Ti₃O₅ and Al₂TiO₅ Crystals Flotation Characteristics from Ti-bearing Blast Furnace Slag: A Density Functional Theory and Experimental Study

Abstract

Anosovite crystalline is an ideal mineral for flotation from the Ti-bearing blast furnace (TBBF) slag. Ti₃O₅ crystal and Al₂TiO₅ crystal are two kinds of anosovites, and the Al element significantly affects the electronic structure and flotation performance of anosovite. The floatability of Ti₃O₅ and Al₂TiO₅ crystals were studied by Mulliken populations, energy bands, and density of states (DOS). In addition, the flotation experiment of the two kinds of anosovite crystals (Ti₃O₅ and Al₂TiO₅) was conducted and proved that the density functional theory (DFT) calculation results were accurate. Compared with Ti₃O₅ crystal, the Fermi energy level of Al₂TiO₅ crystal shifts around 2 eV in a negative direction by DOS analysis, which is beneficial to flotation. And Al₂TiO₅ crystal possesses a larger value of bond population, which is 0.41, for Ti-O bonds than Ti₃O₅ crystal and the bond length of Ti-O in Al₂TiO₅ crystal is shorter, therefore Al₂TiO₅ crystal shows a stronger covalency. The changes of the Fermi energy level and the covalency bonds in Al₂TiO₅ crystal both demonstrated that doping the Al component into the Ti₃O₅ crystal was beneficial to improve the flotation effect. Moreover, the Al₂TiO₅ crystal had a higher flotation efficiency compared to the Ti₃O₅ crystal when the dosages of salicylhydroxamic acid (SHA) and sodium oleate were the same. Therefore, both DFT calculation and experiment show that the flotation effect of the Al₂TiO₅ crystal is better than that of the Ti₃O₅ crystal.

1. Introduction

In southwest of China, there is a large amount of high Ti-bearing vanadium-titanomagnetite and most of which is used for ironmaking process in blast furnace. Therefore, Ti-bearing blast furnace (TBBF) slag is becoming one of the most important secondary resources of titanium, which contains approximately 20–26% TiO₂ [1,2,3,4]. However, titanium is difficult to separate and utilize due to its dispersity in different fine minerals (about 10 μm). Some researchers [5,6] chose perovskite (CaTiO₃) as a Ti enrichment phase for the separation of titanium element. In theory, the amount of TiO₂ in perovskite is about 59%, with a density similar to the glassy phase in the slag, so separating and extracting Ti is difficult from perovskite phase in TBBF slag [7,8]. Compared with perovskite, anosovite crystal contains more than 70% TiO₂ (M_xTi_3−xO₅ and 0 ≤ x ≤ 2, where M represents the divalent Mg, Fe, Ti or trivalent Ti, Fe, Al, etc.) with a density (4.20 g/cm⁻³) significantly higher than that of glass phase (2.81 g/cm⁻³) [7]. Therefore, anosovite crystal may be a better alternative for recovering titanium from TBBF slag.

In our previous research [9,10,11,12,13,14,15,16], Ti-bearing ore sintering and Ti-bearing slag characteristics were studied systematically. B₂O₃ was chosen as the modifier for obtaining the Ti-rich phases, and the results showed that the rise of the (Mg + Al)/(Ca + Si) ratio and B₂O₃ in slag were beneficial to form [TiO₆]⁸⁻ octahedral units and thus beneficial to the precipitation of anosovite from the TBBF slag [14,15]. In addition, Anosovite was successfully obtained from TBBF slag, and the lower cooling temperature and extension of the holding time promoted the coarsening of anosovite crystals [16].

Floatation technology has been applied maturely for mineral separation, and its cost is low. The separation of anosovite crystal from the gangue minerals by flotation is influenced by the electronic structures of mineral crystals [17,18], and the floatability of each mineral is closely related to their electronic structure. Mg and Al had positive effects on the stability of Ti₃O₅ crystal structure [15], whereas experimental methods were challenging to explain the microscopic mechanisms of this phenomenon [19]. Density functional theory (DFT) [20,21], widely and successfully applied in simulations throughout engineering and sciences, a supplement of modern experimental methods, can be used to analyze mineral crystal structures from atom and electron aspects [22]. Related research was conducted on magnesium-bearing anosovite floatation from Ti-bearing electric slag, and the results showed that Mg entering the Ti₃O₅ lattice changed the crystal and electronic structures of Ti₃O₅. Changes in the electronic structures of minerals are bound to affect the adsorption of reagents on the surfaces, which caused the appearance of defects in the crystalline Ti₃O₅ and change the flotability of minerals, which was beneficial to floatation [23]. However, besides the magnesium element, there was a certain content of aluminum in the TBBF slag [14,15,16], which may affect the crystalline structure of Ti₃O₅ crystal. Still, the flotation of anosovite crystals from Ti-bearing blast furnace slag is still not reported in the literature.

Therefore, the electronic structures of Ti₃O₅ and Al₂TiO₅ crystals were calculated by DFT, and the influence of Al element on crystal structures of Ti₃O₅ was studied. In addition, the calculation results were compared with flotation experiment results. and the DFT calculation results provide clarification of the experimental results. It is expected that theoretical support for the separation process of TBBF slag through flotation aluminum-bearing anosovite crystal can be established.

2. Experimental

2.1. Materials

The single mineral sample of anosovite (Ti₃O₅) crystal was obtained from titanium production in Sichuan province in China, and its purity was up to 97.12%, with only small amount of Al₂O₃, MgO, Cr, etc. dissolve in it. Tt is therefore marked as Ti₃O₅ crystal and used for the flotation experiment. Besides, the anosovite crystal was doped with aluminum, and the chemical composition is mainly Al₂TiO₅, the purity of which is 92.63%. Ti₃O₅ and Al₂TiO₅ crystals (each 4 g) were used as raw materials for the flotation experiments in this study.

2.2. Analysis and Experiment Method

2.2.1. Construction Analysis of Ti₃O₅ and Al₂TiO₅ Crystalline Models

DFT is a high-precision quantum chemical calculation tool that uses electron density instead of wave function as the research object. The Cambridge Sequential Total Energy Package (CASTEP) module in Materials Studio software [24,25] is on the basis of the theory of plane wave pseudopotential and the number and type of atoms can be used to predict and calculate the crystal lattice parameters, band structure, solid density, charge density and wave function. The interaction between ionic core and valence electrons was described using the Ultra-soft pseudopotentials [26]. The selected valence electron configurations in this study were Ti3s²3p⁶3d²4s², Al3s²3p¹, and O2s²2p⁴. Furthermore, the sampling accuracy of the Brillouin zone was defined by a 2 × 2 × 1 k-point set. The Pulay density mixing method (PDMM) is used for self-consistent electronic minimization, in which the convergence tolerance energy and maximum force tolerance on each atom was set to 1.0 × 10⁻⁶ eV/atom and 1.0 × 10⁻² eV/Å for the energy calculation.

The crystal structure diagrams of Ti₃O₅ [23] and Al₂TiO₅ formed after the solid solution of Al into Ti₃O₅ crystals were shown in Figure 1. Relevant research results showed that Al-O bond and Ti-O bond with stronger covalency existed in Al₂TiO₅ crystal formed after the solid solution of Al into Ti₃O₅ crystal, which was more stable than Ti₃O₅ crystal [22]. Both crystal Ti₃O₅ and Al₂TiO₅ belong to the orthorhombic with space group Cmcm. The lattice parameters a, b, and c of Ti₃O₅ are 9.84, 3.76, and 9.86 nm, respectively, which for the Al₂TiO₅ crystal are 9.84, 3.76, and 9.861 nm, respectively. The lattice parameters $\alpha$, $\beta$, and $\gamma$ of Ti₃O₅ and Al₂TiO₅ are 90°.

2.2.2. Flotation Experimental

The flotation behavior of anosovite crystal was carried out in a small hanging laboratory flotation machine with an effective volume of 60 mL and an impeller speed of 1900 r/min. The water in flotation process was deionized water. The test procedure was as follows: 4 g of anosovite sample with a particle size of about 180 mesh was placed in a 60 mL flotation cell. Then, the collector and frother were introduced in the flotation machine and then stirred for 2 min and 3 min, respectively. Finally, the flotation pulp scraped for 4 min. The pH adjuster was hydrochloric acid and sodium hydroxide, and the test temperature was 20 ± 5 °C. After collecting and drying, the recovery of the product was calculated.

3. Results and Discussion

3.1. Energy Bond Structures of Ti₃O₅ and Al₂TiO₅ Crystals

Figure 2 and Figure 3 show the band structure of two different anosovites of Ti₃O₅ and Al₂TiO₅. The Fermi energy level is uniformly used as the energy zero point in the Figure 2. It can be seen intuitively that the energy band of Ti₃O₅ is divided into five parts from −60 eV to 10 eV.

From Figure 3, there are five parts for the valence band of Al₂TiO₅ crystal, and the band structure of Al₂TiO₅ is relatively close to that of Ti₃O₅. This may be due to the same valency of Al³⁺ and Ti³⁺, so Al entering into Ti₃O₅ crystal has relatively little impact on its energy band structure. However, the energy of all Al₂TiO₅ band increased about 2 eV compared with that of Ti₃O₅, indicating that he valence and conduction bands have increased in energy. Moreover, the anions adsorption on the surface of minerals is promoted by the movement of Fermi energy level and thus the adsorption probability increases [27]. Therefore, the adsorption of anosovite on the mineral surface is affected by the site of the Fermi energy level. Changes in the electronic structures of minerals are bound to affect the adsorption of reagents on the surfaces, which change the flotability of minerals. A decrease in the Fermi level may result in an increase in the number of holes on the surface of the Al₂TiO₅, which in turn may increase the adsorption or adsorption rate of the anion collector on the crystal during the flotation process, as well as the adsorption stability, which is conducive to the flotation process [28,29]. Therefore, doping Al component into Ti₃O₅ should be beneficial to improve the flotation effect.

3.2. DOS Distributions of Ti₃O₅ and Al₂TiO₅ Crystals

The distributions of electronic DOS of Ti₃O₅ and Al₂TiO₅ crystals are shown in Figure 4 and Figure 5. From Figure 4, the valence band of Ti₃O₅ consists of five parts.

From the DOS of each atom, it can be seen that the valence band peak for Ti₃O₅ crystal at approximately −59.1 eV, which is attributed to orbit Ti4s electrons. Moreover, almost the entire valence band peak at −35.0 and −20.0 eV come from Ti3p orbit and O2s orbit electrons, respectively. The above-mentioned valence bands are all with very sharp peaks and narrow energy bands, which show that the electron is distributed comparatively locally. The electronic peak ranged from −8.0 to -4.0 eV is contributed the O2p and part of Ti3p and Ti3d orbit. The O2p orbit exhibits a very high electronic peak due to the strong localization of electron in the orbit. The overlap of the electronic peaks of the O2p and Ti3d orbit at −8.0 ~ 4.0 eV, and the Fermi energy level indicates that electrons of the Ti3d to O2p orbit have a transition phenomenon and can interact with each other. The results are consistent with Wang et al [23]. There are some changes in the valence band of Ti₃O₅ after Al entering Ti₃O₅ lattice. From Figure 5, Al3s and Al3p orbits appear on the peaks of Al₂TiO₅ valence band at about −18, −6, and −0 eV and the Fermi energy level. Therefore, in addition to the contribution of Ti3d and O2p orbits, the valence band peak at the Fermi energy level of Al₂TiO₅ crystal also comes from Al3s and Al3p orbits. Moreover, the electronic peaks of the O2p, Ti3d, Al3s, and Al3p orbit overlap at the Fermi energy level, demonstrating that they can interact with each other.

3.3. Distributions of Ti₃O₅ and Al₂TiO₅ Atomic Charges

The distributions of electronic states of atoms were from Mulliken population analysis. The valence electrons distributed around atoms are known as atom populations and the overlapped electron charges distributed between two atoms are called bond populations. With the help of Mulliken population analysis [30,31,32,33], the charge distribution, transfer distribution and bonding between atoms can be understood.

Table 1. Average Mulliken populations of atoms in Ti₃O₅ crystal.

Species	Atomic Populations (Mulliken)			Total/e	Charge/e
	s	p	d
O	1.84	4.83	0	6.67	−0.67
Ti	2.26	6.31	2.30	10.87	+1.12

is the calculation results of the Mulliken population analysis of Ti₃O₅ crystal, which is similar to Wang et al. [23]. It can be seen from the average charge of the atom that the electrons number in the Ti atom is 10.87e and lost 1.12e, which indicated that the Ti in Ti₃O₅ crystal is a donor of electrons. Meanwhile, the number of electrons localized on the O atom is 6.67e and obtained 0.67e, indicating that O is acceptor of electrons. Besides, the calculation of Mulliken populations of Ti–O bonds in Ti₃O₅ crystal shows that the value of bond population for Ti–O is 0.367 and the bond length is 2.038 Å. Therefore, the Ti–O bond inside the Ti₃O₅ crystal is covalent, which has directionality, saturation and certain hydrophilicity.

Table 2. Average Mulliken populations of atoms in Al₂TiO₅ crystal.

Species	Atomic Populations (Mulliken)			Total	Charge
	s	p	d
O	1.85	5.08	0	6.93	−0.93
Al	0.51	0.87	0	1.38	+1.63
Ti	2.22	6.22	2.17	10.61	+1.39

and

Table 3. Mulliken populations of bonds in Al₂TiO₅ crystal.

Bond	Population (e)	Length (Å)
Ti-O	0.41	1.98
Al-O	0.46	1.85

are the calculation results of the Mulliken population analysis of Al₂TiO₅ crystal. It can be seen that Ti and Al lose 1.39 e and 1.63 e, respectively, so the Ti and Al have 1.39 e and 1.63 e charges, respectively. O obtained 0.93e and have the −0.93 e of charges. The value of population and length for Ti-O bond in Al₂TiO₅ crystal are 0.41 e and 1.98 Å, respectively, while that of Al-O in Al₂TiO₅ crystal are 0.46 Å and 1.85 Å, respectively.

Compared with Ti₃O₅ crystal, O atom obtains more electrons in Al₂TiO₅ crystal because Ti atom gets electrons more easily than Al atom. From Table 1 and Table 2, O atom in Ti₃O₅ and Al₂TiO₅ crystals obtains 0.67 and 0.93 electrons, respectively. Moreover, Al₂TiO₅ crystal possesses a larger value of bond population for Ti–O than Ti₃O₅ and the bond length of Ti–O in Al₂TiO₅ crystal is shorter compared with Ti₃O₅; therefore, the covalency of crystalline Al₂TiO₅ is stronger than Ti₃O₅. In addition, compared with the Ti–O bond, the value of bond population for Al–O is larger, which indicates that the covalency of Al-O bond in the Al₂TiO₅ crystal is greater than that of the Ti–O bond.

3.4. Flotability of Ti₃O₅ and Al₂TiO₅ Crystals

The electron structures of anosovite crystalline display obvious difference between Ti₃O₅ and Al₂TiO₅ crystal for the doping of aluminum into Ti₃O₅ crystal. In addition, they have a significant difference on the chemical compositions, so it is reasonable to assume that the flotabilities of Ti₃O₅ and Al₂TiO₅ crystal is different. Therefore, the flotation experiments on the Ti₃O₅ and Al₂TiO₅ crystals used sodium oleate, SHA were conducted, and the experiment results are given in Figure 6 and Figure 7. When the dosage of sodium oleate was 8.0 × 10⁻⁵ mol/L, the recovery rate of Ti₃O₅ crystal reaches the highest value of 72.21%, while that of Al₂TiO₅ crystal reaches the maximum value of 79.24%. When using SHA as collectors, the best recovery rate of Ti₃O₅ and Al₂TiO₅ crystals were 80.12% and 84.15%, respectively. For comparison [34], the anosovite of Mg_0.09Ti_2.91O₅ from Ti-bearing electric slag has a higher recovery, which can reach a maximum recovery of 93.26% at an SHA dosage of 4 × 10⁻⁵ mol/L. It can be seen from Figure 6 and Figure 7 that the recovery rate of anosovite crystal is higher when SHA is used as the collector, indicating that sodium oleate had a stronger ability to capture anosovite. More importantly, the recovery rate of Al₂TiO₅ crystal has a higher recovery rate compared with Ti₃O₅ crystal when the flotation conditions are the same. Therefore, doping the Al component into the Ti₃O₅ crystal should be beneficial to improve the flotation effect from TBBF slag.

4. Conclusions

In this paper, DFT is used to study the effects of Al on anosovite crystal structure, electronic structure, and flotation behavior. Moreover, the flotation experiment on Ti₃O₅ and Al₂TiO₅ crystals are conducted, and DFT calculation results are accurate. The main conclusions are as follows:

(1) The energy of all Al₂TiO₅ band increased about 2 eV compared with that of Ti₃O₅, indicating that the valence and conduction bands have increased in energy. The energy band shifts to the positive direction promotes the flotation effect.

(2) Compared with Ti₃O₅ crystal, O atom obtains more electrons in Al₂TiO₅ crystal because the Ti atom receives electrons more easily than Al atoms in the Al₂TiO₅ crystal. The Al₂TiO₅ crystal possesses a larger Ti–O bond population than Ti₃O₅, and the Ti–O bonds length is shorter; therefore, the covalency of Al₂TiO₅ crystal is stronger. Moreover, the Al–O bonds covalency in the Al₂TiO₅ crystal is greater than that in the Ti₃O₅ crystal.

(3) The Al₂TiO₅ crystal has a higher recovery rate compared with the Ti₃O₅ crystal when the flotation conditions are the same. Therefore, doping an Al component into the Ti₃O₅ crystal should be beneficial for improving the flotation effect from TBBF slag.