grizzly
A grating used to sieve stones by size, so as to keep stones out of the sluices (in hydraulic mining) or to control stone size in the feed to the ore crusher.
grizzly: fixed bars that pre-sort stones by size
A grizzly is a stationary framework of parallel bars or rails spaced to allow material of specific size to pass through while rejecting larger stones. The bars typically run at an angle of 35 to 45 degrees from horizontal, and spacing ranges from 2 to 6 inches depending on the application. In hydraulic mining, a grizzly sits ahead of the sluice box to prevent large stones from entering and choking the flow. In hard rock mining, it functions as a fixed pre-crusher classifier, breaking the incoming ore stream into a more uniform size range before grinding mills.
The structure is simple and robust: usually made from steel angles, heavy timber, or cast iron, with parallel bars welded or bolted at even intervals. Material is fed from above, typically by gravity or conveyor. Stones small enough to fit between the bars drop straight through to the next stage of processing. Oversized material either rolls off the downslope end or backs up and recirculates, depending on the installation. Because there are no moving parts, maintenance is minimal and operating costs are low compared to screened classifiers.
Grizzlies in grinding circuits
When installed ahead of an ore crusher, the grizzly protects the mill from boulders and reduces the variability of stone size entering the grinding chamber. This extends crusher life and improves throughput consistency. A grizzly with 4-inch bar spacing, for example, will pass material under 4 inches and reject anything larger. The oversized fraction must then be handled separately, either crushed or returned to feed.
Problems arise when material becomes wet or sticky, causing fines to bridge across the bars instead of dropping through, or when bars bend or break under impact from falling stone. Regular inspection of bar spacing and alignment is necessary; even small deviations reduce separation efficiency. Grizzlies work best with angular, granular material; clay-rich or cohesive ore tends to blind the grates and reduces throughput.
The term likely derives from the German word gries, meaning gravel or grit, and was adopted into English mining vocabulary in the 19th century. Grizzlies remain a standard first-stage screening device in many small to medium mining operations, and the name has persisted despite competition from modern vibratory and rotary screens that offer finer classification and higher capacity.