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SURFACE CHEMISTRY SURFACE CHEMISTRY INTRODUCTION It deals with the study of phenomena that occur at the surfaces or interfaces of substances, like adsorption, heterogeneous catalysis, formation of colloids, corrosion, crystallization, dissolution, electrode processes, chromatography etc. Surface chemistry finds its applications in industry as well as in daily life. • What is surface? • The physical boundary of any condensed phase like liquid or solid is considered as surface. It separates one phase from the other. It can be considered as series of points which make a plane or layer where one phase ends and the other begins. The surface may be uni-layered or multilayered. • The interfaces that exist between two immiscible liquids like oil and water; between a metal and a gas like platinum and hydrogen; a liquid and a gas etc., are some examples. • The interfaces between two phases can be represented as: phase1/phase2 or phase1-phase2. • Applications of surfaces: • Most of surface chemistry involves the interaction of surfaces of one system with the particles of other system. Surfaces play active role in catalysis, colloid formation, electrode reactions, chromatography etc. In the current chapter we deal with the surface phenomena and related topics like: • 1) Adsorption • 2) Catalysis • 3) Colloids Adsorption • Process in which a gas, liquid, or solid adheres to the surface of a solid or (less frequently) a liquid but does not penetrate it, such as in adsorption of gases by activated carbon (charcoal). In comparison, a gas or liquid taken-in during absorption penetrates or mixes with the absorb Or • The phenomenon of concentrarion of molecules of a gas or liquid at the surface of solid. • The substance that concentrate the surface is adsorbate. • The solid on whose surface the concentration occurs is adsorbent. ADSORPTION TYPES OF ADSORPTION • Adsorption is two types. They are 1. Physical adsorption 2. Chemical adsorption 1.Physical adsorption:-The adsorption takes place due to the adsorption of gas molecules on the solid surface by Vander Walls force of attraction. This is also called as Vander walls Adsorption. • Chemisorption:- The type of adsorption is due to the attack by ionic or covalent compounds • Ex :- hydrogen gas is chemisorbed on nickel. At first hydrogen molecule is adsorbed by Vander walls force f attraction then dissociate. Thus hydrogen atom is adsorbed on nickel What is Adsorption Isotherm? The relation between equilibrium pressure of the gas and the amount of gas adsorbed on the solid adsorbent at any constant temperature. It is given in the form of graph or equation. It is two types. They are 1.Freundlich Adsorption isotherm 2. Langmuir Adsorption Isotherm Freundlich Adsorption Isotherm • In 1909, Freundlich gave an empirical expression representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation or simply Freundlich Isotherm.X/M=KP • Where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p and k, n are constants whose values depend upon adsorbent and gas at particular temperature. Though Freundlich Isotherm correctly established the relationship of adsorption with pressure at lower values, it failed to predict value of adsorption at higher pressure. LANGMUIR ADSORPTION ISOTHERM Langmuir derived a simple adsorption isotherm based on theoritical considerations. In 1916, Irving Langmuir proposed another Adsorption Isotherm which explained the variation of Adsorption with pressure. Based on his theory, he derived Langmuir Equation which depicted a relationship between the number of active sites of the surface undergoing adsorption and pressure Assumptions of Langmuir Isotherm • • Langmuir proposed his theory by making following assumptions. 1. Fixed number of vacant or adsorption sites are available on the surface of solid. 2. All the vacant sites are of equal size and shape on the surface of adsorbent. 3. Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released during this process. 4. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. A(g) + B(s) ADSORPTION AB DESORPTION • Where A (g) is un adsorbed gaseous molecule, B(s) is unoccupied metal surface and AB is Adsorbed gaseous molecule. • 5. Adsorption is monolayer or uni layer. Derivation • Langmuir Equation which depicts a relationship between the number of active sites of the surface undergoing adsorption (i.e. extent of adsorption) and pressure. • To derive Langmuir Equation and new parameter ‘ θ ’ is introduced. Let ‘ θ ’ is the fraction of the covered surface area (no.of active sites) & (1- θ) is the fraction of Naked surface area. • Rate of adsorption of molecules on the surface of the adsorbent = ka(1 - ) • Rate of desorption = kd • At equilibrium state the rate of disorption is equal to the rate of adsorption • i.e, kd = ka(1 - ) • • • • • • • • • • • = ( KaP/Kd + KaP) On multiplication of each factor with Kd , then = (Ka/Kd)P/1+(Ka/Kd)P Or = Kp /1+ Kp (since K = Ka/Kd) K is also known as adsorption co efficient. The amount of gas adsorbed per gram of the adsorbent is directly proportional to Hence x α Kp /1+Kp X = Kl .Kp/1+Kp →1 Kl is new constant Eq(1) gives the relation between the amount of gas adsorbed to the pressure of the gas at any constant temp is known as Langmuir Adsorption isotherm. • In order to test the Langmuir Adsorption isotherm eq(1) is rearranged • So, that kII = 1/k + p/kII →(2) (kII=kI/k) • eq(2) is similar to an equation for straight line. • Thus the plot between p/x and p, straight line is obtained with intersept 1/kI • • • • • Failures :The isotherm is fails at high pressures. Langmuir Adsorption isotherm is written as p/x = 1/kI +p/kII if the pressure is low the factor p/kII is ignored & assuming the form x = kIp. if the pressure is low the factor 1/kI is ignored & assuming the form x = kII APPLICATIONS OF ADSORPTION • Adsorption finds extensive applications both in research laboratory and in industry. A few applications are discussed below: • In preserving vacuum: In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enters into the annular space either due to glass imperfection or diffusion though glass is adsorbed. • In gass masks: All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the atmosphere are preferentially adsorbed and the air for breathing is purified. • In clarification of sugar: Sugar is decolorized by treating sugar solution with charcoal powder. The latter adsorbs the undesirable colors present. • In paint industry: The paint should not contain dissolved gases as otherwise the paint does not adhere well to the surface to be painted and thus will have a poor covering power. The dissolved gases are therefore, removed by suitable adsorbents during manufacture. Further, all surfaces are covered with layers of gaseous, liquid or solid films. These have to be removed before the paint is applied. This is done by suitable liquids which adsorbs these films. Such liquids are called wetting agents. The use of spirit as wetting agent in furniture painting is well known. • In chromatographic analysis: The selective adsorbent of certain substances from a solution by a particular solid adsorbent has helped to develop technique for the separation of the components of the mixture. This technique is called chromatographic analysis. For example: in column chromatography a long and wide vertical tube is filled with a suitable adsorbent and the solution of the mixture poured from the top and then collected one by one from the bottom. • In catalysis: The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the solid catalyst. As a result, the concentration of the reactants increases on the surface and hence the rate of reaction increases. The theory is also able to explain the greater efficiency of the catalyst in the finely divided state, the action of catalyst promoters and poisons. PHASERULE Introduction Two or more different phases present in equilibrium with one another, constitute a heterogenous system. Such heterogenous system can be conveniently studied with the help of a generalisation called Gibbs Phase rule. It is applicable to all heterogenous systems and is also free from exceptions which are common features of all other generalisations of physical chemistry. This rule was deduced on the basis of thermodynamic principles by J. Willard Gibbs . This rule predicts qualitatively the effect of temperature, pressure and concentration on a heterogenous equilibrium. Phase Rule Gibbs phase rule may be stated as follows : "In a heterogeneous system in equilibrium, the number of degrees of freedom plus the number of phases is equal to the number of components plus two". Mathematically, F + P = C + 2 where F = number of degrees of freedom C = number of components P = number of phases Phase Defination: “ The physically distinct, homogenous and mechanically separable part of a system are called phases”. Examples: (i) A gaseous mixture constitutes a single phase since gases are completely miscible. Air is a mixture of N2, O2, CO2, water vapour etc. Which constitute a single phase. (ii) Two or more liquids which are miscible with one another constitute a single phase as there is no bounding surfaces separating the different liquids. e.g., water and alcohol, benzene & chloroform constitute one phase system. (iii) A system consisting of a liquid in equilibrium with its vapour constitute a two phase system Component Defination : The number of component of a system at equilibrium is defined as the minimum number of independently variable constituents which are required to express the composition of each phase in the system. Component Examples (i) Sulphur system: Consists of four phases namely monoclinic sulphur, rhombic sulphur, liquid sulphur and sulphur vapour. The composition of each phase of the system can be expressed in terms of sulphur only, so, it is a one component system. (ii) Water system: It is a one component system because the composition of each of the three phases present can be expressed as H20. (iii) Na2SO4 + water system: Certain salts are capable of existing as hydrates with different number of water molecules of crystallization. These hydrates correspond to different solids and hence to different phases. The system is a two component , because the composition of each phase of the hydrates is completely described in terms of the anhydrous salt and water alone. e.g., Na2S04 + water Degrees of Freedom : Defination : The degree of freedom or variance of a system is defined as the minimum number of variable factors such as temperature, pressure and concentration which should be arbitrarily fixed in order to define the system completely. Examples (i) For a given sample of any gas PV = nRT. Any two of the three variables P, V, T define the system completely. Hence the system is bivariant or it has two degrees of freedom. (ii) A gaseous mixture say N2 and O2 gases (mixed 50% each), is completely defined when three variables temperature, pressure and concentration are specified. Thus, the degrees of freedom is three or the system is trivariant. Phase Rule Conclusion· (i) The greater the number of components in a system, the greater is the degree of freedom for a given number of phases. (ii) The greater the number of phases, the smaller is the number of degrees of freedom. (iii) The number of phases is maximum when the number of degrees of freedom = Zero, for a given number of components. Thus, for one component system, P max. = 3 . two component system, P max. = 4 three component system, P max. = 5 Phase Rule Advantages of Phase' Rule : (i) It provides a simple method of classifying equilibrium states of systems. (ii) The phase rule confirms that the different systems having the same number of degrees of freedom behave in same manner. (iii) It is applicable only to macroscopic systems and not concerned with molecular structure. (iv) It predicts the behaviour of systems with changes in the variables that govern the system in equilibrium. (v) It predicts under given conditions whether a number of substances taken together would remain in equilibrium as such or would involve interconversion or elimination of some of them. (vi) It takes no account of nature of the reactants or products in phase reactions. Phase Rule (vii) It finds extensive use in the study of many heterogenous systems. In particular it has been found extremely useful in the extraction of metals. Limitations: (i) The phase rule is applicable to heterogeneous systems in equilibrium, so, it is therefore of no use for such systems which are slow in attaining the equilibrium state. (ii) It is applicable to a single equilibrium state, so it never gives information about the other possible equilibrium in the system. (iii) In Gibbs phase rule, various variables are temperature, pressure and composition. It does not take in account the electric and magnetic influences. For consideration of such variables, the factor 2 of the Phase rule has to be adjusted accordingly. Phase Rule (vi) All the phases in the system must be present under the same temperature, pressure and gravitational force . (v) No solid or liquid phases should be finely divided, otherwise deviation occurs. Phase diagrams : The number of phases that exist in equilibrium depends upon the conditions of temperature and pressure or temperature and composition, pressure being constant. These conditions are determined experimentally and interdependence of values of the variables can be shown graphically using appropriates coordinates. These diagrams are termed phase diagram. A phase diagram is the sum total of the description of the behaviour of the phases under equilibrium. It is very easy to describe the phase behaviour of a system by such diagrams and to investigate the conditions in which various phases will constitute the system . Phase Rule Application of Gibbs Phase Rule to One Component System: From the mathematical expression, F=C–P+2 When C = 1, P = 1 F = 1-1+2 =2 Hence, all one component systems can be completely described graphically by stating only two variables, pressure and temperature on appropriate axis. Phase Rule The Water System : It is a one component system. Water exists in three possible phases viz. ice (solid) , water (liquid), and vapour (gas). These three single phases may form four possible equilibria. (i) Solid Liquid (ii) Liquid Vapour (iii) Solid Vapour (iv) Solid Liquid Vapour The Phase diagram of water system is given as Phase Rule Phase Diagram of water system Phase Rule The phase diagram consists of : (i) Stable curves: three OB, OA and OC (ii) Metastable curve: one OA' (iii) Areas: three AOB, COB and AOC (iv) Triple point: One O (i) Stable curves OA : It is known as vapour pressure curve of water. The curve OA starts from point O i.e., freezing point of water, 0.0098°C under 4.579 mm of Hg pressure and ends at A, the critical temperature (3740C at 218 atm.). Above critical temp. on the vapour phase exists whatever may be the value of pressure. The vapour pressure of water increases with increase in temperature. Phase Rule The rate of increase of its vapour pressure with temperature is relatively higher at higher temperatures and therefore the curve OA slants upwards and slopes away from the temperature axis. From phase rule, F=C–P+2 = 1- 2 + 2 =1 The water vapour system is univariant Phase Rule OB : It is the sublimation curve of ice. Along this curve, solid ice is in equilibrium with its vapour. This curve is not the prolngation of curve A but falls of more steeply. Curve OB starts From the temperature 0.0098°C above which solid water i.e., ice cannot exist. The curve terminates at B i.e., absolute zero (- 273°C). At this temperature, no vapour can exist and, therefore, only ice is left. But on other points of the curve OB, ice is in equilibrium with vapour. Hence, there are two phases. According to phase rule, F=C–P+2 =1–2+2 =1 Thus, the system is univariant. This means that for each temperature; there may be one pressure and for each pressure there may be one temperature. Phase Rule OC : This curve is the melting point curve or fusion curve of ice. Along this curve two phases, ice and water are in equilibrium. The inclination of OC line towards the pressure axis indicates that the melting point of ice is slightly lowered by increase of pressure. (According to Le Chatelier's principle the increase in pressure causes the water - ice equilibrium to shift in such a direction that there is a decrease in volume.) As the melting point of ice is accompanied by decrease in volume, it should be lowered by the increase of pressure. The curve OC starts from point O but there is no limit for this curve. It goes upto a point corresponding to 2000 atm. and 20°C. According to phase rule, F=C–P+2 =1-2+2 = 1 Thus, the system is univariant. This means that for any given pressure, melting point must have one fixed value. Phase Rule (ii) Metastable Curve OA' : It is a metastable curve shown in continuation of AO. When water is cooled below its freezing point (when it is not vigorously stirred) without separation of ice, the water is said to be super cooled. The vapour pressure curve of liquid water AO extends below O as shown by the dotted curve OA'. Along curve OA' liquid water coexists with vapour and vapour pressures are different than over the solid. This equilibrium is called metastable equilibrium as slight disturbance brings it to the stable region OB of the phase diagram. Phase Rule (iii) Areas The areas give the conditions of temperature and pressure under which single phase - ice (solid), water (liquid) and vapour (gas) can exist. It is necessary to specify both temperature and pressure to define a system within this area. In the area BOC, AOC andAOB exists, ice (solid), water (liquid) and vapour (gas) respectively. In these areas, the degrees of freedom for the system is two or they are bivariant (iv) The Triple point O The point O at which the curves AO, BO and CO meet is called the triple point. At this point all the three phases viz , ice, water and vapour co-exist. Thus, P =3. According to phase rule, at triple point O, F = C –P + 2, =1-3+2 , F=0 Phase Rule Thus, the degree of freedom at triple point is zero, which indicates that there is only one set of variables P, T at which all the three phases coexist. If any of the variables is changed, then the number of phases decreases. For example, if the temperature is raised, heat causes more and more of the solid (ice) to melt but no change in temperature or pressure of the system occurs till the whole of the solid has completely changed into liquid (water) and the system becomes a two phase system. On applying the pressure to the system, vapours start condensing to liquid or solid phase. As long as there are three phases, temperature and pressure remains same. The triple point O is a self defined point corresponding to 0.0075°C temperature and 4.579 mm of Hg pressure (difference from the ordinary freezing point,(0.0000C at 760mm pressure) because freezing temperature is lower than triple point temperature due to effect of high pressure and dissolved air). In some systems, an equilibrium exists between solid liquid phases and gaseous phase is practically absent. Hence the effect of pressure on such system can be neglected. Then it is necessary to take into account only two variables viz. temperature and concentration. Such system showing solid-liquid equilibrium is called condensed system and phase rule applied to such systems is as follows: F = C – P + 1 … known as condensed phase rule. It is a two-component system. The phase diagram of the Pb-Ag system is shown in the fig Curve AO: Point A is the melting point of pure Silver. Curve AO shows melting point depression of silver by the addition of lead. At any point in this curve there is equilibrium between solid Ag and liquids part. According to reduced phase rule equation. The system is univarient. F’=C-P+1; F’=2-2+1; F’=1 Curve BO: Point B is the melting point of pure lead, (327oC), curve BO shows the melting point depression of lead on gradual addition of silver to it. Along this curve solid lead and solution co – exist and hence the system is univariant. Point O: The two curves AO & BO meet at point O, where three phases solid pb, solid Ag and their solution co-exist, according to condensed phase rule the system is invariant. F’=C-P+1; F’=2-1+1; F’=2 The point ‘O’ is known as eutectic point, its composition (Ag=2.6%; Pb=97.4%) and temperature (3030C) is known as eutectic composition and eutectic temperature respectively. Further cooling below the eutectic temperature respectively. Further cooling below the eutectic temperature will cause simultaneous crystallization of a mixture of lead and silver. Area AOB: Consists of only one phases namely pb-Ag solution. According to reduced phase rule equation. F’=C-P+1; F’=2-1+1; F’=2 The system is bivarient i.e., both T & composition has to be specified to define the system. Let us consider a point p, which represents a sample of lead containing less than 2.6% silver. On cooling the temperature falls gradually till point p’. On further cooling lead begins to separate and the concentration of Ag increase in the solution till the point O is reached, after that whole mass solidifies (2.6% Ag; 97.4% pb). This process is utilized in the pattinson’s process of desilverization of lead. Below the eutectic point (O), area COEF consists of solid and eutectic compound, where crystalline silver and eutectic compounds are stable. Similarly the area ODFG consists of solid Pb and eutectic compound, where crystalline lead eutectic compounds are stable ABRASIVES • ABRASIVES (Def): • An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. Mohs Hardness Scale Hardness is the imp quality of a abraseive material Mho scale is used to measure the hardness It is incresed in the following order(from 1 – 10) Talc1Gypsum2Calcite3Fluorite4Apatite5Orthoclase6 Quartz7Topaz8Corundum9Diamond10 Types of Abrasives These are two types 1. Natural Abrasives 2. Artificial or synthetic Abrasives Natural Abrasives Diamonds: It is the best hardest material occurs in nature. It is stand on top in abrasive power. It is used in making drill points. To drill a rock or to cut, polish these diamonds are used in the drill tips and polishing wheels. Grinding wheels are made with diamond tips are used for cutting the rock into slices. Corundum: It is made of aluminium oxide with chemical formula Al2O3 that exist in crystalline form. Heavy abrasive wheel used in metal industry are made from corundum. Emery: It has high scratch hardness. It has a character of consistent break down of it crystal structure under pressure. It is perfect mixture of black magnetite (Fe2O3) and corundum. Granet: It occupy at 6.5 to 7.5 reading on Mohr’s scale of hardness. It is vastly applied in making of sandpaper used for surface smoothing and wood polishing. It composition is mainly of silicate minerals. Pumice: In lithography, it is widely used in polishing and cleaning stones. It is a composition made of silicates of aluminium, sodium and potassium. It has pale grey texture. Porous blocks are the main source for pumice. Quartz: It stands seventh position in Mohr’s hardness scale. It is applied for sand paper making. It is quite brittle in nature due to the shell like structure of silica crystal. Sand stone: It is used in making of sharpening stones, pulp stones and grindstones. Flint: It is widely used as mill grinding stones. Due to its conchoidal fracture, it appears in light colour. It is a modified from silica. Diatomaceous earth and Tripoli: Diatomite is obtained from diatoms a unicellular organism, which have siliceous skeleton. This siliceous skeleton is deposited over time and form diatomaceous earth. Similar to this is a Tripoli but difference in the appearance. It has property that it sinks in water at once but diatomite floats for some time and then sinks. These two products are starting materials for make polishing cream used for metals. synthetic types of abrasives Silicon carbide: It is manufacture in an electrical furnace with raw materials of coke, sand and sawdust. Fused alumina or alundum (Al2O3): It is made by arc resistance furnace from bauxite ore. Calcium carbide (CaC2): It is manufacture in an electric furnace at temperature ranging from 2000-2200 degree centigrade. Carbon and quick lime are used as raw materials. Boron carbide (B4C): It is also called as norbide. It is manufactured in resistance furnace at 2600 degree centigrade. Boric oxide and coke are used as raw materials. REFRACTORIES Refractories Any material which can withstand high temperature Importance in construction of 1. Metallurgy 2. Engineering 3. Chemical industries 54 Introduction Refractory is any material which can withstand high temperature, without softening or suffering a deformation in shape. Main Objective 1. To confine heat e.g. to resist loss of heat. 2. To resist abrasive and corrosion action of molten metals, slags and gases at high temperatures, without undergoing softening or distortion in shape. Uses 1. Construction of the linings of the furnaces, tanks, converters, Kilns, crucibles, ladles etc. 2. Manufacture of metals (Ferrous or non-ferrous), cement, glass, ceramics, paper, steel etc. 55 Characteristics of Refractories A good refractory posses following characteristics: 1. Be infusible at the temperature to which it is liable to be exposed. 2. Chemically inerts towards corrosive action of gases, metallic liquids, and slags. 3. Resist the abrasive action of flue gases, flames, etc. 4. Be able to withstand the overlying load of structures at the operating temperature. 5. No crack 6. No loss in size. 7. Expand and contract uniformly, with temperature rise and fall respectively. 56 Classification of Refractories 1. Acid Refractories: important members of this group is Alumina, Silica gel fireclay refractories. 2. Basic Refractories: Cao, MgO 3. Neutral Refractories: made from weakly acid/basic materials like Chromite(FeO. CrO2), Zirconia (ZrO2) Important members of this group are Graphite, Chromite, Zirconia and carborundum (SiC) refractories. 57 Properties of Refractories 1. Refractoriness 2. Strength or Refractoriness-under load 3. Dimensional Stability 4. Chemical Inertness 5. Thermal Expansion 6. Thermal Conductivity 7. Porosity 8. Thermal Spalling 9. Resistance to abrasion or corrosion 10. Electrical conductivity 11. Heat capacity 12. Texture 13. Permeability 58 Properties of Refractories (contd.) 1. Refractoriness Ability of a material to withstand the heat, without appreciable deformation or softening under particular service conditions. In general, measured as the softening or melting temperature of the material. As most of the common refractory materials are mixtures of metallic oxides, so they do not have a sharp fusion temperature. Pyrometric Cones Test (Segar Cones Test) The softening temperature of the refractory material are, generally, determined by using Pyrometric cones test. Expressed in terms of Pyrometric cone Equivalents (PCE). Softening temperature (Material to be used as refractory) >> Operating temperature 59 Pyrometric Cones Test (Segar Cones Test) The refractoriness is, usually, determined by comparing the behaviour of heat on cone of material to be tested with that of a series of Segar cones of standard dimensions. Segar cones melt or fuse at definite temperature when heated under standard conditions of 10°C / min. Segar Cone Pyramid Shaped having triangular base 38 mm high and 19 mm long sides So the temp. at which the fusion or softening of the test cones occurs is indicated by its apex touching the base. The PCE value of the given refractory is taken as the no. of the standard cone, which fuses along with the test cone. 60 Segar Cones Number Fusion temperature 1 2 3 4 5 6 7 8 9 1110 1120 1140 1160 1180 1200 1230 1250 1280 61 Properties of Refractories (contd.) 2. Strength or Refractories-under load (RUL): Refractories used in industrial furnaces have invariably to withstand varying loads of the products, being manufactured at high operating temperature. It is, therefore, essential that refractory materials must also possess high mechanical strength, even at operating temperature, to bear the maximum possible load, without breaking. Some refractories like FIRECLAY, High Alumina Bricks softens gradually over the range of temperature, but under appreciable load, they collapse, far below their true fusion point, as determined by segar cones. On the other hand, other refractories such as Silica Bricks softens over a relatively narrow range of temperature and exerts good load bearing characteristics close to their fusion points. 62 R.U.L. Test Refractories-under load Test R.U.L. test is performed by applying a constant load of 3.5 or 1.75 kg/cm2 to the refractory specimen (of size 5 cm2 and 75 cm high) and heating in a carbon-resistance furnaces at a standard rate of 10°C / min. The record of the height of the specimen vs. temperature is made by a plot, until the test-piece deforms or collapses by 10%. The R.U.L. is expressed as the temperature at which 10% deformation takes place. 63 Properties of Refractories (contd.) 3. Dimensional Stability Resistance of a material to any volume changes, which may occur on its exposure to high temperature, over a prolonged time. These dimensional changes may be permanent (irreversible) or reversible. Irreversible changes may result either in the contraction or expansion of a refractory. The permanent contraction is due to the formation of increasing amounts of liquid from the low fusible constituents of the refractory brick, when it is subjected to a long period of soaking at the high temperature. The liquid gradually fills the pores of the refractory body, causing a high degree of vitrification and shrinkage. 64 Properties of Refractories (contd.) 4. Chemical Inertness A refractory should be selected that is chemically inert in use and does not form fusible products with slags, fuel ashes, furnace gases, etc. usually, the environment in most furnaces are either acidic or basic. It is not recommended to employ Acid refractory in contact with an alkaline (basic) product or vice-versa. 65 Properties of Refractories (contd.) 5. Thermal Expansion Solid materials, on heating, expands and on cooling it contracts. So in the designing of the practical furnaces, a refractory material should have least possible thermal expansion as the expansion affects all dimensions (e.g. length, area, volume) of the body. 6. Thermal Conductivity In industrial operations, refractory materials of both high thermal conductivity and low thermal conductivity are required, depending upon the type of the furnaces. In most cases, furnaces is lined with refractories of low heat conductivities to reduce the heat losses to the outside by radiation; otherwise maintenance of high temp. inside furnaces will become difficult. 66 Properties of Refractories (contd.) 6. Thermal Conductivity A good heat conductivity of the refractory material is desirable for effective heat transmission in furnace construction. The densest and least porous brick have the highest thermal conductivity, owing to the absence of air-voids. On the other hand, in porous bricks, the entrapped air in the pores, acts as a non-heat conducting material. For making porous refractory bricks, the refractory material is mixed with a liberal amount of carbonaceous material, then mould into bricks and burnt. The carbonaceous material burns off; leaving behind minute voids, which enhances the insulating quality. 67 Properties of Refractories (contd.) 7. Porosity All refractories contain pores, either due to manufacturing methods or deliberately made ( by incorporating saw-dust or cork during manufacture). Porosity is the ratio of its pore’s volume to the bulk volume. W- D X 100 W-A W = Wt. of saturated specimen. D = Wt. of Dry specimen. A = Wt. of saturated specimen submerged in water. P= Porosity is an important property of refractory bricks, because it affects many other characteristics, e.g. chemical stability, strength, abrasionresistance and thermal conductivity. In a porous refractory, molten charge, slags, gases etc. are likely to enter more easily to a greater depth and may react and reduces the life of the refractory material. 68 Properties of Refractories (contd.) 7. Porosity Porosity decreases Strength resistance to abrasion resistance to corrosion/ penetration by slags, gases ec. Porosity increases resistance to thermal spalling ( i.e. thermal shock-resistance The densest and least porous brick have the highest thermal conductivity, owing to the absence of air-voids. In porous bricks, the entrapped air in the pores, acts as a non-heat conducting material. A good refractory, in general , should have low porosity. 69 Properties of Refractories (contd.) 8. Thermal Spalling Breaking, cracking, peeling off or fracturing of a refractory brick or block, under high temperature. So good refractory must show a good resistance to thermal spalling. Spalling is caused by rapid changes in temperature, which causes uneven expansion and contraction within the mass of refractory, thereby leading to development of internal stresses and strains. Spalling may also be due to slag penetration into the refractory brick, thereby causing variation in the coefficient of expansion. Spalling can be decreased by • Using high porosity, low coefficient of expansion and good thermal conductivity refractory bricks. • Avoiding sudden temp. changes. • By overfiring the refractories at high temp. for a sufficiently long time, whereby mineral inversion et. takes place making the material less susceptible to uneven expansion or contraction, when heated. 70 Properties of Refractories (contd.) 9. Resistance to abrasion or erosion good refractory must show a good resistance to abrasion or erosion. 10. Electrical conductivity good refractory must show a low electrical conductivity. Except graphite, all other refractories are poor conductors of electricity. 11. Heat capacity Heat capacity of any substance depends on (a) Thermal conductivity (b) Specific heat (c) Specific gravity 71 Properties of Refractories (contd.) 12. Texture Course or light –textured bricks, because of their large porosity, are light in weight and hence, they are more resistant to sudden changes in temperature. However, their crushing strength is low. Such bricks are more susceptible to the action of abrasion and corrosion. on the other hand, fine or dense-textured bricks possess low porosity and hence are light in weight. These are not so resistant to sudden changes in temp. However, such bricks are less susceptible to action and corrosion. 72 Properties of Refractories (contd.) 13. Permeability Measure of rate of diffusion of gases, liquids and molten solids through a refractory. Permeability depends upon the size and number of connected pores. Permeability α temperature α 1 --------------------------Viscosity of molten material 73 Manufacture of Refractories Consist of following steps 1. Crushing: Raw material in the form of big lumps are crushed to about 25 mm size. 2. Grinding: The crushed material are grinded in grinding machine down to 200 mesh size. 3. Screening: Purify the refractory raw materials and remove unwanted materials from the raw materials and this is done by (a) settling (b) magnetic separation (C) Chemical Methods 4. Storage: After screening and mineral dressing, pure material is stored in storage bins with bucket elevators. 5. Mixing: It is done so that proper distribution of the plastic materials throughout the mass takes place. This makes moulding easier. 6. Moulding: Moulding may be done either manually or mechanically by the application of high pressure. Hand- moulding produces refractories of low density and low strength. Mechanical- moulding produces refractories of high density and strength. 74 Manufacture of Refractories (contd.) Consist of following steps In order to increase the density and strength of refractory by mechanical moulding, the de-airing of refractory material is essential. De-airing is done by: (i) Applying vacuum through vents in the moulds (ii) by allowing air inside the void space in the refractory to go out by decreasing the rate of pressure application and release of air. (iii) By double-pressing: the material is first pressed and allowed to crack. Then, it is pressed again so as to close the voids. 7. Drying: Removal of moisture is done under well set conditions of humidity and temperature, depending upon the type of refractories. Drying is usually carried out in tunnel dryers. 8. Firing: To stabilize and strengthen the structure of refractories, Firing is done. The bricks are , generally, fired at a temperature as high as or higher than their use temperature. It is done in tunnel Kilns or Shaft Kilns or rotary kilns. Firing temperature: 14800C for high-fired super duty bricks 17000C for kaolin bricks 18700C for basic bricks 75