X-ray diffraction analysis of refractory materials used in U.S.Steel Košice

M. Černík, L. Hrabčáková, S. Štulrajter, P. Vranec

¹U.S.Steel Košice, s.r.o., Vstupný areál U.S. Steel, 044 54 Košice

mcernik@sk.uss.com, lhrabcakova@sk.uss.com, sstulrajter@sk.uss.com, pvranec@sk.uss.com,

Different types of refractory materials are used in the refractory lining of the heat aggregates for the steel production. Among the basic raw materials used for their production Corundum (Al₂O₃), Spinel (MgO·Al₂O₃), Periclase (MgO) and others are of great matter. The refractory materials are in direct contact with the slag and steel, where at the interface area, mainly in the case of the slag, the refractory material is directly influenced. This influence is significantly dependent on the type of the refractory material, operational temperature, exposition time and corrosion activity of slag and steel. Generally, by the long-term operation the refractory material is worn, what is not the subject of investigation, but also the structural changes take place. It can be stated that the Corundum is stable and it does not change its crystal structure even after a long-term exposition. Spinel-like structures are however more interesting as the crystal structures is often influenced by additional atoms, where the significant changes in lattice parameters occur. Large amount of measured materials from plant including Spinel structures were divided into two groups, i.e. castables and castables influenced by slag on its surface.

Standard lattice parameter of the Spinel MgAl₂O₄ with the space group Fd-3m (227) is according to ICDD card No. 82-2424 a_o = 0.80887 nm and according to ICDD card No. 77-0435 a_o = 0.80806 nm. Materials investigated here include real structures that are often different from the pure materials that are considered as standards. Therefore, the lattice parameter of the Spinel is in many cases different for the analyzed refractory materials containing Spinel from the plant. In many cases of these real materials the presence of two types of Spinel with different lattice parameters was identified. The Spinel structure is in these cases influenced mainly by temperature and chemical elements present in such material. It is necessary to mention that the literature describes the pressure dependence on the lattice parameter. However, our materials are not exposed to high pressures; these are operated at high temperatures and are in contact with liquid iron and liquid slag.

It has been found from many different measured and evaluated data that the lattice parameter in the structure of material which is in contact with liquid slag increases to higher values. The most probable reason is the influence of the slag elements such as Na, Ca, Mg, Al, Si and Fe on the Spinel structure. The lattice parameter takes on higher values which decrease to the standard values by the distance change from surface towards the basic material.

The subject of our investigation was sample including Spinel structure with the lattice parameter of significantly lower value than the standard. Such crystal structure is less described in the literature. The TOPAS software was used for the determination of phase composition content, Fig. 1. Sample 1 contained Corundum, Spinel, Calcium Aluminium Oxide and Silicon Carbide – Moissanite. Table 1 and 2 shows phase composition content and calculated lattice parameter of the Spinel. Table 3 shows the comparison of chemical analysis of Sample 1 with the chemical composition calculated from the phase content. For the simulation of the operational characteristics of the material, sample 1 was annealed at 1500 °C 5 hour in the oxidation atmosphere. After the annealing the surface of the sample was black (Sample 2) and few millimeters under the surface the sample remained white (Sample 3). The content of Corundum increased and the content of Spinel, Calcium Aluminium Oxide and Silicon Carbide – Moissanite decreased (Table 1). The formation of Anorthite was observed for the Sample 2. The original sample contained two types of Spinel with different lattice parameters, Table 1. First Spinel with the lattice parameter of 8.0856·10^-10 m can be considered as standard Spinel while the second Spinel with the lattice parameter of 8.0546·10^-10 m cannot be considered as standard. Chemical composition of standard Spinel with the cubic structure can be stated by chemical formula MgAl₂O₄; non-standard Spinel, non-stechiometric MgO·nAl₂O₃, where n = 1.0 – 7.3 [1]. In the stechiometric crystals (n = 1.0) the Mg²⁺ ions are in tetrahedral positions and Al³⁺ ions are mainly in octahedral positions. When the crystal structure deviates from the stechiometric ratio (n > 1) the excess of vacant cations is formed mainly in octahedral positions. By the increase of Al₂O₃ content in the non-stechiometric Spinel the lattice parameter decreases. The substitution of the tetrahedral Mg²⁺ ions by Al³⁺ ions can be described by the following reaction:

Mg²⁺ = I_1/3 + Al³⁺, where I_1/3 are the unoccupied vacant cations.

In the limiting case the defect Spinel correspond to γ – Al₂O₃. Dependence of the lattice parameter of non-stechiometric Spinel a = f(n) was deduced by several authors, see [1].

Annealing in the oxidation atmosphere leads to the reversible transformation of the Spinel to Corundum in the extreme case. Incomplete inversion process leads to non-stechiometric Spinel MgO·nAl₂O₃, where the lattice parameter of the Spinel is directly dependent on n, a = f(n).

Nevertheless, there are still several questions that are unanswered, such as:

· How the physico-metallurgical characteristics of the Spinel depleted by Mg change?

· What are the physico-metallurgical characteristics of the cubic γ – Al₂O₃ in contrast to Corundum for the extreme case?

· What are the physico-metallurgical characteristics of the Spinel enhanced by elements such as Fe and others?

Figure 1. Sample1, detail of the refined diffraction pattern.

Figure 2. Sample 2, detail of the refined diffraction pattern.

Table 1. Phase compositions of the samples.

*Identified phase composition*			Sample 1	Sample 2	Sample 3
Chemical Formula	Mineralogical name	Space Group	Content [wt%]	Content [wt%]	Content [wt%]
Al₂O₃	Corundum	(167) R-3c	67.6	80.9	77.4
MgAl₂O₄	Spinel	(227) Fd-3m	11.1	4.9	11.1
MgAl₂O₄	Spinel	(227) Fd-3m	13.1	8.2	9.8
CaAl₂O₄	Calcium Aluminium Oxide	(014) P21/n	3.5	0.3	0.2
SiC	Moissanite	(186) P63mc	4.8	0.4	1.6
CaSi₂Al₂O₈	Anorthite	(002) P-1	-	5.3	-

Table 2. The lattice parameters of Spinels and Corundums.

Chemical Formula	MgAl₂O₄	MgAl₂O₄	Al₂O₃
Mineralogical name	Spinel	Spinel	Corundum
Space Group	(227) Fd-3m	(227) Fd-3m	(167) R-3c
Lattice parameter	A [10.E^-10 m]	A [10.E^-10 m]	A [10.E^-10 m]	C [10.E^-10 m]
Sample 1	8.0546	8.0856	4.7592	12.9923
Sample 2	8.0504	8.0643	4.7591	12.9920
Sample 3	8.0023	8.0174	4.7589	12.9913
standard	8.0898		4.75	12.982

Table 3. The element content recalculated from phase content of samples.

Sample

Analysis

[%]

1 - Chem

45.9

43.9

3.4

1.1

1.3

3.3

0.03

0.05

1 - X-ray

46.2

44.2

4.2

0.9

1.5

3.4

1 G.I. Belykh, V.T.Gritsyna , L.A.Lytvykov, V.B.Kolner:Transformation og spinel crystals structure MgO.nAl2O3 under high-temperature annealing, Functional Materials 216, No.3 (2009).