Limestone process flow for the preparation of raw

Limestone and other
reoccurring raw substances are normally received into the plant in the size
range of 1 to 2 mm from the quarry. Production of a mixture of raw materials
with a maximum size of about 0.2 mm is done by breaking down the sizes of the
raw materials derived from the quarry. The ratio of reduction to which these
raw materials need to be 1000-2000:0.2 (Innovations in Portland Cement
Manufacturing,2009). This breaking down of raw materials is difficult to be
achieved by one stage operation. Therefore, after the reduction stage it should
be ensured that raw mix has a good homogeneity to achieve superior quality
product and increase in the efficiency of the plant. Stocks need to be
regularly built to keep the continuity of the manufacturing process at various
points.

The process flow for the
preparation of raw materials is a multi-stage process and involves the
following steps:

·      
Single stage Crushing, two-stage Crushing,
Three-stage crushing.

·      
Pre-homogenization and Crushed materials
storage.

·      
Grinding

·      
Proportioning, homogenization, and storage
of fine material

·      
Quality control in raw meal preparation

2.2.1.1
Crushing

Crushing is the basic
process which involves reduction in the size of raw materials and this process
is carried out by a mechanical equipment inside the plant. Crushing is done in
three stages:

Name

Product Size

Size Range(mm)

Coarse

Highest

200-150

Intermediate

Medium

70-40

Fine

Small

25-5

 

Compression, attrition,
impact combined or individually affect the crushing process in a combination or
each individually. Primary reduction of raw materials in a cement plant is done
by crushers namely single or twin rotor or impact crushers. In case of raw
materials which are hard other type of crushers are used named as Jaw crushers,
to break down abrasive materials the jaw crushers are used along with roll
crushers.

2.2.1.2 Pre-Homogenization and
Crushed materials storage

The necessity of pre-homogenizing
the raw materials in a cement production after the secondary crushing is very
important as it evens out the materials as their chemical composition varies.
Limestone and component of aluminosilicate are the main raw materials for which
this unit operation is carried out.

Another name for the pre
-homogenizing systems is “blending beds”.  The ratio between the input significant
chemical parameter of a given stockpile (Sin) to the standard deviation of the
output parameter for the same stockpile (Sout) is known as the Blending
Effect(H).

Blending Effect (H) =
Sin/Sout

 Measuring the homogenizing effects which are
greater than 1:10 is not possible as inaccuracies take place during the analysis
and sampling of the lumpy materials. However, in practice, the efficiency of
blending of these systems is within the required range (Innovations in Portland
Cement Manufacturing,2009).

2.2.1.3 Grinding

Two types of grinding
namely dry and wet grinding further crush the preblended raw materials into a
fine mixture. Most common type of grinding used is dry grinding as wet grinding
process is energy intensive which leads to excessive cost of energy usage.

Balls Mills are used
majorly used all over the world for grinding process. These mills consist of
rotating cylinders which are full of steel balls, as the material to be grinded
enters these cylinders due to the attrition from the grinding balls present in
it there is an impact on the material and the grinding takes place. The peak
usage of the ball charge energy for fine grinding determines the output and
efficiency of the ball mills (Innovations in Portland Cement
Manufacturing,2009).

2.2.1.4 Homogenization of raw meal

The major step in the
clinker production process is the homogenization of the raw meal before the
calcination process. All over the world there are 4 methods which are widely
known in the cement industry for the raw meal homogenization. Firstly, in the
wet process comes the slurry mixing which is the most traditional method, all
the other methods that include gravity systems, mechanical systems come under
the dry process plants (Innovations in Portland Cement Manufacturing,2009).

In the homogenization of
slurry, the slurry from the is transported to the slurry tanks, the tracking of
the average composition of the slurry is done by chemical analysis of on-spot
samples. As the slurry tank is filled about 2/3th it is compresses with air for
the homogenization of slurry. Before entering the kiln, the slurry from the
tank is again sent into a basin for further homogenization which is done by
using mechanical and pneumatic components. Working of the basin is related to
the working of an ideal blender.

Multiple storage slurry
tanks are installed with regulated facilities of withdrawal and are all present
in a mechanical system. Regular withdrawal of the material from the tank or
silos at variable rates is done to achieve blending. Usage of this type of
mixing uses less power, but the power consumption increases as these systems
require high material handling. Adding to the information, the number of silos
required in this system is more compared to other systems. Summing up all these
shortcomings this type of homogenization system is not widely used in the
cement industry (Innovations in Portland Cement Manufacturing,2009).

Air fluidization method
based on pneumatic unit is the most common type of homogenization method used
over the past several decades. In this system, the raw meal almost behaves like
a fluid due to the air which is introduced through a permeable medium in the
bottom of the silo. This method is done to increase the blending efficiency for
a dry material, in some cases this method is also the highest power consumer (Innovations
in Portland Cement Manufacturing,2009).

Reduction in power
consumption is only way which paved for the homogenization through the gravity
approach.

2.2.1.5 Quality Control in Raw Meal
Preparation

Onsite and offsite
equipment control almost all off the above unit operations. An important part
of the raw materials preparation facility is the control laboratory which needs
to be equipped with proper software and hardware. (Innovations in Portland
Cement Manufacturing,2009).

The quality of the
finished product is dependent on the preparation of a proper and uniform kiln
feed and requires good monitoring about the quality of the raw materials
entering the kiln, the preblending stockpile composition, size distribution of
the particles and the variation in the chemical composition of the raw mill
output and the consistency and the efficacy of the homogenization system in
terms of the output which forms the kiln feed. Sampling arrangements are used
to control and monitor the system with the help of computer software and specialized
instruments.

2.2.2 Kiln Operation Systems

After the process which
includes the selection, blending, homogenization and grounding the raw
materials into a fine uniformed kiln feed and for the clinkering reactions to
take place this feed is subjected to tremendous heat. Starting from the extraction
of the kiln feed from the storage which is then weighed and moved to the kiln,
and the extraction of the clinker on its way to the clinker storage from the
cooler, this entire process is known as the Pyro-Processing stage and schematic
diagram given below represents the process.