Clean preparation of rutile from Ti-containing mixed molten slag by CO 2 oxidation

: The e ﬀ ects of SiO 2 and CO 2 on the crystallization action of Ti-containing mixed molten slag (molten Ti-containing blast furnace slag and molten Ti slag) were discussed by thermodynamic calculation and speci ﬁ c experiments. The results of thermodynamic calculation indicated that the increase of SiO 2 addition mass and CO 2 oxidation time can promote the transformation of anosovite and sphene to rutile. The experiment results showed that the phase composition of modi ﬁ cation slag was only rutile under the SiO 2 addition mass of 110 g and the CO 2 oxidation time of 180 s. Moreover, the formation theory of rutile was investigated. Using CO 2 as an oxidizing gas can not only prepare rutile but also achieve carbon neutrality, which is a clean preparation method.


Introduction
China has rich Ti resources, which mainly are present as V-Ti magnetite.Its total reserve is approximately 10 billion tons, 90% of which are located in the Panzhihua and Xichang regions of Sichuan province.After beneficiation, 50% of titanium components enter the V-Ti magnetite concentrate.After blast furnace ironmaking, the Ti components in the concentrate enter the slag to form Ti-containing blast furnace slag (20-25% TiO 2 ).The multipurpose use of Ti-containing blast furnace slag has been a technical difficulty in China.Because the Ti compositions in this slag are dispersedly distributed in different fine Ti-containing phases, conventional beneficiation methods make it difficult to separate the Ti-containing phase from the gangue phase.Therefore, the multipurpose use of Ti-containing blast furnace slag is extremely difficult.
To realize the multipurpose use of Ti-containing blast furnace slag, a lot of related studies have been carried out.According to the classification of products, research methods can be divided into the following categories: building materials [1], titanium alloy [2], photocatalytic materials [3], titanium white [4], titanium tetrachloride [5], perovskite [6], anosovite [7,8], and rutile [9][10][11][12][13][14][15][16].Although the previous methods can realize the multipurpose use of Ti-containing blast furnace slag, they generally have the disadvantages of high energy consumption and high cost.So far, Ti-containing blast furnace slag can only be stacked in the slag yard, causing the squander of Ti resources and environmental pollution.
To resolve the above-mentioned problems, the platform technology [17] for molten slag metallurgy, mineral regeneration, and resource recycling were proposed by Professor Zhang Li of Northeastern University.In view of the mineral characteristics of metallurgical molten slag, a separation technology for oxide mineral settling was proposed for the first time to realize the efficient utilization of physical heat and the efficient recovery of minerals.The platform technology is developed by the interdisciplinary integration of geology, minerals, materials, metallurgy, and thermal energy.It has the characteristics of short process, low cost, cleanness, low carbon, and high efficiency.It does not require heating or a small amount of heating and realizes energy utilization, mineral regeneration, resources recycling, and environmental protection.
Based on the platform technology, the clean, high-efficiency, and low-carbon utilization technology of Ti-containing mixed molten slag (molten Ti-containing blast furnace slag and molten Ti slag) was proposed.This technology consists of two parts: 1. Mineral regeneration: Taking full the merits of the high physical heat and chemical activity of molten Ti-containing blast furnace slag and molten Ti slag, the two molten slags are mixed.With the help of oxidation and The previous studies [18][19][20] have achieved the transformation of Ti components in Ti-containing mixed molten slag to rutile by O 2 oxidation.In this article, the preparation of rutile from Ti-containing mixed molten slag by CO 2 oxidation was first proposed.Using CO 2 as an oxidizing gas can not only prepare rutile but also achieve carbon neutrality, which is a clean preparation method.
The objective of this article is to propose a method for the clean preparation of rutile from Ti-bearing mixed molten slag by CO 2 oxidation.The effects of SiO 2 and CO 2 on the crystallization action of the Ti-containing mixed molten slag were studied by thermodynamic calculation and specific experiments.Furthermore, the formation theory of rutile was investigated.
2 Materials and methods

Materials
Ti-containing blast furnace slag and Ti slag were gotten from the Panzhihua (Sichuan province, China).The chemical components and phase compositions of Ti-containing blast furnace slag and Ti slag are illustrated in Table 1 and Figure 1.
As illustrated in Figure 1, the phase components of Ti-containing blast furnace slag were perovskite, magnesiaalumina spinel, diopside, and akermanite.The phase components of Ti slag were anosovite and anorthite.

Experimental procedures 2.2.1 Thermodynamic calculation
The schematic diagram of experimental process is shown in Figure 2. As illustrated in Figure 2, the effects of SiO 2 and CO 2 on the crystallization action of Ti-containing mixed molten slag were calculated by the equilibrium module of software Factsage with FToxid and FactPS databases (version 7.1).The chemical compositions of Ti-containing mixed molten slag, the addition mass of SiO 2 , and CO 2  oxidation time were input into the software.Then, it will output the type and mass of precipitation phases.

Modification experiments
Based on the above-mentioned thermodynamic calculation, 278 g of Ti slag, 222 g of Ti-containing blast furnace slag, and a definite mass of silicon dioxide (50, 70, 90, and 110 g) were placed in a crucible at 1,460°C for 30 min.Whereupon, CO 2 was introduced into the mixed molten slag with a given time (60, 120, 180, and 240 s) and a flow rate of 4 L•min −1 .
Afterward, the modification slag was cooled to room temperature with a cooling rate of 6°C•min −1 .The SiO 2 was of analytical grade, and the purity of CO 2 was 99% (mass fraction).The schematic diagram of the modified experimental device is shown in Figure 3.As shown in Figure 3, the heating equipment was a vertical MoSi 2 furnace with a B-type thermocouple.
It was estimated that the overall absolute temperature accuracy of the furnace was ±3°C.

Characterization
The phase components were determined by X-ray diffraction (X'PERT PROMPD/PW3040).The microstructure and element distribution were determined by scanning electron microscopy (TESCAN VEGA III) equipped with an energy-dispersive spectrometer (INCA Energy 350).
3 Results and discussion

Thermodynamic calculation on CO 2 oxidation time
The addition mass of silicon dioxide was fixed at 50 g.After that, the influence of CO 2 oxidation time on the crystallization action of the mixed molten slag was investigated by software.The calculation results are illustrated in Figures 4-6.As illustrated in Figure 4a and b, the main titanium-bearing mineral phases are anosovite ((AO•2TiO 2 ) m •(B 2 O 3 •TiO 2 ) n ) and sphene (CaSiTiO 5 ) while the CO 2 oxidation times are 0 and 60 s.As illustrated in Figure 4c-f, the titanium-bearing mineral phases are rutile, sphene, and anosovite, while the CO 2 oxidation times are 120, 180, 240, and 300 s.It can be seen that other phases begin to precipitate when the mass of rutile precipitation reaches the maximum value.Moreover, as shown in Figure 5, the crystallized temperature of rutile rises as CO 2 oxidation time improves.To sum up, the improvement of CO 2 oxidation time can promote the crystallization of rutile crystals and inhibit the crystallization of other crystals.
As illustrated in Figure 6, with the CO 2 oxidation time improving from 0 to 180 s, the mass of rutile precipitation markedly rises, and the mass of anosovite and sphene precipitation decreases rapidly.As the CO 2 oxidation time improves to 300 s, the mass of rutile precipitation no longer improves, and the mass of anosovite and sphene precipitation no longer decreases.Therefore, the optimal CO 2 oxidation time is 180 s.It can be seen that the improving of CO 2 oxidation time can promote the transformation of anosovite and sphene to rutile.

Experimental verification on CO 2 oxidation time
The addition mass of silicon dioxide was fixed at 50 g.After that, the influence of CO 2 oxidation time on the crystallization action of the mixed molten slag was investigated by specific experiments.The experiment results are shown in Figures 7 and 8 and Table 2.
As illustrated in Figure 7, the phase components of the modification slag were unchanged while the CO 2 oxidation time was 60-240 s, that is, rutile and anosovite.Nevertheless, the diffraction peaks of anosovite reduced and the peak intensity significantly decreased when the CO 2 oxidation time increased from 60 to 180 s, meaning that the improvement of CO 2 oxidation time promoted the transformation of anosovite to rutile.It can be seen from Figure 8 and Table 2 that P3, P6, P9, and P12 were rutile.P2, P5, P8, and P11 were anosovite.P1, P4, P7, and P10 were matrix phases.The above-mentioned results imply that the phase components of the modification slag were rutile and anosovite while the CO 2 oxidation time was 60-240 s.Moreover, as the CO 2 oxidation time improved from 60 to 240 s, the Ti content of rutile reduced from 66.13 to 60.87 wt%, and the O content of rutile increased from 33.87 to 39.13 wt%.When the CO 2 oxidation time was 240 s, the mass ratio of Ti with O was 60.87/39.13.The ratio was close to the Ti/O mass ratio (6/4) of TiO 2 , meaning that the improvement of CO 2 oxidation time   accelerated the transformation of anosovite to rutile.To sum up, the experimental results were consistent with the thermodynamic calculation results; that is, the improvement of CO 2 oxidation time accelerated the transformation of anosovite to rutile.

Thermodynamic calculation on the addition mass of SiO 2
The CO 2 oxidation time was fixed at 180 s.After that, the influence of the addition mass of silicon dioxide on the crystallization action of the mixed molten slag was investigated by FactSage software.The calculation results are shown in Figures 9-11.
As illustrated in Figure 9a-f, the Ti-bearing mineral phases are rutile, sphene, and anosovite while the addition mass of silicon dioxide is 30-130 g.Moreover, it can be seen that other phases begin to precipitate when the mass of rutile precipitation reaches the maximum value.As illustrated in Figure 10, the crystallization temperature of rutile raises as the addition mass of silicon dioxide improves.To sum up, the improvement of the SiO 2 addition mass can promote the crystallization of rutile crystals and inhibit the crystallization of other crystals.As illustrated in Figure 11, when the addition mass of silicon dioxide improves from 30 to 110 g, the mass of rutile precipitation observably improves, and the mass of anosovite and sphene precipitation decreases rapidly.With the addition mass of silicon dioxide improving to 130 g, the mass of rutile precipitation remained unchanged, and the mass of anosovite and sphene precipitation also remained unchanged.Thus, the optimum addition mass of SiO 2 is 110 g.It can be seen that the improvement of the SiO 2 addition mass can promote the transformation of anosovite and sphene to rutile.

Experimental verification on the addition mass of SiO 2
The CO 2 oxidation time was fixed at 180 s.After that, the influence of the addition mass of silicon dioxide on the crystallization action of the mixed molten slag was investigated by relevant experiments.The experiment results are shown in Figures 12 and 13 and Table 3.
As illustrated in Figure 12, the phase component of the modification slag was unchanged when the addition mass of silicon dioxide improved from 50 to 90 g, i.e., rutile and anosovite.However, the diffraction peaks of anosovite reduced and the peak intensity significantly decreased.With the addition mass of SiO 2 improving from 90 to 110 g, anosovite phase disappeared, and the Ti-bearing phase of the modification slag was only rutile.Furthermore, it can be seen from Figure 13 and Table 3 that P2, P5, and P8 were all anosovite.P3, P6, and P9 were all rutile.P1, P4, and P7 were matrix phases.The above results imply that the phase components of the modification slag were rutile and anosovite when the addition mass of silicon dioxide was 50-90 g.With the addition mass of silicon dioxide improving from 90 to 110 g, the anosovite phase disappeared, and the Ti-bearing phase of the modification slag was only rutile.To sum up, the experimental results are consistent with the thermodynamic calculation results; that is, the improvement of the SiO 2 addition mass promoted the transformation of anosovite to rutile.
It can be seen from the above results that the thermodynamic calculation contained different phases, but the experiment did not.This is because theoretical calculation only considered thermodynamic conditions and ignored kinetic conditions.Because the purpose of this article was to obtain rutile, we only focused on the transformation of titanium-containing phases.According to the results of thermodynamic calculation, the main Ti-bearing phases (due to the very low mass of Ti 20 O 39 and perovskite, they were ignored) were rutile, anosovite, and sphene.According   to the results of experiments, the main Ti-bearing phases were rutile and anosovite.It can be seen that the theoretical calculation results were consistent with the experimental results.The only difference is that the titanium-bearing phases of the experimental results did not include sphene.This may be because thermodynamic calculation ignored the effect of kinetics conditions.

Theory of rutile precipitation
In order to investigate the theory of rutile precipitation, the standard Gibbs free-energy changes (ΔG Φ ) of relevant reactions were calculated by the reaction module of Factsage software.The calculation results are illustrated in Figure 14.

Conclusions
1.The improvement of SiO 2 addition mass and CO 2 oxidation time can promote the transformation of anosovite to rutile.2. The optimal experiment conditions were the SiO 2 addition mass of 110 g and the CO 2 oxidation time of 180 s, and the phase composition of slag was only rutile under the above conditions.3. Using CO 2 as an oxidizing gas can not only prepare rutile but also achieve carbon neutrality, which is a clean preparation method.

Figure 3 :
Figure 3: Schematic diagram of modified experimental device.

Figure 6 :
Figure 6: Influence of CO 2 oxidation time on the final mass of Ti-containing precipitation phases.

Figure 5 :
Figure 5: Effect of CO 2 oxidation time on the crystallized temperature of rutile.

Figure 7 :
Figure 7: Phase components of the modification slags with diverse CO 2 oxidation times.

Figure 11 :
Figure 11: Effects of the addition mass of SiO 2 on the final mass of Tibearing precipitation phases.

Figure 10 :
Figure 10: Effects of the addition mass of SiO 2 on the crystallized temperature of rutile.

Figure 12 :
Figure 12: Phase components of the modification slags with diverse addition mass of SiO 2 .
time improves.In other words, the precipitation of anosovite is inhibited.It is well known that anosovite is a solid solution based on Ti 3 O 5 .Thus, CO 2 oxidation can eliminate low-valent titanium oxides including Ti 3 O 5 and promote the transformation of anosovite into rutile.To sum up, reactions (3-7) are restrained, and Ti compositions are present as rutile while CO 2 and SiO 2 are concurrently added to the mixed molten slag.

Table 2 :
Element distribution of every point in Figure8

Table 3 :
Element distribution of every point in Figure13