144 TRANSACTIONS OF THE 
of one per cent of sulphur. The roasted ore is leached 
with dilute sulphuric acid and after neutralizing the 
solution, precipitating the ferric hydrate, settling and 
filtering, and removing other metals, the purified solu- 
tion passes to electrolytic cells where the metallic zinc is 
deposited on aluminum cathodes. This method, although 
it may be limited in its application to zinc ores, especially 
as it requires cheap hydro-electric power, has permitted 
the treatment of complex zinc ores which formerly were 
not amenable to the usual methods of concentration and 
retort smelting without prohibitive losses and high costs. 
Hydrometallurgical processes by which the metal is 
recovered from solutions in the form of a crystallized 
salt might also be mentioned. Concentration of the 
metals by this method is accomplished ‘by evaporation 
which may require redissolving of the crystallized salts 
in order to free the crystals from impurities and recrystal- 
lizing after clarifying the solution. 
The Bureau of Mines has done a great deal of 
research work in applying hydrometallurgical processes 
to the treatment of low-grade lead ores, and to the 
recovery of zinc from low-grade and complex zinc ores. 
Such work is valuable in that commercial processes may 
be developed by which the metals may be recovered 
from ores that are not amenable to present methods of 
treatment. It is believed that many of the present com- 
plex ore-treatment problems will be solved by hydro- 
metallurgical processes. 
