Potentiodynamic Polarization Behavior and Microscopic Examination of Tungsten Carbide Hard Metal Materials in Supported Ammoniacal Medium

Abstract

Electrochemical methods for the recycling of tungsten carbide (WC-10Co) resources suffer from passivation in the acidic medium caused by WO3 and also in NaOH electrolytes, due to hydroxide formation. We found that an ammoniacal solution is a promising electrolyte for sustainable electrochemical dissolution of both tungsten (W) and cobalt (Co). The ammoniacal medium performs greatly when supported with Cl-, SO4 2and CO3 2ions. Poor dissolution/corrosion tendency of WC-10Co in a diluted NH4OH solution enhanced many folds in the presence of Cl-, SO4 2and CO3 2ions. Among these supporting ions, Clemerged as the most suitable for the electrochemical leaching of W and Co from the WC-10Co, accompanying the least noble behavior of WC-10Co. An electrolyte composed of 150 g/L of ammonia and 5% (w/v) of NH4Cl yielded the maximum anodic current density. Microscopic examination of the electrochemically treated samples shows scattered active sites responsible for the oxidative dissolution of WC-10Co. The usefulness of W and Co dissolution in ammonia-additive salt followed the order NH4OH-NH4Cl>NH4OH-(NH4)2SO4>NH4OH-(NH4)2CO3..

Recovery of Strategic Metals from Tungsten Carbide-Cobalt Bonded Waste by Electrochemical Processing

“…So, an increase in NH4Cl up to 10 %, in a 150 g/L NH3 solution, enhanced the anodic dissolution. SO4 -2 and CO3 -2 have a similar behaviour, but they are less effective than the Clions [39]. In summary, if the NH3 solution concentration increases from 50 to 150 g/L, the corresponding OCP value decreases.…”

“…When it is likely that a stable passive layer is formed, the Clions turn out to be highly aggressive, and penetrate through it, by attacking W and Co (which is responsible for local passivation), forming complex products and decreasing the possibility of the layer formation. Similarly, SO4 -2 and CO3 -2 have done the same thing, but they were less effective than Clions [39]. Therefore, such additives (containing aggressive ions) decreased the possibility of a stable passive layer being formed, and the resultant maximum anodic dissolution current was obtained at minimum E (Table 2).…”

“…Therefore, W formed WO4 2− , through the anodic oxidation reaction in the NH3 solution, or due to the NaOH solution high pH (~11). Further, WO4 2− reacted with NH4 + , which is the soluble product, and the following chemical reaction can be expected [39,40].…”

“…Hence, at higher E and pH values, [Co(NH3)6] +2 complex became more stable than Co 3+ , which is also relatively stable at lower pH values [41]. Hence, at higher E and low pH (~ 7) values, Co +2 ions also oxidized into the very low oxidation E of Co +3 ions (-1.82 V), forming the unstable amine-based complex [Co(NH3)6] +3 [39]. 150 g/L NH3 with the aggressive anions Cl -, SO4 2and CO3 2-(separately obtained in the presence of NH4Cl, (NH4)2SO4, and (NH4)2CO3,) have significantly influenced the abovementioned reaction, by avoiding the stable Co passive layer formation.…”

“…150 g/L NH3 with the aggressive anions Cl -, SO4 2and CO3 2-(separately obtained in the presence of NH4Cl, (NH4)2SO4, and (NH4)2CO3,) have significantly influenced the abovementioned reaction, by avoiding the stable Co passive layer formation. Therefore, enhanced anodic current was due to the higher anodic dissolution, by the formation of [Co(NH3)6]Cl2, [Co(NH3)6]SO4 and [Co(NH3)6]CO3 complexes, and the following chemical reaction can be expected [39].…”