The separation of fine sulfides becomes increasingly difficult at fine particle sizes. Potential fine size separation methods are froth flotation and magnetic separation. The mineral sulfides, with the exception of pyrrhotite, are either weakly paramagnetic (e.g. chalcopyrite) or diamagnetic (e.g. molybdenite). Those which are paramagnetic can only be separated using high-gradient magnetic separation (hgms) at high fields (up to 2 Tesla) and low flow rates (e.g. 1 cms-¹).
Arsenopyrite (FeAsS), chalcopyrite (CuFeS2) and marcasite (FeS2) were leached and the susceptibility of the leach product measured. From the measured sulfate in solution, the percentage of the particle that had reacted was calculated and hence the coating thickness was estimated.
Marcasite on leaching at 87.5°C for 60 minutes (930 kPa air, 0.5 M NaOH, -25 + 10 µm particles) gave a leach product with a susceptibility of 2.27 x 10-³.
Mixed Sulfides fall into two categories, mixtures of iron-containing sulfides and non-iron-containing sulfides. The magnetic profiles for various mixtures after leaching (100°C, 60 min, 930 kPa air, 0.5M NaOH). In all cases the mixtures are of equal masses. Curve 1 is for pyrite/arsenopyrite. .The magnetic fraction up to 600 mA (- 50 wt%) was primarily pyrite (by XRD); the non-mags was arsenopyrite. The calculated susceptibility of each mineral in the leached mixture is less than that of leaching a single mineral. For example, arsenopyrite in the mixture gave K = 0.8 x 10-³ down from 1.8 x 10-³ for the pure mineral.
It has been shown that the magnetic susceptibility of the iron-containing sulfide minerals, arsenopyrite, chalcopyrite, and marcasite can be enhanced by oxidative alkaline pressure leaching in a similar manner to pyrite. It is presumed that the reason is that identified for pyrite.
The magnetic susceptibility of marcasite, arsenopyrite and chalcopyrite, in addition to pyrite can be enhanced by oxidative alkaline pressure leaching. Probably all iron-containing sulfides will show a susceptibility increase under these conditions. A thin layer of magnetic iron oxides is probably the cause.