METHOD FOR RECOVERING METAL ZINC FROM SOLID METALLURGICAL WASTES

Abstract

A method for recovering metal zinc from a solid metallurgical waste containing zinc and manganese, may include: (a) bringing the solid metallurgical waste into contact with an aqueous leaching solution comprising chloride ions and ammonium ions to produce at least one leachate including zinc ions and manganese ions and at least one insoluble solid residue; (b) cementing the leachate, by adding metal zinc as a precipitating agent, to eliminate at least one metal other than zinc and manganese possibly present in the leachate as ions and producing a purified leachate; (c) subjecting the purified leachate to electrolysis in an electrolytic cell including at least one cathode and at least one anode immersed in the purified leachate to deposit metal zinc on the cathode and producing at least one exhausted leachate, and, before the electrolysis, precipitating manganese ions by oxidation with permanganate ions and subsequently separating a precipitate including MnO.sub.2.

Claims

1. A method for recovering metal zinc from a solid metallurgical waste comprising zinc and manganese, the method comprising: (a) bringing the solid metallurgical waste into contact with an aqueous leaching solution comprising chloride ions and ammonium ions to produce at least one leachate comprising zinc ions and manganese ions and at least one insoluble solid residue; (b) cementing the leachate by adding metal zinc as a precipitating agent, to eliminate at least one metal other than zinc and manganese optionally present in the leachate as ions and producing a purified leachate; (c) precipitating manganese ions by oxidation with permanganate ions to form a precipitate comprising MnO.sub.2, and subsequently separating the precipitate; (d) electrolyzing purified leachate to electrolysis in an electrolytic cell, comprising a cathode and at least mean anode immersed in the purified leachate, to deposit metal zinc on the cathode and producing at least one exhausted leachate, wherein the precipitating is conducted, before the electrolyzing.

2. The method of claim 1, wherein the precipitating (c) is carried out after the cementing (b) and before the electrolyzing (d).

3. The method of claim 1, wherein of the precipitating (c) is carried out in the bringing (a), by adding the permanganate ions to the leaching solution.

4. The method of claim 1, wherein at least one part of the exhausted leachate exiting from the electrolyzing (d) is recycled as a leaching solution to the bringing (a).

5. The method of claim 4, wherein the precipitating (c) is carried out on the at least one part of exhausted leachate recycled as leaching solution to the bringing (a), after the electrolyzing (c) and before the bringing (a).

6. The method of claim 1, wherein the permanganate ions are in the form of an aqueous solution.

7. The method of claim 1, wherein the permanganate ions are added in the precipitating (c) is adjusted in quantity, continuously or discontinuously, so as to maintain the value of the redox potential of the leachate exiting from the precipitating (c) in a range of reference values.

8. The method of claim 1, wherein the precipitate further comprises an iron oxide.

9. The method of claim 1, wherein the precipitate is washed with an acid aqueous solution having a pH in a range of from 1.5 to 3.

10. The method of claim 1, wherein the leaching solution has a pH in a range of from 5 to 9.

11. The method of claim 4, wherein the exhausted leachate is fed to the bringing (a) after being treated to remove at least partly: calcium ions, magnesium ions, halide ions, alkali metal ions, alkaline and/or alkaline-earth metal ions, and/or water.

12. The method of claim 1, wherein the leaching solution in the bringing (a) comprises anions capable of forming insoluble calcium and/or magnesium salts.

13. The method of claim 1, wherein the anode is an activated metal anode.

14. The method of claim 1, wherein the anode is a graphite anode.

15. The method of claim 1, wherein the cementing (b) is carried out continuously in at least one rotary reactor.

16. The method of claim 1, wherein the precipitating (c) comprises: (i) dosing permanganate ions to the leachate comprising zinc ions and manganese ions; (ii) measuring at least pH, redox potential and optionally temperature of the leachate; (iii) periodically calculating a precipitation redox potential value by a calibration curve which correlates the precipitation redox potential to at least pH values and optionally the leachate temperature; and varying dosage of the permanganate ions so as to bring the redox potential value of the leachate to the calculated precipitation redox potential value.

17. The method of claim 16, wherein the calibration curve is obtained by redox titration of the leachate at two or more different pH values and two or more different temperature values.

18. The method of claim 16, wherein the anode is an activated metal anode.

19. The method of claim 1, wherein the permanganate ions are in the form of an aqueous solution of KMnO.sub.4.

Description

DESCRIPTION OF THE FIGURES

[0062] The characteristics and advantages of the process according to the present invention will be more evident from the following description referring to the attached FIG. 1, which is a schematic representation of an embodiment of the method according to the present invention. The following description and the following examples of embodiment are provided for the sole purpose of illustrating the present invention and are not to be understood in a sense limiting the scope of protection defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0063] With reference to FIG. 1, a system 100 comprises a unit for leaching 101 metallurgical wastes, a cementing unit 103 for the removal of metals other than zinc and manganese, an oxidation unit for the removal of Mn.sup.2+ ions soluble in the form of a precipitate comprising MnO.sub.2 105, an electrolyte recycling tank 107, an electrodeposition unit 109 for the electrodeposition of zinc, a carbonation unit 111 and an evaporation unit 113 for the regeneration of the exhausted electrolyte. The following description of the method according to the present invention relates to a mode of carrying out continuously said method and in a steady state condition.

[0064] During the implementation of the method according to the present invention, the metallurgical wastes containing zinc and manganese 115 are fed to the leaching unit 101, where they are brought into contact with a leaching solution comprising NH.sub.4+ ions and Cl.sup.− ions which is fed for example in the form of ammonium chloride solution 116.

[0065] Preferably, metallurgical wastes include EAF, CZO dust and other wastes containing zinc in oxidized form generated by metallurgical processes, such as ash, slag, and sludge. More preferably, metallurgical wastes comprise at least one of: EAF, CZO dusts and mixtures thereof.

[0066] Zinc and manganese can be present in metallurgical wastes in the form of metal, oxide and/or alloy. The zinc content in metallurgical wastes is preferably within the range 15%-70% by weight. The Mn content is preferably within the range of 0.1%-10% by weight, more preferably 0.5%-5% by weight.

[0067] In addition to manganese, metallurgical wastes can contain other contaminants, such as halides (in particular fluorides) and metals (in particular Pb, CD, Cu, Fe, Ni, AG, alkaline and alkaline-earth metals, in particular Na and Ca). The overall concentration of metal contaminants and fluorides in metallurgical wastes varies depending on the origin of the wastes. Preferably, the overall concentration of metal contaminants, excluding manganese, is within the range 2%-5% by weight, while the overall concentration of halogens is within the range 2%-10% by weight (expressed as X.sub.2, where X is a halogen atom, for example Cl or F), said percentages being referred to the weight of the metallurgical waste.

[0068] The leaching step generates a biphasic reaction product comprising an insoluble residue 117 and a leachate 119 comprising zinc ions and manganese ions. The leachate 119 further comprises the other metal contaminants present in the metallurgical wastes which are dissolved during leaching. The dissolved metals are present in the leachate in the form of ions, in particular chlorine-ammoniacal complexes which are formed, for example, according to reaction 1 shown previously.

[0069] The ammonium and chloride ions are preferably contained in the leaching solution in a variable concentration within the range 100 g/l-600 g/l expressed as ammonium chloride.

[0070] Preferably, the pH of the leaching solution is within the range 5-9, more preferably within the range 5.2-7.5, more preferably within the range 6-7. Under these pH conditions, the leaching of the iron contained in the treated metallurgical waste is minimized. The pH of the leaching solution can be controlled by adding an aqueous solution of NH.sub.3.

[0071] Leaching is preferably carried out at a temperature within the range 50° C.-90° C., more preferably 60° C.-80° C.

[0072] At the end of the leaching, the insoluble residue 117 is separated from the leachate 119, for example by decantation and/or filtration. The insoluble residue consists mainly of zinc ferrite and iron oxides. The insoluble residue may further comprise CaF.sub.2 deriving from the precipitation of the fluoride ions and calcium ions present in the treated metallurgical waste. The insoluble residue can be sent to disposal as a waste or more advantageously recycled to an EAF furnace for the production of steel or to a process for the production of CZO.

[0073] In one embodiment, the oxidation of soluble manganese ions and possibly soluble iron ions is carried out by adding MnO.sub.4.sup.−118 ions to the leaching solution. In this case, the insoluble residue 117 also comprises a precipitate of MnO.sub.2 and optionally of Fe(OH).sub.3.

[0074] In the cementing unit 103, the leachate 119 is subjected to a cementing treatment to remove contaminants consisting of dissolved metals other than zinc which, otherwise, might be co-deposited with the metal zinc during the electrodeposition step.

[0075] Cementation (or precipitation by chemical shift) is the reaction through which a first metal is precipitated in the elemental state from a solution that contains it in the form of ions by adding, to the solution, a second metal in the elemental state (precipitating agent) having a lower reduction potential (or more negative) than the reduction potential of the first metal.

[0076] In the cementing unit, metal zinc is used as a precipitating agent 123 to precipitate dissolved metals having a higher reducing potential than zinc in the electrochemical series. The metal zinc is added in dust form to the leachate in a quantity in excess of that of the metals to be precipitated, for example in a quantity from 30% to 200% in excess of the stoichiometric quantity necessary to precipitate the metal ions contained in the leachate. The quantity of soluble zinc ions resulting from the addition of metal zinc is negligible compared to the quantity of zinc ions resulting from leaching metallurgical wastes.

[0077] As said, metal zinc used as a precipitating agent, in addition to zinc in the elemental state, can contain iron impurities in significant quantities, for example up to 3-4 g of iron per kg of zinc. Since iron introduced into the leachate can be removed together with manganese, it is possible to use metal zinc even of not particularly high purity as a precipitating agent. Preferably, the metal zinc contains iron in a quantity up to 0.1% by weight, up to 0.5% by weight or up to 1% by weight (concentration expressed in terms of iron in the elemental state referred to the weight of the precipitating agent).

[0078] Cementation can be performed in one or more stages in sequence, depending on the total content and the type of metal contaminants to be removed.

[0079] Cementation can be carried out with the techniques and devices known to those skilled in the art. In a preferred embodiment, cementation is carried out continuously in a revolving reactor. This reactor and the relative methods of use are known to those skilled in the art.

[0080] The cementing step generates a biphasic product constituted by a purified leachate 125 and a solid product (cement) 127. The purified leachate 125 comprises zinc ions and a residual quantity of metal ions other than zinc that were initially present in the incoming leachate 119. The cement 127 comprises the precipitated metals in the elemental state other than zinc having a higher reduction potential than zinc, in particular Pb, Cd, Cu, Ag, and unreacted metal zinc. In the purified leachate 125, the concentration of manganese ions present in the leachate remains substantially identical to the concentration in the incoming leachate 119, since the reduction potential of the Mn.sup.2+/Mn pair is lower than that of the Zn.sup.2+/Zn pair under the conditions in which cementation is carried out.

[0081] Preferably, the total concentration of ions of metals other than zinc, including manganese, in the leachate 119 entering the cementing unit 103 is within the range 100 mg/l-3,000 mg/l. Preferably, the total concentration of ions of the metals other than zinc, excluding manganese and iron, in the purified leachate 125 is within the range of 0.5 mg/l-2 mg/l. Preferably, in the purified leachate 125 the concentration of manganese ions is within the range 10 mg/l-2,000 mg/l, more preferably within the range 20 mg/l-1,500 mg/l. Preferably, in the purified leachate 125 the concentration of iron ions is within the range 1 mg/l-50 mg/l.

[0082] In accordance with the embodiment shown in FIG. 1, the purified leachate 125, after being separated from the metal cement 127, for example by decanting and/or filtration, is subjected to an oxidation treatment in the oxidation unit 105 to oxidize the manganese ions in solution and to form insoluble MnO.sub.2. The oxidation of manganese ions is obtained by adding permanganate ions 129 to the purified leachate 125. The addition of permanganate ions 129 in the oxidation unit 105 may be made alternatively or in combination with the addition of permanganate ions 118 in the leaching unit 101.

[0083] The oxidation reaction of manganese ions in solution can take place, for example, according to the following scheme:

3 Mn(NH.sub.3).sub.xCl.sub.2+2 KMnO.sub.4+2 H.sub.2O.fwdarw.5 MnO.sub.2+4 HCl+2 KCl+3x NH.sub.3  (12)

[0084] where x is an integer within the range 1-6.

[0085] In the presence of soluble iron ions in the leachate, the formation of the MnO.sub.2 ions is accompanied by the reaction of the MnO.sub.4.sup.− ions with the iron ions with formation of insoluble iron hydroxides and of further MnO.sub.2, for example according to the following reaction:

3 Fe(NH.sub.3).sub.xCl.sub.2+KMnO.sub.4+7 H.sub.2O.fwdarw.MnO.sub.2+3 Fe(OH).sub.3+5 HCl+KCl+3x NH.sub.3  (13)

[0086] where x is an integer within the range 1-6.

[0087] The oxidation step carried out in the unit 105 generates a biphasic reaction product comprising an insoluble residue 131 and a treated leachate 133 having a reduced concentration of manganese ions and iron ions with respect to the concentration in the incoming leachate 125.

[0088] The insoluble residue 131 comprises the precipitated manganese in the form of MnO.sub.2 and optionally the iron oxides and hydroxides precipitated during the oxidation step. Since generally the concentration of iron ions in the leachate subjected to oxidation with permanganate ions is relatively low with respect to the concentration of manganese ions, the resulting MnO.sub.2 has a high degree of purity (equal to or higher than 95% by weight and up to 99% by weight) and it is therefore reusable as a raw material in other industrial processes.

[0089] In one embodiment, the precipitate 131 comprising MnO.sub.2 is washed with an acid aqueous solution, having for example pH within the range 1.5-3. This washing allows removing any iron oxides and hydroxides from the MnO.sub.2 precipitate, thus increasing the degree of purity of the obtained MnO.sub.2.

[0090] The permanganate ions 129 and/or 118 are preferably added in the form of an aqueous solution, for example an aqueous solution of KMnO.sub.4. In a preferred embodiment, the quantity of added MnO.sub.4.sup.− ions is adjusted so as to maintain substantially the redox potential value of the treated leachate 133 exiting from the unit 105 constant.

[0091] The dosage of the MnO.sub.4.sup.− ions can be adjusted, for example, by periodically or continuously measuring the redox potential of the treated leachate exiting from the oxidation unit 105 and by adjusting the dosage of the oxidizing agent (manually or automatically) so as to maintain the redox potential value of the treated leachate within a predetermined range (reference range). The reference range can be determined experimentally by those skilled in the art for the particular plant in which the method according to the present invention is carried out, such range of values being able to be influenced mainly by factors such as the composition of the leachate, temperature, pH, material forming the electrodes.

[0092] The leachate 133, substantially free of manganese ions and iron ions, exiting from the oxidation unit 105 is fed to the electrodeposition unit 109 for the recovery of zinc.

[0093] Irrespective of the point of the process in which the precipitation of manganese ions and the removal of the precipitated MnO.sub.2 is carried out, preferably, the residual concentration of manganese ions in the leachate circulating in the cell is lower than 2 mg/l. Preferably, the residual concentration of iron ions in the leachate circulating in the cell is lower than 1 mg/l.

[0094] It has been observed that in some cases the addition of permanganate ions does not allow to guarantee the ideal condition of concentration of Mn.sup.2+ ions in the electrolytic cell, that is, to respect the condition of concentration Mn.sup.2+<2 mg/l or lower, and therefore a higher current efficiency of the cell. This drawback can occur both when the dosage of the permanganate ions is carried out by keeping the redox potential of the leachate constant, even in a continuous and automated way, and when the permanganate ions are dosed in stoichiometric excess with respect to the concentration of manganese ions and iron ions to be precipitated.

[0095] The dosage of the permanganate ions in stoichiometric excess, in principle, has the advantage of precipitating these two impurities substantially completely, without increasing the concentration of manganese ions in the leachate. The unreacted permanganate ions, in fact, are destined to be converted into MnO.sub.2 by reacting with ammonia and thus removed from the leachate in the form of precipitate. However, the presence of impurities in the leachate, in variable and unpredictable concentrations, which oxidize in the presence of the permanganate ions together with the slow kinetics of the reaction of conversion of the permanganate ions into MnO.sub.2 in the presence of ammonia, lead to an incomplete precipitation of MnO.sub.2 and, consequently, to the permanence of a residual concentration of manganese ions in the leachate which upon reaching the cell can adversely affect the electrodeposition process, in particular if metal anodes are used.

[0096] The Applicant has now found that it is possible to overcome this drawback by adjusting the dosage of the permanganate ions so as to maintain the redox potential of the leachate at an optimal value—hereinafter also indicated “precipitation redox potential” or “Redox.sub.ppt” corresponding to the value in which the added permanganate ions completely oxidize all the oxidizable species present in the leachate, to the specific pH value thereof, preferably to the specific pH and temperature values thereof.

[0097] The precipitation redox potential can be determined experimentally, either on the plant or in the laboratory, by carrying out a series of redox titrations of aliquots of the leachate containing the manganese and/or iron ions to be removed, using a solution of permanganate ions as a titration agent; the aliquots of the leachate are subjected to titration to different pH values to take into account the possible variations of the values of this parameter during the process; the pH of the aliquots of leachate to be titrated can be adjusted by the additions of a basifying agent (e.g. NH.sub.4) or acidifying agent (e.g. HCl) in order to reach the desired pH.

[0098] Preferably, at least two, more preferably at least three, even more preferably at least four, samples having different pH values are prepared. Typically, the number of samples is within the range from 2 to 8. Preferably, the titration of these samples is carried out by keeping the sample at the operating temperature of the process, e.g., 70° C.

[0099] Preferably, the aliquots of the leachate are subjected to titration to different pH and temperature values to take into account the effects of the variations of both operating conditions on the precipitation redox potential.

[0100] To this end, at least two samples having different pH values are preferably prepared, each of which is titrated to at least two different temperature values, so as to have at least four experimental values of precipitation redox potential. More preferably, the number of samples prepared is at least three, even more preferably at least four. Preferably, each sample is titrated to at least three different temperatures, preferably, each sample is titrated to at least four different temperatures.

[0101] The experimental Redox.sub.ppt values are obtained by determining the inflection point of the titration curve, i.e., the inflection points of the graph which reports the redox potential values of the solution as a function of the volume of titrating agent added.

[0102] The experimental values of redox potential, pH and optionally temperature are mathematically interpolated to obtain a calibration curve Redox.sub.ppt=f(pH) or f(pH, T), which correlates the precipitation redox potential to pH and optionally to the temperature (T) of the leachate. The interpolation can be carried out by means of known mathematical methods, for example by means of a three-dimensional polynomial function.

[0103] Using the calibration curve, the precipitation redox potential can be calculated based on the pH values and possibly on the temperature of the leachate measured during the execution of the process. By periodically repeating the procedure for determining the Redox.sub.ppt value, it is possible to modify the dosage of the permanganate ions to guarantee the optimal precipitation conditions of the manganese ions, thus avoiding to dose the permanganate ions in defect with respect to the manganese ions, with consequent incomplete precipitation of the manganese ions from the solution, or in excess, with consequent entrainment of the not converted manganese species into MnO.sub.2 to the electrolysis cell.

[0104] The Redox.sub.ppt value may vary as a result of different factors and parameters of plant conduction, such as pH, temperature, composition of metallurgical wastes, etc. however, it has been observed that optimizing the Redox.sub.ppt value on the basis of pH, preferably pH and temperature, of the leachate is sufficient to obtain the substantially complete precipitation of the manganese ions.

[0105] The calibration curve of the Redox.sub.ppt parameter, if necessary or desired, can be however determined by taking into account also other parameters of the plant conduction, in addition to pH, such as current density applied to the electrodes, content of iron ions in the leachate, presence of other redox pairs (e.g., Au/Au.sup.+, Ag/Ag.sup.+), etc., in a manner similar to that described above for pH and temperature.

[0106] In general, the Redox.sub.ppt value can vary in wide ranges. In at least one embodiment, the Redox.sub.ppt value varies within the range 400-650 mV (measured with a Pt-based electrode relative to a reference electrode, such as a saturated calomel electrode or AgCl). The pH preferably varies within the range from 5.2-7, more preferably from 5.5-6.5. The temperature preferably varies within the range from 60° C.-80° C.

[0107] The aforesaid method for controlling the conditions of precipitation of manganese ions can be applied to the addition of permanganate ions irrespective of the position in which this addition is carried out in the process for treating metallurgical wastes, for example in the leaching solution, in the purified leachate or in the exhausted leachate.

[0108] Advantageously, the aforesaid method for controlling the conditions of precipitation of manganese ions can be carried out in combination with a continuous and automatic dosing system of the permanganate ions.

[0109] In one embodiment, the dosing system comprises: a device for dosing the permanganate ions (e.g. pump for feeding a KMnO.sub.4 solution); a redox sensor for measuring the redox potential of the leachate to be treated with the permanganate ions; a pH sensor and optionally a temperature sensor for measuring these two parameters on the leachate to be treated; a control unit (e.g. a programmable logic unit, PLC) connected to the sensors to receive and process the results of redox potential, pH and temperature measurements. The control unit is also connected to the dosing device to control the quantity of permanganate ions dosed in response to a set Redox.sub.ppt value. The logic unit is programmed with the calibration curve Redox.sub.ppt=f(pH) or f(pH, T) experimentally determined to calculate and periodically set a Redox.sub.ppt values to be maintained in the leachate on the basis of the pH values and optionally temperature values detected by the sensors during the process.

[0110] During the process, following a permanganate ion dosing, the sensors send the redox potential, pH and optionally temperature values measured on the leachate to the control unit. The control unit calculates the optimal Redox.sub.ppt value based on the programmed calibration curve and sets this value as the set point value to be maintained in the leachate. The control unit then controls the dosing device to feed the permanganate ions so as to bring the redox potential of the leachate to the set Redox.sub.ppt value (for example, by increasing or reducing the quantity of permanganate ions dosed). The aforesaid control process is repeated periodically, possibly continuously and in an automated mode.

[0111] In one embodiment, the method according to the present invention thus comprises:

[0112] a. dosing permanganate ions to the leachate comprising zinc ions and manganese ions;

[0113] b. measuring at least pH, redox potential and optionally temperature of said leachate;

[0114] c. periodically, calculating a precipitation redox potential value (Redox.sub.ppt by means of a calibration curve which correlates the precipitation redox potential to at least pH values and optionally the leachate temperature; [0115] varying the dosage of the permanganate ions so as to bring the redox potential value of the leachate to the calculated precipitation redox potential value (Redox.sub.ppt).

[0116] The electrodeposition unit 109 comprises at least one electrolytic cell (not shown in the FIGURE) comprising at least one cathode and at least one anode immersed in the leachate to be electrolysed.

[0117] In accordance with the scheme of FIG. 1, the leachate 133 to be electrolysed, before being fed to the electrolytic cell, is accumulated in a recycling tank 107. A leachate stream 135 is withdrawn from the recycling tank 107 and is circulated in the electrolytic cell of the electrodeposition unit 109. During electrolysis, the application of an electrical potential difference to the electrodes causes the reduction of the zinc ions present in the leachate and the formation of a metal zinc particulate, which adheres to the cathode surface.

[0118] The exhausted leachate 137, whose concentration of zinc ions is reduced compared to the incoming leachate 133, exiting from the electrolytic cell is recirculated again to the recycling tank 107 where it is mixed with the leachate 133 coming from the oxidation unit 105.

[0119] In one embodiment, an aliquot 159 of the leachate present in the recycling tank 107 is withdrawn and recycled to the leaching unit 101, where it is enriched with zinc ions following the leaching of further metallurgical wastes, so as to carry out the zinc recovery process continuously.

[0120] When the recovery process of metal zinc in continuous mode is in steady state conditions:

[0121] (i) the mass per unit time of metal Zn deposited to the cathode (current 143) is preferably approximately equal to the difference between the mass in the unit of time of Zn.sup.2+ ions entering the recycling tank 107 (current 133) and the mass in unit of time of Zn.sup.2+ ions in the exhausted leachate 137 which is recirculated to the recycling tank 107;

[0122] (ii) the volumetric flow rate of the recirculated leachate in the electrolytic cell (streams 135, 137) is preferably about equal to the volumetric flow rate of the recirculated leachate 159 to the leaching unit 101 (streams 159, 119, 125, 133). Under steady state conditions, the concentration of Zn.sup.2+ ions in the tank 107 is therefore substantially constant.

[0123] Electrolysis may be carried out in an open cell according to the techniques known to those skilled in the art, for example as described in U.S. Pat. Nos. 5,534,131A and 5,534,131A.

[0124] The composition of the electrolytic solution, which contains Cl.sup.− and NH.sub.4.sup.+ ions, allows obtaining the deposition of metal zinc to the cathode and the evolution of gaseous chlorine to the anode. The gaseous chlorine which has just formed, and which is still adsorbed on the electrode reacts rapidly with the ammonium ions present in solution around the anode, regenerating ammonium chloride with evolution of gaseous nitrogen. The electrochemical reactions that take place during electrolysis are the reactions (3) to (6) illustrated above. Since the electrolysis reaction consumes NH.sub.3, this is optionally integrated in the process by feeding it to the electrolytic cell (FIG. 1, arrow 141), for example in the form of an ammonia aqueous solution.

[0125] The zinc deposited on the cathode is separated from the latter (FIG. 1, arrow 143) and optionally processed, for example by melting to dispose it in the form of ingots; the metal zinc can also be recovered in dust form, a part of which can be used as a precipitating agent in the cementation step.

[0126] In one embodiment, the electrolytic cell comprises at least one graphite anode.

[0127] In another embodiment, the electrolytic cell comprises at least one activated metal anode. The activated metal anodes usable for the purposes of the present invention are known to those skilled in the art and commercially available.

[0128] Preferably, the aforesaid activated metal anode comprises at least one electrically conductive substrate (e.g., Ti, Nb, W and Ta) covered with a catalytic coating layer comprising one or more noble metals and/or one or more oxides of a noble metal.

[0129] The cathode can be made of various materials, such as titanium, niobium, tungsten, and tantalum. Preferably, the cathode is made of titanium.

[0130] In order to control the concentration of impurities in the leachate circulating in the continuous process, the leachate contained in the tank 107 is preferably subjected to a regeneration treatment to remove, in particular, at least one of the following components: calcium ions, magnesium ions, halide ions, alkaline and/or alkaline-earth metal ions, water.

[0131] The control of the concentration of these impurities allows controlling the formation of incrustations (in particular calcium and magnesium salts) on the heat exchangers used in the plant.

[0132] In one embodiment, the leachate regeneration treatment comprises a carbonation step. For this purpose, an aliquot 139 of the leachate present in the tank 107 is fed to the carbonation unit 111, where, by adding at least one precipitating agent 145 selected from: carbonate of an alkaline and/or alkaline-earth metal, hydrogen carbonate of an alkaline and/or alkaline-earth metal, and mixtures thereof (e.g. Na.sub.2CO.sub.3 and/or NaHCO.sub.3), the calcium ions and magnesium ions are removed, causing them to precipitate in the form of the respective insoluble carbonate and/or hydrogen carbonate salts (reaction 7). The insoluble precipitate 147 thus formed is separated, for example by filtration, from the supernatant solution 149 which is sent to the tank 107.

[0133] In an alternative embodiment, the control of the concentration of calcium ions and magnesium ions in the leachate circulating in the process can be carried out in the leaching unit 101 by adding anions capable of forming insoluble calcium and/or magnesium salts under the pH and temperature conditions of the leachate.

[0134] Preferably, the aforesaid anions are selected from: sulphate, carbonate, and oxalate.

[0135] Preferably the anions are sulphate anions SO.sub.4.sup.2−, which can be added to the leachate in the leaching unit, for example in the form of an aqueous solution of sulphuric acid. The carbonate and oxalate anions can be added to the leachate in the leaching unit, for example in the form of an aqueous solution of sodium oxalate or sodium carbonate. The sulphate anions form a precipitate comprising calcium sulphate and magnesium sulphate, which is removed together with the insoluble residue 117. The sulphuric acid solution can be an aqueous solution of the type available on the market, having for example a concentration within the range 20-96% by weight. In view of the composition of the ammonium chloride-based leaching solution, the addition of sulphuric acid in the quantity necessary to precipitate calcium ions and magnesium ions does not result in significant changes in the pH of the solution present in the leaching unit 101.

[0136] It should be noted that the carbonation unit in the EZINEX® process according to the state of the art also performs the function of controlling the concentration of Mn.sup.2+ ions in the leachate circulating in the process. Since the method according to the present invention provides for the substantially complete removal of soluble manganese ions from the leachate by oxidation with permanganate ions, when the control of the concentration of calcium ions and magnesium ions is carried out through their precipitation in the leaching unit, it is possible to eliminate the carbonation unit, thus reducing the size of the plant and simplifying the management thereof.

[0137] In one embodiment, the regeneration treatment comprises a step of heat treating the leachate. For this purpose, an aliquot 155 of the solution present in the tank 107 is fed to the evaporation unit 113 where part of the excess water accumulated during the process (dilution water of the reagents, washing water of the filtration residues) is removed by thermal treatment. The removed water is driven away in the form of a vapour stream 151. Water evaporation may cause precipitation of alkali and/or alkaline-earth metal halide salts (e.g., NaCl and KCl), which are separated (arrow 153) from the supernatant by sedimentation and/or filtration. The supernatant solution 157 comprising the concentrated leachate is sent to the tank 107.

[0138] The following experimental example is provided below to further illustrate the features and advantages of the present invention.

Example 1

[0139] The efficiency of the method described herein has been tested on a pilot plant realised according to the scheme of FIG. 1. The productivity of the pilot plant, in the absence of the oxidation unit, was about 8 kg/h of metal zinc.

[0140] The test was carried out by circulating the leachate in the plant with a flow rate of about 600 l/h.

[0141] The oxidation unit comprised a tank containing an aqueous solution of KMnO.sub.4(40 g/l) and a pump for withdrawing the solution from the aforesaid tank and for mixing it with the leachate circulating inside the oxidation unit. The oxidation unit also comprised a filter press to separate the solid MnO.sub.2 particulate formed after the addition of KMnO.sub.4.

[0142] The feeding flow rate of the KMnO.sub.4 solution to the leachate was adjusted so as to maintain the redox potential of the latter constant. The pump flow rate was adjusted automatically, through a pump control device, on the basis of the redox potential of the leachate exiting from the oxidation unit. The pump control device was configured to activate and modulate the flow rate of the KMnO.sub.4 based on the redox potential value measured for the leachate exiting from the oxidation unit so as to keep it at a value of 300 mV (electrode of Pt measurement; saturated calomel reference electrode).

[0143] The leachate entering the oxidation unit contained 357 mg/l of manganese ions 6 mg/l of dissolved iron ions. During the test, the average KMnO.sub.4 feeding flow rate was about 10.5 l/h. The duration of the test was 2 hours.

[0144] 1320.5 g of particulate were recovered from the oxidation unit by pressure filtration. The particulate, after washing with water and drying, weighed 1139.6 g. After drying, the dried particulate contained 62.3% by weight of manganese, equivalent to 98.6% of MnO.sub.2, and 0.91% of iron oxides/hydroxides. The filtered leachate entering the electrolysis unit had a total content of dissolved manganese ions and iron ions lower than 1 mg/l. Visual inspection showed no significant presence of particulate in the cell during electrolysis.

[0145] The electrolysis unit comprised two electrolytic cells connected in series each comprising five titanium cathodes (each having a working surface of 1 m.sup.2) and 6 graphite anodes.

[0146] Upon 2 hours of electrolysis carried out with a current density of 350 A/m.sup.2, a total deposit of 16.76 kg of metal zinc (current efficiency 98.2%) having a purity degree equal to 99.992% was recovered at the cathodes.

[0147] The current efficiency, i.e., the ratio between the quantity of zinc deposited and the quantity of zinc theoretically depositable according to Faraday's law, passed from an average of 94%-95% (with a maximum value of 96%) for the process carried out in the absence of the oxidation unit to a value stably equal to or higher than 98% in the presence of the oxidation unit according to the present invention.

Example 2

[0148] The test of example 1 was repeated by adjusting the feeding flow rate of the KMnO.sub.4 solution to the leachate so as to maintain the redox potential constantly at the optimal Redox.sub.ppt value determined on the basis of the pH and T values of the leachate. For this purpose, a calibration curve was prepared by titrating 3 aliquots of leachate with a solution of KMnO.sub.4 (3.16 g/l), each at pH=5.2, 6.0 and 7.0 and temperature=60° C., 70° C. and 80° C.

[0149] The following table shows the experimental Redox.sub.ppt value (end-of-titration points) obtained for each sample.

TABLE-US-00001 TABLE Calibration curve of Redox.sub.ppt = f(pH, T) pH 5.2 6.0 7.0 Temp. 60° C. 602 521 443 70° C. 572 491 420 80° C. 583 496 404

[0150] The experimental Redox.sub.ppt values were mathematically interpolated by means of a polynomial function obtaining a calibration curve Redox.sub.ppt=f(pH, T), with which the pump control unit was programmed. By performing the dosage of the permanganate ions by continuously adjusting the redox potential value of the leachate to the Redox.sub.ppt value, it was possible to feed a leachate containing a Mn concentration of around 0.2 mg/L to the electrolysis cell. Under these conditions zinc was electrodeposited with a current efficiency equal to 99.2%. Moreover, the electrolysis solution showed no traces of dust, remaining perfectly clear.