Which index most directly expresses the grinding characteristic (grindability) of solid fuels and minerals in routine mill selection and design?

Introduction / Context:
Grinding indices quantify how easily a material can be ground. For coal and some minerals, a quick lab test provides a comparative measure that guides mill selection and expected power demand.

Given Data / Assumptions:

• Coal and similar fuels are frequently specified by HGI.
• Other indices (angle of repose, abrasion, shatter) describe different properties.

Concept / Approach:
The Hardgrove Grindability Index (HGI) ranks materials on a relative scale—higher HGI indicates easier grinding. It is widely adopted in power and cement industries to compare coals and anticipate mill throughput and power needs.

Step-by-Step Solution:

Verification / Alternative check:
For mineral crushing/grinding circuits, Bond Work Index is also common; however, the question emphasizes routine characterisation where HGI is standard for coals.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing HGI with Bond WI; using angle of repose to predict mill power.

Final Answer:
HGI (Hardgrove Grindability Index)