Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR TESTING MATERIAL UTILIZING
DIFFERENTIAL TEMPERATURE MEASUREMENTS
The present invention relates to testing of materials, and more
particularly relates to a method and apparatus which utilize differential
temperature
measurements to determine characteristics of a material, such as the alumina
concentration and sodium fluoride to aluminum fluoride ratio of an aluminum
smelting bath.
Aluminum is conventionally produced by smelting in a Hall bath.
During the smelting operation, it is desirable to control parameters such as
the
temperature of the bath and to determine the composition of the molten
aluminum
in the bath. However, the methods that are currently used to measure the
alumina
concentration and sodium fluoride to aluminum fluoride ratio (NaF:AIF3) of the
bath
are confined to laboratory batch tests. Such laboratory testing causes control
measurements to be made several hours after sampling, with little indication
of
current process conditions.
A probe that measures the bath and liquidus temperatures of a Hall
bath during processing has been developed by Heraeus Electro-Nite, and is
commercially available under the designation Cry-O-Therm. The probe includes a
single thermocouple that is submerged in the molten bath. The Cry-O-Therm
vibrates the sample during cooling to cause nucleation of the bath during
cooling.
An abrupt change in the slope of the cooling curve while the sample is being
vibrated is taken as the liquidus. The difference between the pot temperature
and
the liquidus is taken as the super heat of the bath. However, this type of
probe
does not measure the NaF:AIF3 ratio, which is critical for accurately
determining the
liquidus and alumina concentration.
The present invention has been developed in view of the foregoing
and to address other deficiencies of the prior art.
A method and apparatus for timely and accurate measurement of
material parameters are provided. A test sensor measures the temperature of a
sample of material as it is heated up and/or cooled down. A reference sensor
is
used to obtain differential temperature measurements as the temperature of the
test
sample is varied. A differential temperature trace is generated and analyzed
in
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order to determine various characteristics of the material being tested. In
one
embodiment, the sodium fluoride to aluminum fluoride ratio (NaF:AIF3) and
alumina concentration in a Hall bath aluminum smelting operation are
determined in
order to efficiently control smelting of aluminum metal. In this embodiment,
bath
temperature and liquidus temperature may be measured and compared in order to
determine the amount of superheat of the bath and to prevent the operation of
smelters at higher temperatures than necessary.
An aspect of the present invention is to provide a method of testing a
molten material. The method includes the steps of contacting the molten
material
with a reference sensor and a test sensor, removing the reference and test
sensors
from the molten material, measuring a temperature difference between the
reference
and test sensors as the sensors cool down, and determining at least one
characteristic
of the material based on the differential temperature measurement.
Another aspect of the present invention is to provide a material
testing probe comprising a support member, a reference material carried by the
support member, a reference material temperature sensor in communication with
the
reference material, a test sample container supported by the support member, a
test
sample temperature sensor carried by the support member and an analyzer which
determines the temperature difference between the reference material and the
test
material.
These and other aspects of the present invention will be more
apparent from the following description.
Fig. 1 is a partially schematic plan view of an aluminum smelting
probe in accordance with an embodiment of the present invention.
Fig. 2 is a partially schematic sectional side view of a portion of an
aluminum smelting probe in accordance with an embodiment of the present
invention.
Figs. 3-8 are graphs of differential temperature signals versus time for
aluminum smelting baths having different NaF:AIF3 ratios, showing increases in
liquidus temperatures for test samples having higher NaF:AIF3 ratios.
Figs. 9-11 are graphs of differential temperature signals versus time
for aluminum smelting baths having different alumina concentrations, showing
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differences in the integrated area under a portion of each curve for test
samples
having different alumina concentrations.
The system of the present invention utilizes differential temperature
measurements to determine characteristics such as the composition of a
material.
For example, during aluminum smelting operations, the ratio of certain
constituents
in the bath, e.g., NaF:AIF3 ratio, and the alumina (A12~3) concentration may
be
determined. While the testing of molten aluminum is primarily discussed
herein, it
is to be understood that the testing of other liquidus and/or solid materials
is also
contemplated by the present invention.
Fig. 1 illustrates a test probe 10 in accordance with an embodiment
of the present invention. The probe 10 includes a reference sensor 20 and a
test
sensor 30 connected to one end of a conduit 38. An analyzer 40 is positioned
at the
opposite end of the conduit 38, and is electrically connected to the reference
sensor
and test sensor 30. In the embodiment shown in Fig. 1, the analyzer 40
includes
15 a display 42 which shows information obtained from the reference sensor 20
and
test sensor 30. A handle 44 is connected to the conduit 38 in order to
facilitate
placement of the reference sensor 20 and test sensor 30 in the desired testing
location.
Fig. 2 illustrates a portion of a probe in accordance with an
20 embodiment of the present invention. The reference sensor 20 which is
located at
one end of the probe, includes a housing 21 and a cap 22 defining a generally
cylindrical chamber. A reference material 23 is provided inside the chamber
formed
by the housing 21 and cap 22. The reference material 23 may by any suitable
material from which a temperature reading may be obtained and compared with a
temperature reading from a test sample, as more fully described below. The
reference material 23 is preferably provided in the form of a solid material
in the
temperature ranges to be tested. However, liquid and/or gaseous reference
materials
may also be used. The reference material 23 preferably does not undergo a
phase
transformation in the testing temperature range in order to provide a
substantially
constant baseline temperature readings throughout the testing procedure.
Suitable
reference materials include metals, ceramics, calcined alumina and
refractories. For
example, the reference material 23 may comprise a metal such as 1199 aluminum,
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stainless steel or refractory metals. Furthermore, the reference material 23
may
comprise a ceramic such as metal grade alumina (MGA). For testing aluminum
smelting baths, metal grade alumina is particularly preferred.
As shown in Fig. 2, a reference thermocouple 24 extends into the
housing 21 in contact with the reference material 23. A refractory fitting 25
extends through a wall of the housing 21 and surromlds an electrical lead 27
of the
reference thermocouple 24. The refractory fitting 25 extends through a
connecting
collar 26 which is fastened to a hollow support member 28.
In the embodiment shown in Fig. 2, the lest sensor 30 is generally
cup-shaped and includes a housing 31 defining a substantially conical interior
test
chamber 32. A test thermocouple 33 extends into the test chamber 32. The test
thermocouple 33 is secured to the housing 31 by means of cement 34 and a
refractory fitting 35. The refractory fitting 35 extends through a connecting
collar
36 mounted on the hollow support member 28. A test thermocouple lead 37
extends through the cement 34 and refractory fitting 35 into the interior of
the
support member 28. The reference thermocouple lead 27 and the test
thermocouple
lead 37 extend from the interior of the support member 28 into the conduit 38
and
are preferably connected to an analyzer 40, as shown in Fig. 1. The conduit 38
may be any suitable length. For example, where the probe 10 is used for
testing
aluminum smelting baths, the conduit 38 may be preferably from about 0.5 to
about
10 feet in length.
The various components of the probe are made of any suitable
materials. For example, the housings 21 and 31, cap 22, connecting collars 26
and
36, support member 28 and conduit 38 may be made of metal such as stainless
steel, inconel, monel or aluminum. The refractory fittings 25 and 35 may be
made
of fiberfrax rope, or the lilce. The cement 34 may be, for example, a mixture
of
graphite, solvent and binder sold under the designation C34 that has been
cured to a
temperature of about 150°C.
In accordance with an embodiment of the present invention, the probe
sensors 20 and 30 may be submerged into a molten bath with the configuration
shown in Fig. 2 in which the test sensor 30 is an open cup being open to
collect a
sample and measure the temperature of the molten bath. The reference sensor 20
is
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closed and contains smelting grade alumina 23, or another thermally inert
material,
which acts as a stable reference material. After a stable pot temperature is
noted,
the probe sensors 20 and 30 are removed from the bath with the open cup filled
with molten metal. The test and reference samples are then cooled, e.g., to
250°C,
via ambient air, convection or other means. Convection cooling is particularly
preferred for many testing operations in order to expedite the test process.
During
the cooling process, the difference in temperature between the test sample and
the
reference sample is recorded as delta temperature (~T), for example, using a
meter
comprising a conventional voltage amplification board and data logger. The
profile
obtained from this configuration is a plot- of microvolts versus temperature
and/or
time. The thermal arrests indicated by peaks and valleys are indicative of the
formation of different phases forming as the sample cools, as more fully
described
below.
In accordance with an embodiment of the present invention, it has
been observed that in an aluminum smelting bath sample, the magnitude of the
differential temperature transition at approximately 400 to 500°C is
directly
correlated to NaF:AIF3 ratio, with constant alumina concentration.
Furthermore,
with varying alumina concentrations, the ratio of NaF:AIF3 in cryolite may be
correlated to another differential temperature peak, between about 600 and
800°C,
in the cooling profile (transition/thermal arrest). Also, the differential
temperature
profile shows the liquidus or temperature at which the bath begins to freeze
by
means of a first minimum point of the differential temperature trace during
cooling.
This first minimum or valley of the differential temperature trace
typically occurs at a temperature greater than 900°C. In addition, the
probe may be
used to measure the temperature of the smelting bath, and the amount of
superheat
of the bath may be determined by subtracting the liquidus temperature from the
bath
temperature. The present system thus allows the calculation of parameters such
as
alumina concentration, NaF:AIF3 ratio and superheat.
The following examples illustrate various aspects of the present
invention, and are not intended to limit the scope thereof.
EXAMPLE 1
A probe similar to that shown in Figs. l and 2 is used to measure a
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series of aluminum smelting baths. In each bath, the alumina concentration is
held
constant at about 5.0 weight percent while the NaF:AIF3 ratio is varied from
1.00 to
1.25 in increments of 0.05. During the testing procedure each bath is held at
a
temperature above the expected liquids temperature. A sample of Hall Cell Bath
was obtained. The sodium Fluoride to Aluminum Fluoride ratio was determined
via
X-ray diffraction and pyrotitration methods. The amount of alumina was
determined via a LECO oxygen analyzer. The ratio of the sample was 1..14 and
the
alumina concentration was 3.97 percent. The bulk of the sample was placed in a
crucible furnace and melted under a nitrogen atmosphere. Sodium fluoride and
or
Aluminum Fluoride was added to the melt to adjust the ratio up or down in the
range of 1.0 to 1.25. Smelting or metal grade alumina was added to as needed
to
adjust the alumina from the 3.97 percent starting point up to 6.0 percent.
After
adjusting the ratio and/or the alumina concentration to the desired level, a
DTA
probe was submerged in the bath. The probe remained submerged in the bath
until
a stable temperature was measured by the probe and a full cup of the bath was
captured. Upon measuring a stable temperature the bath filled cup of the probe
was
removed from the melt and air cooled to at least 400°. The DTA profile
was
recorded as the sample cooled. Differential temperature profiles are shown in
Figs.
3-8 for each of the test runs. In each run, when the reference sample and test
sample are simultaneously removed from the bath, the difference in temperature
between the reference sample and the test sample is plotted as the samples
cool
from the molten bath temperature to a temperature of about 250°C. Each
differential temperature trace includes an initial valley which indicates the
liquids
temperature of the test sample, i.e., the temperature at which the molten
sample
begins to freeze. The following table lists the liquidus temperature fox each
test
sample in comparison with the NaF:AIF3 ratio for each sample.
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TABLE 1
NaF:AIF3 Liquidus Temp. (C)
1.00 918
1.05 926
1.10 939
1.15 950
1.20 957
1.25 961
As can be seen from Table 1 and Figs. 3-8, as the NaF:AIF3 ratio
increases from a value of 1.00 to a value of 1.25, the liquidus temperature,
as
measured by the differential temperature valley, increases from 918 to 961
°C.
Based upon this demonstrated relationship between liquidus temperature and
NaF:AlF3 ratio for the various test samples, the probe of the present
invention may
be calibrated to determine and display the NaF:AIF3 ratio for a particular
test
sample.
EXAMPLE 2
A probe similar to that shown in Figs. 1 and 2 is used to measure a
series of aluminum smelting baths. In each bath, the NaF:AIF3 ratio is held
constant
at a value of 1.10, while the alumina concentration is varied from 3.97 to 6.0
weight percent. During the testing procedure, each bath is held at a
substantially
constant temperature of 971 ~ 5°C. Differential temperature profiles
are shown in
Figs. 9-11 for each of the test runs. As shown in Figs. 9-11, after each
differential
temperature trace goes through an initial valley, the differential temperature
signal
becomes positive and yields a trace having several peaks as the sample cools
down.
For example, in each trace, a differential temperature peak occurs within the
temperature range of from about 600 to about 700°C. Another temperature
peak
occurs in each trace between about 400 and 500°C. In accordance with
the present
invention, such differential temperature peaks can be used to determine the
alumina
concentration of each test sample. For example, as shown by the cross-hatched
regions of Figs. 9-11, the differential temperature trace may be integrated
between
400 and 500°C to provide a value indicative of the alumina
concentration of each
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sample. In Fig. 9, an alumina concentration of 3.97 weight percent yields an
integrated value of -5264. In Fig. 10, an alumina concentration of 5.0 weight
percent yields an integrated value of -4800. In Fig. 11, an alumina
concentration of
6.0 weight percent yields an integrated value of -4231. It can therefore be
seen that
as the alumina concentration increases, the integrated differential
temperature curve
between 400 and 500°C also increases. In accordance with a preferred
embodiment
of the present invention, this correlation may be used to calibrate the test
probe in
order to directly analyze and display the alumina concentration of a test
sample.
Other regions of the differential temperature traces may also be analyzed in
accordance with the present invention. For example, the differential
temperature
peak appearing between 600 and 700°C in Figs. 9-11 may be integrated or
other-
wise analyzed to determine alumina concentration or the like. Also the overall
appearance of the pro files can be used to tack Hall Cell Chemistry during
production.
In addition to measuring the NaF:AIF3 ratio and alumina
concentration in aluminum teat samples, the probe of the present invention may
also
be used to measure other parameters. For example, the probe may measure the
pot
temperature of the aluminum smelting bath while the probe is submersed
therein.
Thus, the reference sensor 20 and/or test sensor 30 shown in Figs. 1 and 2 may
be
used to directly measure the pot temperature. Furthermore, the probe may be
used
to determine the superheat or over temperature of the bath by determining the
liquidus temperature of a test sample and then subtracting that value from the
measured pot temperature.
The present invention thus provides the ability to measure parameters
such as pot temperature, bath ratio and alumina concentration on a real-time
basis
for improved process control, thereby resulting in increased efficiency of
aluminum
production. Also, the present system lessens the need for laboratory
measurements
for alumina and NaF:AIF3 ratio in samples taken from the process.
The method and apparatus of the present invention may be used to
test other types of materials. For example, other types of molten metals may
be
analyzed. Furthermore, the system of the present invention may be used to test
solid materials such as metals undergoing heat treatment or refractory
materials
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undergoing curing processes. As a particular example, phase transitions may be
moiutored in aluminum alloys during the heat treatment thereof. In this
embodiment, the test sensor contacts the aluminum alloy during the heat
treatment
process. The reference sensor preferably includes a reference material that
does not
undergo a phase transformation in the measured heat treatment temperature
range.
An example of a suitable reference material is 1199 aluminum.
In this embodiment, the reference material is preferably sized so as to
simulate an infinitely large heat transfer element. The differential
temperature
measurements may be made as the aluminum alloy heats up during the heat
treatment temperature regime. Typically, as the aluminum alloy heats up, the
differential temperature trace hits a positive peak indicative of
precipitation and then
hits a negative valley indicative of dissolution. Thereafter, the differential
temperature trace rises and levels off, indicating the occurrence of
solubility
equilibrium.
For many aluminum alloys, once solubility equilibrium occurs, as
indicated by the leveling off of the differential temperature trace, the
purpose of the
heat treatment process has effectively been achieved. However, in conventional
heat
treatment processes, it is not possible to accurately determine the exact time
at
which solubility equilibrimn is substantially completed. Accordingly, standard
heat
treatment processing times are often used which are longer than necessary to
achieve the desired heat treatment results. In accordance with this embodiment
of
the present invention, shortened heat treatment processing times are possible
because
the completion of solubility may be accurately determined. Processing costs
may
therefore be substantially reduced by decreasing both time and energy
requirements
for heat treatment operations.
Whereas particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to those
skilled in the
art that numerous variations of the details of the present invention may be
made
without departing from the invention as defined in the appended claims.
