WO2024013653A1

WO2024013653A1 - Method for direct reduction of iron oxide-based material for the production of steel, iron sponge or cast iron

Info

Publication number: WO2024013653A1
Application number: PCT/IB2023/057092
Authority: WO
Inventors: Silvio RANCATI
Original assignee: Elsafra Ii S.P.A.
Priority date: 2022-07-11
Filing date: 2023-07-11
Publication date: 2024-01-18

Abstract

A method for the direct reduction of iron oxide-based material for the production of steel, iron sponge or cast iron is described, comprising the steps of: a) producing, by forming at a temperature not exceeding 45°C, briquettes made of a first iron oxide-based material, a second carbon-based material and a third material acting as a binder, wherein said second material is biochar; and b) heating the briquettes thus obtained within a reduction furnace (10) equipped with electrical resistors (12) and/or at least one oxy-fuel burner (14), at a temperature between 1050°C and 1450°C, so as to permit the reduction of the iron oxides contained in the briquettes by reaction with the carbon contained in the briquettes.

Description

METHOD FOR DIRECT REDUCTION OF IRON OXIDE-BASED MATERIAL FOR THE PRODUCTION OF STEEL, IRON SPONGE OR CAST IRON

Technical field of the invention

The present invention relates generally to technology for the direct reduction of iron oxide-based material, i.e. , material containing iron oxides, for the production of steel, iron sponge or cast iron. More specifically, the present invention relates to a method for direct reduction of iron oxide-based material in the form of briquettes obtained by mixing such material with biochar (also called biogenic charcoal), within a reduction furnace equipped with electrical resistors and/or oxy-fuel burners.

State of the art

The entire world steel production, about 2 billion tonnes per year, is based on the following two ways of obtaining crude steel: 1) remelting scrap iron in electric arc furnaces, and 2) reducing iron ore in shaft furnaces (blast furnaces) or by direct reduction in natural gas and/or hydrogen furnaces.

Electric arc furnaces mainly work with iron scrap, the quality of which decreases after each smelting cycle due to the progressive metallurgical contamination of the scrap loaded into the furnaces; clean scrap must therefore be used to dilute the metal bath during each smelting cycle. On the other hand, the amount of clean scrap iron available is not sufficient to saturate furnace capacity and thus support global economic growth.

The prospective goal in the coming years will be to produce steel with a carbon-neutral footprint, i.e., without net carbon dioxide (CO2) emissions. This will lead to increased pressure on the demand for scrap iron, as this is the raw material used by electric furnaces, which has a significantly lower net CO2 emission than integrated cycles based on the combination of blast furnaces and converter furnaces. Consequently, due to the progressive increase in dirty scrap, the amount of clean iron scrap available will not be sufficient to saturate furnace capacity and thus support global economic growth.

It will therefore be necessary in the future to develop methods and technologies for producing ferrous materials (steel, iron sponge, cast iron) that can replace scrap in electric furnaces and at the same time reduce the carbon footprint of the extraction of iron from ore through traditional methods. Steel production from iron ore is, however, a necessary condition for meeting world demand, as iron production from recycled scrap is insufficient in quantity and inadequate in quality.

The main drawbacks of current iron ore reduction methods using shaft furnaces (blast furnaces) are the high costs of the plants and the high CO2 emissions caused by the production of coke and its use as a reducing agent.

For these reasons, production from iron ore is moving towards greater use of direct reduction methods of materials containing iron oxides (pre-reduction), which inherently have a lower environmental impact. Unfortunately, direct reduction methods currently in use entail intensive consumption of natural gas and hydrogen, which are difficult to obtain and therefore extremely expensive in many geographical areas, thus making prereduced production competitive only in a small number of countries.

Another drawback associated with current direct reduction methods is the high cost of building the plants, which is also associated with high operating costs, as the operation of these plants requires the use of natural gas, the price of which has risen considerably in recent times, while the use of hydrogen produced by electrolysis of water involves energy costs (4.5-5 kWh per m³ of hydrogen produced) and consumption of fresh water that are not compatible with a significant reduction in the environmental impact of the production methods.

As regards the issue of CO2 emissions, it should be noted that steel production by direct reduction methods based on the use of coal generates about 2.5 t CO2 for every tonne of steel produced, while steel production by direct reduction methods based on the use of natural gas generates about 0.77 t CO2 for every tonne of steel produced. In addition to being the main cause of ongoing climate change, the emission of large amounts of CO2 into the atmosphere leads to additional costs for steel producers related to the so- called carbon tax, which can be quantified in several tens of Euros per tonne of CO2 produced. The costs for the CO2 emission in the coming years will be around 10 Euro per tonne of CO2 produced, with a possible further increase in order to stimulate the decarbonisation of hard-to-abate sectors, such as the steel industry.

Examples of methods for the direct reduction of iron oxide-based material are known from the documents discussed below.

US6409964B1 discloses a method for producing pellets with sufficient hot and cold mechanical strength for use as feedstock for direct reduction furnaces. The pellets are made from iron-based material from steelmaking processes (mainly lump iron ore) and carbon material, bound together by means of a binder with a high alumina hydrate content.

Due to the pelletising method itself, the pellets require significant consumption of water and natural gas, as well as the use of fine particulate as a starting material, which entails the use and maintenance of costly dedusting plants. In any case, the presence of such fine dust entails risks to human health if it is dispersed into the environment during storage, handling and pelletising.

US6409964B1 aims at solving this drawback by using iron oxide particulate with a particle size between 0.1 and 6 mm and minimising the fraction with a particle size below 0.1 mm. This, however, forces to use ore with a controlled particle size derived from special preparation and treatment processes, thereby limiting the possibility of using scrap and by-products of the steelmaking process (or other metallurgical processes), as they typically have excessively finer particle sizes (< 0.01 mm).

US6409964B1 also generically mentions the use of briquettes, as an alternative to pellets, without however providing specific indications on the nature and characteristics of such aggregates, in particular on the type of binders used to consolidate the briquettes.

WO2021/237281A1 discloses a method for the production of briquettes by direct reduction processes, wherein the briquettes are made of iron ore and lignocellulose, with weight percentages of at least 55% and at least 30% respectively, have a volume comprised between 5 and 20 cm³, and have a compressive strength greater than 500 N (prior to the reduction processes).

WO2021/184078A1 discloses a method and a plant for the production of pre-reduced iron (commonly known by the acronym DRI, from Direct Reduced Iron) by heating iron ore briquettes and biomass in a static furnace by means of gas combustion (air/oxygen mixture with an oxygen content of more than 25%, temperature between 700 and 1100°C and holding time of 10-100 h), with metallisation of the product between 80 and 99%.

The methods known from WO2021/237281A1 and WO2021/184078A1 require iron ore as a starting material, which evidently limits the circularity of these methods. At the same time, the biomass used in these known methods results in an increased wet fraction in the briquettes produced. This wet fraction, although chemically irrelevant in the whole process, nevertheless leads to an increase in the energy required to smelt the briquettes, as the energy required for the evaporation of the wet fraction is added, and also causes a decrease in the reaction kinetics for the same power output.

For the above reasons, it is evident that these known methods do not achieve the goal of total abatement of CO2 produced.

EP1996735A2 discloses a method for the direct reduction of iron ore-based material in a reduction furnace, wherein the iron ore-based material is mixed with a reducing material to form briquettes or pellets. The aim of this document is to provide a new composition and method for the production of metallic iron from iron ore that does not require coal, charcoal or coke, and to achieve this it proposes to use as a reducing material a material essentially deprived of free carbon, in particular a biomass. For example, the following materials are proposed as biomass: paper, cellulose, shredded wood, wheat flour, corn meal, grass clippings, etc. This solution implies that the resulting briquettes are characterised by a high moisture content, due to the high amount of water contained in the biomass that is used to produce the briquettes. A drawback related to the high moisture content of the briquettes is that, during the reduction process, the briquettes tend to shatter due to the formation of steam, resulting in a greater dispersion of dust and in a decrease in the quality of the material to be used in the subsequent smelting process.

WO2021/214167 discloses a method for manufacturing briquettes to be used as material with which to feed reduction furnaces for steel production. The briquettes contain ironbased material, preferably iron ore, and pyrolysed biomass, preferably charcoal (or biochar), and are obtained by sintering process, i.e., by high-temperature forming or agglomeration process. According to this known solution, the biochar contained in the material mixture from which the briquettes are made serves as fuel for the sintering process and thus allows to achieve the goal of performing the sintering process with less fossil fuel consumption. The use of a sintering process to manufacture briquettes has the consequence that the resulting briquettes are completely deprived of biochar, since this material has been consumed during the sintering process to agglomerate the iron ore contained in the initial mixture. Therefore, for the production of steel, it will be necessary to load the furnace with a reducing agent, together with the briquettes, in particular coke in the case of a blast furnace, natural gas and/or syngas and/or hydrogen in the case of a direct reduction furnace, carbon in the case of an electric furnace.

Summary of the invention

It is an object of the present invention to provide a method for the direct reduction of iron oxide-based material by heating, within a reduction furnace, briquettes obtained by mixing such material with carbon-based material, which allows to reduce CO2 emissions into the atmosphere, which is more cost-effective than the prior art discussed above, and which, more generally, is not affected by the disadvantages of the prior art discussed above.

This and other objects are fully achieved according to the present invention by virtue of a method as defined in the appended independent claim 1.

Further advantageous features of the invention are set forth in the dependent claims, the subject-matter of which is to be understood as forming an integral part of the present description.

In summary, the invention is based on the idea of producing, by forming at a temperature not exceeding 45°C, briquettes made of a first iron oxide-based material, a second carbon-based material consisting of biochar (i.e. , charcoal of biogenic origin) and a third material acting as a binder, and to use a reduction furnace equipped with electrical resistors and/or oxy-fuel burners to heat said briquettes, in particular to a temperature comprised between 1050°C and 1450°C, so as to cause the reduction of the iron oxides contained in said first material of the briquettes by reaction with the carbon contained in the biochar forming said second material of the briquettes.

Thanks to this solution, it is possible to reduce iron oxides to metallic iron and bring them to fusion, without the use of reducing agents obtained from fossil sources, with high yields due to the use of electricity or oxy-fuel burners supplied with biomethane and/or hydrogen as a heat source, generating a minimal amount of waste and a carbon-neutral footprint due to the biogenic origin of the carbon (acting as a reducing agent) used in the production of the briquettes.

Brief description of the drawings

Further features and advantages of the present invention will become clearer from the following description, given purely by way of non-limiting example with reference to the accompanying drawings, wherein Figures 1 and 2 are a front view and a side view, respectively, which schematically show a reduction furnace forming part of a plant for the direct reduction of iron oxide-based material according to an embodiment of the present invention.

Detailed description

The method according to the present invention is based on the idea of heating, within a reduction furnace, briquettes obtained by mixing and forming, at a temperature not exceeding 45°C, iron oxide-based material and carbon-based material consisting of biochar (i.e., charcoal of biogenic origin).

Iron ore of various origin and quality naturally present in mines (such as in particular hematite, i.e., Fe2Os) and/or the main by-products of steel production and processing methods (for example rolling, forging or drawing) can be used as iron oxide-based material, also in order to ensure better circularity in the use of steelmaking process residues.

It is also possible to use iron ore otherwise unsuitable for loading in blast furnaces or preheaters, since - as will become clear from the following description - the method according to the invention does not work in vertically extending furnaces and does not require high mechanical strength of the iron ore or coal used. In fact, as far as coal is concerned, in the case of furnaces with high vertical extension (for example cupola furnaces and blast furnaces), the material that is charged must exhibit high crushing resistance characteristics to avoid the structural collapse of the charge during descent into the furnace; in particular, this role is covered by the coke which must exhibit adequate crushing resistance characteristics and must not be characterised by excessive reactivity leading to premature consumption in the upper part of such furnaces. In the method according to the present invention, on the other hand, the coals used must not exhibit special mechanical characteristics. Furthermore, there are no limitations as to the rate of chemical reactivity of the coals which can be used in the method according to the present invention. As far as iron ore is concerned, the one charged in blast furnaces must be in the form of sintered material or in the form of pellets, which must neither disintegrate to form dust (which would decrease the permeability of the charge to rising wind) nor stick to the refractory material of the furnace or chemically attack it; in the case of preheaters, only pellets (DRI grades or DRI premium quality) are permitted, which must neither disintegrate to form dust (which would reduce the permeability of the charge to rising wind) nor stick to the refractory material of the preheater or chemically attack it. In contrast, the method according to the present invention does not pose any restrictions on the quality of the ferrous material that is agglomerated with the carbonbased material to form the briquettes with which to feed the reduction furnace.

The carbon-based material, which is used as a reducing agent for the reduction of iron oxides contained in the briquettes, is biochar. In particular, the biochar is obtained by biomass torrefaction (preferably carried out at a temperature of about 350°C) and/or by biomass pyrolysis (preferably carried out at a temperature between 550°C and 750°C or at a temperature between 950°C and 1050°C). Additionally, or alternatively, biochar is obtained by torrefaction and/or pyrolysis of bioplastics (also known as biopolymers).

The use of biochar reduces, of course, the environmental impact of the method. In addition, the use of biochar obtained by torrefaction or pyrolysis of biomass makes it possible to reduce the amount of energy required for the reduction and smelting process, as well as to reduce the amount of carbon-based material required for the reduction of iron oxides, since the reducing agent (carbon) is more concentrated and in a more reactive form than when using biomass. In addition, due to the chemical nature of the biochar obtained by torrefaction and/or pyrolysis, the fraction of sulphur contained therein is negligible, if not even null, and consequently the desulphurisation process of the metal bath prior to tapping and solidification is not strictly necessary, which shortens the process compared to the traditional production of cast iron. A further advantage is due to the fact that there is no need to operate within the reduction furnace with desulphurising slag, which reduces energy consumption, as energy is only required for the reduction and smelting of the briquettes.

Preferably, the ratio by weight of iron oxides to biochar in each briquette, calculated according to the relation x(FeO_x)/(x(C) + x(FeO_x)) 100, where x(FeO_x) is the weight of iron oxides contained in each briquette and x(C) is the weight of biochar contained in each briquette, is between 68% and 84% depending on the type of biochar used. For example, in the case of biochar obtained from the pyrolysis of biomass at 350°C, this ratio will be 75%, in the case of biochar obtained from the pyrolysis of biomass at 550- 750°C, this ratio will be 80%, while in the case of biochar obtained from the pyrolysis of biomass at 950-1050°C, this ratio will be 84%.

The briquettes contain, in addition to the iron oxide-based material and the biochar, a binder whose function is to ensure the agglomeration or consolidation of the briquettes, ensuring mechanical strength values (at least greater than 4 MPa, preferably greater than 10 MPa) suitable for the storage and/or handling of the briquettes. The quantity of binder corresponds by weight to no more than 5-8% of the weight of the briquette. The binder contains organic material of biogenic nature, for example maize starch, potato starch and/or rice starch, molasses, gum arabic, biopolymer. The amount by weight of organic material of biogenic nature used as a binder is no more than 5% of the weight of the briquette.

In addition to an organic material of biogenic nature, an inorganic material, in particular bentonite, may also be used as a binder. In this case, the amount of inorganic material by weight is no more than 3% of the weight of the briquette.

The use of such binders leads to the following main advantages during the process of smelting and reduction of the briquettes: 1) absence of chemical elements that would lead to the formation of slag on top of the metal bath, 2) increase in the efficiency of the reduction of the oxides, as a result of the generation of carbon monoxide and hydrogen, and 3) as a consequence of the second point, decrease in actual CO2 emissions in proportion to the amount of hydrogen generated. In addition, the carbon and hydrogen atoms of the organic binder may also participate in the reduction process.

The briquettes may also contain, in addition to the biochar, metals capable of having a reducing action on iron oxides (for example Mn, Si, Al, Ti, Ca, Mg). However, the stoichiometric ratio must ensure that there are sufficient atoms of the reducing agent to allow the complete removal of the oxygen atoms combined with the iron, according to the following reactions:

1/3 Fe₂O₃ + Mn 2/3 Fe + MnO 2/3 Fe₂O₃ + Si 4/3 Fe + SiO₂

Fe₂O₃ + 2AI 2 Fe + AI₂O₃

2/3 Fe₂O₃ + Ti 4/3 Fe + TiO₂

1/3 Fe₂O₃ + Ca — > 2/3 Fe + CaO

1/3 Fe₂O₃ + Mg 2/3 Fe + MgO.

The production of briquettes is carried out through the following steps.

Firstly, iron oxide material and biochar with a particle size of less than 500 pm are mixed together. If the starting material has a larger particle size distribution, it is first ground and/or crushed in order to reduce its size below the 500 pm limit. After mixing, the binder is added and the resulting mixture is fed into a special mould to obtain briquettes of the desired shape and size. In order to maximise the mixing efficiency and thus obtain a mixture that is as homogenous as possible, mechanical mixers can be used.

The final geometry of the briquette may be of various shapes (for example cylindrical, "pillow-like", spherical) and sizes in order to meet the dimensional and mechanical requirements for proper storage and handling of the product. In particular, the briquettes will have a volume of no more than 65 cm³. For example, the briquettes may be made as parallelepipeds with dimensions 50 mm x 50 mm x 25 mm.

Although it is possible to use variable particle sizes to achieve the required mechanical strengths, the highest mechanical strength values have been found where the largest weight fraction (i.e., the iron oxide-based material) has a larger particle size (in particular, a particle size between 63 pm and 125 pm) than the carbon-based material (in particular, a particle size smaller than 63 pm).

The briquettes are formed at room temperature, or more generally at a temperature not exceeding 45°C. Therefore, they do not undergo any heating before being loaded into the reduction furnace.

The briquettes thus produced can be stored and handled at a second site or used at the briquetting site itself, for the production of cast iron and/or iron sponge, in case the reduction furnace is installed at the briquetting site. This second option is preferable because, by minimising the distance that the briquettes have to travel to be transported from the production site to the utilisation site, it allows to reduce the total environmental impact of the process, in terms of both pollutant emissions and CO₂ production.

As mentioned above, the briquettes thus obtained are finally fed into a reduction furnace, where they are heated to a temperature between 1050°C and 1450°C, preferably a temperature between 1400°C and 1450°C, in order to cause the softening or smelting of the iron oxides contained in the briquettes and the reduction of these oxides by reaction with the carbon contained in the briquettes (as well as with any additional carbon injected into the furnace). Specifically, the reduction between iron oxides and carbon occurs according to the following reaction:

1/3 Fe₂O₃ + C 2/3 Fe + CO.

In addition, the carbon and hydrogen contained in the organic material of biogenic nature used as a binder for the production of the briquettes can also act as reducing agents for the reduction of the iron oxides, according to the following reactions:

1/3 Fe₂O₃ + C 2/3 Fe + CO, and

1/3 Fe₂O₃ + H₂ 2/3 Fe + H₂O.

If the briquettes also contain metallic residues (for example, Mn, Si, Al, Ti, Ca, Mg) capable of having a reducing action on the iron oxides, then the reduction of the iron oxides will also take place due to these metallic residues, according to the reactions indicated above.

An example of a reduction furnace that can be used for carrying out the present invention is schematically shown in Figures 1 and 2, where it is generally indicated by 10. With reference to these figures, the reduction furnace 10 differs from conventional shaft furnaces (blast furnaces) in that it extends horizontally, instead of vertically. This makes it possible to use briquettes with a low compressive strength, for example of the order of 10 M Pa, since there is no layering of the charge in the furnace, which loads with its mass the underlying layers in contact with the liquid phase. This implies absolute freedom in the supply of the iron oxide-based material (which can also be low-quality material), as well as of the carbon-based material (which can also be material in powder form), and thus makes the process essentially independent of the quality of the raw material.

Given the nature of the process, which also provides for the possible formation of a molten bath, the walls and basin of the reduction furnace 10 must be made of refractory materials capable of resisting corrosion and fluid-dynamic erosion (Marangoni effect) during the operating period. In particular, depending on the presence or absence of slag above the molten bath, it is preferable to use refractory material coated with graphitised carbon, or silicon carbide bricks with graphitic binder, similar to what is used in modern blast furnaces, or even alumina- and/or mullite-based material alloyed with SiAION, traditionally used in blast furnace basins.

The reduction furnace 10 operates by means of electrical resistors 12, which may be silicon carbide (SiC) electrical resistors and/or molybdenum disilicide (MoSi₂) electrical resistors and/or graphite electrical resistors (in the event that a controlled atmosphere with low oxygen concentration, namely with a partial pressure of oxygen below 0.05 atm, is present inside the reduction furnace). Preferably, the electrical resistors 12 are supplied with electricity from renewable sources, so that the carbon footprint of the method is reduced - if not even eliminated.

As an alternative, or in addition, to the electrical resistors 12, the reduction furnace may be provided with one or more oxy-fuel burners 14, fuelled with hydrogen and/or biomethane. Also in order to increase energy efficiency, it is advantageously contemplated that the reduction furnace exploits the combustion heat from the recovery of the gases (mainly CO) present at the steel production and processing site. In addition, or as an alternative, the CO produced inside the furnace as a result of the reduction reaction can be exploited, through its post-combustion, to decrease the energy delivered to the reduction furnace.

Still with reference to Figures 1 and 2, the reduction furnace 10 has a feed door 16 for feeding the briquettes, and one or more porous wall or ducts 20 for insufflating inert gas with which to control the atmosphere in the furnace.

The gaseous environment within the reduction furnace may be formed by ambient air, by Ar (argon), by mixtures of Ar and CO2, by mixtures of N2 and CO2, by mixtures of N2, CO and CO2, by CO2, or by mixtures of CO and CO2. Specifically, the gas mixture within the furnace is adapted depending on the availability and cost of these gases. Furthermore, depending on the carbon material used as a reducing agent, one of these mixtures can be selected in order to maximise the carbon activity for the reduction of the iron oxides. For example, in case of biochar with low concentrations of fixed carbon, the use of a gas consisting mainly of Ar and/or N2 and/or CO2 mixtures enriched with CO is preferable, whereas in case of biochar with high concentrations of fixed carbon, the use of ambient air is preferable.

In light of the above description, the advantages achievable with the present invention are apparent.

In addition to contributing to the reduction in the production costs, the invention also has the great advantage that it can be carried out using plants that are much more limited in size and much less expensive than those available today on the world market. In addition, the invention makes it possible to reduce, if not even eliminate, the carbon footprint of the steel, iron sponge or cast iron production cycle, thus making the process fully sustainable not only from an economic point of view but also from an environmental point of view.

Moreover, due to the fact that the briquettes are produced by forming without heating (at room temperature or at a temperature not exceeding 45°C), when the briquettes are fed into the reduction furnace they still contain biochar (unlike the solution described in WO2021/214167, where the briquettes are produced by sintering process) and thus the biochar contained therein can react with the iron oxides, reducing them, without the need to introduce reducing agents, such as coke, natural gas, syngas, hydrogen or carbon, into the furnace, in addition to the briquettes.

The present invention has been described herein with reference to preferred embodiments thereof. However, it is to be understood that other ways for carrying out the method of the present invention may be envisaged, which share the same inventive core with those described herein, as defined by the appended claims.

Claims

1. Method for the direct reduction of iron oxide-based material for the production of steel, iron sponge or cast iron, the method comprising the steps of: a) producing briquettes made of a first iron oxide-based material, a second carbon-based material and a third material with a binding function, wherein said second material is biochar; and b) heating the briquettes thus obtained within a reduction furnace (10) provided with electrical heating elements (12) and/or at least one oxy-fuel burner (14), at a temperature comprised f between 1050°C and 1450°C, so as to cause the reduction of the iron oxides of said first material of the briquettes by reaction with the carbon contained in the biochar forming said second material of the briquettes; wherein said step a) of production of the briquettes is carried out by forming at a temperature not exceeding 45°C.

2. Method according to claim 1 , wherein said step b) of heating the briquettes is performed at a temperature comprised between 1400°C and 1450°C so as to cause smelting of the iron oxides contained in the briquettes.

3. Method according to claim 1 or claim 2, wherein the ratio by weight of the iron oxides to the biochar in each briquette, calculated according to the relation x(FeO_x)/(x(C) + x(FeO_x)) 100, wherein x(FeOx) is the weight of the iron oxides contained in each briquette and x(C) is the weight of the biochar contained in each briquette, is between 68% and 84%.

4. Method according to any one of the preceding claims, wherein the first material comprises iron ores, in particular hematite (Fe2Os), or by-products of steel production and processing, in particular blast furnace sludge, rolling/forging/spinning slag and oil by-products, or a combination thereof.

5. Method according to any one of the preceding claims, wherein the biochar forming said second material of the briquettes is at least in part biochar obtained by biomass torrefaction, carried out in particular at a temperature of 350°C, and/or biochar obtained by biomass pyrolysis, carried out in particular at a temperature between 550°C and 750°C or at a temperature between 950°C and 1050°C.

6. Method according to any one of claims 1 to 5, wherein the biochar forming said second material of the briquettes is at least in part biochar obtained by torrefaction of bioplastics and/or biochar obtained by pyrolysis of bioplastics.

7. Method according to any one of the preceding claims, wherein said third material contains organic material of biogenic nature, such as corn starch, potato starch, rice starch, molasses or gum arabic.

8. Method according to claim 7, wherein the ratio by weight of said organic material of biogenic nature to the total weight of each briquette is no more than 5%.

9. Method according to claim 7 or claim 8, wherein said third material further contains bentonite, in particular with a ratio by weight to the total weight of each briquette of not more than 3%.

10. Method according to any one of the preceding claims, wherein said step b) of heating the briquettes is carried out by feeding the electrical resistors (12) of the reduction furnace (10) with electricity from renewable sources and/or by feeding said at least one oxy-fuel burner (14) with hydrogen and/or biomethane.