Review on the Phase Equilibria in Iron Ore Sinters

Abstract

Sintering process is a commonly used pre-treatment process for iron containing burden materials with an aim to produce porous, agglomerated sinter material with suitable properties to be charged into the blast furnace. During the sintering process the material undergoes a series of reactions, during which the conditions vary considerably. These changes in temperature and state of oxidation cause changes in the mineralogical composition of the material and although the sintering process does not completely reach the chemical equilibrium, it is important to understand the phase equilibria of the sinter system in order to analyse and control the effect of various factors on the sintering process.

The purpose of this paper is to give a review on the research related to phase equilibria in iron ore sinters. The main components of the sinter are FeO, Fe₂O₃, SiO₂, CaO, Al₂O₃ and MgO and by studying the phase equilibria of this system, the behaviour of sinters can be evaluated. Based on the experimental data, oxide databases have been created to provide thermochemical data of all the necessary compounds within this system. Concerning the solutions, more research is required related to SFCA phases. These databases are commercially available with thermochemistry software and can be used to compute phase diagrams illustrating the effect of different factors on the phase equilibria within the FeO–Fe₂O₃–SiO₂–CaO–Al₂O₃–MgO system. Phase diagrams provide a useful tool to study the behaviour of the material in both sintering process itself as well as in the following reduction processes such as the blast furnace.

1. Introduction

Sintering is a thermal agglomeration process that is applied to a mixture of iron ore fines, recycled ironmaking by-products, fluxes, slag-forming agents, and solid fuel (coke).^1,2,3,4) In the sintering process the sinter mixture undergoes a series of reactions,^2,4,5) during which the process temperature rises up to 1300–1480°C⁴⁾ or to 1300–1375°C⁵⁾ at maximum. The purpose of the sintering process is manufacturing a porous product with suitable thermal, mechanical, physical and chemical characteristics to be used as blast furnace feed to produce hot metal.^1,2,3) The bonding between the particles is caused by partial melting and recrystallization, and therefore no additional binder needs to be added in this process.⁴⁾ Sinter performs well in the blast furnace, particularly if it is made with fluxes added before sintering and is sized to 25 mm × 6 mm before charging into the furnace.⁶⁾ Sinter burdens in a blast furnace process are prevalent in European⁷⁾ and Asian^7,8) blast furnaces, while pellet burdens are used more commonly in Scandinavia and North America.⁷⁾ Both the sintering process and the sinter material have been widely studied in the iron and steelmaking industry as reported by e.g. Fernández-González et al.^1,2,3,9) in their recent review papers.

The sintering in iron and steel industry is most commonly implemented on a belt furnace, which can be divided into four different sections: cold and wet zone (below 100°C); drying zone (100–500°C); reaction zone (up to 1300–1480°C); and cooling zone. The cold and wet zone is formed by the mix to be sintered. The vaporisation of the moisture and subsequent dehydration of hydroxides take place in the drying zone. The main reactions occur in the reaction zone and those are: coke combustion (exothermal); decomposition of carbonates (endothermal); solid phase reactions; reduction of iron oxides; re-oxidation of iron oxides; and formation of the sintered mass. Cooling and re-crystallization of the sintered product take place in the cooling zone.²⁾ As material proceeds in the sintering belt, it gradually undergoes all these stages in such a way that all the above mentioned zones can be identified on a cross-section of the material on a certain location in the belt. The goal is to have material thoroughly sintered at the end of the belt.

The sintering process is very short and the conditions in the different parts of the sintering bed differ particularly with respect to the temperature and partial pressure of oxygen.⁶⁾ Hence, the mineral composition of sinter is not in a state of equilibrium, and there is a large variation in mineralogy between different sinter pieces. In addition, part of the minerals of the sinter mixture remain almost totally unchanged throughout the whole sintering process. Such minerals are the coarsest magnetite and hematite grains and often the quartz and olivine grains.¹⁰⁾ Despite the non-equilibrium and inhomogeneous nature of the sintering process, it is nevertheless important to understand the phase equilibria of the sinter system in order to analyse and control the effect of various factors (such as temperature, composition and state of oxidation) on the sintering process.¹¹⁾ The purpose of this paper is to give a review on the research related to phase equilibria in iron ore sinters with the main focus on the assessment of thermochemical data needed to model sinter system.

2. Iron Ore Sinter

Sinter is a very heterogeneous material and its properties vary considerably depending on the type of raw materials and blend. Usually the raw material blend consists of iron ore fines (with either hematite, Fe₂O₃, or magnetite, Fe₃O₄, as the main component), other iron containing fine materials (such as steelmaking dusts), slag forming substances (such as limestone, serpentine or silica sand), and fuels (coke breeze).^1,10,12) Although iron oxide is the dominant component, sinters are commonly classified based on the amounts of other components. For example, Geerdes et al.⁷⁾ classifies the sinter to three different types based on the short basicity (B2 = Y_CaO/Y_SiO2): acid (B2 < 1.0), fluxed (1.0 < B2 < 2.5) and super-fluxed (B2 > 2.5) sinter, of which the most common type is the fluxed sinter. Y_i in the formula refers to the mass fraction of the component i.

Mochón et al.¹³⁾ point out that it is important to have a sinter with a high iron content, a low gangue content and short basicity of 1.6−2.1. Wang et al.¹⁴⁾ continue that a high-quality sinter should have characteristics of high strength, low reduction degradation and easy reduction. These metallurgical properties are closely related to the structural characteristics of the sinter. For example, the pore size and distribution of the sinter greatly influence the strength and reduction.¹⁴⁾

2.1. Chemical Composition

Chemical compositions of sinters may be presented in various ways depending on the methods that have been used in chemical analysis. The main characterisation method for chemical composition is X-ray fluorescence (XRF). As mentioned above, iron is the element with the largest quantity in all sinters and despite its existence in sinters as oxides its amount is usually presented as “Total Fe (Fe_tot)”, which refers to elemental composition as mass%. To illustrate the distinction between divalent and trivalent iron, the amount of divalent iron (as FeO) is usually shown separately. It should be noted that this should not be confused with the mineralogical composition, and divalent oxide is most likely to appear as a part of other minerals than wüstite. Furthermore, the amounts of the most other components are expressed as contents of corresponding oxides, although there are some exceptions such as sulphur.

Typical compositions of iron ore sinters are shown in Table 1 after Pettersson et al.,¹⁵⁾ Fernández-González et al.³⁾ and Cores et al.,¹⁶⁾ whereas Table 2 collects examples of sinter compositions (both production and laboratory sinters) from various sources.^{10,12,15,16,17,18,19,20,21)} Table 2 shows that sinters from different production sites are chemically quite similar. However, Heinänen¹⁰⁾ emphasizes that some seemingly insignificant differences in the chemical composition are in fact significant in regard to sinter properties, for example the contents of Fe, MgO and Al₂O₃ as well as basicity.

Table 1. Typical chemical compositions of ironmaking sinters (in mass%).

Site	Value type	Fetot	FeO	SiO2	MgO	Al2O3	Alkalis	P	B2
European countries15)	Typical	~56.5	7.3±0.5	5.9±0.2	1.0±0.2				2.0±0.1
Western European countries3)	Typical	>56			1.65	1.35			1.7
Western European countries16)	Minimum	51	4.0	5.3	0.7	0.6
Western European countries16)	Maximum	61	11	5.4	2.2	1.8	0.11	0.04

Table 2. Examples of chemical compositions of iron ore sinters (in mass%).

		Major components							Minor components							Basicity
Reference	Fetot	FeO	Fe2O3***	CaO	SiO2	MgO	Al2O3	Sum	MnO	TiO2	S	P	V	Na2O	K2O	B2
Production sinters
Iljana et al.17)	60.6	11.1	74.3	7.12	3.35	1.99	0.58	98.5							0.050	2.13
Honeyands et al.18)	57.38	5.45	76.0	9.24	5.56	0.95	1.80	99.0				0.06				1.66
Honeyands et al.18)	56.73	5.91	74.5	10.04	5.43	1.76	1.87	99.6				0.07				1.85
Heinänen10)	59.9	10.9	73.5	7.13	4.63	2.20	0.76	99.2								1.54
Heinänen10)	59.6	11.5	72.4	7.07	5.47	1.72	0.84	99.0								1.29
Heinänen10)	60.0	11.1	73.5	6.76	5.51	1.50	0.75	99.1								1.23
Heinänen10)	57.0	5.6	75.3	10.18	5.86	1.95	1.10	100.0								1.74
Heinänen10)	57.1	6.1	74.9	9.62	5.71	1.79	1.51	99.6								1.69
Heinänen10)	56.8	6.3	74.2	9.62	5.76	1.92	1.23	99.0								1.67
Heinänen10)	58.0	4.9	77.5	8.80	5.66	1.66	1.05	99.6								1.55
Average								99.2
Laboratory sinters
Higuchi et al.12)	62.7	9.60	79.0	5.20	3.61	1.13	0.96	99.5								1.44
Higuchi et al.12)	57.7	4.51	77.5	9.02	4.93	0.97	1.72	98.6								1.83
Honeyands et al.18)	56.87	4.45	76.4	9.83	4.95	1.79	1.73	99.1				0.05				1.99
Pownceby et al.19)			77.1	14.16	3.48	0.12	4.56	99.4		0.15		0.02**		0.10		4.07
Umadevi et al.20)	54.8	9.3	68.0	12.0	5.94	1.48	3.60	100.3						0.050	0.090	2.02
Umadevi et al.20)	54.0	9.6	66.5	12.6	6.20	1.85	3.65	100.4						0.530	0.080	2.03
Umadevi et al.20)	53.7	9.8	65.9	13.0	6.30	2.18	3.72	100.9						0.547	0.090	2.06
Umadevi et al.20)	53.2	10.2	64.7	13.6	6.60	2.54	3.83	101.5						0.523	0.876	2.06
Yan et al.21)	57.87	8.98	72.8	9.82	4.79	1.57	1.89	99.8							0.111*	2.05
Pettersson et al.15)	56.06	7.08	72.3	11.7	5.92	1.04	1.14	99.2	0.47*	0.07		0.028	0.004			1.98
Pettersson et al.15)	56.32	7.21	72.5	11.7	5.88	1.03	0.96	99.3	0.39*	0.124		0.027	0.003			1.99
Pettersson et al.15)	56.16	7.81	71.6	11.8	5.88	0.99	1.07	99.2	0.37*	0.108		0.045	0.015			2.01
Pettersson et al.15)	56.02	7.17	72.1	11.9	5.96	1.07	1.02	99.3	0.39*	0.096		0.026	0.02			2.00
Cores et al.16)	57.5	5.5	76.1	9.15	5.7	1.55	1.16	99.2	0.64		0.012	0.034		0.041	0.066	1.62
Cores et al.16)	56.3	5.7	74.2	10.45	5.6	1.71	1.35	99.0	0.72		0.013	0.041		0.038	0.060	1.87
Cores et al.16)	57.1	3.5	77.8	9.25	5.6	1.63	1.17	98.9	0.66		0.010	0.030		0.039	0.064	1.59
Cores et al.16)	56.0	3.2	76.5	11.10	5.4	1.53	1.15	98.9	0.64		0.017	0.031		0.038	0.065	2.02
Cores et al.16)	57.1	5.0	76.1	9.20	5.8	1.44	1.24	98.8	0.65		0.012	0.041		0.038	0.058	1.59
Cores et al.16)	56.2	4.6	75.2	10.90	5.7	1.58	1.15	99.2	0.68		0.015	0.039		0.039	0.058	1.93
Cores et al.16)	57.5	3.3	78.5	9.10	5.4	1.51	1.20	99.1	0.66		0.011	0.038		0.037	0.060	1.70
Cores et al.16)	56.7	4.3	76.3	10.40	5.5	1.50	1.17	99.2	0.70		0.018	0.040		0.037	0.056	1.90
Cores et al.16)	57.3	5.7	75.6	9.35	5.4	1.70	1.22	99.0	0.74		0.011	0.042		0.039	0.071	1.73
Cores et al.16)	56.6	5.6	74.7	10.50	5.3	1.68	1.17	99.0	0.72		0.014	0.039		0.037	0.071	1.98
Cores et al.16)	57.6	3.7	78.2	8.80	5.4	1.60	1.19	98.9	0.80		0.013	0.040		0.036	0.072	1.64
Cores et al.16)	56.9	3.6	77.4	10.35	4.9	1.61	1.16	99.0	0.71		0.018	0.040		0.031	0.068	2.13
Cores et al.16)	58.0	4.7	77.7	8.60	5.2	1.62	1.20	99.0	0.75		0.012	0.040		0.032	0.066	1.67
Cores et al.16)	56.5	5.0	75.2	10.13	5.7	1.66	1.10	98.8	0.73		0.014	0.036		0.036	0.072	1.82
Cores et al.16)	56.7	3.5	77.2	9.05	5.0	1.56	1.19	97.5	0.79		0.012	0.041		0.040	0.075	1.81
Cores et al.16)	57.0	4.5	76.5	9.90	5.5	1.49	1.17	99.1	0.77		0.015	0.039		0.039	0.076	1.80
Cores et al.16)	57.0	5.7	75.2	10.35	5.9	1.75	1.02	99.9	1.04		0.013	0.036		0.036	0.065	1.75
Cores et al.16)	55.5	5.2	73.6	10.85	5.5	1.65	0.99	97.8	0.96		0.022	0.030		0.042	0.070	1.97
Cores et al.16)	56.2	5.5	74.2	10.85	5.7	1.55	0.91	98.8	0.94		0.017	0.031		0.038	0.064	1.90
Cores et al.16)	56.0	3.8	75.8	10.80	5.6	1.65	1.06	98.8	0.93		0.023	0.035		0.045	0.078	1.93
Cores et al.16)	55.4	3.9	74.9	11.10	5.8	1.68	1.11	98.5	0.99		0.020	0.029		0.042	0.078	1.91
Cores et al.16)	54.9	3.5	74.6	11.55	6.0	1.68	1.15	98.5	0.93		0.025	0.032		0.041	0.079	1.94
Cores et al.16)	55.7	5.5	73.5	11.55	5.6	1.88	1.12	99.2	0.85		0.015	0.035		0.041	0.092	2.01
Cores et al.16)	56.1	5.3	74.3	10.10	5.3	1.70	1.18	97.9	0.80		0.012	0.034		0.039	0.091	1.91
Cores et al.16)	56.5	5.2	75.0	10.10	5.2	1.69	1.22	98.4	0.97		0.014	0.032		0.042	0.088	1.94
Cores et al.16)	55.3	4.4	74.2	11.00	5.7	2.01	1.27	98.6	0.80		0.015	0.037		0.044	0.095	1.95
Average								99.1

*  Reported as elemental compound in the original publication but converted here to oxide compound.

**  Reported as oxide compound in the original publication but converted here to elemental compound.

***  Calculated here taking into account the contents of Fe_tot and FeO reported in the original publication.

Because world’s reserve of high-grade iron ores is declining, the gangue minerals of iron ore, such as SiO₂ and Al₂O₃, have been increasing.^22,23) Ores with higher gangue contents do not allow to produce sinters with lower SiO₂ contents and correspondingly higher Fe contents.²⁴⁾ In general, as the Al₂O₃ content of iron ores increases, both the sintering performance and the quality of the sinter product deteriorate.²²⁾ For example, the SiO₂ content in the sinter of Thyssenkrupp Steel Europe AG has risen from values of under 5.4 mass% to 5.9 mass% during the years 2000−2015, along with a decrease of the Fe contents in sinter from 57.5 mass% to 55 mass%.²⁵⁾ Quite similarly, the contents of SiO₂ and Al₂O₃ ranged from 5.2 mass% to 7.7 mass% and from 1.0 mass% to 2.4 mass% in a 3-year period from 2012 to 2015 at Redcar blast furnace owned by SSI UK.²⁶⁾ To counter the adverse impact of alumina on sinter quality, MgO fluxes are often used to improve the high-temperature properties of sinters.²³⁾

The chemical compositions of iron ore sinters collected in Tables 1 and 2 show that the major components in the sinter system are FeO, Fe₂O₃, CaO, SiO₂, MgO and Al₂O₃. With this kind of a six-component system, approximately 99% of the system can be described.

2.2. Mineralogical Composition

In principle, the chemical components of the sinter presented in chapter 2.1 form a large variety of mineral compounds. Thus, the mineralogy of iron ore sinters is quite complex.²⁷⁾ The characterisation of the phases in sinter is usually performed manually by an experienced mineralogist (point counting) using a microscope.²⁸⁾ With the advancement of science and technology, there are an increased number of methods to characterise the composition and structure of mineral phases in sinter. The current main characterization methods are optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and serial sectioning and 3D reconstruction.¹⁴⁾ The SEM can be automated and equipped with a motorized and computer-controlled multiple sample stage as introduced by Wang et al.²⁹⁾ In addition to manual methods, micro image processing software packages, such as VisuMet developed by K1-Met, are used nowadays to identify the main mineral phases in sinter.³⁰⁾

Sinters are primarily composed of iron oxides^19,31) (hematite, Fe₂O₃ and magnetite, Fe₃O₄), complex calcium ferrite phases^19,31) (mainly silico-ferrite of calcium and aluminium, SFCA), crystalline calcium-rich silicates,¹⁹⁾ residual slag³¹⁾ solidified as amorphous phases, and non-assimilated non-ferrous particles,¹⁹⁾ which are not reacted during sintering. Among them, hematite and SFCA are in main proportion.³¹⁾ Iron oxides exist either in an unreacted primary form or as secondary phases formed by reactions or precipitation from the melt.^27,31) Hsieh and Whiteman^32,33,34) have found that the gas atmosphere used in sintering and the compositions of raw materials have significant effects on the mineral phases that are formed.

Different phases verified in the sinter have been collected from literature to Table 3, in which the sinter phases have been classified into iron oxides, calcium ferrites, amorphous silicates, crystalline silicates and non-assimilated non-ferrous particles. The classification has been modified after Mezibricky et al.³⁵⁾ In addition to mineral phases, there are different-sized pores^{8,12,27,28,36,37,38)} in the sinter structure, which also have a marked effect on the sinter properties.¹⁴⁾

Table 3. Sinter phases reported in the literature.

Phase	Formula	Reference
Iron oxides
Hematite	Fe2O3	12,18,27,30,34,35,36,37,38,39,40)
Primary hematite (non-assimilated or residual)		8,16,18,28,41)
Secondary hematite (precipitated)		8,16,18,28,41)
Magnetite	Fe3O4	12,18,27,28,30,34,35,36,37,38,39, 40,41,42)
Primary magnetite (non-assimilated or residual)		16)
Secondary magnetite (precipitated)		16)
Wüstite	Fe1−yO	27,35,39)
Calcium ferrites
SFCA		8,12,16,18,27,28,36,38,40,42)
SFCA-I		18,27,28,42)
Calcium ferrites	e.g. Ca4Fe9O17, CaFe5O7, Ca2Fe22O33	30,34,35,37,38,40,41)
Monocalcium ferrite, CF	CaFe2O4	27)
Dicalcium ferrite, C2F	Ca2Fe2O5	27)
Brownmillerite	Ca2(Al,Fe)2O5	35)
Calcium iron oxides		39)
Amorphous silicates
Slag		37)
Glass		18,19,27,28,30,42)
Glassy silicate/siliceous glass		30,34,41)
Silicate		38)
Crystalline silicates
Fayalite	Fe2SiO4	37)
Calcium silicate		37)
Dicalciumsilicate/larnite, C2S	Ca2SiO4	18,27,35,39)
Fe–Ca-olivine	(Ca,Fe,Mg)2(Si,Al)O4	27)
Kirschsteinite	CaFeSiO4	27)
Hedenbergite	CaFeSi2O6	27,35)
(Pseudo)wollastonite	CaSiO3	27,35)
Non-assimilated non-ferrous particles
(Relict) flux		8,34)
Quartz	SiO2	18,27,35,39)
Calcite	CaCO3	39)
Dolomite	MgCa(CO3)2	35,39)
Lime	CaO	35,39)
Gangue		16)

As earlier stated, the chemical composition together with the process conditions determine the mineral compounds in the sinter. According to Hsieh and Whiteman,³⁴⁾ an increase in short basicity favours the formation of calcium ferrite. The amount of calcium ferrite or SFCA phases is also increased with the increase of Al₂O₃ content, and simultaneously the amounts of amorphous silicates and reoxidised hematite decrease. Use of dolomite increases the MgO content, which leads to a decrease in the amount of calcium ferrites. Otherwise, if the increase in the amount of MgO is due to serpentine addition, the content of calcium ferrite increases considerably.³⁴⁾

2.2.1. Iron Oxides

Hematite, Fe₂O₃, is a dominant mineral in sinter and is always present in varying proportions. The hematite in sinter can be classified into: primary hematite, which is relict hematite from the iron ore concentrate; and secondary hematite, which is crystallized from the melt.¹⁰⁾ If coarse and dense hematite exists in the iron ore, some unreacted primary hematite may exist in the sinter.³⁴⁾

Magnetite, Fe₃O₄, is a dominant mineral in sinter and similarly to hematite the amount of magnetite depends largely on the magnetite/hematite ratio in the sinter mix. Sinter contains several different types of magnetite and they can be classified into: primary magnetite, which is relict magnetite originating from the iron ore concentrate; secondary magnetite, which is magnetite crystallized from the melt; and magnetite reduced from hematite.¹⁰⁾

Wüstite is relatively rare in industrial sinters and only small amounts of it can be found in acid sinters.¹⁰⁾

2.2.2. Calcium Ferrites and SFCAs

According to several authors, no pure calcium ferrites are present in sinter as calcium ferrites contain so much dissolved silicon and aluminium that they can be called SFCA (silico-ferrite of calcium and aluminium) phases. Those phases are key bonding minerals within industrial iron ore sinter.^43,44) Needle-like structures of calcium ferrites have a relatively open structure and are easily accessible for reduction gas in the blast furnace.⁷⁾

Several authors reported on the existence and the importance of calcium ferrites as mineral phases in sinter already in the late 1950s and early 1960s.⁴⁰⁾ However, due to limited observation techniques and analytical methods of that time, calcium ferrites were considered as belonging to relatively simple systems of CaO–Fe₂O₃ and CaO–FeO–Fe₂O₃.⁴⁰⁾ A revolution occurred at the beginning of the 1960s, when Hancart⁴⁵⁾ discovered ‘impure ferrites’ when exposing iron ore sinters to X-ray diffraction. A couple of years later, with the use of electron probe microanalysis (EPMA), Hancart et al.⁴⁶⁾ determined the chemical composition of those complex ferrites. Those authors proposed a general formula for the calcium ferrites existing in iron ore sinters xFe₂O₃·ySiO₂·zAl₂O₃·5CaO with x + y + z = 12.^40,47) The abbreviation SFCA was then created to denote the calcium ferrites found in iron ore sinter.⁴⁰⁾

The SFCA in iron ore sinter has been categorized in the literature on the basis of composition, morphology and crystal structure into two main types.⁴⁴⁾ The first is a high-Fe, low-Si form called SFCA-I, which has a characteristic platy (also described as acicular) morphology.⁴⁴⁾ Mumme et al.⁴⁸⁾ reported that an SFCA-I phase in an industrial sinter contained 84 mass% Fe₂O₃, 13 mass% CaO, 1 mass% SiO₂ and 2 mass% Al₂O₃, and also a synthesized SFCA-I material with the composition 83.2 mass% Fe₂O₃, 12.6 mass% CaO and 4.2 mass% Al₂O₃. The second SFCA type is a low-Fe form called SFCA, which is described as having a prismatic or columnar morphology.⁴⁴⁾ The SFCA phase found in industrial sinters typically contains 60–76 mass% Fe₂O₃, 13–16 mass% CaO, 3–10 mass% SiO₂, 4–10 mass% Al₂O₃ and 0.7–1.5 mass% MgO.⁴⁴⁾ The SFCA and SFCA-I phases are formed in iron ore sintering operations.⁴⁹⁾ Additionally, Mumme⁵⁰⁾ has described a third SFCA homologue, denoted SFCA-II (A₄T₁₄M₁₆O₄₈), synthesized in the ternary CaO–Fe₂O₃–Al₂O₃ system. It is not known if SFCA-II is formed in industrial sinter.⁴²⁾ These SFCA (A₂T₆M₆O₂₀), SFCA-I (A₃BM₈T₈O₂₈) and SFCA-II (A₄T₁₄M₁₆O₄₈) phases are homologous series based on that of aenigmatite.^48,50,51) In the structure formula, A = Ca²⁺, B = Ca²⁺ and Fe²⁺, and M and T indicate the octahedral and the tetrahedral cation sites, respectively.¹¹⁾

2.2.3. Amorphous and Crystalline Silicates

Silicates in the sinter can either be in the crystalline or amorphous form. Crystalline silicates (e.g. hedenbergite, kirschsteinite, dicalciumsilicate) are formed in the sintering process and are usually rich in calcium.^10,19) Amorphous silicates do not have crystal structure and those have solidified from residual slag. The amorphous silicates are primarily composed of CaO, FeO, Fe₂O₃ and SiO₂.¹⁰⁾ If the iron oxides exists either in the divalent or trivalent form, depends on the formation conditions such as temperature, composition and oxygen partial pressure.

2.2.4. Non-assimilated Non-ferrous Particles

Non-assimilated non-ferrous particles are minerals (e.g. quartz, olivine, lime) in the raw material mix, which have not reacted during sintering. In addition to non-ferrous phases, iron oxides (hematite and magnetite) can partly stay unreacted during sintering.¹⁰⁾

3. Phase Equilibria in Sinter Systems

Based on the chemical and mineralogical compositions presented in chapter 2, it can be concluded which components should be considered when studying sinter systems with phase diagrams. A compilation of the systems of interest is presented in Table 4. In order to study the complete sinter system taking into account all major components, a six-component system with FeO, Fe₂O₃, CaO, SiO₂, MgO and Al₂O₃ should be considered.

Table 4. The most significant components in the sinter system.

Studied system	Main components of interest	Additional components of interest
Iron oxides	FeO, Fe2O3
Calcium ferrites	CaO, Fe2O3	FeO
SFCA	FeO, Fe2O3, SiO2, CaO, Al2O3
Silicates	SiO2, CaO, FeO	Al2O3, MgO
Complete sinter system	FeO, Fe2O3, CaO, SiO2, MgO, Al2O3	TiO2, MnO, Na2O

In addition to the sinter and its behaviour in ironmaking processes such as the blast furnace, the understanding of the FeO–Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ system is crucially important in various metallurgical and other industrial applications (such as studying cements, slags, ash, inclusions, glasses or refractory materials).^52,53,54,55) Hence, it is not surprising that the phase equilibria of these systems have been studied comprehensively. In order to collect a more comprehensive review on the studies of the above mentioned FeO–Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ system, some studies which have originally focused on other topics than sintering, have also been included in this study due to the fact that their results are applicable for studying the sinter system, too.

Phase equilibria of different systems can be studied either experimentally or computationally. Experimental studies (with either equilibrium or dynamic measurements) are required when starting to study new systems with components whose thermodynamic behaviour is not known. Experimental measurement of the phase equilibria and construction of phase diagrams (especially with equilibrium method) is time-consuming, expensive and requires extreme accuracy. Furthermore, the addition of new components in the already studied systems requires a new set of experiments. Therefore, computational methods in the construction of phase diagrams are more common as long as sufficient amount of thermochemical data with sufficient accuracy is available from the previous experimental studies. Nowadays, it is possible to construct innumerable amount of phase diagrams of multicomponent oxide systems using thermochemical software and their databases. These software enable the construction of subsystem sections of the multicomponent systems, for which the complete phase diagram would be impossible to visualise. The most essential – and basically the only – requirement for this computational approach is the availability of accurate thermochemical data for the system of interest.

A CALPHAD method (originally “Calculation of phase diagrams”; later expanded to refer to “Computer coupling of phase diagrams and thermochemistry”) in which thermochemical data (i.e. suitable mathematical expressions to model the non-ideality of solution phases as well as values of the parameters included in these models) is optimized based on all available experimental data and then used to simulate computationally the experimentally constructed phase diagrams is nowadays widely used in many applications. One of its strengths is the ability to predict the behaviour of more complex systems with the extrapolation of the data of lower order subsystems.^52,56,57)

The experimental determination of phase equilibria and phase diagrams is still important when studying systems with new sets of components, when studying known systems in new conditions or when adjusting existing models with more detailed or accurate data. However, it is nowadays more convenient and common to construct the phase diagrams for each individual case computationally rather than collect and use vast libraries of existing phase diagrams, each of which was determined experimentally. A comprehensive collection of experimentally determined phase diagrams of lower order subsystems of the FeO–Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ system (among various other oxide systems) was published in 1995 by VDEh. This “Slag Atlas” collects the results of the studies that had been published until mid-1990s’.⁵⁸⁾ Table 5 lists all the phase diagrams available in the “Slag Atlas” for the subsystems of the sinter system together with the references to the experimental phase diagram studies of these systems subsequent to 1995. It should be noted that the “Slag Atlas” is a review compilation in itself and references to original studies are not repeated here since they can be found in the “Slag Atlas”.

Table 5. Phase diagrams available for the subsystems of the FeO–Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ system (based on Slag Atlas⁵⁸⁾ with additions from more recent sources).

Components						Type of diagram	Reference
FeO	Fe2O3	CaO	SiO2	MgO	Al2O3
Binary subsystems
X	X					Binaries: Fe–O; FeO–Fe2O3. T–p(O2) for Fe–O system.	Slag Atlas58)*
X		X				cf. ternary FeO–Fe2O3–CaO system.**	Slag Atlas58)*
X			X			cf. ternary FeO–Fe2O3–SiO2 system.**	Slag Atlas58)*
X				X		cf. ternary FeO–Fe2O3–MgO system.**	Slag Atlas58)*
X					X	cf. ternary FeO–Fe2O3–Al2O3 system.**	Slag Atlas58)*
	X	X				cf. ternary FeO–Fe2O3–CaO system.**	Slag Atlas58)*
	X		X			cf. ternary FeO–Fe2O3–SiO2 system.**	Slag Atlas58)*
	X			X		cf. ternary FeO–Fe2O3–MgO system.**	Slag Atlas58)*
	X				X	cf. ternary FeO–Fe2O3–Al2O3 system.**	Slag Atlas58)*
		X	X			Binary: CaO–SiO2.	Slag Atlas58)*
		X		X		Binary: CaO–MgO.	Slag Atlas58)*
		X			X	Binary: CaO–Al2O3.	Slag Atlas58)*
			X	X		Binary: MgO–SiO2.	Slag Atlas58)*
			X		X	Binary: Al2O3–SiO2.	Slag Atlas58)*
				X	X	Binary: MgO–Al2O3.	Slag Atlas58)*
Ternary subsystems
X	X	X				Pseudobinaries: FeOx–CaO (in eq. with met. Fe; in air; in pure oxygen).	Slag Atlas58)*
X	X	X				Ternaries: Liquidus projection for FeO–Fe2O3–CaO system. /Iso-T sections for FeO–Fe2O3–CaO system in 1450°C, 1500°C and 1550°C.	Slag Atlas58)*
X	X	X				Iso-T sections for Fe–Fe2O3–CaO system in 1120°C and 1200°C.	Slag Atlas58)*
X	X		X			Pseudobinaries: FeOx–SiO2 (in eq. with met. Fe; in air).	Slag Atlas58)*
X	X		X			Ternary: Liquidus projection for FeO–Fe2O3–SiO2 system.	Slag Atlas58)*
X	X			X		Pseudobinaries: FeOx–MgO (in eq. with met. Fe; in air).	Slag Atlas58)*
X	X			X		Ternaries: Liquidus projection for FeO–Fe2O3–MgO system. /Iso-T sections for FeO–Fe2O3–MgO system in 1300°C, 1600°C and 1750°C.	Slag Atlas58)*
X	X			X		Iso-T: X(Mg)-p(O2) for 1000°C.	Slag Atlas58)*
X	X				X	Pseudobinaries: FeOx–Al2O3 (in eq. with met. Fe; in air; in pure oxygen).	Slag Atlas58)*
X	X				X	Ternary: Liquidus projection for FeO–Fe2O3–Al2O3 system.	Slag Atlas58)*
X	X				X	Iso-T: X(Fe)-p(O2) for 1280°C, 1380°C and 1500°C.	Slag Atlas58)*
X		X	X			cf. quaternary FeO–Fe2O3–CaO–SiO2 system.***	Slag Atlas58)*
X		X		X		cf. quaternary FeO–Fe2O3–CaO–MgO system.***	Slag Atlas58)*
X		X			X	cf. quaternary FeO–Fe2O3–CaO–Al2O3 system.***	Slag Atlas58)*
X			X	X		cf. quaternary FeO–Fe2O3–SiO2–MgO system.***	Slag Atlas58)*
X			X		X	cf. quaternary FeO–Fe2O3–SiO2–Al2O3 system.***	Slag Atlas58)*
X				X	X	cf. quaternary FeO–Fe2O3–MgO–Al2O3 system.***	Slag Atlas58)*
	X	X	X			cf. quaternary FeO–Fe2O3–CaO–SiO2 system.***	Slag Atlas58)*
	X	X		X		cf. quaternary FeO–Fe2O3–CaO–MgO system.***	Slag Atlas58)*
	X	X			X	cf. quaternary FeO–Fe2O3–CaO–Al2O3 system.***	Slag Atlas58)*
	X		X	X		cf. quaternary FeO–Fe2O3–SiO2–MgO system.***	Slag Atlas58)*
	X		X		X	cf. quaternary FeO–Fe2O3–SiO2–Al2O3 system.***	Slag Atlas58)*
	X			X	X	cf. quaternary FeO–Fe2O3–MgO–Al2O3 system.***	Slag Atlas58)*
		X	X	X		Pseudobinary: MS-CMS2.	Slag Atlas58)*
		X	X	X		Ternaries: Liquidus and solidus projections for CaO–MgO–SiO2 system.	Slag Atlas58)*
		X	X		X	Ternary: Liquidus projection for Al2O3–CaO–SiO2 system.	Slag Atlas58)*
		X		X	X	Ternary: Liquidus projection for CaO–MgO–Al2O3 system.	Slag Atlas58)*
		X		X	X	Ternary: Liquidus projection for CaO–MgO–Al2O3 system.	Ohta and Suito59)
			X	X	X	Ternaries: Liquidus projection for Al2O3–MgO–SiO2 system. /Iso-T sections for Al2O3–MgO–SiO2 system in 1350°C, 1450°C and 1470°C.	Slag Atlas58)*
Quaternary subsystems
X	X	X	X			Pseudobinaries: CS-“WS”; C2S-W2S.	Slag Atlas58)*
X	X	X	X			Pseudoternaries: Liquidus projection for FeOx–CaO–SiO2 system (in eq. with met. Fe; in air). /Iso-T section for FeOx–CaO–SiO2 system in 1230°C (in air). /Iso-T sections for FeO–Fe2O3–CaO plane in 1450°C and 1550°C with different SiO2 contents (0, 5, 10, 20, 30 mass%). /Iso-T section for FeO–Fe2O3–C2S system in 1500°C.	Slag Atlas58)*
X	X	X	X			Pseudoternaries: IsoT sections for Fe2O3–CaO–SiO2 system in 1240°C, 1255°C, 1270°C and 1300°C (in air).	Pownceby et al.60)
X	X	X	X			Pseudoternaries: IsoT sections for Fe2O3–CaO–SiO2 system in 1240°C, 1255°C, 1270°C and 1300°C (in p(O2) 5·10−3 atm).	Pownceby and Clout61)
X	X	X	X			Pseudoternaries: IsoT sections for FeOx–CaO–SiO2 system in 1573 K (in p(O2) 2.1·10−3, 1.8·10−6 and 1.8·10−8 atm).	Kimura et al.62)
X	X	X	X			Pseudoternaries: IsoT sections for FeO–CaO–SiO2 and Fe3O4–CaO–SiO2 systems in 1573 K (in p(O2) 10−9, 10−8, 10−7, 10−6, 10−5 and 3.63·10−4 atm). Liquidus projection for Fe3O4–CaO–SiO2 system in p(O2) 10−6 atm.	Henao et al.63)
X	X	X	X			Pseudoternaries: Liquidus projections for FeOx–CaO–SiO2 system in 1523 K and 1573 K (in p(O2) from 1.8·10−8 to 0.21 atm).	Matsuura et al.64)
X	X	X	X			Pseudoternaries: IsoT sections for Fe3O4–CaO–SiO2 system in 1300°C (in p(O2) 10−7 atm).	Shigaki et al.65)
X	X	X	X			Pseudoternaries: Liquidus and solidus projections for FeO–CaO–SiO2 system (in p(O2) 10−6 atm).	Nikolic et al.66,67)
X	X	X	X			Pseudoternaries: Liquidus projections for FeO–CaO–SiO2 system (in p(O2) 10−7, 10−6 and 10−5 atm. /Iso-T sections for FeO–CaO–SiO2 system in 1200°C and 1300°C (in p(O2) 10−7 and 10−5 atm).	Hidayat et al.68)
X	X	X		X		Pseudoternaries: Liquidus projection for FeOx–MgO–CaO system (in air; in eq. with met. Fe). /Iso-T sections for FeOx–MgO–CaO system in 1500°C (in air; in p(O2) 10−3 atm) and 1600°C (in eq. with met. Fe). /Liquidus projections for FeO–Fe2O3–MgO plane with different CaO contents (0, 10, 20 mass%).	Slag Atlas58)*
X	X	X		X		Quaternaries: Liquidus projection for FeO–Fe2O3–MgO–CaO system in 1500°C and 1600°C. /Iso-T section for FeO–Fe2O3–MgO–CaO system in 1500°C (in p(O2) 10−3 atm).	Slag Atlas58)*
X	X	X			X	Pseudoternaries: Liquidus projection for FeOx–Al2O3–CaO system (in air; in p(O2) 10−5 and 10−8 atm). /Iso-T sections for FeOx–Al2O3–CaO system in 1170°C and 1170°C (in air) and 1600°C (in eq. with met. Fe).	Slag Atlas58)*
X	X		X	X		Pseudoternaries: Liquidus projections for FeOx–MgO–SiO2 system (in eq. with met. Fe; in air; in pure oxygen). /Solidus projection for FeOx–MgO–SiO2 system. /Solubility limits of solid phases for FeO–Fe2O3–MgO plane in 1600°C with different SiO2 contents (0, 10, 16, 23 mass%).	Slag Atlas58)*
X	X		X	X		Quaternaries: Liquidus projection for FeO–Fe2O3–MgO–SiO2 system in 1600°C.	Slag Atlas58)*
X	X		X	X		Pseudoternaries: Liquidus projections for FeO–MgO–SiO2 system (in eq. with met. Fe).	Chen et al.69)
X	X		X	X		Pseudoternaries: Liquidus projections for FeO–MgO–SiO2 system (in eq. with met. Fe). /Iso-T sections for FeO–MgO–SiO2 system in 1573 K, 1673 K, 1773 K, 1823 K, 1848 K and 1873 K (in eq. with met. Fe).	Chen et al.70)
X	X		X	X		Pseudobinary: M2S-W2S.	Chen et al.70)
X	X		X		X	Pseudoternaries: Liquidus projection for FeOx–Al2O3–SiO2 system (in air; in eq. with met. Fe). /Solidus projection for Fe2O3–Al2O3–SiO2 system (in air).	Slag Atlas58)*
X	X			X	X	Pseudoternary: MgO solid solution stability region in Al2O3–Fe2O3–MgO system in 1400°C and 1700°C.	Slag Atlas58)*
X		X	X	X		cf. quinary FeO–Fe2O3–CaO–SiO2–MgO system.****	Slag Atlas58)*
X		X	X		X	cf. quinary FeO–Fe2O3–CaO–SiO2–Al2O3 system.****	Slag Atlas58)*
X		X		X	X	cf. quinary FeO–Fe2O3–CaO–MgO–Al2O3 system.****	Slag Atlas58)*
	X	X	X	X		cf. quinary FeO–Fe2O3–CaO–SiO2–MgO system.****	Slag Atlas58)*
	X	X	X		X	cf. quinary FeO–Fe2O3–CaO–SiO2–Al2O3 system.****	Slag Atlas58)*
	X	X		X	X	cf. quinary FeO–Fe2O3–CaO–MgO–Al2O3 system.****	Slag Atlas58)*
		X	X	X	X	Pseudoternaries: Liquidus projections for CaO–MgO–SiO2 plane with different Al2O3 contents (5, 10, 15, 20, 25, 30, 35 mass%). /Liquidus projections for Al2O3–CaO–SiO2 plane with different MgO contents (5, 10, 15 mass%). /Liquidus projections for Al2O3–CaO–MgO plane with different SiO2 contents (34, 35, 36, 37 mass%). /Boundaries of primary crystallization fields of solid phases with varying MgO contents.	Slag Atlas58)*
		X	X	X	X	Pseudobinaries: (C+S)−M with 15 and 20 mass% of A for C/S ratios of 1.1 and 1.3. /(C+S)−A with 5 and 10 mass% of M for C/S ratios of 1.1 and 1.3.	Ma et al.71,72)
		X	X	X	X	Pseudoternaries: Liquidus projection for (CaO+SiO2)–MgO–Al2O3 plane with C/S ratios of 1.1 and 1.3.	Ma et al.71,72)
		X	X	X	X	Pseudoternaries: Liquidus projections for CaO–MgO–SiO2 plane with different Al2O3 contents (15, 20 mass%).	Sun et al.73)
		X	X	X	X	Pseudoternary: Liquidus projection for CaO–MgO–SiO2 plane with 30 mass% Al2O3.	Gran et al.74)
Quinary subsystems
X	X	X	X	X		Pseudoternaries: Liquidus projections for MgO–Fe2O3–C2S, MgO–C2S–C2F and MF–C2S–C2F planes. /Boundaries of primary crystallization fields of solid phases in various subsystem sections.	Slag Atlas58)*
X	X	X	X	X		Pseudoquaternary: Liquidus projection for FeOx–CaO–MgO–SiO2 system in 1600°C in eq. with met. Fe.	Slag Atlas58)*
X	X	X	X	X		Pseudoternaries: Liquidus projections for CaO–SiO2–FeOx–MgO system in 1573 K (in p(O2) 1.8·10−8, 1.8·10−6 and 2.1·10−3 atm) with 5 mass% MgO.	Kimura et al.75)
X	X	X	X	X		Pseudoternary: Iso-T section for FeOx–CaO–SiO2 plane with 0 and 5 mass% MgO in 1573 K (in p(O2) 2.7·10−7 atm).	Hayashi et al.76)
X	X	X	X	X		Pseudoquaternary: Conceptual illustration of liquid-magnetite tie lines in FeOx–CaO–MgO–SiO2 system in 1573 K (in p(O2) 2.7·10−7 atm).	Hayashi et al.76)
X	X	X	X		X	Pseudoternaries: Liquidus projections for FeOx–Al2O3–CaO–SiO2 system with different iron contents (calculated as FeO): 5, 10, 15, 20, 25, 30 mass%. /Liquidus projections for FeOx–Al2O3–CaO–SiO2 system (in air) with different Fe2O3 contents (10, 16.5, 20 mass%).	Slag Atlas58)*
X	X	X	X		X	Pseudoternary: Liquidus projection for Fe3O4–Al2O3–CaO–SiO2 system (in p(O2) 10−7 atm) with 5 mass% Al2O3.	Shigaki et al.65)
X	X	X	X		X	Pseudoternaries: Liquidus projections for CaO–SiO2–FeOx–Al2O3 system in 1573 K (in p(O2) 1.8·10−8, 1.8·10−6 and 2.1·10−3 atm) with 5 mass% Al2O3.	Kimura et al.75)
X	X	X	X		X	Pseudoternaries: Liquidus projections for CaO–SiO2–FeOx–Al2O3 system in 1573 K (in p(O2) 2.5·10−6, 1.0·10−4 and 1.0·10−2 atm) with 2 mass% Al2O3.	Katahira et al.77)
X	X	X		X	X	–	–
X	X		X	X	X	Pseudobinaries: (M+W)-S with 2 and 3 mass% A and M/W ratios of 1.2, 1.4, 1.6, 1.8, 2.0 and 2.2.	Chen et al.78)
X	X		X	X	X	Pseudoternaries: Liquidus projections for FeO–MgO–SiO2 plane with 2 and 3 mass% Al2O3 (in eq. with met. Fe). /Iso-T sections for FeO–MgO–SiO2 plane with 2 and 3 mass% of Al2O3 in 1773 and 1823 K.	Chen et al.78)
X		X	X	X	X	cf. complete FeO–Fe2O3–CaO–SiO2–Al2O3–MgO system.*****	Slag Atlas58)*
	X	X	X	X	X	cf. complete FeO–Fe2O3–CaO–SiO2–Al2O3–MgO system.*****	Slag Atlas58)*
Complete system with all six components
X	X	X	X	X	X	Pseudoternaries: Liquidus projections for CaO–SiO2–FeOx–Al2O3–MgO system in 1573 K (in p(O2) 1.8·10−8, 1.8·10−6 and 2.1·10−3 atm) with 2.5 mass% Al2O3 and 2.5 mass% MgO.	Yajima et al.79)
X	X	X	X	X	X	Pseudobinaries: (C+S)−W with 0, 5 and 10 mass% M; 10, 15 and 20 mass% A and C/S ratio of 1.3. /(C+S)−A with 5 and 10 mass% M; 0, 5, 10, 15 and 20 mass% W and C/S ratio of 1.3. /(C+S)−M with 10, 15 and 20 mass% A; 0, 5, 10, 15 and 20 mass% W and C/S ratio of 1.3.	Jang et al.80,81)
X	X	X	X	X	X	Pseudoternaries: Liquidus projections for (CaO+SiO2)–MgO–Al2O3 plane with 5, 10, 15 and 20 mass% of “FeO” and C/S ratio of 1.3./Iso-T section for (CaO+SiO2)–MgO–Al2O3 plane with 0, 10 and 20 mass% of “FeO” and C/S ratio of 1.3 in 1723 K.	Jang et al.80,81)

*  cf. Slag Atlas⁵⁸⁾ for the list of references to the original articles, from which the diagrams in Slag Atlas⁵⁸⁾ were constructed.

**  Binaries of FeO–X and Fe₂O₃–X (in which X refers to either CaO, SiO₂, MgO or Al₂O₃) are not included in Slag Atlas,⁵⁸⁾ because all the “binary” systems containing iron oxide(s) are actually pseudobinary diagrams in the form of FeO_x–X with a fixed state of oxidation (e.g. in equilibrium with metallic iron, in equilibrium with air or in equilibrium with oxygen).

***  Ternaries of FeO–X–Y and Fe₂O₃–X–Y (in which X and Y refer to either CaO, SiO₂, MgO or Al₂O₃) are not included in Slag Atlas,⁵⁸⁾ because all the “ternary” systems containing iron oxide(s) are actually pseudoternary diagrams in the form of FeO_x–X–Y with a fixed state of oxidation (e.g. in equilibrium with metallic iron, in equilibrium with air or in equilibrium with oxygen).

****  Quaternaries of FeO–X–Y–Z and Fe₂O₃–X–Y–Z (in which X, Y and Z refer to either CaO, SiO₂, MgO or Al₂O₃) are not included in Slag Atlas,⁵⁸⁾ because all the “quaternary” systems containing iron oxide(s) are actually pseudoquaternary diagrams in the form of FeO_x–X–Y–Z with a fixed state of oxidation (e.g. in equilibrium with metallic iron, in equilibrium with air or in equilibrium with oxygen).

*****  Quinaries of FeO–CaO–SiO₂–MgO–Al₂O₃ and Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ are not included in Slag Atlas,⁵⁸⁾ because all the “quinary” systems containing iron oxide(s) are actually pseudoquinary diagrams in the form of FeO_x–CaO–SiO₂–MgO–Al₂O₃ with a fixed state of oxidation (e.g. in equilibrium with metallic iron, in equilibrium with air or in equilibrium with oxygen). Unlike for lower degree subsystems, there is no phase diagrams for the complete FeO–Fe₂O₃–CaO–SiO₂–MgO–Al₂O₃ system in Slag Atlas.⁵⁸⁾ However, this system is studied by Yajima et al.⁷⁹⁾

In complex compounds, A, C, F, M, S and W refer to Al₂O₃, CaO, Fe₂O₃, MgO, SiO₂ and FeO, respectively.

In addition to existing diagrams mentioned in Table 5, designated sections of the multicomponent systems – as well as phase diagrams of more simple subsystems – can be constructed from assessed thermochemical data using thermochemical software and their databases. A list of studies, in which assessment and optimization of thermochemical data has been done with the CALPHAD method for the systems containing FeO, Fe₂O₃, CaO, SiO₂, MgO and/or Al₂O₃, is collected to Table 6. In Table 6 it is also mentioned, which solution models have been used in different studies to describe the non-ideality of molten and solid solution phases such as monoxide, olivine, spinel, pyroxene, mullite and melilite.

Table 6. A list of studies, in which assessment and optimization of thermochemical data has been done for the systems containing FeO, Fe₂O₃, CaO, SiO₂, MgO and/or Al₂O₃.

Components						Method	Solution models					Software	Reference
FeO	Fe2O3	CaO	SiO2	MgO	Al2O3		(Mod.) Quasi-chem. model (MQM)	Two-sublattice model for ionic liq. (RILM)	Sublattice model (based on CEF)	Polynom. model	Others
Binary subsystems
X	X					CALPHAD	Liquid		Spinel	Monoxide		FACT	Decterov et al.82)
X	X					CALPHAD	Liquid					FACT	Hidayat et al.83)
X	X					CALPHAD							Spencer and Kubaschhewski84)
X	X					CALPHAD		Liquid	Solid sol.			TC	Sundman85)
X	X					CALPHAD		Liquid	Solid sol.			TC	Fabrichnaya and Sundman86)
X		X				CALPHAD	Liquid					FACT	Pelton and Blander87)
X		X				CALPHAD	Liquid					FACT	Wu et al.88)
X			X			CALPHAD	Liquid					FACT	Pelton and Blander87)
X			X			CALPHAD	Liquid					FACT	Wu et al.89)
X				X		CALPHAD	Liquid					FACT	Wu et al.88)
X				X		CALPHAD	Liquid					FACT	Wu et al.89)
X					X	CALPHAD	Liquid				Regular (Spinel)	FACT	Eriksson et al.90)
		X	X			CALPHAD					Cell model	MPE	Taylor and Dinsdale91)
		X	X			CALPHAD	Liquid					FACT	Pelton and Blander87)
		X	X			CALPHAD		Liquid	Solid sol.			TC	Hillert et al.92)
		X	X			CALPHAD		Liquid	Solid sol.			TC	Huang et al.93)
		X		X		CALPHAD		Liquid					Zhang and Yong94)
		X		X		CALPHAD	Liquid	Liquid	Solid sol.			TC	Huang et al.93)
		X		X		CALPHAD						FACT	Wu et al.88)
		X		X		CALPHAD						FACT	Blander and Pelton95)
		X			X	CALPHAD	Liquid					FACT	Eriksson and Pelton96)
		X			X	CALPHAD					Cell model	MPE	Zhang et al.97)
		X			X	CALPHAD		Liquid				TC	Mao et al.98)
		X			X	CALPHAD		Liquid				TC	Hallstedt99)
			X	X		CALPHAD		Liquid	Solid sol.			TC	Huang et al.93)
			X	X		CALPHAD	Liquid					FACT	Wu et al.89)
			X	X		CALPHAD	Liquid					FACT	Blander and Pelton95)
			X		X	CALPHAD	Liquid					FACT	Eriksson and Pelton96)
			X		X	CALPHAD					Cell model	MPE	Zhang et al.97)
				X	X	CALPHAD		Liquid	Solid sol.			TC	Zienert and Fabrichnaya100)
				X	X	CALPHAD		Liquid				TC	Mao et al.98)
				X	X	CALPHAD		Liquid	Solid sol.			TC	Hallstedt101)
				X	X	CALPHAD	Liquid		Spinel, monoxide, pyroxene			FACT	Jung et al.102)
Ternary subsystems
X	X	X				CALPHAD		Liquid	Solid sol.			TC	Hillert et al.103)
X	X	X				CALPHAD	Liquid		Spinel		Bragg– Williams (Monox.)	FACT	Hidayat et al.104)
X	X		X			CALPHAD	Liquid					FACT	Hidayat et al.105)
X	X		X			CALPHAD		Liquid	Solid sol.			TC	Fabrichnaya and Sundman86)
X	X			X		CALPHAD	Liquid		Spinel, Olivine, Pyroxene, Monoxide			FACT	Jung et al.106)
X	X				X	CALPHAD		Liquid	Solid sol.			TC	Dreval et al.107)
X	X				X	CALPHAD	Liquid		Spinel, Mullite			FACT	Prostakova et al.108)
X		X	X			CALPHAD	Liquid					FACT	Pelton and Blander87)
X		X	X			CALPHAD		Liquid				TC	Hallstedt et al.109)
X		X	X			CALPHAD		Liquid				TC	Selleby110)
X		X			X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
X			X	X		CALPHAD				Solid sol.		–	Saxena111)
X			X	X		CALPHAD	Liquid					FACT	Wu et al.89)
X			X	X		CALPHAD	Liquid					FACT	Blander and Pelton95)
X			X		X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
	X	X	X			CALPHAD		Liquid				TC	Hallstedt et al.109)
	X	X	X			CALPHAD		Liquid				TC	Selleby110)
	X	X			X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
	X		X		X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
		X	X	X		CALPHAD		Liquid	Solid sol.			TC	Huang et al.93)
		X	X	X		CALPHAD	Liquid					FACT	Blander and Pelton95)
		X	X	X		CALPHAD	Liquid		Olivine, Pyroxene			FACT	Jung et al.112)
		X	X		X	CALPHAD	Liquid					FACT	Eriksson and Pelton96)
		X	X		X	CALPHAD					Cell model	MPE	Zhang et al.97)
		X		X	X	CALPHAD	Liquid		Spinel, Pyroxene, Monoxide			FACT	Jung et al.102)
			X	X	X	CALPHAD	Liquid		Spinel, Pyroxene, Monoxide			FACT	Jung et al.102)
Quaternary subsystems
X	X	X	X			CALPHAD		Liquid				TC	Selleby110)
X	X	X	X			CALPHAD					Cell model	MPE	Chen et al.113)
X	X	X	X			CALPHAD	Liquid		Spinel, Olivine, Pyroxene, Melilite		Bragg- Williams (C2S)	FACT	Hidayat et al.114)
X	X	X			X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
X	X		X	X		CALPHAD		Liquid	Solid sol.			TC	Fabrichnaya115)
X	X		X	X		CALPHAD			Monoxide		Regular	TC	Fabrichnaya116)
X	X		X	X		CALPHAD	Liquid		Spinel, Olivine, Pyroxene, Monoxide		Bragg- Williams (C2S)		Jung et al.55)
X	X		X	X		CALPHAD	Liquid		Spinel, Olivine, Pyroxene, Monoxide		Bragg- Williams (C2S)		Decterov et al.117)
X	X		X		X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53)
X	X		X		X	CALPHAD	Liquid		Spinel, Mullite			FACT	Prostakova et al.108)
	X	X	X		X	CALPHAD			SFCA			MPE	Chen et al.47)
	X	X	X		X	CALPHAD	Liquid		SFCA			FACT	Murao et al.11)
Quinary subsystems
X	X	X	X		X	CALPHAD	Liquid		Solid sol.	Melilite		FACT	Jak et al.53,118)
	X	X	X	X	X	CALPHAD						TC	Tazuddin et al.52)
Complete system with all six components
X	X	X	X	X	X	CALPHAD		Liquid	Solid sol.		Cell model (Liquid)	TC	Andersson et al.119)
X	X	X	X	X	X	CALPHAD			Solid sol.		Associate (Liquid)	MTD	Barry et al.120)
X	X	X	X	X	X	CALPHAD	Liquid		Solid sol.			FACT	Decterov et al.121)
X	X	X	X	X	X	CALPHAD	Liquid		Solid sol.	Solid sol.		FACT	Kongoli and Yazawa122)
X	X	X	X	X	X	CALPHAD					Cell model (Liquid)	CEQCSI	Gaye et al.123)
X	X	X	X	X	X	CALPHAD					GCA (Liquid)	CEQCSI	Lehmann et al.124,125)
X	X	X	X	X	X	CALPHAD					GCA (Liquid)	CEQCSI	Lehmann and Zhang126)
X	X	X	X	X	X	CALPHAD					Cell model (Liquid)	MPE	Zhang et al.127)
X	X	X	X	X	X	CALPHAD					Cell model (Liquid)	MPE	Jahanshahi et al.128)

Abbreviations of thermochemical software: CEQCSI = Calcul d’Equilibre Chimique pour la Sidérurgie (Chemical Equilibrium Calculations for the Steel Industry); FACT = FactSage; MPE = Multicomponent Phase Equilibria; MTD = MTDATA; TC = ThermoCalc.

As seen from Table 6, thermodynamic data has been collected and assessed for the systems containing all six components needed to study the phase equilibria in sinters. This thermochemical data is available in the databases of thermochemical software such as FactSage,^129,130,131) ThermoCalc,¹¹⁹⁾ MTDATA,^54,132) CEQCSI¹³³⁾ and MPE,^127,128) all of which can be used to construct required sections of the FeO–Fe₂O₃–SiO₂–CaO–MgO–Al₂O₃ system. However, it should be noted that although all the components of the FeO–Fe₂O₃–SiO₂–CaO–MgO–Al₂O₃ system are taken into account in all the software mentioned in Table 6, data concerning some of the relevant phases may not be included in all the databases of these software.^{54,55,121,122,123,132,134)} Based on the information provided in the references of Table 6 as well as in the documentation on the software websites, Table 7, which collects the possibilities to model the solution phases potentially existing in iron ore sinters with different software, has been compiled. The vast number of stoichiometric phases included in the databases are not mentioned in the table. Empty spaces in Table 7 indicate that either the particulate phase cannot be modelled with the software in question, or the documentation of the databases lacks the information concerning this phase. Based on the Table 7, it can be seen that the major needs for the further research are in the accurate and reliable modelling of the SFCA phases. It should also be noted that the addition of accurate thermochemical data of SFCA phases to the existing databases also requires more detailed understanding on the composition, crystallography, morphology and formation of SFCA phases as classified in more detail by Nicol et al.¹³⁵⁾ in their review.

Table 7. Possibilities to model phases potentially existing in iron ore sinters with different software.

Solution phase			Software/models
Name*	Formula** (if available)	Examples	CEQCSI	FACT	MPE	MTD	TC
Molten oxide phase	Varies	Molten slag phase	CM/GCA	MQM	CM/GCA	AM	CM/AM/RILM
Monoxide/Halite	X+IIO	Wüstite, lime, magnesiowüstite	Yes****	CEF	RSM with RKM	CEF	CEF
Spinel	X+IIY+III2O4	Magnetite	Yes****	CEF	RSM with RKM	CEF	CEF
Corundum	X+III2O3	Hematite, alumina		CEF	RSM with RKM	CEF	CEF
Olivine	X+II2SiO4	Fayalite, forsterite		CEF	RSM with RKM	CEF	CEF
Pyroxenes	X+IIY+II/+IIIZ+III/+IVSiO6	Hedenbergite	Yes****	CEF	RSM with RKM	CEF	CEF
Garnet	X+II3Y+III2Si3O12	Grossular		CEF			CEF
Melilite	Ca2X+II/+IIIY+III/+IV2O7	Gehlenite, åkermanite	Yes****	CEF	RSM with RKM	CEF	CEF
Cordierite	Al4X+II2Si5O18			CEF			CEF
Wollastonite	(Ca,X+II)SiO3			CEF		CEF	CEF
Dicalcium silicate	(Ca,X+II)2SiO4			CEF		CEF	CEF
SFCA	Varies***			CEF*****	CEF
Calcium-aluminium-ferrites	Several; cf. examples	C3AF, C2AF, CAF, CAF2, CAF3, CAF6 (C=CaO; A=Al2O3; F=Fe2O3)		CEF			CEF

Abbreviations of thermochemical software: CEQCSI = Calcul d’Equilibre Chimique pour la Sidérurgie (Chemical Equilibrium Calculations for the Steel Industry); FACT = FactSage; MPE = Multicomponent Phase Equilibria; MTD = MTDATA; TC = ThermoCalc.

Abbreviations of thermodynamic models: AM = Associate model; CEF = Sublattice model based on Compound energy formalism; CM = Cell model; GCA = Generalized central atom model; MQM = Modified quasi-chemical model; RILM = Reciprocal ionic liquid model (two-sublattice model for ionic liquids); RKM = Redlich-Kister-Muggianu polynomial (for the excess function); RSM = Regular solution model.

*  Some software may use different names to describe the same phase.

**  Possible oxidation states are only mentioned for substitutional elements marked with X, Y and Z. Oxidation states of oxygen, calcium, aluminium and silicon are −II, +II, +III and +IV, respectively.

***  Concerning the formula of SFCA phases, see chapters 2.2.2 and 4 for more detailed discussion.

****  Models not specified in the documentation.

*****  SFCA is not mentioned in the documentation of the FactSage’s database descriptions, but the assessment of the data for SFCA has been reported by Murao et al.¹¹⁾

It is also seen from Tables 6 and 7 that different approaches have been used to model the non-ideality of solution phases in different software. For example, the molten oxide phase is described with the modified quasichemical model (MQM)^{56,87,136,137,138,139)} in FactSage’s (GTT) FToxid database,^{129,130,131,134)} with the associate model (AM)^56,91,120) in MTDATA‘s (NPL) NPLOX database,^54,132,134) with the cell model (CM)^56,140,141) in CEQCSI‘s (IRSID)^{123,125,126,133,134)} and MPE’s (CSIRO)^127,128,134) oxide databases and with either cell model^56,140,141) or associate model^56,91,120) in ThermoCalc‘s SLAG2 and NOX3 databases, respectively.¹³⁴⁾ Furthermore, the generalized central atom model (GCA)^124,126) developed based on the cell model has also been used to model both oxide and metal phases in CEQCSI,¹²⁵⁾ whereas the reciprocal ionic liquid model (RILM),^56,142) which assumes two sublattices in liquid oxide phase has been used in ThermoCalc’s own ION3 -oxide database.^119,134) For solid solutions more uniform approach is used, since the sublattice model based on compound energy formalism (CEF)^{56,143,144,145)} is used in most cases.^132,134) An excellent and more detailed overview on thermodynamic models and their application in different areas of metallurgy has been made by Jung.¹³⁴⁾

In addition to the systems collected to Table 6, thermodynamic data has also been assessed for the systems containing minor components (such as sodium, titanium, manganese, phosphorus and boron) of the sinter systems: e.g. Na₂O–FeO–Fe₂O₃–Al₂O₃–SiO₂ system by Moosavi-Khoonsari et al.,¹⁴⁶⁾ TiO₂–Ti₂O₃–FeO–Fe₂O₃–Al₂O₃–MgO system by Jantzen et al.,¹⁴⁷⁾ MnO–FeO_x–MO_x (MO_x = PO_2.5, SiO₂, AlO_1.5, MgO, CaO) system by Suito et al.,¹⁴⁸⁾ MnO–FeO–CaO–SiO₂–MgO–Al₂O₃ system by Duan et al.,¹⁴⁹⁾ MnO–CaO–MgO–SiO₂–Al₂O₃ system by Zhao et al.,¹⁵⁰⁾ MnO–P₂O₅–FeO_x–CaO–SiO₂–Al₂O₃ system by Zhu et al.¹⁵¹⁾ as well as binary and ternary systems containing B₂O₃ (with MgO, CaO and SiO₂) by Sunkar et al.¹⁵²⁾

4. Discussion on Practical Relevance

Phase diagrams as well as computations with thermodynamic data (presented in chapter 3) provide useful tools to study the behaviour of sinters in varying conditions (i.e. temperature and atmosphere). This chapter illustrates the use of these thermodynamic tools in sinter related research and development using studies about phase stabilities of the FeO_x–SiO₂–CaO(–Al₂O₃) system in sintering conditions as examples. This system has been chosen as an example due to its importance in formation of bonding SFCA phases as explained in chapter 2.2.2.

As mentioned in chapter 2, the raw material undergoes various stages with different temperatures and partial pressures of oxygen during the sintering process. During this cycle, primary hematite is partly reduced into magnetite and then oxidized back into secondary hematite, whereas initial melt rich in CaO and SiO₂ is formed and reacts with hematite to form ferritic bonding phases such as SFC (silico-ferrite of calcia) solid solutions. In a stage with the highest temperature and the most reducing conditions, the amounts of magnetite and silicate liquid phase are the highest, whereas the amounts of hematite and SFC are smaller. During the cooling stage, the liquid solidifies to form both crystalline as well as glassy, amorphous phases.⁶¹⁾

In order to define the conditions needed to maximize the stability of SFC solid solution, which - in turn - is related to the formation of the SFCA (silico-ferrite of calcium and aluminium) needed as a bonding phase in sinters, the iron–rich area of the FeO_x–CaO–SiO₂ system needs to be understood.^{47,60,61,153,154,155)} Experimentally determined phase equilibria of this system in the conditions of the sintering process have been illustrated by e.g. Pownceby and Clout.⁶¹⁾ In their paper⁶¹⁾ they aimed to extend their previous studies of the same system in atmospheric pressures⁶⁰⁾ to obtain a more accurate view on the formation of different phases during the heating stage of the sintering process, in which partial pressure of oxygen is decreased to the magnitude of 10⁻³ atm^32,33) due to the combustion of coke particles (after which the partial pressure of oxygen once again increases during the cooling stage).⁶¹⁾ Temperature range in both of their studies was chosen to be between 1240°C and 1300°C.^60,61)

From the isothermal sections (from 1240°C to 1300°C) of the pseudoternary Fe₂O₃–CaO–SiO₂ system in air presented by Pownceby and Clout,^60,61) it is seen that in the temperatures higher than 1250°C the main phases of the sinter system are liquid solution as well as solid Fe₃O₄, C₂S and C₂F, in which C, F and S refer to CaO, Fe₂O₃ and SiO₂, respectively. It is also seen that the homogeneous stability region of the liquid phase extends from iron oxide rich area to the silica rich concentrations. However, in temperatures below 1250°C this liquid region is divided into two regions: one with higher iron oxide content and higher basicity and the other with higher SiO₂ content and lower basicity. As the partial pressure of oxygen is decreased from the atmospheric values, the region of the silicate melt is enlarged significantly in comparison to the calcium ferritic melt. According to the experiments by Pownceby and Clout⁶¹⁾ in the sintering conditions, the bulk composition of the sinter lies within the three phase region of liquid, magnetite and SFC solid solution. Furthermore, the SFC solid solution, which acts as a major ferritic bonding phase in low-Al₂O₃ sinters, cannot be formed as a single crystalline phase in these conditions. Combined with their experiments in air,⁶⁰⁾ they concluded that in order to maximise the formation of SFC solid solution in lower temperatures, a semi-reduced heating followed by slow, oxidizing cooling is preferable.⁶¹⁾

With an addition of Al₂O₃ into the studied system, a more comprehensive view on the bonding phase formation can be achieved. As initially presented by Hancart et al.⁴⁶⁾ and reported by various other researchers since then, a complex silico-ferrite of calcium and aluminium (SFCA), which can be represented by the formula xFe₂O₃·ySiO₂·zAl₂O₃·5CaO (with x + y + z = 12), is regarded as an essential bonding phase in industrial sinters and must therefore be understood in order to control the sintering process properly^153,154) (c.f. chapter 2.2.2). Pseudoquaternary Fe₂O₃–SiO₂–Al₂O₃–CaO diagrams (assuming conditions to be oxidizing enough to stabilize Fe₂O₃ over Fe₃O₄ and FeO) can be used to illustrate the substitution mechanisms between different cations in the SFCA solid solution. According to Patrick and Pownceby,¹⁵³⁾ substitution mechanisms and solid solubilities can be shown within a plane in which CF₃, CA₃ and C₄S₃ (in which C, F, A and S refer to CaO, Fe₂O₃, Al₂O₃ and SiO₂, respectively) are chosen as end members. According to their results, the greatest range of solubility occurs in the direction of the Al³⁺ ↔ Fe³⁺ exchange (between CF₃ and CA₃), whereas the coupled substitution involving 2 (Al³⁺, Fe³⁺) ↔ (Ca²⁺, Fe²⁺) + Si⁴⁺ (between CF₃ and C₄S₃) is not as extensive.¹⁵³⁾ Alternative suggestions as compositional planes, in which substitutions take place, have been proposed by e.g. Inoue & Ikeda¹⁵⁶⁾ (CF₃, CA₃ and CS as end members) and Dawson et al.¹⁵⁷⁾ (CF₂, CA₂ and CS₃ as end members).

Patrick and Pownceby¹⁵³⁾ also used CF₃–CA₃–C₄S₃ planes to illustrate, how the substitution decreases and stability region becomes smaller as temperature is increased. Both laboratory experiments and analyses from various samples of industrial sinters suggest that the stability of the SFCA solid solution is increased with increasing Al₂O₃ content and the Al₂O₃-rich SFCA is stable at 200°C higher temperatures in comparison to alumina-free SFC phase.¹⁵³⁾ The effects of sinter composition (e.g. contents of Al₂O₃ and TiO₂ as well as CaO/SiO₂ ratio) and conditions (temperature and partial pressure of oxygen) on the stabilities of SFCA solid solutions, liquid phase as well as other stable phases of the FeO_x–SiO₂–CaO–Al₂O₃ system has been studied and described in more detail by Pownceby and Webster and their co-workers in their more recent publications.^{19,154,158,159,160,161,162)} Based on their comprehensive experiments and analyses and with the help of illustrative phase diagrams of the studied systems, Webster et al.¹⁵⁸⁾ summarized the flow sheet illustrating the reaction sequences leading into the formation of the SFCA phases (cf. Fig. 1). It should also be noted that although in this chapter it has so far only been referred to one SFCA solid solution, at least three different type of silico-ferrites of calcium and aluminium have been reported in the literature:^11,47) so-called SFCA-I (A₃BM₈T₈O₂₈) and SFCA-II (A₄T₁₄M₁₆O₄₈) in addition to SFCA (A₂T₆M₆O₂₀), in which A, B, M and T refer Ca²⁺, either Ca²⁺ or Fe²⁺, octohedral cation site and tetrahedral cation site, respectively.¹¹⁾ The existence of the SFCA and SFCA-I phases has been verified in iron ore sinters,⁴⁹⁾ but it is not known whether SFCA-II is formed in industrial sinters.⁴²⁾ From the practical point of view SFCA-I has been considered to be the most desirable bonding phase in sinters because it yields higher strength and better reducibility.⁴⁷⁾ It should also be kept in mind that despite the general chemical formulae presented for SFCA, SFCA-I and SFCA-II phases above, the morphologies of the silico-ferrites of calcium and aluminium are not solely defined by the chemical composition, but are also influenced by other factors such as cooling, melt viscosity and kinetics. As reported in a comprehensive review on the compositions, structures and formation mechanisms of the SFCA phases by Nicol et al.,¹³⁵⁾ a wide variety of SFCA morphologies (e.g. acicular, platy, fibrous, irregularly-shaped, needle-like, dendritic, columnar, tabular, blocky, prosmatic, lath-shaped and crystalline, some of which describe similar morphologies with different names defined by different researchers) have been observed and reported by many researchers, and there still is large inconsistency concerning the classification and naming of silico-ferrites of calcium and aluminium.

Fig. 1.

Summary of the reaction sequences in the formation of SFCA-I and SFCA phases during heating and cooling in the range of 25−1350 °C and p(O₂) of 5·10⁻³ atm. A zone, in which Fe₃O₄ and melt coexist, is located between the heating and cooling zones. Heating zone is further divided into three subsections. Based on Webster et al.¹⁵⁸⁾

In order to take the SFCA phases into account in phase stability simulations it is necessary to have accurate assessments of thermochemical data for these phases based on the phase stability experiments and analyses that have been made. This kind of data assessment have been reported within the context of the FactSage software^129,130,131) by e.g. Murao et al.¹¹⁾ and within the context of the MPE software^127,128) by e.g. Chen et al.⁴⁷⁾ A compound energy formalism¹⁴³⁾ with three sublattices has been used to describe the SFCA phase in thermodynamic modelling.⁴⁷⁾

Finally, once thermochemical data has been assessed for a system including all the major components in sinters, this data can be used not only to study the phenomena during the sintering process, but also to estimate the behaviour of sinter during the reduction processes such as blast furnace. For example, Iljana et al.^17,163,164) have reported a good correspondence between computational results for FeO_x–SiO₂–CaO–MgO–Al₂O₃ system and laboratory experiments when studying the behaviour of sinter in the cohesive zone of a blast furnace as seen from Fig. 2.

Fig. 2.

FactSage-computed proportions of the existing phases in the basic sinter as temperature rises in a blast furnace. The experimental estimations of the proportions of the slag phase at the end of the reduction-softening test have been marked with black dots.¹⁷⁾

5. Summary

Sintering process is a commonly used pre-treatment process to manufacture a burden material for the blast furnace. During the sintering process the material undergoes a series of reactions, during which the varying conditions (i.e. temperature and state of oxidation) cause changes in the mineralogical composition of the material. The main purpose of the sintering process is to produce porous, agglomerated sinter material with suitable chemical, physical and mechanical properties to be charged into the blast furnace. Although the sintering process is very fast and the material does not fully reach the chemical equilibrium during the process, it is nevertheless important to understand the phase equilibria of the sinter system in order to analyse and control the effect of various factors on the sintering process.

Despite the compositional variations in different sinters, the main components of practically all sinters are FeO, Fe₂O₃, SiO₂, CaO, Al₂O₃ and MgO, which cover approximately 99% of the sinter chemistry in total. Hence, a reasonable comprehensive view on the behaviour of sinters can be obtained by studying the phase equilibria within this six-component system. Based on the experimental data (i.e. phase diagrams, solubilities, etc.), oxide databases including all the relevant components - although not necessarily data for all the relevant phases (such as SFCA) - have been created with a CALPHAD method. These databases are commercially available with computational thermochemistry software and can thus be used to compute innumerable amount of phase diagram sections in order to illustrate the phase equilibria within the FeO–Fe₂O₃–SiO₂–CaO–Al₂O₃–MgO system. As long as all the relevant data is available in the databases, these computed phase diagrams provide a useful tool to study the behaviour of the material in both sintering process itself as well as in the following reduction processes such as the blast furnace.

Acknowledgements

The authors wish to thank their colleagues Mr. Pekka Tanskanen and D.Sc. (Tech.) Ville-Valtteri Visuri for fruitful discussions and useful comments concerning the manuscript. The patience and the helpfulness of the personnel of the National Repository Library in Kuopio is also acknowledged with great gratitude.

Abbreviations

AM: Associate model

BF: Blast furnace

CALPHAD: Calculation of phase diagrams; Computer coupling of phase diagrams and thermochemistry

CEF: Compound energy formalism

CEQCSI: Calcul d’Equilibre Chimique pour la Sidérurgie (Chemical Equilibrium Calculations for the Steel Industry)

CM: Cell model

CSIRO: Commonwealth Scientific and Industrial Research Organisation

EDS: Energy dispersive spectroscopy

EPMA: Electron probe microanalysis

FACT: FactSage

GCA: Generalized central atom model

GTT: GTT-Technologies

IRSID: Institut de Recherché de la Sidérurgie

MPE: Multicomponent Phase Equilibria

MTD: MTDATA

MQM: Modified quasi-chemical model

NPL: National Physical Laboratory

OM: Optical microscopy

RILM: Reciprocal ionic liquid model

RKM: Redlich-Kister-Muggianu polynomial

RSM: Regular solution model

SEM: Scanning electron microscopy

SFC: Silico-ferrite of calcia

SFCA: Silico-ferrite of calcium and aluminium

TC: ThermoCalc

TEM: Transmission electron microscopy

XRD: X-ray diffraction

XRF: X-ray fluorescence

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