Characterization of a refractory arsenical silver ore by mineral liberation analysis (MLA) and diagnostic leaching

In this study, a detailed characterization of an extremely complex arsenical silver ore using mineral liberation analysis and diagnostic leaching for identification and occurrence of silver phases is presented. Mineralogical analysis of the ore has shown that silver is present mainly as acanthite and to a small extent as Fe-Pb-Ag-Zn arsenate/oxide. Acanthite was observed to occur as very fine grains (53% finer than 7 mu m, d(80): 11 gm) associated with various Pb-Fe-Sb arsenates and silicates (quartz, feldspar, kaolinite, mica/chlorite and clay). Around 33.1% of acanthite grains are present as free grains whilst 12% are locked-up within associated phases. Cyanide leach tests indicated that the extraction of silver from the ore was limited to 71% Ag over a cyanide leaching period of 24 h. This low extraction was presumably due to the limited exposure of fine acanthite grains to cyanide solution since ultra-fine grinding of the ore led to an enhanced silver extraction. It can be inferred from the findings of diagnostic leaching tests that the refractoriness of the ore can be attributed essentially to the encapsulation of acanthite within HCl-soluble phases (in particular, complex Pb-Fe-Sb-Zn oxides, arsenates and sulphates) (22%) and HF-soluble silicates (6%). It can be inferred from these findings that ultrafine grinding (even finer than few microns) or acid leaching can be used for achieving high silver extractions. This study also highlights the effective utilization of MLA in combination with diagnostic leaching for comprehensive characterization of refractory gold/silver ores and providing invaluable insights into their refractory behaviour.