Raw Grinding System: Complete Technical Guide

The raw grinding system is the train of equipment between the quarry stockpile and the kiln preheater: the machines that dry, grind, dose and classify the raw material into the raw meal of the kiln: this system must deliver not only the tons per hour but also the chemistry: the raw meal of the correct LSF, the silica and the alumina ratios, the fineness that the kiln can burn and the homogeneity that keeps the burning zone stable: the raw grinding is the least spectacular department of the cement plant but the one where the errors of the day travel directly to the quality of the clinker and the fuel consumption of the kiln.

The Complete Cement Technical Package (931 files including the grinding technology, the mill books, the Excel calculators, the chemistry modules and the operator courses: $249.99 one-time: instant download via the PayPal payment) includes this full raw-grinding-system file: the guide to the design, the operation and the optimization of the raw mill plant: this article walks the file: the grinding principles and the mill types, the drying duty of the raw material, the classifier and the recirculation, the fineness targets, the chemistry control loop, the interplay with the kiln and the economic figures: the reader who follows the guide understands what the operators and the engineers of the modern dry-process plants do with the raw mill belt.

The raw mill is special in the cement plant for one reason: it is the only mill that must do three jobs at once: grind the material, dry it (the moisture of the raw feed reaches 4 to 20 percent depending on the quarry) and classify it to the target fineness: the vertical roller mill was designed exactly for this combination, and most of the modern plants grind the raw meal in the VRM, while the older and the wet plants still run the ball mills: this article covers both with the modern practice at the center: the parameters, the control and the troubleshooting of the raw grinding system in the daily life of the plant.

1. The Place of the Raw Grinding in the Process: The Chain from the Crusher to the Kalziner

The raw meal travels a long chain before the kiln, and each element of the chain has the role in the final quality:

• The crusher: the quarried rock is crushed to a size of 25-80 mm: the coarse cut of the raw feed: the moisture reduction of the exterior;
• The raw storage and the blending: the stockpiles prehomogenize the material: the layers of the quarry are stacked horizontally and reclaimed vertically, the chemistry of the pile homogenized;
• The raw mill: the grinding to the fineness of 12-14% R90 (residue on the 90 micron screen) together with the drying of the moisture: the heart of this file;
• The raw meal silos: the aerated homogenization, the last correction of the chemistry, the buffering between the mill and the kiln;
• The kiln feed: the continuous dosing from the silo to the top of the preheater: the rate of the kiln and the chemistry of the feed are the constant of the process;

The raw mill in this chain works empirically as a buffer: it must keep the kiln fed without interruption, whatever the quarry does: the efficiency of the raw mill therefore decides the availability of the whole kiln, and the modern plants run the raw mill at the average load 85-95% with the kiln at 100% percent, meaning the mill runs the extra hours to build the inventory in the silos.

2. The Mill Types of the Raw Grinding: The VRM vs the Ball Mill

Two machines grind the raw material of the dry process of the world: the vertical roller mill (VRM) and the ball mill (tube mill):

Parameter	Vertical roller mill (VRM)	Ball mill (air swept)
Specific energy to 12-14% R90	16 – 22 kWh/t	24 – 30 kWh/t
Drying capacity	excellent, with the kiln gas up to 20% moisture	good, up to 10-12%
Footprint and foundation	small, light	large, heavy
Noise	low	high
Maintenance	grinding table and roller tyres, hydraulic system	balls, liners, diaphragm
Suitability	dry process, the standard of the last 30 years	wet plants and old dry plants

The decision of the file: the modern dry plant chooses the VRM almost invariably: the energy saving of 20-30% compared to the ball mill, the drying capacity and the smaller footprint justify the capital: the ball mill remains for the raw where the plant continues the old line or needs the extreme flexibility of the feed: the normalization of tools the economic comparison is delivered in the file with the payback calculation.

3. The Construction of the Vertical Raw Mill: The Components of the Machine

The VRM is a compact machine: the grinding table at the bottom, the 2-4 grinding rollers above, the classifier of the top, and the whole train mounted inside a tower-like mill body:

• The grinding table: the rotating bowl of the welded steel with the wear segments (the tyres, the table liners of the high chromium iron): the material is fed at the center of the table by the feed chute with the rotary feeder;
• The rollers: the two or four cylindrical rollers (the tyres) that press the material against the table: the hydraulics of the system maintain the pressure of 4-10 MPa in the accumulator, adjustable per the hardness of the feed;
• The nozzle ring: the annular air nozzles around the table that lift the ground material into the gas stream: the velocity 40-100 m/s, and the hot gas feeds the mill through them simultaneously drying and conveying;
• The classifier (dynamic separator): the rotating cage on the top, which returns the oversize to the table and passes the fines with the air, the fineness controlled by the speed of the rotor;
• The mill body and the drive: the casing of the gas-tight, the central bevel gear and the auxiliary drive for the starting so:

The VRM grinds exactly at the “the material bed” principle: the rollers press the layer of material on the table, the breakage happens in the bed by the compression, not by the free hits of the ball mill: this explains the lower energy (the compression is the most efficient size reduction) and also the sensitivity: without the bed of the correct height, the metal touches the metal and the mill vibrates.

4. The Drying Duty: The Moisture of the Raw Material and the Kiln Gas

In the dry process, the raw material arrives at the mill with the moisture of its character: the alluvial clays 15-25% water, the limestones 2-6%, the marls 4-10%: the mill must dry all this water to the product of below 1% for the pneumatique transport and the storage:

• The drying media: the main dryer is the kiln exhaust gas (280-360 C at the preheater exit) drawn through the mill: this kills the entire drying duty when the kiln runs, at the full capacity:
• The auxiliary heat: in the winter or at the high moisture, the hot air generator (the HAG) added the separate burner to support: the gas temperature at the mill inlet is then controlled at 300-400 C:
• The gas volume: the flow through the mill is immense: 40-80 m3/s per plant, the pressure differences the gas velocity at the nozzle ring: the false air of the mill must stay under 10% or the drying fails:

The operation rule of the drying: the moisture of the final raw meal must be below 0.5-1% or the raw meal plugs the silo and the kiln feed: the mill as the exit temperature is controlled in the range of 80-110 C: the trend of the exit temperature is the operation’s early warning of the moisture change of the quarry: the file gives the nomograms of the drying capacity of the mill as a function of the gas volume, the temperature and the moisture.

5. The Classification Inside the Mill: The Fines and the Returns

The VRM classifies internally: the ground powder is carried by the gas to the top classifier, which separates the fines and returns the coarse; the return falls back to the table center and regrinds: the circuit of the mill is the closed internal loop:

• The classifier: the dynamic rotor (the cage) with the adjustable speed: the fines pass the rotor blades, the coarse is thrown back to the mill: the fineness control instrument of the raw mill;
• The variables: the rotor speed (e.g. the 300-700 rpm for the typical cage), the gas flow of the mill and the damper position: the higher the rotor speed, the finer the product:
• The internal circulating load: the returns in the VRM, the tons per hour of the mill bed: 200-500% of the feed for the raw meal at 122-14% R90; the higher at the finer grinding;
• The drying: the gas at the exit of the classifier passes the filter (the bag house or the electrostatic) and the product powder is collected, while the vent gas is treated:

The performance of the classifier decides the mill capacity: a mill that vibrates because of the wrong arrangement of the internal recirculation is a mill that loses the tonnage: the file describes the exact sampling: the mill inlet, the differential pressure and the classification curve of the raw meal product, with the tables of the expected curves for the healthy machine.

6. The Fineness of the Raw Meal and Its Effect on the Burning

The raw meal fineness is the second quality parameter of the raw mill, after the chemistry: the target of the modern dry process is about 12-14 percent residue on the 90 micron screen and 1.5-2.5 percent on the 200 micron in the vertical mills:

• The burnability link: the coarse quartz and the coarse calcite particles of the raw meal do not react fully in the kiln: the coarse particle of 200 micron remains partly unburnt and the free lime rises;
• The energy cost: the finer raw meal costs the mill more energy per ton: the rule of thumb of the industry: 1% reduction of the R90 residue costs about 1-2% of the mill energy, and the effect on the kiln fuel is smaller;
• The optimum: the plant balances the mill energy against the fuel consumption of the kiln: the optimum R90 of the dry process is typically 10-14%: below 8% the mill pays more than the kiln saves;
• The distribution: the particle size distribution of the raw meal matters as much as the residue: the VRM produces a narrow distribution of the hard quartz particles that resist the burning;

Raw meal parameter	Target of the dry process	Effect of the deviation
R90 %	10 – 14	coarse = kiln problems, free lime
R200 %	1.0 – 2.5	the coarse tail of the burning
Moisture %	under 0.5 – 1.0	the plugging of the transport
LSF of the meal	target +/- 1.5%	the burning zone control

The raw meal fineness is therefore the balance instrument between the mill and the kiln: the file provides the optimization spreadsheets: for each target of the R90, the mill energy, the kiln fuel, and the total costs of the day: the modern plants shift the raw meal fineness seasonally with the moisture: the basic tuning of the whole system.

7. The Chemistry Control of the Raw Meal: The Dosage, the Analyzer and the Loop

The raw mill receives the material from the stocks by the weigh feeders, and the chemistry of the output is the chemistry of the feed mix: the control system works the loop:

• The analyzers: the X-ray fluorescence (XRF) analyzers measure the key oxides of the raw materials or the raw meal: the online PGNAA (prompt gamma)/XRF belt analyzers at the mill feed measure the chemistry continuously:
• The dosage: the control loop of the raw mill: the proportions of the limestone, the marl, the clay and the sand are the manipulated variables: the LSF, the silica rate and the alumina rate the controlled: the adjustment interval is 10-30 minutes per the analyzer:
• The targets: the raw meal of the kiln: LSF 0.92-0.98 (typical 0.96), the SM 2.2-2.7, the AM 1.3-1.7: the aim of the control: the hourly LSF of the raw meal within +/-1.5% (the modulus of the kiln operation);
• The homogenization: the buffer of the silos: the air-agitated silos smooth the remaining deviations: the raw meal from the silo reaches the kiln with the stable chemistry even when the mill fluctuates:

The chemistry chain of the raw meal is not complete without the sampling discipline: the auto sampler at the mill outlet, the preparation and the shift analysis: the file includes the control charts (the Shewhart) that the plant uses to monitor the LSF and the limits of the correction: the raw meal chemistry, the feed forward of the kiln control, is the quality chain that decides the burning.

8. The Operation of the Raw Mill: The Start, the Steady and the Transitories

The daily operation of the raw mill is a set of the maneuvers that the operators of the file rehearse with the DCS and the plant:

• The start: the mill is started empty: the gas flow first, then the main drive at the low speed, the grindin lowers the pressure in the stages, the feed is added when the mill reaches the motion: the approx 10-15 minutes of the start:
• The steady state: the feed rate is adjusted so that the differential pressure across the mill stays in the working band (typically 4,000-6,000 Pa in the raw VRM; at the same time the exit temperature at 85-105 C):
• The load control: the mill power at 90-100% of the max in the full load: the separate schedule: the operators plan the stops (“mill silent”) only in the electric rates, never at the kiln’s milk:
• The emergency stop: the vibration protection: at the vibration of the mill, the rollers are lifted hydraulically within seconds, the feed stops and the table rotates empty: the correct restart after the event is a trained procedure:

The operators of the raw mill also guard the daily housekeeping: the spillage of the feed chutes, the jamming of the reject material, the cleaning of the filter: the file captures the operator logs of the most repetitive events with the checklists, and the KPIs of the shift: the tons per hour, the kWh/t, the fineness drift, the alarm count.

9. The Wear and the Maintenance of the Raw VRM

The components of the raw mill that wear and the maintenance intervals of the modern plants:

Component	Wear behavior	Typical life	Maintenance
Table liners (segments)	grooves, the wear in the outer ring	8,000 – 15,000 h	ovaWheels the rebuild or the reversal
Roller tyres	the flatness, the cracks of the surface	6,000 – 12,000 h	the grinding (the veining) in situ
Nozzle ring	the erosion of the hard materials	2-3 years	the rebuild or the segments
Classifier rotor blades	the tip erosion	12,000 – 20,000 h	the reversal or the replacement
Hydraulic accumulators	the nitrogen charge, the seals	on the check	the quarterly checks

The wear of the tyres has the direct effect on the grinding: the worn profile changes the grinding angle and the bed, so the mill draws the higher power for the same output: the tyre grinding (the re-profiling of the roller surface in place) is a fast and inexpensive measure that the plants carry every 6-12 months: the wear measurement of the raw mill is the discipline of the millimeter.

10. The False Air and the Gas Gap of the Raw Mill

The raw mill works under the negative pressure, and the leakage of the ambient air into the circuit (the false air) is the silent thief of the drying and the classification:

• The leak points: the rotary feeders of the VRM, the joints of the ducts, the filter compartments, the flanges and the damper seals: the false air of the raw system can reach 10-30 percent of the total gas;
• The effects: the false air cools the gas and reduces the drying capacity, the gas velocity falls and the classification weakens, the fan power rises with the volume, and the exit temperatures drift: the mill loses output at the same power;
• The measurement: the O2 content of the gases: an O2 of 3 percent in the kiln gases before the mill and 8 percent at the mill exit indicates a substantial leak; the file explains the O2 balance method with the worked example;
• The campaigns: the systematic chemical campaign of the raw mill system is among the highest-payback measures, the file provides the audit sheets, the measurement points and the repair instructions;

The typical result of the false-air campaign of the raw mill: the sealing of 10 percent of the leaks restores 3-6 percent of the mill output and about 1-2 kWh/t of the specific energy, with the maintained quality of the raw meal: the numbers of the energy begin with the measuring of the leaks, and the file gives the exact method of the O2-probe measurement to the reader and the tables of the calculation.

11. The Raw Mill and the Kiln: The Integrated Operation of the Gas

The raw mill and the kiln share the gas circuit, and their operation is coupled in the modes of the plant:

• The mill and the kiln in the compound mode: the kiln exhaust gas (300-360 C) is routed through the raw mill for the drying: the gas carries the dust and the alkali from the kiln, and the raw meal binds a part of the sulfur and chlorine measured as the “scrubbing” of the system;
• The mill stopped (direct mode): the kiln gas bypasses the mill and goes to the filter directly: the temperature of the powder tie the filter rises toward the kiln: the tail of the CO and the filter operation change;
• The interplay: the start and the down of the raw millis the game of the temperatures: the raw mill offline with the kiln: the preheater exit gas goes hotter to the filter, the dust climbs: the file covers the manual of these switching modes as the operator action;
• The calciner side: in the calciner kiln the fuel split (the 40/60) is adjusted to hold the same preheater temperatures with the different gas volumes of the raw mill: the engineer of the package knows this balance of the heat;

The coupling of the two systems is the theme of the modern plants: the aligned planning of the raw mill stops with the kiln (the weekends, the peaking) and the storage capacity of the raw meal silo determines the allowed time of the raw equipment stop: the file teaches the philosophy: the silo of the raw meal is the friend of the kiln, and the capacity of 6-12 hours of the buffer is the freedom of the whole plant.

12. The Economics of the Raw Grinding: The Energy, the Maintenance and the Decision

Raw grinding is about the 4-5 percent of the plant value but 10-12 percent of the energy: the economics of the file:

Item	VRM	Ball mill
Specific power (kWh/t raw meal)	16 – 22	24 – 30
Share of the total cement plant electricity	~10%	~13%
Maintenance cost (USD/t raw)	0.2 – 0.5	0.4 – 0.7
Capacity of the most common plants (t/h)	150 – 400	100 – 200

The decision of the raw mill equipment is thus the decision of the whole plant energy: the table of the file compares the total cost horizontal (the investment, energy, maintenance, availability) over the 15-year life: the conclusion: the greenfield plants of the 21 century escolher the VRM; the plants with old ball mills may retrofit the pregrinder + ball configuration: either way, the file measures the specific power budget and returns the payback of each path.

13. The Raw Mix Control Loop: From the Quarry Block to the Kiln Feed

The raw grinding system is commanded by the raw mix control, and the plant’s quality systems close the following loop each day:

• The quarry block data: every block of the quarry is characterized (the CaCO3 of the limestone, the SiO2 of the marl, the moisture, the grindability): the block plan of the week anticipates the material that the mill will receive;
• The stockpile strategy: the preblending beds (the chevron or the window stacking) empty the layers vertically, mixing the days of the quarry into the pile: the claim of the pile is the claim of the raw chemistry;
• The mill dosage: the feeder setpoints of the raw mill follow the analyzer (the XRF spectra or the PGNAA) with the 15-30 minute corrections: the modern plants also hold the online moisture sensor in the loop;
• The quality gates: the laboratory controls the raw meal every 1-2 hours, the autosampler sends the powder to the XRF: the process engineer of the shift, the target corrections on the DCS;

Control parameter	Target	Action band
CaCO3 of the raw meal	own target (e.g. 78.5%)	+/-0.6%: adjustment of the feeder
LSF	0.94 – 0.97	+/-0.02 limit
SM (silicic ratio)	2.2 – 2.7	veiled by the silica addition
AM (alumina ratio)	1.3 – 1.7	veiled by the bauxite/iron

The raw meal of the day is not yesterday’s number: it is the moving balance of the quarry, the pile, the dosing and the mill: the file systematizes it into the “one-page” monitoring form: the quality of the raw meal, the control chart and the log of the events: the discipline of the raw grinding is the discipline which the kiln rewards with the stable burning zone and the constant clinker.

14. The Frequently Asked Questions

Why is the raw meal residue R200 so important?

The R200 (residue on the 200 micron sieve) indicates the very coarse tail of the raw meal: the coarse quartz and the coarse calcite that do not react completely in the burning zone: an R200 of above 2.5% leads to a higher free lime and the lower burnability: with the 45% of the LSF the mills operate the residue control of both the R90 and the R200: the file describes the hourly tests of the laboratory pulp.

What is the difference of the raw mill for the wet vs the dry process?

The wet process (the slurry) grinds 30-40% water ball milling, without the drying: the dry process dries and grinds in the same machine: the wet plants are the heritage of the 20th century: the transition to the dry was the biggest energy saving in whole cement: the file gives both the grinding systems, but the emphasis is on the dry system that the modern plants use.

What does the “false air” do to my raw mill?

The false air is the cold air leaking into the gas circuit: it cools the gases and reduces the drying ability; it dilutes the classification flow; and it increases the fan work: the plant with high false air loses 10-20 percent of the drying and rises the exit temperature: the leak measurement of the O2 and the campaigns of sealing are the standard: the file includes the leaks form.

The raw meal silo empties during the kiln at 90%: what should the process engineer control?

The answer is the raw planning: the silo level is the material speed of the plant: the engineer plans the raw mill operation to build the stock at the kiln-down times and to keep the buffer above the 30-40% at the peak kiln loads: the capacity of the silo (the aerated 2,000-4,000 t) and the operating windows of the power price determine the schedule: the file has the planning tables of the mill hour calendar.

Which mill for the raw meal with the high quartz or the high clay?

The abrasive quartz (the hard sand horizons) wears the VRM tyres more than the ball mill liners: the clay-rich feed binds the table with the high moisture: the file’s selection matrix: the plant with the hard quartz ITs the roller of the small tyres or the alternative of the feeding and the closed: the plant with the clay prefers the VRM of the high drying: the material-specific optimum is in the tables of the trial evaluation.

15. The Conclusion

The raw grinding system is the first link of the cement chain, and the management of the raw meal quality determines the burning, the energy and the clinker of the plant: the file so teaches the whole range: the equipment, the operating parameters, the chemistry and the integrated view: the engineer who enters the raw mill knowledgeable and the operator who follows the procedures of the file: the company and its goals: the meal: the mill: the kilos: the kiln: the finished: the whole is the flow of the industry, and the raw grinding is the first foundation.

The Complete Cement Technical Package contains the file of the raw grinding system with all the tables, the calculators, the operating procedures and the audit forms: the 931 files: the 249.99 one-time: the instant download: the technical universe of the cement: the raw grinding now: the engineer of the cement with the manual of the precise: the package: the cementequipment.org.

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