A fundamental investigation into the microwave assisted leaching of sulphide minerals
Microwave assisted leaching has been investigated in an attempt to improve both the yield of extracted metal and reduce processing time. This is especially pertinent in view of the increased demands for metal and more environmentally friendly processes. This work reports a fundamental study on the influence of microwave energy on the dissolution of sulphide minerals. Chalcopyrite and sphalerite were chosen as model materials due to their economic importance and the diversity of their heating behaviour in a microwave field (chalcopyrite being an excellent microwave heater and sphalerite being an extremely poor microwave receptor). Chalcopyrite leaching has been carried out in ferric sulphate and ferric chloride under both microwave and conventional conditions. Conventionally, it was found that chalcopyrite dissolution in ferric sulphate seems to be limited by surface reaction control. More importantly, it has been shown that specific fracture planes on chalcopyrite particle surfaces experience selective leaching, which was revealed by SEM and ToF-SIMS surface analysis. The preferential attack on particular planes is speculated to be linked to different chemistry of some cleavage planes within the chalcopyrite crystal. In the ferric chloride system, however, it was found that cupric chloride, a reaction product of chalcopyrite with ferric sulphate, may play an important role in the dissolution process. Leaching of both chalcopyrite and sphalerite in ferric sulphate under microwave conditions has shown enhanced recoveries of metal values compared to that produced conventionally. It has been demonstrated that the enhanced copper recovery from chalcopyrite during microwave treatment is as a result of the selective heating of the mineral particles over the solution which was found to be highly lossy. In addition, it is suggested that high loss leaching solutions will develop a superheated layer close to the periphery of the reaction vessel (due to the small penetration depth) which creates localised heating compared to the bulk solution temperature. The enhanced recovery of zinc from sphalerite seems to occur as a result of only the presence of the superheated layer. If leaching takes place within this layer, an apparent rate increase will be noted with respect to the measured bulk temperature. The hypotheses of selective heating (for chalcopyrite) and the effect of penetration depth (for chalcopyrite and sphalerite) were supported by the negligible difference between the activation energy values under microwave and conventional conditions for both chalcopyrite and sphalerite. Furthermore, the measurements of dielectric properties of the leaching solutions have shown that such solutions are highly lossy and characterised by a penetration depth of an order of about 3 mm. Finally, numerical electromagnetic simulations showed that chalcopyrite particles could be heated selectively when micro-waved within highly lossy leaching solutions due to their high conductivity. It is concluded that the dielectric properties of both the solid and liquid phases, the dimensions of the reactor and the position of solid particles within the reactor determine the leaching outcome. More importantly, it is likely that the enhanced recoveries observed are not likely to be as a result of a so called "non-thermal microwave effect" but rather as a result of thermal effects.