CO2 Sequestration Potential of Steel Slags at Ambient Pressure and

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Ind. Eng. Chem. Res. 2008, 47, 7610–7616

CO2 Sequestration Potential of Steel Slags at Ambient Pressure and Temperature Danielle Bonenfant,† Lynda Kharoune,‡ Se´bastien Sauve´,§ Robert Hausler,† Patrick Niquette,† Murielle Mimeault,| and Mourad Kharoune*,⊥ STEPPE, De´partement de Ge´nie de la Construction, De´partement de Ge´nie Me´canique, and De´partement de Ge´nie de la Production Automatise´e, E´cole de Technologie Supe´rieure, 1100, Notre-Dame Ouest, Montre´al, Que´bec, Canada H3C 1K3, De´partement de Chimie, UniVersite´ de Montre´al, P.O. 6128, Succursale Centre-Ville, Montre´al, Que´bec, Canada H3C 3J7, and Department of Biochemistry and Molecular Biology, College of Medicine, Eppley Cancer Institute, 7052 DRC, UniVersity of Nebraska Medical Center, Omaha, Nebraska 68198-5870

A study of carbon dioxide sequestration has been performed in aqueous electric arc furnace (EAF) and ladle furnace (LF) slag suspensions, in leached hydrated-matrixes, and in leachates to estimate their intrinsic sequestration potential at ambient conditions (temperature of 20 ( 1 °C and atmospheric pressure). The CO2 sequestration was tested in aqueous suspensions of steel slags at a liquid-to-solid ratio of 10 kg/kg as well as in leached hydrated-matrixes and leachates isolated from these fresh slag suspensions after three consecutive leachings. The sequestration assays were performed at 20 °C with a flow rate of 5 mL/min of a CO2 concentration of 15.00 vol %. The results have revealed that the CO2 sequestration capacity of the LF slag suspension (24.7 g of CO2/100 g of slag) is 14 times superior to that of the EAF slag suspension. This greater CO2 sequestration capacity of the LF slag suspension may be associated in large part to its higher content of portlandite, which reacts with CO2 relative to the EAF slag suspension. Moreover, the separation of hydratedmatrixes and leachates significantly enhanced the CO2 sequestration capacity of EAF slag while a slight decrease was observed for the LF slags. This may be due to an obstruction of the CO2 binding sites of LF slag hydrated-matrixes following the accumulation of calcium carbonate. Taken together, these results suggest that EAF and LF slags could be used for the CO2 sequestration and given a good yield as well in aqueous suspension as in separated matrixes and leachates. 1. Introduction The global warming associated with an increase of the greenhouse gas emissions represents a worldwide problem that is due in major part to the industrial combustion of fossil fuels.1,2 Each year, more than 7 Gtons of anthropogenic CO2 are released in the atmosphere including a major contribution from the production of steel by the steelworks.3 The worldwide production of steel was evaluated at 752 millions of tons in 1996.4 The steelworks are great emitters of CO2; their production generates 0.28-1 ton of CO2/ton of steel in the atmosphere.5 The steelworks also generate ∼350 millions of tons of steel and iron slags each year all the world over.6 There are four types of steel slags that are named based on their process production including the blast furnaces slag, basic oxygen furnace steel slag, electric arc furnace (EAF) steel slag, and ladle furnace (LF) slag.6-8 The steel slag is a consolidated mix of many compounds, including principally calcium, iron, silicon, aluminum, magnesium, and manganese oxides that are present under diverse phases (Table 1).6-10 Huijgen et al.9,10 have established that the fresh steel slags contain three major phases of calcium: portlandite (Ca(OH)2), Ca-(Fe)-silicate, and Ca-Fe-O, and several mineral phases including Mg-Fe-O, Fe-O, and trace of calcite (CaCO3). The great content of basic * To whom correspondence should be addressed. Phone: (514) 396 8640. Fax: (514) 396 8595. E-mail: [email protected]. † STEPPE, De´partement de Ge´nie de la Construction, E´cole de Technologie Supe´rieure. ‡ De´partement de Ge´nie Me´canique, E´cole de Technologie Supe´rieure. § Universite´ de Montre´al. | University of Nebraska Medical Center. ⊥ De´partement de Ge´nie de la Production Automatise´e, E´cole de Technologie Supe´rieure.

oxides confers to steel slags a high alkanity (pH ∼12) and a high total theoretical CO2 sequestration capacity that has been evaluated at ∼0.25 kg of CO2/kg of slag on the basis of the total calcium content.9 This result further valorizes the potential to use the steel slags in the technologies for the CO2 sequestration. Although several reaction mechanisms have been proposed to describe the CO2 sequestration by carbonation, few mechanisms were developed about aqueous steel slag carbonation.9-14 In general, the aqueous carbonation is a complex process in which the CO2, after dissolution in the aqueous medium, reacts with dissolved metal-containing mineral to form carbonate species and bicarbonate ion (HCO3-).9,11,15,16 Huijgen et al.9,10 have notably developed a mechanism to describe the aqueous steel slag carbonation at high pressure and temperature. These authors have observed that the calcite is the single form of carbonate that was detected in significant amounts in the carbonated steel slag. Moreover, the calcite detected was formed by a partial conversion of the Ca phases (portlandite, ettringite, and Ca-(Fe)-silicates) during the aqueous steel slag carbonTable 1. X-ray Fluorescence Analyses of EAF and LF Slags concentration (wt %)

a

component

EAFa

LFa

FeO MnO CaO SiO2 MgO Al2O3 P2O5 TiO2 S K2O

34.2 2.50 32.8 14.6 10.0 5.10 0.30 0.89 0.07 0.01

4.30 2.18 58.1 26.4 6.20 4.60 0.05 0.74 0.15 0.03

EAF, electric arc furnace slag; LF, ladle furnace slag.

10.1021/ie701721j CCC: $40.75  2008 American Chemical Society Published on Web 09/26/2008

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7611 Table 3. Maximal Concentrations of Reactive Elements Leached in the LF and EAF Slag Suspensions concentration (mg/L)a

Figure 1. Schematic diagram of the carbonation reactor for the CO2 sequestration in EAF and LF slag suspensions and leachates. Table 2. Acidic Neutralizing Capacities of Suspensions, Leached Hydrated-Matrixes, and Leachates of EAF and LF Slags slags

material

acidic neutralizing capacitya (mol of HNO3/kg of slag)

EAF

suspension (fresh slag) leached hydrated-matrix leachate suspension (fresh slag) leached hydrated-matrix leachate

2.12 2.98 0.17 5.85 4.67 0.56

LF

ation. On the basis of these observations, Huijgen et al.9,10 have proposed that, at high CO2 partial pressure and temperature (carbonation degree 60%, pCO2 ) 20 bar, T ) 150 °C, d < 106 µm, L/S ) 10 kg/kg), the aqueous carbonation occurs in two steps including the leaching of calcium into the solution and the precipitation of CaCO3 at the surface of slag particles.9 The steel slag carbonation begins by a rapid carbonation of Ca(OH)2 present in the fresh steel slag that is followed by the diffusion of calcium from calcium silicate toward the surface of steel slag material. This calcium is subsequently leached. Then, the calcium in solution is carbonated and precipitated as a form of CaCO3 on the outside surface of the particles. Its has been suggested that the carbonation reactions of Ca phases that occur during the carbonation of steel slag are as follows:10,17 (1)

Ca6(Al)2(OH)12(SO4)3·26H2O + 6CO2 f CaCO3 + 2Al3++3SO24 + 31H2O (2) Ca-silicates + CO2 f CaCO3+SiO2

LF

Ca Na K Mg Fe Mn Pb Zn Cu Al Cd Ni Cr

670.9 2.379 1.079 0.0054 0.723 0.008 0.231 0.0078 0.0028 0.861 0.0004 0.0003 0.0036

EAF 304.9 11.66 16.19 0.0004 0.018 0 0.0142 0.0371 0.0006 246.6 0 0 0.003

a The leaching was effectuated in deionized water at 20 ( 1 °C under an agitation of 300 rpm during 24 h at atmospheric pressure.

2. Experimental Section

a The acidic neutralizing capacity was measured to pH 8.3 in 10.0 ( 0.1 g of fresh slags in suspension in 100 mL of deionized water; mixture of 10.0 ( 0.1 g of leached hydrated matrix and 100 mL of deionized water; 100 mL of leachate, after a titration with HNO3 1 N.

Ca(OH)2+CO2 f CaCO3+H2O(l)

reactive element

(3)

Interestingly, the leaching experiment done by Huijgen and Comans10 has shown that the carbonation of Ca phases into CaCO3 has a limiting effect by reducing the leaching of alkaline earth metals. The particle size of slag (d), the dissolution of metals into the leachate, the conversion of dissolved CO2, and the precipitation of carbonates at the surface of slag particles represent the other factors that may limit the CO2 sequestration.9,18-21 The principal objective of the present investigation is to evaluate the intrinsic sequestration potential of EAF and LF slags as well as their leaching capacities at ambient conditions. The CO2 sequestration capacities were determined by carbonation of EAF and LF suspensions, and their leached hydratedmatrixes and isolated leachates collected after three consecutive leaching at ambient conditions (temperature of 20 ( 1 °C and atmospheric pressure).

2.1. Materials. The EAF and LF slags were provided from the same production process using electric arc furnace by Mittal Canada Co. Inc. (Contrecoeur, QC, Canada). Concentrated nitric acid (trace metal grade 68-70 wt %) was purchased from Fisher Scientific Co. (Nepean, ON, Canada), and extra dry oxygen and a mixture of 15.00 vol % CO2 and 85.00 vol % N2 gas were from Praxair Production Inc. (Montre´al, QC, Canada). All these products were used without additional purification. 2.2. Materials Preparation. Nonhydrated (fresh) EAF slag was produced from ∼75 wt % mineral iron and 25 wt % scrap iron in an electric arc furnace. LF slag was obtained after refining of steel in a ladle furnace. The EAF slag used was under crystalline form and possessed a specific gravity of ∼3.7 g/cm3 established according to the particle size (d ) 0.315-0.630 mm). At the exit of the furnace, the EAF slag was crushed at the desired particle size and stored in open air. The LF slag used was solid grains with a specific gravity of ∼3.0 g/cm3 and a particle size between 0.160 and 0.315 mm. The EAF and LF slag samples was achieved by crushing and sieving to obtain a particle size of 38-106 µm. The EAF and LF slag suspensions were prepared by mixing 20.0 ( 0.1 g of fresh slags (d ) 38-106 µm) and 200 mL of deionized water in 250-mL Pyrex Erlenmeyers flasks. The slags suspensions were stirred during 24 h at 20 ( 1 °C, at atmospheric pressure, and under an agitation of 300 rpm in an Innova 4230 Refrigerated-Incubator Shaker from New Brunswick Scientific Co. (Fischer, Ottawa, ON, Canada). The three consecutive leachates and leached hydrated-matrixes designated as first, second and third leachates, and the leached hydratedmatrixes of EAF and LF slags were prepared with three consecutive leaching of EAF and LF slag suspensions. The leached hydrated-matrixes and leachates were collected from each leaching after a 20-min decantation at ambient temperature and a filtration with Filtropur cellulose acetate sterile filters (0.45-µm pore size) from Sarstedt Aktlengesellschaft & Co. 2.3. Materials Characterization. The measures of the chemical composition of steel slags (fresh and nonhydrated EAF and LF slags) were effectuated by X-ray fluorescence using a sequential X-ray spectrometer Philips PW2400. The measures were made in triplicate on different slag samples. The acidic neutralizing capacities of suspensions, containing (1) 10.0 ( 0.1 g of fresh EAF and LF slags and 100 mL of deionized water, (2) 10.0 ( 0.1 g of the first leached hydratedmatrixes and 100 mL of deionized water, and (3) the first leachates prepared as indicated in section 2.2, were measured

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Figure 2. Variation of pH (A) and conductivity (B) as a function of leaching time of aqueous fresh LF (9) and EAF (b) slag suspensions. The results were measured after leaching of steel slag suspensions at an agitation of 300 rpm and 20 ( 1 °C.

by titration with 1 N HNO3 using an automatic TIM865 Titration manager and the Titramaster 85 V.1.3.0. software from Radiometer Analytical (Lyon, France). The nitric acid was prepared by a dilution of concentrated nitric acid in deionized water. All the assays have been made in duplicate. The pH and electrolytical conductivities of the slag suspensions, prepared as indicated in section 2.2 and leached at intervals from 2 min to 24 h, were measured with a pH-meter PHM 250 Ion Analyzer and a CDM Conductivity Meter from MeterLab Co. (Radiometer Analytical). All the assays were made in triplicate. The concentrations of the different elements were measured in the first leachates of EAF and LF slags obtained after 30, 60, 120, 360, 600, and 1440 min according to the slag tested, by ICP-AES using an ICP-AES IRIS Advantage system (Thermo Scientific). The samples were previously filtered using Filtropur cellulose acetate sterile filters (0.45-µm pore size) from Sarstedt Aktlengesellschaft & Co, and acidified to 5 vol % of HNO3 before each ICP-AES measurement. 2.4. Materials Carbonation. The carbonation was performed with a gas mixture containing 15.00 vol % CO2 and 85.00 vol % N2 at a gas flow rate of 5 mL/min in the following materials: fresh slag suspensions, the leached hydrated-matrixes, and the leachates collected from three consecutive leaching (section 2.2). The assays were realized at 20 ( 1 °C during 2, 10, 20, and 30 min and 1, 2, 3, 6, 8, 10, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48,

Figure 3. Variation of CO2 sequestration (A), variation of pH (B) and conductivity (C) in aqueous fresh LF (9) and EAF (b) slag suspension effectuated with a 15.00 vol % CO2/N2 mixture and measured at a CO2 flow rate of 5 mL/min, an agitation of 200 rpm, and 20 ( 1 °C.

and 72 h according to the material tested, at atmospheric pressure and under an agitation of 200 rpm. The leached hydratedmatrixes extracted from three consecutive leaching steps, were dispersed at the bottom of Pyrex Erlenmeyer flasks to form a uniform layer before carbonation. The CO2 sequestration was made in Pyrex Erlenmeyers flasks of 250 mL open to the atmosphere (Figure 1). The CO2 was injected from a gas mixture reservoir, and the gas flow rate was controlled by using a flowmeter with a stainless steel float from Cole-Parmer Instrument Co. (Vernon Hills, IL). The uniform repartition of gas in the aqueous steel slag suspensions, leachates, and hydrated-

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7613 Table 4. Results for pH, Electrolytical Conductivity, and CO2Sequestration Capacity of Suspensions, Leached Hydrated-Matrixes, and Leachates of the EAF and LF Slags pHa material EAF suspension 1st leachate 2nd leachate 3nd leachate 1st leached hydrated-matrix 2nd leached hydrated-matrix 3nd leached hydrated-matrix LF suspension 1st leachate 2nd leachate 3nd leachate 1st leached hydrated-matrix 2nd leached hydrated-matrix 3nd leached hydrated-matrix

electrolytical conductivitya(mS/cm) initial final

CO2 sequestration capacitya (g of CO2/100 g of slag)

initial

final

11.73 11.77 11.80 11.54 11.95 12.05 12.30

11.01 6.97 6.69 6.78 10.70 10.35 9.73

1.70 1.60 1.57 1.11

0.41 1.05 0.96 0.63

1.74 1.11 0.76 0.72 2.57 1.74 0.76

12.50 12.59 12.38 11.96 12.79 12.58 12.26

9.00 7.78 7.08 6.86 10.18 10.22 9.52

7.62 7.26 3.39 2.37

0.96 0.79 0.54 0.59

24.7 1.40 0.81 0.53 8.29 7.53 4.08

a The CO2 sequestration was performed with a 15.00 vol % CO2/N2 mixture at a flow of 5 mL/min and 20 ( 1 °C under an agitation of 200 rpm at atmospheric pressure.

Figure 4. Variation of CO2 sequestration in first (b), second (9), and third (2) leachates of LF (A) and EAF (B) slags, and first (b), second (9), and third (2) leached hydrated-matrixes of LF (C) and EAF (D) slags effectuated with a 15.00 vol % CO2/N2 mixture and measured at a CO2 flow rate of 5 mL/min, an agitation of 200 rpm, and 20 ( 1 °C.

matrixes was assumed by using a porous diffuser located within each suspension and leachate or in the Erlenmeyer flasks for the leached hydrated-matrixes. The pH and electrolytical conductivity were measured according to the techniques described in section 2.3, and the content of carbon was measured in each sample using an Apollo 9000 TOC Combustion Analyzer from Dohrmann Division Co. (Santa Clara, CA). The carbon analysis were made using a furnace with a continuous

flux of oxygen at 800 °C for combustion and a tube containing cobalt oxide on alumina for catalysis. The analyzer was calibrated with a solution of potassium biphthalate at an organic carbon concentration of 2000 ppm. The concentrations in organic carbon of slag suspensions and leachates were measured after the injections of 20-40 µL while the values for hydratedmatrixes were measured after the injections of amounts of hydrated-matrixes corresponding to 10-40 µL of nonhydrated

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Figure 5. Variation of pH of first (b), second (9), and third (2) leachates of LF (A) and EAF (B) slags, and first (b), second (9), and third (2) leached hydrated-matrixes of LF (C) and EAF (D) slags measured at a CO2 flow rate of 5 mL/min, an agitation of 200 rpm, and 20 ( 1 °C.

matrixes. The fresh slag samples and leachates were stirred with a magnetic stir-bar at 500 rpm before the measurements, and the amounts of hydrated-matrixes analyzed were weighed using an analytic balance. The concentrations in organic carbon of fresh slags, leachates, and leached hydrated-matrixes were evaluated by subtraction of the amounts of carbon measured before and after the carbonation of these samples. The samples were discarded after each analysis. 3. Results and Discussion 3.1. Chemical Compositions of EAF and LF slags. The chemical compositions of steel slags are indicated in Table 1. The EAF and LF slags are rich in Fe and Ca, and the dominant Ca phase, which was found to be constituted by portlandite, corresponds to amounts of CaO content in these slags. The high portlandite content and alkalinities (pH 11.7 and 12.8) of slags, and more particularly for LF slag, provide them with a great potential for CO2 sequestration. 3.2. Determination of the Acidic Neutralizing Capacities of Fresh Slags and Their Leached Hydrated-Matrixes and Leachates. The acidic neutralizing capacities of fresh EAF and LF slags, and their leached hydrated-matrixes and leachates, were measured by titration to pH 8.3 (pH of a CaCO3 saturated solution at atmospheric pressure) and are indicated in Table 2.22 The native pH of fresh slags and their leached hydrated-matrixes varied between 11.7 and 12.8 and are in agreement with what is expected from the chemical equilibrium of a saturated solution of portlandite (Ca(OH)2) (pH 12.4

calculated with Geochemist’s Workbench 4.0).9,23 The results presented in Table 2 indicate that the acidic neutralizing capacities of fresh EAF and LF slags are 2.12 and 5.85 mol of HNO3/kg of slags, respectively. These values of acidic neutralizing capacities combined with the content of portlandite and native pH of these steel slags suggest that their CO2 sequestration capacities increase with their contents of portlandite. However, these acidic neutralizing capacities correspond to the CO2 sequestration capacities of 0.05 and 0.13 kg of CO2/kg of slags associated with the amount of Ca available for CO2 sequestration at ambient pressure and temperature. These values are low compared to the total theoretical CO2 sequestration capacities based on the slag’s total amount of calcium evaluated at 0.23 and 0.42 kg of CO2/kg of EAF and LF slags, respectively. These values as well as those shown in Table 3 indicate that these slags contain small amounts of calcium (EAF ) 21.7 wt %; LF ) 31.0 wt % total calcium) available for leaching in water and for CO2 sequestration at 20 °C under atmospheric pressure. These data are in agreement with those reported by Huijgen et al.9 who measured an acidic neutralizing capacity of 4.0 mol of HNO3/kg of fresh slag in the steel slag containing 31.7 wt % CaO, corresponding to a sequestration capacity of 0.09 kg of CO2/kg of steel slag. 3.3. Determination of pH, Conductivity, and Concentrations of Reactive Elements Liberated during the Leaching of Steel Slags. The variations of pH and electrolytical conductivity of the fresh EAF and LF slag suspensions and the maximal concentrations of mineral released during the leaching

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7615

Figure 6. Variation of the conductivity of (A) first (b), second (9), and third (2) leachates of LF (A) and EAF (B) slags measured at a CO2 flow rate of 5 mL/min, an agitation of 200 rpm, and 20 ( 1 °C.

are indicated in Figure 2 and Table 3. The curves presented in Figure 2 show that the maximal values of pH and conductivity are higher for the LF slag than those obtained for the EAF slag suspensions. These results may be due to the greater amounts of leached Ca2+ and OH- in the LF slag than in the EAF slag suspension, which can enhance the pH and electrolytical conductivity values of the LF slag leachate as compared to those of the EAF slag. However, the presence of a great amount of aluminum in the leachates of the EAF slag also may enhance its electrolytical conductivity (Table 3). 3.4. Determination of the CO2 Sequestration Capacity of Fresh Steel Slag Suspensions and Their Leached Hydrated-Matrixes and Leachates. The results shown in Figure 3A and Table 4 indicate that the carbonation in the fresh EAF and LF slag suspensions reaches the maximum values of 1.74 and 24.7 g of CO2/100 g of slag after 24 and 40 h, respectively. The CO2 sequestration capacity, which is much higher for the LF slag than the EAF slag suspensions, may be associated with the presence of greater amounts of Ca2+ and OH- leached and particularly with a greater amount of portlandite in the LF slag suspension (Tables 1 and 3). The carbonation of the other metals analyzed may be considered as negligible since no significant amounts are leached, they are not present in alkaline mineral forms, and they did not produce stable carbonate species (Table 3). The unfavorable conditions of reaction including too slow kinetics of the carbonation reaction at ambient pressure and temperature may also have contributed to lack of carbonation for the other metals. On the other hand, the results presented in Figure 3B,C indicate that a higher decrease of pH

and electrolytical conductivity values occurred in the fresh LF slag than the EAF slag suspension. These data suggest the formation of greater amounts of CaCO3 and HCO3- in the LF slag suspension during the carbonation. This result is in agreement with the greater CO2 sequestration capacity of the LF slag suspension. The results of carbonation of three leachates and leached hydrated-matrixes of EAF and LF slags are shown in Figures 4-6 and Table 4. The curves presented in Figure 4A,B indicate that the CO2 sequestration in the leachates of the EAF and LF slags rapidly reached the maximums after 6 h. The CO2 sequestration capacities of leachates are lower than those of the fresh EAF and LF slag suspensions (Figure 3, Table 4). These low CO2 sequestration capacities may be associated with a low leaching of Ca2+ and OH-. This observation is in agreement with the low acidic neutralizing capacities observed for the first leachate of the EAF and LF slags. This also supports the lower decreases of electrolytical conductivity that occur in leachates as compared to those of the fresh slag suspensions (Tables 2-4). In contrast, the higher decrease of pH values that occurred in leachates, as compared to those observed in the fresh slag suspensions, can be due to the absence of the matrix of which alkalinity may contribute to enhance the pH and thereby reduce its decrease in the fresh slag suspensions (Figures 2 and 5). The CO2 sequestration in the leached hydrated-matrixes of EAF and LF slags reached the maximums after 12 and 72 h (Figures 4C,D). These data indicate that the CO2 sequestration in the leached hydrated-matrixes of these slags is slower than those observed in leachates. Nevertheless, the CO2 sequestration capacities of leached hydrated-matrixes of EAF and LF slags are from 2 to 9 times higher than their leachates (Table 4). These results combined with the high alkalinity and acidic neutralizing capacities of leached hydrated-matrixes suggest that a great part of the CO2 sequestration potential stays in their matrixes after the leaching (Table 2). In parallel, the addition of the CO2 sequestration capacities obtained for three leachates and leached hydrated-matrixes of EAF slag gives a CO2 sequestration capacity (7.66 g of CO2/ 100 g of slag) 5 times higher than that observed for the fresh EAF slag suspension (Table 4). This indicates that the leachates and matrixes of this slag are more effective to react with the CO2 when they are separated. In the case of the LF slag, the addition of the sequestration capacities of its leachates and leached hydrated-matrixes gives a CO2 sequestration capacity slightly inferior to that of the fresh LF slag suspension. This decrease of CO2 sequestration capacity may be caused by the formation of a carbonate layer, due to the large amounts of CaCO3 formed, that blocks the CO2 binding sites at the surface of matrixes.18,19 Moreover, the dispersal of leached hydratedmatrixes at the bottom of the Pyrex Erlenmeyers flasks may also limit the CO2 sequestration on the exposed surfaces of matrixes and thereby decreased their sequestration capacities. 4. Conclusions The present study has revealed that the chemical compositions and leaching capacities of steel EAF and LF slags may directly affect their CO2 sequestration capacities at ambient conditions (temperature of 20 ( 1 °C and atmospheric pressure) even if they are provided from the same production process. The results have effectively revealed that aqueous fresh LF slag suspension contains a great amount of portlandite, which confers to them higher alkalinity and greater leaching capacities of Ca2+ and OH- than those observed for EAF slag. These characteristics may confer to the LF slag suspension a liquid-to-solid ratio of

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10 kg/kg and a CO2 sequestration capacity that is 14 times higher than that obtained for the fresh EAF slag. The separation of leached hydrated-matrixes and leachates of the EAF and LF slags does not affect their alkalinity. Moreover, the effectiveness of the CO2 sequestration in leachates and hydrated-matrixes isolated from the EAF slag is higher when they are separated than in the slag suspension. In spite of this, the effectiveness of the CO2 sequestration in leachates and hydrated-matrixes of the EAF slag stays lower than that observed for fresh LF slag suspension. In contrast, the separation of hydrated-matrixes and leachates from the LF slag suspension leads to a decrease of the CO2 sequestration capacity. These results suggest that the high carbonation of the LF slag can significantly limit the CO2 sequestration capacity of hydratedmatrix when they are isolated from the steel slag suspension. Hence, together these results presented in this investigation indicate that the great alkalinity and content of portlandite probably contribute mainly confer to steel slags their high CO2 sequestration capacities. Moreover, the CO2 sequestration could be made with a good yield in aqueous EAF and LF slag suspensions as well as in a process where the carbonation is performed by using the separated leachates and leached hydrated-matrixes. Acknowledgment This work was supported by the grants from Natural Resources Canada in collaboration with Mittal Canada Inc. and MultiServ (Harsco). Michel Piche´ and Jean Lavoie are also acknowledged for providing the slags and information about steelmaking process. Literature Cited (1) IPCC. Policymaker’s summary of the scientific assessment of climate change; Report to IPCC from working group, Meteorological Office, Branknell, 1990. (2) Riemer, P. W. F.; Ormerod, W. G. International perspectives and the results of carbon dioxide disposal and utilization studies. Energy ConVers. Manage. 1995, 36, 813. (3) Liu, Z.; Zhao, J. Contribution of carbonate rock weathering to the atmospheric CO2 sink. EnViron. Geol. 2000, 39, 1053. (4) Robert, C. Instantane´s techniques N°2 (Nouvelle Se´rie) spe´cial 50e`me anniversaire; Techniques de l’inge´nieur Ed: Paris, Juin 1996. (5) Kharoune, M. Se´questration du CO2 par carbonatation mine´rale en re´acteurs dynamiques. 7ie`me Salon International des Technologies Environnementales; AMERICANA: Montre´al, 2007. (6) Miklos, P. The utilization of electric arc furnace slags in Denmark. Euroslag. Engineering of slags; A Scientific and Technological Challenge. 2nd European Slag Conference: Düsseldorf, 2000.

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ReceiVed for reView December 17, 2007 ReVised manuscript receiVed June 12, 2008 Accepted July 28, 2008 IE701721J