METHOD AND DEVICE FOR FEEDING SUPPLEMENTARY METAL IN SOLID STATE TO A BATH OF MOLTEN METAL

Abstract: A method and a device for feeding supplementary metal to a bath of molten metal (2), for coating components of steel, comprise a feed pipe (9, 10) which is insertable into the bath and has an inlet (11) which is positionable on a level below the bath surface (17), an outlet (12) and feeding means (18) situated in the feed pipe for sucking in molten metal from the bath via the inlet (11) together with the supplementary metal (4) for melting thereof while it passes through the pipe.

Description: METHOD AND DEVICE FOR FEEDING SUPPLEMENTARY METAL IN SOLID STATE TO A BATH OF MOLTEN METAL Technical field of the invention The present invention relates to a method for feeding supplementary metal in solid, preferably granular and/or particulate state to a bath of molten metal, said supple¬ mentary metal being supplied from a store down through a feed pipe which is inserted in the bath and has an inlet positioned on a level below the bath surface, at the same time as molten metal from the bath is sucked into the pipe inlet so as to be mixed with the supplementary metal which the supplementary metal passing through said feed pipe is is being molten while passing through said feed pipe. The invention also relates to a device for carrying out the method. Background of the invention To prevent rusting, components of steel are frequent¬ ly coated with a molten plating or coating metal, such as zinc (hot-dip galvanising), tin (hot-dip tinning), lead, aluminium or different types of alloys. This occurs by dipping the components into a bath of the molten coating metal. Owing to the high temperature in this context, the metal bath surface is oxidised by the oxygen in the air. The oxide contaminates the surface of the coating metal and must be removed in a scumming operation in which the oxide or the so-called ash is accumulated at one end of the bath and ladled out in accumulated state. Varying amounts of the coating metal accompany the ash and represent a waste which unfavourably increases the cost of the process. There are many different techniques of preventing the waste or returning the lost coating metal to the bath. One technique is to grind the ash and separate usable coating metal from the useless oxide, whereupon the metal is returned to the bath in the form of granulates. A specific problem is that the granular supplementary metal may contain small air bubbles at the same time as the surface tension of the molten metal bath is rather high. This implies that it is difficult to make the supplementary metal sink into the bath and melt. When feeding supplementary metal to a metal bath, the melt is cooled locally, and to prevent so-called liquation, i.e. cold, substantially solidified portions, in the bath, the supplementary metal must be accurately mixed and distri¬ buted. A certain amount of different kinds of impurities always collect on the bottom of the kettle, and there may also be a so-called bottom protection metal, e.g. lead, which serves to protect the cattle from being attacked. It is therefore important, especially when admixing supple¬ mentary metal, not to provide agitation of the bottom layer so that foreign substances enter the bath working area intended for immersion of objects, and thus to ensure that the metal in this area is kept pure. When admixing supplementary metal, oxide is supplied to and/or formed in the melt. This oxide must be given an opportunity of being separated in a quiet zone outside the working area above the outgoing flow of metal and at a safe distance from the inlet zone surrounding the upper part of the mixer. These oxides and other light impurities are accumulated on the surface of the metal bath. These substances must be prevented from mixing with the metal bath so as to avoid that defects are formed on the finished products. NO 1,357,111 and US 3,741,754 disclose devices for feeding supplementary metal to a melt. These devices resemble each other in many respects and therefore also suffer from largely the same drawbacks and deficiencies. For example, the metal flow as formed is directed down¬ wards and cannot be directed in a different direction. This causes a flow of metal directed towards the bottom, which will carry along the impurities on the bottom, possibly also bottom protection metal, which will contaminate and deteriorate the pure melt in the working zone. Moreover, the devices according to the above patent specifications do not allow separation of the added oxides in a quiet zone at a safe distance from the inlet zone. On the other hand, there is an immediate risk that metal cir¬ culates locally between the bottom and the surface in the vicinity of the mixer where the concentration of oxides and impurities increases and the oxides and impurities are mixed with the melt. Moreover, the risk of liquation in the metal bath is great, since also the concentration of supplementary metal increases in this local circulation, which results in a fall in temperature. Further the Nor¬ wegian device is designed such that the contaminating material on the metal surface in the kettle, in the form of, inter alia, oxides, is not prevented from being carried along into the mixing zone where it is mixed in and contaminates the melt. Brief description of the inventive idea The present invention aims at eliminating the above- mentioned drawbacks and, to this end, relates to a method for feeding supplementary metal to a bath of molten metal, said method being characterised in that the metal flow is caused to flow not only downwards via a pipe section which in known manner is substantially vertical, but also a distance in the horizontal direction via a substantially horizontal pipe section, before being allowed to flow into the metal bath. In addition the method, the invention also relates to a device for feeding supplementary metal to a bath of molten metal. The features of the inventive device are stated in more detail in the appended claims. Brief description of the accompanying drawing The only drawing is a schematic view of an inventive device in connection with a bath of molten coating metal. Detailed description of a preferred embodiment of the invention In the drawing a kettle 1 contains a bath of molten coating metal 2, e.g. zink, tin, lead, aluminium or an optional alloy. A storage container 3 is intended for storing a supplementary metal 4 in solid, preferably granular and/or particulate state. In the area of the con¬ tainer bottom there is mounted a discharge member 5, for example in the form of a screw, which can be driven at constant or variable speed by means of a suitable drive mechanism 6. In practice, the container 3 can be supported by a frame (not shown) which can be put on the floor beside the kettle 1. To the container bottom there is connected a discharge channel 7 in the form of a chute or pipe, for discharging the metal granulates in a controll¬ able flow. The inventive device which is designated 8 in its entirety, comprises a feed pipe which in the embodiment shown is composed of a substantially vertical pipe section 9 and, extending from the lower part thereof, a sub¬ stantially horizontal section 10. The upper portion of the vertical pipe section 9 is formed with an inlet 11. The outlet of the horizontal pipe section 10 is designated 12. About the upper portion of the pipe section 9 there is mounted a collar 13 of greater diameter or width than the pipe. The upper end of the collar is positioned consider- ably higher than the pipe inlet 11, while the lower end of the collar suitably extends a distance below the inlet 11. Between the collar 13 and the narrower pipe 9 there is formed an annular gap 14 through which the molten metal 2 can pass. The device 8 in its entirety is supported by a schematically illustrated holder 15 which in practice can in turn be supported by the same frame (not shown) as supports the container 3. It is schematically indicated at 16 that the holder and/or the device 8 are vertically adjustable relative to the kettle 1, which renders it possible to set the inlet 11 of the pipe 9 on an optional, desired level relative to the surface of the bath. As illustrated in the drawing, the pipe 9 is set such that its inlet 11 is positioned on a level below the surface 17 of the bath of molten coating metal 2. Since the inlet of the pipe can be set on an optional level below the bath surface, the flow of molten metal through the pipe can also be varied. In the feed pipe 9, 10 there is mounted a mechanical type feeding means, in this case in the form of a centri¬ fugal pump impeller 18 which, via a shaft 19, is connected to a suitable drive mechanism, for example a motor 20 and a transmission 21. The feeding means 18 may also be a screw, propeller or the like. By the feeding means 18 it is possible to provide a flow of molten metal through the feed pipe 9, 10, more precisely in such manner that the molten metal is sucked in through the inlet 11 and dis- charged through the outlet 12 after having passed the entire pipe. According to a preferred embodiment of the invention, the horizontal pipe section 10 is pivotable and adjustable in optional directions relative to the device as such. The flow of metal can then be directed such that the under¬ lying contaminated layer is not touched and the oxide which is possibly added to the melt, rises to the surface at a desired point and at a safe distance from the inlet of the mixer. Adjustability can conveniently be provided by a screw joint between the horizontal and vertical pipe sections, the screws being equidistantly spaced from each other along a circle. As a result, the direction of the pipe can readily be changed in that the screw holes of the vertical and horizontal pipe sections are matched in different ways. The invention is applied in the following manner. From the container 3, the screw 5 discharges an amount of supplementary metal which is fixed per unit of time and is allowed to fall via the chute 7 into the area of the inlet 11 of the pipe 9. At the same time, the centrifugal pump impeller 18 is rotated while providing a flow of molten metal which is sucked in via the inlet 11 and discharged through the outlet 12. By controlling one or more of three parameters, viz. the speed at which the screw 5 rotates, the speed at which the pump impeller 18 rotates and the above-mentioned possibility of controlling the level of the inlet below the surface of the bath, the amounts of molten coating metal 2 entering the pipe 9 and solid supplementary metal 4 can be adapted to each other such that an energy content is produced in the pipe 9,10, which is sufficient to melt all the supplementary metal as it reaches the bath, at the latest. In actual practice, the amount of molten metal sucked in per unit of time should thus be at least 10 times larger than the amount of supplementary metal which is simultaneously fed into the pipe. The amount of molten metal can advantageously be 50- 100 times larger that the amount of supplementary metal. In practice, for example 1 kg supplementary metal can thus be supplied per second, at the same time as more than 50 kg molten coating metal is sucked into the pipe 9. While passing through the pipe 9,10, the granular or particlate supplementary metal is intensively mixed with the molten coating metal and is effectively molten therein, and the fact that the molten metal is sucked down in the pipe 9 by the feeding means 18 ensures that the supplementary metal is effectively mixed with the bath and does not remain on the surface thereof. The existence of the collar 13 which extends a considerable distance into the bath, further ensures that only the pure and hot metal which is considerably below the surface of the bath is sucked in via the inlet 11, and the oxide or ash on the surface of the bath is prevented from being sucked down in the pipe. The bath can be replenished periodically with the required amount of supplementary metal, and after each replenishment the device described above is removed from the bath. This can be carried out by e.g. a fork truck or the like which simply lifts and carries away the frame on which both the holder 3 and the device 8 are mounted. Possible modifications of the invention The invention is of course not limited to the embodiment described above and shown in the drawing. Thus, the inventive idea is in no way restricted to returning of metal obtained by scumming of the existing bath and treated, but can also be used to feed any solid material whatsoever, for example conventional substitute metal, to the bath of molten coating metal. In practice, the schematically illustrated holder 15 may consist of a parallel arm system whose arms are pivotable relative to the associated frame, for example by means of a double- acting hydraulic cylinder or a screw, the chute 7 also being movably pivotable relative to the container 3, thereby to accompany the device 8 in the vertical movements thereof relative to the frame.

Claims: CLAIMS 1. A method for feeding supplementary metal in solid, preferably granular and/or particulate state to a bath of molten metal, said supplementary metal (4) being supplied from a store (3) down through a feed pipe (9,10) which is inserted in the bath (2) and has an inlet (11) positioned on a level below the bath surface (17), at the same time as molten metal from the bath is sucked into the pipe inlet (11) so as to be mixed with the supplementary metal which is being molten while passing through said feed pipe, c h a r a c t e r i s e d in that the metal flow is caused to flow not only downwards via a pipe section (9) which in known manner is substantially vertical, but also a distance in the horizontal direction via a substan¬ tially horizontal pipe section (10), before being allowed to flow into the metal bath. 2. The method as claimed in claim 1, c h a r a c - t e r i s e d in that said feed pipe (9,10) sucks in an amount of liquid metal which is at least 10 and preferably 50-100 times larger than the amount of supplementary metal which is simultaneously supplied, thereby providing in said pipe an energy content which suffices for melting all the supplementary metal as it reaches the bath, at the latest. 3. A device for feeding supplementary metal in solid, preferably granular and/or particulate state to a bath of molten metal, comprising a feed pipe (9,10) which is insertable into said bath (2) and has an inlet (11) which is positionable on a level below the bath surface (17) and means (18) for sucking in molten metal from the bath via the inlet together with the supplementary metal (4) for melting thereof while it passes through said pipe, c h a r a c t e r i s e d in that in that said feed pipe comprises, in addition to a first section (9) which in per se known manner extends vertically, a substantially horizontal pipe section (10) which extends from the lower part of the vertical pipe section and through which the molten metal must pass before entering the metal bath. 4. The device as claimed in claim 3, c h a r a c ¬ t e r i s e d in that the horizontal pipe section (10) is pivotable and adjustable in the horizontal plane. 5. The device as claimed in claim 3 or 4, c h a r¬ a c t e r i s e d in that said means comprises a feeding means (18), for example in the form of a centrifugal pump impeller, positioned inside the feed pipe (9,10). 6. The device as claimed in any one of claims 3-5, c h a r a c t e r i s e d in that the feed pipe (9,10) is vertically adjustable relative to the surface of the bath.

Inventors: LARSSON MATS, LARSSON, MATS
Applicants: BIRSTAVERKEN AB
Application year: 1989
Main classification: C23C2
IPC codes: C23C2