Recovery of zinc by electrochemical remediation of zinc oxide-containing waste
Advancing the chemical engineering fundamentals
Electrochemical Engineering (T2-14P)
Keywords: Metal recovery,electroleaching,electrodeposition, zinc oxide, sulphate media
Industrial activity generates large amounts of solid waste containing heavy metals. In particular, steel processing is the source of tremendous amounts of zinc oxide, which need to be treated to avoid environmental issues and recovery of the metal. The present work deals with zinc beneficiation of industrial waste by electrochemical remediation using devices of simple, reliable technology. The technique is based on application of an electrical field to the solid waste to be treated. Protons are generated at the anode by water oxidation and migrate toward the cathode: they pass through the packed bed of metal oxide/salt particles which are placed between the two electrode compartments: the solid is leached by protons and the metal cations formed migrate to the cathode where deposition occurs.
The example worked the recovery of zinc from zinc oxides particles dispersed at 10 wt. % in sand to emulate the treatment of a real industrial waste. Experiments have been carried out in a three-compartment cell separated by polymeric inert cloths with 53 cm2 vertical electrodes for the batch treatment of 147 g ZnO-containing sand in sulphate/sulphuric media. Operation of the overall process involves the occurrence of lumped physicochemical processes: protons generation at the anode, leaching of the oxide, migration of the cations to the cathode, diffusion of the cations to the anode, zinc electrodeposition… Any of these processes have been investigated separately to determine the operating conditions allowing the various constraints of the process to be fulfilled: (i) complete leaching of the reactive solid, (ii) acceptable losses of zinc cations at the anode side, (iii) high deposition efficiency, (iv) formation of compact zinc deposit at the cathode, and (v) low pH throughout the cell to avoid formation of Zn hydroxide, that may occur in case of significant hydrogen evolution.
The electrochemistry of Zn deposition from sulphate media has been investigated in a Hull cell, which led to current density to be applied and the optimal ranges of sulphuric acid and zinc sulphate concentrations in the cathode chamber. The results were then applied to the actual three-compartment cell:
• In a first step, the central compartment was filled by a zinc sulphate/sulphuric solution. Batch runs of migration-electrodeposition were conducted and analysis of the results yielded the suitable initial compositions of the solutions in the three compartments: in particular, slight amounts of zinc cations have to be introduced in the cathode chamber to allow high zinc deposition from the early stages of the run. Stirring the liquid in the cathode chamber prevented from zinc dendrite formation
• Then, electrochemical leaching was coupled to migration and electrode phenomena. Gas sparging and mechanical agitation of the bed allowed better homogenisation of the bed and higher transfer yields. Zinc oxide can be quantitatively leached from the bed and more than 75% of zinc could be recovered in the form of metal at the cathode.
The method is to be applied to real metal oxides such as Waelz oxides. Moreover the technique can be used successfully for the treatment of mixtures of metal oxides, where selective leaching and/or deposition steps have to be carried out for the production of single metals from complex, hardly valuable solids.
Presented Wednesday 19, 13:30 to 15:00, in session Electrochemical Engineering (T2-14P).
