Fundamental aspects of nickel electrowinning from chloride electrolytes

Nickel electrowinning from chloride electrolytes is an innovative and efficient process developed and commercialized mainly by Falconbridge Ltd. Several fundamental aspects related to this process have been addressed in this thesis, including the thermodynamic study of nickel electrolytes, the measurement and modelling of the cathode surface pH during nickel electrowinning and the kinetic study of nickel reduction and hydrogen evolution. The major apparatus and equipment used include a surface pH measuring device, an EG&G rotating disc electrode, a SOLARTRON 1286 Electrochemical Interface and a RADIOMETER titrator system. All of the experiments were carried out via computer control. The thermodynamic study includes the activity coefficient of the hydrogen ion and the spe ciation of nickel electrolytes to obtain a better understanding of the properties of nickel electrolytes. The activity coefficient of the hydrogen ion (yH⁺) was measured using a combination glass pH electrode. It was found that yH⁺ was greater than 1 in concentrated NiCl₂ solutions and increased significantly with increasing NiCl₂ concentration. The addition of NaC1 increases yH⁺, whereas the addition of Na₂SO₄ decreases it. Theoretically, several useful equations were derived based on Meissner’ sand Stokes-Robinson’s theories to calculate the single-ion activity coefficients including yH⁺. These equations are the two-parameter (q and h) functions, capable of predicting with rea sonable accuracy single-ion activity coefficients in any concentrated pure electrolytes and in mixed electrolytes of the type 1:1 + 1:1, 2:1 + 1:1 and 2:1 + 1:1 + 1:1. The accuracy of the calculations may be further improved when the Meissner parameter q is adjusted properly and the effect of ionic strength on the hydration parameter h is taken into account. A series of speciation diagrams for nickel species was plotted with yH⁺ and the effect of the ionic strength on the equilibrium constants being taken into account. It was discovered that the predominant nickel species in the acidic region are Ni²⁺ and NiCl⁺ in concentrated pure NiCl₂ solutions and Ni²⁺, NiCl⁺ and NiSO₄ in concentrated sulfate-containing NiCl₂ solutions. The traditionally accepted electroactive species NiOH is negligible until the NiCl2 concentration is lowered to the order of 10⁻⁶ M. When the pH increases, the formation of insoluble Ni(OH)₂(s) should be expected if the NiCl₂concentration is higher than 10⁻⁶ M. The pH where Ni(OH)₂(s) starts to form decreases with increasing NiCl₂ concentration and temperature. A limited number of electrowinning tests were carried out under conditions similar to those employed in the industrial process in order to obtain information concerning the current efficiency of nickel deposition. It was found that higher nickel concentration, higher pH and the addition of NaCl, H₃B0₃ and NH₄C1 improved the current efficiency ofnickel deposition. However, the addition of sulfate decreased the current efficiency of nickel. In 0.937 M NiCl₂ at 60°C, the pH may go as low as 1.5 for a current efficiency above 96 %. Nickel deposition was also found to be a steady-state process since the amount of acid added to the electrolyte at a constant pH increased linearly with time. To acquire data on the cathode pH behaviour during nickel deposition, the cathode surface pH was measured using a flat-bottom combination glass pH electrode and a fine mesh gold gauze as cathode. Nickel was deposited on the front side of the gold gauze and the pH electrode was positioned in the back and in direct contact with the nickel-plated gold gauze. The cathode surface pH was always found to be higher than the pH in the bulk electrolyte, and if the current density was suf- ficiently large, it would eventually reach a level causing precipitation of insoluble Ni(OH)₂(s) on the cathode surface. Lower bulk pH, higher nickel concentration, higher temperature and the addition of H₃B0₃ and NH₄Cl effectively depress the rise of the cathode surface pH. Additions of NaCl and Na₂SO₄ also depress the rise of the cathode surface pH but to a much smaller degree. Also, agitation of the electrolyte decreases the cathode surface pH. In order to predict the cathode surface pH, mathematical modelling in the case of 0.937 M NiCl₂ and 2 M NiCl₂ was carried out. The model was in reasonably good agreement with the experimental data. Nickel deposition and hydrogen evolution were studied using a rotating disc electrode. The hydrogen evolution was found to be affected strongly by the RPM. The rate of nickel deposition was first order with respect to the activity of nickel ion and zero order with respect to the activities of chloride and hydrogen ions. The rate of hydrogen evolution was found to be first order with respect to the activity of hydrogen ion and to be zero order with respect to the activities of nickel and chloride ions. These findings indicate that nickel deposition and hydrogen evolution proceed independently. The Tafel slopes obtained from the partial polarization curves were 94 mV/decade for nickel deposition and 112 mV/decade for hydrogen evolution. Hydrogen evolution was also studied using a rotating nickel-coated Pt disc electrode in 2.5 M NaCl solution in the absence of nickel ions. The rate of hydrogen evolution was first order with respect to the activity of hydrogen ion and zero order with respect to the activity of chloride ion. According to the relationship between the limiting current density and the square root of rotational speed, hydrogen evolution was mass transfer controlled under the limiting conditions and the buffering actions of H₃B0₃ and NH₄Cl were negligible. The magnitude of the limiting current density at a given pH or a given acidity in the presence of sulfate can be well explained considering the activity coefficient of the hydrogen ion. Further studies of nickel electrowinning should be directed towards hydrogen evolution on the nickel substrate in nickel-containing electrolytes, focusing on the hydrogen bubble’s nucleation, growth, coalescence and detachment. The use of addition agents affecting hydrogen evolution by way of adsorption, change in interfaciai tension or destruction of atomic hydrogen is worth investigating. The identity of intermediate species during nickel reduction is not clear. The identification of these species would be quite rewarding in clarifying the mechanism of nickel reduction. The nucleation of nickel and crystal growth in the initial stages of deposition on various substrates including titanium, stainless steel, copper and nickel are other important aspects of nickel electrowinning which should be investigated.

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