PKODUCING SODA PULP FKOM ASPEN. 3 



barked or peeled wood is then cut diagonally with the grain into 

 slices or chips about one-half to three-fourths inch thick by means 

 of a machine called a " chipper." These pieces are then further 

 broken up by means of a disintegrator, or " shredder," and the 

 resulting chips are conveyed to storage bins, usually above the 

 digesters. An intermediate screening operation to remove dust 

 and dirt and to secure uniform chips is sometimes given them. 



On account of the strong solvent power of the cooking liquors used 

 in the soda process it is not necessary to remove completely the knots 

 or decayed portions of the wood. At some mills, however, the 

 chips before being stored are sorted into different grades from which 

 different qualities of pulp are produced. In the case of peeled wood, 

 delivered as such to the mill, the outer shavings, if the wood is re- 

 cleaned, are kept by themselves and converted into a lower-grade 

 product. In some of the older mills the knots were removed from the 

 peeled logs with a boring machine; and later the chips were picked 

 over by hand. Such procedures, however, have now been practically 

 abandoned in America. 



THE COOKING PROCEDURE. 



The digesters used in soda-pulp making are either rotary or sta- 

 tionary, and may be either cylindrical or spherical in shape. The 

 present tendency in new installations is towards stationary, vertical, 

 cylindrical digesters heated by live steam which enters at the bottom 

 of the digester in such a manner as to carry the cooking liquor through 

 a pipe to the top of the vessel and spray it over the chips. This 

 insures good circulation. The chips and cooking liquors are charged 

 through a manhole at the top of the digester, the bottom of which is 

 provided with a "blow-off" pipe and valve for discharging the pulp 

 after the cooking is complete. Such digesters are from 15 to 50 feet 

 high by from 4 to 9 feet in diameter. The larger sizes have been 

 lately introduced; in the past the most common size held about one 

 cord of wood and was 16 feet high by 5 feet in diameter. At the time 

 of the 1905 census the average American digester produced about 

 1 ton of pulp per cook, and the total combined capacity of the 208 

 soda digesters in operation then was 222 tons of pulp per cook. 



As soon as the charging of chips and caustic soda cooking liquors 

 is complete, steam is turned into the digester until a certain cooking 

 pressure or temperature is reached. This temperature varies at 

 different mills, but one corresponding to 110 pounds steam pres- 

 sure per square inch is probably the most common at present. The 

 pressure is continued from three to eight hours or more. 1 



WASHING OF PULPS AND RECOVERY OF COOKING CHEMICALS. * 



After the digestion process is completed the pulp in the digester 

 is generally forced out under pressure or "blown" through a pipe 



1 The detailed cooking conditions employed at various mills are shown in the appendix, Tables 16 and 17, 



