Comminution: Complete Technical Guide

Comminution is the collective name of the size reduction processes of the mineral industry: the crushing of the quarry rock in the jaw crushers, the grinding of the raw mix in the mills and the fine grinding of the cement in the finish circuit: comminution is the most energy-hungry department of the cement plant: the crushing and the grinding consume roughly two thirds of the electrical energy of the whole process: the equipment of the comminution is the largest capital investment of the plant after the kiln system.

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 comminution guide with the equipment selection tables, the energy calculations and the circuit diagrams: this article walks the file: the theory of the size reduction, the families of the crushers, the grinding mills, the circuits and the energy management: the reader closes the page with the complete map of the comminution department of the cement plant.

The principles of the comminution govern every mill of the plant, from the hammer crusher at the quarry to the finish mill at the cement silo: this page is organized so the theory comes first, the crushing machinery second, the grinding machinery third and the circuits and the economy last: the file itself follows the same order: the engineer, the student and the production specialist each find the level that serves them.

1. The Laws of the Size Reduction: The Energy of the Breakage

The size reduction is described by the classical laws that relate the energy consumed to the size achieved, and the three laws of the comminution frame the whole science:

• The law of Rittinger: the energy consumed is proportional to the new surface area created: the law of the fine grinding, where the surface is the dominant term: the early formulations of the nineteenth century still describe the finest stage of the modern grinding;
• The law of Kick: the energy consumed is proportional to the volume of the material deformed, which depends only on the reduction ratio and not on the absolute sizes: the law of the coarse crushing: the energy of the jaw crusher follows this logic, where the break is dominated by the deformation of the whole particle;
• The law of Bond: the middle theory that fits the industrial grinding range: the energy required is proportional to the difference of the inverse square roots of the product and the feed sizes: the work index Wi of Bond is the standard measure of the grindability of every material: W = 10 × Wi / radic;P − 10 × Wi / radic;F: the design equations of the mills are built around this law;

The three laws are the calibration of the mill engineer: the crushers follow the Kick logic, the ball mills the Bond arithmetic and the fine grinding the Rittinger surface: the practical engineer applies the Bond law for the sizing of the grinding equipment and the measured work indices of the plant: the file explains the derivations, the limits and the industrial applications of the three laws with the worked examples of the power calculations.

2. The Stages of the Comminution: From the Mountain to the Micron

The size reduction of the cement process is executed in the ordered stages, each with its own equipment and its own economics:

Stage	Feed size	Product size	Typical equipment
Primary crushing	Up to 1500 mm	100–250 mm	Jaw and gyratory crushers
Secondary and tertiary crushing	100–250 mm	20–60 mm	Impact, hammer and cone crushers
Raw grinding	20–80 mm	90–180 µm	Ball mills, vertical roller mills, roller presses
Finish grinding	Clinker up to 40 mm	300–450 m2/kg Blaine	Ball mills, vertical mills, closed circuits

The stages are chosen so that each machine operates in its optimal range: the roller press grinds efficiently at the high pressures but cannot accept the meter-sized rocks; the jaw crusher breaks the mountains but produces nothing finer than the gravel: the cascade of the stages keeps every machine in its envelope of the efficiency: the energy per ton rises dramatically from the coarse to the fine stages, and the plant design seeks to shift the reduction towards the cheap coarse stages wherever the process allows.

3. The Crushers of the Coarse Stages: The Jaw, the Gyratory and the Cone

The primary crushers receive the quarry product and the secondary crushers shape the mill feed: the pressure breakage family of the coarse comminution:

• The jaw crusher: the moving jaw presses the rock against the fixed jaw with the eccentric drive: the variety of the sizes and the simple robust construction: the classical primary of the small and the medium plants: the feed sizes up to 1500 millimeters and the capacities to 2000 tons per hour in the large machines;
• The gyratory crusher: the conical mantle rotates eccentrically inside the concave bowl: the continuous crushing action without the idle stroke of the jaw: the primary of the largest capacities: the dedicated machine of the big limestone quarries feeding the 10,000 ton-per-day plants;
• The cone crusher: the gyratory geometry optimized for the secondary and the tertiary duties: the hydraulic adjustment of the setting and the automatic tramp relief: the shaping of the aggregate and the controlled products of the crushed rock: the cone crusher produces the cubical particles that the mills and the concrete like;
• The feed control and the protection: the rock feeders, the finger bars and the metal detectors that protect the crushers from the tramp steel of the quarry: the hydraulic relief systems that pass the unbreakable pieces: the protection devices define the availability of the crushing department;

The jaw, the gyratory and the cone crushers break the rock by the compression between the manganese steel surfaces: they are the workhorses of the coarse stage with the high throughputs and the moderate energies per ton: the file documents the selection criteria, the setting adjustments, the capacities and the wear management of each family, with the comparison tables that the plant designer uses for the choice of the primary crusher.

4. The Impact and the Hammer Crushers: The Breakage on the Fly

The impact family breaks the rock by the kinetic energy of the hammers and the anvils instead of the slow compression:

• The hammer crusher: the rotor with the hinged or the rigid hammers flings the rock against the breaker plates: the single-stage reduction of the soft to the medium limestone from the meter size to the mill feed in one machine: the many cement plants crush the whole raw material with the single hammer crusher: the wet and the sticky materials are handled with the rollers of the feed system;
• The impact crusher (PFC): the primary impact crusher with the large feed openings: the cubical products and the high reduction ratios: the modern alternative to the jaw for the soft and the medium rocks: the rotor speed and the anvil settings tune the product grading;
• The secondary impact crusher: the machines of the aggregate and the raw material shaping: the hydraulically adjustable curtains and the discharge grates: the product size control and the wear parts that can be rotated and replaced from the top of the machine;
• The wear economy: the impact wear of the hammers is proportional to the abrasive content of the rock: the quartz-rich materials punish the impact family and send the designer to the compression crushers: the abrasiveness of the limestone is the decisive factor between the impact and the pressure breakage: the test of the abrasion in the laboratory decides the family of the machine;

The impact crushers dominate the cement practice because the typical limestone is soft enough for the hammer breakage at the lowest capital cost: the single-stage hammer crusher replaced the two- and three-stage plants of the older generation: the file documents the hammer crusher selection, the rotor design, the adjustment of the breaker plates and the wear life data of the actual plant operation: the impact family is the economical choice where the rock allows it.

5. The Tube and the Ball Mills: The Classical Grinding Machines

The tube mill is the most recognizable machine of the cement plant: the slow-turning steel cylinder filled with the grinding media that crushes the material by the tumbling fall:

• The construction: the shell of the rolled steel plates, the mill heads with the trunnion bearings and the replaceable liners of the chrome steel or the high-chromium iron: the mill rotates at the speed of 65 to 75 percent of the critical speed, where the media are lifted to the shoulder and cataracted onto the material bed;
• The grinding media: the steel balls of 25 to 90 millimeters in the first compartment and the smaller media in the later compartments: the graded charge is the grinding machine itself: the ball charge is lost to the wear and replenished continuously: the grinding media consumption of the finish mill sits between 400 and 800 grams per ton of cement in the ordinary practice;
• The compartments and the diaphragms: the two- and three-compartment mills separate the coarse and the fine duties inside one shell: the intermediate diaphragm holds the material back and keeps the graded media in their compartments: the material travels from the feed end to the discharge grate and exits through the holes of the outlet diaphragm;
• The closed circuit: the mill operates with the classifier in the closed circuit: the oversize of the separator returns to the mill inlet: the circulating load of 150 to 300 percent keeps the mill always working on the coarse material and the classifier polishing the product: the closed circuit is the standard of the finish grinding;

The tube mill is the most flexible, the most proven and the most energy-hungry grinding machine of the industry: its specific energy approaches 30 to 38 kilowatt-hours per ton of cement at the typical fineness: the file documents the geometry, the media grading, the liner design and the circuit operation of the tube mill with the full design calculation examples: the classical machine is still the reference of the industry.

6. The Roller Press and the Vertical Roller Mill: The Grinding on the Bed

The modern grinding family breaks the material between the rollers instead of the tumbling media, and the two bed-grinding machines dominate the new plants:

• The roller press (high-pressure grinding rolls): the two counter-rotating rollers press the material bed at the pressures of 50 to 150 bar: the particles crack against each other inside the pressed cake: the cake contains the large portion of the finished fines that the de-agglomerator separates: the roller press consumes about 40 to 50 percent less energy than the equivalent ball mill: the pre-grinding and the finish grinding applications with the ball mill as the finisher;
• The vertical roller mill (VRM): the grinding table rotates under the stationary rollers that crush and shear the material bed: the integrated drying with the hot gases of the kiln and the integrated classification with the dynamic separator inside the mill body: the vertical mill grinds, dries and classifies in one machine: the standard of the raw grinding of the modern plants, and increasingly the standard of the finish grinding as well;
• The Horomill and the ring-roller mills: the cousins of the family that grind between the rings and the rollers: the niche applications of the fine cement grinding with the low energy: the family continues to evolve around the bed-grinding principle;
• The mill dynamics: the bed height, the hydraulic pressure of the rollers, the table speed and the nozzle ring velocities are the tuning parameters of the vertical mill: the vibration of the mill is the alarm of the process: the modern control systems actuate the feed and the pressure to keep the bed stable;

The bed-grinding machines cut the energy of the comminution by more than a third and occupy a fraction of the ball mill’s floor space: their sensitivity to the material moisture and the particle size demands the controlled feed: the file compares the roller press and the vertical mill against the tube mill with the energy tables, the operating parameters and the selection logic of the plant design: the new plants of the industry are built around these machines.

7. The Principles of the Breakage: The Four Mechanisms of the Size Reduction

All the comminution machines of the plant break the material by the combination of the four fundamental mechanisms:

• The compression: the slow squeezing of the particle between the two surfaces: the mechanism of the jaw, the gyratory and the cone crushers, the roller press and the bed-grinding mills: the most energy-efficient mechanism for the coarse and the inter-particle breakage: the controlled pressure of the bed grinding multiplies the crack initiation inside the particles;
• The impact: the sudden application of the load by the falling hammer or the flung particle: the mechanism of the hammer crushers and the cataracting balls of the tumbling mills: the efficiency depends on the impact velocity and the mass of the striking body: the impact breakage of the fine particles wastes energy in the air drag and the elastic deformations;
• The attrition: the rubbing of the particles against each other and against the grinding media: the dominant mechanism of the fine grinding at the low speeds: the rounding of the particles and the production of the fine dust: the attrition acts together with the impact inside the ball mill, and its share rises as the particles become finer;
• The abrasion: the scratching of the particle surfaces by the moving bodies: the energy consumed at the surfaces without the breaking of the volume: the mechanism of the liner wear and the unwanted energy loss: the comminution engineer designs the circuits to minimize the pure abrasion and to maximize the compression and the impact on the volume;

The four mechanisms explain the ranking of the machines: the compression-based machines (the roller press, the vertical mill) transfer the energy into the particle volume and waste little, while the tumbling mills necessarily combine the useful impact with the wasteful abrasion: the design of the modern comminution circuits follows the mechanism economics: the file explains the mechanisms with the illustrations and the measured energy shares of each machine family.

8. The Closed Circuits and the Classification: The Quality of the Product

The grinding circuits of the plant are classified by the path of the material, and the closed circuits dominate the finish grinding:

• The open circuit: the material passes through the mill once: the product of the single pass with the wide particle size distribution: the simplest and the cheapest circuit of the tube mills with the long retention: used for the raw grinding where the kiln tolerates the wide distribution;
• The closed circuit: the mill discharges into the classifier, the separator returns the oversize to the mill and the finished product leaves the system: the circulating load of the plant: the closed circuit produces the narrow particle size distribution with the controlled residues: the standard of the finish grinding: the separator shapes the cement for the strength and the water demand;
• The classifiers: the first-generation static separators with the adjustable vanes, the second-generation with the improved air sweeping and the third-generation dynamic separators with the rotor cages: the modern dynamic separators classify at the high efficiencies with the sharp cuts: the bypass of the separator, the fraction of the fines that return with the oversize, is the key efficiency parameter;
• The air swept mills: the raw mills are swept by the hot gases that dry and convey the material: the gas velocity inside the mill carries the fines to the top separator: the air-swept system integrates the drying and the classification in the single loop: the system of the wet raw materials of the world;

The classifier is the quality instrument of the grinding department: the same mill with the different separator settings produces the cement of the very different performances: the residues on the 45 and the 90 micron sieves and the Blaine of the product are steered by the separator speed: the file documents the classifier families, the efficiency calculations and the tuning of the circuits with the working data of the real plants.

9. The Energy of the Comminution: The Biggest Electric Bill of the Plant

The comminution consumes about 60 percent of the electrical energy of the cement plant, and the energy management of the grinding is the economics of the whole process:

• The theoretical vs the real energy: the theoretical energy of the new surface creation is only a few percent of the energy consumed by the real mill: the rest is lost in the heat, the sound, the wear and the elastic deformations: the thermodynamic efficiency of the tumbling mill is famously low, around one to three percent: the inefficiency is the opportunity of the industry;
• The specific energy of the circuits: the measured kilowatt-hours per ton of the raw grinding sits between 15 and 25, of the finish grinding between 28 and 40 at the ordinary fineness: the roller press and the vertical mill cut these numbers by a third: the energy benchmarks of the industry compare the circuits of the peers;
• The fineness pressure: the specific energy rises steeply with the fineness of the product: the exponent of the Blaine curve means the last hundred m2/kg cost as much as the first thousand: the over-grinding of the cement is the silent waste of many plants: the optimization of the target fineness against the strength specification is the daily economy of the mill;
• The saving measures: the pre-grinding with the roller press, the high-efficiency separators, the mill ventilation, the media grading optimization and the grinding aids: each measure shaves the specific energy: the grinding aids in particular reduce the coating of the media and the agglomeration, adding the percentage points of the capacity at the small cost: the energy audit of the grinding department is the first chapter of the mill modernization;

The energy of the comminution is measured, benchmarked and managed like the currency of the plant: a reduction of the specific energy by ten percent in the finish grinding of a million-ton plant saves about a gigawatt-hour of electricity per month: the file carries the energy audit methodology, the benchmark tables and the saving catalogue that the mill engineers apply in the modernization projects.

10. The Wear and the Materials of the Machines: The Cost of the Friction

The comminution machines consume not only the energy but also the steel of their working surfaces, and the wear management is a standing cost center:

• The wear mechanisms: the abrasive friction of the moving particles against the steel surfaces: the gouging and the scratching of the crusher jaws, the ball wear of the tumbling mills and the erosion of the roller surfaces: the wear rate correlates with the hardness, the shape and the abrasiveness of the feed material;
• The wear materials: the manganese steel of the crusher jaws that work-hardens under the impact, the high-chromium white iron of the mill liners and the grinding balls, the composite rollers of the vertical mills with the welded wear layers: the metallurgy of the wear parts is a specialized science of the industry;
• The wear life measurement: the grams of the media consumed per ton of the material, the millimeters of the liner loss per thousand operating hours: the wear data of the plant feed the prediction models and the replacement schedules: the routine measurement of the ball charge volume and the liner profile is the discipline of the mill crew;
• The abrasive materials: the quartz-bearing rocks, the slag and the hard clinker punish the machines: the abrasiveness of the raw material is measured by the laboratory abrasion test and written into the selection of the machinery: the plant with the flinty limestone invests in the higher chromium grades and the tighter feeding;

The wear parts are a predictable annual budget of the plant, and the engineering of the wear is as important as the engineering of the breakage: the file tabulates the wear rates of the machines, the metallurgies of the parts and the measurement procedures: the mill engineer who controls the wear controls the maintenance bill of the department.

11. The Material Properties that Govern the Grindability

The behavior of the material in the comminution machines is described by the family of the measured properties that the designer must know:

• The work index: the Bond grindability of the standard mill test: the energy required to grind from the infinite feed to the 80 percent passing 100 microns: the clinker at 13 to 16 kilowatt-hours per ton, the raw limestone at 8 to 12: the work index is the single number that sizes the grinding machines;
• The hardness and the abrasiveness: the Mohs hardness of the minerals and the abrasion index of the standard test: the hardness decides the breakage and the abrasion decides the wear: the two properties often move together but not always: the test drives the metallurgy selection;
• The moisture: the surface moisture of the feed raises the energy, coats the media and closes the separators: the drying duty of the raw mill is defined by the moisture of the feed: the materials above eight percent moisture demand the serious drying capacity of the gas system;
• The brittleness and the crack behavior: the crystalline versus the amorphous structures and the pre-existing fractures of the rock: the brittle rocks break cleanly and the tough rocks deform: the fracture mechanics explain why the same chemistry can grind differently from the different quarries;
• The temperature: the heat of the grinding environment: the ball mills heat the cement above the dehydration point of the gypsum if the ventilation fails: the temperature sensitivity of the material defines the cooling requirements of the circuit;

The measured properties of the material are the inputs of every design equation of this guide: the same ton of rock behaves differently in the crusher, the mill and the separator depending on these five properties: the file explains the laboratory tests that measure each property and converts the results into the design parameters of the machines.

12. The Comminution Circuits of the Cement Plant: The Three Grinding Departments

The cement plant operates three distinct grinding departments, each tuned to its own product:

• The raw grinding: the 60 to 80 percent passing 90 micron raw meal: the feed of the kiln with the chemical modules controlled by the blending: the vertical roller mill with the drying gases is the modern standard: the energy objective is the minimum cost per ton of the acceptable reactivity;
• The finish grinding: the cement at the 300 to 450 m2/kg Blaine with the controlled residues and the temperature limits: the closed circuit ball mill remains the workhorse of the world, with the roller press pre-grinding and the vertical mills taking the new installations: the finish grinding separates the profitability of the product: the target fineness, the strength and the cost are balanced every day;
• The coal and the fuel grinding: the coal mill dries, grinds and conveys the fuel to the burner: the fineness of the coal residue on the 90 micron sieve below two percent guards the complete combustion and the flame stability: the inert atmosphere and the explosion protection systems are the specialty of the fuel grinding: the vertical mill with the nitrogen-inerting is the common solution;

The three departments share the same science and differ in the objectives: the raw mill feeds the chemistry, the finish mill feeds the customer and the coal mill feeds the flame: the file documents each circuit with its flow sheets, its operating targets and its typical problems: the plant engineer who masters the three departments commands the whole comminution of the industry.

13. The Often Asked Questions

Why is the crushing cheaper than the grinding?

Because the coarse breakage follows the volume-based mechanism of the Kick and the compression laws, while the fine breakage pays for the immense new surface areas: the energy per ton of the crushed product is a fraction of the energy of the ground product: the design principle of the industry is to do as much of the size reduction as possible in the cheap crushing stages.

Which is more energy efficient: the ball mill or the vertical roller mill?

The vertical roller mill and the roller press consume about 30 to 50 percent less energy than the equivalent ball mill on the same duty, because the bed compression transfers the energy into the particle volume instead of the tumbling media: the modern raw mills are nearly all vertical mills, and the finish grinding increasingly follows.

What does the work index of Bond measure in practice?

The grindability of the material: the kilowatt-hours per ton required to reduce the material to the 80 percent passing 100 microns in the standard laboratory mill: the index is measured for every new material source and used in the Bond equation to calculate the power of the industrial mills.

Why does the cement mill operate in the closed circuit?

Because the closed circuit with the separator produces the narrow particle size distribution and the controlled residues: the oversize returns to the mill until it reaches the fineness, so the product is uniformly ground: the cement of the uniform fineness gives the predictable strength and the water demand that the concrete industry needs.

Is the hammer crusher suitable for every limestone?

No: the hammer breakage suits the soft to the medium rocks with the low abrasiveness; the hard and the abrasive limestones punish the hammers and drive the designer to the jaw, the gyratory or the cone crushers: the abrasion test of the rock decides the family of the crusher before the investment.

Does the file of the package include the calculation spreadsheets of the mills?

The Complete Cement Technical Package includes the Excel tools of the grinding: the Bond power calculation, the media grading and the separator efficiency workbooks are part of the 931 files: the engineers type the material data and receive the mill design parameters in seconds: the guide of the comminution works hand in hand with the calculators of the package.

14. Conclusion

Comminution is the biggest consumer of the electrical energy and the heaviest machinery of the cement plant: from the jaw crusher of the quarry to the separator of the finish circuit, the size reduction follows the same science of the breakage laws and the same economics of the energy and the wear: the modern machines of the bed grinding cut the energy of the process by the third, and the closed circuits shape the product of the market: the comminution guide of the package carries the theory, the machines, the circuits and the numbers that the mill engineer needs: the crushing and the grinding of the plant, understood completely.

The Complete Cement Technical Package includes this comminution guide with the equipment tables, the energy calculations and the circuit diagrams: the one-time 249.99: the instant download: the 931 files of the library of cement: the science of the size reduction, documented from the mountain to the micron: the knowledge of the package, the efficiency of the mill.

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.