REFRACTORIES

Refractory: Refractory is any material that can withstand high temperature, without softening or suffering a deformation in shape. These are the essential material of construction in metallurgy, engineering, chemical industries and without their use, it is impossible to maintain required temperature. Objectives To confine heat (resist loss of heat) to resist corrosive action of molten models

Characteristics Should be infusible Chemically inert, Withstand load of structure No crack and suffer loss in size Expand and contract uniformly Classification On basis of the chemical properties refractories are classified into three types.

1. Acid refractories: Refractories which consist of acidic material like (Al2O3) and silica(SiO2). These are not attacked by acidic material but easily attacked by basic material. 2. Basic refractories: Refractories which consist of basic material like CaO, Mgo etc. These are not attacked by basic material but easily attached by acids. 3. Nautral refractories These are made from weakly acid or basic material like ‘C’, cromite (Feo, Cro2), zirconia (ZrO2), Sic( carborundum) etc.

Conditions leading to failure of a refractory: Using a refractory that has less operating temperature. Using lower duty refractory (than actual load) Using bricks of higher thermal expansion. Rapid change in temperature. Using heavy-weight refractory bricks. Using bricks which are not properly fired. Using acidic/ basic refractory in furnace in which basic/ acidic reactants and / or products are being processed.

INSULATORS The substances which are capable of retarding or prohibiting the flow of heat or electricity or sound through temp. Thermal insulators: The substances having extremely low thermal conductivity. These are used to prevent the loss of heat. Uses: In refrigerators, oven boilers, steam carrying pipes. Characteristics Thermal conductivity is extremely low. It should be fire proof. It should be water proof. Should have low density Chemically stable. During the use, it should be odourless. Should available at economical price. Thermal insulators are classified into two types.

Insulation (materials are insulators) Organic thermal insulators: These materials possess very large no.of fine pores, have low specific gravity and these are naturally occurring compounds. Ex:- leather, wool, silk, wood pulp, saw-dust. Inorganic thermal insulators: These preferred as high insulating materials. Ex:- Asbestos, Glass wool, hydrous calcium silicate. Electrical Insulators Or Dielectrics The materials, which are used to prevent the loss of electricity through certain parts in an electrical system. These substances have two functions Insulation (materials are insulators) Storage of charge (materials are dielectrics)

Characteristics: A good insulator possesses low electrical conductivity or high resistivity. Dielectrics used in capacitors should have high dielectric constant. Low porosity Least thermal expansion Chemically inert. Engineering Applications of Electrical Insulators: Simple gases: Air:- Provides insulation between the overhead transmission lines. Nitrogen:- Used in transformers to replace the harmful oxidizing atmosphere. Electronegative gases: (SF6 & CCl4) used in electrical devices like capacitors.

Silicon rubber:- In ship wiring, motor winding insulations. Liquids: Mineral Oil:- In transformers for cooling. Chlorinated hydrocarbons:- As transformer fluids to reduce fire hazard. Silicone fluids:- Used as coolants in radio, pulse and air craft transformers. Organic esters:- Used in high frequency capacitors. Solids Silicon rubber:- In ship wiring, motor winding insulations. Butyl rubber:- for insulation in underground cables. Synthetic resins: Polystyrene:- used as dielectric in D.C. PVC:- used in flexible wire covering Epoxy resin:- used in making insulators for high voltages. Glasses: used for wire coverings, line insulations.

Glass Glass: it is an amorphous, hard, brittle, transparent, or translucent, super cooled liquid of infinite viscosity, obtained by fusing a mixture of metallic silicates, most commonly of Na,K,Ca. and Pb it posses no sharp melting point, definite formula or crystalline structure, it may be represented as xR2O.yMO.6SiO2, where R is an atom of an monovalent alkali metal like Na,K,, etc, M is an atom of a bivalent metal like Ca,Pb,,Zn, etc.,x and y are whole numbers.thus, approximate composition of an ordinary glass ( called sod-lime glass) is N2O.CaO.6SiO2 In some glasses, SiO2is replaced by Al2O3.B2O3.P2O5, etc.

General properties of glass: It is amorphous . It has no definite melting point. It can absorb light and reflect light. It is a good electrical insulator. It is effected by alkalis, It is not effected by air, water or acids or chemical reagent, except HF , which converts its silica into SIF4,etc.

Types of glasses: Soft-glass or soda-lime: the raw materials are silica(sand), calcium carbonate and soda ash their approximate composition is Na2O.CaO.6SiO2 they are low in cost, resistant to devitrification and relatively resistant to water. They melt easily and hence, can be hot-worked easily. Such glasses are , however attacked by common regents like acids.  

Uses: They are widely used as window glasses, electric bulbs, plate-glasses, bottles, jars, building blocks, and cheaper table wares where high temperature-resistance and chemical stability are not required.

Hard-glass or Potash-Lime: it is obtained from silica(sand), calcium carbonate, and potassium carbonate their approximate composition is K2O.CaO.6SiO2. they posses high melting point, fuse with difficulty and are less acted upon by acids, alkalis and other solvent than ordinary glasses.

Uses: These glasses (costlier than soda-lime glasses) are used for chemical apparatus, combustion tubes, etc., which are to be used for heating purposes

Manufacture of glass Raw materials: chief source of incorporating: (a)sodium is soda, Na2CO3. (b)potassium is potash,K2CO3. (c) calcium or limestone, chalk and lime. (d) lead or lithage, and red lead. (e) silica or quartz, white sand, and ignited flint. (f) zinc or zinc oxide. (g) borate are borax and boric acid, (h) cullets or pieces of broken glass to increase the fusibility. (i) colours: yellow- ferric salt; green- ferrous and chrmioum salts; blue- cobalt salts; purple- manganesedioxide; red- nickel salts or Cu2O;lemon-yellow-CdS; fluorescent greenish- yellow- uranium oxide; opaque milky white- Cryolite(Na3AlF3) or calcium phosphate.

Manufacturing steps: Melting : Raw marerials, in proper proportions(e.g sand, soda, ash and limetone for common glass) mixed with cullets, re finely powdered an intimate mixture (called “batch”) is fused in either fireclay pots(in case of high-grade glass) or tanks that form part of n open-hearth furnace, in which heating is done by burning producer gas and air mixture over the charge. At high prevailing temperature (about 1800oC), the charge melts and fuses. The melting process in case of ordinary soda-glass involves the following series of reactions:

CaCO3+SiO2  CaSiO3 + CO2 Na2CO3 +SiO2  Na2SiO3 + CO2 When all the carbon dioxide has escaped out of the molten mass , decolorizes (such as MnO2 or nitre) are added to do away with ferrous compounds and carbon , if present. If a coloured glass is desired , the colouring salts are adde at this stage. Heating is continued, till the molten mass is free from bubbles and glass- balls and then cooled to about 8000C.

Forming and shaping : molten glass is then worked into articles of desired shapes by either blowing or moulding or pressing between rollers. Annealing: Glass articles are then allowed to cool gradually to room temperature by passing through different chambers with descending temperatures. If allowed to cool rapidly, glass being bad conductor of heat, the superficial layer cools down first; leaving the interior portion in a state of strain. Owing to this unusual expansion the articles are likely to crack to pieces. The longer the annealing period, the better the quality of the glass.

Finishing: All glass articles, after annealing, are subjected to finishing processes such as cleaning, grinding, polishing, cutting, sand-blasting, etc.

Ceramics Ceramics (from greek word meaning burnt stuff) are inorganic ,non metallic materials that are processed and /or used at high temperature. They include silicates, metallic oxides and their combination. Ceramics can be grouped into three broad divisions-clay products refractories and glasses, according to their common characteristic features. Clay products can be sub divided into three main types, namely:

Manufacture Process of structural clay product: 1. The structural clay products , all of which contain iron oxides. They are used for bricks , tiles and similar products. Manufacture Process of structural clay product: The powdered raw materials are mixed with water to increase plasticity. The sticky and plastic clay is moulded in the desired shapes using stiff mud process or soft mud process. The moulded articles are then dried in outdoors and then fired at 8750-11000C for about 7 days.

2. The white wares :- which are paler substances such as porcelain ,and china clay, feldspar and quartz and so it is called triaxial ware. The three raw materials are powdered and made into paste by mixing suitable amount of water and subjected to desired shape by moulding process. This is followed by careful drying and then firing at 13500C to 15000C.

Whitewares Crockery Floor and wall tiles Sanitary-ware Electrical porcelain Decorative ceramics

Whiteware: Bathrooms

3. Chemical stone wares : which have been specially –treated to be hard ,resilient and non porous. A typical formulation of producing stoneware is as follows. Clay-50%, Feldspar-20%, Flint 15%, Kaolin- 5% and grog- 10%. The shaping is done by casting or moulding. The shapes are dried and fired at 10000C. Uses of Stone wares: Stone wares are used for the construction of sanitary fixtures, pipes, piping vessels, drainage pipes.

Applications of Ceramics: The older ceramics refer white wares are widely used in tiles, sanitary ware. Insulators and high frequency applications. The newer ceramics like nitrides, singles oxides, mixed oxides, silicates have high hardness and high resistance to heat and oxidation. Therefore these are used in Refractories for industrial furnaces and fuel nuclear reactors. Ceramics are now used in electrical and electronic industries for insulators, semi-conductors, and super conductors. Yattaralox is a type of ceramic is useful in optical applications because it is transparent as window glass and can resist very high temperatures.