Modelling the performance of grates and pulp lifters in autogenous and semi-autogenous mills

Autogenous (AG) and Semi-autogenous (SAG) mills have been used since the early part of the Id^ century. One of the important components of these mills is the discharge assembly which comprises a grate and, in the majority of mills, an array of pulp (pan) lifters. However, despite the importance of grates and pulp lifters the literature contains very little information on their design and performance. This lack of published information coupled with slurry transport problems experienced at a number of milling operations in Australia, prompted a research programme to be instigated in this area. This thesis covers the latest stage in this programme and concentrates particularly on the pulp lifters with particular reference to the influence of design, size and their interaction with the grates. To provide a comprehensive set of data with which to evaluate and understand pulp lifter performance a test-work programme was conducted using a combination of laboratory, pilot scale and full-scale mills. In total 760 experiments were conducted using laboratory and pilot scale mills during which the influence of mill speed, charge level, pulp lifter size, pulp lifter shape, grate open area and grate design were investigated. A total of 17 full scale mill data sets were also compiled which provided details of the operating conditions of each mill together with mill, grate and pulp lifter design information. These data were augmented with a further programme in which flow through a transparent grate/pulp lifter assembly was visually studied and photographed. The laboratory and pilot mill data were used to model the relationship between flowrate and hold-up of water inside these mills and the influence of grate and pulp lifter design on this response. As these data related to water-only conditions the industrial scale data were used to calibrate the model with respect to slurry. At the same time the data confirmed the influence of mill size which was apparent from the difference in performance between the laboratory and pilot scale mills. The resultant model can accurately predict the slurry hold-up in industrial-scale mills for a wide range of grate and pulp lifters designs. The insight gained from studying the role of the pulp lifters in removing slurry was used to develop a new more efficient design - the Twin Chamber Pulp Lifter (TCPL). The effectiveness of this new design was determined using laboratory and pilot scale mills and was subsequently confirmed at the industrial scale in one of Alcoa's mills. The installation of the TCPL in this mill eradicated its slurry transport problems and resulted in a significant increase in circuit throughput. A world-wide patent was granted for the TCPL.