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54 MATERIALS PERFORMANCE May 2000
The importance of calcium phosphates (Ca/P) as the major constituents of in-dustrial mineral deposits has stimulated extensive research on the precipitation and inhibition of these com-
pounds. In cooling, boiler, geothermal, and desalination applications, the insulating nature of scales on equipment surfaces results in decreased system efficiency and premature equipment failure. In addition to scaling problems, cooling water sys-tems and boilers constructed of carbon steel also experience corrosion problems caused by dissolved oxygen. Effective water treatment programs must control scale, corrosion, particulate matter, and microbiological growth.
Many different inhibitors have been developed to provide corrosion protec-
tion. For many years, chromate-based corrosion inhibitors were considered the standard in the water treatment industry. However, chromates are toxic, and envi-ronmental regulations have caused their use to decline considerably since the 1960s. The mild steel corrosion inhibitors that have been displacing chromate include molybdates, silicates, polyphosphates/phosphates, phosphonates, and zinc salts. From the perspective of versatility, cost, and performance, stabilized phosphate and all-organic cooling water treatment (CWT) programs incorporating phosphates and phos pho nates have become the new per-formance standards.
The key to the performance of phos-phate and phosphonate-based treatment programs lies in the proper selection and performance of polymeric inhibitors of Ca/P and calcium phos phonate deposi-tion. These polymers have a dual purpose: 1) control the thickness of the Ca/P film on the metal surface and 2) prevent pre-cipitation in the recirculating water.
A variety of polymers have been de-veloped and are used extensively as Ca/P inhibitors. Studies have shown that a polymer’s performance strongly depends upon its composition, molecular weight, and various impurities (soluble and in-soluble) present in the cooling water.1-2 A previous publication addressed the adverse effect of cationic flocculating polymer on the performance of Ca/P-inhibiting polymers.3 This article is based on a recent BFGoodrich publication4 and specifically addresses the effect of water impurities (e.g., metal ions, clay) on the inhibitory power of various polymers. It is hoped that the quantification of the effect of water impurities on polymer performance will help facilitate polymer selection and optimization of water treatment programs.
Experimental ProceduresThe chemicals used to prepare the
solutions were Fisher Scientific ACS re-agent grade. Stock solutions of known concentrations of calcium chloride (CaCl2) and disodium phosphate (Na2HPO4), hydrochloric acid (HCl), sodium hydrox-ide (NaOH), and various polymers were
Effect of Cooling Water Impurities
on Deposit Control Polymer Performance
Zahid amjad, RobeRt W. Zuhl, and john F. ZibRida, The BFGoodrich Co.
The performance of polymeric inhibitors in treating recirculating cooling water systems is influenced by many factors, including pH, temperature, makeup water quality, and heat exchanger metallurgy. Impurities such as metal ions and suspended matter impact the performance of polymeric inhibitors used in phosphate-based treatment cooling water programs.
May 2000 MATERIALS PERFORMANCE 55
prepared and used to make test solutions. The dilution water used was double deion-ized and distilled. The polymeric inhibitors were selected from commercial materials. All inhibitor solutions were prepared on dry weight basis.
The supersaturated Ca/P solution was made by the slow addition of the phosphate stock solution to the desired water. The polymer and other ionic con-stituents were added, and the solution pH was adjusted, if necessary. After a 30-min equilibration period, the calcium stock solution was added, making up the total volume to 600 mL. In experiments involving the effect of soluble metal ions (i.e., iron [III], manganese, zinc), a known amount of stock solution of metal salt was added to the test solution following temperature stabilization but prior to pH adjustment and before the introduction of the calcium solution. For all experiments the reaction period was fixed at 22 h. Dur-ing each experiment, the test solution pH was maintained (within ±0.01 pH units) using the pH-stat unit.3 Unless specified, the standard test conditions for the Ca/P inhibition were 140 ppm calcium, 9.0 ppm phosphate, pH 8.50, 50°C, and 22 h.
The reaction progression was deter-mined by spectrophotometric analysis of a filtered (0.45 µm) aliquot of the test so-lution for the phosphate ion.3 The efficacy of the polymer was calculated according to the method described in previous pub-lications.1,3
Results and DiscussionDuring the last 2 decades many poly-
mers with different molecular architecture have been developed and tested as pre-
cipitation inhibitors. The most effective Ca/P and Ca/P-phosphonate inhibitors are co- or terpolymers containing acrylic acid, methacrylic acid, maleic and/or other comonomers with different functional groups (i.e., ester, hydroxyl, sulfonic, am-ide, etc.). These polymers typically also function as metal ion stabilizers and disper-sants for particles suspended in water. The ability of the polymer to act as a dispersant, metal ion stabilizer, and/or precipitation inhibitor largely depends upon polymer composition, molecular weight, and ionic charge. Table 1 lists the polymers evalu-ated herein.
EffEct of PolymEr DosagE at stanDarD conDitionsThe ability of a polymer to inhibit Ca/P
formation is a very important aspect of phosphate and phosphonate-based wa-ter treatment programs. Figure 1 shows the threshold inhibition as a function of polymer dosage for Polymers A, B, and C. Comparison of the three performance curves in Figure 1 clearly shows the supe-rior Ca/P inhibition of Polymer B (terpoly-mer) as a function of dosage. The data in Figure 1 provide the baseline performance and reference point for Polymers A and B compared to data obtained with the incorporation of various water impurities, as discussed below.
EffEct of mEtal ions on ca/P inhibition
Corrosion by-products, treatment pro-gram components, water impurities, and metal ions present in cooling water sys-tems may influence polymer performance. Therefore, it is important to quantify the effect of different metal ions such as iron, copper, manganese, and zinc on the performance of polymers used in CWT programs.
TAbLE 1
DEPOSIT CONTROL POLyMERS EvALuATED Polymer Composition A Acrylic acid/sulfonic acid copolymer(A) B Acrylic acid/sulfonic acid/sulfonated styrene terpolymer(B) C Acrylic acid homopolymer(C) D Acrylic acid/sulfonic acid/nonionic terpolymer(D) E Acrylic acid/sulfonated styrene copolymer(E)
(A)GOOD-RITE® K-775† acrylate copolymer (B)GOOD-RITE® K-798† acrylate terpolymer (C)GOOD-RITE® K-752† polyacrylate (D)Other terpolymer (E)Other copolymer †Trade name.
FIguRE 1
Effect of polymer dosage on Ca/P inhibition.
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50
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30
20
10
0
Thre
shol
d In
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(%)
1614121086420
Polymer Dosage (ppm)
[Ca] = 140 ppm, [PO4] = 9 ppmpH = 8.5, T = 50°C, t = 22 h
Polymer APolymer BPolymer C
56 MATERIALS PERFORMANCE May 2000
100
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40
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0
Thre
shol
d In
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tion
(%)
Polymer A Polymer B Polymer D Polymer E[Ca] = 140 ppm, [PO4] = 9 ppm, pH = 8.5, T = 50°C, t = 22 h, 10 ppm polymer
No metal3 ppm Fe5 ppm Mn5 ppm Cu5 ppm Zn
Effect of iron (iii)Soluble iron, or the ferrous ion, often
is encountered in natural water systems. Their source either is the water itself (at low concentrations, usually <3 ppm) or corrosion of the iron piping, well heads, or other vessels in the system. The effect of iron (III) has been reported as a function of polymer dosage on the Ca/P threshold inhibition (TI) performance of Polymers A and B.4 These data (Figures 1 through 3 in reference 4) show that 3 ppm iron (III) greatly impacts performance, necessitating higher Polymer A and B dosages (50 and 33% higher, respectively) to achieve 90% threshold inhibition of Ca/P. The data
clearly show that Polymer B is more toler-ant of iron (III) than Polymer A.
Figure 2 herein shows a comparison of the iron tolerance of several commercial polymers in the presence of 1.0 and 2.5 ppm iron (III). These data support the conclusion that terpolymers compared to homo- and copolymers exhibit superior performance in preventing Ca/P precipita-tion in the presence of iron.
Effect of manganese, copper, and Zinc ions
Manganese and copper ions occasion-ally are an impurity of water sources. Cop-per ions also may be used as an algaecide
in open cooling basins. Zinc salts often are added in the water treatment formula-tions for corrosion control. Zinc ions also can be introduced to circulating water by corrosion of a galvanized cooling tower.
Figure 3 shows the Ca/P threshold inhibition of different polymers in the presence of 5 ppm of manganese and 5 ppm copper ions. It can be seen that the efficacy of copolymers (Polymers A and E) is dramatically reduced, even in the pres-ence of 5 ppm manganese ions. However, the performance of terpolymers (Polymers B and D) is only slightly affected by the presence of 5 ppm manganese and copper ions. These results further illustrate the superior inhibiting power of terpolymers in preventing Ca/P precipitation in the presence of metal ion impurities.
Figures 6 and 7 in reference 4 show the effect of zinc ions as a function of polymer dosage on the Ca/P threshold in-hibition of Polymers A and B, respectively. The data clearly indicate that zinc ions alone did not provide any significant Ca/P threshold inhibition (<10% TI). However, the presence of zinc ions greatly enhanced the Ca/P inhibition of Polymers A and B. Figure 3 herein shows the influence of 5 ppm zinc ions in combination with 10 ppm Polymer A or B dosages. These results suggest that a copolymer (Polymer A) containing a higher amount of carbox-ylic groups provides more synergy with zinc ions than the terpolymer (Polymer B). Additional testing in the laboratory with the polymers containing different levels of carboxylic groups supported this observation. The synergistic effect of zinc ions on Ca/P inhibition is very clear from these tests, and is consistent with the zinc ion characteristic of Ca/P crystal growth inhibition reported in the literature.5 A similar inhibitory effect of zinc ions on calcium carbonate (CaCO3) crystal growth also has been reported.6
EffEct of susPEnDED soliDs on ca/P inhibition
Polymers are known to adsorb onto particles that may be present in the water. The adsorption of the polymers onto par-ticles and the resultant interaction between
1009080
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Thre
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d In
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tion
(%)
Polymer EPolymer DPolymer BPolymer A
[Ca] = 140 ppm, [PO4] = 9 ppmpH = 8.5, T = 50°C, t = 22 h10 ppm polymer 0 ppm Fe
2.5 ppm Fe1 ppm Fe
FIguRE 2
Ca/P inhibition of polymers in the presence of the iron (III) ion.
Ca/P inhibition of polymers in the presence of iron (III), manganese ion, copper ion, or zinc ion.
FIguRE 3
May 2000 MATERIALS PERFORMANCE 57
10090807060
504030
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Thre
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tion
(%)
250 mg/L Clay100 mg/L Clay50 mg/L ClayNo Clay
[Ca] = 240 mg/L, [PO4] = 15 mg/LpH = 9.0, T = 50°C, t = 22 h
25 ppm Polymer A17.5 ppm Polymer B
1009080
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Thre
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d In
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tion
(%)
250 mg/L Clay100 mg/L Clay50 mg/L ClayNo Clay
[Ca] = 140 mg/L, [PO4] = 9 mg/LpH = 8.5, T = 50°C, t = 22 h 10 ppm Polymer A
7.5 ppm Polymer B
particles is the mechanism of dispersion. However, when a polymer is dispersing particles, it is not as available to inhibit the formation of salt scales. In addition, suspended solids could act as nuclei for the formation of Ca/P. The magnitude of this effect was tested with dixie clay.
Effect of clay ParticlesDixie clay is a kaolin clay (aluminum
silicate hydrate) that has a large sur-face area (27 m2/g BET surface area— Micrometerics ASAP 2400). Because of its surface area, dixie clay was reasoned to be especially detrimental to the polymer inhibition of Ca/P precipitation, and therefore was used as a model suspended particle. Ca/P precipitation experiments were run at the standard condition with varying clay concentrations to determine the loss of polymer efficacy. The clay (adjusted to 50 to 250 mg/L clay in the final solution) was added before the CaCl2 solution and initiated the experiment. This range of particulate concentrations covers more than usually is seen in an operating cooling tower.
Figure 4 shows the results with and without dixie clay and either 10 ppm Polymer A or 7.5 ppm Polymer B. The presence of 50 mg/L clay slightly reduces the efficacy of Polymer A but has no ef-fect on the performance of Polymer B. Increasing the clay concentrations to 100 and 250 mg/L affects the Ca/P threshold inhibition performance of both polymers and is significant at the higher level. The impact of dixie clay on the Ca/P threshold inhibition of polymer E also was evaluated. The data4 show that 10 ppm dosages of Polymers A and E provide almost the same (~65%) Ca/P inhibition in the presence of 100 mg/L clay. However, Polymer B, even at a 25% lower dosage (7.5 ppm), provided excellent (85%) Ca/P TI in the presence of 100 mg/L clay.
Figure 5 shows the results for Polymers A and B that previously were reported for a highly stressed system.7 In this case, the calcium, phosphate, and polymer concen-trations were increased. The results parallel those in Figure 4 wherein Polymer B at a lower dosage outperformed Polymer A.
Ca/P inhibition of co- and terpolymer in the presence of varying concentrations of dixie clay.
Ca/P inhibition of co- and terpolymer in the presence of varying concentrations of dixie clay under highly stressed conditions.
SummaryCooling waters contain a number of
contaminants that may be antagonistic to acrylate polymers used in the water treatment programs. By understanding the effect of these contaminants on the polymer performance, the water treatment technologists can compensate for the ef-fects of these contaminants. The results presented in this article suggest:
• The presence of metal ions [e.g., iron (III), copper (II), manganese (II)] in cooling water systems is detri-
mental to Ca/P inhibition of acrylate polymers.
• The zinc ion exhibits a synergy with the polymer allowing enhanced Ca/P inhibi-tion at lower polymer dosage than would be expected to be necessary.
• The presence of clay particles ad-versely impacts the performance of poly-mers. Terpolymers seem to be very resilient to the presence of clay in water.
• Overall, terpolymers appear to have better performance because of their supe-rior tolerance of metal ions and excellent Ca/P inhibition properties.
FIguRE 4
FIguRE 5
58 MATERIALS PERFORMANCE May 2000
References 1. R.W. Zuhl, Z. Amjad, The Role of Polymers in Water
Treatment Applications and Criteria for Comparing Alterna-tives, Association of Water Technologies 1993, Sixth Annual Convention, Las Vegas, NV, Nov. 1993.
2. J.E. Hoots, G.A. Crucil, Role of Polymers in the Mechanisms and Performance of Alkaline Cooling Water Programs, CORROSION/86, paper no. 13 (Houston, TX: NACE, 1986).
3. Z. Amjad, et al., Polymer Performance in Cooling Wa-ter: The Influence of Process Variables, CORROSION/96, paper no. 160 (Houston, TX: NACE, 1996).
4. Z. Amjad, et al., The Influence of Recirculating Water Impurities on the Performance of Calcium Phosphate Inhib-iting Polymers, CORROSION/99, paper no. 118 (Houston, TX: NACE International, 1999).
5. P. Koutsoukos, “Influence of Metal Ions on the Crys-tal Growth of Calcium Phosphates,” Calcium Phosphates in Biological and Industrial Systems (Part III, Chapter 7, p.145), ed. Z. Amjad (Boston, MA: Kluwer Academic Pub-lishers, 1998).
6. P. Coetzee, et al., “Scale Reduction and Scale Modi-fication Effects Induced by Zn and Other Metal Species in Physical Water Treatment,” Water SA 24, 1 (1998): p. 77.
7. J.F. Zibrida, et al., Factors Influencing Cooling Water Polymer Selection, Water Soluble Polymers: Solution Proper-ties and Applications Symposium, American Chemical Society National Meeting, Las Vegas, NV, Sept. 1997.
This article is based on CORROSION/99 paper no. 118, presented in San Antonio, Texas.
ZahId amjad is a R&d Fellow at The BFGood rich Co., 9911 Brecksville Road, Cleve-land, Oh 44141. he has more than 25 years of experience in water treatment. amjad has authored more than 100 publications, has a Ph.d., holds more than a dozen patents, and is a NaCE member.
ROBERT W. Zuhl is The BFGoodrich Co.’s business manager—Water Treatment Chemi-cals. a NaCE member, he has more than 20 years of water treatment experience.
jOhN F. ZIBRIda, formerly with The BFGo-odrich Co., is the founder of Zibex, Inc., 87 lakeshore drive, duluth, Ga 30136. he is a manufacturer’s representative for BFGoodrich’s water treatment chemicals. Zibrida is a NaCE member and has more than 15 years of water treatment experience.
