The comminution process has exceedingly low efficiency because it is highly irreversible. An outline of energy analysis for comminution is presented. The application refers to an ore-processing plant, which consists of a series of crushers feeding a traditional ball mill that delivers products to a downstream metallurgical process. For optimization, the design characteristics are fixed, i.e. decision variables can only be operational parameters. The chosen decision variable is the size of the feed (F) to the mill. In practice, the mill operator may control the feed granulometry and keep the product size constant by using a constant ball charge. The objective cost function is the sum of energy costs at the crusher and mill, which depend only on F. The exergy consumption of the crusher and mill are evaluated using the Bond correlation, including pertinent correction factors. Optimization leads to a 10% saving in overall energy costs.