Diluent effects, extraction

Organic extractants facilitate the transfer of the metal ions from the aqueous phase to the organic phase in solvent extraction. Based on the nature of the organic extractant, the metal ion, and the diluent, effective separation methods can be devised. Uranium extraction into diethyl ether from nitrate medium by salting out is perhaps one of the first uses of solvent extraction for large-scale actinide processing (9). In this case, ether not only acts as the diluent, it also acts as the extractant, which works according to the solvation mechanism (discussed below). 

Srinivasan, T.G., Vasudeva Rao, P.R., Sood, D.D. 1998. Diluent and extractant effects on the enthalpy of extraction of uranium(VI) and americium(in) nitrates by trialkyl phosphates. Solvent Extraction and Ion Exchange 16(6) 1369-1387. 

Previously, we saw that the extractants used for the extraction of strontium were DC18C6 derivatives or dicarbollide derivatives. Strontium can be effectively extracted with a synergistic mixture of dicarbollide and polyethylene glycols or crown ethers. A drawback of this process is the use of high-polarity diluents, such as nitrobenzene or chlorinated compounds, in order to solubilize the mixture of the extractants.127... 

These competing effects of third-phase formation with changing the chain length of both diluent and extractant can be understood together using the description of reverse micelles interacting through a sticky hard-sphere potential as shown by... 

Extraction of U(VI) and Pu(IV) from 1 to 8 mol L 1 HN03 solutions by radiolytically degraded DCH18C6 in toluene was studied (256). A decrease in the distribution ratios for both U and Pu was observed for irradiation in the range 0.010-0.071 MGy, with a higher effect for Pu(IV). For 0.2 mol I. DCH18C6-toluene solution in contact with 3 mol I. nitric acid, D, decreased from 0.21 to 0.12 and /.)hl from 64.3 to 6.42 after a dose of 0.07 MGy. This behavior was explained by both diluent and extractant degradation. 

Marcus, Y., Diluent effects in solvent extraction. Solv. Extr. Ion Exch., 1989, 7 567-575. 

With these mure basic extractants the nature of the diluent becomes quite important. Blumberg and Gai heve interpreted dilnent effects in terms of Lewis acidity and hasicity for extraction of mineral acids by tertiary amines, Frolov et al.7 interpreted diluent effects for extraction of acetic acid by amines in lerma... 

Mohapatra, RK., Lakshmi, D.S., and Manchanda, V.K. 2006. Diluent effect on Sr(II) extraction using di-tert-butyl cyclohexano 18 crown 6 as the extractant and its correlation with transport data obtained from supported liquid membrane studies. Desalination 198 166-172. 

Although effective extractants, both aliphatic amines and organophosphorous compormds proved toxic to microorganisms. To reduce their toxic action, these extractants are blended, in adequate proportion, with low toxidty organic solvents, thus allowing for their use in extractive fermentations. The effect of diluents on the distribution coeffident was different according to the extractant used, as observed by Choudhury and co-workers [96], while evaluating the extractive effidency of Ahquat 336 and trioctyl amine (TOA) in lactic add extractive fermentation. 

The character of the diluent in a solvent extraction separation scheme generally can have a profound effect on the overall degree of extraction, but the effect of diluent on separation factors is usually much more subtle. Alteration of the diluent modifies metal-ion extraction equilibria primarily by its effect on the solvation of the hydro-phobic metal complex. Although few recent studies have been made of diluent effects in lanthanide/actinide separation (with no truly systematic investigations), enough historical reports exist to make some general observations of the effect of diluent on lanthanide/actinide separation. 

Long-chain, aliphatic amines ate effective extractants for separation of carboxylic acids from dilute aqueous solution (Yang et al., 1991). Generally, the amine extractants are dissolved in a diluent, an organic solvent that dilutes the extractant. It controls the viscosity and density of the solvent phase. In order to improve the amine s solvation power, diluents such as oleyl alcohol, chloroform, methyl isobutyl ketone, and 1-octanol have been used. The diluents affect the basicity of the amine, the stabiUty of the acid amine complex formed and its solvation power. The pH of the aqueous phase is an important parameter for the reactive extraction of organic acids (Kahya et al., 2001). In the present study, various pure diluents are used for extraction of propionic acid from aqueous solution. On the basis of distribution coefficients, reactive extraction is also carried out with amine extractant for the recovery of propionic acid. 

However, when the contribution of the secondary nucleation term A is dominant (at low undercoolings), the diluent effect of the amorphous component must be taken into account, that is, an additional entropic effect to the nucleation barrier due to the reduced probability of extracting crystalline polymer sequences from the entangled melt at the growth front. Following the treatment of Boon and Azcue for a polymer-diluent mixture [35], the activation energy associated with the secondary nucleation in a miscible blend can be expressed as ... 

The use of an extractant depends on loading capacity, extraction rate, pH range, and the cost of the reagent and the diluent. Loss of the extractant must be minimised because of its high cost. Organic losses to the aqueous phase are also undesirable because of the deleterious effect on cathode deposits. Advances in SX—EW processes are described in Reference 38. 

The effects of different diluents on the extraction and stripping properties of a solvent can be determined by similar mixing tests as described above. 

The results of these bioassays on extraetants and modifiers are shown in Table 7.7a and for diluents in Table 7.7b. It is interesting to note the wide range of toleranee limits that are shown for the various extractants. Many show a toxic effect below their solubility level. Also, the toxicity of the Solvesso 150 and Isopar L diluents was acute compared with the other diluents examined. 

Much of the optimization of the solvent extraction plant can be achieved in the pilot plant testing. As noted earlier on the subjeet of proeess design, one must investigate the dependence of the dispersion and eoaleseence char-aeteristies and their effect on extraction and phase separation. Also, such variables as metal concentration, equilibrium pH (or free aeidity or free basieity), salt concentration, solvent concentration (extraetant, diluent, and modifier), and temperature have to be studied to determine their effect on mass transfer. Although many of the variables can be tested in the pilot plant, many circuits are not optimized until the full-scale plant is in operation. 

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