Effect of optimized three-component antiscalant mixture on calcium carbonate scale deposition

Water used in the cooling water systems usually contains particles, organic maters and scale-forming irons containing Ca²⁺, Mg²⁺, CO₃²⁻, SO₄²⁻ and HCO₃⁻ . The constituents could be concentrated many times because of the evaporative loss of water. The elevated concentration and high water temperature could cause severe mineral deposition and adhesion onto the heat transfer equipment surfaces (mineral scaling), along with the problems of corrosion (Shen et al. 2013; Wei et al. 2016) and biofouling (Wang et al. 2013). It is well known that calcium carbonate, in particular, is the predominant mineral scale compound in cooling water systems (Zhao et al. 2014). The problem of scaling in cooling water systems during recirculation poses great challenges from both economical and technical points of view, decreasing system efficiency and increasing frequency of chemical cleaning.

To mitigate the problem of mineral scaling, chemicals and antiscalants including but not limited to polyacrylic acid (PAA), polyacrylamide, hydrolyzed polymaleic anhydride (HPMA), 1-hydroxyethane-1,1-diphosphonic acid (HEDP) and polyphosphates were widely used (Al-Roomi et al. 2015; Chen et al. 2015; Liu et al. 2015; Mithil Kumar et al. 2015; Asghari et al. 2016; Bu et al. 2016; Wang et al. 2016). These antiscaling chemicals control mineral scale through mainly two antiscaling mechanisms: one is that antiscalants could keep more scale-forming positive ions (e.g. Ca²⁺ and Mg²⁺) in the solution from being precipitated through complexation action (Eriksson et al. 2007); the other is that the antiscaling chemicals could interact with mineral nuclei to disrupt the crystallization process and keep the crystal particles dispersed in the aqueous suspension, rendering them less prone to sedimentation or adhesion onto the equipment surfaces (Shakkthivel & Vasudevan 2006; Eriksson et al. 2007). HEDP, HPMA and PAA have been widely used as scale inhibitors, corrosion inhibitors or dispersants. PAA has the ‘threshold effect’ of CaCO₃ inhibition which could absorb the growing crystal phases of the nuclei and prevent vaterite transforming to aragonite or calcite, which results in the distortion and retardation of the crystal growth (Tang et al. 2008; Kavitha et al. 2011). HEDP could modify the structure of calcium carbonate by incorporating into the crystals and thus decrease scale formation on the heat exchanger surfaces (Marín-Cruz et al. 2006). A. Martinod et al. investigated the effect of HPMA on the growth of calcium carbonate particles in the micrometer size range on stainless steel surface, HPMA could affect the growth of CaCO₃ crystals due to adsorption of carboxylate ions on the nuclei of calcium carbonate (Martinod et al. 2008). In addition, we synthesized HPMA and investigated the effect of the synthesized HPMA on the morphologies and crystal structures of calcium carbonate precipitates in our previous work (Shen et al. 2012). Mixtures of different kinds of scale inhibitor according to a certain mixing ratio may have much higher scale inhibition efficiency. Experiments and optimization are usually necessary to find a good mixing ratio. Reports on the optimization of the mixture of HEDP, PAA and HPMA were not easy to find. And the synergetic effect of HEDP, HPMA and PAA on the crystal of calcium carbonate is still lacking.

In the present study, the mixture of HEDP, PAA and synthesized HPMA was optimized by using simplex lattice mixture design according to the Design-Expert software, based on calcium carbonate precipitation method. The morphologies and crystal structures of precipitates and deposits were investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The synergetic effect of HEDP, HPMA and PAA on the crystal of calcium carbonate was discussed.

Chemicals are analytical reagents which were used as received from commercial suppliers without further purification, unless otherwise specified. HEDP and PAA were bought from Nanjing Naco Water Treatment Technology Co, Ltd.

The synthesis process of HPMA was discussed in our previous work in detail (Shen et al. 2012).

The inhibition ability of the mixture of HEDP, PAA and synthesized HPMA was optimized by using simplex lattice of Design-Expert software (Stat-Ease, Inc.) under the condition of calcium carbonate precipitation test. The experimental design was carried out with some constraints (Table 1, A, B and C is for HEDP, HPMA and PAA, respectively). A special cubic mixture model was used to describe the relationship between the inhibition efficiency and the components (Gorman & Hinman 1962).

There are two methods to evaluate antiscalants in the present study. One is calcium carbonate precipitation test using ethylenediaminetetraacetic acid (EDTA) titration under static condition. The other is calcium carbonate deposits weighing test through weighing the stainless steel specimen after mineral scaling test under stirring condition.

The calcium carbonate precipitation test was designed to provide a quantitative measure of the abilities of scale inhibitors to prevent the precipitation of Ca²⁺ from bulk solution. The initial concentration of Ca²⁺ was 240 mg/L in the calcium carbonate precipitation test. The details were described in our previous work in detail (Shen et al. 2012).

The precipitates in the test were carefully collected and dried in room temperature for morphology and crystal structure analysis. The XRD pattern was recorded on a Bruker D8 Advance XRD instrument (Cu, Kα), the diffraction angle (2θ) in the range of 20–80° was scanned (Bruker, Germany). SEM images were obtained using JEOL JSM-6380LV electron microscope (JEOL JSM-6380LV, Japan).

The calcium carbonate scale deposit weighing test was carried out in a 500 mL five mouth flask. To avoid the evaporation of the solution, the 40 cm long glass tube (Ф = 3 mm) plugged by the rubber stopper was used as a reflux condenser. The prepared 304 stainless steel specimen was fixed in the flask after being weighed. Three parallel specimens were used for one test. The flasks were placed in a water-bath at 80 °C constantly for 24 h. During the test, the agitation speed was kept at 200 r/min so that the liquid velocity on the specimen surface was almost 0.6 m/s. The stainless steel specimens were carefully removed and placed in a drier at room temperature without disturbing the deposits on the surface. The specimen was measured using analytical balance (Mettler AE163). Final weighing was performed only after a constant mass was achieved (mass measurement variation <0.05 mg/h). Three parallel specimens were measured and the average value was reported as the mineral mass on the specimen. The mass of deposits on the test specimen was calculated based on the specimen mass before and after the test. The scale inhibition efficiency for scale deposits was calculated as: ⁠, where m₁ is the mass of deposits on the specimen surface during scaling test with adding antiscalant; m₀ is the mass of deposits on the specimen surface during scaling test for the control sample and η_s is the antiscale efficiency of the calcium carbonate scale deposit test.

Following the weighing test, the stainless steel specimens were carefully collected in a drier for morphology analysis of the calcium carbonate scale deposit. SEM images were obtained using JEOL JSM-6380LV electron microscope (JEOL JSM-6380LV, Japan).

The antiscale efficiency of the synthesized HPMA increased from 52% to 71% when concentration increased from 2 mg/L to 6 mg/L, and then increased slowly to 78% when the concentration increased to 30 mg/L. In the presence of HPMA, Ca²⁺ could adsorb onto the main chain of HPMA molecular through chelate reaction (Eriksson et al. 2007). The formation of soluble complexes of HPMA-Ca²⁺ through carboxylic acid groups increased the concentration of Ca²⁺ in the bulk solution and, thereby, decreased the rate of calcium carbonate precipitation (Lin et al. 2005). Otherwise, HPMA may adsorb onto the active sites of the crystal nucleus, thereby disturbing the growth of the crystal of calcium carbonate.

The optimum mixture of HEDP, PAA and synthesized HPMA at the mass ratio of HEDP/HPMA/PAA = 10/80/10 showed much higher efficiency than HPMA. The antiscale efficiency of the mixture showed similar trend to HPMA. The antiscale efficiency of the optimum mixture increased from 57% to 80% when the concentration increased from 2 mg/L to 10 mg/L, and then increased slowly to 84% when the concentration increased to 30 mg/L. HEDP and PAA enhanced the effect of the synthesized HPMA on Ca²⁺. Both the synthesized HPMA and the optimum three-component antiscalant mixture showed ‘threshhold’ effect under the condition of the calcium carbonate precipitation test. Even less than 10 mg/L, the synthesized HPMA and the optimum mixture provided excellent antiscale efficiency. And the efficiency increased insignificantly when the concentration increased from 10 mg/L to 30 mg/L.

The two occurring mineral phases, vaterite and calcite, could clearly be distinguished by their characteristic morphologies. Vaterite could be recognized as framboid spherical shape (Nehrke & Van Cappellen 2006) while calcite as rhombohedra crystallites (Weiner et al. 2005). Figure 4(a) shows regular shaped rhombohedra in the absence of antiscalant corresponding to calcite phase. However, the regular shaped rhombohedrons disappeared in the presence of synthesized HPMA (Figure 4(b)) or the optimum antiscalant mixture (Figure 4(c)).

The precipitates collected from the experiments without HPMA added showed the characteristic of regular shaped rhombohedra, and the particles' size was uniform (Figure 4(a)). In contrast, the morphologies of the precipitates changed to framboid spherical shape without sharp edges and acute corners in the presence synthesized HPMA. HPMA molecular could react crystal nucleus of CaCO₃, and then affect the growth of CaCO₃ crystals (Neira-Carrillo et al. 2008). The carboxylate groups on the main chain could react with the active sites on the crystal surface and thus the HPMA molecule could inhibit the CaCO₃ crystal growth by binding the crystal nucleus. Otherwise, through the interaction between carboxylate groups and the active sites of the crystal surface, HPMA molecular chain could change the stereochemical orientation of CaCO₃ growth (Reddy & Hoch 2001). The resulting morphology of crystals is an expression of different growth rates in the various crystallographic directions. Accordingly, precipitates showed framboid spherical shape with a lot of interspaces and the sharp edges and acute corners disappeared totally. HPMA could decrease the adhesion of the precipitates onto the surface of the conical flasks.

The effect of the optimum three-component antiscalant mixture on the calcium carbonate crystal was similar to that of synthesized HPMA. It was, perhaps, because the synthesized HPMA was the main part of the mixture (80%). However, the precipitate in the presence of the mixture showed a loose cotton-shaped appearance with a lot of interspaces and the sharp edges and acute corners disappeared absolutely. The synthesized HPMA and the optimum mixture could decrease the adhesion of the precipitates onto the surface of the conical flasks.

The mixture of HEDP/HPMA/PAA at the mass ratio of 10/80/10 showed greater influence on the morphologies and crystalline phase of CaCO₃ precipitates. It was demonstrated that a spot of HEDP and PAA enhanced the effect of synthesized HPMA on the crystal of calcium carbonate. The functional groups of HEDP and the COOH groups may react onto the different active points of the crystal nucleus, and thus influence the crystal growth in different ways.

In contrast with calcium carbonate precipitation test, heat exchanger simulated tests were more reasonable to evaluate the scale inhibitors for cooling water systems. The calcium carbonate scale deposit weighing test was designed to investigate the effect of the synthesized HPMA and the optimum three-component antiscalant mixture on the calcium carbonate scale deposit formation on stainless steel surface, which used water bath and electric mixer to simulate industrial conditions.

The mixture of HEDP, and PAA and synthesized HPMA was optimized by using simplex lattice mixture design of Design-Expert software through calcium carbonate precipitation method. The optimum mass ratio of HEDP, PAA and synthesized HPMA was obtained at 10/10/80.

Under the calcium carbonate precipitation test, the optimum showed much better antiscale efficient (78% for synthesized HPMA and 84% for the optimum mixture). The synthesized HPMA could disturb the crystal growth of calcium carbonate and then affect the morphology and crystal structure of the calcium carbonate crystal precipitates. The optimum showed a much stronger effect on the morphology and crystal structure of the calcium carbonate crystal precipitates.

Under the calcium carbonate scale deposit test, the optimum antiscalant mixture showed excellent performance on controlling the calcium carbonate deposition onto the stainless steel surface. The antiscale efficiency increased significantly from less than 40% to more than 95% when increasing the antiscalant concentration from 10 mg/L to 30 mg/L.

This work was supported by the National Natural Science Foundation of China (No. 21407043); the Education Department of the Henan Science and Technology Fund Project (No. 14A610003 and 16A610009) and the Fundamental Research Funds for the Central Universities (No. 30916014102).