The present invention relates to a method and an equipment for treatment of molten aluminium metal, in particular purification of the metal with respect to Na.

Commonly, aluminium metal is produced in electrolysis cells of Hall-Heroult type. Liquid metal that is produced by the cell is regularly tapped in batches via a drain tube that is dipped below the level of molten metal in the cell. The drain tube is at its opposite end connected to a tapping/transport crucible that is underpressurised for suction of metal.

The liquid metal is transported in the tapping/transport crucible to a casting furnace, or as an intermediate step to a holding furnace.

The invention is in the field of sodium (Na) removal from pot-room Al metal transported in crucibles.

There are several ways of removing Na in Al pot-room metal, e.g. treatment with addition of AIF3 and argon in crucible, and casthouse furnace treatment with the addition of salt.

These treatments typically involve the addition of active fluxing agents, e.g. AIF3. Transfer of fluorine from crucible to casthouse furnace is an environmental issue and should be reduced as much as possible.

Depending on requirements of the final product produced by the casthouses, there are limits for the contents of impurities. For instance, the molten metal in the electrolysis cells may contain 80 - 200 ppm of sodium (Na), while the threshold for instance for some cast products can be typically 1 - 15 ppm Na. This requires that sodium is removed in advance of the casting operation in a separate purification operation.

Previously, sodium has been removed from a casting furnace after the furnace being filled up with molten aluminium. As large furnaces, that may contain typically 50-60 tons of metal, are basically not efficient reactors, it is more efficient to remove sodium from the melt while it is still residing in the tapping/transport crucible, which typically contains 4-9 tons of metal.

Removal of alkali metals from an aluminium melt can be done by bringing the tapping/transport crucible to a fluxing station before its content is filled into a furnace. In the fluxing station there is added a reactant the can either be a gas (CI2) or a powder (Cl based salt or fluoride (AIF3)). Often such fluxing stations are placed in the casthouse and the tapping/transport crucible needs to be taken away from the tapping/transport vehicle and handled into the fluxing station by separate handling equipment.

By applying a pulverulent reactant, it can either be introduced in the melt via a hollow axle or with a lance, or by being spread at the surface of the melt. Common for these solutions is that a strong stirring of the melt must be created to be able to distribute the powder thoroughly in the melt. It is also crucial that the stirring action is performed over a certain time lapse to take full benefit of the reactant and thereby achieving a good purification effect.

Further, in the prior art there are known patent publications that describe treatment of molten metal with gas, based on the principle of supplying the gas to and dispersing the gas in the molten metal using a rotor. An example of such a rotor is shown and described in the applicant’s own EP 0151434 B1 , in which the gas is supplied via a drilled hole in the rotor shaft of the rotor, which consists of a hollow, cylindrical rotating body, and in which the gas is supplied to and dispersed in the liquid (molten metal) via holes in the rotating body.

Further, in US 8, 128,726 B2 there is disclosed a method and a device for the treatment of molten aluminium metal by admixture of powder in a liquid metal flowing through a drain tube connected with and influenced by sub pressure in a crucible to which the liquid metal is to be transferred.

US 3,895,937 relates to a method and apparatus for producing light metal alloys, in particular aluminium alloys, in which the desired alloying elements are first introduced into a vacuum furnace which is then evacuated, whereafter light metal melt is introduced into the furnace chamber by suction in the form of a free-falling metal jet which is thereby subjected to a vacuum treatment for reducing the contents of impurities, such as hydrogen, sodium, oxides and other non-metallic particles, therein. The free-falling metal jet is given such a configuration with respect to composition and flow pattern, such an average length as well as such velocity and direction that the alloying elements are readily dissolved and mixed into the melt in the vacuum furnace, whereby an alloy of desired quality is ready for casting immediately after termination of the vacuum treatment and alloying process.

RU 2337980C discloses refining of aluminium in a transport ladle before pouring of metal into casting metal mixers. Blending is implemented by a travelling electromagnetic field created by an electromagnetic field source which is installed against an external wall of the ladle in a fluxing station.

According to one aspect of the present invention molten metal can be purified in an apparatus or purifying station while still being in the tapping/transport crucible, that will say after the tapping/transport has finished but before the molten metal is filled into a furnace. In one other aspect the metal can be purified in the furnace.

According to one aspect of the invention, the purifying station comprises means for stirring the metal in the crucible without involving submerged rotating parts.

According to one aspect of the invention, the purifying of the metal is done without active or passive reactants. An active reactant reacts chemically with Na, whereas a passive reactant is inert.

According to one aspect of the invention it is based upon exposing the molten aluminium to under-pressure (vacuum) in the atmosphere above the molten metal in the crucible to enhance evaporation of Na from the melt in the crucible. More specifically, the under-pressure could be created with a vacuum pump. With the use of a compressor coupled to an ejector the evaporated atmosphere over the metal surface is removed continuously. The crucible can be exposed to under-pressure just after tapping from the cells, to the electro-magnetic stirring (EMS) station for treatment and finally to the casthouse furnace. This exposes the metal to under-pressure a longer time, reducing the effective treatment time in the EMS station.

According to one additional aspect of the invention, equipment comprising means for generating forced convection, e.g. through EMS is arranged in close vicinity of the crucible to stir the melt preferably in a vertical or horizontal direction, or a combination.

According to one aspect of the invention the equipment will ensure that the complete metal volume in the crucible is exposed to the under-pressure as long as the melt treatment lasts.

The under-pressure (sub-atmospheric pressure) ensures a high evaporation rate and the forced convection ensures that the total metal volume in the crucible is treated.

It has been found that exposing the melt in the crucible to a sufficiently low under pressure in combination with sufficient stirring can remove alkali metals as Na to a high extent.

The equipment according to the present invention will demand little maintenance due to no moving parts needed to generate a stirring action of the molten metal. Further it will demand less space compared to commonly available fluxing stations, due to its relatively slim design. It can be arranged in or close to the electrolysis hall and thus space in the casthouse can be available for other purposes.

The crucibles can be provided with a lid for the sub pressurisation or the equipment can have a special lid to be placed on top of the crucible for sub pressurisation during metal treatment.

The invention is in particular favourable for the removal of sodium (Na) from aluminium melts.

According to the invention it is provided a method and equipment for treatment of molten aluminium metal. More specific it relates to Na removal in a crucible with molten aluminium metal where an electromagnetic coil (EMS) generates a stirring action of the molten aluminium metal inside the crucible and where the crucible can be provided with a lid and the atmosphere below the lid is continuously evacuated during stirring of the metal where Na is removed effectively without use of active or passive reactants.

These and further advantages can be obtained by the invention as defined in the accompanying claims.

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

Fig. 1 discloses Na removal by FactSage™ Modelling,

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,

Fig. 3 discloses the crucible of Fig. 2, seen at a left side view,

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.

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.

As previously stated, there are several methods used to removing Na in Al pot-room metal, e.g. treatment with addition of AIF3 and argon in crucible, and casthouse furnace treatment with the addition of salt.

Commonly such treatments involve the addition of an active reactant, such as AIF3. T ransfer of fluorine in the melt from crucible to casthouse furnace is an environmental issue and should be reduced as much as possible. The current idea is based upon eliminating the use of reactants e.g. AIF3, 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.

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.

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.

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

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 Figure 1 , -1 on log scale corresponds to 100 mbar, -2 on log scale corresponds to 10 mbar.

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

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.

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.

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

The environmental advantages are very important when eliminating or reducing the consume of AIF3 to a very little amount. AIF3 is assumed to be one source of fluorine emissions in the casthouse.

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 AIF3 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.

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

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.

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.

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.

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.

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.

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

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.

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 .

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.

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

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.

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.

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.

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.

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:

No addition in crucible of active or passive agents: • No consumption of AIF3 which can be estimated to 0.5-1.0 kg AIF3 per ton aluminium.

• No or reduced fluorine in off-gas from casthouse.

• Lower fluorine in casthouse dross.

• Reduced fluorine problem for dross processor.

No use of injection carrier gas or other passive agents in crucible

• Reduced cost for removing Na No use of mechanical stirring in crucible

The overall cost with vacuum and EMS treatment to remove Na versus today’s standard equipment is assumed to be considerably lower.