Method and facility for treating smelting residues from the metal industry

Abstract

The invention relates to a method for treating aluminous slag comprising aluminium, the method including: - a step (100) of preparing a solid feedstock comprising the aluminous slag and red mud, - a step (200) of mixing the solid feedstock with an aqueous solution containing water to cause a corrosion reaction of the aluminium contained in the aluminous slag, the corrosion reaction producing: - a gaseous product containing dihydrogen and - a reaction fluid composed of a liquid product and a solid product, - a step (300) of recovering the products, the recovery step including the sub-steps consisting in: - removing the solid product from the reaction fluid, the solid product comprising an aluminium oxide and / or an aluminium hydroxide, and - extracting the gaseous product containing dihydrogen.

Description

[0001] METHOD AND INSTALLATION FOR THE TREATMENT OF MELTING RESIDUES FROM THE METALLURGICAL INDUSTRY - FIELD OF THE INVENTION

[0002] The present invention relates to the general technical field of waste inerting solutions, enabling the long-term immobilization, through stabilization, of the compounds contained in this waste.

[0003] In the context of the present invention, "waste" means a secondary material from the metallurgy industry, and in particular from the production of aluminium.

[0004] BACKGROUND OF THE INVENTION

[0005] 1. Principle of slag

[0006] Most metal manufacturing processes generate large quantities of waste, which is often poorly managed or massively stockpiled.

[0007] This waste presents a dual challenge:

[0008] On the one hand, they are potential nuisances to the environment and health, and

[0009] On the other hand, they can constitute a valuable resource for certain industries.

[0010] This is particularly the case for the aluminium industry, which produces large quantities of waste each year, the most problematic of which is aluminous slag.

[0011] Aluminum slag (or "aluminum dross") — also known as "dross" in English — is generated by aluminum foundries when they melt metal and it comes into contact with oxygen in the air.

[0012] In particular, when a mass of aluminum is kept molten in a production furnace, aluminous slag forms on the surface of this molten aluminum. This aluminous slag acts like a sponge, absorbing the metallic aluminum on which it floats. To limit the amount of metallic aluminum absorbed by the aluminous slag, it is periodically removed by a skimming operation.

[0013] The aluminous slag thus removed is typically a pasty or granular material containing a substantial amount of free aluminum in metallic form as well as aluminum oxide and other non-metallic compounds.

[0014] Aluminous slags are generally classified into two categories based on their aluminum content:

[0015] "White dross" type slags when they are composed of a mixture of aluminum oxide and metallic aluminum, and whose metal content varies between 40 and 80%, and "black dross" type slags when they are composed of a mixture of salt, oxides and metallic aluminum, and whose metallic aluminum content varies between 5% and 40% metallic aluminum.

[0016] 1.1. Slag of the "white sludge" type

[0017] "White slag" type slags — or primary slags — are generally considered to be valuable waste.

[0018] Indeed, "white slag" type slags can be treated by processes that allow the extraction of a substantial amount of the metallic aluminum they contain.

[0019] For example, document WO 1992 / 009,708 describes a process for recovering aluminum from "white slag" type slag, comprising the following steps:

[0020] introduction of the slag to be treated into a rotary or oscillating kiln,

[0021] heating the introduced slag to a temperature higher than the melting point of aluminum while rotating or oscillating the furnace to promote the separation of the aluminum,

[0022] removal of the separated aluminum from the oven, then

[0023] removal of remaining slag.

[0024] At the end of such a process, the remaining slag is of the "black grime" type.

[0025] 1.2. Slag of the "black sludge" type

[0026] "Black dross" type slags — or secondary slags — which contain higher concentrations of salt oxides and other impurities, are considered to be:

[0027] hazardous waste, due to its potential to release flammable (H2 and CH4) and toxic (NH3, H2S, PH3, etc.) gases upon contact with water and its potential for aqueous contamination due to the leachable toxic salts it contains, and as

[0028] non-recoverable waste due to the small quantities of metallic aluminum they contain, which makes their exploitation too costly on an industrial scale.

[0029] Currently, ensuring both legal compliance and long-term safety of these "black sludge" type slags is therefore a major challenge for our society.

[0030] 2. Existing slag treatments

[0031] Slag treatment processes have already been proposed to neutralize the slag and recycle the metallic aluminum it contains.

[0032] Such processes are based on the alkaline treatment of slag to neutralize it through a hydrolysis reaction of the aluminum it contains (a so-called "hydrometallurgical" reaction). For example, document WO 2023 / 039490A1 describes a process for obtaining solid products and combustible gas from aluminum waste, the process comprising the following steps:

[0033] obtaining a reactive mass comprising aluminum remelting waste or a derivative thereof; application of a solvent to the reactive mass to generate a solution and a first solid product; separation of the solution from the first solid product;

[0034] application of a reagent to a part of the first solid product to initiate a reaction, the reagent being different from the solvent, the reaction generating a combustible gas and a second solid product; separation of the reagent from the second solid product.

[0035] To increase the pH of the solution, the reagent used can be sodium hydroxide and / or potassium hydroxide.

[0036] One drawback of these processes concerns the cost and environmental impact associated with the use of large quantities of sodium or potassium hydroxide.

[0037] Another drawback of these processes is the slowness of the hydrolysis reaction, which makes them difficult to implement on an industrial scale.

[0038] Another drawback of these processes is their limited effectiveness. Indeed, these processes only allow for the recycling of a small amount of the metallic aluminum contained in the slag. This results in only partial neutralization of the treated slag.

[0039] Document EP 4464659 describes an installation for the production of dihydrogen comprising: a reaction vessel intended to contain an oxidizable material,

[0040] an alkaline solution feeding system,

[0041] a pure water supply system,

[0042] a collection system (4) for dihydrogen downstream of the reaction vessel.

[0043] Document WO 2023 / 039490 describes a process for obtaining solid products and combustible gas using aluminum waste, the process comprising:

[0044] the pretreatment of a reactive mass – the reactive mass comprising aluminum remelting waste – by applying a solvent to generate a solution and a first solid product, the separation of the solution from the first solid product,

[0045] the application of a reagent to the first solid product to initiate a reaction generating a combustible gas and a second solid product.

[0046] Document EP 0576416 describes a process for producing aluminum according to the Bayer technique using slag dust (or "dross") containing aluminum, said slag being treated with caustic soda. 3. Purpose of the invention

[0047] One aim of the present invention is to propose a slag treatment process that overcomes at least one of the aforementioned drawbacks.

[0048] In particular, one object of the present invention is to provide a slag treatment process that is more efficient than prior art processes in that it allows the transformation of metallic aluminum contained in slag (in particular "black dross" type slag) into alumina with a hydrogen conversion / yield of at least 70%, preferably 80%, and even more preferably 90%, and / or

[0049] in which the rate of hydrolysis reaction of metallic aluminium contained in the slag is accelerated.

[0050] Another objective of the present invention is to provide a less expensive process for treating waste from metal manufacturing — and in particular from aluminum manufacturing — in which the products obtained by implementing the process include:

[0051] neutralized products that present a limited risk to the environment and health,

[0052] valuable products with potential economic value.

[0053] In 2022, aluminum slag production in Europe amounted to 653,000 tonnes / year, including:

[0054] 28% of slag of the "white grime" type, and

[0055] 72% of slag of the "black grime" type.

[0056] The process described below is therefore particularly relevant for limiting the environmental impact linked to the production of metals, and in particular to the production of aluminium.

[0057] BRIEF DESCRIPTION OF THE INVENTION

[0058] To this end, the invention proposes a process for treating aluminous slags comprising aluminum, remarkable in that the process comprises:

[0059] a step of preparing a solid feedstock comprising aluminous slag and red mud, a step of mixing the solid feedstock with an aqueous solution containing water to induce a corrosion reaction of the aluminum contained in the aluminous slag, said corrosion reaction producing:

[0060] o a gaseous product containing dihydrogen and

[0061] o a reaction fluid composed of a liquid product and a solid product,

[0062] a product recovery step, said recovery step including the substeps of: collecting the solid product from the reaction fluid, said solid product comprising an aluminium oxide and / or an aluminium hydroxide, and

[0063] to extract the gaseous product containing dihydrogen.

[0064] Therefore, it is proposed:

[0065] to process aluminous slag, which is a primary industrial waste product,

[0066] using red mud, which is a second industrial waste product.

[0067] These red muds are used as a reagent in the corrosion reaction of metallic aluminum contained in aluminous slags.

[0068] In addition to the neutralization of aluminous slags, the process according to the invention makes it possible to produce a gas containing hydrogen H2 (which is a valuable product).

[0069] As will be explained in more detail later, the use of red mud allows:

[0070] not only to reduce the production costs of gaseous hydrogen by using an industrial waste product (i.e., red mud) as a "reaction promoter" to react the aluminum particles contained in aluminous slag with the water contained in the aqueous solution,

[0071] but also to improve the yield of the hydrolysis reaction by increasing the amount of aluminum particles consumed,

[0072] and finally, to neutralize the red mud and aluminous slag, making them less polluting. The process according to the invention is therefore particularly beneficial for the environment.

[0073] Preferred, but not limiting, aspects of the process according to the invention are as follows:

[0074] Red mud may contain a mineral compound including:

[0075] o 10 to 50 percent by weight of ferrous compounds,

[0076] o 12 to 35 percent by weight of aluminium compounds,

[0077] o 5 to 17 percent by weight of silicon compounds,

[0078] o 2 to 21 percent by weight of titanium dioxide (TiO2),

[0079] o 0.5 to 10 percent by weight of calcium compounds,

[0080] o additional impurities if applicable;

[0081] The solid load preparation stage may include the following sub-steps:

[0082] o a sub-step of reducing aluminous slags and red muds to a desired particle size, possibly by grinding and sieving, the desired particle size being between 1 µm and 10 mm, preferably between 1 and 2000 µm, and even more preferably between 1 and 200 µm,

[0083] o a sub-stage combining aluminous slag and red mud; the weight ratio between:

[0084] the quantity of red mud, and

[0085] o the quantity of aluminous slag

[0086] contained in the solid charge can be between 0.2 and 5, preferably between 0.25 and 4, even more preferably between 0.5 and 2, in particular between 1 and 1.5;

[0087] The substep of withdrawing the gaseous product containing dihydrogen may include a filtration operation of the gaseous product to separate the dihydrogen from toxic gases contained in said gaseous product.

[0088] The invention also relates to an installation for processing aluminous slag comprising aluminum, notable in that the installation includes:

[0089] a reaction chamber,

[0090] a loading unit upstream of the reaction vessel, the loading unit including:

[0091] o an aqueous solution feeding system, the aqueous solution containing water, and o a solid feed supply system, the solid feed being composed of aluminous slag and red mud,

[0092] a recovery unit downstream of the reaction vessel, the recovery unit including:

[0093] o a system for collecting gaseous product(s),

[0094] o a system for harvesting solid product(s),

[0095] a control unit to operate the loading unit, the reaction vessel, and the recovery unit, said control unit being configured to control:

[0096] o the preparation of the solid feed comprising aluminous slag and red mud, o the mixing of the solid feed with the aqueous solution in the reaction vessel to induce a corrosion reaction of the aluminum contained in the aluminous slag, said corrosion reaction producing:

[0097] ■ a gaseous product containing dihydrogen and

[0098] ■ a reaction fluid composed of a liquid and a solid product, o the recovery of the products by:

[0099] ■ taking the solid product from the reaction fluid, said solid product comprising an aluminium oxide and / or an aluminium hydroxide, and in

[0100] ■ drawing off the gaseous product containing dihydrogen.

[0101] Preferred, but not limiting, aspects of the installation according to the invention are as follows:

[0102] Red mud may contain a mineral compound including: 10 to 50 percent by weight of ferrous compounds,

[0103] o 12 to 35 percent by weight of aluminium-based compounds,

[0104] o 5 to 17 percent by weight of silicon-based compounds,

[0105] o 2 to 21 percent by weight of titanium dioxide (TiO2),

[0106] o 0.5 to 10 percent by weight of calcium compounds,

[0107] o additional impurities if applicable;

[0108] The loading unit may also include a crushing system, upstream of the supply system, to reduce aluminous slags and red muds to a desired particle size, the desired particle size being between 1pm and 10mm, preferably between 1 and 2000pm, and even more preferably between 1 and 200pm.

[0109] The invention also relates to a solid feedstock for the treatment of aluminous slag, said solid feedstock including said aluminous slag to be treated, notable in that the solid feedstock further comprises red mud. This red mud may comprise a mineral compound including:

[0110] 10 to 50 percent by weight of ferrous compounds,

[0111] 12 to 35 percent by weight of aluminium compounds,

[0112] 5 to 17 percent by weight of silicon compounds,

[0113] 2 to 21 percent by weight of titanium dioxide (TiO2),

[0114] 0.5 to 10 percent by weight of calcium compounds,

[0115] additional impurities if applicable.

[0116] The invention also relates to a thermochemical process for producing hydrogen from water and aluminous slag, based on the corrosion of the aluminum contained in the aluminous slag, in which at least one of the following reactions is carried out:

[0117] - 2Al(s) + &H2O(l) -> 2Al(OH)3(s) + 3H2(g) (1)

[0118] - 2Al(s) + 4H2O(l) ^ 2AlO(OH)(s) + 3H2(g) (2)

[0119] - 2Al(s) + 3H2O(l) -> Al2O3(S) + 3H2(g) (3)

[0120] remarkable in that the process includes:

[0121] a step of preparing a solid feedstock comprising aluminous slag and red mud, a step of mixing the solid feedstock thus prepared with an aqueous solution containing water, and

[0122] the recovery of a gaseous product containing hydrogen.

[0123] BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features of the invention will become clearer from the following description of several embodiments, given by way of non-limiting examples, based on the accompanying drawings in which:

[0124] Figure 1 is a schematic representation of a process for treating aluminous slag; Figure 2 is a schematic representation of an installation for implementing the process for treating aluminous slag;

[0125] Figure 3 is a schematic representation of a processing method implemented for carrying out different series of experiments;

[0126] Figure 4 is a graph illustrating the rate of conversion of aluminium to hydrogen over time for different solid feeds, each solid feed including a respective additive different from the additive contained in the other solid feeds;

[0127] Figure 5 is a schematic representation of an aluminum compound;

[0128] Figure 6 is a graph illustrating the rate of conversion of aluminium to hydrogen as a function of time for different solid feeds, each solid feed including respective aluminous slags different from the aluminous slags contained in the other solid feeds;

[0129] Figure 7 is a graph illustrating the rate of aluminum to hydrogen conversion as a function of time for different solid feedstocks, each composed of the same aluminous slag and the same red mud, with the quantity of red mud contained in each solid feedstock being different. Figure 8 is a graph illustrating the rate of aluminum to hydrogen conversion as a function of time for different solid feedstocks, each solid feedstock including red mud of different compositions.

[0130] Figure 9 is a graph illustrating the rate of conversion of aluminium to hydrogen over time for different solid feedstocks including treated (notably washed and filtered) red muds of different compositions.

[0131] DETAILED DESCRIPTION OF THE INVENTION

[0132] We will now describe different examples of embodiments of the invention with reference to the figures. In these different figures, equivalent elements are designated by the same numerical reference.

[0133] 1. Process

[0134] 1.1. General presentation

[0135] With reference to Figure 1, the process according to the invention comprises the following steps:

[0136] the preparation 100 of a solid feed comprising aluminous slag and red mud, the mixing 200 of the solid feed with an aqueous solution, the recovery 300:

[0137] of a solid product, and

[0138] o of a gaseous product containing dihydrogen.

[0139] Thus, the process includes a step 200 consisting of bringing an aqueous solution into contact with a solid charge composed of aluminous slags and red muds.

[0140] The aqueous solution contains water composed of water molecules H2O and mineral salts, such as distilled water, demineralized water, or osmosis water, etc.

[0141] 1.1.1. Main hydrolysis reaction

[0142] Bringing the aqueous solution into contact with the aluminous slags and red muds causes a reaction known as "hydrolysis".

[0143] During this hydrolysis reaction, the aluminum in the slag reacts with the water in the aqueous solution. The red mud acts as a catalyst, raising the pH of the aqueous solution to approximately 11.5 to 12.0.

[0144] More specifically, during the hydrolysis reaction, water molecules decompose under the action of the aluminum contained in the slag, which produces:

[0145] a solid containing an aluminum hydroxide or oxide (depending on the operating conditions of the reaction medium), and

[0146] a dihydrogen gas H2, more commonly called “hydrogen”.

[0147] In particular, the following first, second, and third hydrolysis reactions — known as "main reactions" — can occur depending on the reaction temperature:

[0148] - 2Al(s) + &H2O(l) -> 2Al(OH)3(s) + 3H2(g} (1)

[0149] - 2Al(s) + 4H2O(l) 2Al0(0H)(s) + 3H2(g) (2)

[0150] - 2Al(s) + 3H2O(l) -> Al2O3(S) + 3H2(g) (3)

[0151] These main reactions (1), (2), (3) are exothermic, and thermodynamically favorable between ambient temperature (25 °C) and the melting point of aluminum (660 °C):

[0152] When the reaction temperature is between 25 °C and 280 °C, the first hydrolysis reaction (1) predominates, with Al(OH)3 being the most stable product.

[0153] When the reaction temperature is between 280 and 480 °C, the second hydrolysis reaction (2) predominates, with AIO(OH) being the most stable product.

[0154] when the reaction temperature is above 480 °C, the third hydrolysis reaction (3) predominates, with Al2O3 being the most stable product.

[0155] 1.1.2. Secondary Neutralization Reactions In parallel with the main hydrolysis reaction, secondary reactions occur. These secondary reactions induce the production of:

[0156] of a solid containing aluminum hydroxide, and

[0157] toxic gases.

[0158] These secondary reactions include the following:

[0159] - AlN(s) + 3H2O(l) -> Al(OH)3(s) + NH3(g) (4)

[0160] - AUC3(S) + 12H2O(l) -> 4Al(OH)3(s) + 3Œ4(g) (5)

[0161] - AlP(s) + 3H2O(l) - Al(OH)3(s) + PH3(g) (6)

[0162] - Al2S3(s) + &H2O(l) -> 2Al(OH)3(s) + 3H2S(g) (7)

[0163] Thus, the secondary reactions — which take place simultaneously with the main hydrolysis reaction(s) — make it possible to neutralize the aluminous slags by eliminating, in gaseous form, the nitrides, carbides, sulfides and phosphides contained in the aluminous slags.

[0164] 1.1.3. Products obtained

[0165] The mixing step 200 therefore allows the water contained in the aqueous solution to react with the aluminum compounds (metallic aluminum, aluminum nitride, aluminum carbide, aluminum phosphide, aluminum sulfide, etc.) contained in the aluminous slag to obtain:

[0166] o a solid product comprising aluminum oxide and / or aluminum hydroxide, and o a gaseous product containing dihydrogen and toxic gases (NH3, CH4, PH3, H2S, etc.). Since the solid product is primarily composed of aluminum oxide / hydroxide, it has a limited capacity:

[0167] of the release of gas (flammable or toxic) upon contact with water, and

[0168] of aqueous contamination.

[0169] This solid product can therefore be considered inert.

[0170] The gaseous product can be treated by any technique known to those skilled in the art (for example, by membrane separation) to:

[0171] on the one hand, extract the dihydrogen gas H2 from the gaseous product (this dihydrogen gas H2 being usable), and

[0172] On the other hand, it retains the toxic gases it contains.

[0173] before being released into the atmosphere.

[0174] Thus, the process according to the invention makes it possible to neutralize aluminous slags by making them less polluting.

[0175] 1.2. Aluminous Dross. Aluminous slag is produced during aluminum smelting processes. More specifically, during conventional aluminum smelting operations, oxides, nitrides, and other non-metallic impurities accumulate on the surface of the molten aluminum bath. Before the molten aluminum bath is drained, these impurities are removed (or skimmed) from the surface of the melt. Significant quantities of metallic aluminum are inevitably carried away and removed with the non-metallic impurities.

[0176] This mixture of impurities, free aluminum and aluminum alloy is known to those skilled in the art as "aluminum slag" or "aluminum dross".

[0177] These masses of slag, when extracted from the kiln, appear as small pieces whose dimensions vary between values ​​less than or equal to 3 cm up to values ​​greater than or equal to 35 cm for example.

[0178] The amount of free metal and / or alloy contained in aluminous slag can vary from 0% to 95% by weight, this amount depending on many factors such as:

[0179] the composition of the alloy to be melted,

[0180] the fusion process used, and

[0181] The care with which the slag is skimmed or scraped from the surface of the molten aluminum bath. As mentioned previously, depending on the amount of aluminum it contains, an aluminous slag can be classified as:

[0182] slag of the "white slag" type (with a metal content that varies between 40 and 80%), or of the "black slag" type (with a metal content that varies between 5 and 40%).

[0183] These aluminous slags of the "black dross" type are considered — by those skilled in the art — as non-recoverable waste.

[0184] The process according to the invention proposes to use these aluminous slags of the "black dross" type to produce hydrogen gas.

[0185] Of course, the process according to the invention can also be implemented using aluminous slags of the "white dross" type.

[0186] 1.3. Red mud

[0187] Red mud is produced during the manufacture of alumina and aluminum from bauxite using the Bayer process. This red mud consists of insoluble impurities from the bauxite after the bauxite has been treated with caustic soda.

[0188] More specifically, the term "red mud" is used in the aluminum industry to refer to the by-product resulting from the "digestion" of bauxite by caustic soda according to the Bayer process, and whose principle is based on the selective solubility of alumina in caustic soda under operating conditions appropriate to the particular bauxite being treated.

[0189] Red mud (bauxite residue) contains:

[0190] not only the components of bauxite that are insoluble in caustic soda, such as iron oxide and titanium dioxide,

[0191] but also other compounds such as a hydrated sodium aluminosilicate similar to the natural mineral called "cancrinite".

[0192] There is no universal agreement on the composition of red mud.

[0193] Indeed, the composition of each red mud varies greatly depending on the origin of the bauxite used, and in particular its aluminum, mineral, and various metal content. Red muds notably contain cancrinite and hematite, which readily bind to heavy metals.

[0194] The industrial process of manufacturing alumina and aluminum also influences the composition of red mud. For example:

[0195] A sample of red mud produced in Hungary following the implementation of the Bayer process has the following composition:

[0196] o 40 to 45% Fe2O3 giving a red color,

[0197] o 10 to 15% of Al2O3,

[0198] o 10 to 15% SiO2,

[0199] o 6 to 10% CaO,

[0200] o 4 to 5% TiO2,

[0201] o 5 to 6% Na2O,

[0202] o traces of cadmium, chromium, mercury, nickel, lead, arsenic and zinc, a sample of red mud produced in France by treatment, with caustic soda, of a bauxite from Weipa presents the following composition:

[0203] o 34.5% of Fe2O3,

[0204] o 22.9% of AI2O3,

[0205] o 16.2% of SiO2,

[0206] o 1.8% CaO,

[0207] o 8.4% of TiO2,

[0208] o 8.7% Na2O.

[0209] In the following, for the purposes of this invention, "red mud" refers to the waste produced following the treatment of bauxite with caustic soda. Such "red mud" comprises a mineral compound that may have the following composition: 10 to 50 percent by weight of ferrous compounds,

[0210] 12 to 35 percent by weight of aluminium compounds,

[0211] 5 to 17 percent by weight of silicon compounds,

[0212] 2 to 21 percent by weight of titanium dioxide (TiO2),

[0213] 0.5 to 10 percent by weight of calcium compounds,

[0214] additional impurities in trace amounts.

[0215] In the case of so-called "active" red mud, this mineral compound is contained in a liquid phase with a pH greater than or equal to 10 (the liquid phase representing 30% by weight of the red mud). For the purposes of this invention, "inactive red mud" refers to a slurry that has reached a high solids content, such that it no longer flows easily in a fluid manner: such red muds have pseudo-plastic properties and are not easily pumped. Conversely, "active red mud" refers to a slurry that is capable of flowing in a fluid manner.

[0216] 1.4. Remarks

[0217] The thermodynamics of reactions (1) to (7) shown above (in sections 1.1.1 and 1.1.2) indicates that aluminum should react spontaneously with water. However, in practice, a piece of aluminum immersed in water will not react, whether the water temperature is 25 °C or 100 °C. This is because, as illustrated in Figure 5, aluminum has a thin, coherent, and adherent layer of aluminum oxide (Al₂O₃, or "alumina") B1 on its surface. This aluminum oxide B1 layer prevents the main hydrolysis reaction (1), (2), (3).

[0218] To initiate and maintain the hydrolysis reaction of aluminum (contained in aluminous slag) with water, the following is necessary:

[0219] to remove the aluminum oxide layer Bl, and

[0220] to continuously disrupt the B2 hydroxide layer that forms during the aluminum hydrolysis reaction.

[0221] The process according to the invention has made it possible to identify that red mud is capable of:

[0222] to greatly accelerate the hydrolysis reaction of "black sludge" type slag,

[0223] to increase the pH of the solution to between approximately 11.5 and 12, and also

[0224] to limit the phenomenon of slow diffusion of the aluminium hydroxide layer.

[0225] As mentioned above, red mud is a waste product generated by the Bayer process in the aluminum industry. This type of waste is highly problematic because no existing process allows for its reuse. The invention proposes a viable industrial process for treating aluminous slag, enabling the reuse of red mud.

[0226] We will now describe an installation for implementing the process according to the invention.

[0227] 2. Aluminous slag processing plant

[0228] With reference to Figure 2, an example of an installation for the treatment of aluminous slag is illustrated. The installation includes:

[0229] a reaction chamber 1,

[0230] a loading unit upstream of the reaction vessel 1, the loading unit including:

[0231] o a 2-pronged aqueous solution feeding system,

[0232] o a solid-state 3-load supply system, and

[0233] o a system for supplying 4 an inert gas such as nitrogen (N2),

[0234] a recovery unit downstream of reaction vessel 1, the recovery unit including:

[0235] o a collection system for 5 gaseous product(s),

[0236] o a system for harvesting 6 solid product(s), and

[0237] o a collection system for 7 liquid product(s),

[0238] a control unit 8 to control the different components of the loading unit, the reaction vessel 1, and the recovery unit.

[0239] Reaction chamber 1 allows the implementation of a corrosion reaction of aluminous slags with the water contained in the aqueous solution.

[0240] The loading unit allows the following to be injected into reaction chamber 1:

[0241] the aqueous solution and the solid charge at the beginning of the reaction in order to fill the reaction chamber 1, and the inert gas (before and / or during the reaction) in order to increase the pressure in the reaction chamber 1. The recovery unit allows the recovery of the different liquid, solid and gaseous products during the main hydrolysis reaction and the secondary reactions described above.

[0242] 2.1. Reaction chamber

[0243] Reaction chamber 1 is configured to contain the reactants, namely:

[0244] aluminous slag (oxidizable material),

[0245] red mud (alkali), and

[0246] the aqueous solution composed of water.

[0247] The reaction chamber 1 comprises a reaction chamber Cl adapted to withstand pressures exceeding 30 bar, and in particular exceeding 60 bar. Specifically, reaction chamber Cl is adapted to withstand pressures exceeding 150 bar, preferably exceeding 200 bar, and even more preferably exceeding 350 bar. Furthermore, reaction chamber Cl is configured to resist corrosion. Finally, since the corrosion reaction of aluminum 11 is exothermic, reaction chamber Cl is configured to withstand temperatures of 200 °C or higher. To meet these various requirements, the material constituting reaction chamber Cl can be nickel or a nickel alloy.

[0248] The corrosion reaction of aluminum contained in aluminous slag with water in the presence of red mud induces the production of aluminum oxides and hydroxides. Reaction chamber 1 is configured to induce the precipitation of these aluminum oxides and hydroxides as aluminates.

[0249] Advantageously, the reaction chamber 1 can include a heat accumulator (not shown) extending over the side wall(s) of the reaction chamber Cl. This helps to limit the risks of temperature increase in the reaction chamber 1 beyond 250 °C.

[0250] Indeed, such a calorie accumulator is capable of:

[0251] absorb the energy dissipated as heat during the corrosion reaction of the aluminum contained in the aluminous slag, particularly when the temperature in the reaction chamber Cl increases and reaches a temperature threshold,

[0252] to release in the form of heat the energy absorbed when the temperature in the reaction chamber Cl decreases below the temperature threshold.

[0253] Thus, the calorie accumulator allows:

[0254] to store thermal energy during the corrosion reaction of the aluminum contained in the aluminous slag in order to limit heating of the walls of the reaction chamber 1, and to release it later.

[0255] In some embodiments, the calorie accumulator may include:

[0256] a partition intended to come into thermal contact with the side wall(s) of the reaction chamber Cl, the partition including a lower panel, an upper panel and a pair of lateral panels between the lower and upper panels,

[0257] a phase change material (or PCM) contained within the partition.

[0258] PCM is a material capable of changing its physical state (solid / liquid) within a limited temperature range (e.g., between 200 °C and 250 °C). Thus, PCM has the unique characteristic of transitioning from a liquid to a solid state at a temperature close to 200 °C. The solidification reaction (i.e., the change from liquid to solid) is exothermic. The liquefaction reaction (the change from solid to liquid) is endothermic. Therefore, integrating such a heat accumulator allows for better temperature control within the reaction chamber 1.

[0259] The reaction chamber 1 also includes a mixing unit 11 for the reaction fluid between the reaction chamber Cl and the settling chamber C2.

[0260] In the embodiment illustrated in Figure 2, the mixing unit 11 consists of:

[0261] a propeller including a shaft and two (or four) blades at one end of the shaft, the propeller being immersed in the housing, between the reaction chambers Cl and the settling chamber C2; alternatively, the propeller can be replaced by any shearing device known to those skilled in the art, such as a turbine (with a flat disc and straight blades),

[0262] A motor mounted on the shaft to drive the propeller in rotation. In this case, the propeller.

[0263] The reaction chamber 1 may include a support—such as a grid—for holding the aluminous slag. In other embodiments, the reaction chamber may be without a support. Advantageously, the reaction chamber 1 may include a fill sensor to detect the presence of reagent (aluminous slag, aqueous solution, red mud) in the reaction chamber 1, and to indicate to the control unit the time at which a maximum volume of reagent that the reaction chamber 1 can hold is reached, in particular during the filling of the reaction chamber 1 with the aqueous solution and the solid feedstock, as will be described in more detail later.

[0264] 2.2. Loading Unit

[0265] As previously mentioned, the loading unit includes:

[0266] o a 2-pronged aqueous solution feeding system,

[0267] o a solid-state 3-load supply system, and

[0268] o a system for supplying 4 an inert gas such as nitrogen (N2),

[0269] The loading unit may also include, upstream of the supply system 3, a grinding system (not shown) to grind the aluminous slag and / or red mud to obtain material grains with a desired particle size, typically between 1 µm and 10 mm, preferably between 1 and 2000 µm, and even more preferably between 1 and 200 µm. Reducing the particle size of the aluminous slag and / or red mud to such a size allows (by increasing the available aluminum surface area and therefore the reaction kinetics) an increase in the amount of aluminum consumed during the main hydrolysis reaction and the secondary reactions.

[0270] 2.2.1. Aqueous solution feeding system The aqueous solution feeding system 2 allows the injection of aqueous solution into the reaction chamber 1.

[0271] The supply system 2 includes a tank adapted to contain the aqueous solution. This tank can have different shapes and be made of different materials suitable for resisting corrosion (such as nickel, a nickel alloy or a stainless steel alloy).

[0272] The tank is brought to atmospheric pressure via a vent in its upper wall. This vent prevents a vacuum from forming inside the tank when the aqueous solution is injected into reaction chamber 1 at the start of the reaction. The vent also allows the tank to be filled with the aqueous solution.

[0273] The feed system 2 also includes one (or more) conduit(s) enabling fluid communication between the vessel and the reaction chamber 1. Each conduit can be associated with an electrically controllable circulation valve (i.e., solenoid valve) to allow (when the circulation valve is open) or prevent (when the circulation valve is closed) the passage of the aqueous solution between the vessel and the reaction chamber 1. This ensures:

[0274] the filling of reaction chamber 1 with the aqueous solution at the start of the reaction,

[0275] emptying reaction chamber 1 of the aqueous solution at the end of the reaction.

[0276] The feed system 2 may include a pump — for example, a peristaltic or roller pump — for circulating the aqueous solution between the vessel and the reaction vessel 1. Of course, the feed system 2 may include other components known to those skilled in the art, such as:

[0277] a safety valve to limit the risk of overpressure in the tank,

[0278] a hatch (not shown) for the introduction of sodium hydroxide or potassium hydroxide, for example, packaged in pellet form,

[0279] a drain hole (not shown) to empty the aqueous solution contained in the tank, etc.

[0280] 2.2.2. Solid feed supply system

[0281] Supply system 3 allows the injection of the solid feedstock into reaction vessel 1. Supply system 3 includes:

[0282] a receiving bowl designed to contain the solid load composed of aluminous slag and red mud,

[0283] a distribution channel for the circulation of the solid charge, and

[0284] one (or more) sealed valve(s) to allow or stop the flow of the solid charge. The receiving bowl includes a base in which a through-hole is provided, connected to one end of the distribution channel, the other end of the distribution channel opening into the reaction chamber 1.

[0285] The valve(s) is / are mounted on the distribution channel.

[0286] When the device is used at atmospheric pressure, the supply system 3 may include a single valve. The control unit 8 allows the valve to be switched from an off state to an on state (and from an on state to an off state):

[0287] the valve blocking the passage of the solid charge between the receiving bowl and the reaction vessel 1 in the deactivated state,

[0288] the valve allowing the passage of the solid charge between the receiving bowl and the reaction chamber 1 in the activated state.

[0289] When the installation is operated at a pressure higher (or lower) than atmospheric pressure, the supply system may include first and second valves, with the second valve being closer to the reaction vessel 1 than the first valve. These first and second valves define a sealed compartment between the receiving vessel and the reaction vessel 1. The control unit 8 allows the first and second valves to be operated.

[0290] More specifically, for filling the compartment, control unit 8 activates the first valve and deactivates the second valve. The solid feed (aluminous slag + red mud) flows by gravity from the receiving bowl into the compartment. When the compartment is full, control unit 8 deactivates the first valve and then activates the second valve. The solid feed then flows by gravity or via a hopper from the compartment to the reaction vessel 1. Once the compartment is emptied, control unit 8 deactivates the second valve, and a new filling cycle can be initiated (activation of the first valve to fill the compartment, etc.). The presence of two valves allows the reaction vessel 1 to be maintained at a pressure higher (respectively lower) than atmospheric pressure.

[0291] 2.2.3. Gas Supply System

[0292] The gas supply system 4 allows the injection of gas into the reaction chamber 1.

[0293] Supply system 4 includes:

[0294] one (or more) gas injection channel(s),

[0295] one (or more) valve(s) mounted on the channel(s), and

[0296] a gas supply source (such as nitrogen) connected to the channel(s).

[0297] The control unit 8 is configured to control the opening and closing of the valve(s) to allow or prevent the injection of inert gas into the reaction vessel 1. The injection of an inert gas increases the pressure in the vessel for the implementation of the main hydrolysis reaction and secondary reactions. The reader will appreciate that the hydrogen produced during a previous implementation of the treatment process according to the invention can also be used as an inert gas to increase the pressure in the vessel.

[0298] Such a gas supply system is known to those skilled in the art and will not be described in further detail hereafter.

[0299] 2.2.4. Grinding system

[0300] The grinding system reduces aluminous slag and red mud to powder in order to obtain material grains with a desired particle size.

[0301] The grinding system can be of any type known to a person skilled in the art.

[0302] For example, the grinding system can be of the "jet mill" type (also known as a "fluidized bed mill"). Such a "jet mill" allows granular or powdery materials to be ground by collision (attrition).

[0303] In this case, the grinding system includes:

[0304] a chamber designed to contain aluminous slag and red mud, which can be introduced into the chamber continuously or intermittently, and

[0305] Fluid ejection nozzles (for a mixture of a neutral carrier gas, nitrogen or argon, and the particles to be ground) are used. The chamber has a substantially cylindrical body and a truncated conical bottom. An opening is provided in the upper part of the chamber for the discharge, to the supply system 3, of aluminous slag and red mud grains of a size that meets a desired particle size distribution. The fluid ejection nozzles are supplied with pressurized fluid (an inert gas or vapor, such as nitrogen, argon, or air) via fluid sources (such as a compressor supplying the nozzles with fluid at a pressure between 7 and 30 bar), which are not shown.These ejection nozzles, housed in the side wall of the chamber, are configured to generate substantially horizontal centripetal jets within the aluminous slag and red mud contained in the chamber in order to cause crushing zones by impacts between the aluminous slag and the red mud (attrition).

[0306] These centripetal jets allow the finest grains (whose dimensions meet the desired particle size) to rise and are then continuously extracted from the upper part of the chamber, while the larger grains (whose size exceeds the desired particle size), because they are insufficiently ground, fall laterally into the chamber.

[0307] Advantageously, the grinding system may include a separator at the chamber outlet. This separator separates the larger grains, which can then be carried along with the fine grains of the desired particle size towards the chamber outlet. These larger grains, whose size exceeds the desired particle size, can be reintroduced into the chamber using any known technique for further grinding.

[0308] Such a grinding system is familiar to those skilled in the art. Indeed, the fluidized bed grinding technique is described in particular in document WO 2012 / 014985. The grains ground to the desired particle size are then moved to the supply system 3.

[0309] Of course, the crushing system can be of any other type known to a person skilled in the art, such as a hammer mill consisting of a rotor equipped with hammers and / or cutting tools, mounted in a crushing chamber equipped with one (or more) screening grid(s) whose mesh diameter is adapted to the desired particle size for aluminous slags and red muds.

[0310] 2.3. Recovery Unit

[0311] As mentioned previously, the recovery unit includes:

[0312] a collection system for 5 gaseous product(s),

[0313] a harvesting system for 6 solid product(s), and

[0314] a collection system 7 of liquid product(s) I.

[0315] 2.3.1. Collection System

[0316] Collection system 5 is configured to:

[0317] to recover the gases generated during the corrosion reaction of the aluminum contained in the aluminous slag,

[0318] filter said gases to separate hydrogen from other compounds contained in the generated gases, connect the installation to a storage tank 52 of the hydrogen produced during the corrosion reaction.

[0319] In particular, collection system 5 is suitable for:

[0320] cool the gases generated in the reaction vessel 1,

[0321] condense the water vapor contained in the said gases, and return the condensed water:

[0322] o in reaction chamber 1 during the implementation of the main hydrolysis reaction and secondary reactions, or

[0323] o in collection system 7 at the end of the reaction.

[0324] For cooling the gases and extracting the water vapor they contain, the collection system 5 includes a heat exchanger 51.

[0325] The heat exchanger is configured to withstand high pressures (pressures above 30 bar, especially above 60 bar, preferably above 150 bar, more preferably above 250 bar and even more preferably above 350 bar), and high temperatures (temperatures above or equal to 200 °C, especially above or equal to 250 °C).

[0326] Heat exchanger 41, for example, is of the air-cooled type. For example, heat exchanger 41 may include:

[0327] a coil (not shown) inside which the gases from the reaction vessel 1 circulate, the external faces of the coil walls including cooling fins,

[0328] a blower (not shown) — such as a fan — to promote heat exchange between the gases circulating in the coil and the outside air.

[0329] Alternatively, the heat exchanger 41 can be of the type cooled by heat transfer fluid.

[0330] The collection system 5 also includes a temperature sensor in communication with the control unit 8, the control unit 8 activating the heat exchanger when the temperature measured by the temperature sensor is above a predefined threshold value (e.g. 50 °C).

[0331] The heat exchanger condenses the water vapor contained in the gases exiting reaction vessel 1 to form water. This liquid water (or condensate) is separated from the hydrogen by a condensate separator located at the outlet of the heat exchanger.

[0332] During the implementation of the main and secondary reactions, the liquid water (or condensate) is reintroduced into the reaction chamber Cl of the reaction vessel 1. At the end of the reactions, the pure water is introduced into a tank of the collection system 7.

[0333] For this purpose, the heat exchanger 51 is in fluidic communication — via one (or more) pipe(s) — with the reaction chamber 1 on the one hand, and with the collection system 7 on the other, one (or more) controlled orientation valve(s) — for example one (or more) solenoid valve(s) — arranged along the pipe(s) allowing the circulation of the condensed water to be directed towards the reaction chamber 1 or towards the collection system 7.

[0334] Optionally, the collection system 5 can include a liquid sensor and a safety valve controlled downstream of the heat exchanger. The liquid sensor and the safety valve are configured to communicate with the control unit 8.

[0335] Especially :

[0336] The liquid sensor detects the presence of corrosive liquid downstream of the heat exchanger; when a corrosive liquid is detected by the liquid sensor, it transmits a signal to the installation's control unit.

[0337] The safety valve allows a passage to be opened or closed to the storage tank for the hydrogen produced: if a corrosive liquid is detected by the liquid sensor, the control unit 8 commands the closure of the safety valve to prevent the spread of said corrosive liquid to the hydrogen storage tank.

[0338] If no corrosive liquid is detected by the liquid sensor, control unit 8 commands the opening of the safety valve to allow the gas to flow to the hydrogen storage tank. The presence of a liquid sensor associated with a safety valve, the opening and closing of which is controlled by control unit 8, helps to limit the risk of damage to the hydrogen storage tank from corrosive liquid.

[0339] To allow the circulation of gases between the heat exchanger and the hydrogen storage tank, the collection system 5 includes a gas circulation line connected to the heat exchanger on one side and to the hydrogen storage tank 52 on the other.

[0340] However, before being stored in storage tank 52, the hydrogen must be treated to achieve a specific purity and / or moisture content, which depends on the technical specifications of the storage tank and / or the intended application. Therefore, the collection system 5 includes a gas scrubber (not shown) mounted along the gas circulation line between the heat exchanger and the hydrogen storage tank. This scrubber may contain a desiccant (such as silica gel, CaO, CaCl2, or others), and / or activated carbon, and / or a particulate filter, and / or a hydrophobic membrane, and / or a palladium membrane, etc.

[0341] Advantageously, the collection system 5 can include a pressure relief valve (or pressure reducer) mounted along the gas circulation line. This pressure relief valve allows the gas pressure in the collection system 5 to be regulated to match the desired pressure for the hydrogen storage tank (e.g., hydrogen storage at a pressure of 150 bar, or 200 bar, or 350 bar, etc.).

[0342] Finally, the collection system 5 may include an isolation valve downstream of the scrubber to open / close (manually or automatically) the passage between the scrubber and the hydrogen storage tank. This allows the gas circulation line to be closed in order to disconnect the hydrogen storage tank when it is full and replace it with an empty storage tank.

[0343] 2.3.2. Harvesting System

[0344] The harvesting system 6 is configured to contain the aluminates produced during the implementation of the main hydrolysis reaction and the secondary reactions.

[0345] For this purpose, the harvesting system 6 includes a settling chamber C2 in which the aluminum oxides and hydroxides (produced in the reaction vessel 1) precipitate as aluminates. The reaction chamber Cl can be mounted on the settling chamber C2. In particular, the reaction chamber Cl and the settling chamber C2 can be constructed within a single cylindrical housing consisting of a base, a top cover, and a side wall between the base and the cover. Integrating the reaction chamber Cl and the settling chamber C2 into the same housing limits the risk of leaks due to a lack of sealing in the reaction vessel 1. This also reduces the overall size of the reaction vessel 1. Advantageously, the reaction chamber Cl can be located above the settling chamber C2 (along a vertical axis A-A').This prevents the circulation of aluminates in the reaction chamber Cl once they have precipitated: the aluminates are deposited in the decantation chamber C2, for example at the level of a removable reservoir to facilitate their extraction from the reaction chamber 1 at the end of the reaction.

[0346] Alternatively, the reaction chambers Cl and C2 settling chambers can be housed in separate, distinct units connected by piping to allow the reaction fluid to circulate between them. This allows the use of reaction chambers Cl and C2 of different shapes and sizes. It also reduces the overall height of the reaction vessel 1.

[0347] Advantageously, the dimensions of the reaction chambers Cl and C2, as well as the characteristics of the mixing unit, can be chosen so that:

[0348] The circulation velocity of the reaction fluid in reaction chamber Cl should be between 5 and 15 cm / s, preferably between 6 and 15 cm / s, and even more preferably between 6 and 10 cm / s; the circulation velocity of the reaction fluid in the settling chamber should be between 0.1 and 4 cm / s, preferably between 1 and 3 cm / s, and even more preferably between 2 and 3 cm / s. The fact that, in reaction chamber Cl, the circulation velocity of the reaction fluid is greater than or equal to 5 cm / s (preferably greater than or equal to 6 cm / s) facilitates the removal of the by-products of the aluminum corrosion reaction contained in the slag from reaction chamber Cl (and therefore limits the risk of the corrosion reaction being slowed down by the alumina).The fact that, in the settling chamber C2, the circulation speed of the reaction fluid is less than or equal to 4 cm / s (preferably less than or equal to 3 cm / s) allows:

[0349] on the one hand, to promote the precipitation of sodium aluminate formed during the corrosion reaction of the aluminum contained in the slag, and

[0350] on the other hand, to promote the separation (by decantation) of the solid particles (alumina) contained in the reaction fluid; the solid particles thus separated are advantageously collected in a removable base of the collection system 6; this base is advantageously removable to allow its separation from the decantation chamber in order to evacuate the solid by-products it contains at the end of the implementation of the main and secondary reactions.

[0351] To obtain different circulation velocities between the reaction chamber Cl and the settling chamber C2 (and in particular velocities of the order of 6 cm / s in the reaction chamber Cl and 3 cm / s in the settling chamber C2), the diameter of the reaction chamber Cl can be reduced relative to the diameter of the settling chamber C2 (in the case of cylindrical chambers).

[0352] In some embodiments, the harvesting system 6 may include a cyclone (or hydrocyclone) separation system to facilitate the separation of the reaction fluid from the solid particles it contains. Such a cyclone separation system (not shown) is known to those skilled in the art and will not be described in further detail hereafter.

[0353] Optionally, the harvesting system 6 can include a cooler (not shown) mounted on the walls of the settling chamber C2 to lower the temperature of the reaction fluid. This promotes the precipitation of aluminum oxides and hydroxides contained in the reaction fluid as aluminates.

[0354] 2.3.3. Collection System

[0355] Collection system 7 is configured to collect liquid products at the end of the reaction.

[0356] For this purpose, the collection system includes a tank connected on one side to the reaction chamber 1 and on the other side to the heat exchanger of the gas collection system 5 in order to collect the condensate obtained at the heat exchanger.

[0357] Since such a collection system is known to those skilled in the art, it will not be described in further detail hereafter.

[0358] 2.4. Control Unit

[0359] Control unit 8 allows the various components to be controlled:

[0360] of the loading unit,

[0361] of reaction chamber 1, and

[0362] from the recovery unit,

[0363] such as valves or sensors.

[0364] The control unit 8 is, for example, computer(s), processor(s), microcontroller(s), microcomputer(s), programmable logic controller(s), application-specific integrated circuit(s), other programmable circuits, or other devices that include a computer, such as a workstation. The control unit 8 includes one or more memory units, such as ROM / RAM, CD-ROM, USB flash drive, or central server memory. This memory (or these memory units) allows the storage of program code instructions for the execution of the process illustrated in Figure 1 and / or the process illustrated in Figure 3.

[0365] 3. Experiments

[0366] As will become clear later, the use of red mud—as a promoter in the implementation of the main and secondary reactions—improves performance in the treatment of aluminous slag. Indeed, the treatment performance obtained with red mud is superior:

[0367] to those obtained with other additives commonly known to those skilled in the art, on the one hand, and also to those obtained using the various main components constituting red mud, on the other. Furthermore, the use of red mud makes it possible to treat aluminous slags of different compositions with similar efficiency.

[0368] To demonstrate these advantages, the procedure described in point 3.1 was implemented for different series of experiments, the results of which are presented below:

[0369] A first series of experiments was conducted to test red mud as an additive (see point 3.2.1.), a second series of experiments was conducted to test the flexibility of red mud as an additive on different types of aluminous slag (see point 3.2.2.),

[0370] a third series of experiments made it possible to identify the optimal quantities of red mud to be used as an additive (see 3.2.3.).

[0371] For these different series of experiments, the red muds used are so-called "inactive" red muds supplied by the company Mytilineos, and come from an alumina production plant located in Greece.

[0372] To facilitate comparison between the different series of experiments:

[0373] the reaction time — for each experiment — was set at seven hours, and

[0374] The initial temperature of the reaction chamber — for each experiment — was set at 25 °C. 3.1. Procedure implemented

[0375] The procedure implemented for the different series of experiments is illustrated in Figure 3.

[0376] This process includes a first step 100 of solid feed preparation in which aluminous slag and red mud are combined to form the solid feed. Prior to (or subsequent to) the combination substep, the preparation step may include a substep of reducing the aluminous slag and red mud to a desired particle size. This powder reduction substep can be carried out by any technique known to those skilled in the art (grinding, sieving, etc.). It yields aluminous slag and red mud particles ranging from 1 µm to 10 mm, preferably from 1 to 2000 µm, and even more preferably from 1 to 200 µm. In a second step 200, the solid feed is mixed with the aqueous solution. More precisely, the solid feed is introduced into the reaction chamber 1 via the supply system 3.Once introduced, the aqueous solution (water) is injected into the reaction chamber 1 by the feeding system 2. The mixture composed of the solid feed and the aqueous solution is stirred by the mixing unit 11 to form a homogeneous suspension.

[0377] The corrosion reaction of the aluminium contained in the aluminous slags (i.e. main hydrolysis reaction) and the secondary reactions begin: hydrogen (and toxic gases) is (are) generated continuously, the reaction solution (solid charge + aqueous solution) circulates between the reaction chamber Cl of the reaction vessel, and the settling chamber C2 of the collection system 6.

[0378] In parallel or successive to the stirring substep, the pressure inside the reaction chamber Cl can be increased by introducing an inert gas (argon, or nitrogen for example) through the supply system 4. Simultaneously (or consecutively) with the increase in pressure in the reaction chamber Cl, the temperature (inside said reaction chamber Cl) can be increased, for example by means of a heating system for the reaction chamber 1.

[0379] In a third step 300, the various products obtained through the execution of the main and secondary reactions are collected.

[0380] In particular, during the execution of the main hydrolysis reaction and secondary reactions, hydrogen, toxic gases, aluminum oxides and aluminum hydroxides are produced in the reaction chamber Cl.

[0381] Hydrogen and toxic gases escape to the gas collection system 5, while aluminum oxides and hydroxides are transported in the reaction fluid to the C2 settling chamber of the harvesting system 6, where they precipitate as aluminates.

[0382] The hydrogen and toxic gases discharged to collection system 5 are saturated with water vapor and are at a very high temperature. As they pass through the heat exchanger, the hydrogen and toxic gases are cooled (to approximately 60 °C), and the excess water vapor they contain is condensed into pure liquid water. This pure liquid water is reintroduced into reaction chamber C1 of reaction vessel 1. The cooled gases (hydrogen + toxic gases) are then filtered by the gas scrubber. At the scrubber outlet, the hydrogen is stored in storage tank 52, while the toxic gases are retained within the scrubber. At the end of the reaction, the aluminum contained in the aluminous slag has been consumed, and the solid by-products are located in the lower part of settling chamber C2.

[0383] The installation is cooled, brought back to atmospheric pressure, and purged of the aqueous solution, which is recovered in the collection system 7 contained in the reaction chamber 1.

[0384] The lower part of the C2 settling chamber is then detached to remove the solid by-products. 3.2. First series of experiments

[0385] 3.2.1. Introduction

[0386] The treatment process described above was implemented on different types of solid feedstock. These different types of solid feedstock differ only in the additive used for treating the aluminous slag (Al-Si-Cu). In particular, each of the solid feedstocks tested consists of aluminous slag of the "black dross" type with the same composition. This composition (estimated from X-ray diffraction (XRD) spectrometric analyses and from microwave plasma atomic emission spectrometric (MP-AES) analyses) is as follows:

[0387] Metallic aluminum (Al): 36.0%

[0388] - Alumina (Al2O3): 14.2%

[0389] Aluminum nitride (AIN): 7.9%,

[0390] - Halite (NaCl): 6.8%

[0391] - Sylvite (KCI): 18.1%

[0392] - Spinel (MgAl2O4): 15.1%

[0393] - Other: 3.7%.

[0394] The different solid loads tested are as follows:

[0395] an initial solid load composed of black grime and red mud,

[0396] a second solid charge composed exclusively of black dross (Al-Si-Cu),

[0397] a third solid charge composed of black dross and sodium hydroxide (NaOH) known as "caustic soda"; sodium hydroxide or "caustic soda" is an additive known to those skilled in the art for carrying out a hydrolysis reaction,

[0398] a fourth solid charge composed of black dross and a mixture of iron oxide (Fe2O3) and sodium hydroxide (NaOH); iron oxide being one of the main components of red mud,

[0399] a fifth solid charge composed of black dross and gibbsite (Al(OH)3), gibbsite being another of the main compounds of red muds, a sixth solid charge composed of black dross and gamma-alumina, alumina (Al2O3) corresponding to a less hydrated form of gibbsite and exhibiting a crystalline structure comparable to that of gibbsite,

[0400] A seventh solid charge consisting of black dross and a mixture of gamma-alumina and caustic soda, and an eighth solid charge consisting of black dross and a mixture of iron oxide, gibbsite, and caustic soda. The second solid charge (i.e., aluminous slag without additives) and the third solid charge (aluminous slag mixed with caustic soda) provide a reference basis that can be compared to the use of red mud.

[0401] The use of sodium hydroxide in the third solid charge allows the pH of the solution to be adjusted to 12. This pH is approximately the same as that generated by the use of red mud as an additive.

[0402] 3.2.2. Results

[0403] Figure 4 illustrates the conversion rate of aluminum contained in "black dross" type slag as a function of the additive used, namely:

[0404] no additives,

[0405] soda,

[0406] red mud,

[0407] iron oxide and sodium hydroxide,

[0408] gibbsite

[0409] alumina,

[0410] alumina and soda,

[0411] iron oxide, gibbsite, and soda.

[0412] This conversion rate, for the different additives tested, is summarized in the table below for a seven-hour cycle.

[0413]

[0414]

[0415] When an additive is used, the lowest conversion rate is obtained with the conventional alkaline solution process including sodium hydroxide. A large portion of the metallic aluminum in "black dross" slag has not yet reacted. There are two ways to increase the final conversion rate: increase the reaction time or increase the amount of sodium hydroxide used. Both of these solutions have drawbacks. Indeed:

[0416] a longer reaction time, and

[0417] a larger quantity of sodium hydroxide used,

[0418] increase operating costs as well as the risk of corrosion of the installation.

[0419] Those skilled in the art will appreciate that the amount of aluminum consumed to produce hydrogen is maximized when red mud is used as a promoter for carrying out the main and secondary reactions described in point 1.

[0420] In particular, Figure 4 shows that the addition of red mud to the hydrolysis reaction of "black sludge" type slag increases significantly:

[0421] the reaction speed, and

[0422] the conversion rate.

[0423] For a 7h cycle, the amount of hydrogen produced per mass of aluminium is double that obtained using sodium hydroxide as an additive (alkaline solution classically used in the prior art).

[0424] Iron oxide and gibbsite are the main components of red mud. As illustrated in Figure 4, none of these compounds, taken alone or in combination, as an additive (for carrying out the main and secondary reactions), allows the treatment of aluminous slag with an efficiency equivalent to that of red mud.

[0425] The test using iron oxide as an additive aimed to determine whether the compound was responsible for the accelerated reaction rate. The results show that the use of iron oxide (Fe2O3) as an additive does play a role in accelerating the reaction rate, but to a lesser extent compared to red mud.

[0426] The test using gibbsite as an additive aimed to determine the compound's effectiveness in terms of conversion rate. The results show that using gibbsite as an additive results in a higher final conversion rate than using iron oxide. However, the reaction rate is considerably lower. The test using a mixture of the main compounds present in red mud (i.e., iron oxide, gibbsite, and sodium hydroxide) aimed to determine whether this mixture had a combined effect on accelerating the reaction rate and increasing the conversion rate. The mass ratio of ferric oxide to gibbsite remained constant, and the pH was adjusted using sodium hydroxide to match the new additive.These "artificial red muds" did not reproduce the same effect as red muds: the mixture reproduced the behavior of the individual compounds, since it followed the behavior of ferric oxide during the initial phase of the reaction and that of gibbsite during the final phase of the reaction.

[0427] 3.3. Second series of experiments

[0428] 3.3.1. Introduction

[0429] The red muds were tested with samples of different slags (hereafter referred to respectively as "Al-Si-Cu", "Al-Si" and "It") in order to evaluate their flexibility as an additive.

[0430] The three slag samples of the "black dross" type "Al-Si-Cu", "Al-Si" and "It" contain different amounts of metallic aluminum. Their respective compositions are detailed in the table below.

[0431]

[0432] 3.3.2. Results

[0433] The results of the reactions without reaction promoter and with red mud as an additive for the three samples of aluminous slag are illustrated in Figure 6.

[0434] The table below shows the conversion rates obtained for the different slags.

[0435] To assess the effectiveness of the red mud used, the treatment of each type of aluminous slag was carried out:

[0436] without additives on the one hand,

[0437] by using red mud as an additive on the other hand.

[0438]

[0439] As illustrated in Figure 6, all results indicate that red muds are very effective reaction promoters.

[0440] Indeed, the conversion rate is much higher when red mud is used as an additive, compared to the conversion rate obtained when the reactions (primary and secondary) are carried out without an additive. Furthermore, this conversion rate is high regardless of the composition of the aluminous slag. The effectiveness of red mud (as a reaction promoter) is therefore independent of the type of aluminous slag being treated.

[0441] The fact that red muds accelerate the reaction rate, regardless of the type of aluminous slag, guarantees the viability of the process.

[0442] The results obtained in Figure 6 therefore teach a person skilled in the art that the treatment process according to the invention can be extrapolated for the treatment of other materials containing a variable amount of metallic aluminum.

[0443] For example, the process described above can be used for the treatment of saline slag (or "sait cake" according to Anglo-Saxon terminology) produced by pyrometallurgical treatment of "white dross" type slag or fly ash (or "flyash" according to Anglo-Saxon terminology) recovered during waste incineration.

[0444] Thus, this second series of experiments shows that red mud is effective in promoting both the primary and secondary reactions, regardless of the type of aluminous slag being treated. Figure 6 also demonstrates to those skilled in the art that for certain types of slag, a 7-hour cycle is unnecessary, as a conversion rate exceeding 90% can be achieved with a treatment cycle of only 3 or 4 hours, or even 2 hours for "It" type slag. This significantly reduces operating costs and equipment size. It also indicates that the process can be optimized by adjusting certain parameters such as pressure, temperature, the proportion of additives, the stirring speed, and the concentration of the reaction solution. 3.4. Third series of experiments

[0445] 3.4.1. Introduction

[0446] Finally, a third series of experiments was carried out to determine the limit quantities of red mud to be used allowing optimal treatment of aluminous slags.

[0447] In this third series of experiments, varying amounts of red mud were introduced into the reaction chamber to study the effect of this red mud on the acceleration phenomenon.

[0448] Thus, the solid loads tested in this third series of experiments differ in their red mud concentration. Each of the solid loads tested contains aluminous slag of the "black sludge" type with the same composition (Al-Si). This composition (estimated from X-ray diffraction (XRD) spectrometric analyses and from microwave plasma atomic emission spectrometric analyses (MP-AES)) is as follows:

[0449] Metallic aluminum (Al): 28.0%

[0450] Alumina (Al2O3): 26.4%

[0451] Aluminum nitride (AIN): 11.0%,

[0452] - Halite (NaCl): 6.1%

[0453] - Sylvite (KCI): 13.0%

[0454] - Spinel (MgAl2O4): 14.7%

[0455] - Other: 0.8%.

[0456] For the experiments in this third series, the only parameter modified concerns the weight ratio between the quantity of red mud used and the quantity of aluminous slag to be treated (additive / dross ratio).

[0457] 3.4.2. Results

[0458] The evolution of the primary and secondary reactions:

[0459] without a reaction promoter

[0460] with a weight ratio between the quantity of red mud used and the quantity of aluminous slag to be treated of 0.25,

[0461] with a weight ratio between the quantity of red mud used and the quantity of aluminous slag to be treated of 1.43,

[0462] with a weight ratio between the quantity of red mud used and the quantity of aluminous slag to be treated of 4.00

[0463] are illustrated in figure 7.

[0464] For a 7-hour cycle, the results obtained are detailed in the table below for the four solid loads studied (without additive, with 0.25, 1.4 and 4.0 red mud additives).

[0465]

[0466] The above results show that the reaction rate and total hydrogen production vary depending on the amount of red mud contained in the solid feed.

[0467] A higher additive-to-slag ratio improves reaction efficiency. In particular, the more red mud the solids feed contains, the better the treatment process performance.

[0468] For the different quantities of red mud tested, it appears that the conversion rate of the aluminum contained in the slag into oxide and hydroxide is greater than 90%, which, regardless of the quantity of red mud used, is higher than the conversion rates obtained with any additive known to a person skilled in the art.

[0469] The performance of the main and secondary reactions increases without an upper limit being determined.

[0470] The minimum and maximum quantities of red mud used therefore depend more on factors associated with the cost of slag treatment than on the performance of the implementation of the main and secondary reactions:

[0471] If too much red mud is introduced into reaction vessel 1, then less aluminous slag can be treated, which increases the costs of treating the aluminous slag; similarly

[0472] If too small a quantity of red mud is introduced into reaction chamber 1, then the rates of the main and secondary reactions are reduced, which also increases the costs associated with the treatment of aluminous slag.

[0473] Thus, the inventors consider it preferable that the weight ratio between the quantity of red mud and the quantity of aluminous slag (contained in the solid charge) be between 0.2 and 5, preferably between 0.25 and 4, even more preferably between 0.5 and 2, in particular between 1 and 1.5.

[0474] 3.4.3. Other experiments

[0475] Additional experiments were conducted to demonstrate the benefits of using red mud for neutralizing aluminous slag. One of these experiments involved using red mud samples of different compositions:

[0476] A first sample (RM A) originates from a production site based in Greece,

[0477] a second sample (RM B) originates from a production site based in Germany.

[0478] The compositions of these two samples are given in the table below.

[0479]

[0480] Figure 8 illustrates the conversion rate of aluminum contained in "black dross" (Al-Si-Cu) slags as a function of the additive used, namely:

[0481] the first sample (RM A) originating from Greece,

[0482] the second sample (RM B) originating from Germany, and

[0483] an alkaline solution (serving as a reference).

[0484] Figure 8 shows that:

[0485] the rate of the hydrolysis reaction of "black sludge" type slag, as well as

[0486] the rate of conversion of aluminium (contained in slag of the "black sludge" type) into hydrogen, is increased significantly regardless of the type of red mud (RM A, RM B) used.

[0487] This experiment proves that the acceleration effect is not linked to a particular sample, and that even at different concentrations, red mud acts as an accelerator of the hydrolysis reaction. Another such experiment concerns the treatment—by washing on the one hand and filtration on the other—of the first and second red mud samples (RM A, RM B) from production sites located in Greece and Germany, before their use in neutralizing aluminous slags.

[0488] This other experiment was carried out to demonstrate that the solid fraction of red mud plays a significant role in the hydrolysis reaction of aluminous slag.

[0489] Specifically, the two red mud samples were washed and filtered to remove soluble compounds. The water-to-salt ratio used in these operations was 2 to 1, and they were repeated three times to ensure complete removal.

[0490] The washed solids and the initial filtrate were recovered and used as additives in the slag hydrolysis reaction, maintaining the same operating parameters as in previous tests.

[0491] In particular, the different solid loads tested are as follows:

[0492] an initial solid charge composed of black grime,

[0493] a second solid load composed of black sludge and red mud from the first sample (RMA), a third solid load composed of filtered black sludge and red mud from the first sample (RMA Filtrate),

[0494] a fourth solid load composed of black dross and washed red sludge from the first sample (Washed RMA),

[0495] a fifth solid load composed of black dross and red mud from the second sample (RMB),

[0496] a sixth solid load composed of black dross and filtered red sludge from the second sample (RMB Filtrate),

[0497] a seventh solid load composed of black dross and washed red mud from the second sample (Washed RMB).

[0498] The results of this other experiment are illustrated in Figure 9 and demonstrate that the accelerated hydrolysis reaction is due to a combination of the soluble and solid fractions of the red mud. Thus, the hydrolysis reaction in red mud is a solid-solid-liquid reaction based on a complex, multi-step mechanism that cannot be reproduced with a simple alkaline solution, as demonstrated by the various experiments performed.

[0499] 4. Conclusions

[0500] In conclusion, red mud is a very effective and flexible additive for the neutralization of aluminous slags, particularly slags of the "black sludge" type.

[0501] The use of red mud in the treatment process according to the invention makes it possible to greatly accelerate the hydrolysis reaction of aluminous slags.

[0502] In addition, the treatment process is very flexible, since the use of red mud allows any waste containing metallic aluminum to be treated with high efficiency.

[0503] For the same reaction time and under similar operating conditions, the conversion rate of aluminum contained in aluminous slags into aluminum oxide and hydroxide is doubled using red mud compared to the conversion rate obtained using soda (NaOH).

[0504] The phenomenon of accelerated reaction rate is not fully understood, as the exact mechanism of interaction between red mud and the aluminum contained in aluminous slag has not been elucidated. It appears that using the main components of red mud (i.e., "artificial red mud") as an additive does not allow the treatment process to achieve the same performance as with natural red mud.

[0505] The reader will have understood that many modifications can be made to the invention described above without materially departing from the new lessons and advantages described here.

Claims

DEMANDS 1. A process for treating aluminous slag comprising aluminum, characterized in that the process comprises: a preparation step (100) of a solid feedstock comprising aluminous slags and red muds, a mixing step (200) of the solid feedstock with an aqueous solution containing water to induce a corrosion reaction of the aluminum contained in the aluminous slag, said corrosion reaction producing: o a gaseous product containing dihydrogen and o a reaction fluid composed of a liquid product and a solid product, a product recovery step (300), said recovery step including the substeps consisting of: to collect the solid product from the reaction fluid, said solid product comprising an aluminum oxide and / or an aluminum hydroxide, and to extract the gaseous product containing dihydrogen.

2. A treatment process according to claim 1, wherein the red mud comprises a mineral compound including: 10 to 50 percent by weight of ferrous compounds, 12 to 35 percent by weight of aluminium compounds, 5 to 17 percent by weight of silicon compounds, 2 to 21 percent by weight of titanium dioxide (TiO2), 0.5 to 10 percent by weight of calcium compounds, additional impurities if applicable.

3. A processing method according to any one of claims 1 or 2, wherein the solid feed preparation step comprises the following substeps: a sub-step of reducing aluminous slags and red muds to a desired particle size, possibly by grinding and sieving, the desired particle size being between 1pm and 10mm, preferably between 1 and 2000pm, and even more preferably between 1 and 200pm, a sub-step of combining aluminous slags and red muds.

4. A treatment process according to any one of claims 1 to 3, wherein the weight ratio between: the quantity of red mud, and the quantity of aluminous slag contained in the solid charge is between 0.2 and 5, preferably between 0.25 and 4, even more preferably between 0.5 and 2, especially between 1 and 1.

5.

5. Processing method according to any one of claims 1 to 4, wherein the substep of withdrawing the gaseous product containing dihydrogen includes a filtration operation of the gaseous product to separate the dihydrogen from toxic gases contained in said gaseous product.

6. Installation for the treatment of aluminous slag comprising aluminum, characterized in that the installation comprises: a reaction chamber (1), an upstream loading unit of the reaction vessel (1), the loading unit including: o an aqueous solution supply system (2), the aqueous solution containing water, and o a solid feed supply system (3), the solid feed being composed of aluminous slags and red mud, a recovery unit downstream of the reaction vessel (1), the recovery unit including: o a collection system (5) for gaseous product(s), o a harvesting system (6) for solid product(s), a control unit (8) for operating the loading unit, the reaction vessel (1), and the recovery unit, said control unit being configured to control: o the preparation of the solid feed comprising aluminous slag and red mud, o the mixing of the solid feed with the aqueous solution in the reaction vessel (1) to induce a corrosion reaction of the aluminum contained in the aluminous slag, said corrosion reaction producing: ■ a gaseous product containing dihydrogen and ■ a reaction fluid composed of a liquid and a solid product, o the recovery of the products by: ■ taking the solid product from the reaction fluid, said solid product comprising an aluminium oxide and / or an aluminium hydroxide, and in ■ drawing off the gaseous product containing dihydrogen.

7. Installation according to claim 6, wherein the red mud comprises a mineral compound including: 10 to 50 percent by weight of ferrous compounds, 12 to 35 percent by weight of aluminium-based compounds, 5 to 17 percent by weight of silicon-based compounds, 2 to 21 percent by weight of titanium dioxide (TiO2), 0.5 to 10 percent by weight of calcium compounds, additional impurities if applicable.

8. Installation according to any one of claims 6 or 7, wherein the loading unit further comprises a grinding system, upstream of the supply system, for reducing aluminous slags and red muds to a desired particle size, the desired particle size being between 1pm and 10mm, preferably between 1 and 2000pm, and even more preferably between 1 and 200pm.

9. Solid feedstock for the treatment of aluminous slag, said solid feedstock including said aluminous slag to be treated, characterized in that the solid feedstock further comprises red mud.

10. Solid feedstock according to claim 9, in which the red mud comprises a mineral compound including: 10 to 50 percent by weight of ferrous compounds, 12 to 35 percent by weight of aluminium compounds, 5 to 17 percent by weight of silicon compounds, 2 to 21 percent by weight of titanium dioxide (TiO2), 0.5 to 10 percent by weight of calcium compounds, additional impurities if applicable.

11. A thermochemical process for producing hydrogen from water and aluminous slag, based on the corrosion of the aluminum contained in the aluminous slag, in which at least one of the following reactions is carried out: - 2Al(s) + &H2O(l) -> 2Al(OH)3(s) + 3H2(g) (1) - 2Al(s) + 4H2O(l) ^ 2AlO(OH)(s) + 3H2(g) (2) - 2Al(s) + 3H2O(l) -> Al2O3(S) + 3H2(g) (3) characterized in that the process comprises: a step of preparing a solid feedstock comprising aluminous slag and red mud, a step of mixing the solid feedstock thus prepared with an aqueous solution containing water, and the recovery of a gaseous product containing hydrogen.

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