Refractories & Lime Production Pant Pakistan Steel.

  

Aqil Khan

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.

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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.

 Clays are sedimentary materials comprised of fine particles (< 2 microns in diameter) of crystallized aluminum hydro-silicates (Al2O3.2SiO2.2H2O) with minor proportions of other constituents such as compounds of iron, calcium, magnesium, titanium, sodium, potassium, lithium. Pakistan steel employs alumino- silicate refractories of various chemical and mineral compositions. 

  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).

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.

Bauxite is classified according to its intended commercial application, such as abrasive, cement, chemical, metallurgical and refractory grades.

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 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.

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.

  

Pakistan steel uses magnesite chrome XM1, XM4, XM8 bricks for LD-converter safety lining in the conical and cylinderical portion and large heavy duty XMKK bricks for converter bottom lining.

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 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.

  These heavy duty refractory bricks are mainly used in steel making furnaces worldwide to minimize down time and to achieve optimal production.

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.

Acquiring the capability of production of high quality refractories discussed above will not only fulfill the requirements of Pakistan steel, other local steel industries, Reko - Diq copper project, cement and glass industry in Pakistan but will also assist Pakistan in emerging as an exporter of high quality refractories like Turkey, India and China.