Method for recovering zinc

Abstract

The invention relates to a zinc leaching process comprising the steps of:-providing a material containing zinc in oxidized form,-leaching the material in an alkaline medium, forming a solid residue and a supersaturated zinc solution (i) or a zinc-rich solution (ii):-separating (A ') the solid leach residue from the supersaturated zinc solution (i) or from the zinc-rich solution (ii),-separating (A') the solid leach residue from the supersaturated zinc solution (i) or from the zinc-rich solution (ii), -heating the supersaturated zinc solution (i), precipitating zinc oxide,-separating (D ') zinc oxide.

Description

[0001] The present invention relates to a process for the recovery of zinc from a zinc-containing material.

[0002] The Waelz process extracts zinc from ores or secondary materials and recovers zinc in the form of zinc oxide enriched compared to the original ore or secondary material. The enriched zinc oxide is sold to zinc smelters to produce metal by conventional methods.

[0003] However, the Waelz process is a hot process that consumes a lot of energy and emits a lot of carbon dioxide.

[0004] There is also the acid leaching of oxidized ores, including purification and electrolysis to produce zinc directly in the form of metal.

[0005] One drawback is the use of large amounts of acid in this process. In addition, certain impurities (Mg, Mn, F, etc.) that are harmful to electrolysis and difficult to remove can be present in the ore and contaminate the leachate. Finally, the investment in capital and human resources of an electrolysis plant is very large and only makes sense in the case of very large deposits.

[0006] Document WO 2020 / 019834 proposes the leaching of zinc-poor rocks with ammonia and ammonium carbonate. After filtering the residue, a first direct addition of lime is carried out to obtain a first precipitate which is filtered. Then, a second addition of lime causes a second precipitate to come out. The second precipitate is calcined to produce a mixture containing zinc oxide, oxides and / or calcium carbonate. One drawback is the use of ammonia, which involves environmental and safety issues. In addition, the zinc concentration in the final product remains low.

[0007] Processes including caustic soda (sodium hydroxide) leaching, purification and electrolysis have been proposed. The advantage of sodium hydroxide leaching is a higher selectivity than acid leaching. In addition to the investment required for an electrolysis plant, another drawback is that during the electrolysis of the sodium zincate solution, zinc metal is recovered in the form of dust, while in an acid medium, zinc metal is recovered in the form of a solid cathode. The metal dust must be melted to be cast into ingots. However, this melting process leads to very expensive metal losses.

[0008] In 1965, Merrill and Lang studied several methods for recovering zinc from caustic soda oxidized zinc ore leachates. In addition to electrolysis, zinc can be precipitated by carbonation, sulfidation or dilution and recovered as a marketable concentrate.

[0009] However, all these methods have drawbacks. Carbonation aims at reducing the solubility of zinc by bubbling carbon dioxide through caustic soda to convert it into soda ash. Zinc then precipitates as an oxide. The drawback is the high consumption of caustic soda which is prohibitive. Caustic soda can be regenerated by reaction of soda ash with lime. However, the lime consumption is very high, about three or four tons of quicklime per ton of zinc. Precipitation by sulphidation involves the precipitation of zinc sulphide by reaction of a sodium zincate solution with elemental sulphur. This method also leads to the consumption of large amounts of sodium hydroxide which reduces the profitability. Finally, zinc can be precipitated as an oxide by dilution. This method requires the addition of large amounts of water. In order to recover the caustic soda, all this water has to be evaporated which is very energy consuming.

[0010] Document WO 2013 / 036268 proposes a method for treating a scrap containing for example 65% of zinc by leaching with a NaOH solution. Most of the scrap dissolves in the solution. The resulting zinc-rich liquid is then separated from the scrap by filtration. The method comprises the addition of zinc metal in the form of dust to the liquid from which lead, copper, tin and cadmium are separated. Zinc oxide is then precipitated after addition of methanol which acts as an anti-solvent. The drawback of this method is the use of an anti-solvent which has to be distilled from the zinc- depleted solution in order to recover the NaOH.

[0011] In order to recover zinc from materials which cannot be sufficiently enriched by simple physical methods to be sold to smelters, there is indeed a need to provide a zinc recovery method which is energy efficient, requires little reagents, emits little carbon dioxide and does not require large investments from the user.

[0012] To solve this problem, the present invention provides a zinc recovery method comprising the following steps:

[0013] - providing a material containing zinc in oxidized form,

[0014] - leaching the material in a basic medium, forming a solid residue and a zinc supersaturated solution (i) or a zinc-rich solution (ii),

[0015] - separating (A') the solid leaching residue from the zinc supersaturated solution (i) or the zinc-rich solution (ii),

[0016] - optionally:

[0017] • adding a calcium compound to the zinc-rich solution (ii) or to the zinc supersaturated solution (i), the calcium compound being preferably selected from lime, calcined dolomite and combinations thereof, forming a slurry containing solid calcium zincate and a zinc-depleted basic solution,

[0018] • separating (B') at least a part of the zinc-depleted basic solution from the calcium zincate slurry,

[0019] • heating said calcium zincate slurry, consisting of solid calcium zincate and the remaining zinc- depleted alkaline solution, forming a zinc supersaturated solution (iii) and a solid material,

[0020] • separating (C') the solid material obtained after the heating step, saving (retaining) the zinc supersaturated solution (iii), and optionally recycling the solid material to the calcium compound addition step,

[0021] - heating said zinc supersaturated solution (i) obtained after said separation (A') or heating the zinc supersaturated solution (iii) obtained after the separation step (C') of the solid material, if any, precipitating (depositing) zinc oxide, preferably as the main solid in the zinc-depleted solution,

[0022] - separating (D') the zinc oxide formed during the heating of the zinc supersaturated solution (i) or (iii),

[0023] - optionally, recycling at least a portion of said zinc-depleted (alkaline) solution formed in at least one of the process steps, by adding it to the leaching step of the alkaline medium or to said zincate slurry, prior to heating and zinc supersaturation.

[0024] From a zinc-containing material, the process can easily and efficiently form a zinc supersaturated solution (i; iii) from which zinc oxide is produced upon heating, which can be sold to zinc smelters to produce metallic zinc or as specialized zinc oxide, in particular for the vulcanization of elastomers, the production of enamels and the zinc oxide for animal feed.

[0025] The heating step causes the precipitation of zinc oxide in the solution, which is then depleted in zinc. This spontaneous precipitation of zinc oxide is a key aspect of the present invention.

[0026] Preferably, the zinc oxide formed after the heating step is the main product present in said zinc-depleted solution.

[0027] Indeed, the heating step, which causes the precipitation of zinc oxide, results in the precipitation of solids, the majority of which is formed by this zinc oxide, preferably at least 80% by weight of zinc oxide, preferably at least 90% by weight of zinc oxide, more preferably at least 95% by weight of zinc oxide, relative to the total weight of the precipitate formed.

[0028] Thus, compared to known processes, the process according to the present invention is more economical, less energy-consuming, requires smaller amounts of reagents and produces less CO2 (more than 70% reduction) while ensuring a high zinc yield.

[0029] It is also preferred to produce a zinc-rich solution (ii) to which a calcium compound is added, forming a calcium zincate slurry. After removing a portion of the depleted solution, heating the calcium zincate slurry produces a zinc supersaturated solution (iii) from which zinc oxide will be produced.

[0030] Advantageously, a zinc oversaturated solution (i) can also be used and a calcium compound is added to form a calcium zincate slurry. This provides a zinc oversaturated solution (iii) when the thickened calcium zincate slurry is heated according to the above described steps after removal of a part of the depleted solution.

[0031] It should be noted that when the zinc oversaturated solution (iii) is obtained by the above described method, this zinc oversaturated solution (iii) has a higher zinc concentration than the initially provided oversaturated solution (i) or zinc rich solution (ii). The effect of adding this step is to obtain a more concentrated zinc solution or to obtain a purer solution.

[0032] By separating a part of the depleted solution prior to the heating and zinc oversaturation step, the zinc depleted (alkaline) solution produced in the process, preferably the zinc depleted (alkaline) solution produced by precipitation of zinc oxide, can be recycled to the leaching step or to the concentrated calcium zincate slurry.

[0033] Preferably, the precipitation of calcium zincate by adding a calcium compound to said zinc rich solution (ii) or to said zinc oversaturated solution (i) is performed by applying a temperature of 70°C or below or by applying a temperature of 0°C to 70°C, preferably 50°C or below, more preferably 30°C or below.

[0034] Advantageously, the precipitation of calcium zincate after adding a calcium compound to said zinc rich solution (ii) or to said zinc oversaturated solution (i) is performed within a time of 1 to 4 hours.

[0035] Preferably, seeds comprising hydrated calcium zincate can be added to said zinc rich solution (ii) or to said zinc oversaturated solution (i) followed by the addition of a calcium compound, preferably selected from the group consisting of lime, calcined dolomite and combinations thereof, resulting in the formation of a slurry comprising solid calcium zincate and a zinc depleted alkaline solution. Thus, the addition of seeds can be performed prior to the possible addition of the calcium compound.

[0036] More preferably, the step of precipitating zinc oxide in a zinc depleted solution upon heating said zinc oversaturated solution (i) obtained after said separation (A') or upon heating said zinc oversaturated solution (iii) obtained after said separation step (C') of the optional solid material is performed in the presence of zinc oxide used as a precipitation seed.

[0037] According to a particularly preferred embodiment, the step of precipitating zinc oxide, preferably as the main solid, in a zinc depleted solution upon heating said zinc oversaturated solution (i) obtained after said separation (A') or upon heating said zinc oversaturated solution (iii) obtained after said separation step (C') of the optional solid material is performed at a temperature of 70°C or above, preferably 90°C or above, optionally for a time of at least one hour, which can advantageously be up to 8 hours.

[0038] According to an advantageous embodiment, a calcium compound and / or a magnesium compound is added in stoichiometric quantity in the leaching step to precipitate the impurities chosen from silica, alumina or carbonates. This allows the formation of calcium silicates, calcium aluminates and calcium carbonates.

[0039] It can also be preferred to add the calcium compound and / or the magnesium compound to the zinc oversaturated solution (i) or to the zinc-rich solution (ii) obtained after separation of the solid residue. In this way, the impurities, such as silica, alumina or carbonates, are separated from the zinc-containing solution and recovered in the form of separate solids. The calcium compound and / or the magnesium compound is preferably chosen from quicklime, slaked lime, dolomite, calcined dolomite, magnesium oxide, limestone and mixtures thereof.

[0040] The presence of silica in the oversaturated solution can inhibit the precipitation of zinc oxide upon heating. Optionally, if the zinc oversaturated solution (i) or the zinc oversaturated solution (iii) contains silica, a specific purification step can be performed prior to the precipitation of zinc oxide, for example by contacting the oversaturated solution with silica seeds to promote the precipitation of the dissolved silica. Once the silica concentration is reduced, the zinc oversaturated solution can be sent to the zinc oxide precipitation step.

[0041] Advantageously, the leaching step is performed at a temperature below 90°C, preferably below 70°C, for an optional time less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours, preventing the premature precipitation of zinc oxide.

[0042] More advantageously, if the oversaturated solution (i) is produced in the leaching step, this step is performed at a temperature below 90°C, preferably below 70°C, for an optional time less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours, preventing the premature precipitation of zinc oxide.

[0043] This embodiment is preferred and can be applied to all variants of the process.

[0044] According to a particularly advantageous alternative embodiment, the leaching step can be performed at a temperature above 50°C, preferably above 90°C, more preferably above 150°C, for an optional time less than 4 hours, preventing the precipitation of calcium zincate. This advantageous alternative embodiment is also preferred when the zinc-rich solution (ii) is produced.

[0045] This alternative embodiment is also advantageous when the material contains zinc as well as calcium, or when a calcium-containing compound is added to the leaching step. A temperature above 50°C is applied, preferably for a time less than 4 hours, preventing the premature precipitation of calcium zincate at this step of the process.

[0046] Preferably, the step of heating the calcium zincate slurry consisting of calcium zincate and the remaining zinc-lean alkaline solution is carried out at a temperature above 50°C, preferably at a temperature between 80°C and 95°C. This heating step is preferably carried out for a time between 0.1 and 4 hours, preferably between 0.5 and 2 hours, more preferably between 0.5 and 1 hour.

[0047] According to another embodiment, the leaching can be carried out in several steps, preferably in two countercurrent steps, as described below:

[0048] - a first leaching of the material in an alkaline medium, yielding a solid zinc-lean residue and a partially zinc-rich solution,

[0049] - a first separation of the solid leaching residue,

[0050] - a second leaching with the partially zinc-rich solution, yielding a zinc-rich solution or a zinc supersaturated solution and a solid material,

[0051] - a second separation of the solid material produced during the second leaching and the recovery of the zinc supersaturated solution (i) or the zinc-rich solution (ii),

[0052] - optionally, recycling the solid material recovered after the second separation to the first leaching process.

[0053] Preferably, a calcium compound or a magnesium compound is added during the first leaching process.

[0054] Preferably, the second leaching process is carried out without addition of a calcium compound.

[0055] Preferably, the first leaching is carried out at a temperature higher than or equal to 60°C, preferably higher than or equal to 70°C. This prevents the precipitation of calcium zincate.

[0056] According to an advantageous embodiment, the second leaching is carried out at a temperature below 70°C, preferably below 60°C, preventing the precipitation of zinc oxide and minimizing the dissolution of impurities.

[0057] Advantageously, the second leaching is carried out in a short time, i.e. less than 2 hours, preferably less than 1 hour, preventing the precipitation of calcium zincate and minimizing the dissolution of impurities.

[0058] Preferably, the solution purification step can be increased by cementation of a metal less noble than zinc onto zinc dust added to the solution.

[0059] According to an even more advantageous embodiment, the calcium compound and / or the magnesium compound is chosen from lime, calcined dolomite, magnesium oxide and mixtures thereof. These substances are involved in the removal of the impurities mentioned above, preferably after the leaching step is completed.

[0060] However, when a calcium compound is added in order to form a calcium zincate slurry, the calcium compound is chosen from lime, calcined dolomite and combinations thereof.

[0061] The step of cementation described in the present application can be included at any stage of the process.

[0062] Preferably, prior to the possible recycling step described above, the less noble metal impurities than zinc in the zinc-depleted solution produced in the process are removed by cementation on the metal dust, preferably zinc metal dust.

[0063] According to a preferred variant, the less noble metal impurities than zinc in the zinc supersaturated solution (i), the zinc supersaturated solution (iii) or the zinc-rich solution (ii) produced in the process are removed by cementation on the metal powder, preferably zinc metal powder.

[0064] Advantageously, the at least one solid separated from the solution containing zinc and alkali hydroxide is washed, preferably with water, in order to recover zinc and alkali hydroxide from the solution impregnated in said at least one solid. The washing solution is preferably recycled in one or more steps of the process according to the application. The water balance of the process is balanced by make-up water by purging, water evaporation, reverse osmosis or a combination of these techniques. The purging volume is preferably adjusted to maintain the concentration of the most difficult to remove impurities in the solution, such as alkali chlorides, at an acceptable level. To complete the balance, the excess water is preferably removed by multiple-effect evaporation or mechanical vapor compression, minimizing the energy consumed.

[0065] Advantageously, prior to the zinc oxide precipitation step, the zinc supersaturated solution (i) or the zinc supersaturated solution (iii) or the zinc-rich solution (ii) produced in the process is purified, reducing its silica concentration to less than 1 g / L, preferably less than 0.5 g / L, even less than 0.3 g / L.

[0066] This purification can be carried out by contacting the solution with silica precipitation seeds or any other equivalent method.

[0067] The zinc recovery process according to the application can be operated continuously, in particular by feeding several steps and allowing continuous recycling, reusing as much as possible the products produced in the process.

[0068] Preferably, calcium zincate seeds are added to the precipitation of said calcium zincate slurry.

[0069] Figure 1 Schematic representation of an embodiment of the process according to the application.

[0070] Figure 2 Schematic representation of a variant of the process according to the application.

[0071] Figure 3 This is a schematic diagram of a preferred embodiment of the method according to the present invention.

[0072] Other features and advantages of the invention will become apparent from the following non-limiting description, with reference to the accompanying drawings and embodiments.

[0073] Therefore, the present invention aims to extract zinc from known primary (ore) or secondary materials and recover zinc in a higher-value concentrate.

[0074] According to the present invention, the material containing zinc in its oxidized form is preferably any raw material or secondary material containing zinc that can be leached in a caustic soda solution. The raw material may include at least one so-called oxidized ore containing zinc in its oxidized mineral form (such as smithsonite, hemimorphite, zeolite, or zinc monazite), or zinc in the form of zinc carbonate and / or zinc silicate and / or calcium zincate. The secondary material may be derived from lead metallurgy (slag), dust generated from scrap metal recycling (EAFD), etc.

[0075] Within the scope of this invention, zinc-containing materials may be selected from ores (raw materials), secondary materials, and mixtures thereof.

[0076] 0 In this invention, the leaching step involves adding an alkaline medium obtained by adding a solution containing an alkaline compound.

[0077] The alkaline compound is preferably selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide, and mixtures thereof.

[0078] According to a preferred embodiment, the concentration of the alkaline compound selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof is preferably from 100 g / L to 300 g / L, more preferably from 100 g / L to 250 g / L, more preferably from 100 g / L to 200 g / L, and advantageously from 130 g / L to 200 g / L.

[0079] Within the scope of this invention, the addition of an anti-solvent is not required. In other words, less than 80% by volume may be added to the zinc supersaturated solution (i) or zinc supersaturated solution (iii) relative to the volume of the zinc supersaturated solution (i) or zinc supersaturated solution (iii). Alternatively, an anti-solvent may be added, but this has insufficient effect on the precipitation of zinc oxide as described within the scope of this invention.

[0080] 0 According to a preferred embodiment of the method of the present invention, the formation of zinc oxide precipitate in the solution is without

[0081] The process is carried out with the addition of an antisolvent (e.g., ethanol or methanol). In this document, the phrase "without the addition of an antisolvent" means that any amount added has little or no effect on the precipitation of zinc oxide as described in this invention.

[0082] The step of leaching the zinc-containing material is preferably carried out in a solution containing caustic soda, the concentration of caustic soda being preferably between 130 g / L and 200 g / L.

[0083] The caustic soda-based solution can also contain zinc, which is dissolved in the medium in the form of sodium zincate (Na2Zn02).

[0084] The caustic soda-based solution can also contain impurities, such as traces of silicon, aluminium, lead or soluble salts (carbonates, chlorides, etc.).

[0085] Advantageously, the leaching temperature is between 0°C and 300°C. If the leaching temperature exceeds the boiling point of the solution at atmospheric pressure, the leaching is carried out under the saturation vapour pressure, which prevents boiling, and the leaching is carried out in the liquid phase. The leaching temperature is chosen in order to maximise the leaching rate of zinc and even other valuable metals, on the one hand, and to minimise side reactions, such as the dissolution of impurities or the excessive consumption of caustic soda by gangue, on the other hand.

[0086] Thus, advantageously, when the material contains Ca, the leaching step is carried out at a temperature above 50°C, optionally for a time of less than 4 hours.

[0087] Advantageously, the leaching step is carried out at a temperature of less than 90°C, preferably less than 70°C, optionally for a time of less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours, in order to prevent the premature precipitation of zinc oxide.

[0088] More advantageously, if an oversaturated solution (i) is produced in the leaching step, the leaching step is carried out at a temperature of less than 90°C, preferably less than 70°C, optionally for a time of less than or equal to 3 hours, preferably less than or equal to 2.5 hours, more preferably less than 2 hours, in order to prevent the premature precipitation of zinc oxide.

[0089] This embodiment is preferred and applies to all variants of the process.

[0090] According to one 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 time of less than 4 hours, in order to prevent the precipitation of calcium zincate. This advantageous alternative embodiment is also preferred when a zinc-rich (ii) solution is produced.

[0091] This alternative embodiment is also advantageous when the material contains zinc as well as calcium, or when a compound containing calcium is added to the leaching step. This prevents the premature precipitation of calcium zincate in this step of the process, the temperature above 50°C being applied, preferably for a time of less than 4 hours.

[0092] This alternative embodiment is also advantageous when the material contains zinc as well as calcium, or when a compound containing calcium is added to the leaching step. This prevents the premature precipitation of calcium zincate in this step of the process, the temperature above 50°C being applied, preferably for a time of less than 4 hours.

[0093] Leaching can be carried out in one step or in several steps (for example, in two steps, see Figure 3 ). In the latter case, the leaching step is preferably carried out countercurrent to an intermediate solid-liquid separation step. At the end of the leaching, the zinc-rich 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 to recover the zinc and the basic compound, preferably selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof or caustic soda, in the impregnation solution. The solids are then removed while the solution is passed to the zinc precipitation step, or to an optional intermediate purification step.

[0094] Thus, the zinc-super saturated solution (i) or the zinc-rich solution (ii) obtained is used in the following steps:

[0095] Alternatively:

[0096] • adding a calcium compound to the zinc-rich solution (ii) or to the zinc-super saturated solution (i) to precipitate solid calcium zincate in a zinc-depleted basic solution,

[0097] • separating (B) at least a part of the zinc-depleted basic solution from the calcium zincate slurry,

[0098] • heating the calcium zincate slurry consisting of solid calcium zincate and the remaining zinc-depleted basic solution to form a zinc-super saturated solution (iii) and a solid material,

[0099] • separating (C) the solid material obtained after the heating step, preserving the zinc-super saturated solution (iii) and optionally recycling the solid material to the calcium compound addition step,

[0100] - heating the zinc-super saturated solution (i) obtained after the separation (A) or heating the zinc-super saturated solution (iii) obtained after the optional separation (C) of the solid material to precipitate zinc oxide, preferably as the main solid in a zinc-depleted solution,

[0101] - separating (D) the zinc oxide formed during the heating of the zinc-super saturated solution (i) or of the zinc-super saturated solution (iii),

[0102] - optionally, recycling at least a part of the zinc-depleted (basic) solution formed in at least one of the process steps by adding it to the leaching step of the basic medium or to the concentrated zincate slurry, preferably after removing at least a part of the zinc-depleted solution and before heating and zinc supersaturation.

[0103] In the process according to the application, the amount of ore or secondary material introduced to leaching is chosen so as to obtain the highest possible concentration of zinc in the pregnant solution, while maximizing the leaching rate. If the zinc contained in the material to be leached is present in the form of zinc oxide, the maximum concentration of zinc in the pregnant solution cannot exceed the solubility limit of zinc oxide, since zinc oxide is the least soluble zinc compound in a solution comprising a basic compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof, preferably at a concentration of 130 g / L to 200 g / L.

[0104] 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 zinc carbonate and / or zinc silicate and / or calcium zincate, it is possible to obtain a concentration of zinc in the leaching solution which is greater than the solubility limit of zinc oxide. In order to obtain the highest possible degree of zinc supersaturation while obtaining a high leaching rate, it is generally possible to carry out a plurality of countercurrent leaching steps (see the schematic diagram in Figure 3

[0105] The advantage of leaching with caustic soda is the strong selectivity. Unlike in acidic media, iron, calcium and magnesium are not soluble in alkaline media.

[0106] However, the zinc-rich leach solution can contain impurities such as silica, alumina, carbonates or lead. Silica, alumina and carbonates can be precipitated by adding lime, the concentration of silica and alumina being up to 1 g of lime per liter, or even lower, and the concentration of carbonates being up to several tens of grams of lime per liter, depending on the concentration of the basic compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof.

[0107] The purification by addition of lime is preferably carried out at a temperature above 50°C, even above 60°C, preventing the precipitation of calcium zincate. The vigorous stirring of the zincate slurry and the use of fine-powdered lime promote a high purification yield. The purification can be carried out in the presence of zinc-poor material, i.e. before or after the separation of the leaching residue. In the first case, the precipitates of silica and / or alumina and / or carbonates are mixed with the leaching residue, while being separated from the zinc-rich solution. In the second case, they are separated from the pregnant solution and recovered without being mixed with the residue. In addition to removing impurities which can contaminate the final zinc-rich product, the precipitates of silica, alumina and carbonates allow the regeneration of the basic compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide and mixtures thereof in free form.

[0108] ​It should be noted that these advantageous methods can be combined with each other, whether for the first or second variation of the invention.

[0109] Furthermore, this "reverse flow" method can also be applied to the first and / or second variations of the method according to the present invention.

[0110] Metals less reactive than zinc, such as lead, copper, or silver, that enter the solution along with zinc during the leaching process can be removed, in whole or in part, from the solution by displacement precipitation (preferably on zinc powder).

[0111] Some impurities, particularly sulfates, chlorides, or alkali metal fluorides, are more difficult to remove. They are typically permissible in the leaching solution to a certain extent, approximately tens of grams per liter. Their accumulation in the leaching circuit can be controlled by purging.

[0112] Then, the zinc contained in the enrichment solution, whether or not it has been purified, can be recovered by precipitation. Zinc is recovered from the supersaturated solution by breaking the supersaturation (preferably as an oxide). To achieve this, the solution temperature is raised to above 70°C, preferably above 100°C, and maintained for several hours. Zinc then precipitates as an oxide until the solubility limit of zinc oxide in a solution containing an alkaline compound is reached, preferably selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide, and mixtures thereof. Precipitating seed crystals are preferably added to accelerate precipitation.

[0113] For the purposes of this invention, the precipitated seed crystal is defined as zinc oxide crystal, derived from, for example, a portion of the precipitated slurry at the top of the substrate.

[0114] The precipitated zinc oxide is recovered by filtration or any other solid-liquid separation method. It is preferable to wash the solid to recover any alkaline compounds impregnated therein, preferably selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide, and mixtures thereof. The zinc-poor solution can advantageously be recycled back to the leaching process, minimizing the consumption of alkaline compounds during this process, preferably selected from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide, and mixtures thereof.

[0115] The separation step included in the method according to the present invention is liquid / solid separation, and various suitable separation methods are familiar to those skilled in the art.

[0116] The wash water from various solids can also be recycled at each stage of the method. The water balance of the loop is then balanced through one or more evaporation steps and / or discharge.

[0117] The present invention provides the use of a calcium compound (preferably lime) that can precipitate a calcium zincate slurry consisting of solid calcium zincate and an alkaline zinc-poor solution.

[0118] This is particularly preferred when the zinc concentration of the enriched solution, whether or not it has been purified, is below the solubility limit of zinc oxide. In this case, it is preferred to precipitate the calcium zincate and then form the zinc supersaturated solution (iii) as described in the present application.

[0119] Calcium zincate has a relatively low and highly temperature-dependent solubility in solutions having a caustic soda content below 250 g per litre. In order to precipitate zinc from a zinc-enriched sodium zincate solution, a preferably finely divided calcium compound, such as lime, is added to the solution at temperatures below 80°C, preferably below 50°C. Advantageously, the slurry is stirred vigorously under high shear, achieving complete reaction of the lime, preferably using a circumferential speed of the stirring device of at least 5 m / s.

[0120] The amount of calcium compound, such as lime, added is preferably stoichiometric to the zinc to be precipitated, i.e. 1 mole of calcium to 2 moles of zinc to be precipitated.

[0121] The zinc-depleted solution can advantageously be recycled to the leaching process, thereby minimising the consumption of the alkaline compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof. The calcium zincate can be washed with water to recover the said alkaline compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof, impregnated in the solid. The wash water can be recycled for use at any stage of the process.

[0122] After drying, the calcium zincate has a zinc content of about 40%. Smelters can use it to produce zinc metal without the need for pre-roasting as with sulphidic concentrates or Waelz oxides.

[0123] The calcium zincate can also advantageously be re-leached in a solution containing an alkaline compound, preferably selected from the group consisting of sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof, preferably containing 130 g / l to 200 g / l caustic soda, preferably heated at a temperature above 70°C, resulting in a zinc supersaturated solution. During the leaching process, the calcium oxide, one of the components of the calcium zincate, does not dissolve. After leaching, the calcium oxide is separated from the enriched solution and can advantageously be recycled for use in the precipitation of calcium zincate or for use in leaching to precipitate impurities, such as silica.

[0124] The zinc supersaturated solution can then be heated to a higher temperature in the presence of a precipitating seed (zinc oxide crystals), causing zinc oxide to precipitate.

[0125] The zinc oxide resulting from the leaching of calcium zincate is very pure; generally, it contains more than 95%, preferably more than 97%, more preferably more than 98%, advantageously more than 99% of zinc oxide. It can be used to produce metallic zinc or as a specialized zinc oxide, in particular for the vulcanization of elastomers, the production of enamels and zinc oxide for animal feed.

[0126] According to a first variant of the process, the following steps are provided:

[0127] - providing a material containing zinc in oxidized form,

[0128] - leaching the material in a basic medium, forming a solid residue and a zinc oversaturated solution (i),

[0129] - separating (A') the solid leaching residue from the zinc oversaturated solution (i),

[0130] - heating the zinc oversaturated solution (i) obtained after said separation (A'), precipitating zinc oxide (preferably as the main solid) in a zinc- depleted solution,

[0131] - separating (D') the zinc oxide formed during the heating of the zinc oversaturated solution (i).

[0132] According to a preferred embodiment of the first variant, after the formation of the solid residue and the zinc oversaturated solution (i), and optionally after the separation step (A'), the process can further comprise the following successive steps:

[0133] - adding a calcium compound to the zinc oversaturated solution (i), forming a zincate slurry consisting of solid calcium zincate and a zinc-depleted basic solution,

[0134] - separating (B') at least a part of the zinc-depleted basic solution from the calcium zincate slurry,

[0135] - heating the calcium zincate slurry consisting of solid calcium zincate and the remaining zinc-depleted basic solution, forming a zinc oversaturated solution (iii) and a solid material,

[0136] - separating (C') the solid material obtained after the heating step, preserving the zinc oversaturated solution (iii), and optionally recycling the solid material to the calcium compound addition step.

[0137] The zinc oversaturated solution (iii) obtained is then thus heated, precipitating zinc oxide (preferably as the main solid). By separating (D') the zinc oxide and the zinc-depleted solution formed during the heating of the zinc oversaturated solution (i), it becomes possible to provide zinc oxide according to the application.

[0138] In particular, the oversaturated solution (iii) formed in the preferred embodiment of the first variant has a higher zinc saturation and is purer than the initial oversaturated solution (i).

[0139] Preferably, at least part of the zinc-lean (basic) solution formed in at least one process step can be recycled by adding it to the leaching step of the basic medium.

[0140] Advantageously, when implementing the first variant of the process according to the application, it is necessary to produce a zinc oversaturated zincate solution. To this end, the zinc- containing material is leached with a basic 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 basic solution without triggering precipitation. The zinc-lean solid residue is separated by filtration. The zinc-rich solution can be heated above 70°C, preferably in the presence of seeds (zinc oxide particles). Under these conditions, zinc oxide precipitation is observed over time. After recovery of the zinc oxide by filtration, the zinc-saturated basic solution can advantageously be recycled for leaching.

[0141] According to the second variant of the application, the following steps are provided:

[0142] - providing a material containing zinc in oxidized form,

[0143] - leaching the material in a basic medium, forming a solid residue and a zinc-rich solution (ii),

[0144] - separating (A') the solid leaching residue from the zinc-rich solution (ii),

[0145] - adding a calcium compound to said zinc-rich solution (ii), precipitating a zincate slurry containing solid calcium zincate and a zinc-lean basic solution,

[0146] - separating (B') at least part of the zinc-lean basic solution from the calcium zincate slurry,

[0147] - heating said calcium zincate slurry consisting of calcium zincate and the remaining zinc-lean basic solution, forming a zinc oversaturated solution (iii) and a solid material,

[0148] - separating (C') the solid material obtained after the heating step, conserving the zinc oversaturated solution (iii), and optionally recycling the solid material to the calcium compound addition step,

[0149] - heating said zinc oversaturated solution (iii) obtained after the separation step (C') of the solid material, precipitating zinc oxide, preferably as the main solid in the zinc-lean solution,

[0150] - separating (D') the zinc oxide formed during heating of the zinc oversaturated solution (iii).

[0151] Preferably, at least a portion of the zinc-lean (basic) solution formed in at least one step of the process according to the second variant is recycled by adding it to the leaching step of the basic medium, or to the zincate slurry, after removal of at least a portion of the zinc-lean solution, and before heating and zinc supersaturation.

[0152] This second variant according to the application makes it possible to leach zinc in a basic solution whose zinc content is below its solubility limit. In this case, zinc can be precipitated from the rich solution by adding fine-powdered lime, preferably at a temperature below 50°C. The lime reacts with the zincate solution to form insoluble calcium zincate, which can be recovered by filtration. This calcium zincate can be sold as is, optionally after washing and drying, or re-leached in a basic solution, optionally saturated with zinc, preferably at a temperature above 50°C, yielding a zinc-supersaturated solution (iii). The lime remains insoluble and can be recovered by filtration. Advantageously, the lime 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 zinc oxide, preferably as the main solid. The zinc-lean basic solution is advantageously recycled to the calcium zincate leaching process.

[0153] The recovered zinc oxide has a high purity (95% to 100% purity) and is a suitable material for the production of metallic zinc, as determined by methods known to the person skilled in the art.

[0154] It can be seen that, whatever the chosen embodiment, the common steps are to form a supersaturated solution, heat this supersaturated solution, to provide zinc oxide according to the application.

[0155] The above embodiments can also be combined.

[0156] Figure 1 An exemplary embodiment is shown in relation to the first variant of the process according to the application.

[0157] A material containing zinc in oxidized form (B) is provided and leached (1) in a basic medium, forming a zinc-supersaturated solution (i). Lime (A) can be added to this leaching step (1) to remove impurities. A solid / liquid separation (2) of the solid residue (C) is then performed and the collected (3) supersaturated solution (i) is heated (7), optionally in the presence of precipitation seeds (F) (zinc oxide crystals), to form zinc oxide in a zinc-lean basic solution.

[0158] Before heating the zinc supersaturated solution (i), a purification step can be performed on the less reactive metals than zinc (e.g. Ag, Pb, Cu, etc.). Thus, a cementation (4) is performed on a metal dust (D) such as a zinc metal dust (D). After a solid / liquid separation step (5), the purified zinc supersaturated solution (6) is recovered to perform the heating step (7) described above.

[0159] After heating (7) and addition of precipitation seeds (F), a solid / liquid separation (8) is performed, thus recovering the zinc oxide (G). If no lime is added in the leaching step (1), the zinc- depleted alkaline solution (9) is optionally purified by addition of a calcium compound (H) such as lime (H). After separation (11) of the solid impurities (I), the purified zinc-depleted solution (12) is recovered and optionally recycled to the first leaching step (1).

[0160] Figure 2 For the exemplary embodiment related to the second variant of the process according to the application, calcium zincate is present.

[0161] The zinc-containing material (A) is introduced and leached (1) in an 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 performed, the solid residue (B) is removed and the zinc-rich solution (3) is thus conserved. Next, a calcium compound (C) (lime) is added to the zinc-rich solution, forming a slurry comprising calcium zincate (4) in precipitated form and a zinc-depleted alkaline solution. The addition of the calcium compound (C) is preferably performed at cold or a temperature below 70°C. At least a part of the zinc-depleted alkaline solution (D) is separated (5) so that it is recycled to the leaching step (1). The calcium zincate slurry (6) is heated to a temperature above 70°C (7), forming a zinc supersaturated solution (iii) and a solid material. A solid / liquid separation (8) of the solid material (E) is performed, which makes it possible to extract the calcium compound (insoluble lime), which can advantageously be recycled to the step of addition of said compound, as shown. The heating (10) of the zinc supersaturated solution (9) after the separation step (8) makes it possible to precipitate zinc oxide in a zinc-depleted solution. It is also advantageous to add precipitation seeds (F) (zinc oxide crystals) in this step. Then, the separator (11) of the zinc oxide (G) formed is collected and the zinc-depleted alkaline solution (H) is optionally recycled to the heating step (7). A part of this zinc-depleted alkaline solution can advantageously be reintroduced in the first step of the process (1). Figure 2

[0162] Figure 3 ​An extraction process is shown, called "counter-current" leaching, in which the leaching step is carried out in two steps. Thus, the zinc-containing material (A) is fed to a first leaching step (1) in a basic medium (aqueous NaOH solution). This allows the formation of a zinc-lean solid residue and a partial zinc-rich solution. Next, a first separation (2) of the solid leaching residue (C) is carried out, the partial zinc-rich solution is saved (3). The partial zinc-rich solution is then used for a second leaching operation, in the presence of a solid material, to form a zinc-rich solution. The zinc-rich solution (E) is separated (5) and the solid material is recharged in the first leaching step (1). Advantageously, a zinc-containing material, optionally a zinc-rich material, can be added to the second leaching step (4). Finally, the zinc-lean basic solution (B) produced in the zinc oxide precipitation step can also be recycled to the first leaching step (1).

[0163] The above figure describes the general steps of a variant of the process according to the application.

[0164] All the time and temperature features mentioned above also apply in particular to the steps described for these three figures.

[0165] According to the application, the material containing zinc in oxidized form is preferably any raw or secondary material containing zinc, which can be leached into a solution containing a basic compound, preferably chosen from sodium hydroxide, potassium hydroxide, alkali metal carbonates, lithium hydroxide and mixtures thereof. This raw material can include at least one so-called oxidic ore containing zinc in the form of zincite, hemimorphite, hydrozincite or willemite. The secondary material can be derived from lead metallurgy (slags), dusts from scrap metal recycling (EAFD), etc.

[0166] The raw material used in the process according to the application can advantageously comprise minerals chosen from (but not limited to) zinc hydroxide (Zn(OH)2), zinc oxide (ZnO), zincite (ZnCO3), hemimorphite (Zn4Si2O7(OH)2(H2O)), hydrozincite (Zn5(CO3)2(OH)6), willemite (Zn2SiO4) and gahnite (ZnAl2O4).

[0167] For the purposes of the present application, the term "for" can be used to denote a limiting step. Thus, if necessary, the expression "for" can be replaced with an expression denoting a limitation of the scope according to the term used after "for", for example by denoting "to form".

[0168] Within the scope of the present application, several chemical reactions can occur, listed as follows. Obviously, the person skilled in the art will be able to adapt the reactions according to the raw material used.

[0169] Zincite leaching

[0170] ZnCO3 + 4NaOH = Na2ZnO2 + Na2CO3 + 2H2O

[0171] Lepidocrocite leaching

[0172] Zn4Si2O7(OH)2(H2O) + 12NaOH = 4Na2ZnO2 + 2Na2SiO3 + 8H2O

[0173] Precipitation of calcium zincate

[0174] 2Na2ZnO2 + CaO + 7H2O = CaZn2(OH)6.2H2O + 4NaOH

[0175] Resolubilization of calcium zincate

[0176] CaZn2(OH)6.2H2O + 4NaOH = 2Na2ZnO2 + Ca(OH)2 + 6H2O

[0177] Lime precipitation of carbonate

[0178] Na2CO3 + Ca(OH)2 = 2NaOH + CaCO3

[0179] Lime precipitation of silica

[0180] Na2SiO3 + Ca(OH)2 = 2NaOH + CaSiO3

[0181] Precipitation of zinc oxide

[0182] Na2ZnO2 + H2O = ZnO + 2NaOH

[0183] Example 1 and Example 2 discuss experiments involving leaching oxidic ores, producing a zinc supersaturated alkaline solution.

[0184] Example 3 relates to the precipitation of zinc oxide from the supersaturated alkaline solution.

[0185] Example 4 and Example 5 illustrate experimental results involving leaching of lead residues with an alkaline solution, producing a zinc undersaturated zinc rich solution (ii).

[0186] Example 6 discusses the precipitation of calcium zincate by adding lime to the leach solution of Example 5.

[0187] Example 7 relates to the production of a zinc supersaturated solution by leaching calcium zincate.

[0188] Example 1

[0189] A zinc oxide ore sample 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. Then 300 g of this ground ore was leached into 2 litres of a solution containing 175 g / L NaOH, 65 g / L Na2CO3 and 10 g / L Zn (added as ZnO). The slurry was heated to 70°C and 30 g of finely ground lime (CaO) was added to the slurry at the start of the leach. The slurry was continuously stirred for 2 hours and then filtered through a Buchner funnel. The filtrate was separated and the concentrations of zinc and silicon were determined spectrometrically. The filter cake was washed with 250 mL of water, dried and its zinc content determined spectrometrically after acid digestion. The results of the test are shown in Table 1 below.

[0190] Table 1 : Leach test conditions and results

[0191]

[0192]

[0193] The leach filtrate contained 28.5 g / L of zinc and 0.85 g / L of silicon, while the depleted residue contained only 3.6% of zinc. The zinc leach recovery was 77%. The zinc in the filtrate was slightly supersaturated.

[0194] Example 2

[0195] Example 2 150 g of fresh zinc oxide ore, ground to a particle size of less than 80 μm, identical to that used in Example 1, was leached into 1 litre of the filtrate from the leach test in Example 1. The slurry was heated to 50°C and stirred for 30 minutes and then filtered through a Buchner funnel. The filtrate was separated and the concentrations of zinc and silicon were determined spectrometrically. The filter cake was washed with 125 mL of water, dried and its zinc content determined spectrometrically after acid digestion. The results of the test are shown in Table 2 below.

[0196] The leach filtrate contained 41.3 g / L of zinc and 1.51 g / L of silicon, while the depleted residue contained 7.8% of zinc. The zinc leach recovery was 55%. This yield was lower than that of Example 1, but the zinc in the filtrate was highly supersaturated (i.e. a zinc supersaturated solution) here.

[0197] In a second step, to illustrate a counter-current leach scenario, 121 g of the leach residue was leached again into 840 mL of the zinc oxide precipitation filtrate from the supersaturated solution. 15 g of lime was added to the slurry which was then heated to 80°C and stirred for 2 hours and then filtered through a Buchner funnel. The filtrate was separated and the concentrations of zinc and silicon were determined spectrometrically. The filter cake was washed with 120 mL of water, dried and its zinc content determined spectrometrically after acid digestion. The results of the test are also shown in Table 2 below.

[0198] Table 2:Conditions and results of the leaching test in Example 2

[0199]

[0200]

[0201] The filtrate from the second leaching step contained 26.7 g / L of zinc and 0.75 g / L of silicon, while the residue contained only 3.7% of zinc. The zinc leaching rate reached 46%. Carbon analysis of the filtrate showed that the carbonate content in the solution decreased from 37 g / L CO2 to 29 g / L after the addition of lime during the leaching process. The total zinc yield from the ore leaching was 76% after the two steps. The two-step countercurrent leaching produced a zinc-rich solution (41.3 g / L).

[0202] Example 3

[0203] A 90 g of pure zinc oxide was introduced as a precipitation seed into 900 mL of highly supersaturated zinc leach filtrate from the leaching of an oxide ore containing 22.2% zinc. The slurry was heated to boiling (>90°C) in a flask and stirred. The outgoing steam was condensed in a water-cooled column and returned to the flask, reducing liquid losses. After 8 hours of boiling, the slurry was filtered through a Buchner funnel. The filtrate was separated and the zinc concentration was determined by spectroscopy. The filter cake was washed with 100 mL of water and dried. The results of the test are shown in Table 3 below.

[0204] Table 3: Conditions and results of the zinc oxide precipitation test

[0205]

[0206]

[0207] The filtrate contained only 20.8 g / L of zinc. 111.6 g of dry solid was recovered. X-ray analysis showed that the solid consisted only of zinc oxide.

[0208] Example 4 and Example 5

[0209] A lead smelter slag sample 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 the ground smelter slag was then leached into 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 slurry was heated to 90°C and continuously stirred for 4 hours, then filtered through a Buchner funnel. The filtrate was separated and the concentrations of zinc and silicon were determined by spectroscopy. The filter cake was washed with 250 mL of water, dried and its zinc content was determined by spectroscopy after alkaline fusion and acid digestion.

[0210] The test was repeated, but this time the slurry was heated to 200°C in a pressurized reactor (Example 5). Prior to filtration, the slurry was cooled to about 90°C using a water-cooled coil. The results of the two tests are shown in Table 5 below.

[0211] Table 4: Leaching test conditions and results for Example 4 and Example 5

[0212]

[0213]

[0214] The yield of zinc was 68% at 200°C, compared to only 53% at 90°C. The filtrate contained 20.7 g / L and 23.5 g / L of zinc, respectively. The filtrate was not saturated with zinc.

[0215] Example 6

[0216] Example 6: Calcium zincate precipitation test 4.6 g of finely ground lime (CaO) was added to 800 mL of the filtrate from the leaching test of Example 5. The slurry was then stirred with a high shear mixer at room temperature for 6 hours prior to filtration through a Buchner funnel. The filtrate was separated and the zinc concentration was determined spectrophotometrically. The filter cake was washed with 50 mL of water, dried, and its zinc content was determined spectrophotometrically after acid digestion. The results of the test are shown in Table 5 below.

[0217] Table 5: Calcium zincate precipitation test conditions and results

[0218]

[0219] Example 6: Calcium zincate precipitation test 4.6 g of finely ground lime (CaO) was added to 800 mL of the filtrate from the leaching test of Example 5. The slurry was then stirred with a high shear mixer at room temperature for 6 hours prior to filtration through a Buchner funnel. The filtrate was separated and the zinc concentration was determined spectrophotometrically. The filter cake was washed with 50 mL of water, dried, and its zinc content was determined spectrophotometrically after acid digestion. The results of the test are shown in Table 5 below.

[0220] Example 7

[0221] Example 6: Calcium zincate precipitation test 4.6 g of finely ground lime (CaO) was added to 800 mL of the filtrate from the leaching test of Example 5. The slurry was then stirred with a high shear mixer at room temperature for 6 hours prior to filtration through a Buchner funnel. The filtrate was separated and the zinc concentration was determined spectrophotometrically. The filter cake was washed with 50 mL of water, dried, and its zinc content was determined spectrophotometrically after acid digestion. The results of the test are shown in Table 5 below.

[0222] Example 6: Calcium zincate precipitation test 4.6 g of finely ground lime (CaO) was added to 800 mL of the filtrate from the leaching test of Example 5. The slurry was then stirred with a high shear mixer at room temperature for 6 hours prior to filtration through a Buchner funnel. The filtrate was separated and the zinc concentration was determined spectrophotometrically. The filter cake was washed with 50 mL of water, dried, and its zinc content was determined spectrophotometrically after acid digestion. The results of the test are shown in Table 5 below.

[0223] Within the scope of the present application, any singular article, such as "a" or "an", can be replaced by a plural article, such as "at least 2", "at least 3", "a plurality" and the like.

[0224] The words "comprise / comprising" or any other variation such as "include / includeing" or "contain / containing" can not exclude other elements or steps, from the list of those specified, that are essential to the proper practice of the application.

[0225] It is understood that the application is in no way limited to the embodiments described above and that many modifications thereof are possible without departing from the scope of the appended claims.

Claims

1. Process for the recovery of zinc comprising the following steps: - providing a material containing zinc in oxidized form, - leaching said material in an alkaline medium, forming a solid residue and a zinc oversaturated solution (i) or a zinc rich solution (ii), - separating (A') a solid leaching residue from said zinc oversaturated solution (i) or said zinc rich solution (ii), - optionally: • adding a calcium compound to said zinc rich solution (ii) or to said zinc oversaturated solution (i), precipitating a zincate slurry containing solid calcium zincate and a zinc depleted alkaline solution, • separating (B') at least a part of the zinc depleted alkaline solution from the calcium zincate slurry, • heating the calcium zincate slurry consisting of solid calcium zincate and the remaining zinc depleted alkaline solution, forming a zinc oversaturated solution (iii) and a solid material, • separating (C') the solid material obtained after the heating step, preserving said zinc oversaturated solution (iii), and optionally recycling said solid material to the calcium compound addition step, - heating said zinc oversaturated solution (i) obtained after said separation (A') or heating said zinc oversaturated solution (iii) obtained after the separation step (C') of optionally said solid material, precipitating zinc oxide, preferably as the main solid in a zinc depleted solution, - separating (D') the zinc oxide formed during the heating of said zinc oversaturated solution (i) or said zinc oversaturated solution (iii), - optionally, recycling at least a part of the zinc depleted (alkaline) solution formed in at least one of the process steps by adding it to the leaching step of the alkaline medium or to said zincate slurry prior to the heating and zinc oversaturation.

2. Process according to claim 1, wherein the addition of the calcium compound to said zinc rich solution (ii) or to said zinc oversaturated solution (i) to precipitate calcium zincate is performed by applying a temperature of 70°C or below or by applying a temperature of 0°C to 70°C, preferably 50°C or below, more preferably 30°C or below.

3. Process according to any one of the preceding claims, wherein the step of heating said zinc oversaturated solution (i) obtained after said separation (A') or heating said zinc oversaturated solution (iii) obtained after the separation step (C') of optionally said solid material, precipitating zinc oxide in a zinc depleted solution, is performed in the presence of zinc oxide used as a precipitation seed.

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

5. The method according to any of the preceding claims, wherein, In the leaching step, a calcium compound and / or a magnesium compound is added in stoichiometric amounts to precipitate impurities selected from the group consisting of silica, alumina or carbonates.

6. Process according to any one of the preceding claims, wherein the leaching step is performed at a temperature of 90°C or below, preferably 70°C or below, optionally for a time of less than 2 hours.

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

8. The process according to any one of the preceding claims, wherein a calcium compound and / or a magnesium compound is added to the zinc oversaturated solution (i) obtained after separation of the solid residue, preferably before zinc oxide formation.

9. The process according to any one of the preceding claims, wherein the leaching step is carried out in several steps, preferably in two steps, comprising the following steps: - a first leaching of the material in a basic medium, producing a solid zinc- depleted residue and a partially zinc-enriched solution, - a first separation of the solid leaching residue, - a second leaching with the partially zinc-enriched solution, producing a zinc- enriched solution or a zinc oversaturated solution and a solid material, - a second separation of the solid material produced during the second leaching and recovery of the zinc oversaturated solution (i) or zinc-enriched solution (ii), - optionally, recycling of the solid material recovered after the second separation to the first leaching process.

10. The process according to claim 9, wherein a calcium compound is added during the first leaching process.

11. The process according to claim 9 or 10, wherein the first leaching is carried out at a temperature higher than or equal to 60°C, preferably higher than or equal to 70°C.

12. The process according to any one of claims 9 to 11, wherein the second leaching is carried out at a temperature lower than 70°C, preferably lower than 60°C, or for a time of less than 2 hours, preferably less than 1 hour.

13. The process according to any one of the preceding claims, wherein the calcium compound and / or magnesium compound is selected from lime, calcined dolomite and magnesium oxide.

14. The process according to any one of the preceding claims, wherein the less active than zinc metal impurities in the zinc-depleted solution produced in the process are removed by performing a cementation precipitation on a metal dust, preferably a zinc metal dust, preferably before the above-mentioned optional recycling step.

15. The process according to any one of the preceding claims, wherein the less active than zinc metal impurities in the zinc oversaturated solution (i), zinc oversaturated solution (iii) or zinc-enriched solution (ii) produced in the process are removed by performing a cementation precipitation on a metal dust, preferably a zinc metal dust.

16. The process according to any one of the preceding claims, wherein calcium zincate seeds are added to the precipitation of the calcium zincate slurry.

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