Precipitation from solutions niobium hydroxide

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. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

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. 

Niobium(V) chloride. Hydrous niobium(V) oxide (0.75 g. Nb) is precipitated from acid solution by the addition of ammonium hydroxide, thoroughly washed by centrifugation with water (two 15-ml. portions), 0.5 M nitric acid (two 10-ml. portions) to remove adsorbed ammonium ion, and acetone (three 20-ml. portions) and vacuum-dried at room temperature. If the initial hydroxide precipitation is carried out from hydrofluoric acid solution, an appreciable quantity of the hydrous oxide may dissolve in the nitric acid washes, presumably because of the presence of traces of fluoride. However, reprecipitation and treatment as above reduces losses at this stage. The dried hydrous oxide is placed in a 40-ml. centrifuge tube fitted with a standard-taper outer joint, and 10 to 15 ml. of freshly distilled thionyl chloride is added slowly, since the initial reaction may be vigorous. The vessel is stoppered loosely, and the reaction is allowed to go to completion at room temperature (24 to 48 hours). Any traces of undissolved hydrous oxide, usually very small, and any yellow crystalline compound (see Discussion) are removed by centrifugation, and the penta-chloride is isolated by vacuum evaporation of the thionyl chloride at room temperature and pumping for several hours at 10 mm. If necessary, the product is further purified by vacuum sublimation in a sealed tube ( 150°). The yield, based on dried hydrous oxide, is 90 to 95%. Anal. Calcd, for NbCU Nb, 34.39 Cl, 65.61. Found Nb, 34.27 Cl. 

Two niobium chemicals are industrially used as feedstock to prepare the niobium metal niobium heptafluorotantalate (K NbF,) and niobium pentoxide (Nb Os)- Niobium hepta-fluorotantalate is obtained by adding potassium fluoride, KF, to the purified stripping solution in order to precipitate the insoluble crystals. The settled crystals of K NbF, are easily removed from the solution by centrifugation and filtration. Once separated, the crystals are dried. Niobium pentoxide is prepared by precipitation of niobium hydroxide, NbjOj.xHjO, by adding ammonia gas, NH, to the stripping solution containing niobium. The settled precipitate is then filtrated, washed with deionized water, dried, and calcinated, giving off water, to obtain the anhydrous niobium pentoxide. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

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 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... 

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. 

When a sample of the niobium (V) chloride-pyridine or niobium(V) bromide-pyridine reaction mixture was dissolved in dilute acid, filtered to remove precipitated niobium oxides, and treated with concentrated sodium hydroxide solution, the same set of spectra were observed for the resulting solution as for the l-(4-pyridyl) pyridinium dihalides. The spectra before and after heating the solutions from the reaction mixtures are shown in Figure 2. Here also the peaks occurred at 432 and 365 m/x, with the 432-m, peak absent after heating. 

Moore found that the hexavalent actinides U(VI) and Pu(Vl) could be quantitatively extracted from 1 M acetic acid solutions and 1 M acetic acid - 0.1 M nitric acid solutions by 5% TIOA-xylene. Of the fission products only Ru, Zr, and Nb extracted appreciably, and these could be scrubbed with 5 M HCl. A preliminary ferric (or uranyl) hydroxide precipitation in the presence of niobium carrier improved the decontamination from these elements. The U and Pu were leached from the insoluble NbgOg with 1 M acetic acid. The uranium could be stripped with dilute HNOg or HCl, NH OH or ammonium bicarbonate. The Pu(VI) could be stripped by these reagents or reductively stripped, since PuCIV) and Pu(in) do not extract under these conditions. 

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