WO2007039019A1

WO2007039019A1 - Moulded filter and process for making same

Info

Publication number: WO2007039019A1
Application number: PCT/EP2006/008207
Authority: WO
Inventors: Swati Agarwal; Jaideep Chatterjee; Bhupesh Sadanand Desale; Samiran Mahapatra
Original assignee: Unilever N.V.; Unilever Plc; Hindustan Lever Limited
Priority date: 2005-09-19
Filing date: 2006-08-21
Publication date: 2007-04-12

Abstract

The present invention relates to moulded filters, for use in gravity water filtration devices and a process for making such moulded filters. It is an object of the invention to provide filter media that provide high and consistent flow rates in gravity-fed water purification devices while ensuring effective removal of particulate contaminants. The invention relates to a process for producing said filters comprising the steps of: mixing 1 part by weight polymeric binder having a Melt Flow Rate less than 5 g/10 minutes, with 1 to 20 parts by weight filter medium in presence of a fluid wherein amount of said fluid is not more than 4 times the amount of said filter medium, moulding the mix obtained in step (a), optionally, compressing said mould, heating said mould to a temperature in the range 150°C to 350°C and demoulding the moulded filter. The invention also relates to moulded filters for use in gravity water filters.

Description

Moulded filter and process for making same

Technical field

The present invention relates to moulded filters, for use in gravity water filtration devices and a process for making such moulded filters.

Background and prior art

The usage of water by human beings can be classified into two categories depending on the purpose: drinking and non- drinking. It is needless to say that drinking water quality is of the utmost importance and should not be compromised upon.

Water to be used for drinking may already be polluted at the source or become polluted during transportation to consumers' homes through leaking pipelines. Thus, boiling or purifying treatments may be necessary before water is to be consumed. Pollutants may include particulate material, chemical contaminants and microorganisms such as bacteria, viruses and protozoa. The latter are known to cause most of the water-borne diseases.

From the many ways in which water can be made fit for consumption, filtration through conventional water filters is an important method.

Water filtration devices can be classified based on whether water to be treated flows under pressure or under the influence of gravity alone. The former method is generally referred to as online method, while the latter one is called batch method. In batch filtration, usually used in domestic water purification devices, the filtration rate is largely dependent on the efficiency of the filter medium. When input water quality is bad, i.e. it contains high levels of particulate contaminants or dissolved chemical impurities that form precipitates during filtration; it is generally observed that the filtration rate drops considerably over time, due to clogging of the filter medium. This necessitates replacement and/or regular cleaning of the filter medium. Thus, innovators in this field of technology have been constantly trying to develop filters, which can provide higher and consistent filtration rates, without clogging over time, while ensuring the removal of particulate contaminants and microorganisms.

Typically a domestic (batch type) gravity-fed water filtration device has two chambers, an upper one and a lower one. A filter element is positioned between the two. Water to be filtered is poured at regular intervals into the top chamber, and purified water flows into the lower one from where it can be dispensed through a tap. In such devices carbon based filter elements are often used. Such filters are either cartridge type containing loose granular carbon in a container or the sintered (moulded) type where carbon particles are bound together with a suitable binder to form a solid filter element. Carbon based moulded filters and various processes for preparing the same are well known in the art .

A wet process for preparing carbon filters has been described in US3,611,678 (Filtrona Corporation, 1971) using carbon particles in the range of 80 to 325 mesh along with 50-90 parts thermoplastic or thermosetting binder. The method includes mixing the dry ingredients and sufficient water, charging to a mould, applying sufficient pressure, and curing the resulting filter. The filter has voids in a sufficient amount to provide a pressure drop. These filters are intended for removing odors from air and are to be utilized in air conditioning equipment.

US5017318 discloses a dry process for preparing double-shell carbon filters.

CA2,396,510 (TYK Corp. 2003) describes a process for preparing carbon filters using particles of size 35-200 μm with 50% or more ceramic binder, wherein water is sprayed onto this mixture before it is sintered.

The technical problems in designing air filters and filters for water treatment are different and the same holds for their solutions. Also, air filters and water treatment filters have different properties and different requirements. One has to also consider several factors like particle size range of the filter medium, particle size of the binder, selection of the appropriate binder and the right amount of solvent that needs to be added, so that the final product gives the desired performance.

Summary and objects of the invention

The present invention provides moulded filters having uniform distribution of binder and filter medium throughout the filter thereby providing proper binding of filter medium with the binder and uniform and consistent flow rate over time. The invention also provides a novel process for producing such filters.

It is therefore an object of the invention to provide filter media that provide high and consistent flow rates in gravity-fed water purification devices while ensuring effective removal of particulate contaminants

It is another object of the invention to provide a moulded filter where the flow rate of the filter after use can be restored to the original flow rate by known methods of restoration, e.g. back flushing.

Yet another object of the present invention is to provide a moulded filter that is capable of giving 3 -log reduction of cysts .

Yet further object of the present invention is to provide a process for preparing a moulded filter having uniform distribution of filter medium and binder.

Detailed description of the invention

According to an aspect, the present invention relates to a moulded filter for use in gravity water-filters comprising, a) 50 to 95% by weight filter medium having particle size such that not more than 5% by weight of medium passes through a sieve of 150 mesh (lOOμm) and not more than 5% is retained on a sieve of 12 mesh (1680μm) b) 5 to 50% by weight polymeric binder having a Melt Flow Rate less than 5 grams/10 minutes wherein the variation in the amount of binder present in each cm³ cell unit is not more than 3% by weight of the total filter mass in that cell unit.

Thus, in other words, the range of weight percent values of said binder as measured in every unit cell of 1 cm³ of said moulded filter is not more than 3%. Preferably, the range of weight percent values of said binder as measured per unit cell of 0.5 cm³ and more preferably per unit cell of 0.1 cm³ of said moulded filter is not more than 3%.

The filter medium particle size is preferably chosen such that not more than 5% by weight of filter medium passes through a sieve of 75 mesh (195μm) and not more than 5% by weight is retained on a sieve of 30 mesh (595μm) . It is particularly preferred that less than 1% filter medium passes through a sieve of 200 mesh (74μm) .

The filter medium according to the invention is preferably selected from the group consisting of diatomaceous earth, glass bead, polystyrene resin bead, and activated carbon, more preferably the filter medium is activated carbon.

Activated carbon is preferably selected from one or more of bituminous coal, coconut shell, wood and petroleum tar. Surface area of the activated carbon preferably exceeds 500 m²/g, more preferably exceeds 1000 m²/g. Preferably, the activated carbon has a size uniformity co-efficient of less than 2, more preferably less than 1.5. The carbon preferably has a Carbon Tetrachloride (CCl₄) number exceeding 50%, more preferably exceeding 60%. The activated carbon preferably has an Iodine number greater than 800, more preferably greater than 1000. The term polymeric binder according to the invention is a binder having melt flow rate (MFR) less than 5 g/ 10 minutes, preferably less than 2 g/ 10 minutes, more preferably less then 1 g/10 minutes. Bulk density of the binder used as per the invention is preferably less than or equal to 0.6 g/cm³, more preferably less than or equal to 0.5 g/cm³, and most preferably less than or equal to 0.25 g/cm³.

The melt-flow rate (MFR) is measured using ASTM D 1238 (ISO 1133) test. The test measures the flow of a molten polymer through an extrusion plastometer under specific temperature and load conditions. The extrusion plastometer consists of a vertical cylinder with a small die of 2 mm at the bottom and a removable piston at the top. A charge of material is placed in the cylinder and preheated for several minutes. The piston is placed on top of the molten polymer and its weight forces the polymer through the die and on to a collecting plate. The temperature for testing the present polymeric binder material is chosen at at 190⁰C and the load at 15kg. The amount of polymer collected after a specific time interval is weighed and normalized to the number of grams that would have been extruded in 10 minutes: melt flow rate is thus expressed in grams per 10 minutes.

The binder is preferably a thermoplastic polymer having the low MFR values above described. Suitable examples include ultra high molecular weight polymer preferably polyethylene, polypropylene and combinations thereof, which have these low MFR values. The molecular weight is preferably in the range of 10⁶ to 10⁹g/mole. Binders of this class are commercially available under the trade names HOSTALEN from Tycona GMBH, GUR, Sunfine (from Asahi, Japan) , Hizex (from Mitsubishi) and from Brasken Corp (Brazil) . Other suitable binders include LDPE sold as Lupolen (from Basel Polyolefins) and LLDPE from Qunos (Australia) . The weight ratio of filter medium to binder is preferably in the range of 2:1 to 10:1.

The moulded filter of the invention provides for removal of chemical contaminants and more importantly provides for effective removal of at least 3 -log i.e. 99.9% of cysts such as Giardia lamblia, Cryptospordirium parvum and Entamoeba histolica. Thus, if the input water contains 1000 cysts, the output water will contain at the most only 1 cyst.

Contrary to the filter media of the prior art, the moulded filter of the present invention does not require frequent washing and reverse flow flushing at regular intervals to ensure reusability. However, when necessary, reverse flushing can be done under tap water or by reversing the carbon block within the gravity filter device. Back flushing can help in increasing the flow rate.

By way of the above moulded filter of the invention it is possible to attain average flow rates of filtered water in the range of 100-700 ml/min, at an average head of 15 cm under gravity, without compromising on the requirements of removal of particulates including microorganisms and chemical contaminants. An average flow rate in the range of 120 to 300 ml/min is acceptable.

Moulded filters of prior art, in which the final product contains particles of filter medium (primarily activated carbon) interspersed with binder particles suffer from a problem of differential distribution of these two particulate materials. That is, some areas have a high local concentration of carbon particles, while others have a high local concentration of binder particles. This leads to improper binding. As a result, the filter chokes faster. On the other hand, the present invention provides a moulded filter wherein the range of weight percent values of the polymeric binder as measured in every unit cell of one cubic centimetre, preferably 0.5 cubic centimetres and more preferably 0.1 cubic centimetre of said moulded filter is not more than 3% .

The term "range" as used above means "statistical range", which is defined for a set of data as the difference between the highest and lowest values in the set . For the purpose of this invention "range" means the numerical difference between the largest value and the smallest value of the binder content as measured in every unit cell of one cubic centimetre of the moulded filter. The range of 3% can be evenly or unevenly distributed around the average binder content .

For example, if a moulded filter is made such that the filter contains 70% activated carbon and 30% polyethylene binder, then in every cubic centimetre of the moulded filter, the measured binder content could be any value in the range of 28.5% to 31.5% or 29% to 32%.

The binder content can be measured by any known method and is preferably measured by thermo-gravimetric analysis. According to another aspect, the present invention relates to a process for preparing a moulded filter according to the invention. The moulded filters obtained by the process are substantially identical with respect to the distribution of filter medium and binder throughout the filter. This overcomes the problem of prior art processes wherein reproducibility of the process is not high; i.e. the process leads to filters having high variation in the local concentration of filter medium or binder.

The process for making a moulded filter comprises the steps of: a) mixing 1 part by weight polymeric binder having Melt Flow Rate less than 5 g/10 minutes, with 1 to 20 parts by weight filter medium in presence of a fluid wherein the amount of said fluid is not more than 4 times the amount of said filter medium by weight; b) moulding the mix obtained in step (a) ; c) optionally, compressing said mould; d) heating said mould to a temperature in the range of

150⁰C to 350⁰C; e) demoulding the moulded filter.

Preferably one part by weight polymeric binder is mixed with 2 to 10 parts by weight filter medium. The fluid is preferably not more than 3 times, more preferably from 0.5 to 1.5 times the amount of the filter medium.

Mixing is preferably done in vessels which include an agitator, mixer with dulled impeller blades, ribbon blender, rotary mixer, sigma mixer or any other low shear mixer that does not significantly alter the particle size distribution. The mixing is carried out to prepare a uniform mix of the filter medium. It is essential, according to the invention that this step of mixing is carried out in the presence of a fluid. The term "fluid" for the purpose of this invention includes solvents preferably selected from the group consisting of water, ethyl alcohol and mixtures thereof. According to the most preferred aspect, the fluid is water. The fluid used is not more that 4 times the amount of filter medium by weight, and preferably not more than 3 times, most preferably from 0.5 tol .5 times. The binder is then added to the above mixture and is further mixed. Mixing is preferably carried out for at least 15 minutes, more preferably 20 to 60 minutes. The most preferred mixer is a sigma mixer.

The above mixture is optionally vibrated for a short period, e.g. for 3-10 minutes to compact the mixture before moulding. The vibratory compaction is preferably carried out in a vibrator having a frequency in the range of 30-100Hz. This process step is preferably carried out for a period of at least one minute, more preferably for 3 to 10 minutes.

The mass, whether or not compacted by vibration, is then placed in a mould of pre-selected size and shape and subjected to a pressure of not more than 12 Kg/cm², preferably from 3 to 10 Kg/cm² and most preferably from 4 to 8 Kg/cm². The pressure is preferably applied using either a hydraulic press or a pneumatic press, more preferably a hydraulic press.

The mould is made of aluminum, cast iron, steel or any material capable of withstanding temperatures exceeding 400⁰C. A mould release agent is preferably coated on the inside surface of the mould. The mould release agent is preferably selected from silicone oil, aluminum foil, or the mould can be coated with suitable materials like Teflon or any other commercially available mould release agent that has little or no adsorption onto the filter medium.

The mould is then heated to a temperature of 150⁰C to 350⁰C, preferably in the range of 200⁰C to 300⁰C. The mould is kept heated for more than 60 minutes, preferably 90 to 300 minutes. The mould is preferably heated in a non-convection, forced air or forced inert-gas convection oven.

The mould is then cooled and the moulded filter released from the mould.

According to yet another aspect, the invention relates to a water filtration device, which comprises the moulded filter of the invention. The device comprises a sediment filter that is washable and removable and is preferably made of a woven or non-woven fabric, more preferably a non-woven fabric having micro porous structure. This sediment filter ensures filtration and retention of particles generally above 3 microns. The sediment filter can be washed and rinsed under flowing tap water or by using a small amount (0.1-10 g/L) of fabric wash detergent in water. This facilitates wide and extensive application of the moulded filter of the invention.

According to this aspect of the invention the carbon block filter media are adhered to a base plate with an orifice for the water exit and additionally comprises a detachable cover. The base plate can be made of polypropylene, polyethylene, or copolymers like Acrylonitrile-Butadiene- Styrene and Styrene-Acrylonitrile . The detachable cover is preferably made of: polypropylene, polyethylene, or copolymers like Acrylonitrile-Butadiene-Styrene and Styrene- Acrylonitrile .

The moulded filter can be of any desired shape depending on the end application or use. Suitable shapes include flat circular discs of low thickness, square discs of low thickness, low height tapered flat discs, cylindrical, solid conical, hollow hemispherical and hollow conical shapes.

The inclusion of the granular adsorbent particles in the water filter enables filtration of a significantly higher amount of input water over extended time period thereby ensuring more efficient utilization of the carbon block filter media of the invention. Additionally, the granular adsorbent particles in the water filter enables effective filtration of highly contaminated water containing high amounts of fine particles like dust and dissolved impurities like iron and aluminum salts.

Further details of the invention, its objects and advantages are explained hereunder in greater details with reference to the following non-limiting examples. It would be apparent to a person skilled in the art that many such examples are possible and the examples given under are for illustrative purpose only. These should not be construed so as to limit the scope of this invention in any manner. Examples

Example 1

Demonstration of uniform binder distribution:

A moulded carbon filter according to the invention (Example Ia) was prepared by using 10Kg activated carbon, supplied by- Active Carbon (India) and 2Kg ultra high molecular weight polyethylene with MFR - Og/10 minutes, and having bulk density of 0.22 g/cm³ from Asahi Corporation (Japan) . The activated carbon had 5% of the particles passing through 150 mesh. 1OL water was added to carbon particles followed by the binder. The composite was mixed in a sigma mixer for 30 minutes and 23Og was transferred to a mould. The mould was then subjected to a hydraulic pressure of 3 kg/cm² followed by heating to 250⁰C for 150 minutes and cooling for 1 hour.

In the conventional process no water was added. A conventional moulded carbon filter was prepared (Example Ib) according to the process as described without water.

The two filters so prepared were used to compare the binder content, which was measured using a thermo-gravimetric analyzer (TGA) .

A set of 6-7 different cubes of 1 cubic centimeter of the carbon block under example Ia and Ib were cut into 8 equal smaller cubes and transferred to the sample holder of the TGA. Analysis was performed holding the sample at 100⁰C for 10 minutes and then heated to 700⁰C under nitrogen at the rate of 10°C/min. The weight loss was calculated between 400- 600⁰C as under these conditions nearly 100% of the binder gets burnt, whereas less than 2% of the carbon gets affected. Therefore the weight loss determined is equal to the weight of the binder.

Table-1 below gives the range of wt% values of the binder as measured by TGA for various lcm³ cubes of the filters prepared as in Examples Ia and Ib.

Table-1

It was observed that making the moulded filter using conventional process gave a moulded filter with large variation in range of % binder content. It can be readily seen that the range in binder content as measured in the filter made using inventive process was significantly low.

Example 2

Demonstration of the efficacy of the process according to the invention:

A comparison of the efficacy of moulded filters made by conventional process against the process of the present invention is given below, as regards the flow rate using average head height of 15 cm and the cyst removal efficacy. Data has been summarized in table-2 and 3. Different carbon to binder ratios were tested as indicated in the table.

Procedure for determining cyst removal efficiency:

Performance of moulded carbon filters made by conventional process against the process of the present invention for cyst exclusion was evaluated by determining its ability to remove cysts/oocysts from the spiked water passed through it. This was determined by passing test water spiked with targeted level of live/irradiated oocysts through the moulded filters and enumerating them in the output samples. Cyst removal was determined as the difference in number of cysts analysed in the input and output water samples and is expressed as percent reduction or reduction log 10 number. Table-2

Table-3

The data shows that the flow rate of the moulded filter block prepared according to the invention is significantly superior to blocks prepared by the conventional method but there is no significant effect in terms of cyst removal.

The invention thus provides for a carbon block filter media, a process for preparing the same and water filters which can be prepared using such carbon blocks which provide for vastly improved filtration efficiency of water under gravity flow conditions while providing the desired high flow rate.

Claims

1. A process for making a moulded filter for use in gravity water filters comprising the steps of : a) mixing 1 part by weight polymeric binder having a Melt Flow Rate (MFR) less than 5 g/10 minutes, with 1 to 20 parts by weight filter medium in presence of a fluid wherein amount of said fluid is not more than 4 times the amount of said filter medium; b) moulding the mix obtained in step (a) ; c) optionally, compressing said mould; d) heating said mould to a temperature in the range 150⁰C to 350⁰C; e) demoulding the moulded filter, wherein said polymeric binder is ultra high molecular weight polyethylene or ultra high molecular weight polypropylene or a mixture thereof.

2. A process according to claim 1, wherein not more than

5% filter medium passes through a sieve of 150 mesh and not more than 5% is retained on a sieve of 12 mesh.

3. A process according to any one of claims 1 or 2, wherein said filter medium is activated carbon.

4. A process according to any one of claims 1 to 3 , wherein said fluid is water.

5. A process according to any one of claims 1 to 4 , wherein amount of said fluid is not more than 3 times the amount of said filter medium.

6. A process according to claim 5, wherein amount of said fluid is 0.5 to 1.5 times the amount of said filter medium.

7. A process according to any one of claims 1 to 6, wherein the MFR of said binder is less than 2 g/10 minutes .

8. A process according to claim 16, wherein the MFR of said binder is less than 1 g/10 minutes.

9. A process according to any one of claims 1 to 8 , wherein the temperature of the mould is from 200⁰C to 300⁰C.

10. A process according to any one of the preceding claims 1 to 9, wherein the mix is compressed by applying a pressure of not more than 12 Kg/cm² before heating the mould .

11. A process according to claim 20, wherein the pressure is from 3 to 10 Kg/cm²

12. A process according to any one of claims 1 to 11, wherein the binder and the filter medium are mixed for at least 15 minutes.

13. A moulded filter obtainable by the process of any one of claims 1 to 12, comprising: a) 50 to 95% by weight filter medium having particle size such that not more than 5% medium passes through a sieve of 150 mesh and not more than 5% is retained on a sieve of 12 mesh; b) 5 to 50% by weight polymeric binder having Melt Flow Rate less than 5 g/10 minutes; wherein the range in weight percent values of said binder as measured in every unit cell of 1 cm³ of said moulded filter is not more than 3%.

14. A moulded filter according to claim 13, wherein not more than 5% medium passes through a sieve of 75 mesh and not more than 5% is retained on a sieve of 30 mesh.

15. A moulded filter according to any one of claims 13 or

14, wherein less than 1% medium passes through a sieve of 200 mesh.

16. A moulded filter according to any one of claims 13 to

15, wherein said filter medium is activated carbon.

17. A moulded filter according to any one of claims 13 to 16, wherein MFR of said binder is less than 2 g/10 minutes .

18. A moulded filter according to claim 17 wherein MFR of said binder is less than 1 g/10 minutes.

19. A moulded filter according to any one of claims 13 to

18, wherein said polymeric binder is ultra high molecular weight polyethylene or ultra high molecular weight polypropylene or a mixture thereof .

20. A moulded filter according to any one of claims 13 to

19, wherein the range in weight percent values of said binder as measured in every unit cell of lcm³ of said moulded filter is not more than 2%.

21. A moulded filter according to any one of claims 13 to

20, wherein ratio of filter medium to binder is from 2:1 to 10:1 parts by weight.

22. A water filtration device comprising: a) a sediment filter capable of removing suspended particulate material above 3 microns; b) a moulded filter comprising filter medium and binder as claimed in any one of the preceding claims; c) a base plate with an orifice for exit of water, to which the moulded filter is adhered; d) detachable cover to hold the entire filter as one integral unit .