PREPARATION OF FLUORINE John T. Pinliston, Jr. T H E HARSHAW CHE3IICAL CORIPANY, CLEVELAND, OHIO
A CO\IJIERCIiLLY practical fluorine cell of t h e diaphragm type drawing appro\iniatelj 1000 amperes has been deielopecl. Ordinarj carbon steel is suitable for t h e major parts of thic ccll, incliicling t h e cell bodj, hot water jac.Let, hjclroflnoric ac.icl feed line, cathode, and solid part of the diaphragni. 'lhe anodes are carhon rods impregnated with copper. l'hc lower portion of t h e diaphragm is a RIoiiel screen. The electrolj t c ic fused KF.2 €IF \tith 1.0 to 1.57, lithirtni fluoride as a n additi\e. 'The cell operates
a t 95" to 115' C. with a n anode ciirrcmt efficiency close to 9570 and a n oier-all potential dropoC9.0 to 9..5 lolts. Such large cells h a l e operated continuousl? with i e r j little a t tention for more t h a n 13 months. R i t h t h e experience gained with t h e large cell, a 50-ampere laboratory cell which generates close to 35 grams of fluorine per hour has been constructed. ilthoiigh extensiie life data are not j e t aiailable, it no\\ appears t h a t t h e sinall cell will be j u s t ;I* witisfactor? as t h r large one.
inch thicli ant1 Wrforated t i ) permit easier circulation of electi,olytc', \vas found cntirely satisfactory. The choicci of :t suitalde porous t l i a p h i q p for the separation of the annile and rathod(, compaI'tinent bc91o!v the liquid level of the clectrolyto vas uomewhat more difficult,. T o varying extents, of s t c d ivool niounicd ljetnwm espancled nietal screens, 1 srr~>('ii.,p,,i'foratid copper sheet, copper screen, antl A1onc.l $cre('ii c ~ ~ u l:ill t l tic: uied. I t \vas soon apparent t h a t the poroqity of t ! i i s (1i;iiihlagiiirould be v u ? - high, antl yet mixing of thc anode and c:itliode gaqc~swould t)c prevented. .kccord\vas c.11o-cm The niethocl of mounting thc i n d y :in &mesh -rrc~>ii scwi~iiturncd out to tw highly important from a. corrosion standpoint. Jl~liciinuts ant1 l~olt;~i-c'rrused to attach the screen t o the
ride dissolvcd in a g e n t excebs oi anhydrous liytlvofiuoric acid; (b: a cell operating a t 7.5' t o 120" C. with a nickel or carlmn ancdo in an electrolyte whose composition \vas very close to liF.2 H F ; and (c) a high temperature cell operating \vitli a carbon anode in a fused potasFigure 1. Typical Section sium bifluoride clectrolyte. of Cell Assembly In our handp, a t least, such cells required constant cspcrt attrntion to pcrmit even intermitten1 fluorine generation. Our prohlem tlemantled a large cell n.hich would operate continuously over estencted period. of time n.itli little attention. T h e solution of this uroblixni involved, among other things, finding w i t able materials of construction, learning the best methods of construction, the development, of a thoroughly practical electrolyte, and the determination of suitable operating conditions. After a small amount of preliminary work i t was decided t o focus attention on a cell of type b operating at a n intermediate temperature. The low temperature cell v a s rejected because of the difficulty anticipated in finding a n operable anode which would not corrode and foul the electrolyte and because of the apparent necessity for refrigeration. Corrosion problems and the apparent requirement of copper or None1 for the cell body caused the high temperature cpll to be abandoned.
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By far the, major part of the cell was made u p of ordinary carbon steel. This material vias found suitable for the cell body proper, the hot water jacket, the hydrofluoric acid feed line, and t h a t part of the diaphragm which extended from the head of the cell down into the electrolyte for a depth of about 2 inches. Under proper operating conditions a steel cathode, made from a plate
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 39, No. 3
tion to desired shape, (6) operation on an electrolyte containing traces of water which re,>ulred in the removal of this water, ( 7 ) high cwrciit efficiency, and (8) density great enough 1 0 allow the anode to sink in the electrolyte, if 1)rc~aliage occurs. Siciicl, the first matcyial studied seriou-ly, requirements 1, 2, 4, 5, 6, and 8. 1 1 % -(,rious disadvantages lie in a low current c , f f i c h c y (60 to 7 5 5 ) and the fact that it cwrlodes gradually to foul the clcctrolyte. On a -1iort-term basis, nickel anodes are satisfacI C J I '1Iuch ~. of the early work v a s carried out, w i t t i ctllls having nickel anodes. Because of tiic. ability of nickel anodes to function in w t c,lvcirolytcs, it has been customary to put it fc>w niclic>lanodes even in the latest desigiictl (,cxllsto permit easier starting of operation. I n tiiv search for anodes n-hich ~vould riot foul the rlectrolyte and vould have better currc'nt rfficiency, a study vias made of several types of carbons. Graphite was found t o operate s:iri.?fartorily for a short time, but se cjries, a t least, were subject to smlling which resultctl in breakage and loss of electrical contact, hltliough anhydrous hydrogen fluoride and the electrolyte mixture had no apparent effect 011 it, a graphite rod exposed for 26 hours t o a ctream of fluorine a t 100' C. gained over BmO i n weight, 2 ( 5 in diamc,tcr, and 0,9Tc in length. .1 Iinrcl, ungraphitized carbon rod of the same size mntle from pctroleuni coke by the Sational ( 'arbon Company, called Type GA carbon, was found t o gain only 0 . 2 8 5 in weight and to buffer no measurable change i n dimensions under I kiese same conditions. I n its favor, the G.1 carbon anode met reyuirenic,ntc 1, 3, and 7 . Its most serious fault lay in t hc difficulty involved in connecting it electricull>-, its l o x physical strt.ngtli, its inability to operate satisfactorily on an electrolyte containing traces of water, and the fact that the carhon anode, if broken, would not sink in the electrolyte. Occasionally floating pieces of car\)on anodes would short out a cell, forcing its 'immediate shutdown. \TIien all factors involved \?-ere evaluated on a .semiquantitative basis, it was found that, for good calcctrolytes, carbon anodcs were more suitable than nickel anodes. Cells containing primarily Figure 2 . I t ~ \ e r t e dIIead of Cell with Esseiitinl component^ carbon anodes, with a few nickel ones to facilitate >tarring,have operated satisfactorily and unintvrruptedly for as long as 4 5 million ampere-hours. l i i :in iitic3nipt t o develop an anode having all of the good propsolid part of tlic diaphragm, the tiolt heads c l u x t o tlic cathodc C ~ ,ic,b I cii cwbon without some of its disadvantages, experiments were scverelv rorrodc~l. All diaphragm tlifGculties m r c eliniiy(>rvcarried out with ungraphitized carbon rods n-hich had been natcdin thc final arrangcxnient-a Monc.1 screen silver-soldered into imprcgnatcd n-ith copper. These were found to be a distinct im.it slut in t l i c x lon-t,r c~lgc'of the solid upper part of the diaphragm. 1irovenient over the unimprcgnated carbon. I n addition to all of A gas \Ionel n-cld \vas used to make the screen-to-screen joint. ihc, advantages of the latter, the impregnated anodes permitted The a n o d c ~\vcrc the principal source of difficulty; in the later ca-ic,r ant1 inore satisfactory electrical connections, had a t least stages of tlic clevc~lopnientof this cell, when the corrosion arid ,!Or; greater physical strength, and were not nearly so sensitive c4ct.t rolyi c prol)lenis had been solved, the operating cliaracterisi o changes in the hydrofluoric acid concentration of the electrot ic-s, life, and goiioral suitability were almost entirely a function Ivt(.. .Ilw) if breakage did occur, the detached portions of the of 1 tit. anode niatclrial. :ili
