The present invention relates to a shield for electromagnetic agitators for agitation of molten metal in ladles or other containers used in metallurgic industry, in order to protect the agitator against heat radiation, spatter of liquid metal or slag, and mechanical strain, which shield is built-up from several layers of heat insulating materials comprising a bottom layer and an external layer in the shape of a cast.

An electromagnetic agitator is commonly built like a folded- out portion of a rotating machine. The core is composed of steel plates or sheet steel. Grooves are provided in the core with one or a number of insulated water cooled windings mounted in said grooves. The windings are supplied with alternating current of a low frequency. The generated alternating field will penetrate and cause an agitating effect in the molten metal. The agitator is placed as closely as possible to the wall of the container of molten metal so that a maximum agitating effect is achieved.

The electromagnetic agitator must be protected against common operational strains that may occur in metallurgical industry. Among these are mentioned: high temperature, dirt and spatter of liquid metal or slag, and mechanical stress which may be due to impacts, e.g. occurring in connection with transport of ladles close to the agitator or when the agitator is positioned close to the container of liquid metal. There is also dust and smoke against which the windings must be protected.

It is known to protect an agitator against operational strains by a cast of fireproof material. Such fireproof material was then cast directly onto winding and core to join them to an assembly. The surface of such a cast was generally protected by a plate of non-magnetic material, generally austenitic steel.

The above mentioned design has several disadvantages. Said cast being an integrated part of coils and core, it is very difficult to do repair work or replace any defective parts of such an agitator. Eddy currents are induced in the cover plate, causing it to be heated. Such losses will reduce the efficiency of the agitator. Heating reach levels which reduce operational reliability.

The object of the present invention is to avoid the above- mentioned disadvantages and to achieve a shield which is easy to handle and to repair.

This is achieved by a shield which is characterized by the features given in the claims.

The shield according to the invention is built-up from a frame work with a bottom to form a casting mould and consists of several dismantable frame units, i.e. at least two. The external insulation is cast inside said frame. The following advantages result from having a shield which is dismantable in the form of frame units. There will be access to windings, core, and cooling system in the agitator for inspection as well as for repair and replacement of any defective parts. Also, the shield or parts of it may be replaced or, if desired, dismantled and repaired separately in case it is damaged. Small injuries may be repaired on the spot.

A shield according to the invention is built from several layers of heat insulating materials. Outermost there is the cast provided the shield with its very good properties of heat resistance. Such a cast is not especially heat insulat¬ ing when materials that are common commodities to day are used. By placing an intermediate layer of especially heat insulating materials between bottom plate and cast, the desired heat insulating properties may be achieved. Especial¬ ly heat insulating materials, e.g. fibre insulation, may show

from 10 to 20 times less heat conductivity than the cast. Additionally, an intermediate layer will act temperature equalizing, so that hot spots are avoided on the underside of the shield. The thickness of the cast and, thus, of the entire shield may be reduced to safe values in view of necessary and desired mechanical properties. The thickness of the shield is an important parameter in view of the entire efficiency of the plant. The operative time of an agitator may be increased as compared to previously known designs and safety against buring through due to liquid metal or slag is considerably improved.

The materials from which the shield is built have so weak conductivity that eddy currents are not induced in them. Thus, local heating and losses are avoided in the shield and the field from the agitator will not be influenced.

The invention will be disclosed in more detail below with reference to embodiments shown in the drawings.

Figure 1 is a top view of an agitator with a shield, which is placed adjacent to a container with molten metal;

Figure 2 is an elevational view of an embodiment of a shield;

Figure 3 is a sectional view of the joining of two members of a shield.

Figure 1 shows an agitator, designated 1, which is placed adjacent to a container, e.g. a ladle 2 with liquid metal. Shield 3, which is dismantable, is attached to the agitator. It is shown to be placed between the agitator and the ladle. A shield may be mounted to protect the agitator on the faces that are most subjected to high temperature, spill and spatter of liquid metal at the same time as there are strong magnetic fields present.

The geometry of the agitator is adapted to the container in which agitation is desired. It may be designed to form one

member or it may be composed of a certain number of members. In Figure 1 the design of the agitator will render it most suitable to have a shield that is divided into three separate straight members.

Figures 2 and 3 show a top view, and an elevation, respect¬ ively of shield comprising three straight dismountable members, and a sectional view of the shield. The shield comprises an external frame 11 made of profiles. The material of said profiles may be non-magnetic steel or other metals. Profiles of fibre reinforced plastic or similar non-metallic materials may also be considered for application here. Onto the frame a bottom 13 is mounted, which may consist of a plate, sheet or woven material or yarns having sufficient mechanical strength to serve as a bottom for the insulation placed on top of it. The bottom may also comprise a combinat¬ ion of materials as mentioned. In areas where the bottom is well supported by the underlying core, part of the plate may, e.g. be replaced by a woven material or yarns. Materials are chosen which, preferably, have low conductivity to avoid induction of eddy currents in them. In this manner losses and hot spots are avoided. Among suitable materials micanite plate, asbestos cement plate, glassfibre, plate and glass fibre sheet may be mentioned.

Preferably, a flexible plate 15 is provided between the bottom plate and core with windings in the agitator. Said plate will equalize uneaveness and distribute and transfer mechanical forces between shield and agitator. Furthermore, said plate will protect the agitator against dust. The material of such a plate may, e.g. be silicon rubber.

Frame 11 and bottom 13 form a mould. In said mould the external insulation 9 is cast. The casting materials used are fireproof and have good mechanical properties. The shield is advantageously made as thin as possible in order to enhance the efficiency of the agitator. In order to improve the

mechanical properties of the cast, reinforcement in the shape of wire 17, 18 may be embedded in the vasting material. Such reinforcements wires may be secured to the frame, 17, or they may extend loosely in the cast. If metal wire is used, it is placed so as not to form closed circuits where circulated currents and heating might occur. The wires should, preferably, have the same coefficient of expansion as the casting material.

The casting material may be a lining material, a ceramic casting material or refractory concrete. In addition to wire reinforcement fibre reinforcement of the cast will be advantageous. This makes for substantial improvement of the mechanical properties. For fibre reinforcement, e.g. steel fibres, glass fibres or similar materials may be used.

To enhance the heat insulating properties of the shield an intermediate layer 14 in the shape of an insulating layer may be provided between bottom plate and cast, e.g. a fibrous insulating plate or a fibrous insulating mat. Fibre insulat¬ ion and similar materials show much better heat insulating properties than does the cast material. The thickness of such a plate may be adapted to the desired insulating properties of the shield.

In order to limit the area of each cast member one or a number of separating profiles 16 may be mounted. Such profiles may, e.g. be H-profiles and may be provided with expansion gaps. Due to reduction of the area of the cast members excessive casting tensions are avoided. The expansion gaps will permit the cast members to expand during heating, so that thermal stresses are avoided.

Figure 3 shows a method for mounting shields on an agitator. The Figure is a sectional view of the joining between two shield members along line A-A in Figure 2. Securing bolts 19 are mounted on the agitator. A cover plate 12 or profile of

non-magnetic steel or another material urges the shield members towards the agitator. The shield frames are very accurately shaped to ensure the dust-sealing function of the shield. If desired, a dust-seal may be mounted, e.g. between cover plate 12 and frames.

The above design may be modified in various manners within the scope of the following claims.