Acid leaching of malachite in synthetic mixtures of clay and zeolite-rich gangue. An experimental approach to improve the understanding of problems in heap leaching operations

Column leaching experiments with sulphuric acid have been carried out on synthetic rock aggregate, thus relating leachate composition to one specific gangue or ore phase. Rock aggregates were prepared using quartz (57%) and short term reactive gangue (42%, SAZ and SWY montmorillonites, KGA-1 and KGA kaolinites, NAU-Nontronite and IMT illite from the CMS repository, and from Chile a bentonite rock, a mordenite and a laumontite-rich rock) and malachite (1%). Short leaching experiments (approximately 4 h) and 4 weeks post-leaching repose resulted in partial dissolution of the smectite gangues and minor dissolution of illite. Kaolinites caused temporary clogging of the columns. Mordenite proved to be relatively stable, while laumontite with a lower Si/Al ratio dissolved. Precipitation of alunogen, meta-alunogen, coquimbite and goldichite was observed, with the sulphate species depending on the cations released from the gangue. Cu-retention in residue reached a maximum of 20% for smectite gangue (bentonite rock); Cu recovery was improved by water elution. Column leaching experiments for single (short term) reactive gangues represent a realistic option to translate knowledge available from single mineral dissolution experiments to the reality of metallurgical extraction, although strict guidelines are needed for test set up, timing of sampling and analytical procedures.