INVESTIGATION OF THE PROCESS OF COPPER CEMENTATION WITH IRON FILINGS

S. A. Babenko, S. A. Pinigyn, R. I. Tasoev

(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)

Many enterprises of the radio-engineering industry, in the manufacture of printed electrical circuits, use a ferric chloride solution for etching copper. The spent etching solutions (SES) obtained as a result of etching contain large amounts of copper and iron. Usually, after neutralization with milk of lime, these solutions are discharged into a dump. In this process such a valuable metal as copper is lost, and the discharge of SES leads to environmental pollution. Therefore, solving the problem of utilizing SES is of great practical importance.

For the utilization of SES containing copper, a method is proposed for cementing copper with iron filings, followed by crystallization of the remaining solution of iron salts. The studies were carried out on SES having the following composition: $\mathrm{Cu}^{2+}$—60 g/l, $\mathrm{Fe}^{2+}$—96 g/l, $\mathrm{Fe}^{3+}$—70 g/l, pH—1.5–1.7. Preliminarily ground iron filings with a specific surface area of 15.3 cm²/g were used as the cementing agent.

Experiments on cementation were carried out as follows. Into a glass beaker placed in a thermostat, 250 ml of SES was poured and 33 g of iron filings was added (one-and-a-half-fold consumption relative to the theoretically necessary amount). Stirring was performed with a glass-blade stirrer at rotational speeds $n = 0$, $n = 500$ rpm, $n = 1750$ rpm. The temperature throughout the entire process was maintained at a specified level. At certain time intervals, samples of the solution were taken and analyzed for the content of copper and divalent iron. Copper was determined by the iodometric method, and iron by the bichromate method.

The experience of previously conducted studies [1–5] showed that most heterogeneous reactions, including copper cementation, proceed according to the laws of first-order reactions; therefore, to describe the kinetics of the process, formula [1] is applicable:

\[ K = \frac{2.3V}{\tau F} \lg \frac{C_0}{C}, \]

where $K$—rate constant;
$\tau$—time from the beginning of copper cementation;
$C_0$—initial copper concentration;
$C$—copper concentration at time $\tau$;

$F$ — surface area of the precipitant;
$V$ — volume of the solution.

To determine the quantitative dependence of cementation on temperature, experiments were carried out at a rotation speed of $n = 500$ rpm, at temperatures of 25, 30, 35, 40, 50, 60°C and cementation times of 30, 60, 90, 120 min.

On the basis of the experimental results a graph was constructed (Fig. 1). Since the change in the surface of the shavings during the cementation process is unknown, the graph was plotted in the coordinates $\lg v - 1000/T^\circ K$, where $v$ is the specific reaction rate calculated at a copper extraction degree of 50%, thereby ensuring an identical surface of the shavings participating in the reaction.

From the tangent of the angle of inclination of the curve, the activation energy of the process of copper cementation by iron shavings from OTR was calculated; it is equal to $E = 65.6$ kJ/mol. The temperature coefficient of the reaction rate in the temperature interval 25–35°C is equal to 2.352 and decreases at 60°C to 1.736. The high value of the activation energy and the large temperature coefficients indicate that the cementation process at $n = 500$ rpm proceeds in the kinetic region. As can be seen from Fig. 1, the rate of the cementation reaction increases sharply with increasing temperature, which makes it possible to regard it as one of the principal factors in intensifying the cementation process.

Fig. 1. Dependence of the logarithm of the rate of the cementation reaction on the reciprocal temperature

In the following series of experiments, the dependence of the reaction-rate constant on the concentration of hydrochloric acid was investigated (the concentration varied from 0.12 to 1 g/l) under the following conditions: temperature 25°C, $n = 500$ rpm, cementation time 30, 60, 90, 120 min.

Table 1

Cementation of copper from OTR by iron shavings at various concentrations of $\mathrm{Fe}^{3+}$

The graph shown in Fig. 2 indicates that, with an increase in the content of free hydrochloric acid, the rate increases.

cementation reaction. However, at a hydrochloric acid concentration above 0.32 g/l, the increase in the reaction rate decreases. It is known [4] that an increase in the concentration of free HCl above 15 g/l leads to a sharp decrease in the cementation rate, which is explained by polarization of the surface of the iron filings by hydrogen evolved during dissolution of the iron.

Fig. 2. Dependence of the cementation rate constant on the content of free hydrochloric acid in the solution

The study of the effect of changing the initial concentration of Fe³⁺ ions on the cementation rate was carried out under the following conditions: temperature 25°C, \(n = 500\) rpm, cementation time 30, 60, 90, 120 min, \(F^{3+}\) concentration—70, 80, 90 g/l. As can be seen from Table 1, changing the concentration of Fe³⁺ within the range from 70 to 90 g/l has no substantial effect on the cementation rate of copper.

Table 2

Results of experiments on copper cementation from OTR
at different numbers of stirrer revolutions

To determine the effect of mixing intensity on the cementation process, experiments were conducted at \(n = 0\), \(n = 500\) rpm, \(n = 1750\) rpm, cementation times of 30, 60, 90, 120 min, and a temperature of 25°C.

The data presented in Table 2 indicate that, with increasing mixing intensity, the cementation rate increases to a significant extent. This indicates a change in the region in which the process proceeds.

process. If, without stirring, the process experiences strong diffusion inhibition, then as the number of revolutions increases it decreases until it disappears completely. The high value of the activation energy and of the temperature coefficient confirm this.

Conclusions

1. The effect of temperature on the process of copper precipitation has been studied, and the dependence of the cementation rate on temperature has been established. As the temperature rises, the cementation rate increases significantly; therefore, the cementation process should be intensified by increasing the temperature.

2. The effect of the content of free hydrochloric acid on the copper cementation process has been studied. Increasing the HCl concentration to 1 g/l leads to a twofold increase in the cementation rate at a temperature of 25°C.

3. A change in the initial concentration of $\mathrm{Fe}^{3+}$ within the range from 70 to 90 g/l has no substantial effect on the copper cementation process.

4. The intensity of stirring of the solution has a substantial effect on the cementation rate.

5. On the basis of the experimental data obtained, the optimum conditions for the process should be considered to be raising the temperature of the OTR entering cementation to 60°C under intensive stirring $(\mathrm{Re}_m = 248000)$.

Literature

1. I. N. Plaksin, N. A. Suvorovskaya, O. K. Budnikova. On the theory of the precipitation of metals from solutions by metallic precipitants. Izv. AN SSSR OTN, 1948, No. 1, 131–138.

2. I. N. Plaksin, N. A. Suvorovskaya. Investigation of the kinetics of precipitation of metals from solutions. Izv. AN SSSR OTN, 1949, No. 3, 407–412.

3. A. S. Shakhov. Certain questions of the kinetics of the substitution reaction of copper and iron with metallic zinc in chloride solutions. Zhurnal prikladnoi khimii, 1936, Vol. VIII, Issue 4.

4. M. L. Episkoposyan, I. A. Kakovskii. Study of the kinetics of cementation of copper and silver by metallic iron from chloride solutions. Tsvetnye metally, 1965, No. 10, 15–20.

5. I. N. Plaksin, D. M. Yuksanov. Hydrometallurgy. Moscow, Metallurgizdat, 1949, pp. 128–146.