Removing inclusions from molten metal

The invention provides a method of removing inclusions from molten metal, particularly aluminum, by PA1 (a) contacting the molten metal with a medium which retains metal-non-wettable inclusions. The medium may be liquid such as a fused salt mixture; or solid such as a filter or metal-non-wettable ceramic materials or a bed of granules e.g. of tabular alumina, PA1 (b) passing the molten metal through a filter of metal-wettable material, e.g. a refractory hard metal such as titanium diboride. The metal-wettable filter attracts and holds metal wettable inclusions within itself, and may also prevent by surface tension the entry of fused salt droplets.

This invention relates to treatment of molten metals, particularly 
aluminium and alloys containing a major proportion of aluminium 
(hereinafter referred to as "Al") for removal of inclusions, particularly 
non-metallic inclusions. The method is particularly suitable for effecting 
such removal by in-line treatment of molten Al flowing in a transfer 
trough. In its most preferred form it is intended for treatment of Al 
flowing in such a trough from a furnace to a casting station to remove 
inclusions immediately before casting e.g. ingots for further working. The 
invention is particularly useful where low evolution of fume during such 
treatment is sought. 
British Patent Specification No. 935191 describes a method of removing 
suspended oxides from molten Al by pouring a stream of the molten metal 
into a molten salt mixture which is capable of wetting, coalescing and 
retaining the suspended metal oxides and effecting intimate contact 
between the metal stream and the molten flux. Suitable molten salt 
mixtures for the treatment of Al are well known in the art, for example 5 
to 20 weight % NaF or cryolite in a 50:50 mixture of NaCl and KCl. The 
method was effective in removing non-metallic inclusions such as suspended 
aluminium oxide from molten Al. However, it was found that complete 
separation of treated molten metal from molten salt was not obtained, such 
that the metal exiting from the treatment unit contained a suspension of 
fine salt particles. These were even more deleterious to the casting 
operation and cast metal quality than the inclusions originally present. 
Viscous salt particles clogged nozzles of casting machines, obstructing 
metal flow and resulted in ingots being "lost" during casting. Fine salt 
inclusions in the cast metal resulted in impaired working, mechanical and 
corrosion properties. As a result, this method of treatment is no longer 
used commercially. 
British Patent Specification No. 1266500 describes a two-step method for 
removing non-metallic inclusions from molten metal. The first step 
involves flowing the metal through a multiplicity of flux-lined channels, 
desirably provided by a bed or layer of coarse refractory granules coated 
with a molten salt mixture. The patent acknowledges that, while this step 
is effective in removing non-metallic inclusions, it is less so in 
removing intermetallic particles. So the second step involves flowing the 
metal through a bed of uncoated refractory granules, whereby the 
intermetallic particles settle out in the interstices. The granules are 
preferably of alumina, but various other refractory materials are 
suggested, all of which are non-wetted by Al. 
This bed of uncoated refractory granules is initially effective to entrap 
and retain molten salt particles, but there are serious disadvantages in 
use. The filter tends to clog as viscous salt particles accmulate, while 
channels are formed which reduce the effectiveness of retention. Moreover, 
intermetallic particles are not securely held by the filter and are liable 
to be flushed through by a subsequent surge of molten metal. 
Some producers simply pass molten Al through a bed of refractory granules. 
These are effective to filter out non-wetted inclusions, but suffer from 
some quite severe disadvantages. Since refractory granules are used which 
are not wetted by Al, the interstices have to be quite large and/or the 
applied pressure has to be quite high in order to achieve an adequate flow 
of molten metal. Metal-wetted inclusions may be trapped in the interstices 
of the bed, but have no affinity for the filter medium, and are liable to 
be flushed through by a subsequent surge of molten metal. 
There are commercially available filters formed of ceramic foams. These 
also are non-wetted by Al and suffer from the disadvantages noted in the 
previous paragraph. 
Our European Patent Specification No. 68782 A2 describes an electrolytic 
reduction cell for Al including a metal-wettable filter located in the 
molten metal pool on the floor of the cell. The filter uses the surface 
tension forces existing between the metal--to prevent passage of suspended 
droplets of electrolyte. For this purpose, the apertures in the filter are 
made smaller than the electrolyte droplets. Provided that the pressure 
difference across the filter is kept below a maximum figure, which varies 
inversely with the size of the apertures, electrolyte droplets are 
retained upstream of the filter and do not enter it at all. The European 
specification is not concerned with removal of metal-wettable inclusions, 
which are indeed not present in molten metal within the reduction cell. 
The present invention provides a method of removing inclusions from molten 
metal which method comprises 
(a) contacting the molten metal with a medium which retains 
metal-non-wettable inclusions, 
characterized by also 
(b) passing the molten metal through a filter of metal-wettable material, 
steps (a) and (b) being performed in sequence in either order. 
While the method is applicable in principle to metals generally, it is 
particularly valuable in the treatment of aluminium (Al) and alloys 
thereof. The major non-metallic inclusions normally found in liquid Al 
include Al.sub.4 C.sub.3, TiB.sub.2, (Ti-V)B, MgO and Al.sub.2 O.sub.3. 
Metallographic analysis shows that these compounds have a strong tendency 
to form aggregates or clusters. These clusters can be constituted either 
of a single specie (e.g. TiB.sub.2) or of a mixed type (e.g. Al.sub.4 
C.sub.3 -TiB.sub.2). 
Some of the inclusions, e.g. TiB.sub.2 and (Ti-V)B, are easily wetted by 
Al. Others, e.g. Al.sub.2 O.sub.3, and Al.sub.4 C.sub.3, are not. It is 
observed that the inclusions which are wetted have a stronger tendency for 
agglomeration. Borides may be added as grain refiners in the form of 
particles of up to 2 microns and may form clusters of up to 30 microns. 
Oxide inclusions may have particle sizes ranging up to 100 microns. There 
appears to be a tendency for the larger non-wetted oxide particles to 
become coated with the smaller wetted boride particles, forming mixed 
clusters which are readily wetted by Al. 
The key to the method is the use of a filter of metal-wettable material. 
Filters of this kind are described in European Patent Specification No. 
68782 A2 as noted above. But for use in the method of this invention, the 
metal-wettable filter may need to perform a different function than, and 
to differ structurally from, the known filters, as described in more 
detail below. 
The nature of the medium which retains metal-non-wettable inclusions, as 
used in step (a), is not critical. One possible medium is a molten flux as 
described in the British Patent Specification No. 935191 noted above; or a 
bed of refractory granules coated with molten flux as described in the 
British Patent Specification No. 1266500 noted above. Another possible 
medium is a solid filter or bed of metal-non-wettable material as 
presently used commercially. With the proviso that the last stage of the 
filtration should involve a solid, rather than a liquid, filter medium, 
various combinations are possible as exemplified by two embodiments: 
(i) In step (a) the molten metal is contacted with a molten salt mixture 
which is capable of wetting and retaining inclusions, and in step (b), 
which is performed after step (a), the molten metal is passed through a 
metal-wettable filter having apertures sized to prevent the passage of 
molten salt droplets. 
(ii) In step (a) the molten metal is passed through a filter of solid 
non-metal-wettable material. 
These two embodiments of the invention will each be described in turn. 
In embodiment (i), the molten metal is contacted in step (a) with a molten 
salt mixture which efficiently wets and retains non-metal-wettable 
inclusions, and less efficiently retains metal-wettable inclusions. The 
metal stream entering step (b) contains molten salt droplets and also some 
metal-wettable inclusions. The apertures of the metal-wettable filter of 
step (b) are smaller than the molten salt droplets, which are accordingly 
prevented by surface tension forces from entering and passing through the 
filter. The metal-wettable inclusions which do enter the filter have an 
affinity for the filter material and are retained thereon, in such a way 
that there is no risk of their being dislodged by a subsequent surge of 
molten metal. 
In step (a), the molten metal is contacted with a molten salt mixture which 
is capable of wetting and retaining inclusions. The contact should be 
sufficiently intimate to ensure transfer of inclusions from one phase to 
the other, and may suitably be turbulent. Contact may be effected by the 
method of British Patent Specification No. 935191, namely pouring the 
molten metal into a bath of the molten salt which contains also a 
spreading device to sub-divide the stream of molten metal. Alternatively, 
contact may be effected by one of the methods of British Patent 
Specification No. 1266500, e.g. by passing the molten metal through a 
perforated refractory screen positioned either within or above the molten 
salt layer, or by causing the molten metal to flow through a multiplicity 
of channels lined with molten salt, formed e.g. by a layer of refractory 
granules coated with the molten salt. Alternatively, the molten salt layer 
may be filled with refractory shapes designed to assist metal/salt contact 
without impeding metal flow. Metal-flux contact can also be achieved by 
use of a mechanical stirring device such as a rotary impellor, a gas 
dispersing system dependent on introduction of an inert sparging gas, or 
electro-magnetic stirring, or any combination of thereof. The nature of 
the molten salt is not critical, and may be conventional. Suitable molten 
salt mixtures include one or both of KCl and NaCl together with one or 
more of NaF, sodium and potassium aluminium fluoride, BaCl.sub.2 
MgCl.sub.2 and CaF.sub.2. 
The surfaces of the filter used in step (b) must be resistant to attack 
both by the molten metal and by the molten salt, and also must be wetted 
by the molten metal in preference to the salt. For Al, there are several 
materials of which the filter may be constructed: 
(a) Titanium diboride, other borides such as zirconium diboride and niobium 
diboride, and other similar substances which are generally known as 
refractory hard metals. 
(b) A composite refractory of alumina and titanium diboride, for example as 
described in our co-pending British Patent Application No. 8236931 filed 
on 30th December 1982. 
The filter may be formed wholly of such material, or alternatively a 
coating of such material may be applied to a ceramic base, such as fused 
alumina, or a strength-providing metal base. 
The filter may take a variety of forms such as apertured plates, honeycomb 
grids, parallel bars, ceramic cloths, ceramic felts, packed beds of 
correctly sized particles. However, structures of robust construction such 
as arrays of paralleled bars, and apertured plates, honeycomb grids, or 
particularly, packed beds are preferred. 
The filter is designed to trap and hold metal-wettable inclusions, a 
function wholly different from anything described in European Patent 
Specification No. 68782 A2. Since the metal-wetted inclusions have a 
positive affinity for the filter medium, it is not necessary that the 
apertures of the filter should be smaller than the inclusions to be 
filtered. The filter may preferably be formed of a bed of granules of 
weight average diameter from 0.5 to 6 mm, particularly 1 to 3 mm. 
The apertures in the metal-wettable filter are also sized to prevent the 
passage of molten salt droplets entrained in the molten metal. The size of 
the molten salt droplets is to some extent dependent on the conditions 
used in step (a). It is preferred that the metal leaving step (a) does so 
under rather quiescent conditions, or alternatively that time is allowed 
for molten salt droplets entrained in the metal leaving step (a) to 
coalesce into larger drops. It may be necessary to use a filter for step 
(b) having apertures in which the diameter or minor dimension is as slow 
as 1 mm or even lower. Preferably, however, the filter will have apertures 
with a diameter of 2 to 4 mm, or essentially rectangular slits with a 
minor dimension of about 2 to 3 mm. 
The molten salt droplets will not pass through the metal-wettable filter 
provided that three conditions are met. The filter must be kept filled 
with metal, and for this purpose it is necessary to maintain a back 
pressure of metal at the outlet side of the filter; this can be achieved 
by providing a column of molten metal downstream of the filter by means of 
an overflow weir in a passage leading e.g. to a casting station. The 
apertures of the filter must, as indicated above, be not larger than the 
diameters of the molten salt droplets. The static pressure difference 
across the filter must not be so great as to overcome the surface tension 
effects on which operation of the filter are based. The value of the 
pressure difference (metal or metal salt head) which can be retained on 
the upstream side of the filter there is substantially in inverse ratio 
with the diameter of the apertures in the filter. As stated in European 
Patent Specificatin No. 68782 A2, the value can be calculated from the 
following formula: 
EQU h.sub.1 =(1/R.sub.1 G)(2g/r+(R.sub.2 -R.sub.1)Gh.sub.2) 
where 
h.sub.1 is the height of the molten salt column above the metal overflow 
weir, 
h.sub.2 is the height of the molten salt column below the weir, 
R.sub.1 is the density of the molten salt, 
R.sub.2 is the density of the molten metal, 
g is the interfacial tension at the metal/salt interface, 
r is the radius of the filter apertures, and 
G is the gravitational constant. 
For a filter aperture of 10 mm diameter, the value of the supportable 
column of metal or salt is about 20 mm; for a filter aperture of 5 mm 
diameter, the supportable column is greater than 30 mm. For filters with 
smaller apertures, the supportable column is correspondingly greater. 
The surface area of the filter needs to be sufficiently great to permit the 
required flow of molten metal without the use of a pressure difference 
which would force molten salt droplets through. By way of example, a 
filter of 1 square meter area constituted by a 5 centimeter thick bed of 1 
mm diameter particles of TiB.sub.2, would permit a metal flow rate of 100 
l/min (equivalent to about 240 kg/min of molten Al) with a pressure head 
drop of 5 cm across the bed. By way of another example, if the openings in 
the filter were of 2 mm diameter, then it can be calculated that 700 would 
be required for a flow rate of 500 kg/min, typical of a large casting 
station. If the distance between centres of each opening was, say, 6 mm, 
then 700 openings could be fitted on a 16 cm.sup.2 plate, and the required 
metal flow achieved by a pressure difference of 3.5 cm of metal. 
If desired, the metal stream may be passed through a conventional filter of 
metal non-wettable material positioned downstream of the metal-wettable 
filter. This filter may be designed to catch any small molten salt 
droplets or any metal non-wettable inclusions that may have passed through 
the metal-wettable filter. A convenient arrangement is a packed bed of 
metal-wettable granules (forming the metal-wettable filter) supported on a 
grid or honeycomb of non-wettable material (forming the non-wettable 
filter). 
Steps (a) and (b) of this embodiment of the method may be carried out in 
different vessels, but are preferably carried out in the same vessel, 
conveniently by flowing the molten metal stream downward while in contact 
with the molten salt mixture, towards a metal wettable filter in the lower 
part of the vessel. The vessel may have a detachable drop bottom to 
facilitate removal of spent metal-wettable balls or granules. When a bed 
of non-wettable shapes is used, this may also conveniently be made 
removable from the rest of the equipment for renewal. The metal-wettable 
filter may form part of a baffle separating incoming from out-going metal. 
The arrangement is preferably such that both the upstream and downstream 
surfaces of the metal-wettable filter are in contact with substantially 
only molten metal. 
In embodiment (ii) of the method, molten metal is passed in sequence in 
either order through a filter of metal-wettable material and a filter of 
metal-non-wettable material. 
While applicants do not want to be bound by theory they believe that the 
metal-wettable filter medium used in this embodiment has an affinity for 
metal-wetted inclusions, and holds these inclusions securely by surface 
interaction within the interstices. The wetted interface (filter 
medium-Al) provides an active surface on to which individual inclusions 
can agglomerate and grow. Due to this surface locking mechanism, the 
clustered inclusions will have less chance of being accidentally dislodged 
from the filter by any subsequent surge of metal flow or other physical 
disturbance. Similarly, it is believed that the metal-non-wettable filter 
medium used has an affinity for metal-non-wetted inclusions, and holds 
these inclusions securely by surface interaction within the interstices. 
The material and structure of the filter may be as described above in 
relation to embodiment (i). 
Since the metal-wetted inclusions have a positive affinity for the filter 
medium, it is not necessary that the apertures of the filter should be 
smaller than the inclusions to be filtered. Indeed, it will often be 
desired to filter out metal-wetted inclusions down to a particle size of 
15-30 microns or even smaller, and it would not be practicable to use a 
filter with apertures so small. The minimum size of metal-wetted inclusion 
retained depends on the aperture of the filter and on the degree of 
convolution of the passages through it. When the filter is formed of a bed 
of granules, it is preferred that the weight average diameter of the 
granules should be from 0.5-6 mm, particularly 1-3 mm. A filter bed of 
such granules having a depth of at least 50 mm, preferably 100-150 mm 
should be effective to filter out undesired metal-wettable inclusions. 
Alternatively, a metal-wettable ceramic felt or blanket of the same 
thickness and aperture size should have similar properties, but with the 
advantage that channelling or blockage, which can arise due to relative 
movement of granules in bed, could not occur. 
The material of which the metal-non-wettable filter is made is not critical 
and may be conventional. Suitable materials for filtering molten aluminium 
include chromite, corundum, forsterite, magnesia spinel, periclase, 
silicon carbide and zircon, with tabular alumina (synthetic corundum) 
being preferred. Also the structure of this filter is not critical. 
Suitable structures include apertured plates, honeycomb grids, parallel 
bars, ceramic cloths, ceramic felts and packed beds of regularly or 
irregularly shaped granules. Preferred are packed beds of granules, which 
may be uniform but are preferably arranged in layers of different sized 
particles. Filtering efficiency is determined by the finest particles 
present, and these should preferably have a weight average diameter of 1.5 
to 8 mm, particularly from 2 to 4 mm. 
Composite filters may simply comprise superimposed layers of different 
grades of filter material. A preferred filter comprises a composite 
upstream layer of granules of metal-non-wettable material, itself 
including two or more layers of decreasing granule size, followed by a 
layer of particles of metal-wettable material. If desired, a downstream 
support layer of granules of metal-non-wettable material may be provided 
and may itself include two or more layers of increasing granule size. This 
arrangement has the advantage that the coarsest material is on the 
outside, so that the layers of finer material, including the 
metal-wettable material are protected from shock and damage. However, many 
other arrangements are possible. For example the layer of metal-wettable 
particles may itself be divided into two or more layers of graded particle 
size. Or layers of metal-wettable particles may be interposed between the 
graded layers of metal-non-wettable granules. Or the metal-wettable layer 
may be positioned upstream of the metal-non-wettable, with a layer of 
coarse particles on the outside to protect the inner layers from damage. 
The thickness of the layers and the total filter area can both be selected 
in relation to the intended metal flow and pressure difference to provide 
a filter having desired replacement life and performance characteristics. 
When TiB.sub.2 and other borides are used as metal-wettable filter media, 
it is important that they should be kept away from contact with oxygen, 
otherwise they lose their metal-wetting properties. This is easily done if 
the method of the invention is operated on a continuous basis. In other 
cases, it may be necessary to ensure that the filter remains immersed in 
liquid or solid metal when not in use, or to exclude atmospheric oxygen 
from the region of the filter. By using an electrically heated filter 
operating under inert atmosphere (Argon or N.sub.2) it is possible to 
preheat the metal-wettable filter without contact with atmospheric oxygen.

Referring to FIG. 1, a vessel 1 contains a bath of molten salt 2 in which 
there is positioned a solid spreading device 3. The molten salt bath 
overlies a layer of molten metal 7. The vessel is divided into two parts 
by means of a baffle 4, the lower part of which is constituted by a 
metal-wettable filter 5. The part of the vessel downstream of the filter 
is filled with molten metal 8 up to the level of a clean metal exit weir 
6. 
In operation, a stream of molten metal enters the upstream side of the 
vessel, impinges against the spreader plate 3, and is broken up into a 
plurality of streams in intimate contact with the molten salt mixture 2. 
During this contact, metal non-wettable inclusions are retained by the 
molten salt, and molten salt droplets become entrained in the molten metal 
which collects at 7 under rather quiescent conditions to encourage 
coalescence of molten salt droplets. On passage of the molten metal 
through the filter 5, molten salt droplets are stopped by surface tension 
forces and metal-wettable inclusions are retained within the filter. The 
molten metal is driven through the filter from 7 to 8 by virtue of the 
difference in the surface levels of the molten salt mixture 2 and of the 
treated metal 8. 
In FIG. 2, the filter 5 is positioned horizontally below the salt layer 2 
in the entry compartment and permits a reduction in the height of the 
treatment vessel 1. 
In FIG. 3, the filter 5 is positioned horizontally in the exit compartment 
and permits a deeper molten salt layer 2 and hence longer metal/salt 
contact time in step 1. 
In FIG. 4, the filter 5 comprises a bed of titanium diboride (or other 
refractory metal boride) balls disposed on the bottom of the vessel 1, 
such that the top of the bed 5 is above the bottom of the baffle 4. The 
bottom end 11 of the vessel 1 is detachable from the remainder of the 
vessel by means of bolts 12 in flanges 13 to facilitate removal of spent 
TiB.sub.2 balls. 
FIG. 5 resembles FIG. 4 except that an alumina honeycomb 14 has been 
positioned immediately downstream of the TiB.sub.2 balls to serve as an 
additional metal non-wettable filter. 
FIG. 6 shows a somewhat different arrangement. A vessel 1 contains a molten 
salt layer 2, provided with a spreader plate 3, overlying a molten metal 
layer 7. One or more tubes 9, of a material resistant to molten salt and 
molten metal, extend from the molten metal layer 7 up through the molten 
salt layer 2 and across to the side wall of the vessel. The open lower end 
of the or each tube 9 is closed by a metal-wettable filter 10. 
Alternatively, the tube or tubes 9 could have been somewhat extended 
downwards and their lower ends located in a shallow well positioned on the 
floor of the vessel 1, which well is either lined with or filled with a 
metal-wettable filter material. Reference is directed to our European 
Patent Specification 68782 A2 for a fuller description of these and other 
constructions of metal-wettable filter. 
FIG. 7 shows a vessel 15 divided into two parts by a baffle 16 with a metal 
entry trough 17 to the side of the vessel upstream of the baffle and a 
metal exit weir 18 from the downstream side. The metal entry trough 
terminates in a distributor 19 having apertures for passage of molten 
metal. A molten salt layer 20 is filled with alumina shapes 21 and 
overlies a layer 22 of fine titanium diboride particles which in turn 
overlies a layer 23 of coarser titanium diboride shapes. This layer 23 
extends beneath the baffle 16 and is overlaid, on the downstream side 
thereof, by a further bed 24 of alumina shapes. The major part of the 
vessel is filled with molten metal, the metal/salt interface lying 
essentially along the boundary between the alumina shapes 21 and the fine 
titanium diboride shapes 22. The top surface 25 of the salt layer should 
be maintained a little below the distributor 19. 
In operation, molten metal passes through the apertures in the distributor 
19 into the molten salt layer 20 where it flows over the alumina shapes 21 
at a low velocity with good metal/salt contact and little or no 
opportunity for salt droplet formation. The metal then flows through the 
thin layer 22 of fine titanium diboride particles, which aid salt 
retention without unduly increasing the pressure drop through the bed, 
through the coarse titanium diboride layer 23, through the alumina bed 24, 
and finally over the weir 18. 
The embodiment of FIG. 8 is similar to that shown in FIG. 7, except that 
the alumina bed 21 is suspended from the distributor 19, on a perforated 
plate 26, such that the assembly can readily be removed upwards for 
renewal of bed material. In operation, the entire molten salt layer 20 is 
positioned within the bed, the top surface being shown as 25 and the 
metal/salt interface as 27, and the plate 26 prevents the bed from 
contacting the metal wettable filter 22/23. The assembly may be formed of 
grey cast iron or of a suitable refractory material. The cross-sectional 
area of the assembly is not critical, though the larger is the 
cross-section the greater is the effective use of the molten salt bath. 
The assembly may be shaped to conform in the space between the baffle 16 
and the outer wall 15 of the treatment vessel. Alternatively, the assembly 
may comprise simply a circular (or other) cross-section bowl with a solid 
side wall. 
Referring to FIG. 9, a downflow sandwich filter comprises three major 
layers, an upstream layer 28 of non-wetted alumina, a midstream layer 29 
of wetted titanium diboride and a downstream support layer 30 of 
non-wetted alumina. The upstream layer is itself formed of three layers of 
granules of progressively increasing fineness, namely a first layer 31 of 
8-16 mm diameter granules, a second layer 32 of 4-8 mm diameter granules, 
and a third layer 33 of 2-4 mm diameter granules. The midstream layer 29 
comprises 1-2 mm diameter titanium diboride particles. The downstream 
support layer is formed of two layers of granules, namely a first layer 34 
of 1.5-4 mm diameter granules, and a second layer 35 of 8-16 mm diameter 
granules. 
The thickness of the various layers is chosen in relation to the desired 
rate of flow of metal per unit area, to provide adequate filtering 
performance and service life without an excessive pressure drop across the 
filter. 
Alternatively, one or more of the layers or indeed all the layers could 
have been formed of particles or granules of up to twice the specified 
diameter. Alternatively again, additional layers of titanium diboride 
particles could have been interpolated between the first and second layers 
31, 32, and/or between the second and third layers 32, 33 of the upstream 
layer 28 of alumina granules.