 |
OPERATING EXPERIENCE OF VERTICAL
ROLLER MILL FOR SLAG GRINDING
Dr. Heinz Ulrich
Schaefer
| SUMMARY
Vertical roller mills are
very well introduced for grinding and drying of raw materials for cement
production all over the world.
The high reliability and
the perfect operational availability of the vertical roller mill leaves
no alternative presently. Therefore it was a consequent step to develop
this system further for the application in grinding plants for clinker,
slag, pozzolana and other materials used for cement production nowadays.
By introducing the vertical
roller mill of the LM ... 2+2 C/S design into the market a most reliable
mill has been offered to cement producers. The newly designed mill uses
the well proven components of roller mills for raw materials, but the installation
of different sized master and slave rollers allows the mill to grind such
fine products like cements and slag. The material bed on the grinding table
is stabilized perfectly without using additives like limestone or spraying
of water into the mill.
Low energy consumption,
low costs for civil construction and low maintenance costs due to the simplicity
of the system makes the LM ... 2+2 C/S an ideal tool in the cement industry.
Perfect cement quality comparable to other grinding systems and very good
quality of concrete produced with the cement made in the vertical roller
mill emphasizes, that the system meets the requirements of the cement industry.
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INTRODUCTION
Many efforts have been made
throughout the last years to improve the efficiency of clinker grinding
systems. High efficiency separators have been developed in order reduce
the circulation load in the grinding system. Roller presses are used in
various set - ups together with ball mills for decreasing the energy consumption.
Roller press systems in
connection with high efficiency separators without ball mills have been
installed for cement production. However many of those solutions did not
reach their aim either in respect of their reliability and operational
availability or in respect of the quality requirements of the product.
During the last years intensive
research and developments were made for the application of vertical roller
mills for grinding of clinker, slag and related materials. Besides the
consideration of mechanical problems due to the hardness and the abrasive
properties of those materials the quality aspect had to be kept seriously
in mind.
No change in quality of
the product was allowed when comparing it with conventional grinding systems.
The result of these efforts was the development of 2+2 C/S design
of the Loesche mill for cement and slag grinding.
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| DESIGN FEATURES
OF MILL TYPE LM .. 2+2 C/S
Basically the well proven
components of the vertical roller mills for raw materials and coal have
been used. The physical properties of clinker and slag however and the
high fineness compared to raw meal or coal dust required a new design in
order to stabilize the relatively thin material bed on the grinding table
and in order to run the mill smoothly without vibrations.
The 2+2 system provided
the most favourable solution for this application. Two different sized
sets of rollers are installed (picture no. 1).
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One set of rollers of small
diameter - the so-called slave rollers - is used for perparation of the
material bed. These slave rollers are of a light steel sheet design. By
means of a hydraulic system their position above the grinding table is
kept on a certain level. No pressure is applied on the slave rollers themselves,
except the counterpressure of the material passing underneath the rollers.
The slave rollers smoothen the surface of the material bed on the grinding
table and they consolidate it and deaerate it slightly. After the material
bed has been prepared in such manner it is easily nipped by the large sized
master rollers.
The master rollers are of
heavy design. Their tires are casted from high wear resistant alloys. A
hydraulic system is used for creating high pressure on the material bed
to be ground. A hydro-pneumatic spring system provides uniform loading.
The 2+2-system is applicable up to capacities of 140 - 150 t/h cement.
For higher capacities, which might exceed 200 t/h the same principle as
above is used but the two sets of different sized rollers consist of 3
slave and 3 master roller each.
A high efficiency separator
is incorporated in the upper part of the mill housing. Mass related surfaces
of cement or ground granulated blast furnace slag between 2800 cm²/g
and more than 4500 cm²/g can be achieved. Particle size distributions
with slopes between 0,8 and 1,2 according to the RRSB - grit are common.
Besides the above highlighted
features many aspects for easy operation and low maintenance have been
considered, like wear protection in the housing, provisions to avoid metal
to metal contact between grinding table and rollers and the hydraulic
swing - out device.
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| GRINDING PLANTS WITH
LM .. 2+2 C/S TYPE MILLS
Grinding plants with this
new generation of cement mills are of very simple design. The matrial is
fed by means of weigh feeders into the mill. A hydraulically operated feed
gate is used for preventing the intrusion of false air into the system.
The material is dried and
ground and after it is discharged from the grinding table it is lifted
by the process air flow to the classifier where grits and product
are separated. The product leaves the mill with the air flow and is collected
in a jet pulse filter. Depending on the heat requirements in the system
the air can be recirculated back to the mill or it can be discarded through
the stack. In case of high heat demands due to drying operation, e.g. in
cases of slag or pozzolana grinding, a hot gas generator might be
used as heat source , if no other sources like cooler waste gases or similar
are available. In case of clinker grinding the hot gas generator will be
used for start-up only. Verse vice the system can be operated with ambient
air in case of excessive clinker temperatures and cement cooling can be
done in the grinding circuit.
Coarse particles which leave
the grinding table and fall down through the nozzle ring are collected
in the lower part of the mill and conveyed to the mill feeder by means
of an external recirculation system (picture no. 2).
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This system is used as well
for the extraction of metallic iron in case of slag grinding.
Picture no. 3 shows the
general arrangement for a grinding plant with a LM ...2+2 C/S mill.
The building structure can
be of a very simple design. The mill is relocated on the ground. Only the
jet pulse filter needs the support of a steel structure. The fan is located
on ground level as well. It is working downstream of the filter in a clean
air enviroment.
The whole grinding system
is completely closed, as the transport of material takes place inside the
ductwork. No open transfer points, conveyors or elevators are required
except for the feeding system. Therefore the whole plant can be kept open
without any building except for the small tower which accomodates the feed
system. Costs for civil construction are considerably low for this reason.
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| QUALITY
OF CEMENT PRODUCED IN A LM .. 2+2 C MILL
Experience with other sophisticated
grinding systems showed in the past, that the cement quality changed compared
to existing ball mill systems. Therefore this matter has been studied in
all details.
In December 1994 the first
LM 46 2+2 C world wide for cement production has been started up.
During the first months of operation clinker from various sources has been
imported for cement production in this plant. From the day of start
- up no problems on the cement quality have been encountered. Later on
the clinker came from a cement plant in which two ball mills grind this
clinker type as well, which provided a perfect chance for a direct comparison
of the quality of the products of both systems.
However 4 -5 % of blast
furnace slag were added into the vertical roller mill at the time of sampling.-The
particle size distributions of both samples have been determined by means
of a CILAS 850 lasergranulometer using alcohol as dispersive agent. The
characteristic parameters of the particle size distriubutions in the RRSB
grit are :
| |
LM
46.2+2 C |
Ball
Mill |
| slope |
0,86 |
0,96 |
| position parameter |
17,3
µm |
15,3
µm |
The slope of the cement
ground in the Loesche mill is much lower compared to the slope of the cement
produced in the ball mill system (picture no. 4).
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This is more favourable
in respect to the water demand of the cement. Compared to other cements
with such a slope the position parameter (63,2 % by mass passing) is slightly
low however.
The mass related surface
area has been determined according to BLAINE:
| |
LM 46.2+2
C |
Ball Mill |
| surface area |
3.320 cm²/g |
3.580 cm²/g |
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The relatively
low position parameters in respect of the mass related surface areas are
required under the consideration that both cements are cements of high
early strength according to the local requirements.
The chemical composition
of the cement samples used for comparison is listed in table no. 1.
It is obvious that both
cements are made out of the same type of clinker. The slight differences
originate from the addition of blast furnace slag.
| sample |
LM 46.2+2 C
|
Ball Mill
|
| sample no. |
1
|
2
|
| lab no. |
96/068
|
96/069
|
| Loi |
1,50
|
1,40
|
| SiO2 |
20,90
|
20,20
|
| Al2O3 |
5,20
|
4,80
|
| Fe2O3 |
3,60
|
3,60
|
| TiO2 |
0,60
|
0,50
|
| CaO |
61,70
|
63,10
|
| CaO, free |
n.d.
|
1,70
|
| MgO |
3,40
|
2,80
|
| SO3 |
2,30
|
2,20
|
| K2O |
0,50
|
0,60
|
| Na2O |
0,40
|
0,40
|
| Na-Eq. |
0,73
|
0,79
|
| Summ. |
100,10
|
99,60
|
Table No.1:
Chemical analyses of cement samples
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The physical
properties of both cement samples have been tested according to DIN EN
196 part 1 and part 3. Regarding
the water demand and the setting time the following results have been obtained:
| |
LM 46.2+2 C
|
Ball Mill
|
| water demand |
27,0 %
|
29,0 %
|
| initial setting |
120 min.
|
180 min.
|
| final setting |
180 min.
|
210 min.
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The lower water demand of
the cement produced in the vertical roller mill can be related to the lower
slope value compared to the cement produced in the ball mill. The setting
times of both tested cements are within the usual range. The development
of strength of both cements has been determined on mortar specimens according
to the above mentioned standards. The development of compressive strength
of both tested cements is fairly good (picture no. 5).
The early strength of the
cement produced in the vertical roller mill is slightly less compared to
the ball mill cement, but it must be kept in mind that the first one contains
4 - 5 % by mass of granulated blast furnace slag and that the particle
size distribution shows a lower slope and that the cement has a lower mass
related surface area. The relatively great difference in the 7 days strength
of both cements is most probably related to the addition of slag. After
28 days the compressive strength of both cements is on the same level.
Both cements meet the requirements
of a high early strength cement class CEM I 42,5 R according to DIN EN
197.
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| QUALITY OF
CONCRETE PRODUCED WITH A CEMENT GROUND IN A LM 46.2+2C
In order to investigate in
the influence of the cement properties on the properties of concrete produced
of the cements ground in different grinding systems a series of concrete
batches have been prepared. Special attention has been drawn on the workability
of the concrete and the possibility of unfavourable early setting of the
concrete. For the preparation of the concrete batches aggregates of particle
size distribution curve A/B32 have been use (picture no.6).
Picture No.
6: Particle size distribution of aggregates
The general aim was to use a
cement quantity of 300 kg/m³ concrete and to achieve a slump of approx.
48 cm. Alltogether 4 concrete batches have been prepared, each of about
25 l.
The concrete batches have
been mixed for 120 s and after that the slump has been determined according
to DIN 1048 after 3 minutes, after 30 minutes and after 60 minutes. After
the last slump test each concrete batch was taken back into the mixer and
after mixing the slump has been determined again (marked as" ## 60 min
"). Picture no. 7 shows the results.
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Concerning early setting
a difference in slump of 3 ± 2 cm after 30 minutes (refered to the
result after 3 minutes ) and of 5 ± 2 cm after 60 minutes ( refered
to the result after 3 minutes ) can be considered as normal.
In this particular case
the concrete made of cement ground in the ball mill shows a significantly
higher trend for early setting than the concrete made with cement produced
in the vertical roller mill. This phenomenon has been encoutered in many
of similar tests before.
Finally water was added
to one of each batches of concrete until the original slump value has been
obtained. After the slump tests specimens for the determination of compressive
strength have been prepared from the concrete batches LM 1, LM 2, BM 1
and BM 2. The actual compositions of the concrete specimens is shown in
table no. 2.
| concrete |
LM 1 |
LM 2 |
BM 1 |
BM 2 |
| cement [kg/m³] |
305 |
302 |
304 |
302 |
| water [kg/m³] |
168 |
178 |
168 |
179 |
| aggregates [kg/m³] |
1.927 |
1.908 |
1.921 |
1.911 |
| concrete density [kg/m³] |
2.400 |
2.388 |
2.393 |
2.392 |
| water/cement ratio |
0,551 |
0,591 |
0,551 |
0,591 |
Table No. 2:
Composition of concrete specimens
used for comprehensive
strength testing
The results of the tests of
compressive strength of the concrete specimens is shown in picture no.
8. The results resemble clearly what could be expected from the tests described
before.
The strength development
after 7 days of the concrete made with cement ground in the vertical roller
mill is slightly less compared to the concrete made with the ball mill
cement. The reason for that is the addition of blast furnace slag into
the cement ground in the LM 46 2+2 C, the lower mass related surface area
and the less steep particle size distribution.
The results obtained after
28 days however show clearly that the concretes made out of cements of
the same clinker but have been produced in different grinding systems reach
easily the same level of compressive strength.
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| ENERGY CONSUMPTION OF
DIFFERENT GRINDING SYSTEMS
The comparison of the specific
energy consumptions between the vertical roller mill and the ball mill
system has been carried out for this particular case. For the energy consumption
figures of the ball mill system two cases can be shown, as this particular
ball mil can be operated with a roller press as a pregrinder if required.
The result is shown in table
no. 3. The table is self - explanatory.
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CONCLUSION
Grinding plants with LOESCHE
mills of the new LM ... 2+2 C/S generation provide cement producers with
grinding systems, which offer perfect cement quality, a wide range of flexibility
and which are very energy efficient systems.
The combination of grinding
and of a perfect drying capability of this type of mill makes it suitable
to grind all kinds of cements, such as cements with granulated blast furnace
slag, pozzolana cements, fly ash cements and other blended cements.
The possibility to switch
within less than one hour from one type of product to the other is a unique
feature of this grinding technology. |
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