ROLLER MILL
Used in cement industry
Vertical Mills used for Pre-grinding of clinker (lumps to coarse powder) Finish grinding (lumps to powder) of – – – –
Coal/Petcoke for kiln Raw materials for kiln Cement, OPC or mixed Slag, pure or mixed
VRM Functions
Vertical Roller Mill (VRM) - Details feed gate (air lock)
mill exhaust product discharge mill outlet duct separator cage rotor separator guide vanes tailings cone separator discharge flap
mill casing
roller with tyre or segments
water injection
rocker arm roller axle table liner
dam ring
hot gas inlet nozzle of louvre ring
grinding table
hydraulic cylinder
material scraper
clutch and motor
speed reducer with table bearing
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
Grinding force = Roller weight + (pressure) Force Material bed
Roller rolls freely with table
Feed material
Table moves with drive Max Particle size in feed: 5-8 % of roller diameter Feed size: f (Roller Ø )
VRM - Material Flow inside Separator: removes fine material
Gas & Product Material Feed Material
rejects coarse material Internal gas flow
Separator Rejects Internal Circulation Large Material falls through to scraper Material
Material & Gas Gas
VRM - Some Common Mill Configurations
G.Pfeiffer
Loesche
FLS Atox
Polysius
VRM - Built-in Separator static separator Static type
conventional rotor type Conventional
cage rotor type Cage Rotor type with guide vanes P
P
Rotor type
F
T
F
F T
P
T F F
F
P
T
Separation control:
1. Vane adjustment 2. Speed control TF
FT
F=
Feed
P=
Product
T=
Tailings
Separators of
rd 3
Generation
(Vertical roller mill) Reject cone
Cage rotor
Guide vane system
VRM - Accessories Rotary valve Reduces false air entering with feed material. Good for dry, small size material.
Weigh belt
Weighs the passing material ensuring a continuous grinding bed in the mill
Metal extraction Metal objects damage the roller & table. The magnet removes them
VRM - External Circulation fresh feed
bucket elevator
louvre ring
scraper
Used for reducing gas flow pressure lost over the mill
(less pneumatic internal circulation) collecting conveyor
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)
Vertical roller mill internals suffering from wear Outlet duct
Inlet chute
Separator Separator cone
Mill housing
Armour & nozzle rings
Rollers and table liners
Scrapers
Wear resistance material alternatives • Nihard 4 %C
% Cr
% Ni
Weldable
< 3.5
< 10
< 6.5
yes
• High Chromium alloys %C
% Cr
% Other
Weldable
Example
< 3.6
< 17
Mo + Ni
yes
Chromodur, FMU18, VegalineV16s
< 2.6
< 20
Mn
No
FMU 52
< 3.5
< 28
< 1.2
No
FMU 46
Wear resistance material alternatives • Metallic inserts roller liner
Soft material base
Hard metallic insert
Wear resistance material alternatives • Ceramic inserts roller and grinding track liners Metallic base (High Cr)
Ceramic insert
Operation and Optimization of Vertical Roller Mills
Raw material characteristics Feed
Size
Product
Fineness
Moisture
Content
Grindability Abrasiveness
Feed Size Ball
Mill Maximum 5% + 25 mm
Vertical
Roller Mills Up to 150 mm
Product Fineness 5% - 25% + 90 µm 1.0% - 2.0%
+ 212 µm
Moisture Content in Raw Materials 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
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/m2) Mill Motor (Kw) 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 70 60
Slag
Cement (OPC)
50 40 30 20 10
B
al lM ill
R M V
al lM ill B
V
R M
0
Mill
Fan, etc.
Grinding systems comparison Power Consumption when grinding to 3800 cm2/gm for 150 tph capacity
CLOSED CIRCUIT BALL MILL
ROLLER MILL PREGRINDER
ROLLER PRESS SEMI FINISH
FINISH GRINDING IN ROLLER PRESS
PRESS PREGRINDER
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
/
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
200 (300)
275 - 300
275 - 300
300 - 350
< 150
450 tph
GRINDING SYSTEM
Roller Press/Pregrinder (kWh/t)
Maximum Output (t/h)
ROLLER
Operational Guidelines for Process Optimization
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
LOWER PRESSURE DROP LESS WEAR OF MILL BODY LINERS
FAN POWER SAVINGS OF 15-30%
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 Mill vibrations, Too high
Mill output, Too low
Action 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. 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 Mill product, Too fine
Increase speed of separator rotor. Check sieving of samples. Decrease speed of separator rotor. Increase feed supply.
Type of problem Mill outlet temperature, Too high
Mill outlet temperature, Too low
Grinding pressure, decreases
Sealing air pressure, Minimum
Action 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. 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 Check hydraulic system. Piping is leaking. Oil pump fault. Oil level in tank for hydraulics minimum. Oil temperature in hydraulics minimum. Malfunction of valves. Filter blocked. Check electrical equipment.
Type of Problem Starting the mill without grinding layer
Action
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 Fee bins almost empty Oil pressure or oil flow through mill gear minimum
Oil pressure or oil flow through the separator gear minimum
Mill fan bearing temperature maximum
Action Feed bins must be filled up. Stop the mill. 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. 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. 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
