Method for detaching and / or recovering a metal

Abstract

The invention relates to a method for detaching and recovering gold particles (12), in particular in the form of flakes, from an object (6) to be treated comprising an external or outer layer made of gold deposited on a substrate, the method comprising the following steps: a) treating the object (6) to be treated with an aqueous reactive solution (8) allowing solid gold particles (12) to be separated from the substrate; and b) separating the solid gold particles (12) by air bubble flotation in said aqueous reactive solution (8).

Description

Method for detaching and / or recovering a metal The invention relates to the recovery of at least one metal, in particular a precious or strategic metal, in the form of metallic particles of a size less than or equal to a few millimeters from manufactured objects or even mineral resources. It notably enables the recycling of production or post-consumer waste and / or components of electrical and electronic equipment. State of the art Numerous technologies exist for leaching metals present in ores as sulfides, oxides, silicates, and even, in the case of gold, in their metallic form. These technologies are used industrially to extract targeted metals. However, these technologies cannot be applied in the context considered here because they lead to non-selective extraction, target non-metallic species, or present significant environmental and health toxicities. One aspect of the invention relates to a method for recovering gold from an object comprising an outer layer of gold deposited on a substrate which comprises, or consists of, nickel. The literature mentions various processes for leaching gold from mineral resources. Among these, the most common are those involving treatment with cyanide and mercury (the latter now banned). Thiosulfates and thiourea, used as early as 1960 in the Soviet Union, are the most frequently cited alternatives to cyanides. Other methods have also been explored, including treatment with chlorine gas or hypochlorite / hypochlorous acid, both chlorinated oxidants. Treatment with chlorine gas, which is used as an oxidant, generates AuCl4 ions. -in solution. This process exhibits gold dissolution kinetics superior to those observed with cyanides. Furthermore, the presence of chloride ions in solution prevents the formation of a passivation layer on the gold surface, which is highly favorable to its dissolution. Regarding the exploitation of mineral resources, this process is hampered by the reactivity of chloride ions with sulfides and carbonates. These phenomena do not pose obstacles when the treated objects do not contain these minerals. However, the handling of chlorine gas is critical due to its toxicity, both to health and the environment. These toxicity problems can be partially circumvented by using hypochlorous acid (HClO) or sodium or calcium hypochlorite as an oxidizing agent. However, calcium hypochlorite leads to very slow leaching kinetics, resulting in a leaching rate of approximately 58% after 46 hours of reaction. A satisfactory leaching rate is obtained for hypochlorous anion concentrations above 10 g / L. But even under these conditions, the leaching kinetics are only acceptable at sufficiently low pH values. A hydrochloric acid concentration of 9 g / L results in the leaching of nearly 70% of the gold after only 5 hours of contact, with the pH value being close to 7.3 at the start of the treatment. Indeed, the pH of the solutions must be maintained to prevent the dissociation of hypochlorous acid and the formation of the ClO₄⁻ ion. -which is a weaker oxidant, be acidic or close to neutral (pH less than 7.5). It has also been shown that dissolving gold in chloride / hypochlorite mixtures results in dissolution rates on the order of 13 mg / cm³ 2 .h, much higher than what is observed with cyanides (2.5 mg / cm³). 2 .h). Research has also focused on dissolving gold present on the surface of components of end-of-life Electrical and Electronic Equipment (EEE), including circuit boards. Reviews on the processing of electronic waste with gold-plated surfaces describe processes that dissolve the gold using common reagents (cyanide, thiosulfate, aqua regia, etc.), which have major drawbacks in terms of both selectivity and toxicity. In particular, some studies have focused on leaching gold with ammonium thiosulfate solutions at 40°C in the presence of copper(II) sulfate and ammonia. Under these conditions, 3000 minutes are required to dissolve all the gold present, while only 50% of the total amount is leached after 1000 minutes. Tests to dissolve gold present in waste materials were also conducted using a mixture of thiourea, iron(III) sulfate, and sulfuric acid. After processing the crushed samples, the gold recovery rate exceeded 90% within 60 to 90 minutes. Iron(III) sulfate initially acts as an oxidant to the gold, which then reacts with the thiourea to form complexes of the type {Au[SC(NH2)2]}2SO4. During this treatment, the following metals were also leached: Cu, Fe, Pb, and Zn, at levels of 68%, 45%, 43%, and 28%, respectively. Therefore, this treatment does not exhibit selectivity for materials containing these metals. The work of Cecchi et al.

[2023] They employ lactic acid combined with hydrogen peroxide to treat organic waste on used electronic circuit board components with a gold coating on a metallic substrate. This treatment leads to oxidation and therefore leaching of the substrate metals, thus allowing the recovery of gold films in the form of gold flakes. However, this treatment has the major drawback of not being selective with respect to base or precious metals, and in particular nickel and / or gold, since it has been shown that lactic acid combined with hydrogen peroxide leads to the almost complete dissolution of the nickel present on the surface of the treated objects and to the dissolution of a significant portion of the gold. The use of copper leaching solutions is also known to enable the recycling of printed circuit boards. US 2022 / 0235434 and US2023 / 0374380 (Lexmark International Inc.) describe a solution based on acetic acid, salt, and an oxidant such as hydrogen peroxide. Thus, one of the objects of the invention is a process for recovering gold from an object comprising an outer layer of gold, which covers or is placed on a metallic substrate, and in particular nickel, not having all or part of these disadvantages and / or allowing a recovery of gold predominantly (> 50% by mass) in solid form. First object of the invention Thus, a first object of the invention is a process for detaching and recovering solid gold, particularly in the form of flakes, from an object to be treated comprising a layer, preferably outer or external, of gold deposited or plated onto a substrate comprising nickel, this process comprising the following steps: a) treating the object to be treated, preferably under agitation, by contacting it with a liquid aqueous solution comprising, consisting essentially of, or consisting of, water, 80 g / l to 300 g / l of acetic acid, 2 g / l to 35 g / l of hydrogen peroxide (H2O2), and 80 g / l to 120 g / l of sodium chloride (NaCl), to obtain solid gold in the form of particles, and in particular gold flakes, and a treated object that is at least partially demetallized, and b) the separation of the solid gold particles from the object being treated and from said liquid aqueous solution. This process is particularly advantageous because, since it recovers the product in particulate form, it offers good selectivity and avoids an additional electrolysis step, such as that used, for example, after cyanide solubilization. Furthermore, the reagents used are relatively safe to handle, and the waste solutions generated by the process are easy to manage. The inventors tested various concentrations of acetic acid alone, ranging from 2% to 100% by volume, or sometimes combined with sodium chloride alone, or with hydrogen peroxide alone. However, these compositions did not remove the gilding from the object, and therefore did not allow for the recovery of gold particles. Only compositions containing all three components were able to remove the gilding and detach the gold particles. The results obtained for each of these "simpler" solutions did not encourage the use of an acetic acid-based solution for removing gold particles. The outer gold layer is preferably a thin layer, but preferably not nanometric. This layer, or film, is generally 0.1 to 50 µm thick. The substrate may comprise or be made of a metallic alloy containing nickel and / or zinc, such as brass, or of a layer comprising or being made of nickel, deposited directly or indirectly on a core. This core may be metallic, but advantageously it comprises or is made of a plastic material such as ABS (acrylonitrile butadiene styrene). When a layer of nickel, or of a nickel-containing material, is deposited on a core, it is preferable that the object to be treated include a layer, or film, of another material that promotes the adhesion of the nickel, for example, to the ABS core. The thickness of this layer may be as described above, for example, 0.1 to 50 microns, and preferably 1 to 5 microns.This other material can be a metallic material and is preferably a copper-based or copper-containing material. Indeed, the aqueous reactive solution is particularly effective for such objects. For the purposes of this invention, a "particle" is defined as a solid body with dimensions of at least 1 µm (micrometer) and less than 1 cm, or even less than 3 mm. Preferably, the particle dimensions range from 10 microns to 2 mm. The particles are preferably only slightly oxidized or not oxidized at all. For the purposes of this invention, a "flake" is defined as a particle having a thickness substantially less than at least one of its two other dimensions, for example, a rod, and preferably less than both of its other dimensions, for example, a flake shape and / or a lamellar particle shape. "Substantially less" is defined as preferably at least three times less, preferably at least five times less, and preferably at least ten times less. This thickness is at least 1 µm. Preferably, the two other dimensions of the flakes range from 10 microns to 2 mm, and more preferably, they are less than or equal to 1.5 mm, preferably 1 mm. By "object to be processed" we mean all types of objects but in particular manufactured objects and, preferably, objects to be recycled. For the purposes of this invention, "material comprising nickel" means a material comprising a non-negligible portion of nickel, in particular more than 2% by mass and preferably more than 5% by mass. "Made of nickel" means a material consisting essentially of nickel, that is to say, of at least 95%, preferably at least 98%, advantageously at least 99% nickel, and particularly pure nickel. In general, treatment with the aqueous solution induces the leaching of nickel and possibly other metals present in the substrate, such as copper and zinc, which causes the detachment and / or delamination of gold particles. The treated object is therefore the object to be treated, from which gold particles and possibly some substrate metals have been removed. It can be partially (e.g., <50% by mass of metal), substantially (e.g., >75% by mass of metal), or completely (e.g., >99% by mass of metal) demetallized. Additionally or alternatively, it can be partially (e.g., <50% by mass of gold), substantially (e.g., >75% by mass of gold), or completely (e.g., >99% by mass of gold) degolded. Preferably, the aqueous solution has a hydrogen peroxide concentration of 20 to 35 g / l, preferably 25 to 30 g / l, and more preferably about 28 g / l). Even more preferably, the concentration of acetic acid is from 250 to 300 g / l, preferably from 275 to 295 g / l, and most preferably it is about 286 g / l. A solution with these preferred concentrations of acetic acid and hydrogen peroxide allows for a particularly rapid detachment of gold particles and / or a particularly effective demetallization, especially on laminated surfaces. In the aqueous solution, demineralized water is preferably used. Preferably, the treatment with the aqueous solution is carried out at a temperature ranging from about 10°C to about 80°C, preferably 15°C to 50°C and preferably 20°C to 30°C, for example about 24°C± 2°C. Preferably, the treatment with the aqueous solution is carried out for a time ranging from approximately a few minutes to approximately 24 hours, preferably from 30 minutes to approximately 20 hours, and even more preferably from 40 minutes to 2 hours, for example, from 50 minutes to 1 hour 40 minutes. The treatment time is advantageously chosen to allow for the most complete removal of the gold layer possible, while avoiding or minimizing the dissolution of the gold. Preferably, treatment with the aqueous solution is carried out under ambient air atmosphere and / or at atmospheric pressure. Preferably, the solid-to-liquid ratio, that is, the mass in grams of the treated object relative to the volume of solution in liters, varies from approximately 12 g / L to 350 g / L, for example, up to approximately 200 g / L. The solid-to-liquid ratio can be around 65 g / L, but under industrial conditions this ratio is substantially higher, for example, around 200 g / L. In general, during separation step b), the gold particles are advantageously first separated from the object being treated, for example by stirring in the presence of the liquid aqueous solution, and then separated from the liquid by solid / liquid separation. Intermediate or subsequent washing and / or rinsing steps, for example with water, can be advantageously implemented. Solid / liquid separation can be carried out by any known conventional method such as filtration, evaporation, decantation, or centrifugation. Filtration is advantageously performed using a hydrophilic filter, so that the gold particles are not captured by the filter. The filtration system is, for example, a Buchner system, which accelerates the filtration process by applying a vacuum or negative pressure. Separation can also be achieved using a moving belt filter which allows both the recovery of the reactive solution in a first zone, the transport of particles to a second zone where these particles can be washed and / or rinsed to remove the reactive solution. Alternatively, the separation is carried out by centrifugation. The particles, possibly washed and / or rinsed, can then be dried. The particles obtained can advantageously be directed to a furnace in which they will be melted for shaping. The liquid from the washing can be used to make a new volume of liquid aqueous solution and / or purified of the metals contained by electrodeposition. In one embodiment, the gold particles are collected by first removing the treated object from the aqueous solution and then filtering the latter. In another embodiment, the object is treated in a reactor with an overflow, and the particles are preferentially gold flakes. The reactor environment is agitated, for example, by air bubbles. The aqueous solution is continuously renewed in the reactor, with the excess being discharged to the overflow. The gold flakes are separated from the treated object by being carried by the air bubbles to the overflow, which then discharges into an external device, such as a filter, allowing the separation of the flakes from the aqueous medium. The object to be treated may consist of a brass-based metal alloy coated with a gold film applied directly to its surface. This object may have various origins, for example, a fragment of a belt buckle or an electronic connector. A total mass of approximately 10 g of object(s), or even approximately 9.5 g of object(s), can be treated in 165 ml of solution. Thus, the solid / liquid ratio during treatment can vary from approximately 58 g / l to approximately 61 g / l. The object to be treated may have a plastic core, for example ABS, covered with copper, nickel and then gold. It can take the form of a branched solid tube, which can, for example, serve as a support for holding objects during an industrial process such as metallization. The total amount of gold present on the surface of the plastic object can be between 300 and 500 mg of gold per kg of object. Generally, the object has a mass of approximately 1 g. Two objects with a plastic core, each weighing 1 g, can be treated in 165 ml of solution. The solid / liquid ratio during treatment is then approximately 12 g / l. Alternatively, fifty plastic-core objects weighing 1 g each can be placed in a total solution volume of 1 to 2 litres; the solid / liquid ratio then varies from approximately 25 g / l to approximately 50 g / l In a first embodiment, the object to be treated consists of a brass-based metal alloy coated with a gold film applied directly to its surface, and the aqueous solution has a hydrogen peroxide concentration ranging from 20 to 35 g / l, preferably from 25 to 30 g / l, and more preferably from about 28 g / l. In this embodiment, the temperature is preferably around 30°C. The treatment in the aqueous solution is preferably carried out for 24 hours or less. In a first variant of this first embodiment, the acetic acid concentration ranges from 80 to 110 g / l, preferably from 90 to 100 g / l, and most preferably from approximately 95 g / l. This results in the detachment of the gold, which is obtained almost entirely in the form of particles. This variant is advantageous because it requires a small quantity of acetic acid. In a second variant of the first embodiment, the acetic acid concentration ranges from 250 to 300 g / l, preferably from 275 to 295 g / l, and most preferably from approximately 286 g / l. This second variant of the first embodiment is particularly advantageous because it allows for complete detachment of the gold with very little gold dissolution. In a second embodiment, the object has a plastic core, for example ABS, covered with copper, nickel and then gold, and the concentration of acetic acid ranges from 250 to 300 g / l, preferably from 275 to 295 g / l, and more preferably it is about 286 g / l. In a first variant of the second embodiment, the aqueous solution has a hydrogen peroxide concentration ranging from 2 to 5 g / l, preferably from 2.5 to 3.5 g / l, and more preferably from about 2.8 g / l. This variant allows for the recovery of gold in the form of particles. In a second variant of the second embodiment, the aqueous solution has a hydrogen peroxide concentration ranging from 20 to 35 g / L, preferably from 25 to 30 g / L, and more preferably from about 28 g / L. The treatment with the aqueous solution can be carried out for a period of about one hour. This second variant is particularly advantageous because it can recover about 80% of the gold in particulate form. In a particularly advantageous example of the second variant of the second embodiment, the aqueous solution treatment is carried out in the presence of ultrasound. This accelerates the detachment of the gold particles, combined with minimal gold dissolution. The result is an improved gold recovery yield in a very short time, approximately one hour. This example is simple and quick to implement, thanks to the use of readily available equipment, namely an ultrasonic cleaner. In a third variant of the second embodiment, the treatment of the object to be treated and the separation of gold particles are carried out in a flotation cell as described below. The particles are preferably flakes. The flotation cell may contain a total volume of aqueous solution ranging from 1 to 2 liters. Plastic-core objects weighing 1 g, preferably 50 objects, are placed in the cell in contact with the aqueous solution, which has a hydrogen peroxide concentration ranging from 20 to 35 g / l, preferably from 25 to 30 g / l, and more preferably from about 28 g / l. The treatment time is advantageously about 1 hour and 30 minutes.The separation of solid gold flakes from the treated objects is carried out as follows: the gold flakes are separated from the remaining object by being carried by air bubbles towards the overflow, then are separated from the aqueous solution by a solid / liquid separation, for example by filtration on a hydrophilic filter, so that the gold flakes are not captured by the filter. According to a third embodiment of the invention, steps a) and b) of the process are carried out during a flotation step as described in relation to the second object of the invention. In one embodiment, it is advantageous to recover the aqueous solution after the objects have been treated, in order to recover the dissolved metal(s), such as nickel, copper, and even gold. This additional recovery step can be carried out using a known method such as chemical precipitation, electrolysis, solvent extraction, the use of ion-exchange membranes, and / or a combination thereof. The choice of method depends on the metal(s) to be recovered. For gold, nickel, and copper, electrolysis is generally the preferred method. Although the process according to the first object of the invention is particularly suited to a substrate having an outer layer of gold, for the detachment and recovery of solid gold, the process also applies to substrates as described above but having an outer layer of a precious metal other than gold, for the detachment and recovery of this other precious metal in solid form and in particular silver, platinum, palladium, rhodium, iridium, osmium and ruthenium. Second object of the invention A second object of the invention is a method for detaching and recovering particles of a solid metal, particularly in the form of flakes, from an object to be treated comprising a layer, preferably outer, of said metal, deposited or plated onto a substrate. The method comprises the following steps: a) treating the object to be treated with an aqueous reactive solution, preferably liquid, enabling the detachment of solid particles of said metal from its substrate; b) separating said solid particles of said metal by air bubble flotation in said aqueous reactive solution; and c) the recovery of said solid particles of said metal. This object of the invention allows a particularly efficient recovery of solid metal, advantageously gold, deposited or plated on objects to be treated and particularly recycled, because it allows the rapid separation of solid gold (or metal) from the reactive medium and thus limits a possible dissolution of the latter in the reaction medium. For the purposes of this invention, the terms "glitter," "coating," and "object to be treated" have the same meanings as before. It should be noted that in the process of the second object of the invention, the object to be treated is not limited to an object comprising, or being composed of, nickel. The gold coating can be deposited on another type of metal, such as, in particular, an alloy like brass, whether or not it contains nickel. The process according to the invention is advantageously implemented by placing the object to be treated in a device such as a flotation cell. A flotation cell comprises a flotation chamber, or reactor, in which the liquid reactive aqueous solution and the object(s) to be treated are brought into contact. For the creation of air bubbles, the device may include an aerated agitator, that is, a mechanical device that creates air bubbles and disperses them in the mixture. Alternatively, and more advantageously, the air bubbles are created by an air injector, which allows air to be introduced into the mixture without requiring mechanical agitation, thus avoiding particle breakage. "Air injector" refers in particular to a glass sinter or an injection nozzle. Glass sintering is particularly preferred for implementing the process according to the invention because it avoids any particle shearing.The flotation cell includes an overflow system, for example an overflow advantageously equipped with a weir. This weir allows, in particular, the removal of a supernatant (for example, foam and / or excess liquid) present on the surface, or near the surface of the aqueous reactive solution and containing the gold particles, by pouring it into a receptacle such as a filter. The aqueous reactive solution allows the metal layer to be detached from the substrate, preferably by leaching the substrate, and in particular by leaching the material to which this gold layer is bonded. Preferably, the reactive solution is a leaching solution, and in particular a solution of a metal or a metal alloy other than gold. Advantageously, this metal (or metal alloy) is the one on which the gold layer is deposited, directly or indirectly. For example, the leaching solution is a solution of nickel, copper, zinc, and / or brass. Preferably, the object to be treated, the aqueous reactive solution, and / or the treatment and / or recovery conditions are as described with reference to the first object of the invention. However, other known aqueous reactive solutions may be used, such as a solution comprising an organic acid like lactic acid and hydrogen peroxide, as mentioned above. Preferably, the object to be treated is of relatively low weight, for example between 1 mg and 50 g, preferably between 10 mg and 10 g and even more preferably between 50 mg and 2 g. An object to be treated of low density (for example less than 2) and / or with a plastic core is particularly suitable for the process according to this second object of the invention. Preferably the aqueous reactive solution has a density greater than or equal to 1, preferably between 1 and 2, more preferably about 1.1. It may be advisable to add an additive to this reactive solution to increase the hydrophobicity of the gold particles. This additive may be a chemical collector (e.g., xanthate or dithiophosphate) to make the particles more hydrophobic, a depressant (e.g., sulfur) or an activator (e.g., potassium xanthate) to adjust the collector's affinity, and / or a foaming agent (e.g., kerosene) to control the size and lifetime of the air bubbles. However, according to a particularly preferred embodiment of the invention, the reactive solution does not contain xanthate, sulfur dithiophosphate, potassium xanthate, and / or kerosene. More advantageously, the reactive solution does not contain a chemical collector, a depressant, an activator, and / or a foaming agent. Indeed, unexpectedly, the gold particles detached from the objects to be treated are particularly well suited to separation by flotation. Gold particles have a size, shape, surface properties, and density that might be perceived as obstacles to the use of air flotation processes, which are generally used to separate different plastics or minerals. However, due to the hydrophobicity of air bubbles, gold flakes trap air bubbles on their surface, which then carry them along in their vertical movement, allowing them to be discharged into the overflow system. The use of a reactive aqueous solution is particularly advantageous because it allows both the detachment of gold particles, especially in flake form, and also has the necessary density for the upward movement of air bubbles and the transport of flakes to the overflow system. It also ensures the complementary dissolution of layered metals other than gold. The flotation chamber can contain a total volume of aqueous solution ranging from 1 to 2 liters, or even several hundred liters for industrial applications. It may have an overflow located in its upper section. Advantageously, the aqueous solution is continuously renewed, preferably at a flow rate of 2 to 20 ml / min for a reactor volume of 1 or 2 liters, or even at higher flow rates depending on the reactor volume, in order to compensate for the liquid discharged through the overflow and to allow for continuous overflow. Once metal particles, particularly gold flakes, are separated from the object being treated and carried by air bubbles towards the overflow system, a separation step takes place. The overflow of the mixture of reactive aqueous solution, possibly containing foam, and metal advantageously carries a portion of the mixture, rich in solid particles, out of the chamber. The metal particles can then be separated from the aqueous solution by a solid / liquid separation, which can be as described previously in relation to the first object of the invention. Similarly, a step involving washing the metal particles, rinsing the metal particles, melting the recovered metal, and / or reusing the aqueous reactive solution, as described above in relation to the first object of the invention, can advantageously be carried out. In particular, the aqueous reactive solution recovered following the solid / liquid separation can be reused, either directly or after purification or the addition of its constituent reagents to restore its reactivity, to fill the flotation chamber. Advantageously, the treatment of the object to be treated with the aqueous solution is carried out for a time ranging from approximately a few minutes (e.g., 5 minutes) to approximately 5 hours, preferably from 30 minutes to approximately 4 hours, and even more preferably from 40 minutes to 3 hours, for example, from 50 minutes to 1 hour and 40 minutes. The treatment time is advantageously chosen to allow for the most complete removal of the metal layer possible, while avoiding or minimizing the dissolution of the metal. Advantageously, the size of the air bubbles is 0.2 to 2 mm, preferably less than or equal to one millimeter, for example 120 microns. The solid-to-liquid ratio during treatment is adjusted by a person skilled in the art, based on their general knowledge and taking into account the specific characteristics of the objects to be treated, particularly their volume and shape, to ensure optimal contact between the object and the aqueous solution. For example, the solid-to-liquid ratio during treatment can vary between approximately 10 g / L and approximately 70 g / L, preferably between approximately 25 g / L and 50 g / L. Advantageously, the flotation chamber is made of a stainless and hydrophilic material. This prevents metal particles, and in particular hydrophobic gold flakes, from adhering to the chamber wall, which would hinder their movement to the overflow system and their recovery. For example, the reactor is made of glass or stainless steel. Advantageously, the flotation chamber is equipped at its bottom with a glass sinter with a porosity suitable for generating air bubbles of one millimeter or less in size. Preferably, the porosity is chosen to avoid the need for a high-pressure gas supply, which is necessary when using very small porosities. Thus, a porosity of 160 to 250 µm is suitable for distributing gases in low-pressure liquids, a porosity of 100 to 160 µm is suitable for distributing and scrubbing gases in liquids, a porosity of 40 to 100 µm is suitable for medium gas filtration, and a porosity of 16 to 40 µm is suitable for fine gas filtration. A porosity of 100 to 160 µm is preferred to avoid the need for high pressure to pass through the sinter. The airflow is advantageously between 1 and 5 l / hour, preferably it is about 1 l / hour. Advantageously, the flotation chamber is equipped with a means of retaining the object being treated. For example, this means could be a basket or a separation grid, advantageously with mesh sizes adapted to retain the object being treated within it. This basket, or separation grid, prevents the object being carried towards the overflow, and thus promotes the separation of metal particles from the object being treated. Preferably, the mesh size ratio of the basket to the size of the object is between 1 / 10 and 2 / 3. This retention means is particularly useful when the objects have a core of plastic polymer that can cause them to float due to a density lower than that of the liquid medium and / or potentially also when they have hydrophobic surface properties. Beyond containing the objects, the basket allows for easy and rapid loading and unloading of the flotation chamber. Advantageously, the basket is equipped with gripping means, such as handles or rings allowing it to be loaded and unloaded, for example using a suitable lifting device. The basket is advantageously made of a non-hydrophobic material, or covered with a non-hydrophobic material, for example stainless steel. Indeed, the implementation of such a system using a basket made of plastic, and therefore hydrophobic, has highlighted the drawback of the presence of a hydrophobic material within it, which leads to the capture of hydrophobic metal particles that, under these conditions, cannot be quickly removed from the flotation chamber by air bubbles and overflow. The flotation cell preferably has a means of controlled supply of aqueous solution to the bottom of the reactor. This means is advantageously a conduit connected to a source of aqueous solution. Thus, the aqueous solution flows from the bottom of the flotation chamber to the overflow system, thereby agitating the solution and accelerating the detachment of the gold flakes. Advantageously, the treatment with the aqueous solution is carried out at a temperature ranging from about 10°C to about 80°C, preferably 15°C to 50°C and preferably 20°C to 30°C, for example about 24°C± 2°C. According to a preferred embodiment of this second object of the invention, the process according to the invention allows the de-metallization, preferably total, of a metallized object whose core is not made of metal. According to another embodiment of the second object of the invention, the process advantageously includes an additional step of recovering at least one metal dissolved in the reactive aqueous solution, for example, nickel, copper, and even gold. This additional recovery step can be carried out by a known method such as chemical precipitation, electrolysis, solvent extraction, the use of ion-exchange membranes, and / or by combining these methods. The choice of method depends on the metal(s) to be recovered. For gold and copper, electrolysis is generally chosen. Although the process according to the second object of the invention is particularly suited to a substrate having an outer layer of gold, the process also extends to a substrate as described above but having an outer layer of a metal other than gold. This may be a precious metal, and in particular silver, platinum, palladium, rhodium, iridium, osmium, and ruthenium. It is also considered that the process applies to the recovery of metals, other than precious metals, applied to substrates in the form of a thin film. In this case, the reactive aqueous solution is suitable for the detachment of particles, and preferably flakes, of this other metal or precious metal. According to one embodiment of this second object, the process for detaching and recovering gold particles, particularly in the form of flakes, from an object to be treated comprising an outer, or external, layer of gold deposited on a substrate, comprises the following steps: a) treating the object to be treated with an aqueous reactive solution enabling the detachment of solid gold particles from its substrate; and b) separating said solid gold particles by air bubble flotation in said aqueous reactive solution. In the process according to this embodiment, the object to be treated can be a manufactured object, and the gold layer is preferably a film with a thickness of 0.5 to 50 microns. In the process according to this embodiment, the aqueous reactive solution may include an organic acid and hydrogen peroxide. In the process according to this embodiment, the air bubbles can have a size ranging from 0.2 to 2 mm, preferably less than or equal to one millimeter, for example 120 microns. In the process according to this embodiment, air bubble flotation can be implemented by placing the object to be treated and the reactive aqueous solution in a flotation cell. In the process according to this embodiment, said flotation cell may include a flotation chamber and said flotation chamber may include a means for retaining the object to be treated, preferably a basket or a separating grid. In the process according to this embodiment, the retention means may be made of or covered with a non-hydrophobic material, for example stainless steel. In the process according to this embodiment, said flotation cell may include an overflow system, for example an overflow advantageously equipped with a weir. In the process according to this embodiment, said reactive aqueous solution may comprise water, from 80 g / l to 300 g / l of acetic acid, from 2 g / l to 35 g / l of hydrogen peroxide (H2O2), and from 80 g / l to 120 g / l of sodium chloride (NaCl). Third object of the invention A third object of the invention relates to a process for the selective recovery of nickel from a treated object comprising a preferably outer nickel layer applied to, or covering, copper, preferably in the form of a copper layer on a core. The process comprises a selective leaching step by contacting the treated object, preferably under agitation, with an aqueous leaching solution comprising 85 g / L to 135 g / L of acetic acid, preferably 95 g / L to 120 g / L, and preferably about 105 g / L. The solution is free of hydrogen peroxide, ferric chloride, and sodium chloride. The result is a leachate enriched in nickel but with little or no copper, and a treated object exhibiting a copper layer. The nickel-enriched leachate is then advantageously separated from the treated object.The nickel-enriched leachate is then optionally treated to allow the recovery of solid nickel. The amount of copper in the nickel-enriched leachate is preferably less than 10% by mass relative to nickel, preferably less than 5%, and advantageously less than 1%. This recovery can be carried out by a known method such as chemical precipitation, electrolysis, solvent extraction, the use of ion-exchange membranes, and / or a combination of these methods. For nickel, electrolysis is generally used. While other acetic acid solutions, including sodium chloride and hydrogen peroxide, or ferric chloride, also dissolve nickel and copper, surprisingly, only the solution according to the invention allows for the selective dissolution of nickel. This process is therefore particularly advantageous for the selective recovery of nickel, and subsequently, potentially, copper. The treated object is thus the object to be treated, from which nickel has been removed by dissolution in the leaching solution. The object to be treated may be partially (e.g., <50% by mass of metal), substantially (e.g., >75% by mass of metal), or completely (e.g., >99% by mass of metal) denickelized (i.e., nickel has been removed, particularly from its surface). For the purposes of this invention, the terms "layer" and "object to be treated" have the same meaning as before. It should be noted that, according to the process of the third object of the invention, the object to be treated does not necessarily comprise a layer of gold but may be an object that previously had an external layer of gold, or of another metal, positioned on the nickel layer and which was removed in a previous step by a de-golding or de-metallizing process, such as, for example, cyanidation, or one of the processes of the first and second objects of the invention described above. Thus, the process according to this third object of the invention can be combined with the processes described previously. In this case, it includes preliminary steps of removing the metal layer, for example gold, by detaching particles or flakes as described previously. A solution having these preferential concentrations of acetic acid makes it possible to obtain a selective leaching of nickel that is particularly effective on a laminated or plated surface. In the aqueous solution, demineralized water is preferably used. Preferably, treatment with aqueous acetic acid solution is carried out at a temperature ranging from about 10°C to about 80°C, preferably 15°C to 50°C and preferably 20°C to 30°C, for example about 24°C± 2°C. Preferably, the treatment with the aqueous solution is carried out for a time ranging from approximately a few minutes to approximately 24 hours, preferably from 30 minutes to approximately 20 hours. The treatment time is advantageously chosen to allow for the leaching of nickel, while avoiding or minimizing possible copper leaching. Preferably, treatment with the aqueous solution is carried out under ambient air atmosphere and / or at atmospheric pressure. According to a preferred embodiment of this third object of the invention, the process according to the invention allows the de-metallization, preferably total, of a metallized object to be treated whose core is not made of metal. According to another embodiment of the third object of the invention, the process advantageously includes an additional step of recovering the metal dissolved in the aqueous acetic acid solution, preferably nickel. This additional recovery step can be carried out by a known method such as chemical precipitation, electrolysis, solvent extraction, the use of ion-exchange membranes, and / or by combining these methods. According to a preferred embodiment, the process includes a subsequent step of leaching, or detaching, the copper from the treated object. According to this process, the object treated with the nickel-selective leaching solution is advantageously separated from said solution, optionally rinsed, and then treated to remove the copper or the copper layer. This step can be carried out using a copper-leaching solution of known means, such as a concentrated nitric acid solution, or alternatively and preferably using one of the solutions previously described under the conditions previously described, particularly with reference to the first object of the invention. Thus, according to a particular variant, the recovery of copper is carried out by treatment, preferably under agitation, of the object being treated, with an aqueous copper leaching solution containing 80 to 110 g / l, preferably 90 to 100 g / l, and more preferably about 95 g / l of acetic acid, 20 to 35 g / l of hydrogen peroxide, preferably 25 to 30 g / l, and more preferably about 28 g / l, and 80 g / l to 120 g / l of sodium chloride (NaCl). Preferably, the treatment with this aqueous solution is carried out at a temperature ranging from about 10°C to about 80°C, preferably 15°C to 50°C and preferably 20°C to 40°C, for example about 30°C± 2°C. Preferably, the treatment with the copper leaching solution is carried out for a time ranging from about a few minutes to about 24 hours, preferably from 30 minutes to about 20 hours, even more preferably from 40 minutes to 6 hours, for example from 4 hours ± 30 minutes. In another variant, the recovery of copper is carried out by treatment, preferably under agitation, of the object being treated, with an aqueous solution containing 280 g / l to 325 g / l of acetic acid, preferably 295 g / l to 315 g / l, and preferably about 305 g / l of acetic acid, and 13 g / l to 28 g / l of ferric chloride, preferably 16 g / l to 24 g / l, and preferably about 19 g / l of ferric chloride. This yields a copper-enriched solution. Preferably, the treatment with this copper leaching solution is carried out at a temperature ranging from about 10°C to about 80°C, preferably 15°C to 50°C and preferably 20°C to 30°C, for example about 25°C± 2°C. Preferably, treatment with the copper leaching solution is carried out for a time ranging from about a few minutes to about 25 hours, preferably ranging from 30 minutes to about 24 hours. According to another variant, the process advantageously includes an additional step of recovering the copper dissolved in the copper leaching solution. This additional recovery step can be carried out by a known method such as chemical precipitation, electrolysis, solvent extraction, the use of ion-exchange membranes, and / or by combining these methods. The preferred method for obtaining solid nickel is electrolytic electrodeposition. Fourth object of the invention A fourth object of the invention is a method for detaching and recovering particles of a solid metal, in particular in the form of flakes, from an object to be treated comprising an outer, or external, layer in said metal, deposited or plated on a substrate. The process comprises the following steps: a) treating the object to be treated with a liquid aqueous reactive solution, enabling the detachment of solid particles of said metal from its substrate; b) separating said solid particles of said metal from said aqueous reactive solution. The treatment with the aqueous solution is carried out in the presence of ultrasound. This process accelerates the detachment of metal particles. Metal dissolution from the particles is minimal because the reaction time can be reduced. This results in improved metal recovery efficiency in a very short time. The process is simple and quick to implement, thanks to the use of readily available equipment: an ultrasonic cleaner. The ultrasound frequency is advantageously between 20 and 30 kHz, preferably around 25 kHz. For the purposes of this invention, the terms "particles," "object to be treated," and "layers" have the same meanings as before. It should be noted that, according to the process of the fourth object of the invention, the object to be treated is not limited to an object containing nickel. The process according to the invention is advantageously implemented by placing the reactor comprising the aqueous reactive solution and the object to be treated in an ultrasonic bath. Preferably, the object to be treated, the aqueous reactive solution, and / or the treatment and / or recovery conditions are as described with reference to the first object of the invention. However, other known aqueous reactive solutions may be used, such as a solution comprising an organic acid like lactic acid and hydrogen peroxide, as mentioned above. Preferably, the object to be treated is of relatively low weight, for example between 1 mg and 50 g, preferably between 10 mg and 10 g and even more preferably between 50 mg and 2 g. Lightweight objects to be treated and / or those with a plastic core are particularly suited to the process according to this second object of the invention. Although the process according to the fourth object of the invention is particularly suited to a substrate having an outer layer of gold, the process also extends to a substrate as described above but having an outer layer of a precious metal other than gold, and in particular silver, platinum, palladium, or rhodium. In this case, the reactive aqueous solution is suitable for detaching particles, and preferably flakes, of this other precious metal. The invention also relates to a method for recovering a plastic core, at the end of the processes described above, when the object to be treated includes such a core. Brief description of the figures The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the attached drawing in which: This is a schematic view of a reactive density-flotation device according to one aspect of the invention used to detach and collect gold flakes. Detailed description Example 1 (Comparative): Partial selective recovery of gold present on the surface of a manufactured brass object. The process was implemented with various brass objects. A set of two objects (total mass of 10 g) made of a brass-based metal alloy coated with a gold film deposited directly onto its surface was treated as described below. These objects are a fragment of a belt buckle and an electronic connector. The set was placed in an Erlenmeyer flask. A mixture of: - 150 ml of a 10 vol.% aqueous solution of acetic acid obtained by diluting glacial acetic acid in demineralized water (the acid represents 10% of the total volume), - 15 ml of a 30 vol.% aqueous solution of hydrogen peroxide (H₂O₂ represents 30% of the total volume of the solution, in demineralized water), - supplemented by the addition of 18 g of sodium chloride, was prepared. The resulting solution contained 95.45 g / l of acetic acid, 28.18 g / l of H2O2 and 109.09 g / l of NaCl. This mixture was then poured into the Erlenmeyer flask under ambient air. The Erlenmeyer flask was sealed, with a vent provided to prevent any potential pressure buildup due to gas release. The object was kept under agitation using a conventional back-and-forth motion. After 24 hours at 30°C, the object was separated from the liquid and weighed. Gold flakes found in the liquid were separated from the liquid, which was then analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The mass of the different metals dissolved was determined. The amount of gold dissolved was extremely small (0.15 µg). Partial demetallization of the object was observed. The mixture allowed for partial detachment of the gold in flake form, accompanied by very low dissolution (approximately 14.5 µg of gold / kg of treated object). This process can potentially allow for easy recovery of gold in flake form. However, under the specific implementation conditions described in the example, it only allows for partial recovery. This recovery of gold in solid form represents almost all, if not nearly 100%, of the gold collected because the dissolution rate of the gold is very low. Example 2 Total selective recovery of gold present on the surface of manufactured brass objects. A set of 2 objects of the same type as those presented in example 1 (total mass of 9.49 g) made of a brass-based metal alloy covered with a gold film deposited directly on its surface was placed in an Erlenmeyer flask. A mixture of: - 150 ml of a 30 vol. aqueous solution of acetic acid obtained by diluting glacial acetic acid in demineralized water (the acid represents 30% of the total volume), - 15 ml of a 30 vol. aqueous solution of hydrogen peroxide (H₂O₂ represents 30% of the total volume in demineralized water), - supplemented by the addition of 18 g of sodium chloride, was prepared and added to the metallic objects. The resulting solution contained 286.35 g / L of acetic acid, 28.18 g of H₂O₂, and 109.09 g / L of NaCl. The Erlenmeyer flask, under ambient air, was sealed with a vent. The object was kept under agitation using a reciprocating motion. After 24 hours at 30°C, the process resulted in the complete detachment of the gold in flake form, accompanied by a very slight dissolution of approximately 0.125 mg of gold per kg of treated objects. The mass of gold recovered was 1.19 µg in solution. The leaching solution treatment resulted in the dissolution of 14.2 g of copper, 0.9 g of nickel, and 51.6 g of zinc per kg of object from the brass, and only 125 µg of gold per kg of object. This confirms the very high selectivity of this step, which allows for the recovery of nearly 100% of the surface gold in flake form when deposited on a brass substrate. Example 3: Selective recovery of gold flakes present on the surface of a manufactured plastic object coated with copper, nickel, and then gold In examples 3, 4, and 5, small objects of similar shape and weight (unit mass approximately 1 g) with a plastic core coated first with copper, then with nickel, and finally with gold, were treated. In this example, the plastic core is made of ABS (acrylonitrile butadiene styrene). These objects are in the form of branched solid tubes, which could, for example, serve as supports for holding objects during an industrial process such as metallization. The total amount of gold present on the surface of the plastic objects was determined to be between 300 and 500 mg of gold per kg of objects. The average value calculated from this range is used in the remainder of this document for leaching efficiency calculations. Example 3a: Selective recovery of gold present on the surface of a manufactured plastic object in the form of flakes using the leaching solution from Example 1. Two plastic-core objects (total mass 2 g) coated with copper, nickel, and then gold were placed in an Erlenmeyer flask. The mixture from Example 1, namely: - 150 ml of a 10 vol. aqueous solution of acetic acid obtained by diluting glacial acetic acid in demineralized water (the acid represents 10% of the total volume), - 15 ml of a 30 vol. aqueous solution of hydrogen peroxide (H₂O₂ represents 30% of the total volume in demineralized water), - supplemented by the addition of 18 g of sodium chloride, was prepared and added to the plastic-core objects in the Erlenmeyer flask. The flask, under ambient air, was sealed with a vent. The objects were kept in motion by a reciprocating motion. After 20 hours of treatment at room temperature with the leaching solution, the objects were completely demetallized. The gold concentration in the solution reached 3661 µg of gold / L, representing a recovery rate of 312 mg of gold / kg of dissolved objects. These results indicate that the gold was not recovered in flake form. The dissolution rates of nickel and copper reach approximately 12 and 26g / kg of treated objects respectively. Example 3b: Selective recovery of gold present on the surface of manufactured plastic objects. As described in Example 3, two plastic-core objects (total mass 2 g) coated with copper, nickel, and then gold were placed in an Erlenmeyer flask. The mixture from Example 2—150 mL of a 30 vol. aqueous acetic acid solution obtained by diluting glacial acetic acid in demineralized water (the acid represents 30% of the acid solution), 15 mL of a 30 vol. aqueous hydrogen peroxide solution (H₂O₂ represents 30% of the total volume of the peroxide solution), and 18 g of sodium chloride—was prepared and added to the plastic-core objects in the Erlenmeyer flask. The flask was sealed under ambient air, with a vent provided. The objects were kept in motion by a reciprocating motion. The treatment was applied for 1 hour, 20 hours, or 24 hours, and each treatment was repeated at least twice. After one hour of treatment at room temperature, the objects were partially demetallized. After 20 hours of treatment, the objects were completely demetallized. After 24 hours of treatment, between 300 and 500 mg of gold per kg of objects were dissolved, whereas the one-hour treatment resulted in 50 to 98 mg of gold per kg of treated objects being dissolved. Furthermore, a one-hour treatment period allowed for the recovery of approximately 81 to 89% of the gold in flake form. In contrast, after 24 hours, no gold was recovered in flake form. The dissolution rates of nickel and copper reach between 15 and 33 and between 20 and 48 g / kg of treated objects, respectively. Example 3c: Selective recovery of gold present on the surface of manufactured plastic objects. As described in Example 3, two plastic-core objects (total mass 2 g) coated with copper, nickel, and then gold were placed in an Erlenmeyer flask. A mixture of 150 mL of a 30 vol. aqueous acetic acid solution (obtained by diluting glacial acetic acid in demineralized water, with the acid representing 30% of the total volume) and 15 mL of a 3 vol. aqueous hydrogen peroxide solution (H₂O₂ representing 3% of the total volume of the peroxide solution in demineralized water), supplemented with 18 g of sodium chloride, was prepared and added to the plastic-core objects in the Erlenmeyer flask. The flask was sealed, with a vent inserted, under ambient air. The objects were kept in motion using a reciprocating motion. The resulting solution contained 286.35 g / l of acetic acid, 2.82 g of H2O2 and 109.09 g / l of NaCl. Objects treated with this leaching solution containing 30% acetic acid and 3% hydrogen peroxide were completely demetallized after 24 hours of treatment at room temperature. Under these conditions, after 24 hours of treatment, 226 mg of gold per kg of objects were released into the solution. These results indicate a gold recovery yield of 48.9%. The dissolution rates of nickel and copper reach 21 and 42g / kg of treated objects respectively. Example 4: Selective recovery of gold flakes present on the surface of a manufactured plastic object coated with copper, nickel and then gold by application of ultrasound. The leaching solution described in Example 3b was used in a process employing an ultrasonic device at room temperature. The step of activating the glitter detachment was carried out as follows. The Erlenmeyer flask containing the reagent solution and the objects to be de-gilded was placed in an ultrasonic bath. The ultrasonic frequency used was 25 kHz. This process allows a strong acceleration of the phenomenon of detachment of the gold flakes, the treated objects being totally demetallized after 1 hour of treatment, whereas in the absence of ultrasound, the phenomenon can require a time that can be estimated at least 8 to 10 hours. Ultrasound degreasing, due to the reduced treatment time, solubilizes only 33 mg of gold per kg of treated objects, compared to 98 mg / kg solubilized without ultrasound. These results allow us to determine the gold recovery yield in flake form, which is 93% with ultrasound and 78% without. Example 5 Selective recovery of gold flakes, present on the surface of a manufactured plastic object coated with copper, nickel, and then gold, by reactive density flotation In this example, a device 2, shown in Figure 1, is used. This device 2 comprises a flotation cell 4 with a flotation chamber in which objects 6 to be degilded and the leaching solution 8 are brought into contact. The flotation chamber 4 is made of a stainless and hydrophilic material (to avoid the risk of hydrophobic gold flakes adhering) (glass, stainless steel, etc.) and is fitted at its bottom with a glass frit 28, with a porosity adapted to the generation of small air bubbles. These air bubbles, not shown, are one millimeter or less in size and move vertically from bottom to top within the flotation chamber due to their density. These air bubbles carry the gold flakes 12 with them in their vertical movement, for the purpose of their removal / extraction from the flotation cell 4 by overflow. This entrainment of the gold flakes can be achieved through a phenomenon referred to here as "density-reactive flotation" (DRF). This phenomenon relies on the hydrophobic surface properties of the gold flakes, the density of the liquid medium (greater than or equal to 1, ideally between 1 and 2), and, in this particular example, the chemical reactivity of this medium towards the metals in the sub-layers beneath the gold film. Thus, the gold flakes are detached from the objects due to the dissolution of the metallic nickel sub-layer immediately beneath the gold film, or, in some cases, due to the dissolution of a copper film or a brass substrate present beneath the gold film. These flakes are then carried along by the upward flow of bubbles generated by the insufflation of air at a controlled flow rate. The airflow is injected through inlet 30. In this example, the airflow rate is between 1 and 5 L / hour. The airflow passes through a glass frit 28 with controlled porosity, ideally chosen from the porosities mentioned below. This avoids the need for a high-pressure gas supply required when using very small porosities. A porosity of 0 (160 to 250 µm) is suitable for distributing gases in low-pressure liquids. A porosity of 1 (100 to 160 µm) is suitable for distributing and scrubbing gases in liquids. A porosity of 2 (40 to 100 µm) is suitable for medium gas filtration. A porosity of 3 (16 to 40 µm) is suitable for fine gas filtration.In addition, the flotation chamber is continuously supplied, advantageously at a controlled flow rate (flow rate between 2 and 20ml / min for a flotation chamber with a capacity of 1 to 2 litres) by the reactive leaching solution 8 used for de-gilding objects 6. The arrival of this solution 8 is symbolized by the reference 10. The flotation cell 4 has an overflow in its upper part to allow for spillage, corresponding to an opening 16 in the upper part of the flotation chamber wall. This spillage flow, symbolized by reference numeral 34, carries the gold flakes 12, which are then conveyed to a device equipped with a filter 14. This filter separates the gold flakes 12 from the reactive solution 8 used, thus preventing the gold from dissolving. To achieve this, the suspension of gold flakes is filtered in a Buchner-type system 24, which accelerates the filtration process by creating a vacuum, symbolized by reference numeral 20, in the container 24 that collects the solution 26, now free of gold flakes. Device 2, as described above, combines the phenomena of chemical detachment of the gold and the entrainment of the gold flakes 12 generated by a flow of microbubbles of air, facilitated by the presence of a medium with a density greater than 1 and close to 1.1. The flotation cell 4 is also equipped with a tube 10 allowing a controlled supply of leaching solution 8 to the bottom of the flotation cell, leading to the overflow of the flotation chamber via the overflow 16. The flotation cell 4 includes a perforated basket 32 ​​or a closed filter with mesh sizes adapted to retain the objects 6 to be de-gilded (basket mesh / object size ratio preferably 1 / 10 to 2 / 3). This closed basket 32, or this separation grid, prevents the objects 6 from being carried towards the overflow 16.It is particularly useful when the objects have a core of plastic polymer, which can cause them to float due to a density lower than that of the liquid medium, and potentially also when they have hydrophobic surface properties. Basket 32, in addition to containing the objects 6, allows for easy and rapid loading and unloading of the device. The basket's weight capacity is adapted to the quantities processed per batch. It can be equipped with rings (not shown) to allow loading and unloading into the flotation cell 4 using a suitable lifting device (also not shown). Semi-continuous batch operation is the typical operating mode for the device considered in this particular example. A de-gilding process according to the invention was carried out in a glass flotation cell, equipped with a retention basket. 50 plastic-core pieces 6 comprising on their surface a layer of copper, nickel and then gold previously described in examples 3 and 4 were placed in the basket 32. 2 liters of the leaching solution 8 of examples 4 and 3b were used, i.e. a solution containing 286.35 g / l of acetic acid, 2.82 g of H2O2 and 109.09 g / l of NaCl. The air flow rate used was 1 L / h, and the air bubbles were submillimeter in size, meaning they were smaller than one millimeter (as described previously). The process was carried out for 90 minutes at room temperature. At the end of the experiment, all the objects were demetallized. More than 99% by mass of the gold was collected as flakes, and less than 1% by mass dissolved. Example 6: Selective recovery of a copper and nickel solution from manufactured objects coated with copper and then nickel. The plastic-core objects in examples 3 to 5 have been de-golded using known industrial methods (cyanidation) to leave the nickel layer visible. These objects were treated in the leaching solution described in Example 1, at a temperature of 30°C. The reaction is very rapid; the solution turns blue within minutes of the objects coming into contact with it. After a 4-hour treatment, the plastic substrate is completely free of metallic layers. The dissolution rates of nickel and copper reach 16.4 and 41.7 g / kg of treated objects respectively. Example 7: Selective recovery of copper and nickel from manufactured objects The plastic-core objects (two objects with a total mass of 2 g) with de-gilded nickel and copper coatings, as described in Example 6, were placed at room temperature in an aqueous solution containing a mixture of 150 mL of acetic acid diluted in demineralized water (30% by mass) and 5 mL of ferric chloride solution (60 g / 100 mL). The resulting solution contained 304.84 g / L of acetic acid and 19.35 g / L of ferric chloride. This allows for a rapid (approximately 24 hours) and complete removal of the metallic coating. This process enables the complete recovery of the copper and nickel content. The nickel and copper dissolution rates reached 37.7 g / kg and 52.6 g / kg of treated objects, respectively. Following the treatment, the plastic substrate was completely free of metallic layers. Example 8: Selective recovery of nickel from manufactured objects. Objects (mass of 1 g) described in Example 6, consisting of a plastic resin core successively coated with copper, nickel, and gold, were initially industrially de-gilded before the treatment described below. These objects were then exposed to an aqueous solution containing 105 g / L of acetic acid in demineralized water at room temperature for 24 hours. The solution contained neither H₂O₂, nor ferric chloride, nor NaCl. The solution consisted of demineralized water and acetic acid. After treatment, the objects were free of the nickel layer deposited on the copper and thus appeared as copper-plated objects. The apparent total dissolution of the nickel, at a level of 27.2 g / kg of treated objects, by this process, is such as to allow for the almost total preservation (dissolution of 1.3 g / kg of treated objects) of the copper layer.Copper can therefore be selectively recovered during a separate leaching step which can be described as described above with reference to examples 6 or 7. The invention is not limited to the embodiments presented, and other embodiments will be readily apparent to those skilled in the art. In particular, it is possible to recover gold flakes by reactive density flotation from objects made of a brass-based metal alloy coated with a gold film deposited directly onto its surface. List of references Teresa Cecchi, Zhaojing Gao, Christophe Clement, Anthony Camus, Andrew Karim, Olivier Girard and Clara Santato, Recovery of gold from e-waste via food waste byproducts, Nanotechnology 34 (2023) 065203 (7pp)- US 2022 / 0235434 - Lexmark International Inc.US2023 / 0374380 Lexmark International Inc.

Claims

A process for detaching and recovering gold particles (12), particularly in the form of flakes, from an object (6) to be treated comprising an outer, or external, layer of gold deposited on a metallic substrate, said process comprises the following steps: a) treating the object (6) to be treated with an aqueous reactive solution (8) enabling the detachment of solid gold particles (12) from its substrate; and b) separating said solid gold particles (12) by air bubble flotation in said aqueous reactive solution (8). The process according to claim 6, wherein the object (6) to be treated is a manufactured object, and the gold layer is preferably a film having a thickness of 0.5 to 50 microns. The process according to claim 6 or 7, wherein the aqueous reactive solution (8) comprises an organic acid and hydrogen peroxide. The method according to any one of claims 6 to 8, wherein the air bubbles have a size ranging from 0.2 to 2 mm, preferably less than or equal to one millimeter, for example 120 microns. The method according to any one of claims 6 to 9, wherein air bubble flotation is implemented by placing the object (6) to be treated and the reactive aqueous solution (8) in a flotation cell (4). The method according to any one of claims 6 to 10, wherein said flotation cell (4) comprises a flotation chamber and wherein said flotation chamber comprises a retention means (32) for the object (6) to be treated, preferably a basket or a separation grid. The method according to claim 11, wherein the retention means (32) is made of or covered with a non-hydrophobic material, for example stainless steel. The method according to any one of claims 6 to 12, wherein said flotation cell (4) comprises an overflow system (16), for example an overflow advantageously equipped with a weir. The process according to any one of claims 1 to 8, wherein said reactive aqueous solution (8) comprises water, 80 g / l to 300 g / l of acetic acid, 2 g / l to 35 g / l of hydrogen peroxide (H2O2), and 80 g / l to 120 g / l of sodium chloride (NaCl). A process for the selective recovery of nickel from an object to be treated comprising an outer layer of nickel applied to, or covering, copper, said process comprising a step of selective leaching of nickel by contacting said object to be treated with an aqueous nickel leaching solution comprising 85 g / l to 135 g / l of acetic acid, preferably 95 g / l to 120 g / l, and preferably about 105 g / l, the aqueous nickel leaching solution being free of hydrogen peroxide, ferric chloride and sodium chloride, to obtain a leachate enriched in nickel but little or no copper, and a treated object having a copper layer. The selective nickel recovery process according to claim 10, further comprising a subsequent step of leaching copper from the treated object having a copper layer in an aqueous copper-leaching solution. The selective nickel recovery process according to claim 11, further comprising, before the copper leaching step, a separation step of the treated object having a copper layer and the nickel-enriched leachate. The selective nickel recovery process according to any one of claims 11 and 12, wherein the aqueous copper leaching solution contains 80 to 110 g / l, preferably 90 to 100 g / l, and more preferably about 95 g / l of acetic acid, 20 to 35 g / l of hydrogen peroxide, preferably 25 to 30 g / l, and more preferably about 28 g / l, and 80 g / l to 120 g / l of sodium chloride. The selective nickel recovery process according to any one of claims 11 and 12, wherein the aqueous copper leaching solution contains from 280 g / l to 325 g / l of acetic acid, preferably from 295 g / l to 315 g / l, and preferably about 305 g / l of acetic acid, and from 13 g / l to 28 g / l of ferric chloride, preferably from 16 g / l to 24 g / l, and preferably about 19 g / l of ferric chloride.

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