Effect of MgO on Phase Development in Hematite-ilmenite Ore Sinter

Abstract

The effect of MgO on phase development in a hematite-ilmenite ore sinter was investigated employing a vertical tube furnace by raising the temperature at 30 K/min to 1553 K. The phases developed after sintering were identified in terms of XRD and EDS analyses. The distribution of metallic elements after sintering were figured out through EPMA mappings. The results showed that the formation of MgO·Fe₂O₃(s) and Mg-rich SFCA increased in the sinter blend, and that more Ti was retained in the Fe-rich phase with increasing MgO. It is believed that the formation of CaO·TiO₂·SiO₂ phase decreased with increasing the addition of MgO.

1. Introduction

There are large deposits of Ti-containing ores around the world but only small amount of the Ti-containing ores are currently being used by adding ilmenite [FeO·TiO₂(s)] or titanomagnetite [Fe_3−xTi_xO₄(s)] into the sinter mix.¹⁾ Titanium (Ti) has been reported to negatively affect the sinter properties due to the formation of perovskite.^2,3,4,5) The perovskite phase is likely to be distributed in the glass phase, which affects the fracture toughness of the glass phase thereby weakening the strength of the sinter.³⁾ It was reported that the increase of TiO₂(s) in the sinter decreased the generation of calcium ferrite and increased CaO·TiO₂(s) content which weakens the role of the bonding phase.⁵⁾ On the other hand, in the sintering of chromium-bearing vanadium titanium magnetite, the perovskite phase was evenly distributed in the silicate phase and improved the reduction disintegration index (RDI) due to the formation of magnesium ferrite.⁶⁾

To effectively utilize the Ti-containing ores, the effects of other metallic elements in these sinters should be clarified. Therefore, the present research aims to investigate the effects of MgO addition on phase development in a hematite-ilmenite ore sinter.

2. Experimental

Table 1 shows the chemical composition of the hematite ore, ilmenite ore and calcium carbonate that had been heated at 1273 K for 6 hrs and cooled to room temperature in argon atmosphere. The ores used had particle size of less than 250 µm. The TiO₂ content in the hematite and ilmenite ores was 0.219 and 18.9 mass%, respectively. The phases in the ores were identified by XRD diffraction using Cu tube at a scan angle of 20 to 80 degree, a rate of 2 degree/min, sampling 0.02 degree at a voltage of 40 kV and current of 40 mA. The major phases identified in the hematite ore were Fe₂O₃(s), 3CaO·3Al₂O₃·SiO₂(s) and SiO₂(s) while the ilmenite ore had Fe₃O₄(s), SiO₂(s), Fe_1.04Ti_0.94O₃(s) and 3CaO·Fe₂O₃·FeO·V_1.5Fe_0.5O₅(s) shown in Fig. 1.

Table 1. Chemical composition of the hematite ore and the ilmenite ore and calcium oxide used (mass%).

Name of Specimen	TFe	FeO	Fe2O3	CaO	MgO	Al2O3	SiO2	TiO2	Cr2O3	V2O3	P	S	LOI
Hematite ore	62.8	0.54	89.19	0.102	0.091	1.49	1.70	0.219	0.017	0.008	0.078	0.0164
Ilmenite ore	43.4	23.0	36.50	1.44	4.48	5.29	5.39	18.9	0.050	0.518	0.074	0.308
CaCO3				93.8	0.584	0.081	0.931						4.604

Fig. 1.

XRD results of the hematite ore and the ilmenite ore.

The blending ratio of the raw material used in making sinter Blend 1 was as in shown Table 2. Blend 1 was designed such that the basicity ratio (CaO/SiO₂) was 2, and the TiO₂ content was intentionally adjusted to be above 2 mass% shown in Table 3. To 16 g of sinter Blend 1, about 1.5 and 3.0 mass% of reagent grade MgO (98% purity) were added to make sinter Blend 2 and sinter Blend 3, respectively. The sinter blends were pressed into cylindrically shaped pellets (6 mm diameter x 6 mm height). The vertical tube furnace with an aluminium tube (65 mm internal diameter) was employed in the sintering experiments by raising the temperature at 30 K/min to 1553 K. The holding time was 50 minutes at 1553 K for the sintering of the pellets. The gas flow rate was 1 L/min (75 vol% N₂, 24 vol% CO₂, and 1 vol% CO) during the heating and holding and cooling was done in the air.⁷⁾ The sample was lowered in the furnace using a platinum wire after attaining target temperature and pulled out to the cold end of the furnace to cool down to room temperature in air after sintering. The sintered pellets were crushed into fine powder for XRD analyses, others mounted and prepared for energy-dispersive X-ray spectroscopy (EDS) and electron probe microanalysis (EPMA) analyses.

Table 2. Blending ratio of the hematite ore, ilmenite ore and CaO (mass%).

Blends	Basicity	Hematite ore	Ilmenite ore	CaCO3
Blend 1	2	82.91	12.44	4.65

Table 3. Estimated chemical composition of the hematite-ilmenite ore blend (mass%).

Blends	TFe	FeO	Fe2O3	SiO2	Al2O3	CaO	MgO	TiO2	CaO/SiO2
Blend 1	57.46	3.31	78.47	2.119	1.897	4.159	0.656	2.414	2

3. Results and Discussion

3.1. Effects of MgO on the Change in Phase

Sintered blend 1 shown in Fig. 2(a) indicates that the main phases formed were Fe₂O₃(s), 3CaO·Fe₂O₃·Si_1.58Ti_1.42O₆(s), MgO·Fe₂O₃(s) and CaO·TiO₂·SiO₂(s). The MgO·Fe₂O₃(s) might have been from the substitution of Mg²⁺ for Fe²⁺ in the Fe₃O₄.The Fe₃O₄ might have originated either from the ilmenite ore or from the reduction of Fe₂O₃.

Fig. 2.

XRD analyses results of sintered Blends: (a) Blend 1, (b) Blend 2 and (c) Blend 3.

The XRD results of sintered Blend 2 in Fig. 2(b) showed that the main phases were CaO·TiO₂·SiO₂(s), Fe₂O₃(s), MgO·Fe₂O₃(s), Fe_1.5Ti_0.5O₃(s) and Ca_0.8Fe_0.2O·SiO₂(s). The peak representing CaO·TiO₂·SiO₂(s) decreased in intensity as compared to that in sintered Blend 1. Further addition of MgO in Fig. 2(c) showed that the part of Ti in CaO·TiO₂·SiO₂(s) was replaced by vanadium leading to the formation of CaO·V_0.7Ti_0.3O₂·SiO₂(s). Some new peaks of MgO·Fe₂O₃(s), CaO·Fe₂O₃(s), 2CaO·Al₂O₃·SiO₂(s), FeO·Fe_0.6Ca_0.4O·2SiO₂(s) and Fe_1.5Ti_0.5O₃(s) were identified.

To provide further information on phase development, EDS point analyses were performed on the sintered blends. Figure 3(a) and Table 4, showed that most of the Ti existed in the Ti-rich calcium iron aluminum silicate phase. XRD results in Fig. 2(a) showed the presence of CaO·TiO₂·SiO₂(s) and 3CaO·Fe₂O₃·Si_1.58Ti_1.42O₆(s) phases.

Fig. 3.

EDS point analyses of sintered Blends: (a) Blend 1, (b) Blend 2 and (c) Blend 3.

Table 4. Summary of possible phases formed with increasing MgO based on EDS analyses.

Blends	Possible phases	Points
Blend 1	Ti-rich calcium iron aluminum silicates	1,5,10
	Ti-rich SFCA	2,4,6,8,9
	SFCA	3,7
Blend 2	Mg-rich SFCA	1
	Ti-rich Mg containing SFCA	2,4,6,7,10
	Ti-rich iron aluminum silicate	3,9
	Ti-rich SFCA	5
	iron silicates	8
Blend 3	Mg-rich calcium iron aluminum silicate	1,2,3,4
	Ti-rich iron aluminum silicate	5,7,10
	Ti-rich Mg containing SFCA	6,8
	Ti-rich Mg containing calcium iron aluminum silicate	9
	Ti-rich iron silicate	11

Increase of MgO shown in Fig. 3(b) and Table 4 produced Mg-rich SFCA. There was considerable amount of Ti in the Fe-rich phase; this phase had a relatively high amount of Si. The XRD results showed the presence of Fe_1.5Ti_0.5O₃(s) and CaO·TiO₂·SiO₂(s) in Fig. 2(b). Sintered Blend 3 in Fig. 3(c) and Table 4 showed that the Mg was concentrated in the calcium iron aluminum silicate phase. High levels of Ti were in the iron aluminum silicate phase. The dark coloured Mg-rich phase around the light coloured Fe-rich phase might have blocked diffusion of the Ti or Ca into the relative adjacent phases hence XRD results confirmed the peak of Fe_1.5Ti_0.5O₃(s) as shown in Figs. 2(b) and 2(c).

3.2. Effect of MgO on the Distribution of Metallic Elements in the Sintered Blends

EPMA results shown in Fig. 4(a), showed that Ca was intensively distributed in the same areas as Si, Ti along with V. The Ca and Ti- rich areas are marked by the red circle. Mg was in the Fe-rich region, which is in good agreement with the XRD results that showed the presence of CaO·TiO₂·SiO₂(s) as shown in Fig. 2(a). Sintering Blend 2 increased the distribution of Mg in the Fe-rich phase shown in Fig. 4(b). Low amount of Ca was in the same area as Ti and Si which might show the formation of low amount of CaO·TiO₂·SiO₂(s) as identified by XRD in Fig. 2(b).

Fig. 4.

EPMA analyses of sintered Blends: (a) Blend 1, (b) Blend 2 and (c) Blend 3. (Online version in color.)

Figure 4(c) showed that more Ti was found in the Fe-rich region. More Mg was present in the Fe-rich phase, and showed some boundary around the highly Fe concentrated Fe-rich phase. Some Al, Si, and small amount of Ca were in the same position as Mg-rich iron phase. Some V existed in the same areas as Ca and Si. XRD results in Fig. 2(c) showed the presence of the CaO·V_0.7Ti_0.3O₂·SiO₂(s) in sintered Blend 3. More Ti was retained in the Fe-rich region with increasing MgO content that might have resulted in the formation of low amounts of CaO·TiO₂-containing phase.

4. Conclusions

(1) The addition of MgO to the sinter blend increased the amount of Mg in the Fe-rich phases.

(2) Increasing MgO content in the hematite-ilmenite ore blend might favor the retention of more Ti in the Fe-rich phase that might lead to less interaction of Ti with Ca hence producing low amount of CaO·TiO₂(s) phase. However, more research is needed to clarify this phenomenon.

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