1 Introduction

The specific electrical energy consumption of a cement pro-duction process is among the key figures in a cement plant as it is one of the main operating cost parameters; about 70 % of this power consumption is used for the grinding processes. The most important factors determining the specific electrical energy demand are the installed grinding and separator technologies, the cement quality and prod-uct portfolio, the environmental standards, the state of the plant and the plant operation itself. Efficient and correctly adjusted classifiers for the raw meal and cement grinding systems constitute effective means of reducing the spe-cific electrical energy demand in the cement production pro- cess. They are also crucial items of equipment for meeting market demands in terms of cement quality, whether this relates to high strength cement, high levels of additions or local constraints on the availability and properties of replace-ment materials.

The map in � Fig. 1 shows some cement producers in Europe, North Africa and the Middle East, where the TSV™ classifier has been installed recently within the framework of modernization projects and which were used for the figures in this article.

2 The TSV™ classifier

The performance of a grinding plant depends on the grind-ing mill used but also to a large extent on the capability of the classifier. The use of a classifier for powdery products with a specified grain size allows the product to be divided into fine and coarse fractions. For grinding in the cement production process the fine fraction is the finished product and the coarse fraction is returned to the grinding mill for re-processing. The evolution of the technology of classifi-ers has seen a transition from the so-called vortex effect

Getting more from the cement ball mill with the Fives FCB TSV™ 3rd generation classifier*)Effizienzsteigerung bei der Zementmahlung in der Kugelmühle durch Einsatz des Fives FCB TSV™-Hochleistungssichters der 3. Generation*)

(static classifier) to a classi-fier equipped with a rotating turbine (dynamic classifier), which at first was a small axial turbine (1st genera-tion) and by the end of this development had become a radial turbine (3rd genera-tion). The evolution of clas-sifiers has followed a policy of improving the sharpness of cut, characterized by bet-ter sorting of the fine and coarse particles, and achiev-ing a lower bypass.

The TSV™ (Turbo Sépara-teur Ventilé, i.e. air-swept turbo-separator) classifier,

developed by Fives FCB in the 1990s, is an advanced 3rd ge- neration classifier in which the separation is carried out inside the turbine blade channels (� Fig. 2). The particles there are submitted to two opposing forces: the centrifugal force due to the turbine rotation and the drag force due to the centripetal flow of gases. The size of a particle at equilibrium between the centrifugal and drag force defines the cut size. Any parti-cle that is coarser than this cut size is rejected from the turbine channel, falls by gravity into the reject cone and is directed back to the mill, while any smaller particle is drawn by the drag force towards the centre of the turbine and carried to the outside. The sharp cut of the TSV™ classifier, which is characterized by a low value of the coef-ficient of imperfection or steep slope of the sepa-ration curve, is achieved by using a patented tur-

bine blade profile (� Fig. 3). The blades are designed in such a way that the cut size is the same regardless of the posi-

Figure 1: Location map of completed TSV™ classifier projects

Figure 2: Cross-section of the Fives FCB TSVTM classifier

T

GAS

Fc

L1L2 L2 > L1

R

V = ω R

Figure 3: Patented blade profile of a TSV™ classifier

*) L. Pottier, Fives FCB, Villeneuve d’ Ascq, France

tion in the channel, i.e. the drag force equals the centrifu-gal force. The separation is therefore not limited to the rotor perimeter but extends to the entire passage between the turbine blades. This is essential to ensure optimum sharp-ness of cut under real operating conditions, which differ from the theoretical conditions. This turbine design, for example, balances the important turbulence of the flows, which are not uniform at the turbine inlet. The intrinsic performance of a classifier for a given product is determined by the Tromp curve (� Fig. 4). The parameters of this curve are the cut size (size for which the separation output is 50 %), the coefficient of imperfection = (d75 – d25) / (2 x d50), the sharpness of separation = d25/d75, the global bypass (limit point) and the maximum bypass. A 1st generation classifier has a coefficient of imperfection of around 0.5 while the values for the TSV™ classifier range from 0.3 to less than 0.2. For the Fives FCB TSV™ the bypass tends to lie between 0 and 10 %, depend-ing on the load, whereas it is commonly ranging from 20 to 70 % with a 1st generation classifier.

3 Increasing the mill output

The first objective when modernizing a grinding circuit by installing a high efficiency classifier is to increase the mill efficiency, with a consequent increase in mill through-put. Reduction of the bypass is the main means of boost-ing the mill throughput when the modernization con- sists of replacing a classifier of an older generation. The high efficiency classifier allows an increased feed of fresh material to the ball mill by avoiding the return of fine

material to the mill and corresponding overgrinding. � Fig. 5 shows the results of the increases in capacity achieved dur-ing the production of CEM I cement at different finenesses according to Blaine. The projects under consideration were either conversion of a 1st generation clas-sifier or conversion from open to closed circuit operation by installation of a TSV™ classifier. The results show that the higher the fineness of the cement produced the greater is the increase in capacity. In par-ticular, the results obtained by the pro-duction of such a high grade product as CEM I 52,5 cement show that the older generations of classifier were definitely not appropriate for production of this kind of product. In projects for conversion of 1st

generation classifiers to TSV™ classifiers it has not been necessary to replace or modify the bucket elevator carry- ing the mill discharge to the classifier inlet, confirming that the reject load is in fact reduced.

The electrical power taken by the mill remains unchanged so the specific electrical energy consumption of the grinding mill is reduced in the same ratio that the capacity increases. The grinding plant as a whole benefits greatly by this saving even if the auxiliary equipment slightly offsets the outcome.

4 Enhancing the quality

Reducing the bypass is not the only way to increase the grinding plant efficiency. Controlling the cement quality appears to be a very attractive means of optimizing the plant utilization, particularly when dealing with the production of blended cements. The TSV™ classifier allows more accurate control of the particle size distribution so its installation pro-vides the opportunity to target cement quality parameters and set points with much tighter margins. The drastic reduc-tion of coarse particles in the finished product also plays a key role in the cement quality.

There are empirical rules that link the fineness and its impact on strength, such as a certain Blaine value with respect to the 1-day strength or the effect of fineness based on the 45 µm residue on the 28-day strength. According to the empirical rule a finished product with a variation of R45 res-idue of 1% affects the 28-day compressive strength by

0.4 MPa. Although these rules are not pre-cise, or even widely supported, they give an indication of the impact of particle size distribution on strength. Some studies [2] even indicate that only the 16 to 24 µm fraction contributes to the development of strength at all ages. The correct grain size distribution is therefore the key for produc-ing a certain grade of cement.

The results of modernization projects applied to the production of blended cements (� Table 1) confirm that increas-ing the capacity of a grinding installa-tion is more attractive when the Blaine value is not the only important param- eter. By reducing the residue on a given mesh size, such as 32 µm, it is pos-

Sele

ctio

n ef

�cie

ncy

[%]

100

90

80

70

60

50

40

30

20

10

01 10 d50 150 1 000

Particle size [μm]

1st generation dynamic classi�er

2nd generation dynamic classi�er

TSVTM classi�er

Figure 4: Tromp curve of a TSV™ classifier

3 000

Bla

ine

3 100

Bla

ine 3 3

00 B

lain

e

4 500

Bla

ine

0

10

20

30

40

50

60

70

OPCOpen circuit to TSVTM

CEM I 32,51st gen. to TSVTM

CEM I 42,51st gen. to TSVTM

CEM I 52,51st gen. to TSVTM

Capa

city

incr

ease

[%]

Constant Blaine Constant strength (28d)

Figure 5: Increase in capacity achieved during production of CEM I cement

tion in the channel, i.e. the drag force equals the centrifu-gal force. The separation is therefore not limited to the rotor perimeter but extends to the entire passage between the turbine blades. This is essential to ensure optimum sharp-ness of cut under real operating conditions, which differ from the theoretical conditions. This turbine design, for example, balances the important turbulence of the flows, which are not uniform at the turbine inlet. The intrinsic performance of a classifier for a given product is determined by the Tromp curve (� Fig. 4). The parameters of this curve are the cut size (size for which the separation output is 50 %), the coefficient of imperfection = (d75 – d25) / (2 x d50), the sharpness of separation = d25/d75, the global bypass (limit point) and the maximum bypass. A 1st generation classifier has a coefficient of imperfection of around 0.5 while the values for the TSV™ classifier range from 0.3 to less than 0.2. For the Fives FCB TSV™ the bypass tends to lie between 0 and 10 %, depend-ing on the load, whereas it is commonly ranging from 20 to 70 % with a 1st generation classifier.

3 Increasing the mill output

The first objective when modernizing a grinding circuit by installing a high efficiency classifier is to increase the mill efficiency, with a consequent increase in mill through-put. Reduction of the bypass is the main means of boost-ing the mill throughput when the modernization con- sists of replacing a classifier of an older generation. The high efficiency classifier allows an increased feed of fresh material to the ball mill by avoiding the return of fine

material to the mill and corresponding overgrinding. � Fig. 5 shows the results of the increases in capacity achieved dur-ing the production of CEM I cement at different finenesses according to Blaine. The projects under consideration were either conversion of a 1st generation clas-sifier or conversion from open to closed circuit operation by installation of a TSV™ classifier. The results show that the higher the fineness of the cement produced the greater is the increase in capacity. In par-ticular, the results obtained by the pro-duction of such a high grade product as CEM I 52,5 cement show that the older generations of classifier were definitely not appropriate for production of this kind of product. In projects for conversion of 1st

generation classifiers to TSV™ classifiers it has not been necessary to replace or modify the bucket elevator carry- ing the mill discharge to the classifier inlet, confirming that the reject load is in fact reduced.

The electrical power taken by the mill remains unchanged so the specific electrical energy consumption of the grinding mill is reduced in the same ratio that the capacity increases. The grinding plant as a whole benefits greatly by this saving even if the auxiliary equipment slightly offsets the outcome.

4 Enhancing the quality

Reducing the bypass is not the only way to increase the grinding plant efficiency. Controlling the cement quality appears to be a very attractive means of optimizing the plant utilization, particularly when dealing with the production of blended cements. The TSV™ classifier allows more accurate control of the particle size distribution so its installation pro-vides the opportunity to target cement quality parameters and set points with much tighter margins. The drastic reduc-tion of coarse particles in the finished product also plays a key role in the cement quality.

There are empirical rules that link the fineness and its impact on strength, such as a certain Blaine value with respect to the 1-day strength or the effect of fineness based on the 45 µm residue on the 28-day strength. According to the empirical rule a finished product with a variation of R45 res-idue of 1% affects the 28-day compressive strength by

0.4 MPa. Although these rules are not pre-cise, or even widely supported, they give an indication of the impact of particle size distribution on strength. Some studies [2] even indicate that only the 16 to 24 µm fraction contributes to the development of strength at all ages. The correct grain size distribution is therefore the key for produc-ing a certain grade of cement.

The results of modernization projects applied to the production of blended cements (� Table 1) confirm that increas-ing the capacity of a grinding installa-tion is more attractive when the Blaine value is not the only important param- eter. By reducing the residue on a given mesh size, such as 32 µm, it is pos-

Sele

ctio

n ef

�cie

ncy

[%]

100

90

80

70

60

50

40

30

20

10

01 10 d50 150 1 000

Particle size [μm]

1st generation dynamic classi�er

2nd generation dynamic classi�er

TSVTM classi�er

Figure 4: Tromp curve of a TSV™ classifier

3 000

Bla

ine

3 100

Bla

ine 3 3

00 B

lain

e

4 500

Bla

ine

0

10

20

30

40

50

60

70

OPCOpen circuit to TSVTM

CEM I 32,51st gen. to TSVTM

CEM I 42,51st gen. to TSVTM

CEM I 52,51st gen. to TSVTM

Capa

city

incr

ease

[%]

Constant Blaine Constant strength (28d)

Figure 5: Increase in capacity achieved during production of CEM I cement

sible to maintain the strength develop-ment while lowering the set point for the Blaine value. The results obtained in an Italian cement plant (� Fig. 6) are partic-ularly revealing. The optimized set points were reached after several iterations of reducing the Blaine value and analyzing the corresponding strength development. Depending on the product this quality enhancement offers two different options: On the one hand, the reduction in Blaine value can be used to increase the mill out-put in accordance with a typical relation-ship Q = k x (Blaine)1/c, where Q is the mill throughput and c a constant factor, ranging from 1.3 to 1.6. A lower Blaine value causes a direct decrease in specific energy consumption and increases the mill throughput. On the other hand, the option of modifying the mix formulation may be preferred – instead of changing the fineness while keeping the strength constant it may be possible to increase the clinker replacement rate provided that there is sufficient margin in the range of cement standards. This is particularly interesting from the economic point of view with regard to the price of additions as compared with clinker production costs, but it is also a way of increasing cement output if the clinker production capacity has already reached its ceiling.

5 Lowering the carbon footprint

Lowering the specific energy consumption is an obvious way of reducing the carbon footprint. The CO2 emissions

Cement typ Blaine value

R 45 µm[%]

R 32 µm [%]

Change [%]

at constant Blaine value

CEM II AM P-Q 3 600 – – + 17

CEM II AL 3 520 – – + 18

at constant R 45 µm

CEM II AM P-Q – 10 – + 21

at constant strength

CEM II AS before with TSV™

––

––

11.2 6.5

–+ 26

CEM II B-LL before with TSV™

5 2004 400

––

14.1 9.4

–+ 57

Table 1: Examples of capacity increases in the production of blended cements after conversion from a 1st generation classifier to a TSV™ classifier

28 days7 days

55504540353025201510

50

2 days

Ball mill + First gen. dynamic separator: 1400 BlaineBall mill + TSVTM: 3 920 Blaine

Com

pres

sive

stre

ngth

[N/m

m2 ]

CEMENTIR TARANTO ITALYProduction of CEM II A-S 42,5 (15 % slag)

Mill output [t/h]Mill consumption [kWh/t]Blaine [cm2/g]R32µm sieve [%]Compressive strength [N/mm2] at 2 days

7 days25 days

Before

4 10011.525

37.349.3

With TSV™58

42.23 9204.925.440.854.1

Figure 6: Development of the cement compressive strength for production of a CEM II A-S 42,5 cement before and after installation of a TSV™ classifier at CEMENTIR TARANTO in Italy

per kWh vary depending on the technology used to gener-ate the electricity: coal-fired power plants emit more CO2 than nuclear power plants or hydro-electric plants. According the GHG protocol in 2009 the CO2 emission rate was 660 kg CO2e/MWh in Poland, 470 kg CO2e/MWh in Egypt and only 404 kg CO2e/MWh in Italy but it exceeded 800 kg CO2e/MWh in countries like India and South Africa. The conversion from open grinding circuit to closed circuit by installing a TSV™ classifier in an Egyptian cement plant, for example, increased the production by 30 %, representing a reduc-tion in grinding plant specific energy of about 5 kWh/t. This means a reduction in CO2 emissions of only 2.5 kg CO2e per tonne of cement.

The advantage of highly efficient classifiers in terms of CO2 abatement actually lies in the quality of the cement and the potential increase in the cement/clinker ratio. One tonne of clinker corresponds to nearly 750 kg of CO2 emis-sions so the replacement of only 1% of the clinker will save at least 6 kg CO2e per tonne of cement for CEM II A cement.

6 Final remarks

Modernization of a ball mill grinding plant with a highly effi-cient classifier will increase the production capacity and auto-matically reduce the specific electrical energy consumption of the grinding plant. However, the energy savings are not in themselves sufficient to justify the project when consid-ered from either the economic or the environmental point of view. In fact, the total cost of this kind of project becomes feasible mainly because the payback is based on the increase in capacity. The other key interest of such a modernization project is the enhancement of the quality of the cement produced.3

[1] World Business Council for Sustainable Development / International Energy Agency: Cement Technology Road-map 2009 – Carbon emissions reductions up to 2050: OECD/IEA & WBCSD 2009.

[2] Tsivilis, S.; Parissakis, G.: A Mathematical Model for the Prediction of Cement Strength, Cement and Concrete Research, Volume 25, Number 1, January 1995, pp. 9–14.

[3] Duhamel, Ph.; Cordonnier, A.; Lemaire, D.: TSV® : The high-efficiency dynamic classifier and its latest develop-ments : SRBII- Seminar, Brussels 28th, March 1996.

[4] Moir, G.: Advanced Concrete Technology, Part 1, New-man, J.; Choo B. S.; Elsevier 2003.

LITERATURE / LITERATUR

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Did we raise your interest?For more information,

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Reprint from CEMENT INTERNATIONAL 11 (2013) No. 5, pp. 54–57