Patent GB-866716-A (GB)
Title: Process for separating and purifying niobium and tantalum
IPC: C22B34/24
Filed: 1957-04-26
Abstract: Niobium and tantalum are separated by selective decomposition of their oxalato complexes of the formula H5[Nb(C2O4)5], H7 [Nb(C2O4)6], H5[Ta(C2O4)5] or H7[Ta(C2O4)6] or complexes formed with complex formulers which are themselves complexes with oxalic acid, such as aluminium, chromium, molybdenum or tungsten oxalic acid, all of which are described and claimed in Specification 866,718. The decomposition of the oxalato complexes is effected according to the procedure of Specification 866,714, that is one or more of the parameters of the liquid system containing the oxalato complexes is altered up to but not beyond the point where crystal nuclei of a niobium or tantalum compound form as determined by the Tyndall effect, the parameter then being maintained unchanged until the separation of the niobium or tantalum fraction is complete.  The decomposition is preferably effected at a temperature of at least 40 DEG C. and in order to stabilize the niobium oxalato complexes whilst the tantalum is being precipitated as hydroxide or basic oxalate, complexes of a di- or tri-basic organic acid described in Specification 566,718 may be added.  Decomposition of the oxalato complexes, if they are prepared in an acid solution, may be effected by ammonium oxalate, the precipitation pH value for tantalum being pH 8 and for niobium pH 6. Such pHs may be maintained by ammonia or hydrochloric acid addition. If the niobium and tantalum oxalato complexes are prepared from alkaline solution using a metal-oxalic acid as complex former, the addition of such metal oxalic acid itself produces the pH change up to the threshold value at which it is maintained by the addition of ammonia.  According to Examples (1) an iron alloy containing tantalum and niobium was dissolved in a nitric/hydrofluoric acid mixture and the niobium and tantalum precipitated by the addition of ammonium hydroxide. After washing with acetic acid to remove iron, the precipitate was converted to oxides by roasting and then melted with potassium bisulphate; the melt was dissolved in aluminium oxalic acid solution at 40-50 DEG C.  Precipitation of the tantalum was then effected by adding ammonium oxalate together with hydrochloric acid at a slow rate with vigorous stirring until the onset of the Tyndall effect.  98% of the tantalum comes down as a first fraction.  (2) Niobium and tantalic acids are dissolved in caustic potash at 60 DEG C. and the solution diluted and the pH adjusted to 9. Aluminium oxalic acid was gradually added with vigorous stirring until a pH of 8 was reached and thereafter maintained by the addition of ammonia. The tantalum precipitate was filtered off and washed with ammonium oxalate solution and the filtrate then adjusted to pH 6 with hydrochloric acid and ammonium oxalate thereby precipitating the niobium.ALSO:Niobium and tantalum are separated by selective decomposition of their oxalato complexes of the formula H5[Nb(C2O4)5], H7[Nb(C2O4)6], H5[Ta(C2O4)5] or H7[Ta(C2O4)6] or complexes formed with complex formers which are themselves complexes with oxalic acid, such aluminium, chromium, molybdenum or tungsten oxalic acid, all of which are described and claimed in Specification 866,718.  The decomposition of the oxalato complexes is effected according to the procedure of Specification 866,714, that is one or more of the parameters of the liquid system containing the oxalato complexes is altered up to, but not beyond, the point where crystal nuclei of a niobium or tantalum compound form as determined by the Tyndall effect, the parameter then being maintained unchanged until the separation of the niobium or tantalum fraction is complete.  The decomposition is preferably effected at a temperature of at least 40 DEG  C. and in order to stabilize the niobium oxalato complexes whilst the tantalum is being precipitated as hydroxide or basic oxalate, complexes of di- or tri-basic organic acids described in Specification 866,718 may be added.  Decomposition of the oxalato complexes, if they are prepared in an acid solution, may be effected by ammonium oxalate, the precipitation pH value for tantalum being pH 8 and for niobium pH 6.  Such pHs may be maintained by ammonia or hydrochloric acid addition.  If the niobium and tantalum oxalato complexes are prepared from alkaline solution using a metal-oxalic acid as complex former, the addition of such metal oxalic acid itself produces the pH change up to the threshold value at which it is maintained by the addition of ammonia.  According to Examples (1) an iron alloy containing tantalum and niobium was dissolved in a nitric/hydrofluoric acid mixture and the niobium and tantalum precipitated by the addition of ammonium hydroxide. After washing with acetic acid to remove iron, the precipitate was converted to oxides by roasting and then melted with potassium bisulphate; the melt was then dissolved in aluminium oxalic acid solution at 40-50 DEG  C.  Precipitation of the tantalum was then effected by adding ammonium oxalate together with hydrochloric acid at a slow rate with vigorous stirring until the onset of the Tyndall effect.  98% of the tantalum comes down as a first fraction.  (2) Niobium and tantalic acids are dissolved in caustic potash at 60 DEG C. and the solution diluted and the pH adjusted to 9. Aluminium oxalic acid was gradually added with vigorous stirring until a pH of 8 was reached and thereafter maintained by the addition of ammonia, The tantalum precipitate was filtered off and washed with ammonium oxalate solution and the filtrate then adjusted to pH 6 with hydrochloric acid and ammonium oxalate thereby precipitating the niobium.