SE542828C2

SE542828C2 - Filter element comprising open voids, material therefor and method for filtering

Info

Publication number: SE542828C2
Application number: SE1850653A
Authority: SE
Inventors: Kjell Svedman
Original assignee: Conpore Tech Ab
Priority date: 2018-05-31
Filing date: 2018-05-31
Publication date: 2020-07-14
Also published as: SE1850653A1; EP3801818A1; WO2019229149A1

Abstract

The present invention relates to a filter element comprising a pervious matrix comprising voids, wherein the voids constitute at least 50 percent by volume of the filter element, the matrix surrounding the voids comprises substantially sharp edges, the filter element comprises an input surface and an output surface, and a distance between the input surface and the output surface is at least 5 cm. The invention further relates to a material comprising a pervious matrix comprising voids, wherein the voids constitute at least 50 percent by volume of the material and the matrix surrounding the voids comprises substantially sharp edges, a method for producing such a material, use of such a material as a filter element, and a method for filtering particles.

Description

FILTER ELEMENT COMPRISING OPEN VOIDS. MATERIAL THEREFOR AND METHOD FOR FILTERING FIELD OF THE INVENTION The present invention relates to the field of filtering particles, and particularly to filter elements, materials for filter elements, and methods for filtering particles.

BACKGROUND OF THE INVENTION Air contamination is a global concern and the effects of the emission of harmful particles are known to be negative for humans as well as for animals, plants and nature as a whole. Significant amounts of harmful particles are released in the air with the flue gas when fuels are burnt, and contribute to the formation of smog. Exposure to smog may lead to severe health problems for humans, particularly pulmonary diseases, such as lung cancer. Smog is also responsible for decreasing radiation from the sun resulting in a low production of e.g. vitamin D, for inhibiting plant growth and for acidification of soil and water.

Reduction of harmful emissions from the industry is thus desirable, and different emission reduction technologies for industrial processes have evolved. Some drawbacks of the currently existing technologies are, however, that these often require large investment costs, and that at least parts of the emission reducing process, when for example flue gas is to be cooled or heated, is highly energy consuming.

SUMMARY OF THE INVENTION An object of the present invention is to overcome, or at least lessen the above mentioned problems. A particular object is to provide a filter element which can filter harmful particles from fluids, such as flue gas or water, in an efficient manner. A further object of the invention is to provide a filter element which can be adapted in size in accordance with any site specific requirements.

A further object of the invention is to provide a method for filtering particles in an efficient manner. A further object of the invention is to provide a method for recycling filtering elements which have been used for filtering particles.

To better address this concern, in a first aspect of the invention there is presented a filter element for filtering particles comprising a pervious matrix comprising open voids, wherein the voids constitute at least 50 percent by volume of the filter element. The matrix surrounding said voids comprises substantially sharp edges. The filter element further comprises an input surface and an output surface, and a distance between the input surface and the output surface is at least 5 cm.

The structure of the filter element, comprising a pervious matrix and a large number of open voids allows for e.g. flue gas to pass through from an input surface to an output surface. As flue gas passes through the filter element, the substantially sharp edges of the matrix surrounding the voids act as breaking points for the gas flow, reducing the speed and energy of the same. Particles carried by the gas flow can at such a point be released and instead of continuing through the filter element with the gas flow, be retained in the pervious matrix. A sharp edge should in this context be interpreted as an edge capable of significantly reducing the speed of an airflow passing by, as opposed to a blunt edge, which provides such an effect to a considerably lower degree. According to an embodiment, an angle of the sharp edges is comprised in the range of 0-90°.

A distance of at least 5 cm between the input surface and the output surface of the filter element provides a structurally stable filter element while also providing a travelling distance for the flue gas which is sufficient for retention of at least a part of the particles present therein.

Particle retention in the filter element is based on the principle that the flow in which the particle travels is disturbed by the sharp edges leading to a loss of energy holding the particles to the flow. Particles can thereby fall out of the flow and be retained in the filter element. Based on this principle, particles of different kinds and sizes may be retained in the filter element.

The pervious matrix may be constituted by a single solid material, such as a concrete material. The pervious matrix may also, according to other embodiments, be constituted by particles such as crushed rocks or glass contained within the filter element and in between which open voids are present. Sharp edges of such crushed particles will thus reduce the speed of a flow passing through such that particles contained therein is released from the flow and retained in the filter element In some examples, the voids may constitute at least 55 percent by volume of the filter element, such as at least 60 percent by volume of the filter element, preferably at least 65 percent by volume of the filter element, more preferably at least 70 percent by volume of the filter element, most preferably at least 80 percent by volume of the filter element.

According to an embodiment of the filter element, the open voids comprise a substructure comprising larger primary voids constituted by air pores, and smaller secondary voids constituted by holes in the air pore walls, such that the air pores are not closed but open. This substructure provides a large amount of sharp edges between primary and secondary voids, which in turn provides for a large amount of breaking points at which the particles in the gas flow can be retained. Additionally, airflow entering a pore is generally prone to enter in a rotational movement within the pore. Heavy particles present in the airflow may then be released therefrom as it travels along the wall of the pore and suddenly, when reaching a secondary void, loses support from the wall and falls through the secondary void.

According to an embodiment of the filter element, the filter element is substantially equally pervious in all directions. This provides a well distributed flow through the filter element, allowing a high flow there through.

According to an embodiment of the filter element, the size of the voids is at least 0.1 mm. The size of the void may be a diameter of a substantially circular void. It may also be the distance between two surfaces of the matrix defining a void there between, having an elliptical, rectangular or other geometrical shape. The voids present in the filter element are generally of different size. In an embodiment comprising primary and secondary voids, the size of the secondary voids is at least 0.1 mm whereas the size of the primary voids is larger. Typically, the size range of the primary voids is from 1 to 5 mm. Sizes outside of this range are however also conceivable within the inventive concept.

According to an embodiment of the filter element, the pervious matrix comprises a concrete material. A concrete material with a structure comprising open voids constituting at least 50 percent by volume of the filter element is advantageous for several reasons. One advantage is, considering the large surface which is in contact with the flue gas, that these can react with the carbon dioxide in the flue gas in a carbonation process and thereby reduce carbon dioxide emissions. A further advantage with concrete material is that it is relatively economic and that it is moldable to different dimensions based on site specific requirements. The open voids of the concrete material may also be referred to as pores. And a substructure of primary and secondary voids may correspondingly be referred to as primary and secondary pores. Formation of closed pores in concrete material is often sought after in order to reduce the weight of the material. The formation of open pores in concrete material, constituted by primary pores with secondary pores in the pore walls, according to the invention, has proven to provide surprisingly advantageous filter properties to the filter element. The density of the filter element can be comprised in the range of 300-1200 kg/m<3>, depending on the media and with what pressure to filter.

According to an embodiment, the concrete material is made from a cured concrete mixture comprising an air pore forming agent and optionally a hydrophobic agent. It has surprisingly been found that a pervious concrete matrix comprising open voids constituting at least 50 percent by volume of the filter element can be obtained from a cured concrete mixture comprising cement, water, an air pore forming agent and a hydrophobic agent.

The air pore forming agent may be a tenside of a non-ionic type. The tenside may have a viscosity (Brookfield, 25 degrees C, cps) in the range of 60-90. Examples of such a non-ionic tenside are ethylene oxide adducts, such as a non-ionic tenside obtained by adding ethylene oxide to linear secondary alcohols having alkyl carbon atoms ranging between 10 and 14.

Such non-ionic tensides may be known under the commercial names Softanol<® >(80, 90, 100 or 120) or Surfonyl<® >(500 or 600). Tensides with similar molecular chains and structure are also possible within the inventive concept. The air pore forming agent may, in other embodiments, be a melamine resin or a protein based air pore forming agent made up of protein and synthetic additives. The air pore forming agent may also be an ionic tenside, such as a cationic tenside.

The hydrophobic agent may be a synthetic or natural resin, or derivatives thereof, having molecular weights of usually below 10,000 and a saponification number of 100-250. Such a resin may be at least partially soluble in the concrete mixture, preferably substantially fully soluble in the concrete mixture. The resin may have a viscosity (Brookfield, 25 degrees C, 50 rpm, cps) in the range of 400-1200, such as 600. The resin may be added as a dispersion having a pH in the range of 8.0-9.5, such as 8.5. The resins and their derivatives may comprise one or more aromatic and/or aliphatic groups having at least 10, preferably 16-35 carbon atoms. The groups may be saturated or unsaturated. Preferred resins are such having an acid number from 10-25, such as 20 and a saponification number from 150 to 175.

Examples of suitable resins are various resin acids and mixtures thereof, such as colophonium, and their dimerised derivatives and wholly or partly esterified and/or hydrated derivatives thereof. The resin may be a tall oil rosin. The resin may be a polymer.

In some embodiments the hydrophobic agent comprises dispersible particles of the resin having a mean particle size in the range of 0.4-0.6 ?m, such as about 0.5 ?m. The resin may furthermore have a mean particle size of less than 0.6 ?m. The mean particle size may also be in the range of 0.1-0.6 ?m, such as 0.2-0.6 ?m, preferably 0.3-0.6 ?m. The particle size may also be in the range of 0.5-0.7 ?m, such as 0.5-0.6 ?m, such as 0.6-0.7 ?m. The particle size may also be in the range of 0.2-0.5 ?m. It has been found that the mean particle size of the resin particles affects the solubility of the resin particles in the concrete mixture. A particle size in the range of 0.4-0.6 ?m is believed to further increase the solubility of the resin in the concrete mixture.

A particle size in the range of 0.4-0.6 ?m, such as about 0.5 ?m gives rise to several advantages, some of which will be discussed herein. One advantage is that it gives rise to an at least partial dissolution of resin in the concrete mixture that further improves the homogenous distribution of resin within the concrete mixture. Such a homogenous distribution allows substantially all of the added resin to form part in the adhesion between the aggregate materials and the concrete mixture.

According to an embodiment of the filter element, the filter element comprising a concrete material is recyclable. This means that the filter element can be crushed and the resulting material reused in the production of concrete. The high number of voids in the material allows for a significant reduction of the volume of the filter element when it is crushed. This is particularly advantageous when considering recycling large filter elements, which can be crushed prior to transportation to a plant for recycling, thereby reducing significantly the load to transport. According to an embodiment, the volume of the filter element is reducible by at least 40 percent. The volume may typically be reducible by 60-80 percent.

In accordance with an aspect of the invention, there is provided a filter comprising a plurality of filter elements as herein disclosed. The filter elements can be arranged consecutively, such that a flow exiting the first filter element directly enters the second filter element, the flow exiting the second filter element directly enters the third filter element and so on. The filter elements can be arranged with a space between each filter element. This allows for a flexible filter system in which the filter elements may be replaced individually when needed, due to for example saturation of a filter element.

In accordance with a second aspect of the invention, there is provided a concrete material comprising a pervious matrix comprising open voids, wherein said voids constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said voids comprises substantially sharp edges.

The inventor has surprisingly found that a concrete material as disclosed in relation to the second aspect of the present invention can advantageously be used as a particle filter. The voids allow the concrete material to be pervious to various types of matter, such as liquid matter and gaseous matter. The open voids of the concrete material may also be referred to as pores. The concrete may further comprise a substructure of primary and secondary voids referred to as primary and secondary pores. Formation of closed pores in concrete material is often sought after in order to reduce the weight of the material. However, the formation of open pores in concrete material, constituted by primary pores with secondary pores in the pore walls, according to the invention, has proven to provide surprisingly advantageous filter properties to the filter element.

The concrete material of the present disclosure may be provided by curing a concrete mixture comprising cement, water, an air pore forming agent and optionally other additives such as a hydrophobic agent and/or an aggregate material.

Cement is as hydraulic binder, which by the addition of water forms a paste and cure by hydration. The curing is mainly dependent on the formation of calcium silicate hydrate. The most important silicate-cement-containing composition is Portland cement clinker. The cement of the present disclosure is preferably Portland cement, owing to its good all-round properties. Portland cement comprises tricalcium silicate, tricalcium aluminate and calcium aluminium ferrite. Other examples of suitable types of cement are Portland blast-furnace cement, white Portland cement, low-heat Portland cement and rapid-hardening Portland cement, which are all based on Portland cement clinker. The cement of the present disclosure may also be aluminate cement. Aluminate cement may typically be used in applications which require fireproof materials. The cement of the present disclosure may furthermore be a mixture of Portland cement and aluminate cement. The addition of aluminate cement to the Portland cement may shorten the time required to cure the concrete mixture, and it may also provide a fire-proof concrete.

Suitable air pore forming agents and hydrophobic agents are discussed in relation to the first aspect of the present disclosure.

The concrete mixture may be a pumpable concrete mixture. The term “pumpable” as referred to in the present disclosure means that the concrete mixture is stable enough to withstand the pressures associated with pumping. Furthermore, the concrete mixture has a sufficiently low viscosity to allow pumping. Due to the air pores in the concrete mixture the concrete mixture may be subject to compression during for example pumping.

According to an embodiment, an angle of the sharp edges is comprised in the range of 0-90 degrees, such as in the range of 0-60 degrees. This is advantageous in that the sharp edges are believed to allow the material to filter flue gas from particles efficiently According to an embodiment of the concrete material, the volume is reducible by at least 40 percent. Due to the large amount of voids in the concrete material, the volume may be reduced significantly by crushing the concrete material. This is advantageous in that the material is easier to transport in its crushed state.

According to an embodiment, of the concrete material is made from a cured concrete mixture comprising an air pore forming agent and a hydrophobic agent. The air pore forming agent may be an ionic tenside or a non-ionic tenside. Suitable air pore forming agents and hydrophobic agents are discussed in relation to the first aspect of the present disclosure.

In some examples, the concrete mixture may further comprise additional additives. Such additives may be, but are not limited to, solubilising compounds, such as ethylene glycol and its mono- or dimethyl or ethylethers having a molecular weight of up to 300; and water-retaining and plasticityincreasing additives (plasticisers), such as non-ionic cellulose ethers and polyalkylene glycols having molecular weights above 400.

In accordance with a third aspect of the invention, there is provided a method for producing a concrete material comprising a pervious matrix comprising open pores, wherein said pores constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said pores comprises substantially sharp edges. The method comprises the steps of providing water, an air pore forming agent, and a hydrophobic agent to form a first mixture in a concrete mixer; provide 20-40 percent by weight of a total amount of cement to said first mixture to form a second mixture; agitating said second mixture at a first rotation rate to form a foam; agitating said second mixture at a second rotation rate and adding the remaining 60-80 percent by weight of the total amount of cement to form a concrete mixture; and curing said concrete mixture.

The method is advantageous for several reasons. Firstly, it provides a concrete material comprising at least 50 percent by volume of voids in the concrete material. Secondly, it allows for the creation of sharp edges in the concrete matrix surrounding the voids. Such features allow the concrete material to be configured for use in various filtering applications. Using an air pore forming agent in combination with a two-step agitation treatment allows for the formation of open pores in the concrete matrix, and for the formation of substantially sharp edges surrounding the pores. It furthermore reduces the need for large electric motors to run the concrete mixer. As discussed in relation to other aspects of the present disclosure, such sharp edges are advantageous in that they improve the concrete materials filtering properties.

The first rotational rate may be in the range of 3-9 m/s in terms of circumferential velocity. The second rotational rate may be in the range of 0.7-3 m/s in terms of circumferential velocity.

The air pore forming agent and/or hydrophobic agent may be the air pore forming agent and hydrophobic agent discussed in relation to other aspects of the present invention.

In some examples, the method may further comprise adding other additives to the mixture. Such additives may for example be added with the air pore forming agent and hydrophobic agent. The additives may be but are not limited to, solubilising compounds, such as ethylene glycol and its monoor dimethyl or ethylethers having a molecular weight of up to 300; and waterretaining and plasticity-increasing additives (plasticisers), such as non-ionic cellulose ethers and polyalkylene glycols having molecular weights above 400.

In some examples, the method further comprises providing an aggregate material to the mixture. Aggregate materials are known to a person skilled in the art.

A further method that is suitable for producing a concrete material comprising a pervious matrix comprising open pores, comprises the steps of producing a light weight concrete mixture comprising closed pores, stirring the concrete mixture during the initial state of the hardening phase, and curing the concrete mixture. This process may also produce sharp edges in the concrete matrix surrounding the pores.

In accordance with a fourth aspect of the invention, there is provided a use of a concrete material as a filter element for filtering particles comprising a pervious matrix comprising open voids, wherein said voids constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said voids comprises substantially sharp edges. Such a concrete material can be used as a filter element for filtering particles present in a liquid, such as water, or in a gas, such as flue gas.

In accordance with a fifth aspect of the invention, there is provided a method for filtering flue gas, the method comprising the steps of providing a filter element as disclosed herein; bringing flue gas to said filter element and allowing the flue gas to pass there through, whereby at least a portion of particles present in the introduced flue gas is retained in the filter element.

When filtering flue gas according to the method herein described, the filter element will gradually be filled with particles from the flue gas retained therein. When the filter element is saturated such that it no longer retains particles, it may be replaced by a new filter element. A saturated filter element comprising a concrete material may be recycled, i.e. crushed and reused for concrete production, providing a sustainable method for filtering. The method is also applicable to filtering liquids, such as water, which may contain particles.

According to an embodiment, the method for filtering flue gas comprises the steps of providing a plurality of filter elements as disclosed herein arranged consecutively; bringing flue gas to a first filter element of the plurality of filter elements and allowing the flue gas to pass there through, whereby at least a portion of particles present in the introduced flue gas is retained in said first filter element; bringing gas exiting the first filter element to a second filter element of the plurality of filter elements and allowing the gas to pass there through, whereby at least a portion of particles present in the gas introduced in the second filter element is retained therein; and repeating the previous step for each filter element such that the flue gas passes through all consecutively arranged filter elements, wherein each of the plurality of filter elements is replaceable independently of each other. This method provides the possibility of exchanging filter elements as they become saturated with particles from the flue gas. The first filter element through which the flue gas passes will generally retain more particles than the subsequently arranged filter elements. Therefore, it is particularly advantageous to replace the first filter element with, for example, a subsequently arranged filter element which is not saturated. Each filter element may thereby be used to its full potential.

A filter element comprising a concrete material may, upon saturation of particles, be recycled and used in the production of concrete. That is, it may be crushed into particulate matter which in turn may be used in the production of concrete mixtures. The particles retained in the filter element may, in the recycling process, constitute adhesive agents and/or filler agents for the concrete mixture.

According to an embodiment, the method for filtering flue gas further comprises the step of providing a water flow through one or more of the plurality of filter elements. Such a step would provide a rinsing of the flue gas.

According to an embodiment, the method further comprises the step of providing a humid atmosphere within one or more of the plurality of filter elements. The humid atmosphere may allow particles in the flue gas to adsorb to the walls of the matrix.

According to an embodiment, the method further comprises the step of adding a reactive gas or fluid to one or more of the plurality of filter elements. When the flue gas passes through a filter element to which reactive gas has been added, the reactive gas is blended with the flue gas and allowed to react with elements therein. This can lead to efficient removal of harmful emissions by means of the resulting chemical reactions.

According to an embodiment, the method further comprises the step of rendering one or more of the plurality of filter elements electrically conductive. This could be achieved by inserting electrodes in the filter element.

Electrically conductive filter elements may be particularly advantageous for retaining heavy metal particles.

In a further aspect of the invention, there is provided a method for recycling a filter element as disclosed herein, comprising the steps of providing one or more filter elements used for filtering flue gas; crushing said one or more filter elements, whereby the volume of each filter element is reduced by at least 40 percent; and using the crushed one or more filter elements in the production of concrete.

According to an embodiment of the method for recycling, the particles retained in the one or more filter elements constitute adhesive agents and/or filler agents in the production of concrete.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail and with reference to the appended drawings in which: Figs. 1 a-c show structures of an embodiment of a filter element according to an aspect of the invention; Fig. 2 shows a schematic illustration of a filter according to an aspect of the invention; Figs. 3a-d show micrographs at different magnifications of a filter paper through which flue gas has been allowed to pass; Figs. 4a-d show micrographs at different magnifications of a filter paper through which flue gas has been allowed to pass, wherein said flue gas has passed a concrete filter element according to the present disclosure before reaching said filter paper; and Fig. 5 shows the discoloration of a filter paper exhibited to 30 seconds of flue gas as compared to the discoloration of a filter paper exhibited to 90 seconds of flue gas having previously passed a filter element according to the present disclosure.

DESCRIPTION OF EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Referring to Figs. 1a-c, the structure 1 of an exemplary embodiment of a concrete material for a filter element is shown. The structure 1 comprises a pervious matrix 2 and open voids 3. In this exemplifying embodiment, the pervious matrix comprises a concrete material. The open voids 3 comprises a substructure comprising primary voids 4 and secondary voids 5, see Fig. 1b. The primary voids 4 are constituted by air pores whereas the secondary voids 5 constitute holes in the air pore walls, such that the air pores 4 are open. The secondary voids 5 are thus generally smaller than the primary voids 4. The size of the secondary voids 5 varies and is generally at least 0.1 mm. In Fig. 1c, a wire 10 of a diameter of 0.25 mm is arranged at the surface of the structure 1. The primary voids 4 also vary in size, and is generally comprised between 0.5 and 5 mm. In Fig. 1a, the length of the black line 7 drawn up in the upper left corner is 1.5 mm. The primary voids 4 shown in this embodiment thus has an estimated size of 1-2 mm in diameter. The edges of the open voids 3 are substantially sharp.

The structure 1 is obtainable from a concrete mixture comprising water, cement, an air pore forming agent and a hydrophobic agent. An exemplifying embodiment of the production of the structure 1 is disclosed in the following examples.

Referring now to Fig. 2, a filter 20 comprising a plurality of filter elements 22 is shown. The number of filter elements 22 in the filter 20 can be adapted to the system into which the filter is to be installed. A skilled person thus understands that a filter may comprise one sole filter element 22, or a plurality of filter elements, such as 3-8 or more filter elements 22, according to the inventive concept. The filter elements may be positioned in horizontal rows. The filter elements may also be stacked on top of each other.

Each filter element 22 comprises an input surface 23 and an output surface 24. A distance between the input surface 23 and the output surface 24 is at least 5 cm. That distance, also referred to as the width of the filter element 22, as well as the length and height of the filter element 22 is adaptable to the specific requirements on the site into which the filter is to be installed. As an example only, the width, height and length of a filter element 22 may be 1, 2, and 3 m, respectively. Other dimensions of the filter element 22 are also conceivable within the inventive concept. The filter elements 22 are arranged consecutively such that an output surface 24 of a first filter element 22 is adjacent an input surface 23 of a second filter element 22. In the exemplifying embodiment of Fig. 2, the plurality of filter elements 22 are arranged with a space between each filter element 22, i.e. with a space between an output surface 24 of a first filter element and an input surface 23 of a second filter element. The filter elements 22 are further arranged within a frame comprising airtight or water proof walls, depending on the media to filter, such that the flow is forced to pass through the filter elements 22.

In the exemplifying embodiment of Fig. 2, the filter elements 22 comprises a concrete material having a structure 1 as described herein, examples of which is shown in Figs. 1a-c. Due to the voids of the concrete material, each filter element 22 can be reduced by at least 40 percent when crushed.

To filter a flow containing particles, such as a flow of flue gas, a filter 20 comprising a plurality of filter elements 22 is provided at an outlet of the flow such that the flue gas is brought to a first filter element 22 of the filter 20. The first filter element 22 is thus that arranged closest to the outlet of the flue gas. The second filter element 22 is that arranged adjacent to the first filter element 22, and so on. The flue gas is allowed to pass through the voids of the first filter element 22, whereby the sharp edges of the voids 3 reduces the flow rate of the flue gas and the energy holding the particles in the flow, such that at least a portion of particles present in the introduced flue gas is retained in the filter element 22. The flue gas exiting the first filter element 22 at its output surface 24 is then brought to the adjacent second filter element 22 through the input surface 23 of the second filter element 22, and allowed to pass there through. In the second filter element 22, the flow is further reduced by the sharp edges of the voids 3, whereby particles are released from the flow and retained in the second filter element 22. The procedure continues until the flow has passed through all consecutively arranged filter elements 22 of the filter 20. The gas exiting the output surface 24 of the last, in this exemplifying embodiment the fourth, filter element 22 may be free from any harmful particles and can thus be released in the air. The degree of filtering is dependent on the type of particles contained in the flow to filter. In order to achieve a particle free gas at the outlet surface of the last filter element 22, the dimensions of the filter element 22, as well as the number of filter elements 22 contained in the filter 20 is adjusted. Generally, a filter 20 comprising several filter elements 22 can retain more particles than a filter 20 comprising one filter element 22 of the same size as the several filter elements 22. This is due to the distance the flow needs to travel in order to pass through the filter 20. The larger the distance traveled, the higher number of sharp edges will interfere with the flow and release particles therefrom. Similarly, a filter element 22 of a larger width, or distance between the input and output surfaces 23, 24 can retain more particles than a filter element 22 of a smaller width. Due to the relative isotropy of voids 3 in the structure 1, the dimensions of the height and length of the filter element 22 also influences on the particle retention capacity of the filter element 22.

A filter element 22 or a filter 20 comprising a plurality of filter elements 22 can also be adapted to remove certain particles from a flow. For example, heavy metal particles can be efficiently removed by means rendering the filter element electrically conductive.

Each filter element 22 is independently replaceable in the filter 20. Thus, when a filter element 22 is saturated with particles it can be removed and replaced by a new filter element 22. Generally, the first filter element 22 of a plurality of filter elements becomes saturated first. It may then be replaced by one of the other filter elements 22 of the filter 20.

In the exemplifying embodiment, a saturated filter element comprising a concrete material can be crushed, whereby the volume is reduced by at least 40 percent. It may then be transported for reutilization in the production of concrete.

Examples Example 1. Production of a concrete material for filtering applications For producing a concrete material suitable for use in filtering applications, the following step-wise procedure took place. Into a laboratory mixer having a volume of 40 liters, 8.1 kg of water, 144 g of a non-ionic tenside sold under the trade name Sulfonyl 120 and 12 g of a hydrophobic resin sold under the trade name Aquatac XR4343 was added and agitated slowly for approximately 1-10 seconds to form a mixture. 4.1 kg of Portland cement was supplied to the mixture and agitated for approximately 30-40 seconds at a high rotational rate of approximately 330 rpm (circumferential velocity of approximately 6-7 m/s) which resulted in the formation of foam. Then 7.6 kg of Portland cement was added continously to the mixer during approximately 30-12 seconds during agitation at a lower rotational rate of approximately 170 rpm (circumferential velocity of approximately 3-4 m/s). The agitation continued for further 120-240 seconds until a concrete mixture was obtained. The concrete mixture was casted in a mold to achieve a shape which is suitable for filter applications, and allowed to cure. The material is shown in Figs. 1a-c.

Example 2. Filter test The concrete material was used as filter in a filter test. A laboratory grade filter paper (F1) was positioned onto the inlet of an industrial vacuum cleaner nozzle. The vacuum cleaner was then used to suck flue gas from a flue gas source. After 30 seconds of suction, the vacuum cleaner was turned off and the filter paper was removed. The filter paper was then photographed and investigated using a light optical microscope. Figs. 3a-d show micrographs of the filter at 100x, 200x, 500x and 1000x magnification, respectively. 30 seconds was enough to substantially discolor the fibers of the filter paper.

A similar test was performed with the difference that a filter paper (E1) was positioned onto the inlet of an industrial vacuum cleaner nozzle downstream of a concrete filter element prepared according to Example 1 above. The vacuum cleaner was used to suck flue gas from a flue gas source through the concrete filter element. After 90 seconds of suction, the vacuum cleaner was turned off and the filter paper was then removed. The filter paper was then photographed and investigated using a light optical microscope. Figs. 4a-d show micrographs of the filter at 100x, 200x, 500x and 1000x magnification, respectively. 90 seconds was required to discolor the fibers of the filter paper as compared to only 30 seconds when no concrete filter element was used.

From an ocular inspection of the filter papers shown in Figs. 3a-d and the filters shown in Figs. 4a-d it is clear that a substantial amount of particles present in the flue gas can be retained by the concrete filter element as prepared according to Example 1.

Figure 5 shows the filter papers F1 (top) after 30 seconds of suction without the filter and E1 (below) after 90 seconds of suction with the filter, respectively. Figure 5 clearly shows that the filter paper E1 is substantially less discolored than the filter paper F1, although the filter paper E1 was used for a longer period of time. It is thus clear that the concrete filter element is capable of retaining particles present in flue gas when flue gas is allowed to pass the concrete filter element.

The skilled person realizes that a number of modifications of the embodiments described herein are possible without departing from the scope of the invention, which is defined in the appended claims. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A filter element for filtering flue gas comprising a pervious matrix comprising open voids, characterized in that: said voids constitute at least 50 percent by volume of the filter element; the matrix surrounding said voids comprises substantially sharp edges; and said filter element comprises an input surface and an output surface, and a distance between said input surface and said output surface is at least 5 cm.

2. A filter element according to claim 1, wherein an angle of said sharp edges is comprised in the range of 0-90 degrees.

3. A filter element according to claim 1 or 2, wherein the open voids comprises a substructure comprising larger primary voids constituted by air bubbles, and smaller secondary voids constituted by holes in the air bubble walls, such that the air bubbles are not closed.

4. A filter element according to any one of the preceding claims, wherein said filter element is substantially equally pervious in all directions.

5. A filter element according to any one of the preceding claims, wherein the size of said voids is at least 0.1 mm.

6. A filter element according to any one of the preceding claims, wherein said pervious matrix comprises a concrete material.

7. A filter element according to claim 6, wherein said concrete material is made from a cured concrete mixture comprising an air pore forming agent and optionally a hydrophobic agent.

8. A filter element according to claims 6 or 7, wherein said filter element is recyclable

9. A filter element according to any one of claims 6-8, wherein the volume of said filter element is reducible by at least 40 percent.

10. A filter comprising a plurality of filter elements according to any one of the preceding claims.

11.A concrete material comprising a pervious matrix comprising open voids, characterized in that said voids constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said voids comprises substantially sharp edges.

12. A concrete material according to claim 11, wherein the volume of said concrete material is reducible by at least 40 percent.

13. A concrete material according to claim 11 or 12, wherein said concrete material is made from a cured concrete mixture comprising an air pore forming agent and a hydrophobic agent.

14. A concrete material according to claim 13, wherein said air pore forming agent is an ionic tenside, and said hydrophobic agent is a resin.

15. A method for producing a concrete material comprising a pervious matrix comprising open pores, characterized in that said pores constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said pores comprises substantially sharp edges, the method comprising the steps of: - providing water, an air pore forming agent, and a hydrophobic agent to form a first mixture in a concrete mixer; - provide 20-40 percent by weight of a total amount of cement to said mixture to form a second mixture; - agitating said second mixture at a first rotation rate to form a foam; - agitating said second mixture at a second rotation rate and adding the remaining 60-80 percent by weight of the total amount of cement to form a concrete mixture; and - curing said concrete mixture.

16. Use of a concrete material as a filter element for filtering particles comprising a pervious matrix comprising open voids, wherein said voids constitute at least 50 percent by volume of the material, and wherein the matrix surrounding said voids comprises substantially sharp edges.

17. A method for filtering flue gas, characterized by the method comprising the steps of providing a filter element according to claim 1 ; bringing flue gas to said filter element and allowing the flue gas to pass there through, whereby at least a portion of particles present in the introduced flue gas is retained in the filter element.

18. A method according to claim 17, wherein said filter element is recyclable.

19. A method according to claim 17 or 18, the method further comprising the steps of: - providing a plurality of filter elements according to claim 1 arranged consecutively; - bringing flue gas to a first filter element of the plurality of filter elements and allowing the flue gas to pass there through, whereby at least a portion of particles present in the introduced flue gas is retained in said first filter element; - bringing gas exiting the first filter element to a second filter element of the plurality of filter elements and allowing the gas to pass there through, whereby at least a portion of particles present in the gas introduced in the second filter element is retained therein; and - repeating the previous step for each filter element such that the flue gas passes through all consecutively arranged filter elements, wherein each of the plurality of filter elements is replaceable independently of each other.

20. A method according to any one of claims 17-19, further comprising the step of providing a water flow through one or more of the plurality of filter elements.

21.A method according to any one of claims 17-19, further comprising the step of providing a humid atmosphere within one or more of the plurality of filter elements.

22. A method according to any one of claims 17-19, further comprising the step of adding a reactive gas or fluid to one or more of the plurality of filter elements.

23. A method according to any one of claims 17-22, further comprising the step of rendering one or more of the plurality of filter elements electrically conductive.

24. A method of recycling a filter element according to any one of claims 1-9, comprising the steps of providing one or more filter elements used for filtering flue gas; crushing said one or more filter elements, whereby the volume of each filter element is reduced by at least 40 percent; using the crushed one or more filter elements in the production of concrete.

25. A method for recycling according to claim 24, wherein the particles retained in the one or more filter elements constitute adhesive agents and/or filler agents in said production of concrete.