Introduction

This invention relates to a filter, and in particular to a filter for use in waste water treatment, industrial filtration, oil/water separation and the like filtration processes.

Background of the Invention

Various filter materials are currently available such as, for example, fibrous membranes, foam structures and ceramic membranes. Fibrous membranes are mostly polymeric in nature made using electrospinning, Foam structures usually comprise a polymer e.g, cellulose acetate and nylon filter membranes. Ceramic membranes may comprise alumina, or other metal oxides made using sol-gel techniques. Track etched membranes providing a regular porous structure are also known. More recently filters incorporating nano-technology, in particular graphene and graphene oxide have been proposed, see for example CN 106283894 which discloses a graphene oxide modified filter paper, but to date have only had limited success.

Many conventional ultra-filter membranes are relatively expensive to produce and have issues with aging, low flux and thermal stability. They can also be prone to degradation when subjected to different chemical environments, such as acidic or alkaline conditions, or various solvents (chemical instability). For uses like high temperature or corrosive solvent filtration, specialised filters which tend to be relatively expensive are generally required. A common short-coming of conventional filter membranes is fouling due to their filtration mechanism. This mechanism involves filtering through a porous network of sponges, fibres, or regular shaped etched pores. In this process, the particles get stuck in these porous filter networks which clog the filter membrane. Complete recovery for such filter systems is very hard where pollutants get stuck inside the filter membrane structure. This fouling significantly reduces flow rate/flux. With polymeric fitter membranes many of the polymers used are unstable at high temperature, inorganic solvent, and in acidic or basic environments, Where high temperature or organic solvents are used in a filtration system often alumina or other ceramic filter membranes are used, However, these filter membranes are brittle in nature and sealing these membranes is challenging. Also a relatively slight mechanical deformation can shatter these membranes. They are highly unstable in acidic or basic liquids and tend to be very expensive to produce. The use of nano-technologies such as graphene or other nano-particles in filter systems has been shown to be very effective at a conceptual level, but scalability, cost and mechanical robustness are challenges associated with this sort of filtration system.

The present invention is directed towards providing and improved filter which overcomes at least some of the aforementioned problems and provides a relatively cheap filter which is effective and stable for use in filtration of a wide range of liquids.

Summary of the Invention

A filter for filtering liquids, the filter comprising a porous substrate of glass fibre material coated or impregnated with a graphene filter material and with polytetrafluoroethylene particles binding the graphene filter material to the glass fibre substrate to form a filter membrane having a number of tortuous passageways through the filter membrane to allow through passage of liquid whilst retaining unwanted particles mainly on a surface of the filter membrane.

In one embodiment pores in the glass fibre substrate are capped with 2-dimensional graphene nanosheet material.

In another embodiment the graphene nanosheet material comprises exfoliated graphene nanosheets. In another embodiment the polytetrafluoroethylene particles comprise polytetrafluoroethylene nanoparticles.

In another embodiment there is provided means for applying a voltage across the filter, said means allowing adjustment of the voltage,

In another embodiment there is provided means for delivering an electric current through the filter, said means allowing adjustment of the electric current.

In another embodiment the invention provides a method for filtering a liquid using the filter, including the steps of delivering the liquid through the filter and controlling the flow rate of liquid through the filter by adjusting a voltage applied across the filter or varying an electric current delivered through the filter.

In another embodiment there is provided a filter comprising a porous substrate coated or impregnated with a filter material which is supported by the substrate and reduces the porosity of the substrate.

In one embodiment of the invention the substrate is a glass fibre material and the filter material is a nano-material.

In another embodiment of the invention the filter material is graphene.

In another embodiment of the invention the filter material comprises one or more of graphene, clay/ceramic, metal and polymer material.

In one embodiment of the invention the substrate comprises one or more of glass fibre, metal, polymer and ceramic material.

In another embodiment of the invention the filter further includes a binder material.

In one embodiment of the invention the binder material is a polymeric binder.

In another embodiment of the invention the binder material is polytetrafluoroethylene nano-particles.

Brief Description of the Drawings

The invention will be more clearly understood by the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings, in which:

Fig. 1 is a schematic illustration of the manufacture of a filter according to the invention;

Fig. 2 is a schematic cross-section view of the filter of the invention; and Fig. 3 is a scanning electron microscope image showing a cross section of the filter of the invention.

Referring to the drawings, there is illustrated a filter according to the invention indicated generally by the reference numeral 1. The filter 1 comprises a porous substrate formed by a glass fibre membrane 2 impregnated with graphene and with polytetrafluoroethylene (PTFE) particles 3 binding graphene particles and glass fibre. Thus, a filter membrane 2 of a desired porosity is formed with a number of tortuous passageways 4 formed through the filter membrane 2 to allow through-passage of liquid, as indicated by arrow 5, whilst retaining unwanted pollutant particles 6 mainly on the surface of the filter 1 ,

The manufacturing process is also schematically illustrated in Fig, 1. A glass fibre membrane 10 is soaked or sprayed in graphene 11 and PTFE 12 of various weight percentages. This results in a modified glass fibre membrane 14 impregnated with the graphene 11 and PTFE 12 wherein the pores in the glass fibre membrane 10 will be capped with nano/2D layered material and the PTFE will act as a binder. The roughness of the glass fibre surface and spacing between layers 16 of 2D material creates enough space or passageways 4 so that solvent molecules can pass through whilst large particles or pollutants 6 are retained at the surface of the graphene layer. As there are no conventional pores, very low clogging occurs coupled with a high efficiency, A relatively high flow rate and low fouling can be achieved for particular separation/retention efficiency compared with conventional commercial membranes.

It will be appreciated that commercially available glass fibre is a low cost material and has very high flux/flow and high thermal and chemical stability, but a relatively low separation efficiency. Graphene has very high thermal, chemical and mechanical stability and has a very effective separation efficiency. However, scalability and making free-standing filter membranes of graphene is a huge challenge. PTFE dispersion is low cost and commercially available and has high thermal and good mechanical stability. Combining glass fibre, graphene and PTFE together in the filter membrane of the invention as described herein results in a nano-strata ultra-filtration system with high flux/flow rate, low fouling, high chemical and thermal stability and good mechanical robustness for a particular separation efficiency. Commercial glass fibre membranes are formed from mats of glass fibres each fibre having diameter of a few microns and a length of many hundreds of microns. They have porosities of >95% (i.e. densities of ~200 kg/m ³ ), typical pore sizes of tens of micron and are very cheap to make (see Fig 1A), However, because of the large pore sizes, although such filters do show high flow rates, they only block iarge particles. The filter 1 of the invention reduces the particle size transmitted through the membrane to the filtrate by using exfoliated graphene nanosheets to reduce the effective pore size of the glass fibre membrane 10. Soaking the glass fibre membrane 10 in a dispersion of graphene nanosheets leads to the infusion of nanosheets into the interior of the glass fibre membrane resulting in the formation of a nanosheet network on the glass fibre membrane 10 with small pore size. Importantly, such a nanosheet network is mechanically supported by the glass fibre membrane 10. To achieve this, we prepare few layer graphene nanosheets by liquid phase exfoliation, which is a well-known graphene production technique. However, this graphene network is mechanically very unstable, and any slight agitation will result in flaking of graphene of the membrane. To address this problem, we use a novel solution to insert a polymeric binder into the glass fibre membrane, either with or without the presence of graphene. Instead of using a soluble polymer as a binder we use commercially available polytetrafluoroethylene (FIFE) nanoparticles (PTFE- NPs).

The filter 1 of the invention has a unique nano-strata filtration mechanism. When contaminated water passes through the graphene modified membranes, most of the pollutant particles 6 will be retained on graphene surface (unlike conventional membranes where they would be trapped inside the membranes) and solvent 5 will pass through interlayer graphene channels/gaps 4. As there are no conventional pores, very low clogging occurs and the flow rate is not greatly compromised. Therefore, high flow rate and low fouling will be achieved for a separation/retention efficiency compared to conventional commercial membranes.

By varying hydrophobicity/hydrophilicity these membranes can be used to separate oil/water and emulsion filtration.

Because of the conducting nature of these membranes flow rate can be controlled by passing/varying electric voltage/current through these membranes. • The flakes or filter material can be graphene, clay/ceramic, metal or polymer,

• Supporting materials will not be limited to glass fibre only but can be metal, polymer, or ceramic. · Also the end product (membranes) can be made up of either one of these materials or combinations of two or more materials.

In the specification the terms “comprise, comprises, comprised and comprising" or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope of the appended claims.