Electrolytic deposition and hydrometallurgy of zinc

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921 Excerpt: ...the rate of corrosion on the zinc from the cathodes increases, as mentioned earlier in this chapter. Nearly all chemical reactions increase in speed with rise of temperature. Many reactions in solutions roughly double their velocity for every 10 C. rise in temperature, but for the dissolving of zinc cathodes in cell solution, Hansen (loc. cit.) finds that the velocity doubles for every 21.7 C. rise in temperature. Hansen has made a careful study of the capacity of the zinc cell to dissipate heat and to compare this with the rate at which heat is generated in the cell. In this way it is possible to calculate the temperature at which the cell would reach equilibrium with the atmosphere of the cell room. Usually this is too high a temperature to allow of efficient use of electrical energy, and it is a better plan to cool the electrolyte within the cell circuit artificially. Rise in temperature increases the conductivity of the solution, which means a decrease in resistance, or a lower operating voltage. This, of course, is a favorable effect. Rise of temperature also lowers the hydrogen over-voltage and hence the current efficiency of zinc deposition. The conclusion is therefore inevitable that low temperature is most desirable for the operation of the zinc cell. Acidity.--The effects of acidity of the electrolyte have already been discussed under various headings above in this chapter. These effects are therefore only summarized: 1. A small acidity is necessary to prevent formation of sponge zinc at the cathode. 2. The corrosion of the cathodes increases with the acidity of the solution, especially in the presence of impurities. 3. The resistance of the solution decreases with acidity, and hence voltage likewise drops. This is a very important effect in that ...