The document discusses different cement grinding systems and their effects on cement properties. It describes various grinding systems including ball mills, vertical mills, and roller presses. It explains how the grinding system and separator type influence particle size distribution, which impacts strength development and workability. Drying capacity is also an important process aspect, as insufficient drying can lead to cement hydration issues. The document compares compound and separate grinding, noting that compound grinding of materials with different grindabilities can affect the particle size distribution achieved.

1. Effect of Grinding Systems on Cement
Properties
Radu Geomolean CTL/TT
(former CMS/MPT – Grinding Technology)

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GEO-November 08
Holcim Group Support Course for Cement Applications 2008
Content
 Background information
 Main grinding systems
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 Conclusions

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GEO-November 08
Holcim Group Support Course for Cement Applications 2008
Content
 Background information
 Main grinding systems
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 Conclusions

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GEO-November 08
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Background information
 The grinding system (including separator type) is
influencing directly the Particle Size Distribution
(PSD)
 Strength development
 Workability
 Drying = one of the main process parameters in
grinding
 Effect on the setting behaviour (e.g. because of different
gypsum dehydration)
 Insufficient drying capacity indicated by low mill exit
temperature (< 85 [oC]) or too high exit gas dew points
(> 60 [oC]) can lead to cement hydration (use of wet
additives)

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GEO-November 08
Holcim Group Support Course for Cement Applications 2008
Content
 Background information
 Main grinding systems
 Grinding systems
 Separators (types, basic principle, influence on PSD)
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 Conclusions

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Main grinding systems
 Separate and combined grinding (with / without external
drying)
 Roller press (in circuit with ball mill)
 Vertical mill
 Ball mill
 Horomill
 All grinding systems can grind clinker together with
different types of additives: slag, puzzolana, fly ash,
limestone
 The use of additives is limited by the main process
aspects: drying, grinding, material transport

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Ball Mill EDM (End Discharge Mill)
Clinker Gypsum
Min.
Comp.
[l/h]
[kW]
[l/h]
[t/h]
[°C]
[mbar] [°C]
[t/h] [t/h] [t/h]
[t/h] [t/h] [t/h]
[%]
[%]
[%]
[%]
[%]
[%]
[t] [t] [t]
[°C]
[l/h]
Grinding
aid
[1/min]
[°C]
[kW]
[mbar]
[°C]
[mbar]
[kW]
[mbar]
[°C]
[1/min]
[°C]
[°C]
[°C]
[1/min]
[m3/h] [mg/m3]
Sollwerte
Prozesswerte
[t/h]
[t/h]
[1/min]
[°C]
Product
Process
parameters

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Roller Press
Two stage grinding system
Separator
fines
Mill feed bin
(intermediate
product)
Separator
returns
Final Product
RP fresh
feed
Finish grinding
Final Product
RP fresh
feed
Separator
returns
De-agglomerator
There are many
other possible
options (see next slide)

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Two stage grinding system: RP + HES* & BM + HES*
20t
Filter
SKS
Filter
VSKS
Klinker Gips
Gös
Vor-
prod.
Tr.
prod.
Staub
Product
Res.
Product
*HES = High Efficiency Separator

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VRM (vertical roller mill) for cement grinding
grinding aid DEG
[t]
[kW]
[t/h]
[mbar]
[°C]
[%] [l/min]
[t/h] [t/h]
[%]
[cm2
/g]
sampler
[%R]
[l/h]
[mbar]
[°C]
[kW] [%]
[°C]
Hot gas
[t]
[t] [%] [%] [bar]
[mbar]
[min-1]
[min-1]
[l/h]
to cement
silo
fresh air
Clinker Gypsum
In blue = process
parameters

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Separators - overview
Static separators and
cyclones
No moving parts / fineness
adjustment via mechanical
modification
Dynamic separators
1st Generation
- Counter blades, distributor plate
and internal fan
- fineness control done by
mechanical adjustment or counter
blade speed change
Dynamic separators
2nd Generation
- Features as 1st Gen.
- Internal fan was replaced by
external ones
- Has cyclones
Dynamic separators
3rd Generation
(HES = High
Efficiency Separator)
- Cage rotor instead of counter
blades and distributor plate
- external fan
- fineness control by rotor speed
housing cone
tailings cone
adjusting device
rad.pos.
immersion tube
tailings
feed
fines
adjustable blades
feed
tailings
air
inlet
air+
fines
outlet
inclined plates
baffle plates

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Separation in 1st Generation Separator
Basic function:
 Material enters the separator (10)
 Air stream is generated by internal
fan (9).
 Coarse material goes to the walls
because of centrifugal forces,
generated by the distributor plate
and counter blades, slips down at
the grit cone wall and leaves at
the bottom (11).
 Fine material is sucked into the
outside chamber (1) and exits at
the bottom of the casing (12).
 Product fineness is adjustable
usually by rotational speed of the
plate and counter blades (6+7).
Airflow

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Influence on PSD at 1st generation separators
 Adaptation of Circulation
Load
 Mechanical Modifications
with impact on bypass
and fineness
 Reduction / increase of
number or position of
counterblades
 Gap between casing
and fan

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Separation in 2nd Generation Separator
Basic function:
 Material is introduced through
feed spouts (6) and dispersed in
the circulating air by the rotating
distributor plate (4)
 Particles fine enough to pass the
rotating counterblades (5) are
conveyed by air to external
cyclones (11) where they are
precipitated and discharged=
product (9)
 Coarse particles move downward
to the coarses cone (10)
 The air is recirculated to the
separator via fan (13) and air
vanes (12)

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Separation in 3rd Generation Separator
Basic function:
 Material enters the separator at
the top
 Air stream is generated by
external fan.
 Material falls down between rotor
and guide vanes. Fines are
sucked in. Coarse particles are
accelerated by rotor and stopped
by guide vanes, where they slip
down and leave the casing at the
bottom.
 Fine material exits with airflow at
the lower/upper part of the casing
 Product fineness is adjustable by
the rotational speed of rotor.
Source: Chr. Pfeiffer QDK Separator
Feed
Air
Turning cage rotor
Centrifugal Forces
Returns Air with fines
Air
Guide vanes
Feed
Air
Turning cage rotor
Centrifugal Forces
Returns Air with fines
Air
Guide vanes

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Influence of airflow on the PSD at 2nd and 3rd Generation
Separators
  Airflow  wider PSD and  Blaine
  Airflow  narrower PSD and  Blaine
separation chamber
tailings cone
air vanes
distributor plate
counterblades
feed spout
gearbox
motor
return air duct
dust collecting
fan
air duct to fan
cyclones
tailings outlet
fines outlet
pipe to filter
8 7
14
11
6
11
5
1
9
2
9
10
3
4
13
15
1
15
2
3
4
5
6
7
8
9
10
11
12
13
14
ROTOR TYPE SEPARATOR, MAKES
Feed
Air outlet
+ fines
Coarses
Distrbutor
Guide vanes
Rotor
Drive
Air inlet
(3 Levels)
Sealing
Air inlet
SEPOL
Airflow

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How to influence the PSD in 2nd and 3rd gen. separators
Influence of SEPARATOR LOAD ( keeping the same Blaine)
  Separator load  higher steepness of PSD (narrower)
 “ higher reject rate case”
  Separator load  lower steepness of PSD (wider)
 “ lower reject rate case”
Note: Far too low (Circulation Load < 1.3) or too high
separator loads (Circulation Load > 5) can
significantly reduce the mill performance and/or
the separator efficiency (loss of production)

18. 18
GEO-November 08
Holcim Group Support Course for Cement Applications 2008
Content
 Background information
 Main grinding systems
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 Conclusions

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Drying in Ball Mill - Bottlenecks
Fresh air
Fresh air
Moist additive
Hot clinker
Hot gas
H2O
Product
Air
Dust ladden air
H2O
Hot air
70
80
90
100
110
120
Product
temp. [°C]
Mill length
grinding
heat
water
evaporation
Cooling by
water injection
Finish
Product
Mill
exhaust
air
< 300 °C
> 100 °C
Recirculating air
Drying
Compartment?
Ball Mills as limited in drying capacity. An EDM can
dry max. 4% feed moisture even having a HGG!

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How to improve drying?
 For low quantities (< 10%) of moist additive:
 Use of hot clinker
 Decrease of mill ventilation
 If single pass separator is installed, reduction of fresh
airflow to minimum (maximize recirculation gas amount if
possible)
 For high quantities of (or pure) moist additive:
 External drying or compound drying/grinding (moist
additives) using:
- Kiln or cooler gases
- Hot gas generator (HGG)

21. 21
GEO-November 08
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Content
 Background information
 Main grinding systems
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 PSD in both cases
 Common set-ups
 Quality impacts of compound grinding systems
 Advantages / Disadvantages
 Conclusions

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PSD in Compound Grinding
 Compound grinding of
materials with different
grindabilities  is affecting
the product PSD
CC01-005.dsf Kma 06.06.00
[%R]
acc.
[cm2/g]
Blaine
mill length [m]
longitudinal sieving graph
GA
[g/t]
°C
[m3/h]
[%]
[kW]
[t/h]
[%R,cm2/g]
[°C]
[°C]
[mbar]
[m3/h]
separator tailings
[t/h]
[°C]
[%R]
Fresh feed
[t/h]
[%H2O]
[°C]
[% comp.]
acc.
[mm]
[m]
particle size
1 10 25 50
Blaine: Some of the
components achieve the
necessary fineness
faster inside the mill
(could be pozzolana,
limestone, fly ash) 
 Waste of energy
 Low flexibility in terms
of influencing the PSD
Cumulated
residues

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PSD in Separate Grinding
%R
Fineness
hard
soft
In separate grinding is possible to
manipulate this zone!

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Possible Systems Compound Grinding
Note: Slag and Limestone are usually added to feed

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Common Systems for Separate Grinding
Clinker Gypsum
Min.
Comp.
[l/h]
[kW]
[l/h]
[t/h]
[°C]
[mbar] [°C]
[t/h] [t/h] [t/h]
[t/h] [t/h] [t/h]
[%]
[%]
[%]
[%]
[%]
[%]
[t] [t] [t]
[°C]
[l/h]
Grinding
aid
[1/min]
[°C]
[kW]
[mbar]
[°C]
[mbar]
[kW]
[mbar]
[°C]
[1/min]
[°C]
[°C]
[°C]
[1/min]
[m3/h] [mg/m3]
Sollwerte
Prozesswerte
[t/h]
[t/h]
[1/min]
[°C]
Product
grinding aid DEG
[t]
[kW]
[t/h]
[mbar]
[°C]
[%] [l/min]
[t/h] [t/h]
[%]
[cm2
/g]
sampler
[%R]
[l/h]
[mbar]
[°C]
[kW] [%]
[°C]
Hot gas
[t]
[t] [%] [%] [bar]
[mbar]
[min-1]
[min-1]
[l/h]
to cement
silo
fresh air
Clinker Gypsum
VRM for
cement & slag
grinding
BM for cement
grinding

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Example of blending facility (IBAU)
3
4
1
2
Product
Batch
mixer

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Quality Impacts of Compound Grinding BM
Problem Solution
 Partly hydration of additive (slag);
loss of strength
 Influence on sulphate due to humid
atmosphere in mill
 Separate drying
 Installation of hot gas generator and
drying chamber
 Avoid dew point > 60 [°C]
 PSD not optimum
 ‘Steepness’ (n) of PSD too high/low
 Change of circulation load*
 Too low n’: increase circ. load
 Too high n’: decrease circ. load
 New Separator?
 Strength development insufficient
 Workability not acceptable
 Setting behaviour does not meet
requirements
 Adapt mill exit temperature if related
to gypsum
*Only in certain range possible

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Quality Impacts of Compound Grinding VRM
Problem Solution
 Partly hydration of additive (slag);
loss of strength
 Influence on Sulfate due to humid
atmosphere in mill
 Increase fresh air (if possible)
 Avoid dew point > 60 [oC]
 Install Hot gas Generator
 Particle Size Distribution (PSD) not
optimum
 ‘Steepness’ of PSD too high/low
Change of
 Air Flow
 Roller Pressure
 Bed depth
 Separator
 Strength development insufficient
 Workability not acceptable
 Setting behaviour does not meet
requirements
 Adapt mill exit temperature if related
to gypsum

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Summary and Comparison of grinding processes
- - Very bad
++ Very good
Comparison for compound grinding
Drying Control of achievable
potential PSD fineness
Roller press circuit 0 0 ++
Roller press circuit with hot gas + 0 ++
Horomill - ? ?
Horomill with hot gas + ? ?
Ball Mill + - ++
Ball Mill with hot gas ++ - ++
Vertical mill 0 ++ +
Vertical mill with hot gas +++ ++ +
0
0
0
++
++

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Advantages / Disadvantages of separate grinding
Advantages
 Optimum PSD of additive in
cement
 Various Cement designs
possible
 High flexibility on market
demands
 Lower specific energy
consumption
 Lower clinker factor possible
Disadvantages
 Blending facilities necessary
 Usually drying installation
required
 More silos (Multi chamber silo)
necessary

31. 31
GEO-November 08
Holcim Group Support Course for Cement Applications 2008
Content
 Background information
 Main grinding systems
 Drying in ball mills  major process aspect
 Compound / Separate grinding
 Conclusions

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Conclusions
 In BM circuits the possibilities of influencing the PSD are
limited and very much depending on the separator type
 The Vertical Roller mill is well suited for compound or
separate grinding
 high flexibility for modifying cement properties
 high drying capacity
 Too low mill exit temperatures (< 85 [oC]) or dew point >
60 [oC] during compound grinding (e.g. caused by wet
slag) can lead to process and quality problems
 Separate grinding gives more flexibility in product
design, product optimisation and reaction on quality
variations of clinker or additive