Tantalum rare earth solubility

But first the synthesis had to come John was interested in reduced metal halides, particularly for the post-transition metals cadmium, galHum, and bismuth (his Ph.D. dissertation was on anhydrous aluminum halides and mixed halide intermediates, a good start for what was to come ). However, he was not yet actively interested in rare-earth metals and their remarkable solubility in their halides. But these elements lured him one floor below where Adrian Daane headed the metallurgy section of Spedding s empire. He knew how to produce rare-earth metals with high purity and in sufficient quantity and also how to handle tantalum containers. What if one gave it a tr/ and reduced some rare-earth metal halides (John insists that this term is used correctly) from their respective metals at high temperatures under appropriate conditions. 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Minerals such as euxenite, fergusonite, samarskite, polycrase and loparite are highly refractory and complex in nature. These minerals may be opened up by treatment with hydrofluoric acid. While metals such as niobium, tantalum and titanium form soluble fluorides, rare earth elements form an insoluble residue of their fluorides. Such insoluble fluorides are filtered out of solution and digested with hot concentrated sulfuric acid. The rare earth sulfates formed are dissolved in cold water and thus separated from the insoluble mineral impurities. Rare earth elements in the aqueous solution are then separated by displacement ion exchange techniques outlined above. 

Eigermann W, Mtiller-Vogt G, Wendl W (1978) Solubility curves in high-temperature melts for the growth of single ciystals of rare earth vanadates and phosphates. Phys Stat Sol (a) 49 145-148 Ewing RC (1975) The ciystal chemistry of complex niobium and tantalum oxides. IV. The metamict state. Am Mineral 60 728-733... 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

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