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Cement Industries

Introduction

• Cement– inorganic material – having adhesive and cohesive properties– sets and hardens when mixed with water (hydrolysis and

hydration)• Uses

– buildings, roads, bridges, dam etc • Types

– hydraulic (Portland cement)• harden because of hydration, can harden underwater or when

constantly exposed to wet weather– non hydraulic (Lime and gypsum plaster)

• slaked limes harden by reaction with atmospheric carbon dioxide

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Installed Production Capacity (Mt/yr)Sr. No. Name and location Capacity 2012/2013

1 Lucky Cement Limited - Pezu 3.91 3.73

2 Lucky Cement Limited - Indus Highway Karachi 3.60 3.43

3 Bestway Cement Limited - Chakwal 3.60 3.43

4 Fauji Cement Company Limited - Fateh Jang 3.43 3.27

5 Maple Leaf Cement Factory Limited - Daudkhel 3.37 3.21

6 Kohat Cement Company Limited - Kohat 2.68 2.55

7 D. G. Khan Cement Limited - D. G. Khan 2.11 2.01

8 D. G. Khan Cement Limited - Chakwal 2.11 2.01

9 GharibWal Cement Limited - Jehlum 2.11 2.01

10 Lafarge Pakistan Cement Company Limited - Chakwal 2.05 1.95

11 Pioneer Cement Limited - Khushab 2.03 1.93

12 Attock Cement Pakistan - Hub Chowki Lasbela 1.80 1.7113 Askari Cement - Nizampur 1.58 1.50

14 Bestway Cement Limited - Hattar 1.23 1.17

15 Flying Cement Limited - Lilla 1.20 1.14

16 Dewan Hattar Cement Limited - Hattar 1.13 1.0817 Askari Cement Limited - Wah 1.10 1.05

18 Cherat Cement Company Limited-Nowshera 1.10 1.05

19 Bestway - Mustehkum Cement Limited - Hattar 1.09 1.04

20 Al-Abbas Cement Limited - Nooriabad Dadu 0.95 0.9021 Fecto Cement Limited - Sangjani 0.82 0.78

22 Dewan Hattar Cement Limited - Dhabeji 0.79 0.75

23 Dandot Cement Limited - Jehlum 0.50 0.48

24 Thatta Cement Limited - Thatta 0.49 0.47

0

5

10

15

20

25

30

35

40

45

50

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Mill

ion

tons

Production Consumption Exports Surplus

45 Mt45 Mt

23 Mt23 Mt

10 Mt10 Mt

8 Mt8 Mt

Mt %

Pakistan 45 1.25

India 239 6.64

China 2 137 59.36

Europe 156.3 4.34

USA 73 2.03

World 3600 100

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History

• 1796 Roman cement James Parker – natural cement made by burning certain clay deposits (containing

minerals and calcium carbonate)

• 1817 Artificial cement Louis Vicat– by burning chalk and clay mixture

• 1822 British cement James Frost– similar

• 1824 Portland cement Joseph Aspdin

• 1841 Modern Portland cement William Aspdin

Introduction

• Types– Natural cement : Hydraulic lime

• burning of 20-40% clay, carbonate of lime and small amount of magnesium carbonate

– Artificial cement : • Calcination of calcareous (Ca) material followed by the

clinkering process with argillaceous (Al + Si) material at high temperature

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Based on the application, appearance and constituents

• Acid resistance cement

• Blast furnace cement

• Coloured cement

• White cement

• Rapid hardening cement

• High alumina cement

• Puzzolana cement

• Hydrophobic cement

• Expanding cement

• Low heat cement

• Quick setting cement

• Sulfate resisting cement

Chemical composition of grey cementNomenclature Molecular

formulaCement chemists notation

Concentration range (w/w

%)

Tricalcium silicate 3CaO•SiO2 C3S 40-80

Dicalcium silicate 2CaO•SiO2 C2S 10-50

Tricalcium aluminate 3CaO•Al2O3 C3A 0-15

Tetracalcium aluminoferite 4CaO•Al2O3•Fe2O3 C4AF 0-20

Calcium oxide CaO 0-3

Magnesium oxide MgO 0-5

Potassium sulphate K2SO4 0-2

Sodium sulphate Na2SO4 0-1

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Compounds

• Tricalcium Silicate (C3S)– Hardens rapidly and is largely responsible for initial set and early

strength• Dicalcium Silicate (C2S)

– Hardens slowly and contributes largely to strength increases at ages beyond 7 days

• Tricalcium Aluminate (C3A)– Liberates a large amount of heat during the first few days of

hardening and together with C3S and C2S may somewhat increase the early strength of the hardening cement

• Tetracalcium Aluminoferrite (C4AF)– acts as a flux during manufacturing. Contributes to the colour

effects that makes cement gray.

Chemical composition of white cementCharacteristics Molecular formula Typical concentration (w/w %)

Chemical composition (%) SiO2 22.5-23.8

Al2O3 2.3-6.2

Fe2O3 0.19-0.4

CaO 66.3-68.0

MgO 0.48-1.0

SO3 0.65 – 2.8

F 0.24 – 0.85

K2O 0.12-0.14

Na2O 0.17

Potential compound composition (%)

3CaO•SiO2 70

2CaO•SiO2 19

3CaO•Al2O3 8

4CaO•Al2O3•Fe2O3 1

Except for colour, white cement has the same properties as grey cement.

highly pure limestone, white clays, kaolin, quartz sand, feldspar, diatomaceous earth, low contents of metals such as iron and manganese.

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Principle constituents• Lime

– calcining the lime stone (CaCO3) at temperature that it will slake, when brought in contact with water. It is principal constituent of cement. Proper amount of lime is important as excess of it reduces the strength as well as lesser amount also reduces the strength and makes its quick setting.

• Silica – It imparts strength to cement.

• Alumina – It works as an accelerator and makes the cement quick settling.

• Gypsum (Calcium sulfate) – It retards the setting action of cement but enhances the initial setting time.

• Iron oxide – It provides colour, strength and hardness of cement.

• Magnesia – If present in small amount impart hardness and colour to cement

• Sulfur trioxide – If present in small amount it imparts soundness to cement but excess of it is undesirable

• Alkalis – Most of the alkalis present in raw materials are carried away by the flue gases during heating and cement

contains only a small amount of alkalis. If present in excess causes the dehydration in cement.

Manufacturing process

• The basic chemistry of the cement manufacturing process begins with the decomposition of calcium carbonate (CaCO3) at about 900 °C to calcium oxide (CaO, lime) and liberated gaseous carbon dioxide (CO2); this process is known as calcination.

• This is followed by the clinkering process in which the calcium oxide reacts at a high temperature (typically 1400 – 1500 °C) with silica, alumina, and ferrous oxide to form the silicates, aluminates, and ferrites of calcium which comprise the clinker.

• The clinker is then ground or milled together with gypsum and other additives to produce cement.

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Manufacturing process

• It involves the following steps – Storage and preparation of raw materials

– Burning

– Grinding

– Storage and packaging

Process routes

• The choice of process is, to a large extent, determined by the state of the raw materials (dry or wet). – The dry process, in which the raw materials are ground and dried to

raw meal in the form of a flowable powder. The dry raw meal is fed to the preheater or precalciner kiln or, more rarely, to a long dry kiln.

– The semi-dry process, in which the dry raw meal is pelletised with water and fed into a preheater before the kiln or to a long kiln equipped with crosses.

– The semi-wet process, in which the slurry is first dewatered in filter presses. The resulting filter cake is extruded into pellets and then fed either to a preheater or directly to a filter cake dryer for raw meal production.

– The wet process, in which the raw materials (often with a high moisture content) are ground in water to form a pumpable slurry. The slurry then is either fed directly into the kiln or first to a slurry dryer.

* Wet processes are more energy consuming, and thus more expensive.

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Raw materials

– Clay

– Limestone

– Gypsum

– Water

Dry process• Lime stone or chalk and clay are crushed into gyratory crusher to get 2-5

cm size pieces.• Crushed material is ground to get fine particle in ball mill or tube mill.

Materials after screening stored in a separate hopper. • Typical dry grinding systems used are:

– Tube/ball mill, centre discharge– Tube/ball mill, airswept– vertical roller mill

• Classification of the powder is carried out using air separators.• The powder is mixed in require proportions to get dry raw meal which is

stored in silos and kept ready to be fed into the rotary kiln. Raw materials are mixed in required proportions so that average composition of the final product is maintained properly.

• For raw meal transport to storage silos, pneumatic and mechanical conveyor systems are used.

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Wet process

• Calcareous raw material is crushed, powdered and stored in silos. • Clay is washed with water in wash mills to remove adhering organic

matter. The washed clay is stored separately. • Powdered lime stone and wet clay are allowed to flow in channel

and transfer to grinding mills (closed circuit milling systems) where they are intimately mixed and paste is formed known as slurry.

• Grinding may be done either in ball mill or tube mill or both. When sufficiently fine, the material passes through screens in the wall of the wash mill and is pumped to storage.

• Then slurry is led to correcting basin where chemical composition may be adjusted. The slurry contains 38-40% water stored in storage tank and kept ready for feeding to a rotary kiln.

Clinker burning

• This part of the process is the most important in terms of emissions potential and of product quality and cost. In clinker burning, the raw meal/slurry is fed to the rotary kiln system where it is dried, preheated, calcinedand sintered to produce cement clinker.

• Cyclone preheater– The raw materials are preheated or calcined in preheater

or series of cyclones before entering to the rotary kiln. A preheater, also called as suspension preheater is a heat exchanger in which the moving crushed powder is dispersed in a stream of hot gas coming from the rotary kiln. Common arrangement of series of cyclones is shown in figure.

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Preheater/Precalciner• The heat transfer of hot kiln gases to raw meal takes place in co-current. The raw

materials are heated upto 800 °C within a less than a minutes. About 40% of the calcite is decarbonated during the heat transfer.

• The quality and quantity of fuel used in the kiln can be reduced by introducing a proportion of the fuel into preheater. 50 – 65 % of the total amount of fuel is introduced into preheater or precalciner which is often carried out by hot air ducted from cooler.

• The fuel in the precaliner is burnt at relatively low temperature, there so heat transfer to the raw meal is very efficient. The material has residence time in the hottest zone of a few seconds and its exit temperature is about 900 °C, 90 – 95% of calcite is decomposed. Ash from the fuel burn in the precalciner is effectively incorporated into mix.

• Advantages of precalination– Decrease the size of kiln – Decrease in capital cost – Increase in rate of material passes to the kiln. – Decrease in rate of heat provided which ultimately lengthens the life of refractory lining – Less NOx is formed, since much of the fuel is burnt at a low temperature, and with some

designs NOx formed in the kiln may be reduced to nitrogen.

Clinker burning

• It is essential to maintain kiln charge temperatures in the sintering zone of the rotary kilns at between 1400 and 1500 °C, and the flame temperature at about 2000 °C. Also, the clinker needs to be burned under oxidising conditions. Therefore, an excess of air is required in the sintering zone of a cement clinker kiln.

• The rotary kiln consists of a steel tube with a length to diameter ratio of between 10 : 1 and 38 : 1. The tube is supported by two to seven (or more) support stations (roller bearings), has an inclination of 2.5 to 4.5 % and a drive rotates the kiln about its axis at 0.5 to 5.0 revolutions per minute.

• The combination of the tube’s slope and rotation causes material to be transported slowly along it. In order to withstand the very high peak temperatures, the entire rotary kiln is lined with heat resistant bricks (refractories). All long and some short kilns are equipped with internals (chains, crosses, lifters) to improve heat transfer.

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Clinker burning

• Due to inclined position and slow rotation of the kiln, the material charged from upper end is moving towards lower end (hottest zone) at a speed of 15m/h. In the upper part, water or moisture in the material is evaporated at 400 °C, so it is known as drying zone.

• In the central part (calcination zone), temperature is around 1000 °C, where decomposition of lime stone takes place. After escapes of CO2, the remaining material in the forms small lumps called nodules. – CaCO3 CaO + CO2

Clinker burning

• The lower part (clinkering zone) have temperature in between 1500-1700 °C where lime and clay reacts to yielding calcium aluminates and calcium silicates. The aluminates and silicates of calcium fuse togather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10 mm.– 2CaO + SiO2 Ca2SiO4

– 3CaO + SiO2 Ca3SiO5

– 3CaO + Al2O3 Ca3Al2O6

– 4CaO + Al2O3 + Fe2O3 Ca4Al2Fe2O10

• The hot clinkers coming from burning zone are cooled down by air admitting counter current direction at the base of rotary kiln. Resulting hot air is used for burning powdered coal or oil.

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Clinker coolers• The clinker cooler is an integral part of the kiln system and has a decisive

influence on performance and economy of the pyroprocessing plant. The cooler has two tasks: to recover as much heat as possible from the hot (1450 °C) clinker so as to return it to the process; and to reduce the clinker temperature to a level suitable for the equipment downstream.

• Heat is recovered by preheating the air used for combustion in the main and secondary firing as close to the thermodynamic limit as possible.

• Rapid cooling fixes the mineralogical composition of the clinker to improve the grindability and optimise cement reactivity.

• Typical problems with clinker coolers are thermal expansion, wear, incorrect airflows and poor availability

• There are three types of coolers: rotary, grate and verticle/gravity coolers.– Rotary coolers : tube and planetary coolers– Grate coolers : travelling, reciprocating, third generation grate coolers

Clinker grinding

• Portland cement is produced by intergrinding cement clinker with sulphates such as gypsum and anhydrite.

• Grinding plants may be at separate locations from clinker production plants.

• Commonly used finish grinding systems are:– Tube/ball mills, closed circuit (mineral addition is rather

limited, if not dry or pre-dried)– vertical roller mills (best suited for high mineral additions

due to its drying capacity, best suited for separate grinding of mineral addition)

– roller presses (mineral addition is rather limited, if not dry or pre-dried).

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Typical process flow diagram

Comparison of dry and wet processCriteria Dry process Wet process

Hardness of raw material Quite hard Any type of raw material

Fuel consumption Low High

Time of process Lesser Higher

Quality Inferior quality Superior quality

Cost of production High Low

Overall cost Costly Cheaper

Physical state Raw meal (solid) Slurry (liquid)

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Ordinary Portland cementChemical Analysis Typical EN 197-1 52.N & BS 12-1996 IS 12269

Loss on Ignition % 3.3 5.0% Max. 4.0% Max.Insoluble residue % 0.95 0.95% Max. 2.0% Max.Sulphate as SO3 % 2.5 4.0% Max. 3.0% Max.Chloride % 0.05 0.1% Max. 0.1% Max.MgO % 2.25 --- 6.0% Max.Lime Saturation Factor % 0.93 --- 0.8Min - 1.02% Max.Alkali Equivalent % 0.57 0.60% Max. --Physical PropertiesFineness by Blaine cm2/g 3200-3000 --- 2250Setting & Soundness BehaviourInitial Setting Time (minutes) 180 45 min (Min.) 30 min (Min.)

Final Setting Time (hours) 4:30 h:m -- 600 min (Max.)Lechatlier's Expansion 1:00 mm 10 (Max.) 10 mm (Max.)Autoclave Expansion 0.05% -- 0.80% (Max.)Compressive Strength Performance Mortar EN 196-1 Testing Method

Typical Results EN 197-1 52.5 N & BS 12-1996

Early strength – 2days (N/mm2) 22.5 20.0 Min.

Standard Strength – 28days (N/mm2) 56 52.5 Min.

Typical Results IS 12269 53 grad

3days (N/mm2) 38 27.0 Min

7days (N/mm2) 47 37.0 Min.

Standard Strength - 8days (N/mm2) 56 53.0 Min.