention.

The polymers of the present invention may also be prepared by reacting the polyacrylic acid or derivatives thereof with an appropriate amount of an alkylene oxide having from 2 to 6 carbon atoms such as ethylene oxide, propylene oxide and the like. The reaction takes place at the COOH or COM group of the moieties to provide the hydroxylated alkyl acrylate moiety.

The preferred copolymer prepared either by copolymerization or by reaction of polyacrylic acid or acrylate with the propylene oxide would be composed of units or moieties having the structural formulas ##STR5## where M is as earlier defined and wherein the molar ratio of x to y is preferably 11:1 to 1:2. The copolymer preferably has a molecular weight of from 1,000 to 500,000.

As to the telomeric phosphinocarboxylic acid compounds (II), these are not new. Suitable compounds, and their preparative reaction mechanisms are disclosed in U.S. Pat. No. 4,239,648 (Marshall et al). The entire disclosure of this patent is hereby incorporated by reference.

Accordingly, the telomeric compounds, suitable for use in the present invention are characterized by the formula ##STR6## and salts thereof, wherein Y" is H or a methyl or ethyl residue, Y is H, a straight or branched alkyl residue, having from 1 to 18 carbon atoms, a cycloalkyl residue having from 5 to 12 carbon atoms, an aryl residue, an aralkyl residue, a residue of the formula: ##STR7## wherein Y" has its previous significance and the sum n+m is zero or an integer of at the most 100, or a residue OX wherein X=H or a straight or branched alkyl residue having from 1 to 4 carbon atoms and Y' is a residue OX wherein X has its previous significance.

Salts of the telomeric compounds are compounds wherein some or all of the acidic hydrogen atoms in the compound have been replaced by Group IA, or IIA metals, ammonium ions or quaternized amine radicals.

The preferred telomeric phosphinocarboxylic acid, for use in the present invention, is commercially available from Ciba-Geigy and is presently sold under the trademark "Belclene 500." This particul ##STR8## wherein m+n=about 3 to about 6, and wherein the molecular weight is about 282 to about 498. NMR analysis of "Belclene 500" has revealed that it is a mixture of telomeric phosphinocarboxylic acid compounds of the above formula with an average m+n value of about 4.6.

Exemplary bases which may react with the telomeric phosphinocarboxylic acids, to form either partial or complete salts, include the hydroxides and carbonates of Na, K, and NH.sub.4.sup.+. Organic bases may also be mentioned. Specifically, primary, secondary and tertiary alkyl and substituted alkyl amines in which the total number of carbon atoms does not exceed 12, are suitable. One such example is triethanolamine.

Basically, the telomeric phosphinocarboxylic acid compounds of the present invention may be prepared by reacting various molar ratios of a compound of the formula

wherein Y" has its previous significance, with a hypophosphorous compound of the formula ##STR9## wherein Y' has its previous significance and Y"' is hydrogen, a straight or branched chain alkyl residue having from 1 to 18 carbon atoms, a cycloalkyl residue having from 5 to 12 carbon atoms, an aryl, an aralkyl residue or OX wherein X has its previous significance.

Salts of the above hypophosphorous compound may also be used, wherein the acidic hydrogen atoms have been partially or completely replaced by cations from the salt forming bases referred to hereinabove.

The combined treatment (copolymer I and telomeric phosphinocarboxylic acid II) should be added to the desired aqueous system in an amount effective for the purpose, taking into consideration the respective concentrations in the water of the potential scale and deposit forming species, the pH of the water and the chemical and physical properties of the combined treatment. The criteria for proper treatment of any aqueous system would be apparent to the worker in the art of water treatment. For the most part, the combined treatment will be effective when utilized at levels of from about 0.1 to 500 parts per million of water.

The operable molar ratio of copolymer I:telomeric phosphinocarboxylic acid II, in accordance with the invention, is from about 10:1 to 1:10. The preferred molar ratio I:II is about 3:1 to 1:3.

The combined treatment (I and II) of the present invention is not limited to any specific category of water system. For instance, in addition to boiler and cooling water systems, the combined treatment may also be effectively utilized in scrubber systems and the like wherein the formation of scale forming salts and/or the agglomeration of inorganic matter is a problem. Other possible environments in which the combined treatment may be used include heat distribution type sea water desalting apparatus and dust collection systems in iron and steel manufacturing industries.

The invention will now be further described with reference to a number of specific examples which are to be regarded solely as illustrative, and not as restricting the scope of invention.

One method of evaluating deposit control activity of a material consists of measuring its ability to prevent bulk phase precipitation of the salt at conditions for which the salt would usually precipitate. It is additionally important to recognize that the material being evaluated is tested at "substoichiometric" concentrations. That is, typical molar ratios of precipitating cation to the material being evaluated are on the order of 20:1 and much greater. Consequently, stoichiometric sequestration is not the route through which bulk phase precipitation is prevented. The well known phenomenon is also called "threshold" treatment and is widely practiced in water treatment technology for the prevention of scale (salt) deposits from forming on various surfaces. In the results that follow calcium phosphate, calcium carbonate, and calcium sulfate salts commonly found in industrial water systems under various conditions have been selected as precipitants. The combined treatment of the present invention has been evaluated for its ability to prevent precipitation (i.e., inhibit crystallization) of these salts. The results are expressed as "percent inhibition", positive values indicate the stated percentage of the precipitate was prevented from being formed. Except as where noted to the contrary, the following conditions, solutions, and testing procedure were utilized to perform the calcium carbonate, calcium phosphate and calcium sulfate inhibition tests, the results of which are reported herein below in the following Tables.

Procedure

(1) To about 1800 ml DIH.sub.2 O in a 2 liter volumetric flask, add 20 ml of CaCl.sub.2.2H.sub.2 O solution followed by 2 drops of conc. HCl.

(2) Add 40 ml of Na.sub.2 HPO.sub.4 solution.

(3) Bring volume to 2 liters with DI water.

(4) Place 100 ml aliquots of solution in 4 oz glass bottles.

(5) Add treatment.

(6) Adjust pH as desired.

(7) Place in 70.degree. C. water bath and equilibrate for 17 hours.

(8) Remove samples and filter while hot through 0.2 u filters.

(9) Cool to room temperature and take Absorbance measurements using Leitz photometer (640 nm).

Preparation for Leitz

a. 5 mls filtrate

b. 10 mls Molybdate Reagent

c. 1 dipper Stannous Reagent

d. Swirl 1 minute, pour into Leitz cuvette; wait 1 minute before reading.

(10) Using current calibration curve (Absorbance vs ppm PO.sub.4.sup.-3) find ppm PO.sub.4.sup.-3 of each sample.

Calculation ##EQU1##

Procedure

(1) Add 50 ml of 10.sup.-1 CaCl.sub.2.2H.sub.2 O pre-adjusted to pH 7.0 to a 4 oz. bottle.

(2) Add treatment.

(3) Add 50 ml of 10.sup.-1 Na.sub.2 SO.sub.4 pre-adjusted to pH 7.0.

(4) Heat samples for 24 hours in a 50.degree. C. water bath.

(5) Cool for 30 minutes, at least.

(6) Filter 5 ml through 0.45 u filters.

(7) Add NaOH to pH 12.0 and dilute to 50 ml with DI H.sub.2 O.

(8) Add Ca.sup.+2 indicator (1 level).

(9) Titrate to purple-violet endpoint with EDTA.

Calculation ##EQU2##

Procedure

(1) Add 50 ml CaCl.sub.2.2H.sub.2 O pre-adjusted to pH 9.0

(2) Add treatment

(3) Add 50 ml Na.sub.2 CO.sub.3 pre-adjusted to pH 9.0

(4) Heat 5 hours at 70.degree. C. water bath. Remove and cool to room temperature.

(5) Filter 5 mls through 0.2 u filters.

(6) Adjust samples to pH <1.0 with conc. HCl (1 g Conc. HCl)

(7) Allow to stand at least 15 minutes.

(8) Dilute to 50 mls with DI H.sub.2 O.

(9) Bring pH to 12.0 with NaOH.

(10) Add Ca.sup.+2 indicator (1 level).

(11) Titrate with EDTA to purple-violet endpoint.

Calculation ##EQU3##

Here, it can be seen that at the 2 ppm total actives level, all of the molar ratio AA/HPA:PCA combination treatments were remarkably effective in inhibiting calcium sulfate formation.

With respect to Table IV above, the synergistic effects of the combined AA/HPA and PCA treatment are made abundantly clear. For example, the total inhibition afforded by the AA/HPA-PCA combination is greater than the sum of the inhibition results recorded for the individual components as comparable treatment dosages. The combined treatment is also synergistic in the inhibition of calcium carbonate, as is shown in Table V following.

In order to demonstrate the effectiveness of the combined treatment composition and method in dispersing suspended particulate matter, the following procedures using Fe.sub.2 O.sub.3 and clay, separately, as suspended solids, were undertaken. The results appear in Table VI hereinbelow. In the results, it is noted that increasing .DELTA.%T values indicate better treatment as more particles remain suspended in the aqueous medium.

Procedure

(1) Prepare a suspension of 0.1% Fe.sub.2 O.sub.3 in DIH.sub.2 O.

(2) Adjust hardness to 200 ppm Ca.sup.+2 as CaCO.sub.3 using CaCl.sub.2.2H.sub.2 O solution-8 ml/1000 ml of Fe.sub.2 O.sub.3 solution.

(3) Using overhead mixer, mix suspension 1/2 hour at 1000 rpms.

(4) Remove solution to magnetic stirrer and adjust to pH 7.5 (about 20 minutes to stabilize pH).

(5) return solution to overhead mixer.

(6) Take 90 ml aliquots of suspension and place 4 oz. glass bottle.

(7) Add treatment and DI water to bring total value to 100 ml.

(8) Cap bottle, invert several times and place on reciprocating shaker at a moderate speed of about 40 spm for 1/2 hour.

(9) Place on vibration-proof surface and allow to stand 18 hours.

(10) Without disturbing settled phase, pipet the top 40 mls off the sample. Place in a cell and read %T (at 415 nm).

Calculation

Procedure

(1) Prepare a suspension of 0.1% Hydrite UF in DIH.sub.2 O.

(2) Adjust hardness to 200 ppm Ca.sup.+2 as CaCO.sub.3 using CaCl.sub.2.2H.sub.2 O solution-8 ml/1000 ml of Hydrite solution.

(3) Using overhead mixer, mix suspension 1/2 hour at 1000 rpms.

(4) Remove solution to magnetic stirrer and adjust to pH 7.5 (about 20 minutes to stabilize pH).

(5) Return solution to overhead mixer.

(6) Take 90 ml aliquots of suspension and place 4 oz. glass bottle.

(7) Add treatment and DI water to bring total volume to 100 ml.

(8) Cap bottle, invert several times and place on reciprocating shaker at a moderate speed of about 40 spm for 1/2 hour.

(9) Place on vibration-proof surface and allow to stand 18 hours.

(10) Without disturbing settled phase, pipet the top 40 mls off the sample. Place in a cell and read %T (at 415 nm).

Calculation

While we have shown and described herein certain embodiments of the present invention, it is intended that there be covered as well any change or modification therein which may be made without departing from the spirit and scope of the invention as defined in the appended claims.