Comparison of Various Extractants for Mineral Carbonation of Industrial Wastes

Abstract

When considering domestic conditions in Korea, one suitable technology for carbon sequestration is mineral carbonation. Research is still in its infancy, although many studies have been conducted to investigate various ways of enhancing the carbonation reaction. In this study, blast furnace slag and waste cement were reacted with various acids at the same concentration (0.1 M) under atmospheric conditions, in order to determine their efficiency for mineral carbonation. Hydrochloric acid, acetic acid, oxalic acid, citric acid, EDTA, and ammonium chloride were used as extractants. Based on maximum calcium concentration (2400 mg/L) and selectivity (99%), waste cement reacted better with acids, including EDTA, than slag. This is attributed to the greater specific surface area (porous structure) of cement. Use of EDTA improved extraction ability, but could not reduce usage of caustic reagent in during the carbonation step. Organic acids (oxalic acid and citric acid) had little effect on dissolution. Subsequently, calcium-rich solutions from each type of wastes were reacted with 99.9% CO2 to precipitate calcium carbonate. The solid product obtained from waste cement was white and in the form of uniform particles (under 3 m), i.e., pure calcite, without other elements. In contrast, the product from slag had light brown and heterogeneous particles of lower quality, mainly due to the presence of silicates and aluminates. Based on these results, waste cement was selected as a suitable material for mineral carbonation, and further experiments were conducted with hydrochloric acid, acetic acid, and ammonium chloride as effective extractants. At low acid concentrations (0.1M), the concentration of calcium was similar regardless of acid type. Increasing acid concentration resulted in a linear relationship with the maximum ratio of calcium extraction, but the ratio of increment depended on the type of acid. Although the yield of calcium extracted with NH4Cl was lower than that extracted with HCl and CH3CHOOH, vaterite was precipitated without introducing a basic reagent into carbonation. Moreover, the NH4Cl regenerated during carbonation can be reused for calcium extraction. However, test results using regenerated NH4Cl solution in a cyclical fashion revealed that the process was not perfectly cyclical, but rather the calcium content increased after each precipitation step as the cycle proceeded. It may be necessary to remove residual calcium ions for complete recycling. However, such properties of ammonium chloride indicate that it is a very economical extraction agent compared to other acids. In addition, direct carbonation of synthetic flue gas resulted in greater efficiency, by reducing costs that would otherwise be involved in the carbon capture process. The reaction of waste cement and acids was analyzed using PHREEQC, a geochemical computer program. The simulation was based on thermodynamic equilibrium. Even though the initial pH of CH3COOH was higher than that of HCl for dissociation constants, the equilibrium pH and calcium concentration of extraction solution were similar for the two acids. NH4Cl showed a lower extraction rate, except at 0.1 M. These results were similar to those obtained experimentally. Calcium was present at 93%, which was almost all in Ca2+ form, with some in the form of CaOH+. S was mainly found as SO42- and CaSO4, Al was found as Al(OH)4 and Al(OH)3, and Si was present as H3SiO4- and H4SiO4. Main species of all elements were the same in the Eh-pH diagram. In a periodic fashion, the amount of calcium in the NH4Cl solution fluctuated within specific values, unlike experimental results for calcium accumulation in the extraction solution. There are two possible reasons for these deviations: (1) kinetics, and (2) lack of thermo-dynamic data for the formation of different types of calcium carbonate polymorphs, with the latter requiring further study. One reason was the kinetic/thermodynamic balance controlled by the amount of Ca2+ and CO2 present in the solution, which may explain the observed vaterite formations. However, the kinetic aspect was not considered in this simulation and, may be responsible for the discrepancy between simulated and experimental results. However, it is feasible to use the geochemical model to evaluate the mineral carbonation process with correction factors since the variation between experimental and simulation results remained reasonably constant throughout the test cycle.