The initial piece of equipment in most mill circuits is a feed hopper. The hopper is used in conjunction with a feeder to smooth out material flow surges introduced by loading devices with fixed bucket sizes (front-end loaders, rear-dump trucks, etc.). Hoppers often contain a grizzly in order to reject large oversize material.
The feeder, typically a vibrating tray located under the hopper, transfers gravel at an even rate to the circuit. Although the hopper-feeder combination may appear to be a minor piece of equipment, a steady flow of material through the mill is very important for effective gravity separation.
Hopper capacity and feeder capacity are two separate items. Generally, hoppers are designed to hold enough material to provide a steady flow of gravel despite surges inherent in mining cycles.
Feeders are set to provide the appropriate flow rate to the mill. So even though a hopper may have a 100-yd3 capacity, the feeder might provide material at 20 yd3/h. Feeders are not always used in placer mills.
When they are not used, feed rate is regulated by the size of the opening in the bottom of the hopper. The cost estimation curves in this report calculate hopper-feeder costs based on feeder capacity, which typically equals mill capacity. Factors are provided for situations where feeders are not used.
Reference
[1] S. Stebbins, “Cost Estimation Handbook for Small Placer Mines I.C. 9170,” 1986. Accessed: Aug. 26, 2025. [Online]. Available: https://www.semanticscholar.org/paper/Cost-Estimation-Handbook-for-Small-Placer-Mines-Stebbins/94c188525178d65cc964f7af20fff0182d8c1c14
