Electrochemical recovery of nickel from nickel sulfamate plating effluents

Acidic aqueous nickel(II) sulfamate solutions are widely used for industrial nickel electroplating. The finite lifetimes of these baths are caused by the hydrolysis of sulfamate ions to ammonium ions which raise the stress in the nickel deposits. Spent solutions require treatment to recover the NiII, typically present at ca. 10³ mol m-³ , because consent concentrations for discharge to sewers are ca. 2x10-² mol m-³ . The aim of this project was to develop an electrochemical process and suitable control methods for treating the concentrated effluents by electrodeposition of NiII. The electrochemical reactor designed with a nickel mesh cathode, a Pt/Ti oxygen-evolving anode, and a cation-permeable membrane, was operated at constant current in batch-recycle mode. Elemental nickel was electrodeposited onto the cathode from the catholyte-effluent, separated from the aqueous sodium sulfate anolyte by the membrane, which prevented the oxidation of the sulfamate ions in the effluent and thus restricted the anodic reaction to the evolution of O2. Experiments with this system demonstrated that NiII could be recovered at current efficiencies greater than 90 % if the catholyte-effluent pH was maintained in the range 2.5 - 4.5 and if the magnitude of the applied current was regulated to be smaller than the mass transport limited NiII reduction current. Continuous additions of NaOH into the anolyte during reactor operation effectively decreased the rate of proton migration through the membrane from anolyte to catholyte and prevented the catholyte pH decreasing, so minimising the hydrogen evolution rate, the primary cause of the current efficiency always being less than 100 %. Increase in catholyte pH to values above 4.5 occured when effluents were contaminated with iron(III), present as either Fe2O3 or Fe(OH)3, the reduction of which consumed protons and caused cathode passivation due to formation of Ni(OH)2. The magnitude of the applied current was adjusted during the NiII depletion process to ensure it did not exceed the mass transport limited value for a given solution flow rate at any time. km(NiII) values, used to predict the limiting current densities, were derived from results of separate experiments in which mass transport limited reduction current densities of hexacyanoferrate(III) ions were measured as a function of flow rate in alkaline solution. Using these control methods, which were optimised using a reactor model incorporating mass balances, NiII concentrations in the effluent could be decreased by over 90 %. Using the typically measured average current efficiency of 95 % and the cell potential difference, typically 4.5 V in the presence of a Pt/Ti mesh anode, the specific electrical energy consumption was evaluated as ca. 4 300 kW h (tonne Ni)-¹, equivalent to ca. $650 (tonne Ni)-¹, which is two orders of magnitude lower than the price of elemental nickel, currently $22k tonne-¹. A micro-kinetic model was developed to describe the reduction mechanisms of nickel(II) and protons, considered as sequential one-electron charge transfers via separate adsorbed intermediates, as functions of electrode potential, ion concentrations and mass transport rates. This was coupled with the reactor model to facilitate predictions of reactor performance, based on the kinetic rate coe fficients and transfer coefficients. In order to determine these kinetic parameters the kinetics of nickel(II) and proton / water reduction were measured in sulfamate solutions on a rotating Ni disc electrode and on a Au EQCM as a function of NiII concentration, bulk pH, electrode potential and rotation rate.EThOS - Electronic Theses Online ServiceGBUnited Kingdo