Extraction curves

Fig. 130 Isotherms of tantalum extraction (curve 1) and stripping (curve 2) at 25°C (after Agulyansky et al., [473]).

If P is insufficiently large to enable a quantitative separation to be made by pH control alone, the addition of a masking agent which forms a water-soluble complex more strongly with one metal than the other will shift the extraction curve for the former to a higher pH range with a consequent increase in P, which is now given by... 

Figure 5.8 The extraction curve for the N3P ligand between water and diethyl ether...

Such a plot is shown in Fig. 4.1b for the same system as in Fig. 4.1a. Percentage extraction curves are particularly useful for designing separation schemes. A series of such curves has already been presented in Fig. 1.3. 

Fig. 4.3 Extraction curves for various types of metal chelate complexes, when log Du is plotted against free ligand ion concentration, pA = -log[A ], or against [HA][H ]. From such plots, the general type of metal chelate complex may be identified (a) type MA , (see also Fig. 4.10) (b) type MA (OH)p(HA) (see also Figs. 4.14 and 4.30) (c) type MA (OH)p, (see also Fig. 4.19). (From Refs. 3a and 3b.)...

Note that the denominator only refers to species in the aqueous phase. Thus at constant [HA] ,g, the curvature of the extraction curve and its position along the [A ] axes is only caused by varying [A ], i.e., the aqueous phase complexation. The numerator refers to the organic phase species only, and is responsible for the position of the extraction curve along the D-axis thus at three different constants [HA] g, three curves are expected to be obtained, with exactly the same curvatures, but higher up along the D-scale with higher [HA] ,g concentration see Fig. 4.14. 

From plots of the distribution ratio against the variables of the system— [M], pH, [HA] , [B], etc.—an indication of the species involved in the solvent extraction process can be obtained from a comparison with the extraction curves presented in this chapter see Fig. 4.3. Sometimes this may not be sufficient, and some additional methods are required for identifying the species in solvent extraction. These and a summary of various methods for calculating equilibrium constants from the experimental data, using graphical as well as numerical techniques is discussed in the following sections. Calculation of equilibrium constants from solvent extraction is described in several monographs [60-64]. 

The studies described above have the purpose of identifying the reacting species in a solvent extraction process and developing a quantitative model for then-interactions. The fundamental parameter measured is the distribution ratio, from which extraction curves are derived. Solvent extraction work can still be carried out with simple batchwise (or point-by-point) technique, but continuous on-line measurements give faster and more accurate results. 

Each point-by-point experiment requires a complete set of mixing, separation, sampling, and analysis. This usually leads to scattered results, though it may not be critical, if the D values cover a limited range from 0.1-10. However, the more the D values deviate from 1, the more accurate must be the measurements also the number of points required for a reliable extraction curve usually increases. To reduce the uncertainty and labor involved with the batch technique, the stirred cell technique has become popular. 

Figure 14 illustrates a comparison between solvent extraction yields predicted from adsorption analyses and actual raffinate yields in commercial solvent extraction plants. Few commercial yields have ever equaled those obtained by adsorption analyses and none has exceeded those values hence, curve A (45° angle) represents the ultimate in solvent extraction. Curve B represents solvent extraction in commercial equipment on stocks ranging in viscosity index from +25 to +110, and for viscosity index improvements ranging from 30 to 130. Extremely viscous stocks or those which require a very large viscosity index improvement have been observed to follow more closely lines C and D. 

The rate of extraction can be increased significantly by using ethyl alcohol as entrainer. The extraction curves, in which the total extraction yields are plotted against the specific CO2-solvent-mass passed through the raw material, are shown in Figure 9.6-12. Increasing the alcohol concentration in CO2 from 0% to 10% significantly improved the solubility of phospholipids (phospholipid content of oils increased from 0.026 wt.% to 0.756 wt.%). The use of alcohol as a co-solvent also improved the functionality of protein materials [103]. 

Figure 9.6-12. Effect of alcohol-entrainer content on the extraction curves Extraction of y-linolenic acid...

It can be seen from Figure 11 that, by the appropriate choice of the equilibrium concentration of chloride ion in the aqueous phase, separations between certain pairs of metals can be made, for example between copper(II) and manganese(II) at a chloride concentration of 3.0 M, and between cobalt(II) and nickel(II) at a chloride concentration of 6 to 8 M. Furthermore, the metals can be stripped from the loaded organic phase by being contacted with an appropriate volume of water so that the equilibrium concentration of chloride ion in the strip liquor lies on the lower portion of the extraction curve, where substantial aquation of the extracted chlorometallate occurs... 

Thebaine, codeine and morphine from poppy straw (Papaver somniferum) were extracted with carbon dioxide and various polar modifiers at 20 MPa and 40.5 °C. Kinetic extraction curves for morphine showed that 50% methanol in carbon dioxide was necessary in order to achieve quantitative yields in less than 20 min. A mixture of 25% methanol, 0.22% methylamine and 0.34% water had the same effect as 50% methanol in the catbon dioxide. However, it was also reported that, in spite of its strong extraction power, the methylamine-water mixture had a major drawback in that morphine in the presence of the amine degraded in the presence of light. Hence, carbon dioxide-methanol-water mixtures were investigated increasing the water content in the extraction fluid dramatically enhanced the extraction rate for thebaine [29]. 

An INTERNET compatible database for solvent extraction of metal ions (SEDATA), developed by H. Watarai et al., contains about 9,600 equilibrium constants, including distribution constants, extraction constants, and adduct formation constants, for more than 1,400 ligands and 82 metal ions.50 Raw data points of extraction curves are also incorporated to be reconstructed as a figure. However, SEDATA contains no fields for 2D chemical structures of extractants and allows one to perform a search using only eight fields (classification, metal, valence, reagent, solvent, title of the paper, author, and year). 

Figure 11.10 Metal extraction curves for four metal ions by LIX 64N. The aqueous phase initially contained 1000 ppm metal as the sulfate salt [43]...

The developed method for analysis of dynamical vacuum-extraction curves allows determination of the binding energy and total volume of the traps of different nature, as well as the diffusion constant of hydrogen in the probe under study. 

In the process of analysis, a gradual heating of metal probe inside the extractor is made up to the extraction temperature 400-800°C. This temperature is always lower then the fusion temperature of the probe. The gases emitted in the probe heating are analyzed by a mass-spectrometer. Time dependence q(t) of the hydrogen flux is fixed by digital registration system in the form of extraction curve. Such extraction curve for pure aluminum A8 is shown in Fig. 2. 

Estimation of defect density and volume from extraction curve... 

The high sensitivity of the analyzer AV-1 and the representative statistics (about 30 thousand points on one curve) allows one to discern some maxima on the extraction curve. The maximum position and shape provides information about the binding energy and total volume of defects for individual peaks (e.g. see Fig. 2). 

Approximation of the experimental extraction curve by the calculated curve with a proper choice of the initial concentrations Cn, and constants uitD0i allows one to obtain the activation energy and diffusion constants for hydrogen diluted in the... 

Figure 9. Calculated extraction curve with three maxima.

Figure 10. Experimental extraction curve for the titanium alloy PT-7M.

As a result of treatment of experimental extraction curves, the range of activation energies of the traps for aluminum and aluminum alloys was determined. This range stretches from 0.2 eV to 0.8 eV. Consequently, one can suppose that there is no chemically bound hydrogen in the alloys. For the titanium alloys, the maximum activation energy equals 1.5 eV. 

The sharp maxima observed on some extraction curves (e.g. the peaks 1 and 2 in Fig. 12.) correspond to explosive character of hydrogen emission from the traps-defects. 

When supposing that hydrogen inside the defects is contained at barometric pressure the volume of defects can be determined from the peak areas. The number of hydrogen molecules corresponding to separate small peaks on the extraction curve is about 1011, that corresponds to the defect volume of about 10"6 mm3. For aluminum this volume corresponds to the volume of age dislocation. 

Correlation is found between the shape of extraction curve and the type of alloy. Experiments with probes of various shapes and masses show that the number of maxima on the extraction curve does not change. 

We have developed equipment, which allows obtaining information on the material structure from the hydrogen extraction curve in heating of a probe in vacuum. Accurate determination of the extraction curve provides information both on the hydrogen binding energy in metal and on concentration of spatial microtraps. 

The proposed calculations procedure allows one to approximate the experimental extraction curve and to determine the diffusion constant and activation energy for each peak of the curve. 

The approach to the study of properties of materials considered above does not require preliminary saturation of the studied probes with hydrogen. Natural hydrogen available in a metal carries information on pre-history of the material that will allow obtaining more complete information from the extraction curves in the further development of the method... 

Within the limits of experimental error, the effective diffusity increases with temperature and decreases with pressure as it is observed for molecular diffusivity. In general, the tortuosity factor was found to be constant at different flow rates and for different particle sizes. Figure 3 illustrates a comparison of predicted and experimental extraction curves for both small particles... 

Figure 3 Comparison of predicted and experimental extraction curves for both small and large particles using DCB as a solute and downflow operation P = 20 MPa, T = 313 K, Re = 40...

The pressure of the carbon dioxide (in the range of 90-150 bar) was found to have direct effect on the yield. Figure 4a shows the effect of the pressure on the extraction curves of ibuprofen. All four runs were performed at 38.5 °C. When the pressure was increased from 90 bar to 150 bar the carbon dioxide consumption was reduced to about 1/5 times. The enantiomeric excess of the extracts obtained from single extraction increased by 1.4 times for both acids (Figure 4b). 

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