Phys Chem Minerals (2014) 41:489–496 DOI 10.1007/s00269-014-0659-z
Original Paper
CO2 mineral sequestration by wollastonite carbonation Wenjin Ding · Liangjie Fu · Jing Ouyang · Huaming Yang
Received: 25 May 2013 / Accepted: 18 February 2014 / Published online: 2 March 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract In this paper, we demonstrated a new approach to CO2 mineral sequestration using wollastonite carbonation assisted by sulfuric acid and ammonia. Samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and 29 Si nuclear magnetic resonance. The change in Gibbs free energy from −223 kJ/mol for the leaching reaction of wollastonite to −101 kJ/mol for the carbonation reaction indicated that these two reactions can proceed spontaneously. The leached and carbonated wollastonite showed fibrous bassanite and granular calcium carbonate, respectively, while the crystal structure of pristine wollastonite was destroyed and the majority of the Ca2+ in pristine wollastonite leached. The chemical changes in the phases were monitored during the whole process. A high carbonation rate of 91.1 % could be obtained under the action of sulfuric acid and ammonia at 30 °C at normal atmospheric pressure, indicating its potential use for CO2 sequestration. Keywords Wollastonite · Mineral carbonation · CO2 sequestration · Sulfuric acid · Ammonia Introduction The control of greenhouse gas (GHG) is one of the most challenging environmental issues currently facing the world W. Ding · L. Fu · J. Ouyang · H. Yang (*) School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China e-mail:
[email protected] W. Ding · L. Fu · J. Ouyang · H. Yang Research Center for Mineral Materials, Central South University, Changsha 410083, China
(Bredesen et al. 2004; Barelli et al. 2008; Drage et al. 2009; Mikkelsen et al. 2010; Shukla et al. 2010; Xu et al. 2005). Since the occurrence of the industrial revolution around 1850, the average atmospheric concentration of CO2 has increased from 280 to 370 ppm. As a result, the average global temperature has increased by at least 0.6 °C over the same time period (Stewart and Hessami 2005). The International Panel on Climate Change predicts that the atmosphere will contain up to 570 ppm CO2 by the end of the year 2100, which would result in an approximately 1.9 °C rise in the mean global temperature (Yang et al. 2008). If left uncontrolled, such rising temperatures may lead to a rise in sea levels, or even species extinction. CO2 is considered to be the major GHG contributing to global warming, particularly of those gases emitted from fossil fuel combustion. According to data from 2002, China is the world’s second largest emitter of CO2, just behind the USA, accounting for 13.6 % of total emissions (Meng et al. 2007). Even with the rapid development of energy technologies witnessed and expected in the twenty-first century, fossil fuels, particularly coal, will retain their place as crucial energy sources in China in the coming decades. Methods of sequestration that are currently being considered by industrialized countries include enhancement of terrestrial carbon sinks, as well as geological, ocean, and mineral sequestration (Tang et al. 2013; Voormeij and Simandl 2004). The technique of mineral carbonation involves the stabilizing of CO2 to form carbonates (i.e., a mineral that is stable on a geological timescale) using alkaline metal oxide (Dunsmore 1992; Lackner et al. 1995; Seifritz 1990). Considered a feasible process for CO2 sequestration (Voormeij and Simandl 2004; Hansson and Bryngelsson 2009), the concept around the technology for mineral carbonation is based on the natural weathering of Ca- or Mg-silicates, where the overall reaction can be written as (where M is a divalent species):
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Mx Siy Ox+2y−z (OH)2z + xCO2 → xMCO3 + ySiO2 + zH2 O
(1) This technique is superior to the strategy of storing CO2 in carriers. First, once CO2 is stabilized as a carbonate, there is no possibility for accidental release, whereas this can be a problem in the storage case. In addition, direct carbonation does not produce problematic by-products. Furthermore, should fibrous serpentine tailings (chrysotile) be considered a raw material for the process, mineral sequestration may be used to dispose of unwanted asbestos waste. In addition, this overall carbonation process is exothermic and thus has the potential to become economically viable (Maroto-Valer et al. 2005). However, such a reaction takes place over a geological timescale in nature. For industrial implementation purposes, alternative process routes should achieve the goal of reduced reaction time. Kakizawa et al. (2001) originally presented acetic acid as a medium for indirect carbonation, with a reported maximum carbonation rate of only 20 % under the conditions of 80 °C and 3 MPa for 30 min. By investigating the impact of various parameters, such as reaction temperature, pressure, time, and particle size, Huijgen et al. (2006) found that the carbonation rate could reach up to 45 %. Gerdemann et al. (2007) improved the technique by applying ex situ mineral carbonation, which they demonstrated as a potential option for CO2 sequestration. At T = 185 °C and PCO2 = 150 bar,, the maximum extent of carbonation can finally reach 80 % after 60 min. Daval et al. (2009a, b) discussed the carbonation reactions of wollastonite in experiments conducted at conditions relevant to geologic CO2 sequestration in subsurface environments (T = 90 °C and PCO2 = 25 MPa), with the highest carbonation rate of 85 % achieved. Bao et al. (2009) improved the carbonation rate by coupling reactive crystallization and solvent extraction with the introduction of an organic solvent, tributyl phosphate, and the extent of carbonation increased from 20 to 50 %. In other words, previous studies have improved the carbonation rate by either including some additives or by optimizing the reaction conditions. However, no carbonation rate achieved thus far has surpassed 90 %. Although recent research showed that the yield can reach 95 % (Montes-Hernandez et al. 2012), this result required that the raw materials should be commercial portlandite, which differs from natural silicate minerals, and the reaction time should be up to 1,440 min. Moreover, previous studies have shown that mineral dissolution rates are surface controlled, and the carbonation reaction stops when the magnesium on the surface of the mineral is depleted and/or blocked by mass transfer resistance (O’Connor et al. 2002). Therefore, fast reaction routes under milder regimes in a continuous integrated process need to be developed before mineral carbonation can be considered a cost-effective sequestration technology.
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Herein, wollastonite was used in carbonation experiments because Ca-silicates tend to be more reactive for carbonation than do Mg-silicates (Huijgen et al. 2005; Lackner et al. 1997a, b). We demonstrate a novel approach to CO2 sequestration using wollastonite mineral carbonation, where sulfuric acid and ammonia were employed as the reaction media to speed up the reaction. We improved the carbonation rate at a lower temperature and general pressure compared with previously reported CO2 mineral sequestration routes. We investigated the feasibility of this route using both theoretical and experimental approaches and discuss the microstructural changes in wollastonite and the underlying mechanisms during the course of the carbonation reaction.
Experimental Pristine wollastonite, obtained from Jiangxi, China, was milled to produce wollastonite powder (wollastonite 90 %) with the following chemical compositions: (mass%) SiO2 51.71; CaO 46.06; Fe2O3 0.41; Al2O3 0.41; MgO 1.16; and ignition loss 0.32. Sulfuric acid and ammonia were used without further purification. Carbon dioxide was industry grade of 99.9 % purity. Prior to the experiment, pristine wollastonite was ground to minus 20 μm. Acid leaching was performed by subjecting the pristine wollastonite samples to sulfuric acid solution. Twenty grams of pristine wollastonite was placed into a 500-mL reaction vessel, to which 200 mL of distilled water was added. A magnetic stir bar was used to ensure adequate mixing throughout the reaction. A hot bath was used to maintain the desired reaction temperature before addition of the selected acid-leaching agents. After the desired reaction time, CO2 was then directly injected with a flow rate of 40 mL/min at 0.1 MPa for 60 min. Simultaneously, and at the solution temperature of 30 °C, 10 % ammonia solution was added. After the desired reaction time, the mixture was cooled and filtered to separate the solids from the solution. The solids were then washed with distilled water until they reached a neutral pH, before being dried in an oven at 110 °C for 24 h. The overall reactions for the mineral carbonation of wollastonite through the proposed process are as follows:Acid leaching: CaSiO3 (s) + H2 SO4 (aq) → CaSO4 (aq) + H2 O(l) + SiO2 (s) (2)
Carbonation reaction:
CaSO4 (aq) + CO2 (g) + 2NH4 OH(aq) → CaCO3 (s) + (NH4 )2 SO4 (aq) + H2 O(l)
(3)
Powder X-ray diffraction (XRD) measurements of the samples were performed with a DX-2700 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate
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of 0.02 deg/s with a voltage of 40 kV and 40 mA. Scanning electron microscopy (SEM) was performed with a JEOL JSM-6360LV scanning electron microanalyzer with an accelerating voltage of 5 kV. Fourier transform infrared (FTIR) spectra of the samples in KBr disks were recorded over the range of 4,000–400 cm−1 on a Nicolet Nexus 670 FTIR spectrometer. Solid-state 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded using a Bruker AMX400 spectrometer in a static magnetic field of 9.4 T at a resonance frequency of 79.49 MHz. The carbonation rate of Ca2+ in pristine wollastonite (η) was calculated according to η = 0.4 c2M2/ (c1M1) × 100 %, where M1 was the weight of pristine wollastonite, c1 was the percentage of calcium in pristine wollastonite, M2 was the weight of the carbonation products, Φ Table 1 ΔfGΦ m of reactants and products and ΔrGm of wollastonite acid-leaching and carbonation reactions
Name Reactants CaSiO3 (s) H2SO4 (aq) Products CaSO4 (s) H2O (l) SiO2 (s) ΔrGΦ m (kJ/mol)
ΔfGΦ m (kJ/mol)
Name
ΔfGΦ m (kJ/mol)
−1,499 −742
CaSO4 (s) CO2 (aq) NH4OH (aq)
−1,322 −386 −254
−1,322 −286 −856
CaCO3 (s) −1,129 (NH4)2SO4 (s) −902 H2O (l) −286
−223
Table 2 Factors, levels, and results of acid-leaching experiments
−101
and c2 was the percentage of calcium carbonate in the carbonation products. M1 and M2 were obtained by direct weighing. Pristine wollastonite was first leached using a 20 % HCl solution, and the total Ca2+ content of filtrate in pristine wollastonite (c1) then determined using the EDTA titration method. The Ca-phases in the carbonation products predominantly included CaSiO3, CaCO3, and CaSO4. 250 g/L NaCl solution was first used to dissolve CaSO4, before 0.5 % acetic acid was employed to dissolve CaCO3, while CaSiO3 remained undissolved. By this means, the percentage of calcium carbonate in the carbonation products (c2) was obtained. Results and discussion To illustrate the theoretical feasibility of the route proposed above, the thermodynamic parameters at the standard state in the acid-leaching and carbonation processes of wollastonite were calculated. Table 1 shows the thermodynamic calculation results, which indicate that the Gibbs free energy change (ΔrGΦ m) was −223 kJ/mol for the acid-leaching reaction and −101 kJ/mol for the carbonation reaction. Thus, both leaching and carbonation reactions can proceed spontaneously, and the proposed route is theoretically feasible. From our previous research into acid leaching for talc (Yang et al. 2006), a set of basic parameters were designed to investigate the effects of various experimental conditions on the Ca2+ leaching rate of wollastonite. The corresponding results listed in Table 2 show that the most suitable
Time (min)
Ca2+ leaching rate (%)
No.
Concentration of sulfuric acid (wt%)
Temperature (°C)
Liquid to solid ratio
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
5 10 15 20 25 15 15 15 15 15 15 15 15 15 15 15
60 60 60 60 60 20 40 80 100 60 60 60 60 60 60 60
5 5 5 5 5 5 5 5 5 8 10 12 15 10 10 10
60 60 60 60 60 60 60 60 60 60 60 60 60 30 45 90
46.7 83.5 97.2 97.8 98.3 53.2 68.4 98.6 98.1 63.1 85.7 97.9 98.3 73.2, 88.6 98.3
17
15
60
10
120
97.8
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scheme for leaching Ca2+ from pristine wollastonite with H2SO4 is to use a 15 % sulfuric acid solution, at 60 °C for 60 min, and a liquid to solid ratio of 10:1. Figure 1 shows the XRD patterns of pristine wollastonite, leached, and carbonated samples. The main phase of pristine wollastonite is of wollastonite containing some calcium carbonate and quartz (Fig. 1a), which provides a rich source of calcium for the sequestration of CO2. After treatment with sulfuric acid, the only crystalline phase observed in the acid-leached wollastonite was bassanite, with no clear diffraction peaks of wollastonite (Fig. 1b). Further chemical analysis indicated that the Ca2+ leaching rate of pristine wollastonite was more than 90 %. This improvement over results previously reported (Kakizawa et al. 2001; Kojima et al. 1997; Zhang et al. 2008) is encouraging because a higher leaching rate is beneficial to the subsequent carbonation reaction. Meanwhile, the silica obtained from the leached wollastonite showed an amorphous structure, corresponding to an XRD pattern containing no clear reflection peaks of silicon dioxide (Fig. 1b). After the carbonation reaction, the crystalline phase of the carbonated wollastonite was predominantly made up of calcium carbonate with no other phases present (Fig. 1c), indicating the successful carbonation of pristine wollastonite at a low temperature and ambient pressure under the action of ammonia. SEM with EDS investigations was performed to provide improved understanding of the morphology and microstructure of pristine wollastonite and of leached and carbonated samples (Fig. 2). The micromorphology of pristine wollastonite showed clubbed and sheet (Fig. 2a). Contrastingly, that of the leached sample is of fibers and sheets with rod diameters
