EP2382332A1

EP2382332A1 - Hydrometallurgical method for the reuse of secondary zinc oxides rich in fluoride and chloride

Info

Publication number: EP2382332A1
Application number: EP09801449A
Authority: EP
Inventors: Jean-Luc Roth; Valérie WEIGEL; Stéphanie MICHEL; Ludivine Piezanowski
Original assignee: Paul Wurth SA
Current assignee: Paul Wurth SA
Priority date: 2008-12-22
Filing date: 2009-12-22
Publication date: 2011-11-02
Also published as: EA201100979A1; CA2744941A1; AU2009331468A1; WO2010072773A1; KR20110102461A; PE20120531A1; TW201030153A; CN102257169A; MX2011006718A; US20110268632A1; BRPI0922620A2; JP2012513536A; LU91509B1

Abstract

The present invention relates to a method for eliminating halides, in particular chlorides and fluorides, from secondary zinc oxides, for example Waelz or Primus oxides, including the steps of (1) stripping the secondary zinc oxides with sodium carbonate and separating the dry matter from the base liquid, (2) leaching at least one portion of the dry matter from step 1 using H₂SO₄, preferably until reaching a pH of 2.5 to 4, and separating the dry matter from the acid liquid, and (3) treating the liquid from step 2 by adding Al³⁺ ions and PO₄ ^3- ions and a neutralising agent in order to eliminate the residual fluoride, preferably until reaching a pH of < 4, and separating the liquid from the dry matter containing the fluorides.

Description

HYDROMETALLURGIC PROCESS FOR THE VALORISATION OF SECONDARY ZINC OXIDES RICH IN FLURORIDES AND CHLORIDES

Technical area

The present invention relates to a dehalogenation process of secondary zinc oxides having high levels of chloride and fluoride for a recovery of mainly zinc content and can be applied alone or in addition to a hydrometallurgical treatment line of concentrate. zinc.

State of the art

The industrial sector of steel and metallurgy is at the origin of the production of co-products rich in precious metals (Zn, Fe, Pb). Zinc-rich co-products, for example dust from electric steelworks, are already valued to a large extent, particularly in the Waelz and PRIMUS processes.

[0003] Zinc metal is generally produced from the ore which undergoes different stages of treatment:

> roasting

> neutral leaching and acid in sulfuric acid medium

> precipitation of iron

> purification

> electrolysis

Unfortunately, this method of manufacturing zinc only consumes a small amount (<20%) of secondary zinc oxides from the concentration of steel mill dust. This percentage of secondary can not be higher because of the high levels of fluorides (from 0.1% to 0.4%) and chlorides (from 4 to 12%) which are real poisons as well at the level of the roasting as during the electrolysis step (especially in terms of the quality of cathodic deposition, faradic efficiency and corrosion phenomena of the electrodes and their support). These secondary oxides also contain high levels of zinc of the order of 40% to 70% and therefore there is an obvious stake (economic and ecological) to recycle.

Table 1 below shows a typical analysis of the secondary oxides used:

[0007] Table 1

[0008] The literature mentions hydrometallurgical dehalogenation patents, mainly focused on washing. For example, patent EP0773301 deals with the step of washing zinc oxide in a basic medium with sodium carbonate (60-140 kg / t oxide) at a temperature of between 50 and 90 ^° C. After separation by filtration, the solid is washed and the liquid undergoes a step of precipitation of fluorides in the form of CaF ₂ (addition of Na ₂ S or Ca (OH) ₂ 3Ca 3 (PO ₄ ) ₂ ). The study is carried out from a liquid / solid ratio of 5.

[0009] An analysis of the Waelz oxide used as well as its evolution during the washes are presented in Table 2 below.

[0010] Table 2

The analysis of the solid shows that washing with sodium carbonate, under the conditions presented in the table, can eliminate about 92% of chlorides and 52% of fluorides. Washing with water in turn allows to remove mainly chlorides and a small fraction of fluorides.

In all cases, we find that there is still 0.1% by weight of fluorides and 0.05% by weight of chlorides in the solid.

[0013] Table 3

The analysis of the filtrates makes it possible to see that high concentrations of sodium, potassium, chlorides as well as concentrations of the order of 0.5 mg / l for zinc and 0.9 mg / l for cadmium are present in the filtrate of the sodium carbonate wash.

The process described in EP0834583 (Ruhr-Zink) demonstrates the possibility of eliminating the halides by carrying out two basic washing steps with sodium carbonate (25-50 kg / t oxide), the first step is carried out at a temperature of 90 ^° C. while the second step is carried out in an autoclave under a high pressure and at a temperature of between 110 ^° C. and 130 ^° C.

The result presented for this process (Table 4) shows that the two successive washes with sodium carbonate can remove a significant portion of chlorides and fluorides. [0017] Table 4

However, despite the two successive washes with sodium carbonate, the final content of fluorides is 0.03% and that of chlorides 0.01%.

Finally, a process based on the elimination of fluorides in a zinc sulphate solution, nickel, cadmium, manganese and / or magnesium is described in the patent application EP0132014A2. The two stages of this process are:

> the addition of Al ³⁺ and PO ₄ ^{3 "} ions to the solution so that it contains at least 1 g / L of AI ³⁺ and 3.5 g / L of PO ₄ ^3" to a temperature between 45 ° C and 90 ⁰ C; the amount of PO ₄ ^{3 "} is added in stoichiometric amount relative to that of AI ³⁺ ,

neutralization of the solution at a pH greater than 4 and less than 5.5 with calcium carbonate.

The various examples presented in this patent show that in all cases, it is possible to obtain fluoride concentrations of less than 50 mg / L by performing the two steps: the addition of aluminum at a rate of 2 g / L to 3 g / L and phosphates in stoichiometric amount and neutralization. It has also been shown that increasing the temperature from 50 ^° C. to 90 ° C. makes it possible to improve the filterability of the solid, but not the final concentration of fluorides.

If the starting solution is acidic, a neutralization step will precede the step of adding Al ³⁺ and PO ₄ ^{3 "} ions.

Finally the last example cited in the patent application EP0132014A2 shows the possibility of reducing the fluoride concentration (500 mg / L) in a solution of zinc sulfate at pH = 4.5 using in a first step to the a solution of concentrated sulfuric acid and the precipitate obtained after a neutralization step with the addition of 3 g / l of aluminum per liter of zinc solution followed a second neutralization step with calcium carbonate at 50 ^° C. The solution obtained has a concentration of less than 30 mg / l of fluoride.

In all the examples of this patent, there is a significant use of aluminum which is an expensive reagent.

Object of the invention

An object of the present invention is therefore to provide a method allowing from a feed comprising more than 20% of secondary oxides to obtain a purified zinc solution having a fluoride concentration of less than 50 mg / ml. L, preferably less than 30 mg / L.

According to the invention, this object is achieved by a method according to claim 1.

General description of the invention

In order to solve the problem mentioned above, the present invention provides a process for removing halides, in particular chlorides and fluorides, from secondary zinc oxides, for example Waelz or Primus oxides. , the method comprising the steps of

(1) washing the secondary zinc oxides with sodium carbonate and separating the solid residue R1 from the basic liquid L1,

(2) acid leaching of at least a portion of the solid residue R1 of step 1 by means of H ₂ SO ₄ , preferably to a pH between 2.5 and 4, and separation of the solid residue R2 (containing certain heavy metals such as lead, iron, silver) L2 acid liquid, R2 residue that can be advantageously valued in the lead industry that separates with silver, and

(3) treatment of the liquid L 2 of step 2, by adding Al ³⁺ ions and PO ₄ ^{3 "} ions and a neutralizing agent in order to remove the residual fluoride, preferably at a pH <4 and separating the liquid L3 from the solid residue R3 containing the fluorides and certain heavy metals such as iron and lead.

The process according to the invention makes it possible to significantly reduce the content of halides, namely that of chlorides and that of fluorides, in secondary oxides initially containing significant amounts of these halides. halides, for example, but not exclusively, Waelz or Primus oxides. By eliminating most of the initially present halides and at the same time some undesirable metals such as lead and iron, these secondary oxides can be used and valorized in processes that have been unusable until now because of their sensitivity to halides, in particular the electrolysis.

In addition, it is found that the residual halides are significantly lower than those obtained with the known methods. Furthermore, the performance of the process according to the invention is furthermore obtained by minimizing the operating costs, in particular by avoiding excessively high temperatures (<100 ^° C.), and therefore preferably at atmospheric pressure, and by minimizing the consumption in expensive reagents, namely aluminum. The method according to the invention therefore does not require special installations and can be implemented relatively economically.

Therefore, the process proposed in the present invention makes it possible to reduce the fluoride content to a value of less than 0.02% and the chloride content to less than 0.01% and thus to enhance the zinc of the steel dusts, as well as than other metals (lead, iron, etc.) in a die that can be fed up to 100% by these residues "secondary sources" of zinc.

The washing of step 1 is an important step of the process according to the invention since it allows the elimination of most of the halides. The washing of the secondary zinc oxides with sodium carbonate from step 1 can be further improved if it is carried out in at least two successive substeps and preferably against the current, the first being carried out at a temperature below 80 ^° C, for example between 55 ° C and 65 ° C, preferably at about 60 ^° C, and the last substep of these at least two substeps at temperatures below 100 ° C, for example between 90 ^° C and 100 ° C, preferably at about 95 ° C. At least the last sub-step further comprises a solid-liquid separation.

In a more preferred embodiment, this washing of step 1 is carried out in three sub-steps (three washes possibly followed each by a liquid-solid separation) and the sodium carbonate introduced (at least in part) to the third substep (third wash) is conducted countercurrently with respect to the secondary zinc oxides. During the first washing, the temperature of the solution is less than 80 ⁰ C with an optimum at 60 ⁰ C. After decantation and separation, the solid undergoes a second wash at a temperature below 100 ° C, preferably 95 ° C. After further decantation and separation, the solid undergoes a third washing under the same conditions as during the second washing. As already mentioned, washes are in all cases at atmospheric pressure and therefore do not require special installation, such as an autoclave.

The sodium carbonate used in step 1 is selected from sodium carbonate, sodium sesquicarbonate, sodium bicarbonate and their hydrates. The amount of sodium carbonate may vary from 80 g / kg oxide to 240 g / kg oxide, and preferably 160 g / kg oxide. The pH measured at 20 ^° C. during the wash (s) is generally greater than 8.

In step 2, the residue R1 is treated in the presence of sulfuric acid so as to put in solution most of the zinc and to precipitate, respectively recover a particular part of the lead, iron and if necessary l money present.

The temperature during step 2 is preferably between 50 and <100 ° C and the pH is adjusted between 2.5 and 4, preferably between 2.7 and 3.8 and in particular between 3.0 and 3.5.

In an advantageous embodiment, step 2 is carried out in two or more consecutive reactors so as to refine the pH in the last reactor to the values indicated above. Thus, in the case of a three-reactor cascade, it is preferable to start from a very low pH (pH of approximately 1) and to increase it gradually in the following reactors, so as to obtain the third reactor has a pH of about 3. The pH is preferably adjusted using the solid R1.

The solid residue R2 of step 2 is then separated from the liquid fraction L2 which is conveyed to step 3. The objective of step 3 is to further reduce the fluoride content already substantially reduced in Step 1. The goal is to achieve fluoride concentrations below 50 mg / L, preferably less than 30 mg / L. This objective is achieved by adding the aluminum ions in an amount of less than 1 g / L preferably of the order of 0.5 g / L and the phosphate ions in stoichiometric amount and then neutralizing with a suitable base. The defluorination is markedly improved when the pH is less than about 4. Therefore, in a preferred embodiment of the process, the pH of step 3 is adjusted between 2.5 and 4, preferably between 3.2 and 4, and in particular between 3.4 and 3.8.

This partial neutralization can be achieved by adding a conventional base, such as for example sodium hydroxide, calcium, lime, etc.

However, in an advantageous variant of the process, the neutralizing agent of step 3 is completely or partially replaced by solid residue R1 from step 1. In fact, the inventors have found that it was possible to introduce a portion of the basic residue R1 of step 1 for neutralization purposes, in a proportion of less than 10% by weight, preferably between 1 and 5% of the amount of R1, and thus be able to reduce or even completely avoid the use of expensive conventional neutralizing agents in step 3. This variant therefore further minimizes the operating costs.

In some cases where the iron content is important, it would be advantageous to complete the precipitation of iron at the end of step 3. In this case, a suitable embodiment is to slightly increase the pH at the end of step 3 by partial neutralization at pH values between 5 and 5.5, preferably 5.2, preferably by additional addition of solid R1 from step 1.

As regards the temperature, it has been found that appropriate temperature values are between 40 ^° C. and 80 ^° C., preferably between 50 ° C. and 75 ° C.

[0041] A further aspect of the invention provides, in addition to the partial elimination of halides, iron and lead, also the elimination of other elements such as copper, cadmium, cobalt and nickel.

Therefore, in a further advantageous variant of the above process, it further comprises a step 4 of purification of the liquid L3 of step 3 by reducing less reducing metals than zinc, particularly copper, cobalt, nickel and cadmium, by adding a suitable reducing agent, preferably zinc powder, followed by separation of the solid residue R4 from the purified liquid L4 containing zinc ions.

This step 4 is a purification step of the solution that may be considered when the L3 solution of step 3 contains certain impurities. Indeed, after step 3, there remain in addition Zn ²⁺ ions generally undesirable ions such as Cu ²⁺ , Cd ²⁺ , Ni ²⁺ , Co ²⁺ and Mn ²⁺ . The removal of most of these undesirable ions is carried out by reduction with the aid of a suitable reducing agent having a greater reducing power. Since, on the other hand, it is not desirable to reduce the zinc ions, it is particularly advantageous to use zinc powder (metal), which is preferably fine, especially since the use of zinc powder makes it possible to avoid the introduction of foreign ions and is therefore preferred. The Mn ²⁺ ions that may be present will not be reduced and will remain in solution, but the other ions will be reduced depending on the reaction.

[0044] Zn + M ²⁺ > Zn ²⁺ + M

The purification operation can be carried out in a single step, but it may be necessary or desirable to proceed by several successive purifications before carrying out the solid-liquid separation. Indeed, the difficulty of extracting the elements follows the following order by increasing difficulty: copper, cadmium, nickel, cobalt. If necessary, the temperature can be adjusted in particular by adjusting it. ex. between 45 ° C and 65 ° C for cadmium, and between 70 ⁰ C and 95 ° C for cobalt. The resulting liquid L4 (solution comprising the Zn ²⁺ ions) and the solid R4 are then separated by any appropriate means, for example by filtration. It is also possible to proceed in a single step by using an intermediate temperature of the order of 75 ° C.

Yet another aspect of the invention relates to the recovery of zinc in the form of zinc metal, preferably with a high degree of purity. Therefore, an advantageous embodiment of the invention further provides for a step of electrolysis of at least a portion of the zinc solution in the liquid of the preceding step, i.e. step 3 (L3) or optionally step 4 (L4) to obtain zinc metal and a liquid depleted of zinc. Thus, the L3 solution containing Zn ²⁺ ions, if necessary freed from some of its impurities in step 4, L4, is sent to the electrolysis (step 5). The zinc, deposited on the cathode, is very pure, that is to say at least of so-called HG quality ("High Grade",> 99.98%), preferably of so-called SHG ("Special High Grade") quality. > 99.99%).

The spent electrolysis solution L5 obtained after step 5 nevertheless still contains a significant amount of zinc ions. In an advantageous variant of the process, this zinc-depleted liquid of step 5 is recycled at least partly in stage 2. In fact, the liquid L5 leaving the electrolysis also contains a certain acidity, especially in the form of sulfuric acid and therefore not only optimizes the recovery of zinc by recycling, but can also advantageously complete the acidification performed in step 2.

Nevertheless, even if such a recycling of the used electrolysis solution in step 2 is desirable, it inevitably leads to the fact that certain chemical species (essentially sodium, potassium and magnesium) may accumulate and to jeopardize the course of reactions at subsequent stages if no appropriate action is planned.

Therefore, in addition or even alternatively to the above recycling (of part) of the spent electrolysis solution in step 2, a purge of salts can be carried out by adding a neutralizing agent. , for example a conventional base, up to a pH between 6 and 7 allowing the precipitation of zinc to a residual content of less than 1 g / l. The precipitation of the zinc is followed by an extraction of the zinc thus precipitated from the liquid containing the salts, then this precipitated zinc is recycled in step 2. This step is preferably carried out between 40 ^° C. and 80 ^° C., in particular with a temperature close to 60 ^° C.

Indeed, this way of proceeding eliminates certain elements in solution in the liquid obtained after separation which without such a treatment could not be effectively eliminated, including sodium, potassium, magnesium, but also manganese, which can not be removed by step 4 of reduction purification. This step makes it possible to purify the zinc as much as possible and to keep the other ions in solution. In addition, as mentioned above, the chlorides and to a lesser extent the fluorides are eliminated almost entirely in step 1. Nevertheless, unlike the fluorides whose elimination is completed in step 3, the steps 2 and 3, possibly 4 and 5, do not significantly reduce the chloride content and the constant introduction of a very small residual amount of chlorides at the end of step 1 in a looped process therefore risks lead to an undesirable accumulation of chlorides. Since step 6 also makes it possible to eliminate the chlorides in the separated liquid after precipitation of the zinc, this step effectively prevents not only the accumulation of the metals mentioned above, but also that of the chlorides.

Finally, an important advantage of this step is not only that it avoids losing the zinc contained in the liquid L5, when the salt content too important otherwise compel to deviate from the process, but it also makes it possible to work in more constant and better controlled conditions.

In a preferred embodiment of the process, at least a portion of the solid residue R1 of step 1 is introduced in step 6 as a total or partial replacement of the conventional neutralizing agent. The fraction of R1 introduced in step 6 thus makes it possible to neutralize the solution at a lower cost up to the indicated pH and thus to precipitate the zinc to a residual content of less than 1 g / l. This fraction of R 1 required generally represents between 10 and 60%, preferably between 20 and 55%, more preferably between 45 and 50% by weight of R 1, the remaining fraction being introduced directly in stage 2 and optionally at 1%. Step 3. Therefore, an additional advantage of the variant using the R1 solid is that it does not require the use of expensive reagents.

The solid-liquid separations carried out during the various steps can be carried out by any appropriate means known, for example by decantation, filtration, centrifugation, etc.

[0056] Finally, the main advantage of the process variants presented above is that they can be integrated into a plant operating on the basis of a conventional die comprising the roasting, lixiviation, purification and electrolysis steps. (shown in Figure 2) and that they allow thus economically recovering secondary zinc oxides that have been difficult to use until now.

Brief description of the drawings

Other features and characteristics of the invention will become apparent from the detailed description of an advantageous embodiment presented below, by way of illustration, with reference to the accompanying drawing. This one shows:

Fig. 1: a block diagram of a preferred embodiment of the invention.

FIG. 2: a diagram of integration of the process in a factory operating on the basis of a conventional die.

Example

The secondary oxides used in a process according to the invention obviously have variable contents in different elements, present if necessary in different forms.

In the example described below with reference to FIG. 1, these secondary oxides of departure had the following composition:

Zn -54.8%, Fe -3.6%, Pb -6.7%, Cl -7.2%, F -0.3%, Cu -0.14%, Cd -0.16 %, Ni -0.006%, Co -0.001%, Mg -0.2%, Na -2.8%, K -2.5%, Mn -0.45%, Ag -0.016% (% by weight).

In principle, the removal of halides, in particular chlorides and fluorides present in the dust is carried out in two main steps: step 1 and step 3.

The first step (step 1) of a preferred embodiment of the process is a washing step where the solid undergoes three successive washes with sodium carbonate (160 gNa2COs / kg oxide) at well-defined temperatures for each washing. During the first washing, the temperature of the solution is approximately 60 ^° C. After decantation and separation, the solid undergoes a second wash at about 95 ° C. After decantation and separation, the solid undergoes a third washing under the same conditions as during the second washing. After decantation and filtration, the solid is washed one last time with water. At the end of this step, the solid R1 no longer contains chlorides (eg <0.004% in mass) but still contains a small amount of fluorides less than 0.02% by weight. The liquid L1 obtained during this stage contains mainly chlorides, fluorides, potassium and sodium.

In the example above, the contents of L1 were as follows: Zn -0.1 g / L, Na -40 g / L, K-10 g / L, Pb -0.3 g / L, Cl -28 g / L, F -1, 4 g / L.

The solid R1 then undergoes in step 2 acid leaching with sulfuric acid. The R2 residue obtained contains mainly iron, lead and silver. The experimental values were as follows: 30% Pb, 15% Fe, 7% Zn, 0.07% Ag.

The liquid L2, preferably completed by a portion of the residue R1 (in the example: 3%) is recovered to proceed to the so-called de-fluorination stage 3. This step is composed in principle of a step of adding a precipitating agent in well-defined proportions (Al ³⁺ and PO ₄ ^{3 "} ) and of a neutralization step: the proportions of the precipitating agents are 0, 5 g / L for aluminum and stoichiometric amount for phosphates The temperature during this step is 70 ^° C.

The phosphates are added in molar ratio 1: 1 with aluminum.

The residue R3 of the de-fluorination step is removed from the process and has the following contents: 11, 8% Pb, 10.8% Fe, 6.5% Zn, 1, 1% F. These residues can be advantageously recycled in known processes, such as Waelz, Primus, etc.

Step 4 is a purification step by reduction to the zinc powder to purify the liquid L3 of its content of copper, cadmium, cobalt and nickel and recover them in the solid R4. The experimental composition was as follows: 20% Cu, 32% Cd, 0.9% Ni. The absence of cobalt is explained in this case by the initial low cobalt content in the secondary oxides used. As indicated previously, the manganese is not reduced during this stage and remains in the purified liquid L3 (L4).

The elemental analysis of the liquid L4 was as follows

Zn -147 g / L, Cl -0.3 g / L, F <30 mg / L, Cu -0.1 mg / L, Co -0.2 mg / L, Mg -3.5 g / L, Na -8 g / L, K -6 g / L, Mn -7 g / L. The zinc upgrading step is the step 5 of this process and it is carried out by means of electrolysis, for example as described in the techniques of the engineer (zinc metallurgy (M2 270), paragraph 7.5 electrolysis), which makes it possible to reduce Zn ²⁺ ions to zinc metal in a targeted manner. Zinc metal is deposited on the cathode and is very pure (SHG grade,> 99.99%).

The spent electrolytic solution L5 contained in the above example: Zn -55 g / l, Mg-3.5 g / l, Na -8.5 g / l, K-6.1 g / l, Mn -7.5 g / L Cl -0.37 g / L, F -0.014 g / L, H ₂ SO ₄ -180 g / L.

Part, about 90%, of L5 is then directly recycled in step 2. The remainder of L5 is preferably first subjected to a step 6 of desalting (purging salts) by precipitation of zinc. The solid residue R6 containing zinc is then reintroduced in step 2, while the liquid L6 causes a large part of the elements not removed by the preceding steps, but also chlorides and to a lesser extent fluorides. Experimental levels of L6 were: Zn -0.8 g / L, Mg -2.6 g / L, Na -5.65 g / L, K -2.7 g / L, Mn -5.1 g / L, Cl -0.211 g / L, F -0.005 g / L.

Claims

claims

A process for removing halides, in particular chlorides and fluorides, from secondary zinc oxides, for example Waelz or Primus oxides, comprising the steps of

(1) washing the secondary zinc oxides with sodium carbonate and separating the solid residue from the basic liquid,

(2) leaching at least a portion of the solid residue from step 1 with H ₂ SO ₄ , preferably to a pH between 2.5 and 4, and separating the solid residue from the acidic liquid, and

(3) treatment of the liquid of step 2 by adding Al ³⁺ ions and PO ₄ ^{3 "} ions and a neutralizing agent in order to eliminate the residual fluoride, preferably at a pH <4, in wherein the neutralizing agent comprises solid residue of step 1, and separating the liquid from the solid residue containing the fluorides.

2. The process according to claim 1, wherein the washing of the secondary zinc oxides with sodium carbonate from step 1 is carried out in at least two successive washing sub-steps, the first being carried out at a temperature below 80. ⁰ C, preferably at about 60 ⁰ C, and the last of the at least two substeps at temperatures below 100 ° C, preferably at about 95 ° C, at least the last of the at least two substeps comprising in addition a solid-liquid separation.

3. The method of claim 2, wherein the washing of step 1 is carried out in three sub-steps and the sodium carbonate introduced in the third substep is conducted against the current relative to the secondary zinc oxides.

4. Process according to any one of claims 1 to 3, wherein the neutralizing agent of step 3 comprises solid residue of step 1 in a proportion of less than 10%, preferably between 1 and 5% of the amount of residue from step 1.

5. Process according to any one of claims 1 to 4, wherein the pH at the end of step 3 is increased to a value between 5 and 5.5 to complete the precipitation of iron, preferably by adding solid residue of step 1.

The method of any one of claims 1 to 5, further comprising the step of

(4) purification of the liquid of step 3 by reduction of less reducing metals than zinc, such as copper, cobalt, nickel, cadmium, by addition of a reducing agent, preferably zinc powder, and separation of the solid residue of the purified liquid.

The method of any one of claims 1 to 6, further comprising the step of

(5) electrolysis of at least a portion of the zinc in solution in the liquid of the preceding step to obtain zinc metal and a liquid depleted of zinc.

The method of claim 7, wherein at least a portion of the zinc depleted liquid of step 5 is recycled at least in part to step 2.

The method of claim 7 or 8, further comprising the step of

(6) purging salts and precipitating zinc from the zinc depleted liquid of step 5 by adding a neutralizing agent and recycling the precipitated zinc to step 2.

The method of claim 9, wherein the neutralizing agent of step 6 comprises solid residue of step 1.