Literature Cited Buckiey, P. S., hstrum. Tech., 51 (Aug 1968). Cox, R. K.,Shunta, J. P., Chern. Eng. Prog., 69, 56 (1973). Fertig, H. S., Ross, C. W., paper presented at 22nd Annual iSA Conference, Chicago, Ili., Sept 1967. Khandheria, J., Ph.D. Thesis, Lehigh University, Bethlehem, Pa., 1975.
Sninskey, F. G., “Process Control Systems”, McGraw-Hili. New York, N.Y., 1967. Shunta, J. P., paper presented at Lehigh University, course on Distillation Control, 1974.
Receiued /or reuiew J u n e 11,1975 Accepted October 14,1975
High Voltage Pulsing of a Laboratory Aluminum Electrolysis Cell Constance F. Acton,’ Paul C. Nordlne, and Daniel E. Rosner* Department of Engineeringand Applied Science-Chemical Engineering Section, Yale University, New Haven, Connecticut 06520
A small laboratory aluminum electrolysis cell has been fabricated in an attempt to reproduce experimentally the reported observation that short duration, high voltage pulses applied to a conventional aluminum electrolysis cell can significantly increase the energy efficiency for subsequent aluminum extraction (Diller, 1966) over present practice. First, we investigated current-voltage relationships associated with the electrolysis of fused cryolite-alumina melts in the absence of pulsing. Results obtained were in agreement with those reported by other investigators using similar laboratory cell designs. Then, high voltage pulsing was imposed upon the electrolysis circuit in attempts to achieve the previously reported activation. We observed no effect of kilovolt pulsing on the (post-pulse) conductivity behavior of the cryolite-alumina melt over the parameter range specified herein. Unfortunately, we can presently offer no satisfactory explanation for the differences between these and earlier results (Diller, 1966) based on available documentation.
1. Introduction
1.1. Conventional Aluminum Electrolysis (Hall Process). In traditional aluminum electrolysis for Al(1) extraction from A1203(s),the bauxite ore, impure Al(OHI3, is first refined to pure A1203(s) powder by the Bayer Process. I t is then dissolved in molten cryolite, Na3AlF6, containing various additives a t about 1250 K. When a potential of 5-7 V is applied across the cell aluminum metal is plated out, forming a liquid pool on the cathode t h a t can be tapped off periodically. Some 6-8 kWh/lb of Al(1) are now required to extract aluminum from alumina using this conventional Hall process. 1.2. Electrolysis Following High Voltage Pulsing. Diller (1966, 1969, 1974) has described a method which according t o his experiments, would cut the electrical energy requirement for aluminum production by a factor of about 2. The main idea is to use high voltage pulses to “activate” cryolite. Two different effects of pulsing are described in the 1966 patent. T h e “mobility effect” is reported t o occur when a pulse having a peak voltage of 1 kV in the bath is superimposed codirectionally upon the electrolysis voltage a t Ih-s intervals. The most effective pulse width is reported to be about 1 p s . This relatively low voltage pulse is supposed to enhance the electrical conductivity of the melt. T h e second effect, called the “crystal effect”, is reportedly produced by a 3-5-kV pulse (fired and measured a t the anode) which has a 1-10-ps (or more) pulse width. From 1 to 20 pulses may be fired in a 5-s interval but 6 pulsed5 s is stated to be the optimum firing rate. This effect was named for its presumed ability to cause ionic dissociation and T o whom inquiries concerning this paper should be directed a t Olin Corporation, Metals Research Laboratories, New Haven, Conn. 06540.
“crystal breakdown” in the cryolite melt. A claim of 1003000% increase in current density a t fixed voltage is made by Diller (1969). For the crystal effect to occur, the low (dc) voltage reportedly does not need to be superimposed with the high voltage pulse(s). 1.3. Purpose of the Present Investigation. Following systematic improvements in the energy efficiency for A1 production by the Hall process during the period 19201950, recent gains have been more modest, with the present requirement of 6-8 kWh/lb Al(1) being far greater than the thermochemical minimum values discussed in Section 2. Since increased costs of electrical energy and raw materials are now stimulating activity on alternative techniques of aluminum production (see, e.g., Peacey and Davenport, 1974), it is interesting to inquire if the above-mentioned scheme of high-voltage electrolyte pulsing is capable of “rescuing” the Hall Process from its competitors. While one can set useful model-free thermochemicar bounds on the electrical energy requirements for A1 production via electrolysis (see Section 2 ) , many important details of conventional aluminum electrolysis (e.g., melt constitution, mechanisms of ion transport, and anode polarization, etc.) remain incompletely understood. Since the possible effects of high voltage electrolyte pulsing are even less transparent, and the experiments on which the above-mentioned patent claims are based have to our knowledge never been independently reproduced in another laboratory, we decided t h a t such independent experiments would be both a necessary and timely prelude to any further developments along these lines.
2. Thermodynamic Considerations The laws of thermodynamics set firm limits on the minimum energy required to obtain Al(1) from A120&), regardInd. Eng. Chem., Process Des. Dev., Voi. 15, No. 2 , 1976
285
less of the detailed mechanism of the reduction. I t is useful to examine these limits with reference to what is claimed to be achievable via pulse activation. Consider a “black box” control volume which envelops the Al203-electrolysis (e.g., Hall-cell) pot. The net effect of electrolysis is the consumption of A1203(s),carbon (anode), and electrical energy and the production of Al(1) and oxygen-containing gases [COz(g),CO(g), O*(g)].The minimum electrical energy required to produce a unit mass of Al(1) can be readily calculated from the first law of thermodynamics, subject to the following assumptions: (i) steadystate operation (i.e., continuous addition of reagents, continuous removal of A N ) , and constant bath composition); (ii) no supplementary energy addition; (iii) no energy loss from the system (other than those associated with reaction product removal); (iv) reaction products removed are a t temperature of bath; reactants added [A1203, C(s)] are a t 298 K. Of course, less electrical power can be used to produce aluminum provided the remaining required energy is added as heat. However, the second law of thermodynamics sets a limit on this type of nonadiabatic operation, since the net entropy outflow from the system must exceed the ratio of the heat added to the prevailing bath temperature. One may obtain a precise limit on the maximum amount of energy t h a t may be added in the form of heat. I t is equal to the energy necessary to heat the reactants A1203(s) and C(s) to the bath temperature plus the product of the bath temperature and the entropy of the net reaction a t the bath temperature. The minimum specific electrical energy for nonadiabatic operation is therefore AHT - TAST = AGT, the change in Gibbs free energy for the net reaction at the bath temperature. Using JANAF thermochemical data (Stull et al., 1966), the first- and second-law values of electrical energy consumption and the corresponding cell emf‘s were computed, and they are given in Table I. Since C02 has been observed to be the dominant gaseous product, it is seen that a t least 2.8 kWh/lb of Al(1) of electrical energy must be supplied to the electrolysis cell for present industrial Hall cells. As little as 1.43 kWh/lb of Al(1) might be necessary in experiments of the type reported by Diller (1966), Le., those conducted inside a furnace. The second-law absolute lower limit on electrical energy would be 1.43 kWh/lb of Al(1) (cf. Table I) corresponding to an absolute minimum cell emf of 1.06 V. Therefore, although one previously reported value (1.8 V, Diller, 1966) for pulse activated electrolysis is less than the above-mentioned first-law limit for adiabatic operation, it does not violate the second-law limit, applicable to furnace experiments. 3. Experimental Technique 3.1 Electrolysis Cell. The electrochemical cell, patterned after that of Canmarota and Schlain (1970), was contained in a SS-304 retort closed a t the top and bottom by stainless steel flanges with a Viton O-ring seal. A number of fittings were tapped into the flanges to accommodate the electrode bus tubes, gas lines, thermocouples, potential rods, and support rods. A schematic of the entire cell assembly and furnace is shown in Figure 1. The top half of the electrochemical cell consisted of an anode bus tube to which was connected the anode, anode sleeve/baffle, and cell cover. Four highly polished inconel or stainless steel baffles were provided a t regular intervals between the cell cover and top flange to reduce radiative heat transfer from the center zone to the top flange and Oring seal. Lava (American Lava Corp.) spacers were used to separate the baffles from each other and the top flange. 286
Ind. Eng. Chem., Process Des. Dew, Vol. 15, No. 2, 1976
Table I. First- and Second-Law Energy Requirements and Cell Voltages for A1,0, Electrolysis at 1250 K First law
Second law ___
Anode gas
_ _ _ ~ Electrical energy required, k W h / lb of Al(1) Cell voltage, V
Anode gas
co,
co
0,
co,
co
0,
2.84
3.49
4.16
1.60
1.43
2.99
2.10
2.58
3.08
1.18
1.06
2.21
Lava sleeves were used to electrically insulate the high voltage-carrying (+) anode bus tube from the other (grounded) steel parts of the cell. The high voltage connection and the positive lead from the dc power supply were made to the anode bus tube with a steel braid. A chromel wire insulated in an alumina tube (anode potential rod) was housed inside the anode bus tube to allow measurement of the net cell emf a t the anode with no current passage. The cell cover was connected to two 0.635-cm stainless steel tubes which allowed it to be raised or lowered independently of the anode. Thus, gas flow into the cell could be regulated. Similarly, the anode could be easily raised or lowered into the melt (and thus, the interelectrode distance varied) by means of a vacuum coupling in the top flange. Argon or nitrogen passed out of the cell along with the offgases produced during electrolysis through an exit line in the top flange. The lower half of the cell consisted of a machined graphite crucible with a boron nitride liner. The graphite bottom of the crucible served as the cathode during electrolysis. The crucible assembly was supported by four 0.635-cm threaded stainless steel rods to which four heat shields were attached a t regular intervals as above. Two wells were tapped into the base of the crucible to house the control and measuring thermocouples. One well was additionally used to hold a chromel wire insulated in an alumina sheath (cathode potential rod). The negative lead from the dc power supply was connected to the retort by a connection on the top flange and electrically contacted the cathode primarily via the threaded support rods and secondarily via the two 0.952-cm tubes which housed the thermocouples. The retort was grounded to a water pipe. Dried argon or nitrogen entered the cell through a port in the bottom flange. Gases were passed through a column of anhydrous Cas04 (Drierite Co.) prior to entry into the retort a t a fixed flowrate that was monitored with a calibrated rotameter. The O-ring seals and flanges were cooled using water-filled copper tubing brazed to the top and bottom ends of the retort. 3.2. Equipment. Current readings were made by measuring the voltage drop across a l A/mV precision shunt on a 20-mV digital voltmeter or on a mu,ltiscale L & N strip chart recorder. The emf across the cell was measured using the anode and czthode potential rods; Le., the classical four-terminal method was used. A constant voltage/constant current power supply (Kepco Model JQE, 0-50 A, 0-20 V) was used for the electrolysis and a variable output 0-35-kV rectifier (Westinghouse Model RA-68-A) was used to supply the high voltage required for pulsing. Pulsing was accomplished with a spark gap (EG & G Model G P 31B) fired manually with a trigger module (EG & G Model T M 11A). The circuit diagram used for pulsing is shown in Figure 2. A blocking diode circuit was used in some cases to investigate the effect of pulsing superimposed upon the electrolysis circuit. The high temperature furnace used was a two winding, 17.78-cm i.d. vertically mounted resistance furnace. I t was constructed using 1437 K kanthal ribbon heating elements
COVER ADJUSTMENT ASSEMBLY
S S TUBE
ANODE THERMOCWPLE AND ANODE POTENTIAL
ROO TO MEASURING CIRCUIT VAC UUM REDUCING UNION -HIGH VOLTAGE CONNECTOR FROM SPARK GAP ( + ) GAS OUTLET VACUUM
COUPLING
DC TERMINAL(-)
-
FROM POWER
SUPPLY
BINDING P O S T WATER
OUTLET -LAVA
FURNACE T U B E SUPPORT
INSULATING F I T T I N G
ARM,
- WATER I N L E T
B N INSULATOR B N SPACER INCONEL HEAT B A F F L E RESISTANCE F U R N A C E K A N T H A L HEATING E L E M E N T S FIEERFRAX
INSULATION
B N COVER EN SLEEVE /BAFFLE CRYOLITE
MELT
GRAPHITE ANODE BN LINER -GRAPHITE
CATHODE
GRAPHITE
CRUCIBLE
REFRACTORY
-SS
BRICK
304 FURNACE
TUB
SS SUPPORT ROD
COPPER COIL W A T E h PIPE GROUND JACIIUM
-
COUPLING GAS INLET
dCIlIUM
kt DUCING UNION
WATER I N L E T O-RING -CONTROL
THERMOCOUPLE
TO C O N l H D L L t e
MEASURING THERMOCOUPLE AND CAT H O D t P O T E N T I A L HOD T O MEASllHlNG i ' l t ? ~I 1 1
Figure 1. Furnace and cell assembly (schematic).
(Lindberg). T h e top and bottom quartered sections were controlled by a 20-A Variac and the center zone was controlled by a n on-off type controller (Honeywell Pyrovane). Temperature measurements were made with chromelalumel (Omega Co., Type K ) thermocouples swaged in aluminum or magnesium oxide in stainless steel protection tubes and with ungrounded, unexposed or exposed junction tips. Temperature fluctuations in t h e working area of the center zone were of the order of -+5 K. 3.3. Materials. T h e cathode crucible and the anode were machined from AUC grade (Union Carbide) graphite stock. Hot-pressed grade A (Carborundum Co.) boron nitride was machined into the cathode liner, anode sleeve/baffle and cell cover. Type 304 stainless steel was used for all of the steel parts with the exception of some of the heat shields which were fabricated from 0.635-mm inconel sheet. (Inco-
ne1 was found to be more corrosion resistant than SS 304 for the present application.) Reagent grade CaF2, natural, sized, -60 mesh Greenland cryolite (Chemalloy) and 3-3370p grade A1203 (donated by Reynolds Metals Co.) were used. T h e background gas used was either argon (99.998%, Airco), nitrogen (99.997%, Airco), or prepurified nitrogen (99.998%, Matheson). 3.4. Experimental Results. The first experiment was a conventional electrolysis run designed t o evaluate typical parameters without pulsing. T h e dc voltage supply was used in the constant current mode and the resulting emf was measured. A four-terminal technique was employed by applying current across the electrode bus tubes and measuring the net cell voltage across the electrode potential rods, i.e., eliminating lead and contact resistances. T h e initial cryolite salt mixture consisted of 7.0 wt % Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 2, 1976
287
'
TRIGGER GENERATOR SPARK CHARGING
I - 5 K V DC POWER SUPPLY
KNIFE
11
ANODE THERMOCOUPLE
TIT1I 1 1
o
R C NETWORK
0
CURRENT VIEWINO
RESISTOR-\
LOW VOLTAGE D C POWER SUPPLY WATER PIPE GROUND CATHODE THERMOCOUPLE
Figure 2. Pulsing circuit diagram.
CaF2, 7.0 w t % A1203, and 86.0 wt % Na3AlF6. The anode was 4.13 cm in diameter and 0.86 cm thick. For this experiment prepurified nitrogen was used as the background gas with a flowrate equivalent to 155 cm3/min (STP) of argon. The data from this experiment are given in Table I1 and are shown graphically in Figure 3. The observed dc resistance of the cell, as determined from the slope of the graph, is 0.05 Q , consistent with the nominal electrolyte resistivity. The first attempt a t pulsing was made in the second experiment (cf. Table 11).A 2-kV pulse was fired after an applied current of 12 A had produced a cell voltage of 2.25 V. The dc electrolysis circuit was disconnected during the pulsing. The constant 12-A current was reapplied 5 min after the pulsing, a t which time the cell emf was still holding a t its original value of 2.25 V. The salts and anode used for the first pulsing experiment were those retained from the previous experiment; Le., they were preelectrolyzed. The third experiment (cf. Table 11) involved a more sophisticated pulsing program. Again the salts were retained from the previous experiment. Single pulses of 1, 2, and 3 kV were used with the dc electrolysis circuit off. The time required to reconnect the constant current electrolysis circuit was generally of the order of 30-60 sec after discharge: A multimeter (Simpson) was connected across the capacitor bank to indicate when the capacitors had been fully discharged such that the dc circuit could be safely reconnected. No change in the net cell emf was seen after any single pulse. A series of twenty pulses ranging from 2 to 3 kV a t 50 pF, fired in the space of 1 min, also did not affect the cell emf. The fourth experiment (cf. Table 11) was done using the previously electrolyzed salt mixture from experiment 3. In this experiment 5 or 6 pulses of 2-3 kV were fired within 7-10 s a t a number of current densities. No important change in the cell emf after any of the pulsing sequences was noted. At 45 A the anode effect was noted such that new salts were required to conduct the next experiment. The fifth experiment (cf. Table 11) was run with a newly machined anode of 4.15 cm diameter and 1.02 cm thickness. Unlike all of the previous experiments, regular grade 288
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 2 , 1976
(not prepurified) nitrogen was used. The flowrate was equivalent to 200 cm3 (STP) of argon/min. New salts were added to the pot to bring the composition to about 10.0 w t % A1203, 9.0 wt 96 CaF2, and 81.0 wt % cryolite. Six pulses of 3.1 kV fired from a 50-pF capacitor bank in about 1 min and 20 pulses of 2.5 kV fired in less than 1 min had no appreciable effect on the normal electrical parameters, even after many minutes. For this reason complete inventories on aluminum were not made; however, aluminum deposition was noted upon breaking open the used cell. The sixth experiment (cf. Table 11) was performed using a 100-pF capacitor bank and a blocking diode circuit (cf. Figure 4) to permit superposition of the dc electrolysis current during pulsing. This additional circuitry was designed to allow determination of whether a short-lived activation effect could be seen which might otherwise have been missed. New salts of about 15 wt % alumina were used. T h e interelectrode distance used was 2.46 cm; however, in this case a new graphite crucible was used without a BN liner. A single pulse of 3 kV fired a t 100 pF showed an apparent drop from 2.2 to 2.0 V a t 20 A which lasted for about 7 s; however, additional pulses of 3.4 and 3.65 kV with the same current density showed no similar effect. An oscilloscope photograph of the 4-kV current pulse is compared to a reportedly successful voltage pulse waveform (Diller, 1966) in Figure 4. (It is estimated (by graphically evaluating the integral of 1Yt)dt in Figure 4b) thak the net energy actually applied to the cell is about 60% of the gross energy calculated as (Yz)CV2.)
4. Discussion and Conclusions 4.1. Discussion of Experimental Results. In view of the absence of significant observed effects of high voltage pulsing on aluminum electrolysis in the present investigation it is necessary to ask whether some feature of the experimental configuration or the range of experimental parameters investigated violated conditions spelled out in the above-mentioned patents. However, as shown in Table 111, apart from rise time (Our pulses were equivalent to Diller's in voltage, energy, percent reversal, duration, and frequency, but due to the unusual shape of his pulses (cf. Figure 4a) our rise times were a t least a factor of 2 higher than his.), the present experimental conditions were definitely within the range of parameters which, according to the patents, should have produced "activation" effects. Regarding pulse parameters, our pulse widths and, importantly, power dissipated are within those specified. The conditions explored in the present investigation were deliberately oriented toward the largest effects reported in the patent literature. These include an initial melt of about 7-15 wt % A1203 and 7-10 wt % CaF2, with an anode area of about 25 cm2 and cathode area of about 20 cm2. Interelectrode distance varied from 1.9 to 3.0 cm. Cell voltages from 1.8 to 5.3 V were monitored along with anode current densities from 0.15 to about 1.84 A/cm2. Various pulses from 1 to 5 kV were fired from a capacitor bank of either 50 or 100 pF. This corresponds to gross energy levels of 50 to 400 J. Pulsing frequency was also varied; single pulses, 5-6 pulses/7-10 s, as well as about 20 pulses in less than 1 min were all tried. The pulses were fired either by themselves or superimposed upon the regular dc electrolysis voltage. Pulsing was done at many different current (density) levels. The mean cell temperature was varied between about 1240 and 1290 K. (It should be noted that it was not within the scope of this investigation to correlate all of the pertinent parameters associated with normal electrolysis, e.g., current density and/or cell emf t o tempera-
Table 11. Experimental Data Expt. no. 1a
Current, A 4 8 12 14 16
Cell emf, V
20 4 12
1.82 2.02 2.25 2.33 2.42 2.51 2.62 1.72 i 0.020 2.25 2 0.025
12
2.25
18
Pulse voltage, kV
Time after pulse, min
2.0 8
f
0.025
2.13
2.70
12 40
2.55 i 0.05 5.00
40
5.25
40
5.30
20
2.15 2.40 2.63 5/10 s
17
6 / < 1 min 26
2.5 2.20
i
2 0 / < 1 min
1
0.04 3.0
20 20 20 20
2.25 F 0.04 2.0 2.28 i 0.01 2.28
20
2.25
20 40
2.25 2.85 * 0.12
40 58
2.85 F 0.12 3.6
Single I s 12s
19 3 5 3.4 3.65
2 3
4.0b
...
1286, Cathode
6/10 s
3.1
a
2.5
Single 5
2.0
20
1280, Cathode
2011 min 17
2.0
12
2.9
Single 4
2.0
5.15
1245, Anode
Single
2.0-3.0
45 4 8 12
3.0
Single
3.0
5.00
1249, Anqde
4
1.90
45
2.4 Single
3.0
4
1268, Anode
20
2.23 1.85
1.83
1.9
Single
3.0 2.20
4
1284, Cathode
3
2.20
1.88
2.4
19
2.15
4
Temp, K
Single
2.0 8
Approx. electrode spacing, cm
5
1.0 8
Pulse program
. . .C
Single Single Single
8 5.0
Single
Capacitance experiments 1-5, 50 p F ; experiment 6, 1 0 0 pF. b Cf. Figure 4. C Burned out supply.
ture, interelectrode distance, melt composition, cell geometry, and so forth. After establishing the cell emf a t fixed applied current (and thus, current density), the purpose of this investigation was to simply search for a lowering in cell emf after pulsing.) 4.2 Conclusions. These feasibility experiments have explored a set of experimental conditions under which high voltage pulsing evidently has no significant effect on the normal currentholtage relationship. Yet our experimental
conditions closely approximate or duplicate those stated t o be necessary and sufficient to produce “activation” (Diller, 1966). Thus, it would appear t h a t certain details, not obvious in the references cited, are crucial to the success of “activation”. Alternative explanations cannot be pursued on the basis of existing documentation. Acknowledgments This work was supported in full by Yale University Ind. Eng. Chern., Process Des. Dev., Vol. 15, No. 2 , 1976
289
Somparison of Experimental Conditions for Evaluation of High Voltage Pulse ‘‘Activation” Parameter current, A I ) electrode area, cm’ de lode it density, A/cm‘ de lode Itage, V Total aoolied
This investigation 4-45
4.4 (
24.46-26.83 20.26
1.18 cm2 (Dl), 8.37 cm’ ( D , )
0.15-1.84 0.20-2.22
0.93, 1.86 (D,)
...
5.1 (D2), 5.9 (I
2.05-7.30
...
1233,1273 (D., >2’/2AI,0,, 8-14AI,O, ( D , ) 4 0 A 4 0 , (D,L 9AL0, ( D , ) >1.5A1 (DL),1.7 A1 (D,) 1, 2.54 ( D , ) 35,700 ( D , ) 30 ( D l ) . 600 ( D , ) 11.43 cm W X 11.43 cm H x 24.13 cm L i.D .. ,) 1-5 PI) 1-8 MF/cm’ anode area ( D , ) 2 x 10’ w/kg ( X 1 ps = 2 X lo-’ kW/kg) (D,)
IS
11c
