[Rev. September 2002]
Largely because of action by environmental groups, the cost and availability of timber in the northwest has become a problem for paper mills. Paper makers now make paper that contains fiber extracted from lumber mill sawdust. Because lumber mills normally burn their sawdust, it represents an economic source of raw material for a pulp mill.
In 1996 this situation led to pressing trials using sawdust. The tests were conducted in Tampa with engineers from Fort James trying out three different Vincent screw presses.
The thin, high speed saw blades used at the lumber mills are cooled with water. This process increases the moisture content of the sawdust slightly from the normal 52% found in green lumber. Rainfall adds even greater moisture content. The samples tested in Tampa had 65% moisture (35% solids).
The normal press configuration was found to produce press cake with 42% solids. The horsepower required was in the high range for Vincent presses. Since the Fort James specification called for 45% to 48% solids, a special screw design was used in subsequent testing.
It is noteworthy that the pressing action is only removing free water. We are not breaking open the cells of the wood. A competitive press requiring 3,500 hp could achieve solids closer to 60%. However this broke down the cellulose fibers, reducing the quality of the paper being produced.
Therefore, the final contract called for two VP-30 presses, each with 300 horsepower drives. These subsequently were modified to 400 horsepower. The bulk density of the sawdust usually is low, about 22 pounds per cubic foot. This is compressed dramatically, to the point where water is expelled, by changing the screw shaft diameter just ahead of the first resistor tooth. The pair of presses can handle approximately 100,000 pounds per hour of sawdust.
The presses were supplied with an unusual feature: The resistor teeth were drilled for the possible addition of steam directly into the sawdust as it is being pressed. This technique was used many years ago in both fish meal and citrus peel to achieve lower cake moisture. To date it has not been used at the paper mill.
Unfortunately, the need at the Fort James Clatskanie mill is unique. They have an overloaded sawdust digester. The arrangement does not allow for the addition of sufficient steam when the sawdust is wet. Thus the capacity of the digester drops from 300 TPD in the summer to 240 TPD in the winter rainy season.
Another problem is that James River in Clatskanie has a limited amount of white liquor generating capacity. The excess water in the sawdust requires extra white liquor to overcome a dilution effect.
On the other hand, improvements in the material handling system appear to have resulted in increased digester capacity. Simply being able to feed more tonnage of the damp sawdust into the digester may have been all that was required.
The installation of the VP-30 screw presses never quite succeeded, primarily due to stick-slip vibration that occurs. This vibration occurs when tight pressing is attempted. A variety of mechanical failures have resulted from the vibration, in spite of numerous modifications.