SE469279C - Methods and apparatus for releasing a fiber suspension from light impurities - Google Patents

Description

469 279 2 Irrespective of the origin of the fiber suspension, the contaminants found in the fiber suspension after screening must be removed. Hydrocyclones are used for this purpose.

Hydrocyclones have been used for many years to remove small and preferably chubby particles of higher density than wet fiber from fiber suspensions. These chubby particles are taken out through the hydrocyclone drain for heavy fraction, i.e. the tip drain in the usually conical cyclones. It has therefore been customary to call the fraction taken through the base drain acceptable.

Recently, there have also been methods of removing particles which are not removed in the conventional mode of operation of hydrocyclones. These particles or contaminants consist of elongate particles, such as skewers, flat or flag-like particles, such as pieces of thin plastic foil, and particles with a lower density than cellulosic fibers, such as certain types of hot-melt and self-adhesive tape and foam plastic.

These impurities are referred to in the following description and in the claims as light particles, light impurities or only impurities. Thus, light particles refer not only to particles which are lighter than cellulose fibers, but also to other particles which in a hydrocyclone are taken out through the drain for light fraction.

A method of removing light particles from a fiber suspension is described in Swedish patent specification 311470. According to this patent specification such a separation is obtained if most of the fiber stream entering a hydrocyclone is taken out through the tip opening of the hydrocyclone and the remainder through its base opening. the fraction taken out through the tip opening is the acceptance fraction, i.e. the heavy fraction, and the fraction taken out through the base outlet is the reject fraction, i.e. the light fraction. The stated mode of operation is made possible by the separation being carried out at such area and / or pressure conditions between the outlet of the hydrocyclone at the base and the tip that less than half of the incoming fiber flow passes through the base opening. The process for carrying out the process specified in Swedish patent specification 311470, using a cyclone suitable for this purpose, provides very good separation of the fiber suspension from light impurities when a maximum of about 70% by volume of the amount of suspension fed to the cyclone is taken out. through the drain for heavy fraction, ie the tip drain. However, the separation efficiency, for example expressed as 1 minus the ratio of the number of contaminant particles per unit weight of fibrous material in acceptance and injection, decreases sharply when the amount of acceptance is increased, i.e. the flow through the tip drain. This means that in practice, at least in a primary hydrocyclone, only about 65% by volume of the added suspension is taken as acceptance through the tip drain. The suspension taken out through the base drain, the reject, must be purified to recover valuable fibers contained therein. This purification is usually carried out in up to four recovery steps in the form of cascaded hydrocyclones, which are usually cascaded even with the primary hydrocyclone. Because such large quantities of reject have to be treated, these recycling steps, like the primary step, become very large and costly to operate. Each hydrocyclone stage usually consists of a large number of hydrocyclones connected in parallel.

When 70% by volume of the incoming amount of suspension is withdrawn through the heavy fraction outlet, about 87% by weight of the incoming amount of fibers is generally withdrawn through this drain.

This means that the amount of fiber taken out of the effluent for light fractions is only about 13% by weight, despite the fact that the flow taken out is 30 per cent. Thus there is a significant thickening, i.e. higher concentration of fibers in the suspension through the heavy fraction drain. This means, on the other hand, that from the last recovery step one obtains large amounts of liquid containing relatively low concentration of fibers. These large amounts of liquid must usually be evaporated in order not to load a recipient.

A general object of the present invention is to provide a method of purifying a cellulosic fiber suspension from light contaminants, i.e. contaminants leaving the hydrocyclone through its base drain, with good separation efficiency, whereby it is desired to be able to substantially increase the acceptance flow ie the flow through the drain for heavy fraction, and thereby significantly reduce the amount of reject.

Another object of the invention is to provide a hydrocyclone with which more than 70% by volume of the incoming amount of suspension can be withdrawn through the heavy fraction effluent, the fibers withdrawn through the latter effluent being substantially free of light impurities and that in the recovery steps treated volume flow is significantly lower.

A further object is the use of a hydrocyclone, especially in one or more recovery stages of a hydrocyclone plant, to greatly reduce the size of the secondary stage or stages and / or their number.

A special purpose is that in a system of multicyclone aggregates from a fraction, where light contaminants have been almost completely removed, recycled entrained fibers and at the same time separate the light contaminants in a suspension that contains as little liquid as possible, and this using as little as possible number of separation steps. More specifically, the aim is to achieve a good separation effect for light particles even at reject parts below 30%, preferably below 20% and preferably below 10% of the injection flow.

The present invention thus relates in particular to a method of purifying a fiber suspension, in particular in the forest industry, i.e. the pulp and paper industry, existing fibers, from light contaminants according to the method stated in the preamble of claim 1, which method is characterized by central axial flow in direction towards the heavy fraction drain is prevented at the latest at the end of the hydrocyclone, at which end the heavy fraction drain is arranged. Advantageous and preferred embodiments of the stated method appear from the dependent dependent claims of claim 1. EP-A-0 234 101 discloses a hydrocyclone intended to eliminate light contaminants from fiber suspensions having a conical separation space, at the tip end of which a centrally located finger is inserted, which is intended to prevent a central, axial current directed towards the tip reaches the outlet of the tip.

The object of the invention is to provide an improved method and an arrangement of the step which is initially stated, and which is particularly suitable for use in secondary hydrocyclones, for recovering fibers from the reject stream from a more normal "reverse" hydrocyclone, which gives a cleaner acceptance at the cost of a larger amount of reject.

This object is achieved according to the invention by a method or a device which has the features stated in the characterizing part of claim 1 and claim 4, respectively. A preferred application is stated in claim 19.

According to preferred embodiments of the invention, at least 70% by volume, preferably at least 80% by volume, especially at least 90% by volume of fibrous material fed to a hydrocyclone is taken as a heavy fraction from this hydrocyclone, which heavy fraction is added to the previous step in the cascade. The first step in the cascade may be a hydrocyclone according to the cited Swedish patent specification 311470. In particular the secondary hydrocyclones are hydrocyclones according to the present invention.

The invention will now be described in more detail on the basis of non-limiting exemplary embodiments and with reference to the drawings.

Fig. 1 shows a section through an axis of symmetry of a first embodiment of the present hydrocyclone. Figs. 2-4 show other embodiments in the form of sections through the axis of symmetry, only the area of the heavy fraction drain being shown.

Fig. 5 schematically shows a hydrocyclone plant with three secondary cyclones and a primary cyclone, which cyclones are cascaded.

Figures 6 and 7 show representative curves for the separation efficiency for separation of polyethylene particles, 0.1-0.5 mm HD polyethylene and 0.5-1 mm LD polyethylene, respectively, for hydrocyclones according to the invention and for hydrocyclones according to SE-PS 311470 as a function of the volume flow distribution through the light fraction outlet.

Fig. 1 schematically shows a section through the axis of symmetry of a hydrocyclone 1 according to the invention. The hydrocyclone 1 has a rotationally symmetrical separation chamber 10, which is delimited by a side wall 12; which in the embodiment shown consists of a straight, truncated conical part 12a, which at its wider end or base merges into a cylindrical part 12b. The cylindrical part 12b is at its other end, i.e. the end facing away from the conical part, connected to an end wall 14 as perpendicular to the symmetry or longitudinal axis of the cylindrical part 15. The end wall 14 is provided in its central part with a tubular element 3, which is concentric with the axis of symmetry 15, projects into the separation chamber 10 and projects beyond the end wall 14. The opening 13 of this tubular element 3 forms a drain for light fraction. The frustoconical portion 12a of the side wall 12 tapers in the direction of the cylindrical portion 12b. The symmetry axis 15 of the separation chamber 10 passes through the center of the drain 13 for light fractionation.

The hydrocyclone 1 further has at least one inlet 11 adjacent the end wall 14, usually at least 2, which inlets are arranged tangentially and symmetrically. The inlet 11 can also be helical or helical, so that even with only one inlet 11 a symmetrical vortex in the hydrocyclone is obtained. It is not necessary that the side wall 12 comprises a cylindrical part 12b but may consist of only one truncated conical jacket. At least a large portion of the side wall 12 should be substantially conical. By substantially conical is meant in the present case that the separation chamber at the end wall 14 has the largest diameter and that its diameter decreases in the direction from the end wall 14. This change of the diameter does not have to be linear or continuous. Furthermore, the inside of the side wall 12a may, for example, have annular, helical or otherwise shaped grooves or elevations.

So far, this description corresponds to a conventional conventional hydrocyclone. This appearance of the separation chamber 10 and the basic shape of the hydroyclone are the same for all embodiments of the invention. It is furthermore very advantageous that the ratio between the diameter of the separation chamber 10 at the first end and the diameters at the second end is within a range of 2 to 6, in particular of 2.5 to 4. Furthermore, it is advantageous that the ratio between the diameter of the separation chamber 10 at the first end, i.e. next to the end wall 14, the diameter of the drain 13 for light fraction is in the range 4 to 12, especially of 5 to 8.

In the embodiments shown according to Figs. 1-4, it is common that the separation chamber 10 at its second end, opposite the wall 14, merges into a tip chamber 33 with a circular cross-section. The tip chamber 33 has a conical part which widens from the separation chamber 10 and then merges into a cylindrical part 32. In the embodiment shown in Fig. 4, the blocking means consists of a central body 26 with at least as large a diameter as the minimum diameter of the drain 13 for light fraction. The central body 26 is connected to the side wall 32 of the tip chamber via four radial arms 36, which form a cross.

According to Fig. 4, the central body projects only into the tip chamber 33. The heavy fraction drain consists of four parallel channels, which are delimited by the arms 26, the central body 26 and the side wall 32 of the tip chamber. 469 279 s The hydrocyclone according to Fig. 3 differs from that in Fig. 4 by the blocking member has a different appearance and lacks connecting arms with the side wall 32. The blocking means consists of a central body 27, which consists of two cylindrical parts, the part facing the separation chamber 10 having the larger diameter. The central body 27 may be attached to a device in which the hydrocyclone may be located during operation. This central body 27 can also be inserted into the tip chamber 33.

Figures 1 and 2 show two embodiments, where the tip chamber 33 of the hydrocyclone is provided with an end wall 28 and 29, respectively, which forms the blocking means. Adjacent to the end wall 28, 29 is at least one drain 34, which may be helical or helical. Preferably, there are two drains 34, which are arranged symmetrically and designed tangentially. The end wall 29 of the hydrocyclone in Fig. 1 is provided with a central body 39 with a larger diameter than the drain 13 for light fraction diameters. The central body 39 can extend to different levels in the tip chamber.

In the case where the blocking means is or comprises a central body, it is essential that the end of the central body facing the drain 13 for light fraction has a circular cross-section. The remaining part may have a different appearance, provided that the specified part has a larger radius than the largest dimension of the other part in the radial direction from the axis of symmetry.

The central body can also be a cone or truncated cone, the base of the cone facing the drain 13. It is furthermore very advantageous that the end of the central body facing the drain 13 is larger than the diameter of the drain 13. If the inner diameter of the tubular element 3 varies along the longitudinal direction, the smallest diameter of the drain is meant.

Experiments with different embodiments of the present hydrocyclone have been performed, some representative of which are reported in Table 1. 9 469 2 ”9 In all experiments, bleached softwood sulphate pulp with a concentration of 0.2 percent was used. To the mass were added colored plastic particles of polyethylene as light contaminants. Thereby, the experiments can be made more reproducible. Separate experiments were carried out with light contaminants of two different types: HD polyethylene of 0.1-0.5 mm and LD polyethylene of 0.5-1.0 mm. The injection pressure to the hydrocyclone was 250 kPa in all experiments.

The acceptance pressure was maintained at about 150-160 kPa. The separation was calculated by counting all contaminants in five hand sheets injected and 10 hand sheets accepted.

The results are reported in Table 1, where Rq denotes the volume flow fraction through the effluent 13 for light fraction, and the stated numerical values denote the separation effect calculated on dry matter. The separation effect, E, is calculated as 1 minus the ratio of the number of dots (contaminant particles) per unit weight of dry sheet of the acceptance fraction and of the number of dots per unit of weight of dry sheet of the injection fraction.

Table 1 Rq = 0.05 Rq = 0.10 Rq = 0.20 Rq = 0.30 HDPE LDPE HDPE LDPE HDPE LDPE HDPE LDPE Experiment 1 0.38 0.90 0.89 0.95 0.91 0.97 0.92 0.98 Experiment 2 0 0.69 0.89 0.97 0.91 0.98 0.88 0.98 Experiment 3 0.86 0.97 0.88 0.98 0.92 0.99 0.95 0.99 Experiment 4 0.30 0.74 0.92 0.99 0.94 0.99 0.95 0.95 0.99 Experiment 5 0.03 0.06 0.88 0.98 0.90 0 .99 0.92 0.99 Further attempts were made to illustrate the difference between a hydrocyclone according to the invention and a hydrocyclone according to the Swedish patent specification 311470.

In the experiments, bleached softwood sulphate pulp was used with the addition of contaminant particles of resp. HD polyethylene and LD polyethylene of the same type as in the previously stated experiments. The result of attempts to separate HDPE particles from a fiber suspension with a fiber concentration of 0.2% by weight is shown in Fig. 6. In this case, curve I denotes the result obtained with a central base outlet stop and curve II the result from the hydrocyclone. according to the aforementioned reference. In Fig. 7, the results are reported for a fiber concentration of 0.6 per cent and for impurities of LD-polyethylene. In Figures 12 and 13, E denotes the separation efficiency and Rq the reject flow ratio, i.e. the flow through the effluent for light fraction divided by the flow through the inlet. It is clear from these Figures 6 and 7 that a hydrocyclone according to the invention (curve I) can be used with good separation ability at much smaller rejection amounts than the conventional hydrocyclone (curve II) according to the reference given.

Other experiments have shown that when using a blocking means with a smaller diameter than the smaller fraction of the drain 13 for lighter fraction, a poorer separation efficiency is obtained, especially at low reject volume proportions, than when using blocking means with an equal or larger diameter than the drain 13 for light fraction diameter. It has also been found that the effect is greatly impaired if the base diameter exceeds 125 mm in configurations of the type tested.

Fig. 5 shows a hydrocyclone plant with four cascaded cyclones, pumps and other necessary devices not shown. Incoming fiber flow passes through line 55 to the hydrocyclone 51, which is the primary hydrocyclone or the primary hydrocyclone stage. Each hydrocyclone stage includes a number of hydrocyclones connected in parallel. In the following, hydrocyclone or cyclone alone denotes one or more hydrocyclones connected in parallel. In the primary hydrocyclone, the incoming flow is divided into an acceptance flow through the heavy fraction drain and a reject flow through the light fraction drain 13. The reject stream is passed through a line 57 to a first recovery stage, which consists of hydrocyclone 52, in which hydrocyclone the stream coming from the primary hydrocyclone 51 is again divided into an acceptance stream and a reject stream. The acceptance stream from the first hydrocyclone 52 in the recovery section is passed through line 58 to line 55, in which line this acceptance stream is mixed with the fiber stream entering the primary hydrocyclone 51. The degree of contamination in line 58 from the first secondary hydrocyclone should be of the same order of magnitude as that present in the suspension through line 55.

The reject from the hydrocyclone 52 passes through line 59 to the second hydrocyclone 53 in the recovery step, from which the obtained acceptance is passed through line 60 and joined in the line with the reject from the primary hydrocyclone 51.

The reject from this second hydrocyclone 53 in the recovery step is passed through a line 61 to the third hydrocyclone 54 in the recovery step. In this, the incoming suspension is divided into an acceptance stream, which leaves the hydrocyclone 54 through a line 62 and is combined with the reject suspension from the hydrocyclone 52 in the line 59 before it is fed to the second hydrocyclone 53 in the recovery step. The reject flow from the third hydrocyclone 54 in the recovery step leaves the system through line 63.

Since the use of the hydrocyclone according to SE-PS 311470 results in a better purification at a relatively large rejection rate than with the present hydrocyclone, it is advantageous that in the first or primary hydrocyclone the known hydrocyclone is used to obtain a very pure acceptance. . This is especially true when the incoming suspension has a high fiber content, eg 0.6 percent. Since a reject is obtained as a more dilute suspension than the incoming one, the hydrocyclones in the recovery step will work with lower fiber contents. According to an advantageous embodiment, the hydrocyclone known from the reference and the hydrocyclone present in one or more second steps is used as the primary hydrocyclone.

Claims (4)

10 15 20 25 30 35 469 279/2 Patent claims 1. A method of at least substantially freeing the suspension from elongated or flat, light particles, such as contaminants, such as sputum, plastic fragments and the like, by separating a suspension of fibrous material in liquid in a substantially conical hydrocyclone. the ring is made at such an area and / or back-pressure ratio between the drains for light and heavy fraction, that if a is the ratio between the amount of difficult-to-separate light particles in the drain for light fraction and in the inlet and b the ratio between the amount of fibrous material in the latter drain or inlet , a is larger, preferably substantially larger than b, and in the hydrocyclone an obstacle is arranged for central axial flow through the heavy fraction drain, characterized in that the hydrocyclone is provided with a tip chamber with a circular cross-section, which connects widening at the narrowest part of the hydrocyclone separation space, the central axial flow for heavy freight ion is prevented by the obstacle arranged in the tip chamber at the latest at the end of the hydrocyclone, in which end the heavy fraction drain is arranged, and that no more than 20% by volume of supplied suspension is taken out through the light fraction drain. 2. A method according to claim 1, characterized in that in the tip chamber opposite the mouth of the separation space a central blocking member is arranged therein, which at its end facing the drain (13) for light fraction has a circular cross-section. 3. A method according to claim 2, characterized in that the ratio between the diameter of the separation chamber at the base drain end and the diameter of the base outlet is made to be in the range 4 - 12. 10 15 469 279- / 3 Hydrocyclone plant with several steps each consisting of a number of parallel-connected hydrocyclones, which are cascaded, and intended for separating light contaminants from a fiber suspension supplied to a first-stage inlet, wherein purified suspension is arranged to flow out as a peak flow acceptor, k. by the fact that the first stage is composed of hydrocyclones without a central blocking means, with a maximum of 70% by volume of the added suspension through the heavy fraction effluent, while one or more secondary stages are composed of hydrocyclones, which are provided with each tip chamber having a circular cross-section, within which an obstacle to central axial flow is arranged, and adapted for a reject portion of supplied suspension of less than 20% by volume.