The fate of molybdenum in the residues of a Chilean copper ore processing plant

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Mining residues have become an important subject for secondary mining and re-processing. They are an inevitable consequence of mining activity with the amount of residues steadily increasing. These residues are heterogeneous bodies, affected by different processes during and after deposition, which might even produce an enrichment of certain valuable elements within parts of the residues. Since geochemistry and mineralogy of the ore can differ heavily due to processing, deposition and weathering within the heaps, the formerly applied processes might be less suitable for re-processing, therefore, these processes should be studied carefully for an effective re-processing. This study examines the fate of molybdenum in the processing of copper ore in a Chilean processing plant. Molybdenum shares the path of Cu from grinding and flotation into the sulfide concentrate. During smelting and converting, the sulfide concentrate is separated gravitationally from the slag. Through the subsequent slag cleaning process, the slag is crushed and submitted to flotation again, but this time the Mo in its oxidized state reports to the tailings and, hence, is deposited on the tailings heap. Drill cores of the heap have been analyzed by X-ray fluorescence spectroscopy revealing a layer of significant Mo enrichment with up to 0.3 wt.-% Mo. From this layer, the minerals within the slag particles were analyzed by more than one thousand electron microprobe analyses. The results show that Molybdenum is enriched mostly in the glassy matrix (median of 0.18 wt.-%), while the spinel phase, which generally incorporates most of the Mo according to literature, contains a smaller fraction of the Mo (median of 0.09 wt.-% Mo). Additionally, within the slag, the glass amount is up to a factor five higher than the magnetite, therefore, the glass poses as the main reservoir for Mo in the heap. Considering the general process, the missing step of Mo-extraction from the sulfide concentrate combined with the slag cleaning process for Cu extraction is responsible for the enrichment of Mo in the slag rich part of the investigated heap.