METHODS OF PROCESSING ETCHING SOLUTIONS

S. A. Babenko, V. P. Pishchulin

(Presented by the scientific seminar of the Department of PMAChP KhTF)

Etching solutions have the composition: Fe$^{3+}$ — 70 g/l, Fe$^{2+}$ — 96 g/l, Cu$^{2+}$ — 60 g/l. Such etching solutions are formed as a result of the interaction of a ferric chloride solution (specific gravity 1.36–1.40) with copper in the process of etching printed-circuit boards.

An analysis of studies and of the practice of processing etching and analogous copper-containing solutions shows that copper utilization can be carried out by cementation, by the ammonia method, and by evaporation. The detoxification of the spent solution is most simply achieved by neutralization.

The study of cementation was reduced to finding the optimal cementation conditions as a function of the consumption of iron shavings, temperature, and process time. The research procedure was as follows. A spent etching solution in an amount of 300 ml was poured into a beaker, which was placed in a thermostat where a specified temperature was maintained. Iron shavings were added to the beaker in amounts 1.5 times and more exceeding the stoichiometrically necessary amount for the reduction of copper and trivalent iron. At specified time intervals a sample was taken from the beaker, and in it the content of copper and of divalent and trivalent iron was determined.

Preliminary experiments established that at room temperature (18–20°C) the deposition of copper on iron shavings proceeds slowly even when their amount is considerable. Thus, even a tenfold excess of shavings over the stoichiometric amount reduced the copper content in the etching solution from 60 to 3.0 g/l over the course of 24 hours. Only a one-and-a-half-fold excess and fractional addition of iron shavings over four days (50, 30, 14, 6% of the total amount of shavings on each day) converted 96% of the copper from the solution into cement copper. Even in this case, 2.4 g/l of copper still remained in the solution.

Further studies were carried out with a one-and-a-half-fold excess of iron shavings over the stoichiometric amount. The effect of temperature on the yield of copper from the solution was studied. The experimental results (Fig. 1) show that as the temperature of the solution increases, the rate of cementation, i.e., the reduction of copper on iron shavings, increases sharply. Thus, at a temperature of 20°C, even after 24 hours less than half of the copper contained in the solution was reduced. Heating to 40°C accelerates the process. After 7 hours of interaction, 8.1 g/l of copper remained in the solution; at 60°C a further decrease to 1.5 g/l occurred. Heating to 80°C made it possible, within three hours, to precipitate on

![Graph: residual copper concentration versus cementation time. Vertical axis: “Copper content, g/L”; horizontal axis: “τ, h”. Curves are labeled 1–4.]

Fig. 1. Effect of temperature and cementation time on the residual concentration of copper in the etching solution (1—20°C, 2—40°C, 3—60°C, 4—80°C)

in the turnings 99.87% of all the copper contained in the spent solution and to reduce its amount in the liquid phase from 60 to 0.8 g/L. More complete separation of copper from the solution upon further continuation of the process (temperature 80°C) does not occur.

It should be noted that cementation copper is readily separated from the surface of the iron turnings. A further increase in temperature is inadvisable, since evaporation of the solution increases sharply.

The solution obtained as a result of cementation contains Cu²⁺—0.8 g/L, Fe³⁺—4.1 g/L, Fe²⁺—241.9 g/L, and in this composition it will find no practical application. Therefore, along with the separation of copper by cementation, it is expedient to treat the resulting solution, containing ferrous iron, with chlorine in order to carry out the reaction

\[ 2\mathrm{FeCl}_2+\mathrm{Cl}_2=2\mathrm{FeCl}_3. \]

On the basis of the experiments carried out, a solution was obtained containing Fe³⁺—196 g/L and Fe²⁺—13 g/L, with a specific gravity of 1.46, that is, meeting the requirements imposed on an etching solution for processing boards. The chlorine consumption was 190 g per 1 L of solution.

Under laboratory conditions, the etching solution was processed with ammonia water with recovery of iron and copper. The spent solution was neutralized with 25% NH₄OH; in this process iron hydroxides precipitated, while copper, in the form of a copper–ammonia complex, passed into solution. After separation of the precipitate on a filter, the solution was evaporated. During evaporation, using a dephlegmator and a condenser, it is possible to separate the vapors of water and ammonia into water and an NH₄OH solution, which can be used again for neutralization of the spent etching solution.

As a result of the studies carried out, it was found that for processing 1 L of etching solution, 1.75 L of 25% NH₄OH is required, and 0.75 L of water for washing the precipitate. In this case, 415 g of iron hydroxide and 195 g of copper salts were obtained, which can be used as secondary raw material, as well as 0.9 L of ammonia water returned to the head of the process. Thus, ammoniacal treatment of the etching solution makes it possible to separate copper chloride from iron chloride and to obtain copper and iron compounds in the form of solid products convenient for transportation.

Detoxification and utilization of the spent etching solutio-

... is possible by concentrating it through evaporation. After condensation, the secondary steam has only a weakly acidic reaction and contains no impurities; therefore it may be discharged into the sewer together with the wash waters after their neutralization. The chlorides of iron and copper, in the form of crystalline hydrates, are contained in 1 liter of solution in an amount of 845 g. Since the density of the spent solution is 1400 g/dm³, in order to isolate the indicated compounds in solid form it is necessary to remove 555 g of water from each liter of solution. For evaporation, electric heating with alternating current was used in an apparatus with graphite electrodes. As a result of the investigations carried out, all the necessary data were obtained for calculating an industrial apparatus with direct electric heating.

After evaporation, the mixture of salts cannot be used as a secondary raw material. To determine the area of their application, special studies should be conducted. From the literature data [1] it is known that ferric chloride is successfully used for strengthening soils. For this purpose, it will probably also be possible to use the iron and copper chlorides obtained as a result of evaporation.

The method of studying the neutralization of spent solutions was reduced to finding the optimum consumption of reagents for converting the chlorides of iron and copper into compounds insoluble in water. It was found that, to neutralize 1 liter of spent solution, it is necessary to expend 0.35 kg of NaOH, 0.55 kg of Na₂CO₃, or 0.40 kg of CaO. Owing to the addition of the above-mentioned reagents, a precipitate is formed, the volume of which after one hour of standing is 44% for Na₂CO₃, 75% for caustic soda, and 80% for lime milk.

A comparison of the proposed methods for processing pickling solutions in terms of economy, technological complexity, and safety makes it possible to recommend, for industrial implementation, cementation in the case of copper recovery, and, for detoxification, evaporation by direct electric heating or neutralization with lime milk.

REFERENCES

1. G. F. Bogdanov. On the question of strengthening soils with iron humates. In: Soil Science and Engineering Geology, Publishing House of Leningrad University, 1964.