AN EVALUATION OF SILICA SCALE CONTROL ADDITIVES

AN EVALUATION OF SILICA SCALE CONTROL ADDITIVES

FOR INDUSTRIAL WATER SYSTEMS

Z. Amjad and R. W. Zuhl

Lubrizol Advanced Materials, Inc.

9911 Brecksville Road, Cleveland, OH 44141

ABSTRACT

Deposition of colloidal silica and silicate based salts on equipment surfaces continue to pose serious challenges to industrial water technologists. Currently, a wide variety of non-polymeric and polymeric additives are used to combat the problems of silica scaling. These additives are generally effective as scale inhibitors and dispersants under normal operating conditions. However, polymeric additives may lose some performance activity under thermal stress. This paper investigates the impact of composition and the influence of thermal stress on the performance of polymeric silica polymerization inhibitors.

Keywords: silica scaling, inhibition, polymer thermal stability, industrial water systems

INTRODUCTION

The major cause of industrial water system failures is the deposition of unwanted materials on equipment surfaces. The mineral deposits commonly encountered include carbonates, sulfates, and phosphates salts of alkaline earth metals. The effects of deposits on heat exchangers and reverse osmosis (RO) membrane surfaces result in increased operational costs. The potential for silica scaling occurs when the dissolved silica level in re-circulating water or reject stream in RO system exceeds the solubility limit (~100 mg/L at ambient temperature) for amorphous silica. Colloidal silica formed during silica polymerization can foul RO membrane and heat exchanger surfaces. Additionally, polyvalent metal ions (e.g., iron, aluminum, calcium, magnesium) present in feed water streams can absorb or complex silica and catalyze the precipitation.

The mechanism of silica polymerization is very complex and is believed to occur via the base catalyzed reaction as shown below:

Si(OH)_{4 + OH^{- (OH)3SiO- + H2O}}

Si(OH)3- + Si(OH)4 (OH)3Si – O -- Si(OH)3 + OH-

(dimer)

Dimer Cyclic Colloidal Amorphous Silica (scale)

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The development and application of silica control technology in aqueous systems has been the subject of numerous investigations.^{1-3
Several approaches to control silica scaling have been attempted including (a)
inhibiting silica polymerization, (b) increasing the silica solubility as it
forms, and (c) dispersion of precipitated silica and silicate compounds using
polymeric dispersants.}

Another type of silica deposit commonly encountered in RO systems is magnesium silicate. The precipitation of magnesium silicate strongly depends on solution pH and temperature. In RO systems operating above pH 9, magnesium silicate is very likely to form due to the presence of magnesium hydroxide and silicate ions. Other hydroxide salts such as calcium, strontium, and sodium can react with silicate ions. However, the resulting products are much more soluble and hence less likely to foul RO membranes.

The influence of additives (i.e., anionic, non-ionic, cationic) as silica polymerization inhibitors has been the subject of numerous investigations. Amjad and Yorke4 in their evaluation of polymers reported that cationic-based copolymers are effective silica polymerization inhibitors. Similar conclusions were also reported by Harrar, et al.5 in their investigation on the use of cationic polymers and surfactants in inhibiting silica polymerization under geothermal conditions. Although these cationic-based homo- and copolymers showed excellent performance in terms of inhibiting silica polymerization, they offered poor silica/silicate dispersancy activity. Gallup and Barcelon6 investigated the performance of a variety of organic inhibitors as alternatives to strong acids for geothermal application. Results of their study reveal that brine acidification always out-performed the organic inhibitors.

The use of boric acid and/or water soluble salts to control silica based deposits in cooling water systems operating at 250 to 300 mg/L has been reported.7 Silica inhibition presumably originates from the ability of borate to condense with silicate to form borate-silicate complexes which are more soluble than silica. The performance of a formulated product containing hydroxyl phosphono acetic acid and a copolymer of acrylic acid and allyl hydroxyl propyl sulfonate ether in high hardness water containing high alkalinity, and 250 mg/L silica, has been investigated.8 The inspection of heat exchangers showed essentially no deposits in the presence of a formulated product compared to heavy silica and silicate deposits in the control (no treatment).

The effect of heat treatment on polymer performance as a scale inhibitor and dispersant has been the subject of numerous investigations. Masler9 investigated the thermal stability of several homo-polymers used for deposit control in boiler water treatment applications. It was demonstrated that under the experimental conditions employed (pH 10.5, 250°C, 18 hr) that poly(acrylic acid), P-AA; poly(maleic acid), P-MA; and poly(methacrylic acid), P-MAA, all underwent some degradation. In terms of molecular weight (MW) loss, P-MAA lost slightly less MW than P-AA which lost considerably less than P-MA. Additionally, P-AA and P-MAA had minimal performance changes whereas P-MA displayed a substantial loss in performance. Amjad10 in another study using the similar test method showed that polymer performance as dispersants of hydroxyapatite and iron oxide is affected both by temperature level and exposure time. More recently, Amjad and Zuhl11 in their study reported that heat treatment has a significant effect on a polymer’s inhibitory properties. Heat treatment of co- and ter-polymers has been shown to cause (a) a decrease calcium phosphate inhibition and (b) a slight increase calcium carbonate inhibition.

This paper present results of our investigations conducted to determine the role of polymer composition on silica scale inhibition in industrial water systems. We also investigated the influence of thermal stress on inhibitor structure and silica inhibitor performance. Table 1 lists the inhibitors tested.

EXPERIMENTAL

Reagent grade chemicals and distilled water were used in this study. Silicate stock solutions were prepared from sodium metasilicate, standardized spectrophotometrically, and were stored in polyethylene bottles. Calcium chloride and magnesium chloride solutions were prepared from calcium chloride dihydrate and magnesium chloride hexahydrate, and were standardized by EDTA titration. The polymers used in this study were commercial materials. All experimental results are reported on a 100% active inhibitor basis.

Silicate polymerization experiments were performed in polyethylene containers placed in a double-walled glass cell maintained at a required temperature (40°, 55°, or 68°C). The supersaturated solutions were prepared by adding a known volumes of sodium silicate (expressed as SiO_{2)
solution and water in a polyethylene container. After allowing the temperature
to equilibrate, the silicate solution was quickly adjusted to pH 7.0 using the
dilute hydrochloric acid. The pH of solution 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 solution. The supersaturated silicate solutions were re-adjusted to
pH 7.0 with dilute HCl and/or NaOH and maintained constant throughout the silica
polymerization experiment. Experiments involving inhibitors were performed by
adding inhibitor solutions to the silicate solutions before adding the calcium
chloride and magnesium chloride solutions.}

Figure 1 shows experimental set-up. The reaction container is capped and kept at constant temperature and pH during the experiment. Silicate polymerization in these supersaturated solutions was monitored by analyzing the aliquots of the filtrate from 0.22-μm filter paper for soluble silicate using the standard colorimetric method.^{12 The polymer performance for silicate
polymerization inhibition was calculated according to the following equation:}

[SiO2] sample --- [SiO2] blank

SI = ------------------------------------------- x 100%

[SiO2] initial --- [SiO2] blank

Where:

SI = Inhibition (%) or %SI

[SiO2] sample = Silica concentration in the presence of inhibitor at 22 hr

[SiO2] blank = Silica concentration in the absence of inhibitor at 22 hr

[SiO2] initial = Silica concentration at the beginning of experiment

_{^{Polymer Heat Treatment

A polymer solution was prepared containing 10% polymer (as
active solids) at pH 10.5 using}}

sodium hydroxide to neutralize the polymer. Sodium sulfite was added as an oxygen scavenger. A known amount of polymer solution was retained for characterization and performance testing. The balance was charged to a stainless steel tube. The headspace was purged with nitrogen followed by tightening the fittings. The tube was then placed in the oven maintained at the required temperature [either 150ºC (84 psig), 200ºC (241 psig), or 230^{oC
(390 psig)]. After 20 hr, tubes were removed from the oven, cooled to room
temperature, and the solutions transferred to vials for characterization and
performance testing.}

^{RESULTS AND DISCUSSION
Silicate Polymerization Inhibition by Polymers

Using the experimental protocol described above a series of
silicate polymerization experiments were carried out in the presence and absence
of polymers. Table 1 shows that the polymers tested in the present study vary
significantly in terms of compositions and molecular weights of the polymers
tested vary significantly.

A. Polymer Dosage.
The assessment of polymer effectiveness as a
silica inhibitor was done at similar initial silica super-saturation and in the
presence of varying polymer concentrations. Figure 2 details the silica
concentrations versus time profiles in the absence and in the presence of
varying concentration of co-polymer blend (P8) and suggests: (a) silica
concentration decreases with increasing time indicating the progress in silicate
polymerization and (b) silica concentration at a given time increases with
increasing polymer dosage. For example, silica concentrations in the absence of
polymer at time equal to 0, 1, and 3 hr are 560, 458, and 299 mg/L,
respectively. As illustrated in Figure 2, silica concentration gradually
decreases with increasing time and at 22 hr essentially all (>95%) silicate (monomeric)
has been converted to polymeric silica. Figure 2 also presents the silica
inhibition as a function of P8 concentration. At 22 hr in the presence of 15
mg/L P8, the silica concentration in solution is 190 mg/L compared to 175 mg/L
in the absence of P8. Increasing the P8 concentration from 15 to 25 mg/L results
in >50% increase (from 190 to 291 mg/L) in silica concentration. Furthermore, in
the presence of 75 mg/L P8, >85% of the silicate remains in solution.

Figure 3 presents silica inhibition data for P1 (homo-polymer
of acrylic acid) and P8. The data indicate that P1 compared to P8 is an
ineffective silica inhibitor; e.g., the %SI values at 22 hr in the presence of
50 mg/L inhibitor are 2% for P1 and 80% for P8. In addition, increasing the P1
concentration seven fold (i.e., from 50 to 350 mg/L) exhibits negligible (from 2
to 4%) performance improvement. It is noteworthy that P1 (a well known
polyacrylic acid) that exhibits good to excellent inhibitory properties for both
calcium fluoride13 and calcium sulfate14 salts is a poor silicate polymerization
inhibitor.

B. Solution Temperature.
It is well documented that solubility of scale
forming salts such as calcium carbonate, calcium sulfate, and calcium phosphates
are inversely dependent on solution temperature.15 This solubility-temperature
relationship suggests that the scaling tendency will be higher at the heat
exchanger surfaces than in other parts of the recirculating water systems.
Recently, it was also reported16 that calcium ion tolerance of polymer decreases
markedly as solution temperature increases from 25 to 70°C. However, the
situation is very different for silica supersaturated system in the sense that
silica solubility increases with increasing temperature thus resulting in lower
supersaturation.}

In order to study the influence of solution temperature on polymer performance, a series of silica polymerization experiment experiments were conducted at similar initial silica concentrations, pH 7.0, and several temperatures (i.e., 40, 55, and 68°C). Figure 4 presents the time-dependent silica concentrations profiles in the absence of polymer. It is evident that solution temperature significantly influences silicate polymerization. There are two competing factors contributing to temperature influence: (a) silicate supersaturation decreases with increasing temperature due to increased silica solubility and (b) silicate polymerization increases with increasing temperature.

The influence of solution temperature on silica inhibition was also studied by conducting a series of polymerization experiments in the presence of varying inhibitor dosages. Figure 5 shows the P8 dosage needed to achieve >80% silicate polymerization inhibition at 22 hr as a function of temperature. The marked influence of solution temperature on silicate polymerization is clearly evident from the data in Figure 5 and summary below: Solution Temperature	40°C	55°C	68°C
P8 Dosage Required for >80% Silicate Inhibition	50 mg/L	12.5 mg/L	4 mg/L