Crush More, Grind Less: Feed Size Sets Mill Performance
A gold mine in Liaoning moved its grinding feed from a top size of 15 mm down to 10 mm. Daily throughput rose from 1,150 to 1,500 tonnes. Ball consumption fell from 1.53 to 1.1 kg/t. Specific power dropped from 41.57 to 29.2 kWh/t. Nothing inside the ball mill was touched. That is the whole argument for “crush more, grind less” in one paragraph — and it has a direct line to your liner budget.
Why the mill is the wrong place to break rock
Grinding consumes the majority of comminution energy in every concentrator — Chinese plant surveys put the crushing-grinding complex at 65–75% of plant power, with the ball mill taking the lion’s share. A ball mill breaks the last fraction of particle size at the highest energy cost per tonne of anything in the circuit. Every millimetre of top size you remove upstream is energy the mill never has to spend, ore it never has to lift, and steel it never has to wear off.
The effect is not subtle:
| Operation | Change | Result |
|---|---|---|
| Liaoning gold mine | feed −15 mm → −10 mm | 1,150 → 1,500 t/d; balls 1.53 → 1.1 kg/t; power 41.57 → 29.2 kWh/t |
| An iron ore operation processing 2 Mt/a (Central China) | feed −13 mm → −8 mm | annual throughput 2.0 → 3.0 Mt/a |
| Iron ore concentrator, Inner Mongolia | crushing screen optimization | ball mill 360 → 380 t/h; crusher liner life +15 to +30 days |
The Inner Mongolia case: no new mill, +5.6%
The Bayan-Obo-type iron operation in Inner Mongolia had a different bottleneck: crushing-screening. The fine crusher was being fed unevenly — “more at the front and sides, less in the middle” — so the crushers ran choked in some sectors and starved in others. The fixes were embarrassingly cheap: a cylindrical distribution chute that lets ore build its own protective dead bed, and resetting the secondary crusher eccentric from 52 mm to 47 mm for a finer, better-graded product.
The interesting engineering was in the screen aperture study. Four options were weighed, and the finest was not the winner:
| Screen aperture (lower deck) | −12 mm fraction | Screening efficiency | Verdict |
|---|---|---|---|
| 14 × 25 mm | 79.74% | 81.43% | too coarse |
| 12 × 25 mm | 89.89% | 71.83% | efficiency collapse — blinding |
| 11 × 35 mm | 94.12% | 84.70% | the optimum |
| 11 × 45 mm | 81.67% | 90.87% | slots deform, wire life falls |
Finer is not automatically better. The narrow-but-long 11 × 35 mm slot passed 94.12% minus-12 mm material at acceptable efficiency, while the ultra-fine 11 × 45 mm option destroyed screen life — a trade every screen man knows, but rarely sees quantified.
The payoff landed in both directions at once: ball mill throughput 360 → 380 t/h, and crusher liner life up 33–50% (45 → 60 days on the secondary, 60 → 90 on the tertiary), worth about RMB 260k a year in liners plus RMB 337k in power. The mill and the crushers stopped competing.
The Hebei case: when the ore changes under you
An iron mine in Hebei designed its circuit around an autogenous mill — φ6.7 × 2.6 m plus a φ3.2 × 4.5 m ball mill. Then the ore body turned soft and powdery: fines reached 40% of feed, and +150 mm lumps — the grinding media an autogenous mill depends on — fell from 40% design to 25–30%. Autogenous grinding lost its media; throughput sank to 90 t/h against a 157 t/h design, at 25 kWh/t.
Their recovery is a textbook SABC conversion in five steps:
| Step | Measure | Throughput after |
|---|---|---|
| 1 | add φ100 mm balls at 9% filling — AG becomes SAG | 125 t/h |
| 2 | self-return screen slot 10 → 20 mm (pebbles pass forward) | 160 t/h |
| 3 | dry magnetic separation rejects 60% of pebbles as waste rock | 165 t/h, ball mill power 530 → 490 kW |
| 4 | 37 kW roller crusher re-crushes pebbles to −5 mm | no more pebble ejection, metal recovery +2 points |
| 5 | switch ball mill media to 50 × 45 mm cylpebs | circulating load back under control |
Final state: 170 t/h average (200 on soft ore, 160 on hard), power down from 25 to 14 kWh/t — a 44% cut — and ball consumption at 0.26 kg/t. Total added machinery: one small roller crusher, one magnetic pulley, one electromagnet.
The liner lesson from the same plant: their SAG shell liners went through three material generations in parallel. Plain manganese steel cracked as it thinned (about 8 months). A harder chromium-nickel alloy resisted cracking but ground away in 5 months. The answer was a compromise manganese steel with a trace of chromium-nickel, the impact face thickened from 60 to 70 mm, and the working angle relaxed from 70° to 60° — that set reached 12 months. Material, geometry and thickness had to move together, the same argument we make in our liner selection guide.
The cement-side confirmation
Short mills are even more sensitive. A survey of three plants running φ2.4 × 7 m mills (length-to-diameter ratio under 3) found all three failing to reach design output for the same upstream reasons: coarse feed, variable moisture, leaking air circuits, unstable feeding. The corrective list started with feed size — get below 20 mm, ideally below 10 mm — before a single mill-internal change was justified. One of the three went from 7–8 t/h to 12.5–13 t/h simply by closing the circuit with a classifier.
What this means for liner budgets
Feed size work pays twice on wear: mills lift less mass per useful tonne, and crushers — the other big liner consumer — run evenly fed, which is the single best thing for their liner life too. Before budgeting your next liner campaign, spend an afternoon on the numbers upstream. The cheapest liner upgrade at your plant probably is not a liner.
QM Mill Liners audits feed-size-to-liner-life chains as part of every quotation — send your circuit details and current liner life, and we will tell you where the money actually is — contact here.
FAQ
What does “crush more, grind less” mean? Doing more size reduction in the crushing stage, where energy cost per tonne is lowest, so the ball mill receives finer feed — cutting mill power draw, ball consumption and liner wear per tonne of product.
How does feed size affect ball mill power? Finer feed means the mill spends less energy per tonne on breakage. In the cases above, moving feed from 15 to 10 mm cut specific power roughly 30% at higher throughput.
Does finer crushing extend liner life? Usually yes — the mill lifts and turns less mass per tonne of product, and evenly fed crushers wear their liners 15–50% longer, as the Inner Mongolia case showed.