Method for irrigating a porous substrate using a foam, and the uses thereof

Abstract

The present invention relates to a method for irrigating a porous substrate, comprising the steps of: preparing a foam consisting of a dispersion of air bubbles in a foaming aqueous solution, said foam comprising a liquid fraction of less than or equal to 15% by volume; spreading, over the surface of the porous substrate, a layer of the foam thus prepared, the thickness of the layer being greater than 5 cm; then maintaining the assembly long enough for at least a part of the liquid contained in the foam to infiltrate the porous substrate. The present invention further relates to the use of this method to recover an element of interest contained in a porous substrate and a particular aqueous foam.

Description

[0001] METHOD OF IRRIGATING A POROUS SUBSTRATE BY MEANS OF FOAM AND ITS USES

[0002] DESCRIPTION

[0003] TECHNICAL FIELD

[0004] The present invention relates to the general field of porous substrates.

[0005] Indeed, the present invention proposes a method for irrigating a porous substrate through its surface in a homogeneous manner, by implementing, in contact with the surface, a layer of aqueous foam which has a controlled liquid fraction (or gas fraction) and height.

[0006] Such a process is of particular interest for irrigating porous substrates such as ores, agricultural or horticultural land, with a view to providing them with an aqueous solution (water or water with additives) allowing, depending on the desired application:

[0007] • treatment of the substrate by heap or column leaching, allowing the solubilization or entrainment of recoverable materials (minerals, strategic metals or recyclable / recoverable pollutants);

[0008] • simple watering in the sense of the uniform and controlled supply of water over a given depth of the substrate;

[0009] • nutritive or curative or decontaminating treatment in the case of vegetated or non-vegetated soils.

[0010] STATE OF THE PRIOR ART

[0011] For the processing of ores, including uranium ore, heap leaching, also known as "heap leaching," is used to leach so-called "low-grade" ores. Ores with a high uranium content are treated by reactor leaching.

[0012] Reactor leaching is also used in urban mines when recycling industrial, metallurgical and electronic waste to separate / recycle strategic metals that are particularly sought after by the new technology industry for their electronic, magnetic, optical and catalytic properties. This waste can be a source of precious metals such as, for example, gold, silver, platinum and palladium. Current industrial processes based on hydrometallurgy have an initial pre-treatment step via coarse grinding followed by selective grinding or physical sorting, to separate plastic compounds from metallic compounds. This is followed by a reactor leaching step, requiring the use of a large quantity of reagents and large reactor volumes. Alternative biological leaching processes are also being studied to minimize the environmental impact.However, these processes have low efficiency and require a long action time.

[0013] Heap leaching has been used for processing low-grade uranium ores since the 1950s and involves crushing the ore before placing it in a heap and then irrigating it with an acid leaching solution. Leaching efficiencies are low, around 50 to 70%.

[0014] The most widely used heap leaching process involves firstly building the heap by crushing the ore and then agglomerating it in concentrated sulphuric acid into "pralines" of several hundred pm to a few tens of mm before building a heap several metres high. It is then watered, drop by drop, with a dilute sulphuric acid solution corresponding to the "leaching solution".

[0015] The pile is watered for several months with the acid leaching solution using a drip system fed by "pipes" placed directly on the pile. Industrially, the space between the pipes is several tens of cm, generally between 50 and 80 cm.

[0016] The pre-dissolved uranium, during the agglomeration phase of the ore, in the concentrated acid, is leached by the dilute acid solution which infiltrates and percolates gently by gravity in the porosity of the heap. The average time to pass through the different heaps is generally between 3 and 6 days. The leaching solution loaded with elements of interest (uranium in this case) is finally collected by drainage in a basin before undergoing a series of other treatments aimed at concentrating and then purifying the uranium, and also at recycling the acid for reuse at the top of the heap. Ghorbani et al report the importance of the permeability of the heap during leaching [1].It is recalled that the flow rate of the leaching solution in the heap must be, generally speaking, sufficiently low to allow the reaction between the solution and the target minerals and at the same time sufficiently fast to avoid the accumulation of water on the surface of the heap leading to the formation of puddles.

[0017] Furthermore, local permeability is often inhomogeneous within the heap; preferential paths may form within the heap, leading to accelerated percolation of solutions through it, with some areas of the heap never being leached. This variation in permeability may result from a compaction effect of the heap but is most often related to the nature of the ore and, in particular, to the presence of fine particles of clay or of plant or organic origin. The ore heap can be compared by analogy to a layer of soil (porous medium). Within a porous medium such as sand or soil, the water flow rate or permeation depends on the geometry of the soil pores but also on the differences in hydraulic head present within the soil. Darcy's law is the relationship that links the hydraulic head (pressure) to the water flow rate in a one-dimensional flow. This law is written in the form (I):

[0018] V = ki (I) with “V” corresponding to the flow velocity, “i” to the hydraulic gradient which is equal to “- dh / dx” (one-dimensional flow along the Ox direction), and “k” to the proportionality coefficient also called “permeability coefficient”.

[0019] Two main limitations on the extraction efficiency for a typical “drip” irrigation process using pipes have been identified.

[0020] The first limitation observed industrially in heap leaching of the most permeable ores is that the irrigation of the acid solution by drip irrigation is not homogeneous. On the surface, the heap is better watered near the pipes and there are unwatered "dry" areas on the surface between the pipes. In addition, preferential percolation paths in the porosity of the heap are formed.

[0021] The second limitation is observed in so-called "clogging" ores: the presence of numerous fine particles, and in particular clay particles, induces the appearance of clogging phenomena and creates preferential paths within the heap. They also cause the formation of puddles of solution on the surface of the heap, which can lead to losses of leaching solution through evaporation.

[0022] Leaching processes using foams have also been described in the prior art.

[0023] US Patent 4,080,419 proposes a method for leaching fragmented ore using in situ generated foam [2]. More specifically, a non-ionic surfactant such as 4-nonylphenyl-polyethylene glycol (Tergitol™ NPX) is added to the leaching solution which is dispersed in the heap before air is added, in an upward manner, whereby foam is generated. Note, however, that US Patent 4,080,419 [2] also envisages using a high density foam, obtained by using CO2 or halogenated hydrocarbon vapors (column 5, last paragraph). According to a particular embodiment, the foam is sprayed directly onto the surface of the heap and then, thanks to its high density, diffuses inside the coarse, high porosity heap.

[0024] In US patent 6,099,615 [3], a solution containing a surfactant or a foaming agent is brought into contact with the ore and foam is formed while the ore is crushed, conveyed or agglomerated, i.e., before the formation of the pile. This pre-treatment is used to promote the permeability of the pile and prevent the formation of preferential paths in the pile.

[0025] Similarly, US patent application 2013 / 045052 [4] contemplates the use of a foam in the treatment of an excavation deposit in order to reduce water contamination and facilitate the recovery of valuable constituents. After the foam has been dispersed over the excavation deposit, means such as drilling are used to inject the foam into the deposit, whereby it spreads upward, downward and circumferentially within the deposit. The method described in US patent application 2013 / 045052 [4] therefore requires means, in particular mechanical, to promote the injection of the foam into the deposit.

[0026] Finally, the Applicants have already proposed decontamination or disinfectant foams all comprising a foaming surfactant and a gelling or viscosifying agent, respectively in international application WO 2004 / 008463 [5] and in international application WO 2016 / 202879 [6].

[0027] The inventors set themselves the goal of proposing a method that addresses the technical problems of irrigation processes in general. More specifically, these problems include inhomogeneities in surface treatment and difficulties in controlling irrigation liquid quantities / flow rates, whether for heap leaching of ore or soil treatment. The method of the invention must therefore guarantee, above all, homogeneous treatment of the substrate surface and provide a controlled irrigation flow adapted to the application. The irrigation flow of said method induces sufficient contact time of the solution with the substrate, thus promoting the recovery of elements of interest present in the heap or the inactivation of polluting elements in a soil or the supply of elements of agronomic interest to a soil.

[0028] STATEMENT OF THE INVENTION

[0029] The present invention achieves the inventors' goal. Indeed, they propose a process that is easy to implement and has a low environmental impact because it requires neither expensive technical devices nor dangerous reagents, thanks to which, for example, it is possible to efficiently recover elements of interest in the context of heap leaching, while minimizing the quantities of effluent to be treated subsequently.

[0030] More particularly, the inventors have shown that it is possible to improve the irrigation of a pile, making it possible to optimize the irrigation efficiency and, therefore, the recovery or extraction efficiency of the elements of interest contained in the latter. This improvement is obtained by using a foam consisting of a dispersion of air bubbles in a foaming aqueous solution whose liquid fraction and height used are controlled.

[0031] For the most permeable piles, irrigation by a layer of aqueous foam as implemented in the invention, deposited homogeneously over the entire surface of the pile, guarantees a homogeneous distribution of the irrigation flow on the surface of the pile and avoids unwatered areas, in particular in the case of ores or divided solids from urban mine waste.

[0032] Similarly, for piles with low permeability, irrigation flow rates are sometimes too high at the injection point and puddles form on the surface, even flooding the entire surface. Irrigation with a layer of foam, particularly depending on its liquid fraction and its height, allows the irrigation flow to be modulated by controlling gravity drainage within the foam and thus preventing the formation of puddles on the pile.

[0033] All these advantages are achieved without the need to force the foam into the pile, particularly by injection means such as pipes, pipelines and drilling.

[0034] In this regard, it should be noted that it was not at all obvious to implement a foam consisting of a dispersion of air bubbles in a foaming aqueous solution given the teaching of US patent 4,080,419 [2] which envisages using a foam obtained by using CO2 or halogenated hydrocarbon vapors and capable of exhibiting, thanks to its high density, a rapid downward movement following its deposition on the surface of the pile. The inventors have shown that a downward movement of the liquid contained in the foam can be obtained with an air bubble foam by selecting an appropriate liquid fraction and usage height.

[0035] Remarkably, the properties of the foam implemented in the present invention can be used not only in heap leaching processes but also in agriculture, agronomy and horticulture.

[0036] Indeed, for applications on soil watering in agriculture, agronomy and horticulture or for the nutritive or curative treatment of these soils, the processes by sprinkling or in English "sprinkler" or by drip via pipes are water-consuming and irrigation is, as for the heap leaching of ore, most often inhomogeneous. Irrigation by the "foam technique" according to the invention allows a homogeneous treatment of the surface of the soil in which a controlled supply of water on the first centimeters of the soil such as, for example, the first 10 to 30 centimeters, makes it possible to save the water used and guarantees a homogeneous and effective supply of elements of agronomic interest. In this case, the improvement in the yield obtained via the process according to the invention corresponds to the improvement in the yield of agricultural or horticultural production for a quantity of water supplied.

[0037] Finally, the irrigation foam implemented in the invention can also be used to decontaminate in situ soil contaminated to a given depth by biological agents such as, for example, bacteria, viruses, fungi and toxins, or by chemical agents such as, for example, organophosphorus compounds and organic solvents.

[0038] In general, the present invention relates to an irrigation method consisting of depositing, on a porous substrate such as, for example, a leaching pile or soil, a layer of foam of suitable formulation with a thickness greater than 5 cm (limit not included) and a liquid fraction less than or equal to 15% by volume.

[0039] In other words, the present invention relates to a method for irrigating a porous substrate comprising the steps of:

[0040] - preparing a foam consisting of a dispersion of air bubbles in a foaming aqueous solution, said foam comprising a liquid fraction less than or equal to 15% by volume,

[0041] - spread, on the surface of the porous substrate, a layer of the foam thus prepared, the thickness of the layer being greater than 5 cm, then

[0042] - maintain the assembly for a sufficient time so that at least part of the liquid contained in the foam infiltrates into the porous substrate.

[0043] By "to irrigate a porous substrate", is meant in the context of the present invention a homogeneous supply, by infiltration or percolation, of a liquid into the porous substrate. More particularly, the foam with controlled humidity and height, implemented in the context of the method according to the invention completely covers and wets the surface of the porous substrate and the gravity drainage in the foam therefore irrigates, in a homogeneous manner, the porous substrate. By "porous substrate", is meant a substrate in particulate, fragmentary and / or lumpy form. The constituents of this substrate are aggregates of organic and / or inorganic elements, resulting from natural processes or grinding, crushing and / or agglomeration processes such as agglomeration with sulfuric acid to form mineral pralines. The pores of such a porous substrate correspond to the empty spaces present between the aggregates of elements constituting it.The size of these aggregates can range from a few micrometers to several centimeters.

[0044] Typically, the porous substrate irrigated by the method according to the invention is chosen from the group consisting of earth such as bare earth for agricultural activity, cultivated agricultural land, a green space, a stadium or field for sports activity or a horticultural garden; sand; broken or fractured geological rock formations; ore; ore concentrates; coal; mining residues; mining waste; slag; industrial, metallurgical and electronic waste; products containing strategic metals, such as, for example, nickel, cobalt and manganese, resulting from the recycling of lithium batteries also known by the English expression "black-mass" and a mixture thereof.

[0045] In a particular embodiment, the irrigated porous substrate may be in the form of a pile prepared prior to the implementation of the method according to the invention and by any technique known to those skilled in the art. This embodiment is particularly suitable for the porous substrate likely to contain one or more elements of interest such as metals and ores such as, for example, gold, silver, uranium, platinum, palladium, lithium, nickel, cobalt, manganese, aluminum, zinc, copper, rare earths, niobium, tantalum, scandium and chromium.

[0046] The first step of the process according to the invention consists of preparing an aqueous foam of controlled humidity from a foaming solution.

[0047] The foaming solution used to prepare the aqueous foam used in the process according to the invention comprises, as solvent, water, thus justifying the name of aqueous foaming solution. By "water" is meant tap water, deionized water or distilled water. Advantageously, the aqueous foam used in the process according to the invention may be a neutral, acidic or basic foam, in particular depending on the possible additional active ingredient(s) that it may contain.

[0048] The aqueous foaming solution used to prepare the aqueous foam used in the process according to the invention comprises at least one organic foaming surfactant.

[0049] By "organic surfactant" is meant an organic molecule comprising a lipophilic (apolar) part and a hydrophilic (polar) part. By "foaming organic surfactant" is meant an organic surfactant as previously defined, also having a hydrophilic / lipophilic balance (or HLB for "Hydrophilic-Lipophilic Balance") of between 3 and 8. As a reminder, the HLB value of a surfactant can easily be obtained using the Davies formula [7] and the HLB tables for different chemical groups, available to those skilled in the art.

[0050] In particular, the aqueous foaming solution constituting the aqueous foam used in the invention may comprise a single organic foaming surfactant or a mixture of at least two organic foaming surfactants chosen from non-ionic foaming surfactants, anionic foaming surfactants, cationic foaming surfactants, amphoteric surfactants, surfactants with a Bolaforme type structure, surfactants with a Gemini type structure and polymeric surfactants.

[0051] More particularly, the aqueous foaming solution used in the context of the present invention comprises a single foaming organic surfactant or a mixture of at least two foaming organic surfactants chosen from non-ionic foaming surfactants, anionic foaming surfactants and cationic foaming surfactants. In the mixtures of foaming organic surfactants, at least two surfactants are chosen from the same family or from two different families chosen from non-ionic foaming surfactants, anionic foaming surfactants and cationic foaming surfactants. Those skilled in the art will find information on the anionic foaming surfactants and the cationic foaming surfactants that can be used in the invention in international application WO 2016 / 202879 [6].

[0052] More particularly still, said at least one foaming organic surfactant contained in the aqueous foaming solution used in the context of the present invention is at least one non-ionic foaming organic surfactant. In other words, the aqueous foaming solution used in the context of the present invention comprises a single non-ionic foaming surfactant or a mixture of at least two non-ionic foaming surfactants.

[0053] As a reminder, non-ionic (or neutral) surfactants are compounds whose surfactant properties, in particular hydrophilicity, are provided by uncharged functional groups such as an alcohol, an ether, an ester or an amide, and may contain heteroatoms such as nitrogen or oxygen. Due to the low hydrophilic contribution of these functions, non-ionic surfactant compounds are most often polyfunctional. In the context of the present invention, the non-ionic foaming surfactants are in particular chosen from alkyl alkoxylates; fatty alcohol alkoxylates; fatty amine alkoxylates; fatty acid alkoxylates; oxoalcohol alkoxylates; alkylphenol alkoxylates; alkyl ethoxylates; fatty alcohol ethoxylates; fatty amine ethoxylates; fatty acid ethoxylates; oxoalcohol ethoxylates;alkylphenol ethoxylates such as, for example, octylphenol and nonylphenol ethoxylates; alcohols, α-diols, polyethoxylated and polypropoxylated alkylphenols having a fatty chain comprising, for example, from 8 to 18 carbon atoms, the number of ethylene oxide or propylene oxide groups possibly being in particular from 2 to 50; complex polymers of polyethylene and polypropylene oxides; copolymers of ethylene and propylene oxides; block copolymers of polyethylene and polypropylene oxides such as, for example, POE-POP-POE triblock copolymers; ethoxylated fatty alcohols; condensates of ethylene and propylene oxide on fatty alcohols; polyethoxylated fatty amides preferably having from 2 to 30 moles of ethylene oxide; polyethoxylated ethers preferably having from 2 to 30 moles of ethylene oxide;monoesters (monolaurate, monomyristate, monostearate, monopalmitate, monooleate, etc.) and polyesters of fatty acids and glycerol; polyglycerolated fatty amides containing on average from 1 to 5 and, more specifically, from 1.5 to 4 glycerol groups; oxyethylenated sorbitan fatty acid esters containing from 2 to 30 moles of ethylene oxide; monoesters (monolaurate, monomyristate, monostearate, monopalmitate, monooleate, etc.) and polyesters of fatty acids and sorbitan, polyoxyethylene sorbitan monoesters; sucrose fatty acid esters; polyethylene glycol fatty acid esters; alkylpolyglucosides; N-alkyl glucamine derivatives and amine oxides such as (Cio-Ci4) alkyl amine oxides or N-acylaminopropylmorpholine oxides; polyols (sugar-derived surfactants) in particular glucose alkoxides such as, for example, glucose hexanate;surfactants derived from glucoside (sorbitol laurate) or polyols such as glycerol alcohol ethers; alkanolamides and their mixtures.;

[0054] Advantageously, said at least one foaming organic surfactant contained in the foaming aqueous solution used in the process according to the invention is chosen from the group consisting of alkylpolyglucosides, ethoxylated fatty alcohols and mixtures thereof. Thus, as non-ionic foaming surfactants usable in the context of the present invention, it is possible to use one or more alkylpolyglucosides and / or one or more ethoxylated fatty alcohols and, in particular, one or more alkylpolyglucosides from the Glucopon® family such as “Glucopon® 215 CS” marketed by the company BASF and / or one or more ethoxylated C8-C11 fatty alcohols for the lipophilic part and comprising 8 units derived from partly hydrophilic ethylene oxide of formula: CH3-(CH2)8-IO-(OC2H4)SOH marketed by the company FEVDI.

[0055] In the aqueous foaming solution constituting the aqueous foam used in the process according to the invention, said at least one organic foaming surfactant, i.e. the organic foaming surfactant or the mixture of at least two organic foaming surfactants, is present, per liter of solution, in an amount of between 0.1 and 10 g, in particular between 1 and 9 g, in particular 2 and 8 g and, more particularly, between 3 and 7 g. Thus, in a particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the process according to the invention comprises or consists of:

[0056] - one or more foaming organic surfactants as previously defined and

[0057] - water.

[0058] In this particular embodiment, the irrigation method of the invention is a simple method of watering the porous substrate, i.e. a method making it possible to supply water to the porous substrate in a homogeneous manner.

[0059] In another particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the method according to the invention may comprise, in addition to at least one organic foaming surfactant as previously defined and water, at least one organic, gelling or viscosifying agent.

[0060] Advantageously, such an organic, gelling or viscosifying agent is a biodegradable and pseudo-plastic agent making it possible to further stabilize the aqueous foam and slow down the drainage and permeation flows through the porous substrate.

[0061] The organic, gelling or viscosifying agent(s) that the foaming aqueous solution may contain is / are, more particularly, chosen from water-soluble polymers, hydrocolloids, heteropolysaccharides such as, for example, polyglucosidic polymers with trisaccharide branched chains, cellulose derivatives and polysaccharides such as polysaccharides containing glucose as the sole monomer. As particular examples, the organic, gelling or viscosifying agent(s) that can be used in the context of the present invention is / are chosen from the group consisting of xanthan gum, carboxymethylcellulose and their mixture.

[0062] In the aqueous foaming solution constituting the aqueous foam used in the process according to the invention, said at least one organic, gelling or viscosifying agent, i.e. the organic, gelling or viscosifying agent, or the mixture of at least two organic, gelling or viscosifying agents, is present, per liter of solution, in an amount of between 0.5 and 8 g, in particular between 1 and 5 g and, in particular, between 1.5 and 3 g.

[0063] Thus, in a particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the method according to the invention comprises or consists of:

[0064] - one or more foaming organic surfactants as previously defined,

[0065] - one or more organic, gelling or viscosifying agents, as previously defined, and

[0066] - water.

[0067] In this particular embodiment, the irrigation method of the invention is also a simple method of watering the porous substrate, i.e. a method making it possible to supply water to the porous substrate in a homogeneous manner.

[0068] Alternatively, the aqueous foaming solution constituting the aqueous foam used in the process according to the invention may not contain any organic, gelling or viscosifying agent.

[0069] In another particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the method according to the invention may comprise, in addition to at least one organic foaming surfactant as previously defined, water and optionally at least one organic, gelling or viscosifying agent, as previously defined, at least one active ingredient.

[0070] By "active ingredient" is meant a chemically or biologically active ingredient, in particular capable of facilitating the recovery, detoxification / decontamination and / or immobilization of at least one element of interest as previously defined and / or capable of improving agricultural or horticultural production and yields either directly or by acting on an external factor which affects this production.

[0071] Any active ingredient having any of the abilities listed above can be used in the context of the present invention. This active ingredient may be soluble in the aqueous foaming solution. Alternatively, it may be insoluble in the latter, the mixture between the active ingredient and the aqueous foaming solution thus forming a dispersion. Similarly, it is obvious that depending on the active ingredient(s) present in the aqueous foaming solution, the other components of the latter such as the organic foaming surfactant(s) and possibly the organic, gelling or viscosifying agent(s), must be chosen so as not to be decomposable by the active ingredient(s).

[0072] In a first particular embodiment, the active ingredient that the foaming aqueous solution may contain is capable of facilitating the recovery, detoxification and / or immobilization of at least one element of interest as previously defined. The active ingredients that can be used for this purpose are in particular those described in US patent 4,080,419 [2], US patent 6,099,615 [3] and US patent application 2013 / 045052 [4] and, in particular, those described in Table II of US patent application 2013 / 045052 [4]. In particular, the active ingredient that the foaming aqueous solution used in the invention may contain is chosen from the group consisting of thiourea, a thiosulfate, a cyanide such as sodium cyanide, a bicarbonate such as sodium bicarbonate, an oxidant such as chlorine and an acid such as sulfuric acid optionally associated with sulfate salts.More particularly, the active ingredient that the foaming aqueous solution used in the invention may contain is sulfuric acid in combination with one or more sulfate salts of a metal chosen from the group consisting of magnesium sulfate, iron sulfate and aluminum sulfate. The experimental part illustrates such an active ingredient which is particularly usable for recovering uranium.

[0073] In this first particular embodiment, the foaming aqueous solution is a leaching solution.

[0074] In a second particular embodiment, the active ingredient that the foaming aqueous solution may contain is chosen from the group consisting of fertilizers, biocides and plant protection products. In particular, this active ingredient is chosen from the group consisting of fertilizers, fertilizers, urease and nitrification inhibitors, insecticides, repellents, herbicides, fungicides, bactericides such as, for example, hydrogen peroxide and bleach, sporicides, algaecides, germination inhibitors and chelating agents / complexing agents. In this second particular embodiment, the foaming aqueous solution is a solution useful for treating soil as previously defined.

[0075] A person skilled in the art will be able to determine, without inventive effort, the quantity of the active ingredient(s) to be used in the foaming aqueous solution depending on the nature of the latter and the desired effect.

[0076] In a particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the method according to the invention consists of or comprises:

[0077] - one or more foaming organic surfactants as previously defined,

[0078] - one or more active ingredients, as previously defined, and

[0079] - water.

[0080] In another particular embodiment, the aqueous foaming solution constituting the aqueous foam used in the method according to the invention consists of or comprises:

[0081] - one or more foaming organic surfactants as previously defined,

[0082] - one or more organic, gelling or viscosifying agents, as previously defined,

[0083] - one or more active ingredients, as previously defined, and

[0084] - water.

[0085] As previously mentioned, the aqueous foam used in the process according to the invention has controlled humidity and therefore controlled expansion. As a reminder, a foam is often characterized by its expansion (F) defined, under normal temperature and pressure conditions, by the following formula (II):

[0086] F = (Volgaz + VO I |jquide) / VO I liquid = Foam Vol / Liquid Vol — l / S (II )

[0087] Therefore, the humidity of a foam or liquid fraction (e) corresponds to the inverse of its expansion and is therefore defined by the Volüquide / olmousse ratio. The expressions "humidity", "liquid fraction" or "quantity of liquid" are equivalent in this document and can be used interchangeably. These values ​​are expressed in volume relative to the total volume of the foam.

[0088] The aqueous foam used in the invention has a liquid fraction (or sometimes called humidity) less than or equal to 15% by volume and in particular between 5% and 15% by volume, which corresponds to an overrun greater than or equal to 6.67 and in particular between 6.67 and 20. Note that the volume of liquid (Voliliquid) in the formula above corresponds to the volumes of the different compounds initially mixed to prepare the aqueous foaming solution and, in particular, to the sum of the volume of the organic foaming surfactant(s), the volume of the possible organic, gelling or viscosifying agent(s), the possible active ingredient(s) and the volume of water.

[0089] The preparation of the aqueous foam implemented in the invention comprises a first step consisting of mixing together the water, the foaming organic surfactant(s), the optional organic, gelling or viscosity agent(s), and the optional active ingredient(s), before generating the foam. This mixing can be carried out by adding the components all at once, in groups or one after the other. In a particular embodiment, it may be envisaged to prepare a l ère solution by mixing together the water, the organic foaming surfactant(s) and any organic, gelling or viscosifying agent(s), and adding the active ingredient(s) to this solution only just before generating the foam.

[0090] The second step of this preparation process consists of generating the foam. This step can be carried out by any foam generation system of the prior art and known to those skilled in the art. This involves any device ensuring gas-liquid mixing, in particular by mechanical stirring, by bubbling, by static mixer containing or not containing beads, by microbead tube foam generator, devices described in international application WO 2016 / 202879 [6], or any other device in particular spray nozzle systems or Venturi lance systems allowing high flow rates generally between 1 and 1000 m 3 / h. More particularly, the invention finds its interest in the use of a foam generator which makes it possible to control the humidity of the generated foam. This humidity control is carried out by measuring the flow rate of solution and mixed air. The formulations of the invention easily make it possible to obtain a foam with this latter type of generator whose humidity is less than or equal to 15% by volume and in particular between 5% and 15% by volume.

[0091] The second step of the method according to the invention consists of applying the foam prepared in the first step to the surface of the porous substrate. This application is carried out in the form of a layer more than 5 cm thick.

[0092] For the purposes of the present invention, "surface" means the boundary between the interior and the exterior of the porous substrate, this exterior corresponding here to the atmosphere surrounding the substrate. When the porous substrate is in the form of a pile, the surface on which the aqueous foam is applied is the upper part of the pile and possibly the upper part of the slopes of the pile.

[0093] The steps of preparing and applying the foam can be carried out one after the other or simultaneously. In a particular embodiment, the steps of preparing and applying the foam are carried out simultaneously. In this embodiment, the foam is generated near and in particular above the surface of the porous substrate and is distributed over the latter by gravity. It is also possible to generate and apply the foam at different points on the surface of the porous substrate. The experimental part below illustrates technical means that can be used to do this.

[0094] The thickness of the aqueous foam layer used in the method according to the invention is substantially uniform, excluding the edges of this layer. Advantageously, this thickness is between 5 cm and 30 cm and in particular between 5 and 10 cm. In these ranges, the “5 cm” limit is not included, while the “30 cm” and “10 cm” limits may be included.

[0095] Furthermore, the application of the aqueous foam can be repeated, whereby the foam is renewed in batches, discontinuously or continuously, on the surface of the porous substrate. When the porous substrate is in the form of a pile, this renewal ensures an average permeation / percolation flow at the outlet of the pile. By "average permeation / percolation flow at the outlet of the pile" is meant a flow of 3 to 75 L / (hm 2). For two consecutive batches, it is possible to apply aqueous foam in a layer of the same or different thickness but greater than 5 cm. The foam layer can also be continuously renewed at a suitable flow rate so as to maintain a constant thickness.

[0096] Typically, the application of aqueous foam in a layer thicker than 5 cm on the surface of the porous substrate can be carried out at flow rates of 3 to 75 L / (hm 2 ) and in particular from 5 to 50 L / (hm 2 ).

[0097] The third step of the method according to the invention consists in maintaining the assembly, i.e. the porous substrate and the foam applied to the latter, in the state i.e. without forcing, in particular mechanically, the penetration of the foam inside the porous substrate. To the naked eye, the porous substrate and the foam applied to the latter are maintained in a static state.

[0098] Under such conditions, only the liquid contained in the aqueous foam applied to the surface of the porous substrate infiltrates, by gravity drainage, into the latter. This infiltration occurs mainly in a downward and circumferential manner in the porous substrate.

[0099] It is obvious that the liquid contained in the foam that infiltrates the porous substrate corresponds to the aqueous foaming solution used to prepare this foam. Therefore, the different elements contained in this aqueous foaming solution are found in the liquid that infiltrates the porous substrate.

[0100] Furthermore, the present invention relates to a method for recovering at least one element of interest contained in a porous substrate in the form of a pile, said method comprising

[0101] - irrigation of the porous substrate according to a method as previously defined, and

[0102] - the recovery of the permeation flow at the outlet of the heap and its treatment in order to recover said at least one element of interest.

[0103] In this process, the porous substrate is typically selected from the group consisting of earth as previously defined; sand; broken or fractured geological rock formations; ore; ore concentrates; coal; mining residues; mining waste; slag; industrial, metallurgical and electronic waste; products containing strategic metals from the recycling of lithium batteries and a mixture thereof.

[0104] Likewise, said element of interest is advantageously chosen from metals and minerals and in particular chosen from the group consisting of gold, silver, uranium, platinum, palladium, lithium, nickel, cobalt, manganese, aluminum, zinc, copper, rare earths, niobium, tantalum, scandium and chromium.

[0105] In addition, the aqueous foaming solution used to prepare the foam used during the irrigation step comprises at least one active ingredient capable of facilitating the recovery, detoxification and / or immobilization of at least one element of interest as previously defined.

[0106] The step of recovering the permeation or percolation flow at the outlet of the heap and its treatment in order to recover one or more elements of interest is a classic step in hydrometallurgical processes, particularly those involving a prior heap leaching step. For this purpose, it should be noted that the permeation or percolation flow can be referred to as "leachate". In the latter, the element(s) of interest are in ionic form, dissolved in the leachate.

[0107] The treatment of leachate with a view to recovering one or more elements of interest uses techniques known to those skilled in the art and generally involves a purification step followed by a possible hydrolysis step.

[0108] The purification step allows the separation of the various elements of interest from each other and from the other constituents present in the leachate such as the foaming surfactant(s), the active ingredient(s) and possibly the organic, gelling or viscosifying agents. Different techniques can be used for this purification such as, for example, liquid / liquid extraction by a solvent, solid / liquid extraction, resin extraction, cementation or precipitation. Following this purification, it is possible to recycle the foaming surfactant(s) and possibly the organic, gelling or viscosifying agents at the head of the process to generate a new aqueous foam particularly useful in an irrigation process as defined in the present invention. Finally, the present invention relates to a particular aqueous foam used in the process according to the invention.The latter consists of a dispersion of air bubbles in a foaming aqueous solution consisting of:

[0109] - a foaming organic surfactant, in particular as defined above, or a mixture of foaming organic surfactants, in particular as defined above,

[0110] - possibly an active ingredient, in particular as previously defined, or a mixture of active ingredients, in particular as previously defined, and

[0111] - water, said foam having a liquid fraction of less than 15% by volume, i.e. an expansion greater than 6.67.

[0112] In a first form of implementation, the aqueous foam according to the invention consists of a dispersion of air bubbles in a foaming aqueous solution consisting of two elements which are:

[0113] - a foaming organic surfactant, in particular as defined above, or a mixture of foaming organic surfactants, in particular as defined above, and

[0114] - water, said foam having a liquid fraction of less than 15% by volume, i.e. an expansion greater than 6.67.

[0115] In a second form of implementation, the aqueous foam according to the invention consists of a dispersion of air bubbles in a foaming aqueous solution consisting of three elements which are:

[0116] - a foaming organic surfactant, in particular as defined above, or a mixture of foaming organic surfactants, in particular as defined above,

[0117] - an active ingredient, in particular as previously defined, or a mixture of active ingredients, in particular as previously defined, and

[0118] - water, said foam having a liquid fraction of less than 15% by volume, i.e. an expansion greater than 6.67.

[0119] Due to its composition, the particular aqueous foam according to the present invention is clearly distinguished from the foams described in international applications WO 2004 / 008463 [5] and WO 2016 / 202879 [6] by the absence of organic, gelling or viscosifying agent.

[0120] Everything previously described about the foaming organic surfactants, the active ingredients and their respective quantities as well as the expansion in the context of the aqueous foam used in the irrigation method according to the invention also applies to the aqueous foam according to the invention.

[0121] Other characteristics and advantages of the present invention will become apparent upon reading the examples below given for illustrative and non-limiting purposes and referring to the appended figures.

[0122] BRIEF DESCRIPTION OF THE DRAWINGS

[0123] Figure 1 shows a schematic of the setup for studying the infiltration into the pile of the drainage solution from a batch of foam.

[0124] Figure 2 shows the influence over time of foam height on flow.

[0125] Figure 3 shows a test in successive batches with a foam with a liquid fraction of 5% and a height of 5 cm (4 batches) and 10 cm (1 batch) in a column.

[0126] Figure 4 shows the variation of permeate volume over time with 10 cm of ore and foams with a liquid fraction of 10% and a height of 10 to 30 cm.

[0127] Figure 5 shows the comparison of the uranium extraction yield for foam tests (columns C1 to C5) in accordance with the method according to the invention and drip tests (columns C1 and C-2) according to the prior art with C1: Glucopon 2 g / L, 5 batches of 30 cm of foam and 2 batches of 60 cm; C2: Glucopon 10 g / L, 5 batches of 30 cm of foam and 2 batches of 60 cm; C3: Glucopon 10 g / L, 4 batches of 10 cm; C4: Glucopon 10 g / L, 9 batches of 5 cm; C5: Glucopon 10 g / L, 2 batches of 10 cm and 2 batches of 5 cm; C1 and C-2: conventional drip irrigation with geotextile distributor on the pile.

[0128] Figure 6 shows the partial covering of the NE 34 sand pile with foam by an ejection point with an internal diameter of 8 mm.

[0129] Figure 7 shows the recovery, on real ore, of the foam via an ejection by a pipe flush with the pile.

[0130] Figure 8 shows the covering of the pile by foam ejected at 4 points on the surface of the sand pile 50 cm apart.

[0131] Figure 9 shows the evolution of the flux as a function of time for a continuous 2-h test of foam injection on sand.

[0132] Figure 10 shows a schematic of the liquid fraction gradient within a continuously fed foam dome after two hours.

[0133] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0134] The following examples concern:

[0135] - irrigation by foam on small heap surfaces, in columns of 10 cm diameter on sand, models of ore heaps or on real uranium ore;

[0136] - leaching of uranium by this “foam” process on real ore; and

[0137] - irrigation by covering a larger surface pile: 45 cm mason's sieve or 2.25 m device 2 .

[0138] In the experimental part below and in the corresponding figures, when reference is made to a 5 cm thick foam, it should be understood that this foam has a thickness slightly greater than 5 cm, that is to say a thickness of the order of 5.2 cm, i.e. 5.2 cm ± 0.1 cm.

[0139] Example 1: Irrigation by acid leaching foam of a model pile of sand and kaolinite agglomerated in a column, in discontinuous batches.

[0140] The foaming leaching solution of the example (preferred solution) comprises “Glucopon® 215 CS” marketed by the company BASF (hereinafter Glucopon), stable in sulfuric acid medium (5 to 10 g / L) and the concentrations of acid and sulfate salts used are:

[0141] - Sulfuric acid at 10 g / L,

[0142] - Magnesium sulfate MgSO4 at 30 g / L (i.e. 6 g / l of Mg),

[0143] - Iron sulfate at 55 g / L (i.e. 20 g / L of Fe), and

[0144] - Aluminum sulfate at 190 g / L (i.e. 15 g / L of AI)

[0145] A given volume of acid leaching foam containing air bubbles (batch of foam of a given height 5 to 60 cm) is generated from this solution by a static bead generator which makes it possible to control the liquid fraction of the foam (5 to 10% most often, or 90 to 95% air).

[0146] The volume of foam is deposited directly onto 12 cm of a model pile (mixture of sand and kaolinite) pre-agglomerated in sulfuric acid in a liquid / solid ratio of 8% by volume, contained in a glass column 10 cm in diameter (i.e. 7.8 x 10' 3 m 2). Tests show that the foam deposited by a feed pipe directly onto the pile "rests", under the effect of its own weight, on the pile and wets it evenly.

[0147] The experimental setup shown in Figure 1 allows us to measure, using a balance, the mass of solution passing through the pile as a function of time, in order to calculate the permeation flow at each instant due to irrigation by the foam (derived curves).

[0148] The tests of foam batches of variable height and liquid fraction, carried out on a model pile of sand and kaolinite agglomerated in a column, made it possible to measure the maximum flow obtained during a foam batch and the average flow obtained during successive batches.

[0149] A. Example of maximum flow.

[0150] The permeation flux induced by the foam batch increases with time, passing through a maximum (slope at the inflection point of the curves Figure 2).

[0151] A low liquid fraction (i.e. 5%) with foam batches of height either 5 ± 2 cm or 10 ± 2 cm allows, for example, to obtain maximum flows of the order of 19 ± 8 L / h / m 2 and 29 ± 6 L / h / m 2 These maximum flows correspond to the maximum of each of the slopes in Figure 3.

[0152] Within experimental errors, the double-height foam generates a doubled hydraulic pressure at the interface, which doubles the permeation flux (Darcy's law). A 5 cm high Glucopon foam with a 5% liquid content achieves the lowest maximum fluxes. Other tests (not provided here) also show the proportionality of the permeation flux with the liquid fraction of the foam.

[0153] B. Examples of average flows.

[0154] Three successive batches of 5 to 10 cm of foam at 5% liquid fraction were carried out over several hours under leaching conditions on an agglomerated pile, after a first batch of 30 cm foam height called "saturation batch" which allows the pile to be completely saturated and a percolating pile to be obtained (height of 30 cm to provide enough liquid in one batch).

[0155] The graph in Figure 3 shows a succession of four 5 cm batches and one 10 cm batch. The average flows are calculated over an average duration of 45 min, for each of the 5 batches, which represents the time when the foam remains stable.

[0156] We first notice a similar behavior of the batches over time when they follow one another on the same heap; around 20 L / (hm 2 ) for the maximum flow without the appearance of a puddle. The average permeation flow obtained in semi-continuous mode is 4.4 L / (hm 2) over three times 45 min for 3 batches of foam with a height of 5 cm and a liquid fraction at 5%. These average permeation flows are compatible with industrial flows obtained in drop by drop of the order of 5 L / (hm 2 ).

[0157] Example 2: Leaching of real uranium ore in column process (effect of surfactant content and foam height).

[0158] A test campaign with leaching foams was carried out on agglomerated uranium ore (500 ppm U). Five leaching columns with a diameter of 10 cm contain approximately 1 kg of ore each, i.e. 12 cm in height. The leaching foam solution comprises Glucopon stable in sulfuric acid medium (2 or

[0159] 10 g / L) and the concentrations of acid and sulfate salts used are:

[0160] - Sulfuric acid at 10 g / L;

[0161] - Magnesium sulfate MgSO4 at 30 g / L (6 g / l of Mg)

[0162] - Iron sulfate at 55 g / L (20 g / L of Fe),

[0163] - Aluminum sulfate at 190 g / L (15 g / L of AI).

[0164] Uranium concentration measurements in the permeates were performed by X-ray fluorescence and inductively coupled plasma atomic emission spectroscopy (ICP / AES).

[0165] The protocol used is as follows: upstream of the irrigation test, the ore is pre-agglomerated with a 3M sulfuric acid solution in a liquid / solid ratio of 8% (unsaturated ore) and then introduced into the column. The foam is then generated directly on the surface of the pile using an air foam generator using a 2-2.3 mm bead tube (flow rate of 1 to 2 L / min) connected to the compressed air network. This method makes it possible to control the liquid fraction (5 to 10%) and the height (5 to 60 cm) of the foam on the surface of the pile.

[0166] Leaching by foam layer directly deposited on the uranium pile is carried out discontinuously by replacing, every 45 to 60 min, the foam layer of a given height previously drained by a fresh foam layer.

[0167] For several experiments carried out for example at 10 g / l of Glucopon and a foam with an expansion F fixed and equal to 10 (i.e. 10% of liquid fraction, the expansion being equal to the inverse of the volumetric liquid fraction) with heights varying from 10 to 30 cm and on a pile of real ore 12 ± 2 cm thick (approximately 1.5 kg of ore) made it possible to evaluate the permeation flux of a process according to the invention (Figure 4).

[0168] Average flows calculated over one hour per batch are in the range of 7 to 28 L / (hm 2 ) and the maximum flows vary between 33 and 90 L / (hm 2 ).

[0169] A reduction in the concentration to 2 g / L of Glucopon could also be carried out in order to reduce the cost of the process associated with the use of the surfactant. The behavior of the foam at 2 g / L in Glucopon is similar to that of the foam at 10 g / L with an average flow of 31 L / (hm 2 ) and a maximum flow of 86 L / (hm 2 ).

[0170] Finally, tests to reduce the permeation flux were carried out with a lower foam height (5 cm) and a higher expansion of 20. These conditions made it possible to obtain a very satisfactory average flux of 4.3 L / (hm 2 ) and a maximum flow of 9-12 L / (hm 2 ). These results are entirely comparable to the permeation flows obtained industrially in drop by drop (4 to 6 L / (hm 2 ).

[0171] Uranium leaching

[0172] The leaching efficiency of uranium by foam at 10% liquid fraction was notably measured for the 2 surfactant concentrations (2 and 10 g / L) after 5 batches of foam of height 30 cm and two batches of height 60 cm (column 1 and 2 respectively) making it possible to reach a quantity of liquid in relation to the ore greater than 1 (Figure 5).

[0173] Uranium extraction yields in foam are greater than 50% for a liquid input greater than the initial solid quantity (L / S >1) (Figure 5), and can even reach 65% for foam at 10 g / L of surfactant.

[0174] These two yields are compared to the yields obtained by industrial-type drip irrigation on the same ore which do not exceed 30% (conventional irrigation on Cl and C-2).

[0175] These results show the interest of the process according to the invention: the foam extracts the uranium quickly over the first centimeters of the pile (10 to 14 cm) thanks to homogeneous irrigation of the pile over the first centimeters.

[0176] Example 3: Covering a foam on a pile of sand with a vertical ejection pipe.

[0177] Deposition / coverage tests of a lixivating acid foam with Glucopon and 10% liquid fraction were carried out on a sand pile.

[0178] Three kilograms of NE 34 sand are placed on absorbent paper at the bottom of a 45 cm diameter mason's sieve to form a pile of 0.16 m 2 to a thickness of 3-4 cm. A neutral Glucopon foam at 10% in liquid fraction is deposited Tl via a flexible pipe with an internal diameter of 8 mm fixed above the center of the sieve at a height of 10 cm (Figure 6).

[0179] The foam generation flow rate is 2 L / min for 2 min and allows to generate 4 liters of foam which flow and gradually cover the pile, without touching the edges of the sieve. After 2 min, the final foam disc has a radius of 20 cm and the foam does not therefore rest on the edges of the sieve. The average thickness of foam deposited is approximately 6 ± 1 cm. This result shows the possibility of easily covering the pile with a foam thickness of 5 cm.

[0180] Example 4: Covering of a foam on real ore by ejection through a pipe flush with the pile.

[0181] Figure 7 illustrates the application of the sulfated acid foam of Example 2 at expansion 10 and a thickness of 5 cm onto a pile of real uranium ore with a diameter of 40 cm via a horizontal flush pipe in contact with the pile.

[0182] The covering forms a foam disc whose radius increases over time at a rate of approximately 7 cm / minute. The foam layer heals easily on the surface to eliminate any hollows or air pockets.

[0183] Example 5: Covering of foam on the surface of a pile with a surface area greater than 2 m 2 .

[0184] A neutral foam (Glucopon 10 g / L) with a liquid fraction of 10% is applied to a surface area of ​​1.5 x 1.5 m 2 of white sand piles with a grain size of less than 2 mm. The experiments are carried out in a specific stainless steel tank with a frame and a grid supporting the pile of sand (5 to 10 cm)

[0185] The foam is ejected from the generator onto the pile at 4 points 50 cm apart (Figure 8, representative spacing of pipes for industrial drip irrigation).

[0186] With the continuous ejection of the foam, the four foam domes formed end up completely covering the surface of 2 m2 with a layer of foam of 5 to 15 cm depending on the flow rate and liquid fraction of the foam. The continuous foam feed tests are carried out with an ejection flow rate of 1 L / min per feed point in order to reduce the average height of the foam to

[0187] 3.5% in liquid fraction (Figure 9).

[0188] After one hour, the maximum flow reaches a plateau and the foam dimensions hardly change any more. The flow of 8 L / (hm 2) is calculated here with the total visual surface of the dome. The final dome measures approximately 120 cm resulting from the junction of 4 domes of 60 cm in diameter with an average maximum height of 25 cm by simple geometric measurement. In reality, the dome is made up of a layer of the wettest moss on the surface of the pile then a gradient in liquid fraction over the thickness of the layer where the driest moss is found on the periphery (Figure 10).

[0189] In dynamic conditions, the actual dimensions of the final dome wetting are obtained by destroying the dry foam on the surface of the dome by compressed air. The "wet core" remains stable under air flow. The parameters of the stabilized foam are an average height of fresh foam around 10 cm with a wetting diameter of 45 cm with an injection of 1 L / min per injector. Under these conditions, the maximum stabilized flow increases from 8 L / (hm 2 ) to 14 L / (hm 2). Continuous feed tests (50 cm spacing) show that a flow rate of 2 L / min in foam with 3.5% liquid fraction allows total coverage with the most complete wetting of the surface, with a flow rate of 10 L / (hm 2 ). Adjustments to the liquid fraction, the injection flow rate and the arrangement of the injectors thus make it possible to manage both the coverage and the height of the foam on the surface of the pile as well as the permeation flow at the pile outlet.

[0190] Bibliographic references

[0191] [1] Ghorbani et al, 2011, “Large particle effects in chemical / biochemical heap leak processes - A review”. Undermine. Eng., Special Issue: Bio and Hydrometallurgy 24, 1172-1184. doi:10.1016 / j.mineng.2011.04.002.

[0192] [2] US Patent 4,080,419 in the name of The United States of America as represented by the Secretary of the Interior, published March 21, 1978.

[0193] [3] US Patent 6,099,615 in the name of Golden West Industries, published on August 8, 2000. [4] US Patent Application 2013 / 045052 in the name of Golder Associates Inc., published on February 21, 2013.

[0194] [5] International application WO 2004 / 008463 in the names of CEA and COGEMA, published on January 22, 2004.

[0195] [6] International application WO 2016 / 202879 in the name of the CEA, published on December 22, 2016.

[0196] [7] “A quantitative kinetic theory of emulsion type, I. Physical chemistry of the emulsifying agent" Gas / Liquid and Liquid / Liquid Interface. Proceedings of the International Congress of Surface Activity (1957): 426-438.

Claims

DEMANDS 1. A method for irrigating a porous substrate comprising the steps of: - to prepare a foam consisting of a dispersion of air bubbles in a foaming aqueous solution, said foam comprising a liquid fraction less than or equal to 15% by volume, - spread a layer of the prepared moss, more than 5 cm thick, over the surface of the porous substrate, then - maintain the assembly for a sufficient time so that at least some of the liquid contained in the foam seeps into the porous substrate.

2. A method according to claim 1, characterized in that said porous substrate is selected from the group consisting of soil such as bare soil for agricultural activity, cultivated agricultural land, green space, stadium or field for sporting activity or horticultural garden; sand; broken or fractured geological rock formations; ore; ore concentrates; coal; mining residues; mining waste; slag; industrial, metallurgical and electronic waste; products containing strategic metals from the recycling of lithium batteries and one of their mixtures.

3. Irrigation method according to claim 1 or 2, characterized in that said aqueous foam has a liquid fraction of between 5% and 15% by volume.

4. Irrigation method according to any one of claims 1 to 3, characterized in that the application of aqueous foam is repeated, whereby the foam is renewed in batches discontinuously or continuously on the surface of the porous substrate.

5. Irrigation method according to any one of claims 1 to 4, characterized in that said foaming aqueous solution comprises at least one foaming organic surfactant.

6. Irrigation method according to claim 5, characterized in that said at least one organic foaming surfactant is at least one non-ionic organic foaming surfactant. 7 Irrigation method according to claim 5 or 6, characterized in that said at least one organic foaming surfactant is selected from the group consisting of alkyl polyglucosides, ethoxylated fatty alcohols and mixtures thereof.

8. Irrigation method according to any one of claims 5 to 7, characterized in that said at least one organic foaming surfactant is present, per liter of solution, in an amount between 0.1 and 10 g, in particular between 1 and 9 g, in particular between 2 and 8 g and, more particularly, between 3 and 7 g.

9. Irrigation method according to any one of claims 5 to 8, characterized in that said foaming aqueous solution further comprises at least one organic, gelling or viscosifying agent.

10. Irrigation method according to any one of claims 5 to 9, characterized in that said foaming aqueous solution further comprises at least one active ingredient.

11. Irrigation method according to claim 10, characterized in that said at least one active ingredient is selected from the group consisting of thiourea, a thiosulfate, a cyanide such as sodium cyanide, a bicarbonate such as sodium bicarbonate, an oxidant such as chlorine and an acid such as sulfuric acid optionally associated with sulfate salts.

12. Irrigation method according to claim 10, characterized in that said at least one active ingredient is selected from the group consisting of fertilizers, urease and nitrification inhibitors, insecticides, repellents, herbicides, fungicides, bactericides, sporicides, algicides, germination inhibitors and chelating / complexing agents.

13. A method for recovering at least one element of interest contained in a porous substrate in the form of a heap, said method comprising: - irrigation of the porous substrate according to a method as defined in any one of claims 1 to 11, and - the recovery of the permeation flow at the outlet of the heap and its processing in order to recover said at least one element of interest.

14. Recovery method according to claim 13, characterized in that said at least one element of interest is selected from the group consisting of gold, silver, uranium, platinum, palladium, lithium, nickel, cobalt, manganese, aluminum, zinc, copper, rare earths, niobium, tantalum, scandium and chromium.

15. Aqueous foam implemented in a process as defined in any one of claims 1 to 8 and 10 to 14, said foam consisting of a dispersion of air bubbles in a foaming aqueous solution constituted by: - an organic foaming surfactant or a mixture of organic foaming surfactants, - possibly an active ingredient or a mixture of active ingredients, in particular, and - water, the said foam having a liquid fraction of less than 15% by volume.

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