PRODUCING SODA PULP FBOM ASPEN. 39 



pulp influence the yield and properties of the pulp by influencing the 

 severity of the cooking reactions. 



(2) Severity of cooking is an effect mainly of the amount of caustic 

 soda consumed per unit of wood. Increasing the amount or concen- 

 tration of the chemical or the pressure of cooking produces a quicker 

 reaction and hence one more complete in a given length of time. 

 Increasing the duration results in a more complete reaction because 

 of the longer time allowed for the available caustic soda to be con- 

 sumed. 



(3) Greater severity of cooking is accompanied by a decrease in 

 the yield of crude pulp, and usually of screened pulp. If screenings 

 are present in considerable quantity (due to incomplete cooking), 

 more thorough cooking increases the yield of screened pulp. 



(4) The properties of the pulp are influenced by greater severity 

 of cooking as follows: 



(a) Shives are decreased in number or eliminated. 



(6) Bleaching is rendered more easy and the loss on bleaching becomes less. 



(c) The strength may either decrease or increase, depending upon which cooking 



condition is varied and the degree of variation. 



(d) The color of the unbleached pulp becomes lighter within certain limits, 



beyond which it may, under certain conditions, become darker. 



(5) A good indication of the severity of cooking is the appearance 

 of the individual fibers when examined under the microscope. 



(6) The decreased yields resulting from more severe cooking result 

 in a greater cost of wood and soda ash per ton of pulp. As a rule, 

 the smaller cost of bleaching powder incident to the more easily 

 bleached pulp produced by thorough cooking only partially offsets 

 the greater cost of wood and soda ash. 



(7) While the amount of bleach required decreases with increasing 

 severity of cooking, a point is soon reached where the decrease in 

 bleach required is not commensurate with the decrease in yields. 



(8) Increasing the initial amount of digester liquor increases the 

 condensation and steam consumption (and hence the cost) because 

 of the greater volume to be heated; increasing either the duration 

 or pressure has a similar effect because of the greater losses of heat 

 by radiation. 



(9) Yields (bone-dry basis) of well-separated unbleached pulps as 

 high as 56 or 58 pounds per 100 pounds of wood can be obtained from 

 aspen if the wood is of the best quality. Yields of from 54 to 55 per 

 cent were obtained which required only from 10 to 11 per cent of 

 bleach. The variation in yields obtained by changing the cooking 

 conditions was from 46 to 58 pounds per 100 pounds of wood charged, 

 or about 26 per cent based on the lowest yield. 



(10) Minimum total durations of from 3 to 4 hours may be success- 

 fully applied to the cooking of aspen for bleaching pulps, provided 

 the other cooking conditions are properly maintained. 



