Mechanism of the formation of engineered magnesite: A critical mineral to mitigate CO2 industrial emissions

Abstract

Magnesium carbonate production at the industrial scale is a realistic option to reduce the industrial emissions of CO2. Ultrabasic rocks and/or alkaline mine waste provide magnesium sources and are widely available in the Earth’s crust. Here, we investigated the aqueous carbonation of magnesium hydroxide under moderate temperature (25–90 °C) and pressure (initial pressure of CO2 = 50 bar) using NaOH as the CO2 sequestering agent. From time-resolved Raman measurements, we demonstrate that the aqueous carbonation of magnesium hydroxide can be an effective engineered method to trap CO2 into a solid material and produce large amounts of magnesite MgCO3 (6 kg/m3h), or hydromagnesite Mg5(CO3)4(OH)2.4H2O (120 kg/m3h) at 90 °C or nesquehonite MgCO3.3H2O (40 kg/m3h) at 25 °C. Higher production rates were measured for nesquehonite (at 25 °C) and hydromagnesite (at 60 and 90 °C). However, only the magnesite produced at 90 °C ensures a permanent CO2 storage because this mineral is the most stable Mg carbonate under Earth surface conditions, and it could be co-used as construction material in roadbeds, bricks with fire-retarding property and granular fill. The use of specific organic additives can reduce the reaction temperature to precipitate magnesite. For example, ferric EDTA (ethylenediaminetetraacetic acid) reduces the temperature from 90 to 60 °C. However, more time is required to complete magnesite precipitation reaction at this lower temperature (15 h at 90 °C and 7 days at 60 °C). These results suggests that functionalized organic groups can reduce the energetic barriers during magnesite nucleation.