An optimised iron ore grinding strategy based on balling fundamentals

In order to meet the future demand of pellets for the blast furnace, the output of the grinding circuit of the Tata Steel IJmuiden pellet plant needs to be increased. As the power consumption of the grinding circuit is already at its maximum, the grinding itself and/or the grinding strategy needs to be optimised, leading to a higher circuit output while still achieving the required fineness. Further processes within the pellet plant require the grinding product to be of a certain fineness which is currently determined by measuring the Blaine number of the grinding circuit output, which is a measurement of the available surface within the grind. At the IJmuiden pellet plant, grinding is done in a closed grinding circuit, meaning that only particles smaller than a certain size are allowed to leave the circuit. Particles that are to coarse are screened out of the main flow of material and fed back to the grinding mills. The pellet feeds of the IJmuiden pellet plant are blends of different ores. The compositions of these blends are based on several factors such as availability, price and iron content. Properties that affect the grinding of the ores, such as initial fineness and grindability, however, are not taken into consideration in the selection of ores. Given this fact, a possibility to increase the grinding capacity might be found in treating the feed of the grinding circuit not as a single material, like is done at the moment, but as a collection of individual ores. This approach allows for a blending/grinding strategy to be designed based on overgrinding softer ores and leaving the harder ores coarser while still achieving the required overall fineness. As this would reduce the energy needed to achieve a certain fineness, this would increase the grinding capacity. Based on knowledge gained through a literature study on the binding mechanisms in iron ore green pellets, the strength and plasticity of green pellets is believed to be affected by such variations in individual ore fineness within the ore feed. To study the influence of individual ore fineness on green pellet strength and plasticity, an experimental study was designed that involved balling and testing of green pellets balled from four different feeds that only varied in individual ore fineness. In addition, the influence of strain rate on strength and plasticity of these same pellets was tested. It was found that variations in individual ore fineness had no significant influence on the strength and plasticity of green pellets. Pellet plasticity and strength were found to be strain rate dependent even for very low strain rates. Based on these experimental results, recommendations for further work and possible improvements in pelletising process monitoring were put forward.section Resource EngineeringCivil Engineering and Geoscience