WO2021189159A1

WO2021189159A1 - Air filter comprising copper-based magnetic alloys for reducing microorganisms in polluted air, and production method thereof

Info

Publication number: WO2021189159A1
Application number: PCT/CL2021/050020
Authority: WO
Inventors: Marta Lorena María LÓPEZ JENSSEN; Christopher Gonzalo SALVO MEDALLA; Felipe Aber SANHUEZA GÓMEZ; Helia Magaly BELLO TOLEDO; Emky Héctor VALDEBENITO ROLACK; Cristian Alberto CUEVAS BARRAZA; José Antonio JIMÉNEZ; Ramalinga Viswanathan MANGALARAJA
Original assignee: Universidad De Concepcion
Priority date: 2020-03-27
Filing date: 2021-03-26
Publication date: 2021-09-30
Also published as: CL2020000808A1

Abstract

Disclosed is an air filter comprising copper-based magnetic alloys, which is useful for reducing microorganisms suspended inside hospital facilities and other enclosed spaces with a high microbial load density. The air filter comprises porous plates of copper-based magnetic alloys of the type Cu-Co-Fe, wherein the alloying agents Co and Fe are present in an amount of 5-10% w/w Co and 5-10% w/w/ Fe, or of the type Cu-SmCo5, wherein the alloying agent SmCo5 is 10-15% w/w. Also disclosed is a production method comprising the steps of: obtaining the magnetic alloys by means of mechanical grinding; compacting the porous plates of copper alloys; and preforming by compaction of the porous plates.

Description

AIR FILTER OF COPPER BASED MAGNETIC ALLOYS TO REDUCE MICROORGANISMS IN POLLUTED AIR, AND ITS MANUFACTURING PROCESS.

Technical Sector

The technology is oriented to the environmental area, more particularly, it corresponds to an air filter made of copper-based magnetic alloys, useful for reducing microorganisms in suspension inside hospital rooms and other closed rooms with a high density of microbial load.

Previous Technique

Microbial contamination is one of the most important parameters of air pollution inside clinical settings (Chang et al., 2015). These microorganisms (bacteria and fungi) cause intra-hospital (nosocomial) infections, diseases contracted by contact, respiratory tract and infections during surgery (Chang et al., 2015; Stauning et al., 2018). This is a potential risk, not only for patients, but also for health workers (Cabo Verde et al., 2015). For this reason, nosocomial diseases are a global health concern and, for that same reason, it is vitally important to know the maximum limits of microbial load in a clinical care setting (Bhattacharyya et al., 2015). However, the International Organization for Standardization (ISO) has not established ranges of microbial load in hospital environments, only referring to standardizing the ranges of airborne microbiota in laboratories and the procedures for taking air samples for their microbiological analysis ( Napoli et al., 2012; Aguiar et al., 2014; Cabo Verde et al., 2015).

Numerous studies on the burden of the aerial microbiota from both clinical and other environments have emerged recently. Many of these refer to the aerial microbiota ranges allowed by the health organizations of the respective countries (Chang et al., 2015; Cabo Verde et al., 2015; Fujiyoshi et al., 2017; Svajlenka et al., 2018 ; Stauning et al., 2018). Most are about fungi, with little information about bacteria (Fujiyoshi et al., 2017). Among the aforementioned standards, the British standard of the Healthcare Infection Society (HIS) and the Italian standard of the Italian Institute for Occupational Safety and Prevention (ISPESL) stand out, which coincide in a maximum of 35 CFU nr ³ for empty spaces and 180 CFU nr ³ for occupied spaces; the Portuguese regulation of the Portuguese Ministry of the Environment establishes a maximum of 350 CFU nr ³ , and the Taiwanese recommendation of the Environmental Protection Administration (EPA) estimates a maximum of 1000 CFU nr ³ (Napoli et al., 2012; Chang et al. ., 2015; Stauning et al., 2018; Cape Verde et al., 2015). In Chile, there are no finished studies in this regard, so quantitatively determining the amount of bacteria and fungi, and reducing them in the air of clinical centers is essential. Until now, High Efficiency Air Purifying (HEPA) systems have represented the industry standard for filtering airborne particulate matter within clinical settings. These devices retain 99.7% of the particles larger than 0.3 mhi, which is effective for fungi such as C. albicans, with a diameter of 5 mhi for yeast and 2 mhi for hyphae, and for pathogenic bacteria such as S. aureus whose diameter is 0.5 to 1.5 mhi, E. coli, 0.5 mGh, A. baumanii, 0.9 to 1.6 mhi, P. aeruginosa, 0.5 to 1 mhi, Kocuria rizophila, 1 to 1.5 mhi (Kovács et al., 1999; Dworkin et al., 2006; Day et al., 2017; Lagree et al., 2018). However, HEPAs are difficult to clean and maintain when particulate matter has already accumulated in them (Day et al., 2017). In addition, the pore size is not regular and strongly depends on the disposition and size of the fibers of the polymer from which they are made (Balgis et al., 2017). These factors make HEPA systems more of a problem than a long-term solution. Given this, the design of an air filter with a regular pore size and a material with bactericidal action is of vital importance.

On the other hand, the bactericidal properties of numerous materials have been proven throughout history. Copper (Cu) has positioned itself as one of the fundamental metals worldwide and is present in the daily life of man in all areas. Cu-derived technology is very broad, due to the combination of its anticorrosive properties, electrical and thermal conductivity. Currently, in addition to its excellent anticorrosive properties, electrical and thermal conductivity, Cu possesses antimicrobial properties that other metals do not possess, which have generated increasing interest in exacerbating its applicability throughout the world.

In 2008, the U.S. Environmental Protection Agency (EPA) approved the registration of 275 Cu-base alloys, recognizing their antibacterial property, for use on solid contact surfaces with health applications, positioning copper as the first metal to be certified for this property. What is remarkable, according to clinical studies carried out in the United States and Chile, is that the action of copper is continuous, permanent and highly effective as an antimicrobial metal, capable of eliminating 99.99% of bacteria in a short period of time. This is attributed to the fact that copper is a natural inhibitor of the growth of bacteria, where microbes disappear or inhibit when they come into contact with this metal.

Cu is widely used, mainly in surface coating (Brenner et al., 2003; Marra et al., 011). This metal has been investigated on food-borne pathogenic bacteria, such as Salmonella typhimurium and Listeria monocytogenes; on Salmonella enteric, Campylobacter jejuni, E. coli 0157: H7, methicillin-resistant S. aureus (MRSA) and, recently, copper activity has been reported on microorganisms isolated from bovine mastitis, such as E. coli, coagulase negative Staphylococcus (CNS), S. Uberis and S. aureus (Mittal et al., 2002; Tang et al., 2009; Miaskiewicz-Peska et al., 2011; Salgado et al., 2013; Reyes et al., 2016). In another study, Cu reduced by 83% the bacteria deposited in components made from this metal (World health organization, WHO, 2009). Thus, it has been determined that Cu reduces the infection rate by 58% in patient rooms with components made of copper, compared to rooms with components made of plastic materials and stainless steel, among others. Until now, it has been reported that the bactericidal action of Cu occurs between 15 min and 60 min of contact (Bailo et al., 2016; Galani et al., 2016; González et al., 2017). The bactericidal effect of Cu on biofilms of pathogenic bacteria has also been recorded with up to 6 h of exposure (Singh et al., 2017). In addition, MRSA, one of the main pathogens of health care associated infections (HAI), disappears in less than 1 hour on a pure copper surface, while on stainless steel surfaces MRSA remains viable for over 72 h (Salgado et al., 2013). The studies exposed above, together with various antimicrobial applications such as self-disinfectant fabrics, in latex gloves, mattresses, bedding and paints with copper content for surfaces in the hospital environment and catheter surfaces, show that copper is an effective bactericidal material.

On the other hand, the bactericidal activity of magnetic materials is also known (Staffolani et al., 1991; Háfeli et al., 1997). However, few studies have been carried out in this regard and the advances made are recent. Thus, it has been found that the magnetic field decreased the growth of bacteria typical of the oral cavity bacterial plaque, such as Streptococcus parasanguinis, Staphylococcus epidermidis, Rhodococcus equi and the fungus C. albicans, with the most important effect in the latter species (Brkovic et al., 2014). Also, it has been reported that biofilms of P. aeruginosa are significantly suppressed by magnetic fields, which, in addition, enhance the effect of antibiotics such as ciprofloxacin (Bandara et al., 2015). The limitation of these studies was that they all tested conventional magnetic materials based on ferrite (Fe203), whose magnetic field consists of a remanence of 0.32 T and a low energy range between 2 to 80 KJ nr ³ . High-energy magnetic materials are those whose range exceeds 80 KJ nr ³ , among which samarium cobalt (SmCos) stands out with an energy of 170 KJ nr ³ and a remanence of 0.92 T (Ramanujan, 2009 ).

Studies have shown that magnetic fields on bacteria affect their cellular deterioration. In particular, SmCos was shown to have bactericidal activity on pathogens such as Escherichia coli and Staphylococcus aureus from 24 h to 48 h of exposure (Abdel-Kader et al., 2012). The suppressive effect of microorganisms of SmCos has been little investigated, being mainly tested in the manufacture of orthodontic implants. Thus, SmCos included in dental brackets has been shown to have a suppressive effect in vitro on bacteria typical of the oral microbiota and the fungus C. albicans, the latter being the most sensitive to the mentioned material (Staffolani et al., 1991). In addition, the use of combinations of 2 antibiotics is widely recommended to avoid the selection of bacteria resistant to these agents (Leekha et al., 2011; Paterson et al., 2016).

Some patents related to this technology are detailed below:

1.- Application WO 2018/199579 (Je Dong Hyun et al.) Discloses an antibacterial copper filter, which uses an antibacterial copper network, which is configured in such a way that the filter itself has an antibacterial and sterilizing function to eradicate a virus, a bacteria and a fungus of the filter, where the antibacterial copper member includes a support frame to maintain a shape and a metal mesh arranged within the frame support, and formed by a copper filament with a purity of 99% or more or a copper alloy with a copper content of 60% or more.

2.- Application US 2018/0085697 (Piry et al.) Mentions a filter medium with antimicrobial action and a cabin air filter to filter air inside vehicles, which has at least a first filter layer where they can be retain contaminants, and a second layer adjacent to the first layer containing antimicrobial agents, including antimicrobial copper metals or copper compounds.

3.- US Patent 9,561,458 B2 (Baek et al.) Protects an antibacterial filter that comprises a copper-based sulfur compound. The filter is easily processed, non-toxic, and has excellent antibacterial and deodorizing activity. This filter comprises a porous carrier that includes micropores formed therein and thus allows a fluid to pass through the pores.

4 - Application US 2012/0285459 (Sata et al.) Protects a cleaning and disinfection device that includes a copper foil filter of the air-permeable photocatalytic foil type.

5.- US Patent 5,840,245 (Coombs et al.) Protects air filtering products that contain an antimicrobial agent, which reduces the amount of microorganisms present in polluted air when it is passed through an air filter. Where the product is air porous fiberglass and where an organic silver, copper, gold or zinc antimicrobial compound is found.

6.- Applications WO 2019/173849 (Hong et al.) And US 2019/021704 (Shastry et al.) Disclose the successful manufacture of porous copper-nickel alloy foams, with potentially improved properties and with a broader applicability regarding of pure metallic foams. The processing of the foams is achieved through the reduction of oxide dust or sintering processes of the Cu-Ni solid solution powder, with open pore structures and varied in size and morphology. The foams of these solid solutions with different% Cu-Ni offer superior corrosion resistance compared to pure Cu or Ni foams. 7.- Application CN109778155 (Li Chanjuan) describes a method to prepare copper foam by electrodeposition in four stages, using a polyurethane foam structure that is introduced into a chemical solution of copper with complexing agents followed by a stabilizer. Then copper plating is done, where the foam material is dipped into a copper sulfate solution and then electrified. Finally, the sintering and reduction of the foam is carried out.

Based on these antecedents, there is still a need to develop new alternatives for air filtering in semi-closed places.

Brief description of the figures

Figure 1: SEM images of the morphology, roughness and particle size of the alloys made of Cu-Co-Fe, processed by mechanical grinding Figure 2: Images of the manufactured specimens, where (a) corresponds to the pure copper cylindrical specimen , (b) corresponds to the same specimen, but with a closer approach (50x), where the morphology of the pores is observed; Images (c), (d), (e) and (f) correspond to SEM images with high magnification where the basal and lateral surface of the Cu cylindrical specimen is shown, with the interconnected porosities and the surface junction between the particles of Cu.

Figure 3: X-ray diffraction images (DR-x) of the ground Cu-Co-Fe alloys, where the spectra identify the different solid phases formed and the% pp, their lattice parameter, crystalline structure Figure 4: Graphs of magnetism of Cu-Co-Fe alloys.

Figure 5: Graph of the pressure drop of a magnetic filter.

Figure 6: Design and construction of filter holders.

Figure 7: Assembly of the filtering system.

Figure 8: Graph on the bacterial count in ATS plates, under use of the prototype at low bacterial load in the waiting room of the Primary Health Center.

Figure 9: Graph on the count of bacteria in ATS plates, under use of the prototype at high bacterial load in the waiting room of the Primary Health Center.

Figure 10: Graph of the fungal count on Sabouraud agar plates, under use of the prototype, at low fungal load in the waiting room of the Primary Health Center.

Figure 11: Graph of the fungal count on Sabouraud agar plates, under use of the prototype, at high fungal load in the waiting room of the Primary Health Center.

Disclosure of the Invention

The present technology corresponds to an air filter made of copper-based magnetic alloys, useful for reducing microorganisms in suspension inside hospital rooms and other closed rooms with a high density of microbial load; in addition to the process of making said filter. Plus In particular, this air filter is made up of porous plates of copper-based magnetic alloys, preferably Cu-Co-Fe alloys, where their alloys (Co and Fe) can be found between the ranges 5 - 10% w / w of Co and 5 - 10% w / w Fe. Other magnetic alloys that can be used in porous plates are a Cu-SmCos mixture where the range can vary between 10-15% pp of SmCos. However, it must be taken into account that the commercial value of SmCo5 is much higher than that of Fe and Co and, therefore, increases the cost of porous plates. This air filter advantageously causes cellular deterioration and rapid death on different pathogens such as Escherichia coli, Staphylococcus aureus, Acinetobacter baumannii, Pseudomonas aerug inosa, Candida albicans and Kocuria rizophila.

Obtaining magnetic copper plates is achieved by reinforcing high purity copper powder with a dispersion of very fine particles of a magnetic nature. Where the Cu-Co-Fe or Cu-SmCos alloys are processed by powder metallurgy, through a mechanical grinding of a mixture of powders of the elements Cu, Co and Fe, or Cu and SmCos generating powders of the required composite alloy. This technique makes it possible to obtain stable alloys with a homogeneous composition.

Specifically, the air filter manufacturing process comprises the following stages: a. Obtaining magnetic alloys by mechanical grinding: a.1. Preparation of the mixture of powders of the elements Cu, Co and Fe: the Fe and Co powders must be weighed in the fixed proportion, between 5 - 10% w / w for a maximum content of 15% of these alloys and copper in powder between 90 - 95% w / w. This handling of powders must be carried out inside an oxygen-free chamber, which must be connected to an air extractor pump and argon gas with 99.9% purity, in order to extract air and inject pure argon into the air. minus 3 cycles of air-argon dump and 3 cycles of argon fill, to eliminate the likelihood of contamination and surface oxidation of powders. Next, the Cu-Fe-Co powders must be introduced into a cylindrical plastic container with ceramic balls, also under argon, the cylinder must be hermetically closed and placed in a bar mill at low speed (20 - 30 rpm ) for 15 to 30 min (depending on the volume of powder). This makes it possible to homogenize the mixture of powders and thus reduce the phenomena of chemical segregation of elements in the final consolidated product. a.2. Mechanical alloying process: in a planetary mill composed of martensitic stainless steel jugs of at least 500cc, balls of 20 mm diameter of martensitic stainless steel must be introduced in each jug, the homogenized powder mixture and a dispersing agent of the preferred type ethylene glycol or copolymer of phosphorus and carboxylic acid (PCA) (1-5% vol.), which is dispersed in drops on the balls and the powder mixture. The amount of drops added is according to the volume of Cu-Co-Fe powders that are mixed (for every 50 g of powder, 3% vol. Of drops are required). This mill must operate at a speed between 200 - 250 rpm for 5 - 20 h of grinding effective, with stopping and operating cycles, changing direction of rotation and with a load ratio between ball mass / powder mass equal to 10/1 or 20/1. Dispersants act as a controlling agent of the process to minimize agglomeration of the particles during grinding and thus reduce the cold welding between particles that occurs due to excessive plastic deformation of the ductile particles during the impacts of these between balls and balls with the wall. . These dispersants also act as a lubricant and coolant, and are easily removed in the first phase of sintering. Once the grinding is completed, the jars are removed from the mill containing the balls and the powdered alloy, which are introduced into an oxygen-free chamber, where a vacuum is made and pure argon is applied, repeating the filling / filling cycles. drained to avoid contamination of the oxygen-alloyed mixture, once opened. The powdered alloy content can be stored under vacuum in larger containers for long periods until it is used in the fabrication of the porous plates. b. Compaction of porous copper alloy plates:

The consolidation of the alloys is carried out through hot compaction under an inert atmosphere of pure argon, preferably in a hot press. A graphite matrix is used, where the internal cavity of this and the punch that compacts the magnetic alloy powder have the final diameter of the porous plates. To do this, the copper powders and the corresponding magnetic alloy must be mixed (generating a Cu base compound) with a preferred type spacer agent, NaCl (30 - 50% w / w) with a particle size between 100 to 400 microns. according to the density and size of the required pores, to ensure the efficient passage of air through the filter plate. The mixing of the powders is carried out for 10-20 min, to ensure homogeneity of mixing. 1% by weight of binder (ethanol) is proportionally added dropwise to the powder mixture, and then this mixture must be poured into a hot press graphite matrix. The mixtures are pre-consolidated at different temperatures that vary between 350 - 450 ° C, applying a variable load between 500 - 1000 kg, in an argon atmosphere to prevent the oxidation of the material. Once the samples have been extracted from the hot pressing process, they are subjected to cycles of removal of the spacer agent in hot distilled water at 60 ° C, until verifying by mass difference that all the spacer was removed. c. Compaction preform of porous plates:

It is carried out through conventional sintering at a temperature lower than or equal to 750 ° C for at least 30 minutes in an argon atmosphere, obtaining the porous permeable copper base plate with adequate mechanical resistance for the proposed application.

These porous plates with interconnected and distorted porosity, allow the air to circulate through all the irregular and interconnected caverns upon entering, extending the air path in a sufficient time for the bacteria to be in contact with surfaces for a longer period. made of magnetic copper. By contrast, a porosity of tubular morphology straight and smooth surface, it will not make it easier for bacteria to get trapped on that surface.

Finally, the efficiency of the air filtering system considerably reduces the death time of bacteria, higher than 50% on average compared to pure copper, which will depend on the type of bacteria. The remaining magnetism of the magnetic copper causes cellular deterioration and rapid death on different pathogens, such as Escherichia coli, Staphylococcus aureus, Acinetobacter baumannii, Pseudomonas aeruginosa and Candida albicans. In addition, the porous plates advantageously facilitate the removal and cleaning of the air filter, which is only limited to normal washing and drying, unlike commercial filters that need to be replaced every four months.

Application examples

Example 1. Air filter manufacturing process.

We first proceeded to obtain porous plates from 6 magnetic alloys, which are detailed below, with their respective grinding speed and time (mechanical grinding operation parameters):

A. Cu-5% Co-5% Fe with 156 rpm and 30 h of milling;

B. Cu-5% Co-5% Fe with 156 rpm and 30 h of milling;

C. Cu-10% Co-5% Fe with 250 rpm and 20 h of milling;

D. Cu-5% Co-5% Fe 250 rpm with 20 h of milling;

E. Cu-5% Co-10% Fe with 200 rpm and 20 h of milling;

F. Cu-15% SmCo5 With 180 rpm and 10 h of grinding.

The applied process was similar for all the proposed alloys and is described in detail below as an example for Alloy E: a. Mechanical grinding: a Planetary ball mill, RETSCH model PM400, was used, which allowed working with a maximum of 4 500cc jugs, simultaneously. Only 2 jars were used at a time per grind. Initially, in a plastic container, the powders of the elements Co, Fe and Cu were added (according to the% by weight of the alloy to be developed), which were previously weighed in a precision balance (3 digits after comma) arranged into an internal chamber of the so-called glove box, which was connected to an air extractor pump and argon gas with 99.9% purity, to extract the air and inject pure argon during 3 cycles of emptying air- argon and 3 argon fill cycles. Next, 13 martensitic stainless steel balls of 20 mm diameter were added per 500 cc grinding container / jar and the homogeneous mixture of Cu-10% Fe-5% Co powders (% by weight) from the mill was added. of bars and 2% by volume of ethylene glycol as dispersant / lubricant, which was dispersed in drops, on the balls and the powder mixture. The ball mill operated at a speed of 200 rpm for 20 h of grinding, with stops of 10 minutes every 20 min. grinding and turning change on every restart. A loading ratio between ball mass / powder mass equal to 10/1 was used. Once the grinding was finished, a vacuum was applied in the internal chamber and pure argon was added, repeating the filling / emptying cycles to avoid contamination of the mixture alloyed by oxygen, once opened. Figure 1 shows the SEM images of the morphology, roughness and particle size of the different alloys composed of Cu-Co-Fe and Cu-SmCos, processed by mechanical grinding. b. Compaction of porous copper and copper alloy plates: the consolidation of the alloys was carried out through hot compaction under an inert atmosphere of pure argon, in a press with a built-in furnace, which could work in vacuum, argon and other atmospheres , known as ΉOT PRESS ”or Hot Press. A graphite matrix was used, where the internal cavity of this and the punch that compacted the magnetic alloy powder would be the equivalent to the final diameter of the porous plates. For this, copper and magnetic alloy powders were mixed with 30% w / w NaCl as a spacer agent with a particle size between 100 to 400 microns according to the density and size of the required pores, to ensure efficient passage. of air through the filter plate. The mixing of the powders was carried out for 15 min, to ensure mixing homogeneity. 1% by weight of ethanol was added as a binder dropwise to the mixture, and then this mixture was poured into a graphite matrix from the hot press. The mixtures were pre-consolidated at different temperatures (350 - 450 ° C), applying a variable load between 500 - 1000 kg, in an argon atmosphere to prevent the oxidation of the material. Once the samples were extracted from the hot pressing process, they were subjected to cycles of removal of the spacer agent in hot distilled water at 60 ° C, until verifying by mass difference that all the spacer had been removed. c. Plate compaction preform: it was carried out through conventional sintering at a temperature of 700 ° C for 30 minutes in an atmosphere of extra-pure argon, obtaining a porous plate with a permeable copper base with mechanical resistance.

All the plates-specimens obtained had a diameter of 25.4 mm and a height of 16 mm, which are observed in Figure 2, where (a) corresponds to the pure copper cylindrical specimen and (b) to the same specimen , but with higher magnification (50x). The regular and homogeneous distribution of the pores can be appreciated after the elimination of the sodium chloride salt. In the same Figure 2, but with higher magnification (c, d, e and f), SEM images of the basal and lateral surface of the Cu cylindrical specimen are shown, with interconnected porosities. In particular, Figure 2 (e) shows the surface bond between the Cu particles. Example 2. Evaluation of the properties of air filters.

For the determination of the properties of the alloyed materials in powder elaborated in Example 1, the following tests were carried out:

2.1.- X-ray diffraction (DRx): this test was carried out in order to evaluate the solid phases precipitated and / or generated during mechanical grinding. Specifically, it allowed to evaluate the crystalline structure, the lattice parameters and the% w / w of each phase present in each alloy. The test consisted in placing alloy powder processed by mechanical alloying in a cylindrical sample holder with a diameter of 2 cm and a height of 3 mm. The powdered alloy had to be of fairly homogeneous particle size and morphology. If it was very granular or heterogeneous, it had to be ground in a mortar until obtaining a homogeneous particle size. This sample holder was introduced in a Bruker D4 Endeavor Diffraction equipment, using Cu-Ka radiation under 40kV and 20 mA in a range of 5 - 90 °. The structural properties were studied at the CENIM in Madrid (Spain) by means of Rietveld refinement, obtaining the crystalline structure, network parameters and% by weight of each solid phase. The DR-x patterns with their respective refinements are presented in Figure 3, where specifically, the spectra correspond to the powdered alloys of: Cu-10% Co with 200 rpm and 30 h of grinding (A); Cu-5% Co-5% Fe 156 rpm and 30 h of milling (B); Cu-10% Co-5% Fe with 250 rpm and 20 h of milling (C); Cu-5% Co-5% Fe 200 rpm with 20 h of milling (D); Cu-5% Co-10% Fe with 200 rpm and 20 h of milling (E); and Cu-15% SmCos with 180 rpm and 10 h of milling (F). The DR-x results of the 6 alloys are summarized in Table 1.

Table 1 . X-Ray Diffraction Results for Different Processing Variables

The DR-x results obtained from the 6 alloys developed, where the% pp of Co and Fe alloys varied, in addition to grinding speed and time, indicate: alloy B was very heterogeneous and oxidized forming cuprite (CU2O). The best powdered alloys processed were C, D, E and F, where Co and Fe were dissolved in copper in solid solution; and in particular alloy D, evidenced an FeCo magnetic phase, which increased the remaining magnetism. In addition, alloys C, D and E were cheaper compared to F (SmCo5) and required only 20 hours of grinding compared to A and B, which required 30 hours.

2.2.- Scanning Electron Microscopy (SEM) and Microanalysis with Energy Dispersed Spectrometry (EDS): these tests were carried out in order to evaluate the morphology and particle size of the powdered alloys. With the EDS, a compositional microanalysis was performed on the alloy particles; where the distribution and% of the elements in different areas were evaluated. The average results of the point and area microanalyses performed on the 6 alloys, showed relative chemical homogeneity of Fe, Co and also of SmCos (sample F) between the different zones of each sample analyzed with EDS, with minor variations depending on the area or selected points.

Figure 2 shows the SEM images of the morphology and particle size of the composite alloys processed by mechanical grinding: Cu-5% Co-5% Fe produced under 156 rpm and 30 h of grinding- 500x (A); Cu-5% Co-5% Fe with 156 rpm and 30 h of milling - 500x (B); Cu-10% Co-5% Fe with 250 rpm and 20 h of grinding - 500x (C); Cu-5% Co-5% Fe 250 rpm with 20 h of grinding - 500x (D); Cu-5% Co-10% Fe with 200 rpm and 20 h of grinding - 500x (E); Cu-15% SmCo5 with 180 rpm and 10 h of grinding - 500x (F). It is deduced from these results that the size and morphology of the particles obtained depends on the amount% w / w of alloys (Fe, Co and others), on the time and grinding speed.

2.3.- Magnetism: this test allowed to evaluate the magnetic properties of the powdered alloys.

The magnetic measurement test was performed with a 1 TESLA Vibrating Sample Magnetometer (VSM). This test allowed to evaluate and know the magnetic behavior of the magnetic copper-based alloys. The Hysteresis curve that was obtained for each alloy, and allowed to deduce the Coercive Field and the remanent magnetism, in addition to the saturation magnetism. To know and verify the efficiency of the magnetic alloy, it is important that the remaining magnetism (remains) in the porous plate, is the magnetism that breaks the shell of the bacteria and causes their death. Figure 4 shows the results obtained for the magnetic determination carried out on the Cu-10% Co-5% Fe alloys with 250 rpm and 20 h of grinding - 500x (C); Cu-5% Co-5% Fe 200 rpm with 20 h of grinding - 500x (D); and Cu-5% Co-10% Fe with 200 rpm and 20 h of grinding - 500x (E). Table 2 shows the Coercive Field (He) and remanent magnetism (Br) values obtained for each alloy, according to its chemical composition and grinding parameters. The results indicate that all alloys have adequate remanent magnetism to kill bacteria. The alloys E and F exhibited greater remanent magnetism and eliminated bacteria in less time. It should be considered that alloy E and D are more economical to process.

Table 2. Results of magnetic properties

2.4.- Atomic Absorption Spectrometry: this test was carried out to evaluate the contamination of the alloys by the presence of iron (Fe) and chromium (Cr), such as wear of the stainless steel of the balls and walls of the jar during grinding. The results indicated that there was no contamination by chromium (Cr) and the contamination by iron (Fe) was of the order of 0.7 - 1.0% w / w in the different alloys, attributable to the wear of the walls of the jugs and of the the balls.

Example 3. Evaluation of a prototype for the validation of the air filter.

3.1.- Construction of the air filter prototype.

To validate the filters developed in Example 1, a prototype was designed, sized and built to eliminate microorganisms present in the air. In particular, it was validated for copper filters with a diameter of 25 mm and a thickness of 5 mm. The system was sized to have an overpressure upstream of the filters of 250 to 300 Pa.

A commercial fan was adapted to the restrictions imposed by the bacteria filter, for which a bypass was incorporated in order to determine the properties of the filter to be tested through its pressure drop and the air flow that circulated through the he. This prototype was able to filter a stream of air from 4.22 to 4.77 m ³ / h. The design of the air filtering system was composed of a filter for particles and dust; a fan; then a bacteria filter and finally a bacteria filter air discharge zone. Where the particle and dust filter was located at the fan inlet to protect the magnetic filters from bacteria from the entry of larger particles that could cover them, and thus increase their pressure drop. In addition, the pressure drop graphs of each filter, which are shown in Figure 5, were considered for the design.

Finally, for the prototype, the use of six magnetic filters 25 mm in diameter and 5 mm thick was considered. These filters were installed on a copper plate in order not to have different materials that generated with use (humidity) galvanic cell corrosion in the assembly area, and to make the system more efficient, according to the arrangement shown in Figure 6. Once all the variables of the filtering system had been dimensioned, we proceeded to assemble each of the pieces that made up the design, as can be seen in Figure 7.

3.2. Evaluation of the air filter prototype in a Medical Care Facility.

The prototype of the magnetic filters was evaluated in a health care facility for the determination of the microbial load (bacteria and fungi) of the air. Specifically, for the collection of samples, the Medical Center for Family Health (CECOSF) Centinela, commune of Talcahuano, Bío-Bío Region, Chile was considered. This enclosure has an area of 258 m ² and is intended for 5,000 users per month, from the nearby area.

First, the air in the waiting room of the health facility was filtered, where the sampling was carried out actively (by impact on the plate), using a MAS-100 NT air sampler (Merck, Germany, no. series: 15986), equipment recommended by the Chilean Public Health Institute for microbiological air sampling. To proceed with the air sampling, this equipment was placed in the center of the room, on a table at a height of 1 m to simulate the breathing zone and was programmed to collect 250 L of air at a flow of 100 L min ¹ (Valenzuela 2011; Cabo Verde et al., 2015). The filtered air impacted on plates containing 20 ml of trypticase-soy agar (culture medium for bacteria) and Sabouraud agar (culture medium for fungi).

To study the effect of the prototype air filter based on magnetized copper, it was turned on and allowed to operate for 2 - 3 h; after this time, air samples were taken in the same way as described above. It must be taken into consideration that the MAS-100 NT equipment was at the same height as described above and, in this case, next to the air outlet of the magnetic filter prototype. This experiment was repeated 3 times, and the sampling with each type of culture medium was carried out in triplicate. 3.2.1.- Microorganism count.

The colonies obtained were counted in the solid culture medium plates described in the previous point. Given that the septum of the MAS-100 NT had 300 holes and through them more than one microorganism could enter and impact the plate with solid medium, the counts had to be corrected according to the number of perforations, according to the Feller expression detailed below. continuation (Valenzuela 2011): S -í- 0.5

Pr = 1 n ¿V - r 4- Q, s! (1)

Where Pr is the probable (calculated) number of bacterial colonies on the plate of the sampling instrument, expressed in colony forming units (CFU); res is the observed plate colony count expressed in CFU; and N is the total number of holes in the lid through which the air is received from the area being sampled towards the Petri dish with the solid culture medium (In this case N = 300).

The calculated count (Pr) was given for every 250 L of air sample. Finally, the count calculated for each cubic meter, expressed in CFU nr ^{3, was reported} (Nunes et al., 2005; Yao and Mainelis, 2007; Van Droogenbroeck et al., 2009; Chang and Chou, 2011; Valenzuela 2011; Aguiar et al. ., 2014; Cape Verde et al., 2015; Chang et al., 2015; Stauning et al., 2018). The count data was plotted chronologically.

A low load of microorganisms was considered, when the initial count was under 180 CFU nr ³ according to the British HIS recommendation, which is the most restrictive standard worldwide, and a high load of microorganisms was assumed when the initial count was greater than 1,000 CFU nr ³ according to the Taiwanese standard, which is the least restrictive in the world (Chang et al., 2015; Stauning et al., 2018).

3.2.2.- Statistical analysis of the microbial load of the air in the waiting room of the CECOSF Centinela.

The data obtained at each time, under conditions without filtration and with air filtration through the prototype of an air filter based on magnetized copper, were compared and analyzed using the Student t test (Graph Pad, USA, 2016).

A sustained and significant decrease in the bacterial count was observed after 2 and 3 h after the prototype of the air filter based on magnetized copper was switched on (P <0.05). On the other hand, in relation to the mushroom count, no significant differences were observed with and without the use of the prototype air filter based on magnetized copper (P> 0.05).

On the other hand, a higher load of microorganisms was observed at higher relative humidity both internally and externally, these differences being statistically significant (P <0.05). However, the temperatures are not showed significant differences at low or high microorganism load (P> 0.05). Similarly, no significant difference was observed between the number of people with a low or high load of microorganisms (P> 0.05). 3.2.3.- Bacterial and fungal count in the Health Center room.

The results of the bacterial and fungal count in the CECOSF Sentinel waiting room were expressed in the graphs for low and high bacterial load (Figures 8 and 9, respectively) and for high and low fungal load (Figures 10 and 11, respectively). .

The prototype magnetized Cu air filter significantly decreased the bacterial count of the air in the waiting room at the health center, both at low bacterial load (Figure 9) and at high load (Figure 10). This bactericidal effect coincided with that described in the literature regarding Cu, magnetic materials and their combination. However, this is the first study in which its effectiveness as an air filter has been proven (Souli et al., 2013; Zeiger et al., 2014; Rózahska et al., 2018; Rubín et al., 2018; Zhang et al., 2018). These results strongly suggest that the prototype air filter based on Cu-magnetic alloy is suitable for production.

Claims

1.- An air filter made of copper-based magnetic alloys, useful for reducing microorganisms in suspension inside hospital rooms and other closed rooms with a high density of microbial load, CHARACTERIZED because it comprises porous plates of copper-based magnetic alloys of the type Cu- Co-Fe, where its alloys Co and Fe are present between 5-10% w / w Co and 5-10% w / w Fe; or Cu-SmCos, where the alloying SmCos varies between 10 - 15% pp.

2.- A process for the elaboration of the air filter of magnetic alloys according to claim 1, CHARACTERIZED because it comprises at least the following stages: a.- Obtaining the magnetic alloys by means of mechanical grinding: a.1 Preparation of the mixture of powders of elements Cu, Co and Fe: Fe and Co powders between 5 - 10% pp must be weighed for a maximum content of 15% of these alloys and copper powder between 90 - 95% pp, inside a chamber free of oxygen, which must be connected to an air extraction pump and argon gas with 99.9% purity, to extract air and inject pure argon in at least 3 air-argon emptying cycles and 3 argon filling cycles, to eliminate the probability of contamination and surface oxidation of powders; Next, the Cu-Fe-Co powders must be introduced into a cylindrical container with ceramic balls, also under argon, which must be hermetically sealed and then placed in a bar mill at a speed between 20 - 30 rpm during 15 to 30 min, to homogenize the powder mixture and reduce the phenomena of chemical segregation of elements in the final consolidated product; a.2 Mechanical alloying process: in the jars of a planetary mill, martensitic stainless steel balls with a diameter of at least 20 mm must be placed, in addition to the homogenized powder mixture in stage a.1 and a dispersing agent between 1 - 5% volume, which is added dropwise on the balls and the powder mixture; and where the mill must operate at a speed between 200 - 250 rpm during 5 - 20 h of effective grinding, with stopping and operating cycles, changing direction of rotation and with a load ratio between ball mass / powder mass equal to 10/1 or 20/1; Once the grinding is completed, the jars should be removed from the mill containing the balls and the powdered alloy, and they are introduced into an oxygen-free chamber, where a vacuum is made and pure argon is applied, repeating the filling / filling cycles. emptying to avoid contamination of the mixture alloyed by oxygen, once opened; and where the powdered alloy content is stored under vacuum in containers. b. Compaction of porous copper alloy plates: the consolidation of the alloys is carried out through hot compaction under an inert atmosphere of pure argon in a hot press, where a graphite matrix is used, where the internal cavity of this and the punch that compacts the magnetic alloy powder possess the final diameter of the porous plates; To do this, the copper powders and the corresponding magnetic alloy must be mixed with 30 - 50% w / w of a spacer agent with a particle size between 100 to 400 microns according to the density and size of the required pores, to ensure efficient passage of air through the filter plate; where the mixing of the powders is carried out for 10-20 min to ensure homogeneity of mixing; then 1% by weight of binder must be added in the form of drops to the mixture and then the mixture must be poured into a hot press graphite matrix, where the mixtures are pre-consolidated at different temperatures between 350 - 450 ° C, applying a variable load between 500 - 1000 kg, in an argon atmosphere to prevent the oxidation of the material; Once the samples have been extracted from the hot pressing process, they must be subjected to cycles of removal of the spacer agent in hot distilled water at 60 ° C, until verifying by mass difference that all the spacer was removed. c. Compaction preform of porous plates: it is carried out through conventional sintering at a temperature lower than or equal to 750 ° C for at least minus 30 minutes in an argon atmosphere, obtaining the porous permeable copper base plate.

3. A process for the elaboration of the air filter of magnetic alloys according to claim 2, CHARACTERIZED because the dispersing agent of stage a.2 is ethylene glycol.

4 A process for the manufacture of the air filter of magnetic alloys according to claim 2, CHARACTERIZED in that the dispersing agent of stage a.2 is a copolymer of phosphorus and carboxylic acid, PCA.

5.- A process for the elaboration of the air filter of magnetic alloys according to claim 2, CHARACTERIZED in that the dispersing agent is added 3% vol. drops per 50g of powder.

6.- A process for the elaboration of the air filter of magnetic alloys according to claim 2, CHARACTERIZED in that the spacer agent of stage b is NaCI.

7.- A process for the elaboration of the air filter of magnetic alloys according to claim 2, CHARACTERIZED because the binder of stage b is ethanol.