Dipl.-Ing. Th. Fahrland, Dr.-Ing. K.-H. Zysk, Loesche GmbH, Duesseldorf, Germany
CEMENTS GROUND IN THE VERTICAL ROLLER MILL FULFIL THE QUALITY REQUIREMENTS OF THE MARKET IN DER VERTIKAL-WÄLZMÜHLE GEMAHLENE ZEMENTE ERFÜLLEN DIE QUALITÄTSANFORDERUNGEN DES MARKTES
4Dipl.-Ing.
Th. Fahrland, Dr.-Ing. K.-H. Zysk, Loesche GmbH, Duesseldorf, Germany
SUMMARY
ZUSAMMENFASSUNG
Cement production typically requires the grinding of three separate types of material during the process: the raw materials and coal before the kiln, and the final cement product once firing is complete and the clinker cooled. Looking back on a century or more, ball mill systems were used for all three grinding stages, but the development of more energyefficient vertical roller mills (VRMs) led to their replacement. Initially, this focused on grinding coal and the cement raw materials, with the adoption of vertical roller mills for cement product grinding – with its finer grinding requirements – coming more recently, in the late 1990s. The main reason for the delay in uptake of VRM technology for cement grinding was the concerns of producers that their product qualities would not meet market requirements, specifically in three key areas: water demand, strength development and setting times. Over the past 15 years, however, it could be demonstrated that these concerns are unfounded, and that the quality of cement obtained from VRM grinding is as good as, or in some cases better than, that produced in a ball mill. In consequence, most of the world’s major cement producers now use vertical roller mills for cement grinding with no hesitation.3
Im Zementherstellungsprozess müssen gewöhnlich drei unterschiedliche Materialien fein gemahlen werden: vor dem Zementdrehofen die Zementrohmaterialien sowie die als Brennstoff verwendete Kohle und nach dem Klinkerkühler der gebrannte Klinker zur Herstellung des Endprodukts Zement. Vor mehr als 100 Jahren wurden zur technischen Realisierung dieser drei Mahlprozesse fast ausschließlich nur Kugelmühlen eingesetzt, meist im Kreislauf mit einem Sichter, bis diese Mühlen durch die Entwicklung der energieeffizienteren Vertikal-Wälzmühlen immer mehr verdrängt wurden. Zuerst vollzog sich dieser Wandel bei der Kohlemahlung und der Rohmehlerzeugung, bis in den späten 1990er Jahren die Vertikal-Wälzmühle auch bei der Zementmahlung mit ihren spezifisch hohen Anforderungen an die Mahlfeinheit den Einzug hielt. Der Hauptgrund für die späte Akzeptanz der Vertikal-Wälzmühle bei der Zementmahlung war die verbreitete Skepsis bei den Zementherstellern, dass die Qualität des in der Vertikal-Wälzmühle hergestellten Finalprodukts Zement den Marktanforderungen nicht entsprechen könnte, speziell was die Anforderungen an den Wasserbedarf, die Festigkeitsentwicklung und die Verarbeitungszeiten betrifft. Über die letzten 15 Jahre konnte jedoch der Nachweis erbracht werden, dass die bisherigen Bedenken unbegründet waren und der in einer Vertikal-Wälzmühle erzeugte Zement hinsichtlich seiner Qualität einem Kugelmühlenzement nicht nachsteht, fallweise sogar besser sein kann. Im Ergebnis dieser Entwicklung entscheiden sich deshalb heute die meisten Zementhersteller weltweit ohne jegliche Bedenken bei der Zementmahlung für den Einsatz von Vertikal-Wälzmühlen.3
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(English text supplied by the author)
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Cements ground in the vertical roller mill fulfil the quality requirements of the market In der Vertikal-Wälzmühle gemahlene Zemente erfüllen die Qualitätsanforderungen des Marktes 1 Introduction
matically some possible alternative flowsheets using VRMs and ball mills.
There is no question that vertical roller mills like the Loesche Mill offer significant advantages over ball mills in terms of their energy efficiency. As noted in a current publication (1) the specific power consumption of a ball mill is higher than that of a vertical roller mill (VRM) carrying out the same operations by a factor of between 1.5 and 2, depending on the degree of optimisation of the ball mill. � Fig. 1 illustrates this connexion, as well as showing the increasing energy benefit that can be obtained with a vertical roller mill as the specific Blaine surface area rises. ] t 100 / h 90 W k [ 80 n o 70 i t p m 60 u s 50 n o c 40 r e 30 w o P 20
Ball mill system
With the focus here being on ball mills and VRMs, Table 1 shows some comparative performance parameters for the two systems when used for grinding cement. It has to be remembered that there are major differences in the mechanism of grinding between VRMs and ball mills, in terms of how grinding occurs, the residence time, the level of repeat grinding and recirculation factors, among others. In a VRM, comminution occurs by pressure and shear forces that are introduced via the grinding rollers. In ball mills, comminution is mainly done by impact, with the grinding balls being lifted up by the rotating shell, then dropped back onto the charge and other balls. There is some attrition as well.
Loesche VRM system
There is also a major difference in terms of the average residence time – the time the material particles remain in the mill system before they leave the classifier as product. Including both grinding in the mill body and a circulation factor, the res3 000 3 500 4 000 4 500 5 000 idence time for a VRM is less than one minute, while partiFineness acc. to Blaine [cm 2 /g] cles can remain within a ball mill system for 20 to 30 minutes. Individual cement particles will be ground from one to Figure 1: Specific power consumption of ball mill system v/s vertical three times in a VRM before being offered to the classifier, roller mill system for OPC grinding whereas repeat grinding in a ball mill is virtually uncountaFundamentally, ball mills use proportionately more energy ble because of the grinding mechanism. Finally, while a ball to produce a finer ground product than do VRMs, and while mill will have a recirculation factor of 2 to 3, this increases the energy consumption in a VRM obviously does increase between 6 to 20 for a VRM, depending on the pressure with product fineness, it does so much less rapidly. Indeed, height, the grinding tools configuration, the grindability of the while the difference in energy efficiency is significant when material and the required product fineness. The differences a VRM is used for grinding OPC, the energy benefit is even in all of these parameters are shown in � Table 1. greater in the case of blastfurnace slag which is hard to grind, as can be seen in � Fig. 2. 3 Outlining the issues Having demonstrated the energy advantage of the VRM The first modern Loesche Mill for cement and slag grinding, concept over ball milling, the remainder of this paper a mill with the designation LM 46.2+2, was sold to Taiwan’s focuses on various aspects of cement quality, with the aim Lucky Cement Corp. in 1993 and commissioned in 1994, for of putting to rest once and for all the outdated, disproven grinding cement at its Pu Shin plant. While producers were ideas that cements produced in a VRM differ markedly from initially concerned that the quality of the cement produced those produced in traditional ball mill systems. The article looks first at the various grinding systems that are avail160 able for cement producers today, then takes each of the purBall mill ] ported product quality issues in turn to show that these are t 140 / system h neither justified nor valid. W k
2 Grinding system options Today, cement producers have the option for using a range of different systems for cement grinding. A comprehensive list of all the available options would certainly include traditional ball-mill systems, high-pressure grinding rolls in every kind of design types and their various combinations with ball mills and, of course, VRMs vertical roller mills. All of these systems treat the material to be ground differently, in that the actual grinding needed to achieve the desired characteristics varies from one to the other. � Fig. 3 illustrates sche-
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[ n o i t p m u s n o c r e w o P
120 100 80 60
Loesche VRM system
40 20 3 000
4 000
5 000
6 000
Fineness acc. to Blaine [cm 2 /g]
Figure 2: Specific power consumption of ball mill system v/s vertical roller mill system for slag grinding
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Clinker
Gypsum Limestone
Clinker
Gypsum Limestone
Product
Product
Ball mill system
Vertical roller mill system
Figure 3: Alternative flow sheets using vertical roller mills and ball mills
would not meet their clients’ specifications, results from the first installations showed that the cement qualities are indeed acceptable to the market. From the late 1990s, the majority of cement producers changed their preference towards the vertical roller mill system. Table 1:
Comparative performance parameters for the two systems when used for grinding cement
Characteristics
Ball mill (closed circuit)
Vertical roller mill
Comminution by
Impact and attrition
Pressure and shear forces
20 to 30
<1
Crushings before separation
∞
1 to 3
Circulation factor
2 to 3
6 to 20
~ 50
3 to 6
Residence time [min]
Wear rate [g/t]
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Fig. 4 shows that in this case, ‘n’ for the VRM cement system is steeper than for a cement produced in a ball mill. This is caused by the higher proportion of fine (over-ground) material present in the ball mill cement, which in turn reflects the greater number of impacts and the inherent inefficiency of ball mill grinding. In more general terms, a typical particle size distribution for a ball mill system in closed-circuit operation with a highefficiency, third-generation classifier would be between 0.75 and 0.98. The equivalent would be between 0.82 and 1.05 in a Loesche vertical roller mill system. These ranges may differ when different types of laser sizers are used. The area of concern in the past was that the steeper particle size distribution for VRM cements would lead, for example, to their having higher water demand and lower early strength development. This could, of course, generate problems, especially in precast concrete manufacturing with its sophisticated production processes regarding cycle and stripping times.
The producers’ concerns were centred on three specific areas: that cement produced in a VRM would have a higher water demand when mixed to a workable paste; that the The question then has to be asked as to the reason for the setting times would differ drastically; and that the compres- different slopes in the particle size distribution diagrams. sive strength would be lower when compared to the same Firstly, and as explained above, the different grinding behavcement produced in a ball mill system. The supposed reasons iour of the vertical roller and ball mill systems means that for these concerns were, respectively: a steeper particle size particles remain in a ball mill system for about 20 to 30 mindistribution, the different particle shape produced by a VRM, utes before they leave the classifier as product. They are and a lower gypsum dehydration. In point of fact, operating impacted repeatedly during that time, with the result that experiences since then have shown conclusively that none of some particles are ground more than necessary. By conthese concerns is justified, and that the cements produced by trast, the limited amount of grinding for each particle in the grinding in a VRM meet market requirements in all respects. VRM system before classification avoids any unnecessary over-grinding.
4 Particle size distribution The particle size distribution of a cement is usually plotted on the well known RRSBdiagram. � Fig. 4 shows size distribution curves for cements produced by grinding in VRMs and in ball mills. The inclination of each curve, the slope ‘n’, is measured at the positioning parameter that represents the particle diameter at which the residue, in terms of mass, is 36.8 %. A higher ‘n-’value produces a steeper curve, whereas the lower the slope, the more fine and over-ground particles are in the finished product at a constant Blaine value, 2 measured in cm /g.
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] % s s a m [ ) x ( Q g n i s s a p f o m u S
Slope n = tan α
Position parameter
Particle size [m]
VRM System (Slope n: 0.82 to 1.05)
Ball mill system (Slope n: 0.70 to 0.98)
Figure 4: Different particle size distributions
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Grinding pressure Dam ring height Mill airflow Classifier rotor speed Table speed for very high Blai ne cements
Dam ring
Testwork carried out in 2009 at the Vicat group’s Montalieu plant in France has proved conclusively that these adjustments will deliver the required results. The study involved grinding the same cement once in a closed-circuit ball mill system and once in a grinding plant with a Loesche mill LM 53.3+3. � Fig. 7 shows some results from the project, confirming that the positioning parameter, the slope of the particle size distribution and the product fineness of both cements are in exactly the same range. Furthermore, the water demand for both cements needed to create a cement paste with a standard consistency are the same, reflecting the same slope ‘n’, positioning parameter and Blaine values. This testwork proved in a practical way that there is no difference in material properties due to the differing grinding mechanisms.
Figure 5: The operational parameters
5 Particle shape considerations
> = = p e e t S n e p o l S w o l l a h S = = <
lower
lower
higher
higher
Grinding pressure
Height of dam ring
Mill air flow
Classifier speed
higher
higher
lower
lower
Differences in particle shape have been another area in which vertical roller mill systems have been the subject of scrutiny. The suggestion was made that cement particles coming out of a ball mill are always much rounder then those coming out of a VRM, which are more shallow and elongated. This again, together with fewer fine particles, would result in increased water demand for the cement paste.
Figure 6: Simple adjustment options to achieve the desired product
The roundness or circularity of the particles can be determined by optical methods such as image analyses. Within those measurements, particle shapes are described with values of between 1 and 0. A value of 1 indicates a particle that is perfectly spherical, and as the value approaches 0, it indicates an increasingly elongated, shallow polygon.
If a similar product to one from an existing ball mill system is � Fig. 8 illustrates the particle size distribution of a cement 2 needed, however, the VRM can be adjusted to achi eve this. with a product fineness of about 4 100 g/cm acc. to Blaine. As shown in � Fig. 5, adjustments can be made to the grind- This shows that the size of the smallest particle is about ing pressure, the dam ring height, the mill airflow, the clas- 0.1 µm and the size of the largest particle is about 52 µm sifier rotor speed and, for cements with a very high Blaine for this particular cement. In general, while a wide variety of value, the table speed. � Fig. 6 shows the effects on the cements are ground to different finenesses, the largest parslope of the particle size distribution curve by adjusting each ticles are usually between 45 and 55 µm. of these parameters. In addition, 95 % of all the cement particles are below 45 µm A higher grinding pressure will result in more intense grind- in size, again depending on the final product fineness and ing with the development of more fine material, and hence the slope of the particle size distribution curve. a shallower particle size distribution. The higher the dam ring, the longer the material remains on the grinding table, Testwork undertaken by the VDZ in its role as Germany’s and the more it is ground in one cycle. This again results in Cement Research Institute compared the particle shapes the generation of more fines, and a shallower particle size produced in different types of milling systems. � Fig. 9 shows distribution. In addition, the airflow and the classifier set- a plot of the shapes of cement particles from high-pressure tings can be adjusted in order to produce the desired prod- grinding rolls, ball mills and vertical roller mills, with the paruct characteristics. ticle size being shown on the x-axis and the circularity on the y-axis [2].
Technical properties Mill
VRM
BM
LSKS
O-SEPA
3.164
3.152
Blaine
g/cm3 cm2 /g
4 258
4 095
Slope n
-
0.93
0.92
µm
11.7
12.6
%
28
28.5
Unit
Separator Density
Position parameter d' Water demand (Standard consistency)
] % s s a m [ e u d i s e R
Particle size DK [µm]
It is clear from this plot that there is no significant difference in particle shape between the cements produced in all three of these mill types, apart from at a particle size of around 58 µm, where the shapes begin to differ. However, this particle size rarely occurs in most types of cement, and where it does, it only appears in such small proportions that there is no severe influence on the water demand, strength development or other issues.
Figure 7: A particle size distribution as needed can be easily produced within a vertical roller mill system
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5%
hardening. This is because the gypsum is dried more intensively in a ball mill system, resulting in the formation of more hemihydrate and hence higher sulphate solubility.
Fineness: about 4100 Blaine
] % s s a m [ e u d i s e R
Particle size DK [µm]
52µm
Of course, commissioning any new mill requires the system to be optimised, including adjusting the amounts of gypsum and the other additives needed to achieve the required setting and strengthening behaviour. The same applies when changing from a ball mill system to a vertical roller mill system, when the operator needs to increase slightly the amo unt of gypsum put into the cement, or substitutes the gypsum to increase the solubility by hemihydrate or anhydrite.
37µm
Figure 8: Particle sizes in cements mostly below 55 µm and 95% below 45 µm
Blaine-value B 3 000 cm2 /g 1.0 0.8
y t i r a l u c r i C
0.6 0.4 0.2 0.0 0.1
7 Final remarks
VRM Roller press Ball mill
Table 2 presents a summary of the results obtained from the comparative milling tests at the Montalieu plant in France. While key features include the very similar particle size distribution, positioning parameter and the fineness acc. to Blaine, the results show that the setting times for the two cements are the same. Setting begins after about 125 minutes and stops – in both cases – after 175 minutes. In addition, the compressive strength and the strength development measured after 2, 7 and 28 days are basically exactly the same.
1
10
Particle size [m]
58 100 (VDZ 2007)
Figure 9: Similar particle shapes of cement produced on vertical roller mill systems
6 Gypsum dehydration optimisation
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Other optimisation measures include increasing the mill exit temperature and/or reducing the moisture content of the mill gas flow, both of which will enhance the drying process of the added gypsum. As summarised in Fig. 10 these are standard process-optimisation procedures that can also be used to ensure that the cements produced in vertical roller mills have comparable setting times and compressive strength development to those from ball mills. Because it is partly true that cement produced in a vertical roller mill has lower gypsum dehydration, this can be adjusted through standard process optimisation.
Gypsum is mainly added to cement to act as setting regulator, with the precise amount and type of gypsum being This proves that cements produced in a ball mill or in a verchosen in relation to its solubility and the individual clinker. tical roller mill can have the same characteristics and qualA producer will work out the correct proportion and type of ities when required to do so by local markets, particularly in sulphate to be used through product optimisation in order to terms of the water demand of the cement paste, the setfulfil optimal requirements. ting times, the compressive strengths and strength developments of mortars. The energy input into the mill and the hot gases will heat the cement, with the gypsum being partially dried and con- Today, out of the 260 Loesche mills sold around 185 are used verted to hemihydrate, so-called bassanite or plaster. Mixed worldwide for grinding cement products. On a regional basis, together with water, hemihydrate dissolves better than gyp- the largest proportion of the total is in eastern Asia, with sigsum, so the dilution is more reactive in regulating the cement nificant numbers of the machines in Europe and the Amerset. Partial dehydration is intended within the milling process, icas as well. It is also significant that around 60 % of these but if it exceeds or underruns a certain value, it can result in mills are used for grinding more than one grade of product, accelerated setting behaviour. This in turn can lead to prob- with around 40 % being used for more than three products. lems such as with the workability of the cement paste. Indeed, the flexibility of this mill system is such that today As the retention time in ball mills is 20 to 30 times greater than in vertical roller mills, the cement is exposed to the hotgas atmosphere for much longer. In addition, ball mills use much more energy than vertical roller mills in order to grind the same amount of cement, with this additional energy heating the material even further. Because of this, the particle temperatures at the exit from ball mills operated without water cooling are usually about 30 °C higher than those experienced with vertical roller mills. Therefore, if the same cement recipe is ground in both mills, paste from cement ground in a vertical roller mill will exhibit different setting behaviour and strength development during
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Increase gypsum content Add an amount of hemihydrate Add an amount of natural anhydrite Increase mill exit temperature Decrease humidity of mill gas flow resulting in Same setting behaviour, i.e. setting time and compressive strength
Figure 10: Normal optimisation process by works quality department
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Table 2:
Same cements with the same characteristics
Designation
Unit
Separator 2
Vertical roller mill
Ball mill
LSKS
O-SEPA
Fineness acc. to Blaine
cm /g
4 258
4 095
Standard consistency
%
28
28.5
Setting time, begin
min
130
130
Setting time, end
min
175
175
Compressive strength (w/c = 0.5) 2d 7d 28 d
MPa
29.8 38.9 57.1
29.9 38.6 54.1
Specific energy consump tion (measured at shaft)
kWh/t
28.6
39.7
there are several mills used to grind five or six cement products. Cement producers worldwide appreciate the higher energy efficiency of the vertical roller mill. In addition, the inherent flexibility of this mill system means that product specifications can be changed quickly and easily in contrast to a ball mill.
LITERATURE / LITERATUR
[1] Pohl, M.; Obry, C.; Zysk, K.-H.: Operating experience with a vertical roller mill for grinding blastfurnace slag and composite cements. CEMENT INTERNATIONAL 10 (2012) No. 2, p. 56–69. [2] VDZ: Activity Report 2005–2007. Ed.: Verein Deutscher Zementwerke e.V., Forschungsinstitut der Zementindustrie, Düsseldorf, 2009, p. 38.
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Loesche – worldwide presence Loesche is an export-oriented company run by the owner, which was established in 1906 in Berlin. Today the company is internationally active with subsidiaries, representatives and agencies worldwide. Our engineers are constantly developing new ideas and individual concepts for grinding technologies and preparation processes for the benefit of our customers. Their competence is mainly due to our worldwide information management. This ensures that current knowledge and developments can also be used immediately for our own projects. The services of our subsidiaries and agencies are of key importance for analysis, processing and solving specific project problems for our customers. Loesche GmbH Hansaallee 243 40549 Düsseldorf Tel. +49 - 211 - 53 53 - 0 Fax +49 - 211 - 53 53 - 500 Email:
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