WO2024194463A1

WO2024194463A1 - Method for recovery of zinc

Info

Publication number: WO2024194463A1
Application number: PCT/EP2024/057778
Authority: WO
Inventors: Antoine Roger Paul MASSE; Noël Jean Joseph MASSON
Original assignee: Belzinc
Priority date: 2023-03-22
Filing date: 2024-03-22
Publication date: 2024-09-26
Also published as: BE1031460B1; BE1031460A1

Abstract

The invention relates to a zinc leaching method which comprises the following steps: - supplying a material that contains zinc in oxidized form, - leaching the material in an alkaline basic medium with formation of a solid residue and of a supersaturated zinc solution (i) or a zinc-rich solution (ii): - separating (A') the solid leaching residue from the supersaturated zinc solution (i) or from the zinc-rich solution (ii), - heating said supersaturated zinc solution (i) in order to precipitate a zinc oxide, - separating (D') the zinc oxide.

Description

ZINC RECOVERY PROCESS The present invention relates to a process for recovering zinc from a zinc-containing material. The Waelz process makes it possible to extract zinc from an ore or a secondary material and to recover it in the form of a zinc oxide richer than the original ore or secondary material. The enriched zinc oxide is sold to zinc smelters to produce metal by conventional methods. However, the Waelz process is a thermal process that consumes a lot of energy and emits a lot of carbon dioxide. There are also processes for acid leaching of oxidized ores that provide for purification and electrolysis to directly produce zinc in the form of metal. One of the disadvantages lies in the large quantity of acid used during the process. In addition, certain impurities that are harmful to electrolysis and very difficult to remove may be present in the ore and contaminate the leaching solution (Mg, Mn, F, etc.). Finally, the financial and human investment for an electrolysis hall is very significant and can only be justified for very large deposits. Document WO 2020/019834 proposes an attack on a zinc-poor rock with ammonia and ammonium carbonate. After filtration of the residue, a first direct addition of lime is made to obtain a first precipitate which is filtered. Then a second addition of lime leads to the precipitation of a second precipitate. Calcination is carried out on the second precipitate to provide a mixture which contains zinc oxide, oxides and/or zinc carbonate. calcium. One of the disadvantages is the use of ammonia, which leads to environmental and safety problems. In addition, the zinc concentration in the final product remains low. Processes involving caustic soda (sodium hydroxide) leaching, purification and electrolysis have been proposed. The advantage of caustic soda leaching is that it is more selective than acid leaching. In addition to the investment required for the electrolysis hall, another disadvantage is that during the electrolysis of sodium zincate solutions, the zinc metal is recovered in powder form, whereas it is recovered in the form of massive cathodes in an acid medium. The metal powder must be melted to cast ingots. However, this melting results in very costly losses of metal. In 1965, Merrill and Lang investigated several methods for recovering zinc from leaching solutions of zinc ores oxidized by caustic soda. Apart from electrolysis, zinc can be precipitated by carbonation, sulphurisation or dilution and recovered in the form of a marketable concentrate. However, all these methods have disadvantages. Carbonation aims to reduce the solubility of zinc by converting caustic soda into soda ash by bubbling carbon dioxide. The zinc then precipitates in the form of oxide. The disadvantage is that the consumption of caustic soda is prohibitive. It can be regenerated by reacting soda ash with lime. However, the consumption of lime is very high, of the order of three or four tonnes of quicklime per tonne of zinc. Sulphurisation consists of the precipitation of zinc sulphide by reacting the sodium zincate solution with elemental sulphur. This process also leads to a high consumption of soda which affects profitability. Finally, zinc can be precipitated in the form of oxide by dilution. This method requires very substantial additions of water. To recover the caustic soda, all of this water must then be evaporated, which is extremely energy-intensive. Document WO 2013/036268 proposes a method for treating waste that contains, for example, 65% Zn by attacking it with a NaOH solution. Most of the waste is dissolved in the solution. The zinc-rich liquor generated is then separated from the waste by filtration. This method involves adding Zn metal in powder form to the liquor in order to separate the Pb, Cu, Sn and Cd from it. Then, the Zn oxide precipitates following the addition of methanol which acts as an anti-solvent. A disadvantage of this method is the use of an anti-solvent which must be distilled from the zinc-depleted solution in order to recycle the NaOH. In order to recover zinc from materials that cannot be sufficiently enriched by simple physical methods to be sold to smelters, there is a real need to provide a zinc recovery process that is energy efficient, requires the use of few reagents, emits little carbon dioxide and does not require a heavy investment for the user. To solve this problem, the present invention provides a zinc recovery process that comprises the following steps: - Supplying a material that contains zinc in oxidized form, - Leaching the material in an alkaline basic medium with formation of a solid residue and a zinc-supersaturated solution (i) or a zinc-rich solution (ii), - Separation (A') of the solid leaching residue from the zinc-supersaturated solution (i) or from the zinc-rich solution (ii), - Optionally: • addition of a calcium compound, preferably selected from the group comprising lime, calcined dolomite and combinations thereof, to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) to form (with formation of) a pulp which contains solid calcium zincate and a basic zinc-depleted solution, • separation (B') of at least a portion of the basic zinc-depleted solution from the calcium zincate pulp, • heating said calcium zincate pulp consisting of the solid calcium zincate and the remainder of the basic zinc-depleted solution with formation of a zinc-supersaturated solution (iii) and a solid matter, • separation (C') of the solid matter obtained after the heating step to preserve (with preservation of) the zinc-supersaturated solution (iii) and optionally recycling this solid material to the step of adding a calcium compound, - Heating of said zinc-supersaturated solution (i) obtained after said separation (A') or of the zinc-supersaturated solution (iii) optionally obtained after the separation step (C') of said solid material to precipitate (precipitant), preferably predominantly, a zinc oxide in a zinc-depleted solution, - Separation (D') of the zinc oxide formed during the heating of the zinc-supersaturated solution (i) or (iii), - Optionally, recycling at least part of said zinc-depleted (basic) solution formed during at least one of the process steps by adding it to the basic medium leaching step or to said zincate pulp before heating and zinc supersaturation. From a material that contains zinc, the process makes it possible, in an easy and efficient manner, to form a solution supersaturated with zinc (i; iii) which after heating produces a zinc oxide that can be sold to zinc foundries (smelters) to produce zinc metal or as technical zinc oxide in particular for the vulcanization of elastomers, the production of enamels, animal feed. The heating step makes it possible to precipitate the zinc oxide in the solution which is therefore depleted in zinc. This spontaneous precipitation of zinc oxide constitutes one of the key aspects of the present invention. Preferably, the zinc oxide formed after the heating step constitutes the product predominantly present in said zinc-depleted solution. Indeed, the heating step which causes the precipitation of zinc oxide leads to a precipitation of a solid, the major part of which is formed of this zinc oxide, preferably at least 80% by weight of zinc oxide, preferentially at least 90% by weight of zinc oxide. of zinc, more preferably at least 95% by weight of zinc oxide, relative to the total weight of precipitate formed. The process according to the present invention is thus more economical, less energy-intensive, requires smaller quantities of reagents, produces less CO ₂ (reduction of more than 70%) compared to known processes, while ensuring a high zinc production yield. It is also possible, preferably, to produce a solution rich in zinc (ii) to which the calcium compound is added in order to form (with formation of) a calcium zincate pulp. After removing part of the depleted solution, a step of heating the calcium zincate pulp makes it possible to produce a solution supersaturated in zinc (iii) which will make it possible to obtain zinc oxide. Advantageously, it is also possible to use the solution supersaturated in zinc (i) and to add the calcium compound to it in order to form (with formation of) the calcium zincate pulp. After removing a portion of the depleted solution, when the thickened calcium zincate pulp is heated according to the steps set out above, this provides a zinc (iii) supersaturated solution. It should be noted that when the zinc (iii) supersaturated solution is obtained by the route set out above, this zinc (iii) supersaturated solution has a higher zinc concentration than the supersaturated solution (i) or the zinc (ii) rich solution that was initially provided. The effect of adding this step is either to obtain a more zinc-concentrated solution or to obtain a purer solution. The zinc (basic) depleted solution that was generated during the process, preferably by the precipitation of zinc oxide, can be recycled either to the leaching step or to the zincate pulp. calcium thickened by separating a portion of the depleted solution prior to the heating and zinc supersaturation step. Preferably, the precipitation of calcium zincate by adding the calcium compound to said zinc-rich solution (ii) or to said zinc-supersaturated solution (i) is carried out by applying a temperature of less than 70°C or between 0 and 70°C, preferably less than 50°C, more preferably less than 30°C. Advantageously, the precipitation of calcium zincate following the addition of the calcium compound to said zinc-rich solution (ii) or to said zinc-supersaturated solution (i) is carried out over a period of time of between 1 and 4 hours. Preferably, seeds comprising hydrated calcium zincate may be added to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) followed by an addition of a calcium compound, preferably selected from the group comprising lime, calcined dolomite and combinations thereof, leading to the formation of a pulp which contains solid calcium zincate and a basic zinc-depleted solution. This addition of seeds may thus take place before this possible addition of the calcium compound. More preferably, the step of heating said zinc-supersaturated solution (i) obtained after said separation (A') or of the zinc-supersaturated solution (iii) optionally obtained after the separation step (C') of said solid material to precipitate a zinc oxide in a zinc-depleted solution is carried out in the presence of zinc oxide, serving as precipitation seeds. According to a particularly preferred embodiment, the step of heating said zinc-supersaturated solution (i) obtained after said separation (A') or of the zinc-supersaturated solution (iii) optionally obtained after the separation step (C') of said solid material to precipitate (precipitant), preferably predominantly, a zinc oxide in a zinc-depleted solution is carried out at a temperature above 70 °C, preferably above 90 °C, optionally for a period of time of at least one hour, which can advantageously be up to 8 hours. According to an advantageous embodiment, during the leaching step, an addition of a calcium and/or magnesium compound in a stoichiometric amount is carried out to precipitate (precipitant) impurities chosen from the group comprising silica, alumina or carbonate. It is thus possible to form calcium silicates, calcium aluminates and calcium carbonates. A calcium and/or magnesium compound can also preferably be added to the zinc-supersaturated solution (i) or to the zinc-rich solution (ii) obtained after said separation of said solid residue. In this way, impurities, such as silica, alumina or carbonate, are separated from the zinc-containing solution and recovered in a separate solid. The calcium and/or magnesium compound is preferably selected from the group consisting of quicklime, hydrated lime, dolomite, calcined dolomite, magnesia, limestone and mixtures thereof. The presence of silica in the supersaturated solution may inhibit the precipitation of zinc oxide upon heating. Optionally, if the zinc-supersaturated solution (i) or (iii) contains silica, a specific purification step may be conducted prior to the precipitation of zinc oxide, for example, by contacting the supersaturated solution with silica seeds so as to force the precipitation of the dissolved silica. Once the silica concentration has been reduced, the zinc-supersaturated solution may be fed to the zinc oxide precipitation step. Advantageously, the leaching step is carried out at a temperature below 90°C, preferably below 70°C, optionally for a period of time less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours to prevent early precipitation of zinc oxide. More advantageously, if a supersaturated solution (i) is generated during the leaching step, the latter is carried out at a temperature below 90°C, preferably below 70°C, optionally for a period of time less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours to prevent early precipitation of zinc oxide. This embodiment is preferred and can be applicable to all variants of the process. According to a particularly advantageous alternative embodiment, the leaching step can be carried out at a temperature above 50°C, preferably above 90°C, more preferably above 150°C, optionally for a period of time less than 4 hours to prevent precipitation of calcium zincate. This advantageous alternative embodiment is also preferred when the zinc-rich solution (ii) is generated. This alternative embodiment is also advantageous when the material contains zinc and also calcium, or when a calcium-containing compound is added to the leaching step, a temperature above 50°C is applied, preferably for a period of time less than 4 hours. This makes it possible to prevent early precipitation of calcium zincate at this stage of the process. Preferably, the step of heating said calcium zincate pulp consisting of calcium zincate and the remainder of the basic solution depleted in zinc is carried out at a temperature above 50°C, preferably between 80°C and 95°C. This heating step is preferably carried out for a period of time between 0.1 and 4 hours, preferably between 0.5 and 2 hours, more preferably 0.5 and 1 hour. According to an additional embodiment, the leaching can be carried out in several stages, preferably in two countercurrent stages, as set out below: - A first leaching of the material in an alkaline basic medium with the production of a solid residue depleted in zinc and a solution partially enriched in zinc, - A first separation of the solid leaching residue, - A second leaching by the solution partially enriched in zinc with the formation of a solution enriched in zinc, or a solution supersaturated in zinc, and a solid material, - A second separation of the solid material generated during the second leaching and recovery of the solution supersaturated in zinc (i) or of the solution rich in zinc (ii), - Optionally, recycling of the solid material recovered after said second separation to the first leaching. Preferably, an addition of a calcium or magnesium compound is carried out during the first leaching. Preferably, the second leaching is carried out without adding a calcium compound. Preferably, said first leaching is carried out at a temperature greater than or equal to 60°C, preferably greater than or equal to 70°C. This makes it possible to avoid the precipitation of calcium zincate. According to an advantageous embodiment, the second leaching is carried out at a temperature below 70°C, preferably below 60°C, so as to avoid the precipitation of zinc oxide and to minimize the dissolution of impurities. Advantageously, the second leaching is carried out for a short period of time, i.e. less than 2 hours, preferably less than 1 hour, so as to avoid the precipitation of calcium zincate and to minimize the dissolution of impurities. Preferably, it is possible to add a step of purifying the solution by cementation of metals more noble than zinc on zinc dust added to the solution. According to an even more advantageous embodiment, the calcium and/or magnesium compound is chosen from the group comprising lime, calcined dolomite, magnesia and mixtures thereof. This group relates to the removal of impurities mentioned above, preferably after having carried out the leaching step. However, when the addition of calcium compound takes place for the purpose of forming the calcium zincate pulp, the calcium compound is selected from the group comprising lime, calcined dolomite and combinations thereof. The cementation step set forth in the context of the present invention can be inserted at any stage of the process. Preferably, the zinc-depleted solution generated in the process is purified of metallic impurities more noble than zinc by cementation on a metallic powder, preferably a zinc metal powder, preferably before the abovementioned possible recycling step. According to a preferred variant, the zinc-supersaturated solution (i) (iii) or the zinc-rich solution (ii) generated in the process is purified from metallic impurities more noble than zinc by cementation on a metal powder, preferably a zinc metal powder. Advantageously, at least one of the solids separated from the solutions containing zinc and alkali hydroxides is washed, preferably with water, so as to recover the zinc and the alkali hydroxides from the solution impregnated in said at least one solid. The washing solutions are preferably recycled in one or more of the steps of the process according to the invention. The water balance of the process is balanced by compensating for water inputs either by bleeding, or by water evaporation, or by reverse osmosis, or by a combination of these techniques. The volume of the bleeding is preferably adjusted to control the concentration in the solution of the most difficult to remove impurities, such as alkali chlorides, at an acceptable level. To finish balancing the balance, the excess water is preferably eliminated by multiple-effect evaporation or mechanical vapor compression so as to minimize the amount of energy consumed. According to an advantageous embodiment, the zinc-supersaturated solution (i) or (iii) or the zinc-rich solution (ii) generated in the process is purified so as to reduce its silica concentration to a content of less than 1 g/L, preferably less than 0.5 g/L, and even less than 0.3 g/L before the zinc oxide precipitation step. This purification can be carried out by contacting the solution with silica precipitation seeds or any other equivalent method. The zinc recovery process according to the present invention can be operated continuously, in particular by reusing as much as possible the products generated in the process by feeding several stages and allowing continuous recycling. Preferably, calcium zincate seeds are added to the precipitation of said calcium zincate pulp. Figure 1 is a schematic view of an embodiment of the process according to the present invention. Figure 2 is a schematic view of a variant of the process according to the present invention. Figure 3 is a schematic view of a preferred embodiment of the process according to the invention. Other features and advantages of the present invention will be drawn from the non-limiting description which follows, and with reference to the drawings and examples. The present invention thus aims to extract zinc from a material called raw or secondary material and to recover it in a concentrate with higher value. According to the invention, the material that contains zinc in oxidized form is preferably any raw or secondary material containing zinc that can be leached in a caustic soda solution. This raw material may include at least one so-called oxidized ore that contains zinc in the form of an oxidized mineral such as smithsonite, hemimorphite, hydrozincite, willemite or in the form of a zinc carbonate and/or a zinc silicate and/or calcium zincate. The secondary material may come from lead metallurgy (slag), dust from scrap metal recycling (EAFD), etc. In the context of the present invention, the zinc-containing material may be selected from the group comprising ores (raw materials), secondary materials and mixtures thereof. In the present invention, the leaching step takes place in an alkaline basic medium obtained by adding a solution which comprises an alkaline basic compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, alkali metal carbonate, lithium hydroxide and mixtures thereof. According to a preferred embodiment, the alkaline basic compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, alkali metal carbonate, lithium hydroxide and mixtures thereof, has a concentration of between 100 and 300 g/L, preferably between 100 and 250 g/L, more preferably between 100 and 200 g/L, advantageously between 130 and 200 g/L. The addition of an anti-solvent is not required in the context of the present invention. In other words, less than 80% by volume may be added to the zinc-supersaturated solution (i) or (iii) relative to the volume of said zinc-supersaturated solution (i) or (iii). Alternatively, an anti-solvent may be added without having sufficient effect in the precipitation of a zinc oxide as described in the context of the present invention. According to a preferred embodiment of the process according to the invention, the formation by precipitation of zinc oxide in solution is carried out in the absence of an added amount of an anti-solvent, for example ethanol or methanol. In this context, the expression "in the absence of an added amount of an anti-solvent" means that an amount possibly added will have little or no effect on the precipitation of zinc oxide as described in the context of the present invention. The step of leaching the material containing zinc is preferably carried out in a solution that contains caustic soda, preferably at a concentration of between 130 and 200 g/L. The caustic soda-based solution may also contain zinc, soluble in this medium in the form of sodium zincate (Na2ZnO2). The caustic soda-based solution may also contain impurities such as traces of silicon, aluminum, lead or soluble salts (carbonates, chlorides and others). Advantageously, the leaching temperature is between 0 and 300°C. If the leaching temperature exceeds the boiling point of the solution at atmospheric pressure, the leaching is carried out under saturated steam pressure in order to prevent boiling and to carry out the leaching in the liquid phase. The leaching temperature is chosen, on the one hand, so as to maximize the leaching yield of zinc, or even of other recoverable metals, and on the other hand, so as to minimize parasitic reactions such as the dissolution of impurities or the excessive consumption of caustic soda by the gangue. Thus and advantageously, when the material contains Ca, the leaching step is carried out at a temperature above 50°C, possibly for a period of time of less than 4 hours. Advantageously, the leaching step is carried out at a temperature below 90°C, preferably below 70°C, possibly for a period of time of less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours to prevent early precipitation of zinc oxide. More advantageously, if a supersaturated solution (i) is generated during the leaching step, the latter is carried out at a temperature below 90°C, preferably below 70°C, optionally for a period of time less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours to prevent early precipitation of zinc oxide. This embodiment is preferred and applicable to all variants of the process. According to a particularly advantageous alternative embodiment, the leaching step can be carried out at a temperature above 50°C, preferably above 90°C, more preferably above 150°C, optionally for a period of time less than 4 hours to prevent precipitation of calcium zincate. This advantageous alternative embodiment is also preferred when a zinc-rich solution (ii) is generated. This alternative embodiment is also advantageous when the material contains zinc and also calcium, or when a calcium-containing compound is added to the leaching step, a temperature above 50 °C is applied, preferably for a time period of less than 4 hours. This prevents early precipitation of calcium zincate at this stage of the process. The leaching can be carried out in one or more stages (e.g. in 2 stages, see figure 3). In the latter case, the leaching steps will preferably be carried out in countercurrent with intermediate solid-liquid separation steps. At the end of the leaching, the zinc-enriched solution is separated from the depleted solids by vacuum filtration, pressure filtration, centrifugation, decantation or any other suitable technique. The solid residue is preferably washed with water so as to recover the zinc and the alkaline basic compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, alkali metal carbonate, lithium hydroxide and mixtures thereof or caustic soda in the impregnated solution. The solid is then removed while the solution advances to the zinc precipitation step, or optionally, an intermediate purification step. Thus, the zinc-supersaturated solution (i) or the zinc-rich solution (ii) obtained is used for the following steps: Optionally: • adding a calcium compound to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) to precipitate (precipitate) solid calcium zincate in a zinc-depleted basic solution, • separating (B) at least a portion of the zinc-depleted basic solution from the calcium zincate pulp, • heating said calcium zincate pulp consisting of solid calcium zincate and the remainder of the zinc-depleted basic solution with formation of a zinc-supersaturated solution (iii) and a solid matter, • separating (C) the solid matter obtained after the heating step to preserve the zinc-supersaturated solution (iii) and optionally recycling this solid matter to the calcium compound addition step, - Heating said zinc-supersaturated solution (i) obtained after said separation (A) or of the supersaturated zinc solution (iii) optionally obtained after the separation step (C) of said solid material to precipitate (precipitant), preferably predominantly, a zinc oxide in a zinc-depleted solution, - Separation (D) of the zinc oxide formed during heating of the zinc-supersaturated solution (i) or (iii), - Optionally, recycling at least a portion of said zinc-depleted (basic) solution formed during at least one of the process steps by adding it to the basic leaching step or to said thickened zincate pulp, preferably after removal of at least a portion of the zinc-depleted solution and before heating and zinc supersaturation. In the process according to the invention, the amount of ore or secondary material introduced into the leaching is chosen so as to obtain the highest possible zinc concentration in the enriched solution, while maximizing the leaching yield. If the zinc contained in the material to be leached is present in the form of zinc oxide, the maximum zinc concentration of the enriched solution cannot exceed the solubility limit of zinc oxide because it is the most insoluble zinc compound in a solution comprising an alkaline basic compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and mixtures thereof, preferably at a concentration of between 130 and 200 g/L. Particularly advantageously, when the zinc contained in the material to be leached is not present in the form of zinc oxide, but for example in the form of a zinc carbonate and/or a zinc silicate and/or calcium zincate, a zinc concentration greater than the solubility limit of zinc oxide can be achieved in the leaching solution. To achieve the highest possible zinc supersaturation while still achieving high leaching efficiency, it is often practical to perform several leaching steps in counter- current (see diagram in Figure 3). The operating conditions of the different leaching stages may be different. For example, the temperature of the leaching of the fresh material may be colder than that of the leaching of the depleted material so as to prevent the re-precipitation of zinc oxide. Similarly, the duration of the leaching of the fresh material may be shorter in order to prevent the formation of insoluble calcium zincate. Leaching with caustic soda has the advantage of being fairly selective. Iron, calcium or magnesium do not pass into solution in a basic medium, unlike what happens in an acidic medium. The leaching solution, enriched with zinc, may however contain impurities such as silica, alumina, carbonate or lead. Silica, alumina and carbonate can be precipitated by adding lime up to concentrations of the order of a gram per liter, or even less, for silica and alumina, and up to concentrations of a few tens of grams per liter for carbonate depending on the concentration of basic alkaline compound, preferably chosen from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and mixtures thereof. Purification by adding lime is preferably carried out at a temperature above 50°C, and even above 60°C, so as to prevent precipitation of calcium zincate. Vigorous stirring of the zincate pulp and the use of finely divided lime promote high purification yields. Purification can be carried out in the presence of the zinc-depleted material, i.e. before separation of the leaching residue, or after. In the first case, the precipitate of silica and/or alumina and/or carbonate is mixed with the leaching residue and separated at the same time from the zinc-enriched solution. In the second case, it is separated from the enriched solution and recovered without being mixed with the residue. In addition to the removal of impurities that could contaminate the zinc-rich final product, the precipitation of silica, alumina and carbonate causes the regeneration of an alkaline basic compound preferably chosen from the group comprising sodium hydroxide, potassium hydroxide, lithium hydroxide and their mixtures in free form. It should be noted that these advantageous modes are all combinable with each other, whether for the first variant and/or the second variant according to the invention. Furthermore, this “countercurrent” mode can also be applied to the first and/or the second variant of the process according to the invention. Metals more noble than zinc that pass into solution with zinc during leaching, such as lead, copper or silver, can be eliminated from the solution, in whole or in part, by cementation, preferably on zinc powder. Some impurities, in particular sulfates, chlorides or alkali fluorides are more difficult to eliminate. They are generally tolerated in the leaching solution up to a certain level, of the order of a few tens of grams per liter. Their accumulation in the leaching circuit can be controlled by performing a bleeding. The zinc contained in the rich solution, purified or not, is then recovered by precipitation. The zinc is recovered from the supersaturated solution preferably in the form of oxide by breaking the supersaturation. To achieve this, the temperature of the solution is raised for several hours to more than 70°C, and preferably to more than 100°C. The zinc precipitates then in the form of oxide until reaching the solubility limit of zinc oxide in the solution comprising the basic alkaline compound, preferably selected from the group comprising a compound selected from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and mixtures thereof. Precipitation seeds are preferably added to accelerate precipitation. By precipitation seeds for the purposes of the present invention, we mean zinc oxide crystals, originating for example from the recycling at the top of a portion of the precipitation pulp. The precipitated zinc oxide is recovered by filtration or any other solid-liquid separation process. The solid is preferably washed to recover the basic alkaline compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and mixtures thereof impregnated in the solid. The zinc-depleted solution can advantageously be recycled to the leaching, so as to minimize the consumption of the process in the alkaline basic compound, preferably chosen from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and their mixtures. The separation steps included in the process according to the invention are liquid/solid separations and the person skilled in the art knows different suitable separation methods. The washing waters of the different solids can also be recycled at each step of the process. The water balance of the circuit is then balanced by means of one or more evaporation steps, and/or bleeding. The invention provides for the use of a calcium compound, preferably lime, which makes it possible to precipitate a calcium zincate pulp consisting of solid calcium zincate and a basic solution depleted in zinc. This is particularly preferred when the zinc concentration of the rich solution, purified or not, is below the solubility limit of zinc oxide. In this case, it is preferable to precipitate this calcium zincate to then form a solution supersaturated with zinc (iii) as described in the present invention. Calcium zincate has a relatively low and very temperature-dependent solubility in solutions containing less than 250 grams per liter of caustic soda. To precipitate the zinc from the rich sodium zincate solution, a calcium compound (e.g. lime), preferably finely divided, is therefore added to the solution at a temperature below 80°C, preferably below 50°C. The agitation of the pulp is advantageously carried out vigorously, with high shear, to achieve complete reaction of the lime, preferably by means of a peripheral speed of the stirring rotor of at least 5 m/s. The amount of the calcium compound (for example lime) added is preferably equal to the stoichiometric amount of the zinc to be precipitated, i.e. one mole of calcium for two moles of zinc to be precipitated. The zinc-depleted solution can advantageously be recycled to the leaching, so as to minimize the consumption of the basic alkaline compound, preferably chosen from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and their mixtures of the process. The calcium zincate can be washed with water to recover said basic alkaline compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, alkali metal carbonate, lithium hydroxide and mixtures thereof impregnated in the solid. The wash water can be recycled at any stage of the process. After drying, the calcium zincate has a zinc content of around 40%. It can be used by smelters to produce zinc metal without requiring prior roasting like sulphide concentrates or Waelz oxides. The calcium zincate can also advantageously be relixiviated in a solution comprising the basic alkaline compound, preferably selected from the group comprising sodium hydroxide, potassium hydroxide, alkali metal carbonate, lithium hydroxide and mixtures thereof, preferably containing between 130 and 200 grams per litre of hot caustic soda, preferably at a temperature above 70°C, so as to produce a solution supersaturated with zinc. During leaching, calcium oxide, one of the constituents of calcium zincate, is not dissolved. After leaching, it is separated from the enriched solution and can advantageously be recycled to the precipitation of calcium zincate or to the leaching to precipitate impurities such as silica. The zinc-supersaturated solution can then be brought to a higher temperature in the presence of precipitation seeds (zinc oxide crystals) so as to force the precipitation of zinc oxide. The zinc oxide produced from the leaching of calcium zincate is very pure, typically it contains more than 95%, preferably more than 97%, more preferably more than 98%, advantageously more than 99% zinc oxide. It can be used to produce zinc metal or as technical zinc oxide, in particular for the vulcanization of elastomers, the production of enamels, animal feed. According to a first variant of the process, the following steps are provided for: - Supply of a material which contains zinc in oxidized form, - Leaching of the material in an alkaline basic medium with formation of a solid residue and a solution supersaturated with zinc (i) - Separation (A') of the solid leaching residue from the solution supersaturated with zinc (i), - Heating of said solution supersaturated with zinc (i) obtained after said separation (A') to precipitate (precipitant), preferably predominantly, a zinc oxide in a solution depleted in zinc, - Separation (D') of the zinc oxide formed during heating of the solution supersaturated with zinc (i). According to a preferred embodiment of the first variant, the method may further include the following successive steps, after formation of the solid residue and the zinc-supersaturated solution (i) and optionally after the separation step (A'): - addition of a calcium compound to the zinc-supersaturated solution (i) to form a zincate pulp which consists of solid calcium zincate and a basic solution depleted in zinc, - separation (B') of at least a portion of the basic solution depleted in zinc from the calcium zincate pulp, - heating said calcium zincate pulp consisting of solid calcium zincate and the remainder of the basic solution depleted in zinc with formation of a zinc-supersaturated solution (iii) and a solid material, - separation (C') of the solid matter obtained after the heating step to preserve the zinc-supersaturated solution (iii) and optionally recycling this solid matter to the step of adding a calcium compound. Thus, the zinc-supersaturated solution (iii) obtained is then heated to precipitate (precipitant), preferably mainly a zinc oxide. A separation (D') of the zinc oxide formed during the heating of the zinc-supersaturated solution (i) and the zinc-depleted solution makes it possible to provide the zinc oxide according to the invention. In particular, the supersaturated solution (iii) formed within the framework of the preferred embodiment of the first variant has a higher zinc saturation compared to the initial supersaturated solution (i) and it is purer. Preferably, it is possible to recycle at least part of the zinc-depleted (basic) solution formed during at least one of the steps of the process by adding it to the basic leaching step. Advantageously, when implementing the first variant of the process according to the invention, it is necessary to produce a zinc-supersaturated zincate solution. To do this, the material containing the zinc is leached by an alkaline solution, optionally saturated with zinc. Under these conditions, it is observed that the zinc content can exceed the solubility limit of zinc oxide in the alkaline solution, without the latter precipitating. The solid residue, depleted in zinc, is separated by filtration. The zinc-rich solution can preferably be heated to a temperature above 70°C in the presence of crystallization seeds (zinc oxide particles). Under these conditions, the precipitation of zinc oxide is observed over time. After recovery of zinc oxide by filtration, the zinc-saturated alkaline solution can advantageously be recycled to leaching. According to a second variant of the invention, the following steps are provided: - Supply of a material which contains zinc in oxidized form, - Leaching of the material in an alkaline basic medium with formation of a solid residue and a zinc-rich solution (ii) - Separation (A') of the solid leaching residue from the zinc-rich solution (ii), - addition of a calcium compound to said zinc-rich solution (ii) to precipitate (precipitant) a zincate pulp which contains solid calcium zincate and a basic solution depleted in zinc, - separation (B') of at least a portion of the basic solution depleted in zinc from the calcium zincate pulp, - heating said calcium zincate pulp consisting of calcium zincate and the remainder of the basic solution depleted in zinc with formation of a solution supersaturated in zinc (iii) and a solid material, - separation (C') of the solid material obtained after the heating step to preserving the zinc-supersaturated solution (iii) and possibly recycling this solid material to the step of adding a calcium compound, - Heating said zinc-supersaturated solution (iii) obtained after the separation step (C') of said solid material to precipitate (precipitant), preferably predominantly, a zinc oxide in a zinc-depleted solution, - Separation (D') of the zinc oxide formed during heating of the zinc-supersaturated solution (iii). Preferably, recycling at least a portion of said zinc-depleted (basic) solution formed during at least one of the steps of the process according to the second variant by adding it to the basic leaching step or to said zincate pulp after removal of at least a portion of the zinc-depleted solution and before heating and supersaturation with zinc. This second variant according to the invention makes it possible to leach the zinc in an alkaline solution whose zinc content is below its solubility limit. In this eventuality, the zinc can be precipitated from the rich solution by adding finely divided lime, preferably at a temperature below 50°C. The lime reacts with the zincate solution to form an insoluble calcium zincate which can be recovered by filtration. This calcium zincate can be marketed as such, optionally after washing and drying, or relixivated in an alkaline solution, optionally saturated with zinc, preferably at a temperature above 50°C so as to produce the solution supersaturated with zinc (iii). The lime remains insoluble and can be recovered by filtration. Advantageously, it can be recycled to the calcium zincate precipitation step. The supersaturated solution can then be heated to a temperature above 70°C, optionally in the presence of precipitation seeds (zinc oxide crystals) to precipitate (precipitant), preferably predominantly, the zinc oxide. The alkaline solution depleted in zinc is advantageously recycled to the calcium zincate leaching. The recovered zinc oxide is very pure (95-100% purity) and constitutes a material of choice for the production of zinc metal, determined by a method known to the person skilled in the art. As can be seen, regardless of the embodiments chosen, the common step is to form a supersaturated solution which will be heated to provide the zinc oxide according to the invention. The embodiments set out above can also be combined with each other. Figure 1 illustrates an exemplary embodiment linked to the first variant of the method according to the invention. A material containing zinc (B) in oxidized form is supplied and leached (1) in an alkaline basic medium to form a solution supersaturated in zinc (i). An addition of lime (A) can be carried out at this leaching step (1) to remove impurities. A solid/liquid separation (2) of the solid residue (C) is then carried out and the supersaturated solution (i) collected (3) is heated (7), optionally in the presence of precipitation seeds (F) (zinc oxide crystals) to form zinc oxide in a basic solution depleted in zinc. Before heating the zinc-supersaturated solution (i), it is possible to carry out a purification step for metals that are more noble than zinc (for example, Ag, Pb, Cu, etc.). Thus, a cementation (4) on a metal powder (D), such as a zinc metal powder (D), is carried out. After a solid/liquid separation step (5), the purified zinc-supersaturated solution (6) is recovered to carry out the heating step (7) mentioned above. After heating (7) and adding precipitation seeds (F), a solid/liquid separation (8) takes place and allows the recovery of zinc oxide (G). The basic zinc-depleted solution (9) is optionally purified (10) by adding a calcium compound (H), such as lime (H), to extract the impurities, provided that the addition of lime to the leaching step (1) does not take place. Following the separation (11) of the solid impurities (I), the purified zinc-depleted solution (12) is recovered and optionally recycled in the first leaching step (1). Figure 2 is an exemplary embodiment related to the second variant of the process according to the present invention – presence of calcium zincate. A material (A) containing zinc is brought to carry out a leaching (1) in a basic alkaline medium (aqueous NaOH solution) which makes it possible to form a zinc-rich solution (ii) in the presence of a solid residue. A solid/liquid separation (2) is carried out to remove the solid residue (B) and thus preserve the zinc-rich solution (3). Then, a calcium compound (C) (lime) is added to the zinc-rich solution which leads to the formation of a pulp which comprises calcium zincate (4) in the form of a precipitate and a basic zinc-depleted solution. The addition of the calcium compound (C) is preferably carried out cold or at a temperature below 70 °C. A separation (5) of at least a portion of the basic solution depleted in zinc (D) takes place, and allows recycling thereof to the leaching step (1). The calcium zincate pulp (6) is heated to a temperature above 70°C (7) to form a solution supersaturated in zinc (iii) and a solid matter. A solid/liquid separation (8) of the solid matter (E) is carried out, which allows extraction of the calcium compound (insoluble lime) which can advantageously be recycled to the step of adding said compound as illustrated in FIG. 2. Heating (10) the solution supersaturated in zinc (9) after the separation step (8) allows precipitating a zinc oxide in a solution depleted in zinc. It is also advantageous to add precipitation seeds (F) (zinc oxide crystals) to this step. Then, a separation (11) of the zinc oxide (G) formed is collected and the basic solution depleted in zinc (H) is optionally recycled to the heating step (7). A part of this basic solution zinc-depleted material can advantageously be reintroduced in the first step of the process (1). Figure 3 illustrates a leaching process called "countercurrent" and whose leaching step is carried out in 2 steps. Thus, the zinc-containing material (A) is supplied to the first leaching step (1) in an alkaline basic medium (aqueous NaOH solution). This allows the formation of a zinc-depleted solid residue and a partially zinc-enriched solution. Then, a first separation (2) of the solid leaching residue (C) is carried out to preserve the partially zinc-enriched solution (3). This is followed by a second leaching by the partially zinc-enriched solution with formation of a zinc-enriched solution in the presence of a solid material. The zinc-enriched solution (E) is separated (5) and the solid material is re-supplied to the first leaching step (1). Advantageously, zinc-containing material (optionally a zinc-rich material) can be added to the second leaching (4). Finally, preferably, the basic solution (B) depleted in zinc generated during a zinc oxide precipitation step can also be recycled to the first leaching (1). The figures above describe the general steps of the variants of the process according to the invention. All the characteristics of duration and temperature cited previously also apply specifically to the steps described for these 3 figures. According to the invention, the material which contains zinc in oxidized form is preferably any raw or secondary material containing zinc which can be leached in a solution comprising an alkaline basic compound, preferably chosen from the group comprising sodium hydroxide, potassium hydroxide, carbonate of an alkali metal, lithium hydroxide and their mixtures. This raw material may include at least one so-called oxidized ore which contains zinc in the form of smithsonite, hemimorphite, hydrozincite, willemite. The secondary material may come from lead metallurgy (slag), dust from scrap metal recycling (EAFD), etc. The raw material used in the process according to the invention may advantageously contain a mineral chosen (without being limited thereto) from the group comprising zinc hydroxide (Zn(OH)2), zinc oxide (ZnO), Smithsonite (ZnCO3), hemimorphite (Zn4Si2O7(OH)2(H2O)), hydrozincite (Zn5(CO3)2(OH)6), willemite (Zn2SiO4) and ghanite (ZnAl2O4). For the purposes of the present invention, the term "for" may be used to indicate a limiting step. Thus, it is possible to replace if necessary the expression "for" by an expression which limits the scope according to the term used following the "for", for example by indicating "with formation of". Within the scope of the present invention, several chemical reactions can take place and are listed below. It is obvious that the person skilled in the art will be able to adapt the reactions according to the raw material used. Leaching of smithsonite ZnCO3 + 4 NaOH = Na2ZnO2 + Na2CO3 + 2 H2O Leaching of hemimorphite Zn4Si2O7(OH)2(H2O) + 12 NaOH = 4 Na2ZnO2 + 2 Na2SiO3 + 8 H2O Precipitation of calcium zincate 2 Na2ZnO2 + CaO + 7 H2O = CaZn2(OH)6.2H2O + 4 NaOH Redissolution of calcium zincate CaZn ₂ (OH) ₆ .2H ₂ O + 4 NaOH = 2 Na ₂ ZnO ₂ + Ca(OH) ₂ + 6 H ₂ O Precipitation of carbonate by lime Na ₂ CO ₃ + Ca(OH) ₂ = 2 NaOH + CaCO ₃ Precipitation of silica by lime Na ₂ SiO ₃ + Ca(OH) ₂ = 2 NaOH + CaSiO ₃ Precipitation of zinc oxide Na2ZnO2 + H2O = ZnO + 2 NaOH Examples 1 and 2 show attempts to leach an oxidized ore to produce a basic solution supersaturated with zinc. Example 3 refers to the precipitation of zinc oxide from a supersaturated basic solution. Examples 4 and 5 illustrate the results of tests on the leaching of a lead slag with a basic solution to produce a zinc-rich solution unsaturated with zinc (ii). Example 6 shows the precipitation of calcium zincate by adding lime to the leaching solution of Example 5. Example 7 relates to the production of a zinc-supersaturated solution by leaching calcium zincate. EXAMPLE 1 A sample of oxidized zinc ore containing 14.7% zinc, mainly as smithsonite, 25.0% iron, 5.6% calcium and 4.8% silicon was ground to a particle size of less than 80 µm. 300 g of this crushed ore were then leached in 2 liters of a solution containing 175 g/L of NaOH, 65 g/L of Na2CO3 and 10 g/L of Zn added in the form of ZnO. The pulp was brought to 70°C and 30 g of lime (CaO) finely ground were introduced into the pulp at the beginning of leaching. The pulp was stirred continuously for 2 hours before being filtered on a Buchner funnel. The filtrate was separated and the zinc and silicon concentrations were determined by spectrometry. The filter cake was washed with 250 mL of water, dried and its zinc content was determined by spectrometry after digestion in acid. The results of the test are shown in Table 1 below.

Table 1: Conditions and results of leaching test no. 1 Mass of crushed ore (g) 300 Addition of CaO (g) 30 Volume of etching solution (mL) 2000 Composition of etching solution NaOH (g/L) 175 Na2CO3 (g/L) 65 Zn (g/L) 10 Temperature (°C) 70 Leaching time (min) 120 Mass of dry residue (g) 285 Zn content of residue (%) 3.6 Volume of filtrate (mL) 1780 Zn content of filtrate (g/L) 28.5 Si content of filtrate (g/L) 0.85 Zinc leaching yield (%) 77 The leaching filtrate contained 28.5 g/L of zinc and 0.85 g/L of silicon while that the depleted residue contained only 3.6% zinc. The zinc leaching yield reached 77%. The filtrate is slightly supersaturated with zinc. EXAMPLE 2 150 g of the same fresh oxidized ore as in Example 1 and ground to a particle size of less than 80 µm were leached in 1 liter of the filtrate from the leaching test of Example 1. The pulp was heated to 50°C and stirred for 30 minutes before being filtered on a Buchner funnel. The filtrate was separated and the zinc and silicon concentrations were determined by spectrometry. The filter cake was washed with 125 mL of water, dried and its zinc content was determined by spectrometry after digestion in acid. The results of the test are shown in Table 2 below. The leach filtrate contained 41.3 g/L zinc and 1.51 g/L silicon while the depleted residue contained 7.8% zinc. The zinc leaching yield reached 55%. This yield is lower than that of Example 1 but the filtrate is here highly supersaturated in zinc (i.e., zinc-supersaturated solution). In a second step and in order to illustrate a countercurrent leaching scheme, 121 g of the leach residue were relixivated in 840 milliliters of a zinc oxide precipitation filtrate from a supersaturated solution. 15 g of lime were added to the pulp, then it was heated to 80°C and stirred for 2 hours before being filtered on a Buchner funnel. The filtrate was separated and the zinc and silicon concentrations were determined by spectrometry. The filter cake was washed with 120 mL of water, dried and its zinc content was determined by spectrometry after digestion in acid. The results of the test are also shown in Table 2 below. Table 2: Conditions and results of the leaching tests of example 2 Leaching step 1 2 Mass of crushed ore (g) 150 Mass of residue (g) 121 Addition of CaO (g) 0 15 Volume of attack solution (mL) 1000 840 Composition of the attack solution N _{aOH (g/L)} ^{171 154} N _{a2CO3 (g/L)} ^{69 90} Z _{n (g/L)} ^{28.5 20.8} S _{i (g/L)} ^{0.85 na} Temperature (°C) ^{50 80} _{Leaching duration (min)} ^{30 120} Mass of dry residue (g) ^{126 140} _{Zn content of the residue (%)} ^{7.8 3.7} Volume of filtrate (mL) ^{920 760} _{Zn content of} the residue (%) _{Filtrate (g/L)} ^{41.3 26.7} Si content of filtrate (g/L) ^{1.51 0.75} Leaching efficiency (%) ^{55 46} The filtrate from the second leaching stage contained 26.7 g/L zinc and 0.75 g/L silicon, while the residue contained only 3.7% zinc. The zinc leaching yield reached 46%. A carbon analysis of the filtrate showed that during leaching, the carbonate content of the solution decreased from 37 g/L CO2 to 29 g/L due to the addition of lime. The overall zinc yield from the ore leaching after the two stages was 76%. The two-stage countercurrent leaching results in a zinc-rich solution (41.3 g/L). EXAMPLE 3 90 g of pure zinc oxide were introduced as precipitation seeds into 900 mL of a highly zinc-supersaturated leach filtrate from the leaching of an oxidized ore containing 22.2% zinc. The pulp was brought to boiling (>90°C) in a flask and kept stirring. The vapors emitted were condensed in a water-cooled column and returned to the flask so as to minimize liquid losses. After 8 hours of boiling, the pulp was filtered on a Buchner funnel. The filtrate was separated and the zinc concentration was determined by spectrometry. The filter cake was washed with 100 mL of water and dried. The results of the test are given in Table No. 3 below.

Table 3: Conditions and results of the zinc oxide precipitation test Initial volume of solution (mL) ⁹⁰⁰ Initial Zn content (g/L) ^40.4 Initial Si content (g/L) ^0.08 Addition of seeds (g of ZnO) ^90.0 Temperature (°C) ¹⁰⁵ Duration (h) ⁸ Mass of dry solid recovered 111.6 (g) Volume of filtrate (mL) ⁸⁶⁰ Zn content of the filtrate (g/L) ^20.8 The filtrate contained only 20.8 g/L of zinc. 111.6 g of dry solid was recovered. An X-ray analysis showed that the solid consisted exclusively of zinc oxide. EXAMPLES 4 and 5 A sample of lead furnace slag containing 9.4% zinc, 17.4% iron and 13.6% silicon was ground to a particle size of less than 80 µm. 200 g of ground slag were then leached in 1 liter of a solution containing 175 g/L NaOH, 65 g/L Na2CO3 and 10 g/L Zn added as ZnO (Example 4). The pulp was heated to 90°C and stirred continuously for 4 hours before being filtered on a Buchner funnel. The filtrate was separated and the zinc and silicon concentrations were determined by spectrometry. The filter cake was washed with 250 mL of water, dried and its zinc content was determined by spectrometry after alkaline fusion and digestion in acid. The test was repeated but this time by bringing the pulp to a temperature of 200°C (example 5) in a pressure reactor. Before filtration, the pulp was cooled to about 90°C using a coil water-cooled. The results of the two tests are shown in Table 5 below. Table No. 4: Conditions and results of the leaching tests of examples 4 and 5 Example Example 4 5 Mass of crushed slag (g) 200 200 Volume of etching solution (mL) 1000 1000 Composition of etching solution NaOH (g/L) 175 175 Na2CO3 (g/L) 65 65 Zn (g/L) 10 10 Temperature (°C) 90 200 Leaching time (min) 240 240 Mass of dry residue (g) 216 204 Zn content of residue (%) 4.1 3.0 Volume of filtrate (mL) 850 870 Zn content of filtrate (g/L) 20.7 23.5 Si content of filtrate (g/L) 0.69 0.76 Leaching efficiency (%) 53 68 The zinc yield reached 68% at 200°C while it was only 53% at 90°C. The filtrates contained 20.7 g/L and 23.5 g/L of zinc, respectively. They were not saturated with zinc. EXAMPLE 6 4.6 g of finely ground lime (CaO) was added to 800 mL of the filtrate from the leaching test of Example 5. The pulp was then stirred at room temperature for 6 hours, with a high shear stirrer, before being filtered on a Buchner funnel. The filtrate was separated and the zinc concentration was determined by spectrometry. The filter cake was washed with 50 mL of water, dried and its zinc content was determined by spectrometry after digestion in acid. The results of the test are shown in Table 5 below. Table 5: Conditions and results of the calcium zincate precipitation test Initial volume of solution (mL) 800 Initial Zn content (g/L) 23.5 Addition of CaO (g) 4.6 Temperature (°C) ambient Duration (h) 8 Mass of dry solid recovered (g) 30.3 Volume of filtrate (mL) 780 Zn content of filtrate (g/L) 12.4 Zn content of solid (%) 39.9 30.3 g of dry solid containing 39.9% zinc were recovered. X-ray analysis showed that the solid consisted of hydrated calcium zincate. The filtrate contained only 12.4 g/L of zinc, which is significantly lower than the solubility of zinc oxide in the absence of lime. EXAMPLE 7 25 g of hydrated calcium zincate from the test of Example 6 were suspended in 500 mL of a solution containing 175 g/L of NaOH and 65 g/L of Na ₂ CO ₃ . The pulp was heated to 90°C and stirred for 2 hours before being filtered on a Buchner funnel. The filtrate was separated and the zinc concentration was determined by spectrometry. The filter cake was washed with 20 mL of water, dried and its zinc content was determined by spectrometry after digestion in acid. 5.7 g of dry solid were recovered. The solid consisted almost exclusively of lime and contained only 2% zinc, showing a zinc leaching efficiency greater than 98%. The zinc content of the filtrate was 36.8 g/L, which was significantly higher than the saturation concentration of zinc in this solution. In the context of the present invention, any singular article such as, for example, "a", "an", "the", "of", "of" can be replaced by an article which designates a plural such as for example "at least 2", "at least 3", "several", "the" etc. The word "comprise", "contains" or any equivalent term or derivatives can be replaced by "consisting of" in order to define a list or possibilities of exclusive selection so as not to include other elements not cited in the expression used. It is understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the appended claims.

Claims

CLAIMS 1. A process for recovering zinc which comprises the following steps: - Supplying a material which contains zinc in oxidized form, - Leaching the material in an alkaline basic medium with formation of a solid residue and a zinc-supersaturated solution (i) or a zinc-rich solution (ii), - Separating (A') the solid leaching residue from the zinc-supersaturated solution (i) or the zinc-rich solution (ii), - Optionally: • adding a calcium compound to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) to precipitate (precipitant) a zincate pulp which contains solid calcium zincate and a basic solution depleted in zinc, • separating (B') at least a portion of the basic solution depleted in zinc from the calcium zincate pulp, • heating said calcium zincate pulp consisting of solid calcium zincate and the remainder of the basic solution depleted in zinc with formation of a solution supersaturated in zinc (iii) and of a solid matter, • separation (C') of the solid matter obtained after the heating step to preserve the solution supersaturated in zinc (iii) and optionally recycling of this solid matter to the step of addition of a calcium compound, - Heating of said solution supersaturated in zinc (i) obtained after said separation (A') or of the solution supersaturated in zinc (iii) optionally obtained after the separation step (C') of said solid matter to precipitate (precipitant), preferably predominantly, a zinc oxide in a zinc-depleted solution, - Separation (D') of the zinc oxide formed during heating of the zinc-supersaturated solution (i) or (iii), - Optionally, recycling of at least part of said zinc-depleted (basic) solution formed during at least one of the steps of the process by adding it to the basic leaching step or to said zincate pulp before heating and zinc supersaturation.

2. A method according to claim 1, wherein the addition of the calcium compound to said zinc-rich solution (ii) or to said zinc-supersaturated solution (i) to precipitate calcium zincate is carried out by applying a temperature below 70°C or between 0 and 70°C, preferably below 50°C, more preferably below 30°C.

3. A method according to any one of the preceding claims, wherein the step of heating said zinc-supersaturated solution (i) obtained after said separation (A') or of the zinc-supersaturated solution (iii) optionally obtained after the separation step (C') of said solid material to precipitate a zinc oxide in a zinc-depleted solution is carried out in the presence of zinc oxide, serving as precipitation seeds.

4. A method according to claim 3, wherein the step of heating said zinc-supersaturated solution (i) obtained after said separation (A') or the zinc-supersaturated solution (iii) optionally obtained after the separation step (C') of said solid material to precipitate a zinc oxide in a zinc-depleted solution is carried out at a temperature above 70°C, preferably above 90°C, optionally for a period of time of at least one hour.

5. Method according to any one of the preceding claims, in which, during the leaching step, an addition of a calcium and/or magnesium compound in a stoichiometric quantity is carried out to precipitate impurities chosen from the group comprising silica, alumina or carbonate.

6. A method according to any preceding claim, wherein the leaching step is carried out at a temperature below 90°C, preferably below 70°C, optionally for a time period of less than 2 hours.

7. A method according to any one of claims 1 to 5 or according to claim 6, in which the leaching step is carried out at a temperature above 50°C, optionally for a time period of less than 4 hours.

8. A method according to any preceding claim, wherein a calcium and/or magnesium compound is added to the zinc-supersaturated solution (i) obtained after said separation of said solid residue, preferably before formation of zinc oxide.

9. A method according to any one of the preceding claims, in which the leaching step is carried out in several steps, preferably in two steps, comprising the following steps: - A first leaching of the material in an alkaline basic medium with the production of a solid residue depleted in zinc and a solution partially enriched in zinc, - A first separation of the solid leaching residue, - A second leaching by the solution partially enriched in zinc with the formation of a solution enriched in zinc, or a solution supersaturated in zinc, and a solid material, - A second separation of the solid material generated during the second leaching and recovery of the solution supersaturated in zinc (i) or of the solution rich in zinc (ii), - Possibly, recycling of the solid material recovered after said second separation to the first leaching.

10. Method according to claim 9, in which an addition of a calcium compound is carried out during the first leaching.

11. Method according to claim 9 or 10, wherein said first leaching is carried out at a temperature greater than or equal to 60°C, preferably greater than or equal to 70°C.

12. A method according to any one of claims 9 to 11, wherein the second leaching is carried out at a temperature below 70°C, preferably below 60°C, or for a duration of less than two hours, preferably less than one hour.

13. Method according to any one of the preceding claims, in which the calcium and/or magnesium compound is chosen from the group comprising lime, calcined dolomite and magnesia.

14. A method according to any preceding claim, wherein the zinc-depleted solution generated in the method is purified of metallic impurities more noble than zinc by cementation on a metal powder, preferably a zinc metal powder, preferably before the abovementioned possible recycling step.

15. A method according to any preceding claim, wherein the zinc-supersaturated solution (i) (iii) or the zinc-rich solution (ii) generated in the method is purified from metallic impurities more noble than zinc by cementation on a metal powder, preferably a zinc metal powder.

16. A method according to any preceding claim, wherein calcium zincate seeds are added to the precipitation of said calcium zincate pulp.