48 FOREST PRODUCTS 



still far more important in its yearly output. For some specialities, sul- 

 phate paper is regarded with great favor. In white papers from bleached 

 sulphate pulp, the product is soft and pliable in contrast with the harder 

 and more " rattling " sulphite papers. However, for the future, the use 

 of sulphate pulp for kraft papers has the greatest promise. 



The process may be described briefly as follows: 1 



After the reduction of the wood billets to the form of chips as has 

 been described in connection with the sulphite process, they are digested 

 under pressure in a liquor containing a solution of various sodium com- 

 pounds. In the ordinary operation, according to this process, these 

 compounds consist of sodium hydroxide, sodium sulphide, sodium car- 

 bonate, and sodium sulphate. Of these compounds the first two are 

 the active agents in the digesting process and combine with about 50 

 per cent of the weight of the dried wood to soluble organic sodium salts. 

 The time required for cooking depends upon the nature of the wood 

 and the character of the pulp desired. After cooking, the pulp is sep- 

 arated from the waste liquor by washing in large tankb. The liquor is 

 later evaporated and the residue is partly burned in rotary furnaces and 

 after being subjected to high temperatures, the sodium sulphate is added 

 to replace the soda lost during the recovery process. 



After cooking and washing, the pulp is run through press rolls and 

 formed into bundles. Then, after drying, it is sent to the pulp mill. 



THE MANUFACTURE OF SODA PULP 



The manufacture of wood pulp by the soda process was discovered 

 about 1880. The preparation of the wood for use in the soda process is 

 exactly the same as has been described in connection with the sulphite 

 pulp. That is, the wood is barked and then chipped and screened. 



This process lends itself especially to the reduction of various hard- 

 woods and pine. In 1916, 394,577 cords of aspen were reduced by this 

 method. In fact aspen composes more than one-half of all of the pulp 

 wood reduced by this method. Other hardwoods, such as beech, birch, 

 maple, yellow poplar, gum and cottonwood are also frequently reduced 

 by the soda process. In 1916 there were 707,419 cords reduced by this 

 method. 



The Digesters. 



The object of boiling the wood under pressure with chemicals is to 



1 Taken partly from an article on the manufacture of sulphate pulp by Carl Moe, in 

 Paper, July 26, 1916. 



