Acid-bacterial and ferric sulfate leaching of pyrite single crystals

Abstract

Pyrite single‐crystal cubes were cut, polished. and x‐rayed to produce orientations of (100), (110), (111), and (112). These crystallographically developed surfaces then were prepared to expose an area of 1 cm², and the remainder of the crystal was coated with an acid‐resistant silicone cement. Crystals with representative orientations then were leached in ferric sulfate solutions adjusted to a pH of 2.3 with H₂SO₄ containing up to 6 × 10³ ppm of Fe³⁺ at 30 and 55°C. Leaching was also conducted in acid‐bacterial lixiviants containing Thiobacillus ferrooxidans at 30°C and a thermophilic microorganism at 55°C. Surface corrosion and pitting associated with pyrite leaching were examined by scanning electron microscopy. Pyrite leaching in ferric sulfate solutions was observed to be different when compared to acid‐bacterial leaching. Ferric sulfate leaching required nearly 2 × 10³ ppm of Fe³⁺ at 30°C while acid‐bacterial leaching at 30°C occurred without additions of Fe³⁺, and values of Fe³⁺ never exceeded 10² ppm. Because of precipitate formation, an accurate assessment of the role of crystallographic orientation on the leaching of pyrite is difficult.