Three Chambers to Two: A +19.2% Throughput Case
A φ3.4 × 9 m coal mill on a 2,500 t/d clinker line was rated for 24 t/h and delivered 20. In 2017 the plant removed a partition plate and gained one tonne per hour. In 2021 the same mill gained six. The difference between +5% and +19.2% was not the number of chambers — it was everything that has to move when a chamber disappears. This is the anatomy of that second decision.
The shallow fix, 2017
The original layout was a 3 m drying chamber, a 2.5 m coarse-grinding chamber and a 3.5 m fine chamber, separated by a single-wall diaphragm that kept deforming and blinding. The 2017 fix took out that single diaphragm and patched the liner gap — and stopped there. The double diaphragm, wave liners and ball charge all stayed as-built. Result: 21 t/h. The concept was right; the follow-through was absent.
The deep fix, 2021: four changes that move together
| Element | Before | After |
|---|---|---|
| Double diaphragm position | after 5.5 m | moved forward 250 mm — drying 3 → 2.75 m, grinding 6 → 6.25 m |
| Drying lifters | 300 mm | 500 mm plates — more heat-exchange surface |
| Shell liners | mixed wave/groove, as-built | 25 rings re-laid: reverse-wave ring at the diaphragm, then staged wave + classifying rings alternating |
| Ball charge | 56 t, 25.1% filling | 50.5 t, 21.5% filling, more φ25 mm small balls |
The liner re-layout is the heart of it. Removing a diaphragm does not remove the job the diaphragm did — it just hands that job to the liners. With one chamber where there were two, balls of every size roam the full 6.25 m length unless something sorts them. The plant alternated single rings of plain wave liner with single rings of classifying liner, so the charge experiences a continuous axial sorting: large balls stay working near the feed end, small balls migrate toward the discharge. A stopped-mill inspection after commissioning found the ball sizes distributed large-to-small along the axis with no back-migration.
The reverse-wave ring at the old diaphragm position stops balls from firing straight into the classifying sections, and a wind baffle in the drying compartment (φ900 mm disc, centred on the inlet duct) rebalanced the internal airflow that losing a chamber had left uncontrolled — pressure drop fell from 2,100 to 1,750 Pa.
The results
| Metric | Before (2020) | After (2021 rework) |
|---|---|---|
| Throughput | 20 t/h | 26 t/h (+19.2%) |
| 80 µm sieve residue | > 6% | 5% |
| Specific power | 17.72 kWh/t | 16.14 kWh/t |
| Auxiliary system power | ~20,100 kWh avg | ~10,975 kWh (−7.1%) |
One extra tonne per hour became six. Anyone quoting the 2017 experience (“we removed a chamber, it barely helped”) stopped at step one.
The independent confirmation
A cement plant in Inner Mongolia ran the mirror-image experiment on a φ3.4 × (6+3) m air-swept coal mill: two chambers merged into one, with the partition removed and its screening function replaced by classifying liners — corrugated-step lifting liners at the feed end, combined small-wave classifying liners behind them, plus a distribution grate to govern air velocity. Result: +5 t/h, pressure drop −800 Pa, charge reduced by 9 t, motor current down 9 A, about RMB 190k a year in power — and no loss of product fineness.
Two plants, opposite directions in chamber count, same law: the gain from any chamber change is capped by how well the liners can classify.
The diaphragm itself: slots and gaps
If you keep your diaphragms, the slot spec is doing more work than most mills admit. The industry trend runs from slots above 8 mm down to 6 mm and, in fine-grinding applications, 2–3 mm — narrower slots force the crushing chamber to actually finish its job before material passes. On one φ3.6 × 8.5 m two-chamber cement mill, bringing the effective opening down to 2–3 mm cured chronic mill choking (“full grinding”) and eliminated recurring coarse discharge.
But there is a trap the same author flagged, and it swallows most of the benefit in the field: installation gaps eat design slots. Adjacent diaphragm plates bolted with a gap between them, or daylight between the plates and the centre ring, will pass whatever the design slots were meant to stop. A 2 mm slot next to a 5 mm erection gap is a 5 mm slot. Slot width is a tolerance problem, not just a drawing number — every mill has to find its own optimum for its ore, media and ventilation, and then defend it at installation.
That connects directly to how we spec liner packages: chamber geometry, slot condition and liner profile are one system, and worth auditing together with the liner cost-per-tonne review before any reline.
QM Mill Liners supplies classifying liner sets and diaphragm plates machined to defend your design slot width at assembly — send your chamber drawing and we will quote the full package, contact here.
FAQ
Why convert a three-chamber mill to two chambers? Fewer partitions mean lower internal resistance, longer effective grinding length and simpler media management — but only if the liners take over the classification job, as the +19.2% coal mill case showed.
What do classifying liners do? Their profile sorts grinding media along the mill axis — large balls toward the feed end, small toward the discharge — replacing the size separation a diaphragm used to perform, and preventing back-migration of media.
How narrow should diaphragm slot openings be? Industry practice runs 8 mm down to 2–3 mm for fine grinding, but the working width is set by installation quality: gaps between plates at erection determine what actually passes, so tolerance control matters as much as the design value.