Balucan et al_2013_Thermal Activation of Antigorite for Mineralization of CO2_Paper and SI

Thermal Activation of Antigorite for Mineralization of CO2

Reydick D. Balucan and Bogdan Z. Dlugogorski*

Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia

*S Supporting Information

ABSTRACT: This contribution demonstrates the sensitivity of antigoritedehydroxylation to treatment conditions and discusses the implications ofthe observations for scientific (i.e., dehydroxylation kinetics) andtechnological (i.e., energy efficient conditions and design of practicalactivation reactors) applications. At present, the energy cost ofdehydroxylation of serpentinite ores represent the most importantimpediment for a large scale implementation of sequestering CO2 bymineralization. We have analyzed changes in antigorite’s derivativethermogravimetric curves (DTG) and deduced factors affecting the massloss profiles. The imposed heating rate, type of purge gas, type ofcomminution and sample mass all influence the dehydroxylation curve.However, the results show no influence of material of construction of theheating vessel and flow rate of the purge gas. We report an important effectof oxidation of Fe2+ under air purge gas that occurs prior to dehydroxylation and leads to formation of hematite skins onserpentinite particles, slowing down subsequent mass transfer and increasing the treatment temperature. From the processperspective, 75 μm particles afford optimal conditions of temperature and rate of dehydroxylation. Overall, the practicalconsiderations, in thermally activating serpentinite ores for storing CO2 by carbonation, comprise rapid heating, proper sizereduction, prior demagnetisation, and fluidization of the powder bed.

1. INTRODUCTION

Accurate measurements of the dehydroxylation of serpentineminerals during thermal treatment allow deriving thethermokinetic parameters and calculating the necessary heatrequirements. Such measurements may also serve to developnew technologies for activating serpentines, and to designequipment items, for implementing CO2 sequestration bymineralization at a realistic scale. However, despite thenumerous thermal studies on serpentine minerals1−15 only asmall number of investigations examined the effect of treatmentconditions.9,14,16,17 To the best of our knowledge, no studyevaluated the influence of these conditions for preparingactivated serpentine minerals for their carbonation. Inves-tigations are needed to identify and quantify the effect of thetreatment parameters on the thermal activation of serpentines.Such investigations must supply information of kinetics ofserpentine dehydroxylation, that, in combination with heattransfer parameters, could serve to design unit operations(equipment items) for testing the viability of mineralcarbonation at a pilot plant scale.Outstanding questions include the determination of the

suitable feedstock for heat activation (crushed versus ground),the appropriate operational sequence and cost efficiency(kWhe) for each option. Operational viability and costefficiency of thermal activation of serpentines for mineralcarbonation must dictate the material’s particle size,14 andhence comminution technology. Small scale experiments couldassist in identifying the practical particle size among those

currently used (−38 to −75 μm)18−21 in serpentinecarbonation at a laboratory scale.The influence of the material of construction of a unit

operation and type of a purge gas on efficiency of theprocessing operation remain poorly understood. Both variablesrequire attention due to variable mineralogical and chemicalcomposition of serpentinite rocks.22−24 Not only do these rockshost the rock-forming serpentine minerals (lizardite, antigorite,and chrysotile), they may retain their relict peridotitic minerals(forsterite and enstatite) as well as contain various amounts ofmetal oxides associated with the serpentinisation process (i.e.,the exothermic hydration of the peridotitic minerals). Cationssuch as Fe2+, Fe3+, and Al3+ are incorporated into the octahedraland tetrahedral sheets of the serpentine minerals.23,25−28

Normally, Fe2+ replaces Mg2+ in octahedral sheet, whereasFe3+ and Al3+ may appear both in octahedral and tetrahedralsheets, replacing Si4+ in the latter.29 The oxidation of Fe2+ wasreported to influence the dehydroxylation process.30,31 It istherefore of practical significance, to the design of adehydroxylation reactor, to understand the suitability of arefractory vessel (e.g., alumina) and purge gases (e.g., CO2,water vapor or air).

Special Issue: Carbon Sequestration

Received: September 11, 2012Revised: October 28, 2012Accepted: November 28, 2012Published: December 6, 2012

Article

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The present study examines antigorite, the most thermallystable serpentine mineral1 which contains the highestproportion of oxidizable Fe2+ among the three commonpolymorphs, and is expected to present higher energyrequirements for thermal processing than those of lizarditeand chrysotile. Because of this consideration, antigoriteprovides a conservative benchmark of energy requirement forthermal processing of serpentines. Previous evaluationsassumed electrical heating rather than the direct use of thermalheat for activation, leading to unrealistic estimates of energyintensity of CO2 storage by mineral carbonation.18,32

The overall scientific objective of this work was to gaininsights into the behavior of antigorite undergoing dehydrox-ylation, and, in particular, to determine how the rate ofdehydroxylation is affected by the experimental conditions andmineral preparation. Specific objectives were (a) to quantify theeffect of hematite formation from oxidizable Fe2+ present in themineral on the rate of dehydroxylation and to investigate theinfluence of the atmosphere (i.e., oxidative vs reductive) on therate, as we hypothesized that the appearance of a hematite layermay add mass transfer resistance to the removal of water vaporfrom the mineral; (b) to investigate the influence ofcomminution type (wet vs dry grinding) on the dehydrox-ylation of antigorite and examine the feasibility of dehydrox-ylation of crashed but unground mineral, as there is mountingevidence that surface properties of mineral grains may modifytheir thermal behavior; (c) to study the relationship betweenthe rate of dehydroxylation and the rate of heating, and thedependence of the rate of dehydroxylation on particle size; aswell as (d) to investigate the effect of liberated water vapor, aswater vapor may engender backward reactions and its presence

may force the dehydroxylation to take place closer tothermodynamic equilibrium delaying the onset of the process.

2. EXPERIMENTAL SECTION

Antigorite, obtained near Bingara in the Great Serpentinite Beltin NSW, Australia (location: 30.122217 S and 150.635966 E)was prepared into various sample fractions by crushing (sampleE) and subsequent grinding (A, B, C, D). The wet groundsamples (A, B, C) were demagnetised prior to heat activation,while the dry ground (D) and crushed (E) samples were usedwithout further demagnetisation. Ground materials (A, B, C,D) are all in powder form while a single shard represents thedry crushed sample (E). Table 1 details the chemical andmineralogical composition as well as the particle sizes of thesample fractions.X-ray powder diffraction (XRPD) in a Philips X’Pert Pro

multi purpose diffractometer, using Cu Kα radiation in therange of 6−90° 2θ, with a step size of 0.02° and collection timeof 1 s step−1, afforded the identification of crystallinecomposition (Figure S4, Supporting Information). Thediffraction patterns were automatched against the InternationalCenter for Diffraction Data using X’pert Highscore, andconfirmed visually for validity.34 Chemical composition of thenatural antigorite was characterized via X-ray fluorescence byAmdel Laboratories, whereas a Spectro X’lab 2000 at theUniversity of Newcastle EM-X-ray Unit was used to analyze thedemagnetised samples. Particle size distribution was obtainedvia low angle laser light scattering (LALLS) using a MalvernMastersizer 2000 laser sizer in aqueous media. Micrographs ofthe gold coated samples of the starting and quenched materials

Table 1. Chemical, Mineralogical and Physical Properties of the Sample Fractions; all Particle Sizes are in μm

A B C D E

demagnetisedfractions

magneticfractions

chemical composition of starting material, % weightc SiO2 43.2 41.8MgO 38.2 38.2FeOa 3.08 3.08Fe2O3

b 1.74 3.79Al2O3 1.04 0.89CaO 0.13 0.05Na2O 0.11 0.05LOI 11.9 12.0

mineral composition (International Center for Diffraction Data reference code) starting material antigorite-8.0 M (00−007−0417), dimagnesium oxidedihydroxide (01−070−9187)

antigorite-8.0 M(00−007−

0417) periclase(01−077−

2364), triirontetroxide(01−089−0691)

quenched materiald forsterite (00−034−0189), enstatite (00−019−0768)

particle size of starting material, μm (±3.8 μm) d3,2 5.31 4.85 4.37 2.87d4,3 44.2 31.0 17.9 6.35d90 125 86.4 47.2 14.2d50 15.7 11.8 9.03 4.23 ≥2000d10 1.82 1.73 1.69 1.39d80

e 75.0 52.0 31.0 10.0aObtained from titration, represents the Fe2+ content of the primary rock. bRepresents magnetite (Fe3O4) produced from serpentinisation. cTracecomponents <0.05% are: SO3, TiO2, K2O, organic C and inorganic C. dAfter heating to 1000 °C. eDerived from the power trendline of the d10, d50,and d90 value.

Environmental Science & Technology Article

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were collected using a Philips XL30 scanning electronmicroscope (SEM) operated at 15 kV at 20000× magnification.A Setaram Setsys Evolution 1200 thermogravimetric

analyzer-differential scanning calorimeter (TGA-DSC) usingSeftSoft 2000 software recorded the thermogravimetric (TG)and derivative thermogravimetric (DTG) curves. The replicateruns were acquired at random dates within a period of threemonths to check for instrumental drift and ensure experimentalreproducibility for the entire duration of the study. Theexperimental conditions for the control experiment are asfollows: 5.5 ± 0.1 mg wet ground, d80 = 75 μm demagnetisedantigorite (Sample A), open cylindrical alumina crucible (5 mmØ, 8 mm height, 0.10 cm3 capacity), heated from 30 to 1000 °Cat a heating rate (β) of 10 °C min−1, under argon purge gasflowing at 20 mL min−1. The samples, once loaded, wereslightly tapped twice in order to distribute the powder evenlywhile avoiding the compaction of the sample bed. Table 2summarizes the conditions used to evaluate the effects of theexperimental variables on antigorite dehydroxylation.

3. RESULTS AND DISCUSSION

Antigorite dehydroxylation, as seen in the DTG curve in Figure1, covers a wide temperature region varying from ∼500 to 800°C. This temperature region defines the removal of structurallybound water, constituting 11.43 ± 0.03% w/w antigorite(Δm105−850). The mass loss of <0.5% w/w from 105 to 500 °Cindicates negligible amounts of either adsorbed moisture ordeformed hydroxyl groups. It must be noted that, fulldehydroxylation is attained at temperatures in excess of 850°C, as such all the quenched products (heated up to 1000 °C,then cooled) are fully dehydroxylated. Weight normalized DTGcurves show the peak temperatures, Tp, at 715 ± 2 °C (Tp1)and 736 ± 2 °C (Tp2). The DTG curve of this sample is typicalfor antigorite, whereby peak temperatures (Tp) are in excess of720 °C. Overall, the dehydroxylation profile of this antigoritespecimen (sample A) is in agreement with the previouslyreported thermal profiles of other antigorites.1

Although the DTG curves generated by the small samplessize (5.5 mg) are uncontaminated by heat and mass transferlimitations, the curves’ attributes are not as smooth as thoseobtained at higher masses >30 mg.1 The pertinent thermalsignatures include the serpentine doublet which comprises alow temperature shoulder, Tsh and the first peak temperature,Tp1, as well as antigorite’s diagnostic high temperature peak, Tp2(Table S4, Supporting Information). The low temperatureshoulder, Tsh, encompasses 635 to 679 °C and is commonamong serpentine minerals, whereas the Tp’s are shifted tohigher temperature with respect to lizardite and chrysotile.From the present results and those of Viti1 for antigorite, ourspecimen’s Tp1, falls around 710 to 720 °C, whereas thediagnostic peak, Tp2, lies between 730 and 760 °C. At eachrespective Tp, the mass loss rate maxima, -(dm/dt)/mo_max_Tp1and -(dm/dt)/mo_ max_Tp2, are roughly identical at 1.7 × 10−4 ±1.4 × 10−6 s−1 (for both Tp1 and Tp2).The quenched material, shown in Figure 2, indicates that full

dehydroxylation (∼1000 °C) of antigorite results in arecrystallized solid. The fully dehydroxylated mineral shows

Table 2. Conditions Employed to Evaluate the Effect of Experimental Variables

heatingmaterial

gas flow rate,mL min‑1

particle size d80, μm(±3.8)

sample mass, mg(±0.1)

purgegas

heating rate,°C min‑1

type of comminutedsample

control alumina 20 75 5.50 argon 10 wet groundeffect of heating material alumina 20 75 5.50 argon 10 wet ground

platinumeffect of purge gas flowrate

alumina 20 75 5.50 argon 10 wet ground200

effect of particle size alumina 20 31 5.50 argon 10 wet ground5275

effect of sample mass alumina 20 75 2.75 argon 10 wet ground5.5011.037.7

effect of purge gas alumina 20 75 5.50 argon 10 wet groundCO2

aireffect of heating rate alumina 20 75 5.50 argon 10 wet ground

2030

effect of comminution alumina 20 75 37.8 argon 10 wet ground10 35.6 dry ground

≥2000 49.9 dry crushed

Figure 1. TGA-DTG profile of the antigorite showing a typicalreplicate runs for the control experiment. See text for detaileddiscussion.




an evident morphological change suggesting that it hasundergone significant structural reorganization and subsequentparticle contraction. The mineral phases identified in thedehydroxylated material (Table 1), shows forsterite andenstatite regardless of material preparation, be it eitherdemagnetised (A, B, C) or magnetic fractions (D, E). This is

as expected, as our previous study on this particular antigoritespecimen33 showed forsterite and enstatite already in existenceby 725 and 825 °C, respectively.With forsterite present by 725 °C, prior to full dehydrox-

ylation, it is expected that antigorite dehydroxylationdecelerates due to structural reorganization and subsequent

Figure 2. SEM micrographs (a) untreated and (b) heat treated antigorite under argon purge gas showing the general change in particle morphology.

Figure 3. Antigorite DTG curves in various (a) types of heating material, (b) rates of purge gas, (c) particle sizes, (d) sample mass, (e) types ofpurge gas, (f) heating rate (g) type of comminution, and (h) heating rates in both inert and oxidizing gas.





particle contraction. External factors such as those evaluated inthis study could further influence the mass loss behavior, riskingfurther deceleration due to the combined effect of theseexperimental variables and solids recrystallization. For example,the entrapment of water vapor due to passivation by hematitemay further delay antigorite Tp’s to higher temperatures.It is therefore essential that fundamental kinetic studies and

cross-comparison of reported kinetic parameters recognize bothinternal (solids recrystallization) and external (experimentalvariables, especially partial pressure of H2O) effects. This meansthat, ideally, comparison of kinetic parameters is possible forstudies employing the same experimental conditions (i.e., sameparticle sizes and sample mass). This explains the largevariations in the reported kinetic parameters for serpentineminerals, where, activation energy, Ea ranges from 160 to >543kJ mol−1 and pre-exponential factor, A, varies from 1 × 10−8 to1 × 108 s−1.2−5,9−11,15,33

Figure 3 shows the DTG curves of antigorite under differentexperimental conditions. Variations in sample mass, purge gas,type of comminution, and heating rates result in a markeddeparture from the mass loss profile of the base case, whilechanges in the type of heating vessel, purge gas flow rate andparticle size do not influence the DTG curves. The cruciblematerial (alumina vs platinum) does not affect the mass-loss

profiles (Figure 3a). This indicates that a wide range ofmaterials could be appropriate for constructing the dehydrox-ylation reactor. In a practical sense, serpentine heat treatment(i.e., 25 to >800 °C) could involve a vessel made either of arefractory material (i.e., alumina) or a relatively inert material(i.e., stainless steel), with the material selection based onsurface erosion rates during treatment.The identical thermal responses for both purge gas flow rates

(Figure 3b) imply that, at 20 mL min−1, the dehydroxylationreaction is neither hindered by entrapment of liberated watervapor within the sample matrix nor limited by any build up ofproduct gas above the sample bed. Furthermore, this suggeststhat, for the present set of experimental conditions (i.e., 5.5 mgof 75 μm antigorite heated in alumina crucible from 25 to 1000°C at 10 °C min−1 under argon purge flowing at 20 mL min−1),the dehydroxylation reaction proceeds far from equilibrium;that is, irreversibly, with a negligible rate of reverse reaction. Inpractical situations, with water vapor accumulating in the purgegas, this deceleration in the reaction may need to be included inthe design calculations.8,9

Figure 3c shows similar thermal curves for the particle sizestypically employed in mineral carbonation. Unless required bythe carbonation reaction, the result indicates that, stage 2grinding, to produce feedstocks −38 μm in size at cost of about

Figure 4. Deviations in (a) practical activation temperature, T‑H2O and (b) mass loss rate, r‑H2O of the antigorite sample; β denotes the heating rate.The deviations were assessed based on the established practical activation temperature, T‑H2O = 725 ± 2 °C and mass loss rate, r‑H2O = 1.7 × 10−4 ±1.4 × 10−6 s−1.




63.5 kWhe (tonne mineral)−1,18 is unnecessary due to similarthermal curves generated by particles of 31 to 75 μm in size. Itis indeed more practical and cost efficient to subject largerparticles (75 μm) to thermal treatment. Under this option, thetotal energy requirement, for prior size reduction to producefeedstocks 75 μm in size, amounts to 11.8 kWhe (tonnemineral)−1. This estimate accounts for electrical power requiredfor both crushing (1.8 kWhe (tonne mineral)−1) and stage 1grinding (10 kWhe (tonne mineral)−1, to −75 μm).18 Thistranslates to energy savings of about 53.5 kWhe by avoidingstage II grinding.The treatment conditions reported in Figures 3d−h influence

antigorite’s mass loss profile. Employing larger sample mass anddifferent oxidizing purge gases results in minor variations, whilethe use of a crushed sample with no demagnetisation andemploying high heating rates significantly alter the mass-lossrate.The process parameters that resulted in significant deviations

in thermal profiles of antigorite were analyzed in detail to assessthe extent of their influence. To simplify the evaluation, werepresented the thermal profile (Table S5, SupportingInformation) by an average value of the two peak temperatures,Tp_mean, and the average value of the maximum mass-loss ratefor the temperature doublet, −(dm/dt)/mo_max_mean. Thesevalues, redesignated as T‑H2O and r‑H2O, signify the practicalactivation temperature (i.e., temperature at which dehydrox-ylation rate is fastest) and its corresponding mass-loss rate,respectively.Figure 4 summarizes the extent of deviations from the

characteristic T‑H2O and r‑H2O of the sample and are expressedin terms of ΔT (°C) and % change, in that order, with respectto the base case, for which T‑H2O = 725 ± 2 °C and r‑H2O = 1.7× 10−4 ± 1.4 × 10−6 s−1. While all deviations favor increase inT‑H2O (11−39 °C), r−H2O may either decrease by as much as10% or increase by as high as 192%. This implies that changesin the processing parameters result in delays in the removal ofevolved gases, and could either enhance or deteriorate the rateat which escaping product gas leaves the sample matrix. Theprobable factors explaining this type of behavior and thepossible implications of the present observations to design ofdehydroxylation reactor are discussed in detail in thesubsequent paragraphs.3.1. Effect of Sample Mass. Mass increase by ∼600%

elevated the T‑H2O by 11 °C (Figure 4a) as well as increased ther‑H2O by 10% (Figure 4b). The new Tp mean of 736 °C for 38mg sample of antigorite falls between the previously reportedTp for ∼30 and 44 mg of antigorite at 731 and 749 °C,respectively.1,13 A slight increase in r‑H2O by 4% with 100% massincrease could be explained in terms of competing effects of theincrease in number of hydroxyl sites and reduction in gaspermeability within the sample matrix that elevates H2Oconcentration in void spaces. This means that the evolved gasbecomes entrapped in the stationary matrix, delaying its escapeinto the bulk carrier gas. Evolved water vapor entrained withinthe powder bed induces the reverse reaction, slowing down thedehydroxylation process. This phenomenon, known as thedepth effect, operates similarly in the thermal decomposition ofcarbonates (50−300 mg),35 owing to increased levels of CO2that force the reverse reactions.We estimated the sample loading and bed height, based on a

tap density of 1.53 g mL−1 sample and crucible capacity of 0.1mL. While sample mass below 11 mg takes no more than 15%of the crucible’s loading capacity, the ∼38 mg sample occupies

as much 40%. The latter corresponds to a bed height of 3.2mm, which is almost half of the entire crucible height of 8 mm.This means that the evolved gases need to negotiate more thantwice the distance through the sample matrix. The entrapmentof water vapor within the sample bed increases the local partialpressure of water vapor, PH2O. Since serpentine dehydroxylationkinetics is highly dependent on PH2O,

9 one would expect thedelays in Tp, exactly as observed in this study. These resultssuggest deployment of fluidized bed reactors for activatingserpentine minerals, in preference to moving bed reactors.

3.2. Effect of Purge Gas. The use of an oxidizing gasincreases heating requirements and slows down the mass lossrate. Despite the perceived simplicity of air activation, the delayin T‑H2O by 17 °C (Figure 4a) and the decrease in r‑H2O by 10%(Figure 4b) makes this purge gas less desirable for operating adehydroxylation reactor. Based on the obvious decoloration ofthe sample to reddish hue, we suggest that the formation ofhematite layer on the surface of serpentine grains limits theremoval of the liberated water. Hematite formation inserpentine minerals had been described by other workers as“fully ferric chrysotile”,31 hematite formation in carlosturanite,36

and oxidized form of magnetite during lizardite heat treat-ment.37 Under oxidizing atmosphere, the transformation31,36,38

of Fe2+ to Fe3+ most often concludes prior to the onset ofdehydroxylation.Besides the oxidation of magnetite, Fe2+, present in the

octahedral sites in serpentinite, oxidizes to Fe3+. Also, Fe3+

present in tetrahedral sites migrates out to octahedral sites.31,39

By conservation of charge, a third of the converted Fe3+ fromFe2+ must migrate toward the surface of serpentinite grains toform Fe2O3 layers, as observed by the reddish hue on surfacesof serpentine activated under air. Based on the chrysotilestudies by MacKenzie and MacGavin,31 iron present in thehematite layers corresponds to about 10% of the initial ironcontent of the mineral. With antigorite having relatively higherproportions of Fe2+ than lizardite or chrysotile,40 the effect ofan oxidizing gas is probably more pronounced for antigoritethan for the other two polymorphs of serpentine.The removal of Fe3O4 via demagnetisation prior to heat

activation reduces the severity of hematite formation. This isexemplified in the slightly faster mass loss rate of the partiallydemagnetised sample (Sample A, despite its relatively largersize) as compared to the nondemagnetised Sample D. As canbe seen in Figures 3g and 4b, the mass loss rate for themagnetic sample (Sample D) slightly decreases by about 3%.However, the ARC (Albany Research Center, now U.S.Department of Energy’s National Energy Technology Labo-ratory) investigations reported that oxidizing gas used duringheat treatment of a partially demagnetised antigorite appearednot to influence the serpentinite conversion during carbo-nation.37 This means that a sufficiently high amount of Fe2+,was removed by demagnetisation prior to thermal treatment,resulting in formation of minor layers of Fe2O3 duringactivation that had no influence on the conversion of activatedserpentine during its carbonation. But the findings of Connor etal. might be specific to their serpentine mineral (Section 4,Supporting Information).In a practical situation, the use of CO2 as purge gas can

provide similar efficiency as that of an inert gas. A possiblescenario may involve bleeding some CO2 delivered from acapture plant to satisfy the purge requirements of the activationunit. After exiting the activation unit (calciner), mixture of CO2and steam could be routed through a heat exchanger for



extraction of heat from the gas. Steam could be thencondensed, separated and used in the subsequent processes(i.e., water input for carbonation), prior to recycling of CO2 tothe activation unit or its use in the carbonation reactor. Whileair is readily available for actual operations, it is not thepreferred purge gas for activating serpentines.Overall, we conclude that a diffusive barrier of Fe2O3 coating

the antigorite surface forces the temperature shift in T‑H2O tohigher values and the decrease in the r‑H2O. This effecthighlights a need for demagnetisation, especially if thermaltreatment involves an oxidizing gas such as air. In other words,demagnetisation serves the dual purpose of removing valuableminerals of iron and chromium for cost offsets, and to decreasethe energy load required for activation.3.3. Effect of Heating Rate. Figures 4a,b shows that the

increase in heating rate by 100% (20 °C min−1) and 200% (20°C min−1) results in impediment in T‑H2O by 11 and 22 °C,respectively. Despite these delays, the increase in r‑H2O (Figure4b) is extremely high, at 89% and 192%, respectively (Section5, Supporting Information). As can be seen in Figure 5a, the

response in both T‑H2O and r‑H2O with increased heating ratesfollows an increasingly linear trend. The shifts in T‑H2O andr‑H2O toward higher values, as function of the increasing heatingrate, arise as a result of the dependence of the mass-loss rate onthe Arrhenius expression.Further analysis indicates that while higher heating rates

elevate T‑H2O, a significant increase in r‑H2O drastically reducesthe processing time, to reach target degree of dehydroxylation.

Figure 5b illustrates the decrease in processing time to producethermally treated antigorite with 20% residual OH content, %OHres. The required treatment time appears to scale with theheating rate as t = 630/β0.942, based on the measurementscollected between 10 and 30 °C min−1. Figure 5b extrapolatesthe treatment time to higher heating rates (40−100 °C min−1).We conclude this section by noting that while slow heatingrates are essential for investigating intrinsic kinetics, operationof a practical reactor necessitates more rapid thermalprocessing.

3.5. Effect of Comminution Type. Figures 4a,b show thatwhile the r‑H2O of the crushed sample increases by 35%, theT‑H2O is 36 °C higher than that of the ground sample. This is asexpected for a crystalline solid, evolving water en-bulk atrelatively higher temperature due to the imparted structuralrigidity of the crystal lattice structure. Further comminution(i.e., grinding) could disrupt this structure,41 rendering thematerial susceptible to thermal and/or chemical decompositionat relatively lower temperatures. The apparent shift in the Tshlocation to ∼725 °C also suggests that, this thermal feature isneither contaminant chrysotile nor partially amorphisedmaterial but an intermediate phase associated with thermaldehydroxylation. This is because the reported peak temperatureof high purity chrysotile at ∼650 °C,1 and even ourexperimentally determined Tp’s of chrysotile fiber (FigureS11, Supporting Information, also shown as the table of contentgraphic) at ∼690 °C are significantly lower.While the direct use of crushed samples in thermal activation

may be possible, the apparent savings in electrical power isnegated by the likelihood of further comminution after thermalactivation. This is because dehydroxylation at especially hightemperatures (approaching 820 °C) may induce the formationof enstatite. Further comminution is also necessary to increasesolid’s surface area. In general, the smaller particles fromground samples represent the preferred feed for thermaldehydroxylation due to the relative ease of dehydroxylation.While there is no significant differences in the thermal profileamong the two types of grinding methods (wet grinding ∼21min; dry grinding ∼1 min), the significantly faster grind timerequired to reduce the particle size makes dry grindingattractive. It must also be noted that although wet groundsample initially contains magnesium hydroxide species (Table1), these hydroxides dehydroxylate prior to antigorite. Hence,the presence of Mg(OH)2 does not have any discernible effecton the subsequent serpentine dehydroxylation process.

3.6. Environmental Technology Implications toSerpentine Activation for CO2 Storage by Mineralisa-tion. The evaluation of the effects of sample mass, purge gas,heating rate, and communition type on the behavior ofantigorite has provided new (i) knowledge of the dehydrox-ylation kinetics of this mineral at elevated temperatures, and (ii)scientific underpinnings for designing larger unit operationsneeded to for scaling up the process of sequestering of CO2 bymineralization. Particles of less than 75 μm in size affordoptimal conditions of temperature and rate of dehydroxylation,and allow savings in electrical power for the size reductionstage. The detrimental effect of oxidation of Fe2+ to Fe3+, owingto formation of hematite layers on activated particles, suggestsCO2 as preferred purge gas.We highlight the need to fluidize the powder bed to avoid

the entrapment of liberated water that engenders the reversereaction and results in higher processing temperature.Otherwise, the effect of bed height on inducing the reverse

Figure 5. (a) Practical activation temperatures (T‑H2O) and estimatedmass loss rates (r‑H2O) with respect to the increase in heating rate, β.(b) Required time at various heating rates (β) for the production ofthermally treated antigorite containing 20% OHres.




reactions must be included in the design of a moving bedreactor, as the results suggest that the build-up of localized PH2Ois likely to occur in moving bed reactors with increase in bedheight. We also recommend the rapid thermal treatment as apractical way to increase the throughput, and minimize thereactor’s size.

■ ASSOCIATED CONTENT*S Supporting InformationFurther information on the material standard pretreatmentprocess, mineralogy, thermal properties and details on thedeviations in T‑H2O and r‑H2O are found in Sections 1−3 of theSupporting Information. Sections 4−7 provide additional noteson the effects of purge gas, heating rate and comminution type.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +61 2 4985 4433; fax: +61 2 4921 6893; e-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study was funded by an internal grant (ref. No.G0189103) from the University of Newcastle. We gratefullyacknowledge valuable discussions with Prof. Eric Kennedyduring the course of this research. The first author thanks theUniversity of Newcastle for a Postgraduate Research Scholar-ship. Material and analytical assistance from Prof. Erich Kisi, Dr.Judy Bailey, Ms. Monica Davis, and Ms. Jennifer Zobec (EM-X-ray Unit) are greatly appreciated.

■ REFERENCES(1) Viti, C. Serpentine minerals discrimination by thermal analysis.Am. Mineral. 2010, 95, 631−638.(2) Inque, T.; Yoshimi, I.; Yamada, A.; Kikegawa, T. J. Time-resolvedX-ray diffraction analysis of the experimental dehydration of serpentineat high pressure. J. Mineral. Petrol. Sci. 2009, 104, 105−109.(3) Llana-Funez, S.; Brodie, K. H.; Rutter, E. H.; Arkwright, J. C.Experimental dehydration kinetics of serpentine using porevolumometry. J. Metamorph. Geol. 2007, 25, 423−438.(4) Candela, P. A.; Crummett, C. D.; Earnest, D. J.; Frank, M. R.;Wylie, A. G. Low pressure decomposition of chrysotile as a function oftime and temperature. Am. Mineral. 2007, 92, 1704−1713.(5) Perrillat, J. P.; Daniel, I.; Koga, K. T.; Reynard, B.; Cardon, H.;Crichton, W. A. Kinetics of antigorite dehydration: A real-time x-raydiffraction study. Earth Planet. Sci. Lett. 2005, 236, 899−913.(6) McKelvy, M. J.; Chizmeshya, A. V. G.; Diefenbacher, J.; Bearat,H.; Wolf, G. Exploration of the role of heat activation in enhancingserpentine carbon sequestration reactions. Environ. Sci. Technol. 2004,38, 6897−6903.(7) MacKenzie, K. J. D.; Meinhold, R. H. Thermal reactions ofchrysotile revisted: A 29Si and 25Mg MAS NMR study. Am. Mineral.1994, 79, 43−50.(8) Tyburczy, J. A.; Ahrens, T. J. Dehydration kinetics of shockedserpentine. In Proceedings of the 18th Lunar and Planetary ScienceConference, Houston, Texas, March 16−20, 1987; Houston, Texas,1988.(9) Brindley, G. W.; Narahari, A.; Sharp, J. H. Kinetics andmechanism of dehydroxylation processes: II. Temperature and vaporpressure dependence of dehydroxylation of serpentinite. Am. Mineral.1967, 52, 1697−1705.

(10) Weber, J. N.; Greer, R. T. Dehydration of serpentine: Heat ofreaction and reaction kinetics at PH2O = 1 atm. Am. Mineral. 1965, 50,450−464.(11) Brindley, G. W.; Hayami, R. Kinetics and mechanisms ofdehydration and recrystallization of serpentine: I. In Proceedings of the12th National Conference on Clays and Clay Minerals, Atlanta, Georgia,September 30 - October 2, 1963; Atlanta, Georgia, 1964; pp 35-47,49−54.(12) Ball, M. C.; Taylor, H. F. W. The dehydration of chrysotile in airand under hydrothermal conditions. Mineral. Mag. 1963, 33, 467−482.(13) Franco, F.; Perez-Maqueda, L.; Ramirez-Valle, V.; Perez-Rodriguez, J. Spectroscopic study of the dehydroxylation process ofsonicated antigorite. Eur. J. Mineral. 2006, 18, 257−264.(14) Martinez, E. The effect of particle size on the thermal propertiesof serpentine minerals. Am. Mineral. 1961, 46, 901−912.(15) Cattaneo, A.; Gualtieri, A. F.; Artioli, G. Kinetic study of thedehydroxylation of chrysotile asbestos with temperature by in-situXRPD. Phys. Chem. Minerals. 2003, 30, 177−183.(16) Maroto-Valer, M. M.; Fauth, D. J.; Kuchta, M. E.; Zhang, Y.;Andresen, J. M. Activation of magnesium rich minerals as carbonationfeedstock materials for CO2 sequestration. Fuel Process. Technol. 2005,86, 1627−1645.(17) Li, W.; Li, W.; Li, B.; Bai, Z. Electrolysis and heat pretreatmentmethods to promote CO2 sequestration by mineral carbonation. Chem.Eng. Res. Des. 2009, 87, 210−215.(18) Gerdemann, S. J.; O’Connor, W. K.; Dahlin, D. C.; Penner, L.R.; Rush, H. Ex- situ aqueous mineral carbonation. Environ. Sci.Technol. 2007, 41 (7), 2587−2593.(19) Huijen, W. J. J.; Comans, R. N. J. Carbon Dioxide Sequestrationby Mineral Carbonation: Literature Review, ECN-C-05-022; EnergyResearch Centre of the Netherlands: The Netherlands, 2005.(20) Huijen, W. J. J.; Comans, R. N. J. Carbon Dioxide Sequestrationby Mineral Carbonation: Literature Review, ECN-C-03-016; EnergyResearch Centre of the Netherlands, The Netherlands. 2003.(21) Sipila, J.; Teir, S.; Zevenhoven, R. Carbon dioxide sequestration bymineral carbonation-Literature review update. Report VT 2008−1; AboAkademi University, Heat Engineering Laboratory: Turku, Finland,2 0 0 8 ; h t t p : / / w e b . a b o . fi / ∼ r z e v e n h o /MineralCarbonationLiteratureReview05-07.pdf.(22) Davis, M. The CO2 Sequestration Potential of the UltramaficRocks of the Great Serpentinite Belt, New South Wales. HonoursThesis, The University of Newcastle, Newcastle, 2008.(23) Moody, J. B. Serpentinization: A review. Lithos 1976, 9, 125−138.(24) O’Hanley, D. S.; Wicks, F. Conditions of formation of lizardite,chrysotile and antigorite, Cassiar, British Columbia. Can. Mineral.1995, 33, 753−773.(25) Auzende, A. L.; Guillot, S.; Devouard, B.; Baronnet, A.Serpentinites in an Alpine convergent setting: Effects of metamorphicgrade and deformation on microstructures. Eur. J. Mineral 2006, 18,21−33.(26) O’Hanley, D. S. Serpentinites: Records of Tectonic and PetrologicalHistory: Oxford University Press: Oxford, United Kingdom, 1996.(27) O’Hanley, D. S.; Dyar, M. D. The composition of lizardite 1Tand the formation of magnetite in serpentinites. Am. Mineral. 1993, 78,391−404.(28) O’Hanley, D. S.; Dyar, M. D. The composition of chrysotile andits relationship with lizardite. Can. Mineral. 1998, 36, 727−739.(29) Burzo, E. Serpentines and related silicates. In Phyllosilicates.;Springer-Verlag: Berlin Heidelberg. 2009; col. 27I5b.(30) O’Connor, W. K.; Dahlin, D. C.; Nilsen, R. P.; Rush, G. E.;Walters, R. P.; Turner, P. C. In Carbon Dioxide Sequestration by DirectMineral Carbonation: Results from Recent Studies and Current Status, 1stAnnual DOE Carbon Sequestration Conference, DOE/ARC-2001-029;National Energy Technology Laboratory, United States Department ofEnergy: Washington, DC, May 14−17, 2001.(31) MacKenzie, K. J. D.; McGavin, D. G. Thermal and mossbauerstudies of iron-containing hydrous silicates. Part 8. Chrysotile.Thermochem. Acta 1994, 244, 205−221.



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http://web.abo.fi/<rzevenho/MineralCarbonationLiteratureReview05-07.pdf

http://web.abo.fi/<rzevenho/MineralCarbonationLiteratureReview05-07.pdf

(32) Khoo, H. H.; Tan, R. B. H. Life cycle evaluation of CO2recovery and mineral sequestration alternatives. Environ. Prog. 2006,25 (3), 208−217.(33) Balucan, R. D.; Kennedy, E. M.; Mackie, J. F.; Dlugogorski, B. Z.Optimization of antigorite heat pre-treatment via kinetic modeling ofthe dehydroxylation reaction for CO2 mineralization. Greenhouse GasSci. Technol. 2011, 1, 294−304.(34) Wicks, F. J. Status of the reference x-ray powder-diffractionpatterns for the serpentine minerals in the PDF database-1997. PowderDiffr. 2000, 15 (1), 42−50.(35) Sharp, J. H.; Wilburn, F. W.; McIntosh, R. M. The effect ofprocedural variables on TG, DTG and DTA curves of magnesite anddolomite. J. Therm. Anal. 1991, 37, 2021−2029.(36) Compagnoni, R.; Ferraris, G.; Mellini, M. Carlosturanite, a newasbestiform rock-forming silicate from Val Varaita, Italy. Am. Mineral.1985, 70, 767−772.(37) O’Connor, W.; Dahlin, D. C.; Nilsen, R. P.; Rush, G. E.;Walters, R. P.; Turner, P. C. In CO2 Storage in Solid Form: A Study ofDirect Mineral Carbonation. 5th International Conference on GreenhouseGas Technologies, DOE/ARC-2000-01; Cairns, Australia, August 14−18, 2000.(38) Gallagher, K. J.; Feitknecht, W.; Mannweiler, U. Mechanism ofoxidation of magnetite to γ-Fe2O3. Nature 1968, 217, 1118−1121.(39) Malysheva, T. V.; Satarova, L. M.; Polyakova, N. P. Thermaltransformations of layer silicates and the nature of iron-bearing phasein CII-type Murray carbonaceous chondrite. Geochem Int. 1977, 14,117−128.(40) Page, N. J. Chemical differences among the serpentine“polymorphs”. Am. Mineral. 1968, 53, 201−215.(41) Drief, A.; Nieto, F. The effect of dry grinding on antigorite fromMulhacen, Spain. Clay Clay Mineral. 1999, 47 (4), 417−424.



S1

Supporting Information

Environmental Science & Technology

28-Oct-2012

Thermal Activation of Antigorite

for Mineralisation of CO2

Reydick D. Balucan1 and Bogdan Z. Dlugogorski

1*

1Priority Research Centre for Energy

The University of Newcastle, Callaghan, NSW 2308, Australia

*corresponding author:

[email protected]

Number of pages: 21

Figures: 11

Tables: 5

Table of Contents:

Section 1. Standard Pretreatment

Section 2. Material and Thermal Characteristics

Section 3. Deviations in T-H2O and r-H2O

Section 4. Effect of Purge Gas

Section 5. Effect of Heating Rate

Section 6. Combined Effect of Heating Rate and Purge Gas

Section 7. Effect of Comminution

Section 1: Standard Pretreatment

S2

Antigorite (Sample BI4 obtained near Bingara in the Great Serpentinite Belt in NSW, Australia, at the

location of 30.122217 S and 150.635966 E), was dry crushed with a jaw crusher to ~5 mm, then in a roll

crusher to reduce its size to ~2 mm. This 2 mm dry crushed sample (Sample E) was further reduced in

size by wet grinding in a rod mill (13 rods, 60 % solids, 21 min grind time) specifically to target ground

material with 80 % passing the -75 µm sieve. From this wet ground sample, we extracted three (3) size

fractions during the sieve analysis at ~ 10 g each and designated these as Samples A (-75 + 53 µm), B (-

53 + 38 µm) and C (-38 µm). Each of these ~ 10 g portions was then wet demagnetised by repeated

removal of the magnetic particles appended on a PTFE magnetic stirring bar (107 mm × 27 mm) using a

PTFE retriever. The demagnetised slurries were dewatered and oven dried for 1 day at 110 °C.

On the other hand, a ring mill (tungsten carbide rings, ~ 60 % solids, 1 min grind time) served to dry

ground the crushed sample to produce a size fraction of d80 = 10 µm (Sample D). The wet ground

samples (A, B, C) were demagnetised prior to heat activation, while the dry ground (D) and crushed (E)

samples were used without further demagnetisation. Ground materials (A, B, C, D) are all in powder

form whilst a single shard represents the dry crushed sample (E).

S3

Figure S1. Standard pretreatment process employed to the antigorite sample BI4. This pretreatment

stage comprises crushing, homogenisation, grinding, and wet demagnetisation.

S4

Table S1. Trial wet grinds to establish the grind time for 80 % passing 75 µm.

Seive,

µm

Trial Grind 1

Seive,

µm

Trial Grind 2

Seive,

µm

Trial Grind 3

Duration 6 min Duration 12 min Duration 24 min

%

mass

%

passing

%

mass

%

passing

%

mass

%

passing

710 3.04 97.0 710 710

600 4.30 92.7 600 600

500 12.3 80.4 500 500

400 11.6 68.7 400 400

300 10.8 57.9 300 1.09 98.9 300 100

250 6.65 51.3 250 3.85 95.1 250 100

180 7.99 43.3 180 14.1 81.0 180 0.05 100

106 9.77 33.5 106 24.9 56.1 106 1.43 98.5

90 2.29 31.2 90 4.83 51.2 90 2.00 96.5

75 2.27 29.0 75 4.69 46.6 75 4.14 92.4

63 63 63 5.37 87.0

53 53 53 6.27 80.7

38 3.94 25.0 38 7.96 38.6 38 5.04 75.7

-38 25.0 -38 38.6 -38 75.7

S5

Figure S2. Wet grind time curve of the P80 values for the three trial grinds. The estimated time to

achieve P80 75 µm is about 21 min.

496

53

176

y = 9155.7x-1.6124

r2 = 0.9982

0

100

200

300

400

500

0 5 10 15 20 25

t , min

P8

0,

µm

X

S6

Table S2. Sieve analysis of the final grind designed to achieve P80 of 75 µm.

Seive, µm

Final Grind

21 min

% mass % passing

710

600

500

400

300 100

250 0.75 99.3

180 3.00 96.3

106 7.00 89.3

90 3.69 85.6

75 4.97 80.6

63 4.03 76.6

53 4.70 71.9

38 7.15 64.7

-38 65.2

S7

Table S3. Resultant particle size and power requirements of the standard pre-treatment stages.

Size Reduction

Stages d80, µm

Power requirements,

kWhe tonne-1

Crushing 2000 1.80

Stage 1 Grinding 75 10.0

Stage 2 Grinding 37 63.5

Stage 3 Grinding < 10 136

S8

Section 2: Material and Thermal Characteristics

Figure S3. Particle size distribution plots showing the (a) trendlines used to calculate and designate the

samples in terms of d80 from the measured d10, d50 and d90 values, (b) cumulative volume distribution,

and (c-f) the differential volume distribution of the various size fractions of the antigorite sample.

A, wet ground (d 80,75 µ m)

02468

10

0 50 100 150 200 250 300Particle size, µ m

Dif

fere

nti

al

vo

lum

e

dis

trib

uti

on

, %

c

B, wet ground (d 80,52 µ m)

024

68

10

0 50 100 150 200 250 300Particle size, µ m

Dif

fere

nti

al

vo

lum

e

dis

trib

uti

on

, %

d

C, wet ground (d 80,31 µ m)

02

46

810

0 50 100 150 200 250 300

Particle size, µ m

Dif

fere

nti

al

vo

lum

e

dis

trib

uti

on

, %

e

0

20

40

60

80

100

0 50 100 150 200 250Particle size, µ m

Cu

mu

lati

ve

vo

lum

e d

istr

ibu

tio

n,

%

A (wet ground, d80 75 um)

B (wet ground, d80 75 um)

C (wet ground, d80 75 um)

D (dry ground, d80 75 um)

b

d 80, 10 µm)

d 80, 31 µm)

d 80, 52 µm)

d 80, 75 µm)

y = 1.0848e0.0529x

r2 = 0.9999

y = 1.2060e0.047x

r2 = 0.9989

y = 1.0961e0.0416x

r2 = 1.0000

y = 0.9984e0.0291x

r2 = 0.9994

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90

d x

Pa

rtic

le s

ize

, µ

m

A (wet ground, d80 75 um)

B (wet ground, d80 52 um)

C (wet ground, d80 31 um)

D (dry ground, d80 10 um)

a

d 80, 10 µm)

d 80, 31 µm)

d 80, 52 µm)

d 80, 75 µm)

D, dry ground (d 80,10 µ m)

02

46

810

0 50 100 150 200 250 300Particle size, µ m

Dif

fere

nti

al

vo

lum

e

dis

trib

uti

on

,

%

f

S9

Figure S4. X-ray powder diffraction of antigorite showing both (a) natural, dry ground sample D and

(b) wet ground, demagnetized sample A.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

5 10 15 20 25 30 35 40 45 50 55 60 65 70o 2θ

inte

nsi

ty,

cou

nts

*

*

*** *

*

*

* Antigorite

** **

b

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

5 10 15 20 25 30 35 40 45 50 55 60 65 70o 2θ

inte

nsi

ty,

cou

nts

+ +

*

**

*

*

* Antigorite

+ Magnetite

**

+*

*

*

* **

a

S10

Figure S5. DTG curves of the control experiments. The DTG peak temperatures and shoulder are

designated as TP1, TP2 and Tsh, respectively.

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

100 200 300 400 500 600 700 800 900

T, o

C

-(d

m/d

t)/

mo,

s-1 Run 1T p2: 734

oC, 170 s

-1 T p1: 716

oC, 172 s

-1

T sh start: 629 oC, 60 s

-1

T sh end: 683 oC, 110 s

-1

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

100 200 300 400 500 600 700 800 900

T, o

C

-(d

m/d

t)/

mo,

s-1

Run 2

T p2: 737 oC, 168 s

-1T p1: 713

oC, 174 s

-1


-1

T sh end: 675 oC, 96 s

-1

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

100 200 300 400 500 600 700 800 900

T, o

C

-(d

m/d

t)/

mo,

s-1

Run 3

T p2: 738 oC,162 s

-1 T p1: 713

oC, 168 s

-1


-1

T sh end: 674 oC, 92 s

-1

S11

Table S4. Summary of the DTG profile for the antigorite specimen.

TGA

Runs

mass

loss, % Tp,

oC –(dm/dt)/mo_max, s

-1†

∆m 105-

850

Tsh

start

Tsh

end

Tsh

range Tp1 Tp2

Tsh

start

Tsh

end

Tsh

range Tp1 Tp2

Run 1 11.46 629 683 54 716 734 60 110 50 172 170

Run 2 11.41 641 675 34 713 737 53 96 43 174 160

Run 3 11.40 631 674 43 713 738 47 92 45 168 162

mean 11.43 635 679 44 715 736 57 103 47 173 169

σ 0.03 8 6 14 2 2 5 10 5 1 1

† listed values are to be multiplied by 1.0 × 10

-6

S12

Table S5. Summary of antigorite’s thermal profile.

Thermal profile T-H2O, °C

(Tp_mean) †

r-H2O, s-1

-(dm/dt)/mo_max_mean††

Tp1 725 1.7 × 10

-4

Tp2

σ 2 1.4 × 10-6

† Average value of Tp1 and Tp2.

†† Average value of the -(dm/dt)/mo_max_mean from each Tp1 and Tp2.

S13

Section 3. Deviations in T-H2O and r-H2O

Figure S6. Analysis of the deviations in the practical activation temperature, T-H2O and its respective

estimated mass lost rate, r-H2O resulting from variations in sample mass.

-10

4

15

-6-4-202468

1012141618

decreased by

50% (2.75 mg)

control:

(5.50 mg)

increased by

100%

(11.0 mg)

increased by

590%

(37.7 mg)

ΔT

-H2

O,

oC

a

mass decreased by

50 % (2.75 mg)

mass increased by

100 % (11.0 mg)

mass increased by

~600 % (37.7 mg)

delayed by 11 oC

3

0

6

12

-3

-1

1

3

5

7

9

11

13

decreased by

50% (2.75 mg)

control:

(5.50 mg)

increased by

100%

(11.0 mg)

increased by

590%

(37.7 mg)

% c

ha

ng

e, r

-H2

O

b

mass decreased by

50 % (2.75 mg)

mass increased by

~600 % (37.7 mg)

mass increased by

~100 % (11.0 mg)

faster by 1 %

faster by 4 %

faster by 10 %

S14


estimated mass lost rate, r-H2O resulting from variations in purge gas.

01

21

-6-4-202468

1012141618202224

control: argon carbon dioxide air

ΔT

-H2

O,

oC

a

CO2 Aircontrol: Argon

delayed by 17 oC

0

3

-12

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

control: argon carbon dioxide air

% c

ha

ng

e, r

-H2

O

b

CO2 control: Argon Air

faster by 1 %

slower by 10 %

S15


estimated mass lost rate, r-H2O resulting from variations in heating rate.

0

91

194

-20

30

80

130

180

230

control:

10 oC/min

in argon

heating rate

increased by 100% in

argon

heating rate


argon

% c

ha

ng

e, r -

H2

O

b

control:

10 oC min

-1, Argon

increased by 100 %,

(20 oC min

-1), Argon

increased by 200 %,

(30 oC min

-1), Argon

faster by 89 %

faster by 192 %

0

15

26

-6

-2

2

6

10

14

18

22

26

30

control:

10 oC/min

in argon

heating rate


argon

heating rate


argon

ΔT

-H2

O,

oC

a

10 oC min

-1, Argon

increased by 100 %,

(20 oC min

-1), Argon

increased by 200 %,

(30 oC min

-1), Argon

delayed by 11 oC

delayed by 22 oC

S16


estimated mass lost rate, r-H2O resulting from variations in the type of comminution.

0-5

37

-20

-10

0

10

20

30

40

50

control : wet ground dry ground dry crushed

% c

ha

ng

e, r

-H2

O

b

faster by 35 %

slower by 3 %

0-3

40

-6-226

10141822263034384246

control : wet ground dry ground dry crushed

ΔT

-H2

O,

oC

a

delayed by 36 oC

S17


estimated mass lost rate, r-H2O resulting from variations in heating rate in both argon and air atmosphere.

0

2126

43

-6

4

14

24

34

44

54

control:

10 oC/min

in argon

10 oC/min

in air

heating rate

increased by

200% in argon

heating rate

increased by

200% in air

ΔT

-H2

O,

oC

a

heating rate

increased by 200 %,

(30 oC min

-1), Argon

heating rate

increased by 200 %,

(30 oC min

-1), air

(10 oC min

-1),

Argon

(10 oC min

-1),

air

delayed by 17 oC

delayed by 22 oC

delayed by 39 oC

0 -12

194 191

-30

20

70

120

170

220

control:

10 oC/min

in argon

10 oC/min

in air

heating rate

increased by

200% in argon

heating rate

increased by

200% in air

% c

ha

ng

e, r

-H2

O

b

heating rate

increased by 200 %,

(30 oC min

-1), Argon

heating rate

increased by 200 %,

(30 oC min

-1), air

control:

(10 oC min

-1),

Argon

(10 oC min

-1),

air

slower by 10 %

faster by 192 % faster by 189 %

S18

Section 4. Effect of Purge Gas

In the work of O’Connor et al,(1) removal of 54 % of the magnetite content, resulted in the reduction of

the total oxidisable species to about 57 % of its original content.(37) The removal of similar amount of

magnetite in our sample (54 % the total magnetite content, Table S1, Fe2O3*), however, only decreased

the total oxidisable Fe to 70 % of its original value (Fe2O3_total 6.87 % down to 4.82 % w/w antigorite).

Although similar amounts of magnetite were removed in both studies, our demagnetised sample

contained more oxidisable Fe due to a relatively higher amount of Fe2+

in octahedral sites than in

magnetite grains. As such, partial demagnetisation (~ 54 % of the original Fe3O4 content remains) may

be sufficient for the mineral used by O’Connor et al., but it seems insufficient for the antigorite of the

present investigation.

1. O'Connor, W.; Dahlin, D. C.; Nilsen, R. P.; Rush, G.E.; Walters, R.P.; Turner, P. C. In CO2

storage in solid form: A study of direct mineral carbonation. 5th International Conference on

Greenhouse Gas Technologies, DOE/ARC-2000-01. Cairns, Australia, Cairns, Australia, August 14-18,

2000.

S19

Section 5. Effect of Heating Rate

Based on our energy studies, this energy demand corresponds to ~15 kJ (kg antigorite)-1

for every 10 °C

delay in Tp. Overall, the negative cost implications of this delay are minor in comparison to the offset in

process intensification (throughput), as a consequence of increased mass-loss rate. The practicality of

implementing rapid thermal processing warrants further examination to gain insights into heat transfer

during serpentine activation, and to avoid heat transfer becoming a step limiting phenomenon.

S20

Section 6. Combined Effect of Heating Rate and Purge Gas

Figures 3a-b emphasise the cumulative effects of high heating rates and the use of an oxidising gas.

Both effects elevated T-H2O, but exerted contradictory influence on the rate of dehydroxylation. The

highest deviation in the T-H2O by 39 °C represents the additive effect of the increase in heating rate by

200 % (increase in T-H2O by 22 °C) and application of air as purge gas (increase by 17 °C). Although

the higher heating rates significantly improve r-H2O (by almost 200 % for 30 °C min-1

), the application of

air activation of non-demagnetised samples results in overall slowing down of the activation rate (by 3

% in comparison to the base case).

S21

Section 7. Effect of Comminution

Figure S11. Thermal profile of chrysotile asbestos.

Chrysotile Fibre

-1.8

-1.55

-1.3

-1.05

-0.8

-0.55

-0.3

-0.05

0.2

0.45

0.7

0 100 200 300 400 500 600 700 800 900

T , oC

He

atf

low

, m

W m

g-1

0.E+00

1.E-03

2.E-03

3.E-03

4.E-03

5.E-03

6.E-03

7.E-03

8.E-03

9.E-03

1.E-02

DT

G,

-(dm

/dt

), m

g s

-1

Heatflow

DTG

Exo

697

810

697

Sample Mass: 22 mg

Heating Rate : 10 oC min

-1

Purge Gas: Argon

Gas Flow rate: 20 mL min-1

Balucan et al_2013_Thermal Activation of Antigorite for Mineralization of CO2_Paper and SI

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