Solid material comminution or disintegration – Processes – Selective or differential comminution of mixed or bonded solids
Patent
1994-12-06
1996-11-26
Eley, Timothy V.
Solid material comminution or disintegration
Processes
Selective or differential comminution of mixed or bonded solids
241 28, B02C 904, B02C 2312
Patent
active
055776710
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The object of the invention is a method for manufacturing low bark content wood chips from whole-tree chips, in which method there are two or more sequential separation stages, which can be divided into pre-separation leading to a bark content of less than 10% and a final cleaning, and in which the pre-separation includes at least pneumatic separation as well as fines sieving and the final cleaning includes sorting based on colour separation or generally speaking on optical separation. In particular the invention is directed towards a cleaning method for birch and pine whole-tree chips.
BACKGROUND OF THE INVENTION
In present timber harvesting methods a considerable amount of the timber mass, in this case timber fibre biomass, is left in the forest, because it is unprofitable to collect it. When industry in Finland uses annually about 50 million m.sup.3 of stemwood with bark, about 23 million m.sup.3 of felling waste remains in the forest, of which it is estimated that half could be brought to the mill by using new harvesting technology.
Up until now cellulose has been manufactured from stemwood with bark, from which the bark can easily be removed by means of drum debarking. The exploitation of waste remaining in the forest cannot economically be connected to this chain.
So far it has not proved possible to use the mass obtained from forest waste, i.e. from crushed small trees and branches, in the manufacture of cellulose, because after even modern cleaning methods the bark content of the mass has been too great. Fines sieving achieves a bark content of only about 10%. It is true that patent publications U.S. Pat. No. 4,266,675, CH 643 160, SU 756 460, and SU 531 230 present various kinds of particle and even chip mass separators which operate on the principle of recognizing the differences in colour of the particles, in this case of the chips. These do not always give satisfactory results in the sorting of whole-tree woodchips with yield staying low and the remaining bark content staying high. It is not possible to achieve both a good yield and a high degree of cleanliness by purely adjusting the selection criteria of the separator.
Because timber mass used for cellulose has a considerably higher value than that used as fuel, there has been a obvious attempt to find a suitable cleaning method, by means of which chips suitable for cellulose manufacture with a bark content of less than 1% and a high yield can be obtained from whole-tree, i.e. forest chips.
SUMMARY OF THE INVENTION
The intention of this invention is to create a new kind of method, by means of which the above aims can be achieved. From the point of view of separation in accordance with the invention it is essential that the forest chips are first treated by grinding in order to remove the bark and by means of a good pre-separation method before colour difference sorting, because this is not able to remove a high bark content from the mass. At this stage pneumatic separation is essential, because light and thin particles, such as birch bark, cannot be separated in any other way and are a great inconvenience in later separation processes and especially in cellulose manufacture. Grinding creates internal cracks in the chips, in which case chemicals are absorbed more rapidly during cellulose cooking. Grinding is carried out alternatively either by a large-tolerance grinder or a vibrator cone crusher. At the present moment the former appears the better of the two.
In addition to bark colour difference separation, separators based on the shape and density of chips can be advantageously used, by means of which knot pieces among others can be removed.
BRIEF DESCRIPTION OF DRAWINGS
In what follows the invention is illustrated with the aid of the accompanying figures in which
FIG. 1 shows the chip cleaning schematically
FIG. 2 shows one kind of colour difference sorter
FIG. 3 shows schematically a simplified version of the cleaning equipment
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
The following is a table illustr
REFERENCES:
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patent: 3254847 (1966-06-01), Brandts et al.
patent: 3337139 (1967-08-01), Lloyd et al.
patent: 3826433 (1974-07-01), Erickson et al.
patent: 4266675 (1981-05-01), Barwise
patent: 4332353 (1982-06-01), Lario
patent: 4992949 (1991-02-01), Arden
patent: 5335790 (1994-08-01), Geiger et al.
Edelmann Kari
Seppanen Veli
Eley Timothy V.
Valtion teknillinen tutkimuskeskus
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