intro (answer paragraph)
An air separator (also called an air classifier or cement mill separator) is the device on top of a closed-circuit ball mill that receives the mill’s discharge mixture of fine powder + coarse grains and separates them by air classification: the fine fraction (finished cement, typically < 30–45 µm) is carried away with the classification air, while the coarse fraction is returned to the mill inlet for another pass. Its diagram matters because the machine is the only “brain” of the grinding circuit: separator setting = fineness (Blaine), separator efficiency = tons per hour at that fineness.
The diagrams in this article are drawn from the “15 Air Separators” module of the ACMC/ASEC grinding course included in the site’s technical package — the same course pages shown as feature images. Read with the parts list below, the diagram tells you exactly which port to open and which vanes to turn to get a finer product.
2. Where the separator sits in the grinding circuit
- Closed-circuit ball mill: mill → elevator → air separator → fine to silo; coarse returns to mill. The airflow is: separator fan → separator → back through the mill and filter.
- The separator drives the Blaine of cement: coarser set-point → higher t/h, lower strength; finer set-point → lower t/h, higher Blaine.
- Circuit types: first-order (mill + static separator), second-order (mill + dynamic separator + separate fan), third-generation (mill + high-efficiency separator). The third is standard today; the course’s historical sequence (Sturtevant 1st gen → 2nd gen → 3rd gen) explains why each step existed.
Typical circuit numbers (4,000 t/d plant, finish mill): mill capacity 100–150 t/h; circulating load 200–400% (that is 2–5× the feed passing the separator per hour); separator feed 250–600 t/h; product 90–120 t/h; rejects 150–450 t/h back to the mill inlet. These are the numbers you compute with the mass-balance sheets in the “Operation of Separators” workbook.
3. The air separator diagram — parts list (the labels on the drawing)
Reference the diagram (feature: package_shots/air_separator_p22.png, course page) — every part below maps to a number on it:
| # | Part (label) | Function |
|---|---|---|
| 1 | Feed inlet / spout | accepts mill discharge (or air-swept feed) |
| 2 | Distribution cone / plate | spreads feed evenly into the classification zone |
| 3 | Classification zone | where air meets the falling material curtain |
| 4 | Rotor (dynamic separators) | rotates (100–300 rpm), creates the “separating” vortex |
| 5 | Guide vanes (adjustable louvers) | set the tangential air velocity / cut size |
| 6 | Air inlet duct | from the separator fan |
| 7 | Fine product outlet | fines + air go to bag filter / collecting cyclone |
| 8 | Coarse (rejects) outlet | returns to mill inlet |
| 9 | Separator fan | supplies classification air (volume + pressure) |
| 10 | Damper / flow control | adjusts air rate in the circuit |
The two physics at work: (a) air drag lifts fines upward; (b) the rotating vane system creates a tangential velocity that flings coarse particles outward — the balance between air drag and centrifugal force sets the cut size (d50). Adjusting vane angle / rotor speed shifts d50 → coarser or finer.
4. Static vs dynamic separators (the two families)
| Feature | Static separator (old; 1st/2nd gen) | Dynamic separator (3rd gen high-efficiency) |
|---|---|---|
| Moving parts | none (only vanes) | rotor (variable speed) |
| Cut-size control | vane angle only | rotor speed + vanes |
| Efficiency | 40–60% | 70–90%+ |
| Blaine precision | coarse setting | sharp cut, lower variation |
| Typical power | low (only fan) | rotor drive + fan |
| Use today | older plants, drying/feed circuits | standard in modern finish mills |
Static diagram: feed falls through a stationary vane ring; air passes inward. Dynamic (3rd gen): feed lands on the rotor surface; the rotor speed (via a drive) directly controls cut size — that’s the machine most “high efficiency separator” pages describe (see the Othman “High Efficiency Separator” book in the package).
5. High-efficiency separator (3rd gen) diagram details
The modern design (Othman’s course, High Efficiency Separator.pdf) adds:
– Feed at top → drop onto a distribution plate → particles spread as a thin curtain
– Rotor with vertical blades — the separating zone; fines pass through the blades to the inner cone
– Guide vanes around the rotor (2nd air supply below) → create the “counterflow” classification
– Fines duct to the bag filter; coarse falls through the inner cone to the return duct
– Air volume split — usually 1/3 through the mill, 2/3 fresh air to control temperature & moisture
6. The air quantity — separator fan sizing (with formula)
Classification needs 2–2.5 m³ of air per kg of cement per hour typically (1.5–3.0). Vent sizing:
Q = (t/h × 1,000 kg × specific air demand m³/kg·h)
Example: 100 t/h mill → 100,000 × 2.0 = 200,000 m³/h classification air. Fan static pressure: 2,500–4,500 Pa depending on circuit resistance (filter, ducts, vanes). Most plants run a dedicated separator fan on the bag-filter side; vane throttling wastes energy — adjust speed/vanes instead.
Air balance rules of thumb:
| Parameter | Typical value | What it controls |
|---|---|---|
| Air-to-fines ratio | 1.5–3.0 m³ air per kg product | cut size & d50 precision |
| Secondary air share (3rd gen) | 30–50% of total | sharpness of separation below rotor |
| Mill sweep air | 1.2–1.8 m³/kg | mill temperature + early fines removal |
| Fan static pressure | 3,000–5,500 Pa | duct/filter/vanes resistance |
| Rotor speed range | 1:3–1:5 turndown | control range in daily operation |
High separator efficiency (≥ 80%) in the 3rd generation comes from balancing all five — the course’s efficiency-vs-vane-angle charts summarize the trade-off.
7. Efficiency & troubleshooting (short table)
| Symptom | Cause | Fix |
|---|---|---|
| Too coarse despite fine set | vane wear / rotor gap | re-laser gaps, adjust vanes |
| Low Blaine | high feed rate / overloading | reduce feed or increase air |
| Clogged fine duct | condensation / high moisture | preheat air, check dew point |
| High rejects | vane angle too coarse | close vanes slightly |
| Vibration | rotor imbalance / build-up | clean & rebalance |
Separator efficiency formula (common plant form): η = (fines in finished / fines in feed) with the rejects stream — see the course for the full mass balance version.
The Tromp curve (separation curve) explained:
For a given air separator the separation performance is usually drawn as a Tromp curve: the probability (0–100%) that a particle of a given size in the feed reports to the coarse product, plotted against particle size. The curve tells the operator two numbers:
- d50 (cut size): the particle size where 50% of it goes to coarse, 50% to fines — the conventional “cut”.
- Sharpness index: typically “cut anomaly” measured as the ratio d75/d25; a sharp separator curve has d75/d25 → 1 (that is, nobody misplaces a 10 µm into the coarse and a 60 µm into the fines).
A healthy 3rd-gen separator yields a nearly vertical Tromp curve; a worn static separator shows a crazy and long “fish hook” (coarse contamination in the product, fines lost in the rejects). This one graph — not just the set point — is what a plant takes home to tune vanes and rotor.
8. Efficiency, quality and troubleshooting (short table)
| Symptom | Cause | Fix |
|---|---|---|
| Too coarse despite fine set | vane wear / rotor gap | re-laser gaps, adjust vanes |
| Low Blaine | high feed rate / overloading of the classifier | reduce feed or raise air flow |
| Clogged fine duct | condensation / excessive moisture | preheat air, control dew point |
| High rejects | vane angle too coarse / low air | close vanes slightly |
| Vibration | rotor imbalance / build-up | clean & rebalance |
Separator efficiency in the common plant form η = (fines in finished / fines in feed) is what the “Operation of Separators” workbook asks you to compute weekly — a fall of more than 5 points typically precedes lining or rotor-gap wear.
What is an air separator? A classifier on a closed-circuit mill that separates fine powder from coarse by air drag vs centrifugal force; fines → product, coarse → mill return.
How does an air separator work? Feed falls into an airstream; fine particles are carried up and out; coarse particles fall back (static) or are flung outward by a rotating rotor (dynamic).
What is the difference between static and dynamic separator? Static has fixed vanes and no moving parts; dynamic has a speed-controlled rotor for a precise, adjustable cut size.
Where is the air separator used? Cement finish/raw grinding circuits — and anywhere a powder must be split at a cut size (also coal grinding, raw meal, drying circuits).
What is d50 of a separator? The particle size at which 50% of the feed goes to each product — the operating point the operator sets with vanes/rotor.
Does the separator affect cement strength? Yes: it sets the fineness (Blaine) and particle size distribution, which control early hydration, water demand and 28-day strength (link to 28-days article).
What do ‘first/second/third generation’ mean? First generation = static separator, no moving parts; second = dynamic, rotor + separate fan; third = high-efficiency separator with sharper Tromp and usually a second air zone. Modern finish mills are all third-generation; older courses still explain the earlier types because many lines run them.
Where does the separator fit in energy? The separator and its fan represent roughly 10–20% of finish-grinding electricity in a modern circuit — and an efficient separator typically saves 5–10 kWh/t of cement compared to a coarse static classifier at the same Blaine. That is why every energy audit in the 04_GRINDING courses starts at the separator.
How do I read a separator mass balance? Feed = fines + rejects (and the air carries the fines). Measure feed, rejects and product for 1 hour, then: circulating load = rejects ÷ product; separator efficiency = (fines in product × 100) ÷ (fines in feed). The “Operation of Separators” workbook walks this sheet row by row.
How often should the rotor/vanes be checked? Weekly visual (build-up, vane wear), monthly gap/clearance, quarterly a full air balance — the troubleshooting table above lists the symptoms each inspection catches.
What causes separator fires? Static from the dry powder + hot dust; the classic dimension is mill discharge hotter than 110–120°C in coal or fuel-fines circuits. The grinding course devotes a section to fire protection (CO2/N2 inerting) for exactly this risk.
Why does the separator matter for the cement grade? The same clinker can be ground to 3,200 or 4,200 Blaine; the separator is what decides. OPC 43/53 grade targets and PPC fineness ranges are achieved in the separator, not in the mill — which is why “separator” and “cement fineness” are inseparable keywords in any grade discussion (see the types-of-cement article).
Deep-dive extras from the technical package
- “15 Air Separators” (ACMC grinding course, 35 pp, scanned) — history: 1st gen Sturtevant-type, 2nd gen, and the cyclone/side-feed designs; diagram p.22 is the feature.
- “High Efficiency Separator” (Othman, 23 pp) — 3rd gen construction details, rotor/vanes, air circuit calculations.
- “Operation of Separators” (30 pp) — field tuning, mass balance, efficiency curves, troubleshooting case histories.
- All three PDFs ship with the technical package (04_GRINDING folder).