Protection for externally heated cast iron vessel used to contain a reactive molten metal

An externally heated cast iron vessel, intended for containing a reactive molten metal, such as aluminum, is made resistant to attack by the molten metal, thereby increasing its useful service life and minimizing contamination of the melt, by lining the inside of the cast iron shell with a plurality of inert self-supporting, refractory plates, of for example, graphite, in such manner that the plates are free to move along their joints as well as relative to the shell upon thermal expansion, and permitting the molten metal to penetrate behind the lining through the joints and crevices therein opened by thermal expansion, thereby producing a refractory layer, in situ in the space between said lining and the inside surface of said cast iron shell, comprising a solid (FeAl.sub.3) reaction product of iron and said molten metal.

BACKGROUND 
The present invention relates to a process for making an externally heated 
cast iron vessel intended for containing reactive molten metals, such as 
aluminum, resistant to attack by the molten metal; thereby increasing the 
useful service life of the vessel and minimizing contamination of the 
melt. The present invention also relates to the vessel thus produced. 
In refining molten aluminum and other reactive metals, it is often 
desirable to use a vessel which is externally heated. Cast iron vessels 
are desirable because they have high thermal conductivity, can be cast in 
any desired shape and have a relatively low coefficient of thermal 
expansion. The problem, however, with cast iron is that it is corroded by 
molten aluminum. It is well known in the art that aluminum is a powerful 
solvent in its molten state, and that consequently care must be exercised 
in selecting materials with which it will come in contact during various 
processing steps such as melting, alloying, degassing, fluxing, 
filtration, transfer and casting. Improper selection of such material may 
cause contamination of the melt by reduction or solution of the container 
as well as deterioration of the container. It is normal commercial 
practice therefore to coat cast iron objects which are to be used in 
contact with molten aluminum, such as with, for example, a wash of red 
mud, zirconium silicate, mica, iron oxide or titanium oxide. Sodium 
silicate may be added to the wash coating to improve its adherence to the 
cast iron. Such coatings are generally applied by brushing or spraying on 
to these portions of cast iron surface which will come in contact with the 
melt. However, these coatings wear off easily. The problem of the limited 
service life for externally heated cast iron vessels used for containing 
molten aluminum has not been satisfactorily solved by the prior art. 
OBJECTS 
It is therefore an object of this invention to provide a process for 
producing an externally heated cast iron vessel for continuous use with 
reactive molten metals such as aluminum which has long service life and 
which causes minimum contamination of the molten metal with iron. 
It is another object of this invention to provide an externally heated cast 
iron vessel having improved service life, which is able to contain 
reactive molten metal, such as aluminum without contaminating the melt. 
SUMMARY 
The above and other objects which will be apparent to those skilled in the 
art are achieved by the present invention, one aspect of which comprises: 
a process for making a vessel, comprised of an externally heated cast iron 
shell, resistant to attack by reactive molten metal contained therein, 
comprising the steps of: 
a. lining the inside surface of said shell with a plurality of 
self-supporting, refractory plates which are inert with respect to said 
molten metal, in such manner that said plates are free to move along their 
joints relative to each other, as well as relative to the inner surface of 
said shell, upon thermal expansion of said vessel, 
b. filling the vessel with said reactive molten metal, 
c. maintaining the temperature of said vessel at a value at least equal to 
the melting point of said molten metal by externally heating said vessel, 
d. permitting said molten metal to penetrate behind said lining through the 
joints and crevices therein opened by thermal expansion, and thereby 
e. producing a refractory layer, in situ in the space between said lining 
and the inside surface of said cast iron shell, comprising a solid 
reaction product of iron and said molten metal, thereby preventing further 
direct contact between the molten metal in the vessel and any fresh cast 
iron surface of said shell. 
Although the process and apparatus described above are particularly 
suitable where the molten metal is aluminum, the invention is also 
applicable to other reactive molten metals, such as zinc, tin and lead. It 
is to be understood that the term, aluminum, as used in the present 
specification and claims, is intended to include the alloys of aluminum as 
well as pure aluminum. 
The term plate, as used herein is not meant to be restricted to flat plates 
of, for example, graphite, but rather is intended to include machined or 
even cast component parts of any refractory material which is inert toward 
the molten metal. The term, plate, is also meant to distinguish the 
structure of the lining from monolithic or unitary structures. 
Another aspect of the present invention comprises: 
an externally heated vessel for containing reactive molten metal comprising 
in combination: 
a. a shell of cast iron, provided with 
b. a lining comprised of a plurality of self-supporting, refractory plates 
on the inside surface of said shell which are inert with respect to said 
molten metal, said plates being free to move along their joints relative 
to each other, as well as relative to the inner surface of said shell, 
upon thermal expansion of said vessel, and 
c. a refractory layer comprising a solid reaction product of iron and said 
molten metal produced in situ in the space between said lining and the 
inside surface of said shell. 
If the vessel is intended for use with molten aluminum, then it is 
preferably made of grey cast iron, containing from about 0.2 - 1.5% 
chromium, and the lining is then preferably made from a plurality of 
self-supporting graphite plates.

DETAILED DESCRIPTION 
FIG. 1 illustrates the aluminum refining system, disclosed in greater 
detail (as FIG. 3) in the aforementioned parent applications, the entire 
disclosures of which are incorporated herein by reference. The vessel of 
FIG. 1 comprises a cast iron shell 31 which is maintained at its operating 
temperature by conventional heating means located in well 32, and an outer 
refractory shell 33 for insulation against heat loss. The inner surface of 
the cast iron shell 31 is lined with graphite 34 or other refractory 
material which is inert to molten aluminum. Shell 31 is provided with a 
cover 36 which rests upon flange 39. Metal 38 enters the vessel through 
inlet port 40. Inside the vessel metal 38 is sparged and agitated by the 
action of inert gas injected into the melt through the rotating gas 
injector 35. Arrows 50 show the overall circulation pattern of the molten 
aluminum in the vessel caused by the rotating gas injector. The refined 
molten metal leaves the vessel through discharge port 44 situated below 
the metal surface 42 in wall 45. The metal passes through well 46 and 
leaves the refining system through exit trough 47 to a casting station. 
The graphite lining 34, in accordance with the present invention, consists 
of a plurality of graphite plates, which upon being heated to operating 
temperature will have sufficient spaces between adjoining plates to permit 
the metal 38 to penetrate behind the plates, formming a thin film of 
molten aluminum which on coming in contact with the cast iron shell 31 
will form the FeAl.sub.3 layer (not shown) as hereinafter described. 
FIGS. 2 and 3 disclose a two-chambered vessel comprised of a cast iron 
shell 51 lined on the inside with a plurality of graphite plates 42 and 
silicon carbide plates 56. Separate plates form the bottom and the side 
walls of the lining. The outside of the cast iron shell 51 is surrounded 
with a heating chamber 53 which may contain any conventional heating means 
such as, for example, electric coils. The heating chamber 53 is in turn 
surrounded with refractory insulation 54. Baffle plate 55 which separates 
the chambers is likewise made of a graphite plate. The direction of the 
flow of molten aluminum is shown by the arrows, arrow 60 showing the inlet 
section and arrow 61 the exit from well 62 which is preferably made of a 
plurality of silicon carbide plates 56 and 57. Rotating gas injectors 63 
and 64, respectively, are mounted in the cover 65 of the vessel. Metal 
return pipe 68 is likewise of graphite. 
FIG. 4 is a schematic representation of an enlargement of a segment of the 
wall of either FIG. 1 or FIG. 2 illustrating the cast iron shell 72, 
graphite plate 71 and therebetween the refractory lining formed in place, 
comprising the iron-saturated molten aluminum film 73 containing the 
precipitated FeAl.sub.3 phase 74 which covers the surface of the cast iron 
shell 72. The small scale of FIGS. 1 and 2 prevents this layer from being 
shown in those Figures. 
When assembling the vessel, the graphite plates are placed within the cast 
iron shell at room temperature, and fit as closely as possible to each 
other, as well as to the wall of the shell. After assembly of the graphite 
plates, all cracks or spaces between abutting plates are cemented with 
graphite cement. However, when the vessel is heated to its intended 
operating temperature (about 700.degree. C. for aluminum) these joints 
open up due to the differential thermal expansion between the cast iron 
and the graphite so that when the molten aluminum is introduced into the 
vessel, it will penetrate through these crevices in the lining and fill 
the space between the casting and the lining. On heating from room 
temperature to 700.degree. C., graphite expands only about 12% as much as 
iron along the grain, and about 27% as much as iron across the grain. In 
addition to graphite plates of silicon carbide or precast forms of either 
material may also be used. These plates may simply be cut to fit snugly 
into the shell or may be keyed or grooved to interlock. 
Preferably the vessel is heated to its desired service temperature (e.g. to 
molten aluminum temperature) before the aluminum is introduced into the 
vessel. During heating of the vessel, the cast iron shell and the plates 
which make up the inert lining expand. Thermal expansion of the lining is 
unrestricted, that is, the plates are free to move relative to each other, 
as well as to the cast iron surface. The expanding components of the 
lining are permitted to move along their joints or abutting surfaces, that 
is along lines predetermined by design. This freedom of movement and the 
higher thermal expansion of cast iron prevents random cracks from being 
produced in the lining at places other than joints or the abutting 
surfaces of the plates during thermal expansion of the vessel. 
A very small quantity of the molten aluminum introduced into the heated 
vessel is permitted to come in contact with the cast iron surface by 
penetration through the crevices opened up along the joints of the plate 
lining by their thermal expansion. The width of these crevices may be 
minimized during installation of the lining at room temperature by 
matching the plates of the lining to each other as accurately as possible. 
In the case of graphite plates, a light application of graphite cement on 
the abutting surfaces is advantageous for establishing a tighter fit. 
Reduction of clearances between the plates, however, cannot be carried so 
far as to prevent their relative movement. The purpose of minimizing 
clearance between the plates is to prevent the crevices at the joints from 
growing too wide on thermal expansion. Contrary to expectations and the 
teachings of the prior art, this seepage of the reactive metal to the cast 
iron surface initiates the process, which under controlled conditions, 
ultimately inhibits the corrosion of the cast iron by molten aluminum, and 
by so doing leads to unexpectedly long vessel life. 
When the molten aluminum behind the lining contacts the cast iron surface, 
it dissolves some iron from the cast iron matrix. Since the volume of the 
aluminum which penetrates behind a well-fitting lining is very small, 
compared to the area of contact with the cast iron, the iron dissolves 
into what can be pictured as a thin molten aluminum film, sandwiched 
between an externally heated cast iron wall and an inert graphite lining. 
The high temperature and the extent of contact area between the cast iron 
shell and the aluminum promotes rapid solution of the cast iron until the 
saturation limit is reached. The saturation concentration of iron in 
aluminum is a function of the temperature and of the composition of the 
aluminum alloy. In pure aluminum the saturation concentration of iron is 
approximate by the following equation, which is valid for the temperature 
range (655.degree. C. - 750.degree. C.) normally encountered in practice: 
EQU c = -13.8 + 0.024 .times. t 
where: c = the concentration of iron in aluminum (wt. -%), and t = 
temperature of the aluminum (.degree.C.). 
From this equation it can be calculated that at 700.degree. C., the 
concentration of iron that will dissolve in aluminum is only about 3%. 
That is, a relatively small amount of iron can establish saturation in the 
molten aluminum film. At this saturation concentration, an intermetallic 
solid phase, corresponding to the stoichiometric formula FeAl.sub.3 
precipitates. This iron-aluminum phase is stable up to a decomposition 
temperature 1160.degree. C., and since it is an iron rich phase, it starts 
to form on or in the vicinity of the cast iron surface. Precipitation of 
the FeAl.sub.3 phase continues until all the aluminum layer enclosed 
behind the inert lining reaches saturation. At this point an equilibrium 
state is reached; no additional iron is dissolved and no additional 
FeAl.sub.3 phase is formed. Further attack on the cast iron surface is now 
inhibited by the presence of the iron rich FeAl.sub.3 intermetallic 
phase. A change in this equilibrium state is possible only if the iron 
concentration in the aluminum film drops below the limit. This could occur 
for example, if dissolved iron excapes from the iron saturated aluminum 
layer by diffusion through the crevices in the lining. If this were to 
happen, the FeAl.sub.3 phase would assume a scavanging role by going into 
solution to re-establish equilibrium. In an overall balance, the rate of 
corrosion of the cast iron surface, following the initial formation of the 
protective intermetallc layer, is determined by the rate of mass transfer 
through the crevices in the graphite lining possibly by the rate of 
diffusion of dissolved iron from the molten aluminum layer enclosed behind 
the lining. These rates, however, are very small so that the corrosion of 
the cast iron shell is extremely small, resulting in the unexpectedly long 
service life of the vessel. 
The above described mechanism underscores the several important functions 
served by a self-supporting inert graphite plate lining. The inert lining 
forms a mechanical barrier against the chemical dissolution of the 
intermetallic refractory phase by the bulk of the molten aluminum metal 
contained in the vessel. It is advantageous to keep the size of the 
crevices small between the plates of the lining, since they represent the 
only avenues of communication between the iron-saturated layer behind the 
lining and the bulk of the metal in the vessel. The lining also prevents 
mechanical erosion of the protective FeAl.sub.3 layer by the flow of the 
molten metal. This protection is particularly important when the metal in 
the vessel is in turbulent flow or vigorously stirred, as for example, 
during the refining process described in U.S. Pat. No. 3,743,263 
previously referred to. Not directly related to the mechanism of formation 
of the refractory layer, but still of great practical importance, is the 
fact that the material of the self-supporting lining can be selected from 
materials, such as graphite or silicon carbide, which are not only truly 
inert to and not wetted by aluminum, but are also good thermal conductors. 
The present invention makes utilization of these materials possible in the 
form of relatively thin self-supporting plates. Consequently, large 
vessels can be lined with such materials without running into prohibitive 
costs. 
Although the FeAl.sub.3 phase can always be found in the refractory layer 
formed between the cast iron and the graphite lining, other phases may 
also be present when commercial aluminum alloys are processed. For 
example, in the case of silicon containing aluminum alloys, an 
intermetallic phase corresponding to a stoichiometric composition of 
Fe.sub.3 SiAl.sub.12 precipitatates at relatively low iron concentrations, 
if the molten metal film behind the inert lining becomes enriched with 
silicon above about 0.7 wt.-% silicon. This phase provides protection for 
the cast iron surface by essentially the same mechanism as FeAl.sub.3. The 
decomposition temperature of this phase (860.degree. C.) is also 
significantly above the normal temperatures encountered in refining molten 
aluminum. 
Besides the iron itself, the alloying elements of cast iron may also 
contribute to the formation of a protective refractory layer. For example, 
the silicon for the aforementioned intermetallic phase can be supplied by 
the cast iron, since cast iron commonly contains silicon. Another alloying 
element which forms an intermetallic phase with aluminum is chromium. At 
700.degree. C. a solid phase CrAl.sub.7 precipitates from molten aluminum 
if the concentration of chromium exceeds about 0.7 wt.-% chromium. The 
decomposition temperature of CrAl.sub.7 is about 725.degree. C. 
EXAMPLE 
A vessel as shown in FIGS. 2 and 3 was constructed of a cast iron shell 
containing 0.6% chromium and lined with 11/8 inch thick graphite plates on 
the sides, and 2 inch thick graphite plates on the bottom. The metal inlet 
and outlet areas of the shell were lined with silicon carbide plates. The 
vessel was preheated to 700.degree. C. before being filled with molten 
aluminum. The vessel was externally heated by electric power, and the 
temperature of the aluminum was kept at about 700.degree. C. throughout. 
The melt was violently stirred by driven impellers and gas bubbles, since 
the vessel was used to carry out the aluminum refining process described 
in U.S. Pat. No. 3,743,263. Over a continuous period of six months of 
field testing under conditions of actual commercial operation, the 
graphite lining was not wetted, chemically attacked or eroded either by 
the aluminum or by the dross. Consequently, the vessel did not require 
periodic cleaning or repairs. This length of continuous operation under 
the turbulent flow conditions of molten aluminum is far in excess of the 
service life of externally heated cast iron vessels made by prior art 
techniques. 
The advantages of a vessel made in accordance with the present invention 
are numerous. The present invention enables an externally heated cast iron 
vessel to have a significantly longer service life than was obtainable by 
the prior art. The molten metal in the vessel is not contaminated by the 
cast iron shell. The metal in the vessel may be in turbulent flow without 
causing damage to the protective layer. And heat transfer through the 
vessel wall is facilitated since all three components of the vessel walls, 
namely the cast iron shell, the intermetallic layer and the graphite 
lining are all good conductors of heat.