Mineral carbonation offers an alternative option for sequestering CO₂. One major benefit of mineral carbonation is the environmentally friendly and virtually permanent trapping of carbon dioxide in the form of carbonate minerals. This option has an additional advantage in that, unlike geological sequestration routes, it provides long-term sequestration without need for post-monitoring processing once the carbon dioxide has been fixed.

CO₂ sequestration by mineral carbonation can be viewed as an acid-base reaction where dissolved CO₂ reacts with a solid base. Calcium and magnesium are common in nature, which are typically found in silicate minerals such as wollastonite and serpentine. Therefore, these minerals were intensively used in earlier studies on mineral carbonation [1] [2] [3] [4] [5] . However, utilization of these minerals requires large-scale mining operations, which often face criticism over cost and environmental issues related to exploitation of natural resources. In light of this, there has been increasing interest in mineral carbonation using alkaline industrial wastes such as steel-making slag, waste concrete, asbestosmi- ning tailings, and coal fly ash [6] [7] [8] [9] . However, in general, mineral car- bonation is considered to be more expensive than geological storage mainly due to the cost and energy required for pre-treatment, which includes fine grinding and extraction of Mg and Ca from source minerals, as shown in Table 1 [10] .

Therefore, in recent years, various studies have been undertaken seeking diffe- rent routes to enhance the carbonation process while minimizing energy consumption and costs [11] . Two main routes can be distinguished: 1) direct carbo- nation and 2) indirect carbonation. Direct carbonation involves a one-step direct reaction of gaseous carbon dioxide with particulate metal oxide, either as a direct gas-solid reaction or in an aqueous medium. Indirect carbonation is conducted in two steps: extraction of Mg or Ca from the minerals using acids or other solvents, followed by carbonation where the extracted components are reacted with CO₂.

In indirect carbonation, excess acid is required to achieve high extraction efficiency, and in turn, the pH of the extraction solution can be very low [12] [13] . This results in an unfavorable condition for the subsequent carbonation reaction, which is favored by basic pH conditions. Therefore, it is necessary to raise the pH before carbonation. If a caustic reagent is used for this purpose, a considerable amount of reagents is required for the extraction and carbonation steps. Alternately, organic acids such as EDTA are added to enhance the dissolution of alkaline metals from silicates in mild acid conditions [14] . This would reduce the usage of caustic reagent in the second step. However, organic acid is very expensive and makes it more difficult to recover and recycle the acid used in the leaching step. Another approach towards reduction of energy and acid consumption is the use of acetic acid. This is a less severe extraction medium and offers the additional advantage of the ability to recover the acetic acid in the second step [15] [16] .

There have been other attempts to develop an extraction agent with potential for full chemical recycling. Kodama et al. [17] developed a process that uses NH₄Cl as an extracting agent. It was reported that NH₄Cl could dissolve calcium ions efficiently and selectively from slag. After removing the slag residue, injection of CO₂ into the leaching solution resulted in precipitation of pure calcium carbonate with NH₄Cl regenerated during carbonation. A caustic reagent was not required for carbonation in this process as the solution pH was stabilized in the alkaline region during extraction. This process seems to be the most promising given the potential for reagent recovery and reuse. Therefore, it was investigated in this study as a preferred method for CO₂ sequestration.

A number of potentially suitable types of industrial waste are generated in large quantities in Korea such as slag and waste concrete. Over 20 million tons of steel slag is generated in Korea each year. Since the calcium content of this slag is typically over 40 wt.%, this material has a great potential for CO₂ sequestration. However, it contains many potentially hazardous heavy metals that can leach out during extraction. This could complicate the process and the resulting carbona- ted products may not be pure enough to be of value in other industries such as the paper industry. Moreover, residues after extraction can be environmentally hazardous and may require additional treatment. The amount of waste concrete generated in Korea was about 90,000 tons per day in 2010. In order to meet the requirements of the mandatory recycling law, there are about 300 construction waste processing plants in Korea. These plants have typical facilities such as sorting and crushing units to produce mainly recycled aggregates. For every ton of waste concrete, 530 kg of powdery waste cement is produced [18] . This material contains over 40 wt% Ca (as CaO) and therefore is suitable for mineral carbonation due to the availability of fine-grained Ca bearing materials.

The objective was to experimentally analyze mineral carbonation of waste cement in a cyclic manner. To achieve this, the degree of extraction was compared to that of other common acids (acetic acid and hydrochloric acid). Additionally, the potential for direct carbonation with flue gas was evaluated to develop a cost effective method of CO₂ sequestration without carbon capture.

The waste cement used in this study was fine residue recovered from aggregates obtained through crushing demolition concrete waste and size separation. Sieve size analysis indicated particles’ median diameter of 33.4 μm. The specific surface area was 13 m²/g determined by BET gas adsorption method (GEMINI V, Micromeritics). The chemical composition measured by X-ray Fluorescence (XRF) (S4 PIONEER, Bruker AXS) and Powder X-Ray Diffractometry (XRD) (D8 Advance, Bruker) show that waste cement is rich in Si (40.4 wt.% SiO₂), Ca (36.7 wt.% CaO), Al (9.8 wt.% Al₂O₃), Fe (5.1 wt% Fe₂O₃), K (2.3 wt.% K₂O), and Mg (1.9 wt.% MgO) along with minor elements: Na, Ti, and Mn (Figure 1). A Field Emission-Scanning Electron Microscopy (FE-SEM) image (SUPRA 55 VP, Carl Zeiss) of the waste cement particles shows a distinct porous structure that may allow rapid penetration of liquid in solid surface (Figure 2). All of these properties indicate that waste cement is a good material for mineral carbonation.

Ammonium chloride (99.5%, Sigma-Aldrich), acetic acid (1 M, Sigma-Al- drich), and hydrochloric acid (1 M, Sigma-Aldrich) were used to extract Ca ions from the waste cement. The reactions of the waste cement (represented by calcium silicate minerals) with three acids are formulated as shown below:

Hydrochloric acid:

CaSiO₃ + 2HCl → CaCl₂ + SiO₂ + H₂O (1)

CaCl₂ + CO₂ + H₂O → CaCO₃ + 2HCl (2)

Acetic acid:

CaSiO₃ + 2CH₃COOH → Ca(CH₃COO)₂ + SiO₂ + H₂O (3)

Ca(CH₃COO)₂ + CO₂ + H₂O → CaCO₃ + 2CH₃COOH (4)

Ammonium chloride:

2CaSiO₃ + 4NH₄Cl → 2CaCl₂ + 2SiO₂ + 4NH₃ + 2H₂O (5)

2CaCl₂ + 2CO₂ + 4NH₃ + 2H₂O → 2CaCO₃ + 4NH₄Cl (6)

For acetic and hydrochloric acid, it is expected that the dissolution amount of Ca ions will increase as the acidity increases. However, carbonation reaction favors basic pH conditions so that addition of basic reagents is required to raise the pH of the solution to at least 8 - 9 in the precipitation of carbonate minerals. In the case of ammonium chloride, the pH of solution in the extraction step becomes alkaline due to the NH₃ generation. This enhances the absorption of CO₂ into the solution and precipitation of CaCO₃ without addition of basic reagents in the carbonation step. Furthermore, during precipitation of CaCO₃, the acidic extraction solution and NH₄Cl are regenerated.

Calcium extraction experiments were carried out as follows: 50 g of the waste cement was mixed with 500 mL of solutions containing various amounts of the agents (0.1 - 0.5 M) in a 1 L beaker. The slurry was stirred with a magnetic stirrer at 400 rpm for up to one hour in ambient conditions (25˚C, 1 atm). The liquid samples were obtained at regular time intervals and were analyzed for concentrations of various ions by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (ICP-730 ES, Varian).

After one hour of extraction, the slurry was immediately filtered through 6 μm filter paper to separate the solid residue. The filtrate was then poured into a 500 mL flask for carbonation experiments. For the filtrates from HCl and CH₃COOH leach, the solution pH was adjusted to, and maintained at pH 10 - 12 in ambient condition through the drop addition of 1 M NaOH solution. After pH was stabilized, pure CO₂ gas (99.9%) was introduced at a constant flow rate (100 mL/min) into the flask for 2 hours to supply carbon dioxide. The pH of the solution was continuously measured to monitor the progress of the carbonation reaction. After the completion of carbonation, the slurry was filtered and the filtrate was analyzed for Ca and other ions using ICP-OES. The precipitate was collected, washed, and dried before being analyzed by XRD and SEM.

In order to evaluate the recyclability of the NH₄Cl solution, additional tests were conducted in a cyclic manner. First, complete extraction and precipitation were conducted in the first cycle using the same procedure described above. Then a second cycle was performed using the filtrate recovered after completion of carbonation in the first cycle as extractant, without adding new reagents. These tests were repeated for several cycles and the intermediate products (precipitated calcium carbonates) were dried, weighed and subjected to chemical analysis.

In addition, carbonation experiments were carried out using synthetic gases of various gas compositions in the same way to evaluate the possibility of carbonation with the flue gas (mixture of CO₂-N₂-SO₂-NO_x). The left column of Table 2 shows the compositions of the synthetic gases mixed and tested in this study along with that of the real flue gas emitted from coal power plants in Korea. The syngas was supplied from bottled gases mixed to the designed composition for injection into the flask for carbonation reaction.

Understanding the chemical reactions involved in any chemical process is important to optimize and control the process’ variables and parameters. Therefore, we wanted to determine if it was feasible to use a geochemical code, PHREEQC [23] , to represent the mineral carbonation processes using NH₄Cl solution in a cyclic fashion. PHREEQC is capable of simulating complex reactions between dis- solved gases, aqueous solutions, and minerals. The geochemical program PHREE- QC takes into account the concentration of ions in water, such as calcium, sodium, sulfate, etc., and then calculates how much of these ions and minerals formed are dissolved in the water, and if they will precipitate out of solution to form minerals. Calculations are made using thermodynamic databases, which include a wide range of data for mineral phases and compounds. The PHREEQC simulation is also capable of incorporating kinetic equations for chemical reactions and determining speciation at any designated time.

For the simulation, it is first required to define the mineralogical phases constituting the starting material. Ordinary Portland Cement (OPC) generally consists of calcium silicates (Ca₃SiO₅ and Ca₂SiO₄), aluminate (Ca₃Al₂O₆), and ferrite (Ca₄(Al_xFe_1−x)₄O₁₀). These components react with water to form hydration products such as amorphous CSH (calcium silicate hydrate), portlandite, ettringite, etc. XRD analysis of the studied material identified the following crystalline constituents: portlandite, calcite, quartz, albite, microcline, and ettringite. However, it is difficult to determine the amount of different phases analytically since XRD only shows materials with crystal structure. Therefore, the amount of amor- phous CSH was determined using thermo-gravimetric analysis (TGA). Figure 9 shows the results of TGA. An abrupt weight loss near 450˚C is associated with the dehydration of portlandite. Weight loss occurring between 700˚C and 800˚C related to the decomposition of calcium carbonate, and a continuous weight loss over the whole range from 105˚C to 500˚C is due to the dehydration of calcium silicate hydrates [24] . When the weight loss in each region was calculated, the content of CSH and portlandite were estimated to be 4.9% and 5.6%, respectively. The content of portlandite was compared with XRD results and normalized to give the estimation of the composition of the sample as shown in Table 4. The CSH composition was assumed to be Ca_1.8SiO_3.8H₂O, which was found to be a good approximation for modeling concrete leaching [25] .

The hydrolysis equilibrium constants of quartz, calcite, microcline, and albite were estimated using the PHREEQC databases. However, CSH, portlandite, and anhydrite were not included in the PHREEQC database, and therefore appropriate values were obtained from research literature [26] [27] and were added to the database. The expected secondary carbonate mineral assemblage due to CO₂ gas injection at 1 atm was based on the PHREEQC database. Table 5 gives the complete list of the initial phase and the newly formed phases with their stoichiometry

reactions and equilibrium constants. Assuming that all reactions have reached equilibrium, extraction and carbonation were calculated through EQUILIBRIUM_ PHASES statement.

Figure 10 shows the pH and the amount of calcium at different stages along the extraction and carbonation process. After the first stage of extraction, the calcium amount from the simulation was 1.3 times higher than that found in the experimental results. This indicates that not all of the calcium was extracted from the waste cement particles, which is to be expected because the particle co- re may still remain intact. After the first stage of carbonation by injecting CO₂ into the solution, the simulation shows 95% calcium removal from the solution by precipitation, whereas in the experiment, only 70% of the calcium was removed.

At the subsequent stages of the extraction and precipitation process, the simulation results showed that the concentration of Ca in the solution rises and falls to the same values in a periodic fashion. At the same time, the experimental results show that the amount of Ca present after the precipitation step increased with each cycle. Correspondingly, the maximum amount of Ca remaining after extraction also increased with each cycle. One possible reason for such aphenomenon is the kinetic/thermodynamic balance as controlled by the amount of Ca²⁺ and CO₂ present in the solution. Calcium carbonate can occur in the form of three anhydrous crystalline polymorphs: vaterite, aragonite, and calcite. Vaterite is not stable under standard conditions and transforms easily into aragonite or calcite when in contact with water. Yet, under particular conditions, vaterite forms and persists without transforming. Brecevic and Kralj [26] reported that vaterite particles were obtained in a very narrow region of Ca²⁺ and CO₂ concentrations at a pH from 9.3 to 9.9. Dickinson et al. [28] reported that the correlation between pCO₂ and Ca²⁺ was one of the most important factors in determining the calcium carbonate precipitation reaction. When they were in complete balance, calcite formed. If the balance was tilted towards high pCO₂, vaterite for- mation was induced but the CaCO₃ precipitation was kinetically controlled. The

precipitation conditions under which the current investigation was conducted were in the range of the aforementioned conditions, which may explain vaterite formations. However, the kinetic aspect was not considered in this simulation, which may be the reason for the discrepancy between the simulation and the experimental results. Currently, databases of reaction kinetics are not sufficient to allow calculation of the vaterite precipitation in detail. However, the results show that it is feasible to use the 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. There are two possible reasons for the deviations: 1) kinetics, and 2) lack of thermodynamic data for the formation of different types of calcium carbonate polymorphs, which requires further study.

Mineral carbonation was conducted using waste cement with NH₄Cl as an extrac- ting agent for preferential carbon dioxide sequestration. The main advantage of using NH₄Cl was that it was regenerative, and it did not require the addition of basic reagents for the precipitation of calcium carbonates. Furthermore, it was de- monstrated that NH₄Cl solutions could sequester carbon dioxide directly from flue gas without the additional expense of a CO₂ capturing step. PHREEQC simulation showed that the calcium extraction and precipitation should operate in a fully periodic fashion. The experimental results showed that the conversion of the precipitation reaction was not fully attained so that the calcium ion concentration increased as the cycle continued. This was possibly due to kinetic/ther- modynamic balance as controlled by the amount of Ca²⁺ and CO₂ present in the solution. However, it is feasible to use the geochemical model to evaluate the mi- neral carbonation process with the correction factors since the deviation between the experimental and the simulation results remained fairly constant throughout the cycle.

Mun, M., Cho, H., Kwon, J., Kim, K. and Kim, R. (2017) Investigation of the CO₂ Sequestration by Indirect Aqueous Carbonation of Waste Ce- ment. American Journal of Climate Change, 6, 132-150. https://doi.org/10.4236/ajcc.2017.61008