Electrodeposition of Zinc

It is difficult both to deposit zinc completely by electrolysis and to obtain good deposits. The frequent sponginess, looseness, or porosity of electrolytically deposited zinc has been ascribed to the presence of oxide or hydride. Occluded hydrogen may make the deposit brittle, and if the temperature rises to about 80° C. the zinc adheres badly to the cathode. Zinc is usually prepared electrolytically from sulphate solutions.

To obtain satisfactory deposits by electrolysis the current density at the cathode should be uniform and the acidity should be constant. High-current densities promote efficient deposition: investigation has steadily raised the suitable current density from about 1 amp. per sq. dcm. to about 20 amp., and up to 50. More positive metals than zinc should be absent. Copper and cadmium, if present in the electrolyte, are deposited with the zinc. The presence of iron is said to contaminate the zinc very little under efficient conditions for electrolysis, though Mcintosh mentions a low iron content in the electrolyte as one condition for good deposition. Mcintosh also says that colloids should not be present, but they increase the efficiency according to Hansen, and Pring and Tainton say that a little colloidal matter favours the electrolysis by producing bright, adherent deposits and permitting a higher current density which secures greater current efficiency. Neutral solutions, or solutions as neutral as possible, have been advocated, but acid solutions seem to be effective if the other conditions are good.

Zinc can be deposited from commercial solutions, according to Pring and Tainton, with an efficiency of 95 per cent, if the concentration of sulphuric acid is about 15 grm. per 100 c.c., the current density is 20-50 amp. per sq. dcm., and the P.D. is 5 volts between platinum electrodes or 3 volts between electrodes of zinc. According to Mcintosh, the zinc concentration should be high.

Palmer and Wejnarth have recently described an electrolytic method of extracting the zinc from solutions prepared from the ore by using a current density of 2-6 amp. per sq. dcm. at the cathode. The anode is lead and the cathode aluminium.

The American electrolytic methods, which are in extensive use, are similar to this method. Since high-current densities of upwards of 100 amp. per sq. foot are required for efficient deposition, they only attempt to deposit a portion of the zinc in the electrolyte, and the residual liquid is then again enriched from fresh ore. The electrolyte contains from 5-7.5 per cent, of zinc, a current density of about 25 amp. to the sq. foot is employed, and, though the optimum acidity is 25-30 grm. sulphuric acid per 100 c.c., the electrolyte is acidified with from 2-2.5 per cent. The temperature may be 30°-45° C., and a little glue is sometimes added, though the electrolyte must be otherwise very pure. The colloidal matter raises the over-voltage of hydrogen, and, by making the hydrogen bubbles more readily detachable, keeps the deposit smooth. The lead anodes and aluminium cathodes must also be very pure. Between 3 and 4 volts is a usual P.D.

Cyanide-plating solutions are the most effective, and zinc deposited from these provides a better rust-proof coating than the deposit from sulphate solutions. Good results are obtained with a current density of 2 amp. per sq. dcm., a temperature of 40° C., and moderate agitation of the electrolyte.

Rotating cathodes are not much used in the electrolytic preparation of zinc, but they are commonly used, to secure effectiveness, in its electrolytic estimation. Though zinc is a difficult metal to estimate by electrolytic methods, the estimation can be performed by using an electrolyte containing 2 grm. sodium sulphate and 1 grm. sodium acetate for every gram of zinc sulphate.

According to Engelenberg, zinc and cadmium can be separated quantitatively by electrolysis in hydrochloric acid solution containing hydroxylamine or hydrazine sulphate.

Accurate results can be secured by depositing the zinc on a weighed mercury cathode. The atomic weight of zinc has been determined by thus estimating the zinc in zinc chloride and zinc bromide.

Attempts have been made to estimate zinc by electrolysis in solutions containing (a) oxalate and free oxalic or tartaric acid; (b) acetate and free acetic acid; (c) caustic alkali or ammonia; (d) cyanide. But if the difficulty of depositing all the zinc is surmounted, the results then, according to Spear, tend to be too high through the inclusion of zinc oxide or hydroxide with the deposited metal. The difficulty of complete precipitation also increases, he adds, with the concentration of the hydroxyl ions.