Mg2+ content control of aragonite whisker synthesised from calcium hydroxide and carbon dioxide in presence of magnesium chloride

Abstract

This research is concerned with thermodynamic control of inorganic whisker synthesis in existence of some additives. The polymorphism and morphology control of calcium carbonate (CC) is an active research area because of wide applications of CC. Magnesium chloride has been used to improve the formation of aragonite whisker synthesised from Ca(OH)2 and CO2. However, the reaction kinetics and the purity of final products affected by MgCl2 have not been theoretically studied. In the present research quantitative thermodynamic calculation showed that MgCl2 can decrease relative supersaturation by several orders of magnitude and so is favourable to the formation of aragonite. High reactant ratio of MgCl2/Ca(OH)2 will give a marked decrease in pH within the transformation of Ca(OH)2 into Mg(OH)2, but a small decrease in pH in the following CaCO3 formation, which is disadvantageous to the control of the synthesis with pH. Both solid Mg(OH)2 and MgCO3 may exist in aragonite product, but can be avoided by pH control. Maximum MgCO3 content in aragonite product is <1·00 wt-% and MgCO3 can be removed by slightly decreasing pH using excessive CO2 adsorption, which will result in a negligible decrease in Ca2+ conversion and an increase in carbonate ion concentration. High carbonate ion concentration, unfortunately, is disadvantageous to the reuse of MgCl2 solution. The present study gives a guidance of synthesising aragonite in the presence of inorganic salts such as MgCl2.