Effect of conductive copolymers on scale formation of gypsum

Introduction

The precipitation and dissolution of calcium sulfate dihydrate (CaSO₄·2H₂O, gypsum) is of particular interest because of its importance in desalination, in water treatment processes, in geothermal and oilfield drilling, and phosphoric acid production. The calcium salts lead to the scale formation on metallic surfaces. The scale formation reduces heat transfer effectiveness over time, leading to unacceptable heat transfer rates [1], [2], [3], [4].

Several methods of preventing of deposits formation have been developed. One of the most effective methods for controlling the unwanted deposition is addition of polymeric compounds at very low concentration. Studies have shown that inhibitory efficiency of the polymers strongly depends on polymer structure such as nature of their functional groups, molecular weight, and chemical composition [5], [6], [7], [8].

Despite all these investigations, the effect of inhibitors on deposition formation is still not yet fully disclosed. The presence of organic molecules [9], inorganic cations [10], macromolecules [11], polyelectrolytes [12], [13] and cations [14] in a crystallizing solution are known to play an important role in the crystallization process. They may affect nucleation or crystal growth stages of mineral formation.

A large number of studies have appeared in the literature on the effects of functional groups like carboxylic acids, sulfonic acids, phosphoric acids, copolymers of acrylic acids, and high molecular weight polymers with acidic groups on crystallization of sparingly solubleinorganic salts process [8], [15], [16]. Although there are many reports about water soluble polymers, there is only one example of the mineralization control by a rigid carboxylated polymer in literatüre. Neira-Carrillo et al. [17] investigated the influence of conductive polymers based on carboxylated polyaniline on CaCO₃ crystallization. Their results show that the structural placement of the carboxy group in the polymer is determinant of its influence on the crystallization.

In this work, the effects of conductive copolymers of vinylphosphonic acid (VPA) and 4-vinylimidazole [poly(4-VIm-co-VPA)] (Fig. 1) on CaSO₄·2H₂O crystallization, was investigated by using spontaneous crystallization method. The degree of inhibition of cystallization was measured as an increase in induction time and the reduction in crystallization rate. The inhibition increases with VPA content of the copolymer. The fit of the Langmuir adsorption model to the experimental data support a mechanism of inhibition through molecular adsorption of polymers on the surface of growing crystals.

Fig. 1:

Chemical structure of vinylphosphonic acid (VPA) and 4-vinylimidazole [poly(4-VIm-co-VPA)] copolymer.

Experimental procedure

Results and discussion

Table 1 summarize the conductive copolymers used in this study and the effect of added copolymer on the crystallization rate. The results show that the presence of a small amount of copolymers resulted in an initial inhibition, followed by precipitation at a rate comparable to the rate of crystallization from pure solutions. The effect of copolymer on the rate of crystallization was determined over a concentration range of 0.5 mg/L–2 mg/L.

The prolongation of induction period and increasing of R₀/R_i ratio depends on the nature of the inhibitor and its effective concentration at a given condition. For a given inhibitor, generally the higher the inhibitor concentration, the longer induction period and bigger R₀/R_i ratio. According to Table 1, is clear that the effectiveness of poly(4-VIm-co-VPA) copolymers increases with increasing VPA content of copolymers. Induction time increases from 85 min to 136 min for copolymers of poly(4-VIm-co-VPA) at 1 mg/L as the VPA content increases from 48 to 58. The experimental results in the present study show that not only induction time but also R₀/R_i ratio increases with increasing VPA content. But increasing is not significiant as induction time for R₀/R_i ratio. R₀/R_i ratio increases from 1.023 to 1.260 as the VPA content increases from 48 to 58.

Figure 2 shows the plots of t_i values as a function of polymer concentration for used copolymers. The crystallization in the presence of 0.5 mg/L of poly(4-VIm-co-VPA) copolymer which has higher VPA content is preceded by an induction period of 120 min and R₀/R_i ratio of 1.171. t_i value increased to 163 min and R₀/R_i value changed as 1.503 at 2 mg/L. This means that duration of induction period and R₀/R_i ratios are increased by increasing copolymer concentration.

Fig. 2:

Effect of copolymer concentrations on induction period.

The influence of concentration and VPA/4-Vlm ratio of copolymers on CaSO₄·2H₂O crystallization is shown in Fig. 3. Inhibition effect increases with concentration and VPA/4-Vlm ratio of copolymers.

Fig. 3:

Effect of VPA/4-Vlm ratio of copolymers on growth rate at 30°C.

Adsorption mechanism

The Langmuir model, which was developed for the adsorption of ideal gasses onto solid surfaces, can be used successfully to describe the adsorption of polymers which inhibit crystal growth on crystal surface. It can be explained by preferential adsortion of inhibitor at the active sites of the crystal surfaces.

Since the amounts of polymer in solution are small, it is thought that the inhibition is most likely occured by blocking of the active growth sites on growing crystal surfaces rather than binding to calcium ions in the solution [13]. This assumption is interpreted by applying the kinetic results in a langmuir isotherm [20].

The fraction of active growth sites on the crystal faces covered by polymers, θ, for adsorption model is described by the following equation [21], [22], [23]:

(1)θ=KLCi(1+KLCi)

where K_L is the adsorption or affinity constant which is the ratio of the rate constants for adsorption and desorption, k_ads/k_des, and C_i is the concentration of polymer. Depending on the surface coverage, θ, the crystallization rate in the presence of polymer, R_i , is defined by Eq. (2)

(2)Ri=R0(1−θ)

where R₀ is the crystallization rate in the absence of polymer. Combination of Eq. (1) and Eq. (2) are shown in Eq. (3).

(3)R0R0−Ri=1+kdeskads1Ci 

The linearity of the plots of Eq. (3) for gypsum crystal growth in the presence of copolymer (Fig. 4) suggests that the reason of inhibitory effect is adsorption of copolymers at active growth sites. k_ads/k_des can be obtained from the slope of the resulting straight line. The values of the affinity constant calculated for C1, C2, and C3 copolymers are 0.02 (L/mg), 0.22 (L/mg), 0.42 (L/mg), respectively. The results showed that the higher affinity was obtained when the VPA content of the copolymers increased. It may be explained that anionic copolymers containing high VPA can inhibit gypsum crystal growth and that such inhibition is directly linked to fractional coverage of adsorption sites because of their higher anionic charge density [24].

Fig. 4:

Langmuir-type adsorption isotherm for the effect of copolymer.

Photographs were also taken by scanning electron microscope (SEM) for the subsequent visual analysis in order to assess the effects of copolymers on crystal shape and size (Fig. 5). The presence of copolymers with different VPA contents in supersaturated solutions affects not only the kinetics of crystal growth but crystal size as well, as shown in Table 2. The needle-like crystals precipitated with average length of around 76.75 μm and width of 2.96 μm in the absence of copolymers. The average length and width of the crystals grown in the presence of the C3 copolymer was less than that of the control sample at 0.1 mg/L copolymer concentration. The average length of the needle-like crystals was reduced to 35.34 μm and width of 1.99 μm. When the copolymer concentration was increased to 2 mg/L, the average length of the needle-like crystals decreased to length of 32.58 μm.

Fig. 5:

SEM of (a) control (b) crystals grown from a solution containing 0.1 mg/L C2 copolymer (c) crystals grown from a solution containing 0.1 mg/L C3 copolymer (d) crystals grown from a solution containing 2 mg/L C3 copolymer.

Table 2:

properties of CaSO₄·2H₂O crystals grown from solution at the end of 5 h processing time.

Copolymer	Polymer concentration (mg/L)	Needle		Plate
		L (μm)	W (μm)	L (μm)	W (μm)
Control	–	76.75	2.96	48.46	7.47
C1	0.1	47.48	2.56	35.80	5.59
C2	0.1	35.34	1.99	48.39	7.76
C3	2	32.58	2.04	36.97	8.77

FT-IR spectra was used to analyse the molecular structure of crystals (Fig. 6). The bands near 3600 and 3400 cm⁻¹ are characteristic of H₂O vibrations. A weak absorption is observed at 1000 cm⁻¹ that ascribed to coupling of molecular vibrations. A strong doublet is seen near 600 and 669 cm⁻¹ due to the (SO₄)²⁻ bending vibrations. The strongest bands are observed as a doublet near 1115 cm⁻¹ due to (SO₄)²⁻ stretching vibrations. The (SO₄)²⁻ bending vibration is attributed to the band near 420–460 cm⁻¹ [25].

Fig. 6:

FT-IR spectra of crystals grown from a solution containing copolymer.

Conclusion

According to experimental results, poly(4-VIm-co-VPA) copolymers which have different VPA content tested in this study are effective inhibitors for the formation of CaSO₄·2H₂O. The presence of copolymers inhibited the crystal growth of CaSO₄·2H₂O possibly through adsorption onto the active growth sites for crystal growth due to their charge and hydrophilic effects. The copolymer concentration and VPA content are found to be important parameters for the controlling of crystallization of CaSO₄·2H₂O.

The fit of the Langmuir adsorption model to the experimental data supports a mechanism of inhibition through molecular adsorption of polymers on the surface of growing crystals. The crystal size of CaSO₄·2H₂O was affected by presence of coopolymers. It was observed that copolymers reduce the particle size of crystals.

The results indicate that an increase in copolymer concentration results in an increase of induction period. The effectiveness of copolymer increases with increasing VPA/4-Vlm ratio of copolymers.