GB2036603A

GB2036603A - Recovering cellulose fibres from waste material

Info

Publication number: GB2036603A
Application number: GB7939081A
Authority: GB
Original assignee: Celleco AB
Current assignee: Celleco AB
Priority date: 1978-11-17
Filing date: 1979-11-12
Publication date: 1980-07-02
Also published as: SE7811866L; FR2441680A1; FR2441680B1; IT1127240B; SE431570B; DE2946160A1; IT7927342A0; GB2036603B

Abstract

In a method of recovering cellulose fibres from waste paper etc., a dispersion of the raw material in water is separated by flotation into a first fraction, rich in fibres, and a second fraction, enriched in hydrophobic impurities, mainly printing ink, and the second fraction undergoes further treatment including separation by sieving into one fraction, rich in fibres, which is recirculated to the flotation stage, and another fraction which is further enriched in hydrophobic impurities and further purified. As shown, the second fraction from the flotation stage can be separated by a hydrocyclone separator plant into a fraction rich in fibres, which is further separated by sieving, a fraction enriched with hydrophobic impurities which is mixed with the fraction having passed through the sieve, for further purification, and a third fraction of heavy impurities, which is rejected. <IMAGE>

Description

SPECIFICATION Recovering cellulose fibres from waste material Recovery of cellulose fibres from waste paper, board etc. has gained increasing importance during recent years with rising wood prices. Different methods for recovery of the cellulose fibres from such raw materials have been suggested and tested. It has become obvious, that it is quite difficult to recover efficiently fibres from printed paper which, from a raw material point of view, is contaminated firstly with printing ink, but also with other impurities like pieces of plastic, cork, rubber etc.

The principle of flotation is often utilized in plants for recovery of fibres from waste paper. Such plants are usually designed as shown diagrammatically in Figure 1.

Basically, the recovery in such plants is performed by more or less efficient separation of dispersed fibres viz. printing ink and other light impurities in one primary flotation stage, in which the printing ink and other light impurities are transferred to a liquid, hydrophobic fraction in the form of a foam, which also contains a certain amount of fibres. Most of the fibres remain in the water fraction, which is fed to a plant for further purification of the fibres. The hydrophobic fraction is fed to a secondary flotation stage, in which some of the fibres are recovered and are recirculated to the process, while a further enriched hydrophobic fraction is further purified in apparatus for this purpose. It has become evident, however, that such a secondary flotation stage is not efficient in recovering fibres, as these tend to float with the hydrophobic fraction.

This invention aims at a more effective recovery process for fibres and accordingly provides a method of recovering cellulose fibres from raw material consisting of waste paper and the like, wherein the raw material is dispersed in water to form a mixture of fibres, hydrophobic impurities, such as printing ink, and light impurities, such as pieces of plastics and/or cork, and the dispersion is separated in a flotation stage into a first fraction which is rich in fibres and fed to apparatus for purification of the fibres, and a second fraction which is enriched in hydrophobic impurities and passed for further treatment including separation by filtering into one fraction which is rich in fibres and recirculated to said flotation stage, and another fraction which is further enriched in hydrophobic impurities and passed for further purification.

The filter means can take different forms, but should include at least one filter of such design that a layer of fibres does not collect on the filter during the filtering process. As examples of suitable filters, bow sieves and apparatus marketed by A B Celleco under the Trade Mark Microsorter R and disclosed in their Brochure No. 87533 E - 2, are mentioned. The latter apparatus comprises one horizontal, flexibly suspended filter with meshes of 30 - 1000 u width.

The dispersion which is to be separated is sprayed in a divergent cone against the filter. The fibres do not pass the filter, while at least the impurities of smaller size than the mesh width, pass through the filter.

For removing light impurities, like pieces of plastic, cork etc., which may accompany the hydropho bicfraction from the primary flotation stage, it is often suitable to separate this fraction by a hydrocyclone separator plant into one fraction, rich in fibres, for further separation, and another, hydrophobic fraction, which is preferably combined with the material fraction having passed through the filter means, for further purification. The advantage is that hydrophobic, "light" impurities are discharged from the process, and at a stage in the process where their concentration is relatively high, which has advantages from an energy point of view.The method according to the invention has the advantage that the flotation stage can be optimized considering the best possible separation of hydrophobic compounds, and particularly long fibres are recovered extensively, while non-desirable, so called noil fibres and certain mineral impurities are to some extent removed from the fibre fraction.

The invention will now be explained in more detail with reference being made to the accompanying drawings, in which: Figure lisa diagram showing a conventional plant for recovering fibres from waste paper and comprising primary and secondary flotation stages, the operation of this plant having been described above; Figure 2 is a flow sheet diagram of a plant for carrying out a method according to the invention and comprising a primary flotation stage, a hydrocyclone separator plant for separation of a hydrophobic light fraction, and a filtering means; and Figure 3 shows schematically a plant for carrying out a method according to the invention and comprising a primary flotation stage, and a filter device in the form of an apparatus of the type Microsorter R.

In the plant of Figure 2, the raw material after certain preparatory steps for dissolving a dispersion and prepurification is fed in the form of a c:a 0.9% (weight) fibre suspension to the primary flotation stage, in which there is separated a small flow of a hydrophobic fraction containing printing ink and pieces of plastic, cork etc. This fraction is fed to a hydrocyclone separator plant including one or more hydrocyclone separators coupled in parallel. The fibres are discharged from the conical outlets of the hydrocyclone separators and the light, hydrophobic impurities are discharged through their central outlets.Thus, there is obtained one fraction containing printing ink and a certain amount of fibres, which fraction is fed to a filtering means of a type in which there is no fibre layer built up and from which a fibre fraction is discharged for recirculation to the primary flotation stage. The hydrophobic light fraction from the hydrocyclone separators is combined with the hydrophobic fraction from the filtering means, and is taken for further purification. Alfernatively, the hydrocyclone separator plant can comprise conventional hydrocyclone separators which above the usual conical and central outlets are provided with third outlets arranged coaxiallywithin the central outlets.

Such hydrocyclone separators function in such a way that a fraction rich in fibres is discharged through the central outlet, a heavy fraction consist ing of sand particles etc. is discharged through the conical outlet, and pieces of plastic, cork etc. are discharged through said third outlet.

If hydrocyclone separators of the latter type are used in the plant shown in Figure 2, a heavy reject fraction, indicated by a dashed arrow, is obtained.

In Figure 3 there is shown a plant, consisting of a flotation unit 1, a filter device 2 in the form of an apparatus of the type Microsorter R, and a purification stage 3 for the fibre fraction, consisting of a number of hydrocyclone separators, coupled in parallel, of which only one is shown in the drawing.

A line 4 for the fibre fraction is connected via an intermediate tank 5 and a pump 6 to the purification stage 3.

For the hydrophobic fraction there is a line 7, which via an intermediate tank 8 and a pump 9 is connected to the filter device 2 which comprises a horizontally suspended filter element 10 for separating the feed flow into one fraction, enriched in fibres, discharged through side outlet 11, and one impurity fraction, which having passed through the strainer is discharged through outlet 12. The fibre-rich fraction is recirculated via an intermediate tank 13, a pump 14 and a line 15 to the flotation unit 1.

In operation, a suspension containing c:a 0.9% (weight) fibres is fed to the flotation unit 1, from where a hydrophobic fraction, forming about 5-20% (volume) of the feed flow is separated and transferred to filter device 2 in the form of a 2% (weight) fibre fraction with impurities. A substantial part of the impurities is separated in the filter device, and a flow enriched in fibres is recirculated to the flotation unit.

The fraction, rich in fibres, containing c:a 0.7% (weight) fibres, is fed from flotation unit 1 to purification unit 3 for further purification.

Claims

1. A method of recovering cellulose fibres from raw material consisting of waste paper and the like, wherein the raw material is dispersed in water to form a mixture of fibres, hydrophobic impurities, such as printing ink, and light impurities, such as pieces of plastics, and/or cork, and the dispersion is separated in a flotation stage into a first fraction which is rich in fibres and fed to apparatus for purification of the fibres, and a second fraction which is enriched in hydrophobic impurities and passed for further treatment including separation by filtering into one fraction which is rich in fibres and recirculated to said flotation stage, and another fraction which is further enriched in hydrophobic impurities and passed for further purification.

2. A method of claim 1, wherein the said second fraction is separated by filtering into said one and another fractions.

3. A method according to claim 1, wherein said second fraction is separated by a hydrocyclone separator plant into fractions comprising a fraction which is rich in fibres and a hydrophobic fraction, said fibre-rich fraction being separated by filtering into said one and another fractions.

4. A method according to claim 3, wherein the hydrophobic fraction from the hydrocyclone separator plant is mixed with said another fraction separated by filtering, for fu rther purification of the two fractions.

5. A method according to claim 3 or 4, wherein a third fraction of heavy impurities is obtained from the hydrocyclone plant and is rejected.

6. A method according to claim 1 and substantially as herein described with reference to Figure 2 or Figure 3 of the accompanying drawings.