utilization of

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UTILISATION OF ELECTRICAL ENERGY

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# Advantages of Electrical Heating:-

Heating is required for domestic purposes such as cooking and heating of buildings, as well as for industrial purposes such as melting of metals, hardening and tempering, case hardening, drying and welding. Practically all the heating requirements can be met by some form of electric heating equipment. The main advantages of electric heating over other systems of heating are given below:

1. Economical: - Electric heating is economical as electric furnaces are cheaper in initial cost as well as maintenance cost. It does not require any attention so there is considerable saving in labor cost over other systems of heating. Electrical energy is also very cheap as it is being produced on large scale.

2. Cleanliness: - Since dust and ash are completely eliminated in electric heating system, so it is clean system and cleaning cost are rendered to minimum.

3. Absence of Flue Gases: - Since no flue gas is produced in the system, so there is no risk of atmosphere or objects being heated and operation is, therefore, hygienic.

4. Ease of Control: - Simple, accurate and reliable temperature control can be had either by hand operated or by fully automatic switches. Desired temperature or temperature cycle can be has accurately in electric heating system, which is not convenient in other heating systems.

5. Automatic Protection: - Automatic protection against over-currents or over heating can be provided through suitable switchgears in the electric heating system.

6. Upper limit of Temperature: - There is no upper limit to the temperature obtainable except the ability of the material to withstand heat.

7. Special Heating Requirements: - Certain requirements of heating such as uniform heating of material or heating of one particular portion of the job without effecting others, heating of non-conducting materials, heating with no oxidation, can be met only in the electric system.

8. High Efficiency of Utilization: - The overall efficiency of electric heating is comparatively higher since in this system of heating, the source can be brought directly to the point where heat is required, thereby reducing the losses. Further there is no product of combustion in which heat losses are involved. It has been practically ascertained that 75 to 100% of heat produced by electric heating can be successfully utilized whereas in case of gas, solid fuel and oil heating the efficiencies are 60%, 30% and 60% respectively.

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9. Better Working Conditions: - Electric heating system produces no irritating noise and also the radiating losses are low. Thus working with electric furnaces is convenient and cool.

10. Safety: - Electric heating is quite safe and responds quickly.

# Classification of Electrical Heating Methods:

Electric heating can be broadly classified as (i) power frequency heating and (ii) high frequency heating.

Power frequency heating can be further classified as (i) resistance heating and (ii) arc heating

Resistance heating can be further classified as (i) direct resistance heating, (ii) indirect resistance heating and (iii) infra-red or radiant heating.

Similarly arc heating can be further classified as (i) direct arc heating and (ii) indirect arc heating.

High frequency heating can be classified into (i) induction heating and (ii) dielectric heating.

Induction heating can further be classified as (i) direct induction heating and (ii) indirect induction heating.

1. Direct Resistance heating: - Electric current is made to pass through the body to be heated. This principle of heating is employed in resistance welding and electrode boiler for heating water.

2. Indirect Resistance heating: - Electric current is made to pass through a wire or other high resistance material forming a heating element; heat so developed is transferred from the heating element to the body by the agency of radiation or convection. Normally this method is used in immersion heaters, resistance ovens, domestic and commercial cooking and heat treatment of metals.

3. Infra-Red or Radiant heating: - Heat energy from an incandescent lamp is focused upon the body to be heated up in the form of electromagnetic radiations. This is employed to dry the wet paints on an object.

4. Arc heating: - The arc between two electrodes develops high temperature depending upon the electrode material. The electric arc may be used in the following different ways:(i) By striking the arc between the charge and the electrode or electrodes. In this

method the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnaces.

(ii) By striking the arc between the two electrodes. In this method the heat is transferred to the charge by radiation. The furnaces operating on this principle are known as indirect arc furnaces.

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(iii) By striking an arc between an electrode and the two metallic pieces to be joined, as in arc welding.

5. Direct Induction heating: - In this method of heating the currents are induced by electromagnetic action in the body to be heated. The induced currents when flowing through the resistance of the body to be heated develop the heat and thus raise the temperature. In induction furnace heat is used to melt the charge and eddy current heaters used for heat treatment of metals are other forms of direct induction heating.

6. Indirect Induction heating: - In this method of electric heating the eddy currents are induced in the heating element by electromagnetic action. Eddy currents setup in the heating element produce the heat which is transferred to the body to be heated up, by radiation and convection. Certain types of induction ovens used for heat treatment of metals operate on this principle.

7. Dielectric heating: - In this method of electric heating use of dielectric losses is made to heat the non-metallic materials. Non-metallic material to be heated is placed between two metal electrodes across which a high voltage having high frequency is applied; the heat is developed owing to the dielectric losses taking place.

# RESISTANCE HEATING:-

Resistance heating is based upon the I2R effect. This method of heating has wide applications such as heat treatment of metals, drying and baking of potteries, stoving of enameled ware and commercial and domestic cooking. Temperature up to about 1,0000C can be obtained in ovens employing wire resistances for heating elements. There are two methods of resistance heating.

1. Direct resistance heating: - In this method of heating, the material or charge to be heated is taken as resistance and current is passed through it. The charge may be in the form of powder, pieces or a liquid. Two electrodes are immersed in the charge and connected to the supply in case of availability of direct current or single phase ac supply and three electrodes are immersed in the charge and connected to supply in case of availability of 3-phase ac supply. When some pieces of metals are to be heated some highly resistive powder is sprinkled over the surface of pieces to avoid direct short-circuit. The current flows through the charge and heat is produced. This method has high efficiency since heat is produced in the charge itself. As the current in this case is not easily variable, therefore, automatic temperature control is not possible. However, uniform and high temperature can be obtained. This method of heating is used in salt bath furnaces and in the electrode boiler for heating water.

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2. Indirect Resistance heating: - In this method of heating, the current is passed through a wire or other high resistance material forming a heating element. The heat proportional to I2R loss produced in the heating element is delivered to the charge by one or more of the modes of transfer of heat viz. conduction, convection and radiation. If the heat transfer is by conduction the resistor must be in contact with the charge. An enclosure, known as heating chamber, is required for heat transfer by radiation and convection for the charge. For industrial purposes, where a large amount of charge is to be heated, the heating element is kept in a cylinder surrounded by jacket containing the charge. This arrangement provides a uniform temperature. Automatic temperature control can be provided in this case. This method of heating is used in room heaters, immersion water heaters and in various types of resistance ovens employed in domestic and commercial cooking, and salt bath furnaces.

# Design of heating element: -

Knowing the electrical input and its voltage the size and length of the wire required as the heating element to produce the given temperature can be calculated. The wire employed may be circular or rectangular like a ribbon. The latter permit the use of higher wattage per unit area and are used in ovens, vulcanizers, toasters etc. The heating element on reaching a steady temperature will dissipate the heat from its surface equivalent to electrical input. Since generally the heat will be dissipated from the heating elements at high temperatures, it is reasonable to assume that the whole of the heat energy is dissipated solely by radiation.

Heat dissipated according to stefan’s law.

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# Efficiency and losses: -

The heat produced in the heating elements is also to overcome the losses occurring due to (i) Heat used in raising the temperature of oven or furnace;

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(ii) Heat used in raising the temperature of the containers or carriers;(iii) Heat conducted through the walls;(iv) Escapement of heat due to opening of the door; in addition to heat required to

raise the temperature of the charge to the required value.

The efficiency of the oven =

The heat required to raise the temperature of the charge to the required value, W= mass of charge in kg*specific heat of charge in J/kg/0C*temperature rise = mst Joules

The heat used for rising the temperature of the oven or furnace can be calculated in a similar way by knowing the weight of the refractory material and its specific heat. In case the oven is used continuously this loss becomes negligible.

Heat used for raising the temperature of container is calculated in exactly the same way as for oven or furnace. The container usually has to be heated up afresh for each charge.

Since the heat is continuously conducted through the walls so this source of heat loss is most important. It can be determined knowing the mean value of the inside and outside surface areas A in square meters; the thickness of walls t in meter; inside and outside temperatures T1 and T2 in 0C and thermal conductivity of walls k in MJ per hour per square meter, per meter, from the following expression

Heat loss by conduction through walls = kA (T1- T2)/t MJ/hour.

Though there are no specific formulae for determination of loss occurring due to opening of door for inspection of the charge, however, this loss may be taken as 0.575 to 1.15 MJ/m2 of the door area if the door is opened for a period of 20 to 30 seconds.

The efficiency defined above lies between 60 and 80 percent. A more convenient figure for expressing the behavior of an oven, however, is the energy required per tone of charge treated. Typical values for different processes are given below

Process Energy Consumption in kWh per tone of charge

Baking of breadAnnealing of copperAnnealing steel

50-100100-200200-250

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CarburizingVitreous enameling of sheet steel

250-500400-700

# Induction heating

In induction heating effect of currents induced by electo-magnetic action in the charge is employed. The heat developed depends on the power drawn by the charge. The power consequently depends upon the voltage and resistance of the charge, because power drawn is equal to V2/R. So to develop heat sufficient to melt the charge, the resistance of the charge must be low, which is possible only with metals, and voltage must be higher, which is obtained by employing higher flux and higher frequency. Magnetic materials, therefore, can be easily treated that non-magnetic materials because of their higher permeability.The various types of induction furnaces are described as follows:

1. Core type Furnaces: - The core type furnace is just like a transformer having primary connected to the supply and the charge to be heated as secondary.(a) Direct Core Type Induction Furnace: - Direct core type induction furnace consists

of an iron core, crucible of some insulating material and primary winding connected to an ac supply. The charge is kept in the crucible, which forms a single turn short-circuited secondary circuit. The current in the charge is very high, of the order of several thousand amperes.This type of furnace has following drawbacks:

1. As magnetic coupling between the primary and secondary circuit is poor, therefore, leakage reactance is high and power factor is low. This difficulty, however, is overcome by employing supply of frequencies as low as 10 Hz for operation of such furnaces. For obtaining low frequency supply motor-generator set or frequency changer is required, which involves extra cost.

2. If normal frequency supply is employed for operation of such furnaces, the electromagnetic forces cause severe stirring action in the molten metal. Low frequency supply, therefore, is also necessary from this point of view.

3. If the current density exceeds about 5 amps per mm2 the pinch effect due to electromagnetic forces may cause complete interruption of the secondary circuit and so of supply.

4. The crucible for the charge is of odd shape and inconvenient from the metallurgical point of view.

5. For functioning of the furnace the closing of the secondary circuit is essential which necessitates the formation of complete ring of the charge around the core. For starting the furnace, either molten metal is poured into the crucible or sufficient molten metal is allowed to remain in the crucible from the previous operation. This is required otherwise, the secondary will remain open-circuited and no current will circulate and no heating will take place. However, once the melting starts additional metal can be added and the

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furnace tapped periodically. Also in order to close the secondary circuit, an iron ring may be placed in the crucible or the lining may be of graphite.

Such furnaces are not suitable for intermittent services or where different types of charges are to be melted.

(b). Vertical Core type induction furnace: - The furnace of this type, known as Ajax Wyatt vertical core type furnace employs a vertical channel instead of horizontal one for the charge. The convection currents keep the circulation of molten metal round the V portion. As V-channel is narrow, so even a small quantity of charge is sufficient to keep the secondary circuit closed. Hence the chances of discontinuity of the circuit are less. Due to pinch effect the adjoining molecules carrying current in same direction will try to repel each other, but because of the weight of the charge they will remain in contact and chances of interruption will be reduced.

The output of the furnace depends upon the type and dimensions of the channels used. In certain furnaces instead V-shaped channels U-shaped channels or rectangular channels are employed.The inside of the furnace is lined depending upon the charge. Clay lining is used for yellow brass. For red brass and bronze an alloy of magnesia and alumina or corundum having high contents of alumina is employed.The shell of the furnace is of heavy steel. The top of the furnace is covered with an insulated cover which can be removed for charging. Necessary hydraulic arrangements are usually made for tilting the furnace to take out the molten metal.

Advantages:-1. Highly efficient heat, low operating costs and improved production.

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2. Accurate temperature control, uniform castings, reduced metal losses and reduction of rejects.

3. Absence of crucibles.4. Consistent performance and simple control.5. Ideal working conditions in a cool atmosphere with no dirt, noise or fuel.6. Absence of combustion gases resulting in elimination of the most common source of

metal contamination.7. Comparatively high power factor with normal supply frequency since primary and

secondary are both on the same central core.However, it is to be noted that the Vee must be kept full of charge in order to maintain continuity of the secondary circuit. For this reason, this type of furnace is suitable only for continuous operation. These furnaces are widely used for melting and refining of brass and other heavy non-ferrous metals. Its efficiency is about 75%. Standard sizes of these furnaces range 60 to 300 kW, all single phase, 50 Hz in this country for standard voltage up to 600V.

(C)Indirect core type induction furnace: -

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In such a furnace an inductively heated element is made to transfer its heat to the charge by radiation. In this type of furnace the principle of induction has been utilized for providing heat treatment of metallic and other charges in addition to its use for melting metals.

It consists of an iron core linking with the primary winding and secondary also. In this case secondary consists of a metal container forming the walls of the oven proper. Primary winding is connected to the ac supply, inducing currents and heating the metal container. Heat is transmitted to the charge by radiation. It is advantageous in respect of temperature control without use of external control equipment. It consists of part AB of the magnetic circuit situated in the oven chamber and consisting of a special alloy which losses its magnetic properties at a particular temperature and regains them when cooled to the same temperature. As soon as the oven attains the critical temperature, the reluctance of the magnetic circuit increases many times and the inductive effect correspondingly decreases, thereby cutting off the heat supply. The bar AB is detachable type and can be replaced by others having different critical temperatures between 4000C and 10000C, according to needs.From the mode of transmission of heat it will be seen that this furnace is directly in competition with resistance oven; but has comparatively poor power factor.

2. Coreless Induction Furnace: -It essentially consists of three main parts (i) the primary coil (ii) the refractory container and (iii) the frame which includes supports and a tilting mechanism.The distinctive features of this furnace are the absence of a continuous iron path for the magnetic flux and small quantity of refractory material in comparison with other types of melting furnaces in construction.Standard performed crucibles are employed for small furnaces. The base and the wall around the crucible are made by ramming granular refractory material. The top of the wall is sealed with refractory cement. The containers of the large furnaces are made in place, the procedure being

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same as for the smaller furnaces except that a hollow collapsible form is substituted for the crucible to form the receptacle. Acid or basic materials are used as per requirements.For floor level mounting, the electrical connections are made by knife contacts in order to make the handling of the furnace as a ladle for pouring. For platform mounting the electrical connections are made with flexible cables. This is done as with this arrangement, power can be left on the furnace while pouring- a feature often desirable when a no. of castings are poured from one heat. A variation is the lift-coil furnace made in the smaller sizes. The primary coil is lowered and raised over the load crucible.The charge is put into the crucible and primary winding coil is connected to high frequency ac supply. The flux created by primary winding sets up eddy currents in the charge which tend to flow concentrically with those in the inductor. These eddy currents heat up the charge its melting point and also set up electromagnetic forces producing stirring action which is essential for obtaining uniform quality of metal. Because of high frequency employed, which is necessary to induce the required voltage in the secondary, the skin effect produces heat in the primary winding coils. The primary winding coils are, therefore, made from hollow tube and are cooled by circulation of water through it. Insulated supporting structure is employed for such furnaces; otherwise stray magnetic field outside the primary will setup emf in it, which will result in circulation of eddy currents in it and so reduction of efficiency.Standard sizes of coreless induction furnaces for melting non-ferrous metals and alloys range from 50 kg to 500 kg holding capacity.The operating voltage varies from 1,000 to 2,000 V for larger sizes and 10,000 V for the smaller sizes.The choice of frequency of operation plays vital role. It is governed by the factors, material to be heated and thickness of cylinder layer at the outside edge of the crucible. The frequency of the primary current can be ascertained by using penetration formula. Accordingly,

The exact theory also shows that for efficient operation the ratio of radius of piece of material in the charge to the thickness should be greater than 3. If it is taken as 4, then the expression for the frequency, for efficient operation, becomes

In most of the modern coreless induction furnaces the frequencies in the range 500 to 1,000 Hz are used. However, for smaller units for melting small quantities of finely divided metal frequencies up to 100 kHz or even 1,000 kHz are used.

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The refractory container makes necessary a large air gap with consequent low power factor. Static capacitors are, therefore, invariably employed in parallel with such a furnace in order to improve the pf. Since in case of coreless induction furnace operation pf does not remain constant, capacitance in the circuit during heat cycle is varied to maintain power factor approximately unity.The choice of frequency is influenced also by the cost of the capacitors, used for power factor improvement, which decreases with the increase of frequency, and the cost of the converting apparatus, which increases with frequency.

Advantages of coreless induction furnaces: - The advantages of a coreless induction furnace over other types are given below:

(i) Low operating cost.(ii) Low erection cost.(iii) Automatic stirring action produced by eddy currents.(iv) Absence of dirt, smoke, noise etc.(v) Simple charging and pouring.(vi) Possibility of operating the furnace intermittently, as no time is lost in warming

up.(vii) Precise control of power.(viii) Less melting time.(ix) Possibility of employing vacuum heating necessary for precious metal melting.(x) No contamination of charge and very accurate control of composition so most

suitable for production of high grade alloy steels.

The coreless induction furnace is mainly employed as a metal melting unit. An important application of this furnace is the production of carbon free ferrous alloys. Various special uses are: vacuum melting, duplexing steel, heating of charges of non-conducting materials by the use of conducting crucibles etc. The energy consumption is 600 to 1,000 kWh per tonne of steel.

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# HIGH FREQUENCY EDDY CURRENT HEATING

The articles to be heated are placed within a high frequency current carrying coil,, alternating magnetic field is set up, eddy currents are induced in the article and heating is, therefore, affected. The high frequency carrying coil is known as heater coil or work coil, the material which is to be heated is known as charge or load and the process employed is referred to as high frequency eddy current heating.The eddy current loss is primarily responsible for the production of heat, but hysteresis loss also contributes to it, though to a little extent, in the case of magnetic materials.

Since the eddy current loss is proportional to the square of the product of supply frequency and flux density, therefore, by controlling the frequency and the flux density the amount of heat can be controlled.However, it is observed that higher the frequency employed in induction heating, the greater will be the tendency of the induced heating currents to remain at the surface of the material being heated. This property is called skin effect. This has a strong effect upon the uses and limitations of high frequency eddy current heating.

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Induced eddy current is of greatest magnitude at the surface of the material to be heated and its value decreases with the distance normal to the surface as we go inside the material.Since the depth of penetration of the eddy currents into the charge is inversely proportional to . therefore eddy current heating can be restricted to any desired depth of the material to be heated by judicious selection of frequency of the heating current. The supply frequency is usually employed between 10 to 400 kHz.To be effective the coil must be so shaped to provide as tight a magnetic coupling as possible between itself and the work, the turns of the coil being parallel to the paths in which it is desired that the eddy currents shall flow. With the higher frequencies it is possible to achieve power inputs to the charge of 1.6 kW or more per square centimeter of surface.

Advantages of Eddy Current heating: -(i) It is quick, clean and convenient method.(ii) There is little wastage of heat, as heat is produced in the body to be heated up

directly.(iii) It can easily take place in vacuum or other special atmosphere, where as other

conventional types of heating are not possible in such places.(iv) The control of temperature is very easy.(v) The heat can be made to penetrate into the metal surface to any desired depth.(vi) Unskilled labour can also operate the equipment.(vii) The area of surface over which heat is produced can be accurately controlled.(viii) The amount of heat produced can be accurately controlled by suitable timing devices.(ix) The work coils are not required to fit closely around the object being treated. It

enables the same coil to heat many different objects of different shapes and sizes and in fact often obviates the necessity of requiring odd shaped coils to heat irregular shaped objects.

From the economic point of view, the generation of heat is costly, efficiency of equipment is quite low and initial cost of equipment is also high.

Applications of High Frequency Eddy Current heating: - The important applications of high frequency eddy current heating are: -

(a) Surface hardening: - It is very important application of high frequency eddy current heating. The bar whose surface is to be hardened, by heat treatment, is surrounded by the work coil. An alternating current of high frequency is made to pass through the work coil. The desired depth of penetration is obtained by judicious selection of the frequency of the heating current in the work coil. After about a few seconds, when the surface has reached the proper temperature, the ac supply is cut off and the bar is at once dipped in water. In the case of hardening done by other methods, the heat treatment of the inner metal is disturbed which produces scales on the surface and also wraps a long piece. This is

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avoided in this method of surface hardening. Such treatment due to induction heating reduces the cost, labour and time considerably.

(b) Annealing: - In conventional method of annealing the process takes long time, which results in scaling of metal,- a great short-coming. But in eddy current heating method, the time taken in the process is less, thus, there is no scale formation. By this method a temperature of the order 7500C can be attained in one minute up to the depth of 25mm.

(c) Soldering: - For soldering it is necessary that required amount of heat is produced at the soldering point and the remaining portion of the solder may remain cold. Eddy current heating can be economically employed for soldering precisely for high temperature soldering, Silver, copper and their alloys are used as solders. By induction heating heat develops very rapidly melting the solder which runs into the joint sealing it properly.

Other applications are welding, drying of paints, melting of precious metals, sterilization of surgical instruments and forging of bolt-heads and rivet heads.

# ARC FURNACES

When a high voltage is applied across an air gap, the air in the air gap gets ionized under the influence of electrostatic forces and becomes conducting medium. Current flows in the form of a continuous spark, called the arc. Arc drawn between two electrodes produces heat and has a temperature between 1,0000C and 3,5000C depending on the material of the electrodes used. The use of this principle is made in electric arc furnaces.

Usually arc furnaces are of cylindrical shape but recently conical shaped shells have been used. Even in conical shapes the horizontal cross-section is cylindrical. The conical shape has the advantage of large surface area per unit bath volume. It consumes less power and the radiation losses are reduced. Melting time is also reduced.

The furnaces may be door-charge type or the top-charge type. The electrodes used in arc furnaces of three types namely carbon electrodes, graphite electrodes and self baking electrodes. Material of the electrodes namely carbon and graphite has been selected on account of their electrical conductivity, insolubility, infusibility, chemical inertness, mechanical strength and resistance to thermal shock. The size of these electrodes may be 18cm to 27cm in diameter. With graphite or carbon electrode, the temperature obtainable from the arc is between 3,0000C and 3,5000C. The trend is toward the general use of graphite electrodes. Carbon electrodes are used with small furnaces for manufacture of Ferro-alloys, aluminum, calcium carbide, phosphorous etc. Self baking electrodes are employed in Ferro-alloys and electro-chemical furnaces and in electrolytic production of aluminum. Owing to lower resistivity of graphite, graphite is required half in size for the same current resulting in easy replacement. Graphite begins to oxidize at about 6000C where as carbon at about 4000C. Under average conditions the consumption of graphite electrodes is about one half of carbon electrodes. Electrode consumption for steel-melting furnaces varies between about 4.5 kg and 9 kg of electrodes per tone of steel with carbon

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electrodes, and between about 2.27 and 6.8 kg for graphite electrodes. Carbon electrodes are very cheap and costless than one half as much for same weight as graphite electrodes. The large area of carbon electrodes allow for more uniform heating. However, the arc has to be brought nearer the surface of the refractory lining and, therefore, the life of the refractory is shortened. Carbon electrodes are made of anthracite coal and coke where as graphite electrodes are obtained by heating the carbon electrodes to a very high temperature. Self baking electrodes are made of special paste, whose composition depends upon the type of process for which it is used, contained in thin steel cylinder. The flow of current produces heat and the paste is baked and formed into an electrode.

1. Direct arc furnace: - In a direct furnace charge acts as one of the electrodes and the charge is heated by producing arc between the electrodes and the charge. Since in a direct arc furnace, the arc is in direct contact with the charge and heat is also produced by flow of current through the charge itself, the charge can be, therefore, heated to highest temperature. In case of a single phase arc furnace two electrodes are taken vertically downward through the roof of the furnace to the surface of the charge and in a 3-phase furnace three electrodes put at the corners of an equilateral triangle, project on the charge through the roof and three arcs are formed. The current passing through the charge develops electromagnetic field and necessary stirring action is automatically obtained by it. Thus uniform heating is obtained.

It is commonly used for production of steel. The usually size of such a furnace is between 5 and 10 tones, though 50 and 100 tone arc furnaces have also been developed. The main advantage of direct arc furnace over cupola method for production of steel is that purer production is obtained and the composition can be exactly controlled during refining process. Another advantage is that arc furnace can operate on 100% steel scarp which is cheaper than pig iron where as the cupola requires a proportion of pig iron in cupola charge. This is the reason, that direct arc furnaces even being costlier in initial as well as

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operating cost is preferred. Though this furnace is employed both for melting and refining, but due to its higher cost it is used for refining than melting. The power factor is about 0.8 lagging. For 1 tone furnace power required is about 200 kW and energy consumed is 1,000 kWh/tone.

2. Indirect arc furnace: - In this case arc is formed between two electrodes above the charge and heat is transmitted to the charge solely by radiation. In this case the temperature of the charge is, therefore, lower than that in case of direct arc furnace. Since in this furnace current does not flow through the charge, so there is no stirring action and the furnace is required to be rocked mechanically. That is why the furnace is made of cylindrical shape, with the electrodes projecting through the chamber from each end and along the horizontal axis. By rocking action there is thorough mixing of the charge. The life of the refractory lining also increases since the molten metal come in contact with the lining and takes away some of its heat thus preventing it from attaining excessive temperature. The efficiency is increased because the charge is heated not only by radiation from the arc but by conduction from the heated refractory during rocking action. Its construction limits the number of electrodes to two, single phase supply is required. The size of the furnace is thus limited by the amount of single phase load, which can be taken from one point. The arc is produced by bringing the electrodes into solid contact and then withdrawing them. Power input is regulated by adjusting the arc length by moving the electrodes.

An electric motor is employed for operating suitable grinders and rollers to provide rocking action to the furnace. At start, the rocking action is carried through an angle of 15 to 200 and as the melt proceeds the angle is increased to about 2000C at a frequency of about 2 cycles per minute. Such furnaces have got following advantages:(i) Low metal losses: - Since furnace chamber is closed and a reducing atmosphere

above the metal is produced due to carbon arc, therefore, metal losses due to oxidation and volatilization are quite low.

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(ii) Economical: - Melting is rapid and takes place in a completely closed chamber resulting in small heat losses and low power consumption. Higher production rate gives low labor costs resulting in lower overall production cost per tone of molten metal.

(iii) Sound Castings: - The agitation produced by the rocking action of the furnace and the absence of combustion gases ensure product free blow holes, inclusions and segregations. Higher pouring temperature may readily be attained. Thus sound castings in thin and intricate designs can be produced.

(iv) Flexible: - Single furnace is capable of handling small or large heat of widely differing analysis. Interchangeable furnace shells can be used for different alloys and can be substituted in a few minutes.

# POWER SUPPLY AND CONTROL

An arc furnace used for melting and refining of steel requires power of about 500 kW per tone for small furnaces and of about 200 kW per tone for very large furnaces. The energy required is usually between 600 and 800 kWh per tone. For the arc furnaces used for refining only electrical energy consumption is about 100-120 kWh/tone. Thus it is seen that the power consumption of the arc furnace is very high. The arc voltage lies between 50 and 150 volts, the current required to give the above mentioned power is, therefore, of the order of several hundred or thousand amperes. The reasons for employing low voltage high current power supply for the arc furnaces are as follows:

(i) Heating effect is proportional to the square of the current, therefore, to achieve higher temperatures heavy currents are essential.

(ii) The maximum secondary voltage is also limited to 275V because of insulation and safety considerations.

(iii) By using low voltage and high currents the electrodes are kept very near to the charge as the arc is of small length. Thus arc remains away from the roof and, therefore, life of the roof refractory is increased.

(iv) Higher voltage causes higher voltage gradient between the electrode and the charge causing nitrogen of furnace atmosphere ionized and absorbed by the charge, which produces embrittlement.

Thus a transformer having low voltage and high current on the secondary and of special design having mechanical rigidity to enable the windings and the electrical insulation to withstand the heavy mechanical stresses set up by the high current surges is required for the arc furnace. The transformer used with arc furnace is of oil immersed water cooled type. A typical specification for a 3-phase arc furnace transformer includes an extended primary winding with taps there in for the secondary voltage range 235-220-205-190-175-160 volts, with the primary winding

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connected in delta. This voltage range is extended by changing the connections of the primary windings from delta to star giving 58% voltage from each tap. Both core type and shell type transformers can be used, but the latter type is preferred because it facilitates the bringing out and bracing of the heavy current leads.

The power input can be controlled by raising or lowering the electrodes which results in variation of the arc resistances RA. The power input can be also controlled by changing the tapping of the transformer which results in variation of voltage across the furnace. For complete control of furnace temperature and to achieve best operating conditions both voltage and electrode controls are employed.

Condition for maximum output

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# DIELECTRIC HEATING

When non-metallic parts such as wood, plastics, bones are subjected to an alternating electrostatic field dielectric loss occurs. In dielectric heating use of these losses is made. The material to be heated is placed as a slab between metallic plates or electrodes connected to high frequency ac supply. For producing sufficient heat frequency between 10 and 30 MHz is used. Even though voltage up to 20 kV has been used but from personnel safety point of view voltages between 600 V and 3 kV are in common use. The necessary high-frequency supply

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is obtained from a valve oscillator, as in the case of high-frequency eddy current heating. An overall efficiency is about 50%.

The current drawn by the capacitor, when an ac supply voltage is applied across its two plates, does not lead the supply voltage exactly 900 and there is always an in-phase component of current. Due to this in-phase component of current, heat is always produced in the dielectric material placed in between the two plates of the capacitor. The electric energy dissipated in the form of heat energy in the dielectric material is known as dielectric loss. The dielectric loss is directly proportional to the frequency of ac supply given to the two plates of the capacitor. The physical conception of the dielectric loss is just as a molecular friction in the dielectric material when an ac electrostatic field is applied to it.

Current through the capacitor,

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The dielectric heating depends upon the values of the frequency and the voltage. By varying one of these two quantities, the rate of dielectric heating can be varied.

The capacity of the condenser can be calculated from the following relation

Where is relative permittivity of dielectric, is absolute permittivity of vacuum and equals

F/m, t is the thickness of dielectric in meters and A is the surface area of plates in m2.

Advantages:

(i) If the material to be heated is homogenous, and the alternating electric field is uniform, heat is developed uniformly and simultaneously throughout the entire mass of the charge.

(ii) As materials heated by this process are non-conducting, so by other methods heat cannot be conducted to inside so easily.

Applications:

1. Preheating of plastic performs: - The raw material in the form of tablets or biscuits, commonly called plastic performs, is required to be heated uniformly before putting them into the hot moulds so that whole mass becomes fluid at a time, otherwise if the raw material is put directly into the moulds, usually heated by steam, the outer skin of the performs will become hot and start curing while the core of the material has not reached fluid temperature resulting in unequal hardening of the plastic and improper filling of moulds corners. Difficulty arises due to the fact that plastic raw material once cured cannot be softened again satisfactorily. Any method of heating depending upon conduction of heat from surface to the core would miserably fail because plastic is bad conductor of heat. Dielectric heating is the only method which can be used for pre-heating of plastic performs to proper temperature uniformly.

2. Gluing of wood: - Dielectric heating is most commonly used for gluing of wooden sheets or boards as in this method of gluing the moisture contents of the wooden sheets remain unaltered. It is due to the fact that heat can be applied to the desired surface. Main difficulty in using animal glues is of long curing time and that parts to be joined are to be kept under mechanical pressure after application of glue for a period of about 24 hours. Mechanical pressure may be applied in gluing of wood by dielectric heating in order to secure better adhesion. Because of higher loss factor of glue as compared to that of wood most of the heat developed goes into the glue and very little heat is wasted.

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High frequency dielectric heating is very economical for obtaining curved wood sections such as radio-cabinets, furniture etc. The curves obtained by this method are stable.

3. Baking of foundary cores: - In foundaries resin type thermo-setting binders are employed as they set almost instantaneously when brought to polymerizing temperature. The dielectric heating evaporates water rapidly from the core mix and at the same time raises the temperature of the core material to polymerization point. Hence dielectric heating is most suitable for baking foundary cores mixed with thermosetting resin type core binders.

4. Diathermy: - Dielectric heating is also employed for heating tissues and bones of the body required for the treatment of certain types of pains and diseases.

5. Sterilization: - The dielectric heating is quite suitable for sterilization of bandages, absorbent cotton, sterile gauge, instruments etc.

6. Textile industry: - In textile industry the dielectric heating is employed for drying purposes.

# ELECTRON BEAM WELDING

Basically, electron beam welding in vacuum utilizes the kinetic energy of electrons travelling with high velocity in a high vacuum. When the electrons strike the surface of the metal, they give up the bulk of their energy as heat, and this goes to melt the metal. If the work is done in a high vacuum, no electrodes, gases, or filter metals need contaminate it and pure welds can be made. Moreover, high vacuum is necessary around the filament so that it will not burn up and will also produce and focus a stable beam.

In all types of electron beam machines, a tungsten filament which serves as a cathode emits a mass of electrons that are accelerated and focused to a 0.25-1 mm diameter beam of high energy density up to 0.5-10 kW/mm2. The temperature produced is about 2,5000C. This is sufficient to melt and vaporize the work piece material and thus fills a narrow weld gap even without a filter rod.

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The speed of the beam is stepped to one-half to two-third of the speed of light by passing it through a high-voltage electrostatic field. An electromagnetic lens is used to obtain correct focusing of the beam.

A similar process, known as non-vacuum electron beam welding is rapidly coming into use today i.e., many welds are now made without elaborate arrangement required for vacuum electron beam welding. In this case, the vacuum chamber in which the beam is created is evacuated to a lower pressure. In situation where contamination of the work-piece must be held to a minimum, the beam should be passed through argon or helium. To make the chamber high vacuum, it takes about 5 to 30 minutes to evacuate the air, depending on the size of the chamber. A medium size electron beam welder operates below 60 kV. The welding head or the work is moved by numerical control or by hand.

The advantages of electron beam welding are that the welds are clean, with no porosity since there is no air; no shielding gas is required; and as the energy input is in a narrow, concentrated beam, distortion is almost eliminated. The speed may be as fast as 2,500 mm per minute, and it will weld or cut any metal or ceramic, diamond, sometimes as thick as 150 mm.

The major advantage of electron beam welding is its tremendous penetration, which occurs when the highly accelerated electron hit the base metal. It will penetrate slightly below the surface and at that point release the bulk of its kinetic energy, which turns to heat energy. This brings about a tremendous temperature rise at the point of impact. The succession of electrons striking the same place causes melting and then evaporation of the base metal. This creates metal vapors, but the electron beam travels through the vapor much easier than solid metal. This causes the beam to penetrate deeper. The depth to width ratio can exceed 20:1. With the increase in power density, penetration is increased. Penetration also depends on the beam current. With the increase in beam current, penetration is increased. The other variable, travel speed, also affects penetration. As travel speed is increased, penetration is reduced.

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Since the electron beam has tremendous penetrating characteristics, with the lower heat input, the heat affected zone is much smaller than of any arc welding process. In addition, because of almost parallel sides of the weld nugget, distortion is greatly reduced. The cooling rate is much higher, and for many metals that are advantageous; however, for high carbon steel this is a drawback and cracking may occur.

Almost all metals can be welded with the electron beam welding process. The metals that are most often welded are super alloys, the refractory metals, the reactive metals, and the stainless steel. Many combinations of dissimilar metals can also be welded.

One of the disadvantages of the electron beam process is its high capital cost. The price of equipment is very high, and it is expensive to operate, due to the requirement for vacuum pumps. In addition, fit up must be precise and locating the parts with respect to the beam must be perfect.

Today automobile, airplane, aerospace, farm and other types of equipments including ball-bearing over 100 mm are being welded by the electron beam process.

# ELECTRODES

An electrode is a piece of wire or rod, with or without flux covering, which carries current for welding. At one end it is gripped in a holder and an arc is set up at the other end.

Either non-consumable or consumable electrodes may be employed in arc welding.

Non-consumable electrodes may be of carbon, graphite or tungsten which do not consume during the welding operation. Consumable electrodes may be made of various metals depending upon their purpose and the chemical composition of the metals to be welded.

In general, the electrodes can be divided into three categories depending upon outer aspects as well as their technological properties.

Bare electrodes are most commonly used in automatic and semi-automatic welding. In using the plain or bare electrodes, as the globules of the metal pass from the electrode to the work, they are exposed to the oxygen and nitrogen in the surrounding air. This causes the formation of some non-metallic constituents which are trapped in the rapidly solidifying weld metal and thereby reduces the strength and ductility of the metal.

Lightly coated electrodes have a coating layer several tenths of a millimeter thick. This coating usually consists of lime mixed with soluble glass which serves as a binder. The primary purpose of the light coating is to increase the arc stability, so they are also called the stabilizing coatings. No attempt is made to prevent oxidation and no slag is formed on the weld, nor are the

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mechanical properties of the weld metal improved. For this reason lightly coated electrodes may only be used for welding non-essential jobs.

Heavy coated electrodes, sometimes referred to as shielded arc electrodes, are used to obtain a weld metal of high quality, comparable with, and even superior to, the parent metal in terms of mechanical properties. Heavy coatings employed are composed of ionizing, deoxidizing, gas generating, slag forming, alloying and binding materials. These electrodes have the following distinct advantages:

(i) The physical and metallurgical properties of the weld can be influenced by adding alloying components to the covering.

(ii) Basic salts of silicon, magnesium and calcium in the covering form slag which floats on the surface of metal and prevents rapid cooling of the weld. Thus weld does not become brittle.

(iii) The weld metal is protected from oxidizing action of atmospheric oxygen and nitrifying action of nitrogen of air due to gases formed from the covering material.

(iv) In case of ac supply arc cools at zero current and there is a tendency of deionizing the arc path. Covering gases keep the arc space ionized.

(v) During welding the covering extends beyond the core wire. This directs the arc and concentrates the arc stream, reduces thermal losses and causes increase in temperature of electrode tip.

There may also be powder cored electrodes which have a good portion of the source of metal for depositing in the joint located as powdered iron mixed in with the flux coating. This makes the electrodes much larger on the outside for the same diameter core wire as standard flux-coated electrodes. These electrodes are well suited to down-hand welding and are said to deposit more metal in a given period time than standard flux covered electrodes.

Both bare and coated electrodes, for manual arc welding, are made in the shape of rods up to 12 mm in diameter and 450 mm long. Semi-automatic and automatic welding use electrode wire in coils.

# ENERGY STORAGE WELDING PROCESSES

To meet the demand of heavy current of very high conductivity metals such as aluminum and magnesium energy storage welding circuits are used. There are basically two such circuits namely electro statically stored energy circuits and electro magnetically stored energy circuits.

1. Capacitor Discharge Welding Circuit: - Condenser C is charged to about 3,000 volts from grid controlled rectifier. When the condenser is connected to the primary of welding transformer by ignition contractor, it will discharge and thus high transient current will be

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produced in the secondary to weld the material. The worth noting points in connection with this circuits are:(i) As the voltage of condenser approaches the voltage of the source of supply,

charging rate becomes lower, therefore to charge condenser to about 3,000 V at high charging rate voltage of about 5,000 to 6,000 V will be required. A voltage regulating circuit cuts off the rectifier from the bank when the voltage of the bank becomes 3,000 V.

(ii) If there is residual magnetism near situation, it will result in low rate of change of flux linkages in the secondary and, therefore, in production of low heat. Hence in the welding transformer core flux should not be present.

2. Magnetic Energy Storage Welding Circuit: - In this type of welding, energy stored in magnetic circuit is used in the welding operation. The dc voltage of the rectifier is suitably controlled so that the current in the primary of the transformer rises gradually without inducing large current in the secondary. This is necessary to avoid preheating of metals at the weld joint. Preheating in aluminum, magnesium etc. is undesirable as it causes deformation.When sufficient energy has been stored up in the transformer core, the contactor opens, dc flow ceases and there is a rapid collapse of magnetic field. The decay of flux induces heavy currents in the secondary of the transformer for welding.The kVA demand on the line in magnetic energy storage welding is higher as compared to that in capacitor discharge welding but a high voltage rectifier and costly capacitor bank are not required.

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# ELECTRIC WELDING EQUIPMENTThe electric welding sets may be either dc or ac type.1. DC Welding Sets: - Such sets are of two types namely

(i) Generator type(ii) Rectifier type

Generator type welding set consists of a differential compound wound dc generator, giving dropping volt-ampere characteristic, driven by any type of prime-mover. In a differential compound wound dc generator the terminal voltage falls automatically with the increase in load current. The control may be obtained by tapping the series field or by providing a suitable shunt across the series field winding. The open-circuit voltage is adjusted from shunt field.

If supply from existing dc distribution system is to be used for welding then ballast is put in series with the equipment and control is obtained by varying this external resistance. This method is also suitable when a number of operators are working on the same supply system. In such cases each operator is provided with separate ballast. The special field of this method is where the service of each arc welding circuit is infrequently in use. In such cases the loss in the series resistor is less than the no-load loss of a generator type unit.

Another type of dc welding set is a dry type rectifier used in conjunction with a multi-phase, high leakage reactance transformer. Many of these rectifier type welders use selenium rectifiers which are forced air cooled. Rectifier type welders are said to combine some of the desirable arcing characteristics of dc welding, such as easy arc starting, with those of welding transformers, such as reduced no-load losses. DC voltage is controlled by regulating the transformer output in this case.

2. AC Welding Sets: - Single phase or 3-phase step-down transformers which provide low voltage power for welding with some means of output control. One or two taps are provided on the primary side to take care of the voltage variations. The secondary winding of an air-cooled set is generally made of bare wound on edge copper strip. The welding transformers may be air-cooled or oil-cooled type. The air-cooled sets are lighter in weight and permit the use of insulating materials of the highest thermal

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classification. They are also cheaper, less hazardous and easier to maintain. On the other hand, the oil-cooled sets are more compact. Being heavy, they are more suitable for stationary applications in workshops. Synthetic liquid filled sets may be employed in hazardous locations. Some machines are provided with an arc booster that provides a momentary surge of current to give an arc a good start when it is struck.

In a transformer type welding machines the current control is achieved by using (a) magnetic shunt or (b) a choke coil or reactor placed in series with the primary or secondary winding or (c) tap changing switch in the primary winding.

In the magnetic shunt type, an adjustable gate of iron laminations is placed between the primary and the secondary windings. Part of the flux is diverted through this gate without linking with the secondary winding, thus providing a simple and reliable means of step less voltage control.

The choke coil type regulator is a variable reactance with a movable iron plunger. The reactor is such that it operates well below the saturation point. Though it improves the arc stability but reduces the power factor.

The tap-changer type regulator is prone to give trouble on account of arcing on the contacts and the moving finger.

The use of series resistance can also be made for current control but efficiency is reduced.

# ELECTROLYSIS –BASIC PRINCIPLE

In nature atoms of electrolyte are closely bound together but bond becomes weaker, when dissolved and the molecules of the electrolyte split up into two types of ions carrying electric charges, called the cations and anions, and moving freely in the solution. Now if two electrodes are dipped into the electrolyte and connected to the dc supply, ions associated with the positive charge (cations) and moving freely in the solution are attracted by the cathode and the ions associated with negative charges (anions) and moving freely in the solution are attracted by the anode.

For example when copper sulphate (CuSO4) is dissolved in water, immediately it gets dissociated into +vely charged copper ions (Cu++) and negatively charged sulphions (SO4

--) moving freely in the solution and if a potential difference is applied between the two electrodes immersed in the solution the +vely charged copper ions will move towards the cathode and the –vely charged sulphions will move towards the anode. Each of the positively charged copper ions reaching the cathode will take two electrons from it and become a metallic atom of copper, and similarly each of the –vely charged sulphions reaching the anode will give up two electrodes to it and cease to

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anion. Thus the electrons will move from anode to cathode in the external circuit and constitute the flow of current from anode to cathode in the electrolyte. Thus the function of source of supply seems only to serve as an electron pump pumping electrons from +ve side of the supply and supply to the –ve side of the supply.

As mentioned above each +vely charged copper ion on reaching the cathode takes two electrons from it, becomes atom of metallic copper and deposit there. Similarly each –vely charged sulphion on reaching the anode gives up two electrons to it and becomes SO4 radical but since SO4 radical cannot exist in the electrical neutral state so it will attack the anode and will form the corresponding sulphate of the material of the anode; for example if the anode is of copper then copper sulphate will be formed, but if the anode is of such a material which cannot be attacked by SO4, for example if the anode is of carbon the SO4 will react with water and form sulphuric acid and liberate oxygen according to chemical equation

The whole process described is called the electrolysis and the effect is that the copper gets dissolved from the anode and deposited to the cathode. During the process there is no accumulation of charge at any point in the circuit and the mass of copper deposited at the cathode is exactly equal to that removed from the anode.

# FARADAY’S LAW OF ELECTRO-DEPOSITION

The laws governing the electrolytic processes were formulated by Michael Faraday, an English Scientist, and are known after his name. These may be stated as below:

Faraday’s First Law: - According to this law the chemical deposition due to flow of current through an electrolyte is directly proportional to the quantity of electricity passed through it.

i.e., mass of chemical deposition,

m α Quantity of Electricity, Q

or m α It

or m = ZIt

where I is the steady current in amperes flowing through the electrolyte for t seconds and Z is a constant of proportionality and is known as the electro-chemical equivalent of the substance.

If I= 1A and t= 1 second, then Z = m

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Thus electro-chemical equivalent, Z, of a substance is defined as the amount of the substance deposited on passing a steady electric current of 1 A for one second through its solution. It is usually expressed in terms of milligrams per coulomb. The SI unit of electro chemical equivalent Z is the kilogram per coulomb.

Faraday’s Second Law: - This law states that when the same quantity of electricity is passed through several electrolytes, the mass of the substance deposited are proportional to their respective chemical equivalents or equivalent weights.

# CURRENT EFFIECIENCY

Owing to impurities, which cause secondary reactions, the quantity of substance or substances liberated is slightly less than that calculated from Faraday’s laws. This is taken into account by employing a factor, called the current efficiency.

The current efficiency is defined as the ratio of the actual quantity of substance liberated or deposited to the theoretical quantity, as calculated from Faraday’s laws.

Its value usually lies between 90 and 98%.

In certain cases this efficiency is very low. For example in chromium plating it is roughly 12 to 15 percent. It is because only 15 percent of the total current passed through the electrolyte consisting of some chromium acid solution is used in depositing chromium and the rest is wasted in producing oxygen and hydrogen gases, which for the purpose in hand, are useless.

# ENERGY EFFICIENCY

On account of various secondary effects and reactions the substance deposited by a given quantity of electricity is less than that determined theoretically from Faraday’s Laws. Voltage required is also higher than that determined theoretically. Hence actual energy consumption will be higher than that determined theoretically for depositing a given quantity of the substance.

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The ratio of theoretical energy required to the actual energy required for depositing a given quantity of metal is known as energy efficiency.

# EXTRACTION OF METALS

Extraction of metal is an electro-chemical process used for the production of metal with commercially acceptable purity. There are two methods of extraction of metals depending upon the physical state of the ore. In one of the processes the ore is treated with a strong acid to obtain a salt and the solution of such a salt is electrolyzed to liberate the metal. The second process is employed when the ore is available in molten state or can be fused and in this process the ore, which is in a molten/fused state, is electrolyzed in a furnace.

Metal to be Extracted Treatment of Ore Solution Energy Consumption in kWh/Tonne

AluminumCopper

Magnesium

Sodium

Zinc

------Roasted and treated with sulphuric acid

------

--------

Treated with concentrated

sulphuric acid

Fused cryoliteCopper Sulphate

Fused magnesium chloride or carnaliteFused sodium hydrate or sodium chloride and sodium nitrateZinc chloride and zinc sulphate

20,000-25,0002,000-2,500

17,000-20,000

10,000-20,000

3,000-5,000

The methods adopted for extracting zinc and aluminum is explained below:

1. Extraction of Zinc: - This is an example in which an aqueous solution of the salt is used. The ore, consisting largely of zinc oxide, is treated with concentrated sulphuric acid, roasted, and passed through various chemical processes in order to remove impurities by precipitation. The zinc sulphate solution so obtained is then electrolyzed. The electrolysis of zinc sulphate is accomplished in large lead-lined wooden boxes having a number of aluminum cathodes and lead-anodes. Zinc gets deposited on the cathodes and is removed periodically. The current density on the cathodes is about 1,000 A/m2. The voltage per

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cell is about 3.5V and usually 100 to 150 cells are used in series requiring a pressure of roughly 500V. Energy consumption is 3,000 to 5,000 kWh/tone.

2. Extraction of Aluminum: - This is an example of fused electrolyte process. Aluminum is produced from bauxite containing aluminum oxide or alumina, silica and iron oxide. The bauxite ore is first reduced to aluminum oxide by chemical treatment and then it is dissolved in fused cryolite. Cryolite is a solution of aluminum fluoride and fluoride of either of sodium, potassium or calcium. The mixture thus obtained is electrolyzed. The fusion and electrolysis are accomplished in a large shallow rectangular steel bath lined with carbon; carbon anodes projecting downwards into the bath and the bottom of the bath forms the cathode. The charge is melted by the arc struck between the carbon anodes and cathodes and is then maintained in a molten state by the heating action of the electric current flowing through the charge. The liquid metal deposits at the cathode and settles at the bath bottom and is periodically siphoned out into large ladles from which it is poured into pig or ingot moulds. Fresh alumina is fed into the bath at short intervals to replace that which has been decomposed by the current and the process is, therefore, a continuous one. The aluminum obtained by this process is 99.5% pure.

A furnace having an area of about 15 square meters will need a voltage of about 6 volts and a current of about 40,000 amperes. Energy consumption is 20,000 to 25,000 units/tone. Almost the whole of the aluminum required in the present day’s industry is produced in this way. As the electrolytic process requires large amount of electric power and process is continuous, so such plants are installed near hydroelectric electric power stations. The high temperature (1,0000C) necessary to keep the ores in a fused state is maintained by the ohmic losses due to the current flowing through the electrodes and electrolyte.

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# ELECTROPALTING

Electroplating is an art of depositing a superior or a more noble metal on an inferior or a base metal by means of electrolysis of an aqueous solution of a suitable electrolyte. For example, metals like iron which are easily corroded by atmospheric air, moisture and carbon dioxide, are coated electrolytically with deposits of nickel or chromium which are more resistant to chemical attack. Picture frames and machinery parts are often chromium plated to protect them from corrosion and at the same time to give them a good polish.

Sometimes, electroplating is done, with a view to repair worn out parts of machinery. In such cases the suitable material is deposited electrolytically on the effected parts of the machinery.

Electroplating is also done occasionally for ornamentation and decoration purposes. For example, several articles made of copper or its alloys, such as table wares, decoration pieces, are coated with silver or gold.

The electrolytic deposits are crystalline in nature. The crystals must be very fine in order to get firm, coherent and uniform deposits. For this purpose, suitable electrolytes should be used in the electrolytic bath and current density used should have am appropriate value. The temperature should also be maintained at a proper level. By experiments, a certain optimum value of current density and temperature has been worked out for each electrolyte. For example, for Na3[Cu(CN)4] bath, the optimum current density lies between 32-65 A/m2

and optimum temperature is 450C. The optimum conditions for [Ag (CN)2] bath, are 42 A/m2 of current density and 200C as the temperature. The articles to be coated with the noble metals should be in as high state of purity as possible. These conditions are briefly described below:

Preparation for Plating: The preparation of an object for plating may involve any or all of the following operations:

1. Removal of oil, grease, or other organic material.2. Removal of rust, scale, oxides, or other inorganic coatings adhering to the metal.3. Mechanical preparation of the surface of the metal to receive the deposited metal, by

polishing, buffing etc.

For the first, soaps, hot alkali solutions, or organic solvents such as gasoline or carbon tetrachloride are used, for the second, various acids, alkali and salt solutions, mechanical abrasion, and electrolytic cleaning, and for the third mechanical abrasion and polishing are used.

Cleaning Methods: In case the object to be electroplated is not cleaned, polished and degreased, the deposit formed may not be well adherent to the base metal and is likely to peel off. For smooth, bright and strong deposit, the surface upon which a layer of a noble metal is required,

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should be thoroughly cleaned first mechanically by grinding or scratching or sand blasting and then chemically by treatment with hot alkalies or with dilute acids or with organic solvent.

Electrolytic Bath: The electrolyte used in the electrolyte bath depends upon the nature of the metal to be deposited.

For copper plating two types of electrolytic baths are used. In acid type bath, solution is made of 150-200 gm of copper sulphate and 25-35 gm of sulphuric acid per 1,000 cc of solution. Current density used is 150-400 A/m2 and temperature of 25 to 500C. Deposit obtained is thick and rough requiring polishing. In cyanide bath solution is made of 25 gm of copper cyanide. 28 gm of sodium cyanide, 6 gm of sodium carbonate and 6 gm of sodium bisulphate per 1,000 cc of solution. Current density used is 50-150 A/m2 and the temperature required is 25-400C. It provides thin and smooth deposits. Copper anodes are used in both of the baths.

For silver plating solution consisting of 24 gm of silver cyanide, 24 gm of potassium carbonate and 36 gm of potassium cyanide per 1,000 cc is used. The required current density and temperature are 50-150 A/m2 and temperature of 20-350C respectively.

For gold plating solution used consists of 18gm of potassium gold cyanide, 12 gm of potassium cyanide, 6gm of potassium sulphate and 12gm of caustic potash per 1,000 cc. Anode employed is of stainless steel. Current density of 50-150 A/m2 and temperature of 50-700C are used.

For chromium plating solution most commonly used consists of 180-300 gm of chromic acid and 2-3 gm of sulphuric acid per 1,000 cc. Current density employed is 1,500-2,500 A/m2 and the working temperature is 35-500C. Current density used is higher for hard chromium plating than for decorative plating. Anodes are of antimonial lead. Vats are used for chromium plating are of steel with lead lining. Chromic acid is added in the solution when required. Arrangements for removal of the fumes are also to be provided.

For nickel plating the solution consists of 180-240 gm of nickel sulphate, 36 gm of nickel chloride and 24 gm of boric acid per 1,000 cc. The current density used is 100-200 A/m2. The working temperature is 25-400C. Anode is of pure nickel.

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# POWER SUPPLY FOR ELECTROLYTIC PROCESSES

Power supply required for electrolytic processes is direct current and at very low voltage. The power required for electro-deposition is usually very small and can be obtained either by employing a motor-generator set consisting of a standard induction motor driving a heavy current low voltage dc generator or by employing the copper oxide rectifier. The latter is preferred because of low maintenance cost, occupying less space and higher operating efficiency. Mercury arc rectifier cannot be used because it has low efficiency at low output dc voltage on account of constant voltage drops at cathodes and anodes. The plate rectifier unit is usually placed along with its transformer in the oil so that it may be protected from the corrosive fumes of the electrolyte. Recently the solid state rectifying devices employing germanium and silicon diodes have been developed for use. These solid state devices occupy very small space even as compared to metal rectifiers. Output dc voltage can be controlled by controlling the excitation of the generator in case of motor-generator set and by means of continuously variable autotransformer in case of rectifier supply. This method of control is suitable where only one bath is being supplied. In case, more than one bath is supplied, a variable resistance is connected in series with each bath so that the supply to each bath can be controlled independently. With the development of SCRs or thyristors, which can control output voltages of power supplies, their use in power supplies for electrochemical processes has increased. They are also compact and light in weight, even cooling attachments are included. Voltage control is by using output transformers.

Power supply required for extraction and refining of metals and large scale manufacture of chemicals is in very large amounts. Since most of the processes are continuous, therefore, have a load factor of 100 percent. Because of power requirements in huge amount and at 100 percent load factor, such plants are located near the hydroelectric power stations or atomic power stations even if extra transportation of raw material is necessitated. The advantage of a high load factor is greater with such stations than with steam stations and also transmission costs are eliminated. Nangal fertilizer factory producing calcium ammonium nitrate and heavy water and utilizing power of 180 MW from left bank Bhakra power house and Shri Ram Fertilizer factory located at Kota are instances in the support of the above statement.

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The voltage of each cell is about 10 volts, but if many cells are connected in series, current of the order of several thousand amperes will be required at the voltage of the order of 500 to 800 volts. Thus by employing heavy current motor-generators, rotary convertors or even mercury arc rectifiers, the required supply may be obtained from the modern grid network.

# ANODIZING

Anodizing is a process of anodic oxidation in which a thin uniform passive film is produced artificially by the passage of electric current. Passive film is formed to protect the base metal from further corrosion. In the process of anodizing electric current of 10 to 30 amperes per square meter is passed through an electrolyte of 3% solution of chromic acid for a period of half an hour to one hour, the article to be anodized is thoroughly cleaned and made anode and carbon rod is made cathode. Aluminum and magnesium have capacity of producing such passive films and, therefore, anodizing is applicable to them.

# ELECTROPOLISHING

Closely allied to bright dipping is electro polishing, which utilizes anodic treatment in specially formulated electrolytes to bring up a polished surface on such metals as stainless steel. Electro polishing is also useful as a tool in preparing metallic surfaces for microscopic examination. Both bright dipping and electro polishing depend on the more rapid eating away by the solution of micro-projections on the metal, so that smoother surface results.

utilization of

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