Gravity separation of a UG-2 ore secondary sample for the reduction of chromite minerals

The efficiency of secondary grinding of UG-2 (Upper Group 2) platinum ores is currently limited by the use of open-circuit grinding in many plants, which is inherently inefficient, due to short-circuiting of coarse particles and possible separation by gravity in the mill. The chromium oxide (Cr2O3) grade in UG-2 flotation concentrates is generally above 3% and this contributes to low efficiencies during PGM smelting. y separation of the chromite and silicate particles prior to secondary grinding was investigated with a spiral concentrator. Laboratory cleaner spiral tests were conducted on a plant sample from a secondary ball mill feed, to determine whether a chromite-rich stream could be removed and discarded from the process prior to milling. Analysis of the chromite-rich stream showed that it could not be discarded, as a significant proportion of the Platinum Group Elements (PGEs) in the feed to secondary milling (∼16%) would be discarded with about 55% of the chromite minerals. Pilot plant cleaner spiral test work confirmed the trends observed. ernative strategy of separate coarse grinding of the gravity concentrate, followed by combined flotation, resulted in a 32% reduction in the Cr2O3 entrainment to secondary rougher flotation concentrate, and most significantly, an increase of 3.7% in secondary recovery of PGE’s with the same overall grinding energy as the standard process.