METHOD FOR THE CLEANING OF THE FILTER PLATES IN A SUCTION DRYER

Abstract: Method in a suction dryer (10), which comprises a basin (12) for the suspension to be suction-dried, in connection with which said basin one or several filter-plate discs (14a) revolve, onto whose faces a filtrate cake is gathered and dewatered by the effect of a suction flow passing through the filter plates (14a), the said filtrate cake/cakes being scraped or otherwise detached from the faces of the filter plates (14a). For the purpose of cleaning the porous structure and faces of the filter plates (14a), an ultrasonic detector or detectors (30) are provided in connection with the basin (12) for the suspension to be filtered, to which said detectors an electric power of ultrasonic frequency is fed out of an ultrasonic generator (32). For the time of the cleaning intensified by ultrasound, the suspension basin (12) is filled with a connecting and cleaning liquid, e.g. water. The filter-plate discs (14) are rotated in the said connecting and cleaning liquid by means of the same equipment (14, 13, 18) by means of which the rotating of the filter-plate discs (14) takes place in connection with the suction drying.

Description: Method for the cleaning of the filter plates in a suction dryer The invention concerns a method in a suction dryer, which comprises a basin for the suspension to be suction-dried, in connection with which said basin one or several disc-shaped components revolve, which consist of filter plates, onto whose faces a filtrate cake is gathered and dewatered by the effect of a suction flow passing through the filter plates, the said filtrate cake/cakes being scraped or otherwise detached from the face or " faces of the filter plates. In prior art, various suction dryers are known which comprise filter discs composed of finely porous filter plates. The said discs are rotated in the sus¬ pension to be dewatered, and by the effect of suction, a filtered cake is gathered on the plane faces of the filter plates, which said cake is scraped off the faces of the filter plates. In respect of recent development of the suction dryers concerned, reference is made to the applicant's earlier Finnish Patent Applications Nos. 870706, 870707, and 871312, reference being made to the constructional details described in the said applications. The said, usually ceramic, filter plates have a tendency to be blocked, because their pore structure is successively filled with finely divided material. In a way known in prior art, attempts have been made to clean the filter plates by through them passing cleaning liquid in the direction opposite to the circulation of the liquid to be removed, said cleaning liquid usually consisting of water. Moreover, various brush devices have been used for outside cleaning of filter plates. However, by means of these prior-art methods and devices, it has not been possible to retain the permeability of the filter plates for a sufficiently long period of time, but it has been necessary to replace the filter plates unnecessarily frequently, in relation to their durability in other respects, which increases the filter costs substantially and causes standstills in the process. The general object of the present invention is to provide improvements in the drawbacks discussed above. A particular object of the present invention is to provide such a method for the cleaning of suction filter plates wherein no substantial alterations have to be made to the suction filter device itself and which said cleaning method can be used smoothly when required, at suitable intervals, so that the natural f nctions and embodiments of equipment of the suction dryer can be utilized efficiently in the cleaning. In view of achieving the objectives given above and those that will come out in the following, the method in accordance with the invention is mainly characterized in - that for the purpose of cleaning the porous structure and faces of the said filter plates, an ultrasonic de¬ tector or detectors are provided in connection with the basin for the suspension to be filtered, to which said detectors an electric power of ultrasonic frequency is fed out of an ultrasonic generator, - that for the time of the cleaning intensified by ultrasound, the suspension basin is filled with a con¬ necting and cleaning liquid, preferably water, and - that the filter-plate discs are rotated in the said connecting and cleaning liquid by means of the same equipment by means of which the rotating of the filter- plate discs takes place in connection with the suction drying. In the following, the invention will be de¬ scribed in detail with reference to an exemplifying e - bodiment of the invention illustrated in the figures in the accompanying drawing, to whose details the invention is by no means strictly confined. Figure 1 is a schematical side view of a suction dryer arranged to be cleaned by means of the method in accordance with the invention. Figure 2 shows the same as Figure 1 , viewed from the end of the suction dryer. The suction dryer 10 shown in Figures 1 and 2 comprises a frame part 11 and a suspension basin 12 arranged on the frame, the suspension to be dewatered, such as dressed metal ore, peat suspension, or equi- valent, being passed into said basin. A horizontal shaft 13 is fitted as revolving on the frame part 11 , said shaft being driven by a motor M by the intermediate of a worm gear 18. To the shaft 13, filter discs 14 in a vertical plane are mounted as fixed, said discs being provided with ceramic filter plates 14a, e.g., of the sort described in the applicant's Finnish Patent Applications Nos. 853963 and 870979. The interior spaces in the filter plates 14a of the filter discs 14 com¬ municate via radial suction tubes 15 with suction tubes placed in connection with the shaft 13, said suction tubes communicating with a distributor head 16 , which distributes the effect of negative pressure introduced through the suction tube 17 to the filter plates 14a of the discs 14 that are submerged in the suspension present in the basin 12 or on whose faces filtered cakes are being dewatered. The dewatered cakes are scraped off the faces of the plates 14a in the filter discs 14 by means of doctors 20 , whose blades 20a are placed one opposite the other, as is shown in Fig. 1, on both of the opposite active faces of the filter plates 14a. The blades 20a of the doctors 20 are connected by means of intermediate arms to guides 19, by whose intermediate the doctor blades 20, placed in pairs one opposite the other, are loaded with an appropriate force. In respect of the various details of the filter plates 14a and of the suction dryer, reference is made further to the applicant's earlier Finnish Patent Applications Nos. 853963, 870706, 870707, 870979, and 871312. In connection with the suspension basin 12, a 5 cleaning system in accordance with the invention is pro¬ vided for the filter plates 14a of the discs 14, pref r¬ ably ceramic, finely porous filter plates. The cleaning system includes ultrasonic detectors 30 , which are permanently connected to the sub-bottom of the basin 12 10 and to which electric power is passed by the intermediate of the conductor 31 from an ultrasonic generator 32. The ultrasonic generator 32 and the operation of the whole system are controlled by the control unit 33 by the intermediate of the connection 34. From the control unit T5 33, a connection 35 may.also start, via which the opera¬ tion of the motor M rotating the filter discs 14 and possibly the operation of other devices, such as the magnetic valve 21 of the cleaning-water pipe 22, are controlled, through which said pipe 22 clean, warm 20 cleaning water can be passed into the suspension basin 12. The devices controlled via the connection 35 of the control unit 33 may also include the water supply means, by means of which, through a distributor piece 16 and pipes 15, the recirculation of the cleaning water 25 is passed through the faces of the filter plates 14a towards the basin 12. In its details, the cleaning method in accord¬ ance with the invention is accomplished, e.g., as follows. The suspension basin 12 is emptied of the suspension to 30 be filtered, and clean, warm water is passed into the suspension basin 12 through the pipe 22 and the magnetic valve 21. When the basin 12 is appropriately filled with cleaning water, to which washing agents, such as hydrochloric acid, are added if necessary, the motor M 35 ■ is started to rotate the shaft 13. At the same time, the recirculation of cleaning liquid, taking place through the filter plates 14a, can be started. The ultrasonic generator 32 is started by means of the con¬ trol unit 33, and a sufficiently intensive field of ultrasonic oscillation is applied to the ceramic plates 14a or equivalent on the revolving filter discs 14 by means of the detectors 30. In ultrasonic cleaning, the cleaning water present in the basin 12 acts as the connecting liquid for the ultrasonic field, which said water spreads the field of oscillation efficiently to the filter plates 14a' while they revolve at the same time, so that the cleaning effect can be spread uniformly over all the plates 14a on the discs 14. Owing to a sufficiently intensive ultrasonic field, cavitation and implosion are produced in the cleaning liquid present in the basin 12, which produce an efficient deep-cleaning effect on the faces of the filter plates 14a to be cleaned. Cavitation depends on surface tension. If necessary, the cavitation is in¬ creased by raising the temperature of the water present in the basin and/or by using washing agents. The frequency of the ultrasonic energy supplied through the conductor 31 is, as a rule, of an order of 15 to 300 kHz. The detectors 30, which are preferably placed symmetrically at both sides of the vertical centre plane of the basin, are, e.g., agnetostrictive, piezo¬ electric or equivalent oscillators, which are arranged to operate, e.g., at or near the resonance frequency. In preliminary tests, it has been noticed that a suitable power of ultrasonic electricity is about 100 to 500 W/m 2 , preferably about 200 to 300 W/m 2 , i.e÷ the said power is given per unit of area of the filter plates 14a to be cleaned. It has been noticed that a suitable cleaning time, e.g., with suction dryers 10 that dewater dressed metal ores is about 1 hour once in a week, so that the amount of ultrasonic energy to be used at one time of cleaning is of an order of about 0.1 to 0.5 kWh/m 2 . In plants in which there are several suction dryers of the sort shown in Figures 1 and 2 or equiva¬ lent, the ultrasonic generator 32 may be arranged dis- placeable and the ultrasonic detectors 30 be mounted permanently on the bottom of the basins 12 of the suction dryers 10. In such a case, the cleaning with ultrasonic intensification is carried out so that the ultrasonic generator 32 is brought to the suction dryer 10 to be cleaned and the plug of the conductor 31 is inserted into the socket 3a, and the cleaning is carried out either by means of the control unit 33 or by manual control. In plants in which there is quite a high number of suction dryers 10 and/or in which cleaning is required at . relatively short intervals, ultrasonic clean- ing can be arranged automatically, e.g., so that one ultrasonic generator 32 is connected by the intermediate of a multiplexor (not shown) alternatingly to the various suction dryers 10 to be cleaned, in which said devices the other necessary operations, such as supply of washing water into the suspension basin 12 and re¬ circulation of the cleaning water back through the filter plates 14a , take place by means of the control unit 33 by automatic control or, in the necessary respects, by manual control. In the following, the patent claims will be given, whereat, within the scope of the inventive idea defined in said claims, the various details of the in¬ vention may show variation and differ from the details described above for the sake of example only.

Claims: WHAT IS CLAIMED IS: 1. Method in a suction dryer (10), which com¬ prises a basin (12) for the suspension to be suction-dried, in connection with which said basin one or several disc¬ shaped components (14) revolve, which consist of filter plates (14a), onto whose faces a filtrate cake is gathered and dewatered by the effect of a suction flow passing through the filter plates (14a), the said filtrate cake/ cakes being scraped or otherwise detached from the face or faces of the filter plates (14a) , c h a r a c t e r ¬ i z e d in - that for the purpose of cleaning the porous structure and ~ faces of the said filter plates (14a), an ultrasonic detector or detectors (30) are provided in connection with the basin (12) for the suspension to be filtered, to which said detectors an electric power of ultrasonic frequency is fed out of an ultrasonic generator (32) , ' - that for the time of the cleaning intensified by ultrasound, the suspension basin (12) is filled with a connecting and cleaning liquid, preferably water, and - that the filter-plate discs (14) are rotated in the said connecting and cleaning liquid by means of the same equipment (14,13,18) by means of which the rotating of the filter-plate discs (14) takes place in connection with the suction drying. 2. Method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that in connection with cleaning with ultrasonic intensification, cleaning water is passed through the filter plates (14a), in the direction opposite to the circulation of the liquid to be filtered off, through the porous filter layers in the filter plates (14a) towards the suspension basin (12). 3. Method as claimed in claim 1 or 2, 2 c h a r a c t e r i z e d in that about 1 00 to 500 W/m , 2 preferably about 200 to 300 W/m , of ultrasonic electric power is used per area unit (m 2 ) of filter plates (14a). 4. Method as claimed in any of the claims 1 to 3, c h a r a c t e r i z e d in that the suspension basin (12) in the suction dryer (10) is filled with heated water before the cleaning with ultrasonic inten- • sification is carried out, to which said water a washing agent, such as hydrochloric acid, has been added if necessary. . Method as claimed in any of the claims 1 to 4, c h a r a c t e r i z e d in that the frequency of the ultrasonic electric power used for the cleaning is of an order of 15 to 300 kHz. 6. Method as claimed in any of the claims 1 to 5, c h a r a c t e r i z e d in that as ultrasonic detectors (30) or detector, magnetostrictive, piezo- electric or equivalent oscillators are used, which is/are fitted permanently onto the bottom of the suspension basin (12) . 7. Method as claimed in any of the claims 1 to 6, c h a r a c t e r i z e d in that the'method is applied by using a preferably displaceable ultrasonic generator (32) common of several suction dryers (10), which said generator is brought to the suction dryer (10) to be cleaned and to which said generator the electric cable (31) of permanently installed ultrasonic detectors (30) is connected. 8. Method as claimed in any of the claims 1 to 6, c h a r a c t e r i z-e d in that in the method an ultrasonic generator common of several suction dryers (10) is used, which is connected by the intermediate of a multiplexor or equivalent, being controlled by the automatic control unit (33) or by means of manual control, at certain intervals alte-rnatingly to each of the suction dryers (10) to be cleaned. 9. Method as claimed in any of the claims 1 to 8, c h a r a c t e r i z e d in that in the method a control unit is applied which controls the operation of the motor (M) of the filter discs and/or the operation of the switching devices and the washing-water supply and valve devices and/or the recirculation of washing liquid taking place through the filter plates (14a).^ 10. Method as claimed in any of the claims.1 to 9, c h a r a c t e r i z e d in that the method is applied in a suction dryer in which a V-section suspen¬ sion basin is used as the suspension basin '(12) , wherein ultrasonic detectors (30) are fitted permanently on the preferably plane bottom parts of the suspension basin (12) placed at both sides of the bottom edge, said ultrasonic detectors (30) extending over a substantial portion of the length of the suspension basin (12) in the direction of the shaft (13) of rotation of the filter discs (14) .

Inventors: EKBERG BJARNE, NORRGAARD GOERAN, EKBERG, BJARNE, NORRGAARD, GOERAN
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1988
Main classification: B01D33
IPC codes: B01D24, B01D33