Inclined Settlers for Separation of Magnesium Hydroxide and Iron Oxides for Mineral Carbonation

One of the options under development for carbon capture, utilization and storage (CCUS) is referred to as the Åbo Akademi (ÅA) route, which is a two-stage route using serpentinite rock and ammonium sulphate to produce magnesium hydroxide that is subsequently carbonated in a gas-solid reactor. An issue for the process is to find a suitable way to precipitate and separate magnesium hydroxide and iron oxides from the obtained aqueous solution. Research presented in this paper will give some results from the separation of precipitated magnesium and iron oxides and hydroxides, as well as magnesium (hydro)carbonates (which are produced in the case where CO2 is fed to the solution at the same time). Experiments were done with analytical grade magnesium and iron sulphates as well as with serpentinite rock-derived magnesium and iron sulphate solutions. Inclined settlers were used and solutions were pumped into the settlers at different flow rates. Precipitation of the species was done either in a mixer before the solution was pumped to the settler or in the inclined settler itself. Most experiments were run at ambient temperature. Recirculation was used in some experiments in order to try to increase the precipitation and sedimentation efficiency. It was shown possible to achieve continuous separation of the solids from the suspensions. Increased temperature did not seem to affect settling of the magnesium hydroxide particles. Separation in inclined settlers would dictate very low pumping rates to avoid a too turbulent environment in the settling zone. Sedimentation was enhanced by applying wet (aqueous) carbonation thus achieving a magnesium (hydro)carbonate product with larger particle size than the suggested magnesium hydroxide separated according to the conventional ÅA route. Effort needs still to be put to optimizing the separation step in order to create a continuous flow combined with preceding process steps.