CH. IX] Flue Dust 155 



because they contain so much combustible gas that 

 they can be used for heating the air blast and the steam 

 boilers with which to run the works. 



In order that these gases should not clog the burners 

 they must be freed as far as possible from the dust 

 mechanically carried over. They are therefore either 

 washed through water or passed into a dust catcher, a 

 large box hung with sacks in which the dust can settle 

 and from which it can be removed as frequently as neces- 

 sary. Another method consists in causing the gases to 

 pass between electrified plates when the dust is com- 

 pletely thrown down. Three grades of dust are obtained : 



1. The black dust from the main collectors, con- 

 taining the soot and about 3 per cent, of potash (KgO) 

 mainly as sulphate and silicate; 



2. A red or whitish brown dust collected from the 

 boilers or stoves which is derived from the black dust 

 by the burning away of most of the soot : it is therefore 

 richer in potash and contains some 15 per cent, of KgO, 

 again mainly as sulphate and silicate ; 



3. An intermediate grade. 



More potash volatilises if a certain amount of salt is 

 added to the charge in the furnace: in this case it 

 appears as muriate. 



Some samples of flue dust contain cyanides and sulpho- 

 cyanideswhichwouldbe harmful if present inlarge propor- 

 tions. Tests should alwaysbe made for these substances^. 



^ A simple test is as follows. Place about 1 gram of the dust in a 

 200 c.c. flask with 100 c.c. of water, (clean tap water will do,) and shake 

 for half a minute. Filter 10 c.c. into a test-tube, add a few drops of dilute 

 sulphuric acid (1 in 4) and 1 c.c. of iron alum solution. If nothing happens 

 the material can safely be used. If the solution turns red sulphocyanides 

 are present; if it turns greenish brown ferrocyanides are present : in neither 

 case should the dust be used until it has been examined by an analyst. 



