Si29 Solid State MAS NMR Study on the Fate of Silicon in Mineral Carbonation of Serpentine: A Journey from Mining to End Products

Low intrinsic reactivity of raw serpentine, relatively low magnesite yields achieved and accumulation of reaction by-products are considered the main challenges of mineral carbonation process in terms of developing a large scale commercially viable process. Several steps are necessary to maximise the reactivity of the feedstock, one of the most important involving physical and thermal activation of the mineral. Moreover, the development of an effective route for utilization of the by-products is considered essential to offset the cost of the process.

Serpentine rock contains approximately equimolar quantities of magnesium and silicon (80-85 wt% of mineral in total), with minor quantities of iron and aluminium. The primary aim of using these rocks in the mineral carbonation processes is to extract magnesium using CO₂, under aqueous conditions, and then to react dissolved magnesium with carbonate ions to form stable solid carbonates. Silicon also plays an important role in the activation and the reactivity of feedstocks as well as being a potentially valuable by-product of the process. Developing an understanding of the fate of silicon and the transformation of silica tetrahedral sheets present in raw serpentine to its textural properties as end products is an important factor in the development of mineral carbonation.

The aim of this work is to study the changes in silica tetrahedral sheets of raw lizardite (polymorph of serpentine) during heat activation, dissolution and carbonation processes using Si29 solid state MAS NMR. The main advantage of this analytical tool is that it provides information about the chemical environment of silicon independent of the particle size distribution and the level of crystallinity of the material being analysed. The silicon content of both crystalline and amorphous materials, at any particle size, can be detected and characterised by this technique.

Powdered raw lizardite (<75 µm) collected from the Great Serpentine Belt in NSW Australia was initially analysed by solid state NMR. The spectrum of raw lizardite showed that it was primarily composed of silica tetrahedral sheets having a strong band at -93 ppm (Q₃). Material heat activated at 630 °C for 4 hrs exhibited bands at -63 (Q₀), -71 (Q₁), -78 (Q₁’) and -93 (Q₃) ppm, attributed to the formation of forsterite, intermediate phases and unreacted raw lizardite remaining in the material respectively.

During single stage aqueous carbonation of heat activated lizardite (reaction taking place at 150 bar CO₂, 150 °C, 7 hrs) forsterite and other intermediate phases are absent, and new bands at -105 and -110 ppm, attributed to precipitated amorphous silica (Q4), appear. In the two stage mineral carbonation process, when heat activated lizardite was dissolved in weak acid (CO₂+water) at 45 °C, the solid residue remaining after dissolution consisted of forsterite, unreacted lizardite and amorphous silica based on 29Si NMR analysis. Treating this residue with a 2M solution of nitric acid at ambient conditions dissolved the forsterite and unreacted lizardite phases, as the only band in the NMR spectrum of solid residue following exposure following strong acid treatment was attributed to an amorphous silica phase.