Comminution Engineering: Complete Technical Guide

Comminution engineering is the discipline of size reduction: the art and the science of breaking rock down to the powder that the kiln burns and the cement the market buys: in a cement plant every ton of clinker has passed through two or three crushing stages and at least one grinding stage: the total electrical energy spent in comminution approaches two-thirds of the plant’s entire power demand: the discipline is therefore not a laboratory curiosity but the biggest single item of the operating cost of the plant: the engineer who masters comminution masters the economics of the whole process.

This file of the Complete Cement Technical Package is the focused reference on comminution engineering for the cement industry: the terminology, the physics of the breakage, the equipment family, the design arithmetic and the operating practice: it is written for the process engineer, the production manager, the new graduate and the maintenance planner: 931 files including this guide are delivered in the package at a $249.99 one-time payment with instant download via PayPal: the reader of this article follows the file section by section, and the sections carry the working numbers that the plant lives by.

The article walks the subject from the molecular scale to the motor scale: the stress that splits the particle, the law that predicts the power, the machines that do the work and the measurements that control it: the beginner finds the vocabulary first, the engineer finds the formulas, and the operator finds the troubleshooting tables: every section is written to be used, not admired.

1. The Terminology of Comminution: The Vocabulary of the Plant

Comminution borrows its language from mechanics, geology and process engineering: the misreading of a single term changes the result of an entire calculation, so the definitions come first: the file fixes the vocabulary that the rest of the text relies on.

• Comminution: the general term for all size-reduction processes: crushing and grinding: the reduction of the solid material to smaller particles by mechanical stressing;
• Crushing: the coarse reduction stages, from run-of-mine rock down to about 5 to 50 millimetres: the machines are called crushers;
• Grinding: the fine reduction stages, from crushed feed down to the powder of 1 to 100 micrometres: the machines are called mills;
• Feed size, F80: the aperture at which 80 per cent of the feed mass passes, the standard description of the feed coarseness;
• Product size, P80: the aperture at which 80 per cent of the product mass passes: the target of the stage;
• Reduction ratio: the ratio F80 divided by P80 of the stage: the measure of the work done by that machine;
• Breakage: the single event of the particle fracture inside the machine: the elementary unit of all the population models;
• Selection function: the probability of a particle entering the mill being broken in the unit interval: the mathematical image of the machine;
• Classification: the separation of a ground product into fines (finished) and oversize (return to the mill): the partner of every grinding stage;
• Circulating load: the ratio of the oversize returned to the mill to the new feed, expressed usually in per cent: the heartbeat measurement of the closed circuit;
• Specific energy: the energy consumed by the machine per ton of material, in kilowatt-hours per ton: the universal currency of the comminution comparison;

The vocabulary above is the skeleton: the following sections put the muscle of the numbers on these bones: each term recurs in the calculations and the tables, and the reader who holds the terms holds the key to the rest.

2. The Laws of Size Reduction: The Physics of Breaking

Three classic laws attempt to relate the energy consumed to the size reduction achieved: each is an empirical statement of the same intuition: that more fine product requires more energy per ton: the three laws differ in the exponent attached to the size, and the engineer must know when each holds: the history of the laws is the history of the mineral processing:

• The law of Rittinger (1867): the energy required is proportional to the new surface area created: E = K×(1/P − 1/F): best suited to the fine grinding, where surface creation dominates;
• The law of Kick (1885): the energy required is proportional to the volume reduction: constant work for equal reduction ratios: the crushing stage is the near-home of the law: for the coarse crushing the most convenient;
• The third theory of Bond (1952): the energy is proportional to the square root of the size change: E = 10Wi(1/√P − 1/√F): the middle exponent: the accepted design basis of the entire industry;

No single law is exactly right across the whole size range, and the modern view sees the exponent as a continuous variable that shifts from the Kick region (coarse crushing) to the Rittinger region (fine grinding): the practical industry has standardized on the Bond equation because it agrees with the plant data in the grinding region where most of the energy is spent: the file documents the three laws, the boundary of each and the working examples of the Bond calculation, the platform on which every mill power number of the package is built.

3. The Bond Index Numbers of the Cement Raw Materials

The Bond work index is the single most quoted number of comminution engineering: it expresses the hardness of the material in kilowatt-hours per ton under a defined test: the higher the number, the harder the grind, the higher the mill power and the cost: the cement industry works within a well-known band, and the table below is the engineer’s pocket reference:

Material	Bond work index, kWh/t	Typical F80 for this table	Note
Limestone, soft	7 – 9	12 – 20 mm	raw mill feed
Limestone, hard	10 – 12	12 – 20 mm	quarry dependent
Raw mix blend	10 – 13	20 mm	the design number of raw mills
Cement clinker	12 – 16	3 – 5 mm	the design number of finish mills
Gypsum	7 – 9	1 – 3 mm	the easy companion of the clinker
Slag, granulated	15 – 22	1 – 5 mm	the hardest of the cement mill feeds
Coal	10 – 13	25 mm	depends on the volatile content
Pozzolana / ash	8 – 11	1 mm	the easy additive

The work index the estimator opens first when a new mill motor is discussed: the specific energy of grinding scales with the Wi linearly: the feed hardness drift within one deposit is tracked by the same seconds of the index: the tables of the file additionally give the rod mill indices and the extended database of the quarries, so the engineer places his own material between the rows instead of outside them.

4. The Fracture Mechanisms: How the Rock Actually Breaks

The comminution machine works by imposing stress on the particles and the way the stress is applied is the physics backbone of the equipment selection: the breakup of the rock is the propagation of the crack system through the flaws of the material, and the four basic stressing modes are present in all the machines, in different proportions:

• Compression: the particle is squeezed between the two surfaces: the slow, high-force path: the jaw crusher, gyratory, cone, HPGR and the rollers act by compression: energy efficient at the coarse end;
• Impact: the particle hits the surface or the projectile at speed, or is hit by the projectile: the hammer mill and the impact crusher live on it: fast, strong for brittle rock, and it shifts the breakage location away from the machine surfaces;
• Attrition and shear: the rubbing of one particle on the other or on the surfaces: the breakage by the peeling of the fines from the particle: inefficient in the pure form, but the always-present companion of tumbling;
• Bending and tension: the stress concentrations of the geometry, important in the pre-weakening of the impact events at the coarse end;

The industrial rule of thumb: the compression machines are the masters of the coarse end where the energy of the breakage itself is small, and the tumbling mills use the whole volume as the grinding chamber, loosening the individual events to the mass process: the selection of the machine is largely the selection of the mechanism, because each mechanism returns a different ratio of new surface to energy and a different product shape:

Mechanism	Typical machines	Favourable size range	Shape of the product	Energy level
Compression	Jaw, gyratory, cone, HPGR	100 – 100 mm to 1 mm	Cubical to elongated	low
Impact	Impact crusher, hammer mill	100 mm to 1 mm	Cubical, tight distribution	medium
Tumbling shear	Ball mill, rod mill	20 mm to 0.05 mm	Rounded, wide distribution	low (3 – 7 %)
Compression fine	HPGR corners, vertical roller mill	50 mm to 0.03 mm	Angular, brittle	medium (up 20 %)

5. The Crushers of the Cement Plant: The Coarse End of the Process

The crushing department of the cement plant is the entry gate of the process: the raw material leaves the quarry at run-of-mine and must arrive to the raw mill at the merchantable size the mill will serve: the equipment line is chosen by the capacity, the hardness of the stone, the moisture of the feed and the shape of the site, and each machine of the family has its fixed role:

• The jaw crusher: the primary brute, fed with the largest boulders directly from the dumpers: the swing jaw presses the rock against the fixed plate with the force of hundreds of tonnes: simple, robust, forgiving of the oversized feed: typical sizes 1000 by 1200 millimetres and above;
• The gyratory crusher: the conical head nutates inside the ring of the concave: the largest primary crushers of the world: the continuous discharge, the first choice of the high-capacity plants;
• The cone crusher: the secondary and tertiary: the compression in the annular gap with the adjustable closed-side setting, producing the well-shaped aggregates and the fine feed for the raw mill;
• The impact crusher / hammer mill: the rotor with the hammers strikes the rock and throws the one against the impact plates: the short production time, high ratio, the product with the cubic shape: moisture-tolerant up to the environmental limits: the favourite of the raw plants with the softer limestone;
• The roll crushers and the high-pressure rolls (HPGR): the minerals shear between the two counter-rotating rolls at the controlled pressure: the modern high-pressure grinding rolls compress the full bed of material, creating the micro-cracks inside the particles, typically before the ball mill;

The tables of the file give the selection criteria: the feed top size versus the gap, the throughput per hour per machine size, the reduction ratio per stage and the power per ton of each type; the crusher maintenance is the daily subject of the mechanical crews: the wear plates, the setting, the jaw movement numbers and the gap control:

Crusher type	Feed size F	Product P80	Reduction ratio	Nominal capacity range	Finest use
Jaw	up to 1 m	100 – 150 mm	4 – 8	50 – 800 t/h	Primary
Gyratory	up to 1.5 m	100 – 250 mm	4 – 10	200 – 2500 t/h	Primary, mega plants
Cone	150 – 350 mm	15 – 40 mm	4 – 10	100 – 1200 t/h	Secondary / tertiary
Impact / hammer	up to 600 mm	5 – 40 mm	6 – 25	50 – 800 t/h	Primary raw, coal
Roll / HPGR	30 – 80 mm	2 – 6 mm	3 – 8	100 – 1500 t/h	Pre-grinding of clinker or raw

6. The Grinding Mills: The Heart of the Fine Reduction

Below the crushers, the comminution passes into the mills: the ball mill, the vertical roller mill and their cousins: the selection of the mill type is one of the most strategic decisions of the plant, because the grinding section runs with the highest operating cost of the factory:

• The ball mill (tube mill): the horizontal rotating cylinder, half to center filled with the steel balls, grinding by the tumbling and the cataracting of the charge: the universal machine of cement grinding, works in the dry or the wet systems, one chamber or two, with the partition between: 100 years of accumulated know-how;
• The rod mill: the long cylinder with rods instead of balls: the coarse grinding to the 1 – 0.3 mm size in the raw sections: the first grinding stage of the older plants;
• The vertical roller mill (VRM): the rotary table carries the material under the rollers that grind it against the table: drying, grinding and classification in one machine: the first choice of the new plants for the raw material preparation and for the coal, and the common choice for the final cement grinding in the modern installations;
• The horizontal roller mill / HPGR: the compressed bed of the rollers, returns the micro-cracked product to the ball mill or the VRM;
• The stirred media mill: the fine and ultra-fine grinding of the fly ash, the micronized products and the additives;

The choice among these machines follows the properties of the feed G, the moisture, the abrasiveness, the required fineness and the electrical tariff of the plant: the VRM combines drying and grinding and therefore saves a complete drying department; the ball mill remains the champion of the flexibility and the product quality of the cement; the tables of the file compare the specific energy of each mill family for the same products:

Mill type	Drying	Product fineness	Specific energy vs ball mill	Best for
Ball mill, 2-comp	Poor	BSS 300 – 5000	100 % (reference)	Cement, wet raw
Vertical roller	Excellent (in-mill)	same range to finer	70 – 85 %	Raw, coal, cement
HPGR + ball	Limited	same	75 – 90 % overall	Clinker pre-grind
Rod mill	Poor	coarse (1 mm)	n/a (coarse stage)	Raw coarse

7. The Grinding Circuits: Open, Closed, and the Circulating Load

The mill alone is only half of the comminution installation: the classifier that divides the stream into the finished and the returned stands beside it, and the two together form the circuit whose everyday behavior and physics the operators feel in the motor loads:

• The open circuit: the mill discharge is the product: simple, suitable when overgrinding risk is low, e.g. the first chambers and coarse duties: the one-pass mill;
• The closed circuit: the discharge goes to the classifier, the coarse fraction returns to the mill, the fine fraction leaves as the product: at the steady state the mass of the recycle is several times the fresh feed: the circulating load is the heart;
• The classical circuit numbers: the closed circuit ball mill runs 150 – 300 % circulating load; the VRM runs with the internal classifier and the high internal recirculation; the circulating load is adjusted by the classifier speed and the millisfeed;
• Why the closed circuit wins: every particle is examined by the classifier and leaves the system only when it has reached its required fineness: the vast majority of the energy is spent on the oversized, not on the re-grinding of the fine product: the specific energy is reduced by 10 to 25 % versus the open circuit and the particle size distribution is steeper;

The millis operation table of the plant: the classifier speed up means the product finer, the recycle up and the millis specific energy up: the classifier speed down, the product in the tail: the instability of the closed loop appears when the classifier jams, the millis vent blocked or the moisture of the feed flange rises: the file includes the classical “spreadsheet of the closed circuit” that maps the flowrate of the feed, the recycle and the product from the sieving data, so the operator knows the numbers his mill is living on before he pushes any button.

8. The Evolution Chain in the Cement Plant: From the Quarry to the Silo

Comminution engineering is not one machine but the chain of the machines, and each product of the plant travels its own chain with the distinct decisions:

• The raw material chain: the quarry blocks to the primary crusher (jaw/gyratory) to the secondary impact cone to the raw stockpile to the raw mill (VRM or ball) to the kiln feed: the design F80/P80 of each stage fixed by the next machine;
• The coal chain: the raw coal to the coal mill (VRM or ball, often with drying from the kiln gas) to the pulverized coal silo to the kiln burners: the coal fineness is critical for the flame;
• The clinker chain: the cooled clinker to the clinker ball mill or the pre-grinder (HPGR) and the finish mill, the cement silos: the most energy-sensitive comminution train of the plant;
• The additive chain: the gypsum, the slag, the ash, the limestone filler are ground together with the clinker in the finish mill: the hardness differences of the feeds complicate the breakage and the control;

The common denominator of the four chains: the engineer sits at the desk of the process design with the flow, the F80/P80 and the Bond Wi of each branch, and by the law of the section 3 the power of every stage arrives: the total installed comminution power of a 5000 tpd plant is of the order of 25 to 40 MW, the figure that must be predicted before the first concrete is poured: the file walks the four chains with the real sizes, the typical mill products and the budget of energy by branch.

9. The Dimensioning of the Grinding Installation: From the Law to the Motor

The dimensioning day of a new grinding plant runs through the routine steps below, and the same routine serves the audit of the existing millis:

• Step 1 — the material data: the Bond work index of the raw material measured or extracted from the table, the feed F80, the target product P80;
• Step 2 — the theoretical energy: the Bond equation produces the specific energy in kWh/t of the comminution itself;
• Step 3 — the reductions: the efficiency factors of the wet/dry, the open/closed circuit, the size-of-product correction bring the theoretical value to the industrial level;
• Step 4 — the throughput: tons per hour required, the energy per ton and the reserve margin give the net power demand of the millis;
• Step 5 — the mill tailoring: the drum speed, the media charge, the millisfeed moisture, the classifier speed: the full shell and the liners, the drive:
• Step 6 — the check: the pilot run or the reference of an identical installed mill: the confirmation loop before the investment;

The script of the six steps is the same arithmetic in every textbook; what the file adds is the complete set of the tables: the correction factors at every feed size, the Wi table of the local raw materials, the capacity formulas of the drums and the worked examples for the raw mill and the cement mill: the machine dimensioner runs the whole calculation from the Wi capture to the motor in one sheet, and tables the answer the management asks: the kilowatts, the tons per hour and the cost per ton of the installed mill.

10. The Media and the Shell: The Balls, the Rods and the Liners

The working tool of the ball mill is the grinding media, and the state of the media decays the performance of the whole circuit: a second of media observation reads the health of the grinding:

• The ball size logic: the coarse feed calls for the large balls (transmit the big impacts), the fine grinding for the small balls (many contacts per mass unit): the two-chamber mill owns the graded charge: 90 mm balls at the first chamber for the raw, 17 to 25 mm balls at the second for the cement, the design rules documented in the media tables;
• The charge level: the classic 30 to 36 % of the mill shell volume: the high level wears the liners and raises the power, the low level kills the capacity: the level is measured by the power pick-up of the millis motor or by the sound of the first chamber;
• The ball wear: the specific wear per ton of the ground product, typically 200 – 500 g/t of the cement for the forged balls, 100 – 250 g/t for the high-chromium white iron: the ball losses are the hidden cost item of the grinding section;
• The liners: the manganese steel for occasions… the trough of the millis the protection of the shell and the lifter that raises the charge: the lifter profile (wave, cement-included lifters) controls the throughput; worn liners collapse the ball trajectory and the millis specific energy;

The measurement in the file: the table of the charge levels for the chamber/length, the monthly method of the media level by the sound or by the power, the documentation of the ball refilling: lasts the millis history: performance of the millis after the last halves: high-chromium balls: the expected tonnage before the resling.

11. The Operation of the Grinding Circuit: The World of the Control Room

The operators of the circuit live on the arithmetic of the set-points, and the comminution engineering defines the owner of the chest: the control philosophy of the millis:

• The mill feed rate: the master variable: the throughput is pushed to the upper boundary at which the classifier overloads or the millis power falls;
• The sound of the first chamber: the power of the first chamber the index of the charge level: the classic “mill sound” control: the balls on the liner of the empty millis sound hollow (empty) versus the dense thud of the full charge;
• The millis power: the plateau of the power curve marks the optimum charge; the peaks at too low fill gives instability: the power draw sign-marks both the charge and the feed state
• The classifier speed: the setter of the fineness and the recirculation: product
• The temperature: the millis discharge, the internal temp: above 120 °C the gypsum dehydrates to the hemihydrate: the setter of the millis performance and the cement quality: the water spray control;

The operator cradle of the file: the fundamentals (the ratio of the ball size distribution to the fineness, the cyclone pressure), the start-stop sequences, the stress conditions (the empty millis, the bent heavy): and the control of the quality: the millis fineness by the 45-micron residue and the Blaine, both hourly: comminution engineering at the control desk is the fine tuning of the feed, the separator and the temperature to keep the finish shift in the spec at the least energy.

12. The Efficiency of the Comminution: The Second Law Is Showing the Waste

The conscience of the discipline: the raw part of the energy in the grinding is wasted in heat, noise and wear: the law of the calibre to the whole energy economy:

• The measured efficiency of the comminution: of the total electrical energy absorbed by the millis, only 1 to 3 % (ball mill ~5 %, VRM 10–20 %) actually creates the new surface of the powder; the rest disappears as heat, sound and the elastic deformation of the balls and the liners;
• The consequence of the grinding energy rule: the commiss generation of the plant represents 60 % of the electricity; any percent saved in the comminution is the largest possible reduction on e.g. the electrical bill of the plant: the grinding section is the first station of the energy audit;
• The levers in order of the benefit: the closed circuit (10-25 %), the VRM (15-30 % lower specific), the HPGR pre-grinding, the finer crusher product (cheap commin vs what is done in the mill), the classifier improvement, the media and the liner optimization, the exact steel cost;
• The tool: the file includes the Excel energy balance of the grinding circuits: the input kWh of the plant measured by month against the tonnage and the fineness, the specific energy curve: the efficiency of the month shows the drift of the charge, the liners, the clinker quality: early detection of the inefficient circuit;

The efficiency gap is also the noise: it is hard to break the mineral directly: the industry made the biggest gains by giving the mill feed less of a task (pre-crush, feed F80 down) and by the classification cutting the mistakes: the comminution engineering of the future belongs to those who manage the waste, the waste of the overgrinding, the waste of the heat, the waste of the re-circulation.

13. The Troubleshooting of Comminution: The Symptoms and the Causes

When the circuit leaves the norms, the engineer has the list of the classic symptoms, and the file pairs each symptom with the logical diagnosis before the millis is opened:

Symptom	Most likely cause	First check	Corrective action
Millis power drops suddenly	Feed blocked or media level low	Feed belt, cyclic sound	Restore the feed, reload the media
Power rises, tonnage falls	Charge overfilled or ball broken/sized	Power curve vs load	Sample the charge, adjust the feed
Product too coarse	Separator speed low / screen blocked	Classifier speed, sieve	Raise speed, clean the screen
Product finer than the spec	Overestimation of the recirculation	Fineness scan	Reduce the classifier speed
High millis outlet temp	Over feed or the gypsum dehydrated	Temperature record	Slash the feed, water in the circuit
Sudden vibration	Liner washed, ball broken, unbalanced	Vibration reading, sound	Stop, inspect the shell and the liners
The specific energy is climbing	Wear of media, harder feed	Wi test, ball charge measure	Re-ball, review the quarry

The diagnosis discipline pays: the operators that document the symptom (time, power, sound) with the cause are the plants that repair the millis before the failure: the file keeps the logbook of the his -the gold of the maintenance intelligence.

14. The Advanced Comminution: The Trends of the Engineering

The last section of this document positions the discipline in the direction of the decade:

• The continuous fine grinding with the stirred mills: the tower mill, the Deswikmills and spin: the energy per new surface 30-50 % less than that of the ball mill at the fine end: used beyond the cement for the ultrafine slag and the additives;
• The ore sorting and the precrush the jig: the cheap sorting of the feed the raw and the clinker fractions before the millis: the energy saved on the rare;
• The use of waste heat in the drying of the raw during grinding (already in the VRM): the combined dry-and-grind installation saves a full drying plant;
• The digital twin of the mill: the model of the millis simulation of the load, the liners, the classifier, the full circuit (for the continuous);

The trend of the industry is clear: the less energy the comminution uses, the more the grinding section is the profit center: the engineering discipline of the breakage has never been so valuable as in the year of energy the cement must make every ton within the budget of the carbon: the file closes by pointing the reader to the complete package that contains the deeper references: the Bond calculations, the mill dimensioning, the media: the database of the full cement course: 931 files at the single payment: the comminution page is the first page of the same library.

15. Frequently Asked Questions

What is the difference between the crushing and the grinding?

The crushing is coarse: the reduction from the block sizes to the top-size of 5-50 mm by the compression and impact machines; the grinding is the fine stage under one kilometre, in the tumbling or the roller mills, by the mass and the colony of the bodies: the two stages are the same desired process, grown to different sizes, run by different machines: communicated: the one field of the same discipline.

Which comminution law should the designer use?

The Bond law of the 1952 is the industrial standard for the mill design: the Rittinger for the fine grinding of the research, the Kick for the estimation of the crushing energy: the modern design applies the Bond law to the whole size range and corrects the exponent of the very fine and very coarse zones: the file gives the areas of each law and the corrections of the Bond equation, so the engineer picks the tool of the job.

Why is the closed circuit with a classifier more efficient?

Because the classifier returns the oversize to the mill and lets the finished leave: the energy of the millis is spent only on the particles that still need the grinding, not on the already-fine powder (over-grinding is paid and produces the problematic agglomeration): the closed circuit also sharpens the particle size distribution of the cement, improving the strength: the recirculation of 150-300 % is the price of the efficiency.

How is the specific energy of a grinding circuit measured?

Divide the consumed electrical power at the millislil meter (total kWh of the period) by the tons of the product in the same window: the figure in kWh/t is the specific energy of the circuit; the comparison to the theoretical Bond energy gives the “efficiency” (typically 15-25 %); the trend of the specific energy shows the state of the media, the liners and the feed hardness: the daily index of the health of the comminution.

What is the best ball top size for a cement finish mill?

The answer belongs to the feed size and the mill chamber: for the cement ball mill with the feed 4-5 mm, the first chamber uses 60-90 balls, the second chamber 15-25: the recommended table of the file take into the initial charge (full) and the refill: the oversized balls waste the energy, the undersized fail the breakage: the correct sizing is the monthly item on the media budget, the single most cost-effective improvement of the comminution section.

16. Conclusion

Comminution engineering is the discipline of the energy and the steel: the discipline that turns the rocks of the quarry into the powder of the kiln: the breakage laws, the sizes tables, the machines, the circuits and the troubleshooting of this document cover the full path: from the boulder to the cement particle: the engineering of comminution is the engineering of the difference between the cost and the loss, and the plant that masters the comminution masters its energy budget, its product quality and its profitability:

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