METHOD AND EQUIPMENT FOR TREATMENT OF MOLTEN ALUMINIUM METAL

Abstract

A method and an equipment for the treatment of molten aluminium metal, in particular Na removal in a tapping/transport crucible (1) provided with a lid (6). The atmosphere above the molten metal is sub-pressurized to a sub pressure between 300-10 mbar and subsequently exposed to a stirring action generated by an electromagnetic coil (EMS) arranged towards one side of the tapping/transport crucible (1), whereby Na can be removed from the molten aluminium metal without any addition of active or passive agents.

Claims

1-12. (canceled)

13. A method for the treatment of molten aluminium metal for Na removal in a tapping/transport crucible (1) by generating a stirring action of the molten aluminium metal, the tapping/transport crucible (1) has a mantle wall of a material that allows an electromagnetic field generated by an electromagnetic coil (EMS) brought in vicinity of said material to generate a stirring action of the molten aluminium metal inside the tapping/transport crucible (1) when energizing the electromagnetic coil (EMS), wherein the tapping/transport crucible (1) is adapted to fit a lid (6) or cover in a substantial air tight manner where the lid is provided with means for continuously evacuating the atmosphere below the lid to a sub pressure between 300-10 mbar during stirring of the aluminium metal, where Na is removed from said space without application of fluxing agents.

14. The method according to claim 13, wherein the atmosphere is evacuated to a sub pressure between 200-10 mbar.

15. The method according to claim 13, wherein the atmosphere is evacuated to a sub pressure between 100-10 mbar.

16. The method according to claim 13, wherein EMS coil is operated to generate forced convection in the upper part of the melt in the tapping/transport crucible (1) to maximize the exposure of volatile components to the sub pressurized space.

17. The method according to claim 13, wherein the EMS coil is operated to create highest metal velocity in the bottom part of the crucible to lift metal to the surface region.

18. The method according to claim 13, wherein the temperature in the metal during treatment is approximately 900? C.

19. Equipment for the treatment of molten aluminium metal for Na removal in a tapping/transport crucible (1) and generating a stirring action, the crucible comprises an outer mantle wall (4), arranged adjacent an electromagnetic coil (EMS) in the metal treatment equipment and that a stirring action of the molten aluminium metal takes place by energizing the electromagnetic coil, wherein the tapping/transport crucible (1) is provided with a lid (6) or cover having means for tightly adapt to it and further having means for continuously evacuating the atmosphere below the lid to a sub pressure between 300-10 mbar during the stirring of the aluminium metal for removal of Na without application of fluxing agents.

20. The equipment according to claim 19, wherein the tapping/transport crucible is made by a mantle wall (4) made out of a non-ferromagnetic material, such as austenitic stainless steel.

21. The equipment according to claim 19, wherein the lid (6) of the tapping/transport crucible is provided with a seal and locking mechanism that secures gas tight connection to the crucible.

22. The equipment according to claim 19, wherein the lid (6) of the tapping/transport crucible is provided with a gas connection for connecting to a vacuum pump controlled by a control station.

23. The equipment according to claim 19, wherein the lid (6) is provided with an ejector that allows the continuous removal of the atmosphere in the crucible.

24. The equipment according to claim 19, wherein the lid (6) is provided with a controllable inlet (10) for ambient air to enter the inside of the crucible.

Description

[0029] The present invention will be described further by examples and Figures where:

[0030] FIG. 1 discloses Na removal by FactSage? Modelling,

[0031] FIG. 2 discloses from one side a tapping/transport crucible with a lid provided with equipment for evacuating atmosphere inside the crucible and connected with an EMS,

[0032] FIG. 3 discloses the crucible of FIG. 2, seen at a left side view,

[0033] FIG. 4 discloses a tapping/transport vehicle with a crucible at an electromagnetic stirring station including an electromagnetic spool (EMS), the crucible having a lid for sub pressurisation.

[0034] The invention is in the field of sodium (Na) removal from pot-room Al metal in crucibles, in particular transport crucibles. The crucible may be of a stationary type receiving molten metal from electrolysis and further arranged for Na removal according to the invention, before the metal is transferred to a casting furnace. In an alternative, a casting furnace may be provided with Na removing equipment, according to the invention.

[0035] As previously stated, there are several methods used to removing Na in Al pot-room metal, e.g. treatment with addition of AlF.sub.3 and argon in crucible, and casthouse furnace treatment with the addition of salt.

[0036] Commonly such treatments involve the addition of an active reactant, such as AlF.sub.3. Transfer of fluorine in the melt from crucible to casthouse furnace is an environmental issue and should be reduced as much as possible.

[0037] The current idea is based upon eliminating the use of reactants e.g. AlF.sub.3, gas (chlorine) or salts to remove Na from pot-room metal, by applying under-pressure (vacuum) in combination with removing the atmosphere above the melt surface in the crucible to enhance evaporation of Na from the melt in the crucible. In addition, forced convection, e.g. through EMS (electromagnetic stirring) mounted on the crucible to stir the melt will be used so that the complete metal volume in the crucible is circulated and exposed to the under-pressure as long as the melt treatment lasts.

[0038] The under-pressure (vacuum) and the continuous removal of the atmosphere above the melt surface ensure a high evaporation rate and the forced convection ensures that the total metal volume in the crucible is treated.

[0039] The pot-room metal temperature is generally high during crucible treatment, typically above 900? C., resulting in relatively high Na vapour pressure over the melt. Using under-pressure (vacuum) in combination with removing the atmosphere over the melt surface will further increase the Na evaporation rate. The removal of the atmosphere ensures that there is no back diffusion of Na into the melt. Typically, an under-pressure of 100 mbar is sufficient to allow for a high Na removal rate within a normal treatment time of 10 minutes. The lower the under-pressure, the higher the Na removal rate for a certain treatment time, the shorter the treatment time will be to achieve a certain Na removal rate.

[0040] The efficiency of the process has been verified by modelling of Na removal with evaporation from Al with a modelling tool named FactSage.

[0041] According to FIG. 1, FactSage modelling has shown a strong Na removal rate with increased under-pressure. As an example, at metal temperature of 940? C. and under-pressure of 100 mbar, FactSage predicts acceptably high Na removal efficiency. However, FactSage does not predict kinetics, i.e. how long time the reduction will take. In the FactSage diagram of FIG. 1, ?1 on log scale corresponds to 100 mbar, ?2 on log scale corresponds to 10 mbar.

[0042] Based upon these modelling results, full scale trials have shown 80% Na removal rate after 10 minutes treatment time.

[0043] Follow-up full-scale trials showed Na removal rates of 67% after 5 minutes treatment time (two crucibles), 76% after 10 minutes treatment time (two crucibles), and 80% after 15 minutes treatment time (two crucibles), all with EMS with 85 Amperage current intensity and 180 mbar under-pressure. The metal temperature during the trial was approximately 900? C.

[0044] Further, full-scale trial filling a furnace with vacuum treated pot-room metal shows that the Na level during casting from the furnace ended at 7 ppm. Typical customer specification in Al extrusion ingots is maximum 15 ppm Na, however stricter requirements are seen, e.g. for rolled products.

[0045] The sub-pressurized atmosphere in combination with removal of the atmosphere in the crucible and the use of EMS increase kinetics of Na evaporation and can ensure that all metal in crucible is treated in short amount of time.

[0046] High pot-room metal temperature ensures high evaporation, typically >900? C. At temperatures >883? ? C. Na is a gas (boiling point: 882.9? C.).

[0047] The environmental advantages are very important when eliminating or reducing the consume of AlF.sub.3 to a very little amount. AlF.sub.3 is assumed to be one source of fluorine emissions in the casthouse.

[0048] By the use of the invention, strongly reduced fugitive fluorine emissions are expected, both from casthouse furnaces and from dross processor. The elimination of the use of AlF.sub.3 in order to remove Na from pot-room metal gives an important contribution in reduction of fluorine emissions and reduces the need for gas treatment. In addition, there are financial gains due to the relatively high cost for the fluxing agents.

[0049] According to the invention as shown in FIGS. 2 and 3, the stirring of the molten metal in the tapping/transport crucible 1 is done by application of an electromagnetic coil (EMS) that due to the electromagnetic fields can generate a stirring or formation of vortexes of the molten metal. The EMS equipment can simply be a vertically arranged structure such as a column or a similar structure that carries the electromagnetic coil (EMS).

[0050] In general, the steel quality of the crucible should preferably be a non-ferromagnetic material, preferably austenitic steel.

[0051] The wall or mantle of the tapping/transport crucible can in general be of a steel quality as mentioned, and can also be provided with an inner liner of refractory material.

[0052] Further, during treatment and for the purpose of removal of Na, the system must be able to evacuate at least partly the atmosphere that is present in the crucible. That will say the space below its lid 6 and above the metal melt.

[0053] This evacuation can be done by exposing the atmosphere to a sub-pressure and allowing ambient air to replace the gas containing Na that is evacuated. That will say the evacuation is done in a continuous manner for continuous removal of Na.

[0054] As shown in FIGS. 2 and 3 the evacuating system can basically comprise an ejector 8 having a suction tube 7 communicating with the inside of the crucible 1 below the lid 6 thereof. Further, referring to FIG. 3, the ejector 8 is provided with an inlet CA for compressed air and one outlet for evacuated gas, EG. An arrow inside the suction tube 7 indicates flow/removal of gas from the crucible's inside, which basically is separated in a lower metal part M, and an upper gas part, G.

[0055] To replace evacuated gas, the lid is provided with an inlet tube 10 that allows ambient air to flow into the inside of the lid 6. The inflow can be controlled by a valve 11, which also will influence the level of the sub-pressure that will be achieved as well as the removal rate of the atmosphere.

[0056] The evacuated gas EG can be captured and removed by a suction system arranged above the lid of the crucible.

[0057] As shown in FIG. 4, the evacuating system of the tapping/transport crucible can be arranged onboard a tapping/transport vehicle 5 provided with a compressor for operating the ejector 8. The vehicle 5 may also have provisions for handling a lid to be placed upon the crucible, and in general comprise similar equipment as described above.

[0058] As the stirring action is to be performed, the tapping/transport crucible 1 is placed towards the electromagnetic coil (EMS). By energizing the coil with electrical current, a strong stirring action of the molten metal in the tapping/transport crucible 1 is generated. The tapping/transport crucible 1 can rest in the tapping/transport vehicle 5 during this operation. The EMS can be arranged in a column or a similar structure and further in a manner where it can be moved horizontally towards the tapping/transport crucible 1.

[0059] The columns with electromagnetic coils EMS will demand very little space. As the treatment or purification now can be done in the electrolysis hall where the electrolysis cells are placed or at least in close vicinity to the hall, the off suction of gas from the tapping/transport crucibles, can be handled by a much simpler duct system.

[0060] The time necessary to perform the purification operation can be reduced as there will be no time-consuming dosing action of fluxing agents needed.

[0061] The sub-pressure can extract gas components such as Na during purification through a hole 7 in a lid 6 of the tapping/transport crucible 1.

[0062] Regarding the stirring pattern of the liquid metal, it can be changed by changing the electrical input, such as current intensity and polarity, to the EMS. A pre-requisite is that as much as possible of the melt is exposed to the under-pressure in the crucible atmosphere during treatment, and the EMS can be controlled according to this.

[0063] The EMS is utilized to create a powerful stirring of the metal while the crucible is attached to the transport vehicle without removing the crucible lid. The transport vehicle will position the crucible close to the EMS for stirring.

[0064] In an alternative, the EMS is utilized to create a powerful stirring of the metal when the crucible is removed away from the transport vehicle and positioned close to the EMS.

[0065] Forced convection (e.g. EMS) in crucible with pot-room metal is used to obtain forced movement of metal. Stirs metal so that all crucible pot-room metal is exposed to sub-pressurized atmosphere.

Advantages of the Present Invention

[0066] No addition in crucible of active or passive agents: [0067] No consumption of AlF.sub.3 which can be estimated to 0.5-1.0 kg AlF.sub.3 per ton aluminium. [0068] No or reduced fluorine in off-gas from casthouse. [0069] Lower fluorine in casthouse dross. [0070] Reduced fluorine problem for dross processor. [0071] No use of injection carrier gas or other passive agents in crucible [0072] Reduced cost for removing Na [0073] No use of mechanical stirring in crucible [0074] The overall cost with vacuum and EMS treatment to remove Na versus today's standard equipment is assumed to be considerably lower.