Identification of suitable conventional cooling methods for direct aqueous carbonation of blast furnace slags and their mechanism

Abstract

The iron and steel industries generate large amounts of unavoidable CO2 emissions as well as considerable quantities of slags. More than one-half of the emitted CO2 is produced in blast furnaces during ironmaking, and thus it is meaningful to use blast furnace slags to capture CO2 while addressing the byproducts and flue gas of ironmaking. Mineral carbonation of slags is a promising route to achieve carbon neutrality and effective slag utilization. To exploit slag more effectively and capture CO2 in flue gas, an in-depth investigation into the carbonation of blast furnace slags generated with different cooling methods was conducted. The effects of the solid–liquid ratio and introduced CO2 concentration on carbonation were determined. The CO2 uptake capacity of air-cooled slag (0.04 g/g) was greater than that of water-quenched slag. The CO2 uptake capacities of the two slags were comparable with those of slags in previous works, indicating the potential of the two slags for CO2 sequestration and utilization even with low-energy input and this fact suggests that this process is feasible.

1. Introduction

Climate change is an urgent issue that requires considerable efforts to achieve alleviation. Emissions of greenhouse gases significantly contribute to this problem, with CO2 emissions accounting for approximately three-quarters of these emissions [1]. Reducing CO2 emissions is therefore a top priority for addressing climate change and achieving the goal of carbon neutrality. Of note, the iron and steel industries account for approximately 9% of greenhouse gas emissions and are regarded as primary targets [2]. Recent studies have formulated decarbonatization countermeasures and pointed out that carbon neutrality in the steel industry can only be achieved through the simultaneous development of multiple solutions, including electrification, hydrogen energy development, waste energy recycling, and CO2 sequestration and utilization [3–4].

In the iron and steel industries, considerable quantities of slags are generated as byproducts of pig iron and steel production. The amount of generated blast furnace slags is approximately 30% of the total amount of pig iron produced, reaching 21.8 million metric tons in Japan in 2021 [5]. Generated blast furnace slags can be simply divided into water-quenched slag and air-cooled slag according to the different cooling methods. Water-quenched slag is popularly used to replace Portland cement in cement manufacturing [6]. This utilization is promising because it decreases the amount of CO2 generated from clinker production, which indirectly contributes to reducing CO2 emissions [7]. Air-cooled slags are generally used in roadbed applications [8]. Slag utilization via mineral carbonation, which also uses and sequesters CO2, is a promising route to achieving the goal of carbon neutrality [9]. Mineral carbonation uses alkali Ca or Mg to sequester CO2 gas as stable solids (i.e., CaCO3 or MgCO3) [10–11]. During mineral carbonation, alkaline wastes or byproducts, such as concrete fines [12–14], concrete sludge [15–16], coal fly ash [17–19], mining waste [20], and steelmaking slag [21–23], react with CO2 to generate carbonated products [24]. This method treats both wastes and exhaust CO2 gas from industries, which considerably reduces CO2 emissions [25–28].