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AND CERAMICS INDUSTRY |
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Introduction on Ceramics industry
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The Refractories
are structural materials and these are very essential
to most process industries. Refractories industry provides
the materials required to achieve energy conservation
in other industries.
The vast improvements achieved during the last 10-15
years both in the quality of refractories and improvements
in the processes in which these refractories are used
have resulted in drastic reduction in their consumption
itself. This has also resulted in the shift to better
and newer types of refractories and reduction in consumption
of conventional refractories.
Refractories can be broadly classified into two categories
- Shaped Refractories
a. Fired Refractories
b. Chemically, hydraulically bonded and fused cast
Refractories
- Unshaped Refractories
These are granular or monolithic refractories used
in the building up of new furnace walls or in their
maintenance. These materials are applied insitu
through, casting, guniting, ramming etc.
In some cases they are precast into shapes and installed
in area of use to be fired insitu.
Manufacturing process involves converting the mined
or beneficiated or synthesized raw materials into
required grain sizes and mixing them with binders
and then shaped into bricks or shapes, which are
fired (sintered) to high temperature to achieve
desired dense structure.
In the case of monolithic refractories different
fractions of prepared refractory materials with
appropriate bonding agents, and other ingredients
like plasticizer, surfactants, wetting agents, dispersants
etc are mixed and packed for shipping.
Raw material preparation is an essential requirement
for both types of refractories. First step is benefication
of the mined material if it is associated with undesirable
compounds or the content of required element is
low.
Next step is to thermally treat-calcine-the material
to complete all the changes in the material or remove
water of crystallization and sinter to increase
the density. The material is then subjected to crushing
and grading operation to get required grain sizes.
The areas to be seen in detail for energy conservation
in ceramics industry are
- raw material section
- pressing section
- Raw material section
Specific energy consumption for different types
of refractories and energy requirement for production
of sintered products (starting materials) and shaped
products in GJ/t is shown in the table below
| Raw material
|
GJ/t
|
Shaped
Product |
GJ/t |
| Natural MgO sint |
20 - 30 |
fireclay |
2 - 6 |
| Seawater MgO, sint |
40 |
High Alumina |
6-12 |
| MgO, fused |
95 |
Chrome Mag
|
- |
| Corundum, sint |
10 |
a) Direct bonded
|
6-14 |
| Corundum, fused |
30-35 |
b) Chemical bonded |
0.5-2 |
The above table shows that preference should be
given wherever feasible for shaped product
Resin bonders
Another trend is to utilise chemical bonding using
resin bonder (chemical bonders). The temperature
requirements for drying these types of bricks is
only 200 to 400oC when compared to 1200 to 1600oC
required for bricks with conventional binders.
Machinery
Refractories
Depending on hardness of material, different types of
crushing and grading equipment are used.The improved
machinery now available are
Higher width Jaw crushers, Rock-on Rock crushers,
which obviate the use of conventional Gyratory-Cone-Crushers
and Roll Crushers, are the better choice.
In case of relatively softer materials, impact mills
and vibro-grinding mills provide the required range
of particle sizes.
All refractory materials need to be free from metallic
Iron introduced during the comminuting operations.
The new rare earth magnetic separators provide excellent
alternatives, both in terms of productivity and
energy consumed, compared to Induced Electro Magnetic
Separators.
Mixers are the most essential piece of equipment
in the manufacture of refractories-shaped or unshaped.
The new generation high intensity mixers, though
require higher power to operate, take less time
to make a better mix. Conventional Counter current
mixers would have taken 40-45 minutes to produce
the consistency now possible in 10 minutes on these
high intensity mixers.
Tiles Industry - Ball Mills
Ball mills are one of the major consumers of power
in tiles industry. Apart from selecting right type
of ball mills, the following points have to be considered
for energy efficiency
- optimisation of grinding cycle time
- amount of water in the mix
- electrical motor can be of dual speed or
fitted with fluid couplings to reduce electrical
load.
Spray dryers
Spray dryers consume considerable thermal energy in
tiles industry. The spray nozzle plays a major role
in the performance of the spray drier. Hence select
a spray drier with a properly designed nozzle. The other
points to be taken care are
- proper insulation for the spray drier
- proper sizing of exhaust fans
- Installation of Variable Speed Drives (Variable
Frequency Drives) for exhaust fans.
Presses
Presses are the major consumers of power in ceramic
industry. Several developments have taken place
in this area. In the case of tiles industry, high
pressure hydraulic presses give good results in
terms of productivity and energy efficiency. In
refractory industry, hydraulic presses are essential
to achive higher quality consistency and productivity.
Though friction screw presses are the choice, when
the capital is scarce it is advisable to go in for
hydraulic presses. For small volume high precision
refractories, specialised vibro-pneumatic presses
may be a better alternative to conserve energy and
capital. Iso-static presses are also finding increasing
application in the production of sophisticated concast
(CC) refractories like sub merged nozzles and shrouds
and also in the production of industrial ceramics.
The bricks are flat and uniform with iso-static
presses and the presses are also compact. In sanitary
ware plants, press casting can yield 6 to 7 times
high production with less space.
The recommendations are
Tile industry - Install high-pressure hydraulic
presses
Refractory industry - Install hydraulic press for
general refractories
Install iso-static presses for specialised refractory
application (CC refractories, glass tank refractories)
Sanitary ware plants - Install pressure casting
for more productivity
Kilns and dryers
Kilns and Dryers are the major consumers of energy
(thermal energy) in ceramics industry. The industry
has seen the most energy inefficient down draft
kiln to the modern kilns like tunnel / shuttle and
roller kilns.
Refractory industry
A typical comparison of specific energy consumption
of different types of kilns is as follows :
| Type
of Ceramic |
In periodic
Kilns
(KJ/kg)
|
In Tunnel
Kilns
(KJ/kg) |
fire bricks
(fired at 1200 - 1400oC)
|
6000 - 8000 |
2500 - 3500 |
High Alumina Refractories
(fired at 1400 - 1600oC)
|
12000 |
3500 |
Basic Refractories
(fired at 1600 - 1750oC)
|
12000 - 16000 |
6000 - 7000
|
For high volumes, Tunnel kilns are very efficient and
for low volumes, shuttle kilns with high velocity burners
can be preferred. The major losses in kilns occur because
of
- Insulation loss
- Exhaust loss
- Intermittent operation
The suggestions are as follows
- Insulate the furnace with proper insulating
material such as ceramic fibre, hot face insulating
refractory.
- Proper combustion control system is essential
to optimise fuel consumption.
- Waste heat recovery system, typically the
gases from cooling zone area are sent to heating
zone.
Sanitary ware
In Sanitary ware industry also roller kilns are
finding a place. The number of patterns must be
less to make the operation less complicated. The
operating temperature is 1200oC. The less weight
skaters are beneficial as temperatures are easier
to achieve. The typical specific thermal energy
consumption should be not more than 1100 - 1200
Kals/Kg of sanitary ware.
It is suggested to install roller kilns for sanitary
ware industry.
Build cars with low thermal mass to improve overall
efficiency. The suggested material is cordierite.
Tiles industry
The preferred Kilns in Tiles industry are roller
kilns. The typical operating temperatures are 1150
to 1300oC. These types of kilns are package type.
The features are
- The rollers are ceramic rolls
- Good control system (computerised controls)
- High velocity burner
- Good insulation practise (Ceramic fibre
to be sandwiched between refractories to avoid ceramic
fibre falling on tiles)
In the modern kilns, drying operation also takes
place (before it enters kilns section). The typical
specific thermal energy consumption should be not
more than 600 to 700 Kcals/Kg of tile.
Installation of roller kilns for tiles industry
optimises thermal energy consumption.
Pre-drying
At present, mostly pre-drying is carried out
with electrical heaters. The temperature requirements
are 100 to 140oC. These temperatures can be achieved
by circulating thermic fluid in thermic fluid heaters.
This modification reduces the energy costs by at least
60%.
Hence installation of thermic fluid heaters for
pre-drying section saves substantial cost on energy.
Choice of fuel
The type of fuel used can be gaseous fuels such
as natural gas. The efficiencies are better with
gaseous fuels. A direct fired system gives good
results when compared to muffle type.
Recycling of refractory
High grade refractory material can be recycled as
low grade refractory material. For example, refractory
material of 1800oC can be recycled for 1600oC applications.
Recycling of refractory gives lot of environmental benefits.
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