Operation Of Separators: Complete Technical Guide

The separator (the classifier of the air-swept mill) is the brain of every closed grinding circuit in the cement plant: the raw mill, the finish mill and the coal mill each depend on the same instrument: the machine that separates the fine product from the coarse returns, so that the grinding energy is spent only on the particles that genuinely need size reduction: when the separator runs well, the mill operates at its design output with a stable fineness; when the separator drifts, the mill output falls, the specific power rises and the quality department complains: the operation of the separator is therefore the craft that sits between the mill and the quality of the cement, and it is one of the most valuable skills in the plant.

The Complete Cement Technical Package (931 files including the separator handbooks, the grinding practices, the Excel calculators and the course modules: $249.99 one-time: instant download via the PayPal payment) contains the full operation-of-separators file with its tables, its control graphs and its troubleshooting matrices: this article walks the file chapter by chapter: the types of separators, the mechanics of the classification, the key parameters (the rotor speed, the air flow, the fineness targets), the control strategy, the optimization of the circuit, the measurement of the classification efficiency and the daily troubleshooting: by the end of the article the reader should be able to run the separator of his plant like the experienced operators of the package do.

The separator deserves this attention because it decides the economics of the grinding department: a perfect separator can lift the output of an old ball mill by 10 to 15% without a single euro of mechanical investment, while a neglected one quietly steals 5% of the production and degrades the cement particle size distribution in the direction of the poor strength development: this article explains both the physics and the practice, with the real numbers of the industry throughout: the finenesses (R90, R45, Blaine), the bypass fractions, the circulating loads and the rotor peripheral speeds.

1. The Role of the Separator in the Closed Grinding Circuit

The grinding in closed circuit works by the simple loop: the mill receives the fresh feed plus the coarse fraction returned by the separator (the rejects), and the separator sorts the mill discharge into the fine product and the coarse return:

• The mill feed: fresh feed plus the coarse returns, called the circulating load: in a typical ball mill circuit, 100 to 300% of the fresh feed;
• The mill discharge: the completely ground material, drawn continuously to the elevator and to the separator;
• The fines (the product): the particles fine enough for the cement, carried to the silo of the finished product: their fineness must sit on the target (e.g. 3,300 to 3,700 cm2/g Blaine);
• The coarse returns: the oversize goes back to the mill inlet, either with the feed stream or to the second compartment, offering the mill the second grinding pass;

The open-circuit mill grinds everything to the final fineness in one pass, and the overgrinding of the already-fine material wastes energy: the separator converts the mill into an energy-efficient two-stage system: the table below compares the typical performance of the same 110 t/h mill in open and in closed circuit with a high-efficiency separator:

The economics are unmistakable: the closed circuit with a modern separator raises the output by 20 to 30 percent at the same installed power and improves the cement strength through the narrower particle size distribution: this understanding of the circuit architecture is the foundation of every section that follows.

2. The Types of Separators: From the Static Cage to the High-Efficiency Turbines

The separator family of the cement industry can be grouped into three generations:

• The first generation (static separators): the air separators working on the gravitational and the centrifugal principle in a stationary cone: simple, cheap, but coarse, the classification poor and the bypass high (25-35%); still used for the preliminary classification of the raw meal;
• The second generation (mechanical separators, the Sturtevant types): a driven cage or rotor with adjustable tail valves: better cut than the static, used in many plants of the region for both raw and finish grinding;
• The third generation (high-efficiency separators): the rotor cage with the adjustable speed (the O-Sepa, the SEPOL, the Krupp-Polysius and their like): the feed enters the annular distribution, the fines are carried out by the classification air: fineness in the hands of the rotor speed and the fan: classification efficiency 65-85% against the 40-55% of the old equipment;

The choice of the separator generation determines the optimization potential of the circuit, and the file guides the plant in the upgrading decision: the replacement of an old mechanical separator with a high-efficiency unit of the same diameter typically costs less than a new mill compartment and yields a similar capacity gain: the file includes the selection calculations and the cost-benefit tables.

3. The Physics of the Classification: The Cut Size, the Sharpness and the Bypass

The separator sorts the particles by the balance of the centrifugal and the aerodynamic (drag) forces: a particle inside the classifying zone experiences:

• The centrifugal force: pushes the particle outward in proportion to the rotor speed squared and to the particle mass (density x diameter cubed);
• The drag force of the air: carries the fine particles inward toward the exit: proportional to the air velocity and to the particle diameter;
• The balance: particles smaller than the cut size (the separation size of the d50) follow the air and leave as the product, particles larger than the cut leave through the coarse outlet and return to the mill;

Classification is perfect in the theory and imperfect in the practice: the real classifiers operate with the Tromp probability curve, and the parameters that matter are:

• The cut size d50: the particle diameter at which 50 percent of the material goes to each side: the target fineness of the cement is set by the cut (e.g. d50 of 25-35 um at the Blaine 3,500);
• The sharpness (the steepness of the curve): measured as the ratio of the sizes at the 75% and the 25% of the yield: the steepetranean ratio d75/d25 of 1.4-1.8 on the good machines: the sharper the transition, the more uniform the product;
• The bypass: the fraction of the feed that leaves with the fines without being classified: in a perfect machine zero, in the real machines 5-15%: high bypass is the disease of the circuit;

The understanding of the three parameters unlocks the reading of the separator results: the fineness of the product is set by the cut size, the quality of the product (the strength and the water demand) is decided by the sharpness, and the electrical consumption of the circuit by the bypass: the file explains how to derive the three from the sampling campaign that every plant can run in one day.

4. The Main Parameters of the Operation: The Rotor Speed and the Air Flow

The two handles of the operator on the separator are the rotor speed and the classification air flow (with the secondary air): the physics of the control:

• The rotor speed: raising the revolutions raises the centrifugal forces, so the cut size falls, the product gets finer, the coarse return grows, the mill’s circulating load rises: the relationship in the typical plant: a 5-10% increase in the rotor speed changes the fineness of the product by 150 to 300 cm2/g Blaine of the target;
• The classification air flow: more air raises the drag, the cut size increases, and the product coarsens: the rule of the thumb: 1% increase of the air flow raises the R90 of the product by about 1-2%;
• The balance of the two: the same fineness can be reached at many rotor/air combinations, but the combination that gives the lowest specific energy and the highest output is the “sweet point” that the experienced operator holds;

The file offers the control table of the practical operator:

5. The Classification Curve: The Sampling and the Interpretation

To know the separator from its healthy state, the plant samples: the mill discharge (the feed of the separator), the fines (the product) and the coarse returns: all three must be taken at the same moment under the steady operation, then sieved on the same screens (typically the 45, 63, 90 and 150 micron): the results reconstruct the classification curve:

• The recovery curve: the percentage of each class of the feed that reports to the fines: the curve rises from 0% at the coarse classes to 100% at the fine classes;
• The cut size: read from the curve at the 50% point, e.g. d50 = 30 um for the OPC at 3,400;
• The bypass: the plateau of the curve at the coarse end, e.g. 8%: the bypass of the fine dust that never got returned to the mill;
• The sharpness index: the ratio of d25 to d75 of the curve: values above the 1.6 indicate the excellent sharpness;

The monthly sampling is the stethoscope of the classifier: a raise of the bypass from 8 to 15% shows the wear of the rotor blades or the leak of the sealing, and the drift of the cut size without the change of the setpoints points to the air flow loss (the dirty fan, the folded damper): the file includes the spreadsheets of the curve construction and the baseline graphs of the healthy separator.

6. The Control of the Fineness: The Blaine and the Residues in the daily practice

Two fineness parameters govern the quality of the cement: the Blaine specific surface (the cm2/g) and the residue on the 45 and the 90 micron screens (R45 and R90): the separator must deliver both on the target, and the control actions differ:

• The Blaine: responds mainly to the rotor speed: the fast rotor = the finer surface area: the target range of the OPC 3,300-3,700 cm2/g, the control band of the plant +/- 50 cm2/g;
• The R45: the residue on the 45 micron screen, the coarse tail of the cement: affected by the air flow and by the sharpness of the cut: the target of the OPC I: 5-12% R45;
• The R90: the very coarse particles, the sand fraction: 0.5-2.5% typical: the high R90 is the sign of the bypass input or of the broken rotor blades;

Note that the Blaine and the residue are not the same quality: two cements of equal Blaine can differ in the R45, and that difference shows in the strength and the water demand: the operation of the separator must therefore satisfy both numbers in parallel, and the article’s control table shows the effect of each valve on each parameter.

7. The Air Flow System: The Fan, the Dampers and the Pressure Drops

The classification air is moved by the dedicated fan (or by the mill fan in some circuits), and the quantity of air is controlled by the inlet/outlet dampers or the fan speed:

• The air to the separator: measured in Nm3/h or in m3/h with the gas density: the typical high-efficiency separator needs 1.3 to 1.7 m3 of air per kg of feed for the OPC at 3,500;
• The static pressure: the fan develops 3,000 to 6,000 Pa across the separator-fashion: the load and the blockage of the rotor raise the drop;
• The dust collection: the fines leave with the air to the filter-bag/cyclone: the filter condition (the pressure drop 800-1,500 Pa) affects the effective air volume of the classifier:
• The false air: the air entering at the joints and the drains bypasses the classification and dilutes the system: every 1% of false air costs 1-2% of the classification efficiency:

The air discipline of the plant is the discipline of the whole circuit: the monthly measurement of the fan flows and the quarterly leak check of the ducts are the routine of every gated plant, and the file includes the tables with the normal ranges of the pressure and the temperature of each separator input of the package plant.

8. The Circulating Load: The Amount of the Return and Its Meaning

The circulating load is the tons per hour that the separator returns to the mill, expressed as the percentage of the fresh feed: the value of the real cement circuits sits typically between 100% and 300%:

• The low load (50-100%): the circuit underloads itself: the mill receives little return, the bed of the grinding is thin, and the output is below the design;
• The medium load (150-250%): the healthy working band, where the mill operates with the stable bed and the efficient separator gets the maximum output per kWh;
• The high load (over 300%): the circuit is overloading: the elevator and the separator exceed their capacity, the product coarsens, and the system must be calmed by the rotor speed reduction;

The circulating load is the mirror of the efficiency: a modern circuit running the OPC at 3,400 Blaine with 220% load should produce at 100-120 t/h per rating: the file explains how to measure the classifier load on the belt scale (the continuous weighing of the returns) and how to use the hourly trends of the load, the fineness and the mill amperage to place the circuit at the optimum.

9. The Separators in the Raw Meal and the Coal Circuits

The separator is not only a finish mill instrument: the same family of machines serves the raw meal circuit (in the vertical or the ball raw mills) and the coal mill circuit, with the different target finenesses:

The coal classifier adds the safety dimension: the classification in the coal mill happens in the closed nitrogen conditions, the differential pressures are watched continuously, and the air sweep of the mill is the classifier air: the coal fineness of R90 1-3% directly affects the flame and the burning of the kiln, so the separator of the coal mill is the precision instrument of the firing line: the file explains the operating procedures of this service with the safety precautions of the package.

10. The Troubleshooting of the Separator: The Symptoms and the Cures

The daily life of the separator brings the repeated symptoms, and the file organizes the table of the diagnosis:

The first rule of the troubleshooting: never change the rotor speed before the mechanical checks of the machine: half of the “separator problems” of the plant are really the problems of the dosing belt or of the moisture of the feed, and the file shows the decision tree that the operator follows to isolate the mechanical from the process cause.

11. The Optimization of the Circuit: The Excel Model of the File

The final chapter of the file is the optimization calculation: with the measured bypass, the cut size and the sharpness, how much can the circuit produce at the constant Blaine? The calculation of the file:

1. The feed analysis: the size distribution of the mill discharge on the standard screens;
2. The separator curve: the extraction of the cut, the sharpness and the bypass;
3. The simulation: the circuit model computes for each rotor speed the resulting fineness and the return load;
4. The optimum: the configuration with the highest output at the Blaine target and the load under 250%
5. The pacing: the setpoints are implemented and the sampling confirms the results after 3 days;

The example of the file: the circuit at 52 t/h at Blaine 3,400 with the bypass 18%: after the renewal of the rotor blades the bypass fell to 9% and the output rose to 58 t/h: the specific power fell from 42 to 38 kWh/t: the before/after table completes the study, and the Excel of the calculation is delivered with the other grinding calculators of the Complete Cement Technical Package.

12. The Mechanical Condition: The Rotor, the Blades, the Wear and the Seals

The separator is a mechanical machine before it is a process instrument, and its condition determines its performance: the monthly inspection route of the file covers the elements that wear and the elements that leak:

• The rotor blades: the outer edges of the rotor blades wear by the abrasion of the particles: the eroded blade loses its guiding effect and the classification becomes turbulent: the wear limit is checked by the template: typical life of the blades 10,000 to 20,000 operating hours depending on the feed abrasiveness (the quartz, the clinker of 2-3% free lime);
• The sealing strips: the gap between the rotor and the housing is sealed by the flexible strips: the worn strips open the short-circuit path of the bypass: the check of the clearance and the replacement at every 4,000-6,000 hours saves the efficiency the company does not see otherwise;
• The guide vanes and the inner cone: the material impact zones, the inspection of the erosion pits that change the aerodynamics of the inlet, repair by the weld overlay of the hard layer;
• The fan and the ducting: the impeller wear, the accumulation, the vane clearances: the amperage trend of the fan is the free sensor of the clogging of the whole air path;
• The drive: the belt tension and the coupling alignment, the gearbox oil condition: the vibration measurements of the rotor (the threshold of the alarm 4.5 to 7.1 mm/s RMS on the speed 1x):

The maintenance plan of the separator is short and cheap but strictly scheduled: the plants that treat the separator as a maintenance item of the FILTER class (once a year) lose the 10-15% of the year’s efficiency; the plants that follow the file’s plan hold the classifier efficiency stable within 2-3 percentage points through the whole year: the file provides the inspection forms for every machine of the package.

13. The Energy Balance of the Classification: the kWh that the separator saves

The separator is not an energy consumer of the same class as the mill, but its quality decides the energy of the mill: the mill grinding efficiency in the closed circuit is the function of the circulating load; the finer the returned stream, the more the balls waste their energy on the fine particles:

The table is the heart of the file’s economic argument: the quality of the separator drives 2 to 6 kWh/t of the finish grinding: at the 1,000,000 tons per year and the 0.08 US dollar per kWh the difference between the healthy and the neglected separator is 160,000 to 480,000 US dollars per year of electricity: the maintenance of the separator is not a cost: it is an investment with the return in the months: the file pushes the audit of the separator into the daily control loop of the plant.

14. The Separator in the Automatized Control Loop of the Modern Plant

The modern cement plant runs the finish grinding through the distributed control system, and the separator is the actuator of the fineness loop: the architecture of the control explained in the file:

• The fineness feedback: the online Blaine/fineness analysers (the laser diffraction or the air permeability instruments) measure the product continuously and the target of +-50 cm2/g is maintained by the automatic adjustment of the rotor speed: the response time of the loop is 5 to 15 minutes, fast enough to compensate the drift of the clinker grindability;
• The mill power control: the amperage of the mill motor (or the power in kW) is held in the band of 85-95% of the rating by the adjustment of the fresh feed: the circulating load is the internal variable of the loop and the return belt scale feeds the model;
• The multivariable coordination: the advanced loops (model predictive control) optimize the circuit: the product fineness, the load, the separator fan and the mill temperature are the four controlled; the rotor speed and the feed the two manipulated: the energy savings versus the manual operation documented at the plants that installed the system: 3-6% of the grinding energy in the stable production;
• The sensors of the separator: the differential pressure of the machine (the DP of the classifier 800-2,400 Pa), the vibration, the fan amperage, and the temperature of the feed: the readings in the datalog and the alarms of the limits;

The automation does not replace the operator: it amplifies him: the process engineer still reads the sampling curves, the maintenance planner still schedules the blade renewal, and the shift operator still witnesses the blowing of the filter: the file positions the separator as the well-controlled and the well-measured machine, and the chapter closes with the configuration tables of the loop parameters that the plant can copy into the DCS of the mill.

15. The Frequently Asked Questions

What is the correct rotor speed of my high-efficiency separator?

The peripheral rotor speed of the typical HE separator sits in the range 15-35 m/s for the OPC and higher for the fine cement: the manufacturer’s table gives the base value and the operator adjusts against the fineness: the practical law: 1% more rotor speed moves the R90 about 1-2 percent of the setpoint (the same speed, the same mill).

Is it better to adjust the fineness with the speed or with the air flow?

The rotor speed is the first handle: the speed control is the finest and the most direct: the air flow is the second: the flow change also affects the dust collection and the circulating load: the rule of the practice: set the air near the rated value (1.3-1.7 Nm3 per kg of the feed) and control the fineness with the rotor: if the air must come in, check the mechanical condition first.

What is the bypass of the separator and why does it matter so much?

The bypass is the amount of the feed that passes through the machine without the separation: in the worn or leaked machine the unwanted fine recirculates, the mill grinds the same energy of the dust again, and the efficiency dies: the mechanical bypass of the healthy unit is 5-10%, the worn: 20-30%: the file tells the measurement from the curve and the repair of the seals, often the biggest savings of the grinding department for the modest cost.

Why does the coal separator need the different rules than the cement?

Because the coal is a fuel: the classification happens in the closed inert atmosphere with the oxygen under about 12%, the temperatures are controlled, the explosion protection is installed, and the fineness must be at R90 1-3% for the complete combustion in the kiln: the same physics of the cut, the different procedure of the margin, the separate section of the file treats it fully.

How much of the mortar water depends on the separator settings?

Significantly: the cement with the wide particle size distribution needs more water for the same workability, and the tighter the distribution of the sharpeness, the better the flow of the concrete, at the same strength: the plant that tunes the bypass from 18 to 9 usually observes the water demand of the concrete down by 2-5 kg per cubic meter, with the corresponding savings in the prime and the durability: the quality argument of the separator, quantified in the closing table of the file.

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.