Electrowinning Process: Part Two
The electrowinning process is a key method for precious metal recovery and can mean big business especially when processing gold plated components.
The gold is actually recovered from aurodicyanide solutions produced by the elution of loaded carbon and this is responsible for driving the resulting electrochemical reactions.
Electrowinning is an attractive method to recover gold from the relatively concentrated aurodicyanide solutions produced by the elution of loaded carbon, provided that the plated gold can be easily removed using high pressure water sprays.
The following electrochemical reactions are of relevance during the gold electrowinning process (E values quoted for metal ion concentrations of 10-4 mol.dm-3, NaCN concentration of 0.2% and NaOH concentrations of 2%):
The gold, which is present in the solution in the form of aurodicyanide (Au(CN)2-), is reduced to metallic gold, according to Reaction [1] at potentials more negative than the reversible potential. Reaction [2], representing oxygen reduction in alkaline solutions, is another cathodic reaction competing with gold deposition. This is mainly because the electrolyte is likely to be saturated with oxygen due to the oxygen evolution occurring at the anode. Reaction [3] represents the evolution of hydrogen, which occurs at a significant rate at potentials more negative than -0.96 VSHE when working at pH values above 10.
The evolution of hydrogen should be under kinetic control at a significant range of potentials more negative than -0.96 VSHE and therefore consumes a high proportion of the cathodic current. The major anodic reaction is the oxidation of water to oxygen, also indicated by Reaction [2].
The reduction reaction of cuprous cyanide (Cu(CN)32-) to metallic copper is indicated in Reaction [4]. The more negative potential for the reduction of cuprous cyanide to metallic copper than that for the reduction of aurodicyanide to metallic gold, signifies that gold should plate preferentially to copper at these conditions. However, copper may codeposit with gold if high overpotentials are applied, or when the copper concentration is high relative to that of gold.
On the other hand, the Ezinex process has been commercialized in Italy for the electrowinning of zinc from zinc ammonia chloride solutions. In this process, zinc is electrodeposited at the cathode and ammonia is oxidized at the anode. Titanium cathodes are used for zinc plating and graphite electrodes are used for anodic oxidation of ammonia. The electro-deposition of nickel from ammonium chloride solutions may offer alternatives to diminish the hydrogen reduction on acid sulfate or acid chloride electrowinning. The nickel deposition at the cathode can be expressed by the reaction [5]:
In R. Cruz-Gaona et al. paper a fundamental electrochemistry study of cathode processes during the electrowinning of nickel from ammonia-ammonium chloride solutions is presented. Several solution compositions were used in this study. The base composition was 0.2 M NiCl2, 4 M NH4Cl, with variable amount of NH3 (added as NH4OH). An analysis of the effect of cathode substrate was carried out by cyclic voltammetry, this analysis showed that when a titanium electrode was used better deposit properties and electrochemical behavior was obtained than when platinum and glassy carbon were used.
The effect of the ammonium chloride on the electrochemical behavior of nickel from chloride medium was studied (Figure 1). From this analysis was observed that electrodeposition of nickel from ammonium chloride solution exhibits important advantages over the electrodeposition process from simple chloride solutions.
The more relevant feature was the non-formation of the nickel oxide/hydroxide formed in the chloride media (peak A).
This fact is mainly due to the formation of nickel complexes and the stabilization of the protons by ammonia. This would be expected to lead to high current efficiency in the nickel electrowinning from ammonium chloride solutions. The effect of the ammonia concentration and system temperature was determined by voltammetry and chronopotentiometry techniques. This study showed the occurrence of two processes during the electro-reduction process. These processes were considered to be the nickel reduction and the hydrogen evolution.
Figure 1: Cyclic voltammetry of 0.2 M NiCl2 and 0.2 M NiCl2-4M NH4Cl using Titanium electrode. ν = 20 mV/seg.
Finally a study of current density and current efficiency was done at different temperatures and potentials in order to analyze the phenomena involved in the electrowinning process of nickel (Figure 2). From this data they observed that the current density obtained increases as the temperature increases. It was also observed that the current density obtained for each temperature depends on the applied potential, but this does not follow a particular trend (Figure 2). Hence, for 30 and 50°C the minimum current density was obtained at -0.9V, while for at 70°C the maximum current was obtained at this same potential. These results confirm that at least two processes are involved in the electrowinning of nickel, and that the dominant process depends on the applied potential.
The data obtained for the C.E. study made clearer the two processes involved in the electrowinning of nickel. At –0.9V the C.E. decreases as the temperature increases and at –1.0V the C.E. increases as the temperature increases. Therefore, the process occurring favorably at -0.9V, which provokes a decrease in the current efficiency with the temperature increment, could be the hydrogen evolution. The nickel electro-deposition is favored at –1.0 V and increases as the temperature increases, which gives high current efficiencies.
Figure 2: Effect of the temperature and the applied potential in the current efficiency of the nickel electrowinning. Solution: 0.4 M Ni2+ and 0.8 M NH3 in ammonium chloride.