Method for forming foamy slag in an electric arc furnace

Abstract

The present invention relates to a method for forming foamy slag in an electric arc melting furnace during the production of iron-based alloys, comprising the steps of: a. melting a metal charge in said electric arc melting furnace to obtain a molten metal bath comprising a floating slag layer; and b. introducing a foamy slag forming agent into said furnace to foam said floating slag, wherein said foamy slag forming agent is a granular composite material comprising at least one thermoplastic polymeric material and at least one bio-based carbonaceous material.

Description

[Technical field]

[0001] The present invention relates to a method for forming foamy slag in an electric arc furnace. In particular, the method according to the invention makes it possible to obtain foamy slag while reducing the environmental impact. [Background technology]

[0002] One of the main technologies for producing iron-based alloys, especially steels, is the Electric Arc Furnace (EAF) technology, which uses metal charges such as iron scrap from a wide variety of steels that have reached the end of their life cycle, and / or other metallic materials, e.g., DRI (Direct Reduced Iron), HBI (Hot Formed Reduced Iron), cast iron and ferroalloys, and possibly other metallic materials (ores or metal oxides) as raw materials for the production of new iron-based alloy products.

[0003] In an electric arc furnace, the metal charge is melted inside a crucible by heat generated by an electric arc that sparks between the metal charge and one or more graphite electrodes located near the charge. According to another technique, the metal charge is heated and then continuously fed into the crucible of the electric arc furnace where it melts as a result of both contact with the molten metal bath and the electric arc.

[0004] Once melting is complete, the molten metal bath is refined in the crucible to reach the desired chemical composition and finally tapped from the crucible into a ladle to begin further processing to obtain the finished product.

[0005] To facilitate the melting process, oxygen and other fuels, such as fossil coal and / or coke, are usually introduced into the furnace to provide chemical energy to the system and reduce the large electricity consumption of the furnace. Anthracite and coke are either added in coarse size to the scrap metal charge to be melted or injected in finer size through a peripheral injection system that is often installed in electric arc furnaces. On the other hand, gaseous oxygen is blown into the molten metal bath to facilitate the dephosphorization and decarburization of the metal bath. In fact, gaseous oxygen reacts with the elements present, in particular iron, aluminum, silicon, manganese and phosphorus, to produce the corresponding oxides, which migrate towards the surface of the bath and form a floating slag layer there. In addition to trapping undesirable elements in the iron-based alloy, the slag also causes foaming, increases the energy efficiency of the process, limits electrode consumption and protects the furnace refractories and panels, which are cooled by forced water circulation, from the direct radiation of the electric arc. In addition, the foamy slag prevents the risk of nitrogen generated by the interaction of the electric arc with the air being introduced into the molten metal bath, and also reduces the nuisance noise generated by the arcing between the electrode and the metal bath.

[0006] Foaming of the slag is achieved by entraining gas in the slag, increasing its apparent volume. The gas is generated in situ by injecting a foaming slag former, e.g., fossil coal or coke, into the slag or into the molten metal bath near the surface where it contacts the slag. The iron oxide, particularly FeO, produced as a result of the gaseous oxygen injection reacts with the carbon of the fossil coal or coke to produce liquid metallic iron and gaseous carbon monoxide, which forms the foaming slag, thus recovering metallic iron that would otherwise escape the furnace in oxide form with the slag. The foaming slag former is injected in the form of a fine powder through one or more lances that use a gaseous fluid, usually compressed air, as a medium for the foaming slag former.

[0007] A major limitation of the slag foaming technique, and more generally of the production of ferrous alloys in electric arc furnaces, is the environmental impact resulting from the use of fossil raw materials such as coal and coke, which emit large amounts of carbon dioxide into the atmosphere.

[0008] In order to limit the environmental burden of carbon dioxide emissions, it is known in the state of the art to use polymeric materials obtained from waste recovery, such as plastics and rubber, both as fuels and foaming slag formers, as partial or complete replacements of coal and coke. However, although the use of these materials has the advantage of saving waste and scrap from industrial processes and consumer goods, it only provides limited improvement in the overall balance of carbon dioxide and other climate change gas emissions from ferrous alloy manufacturing processes.

[0009] In the state of the art, it is also known to use, for similar purposes, materials of biological origin, such as charcoal or other products obtained by pyrolysis or gasification of biomass (collectively called "char" or "biochar" when obtained from biological materials and sourced and processed in an environmentally sustainable manner), at least partially to replace materials of fossil origin. Biochar derived from renewable resources actually improves the overall emission balance of the iron-based alloy production process in the EAF, due to its carbon dioxide emission neutrality (i.e. carbon neutrality) based on the fact that biochar is of biological origin and therefore has zero net emissions of climate change gases overall when obtained from the sustainable exploitation of biomass.

[0010] However, biochar has several drawbacks when used as a foaming slag former. First, its effectiveness is less than that of fossil coal and coke. This is because biochar has a limited ability to penetrate and disperse in slag and molten metal baths due to its relatively low density. Biochar also has limited surface wettability by slag and molten metal, making it less reactive than materials of fossil origin. Furthermore, biochar has low mechanical compactness and can break down into fine powders, which can cause clogging problems in the pneumatic conveying system that removes the material from storage and transports it to the lances installed near the furnace. Furthermore, biochar has a low ability to penetrate slag, and its low density, combined with its poor reactivity, makes it easy for it to become mixed into the fumes present in the furnace before it can react with the slag and molten metal baths. Biochar also has a tendency to break down during handling and storage, also due to its low mechanical compactness, forming additional fine light powders that can easily spread in the working environment, resulting in safety issues for operators. Finally, biochar is a hygroscopic material and therefore tends to absorb moisture from the atmosphere, which requires appropriate storage measures throughout the supply chain, since excessive amounts of moisture should be avoided in the furnace for reasons of energy efficiency, plant safety and to avoid introducing hydrogen into the metal bath.

[0011] The use of materials from waste recovery and carbon sources replacing fossil-derived carbon in metallurgical processes in EAF furnaces (electric arc furnaces) is described, for example, in US Pat. No. 8,021,458, which describes a method for foaming slag in an electric arc furnace, using a carbon-containing polymer, possibly in the form of a physical mixture with a second carbon source (e.g. graphite or coke), as a foaming slag former. In US Pat. No. 8,021,458, the effectiveness of the aforementioned physical mixture was tested in the laboratory by reacting the two components in a dripping tube furnace and analyzing the resulting carbonaceous residue. The interaction of the residue with the slag was evaluated by contacting a sample of the aforementioned mechanically pressed residue with the slag at the melting temperature of the slag.

[0012] US 2011 / 0239822 describes a method for producing iron-based alloys in an EAF. A physical mixture of a carbon-containing polymer (e.g., recycled tire rubber) is used together with a second carbon source (e.g., coke). The physical mixture of the two materials is injected into the furnace both as a supplemental fuel and as a foaming slag former.

[0013] US Patent No. 5,554,207 describes the use of a water-insoluble thermoplastic polymer in combination with fine metal particulate matter in a converter steel or EAF manufacturing process. The thermoplastic polymer is preferably a polymer derived from the recovery of post-consumer waste, while the metal particulate matter is obtained by filtration of the combustion fumes of a melting furnace. The two materials are mixed together under heat, for example in an extruder, to form an agglomerated product in which the thermoplastic polymer acts as a binder for the metal particles. The agglomerated product is added to a post-consumer scrap iron charge and then used as a medium for recovering the valuable metals in the melting furnace and utilizing the thermoplastic material as fuel.

[0014] WO 2012 / 019216 describes the use of a composite product comprising a thermoplastic material and a carbon-containing material in high temperature processes, for example EAF furnace processes. As an alternative to or in addition to the carbon-containing material, the composite product may also comprise a metal-containing material. In an embodiment, the composite material is prepared by extrusion in the form of a block of relatively high mass, of the order of about 3 kg. The block can be used in the steelmaking process as a supplementary fuel in addition to the scrap charge. Alternatively, the composite product can be used as a building material or a protective material.

[0015] Irshad Mansuri et al. analyzed the potential for recycling waste plastics such as compact discs (polycarbonate), carbon fiber reinforced polymers and bakelite in EAF furnaces in the paper "Recycling Carbonaceous Industrial / Commercial Waste as a Carbon Resource in Iron and Steelmaking," Steel Research Int. Vol. 87 (2016) No. 9999 (DOI: 10.1002 / srin.201600333). The paper mentions the use of a general composite containing carbon from biochar instead of traditional fossil carbon sources, but does not specify the exact composition of the composite.

[0016] Irshad Mansouri et al. cited in "Biomass as a Source of Renewable Carbon for Iron and Steelmaking" by Terry Norgate et al., ISIJ International, Vol. 52 (2012), No. 8, pp. 1472-1481, describes the use of direct reduced composites formed from iron ore and biomass as blast furnace feedstock in an integrated cycle process. It also describes the use of biomass as a replacement for fossil carbon sources in the foaming process of slag in EAF furnaces. Summary of the Invention [Problem to be solved by the invention]

[0017] In light of the above state of the art, the Applicant has set out to overcome one or more of the above drawbacks affecting the known methods of foaming slag in an electric arc furnace. In particular, the Applicant has set out to provide a method for effectively producing foamed slag and at the same time reducing the environmental impact. A further object is to provide a method for producing foamed slag that is easier to achieve than the methods of the prior art and that makes it possible in particular to overcome the drawbacks of the prior art related to the use of biochar as a foaming slag forming agent. [Means for solving the problem]

[0018] Applicant has now found that these and other objects, which will be explained in more detail in the following description, can be achieved by a method of forming a foaming slag in an EAF furnace during the manufacturing process of an iron-based alloy, in which the foaming of the slag is carried out by injection of a granular composite material comprising a thermoplastic polymer and a carbonaceous material of biogenic origin, the thermoplastic polymer preferably being obtained from the recovery of post-consumer or post-industrial waste and products of plastic materials. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 shows the results of thermogravimetric analysis of polymer waste containing LDPE as the main component. [Diagram 2] FIG. 2 shows the results of thermogravimetric analysis of biochar produced by gasification. [Diagram 3] FIG. 3 shows the results of a thermogravimetric analysis of a composite material described herein that includes the polymeric material of FIG. 1 and the biochar of FIG. 2 in a 40:60 mass ratio on a dry basis. [Figure 4] FIG. 4 shows the results of thermogravimetric analysis of polymer waste containing LDPE and HDPE as main components. [Diagram 5] FIG. 5 shows the results of thermogravimetric analysis of biochar produced by pyrolysis. [Figure 6] FIG. 6 shows the results of a thermogravimetric analysis of a composite material described herein that includes the polymeric material of FIG. 4 and the biochar of FIG. 5 in a 45:55 mass ratio on a dry basis. [Figure 7] FIG. 7 shows the results of thermogravimetric analysis of biochar produced by torrefaction. [Figure 8] FIG. 8 illustrates the results of a thermogravimetric analysis of a composite material described herein that includes the polymeric material of FIG. 4 and the biochar of FIG. 7 in a 50:50 mass ratio on a dry basis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] It has been observed that the aforementioned composite materials, due to their relatively high granular density, can be more easily injected into the furnace than mixed injection materials of the individual components or their physical mixtures and therefore can penetrate deeper into the slag and / or molten metal bath, resulting in an improved effectiveness of the slag foaming action.

[0021] Additionally, the granular composite material is less likely to become entrained in the combustion fume stream drawn into the furnace recovery system than if its components were used individually or in loosely agglomerated form.

[0022] The above-mentioned granular composite material also allows the introduction of materials with high carbon and fixed carbon (char) content into the EAF furnace at the same time as materials with high volatile and hydrogen content (polymeric materials, e.g. polyolefin-based polymeric materials), which promotes reactivity towards the slag, due to both the intense mass exchange caused by the volatile fraction and the high reactivity of hydrogen, and the formation of small gas bubbles that have a stabilizing effect on the structure of the foaming slag. Also, the two materials (char and polymer) are in direct contact with each other as a result of weak cohesion, which promotes chemical interaction. This direct contact also promotes the decomposition of charred hydrogen (which arises from the scission of the polymer chains) due to the catalytic effect of the char, resulting in the production of solid carbon. In this way, the solid carbon can be deposited on the surface of the char itself, increasing the surface roughness of the char and therefore its wettability compared to the slag and liquid metal. This also overcomes the problems associated with the poor wettability of biochar and therefore its poor reactivity with the slag.

[0023] Furthermore, the use of thermoplastic materials and bio-based carbonaceous materials in granular aggregate form allows the utilization of the high surface area and high porosity characteristic of bio-based carbonaceous materials, facilitating the gasification reactions that occur at the solid-gas interface. In fact, the current state of the art is unable to utilize the porosity of bio-based materials effectively due to their low density, and therefore some of the problems that occur in the furnace are entirely dependent on this porosity.

[0024] The use of granular composites allows the control and optimization of the surface / particle volume ratio, influencing the oxidation and volatilization kinetics of the material during the injection process into the furnace and the reaction process in the slag by acting on the heat exchange and reaction surfaces.

[0025] Therefore, the improved effectiveness of composite materials in the slag foaming process can reduce the environmental impact of the production process of iron-based alloys in electric arc furnaces, effectively reducing the emissions of climate change gases, especially carbon dioxide from fossil carbon sources, as well as the consumption of raw materials and energy.

[0026] Additionally, the dense nature, low moisture absorption, and granular morphology of the composite material allows the material to be transported and stored without significant diffuse emissions of particulate matter into the working environment, and limits the risk of moisture uptake during storage.

[0027] Furthermore, for example, by hot extrusion of the thermoplastic and bio-based carbonaceous materials, the composite materials can be prepared into granules of various shapes and a wide size range, which can be easily prepared into optimal granule sizes for injection into furnaces using equipment commonly used to inject fossil coal or biochar, and the improved mechanical compactness also avoids the clogging problems of the equipment and pneumatic conveying systems mentioned above associated with the fineness of the powder of the materials.

[0028] Thus, according to a first aspect, the present invention provides a method for forming foamy slag in an electric arc melting furnace during the production of an iron-based alloy, comprising the steps of: a. melting a metal charge in said electric arc furnace to obtain a molten metal bath containing a floating slag layer; b. introducing a foaming slag forming agent into the furnace to foam the floating slag; wherein the agent is a granular composite material comprising at least one thermoplastic polymeric material and at least one bio-based carbonaceous material.

[0029] In accordance with the present invention, the foaming slag forming agent is a granular composite material comprising at least one thermoplastic polymeric material and at least one bio-based carbonaceous material.

[0030] For purposes of this specification and the appended claims, the term "composite material" means a weakly cohesive product comprising at least one thermoplastic polymeric material and at least one bio-based carbonaceous material, where the thermoplastic polymeric material acts as a binder for the bio-based carbonaceous material.

[0031] The thermoplastic polymeric material may be any polymeric material that is solid at room temperature, preferably substantially free of halogens (especially fluorine and chlorine) and suitable to act as a binder for the bio-based carbonaceous material to form a dense granular composite material. For this purpose, the polymeric material should be convertible into a fluid polymeric phase by heating, for example at a temperature in the range of 100°C to 300°C, preferably in the range of 150°C to 250°C.

[0032] Preferably, the thermoplastic polymer material comprises a polyolefin-based polymer. Preferably, the thermoplastic polymer material includes polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and mixtures thereof. The polyethylene can be either low density polyethylene (LDPE) or high density polyethylene (HDPE).

[0033] The thermoplastic polymeric material is preferably obtained from recycled polymeric materials, i.e. from the recovery of waste that has reached the end of its life cycle comprising thermoplastic polymeric materials (so-called post-consumer recycled products) or waste from a polymeric material manufacturing process (so-called post-industrial recycled products). Preferably, the polymeric material is a material that is at least partially obtained from renewable resources, such as bioplastics.

[0034] Examples of post-consumer recycled products from which polymeric materials suitable for the purposes of the present invention can be obtained are products obtained from separated collections of municipal waste (e.g. food films and packaging, jars, bottles, containers, etc.) or agricultural film waste and scrap. Examples of post-industrial recycled products are waste products from the manufacturing processes of the above mentioned products. These products are usually subjected to one or more pre-treatments such as sorting, washing, shredding, sieving, pressing and extrusion before being used in the metallurgical production cycle.

[0035] In one embodiment, the thermoplastic polymer material is part of the material remaining at the end of the processing and sorting process of plastics resulting from separate collection of municipal waste, this part also known as Plasmix.

[0036] The use of Plasmix for the purposes of the present invention is particularly advantageous due to its high availability and the fact that, in the state of the art, Plasmix is ​​primarily intended for energy recovery by incineration and landfill disposal.

[0037] The thermoplastic polymer materials used to prepare the granular composites are usually in the form of flakes, powders or granules of highly variable shapes, with maximum sizes ranging from 0.3 mm to 40 mm.

[0038] The carbon content of the thermoplastic polymer material is preferably 50% by weight or more, more preferably 65% ​​by weight or more, based on the weight of the thermoplastic polymer material. Preferably, the carbon content is in the range of 50% to 90%, more preferably 70% to 90%, based on the weight of the thermoplastic polymer material.

[0039] The hydrogen content of the thermoplastic polymer material is preferably 5% by weight or more, more preferably 10% by weight or more, based on the weight of the thermoplastic polymer material. Preferably, the hydrogen content is in the range of 5% to 15% by weight of the thermoplastic polymer material.

[0040] Thermoplastic polymer materials, especially those obtained from waste recovery, may contain impurities, such as metallic elements (e.g. aluminum), dyes, pigments and other additives, or impurities formed from materials of other nature (e.g. sand), that are commonly used in the manufacture of polymer materials.

[0041] The amount of thermoplastic polymer material present in the composite can vary over a wide range and can be determined based on the needs for use in the iron-based alloy manufacturing process. Preferably, the thermoplastic polymer material is present in an amount in the range of 10% to 90% by weight, more preferably in the range of 30% to 70% by weight of the composite.

[0042] For the purposes of the present invention, a biogenic carbonaceous material (hereinafter also referred to as "carbonaceous material") is an organic carbon-containing material produced from a living organism, whether animal or plant. Preferably, the carbonaceous material is an organic material of plant origin. More preferably, the carbonaceous material is a char. Char is a product obtained by thermochemical conversion of biomass in the absence of oxygen, for example by pyrolysis, torrefaction, steam explosion, gasification or hydrothermal carbonization processes. The treatment of biomass with these thermochemical conversions makes it possible to obtain products with a higher carbon content, in particular a higher fixed carbon content, and a higher calorific value than untreated biomass. Preferably, the biogenic carbonaceous material is a "biochar", i.e. a char produced by a process considered to be environmentally sustainable, including the use of waste from the processing of biomass, for example obtained from the proper management of forest resources.

[0043] The carbon content of the biological carbonaceous material is preferably 50% by weight or more, preferably 60% by weight or more, and more preferably 75% by weight or more, based on the weight of the carbonaceous material. Preferably, the carbon content is within the range of 50% to 95%, more preferably 60% to 95%, and even more preferably 75% to 90% based on the weight of the carbonaceous material.

[0044] The other elements present in the char are primarily hydrogen, oxygen and sulfur.

[0045] According to a preferred embodiment, the chemical composition of the char is as follows (weight percentages on a dry basis, referred to as char weight): 75%~90% carbon, 0.5% to 4% hydrogen, Ash content: 2% to 8% 5% to 15% oxygen, 0%-3% sulfur.

[0046] An advantageous feature of char is its relatively low ash content compared to coal and coke of fossil origin. Indeed, ash can hinder the oxide reduction mechanism, since it forms liquid or solid interfaces that prevent contact between the reactants. Furthermore, ash can locally change the viscosity of the slag and therefore the ability of the slag itself to retain gas bubbles within it and form stable bubbles.

[0047] In a preferred embodiment, the char is obtained by torrefaction or steam explosion processes. Preferably, the torrefaction process involves heat treatment of the starting organic material at a temperature between 200°C and 350°C in oxygen depletion conditions. In torrefaction and steam explosion processes, the thermochemical conversion of the organic material is carried out at relatively low temperatures compared to pyrolysis, so the char production yield of such processes is significantly higher compared to pyrolysis or gasification (torrefaction can produce up to 0.5-0.9 kg of char per kg of starting dry material). Also, torrefaction and steam explosion processes are easier to implement, since the amount of gaseous by-products to be handled is relatively small.

[0048] Compared to chars from pyrolysis or gasification, chars from torrefaction and steam explosion generally have lower total and fixed carbon contents, higher volatile fraction contents, and lower heating values. In preferred embodiments, the chars have one or more of the following characteristics: Total carbon (dry basis): 50-70%; Fixed carbon (dry basis): 18-65%; · Volatile fraction (dry basis): 30-80%; -Heat output: 19~30MJ / kg.

[0049] Due to its characteristics, char obtained from torrefaction or steam explosion is a material of biological origin that is practically unused in the steel industry according to the state of the art due to its high ignition potential and therefore a major safety hazard. However, when used in the composite material according to the invention, it can be advantageously utilized as a foaming slag former. The invention thus makes it possible to expand the variety of carbon sources that can replace the fossil carbon sources available today.

[0050] Generally, the bio-based carbonaceous material is in the form of flakes or powder or pellets, depending, for example, on the starting biomass and the preparation process (pyrolysis, torrefaction, etc.). The bio-based carbonaceous material may also be processed, for example by drying and / or grinding, to obtain a size and moisture content suitable for subsequent weak agglomeration with the polymer.

[0051] Typically, the bio-based carbonaceous material is used to prepare a composite material in the form of a powder or flakes or pellets having a maximum size equal to the maximum of 15 mm, more preferably equal to the maximum of 10 mm, and even more preferably equal to the maximum of 5 mm. Preferably, the maximum size of the powder or flakes is in the range of 1 to 10 mm, more preferably in the range of 2 to 5 mm.

[0052] Bio-derived carbonaceous materials obtained by torrefaction or steam explosion are generally commercially available in pellet form. The pellets can be used as is to prepare the composite material according to the present invention. Preferably, the maximum size of the pellets is equal to the maximum of 50 mm, more preferably equal to the maximum of 40 mm, and even more preferably equal to the maximum of 20 mm. Preferably, the maximum size of the pellets is in the range of 1 to 50 mm, more preferably equal to the maximum of 1 to 40 mm, and even more preferably equal to the maximum of 20 mm.

[0053] The amount of carbonaceous material present in the composite can vary over a wide range and can be selected based on the needs for use in the iron-based alloy manufacturing process. Preferably, the carbonaceous material is present in an amount in the range of 10% to 90% by weight, more preferably in the range of 30% to 70% by weight of the composite.

[0054] Preferably, the weight ratio of biogenic carbonaceous material to polymeric material is in the range of 0.1 to 9, preferably in the range of 0.4 to 2.4.

[0055] The composite material may also include one or more additives. Additives can be incorporated into the composite material to improve the performance of the granules for injection into the EAF furnace and / or to improve the granule manufacturing process. For example, lubricant additives, such as calcium stearate, can be added to improve the flowability of the polymer and promote the incorporation of char into the molten polymer. Steel refining additives, such as quicklime, can be introduced to increase the basicity of the slag, or recycled rubber powder (obtained, for example, by crushing tires) can be introduced to further promote slag foaming. It is also possible to use additives commonly used in the manufacture of polymeric materials, such as pigments, dyes, plasticizers, antioxidants, etc. The additives can be present in the composite material in an amount ranging from 0 to 50% by weight, preferably in an amount ranging from 0.1% to 10% by weight, based on the weight of the composite material.

[0056] The composite material according to the invention is in granular form. The term "granular" means that the components of the composite material are strongly agglomerated to each other to form finely divided units (granules). The granules can be of a wide variety of shapes and sizes. They can be, for example, pellets, compacts, cylinders, spheres or other forms of agglomerates, or even irregular shapes.

[0057] Preferably, the bulk density of the granules is 200 to 1000 kg / m 3 (ASTM D1895B compliant), and even more preferably 300-900 kg / m 3 is within the range.

[0058] Preferably, the maximum size of the granules is a maximum size equal to the maximum of 15 mm, more preferably equal to the maximum of 10 mm, even more preferably equal to the maximum of 5 mm. For the purposes of the present invention, this means that the granules can pass through a sieve with a mesh size of 15 mm square, preferably 10 mm square, more preferably 5 mm square.

[0059] Preferably, the maximum size of the granules is a maximum size equal to at least 1 mm, more preferably equal to at least 2 mm, even more preferably equal to at least 3 mm, and even more preferably within the range of 1 mm to 15 mm.

[0060] For the purposes of the present invention, the term "maximum size" refers to the characteristic size of a granule, such as the diameter, length, width or thickness, which is the largest relative to other sizes.

[0061] Granular composite materials may be prepared using techniques known in the art, for example in the art of preparing granules and agglomerates of polymeric materials.

[0062] In general, the preparation process involves heating a thermoplastic polymeric material to its melting temperature and then mixing it with a carbonaceous material to form a fluid homogeneous composite, which is then cooled until it solidifies.

[0063] Alternatively, a homogeneous mixture of the two materials can be prepared in the solid state, then the mixture is heated to a temperature high enough to melt the polymeric material, forming a fluid homogeneous composite, which is then cooled until it solidifies.

[0064] In a preferred embodiment, the heating and mixing step of the two components is carried out in an extruder. In the extruder, the two components can be fed as a physical mixture or separately. In the latter case, the polymeric material is first heated in the extruder body and then mixed with the carbonaceous material, which can be introduced into the extruder through a side inlet. The mixed composite material is then formed into the desired shape (e.g., cylindrical) as it is extruded through the holes of the extrusion die, after which it is cooled (e.g., with air or water) and cut into granules of the desired size.

[0065] Alternatively, other mixing / extrusion techniques, such as continuous mixing, can be used.

[0066] According to the invention, the granular composite material can be used as a foaming slag former in processes for producing iron-based alloys in electric arc furnaces, both in discontinuous mode (conventional processes with discontinuous feeding of metal charge) and in continuous mode (e.g. processes with continuous feeding of preheated metal charge). To this end, the composite material is introduced into the EAF in the presence of floating slag during or after the melting stage of the metal charge. The formation of floating slag can be induced by introducing slag-forming compounds such as quicklime, dolomite and magnesite into the furnace, which may be charged together with the metal charge to be molten or may be injected into the furnace subsequently during melting. Melting of the charge is generally also supported by blowing gaseous oxygen into the furnace.

[0067] The introduction of the composite material as a foaming slag former can be accomplished using techniques and equipment known to those skilled in the art. Preferably, the granular composite material is introduced into the EAF furnace by injection using one or more lances. The lances typically extend into the furnace through openings in the furnace sidewall or roof. The lances typically use a gas flow (e.g., compressed air) to transport the granules.

[0068] Preferably, the granular composite material is dispersed in the floating slag layer and / or in the molten metal bath adjacent to the floating slag layer. This operation is generally carried out when the melting of the metal charge is in an advanced stage and / or when melting has been completed.

[0069] Once injected into the furnace, the composite granules come into contact with the slag, triggering multiple chemical reactions that cause the slag to bubble and at the same time reduce the iron oxide to liquid metallic iron. The reaction of the composite in the slag occurs in two stages. In the first stage, some of the polymeric material leads to an endothermic decomposition process with the general production of hydrocarbons, solid carbon, carbon monoxide and hydrogen that partially reduces the iron oxide, followed by the second stage, the oxidation of the biogenic carbon. The endothermic stage serves to cool the slag and increase its viscosity, promoting the stabilization of the bubbles.

[0070] Without wishing to be bound to any particular theory, it is believed that after introduction of the granules into the furnace, the polymeric material is converted very quickly to release particles of carbonaceous material; therefore, the polymeric carbonaceous material and the bio-based carbonaceous material are believed to undergo various chemical reactions, as shown below.

[0071] Generally, the chemical reactions between the carbonaceous material and the slag that result in slag foaming are primarily as follows:

[0072]

number

[0073]

number

[0074]

number

[0075] When the carbonaceous material comes into contact with the slag, it reduces the iron oxide to liquid metallic iron, simultaneously producing gaseous carbon monoxide (reaction 1). The particles of the carbonaceous material are then enveloped in a gaseous environment of carbon monoxide, which continues its reducing action on the surface of the slag, thereby producing carbon dioxide and further liquid metallic iron (reaction 2). As the carbon dioxide is produced, it then diffuses through the gaseous environment towards the carbonaceous material particles, causing a gasification reaction to produce carbon monoxide (reaction 3).

[0076] For polymeric materials, such as polyolefins, the following reaction can instead be considered:

[0077]

number

[0078]

number

[0079]

number

[0080]

number

[0081]

number

[0082]

number

[0083] First, the polymer chains of the polymeric material are cleaved to form hydrocarbons and shorter hydrocarbon chains (reaction 4). These then decompose to produce solid carbon and hydrogen gas according to reaction 5. They can also react with carbon dioxide (reaction 6) or with the iron oxides of the slag (reaction 8) to form carbon monoxide, hydrogen, and react with the slag to produce metallic iron.

[0084] In reactions 5, 6 and 8, hydrogen is produced as a reaction product, which then acts as a reducing agent. According to reaction 7, hydrogen can reduce iron oxide at a faster reaction rate than carbon monoxide. Hydrogen also contributes to the formation of many small gas bubbles, which in turn has the effect of stabilizing the foamy slag. This is because hydrogen promotes the retention of the gas phase inside the slag. Also, reaction 7 produces water, which, like carbon dioxide, can gasify solid carbon according to reaction 9 to produce hydrogen and carbon monoxide.

[0085] When the volatile fraction content of the biogenic carbonaceous material is relatively high, as is the case for biochars produced by torrefaction, it releases significant amounts of gaseous species that also contribute to the mechanisms of slag foaming and iron oxide reduction.

[0086] The operational steps of the ferrous alloy production process which occur before and after the floating slag forming step are conventional operations carried out in accordance with known techniques.

[0087] Initially, for example, the metal charge to be molten may be introduced into the furnace in one or more loading operations, possibly with an intermediate melting step, or alternatively, the metal charge may be fed into the furnace in a continuous mode after preheating, as is known in the art.

[0088] Once the chemical composition of the molten metal bath and its temperature have been optimized, the molten iron-based alloy metal is removed from the furnace and separated from the slag, and the iron-based alloy thus obtained is then transferred to further processing for conversion into the final finished product.

[0089] The following examples are provided purely for the purpose of illustrating the present invention and should not be considered as limiting the scope of protection defined by the appended claims.

[0090] In the Examples, reference is also made to the accompanying drawings, in which: EXAMPLES

[0091] Example 1 A foaming slag former according to the present invention was prepared as follows. The following was fed into the twin screw extruder: · 60kg of polymeric material derived from waste (LDPE 90w / w%); · 40 kg of biochar.

[0092] The biochar from gasification had a composition of >70% carbon, <6% ash, and <8% moisture. The biochar was in the form of flakes or powder with a maximum size of 5 mm and primarily (at least 50% by weight) less than 2 mm in maximum size.

[0093] Inside the extruder, the polymeric material was melted at a temperature of about 190 °C and then mixed with the biochar, which was fed at three points arranged sequentially along the side wall of the extruder. In this way, the biochar was crushed and the water was evaporated simultaneously, resulting in the two materials becoming weakly agglomerated. Finally, the weakly agglomerated material was extruded through a die with a circular cross section of 4 mm in diameter.

[0094] The extruded composite was cooled and then cut into cylindrical granules with lengths of 3-4 mm.

[0095] This granular composite material was found to have the following characteristics: Bulk density: 420kg / m 3 ·Weight moisture content: 1.2%. The granules also exhibited good mechanical compaction.

[0096] The effectiveness of the granular composite was evaluated by thermogravimetric analysis (11.5 grams sample, heated from 25° C. to 750° C., heating rate: 25° C. / min).

[0097] In Figures 1 - 3 we report the curves of weight loss rate (TG%), heat released (heat flow) and mass change rate (dTG) recorded for the polymeric material (Figure 1), the biochar (Figure 2) and the granular composite (Figure 3).

[0098] Comparing Figures 1 to 3, it can be seen that the mass loss curve of the composite material (Figure 3) can be approximately obtained by superimposing the curves of the polymer material (Figure 1) and the biochar (Figure 2).

[0099] In Figure 3, in the section between 300 and 400 °C, a weight loss of -2% to -8% is observed, while in the section between 400 and 500 °C, the polymer undergoes intense decomposition, reaching a weight loss equal to about -48%. In the section between 500 and 550 °C, the volatilisation decreases as it did in the weakly agglomerated polymeric material (Figure 1), before proceeding back to an almost linear increase as in the case of biochar (Figure 2). At 750 °C, the combustion is not yet complete and 23% of the initial mass is still present.

[0100] The heat flow of the composite (Figure 3) shows a first endothermic peak at about 125°C, corresponding to the melting of the thermoplastic polymer (see Figure 1), and a further endothermic peak in the range 450°C-500°C, which can be attributed to the decomposition of the polymer and its volatilization (see Figure 1). In the range 500-600°C in Figure 1, an exothermic peak is observed which can be attributed to the combustion of the gases produced by the volatilization of the polymer, which can also be seen in Figure 3 for the composite.

[0101] Overall, the thermal analysis shows how the release of heat energy due to the oxidation of biochar is limited by the endothermic decomposition of the polymer. This behavior facilitates the injection mechanism of the composite material into the furnace and reduces the loss of material due to combustion and volatilization of biochar that is commonly observed when attempting to use biochar in a pure, non-aggregated form.

[0102] Thermal analysis shows that the polymer fraction absorbs energy during melting and decomposition to cool the slag and increase its viscosity, which consequently increases its ability to retain the gas bubbles necessary for foaming. The gas released by the polymer can therefore perform the reducing action effectively, mainly at 400°C to 500°C. Furthermore, as a result of the initial thermo-oxidative protection performed by the polymer, the volatile fraction of the biochar can contribute to the foaming formation and the reduction of oxides in the slag. Subsequently, at higher temperatures, a significant fraction of the residual solid carbon, whose presence is evidenced in the thermal analysis by the stabilization of the heat flow that can be observed from a temperature of about 600°C, can also act as a reducing or recarburising agent. The reducing and recarburising action is also promoted by the intense mass exchange resulting from the substantial release of gas by the granules of the composite material.

[0103] Example 2 A second foaming slag former according to the invention was prepared as described in Example 1 starting from the following ingredients: Polymeric materials from post-consumer waste, including LDPE and HDPE (approximately 82% by weight; the remainder foreign matter), · Commercially available biochar obtained by pyrolysis of woody biomass.

[0104] The polymeric material was in the form of granules.

[0105] The biochar in pellet and powder form had the following characteristics: Fixed carbon (dry basis):>90% Volatile fraction (dry basis): 3%-7% ·Ash content (dry basis):<3% ·Moisture content: approx. 1% Calorific value: 34MJ / kg Bulk density: approx. 400kg / m3

[0106] Composite materials were prepared using polymeric material and biochar in a mass ratio of 45:55 on a dry basis.

[0107] The composite material was extruded into cylindrical lentil-shaped granules with a diameter of about 5 mm, a maximum thickness equal to about 3.6 mm, and a bulk density equal to about 610 kg / m3.

[0108] The granular composite had the following characteristics: Lower heating value (dry basis): 37MJ / kg; ·Weight moisture content: <1%.

[0109] The effectiveness of the granular composite was evaluated by thermogravimetric analysis (11.5 grams sample, heated from 25° C. to 750° C., heating rate: 25° C. / min).

[0110] In Figures 4 - 6 we report the weight loss (TG%), heat released (heat flow) and mass change (dTG) curves recorded for the polymeric material (Figure 4), the biochar (Figure 5) and the granular composite (Figure 6).

[0111] In Figure 6, the trend of mass loss is similar to that of the composites described above (Example 1, Figure 3). The most rapid mass loss occurs at the transition from 400°C to 500°C, decreasing from -1% to -25%. A slower oxidation mechanism then results in a mass loss of 46% when reaching 750°C.

[0112] The residual solids content is significantly higher compared to the composite in Figure 3 (54% vs. 23%), due to the higher biochar content and more solid residues in the polymer fraction (Figure 4).

[0113] Compared with the heat flow of the composite of FIG. 3, the heat flow of this composite shows negative values ​​up to 400° C., but becomes positive above 300° C. in FIG. 3. A similar series of endothermic reactions occurs around 450° C., but what can be highlighted for the composite of Example 2 (FIG. 4) are two important energy release peaks around 480° C. and 520° C. The trend of the curve above 550° C. is rather similar to that of the composite of Example 1 containing biochar from gasification of FIG. 2 and polymeric material of FIG. 1, but the value of the heat flow is equal to half of that of Example 1.

[0114] The composite material of Example 2 was also tested in a steel plant, where several advantages were identified over the separate use of thermoplastic polymer and biochar according to the prior art. In particular, the composite material according to the invention completely replaced the anthracite coal used to foam the slag in the steelmaking cycle in the EAF furnace (substitution ratio composite:anthracite=1:1 by weight). The quality of the foamed slag obtained with the composite material was found to be completely equivalent to that obtained with anthracite coal (excellent coverage of the electric arc). During the cycle, no anomalies were observed in terms of flame generation, fumes and excessive temperature rise on the cooling panels of the furnace.

[0115] Regarding CO2 emissions, taking into account the carbon content of anthracite (92% by weight), each kg of anthracite used generates a CO2 equivalent of 3.37 CO2.

[0116] Using the composite material according to Example 2 instead of anthracite (1:1 substitution ratio) resulted in a saving in CO2 emissions equal to 66%.

[0117] Example 3 A third foaming slag former according to the present invention was prepared as described in Examples 1 and 2 starting from the following ingredients: · Polymeric materials from post-consumer waste, including LDPE and HDPE (approximately 82% by weight, the remainder foreign matter); · Commercially available biochar obtained by torrefaction of woody biomass.

[0118] The polymeric material was in the form of granules.

[0119] The biochar in powder form had the following characteristics: Carbon content (ash-free dry basis): 60%~70% Fixed carbon (ash-free dry basis): 35%~45% Volatile fraction (ash-free dry basis): 55%-65% ·Ash content:<4% ·Moisture content:<3% Calorific value: 21.5~23.5MJ / kg -Bulk density: approx. 225kg / m3.

[0120] Composite materials were prepared using polymeric material and biochar in a 50:50 mass ratio on a dry basis.

[0121] The composite material was extruded into cylindrical lentil-shaped granules with a diameter of about 7 mm, a maximum thickness of about 4.5 mm, and a bulk density of about 420 kg / m3.

[0122] The granular composite had the following characteristics: Lower heating value (dry basis): 32MJ / kg; ·Weight moisture content: approx. 1%.

[0123] The effectiveness of the granular composite was evaluated by thermogravimetric analysis (11.5 grams sample, heated from 25° C. to 750° C., heating rate: 25° C. / min).

[0124] Figures 4, 7 and 8 report the weight loss (TG%), heat released (heat flow) and mass change (dTG) curves recorded for the polymeric material (Figure 4), biochar (Figure 7) and granular composite (Figure 8).

[0125] In Figure 8 , the composites show a complex behavior, reflecting what was highlighted for the torrefied biochar in its pure form ( Figure 7 ).

[0126] The composite material first gains mass (+8%) up to about 300 °C. Then there is a mass loss, with the sample at -3% at 400 °C. Between 400 °C and 500 °C, the mass loss is significant, which is due to both the decomposition of the polymer fraction and the decomposition evaporation and oxidation of the biochar. At 500 °C, the residual mass is 63%. Finally, when 750 °C is reached, the residual fraction present is 47%. Combustion is not complete during the test.

[0127] The heat flow trends suggest that the exothermic effect associated with biochar oxidation is countered by the endothermic nature of the polymer decomposition reaction. Between 200°C and 500°C, the behavior is complex, with a series of peaks and valleys, but they are less pronounced and localized than those seen in the composites of Examples 1 and 2 (Figures 3 and 6). Above 520°C, the heat flow stabilizes up to about 620°C, then increases and tends to stabilize around 700°C.

[0128] The composite material of Example 3 was also tested in a steel plant, where several advantages were identified over the separate use of thermoplastic polymer and biochar according to the prior art. In particular, the composite material according to the invention completely replaced the anthracite coal used to foam the slag in the steelmaking cycle in the EAF furnace (substitution ratio composite:anthracite=1:1 by weight). The quality of the foamed slag obtained with the composite material was found to be completely equivalent to that obtained with anthracite coal (excellent coverage of the electric arc). During the cycle, no anomalies were observed in terms of flame generation, fumes and excessive temperature rise in the cooling panels of the furnace.

[0129] Regarding CO2 emissions, taking into account the carbon content of anthracite (92% by weight), each kg of anthracite used generates a CO2 equivalent of 3.37 CO2.

[0130] Substituting the composite material according to Example 3 for anthracite (1:1 substitution ratio) resulted in a reduction in CO2 emissions equal to 62%.

[0131] Overall, the tests carried out in a steel mill using the composite materials described in the examples confirmed several advantages of the invention.

[0132] · The density of the composite material is lower than that of anthracite coal (approximately 900 kg / m3), but up to three times the density of biochar in powder form. This means fewer trucks transporting material to the steel mill, reducing pollution emissions and costs associated with logistics. Steel mills will also be less congested in terms of handling incoming material.

[0133] · Unlike biochar, the composite does not have the problem of hygroscopicity, making it easier to store for long periods of time. From a safety point of view, the weak agglomeration of biochar and polymeric material into mechanically integrated granules solves the problem of the presence of large amounts of ignitable and explosive fines in the working environment, which is characteristic of biochar. For example, the transfer of material from large bags in a silo for injection into the furnace did not result in any appreciable release of powder into the environment. This is also an improvement over the usual practice for anthracite coal. The weak agglomeration solves the problem of biochar's reactivity with air. Due to this reactivity, biochar is a material that is at risk of self-ignition if stored in large quantities for long periods of time and can easily ignite. Thus, the risk in steel plants is minimized by dispersing and capturing the biochar within the polymer matrix.

[0134] · As a result of their physical form, the composite granules are particularly suitable for pneumatic transport from the pressurized tank to the injection lance in the furnace. The granules show good flowability, which allows precise flow rate regulation. This aspect translates into the possibility of optimal control of the injection process, resulting in positive effects in terms of energy consumption and emissions. As a result of the weak agglomeration, the composite solves the problem of the tendency of biochar to form powder fractions of various particle sizes. In fact, these powder fractions tend to clog, especially in the presence of bends or constrictions in the ducts, making it difficult to control the flow rate of the powder feed.

[0135] · Considering the lower bulk density than anthracite, as is the case for pure biochar, the granules of composite material according to the invention also generally require an adaptation of the injection lance. Such modifications may concern the angle of injecting or the use of a secondary entrainment flow (e.g. oxygen jet) allowing effective penetration of the slag material, but in both cases can be easily managed by the skilled person. The composite granules are denser than biochar, thus reducing the problems related to the ability of the material to penetrate the slag. Furthermore, the almost complete absence of a powder phase, characteristic of both anthracite and biochar, limits the loss of material due to the entrainment of these fines in the gases rising from the bath. Such particles may then be wasted due to their tendency to oxidize or volatilize before reaching the slag. In this respect, extrusion allows the control of the surface area to volume ratio, which affects both the heat exchange mechanism undergone by the granules during injection into the furnace and the reactive surface of the particles. It was therefore possible to optimize the effectiveness of the material with respect to injection by controlling the size of the granules. Granules that are too fine, in addition to the possibility of difficult penetration into the slag, tend to heat up quickly with rapid release of volatile fractions or rapid oxidation. Granules that are too large, on the other hand, tend to float on the slag and contribute only partially to the mechanism of iron oxide reduction and foamy slag formation. The fact that no abnormalities occurred in the furnace when replacing anthracite with granules of composite material, which is a foamy slag former, can be seen as showing that the advantages expected from a theoretical point of view were realized in practical application. In particular, no flames higher than normal occurred and the temperatures of the cooling panels and the exhaust gases remained within the historical range. The fact that both the granules produced with biochar from pyrolysis and the granules produced with biochar from torrefaction worked also shows that the polymer effectively protected the biochar in thermal oxidation. Thus, surprisingly, the biochar from torrefaction was also able to reach the slag and released a significant volatile fraction inside it, which exerted a reducing effect.

[0136] · The composite granules are weak aggregates with a uniform composition of biochar and polymer. This maximizes the interaction between the biochar and the polymer and the slag, which are already in full physical contact with each other. In addition to providing thermo-oxidative protection to the biochar as described for the impregnation process, the polymer solves the problem of low reactivity with the slag associated with biological carbonaceous materials. In fact, the problem with the biochar used in the prior art is thought to be due to the presence of a smooth surface at the nanometer and micrometer level, which promotes the formation of a stable gas layer and may therefore inhibit the reduction action of the biochar on the slag. Instead, the large amount of hydrogen and the intense mass exchange associated with the polymer fraction are thought to accelerate the kinetics of the reduction process, especially in the presence of solid carbon, such as that provided by the biochar. Furthermore, the possibility that the hydrocarbon species from the polymer fraction interact with the solid carbon and pyrolyze to form carbon deposits on the surface of the solid carbon allows further promotion of the problem associated with biochar. The fact that anthracite could be completely replaced by the composite granules in the tests carried out suggests that one or more of the above mechanisms did indeed occur. The composite also showed similar effectiveness to that of anthracite in terms of foamy slag quality (good arc coverage) and injection mass, suggesting that even in the presence of the composite, gas bubbles capable of producing a stable foamy slag are formed, despite their different chemical and physical behavior from fossil coal.

Claims

1. 1. A method for forming foamy slag in an electric arc melting furnace during the production of iron-based alloys, comprising the steps of: a. melting a metal charge in said electric arc melting furnace to obtain a molten metal bath containing a floating slag layer; b. introducing a foaming slag forming agent into the electric arc melting furnace to foam the floating slag; Including, The method of claim 1, wherein the foaming slag forming agent is a granular composite material comprising at least one thermoplastic polymeric material and at least one bio-based carbonaceous material.

2. The method of claim 1 , wherein the thermoplastic polymeric material is obtained from the recovery of post-consumer and / or industrial process waste containing polymeric materials.

3. The method according to any one of claims 1 to 2, wherein the thermoplastic polymer material comprises at least one selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polystyrene and mixtures thereof.

4. The method of claim 1 , wherein the bio-derived carbonaceous material is char.

5. 5. The method of claim 4, wherein the char or biochar is obtained by a process of gasification, pyrolysis, torrefaction, hydrothermal carbonization or steam explosion.

6. The method of claim 1 , wherein the thermoplastic polymeric material is present in an amount in the range of 10% to 90% by weight based on the weight of the composite material.

7. The method of claim 1 , wherein the carbonaceous material is present in an amount in the range of 10% to 90% by weight based on the weight of the composite material.

8. The method of claim 1 , wherein the carbon content of the bio-based carbonaceous material is 50% by weight or greater.

9. The biological carbonaceous material has the following characteristics: Total carbon (dry basis): 50-70%; Fixed carbon (dry basis): 18-65%; Volatile fraction (dry basis): 30-80%; ・Heat output: 19-30MJ / kg having one or more of: The method of claim 1.

10. The method of claim 1, wherein the weight ratio of the bio-based carbonaceous material to the polymeric material is in the range of 0.1 to 9.

11. The method of claim 1 , wherein the carbon content of the thermoplastic polymer material is 50% by weight or greater.

12. 2. The method of claim 1, wherein the maximum size of the granules of the foaming slag forming agent is 15 mm or less.

13. 2. The method of claim 1, wherein the maximum size of the granules of the foaming slag forming agent is 1 mm or more.

14. 2. The method of claim 1, wherein step b comprises dispersing the granular composite material in a floating slag layer and / or in a molten metal bath adjacent to the floating slag layer.

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