18. Vertical mills of various suppliers
Loesche
mill
Raymond mil
FLS Atox mill
Gebr Pfeiffer , MPS
Krupp Polysius
Williams mill
CP Mill (claudius Peters)
Babcock E ring mill
21. 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 leaves
the grinding bowl prematurely
22. 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 grindability
properties - 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
23. 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 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 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.
25. Common aspects of operation with regard to the grinding process
and 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 the
time the mill starts to ac on the mass of smaller particles resulting from the
reduction 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
26. Grinding theory of vertical roller mill
Grinding 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. The
material 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.
33. 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 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:
34. 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 circulation
Rate 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.
Separation
The 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 the
Formation of a more stable grinding bed. The raw meal fineness
is easily controlled By the adjustment of the cage rotor speed.
35. 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 , roller
diameter 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.
36. 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 W
Where ,
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
45. • 2.1 vertical roller mill vibration is too large:
(1) Feed uneven, when the mixture into the vertical roller mill
over 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 stabilize
the 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
46. 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
47.
48.
49.
50.
51.
52. 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 first
to 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.
53. 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 pressure
and 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 pressure
can 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.
54. 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 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 pressure can
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.
55. 1.3 The guarantee of the grinding temperature: vertical roller
mill 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 collection
system may cause condensation; If too high, humidity is
not enough that gas cooling, dust collection will also affect the
results.
56. 2. Exception handling
• 2.1 vertical roller mill vibration is too large:
(1) Feed uneven, when the mixture into the vertical roller mill
over 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 stabilize
the 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
57.
58.
59.
60.
61.
62.
63. 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
72. If product has more coarse then classifier seal needs inspection
and rectification. Production drops drastically.
Classifier rotor seal
73.
74. 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 very
Efficient classifier
Size fraction
By pass fraction
75.
76.
77.
78. Nozzle ring velocity
45 – 60 m/s
velocity above roller
V = 1.5 Do to 2.5 Do
Velocity through rotor
vanes = 5.0 m/s
Grinding bed thickness = 1 - 2 % of Do
For cement grinding = 0.2 % of Do
Mill Dp = 350 Do m
Mill exit velocity = 10 m/s
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
operating rpm , N = 56 / Do
5 – 6 m/s
