A PROCESS FOR PURIFICATION OF A FIBRE SUSPENSION

Abstract: A process for separating a fibre suspension wherein undesired impurities, such as printing ink and other contaminating substances, are separated from the fibres in a flotation tank and the fibre suspension in admixture with gas is introduced into the tank via directed jets having a high inlet velocity. The tank has inlet openings for the mixture of suspension advantageously located at or close to the bottom of the tank and the jets of suspension are directed upwards, towards the free surface of the suspension in the flotation tank.

Description: A process for purification of a fibre suspension The present invention relates to a process for separating substances from a fibre suspension and more particularly to such a process wherein impurities, such as for example print - ing inks and other contaminating substances, are separated from the fibres in a flotation tank into which the fibre suspension in admixture with gas is introduced by means of directed jets having a high inlet velocity. Flotation is a separation process which on an industrial scale primarily is utilized in mineral treatment and for purification of recycled fibres in order to separate undesirable components by treatment of a liquid based system of solid materials. Flotation processes can be carried out using methods wherein the essential separation force is either gravitation or centripetal acceleration. The flotation process is based on introduction of a gas, usually air, to which hydrophobic particles are attracted and subsequently can be separated in a foam phase. Different auxiliary chemicals are often used, especially surface active agents, for example in order to improve the hydrophobicity of the particles to be separated, to enhance foam formation etc. In the pulp and paper field, treatment of recycled fibre by processes for removal of ink, often combined with bleaching, for production of recycled fibre pulp with a desired brightness, is a more and more growing area. The raw material, the recycled paper, is beaten with a high intensity while being dissolved to a consistency which usually is around about 15 per cent by weight. Internal friction in the suspension due to high consistency causes strong shearing forces in the suspension which, when the fibres are rubbed against each other, results in release of ink and other impurities. In this preliminary step additions of certain chemicals can be made, for example addition of surface active chemicals for the subsequent flotation process and/or of bleaching chemicals such as hydrogen peroxide for later bleaching steps. After the initial slushing step there normally follows one or several screening steps in order to remove larger particles before the suspension is prepared for the actual removal of the printing ink. The suspension is hereby normally diluted to a consistency of about 1 per cent by weight. Although addition of chemicals usually is made at the slushing, it is also possible to add auxiliary chemicals for the flotation at the dilution step. At the actual flota- tion the fibre suspension and air are introduced separately or in admixture into a flotation tank or cell and separation of the ink particles occurs as the particles adhere to the air bubbles and can be separated in the form of a foam. Subsequent treatment usually comprises certain additional separ- ation steps and dewatering, dispersing, bleaching and dilution. The flotation treatment and optional subsequent steps are often repeated once or several times. To reduce fibre losses in the flotation the separated foam is usually treated in a secondary flotation step. In contrast to flotation processes in the mineral industry there are special problems connected with achieving a good separation at flotation treatment of fibre suspensions, since the fibres form networks which obstruct the transport of the gas bubbles through the suspension. Due to this it has been necessary to work with very low fibre consistencies in the suspensions, which means that extremely large amounts of water have to be handled and this is, of course, a great disadvantage both with regard to economy and environment . Within the field of flotation of fibre suspensions an increase of the fibre concentration in the suspension which is small in terms of percentage thus involves an extremely substantial reduction of the total amount of water and a reduction of the flotation time is likewise an extremely important advantage. One object of the present invention is to make treatment at increased fibre concentrations possible in purification of fibre suspensions, particularly in deinking of recycled fibre. The process of the invention makes this possible with short flotation times and good purification results and thus offers an improved process, both from an economical and en- vironmental point of view. The process according to the present invention is based on the insight of the importance of the rheological properties of the fibre suspension and particularly the breaking up of the fibre network and where the flow field in the tank exceed the yield stress, in order to make it possible to have an early release of the air bubbles and a facilitated transport of the aggregates of bubbles and particles through the medium. The present invention thus relates to a process for separating undesired substances, in particular ink particles, from a fibre suspension, including use of flotation as further specified in the appended claims. The fibre suspension which is separated according to the present invention is particularly a suspension of recycled fibres to be purified from printing ink and other impurities, such as so-called "stickies". The flotation process is hereby as a rule a part of a total treatment procedure for recycled fibres of the principal type described in the introductory part of the specification. The flotation process can of course be applied to other suspensions of cellulose containing fibres, for example purification of white water containing fibres from paper machines. The principle type of flotation which is utilized in the present process is based on gravitation forces with separation of the substances to be removed in a foam phase and not based on separation with a substantial element of centrifugal forces, which is the case at separations in hydrocyclones . The purified fibre suspension which is obtained in the process is withdrawn as "accept" in a known manner. Before the fibre suspension which is to be purified is introduced in the flotation tank or the flotation cell it has been mixed with gas bubbles . The gas used in this type of processes is as a rule always air, for economical and practi- cal reasons, but it is of course principally possible to utilize any other gas. As some examples of gases which may be suitable can be mentioned carbon dioxide and residual oxygen gas, where the latter for example can be obtained from certain bleaching processes in the pulp industry. When the fibre suspension is introduced into the flotation tank it shall be well mixed with gas bubbles. This can be carried out in devices known for the purpose and several such intensive mixers are commercially available. The predominant part of adherence of colour particles and other impurities to the gas bubbles occurs already at the mixing and the bubbles should thus be relatively small in order to achieve a good efficiency in this respect . The admixed amount of gas is usually within the range of from 20 to 50 per cent by volume, based on the suspension, and most often between about 30 and 40 per cent. The actual attraction between the particles and the bubbles can be increased by the presence of surface active agents, eg ethoxylated nonyl phenols, fatty amines etc, which increase the hydrophobicity of the particles. Such auxiliary chemicals, as well as others, for example foaming agents and pH modifiers, can be added in the mixing step but in the treatment of recycled fibre they are suitably added in the slushing step. The flotation process according to the present invention is characteristic in that the mixture of fibre suspension and gas bubbles is introduced in the flotation tank as directed streams having a high inlet velocity, so called "jets". The velocity is above 5 m/s and suitably above 8 m/s. The upper limit for the velocity is not critical but normally it would not be economical to have inlet velocities above 15 - 20 m/s. The fact that the jets have a high velocity and are directed means that they extend a substantial distance into the bulk volume of fibre suspension present in the flotation tank. A jet with an inlet velocity of 10 m/s will for example extend about 1 meter in the flotation tank. As will be discussed in more detail in the following, it is for example preferred that the jets do not end until close to the free surface of the suspension bulk volume when introduction is from the lower part of the flotation tank. The inlet openings in the flotation tank, ie the openings used for introduction of the jets, can be simple holes or inlet pipes, but it is more convenient to use nozzles in order to achieve the desired defined jets. The injection can be made via one or preferably a plural- ity of inlet openings in a tank, which may have any geometric configuration, and for example be rectangular or cylindrical, which contains a defined bulk volume of fibre suspension and the obtained foam phase is withdrawn at the free surface of this bulk volume. The inlet openings can be located in the side walls of the tank at suitable distances below the free surface of the suspension and be directed downwards at an angle. It is, however, preferred that the openings are located at or close to the bottom of the tank and directed upwards, straight or at an angle, towards the free surface of the suspension. The velocity of the jets should be adjusted in such a manner that they end without breaking against any surfaces of the suspension and tank. In the preferred embodiment the velocity of each jet is thus adjusted in such a manner that the jets end at the free surface of the suspension without breaking this or just below said surface. This reduces the risk of back-mixing of the foam with the separated impurities into the purified suspension in the tank. The number of jets is of course dependent on the tank volume and the bulk volume of the suspension. At least three jets are suitably used and according to a preferred embodiment of the invention the number of jets and their velocities are adjusted to form shear surfaces which are as large as possible between the bodies of the jets and the bulk volume of suspension, without interaction between the jets and the boundaries of the bulk volume. As an example it can be mentioned that for a rectangular flotation tank having a length of 14 m, a breadth of 2 m and a height of 1 m, which is a common size at flotation for deinking of recycled fibre, 47 jets having inlet velocities of 9 m/s may be suitable at an approximate consistency of 1.5%. Withdrawal of the foam phase is carried out at the free surface of the suspension in a manner known per se, for example by using an overflow. The purified fibre suspension (the accept) is likewise withdrawn in a manner known per se in this type of process, for example via outlets located at the bottom of the tank. The flotation is often, as customary, carried out with 4 to 6 repetitions to achieve the desired degree of purity. The present invention is, as mentioned above, based on the understanding of the importance of the rheological properties of a fibre suspension. Breaking up the fibre network is here of particular importance to make it possible to have an early release of the air bubbles and a facilitated trans- port of the aggregates of bubbles and particles through the suspension. By considering the rheology in the present process, as described above, an efficient separation is obtained even at relatively concentrated fibre suspensions, where the concentration, which in conventional processes usually is close to 1 per cent by weight, can exceed 1 per cent. Very good results are obtained at fibre concentrations of 1.5 per cent by weight and the concentration can be as high as 2 per cent and even higher. A suitable concentration range is between 0.8 and 2.5% and preferably from 1.5 to 2%. The process thus offers substantial advantages as it makes it possible to have a most considerable reduction of the volumes of li-quid handled and this with short treatment times and very good purification. The theory which is utilized in this invention is "classical" fluid mechanics. For a bubble to rise upwards through the suspension its lifting power must exceed the viscous force from the suspension. The shear stress which must be exceeded is related to the apparent viscosity which arises when the suspension is flowing. The apparent viscosity of the suspension is directly related to the rheology of the suspension and the flow field, which exists in the flow domain. The lifting power required for the bubble to rise in a flowing suspension is related to the state of stress in the surrounding suspension. To be able to optimize the shearing field in the suspension, ie to optimize the transport of bubbles, it is necessary to know the flow field and the rheology of the suspension. By reducing the length of the bubble transport and by controlling the shear field, by means of a correct flow field, a substantially improved bubble transport can be obtained. The parameters given above for the velocity of the jets, the number of jets, their directions etc are based on the insight of the importance of a number of factors. Thus for example small bubbles give a greater efficiency in the attraction between the bubbles and the contaminating, hydrophobic particles which are to be removed. However, small bubbles usually do not have sufficient buoyancy to pass through the fibre network which exists in fibre suspensions. Since the small bubbles in the present process will be released over a larger area in the interface between the surface of each jet and the free suspension bulk volume, a certain coalescence of the bubbles will occur and this results in an increased lifting power. The fibre network itself is also subjected to a mechanical influence in the actual flotation process in the form of a velocity field. The flow field which then arise in the suspension facilitates the transport of the bubbles to the separation zone. Figure 1 is a schematic sectional view of a directed jet having a high inlet velocity into a flotation cell. Figure 2 shows schematically a rectangular cell design and Figure 3 shows schematically a circular cell design. In Fig. 1 the areas are A) inlet opening, B) jet and C) bulk volume. In Fig. 2 and Fig. 3 A) is the distributor which feeds the pulp to the nozzles (air is admixed by means of a mixer between the distributor and the outlet of the nozzle) , B) the jet, C) the surface in the separation zone where the foam accumulates and D) the surface in the reject chamber. The reject in this case flows to the reject chamber via an overflow edge. The invention is further illustrated in the following example, which, however, is not intended to limit the same. Example 1 A suspension of recycled fibre from newspapers and magazines having a concentration of 1.5% and containing flotation chemicals was mixed with 20% air in a static mixer and the mixture was injected into a rectangular flotation tank having a length of 3 m, a breadth of 2 m and a liquid height of 1.1 m. 10 injection nozzles were used with a total flow of 5000 1/min and an inlet velocity of 9m/s. The jets were injected from the lower part of the tank, upwards at an angle towards the free liquid surface without breaking this. The jets were directed towards the intersecting line between the liquid surface and the rear limiting surface of the tank. The brightness for the incoming suspension was 41.5% ISO. The brightness increased to 50.5 % ISO after one step. It should here be mentioned that in order to achieve the same result with a conventional flotation process with an identical suspension the concentration must be lowered to 1.0% and, furthermore, the flotation must be carried out in two steps. As evident the process according to the invention gave a very good deinking and this at a high fibre concentration in comparison with conventional plants. It can also be mentioned that at a normal production of recycled fibre pulp of 500 ton/day an increase of the concentration in the flotation step from 1 per cent by weight to 1.5 per cent by weight involves a reduction of the amount of water of from 50000 m 3 to 33300 m 3 per day.

Claims: Claims 1. A process for separating substances from a fibre suspension including use of flotation, in which the essential separation force is gravitation, and removal of the separated substances from the fibre suspension in a foam phase, characterized in that a mixture of gas bubbles and the fibre suspension to be separated is supplied into a flotation tank through at least one inlet opening from which a directed jet of said mixture having an inlet velocity greater than 5 m/s is emitted, and that the inlet opening is arranged so that said jet does not break the surface of the suspension in the tank . 2. A process according to claim 1, characterized in that the inlet velocity of said mixture jet is greater than 8 m/s. 3. A process according to claim 1 or 2, characterized in that the inlet opening is arranged so that said jet does not impinge on the flotation tank. 4. A process according to any one of claims 1-3, characterized in that the tank has at least three inlet openings for directing jets of mixture into the tank. 5. A process according to claim 4, characterized in that the inlet openings are located at or near the bottom of the tank and that the jets are directed upwards, straight or at an angle, towards the free surface of the suspension in the tank. 6. A process according to claim 5, characterized in that the velocities of the mixture jets are adjusted so that the jets end at or just below the surface of the suspension in the tank. 7. A process according to any one of claims 4-6, characterized in that the number of openings for said mixture jets and the velocities of the mixture jets are adjusted to achieve as large shearing area on the mixture jets as possible. 8. A process according to any one of the preceding claims, characterized in that each inlet opening through which the mixture of suspension and gas is introduced is defined by a nozzle. 9. A process according to any one of the preceding claims, characterized in that the suspension has a fibre concentration of 0.8 to 2.5 per cent by weight. 10. A process according to claim 9, characterized in that the suspension has a fibre concentration of 1.5 to 2 per cent by weight . 11. A process according to any of the preceding claims, characterized in that the fibre suspension is a suspension of recycled fibre which is to be separated from undesired substances, such as ink.

Inventors: ERIKSSON THOMAS, FREDRIKSSON BOERJE, WIKSTROEM TOMAS, ERIKSSON, THOMAS, FREDRIKSSON, BOERJE, WIKSTROEM, TOMAS
Applicants: ERIKSSON THOMAS, FREDRIKSSON BOERJE, SUNDS DEFIBRATOR IND AB, WIKSTROEM TOMAS, ERIKSSON, THOMAS, FREDRIKSSON, BOERJE, SUNDS DEFIBRATOR INDUSTRIES AB, WIKSTROEM, TOMAS
Application year: 1997
Main classification: B03D1
IPC codes: B03D1, D21B1, D21C5, D21F1