Method and apparatus for detecting the position of fluid-fluid interfaces

Precise information is obtainable concerning the position of a fluid-fluid interface (28, 33), such as between a gas and a liquid or between liquids. Such information is important for the control and automation of molten metal refining processes. Specifically, in an electrolytic process for removing magnesium from molten aluminum, knowledge of the vertical position of an interface (33) between the electrolyte and the magnesium and an interface (28) between the electrolyte and the aluminum facilitates the automatic removal of the purified metals when drained or drawn from the furnace. Heat energy is conducted through a thermocouple-heater assembly 34 to a thermocouple (38) located at its tip (36). The equilibrium temperature at the thermocouple junction is dependent upon the heat loss through tip (36). When the tip comes in contact with a fluid of different thermal conductivity, as between molten matter (29) and electrolyte (30), the equilibrium temperature at tip (36) and, thus, of the thermocouple junction will change. This change in temperature is used to determine the level of the liquid-liquid interface between molten matter (29) and electrolyte (30). This information is used to determine when the drawing of further molten material from the furnace is to be terminated, so that aluminum purified in the refining process and previously drawn through an outlet (26) will not be contaminated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for detecting the 
position of fluid-fluid interfaces, e.g., between liquid-liquid and 
gas-liquid interfaces, and, more particularly, to the effecting of such a 
method and apparatus by use of thermal detection means. The present 
invention is suitable for use in the refining of molten metals with 
specific application to the removal of magnesium from scrap aluminum. 
2. Description of Related Art and Other Considerations 
Although the impetus for conceiving the present invention is to provide 
process control in molten metal technologies, specifically, in a process 
for refining scrap aluminum, it is to be understood that the present 
invention is as applicable to any need for detecting the position of 
fluid-fluid interfaces by thermal detection means. 
The removal of magnesium from scrap aluminum has been discussed in several 
publications, of which the following two are of particular interest to the 
present invention, viz., "Electrolytic Removal of Magnesium from Scrap 
Aluminum" JOURNAL OF METALS, Vol. 36, No.7, July 1984 pp 141-43, and 
"Electrolytic Demagging of Secondary Aluminum in a Prototype Furnace" AFS 
Transactions, Vol. 94, pp. 385-390 (1986). The following excerpt from the 
latter article well states the reasons and background for recovering 
aluminum from scrap. 
"The amount of aluminum in an automobile has steadily increased from an 
average of 40 kg in 1976 to an average of 60 kg in 1982 due to efforts to 
achieve higher fuel efficiency by lowering the overall weight of the 
vehicle. Therefore, for a constant supply of aluminum at minimum cost, 
casting producers may consider increasing the use of high magnesium scrap, 
with large potential savings over the purchase of primary aluminum. 
However, to conform with specifications, the production of casting alloys 
such as 319 from high magnesium aluminum scrap would require the removal 
of magnesium in excess of 0.1 wt. %. A chlorination process is most widely 
used by secondary smelters for demagging casting alloys. In this process, 
magnesium is selectively oxidized by chlorine and removed from molten 
aluminum in the form of a magnesium chloride dross. While the process is 
reasonably efficient at high magnesium content, it may create unacceptable 
environmental conditions in the plant. In addition, magnesium is being 
lost in the form of MgCl.sub.2 dross, which being hygroscopic may pose 
disposal problems. 
"Recognizing the need for an efficient and pollution-free demagging 
process, we have been developing the electrochemical process described in 
this paper. This process recovers magnesium in the form of salt-coated 
globules and apparently causes no environmental problems. The process . . 
. consists of covering the molten aluminum scrap with an electrolyte (a 
mixture of alkali and alkaline earth metal halides) and passing a current 
between molten aluminum acting as an anode and inert cathode dipped into 
the electrolyte. On applying a voltage between the electrodes, magnesium 
(being more reactive) dissolves first in the electrolyte from the aluminum 
melt, and concurrently deposits on the cathode. Because of its lower 
density, magnesium floats on the electrolyte and, thus, it is separated 
from the aluminum." 
Inasmuch as the reaction vessel utilized in this demagging process contains 
three liquid layers comprising a top layer of magnesium, a middle layer of 
salt-electrolyte and a bottom layer of aluminum, operators need to monitor 
the levels of each layer during the addition or removal of metal. In 
particular, precise information about the electrolyte-metal interfaces is 
required to permit the removal of purified aluminum from the vessel 
without its being contaminated with the molten salt. 
In the equipment described in the above-referenced AFS Transactions 
publication, the problem of aluminum removal was solved by utilizing two 
vertically placed drain holes, similar to holes 25 and 26 herein shown in 
FIG. 1. As the purified aluminum was drained from the reaction furnace 
into separate collection vessels, the electrolyte appeared at the upper 
drain hole, at which point the draining process was stopped to prevent any 
electrolyte from draining through the lower hole. The procedure was 
inconvenient to use and would be difficult to automate. 
SUMMARY OF THE INVENTION 
The present invention successfully provides the necessary process control 
information in an easier and more dependable manner. The position of an 
interface between fluids, for example, between gaseous and liquid media, 
or between two liquid media, such as between the molten aluminum and the 
electrolyte, which respectively have different heat transfer 
characteristics, is detected by sensing a change in the conductivity of 
heat within the respective interfacing fluids. The term "heat transfer 
characteristics" is intended to encompass, but not be limited to, such 
characteristics as coefficient of thermal conductivity, kinematic 
viscosity, prandtl number and thermal convection. 
Specifically, the method and apparatus embodying the method exploit 
differences in heat transfer characteristics, such as thermal 
conductivity, in adjacent fluid or liquid layers. Preferably, a source of 
heat for heating a probe causes heat to flow into the fluids, whether 
liquid or gas. By measuring the steady state temperature of the heated 
probe, the precise level of the interfacing liquid layers can be 
determined, in particular through the rate of flow of heat energy detected 
by some form of temperature sensing, such as a thermocouple, located in 
the tip of the sensing device. The equilibrium temperature at the 
thermocouple junction is dependent upon the heat loss through its tip. 
When the tip comes in contact with a gas or a liquid of different thermal 
conductivity, the equilibrium temperature at the thermocouple junction 
changes, and this change is used to denote the location of the interface. 
Several advantages are derived from the present invention. Precise 
measurements of fluid-fluid interfaces are obtainable, particularly 
without requiring the use of electrical field or like measuring means, to 
preclude any such field from interfering with the sensing. Level sensing 
can be implemented in corrosive or other hostile environments. In a 
demagging process, removal of the purified aluminum and magnesium can be 
easily automated. 
Other aims and advantages, as well as a more complete understanding of the 
present invention, will appear from the following explanation of exemplary 
embodiments and the accompanying drawings thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1 and 2, a fully sealed furnace or reaction vessel 10 
provides a closed environment for the removal of magnesium from scrap 
aluminum and for enabling purified aluminum to be drawn from closed vessel 
10. The working volume of furnace 10 is divided into a refining zone 12 
and a heating/pouring zone 14. Positioned in refining zone 12 is a cathode 
16 positioned above an anode 18. The cathode and anode are connected to a 
source of direct current 20. Preferably, cathode 16 is formed of mild 
steel, while anode 18 is formed of graphite. A heater 22 is positioned in 
heating/pouring zone 14. A cover 23 in the otherwise closed top Of vessel 
10 is opened so that scrap aluminum in molten form may be placed into the 
furnace. Various holes 24, 25 and 26 are provided in furnace 10 and are 
closable by suitable means. Hole 24 is used as an electrolyte/separated 
magnesium drain, while holes 25 and 26 are used as egresses for removal of 
refined aluminum from the furnace. As will be discussed below, hole 25 may 
be dispensed with, as being useful in practicing the demagging process 
prior to implementation of the present invention. 
In the operation of the process both prior to and after use of the present 
invention, and as more fully detailed in the two publications referred to 
above, scrap aluminum containing magnesium impurities in molten form is 
placed into heating/pouring zone 14 through the opening uncovered by cover 
23, and thus within refining zone 12 to approximately the lowermost 
portion of hole 24, as designated generally by a level symbolized by line 
28. Indicium 29 generally designates molten matter comprising either the 
molten scrap aluminum prior to purification or the purified aluminum 
obtained therefrom. An electrolyte 30 of calcium chloride, magnesium 
chloride, potassium chloride and sodium chloride is placed above the 
molten scrap aluminum to a depth sufficient at least partially to cover 
cathode 16. A space 31 is provided for an inert gas, for example, argon. 
Upon application of electrical energy, the magnesium is ionized and 
collected at the cathode, thereby forming a layer 32 of molten magnesium. 
After a suitable period of time, when the molten scrap aluminum is 
sufficiently purified of the magnesium impurities, one or both tap holes 
25 and 26 are opened in order to draw off the purified aluminum. 
Before use of the present invention and as discussed above, as the level of 
purified aluminum drops and the level of the electrolyte with impurities 
therein reaches tap hole 25, no further aluminum is drawn from furnace 10. 
Because tap hole 26 is positioned lower than tap hole 25, it is possible 
to separate the amount of pure aluminum drawn from the furnace at tap hole 
26 as distinguished from tap hole 25. Therefore, in the process thus 
described, it has been possible to monitor the level which distinguished 
the interface between the pure and impure molten materials. 
In the present invention, however, rather than utilizing a pair of tap 
holes 25 and 26 to determine the level at which impurities are 
discernable, the following thermal sensing system is employed. 
While the preferred heat sensor of the present invention comprises a 
thermocouple, it is to be understood that any form of heat sensing 
apparatus is as applicable. Furthermore, the mechanism of heat transfer 
from or to the molten materials is generically referred to herein as heat 
transfer characteristics, which is intended to encompass such parameters 
as coefficient of thermal conductivity, kinematic viscosity, prandtl 
number and thermal convection. Therefore, these specific terms are 
intended to be taken as illustrative and not limiting of the present 
invention, even though specific use may be employed in the subsequent 
description. 
Accordingly, a thermocouple-heater assembly 34, having a head 35 and a tip 
36, is positioned within furnace 10 and is extended downwardly towards 
bottom 37 of the furnace. Depending upon the stage at which the process is 
being conducted, assembly 34 is positioned within refining zone 12 in 
molten matter 29, and its tip 36 is placed generally at a level where tap 
hole 25 would have been located, if retained. Accordingly, assembly 34 
terminates at a level which is slightly higher than that of tap hole 26. 
The thermocouple-heater assembly extends upwardly to and exits at the top 
of furnace 10 in head 35. 
As best illustrated in FIG. 3, thermocouple-heater assembly 34 includes a 
thermocouple 38 which is coaxially centered within a heater coil 40, for 
example, of nichrome wire. The thermocouple and heater coil are positioned 
within a tube 42 of alumina or other high temperature material which is 
sufficient to withstand the temperatures of molten materials 29 and 30 and 
which is non-electrically conducting. Thermocouple 38 and heater coil 40 
are secured in the alumina tube by a ceramic cement 44. In this 
embodiment, only the thermocouple junction and not its leads is secured 
within the ceramic cement. A temperature display 48 is electrically 
coupled to thermocouple 38 by electrical wires 50. Heater coil 40 is 
energized by a power supply 52 through electrical wires 54. 
A modified form of thermocouple-heater assembly 34 is depicted in FIGS. 4 
and 5, and is denoted by indicium 56. In this embodiment, a thermocouple 
58 is centered within a heater coil 60, and the two are solidly affixed to 
one another by a ceramic encapsulating body 62, so as to space the heater 
coil from the thermocouple by encapsulating both the heater coil and the 
thermocouple, as shown. The total is held within a casing 64, such as of 
alumina. 
Heater coil 60 is simply constructed as coil 40 of FIG. 3. Both comprise a 
nickel-chromium wire having a helically coiled portion 66 extending from a 
first lead 68, and extending to a second lead 70. Both leads 68 and 70 
extend to a power supply, such as power supply 52 shown in FIG. 3. Portion 
66 extends helically downwardly, and encircles thermocouple 58 and its 
wires 59. Portion 66 then terminates at a bottom portion 72, and rises in 
a generally straight line within coiled portion 66 for exit to the power 
supply. 
In operation, prior to commencing purification, and with reference to FIG. 
1, assembly 34 is positioned in the molten scrap aluminum which, at this 
point of the process, constitutes the composition of molten matter 29. 
Power supply 52 is energized to bring the temperature of heater coil 40 or 
60 to a temperature which is greater than that of molten matter 29 and 
molten salt 30, to insure that heat moves from assembly 34 into the molten 
liquids. Upon supply of power to electrodes 16 and 18, magnesium is 
refined from the scrap aluminum and floats above molten salt 30 to its 
position identified by numeral 32. After a period of time, the molten 
scrap aluminum is converted into purified aluminum, which then constitutes 
the composition of molten matter 29. After the electrochemical refining 
process is completed, and when it is desired to draw the purified aluminum 
from refining zone 12, tap hole 26 is opened, to permit the purified 
aluminum to be collected in a collection vessel. During this draining, the 
levels of aluminum layer 29, molten electrolytic salt layer 30 and molten 
magnesium layer 32 drop until interface 28 between the aluminum and 
electrolyte layers passes below tip 36 of thermocouple-heater assembly 34. 
Because thermocouple 38 or 58 is at tip 36, the steady state temperature 
of the thermocouple will change as the rate of heat transfer into 
respective molten electrolyte 30 and molten aluminum 29 changes. Because 
the molten salt has a coefficient of thermal conductivity which is 
different from that of the molten aluminum and because aluminum is a 
better conductor of heat than the salt, heat transfer is at a greater rate 
into the molten aluminum than into the molten salt. These differences in 
the rates of temperature transfer are reflected in the steady state 
temperature of the thermocouple, and are displayed in temperature display 
48, to denote the passage of interface 28 past the thermocouple, at the 
point slightly above the level of hole 26. Accordingly, outlet 26 is 
closed so that no further aluminum will be permitted to be drawn 
therethrough, and thereby to prevent contamination of previously drawn 
aluminum from furnace 10. 
FIGS. 6 and 7 depict the results of experimental uses of the present 
invention, in which all fluids, whether gaseous or liquid, are at the same 
temperature. FIG. 6 illustrates data taken from an experiment where the 
fluids respectively comprise a gas, specifically nitrogen 74, and a 
liquid, specifically a molten salt 76 having a gas-liquid interface 78. 
FIG. 7 shows data comprising three tests in a liquid-liquid environment 
comprising a molten salt 80 and molten aluminum 82. A surface 84 is 
between the molten salt and a gas, and a molten liquid-liquid interface 86 
is between melts 80 and 82. The three tests are represented by the three 
sets of points forming three curves. The vertical axes in the graphical 
representations for the FIG. 6 and the FIG. 7 tests represent the depth of 
the thermocouple junction in the gas or below the surface of the molten 
materials. In FIG. 7, the precise location of the aluminum-salt interface 
was 12 millimeters below the surface of the melt, as indicated by line 86 
of FIG. 7. The horizontal axes represent the equilibrium temperature of 
the thermocouple junction. 
In the experiments, particularly with respect to FIG. 7, the depths of the 
salt and the aluminum layers used were not great and, therefore, the 
sensing device was not completely submerged in either melt at any time 
during the experiment, and also a portion extended into the inert gas 
atmosphere above the melts. Accordingly, heat lost through the sides of 
the sensing device changed when the device was lowered deeper into the 
melt. As a result, the temperature plot of FIG. 7 slopes on both sides of 
interface 86, due to the small depths of the aluminum and salt layers, and 
the exposure of a portion of the probe in the inert gas atmosphere. In 
practice, when the device is completely submerged in the molten materials, 
any slope should be eliminated, except for that portion which passes 
through the interface denoted by line 86 and which may not be linearly 
configured, as depicted, but be curved or stepped. 
The data shown in FIGS. 6 and 7, therefore, clearly demonstrate a change in 
the equilibrium temperature when the sensing device passes through the 
fluid-fluid interface. This change in the equilibrium temperature provides 
the information necessary to define the precise location of the 
fluid-fluid interface. 
Referring now to FIG. 8, a modified thermocouple-heater assembly 90 
includes a pair of thermocouples 92 and 94 which point oppositely from one 
another. Thermocouples 92 and 94 are coupled to a differential temperature 
display 96 by electrical leads 98. A single heater coil 100 is placed 
about both thermocouples within a suitably enclosed container 102 and 
suitably mounted therein such as by a ceramic cement. Heater coil 100 is 
energized from a power source 104 through electrical leads 106. This 
embodiment enables sensing to be obtained in a pair of adjacent molten 
liquids, and permits an expected more precise determination of an 
interface between the two liquids. 
FIG. 9 shows still a further embodiment of an arrangement 110 comprising a 
pair of temperature sensor-heater assemblies 112 and 114 having tips 116 
and 118 which extend downwardly in their furnace, such as in furnace 10 of 
FIG. 1 towards its bottom 37. Heaters 120 and 122, as in the prior 
embodiments, are positioned about the tips. Suitable temperature sensors, 
such as thermocouple junctions, are positioned at respective tips 116 and 
118. Assemblies are secured together in any suitable manner. Depending 
upon the stage at which the process is being conducted, tips 116 and 118 
are positioned within refining zone 12 in molten matter 29, or are 
disposed about interface 28 after liquid aluminum has been drawn from 
furnace 10. Assemblies 112 and 114 terminate at levels which are 
respectively generally level with tap hole 26 and where tap hole 25 would 
have been, if retained, so that tip 116 is at a level which is slightly 
higher than that of tip 118. Connections for the temperature sensors at 
the tips and for heaters 120 and 122 extend upwardly to and from the top 
of furnace 10 where they terminate respectively in a differential 
temperature display 124 and a power supply 126. 
Although the invention has been described with respect to particular 
embodiments thereof, it should be realized that various changes and 
modifications may be made therein without departing from the scope of the 
invention.