Microhardness characterisation of manganese ore minerals – Implications for downstream processing

Abstract

Manganese oxide, silicate and carbonate ores are mostly mined as feedstock for steelmaking where Mn is largely added as ferroalloy, but they are also used for battery and to a lesser extent fertiliser and pigment production. The physical, mineralogical and textural properties of manganese ore minerals are known to influence their thermal properties and thus their high temperature behaviour during sintering or alloy production.Microhardness testing using the Vickers indenter is a potentially valuable characterisation technique to correlate the physical and optical properties of Mn ore minerals with their mineral chemistry and texture when used in conjunction with electron probe microanalysis (EPMA) and helium pycnometry. Microhardness values can also inform potential beneficiation pathways for lower grade Mn ores and/or assist in the prediction of lump: fines ratios during mine planning.This study provides the results of microhardness testing of Mn ore minerals from several different Mn ore types with variable mineralogy and texture. The data indicates that there is a clear link between mineral microhardness and micro- to nano-scale porosity and microcrystallinity, leading to potentially large variations in microhardness for some common Mn ore minerals. For example, cryptomelane with lower reflectivity, interpreted as having higher nano- to micro-porosity and/or differences in microcrystallinity, has significantly lower microhardness (mean 267 kg/mm2) than cryptomelane with higher qualitative reflectivity (mean 629 kg/mm2).EPMA conducted on mineral grains subjected to microhardness testing showed microhardness values did not vary systematically with changes in mineral chemistry but did vary with total element content as determined by EPMA. Low analytical totals were a de facto semi-quantitative measurement of mineral nano- to micro-porosity due to likely beam splitting/dispersion on more microporous samples identified during optical microscopy. Although there was no systematic link seen between microhardness and tetravalent Mn mineral element chemistry in this instance (e.g., K in cryptomelane), cryptomelane with lower reflectivity has lower Mn contents and higher contents of minor elements such as Fe, Al and Si.