A Cell Design for Nickel Electrowinning from Sulfate Electrolyte

A Cell Design for Nickel Electrowinning from Sulfate Electrolyte

The cell for nickel electrowinning from sulfate electrolytes is usually a divided rectangular tank made from concrete and lined with an acid‐resistant material to contain the electrolyte. The cathodes and anodes are located parallel to each other at a fixed interelectrode pitch of typically 100 mm and immersed in the electrolyte.

Cathodes may be in the form of nickel starter sheets that are typically plated on titanium cathode blanks up to a thickness of 1 mm in a separate cell and stripped from the blank to be used in the electrowinning cell as cathode substrate. More recently, permanent titanium cathodes have been introduced at some plants. Nickel plated on titanium typically develops higher internal stresses compared with that on nickel starter sheets but this offers some advantages in terms the of a number of operating sequences in the tank house, which are reduced to only plating on the titanium cathode, harvesting from the cell, washing and stripping the nickel deposit from the titanium substrate, instead of the additional sequence relative to the starter sheet preparation. Small holes may also be drilled through the substrate so that when filled with nickel deposit, they hold the two sides of the plated nickel together to prevent disbanding of the nickel from the titanium. Cathodes are also typically fitted with plastic edges and bottom strips to permit easy separation of the deposit after electrodeposition.

Insoluble anodes are used for nickel electrowinning from sulfate solutions. Typical anodes used are rolled or cast lead generally alloyed with elements such as antimony, strontium, calcium and tin to provide corrosion resistance and mechanical strength. Lead is generally alloyed with silver and/or calcium for zinc electrowinning and with antimony or calcium, tin and silver for copper electrowinning. The durability of the lead anodes in this aggressive environment is achieved by the formation of a compact and electrically conducting lead dioxide layer on the surface. The cathode and anode compartments are separated by membranes made generally of polyester or woven terylene. The mass transfer of the hydrogen ions generated at the anode to the cathode surface is limited by the membranes, by providing a porous barrier to convection and also by allowing a hydrodynamic head to be created by maintaining the height of catholyte slightly higher, e.g., 20 — 25 mm than that of the anolyte so that the solution flows from the cathode compartment through the membrane to the anode compartment.