ROLLER MILL

Used in cement industry

Vertical Mills used for Vertical Mills used for Pre-grinding of clinker (lumps to coarse Pre-grinding of clinker (lumps to coarse

powder)powder) Finish grinding (lumps to powder) of Finish grinding (lumps to powder) of

– Coal/Petcoke for kilnCoal/Petcoke for kiln– Raw materials for kilnRaw materials for kiln– Cement, OPC or mixedCement, OPC or mixed– Slag, pure or mixedSlag, pure or mixed

VRM Functions

Vertical Roller Mill (VRM) - Details

feed gate(air lock)

mill casing

waterinjection

dam ring

louvre ringnozzle of

material scraper

mill outlet duct

speed reducer withtable bearingand motor

clutch

grinding table

hot gas inlet

hydraulic cylinder

table liner

roller with tyreor segments

roller axlerocker arm

discharge flapseparator

tailings cone

separatorguide vanes

cage rotorseparator

product dischargemill exhaust

Working principle

Vertical mills comprise 2-4 conical rollers which are hydraulically pressed onto a horizontal rotating grinding table.

The roller axis is inclined at 15o to the table and, as axes of rollers and table do not intersect in the plane of the table. the relative motion involves both rolling and sliding which enhances comminution.

Feed material is directed onto the centre of the table and is thrown outward by rotation under the rollers and into a rising air current at the periphery which is directed by means of a louvre ring. The air sweep passes through an integral

rotary classifier; fines pass out with the air current while coarse material falls back onto the feed table.

Material drying occurs in air suspension between table and classifier. Circulating load is typically 800%. Roller mills are prone to vibration due to an unstable grinding bed.

A major cause of material instability is fine, dry mill feed which can usually, be mitigated by spraying water directly onto the bed.

VRM - Working principle

Vertical roller mill

Feed size:f (Roller Ø )

Material bed

Feed material

Max Particle size in feed: 5-8 % of roller diameter

Grinding force =

Roller weight + (pressure) Force

Table moves with drive

Roller rolls freely with table

VRM - Material Flow insideGas & Product Material

Feed Material

Separator Rejects

Internal Circulation

Large Material falls through to scraper

Internal gas flow

Material & Gas

Material

Gas

Separator:

removes fine material

rejects coarse material

VRM - Some Common Mill Configurations

G.Pfeiffer Loesche FLS Atox Polysius

static separator conventional rotor type cage rotor typewith guide vanes

F T

P

F F FT T

P

F F

T

P

T F

P

F =

P =

T = Tailings

Product

Feed

F T

VRM - Built-in SeparatorCage Rotor typeConventional

Rotor type

Static type

Separation control:

1. Vane adjustment

2. Speed control

Separators of 3rd Generation (Vertical roller mill)

Guide vane system

Reject cone

Cage rotor

VRM - AccessoriesRotary valve

Weigh belt Metal extraction

Reduces false air entering with feed material. Good for dry, small size material.

Weighs the passing material ensuring a continuous grinding bed in the mill

Metal objects damage the roller & table. The magnet removes them

VRM - External Circulation

Used for reducing gas flow pressure lost over the mill

(less pneumatic internal circulation)

collecting conveyor

fresh feed

louvre ring

scraper

bucket elevator

VRM - Example flow-sheet

Control points

1 gas flow

2 pressure

difference

3 return gas

flow pressure

4 outlet

temperature

5 external

recirculation

VRM MAINTENNACE

Maintenance

Wear - Rollers & Table– - Separator (sealing!) & Housing– Lubrication & Hydraulics (filters and seals)

Outlet duct Inlet chute

Mill housing

Armour & nozzle rings

Rollers and table liners

Separator cone

Separator

Scrapers

Vertical roller mill internals suffering from wear

• Nihard 4

• High Chromium alloys

Wear resistance material alternatives

% C % Cr % Ni Weldable

< 3.5 < 10 < 6.5 yes

% C % Cr % Other Weldable

< 3.6 < 17 Mo + Ni yes

< 2.6 < 20 Mn No

< 3.5 < 28 < 1.2 No

Example

Chromodur, FMU18, VegalineV16s

FMU 52

FMU 46

Wear resistance material alternatives

• Metallic inserts roller liner

Hard metallic insert

Soft material base

• Ceramic inserts roller and grinding track liners

Wear resistance material alternatives

Metallic base (High Cr)

Ceramic insert

Operation and Optimization of Vertical Operation and Optimization of Vertical Roller MillsRoller Mills

Raw material characteristics

Feed Size

Product Fineness

Moisture Content

Grindability

Abrasiveness

Ball Mill Maximum 5% + 25 mm

Vertical Roller Mills Up to 150 mm

Feed Size

5% - 25% + 90 µm

1.0% - 2.0% + 212 µm

Product Fineness

Normal moisture content: 3% - 10% H2O

Possible to dry >20% H2O in vertical mills

Up to 6 to 7%-H2O – Drying with kiln gases

Above 7% H2O – Supplementary heat from auxiliary furnace or cooler

Moisture Content in Raw Materials

OPERATIONAL ASPECTS

Material granulometry Roller pressure Dam ring Louvre ring External circulation

PARAMETERS TO BE MONITORED DURING OPEARATION

Production rate, tonnes/hour Grinding press. (bar) or (kN/m2Grinding press. (bar) or (kN/m2)) Mill Motor (Kw)Mill Motor (Kw) Grinding bed thickness (mm)Grinding bed thickness (mm) Vibration level (mm/s) Pressure drop across the mill Mill outlet temperature Fan flow Rejects (If external recirculation present) Water flow

Additional paramters to be monitered

Operating hours Involuntary downtime hours kWh/tonne (mill motor + fan + seperator) Product fineness on 90/212 microns for raw

mill ,coal mill and blaines for cement mill Feed moisture, % Product moisture, % Feed size

How the nozzle ring velocity corresponds to reject rate and pressure drop

Running with a high velocity in the nozzle ring gives a high pressure loss, but also a low reject rate.

However the lower pressure drop with e.g. 45 m/s is to prefer and will both give considerable power savings on the fan and less wear in the mill body

Operational problems

Vertical roller mill for finish grinding

PROVEN TECHNOLOGY

VERY SUITABLE FOR GRINDING BLENDED CEMENT OR SLAG

COMPACT GRINDING INSTALLATION

ENERGY SAVING - UP TO 30%-40% kWh/Mt

SUITABLE FOR GRINDING MOIST FEED

EASY MAINTENANCE AND OPTIMUM UTILIZATION OF WEARING ROLLERS

VRM versus BALL MILL for CEMENT GRINDING

Blaine Surface, cm2/g: 4000

0

10

20

30

40

50

60

70

Mill Fan, etc.

Cement (OPC) Slag

Grinding systems comparisonPower Consumption when grinding to 3800 cm2/gm for 150 tph capacity

GRINDING SYSTEM

CLOSED CIRCUIT

BALL MILL

ROLLER

PRESS PREGRINDER

ROLLER MILL PREGRINDER

ROLLER PRESS SEMI

FINISH

FINISH GRINDING IN

ROLLER PRESS

OK VRM

Mill (kWh/t) 38.2 29.7 28.0 20.8 / 21.8

Separator and Fans (kWh/t) 4.5 3.9 3.9 6.1 14.2 7.2

Roller Press/Pregrinder (kWh/t) / 4.5 10.0 9.7 20.5 /

Auxiliary (kWh/t) 0.6 1.0 1.0 1.5 1.6 0.2

Total (kWh/t) 43.4 39.0 42.8 38.0 36.3 29.2

Maximum Output (t/h) 200 (300) 275 - 300 275 - 300 300 - 350 < 150 450 tph

Operational Guidelines for Process Optimization

LOWER PRESSURE DROP LESS WEAR OF MILL BODY LINERS

FAN POWER SAVINGS OF 15-30%

Nozzle Ring Velocity versus

Material Rejects and Pressure Drop

N/R Velocity Reject Rate Relative Pressure

Drop

(m/s) (% Fresh Feed) (%)

75-85 Trace 100

60-65 5-10 84

40-45 20-30 60

Air flow

A correct airflow in the mill is important, because the air is transporting the material and also an important factor for efficient separation.

Airflow is kept constant through mill and cyclone/ filter by operating the mill fan with constant power consumption on the motor.

This is normally done by the help of an automatic loop between the mill fan damper position and the power consumption of the mill main motor. Alternatively by the help of an automatic loop between the speed of the mill fan motor and the power consumption of the mill main motor.

Feed

A correct feed rate in the mill is important. if the feed rate is to high and the mill be filled up with material and trip on vibration, because the mill fan don’t have the capacity to transport material out of the mill.

If the feed rate is to low will the mill emptied out and trip on vibration, due to low grinding bed. The feed rate depends on the applied grinding pressure and the grind ability of the material.

The mill differential pressure or the mill motor power consumption is an indication of how much material inside the mill. Normally the feed rate is controlled by the mill motor power consumption through an automatic loop.

Vibrations

A constant and acceptable level of vibrations is important.

If vibrations are to high then the mill be stopped by the safety interlocking of the mill in order not to damage the machine.

The vibrations are minimized by injecting water. The injected amount according to experience.

Operational problems

Changes in feed material properties Equipment problems Deficiencies of control elements Control signal errors External effects

Problems -action

Type of problem Action Mill vibrations, Too high

Reduce feed supply if differential pressure is high. Increase feed supply if differential pressure is low. Check water injection. Reduce grinding pressure. Lift rollers at excessive vibrations.

Mill output, Too low

Check grinding pressure. Check differential pressure. Product fineness very fine. Coarser raw materials. Grindability of raw material changed. Raw materials too wet. Table and roller segments are worn.

Mill product, Too coarse

Increase speed of separator rotor. Check sieving of samples.

Mill product, Too fine

Decrease speed of separator rotor. Increase feed supply.

Type of problem Action Mill outlet temperature, Too high

Hot air amount too high. Hot gas temperature higher than normal. Adjust (close) hot gas damper. Decrease oil for heat generator. Open cold air damper. Increase water injection.

Mill outlet temperature, Too low

Raw material moisture increased. Increase hot air amount. Adjust (open) hot gas damper. Adjust (open) mill fan damper. Increase oil for heat generator. Close cold air damper, if open. Decrease water injection

Grinding pressure, decreases

Check hydraulic system. Piping is leaking. Oil pump fault. Oil level in tank for hydraulics minimum. Oil temperature in hydraulics minimum. Malfunction of valves.

Sealing air pressure, Minimum

Filter blocked. Check electrical equipment.

Type of Problem Action

Starting the mill without grinding layer

Fill mill with material before start. Use automatic program for mill filling. The filling must be done manually by starting transport devices in

correct sequence. Fill in 300 – 500 kg of material.

Heat generator is failing No oil/gas available. Oil/gas filter contaminated. Ignition gas bottle empty. Adjustment of ignition and main burner has changed. Pre-heating of oil not working.

Supply of hot gases from kiln interrupted

Mill operation must be stopped. Check position of kiln (hot) gas damper.

Feeder units disturbed Feed supply from other feeders must be increased. Stop the mill, if feed transport fails.

Type of Problem Action Fee bins almost empty Feed bins must be filled up.

Stop the mill. Oil pressure or oil flow through mill gear minimum

Mill will be stopped automatically. Check leakage in piping. Oil pump fault. Oil level minimum. Electrical fault. Oil temperature too high. Check cooling water supply.

Oil pressure or oil flow through the separator gear minimum

Separator and mill will stop automatically. Check leakage in piping. Oil pump fault. Oil level minimum. Electrical fault. Oil temperature too high. Check cooling water supply.

Mill fan bearing temperature maximum

Automatic stop of fan and mill. Check supply of cooling water. Check greasing of fan bearings. Electrical fault.

Vertical roller mill caluclations

Caluclation of capacity of the mill

Generally speaking, the production capacity refers to grinding capacity and drying capacity of grinding mill.

The material grindability will affect the grinding capacity, the roller pressure and the type of grinding mill.

G=K1 * D2.5 Where G is capacity of the mill K1 is coefficient, which is relevant to the

typer of roller mill, the selected and used pressure, the performance of grinded material. Different specification of roller mill, so the K1 is different.

K1 of Loesche Mills series roller mill is 9.6 and for Atox mills it is 7 and for MPS mills it is 6.6.

D is table diameter Example: for atox 50 mill The capacity of mill is G= 7 * D2.5

= 7* 5^2.5 = 391 TPH~ 400TPH