A method for modelling grate flow characteristics in autogenous and semi-autogenous grinding mills

Since the early 1900’s Autogenous Grind (AG) and Semi-Autogenous Grind (SAG) mills have been an integral part of the grinding circuit in mineral processing plants. One of the major components of these mills is the discharge assembly, which typically includes a grate and pulp lifter. A mathematical model of grate flow has been developed from data obtained from a pilot scale mill.
Measurements were made at a range of mill speeds, flow rates, charge levels and percentage open areas. Both the flow rate and hold-up in the mill were measured. The data was subsequently compared with literature grate flow models. These models were found to be inaccurate. A new model has been developed based on the summation of the flows through individual grate apertures, as determined by Bernoulli’s equation. In contrast to previously posed models, this equation does not contain power law fit terms and therefore its predictive capabilities are not restricted by the conditions of the data set. The grate flow was found to be determined primarily by the hydrostatic pressure and open area, with the mill speed and presence of rocks having minor effect.