Manufacture of Cement

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THE CEMENT FACTORY

1. MANUFACTURE OF CEMENT



The Cement factory

1. Manufacture of Cement

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Table of Contents

1 MANUFACTURE OF CEMENT.............................................................3

1.1 ....MORTAR , CEMENT AND CONCRETE .............................................................3

1.2 ....FROM R AW MATERIAL TO CEMENT ............................................................3

1.3 ....THE MANUFACTURING PROCESS..................................................................4

1.4 ....R AW MATERIALS FOR CEMENT MANUFACTURING .......................................5

1.5 ....R AW MIX MODULI .......................................................................................7

1.6 ....IGNITION LOSS ............................................................................................8

Table of Figures

Figure 1 From Raw Materials to Cement ................................................................4

Figure 2. The Cement Process .................................................................................5

Figure 3 Normal Limits in Raw Mix and Clinker ....................................................6

Figure 4 Raw Material Composition .......................................................................7



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1 MANUFACTURE OF CEMENT

1.1 Mortar, cement and concrete

When limestone, which is composed mainly of calcium carbonate CaCO3, is heated

to temperatures above 700-800oC it will decompose into gaseous carbon dioxide,

C02, and solid calcium oxide or burnt lime, CaO, according to the process

CaCO3 + Heat ↔ CaO + C02

Burnt lime slaked with water will harden when left under atmospheric conditions by

absorbing C02 and thereby reversing the above process. Slaked lime mixed with sand

and used, as a mortar between building bricks has been known since antiquity.

If a small percentage of clay is burnt together with the limestone a different type of

binder will develop, which will hydrate and harden due to the direct influence of

water. This is the hydraulic building material, which we know and use today under

the name of cement.

The Romans already more than 2.000 years ago knew cement making, but the

knowledge was lost with the fall of their empire. It was not until the 19th century,

starting with the British bricklayer John Aspdin's patent in 1824, that manufacturing

of cement was taken up again as an industry.

Cement was originally manufactured in vertical shaft kilns and it was not until the

invention of the rotary kiln and the tube mill that large-scale industrial exploitation

be-came possible.

Cement is mainly used in the form of concrete, i.e. cement to which is added sand or

stones as filler during casting. Concrete has high compression strength but must be

rein-forced with iron bars or similar to obtain acceptable tensile strength.

1.2 From Raw Material to Cement

Cement is manufactured from 75-80% limestone and 20-25% clay, or from rawmaterials containing the same chemical constituents. The raw materials are quarried

and crushed after which they are mixed in the correct proportions. The raw mix is

then ground in a raw mill and subsequently burned in a rotary kiln at a temperature

around 1.450oC.

During the burning process, the raw materials will undergo a number of very

complex chemical reactions and will eventually leave the kiln as cement clinker, i.e.

agglomerates of clinker minerals. The main chemical reactions in the rotary kiln will

be dealt with in the chapter "Clinker burning".

The final product - cement - is obtained by grinding the clinker to a fine powder, in acement mill together with some 3-4% of gypsum. The gypsum is a necessary



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additive in order to retard the setting time of cement. Without the addition of

gypsum, the cement would harden immediately with the addition of water.

Figure 1 From Raw Materials to Cement

1.3 The manufacturing process

The original manufacturing method to produce cement was based on the so-called wet process. In this process the raw mix is ground with addition of up to 40% water

and the finished-ground raw mix leaves the mill and is fed to the mill as slurry. The

process is characterised by simple and uncomplicated installations. Also the

operation of raw mill and the kiln is fairly straightforward. Furthermore, it is easy to

mix - homogenise – the slurry and, finally, the wet process does only produce limited

amounts of dust and was therefore well suited for the early, primitive, de-dusting

facilities.

The energy crisis in the 1970's accelerated the development of the dry process, which

today is used in almost all modern cement plants. The obvious advantage of the dry

process is the fuel saving in the kiln, since there is no water to evaporate from the

kiln feed. Whereas the energy consumption per kilo clinker using the wet process is

in the range of 5,2-5,3 MJ (approx. 1.250 kcal), the dry process energy consumption

is no more than 3 MJ (approx. 700 kcal).

The dry process kiln is considerably shorter than a wet kiln, but both the kiln and

the raw grinding plants are more complicated both in installation and operation.

Furthermore, compared to the fluid slurry, it is much more complicated to

homogenise dry raw meal and the kiln requires highly efficient de-dusting.

Today, all new plants are based on the dry process and many old wet plants are either replaced or, if suitable, converted to dry or semi-dry production.



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Figure 1. The Cement Process

1.4 Raw materials for cement manufacturing

Cement is made primarily from burnt lime, CaO, and oxides of silica, SiO 2, alumina,

Al203 and iron, Fe203.

The raw mix compounds undergo a chemical change during their passage of the high

temperature kiln and are thereby transformed into clinker minerals.

The main clinker mineral, CaO, is not directly available in nature, but is obtained

from limestone or chalk as CaCO3, which decomposes during the calcining phase in

the rotary kiln.

The oxides of silica, alumina and iron are naturally present in clay, sand shale and

marl. If one of the oxides, most often the iron oxide, occurs in insufficient quantity in

the clay, a corrective material must be added to the raw mix.

Fly ash precipitated in dust filters of coal fired power stations is finding increasing

application in the manufacture of cement. Fly ash consists of iron- and alumina-rich

silicates and may to a certain extent substitute the clay component in the raw mix. Its

chemical composition is usually within 35-55% SiO2, 20-30% Al203 and 3-30%

Fe203. Besides, elements like free CaO, MgO, S03 and alkalis K 20 and Na20 may also

occur.



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mix composition. Approximate compositions of typical cement raw materials are

shown in Figure 1.4.

RAW MATERIAL % CaCO3 % Sio2 % Al2O3 % Fe2O3

Rich Limestone 100-96

Lean Limestone 96-90

Limey Marl 90-75

Marl 75-40

0-30 0-10 0-5

Clayey Marl 10-4

Clay 4-0

Fly Ash 2-10

35-70 10-25 5-10

Bauxite 10-16

Iron Ore 45-60

Bl. Furn. Slag 50-70

Pyrite 60-90Figure 2 Raw Material Composition

1.5 Raw mix moduli

Some of the clinker minerals primarily give strength to the cement while others act

as a necessary matrix for the strength-carrying minerals. It is important for the

quality of the cement as well as for the fuel economy and the lifetime of the lining in

the kiln that the proportions between the compounds of the raw mix are correct. This

composition compatibility is governed by some so-called moduli.

Correct formation of the matrix is mainly governed by the relationship between the

silicates and the alumina-ferro oxides - the silica modulus or silica ratio, MS - and by

the relationship between the oxides of aluminas and iron - the alumina modulus or

alumina ratio, MA. We have thus :

( )5,38,13232

2−

+

= NormallyOFeO Al

SiO MS

( )5,35,132

32−= Normally

OFe

O Al MA

The main reaction during the burning phase is the fusion between the basic CaO and

the acid oxides and it is essential that there should be sufficient CaO in order to

combine completely with these. On the other hand, a surplus of CaO must be avoided

since it is harmful to the cement.

The relationship between the total CaO and the total of acid oxides to which it is able

to bind, is the most important single parameter for the cement. The relationship iscalled the Lime Saturation factor and is expressed as



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[ ]%10065,08,18,2 32322

×

×+×+×

=

OFeO AlSiO

CaO LSF

The LSF value is normally in the range of 90 - 100%.

Sometimes the hydraulic modulus MH, expressing a direct relationship between the

ha-sic oxide CaO and the total of the acid oxides, is encountered. The MH is

expressed as

32322 OFeO AlSiO

CaO MH

++

=

The MH is normally in the range of 2,0 - 2,2. The MH modulus has today been

replaced by the LSF, but may still be encountered.

The relationship between the different moduli, clinker quality and burning economy

will be clarified in the chapter covering clinker burning.

1.6 Ignition loss

The raw materials undergo a reduction in weight during burning, partly from loss of

C02 during the calcination of CaCO3 in the limestone and partly from evaporation of

inherent water in the clay. This reduction must be taken into account whencalculating the total consumption of raw materials.

The loss can be calculated from the atomic weights, since this weight remains

unchanged during a chemical reaction. The approximate atomic weights are

Calcium, Ca 40

Carbon, C 12

Oxygen, 0 16

from which the following is derived :

Ca + C + O3 = Ca + O + C + O2

40 + 12 + 48 = 40 + 16 + 12 + 32

and thus

100 parts of CaCO3 = 56 parts of CaO + 44 parts of C02

The clay component loses normally approx. 7% inherent water during burning.

Therefore I kg of a raw mix consisting of 76% CaCO3 and 24% clay totals anignition loss as follows :



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from the CaCO3 : 0,76 × 0,44 = 0,3344 kg C02

from the clay : 0,24 × 0,07 = 0,0168 kg H20

ignition loss : 0,3512 kg total

In other words, 1 kg of raw mix yields a clinker production of (1,0000 - 0,3512) =

0,6488 kg or, alternatively, the manufacture of 1,0 kg clinker will require 1,54 kg of

raw mix.

The ignition loss for cement raw materials is normally around 0,35 to which must be

added a certain amount of dust loss. For first estimates it is common to calculate with

a necessary quantity of 1,6 kg of raw mix for each kg clinker produced. Thus

C R PP ×= 6,1



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Manufacture of Cement

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