EP1019211B1

EP1019211B1 - Method of fluxless melting of magnesium

Info

Publication number: EP1019211B1
Application number: EP98939025A
Authority: EP
Inventors: Jan B. Ronhaug; Stein Erik Berg; Haavard T. Gjestland; Thomas Bergem Reistad; Oystein Solli
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 1997-07-07
Filing date: 1998-07-03
Publication date: 2002-05-02
Anticipated expiration: 2018-07-03
Also published as: JP2001509543A; ATE216932T1; NO973149L; NO304893B1; DE69805195D1; NO973149D0; AU8753498A; WO1999002287A8; EP1019211A1; WO1999002287A1

Abstract

Fluxless melting of magnesium (alloy) metal applying a gas mixture of dry air and SO2 as protection atmosphere is provided where a constant pre-set level of SO2 is maintained by means of two mass flowmeters where one of them is in control of the other.

Description

The present invention concerns a method of fluxless melting of magnesium, and more particularly a method of mixing and controlled feeding of protection gas to melting/holding furnaces.
Magnesium (metal) is known for its high affinity to oxygen and high vapour pressure of molten Mg (alloys) causing operational problems.
Currently SF6 based gas is used by most magnesium die casters for protection against oxidation of molten magnesium metal. Until recently the use of SF6 was not considered to represent any environmental hazard being non-toxic and without negative impact on the working atmosphere in the foundries. However, due to its very high calculated greenhouse warming potential strict restrictions on the use of the gas in applications causing emission to the atmosphere are expected in the near future.
Consequently, a search for an alternative protection medium has focused on switch back to SO₂ originally applied by the industry prior to the introduction of SF6 gas.
It is well known that SO₂ atmosphere strongly reduces the oxidation of molten magnesium by modifying the surface film and that producers of primary metal, sand and die casters have been using SO₂ in gas form or generated (in situ) by adding sulphur powder to furnaces.
SO₂ is, however, toxic. In high concentrations and in contact with humidity it results in enhanced corrosion of steel equipment. The present occupational exposure limit in Norway is 2 ppm over an 8 hours period of time corresponding to a concentration of 5 mg/m³, and corresponding limits in Germany and North America are similar.
Consequently, it is an object of the present invention to provide a new and improved method of mixing and maintaining a pre-set level of SO₂ in an air mix within narrow tolerances.
This object is met by provision of the new method according to the present invention as define in patent claims 1-3.
The new method of mixing and maintaining of the pre-set level of SO₂ in the air mix will now be described and readily understood from the following description under reference to the accompanying drawings, Figs. 1-3, and preferred modes of operation, where

Fig. 1: shows schematically a flow chart of the mixing/feeding process,
Fig. 2: illustrates schematically in a vertical, partial cross-section a feeding arrangement (apparatus) usable in the process of fluxless melting of magnesium, and
Fig. 3: shows schematically the applied furnace/vessel in a horizontal cross-section along line I-I

Referring to Fig. 1 illustrating schematically a flow chart of the mixing/feeding process applicable according to the present invention, a melting furnace 1 is equipped with charging device 2 to feed solid magnesium into the furnace, transferring device, e.g. transfer tube 3 to move subsequently the Mg-melt into and adjacent casting unit (furnace) here depicted as 4. By means of e.g. so-called gas displacement (metering) pump 5 the melt is fed in a controlled manner (doses) into any suitable casting equipment, e.g. the shown high pressure die casting machine 6.
According to the present invention there is used an apparatus arrangement for controlled mixing/feeding of the mixture SO₂/dry air comprises a compressor 13 and a pressure bottle 14 providing (dry) air and SO₂ gas, respectively, into the mixing device 9 comprising a mass flowmeters 10 and 12 for the air and SO₂, respectively, and a control steering unit 11 to ensure an adequate pre-set concentration of SO₂ in the gas mixture. The resulting gas mixture is then conducted through a feeding line 8 and distributed by means of manifolds (flowmeters) 7 and customary valves into the actual (part of) furnaces 1 and 4.
This is achieved by applying a double set of mass flowmeters 10,12 where the mass flowmeter 10 controlling the air flow is also in control of the mass flowmeter 12 so that the concentration of SO₂ in the gas mixture will never exceed the pre-set level. In the case of an emergency situation, e.g. break down on the feeding linelequipment providing dry air, this "master" flowmeter 10 will automatically switch off also feeding of SO₂.
As "dry air" referred to in the description air having a dew point - 30°C is applied.
Using this sophisticated mixing unit ensures that the SO₂ concentration may be kept very low and the distribution of the protection gas to the liquid. Mg surface is so uniform that emissions from furnace atmospheres to surroundings will be negligible and represents no hazard for foundry personnel.
With references to Figs. 2 and 3 illustrating schematically an arrangement of feeding means usable in the process of feeding/distribution of SO₂/air mixture according to the present invention, Fig. 2 shows schematically in a partial, vertical cross-section the furnace 1 as a steel vessel accommodated in a thermally insulated furnace shell 20. The melting furnace is provided with a charging device 2 to introduce solid Mg into the furnace, a baffle plate 15 dividing the furnace into a charging sector and a main body of the melting furnace where the furnace design/configuration should assure a high ratio volume/surface area in order to minimise reaction(s) occurring between molten Mg and the atmosphere. Consequently, all lids and hatches must also be well tightened as illustrated in an exploded view showing details of lid seals 16. The gas mixture is provided to the fumace(s) through a sophisticated distribution system allowing the gas mixture to be evenly distributed close to the melt surface, e.g. by means of a ring tube 17 positioned along the furnace periphery in the vicinity of the melt surface as shown in Fig. 3. Fig. 3 illustrating schematically the applied melting furnace 1 in a horizontal cross-section taken along line I-I in Fig. 2, further depicts the charging zone (device) 2, dividing baffle plate 15 and a lid 19 for eventual removal of dross/sludge from the furnace.
The ring tube 17 is provided with a plurality of apertures ensuring an even distribution of the protective gas mixture by controlled directional impingement of the melt surface.
Experiments conducted in industrial size furnaces with SO₂ air mixtures applying the above method confirm that the SO₂/air protection atmosphere protects effectively molten magnesium surfaces against oxidation.
The following reactions between the metal and the gas mixture may occur: Mg(I) + O₂ + SO₂ → MgSO₄(s) MgSO₄(s) + Mg(l) → MgO(s) + Mg(s) MgS(s) + O₂ + SO₂ → MgSO₄(s) MgO(s) + SO₂ + O₂ → MgSO₄(s)
It is believed that a sort of "self-reparation" of possibly disrupted/cracked layers of MgO(s) + MgS(s) occurs due to a further reaction between magnesium evaporated through such cracked opening and the cover gas.

Example 1

Summary of pilot trials conducted in full scale furnaces in laboratory:
Temperature of the melt	655-690°C.
Gas mixture flow rate /m² melt surface	10l/min
SO₂ concentration in the gas mixture	0.5-1.7%
Duration of trials	47 hours

Example 2

Tests have been carried out on an industrial scale in a hot-chamber casting machine. A mixing unit built according to the present invention provided SO₂/dry air mixture for the furnace. Gas samples were frequently monitored (every four hours) both from the mixing unit, the furnace atmosphere and the working atmosphere as well.

Melt surface area	0.3 m²
Melt temperature	660°C
Flow rate of the gas mixture	7.3 l/min
Duration of trials	3 days
SO₂ concentration in the (dry) air mixture	0.8%
SO₂ concentration in the atmosphere	0.2 ppm

Gas samples of the atmosphere were collected at three differenct locations in the foundry - at operator level, in the vicinity of the charging lid and approximately 3 m above the floor. No significant differences were measured in the gas concentrations between the locations indicating a safe, controlled operation and an adequate ventilation of the foundry hall.
A controlled diluted SO₂/air mixture was provided and maintained during the test periods, something being of crucial importance also for the life time of the applied steel equipment.
It is well known that both SO₂ and SF6 in contact with humidity will accelerate steel corrosion. Deposits and reaction products may be formed on e.g. crucible/furnace walls, something which under unfavourable conditions can lead to metal eruptions from the furnace. Consequently, high concentration of SO₂ combined with high humidity should be avoided.

Claims

Method of fluxless melting of magnesium and magnesium alloy metal comprising mixing of sulphur dioxide (SO₂) and dry air and maintaining a protection gas-mixture atmosphere above molten metal,
characterised in that
a constant pre-set level of SO₂ is maintained in the gas-mixture regardless of variations in actual gas-mixture consumption by means of at least two mass flowmeters (10,12) controlling the flow of dry air and SO₂, respectively, where the dry air flowmeter (10) controls the SO₂ flowmeter (12).
Method according to claim 1,
characterised in that
the concentration of SO₂ in the gas-mixture is in the range from 0.1 to 5 weight%.
Method according to claim 2,
characterised in that
the SO₂ concentration is from 0.2 to 1.5 weight%.