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Development of zirconia electrolyte sensor with auxiliary electrode for the in situ measurement of dissolved silicon in molten iron
Sensors and Actuators,
BI (1990) 203-209
203
Development of Zirconia Electrolyte Sensor with Auxiliary Electrode for the in situ Measurement of Dissolved Silicon in Molten Iron TOSHIAKI
OKIMURA,
KATSUNORI
Kure Research & Development
FUKUI
Laboratories,
and SHIGEAKI
MARUHASHI
Nisshin Steel Co., Ltd., Hiroshima
Abstract In Japan, hot metal pretreatments, such as desiliconization and dephosphorization processes, are applied in most steel making companies. The key to better operation of desiliconization is to use a sensing device that allows the silicon content in molten iron to be rapidly determined. Accordingly, we have investigated a zirconia electrolyte sensor with an auxiliary electrode for the instantaneous measurement of the silicon content in molten iron. To study the effects of the auxiliary electrode, four kinds of silicon sensors coated with various forms of ZrO,/ZrSiO, mixtures as an auxiliary electrode have been tested. These sensors are used for measurihg the electromotive force (e.m.f.) of iron melt in a 300 kg induction furnace. The response time becomes shorter with increasing boundary density of the auxiliary electrode. Furthermore, there appears to be a better correlation between e.m.f. and silicon activity in molten iron with the use of a high boundary density. These results indicate that a kind of equilibrium, in which asio;! is fixed, is formed rapidly with the increase of boundary density at the hot metal/solid electrolyte/auxiliary electrode triphasic interface. Using this sensor, the silicon content in molten iron can be estimated to within f0.015% Si by measuring the e.m.f. and the metal temperature.
(Japan)
sensing device that allows the silicon content in hot metal to be rapidly determined. Several kinds of silicon sensors have been reported so far [3] but their performance is not always good enough from the viewpoint of measuring time, accuracy of silicon measurement and handling. The authors started to investigate zirconia electrolyte sensors with auxiliary electrodes, with which silicon content can be rapidly measured [4, 51. The theory of the oxygen probe is applicable to this sensor. The difference between a silicon sensor and an oxygen sensor is that the silicon sensor has a mixed ZrO,/ZrSiO, auxiliary electrode, which is coated in spots on the outer surface of the zirconia electrolyte. With the use of the auxiliary electrode, an Si-0 equilibrium in which +io, is fixed is formed locally, and the silicon content is determined by the measurement of oxygen potential and metal temperature. The present paper reports the use of a zirconia electrolyte sensor with auxiliary electrode for rapid measurement of silicon activities in hot metal, especially the effects of the coating form of the auxiliary electrode. 2. Theory of Silicon !hsor The oxygen potential is given by the equilibrium between silicon and oxygen in hot metal: Si + 20 = SiOz
1. Introduction
In iron and steel making processes, hot metal produced by the blast furnace inevitably contains silicon, ranging from about 0.1 to 1.5 wt.%. In Japan, desiliconization and dephosphorization processes are applied in most iron and steel works because the production of low-silicon hot metal results in an appreciable cost reduction with the BOF (basic oxygen furnace) process [ 1,2]. The key to better operation of desiliconization is a 09254005/90/$3.50
AG$, = - RT In
(1) aso
hsia2 where AG$, is the Gibb’s free energy of eqn. (I), as+,. is the activity of Si02, hsi is the Henrian activity of silicon in molten iron and a, is the activity of oxygen. Then
The Henrian activities of silicon in iron melts can 0 Elsevier Sequoia/Printed
in The Netherlands
204 Oxygen
sensor
Silicon
MO lead
sensor
wire
aur electrode
reference
electrode
Fig. 1. Concept of silicon sensor compared with oxygen sensor.
be calculated approximately
by:
log bsi = log[ %Si] + cz[ %Si] + e$[ %c]
(4)
From eqn. (3) it is evident that a measurement of oxygen activity, aO, corresponds to an understanding of the Henrian activity of silicon under the condition of constant Usiol. The auxiliary electrode fixes the activity of SiOz at the electrolyte/hot metal interface. Furthermore, stability at high temperature and adherence to the electrolyte are necessary for the performance of the auxiliary electrode. A ZrOz + ZrSiO, mixture satisfies these requirements: (1) ZrO, and ZrSiO, coexist in equilibrium in the temperature range 1473 K to 1873 K. (2) Good adherence is obtained by the use of an ZrO,-based material. Figure 1 shows the concept of the silicon sensor compared with that of the oxygen sensor. In the case of the oxygen sensor, stabilized zirconia is an oxygen anion conductor because of the presence of numerous oxygen anion vacancies. The opencircuit e.m.f. of the galvanic cell incorporating the zirconia electrolyte is a measure of the oxygen potential difference across the electrolyte. On the other hand, in the case of the silicon sensor, the auxiliary electrode is positioned on the outer surface of the zirconia electrolyte for the purpose of hxing the activity of SiO, around the electrolyte, and the oxygen potential in this area is measured. That is to say, the function of the auxiliary electrode is to change the silicon potential to that of oxygen in hot metal.
electrode
Fig. 2. Sensing part of the developed silicon probe.
by 9 mol% MgO (outer diameter 4.5 mm; inner diameter 3 mm; length 30 mm), a mixed ZrOJZrSiO, auxiliary electrode, and MO/MOO, mixture filling with an MO lead wire. The ZrO, + ZrSiO, auxiliary electrode was prepared by mixing 210~ and ZrSiO, in the ratio ZrO,/ZrSiO, = 6/4 by weight. A slurry of this mixture was coated on the outer surface of the zirconia tube, as shown in Fig. 2: spots of 1 to 2 mm diameter and 0.5 to 1 mm thickness were formed on the outer surface of the tube. To study the effects of the auxiliary electrode, its coating form was changed. The authors introduced an index called ‘boundary density’, a parameter that indicates the type of coating of the auxiliary electrode. The boundary density is defined as the total circumference of the auxiliary electrode spots divided by the outer surface area of the zironia tube. Figure 3 illustrates the differences in boundary density. In the case of small boundary density, sparse large spots exist; on the other hand, in the case of large boundary density, there are many closely spaced small spots. The boundary densities of the tested sensors ranged from 0.3 to 1.3 mm-‘. After being dried at 373 K, the zirconia tube with the spot coating was baked at 1673 K in air for 10 hours. This is to ensure tight adhesion
small
-
boundary density
------+
large
3. Experimental Procedure 3.1. Structure of the Silicon Sensor
Figure 2 shows the sensing part of the developed silicon probe. The cell part consisted of a ZrO, tube closed at one end, which was stabilized
Fig. 3. Illustration of the coating forms of the auxiliary electrode with different boundary densities.
205 TABLE I. Experimental conditions
Fig. 4. Outline of silicon probe A, part for detecting e.m.f. and temperature; B, inner paper sleeve; C, splash preventor; D, guide ring; E, heat-resistant resin; F, copper protector and paper cap.
Type
A
B
C
D
Boundary density (mm - ‘)
0.3
0.7
1.0
1.3
Number of test samples
20
20
30
10
Temperature (K)
1573-1773 250
Metal (kg)
between the auxiliary electrode and the zirconia tube. The measuring chain consisted of the following elements: MolMo + MoO,IIZrO,(MgO)
Composition of hot metal (%)
Si
Mn
P
S
4.50
0.10-0.60
0.30
0.080
0.020
(ZrO,
+ ZrSiO, )ISi( in liq. Fe) 1Fe The outline of the silicon probe is given in Fig. 4. The sensor, free MO electrode and thermocouple are fixed with refractory cement. In addition, this cell part is protected by heat-resistant resin and a refractory tube. The refractory tube is composed of an inner paper sleeve and refractory ceramic, with the intention of giving high-temperature endurance and preventing splashing, which is apt to occur during the immersion. 3.2. Experimental Procedure The experimental apparatus and the experimental conditions are given in Fig. 5 and Table 1 respectively. 250 kg of Fe + 4.5 wt.%C alloy was charged and heated to a temperature ranging from 1573 K to 1773 K in the 300 kg induction furnace. After the metallic charge was molten, a PtPtRh13 thermocouple sheath was immersed in the molten Fe + 4.5 wt.%C alloy, and thereafter, temperature measurements were made by using this thermocouple unit. The silicon content of the melt
was changed from 0.1 to 0.6 wt.% by adding Fe-Si alloy. The other components, such as manganese, phosphorus and sulfur, were adjusted to the same level as in the ordinary hot metal in the production line. Measuring instruments such as a digital voltmeter or recorder were connected to a probeholder. The cell part at the top of the silicon probe was immersed in the molten iron in the 300 kg induction furnace. The immersion depth was about 300 mm, and the immersion time was 60 s. The experimental procedure consisted of measuring the open circuit e.m.f.s of the silicon probes and subsequently sampling the molten iron for analytical purposes. In order to clarify the effect of the auxiliary electrode, e.m.f. measurements were also conducted with conventional oxygen sensors. 4. Experimental Results The outer appearances of silicon sensors after being used for e.m.f. measurements are given in Fig. 6. Both sensors, of which the boundary densities were 0.3 and 1.Omm-‘, were immersed in hot metal at 1673 K.
(4 Fig. 5. Experimental apparatus.
C
09
Fig. 6. Outer appearances of silicon sensors after being used for e.m.f. measurements in molten iron at 1673 K. (a) Boundary density = 0.3 mm-‘. (b) Boundary density = l.Omn~-~.
206
In each sensor, the spots of the auxiliary electrode did not come off and they showed close adhesion to the outer surface of the zirconia tube. The zirconia tube also showed good stability and did not have any spalling or changes in quality. Figure 7 shows examples of typical e.m.f. results. Each sensor had a different boundary density: 0.3, 0.7 and l.Omn-‘. These three recordings were obtained during immersion in hot metal at 1773 K. They became stable within 8 to 30 s after immersion of the silicon probe and remained constant. As regards the stability of the e.m.f. curves, obvious e.m.f. plateaux were observed for all the boundary density values. On the other hand, the response performance of the e.m.f. curves depended closely on the boundary density. Figure 8 shows the relation between boundary density and response time at 1573 K and 1773 K. In the range of boundary density < 1.0, the response time became shorter with increasing boundary density, and in the range of boundary density > 1.0, the response time converged to a constant value of 8 s. Furthermore, the response A
boundary
1
10
20
Immersion
- CI
z
1773 K
460
II.
z w
420
_-
I
4 I’
/, ’
I I
0.1
1
0.2
1
0.5
I
0.7
t
1.0
C%sil Fig. 9. Relation between e.m.f. of silicon sensor and silicon content in molten iron at 1573 K to 1773 K.
time became shorter at higher temperature. The response time at 1773 K was 2 to 10 s shorter than that at 1573 K at the same boundary density. Figure 9 shows the relation between e.m.f. of the test sensors (boundary density = 1.Omm-‘) and silicon content in hot metal at 1573- 1773 K. A good correlation between e.m.f. and silicon content is seen. It may be said that the silicon sensor developed in this study is appropriate for the in situ measurement of dissolved silicon in molten iron. The e.m.f. values of conventional oxygen sensors are also plotted in Fig. 9, and are almost independent of the silicon content. It is evident that oxygen sensors are not applicable for measurement of the silicon content in molten iron.
density
I 0
sod
>
30
period
5. Discussion
( set)
Fig. 7. Examples of typical e.m.f. curves obtained during immersion of silicon sensors in molten iron at 1773 K.
5.1. Comparison with the Theoretical e.m.f. Values Considering the transport number of ions, the e.m.f. (open-circuit cell potential) is given by the Nemst equation [6]:
; 2
P,,( ref.) iI4 + Pe iI4 E = g In F Po,(test)-“4 + Pe’14+ Et
40
-
30
-
20
-
10
-
B ‘3 2 ;
-- r
L I...-‘. OO
. boundary
.d
..I...
0.5
1.0 density
1.5 (mm-‘)
Fig. 8. Relation between boundary density and response time at 1573 K and 1773 K.
(5)
where E is the e.m.f. of the cell, R is the gas constant, T is temperature, F is the Faraday constant PO,( ref.) is the oxygen partial pressure at the reference electrode and PO is the oxygen partial pressure when the ionic and n-type electronic conductivities are equal. When the equilibrium between silicon and oxygen in molten iron is obtained, RT PO,( ref.) ‘I4+ PO‘I4 E = 7 In [K( l)ha] - l/4 + POl/4 + E1
(6)
207
The thermochemical data required for the calculation of the theoretical e.m.f. values are as follows [7, 101: Si( 1) = Si( 1 wt.% in liquid iron)
(7)
AG$, = - 119.27 - 25.47T Si( 1) + O,(g) = SiO,(cristobalite)
(8)
AG& = -902.29 - 173.68T MO(S) + O,(g) = Moo,(s)
(9)
AG$, = -576.1 + 0.1692T PO = 4.185 exp( 24.42 - 74370/T)
(10)
Equation (6) was used for the calculation of the theoretical e.m.f. values. The relation between the theoretical e.m.f. values and Henrian activities of silicon in hot metal compared with the measured e.m.f. values is given in Fig. 10. The measured e.m.f. values at boundary density = 1.O mn- ’ are in good agreement with the theoretical values based on eqn. (6). This means that the e.m.f. is determined by the activity of silicon in the hot metal and the metal temperature, or to put it in another way, the activity of silicon in the hot metal could be estimated by the measurement of the e.m.f. and metal temperature. On the other hand, in the case of e.m.f. at boundary density = 0.3 mm-‘, there appeared to be poor correlation between measured values and theoretical values. [%SiI at 4.5wt% C 0.1
0.2
0.0
0.6 0.6 I.1.1
c
520
I
I,,,,,
1
1.0
5.2. Eflects of Auxiliary Electrode As mentioned above, from the reaction equilibrium between silicon and oxygen in hot metal represented by eqn. (2), the following equation is obtained: AG:,, - In asio* ln(hsie2) = RT
(11)
Namely, if asio, is constant, there is a proportional relation between ln(hsia2) and l/T. The measured values of the test sensors at boundary density = 1.0 mm-’ are plotted in Fig. 11 as ln(hsiao2) versus l/T. The theoretical relations at = 1.0, 0.6 and 0.1 are also shown as solid %iO, lines. It is evident that the data at boundary density = 1.0 mm-’ show good agreement with the theoretical equilibrium at aso2 = 0.6. In the case of boundary density=O.3 mm-‘, the measured values varied widely in the aso range 0.1 to 0.6. Considering that the response time became shorter with increasing boundary density, these results indicate that a kind of equilibrium in which the activity of Si02 is fixed is formed rapidly with the increase of boundary density and the rise of metal temperature at the hot metal/solid electrolyte/auxiliary electrode triphasic interface. Thus the auxiliary electrode performs its purpose of fixing asioz if the boundary density is not less than 1.0 mm-’ Figure 12 shows scanning electron micrographs of the auxiliary electrode used in this study. The auxiliary electrode consisted of bright coloured Zr02 grains in which Zr is detected by using an EDX detector, and dark coloured ZrSiO, grains, in which Zr and Si are detected. This indicates that the auxiliary electrode remains a mixture of
1111,111
500
a SiO,=O. 1 -
0.6
1.0
-theoretical
s 960
IL = w
060
440 2
A 420
400
1
1
0.5
n,,,,
I
1
2
I
-16
-
-17
-
-18
-
I,,,,,,
4
6
10
Fig. 10. Relation between the theoretical e.m.f. values and Hemian activities of silicon in hot metal compared with measured e.m.f. values at 1673 K.
5.5
6.0 l/T
Fig. 11. Relation between ln(I~~q,~) and l/T in hot metal.
208
bright
grain
Ur02 1
dark
grain
(ZrSiOsl
bright
dark
Fig. 12. Scanning electron micrographs
5.3. Accuracy of Measurement of Silicon The accuracy of estimation of the silicon content in hot metal obtained from the relation between the estimated silicon content and the analytical silicon content is given in Fig. 13. The measured values are in good agreement with the analytical values, and the standard deviation of 0.7
-
CSSil
--
mean
0.1
ana
value
0.2
Si] Cal
=[%
f
0.3
CS Sil
24
0.4
0.5
grain
of
ZrOz + ZrSiO, during and after immersion in hot metal. There appeared to be no changes in quality of the solid electrolyte.
0.6
grain
0.6
0.7
ana
Fig. 13. Accuracy of estimation of silicon content in hot metal.
the results is 0.015 wt.% Si. The dashed lines show the position of the mean value +2a (a = the standard deviation). For example, when the true value of the silicon content is 0.30 wt.%, the measuring error is approximately f 0.03 wt.%.
6. Conclusions
The authors have investigated a zirconia electrolyte with an auxiliary electrode for the rapid measurement of silicon content in molten iron. To study the effects of the auxiliary electrode, several kinds of silicon sensors with various boundary densities of ZrO,/ZrSiO, as the auxiliary electrode were tested in hot metal in a 300 kg induction furnace. It was confirmed that the coating form of the auxiliary electrode had a great influence on the response performance and the measured values of the silicon activities. The increase of boundary density caused a better response and improved the reliability of the measured value. It can be said that the auxiliary electrode performs its task of Iixing the activity of SiO, when the boundary density is not less than 1.0 mm-‘. The silicon content in molten iron can be estimated within $0.015 wt.% Si by measuring the e.m.f. of this silicon sensor and the metal temperature.
209
References 1 H. Nomiyama, H. Ichikawa, K. Marukawa, M. Anezaki and H. Ueki, Study of optimum silicon content between iron-making and steel-making process on the hot metal pretreatment, Tersu-to-Hugune, 69 (1983) 1738-1745. 2 N. Tsuchiya and H. Taguchi, Present situation of producing pig iron with low silicon content in blast furnace and operational problem to be solved, Teisu-to-Hagane, 69 (1983) 1945 1954. 3 K. Ichihara, D. Janke and H.-J. Engel, A new silicon sensor for hot metal measurements, Steel Res., 57 (1986) 166-171. 4 M. Iwase, H. Kitaguchi, E. Ichise, H. Nakamura, T. Moriya and S. Maruhashi, Rapid determination of silicon contents in hot metal, Tefsu-to-Hagane, 110 (1985) ~1595.
5 M. Iwase, Rapid determination of silicon activities in hot metal by means of solid state electrochemical sensors equipped with an auxiliary electrode, Stand. J. Metall., 17 (1988) 50-56. 6 H. Schmalzried, Z. Electrochem. Ber. Bunsenges, Phys. Chem., 66 (1962) 572-576. 7 G. K. Sigworth and J. F. Elliot, Met. Sci., 8 (1974) 298-310. 8 I. Rarin and 0. Knacke, Thermochemical Properties of Inorganic Substances, Stahleisen, Dilsseldorf/Springer, Berlin, 1973. 9 M. Iwase, M. Yasuda and T. Mot-i, Free energy of formation of MoO at steehnaking temperature from emf measurement, Elecfrochem. Acto, 24 (1979) 261-266. 10 W. A. Fischer and D. Janke, Metallurgische Electrochemie, Stahleisen, Dtisseldorf/Springer, Berlin, 1975.
BI (1990) 203-209
203
Development of Zirconia Electrolyte Sensor with Auxiliary Electrode for the in situ Measurement of Dissolved Silicon in Molten Iron TOSHIAKI
OKIMURA,
KATSUNORI
Kure Research & Development
FUKUI
Laboratories,
and SHIGEAKI
MARUHASHI
Nisshin Steel Co., Ltd., Hiroshima
Abstract In Japan, hot metal pretreatments, such as desiliconization and dephosphorization processes, are applied in most steel making companies. The key to better operation of desiliconization is to use a sensing device that allows the silicon content in molten iron to be rapidly determined. Accordingly, we have investigated a zirconia electrolyte sensor with an auxiliary electrode for the instantaneous measurement of the silicon content in molten iron. To study the effects of the auxiliary electrode, four kinds of silicon sensors coated with various forms of ZrO,/ZrSiO, mixtures as an auxiliary electrode have been tested. These sensors are used for measurihg the electromotive force (e.m.f.) of iron melt in a 300 kg induction furnace. The response time becomes shorter with increasing boundary density of the auxiliary electrode. Furthermore, there appears to be a better correlation between e.m.f. and silicon activity in molten iron with the use of a high boundary density. These results indicate that a kind of equilibrium, in which asio;! is fixed, is formed rapidly with the increase of boundary density at the hot metal/solid electrolyte/auxiliary electrode triphasic interface. Using this sensor, the silicon content in molten iron can be estimated to within f0.015% Si by measuring the e.m.f. and the metal temperature.
(Japan)
sensing device that allows the silicon content in hot metal to be rapidly determined. Several kinds of silicon sensors have been reported so far [3] but their performance is not always good enough from the viewpoint of measuring time, accuracy of silicon measurement and handling. The authors started to investigate zirconia electrolyte sensors with auxiliary electrodes, with which silicon content can be rapidly measured [4, 51. The theory of the oxygen probe is applicable to this sensor. The difference between a silicon sensor and an oxygen sensor is that the silicon sensor has a mixed ZrO,/ZrSiO, auxiliary electrode, which is coated in spots on the outer surface of the zirconia electrolyte. With the use of the auxiliary electrode, an Si-0 equilibrium in which +io, is fixed is formed locally, and the silicon content is determined by the measurement of oxygen potential and metal temperature. The present paper reports the use of a zirconia electrolyte sensor with auxiliary electrode for rapid measurement of silicon activities in hot metal, especially the effects of the coating form of the auxiliary electrode. 2. Theory of Silicon !hsor The oxygen potential is given by the equilibrium between silicon and oxygen in hot metal: Si + 20 = SiOz
1. Introduction
In iron and steel making processes, hot metal produced by the blast furnace inevitably contains silicon, ranging from about 0.1 to 1.5 wt.%. In Japan, desiliconization and dephosphorization processes are applied in most iron and steel works because the production of low-silicon hot metal results in an appreciable cost reduction with the BOF (basic oxygen furnace) process [ 1,2]. The key to better operation of desiliconization is a 09254005/90/$3.50
AG$, = - RT In
(1) aso
hsia2 where AG$, is the Gibb’s free energy of eqn. (I), as+,. is the activity of Si02, hsi is the Henrian activity of silicon in molten iron and a, is the activity of oxygen. Then
The Henrian activities of silicon in iron melts can 0 Elsevier Sequoia/Printed
in The Netherlands
204 Oxygen
sensor
Silicon
MO lead
sensor
wire
aur electrode
reference
electrode
Fig. 1. Concept of silicon sensor compared with oxygen sensor.
be calculated approximately
by:
log bsi = log[ %Si] + cz[ %Si] + e$[ %c]
(4)
From eqn. (3) it is evident that a measurement of oxygen activity, aO, corresponds to an understanding of the Henrian activity of silicon under the condition of constant Usiol. The auxiliary electrode fixes the activity of SiOz at the electrolyte/hot metal interface. Furthermore, stability at high temperature and adherence to the electrolyte are necessary for the performance of the auxiliary electrode. A ZrOz + ZrSiO, mixture satisfies these requirements: (1) ZrO, and ZrSiO, coexist in equilibrium in the temperature range 1473 K to 1873 K. (2) Good adherence is obtained by the use of an ZrO,-based material. Figure 1 shows the concept of the silicon sensor compared with that of the oxygen sensor. In the case of the oxygen sensor, stabilized zirconia is an oxygen anion conductor because of the presence of numerous oxygen anion vacancies. The opencircuit e.m.f. of the galvanic cell incorporating the zirconia electrolyte is a measure of the oxygen potential difference across the electrolyte. On the other hand, in the case of the silicon sensor, the auxiliary electrode is positioned on the outer surface of the zirconia electrolyte for the purpose of hxing the activity of SiO, around the electrolyte, and the oxygen potential in this area is measured. That is to say, the function of the auxiliary electrode is to change the silicon potential to that of oxygen in hot metal.
electrode
Fig. 2. Sensing part of the developed silicon probe.
by 9 mol% MgO (outer diameter 4.5 mm; inner diameter 3 mm; length 30 mm), a mixed ZrOJZrSiO, auxiliary electrode, and MO/MOO, mixture filling with an MO lead wire. The ZrO, + ZrSiO, auxiliary electrode was prepared by mixing 210~ and ZrSiO, in the ratio ZrO,/ZrSiO, = 6/4 by weight. A slurry of this mixture was coated on the outer surface of the zirconia tube, as shown in Fig. 2: spots of 1 to 2 mm diameter and 0.5 to 1 mm thickness were formed on the outer surface of the tube. To study the effects of the auxiliary electrode, its coating form was changed. The authors introduced an index called ‘boundary density’, a parameter that indicates the type of coating of the auxiliary electrode. The boundary density is defined as the total circumference of the auxiliary electrode spots divided by the outer surface area of the zironia tube. Figure 3 illustrates the differences in boundary density. In the case of small boundary density, sparse large spots exist; on the other hand, in the case of large boundary density, there are many closely spaced small spots. The boundary densities of the tested sensors ranged from 0.3 to 1.3 mm-‘. After being dried at 373 K, the zirconia tube with the spot coating was baked at 1673 K in air for 10 hours. This is to ensure tight adhesion
small
-
boundary density
------+
large
3. Experimental Procedure 3.1. Structure of the Silicon Sensor
Figure 2 shows the sensing part of the developed silicon probe. The cell part consisted of a ZrO, tube closed at one end, which was stabilized
Fig. 3. Illustration of the coating forms of the auxiliary electrode with different boundary densities.
205 TABLE I. Experimental conditions
Fig. 4. Outline of silicon probe A, part for detecting e.m.f. and temperature; B, inner paper sleeve; C, splash preventor; D, guide ring; E, heat-resistant resin; F, copper protector and paper cap.
Type
A
B
C
D
Boundary density (mm - ‘)
0.3
0.7
1.0
1.3
Number of test samples
20
20
30
10
Temperature (K)
1573-1773 250
Metal (kg)
between the auxiliary electrode and the zirconia tube. The measuring chain consisted of the following elements: MolMo + MoO,IIZrO,(MgO)
Composition of hot metal (%)
Si
Mn
P
S
4.50
0.10-0.60
0.30
0.080
0.020
(ZrO,
+ ZrSiO, )ISi( in liq. Fe) 1Fe The outline of the silicon probe is given in Fig. 4. The sensor, free MO electrode and thermocouple are fixed with refractory cement. In addition, this cell part is protected by heat-resistant resin and a refractory tube. The refractory tube is composed of an inner paper sleeve and refractory ceramic, with the intention of giving high-temperature endurance and preventing splashing, which is apt to occur during the immersion. 3.2. Experimental Procedure The experimental apparatus and the experimental conditions are given in Fig. 5 and Table 1 respectively. 250 kg of Fe + 4.5 wt.%C alloy was charged and heated to a temperature ranging from 1573 K to 1773 K in the 300 kg induction furnace. After the metallic charge was molten, a PtPtRh13 thermocouple sheath was immersed in the molten Fe + 4.5 wt.%C alloy, and thereafter, temperature measurements were made by using this thermocouple unit. The silicon content of the melt
was changed from 0.1 to 0.6 wt.% by adding Fe-Si alloy. The other components, such as manganese, phosphorus and sulfur, were adjusted to the same level as in the ordinary hot metal in the production line. Measuring instruments such as a digital voltmeter or recorder were connected to a probeholder. The cell part at the top of the silicon probe was immersed in the molten iron in the 300 kg induction furnace. The immersion depth was about 300 mm, and the immersion time was 60 s. The experimental procedure consisted of measuring the open circuit e.m.f.s of the silicon probes and subsequently sampling the molten iron for analytical purposes. In order to clarify the effect of the auxiliary electrode, e.m.f. measurements were also conducted with conventional oxygen sensors. 4. Experimental Results The outer appearances of silicon sensors after being used for e.m.f. measurements are given in Fig. 6. Both sensors, of which the boundary densities were 0.3 and 1.Omm-‘, were immersed in hot metal at 1673 K.
(4 Fig. 5. Experimental apparatus.
C
09
Fig. 6. Outer appearances of silicon sensors after being used for e.m.f. measurements in molten iron at 1673 K. (a) Boundary density = 0.3 mm-‘. (b) Boundary density = l.Omn~-~.
206
In each sensor, the spots of the auxiliary electrode did not come off and they showed close adhesion to the outer surface of the zirconia tube. The zirconia tube also showed good stability and did not have any spalling or changes in quality. Figure 7 shows examples of typical e.m.f. results. Each sensor had a different boundary density: 0.3, 0.7 and l.Omn-‘. These three recordings were obtained during immersion in hot metal at 1773 K. They became stable within 8 to 30 s after immersion of the silicon probe and remained constant. As regards the stability of the e.m.f. curves, obvious e.m.f. plateaux were observed for all the boundary density values. On the other hand, the response performance of the e.m.f. curves depended closely on the boundary density. Figure 8 shows the relation between boundary density and response time at 1573 K and 1773 K. In the range of boundary density < 1.0, the response time became shorter with increasing boundary density, and in the range of boundary density > 1.0, the response time converged to a constant value of 8 s. Furthermore, the response A
boundary
1
10
20
Immersion
- CI
z
1773 K
460
II.
z w
420
_-
I
4 I’
/, ’
I I
0.1
1
0.2
1
0.5
I
0.7
t
1.0
C%sil Fig. 9. Relation between e.m.f. of silicon sensor and silicon content in molten iron at 1573 K to 1773 K.
time became shorter at higher temperature. The response time at 1773 K was 2 to 10 s shorter than that at 1573 K at the same boundary density. Figure 9 shows the relation between e.m.f. of the test sensors (boundary density = 1.Omm-‘) and silicon content in hot metal at 1573- 1773 K. A good correlation between e.m.f. and silicon content is seen. It may be said that the silicon sensor developed in this study is appropriate for the in situ measurement of dissolved silicon in molten iron. The e.m.f. values of conventional oxygen sensors are also plotted in Fig. 9, and are almost independent of the silicon content. It is evident that oxygen sensors are not applicable for measurement of the silicon content in molten iron.
density
I 0
sod
>
30
period
5. Discussion
( set)
Fig. 7. Examples of typical e.m.f. curves obtained during immersion of silicon sensors in molten iron at 1773 K.
5.1. Comparison with the Theoretical e.m.f. Values Considering the transport number of ions, the e.m.f. (open-circuit cell potential) is given by the Nemst equation [6]:
; 2
P,,( ref.) iI4 + Pe iI4 E = g In F Po,(test)-“4 + Pe’14+ Et
40
-
30
-
20
-
10
-
B ‘3 2 ;
-- r
L I...-‘. OO
. boundary
.d
..I...
0.5
1.0 density
1.5 (mm-‘)
Fig. 8. Relation between boundary density and response time at 1573 K and 1773 K.
(5)
where E is the e.m.f. of the cell, R is the gas constant, T is temperature, F is the Faraday constant PO,( ref.) is the oxygen partial pressure at the reference electrode and PO is the oxygen partial pressure when the ionic and n-type electronic conductivities are equal. When the equilibrium between silicon and oxygen in molten iron is obtained, RT PO,( ref.) ‘I4+ PO‘I4 E = 7 In [K( l)ha] - l/4 + POl/4 + E1
(6)
207
The thermochemical data required for the calculation of the theoretical e.m.f. values are as follows [7, 101: Si( 1) = Si( 1 wt.% in liquid iron)
(7)
AG$, = - 119.27 - 25.47T Si( 1) + O,(g) = SiO,(cristobalite)
(8)
AG& = -902.29 - 173.68T MO(S) + O,(g) = Moo,(s)
(9)
AG$, = -576.1 + 0.1692T PO = 4.185 exp( 24.42 - 74370/T)
(10)
Equation (6) was used for the calculation of the theoretical e.m.f. values. The relation between the theoretical e.m.f. values and Henrian activities of silicon in hot metal compared with the measured e.m.f. values is given in Fig. 10. The measured e.m.f. values at boundary density = 1.O mn- ’ are in good agreement with the theoretical values based on eqn. (6). This means that the e.m.f. is determined by the activity of silicon in the hot metal and the metal temperature, or to put it in another way, the activity of silicon in the hot metal could be estimated by the measurement of the e.m.f. and metal temperature. On the other hand, in the case of e.m.f. at boundary density = 0.3 mm-‘, there appeared to be poor correlation between measured values and theoretical values. [%SiI at 4.5wt% C 0.1
0.2
0.0
0.6 0.6 I.1.1
c
520
I
I,,,,,
1
1.0
5.2. Eflects of Auxiliary Electrode As mentioned above, from the reaction equilibrium between silicon and oxygen in hot metal represented by eqn. (2), the following equation is obtained: AG:,, - In asio* ln(hsie2) = RT
(11)
Namely, if asio, is constant, there is a proportional relation between ln(hsia2) and l/T. The measured values of the test sensors at boundary density = 1.0 mm-’ are plotted in Fig. 11 as ln(hsiao2) versus l/T. The theoretical relations at = 1.0, 0.6 and 0.1 are also shown as solid %iO, lines. It is evident that the data at boundary density = 1.0 mm-’ show good agreement with the theoretical equilibrium at aso2 = 0.6. In the case of boundary density=O.3 mm-‘, the measured values varied widely in the aso range 0.1 to 0.6. Considering that the response time became shorter with increasing boundary density, these results indicate that a kind of equilibrium in which the activity of Si02 is fixed is formed rapidly with the increase of boundary density and the rise of metal temperature at the hot metal/solid electrolyte/auxiliary electrode triphasic interface. Thus the auxiliary electrode performs its purpose of fixing asioz if the boundary density is not less than 1.0 mm-’ Figure 12 shows scanning electron micrographs of the auxiliary electrode used in this study. The auxiliary electrode consisted of bright coloured Zr02 grains in which Zr is detected by using an EDX detector, and dark coloured ZrSiO, grains, in which Zr and Si are detected. This indicates that the auxiliary electrode remains a mixture of
1111,111
500
a SiO,=O. 1 -
0.6
1.0
-theoretical
s 960
IL = w
060
440 2
A 420
400
1
1
0.5
n,,,,
I
1
2
I
-16
-
-17
-
-18
-
I,,,,,,
4
6
10
Fig. 10. Relation between the theoretical e.m.f. values and Hemian activities of silicon in hot metal compared with measured e.m.f. values at 1673 K.
5.5
6.0 l/T
Fig. 11. Relation between ln(I~~q,~) and l/T in hot metal.
208
bright
grain
Ur02 1
dark
grain
(ZrSiOsl
bright
dark
Fig. 12. Scanning electron micrographs
5.3. Accuracy of Measurement of Silicon The accuracy of estimation of the silicon content in hot metal obtained from the relation between the estimated silicon content and the analytical silicon content is given in Fig. 13. The measured values are in good agreement with the analytical values, and the standard deviation of 0.7
-
CSSil
--
mean
0.1
ana
value
0.2
Si] Cal
=[%
f
0.3
CS Sil
24
0.4
0.5
grain
of
ZrOz + ZrSiO, during and after immersion in hot metal. There appeared to be no changes in quality of the solid electrolyte.
0.6
grain
0.6
0.7
ana
Fig. 13. Accuracy of estimation of silicon content in hot metal.
the results is 0.015 wt.% Si. The dashed lines show the position of the mean value +2a (a = the standard deviation). For example, when the true value of the silicon content is 0.30 wt.%, the measuring error is approximately f 0.03 wt.%.
6. Conclusions
The authors have investigated a zirconia electrolyte with an auxiliary electrode for the rapid measurement of silicon content in molten iron. To study the effects of the auxiliary electrode, several kinds of silicon sensors with various boundary densities of ZrO,/ZrSiO, as the auxiliary electrode were tested in hot metal in a 300 kg induction furnace. It was confirmed that the coating form of the auxiliary electrode had a great influence on the response performance and the measured values of the silicon activities. The increase of boundary density caused a better response and improved the reliability of the measured value. It can be said that the auxiliary electrode performs its task of Iixing the activity of SiO, when the boundary density is not less than 1.0 mm-‘. The silicon content in molten iron can be estimated within $0.015 wt.% Si by measuring the e.m.f. of this silicon sensor and the metal temperature.
209
References 1 H. Nomiyama, H. Ichikawa, K. Marukawa, M. Anezaki and H. Ueki, Study of optimum silicon content between iron-making and steel-making process on the hot metal pretreatment, Tersu-to-Hugune, 69 (1983) 1738-1745. 2 N. Tsuchiya and H. Taguchi, Present situation of producing pig iron with low silicon content in blast furnace and operational problem to be solved, Teisu-to-Hagane, 69 (1983) 1945 1954. 3 K. Ichihara, D. Janke and H.-J. Engel, A new silicon sensor for hot metal measurements, Steel Res., 57 (1986) 166-171. 4 M. Iwase, H. Kitaguchi, E. Ichise, H. Nakamura, T. Moriya and S. Maruhashi, Rapid determination of silicon contents in hot metal, Tefsu-to-Hagane, 110 (1985) ~1595.
5 M. Iwase, Rapid determination of silicon activities in hot metal by means of solid state electrochemical sensors equipped with an auxiliary electrode, Stand. J. Metall., 17 (1988) 50-56. 6 H. Schmalzried, Z. Electrochem. Ber. Bunsenges, Phys. Chem., 66 (1962) 572-576. 7 G. K. Sigworth and J. F. Elliot, Met. Sci., 8 (1974) 298-310. 8 I. Rarin and 0. Knacke, Thermochemical Properties of Inorganic Substances, Stahleisen, Dilsseldorf/Springer, Berlin, 1973. 9 M. Iwase, M. Yasuda and T. Mot-i, Free energy of formation of MoO at steehnaking temperature from emf measurement, Elecfrochem. Acto, 24 (1979) 261-266. 10 W. A. Fischer and D. Janke, Metallurgische Electrochemie, Stahleisen, Dtisseldorf/Springer, Berlin, 1975.