A STUDY OF KRAFT PULPING WASTES IN RELATION 

 TO THE AQUATIC ENVIRONMENT 



WILUS M. VAN HORN 



THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WIS. j 



Introduction 



Of the several methods for the manufacture of chemical wood pulp, 

 the kraft process now occupies a major position in the industry. Tonnage- 

 wise, the amount of this type of pulp produced in the United States exceeds 

 that produced by the sulphite and soda processes combined. In the pres- 

 ent discussion, it is well to distinguish clearly between the kraft and the 

 sulphite processes. 



In the sulphite process, the liquor used for digesting the wood is an 

 aqueous solution of sulphurous acid in which lime, or some other base, has 

 been dissolved; the final result is, therefore, a solution of a bisulphite of 

 the base containing an excess of sulphurous acid. The nature of this 

 liquor, where calcium is used as the base, as well as the mechanics of the 

 sulphite process, makes it economically unfeasible to recover the spent 

 liquor; it is accordingly wasted to the river and is commonly known as 

 sulphite waste liquor. 



The kraft process, on the other hand, is an alkaline process. The 

 principal constituents of the cooking liquor are sodium hydroxide and 

 sodium sulphide, the latter comprising up to 45% of the total. From 

 the present point of view, the important aspect of the kraft process is that 

 the spent liquor is recovered, for the most part, by a process involving 

 evaporation and combustion. In this treatment, the chemicals are recovered 

 and heat energy is generated as well. 



In order to understand the problem at hand, it is desirable to con- 

 sider briefly the principal features of the kraft pulping process. 



The wood is carefully prepared and cut into small chips, after which 

 it is introduced into large receptacles called digesters. The cooking liquor 

 is added and the digester securely closed. Heat is then gradually applied 

 and the cooking process is started. 



The cooking cycle of any type of chemical pulp production consists 

 of three phases: a period during which the liquor is penetrating the chips 

 and the internal pressure is being brought up to cooking level, a full pres- 

 sure period during which the wood is actually being cooked, and a gassing- 

 down period during which the pressure in the digester is lowered slowly to 

 the point where the pulp can be blown therefrom. 



At the start of a cook, there is a considerable amount of air in the 

 digester which must be removed before the start of the second phase. 



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