The physicochemical bases of the pyrometallurgical processing of the titanomagnetite ores

Introduction

The blast-furnace process is one of the basic stages of the metallurgical treatment of iron ores under the scheme “blast furnace – converter”. The efficiency of blast furnace smelting depends on the iron ore materials and the coke quality. Now the requirements for the raw materials have become harder.

The coke rate as energy carrier (heat and reducing agent source) can be decreased generally in two ways [1]. First, it can be decreased by using the extensive factors, such as increased iron content of burden; utilization of direct coke substitutes (natural gas, oil, pulverized coal, reducing gas, including top gas without carbon dioxide); increased blast temperature; utilization of high potential heat, etc.

Second, the coke consumption can be reduced by using the intensive factors, such as increasing of the utilization heat and reducing the potential of gas as a result of improvement of the iron ore raw materials and coke quality indicators, namely, reducibility, cold and hot strength, softening and melting temperatures of iron ore raw materials, and the coke reactivity index (CRI) and the coke strength after reaction (CSR) index.

The reserves necessary for the first technique have been exhausted. Thus, the main option for decreasing coke consumption and improving the technical and economic parameters of blast furnace smelting is to improve the quality of iron ore materials and coke [2, 3].

At the Institute of Metallurgy of the Ural Branch of the Russian Academy of Sciences, a technique for the analysis of the influence of the qualitative characteristics of iron ore raw materials and coke on the technical and economic indicators of blast furnace smelting, allowing for the definition of the metallurgical value of new types of mineral raw materials, is being developed [4].

The preliminary estimate of the technical and economic indicators of the pyrometallurgical processes of treatment of ores on the basis of trial test results, as it is accepted in practice now, is complicated for technical and economic reasons as negative results or even an emergency in a blast furnace is possible. The method consists of the following steps.

1. In vitro producing of sinter or pellets, definition of the metallurgical properties (strength, reducibility, softening and melting points). Reducibility of raw material is defined according to Russian state standard 17212-84, strength – 15137-84 (ISO 13930), temperatures of softening beginning and melting of iron ore material – 26517-85.

2. Calculated forecasting of coke properties (strength, CRI) based on properties of components of coal mix for coking. Coke reactivity index (CRI, %) and coke strength after reaction (CSR, %) are defined according to Russian state standard R 50921-2005.

3. Definition of technical and economic parameters of blast furnace smelting by means of mathematical models. Mathematical models of blast furnace smelting include the balance logical statistical model [5] and a complex of two-dimensional models (gas dynamics, heat change, reduction, cohesion zone [6]). The mathematical models allow the use of the metallurgical characteristics of iron ore raw materials and coke as the source data, including two-dimensional adaptation. The mathematical analysis allows for the estimation of the possibility of the use of iron ore raw materials and coke as charge in blast furnace smelting.

4. Trial testing with the guaranteed receipt of positive results.

Use of mathematical models allows the definition of the quantitative influence of the quality of iron ore raw materials, including new deposits, on the basic technical and economic parameters of blast furnace smelting.

Metallurgical characteristics of titanomagnetite sinter and pellets

Titanomagnetite sinter and pellets are investigated (iron ore concentrate the same). The chemical composition is shown in Table 1.

Micro X-ray diffraction also has been carried out to study the mineralogical composition of sinter and pellets. Studies of samples are conducted with the X-ray diffractometer XRD 7000 (SHIMADZU, Japan) with automatic program control, with the use of CuK_α-radiation (stress on tube 40 Кv, current on tube 30mA) and a graphitic monochromator. Identification of formed phases in investigated samples is performed using a database of PDF2 (ICDD).

The autoemission electronic microscope TESCAN MIRA 3 LMU (TESCAN, Czech Republic) allows to receive the image of a surface of studied object with high resolution: resolution (In-Beam SE) – 1 nm at 30 kV and 2 nm at 3 kV; resolution (ET type SE) – 1.2 nm at 30 kV and 2.5 nm at 3 kV. It is equipped with system of the X-ray power dispersive microanalysis of Oxford Instruments INCA Energy 350 X-max 80 with the nitrogen-free detector X-max 80 Standart (silicon-drift detecting element, the active area – 80 mm²; resolution on the Mn Kα line – 127 ev).

Sinter

The basic phase in the sinter is Fe₃O₄ (magnetite). The diffraction pattern of the sinter is shown in Fig. 1. It comprises Fe₂O₃ (hematite) and a Ca-containing silicate of difficult composition; Ca_2,3Mg_0,8Al_1,5Fe_8,3Si_1,1O₂₀ is well represented in the sample.

Fig. 1:

Diffraction pattern of the initial sinter (Table 1), X-ray diffractometer XRD 7000, Symbols: v – Fe₃O₄;+ – Fe₂O₃.

* – Ca_2,3Mg_0,8Al_1,5Fe_8,3Si_1,1O₂₀.

The most typical field of sinter microstructure has been chosen. The chemical composition of the basic mineral phases is defined (Fig. 2, Table 2; results are shown in atomic %).

Fig. 2:

Phase composition of sinter (Table 1) in BES mode (×500), electronic microscope TESCAN MIRA 3 LMU.

Table 2:

Results of quantitative analysis in points 1–17 (Fig. 2) with locality 1–2 microns and averaged result (total spectrum), atomic %.

Spectrum	O	Na	Mg	Al	Si	Ca	Ti	V	Fe
Total spectrum	57.88		0.92	1.78	5.58	9.36	1.50	0.29	22.69
Phase	Magnetite Fe3O4
1	53.50		2.17	1.48		1.06	0.26		41.52
2	53.75		2.28	1.48		1.03	0.32		41.15
3	53.53		2.68	1.74		0.75	0.33		40.97
4	53.79		2.14	1.39		1.10	0.32		41.26
Phase	Calcium titanate (perovskite) CaO·TiO2
5	60.09			0.50	3.66	20.18	8.18		7.39
6	61.91			0.57	4.35	19.22	6.21		7.73
7	63.05			0.53	4.58	18.82	5.35		7.67
8	56.23			0.60	4.40	21.92	7.43		9.42
Phase	Dicalcium silicate (SFCA) Ca(Mg, Fe, Mn)O·SiO2
9	57.13			3.07	14.12	18.26	0.95	1.09	5.38
10	62.00			3.31	13.06	15.68	0.77	0.88	4.29
11	58.05			3.72	13.44	17.36	0.81	0.86	5.76
12	59.90	0.39		3.26	13.64	16.59	0.84	1.00	4.38
Phase	Ca2.3Mg0.8Al1.5Fe8.3Si1.1O20
13	55.99		0.40	5.03	4.63	9.17	0.78		24.01
14	55.00		0.33	4.42	3.94	9.07	0.70		26.53
15	55.96		0.46	5.54	4.67	9.25	0.79		23.33
16	56.16		0.37	5.29	4.63	9.04	0.77		23.74
17	55.62			4.59	4.49	9.18	0.83		25.28

1. The most typical field of sinter microstructure has been chosen. The chemical composition of the basic mineral phases is defined (Fig. 2, Table 2; results are shown in atomic %).

In the initial sinter, one can isolate phases of magnetite Fe₃O₄, calcium titanate (perovskite) CaO·TiO₂, lime CaO and dicalcium silicate, one of which is stabilized by aluminum Ca_2,3Mg_0,8Al_1,5Fe_8,3Si_1,1O₂₀. Dicalcium silicate CaO·SiO₂ (phase 4, 5 in Table 3) is not stabilized and as a result will form sinter fines.

Table 3:

Results of quantitative analysis in points 1–7 (Fig. 3), atomic %.

Spectrum	O	Na	Mg	Al	Si	Ca	Ti	V	Fe
Phase	Magnetite Fe3O4
1	53.64		2.11	1.25		1.21			41.78
Phase	Calcium titanate (perovskite) CaO·TiO2
2	64.12			0.56	4.20	18.29	6.05		6.77
Phase	Dicalcium silicate (SFCA) Ca (Mg, Fe, Mn)O·SiO2 (stabilized Al)
3	61.35			3.45	13.66	15.98	0.71	0.83	4.01
Phase	CaO·SiO2 (not stabilized)
4	59.68	0.27			13.92	24.83		0.43	0.87
5	56.10				13.05	29.62		0.48	0.75
Phase	Lime CaO
6	57.69			0.64	4.28	21.11	7.28		9.00
7	56.98			1.31	4.11	19.54	7.52		10.53

Fig. 3:

Phase composition of sinter (Table 1) in BES mode (×2000), electronic microscope TESCAN MIRA 3 LMU.

According to [7, 8] phase, SFCA has a considerable influence on sinter strength.

Pellets

To study pellet mineral composition, micro X-ray structure phase analysis was also performed. In the pellet sample, the most specific part of the micro structure was selected and the chemical composition of the main mineral phases was determined (Fig. 4, Table 4; the results are shown in atomic %).

Fig. 4:

Phase composition of pellets (Table 1) in BES mode (×2000), electronic microscope TESCAN MIRA 3 LMU.

Table 4:

Results of quantitative analysis in points 1–12 (Fig. 4), atomic %.

Spectrum	O	Na	Mg	Al	Si	Ca	Ti	V	Mn	Fe
Phase	Hematite Fe2O3
1	57.59		0.65	0.88			0.98	0.38		39.52
2	58.40		0.44	0.86			0.51	0.30		39.49
Phase	Magnesioferrite MgO·Fe2O3
3	56.20		11.75	5.47					0.42	26.17
4	56.95		11.28	4.74					0.56	26.47
Phase	SiO2+helenite 2CaO·Al2O3·SiO2
5	63.69	0.45	3.20	5.58	18.19	5.83	0.11	0.76		2.19
6	58.53		5.86	4.44	17.31	7.45	0.19	0.31	0.17	5.73
7	58.03	0.31	7.32	3.81	18.72	8.18		0.57		3.06
Phase	Anorthite (Ca, Na)(SiAl)4O8
8	61.90	0.87	0.66	10.81	17.26	6.52				1.98
9	61.18	1.02	0.49	11.54	17.05	6.25				2.48
Phase	Hematite Fe2O3+MgO+Al2O3+SiO2
10	56.07		9.06	4.60	2.79	0.74	0.35		0.51	25.89
11	57.47	0.36	0.94	3.36	5.07	1.64	0.70	0.49		29.96
12	49.82		0.97	2.33	0.75	0.31	1.57			44.26

In pellets, the main phase is hematite. There is also magnesioferrite, helenite and anorthite. An X-ray diffractogram of pellets (Fig. 5) shows the presence of mainly Fe₂O₃ (hematite). Fe₃O₄ (magnetite) is detected very poorly. On the level of instrument threshold, SiO₂ (quartz) and TiO₂ и MgSiO₃ phases are determined.

Fig. 5:

Diffraction pattern of initial pellets (Table 1), X-ray diffractometer XRD 7000. Symbols: v – Fe₃O₄;+ – Fe₂O₃, o – SiO₂; x – MgSiO_3.

Mathematical modelling

As an example, the study results of three different pellet laboratory samples received from iron ore concentrate of one deposit with different titanium dioxide content are shown below. Their chemical compositions are shown in Table 5.

Determination of pellet reducibility was made at the unit which corresponds to requirements of State Standard 17212–84. The calculated rate of reducibility for samples 1, 2 and 3 are equal, correspondingly 0.75, 0.98 and 0.70%.

Calculated degrees of reducibility are provided in Table 6. Results of research of pellets durability at low-temperature reduction on ISO 13930 are given in Table 7, and a temperature interval of a softening in Table 8.

Pellet strength results at low temperature reducibility (LTD_+6,3) as per ISO 13930 for samples 1, 2 and 3 were correspondingly 65.71, 69.92 and 78.58%; softening temperature ranges were correspondingly 1210–1300, 1180–1310 and 1130–1340°С.

Thus, the study of three samples of laboratory iron ore pellets with different content of titanium dioxide showed that pellets (sample 2) with less content of TiО₂ have the best reducibility. Pellets (sample 3, variant 1) with medium content of titanium dioxide have the best strength during reducibility. Pellets (sample 1, variant 2) with high TiО₂ content have the highest onset temperature of softening and lowest softening temperature range.

Based on the chemical composition of iron ore pellets, their reducibility and strength characteristics, the main parameters of blast furnace smelting were calculated (Table 9) by means of a balance logical statistical model [5]. Calculation results of blast-furnace indices for laboratory pellet samples no. 2 and 3 are similar, so that the table shows only results for sample 3. The high content of titanium dioxide in blast furnace slag and the slag low ratio attract attention.

Industrial application

Improvement of processing of titan magnetite ores of the Gusevogorsky field of the Kachkanarsky fields group is made. Stages of 2015 in Tables 10–12: 1 – April-May, base (agglomerate basicity 2.1); 2 – June-July, agglomerate basicity 2,4; 3 – August-September, agglomerate basicity 2,4 with polymeric additive (300 g/t и 500 g/t). According to [9], increase of agglomerate basicity has a considerable influence on forming the phase SFCA and sinter strength.

In Table 13, the change of blast furnace indices is shown.

Conclusion

The method of the influence estimation of the iron ore raw material characteristics on the technical and economic parameters of the blast furnace smelting was reviewed. The examples of assessment of sinter and pellet metallurgical properties of titanium magnetite concentrate were made. The results of blast furnace smelting parameters were calculated. The influence of the phase composition of an agglomerate on its quality is shown. The results of industrial tests on the change of quality of the agglomerate (durability, reducibility, softening and melting temperatures) and its influence on blast furnace indices are given.