APPLICATION OF DIRECT ELECTRIC HEATING FOR EVAPORATION OF PICKLING SOLUTIONS

V. P. Pishchulin

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

Pickling solutions containing 14.9% FeCl₂, 14.6% FeCl₃, and 9.06% CuCl₂, formed as a result of the interaction of a ferric chloride solution with copper during the pickling of boards, are wastes; at present they are neutralized with lime and taken to a dump, contaminating groundwater.

Evaporation of pickling solutions leads to the release of copper and iron chlorides in the form of solid crystalline hydrates; for this purpose it is necessary to evaporate about 560 g of water from each liter of pickling solution. Thus, evaporation yields the solid salts FeCl₂·4H₂O, FeCl₃·6H₂O, and CuCl₂·2H₂O, which can be used for soil stabilization, and clean water as a result of condensation of the secondary steam, which can be used for the needs of the enterprise or discharged into the sewer together with rinse waters [1].

The specific nature of the enterprise, as well as the features and advantages of direct electric heating—such as the high intensity of heat supply, absence of heat transfer, high coefficient of utilization of electrical energy, flexible and fine regulation of heat release, simplicity of the device and compactness of the installation, and high production culture—contributed to the choice of alternating electric current of industrial frequency as the heat carrier [2].

This report presents the results of a study of the electrophysical properties of the system electrode—pickling solution—electrode, necessary for the calculation and design of an electrode-type evaporator. The specific electrical conductivity of the pickling solution during the evaporation process, the limiting voltage drops in the system electrode—pickling solution, the specific limiting resistances, boiling temperatures, and the composition of the solution were determined.

The studies were carried out in a measuring cell with parallel-plate electrodes. Electrode graphite was chosen as the electrode material, since electrode graphite has high electrical conductivity, high corrosion resistance, and is readily machined. A specified volume of the solution under study was poured into the measuring cell. The electrodes were fixed in the cell at a specified distance. Alternating electric current was supplied to them through an ammeter, a voltmeter, and an autotransformer. At established electric current density and solution temperature, the total voltage drop of the electric current across the cell was measured,

and also, using a probe and a high-resistance IV-4 voltmeter, the voltage drop of the electric current in the solution between the electrodes was determined. The difference between the values of the total voltage drop across the cell and the voltage drop across the solution, divided in half, represents the limiting voltage drop at one electrode. The specific limiting resistance was calculated as the quotient obtained by dividing the limiting voltage drop by the current density. The results of the study are given in Table 1.

Table 1

Effect of current density, temperature, and concentration of the pickling solution on the limiting voltage drop and specific limiting resistance

As can be seen from the data obtained, the boiling temperature of the initial pickling solution, containing 343 g/l FeCl₂·4H₂O, 341 g/l FeCl₃·6H₂O, and 161 g/l CuCl₂·2H₂O, is 105°C, while that of the evaporated solution, consisting only of crystalline hydrates of iron and copper chlorides—770 g/l FeCl₂·4H₂O, 766 g/l FeCl₃·6H₂O, and 362 g/l CuCl₂·2H₂O—is 120°C. The limiting voltage drops increase in direct proportion with increasing current density and decrease with temperature. Thus, for example, when the current density is increased fourfold from 0.25 to 1 A/cm², the limiting voltage drops also increase fourfold in the temperature range studied; while at a current density of 0.75 A/cm², as the temperature is increased from 35°C to the boiling temperature of 105°C, the limiting voltage drops decrease from 1.90 to 0.50 V. The specific limiting resistance is practically independent of current density in the investigated range of current densities and decreases with increasing temperature. For the evaporated pickling solution, consisting practically of crystalline hydrates of copper and iron chlorides, at a boiling temperature of 120°C the specific limiting resistance is 0.2 ohm·cm². The evaporated

the solution at the boiling temperature is a mobile liquid which, upon cooling, crystallizes completely.

Using the data of Table 1, the specific electrical conductivity was calculated, since the entire solution is located between plane-parallel electrodes placed exactly opposite one another. The values of the specific electrical conductivity were calculated from the equation

\[ \chi=\frac{i\cdot l}{\Delta U_p}, \tag{1} \]

where \(\chi\) is the specific electrical conductivity of the solution, \(\Omega^{-1}\cdot\text{cm}^{-1}\);
\(i\) is the current density, \(\text{A}/\text{cm}^2\);
\(l\) is the distance between the electrodes, cm;
\(\Delta U_p\) is the voltage drop across the solution, V.

The calculated values of the specific electrical conductivity are given in Table 2.

Table 2

Values of the specific electrical conductivity of the pickling solution

Note. Composition of the solution in experiments 1, 2, 3, 4 — \(\mathrm{FeCl_2\cdot4H_2O}\) — 343 g/l, \(\mathrm{FeCl_3\cdot6H_2O}\) — 341 g/l, \(\mathrm{CuCl_2\cdot2H_2O}\) — 161 g/l. Composition of the solution in experiment 5 — \(\mathrm{FeCl_2\cdot4H_2O}\) — 770 g/l; \(\mathrm{FeCl_3\cdot6H_2O}\) — 766 g/l; \(\mathrm{CuCl_2\cdot2H_2O}\) — 362 g/l.

As the calculated data showed, the specific electrical conductivity increases linearly with temperature and can be determined with an accuracy of \(\pm 2.5\%\) from the equation

\[ \chi_t=\chi_{20^\circ}+0.153\cdot10^{-2}(t-20), \tag{2} \]

where \(\chi_{20^\circ}\) is the specific electrical conductivity of the solution at \(20^\circ\text{C}\), equal to \(0.98\ \Omega^{-1}\cdot\text{cm}^{-1}\);
\(t\) is the temperature of the solution, °C.

On the basis of the data obtained, a technological scheme was proposed for utilization of pickling solutions by evaporation in electrode evaporators; the optimal electric-current density was selected as \(1\ \text{A}/\text{cm}^2\), the electric-field strength as \(6\ \text{V}/\text{cm}\), and the optimal temperature of the evaporated solution at the outlet from the apparatus as \(120^\circ\text{C}\); a calculation of the evaporator was carried out.

Conclusions

1. The possibility of evaporating pickling solutions in evaporators with direct electric heating has been investigated.

2. The electrophysical properties in the electrode—pickling solution—electrode system, necessary for calculating evaporators, have been determined.

3. The optimal conditions for carrying out the evaporation process using direct electric heating have been found.

References

1. S. A. Babenko, V. P. Pishchulin. Ways of processing pickling solutions. Izvestiya TPI, vol. 259, Tomsk, Tomsk University Press (in press).

2. N. I. Gelperin. Evaporators. Moscow, Goskhimizdat, 1947.