Effects of Minor Elements to the Liquidus Temperatures of Blast Furnace Slags

Abstract

The effects of minor elements TiO₂, MnO, Na₂O, K₂O, CaS, and B₂O₃ to the liquidus temperatures of blast furnace slags were experimentally studied. The base blast furnace slag composition in SiO₂–Al₂O₃–CaO–MgO system is fixed at CaO/SiO₂=1.1, 16 wt% Al₂O₃ and 8 wt% MgO. The liquidus temperatures of the synthetic slags have been determined by high temperature equilibration, quenching and Electron Probe X-ray Microanalysis (EPMA) techniques. It was found that within the present investigated composition ranges, the additions of minor elements to the BF slags remain in melilite primary phase field. The minor element decrease the liquidus temperatures of the blast furnace slags in different extents and follows the tendency: B₂O₃ > CaS > Na₂O > K₂O > “TiO₂” > MnO. The present measurements were also compared with FactSage predictions.

1. Introduction

Blast furnace (BF) is remaining the major ironmaking process to produce iron. Recent years, low grade iron ores, which contains high Al₂O₃, are more and more used for BF ironmaking process. The principal components of current iron BF slags are described by the SiO₂–Al₂O₃–CaO–MgO system.^1,2) In addition to Al₂O₃, CaO, MgO and SiO₂, up to 5 wt% of other components, such as TiO₂, MnO, Na₂O, S and etc., are also present in the BF slags.³⁾ Due to their limited concentrations in the BF slags, these components are called as minor elements. Moreover, it was found that the formations of TiCN may have the potential to protect the hearth,³⁾ so that TiO₂ is deliberately added into the BF and the TiO₂ content in the BF slag increases. B₂O₃ is considered as a potential flux to improve overall BF performance, now attracting more and more attentions.⁴⁾

It is essential to understand the liquidus of BF slags with minor elements as the operating temperature of blast furnace may be lowered and solids in the slags can be controlled, which may affect slag tapping and sulphur removal.⁵⁾ However, limited investigations have been carried out to determine the effects of minor elements to the liquidus of CaO–SiO₂–Al₂O₃–MgO system, especially in high Al₂O₃ containing BF slags. Fine and Arac⁶⁾ investigated the effects of minor elements to the liquidus temperature of BF slags with 10 wt% Al₂O₃ using hot-wire microscopy, and claimed that TiO₂ increases the liquidus temperature at BF condition. The phase equilibria in systems CaO–SiO₂–Al₂O₃–MgO–TiO₂^7,8,9) and CaO–SiO₂–Al₂O₃–MgO–MnO¹⁰⁾ were studied by researchers. However, limited useful information was found for the liquidus temperatures of BF slags. No literature has been found for liquidus temperatures in the BF systems containing Na₂O, K₂O, CaS and B₂O₃. The losses of the minor elements, such as S, Na₂O and etc., make the determinations of liquidus temperatures even more difficult at high temperatures.

In the present study, the effects of TiO₂, MnO, Na₂O, K₂O, CaS, and B₂O₃ to the liquidus temperatures of CaO–SiO₂–Al₂O₃–MgO system were investigated respectively.

2. Experimental

The experimental method used in the present study involves high temperature equilibration, quenching and electron probe X-ray microanalysis (EPMA), which has been described in details in the previous papers.^11,12) In brief, the experiments were carried out using a vertical electric resistance furnace. High-purity reagent powders of CaCO₃, SiO₂, MgO, Al₂O₃, TiO₂, MnO, Na₂CO₃, K₂CO₃, CaS and B₂O₃ were carefully weighted according to the experimental plans, and then thoroughly mixed and pelletized. The mixtures were placed in graphite crucibles at high temperatures in a high-purity Ar atmosphere. The samples were equilibrated for predetermined periods depending on the compositions and temperatures. The temperature of the furnace was controlled within 2 K and overall temperature uncertainty is within 5 K.

After equilibrated at designated temperatures and holding time, the samples were directly quenched in water, then dried, mounted and polished for metallographic analysis. A JXA 8200 Electron Probe Microanalyzer with Wavelength Dispersive Detectors (JEOL, Tokyo, Japan). The standards used for analysis were from Charles M. Taylor Co. (Stanford, California): CaSiO₃ for Ca and Si, Al₂O₃ for Al, MgO for Mg, TiO₂ for Ti, Mn₃Al₂Si₃O₁₂ for Mn, NaAlSi₃O₈ for Na, KAlSi₃O₈ for K, CuFeS₂ for S, and BN for B. The analysis was conducted at an accelerating voltage of 15 kV and a probe current of 15 nA. The ZAF correction procedure supplied with the electron probe was applied. In the present study, S may form different sulphides, and Ti⁴⁺ and Ti³⁺ may co-exist in the system under carbon saturation condition. As only elemental concentrations can be measured by EPMA, S and Ti is recalculated to CaS and TiO₂ for presentation purpose only. The samples containing CaS and B₂O₃ were also prepared and sent to do ICP-OES analysis. The compositions of CaS and B₂O₃ measured by ICP confirmed the EPMA measurements. In the present study, the direct determinations of the final compositions of phases after the equilibration minimise the possible changes in the compositions due to the vaporizations.

In the present study, the BF slag base composition was selected to be CaO/SiO₂ 1.1 (in weight), 16 wt% Al₂O₃ and 8 wt% MgO. The liquidus of the base composition has been determined to be 1709 K and the composition locates in melilite primary phase field.¹²⁾ Different minor elements TiO₂, MnO, Na₂O, K₂O, CaS, and B₂O₃ were added respectively into slag, and the concentrations of SiO₂, CaO, Al₂O₃ and MgO decrease proportionally.

3. Results and Discussions

The liquidus temperature of a composition is determined by the measurement of liquid and solid compositions at a given temperature. Series of experiments were undertaken to obtain a fixed SiO₂/CaO/Al₂O₃/MgO ratio due to the precipitations of solid phases. The high temperature experiments also show that “TiO₂” and MnO are stable at high temperatures, while Na₂O, K₂O, S and B₂O₃ will vaporise in small samples. Efforts were made to obtain high concentrations of the volatile elements with the optimum holding time and temperatures. However, the maximum K₂O and CaS concentration obtained in the present study are still less than 5 wt%.

Melilite is the solid solution between akermanite (2CaO.MgO.2SiO₂) and gehlenite (2CaO.Al₂O₃.SiO₂). The compositional analyses of the solid phases show that in all the solid phases, the molar ratios of CaO to (MgO+Al₂O₃) is close to 2, and well agree with the formula of melilite solid solutions. Moreover, it was found that “TiO₂” and B₂O₃ have limited solubilities in melilite, and relatively high concentrations of MnO, Na₂O, K₂O and CaS can be dissolved in melilite solid solutions. Figures 1(a) and 1(b) show two examples of typical microstructures of liquid in equilibrium with melilite in CaO–SiO₂–MgO–Al₂O₃–“TiO₂” system and CaO–SiO₂–MgO–Al₂O₃–K₂O system.

Fig. 1.

Typical backscattered SEM micrographs of slags in (a) CaO–SiO₂–MgO–Al₂O₃–“TiO₂” system quenched at 1653 K and (b) CaO–SiO₂–MgO–Al₂O₃–MnO system quenched at 1703 K.

The effects of “TiO₂” additions to the liquidus temperatures of base composition were determined and shown in Table 1. The result indicates that the addition of 5.9 wt% “TiO₂” does not change the primary phase field and still locates in the melilite primary phase field, which agrees with the study by Zhao et al.⁹⁾ The experimentally determined liquidus was plotted based on experimental results and also compared with the FactSage 7.0 predictions¹³⁾ (Fig. 2). In Fig. 2, it can be seen that the liquidus temperature decreases by 56 K with 5.9 wt% “TiO₂” addition. The comparisons show the predicted liquidus temperatures are lower than the experimentally determined liquidus when the concentrations of “TiO₂” are lower than 5.7 wt%, while the prediction and measurement at “TiO₂” 5.9 wt% are close.

Table 1. Experimental results in CaO–SiO₂–MgO–Al₂O₃–“TiO₂” system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	“TiO2”
1693	Liquid	37.9	34.4	8.1	15.6	4.0	1.10
1683	Liquid	38.2	34.2	8	15.4	4.2	1.12
	Melilite	40.9	28.6	4.4	26.0	0.1
1653	Liquid	37.7	34.2	7.6	14.6	5.9	1.10
	Melilite	40.7	28.9	4.6	25.7	0.1

Fig. 2.

The effect of TiO₂ additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO–“TiO₂” system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L: Liquid only; L + M: Liquid + Melilite).

The experimentally determined liquidus temperatures with 2.6 wt% and 5.3 wt% MnO were shown in Table 2 and plotted in Fig. 3. The figure shows that when MnO addition is less than 5.3 wt%, the system is still located in melilite primary phase field. 5.3 wt% MnO decreases 36 K from the liquidus temperature of the MnO-free system. The FactSage 7.0 predictions¹³⁾ are shown to be lower than the present measurements in Fig. 3.

Table 2. Experimental results in CaO–SiO₂–MgO–Al₂O₃–MnO system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	MnO
1703	Liquid	38.9	35.4	7.5	15.6	2.6	1.10
	Melilite	40.7	29.1	4.1	25.7	0.4
1673	Liquid	37.6	34.1	7.8	15.2	5.3	1.10
	Melilite	40.5	29.3	4.6	25.1	0.5

Fig. 3.

The effect of MnO additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO–MnO system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L + M: Liquid + Melilite).

Table 3 shows the experimentally determined liquidus with 3.1 wt% and 5.1 wt% Na₂O additions. The experimentally determined liquidus was also compared with FactSage predictions¹³⁾ (Fig. 4). As it can be clearly seen in Fig. 4, the FactSage 7.0 predictions¹³⁾ indicate that the system changes from melilite primary phase field to merwinite primary phase field at 4 wt% Na₂O. However, the present measurements show that even with 5.1 wt% Na₂O, the system is still in melilite primary phase field and the present measured liquidus temperatures are higher than the predicted ones.

Table 3. Experimental results in CaO–SiO₂–MgO–Al₂O₃–Na₂O system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	Na2O
1673	Liquid	38.7	34.7	8.0	15.5	3.1	1.12
	Melilite	40.4	29.3	4.4	25.4	0.6
1653	Liquid	37.7	34.9	7.4	14.9	5.1	1.08
	Melilite	39.4	31.1	5.2	23.6	0.7

Fig. 4.

The effect of Na₂O additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO–Na₂O system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L + M: Liquid + Melilite).

The experimentally determined liquidus with 3.6 wt% K₂O was shown in Table 4 and plotted in Fig. 5 in melilite primary phase field. 36 K was decreased by 3.6 wt% K₂O from the liquidus temperature of CaO–SiO₂–MgO–Al₂O₃ system. The liquidus temperature predicted by FactSage 7.0¹³⁾ is 13 K lower than the present measurement with 3.6 wt% K₂O as shown in Fig. 5.

Table 4. Experimental results in CaO–SiO₂–MgO–Al₂O₃–K₂O system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	K2O
1673	Liquid	38.2	35.2	7.6	15.4	3.6	1.09
	Melilite	40.7	29.6	4.8	24.4	0.5

Fig. 5.

The effect of K₂O additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO–K₂O system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L + M: Liquid + Melilite).

Table 5 shows the experimentally determined liquidus with 2.8 wt% CaS additions. The experimentally determined liquidus was also compared with FactSage 7.0 predictions¹³⁾ (Fig. 6). The FactSage predictions show that the primary phase field changed from melilite to CaS when CaS addition is 2.7 wt%. The present investigation shows that with 2.8 wt% CaS, the system is still located in melilite primary phase field. The predicted liquidus temperature at 2.8 wt% CaS is around 40 K higher than the measurement. It seems that CaS has much stronger ability to decrease the liquidus temperatures in melilite primary phase field than the predictions.

Table 5. Experimental results in CaO–SiO₂–MgO–Al₂O₃–CaS system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	CaS
1653	Liquid	38.4	34.8	8.1	15.8	2.8	1.10
	Melilite	40.2	31	5.9	22.6	0.3

Table 6. Experimental results in CaO–SiO₂–MgO–Al₂O₃–B₂O₃ system.

Temperature (K)	Phases	Composition (wt%)					CaO/SiO2
		CaO	SiO2	MgO	Al2O3	B2O3
1573	Liquid	38.2	34.7	7.4	14.8	4.9	1.10
	Melilite	39.7	31.4	6.2	22.7	0.0

Fig. 6.

The effect of CaS additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO–CaS system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L + M: Liquid + Melilite).

The experimentally determined liquidus with 4.9 wt% B₂O₃ is in equilibrium with melilite (Table 4). The present measurement is close to the FactSage 7.0 predictions¹³⁾ as shown in Fig. 7. 4.9 wt% B₂O₃ significantly decreases the liquidus temperature in the CaO–SiO₂–MgO–Al₂O₃ system by 136 K. The predicted liquidus temperature with 4.9 wt% B₂O₃ is 17 K higher than the present measurement (Fig. 7).

Fig. 7.

The effect of B₂O₃ additions to liquidus temperatures of SiO₂–Al₂O₃–CaO–MgO– B₂O₃ system at fixed CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO (L + M: Liquid + Melilite).

The measurements of the effects of minor elements “TiO₂”, MnO, Na₂O, K₂O, CaS, and B₂O₃ to the BF slag system show that all the minor elements investigated will decrease the liquidus temperatures of BF slags. The six liquidus determined in the present studies were summarised and compared in Fig. 8. Table 7 shows the approximate liquidus decrement by adding 1 wt% minor element into the BF slag. It should be noted that the decrement of liquidus temperature from “TiO₂”, MnO and Na₂O are the averages of the two experimental points, and the decrement ability of FeO is predicted by FactSage 7.0.¹³⁾ It can be clearly seen that the ability to decrease the BF slag liquidus temperature follows the tendency: B₂O₃ > CaS > Na₂O > K₂O > “TiO₂” > MnO ≈ FeO. 1 wt% B₂O₃ addition to the BF slag will decrease the liquidus temperature by 27.8 K, and 1 wt% MnO or FeO will only decrease for less than 5 K from the liquidus of BF slag.

Fig. 8.

The effects of adding minor elements to liquidus temperature of blast furnace slag SiO₂–Al₂O₃–CaO–MgO system, initial SiO₂–Al₂O₃–CaO–MgO slag composition CaO/SiO₂ 1.1, 16 wt% Al₂O₃ and 8 wt% MgO.

Table 7. The approximate liquidus temperature decrement.

Minor element	Approximate liquidus temperature decrement per wt% minor element (K)
“TiO2”	7.8
MnO	4.6
Na2O	11.3
K2O	10.0
CaS	20.0
B2O3	27.8
FeO*	4.613)

*:  The decrement ability of FeO is predicted by FactSage 7.0.¹³⁾

4. Industrial Implications

In ironmaking processes, the minor elements commonly exist in the BF slags, and affect the liquidus temperature and fluidity of BF slags. Presences of minor elements in the BF slags do not change the primary phase filed of the BF slags and remain in melilite primary phase, and decrease the liquidus temperatures of BF slags. It was reported by Zhang et al.¹⁴⁾ that the liquidus temperatures of industrial slags are generally 40–50 K lower than those of the synthetic slags prepared according to the industrial slags in the SiO₂–CaO–MgO–Al₂O₃ system. The temperature differences between the industrial slags and the synthetic slags generally agree with the estimations using Table 7. The studies of effects of minor element additions to the liquidus temperatures of BF slags, accompanying with the acuate determinations of liquidus temperatures in the SiO₂–CaO–MgO–Al₂O₃ system,¹²⁾ will help the operator to estimate the liquidus temperatures of the complex BF slags instantly and give more accurate controls for operation temperatures. The understandings of the complex BF slag liquidus temperatures will also give indications for the optimization of BF feeds.

The information on the liquidus temperatures of slags and compositions of melilite solid solutions are useful for the thermodynamic database constructions. The present measurements also clearly indicate that the FactSage prediction should be further improved.

5. Conclusions

The effects of minor elements TiO₂, Na₂O, K₂O, MnO, B₂O₃ and CaS additions to the liquidus temperature of the SiO₂–CaO–MgO–Al₂O₃ system at fixed C/S=1.1, 16 wt% Al₂O₃ and 8 wt% MgO were determined. The EPMA analysis shows that in the present investigations, the BF slag systems with different minor elements additions are still located in melilite primary phase field. The ability of liquidus temperature decrement follows the order: B₂O₃ > CaS > Na₂O > K₂O > “TiO₂” > MnO. The experimentally determined liquidus were also compared with the FactSage predictions.

Acknowledgements

The authors would like to thank the lab assistance from Ms Jie Yu and financial support from Shougang Group, China and Rio Tinto Iron Ore, Australia.

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