Method of separating solids from a liquid, filter fabric therefor and associated method of fabrication

Abstract

The method can include circulating the liquid having the solids suspended therein to a filter fabric made of strands having high-density polyethylene that is crosslinked, and having pores between the strands; and while the liquid has a temperature of 60°C or more, circulating the liquid through the pores, across the filter fabric, as the strands capture at least some of the solids.

Description

METHOD OF SEPARATING SOLIDS FROM A LIQUID, FILTER FABRIC THEREFOR AND ASSOCIATED METHOD OF FABRICATIONTECHNICAL FIELD

[0001] This disclosure relates to the field of liquid filtration for separating solids from liquids, and specifically to filtration processes which occur in harsh conditions such as high temperature and / or extreme pH levels.BACKGROUND OF THE ART

[0002] Liquid filtration is a widely employed process for separating particulate solids from liquids. Liquid filtration generally utilizes a permeable filter fabric to remove solid particles from organic liquids such as oils and fuels. Many industrial plants involved in mineral processing, including alumina, phosphate, iron ore, zinc, and more, require filtration equipment. Additionally, large filtration equipment is common in the chemical industry for producing substances like silica, zeolite, and fertilizers. Among these applications, there are processes operating in harsh environments that demand a filter fabric capable of withstanding high temperatures (95-110°C) and extreme pH levels (approximately 14 or 1). Furthermore, many of these processes generate secondary compounds that adhere to the fabric, impeding the flow rate. Improvements are therefore desired in filter fabrics, particularly it would be desired to be provided with a filter fabric that has a better overall solution to high temperatures, extreme pH conditions and / or adhesive solids.SUMMARY

[0003] Polypropylene (PP) is a material which can be used as a filter material for filtrations that require high temperatures during operation. This is because the temperature can degrade other materials such as polyethylene (PE). It has been found that a filter fabric with improved properties can be produced with crosslinked high-density PE. This filter fabric can outperform PP filters which may have issues with longevity and leaking. The PP filters can degrade at an undesirable rate (too fast). It was found that crosslinked high-density PE could address this problem in some embodiments. Indeed, crosslinked HDPE can be more resistant to high temperatures. The crosslinked high-density PE can be produced starting with a non-crosslinked high-density PE that is already provided as a fabric having a given structure of strands. The fabric can be a woven fabric, a knitted fabric, a non-woven fabric, and can also be extruded or fleeced. The fabric may be a monofilament structure, amultifilament structure or a combination thereof. A particle beam such as an electron beam or a photon (e.g., X-ray, gamma-ray) beam, can be used to crosslink the high-density PE while avoiding increasing the temperature to a point where the structure (e.g. woven structure) loosens or disintegrates.

[0004] Accordingly, in accordance with an aspect, there is provided a method of separating solids from a liquid, the method comprising: circulating the liquid having the solids suspended therein to a filter fabric made of strands having high-density polyethylene polymer that is crosslinked and having pores between the strands; while the liquid has a temperature of 60°C or more, circulating the liquid through the pores, across the filter fabric, as the strands capture at least some of the solids.

[0005] In at least some embodiments, the method of separating forms part of a Bayer process, wherein the filtration is performed on Bayer liquor obtained from refining bauxite ore to produce alumina, at a temperature of more than 105-115°C, in the presence of more than 250 g / L of sodium hydroxide, and the filter fabric is integrated to a leaf filter.

[0006] In at least some embodiments, the filter fabric is a crosslinked high density polyethylene that has a woven structure. In at least some embodiments, the method of separating forms part of a dihydrate process in the context of phosphoric acid production, wherein the solids are gypsum and the liquid is a liquor at a temperature of from 70 to 80 °C, on a pan filter or a vacuum belt filter bearing the filter fabric, wherein the liquor comprises 26 to 30 wt. % of H2SO4 and 26 to 32 wt. % of H3PO4.

[0007] In at least some embodiments, the method of separating forms part of a hemihydrate process in the context of phosphoric acid production, wherein the solids are filtered from liquor at a temperature of from 90 to 110 °C on a pan filter or a vacuum filter bearing the filter fabric, wherein the liquor comprises 40 to 58 wt. % of H2SO4 and 40 to 50 Wt. % Of H3PO4.

[0008] In at least some embodiments, the method of separating forms part of a first carbonatation step in the context of a beet sugar production, the first carbonatation including bubbling carbon dioxide into on beet pulp at a pH from 11.2 to 11.3 at a temperature of about 85 - 87 °C with calcium hydroxide to form a liquor, and the liquor is filtered using a candle filter bearing the filter fabric.

[0009] In at least some embodiments, the method of separating forms part of a second carbonatation step in the context of a beet sugar production, the second carbonatation including bubbling carbon dioxide into a liquor of beet pulp and calcium hydroxide at a pH of 10.2 and a temperature of 92 - 94 °C and filtering the liquor using a candle filter bearing the filter fabric.

[0010] In at least some embodiments, heating is performed during the filtering step to bring the temperature of the liquid to the temperature of 60 °C or more, preferably 80 °C or more.

[0011] In at least some embodiments, the liquid has a pH of less than 2 or more than 13.

[0012] In at least some embodiments, the liquid has a salt content of 200 g / L or more.

[0013] In accordance with an aspect, there is provided a filter comprising: a filter fabric made of strands of high-density polyethylene polymer that is crosslinked and having pores between the strands. In some embodiments, the pores have a size between 5 and 1000 microns. In some embodiments, the filter fabric has a thickness between 40 and 3000 microns. In at least some embodiments, the strands of the high-density polyethylene polymer have a surface that is hydrophilic and functionalized with hydroxy group. In at least some embodiments, the filter fabric is characterized by a shrinking of its length being of less than 10% at a temperature of 80 °C, or less than 5% or around 1%. In at least some embodiments, the filter fabric is characterized by a degree of crosslinking of from 30 to 80 %. In at least some embodiments, the filter further comprises a first volume such as a first vessel operable to receive an influent of a liquid having at least 60°C and having particulate solids suspended therein; a second volume such as a second vessel in fluid communication with the first volume across the filter fabric, the second volume and adapted to receive a filtrate of the liquid. In at least some embodiments, strands are calendared. In at least some embodiments, the high-density polyethylene polymer has a density of from 0.941 to 0.965 g / cm3. In at least some embodiments, the high-density polyethylene polymer comprises from 500 to 1500 monomers.

[0014] In at least some embodiments, the filter as defined herein is integrated to a system for performing a solid separation by liquid filtration, the system for performing solid separation by liquid filtration further having: a heater; and a conduit for circulating thesubstance into fluid contact with the heater, and into fluid contact with the filter fabric, and operable for heating the liquid to a temperature of at least 60 °C.

[0015] In at least some embodiments, the filter as defined herein is integrated to a leaf filter system for an alumina production plant, the leaf filter system further having: a cylindrical shell adapted to receive the filter; the filter comprising a plurality of leafs, wherein each of the plurality of leafs has a mesh frame having a metallic structure supporting a metallic mesh, and wherein the metallic mesh is covered by the filter fabric.

[0016] In at least some embodiments, the filter as defined herein is integrated to a candle filter for a beet sugar production plant.

[0017] In at least some embodiments, the filter as defined herein is integrated to a pan filter for a phosphoric acid production plant.

[0018] In accordance with an aspect, there is provided a method of fabricating a filter fabric, the method comprising: providing a filter fabric made of strands having high-density polyethylene and having pores between the strands; propagating a particle beam onto the filter fabric to initiate crosslinking of the high-density polyethylene. In at least some embodiments, the filter fabric comprises high density polyethylene that has a woven structure, the particle beam is an electron beam and said propagating maintains the woven structure, further comprising cooling the filter membrane when the temperature of the woven structure reaches 50 °C; and repeating the steps of emitting the electron beam followed by the cooling step until the high density polyethylene is crosslinked.

[0019] In at least some embodiments, the method further comprises monitoring a temperature of the filter fabric, cooling the filter fabric when the temperature exceeds a temperature threshold; and repeating the steps of emitting the particle beam followed by the cooling until the high-density polyethylene is crosslinked.

[0020] In at least some embodiments, the temperature threshold is 50 °C. In at least some embodiments, the cooling is performed until a temperature of 35 °C or less is reached.

[0021] In at least some embodiments, the particle beam is an electron beam. In at least some embodiments, the step of emitting an electron beam is performed by a cathode that is heated to emit said electron beam, wherein the electron beam reaches an anode and the filter fabric is positioned between the cathode material and the anode on a path of theelectron beam. In at least some embodiments, the particle beam has an energy of 70 to 12,000 keV. In at least some embodiments, the particle beam is characterized by an absorbed dose in the range of 50 to 400 kGy. In at least some embodiments, the high- density polyethylene has a density of 0.941-0.965 g / cm3. In at least some embodiments, the method is performed until a degree of crosslinking for the high-density polyethylene is from 30 to 80 %.

[0022] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a schematic illustration showing a leaf filter open for cleaning according to an exemplary embodiment.

[0024] FIG. 2 is a schematic illustration of a mesh frame supporting a leaf filter according to an exemplary embodiment.

[0025] FIG. 3 is a schematic illustration of a filter bag with a mesh frame received therein according to an exemplary embodiment.

[0026] FIG. 4 is an optical microscopy image of a polypropylene bag after 70 days of operation.

[0027] FIG. 5 is an optical microscopy of a crosslinked high-density polyethylene (XL- HDPE) bag according to one embodiment after 70 days of operation.

[0028] FIG. 6A is a schematic illustration of a pan filter according to an exemplary embodiment.

[0029] FIG. 6B is a photograph of a pan filter according to an exemplary embodiment.

[0030] FIG. 7 is an illustration of a vacuum belt filter.

[0031] FIG. 8A is a schematic illustration of a candle filter according to an exemplary embodiment.

[0032] FIG. 8B is a photograph of a candle filter according to an exemplary embodiment.DETAILED DESCRIPTION

[0033] The term “fabric” or “filter fabric” as used herein in the context of filtration refers to a structure with interlaced strands such as woven fabrics (e.g., plain, twill and satin), knitted fabrics, or non-woven fabrics (e.g., felt, needle-punched, wet / hydro-entangled, meltblown, heat-bonded, stitch, spunlaced, spunbond). The strands can be monofilament, e.g., made of a single, continuous strand of fiber, or multifilament, e.g., composed of many fine filaments twisted together to form a single strand, and in some cases, fabrics use a combination of monofilament and multifilament. Moreover, different types of fabrics can be made with different strand lengths. For example, many non-woven fabrics are made with short strands whereas many woven fabrics are made with longer strands which may extend continuously from one edge of the fabric to an opposite edge of the fabric. Fabrics are composed of multiple strands that are arranged in a particular microscopic or macroscopic manner. The way the strands are arranged is generally tied to the way the fabric was produced. Fabrics generally have a pore size that is from 5 microns to up to 1000 microns. Accordingly, the term “fabric” should not be confused with “film” or “membrane” when used in the context of filtration. A film or membrane is not made of strands. A film or membrane has a small pore size, for example smaller than 5 microns or smaller than 3 microns. A film or membrane has a thickness that is less than 100 microns. A film or membrane is a homogenous, potentially porous, material that does not have interlaced strands. The thickness of a fabric, by comparison, can for example be more than 40 microns, more than 100 microns, more than 110 microns, more than 200 microns and potentially up to 3000 microns, particularly when consisting of multiple layers such as interlaced layers. Fabrics can be used in liquid filtration, i.e. , for separating a liquid from suspended solids, as will be described in greater detail below.

[0034] Polypropylene (PP) woven fabrics can be used as filter fabrics due to their chemical stability and cost-effectiveness. They can be made available in various weaving patterns, weights, and permeabilities, and be adapted to many industrial applications. However, when used in solutions maintained at high temperatures (e.g. more than 60°C, more than 80°C or around 90°C), polypropylene fabrics are not dimensionally stable. Depending on the temperature of operation and soaking time, the PP filter can shrink by up to 2-5% in width and length, necessitating premature replacement. Additionally, the combination of high temperature and harsh conditions (e.g., extreme pH, in the vicinity of 1 or 14) deteriorates the PP surface, leading to the attachment of adherent compoundsgenerated during chemical processes. This phenomenon is generally known as 'scaling', and this phenomenon significantly reduces the service life of filtration equipment, including filter fabrics. Polyethylene (PE), including high-density polyethylene (HDPE), can be produced from oil or bio-based sources.

[0035] Due namely to its low maximum operating temperature of 80-85°C, PE can be ill suited for use as a filter fabric in harsh conditions. PE is not a single material but a family with distinct members. The polyethylene backbone can be branched and / or linear. Generally, the higher the density of the polyethylene the less branching there is on the backbone. The variations in atomic structure and properties result from different polymerization processes and catalysts used. For instance, low-density polyethylene (LDPE) has a density of 0.917-0.930 g / cm3and long-branched chains, while linear low- density polyethylene (LLDPE) has short-branched chains. Other members include mediumdensity polyethylene (MDPE) and high-density polyethylene (HDPE), which have different densities and chain lengths. HDPE is generally characterized by a density of 0.941-0.965 g / cm3while MDPE generally has a density of 0.931-0.94 g / cm3. Ultra-high molecular weight polyethylene (UHMWPE) stands out due to its extraordinarily long chains. The UHMWPE is composed of chains made of about 100,000 - 250,000 monomers while the HDPE is composed of shorter chains made of only 500 - 1500 monomers. Polyethylene may be used generally but, in many embodiments, there can be advantages to using more specifically high-density polyethylene that has been crosslinked as explained below.

[0036] In many embodiments where a filter fabric is made of strands having HDPE that is crosslinked, the strands can consist solely of HDPE. In some other embodiments where a filter fabric is made of strands having HDPE that is crosslinked, the strands can consist of a mix of HPDE and one or more other polymers (e.g., a HDPE / LLDPE copolymer). For example, the strands can consist of at least 50 %, at least 60 %, at least 70%, at least 80 %, at least 90 %, or at least 95 % by weight of HDPE. The copolymers can be a copolymer of HDPE and LLDPE, a copolymer of HDPE and LDPE or a copolymer of HDPE, LLDPE and LDPE. The advantage of combining LLDPE and / or LDPE with HDPE is that the viscosity of the copolymer in the molten state can be lower than that of HDPE. This may be useful when making yarns of a smaller diameter. The copolymer can combine a strength and rigidity of HDPE with a flexibility and toughness of LLDPE and / or LDPE. The LLDPE and / or LDPE can be included in the copolymer at a weight ratio of up to 50 %, up to 40 %, up to 30 %, or from 10 to 50 %, for example. LDPE, LLDPE and HDPE are mutually soluble in boththe amorphous and crystalline forms. Crosslinking generally occurs in the amorphous form of the polymers.

[0037] Crosslinking polymers involves creating chemical bonds between chains to form a three-dimensional molecular structure. In other words, a non-crosslinked polymer is a two- dimensional structure. The Van Der Waals bonds are the main acting electrostatic forces between polyethylene polymer chains. They are known as weak forces. In contrast, a crosslinked polymer forms a three-dimensional structure created by covalent bonds between polymer chains. The covalent bonds are a much stronger bond and thus a distinct three-dimensional structure is created. It was found that crosslinked polymers can lead to superior properties compared to non-crosslinked ones in the context of filtering solutions, including higher service temperature, improved mechanical properties (e.g. increased tensile strength, improved creep resistance, increased toughness and stress cracking resistance), better heat and chemical resistance, and increased abrasion resistance.

[0038] While PP cannot be crosslinked, members of the PE family can be. Three main processes are used for crosslinking PE. The first two processes involve the use of chemical additives and high temperatures. Heat triggers crosslinking when a mixture of PE pellets and either organic siloxanes or peroxide compounds is melted. The mixture must be rapidly (i.e. within minutes) molded into its final form while still in a molten state. Once crosslinking is complete, reshaping is difficult. In the case of chemically crosslinked woven fabric, crosslinking must be done when drawing the yarns to create crosslinked yarn before weaving. After weaving, a series of thermal and calendaring treatments are necessary to finish the fabric. The finishing aims to remove fabric stresses, to reduce the fabric thickness, to smooth the surface and to decrease the pore sizes. The finished fabric has enhanced filtration properties compared to the raw fabric. However, chemically crosslinked fabrics cannot be finished due to their infusible thermoset yarn. Consequently, the difficulty in finishing the chemically crosslinked fabrics may make these two first processes ill-suited to use for producing a crosslinked PE fabric with a strand structure appropriate for a filter.

[0039] It was found that a third process can be used for crosslinking HDPE fabrics which involves using a particle beam such as an electron beam (or other high energy particles such as high energy photons). It should be noted that the same method can be applied to copolymers of HDPE. In the specific case of an electron beam, the process can be embodied by heating a cathode material to emit electrons, which are attracted by an anode. The HDPEfabric to be crosslinked is placed in the path of these electrons. As the electrons travel, they hit and eject hydrogen atoms from the polymer chains, leaving potential dangling bonds. When chains with dangling bonds come into proximity, they can form chemical links between them, i.e. crosslinking. These bonded chains can further bond with neighboring chains, eventually forming a complex three-dimensional web structure. Unlike chemical additive crosslinking of the above two methods, particle beam crosslinking can be applied to fully finished (e.g., calendared) fabrics. This can allow obtaining a HDPE fabric that has a fully finished fabric structure (e.g., calendared) while also being crosslinked and benefiting from the improved properties of crosslinking. Particle beam crosslinking can be performed with particles having an energy level sufficient to initiate the crosslinking. An electron beam can be used, for instance. Alternately, X rays or gamma rays can be used, which can be emitted as a beam of photons having a level of energy characterized by their wavelength, which can be between 10 nm and 1 pm for instance for X rays and less than 0.01 nm for gamma rays. Gamma rays, in particular, can provide an advantage of very uniform irradiation even in thicker materials, however it can be slower than X rays, require inconvenient radioactive sources and shielding, and pose a risk of oxidative degradation if done in air.

[0040] In the case of an electron beam, the electron beam used for crosslinking can generally be characterized by two main parameters: the energy of the beam (in kilo electron volts or keV) and the absorbed ionizing radiation dose delivered (in kilo grays or kGy). The energy of the beam can be within the range of 70 to 10,000 keV. The depth of penetration varies with energy, from a few millimeters to about 10 centimeters. Using a range of 70-300 keV allows continuous treatment of the material, while an energy range of 1 ,000-10,000 keV may be better suited for treating rolled goods. Accordingly, the specific value will depend on the physical properties of the PE to be crosslinked (thickness of the material, density, length etc.). For PE, the radiation dosage preferably ranges between 50 and 400 kGy, with a preferred range being 100-200 kGy for the PE family. In some embodiments, the electron beam crosslinking process functionalizes the surface of the filter fabric and yields a surface that is hydrophilic thanks to the production of hydroxy groups. The resulting hydrophilic surface can be advantageous, such as by reducing the energy requirement to filter aqueous phases. The surface may react with oxygen and become oxidized, which may confer hydrophilic properties. Moreover, the particle beam can also influence the conformation of the polymer. The structure moves from a linear structure to a three-dimensional structure created by transversal covalent bonds between polymer chains. This can play a significantrole in the material’s shrinking properties in the presence of heat. The shrinking is reduced by more than 10 times after the application of the electron beam. For example, for a filter bag of 5 m the shrinking would change from 150-300 mm to only around 5 mm. Accordingly, the filter fabric can be characterized by a shrinking of less than 10%, less than 5% or around 1 % at a temperature of 80 °C. In some embodiments, the hydrophilicity of the surface can be measured by the water droplet method, i.e. measuring the contact angle of a water droplet deposited on a solid surface. A surface is hydrophobic when its static water contact angle 0 is >90° and is hydrophilic when 0 is <90°.

[0041] In some embodiments, crosslinking can be quantified by the degree of crosslinking which is preferably between 30 to 80 %. There are many methods to determine the degree of crosslinking, for example the degree of crosslinking can be determined by gel content according to the International Standard ISO 10147 (2011).

[0042] Accordingly, a filter fabric made of strands having high-density polyethylene that is crosslinked can be provided. The pores are defined between the strands. Depending on the structure of the strands, the porosity can be varied to adapt the pore size. This may be a woven or non-woven fabric. More generally, the fabric has a given structure from its strand components that define the pore size, and the structure can be adapted based on the desired pore size or other desired features such as mechanical resistance. In an exemplary embodiment, the individual strands having HDPE forming a woven structure can intertwine in a tight manner or in a looser manner, thereby dictating the size of the pores and selectivity of the filter fabric. It should be noted that the woven structure can be made with a monofilament, a multifilament or a combination thereof. In the case of a multifilament, a plurality of filaments of the same material (e.g., HDPE) can be used, or filaments of one or more different materials may be used. The crosslinked HDPE has improved properties compared to non-crosslinked HDPE but also compared to other PP filters. The HDPE is well adapted for filtration operations that occur at 60 °C or more, or 80 °C or more. The present filter fabric degrades less, has better longevity and filter properties compared to the traditional PP fabrics.

[0043] In many embodiments, a filtration occurring at 60 °C or more, or 80 °C or more can mean that a heating is performed such that a temperature of 60 °C or more is reached or in other cases it can mean that the liquid provided on the filter fabric arrives at a temperature of 60 °C or more, or 80 °C or more. In the latter case, no specific heating isneeded but there could still be heating during the filtration. There are many industrial processes that need a filtration performed at a temperature of 60 °C or more, or 80 °C or more such as but not limited to alumina production (Bayer process), phosphoric acid production and sugar production.

[0044] It should be understood that using a filter fabric made of strands having HDPE crosslinked as exposed above can improved properties and resistance to conditions not only of high temperature, but also of high acidity (e.g. pH of less than 2, less than 1.5 or even less than 1), high alkalinity (e.g. pH of more than 13, more than 13.5 or even more than 14) and / or high salt concentrations such as more than 200 g / L of salt content.

[0045] The filter fabric can be provided as part of a system for performing a solid separation by liquid filtration. As explained in greater detail below, the filter fabric can be used for separating solids from a liquid in the context of a Bayer process, a sugar production or a phosphoric acid production among other potential industrial use cases. The filter fabric separates two volumes (e.g., two vessels), a first volume containing the influent, where the solids are suspended in the liquid in a first concentration, and a second volume containing the filtrate, having the solids in a second, significantly lower concentration. Accordingly, the filter fabric is positioned between the first volume and the second volume such that the liquid passes through the filter fabric to reach the second volume. The liquid circulation causes a pressure against the filter fabric, and it is common for the filter to further include a supporting system for the filter fabric, such as a metal mesh. In several industrial contexts, a heater is provided upstream of the filter in the system. The heater can act on the liquid before the filtration and / or can be used to heat the liquid during the filtration. Thus, there can be provided a conduit for circulating the substance into fluid contact with the heater, and into fluid contact with the filter fabric for heating the liquid to a temperature of at least 60 °C or for maintaining the temperature of the liquid at the temperature of at least 60 °C.Example 1 : Fabric for Pressurized Leaf Filter for Alumina Security Filtration in the Bayer Process

[0046] The Bayer process involves the refining of bauxite ore to produce alumina, and involves the filtration of Bayer liquor using a leaf filter configuration, under the conditions of a high temperature of more than 105 °C (e.g. 105-115°C, more typically 106-110°C) and the presence of a high concentration of sodium hydroxide (e.g. more than 200 g / L, more than 250 g / L, more than 300 g / L and up to 450 g / L, 400 g / L or 350 g / L, more typically around 300g / l NaOH). Various filter types, both continuous and batch-operated, are used within pressurized vessels. In batch processes, a typical leaf filter consists of 20-25 frames, each covered by a filtering fabric bag. The total surface area for filtration in a leaf filter ranges from 250 to 600 square meters, with 8-12 filters typically used in a plant producing 1Mt of alumina.

[0047] In a first example, the filter fabric can be incorporated in such leaf filters. An example leaf filter is shown in Fig. 1 , where the leaf filter is shown during a cleaning step, with a cylindrical shell of the filter slid lengthwisely, exposing the internal structure of the leaf filter which houses a plurality of ‘leafs’ (typically about 15-25) of various sizes engineered to adapt to the space available in the cylindrical shell.

[0048] The leafs of the filter each includes a mesh frame having a metal (e.g. steel) structure supporting a mesh (e.g. steel), such as shown in Fig. 2, which is covered by a filtration fabric provided in the form of a bag such as shown in Fig. 3 for use. During operation, the cylindrical shell is slid into position to close the vessel, and the solution intended to be filtered is pumped into it. When the vessel is full, the pressure rises due to a pumping action, and the solution flows through the fabric, across the mesh, and the filtrate is extracted through an outlet pipe. The purified solution, full of solubilised alumina, is ready for production of solid alumina. Filtration by the leaf springs leads to formation of a rocksolid accumulation known as scale on the bags. Accordingly, the bags are consumable items which are subject to regular replacement. The longevity of the bags when using the filter fabric material or fabric as described herein is increased. A typical leaf filter can require 250-500 square meters of fabric to cover all the 15-25 leafs. A typical plant has at least 5 leaf filters, and can thus use about 1000 filter bags per year. Many plants have 10 to 15 filters and can thus use 2000 to 3000 filter bags per year.

[0049] Referring to Fig. 3, the filter bags 11 use an appropriately sized square piece of fabric folded in half along a first longitudinal edge 10, forming a first closed edge 10a. The second longitudinal edge 12 and one of the lateral edges (referred to herein as the distal edge 14) can be closed by sewing or hot welding, which is typically done at the fabric factory. Henceforth, a bag generally has two adjacent sheet portions having a mesh frame spacing therebetween, and having three closed edges 10a, 12, 14 and one open edge (which can be referred to as the proximal edge 16) through which the mesh frame (Fig. 3) could be inserted at the alumina production plant. It is thus traditional that it is at the aluminaproduction plant that the seal is made along the proximal edge 16 and the outlet pipe 18. The closing of the proximal edge 16 is typically done by crumpling the fabric along the portion of the proximal edge 16 extending between a corner 20 and the outlet pipe aperture and stapling it closed. Then, a plurality of bands of fabric are cut from an extra bag using a knife, and one of these is wrapped around the outlet pipe 18 in a manner to overlap the bag, and tied into position using a twisted wire 20 in an effort to form a seal around the outlet pipe 18.

[0050] A typical filtering cycle for the batch process lasts 6-12 hours. After each cycle, the filter is opened, and the cake is removed from each frame by rinsing. Once cleaned and reassembled, the filter resumes operation. After approximately three filtration cycles, the filter is taken out of production for a 10-hour cleaning process, involving soaking in a solution of approximately 450 g / L sodium hydroxide at 85°C. Bag replacement is often prompted by a flow reduction below the plant's designated value. This flow reduction results from sticky solid compounds clogging the pores from cycle to cycle. These solids are composed of a mixture of alumina hydroxide, iron oxides, tricalcium aluminate, sodalite, and other species generated during the Bayer process, forming a solid crust that cannot be removed by sodium hydroxide cleaning. This solid crust is known in the industry as "scale." Accordingly, one advantage of the present filter fabric with crosslinked HDPE is that because the crosslinked HDPE maintained the strand or filament structure, the pore size remains constant, and this helps to ease the cake release (improved non-stick surface) and thus reduce or even prevent build up on the surface of the filter (referred to as scaling).

[0051] PP fabrics are currently recognized as one of the most effective materials for resisting scaling, particularly in alumina security filtration applications. Among various weaving styles, a fabric composed of yarns with a diameter of 200 microns in both warp and weft directions, featuring 85 yarns per cm in warp and 36 yarns per cm in weft, stands out as highly efficient for this purpose.

[0052] Comparing performance, two fabrics were produced with identical construction parameters: one using PP and the other using HDPE. The HDPE fabric underwent crosslinking using a 10,000 keV electron beam at a dose of approximately 200 kGy. This treatment was conducted in air, although it could have been carried out in a controlled atmosphere like nitrogen. Air was preferred due to its unexpected result of imparting a hydrophilic surface to HDPE, which helps to prevent the scaling issue. The treatmentinvolved 8 passes, each giving a 25 kGy dose, with cooling intervals to prevent overheating and alteration of pore size distribution and filtration properties.

[0053] Each type of fabric was used to create 21 bags by stitching panels together, resulting in approximately 13 square meters of effective filtration surface area per bag. Both PP and HDPE filter sets were tested side by side under identical conditions for 70 days, which is the maximum period feasible for PP bags before significant flow reduction due to scaling occurs.

[0054] During the test period, the flow through PP bags gradually decreased by up to 15%. In contrast, there was no reduction in flow observed for the HDPE bags, indicating an absence of scaling on the HDPE fabric. This non-scaling effect was further confirmed by air permeability measurements conducted after the 70-day period (see Table 1 for initial and final values).

[0055] Specifically, the air permeability of PP bags decreased by 90%, while XL-HDPE (crosslinked HDPE) exhibited no significant decrease within the margin of measurement error. This disparity strongly suggests that scaling caused the clogging observed in PP bags but not in XL-HDPE bags.

[0056] Visual confirmation of scaling on PP and its absence on XL-HDPE fabrics was obtained through fabric color and optical microscopy analysis. Fig. 4 shows white and red scaling material on the surface of the PP fabric, whereas Fig. 5 reveals a clean surface on the HDPE fabric, devoid of any such material.

[0057] The non-scaling effect is inherent to the material itself and is independent of the weaving pattern, thread count, or thread diameter.Table 1. Bags air permeability reduction

[0058] The presence of the anti-scaling effect is believed to apply equally to other comparable contexts than alumina production, examples of which are provided below.Example 2: Fabrics for Tilting Pan Filter in Phosphoric Acid Production

[0059] Phosphates are essential ingredients in fertilizers, produced industrially by reacting hot sulfuric acid with ore containing calcium phosphate. This reaction yields either calcium sulfate dihydrate (gypsum) or calcium sulfate hemihydrate, along with a phosphoric acid solution. Separation of the calcium sulfates from phosphoric acid is achieved using either a tilting pan filter (TPF) (see Figs. 6A-6B) or a horizontal vacuum belt filter (HVBF) (see Fig. 7). Both filters can use PP fabrics to separate solid sulfates from a solution at 94°C containing 30% phosphoric acid by weight and 10% sulfuric acid by weight.

[0060] In some embodiments, the phosphoric acid production filtration process is a dihydrate process. To obtain phosphoric acid, gypsum is filtered from liquor at a temperature of from 65 to 85 °C or 70 to 80 °C on a pan filter having the filter fabric. The liquor can comprise about 26 to 30 wt. % of H2SO4 and about 26 to 32 wt. % of H3PO4.

[0061] In other embodiments, the phosphoric acid production filtration process is a hemihydrate process. In such embodiments, the gypsum is filtered from liquor at a temperature of from 85 to 115 °C or 90 to 110 °C on a pan filter having the filter fabric. The liquor can comprise about 40 to 58 wt. % of H2SO4 and about 40 to 50 wt. % of H3PO4.

[0062] Both these latter types of embodiments (i.e. , dihydrate process or hemihydrate process) may use PP fabric. The PP fabric can have 27 yarns per cm with a diameter of 350 microns in the warp and 8 yarns per cm with a diameter of 400 microns in the weft. Like the alumina production process, phosphoric acid production results in the formation of residue containing fluorosilicates, known as scale, which clogs the fabric. This necessitates fabric replacement every 7 to 30 days of production.

[0063] Replacing the PP fabric with, for example, an exact replica made of crosslinked HDPE is expected to significantly reduce scale formation and substantially extend the fabric's service life, while preserving filtration efficiency, similarly to how using the filter fabric of crosslinked HDPE successfully allowed similar advantages in the comparable context of alumina production.Example 3: Fabrics for Candle Filter in Beet Sugar Production

[0064] Beet sugar protection can include two carbonatation steps which involve the use of carbon dioxide and calcium hydroxide to remove impurities from sugar liquor. In the firstcarbonatation, the calcium hydroxide (or lime) is added to the mix of beet pulp and water. The pH in the first carbonatation can be from 11.2 to 11.3 and carbon dioxide is bubbled into solution at a temperature of about 85 - 87 °C. The impurities are precipitated in the form of carbonates which are filtered by candle filters with the filter fabric. In the second carbonatation, the calcium hydroxide (or lime) is added to the mix of beet pulp and water at a pH of 10.2 and carbon dioxide is bubbled into solution at a temperature of about 92 - 94 °C. The impurities again precipitate as carbonates and are filtered out using a candle filter with the fabric filter.

[0065] For example, during the purification of beet sugar, the raw solution containing sugar and impurities can be heated to 85°C and treated with a lime solution (CaO). Carbon dioxide is introduced into the solution to form CaCCh, which precipitates the impurities. The resulting slurry is then processed through a pressurized candle filter to separate the solid impurities from the liquid containing sugar. PP fabrics are a suitable choice for this sugar filtration process. The candle filter may be as shown in Figs. 8A and 8B.

[0066] Among various weaving styles, a fabric with yarns 200 microns in diameter, featuring 85 yarns per cm in the warp and 36 yarns per cm in the weft, has proven highly effective for this application. However, due to the nature of the CaO-CaCCh solid, the PP fabric may become clogged after just one month of production. To address this issue, replacing the PP fabric with the crosslinked HDPE is expected to significantly reduce scale formation and substantially extend the fabric's service life, similarly to how using the filter fabric of crosslinked HDPE successfully allowed similar advantages in the comparable context of alumina production.

[0067] As can be understood, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of separating solids from a liquid, the method comprising: circulating the liquid having the solids suspended therein to a filter fabric made of strands having high-density polyethylene that is crosslinked, and having pores between the strands; and while the liquid has a temperature of 60°C or more, circulating the liquid through the pores, across the filter fabric, as the strands capture at least some of the solids.

2. The method of claim 1 , wherein the method of separating forms part of a Bayer process, wherein the filtration is performed on Bayer liquor obtained from refining bauxite ore to produce alumina, at a temperature of more than 105-115°C, in the presence of more than 250 g / L of sodium hydroxide, and the filter fabric is integrated to a leaf filter.

3. The method of claim 1 , wherein the method of separating forms part of a dihydrate process in the context of phosphoric acid production, wherein the solids are gypsum and the liquid is a liquor at a temperature of from 70 to 80 °C, on a pan filter or a vacuum belt filter bearing the filter fabric, wherein the liquor comprises 26 to 30 wt. % of H2SO4 and 26 to 32 wt. % of H3PO4.

4. The method of claim 1 , wherein the method of separating forms part of a hemihydrate process in the context of phosphoric acid production, wherein the solids are filtered from liquor at a temperature of from 90 to 110 °C on a pan filter or a vacuum filter bearing the filter fabric, wherein the liquor comprises 40 to 58 wt. % of H2SO4 and 40 to 50 wt. % of H3PO4.

5. The method of claim 1 , wherein the method of separating forms part of a first carbonatation step in the context of a beet sugar production, the first carbonatation including bubbling carbon dioxide into on beet pulp at a pH from 11.2 to 11.3 at a temperature of about 85 - 87 °C with calcium hydroxide to form a liquor, and the liquor is filtered using a candle filter bearing the filter fabric.

6. The method of claim 1 , wherein the method of separating forms part of a second carbonatation step in the context of a beet sugar production, the second carbonatation including bubbling carbon dioxide into a liquor of beet pulp and calcium hydroxide at a pH of 10.2 and a temperature of 92 - 94 °C, and filtering the liquor using a candle filter bearing the filter fabric.

7. The method of any one of claims 1 to 6, wherein the filter fabric is a crosslinked high-density polyethylene that has a woven structure.

8. The method of claim 1 or 7, wherein heating is performed during the filtering step to bring the temperature of the liquid to the temperature of 60 °C or more.

9. The method of claim 1 , 7 or 8, wherein the temperature is of 80 °C or more.

10. The method of any one of claims 1 , 7, 8 or 9, wherein the liquid has a pH of less than 2.

11. The method of any one of claims 1 , 7, 8 or 9, wherein the liquid has a pH of more than 13.

12. The method of any one of claims 1 or 7 to 11 , wherein the liquid has a salt content of 200 g / L or more.

13. A filter comprising: a filter fabric made of strands of high-density polyethylene polymer that is crosslinked, and having pores between the strands.

14. The filter of claim 13, further comprising a first vessel adapted to receive a liquid phase, a second vessel in fluid communication with the first vessel and adapted to receive a permeate of the liquid phase, wherein the filter fabric separates the first vessel from the second vessel, and is calendared to form a mesh.

15. The filter of claim 13 or 14, wherein the pores have a size between 5 and 1000 microns between the strands, the filter fabric having a thickness between 40 and 3000 microns.

16. The filter of any one of claims 13 to 15, wherein the strands of the high-density polyethylene polymer have a surface that is hydrophilic and functionalized with hydroxy group.

17. The filter of any one of claims 13 to 16, wherein the filter fabric is characterized by a shrinking of its length being of less than 10% at a temperature of 80 °C.

18. The filter of claim 17, wherein the shrinking is of less than 5%.

19. The filter of claim 17, wherein the shrinking is around 1%.

20. The filter of any one of claims 13 to 19, wherein the filter fabric is characterized by a degree of crosslinking of from 30 to 80 %.

21. The filter of any one of claims 13 to 20 wherein the filter fabric has two or more layers each made of the strands.

22. The filter of any one of claims 13 to 21 , wherein the high-density polyethylene polymer has a density of from 0.941 to 0.965 g / cm3.

23. The filter of any one of claims 13 to 22, wherein the high-density polyethylene polymer comprises from 500 to 1500 monomers.

24. The filter of any one of claims 13 to 23 integrated to a system for performing a solid separation by liquid filtration, the system for performing solid separation by liquid filtration further having: a heater; and a conduit for circulating the substance into fluid contact with the heater, and into fluid contact with the filter fabric, and operable for heating the liquid to a temperature of at least 60 °C.

25. The filter of any one of claims 13 to 24 integrated to a leaf filter system for an alumina production plant, the leaf filter system further having: a cylindrical shell adapted to receive the filter; the filter comprising a plurality of leafs, wherein each of the plurality of leafs has a mesh frame having a metallic structure supporting a metallic mesh, and wherein the metallic mesh is covered by the filter fabric.

26. The filter fabric of any one of claims 13 to 24 integrated to a candle filter for a beet sugar production plant.

27. The filter fabric of any one of claims 13 to 24 integrated to a pan filter for a phosphoric acid production plant.

28. A method of fabricating a filter fabric, the method comprising: providing a filter fabric made of strands having high-density polyethylene, and having pores between the strands; and propagating a particle beam onto the filter fabric to initiate crosslinking of the high-density polyethylene.

29. The method of claim 28 wherein the filter fabric comprises high density polyethylene that has a woven structure, the particle beam is an electron beam and said propagating maintains the woven structure, further comprising cooling the filter membrane when the temperature of the woven structure reaches 50 °C; and repeating the steps of emitting the electron beam followed by the cooling step until the high density polyethylene is crosslinked.

30. The method of claim 28 or 29 further comprising monitoring a temperature of the filter fabric, cooling the filter fabric when the temperature exceeds a temperature threshold; and repeating the steps of emitting the particle beam followed by the cooling until the high-density polyethylene is crosslinked31. The method of claim any one of claims 28 to 30 wherein the temperature threshold is 50 °C.

32. The method of claim 30 or 31 , wherein the cooling is performed until a temperature of 35 °C or less is reached.

33. The method of any one of claims 28 to 32, wherein the particle beam is an electron beam.

34. The method of claim 33 wherein the step of emitting an electron beam is performed by a cathode that is heated to emit said electron beam, wherein theelectron beam reaches an anode and the filter fabric is positioned between the cathode material and the anode on a path of the electron beam.

35. The method of claim 33 or 34, wherein the particle beam has an energy of 70 to 12,000 keV.

36. The method of any one of claims 33 to 35, wherein the particle beam is characterized by an absorbed dose in the range of 50 to 400 kGy.

37. The method of any one of claims 28 to 36, wherein the high-density polyethylene has a density of 0.941-0.965 g / cm3.

38. The method of any one of claims 28 to 37, wherein the method is performed until a degree of crosslinking for the high-density polyethylene is from 30 to 80 %.

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