STEELMAKING USING TYPES OF REFRACTORY



Role of Refractory


Refractory materials have a crucial impact on the cost and quality of steel products. The diversification on steel products and their cleanliness requirement in recent years have increased the demand for high quality refractory. Steelmaking requires high temperatures of the order of 1600 degree centigrade. In addition steelmaking handles high temperature phases like molten steel, slag and hot gases. These phases are chemically reactive; refractory materials are required to produces steels. High quality refractory at a cheaper cost is the main requirement because cost of refractory adds into the cost of product. 


What is a refractory?


● Refractories are inorganic nonmetallic material which can withstand high temperature without undergoing physico chemical changes while remaining in contact with molten slag, metal and gases. It is necessary to produce range of refractory materials with different properties to meet range of processing conditions.

● The refractory range incorporates fired, chemically and carbon bonded materials that are made in different combinations and shapes for diversified applications.


Reason for required Refractory

  • To minimize heat losses from the reaction chamber
  • To allow thermal energy dependent conversion of chemically reactive reactants into products because metallic vessels are not suitable.
ld vessle mac-carbon bricks
Magnesite Carbon Bricks

In steelmaking, the physico chemical properties of the following phases are important:

Slag:    Mixture of acidic and basic inorganic oxides like etc.; temperature varies in between .

Molten steel: Iron containing carbon, silicon, manganese, phosphorous,trampelements,nonmetallic inclusions, dissolved gases like nitrogen, oxygen and hydrogen and different alloying elements like  etc.; temperature 

Gases:   containing solidparticles of  etc.; temperature  


Properties required in a Refractory


The diversified applications of refractory materials in several different types of industries require diversified properties to meet the physico-chemical and thermal requirements of different phases. In some industrial units more than one phase are present e.g. in steel-making vessels slag /metal /gases are simultaneously present in the vessel at high temperatures. In the heat treating furnaces solid/reducing or oxidizing gases are simultaneously present. Below are briefly described the properties of the refractory materials: 

Refractoriness  

Refractoriness is a property at which a refractory will deform under its own load. The refractoriness is indicated by PCE (Pyrometric cone equivalent). It should be higher than the application temperatures.  Refractoriness decreases when refractory is under load. Therefore more important is refractoriness under load (RUL) rather than refractoriness.

Porosity and Slag permeability     

Porosity affects chemical attack by molten slag, metal and gases. Decrease in porosity increases strength and thermal Conductivity.
It is the resistance of the refractory to compressive loads, tension and shear stresses.In taller furnaces, the refractory has to support a heavy load; hence strength under the combined effect of temperature and load, i.e. refractoriness under load is important.
Specific gravity of the refractory is important to consider the weight of a brick. Cost of bricks of higher specific gravity is more that of lower specific gravity. But strength of bricks of higher specific gravity is greater than one with lower specific gravity.


Fire clay bricks
Fire Clay Alumina Bricks


Strength  

It is the resistance of the refractory to compressive loads, tension and shear stresses.In taller furnaces, the refractory has to support a heavy load; hence strength under the combined effect of temperature and load, i.e. refractoriness under load is important.

Specific gravity    

Specific gravity of the refractory is important to consider the weight of a brick. Cost of bricks of higher specific gravity is more that of lower specific gravity. But strength of bricks of higher specific gravity is greater than one with lower specific gravity.


Types of refractory materials 


This can be discussed in several ways, for example chemical composition of refractory or use of refractory or method of manufacture or in terms of physical shape. Below is given type of refractory depending on its chemical composition and physical shape.


(A) Chemical composition
Acid refractory 
The main raw materials used are  and alumino- silicate. They are used where slag and atmosphere are acidic. They cannot be used under basic conditions. Typical refractories are fireclay, quartz and silica.
Basic refractory
   
Raw materials used are  dolomite and chrome-magnesite. Basic refractories are produced from a composition of dead burnt magnesite, dolomite, chrome ore.
For details readers may see the references given at the end of lecture 10.


B) Physical form

Broadly speaking refractory materials are either bricks or monolithic.

Shaped refractories are in the form the bricks of some standard dimensions. These refractories are machine pressed and have uniform properties. Special shapes with required dimensions are hand molded and are used for particular kilns and furnaces. Different types are:
  1. Ramming refractory material is in loose dry form with graded particle size. They are mixed with water for use. Wet ramming masses are used immediately on opening.
  2. Castables refractory materials contain binder such as aluminate cement which imparts hydraulic setting properties when mixed with water. These materials are installed by casting and are also known as refractory concretes.
  3. Mortars are finely ground refractory materials, which become plastic when mixed with water. These are used to fill the gap created by a deformed shell, and to make wall gas tight to prevent slag penetration. Bricks are joined with mortars to provide a structure.
  4. Plastic refractories are packed in moisture proof packing and pickings are opened at the time of use. Plastic refractories have high resistance to corrosion.
Monolithic refractories 

Monolithic refractories are replacing conventional brick refractories in steelmaking and other metal extraction industries. Monolithic refractories are loose materials which can be used to form joint free lining. The main advantages of monolithic linings are

  • Grater volume stability
  • Better spalling tendency
  • Elimination of joint compared with brick lining
  • Can be installed in hot standby mode
  • Transportion is easier
Monolithic refractories can be installed by casting, spraying etc.

Ramming masses are used mostly in cold condition so that desired shapes can be obtained with accuracy.


Refractories are composed of either single or multi-component in organic compounds with non metallic elements.
   a)   Magnesite: Chrome combinations have good resistance to chemical action of basic slag and  mechanical strength and volume stability at high temperatures.
   b)  MagnesiteCarbon refractory with varying amount of carbon has excellent resistance to chemical attack by steelmaking slags.
   c)  Chromite- Magnesite refractory: used in inner lining of BOF and side walls of soaking pits.( basic refractory)
   d)  Magnesite: Basic refractory in nature. Magnesite bricks cannot resist thermal stock, loose strength at high temperature and are not resistant to abrasion.
Neutral refractory Neutral refractory is chemically stable to both acids and bases. They are manufactured from  and carbon.

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