Knots & Shives
In 1994 Vincent presses were used for on-site testing at two paper mills, a James River plant and Buckeye Florida in Perry, FL. The tests at James River were arranged by the Atlanta office of Sandwell Inc., a consulting firm, while the tests at Buckeye were initiated by Agri-Products, a landscaping materials firm. Since then both mills have bought VP-16 presses.
Both plants produce paper pulp, which starts with wood chips that are digested. The mass discharged from the digesters contains pulp fiber, spent chemicals, and waste materials. The liquid is very black in color, hence its name, black liquor. This liquid gives pulp mills their characteristic odor. Conventional practice is to screen out the usable pulp fiber. The screen rejects are waste materials which, along with some black liquor, are sent to landfill.
These screen rejects consist of knots, shives, as well as some good pulp fiber. The shives are little bundles of cellulose fiber that are still bound by lignin that failed to get dissolved in the cooking process. At James River the knots were fairly evident, about the size of quarters, while at Buckeye they rarely appeared. This was a function of the species, as well as age and condition, of the trees being harvested.
The amount of good fiber present in the waste stream varied considerably; at Buckeye it was low, around 25%, while at James River, on the softwood side, it was running unusually high, probably around 80%.
Moisture content of the material coming into the presses from the vibratory screens varied widely. Readings ranged from 76% to 91% moisture. Successful operation is also achieved with some flows in the 96% to 99% moisture range.
Excellent results are evident pressing the knots and shives at both plants. Black liquor, with its chemicals, is a financially important recovery at James River. At this hardwood mill, the rejects are unwashed, coming directly from a continuous digester. At Buckeye the chemicals are less concentrated because the rejects come from the brown stock wash system.
Solids content of the press cake at both mills averages 40% to 50%. Some material can be taken up to 55%. The James River material can be pressed so that it is suitable for a boiler fuel.
Buckeye is entertaining a proposal involving mixing the pressed material with cypress mulch for landscaping application. Another potential use at James River is as a filler in the production of tar paper. In any case landfill problems of contamination and availability are avoided.
The most commanding performance feature of the press is the ability, without operator attention, to handle a wide variety of conditions. Flow rates can be varied from "off" to surge to normal. Tests were run with only plant water going into the press, followed by thin and thick flows. The press rarely wants to purge. There is a tendency to overload and jam on dry material, so reinforced profile bar screens and the heaviest available drives are used for knots and shives.
One characteristic that catches everyone's eye is that knots are disintegrated into small pieces in the Vincent press. This opens the possibility of additional fiber recovery.
In anticipation of future pollution regulations, presses for black liquor applications are offered with vapor barrier construction.