Control of Crystal Shape and Modification of Calcium Carbonate Prepared by Precipitation from Calcium Hydrogencarbonate Solution

Abstract

The control of three modifications and crystal shape of calcium carbonate which precipitated by outgassing dissolved CO₂ in calcium hydrogencarbonate solution was studied. The starting saturated solution (10.3×10^-3mol CaCO₃⋅dm^-3) of calcium hydrogencarbonate was prepared by passing a stream of CO₂ through the suspension of fine limestone powder at 2°C and then diluted with pure water to concentrations of 2.8-8.5×10^-3mol CaCO₃⋅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°C) and concentration (initial supersaturation degree: 20-60) of calcium hydrogencarbonate solution. When calcium carbonate was precipitated by outgassing dissolved CO₂ in calcium hydrogencarbonate solution under stirring with 240rpm, rhombohedral-like calcite about 10μm in size and prismoidal-like aragonite with 2×50μm in size and 99.7% in purity were independently formed at temperatures below 30°C and above 50°C respectively, while no vaterite was observed. However, vaterite was found as an independent phase by adding an alkaline solution of NaOH or NH₄OH into calcium hydrogencarbonate solution without stirring. Especially, NH₄OH 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 20μm 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°C. The uniform hexagonal plate-like crystals of calcium carbonate can be expected as a new functional inorganic filler for paper, plastics and rubber.