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Industrial Waste Could Be Recycled To Meet Calcium Carbonate Needs

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Indirect carbonation is an attractive way of using paper sludge ash, an industrial by-product, to capture atmospheric CO2­ while obtaining calcium carbonate, which is useful in several industries­. In a recent study, scientists from the Korea show, for the first time, how the type and concentration of chelating agent solvents used during indirect carbonation controls the yield, purity, and morphology of the calcium carbonate produced, providing guidelines to serve the differing needs of various industries.

Recycling industrial waste while capturing, using, and storing atmospheric CO2 is a responsible way to meet human needs in the face of climate change. One attractive option for achieving this is indirect carbonation―a process where calcium or magnesium is extracted from industrial by-products using solvents and then used to capture CO2 through a carbonation reaction. Aside from the reduction of atmospheric CO2 levels, indirect carbonation yields highly pure carbonate solids such as calcium carbonate (CaCO3), which are useful in the plastics, paper, paint, cosmetics, and even medical industries. However, different industries require CaCO3 of various yields, purities, and morphologies.

Many studies have focused on maximizing the efficiency of the indirect carbonation of calcium using solvents known as chelating agents, because their calcium solvent (ligand) stability constants may affect not only the calcium extraction efficiency, but also the carbonation efficiency. However, few studies have explored how the stability of the link between calcium and the ligand affects the characteristics of the final product of the reaction, or to scrutinize the effects of the stability constant using various chelating agents. There has been no study so far to find optimal chelating agents with calcium ligand stability constants suitable for maximizing CO2 storage and producing high purity CaCO3 through indirect carbonation. Now, scientists at the National Korea Maritime and Ocean University have not only addressed this knowledge gap, but also come up with guidelines for obtaining CaCO3 of various characteristics, as per the requirements of the different industries.

These scientists, led by Prof Myoung-Jin Kim, tested seven chelating agents with different calcium-ligand stability constants for the indirect carbonation of paper sludge ash, a waste by-product generated in massive amounts in paper mills. Prof Kim explains her motivation: “Though paper sludge ash has not been used much for indirect carbonation, it is high in calcium and has a small particle size, making it an excellent raw material even without any form of pre-treatment.”

In their study, published in the Journal of CO2 Utilization, the scientists found that, according to the chelating agent chosen and its concentration, there exists a sort of trade-off between the amount of calcium recovered, the amount of CO2 captured, and the purity of the CaCO3 obtained. Moreover, they found that the morphology and particle size of the CaCO3 also varied. “For instance,” Prof Kim says, while explaining how the overall results of the study can serve as guideline for selecting the most appropriate chelating agent per the requirements of an industry, “to produce CaCO3 of very high purity―99 % or higher―even in small amounts, it is necessary to select a chelating agent with a low calcium-ligand stability constant. However, to maximize CaCO3 yield regardless of purity, an agent with a stability constant similar to citrate is best.”

Excited about the potential applications of their findings, Prof Kim continues: “We believe our guidelines will be quite helpful, both technically and economically, for the use of indirect carbonation technologies with chelating agents at industrial sites.”

This study will hopefully make recovering and using CaCO3 easier, leading to more meaningful and responsible use of industrial waste. And from the ashes of industrialization, a greener future will emerge.

Kim MJ, Jeon J. Effects of Ca-ligand stability constant and chelating agent concentration on the CO2 storage using paper sludge ash and chelating agent. J. CO2 Util. 2020. doi:10.1016/j.jcou.2020.101202

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