The present invention relates to an apparatus for adding and mixing powder in a molten metal. In particular, the present invention relates to adding aluminum fluoride to molted aluminium to remove sodium and to add alloying elements.

Normally, adding fluoride powder (AIF3) and mixing the fluoride powder with molten aluminium in a smelting plant or foundry remove the sodium from the molten aluminum. A motor driven rotor or a propeller in the bath of molten aluminium is typically used to evenly distribute the powder.

The mixing and sodium removal process is time consuming and equipment intensive and there is a risk that contaminants are stirred into the bath. When using a motor driven rotor, energy must be used to maintain the temperature at a higher level for a longer period to allow sufficient time for the mixing process, and for driving the motor thus increasing the overall energy consumption. These issues are also relevant when adding alloying elements.

Other typical solutions include adding the powder to the molten metal along with a propellant gas, and mixing can in that case be achieved by allowing the powder and gas to float upwards through the metal. The gas is typically argon. In this kind of systems, the inlet nozzle for gas and powder is exposed to the molten metal, and clogging of the nozzle is one out of several problems with this solution.

The present invention relates to a molten metal and powder adding and mixing system and a system for the production of metal according to the accompanying claims. The systems of the invention may reduce the energy consumption, may provide a reliable system with few or no moving parts, may improve the distribution of powder in the molten metal, may provide a solution not relying on propellant gas and may reduce the required time of the molten metal in the crucible. In the system of the invention, the powder is properly distributed in the molten material at an early stage ahead of the crucible, thus reducing the time requirement for the mixture in the crucible. Using a propellant gas adds complexity and cost.

The directions "up", "down", "upper", "lower" etc. in the specification and claims are intended to describe relative directions or locations where the direction of gravity is the reference. The direction "direction of flow" intends to describe the predominant direction of flow to separate this direction from a direction perpendicular to the direction of flow and is a direction defined regardless of an actual flow through the system. The varying cross section in the direction of flow of the mixing chamber thereby excludes that the mixing chamber is a portion of a tube.

Specifically, the invention relates to a molten metal and powder adding and mixing system. The system comprises a powder tank, a mixing chamber with a varying cross section in a direction of flow, a powder inlet, a molten metal inlet, a metal outlet providing a flow path for a mixture of molten metal and powder between the mixing chamber and a crucible, and at least one deflecting portion opposing the inlet. A flow path extends between the powder tank and the powder inlet in the mixing chamber. The powder inlet may be located in an upper portion of the mixing chamber, and the metal inlet may be located in a metal inlet side portion of the mixing chamber. The outlet may be located in a bottom portion of the mixing chamber, and the deflecting side portion opposes the metal inlet side portion of the mixing chamber. The powder inlet is typically in a "dry" portion of the mixing chamber, as a cavity is formed above the molten metal in the mixing chamber. The powder inlet is located in this cavity, allowing the powder to be spread on top of a surface of the molten metal in the mixing chamber. In other words, the mixing chamber will in operation, not be filled completely with molten metal.

The metal inlet in the mixing chamber may be located at a top end of the inlet side portion, adjacent the powder inlet. The powder inlet may be arranged in a "dry area" unexposed to molten metal.

The inlet may be horizontal or at a shallow angle close to horizontal, and may thereby be adapted to lead a jet of molten metal at a horizontal or at a shallow angle close to horizontal into the mixing chamber. This angle facilitates the maintenance of the dry cavity above the molten metal where the powder inlet is located.

The deflecting side portion opposing the inlet side portion of the mixing chamber may form a swirling unit with an outline defining a slight curvature at the top followed by a slightly increased curvature until it reaches a steep curve at an apex portion, and then ease off until an almost flat portion at a bottom portion of the mixing chamber. The mixing chamber shape thereby resembles a common "human nose" and outlet for the mix of powder and molten metal is through the "nostril".

The mixing chamber may be formed between a holding plate and a swirling unit, wherein the molten metal inlet is formed in the holding plate, and wherein the at least one deflecting portion opposing the inlet is formed in the swirling unit.

The mixing chamber may have a rectangular cross section perpendicular to the flow direction.

In one embodiment may the holding plate be integrated in the swirling unit such that the mixing chamber is formed in one unitary structure.

The flow path between the powder tank and the powder inlet may include a first closing valve and a metering nozzle. The powder inlet in the mixing chamber may include a powder spreader.

The task of the powder spreader is to distribute the powder to a powder curtain falling under the effect of gravity onto the surface of the molten metal in the "dry" cavity in the top area of the mixing chamber, finely distributing the powder onto the molten metal surface. The powder inlet may be linear and the powder may fall in a linear curtain through a longitudinal slot, not exposed to molten material.

The inlet may be formed with a tubular curved inlet flange with an attachment portion for a drainpipe.

Furthermore, the invention relates to a system for the production of aluminium. The system includes a drainpipe, a crucible and a crucible cover defining a crucible cavity. The system further includes a powder tank. A mixing chamber is located inside the crucible cavity. The mixing chamber includes a powder inlet, a molten metal inlet and a metal outlet providing a flow path for a mixture for molten metal and powder between the mixing chamber and the crucible. At least one deflecting portion opposes the molten metal inlet, and a flow path is provided between the powder tank and the powder inlet in the mixing chamber.

Locating the mixing chamber inside the cavity defined by the crucible and the crucible cover reduces the heat loss from the mixing chamber to a minimum. The mixing chamber is thereby also exposed to vacuum or nearly vacuum, and leaks in the mixing chamber are unproblematic.

The crucible cover includes at least one connector for connection to a vacuum unit.

The system for the production of aluminium defined above is combinable with a molten metal and powder adding and mixing system with any of the features mentioned above.

The molten metal may be molten aluminium and the powder may be aluminium fluoride.

Short description of the accompanying drawings:

Fig. 1 is a cross section through a crucible with a molten metal and powder adding and mixing system of the invention; Fig. 2 is a cross section of a drain head of fig. 1 in detail, and

Fig. 3 is a cross section of a drain head of fig. 1 in detail, perpendicular to the cross section of fig. 2. Detailed description of an embodiment of the invention with reference to the accompanying drawings:

Fig. 1 is a cross section of molten metal and powder adding and mixing system of the invention with a crucible 5 for molten metal 9 with a crucible cover 7 with a system for adding a powder 10 to the molten metal 9 according to the invention. The system includes a drainpipe 8 attached to an inlet flange 13 on a drain head 6. The inlet flange 13 connects a mixing chamber 4 forming a rotor chamber with the drainpipe 8. A flow path for powder 10 extends between the powder container 1 and the mixing chamber. A closing valve 1 1 followed by a nozzle 2 and a powder spreader 3 at a top of the mixing chamber 4 forms the flow path for the powder 10. A mixing chamber 4 metal outlet 14 allows mixed molten metal and powder to flow into the crucible 5. The outlet 14 is located above a surface 15 of the molten metal bath 9 in the crucible 5. Connectors 16 for connection to a vacuum pump, an ejector mechanism (not shown) or any other vacuum mechanism providing low pressure or vacuum are located in the crucible cover 7. The mixing chamber 4 and the inlet flange 13 form parts of the drain head 6.

In the embodiment shown in figs. 1 -3, is powder fed to the liquid metal during transfer of liquid metal from a furnace to the crucible 5. The vacuum mechanism maintains a low, sub-atmosphere pressure in the crucible 5, thereby "sucking" the molten metal 9 from the furnace into the crucible 5, as the furnace is at

atmospheric pressure. The powder container 1 is sealed and is exposed to the same pressure as the cavity in the crucible. The atmospheric pressure presses the molten metal into the crucible 5 through the drainpipe 8, the inlet flange 13 and the mixing chamber 4.

Low pressure affects the ability of aluminium fluoride powder to remove sodium from liquid aluminium favorably when aluminium fluoride powder is added to remove sodium from liquid aluminium. Fig. 2 is a cross section of the drain head of fig. 1 in detail, also indicating the flow of molten metal with flow lines. The flow lines show how the molten metal in the mixing chamber forming a rotor chamber leads the molten metal and powder into a swirling motion or vortex in the mixing chamber, facilitating the mixing of the molten metal and the powder. The specific mixing chamber shape shown in the drawings provides three "rotors" or vortexes finely distributing the powder in the molten metal. The powder 10 in the powder tank 1 runs through the closing valve 1 1 and the powder spreader 3 at the top of the mixing chamber. A metering nozzle 2 in the flow path between the powder spreader 3 and the opening and closing valve 1 1 ensures addition of the correct amount of powder into the molten metal. A "dry" cavity 12 is formed in the mixing chamber above the molten metal, allowing the powder to be distributed onto a top surface 20 of the molten metal inside the mixing chamber where the powder inlet is located. A holding plate 17 and a swirling unit 18 defines the outer perimeter on the inside of the mixing chamber. The powder spreader 3 is located at a top of the mixing chamber and the swirling unit 18, allowing the powder to be distributed into the molten metal at the top of the mixing chamber. The swirling unit 18 is formed like a "human nose" and the inlet flange enters through the holding plate 17, allowing the molten metal enriched with powder to impinge onto a wall of the swirling unit 18 at the upper part of the nose. The outline of nose shape defines a slight curvature at the top, and the curvature increases slightly until it reaches its steepest curve at its outer extremity at the rightmost portion of the mixing chamber. The curvature eases off until a flat or almost flat portion at a bottom portion of the mixing chamber. The outline then makes a sharp turn downwards to form the outlet 14. The sharp turn downwards forms a step in the outlet 14. The outline is concave or flat along its entire length apart from the sharp downwards turn for the outlet 14. This outline is formed entirely in the swirling unit 18. The holding plate 17 located below the inlet flange 13 opposite the swirling unit 18 also defines a concave curved surface facing towards the swirling unit 18, with the sharpest curvature at the top close to the inlet flange 13. The curved surface of the holding plate 17 extends all the way to the outlet 14. The curved surface of the holding plate imposes an upward force on the molten metal impinging onto the holding plate, thereby facilitating the vortex motion of the molten metal, thus improving mixing. Accordingly, the outlet forms a duct with one substantially flat side on the swirling unit 18, and one substantially curved side on the holding plate opposite the flat side of the swirling unit 18. The outlet 14 is rectangular when seen from below. The flow lines show how the molten metal and powder mixture follows the wall of the swirling unit 18 before impinging on the holding plate 17 above the outlet 14, thereby forming a swirling motion in the nose before exiting out of the outlet 14 and into the molten metal bath in the crucible.

The mixing chamber has thus a varying cross section in a direction of flow of the molten metal from the drainpipe 8. A powder inlet 22, a molten metal inlet 21 and the outlet 14 provides the flow path for the mixture of molten metal and powder between the mixing chamber and the crucible. A deflecting side portion located on the swirling unit 18 opposes the molten metal inlet 21 . The powder inlet 22 is located in an upper portion of the mixing chamber. The molten metal inlet 21 is located in an inlet side portion of the mixing chamber. The outlet 14 is located in a bottom portion of the mixing chamber, and the deflecting side portion opposes the inlet side portion of the mixing chamber 4.

The molten metal inlet 21 of the mixing chamber is located at a top end of the inlet side portion, adjacent the powder inlet 22.

The molten metal inlet 21 is horizontal or at a shallow angle close to horizontal, and is adapted to lead a jet of molten metal at a horizontal or a shallow angle close to horizontal into the mixing chamber.

The swirling unit 18 form an outline defining a slight curvature at the top, followed by a slightly increased curvature until it reaches a steep curve at an apex portion, and then ease off until an almost flat portion at a bottom portion of the mixing chamber 4.

Fig. 3 is a cross section of the drain head of fig. 1 in detail, in a section

perpendicular in to the cross section of fig. 2. Fig. 3, shows the width of the mixing chamber, that the "nose shaped" cross section is uniform across this width and that the rectangular metal outlet 14 provides the outlet from the mixing chamber 4. The opening or closing valve 1 1 and the metering nozzle 2 are located in the flow path for the powder 10 in the powder container 1 . The metering nozzle 2 in the flow path above the powder spreader 3 ensures that the spreader 3 distributes the correct amount of powder to the molten metal. The spreader includes a plow shaped structure to distribute the powder to a powder curtain. In its simplest form, the metering nozzle 2 includes a plate with a hole or aperture allowing a certain amount of powder to pour through. The powder nature of the powder 10 allows the powder to run into the mixing chamber 4 due to gravity in spite of the vacuum or low pressure in the powder container. The swirling unit includes two substantially flat side portions 19 to form a complete enclosure of the mixing chamber.

The powder is typically aluminium fluoride (AIF3), and the molten metal is typically molten aluminium. There are no moving parts in contact with the molten metal, and the powder is completely mixed with the molten metal upon entry into the molten metal bath (molten metal bath 9 on fig. 1 ). Accordingly, the mixing of the powder with the molten metal is quick and no extra energy is required. The cavity inside crucible and crucible cover is exposed to vacuum when the drainpipe is in a draining position in a cell with molten aluminium to suck the molten aluminium through the drainpipe. The valve in the powder tank opens at the same time as the cavity is exposed to vacuum. The powder is metered into the molten metal jet and is mixed with the metal. The shape of the mixing chamber 4 creates axial rotations or turbulences holding the metal at the same time as the powder is mixed into the metal to achieve a homogenous and even distribution of the powder in the metal.

The low number of moving parts provides a relatively uncomplicated and cost effective structure, both in terms of building and operating costs. Apart from the valve, the system can be built without any moving parts. The system is easy to retrofit on existing equipment and involves modest installation costs.

The compact design with the essential components inside the crucible below the crucible cover also provides a solution with negligible temperature loss and thus no increase in hot surface areas on the solution.

The invention was developed particularly for aluminium and aluminium fluoride powder, but the invention can be used for other metals and additives in powder form. Another relevant metal is magnesium.