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Development of an electrochemical sensor for determination of Si contents in molten metal
Solid State lonics 40/41 (1990) 776-778 North-Holland
D E V E L O P M E N T OF A N E L E C T R O C H E M I C A L S E N S O R FOR D E T E R M I N A T I O N OF Si C O N T E N T S IN M O L T E N M E T A L Chikayoshi F U R U T A , Toshio N A G A T S U K A Osaka Sanso Kogyo Ltd., 28-23 Onidaka, 3-Chome, lchikawa, Chiba, Osaka, Japan Katsuhiro IWASAKI, Norio SA1TO NKK ('orporation, 1 Kokan-Cho, Fukuyama, Hiroshima, Japan and M i n o r u SASABE Department of Metallurgy, Chiba Institute of Technology, Narashmo, Chiba, Japan
Principle of electrochemicalsilicon sensor incorporating zirconia eleclrolyteand an auxiliary electrode as wellas its application to Si contents determination of blast furnace hot metal and ferrochromium are described.
1. Introduction
(~) Mo
(~ ~) (~ ~) ® (~) (~
In the external desiliconization processes, which have now been accepted at the major integrated steel works in Japan, rapid d e t e r m i n a t i o n s of silicon contents of blast furnace hot metal are highly required. The present paper d o c u m e n t s the d e v e l o p m e n t of electrochemical silicon sensor incorporating zirconia electrolyte and an auxiliary electrode [1] of Si02 + CaF2 mixture.
Cr.Cr203 ZrO2(MgO) coating Molten metal Mo Quartz tube Thermocoupte
Fig. I. Constitution of the silicon sensor.
2. Experimental aspects log K( 1 ) = log asioz Fig. 1. illustrates schematically the silicon sensor; the cell design is essentially the same as that applied in conventional oxygen probes, except for that the entire surface of zirconia tube is covered by a thin layer of the auxiliary electrode. When the silicon sensor was immersed in molten metal, e q u i l i b r i u m between auxiliary electrode and molten metal can be attained: Si + 2 0 = SiO2, 0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
( 1)
-
-
log as, - 2 log a o .
(2)
From eq. (2), it is evident that so far as the activity of SiO2 is fixed at a given temperature, a measure of oxygen activity, ao, corresponds to a knowledge of silicon activity, a s , in liquid metal. At the initial stages of the present study, the auxiliary electrode consisted of Si02 only. With such a cell design, however, stable EMFs were not obtainable. This would be attributed to the sufficient adhesion could not be obtained between the thin layer of
Ch. Furuta et al. / Development of an electrochemical sensor
1 shows micrographs of the auxiliary electrode of 15 wt% CaF2 before and after EMF measurements. Although relatively dense layer of SiO2 + CaF2 mixture was observed before the measurements, a number of blow holes were detected after the measurements. It was suspected that such blow holes would be created during the solidification of liquid phases within the auxiliary electrode. Based on the results shown in fig. 2, an empirical equation was derived:
EMF= log [%Si ] x b * o CaFz wt'l, "-.. • •
> E
1o
uJ
0
•
--.--
•
--•--15
o t . : . ~ .~e
Q
b
r
10 -37.180 -76.97
0.884
J-t.2.693 -89.169 0.934
I
0"
n
0.032
26
O.O'LZ
........... 20 -L,3.76O -90.435 o.7c~ o.o44
29
26
"~4.\
-10
E = - 0 . 0 0 5 3 log (% Si) +0.00088 T - 0 . 9 7 ,
-"'•2
0
~
-
-
~'l, Si]
Fig. 2. Relationship between electromotiveforceand analyzedSi content in molten pig iron. pure SiO2 and the zirconia electrolyte. Hence, for the subsequent stages, a ratio of CaF2 to SiO2 of from 10 to 30 wt% was examined. With such a mixture, better adhesion to the zirconia tube was expected because the auxiliary electrode would form liquid phase to a small extent. Then, mixture of SiO2+CaF2 at ratios of 10, 15 and 20 wt% were tested in hot metal. The more CaF2 content, the shorter response time. Fig. 2 shows the relation between cell potentials and silicon contents as determined by emission spectroscopy, at temperatures between 1653 K and 1793 K. During these measurements, best accuracy could be obtained with the mixture of 15 wt% CaF2. Photo
(3)
where E is cell potential (mV), (% Si) is silicon concentration in wt% and T is temperature (K). By using this equation in conjunction with EMF measurements, silicon content in hot metal could be determined with a standard deviation of + 0.018% at the silicon concentrations between 0.2 and 0.4 wt%.
3. Industrial applications The silicon sensor was subsequently submitted to in-plant tests. Measurements were conducted at blast furnace hot metal runner, at which the external removal of silicon from hot metal has been conducted. During such in-plant tests it was confirmed that the silicon sensor could detect the silicon contents within 15 s after immersion. Because of such a rapid re-
Coating layer
Solid. electroty
a
777
0.1 mm Photo 1. Microstructure of the solid electrolyteand the coatinglayer.
778
Ch. Furuta et al. / Development o f an electrochemical sensor 100
30
90
•
25
Introduction of the S [ L I C O N S E N S O R
8O
20
70
15 .~
(3_
u u
E ~0 ~
m 60
o
50 /40
l
I
I
1
2
3
i 4
5
i
t
6
7
t
i
8
0
110 ltl
9
'87
Fig. 3. Change o f success rate o f desiliconization and c o n s u m p tion o f desiliconization agent.
]
l e: < 1600"C
//
• ,: > 1600 "C /
4
//
r~ L/%
/
A/
3 o/
/
/
/
/
/ /
/
/
/
/
/ /
An electrochemical sensor incorporating zirconia electrolyte to determine silicon content in molten metal was developed. The sensor is successfully applied to determine silicon content of blast furnace hot metal as well as molten ferrochromium.
/
/
//
e.,'~A / /
/
/
&rill= / ,,%."
2
4. Conclusion
///0// / / /
/ / / 1 ~&
E
sponse time, close control o f the amounts o f desiliconization agents has now become possible. This resuited in a decrease in the consumption of desiliconization agent as shown in fig. 3, which shows the monthly averages for the consumption of desiliconization agents and the success ratio. Additional advantage caused by silicon sensor was accurate regulation of silicon content of hot metal for steel making. The silicon sensor has also been applied in a production of ferrochromium. In such processes silicon contents in metal phase would give a guideline for the removal of sulfur from ferrochromium and appropriate operations for electric arc furnaces. Fig. 4 shows the relation between silicon concentrations in high carbon ferrochromium as determined by EMF measurements and those by conventional chemical analysis made for solidified samples.
O'n-~ = 0.138 /
t
I 3
2 QnQ.
I 4
E °/o Si ]
Fig. 4. Relationship between measured Si content and analyzed Si content in molten pig iron.
Reference
[ 1 ] M. lwase, Scan. J. Metall. 17 ( 1988 ) 50.
D E V E L O P M E N T OF A N E L E C T R O C H E M I C A L S E N S O R FOR D E T E R M I N A T I O N OF Si C O N T E N T S IN M O L T E N M E T A L Chikayoshi F U R U T A , Toshio N A G A T S U K A Osaka Sanso Kogyo Ltd., 28-23 Onidaka, 3-Chome, lchikawa, Chiba, Osaka, Japan Katsuhiro IWASAKI, Norio SA1TO NKK ('orporation, 1 Kokan-Cho, Fukuyama, Hiroshima, Japan and M i n o r u SASABE Department of Metallurgy, Chiba Institute of Technology, Narashmo, Chiba, Japan
Principle of electrochemicalsilicon sensor incorporating zirconia eleclrolyteand an auxiliary electrode as wellas its application to Si contents determination of blast furnace hot metal and ferrochromium are described.
1. Introduction
(~) Mo
(~ ~) (~ ~) ® (~) (~
In the external desiliconization processes, which have now been accepted at the major integrated steel works in Japan, rapid d e t e r m i n a t i o n s of silicon contents of blast furnace hot metal are highly required. The present paper d o c u m e n t s the d e v e l o p m e n t of electrochemical silicon sensor incorporating zirconia electrolyte and an auxiliary electrode [1] of Si02 + CaF2 mixture.
Cr.Cr203 ZrO2(MgO) coating Molten metal Mo Quartz tube Thermocoupte
Fig. I. Constitution of the silicon sensor.
2. Experimental aspects log K( 1 ) = log asioz Fig. 1. illustrates schematically the silicon sensor; the cell design is essentially the same as that applied in conventional oxygen probes, except for that the entire surface of zirconia tube is covered by a thin layer of the auxiliary electrode. When the silicon sensor was immersed in molten metal, e q u i l i b r i u m between auxiliary electrode and molten metal can be attained: Si + 2 0 = SiO2, 0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
( 1)
-
-
log as, - 2 log a o .
(2)
From eq. (2), it is evident that so far as the activity of SiO2 is fixed at a given temperature, a measure of oxygen activity, ao, corresponds to a knowledge of silicon activity, a s , in liquid metal. At the initial stages of the present study, the auxiliary electrode consisted of Si02 only. With such a cell design, however, stable EMFs were not obtainable. This would be attributed to the sufficient adhesion could not be obtained between the thin layer of
Ch. Furuta et al. / Development of an electrochemical sensor
1 shows micrographs of the auxiliary electrode of 15 wt% CaF2 before and after EMF measurements. Although relatively dense layer of SiO2 + CaF2 mixture was observed before the measurements, a number of blow holes were detected after the measurements. It was suspected that such blow holes would be created during the solidification of liquid phases within the auxiliary electrode. Based on the results shown in fig. 2, an empirical equation was derived:
EMF= log [%Si ] x b * o CaFz wt'l, "-.. • •
> E
1o
uJ
0
•
--.--
•
--•--15
o t . : . ~ .~e
Q
b
r
10 -37.180 -76.97
0.884
J-t.2.693 -89.169 0.934
I
0"
n
0.032
26
O.O'LZ
........... 20 -L,3.76O -90.435 o.7c~ o.o44
29
26
"~4.\
-10
E = - 0 . 0 0 5 3 log (% Si) +0.00088 T - 0 . 9 7 ,
-"'•2
0
~
-
-
~'l, Si]
Fig. 2. Relationship between electromotiveforceand analyzedSi content in molten pig iron. pure SiO2 and the zirconia electrolyte. Hence, for the subsequent stages, a ratio of CaF2 to SiO2 of from 10 to 30 wt% was examined. With such a mixture, better adhesion to the zirconia tube was expected because the auxiliary electrode would form liquid phase to a small extent. Then, mixture of SiO2+CaF2 at ratios of 10, 15 and 20 wt% were tested in hot metal. The more CaF2 content, the shorter response time. Fig. 2 shows the relation between cell potentials and silicon contents as determined by emission spectroscopy, at temperatures between 1653 K and 1793 K. During these measurements, best accuracy could be obtained with the mixture of 15 wt% CaF2. Photo
(3)
where E is cell potential (mV), (% Si) is silicon concentration in wt% and T is temperature (K). By using this equation in conjunction with EMF measurements, silicon content in hot metal could be determined with a standard deviation of + 0.018% at the silicon concentrations between 0.2 and 0.4 wt%.
3. Industrial applications The silicon sensor was subsequently submitted to in-plant tests. Measurements were conducted at blast furnace hot metal runner, at which the external removal of silicon from hot metal has been conducted. During such in-plant tests it was confirmed that the silicon sensor could detect the silicon contents within 15 s after immersion. Because of such a rapid re-
Coating layer
Solid. electroty
a
777
0.1 mm Photo 1. Microstructure of the solid electrolyteand the coatinglayer.
778
Ch. Furuta et al. / Development o f an electrochemical sensor 100
30
90
•
25
Introduction of the S [ L I C O N S E N S O R
8O
20
70
15 .~
(3_
u u
E ~0 ~
m 60
o
50 /40
l
I
I
1
2
3
i 4
5
i
t
6
7
t
i
8
0
110 ltl
9
'87
Fig. 3. Change o f success rate o f desiliconization and c o n s u m p tion o f desiliconization agent.
]
l e: < 1600"C
//
• ,: > 1600 "C /
4
//
r~ L/%
/
A/
3 o/
/
/
/
/
/ /
/
/
/
/
/ /
An electrochemical sensor incorporating zirconia electrolyte to determine silicon content in molten metal was developed. The sensor is successfully applied to determine silicon content of blast furnace hot metal as well as molten ferrochromium.
/
/
//
e.,'~A / /
/
/
&rill= / ,,%."
2
4. Conclusion
///0// / / /
/ / / 1 ~&
E
sponse time, close control o f the amounts o f desiliconization agents has now become possible. This resuited in a decrease in the consumption of desiliconization agent as shown in fig. 3, which shows the monthly averages for the consumption of desiliconization agents and the success ratio. Additional advantage caused by silicon sensor was accurate regulation of silicon content of hot metal for steel making. The silicon sensor has also been applied in a production of ferrochromium. In such processes silicon contents in metal phase would give a guideline for the removal of sulfur from ferrochromium and appropriate operations for electric arc furnaces. Fig. 4 shows the relation between silicon concentrations in high carbon ferrochromium as determined by EMF measurements and those by conventional chemical analysis made for solidified samples.
O'n-~ = 0.138 /
t
I 3
2 QnQ.
I 4
E °/o Si ]
Fig. 4. Relationship between measured Si content and analyzed Si content in molten pig iron.
Reference
[ 1 ] M. lwase, Scan. J. Metall. 17 ( 1988 ) 50.