Roller Mills: Complete Technical Guide – Complete Cement Technical Package

Roller Mills: Complete Technical Guide

Roller mills in the cement industry mean above all the vertical roller mill (VRM), and this machine has become the standard of the raw grinding, the coal grinding and, increasingly, the finish grinding of the modern plant: the mill grinds by the pressure of the rollers on a bed of material spread on the rotating table, without the tumbling media of the ball mill: the energy of size reduction drops by 20-40% in comparison with the ball circuit, the drying is integrated in the same machine, and the footprint is a fraction of the ba[/IGNORE]] mill: the roller mill is the centerpiece of the modern grinding department, and the operators need the complete understanding of its physics to run it at the best point.

The Complete Cement Technical Package (931 files including the vertical mill books, the grinding technology, the roller mill manuals, the Excel tools and the course: $249.99 one-time: instant download via the PayPal payment) includes this complete roller-mills file: the guide to the design, the operating parameters, the dynamics and the maintenance of the roller mills: this article walks the file: the grinding principle and the machine architecture, the grinding bed and the roller pressure, the dam ring and the nozzle ring, the classifier, the vibration and its control, the operating philosophy, the wear and the tyres, the mill audits and the troubleshooting: the reader of the article leaves with the complete picture of the roller mill in the cement process.

The vertical roller mill works in a loop with itself: the feed falls on the rotating table, the rollers press the bed, the ground material spills at the edge of the table into the nozzle ring, the gas stream lifts it to the classifier, the fines leave as the product and the coarse returns to the table: the mill is a single machine that is simultaneously the grinder, the dryer and the separator, and its most important parameter is not speed or load but the bed: the thickness and the moisture and the stability of the bed decide everything else: the vibration, the output and the life of the rollers: this article explains the bed logic of the roller mill, the numbers behind it, and the operating practice that keeps it stable.

1. The Family of the Roller Mills in the Cement Process

The roller mill family covers three applications in the cement plant:

The raw mill: the VRM for the raw meal (the topic of the raw grinding file), energy 16-22 kWh/t, drying up to 20% moisture;
The coal mill: the VRM of the fuels, the fineness 1-3% R90, the inert operation, the inevitably smaller (the safety of the fuel grinding);
The finish mill: the VRM of the cement, the fineness 3,300-4,500 Blaine, the specific energy of 26-38 kWh/t with the compact, the limitation: the peak strength of the particle distribution shape (the narrowness vs the ball mill product);

Application	Feed moisture	Fineness	Energy (kWh/t)	Notes
Raw meal	up to 20%	12-14% R90	16 – 22	with the kiln gas drying
Coal/petcoke	up to 15%	1-3% R90	7 – 12 (coal)	nitrogen inert
Cement finish	dry feeds	3,250-4,000 cm2/g	26 – 38	with the additives

The roller mill is not one machine but a family with the same kernel: the disc and the rollers, the gas circuit, the classifier: the file presents the family with the differences of the chapters and the shared chapters of the runtime: the transfer of the skills between the applications is one of the strengths that the commissioning plants rely on.

2. The Architecture of the Vertical Roller Mill: The Main Components

The VRM is a tower: the classifier above, the grinding zone below, and the drive at the bottom:

The grinding table: the rotating bowl under the rollers: the center feed, the rotating speed 20-45 rpm on the table radius, the liner segments of the table as the replaceable wear parts;
The rollers: the 2 to 4 rollers (the tyres) of 1.5-3.5 m diameter, the pressure cylinders with the nitrogen accumulators: the arrangement of the rollers at 45-degree sectors:
The hydraulic loading: the pressure of 4-10 MPa in the system provides the grinding force 80-120 kN per roller non-loading: the accumulator allows the roller to lift on the oversize stones;
The nozzle ring: the annular slots at the disk edge where the gas (the hot kiln gas or the direct heat) rises at 50-100 m/s; the fine material is carried up; the stones and the pyrites fall through into the reject system (“the mill rejects”);
The classifier: the rotor cage of the top, the speed 300-700 rpm typical for the raw and the 500-1,000 for the fine finish: the fines pass to the cyclone/filter;
The drive: the central bevel/planetary gearbox of the 1,500-6,000 kW, the synchronization, the auxiliary drive that allows the slow rotation in the start and the stops;

The key of the vertical design: everything is aligned vertically: the drop of the feed, the press of the rollers, the lift of the gas: the result is a compact tower of about 15-25 m that performs what a chain of machines did before: the plant architect’s favorite building.

3. The Grinding Principle: The Bed, the Compression and the Energy

Why is the roller mill so energy-smart? The answer is in the physics of the size reduction:

The kind of breakage: the compression and the shear of the material bed between the roller and the table: not the random hits of the ball; the crack propagates directly where the force applies;
The energy route: the single compression consumes the minor energy per broken particle, the balls waste most of the impact energy in the sound and the heat;
The numbers: the raw material of a grindability index 12-15: the roller mill uses 16-22 kWh/t, the ball 24-30: the ratio around 0.7-0.8: measured over decades in the plants;
The boundaries: the compression needs the pressure: the pressure requires the force of the hydraulic and the robust bed: the machine has its limits of the hardness and the stickiness;

The four design parameters that decide the mill performance: the roller diameter/table diameter, the roller width, the operating pressure and the table speed: the figure “the mill constant” (the specific grinding force 200-400 kN per m of roller length) is the standard of the design; the file gives the calculation and the range for each application: the pressure of the plant is adjustable, and the operators of the file run the tests at 2 or 3 pressures to find their own optimum.

4. The Bed: The Heart of the Roller Mill

The bed is the layer of the material between the roller and the table: the mill rides on the bed:

The height letter: the bed height of the raw mill is 10-40 mm at the normal situation (the material of the typical: the particle size to the bed length): the compressed bed is smaller;
The stable bed: the mill vibrates when the bed is too thin or too thick: the vibration of 1-3 mm/s is normal, over 4-7 mm/s the protection lifts the rollers;
The moisture paradox: the driest feed builds the thinnest bed; the mother wet product gets the sticky lumps and the overloaded no-length: the metal on the metal on both ends;
The operator handles: the feed rate, the gas temperature and the separator speed: the feed increases the bed, the temperature dries it, and the classifier changes the recirculation:

The operator of the vertical mill reads the bed through the signals: the differential pressure of the mill (the DP 4,000-7,000 Pa for the raw), the vibration, the power and the sound: the modern mills display the “bed thickness” calculated from the roller position: the file teaches the war: the bed of the roller mill is the critical matter of the night shift, and and the parameter envelope table gives the operator the values of the healthy mill.

5. The dam ring, the Nozzle and the Table: The Machine Elements of the Grinding Zone

Three small elements of the machine dominate the behavior of the mill, and these are the adjustments the file teaches the operator:

The dam ring: the adjustable ring at the outer edge of the table: the ring retains the bed: the higher the ring the thicker the milling bed and the more the residence: the typical height of 50-150 mm, the adjustment in the stop duration of the row;
The nozzle ring: the slot between the table and the body: the gas velocity and the direction on the velocity: the numbers: 60-90 m/s the raw, the lower the coal: the blocks of the stones regulated by the velocity;
The pressure of the accumulator: the nitrogen precharge of the cylinders 40-60% of the operating pressure: the softer the cushioning of the oversize;
The speed of the table: the fixed gearbox speed in the majority, the variable in the modern with the frequency: the speed raises the output and the vibrations of the bed;

The tuning of the dam ring is the classical “one-hour tester” of the roller mill: raise the ring, the bed thickens and the vibration calms, the output grows by 5-15 percent if the separator can carry, the energy does not change: lower the ring and the bed thins, the mill dries fast: the file explains which plants run the higher ring (the dry, the fine) and which the lower (the moist, the coarse), with the table of the case studies of the industry.

6. The Control of the Mill: The Automation and the Operator Screen

The roller mill is run by the control system of the plant, and the operator’s screen is modern man’s instrument panel:

The feed controller: the fresh feed follows the setpoint of the production rate: the mill presses the fines: the amplitudes of the power and the DP hold the mill at the rating;
The temperature control: the mill inlet temperature holds the 250-400 C (raw with the kiln gas) and the outlet 80-110: the water of the gas mixing, the hot air damper and the cold air damper are the valves;
The classifier control: the fineness at the target via the rotor speed: the feedback of the lab (R90 hourly) or the online analysers:
The safety chains: the vibration trips, the auxiliary drive, the access interlocks: the sequences of the start after the trip are the encrypted in the logic:

The file describes the “operator loops” in the two versions: the classical manual/semi-auto and the advanced model predictive control: the modern MPC of the raw mill cuts the energy to the reference by 2-5 percent and steadies the quality: the file teaches the DCS-philosophy of the VRM: the one set of the controls: the one operator, the one configuration, the same target.

7. The Vibration of the Roller Mill: The Design System

The vibration is the matter of the vertical mill, and the machine protects itself:

The system: the accelerometers at the table and the gearbox: the operating vibration under the 3.5 mm/s, the alarm 4-6, the trip at 6-8:
The hydraulic lifting: at the trip, the control lifts the rollers by the accumulator pressure (the “plant”) and the feed is stopped; the mill then runs with the empty table for the cool:
The causes: the thin or the thick bed, the tramp metals, the fluffy feed, the clogg of the table: the diagnosis table of the file classifies the vibration waveform the cause;
The recovery: the standard sequence: the mill stopped: the inspection of the vanes and the bell: the new bed: the restart with the feed at 50% and the ramp:

The vibration figures take the losses: the repeated trips of the mill reduce the running hours and hammer the drivers: the file’s vibration chapter gives the complete failure mode analysis of the roller mill: 12 failure modes, each with the measured signature, the cause and the correction:

8. The Wear of the Rollers and the Table: The Tyres, the Segments and the Life

The grinding elements of the roller mill wear, and the wear governs the operating cost of the machine:

The tyres (the rollers): the high-chromium cast or the composite (the hard surfacing): the life of the raw tyres 8,000-15,000 hours: the wear limit of the profile 15-25 mm of the radius:
The table segments: the wear of the outer ring: the life with the raw 10,000-20,000 h: the hard-faced welds and the segments replacement:
The cost per ton: the wear cost of the VRM is 0.05-0.25 USD/t of the product depending on the abrasiveness (the quartz!) the high: the file compares the total media cost with the steel of the ball mill (the balls 0.2-0.8 USD/t of the cement);
The measurement: the wear is checked with the templates and the laser scans during the stops: the history of the wear is the planning tool of the spare tyres:

The principles of the wear management: the reverse of the tyres (the primary/secondary) keeps the uniform profile: the hardfacing of the table segments in the stop extends their life: the intermittent control of the feed moisture and the tramp reduces the numbers of the shocks: the file carries the complete wear book of the VRM of the package, with the pictures and the measurement protocols of the industry.

9. The Operation of the Roller Mill: The Start, the Optimum Load and the Stops

The operating day of the roller mill in the plant runs with the discipline of the procedures:

The start sequence: the gas circuit (the dampers), the auxiliary drive, the main motor at the low speed, the feed valve gradually; the bed builds: the mill goes to the load in the 15-30 minutes;
The stead state: the feed at the target, the power at the 85-95%, the DP at the operating point, the temperature in the envelope: all the values on the screen: the action tables of the file:
The transitions: the feed change of the quarry, the moisture of the winter, the kiln in the modes: the operators have the matrix of the settings per the external conditions;
The stop: the feed off, the main drive continues with the gas until the mill empties (the 10-20 minutes), the auxiliary drive: the access the plant:

The economic target of the operation: the maximum operating hours at the highest feed with the constant fineness: the file teaches the “hours accounting” of the roller mill: the run factor of the good raw mills 92-97%, and the beyond In the file the plant benchmarks its availability vs the best of the region.

10. The Roller Mill in the Finish Grinding: The Modern Cement VRM

The cement VRM is the frontier of the vertical grinding: the last 15 years have seen the systems of the finish grinding pass from the ball mills to the VRM in the new plants:

The particle size distribution: the VRM cement shows the narrower distribution (the sharpness) higher than the ball product: the strength early of the VRM cement is usually the same and the water demand of the concrete often less;
The energy: the 26-38 kWh/t of the finite cement vs the 33-40 of the ball: the saving of 15-25%:
The additives: the VRM grinds the clinker, the gypsum and the fillers: the SPC and the slag cement: the higher moisture of the additives (the up to 5-8%) is handled by the internal drying: the limitations of the moist slag:
The quality control: the Blaine control of the cement VRM is the same tight (the samples of the hourly): the R45 and the mortar strength tests follow the standard:

The finish VRM rides the same principles as the raw: the bed, the pressure, the separator: the file treats the cement VRM as the extension of the family: the operating difference: the fineness is much finer (the d50 15-25 um), the bed is thinner, the vibration management is the master skill: the numbers in the roller mill chapter of the file give the operator of the finish mill the same tables as the raw.

11. The Troubleshooting of the Roller Mill

The troubleshooting table of the file:

Symptom	Cause	Correction
Vibration 5+ mm/s	bed thin, hydraulics low, oversize	reduce the pressure, feed stabilization
DP high, output low	dam ring high, classifier closed	lower the ring, check the rotor
DP low, coarse meal	gas velocity high, classifier slow	reduce the gas, speed the rotor
Outlet hot	feed moisture drop, gas hot	moisture check, cold air damper
Outlet cold, water	gas low, feed moist	raise the gas, the pre dryer
Table power high	charge, bed thick, feed coarse	the feed grain, the crusher

The golden rule of the rip: the roller mill changes one variable at a time: the loss of the output after the modification of the hydraulic means the pressure was the optimum: the file also warns against the “operator tampering” of the stored settings: the factory configuration is the reference, and the deviations must be registered with the reason and the date.

11. The Coal Mill Roller Mill: The Fuel Grinding in the Inert Atmosphere

The vertical roller mill is the most common coal mill of the modern cement plant: the combined drying, grinding and classification of the fuel, in the closed gas circuit where the inert conditions protect the process:

The safety envelope: the coal mill runs with the outlet temperature of 70-95 C (higher for the low-volatile petcoke: 90-110 C) and the oxygen in the mill gas below 12-14%: the inerting valves (the nitrogen or the fire-extinguishing) are tested periodically;
The fineness: the coal R90 of 1-3% for the kiln and the calciner firing, the R200 below 0.5-1%: the coarse fuel particles do not burn completely and the falling of the char costs the fuel money:
The moisture: the drying inside the mill: the coal of the open storage at 8-15% moisture is dried to the powder below 1%: the kiln gas or the hot air generator supplies the heat;
The start and the stop: the coal mill is started with the inert sequence and the mill is purged with the nitrogen… The file provides the complete start-stop logic, the purge, the inerting and the re-start after the trips:

The coal VRM shares with the raw the core operation (the bed, the pressure, the separator) with the essential differences of the walls: the highest risk of the fire if the oxygen above the limit, the dust explosion, the spontaneous heating: the operating file of the package makes the coal roller mill the chapter of the dual concern: the product quality and the plant safety: the mill audit of the file includes the safety inspection list of the coal line that the regulators ask for.

12. The Audit of the Roller Mill: The measurement the energy and the improvements

The plant that wants to know the health of its roller mills performs the audit with the standard measurements of the file:

The mill output test: the material balance at the steady full load: the feed weight, the product and the rejects for 2 hours: the balance closes within 1-2%:
The energy the specific energy (kWh/t) at the constant fineness: the value enters the benchmarking table of the package;
The curve of the fineness: the product, the tailings and the feed of the separator sampled and the classification curve drawn: the bypass and the efficiency of the classifier are read;
The wear measurement: the profile of the tyres and the table with the wear templates: the remaining life planned;
The vibration signature: the measured frequency spectra over the 24 hours stored and compared with the reference;

KPI of the roller mill	Typical	Excellent
Specific energy raw (kWh/t)	18 – 22	under 17
Running factor (%)	88 – 94	above 95
Vibration (mm/s)	2 – 4	under 2
Classifier efficiency	60 – 75%	above 80%

The audit results feed the action plan of the plant: the tired classifier rebuilt, the dam ring adjusted, the pressure rebalanced and the tyres renewed: typically the savings of 3-8% of the mill energy and the output increases of 5-10% after the first audit cycle: the file includes the audit sheets with the exact measurements, the formats and the reporting tables of the results.

13. The Frequently Asked Questions

Why is the roller mill vibrating though the pressure is normal?

Check in this order: the bed (the feed moisture and the rate), the separator settings (the recirculation of the coarse), the tramp steel and the metal detector, finally the table and the roller wear: the vibration flux of the file takes the operator step by step: most of the vibration of the roller mills comes from the feed side: the moisture of the grind and the stones.

Which is better, the roller mill or the ball mill for the raw material?

For the modern raw: the roller: energy, drying, footprint, maintenance and the classifier: the plants of the last 30 years built the VRM raw, and the benchmarks speak: 16-22 kWh/t vs 24-30 and the same fineness: the ball will win only in the special: the very wet process, the extreme hardness, the small capacity below 60 t/h:

The mill roller tyres weeked sooner than the year: the causes?

The wear accelerators: the high quartz and the flint of the raw (the abrasive index above the 0.5), the wrong feed size (the lumps above the 8% of the roller diameter), the low bed (the direct metal contact), the excessive pressure: the audit of the wear contains the three checks and the corrective of the feed distribution: the file documents the wear also of the actual cases;

The cement VRM produces the cement different from the ball mill: the market accepts?

The product difference is real: the narrower distribution, the lower the surface of the same Blaine, the higher the early strength and the similar late: the concrete of the market accepts the VRM cement fully, and the producers add the “particle shaping” (the separate classifier speed profiles) to the tune: the file quotes the experiences of the plants with the finish VRM of the world.

Does the roller mill operate with the slag and the fly ash?

Yes, and gladly: the drying of the slag (10-15% moisture) is done in the mill without a separate dryer, the energy 40-55 kWh/t of the slag cement cement achieving with the 50% of the slag: the fly ash of the low water enters directly the feed: the only care: the fineness of the final and the compatibility of the additive: the file covers the matrix of the mixes of the two:

14. The Conclusion

The roller mill is the machine of the modern cement plant: the raw is ground by the rollers, the coal by the rollers, and the cement more and more by the rollers: the physics of the bed and the pressure, the drying of the gas, the classifier and the vibration make the VRM the most complete mill within the industry, and the file of the Complete Cement Technical Package gives the operators and the engineers the complete military of the vertical machine: from the first principle of the grinding bed to the last repair of the tyre: the reader leaves the file with the full control of the roller mill of her plant.

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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.