PHYSICAL CHEMISTRY

V. B. TIKHOMIROV

ON THE MECHANISM OF EXTRACTION OF COLLOIDALLY PRECIPITATED IRON HYDROXIDE BY FATTY ACIDS

(Presented by Academician P. A. Rehbinder, March 25, 1961)

The mechanism of extraction of metals by fatty acids is usually explained by the formation, in the organic phase, of salts of fatty acids with metals and by exchange chemical reactions, this process being regarded as a typical example of liquid extraction \((^{1,2})\).

The investigations carried out made it possible to establish the existence of another mechanism for the extraction of metals by fatty acids, associated with the fixation in the organic phase of colloidally precipitated particles of hydroxides.

Fig. 1. Effect of pH on the extraction of \(\mathrm{Fe}^{+3}\) by oleic acid (1) and by a 0.05% solution of oleic acid in kerosene (2) from the aqueous phase with an initial iron concentration of 5 g/l.

Fig. 2. Dependence of iron extraction on the ratio between fatty acid (synthetic acids of the \(\mathrm{C}_7\)–\(\mathrm{C}_9\) fraction) and iron in moles of acid per mole of iron, pH of the initial solution 6.0. Designations as in Fig. 1.

Experiments were carried out, in particular, with \(\mathrm{Fe(OH)}_3\) hydroxide obtained by neutralizing an aqueous solution of the salt \((\mathrm{NH}_4)\mathrm{Fe(SO}_4)_2 \cdot 12\mathrm{H}_2\mathrm{O}\) with ammonia. The organic phase consisted of solutions of oleic acid or of synthetic fatty acids of the \(\mathrm{C}_7\)–\(\mathrm{C}_9\) fraction in kerosene.

Comparatively high extraction of iron was observed (Fig. 1) in those cases where minimal doses of fatty acids were introduced into the system, which were clearly insufficient for the chemical reaction of formation of the corresponding salts (0.01 mole of acid per 1 mole of iron). For comparison, the efficiency of the process using the pure acid is shown (30 moles of acid per 1 mole of iron).

Changing the molar ratios (Fig. 2) by decreasing the iron content (0.1–100 g/l) in the system when using a 10% solution of fatty acids in kerosene made it possible to establish the intervals of noticeable manifestation of the proposed extraction mechanism (in the form of a precipitate in the organic phase).

As the initial concentration of iron decreased, the amount of precipitate in the organic phase decreased, with a simultaneous increase in the iron content in the clarified organic phase (filtrate), which was conventionally taken to be equal to the amount of iron extracted with the formation of soluble chemical compounds.

The extraction of iron into the organic phase can be explained by the formation, on the surface of kerosene droplets, of adsorption layers, for example, of the type

\[ \begin{array}{cccc} \mathrm{A} & \mathrm{A} & \mathrm{A} \\ | & | & | \\ \mathrm{Fe} & \mathrm{Fe} & \mathrm{Fe} & \mathrm{Fe} \\ /\ \backslash & /\ | \ \backslash & /\ \backslash & /\ \backslash \\ & \mathrm{O} & \mathrm{OH} & \mathrm{O} & \mathrm{O} \end{array} \]

(where \(A\) is the acid anion), hydrated to one degree or another depending on the pH of the medium. The adsorption layers, having a polymer-chain structure, evidently also fix in the organic phase the colloidally precipitated hydroxide.

The precipitated hydroxide particles are thereby modified by adsorption layers, and the micelles form complexes with the anions of the fatty acid. The fundamental possibility of the extraction mechanism of iron described above is confirmed by the results of studies \((^3)\) devoted to the stabilization of emulsions by solid emulsifiers, in which P. A. Rebinder’s ideas about various types of disperse structures are developed. N. A. Aleinikov and A. M. Makarova \((^4)\), studying iron soaps, found that, depending on the pH of the medium, simultaneous formation of “acid” and “basic” soaps occurs. The same process should also be observed in the extraction, by fatty acids, of colloidally precipitated iron hydroxide. The ability of “basic” soaps to exchange anions can probably be used for extracting certain metals from the aqueous phase in an anionic form.

Received
21 III 1961

REFERENCES CITED

1. L. M. Gindin, P. I. Bobikov et al., DAN, 122, No. 3, 445 (1958).
2. R. W. West, T. G. Lyons, I. K. Carlton, Analyt. chim. acta, 6, 400 (1952).
3. A. F. Koretskii, Candidate’s dissertation, Institute of Physical Chemistry, Academy of Sciences of the USSR, 1960.
4. N. A. Aleinikov, A. M. Makarova, DAN, 124, No. 4, 852 (1959).