Over a period of 7 months, 39 surveys of the DSM screens and SAG mill of the no. 5 mill circuit at Alcoa's Pinjarra refinery were conducted, together with the analysis of the contents of a full mill load. These results were analysed using regression and modelling techniques and the performance of the circuit related to operating conditions.
A literature survey uncovered only one published DSM screen model viz. that of Lynch et al (1977). This uses the reduced efficiency curve and a regression equation which relates the corrected cut point to screen operating conditions. This model was tested using the Alcoa survey data and a new regression equation suggested. Lynch's original data was re-analysed and found to be described by the new equation.
As an alternative to the empirically based Lynch type model a mechanistic form is also presented. It uses the equations describing viscous fluid flow down inclined planes, flow through packed beds and hindered settling of particles to describe the motion of the solids and liquid down and through the screen. The effects of screen blinding, screen wear and screen design factors are incorporated. The model is tested using the Alcoa survey data and found to accurately predict the effects of screen wear and screen aperture. Similar trends in the effects of slurry viscosity and density to those observed in practice are also predicted. The model's absolute accuracy is limited, however. Further investigations into the relative contribution of viscous fluid flow and packed bed flow is indicated.
The SAG mill model of Leung (1987) is reviewed and tested using the Alcoa survey data. Suggestions for improvements are presented including a new mass transfer function which relates the mill discharge rate to design, feed, and charge conditions. This relationship was derived from the analysis of the results of surveys conducted by other authors in this field together with those from the Alcoa survey and the Julius Kruttschnitt Mineral Research Centre data base. In all over 40 sets of data from pilot and full scale mills were used.
Using a modified Lynch model for the DSM screen and Leung's model for the SAG mill, simulations were conducted to determine the effect at Alcoa's Pinjarra refinery of changes in ore type, feed size distribution and mill pulp density.