FI89013B - Method for the manufacture of wood chips with a low bark content from whole tree chips - Google Patents

Abstract

The subject matter of the invention is a method for the manufacture of low-bark wood chips from whole tree chips, which method comprises two or more successive separation phases, which can be divided up into pre-separation to a bark content of less than 10% and a final cleaning, and in which the pre-separation involves at least one pneumatic separation together with chip sifting, and final cleaning consists of sorting on the basis of colour separation. According to the invention the bark is loosened prior to pre-separation from the wood chip pieces by abrasive treatment, which at the same time reduces the particle size of the wood chips. Colour differential sorting comprises two succeeding and so called cascade-coupled phases, that is to say in one phase pure wood chips are separated out and in the second phase the reject containing bark.

Description

89013

METHOD FOR MANUFACTURING LITTLE SHELL CHIPS FROM WHOLE WOOD CHIPS

The invention relates to a method for the preparation of low-bark wood chips from whole wood chips, which method comprises two or more successive separation steps which can be divided into pre-separation to less than 10% bark content and final cleaning, and wherein the pre-separation comprises at least pneumatic separation and . In particular, the invention relates to a method for cleaning birch and pine wood chips.

With current logging methods, a considerable amount of wood pulp, here wood-derived biomass, has been left in the forest because it is unprofitable to recover it. When industry in Finland uses about 50 million. m3 of bark trunk wood, about 23 mill. m3, of which an estimated half could be brought to the plant using a new type of harvesting technology.

20 Until now, the pulp has been made of shelled trunk wood, from which the shell can be easily removed by drum peeling. There is no way to link the exploitation of residues in the forest to this chain.

25 To date, forest chips, ie pulp obtained by crushing small wood and twigs, have not been able to be used in the production of pulp, because even after the current cleaning methods, the bark content of the pulp has been too high. When using a chewing screen, only about 10% of the shell content is achieved. U.S. Pat

30 4,266,675, CH 643 160, SU 756 460 and SU 531 230, however, various sorters of particle masses, even chip pulps, are shown, which operate on the basis of the identification of the color differences of the particles, i.e. chip pieces. In the sorting of whole wood chips, these do not give a satisfactory result both when the yield remains poor and when the residual bark content remains high. By adjusting the sorter selection criterion alone, both good yield and high purity cannot be achieved.

Since the pulp used for pulp is considerably more valuable 40 than the combustion fraction, there has been a clear effort to find a suitable cleaning method to obtain wood chips with a content of less than 1% from whole wood or forest chips in high yield.

The object of the present invention is to provide a novel method by which the above-mentioned objects are achieved. The characteristic features of the method according to the invention are set out in the appended claim 1. For the separation operation according to the invention it is necessary that the wood chips are first treated by abrasive treatment to remove bark and by a good pre-separation method before color separation sorting. Pneumatic separation is necessary here because light particles, e.g. birch ash cannot be separated otherwise and they greatly interfere with the subsequent separation process and especially with the production of pulp 15. In addition to the color difference sorting of the shell, separators based on the shape and density of the pieces are preferably used, with which e.g. twigs off.

In the following, the invention is illustrated by the accompanying figures, in which

Figure 1 shows the cleaning of wood chips in the form of a diagram. Figure 2 shows a color difference sorter. 25 The following table illustrates the abrasive treatment of whole wood chips.

Mäntykokopuuhake

Original Ground P-% p-%

Branched branches 5.4 2.3 30 Loose bark 10.0 11.7

Chips in shell 15.1 2.0

Shelled pieces 59.0 60.7

Bite 10.7 23.3 l, 3 89013 In this example, pine whole wood chips were ground with a disc grinder with a blade spacing of 7 mm. As can be seen from the table above, the rubbing treatment significantly reduces the proportion of bark branches and chips contained in whole wood chips.

5 Although the proportion of pieces of chips without bark did not increase much, this was due to the fact that the size of the pieces of wood chips was initially at an acceptable level and, in the case of abrasive treatment, the size of the pieces fell below the approved level. As a result, it is preferable to select the piece size of the whole wood chip to be treated as oversized, however, because the abrasive treatment reduces the piece size. In this case, the proportion of chips without bark is clearly increased.

Different piece thicknesses are used depending on the factory, usually between 6-8 mm. The proportion of over-thick chips is described by the proportion of those left on the slit-15 sieve. This can be significantly increased because the abrasive treatment reduces the thickness of the piece to an acceptable level. The thickness of the pieces can be adjusted by setting the sharpness of the grinder as desired, preferably in the range of 6-14 mm.

There are a number of methods for pre-separation, from which the above-mentioned pneumatic separation and vibrating screen have been selected in Fig. 1. Pneumatic separation can be considered necessary because certain light fractions, e.g. the leaves and thistle are not otherwise distinguished.

The color difference sorting is performed by a device of a type known per se, in which the belt conveyor transfers the mass under an optical unit which detects an abnormal color on the path of the shell. The piece of shell is deflected from the rest of the mass by means of an air jet as it enters the end of the belt conveyor. The following shows a known type of color difference type, Sortex 4500. Figure 2 below shows the main structure of this color difference sorter. The basic parts of the device are a belt conveyor 1, a control unit 2, an optics unit 3, air ejectors 4, a fluorescent light 35 and a drive mechanism 6. The pre-cleaned pulp chip is fed to a belt conveyor 1 which conveys it at a constant speed under the optics unit 2. The wood pulp is further illuminated with fluorescent light 5 to provide better timing ability. The control unit controls the drive mechanism 6 and in the calculated step 4 89013 the corresponding air ejector 4 is started, which causes the shell-piece to deviate from the direction of the main mass and thus to separate the mass into different fractions.

5 In the experiments with the color difference sorter, wood chips from which needle needles, leaves and debris less than 7 mm, more than 8 mm thick and more than 45 mm fractions were removed were used. The bark content of the samples of both tree species was about 5%. In the sorting, the bark content of the approved fraction with pine reached 0.6% with a yield of 10.5% of the amount of feed. The fraction content of birch-approved fraction was 1.9% and the yield 63.8%. To improve the yield, a second color difference sorter is used, which removes the highly bark-containing (pine 24.7%) reject (proportion 54.7%) and the accepted fraction (bark content 5.1%) is returned to the abrasive treatment. Such a return is necessary because, in most cases, the shell content mentioned is largely due to the chips in the shell, from which the shell must therefore first be removed.

In the case of whole wood chips, pre-separation is required to reduce the bark content to less than 5% by current methods. On the other hand, the final cleaning of the surface board chips can start as early as 10% of the bark content, because the pieces of bark are large.

25 thermal cameras can be used to identify and separate the density of the chips after the chipping stream has first been heated. In this case, the denser pieces, i.e. in practice the branches, appear at a different temperature than the rest of the mass. Pattern recognition is a technique known per se, but requires powerful processors and its own software applied to the sorting it is looking for.

30

Briefly, the significance of the invention can be described in such a way that it is possible to utilize the wood raw material in a more precisely processed form. By means of the invention, pulp chips are obtained in a yield of 70% (0.5% bark content), when the known solutions would achieve a yield of at most 45% with a bark content of 3%. The method according to the invention should therefore be compared with the currently used timber harvesting, where the same bark content of 0.5% is naturally achieved by drum peeling, but the yield of wood chips calculated from the total biomass remains at 40%.

Claims (7)

A method for producing bark-poor wood chips from whole-tree chips, which method has two or more consecutive separation phases, which can be subdivided into a bark content of less than 10% and a final purification, where the pre-separation includes at least one pneumatic separation and tension screening. , and the final purification includes a sort based on color distinction, characterized in that the 10-bark before the pre-separation is detached from the chips with abrasive treatment, which simultaneously reduces the particle size of the chips, and that the color difference sorting contains two consecutive and so-called cascaded phases, i.e. in one phase pure wood chips are separated and in the other phase the bark-containing projections are separated. Method according to claim 1, characterized in that one of the cascade-coupled color difference separators is adjusted to distinguish pure wood chips with a bark content lower than 1% and the rejections from this phase are fed to the second phase input, the part of which is approved by the color separator. Eats are led to treatment and the very bark-containing reject is led out of the process. Method according to claim 2, characterized in that the part of soin approved in the second phase is led back to the treatment soin grinds chips to release the bark contained in the chips. Method according to any one of claims 1-3, characterized in that the near-abrasive treatment is carried out with a disc mill or vibration grain mill. Method according to Claim 4, characterized in that the pitch spacing of the disc mill 35 or the corresponding spacing of the vibration mill is over 6 - 14 mm. 6. A method according to claim 5, characterized in that the thickness of the pieces of whole-wood chips is initially chosen so that the proportion that stops in the factory-specific health screen decreases to one third. Method according to any of claims 1-6, characterized in that, in the final cleaning, separation is also used on the basis of the density of the chips and / or recognition of the mold.