Investigation of the CO<sub>2</sub> Sequestration by Indirect Aqueous Carbonation of Waste Cement

Abstract

Mineral carbonation of CO 2 with fine-grained waste cement was investigated using NH 4 Cl as a recyclable extracting agent. The amount of calcium extracted with NH 4 Cl was not as high as with more commonly used extraction agents such as HCl and CH 3 COOH. NH 4 Cl also exhibited high selectivity in the calcium extraction process, such that calcium ions comprised over 99% of the leaching solution. Another positive benefit of using NH 4 Cl was that precipitation of calcium carbonate by CO 2 injection was possible without the addition of basic reagents. Moreover, the NH 4 Cl regenerated during carbonation can be reused for calcium extraction. However, test results using regenerated NH 4 Cl solution in a cyclic fashion revealed that the process was not perfectly cyclic, but rather the calcium amount after precipitation increased as the cycle proceeded. The geochemical computer simulation PHREEQC was utilized to gain better insight into the cyclic mineral carbonation processes using NH 4 Cl solution. The simulation was based on thermodynamic equilibrium so that the amount of Ca in the solution fluctuated between specific values in a periodic fashion, unlike the experimental results of calcium accumulation in the extraction solution. One reason for this phenomenon was the kinetic/thermodynamic balance controlled by the amount of Ca 2+ and CO 2 present in the solution. However, it was feasible to use a geochemical model to evaluate the mineral carbonation process with the correction factors since the deviation between the experimental and the simulation results remained fairly constant throughout the cycle.

CO2 Mineralization Methods in Cement and Concrete Industry

“…Indirect or multi-stage carbonation is usually an aqueous process involving using acidic reagents to dissolve cementitious material or waste to extract calcium, and then exposing the Ca rich solution to a gas or solution that contains CO 2 to precipitate CaCO 3 [200]. As extracting agents, it was proposed to use either pure water [201][202][203] or organic and inorganic acids like HCl, HNO 3 , CH 3 COOH [204][205][206][207][208][209], alternatively NH 4 Cl and NH 4 OH can be used [206,207]. Using the strong acids allows for the high rate of the calcium extraction from RCP, including the extraction of calcium from limestone aggregates as presented in the RCP [207].…”

“…Using the strong acids allows for the high rate of the calcium extraction from RCP, including the extraction of calcium from limestone aggregates as presented in the RCP [207]. However, the acids application requires the addition of alkalis before the calcium carbonate precipitation step, this is the so-called pH swing method [206][207][208]. After the extraction, the suspension is frequently filtered to separate the calcium rich solution from a residue.…”

“…It was proposed to use this residue as the phosphorus purification agent [210]. The calcium carbonate precipitation step is realized either by reaction of the calcium rich solution with the gaseous CO 2 [201][202][203]206], or with the alkali carbonate solutions [204,205,207,208]. It is possible to directly use the flue gas from a cement plant for these carbonation technologies [202,203].…”

Advancements, Challenges, and Perspectives of Flue Gas Injection in Subsurface Formations: A Comprehensive Review

“…This makes PHREEQC a valuable tool for understanding and modeling the chemical reactions involved in mineral carbonation processes using NH 4 Cl solution. 105 By utilizing thermodynamic databases, PHREEQC can calculate dissolution and precipitation reactions, allowing researchers to optimize variables and parameters in mineral carbonation processes. Moreover, PHREEQC is also used in geosequestration, which involves the capture and storage of CO 2 in geological formations.…”

“…The simulation can also incorporate kinetic equations for chemical reactions and determine speciation at any designated time. This makes PHREEQC a valuable tool for understanding and modeling the chemical reactions involved in mineral carbonation processes using NH 4 Cl solution . By utilizing thermodynamic databases, PHREEQC can calculate dissolution and precipitation reactions, allowing researchers to optimize variables and parameters in mineral carbonation processes.…”