Insights into the roasting kinetics and mechanism of blast furnace slag

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Kinetics, Catalysis, and Reaction Engineering

Insights into the roasting kinetics and mechanism of blast furnace slag with ammonium sulfate for CO2 mineralization Shu Yin, Tahani Aldahri, Sohrab Rohani, Chun Li, Dongmei Luo, Guoquan Zhang, Hairong Yue, Bin Liang, and Weizao Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03109 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Industrial & Engineering Chemistry Research

Insights into the roasting kinetics and mechanism of blast furnace slag with ammonium sulfate for CO2 mineralization Shu Yin a, Tahani Aldahri b, c, Sohrab Rohani b, Chun Li a, Dongmei Luo a, Guoquan Zhang a, Hairong Yue a, Bin Liang a, Weizao Liu a, b * a School of Chemical Engineering, Sichuan University, Chengdu, 610065, China b Department of Chemical and Biochemical Engineering, Western University, London, Ontario, N6A5B9, Canada c Department of physics, Taibah University, Madinah, 41411, Saudi Arabia * [email protected] ABSTRACT A route for the indirect carbonation of different minerals using (NH4)2SO4 (AS) as a recyclable reagent has been recently proposed. For saving energy, the extraction of alkaline components, such as calcium and magnesium, is typically expected to occur at the roasting stage. To enhance the extraction efficiency, the kinetics and mechanism of the reaction between AS and blast furnace slag (BFS), a Ca/Mg-containing mineral, were investigated in this study. The results showed that the reaction consists of two steps: the decomposition of AS into ammonium bisulfate (ABS) and the reaction between the ABS and the BFS. Isothermal kinetics and equal conversion methods were used to determine the apparent activation energy and reaction-controlling steps in different temperature ranges. The results showed that the decomposition of AS was a chemical reaction-controlled step with an apparent activation energy of 102 kJ·mol-1. On the other hand, the reaction between ABS and BFS, which was very quick, was controlled by the ABS supply. During the reaction, a product accumulation layer was generated around the non-reactive BFS particles. However, it was found that ABS can easily pass through this layer and react with the BFS core. The formation of ABS was the limiting step in the roasting process. Keywords: mineralization; blast furnace slag; ammonium sulfate kinetics; mechanism 1 Introduction The excessive use of fossil fuels produces a large amount of CO2. This leads to an increase in CO2 concentration in the atmosphere, which in turn results in global warming.1,2 According to the International Energy Agency (IEA), the annual global CO2 emission has increased dramatically, from less than 0.1 Gt prior to the Industrial Revolution, to 32.31 Gt in 2018, and it is likely to reach 37.2 Gt by 2035.3,4 The Climate Report of the Intergovernmental Panel on Climate Change (IPCC) pointed out that the concentration of CO2 in the atmosphere has increased due to human activities, and reached 400 ppm in 2014. Furthermore, Earth’s global surface temperature is expected to 1

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increase by 3.7 °C (2.6–4.8 °C) by the end of the 21st century, resulting in rising in sea level of about 0.63 m (0.45–0.82 m).5 Under the tremendous pressure of the dramatic climatic changes occurring globally, a reduction in CO2 emission is imperative. Carbon dioxide capture, utilization, and storage6-8 (CCUS), are among the most important methods to stop the rising of atmospheric CO2 concentrations and to mitigate climate change. The carbon dioxide capture technologies include geological storage, oceanic storage, and mineral storage.9 However, geological storage technology is very location-dependent and has high post-storage monitoring costs,10 and oceanic storage can also destroy oceanic ecosystems. CO2 mineral storage mainly uses calcium or magnesium components in minerals to fix CO2 and form stable carbonates. CO2 mineralization has huge storage potential, as many calcium/magnesium silicate minerals and industrial solid wastes exist.11 CO2 mineralization includes direct and indirect carbonate methods. Direct carbonation imitates the natural weathering process, involving a one-step reaction of gaseous CO2 with solid minerals at a high temperature and pressure.12-14 However, direct carbonation has some disadvantages such as low efficiency, slow reaction kinetics, and impure products. Indirect carbonation usually involves two successive reaction steps: (1) extracting the calcium and magnesium components using acidic or weakly acidic chemicals, and (2) further mineralizing CO2 under alkaline or weakly alkaline conditions.15,16 Indirect carbonation has milder reaction conditions and can achieve a higher conversion ratio and produces relatively pure and valuable products. Thus, it has received widespread attention. In the present study, acetic acid,17 sulfuric acid,18,19 nitric acid, hydrochloric acid,20 ammonium nitrate, ammonium chloride, ammonium acetate,21 ammonium sulfate (AS),22-26 and other acidic reagents have been considered to extract calcium and magnesium. A key issue to be addressed in this process is how to regenerate these chemicals using low energy consumption with a net CO2 reduction. Among the above chemicals, only AS can be recycled economically. During carbonation, AS decomposes into acidic ammonium bisulfate (ABS) and alkaline ammonia gas at 250–350 °C. It is then regenerated following the carbonation reaction as shown in Eqs 1, 2, 3 and 4.27 (NH4)2SO4(s)=NH4HSO4(l)+NH3(g)

(1)

(Mg,Ca)xSiyOx+2y+zH2z(s)+2xNH4HSO4(s)=x(Mg,Ca)SO4+ySiO2(s)+(z+x)H2O(g)+x(NH4)2SO4(s) (2) (Mg,Ca)xSiyOx+2y+zH2z(s)+2xNH4HSO4(aq)=x(Mg,Ca)SO4+ySiO2(s)+(z+x)H2O(g)+x(NH4)2SO4(aq) (3) MgSO4/CaSO4(s)+CO2(g)+H2O(l)+2NH3(g)=MgCO3/CaCO3(s)+(NH4)2SO4(aq)

(4)

Different materials, including serpentine,24-29 olivine,30,31 wollastonite,32 and blast furnace slag (BFS),33-37 have been applied to indirect mineralization with AS as the extractant. In the case of serpentine,27 a 68% magnesium extraction efficiency was achieved under optimal conditions (a mass ratio of AS to serpentine of 3:1 roasted at 450 °C for 2 h and followed by a leaching stage). In our previous study,32 the extraction efficiency of calcium from wollastonite was approximately 90% when the mass ratio of AS to wollastonite was 2:1, and roasting was conducted at 390 °C for 2 h 2


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followed by leaching at 50 °C for 1 h. However, the majority of the extraction process took place during the leaching stage (Eq. 3), which had an extraction efficiency of 65%. During the roasting stage, the decomposition of AS (Eq. 1) is an endothermic reaction (ΔHθ350ºC = 112.5 kJ·mol-1), whereas the reaction between ABS and Ca/Mg-containing minerals (Eq. 2) is exothermic (for 1 mol wollastonite, ΔHθ350ºC = –105.3 kJ·mol-1). The heat of the decomposition of AS (Eq. 1) can be partially offset by Eq. 2. Therefore, overall, it is an energy-saving process and it is justifiable to maximize the extraction efficiency of Ca/Mg components during the roasting stage. Our previous study showed that BFS exhibited better reactivity than natural minerals in CO2 mineralization and was possible to recover high value-added aluminum-based by-products at the same time. The schematic flowsheet is shown in Figure 1.35,36 Similarly, the extraction of Ca/Mg components also mainly occurred at the leaching stage. As mentioned above, enhancing the extraction efficiency at the roasting stage saves energy and results in a net CO2 reduction. To the best of our knowledge, no previous efforts have been reported to investigate the kinetics and mechanism of the roasting stage with any type of minerals. Therefore, this paper focuses on the decomposition kinetics of AS and its reaction with BFS during the roasting stage. Through the analysis of the kinetic data and characterization, a mechanism of the reaction process is proposed.

Figure 1. Schematic illustration of mineral carbonation with blast furnace slag.35

2

Experiment 2.1 Material

The water quenching BFS used in this study was provided by Sha Gang Group Company Limited (Zhangjiagang, China). Prior to roasting experiments, the BFS was ground with a planetary ball mill (QM-1SP2-CL, Nanjing Boyuntong instrument technology Co. Limited, Nanjing, China) to obtain particle size distribution between 58-75 μm. All chemical reagents including ammonium sulfate ((NH4)2SO4), silica (SiO2) were analytical grade and obtained from ChengDu Chron Chemicals Co. Ltd of China. A shrinking unreacted core model (SCM) was used to study the kinetics of the roasting reaction between AS and BFS. An excess of reactants was required to ensure that their 3



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concentration can be regarded constant during the reaction. The optimum condition of the kinetic model was satisfied only when the concentration of the reactants was constant.38 However, a layering phenomenon (BFS placed in the bottom and separated from the molten AS or ABS because of its higher density) was observed when the amount of AS added was about 170 % in excess. Therefore, SiO2 was used as an inert material or skeleton to prevent the reactants collapse. In each roasting experiment, 1g of BFS was mixed with AS and SiO2 with a mass ratio (AS/BFS/SiO2) of 8:1:4, and placed in a ceramic crucible. Then the mixture was heated at a rate of 10 °C/min until to the required temperature in a muffle furnace (KSL-1200, Hefei Kejing Materials Technology Co. Limited, Hefei, China). Finally, the sample was cooled down to room temperature in a desiccator. The purpose of this study was to investigate the kinetics and mechanism of the reaction between AS and BFS during the roasting stage. However, it was found that after roasting the residual ABS, the unreacted BFS continued reacting in the subsequent leaching stage.36 To stop the reaction of the BFS, the roasting slag was fast leached with cold (5 °C) deionized water at a mass ratio of liquid-to-solid (L/S) of 100:1 for 5 seconds. Therefore, the residue ABS was dissolved in water and stopped reacting with BFS due to the low temperature and concentration. Then, the slurry was filtrated, obtaining a filter cake and a leachate. To dissolve the extracted Mg and Al completely, a secondary leaching stage of the filter cake was carried out with a mass ratio of L/S of 100:1 at room temperature for 30 min. The slurry was filtered to obtain a secondary leachate. The two leachates were then mixed and the concentrations of Mg2+, Al3+ and NH4+ were measured. The extraction efficiency of Mg and Al was calculated by dividing the measured contents by their contents in the BFS. The AS decomposition ratio (decomposed into gaseous NH3 or N2) was calculated by subtracting the remaining NH4+ in the leachate. To verify that the addition of SiO2 did not have an effect on the decomposition of AS, the thermal decomposition behaviour of AS in the roasting system was analyzed. The weight loss and activation energy of pure AS and the mixture of AS (8g) and SiO2 (mass ratio (AS/SiO2) of 8:5) were measured and compared by TGA. To observe the morphologies of the products obtained at different times, a mixture of BFS (1g), AS and SiO2 at a mass ratio (AS/BFS/SiO2) of 3:1:2.7 was pelleted into cylinders and subjected to backscattered images after roasting at 380 °C for 20, 40, 60 and 80 min. 2.2 Analysis and characterization The concentrations of Mg2+, Al3+ and NH4+ in the leaching solution were analyzed by the same methods in our previous study.36 The thermal decomposition behaviour of AS in the roasting system was analyzed by a thermogravimetric analyser (TGA/DSC, 2/1600, Mettler Toledo, Switzerland). A mixture of AS, SiO2 was placed in alumina crucibles. Pure AS was placed in alumina crucibles as a comparative experiment. The heating rate from 25 °C to 800 °C was 10 °C/min under a nitrogen atmosphere at a flow rate of 100 mL/min. The weight changes were continuously monitored using a data-logging device and a computer. The details of the experimental technique using the thermogravimetric technique are given in a reference.39 The surface morphologies of samples were observed by scanning electron microscopy (SEM, JSM-7500F, JEOL) at an accelerate voltage of 5 kV. The elemental contents of the samples were 4


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analyzed by a combined energy-dispersive X-ray spectrometer (EDS, IS250, Oxford, Japan). The distribution of elements in the inner and outer layers of mineralized slag particles was analysed by focused ion beam (FIB) (FEI Nano Ports, FEI Model 200 SEM FIB system, Hillsboro, USA) technique. A thermogravimetric analyser-infrared spectroscopy (TGA-IR, TENSOR II, Germany) was used for sample analysis. A mixture of AS and BFS was placed in alumina crucibles. Pure AS was used as a comparative experiment. Then, the samples were heated from 25 °C to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere at a flow rate of 100 mL/min. The phase constituents were determined by an X-ray diffraction spectrometer (DX-2007, Danton, China), operated with a Cu Kα radiation source filtered with a graphite monochromator at a wavelength of λ = 1.54 nm. The voltage and anode current were 40 kV and 30 mA, respectively. The scanning range (2θ) was from 10° to 70°. The compositions of BFS were determined by an X-ray fluorescence spectrometer (XRF, XRF-180, Shimadzu, Japan) with a Rh Kα radiation source. 3 Results and discussion The chemical composition of BFS was analyzed by X-ray fluorescence (XRF) and is shown in Table S1. X-ray diffraction (XRD) pattern of the water quenching BFS and annealed BFS is shown in Figure 2. Annealed BFS was obtained by annealing the water quenching BFS at 800 °C for 1 h and cooled naturally to room temperature. The BFS water quenched phase was amorphous but can be converted into crystalline state by annealing. The major constituents are gehlenite (Ca2Al2SiO7, wt. 40%) and akermanite (Ca2MgSi2O7, wt. 60%).

Figure 2. XRD patterns of water quenching BFS and annealing BFS.36

According to our previous study, the roasting reaction between BFS and AS mainly occurs at 200-450 °C.35 During the roasting, AS is decomposed into ABS (Eq. 5) at approximately 200-300 °C. The ABS was further decomposed when the temperature was greater than 350 °C as shown in Eq. 6.40 5



(NH4)2SO4(s)=NH4HSO4(l)+NH3(g) 3NH4HSO4(l)=NH3(g)+3SO2(g)+N2(g)+6H2O(g)

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(5) (6)

The ABS obtained could digest BFS (Eqs 7 and 8).35 Ca2MgSi2O7(s) +6NH4HSO4(l)=2CaSO4(s)+MgSO4(s)+2SiO2(s)+3H2O(g)+3(NH4)2SO4(s)

(7)

Ca2Al2SiO7(s)+10NH4HSO4(l)=2CaSO4(s)+Al2(SO4)3(s)+SiO2(s)+5H2O(g)+5(NH4)2SO4(s) (8) It can be seen that the whole reaction consists of two steps: the decomposition of AS into ABS, and the reaction between ABS and BFS. 3.1 Effect of SiO2 on the decomposition of AS Since ABS has a melting point of 147 °C (HSC Chemistry 6.041), the reaction between BFS and AS can be considered as a gas-liquid-solid reaction. However, a layering phenomenon was observed when there was an excess of AS, which inhibited the contact among the reactants. To prevent the layering phenomenon, an inert material, SiO2, was added to the system. To verify that SiO2 had no effect on the decomposition of AS, a non-isothermal kinetics method was used to compare the decomposition behaviour of AS with and without SiO2. The TGA-DSC curves of the pure AS and the mixture of AS and SiO2 are shown in Figure 3. The temperature intervals at the different weight loss stages and the amount of weight loss are shown in Table S2. Where, ΔT is the temperature interval; ΔW is mass loss of pure AS and AS/ SiO2. As shown in Figure 3, the curves obtained from the two systems have the same tendency. There were three weight loss stages with DSC peak temperatures of 310 °C, 460 °C, and 610 °C for both systems. The first weight loss of the pure AS and AS/SiO2 systems occurred in the temperature ranges of 265–362 °C and 258–363 °C, with weight losses of 17% and 19%, respectively. This indicates that the addition of SiO2 had no significant influence on the first weight loss interval. In the second weight loss interval, the weight loss of the AS/SiO2 system (67%) was lower than that of the pure AS (78%). This may be due to the reaction between ABS and a small amount of the SiO2 to form silicic acid in this stage.42 The silicic acid would have decomposed in the subsequent stage, leading to greater weight loss at temperatures higher than 453 °C.

6


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Figure 3. TG-DTA curves of (a) pure AS and (b) AS/SiO2 (8:5).

The Coast-Redfem integration method was used to determine the apparent activation energy of decomposition of AS43 as shown in Eq. 9. lnG(α')/T2=ln(AR/βEa)-Ea/RT

(9)

where, α' is percentage of weight loss, T is temperature, A is the pre-exponential factor, Ea is the apparent activation energy, R is gas constant, β is heating rate, G(α') is the integral of probabilistic mechanism function. Eq. 10 describes the form of the mechanism function G(α') over the temperature range 235-303 °C adopted from references.44 G(α')=3[1-(1-α')1/3]

(10)

Arrhenius plots (lnG(α')/T2 vs 1/T) are presented in Figure S1. The apparent activation energy Ea, with a correlation factor R2 and fitting temperature interval values ΔT' are listed in Table S3. These values were well-fitted for the pure AS and the AS/SiO2 with R2 values of 0.9985 and 0.9989, respectively. Both apparent activation energy values of the AS and AS/SiO2 were similar and within the range of the chemical reaction controlled mechanism.45 In the analysis of both the weight loss and the apparent activation energy, SiO2 did not have an effect on the decomposition of AS. 3.2 Extraction efficiency of BFS Figure 4 shows the effect of roasting temperature on the extraction efficiency of Mg and Al. At 240 °C, Al was undetectable in the leaching solution. The extraction efficiency of Mg and Al increased monotonically by increasing roasting time in the temperature range 260 to 380 °C. At 260 °C, due to the scarcity of the ABS, the extraction efficiency of Mg and Al were only of 12% and 5%, respectively. The extraction efficiency of the Mg, up to 70%, were similar when the temperature was greater than 340 °C, whereas the maximal value was approximately 57% in the case of Al. It was found that the extraction efficiency of Mg was always higher than that of Al, indicating that the akermanite had a higher reactivity than the gehlenite. These results were 7



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consistent with those of the thermodynamic study.36

Figure 4. Effect of temperature on the extraction efficiency of AS/BFS/SiO2 (8:1:4) system of (a) Mg and (b) Al.

Figure 5 shows the relationship between the percentage of NH3 loss and the roasting interval at different temperatures. By increasing the roasting temperature or the extension of the interval, the percentage of NH3 loss increased almost monotonically. When the temperature was greater than 320 °C, the decomposition of AS rose monotonically and plateaued. In the absence of BFS (Figure 5 (a)), the maximal percentage of NH3 loss was 50% at 380 °C after 30 min. This indicated that AS decomposed completely into ABS (Eq. 5). ABS did not decompose at that temperature. However, in the presence of BFS (Figure 5 (b)), this value increased to 60%. It can be explained by the Eqs 7 and 8, where the obtained ABS was further reacted with BFS to form AS. Then, the resulting AS was decomposed again, increasing the percentage of NH3 loss. From the Eqs 5 and 6, it can be concluded that insufficient ABS was able to react with BFS at low temperatures, and the decomposition of ABS occurred when the roasting temperature was higher than 380 °C. Both of these situations can decrease the extraction efficiency of BFS.

Figure 5. Effect of temperature on the percentage of NH3 loss in (a) AS/SiO2 (8:5) and (b) AS/BFS/SiO2 (8:1:4).

3.3 Roasting kinetics As mentioned above, the reaction between BFS and AS consisted of two steps: the decomposition of AS into ABS and the reaction between ABS and BFS. The kinetics of these steps 8


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was studied separately. Possible controlling steps for the reaction of BFS with AS included: (1) a chemical reaction involving the thermal decomposition of AS into ABS, (2) the diffusion of ABS through a fluid film (external diffusion), (3) the diffusion of ABS through the product layer into the surface of the unreacted BFS (internal diffusion), (4) the chemical reaction between BFS and ABS, and (5) the diffusion of the products. 3.3.1

Kinetics of AS decomposition

To investigate the decomposition kinetics of AS, isothermal kinetics and equal conversion methods were used to determine the apparent activation energy and reaction-controlling steps in different temperature ranges. Decomposition of AS in the temperature range of 260–340 °C is a typical liquid-solid reaction that can be analyzed using the shrinking unreacted core model (SCM). In the decomposition process, heat flux may be considered as a reagent that reacts with AS, and ABS is formed as a product layer.44 According to this assumption, AS particles are spherical, and three processes can be applied to different rate-controlling steps:38 (1) Diffusion of heat through fluid film controlling process: kt=α

(11)

(2) Diffusion of heat through ash layer controlling process: kt=1+2(1-α)-3(1-α)2/3

(12)

(3) Chemical reaction controlled process: kt=1-(1-α)1/3

(13)

where, k is reaction rate constant, t is time, and α is the percentage of NH3 loss. The ash layer refers to the area between the product layer and the unreacted core in which the reaction is in progress.38 Based on the percentage of NH3 loss ratio at time t given in Figure 5, the relationship between t and a was plotted according to Eqs 11, 12 and 13. The results are shown in Figure S2, where the Eqs 11 and 13 exhibited better linear tendency than Eq. 12 at each reaction temperature. The rate constants k for each of the temperatures were calculated from the slopes of linear fitting, which are listed in Table S4 and Table S5. The relationship between k and T can be described using the Arrhenius equation, as shown in Eq. 14. lnk=lnA-Ea/RT

(14)

However, the apparent activation energy obtained by Eq. 11 is not within the range of the gas film diffusion control (121 and 87 kJ·mol-1, respectively, for the absence and presence of a BFS system). Therefore, the decomposition of AS is a chemical reaction-controlled process. The relationship between k and T is shown in Figure S3 according to Eq. 14.45 The apparent activation energy of AS roasted with SiO2 (115 kJ·mol-1) was greater than with BFS (102 kJ·mol-1), indicating 9



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that BFS promoted the decomposition of AS. Over tthe range between 320-380°C, the kinetic data did not fit the known models (shrinking core model, crackling core model,46 Jander model et al). Therefore, the constant conversion method proposed by Coble47 and Ram et al48 was used in this study. The relationship between the apparent activation energy Ea and temperature T is given by the Eq. 15. ln(1/t)=lnA-Ea/RT

(15)

Figure S4 shows the relationship between ln(1/t) and 1/T. The values of apparent activation energy were calculated as 87.14±3.51 and 70.52±2.77 kJ·mol-1, respectively, representing the absence and presence of BFS conditions. The values of apparent activation energy are in the range of typical chemical reaction controlled mechanism.45 The apparent activation energy of AS roasting SiO2 was also greater than AS roasting BFS. The mixture of AS and BFS with a mass ratio of 3:1 and pure AS was analyzed by TGA-IR (Figure 6). The peaks in the range of 900-650 cm-1 and 1580-1490 cm-1 in Figure 6 (c) and Figure 6 (d) correspond to the bending vibration peak of -NH-.49 It indicates that ammonia gas is produced during the roasting. In Figure 6 (e) and Figure 6 (f), two peaks are observed at around 310 and 410°C, respectively, which are attributed to the decomposition of AS and ABS as shown in Eqs 5 and 6. The peak of -NN- starts to appear at 250 °C in the case of pure AS (Figure 6 (e)). Whereas, this value decreases at 225 °C in the mixture sample. Besides, the transmittance of the mixture sample is higher than the pure AS. These observations indicate that the addition of BFS reduces the decomposition temperature of AS and promotes its decomposition. Thus, BFS reduces the activation energy of the AS decomposition reaction, allowing the AS to decompose at a lower temperature. 3.3.2

Kinetics of AS roasting of BFS

The constant conversion method was also used to investigate the kinetic behaviour of the reaction between BFS and AS. Figure S5 shows the relationship between ln(1/t) and 1/T according to the data from Figure 4. As Mg reaches the equilibrium at 360 °C and 380 °C within 50 min, the kinetics of Mg extraction process was studied between 300–340 °C. Similarly, the kinetics of Al extraction and the percentage of NH3 loss were studied between 300–360 °C and 320–380 °C, respectively. The apparent activation energy values of the extraction process of Mg and Al, and the percentage of NH3 loss, (listed in Table S6) were in the chemical controlled regions, and numerically close to each other. It can be explained because the reaction rate of ABS with BFS is extremely fast, and the reaction of BFS with AS is actually controlled by the decomposition of AS (Eq. 5).

10


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Figure 6. GTA-IR spectra of the roasting process. Three-dimensional coordinates of transmittance-wave number-temperature of (a) pure AS and (b) AS/BFS (3:1). Transmittance-wave number of (c) pure AS and (d) AS/BFS (3:1). Transmittance-temperature of (e) pure AS and (f) AS/BFS (3:1).

To prove that the reaction rate of ABS with BFS is extremely fast, the roasting experiment with ABS and BFS was conducted at 320 °C with a mass ratio of ABS, BFS (1g), and SiO2 of 6:1:3. Figure 7 compares the extraction efficiency and the percentage of NH3 loss of the reaction with AS or ABS at 320 °C. The extraction efficiency of Mg and Al plateaued quickly (within 15 min) with the roasting of ABS, whereas they increased almost monotonically in the case of AS. Figure 7 (c) shows that ABS is more difficult to decompose than AS. It indicates that the reaction equilibrium of ABS and BFS is not caused by decomposition of ABS. This phenomenon indicates that the decomposition of AS controlled the reaction of ABS with BFS. 11



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Figure 7. Comparison of AS/BFS/SiO2 (8:1:4) with ABS/BFS/SiO2 (6:1:3) at 320°C. (a) The extraction efficiency of Mg, (b) the extraction efficiency of Al and (c) the percentage of NH3 loss.

3.4 Roasting mechanism To investigate the effect of the roasting time on the roasted product, pellets of AS, BFS and SiO2 with a mass ratio of AS: BFS: SiO2 of 3:1:2.7 were used. The pellets were roasted at 380 °C for 20, 40, 60, and 80 min. The backscatter images of roasted pellets are shown in Figure 8, where the crystallized particles appeared on the surface of the BFS, and with increased roasting time, the crystals became more regular. The crystallized product had two morphologies (Figure 8 (d)): hexagonal prismatic particles and cubic particles. EDS analysis showed that the hexagonal prism particles were Al2(SO4)3 and the cubic particles were CaSO4.

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Figure 8. SEM observation of 380 °C roasting for (a) 20 min, (b) 40 min, (c) 60min and (d) 80 min morphology in backscatter mode. (e) Analysis of the EDS image of area 1 in (d). (f) Analysis of the EDS image of area 2 in (d).

FIB was used to observe the internal structure of BFS particles after the reaction. A cross section and morphology of BFS particles following the reaction are shown in Figure 9. The reaction products were stacked on the surface of the BFS particles in Figure 9 (a). In Figure 9 (b), the cross section of BFS particles and obvious layering phenomenon is observed. Figure 10 shows the mapping and line scanning of the cross section of BFS particles after the reaction. The amounts of Ca, Si, Al and Mg on the external surface were lower than in the interior of BFS particles. This may have happened because sulfates were formed which diluted the relative contents of the Ca, Si, Al, and Mg on the external surface, according to the Eqs 10 and 11. Therefore, the roasted BFS was composed of three parts from external to internal: (1) The product accumulation layer where the fully reacted product is deposited as crystals on the outermost layer of BFS particles. (2) The ash layer:38 13



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ABS infiltrates in this area and BFS is reacting with ABS. (3) The unreacted core. The three parts are shown in Figure 9 (b).

Figure 9. (a) Morphologies of BFS particles after reaction; (b) cross section of BFS particles after reaction.

.

Figure 10. Mapping and line scanning of cross-section of BFS particles after reaction.

The mechanism of the BFS and AS roasting reaction is shown in Figure 11. In the roasting process, AS was mixed with BFS and decomposed to form ABS, being a limiting step in the overall reaction. Since ABS reacted with BFS in a short time, there was a little amount of ABS in the 14


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system. The BFS reacted with ABS to form sulfate products, which were stacked on the surface of the BFS particles. Due to the loose product accumulation layer, ABS easily passed through it and reacted with the unreacted BFS core. Therefore, the entire reaction was controlled by the AS decomposition, or the formation of ABS. The heat transfer efficiency can be enhanced with special reactors or by adding reagents, which could accelerate the decomposition of AS.

Figure 11. Mechanism diagram of AS roasting of BFS: (a) AS mixed with BFS. (b) AS thermal decomposition into ABS. (c) ABS reacts with surface of BFS, surface product of BFS crystallized and peeled off from the surface of BFS. (d) Schematic diagram of BFS particle model during reaction of BFS with ABS. (e) Schematic diagram of the BFS particle model, the product of which is stacked on the surface. (f) Cross-section of BFS particle after reaction. (g) SEM image of product deposited on the surface of BFS particles.

4 Conclusions In this study, the kinetics and mechanism of the reaction between AS and BFS were investigated. The results showed that the entire reaction process consisted of two steps: the decomposition of AS into ABS, and the reaction between ABS and BFS. Isothermal kinetics and equal conversion methods were used to determine the apparent activation energy and reaction-controlling steps in different temperature ranges. The results of isothermal kinetics showed 15



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that the decomposition of AS is a chemical reaction-controlled step. And, the apparent activation energies were calculated to be 115 and 102 kJ·mol-1, respectively, in the absence and presence of BFS at 260–340 °C. The equal conversion methods showed the apparent activation energy values, being calculated as 87.14 ± 3.51 and 70.52 ± 2.77 kJ·mol-1, respectively, in the absence and presence of BFS over the temperature range of 320 to 380 °C. The value of the apparent activation energy and TGA-IR experiments indicated that BFS promoted the decomposition of the AS. A comparative experiment of ABS roasting BFS and AS roasting BFS showed that the reaction between ABS and BFS occurred rapidly (within 15 min). It was controlled by the decomposition of AS, as the apparent activation energies of Mg (72.83 ± 4.82 kJ·mol-1) and Al (68.59 ± 2.91 kJ·mol-1) extraction process were similar to the value of NH3 loss (70.52 ± 2.77 kJ·mol-1) obtained by the equal conversion method. During the reaction, FIB experiments showed that the roasted BFS was composed of three parts from external to internal: (1) the product accumulation layer, (2) the ash layer, and (3) the unreacted core. As the product crystallized completely with the roasting time, it was stacked on the surface of the BFS particles. ABS passed easily through the product accumulation layer and reacted with BFS core. Finally, the formation of ABS was the limiting step for the entire roasting process. Acknowledgements The authors are appreciative of financial support from the National Key Projects for Fundamental Research and Development of China (2016YFB0600904), Sichuan Science and Technology Department Project (2019YJ0111), Sichuan University Postdoctoral Interdisciplinary Innovation Start-up fund project, as well as the State Key Laboratory of V & Ti Resources Comprehensive Utilization (18H0083). We appreciate Ceshigo Research Service Co., Ltd (www.ceshigo.com) for its help with FIB characterization. We also appreciate Dr. Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization. Supporting Information Chemical composition of BFS, temperature interval of TG, linear fit picture and parameters of the material. This information is available free of charge via the Internet at http: //pubs.acs.org. References (1) Goeppert, A.; Czaun, M.; Prakash, G. K. S. Air as the Renewable Carbon Source of the Future: an Overview of CO2 Capture from the Atmosphere. Energy Environ. Sci. 2012, 5, 7833-7853. (2) Tai, C. Y.; Chen, W. R.; Shih, S.-M. Factors Affecting Wollastonite Carbonation under CO2 Supercritical Conditions. AIChE J. 2010, 52, 292-299. (3) IEA. CO2 Emissions from Fuel Combustion-Hightlights 2018. Paris: IEA. 2018. (4) IEA. World Energy Outlook 2018. Paris: IEA. 2018. 16


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Insights into the roasting kinetics and mechanism of blast furnace slag

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