[MciNTOSHl ELECTROLYTIC PRECIPITATION OF ZINC 115 



Series 3. 

 4-5% Zn, 0-003% Fe% 22 Amps, per square foot. 



Additio?2. Deposit. Current Efficiency. 



f21. Very good 96 • 5 per cent 



£22. 0-0025% Cu " 97-4 



f23. 0-04% HCI. " 93-4 



f24. 0-001% Ag.. " • 95-8 



The effect of cobalt, nickel and antimony are shown in the 

 accompanying photographs. The first two metals were present to the 

 extent of 0-01 per cent, so that the selection of ores free from these 

 elements — since they can be removed only with the greatest 

 difficulty— is of importance. The antimony in solution came from 

 antimonial-lead anodes. Unless care is taken in the removal of 

 these substances only the poorest deposits can be obtained. 



The Effect of Iron 

 Iron when present even in small amounts, has a very bad effect on 

 the deposit, particularly when the precipitation is carried on until only 

 a small amount of zinc remains. If the precipitation is stopped at 

 about 2-5 per cent, zinc in a solution containing 0-1 iron, the deposit 

 may be good, but the current efficiency is two or three per cent, lower 

 than in an iron free solution. When manganese is present, the bad 

 effect of iron largely disappears, and perfectly satisfactory deposits 

 can be obtained with 0-1 per cent of iron if an equal amount of 

 manganese be present; but this does not hold with certainty, since in 

 the system of cascades used in practice, the impurities seem to accumu- 

 late in the lower tanks. In general, when a permanganate colour 

 appears in solution the deposit is satisfactory even when considerable 

 amounts of iron are present. Series 4 shows some results on aqueous 

 solutions containing iron and manganese made from pure zinc sulphate. 



Series 4. 



5-0% Zinc. 

 Addition. Deposit. Current Efficiency. 



1. Good. 97 -4 per cent. 



f2. 0-1% Fe Fair 72-3 



f3. 0-1% Fe 



f 0-3%, Mn Very good 96-1 



f4. 0-1% Fe Good 93-7 



f 0-1% Mn 



f5. 0-5% Mn Fair Current sufficient to 



f bring to 0-3% zinc. 



f6. 0- \% Fe Very bad 



