Separating: Complete Technical Guide

Separating is the silent half of the closed-circuit grinding system: while the mill grinds, the separator decides which particles leave as the product and which return as the circulating load, and that single decision sets the mill efficiency, the cement fineness, the particle size distribution and the plant’s kWh per tonne: the presentation “29176758 05 Separating” from the cementequipment.org library documents the separation equipment of the cement grinding circuits: the gravity and the mechanical separators, the modern third-generation dynamic classifiers, the cut size, the Tromp curve, the bypass and the sharpness: this article expands the full logic of the file, from the principles of classification to the operating curves and the formulas that every mill engineer must hold.

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 separating presentation together with the separator operation courses, the high-efficiency separator handbooks and the ball mill optimization tools: this article walks the file for the process engineers and the mill operators who run, audit or replace the separators of their grinding circuits, and it reproduces the formulas and the typical values so that the separator can be discussed with the numbers, not the opinions.

The separator is often the cheapest component of the grinding system in capital terms and the most influential in performance terms: the same ball mill, served by a poor separator, grinds expensive cement with the wide particle size distribution and the high circulating load, and the same mill served by a sharp third-generation classifier produces the right product at the higher capacity and the lower energy: the file teaches the machine that turns the mill from a grinder into a system, and this article carries the whole lesson.

1. The Role of the Separator in the Grinding Circuit: the closed-loop eye of the mill

The closed-circuit grinding system works on a simple idea: the mill grinds the material coarsely and quickly, the separator removes the finished particles and returns the oversize for the re-grinding, so that the mill never wastes its energy re-grinding the already-fine material: the separator is the organ that makes this loop possible: without it the mill must grind everything to the final fineness in one pass, which is the slowest and the most energy-hungry way to make fine cement: with it, the mill operates on the coarse intermediate and the separator finishes the product.

The flow of the closed circuit is the daily picture of every finish mill:

• The mill discharge: everything the mill produces, the fine and the coarse together, arrives at the separator feed, typically at the fineness of 70 to 85 percent passing 90 microns;
• The separator action: the classifier splits that feed into the fine product, the finished cement at the target residue, and the coarse rejects, the oversize that returns to the mill inlet;
• The circulating load: the rejects stream returned to the mill is measured against the fresh feed: the circulating loads of the cement circuits run 150 to 400 percent, and the raw circuits 100 to 250 percent;
• The finished product: only the separator’s fine stream leaves the system to the cement silos or the blending, and its fineness and its particle size distribution are the quality the customer pays for;

The balance of the loop is expressed by the material balance around the separator: the feed equals the product plus the rejects, and every tonne of the rejects that the separator sends back is a tonne the mill must handle again: the efficiency of the separator decides how much of that circulation is useful re-grinding work and how much is the waste, and the file’s analysis tools exist to make that split visible: the separator is the eye of the loop because it sees which particles are ready, and the whole circuit’s economy follows its judgment.

2. The Principles of Classification: gravity, centrifugal force and the fluid drag

All the air separators work by the same physics: the particles in the gas stream are acted on by the opposing forces of the fluid drag, which the air exerts on the particle, and the separating force, which is the gravity or the centrifugal acceleration of the rotating flow: the small, light particles are carried outward and inward with the air and leave with the fine stream, while the large, heavy particles lose the drag battle and settle to the coarse side: the cut size is the particle size at which the two forces balance, and everything the classifier does is the manipulation of that balance.

The settling (Stokes) expression behind the cut size:

v-term = (d2 × (ρp − ρa) × g) / (18 × μ)

Where v-term is the terminal settling velocity, d the particle diameter, rho-p and rho-a the particle and the air densities, g the gravitational or the centrifugal acceleration and mu the air viscosity: the formula shows the two levers of the classifier: the square of the particle size and the acceleration of the separating zone: the modern dynamic separators replace the weak gravity with the strong centrifugal acceleration of the rotor, which allows the sharp classification at the much finer cut sizes with the compact machines: the physics is unchanged, and the acceleration is up.

• The drag force: the fluid drag on the particle grows with the air velocity, the drag coefficient and the cross-section, and it is the force that carries the particle toward the fine outlet;
• The separating force: the gravity in the static classifiers and the centrifugal force in the rotating ones pull the particle toward the coarse side: the stronger the force, the finer the cut;
• The particle trajectory: in the swirling flow each particle follows the trajectory set by the balance of the two forces, and the classifier geometry is arranged so that the balance point is stable and sharp;
• The air flow pattern: the secondary and the tertiary air flows of the modern separator sweep the coarse reject stream and reclaim the fine particles that the primary flow missed, cutting the bypass;

The physical picture the file teaches is the race between the drag and the separating force, and every generation of the separator technology is a refinement of that race: the first generation tried to win it by the shape of the static vanes, the second by the rotating flow surrounding the settling zone, and the third by placing the classifying rotor directly in the flow where the centrifugal acceleration is the strongest: the presentation walks that history because the installed base of the plants still contains all three generations, and the engineer must know which one he is auditing.

3. The Generations of the Separators: from the gravity cone to the third-generation rotor

The classifying equipment of the cement industry evolved in three clear generations, and the presentation maps them with their strengths and their limits: the first generation, the static air separator, is a cone with the internal vanes where the settling velocity does the work; the second generation, the mechanical air separator or the cyclone-type with the fan, adds the controlled air circulation and the internal whizzer blades to sharpen the cut; and the third generation, the high-efficiency dynamic separator, places a cage rotor in the classifying zone to reach the sharpness the earlier machines could not.

The numbers in the table are the efficiency and the bypass that the file uses to justify the modernisation: the old second-generation separators on the cement mills waste part of the mill’s energy by returning fine material with the rejects, and the plants that upgrade to the third-generation classifiers routinely recover 15 to 30 percent of the mill capacity at the same fineness, a payback measured in months rather than years: the third-generation machine does not grind anything itself, but it makes the mill’s grinding count, and that is the economic argument the file makes: the separation is the cheapest capacity increase a cement plant can buy.

4. The Cut Size and the Separation Curve: the fingerprint of the classifier

The behaviour of a separator is summarized by its separation curve, the Tromp curve, which reports what percentage of each particle size in the feed goes to the coarse side: a perfect separator would send all the particles below the cut size to the fines and all the particles above it to the rejects, giving a vertical step at the cut size, but the real separators draw an S-shaped curve that is never vertical: the file devotes its core to the reading of this curve because the curve is the machine’s fingerprint and the starting point of every diagnosis.

The cut size: the d50, or sometimes the d-T, is the particle size at which 50 percent of the particles go to the coarse side and 50 percent to the fines: the cut size is the operating point of the separator, and it is set by the rotor speed, the air flow and the feed loading: the finer the required product, the smaller the cut size, and the separator is set to put the cut just below the target so that the fines contain the finished product.

• The Tromp curve shape: the ideal is the step; the real curve shows the gradual rise from the fine end to the coarse end, and the steepness of that rise is the sharpness of the separation;
• The imperfection: the sharpness is quantified by the imperfection I, defined as (d75 − d25) / (2 × d50), with the good third-generation separators reaching imperfection values of 0.1 to 0.3;
• The bypass: the plateau of the curve at the fine end, where 5 to 20 percent of even the finest particles are carried to the rejects, is the bypass: it represents the short-circuiting of the fine product, and it is the waste the modern separators are designed to cut;
• The fishhook: some curves show a rise at the very fine end, the fishhook effect, caused by the agglomeration of the sticky fine particles: its presence is a signal of the material problems or the poor dispersion;

The presentation teaches the complete curve: the cut size fixes the fineness, the sharpness fixes the particle size distribution, and the bypass fixes the efficiency: the three are separate properties of the same machine, and the operator who wants a better cement must move the right one: raising the rotor speed lowers the cut size and gives the finer product, but it cannot fix the bypass or the sharpness: those belong to the geometry, the dispersion and the air flows, and the file keeps the three disciplines separate in its diagnosis sections.

5. The Efficiency, the Bypass and the Sharpness: the numbers that price the separator

The presentation defines the performance numbers precisely, because the whole value of the separator is in the numbers: the separation efficiency, the bypass and the sharpness (imperfection) are the three quantities that the audit computes from the Tromp curve and that the supplier’s guarantees used to express, and the modern contracts specify all three along with the fineness of the product:

The separation (collecting) efficiency for a given size class:

E-class = (c-coarse × (1 − a-fine)) / (c-fine × a-coarse) × 100

Where the a and c terms are the percentages of each size class in the feed and the rejects stream: the simpler practical form used in the daily work is the overall circuit efficiency, the ratio of the fine product carried out of the system to the fine material available in the feed, so that a perfect separator takes every fine particle straight to the product and the efficiency approaches 100 percent: the third-generation separators operate at 75 to 95 percent, and the old mechanical ones at 55 to 70, with the difference being the money the modernisation recovers.

The bypass deserves the special attention the file gives it, because it is the purest waste in the circuit: every fine particle that falls into the rejects must pass through the mill again, consuming the grinding energy on the material that was already finished: the modern separators attack the bypass with the dispersion of the feed, the secondary and the tertiary air that re-sweep the coarse stream, and the de-agglomeration, and the achieved reduction of the bypass from 30 to 10 percent is the primary reason for the capacity recovery of the modernised circuits: the file quantifies that recovery so the payback calculation is honest.

6. The Air Flows and the Loading of the Separator: the transport numbers

The separator is an air machine: the classification happens in the gas stream, and the air volume, the air velocity and the material loading of that stream set the cut size and the product transport: the presentation treats the air system as the bloodstream of the separator, and it gives the operating numbers that every engineer must know: the separator air volume is sized from the product rate and the required entrainment, and the solids-to-air loading of the classifying zone is the master constraint of the design.

The air-to-fines loading of the separator: the material feed rate to the separator divided by the air volume gives the concentration of the solids in the classifying zone, and the practical designs hold that concentration in a window that balances the productivity against the classification sharpness:

• The loading of 0.5 to 1.0 kg of feed per normal cubic meter of air is the common design range of the high-efficiency separators: too high a loading crowds the classifying zone, the particles collide and the separatio sharpness collapses; too low wastes the fan energy;
• The gas volume vs the product flow: the air volume that carries the finished product to the bag filter or the cyclone is set so that the conveying velocity in the ducts stays 15 to 25 m/s, fast enough to carry, slow enough to avoid the abrasion;
• The cyclone recovery: most circuits recover the product with the cyclones ahead of the bag filter, and the cyclones take the bulk of the load so that the filter receives a light dust: the cyclone efficiency of 90 to 95 percent is normal;
• The fan power: the separator fan is a significant consumer of the circuit energy, and the pressure drops across the classifier, the cyclones, the ducts and the filter set the fan duty: the audit of the air system is an energy audit as much as a quality audit;

The air and the material meet in the separator, and their balance is the operating reality of the plant: the operator who raises the feed rate to gain the production must raise the air to match, or the separator floods and the fineness coarsens; the operator who raises the rotor speed to gain the fineness must check the air, because the finer cut means the more air per tonne of the product: the presentation presents the coupled settings as the control map of the separator, with the feed, the air and the rotor speed as the three axes of the operating plane.

7. The Circulating Load and the Mill Separator Balance

The separator does not work alone: it works against the mill, and the circulating load is the language of their conversation: the circulating load is the ratio of the rejects stream to the fresh feed, and it tells the auditor how many times the average particle passes through the mill: the cement circuits run 150 to 400 percent, and the load is the product of the mill discharge fineness and the separator cut: the coarser the mill discharge, the higher the load, because more of the mill output must be returned.

CL = (rejects / fresh feed) × 100

The presentation derives the circulating load from the separator balance, and it teaches the classic optimisation logic: the optimum circulating load is the one that minimizes the total specific energy of the circuit: too low a load means the mill over-grinds towards the separator feed because the internal fines must be created in the mill; too high a load means the mill spends its energy on the endless re-handling of the coarse rejects: the curves of the file show the optimum in the band where the circuit reaches its minimum kWh per tonne, and the modern plants run their separators with the online particle size analyzers to hold that optimum while the material changes.

• The mill discharge fineness: the classic third-generation circuits run the mill discharge at 70 to 85 percent passing 90 microns, leaving the separator to finish the product and returning the rest;
• The separator efficiency at the load: the load falls as the separator sharpness rises, because a sharp separator takes the product faster and leaves less to circulate;
• The internal recirculation trap: the fine particles that collapse into the rejects through the bypass inflate the load without any useful work, and the bypass reduction is the modernisation’s main weapon on the load;
• The load measurement: the load is measured by the weighers on the rejects stream or computed from the separator balance with the feed, the product and the fineness values: either way, the load is the audit’s thermometer;

The balance discipline of the file is the discipline of the whole circuit: everything that leaves the mill arrives at the separator, everything the separator sends back returns to the mill, and the two machines must be tuned as one: the mill audit without the separator is half an audit, and the separator audit without the mill is speculation: the presentation pairs the two into the complete closed-circuit study that the optimisation engineers perform, and the tools of the package automate the balances that the file teaches.

8. The Design of the Third-Generation Separator: the rotor, the vanes and the geometry

The third-generation separator is a compact machine with a precise inner geometry, and the presentation walks that geometry because the operator who understands it reads the machine: the feed enters from the top through the feed gate, is dispersed by the rotating plate, and falls into the annular classifying zone between the static guide vanes on the outside and the rotating cage on the inside: the air enters centrally, flows outward through the cage, picks up the fine particles, and leaves to the outlet: the coarse particles, unable to pass the cage, fall to the coarse hopper.

• The rotor cage: the rotating cylinder of the radial blades creates the high-velocity field that pulls the fine particles through and rejects the coarse: its speed and its tip velocity set the cut size;
• The guide vanes: the static vanes outside the cage swirl the incoming air and pre-classify the feed so that the rotor sees a prepared stream: the vane angle is a geometric lever of the sharpness;
• The dispersion elements: the feed is de-agglomerated before the classification, because the fine particles that stick together behave as the coarse ones: the air-swept dispersion is the modern answer;
• The airflows: the primary air carries the feed and the product, the secondary air sweeps the coarse rejects to reclaim the fine material, and the tertiary air completes the sweep: the three flows together set the bypass;
• The drives and the seals: the rotor runs on the top drive with the oil lubrication and the labyrinth seals that keep the material from the bearings: the drive power is small against the mill, but the reliability is total;

The rotor tip speed and the geometry together fix the classifying field, and the file presents the cut-size maps that relate the rotor speed, the air volume and the product fineness for each machine size: the maps are the operating manuals of the classifier: the operator finds the speed that gives the target 90-micron or 32-micron residue at his production rate, and the automation connects the online fineness analyzer to the rotor speed for the continuous control: the third-generation separator designed for the cement duty is a precision machine, and the file’s geometry pages are the reference that keeps its settings meaningful.

9. The Operation and the Control: the fineness, the load and the product quality

The operation of the separator in the working circuit is the coordination of the rotor speed, the mill feed, the separator feed, the gas flows and the classifier settings, all held around the point that delivers the product fineness at the lowest energy: the presentation organizes that operation into its control loops, and the modern plants add the online measurement so that the loop closes every minute instead of every laboratory hour:

• The rotor speed loop: the online residue analyzer (or the Blaine estimator) drives the rotor speed to hold the target: the response is fast and the loop is stable, making the rotor the primary fineness controller;
• The feed rate loop: the mill feed is set against the circulating load and the mill power, so that the mill works at its efficient point while the separator holds the quality;
• The air flow loop: the separator air is held against the production rate and the target cut, and the dampers are trimmed when the feed moisture or the material hardness drifts;
• The product quality: the separator’s cut, sharpness and bypass shape the cement’s particle size distribution, which sets the water demand, the strength and the early hydration: the same Blaine from the different separators is not the same cement;

The control philosophy of the file is that the separator is the quality organ and the mill is the productivity organ, and the two must not be set by the same hand without the balance: the plants that only chase the Blaine by the rotor speed and ignore the circulating load and the air find their mill draws more power and their cement grows coarser in the tail: the plants that run the full balance keep the fineness, the load and the energy together on the same target: the file’s control maps show the operator the safe operating region and the limits that protect the circuit from the flooding and the overloading.

10. The Troubleshooting and the Audit of the Separator: the case table

The presentation closes its technical body with the troubleshooting cases, and they form the practical backbone of the file: the symptoms that the operator or the auditor meets in the plant, the causes they point to, and the cures that belong to each: the list below is the condensed case table of the separating file:

The audit that precedes these cures is the sampling study: the feed, the product and the rejects streams are sampled with the proper representativeness, sieved at the standard size classes (45, 63, 90, 125, 180, 250 microns and beyond), and the Tromp curve is computed from the material balance: the audit software of the package draws the curve, reads the cut size, the bypass and the imperfection, and compares them with the design: the diagnosis then follows the curve, not the guess, and the cures in the table are applied to the numbers the audit produced: the separating file is exactly this instrument: measurement, curve, diagnosis, cure.

11. The Separator Selection and the Modernisation: sizing the upgrade

For the plants planning the separator replacement, the presentation closes with the selection logic: the new third-generation separator is sized from the product rate, the target fineness, the available mill circuit and the air system, and the file gives the sizing rules of thumb that the process engineers use for the first layout: the separator capacity is quoted at the product flow (the fresh feed) or at the feed flow (the feed flow includes the circulating load), and the two capacities must never be confused in the enquiry:

• The product capacity: the tonnes per hour of the finished cement the separator must produce: this equals the mill’s fresh feed at the design load;
• The feed capacity: the separator must handle the total feed, the product plus the circulating rejects, which at 200 percent load is three times the product: the machine and the air are sized against the feed;
• The air sizing: the separator air follows the loading rule of 0.5 to 1.0 kg of feed per normal cubic meter, and the fan and the duct are sized for the air at the maximum load;
• The fineness capability: the separator’s fine cut capability must reach the required product: the 3,500 to 4,500 Blaine cements and the residues down to 0.5 to 1 percent on 45 microns need the machines with the high tip speeds and the ample air;

The modernisation economics of the file are honest: the third-generation separator on an existing mill recovers 15 to 30 percent of the capacity, improves the particle size distribution, and reduces the specific energy, with the payback typically under two years at the normal electricity prices: the file presents the expected curves so that the plant evaluates the upgrade against its own fineness and its own energy cost, and it warns that the separator alone cannot fix a mill whose charge and internals have drifted: the complete study, mill and separator together, is the discipline of the package, and this separating presentation is the classifier side of that complete study.

12. The Frequently Asked Questions

What is the difference between the separator efficiency and the bypass?

The efficiency is the share of the available fine material that actually leaves the system as the product, while the bypass is the share of even the finest particles that falls into the rejects and must be re-ground: they are two views of the same waste: a separator with the 30 percent bypass on the fine class necessarily shows the lowered efficiency, and the modern third-generation machines are defined by the low bypass of 5 to 15 percent, which is why they recover the capacity of the old circuits.

How do I read the Tromp curve of my separator?

The Tromp curve plots the probability of each particle size reporting to the coarse side: the d50, where the curve crosses 50 percent, is the cut size and the fineness lever; the steepness of the curve around the d50 is the sharpness (measured by the imperfection); and the height of the curve’s left tail is the bypass: once the curve is drawn from the sampling study, the cut tells you the effective fineness, the steepness the size distribution of your cement, and the tail the waste you can attack with the dispersion and the air adjustment.

What circulating load should my cement circuit run at?

The optimum is the load that minimizes the total specific energy, and the modern third-generation circuits typically run 150 to 300 percent, with the exact figure set by the mill discharge fineness and the separator efficiency: the load that is too low forces the over-grinding in the mill, and the load that is too high wastes the energy on the re-handling: the audit finds the optimum by measuring the circuit energy at several loads and taking the minimum of the curve.

How is the fineness controlled automatically?

With the online particle size analyzer measuring the product residue, typically on the 32 and the 90 micron sieves or the full PSD, and the control system driving the separator rotor speed: the residue deviation is corrected by the rotor speed in a fast, stable loop, while the feed rate and the air hold the mill and the classification balance: the same loop that a good operator runs hourly runs automatically every minute with the online analyzer.

Why does the new third-generation separator improve the cement quality, not just the capacity?

Because the sharpness of the cut shapes the particle size distribution: the sharp separator produces the cement with the narrower spread, the fewer oversize grains and the fewer over-ground fines, which gives the more uniform water demand, the better early strength of the coarser fraction and the improved packing in the concrete: two cements at the same Blaine can behave differently, and the difference lives in the sharpness of the separator that made them.

13. Conclusion

The separating presentation is the classifier half of the closed-circuit grinding discipline: the physics of the drag and the centrifugal force, the three generations of the machines, the cut size, the Tromp curve, the bypass, the sharpness and the efficiency, the air flows and the circulating load, the control loops and the troubleshooting cases: this article walked the file from the settling velocity to the modernisation economics, and the engineer who applies its logic will find his separator delivering the right product at the right energy: the mill grinds, the separator decides, and the circuit earns its keep on the sharpness of that decision.

The Complete Cement Technical Package includes the separating presentation together with the high-efficiency separator handbooks, the separator operation courses and the mill optimisation tools: the one-time $249.99 purchase, the instant download and the lifetime access: the cut, right: the bypass, low: the sharpness, high: the circulating load, optimum: the cement, finished at the lowest cost.

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.