Chloride melt niobium oxides

Niobium Pent chloride. Niobium pentachloride can be prepared in a variety of ways but most easily by direct chlorination of niobium metal. The reaction takes place at 300—350°C. Chlorination of a niobium pentoxide—carbon mixture also yields the pentachloride however, generally the latter is contaminated with niobium oxide trichloride. The pentachloride is a lemon-yeUow crystalline soHd that melts to a red-orange Hquid and hydrolyzes readily to hydrochloric acid and niobic acid. It is soluble in concentrated hydrochloric and sulfuric acids, sulfur monochloride, and many organic solvents. 

Chlorination of ferroalloys (ferroniobium-tantalum) is a more economical and simple alternative [30]. The process is performed on a sodium chloride melt that contains iron trichloride, FeCU. Chlorine is passed through the melt yielding NaFeCl4, which interacts as a chlorination agent with the Fe-Nb-Ta alloy. Chlorination of ferroalloys allows for the production of pure tantalum and niobium pentachlorides, which are used further in the production of high purity oxides and other products. 

Interaction between niobium oxide and fluorides, chlorides or carbonates of alkali metals in an ammonium hydrofluoride melt, yielded monooxyfluoroniobates with different compositions, MxNbOF3+x, where they were subsequently investigated [123-127]. According to DTA patterns of the Nb205 - 6NFL HF2 - 2MF system, (Fig. 18) a rich variety of endothermic effects result from the formation of ammonium monooxyfluoroniobate, its thermal decomposition and its interaction with alkali metal fluorides. The number of effects decreases and separation of ammonium ceases at lower temperatures and when going from lithium to cesium in the sequence of alkali metal fluorides. 

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

V.L. Cherginets, Potentiometric Investigation of Acidic Properties of Niobium(V) and Germanium(IV) Oxides in Chloride Melts (Dep. in ONI1TEKHIM, Cherkassy, 1991) N241-91, p. 7. 

V. L. Cherginets, Potentiometiic studies of acidic properties of niobium (V) and germanium (IV) oxides in chloride melts, Kharkov, 99l Dep in ONIITEKhim (Cherkassy),N 241 (1991). 

Lately, the electrochemical behaviour ofuranium(IV), hafnium(IV) and niobium (V) was investigated in chloride melts by chronopotentiometry and linear sweep voltammetry (LSV) applying the general approach discussed above [30]. In the pure chloride melts, the formation of intermediates is even more pronounced than in fluoride-containing electrolytes, because fluoride ions stabilize the highest oxidation states and thus destabilize the intermediates. 

The results of X-ray powder diffraction study for initial phases and for sediments in chloride melt allow us to conclude that the new niobium phases are not stable in chloride melt. It may be the causes of their absence upon the electrolysis in chloride-oxofluoride melts. They decay into various niobium oxides or into combinations of these oxides and the phase I. In any case the structure loses in part the K and F ions in chloride melt. We propose the following schemes of chemical reactions in the melt ... 

Niobium oxides, provided by the ore treatment, are dissolved in chloride melts (NaCl-KCl or LiCl-KQ) in the form of NbCls [7], and Nb metal is produced by electroreduction in the molten chloride solution. A French company, Cezus, developed this process at the industrial scale in the 1990s [8] for Nb and other refractory metals. Nevertheless, the cathodic process in pure chloride melts was proved to be too complex to be industrially valid, with a series of intermediate steps [8, 9], and provides nonadherent or powder metal layers with a low current efficiency. Better results are obtained in chloride melts containing fluoride ions because of the complexation of Nb in NbF7 ions which are reduced in only two steps Nb —> Nb Nb [10,11], with current efficiencies less than 100 % since Nb reacts readily with Nb cathodic product (proportionation reaction) for giving the intermediate species, Nb. ... 

The data concerning the oxidation state of niobium and the coordination properties of its species in molten halides are incomplete and often contradictory. There is no doubt about the existence of niobium(IV) and (V) species in molten niobium-containing alkali chloride-based mixtures. The only question concerns the stability of NbClg" complex ions under an inert atmosphere. The other disputed moment involves the value of the lowest niobium oxidation state stable in chloride melts. According to the different points of view niobium-containing melts held in contact with the metal can contain Nb ", Nb + or Nb" + ions [1]. 

Thus, the obtained experimental information concerning niobium speciation and coordination properties in chloride melts is incomplete and contradictive, especially in the case of lower niobium oxidation state complexes, and a further investigation of this problem is required. 

Table 4.4.3 Chlorination of niobium oxides by HCI in alkali chloride-based melts...

Under certain conditions even higher oxidation state metal ions are capable of reducing uranyl(VI) species. The reduction is possible when, for example the added metal species has higher affinity for oxygen than uranium(VI). In the presence of beryllium(II) chloro-species uranyl ions are unstable in chloride melts due to formation of very stable insoluble beryllium oxide [28]. Since uranium(VI) does not form stable purely chloride species at elevated temperatures, it decomposes to uranium(IV) chloro-species and chlorine. Zirco-nium(IV), hafnium(IV) and niobium(V) chlorides, despite their quite high affinity for oxygen, do not react with uranyl(VI) species in alkali chloride melts even at 750 °C and when their concentration exceed that of uranium by two orders of magnitude. 

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

The dissolution of niobium pentachloride in molten alkali chlorides was studied in NaCl-KCl-, NaCl-CsCl-and LiCl-KCl-based melts and the progress of the dissolution was followed by in situ spectroscopy measurements. In most instances the spectra contained only the low energy edge of the charge transfer band. The oxidation state of niobium in quenched melt samples of the obtained electrolytes was close to 5.0 (Table 4.4.1). This result is in a good agreement with the literature data [2-6] and indicates that NbClg" species constituted the main product of this reaction. 

Further experiments were performed to ascertain the nature and to characterise the intermediate product of chlorination. The spectra recorded were similar for different temperatures and various types of alkali chloride mixtures (Figure 4.4.6). The results of oxidimetric titrations indicate the predominant formation of niobium(IV) species. Deviations of oxidation state to the higher values result from the partial oxidation of Nb(IV) into Nb(V) by HCI. Oxidation state values lower than four, obtained in the case of NbO chlorination, most likely were caused by niobium(II) oxide particles captured during melt sampling procedure and trapped in the quenched melt samples subjected to the analysis (Table 4.4.3). 

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