Chapter involves mining, crushing, and grinding of raw


Chapter 4

TECHNICAL ANALYSIS

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            The main objective of this part of
the study is to determine the technical viability of the proposed project.
After it has decided that the project would be feasible, the next steps
involves determining the most efficient and economical way of doing it.  Also, the technologies, facilities, and
equipments involved in the production are stated in this section along with
proper waste disposal. Careful selection and application of appropriate
technology are of critical importance as they greatly constitute the main
support of back bone of this study.

            This discussion will also include on
the important technical aspects of the plant’s actual operation including raw
materials to be used, production processes incorporated, productive capacity, and
the needed equipments for manufacturing.

A.   PRODUCT DESCRIPTION AND SPECIFICATION

Manufacturing
of cement involves various raw materials and processes. The preparation of
cement involves mining, crushing, and grinding of raw materials (principally
limestone and clay), calcination of the materials in a rotary kiln, cooling the
resulting clinker, mixing the clinker with gypsum and milling, storing and
bagging the finished cement.

§  RAW MATERIAL PREPARATION

The
prime raw material limestone after blasting in mines is broken into big
boulders. Then it is transported by dumpers to limestone crusher where it is
crushed to 15 to 20mm size. 

§  STACKING OF CRUSHED BULK LIMESTONE

After
crushing, the crushed limestone is piled longitudinally by an equipment called
stacker / reclaimer.  The stacker
deposits limestone longitudinally in the form of a pile. The pile is normally
250 to 300 m long and 8-10 m high. The reclaimer cuts the pile vertically,
simultaneously from top to bottom to ensure homogenization of limestone. The
crushed limestone from pile is transported through belt conveyor to hopper.
Similarly, other raw materials like clay, bauxite, iron ore etc. are also
transported by belt conveyor from storage yard to respective hoppers. All raw
materials are proportioned in requisite quantity through weigh feeders. The
proportioned raw materials are transported by belt conveyor to Raw Mill for
grinding into powder form.

After
grinding, the powdered raw mix, is stored in a raw meal-silo where blending
takes place. Blending is done by injecting compressed air. This powdered
material (Raw mix) is fed to the kiln for burning and Coal also requires
homogenization as it contains different ash.

§  BURNING

The
powdered raw mix is fed into 4 to 6 stage preheater from top by air
pressure.  The hot gases from kiln enter
preheater from bottom. The powdered raw mix slides down through cyclones and
comes in contact with hot air which travels from top to bottom. In preheater
the temperature of raw mix rises to 900

 to
1000

 and
nearly 90% Calcination (removal of CO2 from CaCO3) takes place before entering
the kiln.

Powdered
raw mix enters the kiln at one end and the burner is situated at the opposite
end. The rotary kiln rotates at the speed of 1 to 3 revolutions per minute
(RPM). The raw mix in the kiln melts first into liquid form and then transforms
into nodules due to the effect of the rotation of the kiln. There are two zones
inside the kiln, namely calcining zone and burning zone. The zone where raw mix
enters into the kiln is called calcining zone. Where temperature would be
950-1000

.

Burning
zone starts after this zone where temperature would be 1350-1450

. The hot clinker from kiln discharge is
cooled very quickly and quenched in air with the help of efficient coolers. The
temperature of clinker is brought  to
80-90

 
from 1350

. Fast cooling is very essential to get
good quality clinker. If cooling is not quick, the compound stability in
clinker will be adversely affected resulting in lower strength of cement after
grinding. Clinker from clinker silo is transported to clinker hopper by belt
conveyor.

Similarly,
gypsum, fly ash or any other additive are transported to their respective
hoppers by belt conveyors. All operations of feeding of raw meal, coal,
burning, temperature control and cooling are automatic through fuzzy logic computer
control.  These operations are controlled
from Central Control Room (CCR) which is the nerve center for any cement plant.

§  CEMENT GRINDING

The
proportioning of clinker, gypsum and fly ash is done by electronic weigh
feeders. In modern plants, clinker and gypsum are pre-crushed in a Roller press
and subsequently fed into ball mill for fine grinding. The installation of
roller press technology is very beneficial in terms of both quality and energy
conservation. The cement produced from roller press is showing better particle
size distribution in cement (and hence good strength development) and consumes
less power. The resultant product is called cement and stored in a Multi
compartment silo. This avoids intermixing of products.

§  PACKING OF CEMENT

Cement
is packed in HDPE bags having capacity of 50Kgs. Electronically controlled,
High efficiency rotary packers are used for Packing. Packers are highly precise
with tolerance of +/- 0.5%. It displays the weight of each bag before
discharge. It ensures the weight of packed bag is 50kg. Electronic balances are
installed for random cross check of packed bags. The packed bags are loaded
into trucks or railway wagons.

§  TRANSPORTATION OF CEMENT

Cement
is transferred to various locations as per the market requirement and stored in
go down or the distribution points, this is distributed through trucks and by
rail or by cement Bulkers from the plant.

B.   MATERIAL SELECTION

According
to Ashoka Machine Tools Corporation’s website, they types of machine plants are
the following:

·        
Clinker grinding units

·        
Complete cement plants with
clinker manufacturing

·        
Cement plants with vertical
shaft kilns and

·        
Cement plant with rotary kilns

The table 4.1 presented below is the cement plant equipment
with their type and the material of construction used.

Table 4.1 Summary of
Equipment, Type and Material of Construction

Equipment

Type

Material of Construction

Grinding Mills

Ball and Rod
mills

Steel fabrication

Kilns and Kiln
Shells

Vertical shaft
& rotary kilns

Steel fabrication

Separators

Dynamic High
Efficiency

Steel fabrication

Crushers

Hammer impact

Cast iron, Steel
casting & fabrication

Mill Internals

Liners &
diaphragms

Cast steels and
fabrication

Mill Headers/Ends

Trunion / Mill
headers

Cast steels

Mill Bearings

White metal trunions

Special wear
resistant castings

Feeders

Reciprocating,
weigh feeders

Steel fabrication

Elevators

Bucket belt,
chain

Steel fabrication

Conveyors

Screw & belt
type conveyors

Steel fabrication

Air Slides &
Air Lifts

Air blowing,
pneumatic

Steel fabrication

Process and
Storage Hoppers

MS fabricated

Steel fabrication

Vibrating Screens
and Feeders

Storage Silos and
aeration equipment

Packing equipment

 

As
seen from the table, most of the material of construction used is steel. Steel,
as a material of construction has been known for its strength, toughness,
ductility, weldability and durability. According to the American Foundry
Society, steel castings and fabrications can seem interchangeable because they
share many qualities: cast steels and wrought steels have such similar
mechanical properties that the American Society of Mechanical Engineers Code
doesn’t differentiate between steels on the basis of their manufacturing
process, but by their chemical composition. However, noticeable differences do
exist between the two that can affect the design and cost-effectiveness of a
component.

Wrought
products (rolled or forged) exhibit a characteristic known as directionality.
This characteristic, also known as anistropy, means that a component has strength
and ductility in the working direction but has lower transverse properties.

           Cast
steel products do not exhibit directionality; rather, they can be described as
isotropic. Steel castings can be stressed in any direction without concerns
over the lower strength, ductility and toughness that are exhibited in the
transverse direction of wrought products. Designers of fabrications must be
aware of the directional properties and incorporate them into the component’s
design, or it could become overstressed when a load is applied in the
transverse direction. (American Foundry Society).

    So in choosing between cast steels or
fabricating steels, several things have to be considered.

Castings
allow designers to buy shape cheaply. If the desired component is mostly steel
and the costs involved are mostly material, a fabrication should be used.
However, the more components, the more linear inches of welding required, the
more machining required per individual component, the more attractive casting
becomes. Designs or fabrications that comprise the most pieces and the most
welds are ideal candidates for a casting conversion.

           In
general, castings provide tighter tolerances and better mechanical
performances. They allow the designer to shape the component exclusively for
the project at hand, with no extra pieces or sections. Complex components and
assemblies that can be consolidated into fewer parts become cost-effective as
castings. Limiting assembly reduces cost. Castings often weigh less because the
geometry can be tailored to the actual component requirements instead of being
restricted by the capabilities of bars and sheets.

            The cost bases for fabrications and castings
are different. An increase in steel thickness, shape complexity and stiffening
in a fabrication pushes up costs because of the amount of welding and
nondestructive testing necessary. This also can be affected by the increased
risk associated with the stress relief of highly restrained, heavy sections.
Conversely for castings, castability is enhanced with increased section size,
and with optimum design, the cost/ton will reduce with increased weight. (American
Foundry Society).

C.  
QUANTITATIVE PFD

A simplified mass balance of a cement
production, shown in figure 4.1 had been presented by Semen Andalas
(2006) of LaFarge Indonesia.

Figure 4.1 Simplified Mass Balance Diagram

 

                As seen in the mass balance
diagram, the raw milling processes about 1100 tons of Iron stone, 4200 tons of
limestone, 340 tons of shale and 55 tons of sand and 200 tons of water vapor is
being emitted from the raw mill.

           
From the raw mill, about 5500 tons enters the preheater and the
precalciner. The amount of coal entering the preheater and the precalciner is
about 550 tons and 52000 Normal cubic meters per hour of stack gas are being
emitted from the preheater and the pre calciner. From the preheater and pre
calciner, clinkers of about 3450 tons enter into the cement mill where 1000
tons of pozzolan and 200 tons of gypsum are added.

 The
deficit in the amount of the raw meal and the clinker is due to the bypass dust
or the dust that was rejected from a kiln system in order to reduce the
chlorine or alkali content of the clinker to be produced.

The whole process will produce about 4650 tons
of cement.

           Utlu et al. (2006) conducted a study
using actual operational data from a Turkish cement plant. They have conducted
energy and exergy analysis on a raw mill (RM) with a capacity of 82.9
ton-material hourly. Exergy analysis was described as a thermodynamic method
used for evaluating engineering processes. It was also described by nptel.ac as
a tool being used to indicate how far the system departs from equilibrium
state. It is also defined as the energy that is available for use. It is the
maximum possible work in different processes that would bring systems into
equilibrium, meaning that after the system and the environment are equilibrium,
the exergy is zero.

            Their
main reason for using exergy analysis was to find out the causes and to
estimate the degree of the imperfection of thermal or chemical processes
quantitatively. Exergy analysis is used in industrial ecology to be able to use
energy more efficiently.

The
mass balance equation suggested by Utlu et al. (2006) can be expressed in the
form:

Where:
min = inlet mass flowrate

            mout = outlet mass
flowrate

           There was also a proposed way of computing
the clinker mass when given the mix composition of each raw material. According
to Taylor (1997), the modified Bogue calculation could be utilized to calculate
the mass-balance table showing how all oxide components are distributed among
the phases of the cement production. This method, though only originally used
for cement clinkers, could be adjusted in order to be used for the mass balance
calculation of the cement per se. In fact, this method is widely used in mass
balance computations in the cement industry.

             Even though this is a very helpful tool in
mass balance computations, the precision isn’t very high due to the assumption
that the four main clinker minerals are very pure.  The only composition of the assumed main
clinker materials are as follows:

·        
Alite: C3S, or tricalcium silicate

·        
Belite: C2S, or dicalcium silicate

·        
Aluminate phase: C3A, or tricalcium aluminate

·        
 Ferrite phase: C4AF, or
tetracalcium aluminoferrite

Clinker is made by combining lime and silica
and also lime with alumina and iron. If some of the lime remains uncombined,
(which it almost certainly will) we need to subtract this from the total lime
content before we do the calculation in order to get the best estimate of the
proportions of the four main clinker minerals present. For this reason, a
clinker analysis normally gives a figure for uncombined free lime.
(Understanding Cement, 1990).

The calculation is simple in principle. Firstly,
according to the assumed mineral compositions, ferrite phase is the only
mineral to contain iron. The iron content of the clinker therefore fixes the
ferrite content.

            Secondly, the aluminate content is fixed
by the total alumina content of the clinker, minus the alumina in the ferrite
phase. This can now be calculated, since the amount of ferrite phase has been
calculated.

            Thirdly, it is assumed that all the
silica is present as belite and the next calculation determines how much lime
is needed to form belite from the total silica content of the clinker. There
will be a surplus of lime.

             Fourthly, the lime surplus is allocated to the
belite, converting some of it to alite.(Understanding Cement, 1990).

The Bogue calculations are presented as
follows:

C3S= 4.0710 CaO-7.6024  SiO2-1.4297 Fe2O3-6.7187
Al2O3

C2S=8.6024 SiO2+1.0785
Fe2O3+5.0683 Al2O3-3.0710 CaO

C3A=2.6504 Al2O3-1.6920  Fe2O3

C4AF=3.0432 Fe2O3

In addition, Utlu
et al. (2006) have also described the energy balance as:

Figure 4.2 shows a block flow diagram of a
cogenerated cement production. Cogenerated cement production, otherwise called
as combined heat and power (CHP) is described as the use of a heat engine or
power station to generate electricity and useful heat at the same time.

Figure 4.2 Cogenerated Cement Plant

 

             To
get the mass balance of the materials in the process, the same principle
applies as that of what was discussed earlier from the former plant.

            As for the energy balance, an enthalpy
balance for the system is drawn, taking the reference enthalpy to be 0 kJ/kg at
0oC and 1 atm. The specific enthalpy of various components is obtained
from Peray’s cement handbook. The temperatures of the streams are measured and
the calorific value of coal is obtained from the plant data.  The energy required for the reaction has been
estimated using the correlations given in the handbook. The input energy with
various streams is calculated per kg clinker produced.

            A component wise energy balance was
similarly drawn using the information about the degree of calcination. The
material entering the calciner was 30% calcined and the material leaving the
calciner was 96% calcined. It was assumed that the calcination energy was
uniformly distributed over the temperature range to calculate the calcinations
energy in each component. It was also assumed that the coal entering the
calciner was fully combusted in the calciner and that entering the kiln was
combusted in the kiln.

The energy balance for the entire system was
summarized as a Sankey diagram, as shown in Figure 4.3.

Figure 4.3 Sankey Diagram of the System

 

The values were indicated as percent of the
total energy released from the combustion of coal in the calciner as well as
the kiln. The energy released on combustion of coal was about 3600 kJ/kg
clinker.

           It was observed in the enthalpy flow diagram
that there was a good agreement between the overall energy input to the system
and out of the system with an inconsistency of about 600 kJ/kg clinker that
amounts to about 15% of the input energy. Considering the nature of the data
sources and the simplifications made, the energy balance can be said to be in
good agreement. Some of the sources of error that have not been considered are
the radiation losses predominantly from the kiln shell and  the energy lost with the dust leaving with
the different streams. Table 4.2 provided the summary of the overall
energy balance of the system.

Table
4.2 Overall Energy Balance of the System

 

 

 

D.   EQUIPMENT SPECIFICATIONS

Table 4.3 Preheater
Specifications

PREHEATER

Item: Heater

No. required: 1

Function: Heat entering raw
feed from 50°C to 280°C, heat air exhaust from the cooler, heat flue gas
exhaust from the kiln

Operation:  Continuous

Utilities: Air

 

 

Table 4.4 Kiln Specifications

KILN

Item: Kiln

No. required: 1

Function: Raises materials to a
high temperature

Operation:
Continuous

Size: 8′ diameter X 92′ long

Insulation:
Stainless steel

 

 

 

 

 

Table 4.5 Calciner Specifications

CALCINER

Item: Calciner

No. required: 1

Function: Rotates inside a
heated furnace and performs indirect high-temperature processing

Operation:
Continuous

Size: 10″ diameter X 20′
long

Insulation: Alloy

 

 

Table 4.6 Cooler Specifications

COOLER

Item: Cooler

No. required: 1

Function: Transfers the heat
from the clinker to the combustion air

Operation:
Continuous

Specifications: 3/4 HP
208-230/460V 1725 RPM blower

Size: 28″wide
x 14′ long

Insulation: Stainless steel
conveyor and carbon steel frame

 

 

 

 

Table 4.7 Crusher Specifications

CRUSHER

Item: Primary Crusher and Secondary Crusher

No. required: 2

Function: Crush the raw
materials and limestones for it to become fine particulates

Operation:  Continuous

Size: 78″ diameter x
60″ wide (2000 mm x 1500 mm) rolls

 

 

Table 4.8 Mill Specifications

MILL

Item: Coal Mill, Cement Grinding Mill, and Grinding Mill

No. required: 3

Function: Grinds the hard,
nodular clinker from the cement kiln into the fine grey powder that is cement

Operation:  Continuous

Size: 13′ diameter x 21′ long

Insulation:
Rubber-lined mill

 

 

 

 

 

 

Table 4.9 Electrostatic Precipitator Specifications

ELECTROSTATIC PRECIPITATOR

Item: Electrostatic Precipitator

No. required: 1

Function: Gaseous emissions
cleaning

Operation:
Continuous

 

 

Table 4.10 Storage Specifications

STORAGE

Item: Crushed limestone storage, Clay storage, Coal storage, Clinker
storage, Cement storage, and Additives storage

No. required: 6

Function: Stores the raw
material and products when they are not in use and after they have gone the
process

Size: 7’6″
diameter x 18’8″ long with dished ends

Insulation: Stainless steel

 

 

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