Influence of TiO 2 on viscosity of aluminosilicate melts

Abstract

The structurally related viscosity model proposed in our previous studies has been extended to TiO₂ bearing melts. Experimental data on different systems containing CaO, MgO, Al₂O₃, TiO₂ and SiO₂ were assessed, and good agreement was achieved between the calculated and measured viscosities. It can be concluded that the addition of TiO₂ results in a decrease in viscosity, which is consistent with the experimental findings.

Introduction

With the gradual consumption of high grade raw materials with high concentration of iron, sources of iron bearing materials such as titaniferrous ore, once regarded as uneconomic, are increasingly being used in the electric arc and blast furnace. However, the high content of TiO₂ may result in significant issues such as poor reduction of the ore, change in the liquidus temperature of the slag and slag viscosity deviation from the optimum processing condition. In order to ensure efficient process operations, the variations of thermophysical and thermochemical properties with the addition of TiO₂ should be understood, especially viscosity, which is essential to maintain stable operation.

Much work^1–9 has been done to study the influence of TiO₂ on the viscosity of TiO₂ bearing melts. In the temperature range of 1400–1625°C, Dingwell¹ measured the viscosity of 13 liquids along the CaSiO₃–TiO₂ plain with TiO₂ content varying from 10 to 80 mol.-%. The viscosity decreased with the addition of TiO₂. From the viscosity data of CaO–Al₂O₃–TiO₂–SiO₂ molten slag with CaO/SiO₂ in the range of 0·6–1·20, Al₂O₃ in the range of 0–20 wt-% and TiO₂ in the range of 0–50 wt-%, Ohno and Ross² found that an increase in titania content decreased the viscosity. The viscosity of blast furnace type CaO–MgO–Al₂O₃–TiO₂–SiO₂ slags has been studied by many researchers,^3–8 all of which show that TiO₂ addition lowered the viscosity. However, theoretical models are lacking in estimating the viscosity of TiO₂ bearing slags.¹⁰ In our previous papers,^11–15 a structural based viscosity model that can be well applied to molten slags containing CaO, MgO, FeO, MnO, Al₂O₃, SiO₂, Li₂O, Na₂O, K₂O and CaF₂ was proposed. The aim of the present study is to extend this model to the TiO₂ bearing melts. Before constructing the model, first, the role of TiO₂ on the structure of the slag should be clarified.

Structure of TIO₂ bearing molten slag

From the larger ionic radius of Ti⁴⁺ ion (0·61 Å) relative to Si⁴⁺ ion (0·42 Å),¹⁶ the Ti⁴⁺ ion appears unfit to replace the position of the Si⁴⁺ ion. Thus, in the TiO₂ bearing molten slag, it seems that the Ti⁴⁺ ion may have a high oxygen coordination number in excess of 4 and mainly acts as a network modifier. Based on the X-ray photoelectronic spectroscopy (XPS) results of aluminosilicate melts containing 17 wt-%Al₂O₃ and 10 wt-%MgO, it is found that the fraction of bridging and non-bridging oxygen decreased with TiO₂ addition, but the free oxygen increased with TiO₂ addition.⁴ Thus, the authors concluded that TiO₂ seems to provide higher potential to depolymerise the slag network structure. However, it was found that the Ti⁴⁺ ion can also act as a network former cation with fourfold coordination of oxygen.¹⁷ Mysen and Neuville¹⁸ noted that the oxygen coordination of the Ti⁴⁺ ion varies with TiO₂ content. At low TiO₂ content, Ti⁴⁺ serves predominantly as a network modifier but as a network former at high TiO₂ content. Using a combination of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements, Greegor et al. ¹⁹ found that in the TiO₂–SiO₂ binary system, at TiO₂ concentrations below ∼0·05 wt-%, Ti⁴⁺ is in a rutile-like octahedral coordination; with increasing TiO₂ content, a two site model applies (fourfold and sixfold). At ∼9 wt-%TiO₂, the sixfold/fourfold ratio increases appreciably with increasing TiO₂ content.

According to the above analyses, it can be concluded that the oxygen coordination state of Ti⁴⁺ has a very complex relationship with composition. Thereby, it is hard to incorporate the actual structure changes (with TiO₂ addition) into the viscosity model. For convenience of model treatment, a simplification is adopted, which is shown in the next section.

Model

The temperature dependence of viscosity is described by the Arrhenius law 1 where η is the viscosity (Poise), A is the preexponent factor (Poise), E is the activation energy (J mol⁻¹), R is the gas constant (8·314 J mol⁻¹ K⁻¹) and T is the absolute temperature (K). Compensation effect exists between the logarithm of preexponent factor A and the activation energy E ¹¹ 2 where the E of 572 516 and the ln A of 17·47 are the Arrhenius parameters of pure SiO₂.¹¹ Parameter k in equation (2) is a constant for a specific SiO₂ containing binary system. For a multicomponent system , the value of parameter k is assumed to be the linear addition of the parameter for different binary systems M_xO_y–SiO₂ with the renormalised mole fractions of oxides M_xO_y as the weighting factors. Therefore, parameter k is calculated as follows¹¹ 3 The activation energy E in equation (2) is expressed by the reciprocal function of the linear addition of mole fraction of different types of oxygen ions with different weighting factors. The numbers of oxygen ions can be calculated according to the formulae given in our previous work.^11–14 The influence of TiO₂ on the proportions of other types of oxygen ions is neglected, and the oxygen ion related to Ti⁴⁺ ion is assumed to be only one type O_Ti, the number of which is equal to . Therefore, the activation energy of viscosity can be expressed as follows 4 where are the mole numbers of different types of oxygen ions, and parameter α describes the deforming ability of bond around the corresponding unit. In the denominator of equation (4), the first, second, third, fourth, fifth, sixth and seventh terms represent the contributions of bridging oxygen bonded with Si⁴⁺ ion, free oxygen bonded with metal cation i, oxygen bonded with Al³⁺ ion not being charge compensated, bridging oxygen bonded with compensated Al³⁺ ion, non-bridging oxygen bonded with Si⁴⁺ ion, non-bridging oxygen bonded with Al³⁺ ion and oxygen bonded with Ti⁴⁺ ion, respectively. For the TiO₂ bearing melts, only two parameters κ _Ti and α _Ti are needed to optimise the experimental results of TiO₂ bearing melts based on the parameters of the other components optimised in our previous papers.^11,12 All the assessed parameters are listed in Table 1.

Table 1.

Optimised values of model parameters

i	k _i×10⁵		α _i	α _Al,i
Ca	−2·088	7·422	17·34	4·996	7·115	8·334
Mg	−2·106	6·908	15·54	5·606	3·975
Ti	−0·926		3·032
Al	−2·594		5·671
K	−3·2	16·59

Results

Comparisons between measured and calculated viscosities for different TiO₂ bearing melts are shown in Fig. 1, with the mean deviation (defined as , where η _i,cal and η _i,mea are the calculated and measured viscosity respectively, and N represents the number of the samples) of 22·9%. Next, the different systems will be introduced in turn.

Comparisons between measured and calculated viscosities for different TiO₂ bearing melts

MgO–TiO₂–SiO₂ system

Only two experimental values at 1500 and 1550°C for 35 mol.-%MgO–35%TiO₂–30%SiO₂ slag have been measured in this ternary system.⁹ The calculated viscosities are 0·83 and 0·64 Poise respectively, which are close to the measured values of 0·64 and 0·53 Poise.

CaO–TiO₂–SiO₂ system

Dingwell¹ measured the viscosity of CaSiO₃–TiO₂ with the mole fraction of TiO₂ ranging from 10 to 80% (shown in Fig. 2). The model calculated viscosities agree well with the measured values. From Fig. 2, it is apparent that at a given temperature the addition of TiO₂ causes the viscosity to decrease. The iso-viscosity curves of the CaO–TiO₂–SiO₂ system at 1773 K is plotted by the present model as shown in Fig. 3, in which viscosities data of composition with different TiO₂ contents and different CaO/SiO₂ ratios² are also given for comparison. From Fig. 3, it can be seen that when adding TiO₂ to a specific CaO–SiO₂ melt, there is a decrease of viscosity, which is consistent with the experimental finding of Dingwell.¹ When substituting CaO by TiO₂ along a constant content of SiO₂, viscosity increases, whereas viscosity decreases when substituting SiO₂ by TiO₂ along a constant content of CaO.

Comparisons between measured and calculated viscosities for CaSiO₃–TiO₂ join

Iso-viscosity curves of CaO–TiO₂–SiO₂ system at 1773 K plotted by present model

CaO–Al₂O₃–TiO₂–SiO₂ system

Ohno and Ross² measured the viscosity of the CaO–Al₂O₃–TiO₂–SiO₂ system with TiO₂ content in the range of 0–45 wt-% between 1400 and 1500°C. The viscosity data for composition with 20 wt-%Al₂O₃ are shown in Fig. 4. The present viscosity model can well represent the viscosity variation of this system. It can be seen from Fig. 4 that viscosity decreases with increasing TiO₂ content or CaO/SiO₂ ratio.

Comparisons between measured and calculated viscosities for CaO–Al₂O₃–TiO₂–SiO₂ system

CaO–MgO–Al₂O₃–TiO₂–SiO₂ system

This system is the fundamental slag system for the blast furnace and electric arc processes when smelting TiO₂ bearing ores. Many experimental works^3–8 have been performed on this system, and more than 300 groups of measured viscosities data are collected. Good consistency between the measured and calculated viscosities was obtained, with the mean deviation of 20·0%. Figure 5 shows the variation of viscosity with TiO₂ content and CaO/SiO₂ ratio at 12 wt-%Al₂O₃ and 7 wt-%MgO, and it can be concluded that the same change tendency of viscosity with TiO₂ content and CaO/SiO₂ ratio is obtained as in the CaO–Al₂O₃–TiO₂–SiO₂ system (shown in Fig. 4).

Comparisons between measured and calculated viscosities for CaO–MgO–Al₂O₃–TiO₂–SiO₂ system

Discussion

From the above examples, it can be seen that within a certain composition range, the addition of TiO₂ always decreases the viscosity. In the CaO–TiO₂–SiO₂ ternary system, for example, when adding TiO₂ to CaO–SiO₂ melts, the absolute contents of CaO and SiO₂ decrease but their ratio remains unchanged. From Fig. 3, viscosity decreases when substituting TiO₂ by CaO while keeping the SiO₂ content constant; viscosity increases when substituting TiO₂ by SiO₂ while keeping the CaO content constant. Thus, the ability of increasing viscosity follows the order: SiO₂>TiO₂>CaO. Therefore, when adding TiO₂ to CaO–SiO₂ melts, a decrease of CaO content increases viscosity, while a decrease of SiO₂ content decreases viscosity, but generally, when decreasing the same content of CaO and SiO₂, the viscosity increment is lower than the viscosity decrement for the great influence of SiO₂ on viscosity. Thus, viscosity will decrease when adding TiO₂ to the CaO–SiO₂ melt, regardless of TiO₂ acting as a network former or a network modifier. The same analyses can be applied to other multicomponent silicate systems containing TiO₂.

The parameters optimised in the CaO–MgO–Al₂O₃–TiO₂–SiO₂ system can also be used in the melts containing other components. Viscosities of 17·6 wt-%Li₂O–47%TiO₂–35·9%SiO₂ melt at different temperatures were measured by Dingwell.¹ Figure 6 shows the comparison between the measured and calculated values, which agree well.

Comparisons between measured and calculated viscosities for 17·6 wt-%Li₂O–47%TiO₂–35·9%SiO₂ melt

The present study mainly deals with the case of TiO₂, but in the strong reducing conditions as in the blast furnace, part of TiO₂ will be reduced by carbon to form a series of reduction products such as Ti₂O₃, TiO, TiC, TiN and Ti(C, N). Unlike TiO₂, which behaves as an amphoteric oxide, generally, Ti₂O₃ and TiO behave more like basic oxides. The addition of Ti₂O₃ and TiO could decrease slag viscosity,²⁰ but modelling these two components into the viscosity model still needs much work due to the lack of experimental data. Slags containing TiC and TiN involve solid and liquid two phases for the high melting points of TiC and TiN, which cannot be melted at the smelting temperature, thereby viscosity will increase dramatically when TiC and TiN are present.²¹

Conclusions

The present work mainly discusses the viscosity variation of CaO–MgO–Al₂O₃–TiO₂–SiO₂ molten slag. Our previous viscosity model has been extended to estimate the change of viscosity with composition and temperature. Good agreement was achieved between the experimental measured viscosities and the calculated viscosities. From the model estimation results, it can be concluded that the addition of TiO₂ results in a decrease in viscosity.

Footnotes

Acknowledgements

The authors would like to thank the financial supports from the China Postdoctoral Science Foundation (grant no. 2012M510318) and the Chinese Natural Science Foundation (grant nos. 51174022 and 51234001).

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Influence of TiO 2 on viscosity of aluminosilicate melts

Abstract

Introduction

Structure of TIO2 bearing molten slag

Model

Optimised values of model parameters

Results

MgO–TiO2–SiO2 system

CaO–TiO2–SiO2 system

CaO–Al2O3–TiO2–SiO2 system

CaO–MgO–Al2O3–TiO2–SiO2 system

Discussion

Conclusions

Footnotes

Acknowledgements

References

Structure of TIO₂ bearing molten slag

MgO–TiO₂–SiO₂ system

CaO–TiO₂–SiO₂ system

CaO–Al₂O₃–TiO₂–SiO₂ system

CaO–MgO–Al₂O₃–TiO₂–SiO₂ system