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Carbonation and utilization of basic oxygen furnace slag coupled with concentrated water from electrodeionization
Journal of CO₂ Utilization 25 (2018) 46–55
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Carbonation and utilization of basic oxygen furnace slag coupled with concentrated water from electrodeionization ⁎⁎
Yemei Lia,b, Silu Peic, Shu-Yuan Panc, Pen-Chi Chiangc,d, , Changyi Lua,b, Tong Ouyanga,
T
⁎
a
College of the Environment & Ecology, Xiamen University, Xiamen, 361102, China School of Environmental Science & Engineering, Xiamen University Tan Kah Kee College, Zhangzhou, 363105, China c Carbon Cycle Research Center, National Taiwan University, Taipei City, 10672, Taiwan d Graduate Institute of Environmental Engineering, National Taiwan University, Taipei City, 10673, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct carbonation Basic oxygen furnace slag Concentrated water from electrodeionization Supplementary cementitious materials
In this study, the integrated recycle process of basic oxygen furnace slag (BOFS) coupled with concentrated water from electrodeionization (CW-EDI) which was introduced into the direct carbonation process for the first time was investigated. The effects of different operating parameters on carbonation conversions of BOFS were studied and the reaction kinetics were determined via surface coverage model. Meanwhile, the changes of CWEDI during leaching and carbonation processes were measured. Furthermore, the performance evaluation of BOFS applied as supplementary cementitious materials (SCMs) and a cost and benefits analysis of the whole process was conducted. The results indicated the feasibility of the integrated system to industrial solid waste, sewage, and exhaust treatments and the maximum achievable carbonation conversion of BOFS was approximately 60.9% with R2 of 0.958–0.982. A high level of calcium ion and the alkaline solution hindered the leaching of calcium ion from BOFS, whereas sodium ion was absorbed by BOFS simultaneously. The properties of the cement that used BOFS as SCMs met ASTM requirements. The suggested substitution ratios for fresh and carbonated BOFS in cement mortars were 15% and 10%, respectively. The economic results revealed the superiority of the carbonation and utilization process over traditional treatments in CO2 emission reduction, cost saving and profit gaining.
1. Introduction Carbon dioxide (CO2) capture and storage that includes integrated CO2 treating process from the original gas separation to the final longterm isolation from the atmosphere is less extensive than other mitigation actions for the stabilization of atmospheric greenhouse gas concentrations [1]. Mineral carbonation, also called natural weathering, is a CO2 storage option in which CO2 is fixed and stored in the form of inorganic carbonates that will not be released back into the atmosphere over a long time. Although the weathering reaction is thermodynamically favorable, the reaction time can last for thousands of years due to its slow reaction kinetics. Considering the reaction efficiency, accelerated carbonation, an enhanced version of natural weathering, was proposed. Naturally occurring silicate rocks, such as serpentine [2] and olivine [3], and industrial solid wastes, such as steelmaking slags [4], cement wastes [5], and waste ash [6], are suitable for industrial CO2 fixation due to the existence of alkaline and alkaline earth oxides [7].
⁎
Accelerated carbonation can be conducted through various approaches, such as direct carbonation [8] and indirect carbonation [9]. In direct carbonation, the efficiency of aqueous carbonation at moderate conditions is relatively higher than that of dry carbonation. In general, aqueous carbonation can be simply divided into three steps: (1) cation leaching from the solid phase, (2) CO2 dissolution into the liquid agents, and (3) production of carbonate precipitates [10]. The performance of the carbonation reaction can be influenced by the properties of feedstock [11] and the reaction conditions [12,13], as well as the reactor types. A rotating packed bed reactor applied during the carbonation process also known as High-gravity carbonation, or HiGCarb for short, produces carbonation results with a shorter reaction time than traditional reactors [14]. Stable carbonated products can be reused as construction materials such as supplementary cementitious materials (SCMs), a substitution of clinker in Portland cement or Portland cement in concrete. Using SCMs cannot merely lower cost and environmental impact, especially by decreasing CO2 emission in cement production [15], but also improve
Corresponding author. Corresponding author at: Carbon Cycle Research Center, National Taiwan University, Taipei City, 10672, Taiwan. E-mail addresses: [email protected] (P.-C. Chiang), [email protected] (T. Ouyang).
⁎⁎
https://doi.org/10.1016/j.jcou.2018.03.003 Received 29 October 2017; Received in revised form 2 February 2018; Accepted 2 March 2018 2212-9820/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of carbonation process via a rotating packed bed reactor.
experiments. The synthetic water was prepared using CaCl2 and NaCl (supplied by J. T. Baker) dissolved in tap water on the basis of the real CW-EDI. Furthermore, HCl and NaOH solutions with concentrations of 1 and 0.1 M, respectively, were used to adjust the initial pH of CW-EDI. In addition, high-pressure CO2 gas with a volumetric concentration of 99.5% supplied by Ching-Feng Gas Corporation (Taipei, Taiwan) was used in carbonation process. Ordinary Portland cement (OPC) and standard sand supplied by Gao Ching Corporation (Taipei, Taiwan) were used in the cement replacement experiments.
long-term strength and durability [16]. Moreover, using SCMs is also a pathway to utilizing the by-products generated by industrial manufacturing processes. Although SCMs have been used and studied for decades, the field is still in the stage of development. Recent research has focused on exploring new materials and additives, modifying methods, increasing replacement amount, and developing enhanced test methods [17]. Basic oxygen furnace slag (BOFS) is a by-product from the steelmaking and refining processes. BOFS presents an excellent capacity to remove phosphorous from aqueous solutions [18] and has been widely used in various fields, such as soil improvement, landfills, and marine ecosystem conservation. Nevertheless, the free-CaO contained in BOFS will result in volume expansions and structural damages if BOFS is reused as construction materials. On the contrary, the high CaO content is welcome in the carbonation process [19]. Concentrated water from electrodeionization (CW-EDI), which has high levels of sodium and calcium ions and whose treatment approaches are rarely discussed, is the effluent generated during the electrodeionization process. Further utilization of products generated from the carbonation of CW-EDI may be directly restricted because of its impurity. However, mixing CW-EDI with BOFS before carbonation can simultaneously improve the properties of BOFS and utilize CW-EDI for CO2 mineralization, thereby establishing the waste-to-resource supply chain for a circular economy [20]. Therefore, this study aims to investigate the utilization of CW-EDI in direct carbonation and the feasibility of a comprehensive process for the treatments of industrial sewage, solid wastes, and waste CO2. The integrated recycle procedure of BOFS coupled with CW-EDI, which includes the beaker leaching and HiGCarb processes and utilization of BOFS as SCMs was conducted in the study. Furthermore, a China case of cost and benefit analysis to the HiGCarb process and cement replacement was carried through based on the experiment results.
2.2. Characterization of BOFS X-ray fluorescence (XRF, PW2430, Philips, Netherland) was used to determine the chemical composition of the BOFS, while a scanning electron microscope (SEM, TM3000, Hitachi, Japan) was used to perform the morphological investigation. The change in sample weight with temperature was measured using thermo-gravimetric analysis (TGA, STA6000, PerkinElmer, USA) with a heating rate of 10 °C min−1 at a temperature of 50 °C–950 °C and a N2 flow rate of 19.8 mL min−1. 2.3. Leaching characteristics of BOFS in CW-EDI Beaker leaching experiments were conducted to determine the influences of the leaching time, initial pH, and calcium ion levels of CWEDI on the leaching behaviors of the BOFS. 100 g of the BOFS was added into a beaker containing CW-EDI prepared in advance with a liquid-to-solid (L/S) ratio of 20 mL g−1. The slurry was continuously stirred and sampled and the pH value was simultaneously recorded at special times throughout the experiment. The reaction was continued for 60 min. The liquid part of the sample was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, 700 Series, Agilent Technology) after the solid-liquid separation through a 45 μm membrane filter.
2. Materials and methods 2.4. HiGCarb of BOFS coupled with CW-EDI 2.1. Materials The influence of different operating parameters, such as reaction time, reactor rotation speed, slurry flow rate, and L/S ratio, on the carbonation conversion of BOFS coupled with CW-EDI was evaluated. The rotating packed bed reactor used in this study was made of stainless
The BOFS provided by China Steel Corporation (Kaohsiung, Taiwan) was ground and dried at 105 °C overnight to eliminate moisture. The CW-EDI was used as the liquid phase in the leaching and HiGCarb 47
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steel with a packing zone filled with stainless steel wire. Fig. 1 shows a schematic diagram of the HiGCarb. The BOFS and CW-EDI slurry was pumped into the reactor after adequate mixing in the beaker. Injecting CO2 into the reactor set off the carbonation process, which lasted for 60 min. The reactor rotated and the slurry circulated in the reactorbeaker system with continuous stirring in the beaker during the experiment. After the solid-liquid separation, the solid and liquid samples were analyzed using TGA and ICP-OES, respectively. The carbonation conversion (δCaO , %), which was calculated by Eq. (1), was used to evaluate the carbonation degree of the BOFS. Carbonation conversion was the amount of CO2 actually captured in the dry mass of each sample compared with the theoretical extent of carbonation on the basis of the reactive-oxide content of the fresh BOFS.
δCaO = AcCO2 / ThCO2 × 100
Table 1 Physico-chemical properties of fresh BOFS, carbonated BOFS and ordinary Portland cement (OPC).a Items
Chemical composition
(1) −1
The theoretical CO2 capture capacity (ThCO2, g CO2 (g BOFS) ), whose calculation was based on the XRF result, expressed the potential CO2 capture capacity of the BOFS. Meanwhile, the actual CO2 capture capacity (AcCO2, g CO2 (g BOFS)−1) which was calculated by Eq. (2) on the basis of the TGA result, showed the amount of CO2 captured by the BOFS under a certain operating condition. In this equation, m105℃ (g) is the dry weight of the sample used for TGA analysis. ΔmCaCO3 (g) is the weight loss of CaCO3 in temperatures of 500 °C–850 °C, in which CaCO3 typically decompose into CaO and CO2.
AcCO2 = ΔmCaCO3/(m105℃−ΔmCaCO3)
Physical properties
a b c d
SiO2 (wt.%) Al2O3 (wt.%) Fe2O3 (wt.%) CaO (wt.%) MgO (wt.%) SO3 (wt.%) K2O (wt.%) Na2O (wt.%) Total (wt.%) free-CaO (wt.%) Carbonation conversion (%) d D(v, 0.1) (μm) D(v, 0.5) (μm) D(v, 0.9) (μm) Density (g cm−3)
Fresh BOFS
Carbonated BOFS b
OPC
11.80 4.39 36.07 36.72 4.50 0.37 0.02 0.06 93.93 3.49 2.2
10.75 4.39 36.44 36.98 4.50 0.30 0.02 N.D. c 93.38 0.39 19.0
20.67 4.50 2.90 63.05 2.88 2.63 0.64 0.34 97.61 0.92 –
2.23 9.26 25.63 3.66
1.35 10.91 35.54 2.65
– – – –
Analyzed by CHC Resources Corporation. Carbonated BOFS used for cement replacement experiments. N.D.: not detected. Calculated as Eq. (1) based on the data got from TGA and XRF measurement.
SO3, respectively, in the fresh BOFS determined by XRF. CaCO3 (wt.%) is the weight fraction of CaCO3 measured by TGA and MCO2 (g mol−1), MCaO (g mol−1), MCaCO3 (g mol−1), and MSO3 (g mol−1) are the molecular weights of CO2, CaO, CaCO3, and SO3, respectively. A comparison of the chemical composition of the fresh and carbonated BOFS showed a reduction in free-CaO content, thus revealing the stabilization of the BOFS by the carbonation process. A similar composition of the BOFS and OPC demonstrated the practicability of BOFS used as SCMs. In addition, the MgO and SO3 contents contained in the BOFS and OPC did not allow the mixtures to exceed the ASTM limitation. The physical properties of the BOFS changed after carbonation. The decrease in D(v, 0.1) indicated the generation of small particles and the increment in D(v, 0.5) and D(v, 0.9) showed the expansion of particles. Meanwhile, the density of the carbonated BOFS was 72% of that of the fresh BOFS. Fig. 2(a) and (b) show the TG/DTG curves of the fresh and carbonated BOFS, respectively. The CaCO3 content in the fresh BOFS was 0.62 wt.% in dry weight. This content level corresponds to the weight loss in temperature of 550 °C–800 °C, which significantly increased after carbonation with a DTG peak at 747 °C. In addition, a part of Ca(OH)2 may be attributed to CaO content because of a DTG peak that can be observed in Fig. 2(a) at 445 °C. On the basis of the collective results of XRF and TGA, the theoretical CO2 capture capacity of the BOFS was estimated to be 284 g of CO2 per kg of BOFS.
(2)
2.5. Utilization of BOFS as SCMs in the cement mortars Fresh and carbonated BOFS were used to partially replace OPC at substitution ratios of 0%, 5%, 10% and 15% by weight. Autoclave expansion ratio and compressive strength after curing ages of 3, 7, 28, and 56 days were regarded as the primary indicators with which to evaluate the performance of the BOFS used as SCMs. The autoclave expansion and compressive strength tests were implemented in accordance with ASTM C151 and C109, respectively. The maximum autoclave expansion ratio for Portland cement type I should be less than 0.8% as per ASTM C150. The compressive strength of the cement mortars after curing ages of 3, 7, and 28 (optional requirement) days needed to reach 12.0, 19.0, and 28.0 MPa, respectively. Moreover, toxicity characteristic leaching procedure (TCLP), which is the most widely used assessment method for ecological risk, was conducted for the fresh and carbonated BOFS according to NIEA R201.14C. Hazardous Industrial Waste and Green Building Materials, which are regulated by Taiwan EPA and the Taiwan Architecture and Building Center, respectively, were used to evaluate the results of TCLP. 3. Results and discussion 3.1. Physico-chemical properties of BOFS
3.2. Leaching kinetics of BOFS in CW-EDI
Table 1 presents the physico-chemical properties of the fresh and carbonated BOFS and OPC. The major components of the fresh BOFS were CaO (∼36.7 wt.%) and Fe2O3 (∼36.1 wt.%), with micro amounts of SiO2, Al2O3, MgO, K2O, and Na2O. Although several kinds of metal oxides existed in the fresh BOFS, not all of them contributed to CO2 sequestration. The carbonates generated from K2O and Na2O are soluble. Meanwhile, SiO2, Al2O3 and Fe2O3 cannot react with CO2. Moreover, little MgCO3 could be generated under the mild conditions in this study. Thus, only the calcium species were regarded as the major components that function in CO2 fixation. The ThCO2 of the BOFS used in this study can be calculated by Eq. (3) [5,19].
According to the previous study [21], calcium leaching should be the rate-limiting step for carbonation reaction of steel slag. In this study, a first-order mass loss-based method was used to describe the leaching kinetics of calcium from BOFS into CW-EDI. In view of the initial level of calcium ion in CW-EDI, the model can be integrated by Eq. (4).
Ct = Cmax −(Cmax −C0 ) e−kt
(4)
where t (min) is the leaching time, k (min−1) is the rate constant of leaching, Cmax (mg L−1) is the maximum leaching concentration of calcium ion in the liquid phase, C0 (mg L−1) is the original level of calcium ion in the solution, and Ct is the level of calcium ion at different time. Fig. 3 compares the experimental data and calculated results of
ThCO2 = MCO2/MCaO × (CaO−MCaO / MCaCO3 × CaCO3−MCaO / MSO3 × SO3) (3) where CaO (wt.%) and SO3 (wt.%) are the weight fractions of CaO and 48
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Fig. 2. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of (a) fresh and (b) carbonated BOFS.
Fig. 4. Effect of (a) leaching time and initial level of calcium ion, and (b) the pH of CWEDI on the leaching behavior of calcium ion.
CW-EDI with several levels of initial concentrations of calcium ion and pH. The trends of the calcium ion levels in all conditions increased rapidly in the first 5 min and then gradually approached their respective maximum concentrations. The comparison of ultimate calcium contents contained in the different liquid agents showed that the initial calcium level had a greater influence on calcium leaching than initial pH did. Table 2 presents the model parameters by fitting Eq. (4) with the experimental data shown in Fig. 4. Compared with those in the previous study [10], the particle diameter and initial levels of calcium ion and pH in the liquid phase had different effects on k and Cmax. Introducing CW-EDI positively influenced the improvement of the calcium leaching rate. The level of Cmax rose with the increment in C0 whereas the leaching rate and content decreased at the same time. That is attributed that the rate and content of calcium ion leaching from the BOFS were confined by the high level of calcium ion in the liquid phase. Even the calcium content of the BOFS was much higher than that of CW-EDI. This limitation should be considered because it may affect the further utilization of carbonated BOFS, although additional calcium ion in the aqueous phase can benefit CO2 capture. Additionally, a low initial pH of the CW-EDI (i.e., pH 5 and 6) allowed for Cmax augmentation (i.e., 1425 and 1421 mg L−1). According to the acid-base neutralization, hydrogen ion in liquors was conducive
Fig. 3. Comparison of calculated concentration of calcium ion in the slurry (leaching model) with experimental data.
calcium concentration determined by Eq. (4). The results indicated that the first-order mass loss-based method could successfully describe the calcium ion leaching from BOFS into CW-EDI, as the standard errors were within 10%. Fig. 4 presents the calcium ion leaching from the BOFS in various 49
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Table 2 Key model parameters of calcium ion leaching behavior in various types of liquid agents. ID
Type of liquid phase
Operating conditions
Model parameters
L/S ratio (mL g−1)
Diameter of BOFS (μm )
Initial calcium level (mg L−1)
Initial pH
Cmax(mg L−1)
C0(mg L−1)
k(min−1)
R2
8 8 8 8 5 6 7 8 11.2011.87 11.2011.87 11.2011.87 11.2011.87 11.2011.87
694 1065 1254 1904 1425 1421 1391 1254 2690
10 452 813 1413 798 805 802 813 –
1.683 1.510 0.879 0.595 1.538 1.505 1.797 0.879 1.128
0.952 0.953 0.860 0.896 0.896 0.946 0.910 0.860 0.999
This study This study This study This study This study This study This study This study [10]
2115
–
0.876
0.996
[10]
1310
–
0.684
0.973
[10]
1256
–
0.696
0.973
[10]
876
–
0.708
0.983
[10]
L1 L2 L3 L4 L5 L6 L7 L8 L9
Tap water CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CRW a
20 20 20 20 20 20 20 20 20
25 25 25 25 25 25 25 25 <125
10 451 803 1437 803 803 803 803 147
L10
CRW
20
125-350
147
L11
CRW
20
350-500
147
L12
CRW
20
500-840
147
L13
CRW
20
840-1190
147
a
Reference
CRW: Cold-rolling wastewater.
carbonation reactions of Ca(OH)2 [22,23], was also employed to describe the kinetics of mineral carbonation in aqueous solutions for their reaction behaviors, which were similar to those of Ca(OH)2. According to the reaction and deposition hypotheses of the model, the carbonation reaction can reach the maximum carbonation conversion under a certain condition [24]. The model presented the relationship between carbonation conversion and reaction time (t, min) and can be expressed by Eq. (5).
to calcium ion extraction for the simultaneous generation of hydroxyl and calcium ions in the leaching process. The leaching content of calcium ion was hence improved. The slurry became alkaline during the neutralization process thereby favoring the dissolution of CO2 in the later carbonation reaction. Although a strong leaching behavior of the alkaline matter could be obtained at a low initial pH, the acidity of the solution may not be completely neutralized, thus restricting the generation of carbonate ion. Fortunately, the BOFS used in this study had sufficient neutralization ability to promote the pH of the solution when the initial pH of the liquid agents was 5–8 (Fig. 5(a)). To grasp the leaching behavior of sodium ion from the BOFS, tap water was used as the control liquid phase in this study. Fig. 5(b) shows that no significant change in sodium level had occurred in the control group thereby indicating that no sodium leached from the BOFS. When CW-EDI was used to replace tap water, sodium level remained unchanged after a rapid decrease at the beginning of the leaching process. The reasons may lie in the absorption behaviors of the BOFS and the electrostatic balance between the BOFS and CW-EDI.
δCaO = δmax [1−exp (−ks kp t )]
(5)
where δmax (%) is the maximum achievable carbonation conversion. ks (mol min−1 m−2) is the rate constant of the carbonation reaction, and kp (m2 mol−1) is the rate constant of the surface coverage. Fig. 7(d) and Table 3 present the comparison of the calculated carbonation conversion based on the surface coverage model with the experimental data shown in Fig. 7(a–c) and the fitting results, respectively. The calculated conversions were higher than the experimental ones early on due to the liquid phase used in this study. The abundant calcium ion in the liquid phase caused the generation of solitary CaCO3 thus decreasing the actual conversion of the BOFS. However, the theoretical values, which agreed with the experimental data in the last phase of the experiments, suggested the suitability of the surface coverage model for the HiGCarb of the BOFS in CW-EDI. As shown in Fig. 7 and Table 3, different operating conditions had different effects on the reaction. The carbonation conversion declined with the acceleration of rotation speed. At a high rotation speed (i.e., 1000 and 1250 rpm), the performance of mass transfer was increased to some extent. However, a brief contact between CO2 and slurry led to a low rate of carbonation reaction, thus resulting in a low conversion. δmax occurred at a rotation speed of 250 rpm where ks was faster than kp . δmax reached the highest point at the L/S ratio of 20 mL g−1. When the L/S ratio exceeded 20 mL g−1, ks and kp increased with the L/S ratio because of the violent reaction between the relatively abundant carbonate ion and insufficient calcium ion. However, the rate of surface coverage grew faster than that of carbonation reaction due to the lack of calcium ion, thus resulting in the decrease of δmax . On the contrary, δmax was low at 10 mL g−1 because of the gentle reaction among the reactants. This reaction explains the long processing time needed by mineral carbonation in nature. With the enhancement of slurry flow rate, δmax decreased after
3.3. Carbonation kinetics of BOFS in CW-EDI Fig. 6(a) shows the changes in carbonation conversion of BOFS, pH, and concentration of calcium and sodium ions in the slurry during the carbonation. The carbonation conversion became nearly constant after a sharp ascent. Meanwhile, a mirror symmetry polyline displayed the pH behavior. The tendencies of the levels of calcium and sodium, which were the main metal ions contained in CW-EDI, changed as well in the process, as shown as Fig. 6(b). The concentration of calcium ion rose after the first drop whereas the sodium level remained unchanged. As the main reaction species, calcium ion was consumed by carbonate ion to generate CaCO3, thereby resulting in the increase in carbonation conversion and the reduction of calcium ion level with the introduction of CO2 at the beginning of the reaction. With the continuous dissolution of CO2, the pH of CW-EDI gradually dropped to approximately 6.5, which was beneficial for calcium ion leaching. However, the reaction had no influence on the sodium level, thus indicating that sodium ion did not participate in the carbonation reaction. Fig. 7(a–c) show the influence of reaction time on the carbonation conversion of the BOFS under different operating conditions. The surface coverage model was used to describe the reaction kinetics. The model, which was the most suitable for describing the sulfation and 50
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Fig. 6. Changes of (a) carbonation conversion of BOFS, pH and (b) concentration of calcium and sodium ions in the slurry during the carbonation process. (Operating conL/S ditions: rotation speed = 750 rpm, slurry flow rate = 1.00 L min−1, ratio = 20 mL g−1). Fig. 5. Profiles of (a) pH and (b) concentration of sodium ion in the slurry during leaching experiments.
carbonated BOFS.
peaking at a slurry flow rate of 1.00 L min−1. Whether δmax was promoted or reduced by the improvement of slurry flow rate depended on the competition result of the strengthened of mass transfer and shortened contact time between the slurry and CO2. When the effect caused by the reduction in contact time was overcome by the enhancement of mass transfer, δmax was improved. By contrast, δmax was reduced due to the lack of contact time. Furthermore, δmax appeared at a slurry flow rate of 1.00 L min−1 at which ks was nearly equal to kp . On the premise of sufficient feedstock, calcium and carbonate ions, good maximum carbonation conversion could be approached when the rate of carbonation reaction was faster than that of surface coverage. With the synthesizing effects of rotation speed, L/S ratio, and slurry flow rate, δmax could reach 60.9% with an L/S ratio of 20 mL g−1, rotation speed of 250 rpm, slurry flow rate of 1.00 L min−1 and reaction time of 60 min at room temperature. To confirm the rationale of applying the surface coverage model in this study, the changes in the microstructure of the BOFS before and after carbonation were examined using SEM techniques, as shown in Fig. 8. Each fresh BOFS particle had a smooth surface, whereas the BOFS particles were deposited with carbonated products after carbonation. This phenomenon is in good agreement with those of previous studies [25,26] and the previously discussed physical properties of
3.4. Utilization of carbonated BOFS product as SCMs in cement mortars Table 4 presents the TCLP results of the fresh and carbonated BOFS used in cement replacement. The extractable concentrations of Hg, Pb, Cd, Cr6+, As, Cu, Ag, Cr, Se, and Ba in the fresh and carbonated BOFS were considerably below regulatory levels, thus indicating that both of them are safe enough to be used as SCMs. Stability is an important property in the utilization of SCMs. The free-CaO, MgO, tricalcium aluminate (3CaO·Al2O3, denoted as C3A), and CaSO4 present in cement may cause undesired long-term expansion and volume instability [27]. However, most standards, such as ASTM, have no limitation for the free-CaO content of Portland cement. ASTM C150 limits MgO and SO3 contents by 6.0% and 3.0% or 3.5% (when C3A is 8% or less and when it is more than 8%, respectively), respectively, in Portland cement type I. The expansion caused by free-CaO, MgO, and C3A could be measured by the autoclave expansion test, whereas excessive SO3 content could be determined by a chemical analysis. Fig. 9 illustrates the autoclave expansion of cement mortars blended with different ratios of fresh and carbonated BOFS. The results indicated that adding fresh and carbonated BOFS effectively reduced the expansion of OPC. The cement replaced by 15% of the carbonated BOFS had the capability to restrain expansion. The stable expansion of the 51
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Fig. 7. Effect of (a) rotation speed, (b) liquid-to-solid (L/S) ratio, and (c) slurry flow rate on carbonation conversion of BOFS coupled with CW-EDI and (d) comparison of calculated carbonation conversion (surface coverage model) with experimental data.
curing age. At a substitution ratio of 10%, the mortars replaced by the carbonated BOFS had a greater compressive capacity than that replaced with the fresh BOFS due to the CaCO3 generated by the carbonation reaction. CaCO3, which may provide additional nucleation sites for hydration products, may influence the hydration system as well. The decomposition of ettringite (C3(A,F)·3CaSO4·32H2O) generated from the hydration reaction of C3A or tetracalcium aluminoferrite
cement substituted by the carbonated BOFS with different ratios showed the stabilization of the BOFS after carbonation. In conclusion, the expansion ratios of all the combinations were within the limitation of ASTM C150. Fig. 10 portrays the compressive strength of the cement mortars with different substitution ratios after different curing ages. The compressive strength of all the cement mortars continuously increased with
Table 3 Variation of k and R2 values with operating conditions for surface coverage model. Runs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Operating conditions
Model parameters
Rotation speed(rpm)
L/S ratio(mL g−1)
Slurry flow rate(L min−1)
ks (mol min−1 m−2)
kp (m2 mol−1)
ks kp (min−1)
δmax (%)
250 500 750 1000 1250 750 750 750 750 750 750 750 750 750 750
20 20 20 20 20 10 20 30 40 50 20 20 20 20 20
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.50 0.75 1.00 1.25 1.50
0.389 0.337 0.338 0.275 0.206 0.220 0.338 0.388 0.418 0.429 0.175 0.308 0.338 0.363 0.365
0.338 0.342 0.343 0.374 0.405 0.363 0.343 0.368 0.383 0.388 0.388 0.361 0.343 0.375 0.372
0.132 0.115 0.116 0.103 0.080 0.080 0.116 0.143 0.160 0.167 0.068 0.111 0.116 0.136 0.136
60.90 60.24 59.99 55.10 50.86 56.66 59.99 55.93 53.71 53.05 53.02 56.98 59.99 54.95 55.30
52
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.033 0.025 0.028 0.019 0.016 0.022 0.028 0.037 0.042 0.038 0.017 0.028 0.028 0.026 0.033
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.012 0.011 0.012 0.012 0.016 0.020 0.012 0.014 0.014 0.012 0.023 0.014 0.012 0.010 0.013
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.012 0.009 0.011 0.008 0.007 0.009 0.011 0.015 0.017 0.016 0.008 0.011 0.011 0.010 0.013
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
R2 2.10 1.94 2.14 1.73 2.01 3.05 2.14 2.06 2.01 1.68 3.09 2.25 2.14 1.53 1.96
0.971 0.978 0.973 0.982 0.976 0.962 0.973 0.963 0.958 0.968 0.963 0.968 0.973 0.979 0.968
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Fig. 9. Effect of carbonation and substitution ratio on autoclave expansion ratio of cement. OPC: ordinary Portland cement.
with that mixed with the carbonated BOFS, which performed worse than the cement substituted by 10% of the carbonated BOFS. This outcome was caused by the dilution effect set off by the reduction in the amount of cement. The effective water-to-cement ratio played an important role in the influence of CaCO3 substitution [31]. Meanwhile, the small amount of active silica contained in the carbonated BOFS-cement system due to the higher replacement ratio and dissolution of oxides during the carbonation process also reduced the compressive strength of the system. In summary, mortars substituted by 10% of the carbonated BOFS and 10% or 15% of the fresh BOFS achieved better compressive capacity than pure OPC did. This suggests that the substitution ratios for the fresh and carbonated BOFS as SCMs should be 15% and 10%, respectively, to ensure the engineering performance as well as economic and environmental benefits.
Fig. 8. Scanning electron microscope (SEM) images for (a) fresh and (b) carbonated BOFS.
3.5. Cost and benefit analysis
(4CaO·Al2O3·Fe2O3, denoted as C4AF) with unstable monosulfate (C3(A,F)·CaSO4·12H2O) was replaced by the formation of stable calcium hemicarboaluminate or monocarboaluminate (C3(A,F)·CaCO3·11H2O) in the presence of CaCO3 [28,29]. This phenomenon led to a decrease in porosity, thus increasing compressive strength. The impact would be intensified by the high contents of C3A and C4AF in the cement and the additional alumina provided by the SCMs [30]. However, when the substitution ratio increased to 15%, the mortars mixed with the fresh BOFS showed a significant advantage compared
The experimental results revealed the feasibility of integrated waste treatment that combines the treatments of industrial sewage, solid wastes, and waste CO2 in a comprehensive process. Compared with the traditional aqueous accelerated carbonation process, which uses fresh water, introducing CW-EDI can not only reduce water usage but also improve the quality of CW-EDI. Carbonation products created during the process can be reused as green construction materials. The mixtures not merely reached the requirement of the standards but also performed
Table 4 Toxicity characteristic leaching procedure (TCLP) tests of fresh and carbonated BOFS.a Element
Unit
Fresh BOFS
Carbonated BOFS
Regulatory levels Hazardous Industrial Wastes
Hg Pb Cd Cr6+ As Cu Ag Cr Se Ba a b c
mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1
N.D. (< 0.00028) N.D. (< 0.026) N.D. (< 0.0027) N.D. (< 0.0033) N.D. (< 0.00048) N.D. (< 0.010) 0.030 N.D. (< 0.030) 0.002 0.480
N.D. (< 0.00028) N.D. (< 0.026) N.D. (< 0.0027) N.D. (< 0.0033) N.D. (< 0.00048) N.D. (< 0.010) 0.030 N.D. (< 0.030) 0.002 0.050
0.2 5 1 2.5 5 15 5 5 1 100
Leaching test method in accordance with the regulation of NIEA R201.14C, EPA, Taiwan. N.D.: Not Detected. N.R.: Not Regulated. Leaching limit regulated by NIEA R201.14C, EPA, Taiwan. Leaching limit regulated by Taiwan Architecture and Building Center.
53
b
Green Building Materials 0.05 0.3 0.3 1.5 0.3 0.15 0.05 N.R. N.R. N.R.
c
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Fig. 10. Effect of carbonation and substitution ratio on compressive strength of cement mortar after curing age of (a) 3, (b) 7, (c) 28 and (d) 56 days. OPC: ordinary Portland cement.
better than pure OPC at certain substitution ratios. To promote the application of the whole process, the cost and benefit analysis of carbonation of BOFS and displacement of cement by carbonated BOFS was conducted. China is a big producer of steel, during whose production process 2.3 tonnes CO2 is generated per tonnes of steel [32]. In 2016, China produced 808.4 million tonnes of crude steel, accounting for 49.6% of the world production [33]. Therefore, a China case was evaluated, in the fields of CO2 emission, resource reservation, energy consumption, cost, and profit, as showed in Table 5. In China, 94.8% of steel is produced by BOF, which may discharge 0.15–0.20 tonnes of BOFS per tonnes of steel produced [34]. Therefore, the amount of BOFS that need to be addressed was roughly 115.0–153.3 million tonnes irrespective of BOFS accumulated over the years because of low utilization ratio. The treatment fee of BOFS was about 13.3 USD/ tonnes-BOFS [35]. Based on the experimental results in this study, approximately 19.9–26.5 million tonnes of CO2 will be fixed via the HiGCarb process while generating 134.8–179.8 million tonnes of carbonated BOFS. The energy used for HiGCarb process was estimated to be 80–169 kW h/tonnes-CO2 [36], corresponding to an energy cost of 9.2–19.4 USD/tonnes-CO2, in the case of an industrial electricity price of 0.115 USD/kWh in China. The reduction of CO2 emission may bring a profit from carbon credit which was approximately 8.1 USD/tonnesCO2 [37]. If the carbonated BOFS is sold directly, assuming the price is 6.0 USD/tonnes-carbonated BOFS [38], then the total sale profit should be 809.0–1078.7 USD. When the carbonated BOFS was used in cement replacement, the benefits included resource conservation, CO2 emission reduction, energy consumption reduction and cost reduction. In the
Table 5 Cost and benefit analysis of the HiGCarb process and cement replacement. Items
CO2 emission
Resource reservation Energy consumption Cost
Profit
Units
Captured by HiGCarb Reduction from cement production HiGCarb operation Transportation Limestone Clay HiGCarb operation Reduction from cement production Money saved from BOFS treatment Money saved from cement production HiGCarb operation Transportation Carbon credit
Minimum
a
Maximum
(× 106 )
(× 106 )
MT
19.9
26.5
MT
107.9
179.8
MT MT MT MT kWh kWh
1.2 Undetermined 134.8 27.0 1591.0 14832.6
USD
1528.9
2038.5
USD
6716.1
8953.5
USD USD USD
183.0 515.3 Undetermined c 1006.8 1661.1
b
3.5 c
269.7 71.9 4481.2 19779.8
a The minimum was calculated based on the minimum yield and carbonation conversion of BOFS. b The maximum was calculated based on the maximum yield and carbonation conversion of BOFS. c The value related with transportation depends on the distance between the BOF to the carbonation reactor and the reactor to the cement plant.
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typical cement manufacturing, 1.0–1.5 tonnes of limestone, 0.2–0.4 tonnes of clay and 110 kW h of electricity are used for 1 tonnes of cement production, associated with 0.8–1.0 tonnes of CO2 emission [38,39]. The production cost of cement was assumed to be 49.8 USD/ tonnes-cement [38]. As shown in the Table 5, the amount of CO2 emission, electricity consumption, and cost for operating the HiGCarb process were significantly less than those reduced from cement production. Without considering the cost of transportation, compared with the traditional treatment of BOFS, the HiGCarb process with subsequent reutilization at least reduced 124.3 million tonnes of CO2 emission, saved 7728.7 million USD of cost, and gained 1006.8 million USD from carbon credit. Furthermore, the transportation expense which affects the final amount of CO2 emission and cost shows the importance of the establishment of waste-to-resource supply chain for a circular economy system in which zero waste and pollution is generated during the manufacture period.
[8] A.B. Ghacham, E. Cecchi, L.C. Pasquier, et al., CO2 sequestration using waste concrete and anorthosite tailings by direct mineral carbonation in gas-solid-liquid and gas-solid routes, J. Environ. Manage. 163 (2015) 70–77. [9] S.J. Han, H.J. Im, J.H. Wee, Leaching and indirect mineral carbonation performance of coal fly ash-water solution system, Appl. Energy 142 (2015) 274–282. [10] S.Y. Pan, P.C. Chiang, Y.H. Chen, et al., Systematic approach to determination of maximum achievable capture capacity via leaching and carbonation processes for alkaline steelmaking wastes in a rotating packed bed, Environ. Sci. Technol. 47 (2013) 13677–13685. [11] W.Y. Lin, K.S. Heng, X. Sun, et al., Accelerated carbonation of different size fractions of MSW IBA and the effect on leaching, Waste Manage. 41 (2015) 75–84. [12] W.Y. Lin, K.S. Heng, X. Sun, et al., Influence of moisture content and temperature on degree of carbonation and the effect on Cu and Cr leaching from incineration bottom ash, Waste Manage. 43 (2015) 264–272. [13] M. Tu, H. Zhao, Z. Lei, et al., Aqueous carbonation of steel slag: a kinetics study, ISIJ Int. 55 (2015) 2509–2514. [14] S.Y. Pan, P.C. Chiang, Y.H. Chen, et al., Ex situ CO2 capture by carbonation of steelmaking slag coupled with metalworking wastewater in a rotating packed bed, Environ. Sci. Technol. 47 (2013) 3308–3315. [15] K.H. Yang, Y.B. Jung, M.S. Cho, et al., Effect of supplementary cementitious materials on reduction of CO2 emissions from concrete, J. Clean. Prod. 103 (2015) 774–783. [16] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (2011) 1244–1256. [17] M.C.G. Juenger, R. Siddique, Recent advances in understanding the role of supplementary cementitious materials in concrete, Cem. Concr. Res. 78 (2015) 71–80. [18] C. Han, Z. Wang, W. Yang, et al., Effects of pH on phosphorus removal capacities of basic oxygen furnace slag, Ecol. Eng. 89 (2016) 1–6. [19] E.E. Chang, S.Y. Pan, Y.H. Chen, et al., Accelerated carbonation of steelmaking slags in a high-gravity rotating packed bed, J. Hazard. Mater. 227-228 (2012) 97–106. [20] S.Y. Pan, T.C. Chung, C.C. Ho, et al., CO2 mineralization and utilization using steel slag for establishing a waste-to-resource supply chain, Sci. Rep. U. K. 7 (2017) 17227–17237. [21] J.C.S. Wu, J.D. Sheen, S.Y. Chen, et al., Feasibility of CO2 fixation via artificial rock weathering, Ind. Eng. Chem. Res. 40 (2001) 3902–3905. [22] C.S. Ho, S.M. Shih, C.D. Lee, Infulence of CO2 and O2 on the reaction of Ca(OH)2 under spray-drying flue gas desulfurization conditions, Ind. Eng. Chem. Res. 35 (1996) 3915–3919. [23] C.F. Liu, S.M. Shih, R.B. Lin, Kinetics of the reaction of Ca(OH)2/fly ash sorbent with SO2 at low temperatures, Chem. Eng. Sci. 57 (2002) 93–104. [24] S.Y. Pan, P.C. Chiang, Y.H. Chen, et al., Kinetics of carbonation reaction of basic oxygen furnace slags in a rotating packed bed using the surface coverage model: maximization of carbonation conversion, Appl. Energy 113 (2014) 267–276. [25] E.E. Chang, A.C. Chiu, S.Y. Pan, et al., Carbonation of basic oxygen furnace slag with metalworking wastewater in a slurry reactor, Int. J. Greenh. Gas Control 12 (2013) 382–389. [26] N.L. Ukwattage, P.G. Ranjith, M. Yellishetty, et al., A laboratory-scale study of the aqueous mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. 103 (2015) 665–674. [27] F.M. Lea, The Chemistry of Cement and Concrete, Chemical Publishing Company, New York, 1970. [28] K. De Weerdt, M.B. Haha, G. Le Saout, et al., Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash, Cem. Concr. Res. 41 (2011) 279–291. [29] S. Hoshino, K. Yamada, H. Hirao, XRD/rietveld analysis of the hydration and strength development of slag and limestone blended cement, J. Adv. Concr. Technol. 4 (2006) 357–367. [30] K. De Weerdt, K.O. Kjellsen, E. Sellevold, et al., Synergy between fly ash and limestone powder in ternary cements, Cem. Concr. Compos. 33 (2011) 30–38. [31] D.P. Bentz, Modeling the influence of limestone filler on cement hydration using CEMHYD3D, Cem. Concr. Compos. 28 (2006) 124–129. [32] H. Liu, J. Fu, S. Liu, et al., Calculation methods and application of carbon dioxide emission during steel-making process, Iron Steel 51 (2016) 74–82. [33] WSA, Steel Statistical Yearbook 2017, WSA, Brussels, 2017. [34] B. Das, S. Prakash, P.S.R. Reddy, et al., An overview of utilization of slag and sludge from steel industries, Resour. Conserv. Recycl. 50 (2007) 40–57. [35] S.Y. Pan, K.J. Shah, Y.H. Chen, et al., Deployment of accelerated carbonation using alkaline solid wastes for carbon mineralization and utilization toward a circular economy, ACS Sustain. Chem. Eng. 5 (2017) 6429–6437. [36] S. Pei, S.Y. Pan, Y. Li, et al., Environmental benefit assessment for the carbonation process of petroleum coke fly ash in a rotating packed bed, Environ. Sci. Technol. 51 (2017) 10674–10681. [37] C.E. Bratislava, Carbon Place, European Union, 2014. [38] T. Zhang, Q. Yu, J. Wei, et al., Preparation of high performance blended cements and reclamation of iron concentrate from basic oxygen furnace steel slag, Resour. Conserv. Recycl. 56 (2011) 48–55. [39] S. Kumar, R. Kumar, A. Bandopadhyay, Innovative methodologies for the utilisation of wastes from metallurgical and allied industries, Resour. Conserv. Recycl. 48 (2006) 301–314.
4. Conclusions and recommendations In this study, BOFS coupled with CW-EDI was used in the HiGCarb process. The results indicated that the maximum achievable carbonation conversion of the BOFS with CW-EDI was approximately 60.9%. The surface coverage model described the carbonation kinetics sufficiently. Moreover, calcium ion leaching from BOFS was restricted by the high initial level of calcium ion and alkalinity of CW-EDI. Sodium ion was adsorbed by the BOFS during the leaching process. In addition, all cement mortars with the fresh and carbonated BOFS at different substitution ratios achieved the ASTM requirements for autoclave expansion and compressive strength. The suggested substitution ratios for the fresh and carbonated BOFS in cement mortars were 15% and 10%, respectively. Furthermore, the results of cost and benefit analysis to the HiGCarb process together with cement replacement indicated the superiority of the process in CO2 emission reduction, cost saving and profit gaining. Future research should be consummated with an economic, engineering and environment analysis to the integrated waste treatment in plant trial, to identify the stability, maneuverability and challenge of the project in large scale. Acknowledgements The China Steel Corporation (CSC) in Taiwan was highly appreciated for providing steelmaking slag in this study. High appreciation also goes to the Ministry of Science and Technology (MOST) of Taiwan under Grant Number MOST 106-2811-M-002-018 for the financial support. References [1] IPCC, Carbon Dioxide Capture and Storage, IPCC, New York, 2005. [2] N. Kemache, L.C. Pasquier, I. Mouedhen, et al., Aqueous mineral carbonation of serpentinite on a pilot scale: the effect of liquid recirculation on CO2 sequestration and carbonate precipitation, Appl. Geochem. 67 (2016) 21–29. [3] N.C. Johnson, B. Thomas, K. Maher, et al., Olivine dissolution and carbonation under conditions relevant for in situ carbon storage, Chem. Geol. 373 (2014) 93–105. [4] M. Uibu, R. Kuusik, L. Andreas, et al., The CO2-binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash and steel slag, Energy Procedia 4 (2011) 925–932. [5] D.N. Huntzinger, J.S. Gierke, L.L. Sutter, et al., Mineral carbonation for carbon sequestration in cement kiln dust from waste piles, J. Hazard. Mater. 168 (2009) 31–37. [6] R.R.T. Dananjayan, P. Kandasamy, R. Andimuthu, Direct mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. 112 (2015) 4173–4182. [7] E.R. Bobicki, Q. Liu, Z. Xu, et al., Carbon capture and storage using alkaline industrial wastes, Prog. Energy Combust. 38 (2012) 302–320.
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Carbonation and utilization of basic oxygen furnace slag coupled with concentrated water from electrodeionization ⁎⁎
Yemei Lia,b, Silu Peic, Shu-Yuan Panc, Pen-Chi Chiangc,d, , Changyi Lua,b, Tong Ouyanga,
T
⁎
a
College of the Environment & Ecology, Xiamen University, Xiamen, 361102, China School of Environmental Science & Engineering, Xiamen University Tan Kah Kee College, Zhangzhou, 363105, China c Carbon Cycle Research Center, National Taiwan University, Taipei City, 10672, Taiwan d Graduate Institute of Environmental Engineering, National Taiwan University, Taipei City, 10673, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct carbonation Basic oxygen furnace slag Concentrated water from electrodeionization Supplementary cementitious materials
In this study, the integrated recycle process of basic oxygen furnace slag (BOFS) coupled with concentrated water from electrodeionization (CW-EDI) which was introduced into the direct carbonation process for the first time was investigated. The effects of different operating parameters on carbonation conversions of BOFS were studied and the reaction kinetics were determined via surface coverage model. Meanwhile, the changes of CWEDI during leaching and carbonation processes were measured. Furthermore, the performance evaluation of BOFS applied as supplementary cementitious materials (SCMs) and a cost and benefits analysis of the whole process was conducted. The results indicated the feasibility of the integrated system to industrial solid waste, sewage, and exhaust treatments and the maximum achievable carbonation conversion of BOFS was approximately 60.9% with R2 of 0.958–0.982. A high level of calcium ion and the alkaline solution hindered the leaching of calcium ion from BOFS, whereas sodium ion was absorbed by BOFS simultaneously. The properties of the cement that used BOFS as SCMs met ASTM requirements. The suggested substitution ratios for fresh and carbonated BOFS in cement mortars were 15% and 10%, respectively. The economic results revealed the superiority of the carbonation and utilization process over traditional treatments in CO2 emission reduction, cost saving and profit gaining.
1. Introduction Carbon dioxide (CO2) capture and storage that includes integrated CO2 treating process from the original gas separation to the final longterm isolation from the atmosphere is less extensive than other mitigation actions for the stabilization of atmospheric greenhouse gas concentrations [1]. Mineral carbonation, also called natural weathering, is a CO2 storage option in which CO2 is fixed and stored in the form of inorganic carbonates that will not be released back into the atmosphere over a long time. Although the weathering reaction is thermodynamically favorable, the reaction time can last for thousands of years due to its slow reaction kinetics. Considering the reaction efficiency, accelerated carbonation, an enhanced version of natural weathering, was proposed. Naturally occurring silicate rocks, such as serpentine [2] and olivine [3], and industrial solid wastes, such as steelmaking slags [4], cement wastes [5], and waste ash [6], are suitable for industrial CO2 fixation due to the existence of alkaline and alkaline earth oxides [7].
⁎
Accelerated carbonation can be conducted through various approaches, such as direct carbonation [8] and indirect carbonation [9]. In direct carbonation, the efficiency of aqueous carbonation at moderate conditions is relatively higher than that of dry carbonation. In general, aqueous carbonation can be simply divided into three steps: (1) cation leaching from the solid phase, (2) CO2 dissolution into the liquid agents, and (3) production of carbonate precipitates [10]. The performance of the carbonation reaction can be influenced by the properties of feedstock [11] and the reaction conditions [12,13], as well as the reactor types. A rotating packed bed reactor applied during the carbonation process also known as High-gravity carbonation, or HiGCarb for short, produces carbonation results with a shorter reaction time than traditional reactors [14]. Stable carbonated products can be reused as construction materials such as supplementary cementitious materials (SCMs), a substitution of clinker in Portland cement or Portland cement in concrete. Using SCMs cannot merely lower cost and environmental impact, especially by decreasing CO2 emission in cement production [15], but also improve
Corresponding author. Corresponding author at: Carbon Cycle Research Center, National Taiwan University, Taipei City, 10672, Taiwan. E-mail addresses: [email protected] (P.-C. Chiang), [email protected] (T. Ouyang).
⁎⁎
https://doi.org/10.1016/j.jcou.2018.03.003 Received 29 October 2017; Received in revised form 2 February 2018; Accepted 2 March 2018 2212-9820/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of carbonation process via a rotating packed bed reactor.
experiments. The synthetic water was prepared using CaCl2 and NaCl (supplied by J. T. Baker) dissolved in tap water on the basis of the real CW-EDI. Furthermore, HCl and NaOH solutions with concentrations of 1 and 0.1 M, respectively, were used to adjust the initial pH of CW-EDI. In addition, high-pressure CO2 gas with a volumetric concentration of 99.5% supplied by Ching-Feng Gas Corporation (Taipei, Taiwan) was used in carbonation process. Ordinary Portland cement (OPC) and standard sand supplied by Gao Ching Corporation (Taipei, Taiwan) were used in the cement replacement experiments.
long-term strength and durability [16]. Moreover, using SCMs is also a pathway to utilizing the by-products generated by industrial manufacturing processes. Although SCMs have been used and studied for decades, the field is still in the stage of development. Recent research has focused on exploring new materials and additives, modifying methods, increasing replacement amount, and developing enhanced test methods [17]. Basic oxygen furnace slag (BOFS) is a by-product from the steelmaking and refining processes. BOFS presents an excellent capacity to remove phosphorous from aqueous solutions [18] and has been widely used in various fields, such as soil improvement, landfills, and marine ecosystem conservation. Nevertheless, the free-CaO contained in BOFS will result in volume expansions and structural damages if BOFS is reused as construction materials. On the contrary, the high CaO content is welcome in the carbonation process [19]. Concentrated water from electrodeionization (CW-EDI), which has high levels of sodium and calcium ions and whose treatment approaches are rarely discussed, is the effluent generated during the electrodeionization process. Further utilization of products generated from the carbonation of CW-EDI may be directly restricted because of its impurity. However, mixing CW-EDI with BOFS before carbonation can simultaneously improve the properties of BOFS and utilize CW-EDI for CO2 mineralization, thereby establishing the waste-to-resource supply chain for a circular economy [20]. Therefore, this study aims to investigate the utilization of CW-EDI in direct carbonation and the feasibility of a comprehensive process for the treatments of industrial sewage, solid wastes, and waste CO2. The integrated recycle procedure of BOFS coupled with CW-EDI, which includes the beaker leaching and HiGCarb processes and utilization of BOFS as SCMs was conducted in the study. Furthermore, a China case of cost and benefit analysis to the HiGCarb process and cement replacement was carried through based on the experiment results.
2.2. Characterization of BOFS X-ray fluorescence (XRF, PW2430, Philips, Netherland) was used to determine the chemical composition of the BOFS, while a scanning electron microscope (SEM, TM3000, Hitachi, Japan) was used to perform the morphological investigation. The change in sample weight with temperature was measured using thermo-gravimetric analysis (TGA, STA6000, PerkinElmer, USA) with a heating rate of 10 °C min−1 at a temperature of 50 °C–950 °C and a N2 flow rate of 19.8 mL min−1. 2.3. Leaching characteristics of BOFS in CW-EDI Beaker leaching experiments were conducted to determine the influences of the leaching time, initial pH, and calcium ion levels of CWEDI on the leaching behaviors of the BOFS. 100 g of the BOFS was added into a beaker containing CW-EDI prepared in advance with a liquid-to-solid (L/S) ratio of 20 mL g−1. The slurry was continuously stirred and sampled and the pH value was simultaneously recorded at special times throughout the experiment. The reaction was continued for 60 min. The liquid part of the sample was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, 700 Series, Agilent Technology) after the solid-liquid separation through a 45 μm membrane filter.
2. Materials and methods 2.4. HiGCarb of BOFS coupled with CW-EDI 2.1. Materials The influence of different operating parameters, such as reaction time, reactor rotation speed, slurry flow rate, and L/S ratio, on the carbonation conversion of BOFS coupled with CW-EDI was evaluated. The rotating packed bed reactor used in this study was made of stainless
The BOFS provided by China Steel Corporation (Kaohsiung, Taiwan) was ground and dried at 105 °C overnight to eliminate moisture. The CW-EDI was used as the liquid phase in the leaching and HiGCarb 47
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steel with a packing zone filled with stainless steel wire. Fig. 1 shows a schematic diagram of the HiGCarb. The BOFS and CW-EDI slurry was pumped into the reactor after adequate mixing in the beaker. Injecting CO2 into the reactor set off the carbonation process, which lasted for 60 min. The reactor rotated and the slurry circulated in the reactorbeaker system with continuous stirring in the beaker during the experiment. After the solid-liquid separation, the solid and liquid samples were analyzed using TGA and ICP-OES, respectively. The carbonation conversion (δCaO , %), which was calculated by Eq. (1), was used to evaluate the carbonation degree of the BOFS. Carbonation conversion was the amount of CO2 actually captured in the dry mass of each sample compared with the theoretical extent of carbonation on the basis of the reactive-oxide content of the fresh BOFS.
δCaO = AcCO2 / ThCO2 × 100
Table 1 Physico-chemical properties of fresh BOFS, carbonated BOFS and ordinary Portland cement (OPC).a Items
Chemical composition
(1) −1
The theoretical CO2 capture capacity (ThCO2, g CO2 (g BOFS) ), whose calculation was based on the XRF result, expressed the potential CO2 capture capacity of the BOFS. Meanwhile, the actual CO2 capture capacity (AcCO2, g CO2 (g BOFS)−1) which was calculated by Eq. (2) on the basis of the TGA result, showed the amount of CO2 captured by the BOFS under a certain operating condition. In this equation, m105℃ (g) is the dry weight of the sample used for TGA analysis. ΔmCaCO3 (g) is the weight loss of CaCO3 in temperatures of 500 °C–850 °C, in which CaCO3 typically decompose into CaO and CO2.
AcCO2 = ΔmCaCO3/(m105℃−ΔmCaCO3)
Physical properties
a b c d
SiO2 (wt.%) Al2O3 (wt.%) Fe2O3 (wt.%) CaO (wt.%) MgO (wt.%) SO3 (wt.%) K2O (wt.%) Na2O (wt.%) Total (wt.%) free-CaO (wt.%) Carbonation conversion (%) d D(v, 0.1) (μm) D(v, 0.5) (μm) D(v, 0.9) (μm) Density (g cm−3)
Fresh BOFS
Carbonated BOFS b
OPC
11.80 4.39 36.07 36.72 4.50 0.37 0.02 0.06 93.93 3.49 2.2
10.75 4.39 36.44 36.98 4.50 0.30 0.02 N.D. c 93.38 0.39 19.0
20.67 4.50 2.90 63.05 2.88 2.63 0.64 0.34 97.61 0.92 –
2.23 9.26 25.63 3.66
1.35 10.91 35.54 2.65
– – – –
Analyzed by CHC Resources Corporation. Carbonated BOFS used for cement replacement experiments. N.D.: not detected. Calculated as Eq. (1) based on the data got from TGA and XRF measurement.
SO3, respectively, in the fresh BOFS determined by XRF. CaCO3 (wt.%) is the weight fraction of CaCO3 measured by TGA and MCO2 (g mol−1), MCaO (g mol−1), MCaCO3 (g mol−1), and MSO3 (g mol−1) are the molecular weights of CO2, CaO, CaCO3, and SO3, respectively. A comparison of the chemical composition of the fresh and carbonated BOFS showed a reduction in free-CaO content, thus revealing the stabilization of the BOFS by the carbonation process. A similar composition of the BOFS and OPC demonstrated the practicability of BOFS used as SCMs. In addition, the MgO and SO3 contents contained in the BOFS and OPC did not allow the mixtures to exceed the ASTM limitation. The physical properties of the BOFS changed after carbonation. The decrease in D(v, 0.1) indicated the generation of small particles and the increment in D(v, 0.5) and D(v, 0.9) showed the expansion of particles. Meanwhile, the density of the carbonated BOFS was 72% of that of the fresh BOFS. Fig. 2(a) and (b) show the TG/DTG curves of the fresh and carbonated BOFS, respectively. The CaCO3 content in the fresh BOFS was 0.62 wt.% in dry weight. This content level corresponds to the weight loss in temperature of 550 °C–800 °C, which significantly increased after carbonation with a DTG peak at 747 °C. In addition, a part of Ca(OH)2 may be attributed to CaO content because of a DTG peak that can be observed in Fig. 2(a) at 445 °C. On the basis of the collective results of XRF and TGA, the theoretical CO2 capture capacity of the BOFS was estimated to be 284 g of CO2 per kg of BOFS.
(2)
2.5. Utilization of BOFS as SCMs in the cement mortars Fresh and carbonated BOFS were used to partially replace OPC at substitution ratios of 0%, 5%, 10% and 15% by weight. Autoclave expansion ratio and compressive strength after curing ages of 3, 7, 28, and 56 days were regarded as the primary indicators with which to evaluate the performance of the BOFS used as SCMs. The autoclave expansion and compressive strength tests were implemented in accordance with ASTM C151 and C109, respectively. The maximum autoclave expansion ratio for Portland cement type I should be less than 0.8% as per ASTM C150. The compressive strength of the cement mortars after curing ages of 3, 7, and 28 (optional requirement) days needed to reach 12.0, 19.0, and 28.0 MPa, respectively. Moreover, toxicity characteristic leaching procedure (TCLP), which is the most widely used assessment method for ecological risk, was conducted for the fresh and carbonated BOFS according to NIEA R201.14C. Hazardous Industrial Waste and Green Building Materials, which are regulated by Taiwan EPA and the Taiwan Architecture and Building Center, respectively, were used to evaluate the results of TCLP. 3. Results and discussion 3.1. Physico-chemical properties of BOFS
3.2. Leaching kinetics of BOFS in CW-EDI
Table 1 presents the physico-chemical properties of the fresh and carbonated BOFS and OPC. The major components of the fresh BOFS were CaO (∼36.7 wt.%) and Fe2O3 (∼36.1 wt.%), with micro amounts of SiO2, Al2O3, MgO, K2O, and Na2O. Although several kinds of metal oxides existed in the fresh BOFS, not all of them contributed to CO2 sequestration. The carbonates generated from K2O and Na2O are soluble. Meanwhile, SiO2, Al2O3 and Fe2O3 cannot react with CO2. Moreover, little MgCO3 could be generated under the mild conditions in this study. Thus, only the calcium species were regarded as the major components that function in CO2 fixation. The ThCO2 of the BOFS used in this study can be calculated by Eq. (3) [5,19].
According to the previous study [21], calcium leaching should be the rate-limiting step for carbonation reaction of steel slag. In this study, a first-order mass loss-based method was used to describe the leaching kinetics of calcium from BOFS into CW-EDI. In view of the initial level of calcium ion in CW-EDI, the model can be integrated by Eq. (4).
Ct = Cmax −(Cmax −C0 ) e−kt
(4)
where t (min) is the leaching time, k (min−1) is the rate constant of leaching, Cmax (mg L−1) is the maximum leaching concentration of calcium ion in the liquid phase, C0 (mg L−1) is the original level of calcium ion in the solution, and Ct is the level of calcium ion at different time. Fig. 3 compares the experimental data and calculated results of
ThCO2 = MCO2/MCaO × (CaO−MCaO / MCaCO3 × CaCO3−MCaO / MSO3 × SO3) (3) where CaO (wt.%) and SO3 (wt.%) are the weight fractions of CaO and 48
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Fig. 2. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of (a) fresh and (b) carbonated BOFS.
Fig. 4. Effect of (a) leaching time and initial level of calcium ion, and (b) the pH of CWEDI on the leaching behavior of calcium ion.
CW-EDI with several levels of initial concentrations of calcium ion and pH. The trends of the calcium ion levels in all conditions increased rapidly in the first 5 min and then gradually approached their respective maximum concentrations. The comparison of ultimate calcium contents contained in the different liquid agents showed that the initial calcium level had a greater influence on calcium leaching than initial pH did. Table 2 presents the model parameters by fitting Eq. (4) with the experimental data shown in Fig. 4. Compared with those in the previous study [10], the particle diameter and initial levels of calcium ion and pH in the liquid phase had different effects on k and Cmax. Introducing CW-EDI positively influenced the improvement of the calcium leaching rate. The level of Cmax rose with the increment in C0 whereas the leaching rate and content decreased at the same time. That is attributed that the rate and content of calcium ion leaching from the BOFS were confined by the high level of calcium ion in the liquid phase. Even the calcium content of the BOFS was much higher than that of CW-EDI. This limitation should be considered because it may affect the further utilization of carbonated BOFS, although additional calcium ion in the aqueous phase can benefit CO2 capture. Additionally, a low initial pH of the CW-EDI (i.e., pH 5 and 6) allowed for Cmax augmentation (i.e., 1425 and 1421 mg L−1). According to the acid-base neutralization, hydrogen ion in liquors was conducive
Fig. 3. Comparison of calculated concentration of calcium ion in the slurry (leaching model) with experimental data.
calcium concentration determined by Eq. (4). The results indicated that the first-order mass loss-based method could successfully describe the calcium ion leaching from BOFS into CW-EDI, as the standard errors were within 10%. Fig. 4 presents the calcium ion leaching from the BOFS in various 49
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Table 2 Key model parameters of calcium ion leaching behavior in various types of liquid agents. ID
Type of liquid phase
Operating conditions
Model parameters
L/S ratio (mL g−1)
Diameter of BOFS (μm )
Initial calcium level (mg L−1)
Initial pH
Cmax(mg L−1)
C0(mg L−1)
k(min−1)
R2
8 8 8 8 5 6 7 8 11.2011.87 11.2011.87 11.2011.87 11.2011.87 11.2011.87
694 1065 1254 1904 1425 1421 1391 1254 2690
10 452 813 1413 798 805 802 813 –
1.683 1.510 0.879 0.595 1.538 1.505 1.797 0.879 1.128
0.952 0.953 0.860 0.896 0.896 0.946 0.910 0.860 0.999
This study This study This study This study This study This study This study This study [10]
2115
–
0.876
0.996
[10]
1310
–
0.684
0.973
[10]
1256
–
0.696
0.973
[10]
876
–
0.708
0.983
[10]
L1 L2 L3 L4 L5 L6 L7 L8 L9
Tap water CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CW-EDI CRW a
20 20 20 20 20 20 20 20 20
25 25 25 25 25 25 25 25 <125
10 451 803 1437 803 803 803 803 147
L10
CRW
20
125-350
147
L11
CRW
20
350-500
147
L12
CRW
20
500-840
147
L13
CRW
20
840-1190
147
a
Reference
CRW: Cold-rolling wastewater.
carbonation reactions of Ca(OH)2 [22,23], was also employed to describe the kinetics of mineral carbonation in aqueous solutions for their reaction behaviors, which were similar to those of Ca(OH)2. According to the reaction and deposition hypotheses of the model, the carbonation reaction can reach the maximum carbonation conversion under a certain condition [24]. The model presented the relationship between carbonation conversion and reaction time (t, min) and can be expressed by Eq. (5).
to calcium ion extraction for the simultaneous generation of hydroxyl and calcium ions in the leaching process. The leaching content of calcium ion was hence improved. The slurry became alkaline during the neutralization process thereby favoring the dissolution of CO2 in the later carbonation reaction. Although a strong leaching behavior of the alkaline matter could be obtained at a low initial pH, the acidity of the solution may not be completely neutralized, thus restricting the generation of carbonate ion. Fortunately, the BOFS used in this study had sufficient neutralization ability to promote the pH of the solution when the initial pH of the liquid agents was 5–8 (Fig. 5(a)). To grasp the leaching behavior of sodium ion from the BOFS, tap water was used as the control liquid phase in this study. Fig. 5(b) shows that no significant change in sodium level had occurred in the control group thereby indicating that no sodium leached from the BOFS. When CW-EDI was used to replace tap water, sodium level remained unchanged after a rapid decrease at the beginning of the leaching process. The reasons may lie in the absorption behaviors of the BOFS and the electrostatic balance between the BOFS and CW-EDI.
δCaO = δmax [1−exp (−ks kp t )]
(5)
where δmax (%) is the maximum achievable carbonation conversion. ks (mol min−1 m−2) is the rate constant of the carbonation reaction, and kp (m2 mol−1) is the rate constant of the surface coverage. Fig. 7(d) and Table 3 present the comparison of the calculated carbonation conversion based on the surface coverage model with the experimental data shown in Fig. 7(a–c) and the fitting results, respectively. The calculated conversions were higher than the experimental ones early on due to the liquid phase used in this study. The abundant calcium ion in the liquid phase caused the generation of solitary CaCO3 thus decreasing the actual conversion of the BOFS. However, the theoretical values, which agreed with the experimental data in the last phase of the experiments, suggested the suitability of the surface coverage model for the HiGCarb of the BOFS in CW-EDI. As shown in Fig. 7 and Table 3, different operating conditions had different effects on the reaction. The carbonation conversion declined with the acceleration of rotation speed. At a high rotation speed (i.e., 1000 and 1250 rpm), the performance of mass transfer was increased to some extent. However, a brief contact between CO2 and slurry led to a low rate of carbonation reaction, thus resulting in a low conversion. δmax occurred at a rotation speed of 250 rpm where ks was faster than kp . δmax reached the highest point at the L/S ratio of 20 mL g−1. When the L/S ratio exceeded 20 mL g−1, ks and kp increased with the L/S ratio because of the violent reaction between the relatively abundant carbonate ion and insufficient calcium ion. However, the rate of surface coverage grew faster than that of carbonation reaction due to the lack of calcium ion, thus resulting in the decrease of δmax . On the contrary, δmax was low at 10 mL g−1 because of the gentle reaction among the reactants. This reaction explains the long processing time needed by mineral carbonation in nature. With the enhancement of slurry flow rate, δmax decreased after
3.3. Carbonation kinetics of BOFS in CW-EDI Fig. 6(a) shows the changes in carbonation conversion of BOFS, pH, and concentration of calcium and sodium ions in the slurry during the carbonation. The carbonation conversion became nearly constant after a sharp ascent. Meanwhile, a mirror symmetry polyline displayed the pH behavior. The tendencies of the levels of calcium and sodium, which were the main metal ions contained in CW-EDI, changed as well in the process, as shown as Fig. 6(b). The concentration of calcium ion rose after the first drop whereas the sodium level remained unchanged. As the main reaction species, calcium ion was consumed by carbonate ion to generate CaCO3, thereby resulting in the increase in carbonation conversion and the reduction of calcium ion level with the introduction of CO2 at the beginning of the reaction. With the continuous dissolution of CO2, the pH of CW-EDI gradually dropped to approximately 6.5, which was beneficial for calcium ion leaching. However, the reaction had no influence on the sodium level, thus indicating that sodium ion did not participate in the carbonation reaction. Fig. 7(a–c) show the influence of reaction time on the carbonation conversion of the BOFS under different operating conditions. The surface coverage model was used to describe the reaction kinetics. The model, which was the most suitable for describing the sulfation and 50
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Fig. 6. Changes of (a) carbonation conversion of BOFS, pH and (b) concentration of calcium and sodium ions in the slurry during the carbonation process. (Operating conL/S ditions: rotation speed = 750 rpm, slurry flow rate = 1.00 L min−1, ratio = 20 mL g−1). Fig. 5. Profiles of (a) pH and (b) concentration of sodium ion in the slurry during leaching experiments.
carbonated BOFS.
peaking at a slurry flow rate of 1.00 L min−1. Whether δmax was promoted or reduced by the improvement of slurry flow rate depended on the competition result of the strengthened of mass transfer and shortened contact time between the slurry and CO2. When the effect caused by the reduction in contact time was overcome by the enhancement of mass transfer, δmax was improved. By contrast, δmax was reduced due to the lack of contact time. Furthermore, δmax appeared at a slurry flow rate of 1.00 L min−1 at which ks was nearly equal to kp . On the premise of sufficient feedstock, calcium and carbonate ions, good maximum carbonation conversion could be approached when the rate of carbonation reaction was faster than that of surface coverage. With the synthesizing effects of rotation speed, L/S ratio, and slurry flow rate, δmax could reach 60.9% with an L/S ratio of 20 mL g−1, rotation speed of 250 rpm, slurry flow rate of 1.00 L min−1 and reaction time of 60 min at room temperature. To confirm the rationale of applying the surface coverage model in this study, the changes in the microstructure of the BOFS before and after carbonation were examined using SEM techniques, as shown in Fig. 8. Each fresh BOFS particle had a smooth surface, whereas the BOFS particles were deposited with carbonated products after carbonation. This phenomenon is in good agreement with those of previous studies [25,26] and the previously discussed physical properties of
3.4. Utilization of carbonated BOFS product as SCMs in cement mortars Table 4 presents the TCLP results of the fresh and carbonated BOFS used in cement replacement. The extractable concentrations of Hg, Pb, Cd, Cr6+, As, Cu, Ag, Cr, Se, and Ba in the fresh and carbonated BOFS were considerably below regulatory levels, thus indicating that both of them are safe enough to be used as SCMs. Stability is an important property in the utilization of SCMs. The free-CaO, MgO, tricalcium aluminate (3CaO·Al2O3, denoted as C3A), and CaSO4 present in cement may cause undesired long-term expansion and volume instability [27]. However, most standards, such as ASTM, have no limitation for the free-CaO content of Portland cement. ASTM C150 limits MgO and SO3 contents by 6.0% and 3.0% or 3.5% (when C3A is 8% or less and when it is more than 8%, respectively), respectively, in Portland cement type I. The expansion caused by free-CaO, MgO, and C3A could be measured by the autoclave expansion test, whereas excessive SO3 content could be determined by a chemical analysis. Fig. 9 illustrates the autoclave expansion of cement mortars blended with different ratios of fresh and carbonated BOFS. The results indicated that adding fresh and carbonated BOFS effectively reduced the expansion of OPC. The cement replaced by 15% of the carbonated BOFS had the capability to restrain expansion. The stable expansion of the 51
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Fig. 7. Effect of (a) rotation speed, (b) liquid-to-solid (L/S) ratio, and (c) slurry flow rate on carbonation conversion of BOFS coupled with CW-EDI and (d) comparison of calculated carbonation conversion (surface coverage model) with experimental data.
curing age. At a substitution ratio of 10%, the mortars replaced by the carbonated BOFS had a greater compressive capacity than that replaced with the fresh BOFS due to the CaCO3 generated by the carbonation reaction. CaCO3, which may provide additional nucleation sites for hydration products, may influence the hydration system as well. The decomposition of ettringite (C3(A,F)·3CaSO4·32H2O) generated from the hydration reaction of C3A or tetracalcium aluminoferrite
cement substituted by the carbonated BOFS with different ratios showed the stabilization of the BOFS after carbonation. In conclusion, the expansion ratios of all the combinations were within the limitation of ASTM C150. Fig. 10 portrays the compressive strength of the cement mortars with different substitution ratios after different curing ages. The compressive strength of all the cement mortars continuously increased with
Table 3 Variation of k and R2 values with operating conditions for surface coverage model. Runs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Operating conditions
Model parameters
Rotation speed(rpm)
L/S ratio(mL g−1)
Slurry flow rate(L min−1)
ks (mol min−1 m−2)
kp (m2 mol−1)
ks kp (min−1)
δmax (%)
250 500 750 1000 1250 750 750 750 750 750 750 750 750 750 750
20 20 20 20 20 10 20 30 40 50 20 20 20 20 20
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.50 0.75 1.00 1.25 1.50
0.389 0.337 0.338 0.275 0.206 0.220 0.338 0.388 0.418 0.429 0.175 0.308 0.338 0.363 0.365
0.338 0.342 0.343 0.374 0.405 0.363 0.343 0.368 0.383 0.388 0.388 0.361 0.343 0.375 0.372
0.132 0.115 0.116 0.103 0.080 0.080 0.116 0.143 0.160 0.167 0.068 0.111 0.116 0.136 0.136
60.90 60.24 59.99 55.10 50.86 56.66 59.99 55.93 53.71 53.05 53.02 56.98 59.99 54.95 55.30
52
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.033 0.025 0.028 0.019 0.016 0.022 0.028 0.037 0.042 0.038 0.017 0.028 0.028 0.026 0.033
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.012 0.011 0.012 0.012 0.016 0.020 0.012 0.014 0.014 0.012 0.023 0.014 0.012 0.010 0.013
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.012 0.009 0.011 0.008 0.007 0.009 0.011 0.015 0.017 0.016 0.008 0.011 0.011 0.010 0.013
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
R2 2.10 1.94 2.14 1.73 2.01 3.05 2.14 2.06 2.01 1.68 3.09 2.25 2.14 1.53 1.96
0.971 0.978 0.973 0.982 0.976 0.962 0.973 0.963 0.958 0.968 0.963 0.968 0.973 0.979 0.968
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Fig. 9. Effect of carbonation and substitution ratio on autoclave expansion ratio of cement. OPC: ordinary Portland cement.
with that mixed with the carbonated BOFS, which performed worse than the cement substituted by 10% of the carbonated BOFS. This outcome was caused by the dilution effect set off by the reduction in the amount of cement. The effective water-to-cement ratio played an important role in the influence of CaCO3 substitution [31]. Meanwhile, the small amount of active silica contained in the carbonated BOFS-cement system due to the higher replacement ratio and dissolution of oxides during the carbonation process also reduced the compressive strength of the system. In summary, mortars substituted by 10% of the carbonated BOFS and 10% or 15% of the fresh BOFS achieved better compressive capacity than pure OPC did. This suggests that the substitution ratios for the fresh and carbonated BOFS as SCMs should be 15% and 10%, respectively, to ensure the engineering performance as well as economic and environmental benefits.
Fig. 8. Scanning electron microscope (SEM) images for (a) fresh and (b) carbonated BOFS.
3.5. Cost and benefit analysis
(4CaO·Al2O3·Fe2O3, denoted as C4AF) with unstable monosulfate (C3(A,F)·CaSO4·12H2O) was replaced by the formation of stable calcium hemicarboaluminate or monocarboaluminate (C3(A,F)·CaCO3·11H2O) in the presence of CaCO3 [28,29]. This phenomenon led to a decrease in porosity, thus increasing compressive strength. The impact would be intensified by the high contents of C3A and C4AF in the cement and the additional alumina provided by the SCMs [30]. However, when the substitution ratio increased to 15%, the mortars mixed with the fresh BOFS showed a significant advantage compared
The experimental results revealed the feasibility of integrated waste treatment that combines the treatments of industrial sewage, solid wastes, and waste CO2 in a comprehensive process. Compared with the traditional aqueous accelerated carbonation process, which uses fresh water, introducing CW-EDI can not only reduce water usage but also improve the quality of CW-EDI. Carbonation products created during the process can be reused as green construction materials. The mixtures not merely reached the requirement of the standards but also performed
Table 4 Toxicity characteristic leaching procedure (TCLP) tests of fresh and carbonated BOFS.a Element
Unit
Fresh BOFS
Carbonated BOFS
Regulatory levels Hazardous Industrial Wastes
Hg Pb Cd Cr6+ As Cu Ag Cr Se Ba a b c
mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1
N.D. (< 0.00028) N.D. (< 0.026) N.D. (< 0.0027) N.D. (< 0.0033) N.D. (< 0.00048) N.D. (< 0.010) 0.030 N.D. (< 0.030) 0.002 0.480
N.D. (< 0.00028) N.D. (< 0.026) N.D. (< 0.0027) N.D. (< 0.0033) N.D. (< 0.00048) N.D. (< 0.010) 0.030 N.D. (< 0.030) 0.002 0.050
0.2 5 1 2.5 5 15 5 5 1 100
Leaching test method in accordance with the regulation of NIEA R201.14C, EPA, Taiwan. N.D.: Not Detected. N.R.: Not Regulated. Leaching limit regulated by NIEA R201.14C, EPA, Taiwan. Leaching limit regulated by Taiwan Architecture and Building Center.
53
b
Green Building Materials 0.05 0.3 0.3 1.5 0.3 0.15 0.05 N.R. N.R. N.R.
c
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Fig. 10. Effect of carbonation and substitution ratio on compressive strength of cement mortar after curing age of (a) 3, (b) 7, (c) 28 and (d) 56 days. OPC: ordinary Portland cement.
better than pure OPC at certain substitution ratios. To promote the application of the whole process, the cost and benefit analysis of carbonation of BOFS and displacement of cement by carbonated BOFS was conducted. China is a big producer of steel, during whose production process 2.3 tonnes CO2 is generated per tonnes of steel [32]. In 2016, China produced 808.4 million tonnes of crude steel, accounting for 49.6% of the world production [33]. Therefore, a China case was evaluated, in the fields of CO2 emission, resource reservation, energy consumption, cost, and profit, as showed in Table 5. In China, 94.8% of steel is produced by BOF, which may discharge 0.15–0.20 tonnes of BOFS per tonnes of steel produced [34]. Therefore, the amount of BOFS that need to be addressed was roughly 115.0–153.3 million tonnes irrespective of BOFS accumulated over the years because of low utilization ratio. The treatment fee of BOFS was about 13.3 USD/ tonnes-BOFS [35]. Based on the experimental results in this study, approximately 19.9–26.5 million tonnes of CO2 will be fixed via the HiGCarb process while generating 134.8–179.8 million tonnes of carbonated BOFS. The energy used for HiGCarb process was estimated to be 80–169 kW h/tonnes-CO2 [36], corresponding to an energy cost of 9.2–19.4 USD/tonnes-CO2, in the case of an industrial electricity price of 0.115 USD/kWh in China. The reduction of CO2 emission may bring a profit from carbon credit which was approximately 8.1 USD/tonnesCO2 [37]. If the carbonated BOFS is sold directly, assuming the price is 6.0 USD/tonnes-carbonated BOFS [38], then the total sale profit should be 809.0–1078.7 USD. When the carbonated BOFS was used in cement replacement, the benefits included resource conservation, CO2 emission reduction, energy consumption reduction and cost reduction. In the
Table 5 Cost and benefit analysis of the HiGCarb process and cement replacement. Items
CO2 emission
Resource reservation Energy consumption Cost
Profit
Units
Captured by HiGCarb Reduction from cement production HiGCarb operation Transportation Limestone Clay HiGCarb operation Reduction from cement production Money saved from BOFS treatment Money saved from cement production HiGCarb operation Transportation Carbon credit
Minimum
a
Maximum
(× 106 )
(× 106 )
MT
19.9
26.5
MT
107.9
179.8
MT MT MT MT kWh kWh
1.2 Undetermined 134.8 27.0 1591.0 14832.6
USD
1528.9
2038.5
USD
6716.1
8953.5
USD USD USD
183.0 515.3 Undetermined c 1006.8 1661.1
b
3.5 c
269.7 71.9 4481.2 19779.8
a The minimum was calculated based on the minimum yield and carbonation conversion of BOFS. b The maximum was calculated based on the maximum yield and carbonation conversion of BOFS. c The value related with transportation depends on the distance between the BOF to the carbonation reactor and the reactor to the cement plant.
54
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typical cement manufacturing, 1.0–1.5 tonnes of limestone, 0.2–0.4 tonnes of clay and 110 kW h of electricity are used for 1 tonnes of cement production, associated with 0.8–1.0 tonnes of CO2 emission [38,39]. The production cost of cement was assumed to be 49.8 USD/ tonnes-cement [38]. As shown in the Table 5, the amount of CO2 emission, electricity consumption, and cost for operating the HiGCarb process were significantly less than those reduced from cement production. Without considering the cost of transportation, compared with the traditional treatment of BOFS, the HiGCarb process with subsequent reutilization at least reduced 124.3 million tonnes of CO2 emission, saved 7728.7 million USD of cost, and gained 1006.8 million USD from carbon credit. Furthermore, the transportation expense which affects the final amount of CO2 emission and cost shows the importance of the establishment of waste-to-resource supply chain for a circular economy system in which zero waste and pollution is generated during the manufacture period.
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4. Conclusions and recommendations In this study, BOFS coupled with CW-EDI was used in the HiGCarb process. The results indicated that the maximum achievable carbonation conversion of the BOFS with CW-EDI was approximately 60.9%. The surface coverage model described the carbonation kinetics sufficiently. Moreover, calcium ion leaching from BOFS was restricted by the high initial level of calcium ion and alkalinity of CW-EDI. Sodium ion was adsorbed by the BOFS during the leaching process. In addition, all cement mortars with the fresh and carbonated BOFS at different substitution ratios achieved the ASTM requirements for autoclave expansion and compressive strength. The suggested substitution ratios for the fresh and carbonated BOFS in cement mortars were 15% and 10%, respectively. Furthermore, the results of cost and benefit analysis to the HiGCarb process together with cement replacement indicated the superiority of the process in CO2 emission reduction, cost saving and profit gaining. Future research should be consummated with an economic, engineering and environment analysis to the integrated waste treatment in plant trial, to identify the stability, maneuverability and challenge of the project in large scale. Acknowledgements The China Steel Corporation (CSC) in Taiwan was highly appreciated for providing steelmaking slag in this study. High appreciation also goes to the Ministry of Science and Technology (MOST) of Taiwan under Grant Number MOST 106-2811-M-002-018 for the financial support. References [1] IPCC, Carbon Dioxide Capture and Storage, IPCC, New York, 2005. [2] N. Kemache, L.C. Pasquier, I. Mouedhen, et al., Aqueous mineral carbonation of serpentinite on a pilot scale: the effect of liquid recirculation on CO2 sequestration and carbonate precipitation, Appl. Geochem. 67 (2016) 21–29. [3] N.C. Johnson, B. Thomas, K. Maher, et al., Olivine dissolution and carbonation under conditions relevant for in situ carbon storage, Chem. Geol. 373 (2014) 93–105. [4] M. Uibu, R. Kuusik, L. Andreas, et al., The CO2-binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash and steel slag, Energy Procedia 4 (2011) 925–932. [5] D.N. Huntzinger, J.S. Gierke, L.L. Sutter, et al., Mineral carbonation for carbon sequestration in cement kiln dust from waste piles, J. Hazard. Mater. 168 (2009) 31–37. [6] R.R.T. Dananjayan, P. Kandasamy, R. Andimuthu, Direct mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. 112 (2015) 4173–4182. [7] E.R. Bobicki, Q. Liu, Z. Xu, et al., Carbon capture and storage using alkaline industrial wastes, Prog. Energy Combust. 38 (2012) 302–320.
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