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Process for Heavy Metal Electrowinning Verfahren Zur Elektrogewinnung Von Schweren Metallen Procede D'electroextraction De Metaux Lourds

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Process for Heavy Metal Electrowinning Verfahren Zur Elektrogewinnung Von Schweren Metallen Procede D'electroextraction De Metaux Lourds Europaisches Patentamt (19) European Patent Office Office europeenpeen des brevets EP 0 627 503 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) intci.6: C25C 1/08, C25C 1/16 of the grant of the patent: 01.09.1999 Bulletin 1999/35 (21) Application number: 94201123.0 (22) Date of filing: 23.04.1994 (54) Process for heavy metal electrowinning Verfahren zur Elektrogewinnung von schweren Metallen Procede d'electroextraction de metaux lourds (84) Designated Contracting States: (72) Inventor: Zoppi, Gianni BE DE ES FR GB IT NL CH - 6967 Dino Di Sonvico (CH) (30) Priority: 03.05.1993 CH 35293 (74) Representative: Fusina, Gerolamo et al Ing. Barzano & Zanardo Milano S.p.A, (43) Date of publication of application: Via Borgonuovo, 10 07.12.1994 Bulletin 1994/49 20121 Milano (IT) (73) Proprietor: ECOCHEM AKTIENGESELLSCHAFT (56) References cited: FL-9495 Triesen (LI) EP-A- 0 486 187 DE-A- 2 739 970 FR-A- 936 742 DO CO o CM CO Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice the Patent Office of the Notice of shall be filed in o to European opposition to European patent granted. opposition a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. a. 99(1) European Patent Convention). LU Printed by Jouve, 75001 PARIS (FR) 1 EP 0 627 503 B1 2 Description because it is necessary that the same process can make use of the chlorine it produces. [0001] It is known that, in general, in the electrolysis [0010] This is the case, for example, in the Falcon- of aqueous solutions of chlorides, at the anode chlorine bridge process, which produces electrolytic nickel from is developed, and the cathodic reaction can either be 5 aqueous solutions of chlorides and uses chlorine in or- the development of hydrogen with production of alkalin- der to oxidize the ore. ity, or the precipitation of the metal, according to the po- [0011] In general, according to the prior art, the elec- sition the latter occupies in the series of the electro- trolysis of the aqueous solutions of heavy metal chlo- chemical potentials, according to the following reac- rides did not enjoy those important industrial applica- tions: 10 tions which its potentialities would deserve thanks to the anodic reaction: advantages it offers on energy side, due to the high con- ductivity of chloride solutions, as well as thanks to the anodic potential of chlorine development being lower CI" - e ^ 1/2 Cl2 than of oxygen development. is [0012] The alternative solutions to the anodic chlorine cathodic reaction: development adopted heretofore are, e.g. , the oxidation of Fe2+ to Fe3+, or of Cu+ to Cu2+ which, by occurring at a lower potential than of chlorine development reaction, Me+ MeOH V* + e + H20 -> + H2 avoid the production of the latter, and offer an advantage 20 as regards the cell voltage. An example is the Clear or process, according to which in the cathodic compart- ment Cu is deposited, and at the anode iron and copper are oxidized: these, in their turn, are used in order to Me+ + e -> Me oxidize chalcopyrite, converting sulphide into elemental 25 sulphur and dissolving copper. [0002] At acidic pH values, chlorine gas is developed. [0013] Another solution adopted is of using in the [0003] Under neutral or alkaline pH conditions, chlo- anodic compartment a solution of an oxyacid, e.g., sul- rine, owing to the increase in its water solubility, causes, phuric acid. In this case, in order to separate the anodic by dismutation, the formation of hypochlorite and other from cathodic compartment, an ionic membrane, and oxygen-containing compounds, such as chlorate and 30 the anodic reaction turns into a water oxidation one: perchlorate. [0004] In the case of alkali-metal chlorides at pH<4, 2e 1/2 2H+ chlorine is produced, and at higher pH value alkali-metal H20 - -> 02 + hypochlorites or, in the case of higher anodic potentials, alkali-metal chlorates and perchlorates are produced. 35 [001 4] At the anode oxygen is developed and H+ ions [0005] Large amounts of chemical products are man- through the membrane, reach the cathodic compart- ufactured by this route. ment. [0006] In the case of heavy metal chlorides (Cu, Co, [0015] DE 27 39 970 discloses a cell suitable for the Ni, Zn, Cd, Pb etc.), at a relatively acidic pH the metal electrowinning of heavy metals and actually used for the is deposited at the cathode and chlorine is developed at 40 electrowinning of Zn, which has no separation means the anode. between the anode and cathode. [0007] The anodic compartment of the cell must be [0016] FR 936 742 discloses a process for the elec- kept separated from the cathodic compartment by trowinning of Co from a solution containing Ni, whereby means of a diaphragm or a membrane, and said anodic ammonia-containing complexes are formed in aqueous comportament should be closed in order to make it pos- 45 solution. sible for pure chlorine to be collected, first of all in order [0017] EP 0 486 187 discloses a cell and a method to prevent a so toxicant gas from getting dispersed in for employing the same in the electrowinning of heavy the environment, and, furthermore, in order to prevent metals (copper, nickel, cobalt and the like) from baths chlorine from coming, by diffusion, into contact with the such as etchants, electroless plating baths and the like. deposited metal, dissolving it. 50 [0018] Summarizing the present state of the art of [0008] The split cell, the use of which is mandatory for metal electro winning from chloride solutions, one may these kind of processes, adds a considerable complica- state that, in the case of chlorine production, as well as tion to the electrolysis facility and, in the event when an in the case of alternative anodic reaction, a cell split by ionic membrane is used in order to separate the com- a diaphragm or an ionic membrane should be always partments, it also implies a very high equipment cost. 55 used, with all of the facility complications and the higher [0009] The production of chlorine, parallel to metal costs involved by such a structure. production, constitutes another limitation to the applica- [001 9] The present invention aims at producing metal tion of the electrolysis of chlorides for producing metals, by electrolysis from aqueous solutions, overcoming the 2 3 EP 0 627 503 B1 4 drawbacks displayed by the technology known from the [0029] Another feature of the process is that, with the prior art, which is reminded above. electrolysis occurring at pH values of round 7, the metal [0020] Such a purpose is achieved according to the deposition takes place under much more competitive present invention with a process according to claim 1 . potential conditions than the alternative reaction of hy- [0021] Beside the simplifications as regards the 5 drogen development, with benefits as regards the cur- equipment and the easier facility operations, the proc- rent efficiency. ess according to the present invention makes it possible [0030] The decreased cell voltage and the higher cur- the current efficiency values to be increased and the cell rent efficiency contribute to reduce the energy con- voltage to be reduced, and, consequently, a considera- sumption in metal winning. ble reduction to be attained in energy consumptions per 10 [0031] Another object of the present invention is a each unit of metal produced. suitable facility for implementing the above defined [0022] These considerable advantages and improve- process, which comprises a non-split electolytic cell, e. ments can be obtained according to the present inven- g., one in which the anode and the cathode are not pro- tion as defined in the claims. vided with separation means, such as a diaphragm or a [0023] To the solution containing the chloride of the is membrane means, between both cell compartments. metal to be produced, ammonia and ammonium chlo- [0032] In order to better disclose characteristics and ride is added in order to form the ammino complex of advantages of the invention, an exemplifying, non-limi- Me(NH3)nClm type, which prevents the metal hydroxide tative embodiment thereof is reported in the following. precipitation. [0024] The chloro-ammino complex is thus dissociat- 20 Example: ed into [Me(NH3)n]m+ and mCI\ [0025] When the thus obtained solution is submitted [0033] An amount of 500 g of technical zinc oxide with to electrolysis, at the cathode the metal is deposited and commercial purity was dissolved in 10 I of an aqueous ammonia is liberated from the complex, at the anode the solution with 250 g/l of NH4CI, at the temperature of chloride is oxidized to chlorine, but the resulting chlorine 25 60°C. reacts in the vicinity of the same anode with the ammo- [0034] At reaction end, with all oxide having been dis- nia released and migrated from the anodic region, oxi- solved, 2.5 g of zinc powder was added in order to ce- dizing it to nitrogen, according to the reaction: ment any impurities of Cu, Pb and Cd contained in a small amounts in the oxide. 30 [0035] The purified solution was then circulated at 3CI2 + 2NH3 -> N2 +6HCI 60°C inside a non-split electrolytic cell which contained a cathode consisting of a titanium plate between two in- or soluble anodes of graphite, wherein said solution was kept vigorously stirred by means of air blown under the 35 cathode. 3CI2 +2NH4CI -> N2 +8HCI [0036] By causing a current of 20 A to flow with an initial voltage of 2.7 V (2.85 V under steady-state con- [0026] Thus, elemental nitrogen is developed instead ditions) during 10 hours, 229.6 g of pure zinc was de- of chlorine.
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