Selective Leaching of Steelmaking Slag for Indirect CO2 Mineral

Ind. Eng. Chem. Res. 2010, 49, 2055–2063

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Selective Leaching of Steelmaking Slag for Indirect CO2 Mineral Sequestration Weijun Bao,†,‡ Huiquan Li,*,† and Yi Zhang† National Engineering Laboratory for Clean Production Technology of Hydrometallurgy, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, P.R. China

Indirect CO2 mineral sequestration, which could make CO2 fixate into precipitated calcium carbonate (PCC) of value-added products, is an important technology that is used to reduce greenhouse gas emissions economically. It can be conducted in two steps, one of which has been investigated in the previous paper. In this work, extraction of calcium ions from steelmaking slag using a novel leaching medium, which involves organic solvent tributyl phosphate (TBP), acetic acid, and ultrapure water, was studied. Several operating variables, including stirring speed, phase-volume ratio, organic solvent-to-solid ratio, initial acetic acid concentration, acid-to-slag ratio, reaction temperature, and reaction time were investigated. It was found that the leaching process could be divided into three regions according to the acid-to-slag ratio. The first region below 0.5 g/g was characterized by the acid-to-solid ratio; the second region above 0.5 g/g but below 1.0 g/g was characterized by the acid-to-solid ratio; and the third region above 1.0 g/g was characterized by the acid-to-solid ratio. When the acid-to-solid ratio was below 1.0 g/g, only Ca and Mg could be leached with the maximum leached ratios of 75% and 35%, respectively. Moreover, the leaching behaviors of Ca, Mg, Fe, Al, and Si were greatly affected by reaction temperature and reaction time. Results show that the calcium ions can be effectively and selectively extracted from the steelmaking slag and that the reaction medium can be recovered and recycled with high efficiency. These are the keys to indirect CO2 mineral sequestration. 1. Introduction CO2 mineral sequestration is a potentially important technology that can be used to reduce greenhouse gas emissions on the basis of industrial carbonation of Ca/Mg-silicates.1-4 It can be classified as direct route, wherein mineral materials are carbonated in one step, and indirect route, wherein the reactive components are first extracted from the mineral matrix by the recycling medium and, then, carbonated in a separate step.3 On the basis of the most recent literature review, it has been observed that indirect aqueous carbonation is considered the most attractive route particularly because either the dissolution step or the subsequent precipitation step, or both, can be potentially developed for large-scale applications.5 A popular possible feedstock for mineral CO2 sequestration is in the field of industrial solid waste, which includes waste cement,6-8 coal combustion fly ash,9,10 cement kiln dust,11,12 steelmaking slag,13-20 red mud,21 oil-shale wastes,22 and so on. These materials are generally composed of alkaline and are rich in calcium. Compared to ores, their major advantages are their low cost, widespread availability in industrial areas, and chemical instability, thus proving that they are more reactive compared to primary minerals.13 Moreover, industrial solid waste contains abundant metal elements that can be recovered for further industrial applications. Owing to the rich calcium content of waste cement and steelmaking slag, researchers have proposed producing high-value-added calcium carbonate from solid wastes.8,16 Calcium carbonate produced in the solid waste carbonation process may replace some of the limestone used in the steel or cement industry. If it meets the high standards of synthetic calcium carbonates, it may likewise be used as a filler or coating pigment in plastics, rubbers, paints, and papers, but carried out for a higher price.16 * To whom correspondence should be addressed. Tel.: +86-1062621355. Fax: +86-10-62621355. E-mail: [email protected]. † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

Steelmaking slag is a byproduct of a basic oxygen furnace within 0.15-0.20 t slag per ton of steel product.23 Although it can be used as a substitute for gravel in road construction or lime in agriculture, specificly in China, its utilization ratio is limited to about 36% because of the high alkaline-earth metal (e.g., Ca and Mg) oxide content in this slag, indicating that heavy metal elements are released into the environment.24 It is estimated that the annual amount of steelmaking slag produced in China is above 50 Mt, most of which remain treated as landfill material. Therefore, one of the urgent problems is to seek a new utilization option for cheap and readily available steelmaking slag. Owing to its high calcium oxide content, the slag may be used to produce high value-added calcium carbonate via the indirect aqueous carbonation route that uses acetic acid as a recycling medium. Herein, calcium is extracted in the first dissolution step and, then, carbonated in the second step. In our previous study, we have reported that the second carbonation step could be greatly improved by coupling reactive crystallization with solvent extraction and upon the introduction of organic solvent TBP into the carbonation process.25 By adding organic solvent TBP in the second carbonation step, part of the produced acetic acid could be extracted from the aqueous phase into the organic phase with the formation of (1:1) acid-extractant complexes,26-28 while the recovery of TBP and the recycling of the acetic acid should be further considered. Many methods can be applied to recover TBP from acid-extractant complexes, including removal of acetic acid through reactive distillation29,30 and the conversion of acetic acid into water-soluble chemicals (e.g., calcium acetate.).31 However, the distillation operation consumes too much energy, and thus, it is better to strip acetic acid from the acid-extractant complexes by forming calcium acetate. It is interesting to note that the acetic acid, either in the organic phase or in the aqueous phase, can be used directly in calcium extraction from the steelmaking slag. Moreover, acetic acid had already been employed as an effective reaction medium for the extraction of

10.1021/ie801850s  2010 American Chemical Society Published on Web 01/29/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

Figure 1. Experimental apparatus.

calcium from wollastonite and steelmaking slag, as reported by Kakizawa et al.32 and Teir et al.33 Therefore, to obtain both high-leaching ratio of calcium and the effective recovery of organic solvent, the selective leaching of steelmaking slag for indirect CO2 mineral sequestration by a novel leaching medium, which involves acetic acid mixed with organic solvent TBP and ultrapure water, was investigated in this study for the first time. The effects of several variables, including stirring speed, phasevolume ratio, organic solvent-to-solid ratio, initial acetic acid concentration, acid-to-slag ratio, reaction temperature, and reaction time, were experimentally studied. 2. Experimental Section 2.1. Materials. Steelmaking slag was collected from the steel production process using a 120 t basic oxygen furnace by Jinan Iron & Steel Integrated Co. The slag was dried at 60 °C in an oven and crushed at a particle size smaller than 50 mesh for all of the following experimental runs. Glacial acetic acid (HAc, 99.8 wt %) and TBP (TBP, 98.5 wt %) were of analytical grade, which were purchased from Beijing Chemical Reagent Co., and used as received without further purification. The reaction medium used in the steelmaking slag leaching process was a composite of a water phase and an organic phase. The water phase used only ultrapure water; while the organic phase was prepared by first completely mixing the glacial acetic acid, TBP, and ultrapure water in a stirring container and, then, finally taking the upper organic solvent phase after it was left standing for a few minutes. Therefore, the quantity of the added acetic acid can be determined by both the volume of the organic phase and the initial concentration of acetic acid in the organic phase. 2.2. Apparatus and Procedure. The schematic diagram of the apparatus used in this work is illustrated in Figure 1. The dissolution step was conducted in a three-mouth flask with a temperature monitor and a sampling port. The reactor of the three-mouth flask with a volume of 1 L was surrounded by a temperature-controlled water bath. The leaching medium in this reactor was stirred using a mechanical stirrer, with speed that varied from 300 to 650 rpm. The leaching reaction temperature was monitored at the range of 40-94 °C. However, increasing the reaction temperature to above 94 °C was impossible because of the water bath in the experimental apparatus. In the experimental run, the leaching medium, which involved the organic phase mixed with a certain volume of ultrapure

water, was added to the reactor at the beginning of the experiment and then stirred at a certain speed. The water bath was then heated to a constant temperature. When the monitored temperature in the reactor reached the setting temperature value, the prepared slag was added to the reactor and reaction time was recorded. After 1 h, stirring was stopped and the residual slag was removed from the solution through sedimentation and filtration. The residual slag was collected and washed with ultrapure water, then several times with anhydrous ethanol, and finally kept in an oven at 120 °C for 12 h. The filtrate delaminated quickly, and the superstratum was sampled to determine the residual acetic acid concentration. The underlayer of the filtrate was separated and then mixed to a fixed volume with the lotion from the residual slag and ultrapurity water for analysis. Furthermore, in order to observe separately the different reactions to temperature during the leaching process, the organic and inorganic phases were heated to a certain temperature. Both phases were then added to the reactor and mixed with the prepared slag. During the leaching reaction, samples were taken from the mixed solution 2, 5, 10, 20, 30, 40, 50, and 60 min after the slag was added. The mixed solution samples delaminated quickly, and the superstratum was sampled to determine the residual acetic acid concentration. The underlayer was filtered (using syringe membrane filter, 0.45 µm) and analyzed for the main elements from the slag (e.g., calcium (Ca), magnesium (Mg), iron (Fe), aluminum (Al), and silicon (Si)). 2.3. Analysis. The composition of the slag was analyzed using X-ray diffraction (XRD) (X’pert Pro MPD X-ray diffractometer from PANalytical) and X-ray fluorescence spectroscopy (XRF) (SHIMADZU Lab Center XRF-1700, Japan). The contents of Ca, Mg, Fe, Al, and Si were determined when a 0.05 g steelmaking slag was melted with sodium metaborate at 950 °C for 30 min and then dissolved with hydrochloric acid to a fixed volume. These were finally measured by using inductively coupled plasma-atomic emission spectrometry (ICP-AES)(OPTIMA5300DVinductivelycoupledplasma-optical emission spectrometry manufactured by Perkin-Elmer, USA). The loss on ignition (LOI) of the steelmaking slag was determined when a 5 g slag was put in a high-temperature tube furnace at 900 °C for 2 h under a nitrogen atmosphere. It was then cooled down to measure the weight lost by the slag. The concentration of Ca, Mg, Fe, Al, and Si in the leaching solution was also measured using ICP-AES. The initial and residual acetic acid concentrations in the organic phase were titrated by standard NaOH solution, using anhydrous ethanol as diluents. 2.3. Experimental Data Treatment. The leaching ratios of Ca, Mg, Al, Fe, and Si were calculated using the following equation: leaching ratio(%) )

V0ci × 100; Wsxi

i ) Ca, Mg, Fe, Al, Si

(1) where ci is the concentration of Ca, Mg, Al, Fe, and Si in the leaching aqueous solution; V0 is constant volume of the leaching aqueous solution; Ws is the weight of the added slag; and xi is the element content of Ca, Mg, Al, Fe, and Si in the added slag. The selectivity was calculated based on the leaching of Ca and Mg because both may be used for the fixation of CO2 into carbonate product. The equation is as follows:


selectivity(%) )

2(cCa + cMg) × 100 2(cCa + cMg) + 3(cFe + cAl)

Table 1. Composition of the Prepared Steelmaking Slag As Determined by XRF Analysis

(2) It should be pointed out that the composition of the leaching solution obtained from the steelmaking slag, leached by the novel leaching medium of organic solvent TBP mixed with acetic acid and water, was mostly acetate; for example, calcium acetate (Ca(Ac)2), magnesium acetate (Mg(Ac)2), iron acetate (Fe(Ac)3), and aluminum acetate(Al(Ac)3). Meanwhile, the concentrations of ferrous acetate (Fe(Ac)2) and other metal acetates were negligible because of their low content or instability in an aqueous solution.

components

composition, %

CaO CO2 SiO2 Fe2O3 Al2O3 MgO SO3 F MnO Others

38.7 20.1 15.0 10.3 5.4 5.0 1.8 0.8 0.8 2.1

Table 2. Composition of the Selected Elements in the Prepared Steelmaking Slag As Determined by ICP-AES Analysis

3. Results and Discussion

composition, %

3.1. Feedstock Characterization. The XRD analysis of the used steelmaking slag showed that it consisted of lime (CaO), calcium hydroxide (Ca(OH)2), calcium carbonate (CaCO3), calcium fluoride (CaF2), quartz (SiO2), magnesium hydroxide (Mg(OH)2), wuestite (FeO), calcium iron oxide (Ca2Fe2O5), and larnite (Ca2SiO4) with less akermanite (Ca2MgSi2O7). This indicates that the lime and calcium hydroxide phases were dominant; the slag must have had high alkalinity. The chemical composition of the steelmaking slag, as listed in Tables 1 and 2, was measured by XRF and ICP-AES, respectively. The LOI of the slag was measured at 10.73%, while the CO2 content from the XRF analysis was at 20.1%, indicating that the difference between the two kinds of analysis have resulted from the slag being absorbed in water and CO2 in air during the sample treatment for the XRF analysis. According to the XRF and ICP-AES analyses, the Ca, Mg, Fe, Al, and Si content in the prepared steelmaking slag was high, which indicates that the prepared steelmaking slag was not suitable for building materials but may have potential application for mineral CO2 sequestration, as well as high-value-added calcium carbonate production. 3.2. Leaching Process Fundamental Theory. The leaching process was conducted in a stirred reactor at atmospheric conditions. The leaching medium was a mixture of organic solvent TBP, acetic acid, and ultrapure water. Prior mixing, the acetic acid only existed in the organic phase, showing a formation of (1:1) acid-extractant complexes. When this organic phase contacted with the water phase, the acetic acid was partially stripped and then transformed from the organic phase into the water phase. Therefore, the leaching medium that reacted with steelmaking slag was mainly the acetic acid in the water phase reacting with the steelmaking slag, which formed soluble salts undergoing the following reactions: TBP · HAC(o) f TBP(o) + HAc(w)

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(3)

Ca-resources + 2HAc(w) f Ca(Ac)2 + H2O + SiO2

(4) Mg-resources + 2HAc(w) f Mg(Ac)2 + H2O + SiO2

(5) Al-resources + 3HAc(w) f Al(Ac)2 + H2O + SiO2

(6) Fe-resources + 3HAc(w) f Fe(Ac)3 /Fe(Ac)2 + H2O + SiO2 (7) The leaching behaviors of other metal resources were negligible because of their low content in the prepared steel-

elements

sample 1

sample 2

average

Ca Mg Fe Al Si

25.33 6.94 11.26 3.32 8.48

25.86 6.97 11.07 3.54 8.46

25.60 6.96 11.16 3.43 8.47

making slag. With high acidity, an increase in both the leaching rate of calcium and the extraction rate of magnesium can be observed; meanwhile, aluminum, iron, and silica are dissolved in large quantities. Silica also forms a gel, which decreases the filtration rate or even results in the collapse of the operation.34 Moreover, in order to efficiently recover and recycle the leaching medium, including TBP and acetic acid, for indirect CO2 mineral sequestration, the aqueous solution obtained after the leaching process was directly carbonated for high-value calcium carbonate production. Thus, the dissolved aluminum, iron, and silica (except magnesium) were converted into impurity during calcium carbonate production. Therefore, first, the acidity of the leaching aqueous solution must be controlled to reduce the dissolved aluminum, iron, and silica or have them precipitated from the leaching solution. Second, the residual acetic acid in the organic phase should be as little as possible to recover the organic solvent TBP effectively. 3.3. Leaching Process Variables Analysis. A number of variables affected the leaching behavior of Ca, Mg, Fe, Al, and Si on the steelmaking slag by the novel leaching medium that involved TBP, acetic acid, and ultrapure water. These variables, which included stirring speed, phase-volume ratio, organic solvent-to-solid ratio, the initial acetic acid concentration, acidto-slag ratio, reaction temperature, and reaction time, have been studied in this work. The effects of the individual process variables on the selective leaching of steelmaking slag are discussed below. Stirring Speed. Before studying the effects of other factors that may influence the steelmaking slag leaching, the effect of the stirring speed was first studied. It is known that the speed of stirring affects the external diffusion of the reaction medium and the elements Ca, Mg, Fe, Al, and Si in the leaching process,35 and high stirring speed decreases the external mass transfer resistance, which could improve the reaction rate, and the leaching reaction is hastened. In order to decrease the diffusion resistance in the leaching process, the stirring speed was increased from 300 to 650 rpm in this work. Table 3 shows the experimental results obtained at different stirring speeds at 94 °C after 1 h reaction time in 1.466 mol/L acetic acid using a 30 g slag mixed with 300 mL organic solvent TBP and 100 mL ultrapurity water. As shown in Table 3, with stirring speed increased from 300 to 650 rpm, the decrease of the residual acetic acid concentration in organic phase have not exceeded

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Table 3. Effect of the Stirring Speed on the Leaching Process from the Prepared Steelmaking Slag stirring speed

residual acetic acid concentration in organic phase, mol/L

300 400 500 650

0.169 0.149 0.142 0.141

leaching ratio, % Ca

Mg

72.30 73.30 73.32 73.77

33.68 33.82 34.46 34.58

Table 4. Effect of the Phase-Volume Ratio on the Leaching Process from the Prepared Steelmaking Slag residual acetic acid concentration phase volume ratio in organic phase, mol/L 0.75:1 1:1 3:1 4:1

0.091 0.116 0.142 0.156

leaching ratio, % Ca

Mg

74.08 73.81 73.32 72.80

34.92 34.59 34.46 33.82

0.03 mol/L, and the increase of the leaching ratios of calcium and magnesium have not exceeded 1.5%, considering all unavoidable experimental errors. It could be concluded that the residual acetic acid concentration in the organic phase had little decrease, while the leaching ratios of calcium and magnesium had a little increase when the stirring speed increased from 300 to 650 rpm after 1 h reaction time. That might result in two aspects. For one thing, when the leaching ratios of calcium and magnesium increased, it resulted in more acetic acid consumption and the residual acetic acid concentration in the organic phase decreased. In addition, external mass transfer resistance was decreased when stirring speed varied from 300 to 650 rpm; stirring speed showed very limited influence on reaction rate under the above experimental conditions. Phase-Volume Ratio. The volume ratio between the organic phase including TBP and acetic acid and the aqueous phase including ultrapure water is referred to as the phase-volume ratio in this study. The volume of the organic phase with the initial acetic acid concentration of 1.466 mol/L was kept constant at 300 mL, and the volume of the ultrapure water varied from 75 to 400 mL. Therefore, in this work, the phase-volume ratio varied from 4 to 0.75. Experimental results are shown in Table 4. It was found that with the increase of phase-volume ratio, the concentration of residual acetic acid in organic phase gradually increased, while the leaching ratio of calcium and magnesium decreased. With an increased phase ratio from 0.75 to 4, the increase of the concentration of residual acetic acid in organic phase was observed to be above 0.06 mol/L. Meanwhile, the decrease of the leaching ratios of calcium and magnesium remained at 1.5% and below. It can be concluded that the phasevolume ratio had greater influence on the residual acetic acid concentration in the organic phase than on the leaching ratios of calcium and magnesium. There are two reasons for this. First, with the increase of the phase-volume ratio, the volume of the added ultrapure water decreased, which made the concentration of the leached acetate salts in aqueous solution increase, and prevented acetic acid to be stripped from the organic phase based on the salting out effect. Second, high acidity was preferred for obtaining high leaching ratios of calcium and magnesium. However, the acidity of the aqueous solution in the leaching process was hardly affected by the volume of the added ultrapure water with the phase-volume ratio that varied from 0.75 to 4, which resulted in little change in the leaching ratios of calcium and magnesium. Organic Solvent-to-Solid Ratio, Initial Acetic Acid Concentration, and Acid-to-Slag Ratio. In this study, the novel leaching medium is a mixed phase composed of ultrapure water

Figure 2. Effect of organic solvent-to-solid ratio on the leaching ratios of Ca, Mg, Fe, and Al with (a) initial acetic acid concentration in the organic phase of 0.998 mol/L and (b) initial acetic acid concentration in the organic phase of 1.466 mol/L.

and organic solvent TBP with definite concentration of acetic acid. An important factor that affected the steelmaking slag leaching is the ratio between the quantity of the added slag and the added acetic acid. The quantity of the added acetic acid used in the leaching process was determined by both the volume and the initial concentration of acetic acid in the organic phase. The factors related to organic solvent-to-solid ratio, initial acetic acid concentration in the organic phase, and acid-to-slag ratio shall later be discussed in detail. In this study, the organic solvent-to-solid ratio is the ratio of the volume of the organic phase and the mass of the added slag. Meanwhile, the acid-tosolid ratio is the mass ratio of the added acetic acid and the steelmaking slag at the initial reaction time, combining the factors of the organic solvent-to-solid ratio and the initial acetic acid concentration in the organic phase. Figures 2-4 show the effect of organic solvent-to-solid ratio on the selective leaching of steelmaking slag under the following conditions: reaction temperature of 94 °C, reaction time of 1 h, stirring speed of 500 rpm, phase-volume ratio of 3, and the initial acetic acid concentration in the organic phase of 0.998 and 1.466 mol/L. When the initial acetic acid concentration in the organic phase was 0.998 mol/L, the leaching ratios of Ca, Mg, Fe, and Al varied with solid-to-organic solvent ratio, as plotted in Figure 2a. It shows that with the increase of the organic solvent-tosolid ratio, the leaching ratios of Ca and Mg increased, while their growth rate decreased. The leaching of Fe and Al were hardly detected in aqueous solution with the organic solventto-solid ratio of below 15 mL/g. When the organic solvent-to-


Figure 3. Effect of organic solvent-to-solid ratio on the mass lost by the steelmaking slag and residual acetic acid concentration in the organic phase with (a) initial acetic acid concentration in the organic phase of 0.998 mol/L and (b) initial acetic acid concentration in the organic phase of 1.466 mol/L.

solid ratio was above 15 mL/g, the leaching ratios of Fe and Al increased. It should be noted that the leaching of Si was not detected with the organic solvent-to-solid ratio that varied from 5 to 40 mL/g. Figure 3a shows the mass lost by the steelmaking slag after leaching. Moreover, the residual acetic acid concentration in the organic phase that varied with the organic solventto-solid ratio when the initial acetic acid concentration in organic phase was 0.998 mol/L. From Figure 3a, it could be seen that both the mass lost and the residual acetic acid concentration in the organic phase increased with the organic solvent-to-solid ratio, while their growth rates were seen to be different. When the organic solvent-to-solid ratio was above 15 mL/g, the growth rate of the mass lost decreased with the increase of the organic solvent-to-solid ratio. The growth rate of the residual acetic acid concentration in the organic phase increased initially, but then decreased when the organic solvent-to-solid ratio was above 15 mL/g. Figure 4a shows the selectivity and the Ca and Mg total leaching ratio, which varied with the organic solvent-tosolid ratio when the initial acetic acid concentration in organic phase was 0.998 mol/L. It can be seen that the selectivity was equal to 100% when the organic solvent-to-solid ratio was below 15 mL/g, while it showed minimal decrease when the organic solvent-to-solid ratio increased to above 15 mL/g. The total leaching ratio of Ca and Mg increased with the increase of the organic solvent-to-solid ratio, and it reached as high as 80% with the initial acetic acid concentration of 0.998 mol/L in the organic phase.

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Figure 4. Effect of organic solvent-to-solid ratio on the selectivity and total leaching ratios of Ca and Mg with (a) initial acetic acid concentration in the organic phase of 0.998 mol/L and (b) initial acetic acid concentration in the organic phase of 1.466 mol/L.

On the basis of the above experiment results, it can be concluded that when the initial acetic acid concentration in the organic phase was kept constant, leaching ratios and residual acetic acid concentration increased with organic solvent-to-solid ratio. However, when the organic solvent-to-solid ratio exceeded 15 mL/g, further leaching of Ca and Mg was not easily achieved because the leaching ratios of Ca and Mg, as well as the mass lost by the steelmaking slag, increased slowly. This may be due to the prepared steelmaking slag not only contained the easyreacting Ca-resource and Mg-resource (e.g., CaO, MgO, Ca(OH)2, and CaCO3) but also the difficult-reacting Ca-resource and Mg-resource (e.g., Ca2Fe2O5, Ca2SiO4, and Ca2MgSi2O7). The easy-reacting Ca-resource and Mg-resource dissolved first, which left the difficult-reacting Ca-resource and Mg-resource to form a solid residue. The difficult-reacting Ca-resource and Mg-resource were not completely dissolved though the organic solvent-to-solid ratio, which exceeded 15 mL/g. Moreover, the leaching ratios of Fe and Al increased with the increase of the residual acetic acid concentration in the organic phase. This is a result of the stable existence of the ions of Fe and Al in acidic solutions. In order to investigate the factor of the initial acetic acid concentration in the organic phase, the experimental runs were carried out with the initial acetic acid concentration in the organic phase increased to 1.466 mol/L, while the other conditions were kept constant. Experiment results are also shown in Figures 2-4. Figure 2b shows that when the organic solventto-solid ratio exceeded 12.5 mL/g, the leached Fe and Al were detected. In addition, their leaching ratios increased with organic

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solvent-to-solid ratio. When the organic solvent-to-solid ratio exceeded 12.5 mL/g, the mass lost by the steelmaking slag manifested minimal increase, but the growth rate of the residual acetic acid showed a decreased concentration (shown in Figure 3b). The selectivity was equal to 100% when the organic solvent-to-solid ratio was less than 12.5 mL/g. It decreased, however, with the increase of the organic solvent-to-solid ratio, and the Ca and Mg total leaching ratios reached about 85% when the organic solvent-to-solid ratio was 40 mL/g (shown in Figure 4b). It can be concluded that the increase of the initial acetic acid concentration in the organic phase could promote Ca and Mg leaching with low organic solvent-to-solid ratio. However, when the organic solvent-to-solid ratio was high, the leaching ratios of Ca and Mg were independent of the initial acetic acid concentration. A possible explanation for this may be that although the acidity in the aqueous solution could be improved by increasing the initial acetic acid concentration in the organic phase, the inert Ca and Mg resource in steelmaking slag was still difficult to dissolve by the novel leaching medium. Therefore, to decrease the recycling of the leaching medium, the high initial acetic acid concentration in the organic phase was preferred for indirect CO2 mineral sequestration by the steelmaking slag carbonation. It should be stressed that the initial acetic acid concentration in the organic phase was limited by the crystallization conversion, as well as the initial concentration of calcium acetate aqueous solution in the second carbonation step. The effect of the acid-to-solid ratio on the leaching behaviors of Ca, Mg, Fe, and Al, as well as the residual acetic acid concentration in the organic phase, are shown in Figure 5a and b. It is interesting to observe that the leaching ratios of Ca, Mg, Fe, and Al were mainly dependent on the acid-to-solid ratio, while the residual acetic acid concentration depended on both the acid-to-solid ratio and on the initial acetic acid concentration in the organic phase. Figure 5a shows that when the acid-tosolid ratio was above 1.0 g/g, the leachings of Fe and Al were detected in the aqueous solution, and their leaching ratios increased greatly. On the other hand, the growth rate of the leaching ratios of Ca and Mg decreased with the increase of acid-to-solid ratio. The residual acetic acid concentration in the organic phase increased, while the growth rate increased at first then decreased with the increase of the acid-to-solid ratio, as shown in Figure 5b. When the acid-to-solid ratio was below 0.5 g/g, the residual acetic acid in organic phase was not detected, and it did not depend on the initial acetic acid concentration in the organic phase. When the acid-to-solid ratio was above 0.5 g/g, however, the residual acetic acid concentration in the organic phase tended to be constant at the high acidto-solid ratio, and it greatly depended on the initial acetic acid concentration. Therefore, it can be concluded that the leaching process was greatly affected by the acid-to-solid ratio. According to the acid-to-solid ratio, the leaching process could be divided into three regions. The first region was characterized by the acid-to-solid ratio, which was below 0.5 g/g. In this region, the added acetic acid was completely converted into acetate salt, and the leaching ratios of Ca and Mg were limited to 50% and 20%, respectively. The second region was characterized by the acid-to-solid ratio, which was above 0.5 g/g and below 1.0 g/g. In this region, the leaching ratios of Ca and Mg increased to 75% and 35%, respectively, and the selectivity was 100%, although the residual acetic acid concentration in the organic phase showed little increase with acid-to-solid ratio. The third region was characterized by the acid-to-solid ratio, which was above 1.0 g/g. In this region, the leaching ratios of

Figure 5. Effect of acid-to-solid ratio on the leaching process from the prepared steelmaking slag on (a) leaching ratios of Ca, Mg, Fe, and Al and (b) residual acetic acid concentration in the organic phase.

Fe and Al, and the residual acetic acid concentration in the organic phase, increased with the acid-to-solid ratio. Considering that the aqueous solution obtained in the leaching process should be directly carbonated for high-value calcium carbonate production, the leachings of Fe and Al should be prevented to stop introducing impurity. Thus, the acid-to-solid ratio should be set below 1.0 g/g in the steelmaking slag selective leaching process. Moreover, the residual acetic acid in the organic phase would decrease the carbonation crystallization efficiency in the second carbonation step, and thus, the acid-to-solid ratio should be carefully chosen for both the high leaching ratio of Ca and the low residual acetic acid concentration in the organic phase. Reaction Temperature and Reaction Time. In order to study the progression of the steelmaking slag leaching behavior by this novel leaching medium, experimental runs were carried out under the condition that 50 g of steelmaking slag was added to the reactor. The reactor contained 500 mL organic phase with the initial acetic acid concentration kept at 1.25 mol/L and 800 mL ultrapure water at the reaction temperature of 40, 60, 80, and 94 °C. The effects of the reaction temperature on the residual acetic acid concentration in the organic phase at different reaction times are plotted in Figure 6a. Figure 6a shows that the residual acetic acid concentration in the organic phase decreased with the increase of the reaction time, and it decreased rapidly during the first 5 min. Moreover, the residual acetic acid concentration in the organic phase also decreased with the increase of the reaction temperature. Figure 6b-f shows the


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Figure 6. Effect of reaction temperature and reaction time on (a) residual acetic acid concentration in the organic phase; (b) leaching of Ca; (c) leaching of Mg; (d) leaching of Fe; (e) leaching of Al; and (f) leaching of Si.

concentration of Ca, Mg, Fe, Al, and Si in the leaching aqueous solution that varied with reaction time at different reaction temperatures. Figure 6b and c shows that the concentration of leached Ca and Mg increased with the reaction temperature and reaction time. It increased rapidly in the first 5 min, and its growth rate decreased when the reaction time was extended to 60 min. As shown in Figure 6d, the reaction temperature and reaction time have complicated influence on the concentration of leached Fe in aqueous solution. When the reaction temperature was as low as 40 °C, the concentration of leached Fe increased with the reaction time. It increased quickly during the first stage of reaction time and, then, gradually became stable after 20 min. When the reaction temperature increased to 60 °C, the concentration of leached Fe increased first, and then decreased with the increase of the reaction time. When the

reaction temperature continued to increase to 80 °C, the concentration of leached Fe reached a peak value in the first 5 min, and then decreased rapidly with the increase of the reaction time. The leached Fe could hardly be detected after 60 min of reaction. When the reaction temperature was as high as 94 °C, the variation of the concentration of leached Fe was the same with 80 °C, except that the peak value was higher and the concentration of leached Fe decreased faster than when the leaching process was conducted at 80 °C. Figure 6e and f shows that the concentrations of leached Al and Si varied with reaction temperature at different reaction times and displayed a similar tendency. They decreased with the increase of the reaction temperature, while they increased to a peak value and then decreased with the increase of the reaction time. The peak values decreased with the increase of the reaction temperature. It can

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also be noted that when the reaction temperature was as low as 40 °C, the concentrations of leached Al and Si were still too high after 60 min of reaction. On the other hand, the leached Al and Si were hardly detected after only 30 min of reaction when the reaction temperature was above 80 °C. On the basis of the above-mentioned experiment results, it can be concluded that the high reaction temperature favors obtaining a high leaching ratio of Ca and Mg, as well as a low residual acetic acid concentration in the organic phase. This is because the dissociation of acetic acid from the formation of (1:1) acid-extractant complexes in the organic phase increases with the reaction temperature, which leads to the increase of the acidity in the aqueous solution and also to the increase of the leaching ratios of Ca and Mg. The concentration of the leached Fe, Al, and Si in the aqueous solution greatly increased during the first few minutes. This resulted from the fact that the acidity in the aqueous solution at the initial stage was high, which made Fe, Al, and Si dissolve in large quantities. After the initial stage, the concentration of the leached Fe, Al, and Si decreased with the increase of the reaction time. The reasons for this include the following: First, because of the continued reaction, more acetic acid was consumed to form soluble acetate salts and the acidity in the aqueous solution decreased with the increase of the reaction time. Second, the soluble acetate salts (e.g., iron acetate, aluminum acetate) could be converted into precipitation under the condition of high temperature and low acidity in the aqueous solution. It should be pointed out that some parts of iron were leached in the form of ferrous iron, which was oxidized by oxygen from air in the reactor after a specific reaction period. Such were detected by the color changes of the sampled aqueous solution placed in air after a few hours. Moreover, the color of the residue obtained at 94 °C after 1 h of reaction changed from brown gray to reddish brown, which further indicated that the iron acetate was converted into ferric hydroxide. The element Si was leached in the form of silicic colloid36 and was removed by the adsorption on the surface of the precipitation or the residue under high temperature and low acidity conditions. It can also be concluded that the effect of the reaction temperature on the concentrations of leached Fe, Al, and Si was much greater than the effect of reaction time. This is mainly because the precipitation of Fe, Al, and Si from the aqueous solution usually happens rapidly, and it greatly depends on the reaction temperature, except with the acidity of the solution. Hence, if the reaction temperature is low, it is impossible to prevent the leaching of Fe, Al, and Si by extending the reaction time. It was also determined that the acid-to-solid ratio used in this section was 0.75 g/g, which indicated that the leaching process was in the second region. The leachings of Fe, Al, and Si were not detected after 1 h of reaction time, as shown in Figure 6, which is consistent with the results shown in Figure 5. The high reaction temperature and the long reaction time were seen to be better for the selective leaching of steelmaking slag. 4. Conclusions In this work, the selective leaching of steelmaking slag by a novel leaching medium, which involved TBP, acetic acid, and ultrapure water, was primarily studied for the purpose of indirect CO2 mineral sequestration. Several operating variables were investigated, including stirring speed, phase-volume ratio, organic solvent-to-solid ratio, initial acetic acid concentration in the organic phase, acid-to-slag ratio, reaction temperature, and reaction time. Furthermore, it was found that Ca, Mg, Fe, Al, and Si were the main elements that leached from steelmaking

slag, and were greatly affected by the acid-to-slag ratio, reaction temperature, and reaction time. On the basis of the acid-to-slag ratio, the leaching process can be divided into three regions. The first region was characterized by the acid-to-solid ratio that was below 0.5 g/g. The second region was characterized by the acid-to-solid ratio that was above 0.5 g/g but below 1.0 g/g. Finally, the third region was characterized by the acid-to-solid ratio that was above 1.0 g/g. In the first and second regions, only Ca and Mg could be leached with the maximum leached ratios of 75% and 35%, respectively. The residual acetic acid could never be detected in the first region, while the leaching process in the third region was not suitable for indirect CO2 mineral sequestration. The low reaction temperature and short reaction time would favor the leachings of Fe, Al, and Si, while high reaction temperature and long reaction time would make the leached Fe, Al, and Si precipitate from the solution. This reveals that high reaction temperature and long reaction time are preferred for selective leaching of steelmaking slag for indirect CO2 mineral sequestration. Most importantly, the results from the present study have shown that calcium ions could be effectively and selectively extracted from steelmaking slag, and that the reaction medium could be recovered and recycled with high efficiency for indirect CO2 mineral sequestration. Acknowledgment The authors are grateful for the financial support of National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science and Technology of China (No. 2006BAC02A14), the State Key Development Program for Basic Research of China (Grant No. 2007CB613507), and the National Natural Science Foundation of China (No.50974112). Literature Cited (1) Metz, B.; Davidson, O.; Coninck, H.; Loos, M.; Meyer, L. Special report on carbon dioxide capture and storage; Cambridge University Press, Cambridge, UK, NY, 2005. (2) Lackner, K. S. Carbonate chemistry for sequestering fossil carbon. Annu. ReV. Energy EnViron. 2002, 27, 193. (3) Huijgen, W. J. J.; Comans, R. N. J. Carbon dioxide sequestration by mineral carbonation, literature reView; ECN School Fossiel: Netherlands, 2003. (4) Huijgen, W. J. J.; Comans, R. N. J. Carbon dioxide sequestration by mineral carbonation: literature reView update 2003-2004; ECN School Fossiel: Netherlands, 2005. (5) Sipila¨, J.; Teir, S.; Zevenhoven, R. Carbon dioxide sequestration by mineral carbonation: literature reView update 2005-2007; Faculty of technology heat engineering laboratory report, Åbo Akademi University, Finland, 2008. (6) Yamasaki, A.; Fujii, M.; Kakizawa, M.; Yanagisawa, Y. Reduction process of CO2 emissions by treating with waste concrete via an artificial weathering process. Proceedings of the 6th international conference on greenhouse gas control technologies, Kyoto, Japan, October 1-4, 2002. (7) Iizuka, A.; Fujii, M.; Yamasaki, A.; Yanagisawa, Y. Development of a new CO2 sequestration process utilizing the carbonation of waste cement. Ind. Eng. Chem. Res. 2004, 43, 7880. (8) Katsuyama, Y.; Yamasaki, A.; Lizuka, A.; Fujii, M.; Kumagai, K.; Yanagisawa, Y. Development of a process for producing high-purity calcium carbonate (CaCO3) from waste cement using pressurized CO2. EnViron. Prog. 2005, 24, 162. (9) Martin, B.; Michael, K.; Helge, S.; Stefan, P. Reactivity of alkaline lignite fly ashes towards CO2 in water. EnViron. Sci. Technol. 2008, 42, 4520. (10) Montes-Hernandez, G.; Pr´ez-Lo´pez, R.; Renard, F.; Nieto, J. M.; Charlet, L. Mineral sequestration of CO2 by aqueous carbonation of coal combustion fly-ash. J. Hazard. Mater., in press. (11) Huntzinger, D. N.; Eatmon, T. D. A life-cycle assessment of portland cement manufacturing: comparing the traditional process with alternative technologies. J. Clean. Prod. 2009, 17, 668.

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ReceiVed for reView December 2, 2008 ReVised manuscript receiVed January 5, 2010 Accepted January 19, 2010 IE801850S

Selective Leaching of Steelmaking Slag for Indirect CO2 Mineral

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