Air Separator in Cement Mill: Complete Guide

The air separator is the classification machine of the grinding circuit: the device that receives the mill discharge and splits it into the finished product and the oversize that returns to the mill: the separator decides the fineness, the particle size distribution and the efficiency of every closed-circuit mill in the cement plant: the same classification principle that finishes the raw meal, the cement and the coal: the separator is the silent quality-maker of the grinding sections.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes this air separator guide with the classification theory, the Tromp curve analyses, the separator types and the operation tables: the article walks the file: the principle of the classification, the machine generations, the Tromp curve and its parameters, the circulating load, the operation, the troubleshooting and the performance audits: the reader finishes with the complete professional toolset of the separator engineer.

The separator economics are the handmaidens of the mill economics: a one-point improvement in the separator bypass raises the mill output by several percent, and the classification quality decides the particle size distribution that shapes the cement strength and the water demand: the grinding circuits of the world spend about 35 to 40 kWh per ton of cement, and the separator is the largest single lever of that number: this guide documents the lever completely.

1. The Principle of the Air Classification: The Forces on the Particle

The air separator works by the balance of the forces on the particles in the air stream, and the file opens with the physics of the classification:

• The forces: the drag force of the air (proportional to the particle surface and the air velocity) and the centrifugal force of the rotating flow (proportional to the particle mass and the rotor speed): the fine particles follow the air to the fines outlet: the coarse particles escape the drag and fall to the rejects: the equilibrium particle size is the cut size of the separator;
• The cut size: the d50: the particle size at which 50% of the material goes to the fines and 50% to the rejects: the typical cement separator cut sizes of 20 to 50 microns: the cut size set by the rotor speed, the air flow and the geometry: the single most important number of the classifier;
• The classification of the mixture: the feed of the mill discharge contains the wide size range: the separator must split at the desired cut with the minimum of the misplaced particles: the perfect classifier would give the sharp step function at the cut: the real machines give the S-curves of the Tromp:
• The air circuit: the classifying air enters through the louvres or the distribution plate, carries the fines to the cyclone or the bag filter and returns to the separator: the modern dynamic separators close the air loop internally: the air flow of the circuit 60,000 to 120,000 m³/h in the typical finish mills: the fan power of the system, a significant cost.

The physics section of the file sets the language of the craft: the cut size, the fines, the rejects, the bypass: the operators and the engineers of the package speak the same words from the first chapter: the classification forces, made visible by the numbers of the file.

2. The Generations of the Separators: From the Static to the Dynamic

The history of the air separator is the history of the improvement of this force balance: the file reviews the machine generations:

Generation	Type	Cut sharpness	Specific energy (kWh/t)
1st	Static (gravity) separator	Poor: high bypass	40+ (high mill energy guilt)
2nd	First-generation dynamic	Moderate	38 – 42
3rd	Second-generation dynamic (turboplex)	Good	33 – 38
4th	Third-generation cage rotor (high-efficiency)	Excellent: bypass below 10%	28 – 35 (cement)

The evolution of the file: the static separators (the cone classifiers) gave the poor sharpness and the high bypass of 30 to 50%: the first dynamic separators added the rotating blades: the second generation (the turboplex and the derived designs) improved the dispersion: the modern third generation uses the forced-vortex cage rotor with the tight classification: the high-efficiency separators of the modern plants: the historical table is the reference of the retrofit economics: the old first-generation separator replaced by the third-generation unit lifts the mill output 15 to 30% at the same power: the retrofit studies of the file quantify the upgrading.

3. The High-Efficiency Separator: The Construction of the Modern Machine

The modern high-efficiency separator (the third generation) is the standard of the current plants, and the file details its construction:

• The feed and the dispersion: the mill discharge enters the top and falls onto the distribution plate: the dispersing air and the rotating elements spread the material into the classifying zone: the good dispersion is the prerequisite of the sharp cut: the undispersed lumps pass the classifier as the misplaced rejects;
• The cage rotor: the cylindrical cage of the radial blades rotating at 60 to 150 revolutions per minute (the tip speeds of 20 to 40 m/s): the forced vortex of the cage sets the cut size: the rotor speed, the primary control handle of the fineness;
• The guide vanes: the adjustable stator vanes create the tangential air entry: the guide vane angle and the rotor speed together set the cut: the modern machines adjust the vanes during the operation: the online fineness control of the finish mills;
• The rejects cone and the fines outlet: the coarse material falls to the inner cone and returns to the mill: the fines leave with the air to the collecting cyclone or the filter: the internal air circulation eliminates the external fan ducting: the closed-circuit buffer of the machine;
• The drive and the control: the VFD rotor drive with the fineness setpoint loops: the new machines integrate the separator control with the mill’s expert system: the automatic response to the Blaine targets: the modern classification, an instrumented process unit.

The construction chapter of the file includes the section drawings and the specification tables (the rotor diameters of 1 to 4 meters, the capacities of 20 to 250 t/h, the air flows per the fan curves): the engineer who reads the construction chapter reads the supplier proposals critically: the machine internals, understood completely.

4. The Tromp Curve: The Signature of the Classifier

The classification performance is measured by the Tromp (or the partition) curve, and the file teaches the instrument completely:

• The construction of the curve: the sieve analyses of the feed, the fines and the rejects: the partition value at each size: the percentage of the feed fraction that reports to the rejects: the plotted S-curve: the balance check of the three streams closes within 1 to 2%: the curve, the identity card of the classifier;
• The cut size (d50): the 50% partition point: the position of the curve on the size axis: the d50 of the cement circuits typically 25 to 45 microns: the Blaine target and the residue set the d50 need;
• The sharpness (the Imperfection and the K factor): the I = (d75 − d25) / (2 × d50): the values of 0.2 to 0.5 in the modern machines: the K (the partition slope ratio) of the dynamic separators 2.0 to 4.0: the sharper the curve, the less the oversize in the product and the finer the reject: the sharpness, the quality index of the separation;
• The bypass: the minimum partition value at the fine end: the percentage of the fine material that short-circuits to the rejects: the bypass of 5 to 15% in the good machines: the bypass is the direct efficiency loss of the circuit: the return of the finished-size material to the mill grinds it again: the bypass, the enemy of the capacity;
• The fish-hook effect: the fines-coarse interactions that give the curve its upward bend at the very fine end: the agglomeration of the fines and their rejection: the modern analyses account for the effect: the file’s calculation sheet plots the curve from the three sievings and reads the parameters automatically.

The Tromp section is the analytical core of the file: the Excel tool builds the curve from the residue data of any shift: the file’s worked example (a 4.2 x 13.5 m finish mill with the 220 t/h of the separator feed, the 85 t/h product, the rejects 135 t/h: the bypass 9%, the d50 34 microns, the I 0.32) shows the complete audit: the separator performance, measured like the professionals measure it.

5. The Circulating Load: The Loop Between the Mill and the Separator

The closed circuit is defined by its circulating load, and the file quantifies the loop:

• The definition and the calculation: the circulating load is the ratio of the separator rejects to the finished product: directly from the residues: CL = (a − b) / (c – a) where a is the feed residue, b the fines residue and c the rejects residue at the chosen sieve: the typical cement circuits run 150 to 400%: the raw circuits 100 to 300%;
• The circuit geometry: the mill discharge to the separator, the fines to the silo or the next classifier stage, the rejects back to the mill inlet: the fresh feed joins the rejects at the mill entry: the mill operates on the total feed (the fresh plus the rejects): the 300% circulating load means the mill grinds 4 tons for each ton of product;
• The optimum: the too-low circulation leaves the coarse material too long in the mill (the overgrinding, the pad ding, the low output): the too-high circulation burdens the mill and the separator without the benefit: the optimum of the 200 to 300% in the modern circuits with the well-sharp separators: the optimum shifts with the separator bypass, the mill ventilation and the product spec;
• The fineness split: the separator feed fineness (the Blaine 400 to 700 cm²/g lower than the product), the rejects fineness and the product: the residue pair of the daily control: the 45 micron residue of the feed, the product and the rejects: the daily control charts of the circulating circuit;
• The control of the loop: the separator speed and the mill feed: the expert systems hold the circulating load and the fineness simultaneously: the load feedback of the bucket elevator amps and the differential pressure of the mill: the loops of the modern DCS, documented in the file.

The circulating-load chapter connects the separator to the mill as the single grinding system: the engineers of the package never discuss the separator alone: the mill, the separator and the load, one system: the file’s balance sheets and the control diagrams give the complete system view: the loop, understood and controlled.

6. The Separator in the Finish Grinding: The Cement Fineness Control

The finish mill classification is the most quality-sensitive application of the air separator, and the file treats it with its full importance:

• The fineness targets: the Blaine 3,200 to 4,000 cm²/g (the ordinary OPC at 3,300 to 3,600, the rapid-hardening at 4,000 to 4,500): the 45 micron residue 5 to 15%: the 32 micron residue 15 to 30%: the targets are set by the standard and the market expectations of the strength;
• The particle size distribution: the modern cement quality is decided by the full distribution: the ideal cement contains a low share of the sub-3 micrometer particles (high water demand) and a low share of the above-45 micrometer particles (slow hydration): the sharp separator produces the narrow distribution that optimizes the strength: the narrow-graded cements reach the same strength at a lower Blaine and a lower water demand;
• The strength consequences: the 3 to 32 micrometer fraction hydrates fastest and contributes the most strength: the optimum share of this fraction in the well-separated cement is 60 to 75%: the wide-graded or the over-ground cements waste the strength potential: the separation quality, the performance lever of the cement:
• The sulfate and the temperature balance: the separator returns the hot rejects to the mill: the mill temperature control (the finish mill outlet below 110 to 115 °C) protects the gypsum quality: the false-set risks of the hot cement: the separator’s air and the mill water spray solutions, documented in the file;
• The quality automation: the online particle size analyzers (the laser diffraction on the fines stream) close the loop on the separator speed: the networks of the modern mills hold the Blaine within plus or minus 20 to 40 cm²/g: the automation, the final step of the quality control.

The finish-grinding chapter of the file teaches the bit of the classification in the cement specification: the separation quality is not only an energy matter but the product-quality matter: the file’s distribution analyses and the strength correlations give the laboratory and the process the common language: the cement fineness, engineered by the classifier.

7. The Separators of the Raw Meal and the Coal Grinding

The air separator serves the other mill systems of the plant, and the file dedicates the section to each:

• The raw meal classification: the closed-circuit raw mills use the dynamic separators in the vertical roller mill systems (the integrated classifiers) and in the ball mill circuits: the raw meal targets: the 90 micron residue 12 to 15% and the 200 micron residue 1 to 2%: the coarser raw cut than the cement: the classifier speed, set for the burnability;
• The vertical roller mill classifiers: the integrated cage rotor at the top of the VRM: the classification inside the mill body: the raw meal fineness controlled by the rotor speed and the air flow: the VRM classifier is the same physics in the compact machine: the separate separator disappeared inside the mill;
• The coal mill classification: the coal grinding demands the finest classification of the plant: the pulverized coal for the kiln and the calciner burns fastest when the 90 micron residue is 1 to 3% and the d80 is 40 to 70 microns: the coal mill classifiers (the dynamic types) with the inert atmosphere and the explosion protections: the safety aspects of the coal classification, strict and documented;
• The slag and the additions grinding: the hard slags (the Wi 15 to 20) with the vertical mills and their classifiers: the high fineness demands (the Blaine 4,000 to 4,500 for the CEM III/A cements): the classification economics, applied to the hardest grinds of the plant.

The multi-application chapter of the file gives the complete separator map of the plant: the same physics, the four applications and the four target regimes: the engineer with the complete map optimizes each circuit with the identical toolset: the Tromp curves of the raw, the coal and the finish loops, measured and compared.

8. The Operation of the Separator: The Handles and the Setpoints

The daily operation of the separator is the craft of the mill operators, and the file documents the operator’s instrument panel:

• The rotor speed: the primary fineness handle: the higher the speed, the finer the cut: the speed-fineness curves of the file for the typical machines: the speed setpoints and the Blaine responses: the speed, driven by the DCS or the expert system;
• The air flow: the classification airflow and its distribution: the high air flow coarsens the cut (the drag carries the coarse particles): the air balance with the mill ventilation and the filter: the airflow measurement (the pitot traverses) and the damper control;
• The feed rate and the load: the separator feed (the mill discharge) the sum of the fresh feed and the rejects: the design load of the separator 150 to 250 t/h in the typical finish mills: the overloaded separator loses the sharpness: the load control through the mill feed and the elevator amps;
• The temperature and the moisture: the hot mill discharge (the 100 to 130 °C) and the moisture (the cement below 0.5% H2O): the condensation risks in the winter: the separator dew-point protection and the air heating: the moisture and the temperature disciplines of the operation;
• The monitoring instruments: the pressure differentials of the separator and the filter, the bearing temperatures (the alarms at 75 to 85 °C), the vibration: the daily rounds and the trend logs: the instrumentation of the machine, the operator’s eyes.

The operation chapter of the file includes the complete procedures of the start, the stop, the normal running and the emergency responses: the operator checklists of the shifts: the separator craft, documented for the operators of every level: the daily handles of the classification, presented in the order the shift uses them.

Parameter	Typical value (finish circuit)	Watch point
Rotor speed	60 – 150 rpm	Fineness control loop
Cut size (d50)	25 – 45 microns	Tromp audits
Bypass	5 – 12%	Blade wear
Feed temperature	100 – 130 °C	Gypsum protection
Bearing temperature	Below 75 – 85 °C	Alarm at high level

9. The Troubleshooting of the Separator: The Classic Defects

The separator failures show their symptoms in the fineness and the mill behavior, and the file’s diagnosis tables serve the shift teams:

• The rising bypass (the coarsened product): the causes: the worn rotor blades or the classifier liners, the loading buildup in the reject cone, the air maldistribution: the checks: the Tromp analysis, the internal inspection: the remedies: the part replacement, the airflow rebalance;
• The Blaine fluctuation: the rapid cycles of the fineness: the causes: the feed rate swings, the rejects flap instability, the rotor speed oscillation, the air flow drift: the remedies: the feed stabilization, the control tuning: the fluctuation analysis of the shift logs;
• The separator vibrations: the imbalance of the rotor (the wear and the buildup), the bearing damage, the foundation issues: the vibration monitoring and the rotor balancing: the vibration alarm philosophy of the file: the trip at the sustained high levels;
• The material buildups: the sticky material accumulating in the cone, the air ducts and the vanes: the moisture and the fine-particle agglomeration: the cleaning schedules and the air nozzle solutions: the buildup inspection in the maintenance stops;
• The rejects flow problems: the blocked reject discharge, the flap feeding problems: the rejects circuit check (the elevator or the air slide): the interlock with the mill feed: the reject-flow alarms of the modern systems.

The troubleshooting chapter of the file gives the cause-effect tables with the verification tests and the response times: the separator problems, solved by the systematic method instead of the guesswork: the plants of the package run the diagnosis charts of the file in their maintenance meetings: the classification problems, methodically eliminated.

10. The Audits and the Performance Tests of the Separator

The periodic audit of the separator quantifies its contribution, and the file’s audit procedure is the complete professional method:

• The survey campaign: the simultaneous sampling of the feed, the fines and the rejects over the stable hour: the sieve analyses at the 8 to 10 sizes (32, 45, 63, 90, 125, 180 and 250 microns): the mass balance closure check: the data of the Tromp calculation;
• The parameter extraction: the d50, the sharpness (I and K), the bypass and the fish-hook: the comparison against the supplier’s baseline and the previous audits: the trend of the classifier condition: the machine health, quantified;
• The efficiency index: the classification efficiency at the control size (the percentage of the under-size material recovered in the fines): the 85 to 95% target of the modern machines: the efficiency loss, translated into the mill output loss: the efficiency-to-capacity conversion table of the file;
• The recommendations: the blade replacements, the air adjustment, the vane settings: the priority ranking by the impact: the expected improvements quantified: the audit report format of the package: the professional deliverable of the consulting engineer;
• The frequency: the quarterly full audits for the critical finish circuits and the annual audits for the others: the audits after every major change (the mill relining, the separator rebuild): the audit, the documentation of the machine’s life.

The audit chapter of the file trains the reader to perform the complete performance evaluation: the sampling forms, the calculation sheets and the report templates are the ready instruments: the audit results of the past years show the separator’s aging and the effect of the maintenance: the audits of the file turn the separator performance into the managed number of the plant.

11. The Separator Retrofits: The Modernization Economics

The old separators of the plant are the classic retrofit candidates, and the file quantifies the modernization cases that the plants of the package evaluate:

• The first-generation replacement: the static separator replaced by the third-generation dynamic unit on the same mill: the bypass falls from the 30 to 50% to the 5 to 12%: the mill output rises 15 to 30% at the equal power and the equal fineness: the payback of the typical retrofits 1 to 3 years: the most profitable single investment of the old grinding departments;
• The second-generation upgrades: the turboplex-class separators upgraded with the new rotors and the dispersion systems: the smaller gains (5 to 15%) at the smaller costs: the intermediate step of the budget-constrained plants: the staged modernization path of the file;
• The drives and the control upgrades: the VFD drives and the online fineness controls on the existing machines: the automatic response and the man-power savings: the Blaine stability improvements of the closed-loop operation: the control modernization, the cheapest band of the improvements;
• The civil and the layout constraints: the space, the headroom and the air-duct routes of the existing buildings: the third-generation machines installed in the old towers with the adaptations: the layout studies and the site surveys of the retrofit projects: the engineering realism of the file’s cases;
• The complete case of the file: the 4.2 x 13.5 m finish mill with the 1970s separator at 85 t/h: the retrofit to the high-efficiency unit at 105 t/h with the same Blaine and the same power: the annual value of the 20 t/h at the grinding cost structure of the plant: the project summary table with the capital, the savings and the payback: the retrofit report, complete.

The retrofit chapter of the file is the business case documentation of the classification improvements: the numbers of the capacity, the power and the payback serve the investment committees: the engineering of the retrofit (the layout, the ducts, the control) serves the project teams: the separator modernization, justified and executed by the book: the last major chapter of the guide delivers the improvement economics of the whole classification science.

12. The Frequently Asked Questions

What is the difference between the separator and the screen in the cement circuit?

The screen (the sieve) classifies by the physical size against the aperture; the air separator classifies by the aerodynamic behavior of the particles in the air stream: the screening below 100 microns is impractical in the tonnage flows (the blinding, the capacity), while the air classification works at the 20 to 50 micron cuts effortlessly: the cement industry classifies almost exclusively by the air separators.

What does the bypass of 10% mean for the mill?

The 10% bypass means that 10% of the finished-size material in the separator feed short-circuits into the rejects and returns to the mill: the mill grinds this material again: the bypass of 10% costs roughly 5 to 12% of the mill capacity depending on the circuit: the high-efficiency separators at the 5 to 10% bypass pay for their retrofits through the recovered capacity.

How do I set the cut size of my finish mill separator?

The cut size follows the Blaine and the residue targets: the typical finish cement at the Blaine 3,500 requires the d50 of the separator at 30 to 40 microns: the rule of the thumb: the d50 roughly twice the particle size at the 50% of the product distribution: the rotor speed is the primary handle: the plant’s own speed-fineness calibration curves, refined by the Tromp audits.

Why do the vertical roller mills not need a separate separator?

The vertical roller mills integrate the classifier into the mill body: the cage rotor at the top of the mill classifies the pneumatically-conveyed meal as it leaves the grinding zone: the integrated design saves the separate machine, the ducting and the elevator, at the cost of the classification volume: the same Tromp physics measured with the same methods.

Is the separator included in the package’s Excel tools?

Yes: the Complete Cement Technical Package includes the Tromp curve calculation spreadsheet that builds the partition curve from the sieve data and extracts the d50, the sharpness and the bypass automatically: the 931 files of the library include the calculators, the courses and the books: the audits of this article, executed by the tool.

13. Conclusion

The air separator: the force balance, the machine generations, the Tromp curve, the circulating load, the application regimes and the audits: the complete classification knowledge of the cement grinding circuits: the separator is the quality-maker and the capacity-maker of the mill: the engineer who masters the classification masters the most effective lever of the grinding economics: the circuits of the plant, sharp and efficient: the separators, understood and controlled.

The Complete Cement Technical Package includes the air separator guide with the Tromp tools, the operation tables and the audit templates: the one-time $249.99 purchase, the instant download and the lifetime access: the 931 files of the library: the classification knowledge of the industry, at the hand of the professional.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.