Selective Phase Transformation Behavior of Titanium-bearing Electric Furnace Molten Slag during the Molten NaOH Treatment Process

Abstract

In this paper, selective phase transformation for titanium-bearing phase (Ti-bearing phase) and impurity phase in titanium-bearing electric arc furnace molten slag (Ti-bearing EAF slag) was effectively realized during the molten NaOH treatment process. The phase transformation mechanism based on thermodynamic calculation was investigated in detail by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectrum. It is indicated that the Ti-bearing phases in Ti-bearing EAF slag such as anosovite solid solution and Mg₂TiO₄ can be easily converted to Na₂TiO₃ with NaCl-type crystal structure, whilst the main impurity phase such as MgAl₂O₄ was apt to be decomposed by molten NaOH to form NaAlO₂ and MgO. In addition, with increasing of roasting temperature and time, as well as decreasing of Ti-bearing EAF slag/NaOH mass ratio (R_slag/NaOH), the formed Na₂TiO₃ could be partly changed to NaMO₂ (M=Mg, Ti, Fe) with α-NaFeO₂-type crystal structure, due to the coexisting metal ions like Mg²⁺ and Fe³⁺ in the slag be doped into or even substituted the Ti atoms of Na₂TiO₃ to form NaMO₂ (M=Ti, Fe, Mg).

1. Introduction

Ti-bearing slags such as Ti-bearing blast furnace slag (Ti-bearing BF slag) and Ti-bearing electric arc furnace molten slag (Ti-bearing EAF slag) etc., are considered as valuable secondary resources in China due to their relatively higher TiO₂ contents. Since the Ti-bearing slags are produced from various raw ores by using different treating processes, the main chemical compositions and the phase structure of these slags are basically different. Therefore, how to extract titanium efficiently from the Ti-bearing slags has been attracted much attention, and various approaches have been utilized including acid process,^1,2,3) alkali fusion process,^4,5) carbonization and chlorination process,⁶⁾ selective crystallization process.^7,8,9,10,11) Comparing these methods, the alkali fusion process shows obvious advantage such as unrestricted raw slag materials, and easy recycling of alkali, etc. More importantly, after alkali fusion process, the Ti-bearing phases can be selectively separated from other impurity phases by only water leaching process based on the different water solubility of formed sodium salts instead of complicate dressing method.

However, no matter which Ti-bearing slags were used during the alkali fusion process, the attention of researchers was mainly focused on the extraction efficiency of titanium and the decomposition kinetics of Ti-bearing slags in different alkali systems. None of the efforts have been taken on phase transformation mechanism for titanium and other elements in slags. Zhang et al.¹²⁾ studied the decomposition behavior of Ti-bearing slag in NaOH solution at 493 K for 4 h, and indicated that the hydrothermal product obtained under optimal conditions was Na₄Ti₃O₈. Scott Middlemas et al.¹³⁾ believed that the Ti-bearing phases in Ti-bearing slag could react with molten NaOH at 773 K for 4 h to form Na₂TiO₃. Han et al.⁵⁾ also considered that Ti-bearing slag reacted with molten NaOH yielded only one kind of sodium titanate (Na₂TiO₃). In fact, it should be noted that the titania could react with molten NaOH to form various sodium salts.^14,15,16,17) However, the phase transformation mechanism of the Ti-bearing phases was not clear, especially the influence of metal ions such as Fe³⁺, Al³⁺, Mg²⁺, etc. coexisting in the slags on the formation of Ti-bearing phases were seldom been investigated.

In this paper, Ti-bearing EAF slag was used as raw material to investigate the transformation mechanism of the Ti-bearing phases and impurity phases during the molten NaOH treatment process. The phase transformation mechanism was studied in detail based on thermodynamic calculation and crystal structure analysis. This paper may provide some essential thermodynamic information for Ti-bearing slags or other Ti-bearing materials in the molten NaOH system, and would also give a reference to comprehensive utilization of titanium resource treated by molten NaOH treatment process.

2. Experimental

2.1. Materials and Characterization

All the chemical reagents employed were analytical grade (Sinopharm Chemical Reagent Co. Ltd) and the distilled water was used throughout the experiment. The Ti-bearing EAF slag samples were provided by Panzhihua Steel Company (Sichuan Province, China), which were obtained from vanadium-titanium magnetite concentrate through direct reduction reaction by rotary hearth furnace and smelting separation by electric furnace. The main chemical compositions of the slag were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, TELEDYNE Leeman Labs) and listed in Table 1. The phase structure of the slag was investigated by X-ray diffraction using Cu Kα radiation (λ = 0.154056 nm) with 40 kV, 200 mA and a speed of 10°/min (XRD, M21X, MAC SCIENCE Co. Ltd, Japan). The results shown in Fig. 1 indicated that the Ti-bearing phases of the slag were MgTi₂O₅, Ti₃O₅ and Mg₂TiO₄, while the main impurity phases were MgAl₂O₄ and amorphous CaSiO₃. The valences of transition elements were analyzed by X-ray photoelectron spectroscopy (XPS, Al Kα, AXIS ULTRA), in which C1s binding energy value (284.8 eV) was chosen as standard for baseline correction and baseline subtraction method was Shirley-type. The XPS spectrum of Ti in Ti-bearing EAF slag was shown in Fig. 2. It is shown that about 15% Ti³⁺ and 85% Ti⁴⁺ coexisted in the slag, suggesting that the existence form of titanium element might be Ti₃O₅ or Ti₂O₃ anosovite solid solutions in the slag.

Table 1. Main chemical compositions of the Ti-bearing EAF slag (mass%).

Composition	TiO2	Al2O3	MgO	SiO2	CaO	Fe2O3
Content	50.9	19.4	12.9	8.0	5.4	2.9

Fig. 1.

XRD pattern of the Ti-bearing EAF slag.

Fig. 2.

XPS spectrum of Ti in the Ti-bearing EAF slag. (Online version in color.)

Measurements of Raman spectra were performed on a laser confocal Raman spectrometer (JY-HR800, Jobin Yvon). A blue line (514 nm) of the laser was taken as the excitation source.

Artificially synthesized MgTi₂O₅ was prepared by roasting TiO₂ (anatase, analytical reagent) with MgO at 1873 K for 3 h with TiO₂/MgO molar ratio at 2:1.

2.2. The Concrete Process of Extracting TiO₂ from Ti-bearing EAF Slag

Figure 3 illustrated the general flow sheet of whole extraction process of TiO₂ from Ti-bearing EAF slag, and the detailed procedure were described as follows.¹⁸⁾ It should be noted that this paper was mainly focus on the molten NaOH treatment process to elucidate the Ti-bearing EAF slag phase transformation mechanism clearly.

Fig. 3.

Schematic flow diagram of extracting TiO₂ from Ti-bearing EAF slag.

Firstly, 10 g Ti-bearing EAF slag was ground to about 120 mesh and mixed with NaOH homogeneously at different mass ratios in a nickel crucible. Then, the nickel crucible was placed into a muffle furnace when the temperature reached a preset value, holding for a required time with free access to air. After the molten NaOH treatment process, the nickel crucible was taken out rapidly and then cooled at room temperature. The obtained slag was called alkali fusion slag. For phase analysis, the alkali fusion slag was ground to about 200 mesh rapidly.

Secondly, as for hydrolysis reaction process, 3 g ground alkali fusion slag was dissolved in distilled water with electromagnetic stirring for 1 h. The sodium salts such as Na₂SiO₃ and NaAlO₂ formed in the alkali fusion slag were dissolved in water while the insoluble sodium titanate salts also formed in the alkali fusion slag were separated from the solution by filtration.

Finally, the obtained residue was mixed with HCl solution and refluxed at 373 K for 6 h to perform acidolysis reaction to get metatitanic acid. Then, the samples were calcined in the muffle furnace at 773 K for 1 h to obtain TiO₂.

3. Results and Discussion

3.1. Thermodynamic Analysis

When the Ti-bearing EAF slag was mixed with NaOH and roasted at different temperatures in muffle furnace under atmospheric condition, the related reactions could be described as follows:   

$$\frac{1}{4}\mathrm{M}\mathrm{g}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_5+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=\frac{1}{2}\mathrm{N}{\mathrm{a}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_3+\frac{1}{4}\mathrm{M}\mathrm{g}\mathrm{O}+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$$(1)

  

$$\frac{1}{6}\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+\frac{1}{12}{\mathrm{O}}_2=\frac{1}{2}\mathrm{N}{\mathrm{a}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_3+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$$(2)

  

$$\frac{1}{2}\mathrm{M}{\mathrm{g}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_4+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=\frac{1}{2}\mathrm{N}{\mathrm{a}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_3+\mathrm{M}\mathrm{g}\mathrm{O}+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$$(3)

  

$$\frac{1}{2}\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}{\mathrm{O}}_3+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=\frac{1}{2}\mathrm{N}{\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_3+\frac{1}{2}\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$(4)

  

$$\frac{1}{2}\mathrm{M}\mathrm{g}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2+\frac{1}{2}\mathrm{M}\mathrm{g}\mathrm{O}+\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$$(5)

Therefore, the standard Gibbs free energy change (ΔG°) of the above reactions can be calculated according to Eq. (6).   

$$\begin{array}{l}\mathrm{\Delta }{G}_T^{°}=\mathrm{\Delta }{H}_T^{°}-T\mathrm{\Delta }{S}_T^{°}\\ \hspace{1em}\hspace{1em}=\mathrm{\Delta }{H}_{298}^{°}-T\mathrm{\Delta }{S}_{298}^{°}+{\int }_{298}^{T_1}\mathrm{\Delta }{C}_{p}dT+{\int }_{T_1}^{T_2}\mathrm{\Delta }{C^{\prime }}_{p}dT\\ \hspace{1em}\hspace{1em}-T_1{\int }_{298}^{T_1}\frac{\mathrm{\Delta }{C}_p}{T}dT-T_2{\int }_{T_1}^{T_2}\frac{\mathrm{\Delta }{C^{\prime }}_p}{T}dT\end{array}$$(6)

Where ΔG° is the standard Gibbs free energy change, J/mol, ΔH° is the standard enthalpy change, J/mol, ΔS° is the standard entropy change, J/(mol K), C_p is the specific heat capacity at constant pressure, J/(mol K). Thereinto, $\mathrm{\Delta }{H}_{298}^{°}$ , $S_{298}^{°}$ and C_p could be obtained from the FactPS database of FactSage 6.3 software, respectively, and the related values were listed in Table A1. Figure 4 gave the variation trend of ΔG° ranging from 573 K to 1173 K. It is shown that the ΔG° of all the reactions were negative and decreased with the roasting temperature increasing, indicating that all the reactions could be occurred above 573 K. Moreover, it should be noted that the ΔG° values of the reactions (1), (2) and (3) were more negative than that of the other reactions, while the ΔG° value of the reaction (5) was most positive among all the reactions, suggesting that the Ti-bearing phases in the slag would react with molten NaOH much earlier than other phases, especially the impurity phase MgAl₂O₄ during the molten NaOH treatment process based on thermodynamic calculation. Considering that MgAl₂O₄ was difficult to be decomposed by molten NaOH than other impurity phase, e.g. CaSiO₃, so, the investigation on the transformation of the main impurity phases in this paper was only focus on the decomposition of MgAl₂O₄ during the molten NaOH treatment process.

Fig. 4.

The plot of Δ_rG°~T for reactions between Ti-bearing EAF slag and molten NaOH.

Because NaOH can be used as an ionized solvent at higher temperature above 623 K,¹⁹⁾ it is reasonable to assume that the reactions between Ti-bearing EAF slag and molten NaOH may be considered as a solid-liquid reaction mechanism. In order to make sure the Ti-bearing phases or other impurity phases in Ti-bearing EAF slag convert to their corresponding sodium salts completely, Ti-bearing EAF slag/ NaOH mass ratio (R_slag/NaOH) should be controlled at least below 1 according to the material balance calculation. Moreover, excess amount of NaOH is necessary to maintain the liquidity of the reactants and ensure their sufficient contact areas during the reactions. Based on the aforementioned analysis, all the experiments should be conducted above 673 K with R_slag/NaOH below 1.

3.2. The Effect of Roasting Temperature on the Phase Transformation

The effect of roasting temperature on the Ti-bearing phases transformation was investigated from 773 K to 973 K for 1 h. Figure 5 gives the XRD patterns of alkaline fusion slag obtained under different temperatures with R_slag/NaOH fixed at 1:1. It can be seen that the diffraction peaks of anosovite solid solution and Mg₂TiO₄ completely disappeared while strong diffraction peaks of Na₂TiO₃ (2θ=40.1°) appeared. This case indicated that the anosovite solid solution (Fig. 6(a)) with orthorhombic crystal system was decomposed by molten NaOH and converted to Na₂TiO₃ (Fig. 6(b)) with cubic crystal structure (until cell parameters of a=b=c=4.49 Å, and α=β=γ=90°). Increasing the temperature to 873 K, a new phase named NaMO₂ (M=Ti, Mg, Fe) with hexagonal crystal system (unit cell parameters of a=b=3.04 Å, c=16.26 Å and α=β=90°, γ=120°) appeared, which was derived from α-NaFeO₂-type crystal structure (Fig. 6(c)).¹⁴⁾ Further increasing the temperature to 973 K, the diffraction peaks of NaMO₂ (2θ=16.4°, 41.1°) became stronger, whereas the diffraction peaks of Na₂TiO₃ (2θ=40.1°) turned to weaker, suggesting that relatively higher temperature (>873 K) may be benefit for Ti-bearing phases (e.g. MgTi₂O₅ and Mg₂TiO₄) transformation to NaMO₂ instead of Na₂TiO₃ during the molten NaOH treatment process.

Fig. 5.

XRD patterns of alkaline fusion slags obtained under different temperatures with R_slag/NaOH fixed at 1:1.

Fig. 6.

The phase transformation relationship between Ti-bearing phases (a) MgTi₂O₅, (b) Na₂TiO₃, (c) NaMO₂. (Online version in color.)

It is known that the compositions of Ti-bearing EAF slag are very complex and various elements including Fe³⁺ and Mg²⁺ etc. coexist in the slag system. Therefore, with the roasting temperature increasing, some Fe³⁺ or Mg²⁺ ions would be doped into Na₂TiO₃, or even substituted the Ti atom of Na₂TiO₃ to form a class of layered ionic structure called α-NaFeO₂-type crystal structure.¹⁴⁾ In this paper, the M in NaMO₂ with α-NaFeO₂-type structure phase can be Fe, Mg or Ti elements due to their similar ion radius (Fe³⁺=0.064 nm, Mg²⁺=0.066 nm and Ti⁴⁺=0.068 nm). With increasing the roasting temperature, more and more Fe³⁺ or Mg²⁺ would be doped into Na₂TiO₃ to make Na₂TiO₃ convert to NaMO₂ (M=Fe³⁺, Ti⁴⁺, Mg²⁺), so the amount of NaMO₂ increased whereas the amount of Na₂TiO₃ decreased. According to the analysis above, nearly all the Ti-bearing phases in Ti-bearing EAF slag could convert to Na₂TiO₃ or NaMO₂ during the molten NaOH treatment process, implying the Ti-bearing phase transformation can be realized effectively as illustrated in Fig. 6.

Based on the experimental results, one problem need to be elucidated clearly, that is to say, Fe³⁺ or Mg²⁺ doping into Na₂TiO₃ would promote Na₂TiO₃ to be transformed to NaMO₂ (M=Fe³⁺, Ti⁴⁺, Mg²⁺). In order to confirm the key influence factors on the phase transformation between Na₂TiO₃ and NaMO₂, pure TiO₂ (anatase, analytical reagent) was used as raw material to react with NaOH. Meanwhile, other experimental conditions were controlled the same as that during molten NaOH treatment reaction process. Figure 7 shows the XRD patterns of the product obtained at 973 K for 1 h. It can be seen that the main phases of the product were only Na₂TiO₃ and excess NaOH. None diffraction peaks of NaMO₂ appeared, indicating that TiO₂ reacted with molten NaOH could not be reduced to form NaTiO₂ in air atmosphere.

Fig. 7.

XRD patterns of the product prepared by pure TiO₂ with molten NaOH at 973 K for 1 h.

However, as shown in Fig. 5, when Ti-bearing EAF slag was used as raw material to react with molten NaOH, the diffraction peaks of NaMO₂ did exist at 873 K and their intensities became stronger with the roasting temperature increasing to 973 K. The results were basically different from each other, suggesting that coexisting elements such as Fe³⁺ and Mg²⁺ in the Ti-bearing EAF slag determined the formation of NaMO₂ or Na₂TiO₃. In order to further investigate the transformation mechanism between Na₂TiO₃ and NaMO₂, artificially synthesized MgTi₂O₅ was used as raw material to react with molten NaOH under the same experimental conditions. Figure 8 gives the XRD patterns of the products obtained under different roasting temperatures. It can be clearly seen that the main phases of the product were Na₂TiO₃ and MgO when the temperature was fixed at 773 K, while NaMO₂ with α-NaFeO₂-type crystal structure appeared when the temperature increased to 973 K. So it is reasonable to assume that the synthesized MgTi₂O₅ could be decomposed by molten NaOH according to the following equations:   

$$\begin{array}{l}\mathrm{M}\mathrm{g}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_5+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to 2\mathrm{N}{\mathrm{a}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_3+\mathrm{M}\mathrm{g}\mathrm{O}+2{\mathrm{H}}_2\mathrm{O}\\ 673\mathrm{K}<\mathrm{T}<773\mathrm{K}\end{array}$$(7)

  

$$\begin{array}{l}\mathrm{M}\mathrm{g}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_5+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to \\ 2\mathrm{N}\mathrm{a}\left(\mathrm{M}{\mathrm{g}}_{1-\mathrm{x}},\mathrm{T}{\mathrm{i}}_{\mathrm{x}}\right){\mathrm{O}}_2+\left(2\mathrm{x}-1\right)\mathrm{M}\mathrm{g}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\\ \mathrm{T}>873\mathrm{K}\end{array}$$(8)

Fig. 8.

XRD patterns of the product prepared by artificially synthesized MgTi₂O₅ with molten NaOH under different roasting temperatures for 1 h.

Considering that the M in NaMO₂ is trivalence, it might be assumed that Ti³⁺ should be formed during the alkali fusion process. It is known that X-ray photoelectron spectroscopy (XPS) is often utilized to detect elements and confirm their corresponding valence state. Figure 9 illustrated the XPS of Ti element in the product obtained at 973 K for 1 h. The binding energies of the double peaks are 458.1 eV and 463.6 eV for Ti 2p_3/2 and Ti 2p_1/2, respectively. The energy position of this doublet only corresponds to the Ti⁴⁺ oxidation state, suggesting that none of Ti³⁺ existed in the product.

Fig. 9.

XPS spectrum of Ti in the alkali fusion slag obtain at 973 K for 1 h. (Online version in color.)

More importantly, from Fig. 8, it also should be noted that both diffraction peaks of MgO (2θ=43.1°) and Na₂TiO₃ (2θ=40.1°) decreased a lot, while the diffraction peaks of NaMO₂ (2θ=16.4°, 41.1°) increased correspondingly with roasting temperature increasing from 773 K to 973 K. This phenomenon may be ascribed to that the Mg²⁺ ionized at relatively high temperature could partially dope into or even substitute the Ti atom of Na₂TiO₃ to form solid solution Na(Mg²⁺, Ti⁴⁺)O₂, that is to say, the apparent trivalence of M in NaMO₂ might be displayed by Ti⁴⁺ together with Mg²⁺. The results agreed well with XRD patterns (Fig. 8) and it can be inferred that higher temperature would be benefit for Mg²⁺ doping into/substituting the Ti atoms of Na₂TiO₃, leading to Ti-bearing phase transformation from Na₂TiO₃ to NaMO₂ (M=Mg²⁺, Ti⁴⁺).

In order to verify the assumption, the Raman spectra were used to investigate the change of chemical bonds in the molten NaOH treatment process. Figure 10 is the Raman spectra of synthesized MgTi₂O₅ (Fig. 10(a)) and its corresponding alkali fusion slags obtained under different roasting temperatures (Figs. 10(b) and 10(c)). It can be clearly seen that the Raman spectrum of MgTi₂O₅ shows three peaks at 281, 445 and 605 cm^–1, respectively. Some researchers suggested that the peak near 600 and 445 cm⁻¹ should be ascribed to Ti–O stretching vibrations in 6-coordinated Ti⁴⁺.^20,21) Meanwhile, the peaks near 281 cm⁻¹ correspond to the bending and stretching vibration of Mg–O bonds.²²⁾ When MgTi₂O₅ reacted with molten NaOH at 773 K for 1 h, the peaks of the alkali fusion slag (Fig. 10(b)) didn’t show obvious change comparing to that of MgTi₂O₅ (Fig. 10(a)). However, the intensity of peak at 281 cm⁻¹ decreased a lot, and in the meantime, a weak vibration peak appeared at 800 cm⁻¹ (Fig. 10(b)). This phenomenon may be ascribed to that Ti–O band still existed as 6-coordinated Ti⁴⁺ in the obtained Na₂TiO₃, but the type of Mg–O bonds changed from covalent bond in MgTi₂O₅ to ionic bond in MgO, since MgTi₂O₅ can be decomposed by molten NaOH to form Na₂TiO₃ and MgO (Eq. (7)). Further increasing temperature to 973 K, the bending and stretching vibration peaks of Mg–O bonds at 281 cm⁻¹ and Ti–O stretching vibration 445 and 600 cm⁻¹ disappeared obviously, on the contrary, the intensity of vibration peak at 790 cm⁻¹ increased dramatically (Fig. 10(c)). According to the references,^20,23,24) it is known that the vibration peak at 790 cm⁻¹ should be assigned to Ti–O²⁻ stretching vibrations in TiO₄⁴⁻ tetrahedral monomers. As a result of Mg²⁺ intercalating into Ti–O bonds, titanyl bond did not exist as 6-coordinated Ti⁴⁺, but demonstrated as Ti–O²⁻ in TiO₄⁴⁻ tetrahedral monomers, which combined together to form O–Ti–O or O–(Mg,Ti)–O chain units. According to the analysis of the Raman spectra, it is reasonable to conclude that MgTi₂O₅ can be decomposed by molten NaOH to form 6-coordinated Ti⁴⁺ and Mg²⁺ at 773 K. Then they combined with Na⁺ and O^2– supplied by molten NaOH to form NaTiO₃ and MgO, respectively. However, as the reaction temperature increased to 973 K, some Mg²⁺ would become ionized state and insert into Ti–O bonds, which made 6-coordinated Ti⁴⁺ tetrahedron become to TiO₄⁴⁻ tetrahedral monomers and then form O–(Mg,Ti)–O chain units. During this process, the crystal structure of Na₂TiO₃ changed from NaCl-type crystal structure system to α-NaFeO₂-type crystal structure, further confirming the transformation from Na₂TiO₃ to NaMO₂ (M=Mg²⁺ and Ti⁴⁺).

Fig. 10.

Raman spectra of MgTi₂O₅ and alkali fusion slags obtained under different roasting temperatures for 1 h.

In addition, as for the main impurity in Ti-bearing EAF slag (e.g. MgAl₂O₄), the corresponding phase transformation trend was also shown in Fig. 5. It can be seen that with temperature increasing from 773 K to 973 K, the intensity of diffraction peaks of MgAl₂O₄ decreased obviously, suggesting higher temperature would be benefit for MgAl₂O₄ decomposition by molten NaOH according to the following equation:   

$$\mathrm{M}\mathrm{g}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}=2\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2+\mathrm{M}{\mathrm{g}}^{2+}+{\mathrm{O}}^{2-}+{\mathrm{H}}_2\mathrm{O}$$(9)

Because NaOH can be used as an ionized solvent when the reaction temperature is higher than 623 K, it is reasonable to assume that the solid-liquid reaction process between MgAl₂O₄ and NaOH was a diffusion controlled process.¹⁸⁾ Moreover, MgAl₂O₄ belongs to the cubic crystal system (unit cell parameters are a=b=c=8.09 Å and α=β=γ=90°), and it has a relative stable and dense structure since Al ions uniformly dispersed in Mg–O bonds as shown in Fig. 11(a). Therefore, increasing the amount of NaOH during the molten NaOH treatment reaction, namely, adjusting R_slag/NaOH, would lead to complete phase transformation of MgAl₂O₄ by molten NaOH to form NaAlO₂ and MgO as illustrated in Fig. 11.

Fig. 11.

MgAl₂O₄ decomposed by molten NaOH to form NaAlO₂ and MgO. (Online version in color.)

3.3. The Effect of Ti-bearing EAF Slag/NaOH Mass Ratio (R_slag/NaOH) on the Phase Transformation

Figure 12 gave the XRD patterns of alkaline fusion slags obtained at 973 K for 1 h when R_slag/NaOH was controlled from 1:1 to 1:1.5, respectively. It can be clearly seen that none of the diffraction peaks of anosovite solid solution or Mg₂TiO₄ existed in the alkali fusion slags, while some strong diffraction peaks of Na₂TiO₃ and NaMO₂ appeared accordingly, indicating that all the Ti-bearing phases in Ti-bearing EAF slag could realize the effective phase transformation even when R_slag/NaOH was at 1:1. In addition, it is also found that the intensity of diffraction peaks of MgAl₂O₄ decreased dramatically when R_slag/NaOH was controlled from 1:1.2 to 1:1.5 at 973 K. This case may be ascribed to that much more amount of molten NaOH could ensure sufficient contact areas between reactants (Ti-bearing EAF slag and NaOH), leading to MgAl₂O₄ complete decomposition under the condition.

Fig. 12.

XRD patterns of alkaline fusion slag obtained at 973 K for 1 h with R_slag/NaOH ranging from 1:1 to 1:1.5.

More importantly, it should be noted that with R_slag/NaOH decreasing from 1:1 to 1:1.5, the intensities of diffraction peaks of Na₂TiO₃ (2θ=40.1°) decreased gradually while the intensities of diffraction peaks of NaMO₂ (2θ=16.4°, 41.1°) increased correspondingly, suggesting that some previously formed Na₂TiO₃ would be spontaneously converted to NaMO₂. This phenomenon may be explained as follows: when the R_slag/NaOH was adjusted from 1:1 to 1:1.5, according to Eq. (9), almost all MgAl₂O₄ in the slag could react with enough molten NaOH, liberating amounts of free Mg²⁺. In the meantime, such free Mg²⁺ would prefer to dope into/substitute Ti atoms of Na₂TiO₃ to form NaMO₂ (M=Mg²⁺, Ti⁴⁺), rather than combine with O^2– by ionic bond to produce MgO. This result agrees well with XRD patterns as shown in Fig. 12, that is to say, hardly any diffraction peaks of MgO appeared when MgAl₂O₄ was completely decomposed by molten NaOH. In addition, there were obvious diffraction peaks of NaAlO₂ existed in XRD patterns, indicating that Al atoms in NaAlO₂ could not be substituted by Mg²⁺ ion due to their different ionic radius (Mg²⁺=0.066 nm, Al³⁺= 0.051 nm).

Based on above analysis, in order to make the main impurity phase MgAl₂O₄ convert to water soluble NaAlO₂ completely, and then separate from insoluble sodium titanates (Na₂TiO₃ and NaMO₂) by water leaching, excessive amount of NaOH should be added to maintain a good reaction kinetic condition. Considering the energy consumption and material cost comprehensively, the roasting temperature and R_slag/NaOH should be controlled at 973 K and 1:1.2, respectively during the molten NaOH treatment process.

3.4. The Effect of Roasting Time on the Phase Transformation

Figure 13 illustrates XRD patterns of alkali fusion slags obtained under different time when roasting temperature was controlled at 973 K and R_slag/NaOH fixed at 1:1.2. It is shown that the diffraction peaks of all the Ti-bearing phases in Ti-bearing EAF slag (anosovite solid solution and Mg₂TiO₄) disappeared just within 10 min. Meanwhile, obvious diffraction peaks of corresponding sodium titanates (Na₂TiO₃ and NaMO₂) emerged, suggesting that all the Ti-bearing phases in Ti-bearing EAF slag could be converted to sodium titanates (Na₂TiO₃ and NaMO₂) promptly at the initial stage. Moreover, with prolonging the roasting time from 10 to 45 min, the intensities of diffraction peaks of MgAl₂O₄ decreased. Furthering increasing the time to 60 or 90 min, nearly all of the diffraction peaks of MgAl₂O₄ vanished, implying that relatively longer reaction time (>60 min) would benefit for complete decomposition of MgAl₂O₄.

Fig. 13.

XRD patterns of alkali fusion slag obtained at 973 K under different roasting time with R_slag/NaOH fixed at 1:1.2.

In addition, it is worth noting that the diffraction peaks of NaMO₂ with weak intensity appeared within 10 min. When the roasting time increased to 30 min and further to 90 min, the intensity of diffraction peaks of NaMO₂ increased gradually whereas that of Na₂TiO₃ decreased, and finally, even NaMO₂ became the main phase in the alkali fusion slag as shown in Fig. 13. The same phenomenon also appeared when the alkali fusion process was conducted at different roasting temperatures and R_slag/NaOH. According to the above analysis, it is believed that some trivalence or divalent metal ion like Fe³⁺ or Mg²⁺ existed in Ti-bearing EAF slag could dope into/substitute the Ti atoms of Na₂TiO₃ to form NaMO₂ (M=Ti, Fe, Mg) due to their similar ionic radius (Fe³⁺= 0.064 nm, Mg²⁺=0.066 nm and Ti⁴⁺=0.068 nm). Therefore, NaMO₂ (NaFeO₂, Na(Mg²⁺, Ti⁴⁺)O₂) appeared within 10 min should be ascribed to the doped of these metal ions with low content from Ti-bearing EAF slag. With increasing of roasting time, more MgAl₂O₄ was decomposed to liberate more Mg²⁺, therefore enhanced the chance of Mg²⁺ doping into/substituting the Ti atoms of Na₂TiO₃ to form solid solution Na(Mg²⁺, Ti⁴⁺)O₂. By this way, the amount of Na₂TiO₃ decreased while NaMO₂ increased with roasting time pronging.

Based on the aforementioned analysis, the phase transformation mechanism during the alkali fusion process can be illustrated as Fig. 14. It is shown clearly that the main Ti-bearing phases and impurity phases in Ti-bearing EAF slag can be decomposed by NaOH to form their corresponding sodium salts. Meanwhile, some cations may dope into/substitute the Ti atoms of Na₂TiO₃ to form NaMO₂ (M=Ti, Fe, Mg) due to their similar ionic radius. During the doping or substitution process, these cations originated from two sources. One is existing in the initial Ti-bearing EAF slag such as Fe³⁺ or Ti³⁺, which occupied a small fraction. The other (Mg²⁺) is from decomposition of MgTi₂O₅ or MgAl₂O₄ by NaOH with ionic state at high temperature (>873 K), which occupied high content.

Fig. 14.

The schematic diagram of phase transformation during the molten NaOH treatment process.

4. Conclusion

The present work shows that the Ti-bearing phases and the main impurity phases in Ti-bearing EAF slag can realize phase transformation effectively in molten NaOH fusion process. The phase transformation mechanism was studied systematically by thermodynamic calculation and crystal structure analysis. It is indicated that the Ti-bearing phases in Ti-bearing EAF slag can be decomposed by molten NaOH much earlier and easier than the main impurity phase MgAl₂O₄. The Ti-bearing phases transformed from anosovite solid solution or Mg₂TiO₄ to Na₂TiO₃ with NaCl-type crystal structure easily when the alkali fusion process was above 773 K with Ti-bearing EAF slag/NaOH mass ratio (R_slag/NaOH) below 1:1. However, the main impurity phase MgAl₂O₄ could not be decomposed by molten NaOH to form NaAlO₂ and MgO completely until the alkali fusion reactions were conducted at 973 K for 1 h with R_slag/NaOH below 1:1.2. In addition, with increasing roasting temperature and time or decreasing R_slag/NaOH, the formed Na₂TiO₃ with NaCl-type crystal structure could convert to NaMO₂ (M=Mg, Ti, Fe) with α-NaFeO₂-type crystal structure, due to that some metal ions like Mg²⁺ or Fe³⁺ might dope into/substitute the Ti atoms of Na₂TiO₃ to form NaMO₂ (M=Ti, Fe, Mg).

Acknowledgement

The work was financially supported by the National Natural Science Foundation of China (Nos. 51372019, 51072022 and 50874013), the National Basic Research Program of China (No. 2014CB643401).

Appendices

Table A1. Thermodynamic data.

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