14 BULLETIJSr 404_, U. S. DEPABTMEISTT OF AGEICULTUEE. 



approximately 3 per cent. It became apparent that a finer screen 

 would probably serve as well and effect a saving of small but good 

 hurds. 



Cooking. — Cooking is the technical term for the operation by which 

 fibrous raw materials are reduced to a residue of cellulose pulp by 

 means of chemical treatment. In these tests about 300 pounds of 

 hurds were charged into the rotary with the addition of a caustic- 

 soda solution, such as is regularly employed in pulp miQs and which 

 tested an average of 109.5 grams of caustic soda per liter, or 0.916 

 pound per gallon, and averaged 85 per cent causticity. Sufficient 

 caustic solution was added to furnish 25 or 30 per cent of actual 

 caustic soda, calculated on the bone-dry weight of hurds in the charge. 

 After closing the rotary head, it was started rotating at the rate of 

 one-half revolution per minute, and in about five minutes steam at 

 120 pounds per square inch was admitted at such a rate that the charge 

 was heated in one hour to 170° C, which is the theoretical equivalent 

 of 100 pounds of steam pressure per square inch. It was found, 

 however, that when the temperature reached 170° C. the pressure 

 was usually 115 or 120 pounds instead of 100 pounds, due to air and 

 gases inclosed in the rotary. At this point the rotary was stopped 

 and steam and air relieved until the pressure dropped to 100 pounds, 

 or a sohd steam pressure. The temperature was maintained at this 

 point for the number of hours required to reduce the hurds, which 

 was found to be about five, after which the rotary was stopped 

 and steam relieved until the pressure was reduced to zero, when the 

 head was removed and the stock was emptied into a tank under- 

 neath, measuring 5^ by 6 by 2 feet deep, where it was drained and 

 washed. Samples of waste soda solution or ''black liquor," which 

 were taken from some of the "cooks" for analysis, were drawn whUe 

 the stock was being thus emptied into the drainer. 



Determination of yield. — For determining the yield of cellulose fiber 

 the stock in the drain tank was washed with water until free from 

 waste soda solution, when, by means of a vacuum pump communi- 

 cating with the space between the bottom and the false perforated 

 bottom, the water was sucked from the stock, leaving the fiber with 

 a very uniform moisture content throughout its entire mass and in a 

 condition suitable for removing, sampling, and weighing for a yield 

 determination. Tests have shown that it is possible to sample and 

 calculate the yield of bone-dry fiber within 0.05 per cent of the actual 

 amount. 



It has been found that stocks from different materials vary greatly 

 in their ability to mat in the drain tank, thereby enabling a good 

 vacuum to be obtained, some stocks permitting a 25-inch vacuum to 

 be obtained, while others will not permit more than 5 inches. For 

 this reason the moisture content of the stock will vary from 65 to 85 

 per cent. 



