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Mechanisms of Aqueous Wollastonite Carbonation As a Possible CO2

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Mechanisms of Aqueous Wollastonite Carbonation As a Possible CO2 ECN-RX*-06-056 Mechanisms of aqueous wollastonite carbonation as a possible CO~ sequestration process W.J.J. Huijgen R.N.J. Comans To be published in Chemical Engineering Science (2006) Revisions Made by: ~ -_ E~N Biomass, Coal & Environmental Research W.J.J. Huijgen Checked by: Environmental Risk Assessment February 2006 Mechanisms of Aqueous Wollastonite Carbonation as a Possible CO2 Sequestration Process Wouter J.J. Huijgen1, Geert-Jan Witkamp2 & Rob N.J. Comans1,3 (*) 1) Energy Research Centre of The Netherlands, Clean Fossil Fuels, P.O. Box 1, 1755 ZG Petten, The Netherlands 2) Delft University of Technology, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands 3) Wageningen University, Department of Soil Quality, P.O. Box 8005, 6700 EC Wageningen, The Netherlands (*) Corresponding author: Energy Research Centre of The Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands; phone: +31 224564218; fax: +31 224568163; e- mail: [email protected]. ECN-RX--06-056 1 Abstract The mechanisms of aqueous wollastonite carbonation as a possible carbon dioxide sequestration process were investigated experimentally by systematic variation of the reaction temperature, CO2 pressure, particle size, reaction time, liquid to solid ratio and agitation power. The carbonation reaction was observed to occur via the aqueous phase in two steps: (1) Ca leaching from the CaSiO3 matrix and (2) CaCO3 nucleation and growth. Leaching is hindered by a Ca-depleted silicate rim resulting from incongruent Ca-dissolution. Two temperature regimes were identified in the overall carbonation process. At temperatures below an optimum reaction temperature, the overall reaction rate is probably limited by the leaching rate of Ca. At higher temperatures, nucleation and growth of calcium carbonate is probably limiting the conversion, due to a reduced (bi)carbonate activity. The mechanisms for the aqueous carbonation of wollastonite were shown to be similar to those reported previously for an industrial residue and a Mg-silicate. The carbonation of wollastonite proceeds rapidly relative to Mg-silicates, with a maximum conversion in 15 min of 70% at 200 °C, 20 bar CO2 partial pressure and a particle size of <38 µm. The obtained insight in the reaction mechanisms enables the energetic and economic assessment of CO2 sequestration by wollastonite carbonation, which forms an essential next step in its further development. Keywords environment; chemical processes; reaction engineering; precipitation; carbon sequestration; wollastonite ECN-RX--06-056 2 1. Introduction Various carbon dioxide capture and storage technologies are being studied worldwide in order to mitigate global warming in the relatively short term. Mineral CO2 sequestration is a chemical storage route in which carbon dioxide is bound in a carbonate mineral (e.g., Lackner, 2002; Park and Fan, 2004; IEA GHG, 2005). The basic concept of this technology is deduced from the natural weathering of Ca/Mg- silicates. For wollastonite (CaSiO3), the overall weathering reaction can be written as: CaSiO3 (s) + CO2 (g) ® CaCO3 (s) + SiO2 (s) (1.) Potential advantages of mineral CO2 sequestration are the permanent and inherently safe storage of CO2 due to the thermodynamically stable nature of the carbonate product formed and the vast sequestration capacity because of the widespread and abundant occurrence of suitable feedstock (Lackner, 2002). In addition, carbonation is an exothermic process, ΔHr = -87 kJ/mol for wollastonite (Lackner et al., 1995), which potentially reduces the overall energy consumption and costs of carbon sequestration. However, natural weathering processes are slow with timescales at atmospheric conditions of thousands to millions of years. For industrial implementation, a reduction of the reaction time to the order of minutes has to be achieved by developing alternative process routes. Ca/Mg-silicates that are suitable as feedstock for mineral CO2 sequestration include primary minerals, such as wollastonite (CaSiO3) and olivine (Mg2SiO4), and alkaline solid residues such as steel slag (Huijgen and Comans, 2003). In a previous paper, we have reported the reaction mechanisms of mineral CO2 sequestration by aqueous steel slag carbonation (Huijgen et al., 2005b). In the present study, we have extended our ECN-RX--06-056 3 research to primary minerals. Wollastonite was selected as model feedstock for our carbonation experiments, because Ca-silicates tend to be more reactive towards carbonation than Mg-silicates (Huijgen and Comans, 2003; Lackner et al., 1997), although suitable deposits are limited relative to the world-wide abundance of Mg- silicates (Lackner, 1995). In addition, the choice for a Ca-silicate enables the direct comparison to the carbonation of Ca-rich alkaline solid residues such as steel slag. Several process routes for industrial mineral CO2 sequestration have been reported. The so-called aqueous carbonation route (O'Connor et al., 2005) has been selected as the most promising route in a recent review (Huijgen and Comans, 2003; IEA GHG, 2005). In this process, carbonation occurs in a gas-solid-water slurry, which increases the reaction rate substantially when compared to direct gas-solid carbonation. Process steps within the aqueous carbonation route are: 1. Leaching of Ca: + 2+ CaSiO3 (s) + 2 H (aq) ® Ca (aq) + H2O (l) + SiO2 (s) (2.) 2. Dissolution of CO2 and subsequent conversion of (bi)carbonate species: - + CO2 (g) + H2O (l) ® H2CO3 (aq) ® HCO3 (aq) + H (aq) (3.) 3. Nucleation and growth of calcium carbonate: 2+ - + Ca (aq) + HCO3 (aq) ® CaCO3 (s) + H (aq) (4.) A limited number of studies on wollastonite carbonation for CO2 sequestration has been published so far (Kojima et al., 1997; Wu et al., 2001; O'Connor et al., 2005). These studies have demonstrated that (1) the leaching of Ca from the CaSiO3 matrix (eq. 2) is the rate-limiting reaction step at the conditions applied and that (2) this step can be enhanced by e.g. increasing the specific surface area of the wollastonite. ECN-RX--06-056 4 However, two of these studies (Kojima et al., 1997; Wu et al., 2001) focus on carbonation at low CO2 pressure and low temperature and reported reaction times are, consequently, much too long for industrial application. O'Connor et al. (2005) have studied the carbonation of various silicate minerals at elevated temperature and pressure, including a limited number of experiments with wollastonite, which confirm the higher reactivity of Ca- relative to Mg-silicates. In the present paper, we present an experimental study on the mechanisms of wollastonite carbonation at elevated temperature and pressure in support of the development of a rapid carbon dioxide sequestration process. The rate-determining reaction steps are identified and compared to those reported earlier for other feedstock. Finally, routes for further research on aqueous wollastonite carbonation are indicated. 2. Materials and Methods 2.1 Wollastonite characteristics Wollastonite with size class <7 mm (Casiflux A 7020) was obtained from Ankerpoort B.V., Maastricht, The Netherlands. The particle size distribution of the wollastonite was determined by laser diffraction (Malvern Mastersizer 2000) (Figure 1). Scanning Electron Microscope (SEM) analysis of a wollastonite sample (<106 µm) showed needle-shaped particles consistent with its crystal structure. X-ray powder diffraction (XRD) (Bruker, D8 advance) identified (Fe)-wollastonite, with best fitting formula (Ca0.96Fe0.04)SiO3 and traces of lime (CaO) and calcite (CaCO3), but no crystalline ECN-RX--06-056 5 SiO2 (Figure 2). The calcium carbonate content of the fresh wollastonite (<106 µm), expressed in terms of CO2, was determined by TGA-MS (see below, eq. 5). The lime content was estimated from the amount of dissolved Ca at pH = 8.5 in the pHstat leaching curve of the fresh wollastonite (see also below). The total composition of the wollastonite was determined by grinding a sub-sample to <106 µm. A part of the ground sub-sample was digested with concentrated HNO3/HClO4/HF (in proportions of 5:0.5:4.5) in an autoclave at 190 °C for 10 h. The rest of the sample was digested in a lithium metaborate smelt at 1150 °C during 30 min (1 g at 0.1 g sample). Element concentrations in the resulting solutions were measured by ICP-AES. Table 1 shows the determined composition of the wollastonite. The maximum CO2 sequestration capacity is 329 g CO2 per kg wollastonite (i.e., if all calcium is carbonated). The resulting product would contain 24.8 wt% CO2. The carbonation degree (ζCa) of the starting material is 2.4% of the total amount of Ca, based on eq. 6 below. The pH of a wollastonite-water slurry at a liquid to solid ratio of 10 kg/kg was determined at 10.6 (open to the atmosphere, after 24 h). This value is in good agreement with the natural pH of wollastonite (10.7) as calculated with The Geochemist's Workbench 4.0 (GWB) (Bethke, 2002). The leaching characteristics of Ca and Si from fresh (and carbonated) wollastonite were measured in a pHstat-system. Eight suspensions of ground wollastonite (<106 µm) and nanopure-demineralised water at a liquid to solid ratio of 10 kg/kg were stirred at room temperature in closed Teflon reaction vessels. For seven vessels, the pH was controlled automatically within ±0.2 pH of a pre-set pH value by the addition of HNO3 and NaOH. In one vessel, leaching occurred at the native pH of the sample (i.e., not adjusted by the addition of acid or base). After 48 h, the final pH of the suspensions was determined and the ECN-RX--06-056 6 suspensions were filtered over 0.2 µm membrane filters. The clear filtrates were acidified with concentrated HNO3 (pH < 3) and analysed for Ca and Si by ICP-AES. Solubility products of CaSiO3 (wollastonite), amorphous SiO2 and calcite (CaCO3) were used to calculate the solubility curves of these minerals, as shown in Figure 3. 2.2 Carbonation experiments Three representative wollastonite batches were ground quantitatively to a specific sieve size class by a tungsten-carbide vibratory ring pulveriser (<38, <106 and <500 µm).
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