GB2316015A

GB2316015A - Filtering kaolin particles using a polymer pore filter

Info

Publication number: GB2316015A
Application number: GB9616834A
Authority: GB
Inventors: Paul Wroblewski; Frederick Doran
Original assignee: Scapa Group Ltd
Current assignee: Scapa Group Ltd
Priority date: 1996-08-10
Filing date: 1996-08-10
Publication date: 1998-02-18
Anticipated expiration: 2016-08-10
Also published as: GB9616834D0; GB2316015B

Abstract

Kaolin particles are filtered using a polymer pore filter which comprises a fabric substrate coated or impregnated with a coagulated polymer. The fabric substrate may be woven or non-woven and may be formed from polyester, needle felt or 2/8 PET staple warp. The polymer, which may be coagulated by either heating, using a solvent such as DMF or by utilising a suitable electrolyte, may be a polyurethane, polyester, polyether ketone, caprolactam or carbonate. The polymer is preferably foamed by either chemical or physical means, the foaming occurring simultaneously with, or immediately after, coagulation. A method of filter production is also disclosed.

Description

POLYMER PORE STRUCTURE The present invention relates to a polymer pore structure particularly for use in the filtration of particulate matter from aqueous media. The polymer pore'structure of the invention has particular application in the filtration of kaolin.

It is known to filter kaolin particles from aqueous media utilising rotary drum filters.

A conventional rotary drum filter comprises a circular drum split into a plurality of separate axial sections spaced at predetermined intervals around the drum, each of which are connected to separate filtrate pipes. These pipes lead via the centre of the drum to one or both sides of the drum to a rotary vacuum valve and to liquid filtrate vacuum receivers.

Each section or panel is separated by means of caulking a groove or slot which may be of dovetail design. The filter cloth is fitted around the circumference of the drum as a full size cloth and caulked into one or more of the caulking grooves. Alternatively separate pieces of filter cloth may be fitted to cover one or more of the panels and caulked into the grooves and the complete drum covered in this manner.

The revolving drum is partially immersed in a trough filled with a slurry containing the particulate matter and an aqueous liquid. As the drum enters the suspension the liquid is drawn into the compartments by a vacuum and passed through the cloth into the compartment from where the liquid is conveyed by piping to the outside of the filter.

Solids retained by the cloth form a cake which is then washed by spraying with a liquid, such as water. Removal of the filtered solid cake is achieved by scraper blades known as doctor blades or by means of a doctor roller.

The filter cloth may also leave the drum for the discharge to be effected known as "Belt Discharge System" in which case the filter cloth is not caulked into any of the axial grooves.

Traditionally, fine particulate matter filtration and some grades of kaolin filtration have operated at about 2.0 to 6.0% efficiency, resulting in collection of 94 to 98% solid kaolin particles in liquid filtrate. This is considered to be quite poor retention efficiency.

In virtually all filtration applications, for example kaolin filtration, the principle is that the particles of solids are filtered or retained by the filter fabric on the surface and within the structure until a layer or thickness of particles is formed which then acts as the filter media retaining the remaining particles to be filtered during a prescribed cycle.

Particles of kaolin, TiO2, pigment and other fine particle matter, typically of particle size approximately 2 microns, when filtered with conventional fabrics are filtered with a proportion of the particles passing through the fabric and escaping capture.

An object of the present invention is to replace the conventional filter cloth with filter media which retains the initial layer of solids, thus resulting in an increase of retention efficiency.

It has surprisingly been found that use of a particular coated fabric can improve filtration efficiency of kaolin particles in aqueous media by many orders of magnitude. It is therefore an object of the present invention to provide a coated fabric which maximises the efficiency of kaolin filtration.

According to the present invention, there is proposed the use of a polymer pore structure in the filtration of kaolin particles from an aqueous media, the polymer pore structure comprising a textile substrate and a coagulated polymer.

The polymer pore structure of the present invention has surprisingly been found to improve filtration collection to 99.97% of solids, resulting in kaolin filtration being operated at 0.03% efficiency as compared to 2 to 6% efficiency of previous traditional methods.

The pore structure may be applied to one or both sides or the internal structure of all types of textile fabrics, woven and non-woven.

Preferably the polymer coating of the phase separation fabric comprises a polyether based polyurethane coating. The polymer may also be derived from polyester, polyether ketone, caprolactam or carbonate.

The substrate, which acts as a support fabric for the polymer pore structure, is chosen to be compatible with the pore structure and the chemical conditions of the filtration application.

Preferably the substrate is a woven polyester fabric.

The substrate may comprise a 2/8 PET staple warp, i.e., two intertwined yarns containing eight filaments each, woven in a 2/1 twill weave at 23 to 75 ends per cm with 1/4 PET staple weft, i.e., a single yarn containing four filaments at 9 picks per cm. Such a substrate gives a fabric with a unit weight of approximately 440g/m2 The substrate may be composed of a needle felt i.e., batt on base. It may be chosen from woven fabrics, laminates and combinations of woven and non-woven layers.

It is preferable that a suitable substrate be chosen which allows surface filtration as well as depth filtration, wherein the internal media acts as a phase separation region.

With conventional filter cloths the fabric tends to "blind" which is the build up of particulate matter within the structure of the fabric. These particles remain within the fabric when the cake is removed, this results in the capacity of the filter cloth to pass filtrate being reduced and production rates are consequently not maintained.

The polymer pore structure prevents the build up of a particulate matter within the support fabric and consequently production rates have been found to be maintained.

The substrate may be coated on both sides. Preferably, it is coated on one side only with a polymer coating.

The final coating is sufficiently thick to cover at least one side of the substrate support, preferably at a range of thickness of approximately 0.1-1.0, preferably 0.3-0.8, further preferably at a thickness of 0.5mm.

Preferably, the pore size of the phase separation fabric is approximately 5 to 10 microns and further preferably, approximately 5 microns.

Fabric permeability may be up to 12cfm, preferably 0.2 to 3. Scfrn. It is preferable that the surface of the phase separation fabric be very smooth for good filter cake release properties.

Coagulation of the polymer coating may be carried out in DMF using a 10 to 20% solids solution.

The coagulated polymer is preferably foamed, said foaming occurring simultaneously with or immediately after the coagulation. Foaming may be achieved either by physical means or by using a chemical foaming agent. The foaming agent preferably comprises a low boiling water insoluble halogenated hydrocarbon. The halogenated hydrocarbon preferably has a boiling point in the range from 10"C to 50"C and more preferably in the range from 20"C to 30"C. Preferred foaming agents'include 1 ,2-dibromo- 1, 1 ,2,2,-tetrafluoroethane and trichlorofluoroethane.

The foaming and/or the coagulation of the polymer may be achieved by heating the impregnated coated textile substrate, preferably in the presence of a heat coagulant.

Suitable heat coagulants include vinyl alkyl ether and derivatives thereof, polyacetals, polythio ethers, poly(ethylene oxide) and derivatives thereof, and poly(propylene/ethylene oxide) and derivatives thereof. The heat coagulant may be built into the backbone of the polymer. Usually heating to a temperature of about 70"C results solely in coagulation.

Heating above this temperature will generally also result in foaming provided a foaming agent is present.

Coagulation may also be achieved by means of adding a suitable electrolyte and/or varying the pH of the polymer latex. For example, with cationic polymers coagulation may occur at an alkaline pH and for anionic polymers coagulation occurs at an acid pH.

This may be followed by heating to achieve satisfactory foaming.

The coating may be applied by any coating technique such as knife coating, dip coating, lick coating, screen printing or spraying. Reverse roller techniques may be employed

Claims

CLAIMS 1. A method of filtering kaolin particles from an aqueous media, characterised by the use of a polymer pore filter wherein the polymer pore filter comprises a textile substrate coated or impregnated with a coagulated polymer.
2. A method according to claim 1, wherein the textile substrate is woven or nonwoven.
3. A method according to claim 1, wherein the textile substrate is a woven polyester fabric.
4. A method according to claim 1, wherein the textile substrate comprises a 2/8 PET staple warp woven in a 2/1 twill weave at 23 to 75 ends per cm with a 1/4 staple weft.
5. A method according to claim 1, wherein the substrate comprises a fabric with a unit weight of approximately 440g/m2
6. A method according to claim 1, wherein the substrate is comprised of a needle felt.
7. A method according to claim 1, wherein the substrate is one which allows surface filtration as well as depth filtration.
8. A method according to claim 1, wherein the polymer is comprised of a polyether based polyurethane.
9. A method according to claim 1, wherein the polymer is derived from a polyester, polyether ketone, caprolactam or carbonate.
10. A method according to claim 1, wherein the coagulated polymer is applied to one or both sides of the textile and optionally to the internal structure of the textile substrate.
11. A method according to claim 1, wherein the polymer is coated onto at least one side of the textile substrate and optionally to the internal structure of the textile substrate at a thickness of approximately 0.1-1.0 mm.
12. A method according to claim 1, wherein the polymer is coated onto at least one side of the textile substrate and optionally to the internal structure of the textile substrate at a thickness of approximately 0.3-0.8 mm.
13. A method according to claim 1, wherein the polymer is coated onto at least one side ofthe textile substrate and optionally to the internal structure of the textile substrate at a thickness of approximately 0.5 mm.
14. A method according to claim 1, wherein the polymer pore filter has a pore size of approximately 3-6 microns.
15. A method according to claim 1, wherein the polymer pore filter has a pore size of approximately 4 microns.
16. A method according to claim 1, wherein the permeability ofthe polymer pore filter is up to approximately 12 cfm.
17. A method according to claim 1, wherein the permeability ofthe polymer pore filter is approximately 0.2-3.5 cfm.
18. A method of manufacturing a polymer pore filter for filtering kaolin particles from an aqueous medium, wherein the polymer pore filter is manufactured comprising the steps of coating and/or impregnating a textile substrate with a polymer, heating the substrate, and then coagulating the polymer.
19. A method according to claim 18, wherein the heating step takes place in the presence of a heat coagulant.
20. A method according to claim 18, wherein the heating step takes place in the presence of a heat coagulant such as vinyl alkyl ether and derivatives thereof, polyacetals, polythio ethers, poly(ethylene oxide) and derivatives thereof and poly(propylene/ethylene oxide) and derivates thereof.
21. A method according to claim 18, wherein a heat coagulant is present in the backbone of the polymer.
22. A method according to claim 18, wherein coagulation is achieved by means of adding a suitable electrolyte and/or varying the pH of the polymer latex.
23. A method according to claim 18, comprising foaming of the polymer, wherein foaming occurs simultaneously with or immediately after coagulation.
24. A method according to claim 18, comprising foaming of the polymer, wherein foaming is achieved by using a chemical foaming agent, preferably a low boiling, water insoluble, halogenated hydrocarbon.
25. A method according to claim 18, comprising foaming ofthe polymer, wherein foaming is achieved by using a chemical foaming agent, wherein the foaming agent has a boiling point in the range from 10 C to 60"C and preferably in the range from 40 C to 50"C.
26. A method according to claim 18, comprising foaming of the polymer, wherein foaming is achieved by using a chemical foaming agent, wherein the foaming agent is 1,2. dibromo- 1,1, 2,2-tetrafluoroethane or trichlorofluoroethane.