Producing soda pulp from aspen. 7 



In regard to the time required for cooking there is a wide difference' in practice. 

 However, the most improved plants are now able to effect the complete resolution of 

 the wood in a very short time. In fact, any of the deciduous woods can now be 

 reduced in about four hours. With the improvements in the methods of cooking that 

 have been developed, which enable us to get about twice the work out of a digester 

 that was formerly obtained, a number of special advantages have been found to be the 

 result of these quick cooks. The shorter the time the alkali is in contact with the 

 cellulose, the higher is the yield obtained and the sounder and stronger is the fiber. 

 If all of the cellulose is freed from the lignin at practically the same time, the free 

 alkali will have very little time to react on the weaker celluloses and the fibers will not 

 be broken nor the points and serrations dissolved. Moreover, the fibers from the short 

 cook are not hard to bleach, because the character of the cellulose is uniform. Under 

 conditions of complete saturation with the right proportion of alkali, the lignocellu- 

 loses can be almost instantly dissolved by subjecting the material to the temperature 

 and pressure that is ordinarily used for cooking the fiber. The writer has performed 

 this experiment on a laboratory scale, and the fiber obtained so closely resembled the 

 actual structure of the woody cell that hardly any cellulose could have been dis- 

 solved. 



Clapperton, 1 in 1907, in writing about the soda process, says: 



It is the necessity for employing such high temperatures and pressures (90 pounds 

 per square inch) that constitutes the serious drawback to the alkali process as under 

 the conditions of boiling the strong caustic soda acts on the cellulose, impairing the 

 strength and reducing the yield. 2 The reason why such conditions are necessary is 

 that the soluble acid bodies resolved by the caustic become so oxidized and con- 

 densed that they counteract and weaken the reducing action of the soda, and in order 

 to equalize their resistance higher temperatures and pressures have to be employed. 



Beveridge 3 recently published the results of some of his experi- 

 ments on the effects of varying the cooking conditions in the produc- 

 tion of esparto pulp. He says: 



The treatment of esparto by the soda method is typical of the preparation of paper 

 pulp from nearly all fiber-yielding plants, such as bamboo, straw, wood, etc. The 

 isolation of cellulose is brought about by digesting the prepared plant in an alkaline 

 solution, having for its base caustic soda, at variable temperatures and under variable 

 lengths of time. The chemical reaction which takes place during this digesting proc- 

 ess is not known; that is to say, has not been isolated because of the complicated char- 

 acter of the encrusting substances surrounding the fiber in the plant. The caustic 

 soda in aqueous solution forms soluble compounds with these encrusting bodies and 

 dissolves any silica which forms a part of the plant's structure, so that by subsequent 

 draining, washing, and bleaching the liberated cellulose is obtained in a compara- 

 tively pure state. Cellulose from whatever source it is obtained is, however, soluble 

 in aqueous solutions of caustic soda. Moreover, the solvent action of the caustic is 

 accelerated by heat and by the length of time (within limits) in which the two bodies 

 are heated together. It is therefore apparent that if the maximum yield of cellulose 

 is desired when using this method due regard must be paid to the laws regulating the 

 yield. These laws may be expressed thus: The yield of cellulose obtained from any 

 plant by the caustic-soda method depends upon: 



(1) The proportion of caustic soda (NaOH) used per unit weight of plant; 



(2) The temperature employed; and 



(3) The length of time the digesting operation is continued. 



i Practical Papermaking, p. 33, 2d ed., 1907. 



2 In modern commercial practice even higher temperatures and pressures are employed, and the results 

 of the Forest Service tests do not corroborate Clapperton's statements as to the undesirable effects from using 

 them. 



» Papermaker's Pocketbook, p. 72, 2d ed., 1911. See also Sindall, Manufacture of Paper, p. 77, 1908. 



