Patent GB-1111914-A (GB)
Title: Removal of gels and small particles from viscose
IPC: B01D29/00
Filed: 1966-07-07
Abstract: 1,111,914. Filtering viscose. BUCKEYE CELLULOSE CORPORATION. 7 July, 1966 [7 July, 1965], No. 30489/66. Heading BID. [Also in Division 1B5] Viscose is filtered to remove fibre and gel particles therefrom prior to spinning or extrusion by filtration through a wire mesh filter element 12 sandwiched between perforate cylinders and backwashing said filter with a minor portion of the filtered viscose at least to the extent that the pressure differential across said filter does not exceed a predetermined level. The backwashing pressure differential across said filter and the particulate material deposited thereon may be derived from the pressure of the filtered viscose and the backwashing pressure differential across said filter medium together with the particulate matter deposited thereon may be independently regulated with respect to the pressure differential employed in the initial viscose filtration. The minor portion of the filtered viscose used to backwash the foraminous filter medium may be returned to the cellulose xanthate dissolving stage for subsequent refiltering. Alternatively, a small proportion of carbon disulphide may be added to the minor portion of the filtered viscose used to backwash the foraminous filter medium during return to a retaining tank and prior to subsequent refiltering. Alternately, the minor portion of the filtered viscose used to backwash the foraminous filter medium may be filtered through a backwashable filter clothed with a foraminous filter medium.. In a preferred viscose filtration sequence, viscose is filtered through three successive continuously backwashable filters respectively clothed with a foraminous filter media consisting of stainless steel wire screen filter media having minimum mesh frequencies of 200, 400 and 400 meshes per inch especially 325, 500 and 500 meshes per inch, together with maximum respective screen opening dimensions of 74 microns, 36 microns and 36 microns especially 44 microns, 28 microns and 28 microns. Alternatively, the viscose may be filtered through one continuously backwashable filter clothed with a foraminous filter medium consisting of a stainless steel wire screen filter medium having a minimum mesh frequency of 550 meshes per inch and a maximum screen opening dimension of 26 microns. Suitably, the viscose is filtered through one continuously backwashable filter clothed with a stainless steel wire screen filter medium having a minimum mesh frequency of 500 together with a maximum screen opening dimension of 28 microns, and the minor portion of the filtered viscose used for backwashing is diluted with aqueous sodium hydroxide and filtered through a continuously backwashable filter clothed with a stainless steel wire filter medium having a minimum mesh frequency of 500 together with mesh openings having a maximum dimension of 28 microns. In an alternative method, the viscose is circulated from a retaining tank prior to ripening and deairating, the circulated viscose filtered through a first continuously backwashable filter clothed with a foraminous filter medium, the filtered viscose returned to said retaining tank, and the viscose filtered from said retaining tank through a second continuously backwashable filter clothed with a foraminous filter medium. In such a process, the first continuously backwashable filter may be clothed with a stainless steel wire screen filter medium having a minimum mesh frequency of 325 meshes per inch and a maximum screen opening dimension of 44 microns, and the second continuously backwashable filter may be clothed with a stainless steel wire screen filter medium having a minimum mesh frequency of 500 meshes per inch and a maximum screen opening dimension of 28 microns. In such a process, the viscose may be filtered through at least one continuously backwashable filter clothed with a stainless steel wire screen filter medium, said continuously backwashable filter being placed in series with and prior to at least one conventionally clothed plate and frame filter press. In this form of process, the viscose may be filtered through a continuously backwashable filter clothed with a stainless steel wire screen filter medium having a minimum mesh frequency of 200 meshes per inch and a maximum screen opening dimension of 74 microns, said continuously backwashable filter being placed in series with and prior to two or three successive conventionally clothed plate and frame filter presses. In a preferred viscose filtration sequence as above, the viscose may be filtered through a continuously backwashable filter clothed with a stainless steel wire screen filter medium having a minimum mesh frequency of 325 meshes per inch and a maximum screen opening dimension of 44 microns, said continuously backwashable filter being placed in series with and prior to two successive conventionally clothed plate and frame filter presses, whereby the service life of the conventional filter media used to cloth said plate and frame filter presses is substantially increased. Alternatively, viscose may be filtered through two successive continuously backwashable filters clothed with stainless steel wire screen filter media having respective minimum mesh frequencies of 200 and 325 meshs per inch, together with maximum respective screen opening dimensions of 74 and 44 microns, said continuously backwashable filters being placed in series with and prior to a conventionally clothed plate and frame filter press. As shown, the continuously backwashable filter comprises a cylindrical outer shell 10 closed at the top by cover 26 and at the bottom by header 27. Cover 26 is supported on ring 28. Header 27 has an inlet opening 31 which communicates with the inside of the strainer adjacent one side of the strainer body and is connected to inlet pipe 20. The shell has an outlet opening 21 therein which is connected to a suitably flanged pipe. Header 27 has an opening in the centre connected to the backwash outlet fitting 22. A bearing 32 is fitted into this opening and this bearing received the lower end of shaft 35. Cover 26 has packing in a central opening therein with a packing gland 16 through which shaft 35 of backwash arm 15 extends. Shaft 35 may be equipped with multiple backwash arms if desired. Backwash arm 15 is supported by a bearing in speed reducer 17 and is driven by motor 19 through speed reducer 17 and keyed thereto by means of a shear pin 18. The lower end of shaft 35 is hollow and communicates with backwash outlet 22. Shaft 35 has a slot 36 in one side at an intermediate position. Slot 36 is connected to backwash arm 15. The top of the strainer cage is supported in the body by means of ring 37 and the bottom is supported in a ring 38 which is of L-shaped crosssection and welded to header 37 so that it presents an annular seat 39. The strainer basket is made up of the hollow cylindrical inner cage 13 and the hollow cylindrical outer cage 11 that sandwich the mesh straining element 12 there between. The inner cage and outer cage have holes 43 and 42 respectively that align with each other so that a segment of wire screen straining mesh or filter medium 12 is exposed in each hole. Backwash arm 15 has shoes 25 attached to its outer end which shoes are spaced from each other and provide a space 44 there between. Thus when backwash arm 15 is swung around its axis, which is coaxial with the central axis of the strainer, shoes 25 move in contact with the inside periphery of the inner cage, and the space 44 provides a lower pressure area on the inner periphery of the inner cage to receive particles of material which have been strained out of the liquid on the upstream side of the straining mesh. Since the liquid on the downstream side of the straining mesh will have a higher pressure than the pressure in the space 44, liquid will flow through the straining element in a reverse direction; that is, from the outside of the screen to the inside of space 44 and into the backwash arm 15, thereby carrying the material which has been trapped in the holes 43 and on the screen straining mesh or wire screen filter medium 12 into the backwash arm 15 and out of fitting 22. The rotational speed of backwash arm 15 can be controlled to maintain a maximum or set pressure differential across the wire screen filter medium for optimum viscose filtration. A higher rotational speed will minimise the pressure differential. Backwash arm 15 may be rotated slowly and continuously to result in a substantially constant pressure differential across the wire screen filter medium. The continuously backwashable strainer mechanism may also be made to operate by rotating backwash arm 15 for at least one complete revolution whenever the differential pressure reaches a set maximum for example 50 psig. This intermittent operation may be accomplished by the use of a pressure switch connected to the backwash arm rotating motor 19, or by manual rotation of backwash arm 15. Slot 44 in backwash arm 15 is preferably of a width to cover one vertical row of holes 43. And the shoes 25 are preferably wide enough to prevent viscose leakage from the interior of the strainer basket into backwash arm 15. To provide effective sealing, shoes 25 may be faced with a resilient material e.g. polytetrafluoroethylene, polyoxymethylene resin, polyethylene or polypropylene. A series of plant layouts incorporating one or more continuously backwashable filters of the type described are given together with flow diagrams (Figs. 3 to 9, not reproduced). In addition to stainless steel wire screen filter media, other foraminous filter media which may be used include perforated sheets of stainless steel, polyethylene or polytetrafluoroethylene. Alternatively, backwash arm 15 may be operated continuously at a speed of about 0.044 r.p.m. In this method of operation the pressure differential between the filter