Vertical Roller Mills: Complete Technical Guide

The vertical roller mill is the grinding machine that replaced the ball mill at the heart of the modern cement plant for the raw meal, the coal and, increasingly, the finish cement: it grinds not by the falling media but by the pressure of the rollers rolling over a bed of material on the rotating table, and that single difference changes the physics, the power and the operating envelope of the grinding process: the presentation “29176434 04 Vertical Roller Mills” from the cementequipment.org library documents the machine from the material bed principle to the control parameters: the bed, the roller pressure, the table, the nozzle ring, the separator, the gas flow and the vibration: this article expands the whole file into a complete technical reference.

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 vertical roller mill presentation together with the VRM operation courses, the vertical mill inspection forms and the grinding theory handbooks: this article walks the file for the process engineers, the mill operators and the auditors who run, size or optimize the vertical roller mills of their plants, and it reproduces the formulas and the typical values of the method so that the operation can be discussed with the numbers on the table.

The vertical roller mill owes its success to three numbers: the specific energy of the bed grinding, which is 20 to 40 percent lower than the ball mill energy for the same fineness, the drying capacity, because the mill is a dryer and a grinder in one machine with the kiln gas flowing through it, and the compact footprint that the vertical layout gives to the plant: the file teaches the machine so that the operator exploits those three advantages instead of fighting the vibration and the bed-instability that punish the careless hands: this article is the full walk.

1. The Vertical Roller Mill at a Glance: the machine and its place in the plant

The vertical roller mill, abbreviated VRM or VRM in the industry documents, consists of a horizontal rotating grinding table and two to six rollers that roll over it, each roller pressed onto the material bed by the hydraulic system: the material fed to the center of the table is carried outward by the centrifugal force, passes under the rollers where it is crushed, spills over the edge of the table into the nozzle ring, where the hot gas lifts the fine particles up into the separator, and the coarse rejects fall back onto the table for the next pass: the machine is at once the crusher, the mill, the dryer and the classifier, all in one vertical column.

The place of the VRM in the plant is the transition point of the raw material preparation and the fuel preparation:

• The raw meal grinding: the raw mill VRM dries the moist raw material with the kiln gas, grinds it to the 10 to 14 percent R90 target and delivers the homogenized feed to the blending silo;
• The coal grinding: the coal mill VRM grinds and dries the solid fuel to the fineness the kiln flame needs, typically 1 to 2 percent residue on 90 microns for the kiln coal and 0.5 to 1 percent for the calciner coal;
• The cement grinding: the finish VRM grinds the clinker with the gypsum and the additions to the cement fineness, with 25 to 40 percent lower specific energy than the ball mill, and the modern plants run the VRM finish mills at 3,500 to 4,500 Blaine;
• The slag grinding: the VRM is the standard machine for the separate slag grinding because it reaches the very high fineness with the manageable energy and the stable operation;

The presentation frames the survey around these four applications because the same machine adapts to all of them: the gas temperature changes with the kiln gas availability, the pressure schedule changes with the hardness of the material, and the separator settings change with the required fineness, but the principle of the bed grinding remains the machine’s constant identity: this article carries that identity through every section.

2. The Material-Bed Grinding Principle: why the VRM is efficient

The essence of the vertical roller mill is the grinding on a bed: the material accumulates on the table as a layer of the comminuted particles, and the rollers crush that layer between the roller surface and the table: unlike the ball mill, where the energy is spent lifting and dropping the media, the VRM spends its energy pressing the rollers down onto the material: the measured consequence is the specific energy advantage: the raw meal grinding at 20 to 25 kWh per tonne against the ball mill’s 25 to 30, and the finish grinding at 25 to 35 kWh per tonne against the ball mill’s 35 to 45, savings of 20 to 40 percent that the file quantifies in its comparison table.

The physics of the bed explains the efficiency: the bed grinding converts more of the input energy into the particle fracture because each particle in the bed is loaded from both sides by the pressure on its neighbours, achieving the multi-particle breakage that the single impacts of the ball mill cannot match: the efficiency of the bed is greatest when the bed is neither too thick, which wastes the pressure in the compaction of the coarse layers, nor too thin, which lets the rollers touch the table metal and vibrate the mill.

• The bed thickness management: the stable bed of the raw meal is roughly 10 to 30 millimeters at the roller inlet, set by the feed rate, the roller pressure and the material retention;
• The multi-particle breakage: the individual grains are pressed against the neighboring grains, so the fracture planes are shared and the energy per fracture falls;
• The rejection and the re-grinding: the material that leaves the table too coarse is rejected by the separator and returns to the table for the next pass: the internal classification keeps the coarse material inside the grinding zone;
• The energy flow: the motor torque drives the table, the friction between the rollers and the bed transmits the force, and the particle bed converts it into the fracture: the chain is short and the losses are few;

The bed principle also explains the VRM’s sensitivity: the machine demands a continuous and stable material bed, and anything that breaks the bed: the feed stoppage, the iron particle under the roller, an unstable moisture, triggers the vibration and the mill trip: the operator’s art is keeping the bed alive, and the file’s control sections are the manual of that art: the bed is the heart of the VRM, and every control loop in the machine protects it.

3. The Machine Elements: the table, the rollers, the nozzle ring and the separator

The vertical roller mill is a system of interacting elements, and the presentation walks each of them because the operation and the maintenance speak their language: the four principal elements are the grinding table, the grinding rollers, the nozzle ring and the separator, and each carries its own sizing and its own failure modes:

Element	Function	Typical values / notes
Grinding table	Carries the bed, rotates the bed under the rollers	2.4 to 6.2 m diameter, table speed 25 to 32 rpm
Grinding rollers	Crush the bed under the hydraulic pressure	2 to 6 rollers, 1,600 to 3,000 mm diameter, tire wear parts
Nozzle ring	Accelerates the gas, dries and lifts the fine fraction	Gas velocity 60 to 90 m/s at the ring throat
Separator	Classifies the liftings, returns the coarse to the table	Dynamic third-generation, 40 to 120 rpm rotor
Hydraulic system	Presses the rollers onto the bed	Working pressures 60 to 150 bar depending on the mill

The table is driven by the main gearbox, the largest and the most expensive single component of the mill, and the rollers ride on the bed over the table: between them the nozzle ring accelerates the gas to the throat velocity that decides which particles are carried aloft: the material leaving the table edge falls into the rising gas stream, the fine particles are lifted into the separator and the coarse and the heavy particles drop through the nozzle ring into the rejects hopper to return to the table: the split between the carried and the rejected material is set by the gas velocity and the particle size, and it is the dynamism that keeps the mill’s internal mill recirculation at its proper level.

4. The Grinding Pressure and the Hydraulic System: the force behind the bed

The grinding pressure is the force with which the rollers are pressed onto the material bed, and it is the main lever of the VRM’s power and its fineness: the pressure is generated by the hydraulic cylinders acting on the roller frame, and it is expressed in the operation as the working roller pressure or the specific grinding pressure per unit of the roller width: the pressure must be high enough to fracture the hardest particles in the bed but low enough to keep the bed stable and the vibration down: the file presents the pressure as the master control parameter of the mill.

The specific grinding pressure: the pressure is normally designed in the range of 0.5 to 1.0 megapascal per unit of the projected roller width, and the operating pressure is chosen as a multiple of the mill’s installed hydraulic capability:

• High pressure acts on the bed: raises the grinding fineness and the throughput for the hard materials, but thins the bed and raises the vibration risk when pushed too far;
• Low pressure acts on the bed: protects the mill against the vibration and the metal contact, but reduces the fracture efficiency and raises the recirculation, because the coarse material leaves the table under-ground;
• The hydraulic accumulator: the nitrogen-charged accumulator absorbs the pressure spikes when the large particles pass under the roller, keeping the hydraulic pressure stable and the mill smooth;
• The pressure adjustment loop: the automation raises the pressure when the mill differential pressure and the rejects tell it that the grinding is insufficient, and lowers it when the vibration rises;

The pressure interacts with the feed rate in the classic VRM control balance: the specific energy per tonne falls as the mill approaches its capacity, so the operator wants the highest feed rate the mill can grind, but the feed rate and the pressure are constrained by the bed: too much feed thickens the bed and the mill spits coarse material; too little feed thins the bed and the vibration rises: the control system of the modern VRM solves the couple in real time, and the presentation demonstrates that loop with the pressure, the feed and the differential as the three interconnected signals.

5. The Mill Power and the Specific Energy: the numbers the file teaches

The power of the vertical roller mill is consumed in three places: the drive of the grinding table, the fan that moves the gas through the mill and the separator, and the auxiliary equipment: the presentation separates these because the audit of a VRM finds its energy savings in all three: the table drive carries the grinding load, the fan carries the classification and the drying load, and the separator carries the classification refinement: the total specific energy of the system, at the mill inlet and at the kiln feed silo, is the number the plant buys electricity with.

The grinding power relationship of the VRM follows the same Bond size-reduction logic as the other mills, corrected for the bed grinding:

E = Wi × 10 × [ (1 / √P80) − (1 / √F80) ] × K-vrm

Where E is the specific grinding energy in kWh per tonne, Wi the Bond work index, P80 and F80 the 80-percent-passing sizes in microns, and K-vrm the bed-grinding correction factor in the range 0.6 to 0.8 that captures the VRM energy advantage over the ball mill: the corrected energy matches the measured tables of the industry: the raw meal at 15 to 25 kWh, the cement at 25 to 40, and the slag at 40 to 60 on the total system basis.

Application	Specific energy total kWh/t	Fineness target	Comparison with the ball mill
Raw meal	16 – 24	10 – 14% R90	20 – 30% lower
Coal	15 – 22	1 – 2% R90	30 – 40% lower, plus drying
OPC finish	25 – 38	3,000 – 4,000 Blaine	25 – 40% lower
Slag finish	40 – 60	4,000 – 4,500 Blaine	30 – 45% lower

The energy advantage is paid back in a currency the operators must understand: the VRM finish cement has a slightly different particle size distribution than the ball-milled cement, with the narrower spread and the lower proportion of the very fine particles, which changes the water demand and the strength development of the concrete: the modern plants blend or tune the VRM separator to recreate the well-rounded size distribution, and the presentation covers that tuning because the energy saving is only real when the product quality holds.

6. The Gas Flow, the Drying and the Nozzle Ring: the mill as a dryer

One of the VRM’s great operational facts is that it dries as it grinds: the hot gas from the kiln system, or from the hot gas generator, enters through the nozzle ring and flows upward through the mill, drying the moisture of the feed as the material is carried and classified: the drying capacity of the VRM is large: the raw mills dry feeds with 8 to 20 percent moisture using the kiln gas alone, and the mills with the external heat source dry even wetter materials: the gas flow is therefore a drying instrument as much as a classifying instrument, and the presentation treats the two duties together.

The gas velocity at the nozzle ring is the single number that links the drying to the classification: the standard values run 60 to 90 meters per second at the ring slots, high enough to lift the fine particles and keep the mill internals swept, and the adjusting open area of the ring gives the operator a lever on the internal recirculation: raising the velocity lifts more material and lowers the rejects; lowering it drops more material back to the table and increases the mill load.

• The moisture balance: the material enters at the temperature and the moisture of its storage, the gas brings the drying heat, and the product leaves at the mill outlet temperature that the dew point and the separator demand: the raw mills run the outlet at 90 to 110 C so the meal stays free-flowing;
• The thermal input: the kiln gas at 300 to 450 C supplies the bulk of the heat for the raw mill, drawn through the mill circuit by the mill fan, and the coal and the finish mills run their own gas generators or the waste heat;
• The dew point protection: the mill inlet and the outlet must stay above the dew point of the sulfur compounds, or the condensation and the corrosion attack the bag filter and the mill internals;
• The gas flow control: the mill fan with the damper regulation holds the required flow through the mill, and its power is a significant part of the mill’s total energy bill: the tight gas settings save the fan power as surely as the tight pressure saves the grinding power;

The gas flow is also the transport medium of the rejects: the material that leaves the table and is too heavy to lift falls through the nozzle ring and is swept out as the mill rejects, the mill grits, carrying the moisture and the iron back to the table via the external recirculation or the reject loop: the presentation explains the rejects management because the operating level of the rejects is one of the clearest diagnostic signals of the mill’s health: too high rejects means the gas too weak or the grinding too weak; too low rejects can mean the grinding too fine or the carry-over too strong.

7. The Separator and the Classification: making the fineness target

The material lifted by the gas reaches the separator at the top of the mill, where the dynamic classifier, the third-generation separator with the rotating cage, separates the fine fraction carried to the product and the coarse fraction returned to the table: the separator of the VRM is the same family as the separators of the closed-circuit ball mills, and it obeys the same classification physics: the cut size is set by the rotor tip speed, the gas volume and the air-to-fines loading, and the separator slope decides the sharpness of the cut.

The cut size relationship of the dynamic separator:

d50 ∝ √(Q / (n × D-rotor))

Where d50 is the cut size, Q the gas volume flow through the separator, n the rotor speed and D-rotor the rotor diameter: raising the rotor speed lowers the cut size and gives the finer product; raising the gas flow raises the cut size and carries the coarser material up: the VRM operator tunes the fineness primarily with the rotor speed, and the classification sharpness with the bypass and the separator flows: the typical control statement of the presentation is that the finish fineness responds to the rotor speed in the nearly linear fashion, which makes the automatic control simple and reliable.

• The rotor speed operation: 50 to 100 percent of the range with the rotor tip speeds of 40 to 60 m/s at the design point for the raw meal and higher for the cement;
• The circulating load role: the VRM internal circulation of the coarse fraction is the analogue of the ball mill’s separator recirculation, and its level (often 150 to 400 percent of the feed) sets the load on the table and the gas;
• The separator efficiency: the modern dynamic separators reach the separation efficiencies of 80 to 95 percent at the design point, with the bypass of 5 to 20 percent whose tails spoil the particle size distribution;
• The rejects adjustment: the separator’s internal adjusting vanes and the rotor-leakage gaps set the bypass, and the audit of the classifier is the first step of any VRM fineness problem;

The classification of the VRM is also the difference between the raw and the finish duties: the raw mill wants the homogenous, medium-fineness meal for the kiln, while the finish mill wants the precisely distributed cement, and the same separator adjusts to both by the rotor speed and by the vanes: the presentation carries the classifier maps of both duties, and it emphasizes the too-often-forgotten truth that the separator is a separate machine inside the mill: its power, its wear and its settings deserve the same audit attention as the table and the rollers.

8. The Operation Control: the start, the stable run and the automation

The operation of the vertical roller mill is an exercise in balance: the feed rate, the grinding pressure, the gas flow, the roller damper position and the separator speed must hold a stable point that changes with the material hardness, the moisture and the required fineness, and the control system must move that point without losing the bed: the presentation devotes its operation sections to that balance and to the sequence that protects the machine.

The start-up discipline:

• The bed build-up: before the rollers are loaded, the table must be pre-filled with the material to a bed height that prevents the metal contact: the mills start at the reduced feed and the reduced pressure and ramp both up together;
• The warm-up: the gas flow and the temperature are established slowly so that the internals and the bag filter reach the stable thermal state before the full production;
• The roller lifting: on the start and the stop the hydraulic system lifts the rollers off the table, protecting the grinding surfaces from the metal-to-metal contact during the transient;
• The ramp profile: the feed, the pressure and the separator speed follow the scheduled ramps that keep the mill differential and the vibration inside their windows through the transients;

The stable-run control loops are the daily instruments of the operator: the mill feed is the manipulating signal against the mill differential pressure, the roller pressure is the manipulating signal against the product fineness and the vibration, the gas flow is set against the drying and the classificaation, and the separator speed against the residue: the modern VRMs with the full automation run these loops with the model-predictive controllers, but the presentation insists that the operator must read the same signals the controller reads, because the soft-limit situations, the moisture excursions and the tramp metal events fall to the human judgment when the model cannot see them.

9. The Vibration: the operator’s first enemy and its cures

The vertical roller mill operates in the narrow window between the stable bed and the vibration trip, and the vibration is the single event that most disturbs the VRM operation: the vibration is the mechanical resonance of the mill structure when the forces on the rollers oscillate, and its root causes are the bed conditions: the presentation teaches the operator to read the vibration as the bed’s cry for help, and it maps the causes to the cures:

Vibration cause	Signal	Operating cure
Thin bed, metal contact	High sharp vibration plus the rising mill amps	Increase the feed, lift the rollers, rebuild the bed
Thick bed, spitting	Vibration plus the coarse rejects surge	Raise the pressure, increase the gas velocity, reduce the feed
Tramp metal under the roller	Impulse spikes in the vibration and the pressure	Lift the rollers, purge the tramp metal, check the metal detector
Feed moisture surge	Covered rollers, falling differential, vibration	Raise the gas temperature, reduce the feed, protect the dew point
Worn tires and table segments	Gradual rising vibration and pressure with the wear	Schedule the tire re-profiling or the replacement

The mechanical side of the failure completes the picture: the vibration communicates through the hydraulic oil to the accumulator and to the gearbox, and the plants measure it with the accelerometers on the roller frames and the mill housing: the vibration limits of the modern mills sit near 2 to 3 millimeters per second RMS at the housing and 4 to 8 at the rollers, and the trip values protect the gearbox bearings, the most expensive single part of the machine: the presentation’s message is that the vibration is not a random event but a diagnostic: every vibration excursion has its name, and the operator who reads the name cures the cause instead of masking the symptom.

10. The Wear, the Maintenance and the Inspection of the VRM

The wear of the vertical roller mill concentrates in four zones: the roller tires and the table segments, the nozzle ring and the internal guides, the separator rotor and vanes, and the seals of the air and the material: the presentation closes its technical body with the wear and the inspection practice, and it ties the wear rates to the operation: the harder the material and the higher the pressure, the faster the tires wear, and the worn profile changes the bed and the energy: the tire geometry is the maintenance heart of the machine.

• The roller tires: the outer shells of the rollers wear on their working profile and are re-profiled by the welding build-up or replaced, typically on the 8,000 to 20,000 hour cycles depending on the material abrasiveness;
• The table segments: the replaceable plates on the table wear with the tires and are set back by the build-up welding that restores the profile to the original curve;
• The internal inspections: the mill is stopped for the internal inspection at every major campaign, checking the tables, the tires, the ring slots, the vanes and the seals, with the wear recorded in mm and compared against the wear limits;
• The hydraulic maintenance: the oil quality, the accumulator nitrogen pressure and the cylinder seals are the unsung reliability items, because a leaking accumulator transmits the vibration straight to the frame;
• The gearbox care: the main gearbox runs in the controlled oil system with the condition monitoring, and its overhaul is the longest single shutdown of the mill campaign;

The inspection file of the package pairs with this presentation: the vertical roller mill inspection forms record the tire wear profiles, the table wear, the ring condition, the separator rotor and the hydraulic readings, and the audit then turns the wear data into the energy data: a mill whose tires have worn below the design profile grinds coarser, recirculates more and spends more kWh per tonne, and the energy-distribution logic of the ball mills applies to the VRM as well: the maintenance and the process are two sides of the same machine, and the file keeps both sides in one operating document.

11. The Typical Data Sheet of a Modern VRM

The presentation includes the data sheet pages of the typical VRMs, and the table below summarizes the operating envelope the engineer should hold in mind when evaluating the equipment, the quotation or the performance of a vertical roller mill:

Parameter	Raw mill typical	Finish mill typical	Coal mill typical
Table diameter	3.5 – 6.2 m	3.5 – 5.6 m	2.4 – 3.8 m
Installed power	1,800 – 4,500 kW	2,000 – 5,200 kW	400 – 1,200 kW
Capacity	150 – 600 t/h	100 – 350 t/h	20 – 80 t/h
Product fineness	10 – 14% R90	3,000 – 4,500 Blaine	1 – 2% R90 (kiln coal)
Gas inlet temperature	250 – 450 C (kiln gas)	80 – 120 C	200 – 350 C
Specific energy	16 – 24 kWh/t	25 – 38 kWh/t	15 – 22 kWh/t

The data sheet numbers are the anchor of every discussion in the file: the capacity claims of the suppliers are verified against the specific energy, the fineness and the gas capability, and the operating plants use the same sheet to set their target curves for the monthly review: the engineer who reads the VRM presentation with this table beside him can evaluate whether a quoted mill can dry his wet feed, grind his hard clinker and meet his fineness with his gas temperature: the file is a selection tool as well as an operating manual.

12. The Frequently Asked Questions

Why does the vertical roller mill consume less energy than the ball mill?

Because the grinding is done on the material bed: the rollers press the bed rather than lift and drop the media, so a much larger share of the input energy reaches the fracture of the particles: the measured advantage is 20 to 40 percent lower specific energy for the same fineness, with the biggest saving in the fine grinding where the ball mill wastes the energy on the media wear and the internal friction.

What is the role of the nozzle ring velocity in the VRM?

The nozzle ring accelerates the gas to the 60 to 90 meters per second that lifts the fine and the medium particles into the separator and lets the oversize fall back to the table: the velocity sets the split between the carried and the rejected material, and it is the operator’s lever on the internal circulation and the drying: raising the velocity lifts more and lowers the rejects; lowering it drops more and increases the mill load.

Why does the VRM vibrate and what stops a trip?

The vibration is the symptom of an unstable material bed: too thin a bed lets the rollers touch the metal, too thick a bed stalls the grinding under the coarse spitting, and the tramp metal under the roller gives the sharp impulse peaks: the cures are restoring the bed, adjusting the pressure and the feed, removing the tramp material, and in the final case the automatic lift of the rollers and the mill trip protect the gearbox: the vibration is the bed’s diagnostic, and the operator reads the cause from the pattern.

How is the fineness of the VRM product controlled?

Primarily with the separator rotor speed: raising the rotor speed lowers the cut size and gives the finer product, with the response nearly linear in the operating range: the gas flow, the separator vanes and the circulating load refine the cut, and the automatic control combines the rotor speed with the feed and the pressure to hold the residue or the Blaine target while the material hardness changes.

Can the vertical roller mill dry the wet raw material?

Yes, and this is one of its defining advantages: the kiln gas at 300 to 450 C flows through the mill as the drying and the classifying medium, and the raw mills dry feeds with 8 to 20 percent moisture in the normal operation: the drying capacity is set by the gas temperature, the gas flow and the mill outlet temperature, and the plants with the moisture extremes operate the mill at the reduced output or add the external heat, protecting the dew point of the bag filter.

13. Conclusion

The vertical roller mill is the efficient, drying, compact heart of the modern cement plant: the material-bed grinding that spends its pressure on the fracture, the power formula that prices the energy, the gas flow that dries and the separator that classifies, the pressure and the feed balance that keep the bed alive, and the vibration diagnostics that protect the machine: this article walked the file from the machine elements to the data sheet, and the engineer who applies its operating logic will hold his mill inside its efficiency window, its fineness target and its design life.

The Complete Cement Technical Package includes the vertical roller mill presentation together with the VRM operation courses, the inspection forms and the grinding theory handbooks: the one-time $249.99 purchase, the instant download and the lifetime access: the grinding, on the bed: the energy, saved: the fineness, stable: the vibration, understood: the vertical mill, running at its design point.

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.