Reflecting the importance of primary-crushing performance on the milling process, many operations have transferred primary crushers from the operational control of the mine to the mill. Regardless of who operates the equipment, primary crushing has two customers: serving the mine to maximize load and haul equipment utilization, and also serving the mill to maximize overall comminution efficiency. Many mines select primary crushers based on top size of the designed or anticipated run-of-mine (ROM) ore. This can result in a substantial excess of primary-crushing capacity for smaller operations.
Frequently for large operations, though, primary-crushing capacity becomes an issue not only for milling operation (in terms of capacity and unit power input), but also for mine productivity. As primary crushing becomes taxed, issues that are conventionally dealt with in secondary, tertiary, or pebble-crushing roles become more critical. Management of gap setting,power draw, predicting and extending liner life within a specified performance envelope and productivity issues all become important to the overall comminution process. As with other crushing operations, employing stockpiles(or ore pockets), reclaim, or bins can maximize equipment productivity and efficiency. Most large gyratory installations are designed to accept direct dumping. While this minimizes the capital of the primary-crusher installation, it is inevitable that there is a trade-off with reduced primary-crusher utilization and crusher efficiency. This is a result of feeding the primary crushers based on the frequency that haul trucks present themselves to the crusher, instead of based on optimum crusher operation.
Management of the mill stockpile is critical to maintaining consistent throughput. Most operations reclaim from a stockpile of primary-crushed material. There is generally some degree of stockpile segregation, with coarse material preferentially accumulating towards the outside of stockpiles. Maintaining a live stockpile and balancing multiple reclaim feeders result in the highest average (and most stable) throughput. Maintaining the stockpile at reasonable levels minimizes the effect of load and haul equipment shift changes on downstream operations. The size of the stockpile should be based on anticipated fluctuations in production of primary-crushed material as a result of primary-crusher maintenance, load and haul asset maintenance, mill maintenance downtime requirements, and normal fluctuations due to mine planning and sequencing. Natural stockpile segregation can also offer opportunities to improve overall circuit operation, either via balancing the SAG mill and ball mill circuits, or in cases, by preferential milling of ore from different feeders.
