Effect of the Basicity on the Crystallization Behavior of Titanium Bearing Blast Furnace Slag

Preparation of the synthesis slag

The slag used in this experiment was synthesized with chemical reagent-grade oxides. Pre-melting was applied for mixed powders with total weight of 100 g, including CaO (98 pct), MgO (98 pct), SiO₂ (99 pct), TiO₂ (99 pct) and Al₂O₃ (98 pct) powder. The final composition after mixing is close to the industrial slag composition. Details about preparing the synthesized slag have been described in previous work [13].

In situ observation of the slag crystallization by CLSM at various basicity

(1) Design of slag composition

To study the effect of the basicity on crystallization behavior of the titanium bearing slag, weight percentageof MgO, Al₂O₃ and TiO₂ was kept constant, binary basicity was adjusted from 0.5 to 1.3, and thermodynamics calculation including the basicity of industry BF slag has been carried out. Table 1 shows the chemical compositions of slag with various basicity. Based on the thermodynamics calculation, the basicity is set as 0.6, 0.8 and 1.1 in the CLSM experiments.

(2) The procedure of melting and solidification

The synthetization of slag took place in a platinum crucible (8 mm in inner diameter and a height of 5 mm) under an argon atmosphere in the CLSM. The sample was heated to 1,500 °C with a heating rate of 300 °C/min and held for 2 min. Then it was cooled rapidly to 1,100 °C with a cooling rate of 600 °C/min, and held for certain time to examine the isothermal crystallization of the molten slag. The slag was then cooled to room temperature. Temperatures were measured by a B-type thermocouple welded at the bottom of the Pt holder. The temperature accuracy was confirmed by melting experiments of pure copper (melting point: 1,083 °C) and pure nickel (melting point: 1,453 °C). In order to obtain the crystallization process during isothermal process, video frames were analyzed with Image J.

Characterization

The crystal structures of the slag prepared using the chemical regents at various basicity were characterized by Raman spectroscopy. The cooled samples after crystallization experiments were then characterized by X-Ray Diffraction (XRD, D/MAX2500PC, Rigaku Corporation) and Scanning Electron Microscopy (SEM, AURIGA, Carl Zeiss AG) in order to confirm the phases and morphology of crystals.

Effect of the basicity on the basic structure of the slag

From Figure 1, it can be seen that the peaks position and their shape are similar regardless of the basicity of the slag. It seems that the basicity of the slag has little effect on the crystal structure of the slag. According to Mysen et al. [14], the main envelope curve in the Raman spectra of CaO-MgO-SiO₂ (34.2 mol% of Si) was observed between 800 cm^–1 and 1,150 cm^–1 and the maximum value of the spectral curve was located between 850 cm^–1 and 900 cm^–1. Obviously, the bands composing the envelope stemmed from the three-dimensional network of silicate. With increasing content of silicate, similar results can be obtained [15]. Therefore, when the mass fractions of MgO, Al₂O₃ and TiO₂ are constant, changes of binary basicity of the titanium bearing slag results in unnegligible changes of the crystal structure. Namely, the envelope from 600 cm^–1 to 1,000 cm^–1 is three-dimensional network of silicate in the slag and no obvious changes occur with the variation of the basicity. Therefore, it can be concluded that the molten titanium bearing BF slag has similar structure of three-dimensional network of silicate when the molten slag was cooled to room temperature in air.

Figure 1:

Raman spectra for samples with different basicity of the slag.

Thermodynamics calculation

Figure 2 shows the results of the thermodynamics calculation by Equilibrium module of FactSage 6.2 at various basicity. It can be seen that when the basicity of the slag is lower than 0.6, the main phase is TiO₂. With increasing basicity, perovskite (CaTiO₃) crystallized first at higher temperature rather than TiO_2. Then it gradually becomes the main crystallization phase. The crystallization temperature of TiO₂ and clinopyroxene (CaMgSi₂O₆) is roughly the same at about 1,170 °C. When CaTiO₃ precipitates, the crystallization temperature of TiO₂ become lower than that of CaMgSi₂O₆. The crystallization temperature of CaTiO₃ increases with the increase of the slag basicity. When the basicity was inreased to 1.0, the crystallization temperature of CaTiO₃ exceeds 1,400 °C. Namely, CaTiO₃ is the first phase crystallized from titanium bearing BF slag. CaMgSi₂O₆ is the dorminate phase, particularly when the basicity is lower than 1.0, its composition exceeds 60 % . However, as the slag basicity is higher than 0.9, the cyrstallization temperature of CaMgSi₂O₆ is about 200 °C lower than that of CaTiO₃.

Figure 2:

Theoretical isothermal phase changes of the slag with different basicity during cooling.

Figure 3 shows the effect of the basicity on the crystallization temperature of CaTiO₃, TiO₂ and CaMgSi₂O₆ in the synthesized titanium bearing BF slag. The crystallization temperature of CaTiO₃ increases from 1,200 °C to 1,470 °C when the basicity of the slag changes from 0.6 to 1.3. However, the crystallization temperature of CaMgSi₂O₆ has no obvious change. Both the crystallization temperature and amount of TiO₂ decrease with increasing of slag basicity because titanium oxides in the slag enters into perovskite from rutile.

Figure 3:

The effect of basicity on the crystallization temperature of perovskite, rutile and CaMgSi₂O₆ in Ti-bearing slag.

Figure 4 shows the crystallization process when the basicity is 0.6. When the slag was cooled to 1,100 °C, crystals was observed on the surface of the molten slag. Amounts of the crystals increases with increasing holding time until the molten slag solidifies completely. No obvious morphology change was observed during the crystallization. The XRD results show that the crystallization phase is CaTi₂₁O₃₈, as shown in Figure 5; and the morphology of the crystals is strip, as shown in Figure 6(a).

Figure 4:

The crystallization process of slag with basicity of 0.6: (a) 649.47 s, (b) 656.97 s, (c) 662.54 s, (d) 671.12 s, (e) 838.54 s and (f) 887.64 s.

Figure 5:

The XRD pattern of slag sample with basicity of 0.6 after CLSM experiment.

Figure 6:

The SEM images of slag sample surface with different basicity after CLSM experiment. (a) C/S=0.6, 3,000×, (b) C/S=0.8, 3,000× and (c) C/S=1.1, 2,500×.

Figure 7 shows the crystallization process from molten slag when the basicity is 0.8. The crystallization process is more obvious than that when the slag basicity is 0.6. As the temperature decreases to 1,100 °C, small crystals appear. With increasing of the holding time, some small crystals combine to reform larger crystals and another phase appears and its morphology is diamond. The diamond crystals grow up and the small white crystals gather together. Finally, the small white crystals disapear.

Figure 7:

The crystallization process of slag with basicity of 0.8: (a) 687.77 s, (b) 692.46 s, (c) 693.13 s, (d) 700.44 s, (e) 706.47 s, (f) 715.12 s, (g) 718.14 s, (h) 720.10 s and (i) 725.33 s.

The XRD patterns show that the crystallization phases are complicate, including CaMg_0.39 Al_0.87 Ti_0.48 Si_1.26O₆, CaTiSiO₅, CaMgSi₂O₆, as shown in Figure 8 CaTiSiO₅ belongs to silicate, monoclinic system, and its cross section is diamond. Thus the observed crystals in in-situ experiment are CaTiSiO₅. The micromorphology of the cystals are shown in Figure 6(b). It can be seen that some small dendrites appear.

Figure 8:

The XRD pattern of slag sample with basicity of 0.8 after CLSM experiment.

Figure 9 shows the crystallization process from molten slag when the basicity is 1.1. From Figure 9(a)–9(f), some dendrites appear and then grow up in a specific direction until the molten slag solidifies totally. The XRD results show that crystallization phase is CaTiO₃, as shown in Figure 10, and the morphology is dendrite, as shown in Figure 6(c).

Figure 9:

The crystallization process of slag with basicity of 1.1: (a) 792.86 s, (b) 794.14 s, (c) 795.41 s, (d) 795.81 s, (e) 796.95 s and (f) 816.88 s.

Figure 10:

The XRD pattern of slag sample isothermally crystallized with basicity of 1.1.

The main chemical reactions related with CaO in the titanium bearing BF slag are described as eqs (1)–(7). According to the thermodynamics analysis, as shown in Figure 11, the standard Gibbs free energy changes of chemical reactions (6) and (7) are much smaller than that of reaction (3). Namely, CaO prefers to react with SiO₂ in the presence of Al₂O₃ and MgO, rather than with TiO₂. Therefore, it can be predicted that the activity of CaO on this titanium bearing slag decreases with the decrease of the basicity of the slag. It could be obtained that increasing the slag basicity inhibits the formation of CaTiSiO_5, and CaMgSi₂O₆ until CaTiO₃ forms. The phase diagram of CaO-SiO₂-MgO-Al₂O₃-TiO₂ is constructed by Phase Diagram module of FactSage6.2, as shown in Figure 12.

Figure 11:

Gibbs free energy changes with temperature of the reactions of CaO and other oxides in titanium bearing BF slag.

Figure 12:

Diagram calculated by FactSage of titanium bearing BF slag.

In Figure 12, number 1 to 9 in the diagram represent various basicity from 0.5 to 1.3, as listed in Table 1. From the diagram, it can be seen that when TiO₂ content in the slag keeps a certain value of about 23 %, CaAl₂Si₂O₆ crystallized first. Namely, when the basicity is below 0.5, CaO combines with Al₂O₃ and SiO₂ to form CaAl₂Si₂O₆ rather than with TiO₂ to form CaTiO₃. It can be predicted that the activity of CaO in this titanium bearing BF slag decreases with the decrease of the slag basicity. When the CaO content increases and the basicity exceeds 0.5, CaTiO₃ starts to form. This is different from the results of CLSM experiments. The main reason is that experiment is accomplished under super cooling rate. When the cooling rate is much higher, the mixed phases crystallize, such as CaTiSiO₅, CaMgSi₂O₆, CaMg_0.39Al_0.87Ti_0.48Si_1.26O₆, from the molten slag at low basicity rather than single phase as rutile.