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Content archived on 2022-12-27

TREATMENT OF ELECTROLYTES FROM A ZINC ELECTROLYSIS PLANT BY EED (ELECTRO-ELECTRO-DIALYSIS)

Objective

To build and operate a dialysis cell of industrial size together with the necessary ancillary equipment to test the EED process in long term commercial use. The EED allows a higher yield of zinc connected with considerable energy savings for removal of magnesium from the electrolyte compared with alternative possibilities.
Important technical know-how for electrolytic processes using membranes was generated. Among others the cell with compartments for cathodes and anodes and the membranes fixing system had to be designed and materials and membranes chosen. The membrane IONAC MA 3475 from SYBRON CHEMICALS gave satisfactory results.
Two major technical problems had to be solved:
- insufficient stability of the membrane support grid at the anode side. Newly designed support grids have been constructed and showed a sufficient stability.
- extremely heavy MnO2 deposits at the anodes resulting in obstruction or damming of the liquid stream between anode and membrane and in damaging the membrane structure. An extensive reduction of the MnO2 formation was achieved by introduction of a separate loop for the anolyte by which the Mn concentration can be kept much lower. Impurities introduced by this additional loop did not effect the process, but it was not possible to demonstrate this over the full lifetime of the membranes.
Generally the technical results of the EED including energy consumption are in accordance with the previous pilot and laboratory work, but the process became more expensive and more complicated by the above mentioned additional loop for the anolyte..
Among other alternatives the best process for treating the catholyte from the EED was established to include a selective zinc precipitation and a manganese precipitation as MnS, followed by precipitation of the remaining magnesium.
From the technical point of view it was shown that the treatment of electrolyte by EED and subsequent treatment of the catholyte to maintain the Mg at an acceptable level in a zinc hydrometallurgical plant without generating large amounts of unacceptable residues is now technically reliable.
Economically the process suffered much by the need of additional and more expensive equipment and added operation costs. Calculations of investment and operating costs are given for easy comparison withthe costs of disposing Mg by other means, which often generate large amounts of residue. The future application of the process is only a matter of disposal costs for these unacceptable residues.
Preussag-Weser-Zink GmbH operates in Nordenham (Germany) a plant for the hydrometallurgical-electrolytic production of zinc with a capacity of 110 000 tons of electrolytic zinc per year.
During the electrolysis an enrichment of the magnesium content of the electrolyte taken place. To limit this enrichment, a special treatment of a part of the electrolyte stream is necessary. Per ton of produced zinc generally 0.1 to 0.2 m3 of electrolyte are subjected to this treatment which consists of neutralizing the electrolyte with zinc. This leads to the formation of 30 to 70 Kg per ton of produced zinc, which is costly and energy intensive to dispose of.
Within the framework of this project it is intended to subject a part of the magnesium containing neutral zinc sulfate (neutral lye) as a catholyte to an Electro-Electro-Dialysis (EED). In the EED more than 80% of the zinc is separated in the usual quality at the cathode while a corresponding part of sulfate ions go into the anolyte and arerecirculated into the process.
The zinc which has and been separated at the cathode in the EED is recovered in a second process step by selective precipitation.
EED was developed in the research institute of Minemet in France and pilot testing took place at Preussag-Weser-Zink GmbH, during 12 months. The pilot plant consisted of 2-3 dialysis cells producing daily 3 Kg of zinc per cell. Results from the pilot trials confirmed the previous laboratory work.
The demonstration plant consisted of a dialysis cell with five industrially sized cathodes of 1,2 m2 active surface and additional equipment for the treatment of the catholyte by selective precipitation. The production capacity of the demonstration plant was 50 kg zinc per day. From the laboratory work and the previous pilot tests for a 110,000.
For a 110,000 tons zinc producing plant the estimated energy saving amounts to 1,400 TOE/year, in addition to which 91 000 000 000 KJ/a of primary energy are substituted with 32.3 000 000 000 KJ/a of electrical energy.
On the basis of the above saving, the cost of handling 1 m3 of the electrolyte solution is calculated to be DM 168. compared to the current disposal cost (to a third party) of DM 198.
The process is covered by a joint patent and a cooperation contract covers the relationship between Minemet and Preussag.

Call for proposal

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Coordinator

PREUSSAG-WESER-ZINK GMBH
EU contribution
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Address
RAMMELSBERGER STRASSE 2 AV. A. EINSTEIN
3380 GOSSLAR
Germany

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Total cost
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