Koch-FinalThesis-VerticalMills-2010

HTC

0

Final Thesis

Vertical Mills For Raw and Cement Grinding –

Inspection and Evaluation

From: Josef Koch (Technical Trainee 2009/2010)

Supervisor: Gerhard Gudat (Team Leader of Comminution Department)

Date of Conduction: 02.2010 – 04.2010

Vertical Mills for Raw and Cement Grinding Inspection and Evaluation HTC

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Summary

The industrial development of vertical mills started in the beginning of the 20th century in the

USA. Since then many different designs of vertical mills from many different manufacturers

emerged on the market.

Vertical mills are gas-swept-mills. They are used for fine- and finest-grinding and

simultaneous drying of cement-clinker and raw-material. The advantages and disadvantages

of vertical mills will be described in detail. A comparison between vertical mills and ball mills

will be drawn, too.

As in the area CE/CA mainly mills from the manufacturers Gebrüder Pfeiffer, Loesche and

Polysius are in use, the vertical mills of these suppliers as well as the vertical mill from

FLSmidth will be described in detail.

For optimizing a vertical mill there are many different points to consider. The right dam-ring-

height, the proportion of the separator-speed to fan-speed, the amount of water injected into

the mill or the hydraulic pressure of the grinding rollers are just a few parameters with which

it is possible to control the running of a vertical mill. Based on two examples, the

Polysius RM 46/23 in Slite and the MPS 3705 C in Hannover different possibilities of

optimizing a vertical mill will be discussed.

The introduced optimization charts are a tool for optimizing a vertical mill. By displaying the

grinding process as detailed as possible it is possible to find optimization potentials for

grinding on a vertical mill. The so called optimization charts have gathered these important

parameters and linked their interferences. With these optimization charts it is possible to find

the right measures to enable a smooth operation under the given requirements on the

product.


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Zusammenfassung

Die industrielle Entwicklung der Vertikalmühle begann im frühen 20. Jahrhundert in den USA.

Seit dieser Zeit kamen viele verschiedene Auslegungen der Vertikalmühle von

verschiedenen Herstellern auf den Markt.

Vertikalmühlen sind luftdurchströmte Mühlen. Sie werden für die Fein- und

Feinstzerkleinerung mit gleichzeitiger Trocknung von Zementklinker und Rohmaterial

verwendet. Die Vor- und Nachteile der Vertikalmühlen werden im Detail beschrieben. Ein

Vergleich zwischen Vertikalmühlen und Kugelmühlen wird zusätzlich im Detail behandelt.

Da in der Region CE/CA hauptsächlich Mühlen der Hersteller Gebrüder Pfeiffer, Loesche

und Polysius Verwendung finden, werden die Vertikalmühlen dieser Hersteller detailliert

beschrieben. Zusätzlich wird auf die Vertikalmühle der Firma FLSmidth eingegangen.

Um eine Vertikalmühle zu optimieren gilt es, verschiedenste Einflussgrößen zu beachten.

Die richtige Höhe des Staurandes, das Verhältnis von Sichterdrehzahl zu Gebläsedrehzahl,

die Menge an Wasser, das in die Mühle eingedüst wird oder der hydraulische Druck der

Mahlwalzen sind nur ein paar wenige Faktoren, mit denen man das Laufverhalten einer

Vertikalmühle beeinflussen kann. Basierend auf zwei Beispielen, der Polysius RM 46/23 in

Slite und der MPS 3705 C in Hannover werden verschiedene Möglichkeiten der Optimierung

einer Vertikalmühle dargestellt.

Die vorgestellten Optimization charts sind ein Hilfsmittel, um Vertikalmühlen zu optimieren.

Diese haben die wichtigen Einflussgrößen auf die Mahlung mit Vertikalmühlen

zusammengetragen und deren Querbeziehungen dargestellt. Mit diesen ‘Optimization

Charts’ ist es möglich, die richtigen Maßnahmen zu finden, um eine konstante und

gleichmäßige Mahlung unter den gegebenen Bedingungen an das hergestellte Produkt zu

gewährleisten.


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Contents Summary..................................................................................................................... I

Zusammenfassung ................................................................................................... II

1 Introduction........................................................................................................ 1

2 Vertical Mills....................................................................................................... 2

2.1 Overall description ..................................................................................................2 2.2 Grinding principle....................................................................................................4 2.3 Vertical mills in CE/CA............................................................................................6 2.4 Vertical mills vs. ball mills .......................................................................................8

3 Comparison of different vertical mill types ................................................... 10

3.1 Gebrüder Pfeiffer MPS mill ...................................................................................10 3.2 Loesche LM mill....................................................................................................11 3.3 Polysius mill ..........................................................................................................12 3.4 FLSmidth OK mill..................................................................................................14

4 Comparison...................................................................................................... 16

4.1 Active Grinding Area per time ...............................................................................16 4.2 Top view ...............................................................................................................18

5 Ideas of Optimization....................................................................................... 21

5.1 Polysius Mill RM 46/23 in Slite ..............................................................................21 5.2 Gebrüder Pfeiffer mill MPS 3750 C in Hannover...................................................26

6 Optimization charts ......................................................................................... 29

6.1 Optimization chart: Fineness.................................................................................29 6.2 Optimization chart: Temperature behind mill.........................................................33 6.3 Optimization chart: Vibration.................................................................................35 6.4 Optimization chart: Output ....................................................................................40 6.5 Optimization chart: Mill power...............................................................................42 6.6 Optimization chart: pressure drop .........................................................................44 6.7 Comments on the optimization charts ...................................................................45

7 Conclusion ....................................................................................................... 47

8 Bibliography..................................................................................................... 48

9 Annex................................................................................................................ 49

9.1 Cost calculation between vertical mill and ball mill ................................................49 9.2 Calculating the active grinding area per time ........................................................50 9.3 Pictures of Raw mill 8 in Slite................................................................................51 9.4 Pictures of Cement mill 11 in Hannover ................................................................53



Figure index Figure 1: schematic drawing of a vertical mill .........................................................................3 Figure 2: schematic drawing of a vertical mill with material & gas flow ...................................4 Figure 3: grinding principle of a vertical mill............................................................................5 Figure 4: Picture, drawing and flow-sheet of a Gebrüder Pfeiffer vertical mill .......................11 Figure 5: design of the Loesche grinding rollers ...................................................................11 Figure 6: Picture, drawing and flow-sheet of a Loesche LM vertical mill ...............................12 Figure 7: design of the Polysius grinding roller .....................................................................13 Figure 8: Picture, drawing and flow-sheet of a Polysius vertical mill .....................................13 Figure 9: Design of the FLSmidth grinding roller...................................................................14 Figure 10: Picture, drawing and flow-sheet of an FLSmidth OK vertical mill .........................15 Figure 11: Top view on the grinding table of a Polysius vertical mill .....................................18 Figure 12: Top view on the grinding table of a Loesche vertical mill .....................................19 Figure 13: Material inlet of a Loesche mill ............................................................................19 Figure 14: Top view on the grinding table of a Pfeiffer vertical mill .......................................20 Figure 15: Material inlet of a Gebr. Pfeiffer mill in Hannover.................................................20 Figure 16: Different shapes of a dam ring.............................................................................24 Figure 17: schematic drawing of the gas flow outside of a vertical mill .................................34 Figure 18: changes in mill design for higher output ..............................................................51 Figure 19: mill internals ........................................................................................................51 Figure 20: critical wear spots................................................................................................51 Figure 21: constructive modifications of the mill ...................................................................52 Figure 22: Bad condition of the vertical mill ..........................................................................52 Figure 23: changes of the mill design for a smoother grinding process ................................53



Table index Table 1: Overview of manufacturers of vertical mills in CE/CA ...............................................7 Table 2: general comparison between vertical roller mill and ball mill systems.......................9 Table 3: Calculating the active grinding area per time ..........................................................17 Table 4: basic information of vertical mill in Slite, Sweden....................................................21 Table 5: Blaine and Output for different cement types in Hannover 2009 .............................26 Table 6: side effects of changing parameters of grinding process of a vertical mill ...............46 Table 7: cost comparison between vertical mill and ball mill (Gorazdze) ..............................49

Diagram index Diagram 1: ground material structured into countries in CE/CA..............................................7 Diagram 2: Throughput as function of hydraulic pressure and dam ring height.....................23 Diagram 3: records of the ZM 11 from the control room in Hannover ...................................27 Diagram 4: Optimization Chart: Influence on the fineness....................................................32 Diagram 5: Optimization chart: Influence on the temperature...............................................35 Diagram 6: Vibrations of a VRM due to false grinding bed height (Hannover) ......................36 Diagram 7: Optimization chart: Influence on mill-vibration I ..................................................38 Diagram 8: Optimization chart: Influence on mill-vibration II .................................................39 Diagram 9: Optimization chart: Influence on mill-output........................................................41 Diagram 10: Optimization chart: Influence on mill power......................................................43 Diagram 11: Optimization chart: Influence on the pressure drop ..........................................45


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1 Introduction

Vertical mills have gained an increasing percentage of the raw-meal-production in cement-

plants over the last decade. The mills are used more and more in new cement-plants due to

their cost-effectiveness and their good controllability. The possibility to handle the full hot gas

amount, produced by even the biggest kilns for usage as transport gas as well as drying-gas,

increased the output of vertical mills from 100 t/h to nowadays over 500 t/h in the last

decade. At the same time the mill-drive-power increased from approximately 800 kW to over

3.400 kW.

Vertical mills are gas-swept-mills. They are used for fine- and finest-grinding and

simultaneous drying of cement-clinker and raw-material. With the DIN 24 100, Teil 2

“Mechanische Zerkleinerung; Maschinenbegriffe” there exists a standardized description

“Roller Mill“. The definition describes as follows:

„Machine; in which the grinding table is formed circular. Grinding elements are rolling onto it.

The grinding elements are pressed onto the grinding table either by its self-weight, by

centrifugal force, by springs, by hydraulic- or pneumatic-systems. Both, the grinding table

and the grinding elements are able to be driven.”

By the, in the cement industry commonly used mill-designs, the geometric design of the

grinding elements range from cylindrical rollers to truncated cones, to balls and the

transmission of the pressure range from self-weight to centrifugal force and spring-pressure

to hydro-pneumatic-pressure-systems. [Brundiek 1989]

Objective of this final-trainee-thesis was to gather theoretical basic knowledge about vertical

mills. On the basis of two particular vertical mills ideas for optimizing vertical mills have been

worked out.



2 Vertical Mills

The industrial development of vertical mills started in the beginning of the 20th century in the

USA. Since then many different designs of vertical mills from many different manufacturers

emerged on the market. The theoretical basics as well as information of vertical mills used in

HeidelbergCement Group will be described in this chapter.

2.1 Overall description

At least two fixed grinding rollers (cylindrical or conical shape) are rolling on a horizontal

grinding table. The contact pressure is transferred by weight and hydro-pneumatically. The

grinding table is driven by a gearbox, which also takes up the roller-pressure. By installing an

additional cascade gearbox or a frequency controlled drive it is possible to adjust the

circumferential speed of the table. The material feed is usually carried out in the middle of the

grinding table. The grinding material is transported by centrifugal force to the rim of the

grinding table. On the way to the rim of the grinding table, the material is drawn underneath

the grinding rollers. By self weight of the grinding rollers as well as by additional grinding

force the material is ground (see Figure 3). After grinding and passing the rollers the material

is transported further to the rim of the grinding table. Along the rim of the grinding table a

dam ring is placed (see Figure 19). The height of the dam ring is an important adjustment. By

the height, as well as by the circumferential speed of the grinding table it is possible to adjust

the height of the grinding bed. The higher the dam ring and the lower the circumferential

speed, the higher the grinding bed and vice versa. Between grinding table and mill housing,

a fixed nozzle ring is placed. The ground material passes over the dam ring and falls on the

nozzle ring (Figure 19). Through this nozzle ring, hot gas, produced either by an additional

hot gas burner or coming from the kiln, enters the mill below the grinding table. From there it

passes guiding blades which guide and accelerate the gas. By the area of the nozzle ring,

the speed of the gas flow can be adjusted. Coarse material, which is falling onto the nozzle

ring but can’t be lifted up by the gas flow is falling through the nozzle ring and is transported

Vertical Mills for Raw and Cement Grinding

Inspection and Evaluation HTC


out of the mill. It is also possible to build a vertical mill as circuit mill, which means, that the

coarse particles are transported back into the mill. Material which passes the dam ring and is

fine enough is lifted up. The, from guiding blades guided and accelerated lifting (hot-) gas lifts

the dust in a kind of fluidized bed to the upper part of the mill. Due to an increasing diameter,

the velocity of the gas reduces above the grinding rollers. Coarse particles are falling back

onto the grinding table. The other, finer material is lifted further to the top of the mill. Here,

either a static or a stage-less adjustable dynamic separator separates the coarse material

from the fines. The fineness of the ground material can either be adjusted by the grinding

force of the grinding rollers, by the velocity of the gas flow or by the speed of the dynamic

separator. The higher the grinding force, the lower the speed of the gas flow and the higher

the speed of the separator, the finer the ground material and vice versa. The rejected

particles are falling back onto the grinding table for additional grinding. The fine particles are

passing the separator and are conveyed by the hot gas out of the mill into cyclones or filters.

So, generally speaking, a vertical mill can be divided roughly into three different unit

operations: drive, grinding and separation (see Figure 2).

Figure 1: schematic drawing of a vertical mill

Mill stand

Gear box

Rocker arm

Grinding roller

Separator

Mill outlet

Mill feed inlet

Grinding table

Hydraulic system

Separator drive




Figure 2: schematic drawing of a vertical mill with material & gas flow

2.2 Grinding principle

Grinding in vertical roller mills is carried out by rolling loaded grinding rollers on a grinding

table. Coarse material is broken by the rollers like in a roller crusher. The comminution of the

fine material happens by bruising of the loaded material among each other. After leaving the

grinding table, the partly opened material is exposed to a gas stream with a high velocity and

is torn apart. The ground material is lead to the separator, coarse material back to the

grinding table. Due to the small residence time in the area of grinding (compared to ball mills)

the material bed is freed from finished good, which strains the grinding process

unnecessarily and tends to build agglomerates. Next to a good tapering of the grinding

rollers, the built-up of a stable grinding bed as well as the, for the grinding sufficient, pressure

Drive

Grinding

Separation

Energy

Drying

Hot gas

Material

Waste gases &

Grinding product

Transport




of the grinding rollers is required for grinding in a vertical mill (see Figure 3). [Kohlhaas,

1982]

Figure 3: grinding principle of a vertical mill

As described above, the grinding process follows two different actions:

Feeding of the grinding material between grinding roller and table

The coarser material, which acts as support-grain in the material bed is ground after the

grinding force exceeds the material-specific compressive strength. The comminution of these

parts is supported by the fact, that between the material and the grinding rollers usually point

contact occurs and so the peak stress is many times higher than the compressive strength.

After the breaking of the coarse material, the partly impulsive acting compressive force is

grinding further material. This event continues to the narrowest point between grinding roller

and grinding table. The continuous reduction of the grinding slot up to the zenith of the

grinding rollers causes a proportional increase of the specific grinding pressure. Thus the, by

comminution increased amount of support-grains, are facing increased specific grinding

forces for further grinding.

Compression of the grinding bed

The compression of the grinding bed by simultaneous reduction of the size of the support

grains causes an intensive spatial rearrangement of the different compressed particles. The

so occurring compressive- and shearing forces lead to a further comminution. Grinding by

shear-force is the main reason for finest comminution in vertical roller mills. The shear-force

is supported by the relative movement of the grinding rollers and the grinding table. This

FGrinding Grinding rollers

Grinding table

Side-view: Top-view:

Dam ring




relative movement can also prevent the material to stick onto the grinding table, if the

moisture of the material is too high. [Kohlhaas 1982]

Influences on the grinding process

To enable a smooth and constant grinding process with a high productivity and good quality

it is mandatory to have a constant and good material bed. To gain a good grinding

performance, even with difficult material, regarding the grind-ability, there are several ways to

take an influence on the grinding process.

It is possible to adjust the height of the grinding bed by changing the shape and the height of

the dam ring. The height of the dam ring depends on the flow characteristics of the material

to be ground. Does the material tend to stick, a lower height would be better for the material

flow characteristics on the grinding table. Is the material very fluent, a high dam ring and

sharp corners of the dam ring would be beneficial.

Is the material too fine or too dry, it is possible to insert water into the mill to stabilize the

grinding bed. It has been shown, that for grinding cement on a vertical mill it is necessary to

use grinding aids to achieve the most efficient operation with a stable grinding bed and a low

level of vibrations. As experiences show, water has an effect similar to that of a grinding aid.

[Jorgensen, 2005]

If the material is too soft, it can be advantageous to add coarse, extreme hard material. By

rolling over the grinding material, this coarse material leads to a higher resistance and so the

grinding rollers are lifted slightly up. Due to the hydro pneumatically pre-stressing of the

grinding rollers the rollers are falling back onto the grinding bed and so are leading to a

higher grinding force.

2.3 Vertical mills in CE/CA

In the area central Europe/Central Asia (CE/CA), 26 vertical mills are in use for grinding

different material like raw-material, slag, cement and coal. At the same time approximately

151 ball mills are used in the same area. Table 1 shows an overview of the vertical mills

used in CE/CA in the HeidelbergCement Group.




amount fraction [%]

Gebr. Pfeiffer 15 58

Loesche 6 23

Polysius 2 8

others 3 11

Sum 26 100

Table 1: Overview of manufacturers of vertical mills in CE/CA

As it can be seen, with a fraction of nearly 60 %, mainly mills from the manufacturer

Gebrüder Pfeiffer in Kaiserslautern, Germany are in use. Loesche and Polysius are the two

other important manufacturers for vertical mills in CE/CA. In this table, not all manufacturers

of vertical mills used in the whole group are listed. Raymond mills for example are commonly

used in NAM-area. FLSmidth (cement mill in Union Bridge e.g.) and Claudius Peters (coal

mill in Kakanj e.g.) are two other common vertical-mill manufacturers. There are also vertical

mills from Chinese manufacturers in use (coal mills in Georgia).

0123456789

10

Bosnia-H

erzegovin

a

Czech Republic

Georgia

Germany

Hungaria

Poland

Romania

Ukraine

Am

ount

of V

RM

[-]

CoalRawmaterialCementSlag

Diagram 1: ground material structured into countries in CE/CA As it can be seen in Diagram 1 vertical mills are used in Germany mainly for Raw-material

grinding. In the eastern countries (as well as in plants in NAM-area, not listed here) vertical

mills are commonly used for coal-grinding.

The only vertical mill for cement-grinding in CE/CA is situated in Hannover.




2.4 Vertical mills vs. ball mills

Over the last decades, the vertical roller mill has become the preferred mill for grinding of raw

materials. The grinding efficiency of the vertical roller mill combined with an ability to dry,

grind and classify within a single unit gives the vertical roller mill a decided advantage over

ball mill systems. However, despite these benefits, applications of the vertical roller mill for

cement grinding are less prevalent. The two-compartment ball mill, which operate in a closed

circuit with a high efficiency separator, is thus still the most preferred arrangement for new

cement grinding installations although the vertical roller mill now has emerged as a viable

alternative to the ball mill system and has increased its share of the market for cement mills

over the last decade. There are a number of explanations to this situation which relate to

issues like cost of installation, cost and ease of operation, cost and ease of maintenance,

product quality range, versatility, etc [Jorgensen, 2005].

Comparing vertical mills with ball mills rough overall-assumptions can be made (see Table

2). These assumptions are a general overview but, as the following example shows (see

Annex Table 7) it is necessary to investigate exactly for each installation of a new mill

whether a vertical mill or a ball mill suits best.

The investment cost of the two alternative grinding systems depend on a number of factors

such as the application, whether a separate drying facility is required for the ball mill, system

capacity, requirements with respect to country of origin of the equipment and a number of

other factors. The costs for civil works and erecting show also large regional variations.

Furthermore, the specific details of building requirements vary from plant to plant.

Predominant climatic and weather conditions will have a significant impact on the final

building design, and a ball mill and vertical roller mill installations have significantly different

foundations and layout requirements. Nevertheless, it can be said, that the total costs of

installation is in most cases higher for a vertical roller mill system than for a ball mill system.

The most significant advantage of a vertical roller mill compared to a ball mill system is

related to the specific consumption of electrical energy of the two systems. As examples

have shown [Jorgensen, 2005] the specific energy consumption can be up to 40 % lower for

a vertical mill system than for a ball mill system. The same assumptions can be made for the

operating costs.

As an example shows (see Table 7), the maintenance costs for a vertical mill are much

higher than for a ball mill. Work for remedying progressive wear of the grinding parts of a

vertical mill may involve reversal of roller segments, hard-facing of roller and table segments




and/or eventually replacement of worn out parts. This work is more complicated than just

adding more balls into a ball mill. The wear rate measured in grams per ton is much higher

for a ball mill than for a vertical roller mill. However, the unit costs for wear parts for a ball mill

are much lower than for a vertical roller mill.

The drying capacity of a vertical mill is much higher than for a ball mill system. Based on

manufacturer’s data, vertical mills can dry and grind material with a moisture-content of up to

20 w.-%. Ball mills can grind and dry feed material with a moisture-content of only 2 w.-%.

Another advantage of the vertical roller mill system is, that grinding, drying and separation of

the material takes place in only one unit (see Figure 1). On the one hand, no additional

installation of a dryer is necessary, leading to a significant reduction of the investment costs.

On the other hand, the size of this grinding unit is much more compact than the size for a ball

mill system, leading to a lower required space for installation.

Vertical mill Ball mill

Investment-costs û ü operating-costs ü û Wear û ü Spec. Energy consumption ü û Drying-capacity ü û Size ü û Table 2: general comparison between vertical roller mill and ball mill systems

It appears that quite a number of factors should be taken into consideration when making a

comparative evaluation of a vertical roller mill system and a ball mill system for grinding,

although the cost of electrical energy and total installation costs may be the most significant.

The significance of those factors may vary substantially depending on the location of the

installation, so it is not possible to make a ranking order for the two grinding systems that is

globally applicable. Such an evaluation must be made for the specific project while taking

into consideration the fact that the effects of the various factors depend on local conditions

and specific demands.



3 Comparison of different vertical mill types

As it can be seen in Table 1 there are three main suppliers for vertical mills in the area

CE/CA, Gebrüder Pfeiffer, Loesche and Polysius. In this chapter, the vertical mills of these

suppliers as well as the vertical mill from FLSmidth will be described in detail. Due to the

multitude of different manufacturers and mill types it is not possible to cover all vertical mills.

This is an assumption to classify and introduce the nowadays commonly used mill designs,

whereas this should not be considered as a valuation.

3.1 Gebrüder Pfeiffer MPS mill

Basically, the design of this mill is the same as described in chapter 2.1: yet the three bent

grinding rollers are in a convex shape and are rolling in an accordantly designed grinding

table. According to this design, the material inlet happens from the side onto the grinding

table. The, for grinding the material, required grinding pressure is generated by the self-

weight of the grinding rollers as well as by a hydro-pneumatically pre-stressed spring-system

which is arranged inside of the mill body.

After leaving the grinding table, the ground material passes onto the fixed nozzle-ring and is

loosened and pre-separated by the guided and accelerated lifting gas. By adjusting the

inclination of the nozzles, the ground material is driven in the same direction as the rotational

direction of the grinding rollers. Coarse particles are falling, as well as the by the stage-less

adjustable separator rejected particles, back onto the grinding table for additional grinding.

The fine particles are passing the separator and are conveyed by the hot gas out of the mill

into cyclones or filters (see Figure 4).




Figure 4: Picture, drawing and flow-sheet of a Gebrüder Pfeiffer vertical mill

3.2 Loesche LM mill

The feed material is discharged into the mill via an airlock and a downpipe to the centre of

the rotating grinding table. The material to be ground moves on the grinding track towards

the edge of the grinding table under the effect of centrifugal force and in this way passes

under the hydro pneumatically spring-loaded grinding Master rollers (M-rollers). These

comminute the material while the smaller S-rollers, which each operate between the M-

rollers, support preparation of the grinding bed by de-aeration, crabbing and pre-compaction

(see Figure 5). The rollers are forced upwards as they roll on the material bed.

Figure 5: design of the Loesche grinding rollers

At the same time the functional unit, consisting of M-roller, rocker-arm, spring rod and the

piston of the hydraulic cylinder are deflected. Rotation of the grinding table causes the

ground material from the M-rollers to be thrown outwards over the edge of the table. In the

area of the louver dam ring, which surrounds the grinding table the upwards directed hot gas

Grinding table

S-Roller M-Roller

Packing Grinding Expansion




stream captures the mixture of ground material and material still to be ground and conveys

this to the classifier.

Depending on the classifier settings coarse material is rejected. This falls into the internal grit

return cone and from there is returned to the grinding table for re-grinding under the rollers.

Final product material passes the classifier and is conveyed from the mill with the gas

stream. The mill is driven by an electric motor via a vertical gearbox. A segmented thrust

bearing in the top of the gearbox absorbs the roller forces. Before starting the grinding

process the M-rollers are lifted hydraulically from the grinding track. The support rollers are

also lifted when starting the mill [Loesche].

Figure 6: Picture, drawing and flow-sheet of a Loesche LM vertical mill

3.3 Polysius mill

The essential components of a Polysius Roller mill are the motor, the gear unit (for instance

a planetary bevel unit), the grinding table carried in tilting pad thrust bearings, the two roller

pairs, the housing and the static separator equipped with adjustable guide vanes. A dynamic

separator incorporating a deflector rotor can be used instead of the static separator.

The grinding principle and the basic description of the Polysius mill is the same as described

in chapter 2.1 and 2.2.

The design of the grinding rollers as well as the grinding table in a Polysius mill is special.

The grinding rollers of a Polysius mill are performed as twin rollers. If the inner roller is lifted




up by the material flow and thus the outer roller is pressing all the more onto the material bed

(see Figure 7). This design should lead to a constant output and energy consumption.

Figure 7: design of the Polysius grinding roller

The shape of the grinding table follows the shape of the grinding rollers. Thus the double

groove grinding track increases the retention time of the material on the grinding table. This

is a particular advantage in the case of material with unfavorable grinding properties and a

tendency not to form a stable bed. The double groove grinding track also ensures that the

bed of material is not too deep and minimizes the amount of material passing ungrounded

between the rollers and the grinding table. The basic idea behind this design is to reduce the

specific power requirement [Polysius].

Figure 8: Picture, drawing and flow-sheet of a Polysius vertical mill




3.4 FLSmidth OK mill

Basically, the design of this mill is the same as the ones described before.

The specialness of the FLSmidth mill is the roller and grinding table designs. As shown in

Figure 9, the rollers of the OK mill are spherical in shape with a groove in the middle. The

table is also curved forming a wedge-shaped compression and grinding zone between the

rollers and the table. This dual-lobed design is optimum for clinker grinding because it

supplies two distinct grinding zones – a low pressure zone and a high pressure zone. The

low-pressure area under the inner lobe de-aerates and consolidates the material to be

ground.

Figure 9: Design of the FLSmidth grinding roller

The proper grinding takes place in the high-pressure zone under the outer lobe. The groove

in the middle of the roller facilitates de-aeration of the material without fluidizing it. In order to

further ensure a stable operation with low vibrations the OK mill is provided with a high

efficiency separator. This results in a reduced internal circulation of fine material and a

correspondingly higher feed rate. The material on the grinding track will thus become coarser

and therefore less prone to fluidization [FLSmidth]




Figure 10: Picture, drawing and flow-sheet of an FLSmidth OK vertical mill



4 Comparison

The comparison of the different types of vertical mills can’t be done at first sight. There is not

such a thing like easily accessible operating figures or mill specific dimensions like table

diameter or number of grinding rollers, with which it is possible to make a direct comparison.

So a new calculated, specific figure has to be found with which it is possible to compare all

different vertical mill types.

Due to different designs of the mill interior by the different manufacturers it is also adjuvant to

take a look from the top onto the grinding table to compare the advantages or disadvantages

of the design and arrangement of the rollers, the table and other internals like ploughs or

material inlet chute for example for each mill.

4.1 Active Grinding Area per time

It is not possible to compare different types of vertical mill by figures like installed motor

power, table diameter or roller diameter for example. The interior of vertical mills is so

different and the application of force is conducted in such different ways, that a direct

comparison will not be able to reflect the grinding process in total. For finding a characteristic

figure, with which you can compare vertical mills from different manufacturers it is necessary

to look more into detail. The active grinding area per time for example is a calculated,

specific, characteristic figure with which it is possible to compare different types of vertical

mills. For the installation of a new vertical-slag-mill in Mokra, Czech Republic, such a detailed

consideration has been conducted. The constraints for this mill were a production of 80 t/h at

a fineness of 4.000 Blaine with a grindability of the material of 35 kWh/t. The figures,

gathered from the different manufacturers, as well as the calculated figures are listed in

Table 3. Four different mills from four different manufacturers have been contemplated: a

MPS 4250 B(C) from Gebr. Pfeiffer, a LM 46.2+2 from Loesche, a 38/19 from Polysius and

an OK 30 from FLSmidth.




Vendor

Pfeiffer Loesche Polysius FLS

Mill-type MPS4250 B(C) LM 46.2+2S 38/19 OK 30

Grinding Table

Diameter [m] 4,250 4,600 3,800 4,000

Dia. center line of the track [m] 3,400 3,884 3,170 3,000

Active grinding area [m2] 5,661 8,175 5,278 5,212

Number of revolutions [Rpm] 22,5 24,3 23,5 26,3

Circumferential speed [m/s] 5,01 5,85 4,68 5,50

Circumferential speed out [m/s] 4,01 4,94 3,90 4,13

grinding Area per time of table

[m2/h]

7643

(64,1 %)

11920

(100 %)

7442

(62,4 %)

8215

(68,9 %)

Rollers

Diameter [m] 2,300 2,360 1,900 1,770

With [m] 0,530 0,670 0,530 0,553

Number of Rollers [-] 3 2 4 4

Active grinding area [m²] 11,489 9,935 12,654 12,300

Number of revolutions out [Rpm] 33,26 39,99 39,21 44,53

Grinding Area per time of

rollers [m2/h] 22928 23839 14884 16430

Total active grinding area per

time

[m2/h]

30570

(85,5 %)

35759

(100 %)

22327

(62,4 %)

24645

(68,9 %)

Table 3: Calculating the active grinding area per time

The formulas for calculating the grinding area per time can be found in the Annex chapter

9.2.

The active grinding area per time relating to the grinding table gives information about the

area of the grinding table per unit of time which is rolled over by the grinding rollers. The

higher the active grinding area per time, the better, because more grinding-table-area and so

linked more material is rolled over per time unit. It is not possible to use the active grinding




area per time calculated by the geometric figures from the rollers because there can be

slippage between the rollers and the table. So, a more reliable figure needs to be considered.

The total active grinding area per time can also not been taken into account, because it is

calculated by the active grinding area per time from the grinding-table and the grinding

rollers. Thus the active grinding are per time from the grinding-table remains as

characteristic, specific figure for comparison.

4.2 Top view

Another possibility to compare the different mill-types can be done by a top view onto the

grinding table. The different arrangements of the grinding rollers or additional internals can

have an impact on the grinding process.

Top view on a Polysius mill

In Figure 11 the internal alignment of the mill internals of a Polysius mill can be seen. The

two twin rollers running parallel, the double grooved grinding track, the two material plows

leading the material back onto the grinding table and the material inlet from the side of the

mill housing are the most important internals of a Polysius mill. As it can be seen in Figure 11

the material inlet of a Polysius mill is usually carried out from the side via a chute (see Figure

18). The material is falling onto the centre of the grinding table between the two roller pairs. If

the material is too dry or it has bad inner material support characteristics, it can fall off the

table on the opposite side of the material inlet. It is possible to avoid this by installing material

plows for retention.

Figure 11: Top view on the grinding table of a Polysius vertical mill

Material inlet

Grinding Table twin rollers

plow




Top view on a Loesche Mill

In Figure 12 a Loesche mill Type 3+3 can bee seen meaning it is a mill with 3 Main- (M)

Rollers and 3 Support- (S) Rollers. Between two M-Rollers there is an S-Roller for preparing

a smooth and constant grinding bed. In a Loesche-mill the material inlet is usually conducted

from the top onto the centre of the grinding table. This is carried out via a chute that is

welded onto the separator-grit-cone (see Figure 13). So the material is entering onto the

centre of the grinding table and from there is transported by centrifugal force under each

roller by the same amount. Compared to the Polysius mill, no additional mill-inlets such as

material-retention-plows are needed.

Figure 12: Top view on the grinding table of a Loesche vertical mill

Figure 13: Material inlet of a Loesche mill

Top view on a Gebrüder Pfeiffer mill

In Figure 14 the internal alignment of the mill internals of a Gebrüder Pfeiffer mill can be

seen: the three statically defined grinding rollers, the hydraulic pressure frame, the grinding

table and the guidance for the pressure frame. The material inlet can’t be seen here, but as

mills in Leimen or in Hannover show, the material inlet is conducted in front of one of the

Separator grits

Material inlet S-Roller

M-Roller

Material inlet

S-Roller

M-Roller

Grinding table

Nozzle ring




grinding rollers (see Figure 15). By doing so, it might be possible that not every grinding roller

gets the same amount of material. If the material is ground fine enough by the first roller, the

second and third roller gets less material. This can lead to an uneven grinding process. In

some cases it is also necessary to install stripper plates to create an even and smooth

grinding bed.

Figure 14: Top view on the grinding table of a Pfeiffer vertical mill

Figure 15: Material inlet of a Gebr. Pfeiffer mill in Hannover

Material inlet

Pressure frame

Grinding roller

Grinding table

Material inlet

Grinding roller

Grinding table Nozzle ring

Guidanc



5 Ideas of Optimization

There are many different points to consider according to the optimization of vertical mills. The

right dam-ring-height, the proportion of the separator-speed to fan-speed, the amount of

water injected into the mill or the hydraulic pressure of the grinding rollers are just a few

parameters with which it is possible to control the running of a vertical mill. Based on two

examples, the Polysius RM 46/23 in Slite and the MPS 3705 C in Hannover different

possibilities of optimizing a vertical mill will be discussed in the next chapter.

5.1 Polysius Mill RM 46/23 in Slite

The Raw mill 8 in Slite, Sweden was erected in 1978. Since then there have many different

modifications been done to optimize the running of the mill regarding the output, fineness,

wear and energy consumption. Pictures of the modifications can be found in Chapter 9.3,

page 51 ff. Some basic data of the mill and nowadays production are listed in Table 4.

Manufacturer/Type Polysius RM 46/23

Mill power 2.400 kW

Throughput 395 t/h

Table-/Roller-diameter 4.600 / 2.300 mm

Roller widths 650 mm

Circumferential speed table 25 Rpm

Spec. grinding energy consumption 20 kWh/t

Δp mill 100 mbar

Gas volume-flow 1.060.506 m³/h

Hydraulic pressure of rollers 140 bar

Table 4: basic information of vertical mill in Slite, Sweden




Optimization of output

The first step to optimize the output of the vertical mill was to install two material-ploughs

between the two twin rollers. These internals prevent material to fall directly off the grinding-

table without any grinding by entering the mill too quick. Additionally these ploughs create a

grinding bed of constant height, so that the grinding procedure can go on smoothly (see

Figure 18).

The attempt to increase the output by increasing the table speed didn’t bring much effort.

The table-speed was too high, so the material left the table too quick without any grinding.

This change was undone.

The vertical mill was originally built with a static separator. Thus the second step for

optimizing the output of the mill was to change the static separator to a dynamic separator

with guiding blades and a rotary cage. This was carried out in November 1999. So it was

possible to increase the output of the mill by keeping the same values for the fineness of the

raw-material.

With the new separator it was also possible to install a bigger motor for the fan to increase

the gas-volume-flow. By increasing the volume flow, more material can be put into the mill.

The material, being lifted by the faster gas-stream is coarser than it would be with a slower

gas-flow. But with the new dynamic separator this effect can be reversed and the same

fineness can be kept even with the higher gas-velocity.

Another big issue until nowadays is to find the optimal height of the dam ring. The raw

material from the quarry in Slite reacts very sensitive on changes of the dam ring height as

well as on the shape of the dam ring. Diagram 2 gives an overview of the correlation

between the different dam ring heights and the throughput as a function of different hydraulic

pressure.




340

350

360

370

380

390

400

410

115 120 125 130 135 140 145

hydr. pressure [bar]

Trou

ghpu

t [t/h

]

30 mm40 mm50 mm70 mm

Diagram 2: Throughput as function of hydraulic pressure and dam ring height

Diagram 2 shows that the highest output for the raw mill is with a dam ring of 30 mm height.

It can also be seen that for reaching the same throughput of 400 t/h the hydraulic pressure

increases by 10 bars from using a dam ring with a height of 30 to 40 mm. By changing the

dam ring height of a vertical mill, not only the throughput is changed. If the dam ring is too

low, vibrations, because of a too low grinding bed height can occur and lead to a safety-

shutdown of a mill. On the other hand it is possible to adjust the internal circulation of the

material leading to a change of the particle size distribution especially regarding the

inclination of a particle size distribution curve. So, it is always necessary to consider, which

changes will happen by adjusting the dam ring height.

Yet not only the height of the dam ring, but also its shape plays an important role. The

vertical mill in Slite for example is only running smoothly, when the dam ring has a defined

and sharp corner. The vertical mill in Hannover instead has an continuous increasing dam

ring height, working like an extension of the grinding table (see Figure 16).




Figure 16: Different shapes of a dam ring

Left: shape of the dam ring of the Polysius vertical mill RM 46/23 in Slite Right: shape of the dam ring of the Gebr. Pfeiffer vertical mill MPS 3750 C in Hannover

The shape and the height of the dam ring depend on the flow characteristics of the material

to be ground. Does the material tend to stick, a lower height would be better for the material

flow characteristics on the grinding table. Is the material very fluent, a high dam ring and

sharp corners of the dam ring would be beneficial.

Optimization of fineness

Over the years, there have been many modifications carried out to improve the fineness of

the products, being ground on the vertical mill. One big alteration was the change from a

static to a dynamic separator. With this modification it was possible to increase the output

while keeping the same fineness.

A second step was to overhaul the rotary cage in need of repair. After years of production,

the single blades of the rotary cage have been bent and/or twisted by deformation due to the

torsion moment of the motor and the mass inertia of the rotary cage (see Figure 21). Thus

the old and apparently too small rotary blades have been replaced by new and bigger ones.

So, the dimensions of the old blades changed from 8 mm thickness and 90 mm width to

10 mm thickness and 140 mm width. Additionally a support ring was installed in the central

height of the blades for additional stiffness. With this measure it was on the on hand possible

to reinforce and stiffen the rotary cage without endangering the motor or the bearing to take

damage. On the other hand it was possible to insert more energy into the separation process

which leads to a better performance of the separator.

A third measure to improve the fineness was to reduce the possibility of the ground and lifted

material to bypass the separator, causing coarse particles to leave the mill without

separation. These bypasses are enabled on the one hand by too big rotor sealing gaps (see

Figure 22). The left picture is a top view onto the rotary cage. The sealing gap should be kept

as narrow as possible. If this gap is too wide, as it was in Slite, too coarse material bypasses

the separator at the top, getting into the finished product without getting separated. Solving

Grinding table

Dam ring

Grinding table

Dam ring




this issue is possible by closing this gap either by adjusting the gap or by installing a labyrinth

seal, preventing the coarse material to bypass the separator.

On the other hand, wear spots, especially at the bottom of the separator, caused by the

abrasive, dust loaded hot-gas enable a bypass for coarse material, getting into the finished

product without separation. Solving this issue is possible by removing the wear spots (see

following paragraph).

Optimization of wear condition

The newly installed dynamic separator was built with a distribution plate (see Figure 18). The

coarse material, falling through the nozzle ring was transported via bucket elevator to the top

of the mill and was fed onto the distribution plate. From here, the material was blown by the

hot gas, coming from the bottom of the mill, against the mill housing causing a high wear

rate.

Thus the mill was modified in a way that the coarse material was not longer fed onto the

distribution plate. A new material chute for the coarse material was installed, right above the

material chute for the raw material inlet (see Figure 21). So the coarse material could get

directly onto the grinding table and the wear of the mill housing was reduced by this means.

A second reason for the high wear condition is the too small diameter of the riser duct of the

vertical mill (see Figure 20). By a small diameter of the duct, the velocity of the highly dust

loaded gas is too high and thus the dust particles in the gas-stream are highly abrasive. A

bigger diameter of the duct could solve this problem.

A third way to get rid of such a high wear rate was to cover critical wear-spots inside of the

mill (see Figure 20). Especially in the area of the newly installed dynamic separator there are

critical wear spots. These locations at the separator are especially critical, because they

cause shortcuts for the coarse material, being able to exit the mill without separation and so

creating a bad quality of the finished product. After removing the distribution plate of the new

dynamic separator, for reducing the wear of the mill housing, some sharp corners as well as

some dead spots occurred (see Figure 18 left picture). These spots are very susceptible to

wear, because the flow direction of the highly dust loaded gas-flow is disrupted and thus the

dust particles are working extremely abrasive. So by covering these sharp corners and dead

spots by a casing the wear condition improved.




Optimization of false air situation

Depending on the area where false air occurs there are different effects on the grinding

process. In Slite one big reason for a high false air amount was that the sealing of the

alignments of the separator guiding plates haven’t been at place (see Figure 22). So air from

outside of the mill was sucked inside. By that the pressure drop inside of the mill falls down

and the mill fan needs to work more, leading to an increasing consumption of electrical

energy. Additionally the amount of gas through the mill and thus the velocity of the gas flow

are reduced causing a reduction of the output. These problems were solved just by placing

the sealing in the right position and fix them by welding to prevent further misalignment.

Other spots where false air could enter the mill were at the mill housing and the riser duct.

These areas were generated by wear. With the normal wear repairs this false air spots have

been removed (see Figure 20).

5.2 Gebrüder Pfeiffer mill MPS 3750 C in Hannover

The vertical mill in Hannover produces slag cements. An overview of the Blaine- values and

the output of the different cement types are listed in Table 5.

Cement Type Blaine [cm²/g] Output [t/h]

CEM II B/S 32,5 R 3674,8 70

CEM III A 32,5 N – LH/NA 4741,5 42,1

CEM III A 42,5 N – NA 4701,2 40,4

CEM III B 32,5 N – LH/HS/NA 4950,5 42,4

MC 5 6030,3 48,6

Table 5: Blaine and Output for different cement types in Hannover 2009

As it can be seen in Table 5, the fineness of nearly all cement types exceeds a fineness of

4000 Blaine by far. Usually a fineness of 4.000 Blaine is the maximum for a vertical mill to

grind without problems. For reaching this high fineness, the vertical mill has to be run in

harsh conditions. To increase the specific grinding force the input of the mill is set to a

minimum of sometimes lower than 36 t/h. At the same time the hydraulic pressure of the

grinding rollers is increased to a maximum of approximately 230 bar.

With the combination of a low material input and high hydraulic pressure of the grinding

rollers it is possible, that the grinding rollers penetrate the material bed and are rolling upon

the grinding table. This metal contact leads to high vibrations as seen in Diagram 3.




Diagram 3: records of the ZM 11 from the control room in Hannover

As it can be seen in Diagram 3, the whole grinding process reacts very sensitive on changes

of the grinding bed height. If the material bed is decreasing, the vibrations are increasing.

With the alternating vibrations, the mill power, the pressure drop of the mill and the hydraulic

pressure of the rollers are fluctuating. The vibrations are measured at three places of the mill:

the gear box, the separator and the mill fan. If the vibration of the gear box reaches a

maximum value (10 mm/s in Hannover) the safety circuit of the mill shuts the mill down to

protect it against major damage.

For enabling a smooth and constant running of the mill, different modifications inside of the

mill have been carried out (see Annex chapter 9.4).

As it can be seen in Figure 23 a material deflector plate for generating a smooth and even

grinding bed has been welded directly after the material inlet chute. This plate creates an

optimal material bed for the first grinding roller.

The second modification was the installation of two water injection nozzles after the first and

the second grinding roller. If the material is ground too fine by the first roller, the second roller

can cause vibrations. This is due to a penetration of the material bed by the rollers because

the inner support strength of the grinding bed is too low. Injecting water into this fine material

should increase the stability of the grinding bed and thus ease the grinding of the second

roller.

Grinding bed

height [mm]

Mill vibration [mm/s]

Material

input [t/h]

Mill power [kW]

Gasflow [m³/h]

Hydr. Pressure

of rollers [bar]




A third modification was to install a material trap at the inner wall of the mill. These traps are

installed behind the first and the second roller. They are designed to collect material out of

the gas flow and guide it back onto the grinding table in front of the next roller. By doing so,

more material is drawn under the grinding rollers increasing the grinding bed height and thus

stabilizing the grinding process referring the vibration.

As the current situation of the grinding process of the vertical mill in Hannover shows, all

these modifications did not solve the vibration-problems. This is due to the production of

cements with high fineness of far more than 4.000 Blaine.



6 Optimization charts

For optimizing a vertical mill it is important to know all the different parameters having an

influence on the grinding process. By displaying the grinding process as detailed as possible

it is possible to find optimization potentials for grinding on a vertical mill. The so called

optimization charts have gathered these important parameters and linked their interferences.

With these optimization charts it is possible to find the right measures to enable a smooth

operation under the given requirements on the product. These optimization charts have been

generated for the parameters: fineness, temperature, vibrations, output, mill power and

pressure drop. They are described in the following chapter.

6.1 Optimization chart: Fineness

For describing the parameter fineness there are three possible conditions:

Fineness is in range

If the fineness of a product is within the defined range, there are no problems and thus there

is no need of changing any parameter of the running grinding process.

Fineness is too high

If the fineness of the produced material is too high, this means, that the material is too fine,

the speed of the rotary cage of the dynamic separator needs to be checked. If the separator-

speed is too high, the reduction of the speed should decrease the fineness.

Is the separator-speed reduced to it’s minimum or there is only a static separator installed,

increasing the volume flow and thus the air velocity could also decrease the fineness of the

product. By increasing the gas-velocity, the gas-stream can lift up larger particles and the

diameter of particles being able to pass the separator will also increase.




If the fineness is still too high, one possibility to adjust the fineness is to reduce the hydraulic

pressure of the grinding rollers. By doing so, the specific grinding-energy is reduced and thus

the particle size is increasing.

In some cases vertical mills are equipped with a cascade gearbox. With this gearbox it is

possible to change the speed of the grinding table. If the fineness is still too high, increasing

the speed of the grinding table can lead to coarser particles. By doing so, the circumferential

speed of the table is increased and thus the centrifugal force on the material.

By increasing the speed of the grinding table, the residence time of the material in the area of

the grinding rollers is reduced. In addition, the grinding time, the period in which the material

is strained is also reduced. That leads to an insufficient utilization of the grinding forces and

thus to larger particles.

Is the fineness still too high, false air after the mill can cause this problem. Are there too

many wear spots or gaps between the outlet of the vertical mill and the mill fan, the volume

flow inside of the mill is too low. With a too low volume flow, the size of the particles being

lifted up for separation is decreasing leading to a too high fineness of the product. Solving

the false air situation should solve this problem.

Fineness is too coarse

If the fineness of the produced material is too low, this means, that the material is too coarse,

the speed of the rotary cage of the dynamic separator needs to be checked. If the separator-

speed is too low, the increase of the speed should increase the fineness.

Is the fineness still too low and the separator-speed can’t be increased anymore or there is

only a static separator installed, decreasing the volume flow and thus the air velocity could

also increase the fineness of the product. By decreasing the gas-velocity, the gas-stream can

lift up only smaller particles.

If the fineness is still too low, increasing the hydraulic pressure of the grinding rollers could

increase the fineness. By doing so, the specific grinding-energy is increased and thus the

particle size is decreasing.

The wear condition of the separator can also lead to bad results of the fineness. By a too

high wear condition, bypasses may occur, causing coarse particles to get into the finished




product without separation. Repairing the separator wear should lead to a higher fineness

(see also chapter 5.1).

Some mills have a cascade gear box installed. With this gearbox it is possible to adjust the

fineness by decreasing the speed of the grinding table. By doing so, the circumferential

speed of the particle is decreased and thus the centrifugal force on the material. (explanation

see above).

If none of these measures have gained an increased fineness oft the finished product, the

last possibility to adjust the fineness is to reduce the input. By doing so, the specific grinding

force is increased and thus the compressive strength on a particle is increased, leading to a

higher comminution of the material.




Diagram 4: Optimization Chart: Influence on the fineness

Fineness

OK?

Reduce

Separator speed

Increase

Separator speed

Too fine Too coarse

Increase

Volume flow

Reduce

Volume flow

Separator

Wear?

Increase

Hyd. pressure

Reduce

Table speed

Reduce input

Increase

Table speed

Yes

Repair

No

Reduce

Hyd. pressure

False air

After mill

Repair Yes

Side-effects:

-pressure drop

-output

-vibration

-pressure drop

-vibration

-output

-mill-power

-mill power

-output

-fineness

-pressure drop

Side-effects:

-pressure drop

-output

-vibration

-pressure drop

-vibration

-output

-mill power



6.2 Optimization chart: Temperature behind mill

For describing the parameter temperature behind the mill there are three possible conditions:

Temperature behind mill is in range

If the Temperature behind the mill is within the defined range, there are no problems and

thus there is no need of changing any parameter of the running grinding process.

Temperature behind mill is too low

If the temperature behind the mill is too low, there are little possibilities to increase it.

First of all it needs to be checked, if the hot gas amount is enough for reaching the defined

temperature. If there is sufficient hot gas, increasing the volume flow of the hot gas will also

increase the temperature behind the mill.

Is the amount of hot gas insufficient, reducing the amount of water injected into the mill can

cause an increasing temperature.

If none of the parameters described above lead to an increase of the temperature behind the

mill, the last possibility to increase the temperature is by reducing the feeding of the mill.

Especially during winter times, when the material, fed into the mill is wet, a reduction of the

feed will increase the temperature behind the mill.

Temperature behind mill is too high

If the temperature after the mill is too high, reducing the amount of hot gas is one option to

reduce the temperature. Yet, the grinding process in the vertical mill reacts very sensitive on

changing the amount hot gas. Thus the margin of changing the hot gas amount is very

narrow.

Is the temperature after reducing the hot gas amount still too high or there is no possibility to

reduce the hot gas, increasing the circulating gas is another step in reducing the temperature

behind the mill. The amount of circulating gas can usually be adjusted by the opening or

closing of a flap. Is the temperature after the mill too high, the flap is opened more and so the

hot gas, coming from the kiln, is mixed with colder gas coming out of the mill. The amount of

hot gas entering the mill is not only mixed with the colder circulating gas, but is reduced and

thus leading to a decreasing temperature (see Figure 17). So the total volume entering the




mill is the same and no further changes of the grinding process are done, except lowering

the mill temperature.

Figure 17: schematic drawing of the gas flow outside of a vertical mill

If the temperature after the mill is still too high, the last possibility to reduce it is by inserting

more water into the mill. But the material bed, which is an important parameter of the

grinding process, reacts very sensitive on changes of the water amount. Small changes of

the water amount injected into the mill have a great impact on the running of a mill.

Sometimes vertical mills are equipped with a fresh air flap. These flaps are for emergency

cases if the temperature rises too quickly. These flaps shouldn’t be used for adjusting the

temperature after the mill.

Flap

Fan

dedusting

Vertical mill

Hot gas entering mill

T~ 350 °C

Temperature after mill

T~ 90 °C

Circulating gas




Diagram 5: Optimization chart: Influence on the temperature

6.3 Optimization chart: Vibration

Vibration is a very important parameter regarding the safety of the mill. If the vibrations are

too high and last over a too long period of time, damages on the grinding rollers, the grinding

table and the gear box can occur. For this reason, every mill has a safety shutdown

procedure when vibrations occur.

For describing the parameter vibration there are two possible conditions:

Vibration is in range

If the Vibration of the mill is within an acceptable range, there are no problems and thus there

is no need of changing any parameter of the running grinding process.

Vibration is too high

Is the vibration of a vertical mill too high, there are several possibilities to reduce it.

If no changes of the parameters of the grinding process have happened and yet vibrations

occur, this is a sign for having foreign material inside of the mill. Foreign material, mainly

metal parts, entering the mill can’t be ground by the grinding rollers. So they are rolling over

it, are lifted up and after passing, the rollers are falling back onto the grinding table. This

Temperature

OK?

Hot gas

Enough

Reduce

Hot gas

Too low Too high

Reduce

Input

Increase

Circulating air

Increase

Water injection

Increase

Hot gas

Yes No

Side effects:

-volume flow

-fineness

-pressure drop

-vibration

-mill power

-Vibration

Side effects:

-vibration

-mill power

-mill power

-output

-fineness

-pressure drop

Reduce

Water injection




falling back usually leads to a tremendous increase of the mill-vibration causing a safety

shutdown. For solving this problem, the mill has to be opened and the foreign material to be

removed.

If there is no false material inside of the mill and vibrations occur, this can be due to difficult

material that has to be ground. Is the material too dry, the support of the inner-material

structure of the grinding bed is too low and thus the grinding rollers can break through the

grinding bed causing the mill to ‘rumble’. Insufficient water injected into the mill can be due to

blockage of the water-nozzle, demolished water pipe or false adjustment. So it needs to be

checked, if the water amount injected into the mill is normal compared to the material which

has to be ground.

Is the water amount in the normal range and no foreign material is found a third big reason

for vibrations of a vertical mill can be due to a false grinding bed height (see Diagram 6).

Diagram 6: Vibrations of a VRM due to false grinding bed height (Hannover)

In Diagram 6 the vibrations of the gear box (blue) and the separator (green) in mm/s as well

as the grinding bed height of the different rollers in mm are recorded over the time.

If the grinding bed is too high, the tapering of the grinding rollers is not sufficient and the

material is pushed aside. When the amount of material is too high, the rollers are rolling over,

causing a lifting of the rollers and a following drop back onto the grinding table causing high

vibration. A second effect of a too high material bed is the so called ‘stick and slick effect’.

Stick-slip is caused by the surfaces alternating between sticking to each other and sliding

over each other in. This will lead to a rumbling of the rollers. On the other hand, if the

grinding bed height is too low, the grinding rollers have no contact to the grinding bed or are

breaking through causing a contact between grinding table and grinding rollers causing

vibrations (see Diagram 6).

Mill-vibration Grinding bed




It is possible to adjust the height of the grinding bed by several parameters (see Diagram 8).

Varying the mill feed will have an influence on the grinding bed height. Less material fed into

the mill will decrease the material height and vice versa.

A second reason for affecting the material bed height is changes of the dam ring height. Is

the dam ring too high, too much material is on the grinding table and the material bed is too

high and vice versa. Adjusting the dam ring to the proper height for the material to be ground

can adjust the material bed height.

Another possibility for having a false grinding bed height is a false amount of water injected

into the mill. The effects of a wrong water amount are described above.

Changing the volume flow will also exert an influence on the grinding bed height. Increasing

the volume flow will take more material out of the mill and thus decrease the mill-internal

material circulation. Thus the mount of material getting back onto the grinding table is

reduced.

A wrong temperature of the material on the grinding table will influence the flow

characteristics of the material bed. At a temperature lower than approximately 110 °C the

material will get off the table easily. Increasing the temperature to approximately 130 °C will

also increase the flowability of the material, causing the material to get off the grinding table

more easily.

The last possibility to have an impact on the grinding bed height is the particle size. Too big

or too small particles may influence the grinding process in a negative way. Thus it has to be

checked, if any changes in the granulometry of the material to be ground have occurred.

If the hydraulic pressure of the grinding rollers is high enough and still vibration occurs, the

last reason for vibrations can be a bad wear condition of the grinding elements. If the wear of

the grinding rollers or the grinding table is too high, vibration occurs. This wear has to be

removed either by removing the worn parts or by welding repairs.




Diagram 7: Optimization chart: Influence on mill-vibration I

Vibration

High

Foreign

material

yes

Water injection

normal

Grinding bed height

normal

Hyd. pressure

normal

Wear of

grinding elements

no

OK!

no

yes

yes

no

no

yes

yes

no

remove

check

check

rewelding

yes

Side effects:

-mill power

-temperature

-mill power

-output

-vibration

-fineness

-pressure drop

-fineness

-output

-mill-power

See

Diagram 8




Diagram 8: Optimization chart: Influence on mill-vibration II

Influence of the grinding bed height

Grinding bed height

normal

Volume flow

normal

Check

Input

Check

Dam ring height

Check

Water injection

Material check

-Vibration

-Mill power

no

yes

See

-Diagram 7 (Vibration)

-Diagram 10 (Mill power)

adjust

Side effects:

-fineness

-temperature

-output

-mill power

-pressure drop

-mill power

-temperature

-pressure drop

-output

-fineness



6.4 Optimization chart: Output

For describing the parameter output there are three possible conditions:

Output is in range

If the output of a mill is within the defined range, there are no problems and thus there is no

need of changing any parameter of the running grinding process.

Output is too high

If the output of a mill is too high then there are no problems.

Output is too low

If the output of a vertical mill is too low, there are several possibilities to reduce it.

Increasing the mill feed will also increase the output of the mill. But this will only be possible,

if the hydraulic pressure of the grinding rollers as well as the volume flow for lifting the

particles up to the separator is enough.

After increasing the mill feed, the second parameter to influence the output of a vertical mill is

the hydraulic pressure of the grinding rollers. By increasing this pressure, more material is

ground fine enough to be lifted up and passing the separator.

If the output of a vertical mill is still too low, optimizing the mill power will lead to an increase.

The mill power and its influences will be described in chapter 6.5 in detail.

The last possibility to increase the output is by increasing the volume flow. This increase will

raise the amount of material being able to be lifted up. According to equation 1 coarser

material can pass through the separator. So changing the output by increasing the volume-

flow will always decrease the fineness if no other changes will be made.

If none of these parameters did increase the output, it needs to be checked, if the material

properties, in particular the grindability has changed. Checking these properties have to be

done in laboratory tests.



Diagram 9: Optimization chart: Influence on mill-output

OK?

Output

too high too low

Everything

OK

Increase

Volume flow

Increase

Input

Increase

Mill power

Change in

Grind ability

Reduce

hyd. pressure

Lab test

Grind ability

Side effects:

-temperature

-mill power

-fineness

-pressure drop

-fineness

-vibration

-fineness

-pressure drop

-pressure drop

-vibration

-fineness

See Diagram 10:

Mill power



6.5 Optimization chart: Mill power

For describing the parameter mill power there are three possible conditions:

Mill power is in range

If the power of a mill is within the defined range, there are no problems and thus there is no

need of changing any parameter of the running grinding process.

Mill power is too low

Is the power of a mill too low, increasing the input will also increase the mill power. More

material will be on the grinding table causing the motor to increase the power consumption.

A similar effect will have the increase of the dam ring height. More material will be retained

on the grinding table, leading the motor to increase the power consumption.

Mill power is too high

If the power consumption of a vertical mill is too high, there are several possibilities to reduce

it.

First of all it needs to be checked, if the grinding bed is too high or if the height is within the

normal range. If the grinding bed is within the normal range, the only possibility to reduce the

energy consumption is by decreasing the hydraulic pressure of the grinding rollers. This will

lead to a lower resistance to rolling and thus to a decreasing energy consumption for the

motor turning the grinding table.

Is the grinding bed height normal and the hydraulic pressure is reduced to the possible

minimum, the mill feed needs to be reduced in order to reduce the mill power.

If the mill power is too high and the grinding bed is not within the normal range, it is

necessary to check the parameters influencing the grinding bed height (see Diagram 8).

Adjusting the parameters if necessary should decrease the power consumption of the mill.

If none of these parameters did decrease the mill power, the last possibility is to reduce the

mill feed.




Diagram 10: Optimization chart: Influence on mill power

Mill power

OK?

Increase

Input

Grinding bed

too high

Too low too high

no Reduce

hyd. pressure

yes

adjust

no

Reduce

Input

See:

Diagram 8

no Increase

Dam ring

yes

Side effects:

-temperature

-output

-fineness

-pressure drop

Side effects:

-vibration

-output

-fineness

-pressure drop

-output

-fineness

-pressure drop

-temperature



6.6 Optimization chart: pressure drop

The pressure drop over the mill is an indication for the amount of material inside of the mill.

For describing the parameter pressure drop over the mill there are three possible conditions:

Pressure drop is in range

If the pressure drop of a mill is within the defined range, there are no problems and thus

there is no need of changing any parameter of the running grinding process.

Pressure drop is too low

If the pressure drop of a mill is too low, the material inside of the mill is not sufficient. Thus

the only way to increase the pressure drop is by increasing the amount of material inside of

the mill.

Is pressure drop is still not high enough, additional feed material will lead to an increasing

pressure drop. More feed will lead to a higher amount of dust which in turn will lead to an

increased flow resistance.

As the material inside of the mill is the only reason for the pressure drop over the mill other

optimization potentials need to be adjusted to increase the material feed if the feed isn’t

sufficient.

Pressure drop is too high

If the pressure drop over a vertical mill is too high, too much material is inside of the mill. Is

the pressure drop reaching a critical value (over approximately 80 mbar) too much material is

inside of the mill causing the mill to stop. Thus, the only way to decrease the pressure drop is

by decreasing the amount of material inside of the mill.

Is the pressure drop over the mill too high, increasing the volume flow will decrease it. A

higher volume flow will take more material out of the mill which leads to a decreasing dust

content inside of the mill.

Increasing the hydraulic pressure of the grinding rollers will also decrease the pressure drop.

An increase of the grinding force will lead to a finer comminution of the material. Grinding the

material fine enough will decrease the material circulation inside of the mill; more material will




be fine enough to pass the separator and being transported out of the mill causing the

pressure drop to decrease.

If adjusting the volume flow and the hydraulic pressure didn’t decrease the pressure drop

over the mill, the last possibility to decrease it is by decreasing the mill feed. Thus, less

material enters the mill and less material will lead to a lower pressure drop.

Another item influencing the pressure drop is the free area of the nozzle ring. By uncovering

the covered nozzle area, the speed of the gas-flow is reduced and with it the pressure drop.

Diagram 11: Optimization chart: Influence on the pressure drop

6.7 Comments on the optimization charts

Changing one parameter of the grinding process of a vertical mill can have several

secondary effects. These have to be considered when changing any parameter. In Diagram

7 to Diagram 11 the secondary effects are written in red letters. Changing the volume flow for

a higher fineness of the product might cause vibrations of the mill. A different separator-

OK?

Pressure drop

Increase

Input

too low too high

Other

optimization

Increase

Volume flow

Reduce

Input

Increase

hyd. pressure

70-80 mbar

Side effects:

-fineness

-output

-temperature

-mill power

Side effects:

-fineness

-vibration

-output

-mill power

-output

-fineness

-fineness

-mill power

-output

-temperature

Check

Nozzle ring area




speed for influencing the fineness can have an impact on the pressure drop of a mill.

Increasing the water injected into the mill for reducing the temperature after the mill can

cause a higher mill power. And increasing the hydraulic pressure of the grinding rollers for

lowering vibrations is leading to a higher fineness of the product (see Table 6).

Parameter Influence (increasing) Influence (decreasing)

Volume flow -pressure drop decreasing

-output increasing

-Vibration decreasing

-fineness decreasing

-pressure drop increasing

-output decreasing

-Vibration increasing

-fineness increasing

Hydraulic pressure -pressure drop decreasing

-output decreasing

-Vibration decreasing

-mill power increasing



-output increasing

-Vibration increasing

-mill power decreasing

-fineness decreasing

-Temperature decreasing -Temperature increasing Water injection

-grinding bed height

-mill power

-output

-vibration

Table speed -fineness decreasing

-output increasing

-Mill power decreasing


-output decreasing


Input -fineness decreasing

-Temperature decreasing

-output increasing




-output decreasing

-mill power decreasing

-pressure drop decreasing

Circulating air -Temperature decreasing -Temperature increasing

Dam ring height -output

-mill power

-vibration

Hot gas -Temperature increasing -Temperature decreasing

Table 6: side effects of changing parameters of grinding process of a vertical mill



7 Conclusion

There are many different mills from many different manufacturers in use at

HeidelbergCement. They grind a variety of materials like different cement types, raw-

material, slag and coal.

As both mills in Hannover and Slite show, it is adjuvant to have a tool to find optimization

potentials for improving the grinding process of a mill concerning output, quality and safety of

a mill. The optimization charts are an approach to have a procedure for an investigation or

evaluation of a vertical mill system.

The correctness of these optimization charts have to be proven on a case study.

As experiences show, there is a high acceptance for these optimization charts in the different

plants.

The examples have also shown that recording the history of the mills regarding overhaul

works, would be beneficial. This would easily show, which measures have been done to

improve a mill. This would also reveal which measures brought any enhancement and which

ones didn’t show any effect.



8 Bibliography

Brundiek, Horst Die Wälzmühle- Geschichte und heutiger Stand

Aufbereitungstechnik 30 (1989) Nr. 10

FLSmidth OK vertical roller mill

Gebrüder Pfeiffer Kompetenz in Zement

Jorgensen, MSc S.W. Cement grinding- a comparison between vertical roller mill and

ball mill

Cement international 2/2005 Vol. 3 pages 54-63

Labahn/Kohlhaas Ratgeber für Zementingenieure

6. Auflage, Bauverlag GmbH, Wiesbaden 1982

ISBN: 3-7625-2020-8

Loesche LOESCHE- Mühlen für Zement und Hüttensand

Polysius Polysius Rollenmühlen. Zur Mahlung von …

Schneider, Lohnherr,

Gudat

Rollenmühlen für große Leistungen und schwieriges Mahlgut

Verfahrenstechnik der Zementherstellung, VDZ-Kongress `85,

Fachbereich 3, S. 258-264



9 Annex

9.1 Cost calculation between vertical mill and ball mill

1. General data Cement production [t/ year] 1.200.000 Total grinding capacity [t/h] 200 Operating time [h] 6.000 Mill type Ball Mill Vertical mill 2. Investment costs Total investment costs [%] 100 101,53 3. Operating costs 3.1 Energy costs Mill drive [kW] 7.000 4.680 Mill drive [%] 100 67 Auxiliaries factor from total [%] 0,20 0,40 Auxiliary drives separator + dedusting +slag dryer [kW] 2.158 3.120 Total power [kW] 9.090 7.800 Specific energy mill [kWh/t] 35,0 23,4 Specific energy mill system [kWh/t] 45,5 39,0 Specific energy costs [%] 100 82,71 3.2 Maintenance costs incl. Grinding media and elements [%] 100 136,37 3.3 Total operating cost Specific operating costs [%] 100 99,61 4. Overview Total operating cost per year [%] 100 99,61 Total investment costs [%] 100 101,53 Table 7: cost comparison between vertical mill and ball mill



9.2 Calculating the active grinding area per time

RollerTa bdA ∗∗= π Equation 1

60nDv rT,c

π∗∗= Equation 2

60nAA rag ∗∗= Equation 3

Aa Active grinding area m²

dT Dia. Centre of the track m

bRoller Width of rollers m

vc,T Circumferential speed of table m/s

D Diameter of table m

nr Number of revolutions min-1

Ag Active grinding area pert time m²/s



9.3 Pictures of Raw mill 8 in Slite

Figure 18: changes in mill design for higher output

Left: installation of a new dynamic separator with a distribution plate Right: installation of ploughs to create a smooth grinding bed

Figure 19: mill internals

Left: dam ring Right: nozzle ring with 30° angled guiding blades

Figure 20: critical wear spots

Left: wear-repair done at the riser-duct at the top of the vertical mill Right: wear underneath the old distribution plate

Material inlet

Distribution plate

Grinding table

Plough

Plough

Dam ring

Grinding table

Nozzle ring




Figure 21: constructive modifications of the mill

Left: new material inlet for coarse material Middle: new, bigger blades for the rotary cage with support ring Right: Sealing of critical wear-spots (compare Figure 18, left picture)

Figure 22: Bad condition of the vertical mill

Left: wide gap between mill housing and rotary cage causing bad quality (top view) Right: big gaps in the mill housing (alignment of separator guiding blades) causing high false air amount

Support ring Old blades

New blades

Raw-material

Coarse-material

Mill housing

Mill housing

Rotary cage

Alignment of guiding blades

Gap



9.4 Pictures of Cement mill 11 in Hannover

Figure 23: changes of the mill design for a smoother grinding process

Left: material collection and slide after 1st and 2nd grinding roller Middle: material deflector plate in front of first grinding roller Right: Water injection

Grinding Roller Material inlet

Grinding Roller

Grinding Roller

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