Cement Grinding: Complete Technical Guide

The cement grinding is the closing act of the cement plant: the clinker from the kiln, mixed with the gypsum and the additives, is ground into the fine powder that hardens with the water: the step where the quality of the cement is finally fixed: the particle size, the surface area, the particle size distribution, the temperature and the grinding aids all join the clinker chemistry into the bag of cement: the grinding is the difference between the clinker and the cement, and between the profit and the loss of the plant.

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) contains the full grinding library: the grinding work index according to Bond, the ball data and the mill shell dimensioning, the cement grinding systems, the separators, the mill ventilation and the cement physics: this article walks the whole field: the circuit, the mill, the media, the separator, the quality, the control and the energy: the complete map of the cement grinding.

The grinding consumes about a third of the plant’s electricity and decides the product that the customer measures: the fineness by the Blaine, the strength by the mortar tests and the workability by the setting: the engineer of the finish mill balances the energy, the quality and the output every day: this page is written as the working guide to that daily balance: the machine, the theory and the words of the mill floor.

1. The Role of the Finish Grinding in the Cement Plant

The finish mill is the last unit operation of the cement process, and the quality and the cost of the finished product are written in its operation:

• The size reduction of the clinker: the cooled clinker particles of 5 to 30 millimeters are ground to the powder with the 90 percent passing below 20 to 40 microns: the specific surface of 250 to 500 m2/kg, the Blaine number of the plant;
• The activation of the cement: only the fine particles react with the water at the practical rate: the strength of the cement at 2, 7 and 28 days is locked by the fineness and the particle shape: the grinding is the strength engineering:
• The addition of the gypsum: the clinker alone would flash-set: the gypsum (calcium sulfate) is interground to control the cement setting, and its fineness and the dehydration are the results of the mill temperature:
• The additives: the slag, the fly ash, the limestone and the pozzolans enter the cement: the grinding circuit is the blender of the solid fuels to the composition: the blended cements of the green era:
• The energy of the plant: the grinding of the clinker takes 30 to 40 kWh/t of cement: the finish grinding department between 30 and 40 percent of the plant’s electricity: the mill is both the quality and the energy instrument;

The finish grinding bears the plant’s two most precious letters: the Q of the quality, and the $ of the power bill: the engineer who optimizes the grinding improves the product together with the energy: the two targets of the department are the same pair of the variables: the fineness and the power: the role defines the rest of the article.

2. The Circuit: The Mill and the Separator in the Closed Loop

The grinding circuit of the modern cement mill is a closed loop of the mill and the classifier, and the flowsheet is the first page of the grinding discipline:

• The feed: the clinker from the silo, the gypsum from its bunker and the additives are weighed independently and fed to the mill inlet: the feed composition is the quality instruction of the day;
• The mill: the rotating tube grinds the feed by the falling balls: the mill of the last generation: the first compartment coarse, the second fine: the internals of the double chambers;
• The elevator and the air slide: the ground material leaves the mill outlet and is lifted to the separator: the mechanical elevator and the air slide between the transport steps;
• The separator: the classifier of the third generation separates the fine product from the coarse: the fine passes to the filter and the storage, the coarse returns to the mill inlet as the rejects:
• The circulation: the rejects flow, the circulating load: the relation between the new feed and the circuit tonnage: the circulating load of 200 to 1000 percent of the feed: the property of the mill circuit

The closed circuit is the modern standard: the separator returns the coarse particles to the mill, so no part of the material leaves the circuit until it matches the target: the plant that understands its circulating load understands its mill: the loop: the feed-in, the product-out, the rejects-returned: the circuit of the powder: the economics of the fineness.

3. The Ball Mill: The Compartments, the Media and the Shell

The ball mill is still the most widespread finish mill of the world, and its parts are the daily vocabulary of the mill engineer:

Mill zone	Liner / function	Media charge
Feed head / inlet slide	The inlet trunnion and the feeder	the material and the air enter
First compartment	The lifting liners, the coarse grinding	the large balls of 60 to 90 mm
Intermediate diaphragm	The transfer and the sieve of the particles	Balls of the 30 to 50 mm
Second compartment	The fine wear liners, the fine grinding	the balls of 15 to 30 mm (and the cylpebs)
Outlet diaphragm	The retention and the discharge	The oversize balls retained

The shell of the mill rotates at 70 to 75 percent of the critical speed, the ball charge fills the 30 to 35 percent of the mill volume, and the graded media cascade and cascade over the material: the first compartment breaks the coarse clinker, the second creates the surface: the diaphragm keeps the ball sizes and the particle passage in order: the mill inventory, the balls and the liners, is the capital of the fineness: the monthly ball top-up and the yearly media audit: the measurement of life: the plant: the mill: the charge: the level.

4. The Grindability, the Work Index and the Power of the Mill

The power that the mill must absorb is fixed by the hardness of the clinker and the target fineness: the Bond work index is the number of the contract between the kiln and the mill:

• The definition: the Wi of the final material: the kilowatt-hours required to reduce a short ton from the infinite size to the 80% passing 100 microns: the clinker index of 13 to 16, the slag of 15 to 20:
• The Bond power equation: W = 10 × Wi / √P − 10 × Wi /√F: the 80% passing feed F and product P: the equation yields the kilowatt-hours per ton of the finish target:
• The specific power of the plant: the good modern finish circuits at 35 to 45 kWh/t for the OPC at 3,500 Blaine: the soft limestone 25, the slag blend above 50: the kWh/t of the target:
• The circulating load and the power: the total power of the mill splits between the bed and the rejects: the sweet spot of the circulating load takes the tonnes per hour at the same kWh: the mill audit of the plant:
• The power of the circuit: the mill at the full load, the separator rotor, the fan: the specific energy of the complete circuit: the separator domain carries a large part of the savings potential of the plant:

The kWh/t of the finish is the fused energy of the clinker: the same clinker can grind at 38 or at 50 kWh/t according to the circuit discipline: the Bond number sets the theoretical floor, the separator and the mill charge set the practical: the plant that tracks its specific energy weekly keeps the audit of the finish line: the energy of the grinding, the number of the mill.

5. The Separator: The Commando of the Fineness

The modern separator of the third generation classifies the mill output by the air and the rotor, and its design decides the efficiency of the whole circuit:

• The third-generation separator: the cage rotor with the guide vanes: the tangential and the radial configurations: the adjustable rotor speed separates at the exact cut with the high selectivity:
• The cut size and the bypass: the d50 of the classification and the bypass, the fraction that avoids the separation: the bypass of 10 to 30 percent of the coarse enters the product: the quality: The bypass is the blind of the classification:
• The efficiency: the clean separation unless the near-size particles leak both ways: the Tromp curve of the separator measures the recovery and the bypass: the Tromp curve is the separator’s X-ray:
• The rejects ratio: the rejects carry the fine material of the overgrind: the recirculation: the high efficiency returns the fines to the mill: the recirculation loop: the fineness of the product at the same power:
• The control of the separator: the separator speed setpoints are the levers of the fineness: the fineness feedback to the separator speed closes the automatic loops of the modern plant:

The separator is the quality gate of the finish circuit: a good separator pushes more finish out of the same mill, an indifferent one grinds the same fines over and over: the Tromp analysis of the separator is the weekly instrument of the mill: the loop: the classification, the rejects, the product: the fineness of the plant rides on the rotor speed.

6. The Ventilation of the Mill: The Air, the Temperature and the Dew

The gas of the finish mill has three duties: the removal of the fines, the removal of the heat and the removal of the moisture of the feed:

• The sweep the fines: the air sweeping the mill carries the finished particles to the separator and the filter: without the sweep the powder overgrinds in the mill and the quality is lost: the air velocity in the mill shell of 0.5 to 1.0 m/s:
• The heat removal: the grinding converts most of the motor power into the heat: the cement at the mill outlet 100 to 120 degrees: the water injection or the vent air yields the temperature of the control: the gypsum threshold of 110: the theory of the false set:
• The drying: the moisture of the clinker, the gypsum and the added limestone enters the mill: the vent air evaporates the water and the exhaust leaves through the bag filter: the dew point of the vent gas, the condensation and the coating of the bags: the ventilation keeps the climate dry:
• The mill ventilation system: the fan pulls the air through the mill: the separator dedusting: the sensitivity of the pressure drops to the vent opening: the main instrument: the filter:
• The water spray: the evaporative cooling inside the mill: the water quantity adjusted by the temperature: the water injection: the second ventilation: the temperature, the humidity, the settings:

The ventilation of the mill is the climate of the powder: the temperature of the grinding controls the gypsum and the strength, the humidity controls the coatings of the media, and the air carries everything: the package includes the dedicated mill-ventilation file, which this article accompanies page by page: the temperature, the dew point, the air: the correct atmosphere of the grind.

7. The Gypsum, the False Set and the Setting Chemistry

The small percentage of the gypsum in the finish is the control of the water reaction, and the mill is where the gypsum chemistry is set:

• The dihydrate: the natural gypsum (CaSO4 · 2H2O) in the cement feed between the 2 and 7 percent of the SO3 target, according to the clinker: the SO3 of the cement is the analytical target of the quality lab:
• The mill temperature: above the 130 degrees the dihydrate dehydrates to the hemihydrate (the plaster) in the mill: the plaster reades the hydration: the fast set and the false set of the delivery:
• The false set: the rehydration of the calcined gypsum in the cement: the stiffening of the concrete within minutes, recoverable by the re-mixing: the physical opposite of the flash set, but a delivery problem of its own:
• The flash set: the instant hardening when the gypsum is missing or the C3A reacts: the fatal error of the mix: the setting:
• The SO3 balance: the sulfates of the cement, the alkali of the clinker: the SO3 added for the optimum setting and the development of the grade: the optimum SO3 of the mortar and the concrete: the lab of the mill quality:

The gypsum is the secret ingredient of the cement, and the finish mill is its thermocouple: the mill temperature and the gypsum quality must respect the un-see contract: the result: the cement sets correctly, the truck mixer no surprise: the chemistry of the mill is set every hour by the temperature control and the SO3 of the assay: the false set stories of the industry, most are the mill temperature stories.

8. The Fineness, the Blaine and the Particle Size Distribution

The fineness of the cement is measured in two complementary instruments: the surface by the Blaine and the distribution by the laser:

• The Blaine: the specific surface, the air permeability of the powder bed: the cm2/g (or m2/kg): the OPC of the 3000 to 4500 cm2/g, the modern targets at the higher end:
• The residue on the screens: the samples of the 45 and 90 microns: the standard sieves: the relatively coarse fraction: the focus of the quality of the plant: the R45 of the 2 to 15 percent:
• The particle size distribution: the laser diffraction: the full curve of the size distribution: the steepness of the size distribution: the packaging of the cement particles: the strength of the cement at the same Blaine:
• The 3-32 microns window: the fraction between the 3 and the 32 microns that hydrates the fully and contributes most to the strength: the 65-90 percent of the mass: the target of the grinding curve:
• The two instruments, the one purpose: the Blaine tracks the surface, the laser tracks the distribution: the cement with the same Blaine, different strength: the engineer of the finish reads both: the language of the fineness:

The fineness is the quality of the grind: the same clinker at the higher Blaine shows the strength and the cost: the mill blends the two targets: the limit of the fineness because of the water demand of the concrete: beyond the optimum the paste dries and the strength falls: the fineness is a curve of the optimum, not a race: the lab of the plant finds the optimum for each cement, the mill holds it.

9. The Cement Types and the Standards of the Finished Product

The ground product is classified by the composition and the strength in the standards of the world, and the plant’s grinding schedule follows the norm of the market:

Standard class	Composition / clinker share	Typical Blaine of the product
OPC CEM I (EN 197-1)	95-100% clinker	3200 – 4500
Portland-filler CEM II/A-L	80-94% clinker + limestone	3400 – 4600
Blastfurnace CEM III/A	35-64% clinker + blastfurnace slag	3800 – 5000
ASTM Type I (similar OPC)	Generic construction	3300 – 4000 +
ASTM Type V (sulfate resistant)	Low C3A clinker	3200 – 4000

The standards define the composition, the strength classes (the 32.5, 42.5, 52.5 on the European schemas) and the finiteness of the permissible: the market prices the classes and the mill schedules follow: the same finish mill produces several cements by the change of the feed and the fineness: the changeover: the intergrading and the small batch of the flush: the planning of the mill production: the standards of the sales, the technical reality of the mill.

10. The operation and the Control of the Finish Circuit

The daily operation of the finish circuit is the balancing between the tonnage, the fineness and the power: the control room of the mill holds the levers:

• The feed control: the feed per hour follows the mill power: the load of the mill, the material level: the controller of the mill load: the grinding balance: the fill of the compartments:
• The separator speed: the separator speeds the classification: the fineness setpoints: the quality controller of the circuit: the loop: the product spec of the day:
• The mill temperature: the water injection and the vent air hold the outlet temperature in the safe envelope: the instrument of the setting control: the alarms of the excursions:
• The circulating load reading: the rejects flow of the circuit: the feed and the rejects: the state of the mill: the high rejects with the low power: the empty first comber, the overload second: the diagnosis of the circulation:
• The quality feedback: the automatic samplers feed the Blaine lab in real time (the Malvern, the HELOS): the closed loop of the fineness catches the changes of the clinker: the clinker from the kiln: the final: the mill absorbs the variations:

The finish circuit is the most automated of the plant, and the quality feedback loop the controller: the plant holds the fineness in the spec: the operator, who understands the rejects reading, the water injection and the separator: same instrument the fitter: the control of the mill: the same: the stability of the quality, the output of the plant.

11. The Troubleshooting of the Finish Mill: The Common Problems

The finish mill failure patterns are the classic inheritance of the industry, and the miller of the plant knows them by name:

Symptom	The probable cause	The practical first step
The milled the power rises, the output falls	The overloaded mill, the clogged diaphragm, the ball gradation exhausted	The reduce feed, the mill body taps, the media audit
The product too coarse (the Blaine low)	The open diaphragm, the blocked fine screen, the separator speed low	The separator check, the mill internals inspection
The mill is hot, the cement lumpy	The ventilation insufficient, the water inject failed	The vent increase, the water system checks, the temperature
The coating of the balls and the liners	The moisture, the temperature, the overgrinding fines	The vent, the water, the clinker humidity
The circulating load abnormally high	The diaphragm blocked, the separator bypass high	The pressure of the mill circuit, the Tromp analysis

The troubleshooting: the mill is a machine of the balances: the over- and the under-charge, the separator and the leakage, the temperature and the humidity: the miller reads the symptoms and the marriage of the power, the rejects and the fineness: the tables of the mill are the experimented knowledge of the industry: the resolution of the mill, the restart of the plant.

12. The Energy Saving and the Modern Alternatives: The VRM and the High-Pressure Framing

The modern cement plants compare the ball mill against the vertical roller mill and the rollpress circuits, and the energy difference drives the decisions:

• The vertical mill (VRM) for the cement: the drying, the grinding and the classification in one machine: the specific energy of the VRM circuit runs 20 to 30 percent lower than the ball mill: the limitations: the moisture of the feed, the gypsum dehydration, the vibration sensitivity:
• The high-pressure rollpress: the pre-grinding the clinker at 50-100 bars: the material fractures in the bed: the 20-40 percent savings: the supplementary of the ball mill: the circuits of the world:
• The ball mill with the classifiers: the modern high-efficiency separators have cut the ball circuit’s catalogue: the 25-30 kWh/t at the 3500 Blaine: the continuing role of the ball mills:
• The combined circuits: the press 30-40% and the ball mill 60-70%: the circulating circuit: the capacities of the large lines: the best of the two methods:
• The decarbonized context: the supplementary materials of the low-clinker cements: the grinding of the pozzolan and the slag: the finish line as the green machine of the plant: the tools of the blend design:

The energy of the finish line is the largest electric part of the cement, and the modern techniques trade the capital for the kWh: the mills of the new plants: the press, the VRM and the ball hybrid: the existing ball mills are re-powered with the modern separators and the drives: the file of the package lays the equipment of the comparison tables: the miller of the 2020s: the engineer of the watts and tons per hour.

13. Frequently Asked Questions

What is the difference between the open and the closed circuit grinding?

The open circuit grinds the material once and discharges the product; the closed circuit returns the coarse rejects to the mill through the separator: the closed circuit controls the distribution and the energy, and the modern plants almost always run closed: the open remains in the small outdated installations: the closed is the standard of the finish.

Why is the mill temperature critical for the gypsum?

The finish mill heats the feed above the gypsum threshold: over the 105-120 degrees of the mill the dihydrate loses its crystal water and the hemihydrate forms in the cement: the cement then sets quickly in the mixing or shows the false set: the mill temperature is controlled at the envelope by the ventilation and the water spray: the temperature is a quality instrument, not a comfort number.

What is the difference between the Blaine and the 45-micron residue?

The Blaine measures the specific surface of the total powder mass, the residue measures the coarse fraction on the standard sieve: the cements with the equal Blaine can differ in the residue: the Blaine and the residue together describe the distribution: the surface for the reaction, the residue for the particulate: the two report companions of the finish.

Why is the Blast furnace cement ground finer?

The slag reacts with the water much slower than the clinker; the fineness (3800 to 5000 cm2/g) compensates and the hydration of the surface, the cement reaches the strength: the slag also grinds hard (the Wi above 15), so the split 80% of the electricity of the milling goes to the slag: the finer, wear-resistant cement: the cost of the durability.

What is the effect of the grinding aid on the concrete?

The grinding aids, the amine and the glycol based products, coat the particles and the media, prevent the agglomeration and improve the mill output of the 10 to 20 percent at the same fineness: the aids do not normally affect the concrete properties at the recommended doses: the lab confirms the product certification: the aid of the mill: the chemical optimizer.

Why the cement strength spec needs 28 days?

The 28-day strength is the trade reference of the cement, the point of the standard mortar test EN 196 and the ASTM C-109: the cement manufacturer verifies the 28d with the 2-day acceleration: the ratio: the strength growth of the OPC follows the logs of the grinding: the longer it holds, the stronger the yield: the 28 day of the standard and the cement of the mill, the same.

Does the mill suffer with the humidity of the feed?

Yes: the moisture of the clinker and the gypsum condenses on the media and coats the balls, the coating cushions the blows and the grinding efficiency collapses: the vent air and the dry feed solve it: the cement grinding requires the dry feeds: the clinker silos and the gypsum storage are kept dry, and the humidity of the feed is measured: the water in the mill is the slow death of the grinding.

14. Conclusion

The cement grinding is the fort of the quality and the treasury of the energy: the mill, the separator, the ventilation, the gypsum, the fineness: the system of the end mill: the paper of the finish economy: the Complete Cement Technical Package carries the full grinding library with the Bond’s index file, the mill design tables, the separator texts and the calculator tools: the package of the 931 files: the one-time $249.99: the instant download: the discipline of the finish: the cement quality, made at the mill: the engineer of the fineness: the master of the final, the essential.

The grind of the plant: the quality that the standards demand, the Blaine, the SO3, the water demand of the concrete: the future of the grinding: the VRM and the press roll, the science of the separators: the complete: this guide is the starting point of the discipline: the package file carries the full field: the engineering of the mill, the tables, the career of the grinding: the plant, the cement, 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.