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Figure 8.13 Current and practical minimum annual energy consumption for the U.S. mining and mineral processing sector (USDOE 2007).

resources, emphasis should be on reducing comminution energy. Some possible approaches are:

• Comminute less material: ore sorting, pre-concentration, improved mining practices to reduce dilution by waste.

• Comminute more efficiently: optimize the design of comminution circuits, including process control and use of more energy-efficient comminution equipment (e.g., high pressure grinding rolls, stirred mills).

• Do less comminution: liberate less of the valuable mineral to achieve higher recovery at the expense of lower concentrate grade (i.e., more metal and gangue to be separated in smelting and refining stages).

• Process the ore directly: this is the extreme case of the preceding approach, where no (or very little) comminution is done, either pyro-metallurgically (e.g., direct smelting) or hydrometallurgically (e.g., heap leaching);

• Conduct more comminution in the blasting (mining) stage.

One of the most advanced of these approaches is the development and application of more energy-efficient comminution equipment.

More Energy-efficient Comminution Equipment

Several new technologies may offer energy savings if incorporated optimally into comminution circuits. Two of these are stirred mills and high pressure grinding rolls (HPGR). Figure 8.14 illustrates where these two technologies fit into the overall comminution scheme.

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