PGM converter matte mineral characteristics and effects on downstream processing

There is little in-depth study on the downstream processing characteristics of granulated PGM-containing converter matte, particularly related to grinding and liberation behavior closely associated with leaching, and also as a function of iron endpoint specific mineralogy. Moreover, there is limited physical property data available, such as hardness and breakage of converter matte mineral structures that allows for considering a possible dependence between the mineralogy and the downstream processing characteristics of converter matte. The aim of this study was to investigate a relationship between indentation hardness, breakage and the mineralogy of two different iron end point converter mattes namely Fe_0.15% and Fe_5.17%. This also allowed for a subsequent investigation to relate the mineralogy to the integrated downstream processing characteristics of granulated converter matte.

A Nano-indentation tester was used to measure the indentation hardness of mineral structures. The indentation system also had the ability to test the breakage characteristics of the respective mineral structures by applying a preset load. Laboratory batch grinding tests were conducted at various specific energies with respect to granulated high and low iron mattes. A high resolution field-emission scanning electron microscope was utilized for mineralogical analysis. The perfect mixing ball mill model was subsequently used to assess the breakage rates of matte particles and minerals. A mineral liberation analyzer was used to investigate the liberation characteristics of mineral structures of interest.

The investigations revealed that the minerals and associated boundaries showed relatively different indentation hardness. The indentation-induced breakage of nickel sulfide, copper sulfide and NiCu-alloy structures appeared preferential and related to the iron end point. The softest mineral was found to be copper sulfide, which exhibited the average indentation hardness of 1975 and 2978 MPa within the low and high iron matte respectively. The increasingly harder minerals were nickel sulfide and NiCu-alloy in both low and high iron mattes with mean values around 5000 MPa. The laboratory batch grinding of the converter mattes at specific energy inputs resulted in product size distributions correlated to the underlying mineralogy. Although the trends for the breakage rates was found to be similar for both mattes, the matte with Fe content of 5.17% exhibited higher breakage rates in the specific energy ranges from 5 kWh/t to 25 kWh/t. This indicated that the matte with 5.17% Fe produces finer product than that of the 0.15% Fe matte at the same energy level. Moreover, a higher degree of overall liberation was achieved for copper sulfide and NiCu-alloy present within the high iron matte particles compared to particles within the low iron matte. 40% of particles within the high iron matte are completely liberated at 5 kWh/t specific energy, in contrast to about 20% within the low iron matte. However, Ni extraction achieved during leaching of minerals within the high iron matte was lower as opposed to minerals of the low iron matte. This was attributed to favorable Ni mineralization and chemistry of the low Fe matte which appears to be the most important driver for the downstream processing.