Stripping solutions

Fig. 9. Membrane extraction where the solvent phase is represented by hatched lines and the arrows show the direction of mass transfer, (a) Spherical film (b) emulsion globule where the strip solution is represented by circles and (c) hoUow fiber support.

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

Fluoroboric acid is used as a stripping solution for the removal of solder and plated metals from less active substrates. A number of fluoroborate plating baths (27) require pH adjustment with fluoroboric acid (see Electroplating). 

U02(C02) 3. The pregnant solution is concentrated and purified by ion exchange or solvent extraction, yielding a stripping solution of ca 50 kg/m U Og. Uranium is then precipitated chemically. Pure U Og is obtained by calcination (see Uraniumand URANIUM COMPOUNDS). 

The loaded organic phase is stripped of beryUium using an aqueous ammonium carbonate [506-87-6] solution, apparently as a highly soluble ammonium beryUium carbonate [65997-36-6] complex, (NH 4Be(C02)3. AU of the iron [7439-89-6] contained in the leach solution is coextracted with the beryUium. Heating the strip solution to about 70°C separates the iron and a smaU amount of coextracted aluminum as hydroxide or basic carbonate... 

Heating the ammonium beryUium carbonate solution to 95°C causes nearly quantitative precipitation of beryUium basic carbonate [66104-24-3], Be(OH)2 2BeC02. Evolved carbon dioxide and ammonia are recovered for recycle as the strip solution. Continued heating of the beryUium basic carbonate slurry to 165°C Hberates the remaining carbon dioxide and the resulting beryUium hydroxide [13327-32-7] intermediate is recovered by filtration. The hydroxide is the basic raw material for processing into beryUium metal, copper—beryUium and other aUoys, and beryUia [1304-56-9] for ceramic products. Approximately 90% of the beryUium content of bertrandite is recovered by this process. 

Another example of a cost-effective liquid-liquid extraction process is the one used for recoveiw of uranium from ore leach liquors (Fig. 15-3). In this case the solvents, alkyl phosphates in kerosine, are recovered by liquid-liquid extraclion using a strip solution, and the... 

The liquid membrane (thickness 0.2 cm) was separated from the aqueous solutions by two vertical cellophane films.The electrode compartments were filled with 0.05 M sulfuric acid solutions and were separated by the solid anion-exchange membranes MA-40. Binary mixtures contained, as a mle, 0.04 M Cu(II) and 0.018 M Pt(IV) in 0.01 M HCl. 0.1 M HCl was used usually as the strip solution. 

The copper(II) flux is directly proportional to the cuiTent density up to 10 mPJcrcf. The extraction degree of platinum(IV) into the strip solution is less than 0.1 % per hour of electrodialysis. About 55% of copper(II) is removed from the feed solution under optimal conditions. The copper(II) extraction process is characterized by high selectivity. Maximum separation factor exceeds 900 in the studied system. 

The copper(II) transport rate increases, as a rule, as Cu + initial concentration in the feed solution increases. The increase of the caiiier s concentration from 10 to 30 vol.% results in a decrease of both metal fluxes and in an increase of Cu transport selectivity. The increase of TOA concentration in the liquid membrane up to 0.1 M leads to a reduction of the copper(II) flux, and the platinum(IV) flux increases at > 0.2 M. Composition of the strip solution (HCl, H,SO, HNO, HCIO, H,0)does not exert significant influence on the transport of extracted components through the liquid membranes at electrodialysis. 

Air stripping Solution containing ammonia high pH Ammonia vapor in air... 

The products of the solvent extraction process are tantalum strip solution, niobium strip solution and raffinate - liquid wastes containing impurities and residual acids. 

Another application of tantalum strip solution is in the precipitation of potassium fluorotantalate, K2TaF7, which is used as a precursor in the production of tantalum powder by sodium reduction of melts. 

The raffinate from the selective extraction process contains mostly niobium. The tantalum extract is treated by steam stripping to obtain a tantalum strip solution. The method results in the effective separation and relatively high concentration of tantalum and niobium in the respective strip solutions. 

TBP is less soluble and less dangerous but is more problematic due to its relatively high density. This can lead to poor stratification, especially during the stripping process. Hence, insufficient separation of the strip solution from the extractant can lead to additional contamination, by phosphorous, of the final products [458,462]. 

The tantalum strip solution was used for the preparation, by precipitation and thermal treatment, of tantalum oxide. The product was determined to be of high purity grade. Table 62 presents typical analysis results. 

Stripping Stripping solution - water number of cells 6-7 Vorg.Vaqu = 3 1. 

Table 62. Typical purity of tantalum and niobium oxides prepared from strip solutions after extraction with 2-octanol. Impurity level is given in ppm.

Tantalum and niobium oxides, Ta2Os and Nb2Os, are among the final products obtained from tantalum and niobium strip solutions following liquid-liquid extraction processes. The strip solutions of tantalum and niobium consist of solutions of fluorotantalic and oxyfluoroniobic acids, H2TaF7 and H2NbOF5, respectively. 

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. 

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb205 content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH4)2NbOF5 in water and of Nb metal in... 

In the case of tantalum-containing solutions, a sharp drop in Ta205 concentration was observed also at pH > 10. The precipitated material was identified as a pure amorphous powder, which after appropriate thermal treatment was converted into tantalum oxide. Fig. 137 presents isotherms (20°C) of Ta205 concentration versus pH for solutions with compositions close to those of industrial strip solutions. 

Based on the above approach, the phases that are obtained from the strip solutions by precipitation with ammonium solution can be represented by the general formula xNH4F-yMe2C>5-zH20 or by the following interaction ... 

The way in which ammonia solution is added to tantalum or niobium strip solutions is also important for the quality of the precipitated hydroxides and final oxides. The traditional method by which ammonia is poured into a container of strip solution and the mixture agitated is not optimal. According to this method, the first portion of ammonia is added to a solution of high acidity, the pH of which continues to drop gradually with each addition of ammonia, until the final addition of ammonia is made into a low-acidity solution. This procedure leads to a relatively slow increase in pH that can cause contamination of the hydroxide with crystalline oxyfluoride compounds. 

The opposite process, i.e. pouring the strip solution into the ammonia solution, significantly reduces the fluorine concentration in the hydroxides formed. Bludssus et al. [495] developed a process comprising the introduction of tantalum- or niobium-containing acid solution to an ammonia solution until achieving pH = 9. It is reported that this method enables the production of tantalum or niobium hydroxides with fluoride contents as low as 0.5% wt. with... 

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 essence of the process is the precipitation of ammonium peroxotantalate or peroxoniobate by the addition of solutions of ammonia and hydrogen peroxide, H2O2, to appropriate strip solutions. The process was described in general terms as follows ... 

Nevertheless, the precipitation process can be specified based on data available on the complex structure of the strip solutions. In the case of tantalum strip solutions, the interaction can be written as follows ... 

In the case of niobium-containing strip solutions, the interaction takes place at a lower concentration of added ammonia solution ... 

The presence of ammonia ions in the solution causes no significant differences in the process (strip solution obtained after stripping by ammonium... 

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