Materail Prepration And Raw Milling: Complete Guide & Downlo

Material preparation and raw milling is the first commanded process of the cement plant: the line that turns the quarried rock into the fine, uniform, dry powder called the raw meal: the meal that the kiln burns into clinker: every ton of clinker passes through this line first, and every weakness of the line multiplies downstream: the proportioning errors of the day travel into the kiln feed of the month: the preparation department is not a service unit but the first quality gate of the whole plant.

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 guide with its flow diagrams, the equipment tables and the operating data of the raw preparation plants: the practical reference for the engineers of the raw departments, the designers of the new lines and the students of the process: this article walks the file section by section: the crushing, the storage, the proportioning, the milling, the drying and the homogenization: with the numbers of the real plants: the reader closes the page with the map of the whole line in mind.

The preparation line is usually described as a chain of equal steps, and the description is correct: each step is equally important: the crusher protects the mill, the stockpile smooths the chemistry, the weigh feeders define the mix, and the mill delivers the quality of the meal: the drying air, the classifier and the silo complete the chain: this page follows the same order, so the reader can follow the article with the document in hand, section by section.

1. The Purpose of the Preparation Line: From Quarry Rock to Kiln Feed

The raw preparation exists for one reason: to deliver the kiln a continuous, chemically defined, finely ground, dry powder at the required tonnage: the kiln is the most expensive furnace of the industry and it must be fed without interruption at the target chemistry: the raw meal of the modern dry-process plant is ground to about 10 to 12% residue on 90 micrometers, with a target free lime potential and a stable LSF that the controllers of the kiln trust: the task of the preparation line is to absorb the variability of the quarry and hand the kiln a uniform feed.

The whole line works as one long chain of delays and averaging: the quarry produces rock that varies seam by seam, bench by bench; the crusher reduces and mixes it; the stockpile layers it and blends it; the weigh feeders select it deliberately; the mill grinds and dries it; and the homogenization silo performs the final averaging: each link has its own efficiency of homogenization, and the total smoothing is the product of the smoothing of all the links: the file documents both the individual efficiencies and the way they multiply into the feed stability of the kiln.

• The tonnage: the preparation line handles everything that enters the kiln system: 1.5 to 1.7 tons of raw material per ton of clinker, before the dust losses: a 5,000 t/d plant moves 7,500 to 8,500 tons per day through its raw department;
• The chemistry: the target raw meal is defined by the three modules, LSF, SM and AM, and the preparation must deliver them within narrow tolerances: typical control bands are LSF plus or minus 2 units, SM plus or minus 0.1, AM plus or minus 0.1;
• The fineness: the raw meal is ground to 10 to 14% residue on 90 micrometer, with 85 to 95% passing 212 micrometers: the coarse particles above 400 to 500 micrometers are the classic source of the free lime in the clinker;
• The moisture: the dry-process kiln accepts feed with 0.2 to 1.0% moisture: the raw mill must dry the material from the natural 7 to 20% moisture of the mix down to this low value in the same grinding pass;
• The continuity: the kiln expects the feed around the clock: the preparation line includes the storage days and the redundancy that decouple the single machines from the continuous kiln;

These five requirements define the equipment of the line: the crusher for the tonnage, the stockpiles and the silos for the chemistry and the continuity, the mill for the fineness and the moisture: the sections that follow treat each machine in its turn, with the operating numbers that the plants use every day.

2. The Crushing Circuit: The First Size Reduction of the Line

The quarry delivers blocks of 0.8 to 1.5 meters in size from the blasting rounds: no mill can accept such lumps, and the crusher is the machine that reduces the feed to the 25 to 80 millimeter range that the raw mill accepts: the crusher is selected on the hardness and the moisture of the stone: the soft and sticky marls prefer the impact and the hammer machines, while the hard limestone of the high-strength deposits works best in the jaw and the impact crushers with heavy rotors.

The crushing plant of a modern line is usually a single-stage hammer crusher fed by the dumpers, with a capacity of 800 to 1,200 tons per hour and a product of 90% below 75 millimeters: the semi-mobile crusher inside the quarry reduces the truck cycle and the cost of the haulage: the two-stage arrangement (jaw followed by the hammer or the cone) is reserved for the hardest rocks and the abrasive abrasivity indices above the practical life of the hammers.

Crusher type	Typical capacities t/h	Feed size mm	Product size mm	Best raw material
Single rotor hammer crusher	600 to 1,400	up to 1,500	0 to 80	Limestone, marl, chalk
Impact crusher	200 to 800	up to 700	0 to 60	Soft to medium hard, dry stone
Jaw crusher (primary)	200 to 900	up to 1,500	100 to 250	Hard, abrasive limestone
Cone crusher (secondary)	150 to 500	to 300	20 to 60	Secondary stage of hard rock
Roller crusher	100 to 400	to 150	0 to 40	Clay, chalk, soft sticky feed

The point is that the crusher is sized for the peak of the quarry deliveries, not for the average: the stockpile between the crusher and the mill decouples the two lines and lets the crushing run on the day shift while the milling runs around the clock: the plants that skip the intermediate store pay for the lack with the stoppages of the mill: the file documents the layout options and the surge capacities of 8 to 24 hours that are the practical standard.

3. The Moisture of the Raw Materials: The Data the Drying System Needs

Water is present in every raw material, and the drying system of the mill must be designed from the moisture of the wettest component: the moisture content is a geological property of the deposit, shifted by the weather of the region: the file collects the typical values of the industry so the designer can estimate the drying load before the first laboratory result arrives.

• Limestone: 1 to 5% free moisture in the temperate climates; cave deposits and the karstic limestones may carry 6 to 10% at the wettest season;
• Marl: 8 to 15% in the Mediterranean climates, rising to 18 to 25% in the wet-season stockpile: the marls are the classic sticky material of the raw circuits;
• Clay: 15 to 25% natural moisture after the excavation, and up to 30% in the rainy months: the clay is also the most difficult to feed, to crush and to store;
• Chalk: 18 to 30% moisture in the raw state: the chalk is soft but wet, and the drying load of a chalk-based mix is the highest of the line;
• Sand and iron ore: 2 to 8%: the corrective materials are granular and drain quickly, but their moisture still counts in the mix;

The milling system must then dry the blended mix from its natural moisture to the 0.5 to 1.0% of the kiln feed: the drying can be partly performed in the crusher (the hammer crushers with the hot gas), partly in the stockpile (the natural drainage), and mainly in the mill itself, where the hot gas from the kiln or the hot gas generator evaporates the water during the grinding: the specific drying work of the mill depends directly on this moisture table, and the plants keep a monthly record of the moisture of every component as the planning instrument of the dryer.

4. The Pre-homogenization: The Stacking and the Reclaiming of the Stockpile

The chemistry of the quarry changes within the shifts, and the raw mill cannot average out the variation by itself: the stockpile is the big averaging machine of the line: the long rectangular stockpile is stacked in layers, and then reclaimed end-to-end across the full face of the stockpile, mixing in the reclaimed slice every layer of the stacking period: the result is the smoothing of the chemistry with a documented homogenization factor.

The typical design numbers of the pre-homogenization: the stockpile for the limestone covers 2 to 5 days of the mill consumption; the stacking is done by the boom stacker or the chevron method in 100 to 500 individual layers; the reclaiming by the bridge scraper cuts the full face so every slice contains a sample of all the layers: the mixing factor of the well-run stockpile reaches 5 to 10: the standard deviation of the CaCO3 of the reclaimed material drops to a fifth or a tenth of the deviation of the quarry feed: the file contains the worked example of the standard deviation reduction.

Stacking method	Layers typical	Blending factor	Best application
Chevron stacking	100 to 300	4 to 8	Limestone beds, moderate variations
Windrow stacking	200 to 500	6 to 10	High variation deposits, large beds
Circular stockpile	100 to 250	4 to 7	Compact sites, single component
Reclaim tunnel under stockpile	n/a full face	3 to 5	Small beds, blending by the tunnel hoppers

The second table of the file section adds the operating cautions: the segregation of the coarse and the fine material at the stacking cone, the moisture drainage of the bed, the hard truncated cone at the end of the chevron stacking: the operators rotate the stacking direction and use the front-end loader to respread the end cones: the pre-homogenization is not an optional machine: it is the first and the cheapest averaging stage of the line, and its blending factor multiplies with the factors of the mill and of the silo.

5. The Proportioning: The Weigh Feeders and the Mix Formulation

After the stockpiles, the individual components are extracted at defined rates and combined into the target mix: the proportioning is executed by the weigh feeders, each with its own hopper and extraction belt, controlled from the laboratory line: the X-ray analyzer of the pre-mill samples the blend on the belt, and the control system corrects the setpoints to the target modules: the closed loop is the daily reality of the raw department.

• The weigh feeders: belt weighers with the load cells and the belt speed control, each calibrated at the extraction of 20 to 150 tons per hour per meter of the belt width: the accuracy class of the modern feeders is plus or minus 0.5 to 1.0% of the setpoint;
• The hoppers: equipped with the vibrators or the aeration pads against the arching of the wet clay: the material flow from the hopper lip is the first discipline of the proportioning;
• The sampling: the automatic sampler extracts the belt sample every 30 to 60 minutes into the laboratory, and the X-ray spectrometer returns the CaCO3 and the oxides within 5 to 15 minutes;
• The mix control: the target is a fixed CaCO3 value of the raw mix (typically 76 to 79% for the OPC line), and the controller adjusts the clay and the corrective feeders to hold it;
• The layer of the days: the department logs the daily average mix against the target, and the weekly average of the modules is the compliance report of the line;

The proportioning converts the averaging of the stockpile into a deliberate recipe: the speed of the loop matters: the plants with the fast X-ray and the automatic correction hold the CaCO3 standard deviation of the mill feed below 2 units, while the slower loops drift to 3 to 5 units and compensate at the silo with the cost of the fuel and the free lime: the file presents the control loops and the tuning practice of the weigh feeders in the language of the control engineer.

6. The Raw Mill Systems: The Ball Mill and the Vertical Roller Mill Compared

Two machine families dominate the raw milling of the dry process: the ball mill, usually in the closed circuit with the dynamic separator, and the vertical roller mill (VRM) with the integrated classifier and the gas drying: both grind and dry the material in one pass: the choice between them is decided on the moisture, the hardness, the electricity tariff and the existing plant of the site: the modern lines and the mid-size expansions strongly prefer the VRM for its lower power and its compact footprint.

Parameter	Ball mill system	Vertical roller mill system
Typical unit capacity t/h	60 to 160	150 to 500
Specific power kWh/t (dry grind)	20 to 26	15 to 20
Drying capability (gas temp)	moderate, 250 to 350 C inlet	high, 400 to 500 C inlet
Feed moisture accepted	to 8 to 10%	to 20 to 25%
Coarse feed tolerance mm	to 25	to 75 to 100
Maintenance density	liners and media recirculation	table, rollers, hydraulic system
Footprint	large (mill + separator + cyclones)	compact single machine

The ball mill line grinds the mixed feed with the classical two-compartment charge (the coarse balls of 60 to 90 millimeters in the first compartment, the 15 to 30 millimeter media in the second) and classifies with the third-generation separator: the VRM grinds by the pressure of the rollers on the grinding table and classifies in the internal cage rotor: the electrical saving of the VRM against the ball mill is typically 5 to 7 kWh/t of raw meal, and the drying against the wet feeds is stronger: the file quantifies the comparison in the tables and lets the designer decide with numbers, not with fashion.

7. The Drying Inside the Raw Mill: The Gas, the Dew Point and the Coating

Drying is the invisible task of the raw mill: the water of the feed must leave as vapor in the mill gas, and the mill internals must stay hot enough that the vapor never condenses: the physics of the drying is simple, and the operation of it is a daily watch of the gas temperatures and the dew point at the bag filter.

• The heat source: the kiln exit gas at 300 to 400 C is the free heat of the plant: the raw mill uses it when running, and the hot gas generator or the air heater replaces it when the kiln is down;
• The inlet temperature: the mill inlet gas is 250 to 350 C for the ball mills and 400 to 500 C for the VRMs: the higher temperature is needed to keep the drying capacity at the high throughput;
• The outlet temperature: the mill outlet gas is held at 85 to 110 C, safely above the dew point of 55 to 70 C of the moist gas: the margin protects the bag filter from the condensation and the corrosion;
• The balance: the drying capacity is set by the gas flow and the temperature: raising the feed moisture by 5 points demands roughly 20 to 25% more gas flow or the higher temperature at the same throughput;
• The coating: the condensation of the vapor on the cool internals builds the sticky deposits on the grinding media, the diaphragm and the separator: the coating collapses the grinding rate and the mill is said to be “choked”: the classic remedy is the hotter gas, the lighter feed and the re-established temperature profile;

The plants read the drying state from the outlet temperature and the filter pressure drop: a falling outlet temperature with the constant gas flow is the early sign of the moisture overload: the operators respond by trimming the feed or the heat before the coating forms: the file includes the heat balance of the raw mill with the worked example: the water evaporation at 0.85 kilowatt-hour per kilogram of water in the ball mill systems, and the slightly lower figure in the roller mills with the gas drying.

8. The Grinding of the Raw Mix: The Fineness and the Coarse Grain Rule

The fineness of the raw meal is the quality that burns: the coarse quartz and the coarse calcite grains above 300 to 500 micrometers do not complete the clinkering reactions in the kiln, and the free lime of the clinker rises directly with the fraction of the coarse particles in the meal: the control of the raw milling is therefore the control of the coarse tail distribution, not only the mean fineness.

The operating targets of the raw grinding: 10 to 14% residue on the 90 micrometer sieve, and 1.0 to 2.5% residue on the 212 micrometer sieve for the classic OPC meal: the closed-circuit classifiers sharpen the separation so the fines recirculate and the oversize returns to the mill with the feed: the separator of the raw circuit runs at a cut of 15 to 25 kilograms of the fines per cubic meter of the gas, with the rotor speed of 20 to 30 meters per second and the guide vane settings that hold the bypass below 15%.

Raw meal sieve	Typical residue OPC meal %	Effect on the kiln
90 micrometer residue	10 to 14	Burnability, free lime, fuel consumption
212 micrometer residue	1.0 to 2.5	Coarse calcite and quartz: free lime risk
45 micrometer residue	45 to 60	Indicator of the surface and the reactivity
2000 micrometer residue	0.02 to 0.1	Hazards of the uncrushed nodules and balls

The grinding energy of the raw meal is the second cost of the line: at the Wi of the raw mix of 10 to 13 kilowatt-hours per ton, the ball mill draws 20 to 26 kWh/t and the roller mill 15 to 20 kWh/t: the savings of the classifier sharpness and the correct ball charge are the monthly items of the energy report: the relation between the residue and the specific energy is approximately logarithmic: the last 5% of the residue reduction costs as much as the first 15%: the plants optimize the target to the burnability, not to the habit of the department.

9. The Homogenization Silo: The Final Averaging of the Meal

The mill produces meal that fluctuates around the setpoints with the standard deviation of 2 to 3 units of CaCO3: the kiln needs feed stable to within 0.5 to 1.0 units to burn a consistent clinker: the homogenization silo performs this final averaging: the continuous silo holds 1 to 3 days of the mill output and blends the incoming meal by the aeration of the cone and the continuous extraction from the bottom rings.

The continuous homogenizing silo (CF silo) works with a central feed pipe and the aeration pads at the bottom: the air is directed to the sectors in a rotating sequence, the meal fluidizes, moves to the central outlet and mixes with the older meal of the silo: the mixing factor of such a silo is 5 to 10: the CaCO3 standard deviation of 2.5 units at the inlet falls to 0.5 units at the kiln feed: the air consumption is 0.5 to 0.9 normal cubic meters per ton of meal, and the total silo power is 0.3 to 0.6 kilowatt-hours per ton: the batch silo with the air blending reaches the same mixing with the higher power but the simplest mechanics.

• The storage role: the silos of the line hold 8 to 24 hours of the meal as the buffer between the mill stoppage and the kiln: the operators plan the mill maintenance against the silo level;
• The aeration: the compressed air at 0.1 to 0.2 bar over the meal bed fluidizes the layers: the pads are the disposable parts and their inspection is the weekly duty;
• The mixing test: the tracer test with the lithium or the chloride pulses measures the real mixing factor of the silo: the file describes the test protocol;
• The discharge: the kiln feed bin below the silo holds 15 to 30 minutes of the meal and is weighed by the loss-in-weight feeders to the preheater tower;

The homogenization completes the line: quarry, crusher, stockpile, weigh feeders, mill, silo: the total smoothing of the line is the product of the factors, and the plants that measure each stage can build the full attenuation budget of their feed: the file closes this section with the worked numerical example of the standard deviation through the six stages of the line: the numbers make the design decisions of the homogenization visible.

10. The Operation of the Raw Department: The Daily Log and the Shift Routines

The operation of the raw line is the routine of the station log, and the file turns the routine into a procedure the new shift engineers can execute: the daily log of the raw department records the crushing hours, the mill throughput, the residue, the moisture, the outlet temperature, the separator settings, the silo levels and the laboratory results of the meal.

• The hourly check: the mill power, the feed rate, the outlet temperature, the separator speed and the filter pressure drop: the deviations from the norm are investigated within the shift;
• The sample routine: the belt sample every 30 to 60 minutes, the X-ray result in 10 minutes, and the correction of the weigh feeder setpoints by the controller or the operator;
• The residue check: the laboratory test of the meal every 2 to 4 hours on the 90 and 212 micrometer sieves, plus the daily full residue curve;
• The weekly audit: the ball charge level and the ball size distribution of the mill compartments, the separator vanes and the rotor wear, the state of the diaphragms;
• The moisture audit: the monthly moisture table of all the quarry components, used to forecast the drying load of the following month;

The difference between the good and the average raw departments is visible in the logs: the good ones hold the meal standard deviation below 1.0 unit of CaCO3 and the residue within plus or minus 1 point, and they reach this with the calm corrections, not with the crisis: the file concludes the operational sections with the checklist of the shift handover: the states of the machines, the pending corrections and the open quality issues move with the log, not with the memory of the operators.

11. The Troubleshooting of the Raw Line: The Common Failures and Their Remedies

The failures of the raw line are repetitive, and most of them are prevented by the routine: the file organizes the failure knowledge in the tables that the shift can consult: the table below collects the most frequent symptoms, their first-line causes and the standard remedies of the industry.

Symptom	Most likely cause	Standard first remedy
Mill power rising, throughput falling	Feed too coarse, charge low, moisture high	Check feed size, top up balls, raise gas temperature
Outlet temperature falling	Wet feed overload	Reduce feed or raise heat, avoid coating
Filter pressure drop rising	Condensation, dusty gas, damaged bags	Raise gas temperature, check dew point, inspect bags
Meal residue climbing	Separator wear, vanes changed, mill underloaded	Audit separator, check charge, adjust rotor speed
CaCO3 wandering beyond target	Stockpile segregation, feeder drift, layer changes	Respread the pile, calibrate feeders, re-tune loop
Coating in the mill	Condensation on cool internals	Hotter gas, reduced feed, clean the compartments
Hoppers arching	Wet, sticky feed, wrong hopper angle	Vibrators, aeration, reduce moisture of the feed

The second half of this troubleshooting section lists the mechanical cautions: the crusher hammers wear by the abrasive silica of the stone, the mill liners flip at the uplift of the charge, the separator rotor unbalances with the uneven build-up, and the fans cavitate in the dusty gas: the file pairs each mechanical symptom with the inspection interval and the documented wear limits, so the department plans the stops instead of suffering them: the honest operator starts from the process table above and confirms on the machine before the disassembly.

12. The Energy and the Cost of the Preparation Line

The raw preparation is not the largest electrical consumer of the plant (the finish grinding holds that title), but its efficiency decides the cost share of the department and the design of the whole electrical network: the file collects the benchmark numbers of the industry so the plant can position itself:

• The crushing power: 0.5 to 1.5 kWh/t at the primary stage, rising to 2 to 3 kWh/t for the hard rocks in the two-stage circuits;
• The raw grinding: 15 to 20 kWh/t in the roller mills and 20 to 26 kWh/t in the ball mills, the dominant term of the line;
• The fans: the mill fan and the separator fan add 3 to 6 kWh/t in the ball mill circuits, and 2 to 4 kWh/t in the roller mill systems;
• The homogenization: 0.3 to 0.6 kWh/t for the silo aeration and the transport of the meal;
• The total raw department: 22 to 32 kWh/t of raw meal across the modern plants, against the 25 to 35 of the older ball-mill lines;

At 40,000 tons of raw meal per month and 0.07 US dollars per kilowatt-hour, each kilowatt-hour per ton of saving is worth about 2.8 million dollars over a five-year period of the plant life: the raw department saving projects (the roller mill conversions, the separator upgrades, the charge optimization) therefore pay back in two to four years: the file closes with the worked payback examples and the audit protocol of the department: the energy measure first, the project second, the verification third.

13. The Frequently Asked Questions

What does the term raw milling mean in a dry-process plant?

Raw milling means the grinding and drying of the blended raw material into the fine powder called raw meal: the mill of the dry process receives the proportioned mixture of limestone, clay and the correctives, grinds it to 10 to 14% residue on the 90 micrometer sieve, dries it to below 1% moisture and delivers the meal to the homogenization silo: the same equipment family also grinds coal and cement, but the raw mill always works with the drying duty that the others do not have.

Why is the raw meal ground at only 10 to 14% R90 and not finer?

Because the kiln reactions need the coarse tail controlled, not the surface maximized: the grinding finer than 10% R90 adds 15 to 25% more specific energy to the raw mill (and its electricity dominates the cost) while the burnability gains shrink: the raw meal of 88 to 90% passing the 90 micrometer sieve burns well in the modern precalciner kilns, provided the coarse grains above 212 micrometers stay below 2.5%: the target is the optimization, not the habit.

What happens if the raw mill stops when the kiln is running?

The kiln continues to burn from the silo buffer: the homogenizing silos normally hold 8 to 24 hours of the meal, so the short mill stops are absorbed without the kiln action: the danger is the long stop against the empty silo: the preheater then runs on the thin feed, the kiln must reduce its load or stop, and the whole production is affected: the availability of the raw mill is therefore the scheduled discipline of the plant, with the maintenance alternating between the mill and the kiln runs.

Can a raw mill grind the wet clay without a separate dryer?

Yes, within limits: the vertical roller mill accepts feeds up to about 20% moisture with the hot kiln gas at 400 to 500 C, and the ball mill circuits dry up to about 8 to 10% moisture: beyond these limits the plant adds the separate drying stage (a rotary dryer or the hot-gas crusher) or blends the wet clay with the dry material, because the drying capacity of the mill is a finite budget of gas flow and temperature: the moisture audit of the quarry is the instrument that keeps the mill inside its envelope.

14. Conclusion

The preparation line is the first quality gate of the cement plant: the crushed stone, the blended stockpile, the proportioned mix, the milled and dried meal and the homogenized kiln feed form one continuous chain of averaging, and every link contributes its factor to the stability of the kiln: the engineer who masters the raw department masters the modules, the moisture, the fineness and the chemistry that the whole downstream plant depends on: the tables and the numbers of this guide are the operating vocabulary of the department, and the routine of the log keeps them alive.

The Complete Cement Technical Package includes the raw preparation guides with the flow diagrams, the equipment tables, the operating data and the calculation tools of the line: the one-time $249.99 purchase, the instant download and the lifetime access: the flat knowledge of the industry, organized for the working engineer: the raw department of the plant, documented end to end: the career of the preparation engineer, built on the numbers.

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.