**7. Scale inhibition**

Al-Hamza et al. [113] studied the inhibition effect of poly(acrylic acid) (PAA) with different end groups and molar masses on the formation of CaCO3 scale at low and high temperatures. They showed that the inhibition of CaCO3 precipitation was affected by the hydrophobicity of the end groups of PAA. The best inhibition was found for PAA with hydrophobic end groups of moderate size (6–10 carbons). Also, the scale inhibitors stabilize the less thermodynamically stable polymorph (vaterite) to a degree proportional to their ability to inhibit precipitation. At room temperature, the lowest molar mass of PAA with hydrophilic end group showed good efficiency in the inhibition of CaCO3 precipitation making it suitable to be used as a scale inhibitor in reverse osmosis desalination. However, the lowest molar mass PAA with end groups of moderate hydrophobicity are more suitable as scale inhibitors in MSF desalination. The effectiveness of the inhibitors declined with increasing temperature. Comparable results were found by Li et al. [114] who studied the scale inhibition effect of six varieties of commercial scale inhibitors in highly saline conditions at high temperature. Indeed, they showed that the inhibition efficiency was influenced by scale inhibitor dosage, temperature, heating time and pH. The best scale inhibitor (SQ-1211) could effectively retard scaling at high temperature and when the concentration of Ca2+ was 1600 mg L<sup>1</sup> , the scale inhibition rate reached 90.7% at 80°C and pH 8. The crystal structure of CaCO3 changed from calcite to aragonite. Xu et al. [115] studied the inhibition of CaCO3 scaling on stainless steel surfaces using sodium carboxymethyl cellulose (SCMC). They showed that SCMC exhibited a promising performance of scaling inhibition, and the inhibition efficiency increased with increasing SCMC concentration starting from 50 to 200 mg L<sup>1</sup> . For example, the inhibition efficiency reached 93.2% for SCMC concentration of 200 mg L<sup>1</sup> . The inhibition effect was explained by the formation of a protective film on the stainless-steel surfaces by adsorption of the constituent of SCMC in the presence of SCMC, preventing the deposition of CaCO3. Zuho et al. [116] investigated the feasibility of electrochemical methods to study the scale inhibition performance of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), polyacrylic acid (PAA) on titanium surfaces. They showed that, for electrochemically deposited CaCO3 scale, PBTCA, HEDP and PAA have great inhibitory effects and the order of the scale inhibition efficiency of the three inhibitors is PBTCA > HEDP > PAA. The deposition of CaCO3 on the surface of TA1 metal mainly results in dendritic aragonite crystals and when scale inhibitors were added, the aragonite crystal polymorph was gradually modified to vaterite. Yu et al. [117] designed and prepared phosphorus-free and biodegradable scale inhibitors. In fact, two series of starch*graft*-poly(acrylic acid) (St*-g-*PAA) samples with different grafting ratios and grafted-chain distributions, that are, the number and length of grafted PAA chains

*Effect of Operating Parameters and Foreign Ions on the Crystal Growth of Calcium Carbonate… DOI: http://dx.doi.org/10.5772/intechopen.94121*

on the starch backbone were elaborated. They showed that St*-g-*PAA with relatively low grafting ratio (typically ≤97%) inhibited scale formation more effectively than samples with similar grafted-chain distributions. However, under the similar grafting ratios, samples with higher number of branched chains with shorter grafted chains showed better scale efficiency.

More interesting, Korchef and Touabi [17] showed that high amounts of precipitates were obtained with increasing the initial solution pH, and even when formed in the bulk solution, these precipitates can agglomerate and block up pipes pumps and conducts of water in water treatment devices leading to the complete blockage of fluid cooling. For solution pH lower than 7–7.5, no precipitation of CaCO3 was detected [17]. For that reason, the solution pH should be maintained at low values (pH below 7). This can be easily achieved by avoiding CO2 repelling from the solution. This allows reducing or completely preventing scale in different water treatment processes and favorably affects the water treatment costs and increases the equipment life. In practice, this is achievable by avoiding the contact between water and the atmospheric air. For example, in the desalination units using the solar multiple condensation evaporation cycle (SMCEC) principle, the supersaturation of the feed water increases remarkably with the number of the condensation–evaporation cycles [15] and accordingly, it is expected that iron ions do not inhibit the formation of CaCO3 as scale in the SMCEC desalination units. In that case, it is mandatory to regulate the pH of feed water below 7 to prevent scale formation.

## **8. Conclusion**

Water desalination processes such as reverse osmosis, electrodialysis and distillation are confronted with scale formation which causes enormous energy losses and shortens the equipment life. In most cases, scales are made of CaCO3 which possesses six polymorphs. Environmental conditions play effective roles in the nucleation and growth kinetics of CaCO3 as well as the morphology and polymorphism of the obtained precipitates. In the present chapter, an overview on the effect of operating conditions such as supersaturation, temperature, and pH, on the crystal growth and microstructure of CaCO3 is given. Additionally, I put special emphasis on the effect of foreign ions, especially magnesium, sulphate, chloride, and iron ions. Also, the mechanisms by which these ions affect the crystal growth of CaCO3 were pointed out. The experimental parameters should operate together and control the growth rate and polymorphism of CaCO3. Throughout the present chapter, it was reported that:

