Indian Journal of Chemistry Vol. 52A, March 2013, pp. 342-349 Anomalous transformation of calcite to vaterite: Significance of HEDTA on crystallization behavior and polymorphism at elevated temperatures Shanmukhaprasad Gopi & V K Subramanian* Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India Email: drvksau@gmail.com Received 16 October 2012; revised and accepted 30 January 2013 Phase transformation of calcite to vaterite in the presence of N-(2-hydroxy ethyl) ethylenediamine- N, N, N 1 - triacetic acid (HEDTA) is reported. The effect of HEDTA on the crystallization behavior and polymorphism of CaCO 3 are studied at different temperatures (60 – 240 °C). The samples are characterized by powder X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscope and field emission scanning electron microscopy. The study reveals that above 170 °C, HEDTA facilitates the unusual transformation of calcite to vaterite, which could make it a possible potential scale inhibitor and also a boiler sludge conditioner at elevated temperatures. The present study also provides a novel understanding of the crystallization behavior of CaCO 3 using HEDTA; conversion of a stable polymorph to a less stable one (calcite to vaterite), which can be extended to other systems by the careful optimization of different parameters. Keywords: Inorganic chemistry, Crystal growth, Electron microscopy, Crystal structure, Polymorphism, Crystallization, Calcite, Vaterite, Calcium carbonate Scale formation is a serious problem encountered in boilers, cooling towers and heat exchangers in many industries 1-4 . Hard water and even demineralized water generates adherent deposits of scale on the internal walls of equipment leading to reduced heat transfer efficiency and sometimes shutdown of an industrial plant. Different water treatment techniques such as internal and external treatment methods are adopted for scale inhibition. External treatment such as softening and demineralization not only makes the water more corrosive but also allows some cations to enter in the system. This makes internal treatment inevitable, particularly for operations at elevated temperatures. Internal treatment mainly consists of adding chemical reagents like chelating agents (scale inhibitors) which are capable of preventing or delaying the initiation of scale formation and keeping the precipitated particles in suspension by preventing adhesion between individual particles and between particles and metal surfaces 5-8 . Another major advantage of scale inhibitor is the low dosage concentrations 9 (usually 2-10 ppm), which has very little impact on the feed water quality. Calcium carbonate is one of the major constituents of scale 10,11 and has three anhydrous crystalline forms, viz., calcite, aragonite and vaterite, and, three hydrated forms, viz., amorphous calcium carbonate (ACC), monohydrocalcite (CaCO 3 H 2 O) and ikaite (CaCO 3 6H 2 O) 12 . Amongst the three crystalline polymorphs, the thermodynamically most stable is calcite and the least stable is vaterite 13,14 . The actual crystallization of CaCO 3 starts from thermodynamically unstable ACC, and crystallizes spontaneously when present in aqueous solution at ambient temperatures. ACC will transform to calcite via vaterite at low temperatures (<30 °C) and to aragonite via vaterite at higher temperatures 15 (≥ 40 °C). This process is rapid and governed by many factors such as pH, temperature, additives, etc. Although many reports are available on the morphology and polymorphic composition of CaCO 3 in the presence of different additives and scale inhibitors 5-9,16-18 , very few details are available at elevated temperatures 19 . Various studies suggest that temperature is also an important factor which controls the formation of the final crystalline phase 19-23 . Although calcite presents highest thermodynamic stability under ambient conditions, the thermodynamically less stable aragonite and/or vaterite phase may be stabilized under certain conditions or in the presence of other ions or inhibitors 12 . Earlier report suggests that greater the amount of vaterite a scale inhibitor can produce, the more efficient it will be 10 .