The present invention relates to a method for at least substantially cleansing a fibre material suspension from not-readily separable contaminants in the form of elon¬ gated or flat, lightweight particles, for instance shives, plastic fragments or the like, by subjecting the suspension to a separation process which is effected at an area and/or counterpressure relationship between the light fraction outlet and the heavy fraction outlet such that when a is the relationship between the quantity of not-readily separable lightweight particles in the light fraction outlet and in the inlet, and b is the relation¬ ship between the quantity of fibre material in the last mentioned outlet and inlet respectively, a will be grea ^¬ ter, preferably much greater than b. The invention also relates to a hydrocyclone for carrying out the inventive method, and also to the use of such a cyclone in a system of cascade-connected hydrocyclones operative to cleanse a suspension of fibrous material from light contaminants.

By light contaminants is meant here and in the following particles that have properties which will preferably cause the particles to accompany a fibre suspension through the base outlet when treating the suspension in a conventional conical hydrocyclone. By heavy particles is meant such particles as those which preferably leave the hydrocyclone through its apex outlet.

It has become more and more common in recent times, to recycle _d omestic waste paper for further use. Such domestic waste paper may be plastic coated or waxed. The

bales of collected waste paper will at times also include plastics of other origins, e.g. plastic bags and sacks, in which the waste paper may have been collected, and also foamed plastic, self-adhesive tape, so-called hot melts and other contaminants. These contaminants have to be removed, before fresh paper can be manu¬ factured from the waste. The major part of those con¬ taminants which are not removed prior to introducing the paper to the recovery system are extracted with the aid of screens or with the aid of other devices which incor¬ porate screening elements. Those foreign or con¬ taminating particles which pass through the screening or filtering devices must be extracted by other means.

Fibre suspensions obtained when pulping wood chips or obtained in some other manner are also liable to include particles of plastic material and other contaminants. These other contaminants include, inter alia, shives, i.e. wood which has not been sufficiently defibred. These shives will to a large extent pass through the screens and must therefore be removed in some other way, similar to other undesired particles.

Those contaminants which remain in the fibre suspension downstream of the suspension screening section must be removed from the suspension, irrespective of its origin. Hydrocyclones are used to this end.

For many years, hydrocyclones have been used to extract from fibre suspensions small and preferably short stubby particles of greater density than wet fibre from fibre suspensions. These short stubby particles are removed through the heavy fraction outlet of the hydrocyclone, i.e. through the apex outlet of the conventional conical hydrocyclones. Accordingly, it has become usual to refer

to the fraction taken out through the base outlet as the accept.

In recent times methods have also been proposed for the extraction or removal of those particles which are not removed by the conventional operating methods of cyclones. These particles or contaminants consist of elongated particles, such as shives, flat or flake-like particles, e.g. fragments of thin plastic foil, and particles of lower densities than those of cellulose fibres, e.g. certain types of hot-melts and self-adhesive tape, and foamed plastics. These con¬ taminants are referred to in the following description and in the claims either as light particles or light contaminants or solely as contaminants. Thus, by light particles is not meant solely particles which are light¬ er than cellulose fibres, but also other particles which leave a hydrocyclone through its light fraction outlet. A method for removing light particles from a fibre suspension is described in Swedish Patent Specification No. 311 470. According to this patent such separation is achieved when the major part of the fibre flow entering a hydrocyclone is taken out through the apex outlet of the hydrocyclone and the remainder through its base outlet. The fraction taken out through the apex outlet is the accept fraction, i.e. the heavy fraction, and the fraction taken out through the base outlet is the reject fraction, i.e. the light fraction. This operational method is possible, because separation is effected under conditions in which the area and/or pressure relation¬ ship between the respective base and apex outlets of the hydrocyclones are such that less than half of the incoming fibre flow will pass through the base outlet.

The method taught by Swedish Patent Specification

311 470, using a cyclone suitable for this purpose, results in highly effective separation of light con¬ taminants from the fibre suspension when at most about 70% by volume of the suspension fed to the cyclone is taken out through the heavy fraction outlet, i.e. through the apex outlet. The separation efficiency, for instance expressed as 1 minus the ratio of the number of contaminant particles per unit of weight of fibre mate¬ rial in the accept and inject, falls radically however when the accept quantity increases, i.e. increased flow through the apex outlet. Consequently, in practice only about 65% by volume of the incoming suspension is taken out as accept through the apex outlet, at least in a primary hydrocyclone. The suspension taken out through the base outlet, the reject, must be cleansed in order to recover its valuable-fibre content. This cleansing of the reject is normally effected in up to four recovery stages in the form of cascade-connected hydrocyclones, which are usually also connected in cascade with the primary hydrocyclone. Because of the large quantities of reject which need to be treated, these recovery stages, and also the primary stage, are very large and expensive to operate. Each hydrocyclone stage will normally com¬ prise a large number of parallel-connected hydrocyclones.

When 70% by volume of the incoming suspension is taken out through the heavy fraction outlet, about 87% by weight of the fibres present will exit through this outlet. Accordingly, only about 13% by weight of the fibres present will be taken out through the light fraction outlet, despite the fact that the exiting flow constitutes 30%. The suspension exiting through the heavy fraction outlet is thus significantly thickened, i.e. obtains a higher fibre concentration. This means,

on the other hand, that large liquid quantities having a relative small concentration of fibres is obtained from the last recovering stage. It is normally necessary to evaporate off these large quantities of liquid, in order not to load the recipient.

One object of the present invention is to provide a method for cleansing a cellulose-fibre suspension from light particles, i.e. contaminants which leave the hydrocyclone through its base outlet, with good separa¬ ting efficiency, it being desired to increase con¬ siderably the flow of accept, i.e. the flow through the heavy fraction outlet, and therewith greatly reduce the amount of reject.

Another object of the invention is to provide a hydro¬ cyclone which will enable more than 70% by volume of the incoming suspension to be taken out through the heavy fraction outlet, the fibres exiting through this last mentioned outlet being substantially free from light contaminants and the volume flow treated in the recovery stages being substantially lower.

A further object is one of using a hydrocyclone, and then particularly in one or more recovery stages of a hydrocyclone plant, for greatly reducing the size of the secondary stage or stages and/or their number.

A particular object of the present invention is to recover in a multicyclone system entrained fibres from a fraction from which practically/ all light contaminant have been removed and at the same time separate out the light contaminants in a suspension which contains as little liquid as possible, all with the aid of the smallest possible number of separation stages. More

specifically, it is an object of the invention to effec¬ tively extract light particles even in the case of reject proportions beneath 30%, preferably beneath 20% and more preferably beneath 10% of the inject flow.

The invention accordingly relates in particular to a method for removing light contaminants from a fibre suspension, particularly within the forestry industry, i.e. fibres occurring in the paper pulp and paper industry, in accordance with the method set forth in the preamble of Claim 1, said method being characterized by obstructing the central axial flow towards the heavy outlet fraction at the latest at that end of the hydro¬ cyclone in which the heavy fraction outlet is located. Advantageous and preferred embodiments of the inventive method will be apparent from the claims depending from Claim 1.

The invention also relates to a hydrocyclone for carry- ing out the method according to Claim 1, which hydro¬ cyclone includes an at least substantially rotational- symmetric separation chamber having at least one inlet provided in the chamber side wall adjacent a first end of the separation chamber, a central light fraction outlet provided in an end wall at said first end, and at least one heavy fraction outlet provided at the other end of the chamber, opposite to said first end, and is characterized by a blocking device which is located in the symmetry axis of the spearation chamber and which is operative to prevent at least central axial flow towards and/or through said other end of the hydrocyclone. _Ad vantageous and preferred embodiments of the inventive hydrocyclone will be apparent from the subordinate claims dependent from Claim 7.

The present invention also relates to a further hydro¬ cyclone for carrying out the method according to Claim 1, which hydrocyclone includes at least one substan¬ tially rotational symmetric separation chamber having at least one inlet provided in the chamber wall adjacent a first end of the separation chamber, a central light fraction outlet provided in an end wall at said first end, and at least one heavy fraction outlet provided at the other end of the chamber, opposite said first end, said other end of the separation chamber merging with a apex chamber of circular cross-section, said cyclone being characterized by a blocking device which is located in the symmetry axis of the separation chamber and which is effective in obstructing at least central axial flow towards and/or through said other end of the hydrocyclone, i.e. the other end of the apex chamber. Advantageous and preferred embodiments of this hydro¬ cyclone will be apparent from the subordinate claims dependent from Claim 16.

The invention also relates to the use of a hydrocyclone for separating light contaminants from a liquid fibre suspension in an arrangement of cascade-connected hydrocyclones, in which use suspension entering a hydro- cyclone is divided into a light and a heavy fraction, of which fractions the light fraction is delivered to a downstream hydrocyclone and the heavy fraction is deliv¬ ered to an upstream cyclone in said cyclone cascade, the light fraction deriving from the last hydrocyclone of the cascade array and the heavy fraction deriving from the first hydrocyclone of said cascade array being removed from the sustem. According to preferred embodi¬ ments, at least 70% by volume, suitably at least 80% by volume and preferably at least 90% by volume of the fibre material charged to a hydrocyclone is taken out as

a heavy fraction from said cyclone and returned to the preceding stage in the cascade. The first stage of the cascade may be a hydrocyclone of the kind described in the aforesaid Swedish Patent Specification No. 311 470. Particularly the secondary hydrocyclones are cyclones constructed in accordance with the present invention.

The present invention will now be described in greater detail with reference to the accompanying drawings, in which

Figure 1 is a schematic sectional view taken through the axis of symmetry of a first embodiment of an inventive hydrocyclone; Figures 2-9 and 11 are schematic sectional views taken through the symmetry axes of different embodiments of inventive hydrocyclones, of which solely the region of the heavy fraction outlet is shown; Figure 10 is a sectional view of the hydrocyclone of Figure 1 taken on the line X-X;

Figures 12 and 13 illustrate different degrees of efficiency achieved in the separation of polyethylene particles, 0.1-0.5 mm HD-polyethylene and 0.5-1 mm LD- polyethylene particles respectively when using the inventive hydrocyclone and a hydrocyclone according to SE-PS 311 470, said efficiency being calculated as a function of the volume flow distribution through the light fraction outlet; Figure 14 illustrates schematically a hydrocyclone plant comprising three secondary cyclones and a primary cyclone, said cyclones being coupled in cascade.

Fig. 1 is a schematic sectional view taken through the symmetry axis of a hydrocyclone 1 constructed in accor- dance with the present invention. The hydrocyclone 1

comprises a rotational-symmetric separation chamber 10 which is defined by a side wall 12, which in the case of the illustrated embodiment has the form of a straight, frusto-conical part 12a which merges at its wider end, or base end, with a cylindrical part 12b. The other end of the cylindrical part 12b, i.e. the end distal from the conical part, is connected to an end wall 14 which extends at right angles to the symmetry axis or longitudinal axis 15 of the cylindrical part. The end wall 14 has provided in the centre thereof a tubular element 3 which extends concentrically with the symmetry axis 15 and projects into the separation chamber 10 and outwardly of the end wall 14. The orifice 13 of the tubular element 3 forms a light fraction outlet. The frusto-conical part 12a of the side wall 12 narrows in a direction away from the cylindrical part 12b, wherein the point or apex of said cone defines an opening 2 which forms a heavy fraction outlet. The symmetry axis 15 of the separation chamber 10 passes through the respective centres of the light fraction outlet 13 and the heavy fraction outlet 2.

The hydrocyclone 1 is also provided with at least one inlet 11, normally at least two inlets, located adjacent the end wall 14 and being tangentially and symmetrically arranged. The inlet 11 may also be spirally or helically configured, such as to engender in the hydrocyclone a symmetrical vortex with the aid of solely one inlet 11.

It is not necessary for the side wall 12 to include a cylindrical part 12b, and the side wall may alter¬ natively consist solely of a frusto-conical shell. At least a major part of the side wall 12 should be sub¬ stantially conical. By substantially conical is meant in the present context a separation chamber which has a

largest diameter at the end wall 14 and the diameter of which chamber decreases in a direction away from said end wall 14. This change in diameter need not be linear or continuous. Moreover, the inner surface of the side wall 12a may, for instance, be provided with ring-shaped or helically shaped grooves or promontories, or grooves or promontories of some other configuration.

Up to this point the described hydrocyclone conforms with a conventional hydrocyclone. The aforedescribed configuration of the separation chamber 10 and the fundamental form of the hydrocyclone is the same for all embodiments of the invention. It is also highly advan¬ tageous when the relationship between the diameter of the separation chamber 10 at the first end of the cham¬ ber and the diameter of the chamber at the other end thereof, i.e. the diameter of the opening 2, lies within a range of 2 to 6, particularly between 2.5 to 4. Fur¬ thermore, an advantage is afforded when the ratio between the diameter of the chamber 10 at said first end, i.e. adjacent the end wall 14, and the diameter of the light fraction outlet 13 lies within the range of 4 to 12, particularly in the range of 5 to 8.

The present, inventive hydrocyclone is characterized in that the central axial flow towards the heavy fraction outlet, i.e. the apex outlet, is obstructed at the latest at that end of the hydrocyclone in which the heavy fraction outlet is located. In the case of the first embodiment of the inventive hydrocyclone illu¬ strated in Fig. 1, this is achieved by locating a body 21 centrally in the heavy fraction outlet 2. The body 21 is held centrally in the outlet 2 by three arms 31, which are arranged symmetrically around the body 21 and connect said body to the inner surface of the side wall

12a of the hydrocyclone, as shown in Fig. 10.

The central body 21 is a cylindrical body which may be either longer or shorter than the illustrated body and which will thus project into the separation chamber 10 to a greater or lesser extent. The region of the separa¬ tion chamber 10 located nearest the heavy fraction outlet 2 may be of cylindrical configuration. The cross- sectional area of the central body will have at least the same diameter as the narrowest section of the light fraction outlet in those cases when the inner diameter of the element 3 varies in the longitudinal direction.

The arms 31 joining the central body 21 to the side wall 12 are preferably plate-like elements, the elements 31 having their region of smallest cross-sectional area located in a plane which extends perpendicularly to the radial plane of the separation chamber 10. That part of the element remote from the end wall 14 is, for instance, rounded so as to exert the least possible disturbing effect on the flow pattern of the medium flowing axially through the opening 2. The purpose of the arms 31 is, inter alia, to convert the vortex-like, peripheral flow to the outlet 2 into a flow which is substantially parallel to the axis of the hydrocyclone. Because the three arms 31 connecting the central body 21 to the side wall 12 divide the heavy fraction outlet into three passageways 2a, 2b and 2c, the hydrocyclone has three mutually parallel outlets.

The sole difference between the hydrocyclone illustrated in Fig. 2 and that illustrated in Fig. 1 is that the hydrocyclone of the Fig. 2 embodiment includes a central body 22 which is also cylindrical but which is not connected to the side wall 12 of the hydrocyclone by

means of arms. In this case, the end of the central body remote from the cyclone may be attached to a wall of a device carrying the hydrocyclone, in a manner not shown. In this case, the wall of the arrangement is spaced from the apex end of the hydrocyclone at a distance such as not to influence the outflow of fibre from the apex opening of said cyclone. The extent to which the central body 22 extends into the separation chamber can be varied.

The blocking device of the hydrocyclone illustrated in Fig. 3 has the form of a disc which is spaced from the apex end of the side wall 12. The outlet is formed by the gap defined between the apex end of the side wall 12 and the disc plate 23. The element 23 may also comprise a wall of the arrangement in which the hydrocyclone is placed during operation. The element 23 may also be provided with a cylindrical device 25a, shown in broken lines. The cylindrical device 25a may be provided with screw threads and screwed into a screw-threaded bore provided in the element 23. This will enable adjustments to be made to the extent to which the member 25a extends into the separation chamber 10. The maximum extent of this projection of the member 25a into the chamber should be at most about 33% of the length of said cham¬ ber.

In the case of the embodiments illustrated in Figs. 4 and 5, the hydrocyclone is provided with a second end wall 24 which sealingly closes the other end of the hydrocyclone. The heavy fraction outlet is located in the side wall 12 adjacent said other end. The outlet may comprise at least two mutually symmetrical tangential outlets 40 or at least one spiral or helical outlet 40. The hydrocyclone according to Fig. 5 is provided with a

central body 25 which comprises two cylindrical parts

25a and 25b, the part 25b having a diameter larger than that of 25a. The diameter of the part 25b is at least equal to the smallest diameter of the light fraction outlet 13.

The hydrocyclones according to Figs. 7-9 and 11 differ from the embodiments illustrated in Figs. 1-5 and 10 in that the said other end of the separation chamber 10, i.e. the end opposite the end wall 14, merges with an apex chamber 33 of circular cross-section. Thus, the apex chamber 33 has a conical part which widens in a direction away from the separation chamber 10 and then merges with a cylindrical part 32. In the case of the Fig. 6 embodiment, the blocking device consists of a central body 26 whose diameter is at least equally as large as the smallest diameter of the light fraction outlet 13. The central body 26 is connected to the side wall 32 of the apex chamber by means of four radial arms 36, which form a cross. The central body of the Fig. 6 embodiment projects solely into the apex chamber 33, although it may also project into the separation chamber 10. The heavy fraction outlet comprises four mutually parallel passageways which are defined by the arms 36, the central body 26 and the side wall 32 of the apex chamber.

The hydrocyclone of the Fig. 7 embodiment differs from the Fig. 6 embodiment, in that the blocking device has a different configuration and lacks arms which connect the device to the side wall 32. The blocking device com¬ prises a central body 27 having two cylindrical parts, the part facing towards the separation chamber 10 having the larger diameter. The central body 27 may be attached to an arrangement in which the hydrocyclone can be

placed in operation. This central body 27 may also extend into the apex chamber 33, and also into the separation chamber 10.

Figs. 8 and 9 illustrate two embodiments in which the blocking devices of the hydrocyclones comprise an end wall 28 and 29 provided in respective apex chambers. At least one output 34 is located adjacent to the end walls 28, 29, which outlet may be of spiral or helical con- figuration. Preferably at least two outlets 34 are arranged symmetrically and formed tangentially. The end wall 29 of the cyclone illustrated in Fig. 9 is provided with a central body 39, the diameter of which is greater than the diameter of the light fraction outlet 13. The central body 39 is able to extend to different levels in the apex chamber and also to different levels in the separation chamber, although to a maximum which cor¬ responds to a third of the length of the separation chamber.

The blocking device of the hydrocyclone illustrated in Fig. 11 has the form of a disc-shaped element 30 which is spaced from the other end of the apex chamber 33. This embodiment resembles the embodiment of Fig. 3. The element 30 is spaced from the end of the apex chamber wall 32 such as to obtain a radial opening between the element 30 and said end of the wall 32. The element 30 may also be provided with a central body 41, as illu¬ strated in broken lines. Although the central body 41 is shown to extend into the separation chamber 10, the body may be of shorter length, so as to extend solely into the apex chamber 33.

When the blocking device is or includes a central body, it is important that the end of the central body facing

towards the light fraction outlet 13 has a circular cross-section. The remainder of the central body may have some other configuration, provided that the afore¬ said circular part has a larger radius than the largest dimension of said remaining part in a radial direction from the symmetry axis. The central body may also be a cone or a truncated cone, the cone base facing towards the outlet 13. It is also highly advantageous when the end of the central body facing the outlet 13 has a larger diameter than the outlet 13. When the internal diameter of the tubular element 3 varies along the longitudinal extension of said element, it is the smal¬ lest diameter of the outlet that is meant.

Tests have been carried out on different embodiments of the inventive hydrocyclone, some of the results of which are shown in the following Table. Test 1 was carried out with a hydrocyclone of the form illustrated in Fig. 1, although with the exception that the central body 21 of the Fig. 1 embodiment was replaced with the central body 25 of the Fig. 5 embodiment. The central body was con¬ nected to the side wall 12 by means of three arms 31. The separation chamber 10 had a length of about 50 cm and in the region of the first end adjacent the end wall 14 had a diameter of 80 mm and was provided at this end with two tangential inlets 12. The diameter of the apex opening 2 was 30 mm and the diameter of the light frac¬ tion outlet 13 was 16 mm. The largest diameter of the central body 25 at the end 25b facing the light fraction outlet was 20 mm. The insertion length of the central body 25, i.e. the distance between the end of the cen¬ tral body 25b located in the separation chamber 10 and the pointed or apex end of the separation chamber was 75 mm.

Test 2 was carried out with a hydrocyclone in which the insertion length of the central body was only 40 mm, this being the only difference between the hydrocyclone used in test 2 and the hydrocyclone used in test 1.

The hydrocyclone used in test 3 was identical with the hydrocyclone used in test 1 with the following excep¬ tions: The light fraction outlet 13 had a diameter of 13 mm and the apex opening 2 of the separation chamber had a diameter of 25 mm.

The hydrocyclone used in test 4 was identical with the hydrocyclone used in test 3, with the exception that the insertion length was 50 mm.

Test 5 was carried out with a hydrocyclone which had the configuration illustrated in Fig. 4. The hydrocyclone had the same size as the hydrocyclones used in the other tests and was provided with two tangential outlets adjacent the blocking device, i.e. the other end wall 24. The diameter of the hydrocyclone in the region of the blocking device was 25 mm.

A bleached softwood sulphate pulp having a concentration of 0.2% was used in all of the tests. The pulp was admixed with light contaminants in the form of coloured polyethylene particles. This rendered the tests more reproduceable. Separate tests were carried out with light contaminants of two mutually different kinds: HD- polyethylene of 0.1-0.5 mm and LD-polyethylene of 0.5- 1.0 mm. The hydrocyclone injection pressure was 250 kPa in all tests. The accept pressure was maintained at about 150-160 kPa. The efficiency of the separation processes was estimated by counting all of the conta i- nants present in five sheets hand made from the inject

and ten sheets hand formed from the accept.

The results are shown in Table 1, wherein Rq represents the percentage of the flow passing through the light fraction outlet 13 and the recited numerical values signify the separation efficiency calculated on dry solids content. The separation efficiency, E, is cal¬ culated as one minus the ratio of the number of specks (contaminating particles) per unit weight of dry sheet formed from the accept fraction and the number of specks per unit weight of dry sheet formed from the inject fraction.

Further tests were carried out in order to illustrate the difference between the inventive hydrocyclone and a hydrocyclone constructed in accordance with Swedish

Patent Specification 311 470. The tests were carried out with a hydrocyclone constructed in accordance with the Fig. 1 embodiment, with the central body 21 of this embodiment being replaced with the central body 25 of the Fig. 5 embodiment. The diameter of the cylindrical part of larger diameter was 20 mm. The central body 25 was connected to the side wall 12 by means of three arms 31 and had an insertion length of 75 mm. In other res¬ pects, the hydrocyclone was identical to the described hydrocyclone. The separation chamber had a largest

diameter of 80 mm and a smallest diameter at the apex opening of 30 mm. The light fraction outlet 13 had a diameter of 16 mm. The inject pressure was 230-240 kPa in all tests, and the reject pressure, i.e. the counter pressure externally of the light fraction outlet 13, was 30-40 kPa. A bleached softwood sulphate pulp was used in all tests, said pulp being admixed with contaminating particles of HD-polyethylene and LD-polyethylene respec¬ tively, of the same kind as those used in the earlier tests. The results obtained in tests carried out on the separation of HDPE-particles from a fibre suspension having a fibre concentration of 0.2 percent by weight are illustrated in Fig. 12. Curve I in Fig. 12 illustra¬ tes the result obtained with the inventive hydrocyclone, whereas curve II illustrates the result obtained with the hydrocyclone constructed in accordance with the afore-said publication. Fig. 13 illustrates the result obtained with a fibre concentration of 0.6% and with a suspension containing LD-polyethylene contaminants. In Figs. 12 and 13, the vertical axis E signifies the separation efficiency and the horizontal axis Rq repre¬ sents the reject flow ratio, i.e. the flow through the light fraction outlet divided by the flow through the inlet. It is clearly shown in Figs. 12 and 13 that the inventive hydrocyclone (curve I) can be used with good separation efficiency with significantly smaller quan¬ tities of reject than the conventional hydrocyclone (curve II) according to the aforesaid publication.

Results similar to those in the aforegoing were obtained with hydrocyclones according to Figs. 6-9 and 11.

Other tests have shown that when using a blocking device which has a smaller diameter than the smallest diameter of the light fraction outlet 13, a poorer separation

efficiency is obtained, particularly in the case of low reject volume percentages, than when using blocking devices whose diameters are the same or larger than the diameter of the light fraction outlet 13. It has also been found that the efficiency is significantly impaired when the base diameter in the tested hydrocyclones exceeded 125 mm.

Fig. 14 illustrates a hydrocyclone plant in which four cyclones are connected in cascade. Pumps and other necessary arrangements are not shown in the figure. Fibre suspension is conveyed through a conduit 55 to a hydrocyclone 51, which is the primary hydrocyclone or the primary hydrocyclone stage. Each hydrocyclone stage includes a number of parallel-connected hydrocyclones. In the following the term hydrocyclone or, simply cycl¬ one, alone signifies one or more parallel-connected hydrocyclones. The incoming flow is divided in the primary hydrocyclone into an accept flow, which exits through the heavy fraction outlet, and a reject flow, which exits through the light fraction outlet 13. The reject is passed through a conduit 57 to a first rec¬ overy stage, which comprises a hydrocyclone 52 in which the flow arriving from the primary hydrocyclone 51 is again divided into an accept flow and a reject flow. The accept flow from the first hydrocyclone 52 in the rec¬ overy section is passed through a conduit 58 to the conduit 55, in which latter conduit the accept flow is admixed with the fibre flow entering the primary hydro- cyclone 51. The degree of contamination in the conduit 58 from the first secondary hydrocyclone should be of the same order of magnitude as the degree of cont¬ amination of the suspension passing through the conduit 55.

The reject exiting from the hydrocyclone 52 is conducted through a conduit 59 to the second hydrocyclone 53 of the recovery stage, from which the resultant accept is passed through a conduit 60 and combined in this conduit with reject obtained from the primary hydrocyclone 51. The reject from the second hydrocyclone 53 of the reco¬ very stage is passed through a conduit 61 to a third hydrocyclone 54 of the recovery stage. The suspension entering this third hydrocyclone is divided into an accept flow, which leaves the hydrocyclone 54 through a conduit 62 and is admixed with the reject suspension from the hydrocyclone 52 in the conduit 59, before being returned to the second hydrocyclone 53 of the recovery stage. The reject flow obtained from the third hydro- cyclone 54 of the recovery stage leaves the system through a conduit 63.

Since the hydrocyclone according to the aforesaid refe¬ rence will provide a better cleansing effect in the case of relatively large quantities of reject than the inven¬ tive hydrocyclone, it is advantageous to use the known hydrocyclone in the first or primary hydrocyclone stage, so as to obtain a highly pure accept. This applies in particular when the incoming suspension has a high fibre content, e.g. 0.6%. Since the reject suspension obtained will be more diluted than the incoming suspension, the hydrocyclones in the recovery stage will work with lower fibre contents. In accordance with one advantageous embodiment, the hydrocyclone known from the aforesaid publication is used as the primary hydrocyclone and one or more of the inventive hydrocyclones is/are used in the secondary stage.

In order to show by comparison the advantages of the in¬ vention, the following Examples are given below and in Table II for a prior art system and for a system modified by utilizing hydrocyclones according to the invention.

Example I

In a prior art system for producing hardwood kraft pulp, a primary hydrocyclone cleaning stage for eliminating light impurities is fed with a suspension flow of 96.000 liters per minute at 0.655- fiber concistency. The accept flow is 38.112 liters per minute, with a fiber concintency of 1.^40 , i.e. suitable for feeding to a Fourdrinier machine. The reject from the primary stage is led to a cascade of four secondary hydrocyclone stages, and the accept from the first secondary stage is led back for stock dilution. The feed data are given in Table II.

Example II

In a system designated according to the invention, having the same input and output features for the primary stage , the four secondary stages of Example I were exchanged for two secondary stages provided with hydrocyclones according to the invention. As appears from Table II, the total sum of feed flows in the secondary stages thereby diminishes from 343,200 liters per minute to 167,760 liters per minute, i.e. by a factor of about 2. At the same time, and in spite of halving the number of secondary stages from four to two, the reject flow is reduced by a factor of about 20.

The cost of the installation is roughly proportional to the total sum of feed flows, as more feed flow means that there must be a higher number of hydrocyclones in the respective stages, and more conduits, more pump capacity, etc., are

needed. Further, power consumption will also be reduced accordingly. The Examples therefore show that the invention leads to substantial savings both in capital and operative expenses. To this comes a substantial saving in factory space.

TABLE II

Example I Example II

Feed (1/min) 96,000 96,000 to primary stage

Number of secondary (4) (2) stages

Feeds to secondary 96,000 stages (1/min) 74,000 65,120 50,800 6,640 26,400

Sum total of all 343,200 167,760 secondary feed flows

(1/min)

Reject flow from 13,200 664 last stage (1/min)