Extraction DEHPA

Type 2 facilitation is also known as carrier facilitated transport, since a carrier compound, that is, an extractant or complexing agent, solubilized in the organic phase is used to assist transfer across the membrane. In this simation, the solute of interest reacts with the carrier to form a complex that is only soluble in the membrane phase. The solute is de-complexed by a stripping solution contained in the internal phase. An example of such a process is the removal of a metal ion such as copper or zinc from wastewater by the extractant DEHPA (di-2-ethyUiexyl phosphoric acid, represented as HE) as shown in Figure 25.2. In this case, the carrier also enhances the selectivity as most extractants are specifically designed to extract particular metal ions... 

Perez de Ortiz, E.S., Dias Lay, M. De L., Gruentges, K., Aluminium and iron extraction by DNNSA and DNNSA-DEHPA reverse micelles, Int. Solvent Extraction Conf. (ISEC 96) Value adding through solvent extraction, Ed. Shallcross, D.C., Paimin, R. Prvcic, L.M., Melbourne, Australia, pp.409-411, 1996. 

The extractant is di(2-ethylhexyl) phosphoric acid (DEHPA) in conjuction with trioctyl phosphine oxide (TOPO). Stripping is by ammonium carbonate, and uranium precipitates as ammonium uranyl tricarbonate. The mixture shows a synergistic effect. The mixture is stable, and extracts uranium in hexavalent state. 

The development of more benign alternatives to cyanide for gold-leaching (see Section 9.17.3.1) such as thiourea, thiocyanate, or thiosulfate, which form stable complexes in water has prompted research to identify suitable solvent extractants from these media. Cyanex 301, 302, 272, Ionquest 801, LIX 26, MEHPA, DEHPA, Alamine 300 (Table 5) have been evaluated as extractants for gold or silver from acidic thiourea solutions.347 Whilst the efficacy of Cyanex 301 and 302 was unaffected by the presence of thiourea in the aqueous feed, the loading of the other extractants is severely depressed. Formation of solvated complexes of gold and of an inner-sphere complex of silver has been proposed.347... 

A distribution isotherm is then constructed by plotting the metal concentration in the organic phase against the concentration in the aqueous phase, as a function of the phase ratio. An example of such an isotherm is shown in Fig. 7.1, for the extraction of nickel by DEHPA(Na) at pH 6, showing three different concentrations of extractant [1]. 

The solubility of the sodium salt of DEHPA in basic (NaOH) solution has been reported, together with the effect of temperature on the water solubility of this salt [18], (Figs. 7.12 and 7.13). It is evident that the presence of salts in the aqueous phase depresses the solubility of this extractant in water (Fig. 7.11). This has been confirmed in the extraction of cobalt with DEHPA(Na) at pH 5-6, for which a solubility of the extractant was found to be <50 ppm. Furthermore, the use of DEHPA in the extraction of cobalt from an ammoniacal (pH 11) system containing sodium sulfate showed no apparent loss of extractant after 10 contacts of a DEHPA-ker-osene solvent with fresh aqueous solution [1]. Operation of pilot plants using DEHPA(NH4) and DEHPA(Na) for the extraction of cobalt, at pH 5-6 and at 60°C, showed the loss of DEHPA to be less than 50 ppm [3]. Temperature also has a significant effect on the solubility of DEHPA(Na) (Fig. 7.13). 

In the extraction and separation of zirconium from hafnium in a nitric acid system, using TBP, the system operates best if run at about 10% less than saturation [56]. As saturation of the solvent is approached, a zirconium compound precipitates in the presence of the solvent, causing cruds and emulsions. This problem is also encountered in rare earth circuits using DEHPA. 

The leach liquor is first treated with a DEHPA solution to extract the heavy lanthanides, leaving the light elements in the raffinate. The loaded reagent is then stripped first with l.Smoldm nitric acid to remove the elements from neodymium to terbium, followed by 6moldm acid to separate yttrium and remaining heavy elements. Ytterbium and lutetium are only partially removed hence, a final strip with stronger acid, as mentioned earlier, or with 10% alkali is required before organic phase recycle. The main product from this flow sheet was yttrium, and the yttrium nitrate product was further extracted with a quaternary amine to produce a 99.999% product. 

Fig. 11.20 Effect of DEHPA concentration on extraction of lanthanides. (From Ref. 2.)... 

The rate of extraction of the lanthanides by the 2-ethylhexyl ester of 2-ethylhexylphosphonic acid (e.g., PC88A, P507) is slower than for DEHPA but the higher distribution coefficients allow extraction from more dilute solutions. Also, as acidic stripping is easier than with DEHPA, the reagent has been used commercially to separate lutetium from terbium and ytterbium. 

Other applications of supported liquid membranes have been related to metal speciation. For example, recently a system for chromium speciation has been developed based on the selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two SLM units connected in series. Aliquat 336 and DEHPA were used respectively as carriers for the two species and graphite furnace atomic absorption spectrometry used for final metal determination. With this process, it was possible to determine chromium in its different oxidation states [103]. 

For example, the extraction of nickel with di-(2-ethylhexyl)phosphoiic acid (DEHPA) at a pH below 3 is <10%. If the extraction of nickel is carried out between pH 3-4, the extraction of about 0.3gdm Ni will decrease the pH to below 3 and the mass transfer of nickel will stop. For copper, the extraction is performed between pH 2-3 and thus, an amount of 3 g dm Cu will decrease the pH from 3 to 2 and again extraction will stop. 

An increase of the net transfer of zinc was reported [9] for the extraction with DEHPA (50%) in kerosene of zinc from a feed solution... 

The nickel extraction can be performed with DEHPA under defined conditions. As described earlier for zinc, the extraction of nickel is pH dependent. The Z)-value for nickel decreases drastically with decreasing pH below 3.5. The extraction of each nickel ion (Ni ) liberates two hydrogen ions (H ) from the extractant H DEHPA, which means that only a very small amount of nickel can be transferred to the solvent before the extraction stops. However, by adding a neutralization reagent (NaOH) into the... 

Zinc is extracted and stripped with DEHPA in the conventional way as previously described. The major challenge lies in the extraction and stripping of Cr(III), which requires a higher pH than zinc. Chromium... 

The extraction of vanadium is performed with a mixture of DEHPA and TBP in kerosene (Fig. 14.8). Half of the raffinate, with a somewhat increased acidity, is returned to leaching. The other half is further treated to... 

In this process [27], the metals are extracted successively by the same extractant, employing suitable pH values for each metal. The spent catalyst is first roasted in an air stream at 300°C to remove oil and sulfur deposited on the surface of the catalyst. Then the metals are dissolved in an ammonium carbonate leach liquor at 80°C. Finally, the three metals are isolated and purified from one another using solvent extraction with an alkylmono-thiophosphoric acid (MTPA) with a structure similar to DEHPA. The following flow sheet was suggested (Fig. 14.19) ... 

Berthod et al. [325] employed countercurrent chromatography with diethyl-hexyl phosphoric acid (DEHPA) reverse micelles in heptane as a stationary phase to extract metaUic cations such as Fa +, Ce, Pr, and Nd + (lanthanide series). This technique was suggested for the application of ion filtering and concentration or for deionization of aqueous phases. Ashrafizadeh et al. [326] re-... 

Synergistic extraction, in the system DEHPA and TOPO, is quite interesting for its ability to extract uranium from high concentrations of phosphoric acid (38). This finds application in the recovery of uranium from dilute phosphoric acid medium in... 

Tachimori, S. 1979. Synergistic extraction of americium with MEHPA-DEHPA mixed solvent from nitric acid solution. J. Radioanal. Chem. 49(1) 31-35. 

In the mid-1960s liquid—liquid extraction processes were introduced and today all large-scale commercial production is done in this way. An aqueous solution of the Ln3+ ions is extracted in a continuous countercurrent process into a nonpolar organic liquid containing tri-n-butylphosphine oxide or bis(2-ethylhexyl)phosphinic acid (DEHPA). Typical separation factors for adjacent rare earths using DEHPA are 2.5 per extraction step so that under automatic multistep or countercurrent conditions purities of 99 to 99.9% are routinely achieved. 

BIS(2-ETHYLHEXYL)ORTHOPHOSPHORICACID BIS(2-ETHYLHEXYL)PHOSPHORlC ACID DEHPA EXTRACTANT DI(2-ETHYLHEXYL)PHOSPHATE DI-2(ETHYLHEXYL)PHOSPHORIC ACID DI-(2-ETHYLHEXYL)PHOSPHORIC ACID (DOT) 2-ETHYL-l-HEXANOL HYDROGEN PHOSPHATE HDEHP KYSELINA DI-(2-ETHYLHEXYL)FOSFORECNA... 

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