Stripping Ratio

The stripping ratio is the amount of waste that has to be removed in a surface mine to reach a unit of ore. The U.S. Bureau of Mines' Dictionary of Mining, Mineral, and Related Terms (1968) defines it as "the unit amount of spoil or waste that must be removed to gain access to a similar unit amount of ore or mineral material."

How it works

The ratio is written as waste to ore. For example, a ratio of 3:1 means three tons of waste are moved for each ton of ore mined. Both numbers must be in the same unit.

As an open pit goes deeper, its walls have to be laid back at a stable slope, so each extra bench of ore needs more waste removed above it. The stripping ratio therefore tends to rise as a pit deepens.

Break-even stripping ratio

The break-even ratio is the highest stripping ratio at which ore can still be mined without loss. The 1968 dictionary describes the "economic stripping ratio" as a limiting ratio: a pit is profitable "only if the overall stripping ratio is less than" the limit. The SEC lists "break-even stripping ratio" with net smelter return and pay limit as terms used in a similar way to cut-off grade (17 CFR 229.1300).

What changes the ratio a mine can afford

• Grade: richer ore pays for more waste removal
• Metal price: a higher price raises the break-even ratio
• Mining cost: the cost of moving waste compared with the value recovered from ore
• Pit geometry: wall slopes and the shape of the ore body

Stripping ratio and land

Waste has to go somewhere. A large open pit moves much more waste than ore, and that waste needs dump space. On public land those dumps are part of the operation and must appear on the maps in a plan of operations with the other disposal areas (43 CFR 3809.401). Ground held for dumps and facilities is often covered by mill sites or by mining claims located around the deposit.

Why it matters in research

Stripping ratio is one reason land around a deposit matters as much as the deposit itself. When mining claims are reviewed for an open-pit project, the ground for pit expansion, waste dumps, and leach pads is part of the picture, and so are any conflicting mining claims or withdrawals on that ground.