mill

A mill in mining is a near-vertical or steeply inclined passage driven through rock to convey broken ore from an upper level to a lower one. The ore is loaded at the top and gravity pulls it down through the passage, emerging at the bottom ready for collection and transport to the processing plant. This is distinct from a stope, which is the excavated void where ore is actually broken out of the rock face.

The passage itself is typically lined or reinforced depending on the stability of surrounding rock. Width and height are designed to allow lumps of ore to fall freely without jamming, though in practice operators must manage the size of broken rock and clear blockages when they occur. A steep angle, often 50 to 80 degrees from horizontal, ensures material flows naturally under gravity alone.

Mills differ from raises, which are upward-driven passages used for ventilation, personnel access, or moving material in the opposite direction using mechanical means. A mill is passive; the ore does the work. This makes it one of the cheapest ways to move heavy material inside a mine once the initial driving is complete.

The term comes from the grinding mills of earlier centuries, though the modern mining usage bears no relation to grinding. The association is simply that material passes through the structure. In some regions and older texts, mills are called ore passes, which is more literal and less ambiguous.

Problems arise when ore bridges, creating an arch that blocks flow entirely, or when pieces lodge and cannot be cleared without entering the passage, a dangerous task in confined space. The solution is often to increase the angle or widen the lower section. In operations with many stopes feeding the same mill, congestion and wear are constant management issues.