Increasing Silicate Grade in Crude Ore

Luossavaara-Kiirunavaara AB (LKAB) operates an iron ore mine, three concentration plants and three pelletising plants in Kiruna, Sweden. The current methods of separation at the beneficiation plants are low intensity magnetic separation (LIMS) and reverse flotation (i. e., apatite is floated and magnetite depressed), where the wet LIMS stage is regarded as the crucial part of the separation of silica from the ore. It is increasingly important to understand the Kiirunavaara high-grade iron ore deposit from a mineral processing perspective as well as from a mineralogical and geochemical perspective as the production in the mine is advancing towards deeper levels with higher concentrations of SiO₂ in the ore. The mineral processing parameters such as the natural breaking characteristics, specific energy consumption and degree of liberation of magnetite and silicate minerals are equally important. The intergrowth of magnetite with actinolite is of particular importance for understanding the processes in the processing plants in Kiruna and the flotation behaviour of silicate minerals. The iron ore deposit at Kiirunavaara consists mainly of magnetite and apatite, with an average grade of 63.8% Fe and 0.4% P (estimated from the 3D resource model, LKAB) and with varying, but mostly small, amounts of gangue minerals, mostly silicates and carbonates. Based on mineralogical investigations, actinolite and phlogopite and in many cases also chlorite, titanite, quartz and albite are the most significant SiO₂-bearing minerals in the ore. Currently, the high-grade iron ore deposit of Kiirunavaara has in situ a low grade of silica of approximately 3% SiO₂. However, the SiO₂ grade is expected to increase in the deeper parts of the deposit. It can be assumed that the silicate mineralogy and the SiO₂ grade in the crude ore undoubtedly impact the SiO₂ content of the final products, i. e. the iron ore pellets and/or iron ore fines.

This results in new challenges and requirements for the production at LKAB. Within the framework of the completed “Silica in the Mine” project, which was an important research and development area for LKAB for several years, the entire production chain from “the mine to the mill” and from “the mill to the final product” was evaluated in several subprojects. Important information was gained from the results of these subprojects in order to better understand the problem of the increasing and fluctuating SiO₂ grades in the crude ore and magnetite concentrate. To study the reduction of the SiO₂ content additionally in the magnetite concentrate obtained by WLIMS, a reverse cationic flotation laboratory test programme was initiated in the early spring of 2016. This flotation test work was based on the investigation carried out at the mineral processing laboratory at the Geological Survey of Finland (GTK). To obtain additional information about the problem of SiO₂ and the behaviour of individual silicates during mineral processing, an extensive large-scale sampling programme will be carried out at the different stages of the process at the beneficiation plants (KA1, KA2, KA3) at the Kiirunavaara site.