Precipitation from solutions tantalum hydroxide

The optimal temperature range for the fluorination process was found to be about 230-290°C. The resulting cake was leached with water. The prepared solution was separated from the precipitate by regular filtration and the separated insoluble precipitate was identified as lithium fluoride, LiF. The solution contained up to 90 g/1 Ta205. Solution acidity was relatively low, with a typical pH = 3-4, and was suitable for the precipitation of potassium heptafluorotantalate, K2TaF7, tantalum hydroxide or further purification by liquid-liquid extraction after appropriate adjustment of the solution acidity [113]. 

Carlson and Nielsen (C3) described the pilot and full-scale plant separation of an ore containing more than 30% combined columbium and tantalum oxide using a sulfuric-hydrofluoric acid leach and methyl isobutyl ketone (MIBK) as solvent in pulsed columns. The —200 mesh columbite-tantalite ore was digested with 70% HF until the combined (Ca + Ta)20s in the leach liquid reached 3 Ib/gal at which time it was diluted to 15N free acid and clarified by filtration. This solution was contacted countercurrently in the pulsed column where Ta and Cb were extracted by MIBK. Columbium was stripped from the organic with demineralized water which diluted the free acid in the solvent, making possible the transfer of all the Cb the Ta-loaded solvent was then stripped with demineralized water causing the transfer of Ta to the aqueous phase. The oxides were then precipitated with 28% ammonium hydroxide solution. Conversion to the respective oxides was by calcination of the precipitates. 

The process of separating the intermediate products from the purified solutions, in the form of solid complex fluoride salts or hydroxides, is also related to the behavior of tantalum and niobium complexes in solutions of different compositions. The precipitation of complex fluoride compounds must be performed under conditions that prevent hydrolysis, whereas the precipitation of hydroxides is intended to be performed along with hydrolysis in order to reduce contamination of the oxide material by fluorine. 

The second solution that results from the liquid-liquid extraction process is a high-purity niobium-containing solution. This solution is used in the preparation of niobium oxide, Nb205. The process is similar to the above-described process of tantalum oxide preparation and consists of the precipitation of niobium hydroxide and subsequent thermal treatment to obtain niobium oxide powder. 

Preparation of tantalum and niobium oxides based on the precipitation by ammonium solution of tantalum or niobium hydroxides from strip solutions is the most frequently used method in the industry and consists of several steps. Fig. 135 presents a flow chart of the process. 

The resulting product depends on the precipitation conditions, and in particular, on the over-saturation level of the solution. Formation of ammonium oxyfluorometalate crystalline compounds occurs at a relatively low pH of the solution. From the standpoint of the interactions described in Equation (143), this means that the interaction between NH4F and Me205 (denoted as interaction 1) is stronger than the interaction denoted as interaction 2. In this case, subsequent processes of the hydroxide treatment lead to some defluorination of the product, but the performance of such processes is usually very problematic. Precipitation at high pH values leads to a strong oversaturation of niobium- or tantalum-containing compounds, which in turn... 

Brown et al. [494] developed a method for the production of hydrated niobium or tantalum pentoxide from fluoride-containing solutions. The essence of the method is that the fluorotantalic or oxyfluoroniobic acid solution is mixed in stages with aqueous ammonia at controlled pH, temperature, and precipitation time. The above conditions enable to produce tantalum or niobium hydroxides with a narrow particle size distribution. The precipitated hydroxides are calcinated at temperatures above 790°C, yielding tantalum oxide powder that is characterized by a pack density of approximately 3 g/cm3. Niobium oxide is obtained by thermal treatment of niobium hydroxide at temperatures above 650°C. The product obtained has a pack density of approximately 1.8 g/cm3. The specific surface area of tantalum oxide and niobium oxide is nominally about 3 or 2 m2/g, respectively. 

Application of an excessive amount of ammonia solution in the precipitation of tantalum and niobium hydroxides from strip solutions usually ensures good quality of the products. Nevertheless, the method has two general problems. First, hydroxides containing low levels of fluorine contamination... 

The interaction described in Equation (148), in which C02 separates from the solution and ammonia hydroxide is formed, reduces the acidity of the solution causing precipitation of tantalum or niobium hydroxide. The hydroxide powder precipitated using ammonium carbonate is usually coarser and has better filtering properties. Changing the ammonium carbonate concentration and temperature of the solution allows some control over the particle size and filtering properties of the precipitated hydroxides. 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

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