Aqueous Carbonation of Steel Slag: A Kinetics Study

Abstract

In this study, the kinetics of aqueous carbonation of steel slag in an atmospheric three-phase system containing steel slag, water, and CO₂ gas was studied. Also, some factors likely affecting this process were investigated, such as reaction time and temperature, steel slag particle size (d_0.5), CO₂ flow rate, and the mass ratio of liquid to solid (L/S). The particle size of steel slag and the reaction temperature were found to be the major factors affecting the carbonation degree. The carbonation degree was determined to be 26.4% under the following conditions: reaction time of 3 h, temperature of 60°C, CO₂ flow rate of 600 ml/min, d_0.5=12.8 µm, L/S=10, corresponding to a capacity of 0.264 kg CO₂/kg steel slag. The experimental results were employed to study the reaction mechanism using the shrinking core model. The aqueous carbonation process of steel slag was found to be limited by the diffusion of calcium carbonate through the product layer. The apparent activation energy of the aqueous carbonation of steel slag was found to be 4.8 kJ/mol. It was confirmed that aqueous carbonation of steel slag is an effective approach for enhancing the CO₂-sequestration capacity and reducing the environmental impacts of steel slag.

1. Introduction

The increasing atmospheric CO₂ concentration, mainly caused by fossil fuel combustion, is one of the major concerns related to global warming. It is thus imperative to deal with the critical demand of CO₂ emission reduction. CO₂ sequestration is a promising option to reduce CO₂ and alleviate global warming.^1,2,3) Among the currently popular technologies that may contribute to CO₂ sequestration, such as ocean storage, geological sequestration, and sequestration below sea bed, CO₂ mineral sequestration, which is based on the industrial carbonation of Ca/Mg silicates, is one of the most important methods.^4,5) This method can be classified as i) the direct route (the mineral materials are carbonated in one step) and ii) the indirect route (the reactive components are first extracted from the mineral materials by the recycling medium and then carbonated in a separated step).⁶⁾

Mineral carbonation can be performed with pure oxides (e.g., CaO, MgO) as well as olivine ((Mg,Fe)₂SiO₄) and serpentine (Mg₃Si₂O₅(OH)₄)).⁷⁾ Industrial solid waste, which includes steel slag,^{8,9,10,11,12)} cement kiln dust,^13,14) red mud,¹⁵⁾ and coal combustion fly ash,^16,17) has recently attracted the researchers’ interest as possible feedstock. These materials are generally in alkaline and rich in calcium oxide. The major advantages of industrial solid waste are the low cost and widespread availability in industrial areas.

Steel slag is a byproduct of steel production (0.13–0.20 ton slag per ton of steel).¹¹⁾ The content of free CaO and MgO in steel slag must fall below a certain threshold, because of hydration and natural carbonation phenomena, which result in a slag volume increase and cause the disintegration of the construction material and, hence strength loss;¹⁸⁾ these are the most important factors to be considered for engineering applications of steel slag. Accelerated carbonation has been shown to improve the chemical stability and to enhance the mechanical properties of steel slag, which enable its use for concrete aggregate, road-base and green construction materials.¹⁹⁾

The purpose of this study is to investigate the kinetics of the aqueous carbonation of steel slag in an atmospheric three-phase system containing steel slag, water, and CO₂ gas. The key factors of the aqueous carbonation, such as reaction time and temperature, particle size of steel slag, CO₂ flow rate, and the mass ratio of liquid to solid, were also investigated. SEM, XRD, and FTIR were used to characterize fresh and carbonated steel slag. In addition, the kinetics model of aqueous carbonation process of steel slag was studied using the SCM.

¹Abbreviations:   mass ratio of liquid to solid (L/S); the particle size of steel slag (d_0.5); shrinking core model (SCM).

2. Experimental

2.1. Materials

Steel slag used in these experiments was obtained from the iron and steel plant of China. The chemical composition of steel slag was measured by XRF (AXIOS-MAX, PANalytical, Netherlands). After crushing and carrying out the particle-size analysis, three mean particle sizes of steel slag were chosen for the subsequent tests, i.e., d_0.5=12.8 μm, 22.4 μm, and 118.8 μm. Before each experiment, the sample was dried at 105°C for several hours to eliminate the adsorbed water. A high-pressure cylinder of CO₂ gas with a volumetric concentration of 99.9% was provided. Deionized water was used through the overall carbonation experiments.

2.2. Detection Measurements

Several techniques were used to characterize fresh and carbonated steel slag. XRD patterns were obtained with an X-ray diffractometer (Smartlab 9 kw, Rigaku, Japan), which recorded within an angle range of 5–90° 2θ at room temperature using CuKα radiation with the following measurement conditions: tube voltage, 45 kV; tube current, 200 mA.

Particle samples sputtered with Pt were characterized in terms of structure and elemental composition using scanning electron microscope (SEM; JSM-7001F, Electron Company, Japan) with energy-dispersive spectroscopy (EDS; INCA X-MAX, Oxford Instrument, UK) at an accelerating voltage of 15 kV. The BET surface area of fresh and carbonated steel slag were determined using a low temperature N₂-adsorption BET apparatus (Autosorb, Quantachrome, USA).

FTIR was used in the frequency range of 4000–400 cm⁻¹. Discs were prepared by mixing 1 mg sample with 150 mg KBr in an agate mortar and then pressing the mixture at 20 MPa for 1 min.

2.3. Apparatus and Experimental Details

The schematic diagram of the apparatus used in this work is illustrated in Fig. 1. All the tests were carried out under atmospheric pressure. For each carbonation test, the steel slag sample was suspended with deionized water according to a mass ratio of liquid to solid in four-necked round-bottomed flask. The reactor was surrounded by a thermostat-water bath. When the reactor was heated to a specific temperature, carbon dioxide was bubbled directly into the reactor through a flow-rate controller; the slurry was stirred using a mechanical stirrer with a high stirring speed to disperse the steel slag particles and the CO₂ gas. The operational factors are reaction time , particle size of steel slag, reaction temperature, flow rate of CO₂, and the mass ratio of liquid to solid. After the reaction, the slurry was immediately filtered through a sand-core funnel (G4) and heated in an oven at 105°C for further analysis.

Fig. 1.

Schematic diagram of the apparatus for carbonation of steel slag.

2.4. Reaction Mechanism

The aqueous carbonation process of steel slag (main mineral phase is Ca₂SiO₄) can be described as follows:

Dissolution of calcium   

$${\text{Ca}}_{\text{2}}{\text{SiO}}_{\text{4}}\text{(s)}+{\text{4H}}^{+}\text{(aq)}\to {\text{2Ca}}^{\text{2}+}\text{(aq)}+{\text{2H}}_{\text{2}}\text{O(l)}+{\text{SiO}}_{\text{2}}\text{(s)}$$(1)

Dissolution of CO₂ and subsequent conversion of carbonate species   

$$\begin{array}{l}{\text{CO}}_{\text{2}}\text{(g)}+{\text{H}}_{\text{2}}\text{O(l)}\leftrightarrow {\text{H}}_{\text{2}}\text{CO}{}_{\text{3}}\text{(aq)}\\ \leftrightarrow {\text{HCO}}_{\text{3}}{\hspace{0.17em}}^{-}\text{(aq)}\mathrm{\hspace{0.17em} }+{\text{H}}^{+}\leftrightarrow 2{\text{H}}^{+}+{\text{CO}}_{\text{3}}{\hspace{0.17em}}^{\text{2}-}\text{(aq)}\end{array}$$(2)

Precipitation of calcium carbonate and formation of the product layer of calcium carbonate:   

$${\text{Ca}}^{\text{2}+}\text{(aq)}+{\text{HCO}}_{\text{3}}{\hspace{0.17em}}^{-}\text{(aq)}\to {\text{CaCO}}_{\text{3}}\text{(s)}+{\text{H}}^{+}\text{(aq)}$$(3)

  

$${\text{Ca}}^{\text{2}+}\text{(aq)}+{\text{CO}}_{\text{3}}{\hspace{0.17em}}^{\text{2}-}\text{(aq)}\to {\text{CaCO}}_{\text{3}}\text{(s)}$$(4)

2.5. Carbonation Degree

The aqueous carbonation degree of steel slag was quantified by a CS Analyzer and was calculated using the following equation:   

$$\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{\hspace{0.17em} }\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\%={\text{(m}}_{\text{1}}{\text{c}}_{\text{1}}\text{\%}-{\text{m}}_{\text{0}}{\text{c}}_{\text{0}}\text{\%)}\times {\text{M}}_{{\text{CO}}_{\text{2}}}{\text{/(M}}_{\text{C}}\times {\text{m}}_{\text{0}})$$(5)

Where c₀% and c₁% represent the amount of carbon in fresh and carbonated steel slag, respectively; m₀ and m₁ represent the weight of fresh and carbonated steel slag (kg), respectively; M_CO2 is the molecular weight of CO₂ (kg/mol); M_C is the molecular weight of C (kg/mol).

3. Results and Discussion

3.1. Chemical Composition of Fresh Steel Slag

The chemical composition of the steel slag used in this study is presented in Table 1, which shows that the content of CaO is 41.3% (wt%) and that the other components are SiO₂, Fe₂O₃, MgO, MnO, Al₂O₃, and P₂O₅. The CO₂-capturing capacity of the slag material is mainly attributed to CaO.

Table 1. The main composition of the fresh steel slag (mass fraction, %).

Components	CaO	SiO2	Fe2O3	MgO	MnO	Al2O3	P2O5
Concentarion (wt.%)	41.3	20.9	20.7	6.2	3.3	2.3	2.0

3.2. Effect of the Reaction Time

The effect of reaction time on the carbonation degree of steel slag is shown in Fig. 2. Our results indicate that it increases with the reaction time. In particular, in the first 60 min, the carbonation degree of steel slag is 16%, with a further increase of only 13% at reaction time of 10 h. Thus, the carbonation rate of steel slag rapidly increases in the first 60 min and then gradually decreases as the reaction time is extended. Different pH values were determined during the carbonation process and are displayed in Fig. 3. According to our results, the pH of the slurry is larger than 11 in the initial stage of the reaction, due to the hydrolysis of steel slag; it then gradually decreases to 8.5 after 15 min, finally reaching a stable value of approximately 6.5. These results may be explained by the fact that the carbonate ion is mostly abundant at pH >10.3 and is almost non-existent at pH <8.4. Thus, at a pH ranging between 6.5 and 8.4, the bicarbonate ions are the main species. A higher pH results in a larger amount of carbonate ions in the aqueous phase that can then accelerate carbonation.

Fig. 2.

The effect of reaction time on the carbonation degree of steel slag (Reaction time (t): 0–10 h; d_0.5=22.4 μm; L/S=10; CO₂ flow rate=600 ml/min; T=60°C).

Fig. 3.

The pH values of the slurry during the aqueous carbonation process of steel slag.

3.3. Effect of the Reaction Temperature

The effect of the reaction temperature on the carbonation degree of steel slag is shown in Fig. 4. Our results suggest that the carbonation degree of steel slag increases with an increase in the reaction temperature from 20 to 60°C. However, for reaction temperatures ranging between 60 and 80°C, the carbonation degree shows a slight decrease. The reaction temperature is a complex factor to be considered, i.e., it has a negative effect on the dissolution of CO₂ and a positive effect on the leaching of CaO from steel slag. This latter is enhanced by an increase in the reaction temperature, while a higher temperature would affect the dissolution of CO₂. Taken this into consideration, a temperature of 60°C was chosen as the optimal reaction temperature.

Fig. 4.

The effect of reaction temperature on the carbonation degree of steel slag (Reaction temperature (T): 20°C–80°C; d_0.5=22.4 μm; L/S=10; CO₂ flow rate=600 ml/min).

3.4. Effect of Particle Size of Steel Slag

The effect of steel slag particle size (with the particle size being 12.8 μm, 22.4 μm and 118.8 μm) on carbonation is shown in Fig. 5. Our data suggest that the carbonation degree of steel slag increases with the decrease of particle size; this is due to an increase in the specific surface area of the steel slag particle that occurs when the particle size is decreased. The specific surface areas of fresh and carbonated steel slag with different particle sizes are shown in Table 2. These data show that the specific surface areas of carbonated steel slag are larger than those of fresh steel slag.

Fig. 5.

The effect of particle size of steel slag on the carbonation degree of steel slag (d_0.5=12.8, 22.4, 118.8 μm; L/S=10; CO₂ flow rate=600 ml/min; T=60°C).

Table 2. Specific surface area (m²/g) of fresh steel slag and carbonated steel slag.

Steel slag	d0.5=12.8 μm	d0.5=22.4 μm	d0.5=118.8 μm
fresh	1.61	0.94	0.57
carbonated	18.71	14.82	6.00

3.5. Effect of the Mass Ratio of Liquid to Solid

The effect of mass ratio of liquid to solid, which ranges between 5 and 15 g/g, on carbonation is shown in Fig. 6. Our results indicate that when L/S is increased from 5 to 10 g/g, the carbonation degree of steel slag also increases. A further L/S increase (from 10 to 15 g/g) causes a decrease in the carbonation degree of steel slag, due to the presence of excessive liquid, which lowers the calcium ionic concentration in the liquid phase. Therefore, 10 was chosen as the optimal mass ratio of liquid to solid.

Fig. 6.

The effect of the mass ratio of liquid to solid (L/S) on the carbonation degree of steel slag (L/S=5, 10, 15; d_0.5=12.8 μm; CO₂ flow rate=600 ml/min; T=60°C).

3.6. Effect of CO₂ Flow Rate

The effect of the flow rate of CO₂ (from 200 to 800 ml/min) on the carbonation degree of steel slag is shown in Fig. 7. Our results clearly show that the flow rate of CO₂ has no obviously effect on the carbonation degree of steel slag; due to the limited space of the experimental reactor, we could not identify the extent of this effect. Therefore, an appropriate flow rate of CO₂ according to the practical situation was chosen.

Fig. 7.

The effect of the flow rate of CO₂ on the carbonation degree of steel slag (CO₂ flow rate: 200–800 ml/min; d_0.5=12.8μm; L/S=10; T=60°C).

3.7. Characteristics of Fresh and Carbonated Steel Slag

The XRD images (Fig. 8) indicate that the main mineral phases in the fresh and carbonated steel slag are Ca₂SiO₄ and CaCO₃, respectively. This finding confirms that CO₂ sequestration in aqueous solution with steel slag is feasible.

Fig. 8.

XRD pattern of fresh steel slag and carbonated steel slag.

The SEM-EDS images of fresh steel slag shown in Fig. 9 clearly suggest that it has a smooth and compact surface prior to carbonation. After carbonation, the morphology of the particle surface clearly changes. The SEM-EDS images of the carbonated steel slag (Fig. 9) show a multi-hole and irregular product layer compared to the fresh steel slag, i.e., a loose product layer formed after carbonation. According to the SEM-EDS analysis, the main component of the product layer is CaCO_3, in agreement with the XRD analysis.

Fig. 9.

SEM-EDS images (a) fresh steel slag and (b) carbonated steel slag.

Figure 10 shows the FTIR spectra of fresh and carbonated steel slag. The spectra of the latter show strong bands at 1435 cm⁻¹, which can be assigned to the C-O stretching mode of carbonate; also sharp bands at 875⁻¹ cm and 712 cm⁻¹ appear that can be assigned to the out-of-plane bending mode of CO₃.^2,3,4) In agreement with the XRD and SEM-EDS analyses, these bands indicate that a large quantity of CaCO₃ exist in carbonated steel slag.

Fig. 10.

FTIR spectra of fresh steel slag and carbonated steel slag.

The aqueous carbonation process of steel slag also changes the specific surface area of the steel slag particles (Table 2). In addition, the carbonation reaction increases the specific surface area due to the formation of a multi-hole and loose product layer on the surface of steel slag (Fig. 9).

3.8. Kinetics Model of Aqueous Carbonation

In order to confirm the aqueous carbonation process of steel slag, it is essential to establish a quantitative measurement of the aqueous carbonation kinetics. This process is controlled by the slowest step, which is the rate-limiting step of the aqueous carbonation process. The SCM is typically used for the analysis of heterogeneous solid-fluid reactions. A primary assumption under the SCM is that the reaction first occurs at the steel slag outer region and then gradually moves into the inner part, leaving behind completely reacted product, which is defined as the “ash” layer.⁸⁾ Therefore, an unreacted core of steel slag exists at any time that shrinks in size during the carbonation process. The application of this model to the aqueous carbonation process revealed that the reactions proceed according to the following steps: (1) diffusion of the aqueous carbonic acid ions (CO₃²⁻ and HCO₃⁻) through the liquid boundary layer surrounding the solid particles; (2) penetration and diffusion of the reacting ions through the pores in the product layer (CaCO₃) until they reach the surface of the unreacted core; (3) reaction of calcium ions at the surface of the solid particles with aqueous carbonic acid ions.²⁰⁾

According to the SCM, the following two kinetic equations can be applied to the different rate-controlling steps.

Chemical reaction controlled process,   

$$\text{1}-{\text{(1}-x\text{)}}^{\text{1/3}}={\text{k}}_{\text{r}}\mathrm{\hspace{0.17em} }\text{t}$$(6)

Diffusion through ash layer controlled process,   

$$\text{1}+\text{2(1}-x\text{)}-\text{3(1}-x{\text{)}}^{\text{2/3}}={\text{k}}_{\text{d}}\mathrm{\hspace{0.17em} }\text{t}$$(7)

Where k_r, k_d are the rate constants; x is the carbonation degree of steel slag; t is the reaction time.

The experimental conditions of the kinetics experiments were fixed at the mass ratio of liquid to solid of 10, stirring speed of 250 r/min, flow rate of CO₂ of 600 ml/min, and d_0.5=22.4 μm, respectively. The effect of different reaction temperatures on the carbonation degree of steel slag is shown in Fig. 4. The values of 1−(1−x)^1/3 and 1+2(1−x)−3(1−x)^2/3 were calculated according to Eqs. (6) and (7) and to the data displayed in Fig. 4; these were then plotted against the respective reaction time (Fig. 11). According to these data, the aqueous carbonation process of steel slag can be better expressed by the diffusion through the ash-layer-controlled process rather than the chemical-reaction-controlled process. Therefore, the values of 1+2(1−x)−3(1−x)^2/3 were calculated according to Eq. (7) and to the data shown in Fig. 4 and plotted against the respective reaction time (Fig. 12). A good linear relationship for each carbonation temperature with a correlation coefficient (R²) ranging from 0.979 to 0.987 was obtained, which indicates that the reaction is controlled by the ash-layer diffusion mechanism.

Fig. 11.

Carbonation degree of steel slag versus reaction time fitted by two kinds of kinetics equations.

Fig. 12.

Plots of 1+2(1−x) −3(1−x)^2/3 versus reaction time at different carbonation temperatures.

The relationship between the constant temperature and k_d obtained from Eq. (8) can be expressed by the Arrhenius equation:   

$${\text{k}}_{\text{d}}=\text{Aexp[}-{\text{E}}_{\text{a}}\text{/RT]}$$(8)

Where k_d is the overall rate constant, A is the frequency factor, E_a is the apparent activation energy, R is the gas constant (8.314 J/(K·mol)), and T is the carbonation temperature.

The value of k_d was obtained at different carbonation temperatures; the lnk_d values were then plotted against 1/T, giving a straight line, with a correlation coefficient of 0.972 (Fig. 13). From the slope of this plot, the apparent activation energy of the aqueous carbonation process of steel slag was calculated to be 4.8 kJ/mol.

Fig. 13.

Natural logarithm of reaction rate constant versus reciprocal reaction temperature.

4. Conclusions

This study investigated CO₂ sequestration via carbonation of steel slag in a three-phase system, containing steel slag, water, and CO₂ gas, at ambient pressure in order to reduce CO₂ concentration and enhance the mechanical properties of steel slag. The reaction rates of direct carbonation were quantified to yield the reaction parameters and clarify rate-limiting mechanisms, as listed subsequently:

(1) The chemical and mineralogical characterization indicates that steel slag is rich in calcium oxide, mainly in the form of Ca₂SiO₄, which exhibits a high potential reactivity with CO₂. Based on our experimental results, the aqueous carbonation degree of steel slag is 26.4% under the following conditions: reaction time of 3 h, temperature of 60°C, CO₂ flow rate of 600 ml/min, d_0.5=12.8 μm, L/S=10, corresponding to a capacity of 0.264 kg CO₂/kg steel slag.

(2) The particle size of steel slag and the reaction temperature are the major factors that affect the carbonation degree. Other factors, such as the mass ratio of liquid to solid (L/S) and CO₂ flow rate, have a minor influence.

(3) Results obtained from SEM, XRD, and FTIR indicate that CaCO₃ is formed on the surface of steel slag during the aqueous carbonation process. The specific surface area of the steel slag particles also increases during the process. The product layer of CaCO₃ possesses a highly irregular and multi-hole surface.

(4) The results obtained from SEM-EDS and XRD demonstrate the change in the surface composition and structure of steel slag during the aqueous carbonation process. The kinetics of the aqueous carbonation of steel slag, which can be described by the SCM, is controlled by the diffusion of calcium carbonate through the product layer; the apparent activation energy of the process is 4.8 kJ/mol.

Acknowledgements

The present work has been supported by the National Nature Science Foundation of China (Grant No. 21106167 and No. 51104140) and National Science Foundation for Distinguished Young Scholars of China (Grant No. 51125018 ). The authors also thank Prof. Jiajun Ke for revising this paper.

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