Pulp Chips and Tanbark from Hemlock Slabs By Air-Flotation

Four-foot hemlock (Tsuga canadensis Carr.) slabs with attached bark and wood were banded into 400-500 pound bundles for outside storage and periodic chipping, tests. The slabs consisted of two lots. Lot A was from trees felled in September after the close of the peeling season and Lot B was from trees felled in July and August during the peeling season. Two models of disk type slab chippers were used. The tilted disk model is designed so that the knife enters the bark and then the wood, giving the bark a differential thrust which apparently aids in detaching the bark from the wood. In the vertical disk-type chipper, the knives cut both bark and wood simultaneously with no apparent differential action. Results indicate that the vertical disk chipper was less effective at the lower moisture contents than the tilted disk on Lot B. Both chippers gave equally good detachment on well-seasoned Lot A slabs. On trees felled at the close of the peeling season, a tightening of the bark occurred which made bark separation more difficult even after a year’s seasoning. The chips were screened into five fractions and the three middle fractions (retained on 3/4-inch, 2-mesh and 3-mesh screens) were floated separately on a model BX-100 Sutten Steele and Steele specific gravity separator. Each flotation produced a wood fraction, a middle fraction, and a bark fraction. In the double pass process, a 99 percent pure wood product and an 80 percent bark product could be produced from well-seasoned chips. With chips from less well-seasoned slabs it was only practical to produce 98 percent wood, but the bark fraction remained practically the same. Using the simple pass process, a 95 percent wood product could be obtained directly with less or no discard while producing an 80 percent bark concentrate product. About 7-8 percent of the wood portion consisted of knots which were removed during the flotation process. Bark content of either 99 or 95 percent wood product was still too high to produce a clean bleachable pulp. Thus, it would not be economical to operate the double flotation process and discard a part of wood to obtain the 99 percent wood product. The single pass process therefore appears to be more attractive. Tannin analysis of various bark concentrates produced an average tannin content of about 10 percent. The bark product is considered an acceptable source of hemlock extract.

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