LKAB is one of the leading European companies in exploitation of iron while mining more than 80% of all iron ore in the European Union. The iron products are expected to be high quality regarding their content of impurities. When sulphur content exceeds a certain level in concentrates, expensive desulphurization process is required to avert technical problems in steel and alloys and environmental issues as emissions during the processing of iron ore. In order to prevent or rectify the sulphur impact, it is critical to understand the sources of sulphur in the ore and why they end up in the concentrate. This study focuses on sulphur tracking from concentrates by chemical and mineralogical characterisation of feed and concentrates of LKAB beneficiation plant in Malmberget. Previous studies on sulphur balance during processing have shown that the greatest impact is the sulphur content of the constituent material of the feed. This study aims to shed light on the sulphur fluctuations detected in the Malmberget magnetite concentrate by identifying its source in terms of mineralogy.

For this study, concentrate samples were collected covering episodes with high-sulphur peaks as well as regular low background levels. The scope of the study also includes mineralogical investigation of the drill core samples. Analytical methods such as micro-X-rays fluorescence (µ-XRF), Scanning electron microscopy (SEM) – Energy dispersive spectroscopy (EDS), Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN) – Particle mineral analysis (PMA), Electron probe micro analyzer (EPMA), and X-rays fluorescence (XRF) were used for characterization of the ore and concentrate products.

Selected drill core intervals contain visible sulfide minerals. Different mineralization styles were observed including patchy, dissemination, discontinuous veinlets, and veins. Pyrrhotite occurs preferentially as fine grains (50-150μm) along the edge of coarse pyrite grains (200μm-2mm). Pyrite is the main sulphur bearing mineral in most of samples. Pyrite grade is highly variable, ranging from 0.3 wt% to 13.2 wt% across the analysed drill cores. Pyrrhotite is the second most abundant sulphur bearing mineral. Pyrrhotite grades range from 0.1 wt% to 2.2 wt%. Chalcopyrite occurrences are variable ranging from 0.0 to 0.5 wt%.

In the concentrates, chemical and mineralogical analysis allowed to track the type and trend of the sulphide minerals in size fractions. Pyrrhotite (mostly liberated) and pyrite (mostly locked with magnetite) are the major sulphur carriers in the concentrates. There is an increase in observable pyrrhotite particles present in the samples corresponding to high sulphur events. Pyrrhotite shows generally higher deportment among sulphides in finer fractions and these finer fractions have the highest sulphur grade. Specimens found were pyrite, pyrrhotite and chalcopyrite, as well as some minor sphalerite and cobaltite. Pyrite in polymetallic assemblages as well as fully liberated pyrrhotite are the most common state of the sulphide particles. Sulphur comes from sulphide minerals primarily, while sulphates are successfully removed in the process (with exception of few baryte particles that were spotted). Finally, it was concluded that high sulphur peaks are correlated with increased pyrrhotite content of the monoclinic type in the concentrates.