REFRACTORIES & LIME PRODUCTION PLANT
(Pakistan Steel).
A refractory material is one that is physically and chemically stable at high temperatures but should also have a low thermal conductivity to save energy.
The
iron and steel industry is the largest consumer of refractory materials
worldwide, accounting for around two thirds of refractory usage. The range of materials used as refractories is very wide and
includes such substances as following:
Fireclays and kaolines,
quartzites, bauxites, magnesite, dolomite, graphite, corundum, fused alumina,
chromite, Zirconium oxide, Zirconium silicate and others.
From
chemical point of view, refractories are graded in three classes; Acid, Basic, and neutral. The chemical and mineralogical composition is of
fundamental importance in determining the character and quality of a refractory
material but for ensuring high performance
in specific use, the following key properties play significant role.
.
1.
Refractoriness.
2.
Permanent
expansion or contraction upon reheating.
3.
Porosity
and specific gravity.
4.
Resistance
to compression at furnace temperatures.
5.
Thermal
properties, heat capacity, thermal conductivity and expansion.
6.
Resistance
to sudden temperature change.
7.
Resistance
to slagging action.
8.
Mechanical
strength and resistance to abrasion.
Production facilities at Pakistan Steel.
Fireclay Refractories.
Fireclay bricks are by
far the largest group of refractories used in Pakistan Steel. The most common
raw material for the manufacture of Fireclay bricks ( Chamotte), is the
locally available clays.
Appearance of Tunnel Kiln for massive production of Chamotte
Bricks.
As
per ASTM, there are five standard classes of fireclay bricks: super-duty,
high-duty, medium-duty, low-duty and semi-silica. These classes cover the range
from approximately 18% - 44% alumina and from about 50 – 80% silica. The maximum service
temperature increases with increasing alumina content.
High grade fire clays can withstand temperatures of 1775°C
(3227°F), but to be referred to as a "fire clay" the material must withstand a
minimum temperature of 1515°C (2759°F).
Refractories
& Lime Production shop of Pakistan steel produces fireclay (Chamotte)
bricks of medium
duty class of various shapes and dimensions( few of which illustrated
above) mostly used inside Pakistan Steel at Coke oven plant, Iron making dept.,
Steel making dept., Lime kilns , reheating furnaces of Billet mill & Hot
rolling mill etc.
The
heavy duty
fire clay bricks for capital repairs of various production units are imported
as per Pakistan steel’s standard technical specifications.
Limestone and Dolomite Production.
Limestone and dolomite are essential raw materials for an
Iron & Steel manufacturing plant. Both raw materials are locally available
near villages of Makli and Jhimpir areas of Thatta
district, 70 kilometers away from Karachi (the quarries site of limestone and dolomite
for Pakistan steel).
Rotary Kiln for Lime
& Dolomite Calcination.
As
an important auxiliary material, active lime is used in steel making, not only
help to improve the quality and output of steel but also reduces the
consumption & cost of refractories.
Limestone (CaCo3) firing in a rotary kiln
drives off excess carbon dioxide (CO2) and results in a chemically reactive
lime (CaO).
A high-quality active lime with low loss of
ignition, is obtained to satisfy the slag formation requirement of LD-Converter
steelmaking.
CaCO3
+ Kcal ----- à CaO +
CO2 (+ 760 kcal/kg).
Burnt Dolomite for making Tar-Dolomite Refractory Bricks.
The
only sizeable basic refractory bricks ( the word “ basic”
refers to the brick’s chemistry )
manufacturing plant is present in
Pakistan steel which is unique of its kind in Pakistan, producing Tar dolomite
bricks for LD- Converters and continuous casting ladles.
Locally available raw dolomite (CaCO3.MgCO3) was adopted for cost effective manufacturing of Tar dolomite refractory
bricks for 130 tons LD-Converter at
Pakistan Steel.
CaCO3.MgCO3
+ Kcal -----à Cao.Mgo
+ 2CO2 (+723 kcal/kg).
Since burnt lime & dolomite is not
completely resistant to hydration it cannot be stored long and should be used
as soon as possible.
Tar- Dolomite Refractory Bricks Manufacturing facilities for
LD-Converter.
Burnt
dolomite for bricks making, should contain 50-55% CaO, 35-29.0% MgO, not more
than 5-6 % SiO2 and not more than 5-8% ( Fe2O3 + Al2O3). Dolomite with
calcination losses exceeding 2%, is under-burnt and cannot be employed for
making Converter bricks.
Burnt
dolomite is ground and screened to the required size. Recommended dolomite
sizes are mixed in blade mixer with hot and water free tar in amount 9 -10% and the bricks are pressed & molded under
high pressure on hydraulic press, equipped with devices for mechanized removal.
Hydraulic
press for Tar Dolomite Bricks (Pak. Steel).
The
high refractoriness under load of these bricks at high steelmaking temperatures
1800 0C,
indirectly indicates their strength in the hot state. The storage period for tar-dolomite
bricks in open air without signs of serious hydration in the winter time is 10
– 12 days. The tar-dolomite bricks are kept in plastic covers and encased in
metal pallets to prevent their disintegration
from hydration.
Hydration rigorously reduces the life campaign of LD-Converter lining and hence decreases the availability of Converter for production (increases downtime).
Hydration rigorously reduces the life campaign of LD-Converter lining and hence decreases the availability of Converter for production (increases downtime).
Manufacturing of Tar- Dolomite Bricks for Continuous Casting Steel
Ladles.
As per project ,the working refractory lining
of Steel Casting ladles(SMD) was comprised of Fire clay bricks ( lining life was
9-11 heats), later ( with the efforts of Pakistan
Steel’s engineers) switched over to local Tar-dolomite bricks (25-30 heats) in the year 1988-89, which brought improvements and smoothness
in the steel production cycle.
As
per project, the teeming ladles of steel making department were provided with
sleeve’s stopper and nozzle mechanism which were inferior against resistance to
aggressive slags and were unable to cope up the teeming demand of high quality aggressive
steel grades.
Later
in the year 1984-85, the steel teeming mechanism was replaced with imported hydraulically
operated Ladle slidegate mechanism with its superior quality shaped
refractories (Al2O3 content 70% or higher) shown below.
Refractory Repair Department ( R.R.D).
Pakistan
steel has its own specialized/ trained masonry group known as R.R.D. to carry out
masonry and insulation works inside plant. The R.R. Dept. is associated with Refractories
& Lime Production shop.
The Brick laying or masonry work becomes an
specialized subject when it is carried out with refractory bricks of special shaped,
sized and high characteristics, as per specific engineering schemes/ drawings
of various furnaces and vessels like Coke Oven Batteries, Blast furnaces, Hot
Blast stoves, Lime Kilns, 1300 tons Mixer, LD-Converters, Reheating furnaces of
Rolling mills.
Proper handling of
equipments for refractory bricks application, maintaining proper expansion gaps,
correct dimension (size) of bricks, fixing anchors etc., all are very important
to achieve better campaign life
of refractories in a furnace and to prevent unforeseen accidents,
equipments and production loss.
Need to Switch over to Heavy Duty/ Superior Quality
Refractories in Production Responsive Areas of
Pakistan Steel to obtain uninterrupted Production.
As
a result of higher quality demand in steel, the steel production technology has
improved faster through innovations in materials and manufacturing technology.
Heavy
duty refractories are expensive and any
failure in the refractories during service results in great loss of equipment
and production time. The type of refractories also influences energy consumption
and product quality. Therefore, the choice of refractories best suited to each
application is of supreme importance and must be selected carefully to achieve
a balance between initial installed cost and service performance i.e.
the estimated cost should not exceed the benefits.
High melting, fusion stabilized refractory
materials like fused
alumina and fused magnesia bricks are more superior
bricks and are used especially in steel
making furnaces and ladle metallurgy furnaces to achieve optimum service life
and minimize down time.
Important indigenous refractory minerals need
to be exploited properly.
Mainly the following minerals
are used in production of high quality refractories:
Alumina
(Al2O3) , Chromite (Cr2O3),
Magnesite (MgCO3).
Bauxite:
Bauxite represents mountain rocks, the main
component of which is aluminum oxide.
Bauxite deposits are known to occur in at least 50 countries, with estimated world
reserves of approximately 25 billion tonnes.
In
Pakistan bauxite is found in Kotly, Muzaffarabad, Ziarat, Sibbi, Attock,
Hazara, Loralai and Khushab districts. Bauxite reserves of Pakistan were
estimated about 74 million tonnes by Geological Survey of Pakistan.
Bauxite contains a large number of impurities
such as silica, iron, and titanium oxides, and various other elements in trace
amounts. Free moisture in crude bauxite may range from 5% to 30%. In dried bauxite, most of free
moisture is removed by heating crude bauxite in rotary drying kilns at about
6000F.
Kala
Chitta deposits of district Attock are mainly diaspore and were reported to
have 36.6 – 74.0 % Al2O3 and 7.6 – 37.8 % SiO2. Salt Range deposits of district Khushab have 36.0 – 72.8 % Al2O3 and 8.6 – 46.96
percent SiO2. The deposits in both districts were reported remarkably low in
iron oxide as much as 2%.
The
Salt range mountains in district Khushab, Punjab, Pakistan.
Approximately 90% of world production of Bauxite is intended to manufacture alumina and all commercially produced alumina is obtained by Bayer process.
Metallurgical Grade Bauxite : Metallurgical Grade Bauxite Parameters Specifications: Al2O3 45-47 % min, SiO2 4 - 5 % max, TiO2 3- 4 % max Fe2O3 15-25 % max CaO 0.50- 3 % max Size 0-100 mm Moisture 5 %
max.
Alumina
(aluminum oxide) is white powder produced from bauxite ores by treating them
with caustic soda in Bayer process.
The processing efficiency /conditions
such as the leach temperature in autoclave, holding time and caustic
concentration, are influenced by the type of Bauxite to be processed.
Primary Aluminium metal (99.9% pure) is obtained after
electrolysis in a cryolite solution in Hall Heroult Process.
Hall Heroult Process
Aluminium
metal production is highly electricity – intensive. The worldwide average
energy consumption is estimated to be 15 kWh/ kg of aluminium.
Considering the large energy consumption, the past Aluminium smelters have been mostly built in
countries with readily available low-priced hydroelectric power, such as
Canada, Norway, Venezuela, and Brazil, or in countries with abundant deposits
of low-grade coal for power generation such as Australia or the Republic of
South Africa.HIGH-ALUMINA REFRACTORY BRICKS.
High-Alumina
bricks belong to the Alumina- Silica group containing more than 47.5% Alumina. The
refractoriness of 50% alumina brick is greater than fireclay bricks and
progressively improves as alumina content increases up to 99+%.
High-alumina
bricks are made from bauxite (Al[OH]3) and kaolinitic clays. These raw materials are
roasted to produce a mixture of synthetic alumina.
The primary mineral phases present in fired
high- alumina bricks are mullite and corundum which have melting points of 1850 deg.cent and 2050 deg.cent. respectively.
These
refractories with alumina contents of up to 99+% are useful for service
temperature extending to about 1817deg.cent.
They
are often chosen for their resistant to spalling, impact, abrasion, or load.
They have high refractoriness and execellent corrosion resistance to acidic and
neutral slags at high temperatures.
Considerations for Refractory grade bauxite .
a) Alumina (Al2O3) content should be (>58 % ) in raw ore so as calcined bauxite of 85% Al2O3 could be
obtained.
b) low iron oxide content generally 2.0 % maximum after calcinations.
c) Low TiO2 content (4% maximum) and only
trace amounts of alkalies ( K2O+Na2O < o.3%) and
alkaline earths ( CaO+MgO < 0.5% )
d) >8% silica may be acceptable but it should be present
mainly as a clay mineral (as kaolinite, halloysite etc.)
and not as quartz.
For production of refractory, the diasporic
bauxite is preferred while mixed gibbsitic-boehmitic or boehmitic
ores can also be used.
The refractory grade bauxite is calcined at a temperature of 1600 to 1700° C, during which all free and combined water is removed. The silicate minerals present as clay undergo solid state reactions and are converted to mullite, an aluminium silicate corresponding to the formula 3Al2O3.2SiO2.
The refractory grade bauxite is calcined at a temperature of 1600 to 1700° C, during which all free and combined water is removed. The silicate minerals present as clay undergo solid state reactions and are converted to mullite, an aluminium silicate corresponding to the formula 3Al2O3.2SiO2.
The
material composition controls high-temperature properties of refractoriness,
melting point and wear resistance under heavy-duty operating conditions.
Although
China has internal bauxite resources, it’s internal demand has exceeded this
supply for some time, and the country depends primarily on Australia but also
to a lesser extent on India for alumina imports. This enormous demand for
alumina has pushed up trading prices for this raw material.
Low Creep High Alumina Bricks for Hot Blast Stoves
These low
creeping high alumina
bricks for Hot Blast Stoves of Blast Furnace are made from super bauxites.
High pressure pressing and sintering in high temperature makes the products superior
in working conditions of Refractory Under Load, creep rate and thermal shock resistance.
MAGNESITE-CHROME BRICKS.
MAGNESITE-CHROME BRICKS.
Chromite is the commercial name
for iron chromium oxide (FeCr2O4). World chromite reserves are estimated to
total 7.6 billion tones. The majority located in South Africa 5.5bn tones ,72% of
the world reseves, the rest in Zimbabwe 12%, Kazakhstan 4%, Finland 2%, India
1% and smaller amount in Turkey, Pakistan and elsewhere.
In Pakistan, chromite is mined from the
ultramafic rocks in mainly the khanozai area of Pishine District of Balochistan. Most of the chromite is of
metallurgical grade with Cr2O3 averaging
54%.
Recently,
the biggest user of chromite ore has been China, importing large quantities
from South Africa, Pakistan and other countries. The concentrate is used to
make ferrochromium, which is in turn
used to produce stainless steel and nonferrous alloys, are two of its more
important applications. Other applications are in alloy steel, plating of
metals, pigments, leather processing, catalysts, surface treatments, and
refractories.
When Cr2O3 is added to MgO less than 30% it is known as Magnesite-chrome bricks whereas bricks with Cr2O3 content more than 30% are known as Chrome-magnesite bricks.
Magnesite-chromite refractories are made of a mixture of pure magnesia MgO (65 – 80%)and ground chromite Cr2O3(20 – 35%), have high refractoriness under load, stable and good thermal shock resistant.
When Cr2O3 is added to MgO less than 30% it is known as Magnesite-chrome bricks whereas bricks with Cr2O3 content more than 30% are known as Chrome-magnesite bricks.
Magnesite-chromite refractories are made of a mixture of pure magnesia MgO (65 – 80%)and ground chromite Cr2O3(20 – 35%), have high refractoriness under load, stable and good thermal shock resistant.
Direct bonded XMKK are high temperature fired (1600 deg. cent.) Chrome-Magnesite bricks, the principal crystal phase of which are periclase and magnesia-chrome spinal, featuring with corrosion resistance, spalling resistance and proved to be stable at high temperatures of steelmaking.
Few small private ceramic
companies in karachi, are producing Magnesite - Chrome bricks but due to their limited
production facilities unable to manufacture heavy duty large and
shaped bricks.
MAGNESIA-CARBON BRICKS.
Magnesium oxide (MgO) or magnesia (Periclase), is a white hygroscopic solid mineral mainly obtained from magnesite ( MgCO3) and
brucite (Mg(OH)2, serves for important steelmaking applications worldwide.
Two-thirds of worldwide magesite reserves are found in
China. Turkey ranks second and North Korea ranks third followed by Russia,
Austria, Slovakia, Spain, Greece, India, Brazil, Australia, Saudi Arabia and
later discovered in Pakistan.
Magnesite is the most common source of
magnesia. At
temperatures approximately 400-700 °C, Magnesite (MgCO3) starts to decompose to magnesium oxide and carbon dioxide in rotary kiln.
MgCO3 + Heat - --à MgO + CO2 (ΔH = +118 kJ/mol)
This magnesite is called calcined magnesia. The other forms
of magnesia are dead
burned and fused. Dead burned Magnesia is produced at
higher calcinations temperature 18000C.
Fused magnesia is of more superior quality material obtained
from Dead burned Mgo in
Electric Arc Furnace at temperatures about 2800-2852 OC, a high cost and
energy
intensive process, in which the crystalline structure of magnesia is improved significantly and the magnesia
(MgO) content is increased to nearly 100%.
The cost of production of fused magnesia depends significantly on its quality and drastically increases ( from $400 /tonne up to $1400 /tonne) as the purity increases above 97.5%.
The cost of production of fused magnesia depends significantly on its quality and drastically increases ( from $400 /tonne up to $1400 /tonne) as the purity increases above 97.5%.
The high density heavy duty bricks are shaped on high pressure hydraulic presses more than 2,500-tonne capacity.
In industrially developed countries, large high density Magnesia-Carbon bricks are manufactured from Dead burned magnesia or Electrically fused magnesia crystals by adding high purity graphite and metallic antioxidants as needed.
In industrially developed countries, large high density Magnesia-Carbon bricks are manufactured from Dead burned magnesia or Electrically fused magnesia crystals by adding high purity graphite and metallic antioxidants as needed.
Pure Magnesia has a very high melting point 2,852°C. Ultra high purity (>99 per cent MgO) grades have been used in high-tech applications such as nuclear reactors and rocket nozzles.
With the development of Unix
based computer controlled Electric Arc Furnaces and high pressure hydraulic
presses, an excellent high density Magnesia
- carbon bricks are now produced economically on large scale in China.
Medium quality Magnesia- Carbon bricks were tried by Pakistan Steel during 1990 – 93 in steel making LD-Converters, imported from
four European suppliers which lasted for an average campaign life of 850
heats per campaign (compared to local Tar dolomite bricks 450 - 500
heats/campaign) but their higher cost made them uneconomical.
After
the discovery of Magnesite deposits in Khuzdar district of Balochistan (3.0
million tons) and Kumhar Abbottabad (11.0 million tons), it was planned to
utilize these local resources in Pakistan steel but execution is still awaited.
Subject
to supply of local magnesite of required quality, the next step in achieving
the production of higher quality dead burned magnesite bricks ( Production of electrically fused
magnesia may be deferred subject to availability of affordable power supply ) in
Pakistan Steel, is the up-gradation
of the existing calcination and related
manufacturing facilities, especially the hydraulic press capability of Refractory Production Shop which may be deemed
as part of revamping package of Pakistan Steel.
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