66 ESTIMATION OF ZINC IN COPPER ORES 



Process. —The ore is treated in a casserole with strong 

 nitric acid, which has been ^aturated with chlorate of 

 potash, and the sclution is evaporated to dryness without 

 baking. Solid ammonium chloride is now introduced 

 (abcut 10 grammes) and then about 25cc hot water, and the 

 assay kept warm till the mass is disintegrated, then excesii 

 of ammonia is added, and the solution is filtered. Boiling 

 should be avoided, as there is a tendency to form insoluble 

 basic compounds of zinc. The precipitate is well washed 

 with hot water containing ammonium chloride and 

 ammonia. If the precipitate is bulky it is advisable to 

 assist the washing by bringing the precipitate back into 

 the casserole with the wash bottle (a cone of platinum 

 in the filter is of great assistance). 



The main lot of zinc and copper is now in the filtrate* 

 but it is necessary to retreat the precipitate. If the 

 process has been caiefuDy carried out, only one retreat- 

 ment will be necessary. 



The ammnniacal filtrate (about lOOcc) is rendered 

 just acid with hydrochloric acid, boiled with a little 

 sulphurous acid, and the copper is thrown out with a ten 

 per cent, solution of pctassium thiocyanate and sodium 

 sulphite, added drop by drop to the solution at about 

 60° C. If only a little copper is present, the addition of a 

 little cold water assiots the foi'mation of the precipitate. 



To the filtrate a few drops of methyl orange are added 

 and then 5 E hydrochloric acid till the neutral point is 

 indicated, and then one drop more of the 5 E hydrochloric 

 acid. The solution is now heated to boiling, and the zinc 

 precipitated by addition of ammonium phosphate, the 

 solution being kept warm for a few minutes till the precipi- 

 tate becomes granular. Care must be taken here to avoid 

 bumping. 



The precipitate is readily filtered of), and is then 

 treated, with the filter paper, in a beaker with ISOcc hot 

 water and 5cc strong hydrcchloric acid, and titjated at 

 60° C, with standard potassium ferrocyanide as usual. 



In dealing with ores which contain under one per 

 cent, of copper, a very good alternative method i? to take 

 the ammoniacal filtrate from the iron, and after adding 

 the required excess of acid add a small crystal of sodium 



