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Lalinda Palliyaguru, Ushan S. Kulathunga, Lakruwani I. Jayarathna, Champa D. Jayaweera, and Pradeep M. Jayaweera, A simple and novel synthetic route to prepare anatase TiO2 nanopowders from natural ilmenite via the H3PO4/NH3 process, Int. J. Miner. Metall. Mater., 27(2020), No. 6, pp.846-855. https://dx.doi.org/10.1007/s12613-020-2030-3
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A simple and novel technique for the preparation of anatase TiO2 nanopowders using natural ilmenite (FeTiO3) as the starting material is reported. Digesting ilmenite with concentrated H3PO4 under refluxing conditions yields a white α-titanium bismonohydrogen orthophosphate monohydrate (TOP), Ti(HPO4)2·H2O, which can be easily isolated via gravity separation from unreacted ilmenite. The addition of ammonia to the separated TOP followed by calcination at 500°C completes the preparation of anatase TiO2. Calcination at temperatures above 800°C converts the anatase form of TiO2 to the stable rutile phase. The removal of iron from ilmenite during the commercial production of synthetic TiO2 is problematic and environmentally unfriendly. In the present study, the removal of iron was found to be markedly simple due to the high solubility of iron phosphate species in concentrated H3PO4 with the precipitation of TOP. The titanium content of the prepared samples on metal basis with silica and phosphorous as major impurities was over 90%. Prepared TiO2 samples were characterized using X-ray fluorescence, Fourier-transform infrared spectroscopy, Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, and X-ray diffraction analyses. The photocatalytic potentials of the commercial and as-prepared TiO2 samples were assessed by the photodegradation of rhodamine B dye.
Vanadium–titanium magnetite (VTM) is a polymetallic complex iron ore with a high comprehensive utilization value and is filled with multiple precious elements [1–5]. To date, VTM is mainly smelted using a blast furnace, which is accompanied by a large amount of carbon emissions [6–10]. For the efficient and environmentally friendly utilization of VTM, the author’s team has developed a novel smelting technique based on gas-based shaft furnace direct reduction [11] with the characteristics of low energy consumption, low environmental load, and high product quality [12–17]. VTM is typically fed as pellets into shaft furnaces [18−19]. However, adjacent pellets stick to each other during the reduction, which worsens the smooth flow of raw materials and the distribution of airflow in the shaft furnace. This problem must be solved urgently for the iron and steel industry.
The sticking can be explained as the deoxygenation of iron ore during the reduction, resulting in the continuous aggregation of metallic iron between the contact surfaces of adjacent pellets [17]. This phenomenon can be primarily influenced by reduction temperature, reduction degree, reduction atmosphere, and load pressure [18–22]. In addition, the sticking index (SI) is an important indicator to measure the tendency of sticking behavior. The reduction in temperature has a considerable influence on sticking behavior [19,23–24]. A high temperature is accompanied by a large SI, indicating a strong tendency to stick. Yi et al. [20–21] found that the reduction degree was a direct factor affecting SI, and the rise in reduction degree led to the formation of additional metallic iron, resulting in an increase in SI. The difference in the reduction atmosphere caused remarkably changes in the precipitation morphology of metallic iron in the sticking interface. Under a CO atmosphere, the sticking interface exhibited a coarse, dense metallic iron structure that corresponds to a high SI [21,25–27]. Abdel-Halim et al. [22] investigated the sticking behavior of pellets under the multiple pressures of load and illustrated that the increase in load led to a tight accumulation of particles, which in turn increased the contact points between metallic iron and consequently enhanced the tendency of sticking behavior.
From the perspective of global carbon neutrality, the hydrogen metallurgy process has become a revolutionary technology for steel manufacturing [28–31]. Direct reduction in hydrogen metallurgical gas-based shaft furnaces is currently one of the main directions of low-carbon and green development in the steel industry [32–33]. Nevertheless, further understanding the sticking behavior of pellets during the reduction under hydrogen atmosphere is crucial to effectively utilize the gas-based shaft furnace. In addition, titanium is an important feature and valuable component of VTM, but only a few literature reports are available on the influence of titanium on the sticking behavior and mechanism of pellets under hydrogen atmosphere during reduction.
As part of the foundational work on the novel method of VTM comprehensive utilization, this study aimed to investigate the influence of TiO2 on the behavior and mechanism evolution of the sticking of pellets at different reduction conditions under a hydrogen atmosphere. Furthermore, the sticking mechanism of pellets with different TiO2 additions and reduction temperatures during reduction was illuminated by micromorphological characterization in detail.
A sample of iron ore concentrate provided by a steel enterprise in China was used as the main raw material for this study, and its main chemical composition is detailed in Table 1. Bentonite was utilized as a binder, and its main chemical components are shown in Table 2. TiO2 (analytically pure reagent) was acquired from Sinopharm Chemical Regent Co., Ltd., China. Every single substance had a particle size over 0.074 mm. High-purity hydrogen cylinders (purity: 99.99%) and nitrogen cylinders (purity: 99.99%) were provided by Shuntai Specialty Gases Co., Ltd. (Shenyang, China).
| TFe | FeO | CaO | SiO2 | MgO | Al2O3 |
| 65.69 | 29.38 | 0.47 | 5.05 | 0.62 | 0.30 |
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| SiO2 | MgO | Al2O3 | CaO | Na2O | K2O | LOI |
| 67.45 | 4.61 | 14.47 | 2.47 | 1.68 | 1.19 | 3.34 |
| Note: LOI—Loss of ignition. | ||||||
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The flow diagram of the reduction process of pellets with different TiO2 additions is shown in Fig. 1. First, the iron ore concentrate, TiO2, and bentonite were blended in a planetary ball mill at 200 r/min for 30 min to obtain the uniform mixture, and bentonite was fixed at 1wt%. An experimental balling disk pelletizer with a diameter of 1000 mm, rotation speed of 20 r/min, edge height of 200 mm, and tilt angle of 45° was used to create green pellets. The pellets were then placed in a drying oven at 105°C for 5 h to remove excess moisture. Oxidation–roasting was carried out in a muffle furnace with sufficient air to maintain the oxidizing atmosphere. The dried green pellets were placed in the center of the muffle furnace at 900°C for 10 min for preoxidation. The pellets were roasted at a roasting temperature (1250°C) for a fixed period of time (30 min) and then cooled spontaneously to room temperature. Our previous study found that the mineralogy composition was similar between direct TiO2 addition and actual VTM ore [34]. Thus, this mixing method is reasonable and practical.
The reduction experiment was performed in a high-temperature reduction furnace with a loading of 1.0 kg/cm2 under a hydrogen atmosphere, as shown in Fig. 1. The temperature was controlled and adjusted by a thermocouple and a proportion integration differentiation (PID) controller to ensure that the temperature measurement error was less than 1°C. The loading was designed to simulate the pressure exerted by the upper charge layer of the shaft furnace. A high-purity graphite crucible equipped with 500 g of roasted pellets was placed in the reduction area of the reduction furnace to heat at a setting temperature for the reduction experiment, and 0.6 m3/h inert gas (nitrogen) was pumped into the furnace to maintain an inert atmosphere. The inert gas (nitrogen) was switched to the reducing gas (hydrogen) to start the reduction for a fixed period of time until the reduction furnace reached the setting temperature, and the gas flow rates were maintained at 0.6 m3/h. The reduced samples were cooled to room temperature in a nitrogen atmosphere to obtain clustered pellets.
The sticking index is an important standard to evaluate the sticking behavior of pellets during reduction. The clustered pellets containing more than two pellets were dropped 20 times onto a steel plate surface from a height of 1 m. The percentage of remaining clusters after each drop was calculated, and the number of drops was plotted against the percentage of clusters. SI is defined as the area proportion under the curve [21]. Fig. 2 shows the calculation of the SI of pellets after reduction. A low SI represents a slight sticking behavior [25].
The sticking behavior of pellets during reduction is displayed in Fig. 3(a) and (b). The roasted pellets were individual and scattered (before reduction), and the metalized pellets were aggregated together to form clustered pellets (after reduction). The microstructure of the material determines its macro characteristics. Therefore, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used to analyze the sticking interface to clarify the sticking mechanism during the reduction of pellets. Fig. 3(c), (d), and (e) illustrates that the main phase composition of the sticking interface was a newly formed metallic iron phase. Meanwhile, the liquid phase occurred in the absence of a low melting point.
The effect of TiO2 addition from 0 to 15wt% (the interval is 5wt%) on the SI of pellets during reduction was studied under the reduction temperature of 1100°C. Fig. 4 shows the change trend of SI during reduction with different TiO2 additions. The SI decreased monotonically with the increase in TiO2 addition. The change in the composition of raw materials resulted in SI decreasing from 59.91% to 35.56%. Sui et al. [18] found that the SI of pellets coated with titanium-bearing tailing was significantly lower than that of pellets without coating materials, and this result coincided with Fig. 4. TiO2 improved the sticking behavior of pellets during reduction, which was beneficial to the smooth flow of raw materials and the distribution of airflow during production in the shaft furnace.
The increasing TiO2 addition significantly affected the phase composition of the reduction products. The phase-composition energy spectra of the reduction products were obtained by X-ray diffraction (XRD) detection to further clarify the main phase transformation. Fig. 5 displays the XRD spectra of the sticking interface between adjacent pellets reduced at 1100°C for 60 min with different TiO2 additions.
As illustrated in Fig. 5(a), the diffraction peak of the main phase changed with the increase in TiO2 addition. The main phase composition of the sticking interface without TiO2 was metallic iron (Fe). As shown in Fig. 5(b), the diffraction peaks of FeTiO3 at 2θ of 32.861° (FeTiO3, PDF#01-075-1212) appeared when the TiO2 addition increased to 5wt%. The composition of each phase remained constant, and the intensity of the FeTiO3 diffraction peaks was further strengthened when the TiO2 addition increased to 15wt%. This phenomenon indicated that the increase in TiO2 addition could increase the content of FeTiO3 in the sticking interface. Fig. 5(c) shows that the diffraction peaks of metallic iron were weakened gradually with the increasing TiO2 content in the samples, and the opposite change was observed for the diffraction peaks of FeTiO3. This finding indicated that increasing TiO2 addition would decrease the formation of metallic iron during reduction, mainly due to the metallization of pellets being suppressed by the formation of FeTiO3.
SEM−EDS was used to analyze the microstructural development of the sticking interface for the reduction products with different TiO2 additions under a reduction temperature of 1100°C and reduction time of 60 min to reveal the intrinsic morphology evolution and reduction mechanism of the sticking interface during the direct reduction. The results are shown in Fig. 6.
The connection characteristics between particles on the sticking interface varied significantly with the TiO2 addition. In the absence of TiO2 of pellets (Fig. 6(a)), the particle morphology was dense with a small void volume; the connections of metallic iron between adjacent pellets were developed for sufficient metallic iron clustering, creating the spot of “surface to surface” tight connections. Moreover, the precipitation morphology of metallic iron was mainly pyknotic layered iron [21]. This phenomenon could be explained by the fact that the speed of precipitated metallic iron was accelerated under the hydrogen atmosphere, resulting in the incomplete crystallization of metallic iron. As a result, the pyknotic layered iron was formed.
A large number of sticking points appeared between the metallic iron. All of the above phenomena resulted in a close interconnection between adjacent pellets, corresponding to a high SI. With the increase in TiO2 addition to 5wt% (Fig. 6(b)), the dense interconnection structure between metallic iron was destroyed, as manifested by the decreased interconnection size and strength between metallic iron. In addition, the particle structure showed a larger void volume than those without TiO2 addition. These observations were attributed to the formation of titanium-containing oxides (FeTiO3) that were not further reduced on the sticking surface of pellets. This phenomenon could be further explained by XRD analysis (Fig. 5). The formation of FeTiO3 weakened the aggregation of newly formed metallic iron between the contact surfaces of adjacent pellets, thus shrinking the contact surfaces between metallic iron and resulting in the insufficient development of the interconnection between metallic iron. Furthermore, the formation of FeTiO3 replaced some of the sticking points originally belonging to the metallic iron between the contact surfaces of adjacent pellets. As a result, the amount of connection points between metallic iron decreased. This phenomenon also simultaneously caused a large distance between adjacent metallic iron interconnections; therefore, the void volume of particle structure increased. The aforementioned factors inhibited the sticking behavior of pellets and consequently decreased the SI. Fig. 6(c) displays that the size and amount of interconnection between metallic iron further decreased with the increase in the FeTiO3 phase at the interface of adjacent pellets when the TiO2 addition reached 10wt%. The particle surface evolved to be rough, and the “surface to surface” interconnections of metallic iron also showed the trend of transforming to be “point to point” ones. The SI further decreased at this moment. When the TiO2 addition was further increased to 15wt% (Fig. 6(d)), a small number of interconnections between metallic iron, which were previously developed insufficiently, finally formed and grew in a large void volume structure. This phenomenon ultimately resulted in a low SI.
The cross-section images of SEM and EDS analysis of the sticking interface of pellets (signed as dashed line) at the TiO2 additions of 0 and 15wt% are displayed in Fig. 6(e) and Fig. 6(f), respectively, to further clarify the reasons for the alleviated sticking behavior of pellets during reduction with increased TiO2 addition.
Fig. 6(e) indicates that the interconnected metallic iron exhibited a relatively dense structure and formed a large number of uniform continuous areas on the sticking interface without TiO2 addition. A large SI was obtained at this moment. When 15wt% of TiO2 was added (Fig. 6(f)), the uniformly and continuously distributed dense structure of the interconnected metallic iron in the sticking interface was evidently destroyed and developed into a porous structure, leading to the separation of particles. Furthermore, the aggregated metallic iron particles shrunk with strong dispersibility, and only a part of them eventually aggregated and connected with each other. This finding indicated that the growth and interconnection of metallic iron particles at the sticking interface were suppressed and had a positive effect on relieving the sticking behavior of pellets upon TiO2 addition. Fig. 6(g) is a partially enlarged view of Fig. 6(f), showing the interruption between the interconnections of metallic iron. Fig. 6(h) is the line scan result of Fig. 6(g), showing that both ends of the fracture section of the sticking interface were FeTiO3 phases. EDS analysis of points A and B in Fig. 6(i) and (j) indicated that the metallic iron particles (white) contained no titanium. Much of the titanium was found in the iron oxides, accompanied by a small number of slag phases (dark gray) instead of entering into the metallic iron phases. XRD results (Fig. 5) showed that titanium mainly existed in the form of FeTiO3. The metallic iron particles were closely wrapped by the titaniferous phase of FeTiO3 at the fracture section of the sticking interface between adjacent pellets (Fig. 6(g)), in which the borderlines between metallic iron and FeTiO3 were evident. Nevertheless, this phenomenon negatively affected the aggregation of metallic iron and the formation of interconnection bridges for metallic iron at the sticking interface, resulting in a decreased amount of interconnection points and sizes between metallic iron. Thus, the SI of pellets decreased. The above findings were highly compatible with the results in Fig. 4.
On the basis of these experimental results, a diagram was drawn to visualize how TiO2 inhibits the sticking behavior of pellets during reduction, as shown in Fig. 7. The interconnection points of metallic iron between adjacent pellets were extraordinarily intricate and firm without TiO2 addition. In addition, the void volume between adjacent pellets was small at this point. All these phenomena led to close interconnections between adjacent pellets, resulting in a high SI.
When TiO2 was added, the stable FeTiO3 particles tightly enclosed around the metallic iron grown in the sticking interface of adjacent pellets, preventing the metallic iron from forming an interconnection bridge between each other and achieving an interconnected structure. As a result, partially strong metallic iron interconnections were destroyed, resulting in a decrease in the number and strength of metallic iron interconnections. The increase in void volume at the sticking interface loosened the structure. All of the above phenomena weakened the sticking strength between adjacent pellets. With a further increase in the amount of TiO2 added, the number and strength of metallic iron interconnections further decreased, and a large void volume structure grew, leading to a further decrease in the SI of pellets. Therefore, TiO2 addition was not conducive to the development of the interconnections of metallic iron particles at the sticking interface and finally inhibited the sticking behavior of pellets to a certain extent.
The effect of reduction temperature from 900to 1100°C on the SI of pellets during reduction was studied as shown in Fig. 8(a). The SI showed an upward trend with the increase in reduction temperature at all TiO2 addition amounts. With the reduction temperature rising from 900 to 1100°C, the SI with 0 and 15wt% TiO2 additions increased from 0.71% to 59.91% and from 0.68% to 35.56%, respectively. In particular, all the SIs exhibited a similar slight increase with the reduction temperature ranging from 900 to 950°C. Increasing the reduction temperature from 900 to 950°C without TiO2 addition led to a slight increase in the SI from 0.71% to 3.14%. However, the SI increased sharply from 3.14% to 59.91% when the reduction temperature was further increased from 950 to 1100°C without TiO2 addition. Therefore, the reduction temperature of 950°C was defined as the critical temperature for the sticking tendency of pellets. Moreover, previous study was shown that the sticking force between adjacent pellets was dominantly influenced by the surface energy of newly formed metallic iron, which increased with the reduction temperature [23]. This phenomenon could cause the surrounding metallic iron to continuously accumulate and worsen the sticking behavior of pellets. Thus, SI increased with the increase of reduction temperature, which was consistent with the results in Fig. 8(a).
TiO2 showed different influences on the SI with the change of reduction temperature, as shown in Fig. 8(b). When the reduction temperature was 900°C, all the SIs for every TiO2 addition tended to be zero, demonstrating that the sticking behavior would not occur. The increase in reduction temperature would lead to the increasingly significant influence of TiO2 addition on the SI. When the reduction temperature was further increased to 1100°C, the SI was 59.91%, 51.68%, 45.35%, and 35.56% with TiO2 additions of 0, 5wt%, 10wt%, and 15wt%, respectively. This phenomenon could be attributed to the inherently small SI at low reduction temperature, which covered up the effect of TiO2 on sticking behavior. By contrast, the influence of TiO2 was amplified at a high temperature. Therefore, increasing TiO2 addition could decrease the SI in a relatively high-temperature range.
Fig. 9 shows the XRD spectra of the sticking surface for the analysis of the phase changes at reduction temperatures from 900 to 1100°C. With the increase in reduction temperature, the characteristic diffraction intensity peak of FeO gradually decreased and completely disappeared at 1100°C. This finding indicated that FeO could be fully reduced at 1100°C. On the contrary, the characteristic diffraction peak intensity of iron increased gradually and reached the maximum at 1100°C. The above results showed that the gradually decreasing FeO phase slowly transformed into the iron phase during reduction.
Studying the influence of reduction temperature on the sticking behavior of pellets is of great importance. Therefore, the intrinsic morphology evolution of sticking interfaces at different reduction temperatures was analyzed by SEM−EDS. The morphologies of the sticking interfaces formed at reduction temperatures of 900, 950, 1000, and 1100°C (the reduction time was set at 60 min) is presented in Fig. 10(a), (b), (c), and (d), respectively. The interconnection between particles on the sticking interface also varied with the reduction temperature.
Fig. 10(a) shows that the surface structure of metallic iron particles on the sticking interface was irregular, and the initial shape of the fibrous metallic iron began to change. In addition, the growth direction of this fibrous metallic iron was not unidirectional; that is, it showed a tendency for multidirectional irregular growth. The sticking behavior at 900°C was initiated by the “needles to needles” contact and mutual attachment of fibrous metal irons whose small number and low strength of contact resulted in slight sticking, which mostly appeared at a low temperature. When the reduction temperature rose to 950°C (Fig. 10(b)), the connection points between metallic iron began to increase, and the original “needles to needles” interconnections between the metallic iron gradually grew into a relatively strong “point to point” one. This finding demonstrated that the increase in reduction temperature was conducive to the diffusion and crystallization of newly formed metallic iron. Thereby, the SI of pellets increased compared with that at 900°C. As shown in Fig. 10(c), the interconnection points between metallic iron became complicated and numerous. The development and thickening of these interconnections were gradually dominated by the form of “surface to surface,” which corresponded to a high strength. Thus, the sticking tendency was enhanced. The newly formed interconnections of metallic iron sufficiently agglomerated and grew up between the contact surfaces of adjacent pellets, eventually forming clustered metallic iron agglomerates with the reduction temperature increasing to 1100°C, as shown in Fig. 10(d). These metallic iron agglomerates were numerous and stable, making them difficult to destroy, and were closely connected to the adjacent pellets, resulting in the highest SI. These morphological observations of sticking interfaces corresponded to the analysis in Fig. 8.
The cross-section microstructures between the sticking interface of pellets at different reduction temperatures were analyzed by SEM to further reveal the relationship between sticking behavior and reduction temperature. Fig. 10(e), (f), (g), and (h) illustrates the cross-section microstructures of the sticking interfaces formed at 900, 950, 1000, and 1100°C, respectively.
The above results confirmed that the sticking phenomenon between adjacent pellets was mainly dependent on the sticking of slag phases at 900°C. Only a few metallic iron interconnection points appeared between the sticking interface at this time, as shown in Fig. 10(e). This finding could be interpreted as the loose structure of the sticking interface at this point, and the distance between metallic iron particles of adjacent pellets restrained their attachment, which was the primary reason why the slag phases were the main connection phase of adjacent pellets. Thus, these pellets exhibited a low SI. When the reduction temperature rose to 950°C (Fig. 10(f)), the structure of the sticking interface between adjacent pellets was initially densified when the attachment between metallic iron became the dominant factor in the sticking behavior, leading to the increase in SI. When the reduction temperature was further increased to 1000°C (Fig. 10(g)), the connections between metallic iron gradually thickened, accompanied by an increase in connection points with the steady diffusion and crystallization of metallic iron. In addition, partial aggregation occurred between the metallic iron in the sticking interface, indicating that the sticking behavior between adjacent pellets worsened. Fig. 10(h) shows the cross-section microstructures of the sticking interface between adjacent pellets at 1100°C. The connection strength and points between metallic iron further increased, and the metallic iron closely aggregated, resulting in high densification at the sticking interface and eventually leading to the highest sticking strength.
Fig. 11 shows the SEM−EDS analysis of the cross-section microstructures between the sticking interface of pellets at different temperatures and TiO2 additions. Fig. 11(a) and (c) suggests that the internal structure between the sticking interface was generally unchanged with the increasing TiO2 addition at 900°C. Meanwhile, the slag phase was the main connection phase between adjacent pellets, accompanied by a low sticking strength. Thus, the influence of TiO2 on the internal structure between the sticking interface was not significant at a low reduction temperature. By contrast, Fig. 11(b) and (d) shows that when the TiO2 addition increased from 0 to 15wt% at 1100°C, the strength and quantity of metallic iron connections between the sticking interface decreased, resulting in a decrease in sticking strength. The amount of TiO2 addition was inversely proportional to SI. Therefore, the influence of TiO2 on the internal structure between the sticking interface was significant at a high reduction temperature.
In this work, the influence of TiO2 addition and reduction temperature on the sticking behavior of pellets based on direct reduction of gas-based shaft furnaces were studied. The experimental conclusions are summarized as follows.
(1) In direct reduction, TiO2 can effectively inhibit the sticking behavior of pellets. The SI of pellets decreased with the increase in TiO2 addition. SI decreased from 59.91% to 35.56%, with the TiO2 addition amount increasing from 0 to 15wt%. The mechanism by which TiO2 improved the sticking behavior of pellets was that the metallic iron particles were closely wrapped by FeTiO3 at the sticking interface, leading to the destruction of the original metal iron interconnections. As a result, the number and strength of metallic iron interconnections decreased, and the void volume at the sticking interface increased. Therefore, SI decreased. An inversely proportional relationship was found between TiO2 addition and SI.
(2) The SI of pellets increased with the reduction temperature. The SI was 0.71% and 59.91% at reduction temperatures of 900 and 1100°C, respectively. When the temperature exceeded 950°C, the SI increased sharply with the reduction temperature increasing.
(3) The sticking behavior at low reduction temperature mainly depended on the connection of the slag phase at the interface between adjacent pellets, accompanied by a low sticking strength. The interconnection of metallic iron became the dominant factor when the reduction temperature increased. Meanwhile, the interconnection strength and points between metallic iron further increased, leading to an increase in SI. In addition, the surface energy of newly formed metallic iron increased with the temperature, which also led to an increase in SI.
(4) Compared with that at low reduction temperature, TiO2 had a great effect on the SI of pellets at high reduction temperature. The SI was 0.71% and 0.68% at 900°C when the TiO2 addition reached 0 and 15wt%, respectively, but these changes were insignificant. Meanwhile, the SI decreased from 59.91% to 35.56% at 1100°C when the TiO2 addition increased from 0 to 15wt%, and these changes were significant. Therefore, increasing TiO2 addition could decrease the SI in a relatively high-temperature range.
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