APPARATUS AND METHOD FOR THE SECONDARY REFINEMENT OF METALS

Abstract: Apparatus for use in the secondary refinement of a metal, particularly steel, is disclosed, which includes an elongated vessel (10) of which the length to width ratio is greater than 2, the vessel (10) being provided with at least one channel type inductor (13) adapted for heating the vessel (10), and the vessel (10) being adapted in operation to be located with its long side substantially horizontally disposed. The invention also includes a method of refining a metal such as steel, using the aforesaid apparatus.

Description: APPARATUS AND METHOD FOR THE SECONDARY REFINEMENT OF METALS TECHNICAL FIELD This invention relates to an apparatus and a method for the secondary refinement of metals, particularly steel. BACKGROUND ART In the case of the steel industry, ladle furnaces are used extensively for controlling the temperature of the steel coming from a steel furnace and for refining it prior to casting through continuous casting machines. In such a refining process, a steel casting ladle is positioned under a hood, and electric arc heating applied to the upper surface of the steel in the ladle while argon gas is bubbled through the steel to distribute the heat throughout the contents of the ladle. By the introduction of fluxing agents such as calcined lime, silica, alumina etc to the steel, a layer of slag is formed which floats on top of the steel. The aforesaid bubbling of argon gas is usually effected by introducing such gas through one or more porous elements located on the bottom of the ladle. Such argon gas bubbling is indispensable for ensuring good mass and heat transfer conditions between the steel and the slag. In some cases electromagnetic stirring may be used for obtaining additional benefits. Steel ladles are vertically disposed cylindrical vessels, of which the depth to diameter ratio is normally larger than 1 , and smaller than 2. It will be appreciated that one of the main reasons for having such vessels so vertically disposed, is to maintain the surface area of the bath as small as possible and hence retain the energy of the metal bath inside it. The positioning of the aforesaid porous elements in such a vessel is also of importance in order to obtain the best stirring effects. It is also known that the employment of vessels with smaller depth to diameter ratios, gives rise to poorer mixing conditions, and that in shallower vessels, such as basic oxygen vessels, several of such porous elements are required to obtain effective mixing. A further disadvantage found with such a vertically disposed vessel is that the surface to volume ratio of the metal bath in the vessel is relatively small. Furthermore, since sufficient freeboard must be maintained in the ladle in order to minimise spillage during stirring, it is difficult to remove from such ladle slag that has been saturated by impurities. If this is for example done by tilting the ladle, the metal-slag surface area is increased, thereby reducing the thickness of the slag layer. Furthermore, where paddles are used for scraping the slag towards the lip of the ladle, the substantially circular configuration of the surface of the slag layer puts a restriction on the width of such a paddle in the sense that a paddle which is wide enough to extend over the whole width of the slag layer near such lip, is too narrow in a position further away from such lip, thus allowing slag to escape around its ends. OBJECT OF THE INVENTION It is an object of this invention to provide an apparatus and method for the secondary refinement of a metal, particularly steel, with which the aforesaid problems may be overcome or at least minimised. DISCLOSURE OF THE INVENTION According to the invention apparatus for use in the secondary refinement of a metal, particularly steel, includes an elongated vessel of which the length to width ratio is greater than two, the vessel being provided with at least one channel type inductor adapted for heating the vessel, and the vessel being adapted in operation to be located with its long side substantially horizontally disposed. It will be appreciated that while so horizontally disposed, the aspect ratio of a metal/slag bath inside the vessel, i.e. the ratio of the length of its surface to the width of its surface, will be larger than two. It will further be appreciated that the surface area of such a metal/slag bath will be much larger than what the case is with the conventional vertically disposed vessel so that a much larger contact area is available for reaction with chemicals added to the bath for the secondary refining of the metal and/or for the removal of sulphur from such bath and/or for the interaction between the metal bath and its overlying slag layer. Further according to the invention the at least one inductor is disposed substantially along the centre of the bottom side of the vessel when it is so horizontally disposed. Applicant has found that the fact that the bath in such a vessel is heated from below, coupled to the fact that a relatively low metallo static head is present, together with the resulting convection streams, ensure that a sufficiently strong stirring action is created in the vicinity of the inductor(s) to cause effective slag/metal interaction and homogeneous distribution of the refined metal and impurity saturated slag. Furthermore, the fluxes and alloys which are required in the refining process can be added in an at least semi-continuous stream to the surface of the metal bath directly above the inductor(s), i.e. where the most active stirring motion is achieved. It will be appreciated that any such fluxes etc which are required in the metal bath can also in conventional manner be introduced to the bath through one or more lances passing from above into the bath. Furthermore, if required, additional stirring of the metal bath may be effected by introducing argon and/or nitrogen gas into the bath by means of one or more lances or porous element stirring devices located in the vessel. Also, any additional heating which may be required, may be effected in conventional manner by means of one or more plasma torches and/or three phase electric arc electrodes. Further according to the invention the vessel is of round cylindrical configuration in cross section. Still further according to the invention the vessel is provided towards at least one of its shorter ends with an opening through which a paddle or the like can be inserted for scraping the spent slag layer from the metal bath in the vessel. It will be appreciated that because the slag surface is of substantially rectangular configuration in plan, a paddle can be used which substantially spans the whole width of the slag layer over substantially the whole length of the slag layer. By pushing or pulling such a paddle along the slag layer, the spent slag can be removed from the top of the metal bath in a very effective manner. To facilitate such removal of the slag, at least one of the said shorter ends of the vessel may be provided with a recessed outlet. Preferably the width of the paddle is slightly less than the width of the vessel, the narrow gaps so left between the ends of the paddle and the walls of the vessel ensuring free movement of the paddle in the vessel. Preferably, air or nitrogen may be blown into such gaps to prevent the slag from escaping through the gaps when the paddle is moved along the slag layer. The invention also includes within its scope a method for the secondary refining of a metal using the apparatus disclosed above. BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of apparatus according to the invention will now be described by way of example with reference to the enclosed drawings in which: Figure 1 is a diagrammatic side view of apparatus according to the invention; Figure 2 is a diagrammatic cross sectional view of the apparatus of figure 2, as seen on line I : I in figure 1 ; Figure 3 is a diagrammatic plan view of the apparatus of figure 1 as seen in the direction arrow A in figure 1. BEST MODE OF CARRYING OUT THE INVENTION In a preferred embodiment of the invention apparatus for carrying out the secondary refinement of a metal, such as steel for example, comprises a horizontally disposed refractory lined cylindrical vessel 10 of which the length to diameter ratio is greater than 2. In one embodiment of the invention the length of vessel 10 may, for example, be in the order of 8m and its diameter in the order of 2m, the intention being that it will be filled with liquid metal 11 to a depth in the order of 1 m. It will be appreciated that although vessel 10 is shown in the drawings as being of circular configuration in cross section, it can be of any other suitable configuration in cross section such as, for example, substantially square, triangular, etc. Vessel 10 is provided along its upper wall with an inlet 12 through which fluxes or the like may be charged to it. Vessel 10 is also provided along the centre of its bottom wall with at least one channel type inductor 13 of 3MW capacity. The upper wall of vessel 10 is also provided along each of its opposite ends with an opening 14 through which an elongated slag paddle 15 may be inserted which is operated by means of a handle 16, the width of paddle 15 being slightly less than the diameter of vessel 10. Openings 14 are provided with lids (not shown). Vessel 10 is also provided with a lance 17 which passes from above into metal bath 11 and through which argon and/or nitrogen may be passed into bath 11 for stirring purposes. If required, vessel 10 may also be provided with one or more porous stirring elements which can be fed with argon and/or nitrogen in order to assist in the stirring of bath 11. Only one of these is shown in dotted lines at 18 in figure 1 of the drawings. In operation the molten metal to be refined is charged to vessel 10 through one of the openings 14 to form a bath with a depth in the order of 1m, and the required fluxes introduced through inlet 12. lnductor(s) 13, when switched on, not only serve(s) to heat up bath 11 , but through convection and electromagnetic forces also cause(s) the stirring of bath 11. If required, the aforesaid stirring may be supplemented by passing argon and/or nitrogen through lance 17 (or element(s) 18) into bath 11. As a result of the fluxes melting and reacting with each other, a slag layer 19 is formed on top of bath 11 which then reacts with the molten metal in bath 11. It will be appreciated that because of the horizontal disposition of vessel 10, the surface of the slag/metal bath has the aforesaid advantageous aspect ratio and, because bath 11 is mainly heated from below, all the advantages referred to earlier relating to the efficiency of the chemical refining of the metal and stirring of the bath can be achieved with apparatus according to the invention. When the spent slag layer 19 has to be removed from vessel 10, paddle 15 is inserted through one of the openings 14, and the slag in layer 19 raked or pushed out through recessed outlets (not shown) provided in the said outer ends of vessel 10. The width of paddle 15 is slightly less than the cross sectional diameter of vessel 10 so that when paddle 15 engages slag layer 19, narrow gaps 20 are left between the outer ends of paddle 15 and the inside wall of vessel 10 to allow paddle 15 relatively free to-and-fro movement in vessel 10. By blowing air or nitrogen into gaps 20, slag from layer 19 is prevented from escaping through such gaps 20 during such movement of paddle 15. It will be appreciated that with such an arrangement the disadvantages referred to earlier when such a slag layer has to be removed from the vessel, are overcome or at least minimised. It will be appreciated further that there are no doubt many variations in detail possible with an apparatus and method for the secondary refinement of metals according to the invention without departing from the spirit and/or scope of the claims.

Claims: CLAIMS 1. Apparatus for use in the secondary refinement of a metal, particularly steel, characterised in that it includes an elongated vessel of which the length to width ratio is greater than two, the vessel having at least one channel type inductor adapted for heating the vessel, and the vessel being adapted in operation to be located with its long side substantially horizontally disposed. 2. The apparatus of claim 1 characterised in that the at least one inductor is disposed substantially along the centre of the bottom of the vessel while it is so horizontally disposed. 3. The apparatus of claims 1 or 2, characterised in that the vessel includes an opening towards its upper end through which any fluxes and/or alloys required in the refining process can be added to the vessel in an at least semi-continuous stream to the surface of the metal bath in a position substantially directly above the inductor(s). 4. The apparatus of any one of the preceding claims characterised in that it includes at least one lance passing from above into the bath, through which fluxes and/or alloys may be introduced to the bath. 5. The apparatus of any one of the preceding claims characterised in that it includes at least one lance or porous element stirring device by means of which argon and/or nitrogen gas may be introduced into the bath for effecting additional stirring of the bath. 6. The apparatus of any one of the preceding claims characterised in that it includes at least one plasma torch and/or three phase electric arc electrode for effecting any additional heating which may be required. 7. The apparatus of any one of the preceding claims characterised in that the vessel is of round cylindrical configuration in cross section. 8. The apparatus of any one of the preceding claims characterised in that the vessel is provided towards at least one of its shorter ends with an opening and that the apparatus includes a paddle which can be inserted through said opening for scraping spent slag from the metal bath in the vessel. 9. The apparatus of claim 8 characterised in that the width of the paddle is slightly less than the cross sectional diameter of the vessel so that when the paddle engages the slag layer narrow gaps are left between opposite ends of the paddle and the walls of the vessel for ensuring free movement of the paddle in the vessel. 10. The apparatus of claim 9 characterised in that the apparatus is adapted to have air and/or nitrogen blown into said gaps to prevent slag from escaping through the gaps when the paddle is moved along the slag layer. 11. The apparatus of any one of the preceding claims characterised in that at least one of the shorter ends of the vessel is provided with a recessed outlet through which spent slag may be removed from the vessel. 12. The apparatus of any one of the preceding claims characterised in that the metal comprises steel. 13. A method for the secondary refinement of a metal, particularly steel, characterised in that it is carried out using apparatus as claimed in any one of the preceding claims.

Inventors: FOURIE LOUIS JOHANNES, FOURIE, LOUIS, JOHANNES
Applicants: FOURIE LOUIS JOHANNES, IPCOR NV, FOURIE, LOUIS, JOHANNES, IPCOR N.V.
Application year: 1999
Main classification: C21C7
IPC codes: B22D1, C21C7, C22B9, F27B14, F27D3, F27D7, F27D23