Pulp and Paper 3 1 09 



wood dates from the mid- 1 800' s. In 1854 Watt and Burgess used a sodium 

 hydroxide solution under pressure to produce wood pulp from aspen, initiating 

 chemical pulping and the soda process in this country. Partial recovery of the 

 soda (sodium carbonate) used as makeup chemical was practiced within a few 

 years. Use of the soda process has declined, but less drastic alkali cooking is 

 again of some interest. 



CHEMICAL PULPING REAGENTS 



pH 



1 



14 



CHEMICALS 

 SULFUROUS ACID 

 BISUFITES OF CALCIUM, 

 MAGNESIUM, AMMONIA, 



WATER 



SODIUM BICARBONATE 

 lODIUM SULFITE 



CARBONATE 



SULFIDE 



SODIUM 

 SODIUM 



SODIUM 



PROCESS 

 ACID SULFITE 

 BISULFITE 

 MAGNEFITE 



NEUTRAL SULFITE 



KRAFT 



SODIUM HYDROXIDE 



■^ SODA 



Figure 25-14. — Chemical reagents commonly used for pulping, arranged by pH, and 

 grouped according to pulping process. (Drawing after Whitney 1980.) 



In the acid sulfite process, wood is digested in a solution of sulfurous acid 

 containing alkali or alkaline earth bisulfites. Long the source of fine bleached 

 pulps for stationery and high-grade printing papers, and still viable in certain 

 situations, sulfite pulp production in this country is decreasing rapidly. 



During the 1880's, the German chemist Dahl developed a cooking liquor 

 containing sodium hydroxide and sodium sulfide. This sulfate, or kraft process 

 was cheaper than soda pulping, yielded stronger pulp, and proved capable of 

 pulping most wood species. Production of kraft pulp has grown for a century and 

 now dominates the pulping scene. One important reason for its dominance is its 

 excellent system for chemical recovery. 



Of the semichemical processes, probably neutral sulfite comes closest to 

 being standard. The principal reagent is sodium sulfite, and the pH is kept 

 around 8 or 9 with carbonate and bicarbonate. Neutral sulfite pulping is particu- 

 larly well suited to hardwoods; its product is a preferred grade for corrugating 

 medium. Several variants of the process are now being practiced. 



In days of scarce and costly energy and rigid environmental control, no 

 pulping process can remain viable without an effective system to recover and 

 reuse the pulping chemicals and to generate energy from those organic residues 

 not otherwise used more profitably. 



