Phase Reactions Between Refractory and High-Acidic Synthetic CaO-Ferronickel Slag
Phase Reactions Between Refractory and High-Acidic Synthetic CaO-Ferronickel Slag
0 1.-CD Laboratory for Extractive Metallurgy of Technological Metals, Montanuniversitaet Leoben/Nonferrous Metallurgy , Franz-Josef-Straße 18, 8700 Leoben , Austria. 2.-RHI Feuerfest GmbH-Nonferrous Metals , Wienerbergerstraße 9, 1100 Vienna , Austria. 3.-Montanuniversitaet Leoben / Nonferrous Metallurgy , Franz-Josef-Straße 18, 8700 Leoben, Austria. 4.-
Interactions between high melting synthetic ferronickel slags with acidic character and MgO as refractory were investigated. In order to facilitate the complex composition of real ferronickel slag, a synthetic slag of SiO2-MgOFe2O3-CaO was used. The practical corrosion tests of the MgO refractory were performed in a hot-stage microscope at temperatures of 1650 C under a CO/ CO2 atmosphere for process-oriented conditions. The formed phases between slag and magnesia substrate were analyzed by scanning electron microscope analysis. The results show that, by penetrating the slag into the refractory, the melt dissolves the magnesia and forms Mg- and Ca silicates. Furthermore, a diffusion of Fe from the slag into the magnesia grains can be observed and a transformation to magnesiowustite occurs. Thermodynamic phase calculations using FactSage software confirmed the generation of these minerals. The combination of practical testing and thermodynamic calculations should ultimately provide a path for improving the refractory lifetime and performance.
CHRISTOPH SAGADIN,1,4 STEFAN LUIDOLD,1,5 CHRISTOPH WAGNER,2
ALFRED SPANRING,2 and THOMAS KREMMER3
Refractory stability is an essential factor in the
ferroalloy industry especially for ferronickel
production. The corrosion of the refractory caused by
high melting slags massively influences the furnace
life-time. Reduced production efficiency and furnace
shutdowns are some of the results of inferior
refractory. Therefore, the aim is to select adequate
furnace linings for individual processes to provide
homogeneous wear and predictable refractory
service limits for scheduled maintenance and high
The chemical attack of the refractory by metal,
slag, dust and atmosphere constitutes an important
cause of corrosion. This leads to the disintegration
of refractory furnace walls with simultaneous
penetration by liquid substances, which could change
the mechanical stability. The resistance of the
refractory against corrosion can be achieved by
suitable measures during the production of the
furnace lining or by proper process management.
The chemical reactivity of the liquid slag composed
of ions plays a key role in metallurgical processes.
The deterioration can be described by a four-stage
process: wetting, which is followed by penetration
due to porosity, disruption of the refractory bonds
by chemical reactions, and finally erosion of the
refractory constituents by flowing liquids.
Therefore, the right choice of refractories with high
corrosion and erosion resistance is one of the critical
requirements to be satisfied. During the
manufacture of ferronickel, temperatures of more than
1650 C and high acidic slags are typical for the
smelting process, which stresses the ceramic
magnesia refractory despite its high melting point, good
mechanical properties at elevated temperatures and
ability to withstand hostile conditions.2–8
According to Hu,9 basic magnesia refractory
material has a higher resistance performance than
acidic refractories in the ferronickel process despite
the acidic slags. In detail, FeO and MgO form a
substitution solid solution resulting in a decreased
FeO content in the slag system. Consequently, the
viscosity of the slag increases and penetration
would decline.9 Investigations of corrosion
mechanisms are commonly performed by using a range of
simplified laboratory-scale experiments.10,11 The
current tests combine practical measurements in
laboratory-scale trials with theoretical
thermodynamic calculations to understand the actual
corrosion mechanisms, which is necessary for an
improvement of the refractory performance and
lifetime. The focus of this work is the evaluation of
corrosion of refractory substrates and phase
formation when a synthetically produced slag, based on
data from FeNi producers, interacts with them.
Therefore, the slag is melted in a hot-stage
microscope under a defined CO/CO2 gas mixture to mimic
actual process conditions. Further, the
examinations include investigations via scanning electron
microscopy (SEM) for phase determination and
thermodynamic calculation by using FactSage
7.112 software. The characterization of the corrosion
of refractories by combining hot-stage microscopy
and SEM including energy dispersive x-ray
spectroscopy (EDX) with thermodynamic calculations
provides an important basis for the further
development of the ferronickel process and therefore
METHODS AND MATERIALS
Usually, a hot-stage microscope (HSM) is applied
for investigations of the sintering and deformation
temperature as well as the melting behavior of
different materials like slags, dusts and ashes. The
HSM (EM301; Hesse Instruments, Germany)
consists of several main components: a halogen lamp, a
high-temperature tube furnace, a CCD camera and
a control unit. It is a complete testing system
designed to determine high-temperature
characteristics such as wetting angle, softening behavior and
spherical or hemispherical points of the specimen
materials. In the current research, the HSM served
as the melting and reaction device. Therefore, a slag
powder was pressed into a 3-mm-high and
3-mmdiameter slag cylinder, which was positioned on a
small magnesia refractory substrate. With a
controlled heating rate of 10 K/min, the tube
furnace was heated to 1700 C, which correlates to a
sample temperature of 1650 C. The slag sample
melts, infiltrates and reacts with the magnesia
substrate at this temperature. One hour after
reaching 1650 C, the furnace was automatically
switched off and cooled to room temperature. To
mimic the real ferronickel process in the
laboratoryscale test, a gas mixture of 60 vol% CO and 40 vol%
CO2 was used as atmosphere, corresponding to an
oxygen partial pressure of 1.94 9 10 7 atm at
1650 C (calculated by FactSage). The furnace
purging of the reducing gas mixture amounted to 0.16 l/
min, which is about twice the furnace volume.
Sample Preparation and SEM/EDX Analysis
After the investigation in the hot-stage
microscope, the magnesia substrate including the melted
and infiltrated slag was cut in cross-section by a
diamond grinding wheel. Then, the sample was
embedded in a two-component resin (resin with
hardener, Araldite DBF). A smooth sample surface
was achieved by grinding in several steps with
different SiC foils (800, 1200, 2400 and 4000). A thin
film of sputtered carbon made the sample surface
conductive, which is essential for scanning electron
The microstructure of the slag/refractory
interface was analyzed by a SEM (JEOL JSM IT-300 LV)
equipped with an EDX analyser. The measurements
took place with the following imaging conditions:
20 kV, scan time: 800 ls/pixel, resolution:
2048 9 2048, approximate counts on mapping:
8 9 105 on main element, 1.5 9 106 counts in sum.
To understand the processes of interaction
between synthetic FeNi slag and magnesia
refractory, thermodynamic calculations were carried out
with FactSage 7.1 software. The calculations were
performed by using the modules Phase Diagram and
Equilib as well as the databases FactPS and
FToxide. The main focus of the thermodynamic
investigations is to simulate refractory attack by slag and
its comparison with experimental results.
SiO2, MgO, Fe2O3 and CaO constitute the oxides
for the production of the synthetic slag. The mixture
of oxides as received, each with more the 99% purity
(i.e. composed of 48% SiO2, 19.2% MgO, 28.8%
Fe2O3 and 4% CaO) was homogenized and melted in
a graphite crucible by an induction furnace. After
the casting of the melt, the solid slag was ground in
a swing mill to produce a fine slag powder. Table I
shows the chemical analysis of the high-quality
magnesia plate (14 9 14 9 2 mm) and of the
synthetic slag produced.
The thermodynamic calculation of stable phases
at the slag/refractory interface with FactSage is a
main part of this research. The corrosion phase
model ranges from pure slag (48% SiO2, 19.2% MgO,
28.8% Fe2O3 and 4% CaO) to pure magnesia (MgO).
The calculation was performed for the same
atmosphere (60% CO and 40% CO2) as used by the
practical corrosion test in the HSM. In Fig. 1, the
stable phases at thermodynamic equilibrium
between the slag and refractory material are
displayed as a function of refractory/slag portion and
temperature. This is approximately comparable
with the phase diagrams for MeOx-MgO-SiO2 in
the slag atlas.13
An isotherm section at 1650 C of the
slag/refractory-system is displayed in Fig. 2, which illustrates
the distribution of the components in the phases. At
a low refractory to slag ratio of up to 13%, the slag
consists of a melt and dissolves the refractory, then
the first-phase olivine becomes thermodynamically
stable. The calculations predict that the silicate
called olivine is composed of MgO, SiO2, FeO and
CaO. The chemical composition of the olivine is
approximately constant in the stability zone, except
that FeO decrease with an increasing refractory
content. At about 27% refractory, the next phase,
magnesiowuestite (MeO), appears which consists of
the dominating components MgO and FeO. This
phase has a solid solution range from pure FeO to
pure MgO where the liquidus temperature
significantly increases with rising MgO content. The
concentration of Fe2O3 in the melt is not illustrated
in the diagram due to its too small content. At 0%
slag the ratio of Fe2+/Fe3+ is about 72 and decreases
to a ratio of 12 at about 27% refractory and to 9 at
even higher refractory fractions.
Figure 3 illustrates the distribution of the
elements on individual thermodynamically
stable phases as a function of the refractory/slag
ratio. The bulk of Mg and Si forms an olivine phase
between 13% and 27% refractory in the mixture,
whereas Fe and Ca primarily remain in the melt. As
soon as the monoxide phase reaches stability, the Fe
content of olivine and melt rapidly decreases. The
expulsion of Fe and Mg from the olivine causes an
intensified enrichment of Ca in the olivine on the
basis of exchangeability of Ca, Fe and Mg
SEM/EDS Corrosion Evaluation
In addition to the thermodynamic calculations,
microstructure investigations were carried out by
SEM/EDX. Figure 4 shows a section of the slag/
refractory transition from the slag to the pure
refractory. The upper zone indicates the infiltration
of slag into the refractory. The slag-influenced zone
is approximately 350–450 lm thick, however, the
infiltration depth can vary depending on the open
porosity of substrate. The corresponding spot
analyses are summarized in Table II.
Figure 5 exhibits the element distribution of Mg,
Si, Fe and Ca of the slag/refractory intersection. It
shows a qualitative measurement of these elements,
wherein brighter-colored regions correspond to a
higher element concentration.
Equilibrium Calculation for Slag/Refractory
The equilibrium calculation between two
neighboring phases express an important point. For the
correlation of the experimentally determined
compositions with the phase diagram, the Fe/Mg ratios
in olivine and magnesiowustite were calculated over
the complete slag/refractory range by FactSage. The
measured Fe/Mg ratio of region A from Fig. 4
amounts to 0.1 for olivine (solidified infiltrated slag)
and 0.35 for monoxide (MgO grains with absorbed
FeO). The thermodynamic calculations were done
for a temperature of 1400 C, which describes the
approximate consistency to the practical corrosion
test. Figure 6 illustrates the measured Fe/Mg ratios
as well as the calculated ones as a function of the
RESULTS AND DISCUSSION
The results of the melting trials regarding the
slag/refractory intersection show good agreement
with thermodynamic calculations. The matrix
consists of slag and refractory, where the slag fills up
the pores of the refractory. At the beginning, the
slag infiltrates the substrate along the pores, reacts
and partly dissolves the refractory. Large MgO
grains of the refractory are not dissolved by the slag.
Fe ions from the slag diffuse into the MgO particles
and form magnesiowustite (Mg, Fe)O), meanwhile,
the slag dissolves some magnesia and builds the
silicate olivine. The olivine ((Mg,Fe,Ca)2SiO4)
comprises of Mg, Ca and Fe, which are exchangeable,
The mapping (Fig. 4) shows different ranges of
element concentrations: areas composed of
magnesiowustite (white) indicated by the presence of Mg,
Fe and O; areas containing of Si, Mg, Fe, Ca and O
(green) and areas comprising of Mg and O (orange).
Spots 1 and 6 indicate high Si concentrations in the
slag. They exhibit (Mg, Fe, Ca)/Si ratios of
approximately 2 (2.2 at EDX spot) which coincide with the
stoichiometry of olivine. The chemical analyses of
the EDX spots show contents of Fe and Ca similar to
the thermodynamic analysis. A high FeOx content
in the slag causes a highly enrichment of MgO with
Fe oxide. The analyses on the refractory grains
(Spots 2, 3, 4 and 5) reveal a decreasing Fe content
from top to bottom. The formed magnesiawustite
corresponds to the phase monoxide of the
thermodynamic FactSage database, FToxide. Spot 7
represents a high Ca-containing phase, which was not
attributable to the results of thermodynamic
calculations for 1650 C.
The equilibrium calculation of spot A indicates
that the composition of the analysed olivine matches
to the calculation results for 55% refractory in the
mixture and that of monoxide to 57.5% refractory.
That means that the two neighboring phases
(olivine and magnesiowustite) are close to the
equilibrium (in balance when they are arranged at
the same position).
The very acidic high melting ferronickel slags
challenge ferronickel and refractory producers. To
achieve an understanding how the corrosion
mechanisms at the slag/refractory interface act in
ferronickel manufacturing, an analysis of the
microstructure is unavoidable and constitutes the
main topic of this work. The combination of
theoretical and experimental investigation methods,
namely hot-stage microscopy, inclusive SEM/EDX
analysis and thermodynamic FactSage calculations
provides excellent results.
The thermodynamic calculations were carried out
for the main oxides (FeOx, MgO and SiO2) and CaO
(slag in contact with refractory) as a function of
temperature. The formed phases at the
slag/refractory intersection was the main focus of attention.
The outcome from the practical investigation
methods showed good agreement with the
thermodynamic phase calculations.
The corrosion is caused by penetration of the
molten slag and reaction with the refractory
substrate. It results in the formation of new phases and
a partial dissolution of the magnesia substrate.
The thermodynamic calculations are a
suitable tool for predicting the sequence of phase
transformations in equilibrium and are, therefore,
useful for supporting experimental investigations.
The future work involves corrosion mechanisms of
real refractory substrates by different ferronickel
slag systems, where the presented methodology will
be refined and extended.
Open access funding provided by
Montanuniversitaet Leoben. The financial support by the Austrian
Federal Ministry of Science, Research and Economy
and the National Foundation for Research,
Technology and Development is gratefully acknowledged.
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