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INDUSTRIAL AND ENGINEERING CHEMISTRY
given current, and eventually a condition \vas reachcd in irliich the cell did not operate at all. This polarized state could be rclieved by two rather empirical treatments ivvhich eliminated it as an obstacle to the commercial operat,ion of carbon anode cells: the addition of 2 5 lithium fluoride as a pr is Jvith a n auxiliary nickel anode as preliminary and again a t such times during the life of the cell as the polarization appeared. ACKNOW LEDGJiEXT
The authors take pleasure in aclmo\\.ledginp t h Johnson, who studied the impurities in the clcctl d o p e d methods of purifying it.
Vol. 39, No. 3
( 2 ) Booth, H.S.,"Inorganic Syntheses", T'ol. I, S e w Yolk, M e Graiv-Hill Book Co., iiic.. 1939. (3) Calcort, n'. S.,and Benning, -%, 6. (to Du Pont Cornpaw'), U. 8 . Patent 2,031,456 (March 17, 1936;. (4) Eirieleu>. H. J., J . L'hem. h o c . . 1942, 441. ( 5 ) I'rcdriihagen, Ii., and Iirefft, 0.T., 2 . Elecli'ochem., 35. 670 (1929). Hennc. -4, L., 7.Am. Chem. .Sot., 60,96 (193s). Jones, 1. C . , J . Phys. Chem.. 33, i i O l (1929). Lebenu. P., and Dnmiens, .%,, C'ompt. rt,sd., 181, 917 (1925;. (9) lIc\-ci., I.'.. ailti Qa11dowI T . . Btr.. 54, 759 (1921). (10) Sliller anrl Uigeioir. .I.A m . ( ' h i m . .';oc.. 58, 158.5 (l9:3(i;. (11) S i i i i ~ i i i I~f., ? ( ( 46, , , 2175 (1924).
LITERATURE CITED
(1) Argo, Mathers. Humiston, and Anderson, C ~ L SOC., . 35, 335 (1919).
T
of
FLUORINE J . F. Gall and 13. C. Miller PEU\SYL\
\\I\
SALT \ I i \ l E % C l C R I \ 6 LO., U l \ D \ i O O R ,
COSSTRUCTION details are g i n m for a sinlple and fluorine gellerator of 250 anlperes clesigll c.apilcit?., 'This unit has proTed coiiyeiiient to supply about one-third pound of fluorine per hour for lahortttory s t u d y artd small scale preparative \+ orli. Full operati% in*tructiom are given? and materials of construction and procedure for safe handling of the gas are described.
P4.
\TO
COAIPREHESSIYE cle.criptiim of the pt.epar:itim aiid liandling of elemental fluorine on the 1ni)oratory or +nla11 pilot plant x n i e has been puhliahcd bince those of Simons and of Catly in 1939 (*'Inorganic Pyiithescs", 1-01. I, 1311. 1 3 5 ff., 1IcGr;iv-Hill Book Co., Scn- l o r k , 1930). Since then, e>perially Juring the r e c e n t ~ v a ryear., con>iderat,le advancement has been made in the art of fluorine production; present-day generators of large output c a n be reliably operated, and the gas can be handled safely and confidently. The fluorine cell dcrcrihcd in the following paragraphs is eonveni. ent for the production uf adequate quantities of fluorine for laboratory preparative n-ork. The design i!: based upon several years of experience in fluorine cell development. A
F L U O R I 3 E CELL
Figure 1 .-lio\\-st h e cell. I t consumes 250 aniperei at 8.5 to '3.0 volts, and \\-ill produce about pound of fluoiinr pw hour. The construction is revealed i n Figure 2 . The anode iwulation i i a11 iron phosphate cement developed in this rerearch. Insulation lxtween thc lit1 bearing the gas harrier i.3 prcividid by a gasket of Teflon (polytetrafluoroetliJ-lene). The pa-ket sells betn-een the remaining lid pairs are inade \\.it11 ?/,lz-inch sheet lead, or
f-
Figure 1. Photo of a 250-.impere Fluorine Cell
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1E47
263
Diagrain of Fluorine ('ell ith 2.50- impere Hating
Figure 2 . M
POINTER
MAGNET
=COPPER
TUBE
LIQUID L E V E L
c) Figure 3.
Electrolyte Level Indicator
Lvitli French type gaskets of thin sheet copper rolled oyer a.=hesto* board. T h e anodes are type GA carbon plates (Sational Carbon Company), 2 X 8 X 18 inches, and current is conducted t o them X 3 inch through a l,/r-inch copper support rod and four specially constructed copper hits threaded into the carbon. jig n-as used t o ensure precise anode alignment in setting up the support rods for casting into t h e lid. The separate cathode used in this cell is supported from the main lid and thereby electrically honded t o the cell body. A successful cathode form is a louvered
sheet n-hich directs liquid flow and hydrogen buhhle,~upvard and hack an-a!- from the anode. The cell i, provided n-ith tn-o 11Y4inch I.P.S. (iron pipe size) explosion vents, designed t o relieve at very slight overpressure in the cathode chanlber. The cell i* electrically isolated by rubber hose in the steam connections, hy cast Asplit cement insulators in the nitrogen line, and by a sheet of Teflon polymer between steel flanges in the fluorine line. This cell holds about 500 pounds of electrolyte. A 1:',-inrh I.P.S. steel pipe inserted through the cathode lid t o a point about 8 inches belon- the surface of the electrolyte provides for hydrogen fluoride addition. ; i90 elbow a t the lower end of the pipe serve: to direct the turbulence from the absorption of hydrogen fluoride toward the n.:ill of the cell, and minimize the posiibility of sn-eeping buhhler of hydrogen into the fluorine chamber. Charging i> manually controlled n i t h the aid of a visual level indicator (Figure 3), consinting of an electroformed nickel float bearing a light vertical copper wire. This n-ire carries a small Alniro magnet into :I length of 3/&-inch copper tubing, sealed at its upper end and protruding through the anode lid. A light balanced steel pointel, external t o tlie copper tube follows the movement of the magnet. The electrolyte is normally maintained a t ahout 95-105" C. by tlie use of 1on- pressure steam during stand-hy condition and by ii hlast of air through the jacket for cooling during the p current. Air cooling was used for lahoratory convenience rather than for any other advantage. Provision is niade for blowing nitrogen into either the anode or the cathode gas space for flushing out air in starting or residual gases after shutclon-n. This is a n important, provision for safe operation.
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0
20
60
40
100
80
120
140
Vol. 39, No. 3
160
180
200
TIME IN HOURS
Figure 4. Typical Data for 230-Ampere Fluorine Cell
ELECTROLYTE PREPAR.ATIOK
The electrolyte can be prepared by heating 455 pouiicli r ~ f potassiuin difluoride in the cell tank and pas>iiig in 115 pound- of vaporized hydrogen fluoride. hbsorption of hydrogen fluoride i. rapid, and the mass n-ill soon liquefy. The level should stand n o more than 4.5 inches from the top of the tank. The cell is now assembled n-ith a nickel anode, 10 inches xide, 18 inches long, anti 0.5 inch thick, replacing t h e two carbon anode plates. (.in esti:i anode lid should be kept available for this purpose.) . i f t r i , thorough flushing of t h e cathode chamber, electrol out x i t h about 150 amperes for 8 hour>. Continued nitrogen flushing in the cathode space may be required in this operation t o avoid explosion due t o back diffusion of air at the reduced rurren: and to uneven evolution of gas from the nickel surface. The electrolyte is then analyzed and adjusted within 38 and 4Oyo hydrogen fluoride. At the same time the level is adjusted PO as to stand betn-een 3 and 3.5 inches from the top of the tank when the cell is asembled with carbon anodes (about 5 inches n-hen the cell is open). Withdrary and weigh in a small ~ K A L I - S I S OF ELECTROLYTE. platinum crucible about 0.2 gram of t h e liquid. Dissolve the sample in 50 ml. of standard 0.1 S sodium hydroxide solution, transfer t o a glass beaker, and titrate a t once Kith 0.1 -1-hydrochloric acid, using phenolphthalein indicator. C E L L OPERATIOIV
hfter the preliminai y electrolysis, the nickel anode is replaced by carbon, and the assembly is carefully tested for leaks. For this purpose a slight overpressure of nitrogen is applied alteinatelg to anode and cathode gas spaces and observed for pressure drop on standing. Leaks may be located b y searching foi e s a p i n g acid gas with a rod dipped in aqueous ammonia. The cathode and anode gas spaces are now thoroughly flushed x i t h inert gas, after which the current may he brought immediatelj- t o Its full value of 250 amperes. Fluorine gas of good purity xi11 be obtained in about a n hour after starting. Hydrogen fluoride ipassed in as required t o maintain the correct electrolyte level Cuirent is not interrupted for this operation. I n normal operation the total cell voltage will be about 9.0 volts, and the cathode t o barrier voltage about 3.5 volts. If t h e voltages depart veil much from these values, anode failure or faulty insulation should
he suspected. \Then current flov is discontinued for any reason, the cathode gas space should be flushed a t once wit,h inert gas. If the cell is t o be opened, the anode gas space should also he flushed and the cell alloxed t o stand idle for a t least 24 hour. t o permit diaengagement of lil-drogen which may be trapped in sludge accumulations within the cell. During stand-by periods cell temperature must be maintained above 80" C. at all times to avoid damage due t o freezing of the electrolyte. OBSERVATIOS S
A cell of the type de,crihed TI-as found t,o operate reliably in 110th continuous and intermittent ube over long periods. Cor1.0>ion of the cell components v a s found t o be negligible. S o evidence of attack !vas observed at any point on the steel container or lids. Lead gasketing was attacked enough t o require replacement after each dismantling; the French type gaiketd were unattacked except by accidental spillage of electrolyte on the asbestos portion. The iron phosphate insulating composition was found to be satisfactory. Teflon was unaffected hy exposure to cell operating temperature and hydrogen fluoride, but was sonietime? severely corroded in contact n-ith fluorine, particularly a t points x-here good thermal release was not provided. Sornially, cell operation x a s smooth, and continued fluorine production could be assured without hazard. Certain abnormal conditions, however, xere observed t o give rise t o internal explosions of varying intensity. The cells were designed to withitand or relieve severe shock pressures, and no significant damage \ras observed to result from such explosions. Furthermore, the causes of the several types of explosions have become well under.-toad, and effective means for their prevention have been developed. The sharpest cell explosions result from the ignition, liy stray bubbles of fluorine, of hydrogen-air mixtures in the cathode chamber. Such mixtures may result from leakage of gaskets or fittings, from back diffusion of air through the hydrogen exhaust, or from air present at the start of cell operation. Formation of hydrogen-air mixtures may t,herefore be avoided hg tight sealing; by t h e use of a n inverted U-trap in the hydrogen exhaust and a sufficient length of piping, together with avoidance of cell operation much below rated current capacity; and by careful purging of the cathode gas space with inert gas prior t o the start of electrolysis. Hydrogen-air explosions
March 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
265
may alio occur in dismantling the cell if 10.4 inadequate purging has not follon-erl di+ continuance of operation. Hydrogen 11iay also be trapped in sludge accumulntions m within t h e electrolyte arid 1nuduc.e cxplo5 10.0 sions n-hen t h e electrol>-te i- tli.tui~I~ctlin > anode removal. These latter tv-o t y p of explosion are dangerow, since c.on.ider:ilile scattering of hot electrolyte may ocrur, 2nd a: stmding period of 21 hour; is reconiiiiended 9.6 after -1iutdom-n t o perinit disengagement of accuniulated gas before the cell is opened. Heavy rumbling explosion- are observed \rhen fluorine back pre->urr liecomes high enough t o force the gas underneath and 102 104 I06 108 110 :irountl the harrier. The+e cnwe no serious C E L L TEMPERATURE ' C. c1:im:ipr iinle-,+ permitted to continue. Failure Figure 5 . Variation of Cell 1-oltage w i t h Electrol>-te Tc mperature i u of :,lloc~ror catllotle i l l ~ u ~ a t ~ o n l,e5,1it 25O-Anipere Fluorine Cell i n the formation of liydrogcn or crf fluorine < ) i i the gas barrier, ~ r l i c n :I continuous iiite1)- unsatidactory. For use ivith fluorine at atmo~pliericpwss h n r p cmckling is observed. For t h i j reason a continuo~isindication of barrier potential relative t o anode or cathode is debirable. sure, Teflon packing works iyell, provided the hulk--urface relaInaccuriite alignment of anodes or insufficient elrctrode to barrier tions in the packing are such t h a t thermal releahe i.; good i n the . also cause product int ermisiiig and develop similar event of incipient combustion of the Ijacking. Dia~jhrngm, pe exploiion~. F i ~ l l y organic , impuritier :ucli :Iand other types of packleas valves are satisfactory if the n-ood eliipu, rvhich niay be present in freshly prepared elect g mechanism in contact n-ith the fluorine i.: not readily batches, may cause minor erp1o;ions at the start of eleetrol. iiiade inoperative by fluoride scale. Quantitative operating dat:i xvere obtained for this cell ~ i i d The generator described a h v e will produce a PURIFICBTION. ;-onie typical results are presented in Figures 4 and 5. The-e gas containing about 5% by n-eight of hydrogen fluoride, as a Tvere obtained in a 200-hour coiit iiiuously supervi-ed test run on result of the appreciable vapor pressure of this substance over the the 250-ampere cell. .Is the tn-o top line. of Figure 4 .slio\v, electrolyte. The bulk of the hydrogen fluoride is easily removrd by condensing it out in a trap cooled n i t h solid carbon dioxide and cm,rent was maintained at 250 amperes and divided equally heti\-ern tlie two anodes. The tot:rl cell voltage fluctuated b e t ~ e c i i triclrloroethyleiie, and remaining traces may he ah-orbed in a S.5 and 9.0, and was a function priniarily of cell temperature :ind tube packed TI-ithpelleted sodium fluoride. current. T h e voltage betn-een the cathode and gas barrier ISSTRUIIEZTS. ; i nianonietrr, corering tlie range of plus or fluctuated betn-een 3 and 4 for normal operation. With air coolminus a fen- inches of ivater and sensitive t o '4-inch w t r r h e a d , ing, electrolyte temperature n-as held t o 106-110" C. -4marked i. required for safe operation of the cell. -1 convenient form temperature rice was noted dui,ing each hydrogen fluoride cliargromprises the usual open-end glass L--tube, filled Tvith carbon iiig period. Figure 5 shon-.; cell voltage as a function of cell tetrachloride or sulfuric ncid, and protected by a n ahmrber temperature. From these scattered data, the temperature rh:irged with sodium fluoride to remove hydrogen fluoride, coeficient T V ~ Ofound to be about -0.07 volt per C. During follon-ed by an absorber charged n-ith sodium chloride n-hich n.ill tlie 200-hour run t h e over-all efficiency Jws about 90y6 in hydroabsorb fluorine n-ith liberation of chlorine. .1 flo~vnieterniay be gen fluoride consumption, representing an expected l0y0loss of similarly constructed, \\-ith a pierced platinum disk used as an hydrogen fluoride as vapor in the hydrogen and fluorine gas orifice between the tn-o legs of the T--tube manometer. For this streams. purpose an inclined-tube draft gage is conveiiient, since tlie pressure drop permissible a t the orifice ic small. FLUORISE HAXDLIXG PRECIUTIOSS. Fluorine re11 operation is not dangerouc if certain precautions are rigorouqly follolved: PIPIKG. T h e gas can be conducted in standard steel pipe or 1. .Iroid contact irith electrolyte or hydrogen fluoride. Both copper tubing at atmospheric temperature without noticeable compounds produce severe deep-seated burns on contact n-ith attack except for the formation of n h i t e scale on the interior surthe skin. I n case of espoaure, thorough Ivashing should be face and some corrosion at the exit end of the pipe where it is exinstituted a t once, followed by treatment as for hydrogen fluoride posed t o atmospheric moisture. I n pipes xhich are subject t o burns. Great care should be taken t o protect operators against flexure or vibration, t h e protective fluoride deposit may flake or possible electrolyte splash in the event of a11 esplosion. Exploponder off, and the resultant scale niay accumulate in restricted sion vents and gasket exposures should he xell shielded, and adeparts of t h e line. I n t h e case of iron pipe, oxide scale sometimes quate protective equipment should be worn by operators working present in new pipe Ivill be converted to a poydered fluoride, near the cella. The vapors of hydrogen fluoride rising from the which may similarly cause line stoppages. If a section of iron hot electrolyte are also harmful, and prolonged contact or inpipe is locally heated-e.g., by combustion of dirt or grease in halation are t o be aJ-oided. .\ reipirator or gas mask with t h e joints-continued conibustioii niay ensue, and a substantial canister for acid gas protection :hould be worn by anyone working portion of the pipeline may be destroyed if the passage of fluorine near open cell-. is rontinued. For applications n-here the formation of fluoride 2. -\void contact ivith or irilialation of Huorine. Fluorine ~ c n l eis undesirable or 7%-hereelevated temperatures may be ennil1 c a w e seriou. fled1 burns on contact Tvitfi the skin, and the countered, None1 or nickel pipe i- recommended. hazard of inhalation i* x t least as great as that for chlorine. TALVI:R.Because of the abrading action encountered in the Protective measures :again-t fluorine :Ire not fully developed, and turning of a valve, materials which sho~t.minimum scale forniaentry into zones hearily contnminated n-itli fluorine should be tion are preferred for valve stem and seat, and None1 and nickel avoided. Only air line or oxygen masks can be recommended a t :ire preferred. For the same reason, globe or needle type valves the present time for personal protection. Rubber gloves brought are preferred t o gate valves, and plug cocks are found to he defiin local contact n-it11 a fluorine leak may inflame, and it is strongly
