CO2 Sequestration in Chrysotile Mining Residues ... - ACS Publications

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CO2 Sequestration in Chrysotile Mining ResiduesImplication of Watering and Passivation under Environmental Conditions Gnouyaro P. Assima,† Faïçal Larachi,*,† Georges Beaudoin,‡ and John Molson‡ †

Department of Chemical Engineering, and ‡Department of Geology & Geological Engineering, Université Laval, Québec, QC, Canada G1V 0A6 ABSTRACT: Factors affecting carbon dioxide fixation in chrysotile mining residues (CMR) under environmental conditions were studied by reproducing mineral dissolution and carbonation in laboratory columns packed with CMR particles. Carbonation is very sensitive to water saturation and watering frequency of the CMR porous media. CO2 uptake by dry residues subjected to dry CO2 flow for several days at ambient temperature was below 0.02%. However, an increase by a factor of 20 in CO2 uptake was achieved by periodic addition of small amounts of water with respect to a moistened CO2 stream over dry CMR samples. The highest MgCO3 conversion resulted in nearly 22 mg of CO2 captured per gram of residue, revealing that up to 93% of Mg remained noncarbonated because of surface obstructing processes. Magnesium leaching from CMR was hindered by two concomitant passivation phenomena limiting the residue’s CO2 storage capacity. A unique cyclic voltammetry technique using oxic and anoxic aqueous solutions contacted with CMR fixed beds was implemented to assess the relative importance from CMR-borne iron electrochemical passivation and silica-deposit nonelectrochemical passivation. Passivation around the dissolving CMR particles by iron hydroxide precipitation was found to develop very rapidly in comparison to silica gel polymerization.

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INTRODUCTION The fixation of carbon dioxide as stable solid carbonates by reacting it with magnesium silicates is one among several solutions contemplated for the reduction of its concentration in the atmosphere.1 The process is akin to natural weathering which is the main control of atmospheric CO2 concentrations over geological periods.2 This has triggered over the past few years vivid interest on the vast amounts of ultramafic mining wastes accrued during mining operations as such stockpiles may offer a long-term solid storage buffer of atmospheric CO2.3 An example is the huge store of ca. 2 billion metric tons of chrysotile mining residues (CMR) accumulated in southern Québec with a potential to store up to 700 million metric tons of atmospheric CO2 by mineral carbonation.4 A great deal of research is under way on deploying pre- and postcombustion carbonation reactors for the capture of CO2 produced at its source.5−8 In most available direct carbonation projects, extra energy (elevated temperature/pressure) and chemical reagents are required to improve reaction efficiency and to shorten reaction time,9−13 and up to 90% of Mg conversion was reported.14−16 However, only a few studies have hitherto been devoted to the assessment of the carbonation capacity of magnesium-bearing mining residues subject to environmental (ambient open-air) conditions.17−24 This passive mode of CO2 sequestration, provided it is proven quantitatively attractive, may constitute an additional tool to trap atmospheric CO2 virtually at no cost while leveraging both the emitters’ carbon footprint and trading. Carbonation of magnesium silicate mineral waste piles is a very slow process under environmental conditions.25−27 It involves metal extraction from the mineral source feedstock and its reaction with dissolved CO2 to yield solid carbonates at ambient temperature in one single pot. The extent of carbonation of mining residues under environmental conditions is expected © 2012 American Chemical Society

to be a function of several factors such as matrix compositional elements and the interference they induce on the reactive magnesium, the presence or not of dissolved oxygen, the presence of dissolved and gaseous CO2, and/or the pore water saturation which is determined by the alternating dry and raining episodes, to name just a few factors. The correlation between water content and the undesirable processes affecting the reaction of magnesium with carbon dioxide and the carbonation capacity of mining residues is still not clearly established. The benefits from understanding this correlation are multiple. First, it allows a more realistic assessment of the carbonation capacity of a given residue instead of the current stoichiometric maximum based on Mg content. Second, it has the potential to open up avenues for increasing the outdoor residue carbonation capacity within the constraints allowed for realistic and economical optimization. To provide a preliminary understanding into these issues, this work aims at the following objectives: (i) estimate the amount of CO2 that can actually be sequestered by chrysotile mining residues, (ii) relate Mg dissolution to carbonation of the chrysotile mining residues, and (iii) investigate the surface evolution of residue particles during dissolution and its impact on carbonation efficiency.

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EXPERIMENTAL SECTION Material Characterization. Chrysotile mining residues (CMR) as received from the Black Lake mine (Thetford Mines,

Special Issue: CAMURE 8 and ISMR 7 Received: Revised: Accepted: Published: 8726

November 21, 2011 January 16, 2012 January 18, 2012 January 18, 2012 dx.doi.org/10.1021/ie202693q | Ind. Eng.Chem. Res. 2012, 51, 8726−8734

Industrial & Engineering Chemistry Research

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Figure 1. Battery of direct carbonation packed beds. (a) [1] Direct carbonation of dry residue under dry CO2 stream; [2] direct carbonation of both dry and moist residue under moist CO2 stream; [3] direct carbonation of residue exposed to periodic water showering and moist CO2 stream. (b) Features of carbonation packed-bed cell.

ambient carbonation in an accelerated manner by using CO2rich gas feeds. Experiments were carried out using a battery of five fixed-bed cell replicates (Figure 1a), wherein carbonation was monitored simultaneously subject to dry and moist CO2 streams, and to CO2-air mixtures over four continuous days. Each carbonation cell consisted of a 3 cm I.D. quartz tube filled to a height of 5 cm with sieved CMR samples (Figure 1b). Prior to being packed in the cells, the sieved CMR samples were dried at 110 °C for 48 h to eliminate weakly bound water via condensation/adsorption. Each cell was connected to a CO2 (or 90%:10% CO2+air mixture) cylinder, a pressure regulator, a water saturator, and a volumetric gas flow meter. The CO2 gas stream was injected continuously from the cell bottom through a sintered quartz distributor at a volumetric flow rate of 100 ± 3 mL/min/cell, and was either pure dry or moistened (with or without air) CO2. The CMR layer was topped by a free headspace and a sponge placed in the upper part of the tube used to reduce contact with ambient air. The carbonation setup was placed in a temperaturecontrolled hood at 22 ± 0.5 °C to simulate three outdoor carbonation conditions: (i) dry carbonation where dry CMR was contacted with a dry CO2 stream; (ii) moist carbonation where a saturated humid CO2 stream was contacted with dry residue beds; (iii) moist carbonation where a saturated humid CO2 stream was contacted with partially saturated residue beds. In the latter case, to mimic CO2 sequestration following raining episodes, the beds were showered periodically with different amounts of deionized water while a moist CO2 stream was continuously fed in an upward manner. Native and reacted residues were dried prior to carbonate quantification in an oven at 110 °C for 48 h. They were then calcined under a N2 stream in an induction furnace at 810 °C. The evolving gases were desiccated (anhydrous calcium chloride 4−20 mesh powder, Fisher Scientific) and analyzed using a CO2 infrared analyzer (Advance optima continuous gas analyzer A02000 Series, Uras 14). The conversion of Mg into carbonates, YMg, was expressed as the mass of carbonated Mg per mass of CMR-borne Mg on the basis of CO2 release during calcination assuming 1:1 Mg:CO2 stoichiometry as well as after subtraction of the contribution from CMR indigenous carbonates. The sample variability in terms of standard deviation on the carbonation conversion was at most 0.4% as determined from bed replications. Also, the loss of carbon within the desiccator was estimated to be 0.08 mL/mL of CO2, limiting the global error on Mg conversion to around 5.4%.

Québec) were sieved to isolate the fraction passing the 2 mm sieve size for carbonation, dissolution, and electrochemical tests. Elemental analysis of the residue was carried out by digesting CMR samples using HCl, H3PO4, and HF solutions, from which surplus HF was eliminated using boric acid (H3BO3).28 Dissolved samples were analyzed with a Perkin-Elmer inductively coupled plasma−optical emission spectrometry (ICP-OES 43000DV) to assay Mg, Si, Fe, Ca, Cr, Na, K, Al, Ni, and Ti. The crystalline phases in raw and reacted CMR samples were identified from X-ray diffraction (XRD) spectra registered on a Siemens D5000 X-ray powder diffractometer (Cu Kα radiation) at 1°/min (0.02° step size) over the 5−40° scattering angle range. The observed diffraction peaks were compared to those compiled in the Joint Committee on Powder Dif f raction Standards library. Free brucite entwined within the chrysotile residue was isolated using a method for selectively leaching brucitic magnesium from the CMR.29 A 30 mL solution of NH4OH 35% vol was contacted for 36 h in a glass tube with 10 g of CMR previously dried for 48 h at 110 °C. After settling of the residual solids, the supernatant solution was recovered while the bottom most wet cake was washed five times with 10 mL of NH4OH. All the liquids collected were put together in a glass jar and placed in an oven at 60 °C for 48 h for a complete evaporation. The dry precipitate was dissolved by adding 0.5 M HCl solution. The solution is then analyzed using a PerkinElmer AA-800 atomic absorption spectrometer. The amount of Mg dissolved was assigned to dissolved brucite. Serpentine and magnetite were quantified using electron microprobe analysis on a CAMECA SX-100 instrument. The different elements were quantified by aiming at the CMR with a finely focused 1 μm electron beam (10 kV, 20 mA) for 20 s counting times on peaks and 10 s on background noise. The following reagents were also employed in the iron leaching, carbonation, and electrochemical tests: ethylenediaminetetraacetic acid, EDTA (Sigma-Aldrich, >99% purity), trans-1,2-diaminocyclohexanetetra acetic acid, CDTA (SigmaAldrich, 98.9% purity), FeSO4·7H2O (Laboratoire Mat-Québec, 99% purity), FeCl3·6H2O (Anachemia >97% purity), and FeO(OH) 30−50 mesh powder which was provided from Aldrich Chemical Co. inc. Carbonation Setup. One of the impediments to replicate the evolution of ambient carbonation in the laboratory resides in the slow carbonation kinetics prevailing under natural weathering conditions.5 We deliberately chose to emulate 8727

dx.doi.org/10.1021/ie202693q | Ind. Eng.Chem. Res. 2012, 51, 8726−8734


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Figure 2. Packed-bed experimental setup in (a) leaching and (b) electrochemical configurations. Legend: (1) inlet section; (2) Teflon working section (residue enclosure); (3) outlet section; (4) stainless steel casing; (5) sintered glass plate distributor; (6) peristaltic pump; (7) magnetic stirrer; (8) pH-meter electrode; (9) solution reservoir.

Table 1. Elemental Composition from ICP−OES of Chrysotile Mining Residues element

Si

Mg

Fe

Al

Ca

K

Na

Ni

Cr

Ti

wt %

18.7

16.0

6.1

0.28

0.21

0.16

0.16

0.07

0.04

0.005

Dissolution Setup. A schematic of the room-temperature dissolution setup is shown in Figure 2a. Built from stainlesssteel with interior Teflon lining, the leaching reactor consisted of three sections: inlet, working and outlet sections. The stainless-steel reactor body had contact neither with the CMR powder nor with the aqueous solution. Typically the residue dissolution tests used ca. 3 g of CMR sample encased as a packed bed in the cylindrical working section (25 mm H × 15 mm I.D). The fixed bed in the working section (2) was fed from a 250 mL magnetically stirred glass solution reservoir (9). The solution was circulated upwardly in a closed loop through the fixed bed using a peristaltic pump (6) at a flow rate of 2.5 mL/min. The initial pH values of the solution in the reservoir were varied between 3 and 9 via addition of HCl (SigmaAldrich) and NaOH (Fisher Scientific Canada), while during dissolution, the pH was allowed to freely drift until it stabilized. It was monitored using an Oakton 1000 series pH-meter. For Mg analysis, small aliquots were periodically withdrawn from the reservoir and filtered using a filter-equipped syringe (VWR, 0.45 μm). The concentration of dissolved Mg was measured using a Perkin-Elmer AA-800 atomic absorption spectrometer. Electrochemical Setup. The leaching tests suggested that surface phenomena taking place during wet carbonation were detrimental to the release of Mg via some forms of developing passivation. To expose the role of passivation, the previous dissolution setup was converted into an electrochemical reactor (Figure 2b) to allow following in real time the evolution of the passivating layer during surface dissolution of the mineral particles. The CMR powder, thoroughly mixed with 1 wt % of graphite powder, was packed in the lower part of the working section and was electrically isolated by a sintered-glass filter disk (VWR International, USA) from an upper inert quartz powder layer (P80 ≤ 149 μm Sigma Aldrich, Canada). Blank leaching tests performed with the quartz powder confirmed that it was (electro)-chemically inert thus had no effect on the leaching

behavior of CMR. A platinum spring was inserted inside each layer and linked to an insulated platinum wire that was used to establish, via a carbon brush, electrical contact between each mineral electrode and a multichannel potentiostat−galvanostat (model VSP-27 from Bio-Logic SA). The current/voltage was monitored using a silver/silver chloride (Ag/AgCl in saturated AgCl-KCl solution) reference electrode (+197 mV vs standard hydrogen electrode). A lugging capillary tube housing the reference electrode was placed in the magnetically stirred glass reservoir (9). The reservoir contained 75 mL of a 1.5 M NaCl background electrolyte at initial pH 6.8 which was circulated upflow in a closed loop through the two-layer packed-bed reactor. Oxidation and reduction peaks, generated when scanning the potential between −1 V and +1 V, were obtained using cyclic voltammetry. Prior to starting each sweep, the CMR working electrode was maintained for 10 s at −1 V versus the reference electrode.

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RESULTS AND DISCUSSION Mineralogy of Chrysotile Mining Residue. The elemental analysis shown in Table 1 indicated that Si, Mg, and Fe were the major elements of the mining residue while the remaining assayed elements Al, Ni, Ti, Ca, Cr, Na, and K were minor. The main crystalline phase composing the CMR was Mg3[Si2‑xO5](OH)4−4x which we assigned to chrysotile (Figure 3a). Magnetite [Fe2+Fe23+O4] and brucite [Mg(OH)2] were also present. The other detected crystalline phases were minor and included magnesium carbonate [MgCO3], albite [NaAlSi3O8], chlorite [Mg6Si4O10(OH)8], olivine [Mg2SiO4], phlogopite [KMg3(Si3Al)O10(OH)2], and talc [Mg3(SiO5)2(OH)2]. Indigenous carbonates and free brucite in the raw CMR (Figure 3a insets) contributed, respectively, 0.35 wt % (as measured from CO2 infrared analysis) and 1.3 wt % (as measured from brucite selective dissolution which is very close to the determination of Pronost et al.30) of the sample’s weight. These led to 0.9% and 8728



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Figure 3. XRD spectra in the 5−40° 2θ region (Cu Kα) of (a) native CMR, (b) reacted CMR (4 days, saturated humid CO2 stream, daily watering × frequency = 12.5% per day × 4). Insets show magnified peaks for brucite and magnesite. Intensity in vertical scales shown in panels a and b are identical pairwise: (1) = chrysotile; (2) = brucite; (3) = magnesium carbonate; (4) = magnetite; (5) = silica.

is close to that established from Mössbauer spectroscopy for pure chrysotile fibers.8 The FeIII/FeII ratio in the magnetite-rich regions (Figure 4) was equal to 2.06 (Table 2) confirming the XRD-evidenced magnetite. Carbonation of Chrysotile Mining Residue. Effect of Watering. Passing dry CO2 through a dry bed of residue revealed that unassisted CO2 did not induce any meaningful carbonation reaction (ca. 0.015%) of the residues after 4 days (run 0%-D in Figure 5a). This finding is in accordance with our earlier observations on chrysotile unreactivity to carbonate in H2O-deprived post-4 and precombustion8 effluents. Contacting dry CMR with a CO2 stream saturated with humidity led to a MgCO3 conversion, YMg ≈ 0.4% after 4 days at 22 °C (Figure 5a). At such low temperatures, gas−solid carbonation is very unlikely to have taken place unless condensation of moisture within the microregions in the samples would have formed as a prerequisite to wet carbonation30 to dissolve Mg and CO2. At 22 °C and at saturated water vapor pressure, condensation/adsorption on chrysotile led to a water weight gain of 1.5 wt %.7 Water adsorption isotherm studies support wet carbonation at ambient temperature as a plausible explanation. The role of water to stimulate carbonation was further revealed by initially imbibing the CMR beds with 3, 10, 15, and 25 wt % of deonized water. These amounts corresponded, respectively, to 12, 40, 60, and 100% pore water saturation (fraction of pore volume) of CMR beds prior to sparging gas through the beds. The saturation of gas with humidity prevented the beds from drying, whereas bed wetness prompted Mg dissolution aided by the acidity from absorbed CO 2.

3.6% of initial Mg as MgCO3 and Mg(OH)2, respectively. The electron microprobed distribution of elements over serpentinerich (dark spots 1−8) and magnetite-rich (bright spots 9, 10) microregions is displayed in Figure 4 and Table 2. Intraframe-

Figure 4. Backscattered electron photomicrograph of fresh CMR with analytical spots 1−10 for elemental analysis (Table 2).

work substitutions of Mg with Fe occurred in the serpentinerich regions leading to average atom ratios Mg/Si ≈ 1.47, Mg/Fe ≈ 39.4 and Si/Fe ≈ 26.85 and to substituted chrysotile with formula (FeII0.025Mg0.975)3Si2O5(OH)4. This stoichiometry 8729



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Table 2. Average Composition by Electron Microprobe Analysis of Serpentine and Magnetite from Chrysotile Mining Residues, Black Lake Mine SiO2

a

Al2O3

Cr2O3

MgO

wt % std. devb

42.37 0.67

0.26 0.22

0.09 0.08

41.75 0.90

wt % std- devc

0.083 0.022

0.003 0.004

0.27 0.01

0.34 0.06

CaO

MnO

Serpentine 0.022 0.061 0.008 0.037 Magnetite 0.003 0.38 0.004 0.16

FeO

Fe2O3

1.887 0.533 29.71 0.32

NiO

TiO2

99.45a 1.41

0.205 0.103 67.91 0.19

Total

0.012 0.016

98.72 0.26

Balanced with water. bEight analyses. cTwo analyses.

Figure 5. Mg conversion into carbonates of chrysotile mining residue at ambient conditions. Effect of (a) watering (X%-D, percent of pore saturation with liquid water−dry CO2; X%-M, idem−moist CO2); (b) watering frequency (Y (frequency)*X%); (c) iron passivation for (4 × 25%) watering runs. Deonized water pHi = 6.8.

Exposure for 4 days of these beds to water-saturated CO2 streams and (stagnant) liquid water boosted MgCO 3 conversions to 0.7, 2.2, 3.6, and 5.4%, respectively. The XRD peak intensities of MgCO3 and brucite, respectively, increased and decreased in the carbonated CMR suggesting native brucite and chrysotile alike were partially carbonated (Figure 3b). The more water that was added the more Mg was carbonated (Figure 5a) though carbonation slowed down after 24 h (Figure 6). Unlike dry periods, raining episodes appear to be more beneficial for CO2 capture in CMR (5.4% conversion). Dry residues in contact with moistened CO2 showed modest carbonation (0.4% conversion), but this value was nonetheless more significant vis-à-vis dry CO2 (0.01% conversion). Effect of Watering Frequency. To gain insight into how water content can affect field carbonation, rain events were

Figure 6. Time evolution at ambient temperature of CMR carbonation for runs 100%-M. 8730



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Therefore, transient responses of dissolved magnesium in leachate were monitored in the dissolution setup (Figure 2a) by contacting CMR beds with recirculating solutions for initial pHi 3, 5, and 9 as shown in Figure 7a. No gas was sparged during

mimicked by adding to CMR beds the same cumulative amount of water but at different frequencies (corresponding to 100% of water pore saturation before gas flow): one single initial event for (1 × 100%), one injection every 2 days (2 × 50%), one injection every 32 h for (3 × 33%), and finally one injection every 24 h (4 × 25%). For the same cumulative amount of added water, MgCO3 conversion positively correlated with watering frequency (Figure 5b) reflecting total magnesium conversions of 5.4 (1 × 100%), 6.5 (2 × 50%), 6.7 (3 × 33%), and 6.9% (4 × 25%). Decreasing the watering pore volume and increasing watering frequency seems to enhance CMR CO2 sequestration. This is illustrated with a run performed with adding four times 12.5% of water pore saturation every 24 h which improved MgCO3 conversion to 7.4 wt % (Figure 5b). This improvement is understandable on account of the fact that reducing the amount of water promotes Mg supersaturation and increases pH, which are both favorable for carbonate precipitation. Furthermore, silica gel polymerization around the dissolving CMR particles is favored by large water excess.31 Therefore, the use of smaller amounts of water will have the tendency to slow down densification of the passivating silica layer thus enabling more Mg dissolution. Effect of Iron Hydroxide Passivation. Bleeding a tiny air flow rate along with a water-saturated CO2 stream translated into a drop of carbonation conversion from 6.8 to 4.0 wt %, shown as CMR and CMR(air) bars in Figure 5c. Being the third element in abundance after Si and Mg (Table 1), interference of iron with magnesium dissolution and carbonation is interpreted to be the cause of such loss in CMR carbonation capacity. To identify iron precipitation as a contributing factor to passivation, beds with Fe precursors FeSO4·7H2O, FeCl3·6H2O, FeO(OH) mixed with CMR were prepared. The synthetic composite beds emulated the release of homogeneous ferrous (FeSO4·7H2O) and ferric (FeCl3·6H2O) cations as well as heterogeneous FeIII (FeO(OH)) readily available nearby the Mg-bearing minerals under anoxic conditions. The added masses of Fe varied from 2.5 to 5 wt % per unit mass of CMR to reflect the Fe content in the native residues. The amount of water added corresponded to 100% of pore water saturation before gas flow. The leaching of free Fe3+ from CMR minerals is deleterious to mineral carbonation as is Fe2+ which is prone to oxidation into Fe3+ in the presence of dissolved oxygen. Figure 5c corresponding to runs with (4 × 25%) watering frequency highlights a negative correlation between carbonation conversion and added free (or homogeneous) iron. Compared to CMR carbonation without iron addition, CO2 uptakes decreased by 66% (45%) after adding 5% of free Fe3+ (free Fe2+). Also, reduction of free Fe3+ from 5% to 2.5% barely improved CO2 uptake from 2.3% to 2.8%. These trends are consistent with ferric precipitation before ferrous iron as the cause for reduced CO2 uptake (Figure 5c). However, carbonation conversions of CMR beds were indifferent to FeO(OH) addition. Indeed, independent FeO(OH) dissolution tests between pH 3 and 9 confirmed that iron did not leach even with the aid of EDTA and CDTA chelating agents. Mg Leaching from Chrysotile Mining Residue. The above carbonation experiments indicated that MgCO3 conversion was possibly limited by passivation phenomena as a consequence of silica deposits and iron electrochemicalprecipitation effects hindering further releases of leached magnesium into solution.

Figure 7. Time evolution at ambient temperature of (a) leached magnesium concentration (b) and pH in CMR leachate for several initial solution pH values (pHi = 3, 5, 9) at various ferrous and ferric loadings at pHi = 5 (Fe loading expressed as mass of iron in added sulfate or chloride per CMR unit mass).

dissolution so that the solution across the bed continuously saturated its pores. The extracted Mg was expressed as the mass percent, XMg, of leached Mg per mass of CMR-borne magnesium, that is, typically, 480 mg of Mg. Leached magnesium leveled off at XMg = 1.8 (pHi = 9), 2.4 (pHi = 5), and 5.7 (pHi = 3) wt % with more Mg extraction the lower the initial pHi. Left to drift freely, pH mirrored the Mg dissolution patterns and stabilized in the alkaline range, respectively, around 9.3 (pHi = 9), 9.2 (pHi = 5) and 8.8 (pHi = 3), Figure 7b. A rapid dissolution over the first 30 min was followed at pH ≈ 6.5 and higher by a stage of gradually reduced dissolution rate until a steady state was reached. The ultimate leached Mg outpaced the content of CMR in native brucite (i.e., 1.08%) confirming partial dissolution of chrysotile. Also, Mg recovery was marginally boosted by conducting iso-pH leaching in the acidic range, by gradual addition of 0.1 M HCl. For instance, XMg plateaued near 8 wt % at pH 3.5, Figure 7a. The observed dissolution pattern is reminiscent of surface passivation induced by incongruent dissolution of some CMR components. This was qualitatively verified by the deposition of a silica phase as a product of polymerization of fine silica particles formed during CMR dissolution (Figure 3b) and by the appearance at pH 3 of a yellowish color suggesting iron precipitation. 8731



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Figure 8. Cyclic voltammograms of CMR illustrating current−voltage (redox) behavior as a function of time: (a) progressive CMR passivation in anoxic solution via Si-rich layer formation; (b) fast CMR passivation in oxic solution via Fe(III) hydroxide formation; (c) cleanup of CMR surface in oxic solution via CDTA complexation of iron cations.

Several investigations have shown that magnesium silicates in contact with aqueous media lead to the development of an outer Si-rich layer.6,32−34 Growing inward, and also possibly circumferentially, during particle leaching this product layer hampers fluid accessibility to the reactive surface sites slowing down mineral dissolution. In the long run, hindrance of intralayer mass transport is aggravated by polymerization due to >Si−O−Si< bonding within the Si-rich layer cementing the active surface.34 Being present in the CMR as magnetite grains or incorporated within the magnesium silicate structure (Table 2), iron is likely to interfere with magnesium dissolution. Hence similarly to the above carbonation tests, to determine whether Mg dissolution is sensitive or not to iron interference, synthetic solutions doped with FeSO4·7H2O and FeCl3·6H2O salt precursors at pHi 5 were contacted with CMR beds. Such additions notably reduced the concentration of leached magnesium (Figure 7a) though nearly without change of the final pH (Figure 7b). Ferric precipitation at lower pH than ferrous iron consistently led to less Mg recovery (Figure 7a). This indirect test suggests that if dissolution of CMR-borne Fe (II) and Fe (III) occurred, it would likely have led to iron hydroxide reprecipitation thus limiting Mg dissolution from CMR. Inhibitory Effects of Surface Phenomena. The limited magnesium liberation from CMR, and carbonation thereof, is likely controlled by surface phenomena linked to incongruent mineral dissolution. Visual inspection showed that formation/ densification of a silica layer and oxidation/precipitation of CMR iron occurred simultaneously. To separate the influence of each phenomenon, specific experiments were realized using

cyclic voltammetry. The use of this electrochemical technique has been recently illustrated in the case of nonconductive materials such as magnesium silicates.35,36 Silica Gel Passivation. To study the effect of the formation of a Si-rich layer and densification, cyclic voltammetry was implemented in anoxic conditions by bubbling N2 into recirculating 1.5 M NaCl solutions using the fixed-bed configuration shown in Figure 2b. The voltage scans were performed at t = 0, 30, 90, 120, 240, and 1200 min time intervals (Figure 8a). At each time, the passivating nature acquired by the residue particle surfaces was identified by scanning the potentials at 50 mV/s from −1 to +1 V (vs Ag/AgCl reference electrode). The moment the recirculating solution first hit the CMR bed and returned back to the reservoir was taken as t = 0 and was featured by setting electrical contact between electrodes. Two peaks centered near −0.75 and 0 V emerged in the halfcycle oxidation sweep (Figure 8a). Consistent with Prandi et al. peak assignments,36 these peaks correspond to Fe2+ (and FeII) oxidation in solution (and at the surface of the CMR particles). These peaks only reflect the electrochemical phenomena occurring over the developing CMR silica layer by virtue of the silica layer inertness. The voltammograms first show two prominent oxidation current peaks growing from t = 0 to 30 min to reach, respectively, 1220 and 2050 mA. The increase in intensity of the first oxidation peak is consistent with rapid dissolution, as portrayed in Figure 8a, of the finer CMR particles during the first 30 min of leaching, promoting passage of Fe2+ into the solution surrounding the working electrode. Also, inflation of 8732



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CONCLUSION Carbonation of chrysotile mining residues under ambient conditions was studied. The effect of watering on carbonation was investigated comprehensively. Parameters such as the amount of water added, watering frequency, and passivation phenomena have significant effects on the carbonation efficiency. The packedbed electrochemical reactor proved very helpful to decouple the types of surface passivation phenomena which can occur during residue dissolution in oxic and anoxic solutions. The best MgCO3 conversion efficiency was reached by adding an amount of water equal to 12.5% of the pore volume once every 24 h for 4 days, resulting in nearly 22 mg CO2 captured per gram of residue. While Si-rich gel passivating layer densification was slowed down by regularly adding small volumes of water, iron passivation was found to have a strong impact on limiting CMR carbonation. Working under ambient conditions and without using costly reagents to promote residue dissolution, 0−2 mm grain size CMR revealed that up to 93% of the magnesium would not be available for carbonation because of the slow dissolution of residues and especially the formation of passivating layers of FeIII hydroxide and silica. One further finding from this study was that once formed, carbonates did not dissolve during the subsequent watering episodes. Ultramafic rock ores mined at the Black Lake mine and other chrysotile mining operations are usually stacked in heaps and left barren which is not optimal for CO2 capture. Controlling watering and above all preventing water loss by placing the residues on an impermeable barrier with a shallow slope to recover watering water could prove very helpful to improve the performance of residues for CO2 sequestration. Using finer particles and limiting iron passivation, by means of proper chelating agents, are foreseen to improve both carbonation rate and capacity.

the higher-voltage oxidation peak corresponds to the heterogeneous ferrous species made more readily available on the residue surface by dissolution. After the first 30 min, the working electrode in the packed bed registered peak oxidation currents that decreased progressively with time. Fading of the lower-voltage oxidation peak is symptomatic of the interruption in the release of Fe2+ toward the solution by an irreversible process such as the buildup of an electrochemically inert passivation layer. Also, the low reduction peak currents were almost constant near −637 mA (Figure 8a) during the whole experiment, suggesting ferric iron formation was marginal and that passivation was the result of electrochemically nonreducible species. Depletion of dissolved oxidizing agents (Figure 8a) and disruption of Mg dissolution (Figure 7a) point to the formation of a silica-passivating layer hindering transit of the soluble species.14,28−31 After 1200 min, the iron oxidation peaks became hardly recognizable as the passivating layer of silica completely obstructed the grains and electrode outer surfaces according to the reaction Mg3Si2O5(OH)4 + 6H+ → 3Mg2+ + 2SiO2 + 5H2O. Iron Hydroxide Passivation. In addition to the deposition of silica gel, the precipitation of iron hydroxides is also detrimental to Mg recovery (and thus to furthering carbonation). To highlight the effect of passivation caused by iron precipitation, experiments were carried out by sparging air into the reservoir to ensure saturation in dissolved oxygen of the recirculating 1.5 M NaCl solution. As seen in Figure 8b, the oxidation peaks in oxic conditions are notably less prominent than those registered during the anoxic runs at the same corresponding times (Figure 8a). Such differences arise because of the depleted Fe2+ (FeII) in solution (on residue surfaces) which were mostly oxidized into ferric hydroxide. Unlike in the anoxic runs, the oxidation peak intensity of Fe2+ in solution, that is, the lower-voltage peak, collapsed very quickly over time. This behavior can be ascribed to ferric hydroxide depositing on the working electrode and on the surface of residue particles as well. The measured current corresponding to the reduction of FeIII formed on the surface of residue particles, observed between 100 and −300 mV, was so high that it merged near −300 mV with the reduction of the supporting electrolyte. In this case, the peak height was more prominent (−1320 mA) than that registered during the anoxic tests (−637 mA) revealing that FeIII was more abundant on the residue particles and thus available to be reduced back to FeII. Surface passivation by ferric hydroxide in oxic solutions took over quickly as compared to silica passivation which, in anoxic solutions, was shown to evolve more slowly. The reason why in oxic solutions the oxidation peaks did not disappear completely is because before resumption of each voltammetric cycle the working electrode had to be maintained for 10 s at −1 V where some surface ferric reduction could have taken place. To highlight the extent of iron passivation in alkaline solutions, electrochemical experiments were carried out using a 0.3 M CDTA chelating solution. The oxidation peaks corresponding to surface FeII were virtually nonexistent contrary to the oxidation peaks of soluble Fe2+ which strengthened with time (Figure 8c). Therefore CDTA complexation, especially of trivalent iron, prevented precipitation of recently oxidized surface ferric ions (due to air bubbling) and thus procured further evidence for the role of precipitation of iron species on Mg dissolution and its subsequent carbonation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the Fonds québécois de la recherche sur la nature et les technologies (FQRNT) under the project: Programme de recherche en partenariat contribuant à la réduction et la séquestration des gaz à effet de serre. Financial support from the Natural Sciences and Engineering Research Council Discovery Grants to F.L., G.B., and J.M. is gratefully acknowledged.

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REFERENCES

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