Journal of Hazardous Materials 195 (2011) 107–114 Contents lists available at ScienceDirect Journal of Hazardous Materials jou rn al h om epage: www.elsevier.com/loc ate/jhazmat CO 2 sequestration by carbonation of steelmaking slags in an autoclave reactor E.-E. Chang a , Shu-Yuan Pan b , Yi-Hung Chen c , Hsiao-Wen Chu b , Chu-Fang Wang d , Pen-Chi Chiang b,∗ a Department of Biochemistry, Taipei Medical University, Taipei, Taiwan b Graduate Institute of Environmental Engineering, National Taiwan University, No. 71 Chou-shan Rd., Taiwan c Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan d Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Taiwan a r t i c l e i n f o Article history: Received 13 May 2011 Received in revised form 3 August 2011 Accepted 4 August 2011 Available online 10 August 2011 Keywords: Accelerated carbonation Alkaline solid waste Calcite Surface coverage model Life cycle assessment a b s t r a c t Carbon dioxide (CO 2 ) sequestration experiments using the accelerated carbonation of three types of steelmaking slags, i.e., ultra-fine (UF) slag, fly-ash (FA) slag, and blended hydraulic slag cement (BHC), were performed in an autoclave reactor. The effects of reaction time, liquid-to-solid ratio (L/S), temper- ature, CO 2 pressure, and initial pH on CO 2 sequestration were evaluated. Two different CO 2 pressures were chosen: the normal condition (700 psig) and the supercritical condition (1300 psig). The carbona- tion conversion was determined quantitatively by using thermo-gravimetric analysis (TGA). The major factors that affected the conversion were reaction time (5 min to 12 h) and temperature (40–160 ◦ C). The BHC was found to have the highest carbonation conversion of approximately 68%, corresponding to a capacity of 0.283 kg CO 2 /kg BHC, in 12 h at 700 psig and 160 ◦ C. In addition, the carbonation products were confirmed to be mainly in CaCO 3, which was determined by using scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) to analyze samples before and after carbonation. Furthermore, reac- tion kinetics were expressed with a surface coverage model, and the carbon footprint of the developed technology in this investigation was calculated by a life cycle assessment (LCA). © 2011 Elsevier B.V. All rights reserved. 1. Introduction Carbon sequestration is a promising option for reducing car- bon dioxide (CO 2 ) emissions and alleviating global warming. Both CO 2 captured from emission sources and subsequent transport of the captured CO 2 to isolated reservoirs are essential for car- bon sequestration. Carbon capture is affected by environmental factors, capacity, and cost. Mineral sequestration is a method of carbon capture that accelerates the natural weathering of sili- cate minerals, allowing them to react with CO 2 to form stable products, carbonate minerals, and silica for further usage or dis- posal [1]. In addition, carbonation is an exothermal reaction; thus, energy consumption and costs may be limited by its inherent properties [1,2]. In all cases, the sequestration chemicals must provide base ions such as monovalent sodium and potassium, or divalent calcium and magnesium ions to neutralize the carbonic acid. Other carbonate-forming elements such as iron carbon- ates are not practical due to their unique and precious features [3]. In addition to controlling the reaction conditions, choosing suitable mineral feedstocks and properly designing the reac- tor are crucial to achieving high CO 2 sequestration efficiencies. ∗ Corresponding author. Tel.: +886 2 23622510, fax: +886 2 23661642. E-mail address: pcchiang@ntu.edu.tw (P.-C. Chiang). One possible feedstock for CO 2 sequestration by accelerated carbonation is industrial solid waste, including steelmaking slags, combustion residues, and fly ash, which generally are alkaline and rich in calcium. The use of industrial waste is advantageous because of its low cost and widespread avail- ability in industrial areas [4]. Interest in using industrial alkaline solid wastes as sources of calcium or magnesium oxide for CO 2 sequestration has arisen because these mate- rials are readily available, cheap, and usually produced near large-emission sources of CO 2 [5]. In this study, carbona- tion reactions were performed primarily via the reaction of CO 2 with raw CaO-based materials, and calcium carbonate (CaCO 3 ) was observed to be the predominant carbonation product [6]. The use of this material simultaneously can reduce the amount of waste and neutralize a hazardous material. The objectives of this study were to investigate the car- bonation of several steelmaking slags, including ultra-fine (UF) slag, fly-ash (FA) slag, and blended hydraulic slag cement (BHC), in an autoclave reactor. The effects of the operational conditions, including the type of steelmaking slag, reaction time, liquid-to-solid ratio (L/S), temperature, CO 2 pressure, and initial pH, on the performance of the carbonation pro- cess were evaluated. In addition, reaction kinetics of the carbonation process were tested using a surface coverage model. 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.08.006