CRUSHING AND PULVERIZING MACHINERY. 455 



Krupp ball mill. This mill is described as follows by its agents: 



"The Krupp ball mills are made in twelve different sizes, but for 

 cement purposes in this country the size known as No. 8 has been uni- 

 formly adopted. 



"The mill comprises essentially a cylindrical grinding-drum mounted 

 on a hammered steel shaft running through it and provided with a dust- 

 proof sheet-iron casing. The main shaft is carried on bearings secured 

 to heavy bedplates bolted to a masonry foundation. The grinding- 

 drum is composed of overlapping cast-steel plates (grinding-plates). 

 The sides or head-walls of the drum are of wrought iron, secured to 

 the main shaft by means of naves and lined with steel side-plates. 



"The grinding-plates are strengthened on one half and bent inwards, 

 and in the other half holes are bored and a series of blades or scoops 

 (return scoops) of sheet iron, directed outwards, is provided. At those 

 places where the scoops overlap, channels are left extending right 

 across the whole breadth of the drum and partly stopped by low pro- 

 tection sieves arranged radially on the scoops. 



" The drum is surrounded externally by a fine cylindrical sieve located 

 at a certain distance from the grinding-plates. This sieve is constructed 

 of a number of frames connected together and secured to angle-iron 

 rings, forming the outside limit of the sides of the drum. 



" The sieve frames are supplied in wrought iron. The sieves are 

 covered by a phosphor-bronze or steel-wire gauze; the mesh is adapted 

 to the required degree of fineness of the product desired. 



" To protect the fine sieves, a coarse inner or fore sieve is arranged 

 between the outer sieve and the grinding-plates. This inner sieve is 

 constructed of finely perforated steel plates secured to the lateral flanges 

 of the grinding-plates and to the back of the scoops; in front of the 

 latter and across the whole breadth of the drum apertures are left. 



" The drum contains a charge of Krupp special forged steel balls of 

 various sizes and of a certain specified weight. 



"The material is fed into the drum through a hopper secured to the 

 nave at the front end of the drum. The nave itself is designed so as 

 to form two peculiar slanting spokes which act as a screw conveyor 

 and prevent the balls from being thrown out of the mill. 



" The lower part of dust casing of the mill serves also for collection 

 and discharge of the product ground and is provided with an outlet 

 closed by a slide. 



"In the larger mills a rectangular aperture is provided at the top 

 of the dust casing for the purpose of connecting the mill with an air- 

 shaft arranged above. The connection is made by means of a canvas 



