Mechanism Research for the Influence of TiO₂ Content on the Shape Transformation of Rutile Crystals

Abstract

The isothermal precipitation kinetics of rutile crystals was studied to clarify the mechanism for the influence of TiO₂ content on the shape transformation of rutile crystals. The results indicate that the growth index n values were about 2, 3, and 4 when the TiO₂ contents of the raw materials were 27, 37, and 47%, indicating that the precipitation of the rutile crystal had a one-dimensional, two-dimensional, and three-dimensional growth. Thus, the shapes of the rutile crystals were a cuboid, a cube, and a sphere when the TiO₂ contents of the raw materials were 27, 37, and 47%. An increase in the TiO₂ content of raw materials can encourage the transformation of rutile crystals into spheres, which is beneficial to the settling of rutile crystals in molten slag. It realizes the conversion of rutile from a lean ore to a rich ore and avoids subsequent beneficiation processes.

1. Introduction

Titanium-rich materials are used as the raw materials in the production of titanium white with the chlorination method and a titanium sponge. The titanium-rich materials are chlorinated to obtain intermediate TiCl₄, and then TiCl₄ is oxidized or reduced to obtain titanium white or sponge titanium. In the chlorination process, there are currently two methods to achieve industrial production, i.e., fluidizing chlorination and molten salt chlorination. The molten salt chlorination can treat titanium slag with high calcium and magnesium contents, but a large amount of waste molten salt is produced, such as 200 kg of waste molten salt for 1 t of TiCl₄. Fluidizing chlorination has the advantages of high productivity, a simple process, and less waste. Therefore, fluidizing chlorination is currently the mainstream method for preparing TiCl₄. However, fluidizing chlorination has strict requirements for titanium-rich materials, which need to meet the TiO₂ content > 92% and the CaO and MgO content < 1.2% (in fluidizing chlorination, CaO and MgO are chlorinated into high-melting-point CaCl₂ and MgCl₂, which blocks the sieve plate and hinders the chlorination of titanium-rich materials). At present, the titanium-rich materials that meet the conditions of fluidizing chlorination include rutile and UGS slag. QIT is currently the only company that can successfully use titanium slag with high calcium and magnesium contents to produce UGS slag. However, its technology is strictly confidential to the outside world. In addition, the reserves of natural rutile in the world are very limited. With the continuous development of titanium white and sponge titanium industries, natural rutile is on the verge of exhaustion. Therefore, the production of synthetic rutile that meets the requirements of fluidizing chlorination is a key problem to be resolved in the development of the titanium white and titanium sponge industries.

China has a large number of titanium resources, mainly in the form of vanadium–titanium magnetite. Its total reserve volume is about 10 billion tons, which are mainly situated in the Panzhihua regions. After blast furnace ironmaking, the titanium components in the vanadium–titanium magnetite enter the slag to generate titanium-bearing blast furnace slag (10%–25% TiO₂). The comprehensive utilization of titanium-containing blast furnace slag has always been a technical challenge in China. To achieve the comprehensive utilization of titanium-bearing blast furnace slag, a large amount of related research has been conducted. On the basis of the classification of research products, the methods can be parted into the following types: titanium pigments [1], titanium-containing alloys [2], photocatalysts [3], construction materials [4], TiCl₄ [5], CaTiO₃ [6], anosovites [7], and synthetic rutiles [8,9,10,11,12,13,14,15]. Though the aforementioned ways can facilitate the comprehensive utilization of titanium-bearing blast furnace slag, they typically entail drawbacks, such as high energy consumption and elevated cost.

To resolve the above problems, a new comprehensive utilization technology for titanium-bearing mixed molten slag was proposed by Zhang Li [16]. The core of this technology is to realize the settling of rutile crystals by controlling the TiO₂ content of the molten slag. Previous research [16] has achieved the settling of rutile crystals through specific experiments and related theories and concluded that the settling speed of spherical rutile crystals is the largest. However, the mechanism for the influence of TiO₂ content on the shape transformation of rutile crystals is still unclear.

In this paper, the isothermal precipitation kinetics of rutile crystals is used to clarify the mechanism for the influence of TiO₂ content on the shape transformation of rutile crystals, which provides reference for related research. On the other hand, it offers theoretical guidance for the subsequent optimization of the sphericity of rutile crystals so as to realize the settlement of more rutile crystals. Thus, a rutile-rich ore can be obtained, which greatly reduces the workload of beneficiation and saves production costs.

2. Materials and Methods

2.1. Materials

In this study, Ti-bearing blast furnace slag and titanium slag were used as raw materials. SiO₂ (analytical grade) was used as an additive. The main chemical components of the Ti-bearing blast furnace slag and titanium slag are presented in

Table 1. Main chemical components of Ti-containing blast furnace slag (1) and titanium slag (2) (%, mass fraction).

Raw Materials	CaO	SiO2	TiO2	Ti2O3	Al2O3	MgO
1	26.87	25.13	17.58	3.86	14.08	7.86
2	4.32	8.85	60.72	14.65	2.64	2.02

, while their XRD patterns are shown in Figure 1.

As can be observed from Figure 1, Ti-containing blast furnace slag mainly consists of perovskite (CaTiO₃), akermanite (Ca₂Mg(Si₂O₇)), diopside (CaMgSi₂O₆), and spinel (MgAl₂O₄). Titanium slag mainly contains anosovite and anorthite (CaAl₂(SiO₄)₂).

2.2. Procedures

2.2.1. Experimental Procedure for the Influence of TiO₂ Content on Rutile Shape and Rutile Settling

As shown in Figure 2, the experiments were carried out in a MoSi₂ furnace. The oxidation gas was O₂. According to

Table 2. Experimental program.

No.	Ti-Bearing Blast Furnace Slag/g	Titanium Slag/g	SiO2/%	Oxidation Time/s	Total TiO2 Content/%
1	407	93	18	168	27
2	315	185	10	168	37
3	222	278	8	126	47

, 500 g of Ti-containing blast furnace slag and Ti slag, along with a certain quantity of SiO₂, was placed into a crucible. The crucible was heated to 1450 °C for 20 min to melt the raw materials and additive. Subsequently, oxygen was blown into the molten slag at a flow rate of 5 L/min. After that, the molten slag was slowly cooled to indoor temperature at a cooling speed of 5 °C/min. The cooled slag was named the modified slag. The purpose of the oxidation is to oxidize the low-valent titanium oxides into tetravalent titanium oxide (TiO₂). The proper oxidation time is to oxidize most of the low-valent titanium oxides in the slag to tetravalent titanium oxides. The combined effect of oxidation and SiO₂ makes the titanium components in the slag exist in the form of rutile TiO₂.

2.2.2. Experimental Procedure for Effect of TiO₂ Content on the Isothermal Precipitation Kinetics of Rutile

According to a certain ratio (as shown in Table 2), titanium-containing blast furnace slag, Ti slag, and SiO₂ were placed in a crucible at 1450 °C for 20 min. Subsequently, oxygen was blown into the molten slag at a flow speed of 5 L/min, and the oxidation times are shown in Table 2. Subsequently, the molten slag was quickly cooled to a specified temperature (1430, 1410, and 1390 °C) at a cooling rate of 10 °C/min, and then the temperature was held. A corundum tube was used to take samples at a regular interval (4.0, 2.0, and 1.5 min). Then, the samples were quickly put into cold water for a quenching treatment. The crystallization situation was observed and photographed under a metallographic microscope. With the help of related software, 15 photos with different fields were selected to count the volume fraction of the rutile crystals.

3. Results and Discussion

3.1. Influence of TiO₂ Content on Rutile Shape and Rutile Settling

According to the experimental program in Table 2, modification experiments were carried out. The microscopic morphology of the samples at the upper and lower parts of the modified slag was observed, and then 15 images with different fields were selected to count the volume fraction of the rutile crystals. The results are shown in Figure 3 and Figure 4 [16]. In order to determine the phase composition of the modified slag, the modified slags with different TiO₂ contents were analyzed by XRD, and the results are shown in Figure 5.

Based on previous research [16], the settling speed of sphere rutile crystals > the settling speed of cube rutile crystals > the settling speed of cuboid rutile crystals. As shown in Figure 3a,b and Figure 4, most of the rutile crystals are nearly cuboid. Therefore, the rutile settlement barely occurs. As shown in Figure 3c,d and Figure 4, most of the rutile crystals are nearly cubes. Therefore, the rutile settlement occurs. As shown in Figure 3e,f and Figure 4, most of the rutile crystals are nearly spheres. Therefore, the rutile settlement was obvious. In summary, when the TiO₂ content of the raw materials increased from 27% to 47 wt.%, the morphology transformation of the rutile crystal was as follows: cuboid → cube → sphere. The rutile settling becomes more and more significant with the increase in the TiO₂ content of the raw materials.

The content of residual Ti₂O₃ in the slag was measured. The results are shown in

Table 3. The content of Ti₂O₃ remaining in the slag (No. 1, 2, and 3 in Table 2).

No.	The Content of Ti2O3/%
1	1.14
2	1.27
3	1.23

. The contents of Ti₂O₃ in Table 1 and Table 3 were determined by the potassium dichromate titration method [17]. It can be seen that most of the low-valent titanium oxides in the slag are transformed into tetravalent titanium oxide. Therefore, the titanium components in the slag exist in the form of rutile. Furthermore, it can be seen from Figure 5 that the modified slags with different TiO₂ contents have only a rutile phase. Thus, the light gray particles in Figure 3 are rutile-type TiO₂, and the black phase is a glass phase.

At present, there are many methods for rutile synthesis, and the main methods are shown in

Table 4. Synthesis methods of rutile.

Method	Conditions	Advantages	Disadvantages
Sodium salt roasting–leaching [18]	900–925 °C, 14%–25%H2SO4	High-grade rutile	Corrosion of furnace lining
Phosphoric acid roasting–leaching [19]	1000 °C, 15%H2SO4	Medium-grade rutile	Corrosion of furnace lining
Microwave roasting–leaching [20]	950 °C, 30%H3PO4	Environmental protection in the roasting process	Low-grade rutile
Oxidation–reduction–leaching [21]	800–1050 °C, 15%–22%HCl	Medium-grade rutile	High energy consumption
Oxidation–chlorination–leaching [22]	800–850 °C, 10%–30%HCl	High-grade rutile	High energy consumption
Acid alkali combined leaching [23]	30~180 g/L NaOH, 25 g/L HCl	Medium-grade rutile	More wastewater
Sulfation roasting–water leaching [24]	600–1000 °C	Medium-grade rutile	Environmental pollution

.

As shown in Table 4, the methods in Table 4 have some problems, such as corrosion of furnace lining, low rutile grade, environmental pollution, high energy consumption, and more wastewater. The technology proposed in previous research [16] can not only produce high-grade rutile but also avoid the problems of the existing methods. It has less corrosion on the furnace lining with the additive of SiO₂. It has one high-temperature treatment process, so the energy consumption is not high. Due to leaching with a dilute hydrochloric acid solution, there is less pollution and wastewater. Compared with the traditional rutile synthesis technologies (the methods in Table 4), this technology has significant advantages.

3.2. Influence of TiO₂ Content on the Isothermal Precipitation Kinetics of Rutile

3.2.1. Theoretical Derivation of the Isothermal Precipitation Kinetics Equation of Rutile

The precipitation kinetics of rutile crystals can be studied by JMAK [25,26,27] Equation (1).

$$R=1-e^{-k{t}^n}$$

(1)

where k is a coefficient related to the nucleation rate of the new phase and the crystal growth ratio; n is the crystal growth index; and R is the transition fraction.

Taking the logarithm of Equation (1):

$$\ln \left(1-R\right)=-k{t}^n$$

(2)

Taking the logarithm of Equation (2):

$$\ln \left[-\ln \left(1-R\right)\right]=\ln \left(k\right)+n\ln \left(t\right)$$

(3)

Then, a straight line was drawn according to Equation (3). The n and k can be determined by the slope and intercept of the straight line.

Substituting the ln(k) values obtained at different temperatures into the Arrhenius Equation can obtain the crystallization activation energy.

The Arrhenius [28,29] equation is shown in Equation (4).

$$k=k_0\exp \left(-{\displaystyle \frac{E}{RT}}\right)$$

(4)

Taking the logarithm of Equation (4):

$$\ln \left(k\right)=\ln \left(k_0\right)-{\displaystyle \frac{E}{RT}}$$

(5)

Here, $k_0$ is a constant; R represents the gas constant; and E is the activation energy of the crystallization.

The transformation fraction of rutile crystal can be calculated using Equation (6).

$$R={\displaystyle \frac{f\left(T,t\right)}{f\left(T,eq\right)}}$$

(6)

where $f\left(T,t\right)$ is the volume fraction of the rutile crystal at temperature T; and $f\left(T,eq\right)$ is the volume fraction of the rutile crystal under equilibrium conditions.

3.2.2. Experimental Study on the Isothermal Precipitation Kinetics of Rutile

Through high-temperature quenching experiments, the relationships between the rutile transformation fraction and the time at three temperatures (1430, 1410, and 1390 °C) under different TiO₂ contents (27, 37, and 47%) were obtained. The results are as shown in Figure 6a–c. According to Equation (3), a straight line can be obtained, and then the data were linearly fitted by the Origin software (ver. 2018). The linear fitting line and fitting parameters are shown in Figure 6d–f and

Table 5. Parameter values fitted by Equation (3).

TiO2 Content/%	Temperature/°C	k	n	R2/%
27	1390	0.011	1.93	99.08
	1410	0.0081	2.00	99.41
	1430	0.0065	2.00	99.05
37	1390	0.0066	3.11	99.10
	1410	0.0052	3.17	99.47
	1430	0.0039	3.21	99.25
47	1390	0.0051	3.93	99.93
	1410	0.0039	3.90	99.73
	1430	0.0032	4.05	99.79

. According to Equation (5), a straight line can be obtained, and the data were linearly fitted by the Origin software (ver. 2018). The linear fitting line and fitting parameters are shown in Figure 6g–i and

Table 6. Parameter values fitted by Equation (5).

TiO2 Content/%	k0	E/(kJ/mol)	R2/%
27	9.4 × 10−13	−320.7	96.76
37	1.3 × 10−12	−308.8	99.61
47	1.4 × 10−11	−273.2	96.90

.

As can be seen from Figure 6a–c, the transformation fraction of the rutile crystal first increased rapidly and then tended to be flat with an increase in time. In the meantime, the lower the temperature, the higher the transformation fraction of the rutile crystal. This is because the lower the precipitation temperature, the greater the degree of supercooling and the greater the supersaturation of TiO₂, which is more conducive to the precipitation of the rutile crystal. In addition, with an increment in the TiO₂ content of the raw materials, the time when the transformation fraction of the rutile crystal reached the maximum became shorter and shorter, indicating that an increment in the TiO₂ content of the raw materials can promote the rapid precipitation of the rutile crystal. This is because an increment in the TiO₂ content of the raw materials makes the supersaturation of TiO₂ larger, thereby promoting the precipitation of the rutile crystal.

As can be seen from Figure 6d–f and Table 5, the correlation coefficient R² was greater than 99.00% when the TiO₂ contents of the raw materials were 27, 37, and 47%. Thus, ln[−ln(1 − R)] and ln(t) satisfied the linear relationship, indicating that the isothermal precipitation kinetics of the rutile crystal can be described by JMAK Equation (1). Furthermore, it can be seen from Table 5 that when the TiO₂ content of the raw materials was 27%, the growth index n was about 2, indicating that the rutile precipitation had a one-dimensional growth. Thus, the shapes of the rutile crystals were mostly cuboids. When the TiO₂ content of the raw materials was 37%, the growth index n was about 3, indicating that the rutile precipitation had a two-dimensional growth. Therefore, the shapes of the rutile crystals were mostly cubes. When the TiO₂ content of the raw materials was 47%, the growth index n was about 4, indicating that the rutile precipitation had a three-dimensional growth. Thus, the shapes of the rutile crystals were mostly spheres. The above results fully prove the effect of the raw material TiO₂ content on the shape transformation of rutile crystals in Section 3.1. As can be seen from Figure 6g–i and Table 6, the correlation coefficient R² was greater than 96.00% when the TiO₂ contents of the raw materials were 27, 37, and 47%. Thus, ln(k) and 1/T met the linear relationship, indicating that the activation energy for the precipitation of rutile crystals can be calculated by the Arrhenius Equation (5). As shown in Table 6, the isothermal precipitation activation energy (absolute value) of the rutile crystal gradually decreased with an increase in the TiO₂ content of the raw materials, implying that an increase in the TiO₂ content of the raw materials is conducive to the precipitation of the rutile crystals. In addition, it can be seen from Table 6 that the isothermal crystallization activation energy of the rutile crystal was negative. This is because the coefficient k related to the nucleation rate of the new phase and the crystal growth rate increased with the decrease in the temperature, resulting in the activation energy being negative.

4. Conclusions

• While the TiO₂ content of the raw materials increased from 27 to 47%, the shape transformation of the rutile crystal was as follows: cuboid → cube → sphere.
• The isothermal precipitation kinetics of the rutile crystal can be described by the JMAK Equation.
• When the TiO₂ contents of the raw materials were 27, 37, and 47%, the growth index n was about 2, 3, and 4, respectively, indicating that the precipitation of the rutile crystal had a one-dimensional, two-dimensional, and three-dimensional growth. Thus, the shapes of the rutile crystals were a cuboid, a cube, and a sphere.
• The isothermal precipitation activation energy (absolute value) of the rutile crystal gradually decreased with an increase in the TiO₂ content of the raw materials, implying that an increase in the TiO₂ content of the raw materials is conducive to the precipitation of the rutile crystal.