Classification of Refractories

                                                     Refractories are classified into a number of ways on the basis of (i) chemical nature  (ii) method of manufacturing, (iii) physical form, (iv) porosity/ based on compactness. (v) Classification based on refractoriness

Classification based on chemical Nature

Accordingly to this classification, refractories are of three types namely (i) acid refractories, (ii) basic refractories, and (iii) neutral refractories.

• Acid refractories – These refractories are attacked by alkalis (basic slags). These are used in areas where both slag and atmosphere are acidic. Examples of acid refractories are silica refractories, zirconia refractories and alumino-silicate refractories.
• Basic refractories – Basic refractories are those which are attacked by acid slags but stable to alkaline slags, dusts and fumes at high temperatures. Since they do not react with alkaline slags, these refractories are of considerable importance for furnace linings where the environment is alkaline such as steelmaking operations. The most important basic refractories are magnesite refractories, dolomite refractories, and magnesia-chrome refractories.
• Neutral refractories – Neutral refractories are chemically stable to both acids and bases and are used in areas where slag and atmosphere are either acidic or basic. The common examples of these refractories materials are carbon graphite (most inert), chromite refractories, and alumina refractories. Out of these graphite is the least reactive and is extensively used in metallurgical furnaces where the process of oxidation can be controlled.

Classification based on method of manufacture

The refractories can be manufactured by several methods consisting mainly of (i) dry press process, (ii) fused cast process, (iii) hand molding process

Classification based on physical form

Refractories are classified according to their physical form. These are the shaped and unshaped refractories. The shaped is commonly known as refractory bricks and the unshaped as monolithic refractories.

Shaped refractories are those which have fixed shapes when delivered to the user. The shaped refractories are normally known as refractory bricks. Brick shapes are usually divided into two types (i) standard shapes and (ii) special shapes.  Standards shapes have dimensions which are conformed to by most refractory manufacturers and are generally applicable to kilns and furnaces of the same type.

Special shapes are specifically made for particular kilns and furnaces. This may not be applicable to another furnaces or kiln of the same type. Shaped refractories are almost always machine-pressed, thus, high uniformity in properties are expected. Special shapes are most often hand-molded and are expected to exhibit slight variations in properties.

Unshaped refractories are without definite form and are only given shape during their application. They form joint less lining and are better known as monolithic refractories. These are categorized as plastic refractories, ramming mixes, castables, gunning mixes, fettling mixes and mortars.

Ramming refractory materials are in loose dry form with graded particle size. They are mixed with water for use. Wet ramming masses are used immediately on opening. Ramming masses are used mostly in cold condition so that desired shapes can be obtained with accuracy.

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.

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.

Plastic refractories are packed in moisture proof packings and the packings are opened at the time of use. Plastic refractories have high resistance to corrosion.

Monolithic refractories are replacing conventional brick refractories in steelmaking and other metal extraction industries. These refractories are loose materials which can be used to form joint free lining. Various means are employed in the placement of monolithic refractories like ramming, casting, gunniting, spraying, and sand slinging, etc. The main advantages of monolithic linings are (i) they eliminate joints which is an inherent weakness with brick lining, (ii) hey have greater volume stability, (iii) they have better spalling tendency, (iv) they can be installed in hot standby mode, (v) they have easier transportation and handling, (vi) the method of application is faster and skilled measures in large number are not required, (vii) they offer better scope to reduce downtime for repairs, (viii) they offer considerable scope to reduce inventory and eliminate special shapes, (ix) they are heat saver.

Classification based on compactness/ Porosity

Based on this classification refractories are two types. The first type consists of dense refractories with their true porosity less than 45 %. The second type consists of insulating refractories with their true porosity more than 45 %.

Classification based on refractoriness

Based on the property of refractoriness, the refractories are usually classified usually in four classes. These are namely (i) super-duty, (ii) high-duty, (iii) intermediate duty, and (v) low duty. Super duty refractories have PCE (pyrometric cone equivalent) value ranging from 33-38. High duty refractories have PCE (pyrometric cone equivalent) value ranging from 30-33. Intermediate duty refractories have PCE (pyrometric cone equivalent) value ranging from 28-30, while low duty refractories have PCE (pyrometric cone equivalent) value ranging from 19-28.

Refractory Materials

                                              Refractor Materials, are ceramic materials that has an unusually high melting point and that maintains its structural properties at very high temperature. 

According to ASTM (American Society for Testing and Materials) the definition of Refractories is:

                        It's a non-metallic materials having those chemical and physical properties that made them applicable for structure or as components of system, that are exposed to environments above 1000 °F / 500 °C. 

Refractories are to be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion.

Refractories are used in high temperature processes because of their heat resistant properties and stability at high temperatures. They are normally used as linings for high-temperature furnaces and other processing units such as kilns, incinerators and reactors since they are able to withstand physical wear, high temperatures, and corrosion by chemical agents. They are also used to make crucibles. They find extensive uses in iron and steel industry. Refractories are also employed in great quantities glassmaking industries, power plant, fertilizer industries and cement industries, where they are formed into a variety of shapes to line the interiors of furnaces, kilns, and other devices that process materials at high temperatures. 

  

As above photo indicates refractories can come in various shapes and sizes and also it can be powdery forms. Refractories in shapes called as Refractory Bricks and Blocks. Refractories in un-shape form called as Refractory monolithic.