TANTALUM AND NIOBIUM
Anhydrous Separation Selective Chlorination of Partially Hydrolyzed Chlorides S. 1. MAY, A. W. HENDERSON,
AND
H. A. JOHANSEN'
U. S. Bureau o f Mines, Albany, Ore.
T
H E two metallic elements, tantalum and niobium (also kno\+n as columbium), have proved difficult t o separate because of their closely parallel chemical behavior. As stated by Atkinson, Steigman, and Hiskey ( I ) , aqueous methods of separation of tantalum and niobium require tedious recrystallizations of complex salts t o achieve reasonably pure compounds. Anhydrous methods of separation presented heretofore do not effect a sharp separation and also present problems concerning materials of construction. Since the discovery of tantalum and niobium, early in the 19th century, many methods have been proposed for separating the two elements. The process developed by Marignac ( 1 7 ) in 1866, or variations of it, is the onlv method used commercially. The Marignac separation is based on the difference in solubility in water of the double potassium salts-1 part t o 150 parts by weight for potassium fluotantalate and 1 part t o 12 parts by weight for potassium oxifluoniohate I t is a costly process and entails the use of special coirosion-resistant equipment. .4method presented by Schoeller (26) makes use of the different extents to n-hich ovalotantalic and ovaloniobic acids are affected by hydrolysis. This method is rommonlg used for analytical procedures. Boiling 11ith tannin or tartaric acid favors precipitation of the tantalum salt. Keiss and Landecker ( 3 8 )patented a process whereby tantalum and niobium could be separated from each other by precipitation from an acid solution. Introducing carbon dioxide into the solution containing tantalum and niobium caused the tantalic salt to precipitate. Latpr investigators (6) reported this process to be inefficient and to effect a poor separation. A method introduced by Merrill (19) was based on the discovery that niobium pentovide is soluble in a sulfuric acid-selenium oxychloride solution, while the tantalum pentoxide is nearly insoluble. This method requires a very expensive reagent. According t o Sears (a?),advantage may be taken of the difference in decomposition temperatures of sodium tantalates and mobates and of the difference in solubilities of the compounds formed. -4fter fusion and leaching operations, the insoluble residue mas treated with concentrated sulfuric acid which dissolved the niobium compound and left the insoluble tantalum pentoxide. However, no sharp separation was obtained. An anhydrous separation mas reported by Ruff and Thomas (24). Carbon tetrachloride was used as a chlorinating agent in the treatment of tantalum and niobium oxides. At 200" to 225' C. niobium pentachloride was formed, while the tantalum oxide remained unaffected. However, in a secondary reaction, niobium pentachloride reacted with tantalum oxide to give niobium oxychloride and tantalum prntachloride, so t h a t a sharp separation did not occur. Yntema (41j reported that an incomplete separation could be effected by electrolytic hydrolysis. This separation depended on the precipitation of the insoluble niobic and tantalic acids a t the different H-ion concentrations produced during the course of the electrolysis. Later investigations by Pierce and Yntema 1
Present address, University of Oregon, Eugene, Ore
D e c e m b e r 1954
( 2 3 ) indicated that i t was impossible t o effect a clean separation by merely regulating the pH. Later, Pierce ( 2 2 ) claimed that niobium could be plated from a saturated solut'ion of ignited niobium oxide in sodium carbonate. Under identical conditions of current density and temperature, a similar solution containing tant,alum oxide would not plate out tantalum. This finding was not confirmed, hos-ever Using tantalic and niobic acids as a starting material, Cunningham and Price ( 3 ) claimed a separation by convcrting the earth acids to potassium hexatantalate and potassium hexaniobat,e. Then, by adding potassium bicarbonate to the solution, the t'antalum salt was precipitated. No sharp separation was obtained. A process for separating tantalum and niobium was patented (SO) whereby an oxide or oxidized compound was added to a fused alloy containing tantalum and niobium. The compound displaced tantalum with respect to niobium, and a slag containing tantalum and a n alloy containing niobium r e r e obtained. This evidently refers to a process of enrichment only, and no sharp separation would be expected. Tantalum and niobium may be separated by chlorination (4). The mixed oxides were subjected to chlorine or a chlorine-containing gas a t 1050" C. The niobium was volatilized as niobium chloride or oxychloride, but the tantalum oxide was unaffected. A modification of this process was to heat the mixed oxides wit,h a base before chlorination, thus causing the tantalic acid to combine with the base. The mixt,ure was then treated with chlorine for several hours. Under these conditions, 70 t o 80% of the niobium was volatilized, and practically all the tantalum remained in the residue. No information as to the purity of the product was recorded. Jenness ( 1 2 ) obtained a patent describing the separation of niobium from tantalum by a process involving selective reduction and chlorination. A mixture of sulfur dichloride and chlorine was passed over the mixed oxides a t 200" C.; reduction, chlorination, and volatilization could take place for the niohium but not for the tantalum. 50 information as t o product purity was given, but this process appears t o have merit. Another method involving selective chlorination is that of Kroll and Bacon (14, 28). Treating the mixed oxides a t moderate temperatures with ammonia or nitrogen caused the niobium oxide to become nitrided. The mixture was treated with chlorine at a slightly lower temperatur- and the niobium nitride was preferentially chlorinated T h e niobium was volat'ilized as niobium chloride. Up t o 88% of the niobium and less than 570 of t'he tantalum were removed in one teat. This method offers more promise of a practical separation method t'han most others. Another patent (16, 29) suggested treatment of the mixed oxides with hydyogen t o reduce selectively the niobium pentoxide t o niobium tetroxide. After the partial reduction, the charge was treated with chlorine, and niobium pentachloride was obtained according to the reaction
5Nbz0,
+ 5C12
-+
2NbCI5
+ 4SbaO~
About 20% of the niobium contained in the charge was removed
INDUSTRIAL AND ENGINEERING CHEMISTRY
2495
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT in one cycle.
This method would require many recycling steps
t,o achieve a complete separation.
b u t suffers from t,he disadvant,ages of high cost of corroeion-resistant equipment and hazardous chemicals. K e r n e t (59)proposed a novel niet,hod of separat,ion in which a hydrochloric acid plus ammonium chloride solution of tantalum and niobium TYRS treated with a titanium addit,ion and bot,h the
Fom-ler ( 7 ) claimed a separat,ion by precipitating the tantalum complex potassium tantalum salt from a solution of the hyed oxides in potassium oxalate and oxalic acid. 9hydrolyzing agent Iras added to the solution, and the tantalum complex niobium and titanium precipitated t,lirough the forniatiori of was precipit'ated a t a pH of 4.5to 5.0. It, T T ~ Sfound that niobium mixed crystals of the formulas (SH,), TiCI, - (SH~)sNbOC16. did not form an insoluble precipitate below a pH of 5.0. This The tantalum remained in solut,ion. This method gave tantalum method requires pH ranges that are probably t,oo critical to concont,ainirig 27, each of niobium and tit,anium. A diet.inct tlistrol completely. advantage of t,his method is the furt,her separation required to ob-1 process for separating taiit,aluin arid niobium b y elect,rolytic tain titanium-free niobium. a separation as difficult as that ol tho reduction of iiiohiuiii r r a i reported by Golibersuch and Young original tantaluni-niobium. (f 1 ) . The method t,ooli adrantage of the reduct,ioii of quinqurDespit,e t.he variety of methods of separat,ioii iiivestigat,ed by valent niobium t o the quadri- aiicl trivalent states in a 'i5yo earlicr n-orkers in this field, no niet,hod was found that substailsulfuric acid bath t o precipitate the complex niobium salt from t i d y improved t,he original process of AIarignac (17)-i.e., t,hc a solution contaiiiiiig both tantalum and niobium sulfatcs. separation based on the difference in solubility in water of the A similar process involving reduction of quinqucvaloiit, niobium double potassium salt,s. Unfort,unately, this method produces to quadri- and trivalent niobium has been pateutcd (35). T h e only taritalum ill high purity and a serious materials of coristi,ucniobium in acid solution as reduced x\-it,h zinc dust. Boiling tion problem is encountered. preeipitat,ed the tantalum hydrate, leaving the Ion-cr valent The new method described in this paprr falls into iEie classificaniobium in solutioii. T h e niobium was then reoxidized a n d pretion of an anhydrous separation, alt,hough partial hydrol> ' cipitated by boiling. -1separation with anion exchange resins has been reported hy Iiraus and 5Ioore ( I S ) . It n-as found that the separation in a Partial Hydrolysis of Chlorides I s ~ o ~ ~ 921 hydrochloric acid-0.05dl hydrofluoric acid solution by meanq by Admixfure of Tantalum-Niobium Oxide Hydrate of Doxex-1 n-as TP~!- efficieut xiid probably completc in a 12.5R3-n thetic i~iist~ures of t:ii)talutn and niobium chloridcs t w r c ~ i i column. . T h e e last three methods, which depend on solup r c p n ~ e dh y direct, chlorination of comincrcisl tantalum tion? of tant:ilum and niobium, could be expectcd t,o bc w b j c r t aiid !iiobiuni powder obtaiiied from Falisteel Corp. The to the difficulties arising from hydrolysis Tvhich prevents, in t ions n-crc carried out iii it lioyoeilicate glass tuhe heated by a tuhe casc, the formation of stable solutionP. fur1x1cc. n for use in analytical procedure8 may he effected The follon-ing misturos ~ c r pxpctreif e : b y precipitation of the tantalum as ii, complex iodate salt, according t o Uspeiiskays and Chernikhov ( 3 6 ) . Tlie exact coniMixture S o . TaClr, 7, SbCls, 70 position of the tantrtlum salt n-as not known, but it x i s 4milar 1 50 50 to the ceric complex, 2Ce (IOs)4.8H,O. In the tantalum com2 4:3 57 ples, 1 Ta = 210i. Siobiuin ivas not precipitated finin t,he In addition, a o d a b l e ores containing taiitalum and niobium soliitioii by the addition of potassium iodate. v w e treated to obtain various "natural" mixtures to test tlic .hi anhydrous separation involving solubilities of t h e anhgmethod. Tlie follo.rving natural mixtures of anhydrous chloridw droiis chlorides of tmitalum and niobium in titanium tetrachloride ere prepared: has heen suggested by Taraseuliov and Iioiiiandiii ( 3 3 ) . TanTa Nb talum pentachloride is soliihle in titanium tetrachloride, whereas S a t u i a l Alixtuie ___ T a I_\TI> x 100 Ta Sb niobium pentachloride is less soluble. This method depends o n .1 81 69 maintenance of scrupulous ailhydrous conditions aiid involves B 76 24 further separation of t,itanium from both ttintalum aiid niobium. Schaefer and l'ietrucli ( 2 6 ) reported that, in a mixture of anhyHydrated t,aiitalu~n-niobiu:ii oxides were prepared by hyand nioliiuni chlorides, the niobium pentachloride drous t#ant>alum drolysis of anhydrous chlorides. The hydrated precipii-ate wamay be reduccd with aluminum, hydrogen, tant,alum, 01'niobium dried to constant weight a t rooiii temperature. Two hytiratecl samples of the follo\Ting percentage composition were uecd: t o niobium tetrachloride n-hich is less rolatile, aiid the taiitaluni thus mag h e separated from the niobium hy distillat,ion. Hydrate TasOj I\;byOj Nb TiOn H20 A chromatographic: xparatioii of t,aiitalum and niobium lias A 40.5 20 1.5 29 been developed hy Burstall, Swain, a,nd associates ( 2 ) . T h e B 77 .. ... 23 method is based o n the ext,raction of tantaluiii and niobiuin as Reagent gradc aniiiioriium chloride wai used. Commer.cially fluorides by methyl ct,hyl ketone in the pxsciicc of wtivated bot,tled helium, chlorine, and ammonia v-ere used as rcccivetl. cellulose. The author3 st,atrd that, the t,antnlum m ; t ~be - removed The equipment for separation consists of a 1-inch split t , u h quantitatively if the iiiobiuiii is present in amounts equal t o or furnace n i t h borosilicate glass tubes. *k silica boat held t.hc win excess of the tantalum. Chromatographic separation of action mixtures. A dry box n-as used to minimize unwantc,tl tantalum and niobium may be accomplished by adsorption on hydrolysis effects of the atmosphere in preparing and handling activated alumina from an aiiiinonium osalatc 3oiution. l-Lccordiiip t8hcbatches. to Tiliho~iiirofi'( 3 4 ) . 1nit.ialIy; p:irtial hydro1 of t,hc anhydrous mixed chloridw Rccent,ly. sep:tratioiis have bceri achieved by sol~-cnl \vas achievrd ?q niising the c?itirgcs in nioiat air. Vai,yiiig and AIoore ( 1 6 ) reported fa tioii methods. lAclicot8t~e amounts of hydrolysis could be obtained bj- different t.imcs of c?:r e d t s x i t h t,he a!-stem meth~ltIicotyla~niiie-syienc--h~~drochloricposiire. Holyever, t,he esact cxtent of hydrolysis couki :lot be aeici. Etcverison and Hick- (31) effected a separat,ion by extrscconii~oll~d in t,liis iiiaiiiicr, and an alternat,ive method w:ts tlcti011 into diisoprop~-llcetorie i'rom mineral acid-hydrofluoric acid vised. Tantaluin-iiiobium oxide hydrate x-as found to ~ J C :: I aqueous phases. Werning and associat,es ( 4 0 ) achi ,mitable Rourw of nioist.ur,e for controlled hydrolysis of t
