Influence of variations in feed ore characteristics on autogenous grinding

Sammanfattning: In autogenous mills, the grinding media derives from the feed ore itself. Variations in feed ore properties will therefore affect the grinding charge and thus, the resulting ground product will vary. From a size distribution point of view, the optimum mill performance is achieved when there is a balance between coarse and fine particles in the feed, i.e., the size and amount of large particles have to be enough to break the finer particles. Particle size distribution is not the only throughput-limiting factor in autogenous mills. In addition, ore hardness varies and therefore, causes significant disturbances in the circuit. Different ore characterisation techniques and their application to the field of autogenous grinding are discussed in this thesis. With a better understanding of the influence of variations in feed ore characteristics on autogenous mills, it should be possible to better optimise and develop models of autogenous grinding circuits. In the first part of the thesis, a study of important parameters in the autogenous grinding process is presented. The second part deals with an extensive sampling of the feed to the autogenous mills at the LKAB Kiruna mine in Sweden. The sampling campaign was carried out over a period of six months in order to enclose the variations in composition and particle size distributions. The 10-15 mm fraction from each sample was used in a batch abrasion test. Particle size data, mineralogical compositions and abrasion data were subjected to analyses by multivariate statistics. Principal Component Analysis (PCA) shows that the major influence on the feed ore comes from variations in the iron content, and that the second most important is the changing composition of the gangue. The analysis showed that it is also possible by Partial Least Square (PLS) regression to link the variations in the abrasion test to changes in the feed's composition and its particle size distribution. The third part of the thesis presents a pilot plant campaign in primary autogenous grinding; where the consequences of a partially crushed feed are compared to RoM-feed. The campaign indicates that partially crushed feed results in lower energy consumption (kWh/t), a coarser product and better opportunities to control the mill performance. The last part in the thesis presents how drop weight- and abrasion tests were carried out on a number of samples from different locations in the production system. Based on data from ore characterisation and the pilot plant campaign, a model of the primary autogenous grinding circuit was calibrated. Comparing the simulation results to plant data validated the model. The model was used to simulate the influence of variations in feed ore characteristics on the autogenous grinding circuit.