Lecture # 2

Manufacturing process of Cement

Traditional Process of Cement Manufacturing

1- Aspdin mixed and ground hard limestone and finely divided clay into a slurry

2- The slurry was calcinated in a furnace till CO₂ wasexpelled

3- The calcinated mixture was then ground to a fine powder perhaps at a lower temperature than the

clinkering temperature by Aspdin.

Later in 1845 Isaac Charles Johnson burnt a mixture of clay and chalk at clinkering temperature to make better cement .

Modern ways to Manufacture CementThe process is divided into 4 main parts

– Quarrying and Grinding the raw materials – Mixing them in a intimately in certain proportions

depending upon their purity and composition – Burning the proportioned mix in a kiln at 1300 ⁰C

to 1500 ⁰C also called clinkering – Grinding of clinker and adding of gypsum

1-Quarrying of Raw materials– The raw materials required are calcareous

materials like chalk or limestone and argillaceousmaterials such as clay, marble or shale

– Local availability of raw material is an important concern setting up a cement factory in order to

reduce the transportation cost .The quarried materials are crushed, then

mixed at the plant to be fed to the kiln

2-Mixing and storage of feedMixing is carried out by the either of the

following processes – Wet Process – Dry Process

i) Wet ProcessThe materials in the wet process are mixed with a

water content of about 35% to 50%

ii) Dry ProcessIn this process the materials are mixed in the

powdered form using compressed air. The powdered material behaves like a fluid

when exposed to compressed air in a chamber and mixing becomes possible

This is the most widely used method in the recent years

It reduces the fuel consumption as the slurry is already dry unlike wet process.

Burning or clinkering (wet process) • A typical kiln is a large refractory lined steel tube

165 m long inclined to the horizontal with capacity of 1050 tonnes/day

• At the lower end of tube there is a burner and raw meal is fed from the other end

• The kiln rotates slowly at about 20-86 rph passing material to the lower end

• The water from the slurry needs to be driven off prior to the calcinating process

Zone 1: 0 - 35 min, temp is 800 – 1100 C o

Decarbonation. Formation of 3CaO•Al2O3(C3A) above 900 Co. Melting of fluxing

compounds Al2O3 and Fe2O3. heat

CaCO3 → CaO + CO2

Zone 2: 35 - 40 min, 1100 – 1300 Co Exothermic reactions and the formation of secondary

silicate phases as follows:

heat

2CaO + SiO2 → 2CaO•SiO2

Zone 3: 40 - 50 min, 1300 - 1450 – 1300 Co Sintering and reaction within the melt to form ternary silicates

and tetracalcium alumino-ferrates:

heat+time2CaO.SiO2 + CaO → 3CaO•SiO2

heat+time3CaO.Al2O3 + CaO + Fe2O3 → 4CaO•Al2O3•Fe2O3

Zone 4: 50 - 60 min, 1300 – 1000 Co Cooling and crystallisation of the various mineral phases formed

in the kiln.

The cooler :Immediately following the kiln is a large cooler designed to drop

the temperature of theclinker (as the fused material is now called) from 1000o

C to 150 Co. This is achieved byforcing air through a bed of clinker via perforated plates in the

base of the cooler. Like cement, the clinker will react with water and harden, but because it is

composed of 1-3 cm diameter fragments it is too coarse to be used.

Cement milling To produce the final product the clinker is mixed with gypsum

(CaSO4•2H2O), which is added as a set retarder, and ground for approximately 30

minutes in large tube mills. The cement flows from the inlet to the outlet of the mill (a rotating

chamber), being first ground with 60 mm then 30 mm diameter steel balls. The first grinding

breaks up the material and the second grinds it to a fine powder.