Process water geochemistry and interactions with magnetite at the Kiirunavaara iron mine, northern Sweden

Current global estimates indicate that ca. 2400 million tonnes of iron is produced annually, most of which is used to make steel. Due to the associated costs, environmental challenges and energy consumption, steel is one of the world’s most intensively recycled materials. For example, about 98 % of structural steel is recycled in the USA. However, iron production is still increasing. Costs, prices and environmental demands have led to increased recycling, but also increased attention to the processes used to refine iron ores. This thesis focuses on the impact of variations in process waters on the refinement process at the Kiirunavaara mine, northern Sweden, and possible environmental implications.The water used to process iron ore is seldom considered in its full complexity, if at all. This could be problematic since it has been shown that ions in the water can affect key process steps, including dispersion, flotation, filtration and pelletization. To gain a better understanding of process water variations and their effect on refinement, process water at the Kiirunavaara plant was characterised by ultrafiltration, membrane filtration and ICP analyses, complemented with geochemical modelling. Leachings of the ore concentrate and Zeta potential measurements were performed to better understand process water influence on ore surfaces and different refinement steps effects on the ore surfaces in the refinement process. In addition, fine particles (< 5µm) were studied to obtain information about the unwanted fine particles in the ore concentrate. The findings indicate that Ca, S, Na and Cl are the dominant dissolved ions in the process water. The colloidal fractions of the major ions were < 3%. The main cause of variation in the major ions’ concentrations was probably dilution by rain and meltwater. The high concentrations of Ca and S probably originated from calcite buffering and sulphide oxidation in the drainage area, respectively, while the Na and Cl probably originated largely from fluid inclusions, and N species from undetonated explosives.The results also show that increases in iron production and recirculation of process water have increased elemental concentrations in the process water throughout the system at the Kiirunavaara site. In addition, the surfaces of the ore concentrate carry far more Ca, Mg and Si than the water in its pores. This implies that sorption of Ca, Mg and Si to ore concentrate is the most important transport process in the refinement process and that more Ca, Mg and Si are available for interactions in the process than analyses of water samples analysis indicate. Cl and SO4 are also enriched on or close to the ore surface, but to a lesser extent. The magnetite surfaces are enriched in the beginning of the process chain and subsequent milling, magnetic separation and flotation with the addition of water glass and collector do not significantly change the readily accessible concentrations of ions. However, a small increase in the amount of Si released in leaching tests was observed after the addition of water glass. The zeta potential measurements showed the importance of considering the complex nature of process water in the refinement processes, which has been neglected in most studies. The zeta potential of the magnetite in the process water differed substantially from what could be predicted by only changing pH and Ca and Si concentrations. Zeta potential measurements on silicate particles from the process water showed that addition of water glass or floater caused little or no change. The silicate particles also had low zeta potential, close to that of magnetite, which could lead to entrapment or slime coating of unwanted silica particles in the ore concentrate. Fine silicate particles were enriched in the fine fraction in the ore concentrate. Control for other possible enrichment factors (then entrapment or slime coating) of fine particles (less than 5µm) in the ore concentrate gave no clear picture. Neither the density nor form of different minerals seems to play an important role. However, biotite, the softest mineral, was enriched in the smallest particles.Godkänd; 2013; 20130808 (magwes); Tillkännagivande disputation 2013-09-05 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Magnus Westerstrand Ämne: Tillämpad geologi/Applied Geology Avhandling: Process Water Geochemistry and Interactions With Magnetite at the Kiirunavaara Iron Mine, Northern Sweden Opponent: Docent Lars Lövgren, Kemiska institutionen, Umeå universitet Ordförande: Professor Björn Öhlander, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Tid: Fredag den 27 september 2013, kl 10.00 Plats: F341, Luleå tekniska universite