Fiberboards 276 1 



Strength, and in wet processes where slow drainage is not a problem and where 

 surface quality requirements are not very critical. Untreated chips give maxi- 

 mum pulp yield and minimum pollution of process water. However, the pulping 

 of untreated chips is very energy intensive. 



Steaming or water soaking hardwood chips at atmospheric pressure and tem- 

 perature produces more flexible fibers with fewer breaks; this slightly improved 

 pulp forms (felts) better and yields a stronger mat. One northern hardwood 

 siding mill produces most of its pulp (80 percent aspen; 15 percent birch, oak, 

 ash, and jackpine; 5 percent recycled paper) on atmospheric refiners without 

 cooking. 



Cooking chips in steam or hot water at elevated pressure and temperature 

 darkens them, reduces their yield, and increases pollution of process-water. 

 Specific refining energy is reduced, however, and resulting fibers are strong and 

 pliable. This method is considerably used in the manufacture of high-quality 

 wet-formed SIS and S2S fiberboard. Cooking vessels (digesters) can be de- 

 signed for batch or continuous discharge. 



A batch digester to feed an atmospheric refiner is typically a corrosion- 

 resistant vessel designed for pressures up to about 300 psi. The one illustrated 

 (Fig. 23-14) is about 3 feet in diameter, 20 feet tall, and holds about 120 cubic 

 feet of chips. With bottom ports closed the digester is filled with green chips 

 through the top port. Then, with top and bottom ports closed, the bottom steam 

 valve and top blow-down valve are opened so the digester fills with steam while 

 being purged of air. After purging, the blow-down valve is closed and steam 

 pressure built to desired level and held for prescribed time. Pressure is then 

 reduced by opening the blow-down valve and when pressure has dropped to 25- 

 50 psi the chips are blown into the chip bin by opening the bottom port. The 

 steam escapes to atmosphere through large-diameter blow stacks and cooked 

 chips are conveyed to primary refiners. 



A wet-process hardboard mill reports the following cycles for such a batch 

 digester^: 



Steam Cooking 



Hardboard type Species pressure time 



S2S Populus sp. 



SIS 50/50 oak/birch 



Should the oak component exceed 60 percent, then cooking time must be 

 reduced; and if the oak component is less than 40 percent, cooking time should 

 be increased. With 100 percent southern pine chips the cook might be extended 

 to 6 minutes at 190 psi. 



■Eustis, O. B. 1980. State of the art of the fiberboard industry. Unpublished manuscript. 



