Crystallization of aragonite particles from solution under microwave irradiation A. Rizzuti ⁎ , C. Leonelli Department of Materials and Environmental Engineering, Modena and Reggio Emilia University, Via Vignolese 905, 41100 Modena, Italy Received 29 March 2007; received in revised form 26 October 2007; accepted 19 December 2007 Available online 28 December 2007 Abstract The effect of intense electromagnetic fields, generated inside a multimode microwave applicators, on the crystallization of calcium carbonate polymorphs starting from calcium bicarbonate solutions has been investigated. The identification and quantitative analysis of the calcium carbonate polymorphs has been made by X-ray diffraction using Rietveld-Reference Intensity Ratio (RIR) method. Scanning Electron Microscopy analysis has also been performed to study the morphology of calcium carbonate samples. Crystallization tests have been conducted at atmospheric pressure while adjusting microwave power to maintain solution temperature between 80 °C and 90 °C. For the sake of comparison, calcium carbonate powders have also been prepared using conventional heating either in a oven or on a hot plate with a magnetic stirrer. The experiments conducted using microwave radiation gave in a high percentage yield of aragonite (99%) which has been favoured by increasing microwave exposure times. Low microwave exposure times, on the other hand, favours vaterite formation which crystallized in an unusual flower-like morphology. © 2007 Elsevier B.V. All rights reserved. Keywords: Microwave; Calcium carbonate; Aragonite; Polymorphism; Morphology 1. Introduction During the last two decades it was claimed by a large number of scientific publications [1–4] that microwave technology is an efficient and suitable tool in the chemical synthesis and pro- cessing of materials. As far as chemical synthetic routes were concerned, microwaves were extensively applied for accelerat- ing, in some cases selectively, a wide range of organic reactions, reducing time of syntheses of organic, organometallic and coor- dination compounds as compared with the conventional heating methods [5–11]. Moreover the interactions between materials and the high intensity electromagnetic field generated within a microwave applicator could favour numerous processes. In fact it was widely known [12–16] that the treatment of water under strong magnetic fields can modify the formation of mineral scales, identifiable as calcite crystallisation, which is a persistent and expensive problem in heat exchanges. Since the first commercial device was patented in Belgium in 1945 [17–19], powerful magnetic fields based on permanent magnets were developed with various geometrical configurations and applied to industrial as well as domestic hot water systems. However the scientific community remains sceptical about the use of magnetic filters on flowing water to suppress scale formation as demonstrated by an increasing number of papers which try to explain the still unclear phenomenon [20]. Nevertheless it was demonstrated that after mixing solutions of CaCl 2 and Na 2 CO 3 which were exposed to magnetic fields, the formation of calcium carbonate particles is influenced in such a way as to suppress nucleation rate and to accelerate the crystal growth [15]. It was also observed that the magnetic effect on the formation of calcium carbonate particles is caused mainly by the exposure of sodium carbonate solutions to magnetic fields and verified the existence of the phenomena called “magnetic memory”, described by other authors [20] as the retention time of the antiscaling properties of the treated water for some time after magnetic treatment. Moreover it was observed that increased exposure to the magnetic fields favours the formation of aragonite. In fact aragonite, which results from the transforma- tion of metastable vaterite nuclei [21], exhibits needle-like crystals with a rather weak adhesion to the substrate that is different to that from calcite, mainly responsible for scale for- mation. Notwithstanding the fact that this crystalline evolution has been well demonstrated, the influence of strong magnetic Available online at www.sciencedirect.com Powder Technology 186 (2008) 255 – 262 www.elsevier.com/locate/powtec ⁎ Corresponding author. E-mail address: rizzuti.antonino@unimore.it (A. Rizzuti). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.12.012