Effect of potential and ferric ions on lead sulfide dissolution in nitric acid

The dissolution of the rotating disk electrode (RDE) of natural lead sulfide (galena) and the ground mineral in nitric acid solutions has been studied as a function of electrode potential, HNO3 concentration and temperature. The rate of dissolution producing hydrogen sulfide slowly increases as the potential varies from +0.1 to −0.4 V (Ag/AgCl). The reaction order on nitric acid concentration has been found to be 1.2±0.15 at 0.2 V and 0.9 at −0.4 V (40 °C), and the apparent activation energy is 35 kJ mol−1 in 1 M HNO3 at 0 V, suggesting that the process is controlled by a chemical or electrochemical reaction. At higher biases the RDE of PbS dissolves for the most part anodically, showing the highest rate at ∼0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. The yield of sulfate increases with potential and is small for the leaching of both compact and ground galena, while it reaches 50% in the case of a flotation lead concentrate. Ferric ions catalyze the dissolution of compact and, especially, ground galena, with the peak rate at the potential of immersed platinum electrode of 0.4–0.5 V. The Fe3+/Fe2+ couple is concluded to act as an intermediator for the electron transfer between nitrate ions and the solid, indicating that the dissolution is electrochemical in nature.