Vertical raw mill

K.P.PRADEEP KUMARSr G M OperationKarikkali WorksChettinad Cements Ltd

The principle of vertical millresembles like ammi kallu, attaankallu,Or ennai chekku vandi

History of grinding technology

•Grinding•Separation•Drying •Conveying

Four functions in one machine makes VRM a compact grinding system

Functions of Roller Mill

Vertical mill

General Design

material to air ratio ,0.5 – 0.6 kgs/m3

Vertical mills of various suppliersLoeschemill

Raymond milFLS Atox mill

Gebr Pfeiffer , MPS

Krupp Polysius

Williams mill

CP Mill (claudius Peters)Babcock E ring mill

Raw mills of different designers

Investigation of the feed material

Different materials are characterized by

high resistance to grinding

severe abrasiveness

in homogeneous grindability , moisture content and grain sizes

tendency not to form a stable bed , so that the material leavesthe grinding bowl prematurely

High resistant to grinding - coal below 45 hard groove or petcoke- lime stone above 9 kwh/t at 12% on 90

microns - clinker

Flowabity - poor drawing in behavior- poor bed formation properties - tendency for quartz accumulation on

grinding bowl

Combination of different - low and high grindabilityproperties - moist and dry

- coarse and fine fractions

Abrasiveness - high quartz content,particularly above 90 microns particles

- pyrites- felspars- shelly lime stone

Investigation of the feed material

3.2 How does a vertical mil work?The raw materials are fed directly to the centre of the grinding plate,which, thanks to centrifugal forces and the pushing effect of the incoming materials themselves (both fresh and separator coarse material), are distributed evenly under the grinding rollers. The turning speed of the grinding plate, together with the pressure of the grinding rollers, create the necessary friction for grinding the materials. After being ground, the material is transported to the dynamic separator by the (more or less hot) gas flow, where the selection of the material takes place.The gas flow, besides transporting the material, performs other important tasks:• First material selection; the coarse particles tend to fall back on thegrinding plate.• Thermal exchange; more efficient compared to tubular ball mills,thanks to the complete mix of gases and materials.The coarse material coming from the separator is directed back to thecentre of the grinding plate in order to be ground again.The grinding process starts with the preparation of the so-called grindingbed, where the coarse material is crushed. Then the material in compressedand ground by the grinding rollers. For a stable grinding procedure thefollowing parameters are important:• The vibrations of the grinding plate.• The ∆P of the grinding system.• The height of the grinding bed(formed by a small border on the grinding plate).• The pressure of the grinding rollers.•The speed of the grinding plate.

P.GUNASEKARANMECHANICAL FOREMAN.

Common aspects of operation with regard to the grinding processand generally exhibits the following characteristics.

Compression is the main mechanism of size reduction , although some elements of attrition , shear and impact will be present

Grinding takes place selectively, with most of the initial grinding action applied to the larger particles at the start of the process .Then progressively with thetime the mill starts to ac on the mass of smaller particles resulting from thereduction of the larger , more friable ones

The resistance of the rollers to motion through the product decreases as the material bed becomes shallower .Consequently , the power absorbed by the grinder decreases with the decrease of the crushed flow and vice versa.

The effective grinding in the mill depends on many parameters . Important among them are area , gripping angle, nip angle, maximum grinding pressure ,grinding bed thickness between grinding surfaces , rolling friction factor - µ

Grinding theory of vertical roller millGrinding in roller mill is effected by shear and compaction. The effective grinding In the mill depends on many parameters . Important among them area : the gripping angle , nip angle, maximum grinding pressure , grinding bed thickness between the Grinding surfaces , rolling friction factor- µ

Gripping angle(ligament angle)

It is the angle between the surface of roller and table. Thematerial throughput Between the grinding surfaces is determined

by the maximum angle. From the diagram it Is understood that the increase in the gripping angle will result in a Thicker bed . But the maximum gripping angle is limited by the internal friction and the Stability of the material.

Comminution Process in Roller Mills

Material Track

Grinding Action in Roller Mill

Multiple point contacts by particles on a particle: inter particlecomminution

Comminution Process in Roller Mills

3.2 How does a vertical mil work?

The raw materials are fed directly to the centre of the grinding plate,which, thanks to centrifugal forces and the pushing effect of the incoming materials themselves (both fresh and separator coarse material), are distributed evenly under the grinding rollers. The turning speed of thegrinding plate, together with the pressure of the grinding rollers, create the necessary friction for grinding the materials. After being ground, the material is transported to the dynamic separator by the (more or less hot) gas flow, where the selection of the material takes place.The gas flow, besides transporting the material, performs other important tasks:

• First material selection; the coarse particles tend to fall back on the grinding plate

• Thermal exchange; more efficient compared to tubular ball mills,thanks to the complete mix of gases and materials. The coarse material coming from the separator is directed back to the centre of the grinding plate in order to be ground again. The grinding process starts with the preparation of the so- called grinding bed, where the coarse material is crushed. Then the material in compressed and ground by the grinding rollers. For a stable grinding procedure the following parameters are important:

The material , which flows over the damn ring is caught by the vertical gas flow from the nozzle ring and lifeted up. Coarse particles fall back to the grinding table and Finer ones are swept up to the separator for being classified. The internal circulationRate depends mainly on the grindability of the ground material and can amount up to 15 to 25 cycles.Reducing the gas speed in the nozzle ring adjustment of the open Area leads ti falling through of larger particles. The fall through material has to be Extracted with scrapers and mechanically recirculated to the mill feed.

SeparationThe use of modern separators in roller mils is state of the art.A sharp separation Improves the rawmeal quality and avoids

Over grinding ( saving of energy). Coarse Tailings , fed through the tailings cone to the center of the grinding table, helps theFormation of a more stable grinding bed. The raw meal fineness is easily controlled By the adjustment of the cage rotor speed.

Friction factor µ : Ιt affects the material movement on the table. Material with low Friction factor tends to have high radial velocity in relation with the table speed and has The backward sliding tendency. So certain amount of material falling on the table flows To the table edge without passing under the roller. The µ increases with increasing pressure;However the capacity of the mill does not grow in proportional to the pressure. So it is economical if we design a slightly larger mill with lower Specific pressure that allows a less strained design and an longer life Of the machinery instead of sizing a mill with high grinding pressure.The amount of material passing between the grinding surfaces is The product of grinding bed thickness , roller width , rollerdiameter And the density of the pressed material. For a certain roller speed and material and material , the capacity of the roller mill is directly Proportional to the grinding thickness.

Power taken by the mill

The power taken up by the vertical roller mill is given\by the following expression

N =( I. µ .KT .DR .W.Dm. π.n ) / 60 K WWhere ,I = no.of rollers

µ = friction factor (0.07-0.14 for raw materials , 0.09-0.1 for coal) ΚT = specific roller pressure , kN/m2(400 – 800 for raw and coal) DR = roller diameter , m W = roller width , m Dm = mean diameter of the track , m N = table speed , rpm

For a given mill design, the expression reduces to

N = (constant) .m .KT.Do 2.5

Roller Specific force curve

• 2.1 vertical roller mill vibration is too large:

(1) Feed uneven, when the mixture into the vertical roller millover the powder, the vertical roller mill's loading rate, leading to the vertical roller mill on the thin material, and even disc direct contact with the roller,resulting in mechanical milling; when into the vertical roller mill multimaterial mixture materials for the block, the resulting pressure grinding roll instability, resulting in mechanical milling. The solution is to stabilizethe particle size of materials into the vertical roller mill, the appropriate adjustment of feed rate or reduce the grinding pressure, the assurance required under the premise of the fineness of materials, appropriate to reduce the classifier speed.

(2) Metal piece into the vertical roller mill, check the metal detectors

Controlling Vibrations•Know the vibration limit and optimum grinding bed thickness for your mill•Have reliable, calibrated bed depth monitors•Adjust the mill dam ring height to compensate for table wear•Match grinding pressure to mill throughput – establish relationship between feedrate and hydraulic pressure. Consider control loop bed depth vs hydraulic pressure•Maintain an even bed, avoid big stones in mill, avoid excessive fines in mill feed, avoid uncontrolled feed variations, even distribution of material to each of the rollers, feed to the centre of the table, adjustment of feed chute and or use of guide vanes (ploughs)•Apply stable water spray to the control bed, normally fed to the feed side of each roller, check flow consistency on each spray•Know the critical level of gasflow and pressure drop for your mill•Maintain stable mill gasflow to keep internal circulation consistent –apply automatic control loop•Avoid magnetic and non-magnetic objects entering mill

1.Operation points

• 1.1 Stable Beds: to maintain stability and bed material, which is grinding roller grinding bed basis, the normal operation of the key. Material thickness can be adjusted to adjust the height of retaining ring material, a suitable thickness and their relationship with the correspondence between the vertical roller mill output should be in the debugging stage of the firstto find out. Lower fuel layer thick grinding efficiency, material layer is too thin will cause vibration. Such as roll up,then fine powder generated much thinner material layer; roll reduction, disc material thicker, more appropriate to return the materials,material layer thickening. vertical roller mill speed increase, increase in internal circulation, material layer thickness, reducing the wind speed, reducing the inner loop, material layer thinning. Under normal operation by the vertical roller mill feed roller bed thickness after compaction should not be less than 40 ~ 50ram.

1.2 Control of grinding pressure:

The pressure is affecting the grinding mill output,power mill grinding efficiency and the main factor. vertical roller mill is to impose by means of the material bed crushing pressureand the pressure to increase production increased, but reached a certain critical value does not change, the pressure increase followed by a increase in power, resulting in unit power consumption increase, so the appropriate roll to production and energy consumption both. This value depends on material properties, particle size and volume of feed. In the test production grinding to find the right pressure and wind pressurecan form a reasonable good internal circulation, so that the material layer disc proper, stable, high grinding efficiency. In the production process, when the wind area of a certain ring, the wind speed from the wind decide. Production capacity with the correspondence between, to ensure the grinding effect.

1.2 Control of grinding pressure: the pressure is affecting the grinding mill out put,power mill grinding efficiency and the main factor. vertical roller mill is to impose by means of the material bed crushing pressure and the pressure toincrease production increased, but reached a certain critical value does not change, the pressure increase followed by a increase in power, resulting in unit power consumption increase, so the appropriate roll to production and energyconsumption both. This value depends on material properties, particle size and volume of feed. In the test production grinding to find the right pressure and wind pressure can form a reasonable good internal circulation, so that the material layer disc proper, stable, high grinding efficiency. Inthe production process, when the wind area of a certain ring, the wind speed from the wind decide. Production capacity with the correspondence between, to ensure the grinding effect.

1.3 The guarantee of the grinding temperature: vertical rollermill is drying and grinding system, the ground temperature is a measure of whether the normal drying operation comprehensive index. In order to ensure good drying of raw materials, the grinding of materials is less than 0.5% moisture, the general control of mill outlet temperature at 90 ~ C or so. If the temperature is too low then finished water, causing the efficiency of grinding efficiency and reduce the separator, dust collectionsystem may cause condensation; If too high, humidity isnot enough that gas cooling, dust collection will also affect the results.

2. Exception handling

• 2.1 vertical roller mill vibration is too large:

(1) Feed uneven, when the mixture into the vertical roller millover the powder, the vertical roller mill's loading rate, leading to the vertical roller mill on the thin material, and even disc direct contact with the roller, resulting in mechanical milling;when into the vertical roller mill multimaterial mixture materials for the block, the resulting pressure grinding roll instability,resulting in mechanical milling. The solution is to stabilizethe particle size of materials into the vertical roller mill, the appropriate adjustment of feed rate or reduce the grinding pressure, the assurance required under the premise of the fineness of materials, appropriate to reduce the classifier speed.(2) Metal piece into the vertical roller mill, check the metal detectors

Static separators: Produce wide psd, fineness adjustment by vanes cause high ∆P and poor efficiency

Conventional rotor type: Fineness adjustment by rotor speed, wide psd, low ∆P, tailings fall spread over grinding table

Cage rotor with guide vanes: Operate with high air speed (4.5-6 m/s), produce narrow psd steep slope R200/R90 = 0.02-0.05

Air drag

Centrifugal forceGenerated by rotor cage

Gravitational force

Air drag

Gravitational force

Centrifugal force

Stationary vanes

Rotary cage

Function of classifier

15 – 18 m/s

5 m/s

Classifier

Separator or Classifier

Loeshe mill separator inlet stationary vanes

If product has more coarse then classifier seal needs inspectionand rectification. Production drops drastically.

Classifier rotor seal

1000 500 400 100 90 80 70 60 50 40 30 20 mic

rejects

100%

80

60

30

10

100%

80

60

30

10

1000 500 400 100 90 80 70 60 50 40 30 20mic

1000 500 400 100 90 80 70 60 50 40 30 20mic

Ideal separatorGround material to separator

Sharpness η = d75 / d 25

Value =0.8 – 0.85 is a veryEfficient classifier

Size fraction

By pass fraction

Nozzle ring velocity45 – 60 m/s

velocity above roller

V = 1.5 Do to 2.5 Do

Velocity through rotorvanes = 5.0 m/s

Grinding bed thickness = 1 - 2 % of DoFor cement grinding = 0.2 % of Do

Mill Dp = 350 Do m

Mill exit velocity = 10 m/sCage rotor with guide vanes: Operate with high air speed (4.5-6 m/s),produce narrow psdsteep slope R200/R90 = 0.02-0.05

operating rpm , N = 56 / Do

5 – 6 m/s

Counter measures in wear parts

Atox mill dimensions and characteristics

Atox mill dimensions and characteristics

Foster wheeler energy company

Innovation and patented

Thank you for your kind attention

K.P.Pradeep KumarSr G.M.OperationKarikkali worksChettinad cements Ltd