GB2317506A

GB2317506A - Liquid metal pump having insulated demountable electrodes

Info

Publication number: GB2317506A
Application number: GB9720098A
Authority: GB
Inventors: Anthony James Brown; David James Settle; Geoffrey Charles Eckold; Neil Tyson; David Grainger Taggart
Original assignee: AEA Technology PLC
Current assignee: Ricardo AEA Ltd
Priority date: 1996-09-24
Filing date: 1997-09-23
Publication date: 1998-03-25
Also published as: GB9720098D0

Abstract

A liquid metal pump (5) includes a flow duct (10) which passes through the magnetic field (16) of a magnet (14), with two electrodes (22) locating in apertures in the duct walls directly opposite each other. The demountable electrodes (22) are electrically insulated from the walls of the duct (10) by gaskets (24), and are supported by flanged side ducts (18). A voltage applied between the electrodes (22) causes an electric current to flow through the liquid metal, and the interaction with the field (16) causes the liquid metal to flow.

Description

Liquid Metal Pump The invention relates to a pump for pumping liquid metal, and to the use of such a pump, particularly but not exclusively for pumping molten solder.

A known technique for pumping a liquid metal such as mercury or molten sodium uses a duct, which may be of a metal alloy, passing between the poles of a magnet. An electric current is passed between opposed electrodes on the walls of the duct so as to flow through the metal at right angles to the magnetic field, and so cause the metal to flow. For example GB 567 772 describes a mercury pump with a duct of nickel-chromium alloy, with electrodes of nickel or iron brazed to the duct in slots opposite each other, or fixed to the duct by vitreous enamel. GB 914 908 describes a pump used for mercury, sodium or potassium, with a duct of high resistance metal, with electrodes either inserted into and sealed to opposite walls or electrically joined to the outer surfaces of those walls. Problems have been found to arise when applying this technique to other liquid metals such as molten solder, not only in ensuring sufficient electric current flows through the molten metal rather than the duct wall, but because an insulating layer gradually builds up on the wall or electrode surfaces. To some extent these problems can be overcome by coating the electrode surface with a more readily wetted material, but in the long term such coatings may dissolve in the molten metal.

According to the present invention there is provided a pump for pumping a liquid metal, the pump comprising a duct which in operation contains said liquid metal, the duct extending through a region in which a magnet creates a magnetic field so the longitudinal axis of the duct is transverse to the magnetic field, two side ducts extending transverse to both the axis of the duct and to the magnetic field, and communicating with respective apertures at opposite sides of the duct, electrodes locating in the side ducts, demountably connected to them and sealed to them by gaskets, and means to apply an electric voltage between the electrodes, such that the electrodes are electrically insulated from each other in the absence of the liquid metal.

The duct is preferably of non-ferromagnetic material, so as to avoid shielding the liquid metal from the magnetic field. The preferred material for the duct is stainless steel, as this can withstand elevated temperatures, and does not dissolve in or interact chemically with most liquid metals. The side ducts preferably have flanges at their ends to which the electrodes are connected, the insulating gaskets being provided between a flange on the electrode and the end flange of the side duct. Such gaskets may have to withstand elevated temperatures (depending on the nature of the liquid metal), and would consequently be made of an inert material such as glass fibres.

The force exerted on the liquid metal increases with the magnetic flux density, and with the magnitude of the electric current. The magnet may be a permanent magnet or an electromagnet, but in either case it is easier to achieve a high flux density if the magnet's poles are not far apart; evidently they must be far enough apart to allow the duct to fit between them. In one embodiment the poles might be about 10 mm apart, and about 20 mm square, and the duct be of rectangular cross-section in the vicinity of the poles; and the electrode faces be rectangular, about 20 mm by 4 mm, so as to be the same length as the region of high flux density, and to have as large a cross-sectional area as practicable within the size constraints of the space between the poles.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic perspective view of a pump for pumping liquid metal; and Figure 2 shows an exploded perspective view illustrating the assembly of the electrodes, some components being omitted for clarity.

Referring to Figure 1, a pump 5 for pumping liquid metal, such as molten tin or molten solder, comprises a stainless steel duct leo which passes between the poles 12 of a magnet 14, the duct 10 being of rectangular crosssection in the vicinity of the poles 12 but being circular elsewhere. The separation of the poles 12 is exaggerated for clarity. The magnetic field, indicated by arrows 16, is perpendicular to the longitudinal axis of the duct 10. The part of the duct 10 between the poles 12 has two branch ducts 18 on opposite sides of the duct 10 and also of rectangular cross-section, which communicate with the duct 10 via corresponding rectangular apertures 20 (not shown in Figure 1; see Figure 2). A rectangular electrode 22 locates in each branch duct 18, sealed to the end of the branch duct 18 by an insulating gasket 24, and projects beyond the end of the branch duct 18.

A source of electric current (not shown) is connected between the electrodes 22 in operation, so causing electric current to flow through the liquid metal in the duct 10 transverse to both the longitudinal axis of the duct 10 and to the magnetic field 16. This generates a force to drive the liquid metal through the duct 10. The direction of the liquid flow can be reversed by reversing the direction of the electric current. The current source is a high current low voltage power supply, and by way of example the electrodes 22 might be of cross-section 4 mm by 20 mm, the voltage 0.25 V between the electrodes 22, and the current 400 A.

Referring to Figure 2 there is shown an exploded perspective view during assembly of the pump 5. As shown in Figure 2 the duct 10 communicates via rectangular apertures 20 (only one is shown, in broken lines) with two short, stainless steel, branch ducts 18 which are opposite each other. The ends of the branch ducts 18 remote from the duct 10 have rectangular flanges 26. The stainless steel electrodes 22 (only one is shown) consist of a flat plate with a flange 28 at an intermediate position along its length. The part 30 of the electrode 22 which locates in the branch duct 18 is 0.5 mm clear of the walls of the branch duct 18, and is of such a length that the end face of the electrode 22 is substantially flush with the inner surface of the wall of the duct 10.

Each electrically insulating gasket 24 locates between the flange 26 and the flange 28; this may for example be a glass-fibre/rubber composite gasket such as a Klingersil gasket (trade mark, available from Klinger Ltd. of Bradford, England). The flanges 26 and 28 are clamped together by four bolts 34 which pass through corresponding holes in the flange 28 without touching it, and screw into threaded holes in the flange 26, the head of the bolt 34 being separated from the flange 28 by a washer 35 and an insulating gasket 36 (only one such bolt 34 is shown). After assembly of both the electrodes 22 to the duct 10, tests are carried out to ensure there is no electrical contact between the two electrodes 22, or between either electrode 22 and the duct 10 (in the absence of liquid metal).

It will be appreciated that the pump 5 may be used with a wide variety of different molten metals. In most cases the metal must be maintained at elevated temperature to remain molten, so the duct 10 will be provided with heating elements (not shown) such as trace heating, and also enclosed with thermal insulation (not shown). It will also be appreciated that a pump may differ from that described above, while remaining within the scope of the invention. For example the duct for liquid metal flow might be of a different shape and size, and might be of a different material such as titanium, or a ceramic material, or (for lower temperature operation) a plastic material. The side ducts enclosing the electrodes might be of a different shape, and might be longer or shorter than shown; at an extreme the side ducts might be only as long as the wall thickness and the electrode flanges be clamped directly onto the side walls of the flow duct (with interposed gaskets); in this case bolts or studs might be welded to the outside of the duct around the aperture to locate and attach the electrodes. Similarly, the electrodes might be of a different shape, and of a different material, than described above.

When the pump 5 is used to pump used solder (in a molten state) it has been found that an electricallyinsulating black layer, believed to be of organic material and originating from the flux and oil present in the solder, is gradually deposited on the protruding parts 30 of the electrodes 22. This can be removed by mechanical scraping, for example once a week.

Alternatively the deposition can be prevented by replacing the electrodes 22 with electrodes of the same shape, but of nickel instead of stainless steel, and in which the protruding parts 30 are coated with aluminium oxide leaving the end face exposed.

In use there is a very slight degree of dissolution of nickel from the end face into the molten solder, which ensures the end face does not become coated with the black layer; the rate of dissolution is very much less than if uncoated nickel electrodes are used, as with uncoated electrodes there is considerably greater dissolution at the sides than at the end. In addition the current is more effectively passed through the solder, rather than passing through the wall of the duct, so the solder is pumped faster; in tests using a standard quantity of molten solder, the times taken to transfer the solder using different electrodes 22 were: stainless steel, 30 to 35 sec; uncoated nickel, 30 sec; coated nickel, 18 sec. The aluminium oxide coating may be deposited by thermal spraying, and it is desirable to first deposit a bonding coat or interlayer to minimize the mismatch in thermal expansivity between adjacent materials. It will be appreciated that the coating may be of other electrically-insulating materials, for example other ceramic materials.

It may also be beneficial to coat the inner surfaces of the branch ducts 18 with an insulating coating, for example of alumina, to ensure that the electric current does not tend to flow into the wall of the duct 10 as the end of the electrode 22 gradually dissolves.

Claims

1. A pump for pumping a liquid metal, the pump comprising a duct which in operation contains said liquid metal, the duct extending through a region in which a magnet creates a magnetic field so the longitudinal axis of the duct is transverse to the magnetic field, two side ducts extending transverse to both the axis of the duct and to the magnetic field, and communicating with respective apertures at opposite sides of the duct, electrodes locating in the side ducts, demountably connected to them and sealed to them by gaskets, and means to apply an electric voltage between the electrodes, such that the electrodes are electrically insulated from each other in the absence of the liquid metal.

2. A pump as claimed in Claim 1 wherein the duct is of rectangular cross-section in the vicinity of the magnetic field, and the side ducts and the electrodes are also of rectangular cross-section

3. A pump as claimed in Claim 1 or Claim 2 wherein the end face of each electrode is substantially flush with the inner surface of the wall of the duct.

4. A pump as claimed in any any one of the preceding claims wherein the sides of that part of each electrode which locates within one of the side ducts are coated with an electrically insulating material.

5. A pump as claimed in any one of the preceding claims wherein the sides of that part of each electrode which locates within one of the side ducts are coated with a ceramic material

6. A pump as claimed in Claim 5 wherein the coating is of aluminium oxide.

7. A pump as claimed in any one of the preceding claims wherein the electrodes are of nickel.

8. A pump as claimed in any one of the preceding claims wherein the inner surfaces of the side ducts are coated with an electrically insulating material.

9. A pump substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

10. The use of a pump as claimed in any one of the preceding claims to pump molten solder.