Article pubs.acs.org/EF
Mineral Carbonation of Red Gypsum for CO2 Sequestration Omeid Rahmani,*,† Radzuan Junin,† Mark Tyrer,‡ and Rahmat Mohsin§ †
Department of Petroleum Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Johor, Malaysia ‡ Mineral Industry Research Organisation, Wellington House, Starley Way, Birmingham International Park, Solihull, Birmingham B37 7HB, United Kingdom § Department of Renewable Energy Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Johor, Malaysia ABSTRACT: Reduction of carbon dioxide (CO2) emissions into the atmosphere is a key challenge to mitigate the anthropogenic greenhouse effect. CO2 emissions cause lots of problems for the health of humans and increase global warming, in which CO2 uptake decreases these environmental issues. The mineral carbonation process is an alternative method during which industrial wastes rich in calcium (Ca) or magnesium (Mg) react with CO2 to form a stable carbonate mineral. In this research, the feasibility of CO2 mineral carbonation by the use of red gypsum, as a Ca-rich source, was evaluated using an autoclave mini reactor. Wide-range conditions of procedure variables, such as reaction temperature, reaction time, CO2 pressure, and liquid/ solid ratio, on the rate of mineral carbonation were studied. The results showed that the maximum conversion of Ca (98.8%) is obtained at the condition that has an optimum amount of these variables. Moreover, the results confirmed that red gypsum has high potential to form calcium carbonate (CaCO3) during the process of CO2 mineral carbonation. It was concluded that the mineral carbonation process using red gypsum can be considered to be an interesting, applicable, and low-cost method in industry to mitigate a considerable amount of CO2 from the atmosphere, which is the main issue in the current and coming years. mineral carbonation processing. According to Lee et al.,12 1 tonne of gypsum can sequester approximately 0.26 tonne of CO2 by the precipitation of stable carbonate mineral. In addition, no mining is needed using red gypsum for mineral carbonation processing because it is produced in a fine powder form; therefore, the cost of the carbonation process noticeably reduces. Therefore, the main objective of the current study is to develop a technically applicable and feasible process for CO2 mineral carbonation using red gypsum.
1. INTRODUCTION Carbon dioxide (CO2), along with other gases, is released into the atmosphere during fuel combustion, particularly because of the extensive use of fossil fuels for energy production from coal, oil, and natural gas that has occurred since the industrial revolution.1−8 There are some strategies to sequester CO2 and reduce greenhouse gas emissions. Mineral carbonation is one of the proposed methods for CO2 sequestration because of their high availability and low cost and being environmentally benign.9 Indeed, the costs associated with the industrial scale of mineral carbonation are the main retreat of this strategy;10 however, using industrial alkaline wastes could reduce the due costs.11 In mineral carbonation processing, CO2 reacts with calcium (Ca2+) or magnesium (Mg2+) to form solid carbonates.12 The natural minerals, such as olivine, gypsum, and wollastonite, which are rich in Ca2+ or Mg2+, were usually considered as feedstock candidates.9,13−15 Additionally, many industrial wastes, such as lignite fly ash, mining waste, and steel slag containing large amounts of Ca2+/Mg2+, have been evaluated as potential raw materials for CO2 sequestration processing. Comprehensive efforts have been paid to investigate the efficiency of these solid residues to date, and nearly promising results have been achieved.8,12 However, red gypsum is a new Ca-rich feedstock that has not yet been addressed for mineral carbonation processing. This study used red gypsum to enhance the conversion of Ca into a carbonate form in which stable calcium carbonate (CaCO3) was produced. The gypsum usually takes the form of calcium sulfate dihydrate (CaSO4·2H2O) that has a purity of approximately 95%.12 This industrial waste contains approximately 32% CaO that makes it a potential feedstock for © 2014 American Chemical Society
2. MATERIALS AND METHODS 2.1. Materials. The following raw material and chemicals were used in the experiment: red gypsum (as the main raw material), sulfuric acid (H2SO4), ammonium hydroxide (NH4OH), and CO2. More than 3 kg of red gypsum were obtained from the landfill of Terengganu, Malaysia. To determine the main mineral phase of the red gypsum sample, X-ray diffraction (XRD, X’Pert-MPD Philips) was conducted at 2θ angles from 5° to 70° (Figure 1). The XRD results showed that calcium sulfate or gypsum mineral (CaSO4·2H2O) is the dominant component in the sample. Moreover, a chemical composition analysis of red gypsum was conducted using X-ray fluorescence (XRF, PW-1410 Philips) and inductively coupled plasma−mass spectrometry (ICP−MS, 4500 HP) to determine the major and minor elements, respectively (Table 1). XRF analysis of the samples showed that they consist of CaO, SO3, Fe2O3, and TiO2. The Ca, Fe, and S contents of the samples were high and were considered the major elements. The contents of MnO, ZnO, CuO, and Cr2O3, which were minor elements (conducted by ICP− MS), were less than 1 wt %. Received: June 5, 2014 Revised: July 31, 2014 Published: August 21, 2014 5953
dx.doi.org/10.1021/ef501265z | Energy Fuels 2014, 28, 5953−5958
Energy & Fuels
Article
Figure 1. XRD pattern of the fresh sample of red gypsum. Furthermore, the particle sizes of the red gypsum samples were measured with a particle size analyzer (Micrometrics ASAP-2020), and the samples were sieved to less than 75 μm. The morphology of red gypsum was characterized in field emission scanning electron microscopy (FESEM, SU8200 Hitachi, JSM-6701F). The composition of the final product was also determined by ICP−MS. The collected samples were dried in an oven at 45 °C for 24 h to remove surface water but prevent dehydration. To dissolve the Ca and Fe components in red gypsum, 1.5 mol of H2SO4 with a concentration of 35% was used. Subsequently, 2.1 mol of NH4OH was used to precipitate the Fe components and then the Ca components from solution. Pressurized CO2, with a purity of 99.99% in the cylinder, was introduced into an autoclave reactor for the mineral carbonation process. 2.2. Methods. Indirect aqueous mineral carbonation of red gypsum samples were selected as the main route of the carbonation process because the dissolution and carbonation experiments were conducted in two different stages. The feasibility of indirect mineral carbonation of different industrial wastes (e.g., fly ash, steel slag, and waste cement) has been approved in several studies.16−19 In the first stage, iron (hydr)oxide and other impurities were extracted from red gypsum samples. In the second stage, Ca in the solution reacted with CO2 and then stable carbonate mineral was precipitated (Figure 2). At the beginning of the experiment, 10 g of dried red gypsum with a particle size of
