(80b) Particle Size Characterization in Mineral Carbonation for Understanding Reaction Fundamentals

Mineral carbonation is the conversion of carbon dioxide, in gas form or dissolved in water, to solid carbonates. These may include calcium carbonates, magnesium carbonates, and a variety of other alkaline earth metal carbonates. Alkaline earth metals can be derived from natural minerals, waste residues, or even brines. When performed in a reactor, mineral carbonation leads to the formation of a powder that can be composed of a carbonate alone (e.g. high purity precipitated calcium carbonate), or a mixture of one or more carbonates with other solid phases, such as silica and silicates. Materials characterization thus plays an important role in assessing the potential to use these carbonates in commercial applications (e.g. filler in paper, aggregate in concrete). Materials characterization also aids in understanding fundamental phenomena about the reactions, such as the behavior of the reagents, the formation of products and by-products, and the reaction rate and conversion limitations. In this presentation, I will highlight findings of fundamental nature that I have made on topics related to mineral carbonation, and that were made possible by assessing particle size, particle size distribution and other morphological characteristics.

Laser diffraction analysis (LDA) is a technique that is useful to follow particle size growth or reduction. Thus, it can help elucidate effects of carbonation such as formation of precipitated layer or alteration of microstructure, and of sonication (a technique to intensify the carbonation reaction) such as particle attrition and fragmentation. Of the three average particle diameters commonly obtained by laser diffraction (the D₅₀, D[4,3] and D[3,2]), the Sauter mean diameter (D[3,2]), being surface area sensitive, is the most important when it comes to the susceptibility towards mineral carbonation of powdery materials, since mineral carbonation reactivity is proportional to exposed surface area. This mean diameter is also found to be the most sensitive to sonication effects, as it best detects the formation of micron- to sub-micron sized fragments. The volume-moment mean diameter (D[4,3]), on the other hand, is useful in indicating the degree of erosion of larger particles.

LDA results have been particularly useful in assessing the performance of sonication as a means of process intensification. It is interesting to note that slag particle size reduces after only 30 minutes of sonicated carbonation, while in the case of non-reacting sonication the particle size does not change. This suggests that sonication not only removes the precipitated calcium carbonate layer, but also the depleted silica layer that constituted the original slag particle material, and that this depleted silica layer is weaker than the original silicate material. This has been further substantiated by comparing Scanning Electron Microscopy (SEM) images to the LDA results and observing that two particle size modes form: flaky sub-micron particles and rounded particles roughly 2-10 μm in size, which appears to be remnants of larger particles that have been significantly eroded over time. It is also worth noting that, while SEM provides direct visualization of particles, it is a semi-quantitative method for assessing particle size distribution, while LDA is a quantitative method for doing so, and thus offers added insight not realizable by SEM.

LDA also provides a window into the mechanism that controls the particle size of sonochemically synthesized aragonite precipitates (aragonite is the high-temperature polymorph of calcium carbonate). Larger particle modes are thought to represent needle-shaped particles, while the smaller modes may be fragmented particles, due to sonication, and/or particles that form near the end of the experiments when crystal growth becomes hindered by diminishing calcium concentration in solution. Samples from low temperature synthesis (24 °C and 30 °C) are found to have unimodal distributions below 3 μm. The low temperatures, which hinders crystal growth throughout the experiment, can be attributed to the smaller particle sizes.

Finally, when MgCl₂ is used as an additive during slag slurry carbonation, laser diffraction helps to provide an explanation as to why MgCl₂ addition is only beneficial in the presence of sonication. Particle size reduction can be linked to the formation of acicular aragonitic crystals, whose morphology and arrangement enable them to be more easily cleaved off the surface of the carbonated particles due to sonication compared to the more closely-packed calcitic layers formed in the absence of MgCl₂.