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PmUan .d C em e n t AssodM1DI I l

.Research muiDeve iopmrmt .BrdJdin RDl l0T

E ffects on C em en t o f H igh

E ffic ie ncy S ep ara to rs

. f J y R a ch el/. D e tw ile r



KEYWORDS : cement, classifier, energy efficiency, finish mill, finish grinding, high efficiency, particle size distribu-

tion, separator, Tromp curve

ABSTRACT : High efficiency separators have two major effects on cements: a narrower particle size distribution and

lower milling temperatures. The narrower particle size distribution will result in higher strength for a given specific

surface, but will also increase water demand. Lower milling temperatures may obviate the need for a cement cooler

but will result in less dehydration of the gypsum. Thus it may be necessary to adjust the dosage of the gypsum.

Optimizing the dosage and degree of dehydration of the gypsum will mitigate the effects of the particle size distribu-tion on water demand.

In order to obtain the greatest benefit from a high efficiency separator the operation parameters of the mill circuit

should first be optimized. Tromp curves of the separator feed, fines, and tailings will show whether a high efficiency

separator could provide any benefit. The entire mill circuit must be considered. A high efficiency separator requires a

much greater supply of ambient air, which will generally necessitate additional ducts and fans as well as modifica-

tions to the dust collection system. Modifications to the mill may be beneficial with or without a high efficiency

separator. After the equipment is installed, the feed rate and other milling conditions should be optimized.

The operating conditions that produce the best possible particle size distribution and surface area for the desired

strength gain and water demand must be determined by trial and error. The dosage of gypsum must also be opti-

mized. Itmay be necessary to operate at less than maximum efficiency in order to produce a cement that is acceptable

to the user.

REFERENCE : Detwiler, Rachel J . , E ffe cts o n C eme nt o f H ig h E f fic ie nc y S ep ara to rs, Research and Development Bulletin

RDllOT, Portland Cement Association, Skokie, Illinois, U.S.A., 1995.

MO TS C LE S: broyage final, broyeur de finition, ciment, classificateur, courbes de Tromp, distribution de la taille des

particules, efficacite energique, haute efficacite, separateur

RESUME : Les separateurs a haute efficacite ont deux effets significatifs sur les ciments: une distribution plus etroite

de la taille des particules et des temperatures de mouture plus faibles. Pour une surface specifique donnee, la distribu-

tion plus etroite de la taille des particules a pour effet d' augmenter la resistance mais augmente aussi la demande en

eau. Les temperatures de mouture plus faibles peuvent satisfaire a la demande de ciments plus froids mais il en resulte

une diminution de la deshydratation du gypse. II pourrait ainsi etre necessaire d'ajuster le dosage en gypse.

L'optimisation du dosage et du degre de deshydratation du gypse attenuera toutefois les effets du changement dans la

distribution des particules sur la demande en eau.

Dans Iebut de tire r le plus grand benefice de l'installation d'un separateur a haute efficaci te, ilfaut tout d'abord

optimiser les parametres d/operation du circuit de mouture. En utilisant les courbes de Tromp pour l'alimentation du

separateur, les fins et les residus, on pourra determiner si un separateur a haute efficacite peut apporter une

amelioration. L'ensemble du circuit de mouture doit e tr e c on s id e re . Un separa t eur a haute e f fi cac it e neces si te un

apport d'air ambiant beaucoup plus important qui peut conduire a l 'ajout de gaines de ventilation et de ventilateurs,

aussi bien qu'a des modifications du systeme de depoussierage. Les modifications au broyeur peuvent etre benefiques

que l'on installe ou non un separateur a haute efficaci te. Une fois cet e qu ip em e n t in sta lle , le taux d'alimentation et les

autres parametres du broyage doivent etre optimises.

C'est uniquement par iteration que l'on peut determiner les conditions d'operation qui produiront les meilleures

distributions des particules et surfaces specifiques pour le gain de resistance et la demande en eau desires. Le dosage

en gypse do it aussi etre optimise. II peut aussi etre necessaire d'operer a moindre efficacite dans le but de produire unciment qui soit acceptable pour l'utilisateur.

REFERENCE : Detwiler, Rachel J . , E ff ec ts o n C eme nt o f H i gh E ffic ie nc y S ep ar ato rs , Research and Development Bulletin

RDllOT, Portland Cement Association, [Effets des separateurs a haute efficacite sur Ie ciment, Bulletin de Rechercheet Developpement RDllOT, Association du Ciment Portland], Skokie, Illinois, U.S.A., 1995.

Cover illustratons:Top Left: Particle size analysis by sedimentation. A finely collimated beam of low energy X-rays

and a detector are used to determine the distribution of particle sizes in a cell containing a

sedimentation liquid. Courtesy of Micromerit ics Instrument Corp. , Norcross, Georgia, U.s.A.

Right: High efficiency separator. Courtesy of ABBRaymond, Lisle, Illinois, U.S.A.

Bottom Left: Tromp curve after the installation of a high efficiency separator and optimization of

feed rate. Adapted from Kohan+' Used by permission.

PCA R&D Serial No. 2020



P CA R es ea rc h an d D ev elo pm en t B ulle tin R 011 0

EFFECTS ON CEMENTOF H IGH EFF IC IENCY SEPARATORS

CHAPTER 1

Introduction

The advent of high efficiency separa-

tors in the early 1980's promised sig-

nificantenergy savings, increased pro-

duction capacity, and improved ce-

ment quality. However, the results

obtained in the operation of cement

plants were mixed. This report is a

state-of-the-art review of high effi-

ciencyseparators: the technology and

its application in cement finish grind-

ing mills.

Chapter 2 of this report discusses

how high efficiency separators work

and their advantages over conven-

tional separators. The most signifi-

cant of these are substantial reduc-

tions in power consumption and in-

creases in production rates. Other

advantages include ease of operation

and maintenance of the equipment

and cement that is less prone to false

set or pack set.Chapter 3 discusses the effects of

high efficiency separators on cement.

Since separa tors increase milling effi-

ciency by removing acceptable prod-

uct from the tailings so that the ce-

ment is not over ground, the perfor-

mance of the separator is judged on

the sharpness of the separation. An

ideal separator gives a vertical Tromp

ISBl\' 0-89312-132-0

© Portland Cement Association 1995

b y R ach el J . D etw ile r*

curve, and a real high efficiency sepa-

rator gives a much steeper Tromp

curve than a conventional separator,

the less efficient packing of the par-

ticles. However, the most significant

influence on water demand is the

5 10 20 30 4050 100 200

Part ic le S iz e , m i crons

F ig ure 1.1 . T rom p curves for conve ntiona l a ir sep ara tors , a h ig h

e fficie ncy s ep ara tor, a nd a n ide al s ep ara tor [D uda 1 .1 ].

as illustrated in Figure 1.1. Because

more of the particles are in the 3-30 u r nsize range, the specific surface of the

cement can be reduced by 100 cm2/ g

ormore while maintaining comparable

strengths at all ages (1-28 days). The

water demand of the cement will in-

crease somewhat because of the in-

creased influence of surface forces and

chemical effect of grinding the C3A so

as to expose more surface. This par-

" Senior Engineer, Construction

Technology Laboratories, Inc.,

5420Old Orchard Road, Skokie,

Illinois 60077-1083. Phone: 708-

965-7500, Fax 708-965-6541.

1



E f f e c ts o n C e m e n t o f H ig h E f f i c ie n cy S e p a ra to r s

ticu la r e ffe ct ca n be m itiga te d by a d-

justin g the gypsum add ition to the

f in i sh mi l l.

In a high e fficien cy se pa ra tor

the re is n o re circula tio n of a ir flow .

T his re su lt s in low er t em p e ra tu re s fo r

b ot h t he p ro du ct a nd th e t ailin gs. T he

low er prod uct te m pe ra ture m ay ob-

v ia te th e n ee d fo r a c em e n t co ole r a nd

red uces the risk of pack se t. The

l owe r t a ili ng s t emp e ra t ur e w i ll r es ult

in a low er m illin g te m pe ra ture . T his

m a ke s the m illin g op era tio n m o re e f-

ficie nt b y re du cin g the like lihoo d o f

c oa tin g o f t he b alls , b ut it a ls o re su lt s

in le ss d ehyd ra tion of the gypsum .

T hu s it m a y b e n ece ssa ry t o a d ju st th e

g yp su m d osa ge t o e n su re t ha t t he re is

s uf fic ie n t 5 03in solutio n to con trol

f la sh s et o r e a rly s tiffe n in g b eh av io r.In s om e in st alla tio n s, m e a su re s w e re

t ak en t o in cr ea se t he m i llin g t em p e ra -

tu re in ord er to in cre ase the d eg re e of

d eh yd ra tio n o f t he g yp su m .

C hapte r 4 d iscusse s how to ob-

ta in the m o st be ne fit from a h ig h e ffi-

cie ncy se pa ra tor. T he first ste p is to

d ete rm in e w he th er it is w orthw hile

t o p urc ha se a h ig h e ffic ie n cy s ep ar a-

to r a t a ll. A ca re fu l e va lua tion o f t he

2

e xistin g m ill circu it w hich ha s be en

a djuste d for o ptim u m p erform a nce

w ill s how t he p ot en tia l b en e fit , if a n y.

Sin ce a high e fficie ncy se pa ra tor is

d esign ed to red uce or e lim in ate by-

p a ss ( th e i nc lu sio n o f a c ce p ta b le p ro d -

uct in the ta ilings), the am oun t of

byp ass in th e cu rre nt syste m shou ld

be d e te rm in ed from a Trom p curve .

If it is le ss th an a bo ut 2 0% , e ffo rts to

im prove the perform a nce of the m ill

c irc uit sh ou ld fo cu s o n th e m i ll. Itis

im p ort an t to n ot e t ha t o pe ra tin g w it h

a lo w c ircu la tin g lo ad o r g rin d in g t o a

low specific surfa ce w ill m ake the

system appear m ore e fficien t. The

sta rtin g poin t for the pe rform a nce

eva lua tion m ust be a sa tisfa ctory

p ro du ct a t a sa tisfa cto ry p ro du ct io n

rate .F or system s tha t w ould ben efit

fro m a h ig h e fficie nc y se pa ra to r, t he

n ex t ste p is to se le ct a se pa ra to r o f t he

right capacity. Be fore an d d uring

in st alla tio n , t he wh ole c irc uit m u st b e

c on sid e re d . S in ce t he h ig h e ffic ie n cy

s ep ara to r d oe s n o t re circ ula te a ir, s uf-

ficie nt fre sh a ir m ust be supplied to

the se pa ra to r for p rop er o pe ra tion .

Ad d ition a l ducts an d fan s m ay be

require d . The in crea sed volum e of

d ust -la de n a ir fro m t he se pa ra to r m a y

n ece ssit ate m o difica tio ns t o th e d ust

co lle ct io n sy st em . M od ific at io ns t o

the m ill such a s the in sta lla tion of a

cla ssifyin g lin er a nd a d ia ph ra gm to

c on tro l th e flo w o f m a te ria l b et we en

c ha m be rs a re o fte n b en eficia l w ith o r

w it ho ut a h ig h e fficie nc y se pa ra to r.

T he ba ll cha rge an d gra din g should

a lso b e a dju ste d to suit the n ew m ill-

in g c on d it io ns. O n ce th e e qu ip m en t

is in sta lled , the fe ed ra te an d othe r

m illin g con dition s should be opti-

m i z ed .

C h ap te r 5 g iv es t he co nc lu sio ns

a n d r ec omm en d a tio n s. Itf oc us es o n

a pplica tion of the kn ow le dg e tha t is

a lre a dy a va ila ble , b ut a ls o g iv es s om e

s ug ge st io n s f or fu rt he r r es ea rc h.

R E F E R E N C E

1.1 D ud a, W alte r, H ., C e m e n t - D a t a -

B o o k , V o lu m e 1: I n t e r n a t i o n a l

P r o c e s s E n g i n e e r in g , 3 rd edi tion,

B a uv erla g GMBG, W ie sb ad en

a nd B erlin , 1 98 5.



PCA R es ea rc h a nd De ve lo pm e nt B ulle tin RD II 0

CHAPTER 2

H ig h E fficie ncy S e pa ra tors

Figure 2.1 isa schematic drawing ofa

conventional air separator. Accord-

ing to Duda." in a conventional air

separator the material isfed from thetop onto a rotating distribution plate,

which disperses the material into the

separation zone. Most conventional

separators generate circulating air in-

ternally with a fan system. Thus a

particle of material is subjected to

three forces: (1)centrifugalforce from

the distribution plate, (2) uplift from

the air current, and (3) gravity. The

magnitude of the uplift force is pro-

portional to thesurface area exposed,

or the square ofthe mean dimension

of the particle. The magnitudes of

both thecentrifugalforceand thegrav-

ity force are proportional to the mass

and thus to the cube of the mean

dimension of the particle. With in-

creasing particle size, the centrifugal

and gravity forcesincreasefaster than

the uplift force. Heavier and larger

particles are thrown farther outwards

and settle by the action of gravity. If

they hit the separator wall, the wall

effectforces them down into the in-

ner or tailings cone. Finer particlesare lifted by the air current and pass

between the blades of the auxiliary

and main fans into the outer or fines

cone. The main fan moves the air

from the fines cone into the separa-

tion zone, where the separation of the Tails

fines from the air is accomplished by Product

the reduction in air velocity andchange in direction of the flow. The Figure 2.1. S che matic draw ing o f a c on ve ntio nal air se parato r [D unn 2' 2 ] .

CONVENT IONAL

SEPARATORS

smallest particles are slow to descend;

thus a portion of the fines is always

circulating, and somebecome trapped

in the tailings.

Dunn-" describes two problems,

both forms of bypass, that limit the

efficiencyofconventional separators.

The first is the recirculation of fines

Co nv e ntio na l A ir S e pa ra to r

Feed AirflowFeed

Tails

Product

3



E f fe c t s o n C e m en t o f H ig h E f f i c ie n c y S ep a r at o rs

back into the separation zone. The

secondis the spillage offeed from the

distributor plate directly into the tail-

ings. Spillageisparticularly common

at high production rates when mate-

rial dispersion is not adequate and

thefeedpours offthe distributor plate

directly into the tailings with no

chance for the air current to carry it

upward. The smaller a separator's

bypass, the better its performance.

Herrmanrr" statestheproblem some-

what differently: the conventional

separator "does not provide clean

material separation and the fines dis-

charge ... is incomplete, which re-

sults in re-entrainrnent of fines."

Duda-' alludes to another disad-

vantage of conventional separators:

the difficulty ofadjusting the particlesizedistribution ofthe finished prod-

uct. Four types of adjustment are

possible:

• The auxiliary fan acts against

the intake air current from the

main fan;thiscounteraction can

be controlled by varying the

number of blades in the auxil-

iary fan. A large number of

blades chokesthe upcoming air

current more than a small num-

ber of blades. Making this ad-

justment requires that the fin-

ish mill be shut down.

• The main fan can also be ad-

justed by changing the posi-

tion of the blades, again while

the finish mill is shut down.

The maximum outward posi-

tion of the blades with the air

control valves wide open in-

creases separator capacity but

produces a coarser product.

• The horizontal control valves

allow fine adjustment withoutshutting down the grinding op-

eration byvarying thecross sec-

tion of the ascending air cur-

rent.

• Therotation speed ofthe distri-

bution plate can be adjusted.

However, since a single shaft

drives the rotation ofthe distri-

bution plate and both main and

4

auxiliary fans, any adjustment

of the rotation speed will ne-

cessitate adjustments to the

blades ofboth fans. Increasing

the fineness of the product by

any means reduces the capac-

ity of the separator.

to Dunn.-' II the primary goal of a

high efficiencyseparator is to reduce

or eliminatebypass." Various manu-

facturers of high efficiency separa-

torshavedeveloped differentdesigns,

but essentially they have made three

improvements over conventional

separator design intended either to

reduce bypass or to facilitate adjust-

ment and maintenance. The firstwas

toimprove dispersion ofthe feedma-

terialinto theseparation zone. Dunn"

states that IIthe key togood classifica-

tion isto get the particles dispersed in

H IGH E F FIC IE NCY

SEPARATORS

Figure 2.2is a schematicdrawing ofa

high efficiencyseparator. According

H ig h E ffic ie ncy S e pa ra to r

Feed ----j

,

---- A irflow

------- Feed

- - - - - - Ta ils----- Product

Airf low,Tails

Figure 2.2. S ch em a tic dra w ing of a h ig h e fficie ncy s ep ara tor [Dunn2 .2 ].



the air vortex so that the centrifugal

anddrag forcescan interact effectively

andperform the classifying function."

Agglomeration of fine particles or ex-

cessfeedmaterial that cannot be prop-

erly dispersed will result in bypass,

the inclusion ofacceptable product in

the separator tailings. Schonbach-"

identifies the control of particle mo-

tion asessential tothe improvement of

separator performance. Limiting the

random motion of particles allows

more fines to be removed from the

mill,thus reducing thecirculating load.

Theinternal configuration of the sepa-

rator also helps to guide the circula-

tion of the air current. Duda-' de-

scribeshow somemanufacturers have

installed stationary guide vanes for

this purpose.According to Dunn." the second

design improvement was the elimina-

tion of airflow recirculation. Instead,

all of the air flow is vented from the

separator. Dunn+discusses the situa-

tion in which so much feed material

flows into the separator that further

increases in the feed rate do not in-

crease the rate of production of fines.

Instead the production of tailings in-

creasesby the sameamount as the feed

rate. In this case there are so manyparticles in the separation zone that

they interfere with one another, pre-

venting efficient classification. Byre-

moving the fines from the air stream

before any recycling of the air takes

place, high efficiency separators re-

duce the dust loading of the mill com-

pared with that associated with con-

ventional separators."

The third category of design im-

provement deals with the operation

and maintenance of the mill circuit.

High efficiency separators generally

have the fans outside the separator

itself. The airflow can be adjusted for

"fine tuning" the particle size distri-

butionwithout shutting down the mill

operation, since it is not necessary to

add or remove fan blades. In conven-

tional separators all of the rotating

parts - distribution plate and main

and auxiliaryfans-are usually driven

peA Res ea rc h a nd Dev elo pm en t B ulle tin RO llO

by a common shaft, so their speeds

cannot be independently adjusted. In

high efficiency separators the fans

supplying the airfor the separator are

controlled individually, so it is pos-

sible to adjust the motor speeds to

control the airflow. Various manu-

facturers have also improved their

designs in other ways to reduce wear

and simpHfy the replacement ofparts

(Klumpar and Zoubov-")or to reduce

the overall size of the separator

(Duda,2-1Polsberg").

P OT EN TIA L B EN EF IT S O F

H IGH E F F IC IE NCY

SEPARATORS

R e duce d S pe cific P ow e rConsumpt ion

High efficiency separators were de-

veloped to reduce electrical energy

consumption. According to Scheuer

and Ellerbrock." between 1960 and

1990 the fuel energy required to pro-

duce a tonne of cement in western

Germany dropped from 4. 8 GJto 3 .0

GJ. However, from 1960 to 1985 the

specific electrical energy consump-

tionincreased from 80kWhl tcement

to 110kWhlt cement for a variety of

reasons. These include environmen-

talprotection, measures for reducing

fuelconsumption, switchingfromfuel

oil to coal (which must be ground

before use), automated operation of

cementplants, and theincreased fine-

ness ofmodern cements.

Scheuer and Ellerbrock?"goon to

say that in cement production, com-

minution consumes the most power,

with raw material grinding respon-

sible for 24% and finish grinding for

38% ofthe total electrical energy con-

sumption. The use of a high effi-

ciency separator removes fines from

the grinding circuit, thus relieving

the mill ofthat portion of the cement

which is already fine enough. The

consequent energy saving is greater

the more sharply the separator di-

vides the coarse fraction from the

fines. Ellerbrock and Schiller-'? state

that the design and mode of opera-

tion ofthe separator playa major role

in determining the energy consump-

tion of the grinding plant. Minimiz-

ing the fraction of material that is

carried unclassified into the tailings

minimizes the energy consumption

for two reasons. The first isthat once

the cement is fine enough, further

comminution simply wastes energy.

The second is that, as Kellett-" ex-

plains, thefineparticles promote coat-

ing of the balls, producing a cushion-

ing effect during grinding.

Kuhlmann's':" model analysis shows

that the efficiency of energy utiliza-

tion in grinding could be improved

from the present 24-31 cm2 / J to as

much as 50 cm2/J for cement of the

same class i f t he ma te ri al i s c omp le te ly

sep ara ted in th e cla ssifier [emphasis

added].

Hanke et a1..13 report that the spe-

cific power consumption of a finish

grinding system with low mill vent

air volume and a conventional air

separator is 20-27% higher than that

of a system with high mill vent air

volume, static separator and high ef-

ficiency separator. In a study for the

U.S. Department of Energy,Helmuth-" determined that control

of cement particle size distribution

could reduce grinding energy con-

sumption by 27% .

Brugan-" states that, on average,

the installation of a high efficiency

separator reduces the specificpower

consumption in finish grinding by

20-30% while increasing the capacity

by about 30%. Kellett and Rock2.16

found that by replacing a conven-

tional separator with ahigh efficiency

separator and installing a recycle air

duct, they were able to increase

throughput by 15% and reduce spe-

cificpower consumption by the same

amount. Kershaw and Yardi-" re-

port productivity increases up to 13%

and power reductions of 12 % after

replacement of mechanical air sepa-

rators with high efficiency separa-

tors. Nyman-" found that the grind-

5



E f f e c t s o n C e m e n t o f H i g h E f f i c ie n c y S e p a ra to rs

ing capacity of closed circuit mills

increasedby 15-25%whilepower con-

sumption decreased by at least 8%.

He also found additional benefits as-

sociated with the increased capacity:

rationalization of the mill operation,

decreased wear on the grinding me-

dia and linings, and easier mainte-

nance.

F urth e r Mo dific atio ns to

th e F inish M ill

Often the installation of a high effi-

ciency separator is accompanied by

additional modifications to the finish

grinding circuit. In this case further

improvements are possible. von

Seebach and Schneider+" report that

a capacity increase of 68% and an

energy saving of 25%were achieved

by making several modifications to a

mill circuit in addition to the installa-

tion ofa high efficiencyseparator: an

adjustable diaphragm for theballmill

to control material transfer from the

first to the second compartment, a

static separator for the separation of

product from the mill vent air, and a

high pressure grinding roll. Henz2.20

cites a different example of a modifi-

cation in which the tube mill wasfittedwith an intermediate diaphragm

and a high efficiency separator re-

placed a conventional separator. In

this case there was a 30%increase in

throughput and a 17% reduction in

specific power consumption.

Chesley-" states that the installation

of a high efficiency separator pro-

duces the most significant impact on

both increased production and re-

duced energy consumption of any

modification ofthe mill circuit exceptthe installation of a twin roll press.

He also points out that the greatest

improvement inproductionrates and

energy use results from coupling the

installation of a high efficiency sepa-

rator with modifications to the mill

internals. Schmidt-" reports that in-

stallation of a high efficiency separa-

tor and a new mill lining, along with

6

a modification of the mill ball charge

reduced the specificpower consump-

tionby20%.

Con trolle d P a rticle S iz e

D is trib u tio n (CPSD )

As stated above, the greatest reduc-

tions in specific power consumption

result from adjusting the separator

forthenarrowest possibleparticle size

distribution. Helmuth+" indicates

that in normally ground cements, 20-

40% of the cement is not utilized for

strength development in concrete.

The largest particles cannot hydrate

completely, and the smallest particles

may formaggregations which remain

as highly porous regions, thus limit-

ing the ultimate strength. Control-

ling the particle sizedistribution soas

to limit the number ofextremely fine

and extremely coarseparticles should

result in a more efficient use of the

cement, thus adding to the energy

savings.

A number of studies bear this

out. Onuma-P found that the specific

surface can be reduced by 160cm2/ g

without affecting the cement quality.

Comparing on the basis of constant

cement quality, the production rateincreased by 19%and specific energy

consumption decreased by 8.4kWh/

t. Henz2.24 reported that for cements

having finenesses between 3000and

4000Blaine, the specific surface can

be reduced by about 250cm2/ gwhile

still producing cement of the same

strength as before. He anticipated

that the reduction in surface area

would increase production capacity

by an additional 10-15%. Hanke et

a1.2-13state that for the same strength

development a Type I cement can be

produced with approximately 150-

200cm2/ glower specificsurfacewhen

a high efficiency separator is used.Herrmann" found that for cements

offineness ranging from 3500to4000

cm2/ g,the power consumption ofthe

grinding circuit was reduced by 10-

15%, including the benefit from re-

ducing the cement fineness to pro-

duce the same strength asbefore. He

also pointed out that because a given

mill will often be modified in other

ways when the high efficiency sepa-

rator is installed, the total capacity

increasewill vary frommill tomill. In1987,Weiss and Tresouthick+" esti-

mated the energy savings possible for

various modifications to then-current

U.S.practice in cement grinding (i.e.

primarily closed-circuit ball mills).

Their data are given in Table 2.1.

Mi ll ing Tempe ra ture

Because the high efficiencyseparator

does not recirculate air, its air supply

is likely to be cool ambient air ratherthan hot recycle air. Thus the tem-

perature of the cement is reduced.

For this reason, in some cases the

installation of a high efficiencysepa-

rator may obviate the need for a ce-

ment cooler. Ito et a1.2-26emphasize

the importance of maintaining the

temperature ofboth themilldischarge

T ab le 2.1 Pote ntia l E ne rg y S av ing s in U .S . C em ent G rinding

Method % Energy Savings Total, kWh/year

High efficiency classifier only 15 0.5x 109

Changes in mill configuration only 10 0.3 x 109

Both 1 and 2 27 0.9x 109

Lower Blaine and CPSD 10 0.3 x 109

All of the above 39 1.3x 109



and the finished cement in order to

produce a cement ofhigh quality. They

found that the temperature of the mill

discharge was reduced by20-30°C, and

that of the product by 30-35°C, with


separator. Onuma-" found that the

cementtemperature decreased by 40°C

when a high efficiency separator was

installed. Currier-Flists several ben-

efits in addition to cooling the cement:

(1) gypsum dehydration (to hemihy-

drate), which can cause false set, is

eliminated; (2 ) the separator is auto-

maticallyvented; (3)when a separator

is shut down for internal adjustment

or maintenance,it can be cooled within

minutes by leaving the dust collector

fan on; and (4) the tailings are also

cooled, contributing to lower millgrinding temperatures. Lowering the

temperature inside the mill improves

the grinding efficiency.

Ease ofoperation and maintenance

as described in the previous section

can also be considered among the ben-

efits of high efficiency separators.

REFERENCES

2.1 Duda, Walter H., Cement-Data-

Book ,Volume 1: I n te rna tiona l Pro -c es s En g i ne er in g , 3rd edition,

Bauverlag GMBG, Wiesbaden

and Berlin, 1985.

2.2 Dunn, Mark R, ''Understand-

ing an Air Separator," Portland

Cement Association Mill Grind-

ing Short Course, Skokie, Illi-

nois, 1-5 November 1993.

2.3 Herrmann, Christian., "In-

creased Cement Grinding Effi-

ciency by Using High Efficiency

Separators," presented at the

27th IEEE Cement Industry

Technical Conference, New Or-

leans, Louisiana, February 1985.

2.4 Dunn, Mark R, "What is Clas-

sifier Efficiency?" Portland Ce-

ment Association Mill Grinding

Short Course, Skokie, Illinois,

November IS, 1993.

PCA Re se ar ch a n d De ve lo pmen t Bu ll et in RD ll0

2.5 Schonbach, Bernard H., "High

efficiency separators in roller

mills," Wo rld C em en t, vol. 19,

no. l1,November1988, pp.436-

444.

2.6 Dunn, Mark R., Personal com-

munication, August IS, 1994.

2.7 Klumpar, Ivan V.,andZoubov,

Nicholas N., "New Sturtevant

high efficiency SD classifier at

Keystone Cement," W orld C e-

ment , vol. 16,no. 8,October1985,

pp. 302-307.

2.8 Folsberg,J., "A new generation

ofhigh efficiency separators for

ball mills and roller presses,"

Zement-Kalk-Gips , vol. 44, no. I,

January 1991, pp. 37-41.

2.9 Scheuer, A., and Ellerbrock,

H.-G., "M6glichkeiten derEnergieinsparung der Zement-

herstellung," Zement-Kalk-Gips ,

vol. 45, no. 5, May 1992, pp.

222-230. (English translation in

vol. 45, no. 7.)

2.10 Ellerbrock, H.-G., and Schiller,

B., "Energieaufwand zum

Mahlen von Zement," Z emen i -

Kalk-Gips , vol. 41, no. 2, Febru-

ary 1988, pp. 57-63. (English

translation in vol. 41, no. 4.)

2.11 Kellett, Charles D., "Lehigh's

Experiences with a High Effi-

ciency Separator," Proceedings ,

2 1s t I nte rn at io na l C em en t S emi -

nar ,1985RockProduc ts , pp.197-

230.

2.12 Kuhlmann, K., "Energie-

aufwand beim Mahlen von

Zement in Kugelmiihlen -

Ergebnisse einer Bilanzierung

des Mahlkreislaufs," Zemeni-

Kalk-Gips , vol. 37, no. 8,August

1984, pp. 421-426. (English

translation in vol. 37, no. 10.)2.13 Hanke,H.,Kochmann,H.J.,and

von Seebach, HM., "The Ap-

plication of High Efficiency

Separators in Finish Grinding

Systems," presented at the IEEE

Cement Industry Technical

Conference, Anaheim, Califor-

nia, May 1984.

2.14 Helmuth,RA.,"ImprovedCe-

ment and Energy Savings with

Particle Size Control," pre-

sented at the IEEE Cement In-

dustry Technical Conference,

May 10-14, 1981, Lancaster,

Pennsylvania, Portland Ce-

ment Association report serial

number 1671.

2.15 Brugan,J.M., "High Efficiency

Separators- Problems and So-

lutions," Zement-Kalk-Gips , vol.

41,no. 7,July1988, pp. 350-355.

2.16 Kellet, C.D., and Rock, H.G.,

"Betriebserfahrungen mit dem

O-Sepa Windsichter," Zemeni-

Kalk-Gips , vol. 39, no. 6, June

1986, pp. 312-316. (English


2.17 Kershaw, M., and Yardi, J.,"Analysis ofO-Sepa separators

at Blue Circle, Australia," World

Cement , vol. 20, no. 11, Novem-

ber 1989, pp. 400-405.

2.18 Nyman,K.-E., "Improvements

in cement grinding with Larox

EC-90classifier," Wor ld C ement

Technology, vol. 11, no. 8, Octo-

ber 1980, pp. 407-417.

2.19 von Seebach,M., and Schneider,

L ., "Update on finish grinding

with improved energy effi-

ciency," Wo rld C em en t, vol. 17,

no. 8, October 1986, pp. 336-

346.

2.20 Henz, F., "Optimierung einer

Zementmahlanlage durch

Einbau eines trennscharfen

Sichters mit einfacher Luft-

fuhring - Betriebserfahrun-

gen," Zement-Kalk-Gips , vol. 39,

no.e.Iune 1986. (English trans-

lation in vol. 39, no. 8.)

2.21 Chesley.j.A; A F in ish M i ll S ur-

veyo f theU.S.andCanadianPor t-l an dC emen t I nd u st ry , 1 9 8B, Port-

land Cement Association

Research and Development

Restricted Internal Report

IR002.0lT, July 1988.

7



E f f ec ts o n Cement o f H i gh E f fi ci en cy S e pa ra to rs

2.22 Schmidt, D., "Hochleistungs-

Sichter SEPOL-Erfahrungen

und Betriebsergebnisse im

Zementwerk Hardegsen,"

Zement-Kalk-Gips , vol. 41, no.

10, October 1988, pp. 506-510.

2.23 Onuma, E., "ANew High-Effi-

ciency Classifier as Applied totheCement Industry," Proceed-

in gs, 1 9th In te rn atio na l C e men t

Seminar, 1983, Rock Products,

pp.4O-44.

8

2.24 Henz, Fritz., "Upgrading of a

Finish Mill Circuit with a High

Efficiency Single Pass Separa-

tor," Pr oc ee di ng s , 2 1 st I nt er na -

tional C em ent S em inar, 1985 ,

Rock Products, pp. 243-262.

2.25 Weiss, Stuart J ., and

Tresouthick, Stewart W., E n-erg y S av in gs by Im pro ve d C o n-

trol of the F in ish G rinding P ro-

cess in C em ent M anufacture,

DOE/CS/40419-Tl, March

1987.

2.26 Ito, M., Misaka, T., Furukawa,

T., Sota, Y., and Onuma, E.,

"Cooling effect of the O-sepa

air separator in cement grind-

ing," Zement-Kalk-Gips , vol. 41,

no. 5,May 1988,pp. 214-223.

2.27 Currier,Fred, IIAirSeparators,"

Mill Paper No.4.



peA R e se a r c h a nd D e ve lo pm e n t B u l l e ti n RDll0

CHAPTER 3

Effe ct of H igh E fficie ncy Sepa ra tors on C em ent

P AR TIC LE S IZ E

D ISTR IBUT ION

Rosin-Rammler -Sper l ing-

B e nn ett (R R SB ) D ia gra m s

One way to d epict the pa rticle size

d istrib ution of g ra nula r m a te ria ls is

by use of the Rosin -Ram mle r-

S pe rlin g-B en n ett (RRSB) d ia gra m .

Osbeeck-' e xpre sse s th e e qu ation a s:

R(d) = 100 x exp t- d /d ' )"

w he re R (d ) is the p erce nta ge b y m a ss

o f pa rtic le s w ith d iam e te rs la rg e r th an

d . The position pa ram ete r d ' corre -

sp on ds to th e d ia m e te r w ith a re sid ue

of 36.8% a nd in dica te s the fin en essle vel of the d istribution . The e xpo-

nen t n is a m easure of the spread or

d is pe rs io n o f t he d is tr ib ut io n . F ig ur e

3 .1 s hows a n id e a li ze d RRSBd ia g ram .

Kuhlm ann e t a1.3.2 expla in tha t the

grea te r the slope , the na rrower the

d is tr ib ut io n . F or a g iv en s pe cific s ur -

face , cem en ts w ith a na rrower pa r-

ticle size d istrib utio n h av e a sm a lle r

po sition p ara m ete r a nd th us a h ig he r

proportion of fin e pa rticles. The

stren gth in crea se of cem en ts w ith

e qua l sp ecific su rfa ce a nd p rog re s-

s iv e ly n a rr owe r p a rt ic le s iz e d is tr ibu -

tion is due to an increa se in the pro-

portion of com p le te ly hy dra te d fin e

pa rticles in con sequen ce of the de -

creasing position pa ram ete r. For

e qu al v alu es o f d ', th e 2 -d ay com pre s-

s iv e s tr en g th d e cr ea se s w it h n a rrow -

in g o f the pa rticle size d istrib utio n

be ca use the d ecre ase in the propor-

tio n o f c om p le te ly h yd ra te d fin e p ar-

ticle s w ith in cre asin g n e xce ed s the

in cre ase in th e propo rtio n o f in cip i-

e n tly h yd ra te d c oa rs er p art ic le s.

E lle rb ro ck e t a l.3.3 sta te tha t the

p os itio n p aram e te r is t he p a rt ic le s iz e

a t a cum ula tive m ass d istribution of63 .2% , an d therefore m easures the

prop ortion o f fin es. T he gra die nt n is

the slope of the d istribution m ean

s tra ig ht lin e o n t he RRSBp a rt ic le s iz e

g rid a n d t he re fo re m e a su re s th e w id th

o f th e d istrib utio n, b ein g ste ep er th e

na rrower the d istribution . The

sh arpe r the se le ctiv ity of th e cla ssi-

fie r, the na rrower the pa rticle size

d is trib utio n o f t h e c la ss ifie r fin e s a n d

thus of the ce me nt. The position pa -

ra me ter is sm aller the lon ge r the d u-

ra tio n o f g rin din g. T he m a in e ffe ct o fthe grin din g syste m is on the w id th o f

the pa rticle size d istribution pro-

d uce d. The sa me authors= in dica te

tha t th e po sition pa ra m ete rs o f c om -

m ercia l cem en ts ran ge from 15.0 to

31.31!ID.

' i f .ai: : : s"0

' i i iQ)

II:

Pa rticle S ize a s D ia me te r of Sp he re s, J Jm

Figure 3.1. Ide aliz ed p article s iz e dis trib utio n of p ortla nd cem ent

ground in a n ope n circuit m ill. Th e g ra ph ica l re pre se nta tion of th e

R RS e function pa ra me te rs na nd d' is sh ow n [O sb mc!(3·1].

9



E ff ec ts o n C emen t o f H i gh E ff ic ie nc y S ep ar ato rs

E ffe ct o f P article S iz e

D is trib u tio n o n C em e n t

Qual i ty

Onuma-t attributes the improvement

in cement quality brought about by

the use of a high efficiency separator

partly to a narrowing of the particle

size distribution so that more par-

ticles fall within the range of 3 and 30

urn, the size fraction that contributes

most to strength development.

Nyman'" explains that particles

smaller than 3 urn have no decisive

effect on the final strength, and par-

ticles larger than 30 urn are not fully

hydrated and thus do not fully con-

tribute to the strength. Indocument-

ing plant experience with a high effi-

ciency separator, Kellett" reportedthat the particle size distribution had

a slope n = 1. 2 compared with n =1.0

for the old separator. This meant an

increase in the mass of particles in the

3-30 urn size range from 53% to 67%.

Thus they were able to reduce the

required surface area to produce ce-

ment of a given strength.

Henz" reports that the installa-

tion of a high efficiency separator in

an existing plant together with the

addition of a diaphragm in the tubemill resulted in an increase in the

slope of the particle size distribution

from n = 0.89 with the conventional

separator to n = 0.96 with the high

efficiency separator. However,

Everett" found no significant change

in the characteristics of the finished

product after installation of a high

efficiency separator. Schmidf" re-

ports that after installation of a high

efficiency separator along with some

minor modifications to the mill to

take advantage ofthe increased sharp-

ness of separation, the cement had a

narrower particle size distribution.

This proved to be advantageous from

the standpoint of product quality:

better workability, better setting be-

havior, and higher compressive

strength. The water requirement of

the paste increased slightly. Tests of

standard concretes of equal water I

10

cement ratio and cement content

showed superior compressive

strengths for those made from the

cements from the high efficiency sepa-

rator; there was no significant differ-

ence in bleeding.

Weiss and Tresouthick=" confirm

that the most important impact of the

use of high efficiency separators has

been the narrowing of the particle

size distribution while increasing mill

system capacity by removing more

fines from the system. They found

that product quality was not signifi-

cantly influenced by classifier perfor-

mance for cements with lower spe-

cific surfaces because of the charac-

teristics of the rest of the mill circuit.

For cements with higher specific sur-

faces, however, the characteristics ofthe cement improved with the instal-

lation of a high efficiency separator.

Contrib u tio ns o f th e

P a rtic le S iz e F ra c tions

In their experiments on the particle

size distribution of cements, Tsivilis

et a1,3·11ound that the most important

parameters influencing the strength

of cement are the slope n of the par-

ticle size distribution and the contentof the particle size fraction 3-32 urn.

Within this fraction, the grain size

distribution has a significant effect on

strength. The ideal cement would

have a continuous and steep particle

size distribution with at least 65% of

the cement within the 3-32 urn size

range, a significant portion of which

would be in the 16-24 urn range. The

<3 u rn particles would comprise only

about 10% of the cement, and the

specific surface would be 2500-3000ern?Ig. According to Sumneret a1.,3·12

for ordinary portland cement approxi-

mately 49% of the total specific sur-

face is contributed by the <2 um frac-

tion, which accounts for only 7-9% of

the mass, while less than 2% of the

total specific surface iscontributed by

the >45 urn fraction, which is 10-20%

of the mass. Helmuth':" determined

that the desired minimum size for

cement grains is between 2 and 5 urn.

The >5ummaterial bleeds excessively,

while the 2-5 um material does not

bleed and shows good strength de-

velopment, but tends to produce stiff

mixes. He found no indication of a

significant dependency of his results

on the composition of the clinker.

Brugan':" reports that the instal-

lation of a high efficiency separator

resulted in cements with fine particle

«10 urn and <3 urn) contents about

the same as before, but at lower spe-

cific surface values. The coarsest par-

ticle size fractions had been reduced.

Thus both early- and later-age

strengths were enhanced. Brugan

points out that should it be necessary

to adjust the fineness, a change of 100cm

21g represents a 4% change in ca-

paci ty and a 30% difference in cement

strength. He found that cement from

a high efficiency separator achieved

the same l-day strength with a reduc-

tion of 130em' Ig in fineness as com-

pared with cement produced before

the installation of the separator. He

also reports that product uniformity

is better and much easier to control

with a high efficiency separator.

Kershaw and Yardi+" reported

that the installation ofhigh efficiency

separators resulted in narrower par-

ticle size distributions by reductions

in both the <3 um and the >45 urn

fractions. Inone installation the spe-

cific surface was kept constant and

the strength increased; in another the

specific surface was reduced by 150

em?Ig while the strength was main-

tained.

In their discussion of retrofitting

existing plants with high efficiency

separators and other equipment de-

signed to reduce energy consump-

tion, von Seebach and Schneider+"

state thatthe particle size distribution

ofthe cement from the high efficiency

separator was considerably steeper

than for the original cement: the resi-

due on the 325 mesh was reduced to

2.0% for a fineness of 4140 cm2/g

(Blaine), or 2.5% for 3660 crrr'Ig. Thus



the strength development was supe-

rior to that of the original cement.

They found that narrowing the par-

ticlesizedistribution had a negligible

effecton the early-age strength while

increasing the later-age strength.

W ATER DEM AND

C h em ica l a nd Ph ys ica l

Ef fects

According toSumner et a1.,3.12ements

with narrow particle size distribu-

tions can have significantly different

properties from those ofconventional

cements. Water demand can be in-

creased so much that potential en-

ergy savings can be reduced or even

eliminatedwhen thecementsarecom-

pared on the basis of strength at con-

stant slump. In a comparison of ce-

ments ground todifferent particlesize

distributions, they found that the ce-

ment paste water demand increased

from 25.5% to 29.0% while the con-

crete slump dropped from 40mm to

10mm. In practice the water content

of the concrete made from cement

with a narrow particle size distribu-

tion would have to be increased to

give a constant slump, negating atleast some of the strength advantage

enjoyed by such a cement. They at-

tribute the increase inwater demand

toboth chemical and physical causes:

• the physical influence of the

narrower granulometry, spe-

cifically less efficient packing

factors and greater influence of

surface forces

the influence of the narrow

granulometry on the cement's

chemical behavior and early re-activity

the effect ofless gypsum dehy-

dration asa result oflower mill-

ing temperatures

Since there is an interrelation be-

tween the physical and chemical

mechanisms affectingwater demand

- for example, the amount of C3A

that reacts before the dormant period

•

•

peA R e s e a r ch a n d D e v e lo pm e n t B u ll e ti n RDIlO

depends partly on the specific sur-

faceand granulometry ofthe cement

- theyexperimented with the rheo-

logicalproperties ofgranulated blast

furnace slag "paste" and "concrete"

inorder to examine the physical in-

fluences in isolation. On the basis of

constant volume of granulated ma-terial, slag has a similar or slightly

lower water demand than cement.

They found that while the total in-

crease inwater demand in concrete

associated with narrow granulo-

metry resulted in an increase in the

water / cement ratio of0.05,physical

influences (primarily packing fac-

tors) alone were responsible for an

increase in the water/cement ratio

of only 0.005-0.01.

Increases in the specificsurfaceimprove the ability of the paste to

retain water, thus enhancing the lu-

brication of the aggregate particles.

They found that an increase of 500

cm2/ g inthe specificsurface resulted

in a decrease of 0.01 in the water /

cement ratio. They concluded that

the net effect of switching to a nar-

row particle size distribution at a

lowerspecificsurfacewould increase

the concrete water demand by 0.01

to0.015due tophysical effectsalone.

Kuhlmann et a1,3-2ound that

for constant specific surface the

water required to obtain a standard

consistency increases foranarrower

particle size distribution. Theyat-

tribute this behavior to two causes:

• With a narrower particle size

distribution the rate of con-

version ofC3Aincreases, even

when the gypsum content has

been optimized. Thus more

hydration products are

formed within the first fewminutes after water is added

to the cement, requiring more

water to produce a given

workability.

• Thevolume ofvoids isgreater

for amixture with a narrower

particle size distribution, re-

quiring more water to fill the

voids between the particles.

The same authors!" experimented

with ground limestone to examine

the physical effects separately from

the chemical. UnlikeSumner et a1.,3-

they found that water demand asde-

termined by DIN1164increases with

increasing specific surface and de-

creasing position parameter d' of theparticle size distribution, i.e., when

the cement becomes finer. Ifthe posi-

tion parameter remains constant,

water demand increases as the par-

ticle size distribution becomes nar-

rower. They determined that only a

small proportion ofthe mixwater is

chemically combined as hydrates be-

forethe dormant period. Ofthewater

required for physical reasons, only a

small portion is needed for wetting

the surfaces of the cement particlesand filling the voids between them.

The greater part ofthe water demand

is needed to make the particles mo-

bile in relation to one another. How-

ever, there was wide variation be-

tween cements ofdifferent composi-

tion; cements with high C3Acontents

or highly reactiveC3Aaremore sensi-

tive to variations in particle size dis-

tribution. Cements with lower spe-

cific surface are also less sensitive to

particle size distribution. The same

authors= point out that that portion

of the water demand that can be at-

tributed to chemical effects may be

minimized by optimizing the gyp-

sum content.

Braun and Cebauer'" also found

that clinkers with lower reactivity-

having low to moderate C3A and al-

kalicontents - produce cements that

are less sensitive to grinding condi-

tions. They ground the same clinker

in an open-circuit mill or in a closed-

circuitmillwith ahigh efficiencysepa-rator. Their results show that a nar-

row particle size distribution causes

the cement from the high efficiency

separator to have a higher water de-

mand in paste despite its lower spe-

cificsurface. However, theworkabil-

ity and slump loss of the concrete

were not affected.

11



E ff ec ts o n C eme nt o f H i g h E ff ic ie nc y S ep ara to rs

Paste VS. Mo rta r a nd

Concrete

Helmuth':" emphasizes the impor-

tance of water requirement for ad-

equate flowproperties,bleeding, time

ofset, and drying shrinkage aswell as

strength development. Hepoints out

that these properties must be at least

as good as for conventional cements

if the new cements are to be accept-

able to users. He states that the par-

ticlesize range 2to 5 u m isessential tothe control of these properties. With

little 2-5 u m material the flow was

improved but there was excessive

bleeding and the strengths at early

ages were poor. However, cements

with 21-26%2-5!!ffimaterial had poor

paste flow properties and in somecases failed to reach the expected

strengths at low water/cement ra-

tios. He attributes the differences in

water requirement to differences in

the early hydration reactions of the

C3A. Cementswith differentCsAcon-

tents differed in their bleeding char-

acteristics. The same author+" re-

ported that while cements with con-

trolled particle size distributions

showed higher water demand in

minislump tests of paste flow, theyshowed lower water requirements in

mortar flow tablemeasurements. He

explains that coarseparticles in ordi-

nary cements cause particle interfer-

ence in mortars and concretes, in-

creasing water requirements. In

pastes, however, coarse particles re-

duce the specificsurface and thus the

water requirement. Hence mortars

made from cements with properly

optimized particle size distributions

should have improved flow charac-

teristics even if the pastes are stiffer.

The experience of Kershaw and

Yardi-" bears this out. They found

that the increased voidage of the ce-

ment increases the water demand of

the paste by 3-5%,but in tests of a

standard concrete with a fixed wa-

ter/ cement ratio the slump increased

from 65 mm to 85 mm. Schmidt"

found no Significant difference in

12

bleedingbetween concretesmadewith

conventional cements and thosemade

from cements from high efficiency

separators. Rock3•20 observed that the

water demand for obtaining a stan-

dard consistency increases with in-

creasing slope n of theRRSBdiagram.

Cebauer':" compared a series of

cements ground either in an open cir-

cuit mill or in a closed circuit mill

equipped with a high efficiencysepa-

rator. He then assessed the perfor-

mance of these cements in a standard

concrete with a cement content of300

kg/rn' and a slump of 7.5 ± 1 em.Despite appreciable differences in the

slope (n) and the water demand ofthe

pastes, the water demand of the con-

crete did not change significantly.

Custom e r A cce p ta nce

Kellett" reports that his company's

customers have accepted the cement

produced from high efficiency sepa-

rators quite well, not having observed

any significant change in quality or

workability. Somehave even praised

the improved uniformity of the ce-

ment. Brugan3.14 confirms that the

physical properties and quality of the

cement have been favorably receivedin the marketplace and states that in

some markets the quality of the new

cementhasbecome thepreferred stan-

dard. His experience is that the mor-

tar water demand has increased by 0-

2%, which has not caused problems

for customers. He did citeone excep-

tion: a customer who requested a

blend of the new cement with a con-

ventional cement.

SETT ING

CHARACTER IST ICS

M i ll Ope r a ting

Tempera tures

According to Dunn,3.22ecause of the

complete venting of the air flow

through the separator without recir-

culation, the air supply is generally

cool ambient air rather than hot re-

cycle air, significantly reducing the

temperature ofboth the products and

the separator rejects. Thus the tem-

peratures of the finished cement and

the milling operation are lower than

fora conventional separator.Onuma 3.4

points out that with the lowering of

the temperature come two benefits:

false set due to dehydration of the

gypsum is prevented and there is no

need to aerate the cement in the silo.

Brugan-" concurs, adding that the

lower temperature alsominimizes the

internal coating of the mill aswell as

the tendency for false setor pack set in

the cement. In describing the results

of the installation of a high efficiency

separator in an existing finish mill,

Henz-Preports that theplant hadbeenoperating without a cement cooler,

and high cement temperatures had

resulted inoccasionalcomplaints from

customers. However, tests ofthemill

circuit after the new separator had

been installed showed that even with

clinker temperatures of75°C,theprod -

uct temperature was only60°e. Ito et

a U 2 4 state that a high efficiencysepa-

rator lowers the mill discharge tem-

perature by 20-30°Cand the cement

temperature by 30-35°C comparedwith conventional separators.

Th e Role of Gypsum in

Sett ing

Gypsum is added to clinker in order

to control the hydration of the C3A.

According toMindess and Young=",

in the absence of gypsum the initial

reaction of CsA with water is very

rapid, leading to flash set:

In the presence of gypsum the C3A

hydrates to form ettringite:

C3A + 3CSHz + 26H->C

6AS

3H

32

trica1cium gypsum water ettringitealuminate



If th e su lfa te c on ce n tra tio n is n ot su f-

ficie nt to co m bin e w ith a ll o f t he C3A,

the e ttringite tran sform s to

monosul foalumina t e :

M on osulfoa lum ina te m ay form be-fo re e ttrin gite if the C

3A re acts m ore

ra pid ly w ith th e su lfa te io ns th an th ey

can be supplied by the gypsum to the

m i xw a te r. T he fo rm a t io n o fe tt rin g it e

slows d ow n the hyd ra tion of C3A by

c re atin g a d iffu sio n b arrie r o n its su r-

face . This ba rrie r is broken down

durin g the conve rsion to m ono-

su lfoa lu m in ate , a llo win g the C3A to

react rapid ly aga in . The m ore gyp-

sum there is in the syste m, the lon ge r

the e ttrin gite w ill rem ain stable .Prigione-" em phasizes the im por-

ta nc e o f su fficie nt a va ila ble su lfa te to

a llow the C3A t o c omb in e e x clu siv ely

a s e ttrin gite in ord er to a chie ve n or-

m a l s e tt in g b ehav io r. If the supply of

s ulfa te is in a de qu a te , m o n os ulf oa lu -

m in ate w ill form : if it is excessive ,

secon da ry gypsum will form . C e-

o m en t w ith in sufficie nt gypsum w ill

have a grea te r wa ter d em and due to

the form ation of la rge crysta ls of

m on osulfoalum in ate a nd te tra ca l-

cium a lum in ate hy dra te , w hich form

brid ges be tw een the cem en t gra in s,

p re ve n tin g th em from p ac kin g c lo se ly

an d trapping wa te r be tw een them .

Locher e t a1 .3.27 stud ied the hy-

d ra tion of cem en t w ith and w ithout

g yp sum usin g the sca nn in g e le ctro n

m icroscope . They found tha t in the

absen ce of gypsum the ca lcium a lu-

m in ate hyd ra te s d eve lop in tabula r

form a nd a re un iform ly d istributed

in th e w ate r-fille d spa ce be tw ee n th e

ce me nt gra in s, form in g brid ge s an d

c aus in g r ap id s e tt in g , g e n er al ly w it hi n

a bo ut te n m in ute s. Inth e p re se nce o f

gypsum , e ttrin gite form s on the sur-

fa ce of the cem en t gra in s. The se in i-

tia l re action products d o not form

b rid ge s b etw ee n th e c em e n t p artic le s.

Norm al se tting behavior appears to

b e a re sult o f re cry sta lliza tio n of the

e ttrin gite d urin g th e d orm a n t p erio d.

peA R e se a r ch a n d D e v e lo pm e n t B u ll e ti n RDllO

With a gypsum con ten t of 5-7% by

m ass, a cem en t w ill begin to se t in 2 -

3 hours. The sam e authors-" point

out the im portan ce of re gula tin g the

d osa ge of gypsum to the re activity of

the C3A so tha t it com bines exclu-

sively as e ttrin gite . Sprung et aP 17

sta te tha t w ith a n e xce ss of gypsum ,

th e re cry sta lliza tion o f g yp sum w ill

m a ke th e p aste slig htly stiffe r, n ec es-

s it at in g a m i no r i nc re a se int he am oun t

o f m i x w ate r to m a in ta in c on siste n cy .

For fin e r cem en t the activity of the

C3A in cre ase s, a nd the re quire me nt

for S03 in solutio n in cre ase s a ccord -

in gly. For a given C3A con ten t and

re ac tiv ity , th e c on ve rsio n re ac tio n is

m ore rapid for increa sin g specific

surface and sm a lle r position

pa ram ete r d ' of the RR5B d iagram .B en ste d+ ", ho we ve r, a sso cia te s se t-

ting with the loss of m obility of the

wa te r in the cem en t paste d ue to the

o nse t o f t he fo rm a tio n o f c a lc iu m sili-

c a te hyd r at e.

E ffe ct of D e hy dra tion of

G y ps um on S olu bility

Alsted Nielsen-" lists fou r form s of

ca lcium su lfa te w hich ca n be fou nd in

P ortla nd ce m e nt:

• gypsum , C aS04,2Hp

• pla ste rof P a ris, C aSO 4,1 / 2HP

(hemihydra t e)

• naturalanhydrite.Caikjjtnatu-

ra lly o cc urr in g m i ne ra l)

• solubleanhydrite.CefXj.tfrom

d eh yd ra tion of gy psu m )

A cco rd in g to T ay lo r+ " d eh yd ra tio n

of gypsum in air a t 70-2 00°C yie ld s

first hem ihyd ra te a nd the n y-C a504,

or soluble an hyd rite . A bove about

200°C , anhyd rite (a lso ca lled"in soluble anhyd rite ") is form ed .

Be nste d-" e xpla in s tha t the e xte nt of

d eh yd ra tio n is a fu ne tio no fth e le ng th

of th e h ea tin g pe riod a nd the re la tiv e

h um id ity . T hu s re ducin g th e hu m id -

ity or le ng th en in g the he atin g p eriod

lowers the tem pera ture a t which a

sig nific an t d e gre e o f d eh yd ra tio n o c-

curs. A t tem pera ture s above about

120°C hem ihyd ra te furthe r dehy-

d ra te s to so lu ble a nh yd rite , w hich is

n ot p artic ula rly so lu ble in w ate r b ut,

like hem ihyd ra te , ha s a solubility of

about 6 g/L , a s com pared with gyp-

sum , w hich ha s a solubility of 2 .4 g lL. A lso like he mihyd ra te , soluble

a nhy drite h as a h ig h ra te o f s olu tio n,

producin g a m uch m ore rapid in -

c re a se in t he s ulf at e io n c on ce n tr at io n

to a le ve l su pe rsa tu ra te d w ith re sp ec t

to g yp sum . S in ce th is c an b e fo llo we d

b y ra pid re cry sta lliz atio n o f g yp sum

from s olu tio n , t he p re se n ce o f s o lu ble

an hyd rite can have a sign ifican t e f-

fe ct on the rhe ologica l prope rtie s of

m orta rs an d con cre te , an d m ay even

re sult in fa lse se t. In soluble anhy-

d rite , form ed a t te mpera tures above

about 2 00°C , d issolves m uch m oreslowly than gypsum and does not

form a so lutio n su pe rsa tu ra te d w ith

re spe ct to gypsum . In soluble an hy-

d rite does not cause fa lse set: how-e ve r, b ec au se o fits slo w d isso lu tio n it

m ay give rise to flash se t.

L oc he r e t a l. 3 . 2 B e m ph asize the im -

po rta nce of th e hy dra tio n sta te o n th e

solubility of C a504• Gypsum ,

CaS04,2HP , d issolves re la tive ly

slo wly. H em ihy dra te a nd solub le a n-

hy drite d isso lv e m uch m o re quickly,p ro du cin g a su lfa te -ric h so lu tio n su -

p ersa tu ra te d w ith re sp ec tto g yp sum .

Na tura l an hyd ra te d issolves m uch

m o re slowly in th e m ix in g w ate r. T he y

found tha t when gypsum is used to

c on tro l th e se ttin g b eh av io r th e a va il-

a bility o f d isso lv ed su lfa te is g en er-

a lly in sufficie nt to a llow the C3A to

com bin e e xclusively a s e ttrin gite .

The e arly form ation of m on osulfo-

a lu m in a te c an le ad to p re m a tu re stiff-

e nin g. W ith h em ih yd ra te , h ow ev er,

the re is a bu nd an t sulfa te in solution

and the C3A com bines on ly as

e ttrin gite . A ny e xce ss h em ih yd ra te

w ill rehyd ra te to form gypsum . Re-

crysta lliza tion of the gypsum is re -

sp on sib le for the se ttin g in this case ,

so an excess of gypsum m ay result in

m o re ra pid re cry sta lliz atio n a n d th us

m ore rapid se ttin g. For this re ason

the a m oun t of a va ila ble su lfa te m ust

13



E ff e c t s o n C e m e n t o f H ig h E f f i c ie n c y S e p a r a to rs

be adjusted to the amount and reac-

tivity oftheC3A.Locher et al.3.32har-

acterize the reactivity of the C3A by

the quantity of C3Athat passes into

solution in the first 5-15 minutes after

mixing. Locher et al.3.33eport that the

amount of C3A that initially reacts is

generallybetween 0. 4 and 1.6%ofthemass ofthe cement.

O ptim a l D osa ge of

Gypsum

Sumner et a1.3·12oncur that the level

of gypsum must be optimized, since

an insufficient supply ofS031eads to

uncontrolled hydration of the C3A

and the consequent flash set. On the

other hand, excess 503 leads to re-

crystallization ofthe gypsum and the

consequent false set. They point out

that for agiven clinker the level of503required to achieve the minimum

water demand is lower for a cement

with narrow granulometry. Further,

the general level ofwater demand in

concrete is higher if that concrete is

made with cement having a wider

particle size distribution, as Helmuth

also observed. When the concrete

water demand is higher for cements

having a narrow particle size distri-bution, they attribute it not to the

granulometry but to the reduced

availability of503due tothe reduced

degree ofdehydration ofthe gypsum

in cement milled at lower tempera-

tures. Helmuth':" found that for ce-

ments with controlled particle size

distributions, the optimal 503con-

tent for strength is higher than for

normally ground cements ofthe same

composition. For conventional ce-

ments theoptimum gypsum levelcor-responds toa50/ Alp3ratio ofabout0.6,while forcementswith controlled

particle size distributions the optimal

gypsum dosagecorresponds toa50/

Alp) ratio of about O . B .

Schmidt'<found that with a high

efficiencyseparator the initial setting

time of the cement did not change

significantly,but thefinalsetting time

14

improved slightly. For concretes

made from these cements, the early

stiffening behavior improved.

Helmuth-" observed that if air swept

mills and high efficiency separators

are used to reduce the amount of

clinkerfines,there isconsiderable risk

of removing the gypsum before it isground finely enough or sufficiently

dehydrated to dissolve at rates high

enough to control the hydration of

the C3A. Thus changes in the mill

system operation may be necessary

to prevent problems with excessive

water demand. The experience of

Kershaw and Yardi-" bears this out.

They found that the lower milling

temperatures led to a deficiency in

the level of soluble sulfate. Thus the

setting times were increased to thepoint where they decided to install a

hot-air recycle duct and to feed hot

clinker directly from the kiln as often

aspossibleto stabilizethe setting char-

acteristics of the cement. These ob-

servations are consistent with those

ofSumner et al.3.!2ited earlier in this

section. Theisen-" examined the ef-

fect of gypsum dehydration on set-

ting and strength development and

concluded that ahigh degree ofdehy-

dration isdesirable solong asfalseset

and flash set are avoided. Locher et

a1.3-28lso recommend that the gyp-

sum be dehydrated as completely as

possible to hemihydrate during the

grinding process.

D ry ing Sh rin ka g e

Helmuth':" observed that drying

shrinkage for cements with narrow

particle sizedistributions was insome

cases less than for conventional ce-

ments. Shrinkage was frequently

higher ata given curing time, but this

wasmainlybecause thecementswere

more completely hydrated. Thus they

had developed greater strength and

resistance to shrinkage cracking. In

general, shrinkage increasedwith wa-

ter/cementratioandcuringtime. He

also found that cements with narrow

particle size distributions performed

better over a wide range of mixing

and curing temperatures. Helmuth="

also found a better correlation be-

tween optimum gypsum content for

strength and that for shrinkage for

particle-size controlled cements than

for conventional cements. Kershawand Yardi-" found it necessary to

adjustthe gypsum dosageforstrength

and shrinkage, asshrinkage increased

with increasing gypsum content

above the optimum level.

S ilo S et

Before the advent of high efficiency

separators, Alsted Nielsen-" wrote

that temperatures in the mill could

easily reach 180°C. Thus during

grinding or storage ofthehot cement,

the gypsum releases hydrate water,

converting to soluble anhydrite.

Water from the gypsum in the mill

will be generally drawn away with

the ventilation and not cause difficul-

ties. However, water released from

the gypsum during storage of hot

cement can give rise to the formation

of lumps of cement in the silo. If a

significant portion of the gypsum in

the cement converts to hemihydrate,the cement will be false setting. Ce-

ment in the silo cools very slowly, so

that it remains within IO-20°C of the

exit temperature from the mill for

quite some time. Thus in conven-

tional mill circuits it is often neces-

sary to cool the cement. Frigione-"

explains the phenomenon of silo set

in more detail. The storage of hot

cement in a silosets up a temperature

gradient as heat is lost to the sur-

roundings. Water vapor from thedehydrating gypsum condenses on

the walls of the silo or on the cement

itself and reactswith the cement. The

heatevolved fromthehydrationraises

the temperature in the silo, causing

further dehydration of the gypsum

with theconsequent liberationofmore

water vapor. Lumps and/or sticky

cement may also be caused by crys-



ta lliz atio n o f th e d ou ble sa lt h yd ra te ,

sy ng en ite (CK S2H).

W eiss a nd Tre southick ':" sta te

tha t the reduced tem pera ture of ce -

m e nt fro m h ig h e fficie ncy se pa ra to rs

m ay e lim in ate the n eed for a cem en t

coole r an d w ill red uce the d egree of

gypsum d ehydra tion . D urin g m ill-

in g, gypsum in te rground w ith the

c lin k er c an d e hy d ra te t o h em ih yd ra te

o r a n hy drite . C o sw am i e ta l.P 'fo un d

tha t cem en t w ith hem ihyd ra te had

lon ge r se ttin g tim e s a nd a 10% low er

strength tha n cem en t w ith gypsum .

C em en t w ith an hyd rite , on the othe r

han d , had shorte r se tting tim es an d

highe r stre ngth tha n the ce m en t w ith

g yp sum . O n h yd ra tio n, h em i hy dra te

w ill rehyd ra te to gypsum , while an -

hyd rite w ill n ot. The pre sen ce ofh em i hy dra te in cre as es e ttrin gite fo r-

m a tion d urin g e arly hyd ra tion of the

cemen t .

S TRE NGTH DE VE LO PMEN T

E ffe ct o f P a rtic le S iz e

Distr ibution

A ccord in g to Tsivilis e t a l}l1 an in -

cre ase in th e 3 -3 2 u m fra ctio n le ad s to

an in cre a se in the stren gth . An in -cre ase in th e < 3 u rn fra ctio n co ntrib-

ute s to an in crea se in the ea rly

stre ngth . T he con tribution s of the 3-

16 urn fraction an d of the 16-32 urn

fraction to the strength a re a lm ost

e qu al, w hile th e 16 -2 4 u r n fra ctio n is

m o re sig nifica nt to th e stre ngth th an

the 3-16 um fra ction for ce m en t ha v-

in g specific surface in the range of

3500-4000 em- Ig . T his a pp lie s o nly if

the pa rticle size d istribution is con -

tin uous a nd ste ep. A com parison oftwo cem en ts, on e w ith a fin ene ss of

2500 cm21g an d the othe r w ith a fin e-

n e ss o f 33 20 em - Ig, show s tha tthe 2 8-

d ay stren gth of the coa rse r cem en t

w as highe r be ca use of its highe r pro-

p ortio n o f 16 -2 4 u rn p article s (2 2.6 %

a s com pa re d w ith 12 .8% ). P article s

gre ate r tha n 2 4 u rn in size d o n ot ha ve

a s ig n if ic an t in flu e nc e o n t he s tr en g th .

peA R e s e a r ch a n d D e v e lo pm e n t B u ll e ti n RDllO

B ase d on his in ve stiga tion of the

re la tio nsh ip b etw e en th e p artic le siz e

d istribution of the cem en t and the

ISO m orta r stren gth, Gebaue r+"

foun d tha t pa rticle s 0-8 um in size a re

m ost im porta nt to the 2 -d ay stre ngth

and pa r ti cl e sz -Zdumar emos t impor -

tan t to the 28-day stren gth . H e a lso

found tha t the highe r the slope n of

the R RSB d ia gra m , the highe r the 2 8-

d ay stre ngth for m ost clin ke rs. The

in flu en ce o n th e 2 -d ay stre ng th is le ss

pronoun ced . The sm alle r the posi-

tion pa ram ete r d ', the highe r the 2 -

a nd 2 8-d ay stre ngths for a ll clin ke rs

te ste d. T he re la tio nsh ip b etw ee n pa r-

ticle size d istribution a nd stre ngth

d ep en ds o n th e ch em ica l Iminera logi -

ca l com position of the clin ke r a nd on

th e c alc iu m s ulfa te a d ditiv e.A s a lre ad y sta te d, the fin en ess of

the cem en t is gen e ra lly re duced in

ord e r to take advan tage of the in -

crea se in stren gth of the cem en t.

H en z-" found tha t for cem en ts pro-

d ucin g co m pa ra ble m o rta r stre ng th s

it was n ecessa ry to grin d the cem en t

to 3180 e m ' Ig when usin g a conven -

tion a l sepa ra tor, while w ith a high

e ffic ie nc y s ep ara to r a fin e ne ss o f 2 9 30

. cn f Ig s uff ic e d. H a n ke e t a 1 ,3 -3 7o un d

th at th e s te ep er p artic le s iz e d istrib u-

tion re sults in im prove d stre ngth d e-

v elo pm e nt for ce m en ts p ro du ce d w ith

h ig h e ffic ie n cy se pa ra to rs e ve n w he n

the specific surfa ce is red uced .

Schm id t-? obse rved tha t in gen era l

the 2 -d ay stre ngths w ere the sa me for

cem en ts produced w ith high e ffi-

c ie n cy o r c o nv en tio na l se pa ra to rs, b ut

the 7- an d 28-d ay strengths were

h ig he rfo r th e ce m en ts p ro duce d w ith

h ig h e ffic ie n cy s ep ara to rs. B la sc zy k

e t a 1,3 ·3 8a d th e sa m e clin ke r gro un d

u sin g tw o d iffe re nt m ill circu its, o ne

w ith co nv en tio na l se pa ra to rs a nd th e

othe r w ith a high e fficie ncy se pa ra -

tor. The cem en t from the high e ffi-

cie nc y se pa ra tor h ad a lo we r sp ecific

s urfa ce (4 00 0 cm2/ g com pa re d w ith

4500 cm 2/g) a nd a highe r slope (n =

1.11 a s c om pa re d w ith n = 1 .01 f or t he

co nv en tio na l ce m en t). T he co ncre te

m ad e from the ce m en t prod uce d w ith

the high e fficien cy sepa ra tor had

highe r stren gths a t 2 , 7 , and 2 8 d ays.

Loche r e t aU 39 foun d tha t for a

g ive n sp ecific su rfa ce , th e stre ng th

in cre as es a s th e g ra n ulom etric d is tri-

bution be com e s n arrow er. Sum n er e t

a l,3 .1 2 em ph a siz e th a t t he c oa rs es t p a r-

ticle s have the gre ate st in fluen ce in

re ducin g th e d eg re e o f hy dra tio n, a nd

thus the stren gth . He lm uth-" foun d

tha t re ducin g the m a xim um pa rticle

size to 2 0 urn im prove d the stre ngth

d e ve lo pm e n t , w h ile fu rt he r re d uc tio n

to 1 4 u m d ecre ase d stre ng th d ev elo p-

m en t. It a ppea rs tha t the am oun t of 2 -

5 u m m a te ria l is sig nifica nt in o bta in -

in g in crea sed stren gths a t red uced

m axim um size . He con cluded tha t

the cem en ts were stronge r not be -

cause of an in he ren tly stronge r ce -m en t ge l, but because of m uch m ore

e fficie nt u tiliza tio n o f th e ce m en t b y

m ean s of m ore rapid an d com ple te

hyd ra tion . L oche r e t a 1 .3.39 fo un d th at

the e ffect of fin e r grin d in g on the

ea rly stren gth of cem en t d epend s on

the a lka li con ten t: lower-a lka li ce -

m e nts a re m o re se nsitiv e to th e e ffe cts

o f f in e r g rin d in g .

K uhlm ann e t a1.3.2 foun d tha t the

hig he r w ate r d em a nd d ue to n arro we r

p artic le siz e d istrib utio n d id n ot h av e

any m easurable e ffect on the com -

pre ssive stre ngth of con cre te s m a de

w ith a con stan t w ate r/cem en t ra tio

o f 0 .6 0. H ow e ve r, th e sp littin g te n sile

stre n gth o f th ese c on cre te s in cre ase d

w ith a n arrow in g of the pa rtic le size

d is trib utio n o f th e c em e n t.

E ffe ct o f G y ps um

T he ise n-" e m ph asize s th e in flue nce

of the degree of d ehyd ra tion of thegypsum on the stren gth . The opti-

m um dehyd ra tion an d gypsum con -

ten t depen d on the C 3A and a lka li

con ten ts of the clin ke r. A s the gyp-

sum d osage is in crea sed , the d egree

o f d eh yd ra tio n b eco m es le ss sig nifi-

ca nt. O ptim iza tion of t he d osa ge a nd

d eg re e o f d e hy dra tio n o f t he g ypsu m

can in crea se the stren gth by 5-10% .

15



E f f ec ts o n Cement o f H i gh E f f ic ie nc y S e p ar at or s

Helmuth':" found that increasing the

fineness of the gypsum tends to in-

crease the strength by about 20% at

the highest gypsum contents; at the

lowest gypsum contents the effectis

small.

Qua lity Con trol

Brugan' :" lists among the benefits of

high efficiency separators an in-

creased control of cement strength.

In comparing cements from eight dif-

ferent plants before and after the in-

stallation of high efficiency separa-

tors, he found that the fineness de-

creased by 13to 240cm2/ g while the

7-and 28-day strengths increased. In

most cases the 3-day strengths also

increased, while the l-day strengths

were lower in some cases and higher

in others. He observed that the early-

age strengths did not suffer as much

asmight be expected because the fine

particlecontentwas similar. Asstated

earlier, if necessary the fineness can

be increased by 100 cm2/g, which

represents a3-4%difference in capac-

ity. Thuscontrol ofthe strength does

not result in significant loss of effi-

ciency. Kuhlmann" point out that

changes in fineness have more effecton the early strength than on later-

age strength because of changes in

the rate ofhydration. However, finer

grinding increases the strength only

up toa certain limit. Herholdt etaP40

explain that the fineness of the ce-

ment is a deciding factorin its rate of

reaction, since hydration occurs on

the surface of the cement grain. The

effect of the increased rate of hydra-

tion due to finer grinding is most

apparent at ages up to about 3 days.

Brugan3.14 citesanother benefit of

high efficiency separators in the in-

creased uniformity of the product,

which isimportant inthemarket. The

cement is more consistent in quality

because the mill circuit is easier to

operate and the high efficiencysepa-

rator easier to adjust.

16

REFERENCES

3.1 Osbeeck, Bjarne, "Grinding

Methods, Particle Size Distri-

bution and Properties of Port-

land Cement," F.L. Smidth In-

ternational Cement Production

Seminar.

3.2 Kuhlmann, K., Ellerbrock,

H.G., and Sprung, S.,

"Korngrofsenverteilung un

Eigenschaften vonZementTeil

I: Festigheit von Portland-

zement," Zement-Kalk-Gips , vol.

38, no. 4, April 1985, pp. 169-

178. (English translation invol.

38,no. 6.)

3.3 Ellerbrock, H.G., Sprung,

S., and Kuhlmann, K.,

"Korngrofienverteilung undEigenschaftenvonZementTeil

III: Einfluss des Mahlpro-

zessen," Zement-Kalk-Gips , vol.

43,no. 1,January 1990,pp. 13-

19. (English translation in vol.

40,no. 3.)

3.4 Onuma, Eiichi, "A New High-

EfficiencyClassifier asApplied

to the Cement Industry," Pro-

c ee din g s, 1 9 th I nt er na tio na l C e -

me nt S em in ar , 1983Rock Prod-

ucts, pp. 40-44.

3.5 Nyman, Karl-Erik, "Improve-

ments in cement grinding with

Larox EC-90 classifier," World

C ement T ec hn o lo g y , vol. 11, no.

8,October 1980,pp. 407-417.

3.6 Kellett, Charles D., "Lehigh's

Experiences with a High Effi-

ciency Separator," Proceedings ,

2 1s t I nt ern at io na l C em en t S emi -

nar, 1985 Rock Products, pp.

197-230.

3.7 Henz, F., "Optimierung einer

Zementmahlanlage durchEinbau eiens trennschar-

fen Sichters mit einfacher

Luftfuhrung - Betriebserfah-

rungen," Zement-Kalk-Gips , vol.

39, no. 6, June 1986. (English

translation in vol. 39,no. 8.)

3.8 Everett, Donald K., "Increased

Mill Throughput: a Result of

StaticSeparation atBlueCircle,

Atlanta," Pr oc ee di ng s , 2 2n d I n-

t er nat iona l Cemen t Seminar , 1986


3.9 Schmidt, D., "Hochleistungs-

Sichter SEPOL-Erfahrungen

und Betriebsergebnisse im

Zementwerk Hardegsen,"

Zement-Kalk-Gips , vol. 41, no.

10, October 1988, pp. 506-510.

(English translation in vol. 41,

no. 12.)

3.10 Weiss, Stuart J., and

Tresouthick, Stewart W., E n -

erg y S aving s by Im proved C on-

trol of th e F inish G rin ding P ro -

cess in C em ent M anufacture,

DOE/CS/40419-T1, March1987.

3.11 Tsivilis, S., Tsimas, S.,

Benetatou, A., and Haniotakis,

E., "Study on the contribution

of the fineness on cement

strength," Zement-Kalk-Gips ,

vol. 43, no. 1,January 1990,pp.

26-29.

3.12 Sumner, M.S., Hepher, N.M.,

and Moir,G.K., "The Influence

of a Narrow Cement Particle

Size Distribution on Cement

Paste and Concrete Water De-

mand," 8 th In te rn atio na l C o n-

g r es s o n t he Ch em is tr y o fC emen t,

Rio de Janeiro, September 22-

27,1986, vol. II,pp. 310-315.

3.13 Helmuth, R.A., En e rg y Con se r-

v atio n P o te ntia l o f P ortla nd C e -

m en t P artic le S iz e D istrib utio n

Control, Construction Technol-

ogy Laboratories, Inc., Skokie,

Illinois, Report to the Depart-

ment of Energy, March 1979.

3.14 Brugan,J.M., "High EfficiencySeparators - Problems and So-

lutions," Zement-Kalk-Gips , vol.

41,no. 7,July 1988,pp. 350-355.

3.15 Kershaw,Mark,andYardi,Joe,

"Analysis ofO-Sepa separators

atBlueCircle,Australia," World

Cement , vol. 20,no. 11,Novem-

ber 1989,pp. 400-405.



3.16 von Seebach, Michael, and

Schneider, Lutz, "Update on

finish grinding with improved

energy efficiency," W orld C e-

ment , vol. 17,no. 8,October 1986,

pp.336-346.

3.17 Sprung, S., Kuhlmann, K., and

Ellerbrock, H.G., "Korngroflen-

verteilung und Eigenschaften

von Zement Teil II:Wasserans-

pruch von Portlandzement,"

Zement-Kalk-Gips, vol. 38, no. 9,

September 1985, pp. 528-534.

(English translation in vol. 38,

no. 11.)

3.18 Braun, H., and Gebauer, J ., "Ap-

propriate Grinding Systems for

Superior and Cost Effective Ce-

ments," presented at the Sec-

ond NCB International Semi-nar, New Delhi, 1989.

3.19 Helmuth, Richard, "Effects of

Cement Particle Size Distribu-

tion on the Performance of Con-

crete," R&D Serial No. 1896,

Portland Cement Association,

Skokie, Illinois, September 1989.

3.20 Rock, H.G., "Eigenschaften von

Portlandzement in Abhangig-

keit vom Mahlverfahren,"

Zement-Kalk-Gips, vol. 38, no.

10, October 1985, pp. 606-608.3.21 Gebauer, J ., "Optimization of

Cement with New Technologies

for Comminution," presented

at the Engineering Foundation

Conference on Advances in

Cement Manufacture and Use,

July 31- August 5,1988, Potosi,

Missouri.

3.22 Dunn, Mark R., Personal com-

munication, August 15, 1994.

3.23 Henz, Fritz, "Upgrading of a

Finish Mill Circuit with a High

Efficiency Single Pass Separa-

tor," P ro ce ed in gs 21 st I nte rn a-

t io nal Cemen t S em ina r, 1985 Rock

Products, pp. 243-262.

3.24 Ito, M., Misaka, T., Furukawa,

T., Sota, Y., and Onuma, E.,

"Cooling effect of the O-Sepa

air separator in cement grind-

ing," Zement-Kalk-Gips, vol. 41,

no. 5, May 1988, pp. 214-223.

P CA R ese ar ch a nd D ev elo pm en t B ulle tin RO L lO

3.25 Mindess,Sidney,andYoung,J.

Francis, Concrete , Prentice-

Hall, Inc., Englewood Cliffs,

New Jersey, 1981.

3.26 Frigione, G., "Gypsum in Ce-

ment," A d va nc es in C emen t T ec h-

nology , S.N. Ghosh, editor,

Pergamon Press, 1983, pp. 485-

535.

3.27 Locher, F.W., Richartz, W., and

Sprung, S., "Erstarren von

Zement Teil I: Reaktion und

Gefiigeentwicklung," Zement-

Kalk-Gips, vol. 29, no. 10, Octo-

ber 1976, pp. 435-442. (English


3.28 Locher, F.W., Richartz, W., and

Sprung, S., "Erstarren von

Zement Teil II: EinflufS des

Calci umsulfatzusatzes,"Zement-Kalk-Gips, vol. 33,no. 6,

June 1980, pp. 271-277. (En-

glish translation in vol. 33,

no. 8)

3.29 Bensted,J., "Hydration of Port-

land Cement," Advances in

Cement Technology, S.N.

Ghosh,editor, Pergamon Press,

1983, pp. 307-347.

3.30 AlstedNielsen, H.C.,"Falsches

Erstarren von Portlandzement

und Klumpenbildungen imSilo," Zement-Kalk-Gips, vol.

26, no. 8, August 1973, pp. 380-

384.

3.31 Taylor, H.F.W., C em ent C he m-

istry, Academic Press, London,

1990.

3.32 Locher, F.W., Richartz, W.,

Sprung, S., and Sylla, H.M.,

"Erstarren von Zement Teil III:

Einfluf der Klinkerhers-

tellung," Zement-Kalk-Gips, vol.

35, no. 12, December 1982, pp.

669-676. (English translation

in vol. 36, no. 2.)

3.33 Locher, F.W., Richartz, W.,

Sprung, S., and Rechenburg,

W., "Erstarren von Zement Teil

IV: Einfluls der Losungszus-

ammensetzung," Zement-Kalk-

Gips , vol. 36, no. 4, April 1983,

pp. 224-231. (English transla-

tion in vol. 36, no. 6.)

3.34 Helmuth, R.A., "Improved

Cement and Energy Savings

with Particle Size Control," pre-

sented at the IEEE Cement

Industry Technical Conference,

May 1981, Lancaster, Pennsyl-

vania, PCA Serial No. 1671,

Portland Cement Association,

Skokie, Illinois, 1981.

3.35 Theisen,Kirsten, "Gypsum De-

hydration - Its Effect on Some

Cement Properties," unpub-

lished paper.

3.36 Goswami, G., Mohapatra, B.,

and Panda, JD., "Gypsum De-

hydration During Comminu-

tion and its Effect on Cement

Properties," Journa l o f t heAmer i -

c an C e ram ic S o cie ty , vol. 73, no.

3, March 1990, pp. 721-723.3.37 Hanke, Heinz, Kochmann, H.J.,

and von Seebach, H. Michael,

"The Application of High Effi-

ciency Separators in Finish

Grinding Systems," presented

at the IEEE Cement Industry

Technical Conference, Ana-

heim, California, May 1984.

3.38 Blasczyk, G., Eickholt, H., and

Schneider, L.T., "Zement-

mahlanlagen -Moglichkeiten

der Modernisierung," Zement-Kalk-Gips, vol. 38, no. 9, Sep-

tember 1985, pp. 554-559. (En-

glish translation in vol. 38, no.

11.)

3.39 Locher, F.W., Sprung, S., and

Korf, P., "Der Einfluf der

Korngrofsenverteilung auf die

Festigkeit von Portland-

zement," Zement-Kalk-Gips, vol.

26, no. 8, August 1973, pp. 349-

355.

3.40 Herholdt, Aage D., Justesen,

Chr.F.P., Nepper-Christensen,

Palle, and Nielsen, Anders, edi-

tors, Beton-Bogen, Aalborg

Portland, Copenhagen, 1979.

17



E ff ec ts o n C eme nt o f H i gh E ff ic ie nc y S ep ara to rs

CHAPTER 4

D eriv ing th e M ost B ene fit from a High E fficiency Sepa ra tor

EVA LUA TIN G THE NE ED

FO R A HIG H E FF IC IE NC Y

SEPARATOR

T rom p C urve s

According to Dunn':', the improve-ment in performance resulting from


separator depends on how poorly the

original conventional separator was

performing. Ifthe conventional sepa-

rator was already performing well,

replacing it with a high efficiency

separator gives only minimal im-

provement. If the separator was per-

forming badly, replacement with a

high efficiency separator will have a

significant impact. Thus plant opera-tors will report varying success in

reducing energy consumption by in-

stalling high efficiency separators.

Slegterr= reinforces this idea, point-

ing out that the separator is not grind-

ing, and therefore no other improve-

ment should be expected except in

special circumstances such as when

the previous separator was under-

sized and thus acted as abottleneck in

the circuit, or when there were coat-

ing problems within the mill that the

new separator can help to alleviate.

Dunn" emphasizes the impor-

tance of ensuring that the original

separator (along with the rest of the

mill circuit) has been adjusted for op-

timal performance before evaluating

the need for new equipment. Itmay

be that poor performance is due to

operating conditions rather than in-

adequate equipment. In order to

18

evaluate the performance of a separa-

tor, one makes use ofthe Tromp curve.

Dunn'" describes a means of ana-

lyzing the performance ofa separator

using a Tromp curve. It is essential

that truly representative samples of

the feed, tailings,and product streamsbe obtained for analysis. Mass bal-

ance (i.e. feed rate = product rate +

tailings rate) on the whole sample or

on any single size fraction provides a

check on the representativeness of

the sample (and thus on the quality of

the data). From the particle size analy-

sis of the product, feed and tailings,

Dunn determines the fractional re-

covery for each size fraction on the

basis of both product and tailings.

Comparison of the two values indi-

'# .> :cc:(1)

'( 3

= =wiUe

0' , ; : : 0olIS. . .IL

0

Pe r fe c t C l a ssi fi er

cates the quality of the data; ideally

the sum of the two values will be

100%. Dunn determines a corrected

value for the fractional recovery of

the product for each size fraction and

then plots fractional recoveryvs. par-

ticle size on the Tromp curve, asshown in Figure 4.1.

Chapter 2 defined "bypass" as

either the recirculation of fines back

into the separation zone or the spill-

age of feed from the distributor plate

directly into the tailings. The smaller

the separator's bypass the better its

performance. Dunn':' describes how

to determine from the bypass the need

for a high efficiency separator as

shown on the Tromp curve, Figure

4.2,where bypass =100%- maximum

H i gh E f fic ie n c y C l as s if ie r

75 38 19 9 5

Pa rtic le S i ze , m i cr ons

F ig ure 4 .1 . T ro m p cu rv es fo r a n id ea l cla ss ifie r a nd a h ig h e fficie nc y

c la s s if ie r [ Dunn4.4].



fractional recovery. If the bypass is

large (50-60%) even when the con-

ventional separator has been opti-

mized, then a high efficiency separa-

tor will be of great benefit. On the

other hand, if the bypass is only 10 -

20% the installation of a high effi-

ciency separator is unlikely to pro-

vide significant improvement in the

performance of the system. He em-

phasizes the importance of evaluat-

ingthe performance ofthe mill circuit

to obtain hard evidence of the poten-

tial improvement that could be ob-

tained with a high efficiency separa-

tor. Many cement producers were

disappointed with high efficiency

separators because they had not prop-

erly analyzed the performance of the

grinding circuit before installing thenew separator. Tromp curves need to

be developed for the existing circuit

in order to predict the performance

improvement that will be obtained

with the high efficiency separator.

Knoflicek': ' points out the need

toevaluate the performance ofa sepa-

rator in the light of both the Tromp

curve and the circulating load, since a

relatively low circulating load makes

the Tromp curve appear better than it

would be with a higher circulating

load.

" # .

~cQ)

' u

f f iiiic

o: ; : : :u

f!L L .

600 300

peA Res ea rc h a nd De ve lo pm e nt B u lle tin RO llO

C a lc ula tio n of th e

C ircu la tin g L oa d a nd

Recovery

The circulating load can be calculated

from the information provided for

drawing the Tromp curve. Let

F = rate of feed to the classifier

T = tailings rate

P = product rate

Then, assuming steady-state condi-

tions (that is, no material is gained or

lost in the mill circuit),

F = T + P (4.1)

This is the mass balance relationship.

Similarly, any component of the ma-

terial, such as a chemical constituent

or a size fraction, must also satisfy the

mass balance equation under steady-

state conditions. Letf = % of a given constituent in

the feed

t = % of the same constituent

in the tailings

p = % of the same constituent

in the product

Then the mass balance equation for a

component of the material is

fF = tT + pP (4.2)

The circulating load is defined as

L = TI P x 100% (4.3)

Substituting equation 4.1 into equa-tion 4.2,

Bypass = 40%

P a rtic le S iz e , m i cro ns

Figure 4.2 . Th e b ypass can b e de te rm ined from th e T rom p

curv e [Dunn4.4].

f(T + P) = tT + pP

fT + fp = tT + pP

(f - t)T = (p - f)P

L = TIP = (p -f)/( f - t) x 100%(4.4)

Reference 4.6 gives the derivations and

formulae for various mill circuit con-

figurations, including sample calcula-

tions. Recovery is defined as

R = P I F x 100% (4.5)

As Dunn':' points out, it is simply an-

other way to express the circulating

load, since

L = (100/R -1) x 100% (4.6)

Modify in g th e M ill

Since the mill will now be grinding a

coarser material with a narrower par-

ticle size distribution, the effect of the

new separator on the mill performance

must be carefully considered. In re-

cent years many improvements in the

design of milling equipment have al-

ready increased the classification effi-

ciency of mill circuits. Some closed

circuit mills, when properly adjusted

and operating at optimum circulating

load, may approach the performance

of a mill circuit with a high efficiency

separator. Installation of a high effi-

ciency separator in such a case may

not improve the performance. Forbest performance, the rest of the mill

circuit needs to be examined and pos-

sibly adjusted or modified. Possible

modifications include changes to the

ball charge and size distribution and

diaphragms to separate the chambers

within the mill. These and other modi-

fications are discussed in more detail

inChapter 2. Scheuer and Ellerbrock"

emphasize that significant amounts of

energy can be saved only when the

operation of the ball mill is adapted tosuit the operation of the new separa-

tor. Because the sharper separation

may also have a detrimental effect on

the properties of the cement, saving

energy is permissible only when it

does not reduce the quality of the ce-

ment.

Slegterr" points out that the in-

stallation of high-performance inter-

nal equipmentin the mill brings about

19



E ff ec ts o n C eme nt o f H i g h E ff ic ie nc y S ep ar ato rs

a substantial gain with or without a

high efficiency separator. He cites

two cases in which the mill liners were

replaced with classifying liners and

the ball charge adapted to suit them.

Inone case the output was increased

by 6.8%for the same power consump-

tion. In the other the output was in-

creased by 7.1-15.3% in three mills. A

mill that has been completely rede-

signed in terms of its liners, interme-

diate diaphragm, respective lengths

of its compartment, and the load may

improve by as much as 25% if the

control system set points are suitably

adapted.

S EL EC TIN G T HE R IG HT

SEPARATORDunn':' maintains that a poorly per-

forming separator is readily identi-

fied by the nearly identical nature of

the particle size distributions of the

separator feed and tailings. This phe-

nomenon occurs when the separator

is being overfed, since the dispersion

of the material is not adequate for

good separation. Both conventional

and high efficiency separa tors are sub-

ject to this problem, but the high effi-

ciency separator is more forgiving.Dunn':' discusses two indicators

of a separator's performance, both of

which are shown on the Tromp curve.

The first, as already mentioned, is the

bypass. The second is the steepness of

the Tromp curve. An ideal separator

would have zero bypass and a vertical

slope (Figure 4.1). However, the steep-

ness parameter is less significant than

bypass, since its contribution to the

performance is of little consequence

until the bypass has been nearly elimi-nated. A properly designed separator

can achieve zero bypass if the solids-

to-air ratio in the classifying zone is

kept sufficiently low to prevent par-

ticles from interfering with one an-

other so that some acceptable fines are

rejected into the tailings.

According toDunn=the optimum

high efficiency separator should have

20

a solids-to-air ratio just below the criti-

cal level in order to provide the best

separator performance at the lowest

capital cost. That is, the capacity of

the separator should be such that the

solids-to-air ratio is just low enough

to eliminate bypass. A larger separa-

tor (higher air flow) would provide

no improvement in efficiency despite

the greater cost, while a smaller sepa-

rator (lower air flow) would provide

a lower efficiency for a lower capital

cost. A separator generally operates

with constant air flow. For a separa-

tor operating at or near the critical

solids-to-air ratio, increasing the cir-

culating load while keeping the air

flow constant increases the solids-to-

air ratio and the bypass, thus reduc-

ing the separator efficiency. For thisreason many people believe that a

high circulating load causes bypass

and low efficiency. However, if the

separator is properly sized so that the

solids-to-air ratio is below the critical

point, a high circulating load may not

be a problem. Based on their experi-

ence, some cement companies have

developed benchmarks for sizing

separators. These specify maximum

values for the solids-to-air-, solids-to-

cage-area, and airflow-to-cage-area

ratios as guidelines for economical

choices.

O PT IM IZA TIO N O F T HE

M IL L C IR CU IT

A ir F low a nd S ep ara tor

F ee d R ate

In tests with a full-size industrial turbo

separator, Kuhlmann and Ellerbrock"

found that bypass can be reduced byincreasing the speed of the main fan.

To prevent a consequent reduction of

the specific surface of the fines, it is

necessary to increase the speed of the

counter-vane system. In this way the

selectivity (sharpness of separation,

as seen by the steepness of the slope of

the PSD curve) and cut size remain

approximately the same. The best

results are obtained with the vanes of

the counter-vane rotor extended as

far out as possible (with only a nar-

row gap between them and the wall

of the casing). Variations in the sepa-

rator feed rate affect the fineness of

the fine particles less as the circum-

ferential velocity of the counter-vane

system is higher. At the same time,

however, the proportion of material

passing unseparated into the tailings

increases with increasing feed rate.

Increasing the speeds of the main fan

and the counter-vane results in higher

power consumption. Thus variations

in the feed rate do not significantly

affect the power consumption of the

separator.

Koharr'? discusses details of the

operation of the mill circuit for bestresults. If either the separator feed

rate is too high or the airflow too low,

the fine particle bypass will be higher

than normal. Insufficient air flow is

caused by deficiencies in the bag filter

operation, due to inoperative parts,

incorrect bag cloth specification, or

erratic mill water spray/grinding ad-

ditive control. The blinding of bags

can raise the system pressure and

lower the air flow, thus increasing the

fine particle bypass of the separator.

Proper operation ofthe high efficiency

separator depends ona stable air flow

at the proper level. The appropriate

feed rate can be determined by run-

ning a series of extended tests, each

lasting 24-48 hours, to ensure that

steady-state conditions have been

reached at each feed rate. Byplotting

mill production versus feed rate one

can determine the optimum feed rate

for maximum production.

Kohan'? also discusses the analy-

sis of separator performance using

Tromp curves. He gives three figures

illustra ting the effect of separator feed

rate on performance. Figure 4.3shows

a Tromp curve typical ofthe situation

in which either the air flow is too low

or the feed rate too high. Figure 4.4

illustrates a case in which the feed

rate is too low and the coarse particle

classification ispoor. Figure 4.5 shows



a Tromp curve for a separator with

good performance. Ifanalysis shows

good separator performance, further

attempts to improve the performance

ofthe system should be concentrated

on the mill.

Everett+'?reported that oneprob-

lem encountered during startup of a

newly installed high efficiency sepa-

ratorwas theinability to forceenough

air through the system to satisfy the

design requirements ofthe separator.

The mill itself had a large pressure

drop andwashindering airflow. Add-

ingableed-in vent pipe to the ducting

at the mill discharge increased the

airflow and pulled additional fines

through the separator as they were

discharged from the mill. System

airflow was also increased by open-ingup the mill diaphragm byenlarg-

ing the center partition holes.

Slegten='points out that all of the

elements that make up a mill circuit

are interdependent. Thus if the cir-

cuit is to be modified by a major in-

stallation such as a high efficiency

separator or a roller press, the re-

maining elements of the circuit must

be optimized to suit the new condi-

tions. Headds that adequate ventila-

tion of the mill is required when a

high efficiency separator is installed

in the mill circuit, and that the mill

internal equipment must have the

lowest possible pressure drop.

Dust Collection

Everett':" reports the results of the

installation of a high efficiency sepa-

rator in a cement plant. Several prob-

lemsoccurred during the startup. The

most significant problem occurred

with the dust collector system, which

was connected to the static separator.

The large increase in mill air sweep

forced the dust collector to handle a

much larger volume of air than be-

fore, causing severe wear of the col-

lector bags. Small cracks in the dust

collector soon became large ones, al-

lowing the floor of the dust collector

to build up with dust, partially col-

peA Re se ar ch and Dev elo pmen t Bu lle ti n RD llO

100 J ~ .B E lore , . " " n o .~So pt. 1 ,1995

I /,~

1\

I I . . . . . !. ~ , f o•

. . . . . I

"!

-, /

',- . . . .I,

1\1-1- After ·Se aApri l 8,1 86

Cement

Before O-Sepa

P70tP30 = NtAP50 = NtAABP =51%E60u = 94%WACL =3.5

After O-Sepa

P70tP30 = 2.67P50 = 21uABP =25%E60u = 93%WACL =2.33

100

ParticleSize,microns

Figure 4.3. Tromp curves before and after the installation of a high

efficiency separator. In this case either the air flow is too low or the

separator feed rate too high [Kohan4 . V j .

~80o

20

2 3 10 20 30 50

Cement

Before O-Sepa

P70tP30 = NtAP50 =8uABP =44%E60u =96%WACL =3.15

After O-Sepa

P70tP30 = 1.79P50 =25uABP =4%E60u = 86%

WACL =0.67

100

Figure 4.4. Tromp curves before and after the installation of a high

efficiency separator. Herethe separator feed rate is too low and the

coarse particle classification is poor [KohanU].

100

). . .

. . A l l

/ I·~

i 7

.J ~

Bore p·Sep, -'\II ~

M y3, 985 j I

1 \~ /

V1 " ' " " . . . . . .

-, : . . - ' " I-- -

It

~t rO ~ g ~ 6-h Ip'121

\ /"- . . . . . _ ~ •

? f t . 80

>.. .CI)

>o~ 60

II:

'i-cCI)

E 40CI). . . .UC

20

3 20 30 5010

Particle Size,microns

21



E f fe cts o n C eme nt o f H ig h E ffic ie nc y S ep ara to rs

lapsethe dust collectorbags and cause

premature wearing out of the bags.

They corrected this problem by re-

pairing all of the leaks in the dust

collector tube sheets.

After the installation of the high

efficiency separator and the adjust-

ments to the dust collector and theventilation system described above,

the mill had the following character-

istics compared to the original mill:

Before After

Tonsfhour 50.0 54.0

Blaine fineness, c m 2 / g 3500 3550

Grinding energy, kWh/t 47.5 45.4

Total system air sweep,

ACFM 2000 13,500

Kellett and Rockv" describe theinstallation of a high efficiency sepa-

rator in an Iowa cement plant. They

encountered difficultiesonlywith the

filter medium, which had not been

designed tohandle dust in suchquan-

tity or fineness. Installation of a re-

cycleduct from the system fan to the

separatorremedied this problem. Af-

teroptimization oftheplant, grinding

100

capacity increased by 15% and spe-

cificpower consumption reduced by

15%compared to the performance of

the plant before the installation ofthe

highefficiencyseparator. Atthesame

time the quality of the cement was

improved due to the narrower par-

ticle size distribution.

Cement Characteristics

Kuhlmann':" states that an increase

in throughput,with the consequent

reduction inspecificpower consump-

tion, is possible only if the mill al-

ready produces material with a par-

ticlesizedistributiononlya littlewider

than that required for the cement it-

self. Thus the energy saving effectof

modification of the particle size dis-

tribution is governed not somuch by

the sharpness of separation of the

separator asby themode ofoperation

of the mill.

Incomparing theperformance of

different milling systems on the

basis of equal cement strengths,

Herrmann':" found that for cements

ground to finenesses of 3500-4000

' i f .80

~Q)

>0 60oQ)

a:

iii. . .c

40)

Eeue

20

2 5

oL_-r-'--'-~~~~.--.-.--.-~.-~~

1 5010 20 30

Particle Size, micrometers

Figure 4.5. Tromp curve after the installation of a high efficiency

separator. Here the performance of the high efficiency separator is

optimal; any further attempts to improve performance should be

concentrated on the mill [Kohan4.9].

22

cm2/ g, systems employing high effi-

ciency separators consumed 10-15%

less power than conventional milling

systems. For cements ground to

finenesses of 3000 em!/ g or lower,

there isvery little advantage to using

a high efficiency separator. On the

other hand, for cements ground tofinenesses above 4000em' / g the high

efficiency separator has great advan-

tages over conventional separators.

Mill Characteristics

Weiss and Tresouthick':" also imply

the importance of the interdepen-

dence of the mill circuit elements,

stating that the installation of high

efficiency separators will lead to

coarser tailings and powder loads in

the mills due tobetter removal ofthe

fines from the ball mill discharge

streams. In effect, much of the work

of processing is shifted from the mill

to theseparator, sincegrinding ismore

efficient when the fines are removed

- that is, the degree ofovergrinding

is reduced. High circulating loads

appear to be most appropriate for

operationsusinghighefficiencysepa-

rators. Mills with high length-to-di-

ameter (L/D) ratios and fine ballcharges are designed for long resi-

dencetimes; suchmillconditions may

not be appropriate for high efficiency

separators. Short, high-throughput

mills with coarse ball charges and

high circulating loads may be better

suited to high efficiency separators.

Kohan" alsofound that millswith

low L/D ratios have an advantage

over other types of mill in that they

are more stable in operation and thus

more reliable and easier to operate.The shorter the mill the higher its

natural operating rate of feed to the

separator. The separator's capability

must bematched to the expected feed

rate. Thus the selection and opera-

tion of the high efficiency separator

should be based on both the power

and the L/D ratio of the mill.

According to Ellerbrock and

Schiller.!" the grinding of cement re-



quires a specific energy input of about

40 kWh/t, or almost 40% of the total

electrical energy requirement for ce-

ment manufacture. The greater part

ofthis energy islost. Thus it is impor-

tant to limit the energy losses as much

as possible by appropriate choice of

the grinding process and of the oper-

ating conditions of the mill and sepa-

rator. The principal factors affecting

the energy input for cement grinding

are:

• the set values of the ball mill

• the design and mode of opera-

tion of the separator

• the grindability of the princi-

pal constituents of the cement

The temperature in the grinding com-

partment also affects the energy in-

put. At elevated temperatures thereis a risk that agglomeration, coating

of the grinding media, and caking

will sharply reduce the degree ofcom-

minution attained in the mill. Thus it

can be advantageous to cool the ce-

ment during the grinding operation,

as is done in the high efficiency sepa-

rator.

Schneider and Lohnherr-" sum-

marize the optimization measures that

can be used to increase output, re-

duce energy consumption, and im-

prove cement quality:

• altering the effective lengths of

the grinding chambers

• installing classifying linings

• adjusting the grinding ball

charge and grading

• installing intermediate dia-

phragms to control transfer of

material between chambers

• increasing mill venting

• installing high efficiency sepa-

rators in closed-circuit grind-

ing systems• converting open-circuit grind-

ing systems to closed-circuit

• installing high-pressure grind-

ing rolls

Despite the glowing testimonials

found in the literature, itis clear from

discussions with both suppliers of

high efficiency separators and their

customers that not all installations of

PCA Re se ar ch a n d De ve lo pmen t Bu ll et in RO l lO

this equipment were equally satisfac-

tory. Knoflicek" noted that compa-

nies that had successful installations

of high efficiency separators looked

at the need for additional modifica-

tions to the mill circuit and the ce-

ment, such asoptimization ofthe gyp-

sum content. They also made an ef-

fort to educate their customers about

the differences in the product.

REFERENCES

4.1 Dunn, Mark R, "Understand-

ing an Air Separator," Port-

land Cement Association Mill

Grinding Short Course, Skokie,

Illinois, November 1993.

4.2 Slegten, Bernard., "The Influ-

ence of Ball Mill Performances

in Relation with the High Effi-

ciency Separators and the

Roller-Presses," Proceedings ,

2 3 rd I nt er na ti on a l C ement S em i-

nar, 1987, Rock Products, pp.

309-328.

4.3 Dunn, MarkR, "A method for

analyzing the performance of a

mechanical air separator,"

Wo rld C em en t, vol. 16, no. 8,

October 1985, pp. 326-332.

4.4 Dunn, Mark R, "What is Clas-sifier Efficiency? ," Portland Ce-

ment Association Mill Grind-

ing Short Course, Skokie, Illi-

nois, November 1993.

4.5 Knoflicek, Michael, Personal

communication, February 16,

1994.

4.6 Portland Cement Association,

Cla ss if ie r Te st Manua l, Bulletin

MRB-53,1954.

4.7 Scheuer, A., and Ellerbrock,

H.-G., "Moglichkeiten der

Energieeinsparung bei der

Zementherstellung," Zement-

Kalk-Gips , vol. 45, no. 5, May

1992, pp. 222-230. (English


4.8 Kuhlmann, K., and Ellerbrock,

H.-G., "Untersuchungen zum

Betriebsverhalten von Umluft-

sichtern," Zemen i -Ka l k -Gip s ,

vol. 34, no. 11,November 1981,

pp. 580-585. (English transla-

tion invol. 35, no. 1.)

4.9 Kohan, W.J., "Hochste Kugel-

miihlenleistung mit O-Sepa-

Sichtem," Zement-Kalk-Gips , vol.

43,no.2,February1990,pp.91-

95. (English translation invol.

43, no. 4. )

4.10 Everett, Donald K., "Increased

Mill Throughput: a Result of

Static Separation at Blue Circle,

Atlanta," P ro ce ed in g s, 2 2n d I n-

terna t ional Cement Seminar ,1986 ,


4.11 Kellett, Ch.D., and Rock, H.G.,

"Betriebserfahrungen mit dem

O-Sepa Windsichter," Zement-

Kalk-Gips , vol. 39, no. 6, June

1986,pp. 312-316. (English trans-

lation invol. 39, no. 8.)4.12 Kuhlmann, K., "Bedeutung des

Sichtens beim Mahlen von

Zement - Ergebnisse einer

Bilanzierung des Mahlkreis-

laufs," Zement-Kalk-Gips , vol. 37,

no. 9, September 1984, pp. 474-

480. (English translation in vol.

37, no. 10.)

4.13 Herrmann, Christian, IIIn-

creased Cement Grinding Effi-

ciency by Using High Efficiency

Separators," presented at the

27th IEEE Cement Industry

Technical Conference, New Or-

leans, February 1985.

4.14 Weiss, Stuart J., and Tresou-

thick, Stewart W., E n erg y S av-

ings by Im proved C ontrol of the

F in is h Gr ind ing Proce s s in Cemen t

Manufacture, ooE/CS/40419-

Tl,March 1987.

4.15 Ellerbrock, H.-G., and Schiller,

B., "Energieaufwand zum

Mahlen von Zement," Zement-

Kalk-Gips , vol. 41, no. 2, Febru-ary 1988, pp. 57-63. (English


4.16 Schneider, L.T., and Lohnherr,

L., "Influence ofhigh efficiency

separators on the performance

of airswept mills," presented at

the 9th AFMC Technical Sym-

posium, Kuala Lumpur, 1988.

23



E ff e c t s o n C e m e n t o f H i g h E f f i c ie n c y S ep a r a to rs

CHAPTER 5

Conclus ions a nd R e comm enda tio ns

SUM M ARY A ND

CONCLUS IONS

High e fficien cy sepa ra tors offe r a

n um ber of poten tia l ben efits in the

production of cem en t. The m ost n o-

ta ble in clu de sig nifica nt e ne rgy sa v-

in gs an d increa se s in production .

O the r possible ben e fits a re ea se of

ope ra tion , a djustm en t, a nd m ain te -

n an ce ; sm a lle r e qu ip m en t size for th e

sa me throughput; coole r ope ra tin g

tem pera tures; and m ore con sisten t

p ro du ct q ua lity . H ow e ve r, th ese b en -

e fits d o n otfo llo w a utom atica lly from

the in sta lla tion of a high e fficie ncy

separator . It is necessa ry to take an

in te llige nt, system atic a pproa ch to

o bta in th e b est re su lts.The first step is to de te rm ine

w he th er a h igh e fficie ncy se pa ra tor

would be w orth the cost an d e ffort of

in sta lla tion . D esp ite the g low in g re -

p ort s in th e lit er at ure , p art ic ula rly t ha t

of the e arly 1980's, high efficie ncy

sepa ra tors a re not the solution to a ll

p ro ble m s re la te d to e ne rg y co nsum p-

tion in fin ish grin din g. The prim ary

p urp ose o f t he h ig h e ffic ie nc y se pa ra -

tor is to reduce specific ene rgy con -

sum ption by e lim in atin g bypa ss, the

in clu sio n o f a cc ep ta ble p ro du ct in th e

tai l ings. If bypass is the problem , a

h igh e fficie ncy se pa ra tor is a lo gica l

p art o f th e so lu tio n.

T he e xistin g m ill syste m sho uld

be optim ized for best prod uction .

Then repre sen ta tive sam ples of the

fe ed , prod uct, an d ta ilin gs should be

ta ke n for pa rticle size a na lysis. The

da ta should be checked by m ean s of

24

m a ss ba la nce e qua tio ns to e nsure th at

the sam ples w ere truly repre sen ta-

tive . Then a Trom p curve can be

plotted to show the pe rform ance of

th e e xistin g m ill sy ste m . T he b yp ass

can be com puted from the Trom p

curve : bypa ss = 100% - m axim umf ra c tio n a l r ec ove ry . If the bypass is

large - say 50% or m ore - then a

h ig h e ffic ie n cy s ep a ra to r c ou ld lik ely

be of gre at be ne fit . If the bypass is

sm a ll - say 10-20% - then a high

e fficien cy sepa ra tor w ill n ot be of

m uch use because the m ill system is

a lre ad y "high efficie ncy." The use r

should keep in m in d tha t in recen t

years m any types of m illin g equip-

m en t have been developed or rede -

sig ne d to re du ce spe cific e ne rgy con -sumpt ion . If the system alre ady has

little b yp as s, th e in sta lla tio n o f a h ig h

e fficie ncy se pa ra to r d oe s little to im -

prove the ope ra tion . In tha t case ,

e ffo rts to re du ce th e spe cific e ne rgy

re qu irem e n ts s ho uld b e c on ce n tra te d

o n th e m i ll. G e n era lly s pe a kin g , s ho rt ,

high-throughput m ills w ith coa rse

b all c ha rg es a n d h ig h c irc ula tin g lo ad s

a re b est su i te d to h ig h e ffic ie nc y se pa -

rators.

T he n ext ste p is to se le ct th e righ t

se pa ra to r. T he c ap ac ity o f t he se pa ra -

tor sh ould b e such th at the solid s-to-

a ir ra tio is just low enough to elim i-

n ate b yp ass. A se pa ra to r w ith a la rg er

capacity w ill have a higher capita l

co st w ith ou t p rov id in g a ny be ne fit in

te rm s of e fficie ncy . O ne w ith too low

a capacity w ill provid e a lower e ffi-

cien cy for a low er ca pita l cost. Som e

co m pa nie s ha ve e sta blish ed e m piri-

ca l guid elin es for sizin g the se pa ra -

tor. They specify m axim um va lues

fo r t he s olid s- to -a ir, s olid s-t o-c ag e-

a re a , a n d a ir flow -t o- ca g e -a re a .

Before and during the in sta lla-

tio n o f a h ig h e ffic ie n cy s ep a ra to r, itis

e ssen tia l to consid er the en tire m illc ircu it a s w ell a s its e ffe ct o n th e p ro d-

uct qua lity. Each e lem en t in the cir-

cuit works with - and affects - the

oth ers. T hus som e in te ra dju stm e nt

w ill b e n ece ssa ry for b est re su lts, a nd

som e a dd ition al e quipm en t m ay also

be required . On e m ajor e ffect of the

high e fficie ncy se pa ra tor on the re st

of the circuit is the grea te r d em an d

for fre sh a ir flow in to the se pa ra tor

d ue to the elim in ation of recircula -

tion of a ir w ithin the separa tor. Thev en tila tio n sy ste m m ust prov id e th is

a ir flo w a nd th e d ust ha nd lin g sy ste m

m u st h an d le th e g re atly in cre as ed v ol-

um e of dust-lad en a ir. Both of these

co nd ition s w ill m o st lik ely n ece ssi-

ta te so m e m o difica tion s to the e quip-

men t .

A n ot he r e ffe ct o f th e e lim i na tio n

o f a i r flow r ec irc ula tio n is t he r ed u ce d

te m pe ra tu re of bo th pro duct a nd ta il-

in gs. O n th e po sitive sid e, th e lo we r-

te mpe ra ture prod uct is le ss like ly to

un de rgo fa lse se t or pack se t, and

custo m ers p re fe r ce m en t d elive re d a t

a low er te m pe ra tu re , e sp ecia lly d ur-

ing the sum m er. It m ay even be pos-

sib le to re m ov e th e c em e n t co ole r fro m

the c ir cu it .

H ow ev er, lo we r-te m pe ra tu re

ta ilin gs m e a n lowe r m illin g te m pe ra -

tures, w hich w ill a ffe ct the d egre e of

d ehyd ra tion of the gypsum . Dehy-



drated gypsum (hemihydrate or

soluble anhydrite) is more soluble

than dihydrate gypsum; thus gyp-

sum having a lower degree of dehy-

dration is less effective in controlling

flash set. Depending on the chemis-

try of the clinker and the operating

conditions in the plant, this problemmay be handled in various ways. It

may be possible to control it simply

by adjusting the dosage of the gyp-

sum, or it may be necessary to raise

the milling temperature by feeding

hot clinker directly into the mill and /

or by connecting a hot air duct to the

mill.

Other modifications to the mill

circuit will reduce specific energy re-

quirements with or without the in-

stallation of a high efficiency separa-tor. These include the installation of

a high-pressure twin roll press and

modifications to the mill internals,

such as the addition of a classifying

lining or a diaphragm to control the

flow of material from one compart-

ment to the next. Theball charge and

grading should be adjusted for these

modifications. Grinding aids arecom-

monly used to eliminate pack set and

ball coating, and water spray can be

used to decrease mill exit tempera-

tures.

Once the equipment has been in-

stalled, the whole system must be

optimized. The feedrate isoptimized

bymaking a seriesof 24-48hour runs

atconstant feed rate to determine the

rate at which production is maxi-

mized. The speeds of the main fan

and the counter vanes must be ad-

justed for best production.

In addition to the effect on the

settingcharacteristics, themain effect

of high efficiency separators on ce-ment is a narrowing of the particle

sizedistribution. Because this gener-

ally will increase the proportion of

the cement particles in the 3-30 u r nrange,which isthe range that contrib-

utesmost tothe strength, it iscustom-

ary to reduce the fineness of the ce-

ment by 100cm2/ g or more to keep

the strength the same as before. The

PCA R es ea rc h a nd De ve lo pm e nt B u lle ti n RD I I0

water demand of such cements is

higher than for those produced with

conventional separators. Although

part of the increase in water demand

is due to physical effects such as less

efficientpacking ofthe particles,most

is due to chemical effects such as the

activity oftheC3Awhen more surfacearea isexposed and the availability of

503in solution. Thus the increase in

water demand can be mitigated by

adjusting the dosage and degree of

dehydration of the gypsum.

It is important to note that ce-

ment produced using high efficiency

separators may differ from cement

produced using conventional separa-

tors. Customers need to be made

aware that the particle size distribu-

tion is narrower and that the cementmay have a slightly increased water

demand. Overall, the quality of the

cement is still acceptable and may

actually be more consistent because

of the relative ease of operation and

adjustment of a high efficiency sepa-

rator. However, cement producers

need towork with their customers to

ensure that their product is satisfac-

tory.

The operating conditions that

prod ucethe best possible particle size

distribution and surface area for the

desired strength gain and water de-

mand arenot automaticconsequences

of the installation ofa new separator,

but must be determined by trial and

error. Itmay be necessary to operate

at less than maximum efficiency in

order to produce a cement that is

acceptable to the user. The dosage of

gypsum must alsobe optimized.

RECO MM ENDAT IO NS FO RF UR TH ER R ES EA RC H

1.Case studies of selected plants to

compare the cements from conven-

tional and high efficiencyseparators.

In order to ensure high-quality data,

these would have to be either plants

where one mill circuit has a high effi-

ciencyseparator and one does not, or

plants where a high efficiency sepa-

rator is about to be installed so that

good "before" and "after" data could

be obtained. The clinker would have

to be the same in both cases.

2.Optimization ofmilling operations

to produce the best quality cement

having the lowest specificenergy con-sumption. In this case the "best" ce-

ment is the one that performs best in

concrete. This project could include a

study ofhow toproportion concretes

using cements from high efficiency

separators in order to get the best

results.

3.Operation ofhigh efficiency sepa-

rators with high-pressure twin roll

presses as compared to their opera-

tionwithballmills.Thisprojectwould

examine such questions ashowmuchdeagglomerated press cake could by-

pass the mill and be sent directly to

the separator. Ideally, the project

would include investigations ofmill

circuits before and after the installa-

tion of the high-pressure twin roll

press.

4.Use of grinding aids in finishmills

with conventional andhigh efficiency

separators. This projectwould exam-

ine their effects, if any, on cement

quality and specificenergy consump-

tion and determine their optimal use.

ACKNOWLEDGEMENTS

The research reported in this paper

(PCAR&DSerial No. 2020)was con-

ducted at Construction Technology

Laboratories, Inc. with the sponsor-

ship ofthe Portland Cement Associa-

tion (PCAProject Index No. 93-02a).

The contents of this paper reflect the

views of the author, who is respon-

sible for the facts and accuracy ofthe

data presented. The contents do not

necessarily reflect the views of the

Portland Cement Association.

25



Me tric C onve rs ion T ab le

F ollo win g a re m e tr ic c on ve rs io ns o f th e m e as ur em e nt s u se d in th is te xt.

T he y a re b as ed in m os t c as es o n th e In te rn atio na l S ys te m o f U nits (S I).

1 in .

1 s q in .1 ft

1 sq ft

1 sq ft pe r ga ll on

1 gal

1 k ip = 1 00 0 Ib f

1 Ib

1 Ib p er c ub ic y ard

1 psf

1 psi

N o.4 s ie ve

N o. 2 00 s ie ve

1 b a g o f c em e nt (U .S .)

1 b a g o f c em e nt (C a na dia n)1 b ag p er c ub ic y ard (U .S .)

deg.C

= 25.4 0 m m

= 6 45 .1 6 mm 2= 0 .3 04 8 m

= 0 .0 92 9 m 2

= 0 .0 24 5 m2/L

= 3 .785 L

= 4 .4 48 kN

= 0 .4 53 6 k g

= 0 .5 93 3 k g/m 3

= 4.882 k g / m 2

= 0 .0 0 68 95 MP a

= 4 .7 5 m m

= 75mm

= 94 Ib = 4 2 .6 k g

= 881b = 4 0 kg= 5 5 .8 k glm3

= (deg. F - 32)/1.8



PA LA B RAS C L AVE : cemento, clasificador, eficiencia en energia, molienda final, trituracion final, alta eficiencia,

distribucion del tamafio de particulas, separador, curva de Tromp

S INOPS IS : Los separadores de alta eficiencia afectan al cementa principlamente en dos maneras: se estrecha el rango

de la distribucion del tamafio del las particulas y se reduce la temperatura de molienda. El menor rango de la

distribucion del tamafio de las particulas produce mayor resistencia en relacion a una superficie espedfica, pero

incrementa la demanda de agua. Las menores temperaturas de molienda permiten evitar el uso de refrigerantes de

cemento, pero producen menor deshidratacion de la cal. Consecuentemente, modificaciones en la dosificacion de calpueden requerirse. La optimizacion de la dosis de cal y de su grado de hidratacion ayudara a mitigar los efectos de la

distribucion del tamafio de las particulas en la demand a de agua.

Los parametros de operacion del circuito de molienda deben ser optimizados inicialmente para obtener el mayor

beneficio en el uso de separadores de alta eficiencia. Las curvas de Tromp del suministro, de los finos, y de los residuos

del separador, demostraran si con el uso de un separador de alta eficiencia se puede obtener algun beneficio. Todo el

circuito de molienda debe ser considerado. Un separador de alta eficiencia requiere de un mayor suministro de aire del

ambiente, 1 0 que generalmente requiere de ductos y ventiladores adicionales asi como de modificaciones al sistema de

coleccion de polvos. Algunas modificaciones a la molienda pueden ser beneficas con 0 sin el uso de separadores de alta

eficiencia. Despues de que el equipo es instal ado, el grado de alimentacion y otras condiciones de molienda deben

optimizarse.

Las condiciones de operacion que producen la mejor distribucion del tamafio de las particulas y area superficial, para

obtener la ganacia de resistencia y la demanda de agua deseadas, deben determinarse mediante un proceso de ensayoy error. La dosis de cal tambien debe de optimizarse. Puede ser necesario operar bajo una eficiencia menor que la

maxima para producir un cementa que es aceptable para el usuario.

REFERENC IA : Detwiler, Rachel J . , E ff ec ts o n C eme nt o f H ig h E ff ic ie nc y S ep ar ato rs , Research and Development Bulletin

RD110T, Portland Cement Association, [ E ff ec to s e n e l C emen ta d e lo s S e pa ra do re s d e A lt a E f ic ie nc ia , Boletin de Investigacion

y Desarrollo RDllOT, Asociacion de Cemento Portland], Skokie, Illinois, U.S.A., 1995.

ST ICHWORTER : Zement, Klassierer, Energiewirkungsgrad, Zementmiihle, Zementmahlung, hoher Wirkungsgrad,

Kornverteilung, Sichter, Tromp-Kurve

AUSZUG: Hochleistungssichter haben zwei Hauptauswirkungen auf Zemente: eine engere Kornverteilung und

niedrigere Mahltemperaturen. Die engere Kornverteilung verursacht hohere Festigkeiten bei gleicher

Oberflachenfeinheit: der Wasseranspruch steigt allerdings. Wegen der niedrigeren Mahltemperaturen sind

Zementkuhler ublicherweise nicht erforderlich, allerdings wird dadurch die Gipsentwasserung reduziert. Dann kann es

erforderlich sein, die Gipsmenge zu neu einzustellen. Optimierung der Gipsmenge und den Grad der Cipsentwasserung

vermindert die unerwiinschten Auswirkungen der engeren Kornverteilung auf den Wasseranspruch.

Die Betriebsparameter des Mahlkreises rniissen zunachst optimiert werden, um den groiSten Vorteil durch den Einsatz

von Hochleistungssichtern zu erhalten. Die Tromp-Kurven des Sichteraufgabegutes, des Fertiggutes und der

Sichtergriese zeigen, ob ein Hochleistungssichter Betriebsvorteile bietet. Der gesamte Mahlkreislauf mug mit in die

Analyse einbezogen werden. Ein Hochleistungssichter benotigt eine hohere Frischluftmenge, die ein Anpassen der

Luftleitungen, Ceblaseleistung sowie Modifizierungen am Entstaubungssystem erforderlich machen konnen.

Veranderungen an der Miihle selbst sind oft von Vorteil, ob mit oder ohne den Einsatz von Hochleistungssichtern. Im

Anschluf an die Modifizierungen sind die Milhlenaufgabemenge und die Milhlensteuerung den neuen Bedingungen

anzupassen.

Die Parameter, die die optimalste Kornverteilung und Oberflachenfeinheit fur eine gewilnschte Festigkeitsentwicklung

und den Wasseranspruch liefern, miissen durch Betriebsversuche ermittelt werden. Die Gipsmenge mug auch

optimiert werden. Es ist moglich, dag die Anlange mit reduziertem Wirkungsgrad gefahren werden mug, urn die

besten Zementeigenschaften fur den Verbraucher herstellen zu konnen.

REFERENZ: Detwiler, Rachel J ., E ffe cts o n C eme nt o f H i g h E ff ic ie nc y S ep ar ato rs , Research and Development Bulletin

RD110T, Portland Cement Association [Wirkungen von Hochleistungssichtern auf Zement, Forschungs-und

Entwicklungsbulletin RDllOT, Portlandzernentverband], Skokie, Illinois, U.S.A, 1995.

PCA R&D Serial No. 2020



This publication is intended SOLELY for use by PROFESSIONAL

PERSONNEL who are competent to evaluate the significance and

limitations of the information provided herein, and who will accept

total responsibility for the application of this information. The

Portland Cement Association DISCLAIMS any and all

RESPONSIBILITY and LIABILITY for the accuracy of and the

application of the information contained in this publication to the

ful l extent permitted by law.

Portland Cement Association 5 42 0 O ld O rc ha rd R oad , S ko kie , Illin ois 6 00 77 -1 08 3, (7 08 ) 9 66 -6 20 0, F ax (7 08) 96 6-97 81

An organization of cement manufacturers to improve and extend the uses

of portland cement and concrete through market development, engineer-

ing, research, education and public affairs work.

Printed in U.S.A. RD110.0H

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