炭酸水素カルシウム溶液から沈殿した炭酸カルシウムの形態制御  [in Japanese] Control of Crystal Shape and Modification of Calcium Carbonate Prepared by Precipitation from Calcium Hydrogencarbonate Solution  [in Japanese]

The control of three modifications and crystal shape of calcium carbonate which precipitated by outgassing dissolved CO_2 in calcium hydrogencarbonate solution was studied. The starting saturated solution (10.3×10^<-3> mol CaCO_3・dm^<-3>) of calcium hydrogencarbonate was prepared by passing a stream of CO_2 through the suspension of fine limestone powder at 2℃ and then diluted with pure water to concentrations of 2.8-8.5×10^<-3> mol CaCO_3・dm^<-3> (initial supersaturation degree: 20-60). Characteristics of the precipitated calcium carbonate were determined by means of X-ray diffraction, infrared spectroscopy, scanning electron microscopy and chemical analysis. The formation region of three modifications was affected remarkably by synthetic conditions such as stirring, pH (6.2-8.0), heating temperature (20°-100℃) and concentration (initial supersaturation degree: 20-60) of calcium hydrogencarbonate solution. When calcium carbonate was precipitated by outgassing dissolved CO_2 in calcium hydrogencarbonate solution under stirring with 240 rpm, rhombohedral-like calcite about 10 μm in size and prismoidal-like aragonite with 2×50 μm in size and 99.7% in purity were independently formed at temperatures below 30℃ and above 50℃ respectively, while no vaterite was observed. However, vaterite was found as an independent phase by adding an alkaline solution of NaOH or NH_4OH into calcium hydrogencarbonate solution without stirring. Especially, NH_4OH solution was excellent as additive to form hexagonal plate-like crystals of vaterite as compared with NaOH solution. For example, hexagonal plate-like vaterite crystals 20 μm in size and 99.7% in purity formed from calcium hydrogencarbonate solution with a supersaturation degree of 20 at pH 7.5-7.8 without stirring. These vaterite-crystals were unstable and transformed to the stable phase of hexagonal plate-like calcite by heating at 600℃. The uniform hexagonal plate-like crystals of calcium carbonate can be expected as a new functional inorganic filler for paper, plastics and rubber.