448 SMITH'S INTERMEDIATE CHEMISTRY 



to the salt remedies this difficulty. Magnesium chloride is a very 

 objectionable form of hardness in water, because hot water par- 

 tially hydrolyzes the salt and liberates hydrochloric acid, which 

 attacks and corrodes the iron of the boiler and tubes. Hence 

 sea water can not be used in marine boilers. 



s ZiNcZnJ 



Occurrence and Manufacture. Zinc is found as zinc 

 blende ZnS (in large amounts in Missouri) and smithsonite ZnC0 3 

 (Spain and U. S.). 



In the case of the carbonate ore, the oxide ZnO is first obtained 

 by heating. When zinc blende ZnS is the ore, it is crushed and 

 pulverized, and then roasted (p. 258) to remove the sulphur and 

 leave the oxide: 



2ZnS + 30 2 -* 2ZnO + 2SO 2 . 



The ore is fed in at the top of a huge, box-like furnace (Fig. 108, 

 diagrammatic) through which rush the flames and heated gases 

 from fuel gas burning with an excess of air. 

 The ore is turned over and gradually displaced 

 forward by moving rakes until, at the end, it 

 drops to the next level. Here it is raked in the 

 opposite direction, until it falls to the third 

 FIG. 108 level The Qre collectg at the bottom fully 



oxidized, while the sulphur dioxide in the gases is made into 

 sulphuric acid. The oxide from either ore is then reduced by 

 heating with powdered coal: 



C->Zn + CO. 



This treatment of zinc ores should be carefully considered. 

 Since orete of most metals consist of the carbonate, sulphide, or 

 oxide of the metal, these steps are common to most metallurgical 

 processes. In the case of other metals, only the forms of the 

 furnaces and other details vary. 



