Furnace and a Method for Cooling a Furnace

Abstract

A furnace for conducting a high temperature process under oxidising conditions comprises an outer shell made from a metal, one or more cooling channels formed on or joined to the outer shell and a furnace lining. The furnace lining comprises a backing lining comprising a relatively high thermal conductivity layer positioned adjacent to an inner wall of the outer shell and a working lining positioned inwardly of the layer of relatively high thermal conductivity. The backing lining can comprise a graphite lining or a graphite-containing lining. The rate of heat transfer through the backing lining is sufficiently high to form a protective freeze on the backing lining in the event that the working lining wears away.

Description

FIELD OF THE INVENTION

The present invention relates to a furnace and to a method for cooling a furnace. More particularly, the furnace of the present invention is a furnace in which a high temperature process is conducted under oxidising conditions.

BACKGROUND TO THE INVENTION

Top submerged lance type furnaces are known. An example of a top submerged lance type furnace is a furnace available from Xstrata Technology Pty Limited under the trademark ISASMELT™. FIG. 1 shows a schematic diagram of such a furnace. The furnace 10 shown in FIG. 1 includes a barrel section 12 and an offgas section 14. A bath of molten material 15 is held inside the furnace and a lance 16 is lowered into the bath of material 15 such that the tip of the lance 16 is immersed in the bath 15. Air or oxygen and a fuel, such as fuel oil or coal or coke, is injected through the lance. The fuel is combusted to heat the furnace. These furnaces are used in processes such as copper converting, lead smelting and the like. Such processes are operated under high temperature and under oxidising conditions due to the injection of air or oxygen through the lance into the furnace.

Top submerged lance type furnaces are typically constructed such that they have an outer steel shell with an inside lining of refractory material. The refractory material protects the outer steel shell from the extremely high temperatures experienced inside the furnace. The, inside lining of refractory material, is sometimes divided into an inner and an outer layer. The inner layer is sometimes referred to as the working lining and the outer layer is sometimes referred to as the backing lining. The backing lining comprises of a much more insulating refractory composition compared to the working lining. Throughout this specification, the term “working lining” will be used to refer to the part of the lining that is adjacent the hot contents of the furnace and the term “backing lining” will be used to refer to the part of the lining that is adjacent the outer shell of the furnace.

In a number of furnaces, efforts have been made to cool the outer steel shell (and thereby increase the rate of heat removal from the furnace). Systems that have been used for external shell cooling comprise spray cooling or film cooling systems. In these systems, water is sprayed onto or runs down the external face of the outer steel shell. The water extracts heat from the outer steel shell, thereby cooling the outer steel shell. However, due to the system being exposed to the atmosphere, combined with the relatively high shell and water temperatures, extensive corrosion of the outer steel shell can occur. Regular cleaning and maintenance of the surface of the outer steel shell is required to prevent the insulating corrosion layer that would otherwise form on the outer steel shell from inhibiting the heat transfer from the shell to the cooling water. Even with a clean outer shell surface, the heat transfer coefficient between the shell and the cooling water is relatively low due to the use of low water velocities and pressures.

External shell-mounted forced cooling water systems have been used on various types of furnaces. The external shell-mounted forced cooling water systems typically comprise steel channels welded to or formed on the external surface of the outer steel shell (or furnace steel shell), enabling the flow of water against the furnace steel shell under relatively high pressures and velocities, ensuring a high heat transfer coefficient between the water and the shell. This results in the effective removal of heat from the furnace shell whilst preventing contact between the water, the cooled surface, and the atmosphere. Furthermore, the quality of the water that has passed through the cooling channels can be controlled to prevent or minimise corrosion of the furnace steel shell. As a further safety advantage, as the cooling water channels are mounted or formed externally to the outer steel shell, any leaks that may occur in the cooling water channels result in water running down the outer face of the outer shell. In this regard, it will be understood that it is important that any water leaks not cause water to leak into the interior of the furnace as this could potentially cause the furnace to explode due to the rapid generation of steam from such water leaks.

Throughout the specification, the term “comprising” and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a furnace and a method for cooling a furnace that is appropriate for use in furnaces in which oxidising conditions are encountered within the furnace.

In a first aspect, the present invention provides a furnace in which a high temperature process is conducted under oxidising conditions in the furnace, the furnace comprising an outer shell made from a metal, one or more cooling channels formed on or joined to the outer shell and a furnace lining, the furnace lining comprising a backing lining comprising a relatively high thermal conductivity layer positioned adjacent to an inner wall of the outer shell and a working lining positioned inwardly of the layer of relatively high thermal conductivity.

Throughout this specification, a furnace is to be taken to be operating under oxidising conditions if the partial pressure of oxygen in the furnace atmosphere is greater than 10⁻⁹ atm.

The working lining may be positioned against the backing lining.

In embodiments of the present invention, the backing lining has a thermal conductivity that is significantly higher than the thermal conductivity of the working lining. In some embodiments, the backing lining has a thermal conductivity that is similar to or even higher than the thermal conductivity of the outer shell.

In some embodiments, the backing lining comprises a graphite layer or a layer made from a material including graphite or a layer of a carbon-based material. In some embodiments, the backing lining comprises a graphite layer.

In one embodiment, the outer shell of the furnace comprises a steel shell.

In some embodiments of the present invention, the one or more cooling channels may be welded to an external surface of the outer shell.

The one or more cooling channels may comprise cooling water channels for receiving cooling water. The cooling water channels may receive cooling water that has a high pressure and a high velocity of travel through the cooling water channels.

The one or more channels may be arranged in a serpentine pattern. The one or more channels may comprise a plurality of channels that are spaced apart from each other.

The furnace may comprise a top submerged lance furnace.

The relatively high thermal conductivity backing lining, such as a graphite layer or a layer made from a material including graphite or a layer of a material of graphite or a layer of a carbon-based material may be positioned throughout all of the lining of the furnace. Alternatively, the layer may be positioned in only a portion or portions of the furnace.

The graphite layer or layer made from a material including graphite or layer of a material of graphite or a layer of a carbon-based material may comprise a plurality of graphite tiles or graphite bricks or tiles or bricks made from a material including graphite or a material of graphite or a layer of a carbon-based material that are glued or cemented or otherwise affixed to the inside surface of the outer shell of the furnace. Where a cement or glue is used for this purpose, the cement or glue may be graphite or carbon-based with a high thermal conductivity.

The graphite layer or layer made from a material including graphite or layer of a material of graphite or a layer of a carbon-based material may have the thickness of between 30 and 250 mm, more suitably between 50 and 100 mm. A thickness of approximately 70 mm may be appropriate.

The working lining may comprise any suitable refractory material known to the person skilled in the art. The working lining may have a thickness that is greater than the thickness of the backing lining.

In a second aspect, the present invention provides a method for cooling a furnace in which a high temperature process is conducted under oxidising conditions, the method comprising providing a furnace comprising an outer shell made from a metal, one or more cooling channels formed on or joined to the outer shell and a furnace lining, the furnace lining comprising a relatively high thermal conductivity backing lining positioned adjacent to an inner wall of the outer shell and a working lining positioned inwardly of the backing lining, operating the process in the furnace and passing cooling water through the cooling channels to cool the furnace.

The working lining may be positioned against the backing lining. The working lining may be a refractory based lining.

In embodiments of the present invention, the backing lining has a thermal conductivity that is significantly higher than the thermal conductivity of refractory based working lining. In some embodiments, the backing lining has a thermal conductivity that is similar to or even higher than the thermal conductivity of the outer shell.

In some embodiments, the backing lining comprises a graphite layer or a layer made from a material including graphite or a layer of a material of or including graphite or a layer of a carbon-based material. In some embodiments, the backing lining comprises a graphite layer.

In one embodiment, the method of the present invention is operated such that the maximum temperature reached in the graphite layer does not exceed 500° C., preferably not exceed 400° C., more preferably not exceed 250° C.

In another embodiment, the method of the present invention is operated such that heat is removed from the furnace at a rate of 5 kW/m² under normal operating conditions and a new working lining, up to 25 kW/m² for a worn working lining, and not exceeding a localised heat flux of 120 kW/m² under extreme operating conditions and localised failure of the working lining.

In some embodiments of the present invention, cooling water flows through the cooling channels at an average rate of 1 to 2 m³/h per m² of furnace shell area, and at a minimum velocity in the cooling channels of 1 m/s, preferably above 2 m/s.

Graphite layers are suitably used as the backing lining in some embodiments of the present invention and, for convenience and brevity of description, the present invention will be described hereinafter with reference to a graphite layer. However, it will be understood that the present invention also encompasses layers made from other materials such as a layer made from a material including graphite or a layer of a material of graphite.

In the furnace and method of the present invention, the thermal conductivity of the graphite lining is three to four times higher than the thermal conductivity of the outer steel shell. As a result, the graphite lining layer will conduct and spread heat sideways along the shell before the heat exits the shell into the forced cooling water system. Therefore, the graphite layer will assist in removing heat from the working lining adequately to reduce the wear rate of the working lining due to lower operating temperatures, especially for a worn working lining. Furthermore, this design prevents or minimises the formation of localised hot spots on the shell between the external shell mounted forced cooling channels.

This is in contrast to the prior art linings used in furnaces, for example, top submerged lance furnaces, in which oxidising processes take place. In such furnaces, the working lining is positioned against a more insulating backing lining, which in turn is positioned against the inner wall of the outer steel shell. The thermal conductivity of the insulating backing lining is approximately 150 times less than that of the steel shell. Combining the insulating backing lining with an external shell cooling system will not be advantageous for the side wall lining campaign life because the insulating backing lining will insulate the working lining from the shell cooling system, resulting in a higher wear rate for the working lining due to higher operating temperatures, even for a worn working lining. Furthermore, a localised high heat load on the sidewall could result in a hot spot on the shell between the external shell mounted forced cooling channels. Operating experience has also shown that the temperature of the outer steel shell in such furnaces can approach or even exceed 200° C. This high temperature on the outer wall of the furnace represents an occupational health and safety problem in the working environment for operators of the furnace.

In contrast, using a furnace in accordance with the present invention can result in the temperature of the outer surface of the outer steel shell being in the range of from 40 to 80° C. It will be appreciated that this provides a safer and more comfortable working environment for the operators of the furnace.

In all aspects of the present invention, the furnace may be continuously operated under oxidising conditions. In other embodiments, the furnace may operate under oxidising conditions for a period of time and then operate under reducing conditions. Operation of the furnace may sequence between operation under oxidising conditions and operation under reducing conditions.

Other benefits and advantages arising from the present invention will be described in the following description of a preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a top submerged lance furnace;

FIG. 2 shows a schematic cross sectional view of a side wall lining/cooling system arrangement used in a top submerged lance type furnace in accordance with an embodiment of the present invention; and

FIG. 3 shows a temperature profile through the side wall of the furnace shown in FIG. 2 in the event that the working lining becomes completely worn away.

DETAILED DESCRIPTION OF THE DRAWINGS

It will be appreciated that the drawings have been provided to illustrate features of preferred embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited solely to those features as shown in the drawings.

FIG. 1 is a schematic diagram of a prior art top submerged lance furnace. This figure has been described in the background section of this specification and need not be described further.

FIG. 2 shows a side wall lining/cooling system arrangement for use in an embodiment of a furnace in accordance with the present invention. The furnace may be a top submerged lance type furnace. The side wall lining/cooling system comprises an outer steel shell 30. Cooling water channels 32, 34 are welded to the outside of the outer steel shell 30. The cooling water channels are placed into fluid communication with a source of high pressure cooling water in a manner that will be known to a person skilled in the art.

The furnace lining includes a backing lining in the form of a graphite layer 36. The graphite layer may be formed from a plurality of graphite tiles having a thickness of approximately 70 mm that are glued or cemented to the inside surface of the steel shell 30. The graphite layer may alternatively be made from graphite bricks having a thickness of up to 250 mm or even greater. The backing lining may alternatively be made from a material including graphite or a material of graphite or a layer of a carbon-based material. The cement used for this purpose is suitably graphite or carbon-based and it has a very high thermal conductivity. As will be appreciated by persons skilled in the art, the graphite layer 36 provides a layer having a high thermal conductivity. Indeed, the thermal conductivity of the graphite layer 36 may be three to four times higher than the thermal conductivity of the outer steel shell 30.

The furnace lining also includes a working lining, in this case in the form of a refractory lining 38. The layer 38 constitutes the working lining of the furnace. The hot environment of the furnace is denoted by reference numeral 40. As can be seen from FIG. 2, the working lining 38 is positioned between the hot environment 40 and the graphite layer 36.

As mentioned above, the thermal conductivity of the graphite layer 36 is three to four times higher than the thermal conductivity of the furnace steel shell 30. As a result, the graphite backing lining layer 36 will conduct and spread heat sideways along the furnace steel shell 30 before the heat exit the shell into the forced cooling water channels 32, 34. Therefore, the graphite backing lining layer 36 assists in removing heat from the working lining 38 adequately to reduce the wear rate of the working lining due to the lower operating temperatures in the working lining. This is especially so for a worn working lining. Furthermore, the graphite backing layer 36 prevents or minimises the formation of hot spots on the outer steel shell 30 between the external shell mounted forced cooling water channels 32, 34.

The operating temperature of the furnace can vary between 900° C. to 1600° C. under extreme conditions. Heat transfer to the furnace sidewall is through convection adjacent to the liquid furnace bath, and through conjugate convection and radiation above the liquid furnace bath. The resulting heat flux through the furnace sidewall could vary between 5 and 25 kW/m² depending on the working lining condition and operating conditions. Under extreme operating conditions and in areas where the working lining is damaged or completely worn away, localised heat fluxes of up to 120 kW/m² can be experienced. The operating temperature of the graphite layer will vary between 55 and 110° C. depending on the working lining and operating conditions. Under extreme operating conditions and with the working lining worn back completely, the graphite temperature may rise to a maximum of 400° C. The average temperature of the external surface of the steel shell and cooling water channels will vary between 40 to 80° C. depending on the working lining and operating conditions. The increase in cooling water temperature through the cooling water circuits may vary between 5 and 15° C. The cooling water outlet temperature may reach a maximum of 65° C., depending on inlet water temperatures and heat load.

The present inventor is aware that a similar furnace lining/cooling system in which an external shell mounted forced cooling water system is combined with a high thermal conductivity graphite backing lining has been used in other types of furnaces (such as electric furnaces) in which high temperature processes are conducted under reducing conditions. However, such cooling systems/furnace linings have not been used in furnaces in which oxidising processes take place. The reason that persons skilled in this art have heretofore not considered such furnace linings to be suitable for use in furnaces in which high temperature processes take place under oxidising conditions is that the graphite layer is itself readily oxidisable if it ever becomes exposed to the hot environment of the furnace. Therefore, if wear of the working lining of the furnace takes place to a degree such that the working lining is essentially worn away in a part of the furnace such that the graphite layer is exposed to the hot environment of the furnace, conventional thought was that the graphite layer would very quickly become oxidised by the oxidising conditions to which it was being exposed. Effectively, it was thought that if the graphite layer was exposed to the hot, oxidising conditions inside the furnace, the graphite layer would essentially very quickly burn away. This dilemma is, of course, not of concern in furnaces operated under reducing conditions.

Surprisingly, the present inventor has found that in the event that the working lining 38 becomes completely worn away in a part of the furnace, the rate of cooling through the graphite layer 36 is sufficiently high such that instead of the graphite layer 36 becoming quickly oxidised, a protective and stable freeze layer will form on the hot face of the graphite lining 36, thereby limiting the heat loss through the side wall and protecting the graphite lining 36 from other wear mechanisms, such as erosion and oxidation. The hot face temperature of the graphite lining is maintained well below 500° C., thereby preventing significant oxidation of the graphite taking place in the medium to long term. As mentioned above, this finding is contrary to conventional thinking.

FIG. 3 demonstrates the formation of a stable protective freeze layer on the hot face of the graphite layer in the event that the graphite layer becomes exposed by virtue of the working lining 38 becoming worn away. In FIG. 3, the steel shell 30 and the graphite layer 36 are shown. A stable freeze layer 42, which forms on the graphite layer 36, is also shown. The stable freeze layer may, for example, have a thickness of approximately 15 mm. As can be seen from FIG. 3, the furnace is operated at a temperature of approximately 1100° C. However, due to the quite high rate of heat transfer through the graphite layer, the freeze layer 42 is formed over the exposed graphite layer 36. Typically, the freeze layer is formed within around 30 minutes of the graphite layer becoming exposed. This minimises the amount of oxidation of the exposed graphite layer that takes place. Further, the maximum temperature in the graphite layer is well below 500° C., and typically maintained below 250° C., thereby avoiding further oxidation of the graphite layer. FIG. 3 also shows that there is a steep temperature gradient through the protective freeze layer 42.

The furnace and the method for cooling the furnace in accordance with the present invention has a number of further advantages:

• • When the working lining is new, the furnace and method for cooling the furnace of the present invention does not result in a lot of difference to the lining temperature (when compared with prior art linings used in top submerged lance furnaces). However, it does make a large difference to the external temperature of the steel shell of the furnace. Normally, a top submerged lance furnace, without water channel cooling has an outside steel shell temperature of approximately 200 to 300° C. However, furnaces operated in accordance with the present invention have an outside steel shell temperature of around 40 to 80° C.
  • As the working lining wears away, a cooler temperature is established in the working lining, which reduces the wear rate of the working lining.
  • If the working lining becomes completely worn away, the graphite layer extracts heat away from any developing hotspots within the furnace sidewall and prevents the formation of hotspots on the furnace steel shell. Further, a frozen slag layer forms on the graphite surface, which protects the graphite layer and reduces heat loss through the graphite layer.
  • The externally mounted cooling channels can be spaced from each other such that a large portion of the external surface of the outer shell of the furnace is exposed. This allows for visual inspection of the outer shell to take place. It is also possible to mount thermocouples to the outer surface of the outer shell in order to monitor the temperature of the outer shell. This is not possible if panel cooling (in which water covers the whole outer shell of the furnace) is used.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be appreciated that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

1.-12. (canceled)

13. A method for cooling a furnace in which a high temperature process is conducted under oxidising conditions, the method comprising providing a furnace comprising an outer shell made from a metal, one or more cooling channels formed on or joined to an outer surface of the outer shell and a furnace lining, the furnace lining comprising a relatively high thermal conductivity backing lining positioned adjacent to an inner wall of the outer shell, the backing lining comprising graphite tiles or graphite bricks or tiles or bricks made from a material including graphite or tiles or bricks made from a carbon-based material, and a refractory working lining positioned inwardly of the backing lining, the backing lining having a thermal conductivity that is significantly higher than the thermal conductivity of the refractory working lining, the backing layer having a thermal conductivity that is similar to or higher than the thermal conductivity of the outer shell, operating the process in the furnace and passing cooling water through the cooling channels to cool the furnace such that the maximum temperature reached in the backing lining does not exceed 500° C.

14. A method as claimed in claim 13 wherein the process is operated with a partial pressure of oxygen in the furnace atmosphere of greater than 10−9 atm.

15. A method as claimed in claim 13 wherein the backing lining has a thermal conductivity that is about three to four time higher than the thermal conductivity of the outer steel shell.

16.-17. (canceled)

18. A method as claimed in claim 13 wherein the maximum temperature reached in the graphite layer or the layer of a material of or including graphite or the layer of a carbon-based material does not exceed 400 ° C..

19. A method as claimed in claim 13 wherein heat is removed from the furnace at a rate of 5 kW/m2 under normal operating conditions and a new working lining, up to 25 kW/m2 for a worn working lining, and not exceeding a localised heat flux of 120 kW/m2 under extreme operating conditions and localized failure of the working lining.

20. A method as claimed in claim 13 wherein cooling water flows through the cooling channels at an average rate of 1 to 2 m3/h per m2 of furnace shell area, and at a minimum velocity in the cooling channels of 1 m/s.

21. A method as claimed in claim 13 wherein a temperature of the outer surface of the outer steel shell of the furnace falls in the range of from 40 to 80 ° C.

22. A method as claimed in claim 13 wherein the rate of cooling of the backing lining is sufficiently high such that in the event that the working lining becomes completely worn away in a part of the furnace a protective and stable freeze layer will form on the backing lining to protect the backing lining from being oxidised.

23. A method as claimed in claim 13 wherein the furnace is operated for a period of time under oxidising conditions and is operated for a brief time under reducing conditions.

24. (canceled)

25. A method as claimed in claim 13 wherein the graphite bricks or tiles or bricks or tiles made from a material including graphite or tiles or bricks made from a carbon-based material have the thickness of between 30 and 250 mm.

26. A method as claimed in claim 13 wherein the graphite bricks or tiles or bricks or tiles made from a material including graphite or tiles or bricks made from a carbon-based material have the thickness of between 50 and 100 mm.

27. A method as claimed in claim 13 wherein the graphite bricks or tiles or bricks or tiles made from a material including graphite or tiles or bricks made from a carbon-based material have the thickness of approximately 70 mm.

28. A method as claimed in claim 13 wherein the maximum temperature reached in the graphite layer or the layer of a material of or including graphite or the layer of a carbon-based material does not exceed 250° C.

29. A method as claimed in claim 13 wherein cooling water flows through the cooling channels at an average rate of 1 to 2 m3/h per m2 of furnace shell area, and at a minimum velocity in the cooling channels of above 2 m/s.