Mineralogical characterisation of chromite in the UG2 Reef from Waterval Mine, Western bushveld : implications for minerals processing

[摘要] ENGLISH ABSTRACT: The Bushveld Complex of South Africa contains three of the most important platinumdeposits in the world namely the Merensky Reef, the Upper Group Two (UG2) chromititereef and the Platreef. These three ore bodies are principally beneficiated by froth flotation.During the beneficiation of chromite hosted PGE's by froth flotation, chromite representsthe principal gangue mineral. This is particularly true for the UG2 main seam. An excess ofmore than 3% in mass of chromite in the PGM concentrate is known to result in significantproblems in the downstream processing and extraction of PGEs. The variability in textureand composition of chromite due to its primary crystallization and subsequent modificationby the development of potholes or through IRUP intrusions are thought to influence theflotation behaviour of the UG2 main seam chromitite ore.This study conducted at Waterval Mine investigated the role of mineralogical characteristicsof chromites on the flotation performance of three different environments for the UG2 mainseam: (1) 'normal UG2 main seam; (2) UG2 main seam affected by pothole formation; and(3) UG2 main seam affected by IRUP intrusion. This was achieved through an extensivepetrographic investigation of the chromites from each environment, to individuallycharacterise their primary textures. This was followed by compositional characterisation ofthe chromite from each environment. Finally the flotation performance of the ore from eachenvironment was investigated, using small scale batch flotation experiments, to establishany linkage between the textures, the composition and the flotation performance of thechromite from different environments.In this study it was found that the UG2 normal reef and the UG2 reef affected by potholeformation are both principally characterised by primary mineralogical features comprisingmainly fine‐grained chromite as the cumulate phase and orthopyroxene and plagioclase asintercumulate phases. These two reef types were also found to be identical in thecomposition of the chromites present. In addition, in both of these almost unaltered reeftypes it was found that chromite showed small recoveries by flotation. On the other hand, itwas found that the UG2 affected by IRUP intrusion was affected by post‐magmatic alterationthat had overprinted primary textures and compositional features. This resulted in thereplacement of primary minerals by secondary alteration assemblages. Orthopyroxene wasiiireplaced by serpentine, chlorite, amphibole and talc, while plagioclase is replaced by sericiticalteration. Furthermore, this alteration also resulted in modification of the chromitecompositions. The compositional change in the chromites from the IRUP reef type resultedin Fe and Ti enrichment of chromite with increasing magnetic properties, and Cr, Al and Mgdepletion. The alteration also resulted in the coarsening of chromite in the IRUP affectedmain seam reef particularly at the bottom and the top of the main seam.These compositional and textural modifications, principally the post‐magmatic alteration ofintercumulate orthopyroxene, resulted in a greater recovery of chromite by flotation in theconcentrate from the IRUP affected ore compared to the two other two ore types wherethere was small amount of chromite recovered. The characterisation of the recoveredchromite revealed that the principal reason for chromite flotation was caused by the mineralassociation of chromite with hydrophobic Si, Mg, Fe rich phases, principally alteredorthopyroxene and associated serpentine, chlorite, amphibole and talc.This investigation showed that the difference in mineralogical and flotation performances ofchromite from the different UG2 main seam reef types was caused by the postcrystallisationalteration of cumulate and intercumulate phases due to the emplacement ofIRUPs. Although IRUP affected UG2 main seam ore is not currently processed, it could beprocessed much more rapidly than the other two types of UG2 main seam ores because ofits softer character resulting in shorter milling times. This is most likely a function of thepresence of alteration phases and the presence of coarser chromite grains, as well asalready brecciated chromite grains. Savings associated with the shorter milling time of thisore type are perhaps offset by the cost of the higher dosages of depressant required tosuppress the floatable chromite in this ore type. However, given the energy cost of longermilling times, the cost of the depressant is likely to be insignificant. Moreover, theprocessing of the UG2 main seam ore affected by IRUP intrusion would also require adifferent approach to extraction of the ore to keep it separate from the normal reef ore.

[发布机构] Stellenbosch University