The present invention relates to chemical separation of cellulose fibres and more particularly it relates to the 5. use of high molecular weight polyalkylene oxides for sepa¬ ration of unbleached sulphate fibres from mixed cellulose pulps. The invention can be applied in connection with re¬ cycling of secondary fibres, broke etc., for subsequent production of paper, board, papertoard and similar pro-

10. ducts.

Paper _j board- paperboard etc. is for the most part manufactured from the following kinds of pulps: mechanical pulp - including both pulp prepared by purely mechanical means and thermomechanical pulp --sulphite- and sulphate

15. pulpsj and semi-chemical sulphite pulp. e.g. KSSC-pulp.

Mechanical and thermomechanical pulps contain substantial¬ ly all of the original components of the wood or the raw material, but for the other kinds of pulp, lignin and other non-cellulosic components have been removed to a

20. varying extent by pulping. Depending on the intended field of use, the pulps are subjected to further treatment in the form of bleaching. Bleaching is the term used for chemical treatment of pulps with different kinds of chemi¬ cals, e.g. chlorine, chlorine dioxide, hypochlorite, per-

25. oxide, oxygen, dithionite _. etc.,to produce a brighter pulp and it involves mainly a removal and/or a cherr.ical modifi¬ cation of the lignin.

At the production of e.g. multi-layer paper or paper board, different pulps are used fcr the different layers,

30. and broke and white water from these processes will thus contain mixtures of fibres. Mixed pulps are also obtained in the increased handling of waste paper. According to the invention mixed pulps from such processes can be worked up to different fractions and thereafter be utilized with im-

35. proved economy and in an improved technical r.anner.

Broke and reclaimed fibres from the production of paper, paper board and similar pr-cducts wherein different kinds of pulps are generally utilized in integrated sys¬ tems without preceding separation. Large quantities of

originally high quality pulp suited for the production of products with high economic value are hereby utilized in a very uneconomical manner . Considerable economic advanta ges can be gained by converting pulp blends into pure pul 5 . fractions . Upgrading pulp mixtures also gives advantages in that the subsequent production of paper products can b controlled in a better way as far as chemicals and proces variables are concerned than for systems of unspecific an varying mixtures . As examples of systems wherein differen

10. pulps are used and wherein reclaiming of broke and fibr slurry is valuable can be mentioned e . g. the production o corrugated fibreboard where the corrugated layer is made up from NSSC and/ or recycled fibre pulp , while the facing (linerboard) are made up from unbleached sulphate pulp .

15. Further, there can be mentioned multi-layer products wher in one or more layers have been produced from bleached pulp , to obtain a better appearance or to improve the pri ability , but which contain layers of unbleached sulphate order that they will possess the required strength.

20. In the handling of waste paper, in which paper or board after use , is reclaimed and after slushing and opti nally de-inking, is re-used for the production of paper products , the incoming waste material contains mixtures o pulps of different types and it is here highly desirable

25. that the material, to the greatest possible extent , can b sorted and classified to give the best utilization. The sources of the waste material vary to a great extent , e . g from households , where the largest part is made up from newsprint and magazine paper, to industries and offices ,

30. where the waste is a mixture of printing and writing pape paperboard , sacks , newsprint etc . Collected waste paper is - if sorted at all - sorted manually and it is evident that this sorting cannot give a total grading of all the material in uniform groups . A large part of the material

35. will be classified as "mixed" and used for, or as part of products with lower quality requirements , e . g. as middle- -layer in paperboard such as chipboard . Even in quantitie of waste paper of a comparatively great uniformity , mix¬ tures of cellulose pulps will of course be present as it

is impossible to obtain a total separation of these by manual sorting. As even very small amounts of a foreign kind of pulp may make an otherwise high-quality raw mate¬ rial unsuitable for the intended field of utilization, an 5 - additional separation would be of great importance even in this case.

It has now been found that polyalkylene oxides are capable of flocculating unbleached sulphate fibres (un¬ bleached sulphate fibres (unbleached sulphate, or kraft,

10. pulp) selectively from mixed woodfibre-based pulps.

It is known to use high molecular weight polyalky¬ lene oxides in the paper industry. Like other water-solub¬ le polymers, such as polyacrylamide and polyethyleneimine, they have been used as retention agents and de-watering

15. agents. These agents are added to the paper stock to give an improved retention of fibres, fines, pigments, fillers etc. on the wire and they should, at the same time, facilitate drainage and thereby increase the production capacity. The polyalkylene oxides have further been used

20. for purification of process water in the paper industry and they have then been added to the white water system in flotation type of systems to improve the total flota¬ tion of fibres and other suspended materials as described in the US patent 3,l4l,8l6.

25. As it has now been found that high molecular polyal¬ kylene oxides can flocculate unbleached sulphate fibres essentially selectively a possibility is thus obtained to chemically separate and upgrade mixtures of pulps in re¬ claimed fibres from recovery units, in broke and secondary

30. fibre raw materials to improve the economy in their further use and so that they can be used to produce products where demands on quality and uniformity are higher than now generally is the case.

The invention thus relates to the use of high mole-

35- cular polyalkylene oxides for separation of unbleached sulphate fibres from fibre mixtures being suspended in water. By adding polyalkylene oxides to such systems a selective flocculation of the unbleached sulphate fibres is obtained and the formed flocks can then be separated

by means of known separation methods such as sedimentatio flotation , filtration etc .

The polyalkylene oxides used in the invention are water-soluble nonionic polyalkylene oxides wherein the 5 . alkylene groups are lower alkylεne groups such as ethyle- ne- , propylene- , butylene groups or mixtures of these . Preferably polyethylene oxides are used. The molecular weight of the polyalkylene oxides should exceed 50,000 and can be as high as 20-10 . It is suitably above 100,000 and preferably above 500,000. 10. The 500 , 000

20-10 and preferably w th n he range - - .

The total pulp concentration in the systems wherein the polyalkylene oxides are used should not be too high a there is otherwise a risk that networks of fibres will

15. spontaneously form in a mechanical way. The concentration at which networks of fibres are formed varies with the type of fibres and the degree of beating. The formation o fibre networks can be counteracted by generating shearing forces or turbulence in the systems to disperse the fibre

20. before the addition of the polymer. In most cases it is suitable that the pulp concentration at separation accord ing to the invention does not exceed 0.9 per cent by weig and it should preferably be below 0.5 per cent by wight. The amount of polyalkylene oxide required to obtain

25. a good separation varies mainly with the pH and the ionic strength of the systems and a higher dosage is generally required at a higher pH and/or lower ionic strength. An addition of 0.001-3.0 per cent by weight based on ^* the amount of unbleached sulphate pulp in the fibre mixture

30. is suitable for most systems and the amount added is pre¬ ferably within the range of 0.01-0.5 per cent by weight. For each individual system the amount should be adjusted with respect to pH and ionic strength and over—dosage should be avoided as the flocculation of the unbleached

35. sulphate fibres will then occur so rapidly that there is risk of purely mechanical inclusion of other types of fib res in the flocks. Overdosage should also be avoided in order to avoid loading the obtained pulp fractions with foreign chemicals unnecessarily. As the amounts of poly-

alkylene oxides which are required are small and as the polyalkylene oxides are extremely viscous it is most suit¬ able to add them in the form of very dilute solutions.

With respect to the relationship between added amount 5. and pH for obtaining a good separation mentioned above, it is suitable that the systems have a lower pH, suitably be¬ low 7 and preferably within the range of 3 to 5.

When unbleached sulpahte fibres have been floccula¬ ted from pulp mixtures using polyalkylene oxides accord-

10. ing to the invention the obtained ^locks are separated in a manner known per se, e.g. by flotation, filtration, sedimentation etc., and the resulting pulp is subsequent¬ ly recovered. When the flocculated sulphate fibres are separated the turbulence in the systems should not be too

15. high to avoid the risk that the flocks break up. As an example of a suitable method for separation can be mentio¬ ned so-called micro-flotation utilizing air-bubbles of a smaller size than normally used in flotation and wherein rising ^' air-bubbles mechanically catches the formed flocks.

20. The mechanical separation can be facilitated by the pre¬ sence of surface active substances such as e.g. foaming agents.

Unbleached sulphate fibres here refers to cellulose pulp prepared by conventional treatment according to the

25. sulphate process. This type of pulp is sometimes called kraft pulp due to the strength products prepared therefrom usually have. Cooking is carried out to different yields with respect to the original weight of the raw material and in the term sulphate pulp both so-called low-yield and

30. high-yield pulps are included.

According to the invention unbleached sulphate pulp can be separated from other wood-based pulps, from bleached pulps of different kinds - bleached sulphate pulp, bleached sulphite pulp, bleached mechanical pulps - from unbleached

35- sulphite pulp, mechanical and thermomechanical pulps and so-called NSSC-pulps (neutral sulphite semi-chemical) and mixtures of such pulps. Whenever unbleached sulphate fib¬ res are present in mixtures these fibres can be separated according to the invention and a fractionation of the

/ OMPl

mixtures are thus obtained and this means that subsequent utilization is improved both with respect to technical an economical factors. The polyalkylene oxides are preferabl used for the separation of unbleached sulphate fibres fro 5. fibre mixtures originating from secondary fibre raw mater ials and from broke and recovered fibre material from the production of two- or multilayered paper products in inte grated systems. Example 1

10. In a screening-test the flocculation effect of polyethylene oxide on different types of fibres was in¬ vestigated.

Polyethylene oxide having an average molecular weight of ll.10 (according to the manufacturer) was added

15. to fibre suspensions and the flocculation effect was ob¬ served visually. At the tests the amount of added poly¬ ethylene oxide was varied up to 1 per cent by weight, based on dry fibre, the pH was varied between 3 and 9 and the concentration of NaCl between 0 and 0.1 molar.

20. The following pulps were investigated in the tests:

1. Unbleached sulphate pulp (cooked to h 7 % yield, based on the wood)

2. Unbleached sulphate pulp (cooked to 59 % yield, based 25. on the wood)

3. Groundwood pulp, mixture of bleached and unbleached . Thermomechanical pulp, mixture of bleached and un¬ bleached

5. Semi-bleached sulphate pulp

30. 6. Sulphite pulp, mixture of unbleaced and bleached 7. NSSC-pulp

It was found that the polyethylene oxide did not have any flocculating effect on the pulps 3.-7., while it in all cases flocculated the unbleached sulphate pulps 35• 1• and 2.. Example 2

The separation effect of different polyethylene oxi des on unbleached sulphate fibres under varying condition was investigated in a testing apparatus. The apparatus eo

sisted of a glass cylinder, diameter about 10 cm and height about 20 cm, which in its lower part was equipped with a glass filter disc through which nitrogen gas was ^" bubbled for flotation. 5. The pulp-system was a 50/50 mixture of bleached sulphate and unbleached sulphate, the later being cooked to a yield of ^7 % based on the wood, both having fibres within the 30-50 mesh fraction. The total fibre concentra¬ tion in the system was 200 mg/1.

10. After flotation for 20 minutes the fibre composition in the flotate was measured by means of kappa number deter¬ mination according to SCAN-C 1:77, modified for measuring small amounts .

The results of the separation tests are shown in the

15. tables below.

Table 1

Table 2

Table 3