(i) Derive the differential equation model for batch milling given in the information sheet. Define all terms in this equation. Describe how the selection function Si is measured. Provide a sketch of a typical variation of the selection function with time. Comment on the shape of the graph, particularly for large particle sizes. [12] (ii) Define the critical speed of a mill. Discuss qualitatively the effect of mill speed N and load volume J (fraction of mill volume) on mill power. [6]