AU2004222813B2

AU2004222813B2 - Method of production and application of a steel mesh filter

Info

Publication number: AU2004222813B2
Application number: AU2004222813A
Authority: AU
Inventors: Gilbert Gerald Erskine
Original assignee: STERI FLOW FILTRATION SYSTEMS AUST Pty Ltd
Current assignee: STERI-FLOW FILTRATION SYSTEMS (AUST) Pty Ltd
Priority date: 2003-10-23
Filing date: 2004-10-25
Publication date: 2011-04-07
Anticipated expiration: 2024-10-25
Also published as: AU2004222813A1

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: LGE Pty Ltd Actual Inventor: Gilbert Gerald ERSKINE Address for Service: C/- MADDERNS, 1st Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia Invention title: METHOD OF PRODUCTION AND APPLICATION OF A STEEL MESH FILTER Details of Associated Provisional Application No: 2003905820 dated 23rd October 2003 The following statement is a full description of this invention, including the best method of performing it known to us. I PatAU131I TECHNICAL FIELD This invention relates to metallic filter membranes and their use in filtering liquids. BACKGROUND TO THE INVENTION 5 The filtering of liquids is required in many areas of application ranging from medical uses to industrial chemical applications. One particular area in which filtering of liquids is important is in the wine making process. Filtering of the wine liquid is necessary at various stages to remove impurities such as grape skin particles, finer particulate matter and even bacteria. 10 It has previously been proposed to use a stainless steel woven mesh material as the filter however, this suffers from a number of drawbacks. For example, it is difficult to get hole or pore sizes within the mesh small enough to adequately filter smaller particles. Furthermore, the number of pores present cannot always be controlled, and 15 the effective open area (ie percentage of open pores to steel) is not great (eg a 2 percent open area is typical of current meshes). Furthermore, as the pore size decreases, the mesh becomes very brittle and difficult to use. It is an object of the present invention to provide an improved filter membrane for 20 filtering liquids such as wine. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a metallic filter membrane having a first surface and a second, opposing surface, the membrane 25 having a plurality of apertures extending from the first surface to the second surface, wherein at least some of said apertures increase in cross-sectional area as the at least some of said apertures extend from the first surface to the second surface; and wherein the filter membrane includes at least two layers such that the membrane is built up layer by layer from the first surface to the second surface, and wherein each 30 subsequent layer has apertures of greater area than those of the preceding layer. 2 Preferably each layer is provided by a mixture of stainless steel powder of a given grain size, and a methanol based solution, and wherein the grain size of the stainless powder increases with each subsequent layer. 5 Preferably the stainless steel powder grain size of a first of the layers is 3 to 8 microns, the stainless powder grain size of a second of the layers is 5 to 15 microns and the stainless powder grain size of a third of the layers is 10 to 30 microns. In preference the stainless steel powder is 316L stainless powder. Preferably the 10 mixture also includes nickel powder. Preferably the mixture of the first layer also includes nickel powder of grain size 5 microns, the mixture of the second layer also includes nickel powder of grain size 10 microns and the mixture of the third layer also includes nickel powder of grain size 15 20 microns. In preference the methanol based solution consists of 1000ml of denatured alcohol, 10 grams to 20 grams of Teflon, 7 grams to 23 grams of wax, 2 ml to 9 mm of glycerin and 2 ml to 7 ml of polyethylene glycol, which is mixed with the stainless 20 powder to produce a paint-like consistency. Preferably a first layer is formed from a sheet of stainless steel woven mesh whose state has been changed from solid state to liquid (puddle) state and allowed to cool and harden. 25 Preferably an at least one further layer of stainless steel woven mesh is applied to the first layer and fused together to form an integral piece, and wherein the at least one further layer has an aperture size greater than an aperture size of the first layer. Preferably the metallic filter membrane is a stainless steel filter membrane. 30 3 According to a second aspect of the present invention, there is provided a method of making a filter membrane, the method including fusing together a first layer of metallic woven mesh to at least one further layer of metallic woven mesh, each having respective aperture sizes, wherein the aperture size of the at least one further 5 layer is greater than the aperture size of the first layer and the aperture size of any subsequent layers is greater than the aperture size of the previous layer. Preferably the first layer is heated and pressurised so as to change its state from solid to liquid (puddle) and allowed to cool and harden, before fusing with the subsequent 10 layers. In preference the first layer is rolled between rollers to reduce the aperture size of the first layer. 15 Preferably the metallic woven mesh is stainless steel. According to a third aspect of the present invention, there is provided a method of making a filter membrane, the method including: mixing a metallic powder having a first grain size with a methanol based 20 solution, to produce a first mixture; coating a polished rod with the first mixture to form a first layer; mixing metallic powder having grains of a second size, larger than the first size, with a methanol based solution to provide a second mixture; coating the first layer with the second mixture to form a second layer; and 25 pressurising the coated rod to harden the first and second layers. Preferably a third mixture of the metallic powder having grains of a third size, greater than the first and second sizes, and the methanol based solution to provide a third mixture and coating the second layer with the third mixture. 30 3a In preference the method of producing each of the mixtures further includes the step of mixing in nickel powder. Preferably the nickel powder has grains of increasing sizes for each of the 5 respective mixtures. Preferably the method further includes the step of placing the coated rod inside a polyurethane sheath and filling a space between the urethane sheath and the coated rod with stainless powder before the step of pressurising. 10 Preferably the metallic powder is stainless steel powder. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described with reference to the 15 following drawings in which: Figure 1 shows a side cross-section of a filter membrane according to one aspect of the invention; Figure 2 shows a cross-section of the filter-membrane showing the funnel-shaped apertures; 20 Figure 3A to 3C show progressive steps of making the filter membrane according to another aspect of the present invention; Figure 4 shows a completed filter; and Figure 5 shows a diagram of an apparatus used to filter wine. 25 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Currently, various stainless steel woven mesh products are available and are provided with different aperture or pore sizes. Pore sizes can range generally from 1 micron up to 100 microns. 3b The 1 micron mesh is then rolled between two highly polished stainless steel rollers. The rolling in this manner reduces the aperture size from one micron to 0.2 microns or 0.5 microns depending on the finish that is required. The aperture size may be controlled by the amount of pressure used. The rolling process also provides a much 5 better RA finish on the material. The present invention is applicable to any suitable metallic filters, but is preferably, stainless steel. 10 This rolled material is then placed in a high temperature furnace, and the temperature raised to 450 degrees celsius in normal atmosphere. At this point, ultra dry nitrogen is injected into the furnace to purge and replace the current atmosphere. The nitrogen is introduced into the atmosphere slowly, preferably at a rate of 60 litres per minute per 1m 3 , reducing to 8 litres per minute after the first 16 minutes. At this 15 point, the temperature is raised to 1450 degrees celcius, and the pressure in the furnace is increased to prevent the loss of chromium by it being vaporised from the stainless steel at this high temperature. It is desirable to retain the chromium in the steel as this will prevent the filter from rusting when in use. 20 At the end of this cycle, the oven is turned off and the material is allowed to cool slowly. The process of heating the material to 1450 degrees celsius under pressure, results in the changing of state of the material from solid to liquid (puddle). The stainless steel 25 woven mesh material has thus been changed into a solid but has maintained the matrix from the weave. As a result, good uniformity and controlled pore or aperture size is achieved. It will be appreciated that this process is preferred, but may be omitted, and an unprocessed sheet of desired aperture size used instead. 30 As shown in Figure 1, the resulting sheet material (1) is then layered with three sheets of unprocessed stainless steel woven mesh, one sheet (2) having pore or aperture size of 5 microns, the second sheet (3) of 15 microns and the third (4) of 100 4 microns. This stacked layer (10) is then sandwiched between two pieces of ceramics and once again placed in a furnace (not shown). The temperature is then raised to 450 degrees celsius in normal atmosphere, at which point, again ultra dry nitrogen is injected into the furnace to purge and replace the atmosphere slowly. The 5 temperature is then raised to 1150 degrees celsius as the pressure in the furnace is increased to again prevent the loss of chromium by vaporisation from the stainless steel. At the end of the cycle, the oven is turned off and the processed material allowed to 10 cool slowly. The above process has resulted in all of the layers being fused together forming an integral piece of filter media. This provides a material of increased strength over currently available stainless steel woven mesh while also allowing the provision of apertures which are of uniform size and whose size can be controlled. The resulting material also has a relatively large open area in contrast to other 15 methods of providing filtering materials. Due to relatively increasing pore sizes of the subsequent layers, a plurality of "funnel-like" tubes 11 is formed traversing the filter 10. This is shown in cross section in Figure 2. This reduces the likelihood of particles becoming blocked within 20 the filter and clogging it. While not all apertures from all layers will line up to form an aperture traversing the thickness of the filter membrane 20, many will, providing the apertures 11. 25 Pore sizes of 0.5 microns can filter out yeast cells, while pore sizes of 0.2 microns can filter out bacteria. To construct the actual filter, the material is formed into two pieces and seam welded together. Each piece is then welded end to end to form a tube of specified length. 30 Alternatively, the filter membrane may be formed into any other convenient shape as desired, to suit a particular filtering application. 5 The formed tube may now be used in various filtering applications including the filtering of wines as described with reference to Figure 5. 5 An alternative method of producing a filter membrane in the form of a tube will now be described. Three to eight micron 316L stainless steel powder is mixed with five micron nickel powder to a ration of 70% to 90% stainless powder, with 10% to 30% nickel powder. 10 The use of nickel powder is optional and is dependant upon the application. For example, certain types of nickel have charge related sites, which can be very useful when filtering materials containing colloidal or protein. Nickel also has very good corrosion resistance, for use in harsh environments such as brackish or salty water. 15 This mixture is then added to a methanol based solution made up from 1000 ml of denatured alcohol, 10 grams to 20 grams of Teflon, 7 grams to 23 grams of wax, 2 ml to 9 mm of glycerin, 2 ml to 7 mm of polyethyleneglycol, mixed to produce a paint like consistency. 20 A highly polished rod 25 to a RA finish of 0.2 micron (500 mm) is then coated 21 with this mixture, to a material thickness of between 25 microns and 70 microns. A second mixture is then prepared, consisting of 5 to 15 micron 316L stainless powder with 10 micron nickel powder to a ration of 70% to 90% stainless powder 25 with 10% to 30% nickel powder. Again, the use of nickel is optional dependent on the application. This mixture is then added to a methanol based solution made up from 1000 ml of denatured alcohol, 3 grams to 8 grams of Teflon, 7 grams to 23 grams of wax, 2 ml to 9mm of glycerin, 2ml to 7mm of polyethyleneglycol and is then mixed to produce a paint-like consistency. 30 This mixture is then sprayed on to the top of the previous coating 21, to provide a layer 22 of material thickness of between 25 microns and 70 microns (Figure 3B). 6 A third mixture is then made consisting of 10 to 30 micron 316L stainless powder, with 20 micron nickel powder to a ration of 70% to 90% stainless powder with 10% to 30% nickel powder. Once again, the use of nickel is optional depending upon the 5 desired application. This mixture is then added to a methanol based solution made up from 1000 ml of denatured alcohol, 3 grams to 8 grams of Teflon, 7 grams to 23 grams of wax, 2 ml to 9 mm of glycerin, 2 ml to 7 mm of polyethyleneglycol and is mixed together to 10 produce a paint like consistency as for the previous two mixtures. This mixture is then sprayed onto the top of the previous coating 22 to provide a third layer 23 of material thickness of between 25 microns and 70 microns (see Figure 3C). 15 The rod 25 with the three coatings (21, 22, 23) is then placed inside a polyurethane sheath, having a density of between 70 and 90 duro (shore) (not shown). Thirty to eighty micron stainless powder is then mixed with 40 micron nickel, 70% to 20 90% stainless powder with 10% to 30% nickel powder ratio. Again, the nickel is optional. This mixture is then used to fill the space between the urethane and the coated rod. This forms the structural part of the membrane of the filter. The ends of the sheath are then sealed. 25 The urethane sheath is then dropped into an isostatic press, where a pressure of between 20,000 psi and 85,000 psi is applied and held for between 120 seconds and 180 seconds. The pressure is then reduced to atmosphere over a period of 3 to 5 seconds. This rapid reduction in pressure causes the compressed metal powder to 30 spring back. Due to this keying arrangement, the fine powders are pulled back with the 40 micron fill material. The urethane sheath is then removed from the press. 7 Upon removal from the urethane sheath, the layered metal powders have been compressed to the stage that it is rigid and can be removed from the rod. The compressed metal powder (green sinter) is then placed into a controlled 5 atmosphere furnace where, high vacuums are achieved of 10 to -2 mbar. The furnace temperature is ramped up over a period of 1 to 4 hours. During this heating process, back-fill gas is introduced. This gas is a combination of hydrogen/argon and nitrogen. The furnace undergoes several steps of heat and steep, until it reaches a pre-set temperature of between 1180*C and 1240*C. After a holding time of about 30 10 to 50 minutes, a rapid cooling stage is effected, over a period of between 1.5 hours and 2 hours, depending upon the furnace load at the time. This process provides tubes 20 (see Figure 4) which can be welded together to form a filter membrane of desired length. The use of a highly polished rod gives a mirror 15 finish on the compressed powders. The mirror finish effectively reduces the risk of fouling. Using progressively larger powders has the effect of keying the powders during high pressure isostatic pressing, thus eliminating the risk of laminating. 20 The method also allows the ability to control and vary the micron finish and maintain a consistent open area of for example 30 to 40% open area in the filter membrane. 25 By building the membrane from inside out, gradually increasing powder size, an aperture matrix is formed wherein the cross-sectional area of the apertures increases as the apertures extend from the inside of the tube to the outside. This reduces the risk of plugging, which also reduces pressure drop across the membrane which in turn reduces power input to run a filter machine in which the filter membrane is 30 used. 8 This formed tube may also be used in the application as described with reference to Figure 5. The apparatus of Figure 5 is operated in accordance with the following procedures. 5 On initialisation of power and with both raw product and filtrate lines connected, manual isolation valves in the open position, filtrate volume should be entered in litres per hour or set to continuous. 10 * Product enters and floods break tank via level control. 0 Nitrogen valve opens on filtrate side of the membranes, for example 30 seconds. 0 Feed pump starts and ramps up to 30%, priming the primary side and ensuring that there is no condensable matter in the system. 0 Main circulating pump starts and ramps up, controlled by the mag flow meter on 15 the main return loop (adjustable range). * Feed pump ramps the system up to the running pressure (adjustable). 0 Filtrate valve opens via speed control to fully open. * Filtrate pump senses filtrate pressure from the filtrate transducer and will ramp up and down to maintain a preset pressure. 20 e Initial filtrate flow is monitored and automatically entered into the data logger. 9 Due to build up of particulate material on the membrane the filtrate will slow. This is picked up by the mag flow on the output of the filtrate, at a predetermined flow rate back flush is initiated. Filtrate valve closes, a secondary nitrogen valve opens and forces clean filtrate in a reverse direction back through the membrane, 25 dislodging and replacing the build up of particulate. Normal time span would be 3 seconds, nitrogen valve closes, filtrate valve opens - this action will return flux levels back to pre-coat levels. The stainless steel membrane microfiltration filter is embodied in the form of a 30 cylinder having a wall thickness of typically 5 millimetres and an overall diameter of 10 to 20 millimetres. However, these dimensions can be varied to suit the particular application. 9 While the above has been described in relation to a particular embodiment and to a particular application of filtering wine, it will be understood that the above method and apparatus can be applied to any other filtering application such as the filtering of 5 water. 10

Claims

1. A metallic filter membrane having a first surface and a second, opposing surface, the membrane having a plurality of apertures extending from the first 5 surface to the second surface, wherein at least some of said apertures increase in cross-sectional area as the at least some of said apertures extend from the first surface to the second surface; and wherein the filter membrane includes at least two layers such that the membrane is built up layer by layer from the first surface to the second 10 surface, and wherein each subsequent layer has apertures of greater area than those of the preceding layer.

2. A metallic filter membrane according to claim 1 wherein each layer is 15 provided by a mixture of stainless steel powder of a given grain size, and a methanol based solution, and wherein the grain size of the stainless powder increases with each subsequent layer.

3. A metallic filter membrane according to claim 2 wherein the stainless steel 20 powder grain size of a first of the layers is 3 to 8 microns, the stainless powder grain size of a second of the layers is 5 to 15 microns and the stainless powder grain size of a third of the layers is 10 to 30 microns.

4. A metallic filter membrane according to claim 2 wherein the stainless steel 25 powder is 316L stainless powder.

5. A metallic filter membrane according to claim 4 wherein the mixture also includes nickel powder. 30

6. A metallic filter membrane according to claim 5 wherein the mixture of the first layer also includes nickel powder of grain size 5 microns, the mixture of the second layer also includes nickel powder of grain size 10 microns and the 11 mixture of the third layer also includes nickel powder of grain size 20 microns.

7. A metallic filter membrane according to any one of claims 2 to 6 wherein the 5 methanol based solution consists of 1000ml of denatured alcohol, 10 grams to 20 grams of Teflon, 7 grams to 23 grams of wax, 2 ml to 9 mm of glycerin and 2 ml to 7 ml of polyethylene glycol, which is mixed with the stainless powder to produce a paint-like consistency. 10

8. A metallic filter membrane according to claim 1 wherein a first layer is formed from a sheet of stainless steel woven mesh whose state has been changed from solid state to liquid (puddle) state and allowed to cool and harden. 15

9. A metallic filter membrane according to claim 8 wherein an at least one further layer of stainless steel woven mesh is applied to the first layer and fused together to form an integral piece, and wherein the at least one further layer has an aperture size greater than an aperture size of the first layer. 20

10. A method of making a filter membrane, the method including fusing together a first layer of metallic woven mesh to at least one further layer of metallic woven mesh, each having respective aperture sizes, wherein the aperture size of the at least one further layer is greater than the aperture size of the first layer and the aperture size of any subsequent layers is greater than the 25 aperture size of the previous layer.

11. A method of making a filter membrane according to claim 10 wherein the first layer is heated and pressurised so as to change its state from solid to liquid (puddle) and allowed to cool and harden, before fusing with the 30 subsequent layers. 12

12. A method of making a filter membrane according to claim 10 or 11 wherein the first layer is rolled between rollers to reduce the aperture size of the first layer. 5

13. A method of making a filter membrane, the method including: mixing a metallic powder having a first grain size with a methanol-based solution, to produce a first mixture; coating a polished rod with the first mixture to form a first layer; mixing metallic powder having grains of a second size, larger than the first 10 size, with a methanol based solution to provide a second mixture;coating the first layer with the second mixture to form a second layer; and pressurising the coated rod to harden the first and second layers.

14. A method of making a filter membrane according to claim 13 wherein a third 15 mixture of the metallic powder having grains of a third size, greater than the first and second sizes, and the methanol based solution to provide a third mixture and coating the second layer with the third mixture.

15. A method of making a filter membrane according to any one of claims 13 20 and 14, wherein the method of producing each of the mixtures further includes the step of mixing in nickel powder.

16. A method of making a filter membrane according to claim 15 wherein the nickel powder has grains of increasing sizes for each of the respective 25 mixtures,

17. A method of making a filter membrane according to any one of claims 13 to 16, the method further including the step of placing the coated rod inside a polyurethane sheath and filling a space between the urethane sheath and the 30 coated rod with stainless powder before the step of pressurising.

18. A method of making a filter membrane according to any one of claims 10 to 12 wherein the metallic woven mesh is stainless steel. 13

19. A method of making a filter membrane according to any one of claims 13 to 17, wherein the metallic powder is stainless steel powder. 5

20. A metallic filter membrane according to any one of claims 1-9 wherein the metallic filter membrane is a stainless steel filter membrane.

21. A filter membrane according to claim 1 substantially as herein described with reference to the accompanying drawings. 10

22. A filter membrane according to claim 2 substantially as herein described with reference to the accompanying drawings.

23. A method of making a filter membrane according to claim 10 substantially as 15 herein described with reference to the accompanying drawings.

24. A method of making a filter membrane according to claim 13 substantially as herein described with reference to the accompanying drawings. 14