Description of Invention

The present invention relates to the treatment of molten metal with a fluid. An example of such treatment and of apparatus for c__rrying out the treatment is disclosed in US 4177066 issued December 4th 1979. This patent discloses treatment in which molten aluminium is caused to flow along a path which includes a descending portion and an ascending portion and in which bubbles of argon containing a gaseous halogen or a gaseous derivative of a halogen are introduced into the descending portion of the path. The descending portion of the path for the molten aluminium is defined by a hollow body which includes an upper cylindrical portion and a lower tapered portion. The ascending portion of the path is defined by a riser which is spaced from the body.

The path along which the molten metal flows through treatment apparatus as described in the US patent is generally defined by a body formed of silica-based materials. In order to reduce loss of heat from the molten metal in the path, a body defining the flow path may be surrounded by a fibrous jacket. Nevertheless, in many cases, it is necessary to provide means for supplying heat to metal in the flow path. Typically, electrical heating elements are positioned at the outside of walls defining the flow path so that heat is transmitted through these walls to the metal.

The apparatus described in the US patent includes a number of nozzles which are fixed in a wall surrounding the descending portion of the flow path. It is also known to introduce a gaseous fluid to a body of molten metal by means of a rotor having orifices through which bubbles of gas emerge into the metal as the rotor turns. The rotor requires a substantial space and one advantage of the arrangement of fixed nozzles described in the US patent is that the descending portion of the flow path can be somewhat smaller, for

a given throughput of molten metal, than in a case where the gas is introduced through a rotor. It will be understood that it is desirable minimise the size of the apparatus required for a given throughput of molten metal and, accordingly, the fixed nozzle arrangement is preferred.

According to a first aspect of the present invention, there is provided apparatus for treating molten metal with a fluid and comprising a body ^' defining a flow path for the metal, an inlet for admitting a gas to the flow path and heating means for supplying heat to metal in the flow path, wherein the inlet is fixed with respect to the body and wherein the heating element is at least partly disposed in the flow path and is arranged to be immersed in the molten metal which flows along the flow path.

In the preferred apparatus, the body defines an outlet for the treatment fluid adjacent to an inlet for the molten metal.

According to a second aspect of the invention, there is provided apparatus for treating a molten metal with a fluid and comprising a body defining a flow path which includes a downwardly extending portion and a riser which, in use, receives a flow of molten metal from the downwardly extending portion of the path, the apparatus further comprising feed means for feeding the treatment fluid to the flow path, wherein the body comprises an outer part which surrounds the flow path and a baffle which separates the riser from the downwardly extending portion of the flow path and wherein the thermal conductivity of the baffle is greater than is the thermal conductivity of the outer part of the body.

By describing the thermal conductivity of the baffle as greater, I mean that the rate of flow of heat through the baffle per unit area of surface at the boundary of the downwardly extending portion of the flow path per degree centigrade temperature drop across the baffle exceeds the rate of flow of heat through the outer part of the body per unit area of surface at the boundary of the downwardly extending portion of the flow path per degree centigrade temperature drop across the outer part of the body. The outer part of the body is impermeable to molten metal and has substantial compressive strength.

The outer part of the body may be surrounded by a fibrous jacket

The baffle may be adapted by selection of the composition and/or of the form and dimensions of the baffle to conduct heat at a relatively high rate. The baffle is preferably formed mainly or entirely of graphite.

The outer part of the body preferably has a pair of spaced seats on which opposite marginal portions of the baffle bear and these seats are

^' preferably inclined to each other so that the baffle will have the effect of a wedge, when introduced into the gap between the seats. The baffle is preferably tapered to conform to the seats.

In the preferred apparatus, heating means is provided for supplying heat to the metal in the flow path and the heating means is disposed entirely at one side of the baffle.

The first and second aspects of the invention are preferably used in combination.

According to a third aspect of the invention, there is provided a method of treating molten metal with a treatment fluid wherein the metal is caused to flow along a path which includes a descending portion and an ascending portion, the treatment fluid is mixed with the metal in said path, the ascending portion is separated from the descending portion by a baffle so that the metal flows downwards at one face of the baffle and upwards at an opposite face of the baffle and wherein heat is transmitted to the metal in the ascending portion of the path from the metal in the descending portion of the path via the baffle.

An example of apparatus embodying both the first and second aspects of the invention and which is used in a method according to the third aspect of the invention will now be described, with reference to the accompanying drawings, wherein

FIGURE 1 shows diagrammatically a cross section in a vertical plane through the apparatus,

FIGURE 2 shows certain parts in cross section on the line II-II of Figure 1 and

FIGURE 3 shows a part of Figure 2 on an enlarged scale. The apparatus illustrated in the drawings comprises a hollow body 10 which defines an inlet 11 for molten metal, an outlet 12 for the metal and a flow path extending from the inlet to the outiet The flow path includes a descending portion 13 and an ascending portion 14. The inlet and the outlet are at approximately the same level so that molten metal can be caused to ^* flow along the flow path by an hydrostatic head.

The body 10 extends upwardly somewhat from the level of the inlet 11 and outiet 12 to provide a space 15 above the level of the surface of molten metal in the apparatus to accommodate solid matter separated from the molten metal. An open top of the body is normally closed by a lid 16 which can be removed to permit the separated solid matter to be extracted from the apparatus.

Heating means is provided for heating metal in the flow path. The heating means consists of a single, electrically energisable heating element 17. The heating element is of substantially cylindrical form and is arranged with its length extending from the lid 16 towards a bottom wall of the body 10. The heating element incorporates a sheath which is able to withstand the action of molten aluminium. The sheath may be formed, for example, of silicon carbide bonded by silicon nitride.

The descending portion 13 of the flow path is typically annular, as viewed in plan. As shown in the drawing, the flow path may taper towards the bottom of the descending portion and the flow path may be of uniform transverse cross section at a level above the tapered portion. The heating element 17 is disposed centrally in the descending portion 13 of the flow path. It will be noted that a lower part of a heating element is immersed in the metal contained in the flow path. An upper end portion of the heating element projects above the metal and is sealed to and supported by the lid 16. The conversion of electrical energy to heat energy takes place substantially entirely in that part of the element which is immersed in the metal.

The apparatus further comprises feed means for feeding a gaseous

mixture into the descending portion 13 of the flow path. The feed means includes nozzles, which may be of known form, mounted in respective openings in the body 10 with discharge ends of the nozzles at that face of the body which defines the lower, tapered part of the descending portion 13 of the flow path. A representative nozzle is shown at 19 in Figure 1 and there is also represented diagrammatically a duct 20 for conveying the gaseous mixture to

_* the nozzle 19, a valve 21 for controlling the flow of the gaseous mixture and a pressurised source P of the gaseous mixture.

As shown in the drawing, a drain plug 22 may also be inserted in an opening near to the bottom of the body 10 to permit molten metal to be drained from the flow path on removal of the plug.

The descending portion 13 of the flow path is separated from the ascending portion 14 by a baffle 18. The baffle comprises one or more blocks seated in an outer part of the body 10. As shown in Figure 3, the seats for each block are slightly convergent in a direction from the descending portion 13 towards the ascending portion 14 of the flow path so that the hydrostatic pressure in the descending portion 13 of the path urges the block onto its seats and maintains a substantially liquid-tight joint between the baffle and the outer part of the body. Furthermore, the baffle is arched as shown in Figure 1 and is trapped between the wall 23 above the baffle and the wall 24 below the baffle where laterally spaced portions of the baffle rest on the wall. The arched shape of the baffle ensures that it cannot move away from the seats in a direction towards the descending portion 13. The baffle is spaced upwardly from the bottom wall of the body 10, to provide communication between the portions 13 and 14 of the flow path.

The respective seats formed on the outer part of the body 10 and engaged by opposite marginal portions of the baffle 18 are flat and are mutually inclined at an angle in the region of 10°. The surfaces of the baffle which bear on these seats also are flat and are mutually inclined at the same angle. These surfaces of the baffle engage in face-to-face contact with the outer part of the body at the seats. The seats are mutually divergent in a

direction from the ascending portion 14 towards the descending portion 13 of the path.

The baffle 18 has a higher thermal conductivity than does the outer part of the body 10. The outer part of the body may be formed of a material which is a moderately good thermal insulator and may be surrounded by a fibrous jacket which provides additional thermal insulation for the contents of ^* the feed path. A material having a substantial silica content is suitable. The baffle 18 is typically formed of high density graphite.Graphite has a coefficient of thermal conductivity which is considerably greater than the coefficient of thermal conductivity of silica -based materials known for use in confining molten aluminium

It will be noted that there is no heating element in the ascending portion 14 of the feed path or closely associated with any wall which constitutes a boundary of that portion of the feed path. The sole heating element provided in the apparatus is the element 17 submerged in the metal in the descending portion 13 of the flow path. During operation of the apparatus, heat is imparted from the heating element 17 to the metal in the descending portion 13 and is conducted from the metal in that portion through the baffle 18 to the metal in the ascending portion 14 of the flow path. The rate at which heat is conducted through the baffle 18 is sufficiently high to ensure that metal does not solidify in the ascending portion 14 of the flow path or in the outlet 12 from the apparatus. It will be noted that the baffle 18 is disposed inside and is completely surrounded by the outer part of the body 10. Both the outer part of the body and the baffle are impermeable to molten metal and have substantial mechanical strength, in particular compressive strength.

The outer part of the body 10 defines a substantially horizontal portion 25 of the path for flow of molten metal from the inlet 11 to the downwardly extending portion 13 of that path. Above the molten metal in this horizontal portion of the path, there is a space to receive gases which have bubbled up through the molten metal from the gas inlet 19. Adjacent to the

inlet 11 for the molten metal, there is an outlet 26 for these gases.

The inlet 11 is defined between a wall portion 27 of the body 10 which defines the bottom and the sides of the portion 25 of the flow path and a baffle 28 which extends upwardly from the inlet 11. When the apparatus is in normal use, a lower marginal portion of the baffle 28 is submerged in the molten metal so that the inlet 11 is completely occupied by molten metal and ^' it is not possible for air to enter the apparatus through this inlet. In the event of the level of the surface of the molten metal at the inlet 11 being permitted to fall, for example when flow of metal through the apparatus is to be discontinued, the lower edge of the baffle 28 may become exposed above the surface of the molten metal and air may thus be admitted to the apparatus through the inlet 11. In this case, the incoming air will meet the flow of gas towards the gas outlet 26 and any air which does enter the apparatus through the inlet 11 will be carried directly to the outiet 26 and discharged from the apparatus. In this way, the turbulent surface of the molten metal directly above the descending portion 13 of the flow path is protected from contact with air.

The gas outlet 26 lies immediately adjacent to the baffle 28 and the baffle preferably forms one boundary of the gas outlet. Since the baffle is vertical, there is no substantial horizontal spacing of the gas outlet 26 from the metal inlet 11. The gas outlet 26 is, however, spaced upwardly from the inlet 11 by a substantial distance. It will be noted that the baffle 28 and the gas outlet 26 are spaced horizontally from the descending portion 13 of the flow path by a substantial distance, typically by a distance which exceeds the vertical spacing of the gas outlet 26 from the metal inlet 11. It will be understood that the lid 16 normally seals the top of the apparatus against egress of the gases and that the gas which bubbles up through the molten metal in the descending portion 13 of the path is therefore constrained to flow horizontally towards the baffle 28, before escaping through the gas outlet 26.

The flow path for molten metal extending from the inlet 11 to the descending portion 13 of the flow path is preferably offset from the vertical

axis of the descending portion 13 so that when molten metal is flowing through the apparatus, the molten metal tends to spiral downwardly in the descending portion of the path.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for ^* attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.