Mechanisms of aqueous wollastonite carbonation as a possible CO₂ sequestration process

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 are 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 $200{}^{\circ }\mathrm{C}$, 20 bar CO₂ partial pressure and particle size of $<38\mathrm{\mu }\mathrm{m}$. 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.

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:

$${\mathrm{CaSiO}}_3(\mathrm{s})+{\mathrm{CO}}_2(\mathrm{g})\to {\mathrm{CaCO}}_3(\mathrm{s})+{\mathrm{SiO}}_2(\mathrm{s})$$

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, $\mathrm{\Delta }{H}_r=-87\mathrm{kJ}/\mathrm{mol}$ 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., 2005). In the present study, we have extended our 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 et al., 1995). In addition, the choice for a Ca–silicate enables 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 recent reviews (Huijgen and Comans, 2003, IEA GHG, 2005). In this process, carbonation occurs in a gas–solid–water slurry, which increases the reaction rate substantially compared to direct gas–solid carbonation. Process steps within the aqueous carbonation route are

• Leaching of Ca:

  $${\mathrm{CaSiO}}_3(\mathrm{s})+2{\mathrm{H}}^{+}(\mathrm{aq})\to {\mathrm{Ca}}^{2+}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+{\mathrm{SiO}}_2(\mathrm{s})$$

• Dissolution of CO₂ and subsequent conversion of (bi)carbonate species:

  $${\mathrm{CO}}_2(\mathrm{g})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to {\mathrm{H}}_2{\mathrm{CO}}_3(\mathrm{aq})\to {\mathrm{HCO}}_3^{-}(\mathrm{aq})+{\mathrm{H}}^{+}(\mathrm{aq})$$

• Nucleation and growth of calcium carbonate:

  $${\mathrm{Ca}}^{2+}(\mathrm{aq})+{\mathrm{HCO}}_3^{-}(\mathrm{aq})\to {\mathrm{CaCO}}_3(\mathrm{s})+{\mathrm{H}}^{+}(\mathrm{aq})$$

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. 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 confirms 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.