The CO₂ sequestration by supercritical carbonation of electric arc furnace slag

Highlights

• A supercritical carbonation process was developed to sequester CO₂ using steel slag.

• The main contributing factor to CO₂ sequestration is the slag particle size.

• The optimized process results in 213 gCO₂/kg CO₂ uptake of EAF slag in 48 h.

• Fundamental investigations were performed to elucidate the reaction mechanisms.

Abstract

The current study puts the emphasis on developing a supercritical carbonation process of electric arc furnace slag for the CO₂ sequestration. In this process, the feed is mixed with water and processed with supercritical CO₂ at 40−80 °C and 80–120 bar for 48 h. This process offers several advantages including higher reactivity, faster kinetics, less waste generation, better economic feasibility, and less pretreatment compared with aqueous carbonation. The empirical model established by using a systematic design of experiments methodology is demonstrated to be suitable for calculating the carbonation efficiency as a function of operating parameters and performing the process optimization. With fundamental investigations into carbonation mechanisms, the slag particle size is determined to have the most significant positive impact on carbonation efficiency. The sample with a smaller particle size has a higher total carbonated volume, resulting in higher CO₂ uptake. Fundamental investigations into the carbonation process indicate that the diffusion barrier to the carbonation process is a product calcium carbonate layer formed on the outer shell of the slag particle and the thickness of this layer is constant regardless of the slag particle size. Furthermore, it was revealed that the reaction mechanism of the carbonation process is mainly driven by the hydrogen ion which reacts with dicalcium silicate in EAF slag and produces dissolved calcium ions. In this study, the reaction pathway for supercritical carbonation of EAF slag is elucidated and the results of mechanistic investigations shed light on the diffusion barrier to the carbonation process and the rate-determining step. We expect the results of this study help enable the carbonation of industrial byproducts, in particular, steelmaking slag.

Introduction

Mineral carbonation of industrial wastes is a compelling carbon capture and storage technique to mitigate anthropogenic CO₂ emissions while stabilizing and inertizing waste materials. Electric arc furnace (EAF) slag is the main byproduct of the steelmaking industry and it is a potential source of alkaline oxides such as calcium oxide, magnesium oxide, and iron oxide, which can be transformed into carbonates [1,2]. In general, EAF slag comprises of 25−60 wt% CaO, 3−13 wt% MgO, 7−22 wt% total Fe oxide, 10−18 wt% SiO₂, 1−13 wt% Al₂O₃, and 1−4 wt% P₂O₅ [[3], [4], [5]]. Because of its advantageous properties including high strength, density, and wear resistance, EAF slag finds its wide applications as an aggregate for concrete, road base and fills, cement clinker, asphalt, and railway ballast in the construction industry [6]. However, the hydration of CaO and MgO in the EAF slag, called the “free lime volume instability” process, results in volume expansion and fracture of the construction material made of EAF slag, limiting its application to a fertilizer [[7], [8], [9]]. The carbonation of EAF slag proves to be a great countermeasure to improve the volume stability of the slag and consequentially to produce enhanced construction materials with better inertness and corrosion resistance. Above all, significant amounts of CO₂ emitted from the steelmaking process can be sequestrated at the point sources using the EAF slag carbonation, reducing the carbon footprint. The steelmaking industry generated 1.85 tonnes of CO₂ for every tonne of steel produced in 2018 and it accounts for 7–9 % of the direct CO₂ emissions from global fossil fuel use [9,10]. Because carbonate products provide enough carbon storage capacity over geological time scales, the carbonation of slag has been on the rise as a breakthrough technique to mitigate the overall CO₂ gas emissions resulting from the steelmaking process.

There are several previous studies on the aqueous carbonation of steelmaking slags including EAF slag, basic oxygen furnace (BOF) slag, and ladle furnace (LF) slag, an overview of which is presented in Supplementary Material Table S1a. Although they demonstrate the process feasibility and high stability of the products, the implementation of this process is still limited because of several challenges. Despite being thermodynamically favorable, the slow kinetics of the carbonation reaction results in low CO₂ uptake at a given time. Furthermore, the small particle size and hence high comminution cost are unavoidable for enhancing carbonation efficiency because of the passivating layer formed on the outer shell of the slag particles [11]. These result in low process efficiency and huge energy consumption, hindering the scale-up applications of the aqueous carbonation process. To address these challenges, we performed the carbonation process in supercritical CO₂.

Supercritical CO₂ (sc-CO₂) is one of the most widely used supercritical fluids because it is inert, non-toxic, recyclable, and cost-effective with relatively low critical pressure and temperature (73.6 bar and 31.1 °C, respectively). It has high diffusivity and low viscosity; thus, it can diffuse into smaller pores that the CO₂-dissolved aqueous phase cannot enter. Moreover, sc-CO₂ is known to have a higher reactivity to mineral carbonation than aqueous CO₂ [12,13]. Therefore, supercritical carbonation of EAF slag offers several advantages including higher reactivity, faster kinetics, less waste generation, better economic feasibility, and less pretreatment compared with aqueous carbonation. The supercritical carbonation process has been utilized to sequestrate CO₂ with various minerals and construction materials, in particular, cement paste. However, to the best of our knowledge, no previous study has been devoted to the supercritical carbonation of steelmaking slag. Table S1b shows the details of the previous studies on supercritical carbonation focusing on the feed material, carbonation method, and operating conditions.

During the carbonation process of larnite (Ca₂SiO₄) as an example, the following reactions along with CO₂ dissolution (CO_{2 (g or sc)} = CO_{2 (aq)}) and dissociation reactions (CO_{2 (aq)} + H₂O _(l) = H₂CO_{3 (aq)} = H⁺ _(aq) + HCO₃⁻ _(aq) = 2H⁺ _(aq) + CO₃²⁻ _(aq)) are expected to take place.

Ca₂SiO_{4 (s)} + 4H⁺ _(aq) = 2Ca²⁺ _(aq) + H₄SiO_{4 (aq)}

Ca²⁺ _(aq) + HCO₃⁻ _(aq) = CaCO_{3 (s)} + H⁺ _(aq)

Ca₂SiO_{4 (s)} + 2CO_{2 (aq)} + 2 H₂O _(aq) = 2CaCO_{3 (s)} + H₄SiO_{4 (aq)}

In this mechanism, H⁺ ion renders the Ca²⁺ ion in the matrix to dissolve into the water medium. The free Ca²⁺ ion then combines with HCO₃⁻ ion and forms calcium carbonate, leaving H⁺ ion behind. The overall reaction is presented in Reaction (3).

However, O’Connor et al. [14] proposed a different reaction pathway in which a mineral particle directly reacts with HCO₃⁻ ion, which is then regenerated through the reaction between CO₂ and OH⁻ ion, based on the experimental results showing a much shorter reaction time with the same carbonation efficiency when NaHCO₃ was added to the system (Reactions (4) and (5)).

Ca₂SiO_{4 (s)} + 2HCO₃⁻ _(aq) + 2 H₂O _(aq) = 2CaCO_{3 (s)} + H₄SiO_{4 (aq)}+ 2OH⁻ _(aq)

OH⁻ _(aq) + CO_{2 (aq)} = HCO₃⁻ _(aq)

Regardless of the reaction mechanism, carbonated products and silicic acid (or free silica depending on pH) are produced during the carbonation process. However, it is still unclear which one forms the product layer on the slag particle that mainly behaves as a passivating barrier of the succeeding carbonation [[15], [16], [17], [18]]. Moreover, the kinetics of the carbonation reactions are not fully understood. One of these steps can be a rate-determining step – CO₂ dissolution, CO₂ diffusion through a boundary layer, CO₂ diffusion through product layer, dissolution of calcium or magnesium, and precipitation of carbonates.

To this end, we performed systematic and fundamental investigations on the supercritical carbonation of EAF slag with the objective of sequestrating CO₂. A response surface methodology was utilized to design the experiments, to assess the effect of operating parameters, namely particle size of the slag, CO₂ pressure, reaction temperature, and water to slag ratio, on the carbonation efficiency and to optimize the process. Furthermore, fundamental investigations were carried out to gain a better understanding of the effect of slag particle size and reaction time. The results enabled to unravel of the carbonation reaction mechanisms with emphasis on the type and thickness of diffusion barrier, rate-determining step, and reaction pathway. We believe the findings of this study would help develop comprehensive insight into the carbonation process and thereby contribute to the efficient CO₂ mitigation along with the valorization of steelmaking slag.