Experimental study on calcium carbonate phase transition from amorphous to crystalline forms in a reverse emulsion

• Published in: Journal of crystal growth Vol. 610; p. 127152
• Main Authors: Tallon, Jean, Browning, Barbara, Gagnière, Emilie, Bordes, Claire, Bardin, Franck, Agusti, Géraldine, Chevalier, Yves, Tayakout, Mélaz
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-05-2023; Elsevier
• Subjects: A1. Additive impact; A1. Characterization; A1. Crystal habit; A2. Phase transition; B1. Calcium compounds; B1. Nanomaterials; Chemical and Process Engineering; Engineering Sciences; B1. Calcium compounds; B1. Nanomaterials; A2. Phase transition; A1. Characterization; A1. Additive impact; A1. Crystal habit
• ISSN: 0022-0248; 1873-5002
• DOI: 10.1016/j.jcrysgro.2023.127152

•Industrial monitoring of the amorphous to crystalline CaCO3 phase transition.•A more acidic environment resulted in faster transition kinetics.•Asymmetrical and thermally activated inhibition of calcite crystal growth by HSA.•The inhibition of the calcite crystal growth by HSA can lead to the vaterite formation.
Calcium carbonate (CaCO3) nanoparticles for lubricant additives are typically manufactured in reverse emulsion. The amorphous form is sterically stabilised in oil to create the so-called, ‘overbased detergents’, for motor engine oils, whilst crystalline CaCO3 nanoparticles are produced for specific applications, like lubricating greases. Here, the phase transition of amorphous calcium carbonate to crystalline forms in a reverse water/oil emulsion was studied in a 23L jacketed batch reactor equipped with agitator, temperature and pressure control. During the process, several acids, such as acetic, alkylaryl and 12-hydroxystearic (HSA), were added. The impact of acid type and HSA quantity were studied with all other aspects constant. The amorphous to crystalline phase transition kinetics were measured, the crystal habits were observed and the solid forms were characterized. With no HSA, a more acidic environment resulted in faster transition kinetics from amorphous CaCO3 to calcite. However, the presence of HSA tended to slow down the transition kinetics. Indeed, calcite crystal growth was asymmetrically inhibited by HSA, leading to elongated rod-shaped crystals. Finally, a very high HSA concentration leads to the formation of vaterite crystals, vaterite being a less stable phase than calcite.