AWT-9 (Aug-9) Association of Water Technologies, Inc. Annual Convention & Exposition The Westin Diplomat, Hollywood, Florida August 26 to 29, 29 Silica Control in Industrial Water Systems with a New Polymeric Dispersant Zahid Amjad, Ph.D. and Robert W. Zuhl, P.E. Lubrizol Advanced Materials, Inc. 9911 Brecksville Road Cleveland, OH 44141 Carbosperse K-7 Water Treatment Polymers 29, The Lubrizol Corporation. All rights reserved. Corrections made Oct-21
Abstract Minimizing fouling caused by silica and silicate scales in industrial water systems using silica laden feed water continues to challenge water technologists. The solution chemistry of both silica and metal silicate systems is very complex. At neutral ph, scaling can occur by monomeric silica polymerization resulting in colloidal silica deposition. Operating systems above ph 7 may reduce silica-based scaling problems because silica solubility increases with increasing ph. However, operating systems above ph 8.5 may increase the potential deposition of (a) magnesium silicate (if high silicate and Mg 2+ ions levels are present) and/or (b) other scales (e.g., calcium carbonate whose solubility decreases as ph increases). This study presents results on new silica/silicate control agent. Comparative performance data suggest that a new copolymer blend is an excellent silica polymerization inhibitor and an effective particulate (iron oxide, clay, silica, and magnesium silicate) dispersant. Furthermore, the new polymer is tolerant to high levels of calcium hardness ions. Keywords: silica, polymerization inhibition, scale, inhibition, particulate matter, iron oxide, clay, magnesium silicate, dispersion, fouling, calcium ion tolerance Introduction Providing cost-effective solutions for reducing fouling of heat exchangers and surface equipments by silica and silicate scales in industrial water systems using silica laden feed waters continues to challenge water technologists. In evaporative cooling systems, water technologists must maintain silica at acceptable levels (usually below 18 mg/l in absence of silica/silicate control agents) to avoid silica and silicate deposits. This requires operating systems at low cycles of concentration resulting in consumption of large amounts of water (which increases water cost) and/or incorporation of silica/silicate control agents in the water treatment programs. In reverse osmosis (RO) systems, silica-based fouling problems cause reduced permeate production rates (lower recovery rates), increased energy costs, poor permeate quality, and more frequent membrane cleaning. In geothermal applications, factors such as variable fluid compositions, fluctuating plant operating conditions, and the complex nature of silica polymerization reactions collectively make silica control a challenging problem. Silica fouling in industrial water systems falls into two categories; (a) precipitation fouling and (b) particulate fouling. Precipitation fouling occurs when monomeric silica (or silicic acid, H 4 SiO 4 ) polymerizes at the heat exchanger or RO membrane surface. Particulate fouling, is the accumulation of the colloids, formed during the silica polymerization process, on equipment surfaces. Effective silica/silicate control in industrial water systems necessitates an understanding of the various forms of silica present in water. The silica solubility in water systems is dependent upon several factors including ph, temperature, other ions present, and the silica form(s) present. Figure 1 presents silica solubility as a function of solution ph. As shown, silica solubility increases with ph varying from 12 mg/l at ph 6 to 14 mg/l at ph 9 and increases rapidly as ph is increased from 9.5 to 1.5. 1 Unlike certain silicate salts such as magnesium silicate that becomes less soluble as the temperature increases, silica solubility increases with increasing solution temperature 1 as illustrated in Figure 2. In addition, the presence of various electrolytes also influences the solubility of amorphous silica in aqueous solution. 2 Because silica solubility increases with increasing ph, operating systems at high ph may potentially reduce silica scaling problems. However, water technologists must consider 1/25
the presence of magnesium (Mg 2+ ) and other scaling ions such as calcium, iron, and aluminum. Operating systems above ph 8.5 might cause massive precipitation and deposition of magnesium silicate if high levels of Mg 2+ ions are present and/or calcium, carbonate, and phosphate ions are overlooked. 3 Similarly, calcium carbonate if not controlled, may provide a crystalline matrix that entraps and causes silica growth. 4 Thus, it is of paramount importance that water technologists develop treatment programs that control scale such as calcium carbonate as well as effectively prevent precipitation of amorphous silica and metal silicate scales. Silica (SiO 2 ) exists in many crystalline and amorphous forms. Crystalline silica has a very low solubility in water; on the order of 6 mg/l SiO 2. 5 By contrast, amorphous silica has a much higher solubility in the range of 1 to 14 mg/l SiO 2. 6 The potential for silica scaling increases when the dissolved silica levels in an aqueous system exceeds the amorphous silica solubility limit (in the range of 12 to 14 mg/l at ambient temperature). When dissolved in water, silica forms silicic acid [H 4 Si(OH) 4 ]. However, at silica concentration above 14 mg/l, silicic acid undergoes base-catalyzed polymerization leading to the formation and deposition of colloidal polysilicic acid on heat exchangers and RO membrane surfaces. Silica scaling/fouling of heat exchangers and RO membranes can occur by one of the three possible routes 7 : (a) condensation of monomeric silicic acid on solid substrate containing OH group, (b) polymerized silicic acid or colloidal deposition, and (c) biogenic amorphous silica by living organisms. The problem of silica scaling is exacerbated in presence of low levels of polyvalent metal ions (i.e., aluminum, iron, calcium, magnesium, etc.). In addition, corroded steel pipes and heat exchangers are very prone to silica scaling. The composition and quantity of silica scale as well as the rate at which it forms is dependent on ph, temperature, the ratio of calcium to magnesium, and the concentration of polyvalent metal ions. Furthermore, silica and silicate deposits are particularly difficult to remove once they form necessitating the use of strong chemical cleaners (based on hydrofluoric acid) or laborious mechanical removal methods. Amjad and Yorke 8 in their evaluation of polymers reported that cationic-based copolymers are effective silica polymerization inhibitors. Similar conclusions were also reached by Harrar, et al. 9 when investigating the use of cationic polymers and surfactants in inhibiting silica polymerization under geothermal conditions. Although these cationic-based homo- and copolymers showed excellent silica polymerization inhibition performance, they offered poor silica/silicate dispersancy activity. Gallup and Barcelon 1 investigated the performance of several organic inhibitors as alternatives to strong acids for geothermal applications. Results of their study reveal that brine acidification always out-performed organic inhibitors. Smith 11 used a pilot RO unit to evaluate several polymers as silica scale inhibitors and the results indicate that a proprietary polymer exhibits excellent silica inhibitory activity. Neofotistou and Demadis 12 investigated the performance of several polyaminoamide-based dendrimers as silica inhibitors for cooling water applications and observed that inhibitor performance strongly depends on the branching present in the dentrimer. The performance of a formulated product containing hydroxyl phosphono acetic acid and a copolymer of acrylic acid:allyl hydroxyl sulfonate ether in high hardness waters containing high alkalinity and 225 mg/l silica, has been reported. 13 The inspection of the heat exchangers showed essentially no deposits in the presence of formulated product compared to heavy silica and 2/25
silicate deposits in the control (no treatment). The precipitation and inhibition of metal silicates has been studied by several investigated. 14-16 Results of these studies reveal that precipitation of metal silicates depends on several factors including ph, temperature, and metal silicate supersaturation. Currently, a wide variety of additives are available to water technologists to effectively control mineral scales, corrosion, and microbiologal growth in the industrial water systems. Commonly used scale control agents include phosphonates and acrylic acid and/or maleic acid-based polymers. Phosphonates are the benchmarks for preventing the precipitation of calcium carbonate but they are ineffective against silica polymerization and magnesium silicate. Similarly, homo- and co-polymers that have historically proven to be excellent calcium phosphate and calcium phosphonate inhibitors and/or dispersants exhibit poor inhibitory properties for silica/silicate-based scales. Thus, it is clear that anionic charged additives exhibit poor affinity for silica/silicate-based scales. Our previous studies 17-19 have shown that silica polymerization is affected by ph, temperature, and the presence of polyvalent metal ions (i.e., Ca, Mg, Fe). It was shown that the silica control agent composition also plays an important role in preventing the polymerization and deposition of amorphous silica on RO membrane and heat exchanger surfaces. The present study evaluates a new polymeric silica/silicate control agent as silica polymerization inhibitor agent and particulate matter (iron oxide, clay, silica, and magnesium silicate) dispersant. Comparative performance data for a variety of commercially available polymers are also presented. Experimental Table 1 summarizes the descriptions, compositions, and acronyms of the non-polymeric and polymeric commercially available additives evaluated in the present study including a new proprietary copolymer blend (CP7 or Carbosperse K-XP229 copolymer). The results herein are reported on a 1% active inhibitor basis. Silica Polymerization Inhibition Test Method Reagent grade chemicals and distilled water were used in accordance with Lubrizol s Silica Polymerization Inhibition Test Procedure, 2 schematically shown and discussed below. Silica Polymerization Inhibition Experimental Protocol Distilled Water + Sodium Silicate + Inhibitor Solution Adjust to ph 7. + Calcium and Magnesium Solution Adjust to ph 7. Sample and Filter through.22 micron Analyze for SiO 2 3/25
Silicate stock solutions were prepared from sodium metasilicate, standardized spectrophotometrically, and stored in polyethylene bottles. Calcium chloride and magnesium chloride stock solutions were prepared from calcium chloride dihydrate and magnesium chloride hexahydrate and were standardized by EDTA titration. Silica polymerization experiments were performed in polyethylene containers placed in doublewalled glass cells maintained at 4ºC. The supersaturated solutions were prepared by adding known volumes of sodium silicate (expressed as SiO 2 ) solution and water in polyethylene containers. After allowing the temperature to equilibrate, the silicate solutions were quickly adjusted to ph 7. using dilute hydrochloric acid. The ph of the solutions was monitored using Brinkmann/Metrohm ph meter equipped with a combination electrode. The electrode was calibrated before each experiment with standard buffers. After ph adjustment, a known volume of calcium chloride and magnesium chloride stock solution was added to the silicate solutions. The supersaturated silicate solutions were re-adjusted to ph 7. with dilute HCl and/or NaOH and maintained constant throughout the silica polymerization experiments. Experiments involving inhibitors were performed by adding inhibitor solutions to the silicate solutions before adding the calcium chloride and magnesium chloride solution. The reaction containers were capped and kept at constant temperature and ph during the experiments. Silicate polymerization in these supersaturated solutions was monitored by analyzing the aliquots of the filtrate from.22-µm filter paper for soluble silica using the standard colorimetric method. 2 The silicate polymerization inhibition values were calculated according to the following equation: [SiO 2 ] sample --- [SiO 2 ] blank %SI = ------------------------------------------- x 1% [SiO 2 ] initial --- [SiO 2 ] blank Where: SI = Silica Inhibition (%) or %SI [SiO 2 ] sample = Silica concentration in the presence of inhibitor at 22 hr [SiO 2 ] blank = Silica concentration in the absence of inhibitor at 22 hr [SiO 2 ] initial = Silica concentration at the beginning of experiment Particulate Dispersion Test Methods Particulate dispersion experiments for iron oxide, kaolin clay, silica, and magnesium silicate were conducted in accordance with test procedures described below. All procedures were conducted at room temperature ( 23ºC) and involved adding known quantities of particular matter to a breaker or graduated cylinder containing synthetic water and varying amounts of dispersant. Each particulate material was selected from available commercial or reagent grades. Typical experiments consisted of six (6) to eight (8) tests running simultaneously. At known time ( t ) intervals, transmittance (%T) readings were taken using a colorimeter (Brinkmann PC/91) equipped with a 42 nm filter. The results as % dispersed (%D) were calculated from %T readings as a function of particulate matter dispersed compared to a control 4/25
(experiment run without dispersant). Therefore, greater dispersion was indicated by higher %D values. The % D was calculated using the following equation: Where: % Dispersed (%D) = (1 [T e x (1/%T c ) x 1]) %T e = Percent transmittance of the experimental solution in the presence of dispersant at time t %T c = Percent transmittance of the experimental solution in the absence of dispersant at time t (control) Specific details for each particulate (i.e., iron oxide, kaolin clay, silica, and magnesium silicate) dispersion test method are discussed below. Iron Oxide Dispersion Test Method The experimental test procedure for evaluating iron oxide dispersants was described previously. 21 A known amount (.12 g) of iron oxide was added to synthetic water (6 ml) containing a known amount of dispersant solution in a 8 ml beaker. The synthetic water used in dispersancy test was made by mixing standard solutions of calcium chloride, magnesium chloride, sodium sulfate, sodium bicarbonate, and sodium chloride. The synthetic water composition included 1 mg/l Ca, 3 mg/l Mg, 314 ppm Na, 571 mg/l chloride (Cl), 2 mg/l sulfate (SO 4, ), and 6 mg/l bicarbonate (HCO 3 ). Other test conditions include 2 mg/l iron oxide, ph 7.6±.2, 23 C, and 1 ppm (as active) dispersant. In a typical test, six (6) experiments were run simultaneously using a gang-stirrer set to 1 revolutions per minute. At known time intervals, transmittance readings (%T) were taken with a Brinkmann PC/1 colorimeter using 42 nm filter. Dispersancy (%D) was calculated as described above based on %T readings measured at 3 hr as a function of the amount of the iron oxide dispersed compared to the control. Kaolin Clay Dispersion Test Method The clay dispersion tests were conducted by adding 1. g of clay to known volumes of synthetic water containing dispersant to 1 ml graduated cylinders. The synthetic water contained a mixture of calcium chloride, magnesium chloride, sodium chloride, sodium sulfate and sodium bicarbonate. The experimental solutions (ph 7.6±.2) in the cylinders totaled 1 ml and were comprised of varying polymer dosages ( to 1 ppm as active), 1, mg/l clay, 15 mg/l Ca, 31 mg/l Mg, 353 mg/l Na, 6 mg/l Cl, 29 mg/l sulfate, and 116 mg/l bicarbonate. After clay addition, the cylinders were covered with Parafilm and manually mixed (with a motion similar to repeatedly turning over an hourglass) for 1 minute and then left to stand. Transmittance readings were taken at hourly intervals using a colorimeter as described above. The results from these experiments have shown good (within ±5%) reproducibility. The dispersion values (%D) were calculated as described above. The values reported herein are based on 1 ppm dispersant dosages and readings taken at 3 hr. 5/25
Silica Dispersion Test Method The silica dispersion tests were conducted by adding 6 mg of silica (>99% silicon dioxide) to known volumes of synthetic water in 1 ml graduated cylinders containing varying amounts of dispersant. The synthetic water contained a mixture of calcium chloride, magnesium chloride, sodium chloride, sodium sulfate and sodium bicarbonate. The experimental solutions (ph 7.6±.2) in the cylinders totaled 1 ml and were comprised of varying inhibitor dosages ( to 5 ppm as active), 6 mg/l silica, 15 mg/l Ca; 31 mg/l Mg; 353 mg/l Na; 6 mg/l Cl; 22 mg/l sulfate; 58 mg/l bicarbonate. After adding silica, the cylinders were covered with Parafilm and manually mixed (using a motion similar to repeatedly turning over an hourglass) for 1 minute and then left to stand. A typical test consisted of eight (8) experiments run simultaneously. At regular time intervals transmittance (%T) readings were taken as described above (i.e., using a Brinkmann PC/91 colorimeter equipped with a 42 nm filter). Results from these experiments have shown good (±1%) reproducibility. The % dispersed (%D) values were calculated as described above based on the %T readings at 5 hr as a function of silica dispersed compared to control. Magnesium Silicate Dispersion Test Method The magnesium silicate dispersancy tests were conducted by adding 15 mg of magnesium silicate to known volumes of synthetic water in 1 ml graduated cylinders containing varying amounts of dispersant. The synthetic water used was the same as described above in the silica dispersion test method. The experimental solutions (ph 7.6±.2) in the cylinders totaled 1 ml and were comprised of varying inhibitor dosages ( or 2.5 ppm as active), 1,5 mg/l magnesium silicate, 15 mg/l Ca; 31 mg/l Mg; 353 mg/l Na; 6 mg/l Cl; 22 mg/l sulfate; 58 mg/l bicarbonate. After adding magnesium silicate, the cylinders were covered with Parafilm and manually mixed (using a motion similar to repeatedly turning over an hourglass for 1 minute) and then left to stand. A typical test consisted of eight (8) experiments run simultaneously. At regular time intervals, transmittance (%T) readings were taken using Brinkmann PC/91 colorimeter equipped with a 42 nm filter as described above. Results from these experiments have shown good (±1%) reproducibility. The % dispersed (%D) values were calculated as described above based on %T readings at 2 hr as a function of magnesium silicate dispersed compared to control. Calcium Ion Compatibility Test Method The calcium ion compatibility for polymers was investigated using a procedure described previously. 21 Experiments were performed in glass bottles (125 ml capacity) placed in doublewalled glass cells maintained at 25ºC. Test solutions were prepared by adding known volumes of stock calcium chloride solutions (1 ml,.25m) to known volumes of distilled water. After allowing the test solutions to equilibrate for 3 minutes, known volumes of stock polymer solutions ( to15 ml,.25%) were added to the test solutions. The test solution ph in each glass bottle was adjusted to 9.5 with dilute HCl and/or NaOH. The bottles were capped and continuously stirred (using stirring bars). At 3 minutes, percent transmittance readings were taken using a Brinkmann PC/91 colorimeter equipped with 42 nm filter. Duplicate or 6/25
triplicate experiments were run to verify the reproducibility of the calcium-ion compatibility data. To avoid faulty transmittance signals, extreme care was taken to eliminate air bubbles in the test solutions, especially in the vicinity of fiber optic probe. The calcium ion compatibility was calculated by plotting the percent transmittance as a function of polymer dosage. The inflection points in the % transmittance vs. polymer dosage profiles were used to determine the point where the onset of turbidity occurs. Silica Polymerization Inhibition Silica Supersaturation Results and Discussion Using the experimental set-up described above, a series of experiments were conducted at various silica supersaturation (SS) conditions. Figure 3 shows profiles of soluble silica concentration as a function of time for various silica SS solutions. It is evident from Figure 3 that silica polymerization depends on the silica concentration present in the supersaturated solutions. For highly supersaturated solutions containing 55 mg/l silica, polymerization starts immediately (as evident from the sharp decrease in soluble silica concentration with time) whereas lower silica concentration solutions polymerizes (react) at a much slower rate. At low degrees of silica supersaturation, a decrease in silica concentration is preceded by a slow polymerization reaction (induction time or β). During the induction time, the concentration of soluble silica does not change significantly. However, once polymerization begins, soluble silica begins decreasing. Figure 3 illustrates that the β values for various SS solutions are <8 hr for 45 mg/l, and >2 hr for 3 mg/l silica compared to <5 minutes for the high SS (55 mg/l silica) solutions. For the results presented in this paper, we chose 55 mg/l silica as the SS condition to evaluate the performance of various inhibitors. Inhibitor Concentration Figure 4 shows silica concentration as a function of time and CP6 (Carbosperse K-XP212 copolymer) dosage. The data in Figure 4 indicate that dosage strongly affects the ability of CP6 to inhibit silica polymerization and that 5 ppm and 75 ppm dosages (as active polymer) provide similar performance. The silica conc. (soluble silica) values for CP6 in Figure 4 converted to % silica inhibition values are summarized below and indicate that the inhibitory effect of CP6 increases dramatically as dosage increases to 5 ppm and incrementally improves thereafter: CP6 Dosage: 15 ppm 25 ppm 5 ppm >75 ppm Silica Inhibition: 14% 52% 84% >9% Performance: Poor Fair Excellent Excellent 7/25
Inhibitor Performance Non-Polymeric Inhibitors The use of non-polymeric inhibitors, such as polyphosphates and phosphonates, to control scaling (especially calcium carbonate) is well known. However, under stressed conditions (i.e., high ph, high temperature, high hardness, etc.), these phosphorus-containing inhibitors frequently react stoichiometrically with calcium ions leading to calciumpolyphosphate/phosphonate precipitation. 22 The application of boric acid (BA) and/or its water soluble salts to prevent silica polymerization has been reported. 23 It has been suggested that silica inhibition by borate is perhaps due to the formation of more soluble borate-silicate complexes. To understand the importance of non-polymeric additives as silica polymerization inhibitors, a series of experiments were conducted in the presence of 25 and 35 ppm inhibitor dosages and the results appear in Figure 5. For the non-polymeric additives (i.e., A = HEDP, B = PBTC, C= BA) tested, the data in Figure 5 indicate that these materials are ineffective silica inhibitors. Although HEDP and PBTC are excellent calcium carbonate inhibitors, 24 their poor performance as silica polymerization inhibitors suggests that phosphonate groups do not inhibit silica polymerization in aqueous solutions. Polymeric Inhibitors Many studies have examined the influence of polymer architecture on the precipitation of scale-forming salts. It is generally accepted that polymer performance as scale inhibitors in industrial water systems is strongly affected by polymer molecular weight (MW), composition (monomer types and weight ratios), and polymerization solvent. It has been reported 21 that solvent polymerized polyacrylates are more tolerant to calcium ions than the water polymerized polyacrylates. For carboxylic acid containing polymers, it appears that precipitation inhibition is greatest for polymers whose MW is below 2, DA. However, the optimum polymer MW depends upon polymer composition and the salts being inhibited and/or particulate being dispersed. For calcium phosphate and calcium phosphonate inhibition, acrylic acid and/or maleic acid-based copolymers have been shown to perform better than homopolymers of acrylic acid and maleic acid. In the case of particulate matter (i.e., clay, iron oxide) dispersion, polymers that exhibit good dispersion are typically low (<1, DA) MW and contain both carboxylic acid and sulfonic acid groups. Figure 5 presents performance data for polymeric inhibitors containing different functional groups (e.g., carboxylic acid, sulfonic acid). It is evident that the homopolymers (P1 and P3), copolymer (CP1), and terpolymers (CP2, CP3, CP4, CP5) commonly used in water treatment formulations as deposit control agents for controlling mineral scales and dispersing suspended matter are poor (<1% inhibition) silica inhibitors. The performance data for these commonly used polymers with the exception of a copolymer blend (CP6) and a new silica/silicate control agent (CP7) clearly show that carboxylic acid, sulfonic acid, ester, and non-ionic groups present in the polymers exhibit poor interaction with silane groups present in silica. 8/25
New Silica/Silicate Control Agent As previously discussed, a variety of additives have been developed for silica control with limited commercial success; perhaps due to limited efficiency under the field conditions and/or because of poor cost/benefit considerations. Therefore, water technologists are still seeking a cost effective silica polymerization inhibitor that minimizes the potential for silica scaling in industrial water systems. Figure 6 presents silica inhibition vs. dosage for three (3) distinctively different polymeric inhibitors. CP2 is a patented terpolymer containing carboxylic acid and sulfonic acid groups designed for use in controlling calcium phosphate/phosphonate salts, metal ion stabilization, and particulate dispersion. The composition of CP2 is similar to homopolymers P1 to P5 and copolymers CP1 and CP3 to CP5 in that its composition is dominated (>5%) by carboxylic monomer groups. On the other hand, CP6 and CP7 are polymeric inhibitors (patented and patent pending, respectively) whose compositions are distinctly different (i.e., CP6 & CP7 contain <5% carboxylic acid monomer groups) from the other polymeric inhibitors. The data in Figure 6 as well as those in Figure 5 indicate the following: Additive Silica Polymerization Inhibition Performance CP2 CP2 (as well as P1, P2, CP1, and CP3 to CP5) provides poor (<2%) performance at all dosages tested up to 35 ppm (only up to 5 ppm dosage shown in Figure 6). CP6 CP6 performance increases dramatically from poor to excellent with dosage up to 35 ppm and improves incrementally thereafter. CP7 CP7 performance increases dramatically as dosage increases from 5 ppm (7% SI) to 25 ppm (85% SI) and improves incrementally thereafter. At 12.5 ppm dosages, CP7 provides 35% SI or more than double the %SI values for both CP6 and CP2. The absolute performance and dosage response of CP6 is superior to the other additives tested except CP7 which is dramatically better than all other materials tested. Figure 7 takes a closer look at a subset (7.5 to 5 ppm dosages) of the silica polymerization inhibition vs. dosage data for CP6 and CP7 and reinforces earlier observations herein that CP7 is a significant improvement in silica-polymerization inhibitor technology. - Effect of Iron (III) The presence of iron in re-circulating waters and/or brine in RO system, whether originating from the feed water or as a result of iron-based metal corrosion in the system or the carry-over from a clarifier using iron-based flocculating agents, can have profound effect on the performance of polymers used as scale control and dispersant. It has been previously reported that polymer performance as a calcium phosphate inhibitor and iron oxide dispersant is negatively impacted by iron (III). 25 Figure 8 presents the impact of.5 and 1. ppm soluble iron on the silica polymerization inhibition performance of CP6 and CP7 at 25 ppm dosages. As shown, CP7 retains >55% and >2% SI performance in the presence of.5 and 1. ppm, respectively of Fe (III). The antagonistic effect shown by Fe(III) may be attributed to the formation of the Fe(OH) 3 silica complex. Thus, if iron (III) is encountered in cooling waters, RO, or geothermal systems, a 9/25
polymer that more effectively prevents silica scaling, especially in the presence of soluble iron, should be considered in developing high performance water treatment formulations. Suspended Matter Dispersion Feed waters available for domestic and industrial uses generally contain a variety of dissolved ions (i.e., calcium, magnesium, carbonate, sulfate, etc.), colloids, and suspended matters (i.e., clay, silt, organic debris, corrosion products, etc.). The type, size, and concentration of suspended particles affect their behavior in water systems. In addition, metal ions (i.e., Cu, Mn, Fe) present in feed water may either hydrolyze and/or oxidize under conditions typically encountered in domestic and industrial systems. Maintaining these hydrolyzed and/or oxidized metal ions in soluble and in dispersed forms can prevent the build-up of unwanted deposits on various substrates. The suspended particles typically encountered in industrial water applications generally carry a slight negative charge. Therefore, anionic polymers are normally the most efficient dispersants because these anionic polymers increase the negative surface charge and keep particles in suspension. Cationic polymers can be but are generally not used as dispersants for at least two (2) reasons: 1) Relatively high concentrations are required in order to first neutralize the negative surface charges and then to transfer cationic charge to particles for efficient dispersion. 2) The presence of cationic polymers can adversely impact the performance of anionic inhibitors used as deposit control agents in most water treatment programs. Suspension of clays, metal oxides, pigments, ceramic materials, and other insoluble inorganic particulate solids in aqueous systems through the use of small quantities of synthetic polymers, polyphosphates, and other polyelectrolytes has become an increasingly important area of study with high technological relevance. Dubin 26 in his investigation on the evaluation of polyphosphates, organophosphonates, poly(acrylic acid), P-AA; poly(maleic acid), P-MA; and a copolymer of acrylic acid:maleic acid (P-AA:MA) as dispersants for iron oxide showed that acrylic acid and maleic acid based polymers performed better than polyphosphates and organophosphonates. The effectiveness of polymers as dispersants for clay and iron oxide has been the subject of numerous investigations. It has been reported that polymers containing different functional groups (i.e., carboxyl, amino, sulfonic, amido), are effective dispersants. 27 In addition, it has been shown that polymer MW plays an important role in dispersing suspended matter in aqueous systems. Iron Oxide Dispersion Figure 9 shows the iron oxide dispersion by several polymers as determined by a Lubrizol s standard test method (conditions include synthetic water, ph 7.6±.2, 2 ppm iron oxide, 1 to 2 ppm inhibitor). 27 The data presented in Figure 9 lead to the following observations: 1) The data presented herein as well as those for other experiments not shown herein indicate variations in the performance vs. dosage profiles. As a general rule, polymer type and architecture are the most significant factors impacting performance with the greatest performance increases occurring when dosages are increased from 1 to 2 ppm and incremental if any performance improvements with dosage increases above 2 ppm. 1/25
2) P1 and P3 perform poorly as iron oxide dispersants at 1 ppm dosages and increasing dosages from 1 to 2 ppm had insignificant impacts. 3) All the copolymers except the modified maleic copolymer (CP5) @ 55% and the AA/SA copolymer (CP1) @ 64% provide greater than 7% iron oxide dispersed at 1 ppm dosages. 4) Increasing copolymer dosages from 1 to 2 ppm increases performance to varying degrees with all but CP5 providing excellent (>8%) iron oxide dispersed. The most dramatic performance change (increase) occurred for the new silica-silicate deposit control agent (CP7) which increased from the fourth best (72%) to the best (92%) performing material. Kaolin Clay Dispersion Kaolin clay dispersion by various polymers was investigated by carrying out a series of experiments under standard test conditions. Figure 1 presents clay dispersancy data in the presence of 1 ppm polymer dosages leading to the following observations: 1) The performance of the new silica/silicate control agent (CP7) is comparable to the best performing terpolymer (CP2). 2) The clay dispersancy of CP7 is superior to all polymers tested except CP2. Silica Dispersion As previously discussed, the composition and quantity of a silica deposit and the rate at which it forms is dependent on ph, temperature, the ratio and concentration of calcium and magnesium, and the concentration of other polyvalent ions in the water. It has also been reported that polymerized colloidal silica in the presence of polyvalent metal ions forms flocculated silica. These precipitates can deposit on heat exchanger and RO membrane surfaces resulting in poor system performance. Figure 11 illustrates dispersancy data in the presence of varying (.25 to 5. ppm) dosages of two (2) homopolymers (P1 & P3) and a terpolymer (CP2). Observations from these data and our assessments include: 1) The ranking (from best to worst) of the polymers as silica dispersants is CP2 > P3 > P1. 2) Silica dispersion increases with polymer dosage from.25 to.5 to 1. ppm dosages but improves very little thereafter as polymer dosage increases 5x to 5. ppm. Silica dispersion data for 1 ppm dosages of several inhibitors including phosphonates and polymers (homo, co- and ter-polymers) are presented in Figure 12. The data indicate the following: 1) The phosphonates (A & B) provide poor silica dispersion which is consisted with the observations about phosphonates as dispersants for iron oxide, clay, and hydroxyapatite. The poor activity shown by phosphonates may be attributed to lack of interactions of phosphonate group with silica particles. 2) All the polyacrylates (PAAs) except P5 (>3k MW) outperform P1 [poly(maleic acid)]. 3) The silica dispersion performance of the four (4) PAAs (2k, 6k, 12k, and 345k MW) is inversely related to molecular (i.e., the lower the MW the better the performance). 4) The silica dispersion performance of the 2k and 6k MW PAAs (i.e., P2 & P3) and the modified maleic copolymer (CP4) are comparable to two copolymers promoted as silicasilicate control agents (CP5 & CP6). 11/25
5) The new silica-silicate control agent (CP7) provides silica dispersion comparable to the best of the copolymers tested (CP1, CP2, and CP4). The excellent silica dispersing activity shown by CP7 is very significant as it also is an excellent silica polymerization inhibitor. Magnesium Silicate Dispersion The precipitation and deposition of magnesium silicate on equipment surfaces continues to pose serious operational problems in industrial applications (e.g., recirculating cooling water systems, geothermal plants, closed engine cooling systems) containing high silica and magnesium. Several investigators have reported that acrylic-acid containing copolymers and non-polymeric additives such as borate compounds inhibit silica polymerization/precipitation. However, no proven technology exists for the control of magnesium silicate (except ph and concentration control). It is well known that the magnesium silicate system is highly ph dependent. Below ph 7, magnesium silicate precipitation does not occur because silica is present essentially in an unionized form. As the solution ph is increased (especially, above ph 9), magnesium silicate is very likely to form. In addition, the magnesium silicate system is very complicated due to various species of different compositions that can precipitate depending on the water chemistry (Mg, SiO 2, ph, temperature, etc.). Figure 13 presents magnesium silicate dispersion data for various inhibitors including phosphonate and polymers (homo-, co-, and ter-polymers). The data suggest the following observations and conclusions: 1) Although excellent calcium carbonate inhibitors, the phosphonates (A & B) provide poor silica dispersion which is consisted with the observations about phosphonates as dispersants for iron oxide, clay, hydroxyapatite, and silica. 2) All of the polyacrylates (P2 to P5) outperform the poly(maleic acid) which is marginally better than the phosphonates (A &B). 3) The magnesium-silicate dispersion performance of the polyacrylates (P2, P3, P4, and P6) whose MWs are 2k, 6k, 12k, and 345k MW, respectively) is inversely related to MW (i.e., the lower the MW the better the performance). 4) The silicate dispersion performance of the 2k and 6k MW polyacrylates (i.e., P2 & P3) is better than that of the modified maleic copolymer (CP4). 5) The new silica-silicate control agent (CP7) provides the best magnesium silicate dispersion among the inhibitors tested and for the copolymers the rank order of performance (highest to lowest) is: CP7 > CP2 > CP3 > CP6 > CP5 > CP4. Calcium Ion Compatibility One of the water technologists concerns is the compatibility or tolerance limit of the anionic inhibitors (polymeric or non-polymeric) with hardness ions (i.e., Ca, Mg, Ba, Sr). The calcium ion tolerance or compatibility is defined as the maximum amount of the anionic inhibitor that can be added to the water system without significant precipitation of metal-inhibitor salt. If the compatibility is poor and the dosage of the water treatment formulation used is higher than the recommended level, metal-inhibitor salt precipitation occurs and the overall scale formation becomes faster and could lead to many other issues resulting from premature fouling (e.g., early shut down, wasted time, reduced production, and higher operating costs). 12/25
In our previous laboratory studies, we presented tolerance data for various polymeric and nonpolymeric additives with calcium ions. It was shown that phosphonates exhibit poor calcium ion tolerance. 21 Results on the interactions of various homo-, co-, and ter-polymers reveal that terpolymers generally show better tolerance to Ca ions than co- and homo-polymers. It was also illustrated that several factors such as solution ph, temperature, total dissolved solids, and inhibitor architecture, affect the inhibitor calcium tolerance values. Furthermore, results of another study on the thermal stability of sulfonic acid-containing copolymers show that polymer solution exposure to heat treatment (15 to 24ºC) decreases their calcium ion tolerance. 28 The calcium ion compatibility of several inhibitors was investigated using the procedure described previously. 21 Results presented in Figure 14 suggest the following observations: 1) The calcium ion compatibility of polyacrylates (P2, P3, P4, and P5 whose MWs are 2k, 6k, 12k, and 345k, respectively) is inversely related to MW (i.e., the lower the MW the better the compatibility). 2) Among the homopolymers, P2 performs better than P1 which is better than P3. The calcium ion compatibility of P1 vs. P3 is interesting because P1 exhibits poor performance as a dispersant for clay, silica, and magnesium silicate. The observed performance difference for P1 vs. P3 may be attributed to the former s low molecular weight and poor adsorption behavior for various particulate matter. 3) The calcium ion tolerances of CP4 and CP5 are poorer than the poly(maleic acid), P1, and the 2k MW polyacrylate (P2). 4) CP7 as well as CP2, CP3, and CP6 display excellent tolerance to calcium ions. However, both CP4 (modified maleic copolymer) and CP5 (a silica/silicate control agent) exhibit poor performance compared to the other copolymers suggesting that polymer architecture plays a role in tolerance to calcium ions. New Silica/Silicate Control Agent Use Considerations The information available for considering new silica-silicate control agent (CP7) as a high performance silica control agent in water treatment applications include: CP7 dosage rates as a silica/silicate control agent are expected to vary based upon operating conditions and water chemistry. Dosage requirements decrease with increasing temperature (in the range of 2 to 9ºC) due to silica solubility variability as a function of system temperature. As with any inhibitor, the presence of antagonistic materials may increase dosage requirements and/or adversely impact performance. Lubrizol s laboratory testing indicates that CP7 is an excellent iron oxide dispersant comparable to Carbosperse K-781 and K-798 acrylate terpolymers. CP7 is suitable for use as an overlay to existing water treatment programs where silica control is a primary concern. Lubrizol used test data for the polymeric components to estimate the aquatic toxicity for CP7 and concludes that this product has a relatively low order of aquatic toxicity. The components of CP7 are either listed on or are exempt from the U.S.A. Toxic Substance Control Act Chemical Substance Inventory. CP7 is a partially neutralized low molecular weight (<3k) water soluble copolymer blend supplied as a water white to amber liquid, clear to slightly hazy liquid. The typical properties include 41 to 44% total solids, >96.5% active solids, ph 3.6 to 4.6, Brookfield viscosity <1, cp (25 C). The standard packaging for CP7 is 55-gal plastic drums at 525 lb net; intermediate bulk containers and bulk quantities are also available). 13/25
Lubrizol conducted freeze (ºC for 16 hr) thaw (7 hr) testing (five cycles) with CP7. The polymer exhibited minimal changes in viscosity and/or appearance. Based on limited evaluations, the current guidelines for incorporating CP7 as a component of water treatment formulation blends are as follows: - CP7 may be blended (up to 5/5 solids weight basis) with anionic dispersants such as Carbosperse K-7 homopolymers, acrylate copolymers, and acrylate terpolymers. - The formulating window for CP7 with other water treatment additives (e.g., phosphonates, azoles, molybdates, inorganic neutralization agents [NaOH or KOH], or strong acids [H 2 SO 4, HCl]) is limited. Therefore, we recommend limiting formulations to less than 1% solid solids. Alternatively, if formulations exceed 1% total solids and/or are greater than ph 4.5 use coupling agents and/or organic neutralizing agents. Future Investigations Investigations currently underway or contemplated pertaining to CP7 are as follows: Magnesium silicate scale control. Thermal stability testing. Expanding formulating window knowledge base. Field testing. Developing predictive models for polymer dosage requirements vs. silica SS for various water chemistries. Summary The potential for silica scaling occurs when the dissolved silica in re-circulating water or reject stream in RO systems exceeds the solubility limit at ambient temperature for amorphous silica. To avoid the formation of silica-based deposits, which are very difficult to remove safely and economically, water technologists typically employ conservative operating criteria; e.g., cooling systems limit cycles of concentration and desalination systems limit recovery. These approaches result in either large water consumption (which increases water costs) and/or water treatment chemical program modification to incorporate a silica/silicate control agent. During the last three decades, various proprietary inhibitors and/or formulated products have been developed and have achieved limited commercial success due to either inhibitor inefficiency under the system operating conditions and/or poor cost/benefit ratios. Based on extensive research and applying inhibitor architecture vs. performance profile, a new proprietary inhibitor/dispersant has been developed that exceeds performance of other commercial silica/silicate control agents. The results presented in this paper show that deposit control polymers (homo-, co, and terpolymers) and non-polymeric additives (i.e., HEDP & PBTC) commonly used as inhibitors for mineral scales and as dispersants for suspended matter perform poorly as silica polymerization inhibitors. The performance data for a new silica/silicate control agent clearly show that it is an excellent silica polymerization inhibitor, especially at low dosages. Furthermore, the new inhibitor has excellent calcium ion tolerance. The comparative data also reveal that the new inhibitor/dispersant is an excellent particulate matter (i.e., iron oxide, clay, silica, and magnesium silicate) dispersant. 14/25
Collectively, the data herein indicate that CP7 is a unique multi-functional product that incorporates silica polymerization inhibition and particulate (iron, clay, silica, and magnesium silicate) dispersant properties. Incorporation of CP7 as part of a treatment program for silica limited systems (e.g., cooling) should facilitate operating at higher cycles of concentration thereby reducing make up water requirements, reducing treatment chemicals lost via system blow down, and reducing overall operating costs. Acknowledgements The authors would like to thank Tina Dame for preparation of samples and conducting the freeze/thaw stability testing and The Lubrizol Corporation for supporting the research and allowing us to present the findings at the Association of Water Technologies Annual Convention. References 1. R. K. Iler, The Chemistry of Silica, John Wiley and Sons, New York, NY, (1979). 2. S. H. Chan, A Review on Solubility and Polymerization of Silica, Geothermics, 18 49-56 (1989). 3. W. F. Hann, S. T. Robertson, and J. H. Bardsley, Recent Experience in Controlling Silica and Magnesium Silicate Deposits with Polymeric Dispersants, Paper No. 93-59, International Water Conference, Pittsburgh, PA (1993). 4. J. S. Gill, Inhibition of Silica - Silicate Deposit in Industrial Waters, Colloids and Surfaces, 74, 11-16 (1993). 5. R. Sheikholeslami and S. Tan, Effects of Water Quality on Silica Fouling of Desalination Plants, Desalination 126, 267-28 (1999). 6. R. Sheikholeslami, I. S. Al-Mutes, T. Koo, and A. Young, Pretreatment and the effect of cations and anions on prevention of silica fouling, Desalination, 139, 83-95 (21). 7. K. D. Demadis, A. Stathoulopoulou, and A. Ketsetzi, Inhibition and Control of Colloidal Silica: Can Chemical Additives Untie the Gordian Knot of Scale Inhibition? Paper No. 758, CORROSION/27, NACE International, Houston, TX (27). 8. Z. Amjad and M. Yorke, Carboxylic Functional Polyampholytes as Silica Polymerization Inhibitors, U.S. Patent No. 4,51,59 (1985). 9. J. E. Harrar, L. E. Lorensen, and F. E. Locke, Method for Inhibiting Silica Precipitation and Scaling in Geothermal Flow Systems, U.S. Patent No. 4,328,16 (1982). 1. D. L. Gallup and E. Barcelon, Investigations of Organic Inhibitors for Silica Scale Control from Geothermal Brines-II, Geothermics 34, 756-771 (25). 15/25
11. C. W. Smith, Pilot Test Results Utilizing Polymeric Dispersants for Control of Silica, Water Soluble Polymers: Solution Properties and Application, Z. Amjad (ed.), Plenum Press, New York, New York (1998). 12. E. Neofotisou and K. D. Demadis, Use of Antiscalants for Mitigation of Silica Fouling and Deposition: Fundamentals and Applications in Desalination Systems, Desalination, 167, 257-272 (24). 13. L. A. Perez, J. M. Brown, and K. T. Nguyen, Method for Controlling Silica and Water Soluble Silicate Deposition, U.S. Patent No. 5,256,32 (1993). 14. J. S. Gill, Silica Scale Control, Paper No. 226, CORROSION/98, NACE International, Houston, TX (1998). 15. P. R. Young, C. M. Stuart, P. M. Eastin, and M. McCormick, Silica Stabilization in Industrial Cooling Towers: Recent Experiences and Advances, Technical Paper No. TP93-11, Cooling Technologies Institute 1993 Annual Meeting, Houston, TX (1993). 16. D. L. Gallup, Investigations of Organic Inhibitors for Silica Scale Control in Geothermal Brines, Geothermics, 31, 415-43 (22). 17. Z. Amjad and R. W. Zuhl, Laboratory Evaluation of Process Variables Impacting the Performance of Silica Control Agents in Industrial Water Treatment Programs, Association of Water Technologies (AWT) 28 Annual Convention, Austin, TX, November (28). 18. P. P. Nicholas and Z. Amjad, Method for Inhibiting and Deposition of Silica and Silicate Compounds in Water Systems, U.S. Patent No. 5,658,465 (1997). 19. Z. Amjad and R. W. Zuhl, An Evaluation of Silica Scale Control Additive for Industrial Water System, Paper No. 8368, CORROSION/28, NACE International, Houston, TX (28). 2. Silica Polymerization Inhibition Test Procedure, Lubrizol technical bulletin Silica-PITP, Oct-27. 21. Z. Amjad and R. W. Zuhl, Factors Influencing the Precipitation of Calcium-Inhibitor Salts in Industrial Water Systems, AWT 23 Annual Convention, Association of Water Technologies, Phoenix, AZ, September (23). 22. J. A. Wohlever, Z. Amjad, and R. W. Zuhl, Performance of Anionic Polymers as Precipitation Inhibitors for Calcium Phosphonates, in Advances in Crystal Growth Inhibition Technologies, Kluwer Academic Publishers, New York, NY (21). 23. D. A. Meier and L. Dubin, A Novel Approach to Silica Scale Inhibition, Paper No. 334, CORROSION/87, NACE International, Houston, TX (1987). 16/25
24. Z. Amjad and R. W. Zuhl, Kinetic and Morphological Investigation on the Precipitation of Calcium Carbonate in the Presence of Inhibitors, Paper No. 6385, CORROSION/26, NACE International, Houston, TX (26).. 25. Z. Amjad, J. F. Zibrida, and R. W. Zuhl, Performance of Polymers in Industrial Water Systems: The Influence of Process Variables, Materials Performance, 36 (1) 32 1997). 26. L. Dubin and K. E. Fulks, The Role of Water Chemistry on Iron Oxide Dispersant Performance, Paper No. 118, CORROSION/84, NACE International, Inc., Houston, TX (1984). 27. Z. Amjad, Dispersion of Iron Oxide Particles in Industrial Waters, Tenside Surfactants and Detergents, 36, 5-56 (1999). 28. Z. Amjad and R. W. Zuhl, The Impact of Thermal Stability on the Performance of Polymeric Dispersants for Boiler Water Applications, AWT 25 Annual Convention, Palm Springs, CA, September (25). Table 1: Polymeric and Non-Polymeric Additives Evaluated Additive Composition Acronym HEDP 1-Hydroxyethylidine-1,1-diphosphonic acid A PBTC 2-Phosphonobutane 1,2 4-tricarboxylic acid B BA Boric acid C P-MA Poly(maleic acid) or PMA P1 K-752* 2k MW poly(acrylic acid) or PAA P2 K-732* 6k MW poly(acrylic acid) P3 K-76N* 12k MW poly(acrylic acid) P4 K-72* 345k MW poly(acrylic acid) P5 K-775* Poly(acrylic acid: 2-acrylamido-2-methylpropane sulfonic acid) CP1 or p(aa:sa) K-798* Poly(AA:SA : 2-acrylamido-2-methylpropane sulfonic acid: sulfonated styrene) or p(aa:sa:ss) CP2 Competitive-1 Poly(AA:SA : 2-acrylamido-2-methylpropane sulfonic acid: nonionic) CP3 or p(aa:sa:ni) Competitive-2 Poly(maleic acid:ethylacrylate:vinyl acetate) CP4 Competitive-3 Proprietary acrylic copolymer CP5 K-XP212* Proprietary copolymer blend CP6 K-XP229* New proprietary copolymer blend CP7 *Carbosperse K-7 polymer supplied by Lubrizol Advanced Materials, Inc. 17/25