Improved Process for Heavy Metals Electrowinning

Patentamt Europaisches ||| || 1 1| || || || || || || || ||| ||| || (19) J European Patent Office

Office europeen des brevets (11) EP 0 704 557 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) int. CI.6: C25C 1/08, C25C 1/16, 03.04.1996 Bulletin 1996/14 C25C1/12 (21) Application number: 95202196.2

(22) Date of filing: 12.08.1995

(84) Designated Contracting States: (72) Inventor: Zoppi, Gianni BE DE ES FR GB IT NL CH-6967 Dino Di Sonvico (CH)

(30) Priority: 08.09.1994 CH 2746/94 (74) Representative: Appoloni, Romano et al ING. BARZANO' & ZANARDO MILANO S.p.A. (71) Applicant: ECOCHEM AKTIENGESELLSCHAFT via Borgonuovo 10 FL-9495 Triesen (LI) 1-20121 Milano (IT)

(54) Improved process for heavy metals electrowinning

(57) The conditions for electrowinning Zn, Ni, Co and Cd metals from baths based on chlorinated ammino complexes of Me(NH3)nClm type are substantially improved by the addition of small levels of dissolved Br~, which considerably reduce the cell voltage without exert- ing any negative effects on cathodic current yields.

LO LO o r»- o Q_ LU Printed by Rank Xerox (UK) Business Services 2.9.15/3.4 1 EP 0 704 557 A1 2

Description 2.52 V on an average, with temperature being kept comprised within the range of from 60° ± 2 and the pH value European Patent Application Publication No. 0 627 being kept comprised within the range of from 6 to 8, by 503 in the name of the same Applicant, Ecochem gradually adding NH3 solution. Aktiengesellschaft, relates to a process for electrowin- s An amount of 70.1 g of zinc was obtained with a cur- ning a metal Me selected from Zn, Ni, Co and Cd which rent efficiency of 95.7%. yields an ammino-chlorinated complex of Me(NH3)nClm The calculated d.c. (direct current) energy consump- type which, in aqueous solution, is submitted to electrol- tion was of 2.16 Kwh/kg of Zn. ysis inside a cell not provided with separation dia- phragms or membranes between anodes and cathodes, 10 Example 2 with a pure metal deposit and nitrogen development being obtained at the cathode and at the anode, respec- To a similar solution to the preceding one, 2 g/l of tively. Nitrogen derives from the oxidation of ammonia Br" as KBr, heated at 60°C, was added. contained in the bath by chlorine formed at anode, pro- On dissolution complete, the resulting solution was vided that the pH value of the bath is kept comprised is charged to an electrolytic cell with a graphite anode and within the range of from 6 to 8, with consumed ammonia a titanium plate cathode, not separated by a membrane being added to the bath. diaphragm. The present Applicant has surprisingly found now Through the cell a current of 10 A was flown during that the process disclosed in said patent application can 6 hours, at an average voltage value of 2.27 V. be improved in terms of reduction in cell voltage, and 20 The solution was kept at 60 + 2/-0°C and at pH of 6- hence, of energy saving. 8 by means of the addition of a total amount of 38 g of With the main chemical and electrochemical fea- 31% NH3 solution. tures of the cited process for electrowinning metals An amount of 69.5 g of Zn was obtained, with a cur- selected from Zn, Ni, Co and Cd remaining the same, rent efficiency of 94.9%. the present invention consists in adding low levels of Br~ 25 The calculated d.c. energy consumption was of 1 .96 to the electrolite. The anodic oxidation of bromide into Kwh/kg of Zn. bromine takes place at a voltage which is approximately 300 mV lower than the necessary voltage for chloride Example 3 conversion into chlorine. The Applicant could also observe that, within the pH range taken into considera- 30 An amount of 500 g of technical ZnO, with commer- tion, ammonia contained in the bath is oxidized with N2 cial purity, was dissolved in 10 I of an aqueous solution being produced according to the overall reaction: containing 250 g of NH4CI per litre and 10 g of Br" per litre, and heated at 60°C. 3Br2 + 2NH3 --> 6Br- + 2N2 + 6H* After being purified with 2.5 g of Zn powder, which 35 cements the small impurities of Cu, Pb and Cd, always with a very fast kinetics as compared to the analogous present in commercial oxides, the solution was sent, reaction disclosed in the above cited European Patent after being preliminarily filtered, to the electrolytic cell of Application Publication No. 0 627 503, i.e. the preceding example, kept at 62 ±2°C throughout the test run. 3CI2 + 2NH3 --> 6CI- + 2N2 + 6H*. 40 A current of 1 0 A was flown during 24 hours and the average voltage reading through the cell was of 2.21 V. Therefore, the presence of a low level of bromide in During the test, 152 g of an aqueous solution con- the bath (comprised within the range of from 1 to 1 0 g/l taining 31% NH3 was gradually added in order to keep of Br") makes it possible the cell voltage to be consider- the pH value of the bath comprised within the range of ably reduced and consequently a considerable energy 45 from 6 to 8. saving being achieved in metal electrowinning. The deposited Zn amount was of 280.5 g. In order to illustrate the present invention without lim- The current efficiency was 95.8%. iting it, the following examples are supplied. The calculated d.c. energy consumption was of 1 .89 Kwh/kg of Zn. Example 1 (comparison example) 50so From the above examples, it will be seen that already with the introduction of 2 g/l of Br", an energy saving of An amount of 250 g of pure zinc oxide was dissolved 0.2 Kwh of zinc is obtained, whilst when 10 g/l of Br" is in 5 I of aqueous solution containing 250 g/l of NH4CI, present, an energy saving of 0.27 Kwh/kg of zinc is and the resulting solution was heated up to 60°C. obtained. The solution was then fed to an electrolytic cell with 55ss Larger amounts of Br" ion do not cause any further a graphite anode and a titanium sheet cathode, without decrease in cell voltage. 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 3 EP 0 704 557 A1

Claims

1 . Process for producing Me metals selected from cop- per, zinc, nickel, cadmium and cobalt, in which the corresponding water-soluble ammino complex s Me(NH3)nClm is formed and said complex is submitted, in aqueous solution, to electrolysis inside a cell with no separation means between anode and cathode, characterized in that to said aqueous solution, bromide ion is added. to

2. 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. 15 3. Process according to claim 1, characterized in that said ammino complex Me(NH3)nClm is directly submitted to electrolysis.

4. Process according to claim 1, characterized in that 20 said ammino complex is formed by reacting a suitable compound of said metal and ammonia, or ammo- nium chloride, and the so formed ammino complex is submitted to said electrolysis. 25 5. Process according to claim 1, characterized in that during said electrolysis at cathode said metal Me is deposited with NH3 being released, at the anode bromide is oxidized into elemental bromine and the latter reacts with said ammonia released at the cath- 30 ode and migrated to the region surrounding the anode according to the reaction:

3Br2 + 2NH3 -- > 2N2 + 6Br" + 6H* 35 with N2 thus being developed at the anode.

6. Process according to claim 5, characterized in that said ammonia oxidized into nitrogen gas is replen- ished in the electrolyte by keeping the pH value con- 40 stantly controlled within the approximate pH range of from 6 to 8.

7. Suitable facility for carrying out the process according to one or more of the preceding claims, charac- 45 terized in that it comprises an electrolytic cell without any separation means between anode and cathode.

55 EP 0 704 557 A1

Patent ^ European EUROPEAN SEARCH REPORT AppUC*,i0B Office EP 95 20 2196

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (Int.C1.6) GB-A-2 163 101 (HANS HOLLMULLER C25C1/08 MASCHINENBAU GMBH) 19 February 1986 C25C1/16 * page 1, line 36 - page 2, line 16 * C25C1/12

DE-A-27 39 970 (SOCIETE D' ETUDES POUR LA RECUPERATION ELECTROLYTIQUE DU ZINC) 16 March 1978 * page 14, line1 ine 4 - page 15, line 4 * * page 26, line1 ine 1 - page 27, line 11 * * page 29, line1 ine 26 - page 30, line 6 * * page 31, line1 ine 23 - line 36 *

TECHNICAL FIELDS SEARCHED (Int.CI.6) C25C

The present search report has been drawn up for all claims Place of search Date of conpletioa of the search THE HAGUE 19 January 1996 Groseiller, P CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document