BE1031460A1 - ZINC RECOVERY PROCESS - Google Patents

Abstract

The invention relates to a process for leaching 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): - Separation (A') of the solid leaching residue from the zinc-supersaturated solution (i) or from the zinc-rich solution (ii), - Heating said zinc-supersaturated solution (i) to precipitate a zinc oxide, - Separation (D') of the zinc oxide.

Description

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BE2023/5221

ZINC RECOVERY PROCESS

The present invention relates to a process for recovering zinc from a zinc-containing material.

The Waelz process allows zinc to be extracted from an ore or secondary material and recovered in the form of an oxide.

Zinc richer than the original ore or secondary material. 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 acid leaching processes of oxidized ores which involve purification and electrolysis to directly produce zinc in metallic form.

One of the disadvantages is the large amount of acid used during the process. In addition, certain impurities harmful to electrolysis and very difficult to eliminate 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.

Processes involving caustic soda (sodium hydroxide) leaching, purification and electrolysis have been proposed. Sodium leaching has the advantage of being more selective. 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 metal losses.

In 1965, Merrill and Lang investigated several methods for recovering zinc from leach solutions of zinc ores oxidized by caustic soda. Apart from electrolysis, zinc can be precipitated by carbonation, sulfidation or dilution and recovered as a marketable concentrate.

However, all these methods have drawbacks. Carbonation aims to reduce the solubility of zinc by converting caustic soda into soda ash by bubbling carbon dioxide through it. The zinc then precipitates as an oxide.

The disadvantage is that the consumption of caustic soda is prohibitive. It can be regenerated by reacting soda ash with lime. The consumption of lime is however very high, of the order of three or four tonnes of quicklime per tonne of zinc. Sulphidation consists of the precipitation of zinc sulphide by reaction of the sodium zincate solution with elemental sulphur.

This process also leads to a significant consumption of soda which affects profitability. Finally, zinc can be precipitated in the form of oxide by dilution. This method requires very significant 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 a waste containing, for example, 65% Zn by attacking it with a NaOH solution. Most of the waste is dissolved in the solution. The liquor generated is then separated from the waste by filtration.

This process involves adding Zn in powder form to the liquor in order to separate Pb, Cu, SN and Cd from it. Then, Zn oxide precipitates following the addition of methanol which acts as an anti-solvent. A disadvantage of this process 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 process for recovering zinc which comprises the following steps: - Providing 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, preferably selected from the group consisting of lime, calcined dolomite and magnesia, to said zinc-rich solution (ii) or to the zinc-supersaturated solution (1) to form a pulp which contains solid calcium zincate and a zinc-depleted basic solution,

° 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 the 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, e separation (C'} of the solid matter obtained after the heating step to preserve the solution supersaturated in zinc (ii) and optionally recycling this solid matter to the step of adding a calcium compound, - Heating said solution supersaturated in zinc (i) obtained after said separation (A') or of the solution supersaturated in zinc (ii) optionally obtained after the separation step (C') of said solid matter to precipitate a zinc oxide in a solution depleted in zinc, - Separation (D') of the zinc oxide formed during heating of the solution supersaturated in zinc {i) or (ii), - Optionally, recycling of 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 zincate pulp before heating and supersaturation with zinc.

From a material that contains zinc, the process makes it possible, easily and efficiently, to form a solution supersaturated with zinc (i) 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. It is also possible in a preferred manner to produce a solution rich in zinc (ii) to which the calcium compound is added in order to form a calcium zincate pulp. After removing a portion of the depleted solution, a heating step makes it possible to produce a solution supersaturated with zinc (ii) which will make it possible to obtain zinc oxide.

Advantageously, it is also possible to use the zinc-supersaturated solution (i) and add the calcium compound thereto to form 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 makes it possible to provide a zinc-supersaturated solution (iii). It should be noted that when the zinc-supersaturated solution (iii) is obtained by the route set out above, this solution has a greater supersaturation than the supersaturated solution [i} which was initially provided.

The zinc-depleted (basic) solution which has been generated during the process, preferably by the precipitation of zinc oxide, can be recycled either to the leaching step or to the thickened calcium zincate pulp by separation of a portion of the depleted solution prior to the heating and zinc supersaturation step.

Preferably, the precipitation of calcium zincate by the addition of the calcium compound to said zinc-rich solution (ii) or to said zinc-supersaturated solution (i) is carried out by applying a temperature below 70°C, preferably below 50°C, more preferably below 30°C, advantageously between 0 and 70°C.

More preferably, the step of heating said zinc-supersaturated solution (i) obtained after said separation (A) or of the zinc-supersaturated solution (ii) 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 (ii) 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.

According to an advantageous embodiment, 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. It is thus possible to form calcium silicates, calcium aluminates and calcium carbonates.

A calcium and/or magnesium compound may 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 distinct solid. The calcium and/or magnesium compound is preferably selected from the group comprising quicklime, hydrated lime, dolomite, 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 (fi) contains silica, a specific purification step may be carried out before 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, if a supersaturated solution is generated during the leaching step, this is carried out at a temperature below 90°C, preferably below 70°C, optionally for a period of time less than 2 hours to prevent early precipitation of zinc oxide.

This embodiment is preferred and applicable to all variants of the method.

According to a particularly advantageous alternative mode, the leaching step can be carried out at a temperature above 50°C, optionally for a period of time less than 4 hours to prevent precipitation of calcium zincate.

This alternative embodiment is 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.

According to an additional embodiment, the leaching can be carried out in several stages, preferably in two counter-current 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 (1) or 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 made during the first leaching.

Preferably, the second leaching is carried out without the addition of 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 purification of the solution by cementation of the metals more noble than the

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 and magnesia.

However, when the addition of calcium compound takes place to form the calcium zincate pulp, it is chosen from the group comprising lime and calcined dolomite.

The cementation step disclosed within the framework 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 possible recycling step mentioned above.

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 metallic 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 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 careful evaporation, 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 [|] 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 contact of the solution with silica precipitation germs 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.

Figure 1 is a schematic view of one embodiment of the method according to the present invention.

Figure 2 is a schematic view of a variant of the method according to the present invention.

Figure 3 is a schematic view of a preferred embodiment of the method 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 higher value concentrate.

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 caustic soda solution. 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 can 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 a compound chosen from the group comprising sodium hydroxide, potassium hydroxide and lithium hydroxide.

The step of leaching the material containing zinc is preferably carried out in a solution which contains caustic soda, preferably at a concentration of between 130 and 200 g/L.

The caustic soda solution may also contain zinc, soluble in this medium in the form of sodium zincate (Na2ZNO2).

The caustic soda 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 temperature 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 less than 4 hours.

Generally, the leaching step is carried out at a temperature below 90°C, preferably below 70°C, optionally for a time period of less than 4 hours.

Leaching can be carried out in a single or multiple stages (e.g. in 2 stages, see Figure 3). In the latter case, the leaching stages will preferably be carried out in countercurrent with intermediate solid-liquid separation stages. 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 caustic soda in the impregnated solution. The solid is then discharged while the solution advances to the zinc precipitation stage, or possibly, an intermediate purification stage.

Thus, the zinc-supersaturated solution {(i) or the zinc-rich solution (ii) obtained is used for the following steps:

Optionally: e adding a calcium compound to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) to precipitate solid calcium zincate in a zinc-depleted basic solution, e separating {B} at least a portion of the zinc-depleted basic solution from the calcium zincate pulp, e 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 material, e separating (C) the solid material obtained after the heating step to preserve the zinc-supersaturated solution (ii) and optionally recycling this solid material to the calcium compound addition step, - heating said zinc-supersaturated solution (i) obtained after said separation (A) or of the zinc-supersaturated solution (ii) optionally obtained after the separation step (C) of said solid material to precipitate a zinc oxide in a zinc-depleted solution, - Separation (D) of the zinc oxide formed during heating of the zinc-supersaturated solution {(i) or (ii), - 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 method 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 caustic soda solution, 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, a zinc concentration in the leaching solution can be achieved which is higher than the solubility limit of zinc oxide. In order to achieve the highest possible zinc supersaturation while achieving a high leaching efficiency, it is often practical to carry out several countercurrent leaching stages (see diagram in Figure 3). The operating conditions of the different leaching stages can be different. For example, the temperature of the leaching of the fresh material can 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 can be shorter in order to prevent the formation of insoluble calcium zincate.

Leaching with caustic soda has the advantage of being quite selective. Iron, calcium or magnesium do not go 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 litre, or even less, for silica and alumina, and up to concentrations of a few tens of grams per litre for carbonate depending on the soda concentration.

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 elimination of impurities which could contaminate the final zinc-rich product, the precipitation of silica, alumina and carbonate causes the regeneration of free caustic soda. 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 “counter-current” mode can also be applied to the first and/or second variant of the method according to the invention.

Metals more noble than zinc which pass into solution with zinc during leaching, such as lead, copper or silver, can be removed from the solution, in whole or in part, by cementation, preferably on zinc powder.

Some impurities, including sulfates, chlorides or alkali fluorides, are more difficult to remove. 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 operation.

The zinc contained in the rich solution, purified or not, is then recovered by precipitation. The zinc from the supersaturated solution is recovered 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 then precipitates in the form of oxide until it reaches the solubility limit of zinc oxide in the caustic soda solution. Precipitation seeds are preferably added to accelerate the reaction.

For the purposes of the present invention, precipitation seeds are understood to mean zinc oxide crystals, originating for example from the recycling at the head of a part 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 impregnated caustic soda. The zinc-depleted caustic soda solution can advantageously be recycled to the leaching, so as to minimize the caustic soda consumption of the process.

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 water from the various solids can also be recycled at each stage of the process. The water balance of the circuit is then balanced by means of one or more evaporation stages, 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 lower than 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 (ii) as reported in the present invention.

Calcium zincate has a relatively low and highly 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 calcium compound (e.g. lime) added is preferably equal to the stoichiometric amount of 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 process, so as to minimize the caustic soda consumption of the process. The calcium zincate can be washed with water to recover the impregnated caustic soda. The wash waters can be recycled at any stage of the process.

After drying, calcium zincate has a zinc content of around 40%. It can be used by smelters to produce metallic zinc without requiring prior roasting like sulphide concentrates or Waelz oxides.

Calcium zincate may also advantageously be relixivated in a caustic soda solution, preferably containing between 130 and 200 grams per liter of caustic soda, hot, 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 may advantageously be recycled to the precipitation of calcium zincate or to leaching to precipitate impurities such as silica.

The zinc-supersaturated solution can then be heated to a higher temperature in the presence of precipitation seeds (zinc oxide crystals) so as to force the precipitation of zinc oxide.

Zinc oxide produced from the leaching of calcium zincate is very pure, typically containing over 99% zinc oxide.

I! can be used to produce metallic zinc or as technical zinc oxide, particularly for the vulcanization of elastomers, the production of enamels, animal feed.

According to a first variant of the process, it is planned to carry out the following steps: - Supply of a material which contains zinc in oxidized form,

- Leaching of the material in a basic alkaline medium with formation of a solid residue and a zinc-supersaturated solution (I) - Separation (A') of the solid leaching residue from the zinc-supersaturated solution (i), - Heating of said zinc-supersaturated solution (i) obtained after said separation (A') to precipitate a zinc oxide in a zinc-depleted solution, - Separation (D') of the zinc oxide formed during heating of the zinc-supersaturated solution (ij.

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): - addition of a calcium compound to the zinc-supersaturated solution (i) to form a zincate pulp which is composed 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 composed of solid calcium zincate and the remainder of the basic solution depleted in zinc with formation of a zinc-supersaturated solution (ii) and a solid material, - separation (C') of the solid material obtained after the heating step to preserve the zinc-supersaturated solution (ii) and optionally recycling this solid material to the calcium compound addition step.

Thus, the zinc-supersaturated solution (ii) obtained is then heated to precipitate 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).

Preferably, it is possible to recycle at least part of the zinc-depleted (basic) solution formed during at least one of the process steps 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 solution of sodium zincate supersaturated with zinc. To do this, the material containing the zinc is leached by a solution of caustic soda, optionally saturated with zinc. Under these conditions, it is observed that the zinc content can exceed the solubility limit of zinc oxide in the soda 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 the zinc oxide by filtration, the sodium hydroxide solution saturated with zinc can advantageously be recycled to leaching.

According to a second variant of the invention, it is planned to carry out the following steps: - 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 a zincate pulp which contains solid calcium zincate and a zinc-depleted basic solution, - Separation (B') of at least a portion of the zinc-depleted basic solution from the calcium zincate pulp, - Heating said calcium zincate pulp consisting of calcium zincate and the remainder of the zinc-depleted basic solution with formation of a zinc-supersaturated solution (iii) and a solid material, - Separation (C') of the solid material obtained after the heating step to preserve the zinc-supersaturated solution (iii) and optionally recycling this solid material to the calcium compound addition step, - Heating said solution supersaturated in zinc (iii) obtained after the separation step (C') of said solid material to precipitate a zinc oxide in a solution depleted in zinc, - Separation (D') of the zinc oxide formed during the heating of the solution supersaturated in zinc (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 a soda 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 sodium 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 a soda solution, optionally saturated with zinc, preferably at a temperature above 50°C so as to produce the solution supersaturated with zinc (ii). 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, possibly in the presence of precipitation seeds (zinc oxide crystals) to precipitate the zinc oxide. The sodium hydroxide solution depleted in zinc is advantageously recycled to the leaching of calcium zincate.

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 those 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 zinc-supersaturated solution (ij. An addition of lime (A) can be made 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 [ij] collected (3) is heated (7), possibly 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 (e.g. 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, 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 in 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 method 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 solution depleted in zinc. 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 higher than at 70°C (7) to form a zinc-supersaturated solution (ii) and a solid matter. A solid/liquid separation (8) of the solid matter (E) is carried out, which makes it possible to extract 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 zinc-supersaturated solution (9) after the separation step (8) makes it possible to precipitate a zinc oxide in a zinc-depleted solution. 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 zinc-depleted solution (H) is optionally recycled to the heating step (7). Part of this basic zinc-depleted solution can advantageously be reintroduced to the first step of the process (1).

Figure 3 illustrates a leaching process called “counter-current” and whose leaching stage 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 the 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 above figures describe the general steps of the variants of the method according to the invention.

All the characteristics of duration and temperature mentioned above 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 caustic soda solution. This raw material may include at least one so-called oxidized ore which contains zinc in the form of smithsonite, hemimorphite, hydrozincite, wilemite.

The secondary material can come from lead metallurgy (slag), dust from scrap metal recycling (EAFD), etc.

The raw material used in the process according to the invention can advantageously be chosen (without being limited thereto) from the group comprising zinc hydroxide (Zn(OH)), zinc oxide (ZnO),

Smithsonite (ZNCO3), hemimorphite (Zn45i207(OH]2{H20]}}, hydrozincite (Zns(CO3)>(OH)«), willemite (Zn25i04) and ghanite (ZNAlO;4).

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 depending on the raw material used.

Leaching of smithsonite

ZNCO3 + 4 NaOH = NazZ/NO2 + Na2CO3 + 2 H2O

Leaching of hemimorphite

ZN4Si207(OH)2{H20) + 12 NaOH = 4 NazZnO2 + 2 NazSiO3 + 8 H20

Precipitation of calcium zincate 2 No2ZnO2 + CaO + 7 H20 = CaZn2(OH)6.2H20 + 4 NaOH

Redissolution of calcium zincate

Cazna(OH)6.2H20 + 4 NaOH = 2 Na2ZnO2 + Ca(OH)2 + 6 H2O

Precipitation of carbonate by lime

Na2CO3 + Ca(OH)2 = 2 NaOH + CaCO3

Precipitation of silica by lime

NazSIO3 + Ca(OH)2 = 2 NaOH + CasSiO3

Precipitation of zinc oxide

Na2ZnO2 + H2O = ZnO + 2 NaOH

Examples 1 and 2 present leaching tests of an oxidized ore in order to produce a supersaturated basic solution in

Zinc.

Example 3 refers to the precipitation of zinc oxide from a supersaturated basic solution.

Examples 4 and 5 illustrate the results of leaching tests of a lead slag by a basic solution not saturated with zinc.

Example 6 shows the precipitation of calcium zincate by addition of lime to the leaching solution of Example 5.

Example 7 relates to the production of a zinc supersaturated solution by leaching of 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 um. 300 g of this ground ore was then leached in 2 liters of a solution containing 175 g/L NaOH, 65 g/L Na2CO: and 10 g/L Zn added as ZnO. The pulp was heated to 70 °C and 30 g of finely ground lime (CaO) was 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 No. 1 below.

Table No. 1: Conditions and results of leaching test No. 1

Mass of crushed ore (g)

Addition of CaO (9)

Volume of attack solution 2000. (mL) 2000

Composition of the attack solution

NaOH (g/L)

NazCO: (g/L)

Zn (g/l) | 10

Temperature (°C)

Leaching time [min]

Mass of dry residue {(g)

Zn content of residue (%)

Filtrate volume (mL) 1780

Zn content of the filtrate (g/L)

Si content of the filtrate (g/L)

Zinc leaching yield mn (%) 77

The leaching filtrate contained 28.5 g/L zinc and 0.85 g/L silicon while 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 um were leached in 1 liter of the filtrot 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 given in Table 2 below.

The leaching 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 with zinc.

In a second step and in order to illustrate a countercurrent leaching scheme, 121 g of the leaching 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

Buchner. The filtrate was separated and the zinc and silicon concentrations were determined spectrometrically. The filter cake was washed with 120 mL of water, dried and its zinc content was determined spectrometrically after digestion in acid. The results of the test are also shown in Table 2 below.

Table No. 2: Conditions and results of the leaching tests of example 2

Leaching stage

Mass of crushed ore (g) | 150

Residue mass (g) L | 021

Addition of CaO (g) | 0 | 15

Volume of attack solution (mL) 1000

Composition of the attack solution in

NaOH [9/1]

Na3CO: (g/L] | 69 | 90

Temperature (°C 50 | 8

Leaching time (min)

Mass of dry residue (g)

Zn content of residue (%)

Filtrate volume (mL)

Zn content of the filtrate (g/L)

Si content of the filtrate (g/L)

Leaching yield (7%)

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 the leaching, the carbonate content of the solution decreased from 37 g/L

CO» at 29 g/L by adding lime. The overall zinc yield from the ore leaching after the two stages was 76%. The two-stage countercurrent leaching produces 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 the boil in a flask and kept under 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

Buchner. The filtrate was separated and the zinc concentration was determined spectrometrically. The filter cake was washed with 100 mL of water and dried. The test results are shown in Table

No. 3 below.

Table No. 3:

Conditions and results of zinc oxide precipitation test

Initial volume of solution (mL) | 900

Initial Zn content (g/L)

Initial Si content (g/L)

Addition of seeds (g of ZnO] | 900

Temperature (°C)

Duration {h) | 8

Mass of dry solid recovered (9)

Filtrate volume (mL)

Zn content of the filtrate (g/l)

The filtrate contained only 20.8 g/L of zinc. 111.6 g of dry solid were recovered. 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 um. 200 g of ground slag was 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 fitrate 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 acid digestion.

The test was repeated but this time by bringing the pulp to a temperature of 200°C (example 5) in a pressurized reactor. Before filtration, the pulp was cooled to approximately 90°C using a water-cooled coil. 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 4 5

Mass of crushed slag (g)

Volume of attack solution mL 1000 1000

Composition of the attack solution

NaOH (g/L)

NazCO: (g/L)

Zn (g/L)

Temperature (°C) | 90 | 200

Leaching time (min)

Zn content of residue (%)

Filtrate volume (mL)

Zn content of the filtrate (g/L

Si content of filtrate (g/L) | 069 | 076

Leaching efficiency (%) | 5 | 68

The zinc yield reached 68% at 200°C while it was only 53% at 90°C. The filtrates contain 20.7 g/L and 23.5 g/L of zinc respectively. They are not saturated with zinc.

EXAMPLE 6 4.69 ml 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, using a high shear stirrer, before being filtered on a Buchner funnel. The filtrate was separated and the zinc concentration was determined spectrometrically. The filter cake was washed with 50 ml of water, dried and its zinc content was determined spectrometrically after digestion in acid. The results of the test are given in Table 5 below.

Table No. 5:

Conditions and results of calcium zincate precipitation test

Initial solution volume [ML]

Initial Zn content (g/L)

Addition of CaO (g) [Duration {h) UN 8

Mass of dry solid recovered (9

Filtrate volume (mL)

Zn content of the filtrate (g/L)

Zn content of solid (%) 30.3 g of dry solid containing 39.9% zinc was 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 No2CO:. 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 is 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 "include", "contains" or any equivalent or derivative term may be replaced by "consisting of" in order to define a list or exclusive selection possibilities so as not to include other elements not mentioned 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 may be made thereto without departing from the scope of the appended claims.

Claims (15)

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 (fi), - Separating (A') the solid leaching residue from the zinc-supersaturated solution [i} or from the zinc-rich solution (ii), - Optionally: e adding a calcium compound to said zinc-rich solution (ii) or to the zinc-supersaturated solution (i) to precipitate a zincate pulp which contains solid calcium zincate and a zinc-depleted basic solution, e separating (B') at least a portion of the zinc-depleted basic solution from the calcium zincate pulp, e 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, e 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, - Heating said zinc-supersaturated solution (i) obtained after said separation (A') or of the zinc-supersaturated solution (ii) optionally obtained after the separation step (C') of said solid matter to precipitate a zinc oxide in a BE2023/5221 solution depleted in zinc, - Separation (D'} of the zinc oxide formed during heating of the zinc-supersaturated solution {(i) or (ii), - Optionally, recycling at least a portion 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, preferably below 50°C, more preferably below 30°C, advantageously between 0 and 70°C. 3. A method according to any preceding claim, wherein the step of heating said zinc-supersaturated solution {i) obtained after said separation (A') or of the zinc-supersaturated solution (ii) 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 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 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 BE2023/5221, 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 6, wherein 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 (1) or of the solution rich in zinc (ii), - Possibly, recycling of the solid material recovered after BE2023/5221 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. 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 above-mentioned possible recycling step. 15. A method according to any preceding claim, wherein the zinc-supersaturated solution {i) (ii) 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.