Crushing & Grinding

Comminution reduces ore size to liberate valuable minerals. Learn crushers (jaw, gyratory, cone, HPGR), mills (SAG, ball, rod), reduction ratio, F80/P80, Bond's law and work index, mill power, circuit configurations (SABC, HPGR–ball) and energy efficiency.

Valuable minerals are locked in rock as tiny grains, often < 100 µm. To separate them, the ore must be broken finely enough to free (liberate) the grains. Comminution uses about 3–4 % of the world's electricity and around half of a mine's energy — making it the biggest target for efficiency gains.

An analogy

Comminution is like cracking a walnut: break it just enough to free the kernel — smashing it to dust wastes energy and makes separation harder.

Crushing

Primary crushing (gyratory or jaw) reduces ROM rock from ≈ 1 m to ≈ 150–250 mm; secondary and tertiary cone crushers take it to ≈ 10–20 mm. Crushers break by compression. High-pressure grinding rolls (HPGR) crush a bed of particles between rollers efficiently.

Grinding

Tumbling mills: semi-autogenous (SAG) mills use ore plus some steel balls to grind coarse feed; ball mills grind to final size (often 75–150 µm). Mills operate in closed circuit with hydrocyclones that return coarse particles.

Bond work index

W_i measures ore resistance to grinding (soft 8–10, medium 12–15, hard 16–20+ kWh/t), from a standard lab test. Bond's law estimates energy between two sizes, used to size mills.

Efficiency and geometallurgy

Ore hardness varies through the orebody, so throughput varies. SMC and JK drop-weight tests size SAG mills; simulation (JKSimMet) optimises circuits. Coarse particle flotation, ore sorting and HPGR reduce grinding energy.

Formulas

• Bond's law: W = 10 W_i (1/√P₈₀ − 1/√F₈₀)
• Mill power: P = W × throughput (t/h)
• Reduction ratio: RR = F₈₀ / P₈₀

Worked example: Ball mill energy

Given: W_i = 14 kWh/t, F₈₀ = 10 mm, P₈₀ = 100 µm; Throughput 500 t/h

Find: Energy and power.

• W = 10 × 14 × (1/√100 − 1/√10 000) = 140 × (0.1 − 0.01) = 12.6 kWh/t
• P = 12.6 × 500 = 6300 kW

Answer: 12.6 kWh/t, 6.3 MW

Worked example: Reduction ratio

Given: Primary crusher feed 800 mm, product 160 mm

Find: RR.

• RR = 800/160 = 5

Answer: 5 : 1 (typical for jaw/gyratory)

Common mistakes

• Wrong: Grinding finer 'to be safe'. — Right: Grind only to the liberation size — overgrinding wastes energy and creates slimes.
• Wrong: Using one work index for the whole deposit. — Right: Map hardness variability.

Applications

• Copper concentrators: SAG–ball circuits at 100 kt/day.
• Gold plants: Grinding to liberate gold for leaching.
• Energy reduction: HPGR and pre-concentration projects.

Key takeaways

• Crush then grind to liberate.
• W = 10W_i(1/√P₈₀ − 1/√F₈₀).
• Power = W × throughput.
• Comminution dominates plant energy.

Going further

• SMC and JK tests: Characterise SAG mill competence.
• Circuit simulation: JKSimMet/USIM PAC models.
• Liberation analysis: MLA/QEMSCAN to set target grind.