International Journal of Minerals, Metallurgy and Materials Accepted manuscript, https://doi.org/10.1007/s12613-019-1903-9 © University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020
The formation of calcium titanate in the carbothermic reduction of vanadium titanomagnetite
concentrate by adding CaCO3
Xiao-hui Li1, Jue Kou 2,*, Ti-chang Sun3, Shi-chao Wu4, Yong-qiang Zhao5
School of Civil and Resource Engineering, University of Science and Technology Beijing
100083, China
Corresponding author, [email protected]
Abstract: The formation of calcium titanate in the carbothermic reduction of vanadium
titanomagnetite concentrate (VTC) by adding CaCO3 was investigated. Thermodynamic analysis shows
the feasibility of calcium titanate formation by the reaction of ilmenite and CaCO3 in a reductive
atmosphere, where ilmenite is more easily reduced by CO or carbon in the presence of CaCO3. The
effects of CaCO3 dosage and reduction temperature on the phase transformation and metallization
degree were also investigated in the actual roasting test. Appropriate increase of CaCO3 dosages and reduction temperatures are found to be conducive to the formation of calcium titanate, and the optimum
conditions were CaCO3 dosage of 18wt% and reduction temperature of 1400℃ . Additionally, scanning electron microscopic-energy dispersive spectrometric (SEM-EDS) analysis shows that calcium titanate
produced via carbothermic reduction of VTC by adding CaCO3 is of higher purity with particle size approximate 50μm, and hence the separation of calcium titanate and metallic iron will be the focus in the future study.
Keywords: VTC, CaCO3, carbothermic reduction, metallic iron, calcium titanate
1. Introduction
Calcium titanate, is a typical functional ceramic materials, widely used in electronic devices and
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red luminescent materials owing to its good dielectric characteristics of high dielectric constant, low dielectric loss, and positive temperature coefficient [1], it is also established as one of the important basic materials of ferroelectric ceramics [2]. Various methods for calcium titanate synthesis, including solid phase reaction, mechanical synthesis, sol-gel and hydrothermal synthesis, have been developed in the literatures in past years [3-4]. In solid-state synthesis, calcium titanate can be prepared via
conventional solid-state reaction between TiO2 and CaCO3 or CaO at a temperature of approximately
1623K [5]. This method has been widely used in industrial production owing to its advantages of high
yield, simple process and mature technology, but the process has higher requirements for TiO2 purity
[1]. TiO2 in nature exists mainly in the form of titanate in minerals, and the Ti-bearing minerals that can be used as industrial raw materials are primarily ilmenite, vanadium-titanium magnetite and rutile [6].
TiO2 is mainly prepared from ilmenite in the industry by sulfuric acid or chlorination method owing to the low reserves of natural rutile, but these methods cause serious environmental pollution [7, 8]. It is
obvious that the process for obtaining TiO2 is complex, costly and environmentally unfriendly, and hence it is necessary to determine an environment-friendly and low-cost alternative for the preparation process of calcium titanate.
Vanadium titanomagnetite which contains many useful elements such as Fe, Ti, and V, is an associated and coexisting mineral, with high comprehensive utilization value [9, 10]. The vanadium titanomagnetite deposit in Panxi, Sichuan province, China, is an important source of Fe and Ti, and accounts for approximately 83wt% of the total reserves in the country [11]. However, the vanadium titanomagnetite in Panxi has high Fe and low Ti contents, consequently, during conventional grinding
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process, approximately 53wt% of Ti enters into the Fe concentrate, and blast furnace slag containing
20wt%~25wt% of titanium is formed after blast furnace ironmaking [12,13], leading to underutilization of titanium resources.
In recent decades, significant researches have been carried out to improve the utilization of vanadium titanomagnetite [14, 15]. Several new processes, such as pyro-metallurgical and hydro-metallurgical processing [16, 17] and solvent extraction [18], have been studied. However, these processes are neither techno-economically feasible nor environmentally viable. With its advantages of a relatively short process, simple operation, and low energy consumption, carbothermic reduction has become one of the important processes for ironmaking and titanium recycling [19, 20]. In this process,
Fe is preferentially reduced to metallic iron, while Ti still exists in the form of oxides, which are
conducive to separate them through magnetic separation, and titanium slag of TiO2 content exceeding
40wt% could be obtained after magnetic separation [21-25]. However, the titanium slag obtained from
this process was still not a qualified raw material for the production of TiO2 because of high impurity contents. It is obvious that there remain many problems in the separating of iron and titanium slag by the process of carbothermic reduction.
To further promote the carbothermic reduction of vanadium titanomagnetite, the effects of
additives were also investigated [26, 27]. Jiang et al. [28] demonstrated that the addition of 3wt% CaF2 in the reduction of vanadium titanomagnetite could effectively improve the metallization rate of the reduced product. Jung et al. [29] concluded that the activation energy for the carbothermic reduction of titanomagnetite was greatly decreased with the addition of CaO, which improved the reducibility of
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titanomagnetite. Zhou et al. [30] showed that the addition of CaO could promote the formation of calcium ferrite, which significantly increased the porosity between VTC and coal and promoted the reduction process. It could be found that the addition of calcium compounds were conducive to the carbothermic reduction of vanadium titanomagnetite.
Nowadays, relevant studies [31-33] found that mixing VTC with 12wt% MgO can reduce Fe to metallic iron and transform Ti to magnesium titanate via carbothermic reduction at 1200℃. This provided a new possibility for realizing the separation of Ti and Fe effectively. However, the purity of magnesium titanate was low for Mg and Fe were easy to exist in the form of isomorphism. Therefore,
CaCO3 was chosen as the additive in the carbothermic reduction of VTC in this study, and the feasibility of preparing calcium titanate was investigated by thermodynamic analysis, X-ray diffraction
(XRD), gas analysis and scanning electron microscopy and energy dispersive spectrometry
(SEM-EDS), with the objective of providing a new way for the utilization of Ti resources in VTC and simplifying the preparation of calcium titanate.
2. Experimental
2.1 Materials
The vanadium titanomagnetite concentrate used in this study was obtained from Panxi, Sichuan province, China. It was treated by the low intensity magnetic separation to obtain VTC. Chemical
analysis results are shown in Tab. 1, in which TFe and TiO2 contents are 58.80wt% and 12.75wt%, respectively.
Fig. 1 shows the X-ray diffraction (XRD, Rigaku D/Max 2500, Japan) pattern of VTC, where Fe
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exists mainly in the form of magnetite(Fe3O4), Ti is mainly present in the form of ilmenite (FeTiO3),
and the impurity elements exist mainly in the form of chlorite (Mg, Fe)5(Si3Al2)O10(OH)10.
CaCO3 used in this study was analytically pure, which was sourced from Sinopharm Chemical
Reagent Co Ltd, China.
Tab. 1. Chemical compositions of the VTC (wt%).
Component TFe FeO TiO2 CaO MgO Al2O3 SiO2 V2O5 MnO
Content 56.80 30.14 12.75 0.57 2.44 3.60 2.21 0.56 0.32
Fig. 1. XRD patterns of VTC.
The reductant was bituminous coal with particle size of less than 1 mm, and a composition of
56.66wt% fixed carbon, 6.55wt% ash, 29.54wt% volatile matter and 7.25wt% moisture.
2.2 Experimental procedure
The experiment comprised five main parts: thermodynamic analysis, mixing and pelletizing, reduction roasting, XRD and gas analysis, and SEM-EDS analysis. The specific test flow sheet and crucible embedding method are shown in Fig. 2.
Mixing and pelletizing: 100g of VTC was mixed with 18wt% CaCO3 and water was added to
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produce pellets with diameters of 8~10mm. These pellets were dried at 100℃ for 5h to obtain green pellets.
Reduction roasting: First, three green pellets together with 12.6g of bituminous coal were placed in graphite crucibles (50 mm in diameter and 70 mm in height) with lids. The green pellets were completely embedded in the bituminous coal to ensure sufficient reduction atmosphere. The crucibles were then placed inside a furnace and maintained for 150min at the target temperature. After heating, the crucibles were taken out from the furnace when the temperature decreased to 60℃, and cooled to room temperature in air to obtain roasted pellets.
1
2
3
1-crucible lid 2-pellets 3- bituminous coal
Fig. 2. Test flow sheet and crucible embedding method.
Gas analysis: To further understand the effect of CaCO3 on the reductive atmosphere during the preparation calcium titanate via carbothermic reduction of VTC, the changes in the concentration of
CO and CO2 were analyzed using a gas analyzer (MRU Messgeräte für Rauchgase und Umweltschutz
GmbH, Neckarsulm, Germany).
XRD and SEM-EDS analyses: A roast pellet was cut along the center. One half was ground to
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80wt% -0.074 mm to analyze the changes in mineral composition by XRD (Rigaku D/Max 2500,
Japan). The other half was used as an electron microscope sample to analyze the microstructural changes and the formation and distribution of calcium titanate by SEM-EDS (CAMBRIDGES-360,
VO18, Carl Zeiss, Germany). Next, the metallization rate (Rm) of the reduced product was calculated
using the equation: Rm = ωMFe / ωTFe*100 %.
where: ωTFe is mass percent of total iron in the reduced product, (wt%); and ωMFe is mass percent of metallic iron in the reduced product, (wt%).
3. Results and discussion
3.1 Thermodynamic analysis of calcium titanate preparation via carbothermic reduction of VTC in the
presence of CaCO3
The material analysis showed that Fe and Ti in VTC mainly existed in the form of magnetite
(Fe3O4) and ilmenite (FeTiO3), respectively. The preparation of calcium titanate via carbothermic
reduction VTC in the presence of CaCO3 was mainly proceeded via reaction between Ca and Ti, and
hence a thermodynamic calculation between CaCO3 and ilmenite was carried out.
3.1.1 Thermodynamic analysis of ilmenite reduction by carbon in presence of CaCO3
Initially, the reduction of ilmenite in VTC was mainly a solid-solid reaction as there was no
reductant except carbon in the roasting system. According to the relevant literature [31, 35] that
ilmenite in VTC is mainly reduced by reactions (1) to (5), while reaction (6) becomes he most
possible reduction reaction of ilmenite following the addition of CaCO3.
FeTiO3(s)+C(s)→Fe(s)+TiO2(s)+CO(g) (1)
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2FeTiO3(s)+C(s)→FeTi2O5(s)+Fe(s)+CO(g) (2)
FeTi2O5(s)+C(s)→Fe(s)+2TiO2(s)+CO(g) (3)
3FeTiO3(s)+4C(s)→3Fe(s)+Ti3O5(s)+4CO(g) (4)
FeTiO3(s)+2C(s)→Fe(s)+TiO(s)+2CO(g) (5)
FeTiO3(s)+CaCO3(s)+CO(g)→CaTiO3(s)+Fe(s)+CO2(g) (6)
(CaCO3(s)+C(s)→CaO(s)+2CO(g) CaO(s)+TiO2(s)→CaTiO3(s))
Based on the reaction web of the thermodynamic software, Factsage, the standard Gibbs free
θ energy (rGm ) values of reactions (1) to (6) at temperature are calculated and shown in Fig. 3.
θ Fig. 3. The relationship between rGm and Temperature of reactions (1) to (6).
θ Fig. 3 shows that reaction (5) cannot be carried out spontaneously in theory because their rGm values are always greater than zero when the temperature increased from 400℃℃ to 1500 . Point A is the intersection of reactions (1), (2), (3) and (4), it is obvious that these reactions cannot proceed
θ spontaneously before 950℃ as the rGm of reactions (1) to (4) are greater than zero at this temperature range, whereas these reactions can be carried out spontaneously when the temperature is higher than
950℃ . Similarly, Point B shows that reaction (6) can proceed spontaneously when the temperature
θ higher than 640℃ because the rGm of reaction (6) is less than zero. Moreover, it can be found from
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θ observing reaction curves (1) to (6) that the rGm of reaction (6) is much smaller than those of reactions (1) to (5) when the temperature is higher than 550℃, which indicates that the reaction (6) is more likely to occur than reactions (1) to (5) at the same temperature and it maybe take precedence
over other reactions in the carbothermic reduction of VTC with adding CaCO3. In other words, ilmenite
is much easier to be reduced in the condition of CaCO3 presenting, and calcium titanate could also be formed via reaction (6).
3.1.2 Thermodynamic analysis of ilmenite reduction by CO in presence of CaCO3
As the reaction proceeded, the reduction of ilmenite in VTC was transformed from a solid-solid
reaction to a solid-gas reaction owing to the increasing content of CO in the roasting system. Ilmenite
in VTC was mainly reduced via reactions (8) to (12) without additives, while reaction (13) became
the most possible reduction reaction of ilmenite following the addition of CaCO3, with calcium
θ titanate being produced simultaneously. The reaction web of Factsage was used to calculate the rGm
values of reactions (7) to (13), and the gas phase equilibrium diagram was drawn using the
relationship between CO concentration (ψ(CO)) and reaction equilibrium constant (K) at different
temperatures under the assumption that the summation of CO and CO2 partial pressures to be 1.
Among them, the CO concentration required for reactions (8) to (13) were calculated by Eq. (14), that
required for reaction (7) were calculated by Eq. (15). The results are shown in Fig. 4.
C(s)+CO2(g)→2CO(g) (7)
FeTiO3(s)+CO(g)→Fe+TiO2(s)+CO2(g) (8)
2FeTiO3(s)+CO(g)→FeTi2O5(s)+Fe(s)+CO2(g) (9)
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FeTi2O5(s)+CO(g)→Fe(s)+2TiO2(s)+CO2(g) (10)
3FeTiO3(s)+4CO(s)→3Fe(s)+Ti3O5(s)+4CO(g) (11)
FeTiO3(s)+2C(s)→Fe(s)+TiO(s)+2CO(g) (12)
FeTiO3(s)+CaCO3(s)+CO(g)→CaTiO3(s)+Fe(s) +CO2(g) (13)
p 100 CO2 (CO2 ) K (CO) % p (CO) 1 K CO , (14)
2 2 p (CO) 4 K CO (CO) 50K 1 1 % p 100 (CO ) K CO2 2 , (15)
Fig. 4 shows that the ψ(CO) value curves of reactions (11) and (12) are all close to 100vol% and are approximately coincident in the temperature range of 400℃ to 1500℃, which indicates that reactions (11) and (12) are more difficult to occur in theory. And the ψ(CO) values of reactions (6), (7) and (8) are all higher than that of reaction (9) in this temperature range, which indicates that the CO
concentration required for ilmenite reduction without the addition of CaCO3 is much higher than that
required in the presence of CaCO3. In other words, higher ψ(CO) values (approximately 93vol% and
95vol%) were required for reducing ilmenite to FeTi2O5 or TiO2, whereas lower ψ(CO) values were
required for transforming ilmenite to calcium titanate, which indicated that the addition of CaCO3 promoted the reduction of ilmenite.
Fig. 4. Gas equilibrium phase diagram of reducing ilmenite by CO.
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It was concluded from the thermodynamic analysis that calcium titanate could be prepared during
carbothermic reduction of VTC by adding CaCO3. The addition of CaCO3 would change the
transformation of Ti in VTC, the transformation of Ti occurred as FeTiO3→FeTi2O5 in the absence of
additives, which changed to FeTiO3→CaTiO3 following the addition of CaCO3, and the latter was
more conducive to the reduction of ilmenite.
3.2 Study of factors influencing calcium titanate preparation during the actual roasting test
Thermodynamic analysis showed that it was feasible to prepare calcium titanate in carbothermic
reduction of VTC by adding CaCO3. To determine the effects of CaCO3 dosage and reduction temperature in the actual roasting test, the preparation of calcium titanate in carbothermic reduction of
VTC by adding CaCO3 was investigated under the conditions of bituminous dosage of 60wt% and reduction time of 150 min.
3.2.1 Effect of CaCO3 dosage
Firstly, the effects of CaCO3 dosages on the transformation of Ti and Fe in the roasting product
were studied. The results are shown in Fig. 5.
Fig. 5. Effects of CaCO3 dosages on phase compositions of reduction products.
Fig. 5 shows that CaCO3 dosage has significant influence on the existence of Fe and Ti in the
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reduction products. In the absence of CaCO3, Fe and Ti in the reduction products were mainly present
in the form of metallic iron (Fe) and anosovite (FeTi2O5), respectively. The material analysis revealed
that Fe and Ti in VTC mainly existed in the form of magnetite (Fe3O4) and ilmenite (FeTiO3),
respectively, which indicated that in the absence of CaCO3, magnetite in VTC had been completely reduced to metallic iron, and ilmenite has been reduced to anosovite containing mostly unreduced Fe.
It was obvious that ilmenite had not been completely reduced under this condition. However, after the
addition of CaCO3, the diffraction peaks of anosovite disappeared and those of calcium titanate
appeared in the reduction products, which indicated that CaCO3 added had reacted with ilmenite in
VTC via reaction (13) and formed calcium titanate, further confirmed that the feasibility of preparing
calcium titanate during carbothermic reduction of VTC by adding CaCO3. Moreover, the diffraction peaks of calcium titanate and metallic iron in the reduction product increased with the increase of
CaCO3 dosages, which indicated that the formation of calcium titanate could change the reduction path of ilmenite in VTC and promote it to be directly reduced to metallic iron via reaction (10).
Obviously, the formation of calcium titanate was conducive to the reduction of ilmenite, which was consistent with the results of thermodynamic analysis. Additionally, it could be seen from Fig. 5 that the diffraction peaks of pyroxene appeared in addition to those of metallic iron and calcium titanate
when the dosage of CaCO3 greater than 18wt%, which indicated that 18wt% of CaCO3 was the
optimum value and excessive addition of CaCO3 might hinder the reduction of Fe.
To ensure the comprehensive utilization of Ti and Fe resources in VTC, the process of preparing
calcium titanate during the carbonthermic reduction of VTC by adding CaCO3, should also ensure
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that Fe reduction could achieve good effect. Hence, the chemical analysis of the reduction products
was carried out at different CaCO3 dosages. The results are shown in Fig. 6.
Fig. 6. Effect of CaCO3 dosages on metallization rate in reduction products.
Fig. 6 shows the metallization rate of reduction product at different CaCO3 dosage at a reduction temperature of 1300℃ and reduction time of 150 min. In the absence of additives, the metallization
rate of reduction product was 88.37wt%. In the presence of CaCO3, it first increased and then
decreased with increase in CaCO3 dosage, and peaking was 91.29wt% at a CaCO3 dosage increased to
18wt%. It was obvious that the addition of CaCO3 could improve the metallization rate of the reduction products. This was sufficient to confirm that the formation of calcium titanate had no adverse effect on the reduction of Fe in VTC, on the contrary, it could promote the metallization rate of the reduction products.
3.2.2 Effect of reduction temperature
Thermodynamic analysis revealed that higher temperature was conducive to the reaction of
CaCO3 and ilmenite. To investigate the effect of temperature on the formation of calcium titanate in
carbothermic reduction of VTC by adding CaCO3 in the actual experiment, XRD analysis of the
reduction products at 900℃ to 1450℃ was carried out under the conditions of CaCO3 dosage of
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18wt% and reduction time of 150 min. The results are shown in Fig. 7.
Fig. 7. Effect of reduction temperature on phase composition in reduction products.
Fig. 7 shows that the reduction temperature has significant effect on the process of calcium
titanate formation. At 900°C, Fe in the reduction product mainly existed in the form of metallic iron
(Fe); Ti existed only as ilmenite (FeTiO3), which indicated that ilmenite had not reacted with CaCO3
at this temperature. The diffraction peak of calcium titanate began to appear in the reduction product
when the temperature was raised to 1100°C, which indicated that the increase in temperature
promoted reaction (13) and the formation of calcium titanate in the roasting system, but that ilmenite
could not be completely converted into calcium titanate because the temperature was lower than
required. Although the initial temperature of calcium titanate formation in the actual test was higher
than that theoretically calculated using Factsage, the feasibility of preparing calcium titanate in
carbothermic reduction of VTC by adding CaCO3 was still established. Moreover, the diffraction
peaks of calcium titanate and metallic iron in the reduction products gradually increased with the
increase of reduction temperature and were the strongest at 1400°C. Hence, the temperature was set
as 1400°C for the preparation of calcium titanate via the carbothermic reduction of VTC in the
presence of CaCO3.
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Similarly, to determine whether good reduction effect of Fe in VTC was achieved, chemical
analysis was conducted on the reduction product at different temperatures. The results are shown in
Fig. 8.
Fig. 8. Effect of temperature on metallization rate in reduction products.
Fig. 8 shows the metallization rate of the reduction product at different temperatures at a CaCO3 of 18wt%. The metallization rate of the reduction product increased gradually with temperature, rising from 70.95wt% to 84.88wt%, 91.29wt%, 93.24wt% and 94.52wt% when the reduction temperature was increased from 900°C to 1400℃, representing a total increase of 23.57wt%. However, the metallization rate did not change significantly and was stable at approximately 95wt% at temperatures higher than 1400℃ . This result indicated that the reduction effect of reduction products was already good at 1400℃, and that a continuous increase of temperature had little effect on the metallization rate.
It was concluded from above analyses that it was feasible to prepare calcium titanate during the
carbothermic reduction of VTC by adding CaCO3. The actual roasting test results showed that the formation of calcium titanate had no adverse effect on the reduction of Fe in VTC, on the contrary, it
augmented the metallization rate of reduction product, and the optimum conditions were CaCO3 dosages of 18wt% and reduction temperature of 1400℃ .
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3.3 Gas concentration in reduction system
It could be known from above analyses that the formation of calcium titanate could promote the
metallization rate of the reduction products, indicating that the addition of CaCO3 was conducive to the
reduction of Fe minerals. Zhao [36] showed that the addition of CaCO3 in the direct reduction of
hematite could effectively increase the concentrations of CO and CO2 in the roasting system and
promote the reduction of hematite. It is obvious that higher CO and CO2 concentrations were
conducive to the reduction of Fe minerals. To determine the effects of CaCO3 on the gas concentration
in the roasting system, the concentrations of CO and CO2 in the roasting system with or without the
addition of CaCO3 were analyzed. The results are shown in Fig. 9.
Fig. 9. Changes in the concentration of CO2 and CO in the roasting system.
Fig. 9 (a) shows the changes in gas concentration in the reduction system under different
conditions. The CO and CO2 concentration curves in the reduction of VTC with or without the addition
of 18wt% CaCO3 exhibited the same trend. In the initial stage of the reaction, the concentrations of CO
and CO2 in the roasting system were close to zero in the presence as well as absence of CaCO3 at
temperatures lower than 465℃. This result indicated that the addition of 18wt% CaCO3 had no effect on the carbothermic reduction of VTC at lower reduction temperatures. The carbothermic reduction of
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VTC entered the second stage when the temperature increased from 465℃ to 925℃, and the
concentration of CO and CO2 in the roasting system began to increase slowly at this time. This was because the reduction in the initial stage of the roasting system was dominated by the solid-solid
reaction, wherein VTC was first reduced by C and simultaneously produced CO. At the same time, CO2 could be produced in the roasting system. Generally, as the contact surface for gas reduction was much
larger than that for solid reduction, the CO2 produced could generate CO via the carbon gasification reaction (7). The rate of was reaction (7) very low at lower temperatures, and hence the concentration
of CO2 was higher than that of CO at this reduction stage. The temperature gradually increased as the reaction continued, and the carbothermic reduction of VTC entered the third stage when the temperature was higher than 925°C. At this time, the rate of the carbon gasification increased
significantly, rendering the concentration of CO higher than that of CO2 in this stage.
Similarly, Fig. 9 (b) shows that two peaks are observed in the roasting system whether CaCO3 was added or not, where first peak is mainly due to the release of volatile matter in the roasting temperature
range from 465℃ to 925℃ . Then, the CO/(CO+CO2) volume ratio increased rapidly to the second peak, mainly because the gasification of char, i.e., the Boudouard reaction [22]. Moreover, it can be
observed from Fig. 9 (a) and Fig. 9 (b) that the concentrations of CO and CO2 in the roasting system
with the addition of 18wt% CaCO3 were significantly higher than those in the absence of CaCO3 at different roasting stages, and the carbothermic reduction reaction of the former started earlier than that
of the latter. This was owing to the fact that the addition of CaCO3 would decompose some CO2 during the preparation of calcium titanate via carbothermic reduction of VTC, which could promote
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gasification and enhance the reduction atmosphere in the roasting system.
In summary, the addition of CaCO3 in the carbothermic reduction of VTC played a key role in two
aspects. The first role of CaCO3 was to provide CO2 gas released from the reduction reaction, which could promote the gasification and enhance the concentrate of reduction atmosphere in roasting system.
The second role was to provide CaO which could combine with TiO2 and promote the formation of calcium titanate. This result further confirmed the formation of calcium titanate had no adverse effect on the reduction of Fe in VTC, conversely it could enhance the metallization rate in reduction product.
3.4 Microstructure of the reduction product
The above analyses established the feasibility of preparing calcium titanate in carbothermic
reduction of VTC by adding CaCO3. To further investigate the formation and distribution of calcium
titanate, SEM-EDS analysis was conducted on the reduction product under the conditions of CaCO3 dosage of 18wt%, reduction temperature of 1400°C and reduction time of 150min to observe its microstructure and purity. The results are shown in Fig. 10.
It was seen from Fig. 10 (a) and 10 (b) that the brightest particles in the reduction product were
metallic iron with particle size exceeding 50μm and were distributed widely. The darker particles
were calcium titanate with particle size of approximately 50μm and were distributed uniformly. And
From observing Fig. 10 (a1) and 10 (a2), it could be seen that the distribution of elements Ti and Ca in
reduction product tended to coincident in macroscopic state, which indicated that elements Ti and Ca
existed in the same mineral in reduction product, further explaining that there were no other
Ti-bearing compounds except for calcium titanate in the reduction products. Meanwhile, Fig. 10 (c)
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and (d) shows that there were no impurities in the reduction products except for the component element in the metallic iron and calcium titanate particles, which indicated that the calcium titanate particles produced in this process were of higher purity. This result further confirmed the feasibility of
preparing calcium titanate during carbothermic reduction of VTC by adding CaCO3. Hence, the separation of metallic iron and calcium titanate will be the focus in the future studies. Intending to use the method of grinding and low intensity magnetic separation to separate them for there are significant differences in the magnetic properties between metallic iron and calcium titanate.
Additionally, the effects of grinding fineness and magnetic field intensity on the separation of metallic iron and calcium titanate will be also the key in the in the next tests.
(a) (a1) (a2)
+
2 (CaTiO3) +
1 (Metallic Fe) + 50μm (b)
+ 1 (Metallic Fe) +
+ 2 (CaTiO3)
+ 50μm
cps/eV cps/eV 16 Fe Ca (c) 1.4 (d) 14 1 2 1.2 12 Ti 1.0 10
8 0.8
6 Fe 0.6
4 0.4 Ti C 0.2 2 Ca
0 0.0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
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(a), (b)-SEM images of reduction product; (c), (d)-EDS spectra of point 1 and 2; (a1), (b1)-Surface
distributions of elements Ti and Ca
Fig. 10. SEM-EDS analysis of reduction products under optimum reduction conditions
4. Conclusions
(1) Thermodynamic analysis shows the feasibility of forming calcium titanate via the reaction of
ilmenite and CaCO3 in a reductive atmosphere, in which ilmenite is more easily reduced by CO or
carbon in the presence of CaCO3.
(2) The effects of CaCO3 dosage and reduction temperature on the phase transformation and
metallization degree were investigated in the actual roasting test. The form of existence of Ti is
transformed from FeTi2O5 to CaTiO3 with the addition of CaCO3, and the optimum conditions for the
formation of calcium titanate are CaCO3 dosage of 18wt% and reduction temperature of 1400℃ .
(3) Gas analysis shows that the concentrations of CO and CO2 in the roasting system with adding
CaCO3 are significantly higher in the presence of CaCO3 than without, which is conducive to
enhancing the reductive atmosphere.
(4) SEM-EDS analysis shows that the calcium titanate particles obtained approximately 50μm in
size, are of higher purity and are distributed uniformly, which further confirms the feasibility of
forming calcium titanate in the carbonthermic reduction of VTC by adding CaCO3.
Acknowledgment
The authors wish to express their thanks to the National Natural Science Foundation of China
(Grant No. 51674018).for the finance support for this research.
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