Improved process for heavy metals electrowinning

Abstract: The conditions for electrowinning Zn, Ni, Co and Cd metals from baths based on chlorinated ammino complexes of Me(NH₃) n Cl m type are substantially improved by the addition of small levels of dissolved Br⁻, which considerably reduce the cell voltage without exerting any negative effects on cathodic current yields.

Description: European Patent Application Publication No. 0 627 503 in the name of the same Applicant, Ecochem Aktiengesellschaft, relates to a process for electrowinning a metal Me selected from Zn, Ni, Co and Cd which yields an ammino-chlorinated complex of Ne(NH 3 ) n Cl m type which, in aqueous solution, is submitted to electrolysis inside a cell not provided with separation diaphragms or membranes between anodes and cathodes, with a pure metal deposit and nitrogen development being obtained at the cathode and at the anode, respectively. Nitrogen derives from the oxidation of ammonia contained in the bath by chlorine formed at anode, provided that the pH value of the bath is kept comprised within the range of from 6 to 8, with consumed ammonia being added to the bath. The present Applicant has surprisingly found now that the process disclosed in said patent application can be improved in terms of reduction in cell voltage, and hence, of energy saving. With the main chemical and electrochemical features of the cited process for electrowinning metals selected from Zn, Ni, Co and Cd remaining the same, the present invention consists in adding low levels of Br - to the electrolyte. The anodic oxidation of bromide into bromine takes place at a voltage which is approximately 300 mV lower than the necessary voltage for chloride conversion into chlorine. The Applicant could also observe that, within the pH range taken into consideration, ammonia contained in the bath is oxidized with N 2 being produced according to the overall reaction: 3Br 2 + 2NH 3 → 6Br - + 2N 2 + 6H + with a very fast kinetics as compared to the analogous reaction disclosed in the above cited European Patent Application Publication No. 0 627 503, i.e. 3Cl 2 + 2NH 3 → 6Cl - + 2N 2 + 6H + . Therefore, the presence of a low level of bromide in the bath (comprised within the range of from 1 to 10 g/l of Br - ) makes it possible the cell voltage to be considerably reduced and consequently a considerable energy saving being achieved in metal electrowinning. In order to illustrate the present invention without limiting it, the following examples are supplied. Example 1 (comparison example) An amount of 250 g of pure zinc oxide was dissolved in 5 l of aqueous solution containing 250 g/l of NH 4 Cl, and the resulting solution was heated up to 60°C. The solution was then fed to an electrolytic cell with a graphite anode and a titanium sheet cathode, without any separator means between anode and cathode. Through this cell, a current of 10 A was flown during 6 hours and the voltage reading at the electrodes was of 2.52 V on an average, with temperature being kept comprised within the range of from 60° ± 2 and the pH value being kept comprised within the range of from 6 to 8, by gradually adding NH 3 solution. An amount of 70.1 g of zinc was obtained with a current efficiency of 95.7%. The calculated d.c. (direct current) energy consumption was of 2.16 Kwh/kg of Zn. Example 2 To a similar solution to the preceding one, 2 g/l of Br - as KBr, heated at 60°C, was added. On dissolution complete, the resulting solution was charged to an electrolytic cell with a graphite anode and a titanium plate cathode, not separated by a membrane diaphragm. Through the cell a current of 10 A was flown during 6 hours, at an average voltage value of 2.27 V. The solution was kept at 60 + 2/-0°C and at pH of 6-8 by means of the addition of a total amount of 38 g of 31% NH 3 solution. An amount of 69.5 g of Zn was obtained, with a current efficiency of 94.9%. The calculated d.c. energy consumption was of 1.96 Kwh/kg of Zn. Example 3 An amount of 500 g of technical ZnO, with commercial purity, was dissolved in 10 l of an aqueous solution containing 250 g of NH 4 Cl per litre and 10 g of Br - per litre, and heated at 60°C. After being purified with 2.5 g of Zn powder, which cements the small impurities of Cu, Pb and Cd, always present in commercial oxides, the solution was sent, after being preliminarily filtered, to the electrolytic cell of the preceding example, kept at 62 ±2°C throughout the test run. A current of 10 A was flown during 24 hours and the average voltage reading through the cell was of 2.21 V. During the test, 152 g of an aqueous solution containing 31% NH 3 was gradually added in order to keep the pH value of the bath comprised within the range of from 6 to 8. The deposited Zn amount was of 280.5 g. The current efficiency was 95.8%. The calculated d.c. energy consumption was of 1.89 Kwh/kg of Zn. From the above examples, it will be seen that already with the introduction of 2 g/l of Br - , an energy saving of 0.2 Kwh of zinc is obtained, whilst when 10 g/l of Br - is present, an energy saving of 0.27 Kwh/kg of zinc is obtained. Larger amounts of Br - ion do not cause any further decrease in cell voltage.

Claims: Process for producing metals Me selected from zinc, nickel, cadmium and cobalt, in which the corresponding water-soluble ammino complex Me(NH 3 ) n Cl m is formed, and said complex is submitted, in aqueous solution, to electrolysis inside a cell with no separation means between anode and cathode, characterised in that to said aqueous solution, bromide ion is added, and in that during said electrolysis the pH value is kept within the range of about from 6 to 8. Process according to claim 1, characterized in that said bromide ion is added in a concentration comprised within the range of from 1 to 10 g/l. Process according to claim 1, characterized in that said ammino complex Ne(NH 3 ) n Cl m is directly submitted to electrolysis. Process according to claim 1, characterized in that said ammino complex is formed by reacting a suitable compound of said metal and ammonia, or ammonium chloride, and the so formed ammino complex is submitted to said electrolysis. Process according to claim 1, characterized in that during said electrolysis at cathode said metal Me is deposited with NH 3 being released, at the anode bromide is oxidized into elemental bromine and the latter reacts with said ammonia released at the cathode and migrated to the region surrounding the anode according to the reaction: 3Br 2 + 2NH 3 → 2N 2 + 6Br - + 6H + with N 2 thus being developed at the anode. Process according to claim 5, characterized in that said ammonia oxidized into nitrogen gas is replenished in the electrolyte.

Inventors: ZOPPI GIANNI, ZOPPI, GIANNI
Applicants: ECOCHEM AG, ECOCHEM AKTIENGESELLSCHAFT
Application year: 1995
Main classification: C25C1
IPC codes: C25C1