Fiberboards 2783 



ever, requires addition of 1 ,67 1 gallons of water every minute. All of this water 

 is removed in the washer and partially replaced subsequendy. Following the 

 washer, the consistency is gradually reduced to the forming consistency of about 

 2 percent (fig. 23-25). On the forming machine the mat is formed by drawing the 

 water from the slurry. Some more water is removed in the wet press, and the rest 

 is squeezed out or evaporated in the hotpress. 



Water consumption and water discharge problems can be greatly reduced by 

 partial or total recirculation as indicated by the dotted lines in figure 23-29. Two 

 water cycles are apparent. The primary water cycle recirculates the washer 

 discharge, which is contaminated with sugars. The secondary cycle (machine 

 white water) is kept separate from the first. It carries washed out chemical 

 additives which will build up to a constant level so that chemicals have to be 

 added only at the rate at which they are actually retained in the board. Similar 

 diagrams could be drawn for the other two options of the wet process: insulation 

 board and wet-formed S2S board. For insulation board the hot press would be 

 replaced by the continuous dryer, and for wet-formed S2S board, a dryer would 

 precede the hot press (fig. 23-30). 



Although the amount of water removed in hot press or dryer is relatively 

 small, these two steps impose important limitations on the wet process: only one 

 smooth side in the SIS process, and considerable energy requirements for water 

 evaporation in continuous dryers (insulation board and S2S board). 



FORMING MACHINES 



Sheets or mats are formed when the watery pulp suspension is flowed onto a 

 wire screen which retains the fibers but allows the water to drain. Uniformity of 

 fiber dispersion in the mat depends to a large extent on consistency, as pulp 

 fibers have a tendency to form clumps at higher consistencies when their free 

 movement is restricted. Lampert (1967) found that such floculation may occur if 

 stock consistency in the headbox ahead of the former is greater than about 2.5 

 percent. At very low consistency, e.g., less than y4-percent, fiber movement is 

 unimpeded, but fibers may settle singly without the interweaving necessary for 

 development of maximum sheet strength. Lampert (1967) found that maximum 

 board strength resulted from a headbox consistency of about 1 Vi percent, just 

 avoiding floculation. As de watering proceeds, there occurs a collective sedi- 

 mentation of fibers accompanied by physical interference at the moment of 

 sedimentation resulting in a somewhat three-dimensional network. Optimal 

 consistency in the headbox is influenced by fiber length, type, and cooking 

 conditions. 



Fast drainage enhances productivity and manufacturing economy. Freeness of 

 pulp is the dominant factor, but water temperature also affects drainage rate 

 because viscosity of water decreases as temperature increases. Adverse effects 

 of higher water temperatures on additives limits stock temperatures in the head- 

 box, however. 



