Abstract:
A method and apparatus for accurately measuring superheat, bath ratio and alumina concentration in an aluminum smelting bath. In one embodiment, a reusable probe determines the bath temperature and bath sample superheat. In other embodiments, the probe also determines bath composition including bath cryolite ratio and alumina concentration.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the testing of molten material, more particularly relates to a method and apparatus which uses differential temperature measurements to determine characteristics such as superheat, alumina concentration and sodium fluoride to aluminum fluoride ratio of an aluminum smelting bath.  
         [0003]     2. Prior Art  
         [0004]     Aluminum is conventionally produced in a smelting operation in an electrolytic cell of the established Hall-Heroult design. In a conventional Hall cell, alumina is electrolytically reduced to aluminum in a molten electrolytic bath generally composed of sodium cryolite (Na 3 AlF 6 ) and aluminum fluoride (AlF 3 ) as well as other additives. Alumina (Al 2 O 3 ) is introduced into the molten electrolyte bath, dissolves and reacts to form carbon dioxide and aluminum that accumulates as molten aluminum pad. Control parameters monitored during an aluminum smelting operation include the temperature of the bath and the composition of the molten electrolytic bath. Typically, samples of electrolyte are periodically withdrawn from the Hall cell and analyzed for the concentration of alumina and the ratio of the concentration of NaF to the concentration of AlF 3  (termed the bath ratio) in laboratory batch tests. Such laboratory tests are typically completed several hours or days after the sampling occurs with little indication of current process conditions.  
         [0005]     One probe that has been developed to measure the bath temperature and liquidus temperature of an aluminum smelting bath is described in U.S. Pat. No. 5,752,772 and is available from Heraeus Electro-Nite under the commercial designation of Cry-O-Therm. The probe includes a copper cup surrounded by a cardboard tube and a thermocouple extending into the cup. The probe is submerged in the molten bath and a bath temperature reading is taken. A sample of the bath in the cup is removed and allowed to cool. The temperature of the cooling sample is monitored over time. An abrupt change in the slope of the cooling curve for the sample is taken as the liquidus temperature for the bath. The difference between the bath temperature and the liquidus temperature is determined to be the superheat of the bath. The probe has several drawbacks including its limited utility (no ability to measure the bath ratio) and fragility in the Hall cell environment. The temperature probe may be used only once because the copper cup, cardboard tube and thermocouple of the probe are damaged by exposure to the harsh conditions of the smelting bath. In addition, a portion of the molten aluminum pad produced in the Hall cell occasionally rises up into the smelting bath and contacts the temperature probe. Such direct metal contact destroys the probe before temperature readings can even be made. Likewise, when carbon dust accumulates on the surface of the bath, the probe cannot make an accurate temperature measurement.  
         [0006]     An apparatus for measuring the bath ratio as well as the superheat of an aluminum smelting bath is disclosed in U.S. Pat. No. 6,220,748, incorporated herein by reference. The apparatus includes a test sensor that measures the temperature of a sample of the bath and a reference sensor which measures the temperature of a reference material. The reference material does not undergo a phase change whereas the test sensor detects the temperature of the sample of smelting bath as it cools and solidifies. The temperature differential between the reference sensor and the test sensor is monitored and analyzed to determine various characteristics of the bath. The NaF:AlF 3  ratio and Al 2 O 3  concentration in the bath are determined in order to control smelting of aluminum metal. In addition, the bath temperature and liquidus temperature are measured to determine the amount of superheat in the bath. In the apparatus disclosed in the patent, the reference sensor and the test sensor are positioned at spaced apart locations. It has been found that the accuracy and consistency of the temperature measurements of the spaced apart sensors are insufficient for determining the bath composition.  
         [0007]     Accordingly, a need remains for a molten bath testing probe which accurately determines superheat and bath composition in an aluminum smelting bath.  
       SUMMARY OF THE INVENTION  
       [0008]     This need is met by the molten bath testing probe of the present invention and method of its use. One embodiment of the molten bath testing probe includes a singular (one-piece) metal body having pair of integrally formed receptacles that is submersible into a bath of molten material, e.g. electrolyte, to obtain two samples of the molten material. Temperature sensors are received in each of the sample receptacles. The probe includes an analyzer for determining the temperature of the molten material in the sample receptacles when the receptacles are submersed in the bath and the temperature change at which the molten material in the sample receptacles begin to solidify after the body is removed from the bath. The thermocouples may be K-type thermocouples. The body of the probe is formed from steel and may be repeatedly used for testing the molten bath. The analyzer includes means for determining the superheat of the bath.  
         [0009]     Another aspect of the present invention is to provide a method of testing molten bath that includes steps of submersing a metal body having a pair of integrally formed receptacles into a bath of molten material, filling the sample receptacles with the molten material, removing the body with the filled sample receptacles from the bath, measuring a first temperature of the molten material with the temperature sensors, allowing the molten material to cool while measuring the temperature of the cooling molten material, measuring a second temperature of the molten material when the cooling rate of the molten material changes and determining the difference between the first and second temperatures. When the second temperature is measured at the liquidus temperature for the molten bath, the temperature difference is a measurement of the superheat of the bath. The cooled material within the sample receptacles may be reheated and removed so that the body may be reused.  
         [0010]     Another embodiment of the invention includes a molten bath testing probe having a singular body comprising an integrally formed sample receptacle and reference member. The sample receptacle defines a well for submersing into a bath of molten material and holding a sample of molten bath. The reference member comprises a solid reference material. A sample temperature sensor is received in the sample well and a reference temperature sensor contacts the reference material. An analyzer is included for determining differences between the temperature of molten material in the well and the temperature of the reference material. In use, the testing probe is submerged into a molten bath to fill the sample well and is removed from the bath. Upon cooling, the molten material in the sample well undergoes a phase change and solidifies. The reference material undergoes no phase change. The difference between the temperature of the reference material and the molten material is determined while the molten material and the reference material cool. The rate at which the temperature differential changes as a function of the cooling of the molten material is an indication of the operation of the bath. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a plan view of an aluminum smelting bath probe made in accordance with one embodiment of the present invention;  
         [0012]      FIG. 2  is a sectional side view of a portion of the probe shown in  FIG. 1 ;  
         [0013]      FIG. 3  is a perspective view of the probe body shown in  FIG. 1 ;  
         [0014]      FIG. 4  is a schematic of a testing system incorporating the probe shown in  FIG. 1 ;  
         [0015]      FIG. 5  is a schematic temperature profile produced using the system shown in  FIG. 4 ;  
         [0016]      FIG. 6  is a plan view of aluminum smelting bath probe made in accordance with another embodiment of the present invention;  
         [0017]      FIG. 7  is a schematic view of a portion of the smelting probe of  FIG. 6 ;  
         [0018]      FIG. 8  is a perspective view of the probe body shown in  FIG. 6 ;  
         [0019]      FIG. 9  is a schematic temperature profile produced using the probe shown in  FIG. 6 ;  
         [0020]      FIG. 10  is a temperature differential profile for various bath ratios at constant alumina concentration produced by using a probe as shown in  FIG. 6 ; and  
         [0021]      FIG. 11  is a temperature differential profile for various alumina concentrations at constant bath ratio produced by using a probe as shown in  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout. For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom” and derivatives thereof relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting.  
         [0023]     The present invention is described in reference to testing the temperature of a molten smelting bath. However, this is not meant to be limiting as the present invention is applicable to other testing environments.  
         [0024]      FIGS. 1-4  illustrate a molten material testing probe  2  having a singular metal body  4  including a central portion and a pair of sample receptacles  8  flanking the central portion  6 . The central portion  6  defines a connection port  10  which is internally threaded at  12  to receive externally threaded conduit  14 . The sample receptacles  8  each define a sample well  16  for receiving molten bath. Walls  18  of the sample receptacles  8  are shown as cylindrical, but this is not meant to be limiting as other geometric configurations may be used. A passageway  20  is defined in the central portion  6  and walls  18 . One end of a temperature sensor  22  extends through conduit  14  and a passageway  20  and is received in sample well  16 . The temperature sensors  22  are preferably thermocouples such as K-type thermocouples. The type of thermocouple  22  selected is determined by the surrounding environment and the accuracy requirements. While other thermocouples having greater accuracy may be used, such as a Type-S platinum-rhodium thermocouple, it has been found that calibrated Type-K thermocouples provide sufficient accuracy for controlling an aluminum smelting bath. The other end of each temperature sensor  22  is received in a sheath  24  and the sheathed thermocouples  22  extend out of conduit  14  and through tubing  26 . The distal ends of the thermocouples  22  terminate in electrical connectors  28  for connection to an analyzer  34  and optional printer  36 . The tubing may be of any desired length such as about 0.5 to about 10 feet in length for use in testing aluminum smelting baths. Nut  30  is threadable on conduit  14  to tighten and seal conduit  14  within the port  10 . Nut  32  is threadable on ends of each of the conduit  14  and tubing  26  to join them together. The tubing  26  may be threaded directly onto the threads  12  of the central portion  8  thereby eliminating nuts  30  and  32  and conduit  14 . The components of the testing probe  2  are made of materials suitable for use and reuse in an aluminum smelting bath. A particularly suitable material for the probe body  6  is stainless steel, e.g. alloy 304L.  
         [0025]     In use, the end of the testing probe  2  is placed into a bath of molten material such that the sample receptacles  8  are submerged in the bath and the sample wells  16  fill with molten material. The temperature sensors  22  provide a temperature reading of the molten material while the body  6  is in the bath. After a stable bath temperature is noted, the body  6  is removed from the bath with the sample wells  16  filled with molten material. The molten material samples are cooled, e.g. to about 850° C. via ambient air, convection or other means. While the molten material is cooling, the temperature of the material in each of the sample wells  16  is recorded by the analyzer  34 . The analyzer  34  includes software for plotting a temperature profile of the temperature of the material samples over time and for calculating the superheat of the bath.  FIG. 5  is a schematic plot of temperature detected in the sample wells  16  over time. While the testing probe is within the smelting bath, the temperature is generally constant as indicated at TB. When the testing probe is removed from the bath, the temperature begins to fall as the molten material cools. At the liquidus temperature T LIQ , the molten material begins to freeze. At that stage, the cooling rate for the molten material slows for a period of time until the cooling rate again increases. The analyzer  34  detects the point at which the cooling rate slows and calculates the difference between T B  and T LIQ  as the superheat of the bath.  
         [0026]     The probe  2  may be reused by resubmerging the body  4  in the bath until the solidified material in the sample wells  16  melts. For sample wells  16  holding about 2-3 milliliters of material, remelting is accomplished in about three to four minutes. The testing probe  2  is tipped so that the remelted material in the sample wells  16  pours back into the bath. The testing probe  2  is then ready for use in another testing process.  
         [0027]     Another embodiment of the invention is shown in  FIGS. 6-8 . The testing probe  102  shown in  FIGS. 6-8  is similar to the probe  2  shown in  FIGS. 1-4  but differs in the metal body  104  used in place of body  4 . The probe  102  may be used in connection with the analyzer  34  and printer  36  of  FIG. 4 . The metal body  104  includes a central portion  106  flanked by a sample receptacle  108  and a reference member  109 . The central portion  106  defines a connection port  110  that is internally threaded at  112  to receive the externally threaded conduit  14 . The sample receptacle  108  defines a sample well  116 . While each of the sample receptacle  108 , central portion  106  and reference member  109  are shown as having general cylindrical shape, they are formed together as a single structure of the body  104  and the cylindrical shapes are not meant to be limiting. Reference member  109  includes a solid reference material that is integrally formed with the body  104 . In this manner, the reference material is composed of the same material as that which makes up the central portion  106  and the sample receptacle  108 . A passageway  120  is defined in the central portion  106  extending between the connection port  110  and the sample well  116 . A reference bore  121  is defined in the reference member  109  and is open to the connection port  110 . A pair of temperature sensors  22   a  and  22   b  bearing sheaths  24  extend through conduit  14 . One end of sensor  22   a  extends through the passageway  120  and is received in the sample well  116 . In a similar manner, one end of sensor  22   b  extends into the reference bore  121  and abuts the material of the reference member  109 . The other ends of sensors  22   a  and  22   b  extend out through conduit  14  and tubing  26  and terminate in suitable electrical connectors  28 . The components of the testing probe  102  are made of materials suitable for use and reuse in an aluminum smelting bath. A particularly suitable material for the probe body  104  (including the material of the reference member  109 ) is stainless steel, e.g. alloy 304L.  
         [0028]     In use, the end of testing probe  102  is placed in a bath of molten material to submerge the body  104  thereby filling the sample well  116  with molten material and surrounding the reference member  109  by molten material. The temperature sensor  22   a  provides a temperature reading of the molten material while the probe  102  is in the bath. The temperature sensor  22   b  is not exposed to the bath, but instead detects the temperature of the material of the reference member  109 . After a stable bath temperature is detected with the sensors  22   a  and  22   b , the testing probe  102  is removed from the bath with the sample well  116  filled with molten material. The test sensor  22   a  may be used to determine bath temperature and super heat temperature as described above in reference to use of probe  2 . However, probe  102  has additional functionality.  
         [0029]     Referring to  FIG. 9 , the temperature profile for the cooling material in the sample well  116  is schematically represented with changes in slope of the temperature profile during phase changes. The material of the reference member  109  does not undergo a phase change, and the slope of the temperature profile for the cooling reference member is smooth. During the cooling process, the difference in temperature between the sample and the reference material is recorded as delta temperature (ΔT) as a function of temperature and/or time. A schematic of the change in ΔT over time is shown in  FIG. 9 . The thermal arrests indicated by increases and decreases in ΔT are indicative of the formation of different phases as the test sample cools.  
         [0030]     In particular, the magnitude of ΔT occurring between about 400° C. and the T LIQ  is directly correlated to bath ratio (NaF:AlF 3 ) at a constant alumina concentration. The ΔT at about 700 to 900° C. is correlatable to the alumina concentration at a constant bath ratio. The differential temperature profile also shows the liquidus or temperature at which the molten material begins to freeze by means of a first slope change of the differential temperature during cooling. The peaks and valleys of the ΔT occurring over the temperature range allow determination of parameters such as alumina concentration, bath ratio and superheat.  
         [0031]     The following examples illustrate various aspects of the present invention.  
       EXAMPLE 1  
       [0032]     A probe made in accordance with the embodiment of  FIGS. 6-8  was used to test a series of aluminum smelting baths. In each bath the alumina concentration was held constant at about 2.6 wt. %. A different NaF:AlF 3  ratio was set in each run of the bath from 1.0 to 1.3. During the testing procedure, each bath was held at a temperature above the expected liquidus temperature. A sample of the bath was taken for analysis of the NaF:AlF 3  ratio via x-ray diffraction and pyrotitration methods. The amount of alumina was determined by a LECO oxygen analyzer. After adjusting the bath ratio and the alumina concentration to the desired level, the 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 in the sample well. Upon measuring a stable temperature, the probe with filled sample well was removed from the bath and air cooled to at least 400° C. The ΔT profile was recorded as the sample was cooled and is shown versus bath temperature in  FIG. 10  for each of the test runs. As the NaF:AlF 3  ratio increased from a value of 1.0 to a value of 1.3, the liquidus temperature as measured by the temperature differential increased, and the ΔT peak at about 850 to 930° C. increased while the ΔT peak at about 650 to 700° C. decreased. Based on the demonstrated relationships between ΔT changes and bath ratio for the various test samples, the probe of the present invention may be calibrated to determine and display the bath ratio for a particular bath.  
       EXAMPLE 2  
       [0033]     Example 1 was repeated except that the bath ratio was kept constant at 1.13 and the alumina concentration by weight was set in separate runs at 2.6%, 3.2%, 3.7%, 4.7% and 5.7%. A ΔT profile was produced for each run and appears in  FIG. 11 . As shown in  FIG. 11 , as the alumina concentration changes between 2.6% and 5.7%, the position and magnitude of the peaks and valleys in the ΔT profile changes accordingly. The alumina concentration may be correlated with the area under the ΔT profile between temperatures (e.g. 800-900° C.). Accordingly, the relationship between the ΔT profile and alumina concentration may be used to calibrate the probe of the present invention in order to determine the alumina concentration for a particular test sample.  
         [0034]     It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.