EP0436465A1 - Refining of molten metal - Google Patents

Abstract

The method for cleaning a molten metal (10) works with an active gas introduced into its lower region, consisting of an inert carrier gas (30) and an active, gaseous halogen (20), which is metered into a vessel (12) with the standing or flowing Metal melt (10) are introduced. The active, gaseous halogen (20) is generated in a controlled manner in at least one gas development cell and is introduced into the carrier gas. The active gas mixture is passed directly into the molten metal (10). The halogen gas supply line (22) has no rule organ. The gas development cell for generating the halogen (20) is preferably an exchangeable electrolysis cell (18); gas is only generated when the electrolysis current is switched on, which follows a setpoint curve, proportional to the current strength. <IMAGE>

Description

The invention relates to a method for cleaning a molten metal with an active gas introduced into the lower region thereof, consisting of an inert carrier gas and an active, gaseous halogen, which are introduced in a metered manner into a vessel with the standing or flowing molten metal. The invention further relates to a device for carrying out and using the method.

• Spülgasbehandlung mit einem inerten Gas, beispielsweise Argon und/oder Stickstoff,
• Spülgasbehandlung mit einem aktivierten Inertgas, welches einen Zusatz eines aktiven Gases hat, beispielsweise Chlor oder Freon,
• ausschliessliche Filtration der Schmelze,
• kombinierte Spülgas- und Filtrationsbehandlung,
• Vakuumbehandlung.

In general, the purpose of melt cleaning is to reduce the concentration of both dissolved components and gaseous or solid inclusions to an acceptable level. To this end, a number of smelting processes have been developed in various metal foundries, some of which are mentioned:

• Purge gas treatment with an inert gas, for example argon and / or nitrogen,
• Purge gas treatment with an activated inert gas, which has an addition of an active gas, for example chlorine or freon,
• exclusive filtration of the melt,
• combined purge gas and filtration treatment,
• Vacuum treatment.

Of these methods known to the person skilled in the art, the purge gas treatment with an activated inert gas is of particular interest, at least in the present case.

A pure inert gas has an exclusively physical effect, the metal ions diffuse into the bubbles and vesicles of an inert gas rising in a melt due to their vapor pressure and are carried to the metal surface where the dross forms.

The addition of an active component, for example chlorine, causes a chemical reaction in addition to the physical one. A diluted, gaseous halogen oxidizes the alkali and alkaline earth metals dissolved in the molten metal, which are excreted as halides in the dross after rising. The argon and / or nitrogen used as the carrier gas for the gaseous halogen can simultaneously lower the hydrogen content of the melt.

Apart from the fact that the higher effectiveness when adding an active gas to an inert gas with higher dross losses has to be bought, the environmental compatibility of the halogens used in foundries is increasingly coming into focus. The dilution of the gaseous halogens used with an inert carrier gas has alleviated the problems with regard to the environment and workplace hygiene. With program-controlled process control, the dosing can be carried out so precisely that the gaseous halogen contained in fine bubbles is converted almost completely into a metal halide. The remaining hydrogen halides can be washed out of the exhaust gas with corresponding effort. The main problems of foundries that use processes for cleaning metal melts are hardly at this level anymore.

• Der Vorrat an gasförmigem Halogen muss unter hohem Druck in einem Lagertank ausserhalb von Gebäuden, in einem geschützten Areal, gelagert werden. Nicht nur an den Tank, auch an die Zuliefertechnik und die berstsichere Zuleitung des Halogens zur Schmelze werden erhebliche Anforderungen gestellt.
• Die erforderlichen Reduzierventile für gasförmiges Halogen bedingen hohe Wartungskosten, ausserdem sind gefährliche Manipulationen notwendig.
• Die unvermeidbare Korrosion in den Mess- und Dosiervorrichtungen führt nicht selten zu einer Verfälschung der Anzeige. Der Fachmann bleibt stets im Ungewissen, ob tatsächlich die richtige Menge gasförmiges Halogen zugeführt wird. Eine zu grosse zugeführte Menge kann die Umwelt und den Arbeitsplatz belasten sowie zu Korrosionsschäden führen, eine zu geringe Menge führt zu metallurgischen Unsicherheiten.

Of the gaseous halogens, chlorine is mainly used today, as in the past. This very aggressive gas, which is dangerous in large quantities, brings the metal foundries numerous legal requirements and considerable operational problems:

• The supply of gaseous halogen must be stored under high pressure in a storage tank outside of buildings, in a protected area. Significant demands are placed not only on the tank, but also on the supply technology and the burst-proof supply of the halogen to the melt posed.
• The necessary reducing valves for gaseous halogen result in high maintenance costs, and dangerous manipulations are also necessary.
• The inevitable corrosion in the measuring and dosing devices often leads to a falsification of the display. The expert always remains uncertain whether the correct amount of gaseous halogen is actually being supplied. An excessively large quantity can pollute the environment and the workplace and lead to corrosion damage, an insufficient quantity leads to metallurgical uncertainties.

The present invention has for its object to provide a method of the type mentioned, with which a dilute, active, gaseous halogen can be metered into a molten metal without the above disadvantages. Furthermore, a device for carrying out and using the method is to be created.

With regard to the method, the object is achieved according to the invention in that the active, gaseous halogen is generated in a controlled manner in at least one gas development cell, is introduced into the carrier gas, and the active gas mixture is passed directly into the molten metal.

The difference to the known prior art, which is essential to the invention, is that it is no longer necessary to produce a large amount of halogen, to transport it, to store it outdoors, to lead it into the interior of the building via a burst-proof line and to meter it there, but rather the gaseous halogen in that amount and for as long is produced as it is to be passed into the melt. Dosing does not take place via one or more corrosion-prone dosing devices, but by problem-free variation of the manufacturing parameters.

This becomes particularly clear in a preferred embodiment of the invention, according to which the gaseous halogen is generated in a program-controlled manner in an electrolysis cell, the gas being generated in terms of quantity per unit of time and duration by controlling the current strength of the electrolysis current.

The amount of halogen introduced into the melt per unit of time and, accordingly, the current strength of the electrolysis cell are controlled according to a specified program by the separately measured flow of the carrier gas stream, the metal flow (with molten metal flowing) and / or the concentration of reaction products or unused halogen measured above the treatment vessel .

The gas generation starts when the electrolysis current is switched on and is stopped immediately when the power supply is interrupted. During the electrolysis process, the gas formation rate is directly proportional to the flowing direct current. Since the electrolysis current can be regulated easily and precisely, the metering of the gaseous halogen formed is correspondingly exact and is not hindered by any corrosion processes. The gaseous halogen, for example chlorine, can be added in the correct amount and for the required period of time, there are neither metallurgical uncertainties due to the gas supply being possibly too low, nor unnecessary pollution of the environment and the workplace due to the gas supply being too high. The amount of halogen supplied can be dosed so that it is practically completely used up.

The developed halogen is preferably introduced into a gas stream of pure inert carrier gas and a gas mixture educated. Argon and / or nitrogen are primarily suitable as carrier gases for technical and economic reasons. These inert gases can e.g. B. metered with conventional flow meters, they have no corrosive effect.

0.5-10% by volume of a gaseous halogen is preferably mixed into the carrier gas, in particular 1-3% by volume. This dilution is known per se and is frequently used in conventional processes.

As the program-controlled halogen source for the halogens introduced in gaseous form into the carrier gas, the hydrogen halide in question, e.g. Hydrogen chloride, or an alkali salt of the halogen in question, e.g. Table salt. These halogen sources are preferably dissolved or liquefied in an electrolytic cell of a known type. When the cell is supplied with direct current, gaseous halogen is separated in proportion to the current. Hydrogen, the alkali metal in question or an alkali lye is formed as a by-product. Because of the relatively small amounts of the required gaseous halogen, the by-products are generally not used, but burned (hydrogen) or neutralized (alkali eyes).

As already mentioned, chlorine is by far the most important of the gaseous halogens used to clean metal melts. This is made from hydrochloric acid or table salt as a source of chlorine. It is not uncommon for a foundry customer to explicitly request that the metal supplied be cleaned with chlorine.

Although the gaseous halogen diluted with the inert carrier gas is supplied to the molten metal in practice in a predetermined, constant amount, thanks to the controlled Gas development in a cell, in particular in an electrolysis cell, a target curve for the time course of the amount of gas to be generated can be determined. Depending on the specific need, this can not only run parallel to the time axis, but also increase or decrease linearly, progressively or degressively. The gas can also be supplied in a pulsating manner, with or without gas generation quantities lying between the pulses. With an electrolysis cell in particular, a practically arbitrary desired curve can be defined and followed in a program-controlled manner.

Such flexibility would be completely unthinkable with a known, previously customary regulation of the supply of gaseous halogens to the carrier gas.

When removing dissolved alkali and / or alkaline earth metals from a standing aluminum melt with chlorine, for example, in accordance with a higher concentration of the impurities to be set with chlorine, a higher concentration of chlorine can initially be generated and added to the gas, preferably 3-20 vol. %. Then, in accordance with the falling impurity content of the melt, the chlorine content is gradually reduced, preferably slowly to zero. This ensures that the degree of contamination of the melt is reduced to the desired level in a minimum of time without excess chlorine being released. The gas supply elements can be immersed and pulled out while pure inert gas emerges.

With regard to the device for carrying out the method, the object is achieved according to the invention in that a vessel with a molten metal is assigned at least one gas development cell for producing a gaseous halogen and a gas feed line leading into the molten metal without a control element.

The associated gas development cells, in particular electrolysis cells, are known per se and can be found in any relevant textbook for electrochemistry. It is of importance to the invention that at least one of these cells is assigned to a vessel with a metal melt to be cleaned, and the regulation of a gaseous halogen produced is carried out by controlling the production process, not by regulating elements, for example flow meters, built into the gas supply line to the vessel with the metal melt. These control elements, which are attacked by the aggressive, gaseous halogens and work unreliably because of corrosion damage, are therefore superfluous.

The gas development cell / s assigned to a vessel with a metal melt is / are preferably interchangeable. Thus, on the one hand, the gas development cells can be used for different vessels for a molten metal, and on the other hand, if necessary, a molten metal can be cleaned with a larger amount of gas and / or with different gaseous halogens even when smaller cells are present.

Although the method according to the invention is very generally applicable for cleaning metal melts, it is particularly suitable for cleaning a melt made of aluminum or an aluminum alloy with chlorine. In addition to hydrogen, dissolved alkali and alkaline earth metals, such as sodium, lithium, magnesium and calcium, can practically be completely removed from the melt or reduced to the required level.

In a particularly advantageous manner, the method according to the invention for cleaning a molten metal can be used in a vessel arranged between a casting furnace and a casting machine, in which a filter for removing solid inclusions can be arranged at the same time.

• Fig. 1 ein Gefäss mit der Ausrüstung zur Reinigung von Metallschmelzen im Durchlaufverfahren, und
• Fig. 2 ein Gefäss mit der Ausrüstung zur Reinigung einer stehenden Metallschmelze.

The invention is explained in more detail using the exemplary embodiments shown in the drawing. The cut views show schematically:

• Fig. 1 is a vessel with the equipment for cleaning molten metals in a continuous process, and
• Fig. 2 shows a vessel with the equipment for cleaning a standing molten metal.

The contaminated molten metal 10 is passed through an inlet 34 into a vessel 12. In this vessel there is a deflection wall 14 which extends up to the region of the base, around which the molten metal 10 is guided and, after ascending, is discharged via an outlet 35. The molten metal 10 can still be passed through a filter, not shown, which retains solid inclusions.

A rotor 16 dips into the molten metal 10 from above. Of course, several lances can be arranged in a known manner instead of the rotor.

A gaseous halogen 20, in the present case chlorine, is produced in an electrolysis cell 18 of known type fed by low-voltage direct current, which is passed through the feed line 22 in the direction of the vessel 12 with the metal melt 10. The feed line 22 opens into a further feed line 24 with a metering element 28 for the inert gas 30 located upstream of the branch 26.

Downstream of the branch 26, the common supply line 22, 24 for the inert carrier gas 30 diluted with gaseous halogen is connected to the rotor 16. A rotating spray plate 32 distributes the supplied active gas into small gas bubbles which contain the molten metal 10 of hydrogen inclusions and which is supplied via the inlet 34 Clean dissolved alkali and alkaline earth metals.

The devices known per se for carrying away and disposing of halogens and by-products of the electrolysis cell 18 which are not used in the molten metal 10 are not shown for the sake of clarity.

FIG. 2 shows a standing molten metal 10 in a vessel 12. The generation and supply of the gaseous halogen is carried out in a corresponding manner in FIG. 1.

The gaseous halogen 20 diluted with the inert gas 30 is passed into a distribution chamber 36 arranged below the vessel 12 and from there passes over a flushing plug 38 with a holding device 40 as finely divided bubbles into the metal melt 10.

The alkali and alkaline earth metals reacting with the halogen collect in a dross 42 floating on the molten metal 10 and can be removed therewith.

From both figures the principle of the invention of the direct production of a gaseous halogen and its transfer into the melt without any dosing elements is clearly evident.

Claims (10)

Process for cleaning a metal melt (10) with an active gas introduced into its lower area, consisting of an inert carrier gas (30) and an active, gaseous halogen (20), which is metered into a vessel (12) with the standing or flowing metal melt ( 10) are introduced, characterized in that the active, gaseous halogen (20) is produced in a controlled manner in at least one gas development cell, is introduced into the carrier gas (30), and the active gas mixture is passed directly into the molten metal (10). Method according to Claim 1, characterized in that the gaseous halogen (20) is generated in a program-controlled manner in an electrolysis cell (18), the gas being generated in terms of quantity per unit of time and duration by controlling the current strength of the electrolysis current. A method according to claim 2, characterized in that the amount of halogen (20) introduced into the molten metal (10) per unit of time in accordance with the specified program by the separately measured flow of the carrier gas (30), in the case of a flowing molten metal (10), the metal flow and / or the concentration of reaction products or unused halogen (20) measured above the treatment vessel is controlled. Method according to one of claims 1-3, characterized in that a carrier gas (30) made of argon and / or nitrogen, preferably 0.5-10, as the active gas Vol .-% of a gaseous halogen (20), in particular chlorine, is used. Method according to one of claims 1-4, characterized in that at the end of the cleaning of a standing metal melt (10) is purged with pure inert gas (30). A method according to claim 5, characterized in that at the beginning of the cleaning of the molten metal (10), corresponding to the initially higher content of impurities, an increased amount of gaseous halogen (20) is generated and supplied, preferably in a proportion of 3-20 vol. % in the carrier gas, and the halogen content is successively reduced, in particular slowly to zero. Apparatus for carrying out the method according to one of claims 1-6, characterized in that a vessel (12) with a metal melt (10) has at least one gas development cell for producing a halogen and a gas supply line (22) leading into the metal melt (10) without a control member assigned. Apparatus according to claim 7, characterized in that the gas development cell (s), in particular an electrolysis cell (s) (18), is / are replaceable. Use of the method according to one of claims 1-6 for cleaning a melt (10) made of aluminum or an aluminum alloy with chlorine. Use of the method according to claim 9 for cleaning the melt (10) in a vessel (12) arranged between a casting furnace and a casting machine, also with a filter, for removing solid inclusions.

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