METHOD OF AND APPARATUS FOR SEPARATING HEAVY IMPURITIES FROM FIBER SUSPENSIONS IN CONNECTION WITH PUMPING

Abstract: The present invention relates to a method of and an apparatus for separating heavy impurities from fiber suspensions in a centrifugal pump. The method and the apparatus of the invention are in particular suitable for the treatment of fiber suspensions in the wood processing industry. The stock treated in the pulp and paper industry most often contains heavy impurities, for example pieces of metal or sand. It is a characteristic feature of the method of the invention that the rotary movement of the fiber suspension created by the pump is utilized to separate heavy impurities from the suspension to be pumped to the outer periphery of the rotary flow from which they are removed from the pump separate from the rest of the suspension. The apparatus of the invention is characterized in that the housing (52) of the pump (50) has at least one discharge opening (66) for discharge from the pump of the heavy particles collected at the wall of the housing (52).

Description: METHOD OF AND APPARATUS FOR SEPARATING HEAVY IMPURITIES FROM FIBER SUSPENSIONS IN CONNECTION WITH PUMPING BACKGROUND OF THE INVENTION The present invention relates to a method of and an apparatus for separating heavy impurities from fiber suspensions in a centrifugal pump. The method and the apparatus of the invention are particularly suitable for treatment of fiber suspensions in the wood processing industry. The stock treated in the pulp and paper industry frequently contains heavy impurities, for example pieces of metal or sand. Sand is often transported to the mill with the raw material, i.e. wood or waste paper. Metal may come off from various treatment devices of the mill itself or it may arrive with the raw material. Metal scrap comes into paper mills with waste paper e.g. as staples or paper clips. The common feature of all heavy and hard objects of this kind is that they are very detrimental to the process and they have to be removed. Sand and metal cause trouble by wearing out the process devices and by collecting in the devices and clogging them. Heavy particles are usually separated from the stock by various vortex separators which have been developed different for different applications. We will discuss here, by way of example, two types of them. A conventional low consistency cleaner usually operates at a consistency range of less than 1 % and is a vortex separator similar to the one indicated in Fig. 1 by reference numeral 10. Stock is introduced at a high speed via a tangential connection 12 to the cylindrical portion 14 of the separator where heavy particles are collected at the walls and run along a spiral path down to a conical portion 16 of the separator from the bottom of which they are rejected via a connection 18. The purified lighter stock is collected around the axis of the separator 10, rises up and is discharged via a connection 20 in the upper end of the separator 10. This kind of a vortex separator is a good and an efficient purifier as such but it has two disadvantages, namely low operation consistency and high energy consumption. The energy consumption is high as, due to the low operation consistency, a large amount of water must be pumped through the vortex cleaner and the pressure loss in the apparatus is great. Another apparatus generally used for separating scrap and sand is a high consistency cleaner. The high consistency cleaner 30 illustrated in Fig. 2. is in other respects identical with a conventional vortex cleaner but it is provided with a rotor 32. The rotor usually rotates at the upper end of a conical portion 34 of the cleaner 30 close to a feed connection 36 and an accept discharge 38. The rotor 32 has two functions. One of them is to increase the movement speed of the high consistency stock and thus the turbulence in the suspension to a level at which the scrap and the sand can move in relation to the fibers, i.e. the scrap and the sand are not bound by the fibers and fiber floes. If the turbulence level is too low the fiber network renders the suspension semi-solid whereby the scrap and sand particles cannot easily move in the suspension. The second function of the rotor 32 is to subject the stock suspension to a fast circular movement and thus to create a centrifugal force forcing the heavy- particles to the periphery of the separator. This kind of a cleaner is applicable at a consistency range of 3 to 5%. The drawbacks of the cleaner are its large size and high energy consumption when adequate separation efficiency it strived for. Further, the cleaner, despite its rotor, is not capable of creating pressure, in other words it cannot pass the material further on but the material to be cleaned must be supplied to the apparatus at such a high pressure that the material is also subsequently discharged from the apparatus. OBJECT OF THE INVENTION The object of the present invention is to overcome or minimize the problems of the prior art apparatus in separating sand and scrap from the stock. The apparatus of the present invention is small and its energy consumption is low. SUMMARY OF THE INVENTION The method of the invention is characterized e.g. in that the rotary movement of the fiber suspension created by a pump is utilized in the separation of heavy impurities from the suspension to be pumped which impurities move to the periphery of the rotary flow wherefrom they are removed from the pump as a flow separate from the rest of the suspension. The apparatus of the invention is characterized in that the pump housing has at least one discharge opening for removal of the heavy impurities collected to the wall of the housing. The invention is now described in more detail by way of example, with reference to the accompanying drawings, of which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically illustrates a prior art vortex separator used at low consistencies; Fig. 2 schematically illustrates a prior art vortex separator used at higher consistencies; Fig. 3 is a schematic section transverse to the rotary axis of a preferred embodiment of an apparatus according to the invention; and Fig. 4 is a schematic section parallel to the rotary axis of another preferred embodiment of an apparatus according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 schematically illustrates a centrifugal pump 40 having a pump housing 42, an impeller 44, stock discharge connection or pressure outlet 46, and a discharge connection 48 for heavy particles. The impeller 44, while it rotates at a high speed in the pump housing 42, subjects the stock to be pumped to a high-speed rotary movement. The rotary movement, and the centrifugal force it creates, causes the heavy particles to drift to the outer periphery of the inside of pump housing 42. By designing the housing so as to cause the heavy particles to be collected and flow to the discharge connection 48, the required separation is achieved and there is no need to separately create a centrifugal force field in a separate apparatus for the separation. Thus both the separation of the heavy particles and the subjection of the stock to pressure, i.e. the increase of pressure for the transfer of the stock, can be carried out in one and the same apparatus. The pump illustrated in Fig. 3 can be used in many partial processes of the pulp and paper industry. Fig. 4 illustrates another preferred embodiment of a centrifugal pump 50 according to the invention in which a pump housing 52 has a first portion 54 inside which an impeller 56 is disposed, and a second portion 58 inside which the sealing and the bearings of the pump shaft 60 are provided. The shape of the first portion 54 of the housing 52 is substantially identical with the shape of a conventional centrifugal pump housing with the exception that the said housing portion 54 has no pressure outlet of the pump but the first portion 54 of the housing 52 continues either as a volute or a curving spiral or as successive annular zones to form the second portion 58 of the housing 52 and ends at a tangential pressure outlet 62 of the pump. The first portion 54 of the pump housing 52 is, as is conventional, provided with an inlet of a suction opening/connection 64. It is a characteristic feature of the portions 54 and 58 of the pump housing 52 that their cross-sectional flow area slightly deminishes towards the pressure outlet 62 so as to accelerate the rotating velocity in the spiral of the material to be cleaned/pumped. This improves the separation efficiency of the pump and the pressure of the stock is substantially unchanged even though heavy material is removed via discharge outlets 66 provided in the spiral. The deminishing of the cross- sectional flow area described above can be carried out either as illustrated in Fig. 4, whereby the sealing and the bearings of the pump are enclosed in a substantially cylindrical casing and the diameter of the spiral reduces towards the pressure outlet, or by arranging the casing, which is conical and encloses the said bearings and the sealing, to open towards the pump drive motor 68 in which case the diameter of the pump spiral can be unchanged along its whole length. The embodiment illustrated in Fig. 4 can of course be substituted by an inverted solution wherein the helical housing of the pump is arranged around the suction opening of the pump. In this case, the suction opening can be left cylindrical, if necessary, or it can be conical as described above. There are good grounds for this kind of an arrangement as maintenance of the pump bearings and the sealings and control of the operation is remarkably easier in a pump in which the bearings and the sealings are easily at hand and not inside the spiral housing of the pump. An alternative embodiment of the invention is also a solution in which the spirally projecting housing is replaced by several annular housing portions whereby the heavy particles circulating at the wall of the housing cannot travel from the impeller towards the pressure outlet opening but remain circulating in the said annular portions from which they are easy to discharge. In order to remove the heavy particles the spiral or the annular portions of the housing are provided with discharge openings/connections via which the stock portion containing heavy particles is removed from the main flow and guided to another corresponding separator or for example to a prior art cyclone. Then the accept from the said secondary separation device can be recycled either to the inlet of the primary stage separator or, if the discharge pressure of the secondary stage is adequate, directly to the accept flow of the primary stage. By arranging the discharge appropriately either to an adequate counter-pressure or via an adequately narrow opening it can be guaranteed that the pressure generated by the pump cannot significantly "leak" through this discharge opening. In the following, a few examples are described. In the pulping of waste paper, sand and metal arrive in the pulper with the paper. After the pulping, the stock is usually pumped to a following process step. When the method of the invention of separating sand and scrap is applied to this pumping stage the problems which otherwise would be present in the pumping to the following process step are avoided.- A second application example is the feeding of stock by a pump to a screen. In the screen, there is a rotor rotating close to the screen surface and both the rotor and the screen surface would be heavily worn if sand and metal scrap were to come into the screen. By separating the sand and the scrap in the feed pump of the screen the wearing can be reduced. A third application example is the feeding of stock to a refiner. In a refiner, the stock is treated between two rotating metal blade surfaces. The pitch of the blades is often very small, even less than 1 mm. It is obvious that separation of scrap and sand in the feed pump of the refiner reduces wearing of the refiner blades. A fourth example is the feeding of stock to a stock washer, a store tank or any other space in which the flow rate is low. In this kind of spaces, the heavy particles tend to be settled to the bottom and thus gradually clogg the device. By separating the heavy particles in the feeding pump the problems can be reduced. A feature common to all the above examples is that the stock feed pump is used in an attempt to eliminate or reduce the problems caused by heavy particles in the apparatus. In addition to these examples, more applications are to be found in the pulp and paper industry. As can be comprehended from the above, the method and the apparatus of the invention makes possible the elimination or minimization of the disadvantages of a variety of prior art systems and thus stock treatment systems have become simpler and the life of the apparatus required by the systems remarkably longer.

Claims: CLAIMS: 1. A method of separating heavy impurities from fiber suspensions in connection with pumping, according to which method the fiber suspension is supplied to a centrifugal pump, the pressure of the fiber suspension in the pump is raised and the fiber suspension is discharged from the pump at a pressure higher than the supply pressure, characterized in that the rotary movement of the fiber suspension created by the pump is utilized in separation of heavy impurities from the suspension to be pumped to the outer periphery of the rotary flow from which the heavy impurities are removed from the pump as a flow separate from the rest of the suspension. 2. A method as claimed in claim 1, characterized in that the material to be pumped is circulated in the pump housing for at least one cycle. 3. A method as claimed in claim 2, characterized in that the suspension to be pumped is circulated in the pump housing for at least two cycles. 4. A method of separating heavy impurities from fiber suspensions in connection with pumping, according to which method the fiber suspension is supplied to a centrifugal pump, its pressure in the pump is raised and the fiber suspension is discharged from the pump at a pressure higher than the supply pressure, characterized in that the fiber suspension is supplied from a pulper to a centrifugal pump from which the sand and other heavy impurities which flow in with the suspension are separated by centrifugal force to the wall of the pump volute wherefrom said impurities are discharged at the same time as the pressure of the suspension is raised for transferring the suspension to a following process step. 5. A method of separating heavy impurities from fiber suspensions in connection with pumping, according to which method the fiber suspension is supplied to a centrifugal pump, its pressure in the pump is raised and the fiber suspension is discharged from the pump at a pressure higher than the supply pressure, characterized in that heavy impurities are separated to the spiral wall of the pump from the suspension supplied to the pump, which impurities are discharged from the pump at the same time as the pump is used to raise the pressure of the suspension in order to supply the suspension to a screen. 6. A method of separating heavy impurities from fiber suspensions in connection with pumping, according to which method the fiber suspension is supplied to a centrifugal pump, its pressure in the pump is raised and the fiber suspension is discharged from the pump at a pressure higher than the supply pressure, characterized in that heavy impurities are separated to the spiral wall of the pump from the suspension supplied to the pump, which impurities are discharged from the pump at the same time as the pump is used to raise the pressure of the suspension in order to supply the suspension to a refiner. 7. A method of separating heavy impurities from fiber suspensions in connection with pumping, according to which method the fiber suspension is supplied to a centrifugal pump, its pressure in the pump is raised and the fiber suspension is discharged from the pump at a pressure higher than the supply pressure, chacterized in that heavy impurities are separated to the spiral wall of the pump from the suspension supplied to the pump, which impurities are discharged from the pump at the same time as the pump is used to raise the pressure of the suspension in order to supply the suspension to a stock washer, a stock tank or a corresponding space in which the heavy particles would be settled to the bottom and would encumber the discharge of the tank or a corresponding device. 8. An apparatus for separating heavy impurities from fiber suspensions in connection with pumping, the apparatus comprising a pump housing having a suction inlet opening and pressure outlet opening, an impeller rotatably disposed in the housing, an impeller shaft having bearings and sealing, and a drive means for the shaft, characterized in that the housing (42, 52) of the pump (40, 50) is provided with at least one discharge opening (48, 66) for the discharge from the pump of the heavy impurities collecting at the wall of the housing. 9. An apparatus as claimed in claim 8, characterized in that the housing (52) of the pump (50) comprises a volute (54, 58) connecting its suction opening (64) and discharge opening (62), the volute being arranged to wind around the pump shaft (60) at least for one whole cycle. 10. An apparatus as claimed in claim 8, characterized in that the housing (52) of the pump (50) comprises several adjacent annular housing portions (54, 58) connecting its suction opening (64) and its discharge opening (62).

Inventors: HENRICSON KAJ, PITKAENEN RAIMO, VIKIOE PENTTI, HENRICSON, KAJ, PITKAENEN, RAIMO, VIKIOE, PENTTI
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1990
Main classification: B04C9
IPC codes: B01D21, D21D5, F04D7, F04D29