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Fireclay refractories in pyrometallurgical processes
Applied Clay Science, 2 (1987) 187-192
187
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Fireclay Refractories in Pyrometallurgical Processes JOHN CAMERON
Metallurgical Engineering Department, Queen's University, Kingston, Ont. K7L 3N6 (Canada) (Accepted for publication February 11, 1987)
ABSTRACT
Cameron, J., 1987. Fireclay refractories in pyrometallurgical processes. Appl. Clay Sci., 2: 187-192. In primary liquid metal production, the temperature of processing and the composition of the slagphase largelydetermine the selectionof the refractoriesused to linethe furnace. The objective of this work was to study the effectof temperature and FeO content on the dissolution rate of a clay refractory. 50-g samples of synthetic, prefused iron oxide and silicawere poured into slipcast crucibles of a high-duty fireclay.These were then placed in a preheated silicon carbide-element furnace and held at the required temperatures for known times. The cooled cruciblesand their contents were broken and the amount of crucible wall thinning which had taken place at the slag-refractory interfacewas measured using a low-power microscope and a calibratedgraticule. The study of the effect of process temperature was carried out over the temperature range II00°-1400°C at 50°C intervals.Only two compositions, 65 and 75 wt.% FeO, were chosen for this part of the work. The effectof slag composition was next studied at 1300 °C, the range of slag compositions being 60-80 wt.% iron oxide at 5 % intervalsin the iron oxide-silicasystem. Only one high-duty fireclay composition was studied throughout the tests. The outcome of the work was to show that for the iron oxide-silicacompositions studied a highduty fireclayrefractorycan be used to contain the melts ifthe temperature of the process is maintained below 1300°C and when the slag composition is less than the orthosilicatecomposition (2FeO.SiO2).
INTRODUCTION
In the processing of liquid metals at elevated temperatures it is important to cover the metal with a protective and thermally insulating phase. This slag layer, as it is called, is also capable of acting as a depository for impurities that are removed from the liquid metal during refining. The slag is normally maintained in a molten condition and is primarily composed of a solution of a large number of oxide species. Some of these oxides are deliberately added to serve 0169-1317/87/$03.50
© 1987 Elsevier Science Publishers B.V.
188
as fluxes to maintain the slag in a fluid, molten condition. This helps achieve the subsequent clean separation of the metal from the impurities. These two liquid phases, the slag and the metal, are contained in a furnace or reactor which is itself lined with a higher-melting material - - usually an assemblage of relatively pure, high-melting-point oxides. These oxides are selected because they are capable of remaining in the solid state up to and beyond the operating temperature of the process. In order to have a useful life, however, the lining has to be able to withstand the corrosive effects of the liquids it is to contain. This paper will be limited to a discussion of the chemistry of dissolution that takes place at the liquid-slag/solid-refractory interface. The scope of the work will be limited to a discussion of' the results obtained from a series of experiments that were carried out using a high-duty fireclay refractory crucible composition to contain synthetic slags from the iron-oxide-silica system (Allen and Snow, 1955; Habashi, 1986 ). How these results fit in with industrial practice will feature in the discussion. The dissolution tests were done by pouring prefused iron-oxide-silica slags into high-duty fireclay crucibles (made by the slip-casting process (Kingery, 1958) ) and then placing them in a muffle furnace for a desired length of time at controlled temperatures. The dissolution rates were subsequently determined by breaking the cooled crucibles and measuring the remaining wall thickness using a graticule scale in a low-power microscope. The outcome of the work was to show that the effect of temperature and slag composition on the rate of dissolution of this particular refractory fireclay changes progressively and rapidly at temperatures above 1300 ° C and when the iron oxide content of the synthetic slags is increased from silica saturation-low iron oxide levels up to and beyond the orthosilicate composition (2FeO.SiO2) in the FeO-SiO~ system (Allen and Snow, 1955 ). RESULTS
Table I shows the measured dissolution rates of the clay refractory when it is used to contain two synthetic slags at temperatures between 1100 ° and 1400 ° C, runs being done at 50cC intervals. Slag A was of 65%FeO-35%SiO2, slag B 75%FeO-25%SiO2.50 g of slag was held at each of the indicated temperatures for 1/2 to 3 h to produce a measurable amount of thinning of the crucible wall. The results are normalised to mm/h and shown in Table I. Repeat experiments and multiple sampling of the test crucibles indicated that the dissolution rates could be assessed to _+6% on all but the most severely attacked runs at 1350°and 1400°C: Table II shows the effect of slag composition on dissolution rates at 1300 °C over a range of composition~ from 60 to 80% by weight of iron oxide at 5% intervals. The 80% FeO runs ~esulted in excessive refractory dissolution, with complete slag penetration of the crucible wall taking place before the end of
189
TABLE I Measured dissolution rate of clay refractory in range 1100 °-1400 °C
Slag A (65%FeO,35% SiO2)
Slag B (75% FeO,35% Si02)
Temperature ( ° C)
Temperature ( °C )
Measured dissolution rate
( ___6%,mm/h) 1100 1150 1200 1250 1300 1350 1400
0.038 0.038 0.046 0.038 0.061 0.37 >0.80
Measured dissolution rate
( _+6%, mm/h) 1100 1150 1200 1250 1300 1350 1400
0.053 0.061 0.065 0.107 0.251 >0.80 >0.80
the short-time tests of 30 min duration. These run-outs looked similar to the outcome of the tests done at high temperatures for slags A and B and as reported in Table I. It was difficult to assign a single value to the dissolution rate for such runs as the variation of thickness of the considerable quantity of material removed was not constant along the slag-crucible interface area. This was possibly due to the large temperature gradients within the furnace that resulted upon charging the samples. This allowed portions of the clay crucible wall to heat up in advance of the remainder of the crucible, causing early initiation of the "runaway" slag corrosion at these hotter positions. DISCUSSION
The most prevalent reactive species in base metal processing is undoubtedly ferrous iron oxide (FeO). This oxide comes, for example, from the oxidation of sulphide mattes in both copper and nickel production and from the oxidaT A B L E II
Measured dissolution rate of clay refractoryby FeO-SiO2 liquidslags at 1300 °C Nominal slag composition (wt.% FeO )
Measured dissolutionrate ram/h, + 6 %
60.0 65.0 70.0 75.0 80.0
0.0251 0.0251 0.091 0.251 > 0.800
190
Concentration of Reacting Specie
I C Co
AI
~,
Distance from the refractory-slag interface
Fig. 1. Concentrationprofile within the reactingslag phase at the refractory-slaginterface. tion of some of the liquid iron phase during the oxidation steps of steelmaking. It is the possible reaction of the FeO-rich slags with the chosen refractories that has to be given careful consideration during the operation of these and other base-metal processes. When the refractory is clay-based, it is made up of a physical mixture of silica (SiO2) and mullite (3A1203.2SIO2) as the principal phase assemblages (Aramaki and Roy, 1959). The ability of FeO-rich liquids to react with silica is the mechanism that is responsible for the rapid dissolution of fireclays at elevated temperatures. The more resistant mullite phase, present as part of the eutectic phase, is usually lost by entrainment to the slag phase, where it is eventually dissolved as it is transported around in the liquid phase. FeO and SiO2 form low-melting eutectics at ~ 1180°C. When the slag composition is lean in silica and rich in FeO it is chemically reactive towards the silica when it contacts the refractory. This causes progressive dissolution. Slag A has an FeO activity of only 0.43 compared to the slag B value of 0.66 on the Raoultian scale (Schuhmann and Ensio, 1951; Bodsworth, 1959). Thermodynamically the dissolution reaction will continue in an effort for the system to establish a slag composition which has the same activity as the refractory, i.e. it attains silica saturation. As the mass of slag in contact with the furnace wall is usually less than the mass of refractory available at the slagline, only a limited chemical dissolution may take place even after a significant period has lapsed. This is due to a local saturation effect set up by the chemical boundary layer (see Fig. 1). Equalization of this concentration gradient is diffusion-controlled and this, in turn, is controlled by the temperature of the slag phase. In practice, only a limited amount of chemical dissolution may take place, but a continuous removal of refractory will inevitably result when processing ferrous iron oxide-rich slags in silica-containing refractories. At temperatures below ~ 1300 ° C and as shown in Fig. 2, the rate of disso-
191 8.C
D
t
7.C
/ F
6.0 5.{3 Dissolution Rate ~hr xlO
SLAG B
%
4.0
SLAG A
3.0
/ 1.0
ii00
1200
1300
14do
15i
Temperature in °C Fig. 2. T h e effect of t e m p e r a t u r e on dissolution rates for two slags chosen from range shown in Fig. 3.
lution of fireclay by such slags is relatively low. At higher temperatures, however, the rate of dissolution becomes increasingly more rapid. In these tests, a 50 ° C temperature increase above 1300 ° C, for example, increases the rate of dissolution by a factor of 6 to 10. Similarly, at lower FeO levels of slag composition, Fig. 3 shows that the dissolution rates for the fireclay are increased markedly when the FeO level increases above the orthosilicate composition ( 2FeO.SiO2 which is ~ 70% FeO by weight). These apparent reaction rates indicate that the slag composition has to be maintained at the low range of FeO levels within the composition range that is fully liquid at 1300 °C if the refractory is to have a useful working lifetime. During the pyrometallurgical processing of many of the base metals, where iron oxide is being continuously added to the slag phase, (by the on-going selective oxidation of iron or iron sulphide) silica grains are often added at the hotter regions of the furnace in order to neutralize the FeO that is joining the slag and to thus render the slag of low iron oxide percentage before it makes contact with the furnace refractory wall (Pehlke, 1977). This is an effective measure but it requires careful control of the amount and composition of all materials being charged to the furnace. It also increases the slag volume of the
192 8.0
I
7.0 T - 13OO°c
I
I
6.0
r T
5.0 Dissolution Rate ~ / h r xlO
i
4.0
3.0
2.0
1.0
oJ 60 Weight percentage Fe0 Fig. 3. Dissolution rate versus F e O level in slags from FeO-SiO2 system at 1300 °C.
process and this may be an unattractive feature of the process that is also thermally inefficient. CONCLUSION
In conclusion, therefore, fireclays may be used to contain FeO-rich slags to temperatures up to ~ 1300 °C and at substantial levels of FeO, up to the orthosilicate composition, before they are dissolved in a "runaway" fashion. When higher temperatures and higher FeO levels are to be processed, other refractory systems should be chosen to line the furnace.
REFERENCES Allen, W. C. and Snow, R. B., 1955. The FeO-SiOe Phase Diagram. J. Am. Ceram. Soc., 38 (8): 268. Aramaki and Roy, 1959. Equilibrium diagram of the system alumina silica. Nature, August 1959. Bodsworth, C., 1959. In: J. Iron Steel Inst., 193: 13. Habashi, F., 1986. Principles of Extractive Metallurgy, Vol. 3, Pyrometallurgy. Gordon and Breach Science Publishers. Kingery, W. D. (Editor), 1958. Ceramic Fabrication Processes. Wiley, New York, N.Y. Pehlke, R. D., 1977. Unit Process of Extractive Metallurgy. Elsevier, Amsterdam, 3rd ed. Schuhmann, R. and Ensio, P. J., 1951. Activities in the FeO-SiO~ system. Trans. Am. Inst. Min. Metall., 191: 401.
187
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Fireclay Refractories in Pyrometallurgical Processes JOHN CAMERON
Metallurgical Engineering Department, Queen's University, Kingston, Ont. K7L 3N6 (Canada) (Accepted for publication February 11, 1987)
ABSTRACT
Cameron, J., 1987. Fireclay refractories in pyrometallurgical processes. Appl. Clay Sci., 2: 187-192. In primary liquid metal production, the temperature of processing and the composition of the slagphase largelydetermine the selectionof the refractoriesused to linethe furnace. The objective of this work was to study the effectof temperature and FeO content on the dissolution rate of a clay refractory. 50-g samples of synthetic, prefused iron oxide and silicawere poured into slipcast crucibles of a high-duty fireclay.These were then placed in a preheated silicon carbide-element furnace and held at the required temperatures for known times. The cooled cruciblesand their contents were broken and the amount of crucible wall thinning which had taken place at the slag-refractory interfacewas measured using a low-power microscope and a calibratedgraticule. The study of the effect of process temperature was carried out over the temperature range II00°-1400°C at 50°C intervals.Only two compositions, 65 and 75 wt.% FeO, were chosen for this part of the work. The effectof slag composition was next studied at 1300 °C, the range of slag compositions being 60-80 wt.% iron oxide at 5 % intervalsin the iron oxide-silicasystem. Only one high-duty fireclay composition was studied throughout the tests. The outcome of the work was to show that for the iron oxide-silicacompositions studied a highduty fireclayrefractorycan be used to contain the melts ifthe temperature of the process is maintained below 1300°C and when the slag composition is less than the orthosilicatecomposition (2FeO.SiO2).
INTRODUCTION
In the processing of liquid metals at elevated temperatures it is important to cover the metal with a protective and thermally insulating phase. This slag layer, as it is called, is also capable of acting as a depository for impurities that are removed from the liquid metal during refining. The slag is normally maintained in a molten condition and is primarily composed of a solution of a large number of oxide species. Some of these oxides are deliberately added to serve 0169-1317/87/$03.50
© 1987 Elsevier Science Publishers B.V.
188
as fluxes to maintain the slag in a fluid, molten condition. This helps achieve the subsequent clean separation of the metal from the impurities. These two liquid phases, the slag and the metal, are contained in a furnace or reactor which is itself lined with a higher-melting material - - usually an assemblage of relatively pure, high-melting-point oxides. These oxides are selected because they are capable of remaining in the solid state up to and beyond the operating temperature of the process. In order to have a useful life, however, the lining has to be able to withstand the corrosive effects of the liquids it is to contain. This paper will be limited to a discussion of the chemistry of dissolution that takes place at the liquid-slag/solid-refractory interface. The scope of the work will be limited to a discussion of' the results obtained from a series of experiments that were carried out using a high-duty fireclay refractory crucible composition to contain synthetic slags from the iron-oxide-silica system (Allen and Snow, 1955; Habashi, 1986 ). How these results fit in with industrial practice will feature in the discussion. The dissolution tests were done by pouring prefused iron-oxide-silica slags into high-duty fireclay crucibles (made by the slip-casting process (Kingery, 1958) ) and then placing them in a muffle furnace for a desired length of time at controlled temperatures. The dissolution rates were subsequently determined by breaking the cooled crucibles and measuring the remaining wall thickness using a graticule scale in a low-power microscope. The outcome of the work was to show that the effect of temperature and slag composition on the rate of dissolution of this particular refractory fireclay changes progressively and rapidly at temperatures above 1300 ° C and when the iron oxide content of the synthetic slags is increased from silica saturation-low iron oxide levels up to and beyond the orthosilicate composition (2FeO.SiO2) in the FeO-SiO~ system (Allen and Snow, 1955 ). RESULTS
Table I shows the measured dissolution rates of the clay refractory when it is used to contain two synthetic slags at temperatures between 1100 ° and 1400 ° C, runs being done at 50cC intervals. Slag A was of 65%FeO-35%SiO2, slag B 75%FeO-25%SiO2.50 g of slag was held at each of the indicated temperatures for 1/2 to 3 h to produce a measurable amount of thinning of the crucible wall. The results are normalised to mm/h and shown in Table I. Repeat experiments and multiple sampling of the test crucibles indicated that the dissolution rates could be assessed to _+6% on all but the most severely attacked runs at 1350°and 1400°C: Table II shows the effect of slag composition on dissolution rates at 1300 °C over a range of composition~ from 60 to 80% by weight of iron oxide at 5% intervals. The 80% FeO runs ~esulted in excessive refractory dissolution, with complete slag penetration of the crucible wall taking place before the end of
189
TABLE I Measured dissolution rate of clay refractory in range 1100 °-1400 °C
Slag A (65%FeO,35% SiO2)
Slag B (75% FeO,35% Si02)
Temperature ( ° C)
Temperature ( °C )
Measured dissolution rate
( ___6%,mm/h) 1100 1150 1200 1250 1300 1350 1400
0.038 0.038 0.046 0.038 0.061 0.37 >0.80
Measured dissolution rate
( _+6%, mm/h) 1100 1150 1200 1250 1300 1350 1400
0.053 0.061 0.065 0.107 0.251 >0.80 >0.80
the short-time tests of 30 min duration. These run-outs looked similar to the outcome of the tests done at high temperatures for slags A and B and as reported in Table I. It was difficult to assign a single value to the dissolution rate for such runs as the variation of thickness of the considerable quantity of material removed was not constant along the slag-crucible interface area. This was possibly due to the large temperature gradients within the furnace that resulted upon charging the samples. This allowed portions of the clay crucible wall to heat up in advance of the remainder of the crucible, causing early initiation of the "runaway" slag corrosion at these hotter positions. DISCUSSION
The most prevalent reactive species in base metal processing is undoubtedly ferrous iron oxide (FeO). This oxide comes, for example, from the oxidation of sulphide mattes in both copper and nickel production and from the oxidaT A B L E II
Measured dissolution rate of clay refractoryby FeO-SiO2 liquidslags at 1300 °C Nominal slag composition (wt.% FeO )
Measured dissolutionrate ram/h, + 6 %
60.0 65.0 70.0 75.0 80.0
0.0251 0.0251 0.091 0.251 > 0.800
190
Concentration of Reacting Specie
I C Co
AI
~,
Distance from the refractory-slag interface
Fig. 1. Concentrationprofile within the reactingslag phase at the refractory-slaginterface. tion of some of the liquid iron phase during the oxidation steps of steelmaking. It is the possible reaction of the FeO-rich slags with the chosen refractories that has to be given careful consideration during the operation of these and other base-metal processes. When the refractory is clay-based, it is made up of a physical mixture of silica (SiO2) and mullite (3A1203.2SIO2) as the principal phase assemblages (Aramaki and Roy, 1959). The ability of FeO-rich liquids to react with silica is the mechanism that is responsible for the rapid dissolution of fireclays at elevated temperatures. The more resistant mullite phase, present as part of the eutectic phase, is usually lost by entrainment to the slag phase, where it is eventually dissolved as it is transported around in the liquid phase. FeO and SiO2 form low-melting eutectics at ~ 1180°C. When the slag composition is lean in silica and rich in FeO it is chemically reactive towards the silica when it contacts the refractory. This causes progressive dissolution. Slag A has an FeO activity of only 0.43 compared to the slag B value of 0.66 on the Raoultian scale (Schuhmann and Ensio, 1951; Bodsworth, 1959). Thermodynamically the dissolution reaction will continue in an effort for the system to establish a slag composition which has the same activity as the refractory, i.e. it attains silica saturation. As the mass of slag in contact with the furnace wall is usually less than the mass of refractory available at the slagline, only a limited chemical dissolution may take place even after a significant period has lapsed. This is due to a local saturation effect set up by the chemical boundary layer (see Fig. 1). Equalization of this concentration gradient is diffusion-controlled and this, in turn, is controlled by the temperature of the slag phase. In practice, only a limited amount of chemical dissolution may take place, but a continuous removal of refractory will inevitably result when processing ferrous iron oxide-rich slags in silica-containing refractories. At temperatures below ~ 1300 ° C and as shown in Fig. 2, the rate of disso-
191 8.C
D
t
7.C
/ F
6.0 5.{3 Dissolution Rate ~hr xlO
SLAG B
%
4.0
SLAG A
3.0
/ 1.0
ii00
1200
1300
14do
15i
Temperature in °C Fig. 2. T h e effect of t e m p e r a t u r e on dissolution rates for two slags chosen from range shown in Fig. 3.
lution of fireclay by such slags is relatively low. At higher temperatures, however, the rate of dissolution becomes increasingly more rapid. In these tests, a 50 ° C temperature increase above 1300 ° C, for example, increases the rate of dissolution by a factor of 6 to 10. Similarly, at lower FeO levels of slag composition, Fig. 3 shows that the dissolution rates for the fireclay are increased markedly when the FeO level increases above the orthosilicate composition ( 2FeO.SiO2 which is ~ 70% FeO by weight). These apparent reaction rates indicate that the slag composition has to be maintained at the low range of FeO levels within the composition range that is fully liquid at 1300 °C if the refractory is to have a useful working lifetime. During the pyrometallurgical processing of many of the base metals, where iron oxide is being continuously added to the slag phase, (by the on-going selective oxidation of iron or iron sulphide) silica grains are often added at the hotter regions of the furnace in order to neutralize the FeO that is joining the slag and to thus render the slag of low iron oxide percentage before it makes contact with the furnace refractory wall (Pehlke, 1977). This is an effective measure but it requires careful control of the amount and composition of all materials being charged to the furnace. It also increases the slag volume of the
192 8.0
I
7.0 T - 13OO°c
I
I
6.0
r T
5.0 Dissolution Rate ~ / h r xlO
i
4.0
3.0
2.0
1.0
oJ 60 Weight percentage Fe0 Fig. 3. Dissolution rate versus F e O level in slags from FeO-SiO2 system at 1300 °C.
process and this may be an unattractive feature of the process that is also thermally inefficient. CONCLUSION
In conclusion, therefore, fireclays may be used to contain FeO-rich slags to temperatures up to ~ 1300 °C and at substantial levels of FeO, up to the orthosilicate composition, before they are dissolved in a "runaway" fashion. When higher temperatures and higher FeO levels are to be processed, other refractory systems should be chosen to line the furnace.
REFERENCES Allen, W. C. and Snow, R. B., 1955. The FeO-SiOe Phase Diagram. J. Am. Ceram. Soc., 38 (8): 268. Aramaki and Roy, 1959. Equilibrium diagram of the system alumina silica. Nature, August 1959. Bodsworth, C., 1959. In: J. Iron Steel Inst., 193: 13. Habashi, F., 1986. Principles of Extractive Metallurgy, Vol. 3, Pyrometallurgy. Gordon and Breach Science Publishers. Kingery, W. D. (Editor), 1958. Ceramic Fabrication Processes. Wiley, New York, N.Y. Pehlke, R. D., 1977. Unit Process of Extractive Metallurgy. Elsevier, Amsterdam, 3rd ed. Schuhmann, R. and Ensio, P. J., 1951. Activities in the FeO-SiO~ system. Trans. Am. Inst. Min. Metall., 191: 401.