Sadia Bashir Industrial Chemistry

Assistant Professor Cement Industry

Department of Chemistry
Manufacturing of
Cement
Manufacturing of Cement
Port land cement is made by heating raw materials
around 1200-1400 C. Bogues Compound in Cement is
the complex compound formed when the constituents
of cement are subjected at high temperatures to
form clinkers.
Cement Manufacturing Flowsheet Diagram
Stages in manufacturing of Cement

Stage 1: Raw Material Extraction/Quarry

Stage 2: Grinding, Proportioning, and Blending

Stage 3: Pre-Heating Raw Material

Stage 4: Kiln Phase

Stage 5: Cooling and final grinding

Stage 6: Packing and Shipping

Stage 1: Raw Material Extraction/Quarry

The raw cement ingredients needed for cement production are limestone

(calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, and

bauxite. The ore rocks are quarried and crushed into smaller pieces of

about 6 inches. Secondary crushers or hammer mills then reduce them

to an even smaller size of 3 inches. After that, the ingredients are

prepared for pyroprocessing.

Stage 2: Grinding, Proportioning, and Blending

The crushed raw ingredients are made ready for the cement-making process in

the kiln by combining them with additives and grinding them to ensure a fine

homogenous mixture.

The composition of cement is proportioned here depending on the desired

properties of the cement. Generally, limestone is 80%, and the remaining 20% is

clay. In the cement plant, the raw mix is dried (moisture content reduced to less

than 1%); heavy wheel-type rollers and rotating tables blend the raw mix and then

the roller crushes it to a fine powder to be stored in silos and fed to the kiln.
Stage 3: Pre-Heating Raw Material

Preheaters are used in dry kiln cement production plants to heat the

raw mixture and drive off carbon dioxide and water before it is fed into

the kiln.A pre-heating chamber consists of a series of cyclones that

utilizes the hot gases in order to reduce energy consumption and

make the cement-making process more environment-friendly. The raw

materials are passed through here and turned into oxides to be burned

in the kiln.
 Stage 4: Kiln Phase
The kiln phase is the principal stage of the cement production process. Here,
clinker is produced from the raw mix through a series of chemical reactions
between calcium and silicon dioxide compounds. Though the process is complex,
the events of the clinker production can be written in the following sequence:
 Evaporation of free water.
 Evolution of combined water in the argillaceous components.
 Calcination of the calcium carbonate (CaCO3) to calcium oxide (CaO).
 The reaction of CaO with silica to form dicalcium silicate.
 The reaction of CaO with the aluminum and iron-bearing constituents forms the
liquid phase.
 Formation of the clinker nodules.
 Evaporation of volatile constituents (e. g., sodium, potassium, chlorides, and
sulfates).
 The reaction of excess CaO with dicalcium silicate forms tricalcium silicate.
 Stage 4: Kiln Phase

 The above events can be condensed into four major stages based on the change in
temperature inside the kiln:

 100°C : Evaporation of free water

 100°C -430°C : Dehydration and formation of oxides of silicon, aluminum, and iron

 900°C -982°C : CO2 is evolved and CaO is produced through calcination

 1510°C : Cement clinker is formed

 The kiln is angled by 3 degrees to the horizontal to allow the material to pass through it,
over a period of 20 to 30 minutes. By the time the raw mix reaches the lower part of the kiln,
clinker forms and comes out of the kiln in marble-sized nodules.
Kiln: A kiln is a thermally insulated chamber, a
type of oven, that produces temperatures
sufficient to complete some process, such as
hardening, drying, or chemical changes.
The earliest kilns in which cement was burned in batches were bottle kilns, followed by
chamber kilns and then by continuous shaft kilns. The shaft kiln in a modernized form is
still used in some countries, but the dominant means of burning is the rotary kiln.

These kilns—up to 200 metres (660 feet) long and six metres in diameter in wet process
plants but shorter for the dry process—consist of a steel, cylindrical shell lined with
refractory materials. They rotate slowly on an axis that is inclined a few degrees to the
horizontal.

The largest rotary kilns have outputs exceeding 5,000 tons per day.
 Rotary Kiln
The rotary kiln is divided in to three parts; the upper part, the central part and the lower part. The
processes ongoing in each of the parts are:

 Upper Part:

It is maintained at 400°C. Water or moisture in the feed mixture is evaporated.

 Central Part:

The temperatures are around 1000°C. This part is also known as calcination zone because lime
stone decomposes in this part. Calcium carbonate decomposes to form calcium oxide and carbon
dioxide. After the carbon dioxide escapes remaining material forms small lumps called nodules.

 Lower Part:

The temperatures in this part are between 1500-1700°C. This part is also known as clinkering zone.
The calcareous and argillaceous materials react to form calcium silicates and calcium aluminates.
The aluminates and silicates fuse together to form small and hard stones known as clinkers.
 Rotary Kiln

The process of burning is carried out in rotary kiln and the rotary kiln is lined with

refractory bricks. The prepared mixture stored in the silo is fed into the rotary kiln from

upper side and a burning fuel is fed from the lower side of the rotary kiln in order to

produce hot flame. The fuel can be oil or powdered coal or it even can be hot gases.

There is a temperature gradient inside the rotary kiln with the lowest temperature being

at the upper side and the highest temperature being at the lower side. There is a

countercurrent motion between the feed mixture and the burning fuel. Due to the slow

rotation of the rotary kiln the temperature of the feed mixture rises gradually as it

moves from the upper end to the lower end of the rotary kiln.
Rotary Kiln
Stage 5: Cooling and final grinding
 After exiting the kiln, the clinker is rapidly cooled down from 2000°C to 100°C-200°C
by passing air over it. At this stage, different additives are combined with the
clinker to be ground in order to produce the final product, cement. Gypsum added
to and ground with clinker regulates the setting time and gives the most
important property of cement, compressive strength. It also prevents
agglomeration and coating of the powder at the surface of balls and mill walls.
Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added
as grinding aids to avoid powder agglomeration. Other additives sometimes used
are ethylene glycol, oleic acid, and dodecyl-benzene sulphonate.

 The heat produced by the clinker is circulated back to the kiln to save energy. The
last stage of making cement is the final grinding process. In the cement plant, there
are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred
to these rotating drums and ground into such a fine powder that each pound of it
contains 150 billion grains. This powder is the final product, cement.
Stage 6: Packing and Shipping
Cement is conveyed from grinding mills to silos (large storage tanks) where it

is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities

by trucks, trains, or ships, and only a small amount is packed for customers

who need small quantities.

 Chemical Reactions during Cement
Manufacturing Process

The reactions that take place (after evaporation of free water) between the reactants in the kilning

phase of cement making process are as follows:

 Clay Decomposition:

Si2Al2O5(OH)2 → 2 SiO2 + Al2O3 + 2 H2O (vapor)

KAlSi3O8 (orthoclase) + 0.5 SO2 + 0.25 O2 → 3 SiO2 + 0.5 Al2O3 + 0.5 K2SO4

 Dolomite Decomposition:

CaMg(CO3)2 → CaCO3 + MgO + CO2

KMg3AlSi3O10(OH)2 + 0.5 SO2 + 0.25 O2 → 0.5 K2SO4 + 3 MgO + 0.5 Al2O3 + 3 SiO2 + H2O (vapor)
 Chemical Reactions during Cement
Manufacturing Process

 Low-Temperature Calcite Decomposition:

2 CaCO3 + SiO2 → Ca2SiO4 + 2 CO2
2 MgO + SiO2 → Mg2SiO4
Ca5(PO4)3OH + 0.25 SiO2 → 1.5 Ca3(PO4)2 + 0.25 Ca2SiO4 + 0.5 H2O (vapour)

 Alumina and Oxide Reaction:

12 CaCO3 + 7 Al2O3 → Ca12Al14O33 + 12 CO2
4 CaCO3 + Al2O3 + Fe2O3 → Ca4Al2Fe2O10 + 4 CO2
4 CaCO3 + Al2O3 + Mn2O3 → Ca4Al2Mn2O10 + 4 CO2
 The reaction of Remaining Calcite:
CaCO3 → CaO + CO2
 Sintering:
Ca2SiO4 + CaO → Ca3SiO5
Types of Cement Manufacturing Process
The three processes of manufacture of cement are known on the Basis of raw
material grinding and feeding to Kiln

1. Wet Process

when the raw materials are ground wet and fed to the kiln as a slurry

2. Dry Process

when the raw materials are ground dry and fed to kiln as dry powder

3. Semi Dry Process

when the raw materials are ground dry and then moistened to form nodules that

are fed to the kiln.

Dry Process
Wet Process
Dry Process vs Wet Process
Emissions during Cement Manufacturing

i) Gases and VOCs

ii) Dust

iii) Noise

iv) Bad Odour

Gaseous atmospheric emissions of CO2, NOx ,SO2 ,VOCs are released during clinker formation.

It is estimated that around 4–8 percent of the world’s carbon dioxide (CO2) emissions come from

the manufacture of cement, making it a major contributor to global warming. Some of the

solutions to these greenhouse gas emissions are common to other sectors, such as increasing

the energy efficiency of cement plants, replacing fossil fuels with renewable energy, and

capturing and storing the CO2 that is emitted.