WOOD PULP AND PAPER 35 



ground pulp the fibers are torn away very readily so that the resultant 

 pulp is very much coarser and the fibers longer. 



In this process, the grinding machine includes a grindstone mounted 

 in a vertical position on a horizontal shaft and surrounded by a heavy 

 iron casing. Pockets are provided and the pressure afforded in a way 

 similar to that described for the other process. Even in the presence of 

 sufficient water to prevent the pulp from burning, the temperature rises, 

 commonly, to over 160 F. Hot ground pulp is used largely in the man- 

 ufacture of newspaper. It runs freely on the Fourdrinier wire, since the 

 coarse quality of the fiber permits the water to drain away quickly. 



Much higher yields are secured in the hot ground process owing to 

 the fact that the wood is worn away more rapidly and to the use of coarse 

 stones or stones which are finished to grind the maximum amount of 

 pulp in a given period of time. 



In a few mills in this country and in Canada magazine grinders have 

 been installed which take a charge of 12 cords of 4-ft. wood sticks. The 

 grinding proceeds during a twelve-hour shift without any special atten- 

 tion being necessary on the part of attendants. 1 



Screening. 



The pulp stock, after grinding, is run through a series of screens to 

 remove chips, portions of knots and any foreign material from the pulp. 

 There are many kinds of screens in common use, but they all follow the 

 same general principle. In some stages of the process flat plates, per- 

 forated with fine holes are used. This lets the water and fine pulp go 

 through, but retains the coarse material. Revolving drum screens are 

 also used. The latter are arranged in rows, and are 4 ft. by 4 ft. 6 in. 

 with a lo-in. perforation, through which the pulp passes. The feed-pipe 

 supplies the pulp at the end of each drum. 



In the case of the plates they are vibrated to do the work of screening. 

 In the case of the centrifugal screens, which are the latest form, a cylin- 

 der revolves at a high rate of speed, fine chips being forced through the 

 slits by centrifugal force. 



After the pulp has been screened it is treated in a wet press or lap 

 machine in order to remove the large amount of water with which it is 

 mixed. The material which does not pass through the screens is pumped 

 to a refiner where it is again ground up and submitted to the same 

 screening process until reduced to a fine fibrous condition. 



1 See Paper, June 25, 1513. 



