Plastic and nonmetallic article shaping or treating: processes – Furnace lining formation or repair
Reexamination Certificate
2001-07-18
2004-06-01
Lechert, Jr., Stephen J. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Furnace lining formation or repair
C264S069000, C264S071000, C264S109000, C264S123000
Reexamination Certificate
active
06743382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of installing a refractory lining in an electric induction furnace, such as a coreless or channel induction furnace, and particularly a method of installing a refractory lining in an electric induction furnace using an electric vibrator.
BACKGROUND OF THE INVENTION
Electric induction furnaces are used, for example, in the production of molten ferrous and nonferrous metals. These molten metals typically are used to produce castings in foundries from scrap. Induction melting is accomplished by applying an electric current to copper furnace coils, referred to as the primary winding. The current in the primary winding induces a current in the scrap metal within the furnace, referred to as the secondary winding. The current induced in the secondary winding meets electrical resistance and generates heat. When sufficient heat is generated, the scrap metal melts. Induction heating is used not only for melting metals but also for holding metals in the molten state until the metal is removed from the furnace for the production of castings or other processing Application temperatures typically range from about 1000 F. to about 3200 F.
To contain the heat and molten metal within an induction furnace, specialized refractory materials typically are used to line the furnace. Conventional refractory linings for induction furnaces typically are comprised of silica, fused alumina, fused magnesia, calcined magnesia, fused mullite, calcined fireclay, calcined chamotte, calcined bauxite, and zircon refractory aggregate. The refractory lining is a consumable material that is eroded or otherwise damaged by exposure to the conditions within the furnace. Conventional refractory lining materials tend to have a relatively high consumption rate, which corresponds to a short lining life.
When a certain amount of consumption or damage to the lining has occurred, the operation of the induction furnace must be interrupted to repair or replace the refractory lining. The frequency of the interruption is determined by the consumption rate of the refractory lining for a given process. The duration of an interruption depends on the nature and extent of the consumption. When the consumption or damage is extensive, removal and replacement of the entire refractory lining rather than repair of the eroded or damaged portion may be necessary. Replacement of the lining increases furnace downtime. The total furnace downtime depends on the frequency and duration of the interruptions.
The electric induction furnace may be a coreless or a channel furnace. A coreless furnace includes a generally continuous floor portion and walls that extend upwardly from the periphery of the floor portion. A channel furnace includes a floor/throat portion and walls that extend upwardly from the periphery of the floor portion. The floor/throat portion defines an outlet connected to an inductor. The inductor typically includes a metal container or casing that encloses a refractory lining and bushings. The bushings are hollow and typically are made of copper or steel. An induction assembly is enclosed within each bushing, which serves as the primary winding. The refractory lining defines a passageway for molten metal flowing from the outlet. The molten metal is heated further in the inductor.
Refractory linings for the floor portion and walls of electric induction furnaces typically are installed in a two step process. First, the refractory lining is installed in the floor portion of the furnace. Second, the walls of the refractory lining are installed using a liner form that is positioned on the installed floor. The liner form defines the inner wall of the refractory lining. The inner wall of the furnace defines the outer wall of the refractory lining. Channel furnaces require an additional step for installation of refractory lining in the inductor.
The liner form may be removable or consumable. Removable forms typically are used for refractories designed to have low-temperature, heat-set bonds. Removable forms also are desirable to prevent contamination from melting of a consumable form into the molten metal product. Consumable forms typically are used for higher temperature applications (i.e., greater than about 2000 F.) when the melted form can be used as part of the molten metal product. Consumable forms also are used when removal of a form would not be feasible after refractory installation, for example, in the inductor of a channel furnace.
A refractory lining typically is installed in an inductor casing using a solid or hollow loop or channel form and cylindrical or rectangular bushings. The channel form typically is burned or melted away during a heatup process after the refractory has been installed. The decomposition of the channel form leaves a passageway for molten metal inside the refractory cross-section. The refractory is installed into the inductor casing around the channel form and bushings. Generally, the refractory is placed into the bottom of the inductor casing. When a sufficient amount of refractory has been added to the bottom of the casing, the channel form is placed inside the inductor casing. Refractory is then added around the channel form. When a sufficient thickness of refractory has been added around the channel form, at least one bushing is installed. After installation of the bushing(s), refractory is continually installed between the channel form and the inductor casing and around the bushing(s) within the channel form to the top of the inductor casing.
Proper installation of the refractory is essential to prolonging refractory life. A successful installation is judged by the density of the installed refractory materials, i.e., the amount of material installed into an induction furnace for a given volume. Errors during installation that cause the installed refractory lining to have a less than optimal density will reduce the service life of the lining. Optimal density depends on the effective removal of air trapped within the dry refractory material and compaction of the dry refractory particles to reduce the distance between them. Air trapped within the dry refractory material typically is removed by inducing flow in loosely packed dry refractory material.
Refractory installation requires a skilled labor force and can be labor intensive. An installation can take from about three hours to three shifts or more, depending on the size of the furnace.
Conventional methods for installing a refractory lining require the provision of dry refractory material in loose shallow layers. The maximum depth of the layers is about 3-5 inches depending on the type of refractory. For example, silica refractory generally may be installed in layers having a maximum depth of about 5 inches, dry vibratable refractories (including alumina/magnesia/mullite refractories) generally may be installed in layers having a maximum depth of about 4 inches, and chrome-alumina refractories generally may be installed in layers having a maximum depth of about 3 inches. The density of the refractory lining will be reduced if the layers of dry refractory material exceed the maximum depth for that application because it is difficult to properly perform manual deairing of thicker layers.
Manual deairing involves inducing flow in the dry refractory material so that air trapped within the dry refractory material may escape. This typically is accomplished by forking or spading the entire surface of the layer about four times. The density of the installed refractory lining will be reduced if the dry refractory material is not thoroughly deaired. Operator error due to inattention, undue haste or inadequate training may compromise a successful installation. The forking or spading tool typically weighs about 15-20 pounds, which can result in operator fatigue, which also may compromise a successful installation.
Conventional methods of installing refractory linings also require compaction of the dry refractory material using an electric vibrating tamper or form vibration. An electric vibrating tamper
Doza Douglas K.
Opatt William M.
Allied Mineral Products, Inc.
Lechert Jr. Stephen J.
Porter Wright Morris & Arthur LLP
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