Effect of Basicity on Titanomagnetite Concentrate Sintering

Abstract

The effect of binary basicity (mass ratio of CaO to SiO₂) on the sintering of titanomagnetite concentrate was investigated in an experimental-scale sintering pot. The results indicated that increasing basicity can improve yield, productivity, and reduction index (RI) of the finished sinter. However, sinter strength and reduction disintegration index (RDI_+3.15) have a significant decrease at the basicity varying from 1.0 to 2.0. It also proved that titanomagnetite and titanohematite are the main minerals in sinters, while kirschsteinite and calcium silicate (CS) are the main melted phases. The amount of perovskite increases then slightly decreases with the increasing basicity as silicoferrite of calcium and aluminum (SFCA) began to generate and increase with basicity from 1.5 to 3.0. It is concluded that adjusting sinter basicity is capable to restrain the adverse effect of TiO₂ on sintering of titanomagnetite concentrate.

1. Introduction

Vanadium-bearing titanomagnetite ore, a multi-mineral ore mainly composed of Fe, Ti and V, is abundant in China, Russia, South Africa and USA.¹⁾ More than 7400 million tons of V-bearing titanomagnetite ore are reserved in Panzhihua-Xichang area, Sichuan Province, China.^2,3) Titanomagnetite concentrate has been used for extraction of Fe and V by the route of blast furnace-basic oxygen furnace (BF-BOF) presently in China.^3,4)

Sintering is a main process for agglomeration of titanomagnetite concentrate prior to smelting in BF.¹⁾ Constituents and microstructure of minerals in sinter are closely associated with sintering indexes.⁵⁾ In titanomagnetite concentrate sintering, TiO₂ will react with CaO to form perovskite, which deteriorates the tumbler index (TI) and RDI_+3.15 of the sinter.^{6,7,8,11,12,13)} Therefore, previous works were mainly focused on reducing TiO₂ content in the sintering mixture. Additionally, lack of SFCA complicates to produce the qualified sinter as compared with the conventional iron ore sintering,^9,10,11) which inevitably leads to a worse binding strength, and the poor sinter yield and tumbler index thereafter. Moreover, information about titanomagnetite concentrate sintering is rare in existing English literatures.

Aiming at understanding the sintering characteristics of titanomagnetite concentrate and its difference from conventional iron ore sintering, this study evaluated effects of basicity on titanomagnetite concentrate sintering.

2. Experimental

2.1. Materials

The main chemical composition of the titanomagnetite concentrate used in this study is shown in Table 1. –0.074 mm fraction of the concentrate is 55.20%. The coke breeze, composed of 84.05% fixed carbon, 2.79% volatile matter, and 13.55% ash (air-dry basis), was used as the solid fuel for sintering. Its calorific value is 31.92 MJ·kg^–1. More than 90% of coke breeze can pass a 3 mm-pore standard sieve. Main compositions of quick lime are 76.03% CaO, 4.20% MgO and 5.49% SiO₂.

Table 1. Main chemical composition of titanomagnetite concentrate (mass%).

FeTotal	FeO	TiO2	SiO2	Al2O3	MgO	CaO	V2O5	P	S
53.51	31.71	12.22	3.65	3.45	2.92	0.99	0.54	0.009	0.69

2.2. Procedure of Sintering Pot Experiment

Quick lime was added to adjust the basicity of blends. Table 2 shows the blending ratio of sintering materials and the calculated composition of sinters. Subsequently, the blends were granulated in drum mixer. The granulated blends were charged into a sintering pot with the dimension of Φ 165 mm × 700 mm and ignited at 1150°C for 2 min under 5 kPa suction pressure before sintering proceeded under 10 kPa suction pressure. The pre-ignition bed permeability, sintering speed, sinter yield and productivity were measured to evaluate the sintering performance.^14,15)

Table 2. Blending ratio of blends and calculated composition of sinters (mass%).

Basicity	Blending ratio			Calculated composition of sinters
	Titanomagnetite concentrate	Coke	Quick lime	FeTotal	SiO2	Al2O3	CaO	MgO	TiO2
0.5	94.23	4.5	1.27	51.57	3.92	3.53	1.95	2.87	11.76
1.0	91.61	4.5	3.89	50.49	4.00	3.48	4.04	2.92	11.52
1.5	88.93	4.5	6.57	49.38	4.08	3.43	6.13	2.98	11.26
2.0	86.17	4.5	9.33	48.22	4.16	3.37	8.30	3.04	11.00
2.5	83.34	4.5	12.16	47.01	4.25	3.32	10.58	3.10	10.72
3.0	80.44	4.5	15.06	45.75	4.34	3.26	13.06	3.16	10.43

* Return fines ratios are fixed at 23.08% to blend.

2.3. Characterization of the Sinter

Sinter was tested in an ISO tumble for tumbler index (TI) and crushed into 10–12.5 mm samples respectively to determine RDI referred to GB T13242-91 (China) and RI referred to GB T13241-91 (China). Representative sinter samples were chosen considering granularity and proportion of sinter products, which were heat mounted in resin, polished to mirror finish and examined by an optical microscope for microstructure analysis. Minerals constituent of the samples were analysed using an image analyzer.

3. Results and Discussion

3.1. Effects of Basicity on Sintering Indexes

The permeability of the bed during sintering strongly affects the sintering speed, sinter productivity and quality,¹⁶⁾ which is mainly determined by the permeability of pre-ignition bed. Figure 1(a) shows that the permeability increases with the increasing basicity from 0.5 to 3.0, which appears in a linear relationship. Figure 1(b) shows a significant decrease in TI from 1.0 to 2.0 of basicity and sinter strength improves with increasing basicity. It is different from previous results.^12,13) Figure 1(c) shows that with an increase in basicity from 0.5 to 3.0, the sintering speed increases. Figure 1(d) shows when the basicity is increased, the sinter yield increases. Productivity is determined by sintering speed and sinter yield. Figure 1(e) shows that the productivity increases with the increase in basicity.

Fig. 1.

Effect of basicity on sintering indexes and reduction properties of sinter.

3.2. Effects of Basicity on Reduction Properties of Sinter

Figures 1(f) and 1(g) show reduction properties of sinters. RI increases from 72.55% to 83.55% with increasing basicity from 0.5 to 3.0, which indicates that sinter with a higher basicity is easy to be reduced in the blast furnace. The RDI_+3.15 decreases from 86.2% to 67.5% with the basicity increase from 0.5 to 1.5. However, as the basicity further increases from 1.5 to 3.0, the RDI_+3.15 increase gradually. These results indicated that sinter basicity in the range of 1.0 to 2.5 has a negative effect on RDI_+3.15. It should be noted that the suitable basicity of conventional iron ore sintering is in the range of 1.7 to 2.2, in which RDI_+3.15 increases with increasing basicity due to the improved sinter strength and decreased hematite content.^9,10) Therefore, a higher basicity should be chosen for the sintering of titanomagnetite concentrate, considering the factor of RDI.

3.3. Effects of Basicity on Minerals Constituent and Microstructure of Sinter

According to reflective color of minerals in sinter described in reference,¹⁷⁾ minerals constituent of sinters is shown in Fig. 2. It is indicated that titanomagnetite and titanohematite are the main minerals in sinters, which are accounting for about 80% of the sinter. When the basicity is 0.5, kirschsteinite and CS are the main melted phases, while SFCA and perovskite are not observed. When basicity increases to 1.0 and 1.5, the content of kirschsteinite decreases gradually, while that of CS increases accordingly. Meanwhile, the content of perovskite is observed and increased to 7.9%. SFCA is observed in sinter at basicity 2.0, which accounts for 4.5% of the sinter. When the basicity increases to 2.5 and 3.0 further, SFCA content increases to 9.5% and 13.3%, respectively. Besides, perovskite decreases slightly.

Fig. 2.

Effect of basicity on minerals constituent of sinters.

The microstructures of sinters with basicity 0.5, 2.0 and 3.0 are shown in Fig. 3. When the basicity is 0.5 (Fig. 3 (A)), interweaved titanomagnetite and titanohematite are the main structure in sinter and kirschsteinite and CS are the main melted phases. SFCA and perovskite are not obviously observed in this sinter. When basicity increases to 2.0 (Fig. 3(B)), perovskite is observed in the gaps between joined crystals of titanomagnetite or titanohematite. When the basicity increases to 3.0 further (Fig. 3(C)), the content of SFCA increases remarkably, which closely associated with titanomagnetite.

Fig. 3.

Microstructure of sinters with different basicities. R=CaO/SiO₂, (A): R 0.5; (B): R 2.0; (C): R 3.0. Tm –Titanomagnetite, Th – Titanohematite, G – Kirschsteinite, F – Calcium silicates, P – Perovskite.

From these results, the strength and RDI property of sinter from titanomagnetite concentrate is deteriorated by the increasing perovskite in sinters. However, the adverse effects are offset by the increasing of SFCA at basicity over 2.0. In addition, acidic sintering (basicity lower than 0.5) and ultra high basicity (basicity higher than 2.5) sintering is considered to be better choices for titanomagnetite concentrate sintering with respect to RDI.

4. Conclusions

(1) Increasing basicity can improve titanomagnetite concentrate sintering performances and RI of product sinters. While sinter strength and RDI_+3.15 have a significant decrease at the basicity ranging from 1.0 to 2.0.

(2) Titanomagnetite and titanohematite are the main minerals in sinters, while kirschsteinite and CS are the main melted phases. Perovskite increases then slightly decreases with the increasing basicity. SFCA began to generate and increase with basicity from 1.5 to 3.0.

(3) The adverse effect of TiO₂ on sintering of titanomagnetite concentrate is able to be restrained to a large extent by adjusting the basicity of sinter mixture in a suitable range.

Acknowledgements

The authors wish to express their thanks to the New Century Excellent Talents of the Ministry of Education (NCET-11-0515) for the financial support of this work.

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