4 BULLETIN 80, U. S. DEPARTMENT OP AGRICULTURE. 



connected with the bottom of the digester into a "blowpit" or 

 "balloon;" whence it is transferred to large washing pans. Here it 

 is drained as free as possible from the strong spent cooking liquors, 

 called "black liquors," and washed thoroughly, first with hot, weak, 

 black liquors from the last washings of previous cooks, and lastly 

 with fresh hot water. The first drainings and washings which con- 

 tain the greater part of the alkali cooking chemicals are run to evap- 

 orators, concentrated, and later calcined in furnaces. The burned ash, 

 called "black ash," is leached with water, and the alkali in the form 

 of sodium carbonate is dissolved. The resulting solution is treated 

 with* quicklime (CaO), which changes the carbonate into caustic soda. 

 Modern practice recovers from 88 to 92 per cent of the alkali charged 

 into the digesters. By properly controlling the strength of the black 

 ash solution and mixing various strengths of recausticized solutions, 

 a caustic soda liquor of the desired strength for cooking is prepared. 1 



TREATMENTS GIVEN THE SODA PULP. 



After the pulp has been thoroughly washed it is diluted with a large 

 amount of water and screened to remove uncooked portions. This 

 is accomplished by either flat plate, diaphragm screens, or by such 

 screens in conjunction with centrifugal ones. In the case of aspen 

 or poplar the greater proportion of the water in the pulp is then 

 removed by means of "slushers," "feltless wet machines," or 

 "deckers." The pulp is then treated in a suitable vessel with bleach- 

 ing-powder solution and afterwards thoroughly washed. Very little 

 aspen or poplar pulp is left in the unbleached state, but is usually 

 bleached immediately after it is screened. Those mills making both 

 pulp and paper generally carry the bleached wet pulp directly 

 through the subsequent paper-making operations; but if the pulp 

 is to be sold or stored it is simply run over a paper machine into rolls 

 of dry pulp (about 10 per cent water). 



PREVIOUS INVESTIGATIONS. 



The treatises by Cross and Bevan 2 and by Schwalbe 3 and the 

 recent experiments 4 by Viewig, Miller-Moskan, Miller, Schwalbe, and 

 Schwalbe and Robinoff give much information on the nature of the 

 chemical reactions which take place between caustic soda and cellu- 

 lose under various conditions, and on the formation of decomposition, 

 mercerization, and other similar products from cellulose. • 



1 A few mills still cling to the older practice of not recovering the alkali from the black liquors. Such mills 

 buy the alkali for cooking in the form of caustic soda (NaOH). The cooking solution is produced by dis- 

 solving in water a sufficient quantity of the caustic to give a solution of the desired strength. The black 

 liquors are run to waste, and, although the consumption of cooking chemicals is very high, the mills seem 

 to operate at a profit. 



2 Cellulose, 1903. Also Researches on Cellulose, 1895-1900; 1900-1905; 1905-1910. 



* Die Chemie der Cellulose. 1910-1912. 



* For specific literature references see bibliography in appendix. 



