WO2003035917A2

WO2003035917A2 - Method for processing magnesium containing scrap by melting in a vacuum furnace

Info

Publication number: WO2003035917A2
Application number: PCT/HU2002/000106
Authority: WO
Inventors: Gábor VALLÓ; István Hegedüs; József HAUPT; Tibor JÓNYER
Original assignee: Vallo Gabor; Hegedues Istvan; Haupt Jozsef; Jonyer Tibor
Priority date: 2001-10-24
Filing date: 2002-10-18
Publication date: 2003-05-01
Also published as: AU2002341227A1; HUP0104447A2; WO2003035917A3; HU0104447D0

Abstract

A method, that enables the processing and recycling of magnesium-containing scrap, primarily the highly flammable scrap with large surface-to-volume ratio, (clips, flashing, turnings, chips), by way of melting, whilst maintaining a suitable quality of the metal for die-casting, which method is carried out in a vacuum furnace, rated for low overpressure, vacuum, having a double-jacketed melting hearth and a vacuum-sealed crucible. Magnesium-containing scrap is charges into the crucible of the vacuum furnace and the temperature is raised until the charge reaches the melting point. The sealed melting hearth evacuated space is maintained until the melting process is completed. Then the space above the melt is filled up with protective inert gas maintained under a pressure above the atmospheric pressure, and a flux mixture is introduced onto the bath surface, and the bath of molten metal together with the introduced flux is agitated, whilst preferably continuously introducing protective inert gas into the interior of the said bath. The emerging dross is skimmed off and agitation is continued until no more dross emerges. Then a new charge of scrap is charged into the crucible maintained under inert gas pressure to reach a required melt volume. After seal closing the next gas pressure is discontinued and the vacuum system is opened for refining the magnesium melt.

Description

METHOD FOR PROCESSING MAGNESIUM CONTAINING SCRAP

We, HEGEDUS Istvan, of H-3032 Gyόngyόs, Farkas Tamas u. 3, NALLO Gabor, of H-1 106 Budapest, Bojtocska u. 23/B and HAUPT Jόzsef, of H-3908 Ratka, Kossuth u. 37, all Hungarian citizens, do hereby declare the nature of this invention, for which we request that a patent be granted to us, to be particularly described in and by the following statement:

TECHNICAL FIELD

The present invention relates to methods of melting of magnesium containing materials in a melting furnace. The object of this invention is to provide a method, that enables the processing and recycling of hazardous, highly flammable, magnesium-based scrap with large surface-to- volume ratio, generated in foundry, in particular in casting (clips, flashings), as well as magnesium alloy process arisings (turnings, chips), generated during machining of the cast metal, by way of melting, whilst maintaining a suitable quality of the metal for die-casting.

At the casting of components and the machining of the cast metal, process scrap in volumes reaching to 40-50 % of the input material is generated, about one third of which are scrap with large surface-to-volume ratio and turnings, chips.

BACKGROUND ART

As to present state of the art, a characteristic example of the method of metallurgical processing of magnesium scrap, may in brief be described by the following process steps:

Selected pieces of lumpy scrap of magnesium or its alloys, being entirely free from oils and moisture, is charged into a steel or cast iron crucible of a melting furnace - being at modern technologies an induction furnace. The induction furnace is a low-frequency one, for the sake of agitating effect. Melting furnaces may as well be chosen with a heating system, other than induction. For the sake of casting and manipulating the melt, the melting furnaces are mainly tiltable. The metal in the crucible is heated to 720 °C temperature to melt it. The technological overheating reaches to 850 to 900 °C, depending on the actual composition of the charge.

The magnesium has a strong affinity to the oxygen, the molten metal is therefore protected from combustion by filling up the empty space of the crucible above the bath with protective inert gases [specifications GB395633; GB403891, GB457826 are referred to] and by applying fluxes [specifications GB401672; GB583402, GB590172 are referred to] The fluxes serve the purposes of both the protection and purification of the melt When the metal has become molten at 720 °C temperature, a complex flux-mixture is charged onto the surface of the bath, which is then agitated by paddles of a mechanically rotated agitator, immersed in the bath, or by any other mechanical agitating device [specification GB403891 is referred to] Through the holes of a longitudinally hollow shaft, immersed in the bath, inert gas is introduced into the bath The inert gas is then bubbled through the melt, which brings the oxides and other non-metallic inclusions to the bath surface, [specification WO9317136 is referred to] Following a necessary agitation and purification, which takes 10 to 20 minutes in case of melting lumpy scrap, the fixed dross is skimmed off from the bath surface by traditional skimming ladle or similar device Purified and ladle-conditioned molten metal is then poured into the ladle of a holding furnace, or directly to casting die, through a pouring gate by tipping [specification WO9600799 is referred to], or through a pouring tube by either pumping [specification JP8215830 is referred to], or by introducing overpressure [specification EP0126797 is referred to]

As protective gas, substantially a mixture of 0-2% SO₂ , 60-80% Ar , 18-40% N₂ is used Fluxes are available as merchandise, in compositions set by the producers according to particular alloys Flux mixtures contain - inevitable for the melting - deliquescing chlorides, de-oxidizing agents, refining and protecting fluorides, the latter may also including BeF, a pre-alloy of beryllium By way of example, the flux mixture ELRASAL D of 1G Farbenindustrie AG is referred to, which contains in % volume ratio MgCl₂ 31, MgO 5, CaCl₂ 7, Ca F₂ 21. KC1 25, NaCl 6, H₂O 4, and other components total 2

The magnesium alloys are available in form of casting ingots, with compositions corresponding to standards (e.g. ASTM), identified as e g AZ91, AM50, AM60 As an example, alloy AZ91 contains the following constituents in % volume ratio. Al 8,0-10,0, Mn 0,1 min , Zn 0,3-1,0, Cu 0,2 max.; Si 0,3 max , Fe 300 ppm max ; Ni 100 ppm max , Be 6-12 ppm max , the balance being magnesium.

To the present state of the art, the melting of scrap pieces of magnesium and its alloys with small volume and large surface-to-volume ratio is restricted to charging such scrap into the bath of already molten metal, made of lumpy scrap, in portions, not exceeding in volume 10% of the melted batch. Due to the difference between the densities of molten magnesium metal [1,74 kp/dm³] and the solid scrap pieces of magnesium alloys with small volume and large surface-to- volume ratio [at an average 0,64 kp/dm³], the latter - having a layer of oxide - floats on the surface of the bath instead of sinking into it. Remaining on the surface of the bath without melting in it, such scrap pieces reach to the flashing point and the prevention of the scrap from combustion remains uncertain.

A known method particularly for the recycling the said type of magnesium containing scrap is the distillation or sublimation [literature referred to: Metallurgical Newspaper (HU) 1951 Jakόbi- Emόd-Najk: Refining of magnesium melt; Book and Newspaper Publishing Co. for Heavy Industry (HU) 1951 Sztrelec: Metallurgy of magnesium pp. 387 and 444 about the processing of magnesium fines by distillation]. The said method is suitable for recovering pure magnesium metal only, alloys cannot be maintained. The said method requires expensive machinery, the said chemical process is very time-consuming, and the recovered pure magnesium metal must be re- melted and alloyed in a subsequent process. All that makes the said method uneconomical.

The surface of the said type of magnesium containing scrap is contaminated by hydrocarbons, such as emulsions, lubricants and additives, arising from certain prior processes. When melting such scrap by way of charging it into the bath of molten magnesium or its alloys, made first from lumpy scrap, the said hydrocarbons will rapidly become decomposed to C_nH_m constituents. At a temperature of > 720 °C of the melt and the crucible, the gases, which get very rapidly decomposed according to their individual partial pressure, will violently and explosion-likely combust when they reach their flashing points (butane 365-570 °C; ethylene 425-547 °C; ethane 470-630 °C; benzene 540-740 °C). The combusted gases combust the said type of magnesium containing scrap, that may further combust the whole batch of molten metal. An additional difficulty of melting the said type of magnesium containing scrap is that the carbon constituent of the said decomposed C_nH_m gases is likely to promote a re-oxidation process according to the reaction MgO+C Mg gas+CO. The re-oxidation deteriorates substantially the quality of the metal. The creation of a layer of MgO increases the yield loss of processing.

The surface of the molten metal may be protected against oxidation by filling up the empty space of the crucible above the bath with protective inert gas, certain process steps, such as pouring and agitation, which latter is extensively required in case of processing the said type of magnesium containing scrap, may not be executed under perfect protection. Thus, the risk of combustion of the scrap may not be eliminated for sure.

The said type of magnesium containing scrap, if it remains un-recycled, is a hazardous waste, its oxides are contaminating the environment. The storage and disposal of such waste is money- consuming. A commonly used way of disposal of such magnesium scrap is the burning, which method becomes more and more restricted and prohibited, due to the high environmental risk of burning hazardous waste.

THE PURPOSE AND THE BASIS OF THE INVENTION

The task, that this invention desires to solve, is the elimination of technical and economic disadvantages of known methods, by the elaboration of a method, which enables the melting of magnesium containing scrap, primarily scrap with small volume and large surface-to-volume ratio, avoiding the combustion of the charge of said type of magnesium containing scrap and the distillation or sublimation of pure magnesium metal, eliminating the quantitative and qualitative restrictedness of recycling the said type of magnesium containing scrap.

The method according to this invention is based on the perception, that in order to introduce pieces of scrap of magnesium metal or its alloys, with relatively large surface to volume ratio and having a layer of oxide on them and having a density of 0,64 kp/dm³, from the surface of a bath of molten metal into the bath, having a density of 1,74 kp/dm³, the oxides must be first removed or modified.

> The MgO particles on the surface of pieces of the said type of magnesium containing scrap are getting deformed due to the deliquescing effect of MgCl and CaF₂ constituents of the flux, the surface tension of MgO particles does no longer generate forces directed to the centre, creating spherical shape of metal drops, but through the attenuated layer of oxide the metal drops can join to the surface of the already molten metal.

> Turnings and other process arisings of magnesium metal or its alloys usually contain C_nH_m hydrocarbons, which will become decomposed still below the melting point of the metal. The combustion of hydrocarbons at a crucible temperature above the flashing point of the said hydrocarbons, is safely avoided by way of evacuation of the empty space in the crucible first, and by filling it up with protective inert gas afterwards. Under the protective gas, the fluxes enable the reduction of oxides and the said modification, and the bath with flux can be safely agitated by mechanised or conventional means.

The method according to this invention is further based on the perception, that since one substantial precondition to the production of pure magnesium metal by thermo-reduction method is the evaporation of pure magnesium metal below its partial evaporating pressure, therefore the said evaporation of pure magnesium metal can be avoided by the implementation of vacuum In case of pure magnesium metal, considered at a melting temperature of 650 °C, the said partial evaporating pressure equals to 216 Pa. Keeping the melting temperature at the said value, disregarding the uncertainty of temperature regulation, maintaining a pressure at 216 Pa or above that value in the melting means, vacuum melting can safely be applied, without the evaporation and condensation of pure magnesium metal The application of vacuum method improves the quality of refining of the metal, and it contributes to a perfect degassing of the melt and to the complete removal of non-metallic inclusions from the melt. Thus, an improved quality of secondary metal can be produced at reduced cycle time. The holding of the bath can also be performed safely under vacuum in the crucible.

> The melting of the metal and also other process steps are realised at a reduced temperature under the application of vacuum. This results in reduced cycle time and reduced energy consumption.

DISCLOSURE OF THE INVENTION

The method according to this invention is carried out in a vacuum furnace, rated for low overpressure, vacuum, having a double-jacketed melting hearth and a vacuum-sealed crucible The vacuum furnace may be heated by the implementation of any known method for the transfer of heat. During the said method, we charge magnesium-containing scrap into the crucible of the vacuum furnace and we raise the temperature of the charge until it reaches to the melting point, so that from the beginning of heating up the charge, we establish in the sealed melting hearth evacuated space, which we continuously maintain until the melting process is completed, and at the completion of the melting process we immediately fill up the space above the melt with protective inert gas, and while maintaining a pressure of the said protective inert gas above the atmospheric pressure, we open the crucible, and introduce flux mixture onto the surface of the molten metal, and we agitate the bath of molten metal together with the already introduced flux continuously, whilst preferably we continuously introduce protective inert gas into the interior of the said bath, then we skim off the emerged dross from the surface of the bath, we continue the agitation of the bath until no more dross emerges, then we introduce into the crucible a new charge of magnesium-containing scrap, we still maintain the introduction of the said protective inert gas during the charging and until the completion of the seal-closing of the crucible, and upon the completion of the said closing we discontinue the application of protective inert gas and, parallel to that, we open the vacuum system onto the charge of metal The same process steps shall be repeated in the following melting and refining cycles, until the required melt volume is created, and upon reaching the required melt volume, we hold the molten metal and then we discharge the melt from the crucible by applying any known discharging method

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to this invention may be carried out substantially by the implementation of means, one preferential embodiment and layout of which, by way of example, is hereinafter described, referring to the accompanying drawings, in which

Figure 1 shows a rough sketch of semi-sectional front view of the said embodiment of the said means, suitable for carrying out this invention, and

Figure 2 shows a rough sketch of top view of the said means, as shown in Figure 1

In detail, Figure 1 shows a vacuum furnace, the components of which are a frame (19) on which rests a furnace shell (9), on which an elevating mechanism (11) is mounted, as supported on a pivot (10) A burner (12), as mounted on the furnace shell (9), heats the combustion chamber, surrounded by a refractory jacket (8) and a crucible (1) and a protective casing (15), the latter two forming a double-jacketed melting hearth The products of combustion, produced by the burner (12), are conveyed through a chimney (18) The projections of the crucible (1) and the protective casing (15) over the furnace shell (9) contain a cooling section (7), serving the cooling of a gasket (6), and a vacuum pipe (14), that latter also indicated in Figure 2 The vacuum pipe (14) is connected to the vacuum pump system (not indicated in the drawing) The melting hearth is closed by a furnace cover (2), which adjoins to the gasket (6) Further technological components, such as a discharge tube (3), a thermocouple (13), an agitator (4) and a pressure gas pipe (20) are assembled on the furnace cover (2) A manometer (17) with measuring range from 5 x 10⁵ Pa to 2,5 x 10^"2 Pa and a cock (16) are assembled on the pressure gas pipe (20) Another cock (16) is assembled onto the inert gas inlet of the agitator (4) The agitator (4) immerses vertically into the crucible (1) The agitator (4) has a longitudinally hollow shaft, on which one or more stirring paddles are installed at each of one or more levels Each stirring paddle has one or more longitudinal through-holes. The agitator (4) is mechanically rotated by any suitable driving mechanism, as shown by way of example in Figure 1 The discharge tube (3) is longitudinally hollow with open ends, it immerses vertically into the crucible (1) and it has a curved projection above the crucible cover (2) The discharge tube (3) and the agitator (4) are assembled into the crucible cover (2) with tight joint The crucible cover (2) is seal-closed onto the crucible (1) by the cover locks (5) by releasable fastening

PREFERRED MODE OF CARRYING OUT THE INVENTION

The method, being the object of this invention, is hereinafter described in more detail, in relation to the functioning of the means as shown in Figure 1 and Figure 2, being suitable, by way of example, for carrying out the said method Example 1

The crucible (1) and the protective casing (15) constitute the melting hearth, applied to accommodate the magnesium containing scrap. At the beginning of the process, we charge 35-40 vol. % of the entire batch to be melted, for the creation of an initial bath of molten metal We seal-close the crucible cover (2) by the cover locks (5) The seal-closure between the crucible cover (2) and the gasket (6), the latter cooled by preferably water coolant, which flows or circulates in the cooling section (7), contained in the projections of the crucible (1) and the protective casing (15) over the furnace shell (9), is controlled by the manometer (17), by way of filling up the vacuum-sealed melting hearth through the pressure gas pipe (20) with inert gas at a pressure of 10⁵ Pa, as regulated by the cock (16) We raise the temperature of the magnesium containing charge until it reaches to the melting point at maximum 720 °C, but preferably at 650 to 680 °C, by the utilization of the burner (12) The products of combustion are conveyed from the combustion chamber, surrounded by the refractory jacket (8) and the crucible (1) and the protective casing (15), through the chimney (18) The development of the temperature is sensed by the thermocouple (13) and the capacity of the burner (12) is regulated by an instrument (not indicated in the drawings), according to the actual temperature In the first melting cycle of the process as well as after each stoppage of the melting furnace, we raise the temperature at a rate of 150 to 180 °C per hour At the beginning of heating up the furnace, we stop the filling of overpressure inert gas into the melting hearth, and parallel to that, through the vacuum pipe (14) we establish in the melting hearth an evacuated space until a pressure of 216 Pa or any pressure above that value, controlled by a vacuum gauge (not indicated in the drawings). According to experiments, considering a batch of 1 000 kp to be melted, the creation of the melt in the said first melting cycle takes ∞40 minutes The completion of melting process shall be observed in relation to the increase of temperature and pressure at the same time The evacuated space is continuously maintained until the melting process is completed At the completion of the melting process, we immediately fill up the space above the melt with protective inert gas, preferably to a pressure value of 5 x 10⁴ Pa

While maintaining a pressure of the protective inert gas, introduced into the space in the crucible (1) above the molten metal, above atmospheric pressure, we release and remove the cover locks (5) and open the crucible cover (2) We introduce the referred flux ELRAS AL D or another flux mixture, containing substantially the said chlorides and fluorides of metals, in a volume equal to 5-7 % of the volume of the charge to be actually refined, onto the surface of the bath of molten metal We continuously agitate the bath by the mechanically rotating agitator (4) for a period of time, necessary for refining, deoxidizing, purifying the melt, taking characteristically »30 minutes

During the whole agitation, through the hollow shaft and the adjoining one or more longitudinal through-holes in each of the stirring paddles of the agitator (4), immersing vertically into the bath, we continuously introduce into the interior of the bath a mixture of argon and nitrogen gases, containing preferably 0,5 to 2 vol % argon gas and the balance being nitrogen gas, which promotes the release of non-metallic inclusions from the melt and the degassing of the melt The gathered dross may be skimmed off the surface of the bath with a drossing ladle or any known method and means. We continue the agitation of the bath until no more dross emerges

At the second and any further melting cycles we add a new charge of magnesium containing solid scrap to the already created bath of molten metal, preferably in a volume ratio of 3 5 in favour of the volume of molten metal Based on experiments, the said ratio may be adjusted till 4 ^• 4, depending on the actual composition of the new charge After having introduced the new charge to the crucible (1), we immediately seal-close the crucible cover (2) and fasten it by the cover locks (5) We continue the application of the said protective inert gas mixture into the empty space of the crucible (1) until the crucible (1) is seal-closed by the crucible cover (2) When the crucible (1) has become sealed, we immediately stop the introduction of protective inert gas and, parallel to that, we open the vacuum system through the vacuum pipe (14) into the sealed melting hearth containing the melt and the new charge

The same process steps shall be repeated in the following melting and refining cycles, until the required melt volume, in the described example 1 000 kp, is realised The complete process in this described example takes 4 hours, disregarding the period of time of initial idle heating up

Upon the completion of the melting and refining cycles, we hold the whole batch of molten metal, which we perform preferably through the evacuation of the melting hearth, under vacuum, for a period of time of 1/10 to 1/5 of an hour Following the holding of the bath, we cancel the vacuum in the melting hearth and preferably through the discharge tube (3), at closed state of the crucible cover (2), creating an overpressure of protective inert gas in the crucible, or optionally through tipping by the utilisation of the elevating mechanism (1 1), we discharge the already molten and refined metal from the crucible (1) to the place of further handling as required We continue the discharge of the crucible until preferably 1/4 volume and at least 15 to 20 % of the batch volume remains in the crucible (1)

The crucible (1) shall be completely emptied in case of stoppages and the cleaning of the melting means For the cleaning of the melting means we may preferably tilt the furnace over the pivot (10) into a horizontal position, utilising the elevating mechanism (11) for the tilting

INDUSTRIAL APPLICABILITY

Magnesium alloys have become sought after, primarily due to a rapidly increasing demand from applications in the vehicle industry, considering the tendency toward the reduction of the weight of vehicles, in order to decrease emissions, polluting the environment, through reduced fuel consumption of vehicles Beyond the reduction of vehicle weight, magnesium alloys have more favourable strength and stiffness parameters and a better vibration damping than aluminium alloys, which make them preferred in a great number of applications in the vehicle industry

Claims

WHAT IS CLAIMED IS

1. A method that enables the processing and recycling of magnesium-containing scrap, primarily the highly flammable scrap with large surface-to-volume ratio, generated in foundry, in particular in casting (clips, flashings), as well as magnesium alloy process arisings (turnings, chips), generated during machining of the cast metal, by way of melting, whilst maintaining a suitable quality of the metal for die-casting, during which method we charge magnesium- containing scrap into the crucible of a melting furnace and we raise the temperature of the charge until it reaches to the melting point, we realise the refining and deoxidising of the melt by the application of known fluxes, containing substantially chlorides and fluorides of metals, and by the agitation of the melt together with the flux afterwards, then we skim off the emerged dross from the surface of the bath by known method, and then we discharge the molten metal from the crucible, h e r e i n the method according to this invention is carried out in a vacuum furnace, rated for low overpressure, vacuum, having a double-jacketed melting hearth and a vacuum- sealed crucible, the vacuum furnace may be heated by the implementation of any known method for the transfer of heat, from the beginning of heating up the charge we establish in the sealed melting hearth evacuated space, which we continuously maintain until the melting process is completed, and at the completion of the melting process we immediately fill up the space above the melt with protective inert gas, and while maintaining a pressure of the said protective inert gas above the atmospheric pressure, we open the crucible, and introduce flux mixture onto the surface of the molten metal, and we agitate the bath of molten metal together with the already introduced flux continuously, whilst preferably we continuously introduce protective inert gas into the interior of the said bath, then we skim off the emerged dross from the surface of the bath, we continue the agitation of the bath until no more dross emerges, then we introduce into the crucible a new charge of magnesium-containing scrap, we still maintain the introduction of the said protective inert gas during the charging and until the completion of the seal-closing of the crucible, and upon the completion of the said closing we discontinue the application of protective inert gas and, parallel to that, we open the vacuum system onto the charge of metal. The same process steps shall be repeated in the following melting and refining cycles, until the required melt volume is created, and upon reaching the required melt volume, we hold the molten metal and then we discharge the melt from the crucible by applying any known discharging method.

2. The method according to Claim 1 wherein the lower limit of vacuum in the sealed melting hearth is determined by the partial pressure of magnesium metal, at 650 °C equal to 216 Pa, whilst the upper limit of vacuum is determined by the strength of the melting means applied.

3. The method according to Claim 1 wherein any inert gas or any mixture of two or more such gases may be applied as protective gas.

4. The method according to Claim 1 wherein at the completion of the melting process, the space above the melt is filled up with protective inert gas, preferably till the pressure of 5 x 10⁴ Pa.

5. The method according to Claim 1 wherein to the already created bath of molten metal a new charge of magnesium containing solid scrap may be added preferably in a volume ratio of 3 : 5 to 4 : 4 in favour of the volume of molten metal.

6. The method according to Claim 1 wherein a combined application of vacuum and the agitation of the bath of molten metal is implemented for the degassing of the melt and the removal of non- metallic inclusions therefrom.

7. The method according to Claim 1 wherein any known and applicable method and means may be applied for the agitation of the bath of molten metal.

8. The method according to Claim 1 wherein prior to the discharge from the crucible, the molten metal may preferably be held in a sealed crucible under vacuum.

9. The method according to Claim 1 wherein any known and applicable method and means may be applied for the discharge of the molten metal from the melting means.

10. The method according to Claim 1 wherein the discharge of the crucible may be continued until preferably 1/4 but at least 15 to 20 % melt of the batch volume remains in the crucible.