How Raw Mills Work: Comparing Vertical Roller Mills, Ball Mills, and Roller Presses

Why Raw Material Must Be Ground Fine

Cement clinker requires specific mineral formation at high temperature. If raw particles are too large, components cannot contact sufficiently, slowing reactions and risking incomplete local reactions. Grinding fine serves three purposes:

1. Increase reaction surface area. Finer particles provide larger total surface area, allowing calcium, silicon, aluminum, and iron to react more thoroughly in the kiln.

2. Improve mixing uniformity. Co-grinding multiple raw materials creates a more homogeneous powder mixture, reducing local composition variations and stabilizing calcination.

3. Improve raw meal burnability. For the same chemical composition, raw meal with appropriate fineness and particle distribution completes high-temperature reactions more easily.

However, over-grinding raises power consumption and may reduce output. Plants must balance fineness, burnability, output, and energy use.

Components of a Raw Grinding System

A raw mill is not a single machine but a system comprising:

Raw material proportioning feeders

Conveying equipment

Mill (vertical roller mill, ball mill, or roller press)

Hot gas system

Classifier (separator)

Dust collector

Circulating fan

Product conveying equipment

Automatic control and monitoring system

Basic flow: Proportioned raw materials → enter mill → grinding and drying → classification → qualified raw meal to homogenization silo.

Coarse particles return to the mill for re-grinding, forming a closed loop: grind and classify simultaneously, coarse returns, fines exit.

Why Hot Gas Is Needed

Quarry materials (especially clay, shale) contain moisture, which can cause material adhesion, poor flow, balling in the mill, reduced grinding efficiency, high product moisture, and handling difficulties. Hot gas dries the material during grinding. Sources include:

Kiln exhaust gas (preferred for waste heat recovery)

Waste heat recovery system gas

Independent hot air furnace

Using kiln exhaust gas recovers process heat and reduces extra fuel consumption, making the raw mill a key node in plant heat integration.

First Equipment: Vertical Roller Mill (VRM)

How a VRM Works

Raw material enters the top or center of the mill onto a rotating horizontal grinding table. Centrifugal force moves material outward under grinding rollers. Hydraulic pressure applied to rollers (adjustable online) crushes and shears the material bed. Hot gas enters through a high-velocity nozzle ring at the table periphery, carrying fine particles upward to the integrated dynamic classifier. Coarse particles are thrown off the table edge: some fall back onto the table (internal circulation), others drop into a scraper chamber and are lifted externally back to the mill (external circulation). Qualified fines leave with the gas stream and are collected in the dust collector.

Summary: Table feeds material, rollers crush bed, hot gas dries and conveys, classifier controls product fineness.

Why VRM Combines Multiple Functions

Inside the VRM, mechanical grinding and pneumatic classification occur together. Material continuously undergoes: pressure crushing, bed grinding, hot gas drying, pneumatic lifting, dynamic classification, and coarse return. Thus a single VRM unit accomplishes grinding, drying, and classification.

VRMs handle moist raw materials well, a major reason for their widespread adoption in raw grinding.

What Is Bed Grinding?

Rollers do not press directly on the table but on a layer of material (the bed or material bed ). A stable bed is critical: too thin → insufficient cushioning, increased vibration; too thick → ineffective crushing, reduced efficiency and circulation issues. Factors affecting bed stability include feed rate, raw material size, moisture, roller pressure, table speed, system air flow, material grindability, and external circulation return rate. Stabilizing the bed is a key operational task.

Why VRMs Vibrate

Normal operation allows some vibration, but abnormal vibration threatens equipment safety and production stability. Common causes:

Unstable feed

Bed too thin or thick

Large lumps or metal foreign objects in feed

Sudden feed size changes

Incorrect roller pressure

Air flow fluctuations

Uneven wear of table or rollers

Material too dry, too wet, or grindability changes

Material buildup or balling inside mill

Abnormal vibration reflects loss of balance among feed, pressure, air flow, bed, and material properties. Stabilizing a VRM essentially means stabilizing the bed–air–pressure triangle .

VRM Key Characteristics

Advantages: higher grinding efficiency, lower specific power consumption, strong drying capacity, compact system flow, smaller footprint, integrated grinding/drying/classification, suitable for large-scale continuous production.

Operational/maintenance demands: sensitive to feed stability, requires precise air flow control, roller and table liners wear, foreign objects can cause vibration/damage, maintenance needs specialized skills, parameter re-adjustment needed when switching raw material types.

VRM efficiency depends on stable operation and good maintenance.

Second Equipment: Ball Mill

How a Ball Mill Works

A horizontal rotating cylinder contains steel balls (grinding media). As the cylinder rotates, balls are lifted and then fall or cascade, crushing material by impact, compression, and abrasion. Material moves along the cylinder length, progressively reducing in size. Grinding actions come from: ball impact, inter-ball compression, ball-material friction, and inter-particle interactions.

Contrast: VRM relies on roller pressing a material bed; ball mill relies on steel balls repeatedly impacting and grinding material.

Why Ball Mills Are Long

Material needs sufficient residence time. Some mills use diaphragms to divide the cylinder into compartments. The first compartment uses larger balls for coarse crushing; subsequent compartments use smaller balls for fine grinding. This “coarse then fine” approach lets different ball sizes handle different tasks. Mill length, diameter, ball charge grading, and residence time all affect output and product fineness.

Open-Circuit vs. Closed-Circuit Ball Mills

Open-circuit: Material passes once through the mill to become product. Simple flow but limited fineness control and grinding efficiency.

Closed-circuit: Mill discharge goes to a classifier. Fines become product; coarse returns for re-grinding. Closed-circuit systems promptly remove qualified fines, reducing over-grinding. Modern grinding emphasizes pairing mills with high-efficiency classifiers.

How Ball Mills Dry Material

Hot gas enters the mill or a separate dryer to remove moisture. However, traditional ball mill systems have limited integrated drying capacity compared to VRMs. For high-moisture raw materials, plants may need to: increase inlet gas temperature, increase ventilation, add a dedicated dryer, or pre-treat wet/sticky materials. Insufficient mill ventilation can cause ball coating (see below), blockages, and efficiency loss.

What Are Ball Coating and Ball Packing?

High material moisture, poor temperature control, or electrostatic attraction can cause fine powder to coat ball surfaces. Coated balls lose effective impact and friction. This phenomenon is called ball coating or ball packing . Similar fines can coat liners and diaphragms, increasing ventilation resistance and hindering material flow. Therefore, ball mill operation requires careful control of: feed moisture, mill ventilation, material temperature, ball charge grading, feed rate, and product fineness.

Ball Mill Key Characteristics

Advantages: mature structure and principle, wide adaptability to material variations, relatively intuitive operating state, mature maintenance technology and spare parts supply, some tolerance to operational fluctuations.

Drawbacks: many ineffective collisions during grinding, higher specific power consumption, larger equipment volume and footprint, high grinding media and liner consumption, limited drying capacity for high-moisture materials, noticeable noise during operation.

Note: Although ball mills are rarely chosen for new raw grinding lines, they remain the main equipment for cement finish grinding because cement requires specific particle size distribution and sphericity, where ball milling still offers unique value. Ball mills are not simply “outdated”; they retain value for specific raw conditions, capacities, and retrofit projects.

Third Equipment: Roller Press

How a Roller Press Works Two counter-rotating rolls apply high pressure. Feed material enters above the rolls and is drawn into the nip by gravity and roll surface friction. High pressure crushes particles and induces micro-cracks inside them. The compacted material exits as a sheet or cake ( flake or cake ). This cake cannot be classified directly; it must first pass through a de-agglomerator (disintegrator/classifier) to break it up before classification. Hence every roller press system includes a de-agglomeration device. Why Roller Presses Save Energy Traditional ball mills rely on balls tumbling in a cylinder; energy is lost in ball-ball collisions, cylinder motion, and ineffective friction. Roller presses concentrate pressure directly into the particle bed. Particles crush each other under high pressure (inter-particle comminution), improving energy utilization. Micro-cracks formed during pressing make subsequent grinding easier. Therefore, roller presses serve as pre-grinders or can form complete grinding systems with classifiers. Roller Press System Configurations Pre-grinding: Roller press crushes feed first, then ball mill finishes grinding. Reduces ball mill feed size and exploits micro-cracks to boost ball mill efficiency. Combined grinding: Roller press handles primary crushing; ball mill further refines and adjusts product. Both share the grinding duty. Semi-finish grinding: Part of the roller press product (after classification) goes directly to final product; the rest goes to ball mill for further grinding. Finish grinding: Roller press with de-agglomerator, classifier, and separator completes all grinding without a ball mill for final finishing. For raw grinding, the specific flow depends on raw material moisture, grindability, product requirements, and hot gas conditions. Why Roller Presses Need a Stable Feed Column A sufficient material column above the rolls ensures continuous, uniform feed into the nip. Insufficient feed prevents stable bed formation, reducing crushing efficiency and upsetting operation. Excessive feed fluctuations cause: nip gap variations, load fluctuations, increased vibration, unstable cake quality, uneven pressure on roll sides, and machine skew. Therefore, the feed bin (stabilizing silo) and feeder are critical. Roller press efficiency likewise rests on stable feeding. Roller Press Key Characteristics Advantages: high-pressure bed crushing efficiency, lowers system specific power, can increase ball mill system output, suitable for new plants and energy-saving retrofits, flexible system configurations. Challenges: demands stable feeding, sensitive to metal foreign objects, roll surfaces endure high pressure and wear, uneven material distribution causes skew, system flow and circulating load require precise control, high-moisture/sticky materials increase operational difficulty. A roller press does not save energy in isolation; it must be properly integrated with stabilizing silo, de-agglomerator, classifier, drying, and circulation systems. Comparing VRM, Ball Mill, and Roller Press The fundamental difference lies in the comminution mechanism : VRM: Rollers press and shear material bed on rotating table. Highly integrated grinding, drying, classification; compact; suited for large continuous production. Ball Mill: Steel balls impact and grind in rotating cylinder. Mature technology, good adaptability, but higher specific power consumption. Roller Press: Two rolls apply high pressure to continuous material bed. High crushing efficiency; can be pre-grinder or form combined/finish grinding systems. How to Compare the Three Grinding efficiency: VRM and roller press (bed comminution) generally have higher energy utilization than traditional ball milling. Actual power consumption depends on system design, equipment size, raw material properties, product fineness, and operational skill. Drying capacity: VRM typically has strong integrated drying. Ball mill systems can use hot gas but may be limited by system structure for high-moisture feeds. Roller press drying depends on auxiliary drying/classification equipment. Material adaptability: Ball mills tolerate material variations well. VRMs demand stable feed, air flow, and bed conditions. Roller presses are sensitive to feed stability and uniformity; sticky/wet materials and foreign objects can disrupt operation. Footprint and flow: VRM systems are compact. Ball mill systems with auxiliaries may occupy more space. Roller press units are compact, but full systems need de-agglomerator, classifier, circulation, and drying equipment. Maintenance focus: VRM – rollers, table, hydraulics, vibration. Ball mill – grinding media, liners, diaphragms, drive. Roller press – roll surfaces, bearings, hydraulics, pressure balance. Why No Single “Best” Choice Different plants face different conditions. Selecting a raw mill requires evaluating: Production scale Raw material hardness Raw material grindability Raw material moisture Feed size Product fineness target Available waste gas temperature Site constraints Electricity cost and capital budget Existing equipment base Operational and maintenance capability Spare parts availability For large new lines, VRM may best exploit integration and low power. For plants with existing ball mills, adding a roller press can be an energy-saving upgrade. For variable materials, moderate scale, or mature maintenance setups, ball mills may still offer practical value. Equipment selection is not about “which is more advanced” but finding the system solution best matched to raw material, capacity, investment, and operating conditions . Importance of the Classifier Regardless of mill type, the classifier significantly influences system performance. Its tasks: Allow qualified fines to exit promptly Return coarse particles for re-grinding Control product fineness and particle distribution Modern systems widely use dynamic classifiers (cage rotor separators) ; adjusting rotor speed changes the cut size – classifier speed is one of the most direct levers for product fineness. Low classifier efficiency causes: qualified fines incorrectly returned (over-grinding, energy waste) and coarse particles entering product (off-spec raw meal). Hence a high-efficiency grinding system must not only “grind well” but also “classify accurately.” Many energy-saving retrofits optimize not just the mill but also the classifier, fans, and internal circulation. Why Air Flow Control Is Critical In raw grinding, gas serves drying, conveying, and classification. Insufficient air flow leads to: fines not evacuated, increased internal circulation, inadequate drying, reduced output. Excessive air flow causes: coarse particles carried over, higher classifier load, increased fan power, system resistance changes, higher dust collector load. Proper air flow is not “the more the better” but must match feed rate, raw moisture, product fineness, and equipment condition. How Raw Meal Fineness Is Judged The most common metric is sieve residue – industry practice uses 80 µm and 200 µm residues as primary control indices (80 µm reflects overall fineness; 200 µm monitors coarse particles). Particle distribution and Blaine specific surface are more used in cement finish grinding. In production, a single average fineness number is insufficient. Exceptionally coarse particles, especially those enriched in key oxides, may not react fully in the kiln, hurting burnability and clinker quality. Therefore raw meal quality monitoring covers: Overall fineness Coarse particle content Particle size distribution Chemical composition Compositional uniformity Moisture Good raw meal is not simply “finer is better” but: appropriate fineness, controlled coarse particles, uniform composition, stable moisture. Raw Mill – Kiln Coordination Raw mills often use kiln exhaust gas for drying, creating strong coupling between grinding and kiln systems. When the kiln runs normally, its exhaust provides heat to the raw mill. If the kiln stops but the raw mill must continue, a hot air furnace or alternative operation is needed. Conversely, if the raw mill stops while the kiln runs, kiln exhaust typically bypasses to a humidification tower for water spray cooling before dedusting. Raw mill ventilation and exhaust handling also affect kiln tail gas system pressure balance. This interdependent operation is called kiln-mill coordination . The kiln and mill are not independent; they are linked through gas, heat, and material handling systems. Evaluating Raw Mill Performance Output alone is not enough. A well-run raw grinding system simultaneously meets: Target output Stable raw meal fineness Qualified product moisture Reasonable specific power consumption Adequate heat utilization Stable system ventilation Normal equipment vibration Long run time between shutdowns Controllable maintenance cost Stable raw meal supply to kiln Chasing output at the expense of coarser meal, higher vibration, or soaring power consumption is not true optimization. The goal is a comprehensive balance of quality, output, energy consumption, and reliability . Final Summary A raw mill is not a simple “rock crusher” but a process system that simultaneously performs grinding, drying, mixing, and classification. The main differences among VRM, ball mill, and roller press lie in their comminution principles: VRM: Roller-table bed comminution – high integration, strong drying. Ball Mill: Steel ball impact and attrition – mature, adaptable. Roller Press: Dual-roll high-pressure bed comminution – high efficiency, flexible system configurations. No equipment has absolute superiority independent of conditions. Raw material hardness, moisture, scale, hot gas availability, investment capability, and maintenance level dictate the suitable solution. Truly efficient raw grinding is not just choosing the right mill but ensuring feeding, hot gas, classification, circulation, dedusting, and automatic control form a stable whole. In one sentence: The mill determines how raw material is crushed; the system determines whether raw meal is produced stably and efficiently. Understanding the raw mill reveals where half of a cement plant’s electricity goes and what the primary battlefield for industrial AI optimization looks like. ⚙️