Phase Change Cooling and Crystallization Characteristics of Blast

The chemical composition of the blast furnace (BF) slag is an intrinsic factor of crystallization characteristics. Therefore, a good understanding of ...
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Phase Change Cooling and Crystallization Characteristics of Blast Furnace Slags with Various MgO/AlO Ratios 2

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Bin Ding, Hong Wang, Xun Zhu, Xian-yan He, Yu Tan, and Qiang Liao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01352 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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Phase Change Cooling and Crystallization Characteristics of Blast Furnace Slags with Various MgO/Al2O3 Ratios Bin Ding2, Hong Wang1-2*, Xun Zhu1-2*, Xian-Yan He2, Yu Tan2, Qiang Liao1-2 1 Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing 400030, China 2 Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China E-mail: [email protected], [email protected] Tel: +86-023-65102474

Abstract The chemical composition of the blast furnace (BF) slag is an intrinsic factor of crystallization characteristics. Therefore, a good understanding of the effects of the main components of BF slag on the coupling relationship between the BF slag phase change cooling and crystallization characteristics is essential and crucial to achieve the simultaneous collection of high-performance glassy slag and high-temperature air during the dry heat recovery process. In this paper, differential scanning calorimetry (DSC), X-ray diffractometer (XRD), and scanning electron microscopy (SEM) are adopted to obtain the physicochemical properties, crystal phase content, crystal phase type and morphology of four synthetic BF slags with various MgO/Al2O3 ratios. Moreover, the temperature-time curve inside the slag during the phase change cooling process is used to reflect the crystallization temperature region, crystal phase growth time and growth mode. The results indicate that the primary crystal phase transforms from Merwinite to Akermanite with the MgO/Al2O3 ratio decreases from 1.5 to 1.0, which results in a significant variation in the crystal phase latent heat, crystallization onset and ending temperatures, crystal phase content and growth rate. Moreover, the decrease in the MgO/Al2O3 ratio and the transformation of the crystal type give rise to the decline in the critical cooling rate and the increase in the critical supercooling degree. Furthermore, with a decrease in the average cooling rate, the average growth rate of the crystal phase presents a peak value. 1. Introduction Blast furnace slag (BF slag) is one of the most abundant solid by-products of the iron-making process.1 Generally, 300~350 kg BF slag per ton of pig iron is produced.2 In China, the output of pig iron reached 691 million tons in 2015, meanwhile nearly 235 million tons of BF slag was produced. Moreover, molten BF slag is discharged at a temperature of approximately 1500 °C.3 Thus, the total heat energy carried by the BF slag equals a calorific value of 14.2 million tons of standard coal. Currently, water quenching and self-cooling in the dreg field are the popular treatments of BF slag.4 The cooling rate plays a dominant role in the solid structure of BF slag.5 Specifically, the high value-added glassy phase is present in the BF slag when cooled at a fast cooling rate, which is used for cement manufacturing. In comparison, the crystal phase appears in the slag when it is cooled slowly, which is only used as aggregate for 1

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road construction and landfilling purposes. However, these treatments fail to realize the heat recovery from the slag. Furthermore, the water quenching treatment has shortcomings such as water consumption and toxic gas (SO2 and H2S) emission.6, 7 To realize the multi goals of heat recovery, water saving and pollution mitigation, various dry granulation heat recovery technologies have been developed such as the mechanical crushing method 8, 9, air blast method 10 and centrifugal granulation method 11-13. Among them, the centrifugal granulation method is the most feasible owing to the compact structure, reliable operation, low power consumption and high heat recovery rate.1 The centrifugal granulation heat recovery technology mainly consists of a centrifugal granulation device and waste heat recovery device. The technological process can be described as follows. The melting BF slag is granulated into small droplets using a high-speed rotating atomizer (disk/cup) and rapidly cooled by air in the granulation cabin. Then, the semi-melting particles are further cooled to solidification in a waste heat recovery device (moving/fluidized bed).14, 15 However, this technology faces two opposing problems: the demand for a rapid cooling rate under a higher cooling air flow rate to obtain glassy slag and the requirement to achieve high-grade recovery of heat at a small cooling air flow rate. Thus, the determination of a suitable cooling rate is of significant importance to create a technology that addresses both of these problems. For the BF slag cooling process, it has to be noted that the main components (CaO, SiO2, MgO and Al2O3) of BF slag significantly affect the physical parameters and crystallization characteristics.16, 17 Furthermore, the main components of BF slag vary with time due to the variation of the iron ore composition and blast furnace operating conditions.18, 19 Consequently, a good understanding of the effects of the main components of BF slag on the coupling relationship between the BF slag phase change cooling and crystal phase evolution is essential and crucial to achieve the simultaneous collection of high-performance glassy slag and high temperature air during the dry heat recovery process. Until now, the effects of the main components of the slag on the crystallization behaviors have been widely investigated using differential scanning calorimetry (DSC) 20, X-ray diffraction (XRD) 21-23 and single/double hot thermocouple technique (S/DHTT) 4, 24, 25. Recently, the research of Qin et al.26 indicated that the lower contents of CaO and MgO and higher contents of SiO2 and Al2O3 were beneficial to form the crystal phase in the BF slag. However, it should not be neglected that the variation of other components was inevitable when the content of one component is varied to explore its effect on the crystallization behaviors of slag. This means that it is difficult to isolate the effect of only one component to obtain a comprehensive understanding (the effects of at least two components with sometimes opposing effects) of the BF slag crystallization behaviors. Based on this, the binary basicity (CaO/SiO2) and ratios of SiO2/Al2O3, MgO/Al2O3, etc. were adopted to investigate the effects of main components on the crystallization behaviors of slag. For instance, the crystallization temperature and critical cooling rate of the mold fluxes were increased with an increase in binary basicity.27 This means that the crystallization ability of the mold fluxes was enhanced 2

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with increasing binary basicity.28 Moreover, a certain increase in the SiO2/Al2O3 ratio improves the crystallization ability, but a further increase gives rise to a lower and narrower crystallization temperature region and reduces the crystal phase content.29 Bases on the unique content of the main components: CaO (30~45%), SiO2 (28~38%), Al2O3 (8~24%), and MgO (3~18%) 30, the MgO/Al2O3 ratio presents significant effect on the crystallization behaviors of BF slag with the same binary basicity. However, investigations of the effect of the MgO/Al2O3 ratio have mainly focused on the BF slag viscosity instead of crystallization behaviors. 31-33 Furthermore, in these experiments, the tiny amount of sample limited the detailed investigation of phase change cooling process and the coupling relationships between the phase change cooling and crystal phase growth. In addition, the directional solidification technique is a popular method for exploring the phase change cooling process in materials. In our previous work34, this method, in combination with the DSC and XRD, was adopted to investigate the interplay between the phase change cooling and the crystallization characteristics in a BF slag column. Moreover, a dimensionless correlation was developed to predict the crystal phase content in the phase change cooling process. Unfortunately, until now, additional studies on the effect of the MgO/Al2O3 ratio on the crystallization characteristics of BF slags in a phase change cooling process have not been reported. In this paper, the slag samples with MgO/Al2O3 ratios ranging from 2.0 to 0.5 were prepared to explore the effect of the MgO/Al2O3 ratio on the crystallization characteristics. DSC was applied to obtain the liquidus temperature, glass transition temperature, and latent heat of glassy and crystal phases. The directional solidification technique was applied to measure the temperature evolution in a slag column. XRD was adopted to analyze the crystal phase type and content under various phase change cooling processes. Therefore, the evolution of the crystal phase structure and content, critical average cooling rate, critical supercooling degree and crystal growth mode under various MgO/Al2O3 ratios were experimentally investigated. These investigations may provide theoretical guidance for the dry graduation and waste heat recovery technique in choosing the operating conditions. 2. Experimental Procedure 2.1. Sample Preparation In the present research, the slag samples were prepared by mixing water-quenched BF slag and reagent-grade powders of CaO, SiO2, Al2O3 and MgO. Four slag samples (S1~S4) with the same CaO/SiO2 ratio and various MgO/Al2O3 ratios were designed to investigate crystallization characteristics in the phase change cooling process. As described in table 1, the MgO/Al2O3 ratio was ranked from 2.0 to 0.5, and the binary basicity of these samples was kept at 1.2. Then, the slag samples were heated to 1550 °C at a heating rate of 10 °C·min-1 in a heating furnace (Model SX-G08163, TJZH, China) and kept warm for approximately 1 hour. After that, part of the melting slag was taken out from the heating furnace and quenched with water to obtain a purely glassy slag. Eventually, to obtain a purely crystal phase slag, the remaining melting slag was gradually cooled to 1100 °C in the heating furnace and 3

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kept warm for 12 hours.

Sample S1 S2 S3 S4

Table 1. Major chemical composition of the slag samples Chemical composition (wt %) Binary MgO/Al2O3 basicity ratio Trace CaO SiO2 MgO Al2O3 elements 39.4 32.8 14.4 7.2 6.2 1.2 2.0 1.5 39.4 32.8 13 8.6 6.2 1.2 39.4 32.8 10.8 10.8 6.2 1.2 1.0 39.4 32.8 7.2 14.4 6.2 1.2 0.5

2.2. Experimental System and Methods A directional solidification apparatus was designed to explore the crystallization characteristics of BF slag in a phase change cooling process. As described in Figure 1, the experimental apparatus mainly consisted of a test unit and a measurement and cooling system. In the test unit, the stainless crucible with a diameter of 32 mm and a length of 100 mm was used to contain the testing slag. To realize the directional solidification of slag, the flank and top of the stainless crucible was covered by an insulation layer with a thickness of 80 mm. In the measurement system, ten thermocouples (type B, diameter of 0.5 mm and accuracy of ±0.1 °C) were placed along the axis of the stainless crucible. Moreover, the temperature data were acquired by a data logger (Keithley Instruments, Model 2701) and saved on a computer. In the cooling system, a graphite cooler was used to fix the stainless crucible, insulation layer and nozzle. Moreover, a flowmeter was adopted to control the flow rate of cooling water.

Figure 1. Schematic representation of the experimental setup.

Prior to the experiment, the slag sample was heated to 1550 °C and kept warm 4

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for 1 hour in a heating furnace (Model SX-G08163, TJZH, China). Meanwhile, a flame gun was used to preheat the stainless crucible up to 1100 °C. Then, the melting BF slag was taken out from the heating furnace and rapidly poured into the stainless crucible. After that, the top of the stainless crucible was sealed with the insulation cover, and the cooling water valve was opened simultaneously. Eventually, the experiment ended when the temperature of the slag column decreased to 250 °C. After the experiment, a diamond wire cutting machine (Model WXD-170, Mike, China) was adopted to obtain the slag samples at the position where the thermocouple was placed. Then, parts of these samples were analyzed using an SEM instrument (Model S-4800, Hitachi, Japan) to obtain the morphology and microstructural characteristics. Moreover, elemental information on the crystal phase with various shapes was obtained with an energy-dispersive spectrometer (EDS). After that, the rest of these samples were ground into homogenized fine powder with diameters of 38 µm. Finally, XRD (Model D/max-1200, Rigaku, Japan) was used to determine the crystal phase type of these samples. To calculate the crystal phase content of these samples, standard samples with crystal phase contents of 0%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% and 100% were prepared by mixing the purely glassy and crystal phase slag. Similarly, these standard samples were ground into powder of the same size and tested via XRD. In addition, the glass transition temperature (Tg), liquidus temperature (Tl), latent heat of glassy slag (Lg) and latent heat of purely crystal slag (Lc) were detected using DSC (Q20 DSC, TA, USA). 2.3. Data Processing and Error Analysis The previous research34, 35 declared that the crystallization characteristics of BF slag can be factually reflected by the temperature-time curve. Specifically, fast cooling resulted in the appearance of glassy phase and the release of latent heat in the temperature range between Tl and Tg. That is, the release of glassy phase latent heat had no obvious influence on the temperature-time curve, as described in Figure 2(a). Conversely, the crystal phase is present in the slag and releases more latent heat in a narrower temperature range at a slow cooling rate. This means that the temperature-time curve becomes flat in the crystallization region. Moreover, the growth mode of the crystal phase transforms from the columnar growth to equiaxed growth with a further decrease in the cooling rate, giving rise to the appearance of the recalescence phenomenon. Therefore, as shown in Figure 2(b)~(c), the crystallization onset temperature (To), crystallization ending temperature (Te) and growth time (τe-τo) of the crystal phase can be obtained from the temperature-time curve using the method mentioned in the previous research35.

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Figure 2. The temperature-time curve of BF slag under various cooling conditions

As described in Figure 2, the temperature of the slag underwent complex variations during the phase change cooling process. Therefore, an average cooling rate (v) between the Tl and Tg was adopted to represent the cooling condition of slag and is introduced as v=

Tl − Tg

(1)

τ g −τ l

where τg-τl is the time period when the slag is cooled from Tl to Tg. The crystal phase content (α) of BF slag can be calculated from the percentage of the crystal phase content that accounts for the gross of the glassy and crystal phase in the BF slag.36-38 Moreover, Wang et al.39 proposed a matrix-flushing method for the quantitative multicomponent analysis via XRD. The correlation to calculate the crystal phase content can be expressed as α =

K ∑ Ic

(2)

Ig + K ∑ Ic

where ∑Ic is the gross area of crystal peaks, Ig is the integral area of glassy peaks in the range from 2θ=20° to 40° (as shown in Figure 3a), and K is the glass quotient of BF slag. Moreover, the previous research 22, 39 indicated that K can be expressed as K =

Ig mc ⋅ mg ∑ Ic

(3)

where mc and mg are the mass percent of crystal and glassy phase in the standard sample, respectively. 6

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Then, the XRD patterns of the standard samples were measured to determine the areas of the crystalline and glassy peaks (∑Ic and Ig, respectively), these areas were integrated to obtain the ratio ∑Ic·Ig-1, and ∑Ic·Ig-1 was plotted to extract the slope K, as shown in Figure 3(b). Therefore, the K values of the samples 1~4 in this paper are 0.703±0.02, 0.602±0.01, 0.841±0.01 and 0.770±0.02, respectively.

Figure 3(a) Schematic representation of a portion of the XRD pattern (sample 3, α=20%). 3(b) Relationship between mc/mg and ∑Ic/Ig (sample 3).

In addition, the crystallization rate of the slag peaked with a decrease in temperature under a constant cooling rate, which was attributed to the combined effects of the nucleation rate and the crystal growth rate.40 Moreover, the dependence of the crystallization rate on temperature was more complicated in the phase change cooling process due to the complex variation in the cooling rate. Therefore, an average crystallization rate (vα) was employed to reflect the crystallization rate under various cooling conditions and is expressed as α vα = (4) τe −τo where τe-τo is the time period when the slag is cooled from the crystallization onset temperature (To) to the crystallization ending temperature (Tg), as shown in Figure 2. The experimental error in the computation of v in Eq. (1) is the sum of Tl, Tg, τl and τg. Therefore, based on the fractional uncertainty of Tl, Tg, τl and τg, the largest uncertainty of v is 0.5%. Similarly, the uncertainties of α and vα are 3% and 3.1%, respectively. 3. Results and Discussion 3.1. Physicochemical Properties of BF Slags Based on the method mentioned in the previous research,34 the liquidus temperature (Tl), glassy transition temperature (Tg), and latent heat of the glassy and crystal phases were obtained from the DSC curve, as shown in Figure 4. One can see that the liquidus temperature had a minimum value with a decrease in the MgO/Al2O3 ratio. For instance, the liquidus temperature decreased from 1383.5 °C to 1369.3 °C when the MgO/Al2O3 ratio reduced from 2.0 to 1.5. Then, the liquidus temperature gradually increased to 1413.4 °C when the MgO/Al2O3 ratio decreased to 0.5. By 7

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contrast, the glassy transition temperature varied in a temperature range between 720 °C and 740 °C. This was attributed to the liquidus temperature and glassy transition temperature being governed by the mole fraction of the main components of the BF slag.41 As shown in Figure 4(b), the latent heat of the glassy phase gradually increased from 308.8 to 350 J·g-1 when the MgO/Al2O3 ratio decreased from 2.0 to 0.5. In comparison, the latent heat of the crystal phase was larger than that of the glassy phase. Moreover, the latent heat of the crystal phase increased from 391 to 415.7 J·g-1 when the MgO/Al2O3 ratio decreased from 2.0 to 1.5. Then, it rapidly increased to 495.7 J·g-1 when the MgO/Al2O3 ratio equaled 1.0, which was attributed to the variation in the crystal phase type. Eventually, the latent heat of the crystal phase remained nearly constant when the MgO/Al2O3 ratio decreased further. In conclusion, the latent heat of the glassy phase was controlled by the mole fraction of the main components of the BF slag. The type of crystal phase played a dominant role in the value of the crystal phase latent heat.

Figure 4 (a) Distribution of the liquidus and glassy transition temperatures with various MgO/Al2O3 ratios, (b) Distribution of the glassy and crystal phase latent heat with various MgO/Al2O3 ratios.

3.2. Evolution of the Precipitated Crystal Phase To discuss the effect of the MgO/Al2O3 ratio on the evolution of the crystal phase type and content along the vertical direction, the XRD patterns of samples 1~4 at different positions were obtained and are described in Figure 5. Figure 5(a) illustrates the XRD pattern evolution of sample 1 along the vertical direction. One can see that the crystallization peaks of Merwinite are presented in the XRD pattern of y=1 mm. Then, both the intensity and number of the crystallization peaks increased along the vertical direction. Moreover, a portion of Akermanite and Monticellite precipitated in the position of y=10 mm. After that, both the intensity and number of the Merwinite peaks were nearly constant, as shown in the XRD pattern of y=16 mm. By contrast, the intensity and number of the Akermanite peaks and Monticellite peaks increased gradually. Eventually, the content of Merwinite decreased gradually along the vertical direction. Conversely, the contents of Akermanite and Monticellite increased rapidly, as described in the XRD pattern of y=52 mm. This means that a part of the Merwinite transformed to Monticellite during the phase change cooling process of BF slag. Figure 5(b)~(d) showed the XRD patterns under various MgO/Al2O3 ratios at 8

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positions of y=1, 10 and 52 mm. As described in Figure 5(b), both the intensity and number of the crystallization peaks decreased with a reduction in the MgO/Al2O3 ratio. Moreover, no obvious crystallization peaks can be observed in the XRD patterns of the samples with MgO/Al2O3 ratios less than 1.0. Figures 5(c) and (d) indicate that the crystal phase contents of samples 1 and 2 at the same position were nearly constant and were much smaller than those of samples 3 and 4. Moreover, the primary crystal phase transformed from Merwinite to Akermanite when the MgO/Al2O3 ratio decreased from 1.5 to 1.0. Furthermore, the secondary crystal phase transformed from Akermanite and Monticellite to Akermanite when the MgO/Al2O3 ratio decreased from 2.0 to 1.5. In addition, the number and intensity of secondary crystallization peaks declined with a decrease in the MgO/Al2O3 ratio. Eventually, no obvious secondary crystallization peaks were present in the BF slag when the MgO/Al2O3 ratio was further decreased.

Figure 5 (a) XRD pattern evolution of S1 along the vertical direction (y=1, 10, 16 and 52 mm); (b)~(d) XRD patterns of S1~4 at positions of y=1, 10 and 52 mm. △-Merwinite (Ca3Mg(SiO4)2), □-Akermanite (Ca2MgSi2O7), ◇-Monticellite (CaMgSiO4)

The crystal phase content of samples 1~4 at different position was determined using Eq. (2) and is shown in Figure 6. One can see that the crystal phase content increased rapidly at the position of y