PHYSICAL CHEMISTRY

Yu. P. NIKITIN and O. A. ESIN

ON THE KINETICS OF ION EXCHANGE BETWEEN METAL AND SLAG

(Presented by Academician A. N. Frumkin, April 11, 1958)

The rate of ion exchange between liquid metals (Fe—C, Fe—Si, Fe—P, Ag) and molten slags was studied by the method described earlier ((^1)). From the values found for the diffusion resistance (R_d), the diffusion coefficients (D) of iron and silver ions were estimated using the relation ((^2)):

[
R_d=\frac{RT}{n^2F^2}\frac{1}{C}\sqrt{\frac{2}{D\omega}},
\tag{1}
]

where (C) is the concentration of the potential-determining ions in the slag, (\omega) is the angular frequency of the current, (R) is the gas constant, (T) is the absolute temperature, (F) is Faraday’s constant, and (n) is the charge of the ion.

It turned out that, for slags containing 31% CaO, 54% SiO(_2), and 15% Al(_2)O(_3), the diffusion coefficients of iron ions at 1500°C lie in the range from (2.4) to (3.1\cdot 10^{-6}\ \text{cm}^2\cdot\text{s}^{-1}). The values obtained are close to that previously found ((D=3.5\cdot 10^{-6})) at the same temperature by the radioactive-isotope method ((^3)).

The diffusion coefficient of silver ions in molten sodium borate (15% Na(2)O, 85% B(_2)O(_3)) was found to be (0.6\cdot 10^{-7}) at 840° and (1.42\cdot 10^{-7}\ \text{cm}^2\cdot\text{s}^{-1}) at 940°. Hence the activation energy of the diffusion process, calculated from the usual exponential equation, is 23 kcal/g-atom. The smaller values of (D) are apparently due to the relatively high viscosity of the medium and the low temperatures.}^+}) compared with (D_{\mathrm{Fe}^{2+}

From the values of the reaction resistance (R_r), the exchange currents (i_0) were calculated using the expression ((^2)):

[
i_0=\frac{RT}{nF}\cdot\frac{1}{R_r}.
\tag{2}
]

For iron alloys with carbon, silicon, and phosphorus, and for slags containing CaO, SiO(_2), Al(_2)O(_3), Na(_2)O, B(_2)O(_3), P(_2)O(_5), and small concentrations of FeO and Fe(_2)O(_3), an almost rectilinear dependence was found between (i_0) and the total percentage of iron oxides (see Fig. 1). A similar dependence, at low concentrations of potential-determining ions, has been observed for a number of metals in aqueous and organic solutions ((^{2,4,5})). As is known ((^2)), this fact indicates that ion discharge is the stage determining the rate of exchange.

Introduction of Na(_2)O into the slag leads to an increase in the concentration of FeO in it and to an increase in the exchange current. This is consistent with previously obtained results, according to which the addition of Na(_2)O increases the density of negative charge on Fe—C and Fe—P electrodes ((^6)) and promotes a decrease in the interfacial tension at the boundary under consideration ((^7)).

Measurements carried out for liquid cast iron (4.3% C) and a slag with 31% CaO, 54% SiO(_2), and 15% Al(_2)O(_3) showed that, at a temperature of 1350°C and an iron-ion concentration of 0.36%, the exchange current was 22 ma/cm(^2); at 1550°

and at a concentration of 0.52% it was 50 ma/cm². Hence, taking into account the linear dependence of (i_0) on (C), we find the activation energy of reaction ((E_1)):

[
\mathrm{Fe}\,(\text{cast iron}) = \mathrm{Fe}^{2+}\,(\text{slag}) + 2e,
\tag{a}
]

equal to 23.5 kcal/g-at, and for the reverse process (E_2 = 13) kcal/g-at. Such a relation between (E_1) and (E_2) is apparently due to the fact that the carbon of the metallic alloy creates sharply reducing conditions at the interphase boundary. The influence of changes in the potential jump with temperature and with the concentration of ions ((^8)) on the magnitude of the activation energy ((^9)) has not yet been investigated by us.

For a silver electrode in oxide melts, the results presented in Table 1 were obtained. They make it possible to estimate the activation energy for the direct ((E_1))

[
\mathrm{Ag}\,(\text{metal}) = \mathrm{Ag}^{+}\,(\text{slag}) + e
\tag{b}
]

and reverse ((E_2)) exchange processes. They are (E_1 = 12.8) and (E_2 = 22.8) kcal/g-at. Such a relation of the values (E_1) and (E_2) for silver seems unusual and requires further investigation. It is possible that it is caused by the strong bond of the (\mathrm{Ag}^{+}) cations with the anions of the borate melt.

The values of the double-layer capacitance are approximately the same in all three cases and are close to those obtained earlier for Fe—C. This suggests that on the surface of silver in contact with borate there is also an excess of negative charge.

Fig. 1. Dependence of the exchange current between iron alloys and slags on the concentration of iron ions in the slag

It is noteworthy that, despite the high temperature and the relatively high concentration of potential-determining iron ions in the slag, the values of (i_0) in the melts studied (Fe—C, Fe—Si, Fe—P) are close to those observed in aqueous solutions. Apparently, iron ions are bound to the slag more strongly than metal cations are to an aqueous solution. In addition, silicon, carbon, and phosphorus are concentrated at the interphase boundary ((^1)), which leads to depletion of the metal surface in iron atoms, i.e., to a kind of isolation of the latter from the slag. A similar effect of surface-active substances on the magnitude of the exchange current (a decrease in it) was observed in aqueous solutions ((^4)).

The foregoing is also supported by the extremely low resistance of the reaction between cast iron (4.3% C) and a slag containing calcium carbide (10.8% CaC₂, 55% CaO, 19.8% SiO₂, 3.6% Al₂O₃, 10.8% MgO, and 0.143% Fe) at a temperature of about 1500°. Probably, in this case the potential is determined by the transition not of iron, but of carbon:

[
2\mathrm{C}\,(\text{metal}) + 2E = \mathrm{C}_{2}^{2-}\,(\text{slag}).
\tag{c}
]

High concentrations of it in the surface layer of cast iron and slag promote rapid ionic exchange and sharply increase (i_0). The capillary activity of (\mathrm{CaC}_2) at such an interface follows from the fact of a decrease in the interphase tension ((^{10})).

In this connection one may think that, for pure iron and other metals, the exchange current with the slag must be large at high temperatures. This is also indicated by a comparison of the values of (i_0) for Ag and Fe—C. Their exchange currents are approximately the same, despite the lower temperature and the lower concentration of (\mathrm{Ag}^{+}) ions in the slag.

Ural Polytechnic Institute named after S. M. Kirov
Sverdlovsk

Received
15 III 1958

CITED LITERATURE

(^{1}) Yu. P. Nikitin, O. A. Esin, DAN, 116, 63 (1957).
(^{2}) B. V. Ershler, K. I. Rozental’, Proc. Conf. on Electrochemistry, Publishing House of the USSR Academy of Sciences, 1953, p. 446.
(^{3}) E. S. Vorontsov, O. A. Esin, Izv. AN SSSR, OTN, No. 3 (1958).
(^{4}) V. A. Pleskov, N. B. Miller, Proc. Conf. on Electrochemistry, Publishing House of the USSR Academy of Sciences, 1953, p. 165.
(^{5}) H. Gerischer, K.-E. Staubach, Zs. Phys. Chem., 6, 118 (1956).
(^{6}) O. A. Esin, Yu. P. Nikitin, Proc. Conf. on the Physicochemical Foundations of Steel Production, Publishing House of the USSR Academy of Sciences, 1957, p. 446.
(^{7}) O. A. Esin, ZhFKh, 30, issue 3 (1956).
(^{8}) A. N. Frumkin, V. S. Bagotskii, Z. A. Iofa, B. N. Kabanov, Kinetics of Electrode Processes, Moscow, 1952.
(^{9}) M. I. Temkin, Proc. Conf. on Electrochemistry, Publishing House of the USSR Academy of Sciences, 1953, p. 181.
(^{10}) S. I. Popel’, O. A. Esin, Yu. P. Nikitin, Collected Works of the Ural Polytechnic Institute named after S. M. Kirov, Sverdlovsk, No. 49, 82 (1952).