WO2002089951A1 - Process for regenerating a filtration cartridge for filtering a slurry - Google Patents

Process for regenerating a filtration cartridge for filtering a slurry Download PDF

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Publication number
WO2002089951A1
WO2002089951A1 PCT/US2002/013716 US0213716W WO02089951A1 WO 2002089951 A1 WO2002089951 A1 WO 2002089951A1 US 0213716 W US0213716 W US 0213716W WO 02089951 A1 WO02089951 A1 WO 02089951A1
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WO
WIPO (PCT)
Prior art keywords
depth filter
filter
slurry
particles
liquid
Prior art date
Application number
PCT/US2002/013716
Other languages
French (fr)
Inventor
Zhenwu Lin
Eugenia Vasilopoulos
Jieh-Hwa Shyu
Original Assignee
Mykrolis Corporation
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Filing date
Publication date
Application filed by Mykrolis Corporation filed Critical Mykrolis Corporation
Publication of WO2002089951A1 publication Critical patent/WO2002089951A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B37/00—Lapping machines or devices; Accessories
    • B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D29/00—Filters with filtering elements stationary during filtration, e.g. pressure or suction filters, not covered by groups B01D24/00 - B01D27/00; Filtering elements therefor
    • B01D29/62—Regenerating the filter material in the filter
    • B01D29/66—Regenerating the filter material in the filter by flushing, e.g. counter-current air-bumps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B57/00—Devices for feeding, applying, grading or recovering grinding, polishing or lapping agents
    • B24B57/02—Devices for feeding, applying, grading or recovering grinding, polishing or lapping agents for feeding of fluid, sprayed, pulverised, or liquefied grinding, polishing or lapping agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2201/00—Details relating to filtering apparatus
    • B01D2201/08—Regeneration of the filter
    • B01D2201/085—Regeneration of the filter using another chemical than the liquid to be filtered

Definitions

  • This invention relates to a process and a system for regenerating a depth filter cartridge for filtering a slurry composition.
  • a fluid composition containing a particulate solid component is referred to in the art as a "slurry".
  • the solid component can be any of a variety of materials including solid particles, cell components, flocculating agents, gel particles or the like.
  • Slurry compositions presently are utilized in chemical mechanical polishing (CMP) to polish wafers in VLSI and ULSI integrated circuit devices.
  • CMP chemical mechanical polishing
  • High pH silica slurries are utilized to polish dielectric, polysilicon, copper layers.
  • acidic silica, alumina and ceria abrasive based slurries are utilized to polish metal interconnects.
  • the CMP process uses sub-micron (20-200 nm) abrasive particles such as, silica, alumina, ceria or manganese oxide or the like, at a typical concentration of 1-30% by weight particles.
  • the typical specification for commercial slurries includes percent solids, pH, specific gravity, mean particle size and general (bulk) particle size distribution.
  • a small number of "large" particles have been found which fall outside of the specified size distribution.
  • These particles can be aggregates or agglomerates and maybe formed from agglomeration, settling, shearing or local drying of slurry.
  • These large particles and agglomerates can cause micro-scratches and other defects on planarized wafer surfaces during CMP processing. Slurry filtration to remove these relatively large particles has proven to be beneficial in reducing wafer defects and increasing yields in CMP processes.
  • a vide variety of filter cartridge constructions are utilized to purify fluids.
  • cartridge constructions are designed to remove solids and colloidal particles as well as microorganisms.
  • the basic two separate and distinct types of cartridges used in filtration of gases and liquids are depth filters (typically wound) and surface or screen filters (usually pleated).
  • a depth filter is primarily used to remove most of the contaminants and particles. It is typically utilized upstream of a surface or screen filters.
  • the most important properties for a depth filter are its "dirt holding capacity" or throughput, pressure drop and retention.
  • the filter design allows contaminants and particles to be trapped in stages within the depth of the filter due to the construction of the multiple layers of various media types.
  • a wound depth filter has multiple layers with the most open media (largest micron retention rating), i.e., largest pore size usually the outermost layer, adjacent the liquid inlet with the tightest media at t he core adjacent the liquid outlet will have the least amount of surface area due to the smallest diameter around which it is wrapped.
  • This layer at the core contributes to most of the pressure drop of the cartridge because the media has the highest pressure drop and the least amount of filtration surface area. Likewise, this layer will significantly reduce the capacity of the filter due to both the low filtration surface area and the smallest micron retention rating.
  • depth filters are positioned within a housing, spaced apart from the interior housing walls thereby to forma void volume upstream of the depth filter. This spacing is effected to permit either the introduction of a fluid feed into the entire filter or the removal of the entire permeate from the filter. If this spacing were not maintained, fluid flow through the filter can be severely restricted. As a result, a relatively large fluid hold-up volume occurs in a conventional filter unit.
  • a depth filter construction utilizing such a spacing also is disadvantageous for filtering a slurry since the particles in the slurry can settle out of the slurry on and within the filter. This results in rapid plugging of the filter, particularly at low flow rate point of use applications.
  • a surface or screen filter will retain virtually 100% of the particles or contaminants for which it is rated.
  • the media used in surface or screen filter typically has a high pressure drop and low "dirt holding capacity" or throughput because of its high retention efficiency.
  • the media normally used in a surface filter comprises glass or polymeric microfibers. Particles are retained by size exclusion primarily on the surface of the screen filter rather than within the depth of the filter. Particles smaller than the controlled pore size tend to be trapped within the media of the surface filter. However, as a result of the controlled pore structure, they provide more predictable filtration than depth filters. Screen filters are not useful for filtering a slurry since they will become plugged quickly by the solid particles and gels in the slurry.
  • the present invention comprises a process for regenerating a depth filter such as a stacked depth filter, a wound graded depth filter, or a non-woven pleated filter used to filter a slurry.
  • the filter to be regenerated is loaded with retained particles as a result of filtering a slurry of the particles.
  • the stacked depth filter comprises a depth filter such as a fibrous mass, a plurality of non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt of the like positioned within a housing free of an open void volume upstream of the depth filter.
  • a depth filter such as a fibrous mass, a plurality of non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt of the like positioned within a housing free of an open void volume upstream of the depth filter.
  • open void volume is meant a volume free of a material including materials for forming a depth filter and is not meant to include the void volume of the depth filter.
  • the graded depth filter cartridge construction for filtering a slurry has a filtration medium formed as a depth filter such as a cylindrical seamless fibrous depth filter comprising a non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt or the like or a wound depth filter retained within a housing.
  • a depth filter such as a cylindrical seamless fibrous depth filter comprising a non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt or the like or a wound depth filter retained within a housing.
  • One end of the cartridge is sealed with a cap having a fluid inlet while the opposing end is sealed with a cap having a fluid outlet.
  • the filtration medium is a wound depth filter, it is positioned around a core that extends substantially the length of the cartridge.
  • the depth filter comprises a non- woven fibrous mass, it is compressed to effect the desired percent retention efficiency of the mass.
  • the depth filter also can comprise a layered filter construction having a plurality of filtration media, each having a controlled percent retention rating.
  • the layers of the depth filter are formed of felt layers, of wound or layered flat filtration sheets of a fibrous mass of non- woven polymeric fibers secured together by mechanical entanglement or interweaving of the fibers.
  • the pleated filter is formed by pressing non-woven fibers into a pleated configuration.
  • the resultant pleated non-woven fiber filter then is wrapped around an open core that is positioned within a housing having a fluid inlet and a fluid outlet.
  • the filter cartridges retain undesirably large particles and gel particles that permit passage there through of particles of a slurry having a size within a desired size range.
  • the filter cartridge fully loaded or partially loaded with retained slurry particles is processed to remove the slurry particles by one of three embodiments of this invention.
  • “fully loaded” is meant that, due to retained particles in the depth filter, a slurry cannot be passed through the filter.
  • partially loaded is meant that, due to retained particles in the depth filter, a slurry can be passed through the depth filter but at a flow rate less than or at a pressure drop higher than that obtained with a particle-free depth filter. It is preferred to regenerate a partially loaded depth filter since reduced regeneration times are required to effect regeneration.
  • the filter is reverse flushed with deionized water.
  • reverse flushed or “reverse flush” as used herein, is meant that the flushing liquid is passed through the filter in a direction opposite to the direction in which the slurry flows during the slurry filtration step.
  • the plugged filter is reverse flushed with an aqueous solution having a composition which selectively reacts with the retained particles to reduce the size of the retained particle so that they can be removed from the filter by passing the solution through the plugged filter.
  • the plugged filter is forward flushed with an aqueous solution having a composition which selectively reacts with the retained particles to reduce the size of the retained particle so that they can be removed from the filter by passing the solution through the plugged filter.
  • forward flushed or "forward flush” as used herein is meant that the flushing liquid is passed through the plugged filter in the same direction as the direction of flow of the slurry through the filter during the filtration step.
  • Fig. 1 is a schematic diagram of a system of this invention.
  • Fig. 2 is a graph showing the test result with CMP5 filter of Example 1.
  • Fig. 3 is a graph showing the test result with CMP3 filter of Example 1.
  • DESCRIPTION OF SPECIFIC EMBODIMENTS In accordance with this invention, a partially loaded or fully loaded depth filter which has been utilized to filter a slurry in order to selectively retain undesirably large particles is regenerated either with deionized water or with a reagent which reacts selectively with the retained particles to reduce their size while being unreactive with the depth filter medium. As set forth above, deionized water can be utilized to reverse flush a plugged depth filter to remove retained slurry particles therefrom.
  • the regeneration step can be conducted on a partially plugged filter during off step in the polishing cycle (typically 1-3 minutes). Regeneration can also be performed on a completely plugged filter as well using longer time intervals. Typically, the regeneration step is conducted for a time period between 30 seconds and 30 minutes. Regeneration of the depth filter with deionized water cannot be affected by forward flushing the filter.
  • an aqueous solution containing a composition which reacts with retained slurry particles to reduce the size of the retained slurry particles can be used to reverse flush or forward flush a plugged filter.
  • suitable compositions include an acid to form an aqueous solution, having a pH below about 6 preferably below about 4 or a base to form an aqueous solution having a pH greater than pH9.
  • the reverse flushing or forward flushing with the reactive composition can be affected at flow rate of lx to lOx of that used in the application.
  • Representative suitable acids include hydrochloric acid, sulfuric acid, nitric acid, or the like.
  • Representative suitable bases include sodium hydroxide, potassium hydroxide, ammonium hydroxide or the like.
  • the pH of the reactive acid or base can be adjusted by directing an acid or base through a membrane into a carrier liquid such as water.
  • a carrier liquid such as water.
  • the resultant pH adjusted carrier liquid then is passed through the slurry loaded depth filter to effect particle size reduction to the point where the size reduced retained particles are carried out of the depth filter.
  • the supply of acid or base is stopped while the carrier liquid is continued through the depth filter to remove any reagent therein. Thereafter, the depth filter is utilized to filter additional slurry.
  • the depth filter has a thickness in the direction of fluid flow there tlirough of between about 0.5 and about 18 inches, preferably between about 1 and about. 12 inches to attain effective retention of undesirably large particles while permitting passage there through of particles within a desired size range.
  • the depth filter comprises a plurality of media (layers) each having a different micron retention size so that the permeability or retention of the media layers is largest adjacent a fluid inlet to the cartridge and is smallest adjacent the fluid outlet from the cartridge.
  • the permeability of retention of the media layers is controlled so that particles in the slurry within a desired particle range pass through the cartridge and through the outlet. It has been found, in accordance with the process of this invention, that the useful life of the cartridge is at least about 300 percent longer, preferably at least about 500 percent longer than a filter cartridge with a nonregenerated filter. Thus, the process of this invention permits the use of fewer cartridges for a particular application and at a reduced cost as compared to the cost of filter cartridges of the prior art. Percent retention efficiency and Beta Ratio are measures of the ability of the cartridge to capture and retain particles.
  • Beta Ratio The Beta Ratio concept was introduced by the Fluid Power Research Center (FPRC) at Oklahoma State University (OSU) in 1970. Originally developed for use in hydraulic and lubricating oil filter, the test has been adapted by many cartridge manufacturers to measure and predict the cartridge filter performance in aqueous based fields. Beta Ratio is defined by the FPRC as the number of particles greater than a give size (X) in the feed, divided by the number of particles greater than the same size in the effluent. Both percent retention efficiency and Beta Ratio values are calculated for specific particle size ranges.
  • FPRC Fluid Power Research Center
  • OSU Oklahoma State University
  • Beta Ratio (B) Number of feed particles (size X)
  • the filter medium of the depth filter having the largest micron retention is positioned adjacent an inlet to the filter cartridge.
  • the filter media of the depth filter having the smallest micron retention is positioned adjacent the outlet from the filter cartridge.
  • the micron retention characteristics of a filter can be varied by varying the diameter of fibers used to form the filter and/or the extent of compression of the fibers such as by winding a filter medium sheet tighter or looser around a core. A tighter wound filter medium gives a higher percent retention efficiency.
  • the intermediate filter media are positioned according to percent retention of efficiency so that incoming slurry is passed sequentially through the filter media having progressively smaller micron retention and lastly through the filter media having the smallest micron retention.
  • the overall filter cartridge presents a percent retention efficiency which comprises a progressive gradient from the inlet to the outlet wherein the percent retention efficiency progressively increases.
  • Representative media useful for forming the depth filter include the fibers of polyolefins such as polyethylene, polypropylene, cellulose, cotton, polyamides, polyesters, fiberglass, polytetrafluoroethylene (PTFE), fluoropolymers such as PFA, MFA and FEP or the like.
  • the fibrous depth filter is free of seams and is formed of fibers that produce a fibrous mass of fibers.
  • This embodiment of the depth filter can be characterized by a gradation of micron retention characteristics throughout its thickness in the direction of fluid flow through the depth filter. This gradation can be achieved either by varying the void volume of the cylindrical fibrous depth filter medium as a function of thickness in the direction of fluid flow through the filter or by maintaining a constant volume and varying the size of the fibers as a function of depth filter thickness in the direction of fluid flow through the depth filter, hi either embodiment, all that is necessary is that the gradation of micron retention characteristics is produced.
  • the gradation is effected such that the slurry to be filtered first encounters a layer of the depth filter having the largest micron retention characteristics (i.e., largest pores) and encounters layers having progressively smaller micron retention characteristics, (i.e., smallest pores) prior to being directed tlirough the outlet.
  • the seamless cylindrical fibrous depth filter can be formed by any conventional means such as is disclosed in U.S. Patents 3,933,557; 4,032,688; 4,726,901 or 4,594,202 which are incorporated herein by reference.
  • the pleated filters regenerated by the process of this invention are formed by pressing non-woven fibers into a pleated shape. Representative useful fibers are the same fibers set forth above for the depth filters.
  • the void volume of the cylindrical fibrous depth filter ranges between about 60 and 95 percent and varies no more than about 1 to 2 percent.
  • the fibers range in diameter between about 1.6 and 16 micrometers. !
  • the wound depth filter is formed by winding one or a plurality of filter sheets formed of fibers to form a joint generally cylindrical structure.
  • the filter sheet or sheets have varying pore size such that the micron retention characteristic of a portion of the depth filter as a function of radial position within or on the depth filter.
  • the portion of the wound depth filter positioned adjacent an inlet to the filter cartridge including the wound depth filter has the largest micron retention characteristics while the portion of the wound depth filter having the smallest micron retention characteristics, i.e., the smallest pore size is positioned adjacent the outlet from the filter cartridge.
  • Any intermediate portions of the wound depth filter are positioned according to pore size so that incoming slurry is passed sequentially tlirough portions of the depth filter having progressively smaller micron retention characteristics.
  • Representative media useful for forming depth filters include the fibers set forth above for the cylindrical seamless fibrous filters.
  • the stacked depth filter which is regenerated of the process of this invention can be formed from one or a plurality of separate filter sheets by stacking the sheet or sheets within a housing in a manner such that an open volume within the housing and upstream of the depth filter is avoided.
  • the filter sheet or sheets can have the same pore size or varying pore size such that the micron retention characteristic of a portion of the stacked depth filter varies along the length of the housing.
  • the portion of the filer stack positioned adjacent an inlet to the filter cartridge has the largest micron retention characteristics while the portion of the filter stack having the smallest micron retention characteristics, i.e., the smallest pore size is passed sequentially tlirough portions of the depth filter having progressively smaller micron retention characteristics and lastly through the portion of the filter having the smallest micron retention characteristics.
  • Representative media useful for forming the filter stack include the fibers set forth above for the cylindrical seamless fibrous filters.
  • a system 10 having a slurry supply conduit 12 connected to a slurry source (not shown).
  • the system 10 includes two depth filter cartridges 14 and 16, one of which is utilized to filter slurry while the other of which is being regenerated in accordance with this invention at any given time.
  • Each depth filter cartridge 14 or 16 is provided with a pressure gauge 18 or 20 to measure pressure drop across the cartridge 14 or 16.
  • depth filter cartridge 14 is utilized to filter slurry when valves 22 and 24 are open and valves 26 and 28 are closed.
  • depth filter cartridge 16 is being regenerated with a reactive reagent such as a base having a pH between about 10.5 and 14 when valves 30 and 32 are closed and valves 34 and 36 are open.
  • Deionized water is passed through conduit 40 and mixed with alkali solution 42 obtained by passing deionized water through conduit 44 by pump 46.
  • Pump 46 directs deionized water through conduit 48 and through housing 50.
  • the alkali solution and deionized water mixture is filtered by filter 52, the pH of which is registered by pH monitor 54.
  • the alkali solution passes through conduit 60, valve 34, conduit 62, filter cartridge 16, conduit 64, valve 36 and conduit 66 to drain.
  • Example 1 illustrates the present invention and are not intended to limit the same.
  • Example 1 illustrates the present invention and are not intended to limit the same.
  • This example illustrates the regeneration of a depth filter cartridge having the following filtration characteristics.
  • a diluted silica slurry comprising a 12 percent silica particles by weight having a pH of 10.5 was filtered through a 2 inch long filter cartridge having a percent retention efficiency of approximately 100% for 3 micron particles (CMP3), and through a 2 inch long filter cartridge having a percent retention efficiency of about 100% for 5 micron particles (CMP5).
  • the pressure drop of this filter was measured while flushing by (a) reverse deionizing flush, (b) forward pH 11 flush or (c) reverse pH 11 flush. These flushing procedures were compared with (d) control/no flush and (e) forward deionized water flush. As shown in Figs.

Abstract

The present invention provides devices and processes that filter slurries and regenerate the filters so they can be reused.

Description

PROCESS FOR REGENERATING A FILTRATION CARTRIDGE FOR
FILTERING A SLURRY
BACKGROUND OF THE INVENTION
This invention relates to a process and a system for regenerating a depth filter cartridge for filtering a slurry composition.
A fluid composition containing a particulate solid component is referred to in the art as a "slurry". The solid component can be any of a variety of materials including solid particles, cell components, flocculating agents, gel particles or the like. Slurry compositions presently are utilized in chemical mechanical polishing (CMP) to polish wafers in VLSI and ULSI integrated circuit devices. High pH silica slurries are utilized to polish dielectric, polysilicon, copper layers. In addition, acidic silica, alumina and ceria abrasive based slurries are utilized to polish metal interconnects. The CMP process uses sub-micron (20-200 nm) abrasive particles such as, silica, alumina, ceria or manganese oxide or the like, at a typical concentration of 1-30% by weight particles.
The typical specification for commercial slurries includes percent solids, pH, specific gravity, mean particle size and general (bulk) particle size distribution. However, a small number of "large" particles (>lμm) have been found which fall outside of the specified size distribution. These particles can be aggregates or agglomerates and maybe formed from agglomeration, settling, shearing or local drying of slurry. These large particles and agglomerates can cause micro-scratches and other defects on planarized wafer surfaces during CMP processing. Slurry filtration to remove these relatively large particles has proven to be beneficial in reducing wafer defects and increasing yields in CMP processes. At the present time a vide variety of filter cartridge constructions are utilized to purify fluids. These cartridge constructions are designed to remove solids and colloidal particles as well as microorganisms. The basic two separate and distinct types of cartridges used in filtration of gases and liquids are depth filters (typically wound) and surface or screen filters (usually pleated). A depth filter is primarily used to remove most of the contaminants and particles. It is typically utilized upstream of a surface or screen filters. The most important properties for a depth filter are its "dirt holding capacity" or throughput, pressure drop and retention. The filter design allows contaminants and particles to be trapped in stages within the depth of the filter due to the construction of the multiple layers of various media types. A wound depth filter has multiple layers with the most open media (largest micron retention rating), i.e., largest pore size usually the outermost layer, adjacent the liquid inlet with the tightest media at t he core adjacent the liquid outlet will have the least amount of surface area due to the smallest diameter around which it is wrapped. This layer at the core contributes to most of the pressure drop of the cartridge because the media has the highest pressure drop and the least amount of filtration surface area. Likewise, this layer will significantly reduce the capacity of the filter due to both the low filtration surface area and the smallest micron retention rating.
Presently available depth filters are positioned within a housing, spaced apart from the interior housing walls thereby to forma void volume upstream of the depth filter. This spacing is effected to permit either the introduction of a fluid feed into the entire filter or the removal of the entire permeate from the filter. If this spacing were not maintained, fluid flow through the filter can be severely restricted. As a result, a relatively large fluid hold-up volume occurs in a conventional filter unit. A depth filter construction utilizing such a spacing also is disadvantageous for filtering a slurry since the particles in the slurry can settle out of the slurry on and within the filter. This results in rapid plugging of the filter, particularly at low flow rate point of use applications.
A surface or screen filter will retain virtually 100% of the particles or contaminants for which it is rated. The media used in surface or screen filter typically has a high pressure drop and low "dirt holding capacity" or throughput because of its high retention efficiency. The media normally used in a surface filter comprises glass or polymeric microfibers. Particles are retained by size exclusion primarily on the surface of the screen filter rather than within the depth of the filter. Particles smaller than the controlled pore size tend to be trapped within the media of the surface filter. However, as a result of the controlled pore structure, they provide more predictable filtration than depth filters. Screen filters are not useful for filtering a slurry since they will become plugged quickly by the solid particles and gels in the slurry.
Unfortunately, the filter becomes plugged with retained particles relatively quickly to the extent that the filter must be replaced. This result is undesirable since the filters employed are expensive.
Accordingly, it would be desirable to provide a process for extending the useful life of a filter cartridge for filtering a slurry. Such a process would effectively reduce the cost of filtering a slurry by reducing the requirements of filter cartridge replacement. SUMMARY OF THE INVENTION
The present invention comprises a process for regenerating a depth filter such as a stacked depth filter, a wound graded depth filter, or a non-woven pleated filter used to filter a slurry. The filter to be regenerated is loaded with retained particles as a result of filtering a slurry of the particles.
The stacked depth filter comprises a depth filter such as a fibrous mass, a plurality of non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt of the like positioned within a housing free of an open void volume upstream of the depth filter. By the term "open void volume" is meant a volume free of a material including materials for forming a depth filter and is not meant to include the void volume of the depth filter. The graded depth filter cartridge construction for filtering a slurry has a filtration medium formed as a depth filter such as a cylindrical seamless fibrous depth filter comprising a non- woven fibrous mass, a plurality of non- woven fibrous layers or a fibrous felt or the like or a wound depth filter retained within a housing. One end of the cartridge is sealed with a cap having a fluid inlet while the opposing end is sealed with a cap having a fluid outlet. When the filtration medium is a wound depth filter, it is positioned around a core that extends substantially the length of the cartridge. When the depth filter comprises a non- woven fibrous mass, it is compressed to effect the desired percent retention efficiency of the mass. The depth filter also can comprise a layered filter construction having a plurality of filtration media, each having a controlled percent retention rating. The layers of the depth filter are formed of felt layers, of wound or layered flat filtration sheets of a fibrous mass of non- woven polymeric fibers secured together by mechanical entanglement or interweaving of the fibers.
The pleated filter is formed by pressing non-woven fibers into a pleated configuration. The resultant pleated non-woven fiber filter then is wrapped around an open core that is positioned within a housing having a fluid inlet and a fluid outlet. The filter cartridges retain undesirably large particles and gel particles that permit passage there through of particles of a slurry having a size within a desired size range. h accordance with this invention, the filter cartridge fully loaded or partially loaded with retained slurry particles is processed to remove the slurry particles by one of three embodiments of this invention. By "fully loaded" is meant that, due to retained particles in the depth filter, a slurry cannot be passed through the filter. By "partially loaded" is meant that, due to retained particles in the depth filter, a slurry can be passed through the depth filter but at a flow rate less than or at a pressure drop higher than that obtained with a particle-free depth filter. It is preferred to regenerate a partially loaded depth filter since reduced regeneration times are required to effect regeneration. In the first embodiment, the filter is reverse flushed with deionized water. By the term "reverse flushed" or "reverse flush" as used herein, is meant that the flushing liquid is passed through the filter in a direction opposite to the direction in which the slurry flows during the slurry filtration step.
In a second embodiment, the plugged filter is reverse flushed with an aqueous solution having a composition which selectively reacts with the retained particles to reduce the size of the retained particle so that they can be removed from the filter by passing the solution through the plugged filter. hi a third embodiment, the plugged filter is forward flushed with an aqueous solution having a composition which selectively reacts with the retained particles to reduce the size of the retained particle so that they can be removed from the filter by passing the solution through the plugged filter. By the term "forward flushed" or "forward flush" as used herein is meant that the flushing liquid is passed through the plugged filter in the same direction as the direction of flow of the slurry through the filter during the filtration step. BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of a system of this invention. Fig. 2 is a graph showing the test result with CMP5 filter of Example 1. Fig. 3 is a graph showing the test result with CMP3 filter of Example 1. DESCRIPTION OF SPECIFIC EMBODIMENTS In accordance with this invention, a partially loaded or fully loaded depth filter which has been utilized to filter a slurry in order to selectively retain undesirably large particles is regenerated either with deionized water or with a reagent which reacts selectively with the retained particles to reduce their size while being unreactive with the depth filter medium. As set forth above, deionized water can be utilized to reverse flush a plugged depth filter to remove retained slurry particles therefrom. Generally, the reverse flushing with deionized water at a flow rate between about IX and 10X that used in the application. The regeneration step can be conducted on a partially plugged filter during off step in the polishing cycle (typically 1-3 minutes). Regeneration can also be performed on a completely plugged filter as well using longer time intervals. Typically, the regeneration step is conducted for a time period between 30 seconds and 30 minutes. Regeneration of the depth filter with deionized water cannot be affected by forward flushing the filter.
Also, as set forth above, an aqueous solution containing a composition which reacts with retained slurry particles to reduce the size of the retained slurry particles can be used to reverse flush or forward flush a plugged filter. Representative suitable compositions include an acid to form an aqueous solution, having a pH below about 6 preferably below about 4 or a base to form an aqueous solution having a pH greater than pH9. Generally, the reverse flushing or forward flushing with the reactive composition can be affected at flow rate of lx to lOx of that used in the application. Representative suitable acids include hydrochloric acid, sulfuric acid, nitric acid, or the like. Representative suitable bases include sodium hydroxide, potassium hydroxide, ammonium hydroxide or the like.
The pH of the reactive acid or base can be adjusted by directing an acid or base through a membrane into a carrier liquid such as water. The resultant pH adjusted carrier liquid then is passed through the slurry loaded depth filter to effect particle size reduction to the point where the size reduced retained particles are carried out of the depth filter. After a desired regeneration time, the supply of acid or base is stopped while the carrier liquid is continued through the depth filter to remove any reagent therein. Thereafter, the depth filter is utilized to filter additional slurry.
The filter cartridge construction which is regenerated by the process of this invention comprises (1) a depth filter comprising either (a) a wound depth filter, (b) a stack of depth filters or (c) a cylindrical seamless fibrous depth filter formed from a fibrous mass of fibers. The depth filter has a thickness in the direction of fluid flow there tlirough of between about 0.5 and about 18 inches, preferably between about 1 and about. 12 inches to attain effective retention of undesirably large particles while permitting passage there through of particles within a desired size range. The depth filter comprises a plurality of media (layers) each having a different micron retention size so that the permeability or retention of the media layers is largest adjacent a fluid inlet to the cartridge and is smallest adjacent the fluid outlet from the cartridge. Thus, large particles will be retained adjacent the feed inlet and progressively smaller particles will be retained as the feed passes through the filter cartridge. The permeability of retention of the media layers is controlled so that particles in the slurry within a desired particle range pass through the cartridge and through the outlet. It has been found, in accordance with the process of this invention, that the useful life of the cartridge is at least about 300 percent longer, preferably at least about 500 percent longer than a filter cartridge with a nonregenerated filter. Thus, the process of this invention permits the use of fewer cartridges for a particular application and at a reduced cost as compared to the cost of filter cartridges of the prior art. Percent retention efficiency and Beta Ratio are measures of the ability of the cartridge to capture and retain particles. The Beta Ratio concept was introduced by the Fluid Power Research Center (FPRC) at Oklahoma State University (OSU) in 1970. Originally developed for use in hydraulic and lubricating oil filter, the test has been adapted by many cartridge manufacturers to measure and predict the cartridge filter performance in aqueous based fields. Beta Ratio is defined by the FPRC as the number of particles greater than a give size (X) in the feed, divided by the number of particles greater than the same size in the effluent. Both percent retention efficiency and Beta Ratio values are calculated for specific particle size ranges.
The following equations showed the relationship between Beta Ratio and percent retention efficiency:
%Retention Efficiency = (Number of feed particles-Number of effluent particles) (size X) X 100
Number of feed particles (size X)
Beta Ratio (B) = Number of feed particles (size X Number of effluent particles (size X)
% Retention Efficiency = X 100 B
Beta Ratio (B) = 100
100% Retention Efficiency In one embodiment, the filter cartridge regenerated by the process of this invention, the filter medium of the depth filter having the largest micron retention is positioned adjacent an inlet to the filter cartridge. The filter media of the depth filter having the smallest micron retention is positioned adjacent the outlet from the filter cartridge. The micron retention characteristics of a filter can be varied by varying the diameter of fibers used to form the filter and/or the extent of compression of the fibers such as by winding a filter medium sheet tighter or looser around a core. A tighter wound filter medium gives a higher percent retention efficiency. The intermediate filter media are positioned according to percent retention of efficiency so that incoming slurry is passed sequentially through the filter media having progressively smaller micron retention and lastly through the filter media having the smallest micron retention. Thus, the overall filter cartridge presents a percent retention efficiency which comprises a progressive gradient from the inlet to the outlet wherein the percent retention efficiency progressively increases. Representative media useful for forming the depth filter include the fibers of polyolefins such as polyethylene, polypropylene, cellulose, cotton, polyamides, polyesters, fiberglass, polytetrafluoroethylene (PTFE), fluoropolymers such as PFA, MFA and FEP or the like.
The fibrous depth filter is free of seams and is formed of fibers that produce a fibrous mass of fibers. This embodiment of the depth filter can be characterized by a gradation of micron retention characteristics throughout its thickness in the direction of fluid flow through the depth filter. This gradation can be achieved either by varying the void volume of the cylindrical fibrous depth filter medium as a function of thickness in the direction of fluid flow through the filter or by maintaining a constant volume and varying the size of the fibers as a function of depth filter thickness in the direction of fluid flow through the depth filter, hi either embodiment, all that is necessary is that the gradation of micron retention characteristics is produced. The gradation is effected such that the slurry to be filtered first encounters a layer of the depth filter having the largest micron retention characteristics (i.e., largest pores) and encounters layers having progressively smaller micron retention characteristics, (i.e., smallest pores) prior to being directed tlirough the outlet. The seamless cylindrical fibrous depth filter can be formed by any conventional means such as is disclosed in U.S. Patents 3,933,557; 4,032,688; 4,726,901 or 4,594,202 which are incorporated herein by reference. The pleated filters regenerated by the process of this invention are formed by pressing non-woven fibers into a pleated shape. Representative useful fibers are the same fibers set forth above for the depth filters. Typically, the void volume of the cylindrical fibrous depth filter ranges between about 60 and 95 percent and varies no more than about 1 to 2 percent. Typically, the fibers range in diameter between about 1.6 and 16 micrometers. !
The wound depth filter is formed by winding one or a plurality of filter sheets formed of fibers to form a joint generally cylindrical structure. The filter sheet or sheets have varying pore size such that the micron retention characteristic of a portion of the depth filter as a function of radial position within or on the depth filter. The portion of the wound depth filter positioned adjacent an inlet to the filter cartridge including the wound depth filter has the largest micron retention characteristics while the portion of the wound depth filter having the smallest micron retention characteristics, i.e., the smallest pore size is positioned adjacent the outlet from the filter cartridge. Any intermediate portions of the wound depth filter are positioned according to pore size so that incoming slurry is passed sequentially tlirough portions of the depth filter having progressively smaller micron retention characteristics. Representative media useful for forming depth filters include the fibers set forth above for the cylindrical seamless fibrous filters.
The stacked depth filter which is regenerated of the process of this invention can be formed from one or a plurality of separate filter sheets by stacking the sheet or sheets within a housing in a manner such that an open volume within the housing and upstream of the depth filter is avoided. The filter sheet or sheets can have the same pore size or varying pore size such that the micron retention characteristic of a portion of the stacked depth filter varies along the length of the housing. When utilizing sheets having varying pore size, the portion of the filer stack positioned adjacent an inlet to the filter cartridge has the largest micron retention characteristics while the portion of the filter stack having the smallest micron retention characteristics, i.e., the smallest pore size is passed sequentially tlirough portions of the depth filter having progressively smaller micron retention characteristics and lastly through the portion of the filter having the smallest micron retention characteristics. Representative media useful for forming the filter stack include the fibers set forth above for the cylindrical seamless fibrous filters.
As shown in Fig. 1, a system 10 is shown having a slurry supply conduit 12 connected to a slurry source (not shown). The system 10 includes two depth filter cartridges 14 and 16, one of which is utilized to filter slurry while the other of which is being regenerated in accordance with this invention at any given time. Each depth filter cartridge 14 or 16 is provided with a pressure gauge 18 or 20 to measure pressure drop across the cartridge 14 or 16. As shown in Fig. 1, depth filter cartridge 14 is utilized to filter slurry when valves 22 and 24 are open and valves 26 and 28 are closed. At the same time, depth filter cartridge 16 is being regenerated with a reactive reagent such as a base having a pH between about 10.5 and 14 when valves 30 and 32 are closed and valves 34 and 36 are open. Deionized water is passed through conduit 40 and mixed with alkali solution 42 obtained by passing deionized water through conduit 44 by pump 46. Pump 46 directs deionized water through conduit 48 and through housing 50. The alkali solution and deionized water mixture is filtered by filter 52, the pH of which is registered by pH monitor 54. The alkali solution passes through conduit 60, valve 34, conduit 62, filter cartridge 16, conduit 64, valve 36 and conduit 66 to drain. When regenerating depth filter 14, and filtering with depth filter 16, valves 22 and
24 are closed while valves 26 and 28 are open. Valve 32 and 30 are also open while valves 34 and 36 are also closed. By operating in this manner, a slurry can be continuously filtered while a depth filter can be continuously regenerated. The system of Fig. 1, also can be utilized with more than two depth filter cartridges. The following examples illustrate the present invention and are not intended to limit the same. Example 1
This example illustrates the regeneration of a depth filter cartridge having the following filtration characteristics. A diluted silica slurry comprising a 12 percent silica particles by weight having a pH of 10.5 was filtered through a 2 inch long filter cartridge having a percent retention efficiency of approximately 100% for 3 micron particles (CMP3), and through a 2 inch long filter cartridge having a percent retention efficiency of about 100% for 5 micron particles (CMP5). The pressure drop of this filter was measured while flushing by (a) reverse deionizing flush, (b) forward pH 11 flush or (c) reverse pH 11 flush. These flushing procedures were compared with (d) control/no flush and (e) forward deionized water flush. As shown in Figs. 2 and 3, the pressure drop of 2 psi or less with CMP5 and CMP3 filters and silica slurry was obtained for 1000 minutes with periodic flushing by (a) reverse flush with deionized water (b) reverse flush with pH adjusted water and (c) forward flush with pH adjusted water, hi contrast, a pressure drop of at least 10 psi was obtained after 750 minutes with both (d) no flushing and (e) forward flush with deionized water. Thus, satisfactory depth filter regeneration was obtained only by methods (a), (b) or (c) of this invention.

Claims

WHAT WE CLAIM:
1. A system for filtering a slurry with a depth filter and regenerating the depth filter which comprises:
(a) means for passing a slurry through a depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(b) means for ceasing flow of slurry through said depth filter,
(c) means for passing a liquid through the depth filter after ceasing flow of slurry through said depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said depth filter, (b) a liquid selectively reactive with retained particles in said depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said depth filter in a forward flush direction.
2. The system of claim 1 wherein said liquid passed through said depth filter is deionized water in a reverse flush direction.
3. The system of claim 1 wherein said liquid passed through said depth filter is selectively reactive with said retained particles passed in a reverse flush direction.
4. The system of claim 1 wherein said liquid passed through said depth filter is selectively reactive with said retained particles passed in a forward flush direction.
5. The system for filtering a slurry with a first depth filter and a second depth filter
(a) means for passing a slurry through said first depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(b) means for ceasing flow of slurry through said first depth filter,
(c) means for passing a liquid through the first depth filter after ceasing flow of slurry through said first depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said first depth filter, (b) a liquid selectively reactive with retained particles in said first depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said first depth filter in a forward flush direction
(d) means for passing a slurry through said second depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(e) means for ceasing flow of slurry through said second depth filter, and
(f) means for passing a liquid through said second depth filter after ceasing flow of slurry through said second depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said second depth filter, (b) a liquid selectively reactive with retained particles in said second depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said second depth filter in a forward flush direction.
6. The system of claim 5 wherein said liquid passed through said first depth filter and said second depth filter is deionized water in a reverse flush direction.
7. The system of claim 5 wherein said liquid passed through said first depth filter and said second depth filter is selectively reactive with said retained particles passed in a reverse flush direction.
8. The system of claim 5 wherein said liquid passed through said first depth filter and said second depth filter is selectively reactive with said retained particles passed in a forward flush direction.
9. A process for filtering a slurry with a depth filter and regenerating the depth filter which comprises:
(a) passing a slurry through a depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(b) ceasing flow of slurry through said depth filter, and
(c) passing a liquid through said depth filter after ceasing flow of slurry through said depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said depth filter, (b) a liquid selectively reactive with retained particles in said depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said depth filter in a forward flush direction.
10. The process of claim 9 wherein said liquid passed through said depth filter is deionized water in a reverse flush direction.
11. The process of claim 9 wherein said liquid passed through said depth filter is selectively reactive with said retained particles passed in a reverse flush direction.
12. The process of claim 9 wherein said liquid passed through said depth filter is selectively reactive with said retained particles passed in a forward flush direction.
13. The process for filtering a slurry with a first depth filter and a second depth filter
(a) passing a slurry through said first depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(b) ceasing flow of slurry through said first depth filter,
(c) passing a liquid through the first depth filter after ceasing flow of slurry through said first depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said first depth filter, (b) a liquid selectively reactive with retained particles in said first depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said first depth filter in a forward flush direction
(d) means for passing a slurry through said second depth filter having a percent retention efficiency to effect selective removal of particles having an undesirably large size from the slurry,
(e) means for ceasing flow of slurry through said second depth filter, and
(f) means for passing a liquid through said second depth filter after ceasing flow of slurry through said second depth filter selected from the group consisting of (a) deionized water in a reverse flush direction through said second depth filter, (b) a liquid selectively reactive with retained particles in said second depth filter in a reverse flush direction and (c) a liquid selectively reactive with retained particles in said second depth filter in a forward flush direction.
14. The process of claim 13 wherein said liquid passed through said first depth filter and said second depth filter is deionized water in a reverse flush direction.
15. The process of claim 13 wherein said liquid passed through said first depth filter and said second depth filter is selectively reactive with said retained particles passed in a reverse flush direction.
16. The process of claim 13 wherein said liquid passed through said first depth filter and said second depth filter is selectively reactive with said retained particles passed in a forward flush direction.
PCT/US2002/013716 2001-05-03 2002-05-02 Process for regenerating a filtration cartridge for filtering a slurry WO2002089951A1 (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
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WO2010077718A3 (en) * 2008-12-09 2010-08-26 E. I. Du Pont De Nemours And Company Filters for selective removal of large particles from particle slurries
WO2022179970A1 (en) * 2021-02-24 2022-09-01 F. Hoffmann-La Roche Ag Regeneration and multiple use of depth filters

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US6165048A (en) * 1998-11-10 2000-12-26 Vlsi Technology, Inc. Chemical-mechanical-polishing system with continuous filtration

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US5928492A (en) * 1997-06-05 1999-07-27 Lucid Treatment Systems, Inc. Method and apparatus for recovery of water and slurry abrasives used for chemical and mechanical planarization
US6165048A (en) * 1998-11-10 2000-12-26 Vlsi Technology, Inc. Chemical-mechanical-polishing system with continuous filtration

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Publication number Priority date Publication date Assignee Title
WO2010077718A3 (en) * 2008-12-09 2010-08-26 E. I. Du Pont De Nemours And Company Filters for selective removal of large particles from particle slurries
CN102271782A (en) * 2008-12-09 2011-12-07 纳幕尔杜邦公司 Filters for selective removal of large particles from particle slurries
JP2012511428A (en) * 2008-12-09 2012-05-24 イー・アイ・デュポン・ドウ・ヌムール・アンド・カンパニー Filter for selectively removing large particles from particle slurry
WO2022179970A1 (en) * 2021-02-24 2022-09-01 F. Hoffmann-La Roche Ag Regeneration and multiple use of depth filters

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