Method for manufacturing a stainless steel

Abstract

The invention relates to a method for manufacturing a stainless steel: a. manufacturing (10) a ferroalloy, comprising: - providing (12) plant material having a nickel content of 0.5% or more by dry weight; - thermochemically treating (14) the plant material in order to obtain nickel-rich ash having a nickel content of 10% or more by weight or nickel-rich biochar having a nickel content of 2% or more by weight; - manufacturing (16) briquettes from the nickel-rich ash or from a homogenate of the nickel-rich biochar, comprising adjusting the phosphorus content such that the briquettes have a ratio of the nickel content by weight to the phosphorus content by weight of 40 or more; and - manufacturing (18) a ferroalloy from the briquettes by pyrometallurgy, the ferroalloy having a nickel content of between 4% and 50% by weight; and b) preparing (20) a stainless steel from the ferroalloy.

Description

[0001] Process for manufacturing stainless steel

[0002] The present invention relates to a method for manufacturing a stainless steel from a ferroalloy containing between 4% and 50% by mass of nickel.

[0003] Ferroalloys containing nickel at contents greater than or equal to 4% by mass are usually obtained by pyrometallurgy from nickel-rich ores, such as laterites.

[0004] However, this process is not entirely satisfactory. It is relatively expensive, particularly due to the relatively high price of nickel-rich ore. Furthermore, the extraction of nickel-rich ore has a significant environmental impact, particularly in terms of the profound destruction of soils and the hydrosphere, as well as local flora and fauna. Furthermore, nickel extraction leads to direct CO2 emissions of between 10 and 100 tonnes of CO2 per tonne of nickel equivalent in the production of nickel pig iron.

[0005] One aim of the invention is to provide a method for manufacturing stainless steel with reduced environmental impact and cost.

[0006] To this end, the invention relates to a method for manufacturing a stainless steel, comprising the following successive steps: a. the manufacture of a ferroalloy, comprising:

[0007] - the supply of plant material, said plant material having a nickel content greater than or equal to 0.5% by dry mass;

[0008] - thermochemical treatment of plant material in a thermochemical treatment unit to obtain nickel-rich ash with a nickel content greater than or equal to 10% by mass or nickel-rich biochar with a nickel content greater than or equal to 2% by mass;

[0009] - the manufacture of briquettes from nickel-rich ash or nickel-rich biochar grind, comprising adjusting the phosphorus content such that the briquettes have a ratio of nickel mass content to phosphorus mass content greater than or equal to 40; and

[0010] - the production of a ferroalloy by pyrometallurgy from the briquettes, the ferroalloy having a nickel content of between 4% and 50% by mass; and b. the production of a stainless steel from the ferroalloy.

[0011] The manufacturing process according to the invention may also comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: the thermochemical treatment is a combustion in the presence of oxygen at a temperature between 550°C and 1200°C so as to obtain nickel-rich ash;

[0012] - combustion is carried out between 800°C and 1100°C; adjustment of the phosphorus content includes: a. a mixture of nickel-rich ashes from plants associated with different combinations of species, geographical origins and years of harvest of the plants to obtain a pre-defined ratio of the mass content of nickel to the mass content of phosphorus, for example greater than or equal to 40, in the mixture; and / or b. washing the nickel-rich ashes to reduce the phosphorus content; and / or c. mixing the nickel-rich ashes with other nickel-containing materials so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the nickel-rich ashes;

[0013] - the washing of the ashes is carried out in an acid solution for a period of between 5 minutes and 10 minutes at a temperature of between 5°C and 50°C;

[0014] - the acid solution is an aqueous solution containing 0.1 to 4 mol / L of protons;

[0015] - the ratio of the mass content of nickel to the mass content of phosphorus in the nickel-rich ash or nickel-rich biochar grind is less than 40;

[0016] - the manufacture of briquettes comprises: a. the manufacture of a mixture comprising nickel-rich ash, as well as a binder, a source of reducing agent, a flux and optionally additional materials for supplying alloying elements for the ferroalloy; and b. the compression of this mixture to obtain the briquettes;

[0017] - the additional materials come at least partly from sludge or dust from the metallurgical industry;

[0018] - the thermochemical treatment is a pyrolysis so as to obtain nickel-rich biochar, the method further comprising a step of grinding the biochar to obtain a biochar grind; the adjustment of the phosphorus content comprises: a mixture of nickel-rich biochars originating from plants characterized by different combinations of the species, geographical origins and years of harvest of the plants to obtain a ratio of the mass content of nickel to the mass content of phosphorus pre-defined, and for example greater than or equal to 40, in the mixture; and / or a mixture of the nickel-rich biochar with other materials containing nickel so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the nickel-rich biochar;

[0019] - the manufacture of briquettes includes: the manufacture of a mixture comprising the nickel-rich biochar grind, as well as a binder, a flux and additional materials for adding alloying elements for the ferroalloy; and

[0020] - compression of this mixture to obtain briquettes;

[0021] - the additional materials come at least in part from sludge or dust from the metallurgical industry; the plant material is supplied in the form of ground plant material, in particular in the form of granules comprising the ground plant material; the step of supplying plant material comprises:

[0022] - the supply of dried plants;

[0023] - grinding the dried plants to obtain a ground plant material; and optionally

[0024] - compressing this ground plant material to obtain granules; the method further comprises, during the compression step to form the granules, heating the ground plant material to a temperature of between 60°C and 80°C; the method further comprises, during the compression step to form the granules, adding an adjuvant intended to facilitate the agglomeration of the ground plant material, the adjuvant preferably being water;

[0025] - the plant material is obtained from nickel hyperaccumulator plants, for example chosen from the following species:

[0026] Odontarrhena chalcidica (syn= Alyssum mural)

[0027] Bornmuellera emarginata (syn= Leptoplax emarginata)

[0028] Bornmuellera tymphaea

[0029] Berkheya coddii

[0030] Blepharidium guatemalense

[0031] Phyllanthus rufuschaneyi

[0032] Rinorea aff. bengalensïs. the adjustment of the phosphorus content is carried out in such a way that the briquettes have a ratio of the mass content of nickel to the mass content of phosphorus greater than or equal to 80;

[0033] - the manufacture of ferroalloy by pyrometallurgy from briquettes includes:

[0034] - the introduction of briquettes into an oven;

[0035] - melting the briquettes in the furnace to obtain a liquid ferroalloy, the ferroalloy comprising between 4% and 50% by mass of nickel and casting the ferroalloy in the form of a semi-finished product, for example in the form of ingots or granules; the manufacture of stainless steel from the ferroalloy comprises:

[0036] - the supply of a mixture of ferroalloy and scrap metal, the scrap metal including in particular stainless steel scrap metal;

[0037] - melting this mixture in an electric furnace, in particular an electric arc furnace, to obtain liquid steel; and

[0038] - refining this liquid steel to obtain stainless steel.

[0039] The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the appended figure, which is a schematic view of the process for manufacturing a stainless steel according to the invention.

[0040] The method for manufacturing a stainless steel according to the invention comprises the following successive steps:

[0041] - the manufacture 10 of a ferroalloy, comprising:

[0042] - the supply 12 of plant material; - the thermochemical treatment 14 of the plant material to obtain nickel-rich ash or nickel-rich biochar;

[0043] - the production of briquettes from nickel-rich ash or nickel-rich biochar grind;

[0044] - the manufacture 18 of a ferro-alloy alloy by pyrometallurgy from briquettes; and

[0045] - the production of a stainless steel from the ferroalloy.

[0046] More particularly, the plant material provided in step 12 has a nickel content greater than or equal to 0.5% by dry mass, and in particular greater than or equal to 1% by dry mass.

[0047] The plant material provided in step 12 comprises, for example, a phosphorus content of between 1000 and 4000 mg / kg of plant material.

[0048] This plant material is obtained from nickel hyperaccumulator plants.

[0049] A "nickel hyperaccumulator plant" means a plant capable of storing in its aerial parts a nickel concentration greater than or equal to 1000 pg / g of dry matter. The nickel stored in the aerial parts of the plant comes from the soil in which the plant is grown.

[0050] For example, depending on the geographical area, nickel hyperaccumulator plants are chosen from the following species:

[0051] Odontarrhena chalcidica (syn= Alyssum mural)

[0052] Bornmuellera emarginata (syn= Leptoplax emarginata)

[0053] Bornmuellera tymphaea

[0054] Berkheya coddii

[0055] Blepharidium guatemalense

[0056] Phyllanthus rufuschaneyi

[0057] Rinorea aff. bengalensïs.

[0058] The step 12 of providing the plant material preferably comprises a step of providing dried plants.

[0059] Preferably, the plants are dried to a moisture content of 20% or less. A moisture content above 20% may lead to deterioration, particularly mold growth, of the plants during storage prior to crushing. Plants are best dried in the open air, particularly in the field after harvesting.

[0060] Alternatively, and in particular in the case where the plants are grown in regions in which the air humidity level is high so that the open air drying results in a humidity level of the plants greater than 20%, the drying comprises a step of heating the plants at a temperature and for a duration suitable for obtaining a humidity level less than or equal to 20%, and preferably between 5% and 20%.

[0061] Dried plants also preferably have a moisture content of 5% or higher. This is because such a moisture content is favorable for the agglomeration of dried plants in the form of granules. A moisture content of 5% or higher is obtained in particular by adjusting the drying time of the plants in the open air and / or the temperature and heating time accordingly.

[0062] Dried plants therefore preferably have a humidity level of between 5% and 20%.

[0063] For example, the moisture content of dried plants is approximately 15%.

[0064] According to one embodiment, step 12 of providing plant material comprises:

[0065] - the supply of dried plants as described above; and

[0066] - grinding dried plants to obtain a ground plant material.

[0067] More particularly, during step 12 of providing the plant material, the plant material is provided in the form of granules comprising this ground plant material. According to this embodiment, step 12 of providing plant material comprises a step of manufacturing granules from this ground plant material.

[0068] The pellet manufacturing stage optionally includes the compression of the plant shreds to obtain the pellets.

[0069] In this case, the step of grinding the dried plants to obtain a ground plant material is carried out in such a way as to obtain a ground plant material with a particle size suitable for the manufacture of granules by compression. This particle size is notably between 3 mm and 5 mm. However, other particle sizes are possible depending on the desired dimensions of the granules and the compaction capacity of the plants used.

[0070] The step of compressing the ground plant material to form the granules is carried out in particular by extrusion in a die. The length of the granules is determined by the compression device used, in particular the die. The maximum length of the granules is chosen so as to prevent the granules from breaking during storage and / or transport. For example, the granules have a length of between 20 mm and 40 mm, and for example approximately equal to 27 mm.

[0071] The step of compressing the ground plant material to form the granules optionally comprises heating the ground plant material to a temperature between 60°C and 80°C. This heating step is preferably carried out simultaneously with the compression. It facilitates the agglomeration of the ground plant material into granules.

[0072] Optionally, the compression step further comprises the addition of an adjuvant intended to facilitate the agglomeration of the ground plant material. This adjuvant is preferably water. The quantity of adjuvant added during this step is chosen according to the initial humidity level of the dried plants in order to obtain sufficient agglomeration of the ground plant material.

[0073] According to a first embodiment of the method, the thermochemical treatment step 14 of the plant material is a combustion step to obtain nickel-rich ash. Depending on the composition of the plant material, the nickel-rich ash may contain, in addition to nickel, other elements of interest in the stainless steel manufacturing process, such as, for example, magnesium, silicon, iron or calcium.

[0074] The combustion step is carried out in a combustion unit in the presence of oxygen at a temperature between 550°C and 1200°C. More particularly, the combustion temperature is between 800°C and 1100°C, and is notably approximately equal to 900°C.

[0075] Combustion is carried out in the presence of excess oxygen so as to achieve complete combustion of the plant material. For example, oxygen is supplied via an air inlet provided in the combustion unit.

[0076] The combustion unit is, for example, a furnace or a biomass boiler.

[0077] At the end of this combustion stage, nickel-rich ash is obtained with a nickel content greater than or equal to 10% by mass, and for example between 10% by mass and 40% by mass.

[0078] In the ash, nickel is in the form of nickel oxide NiO x .

[0079] These ashes also have a phosphorus content greater than or equal to 0.5% by mass, and in particular between 0.5% by mass and 5% by mass. In the ashes, phosphorus is typically present in the form of apatite Ca5(PO4)3(OH) and related forms.

[0080] In ash, the ratio of nickel mass content to phosphorus mass content is, for example, less than 40.

[0081] Ash may also contain other chemical elements, including calcium, for example, at a content of between 10% by mass and 40% by mass, and potassium, for example, at a content of between 2% by mass and 15% by mass. Calcium and potassium are typically present in ash as oxides, particularly carbonates, hydroxyapatite Ca5(PO4)3(OH), potassium sulfate (K2SO4) or potassium chloride (KCl).

[0082] The precise composition of the ash depends in particular on the nature of the plant material, the soil in which it was grown, the amendments made to the soil (fertilizers, manure, composts, etc.), the combustion temperature used, as well as climatic conditions.

[0083] Step 16 of manufacturing briquettes from nickel-rich ash includes:

[0084] - the manufacture of a mixture comprising nickel-rich ash, as well as a binder, a source of reducing agent, a flux and optionally additional materials for supplying alloying elements for the ferroalloy; and

[0085] - compression of this mixture to obtain briquettes.

[0086] The binder used is, for example, molasses. Its function is to bind the different, relatively dry, components of the mixture together, in order to enable the production of briquettes.

[0087] The reducing agent comprises in particular carbon, aluminum and / or silicon. It enables, during the further processing of the briquettes in the combustion unit, the reduction of the oxides contained in the briquettes, and in particular the reduction of nickel oxide to metallic nickel. In the case where carbon is used as a reducing agent, the carbon is supplied, for example, in the form of anthracite or biochar.

[0088] Lime is one example of a flux. Fluxing helps remove unwanted impurities from the alloy being manufactured during subsequent processing of the briquettes in the furnace. Specifically, the flux fuses with these impurities to form slag, which separates from the alloy and can then be removed.

[0089] The mixture optionally contains additional materials, other than ash, to provide additional alloying elements. These additional materials include in particular materials containing nickel, iron, molybdenum, calcium and / or chromium. These additional materials are, for example, supplied in the form of sludge or dust from the metallurgical industry and, optionally, metals, for example nickel or molybdenum, added as dopants. In sludge and dust from the metallurgical industry, the elements of interest, such as nickel, iron, molybdenum, calcium and / or chromium, are usually present in the form of oxides.

[0090] Optionally, the mixing manufacturing sub-step of briquette manufacturing step 16 comprises:

[0091] - the manufacture of a first mixture comprising nickel-rich ash, the reducing agent and optionally additional materials for adding alloying elements for the ferroalloy;

[0092] - drying of this first mixture; and

[0093] - mixing this first mixture with the binder and the fondant.

[0094] According to the invention, the step 16 of manufacturing the briquettes includes an adjustment of the phosphorus content such that the briquettes have a ratio of the mass content of nickel to the mass content of phosphorus greater than or equal to 40. Preferably, the adjustment of the phosphorus content is carried out such that the briquettes have a ratio of the mass content of nickel to the mass content of phosphorus greater than or equal to 80.

[0095] Such an adjustment is necessary in the context of the process for manufacturing a stainless steel according to the invention. Indeed, the plant material used in the context of the invention typically comprises a phosphorus content of the order of 1000 to 4000 mg / kg of plant material. This phosphorus comes in particular from the fertilizers used for the growth of the plants forming the plant material. However, phosphorus is undesirable in the context of the manufacture of stainless steels, insofar as it deteriorates their properties. Consequently, according to European standards, the phosphorus content of stainless steels must typically be less than 0.045%.

[0096] In the context of the present invention, briquettes are considered suitable for the manufacture of stainless steels if they have a ratio of the mass content of nickel to the mass content of phosphorus greater than or equal to 40.

[0097] The higher the ratio of nickel mass content to phosphorus mass content, the more nickel from the combustion of plant material as described below can be used in the stainless steel manufacturing process. Therefore, in order to maximize the use of bio-sourced nickel, the ratio of nickel mass content to phosphorus mass content is preferably greater than or equal to 80.

[0098] More specifically, the phosphorus content adjustment step includes:

[0099] (a) a mixture of nickel-rich ashes from plants characterized by different combinations of species, geographical origins and harvest years to obtain a pre-defined ratio of nickel mass content to phosphorus mass content, for example greater than or equal to 40, in the mixture;

[0100] (b) washing the nickel-rich ash from the combustion stage to reduce its phosphorus content; and / or

[0101] (c) a mixture of the nickel-rich ash from the combustion step with other nickel-containing materials so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the ash.

[0102] The composition of the plant, and in particular, the nickel and / or phosphorus content of the plant, depends on the species of the plant, its geographical origin, as well as its harvest year, and in particular the climatic conditions associated with the latter. Therefore, it is possible to adjust the ratio of the nickel content to the phosphorus content of the ash by mixing together ash obtained from plants with different combinations of species, geographical origins and harvest years.

[0103] Each type of ash used to produce the ash mixture according to alternative (a) is in particular obtained by implementing the thermochemical treatment step 14 as described above with a plant material obtained from plants characterized by a combination of species, geographical origins and particular harvest years.

[0104] The mixture of ashes from plants characterized by different combinations of species, geographical origins and harvest years, the washing of the ashes and the mixing of the ashes with other nickel-containing materials are, within the scope of the invention, used either alternatively or in combination, depending on the phosphorus content of the ashes, as well as the quantity of ashes available.

[0105] For example, adjustment of the phosphorus content is achieved solely by washing the ash and / or by mixing ash from plants corresponding to different combinations of species, geographical origins and harvest years in a case where the amount of nickel contained in the ash alone is sufficient to ensure the desired nickel content in the ferroalloy subsequently manufactured by pyrometallurgy. In this case, the use of bio-sourced ash in the stainless steel manufacturing process is maximized.

[0106] Preferably, adjustment of the phosphorus content is achieved solely by washing the ash.

[0107] Alternatively, adjustment of the phosphorus content is achieved by mixing the nickel-rich ash with other nickel-containing materials, for example in the case where the amount of nickel contained in the ash is insufficient on its own to ensure the desired nickel content in the ferroalloy subsequently manufactured by pyrometallurgy.

[0108] Ash washing is carried out before the manufacturing step of the mixture with the binder, the source of reducing agent, the flux and the additional alloying element materials described above.

[0109] For example, ash washing includes:

[0110] - bringing the ashes into contact with an acid solution for a period of time sufficient to reduce the phosphorus content of the ashes;

[0111] - filtration of this mixture to separate the solid fraction, namely the washed ash, from the liquid fraction, namely an acidic aqueous solution also containing the chemical elements extracted from the ash, in particular phosphorus.

[0112] More particularly, the ashes are placed in contact with the acid solution for a period of between 5 minutes and 10 minutes at a temperature of between 5°C and 50°C, and more particularly at a temperature of between 15°C and 25°C, for example approximately equal to 20°C.

[0113] Preferably, the acidic solution is in particular an aqueous solution containing 0.1 mol / L to 4 mol / L of protons, for example an aqueous solution of sulfuric acid, hydrochloric acid, nitric acid or mixtures thereof.

[0114] For example, the acid solution is an aqueous solution of sulfuric acid with a concentration between 0.05 mol / L and 2 mol / L.

[0115] In addition to phosphorus, the liquid fraction obtained after filtration may also contain other chemical elements extracted from the ash, in particular potassium and calcium.

[0116] Optionally, at least some of the chemical elements extracted from the ash, in particular phosphorus and / or potassium, are then extracted from the liquid fraction by any suitable process. These elements can, for example, be reused as fertilizers. The extraction of these chemical elements is carried out by any suitable method, and in particular by decantation after concentration.

[0117] According to a variant, the washing of the ashes comprises, before bringing the ashes into contact with the written solution described above, a preliminary washing of the ashes with water. This preliminary washing step with water makes it possible to extract from the ashes at least part of the potassium contained in the ashes.

[0118] Optionally, after extraction of the chemical elements extracted from the ash from the liquid fraction, the acidic aqueous solution is reused in the ash washing process.

[0119] At the end of the washing stage, ash is obtained whose phosphorus content is reduced compared to its initial content before washing by a reduction factor of around 10.

[0120] The washing step solubilizes the phosphorus present in the ash, without solubilizing the nickel.

[0121] The washed ash is then mixed with the binder, reducing agent, flux and optionally the additional alloying element materials required for the production of the ferroalloy during the mixing step described above.

[0122] The production of ferroalloy by pyrometallurgy from briquettes includes:

[0123] - the introduction of briquettes into a furnace; and

[0124] - melting the briquettes in the furnace to obtain a liquid ferroalloy, the ferroalloy comprising between 4% and 50% by mass of nickel.

[0125] The briquette introduction sub-step optionally includes the introduction of scrap metal. Scrap metal is particularly useful during the start-up phase of the electric arc furnace, as explained later. For example, a quantity of scrap metal of between 10 and 20 tonnes is introduced during this sub-step.

[0126] The melting of the briquettes in the furnace is accompanied by a reduction of the metal oxides contained in the briquettes.

[0127] The melting step is particularly carried out in an electric arc furnace. In this case, the briquette introduction sub-step also includes the introduction of scrap metal. This scrap metal is melted during the start-up phase of the electric arc furnace and forms a molten bed, also called a "bath foot", on the bottom of the electric arc furnace. This "bath foot" is necessary to initiate melting in the electric arc furnace.

[0128] The manufacture of ferroalloy by pyrometallurgy also includes the casting of liquid ferroalloy into semi-finished products, in particular the casting of liquid ferroalloy into ingots in suitable molds or into granules by granulation. Granulation means the casting of liquid ferroalloy into a water jet.

[0129] In the case where the semi-finished products are ingots, these ingots are optionally mechanically split into ferroalloy blocks.

[0130] The ferroalloy thus obtained is an iron-based alloy, comprising between 4% and 50% nickel and, optionally, depending on the composition of the raw materials used, molybdenum and / or chromium. For example, the ferroalloy contains between 0% by mass and 10% by mass of molybdenum and between 10% by mass and 20% by mass of chromium. The nickel content of the ferroalloy is essentially defined by the nickel content of the briquettes, the nickel yield of the process being greater than 95%.

[0131] Finally, the step of manufacturing stainless steel from the ferroalloy comprises: providing a mixture of the ferroalloy with scrap metal, the scrap metal including in particular stainless steel scrap metal; melting this mixture in an electric furnace, in particular an electric arc furnace, to obtain a liquid steel; and refining this liquid steel to obtain the stainless steel.

[0132] The refining stage includes in particular a first refining stage carried out in a converter, in particular an AOD converter ("Argon Oxygen Decarburization" in English), followed by a second refining stage, carried out in a ladle.

[0133] In particular, the refining stages allow the liquid steel to be purified and its composition adjusted to the desired composition for stainless steel.

[0134] At the end of the refining stage, the stainless steel is cast into semi-finished products, particularly slabs or blooms.

[0135] The manufacturing step of stainless steel is known in itself and is therefore not described in further detail.

[0136] The method for manufacturing a stainless steel according to the invention is advantageous. Indeed, thanks to the use of nickel hyperaccumulator plants as a source of nickel for the manufacture of the ferroalloy, it makes it possible to avoid or at least reduce the use of nickel from nickel-rich ores, and thus not only to reduce the costs associated with the manufacture of stainless steels, but also to improve the environmental impact linked to the manufacture of these steels. Indeed, it makes it possible to reduce the degradation of the soil, the subsoil and the hydrosphere associated with the extraction of ores, to participate in the decontamination of the soil thanks to the use of nickel hyperaccumulator plants and to reduce the CO2 emissions associated with the manufacture of the ferroalloy.The method according to a second embodiment differs from the method according to the first embodiment only by the nature of the thermochemical treatment step 14 of the plant material, as well as by the manufacturing step 16 of the briquettes.

[0137] In the second embodiment, the thermochemical treatment 14 is pyrolysis, not combustion. Pyrolysis produces nickel-rich biochar from the plant material, rather than ash as described in the first embodiment.

[0138] Pyrolysis is carried out in a pyrolysis unit.

[0139] At the end of the pyrolysis stage, a nickel-rich biochar is obtained with a nickel content greater than or equal to 2% by mass, and for example greater than or equal to 4% by mass.

[0140] The phosphorus content of biochar is greater than or equal to 0.5% by mass, and in particular between 0.5% and 5%.

[0141] In biochar, the ratio of nickel mass content to phosphorus mass content is, for example, less than 40.

[0142] Biochar means a solid material obtained by pyrolysis of plant material.

[0143] The precise composition of biochar depends in particular on the nature of the plant material, the soil in which it was grown, the amendments made to the soil (fertilizers, manure, composts, etc.), the combustion temperature used, as well as climatic conditions.

[0144] The other process steps described in the process according to the first embodiment are identical in the process according to the second embodiment, except for the differences described below.

[0145] The steps implemented on the ashes obtained at the end of the thermochemical treatment step 14 by combustion in the first embodiment are, in the second embodiment, implemented on a crushed nickel-rich biochar obtained at the end of the thermochemical treatment step by pyrolysis.

[0146] In the second embodiment, the method comprises a biochar grinding step carried out between the thermochemical treatment step 14 and the briquette manufacturing step 16 to obtain the nickel-rich biochar grind.

[0147] In the context of the method according to the second embodiment, the step 16 of manufacturing briquettes from the nickel-rich biochar ground material thus comprises: - the manufacture of a mixture comprising the nickel-rich biochar ground material, as well as a binder, a flux and additional materials for supplying alloying elements for the ferroalloy; and

[0148] - compression of this mixture to obtain briquettes.

[0149] Indeed, in the process according to the second embodiment, the nickel-rich biochar, which contains carbon, acts as a reducing agent for the pyrometallurgy step. It is therefore not necessary to add an additional reducing agent to the mixture, as was described in the first embodiment.

[0150] The binder and the flux have the same characteristics as described above with regard to the first embodiment.

[0151] Additional materials, other than biochar, for providing additional alloying elements. These additional materials include, in particular, materials containing nickel, iron, molybdenum, calcium and / or chromium. These additional materials are, for example, supplied in the form of sludge or dust from the metallurgical industry and, optionally, metals, for example nickel or molybdenum, added as dopants. In sludge and dust from the metallurgical industry, the elements of interest, such as nickel, iron, molybdenum, calcium and / or chromium, are usually present in the form of oxides.

[0152] In this embodiment, the mixing manufacturing sub-step of the briquette manufacturing step 16 optionally comprises:

[0153] - the production of a first mixture comprising nickel-rich biochar and optionally additional materials for adding alloying elements for the ferroalloy;

[0154] - drying of this first mixture; and

[0155] - mixing this first mixture with the binder and the fondant.

[0156] In the method according to the second embodiment, the adjustment of the phosphorus content comprises:

[0157] (a) a mixture of nickel-rich biochars from plants characterized by different combinations of species, geographical provenances and harvest years to obtain a ratio of nickel mass content to phosphorus mass content greater than or equal to 40; and / or

[0158] (b) a mixture of the nickel-rich biochar obtained by the pyrolysis step with other nickel-containing materials so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the ash.

[0159] Each type of biochar used to make the biochar mixture according to alternative (a) is in particular obtained by implementing the thermochemical treatment step 14 as described above with a plant material obtained from plants characterized by a combination of species, geographical origins and particular harvest years.

[0160] As indicated above, the steps of providing 12 plant material, manufacturing 18 the ferroalloy from the briquettes, and manufacturing the stainless steel from the ferroalloy are identical in the first and second embodiments.

[0161] The manufacturing method according to the second embodiment has the same advantages as the manufacturing method according to the first embodiment.

[0162] Furthermore, to the extent that the biochar contains carbon, the method according to the second embodiment has the additional advantage of providing, in addition to nickel, at least part of the reducing agent necessary for the step of manufacturing the ferroalloy by pyrometallurgy.

Claims

CLAIMS 1. Method for manufacturing a stainless steel, comprising the following successive steps: a. the manufacture (10) of a ferroalloy, comprising: - the supply (12) of plant material, said plant material having a nickel content greater than or equal to 0.5% by dry mass; - thermochemical treatment (14) of the plant material in a thermochemical treatment unit to obtain nickel-rich ash having a nickel content greater than or equal to 10% by mass or nickel-rich biochar having a nickel content greater than or equal to 2% by mass; - manufacturing (16) briquettes from nickel-rich ash or nickel-rich biochar grind, comprising adjusting the phosphorus content such that the briquettes have a ratio of nickel mass content to phosphorus mass content greater than or equal to 40; and - the manufacture (18) of a ferroalloy by pyrometallurgy from the briquettes, the ferroalloy having a nickel content of between 4% and 50% by mass; and b. the production (20) of a stainless steel from the ferroalloy.

2. Manufacturing method according to claim 1, in which the thermochemical treatment (14) is a combustion in the presence of oxygen at a temperature between 550°C and 1200°C so as to obtain ash.

3. Manufacturing method according to claim 2, wherein the combustion is carried out between 800°C and 1100°C.

4. Manufacturing method according to one of claims 2 or 3, in which the adjustment of the phosphorus content comprises: a. a mixture of nickel-rich ashes from plants associated with different combinations of species, geographical provenances and years of harvest of the plants to obtain a pre-defined ratio of the mass content of nickel to the mass content of phosphorus in the mixture; b. washing the nickel-rich ashes to reduce the phosphorus content; and / or c. mixing the nickel-rich ashes with other nickel-containing materials so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the nickel-rich ashes.

5. Manufacturing method according to claim 4, in which the washing of the ashes is carried out in an acid solution for a period of between 5 minutes and 10 minutes at a temperature of between 5°C and 50°C.

6. Manufacturing method according to claim 5, wherein the acid solution is an aqueous solution containing 0.1 to 4 mol / L of protons.

7. A manufacturing method according to any one of claims 1 to 6, wherein the ratio of the mass content of nickel to the mass content of phosphorus in the nickel-rich ash or nickel-rich biochar grind is less than 40.

8. A method according to any one of claims 2 to 7, wherein the manufacture of the briquettes (16) comprises: a. the manufacture of a mixture comprising the nickel-rich ash, as well as a binder, a source of reducing agent, a flux and optionally additional materials for supplying alloying elements for the ferroalloy; and b. the compression of this mixture to obtain the briquettes.

9. Method according to claim 8, in which the additional materials come at least in part from sludge or dust from the metallurgical industry.

10. Manufacturing method according to claim 1, in which the thermochemical treatment (14) is a pyrolysis so as to obtain nickel-rich biochar, the method further comprising a step of grinding the biochar to obtain a biochar grind. 1 1. A manufacturing method according to claim 10, wherein the adjustment of the phosphorus content comprises: a. a mixture of nickel-rich biochars from plants characterized by different combinations of species, geographical origins and years of harvest of the plants to obtain a pre-defined ratio of the mass content of nickel to the mass content of phosphorus in the mixture; and / or b. a mixture of the nickel-rich biochar with other nickel-containing materials so as to obtain in the mixture a value of the ratio of the mass content of nickel to the mass content of phosphorus greater than the value of the mass content of nickel to the mass content of phosphorus in the nickel-rich biochar.

12. A method according to any one of claims 10 or 11, wherein the manufacture of the briquettes comprises: the manufacture of a mixture comprising the nickel-rich biochar grind, as well as a binder, a flux and additional alloying element materials for the ferroalloy; and - compression of this mixture to obtain briquettes.

13. Method according to claim 12, in which the additional materials come at least in part from sludge or dust from the metallurgical industry.

14. A manufacturing method according to any one of claims 1 to 13, wherein the plant material is provided in the form of a ground material. plants, particularly in the form of granules comprising ground plant material.

15. Manufacturing method according to claim 14, wherein the step of providing (12) plant material comprises: - the supply of dried plants; - grinding the dried plants to obtain a ground plant material; and optionally - the compression of this plant material to obtain granules.

16. Manufacturing method according to claim 15, further comprising, during the compression step to form the granules, heating the plant ground material to a temperature between 60°C and 80°C.

17. Manufacturing method according to one of claims 15 or 16, further comprising, during the compression step to form the granules, an addition of an adjuvant intended to facilitate the agglomeration of the ground plant material, the adjuvant preferably being water.

18. Method according to any one of claims 1 to 17, in which the plant material is obtained from nickel hyperaccumulator plants, for example chosen from the following species: Odontarrhena chalcidica (syn= Alyssum mural) Bornmuellera emarginata (syn= Leptoplax emarginata) Bornmuellera tymphaea Berkheya coddii Blepharidium guatemalense Phyllanthus rufuschaneyi Rinorea aff. bengalensïs.

19. A method according to any one of claims 1 to 18, wherein the adjustment of the phosphorus content is carried out such that the briquettes have a ratio of the mass content of nickel to the mass content of phosphorus greater than or equal to 80.

20. A method according to any one of claims 1 to 19, wherein the manufacture (18) of the ferroalloy by pyrometallurgy from the briquettes comprises - the introduction of briquettes into an oven; - melting the briquettes in the furnace to obtain a liquid ferroalloy, the ferroalloy comprising between 4% and 50% by mass of nickel and casting the ferroalloy in the form of a semi-finished product, for example in the form of ingots or granules.

21. A method according to any one of claims 1 to 20, wherein the manufacture (20) of the stainless steel from the ferroalloy comprises: - the supply of a mixture of ferroalloy and scrap metal, the scrap metal including in particular stainless steel scrap metal; - melting this mixture in an electric furnace, in particular an electric arc furnace, to obtain liquid steel; and - refining this liquid steel to obtain stainless steel.

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