International Journal of Minerals, Metallurgy and MaterialsAccepted manuscript, https://doi.org/10.1007/s12613-020-2160-7© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Carbothermic reduction of vanadium titanomagnetite with the

assistance of sodium carbonate

Lu-Ming Chen1,2, Yu-Lan Zhen1* , Guo-Hua Zhang3, De-Sheng Chen1,

Li-Na Wang1, Hong-Xin Zhao1, Fan-Cheng Meng1, Tao Qi1

1National Engineering Laboratory for Hydrometallurgical Cleaner Production

Technology, Institute of Process Engineering, Beijing, China.

2University of Chinese Academy of Sciences, Beijing, China.

3State Key Laboratory of Advanced Metallurgy, University of Science and Technology

Beijing, Beijing, China.

ABSTRACT

In the present study, the carbothermic reduction of vanadium titanomagnetite

concentrates (VTC) with the assistance of Na2CO3 was carried out in argon atmosphere

between 1073K and 1473K. X-ray diffraction (XRD) and scanning electron microscope

(SEM) were used to investigate the phase transformations during the reaction process.

By investigating the reaction between VTC and Na2CO3, it was concluded that molten

* Corresponding author. Tel: 010-82544849. E-mail address: zhenzhen9545@126.com (Yu-Lan

Zhen)

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International Journal of Minerals, Metallurgy and Materials https://doi.org/10.1007/s12613-020-2160-7

2

Na2CO3 could break the structure of titanomagnetite by combining with the acidic

oxides (Fe2O3, TiO2, Al2O3, and SiO2) to form the Na-rich melt, and release FeO and

MgO. Therefore, Na2CO3 could accelerate the reduction rate. In addition, the addition

of Na2CO3 was also beneficial for the agglomeration of iron particles and the slag-metal

separation by decreasing the viscosity of slag. Thus, the Na2CO3 assisted carbothermic

reduction will be a promising method to treat VTC at low temperatures.

KEYWORDS: vanadium titanomagnetite; Na2CO3; phase transformation;

carbothermic reduction; slag-metal separation

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1. Introduction

Vanadium titanomagnetite concentrate (VTC), containing several valuable metallic

elements, is a sort of complex iron ore. In China, about 9.66 billion tons of the VTC is

located in Panzhihua-Xichang region [1]. The titanium resources in this region account

for approximately 95% the whole titanium resources in China, and about 54% of the

titanium resources in VTC after physical separation [2]. So, the efficient comprehensive

utilization of VTC is a hot research topic.

At present, the traditional blast furnace process [3] and the direct reduction-electric

furnace smelting (or magnetic separation) [4-6] are two main methods for efficiently

separating and recovering iron, titanium, and vanadium from VTC. However, these two

processes whose principle is essentially the carbothermal reduction of VTC still have

some technological problems. In the blast furnace process, the Ti(C,N) phase is easily

generated, whereas the formed Ti(C,N) particles cause the increase of the viscosity of

the slag and the hard separation of the iron from slag, due to the good wettability with

molten slags [1,7]. In order to reduce the probability of generating Ti(C,N) phase during

the smelting, ordinary iron ores are blended with the VTC concentrate. However, these

operations reduce the concentration of TiO2 in the slag, and lead to the large difficulty

about the recycle of TiO2 [8,9]. Compared to the blast furnace process, after direct

reduction process, the content of titania in the products is much higher, but the problems

of iron-slag separation and direct-utilization of the titanium-rich phase has not been

solved effectively [5,6,10].

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In recent years, our group proposed a new carbothermic reduction method with the

assistance of alkali (Na2CO3 or NaOH) to reduce VTC at low-temperature (~1473K)

[11-13]. By this process, in the obtained titanium-rich slag, titanium mainly exists in

different sodium titanates. Since sodion (Na+) in the crystal of sodium titanate is easily

exchanged with H+, the highly active, titanium-rich slag can be effectively treated by

acid leaching process [14,15]. However, the effects of Na2CO3 addition and the reaction

mechanism of this process, which are the aims of this study, have not been

systematically studied.

Many related studies on the carbothermal reduction process of iron ore with the

addition of Na2CO3 have been carried out. Chen et al. [16] found that the crystal lattice

of raw VTC concentrate was destroyed and pores were formed after adding Na2CO3,

which accelerated the reduction process and reduced the reaction temperature. Another

effect was also proposed that alkaline additives will promote reduction process by

catalyzing carbon-gas reaction (Boudoir reaction) [17,18]. However, the additive

amounts of Na2CO3 in their work were low (< 5 mass%), and Na2CO3 mainly played a

catalytic role in the carbothermal reduction process, rather than directly participated in

the reaction. When the additive amount of Na2CO3 increases, the interaction of it with

the ore could not be ignored. Some investigations have been carried out on the

interaction mechanism of Na2CO3 and ilmenite during the roasting process [19-22], and

it was concluded that Na2CO3 can react with ilmenite at a temperature above 1123K, to

generate different binary ferrite, titanate, and various ternary Na2O-Fe2O3-TiO2 phases

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[23]. However, there was no clear description of microstructure evolution in the

reaction process. Hence, the reaction mechanism between Na2CO3 and VTC is still

unclear and need to be illustrated. In the present study, the carbothermic reduction

reaction of VTC with the assistance of Na2CO3 will be investigated in detail to elucidate

the effect of Na2CO3 during the reaction process. Meanwhile, the optimal reaction

parameters will be studied.

2. Experimental

2.1 Raw Materials

The vanadium titanomagnetite concentrate was obtained from Panzhihua Iron &

Steel Company, China, with the chemical compositions shown in Table 1. It should be

noted that the total iron content (TFe), FeO content, and TiO2 content were determined

by the chemical titration method, while the content of Fe2O3 was calculated by the

difference of total iron and FeO contents. The contents of other components were

examined by X-ray fluorescence (ShimazuXRF-1800, current 140 mA, voltage 60kV).

The XRD patterns of the VTC concentrate were illustrated in Fig. 1, from which it can

be seen that the main mineral phases of the VTC concentrate were titanomagnetite (Fe3-

xTixO4), with the existence of a small amount of ilmenite (FeTiO3) and pyroxene

(Mg2SiO4). The reagent grade graphite and Na2CO3 were purchased from Sinopharm

Chemical Reagent Co., Ltd, China.

Table 1. Chemical composition of vanadium titanium magnetite concentrate (weight

percent)

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TFe Fe2O3 FeO TiO2 SiO2 CaO Al2O3 MgO V2O5 Others

53.99 43.28 30.51 12.24 1.98 0.95 4.54 5.2 0.53 0.77

Fig. 1. XRD patterns of vanadium titanium magnetite concentrate.

2.2 Experimental procedure

Based on the work of Zhang et al. [12], a high vanadium extraction rate and good

slag-iron separation effect were achieved when the mass ratio of Na2CO3-to-VTC (RN/V)

was 60%. Therefore, herein, RN/V was also set to 60%. The mass ratio of C to VTC

(RC/V) was calculated as follows. According to the main compositions of the vanadium

titanomagnetite concentrate (shown in Table 1), in Na2CO3-C-VTC system, the

possible reactions may include: reductions of iron oxides or titanic oxide (Eq. (1-4));

reaction between the acidic oxides (SiO2) or amphoteric oxides (Al2O3, TiO2, Fe2O3) in

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VTC and Na2CO3 (Eq. (5-8)); thermal decomposition of Na2CO3 (Eq. (9));

carbothermic reduction of Na2CO3 (Eq. (9, 10)), and carbon gasification reaction (Eq.

(11)) [24].

FeO + C = Fe + CO(g) (1)

Fe2O3 + C = 2FeO + CO(g) (2)

TiO2 + 2C = Ti + 2CO(g) (3)

TiO2 + 3C = TiC + 2CO(g) (4)

Na2CO3 + SiO2 = Na2SiO3 + CO2(g) (5)

Na2CO3 + Al2O3 = 2NaAlO2 + CO2(g) (6)

Na2CO3 + TiO2 = Na2TiO3 + CO2(g) (7)

Na2CO3 + Fe2O3 = NaFeO2 + CO2(g) (8)

Na2CO3 = Na2O(l, s) + CO2(g) (9)

Na2O + C = 2Na(g) +CO(g) (10)

CO2(g) + C = 2CO(g) (11)

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8Fig. 2. The changes of standard Gibbs free energy as function of temperature: (a)

(a)

(b)

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carbothermic reduction reactions (Eqs. (1-4, 10)); (b) reactions between the oxides in

VTC and Na2CO3 (Eqs. (5-8)).

The changes of standard Gibbs free energy of different reactions were calculated by

Factsage 7.0 [25] and the results are shown in Fig. 2. Lines for Eqs. (1-4,10) in Fig. 2(a)

indicate that the critical temperatures (corresponding to the value at which ) ∆GθT = 0K

for carbothermic reduction of different components follow the sequence: TFeO < TNa2O

. Lines for Eqs. (3) and (4) indicate that the temperature for the reduction of < TTiO2

TiO2 to TiC (1562K) is lower than that to metallic Ti (2038K). In order to guarantee

the reduction of iron oxides and avoid the generation of TiC, the temperature range of

experiment was set as 1073~1473K. In addition, based on Fig. 2(b), it can be also

concluded that the reactions between Na2CO3 and acidic/amphoteric oxides (Eqs. (5-

8)) may occur at the current experimental temperature. Additionally, it also shows that

Na2O can be reduced by C when the reaction temperature is greater than 1286K. The

critical temperature will be even lower in the actual experimental condition due to the

use of Ar as the protecting gas which decreases the partial pressure of gaseous product.

Kim and Lee also concluded that Na2O can be reduced when the temperature exceeded

the melting point of Na2CO3 (~1123K). Therefore, the carbothermic reduction of Na2O

should be taken into account [24].

The changes of Gibbs free energy for the reactions involved TiO2 are calculated and

shown in Fig. 3, from which it can be seen that if only the reduction TiO2 is considered,

the phases of TiO2n-1 will be generated. However, in the present study, the slag is

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composed of composite mineral phases, and it was much more difficult for the

reduction of titanates (Na2TiO3, CaTiO3, and Mg2TiO4) than TiO2 at 1073~1473K

based on the thermodynamic calculations shown in Fig. 3. Hence, the existence of low

valence Ti in the slag is neglected, which is also confirmed by in the XRD results in

Figs. 7-9.

Fig. 3. Variations of standard Gibbs free energy of the reduction reactions involved

TiO2.

The volume fractions (%) of CO and CO2 produced during the reduction process

when mass ratio of C-to-VTC (RC/V) was 25% were detected by an infrared gas analyzer

and shown in Fig. 4, from which it was obvious that the main gaseous product in the

reaction process was CO.

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Fig. 4. Volume fractions of CO and CO2 in gaseous product generated in the isothermal

reduction process of VTC when the mass ratio of Na2CO3/VTM was 60% under 1473K.

Therefore, as pre-determining the amount of carbon addition, for simplicity, only the

gaseous product of CO was considered.

By assuming the main gaseous product is CO, the theoretical minimum of RC/V (Rmin)

can be calculated by Eq. (12), if only considering the reduction of iron oxides (Eqs. (1,

2)). Both, its maximum (Rmax) can be calculated by Eq. (13) due to Eqs. (9-11). Rmin

and Rmax are calculated to be 14.88% and 28.44%, respectively. Accordingly, RC/V were

set to be 15%, 20%, 25%, and 29% to study with the influence of carbon content on

reaction process.

(12)Rmin = mFeO ×1272 + mFe2O3 ×

36160

(13) Rmax = Rmin + RN/V × 24106

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To simplify the description, herein, the sample name is referred to ‘XC_YN_t_T’,

with a certain mass ratio (X/Y/100) of graphite (C), Na2CO3 (N), and VTC, at the

reaction temperature of T K for t hours. A summary of the experimental conditions in

this study can be found in Table 2.

Table 2. Experimental conditions in this study

Composition

Sample

(XC_YN_t_T)RC/V

(X% wt.)

RN/V

(Y% wt.)

Reaction

Time

(t h)

Reaction

Temperatur

e

(T K)

15C_60N_2_1473 15 60 2 1473

20C_60N_2_1473 20 60 2 1473

25C_60N_2_1473 25 60 2 1473

29C_60N_2_1473 29 60 2 1473

25C_60N_2_1073 25 60 2 1073

25C_60N_2_1273 25 60 2 1273

25C_60N_1_1473 25 60 1 1473

25C_60N_0.5_1473 25 60 0.5 1473

25C_60N_0.25_1473 25 60 0.25 1473

0C_60N_2_1473 0 60 2 1473

25C_0N_2_1473 60 0 2 1473

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The schematic diagram of the experimental apparatus is shown in Fig. 5. A horizontal

tube furnace with the heating element of silicon carbide rods was used. The diameter

of the quartz tube is 48 mm and the length of the constant temperature zone of the

furnace is 60 mm. The graphite tube with one end sealed was used to protect the quartz

tube from corrosion at high temperatures due to the generation of Na vapor. The raw

materials (10 g) were firstly ground in an agate mortar for 30 minutes and then moved

into an alumina boat (60mm×15mm×15mm). The alumina boat was put in the graphite

inner tube embedded in the quartz tube, which was then placed in the constant

temperature zone of the furnace. Argon gas (0.3 L/min) was blown for 1 hour in

advance to exclude the air from the graphite tube. When the furnace temperature

reached the desired value, the quartz tube was quickly inserted into the furnace. After

reacting for a certain time, the quartz tube was taken out rapidly from the furnace, and

cooled down to the room temperature in the flowing argon atmosphere.

Fig. 5. The schematic diagram of the experimental apparatus.

After cooling, the generated iron beads were separated from the slag. The phase

composition of the reduced sample was examined via X-ray diffraction (TTR III,

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conducted using a Cu-Kα source, tube current 10 to 300 mA, tube voltage 20 to 60 kV)

in the 2θ range from 10° to 90° with a scanning rate of 15 deg/min. The morphology

was characterized by SEM (Quanta 250, FEI Company, US). It’s worth noted that,

considering the hydration of high sodium oxide slag, the method of dry grinding and

dry polishing was adopted in this study. The volume fractions of CO and CO2 were

recorded by the infrared gas analyzer (XLZ−1090) every 5 s during the reaction process

until its value decreased to 0.1%.

3. Results

3.1 Total mass loss ratio

During carbothermic reduction process of VTC by the assistance of Na2CO3, the total

mass loss ratios (W) in the experiments were calculated by Eq. (14), where w0 is the

initial mass of the sample and wt is the mass of the sample after the reaction time t.

(14)W =w0 - wt

w0× 100 pct.

The total weight loss W of relevant samples was illustrated in Fig. 6. Line 1

(W=35.83%) and Line 2 (W=51.13%) showed the theoretical weight loss rates when

RC/V was 14.88% and 28.44%, respectively. Comparing samples with different RC/V

(15%, 20%, 25%, and 29%), it was found that their W values were all located between

line 1 and line 2, which explained a part of Na2O was reduced by carbon during this

process.

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Fig. 6. The mass loss of different samples after reacting at different temperatures for

different times.

Furthermore, the more the additive amount of carbon, the larger the value of W will be,

since more Na2O was reduced as the amount of carbon increased. In addition, by

comparing the weight loss rate of samples (25C_60N_2_1073, 25C_60N_2_1273 and

25C_60N_2_1473), it can be concluded that the reaction rate increased when the

temperature was elevated. In addition, from curve A, it can be distinctly observed that

the carbothermic reduction reaction was very fast with the assistance of Na2CO3, and

the reduction reaction was almost completed after 30 minutes.

3.2 Phase transformation

The phase compositions of samples XC_60N_2_1473 with various RC/V after reacting

at 1473K for 2 hours was detected and shown in Fig. 7. When RC/V was 15% and 20%,

the main phases of products were Na2O-Fe2O3-TiO2 ternary compound

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(Na0.75Fe0.75Ti0.25O2), magnesium iron oxide (MgO0.239FeO0.761 or MgO0.77FeO0.23),

iron (Fe), and sodium aluminum silicate (Na1.95Al1.95Si0.05O4). It was indicated that part

of iron oxides in VTC was reduced, but there were still unreduced iron oxides in slag

(e.g. Na0.75Fe0.75Ti0.25O2, MgO0.77FeO0.23).

Fig. 7. The XRD patterns of products for samples XC_60N_2_1473 (X=15, 20, 25, and

29, respectively)

Meanwhile, during the reaction process, a large number of sodium ions destroyed the

mineral phases of VTC and formed a liquid phase, such as Na1.95Al1.95Si0.05O4. As RV/C

rose to 25%, the characteristic peaks of sodium iron titanium oxide and magnesium iron

oxides disappeared, but the peaks of graphite appeared, since the amount of graphite

was excessive and sufficient to reduce all the iron oxides in the VTC. The phase

compositions of products (after separating the iron) were composed of Na2O-MgO-

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TiO2 ternary compound (Na12MgTi13O33), perovskite (CaTiO3), and magnesium-

titanium spinel (Mg2TiO4). As increasing RC/V to 29%, Na12MgTi13O33 disappeared

because more Na2O in the melt was reduced by graphite. However, it was hard to reduce

the TiO2 in CaTiO3 and Mg2TiO4 due to their stable crystalline structures [6, 11].

Therefore, a RC/V of 25% was adopted for further experiments.

Fig. 8. The XRD patterns of products for samples 25C_60N_2_T (T=1073K, 1273K,

and 1473K, respectively)

Fig. 8 showed XRD patterns of products for samples (25C_60N_2_T) after reacting

at 1073K, 1273K, and 1473K, respectively. At 1073K, the Na2O-Fe2O3-TiO2 ternary

compound (Na0.75Fe0.75Ti0.25O2) was generated, accompanying the formation of FeO.

When the reduction temperature was ascended to 1273K, Na2CO3, titanomagnetite and

Na0.75Fe0.75Ti0.25O2 phases almost disappeared, along with the appearance of sodium

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titanate (α-Na2TiO3 [14]) and iron (Fe). It indicated that most of iron oxides were

reduced to metallic iron, except those in the stable MgO0.77FeO0.23 solid phase. As the

reaction temperature was further raised to 1473K, all of the iron oxides were reduced

to metallic iron, and the sodium titanate phase transformed into Na2O-MgO-TiO2

ternary compound (Na12MgTi13O33), perovskite (CaTiO3), and magnesium-titanium

spinel (Mg2TiO4). Meanwhile, diffraction peak intensity of metallic iron (Fe) became

weak, suggesting that the separation between iron and slag was better as the temperature

was 1473K. Consequently, the proper temperature was 1473K.

Table 3. Phase compositions of products for samples 25C_60N_t_1473

Sample Identified Phases

25C_60N_0.25_1473 C, Fe, Na0.75Fe0.75Ti0.25O2, Na2TiO3, Na16Ti10O28, Na1.95Al1.95Si0.05O4, MgO

25C_60N_0.5_1473 C, Fe, Na0.75Fe0.75Ti0.25O2, Na16Ti10O28, Na1.95Al1.95Si0.05O4, MgO

25C_60N_1_1473 C, Fe, NaFeTiO4, CaTiO3, Mg2TiO4, Na1.95Al1.95Si0.05O4

25C_60N_2_1473 C, Fe, Na12MgTi13O33, CaTiO3, Mg2TiO4, Na1.95Al1.95Si0.05O4

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Fig. 9. The XRD patterns of products for samples 25C_60N_t_1473 (t=0.25, 0.5, 1, and

2, respectively)

Furthermore, the XRD patterns of products for samples (25C_60N_t_1473) reduced

for various time were shown in Fig. 9, and the identified phases were listed in Table 3.

The Na2O-Fe2O3-TiO2 ternary compounds (e.g. Na0.75Fe0.75Ti0.25O2, NaFeTiO4) were

identified after reacting for 1 hour. However, after 2 hours, the diffraction peaks of the

ternary compounds were disappeared. This phenomenon indicated that some ferric ions

in the slag phase were reduced by graphite. Meanwhile, with the increase of the reaction

time, the relative intensity of diffraction peaks of iron was decreased, and nearly

disappeared after 2 hours, indicating the effective separation of the slag and iron.

Besides, as the reaction went on, the phase containing sodium and titanium changed

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from α-Na2TiO3 to Na16Ti10O28 and then to Na12MgTi13O33, CaTiO3 and Mg2TiO4.

According to the changes of standard Gibbs free energy of reactions between basic

oxides (Na2O, CaO, MgO, and FeO) and TiO2 shown in Fig. 10, it is much easier for

Na2O to react with TiO2. However, Na2O is continuously consumed by graphite during

the smelting process, so that the other titanates may be also generated when Na2O is

not enough.

Fig. 10. Variations of standard Gibbs free energy of the reduction reactions between

basic oxides and TiO2.

3.3 Microstructure evolution

To further analyze the reaction process, the microstructure of samples

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25C_60N_2_1473, 20C_60N_2_1473, 25C_60N_2_1273, and 25C_60N_0.5_1473

were detected and illustrated in Fig. 11(a), (b), (c), and (d), respectively. As shown in

Fig. 11(a), there were metallic iron particles and slag in the products. The iron particles

were agglomerate together, and slag contained four major regions, which were

Na12MgTi13O33, Na1.95Al1.95Si0.05O4, CaTiO3, and Mg2TiO4 phases, respectively. In

addition, there was no iron element in the slag phases, which indicated that all the iron

was metallized.

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Fig. 11. SEM images of samples: (a) 25C_60N_2_1473; (b) 20C_60N_2_1473; (c)

25C_60N_2_1273; (d) 25C_60N_0.5_1473; and (e) EDS analysis results of

corresponding regions.

In Fig. 11(b), only the slag phase existed since iron particles were separated during

grinding and polishing process. In slag phase, region 1 contained 7.97% Na, 2.2% Mg,

12.77% Ti, 28.3% Fe and 48.75% O, which indicated that the lighter white was

MgO0.77FeO0.23 phase. Region 2 consisted of 22.55% Na, 16.58% Ti, 13.66% Fe and

47.20% O, corresponding to the Na0.75Fe0.75Ti0.25O2 phase. Besides, region 3 contained

21.70% Na, 16.64% Al, 4.70% Si, 50.65% O, 1.93% Ti and 4.38% Fe, thus the black

phase was mainly Na1.95Al1.95Si0.05O4. The results showed that there were still lots of

Fe2O3 in slag phase, due to the insufficient graphite addition.

The microstructure of 25C_60N_2_1273 was illustrated in Fig. 11(c). The slag phase

contained 29.70% Na, 8.14% Ti, 1.24% Fe and 55.71% O, which indicated the main

(e)

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phase was sodium titanate (α-Na2TiO3). The small metallic iron particles were

dispersively distributed in the reduced products, so it was hard to separate iron from

slag. Fig. 9(d) showed the microstructure of sample 15C_60N_0.5_1473. Expect for

the iron phase, the slag phase was composed of four regions which were Na16Ti10O28,

Na1.95Al1.95Si0.05O4, CaTiO3, and Na0.75Fe0.75Ti0.25O2, corresponding to regions 1-4,

respectively. Therefore, there were still unreduced iron oxides in the slag after reaction

for 30 minutes.

From the above results, it can be concluded that the phase compositions obtained

from the SEM-EDS analyses were consistent with XRD analysis. Meanwhile, the iron

oxide in sample 25C_60N_2_1473 was completely reduced and had a good separation

effect with slag.

4. Discussion

4.1 Reaction mechanism of VTC with Na2CO3

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Fig. 12. The XRD (a) and SEM images (b) of sample 0C_60N_2_1473.

Reaction between Na2CO3 and VTC was of great importance for the carbothermic

reduction process of VTC, and it was necessary to investigate its reaction mechanism.

Fig. 12(a) and (b) were the XRD and SEM-EDS results of products for sample

0C_60N_2_1473. From Fig. 12(a), it can be seen that the main phases of roasted

products were Na2O-Fe2O3-TiO2 ternary compound (Na0.75Fe0.75Ti0.25O2), ferrous oxide

(FeO), and the Na2O-Al2O3-SiO2 ternary compound (Na1.95Al1.95Si0.05O4). In Fig. 11(b),

the roasted product was composed of three phases which were consistent with the

results of XRD. Interestingly enough, the regions of three phases formed a nucleus-like

structure, with the solid solution phase of FeO and MgO (Region 1) in the center and

ternary compounds (Regions 2 and 3) at the edges. The formation of this structure was

beneficial for the reduction reaction. It was inferred that Na2CO3 melt broke and entered

into the lattices of VTC to form a complex melt with the acidic oxides (Fe2O3, TiO2,

Al2O3, and SiO2) since Na2O was a basic oxide, along with the release of some basic

oxide (FeO and MgO). Similar result had been reported by Holloway et al. [26,27] that

ZnFe2O4 spinel transformed into ZnO and either α-NaFeO2 or β-NaFeO2 in the presence

of Na2CO3. Selivanov et al.’s work [28] also confirmed that Na2CO3 can replaced FeO

and MnO in wolframite. Therefore, the reaction mechanism between Na2CO3 and VTC

can be described as Eq. (15). As the reaction went on, more and more acidic oxides

diffuse to the edge of the melt and combine with Na2O to form different compounds

(during cooling), with the FeO and MgO left in the center to form the core phase.

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(15)Na2CO3 +VTCAr gas, 1473KMelt(Na + , Fe3 + ,Ti4 + ,Al3 + ,Si4 + ) + FeO(Mg2 + ) + CO2(g)

4.2 Effect of Na2CO3 on reduction process

Microstructures of reduced samples of (a) 25C_0N_2_1473 (without the addition of

Na2CO3) and (b) 25C_60N_2_1473 were shown in Fig. 13. For 25C_0N_2_1473, the

generated iron was dispersed and embedded in the slag, which increased the difficulty

of separation. For 25C_60N_2_1473, the iron phase agglomerated to beads and

separated effectively from slag. These phenomena indicated Na2CO3 helped iron

particles grow and separate from slag, owing to the generation of the liquid slag and the

decreases of viscosity of liquid slag [29]. In addition, from the volume fraction changes

of CO generated during the reduction process (shown in Fig. 14), it can be found clearly

that the reaction rate of 25C_60N_2_1473 was much faster than 25C_0N_2_1473,

which indicated that Na2CO3 accelerated the reducing reaction. The reason for the

acceleration may be related to the catalytic effect of Na2CO3 on carbon-gas reaction [30]

and the destruction of mineral structures by Na2CO3 [16].

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Fig. 13. The SEM images of (a) 25C_0N_2_1473 and (b) 25C_60N_2_1473.

4.3 Carbothermic reduction mechanism

From Fig. 11(d), it can be seen that the product of sample 25C_60N_0.5_1473

contained the Na2O-Fe2O3-TiO2 ternary phase (Na0.75Fe0.75Ti0.25O2), which confirmed

that some Fe2O3 entered into the melt during the reduction process. Besides, after two

hours of reduction (15C_60N_2_1473), there was no Fe oxide in the slag. Therefore,

Fe2O3 in the melt had been reduced completely. Accordingly, there were two

mechanisms for the reduction of iron oxide in VTC: direct reduction by graphite, and

reduction from liquid slag. The latter one was a solid (graphite)-liquid (melt) reaction.

Since the activity of iron oxide gradually decreases in the liquid slag as the reaction

goes on which is bad for the reduction reaction, there is an increase in reduction time

relative to that without the addition of Na2CO3, as indicated by Fig. 14. This

phenomenon agreed with conclusion of Pan et al. [31] that the sufficient Na2O retarded

the reduction of Fe2O3 because the formed liquid phase inhibited the diffusion of the

reducing gases.

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Fig. 14. Change of volume fraction of CO generated at 1473 K: (a) 25C_0N_2_1473;

(b) 25C_60N_2_1473.

5. Conclusions

The present study investigated the carbothermal reduction process of vanadium

titanomagnetite with the assistance of Na2CO3 under argon atmosphere, and the

following conclusions were summarized:

(1) The optimal mass ratio of VTC: C: Na2CO3 was 100:25:60 at 1473 K.

(2) During the reaction between Na2CO3 with VTC, acidic oxides (Fe2O3, TiO2,

Al2O3, and SiO2) in VTC entered into molten Na2CO3 to form the Na-rich melt, and the

residual basic oxides (FeO and MgO) was concentrated in the center to form the solid

solution phase.

(3) During the reduction process, Na2CO3 not only accelerated the reduction rate, but

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also promoted the separation of generated iron from slag by decreasing the viscosity of

slag.

Acknowledgements

This work was supported by National Key R&D Program of China

(2018YFC1900500), National Natural Science Foundation of China (21908231,

51774260, 51804289, and 51904286), Key Research Program of Frontier Sciences of

Chinese Academy of Sciences (Grant No. QYZDJ-SSW-JSC021), CAS

Interdisciplinary Innovation Team, and Special Project for Transformation of Major

Technological Achievements in HeBei province (19044012Z).

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