Using nondestructive techniques in mineral carbonation for understanding reaction fundamentals

Abstract

Mineral carbonation of alkaline earth metals is the conversion of carbon dioxide, in gaseous form or dissolved in water, to form solid inorganic carbonates. Alkaline earth metals can be derived from natural minerals, waste residues, or brines. Calcium and magnesium are the two most abundant of these metals, which when carbonated form a variety of calcium and magnesium carbonates of different mineral compositions and crystal structures. Determining inorganic carbon (IC) content of carbonation solid products with high precision is an essential requisite to quantify the amount of carbon dioxide sequestered by a given process at a given reaction time. It also enables understanding fundamental phenomena about the reactions, including the reaction rate, conversion limitations, and the mineral composition of carbonation products and by-products. The most conventional methods to determine IC content utilize destructive techniques based on acid decomposition (e.g. calcimeter) and thermal decomposition (e.g. thermogravimetric analyzer). The main disadvantage of these methods includes chemical usage, long measurement time, overestimation of IC, and limited ability to differentiate the various mineral forms of carbonates. In this regard, nondestructive techniques (NDT) constitute a valuable alternative to the conventional method for IC determination and carbonates characterization; these include spectroscopic methods (NIR, MIR), X-ray spectroscopy (XRD, XRF, XCT); and microscopic methods (SEM-EDS, TEM, RLM, EPMA). While infrared spectroscopy can be used to quantitatively identify carbonate spectral bands, X-ray diffraction quantitatively identifies the presence of specific crystalline carbonate phases. Elemental chemical information is provided by X-ray fluorescence, and microstructure analysis is done using electron probe microanalysis and micro-tomography. SEM provides visualization of crystal and particle size and morphology, while energy dispersive spectroscopy is used to inspect elemental composition of individual particles or intra-particle layers and grains. This paper presents a critical comparative analysis of the use of these NDT in determining IC content and carbonate composition. Improved understanding of the advantages, limitations, and applicability of each method can enable the development of standard protocols for characterization of carbonation products, which is especially required for carbon sequestration accounting in view of climate change mitigation.