Distribution system for molten magnesium

An apparatus for distribution of magnesium or magnesium alloys from a central melting unit (2) into a casting shop with one or more casting machines (3) has a tube furnace (1) extending from a central melting unit (2) into the casting shop. The tube furnace (1) is equipped with outlets (6) at the top for the mounting of a transfer tube (7) for the supply of metal to one or more holding furnaces (4) arranged at each casting machine. The tube furnace is preferably placed inside a steel cover (8) and is positioned just beneath the metal level in the holding furnace (4). The tube furnace (1) and transfer tubes (7) are provided with heating elements (11) with an outer insulation (12). The transfer tubes (7) have an air inlet (9). Two or more tube furnaces could be placed in the steel cover (8). The metal is transferred by the act of gravity.

BACKGROUND OF THE INVENTION 
The invention relates to a system for distribution of molten magnesium or 
magnesium alloys from a central melting unit to several casting machines, 
which would typically be high pressure die casting machines. 
During melting of magnesium alloys in foundries, separate melting furnaces 
for each casting machine are commonly used. This is inconvenient since it 
demands an investment in several melting furnaces. It also requires space 
for and a system for transportation of ingot pallets to each casting 
machine. This transport is most often carried out by truck. In addition, 
the use of only one furnace at each casting station may cause quality 
variations due to considerable temperature fluctuations in the liquid 
metal transferred to the casting machine. To some extent this disadvantage 
has been compensated for by the use of two furnaces at each casting 
machine; one melting furnace and one holding furnace for temperature 
stabilization. Liquid metal is then transferred from the melting furnace 
to the holding furnace by use of a siphon tube. The use of two furnaces 
is, however, more costly, and it still demands the transport of ingot 
pallets to each casting machine. 
One solution to this problem is known from U.S. Pat. No. 4,635,706, which 
describes a molten metal handling system. This system is particularly 
useful for delivering high melting temperature metals in their molten 
state from a crucible to a casting machine. The essential elements of this 
metal handling system are a pump and heated conduits to carry the molten 
metal to the casting station. Optionally, two or more sources of molten 
metal can be linked together to provide a continuous source of metal to 
one or more die casting machines. However, the system is based on the use 
of electromagnetic pumps. This is unnecessarily complex and vulnerable. 
The patent gives no solution as to how the distribution to several casting 
machines should be performed. The direct connection of the transport tube 
to the metering unit does not give any flexibility with regard to the 
choice of metering system. In addition, the transport tubes are situated 
above the melt level in the furnace. This is unfortunate with regard to 
safety in the case of an eventual run-out of magnesium. 
SUMMARY OF THE INVENTION 
The object of the invention is to obtain a safe and flexible distribution 
system for molten magnesium and magnesium alloys. Another object is to 
obtain a system that is reliable in service and that delivers metal of 
high quality at a correct casting temperature. 
These and other objects of the invention are achieved by the process and 
apparatus described below, and the invention is characterized and defined 
by the accompanying patent claims. 
The invention relates to an apparatus for the distribution of magnesium or 
magnesium alloys from a central melting unit to a foundry with one or more 
casting machines. A tube furnace is arranged from the central melting unit 
and into the casting shop. The use of a central melting unit ensures 
uniform temperature of the metal to the casting machines. The tube furnace 
is equipped with outlets at the top for the mounting of transfer tubes for 
the supply of metal to one or more holding furnaces arranged at each 
casting machine. Preferably the tube furnace is positioned just beneath 
the metal level in the furnaces. The molten magnesium or magnesium alloy 
is transferred from the central melting unit to the tube furnace and 
further to one or more holding furnaces through transfer tubes by gravity. 
The tube furnace and transfer tubes are equipped with heating elements and 
an outer insulation zone. The transfer tube has an air inlet for emergency 
stop. A steel cover surrounds the tube furnace. It is also possible to 
place two or more tube furnaces in the steel cover.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention utilizes the advantage that molten magnesium does not attack 
selected steel materials and the possibility of heating steel tubes of 
these materials with electrical resistance windings. 
As shown in FIG. 1, a distribution system consists of a tube furnace 1 
stretching from a central melting unit 2 and into a casting shop having 
several casting machines 3 which are to be fed with liquid metal. The 
length and shape of the tube furnace will be dependent on the number of 
casting machines and the design of the casting shop. Each casting machine 
is provided with a holding furnace 4 containing a metering unit (not 
shown) supplying the machine. The central melting unit 2 can be located in 
a facility separated from the casting machines by a wall 5. In the figure 
two melting units are shown connected to the tube furnace 1. 
The tube furnace 1 is positioned just beneath the metal level in the 
furnaces and is equipped with outlets 6 on its top at regular intervals. 
At each outlet 6 can be mounted a transfer tube 7, ensuring transfer of 
metal from the tube furnace to the holding furnace 4 by the act of 
gravity. In the same way, metal is transferred from the central melting 
furnace(s) to the tube furnace by similar transfer tubes utilizing the act 
of gravity. 
By this design the distribution of the molten alloy from the central 
melting unit to each casting machine will take place fully automatically 
since the act of gravity will ensure equal level in each of the furnaces 
combined in the network. Also, any kind of valves that would be vulnerable 
to mechanical wear are avoided. Metal needs to be melted at the same rate 
as the total consumption in the network. This can be ensured by a level 
detector in the melting furnace controlling the feed of metal to the 
furnace. 
The tube furnace 1 and transfer tube 7 are shown in more detail in FIG. 2. 
For security reasons the tube furnace 1 is placed inside an insulated 
steel cover 8 that would hold the total amount of metal in the tube 
furnace 1 and the transfer tube 7 in case of an outbreak from the tube 
furnace 1. Also, each transfer tube 7 is equipped with an emergency stop 
that comprises an inlet for air 9 on top of the tube 7 and thereby empties 
it and prevents any further transfer between the tube furnace and the 
melting/holding furnaces. 
The tube furnace 1 and transfer tube 7 have an inner steel tube 10 which is 
wound with a heating element 11. A centrally located thermocouple (not 
shown) is provided for temperature regulation. This system is then 
provided with a thin stainless steel foil, to keep the heating elements in 
place and contribute to uniformity of heating. The inner pipe is insulated 
with insulation 12 and has an outer mantle 13. 
One important aspect with the design is that, apart from the melting and 
holding furnaces, the molten alloy has no surface towards the atmosphere. 
Such open surfaces would need protection against metal oxidation and would 
also be vulnerable for contamination of the melt through oxide formation. 
As an example, a system is designed for feeding six casting machines with a 
capacity of 500 kg/hour each. The total melting capacity will be 3000 
kg/hour. In order to distribute this amount over a distance of 50 m, a 
tube furnace with an inner diameter of 150 mm is needed, whereas each of 
the transfer tubes would need an inner diameter of 38 mm in order to 
transfer the amount of 500 kg/hour over a distance of 3 m. These 
dimensions will be sufficient to avoid level differences to build up 
between the melting furnace(s) and the holding furnaces. 
Start-up of the distribution system would include melting of initial metal 
in the melting and holding furnaces followed by heating of the tube 
furnace and filling it with liquid alloy to the same level as the 
furnaces. Eventually, if the tube furnace already contained solid metal, 
this would have to be melted. Transfer tubes prefilled with solid metal 
could then be mounted and heated. As soon as the metal in the transfer 
tube is melted, transfer of metal to the holding furnace begins. If, for 
any reason, one or more of the casting machines in the network should be 
taken out of service, the transfer tubes connecting these machines to the 
network could be solidified and/or removed from service. At longer 
operation stops, for instance over week ends, the metal in the whole 
distribution system could be solidified. 
As a backup system, two or more tube furnaces may be located side by side 
in the same cover box. This would also give the possibility of casting 
different alloys on the network. For the same reasons, several melting 
furnaces should be available in the melting shop, with possibilities of 
connecting to either of the tube furnaces. The melting shop could even 
include refining furnaces for the recycling of scrap that could be 
directly attached to the network. 
The system is also suitable for casting of other metals, for example zinc.