Optical, electron and scanning electron microscopies were used to study the pressure leaching of pyrite in 1N-H₂SO₄. Natural pyrite single crystals from several mines with different grades of imperfections and synthetic pyrite single crystals were used as specimens.
The optical microscopy showed that the high quality natural crystals dissolved uniformly, while the other crystals dissolved with some local corrosion which depends on their imperfections in the crystals. However, the electron microscopy revealed that pyramidal etch hills covered the whole surface of any specimen after the pressure leaching. The pyramidal etch hills were composed of {111} planes regardless of the Miller indices of the surface.
The sulphur droplets liberated by the reaction seem to inhibit the dissolution process at the points where they contact to the specimen, and hence they cause the development of the etch hills. The role of the formation of the pyramidal etch hills in the dissolution kinetics of pyrite was discussed.