Extraction of alkali metals and/or alkaline earth metals for use in carbon sequestration

Abstract

A method of extracting an alkali metal and/or an alkaline earth metal from a mineral including an alkali metal and/or an alkaline earth metal, or a rock containing the mineral, the method including contacting the mineral, with an aqueous composition including formic acid, and to their use in carbon sequestration.

Description

The present invention relates to the extraction of alkali metals and/or alkaline earth metals from minerals, and rocks containing same, and to their use in carbon sequestration.

BACKGROUND

One proposed method to sequester carbon is to react naturally occurring alkali metal and alkaline earth metal (e.g. sodium, potassium, calcium & magnesium) containing minerals with carbon dioxide to form carbonates. Such a method has some significant advantages including that the process is thermodynamically favourable and occurs naturally. In addition, the minerals such as feldspar, olivine, serpentine and wollastonite are abundant. Moreover, the carbonates that are produced from such a method are stable and thus re-release of carbon dioxide into the atmosphere is not an issue. However, conventional carbonation pathways are slow under ambient temperatures and pressures. The significant challenge is to identify an industrially and environmentally viable carbonation route that will allow such a method of sequestering carbon to be economically viable.

Reaction rates for carbonation may be accelerated by decreasing the particle size of the minerals through pulverisation, raising reaction temperature and pressure, changing solution chemistry and using catalysts/additives.

One such attempt to increase reaction rates found that serpentine minerals are activated by heating to between 600 and 750° C. which removes part of the hydroxyl groups and amorphises the mineral structure, significantly increasing their reactivity to CO₂ (Balucan et al., 2010).

The most comprehensive studies so far outline two thermodynamically feasible aqueous mineral carbonation approaches. The first process, developed by the Albany Research Centre, involves direct carbonation in aqueous solutions of 0.64 M NaHCO₃ and 1 M NaCl conducted at 150 bar CO₂ and 155° C. and 185° C. for heat pre-treated serpentine and olivine respectively (O'Connor et al., 2000, 2001).

The second approach (known as indirect carbonation) is based on two principal steps namely dissolution of the alkali metal or alkaline earth metal mineral to obtain the alkali metal or alkaline earth metal ions in solution, and subsequent carbonate precipitation (Oelkers and Scott, 2005). Since the former mechanism is generally assumed to be rate-limiting with respect to the overall carbonation process, many studies have focused on the extraction of calcium or magnesium from native minerals (Park and Fan, 2004; Carroll and Knauss, 2005; Hänchen et al., 2006).

Moreover, extensive research has been undertaken to reveal and subsequently enhance dissolution kinetics. While Golubev and co-workers (2005) inferred that the presence of bicarbonate ions, at concentrations of 0.01-0.1 M and pH 7-8, would enhance dissolution rates, Krevor and Lackner (2009) concluded that sodium salts of citrate, oxalate and EDTA considerably increased the degree of dissolution. Experimental investigations coupled with kinetic modelling have also been performed with a view to estimating dissolution rates of basic silicates at conditions relevant to geologic CO₂ sequestration (Prigiobbe et al., 2009; Daval et al., 2009).

Acid-aided dissolution, where hydrochloric acid was used to leach out magnesium ions, was initially investigated by Lackner et al. (1995). This scenario was highly energy intensive and was thus phased out by a different process using acetic acid as an accelerating medium for the artificial weathering of wollastonite. This acid was selected based on the thermodynamic consideration that the extraction acid must not only be stronger than silicic acid but also weaker than carbonic acid, such that the precipitation of carbonates occurs spontaneously (Kakizawa et al., 2001). Other weak acids have been subject to less detailed studies (Teir et al., 2007; Krevor and Lackner, 2009; Carey et al., 2004; Baldyga et al., 2010)

The present invention seeks to provide an alternative method of extracting alkali metals and alkaline earth metals from minerals and/or rocks containing same.

SUMMARY

According to one aspect the present invention provides a method of extracting an alkali metal and/or an alkaline earth metal from a mineral including an alkali metal and/or an alkaline earth metal, or a rock containing the mineral, the method including contacting the mineral, with an aqueous composition including formic acid.

In one form the mineral is a naturally occurring mineral.

In one form the alkali metal is chosen from sodium or potassium.

In one form the alkaline earth metal is chosen from calcium or magnesium.

In one form the mineral is a silicate. In a further form the mineral is chosen from an aluminosilicate or an alkaline earth metal silicate. In one form the aluminosilicate is a feldspar and includes sodium, potassium and/or calcium. In one form the alkaline earth metal silicate is chosen from olivine, enstatite, forsterite, serpentine and/or wollastonite.

In one form the mineral, or the rock containing the mineral, is crushed prior to contact with the aqueous composition including formic acid. In one form the mineral, or the rock containing the mineral, is ground prior to contact with the aqueous composition including formic acid. In one form the mineral, or the rock containing the mineral, has an average particle size of between 1 μm to 250 μm.

In one form the composition including formic acid has a pH of between 1 and 1.8.

In one form the aqueous composition is at a temperature of between about 20° C. and about 95° C. In a further form the aqueous composition is at a temperature of between about 75° C. to about 85° C.

According to another aspect the present invention provides a process for the indirect carbonation of carbon dioxide the process including the following steps:

• • extracting an alkali metal and/or an alkaline earth metal from a mineral including an alkali metal and/or an alkaline earth metal, or a rock containing the mineral, the method including contacting the mineral with an aqueous composition including formic acid; and,
  • carbonating the alkali metal or alkaline earth metal with carbon dioxide to form a carbonate.

In one form the carbonate is chosen from an alkaline earth metal carbonate and/or an alkali metal aluminium carbonate.

According to another aspect the present invention provides a use of formic acid for the extraction of an alkali metal or an alkaline earth metal from a mineral, or a rock containing the mineral.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The present invention will become better understood from the following detailed description of various non-limiting embodiments and examples thereof, described in connection with the accompanying figures, wherein:

FIG. 1 illustrates the cumulative particle size distribution of wollastonite particles before and after acid dissolution before and after reaction at 80° C., within 3 h and without pH control;

FIG. 2 depicts X-ray diffraction (XRD) results confirming the presence of wollastonite as the major phase;

FIG. 3 illustrates the pH profile for dissolution in 0.1 M formic acid;

FIG. 4 illustrates the pH profile for dissolution in 0.1 M acetic acid;

FIG. 5 illustrates the Extent of Ca extraction with increasing temperature in 3 h (pH 2.0-4.5;

FIG. 6 illustrates the Concentration of aqueous species as calculated at thermodynamic equilibrium, at 80° C. and 1 atm (OLI Analyzer Studio 3.0); symbols are used only to identify each plot;

FIG. 7 illustrates the Solid phase composition and pH profile for the results presented in FIG. 6 (OLI. Analyzer Studio 3.0); symbols are used only to identify each plot;

FIG. 8 illustrates the Dissolution in formic acid at 80° C. and pH 1.04;

FIG. 9 illustrates the Dissolution in acetic acid at 80° C. and pH 1.61;

FIG. 10 illustrates the Dissolution in DL-lactic acid at 80° C. and pH 1.19; and,

FIG. 11 is an Arrhenius plot of the initial dissolution rates.

DESCRIPTION OF EMBODIMENTS, EXAMPLES AND THE ACCOMPANYING FIGURES

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

In the context of this specification, the word “comprising” means “including principally but not necessarily solely” or “having” or “including”, and not “consisting only of”. Variations of the word “comprising”; such as “comprise” and “comprises” have correspondingly varied meanings.

As used herein the term ‘formic acid’ (also referred to in literature as methanioc acid) is used to denote the carboxylic acid of the chemical formula HCOOH.

As used herein the term ‘indirect carbonation’ refers to the two step process of extracting a reactive compound in a first step and subsequently carbonating the reactive compound in a second step.

As used herein the term ‘a rock containing the mineral’ includes any type of rock which may contain a mineral which includes an alkali metal or an alkaline earth metal such as for example peridotites, basalts, harzburgites and limburgites.

It was surprisingly found that the treatment of a mineral, or a rock containing the mineral, including an alkali metal or an alkaline earth metal with an aqueous solution of formic acid resulted in the extraction of alkali metal ions or alkaline earth metal ions in solution at a far greater reaction rate than expected. It was also found that contacting a mineral including an alkali metal or an alkaline earth metal, or a rock containing the mineral, with an aqueous composition including formic acid resulted in a far greater rate of dissolution than if an aqueous composition including acetic acid or lactic acid was employed.

The following are equations which represent aqueous dissolution reactions for some common minerals with formic acid:

Enstatite MgSiO³+2HCOOH→Mg²⁺+2HCOO⁻+SiO₂+H₂O

Serpentine Mg₃Si₂O₅(OH)₄+6HCOOH→3Mg²⁺+6HCOO⁻+2SiO₂+5H₂O

Forsterite Mg₂SiO₄+4HCOOH→2Mg²⁺+4HCOO⁻+SiO₂+2H₂O

Wollastonite CaSiO₃+2HCOOH→Ca²⁺+2HCOO⁻+H₂O+SiO₂

Following the extraction of the alkali metals or the alkaline earth metals by formic acid via dissolution, the resulting aqueous solution may pass to the second step of an indirect carbonation process where the aqueous solution including the alkali metal ions or the alkaline earth metal ions (e.g., K⁺, Na⁺, Ca²⁺ and Mg²⁺) are contacted with carbon dioxide (typically in gaseous form) which then results in the precipitation of carbonates.

These carbonates may be in the form of alkaline earth metal carbonates and/or an alkali metal aluminium carbonate as examples.

The temperature of the extraction reaction may be between 20° C. and 95° C. The highest dissolution reaction rates were found when the temperature of the extraction reaction was maintained at about 80° C.

The pH of the aqueous composition including formic acid may be between 1 and 1.8.

It was also found to be advantageous to crush the mineral, or the rock containing the mineral, prior to being contacted with the aqueous composition including formic acid. In addition to crushing the mineral, or the rock containing the mineral, an additional processing step of grinding the mineral, or the rock containing the mineral, may also be used to break the particles of the alkaline earth metal silicate down to a particle size of between 1 and 250 μm and preferably a particle size of between 10 and 80 μm.

EXAMPLES

The extent of calcium extraction from wollastonite was investigated when the latter was treated with three weak organic acids, namely: acetic, formic and DL-lactic acids, under the effect of increasing reaction temperature. Moreover OLI Analyzer Studio 3.0, a thermodynamic prediction software employing the Helgeson-Kirkham-Flowers-Tangers (HKFT) equation of state and the Bromley equation for solution non-ideality, was used to predict the equilibrium calcium extraction. The R_CO2 value of wollastonite stands at about 2.8.

Experiments were also performed at constant pH to determine the maximum initial (t=0) kinetics of acid digestion of the rock, at low but achievable pH and in the absence of a silica layer that builds on particle surfaces as a result of incongruent dissolution of wollastonite. Finally, wollastonite particles were analysed to examine their morphology before and after reaction.

The wollastonite specimen used for dissolution reactions was procured from New South Wales Pottery Supplies, Australia. Laser particle sizing of the ground and sieved sample, which was performed in aqueous media on a Malvern Mastersizer “E”, indicated a volume mean diameter (VMD or D[v, 0.5]) of 17±1 μm, with D[v, 0.9] and D[v, 0.1] of 56±1 and 2±1 μm, respectively.

FIG. 1 illustrates the cumulative particle size distribution of wollastonite particles before and after acid dissolution. The average density of the, starting material was 2.86 g/cm³ while its specific surface area was determined to be 0.1 m²/g based on a low temperature N₂ adsorption BET method (Micromeritics Gemini). X-ray diffraction (XRD) results confirmed the presence of wollastonite as the major phase (FIG. 2). Diopside and pectolite, appearing as minor phases, are both metasilicates like wollastonite and crystallise in the monoclinic and triclinic systems respectively.

Table 1 lists the elemental composition derived from X-ray fluorescence (XRF). Distributing the elemental abundances among minerals leads to approximate contents of 81.8% wollastonite (CaSiO₃), 9.2% diopside (MgCaSi₂O₆), 4.6% silica (SiO₂), 1.9% pectolite (NaCa₂Si₃O₈(OH)), and possibly 0.7% hedenbergite (FeCaSi₂O₆); possibly with diopside and hedenbergite end members forming a solid solution. The remainder of about 0.5%, after accounting for the loss on ignition, seems to include mostly the aluminosilicate minerals.

TABLE 1
Chemical composition (by weight) of wollastonite derived from
XRF, excluding oxides of less than 0.1% in abundance; the total
composition in the table corresponds to 99.7%.
SiO2	MgO	CaO	Fe2O3	Al2O3	Na2O	LOI*
53.5	1.71	42.7	0.234	0.176	0.222	1.2
*Loss on ignition

Wollastonite (CaSiO₃) dissolution experiments were performed in a three-neck glass reactor, immersed in a temperature-controlled water bath, equipped with a condenser to minimise solution losses due to evaporation. Two series of experiments were carried out in non-pH controlled and pH controlled systems to determine the extent of calcium extraction and the rates of dissolution, respectively.

The first set of experiments were conducted at temperatures ranging from 22° C. to 80° C. in an acidic leaching medium with a concentration of 0.1 M, for a total reaction time of 3 hours. The analytical reagent grade formic and DL-lactic acids were purchased from Sigma Aldrich (Australia) while acetic acid was obtained from Ajax Finechem Pty Ltd (Australia), Acid solutions were prepared, in ultrapure deionised water with electrical resistivity of 18.2 MΩ/cm, by standard volumetric dilution techniques. In-situ pH measurements, with an accuracy of ±0.01, were taken by a Hanna pH probe and meter. Temperature control was achieved by using a water bath. Dispersion of the particles was accomplished through continuous stirring of the slurry by a magnetic stirrer.

In each run, 0.58 g of ground samples of CaSiO₃ were loaded into the batch reactor after heating 100 mL of the diluted acid to the desired temperature. The ratio of acid to CaSiO₃ was fixed according to stoichiometry. Equations 1-3 illustrate the extraction of calcium from CaSiO₃ using formic acid (HCOOH—pK_a 3.75), acetic acid (CH₃COOH—pK_a 4.76) and DL-lactic acid (CH₃CHOHCOOH—pK_a 3.86).

CaSiO₃+2HCOOH→Ca²⁺+2HCOO⁻+H₂O+SiO₂   (1)

CaSiO₃+2CH₃COOH→Ca²⁺+2CH₃COO⁻+H₂O+SiO₂   (2)

CaSiO₃+2CH₃CHOHCOOH→Ca²⁺+2CH₃CHOHCOO⁻+H₂O+SiO₂   (3)

The Reactions constitute an overall description of the dissolution process. In solution, other ions will also exist, such as, for Reaction 1, calcium monoformate Ca(HCOO)⁻ and calcium formate Ca(HCOO)₂, and, Reaction 2, calcium monoacetate Ca(CH₃COO)⁻ and calcium acetate Ca(CH₃COO)₂. OLI Analyzer Studio 3.0 predicted only calcium ion (Ca²⁺) for Reaction 3, due to the absence of thermodynamic data for calcium monolactate and calcium lactate in the database of the software.

At the end of the desired test time, the suspension was filtered through a 0.45 μm PVDF membrane. Analysis of calcium ion concentrations in the filtrate was carried out by ICP-OES, which was calibrated using multielement standard solution that was matched to the filtrate composition. The calibration curves consistently had R² values greater than 0.999 and were linear over the calibrated range. To obtain more accurate results two different wavelengths were considered. The extent of calcium extraction was calculated as the ratio of calcium concentration in the filtrate solution to the initial fraction of calcium in the feed. The filter cake was washed with deionised water prior to drying overnight in an oven set at 105° C. Its properties and surface morphology were subsequently investigated by SEM. The particle size distribution of the reacted particles was also carried out using Malvern Mastersizer.

In order to close elemental balance on calcium, the filter cake was also analysed for its calcium content. Volumes of 4.5 mL of 65% HNO₃, 4.5 mL of 37% HCl and 3 mL of 50% HBF₄ were added to 0.1 g of the dried solid residue prior to digestion in a Milestone start D microwave unit. Complete digestion was achieved after 1 h at 160° C. The liquid was then analysed by ICP-OES.

To obtain estimates of the maximum dissolution rates in the absence of diffusional resistance in pores and cracks of the silica skin, as a function of temperature, another series of experiments was performed in a constant-pH. The investigated reaction temperatures were 40° C., 60° C. and 80° C. In order to obtain maximum possible rates, the dissolution was performed using excess amounts of 5 M acids. Although, this buffered system pH, small amounts of acid (not exceeding 10 mL) had to be added to the leaching medium from time to time to adjust the pH. About 0.4 g of powdered CaSiO₃ was rapidly injected in the batch stirred-vessel containing 100 mL of acid solution. Slurry samples of about 0.6 mL were drawn via a syringe and immediately syringe-filtered through a 0.22 μm membrane at intervals of 5 min within the total reaction time of 1 h. The combined volume of the aliquots from any given experiment represented less than 10% of the total volume. Care was taken to minimise changes in acid concentration during experiments by ensuring that the volumes of aliquots and added acid for pH adjustment were kept within the stated limits. The initial dissolution rates, normalised to the specific surface area of the feedstock, was determined from the change in calcium concentration in the sampled solution, as evaluated by ICP-OES.

FIGS. 3 and 4 illustrate typical pH profiles as wollastonite dissolution proceeds in acidic medium. For all runs, the pH was found to vary in the range of 2.0 to 4.5. This process is characterised by the consumption of protons and the release of cations from the silicate mineral, resulting in the alkalisation of the reaction mixture thus increasing solution pH. Since dissolution is incongruent at such low pH (Schott et al, 2002), the concentration of calcium is expected to be much higher compared to other ions that could leach out, hence allowing an initial monitoring of the reaction course via the pH of the system.

The measurements of FIGS. 3 and 4 are summarised graphically in FIG. 5, where we plot the extraction of Ca²⁺ at the end of 3 h of the process. The process depends on temperature, as both the diffusion and chemical-reaction rates increase with temperature. A linear relationship was observed within the investigated temperature interval where the amount of calcium in solution has risen by more than 55%.

In comparison to acetic and DL-lactic acids, formic acid demonstrated a higher calcium-extracting capability reaching up to 96% after 3 h at 80° C. (FIGS. 3 and 5). Elemental calcium balance was closed by analysing the amount of calcium in solution and in the residual solid phase at the end of the experiment. The results are listed in Table 2.

Furthermore, dissolution in formic acid at 80° C. was modelled using OLI Analyzer Studio 3.0 (FIGS. 6 & 7), neglecting the presence of mineralogical impurities. The input to the software was similar to the initial experimental conditions; a temperature of 80° C., pressure of 1 atm, and wollastonite/formic acid (0.58 g/0.46 g) in stoichiometric ratio, representing 0.2 g of calcium.

The following equilibrium aqueous phase composition was predicted: 0.1 g Ca²⁺, 0.14 g calcium monoformate equivalent to. 0.07 g Ca²⁺ and 0.06 g calcium formate equivalent to 0.02 g Ca²⁺. The total mass of calcium in solution amounts to 0.19 g, which represents 95% of the input mass. A solid phase consisting of only silicon dioxide and pH range of 2.5-5 were also predicted. Experimental data was found to be compatable with results from the simulation, except for the measurements of pH, which appear to be significantly lower in the experimental measurements than in thermodynamic predictions. The difference is as high as 1 pH unit at the end of an experiment.

TABLE 2
Calcium balance for dissolution in formic acid at 80° C. and 3 h.
	Output (g)
	Input (g)	Filtrate	Filter Cake
	Ca2+	0.20 ± 0.03	0.192 ± 0.020	0.006 ± 0.001

As indicated by pH measurements, the reaction rate is initially high but it plateaus with time. This occurs as a consequence of slower kinetic and mass transfer rates. At the beginning of each experiment, both H⁺ and anionic ligands (e.g., HCOO⁻) diffuse only through the liquid film surrounding each particle. However, as SiO₂ skin thickens on the particle surfaces, the diffusion proceeds through the cracks and pores of the skin, significantly slowing down the dissolution process. The other reason for slowing down of the dissolution rate is the dependence of breaking of Ca—O bonds on the activity of protons. The lower the activity (e.g., the higher the pH), the slower is the reaction process.

The dissolution may proceed via the following steps, where —O—Ca—OH denotes surface calcium atoms terminated with hydroxyl groups:

—O—Ca—OH_(s)+H⁺_(aq)→—O—Ca—OH₂⁺_(s)   (4)

—O—Ca—OH₂⁺_(s)→—O—Ca⁺_(s)+H₂O₍₁₎   (5)

—O—Ca⁺_(s)+H⁺_(aq)→—OH⁺—Ca⁺_(s)   (6)

–OH⁺—Ca⁺_(s)→—OH_(s)+Ca²⁺_(aq)   (7)

Clearly, Reactions 4 and 6 are pH dependent. In addition, the ligands themselves may assist in the dissolution of wollastonite, say, via the following reaction

—OH⁺—Ca⁻_(s)+HCOO⁻_(aq)→—OH_(s)+CaHCOO⁺_(aq)   (8)

The effectiveness of ligands depends both on the nature of their functional groups, molecular structure and thermodynamic stability of the transitional surface complexes they form (Pokrovsky et al, 2009). Organic ligands such as acetate, lactate and formate are known to form monodentate complexes on oxides, which upon polarisation, labilise the Ca—O bonds thereby facilitating the removal of calcium atoms from the crystal lattice (Swaddle, 1997). As the calcium-ligand complexes detach the surface, the underlying layers are exposed to further contact with the solvent.

The higher extraction yield of formic acid can also be justified in terms of H⁺ activity as the pH dependency of dissolution has been postulated by many geochemical researchers. Among the three weak acids studied, formic acid is the strongest (pK_a 3.75) and hence dissociates to a larger extent to produce H⁺ ions when in solution. The O—Ca bond is weakened as a result of increased protonation of the lone pairs of electrons in oxygen atoms (Reactions 4 and 6).

In order to determine dissolution kinetics as a function of temperature in acidic medium in the limit of low pH achievable for these acids, we conducted experiments at constant pH or H⁺ activity, for a period of 1 h, at 40° C., 60° C. and 80° C. The mass ratio of wollastonite to acid was maintained as low as 0.02 to ensure constant pH throughout the runs. Representative examples of the temporal evolution of the leaching solution composition at 80° C. and in aqueous solutions of formic, acetic and DL-lactic acids are depicted in FIGS. 8, 9 and 10, respectively.

The kinetic behaviour of the dissolution reactions displays an initial fast rate during the first 10 min followed by a slowdown, observed in all cases. During the early stages, the dissolution rates can be considered to be surface controlled (i.e., film-diffusion or reaction-rate controlled) but the levelling off of calcium concentration in filtrates indicates a diffusion limitation (i.e., pore/crack-diffusion controlled) at the later part of the process (Park et al., 2003, Alexander et al., 2006). This limitation can be attributed to the fact that wollastonite dissolution is strongly incongruent at acidic pH leading to the formation of a passivating, amorphous silica rich layer, which could partly reduce the transport of aqueous species and eventually hinder further dissolution of the mineral (Xie and Walther, 1994; Weissbart and Rimstidt, 2000; Schott et al, 2002; Golubev et al, 2005; Béarat et al, 2006; Huijen et al, 2006; Green and Lüttge, 2006).

The low concentration of dissolved silicon (5-10%) in the experiments also suggests the build-up of silica coating on the rock particles. However, referring to experimental and simulation data (FIGS. 5 & 6), we can deduce that running the experiment for longer reaction times (in this case 3 h) compensated for the passivating effect of the silica layer. In other words, calcium will continue to diffuse out of the crystal lattice, at a slower rate, though, to reach its equilibrium concentration.

We applied the method of initial rates to estimate the dissolution rates in the absence of a passivating layer of amorphous silica. The rate of reaction can be computed by plotting the concentration of calcium in the leaching medium as a function of time and then evaluating the gradient of the curve at time t=0 min. As we were unable measure Ca²⁺ concentration at a very short time (our shortest measurement interval was 5 min), we fitted a sixth degree polynomial to all measurements and applied that polynomial to estimate the rates at t=0 min. Table 3 summarises the rates of dissolution, which have been normalised to the specific surface area at the investigated temperatures. The highest initial rate has been recorded for formic acid at 80° C.

TABLE 3
Initial rates of dissolution in a constant pH system
under the effect of increasing temperature.
	Temperature	% Ca extraction	Initial rate × 105
Acid	(° C.)	within 3 h	(mol m−2 s−1)
Formic acid	40	60	18 ± 5
	60	78	25 ± 7
	80	96	29 ± 8
Acetic acid	40	33	1.8 ± 0.5
	60	63	4.7 ± 1.4
	80	85	13 ± 3
DL-lactic acid	40	37	1.7 ± 0.5
	60	71	5.8 ± 1.5
	80	90	16 ± 4

The calculations are based on an estimate of the mineral surface area which is equal to the total mass of reacting material multiplied by the specific surface area per unit mass of material, as determined by the BET method. While some researchers have assumed that the surface area of the leached layer grew linearly with time (Stillings and Brantley, 1995), others found that the surface area of their reacted wollastonite grains increased according to a power law function (Weissbart and Rimstidt, 2000). The surface area certainly changes as the reaction proceeds but for this study, during the very onset of the reactions, when we made our measurements, it is the same as the BET surface area of the fresh particles.

The observed increase in extent of dissolution with growing temperature can be interpreted by the empirical Arrhenius equation given by)

r=A exp ^(−Eα/RT)   (9)

where r designates the rate of reaction in mol m⁻² s⁻¹, A refers to a pre-exponential factor in mol m⁻² s⁻¹ (which is here a function of pH) and E_α represents the activation energy, in kJ mol⁻¹, defined by

E_α=−2.303 R [∂ log r/∂(1/T)]_pH   (10)

FIG. 11 illustrates an Arrhenius plot with the logarithm of the measured wollastonite dissolution rates on the ordinate and the reciprocal of temperature on the abscissa. The activation energies are derived from the slopes of the straight lines that best fit the points, given by −E_α/2.303 R. The calculated activation energies and pre-exponential factors are presented in Table 4. It should be noted that the term for temperature dependence denotes an apparent global activation energy because the dissolution of minerals is not a single elementary reaction but rather involves a complex series of reactions, each carrying their own activation energy (Lasaga, 1995). The low activation energy for the formic acid implies mass transfer control for diffusion of H⁺ and HCOO⁻ in the aqueous film surrounding the reacting particles. Alternatively, the ligand (HCOO⁻) provides an effective, low activation energy pathway for solubilising Ca²⁺, as suggested by reaction 8.

The results for acetic and DL-lactic acid point to kinetic control of removing Ca²⁺ via protonation of the reacting surface and heterogeneously breaking of O—Ca bonds. For comparison, apparent activation energies of 68, 70 and 74 kJ mol⁻¹, for the dissolution of serpentinite in H₂SO₄, HCl and HNO₃ respectively, have been reported in literature (Teir et al., 2007), indicating lower reactivity of serpentine minerals for the dissolution.

TABLE 4
Apparent kinetic parameters calculated
from the Arrhenius plots in FIG. 11.
Kinetic	Solvent
Parameters	Formic acid	Acetic acid	DL-lactic acid
Activation energy	11 ± 3	46 ± 12	53 ± 15
(kJ mol−1)
Pre-exponential	0.01	10000	700
factor (mol m−2 s−1)

SEM analysis revealed that fresh wollastonite particles consist mostly of fibrous needle-like structures. The dissolution products displayed distinct microstructural features including fractures, cracks and surface unevenness. The greater impact of formic acid can was discerned through more pronounced crazing of the grains. This may be one of the reasons for the dissolution of wollastonite proceeding faster, even in the presence of a silica layer. However, in both cases, the morphology of the original particles was preserved, indicating the formation of amorphous silica deposits on the particle surface while its core gradually disappeared.

To confirm the formation of the silica layer, we measured the particle size distribution of the reacted wollastonite grains. The distribution slightly shifted to the left. From the results shown in FIG. 1, it can be observed that the volume mean diameter of the acetic acid-treated particles decreased by 6% (16±1 μm) whereas those treated with formic acid have shrunk by 12% (15±1 μm), when compared to the original ‘particles (17±1 μm). It can thus be inferred that formic acid was more capable of removing or crazing the silica layer.

The results of the experiments outlined above demonstrate significant differences in the dissolution of Ca²⁻ from wollastonite with formic acid on one hand, and acetic and DL-lactic acids on the other. Even in the absence of the amorphous silica layer on particle surfaces, the dissolution of Ca²⁺ appears to be mass-transfer controlled in the case of formic acid and kinetically controlled in the case of acetic and DL-lactic acids. All rates decrease with time as the silica layer builds up. However, the decrease is significantly less pronounced for formic than for acetic and DL-lactic acids. The SEM microphotographs and particle-size measurements suggest enhanced crazing in silica layer formed in the presence of formic acid and partial dissolution of silica.

For comparison, at 80° C. and similar pH, for the particle distribution investigated (D[v, 0.1]=2±1 μm, D[v, 0.5]=17±1 μm, D[v, 0.9]=56±1 μm), it takes less than 20 min for the extraction of Ca²⁺ by formic acid to reach completeness, whereas acetic and DL-lactic acids only achieve Ca²⁺60-70% extraction in the same time. Further dissolution of Ca²⁺ proceeds extremely slowly. As expected, the rates of dissolution attain their maximal values under the lowest achievable pH (between 1 and 1.6 depending on acid) and the highest temperature (80° C.).

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

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Claims

1-31. (canceled)

32. A method of extracting an alkali metal and/or an alkaline earth metal from a mineral including an alkali metal and/or an alkaline earth metal, or a rock containing the mineral, the method including contacting the mineral, with an aqueous composition including formic acid.

33. A method according to claim 32 wherein the alkaline earth metal is chosen from calcium or magnesium.

34. A method according to claim 32 wherein the mineral is a silicate.

35. A method according to claim 32 wherein the mineral is chosen from olivine, enstatite, forsterite, serpentine and/or wollastonite.

36. A method according to claim 32 wherein the mineral is wollastonite.

37. A method according to claim 32 wherein the mineral, or the rock containing the mineral, is crushed prior to contact with the aqueous composition including formic acid.

38. A method according to claim 32 wherein the mineral, or the rock containing the mineral, is ground prior to contact with the aqueous composition including formic acid.

39. A method according to claim 32 wherein the mineral, or the rock containing the mineral, has an average particle size of between 1 μm to 250 μm.

40. A method according to claim 32 wherein the composition including formic acid has a pH of between 1 and 1.8.

41. A method according to claim 32 wherein the aqueous composition is at a temperature of between about 20° C. and about 95° C.

42. A method according to claim 41 wherein the aqueous composition is at a temperature of between about 75° C. to about 85° C.

43. A process for the indirect carbonation of carbon dioxide the process including the following steps:

extracting an alkali metal and/or an alkaline earth metal from a mineral, or a rock containing the mineral, according to claim 32; and,

carbonating the alkali metal or alkaline earth metal with carbon dioxide to form a carbonate.

44. A process according to claim 43 wherein the carbonate is chosen from an alkaline earth metal carbonate and/or an alkali metal aluminium carbonate.

45. Use of formic acid for the extraction of an alkali metal or an alkaline earth metal from a mineral, or a rock containing the mineral.

46. Use according to claim 45 wherein the alkali metal or the alkaline earth metal is subsequently carbonated with carbon dioxide to form a carbonate.

47. Use according to claim 45 wherein the mineral is a naturally occurring mineral.

48. Use according to claim 45 wherein the mineral is chosen from olivine, enstatite, forsterite, serpentine and/or wollastonite.