Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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54 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 CHEMICAL TREATMENT solution with continuous stirring. In experi- ments involving an additive, a known vol- ume of inhibitor stock solution was added to the sodium carbonate/bicarbonate solution prior to adding the CaCl 2 solution. T h e e x p e r i m e n t a l s o l u t i o n s w e re stored in a water bath maintained at 67 °C. At known times, each solution was filtered through 0.22-µm filter paper and solution Ca 2+ concentration was determined by a standard EDTA (ethylenediaminetetraace- tic acid) titration method. At the end of experiments, CaCO 3 crystals collected on the filter papers were characterized by XRD and scanning electron microscopy (SEM). Experiments involving the impact FIGURE 1 CaCO 3 precipitation in the presence of different dosages of HP1. FIGURE 2 Performance of polymers as CaCO 3 inhibitors (2.5-mg/L polymer). of Fe 2 O 3 were done by adding a known amount of Fe 2 O 3 to CaCO 3 supersaturated solution and manually mixing the solution for 60 s. Additive efficacy as a CaCO 3 inhibitor was calculated using the following equation: Percent inhibition (% I) = [(Ca) e – (Ca) f ]/ [(Ca) i – (Ca) f ] × 100 (1) where (Ca) e = Ca ion concentration in the filtrate in the presence of inhibitor at known time, (Ca) f = Ca ion concentration in the filtrate in the absence of inhibitor at known time, and (Ca) i = Ca ion concentra- tion at the beginning of the experiment. Results and Discussion In all experiments, the calculations for the driving force were made by considering all appropriate equilibria among the cal- cium, carbonate/bicarbonate, and additive species in a solution . A scaling index, defined as the ratio of the ion activity prod- uct over the thermodynamic solubility product, was calculated for each of the potentially forming solid phases. Performance of Additives During the last two decades, a variety of natural ( biodegradable) and synth etic additives have been developed and are cur- rently being used in water treatment for- mulations. The role of these additives (polymeric and non-polymeric) in such for- mulations is to: (a) inhibit the precipitation of scale-forming salts such as CaCO 3 , trical- cium phosphate [Ca 3 (PO 4 ) 2 ], calcium fluo- ride (CaF 2 ), and barium sulfate (BaSO 4 ); and (b) disperse the suspended matter such as clay, Fe 2 O 3 , organic debris, etc. In the former case, the additive inhibits the precipitation of scale-forming salt by adsorbing onto the scale crystallites and preventing their further growth. The addi- tives that fall into this category are usually low MW homopolymers containing a car- boxyl group (–COOH). Dispersants, on the other hand, are copolymers containing dif- ferent functional groups (e.g., –CO OH, s-acrylamide, CONHR; sulfonic acid, –SO 3 H; ester, –COOR, etc.). These polymers func- tion by adsorbing onto particles such as Fe 2 O 3 and silt, and preventing them from set- tling on the equipment surfaces. In the pres- ent investigation, polymers from both cate- gories were tested as CaCO 3 scale inhibitors. Effect of Additive Concentration Figure 1 presents Ca 2+ concentration as a function of time for the precipitation experiments carried out in the presence of dif ferent concentrations of poly(acr ylic acid) (HP1). There are three points worth noting in Figure 1: (a) Ca 2+ concentration decreases with an increase in reaction time, ( b) Ca 2+ c onc entration increase s w ith increasing HP1 concentration , and (c) i n du c ti on ti m e (th e ti m e at w hi ch a

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