Rapid precipitation of magnesite micro-crystals from Mg(OH)2- H2O-CO2 slurry enhanced by NaOH and a heat-ageing step (from ∼20 to 90°C)

This study proposes a simple and novel synthesis route for rhombohedral single crystals (<2µm) 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 (2Molal of NaOH) at ambient temperature (∼20°C), leading to precipitation of platy-compacted aggregates of dypingite (O H OH CO Mg 2 2 4 3 5 5 .) () () after 24h; (2) complete dypingite-to-magnesite transformation after 24h by a simple heat-ageing 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, i.e., it accelerated brucite carbonation by an increase in carbonate ion concentration with time and it promoted the formation of magnesite during the heat-ageing step as illustrated in the following global reaction: O H MgCO CO OH Mg s NaOH aq s 2) (3 2) (2) (2) (+   →  + 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 48h 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 5h. Based on 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.