Grinder reduction ratio definition: for size reduction equipment handling fine products, the standard reduction ratio (based on characteristic diameters) is defined as

Introduction / Context:
Reduction ratio is a concise descriptor of how much a grinder or crusher reduces a characteristic particle size from feed to product. It guides preliminary equipment selection and indicates whether single- or multi-stage reduction is needed to reach a target size distribution.

Given Data / Assumptions:

• Df = characteristic (average) diameter of the feed.
• Dp = characteristic (average) diameter of the product.
• We adopt the conventional ratio definition used in comminution texts.

Concept / Approach:
The standard reduction ratio for grinders is defined as the feed size divided by the product size, i.e., Df / Dp. Values greater than 1 indicate size reduction; the larger the value, the greater the extent of reduction. This ratio can be defined on various characteristic sizes (e.g., average, median, or P80), as long as consistency is maintained across calculations.

Step-by-Step Solution:
Identify the conventional definition: Reduction ratio R = Df / Dp.Confirm that this gives R > 1 for actual size reductions.Select the option explicitly stating Df/Dp.

Verification / Alternative check:
Equipment datasheets and plant reports commonly state crusher and mill reduction ratios in this form, aligning with sizing methodologies and scale-up heuristics.

Why Other Options Are Wrong:
Dp/Df would give values less than 1 for size reduction and inverts the conventional definition.Df - Dp is dimensionally a length, not a dimensionless ratio, and is not standard.“None of these” is incorrect because Df/Dp is the accepted definition.

Common Pitfalls:
Mixing definitions when comparing vendors (e.g., some may quote R on P80 while others on median size). Always confirm which characteristic size is being used.

Final Answer:
Df/Dp