interaction of the refractory lining of the hearth of an electric arc furnace with nonferrous metals...
TRANSCRIPT
UDC 669.18:546.621’17
INTERACTION OF THE REFRACTORY LINING OF THE HEARTH
OF AN ELECTRIC ARC FURNACE WITH NONFERROUS METALS
OF THE SCRAP CHARGE
V. A. Kudrin,1 A. S. Guzenkova,1 S. S. Ivanov,1 and G. A. Isaev1
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp. 53 – 55, April, 2003.
The interaction of the refractory material of the electric furnace hearth with molten nonferrous components of
the metallic mixture is studied. At elevated temperature, the nonferrous components are accumulated by infil-
tration in the furnace refractory and thus may be a source of contamination. The use of scrap charges contain-
ing nonferrous metal impurities constitutes a danger for electric furnaces of small capacity.
The interaction between the refractory lining of steel-
making plants and the metallic mixture containing materials
such as galvanized iron, disused engineering components
made of copper, tin, etc. regarded as a subject of investiga-
tion, has a recent history. The point is that in the ever-in-
creasing application of continuous steel casting technologies,
the share of “pure” steel scrap (free of nonferrous metals and
typically used in the charging of arc furnaces) tends to de-
crease whereas that of obsolete crap, frequently high in unde-
sirable nonferrous metal impurities (copper, tin, zinc, lead,
arsenic, antimony, etc.) tends to increase correspondingly.
This results in significant “contamination” of the commercial
steel product with nonferrous metal impurities. Interactions
in the refractory — nonferrous metal system constitute an is-
sue of special concern in the electrometallurgy where the
greater part of the metallic mixture is typically obsolete
scrap, the main source of nonferrous metal impurities.
The problem of removing nonferrous metals under con-
ventional production conditions remains to be solved despite
the fact that the presence of nonferrous impurities, even at
the level of a few hundredths of a percent, can degrade ap-
preciably the properties of steels, especially detrimental in
the production of high-quality structural steel. The occur-
rence of several impurities (for example, tin and copper) at
higher concentrations is particularly undesirable.
In most cases, prior to producing a relatively homoge-
neous metal melt, the mixture is subjected to “warm-ups”
lasting from several minutes by converter technology to se-
veral hours — in blast furnaces or in low-capacity electric
furnaces. During these operations, low-melting point impuri-
ties (tin, lead, zinc, copper) melt and finally find their way to
the hearth of the furnace. Nonferrous metals with a low melt-
ing point convert to a molten state well before the steel melt-
ing temperature is reached. Therefore, even if the total con-
tent of nonferrous impurities in the charged mixture is low
(several hundredths or even thousandths of a percent), in cer-
tain periods of time (at the onset of melting), the local con-
centration of the impurity melt at its contact with refractory
lining may be quite high, occasionally close to 100%. This
may result in an interaction between the material of the lin-
ing and low-melting nonferrous metals.
Our goal in this study was to see in what a manner the re-
fractory lining may interact with low-melting liquid compo-
nents of the mixture. The model impurities selected were me-
tallic tin (for its low melting point) and metallic zinc (for its
high volatility). The hot surface temperature of the furnace
hearth during a warm-up period was by convention taken to
be 800 – 900°C. The refractory material of the hearth was
chromium magnesite with a total porosity of 16%. The test
specimens cut out of the refractory material had dimensions
of 30 � 30 � 12 mm. The metal chips for testing with a
weight of 2 – 4 g (a VRL-200 analytical balance was used
for weighing) were placed on the refractory surface and cov-
ered with an Alundum crucible to prevent them from oxida-
tion and volatization. The test specimens thus prepared were
heated in a muffle furnace at 800 and 900°C for 15, 30, 45
and 60 min. The maximum metal — refractory contact time
was 60 min, allowing for the fact that, under actual condi-
tions even for low-capacity furnaces, within 60 min after the
onset of melting, a pool of molten phase composed mainly of
iron is formed at the furnace hearth, and the activity of non-
ferrous metals under these conditions is insignificant. The
test specimens heated for a specified time were put out of the
Refractories and Industrial Ceramics Vol. 44, No. 4, 2003
2421083-4877/03/4404-0242$25.00 © 2003 Plenum Publishing Corporation
1 Moscow State Night Institute of Metallurgy, Moscow, Russia.
furnace and allowed to cool; the remaining metal was re-
moved from the refractory surface. To determine the mass of
metal m(Me) infiltrated into the refractory, the specimens be-
fore and after heat treatment were weighed to the nearest
0.0001 g. A PRIM-1M x-ray radiometric instrument
(VNIITFA, [1]) was used to determine the amount of metal
infiltrated into the refractory material.
Relevant data are presented in Figs. 1 and 2 and in Ta-
ble 1. Figure 1 shows that the infiltration of zinc into the re-
fractory at 800°C is active within the first 15 min; later (with
the test time extended up to 60 min), the concentration of
zinc measured at the opposite face of the test specimen re-
mains virtually constant (about 2.0 wt.%). If the temperature
is raised to 900°C, the measured Zn concentration increases
to 2.7 wt.%, that is, the infiltration of zinc into the refractory
material at this temperature proceeds at a higher rate.
The infiltration of tin into the refractory (Fig. 2) likewise
increases as the test temperature is raised from 800 to 900°C.
However, the tin concentration at the specimen’s face oppo-
site to the molten-metal contact face did not exceed 1 wt.%.
The course of time curve for the concentration of infiltrated
tin measured at the opposite face resembles that for zinc: the
infiltration rate is maximum within the first 15 and 30 min.
The total mass of metal sorbed at the refractory surface in-
creases with temperature (Table 1). Assuming that the inter-
action of a fluid metal with a porous refractory materials
obeys the diffusional mass transfer in capillary systems (ca-
pillary infiltration) [2 – 9], the effective diffusion coefficient
Def was calculated from the experimental data. For both zinc
and tin, the value of Def in the temperature range 800 –
900°C is roughly of the same order of magnitude. Still, de-
spite the close values of Def , the sorption of zinc and tin dif-
fers somewhat and tends to increase with temperature.
The results obtained have important technological impli-
cations, especially for small-capacity arc furnaces. We as-
sume that the hearth of the arc furnace has the simple shape
of a truncated cone; using data from [10, 11], the specific
hearth surface per unit mass of molten metal was calculated
as a function of the furnace capacity. As an be seen from
Fig. 3, the specific hearth surface is highest for furnaces of
small capacity. Recall that electric furnaces of small capacity
are best suited for making steels with special quality requi-
rements.
CONCLUSIONS
Our data on the infiltration of zinc and tin into the refrac-
tory material show that, during the warm-up period, low-
melting components of the metallic mixture are accumulated
Interaction of the Refractory Lining with Nonferrous Metals of the Scrap Charge 243
TABLE 1. Calculated Diffusion Coefficients for Zinc
and Tin Infiltrated into the Chromium Magnesite Refrac-
tory
MetalTemperature,
°C
Infiltrated
metal mass
m (Me) � 102, g
Def � 10 – 5,
cm2�sec
Zn 800 7.1 3.46
Zn 900 8.3 3.90
Sn 800 8.7 2.77
Sn 900 9.6 2.77
3.0
2.0
1.0
15 30 45 60
2
1
Zin
cm
ass
frac
tion
(Zn),
%�
Contact time , min�
Fig. 1. Kinetics of infiltration of molten zinc into the refractory:
1 ) at 800°C; 2 ) at 900°C.
15 30 45 60
2
1
Tin
mas
sfr
acti
on
(Sn),
%�
Contact time , min�
1.0
0.8
0.6
0.4
0.2
Fig. 2. Kinetics of infiltration of molten tin into the refractory:
1 ) at 800°C; 2 ) at 900°C.
Sp
ecif
icsu
rfac
e,
10
,m
ton
Sm�
�2
2�
Furnace capacity, tons
20
15
10
50 100 150 200
Fig. 3. Specific surface per ton molten metal plotted as a function of
the furnace capacity.
in the refractory lining. This creates a danger, since in subse-
quent heats the fluid metal comes into contact with the lining
infiltrated with a certain amount of nonferrous metals. The
infiltrated lining may serve as a contaminating “donor” for
the fluid steel in subsequent heats. The use of scrap charges
containing nonferrous metal impurities may constitute a dan-
ger for electric furnaces of small capacity.
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