Comminution and Crushing

Indian iron ore and copper circuits crush to P80 150–25 mm before ball milling. Bond equation standard in MET exam papers — W_i from lab test on representative composite sample.

AG/SAG mills reduce steel media cost for competent ore; jaw+cone+ball still common for hard banded hematite. Power cost at Indian industrial tariff makes W_i optimisation economically significant.

Hartman & Mutmansky comminution chapter links RR to number of crushing stages.

Relation 1 —

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Wills Mineral Processing — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Bond: W = 10 W_i (1/√P80 − 1/√F80) kWh/t — F80, P80 in μm. W_i characteristic of ore — lab ball mill test. Scale-up to industrial mill with efficiency factors 0.8–0.9.

RR = F80/P80 per stage — total RR product of stages. Primary RR 4–6; tertiary may reach 10. Too high RR causes choke and excessive fines.

Crusher types: jaw for primary (upto 1.2 m feed); gyratory high capacity; cone secondary/tertiary with closed side setting controls P80. AG autogenous — ore as media; SAG adds small ball charge.

Energy 40–60% of mine site electrical load — optimise blast fragmentation to reduce W_i effective work (finer ROM from good blast lowers crushing energy).

State assumptions explicitly before using any relation for comminution and crushing — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Mineral Processing viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Mineral Processing papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to comminution and crushing.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Comminution and Crushing appears in beneficiation plants. In Indian mining curricula this topic is tested because it connects theory to comminution and separation.

GATE and semester exams often combine comminution and crushing with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use comminution and crushing?" — answer with a lab, mini-project, or plant visit example if possible.

• P80 and F80 in mm instead of μm in Bond equation
• Forgetting 10 multiplier in Bond W equation
• RR as P80/F80 inverted
• Lab W_i applied without ore variability check

Before attempting comminution and crushing problems, confirm you can:

1. Primary jaw/gyratory; secondary cone
2. AG/SAG mills for wet grinding
3. Energy major cost in comminution

Revise the solved examples in Wills Mineral Processing — Standard reference and one previous-year GATE or university paper for this unit.