244 ELECTROLYSIS. 



formed on the surface of the liquid must be skimmed from time 

 to time. 



When a solution of sulphate of zinc is used, great care must 

 be taken to maintain the liquid in a neutral state ; this is not 

 so much necessary with chloride of zinc, but this latter solution 

 has the inconvenience of generating chlorine at the anode. A 

 suitable liquor, according to M. Luckow, for the direct extrac- 

 tion of the zinc from the blende, is a solution of sea-salt slightly 

 acidulated. 



LETRANGE'S METHOD. This process, also applied to zinc 

 ores, has been tried on a large scale at Komilly and at Saint- 

 Denis, and consists, in principle, 



1st. In moderately roasting the blendes without overheating, 

 so as to obtain zinc sulphate by directing the sulphurous 

 vapours on to some oxidised ores (roasted blende, calcined 

 calamines), in order also to transform them into sulphates. 



2nd. In dissolving the sulphate of zinc thus obtained in 

 order to make it into a concentrated solution. 



3rd. In precipitating the metal of that solution, by using as 

 anodes some plates of graphite, or of a metal which is not 

 attacked by sulphuric acid. 



The characteristic of the system is the use, as a dissolvent, 

 of the sulphuric acid borrowed from the ore itself. The inventor 

 deals with any ore, and particularly with those which are not 

 much sought after, and are even abandoned with the usual 

 processes. The preparation is an economical one, for it is not 

 required to separate the plumbic substances, and in many cases 

 the calcareous gangues may be left when the sulphuric acid 

 produced in excess is to be absorbed in the treatment by reduc- 

 tion of the sulphate. 



The calamine does not require to be calcined. The roasting 

 of the blende must be conducted at a moderate temperature, 

 so as to facilitate the formation of the greatest possible quantity 

 of sulphate of zinc. The sulphurous vapours which are not 

 retained in combination with the ore are brought into contact 

 with the roasted blende and the calamine, for converting their 

 zinc into sulphate or sulphite, which, exposed to the air, is soon 

 sulphated. 



