Rapid Precipitation of Magnesite Microcrystals from Mg(OH)2-H2O-CO2 Slurry Enhanced by NaOH and a Heat-Aging Step (from ∼20 to 90 °C)

Abstract

This study proposes a simple and novel synthesis route for rhombohedral single crystals (<2 μm) of magnesite. This synthesis can be summarized by two main sequential reactions: (1) aqueous carbonation of synthetic brucite (Mg(OH) 2 ) by injection of CO 2 in a highly alkaline medium (2 m NaOH) at ambient temperature (∼20 °C), leading to precipitation of platy-compacted aggregates of dypingite (Mg 5 (CO 3 ) 4 (OH) 2 ·5H 2 O) after 24 h, and (2) complete dypingite-to-magnesite transformation after 24 h by a simple heat-aging step from 20 to 90 °C. The dypingite-to-magnesite transformation implies the simultaneous dehydration and carbonation of a brucitic layer of dypingite coupled with instantaneous formation of magnesite crystals. In this study, the NaOH played a catalytic role; that is, it accelerated brucite carbonation by an increase in carbonate ion concentration with time and it promoted the formation of magnesite during the heat-aging step, as illustrated in the following global reaction: At laboratory scale, magnesite is typically synthesized at high temperature (>90 °C) and its synthesis requires several days or weeks depending on experimental conditions. For this reason, industrial-scale magnesite production has been limited. The proposed magnesite synthesis method, requiring only 48 h and moderate temperature, could easily be extrapolated on an industrial scale. Moreover, a simple and novel synthesis route for the production of fine platy particles of hydromagnesite is reported, with synthesis requiring only 5 h. On the basis of their chemical compositions and textural properties, there are potential applications for both minerals, for example, as a mineral filler and/or as a flame retardant.