328 Kenneth A. Miclde : 



The separation of intimately mixed lead and zinc 



sulphides. 



One of the problems to be overcome in the treatment of 

 lead zinc ores is the separation of the lead and zinc sulphides 

 into suitable products for subsequent, treatment. For in- 

 stance the concentrates from the different flotation processes 

 working at Broken Hill contain varying amounts of zinc blende 

 and galena. When these concentrates have been produced 

 from fairly coarse tailings, a fair separation of the galena and 

 blende can be made by the difference in gravity of the mine- 

 rals, by such appliances as vanners and concentrating tables. 

 However, with concentrates produced from slime, the separa- 

 tion of the lead and zinc sulphides becomes very much more 

 difficult, and cannot be efficiently effected by gravity concen- 

 tration. One method that has been used is as follows : — The 

 mixed sulphides are given an oxidising roast at a low tempera- 

 ture, which hardly affects the zinc blende, but which causes a 

 '^ coat " of lead sulphate to form on the surface of the lead 

 sulphide particles. The product is then treated by a flotation 

 process, the zinc blende floating and the sulphated galena be^ 

 having similarly to gangue, and not floating. This is known 

 as the Horwood process. ^ 



The writer tried various chemical solutions for the purpose 

 of preventing the flotation of the galena in such mixtures of 

 sulphides. Some fairly good results were obtained by using 

 a solution of ferric chloi-ide. Tiie action of the ferric chloride 

 solution is to cause a permanent and not easily detached coat 

 on the particles of galena. The zinc blende is unaffected by 

 the solution. This white coat formed on the surface of the 

 galena particles consists of sulphur and lead chloride. When 

 the ferric cliloride solution is used hot, the action is quicker. 

 Some oxide of iron, thrown out of solution, is sometimes pre- 

 sent in the substance forming the coat when the ferric chloride 

 is allowed to stand on the mixed sulphides for some time. Any 

 lead being dissolved is re-precipitated by the sulphuric acid in 



1 "Treatment of mixed sulphides by the Horwood process." E. .J. Horwood, Proc. 

 Aust. Institute Mining Ensfiiieers, June, 1911. 



