I
R. D. HOLLIDAY and JACK L. HENRY Metals Division Research, Kaiser Aluminum and Chemical Corp., Permanente, Calif.
Anode Polarization and Fluorocarbon Formation in Aluminum Reduction Cells Adequate control of anode-cathode distance prevents concentration polarization and the accompanying fluorocarbon formation
ALUMINUM is produced commercially by electrolytic reduction of alumina in molten cryolite solution. Experimental data on the system are sparse, and composition of the anode gas has not been established under a variety of ceil operating conditions. During normal electrolysis oxide ion concentration in the melt is low, but carbon oxides are the main anode prod: uct. Fluoride ion concentration in the melt is high, but conditions necessary for fluorocarbon formation remain speculative as previous reports of cell gas analyses conflict. Evolution of fluorocarbons during normal operation could significantly affect the materials balance of fluorine and would make it improbable that fluoride ion discharge leads to rapid dewetting of the anode and onset of anode effect. Carbon fluorides were not formed during normal electrolysis of cryolitealumina melts, but carbon tetrafluoride may form when high local anode current densities exist, probably explaining earlier reports of its generation during ostensibly normal operation (7). Such conditions can develop when rigorous control of anodecathode distances is not practicable, as in operating large numbers of multianode cells. Discontinuities in the metal pad result in nonuniform anode consumption and inequalities of current distribution in both Soderberg and multianode cells. Anode current density and surface potential are high at the edges of horizontal-stud Soderberg anodes (9), and reports of carbon tetrafluoride formation during normal operation refer to this type of cell. In normal operation with uniform anode-cathode spacing, several anodes with flat suifaces will attain the condition for carbon tetrafluoride formation. O n one or more surfaces film formation will commence, inducing a
rapid rise in polarization which leads to carbon tetrafluoride formation at other anodes. Film spread is selfaccelerating, and anode effect develops rapidly throughout the cell. Experimental
Fluorocarbon Analysis. Estimation of fluorocarbons by reduction to fluoride ion has been used in all previous studies (7, 7 7 , 74). Reduction is usually incomplete, and removal of fluoride from catalysts and apparatus is extremely difficult (7, 8). Further, identity of the reduced species is not revealed by such analyses. The mass spectrometer is well suited to anode gas analysis. Mass spectra are available for all components, including carbon tetrafluoride and hexafluoroethane when present ( 7 ) , and complete quantitative analysis is possible. In preliminary work, mass spectrometer traces were examined to determine parent masses, mass intervals, peak-free regions, and other clues of gases present. The presence of more complex fluorocarbons was sought, but only carbon tetrafluoride and hexafluoroethane were ever found. Highest sensitivity for estimation of fluorocarbons is achieved by freezing out in liquid air or nitrogen. Complete separation of carbon tetrafluoride and hexafluoroethane is possible as their vapor pressures differ greatly from those of other anode gas components a t low temperatures (72). For the freeze-out technique, the apparatus consisted of a scrubbing train in which acid gases (carbon dioxide, sulfur dioxide, hydrogen sulfide) and carbon sulfides (carbon disulfide, carbon oxysulfide) were absorbed in 50y0 potassium hydroxide solution, a sulfuric acid bubbler and Drierite tube for removal of water vapor and entrained
liquid, and a two-stage precooling and condensing system. Noncondensable gas was removed by evacuating the final condenser, cooled either by liquid air or nitrogen. Pressure measurements were used to compute fluorocarbon concentration, and condensate was identified by accurate gas density measurements. With liquid air coolant, the minimum detectable concentration of carbon tetrafluoride is 0.02% and with liquid nitrogen, 0.003% for typical anode gas. Mass spectrometric analysis of carbon tetrafluoride isolated by freeze-out revealed only insignificant quantities of nitrogen and carbon monoxide. 10,000-Ampere Reduction Cells. Because of disparities between previously reported values for carbon tetrafluoride concentration, attention was focused on control of cell operating variables, particularly current distribution. Earlier work (7, 7 7 , 74) was conducted on commercial 60,000-ampere cells of multianode or horizontal-stud Soderberg anode design, operating in a production line, and cell conditions were not defined. In 10,000-ampere prebake anode experimental cells with individual power supply, anode current, bath temperature, and interelectrode spacing can be maintained with precision. T h e cells were of conventional multianode design with only four anodes instead of the 24 used in 60.000-ampere commercial cells. Anodes and their mounting were identical with those of commercial cells, the only difference being the thicker alumina insulation to compensate for higher relative heat loss. At the center of the cell, gases escaped through a n aperture in the bath crust and were led to atmosphere via a hood, blower, and scrubber, thus ensuring good mixing and complete collection of gas from all anodes. Anode gas samples were led from a steel collrcting chamber VOL. 51, NO. 10
OCTOBER 1959
1289
over the crust aperture into evacuated containers by a 96Yc silica glass sampling tube packed with pelletized sodium fluoride to remove hydrogen fluoride. Polyethylene connections prevented removal of sulfur gases. Power was supplied by reclifiers Ivith Powerstat control, and current was stabilized by saturable rractors in the cell circuit. Cell current remained nearly constant with variation of cell resistance below 6.4 X ohm, but a t anode effect resistance \\-as about 20 times Freater. and voltage drop approached a limiting value. Cell current xias recorded continuously on a 12,000-ampere ammetrr. Current through individual anodes \cas determined from potential drop in a calibrated section of each anode connecting rod: with a maximum error of 2 5 . Potential drop betwern anode bus and cathode collector bars \vas recorded continuously. and frequent ineasurements Icere made a t anodr clamps, between anode rod and anode stub. between anode stub and carbon. and between metal pad and cathode collectors. It was thus possible to determine potential fall bet\veen anode surface and metal cathode pad. 'I'his included bath ohmic drop, decomposition potential of the cell reaction, and polarization potentials at each electrode. Operating Conditions. Crll remperature was measured by a sheathed Chromel-Alumel thrrmocouple \\.irhin 5°C. Samples for alumina analysis and measurement of the \\right ratio of sodium fluoride to alurninuni fluoride were withdrawn via the central hole in the bath crust by a steel samplinq cup. Digestion in boiling aluminum chloride solution was used for determination of alumina within 0.2Yc: ratio \vas drtermined by pyrotitration or indicator methods. usually reproducibly lcithin two units in the second decimal place. During normal operation. decrease in alumina concentration was proportional to cell current, but no systematic change in cell current or voltage was observed. Operating a t 10,000 amperes near 970' C. with a 2-inch anode-cathodr spacing, full anode effect occurred lvhrn alumina content fell below 17~. By trial it was found that concentration polarization could be induced on a n individual anode when alumina content was below about l..??. For a cell current of 10,000 amperes, this occurred when three anodes carried equal load and that of the polarized anode was initially SO to 100 This distribution was attained by adjusting individual anode-cathode distances. .4node-cathode distances were determined within about 5% by dip-stick measurements of total bath depth and anode immersion depth.
1290
Table I Time, Mill
'
+ 73 + 6060 + 43 1 .
f 19 4- 18 - 5 7 2 - 5 - 17 - 57 - 150 - 150
Table II.
N o Fluorocarbons Are Formed in Normal Operation 10,000 amperes; 970' C. .itial> Z I ~ Methodb
Lat
A
-0.02
N
- 0,003
M A A A A M M A A
10.01 -0.02 -0.02 -0.02
M M
-0.02
10.01
LO.01 -0.02 -0.02 zo.01 -:0.01
I(at1o
VaE /A111
\IL( 1 C' /c
1.50 1.50 1.50 1.50 1.14 1.49 1.14 1.50 1.49 1.50 1.50 1.53 1.56
1.0 1.5 1.5 1.5 0.85 1.0 0.85 0.60 5.0' 5.0 4.5c 3.20 3.20
Fluorocarbon Content of Anode Gas Increases with Potential at Anode Effect 970" C.
h p
Cell Potential llrop 1 olt-
500 1,100 1,000 1,000 600 1,000 600 1,200 1,200 62, OOOa 53, oooc
8.7 8.3 8.8 8.0 6.2 8.0 8.6 10.0 12.0 25.0 27.0
( ell Current
Results and Discussion Gas samples were collectrd during normal operation. full anode effect, and concentration polarization of single anodes. Normal Operation. T h i s condition (Table I) implies any stage of the reduction cycle behceen anode effects, with current equally distributed bet\ceen anodes. Sampling covered the period preceding and immediately follo\cing anode effect. Carbon fluorides \cere not generated at bath ratios behveen 1.14 and 1.56 and near 970' C. for anode current density in the production range. Anode Effect. \l'hen anode-cathode distances \vere equal at all anodes. onset of anode effect \cas rapid below 1.0%~alumina. Cell current and voltage fluctuared erratically, indicating an unstable resistive barrier. hlean values of current and voltage depended on Powerstat setting: 1200 amperes a t 12 volts and 600 amperes at 6.25 volts were attained at maximum and minimum. During anode effect, carbon monoxide-carbon dioxide ratio increased greatly, and appreciable quantities of carbon tetrafluoride \yere produced (Table 11). Because of the smaller gas evolution rate, air leakage occurred. Oxygen xias consumtxd i n comhstion of
INDUSTRIAL AND ENGINEERINGCHEMISTRY
M d X
Error
(-14
( (
1.48 1.80 1.85 2.00 2.00 2.50 2.56 8.00 9.86 12.53 14.70
liatl(1 Y a p All s 1.46 1.57 1.14 1.50 1.35 1.50 1.46 1.50 1.50 1.48 1.51
\I>()
r
0.63 0.28 0.85 1.00 1 .oo 1.00 0.63 1.00 1.oo 2.00" 2.34
carbon nionoxidr. but tht. argon 111 nitrogrn ratio \vas close to that for normal air. 'l'ht- fraction o f crll current ustd i n depositing fluoride ions can br rclatrd to the proportions of carbon tr1I.at!ii(iridt~, carbon dioxide? carbon nionoxide. and nitroqrn in thc sa~nplrs. C:alculation must rakes account oi' r h v folio\\ ing possibilitirs:
