Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 2783 – 2786
The 6th International Conference on Applied Energy – ICAE2014
Carbon Dioxide Mineral Carbonation Through pH-swing Process: A Review Amin Azdarpoura,b,*, Mohammad Asadullahb, Radzuan Junina, Muhammad Manana, Hossein Hamidib, Ahmad Rafizan Mohamad Daudb a
Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, 81310, Malaysia b Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, 40100, Malaysia
Abstract The promotion of carbon dioxide (CO 2) reduction methods is due to the fact that the CO2 concentration has been increasing rapidly in 21st century. In order to prevent further damage of the environment caused by greenhouse gases, CO 2 concentration should be stabilized by increasing CO 2 fixation, which can reduce CO2 emission into the atmosphere. M ineral carbon sequestration or mineral carbonation is the process of utilizing minerals, mostly rich in calcium and magnesium (Ca/M g) as the feedstock in reaction with CO2 to produce stable solid carbonates. M ineral carbonation through indirect pH swing process is a very effective method for producing calcium and magnesium carbonates. The Ca/M g ions are extracted out of feedstock using suitable solvents at low pH condition and then in the second step the leached Ca/M g ions are carbonated at elevated pH condition. In this paper the state-of-the-art of the carbonation involving pH swing method is updated.
© Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2014 2014The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: Carbon capture and storage; Mineral carbonation; Greenhouse gases, Carbon dioxide, pH swing, Carbon ation efficiency
1. Introducti on Fossil fuels by providing mo re than 86% of the world energy are the main energy sources in the world. Co mbustion of these huge amounts of fossil fuels emit g reenhouse gases (GHGs) and in particu lar carbon dio xide (CO2 ) into the at mosphere. Continuous increment of g lobal CO2 due to co mbustion of fossil fuels leads to steady rise of global mean temperature. These activit ies have resulted in increasing the CO2 concentration fro m 280 pp m in the 1750s to 398 ppm in 2013. As a consequence, the global climate has
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1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.311
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been changed to unfavorable situation for hu man and an imal being. Th is alarming situation warns to reduce the CO2 pumping into the atmosphere [1-4]. Mineral carbon dio xide sequestration is an exothermic chemical reaction of a metal–bearing o xide (usually calciu m, magnesiu m, or iron) with CO2 to form stable solid carbonates. Carbonation can take place either in -situ or ex-situ [5]. In-situ carbonation is the reaction of CO2 with magnesium and calciu m mineral at underground where CO2 is being injected and ex-situ carbonation is the same reaction takes place above ground in a chemical processing plan t [6,7]. The CO2 mineralization, or mineral carbonation is an artificial rock weathering and was first proposed by Seifritz in 1990 whereas natural rock weathering is a geological t ime scale process [8]. Mineral carbonation provides a permanent and leakage–free CO2 disposal in such that the produced carbonate is environmentally benign and stable [9]. The p roduced carbonates are also profitable because calcium and magnesium carbonates are widely used industrially such as in papers, paints, plastics, adhesives, sealants, cosmetics, flooring, fireproofing and fire– extinguishing industries [10]. Alkaline earth metals such as calcium and magnesium are the most favorable minerals for mineral carbonation. However, these minerals are usually rare in nature due to th eir high react ivity, and they usually appear in the form of silicates. In addition to the natural minerals, industrial solid residues and wastes rich in magnesium and calciu m are also potential materials to be used as carbonation feedstocks [5]. This paper provides an overview of the experimental on mineral carbonation through pH swing process. 2. Why pH swing? In the pH swing process Ca/Mg extract ion and carbonate precipitation rate can be enhanced by the addition of acids and bases. The addition of acid decreased the pH of solution and, therefore, imp roved the metal ion extraction rate, wh ile in the second step; the pH was increased due to the addition of bases to the solution, resulting in the improvement of degree of carbonation and precipitation. It is believed that the optimum pH for the aqueous carbonation is around 10.
3. pH swing experiments Park and Fan (2004) investigated the physical activation of serpentine and then carbonation through a pH swing process. Prepared samples were dissolved in a mixture of orthophosphoric acid, oxalic acid, and EDTA at 70 °C. Then the slurry was filtered in order to separate the solid phase (rich in SiO2 ) fro m solution (Mg and Fe rich). The Fe and Mg exist in a clear solution and thus it is difficult to separate them fro m each other. Ho wever, when NH4 OH was added to the solution Fe was precipitated at pH around 8.6. In this case, NH4OH was added controlly, maintain ing the solution pH 8.6, while Fe(OH) 3 was precipited completely. The precipitated iron hydro xide was removed fro m the solution and the pH was further increased to about 9.5 and CO2 was injected to the Mg rich solution. Finally the precipitated solid that is MgCO3 was collected and further analyzed with TGA and XRD. Although the produced iron oxide and MgCO3 solids were highly pure, the overall cost of process is still very high [11]. Katsuyama et al. (2005) developed a high purity calciu m carbonate production system fro m waste cement through a two-step aqueous process. Calciu m ion was first extracted fro m the cement and water slurry at a high CO2 pressure condition (30 bar) and then in the second step calcium carbonate was precipitated by CO2 pressure reduction to 1 bar at ambient temperature. Experimental results show that even after 10 minutes of reaction, significant amount of calciu m ions were extracted fro m cement slurry. In addition, produced CaCO3 were mo re than 98% pure. They also estimated the overall cost of the
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process and concluded their findings in such a way that the cost of calciu m carbonate production is about US$136/tone [12]. Teir et al (2007) investigated the purity of produced MgCO 3 through a pH swing process. The magnesiu m ions were extracted in the first step using HCl or HNO 3 and then in the second step the solutions contacted gaseous CO2 for carbonate precipitation. In the second step, NaOH was added to the solution to increase the solution pH and improve the precipitation rate. Serpentine dissolution carried out in a glass reactor at the temperature of 70 °C. After this step, solutio ns brought into contact with the CO2 at atmospheric p ressure and amb ient temperature with variab le concentration of NaOH. The HCl showed to be more effective than HNO3 in ext racting magnesium ions fro m serpentine since 93% of Mg was extracted using HCl and this value was 88% when HNO3 was used. Hydromagnesite with approximately 94% purity was recovered as the final product. Although relatively high conversion rate was achieved in both extraction and precipitation steps, the high requirements of NaOH (2.4 tonne per tonne CO2 stored) and make-up acid (2– 4 tonne per tonne CO2 stored) seem to be the largest obstacles to overcome for application of this approach as a CO2 storage process [13]. Kodama et al., (2008) also developed a new p H-swing CO2 mineralization process with a recyclable reaction solution. The process involves calcium carbonation by using a weak base -strong acid system. The pH swing process consists of two steps including calciu m ext raction fro m the reaction of steelmaking slag and ammoniu m chloride in the first step and CaCO3 precipitation in the second step. Samples with average size of less than 63 to 2000 micron were prepared and undergone pH swing process at 40, 60, 80 and 90 °C and atmospheric pressure. After completing the reaction, the samples were collected and analyzed with SEM and XRD. Carbonation through this process resulted in calciu m carbonate production with 98% purity. They also concluded that CO2 sequestration capacity of steel slag is about 0.16 kg CO2 /kg slag [14]. Wang and Maroto-Valer (2011) investigated the CO2 capture and mineral carbonation by using recyclable ammon iu m salts through a pH swing process. They stated that their process integrates CO2 capture with mineral carbonation by emp loying NH 3 , NH4 HSO4 , and NH4 HCO3 in the capture, mineral dissolution, and carbonation steps, respectively. They concluded that the mass ration of Mg/NH4 HCO3 /NH3 is one of the main parameters affect ing carbonation efficiency. The maximu m Mg conversion of 95.5% is achieved with the optimum mass ratio of 1:4:2 in this work [15]. 4. Conclusions The pH swing process was proposed to further increase the carbonation efficiency through indirect pH regulating process. In the first step, the solution pH is lowered to improve the leaching of metal ions fro m feedstocks and then in the second stage solution pH is increased to alkaline conditions to enhance the precipitation rate of calciu m and magnesium carbonates. Experimental researches have shown that pH swing method could be a potential method for produ cing high purity carbonates in industrial scale. However, the method still requires a significant research for process improvements and developments. Acknowledgements This research is financially supported by the Research Management Institute, Universiti Teknologi Mara under the project No. 600-RMI/DA NA5/3/ RIF(383/ 2012) and 600-RMI/DANA 5/3/ RIF(549/2012), and Ministry of Higher Education, Malaysia under the project No. 600-RMI/FRGS/ 5/3(98/2013) and 600RMI/FRGS 5/3 (90/2013). References [1] Azdarpour A, Asadullah M, Jnin R, Manan M, Hamidi, H, Mohammadian, E. Direct carbonation of red gypsum to produce solid carbonates. Fuel Processing Technology 2014; 126: 429-434.
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Biography AMIN AZDA RPOUR awarded his Msc in 2011 fro m the Universiti Teknologi Malaysia. He has been doing his PhD at Universiti Teknologi Malaysia since 2011.
