MELTING FURNACE

• Furnace is used to melt the metal.
• Foundry furnace remelts the metal to be cast.
• A foundry furnace does not convert ore into usable metal as blast furnace.
• A furnace generally forms a high temperature zone surrounded by refractory bricks which can withstand the heat and minimize heat lose by insulating it from the outside environment.
• The metal is placed in this high temperature zone.

Remelting Furnaces for Casting

Metals are generally obtained from blast furnace, steel making furnace or other non-ferrous smelting furnaces.

These metals can not be cast directly into desired shapes of components basically due to following two reasons.

1. These metals are not always in a sufficiently refined state.

2. Practically it is difficult to pour huge quantity of molten metal into mould.

For the above reasons metals obtained from smelting furnaces are first cast into same regular shapes as ingots. These are then remelted in foundries for casting required components.

Various types of remelting furnaces are used for this purpose. They are as follows :

(i) Cupola Furnace

(ii) Electric Furnace

(a) Resistance furnace

(b) Induction furnace

– Core less type/ High frequency induction furnace

– Core type/Low frequency induction furnace

(c) Arc furnace

– Direct arc furnace

– Indirect arc furnace/Rocking furnace

Selection of Remelting Furnace

Selection of remelting furnace depends on following factors.

(i) Initial cost of furnace.

(ii) Fuel cost

(iii) Metal/alloy to be melted.

(iv) Melting / pouring temperature.

(v) Quantity of metal to be melted.

(vi) Cost of melting per unit weight.

(vii) Flexibility of unit i.e., whether can be used for melting other metal or not.

(viii)Speed of melting

(ix) Cost of operation

(x) Degree of cleanliness/pollution.

Cupola Furnace

A cupola furnace is generally used for melting and refining pig iron alongwith scrap for producing cast iron (Fig.)

Features/Advantages of Cupola

– simple operation

– continuous production

– economy of working

– increased output

– low melting cost

– easy tempeature control

– less fuel expensive and easily available

For producing white cast iron (below 2.71% carbon) duplex process is used.

Description of Cupola

Shell

– It is vertical and cylindrical in shape

– Made of sheet thickness 6 – 12 mm inside of which is lined with acid refractory bricks and clay consisting of silicon acid (SiSO2) and Alumina (Al2O3).

– Diameter of cupola varies from 1 – 2 meters and height is 3 – 5 times diameter.

Foundation

– The shell is mounted on a brickwork foundation or cast iron columns.

– The bottom of the shell consists of a drop bottom door for cleaning purposes at the end of melting.

Tuyers

– Air for combustion of fuel is delivered through the tuyers.

– These are provided at height of between 0.6 to 1.2 m above the working bottom.

Wind Belt

– Air supplied by blower is delivered to the tuyers from wind belt/jacket.

– It is a jacket like structure which is a steel plate duct mounted on outside of shell.

Blower

A high pressure fan/ blower is used to supply air to the wind belt through a blast pipe.

Slag Hole

– It is used for removing the slag.

– It is placed at a level at about 250 mm below the centre of the tuyers.

Tapping Hole

– It is used for pouring out molten metal.

– It is located opposite and just below the slag hole.

Charging Hole

– It is used for feeding charge to the furnace.

– Charge is a mixture of metal, coke and flux.

– Normally situated 3 to 6 meters above the tuyers.

Chimney / Stack

– The shell normally continues for 4.5 m to 6 m above the charging hole to chimney.

Zones of Cupola

The entire cupola can be divided into the following sections.

Crucible Zone

It is between top of the sand bed and bottom of the tuyers.

Combustion / Oxidizing Zone

– It starts from top of the tuyer to 150 to 300 mm above it.

– Heat is generated in this zone due to the following reaction.

Reducing Zone

– It starts from the top of the combustion zone upto the top of cokebed.

– CO2 is reduced in these zone.

Melting Zone

– It starts from top of the coke bed and ranges up to a height of 900 mm.

– Temperature is highest in this zone (1600°C).

Preheating / Charging Zone

– It starts from top of the melting zone and ranges upto the charging door.

– Charging materials when fed gets preheated here.

Stack Zone

– It starts from the charging zone upto top of cupola.

– Gases generated are carried out in this zone.

Cross section of a Cupola

Operation of Cupola

Preparation of Cupola

– Clean the slag and repair damaged lining using mixture of fire clay and silica sand.

– Bottom doors are raised up and bottom sand poured.

– The surface of the sand is sloped from all directions towards the tap hole and rammed.

– Slag hole is also formed to remove slag.

Firing the Cupola

– Wood is ignited on sand bottom.

– Then coke is added to a level slightly above the tuyers.

– Air blast is turned on at a slower rate.

– After having red spots on the fuel bed, extra coke is poured upto required height.

Charging the Cupola

– Cupola starts burning properly.

– Then alternate layers of pig iron, coke and flux (limestone) are fed from charging door till cupola is full.

– Flux does two functions (i) prevent oxidation (ii) remove impurities (Flux is normally 2 – 3% of metal charge by weight)

Soaking of Iron

– After charging these get slowly heated up.

– Air blast is kept closed for 45 minutes.

– This causes iron to soak.

Opening of Air Blast

– Air blast is opened after near about 45 minutes.

– Tap hole is kept closed to gather sufficient amount of molten metal.

– The rate of charging should be equal to the rate of melting in order to keep the furnace full for continuous operation.

Resistance Furnace

This furnace works on the principle of resistance heating effect of current (Fig.).

Resistance heating furnace

It states that when a current carrying conductor carries current (I) having resistance (R) for time (t) then the heat produced will be.

Resistance heating of a single electrode provides required heat for melting

Normally silicon carbide is melted in this kind of furnace.

The electrode has reduced diameter at its center.

It offers greatest resistance to current flow as

High heat is generated which is used for melting.

The operation is noiseless and costly due to high cost of electricity.

Induction Furnace

Coreless type / High frequency Induction Furnace

A high frequency induction furnace has a refractory crucible placed inside a water cooled copper coil.

This crucible is kept in required position by ramming dry refractory sand around it.

It can be of two types depending on the method of molten metal extraction. They are

(i) tilting type (Fig.)

(ii) lift coil type (Fig.).

Induction Furnace : tilting type

Induction Furnace : lift coil type.

Operating Principle of Coreless type / High frequency Induction Furnace

Steel pieces (scrap) normally used as charge is fed into the furnace.

High frequency current is supplied to the coils. So the coil acts as primary of transformer and the charge acts as secondary.

Due to electromagnetic induction alternating current is induced in secondary (charge).

These offer resistance to secondary current due to which heat is generated.

Due to conduction, this heat created on skin of charge propagates and melts the charge.

A magnetic field is associated with the secondary current which provides a stirring action in the molten which results in.

– speed up of melting process

– mixing of metal charge uniformly

This type of furnace takes very little time for melting.

Pouring is done by any of the method aforesaid.

It is used /preferred where different metals/alloys are to be melted in small amount.

Advantages of Coreless type / High frequency Induction Furnace

Can melt small quantity (i.e., 1.5 kg to 12 tons) of various types of methods or alloys quickly.

Magnetic stirring produces uniform melt.

Construction is simpler than core type induction furnace.

No warm up time required.

Great control over energy input and furnace atmosphere.

Limitations of Coreless type / High frequency Induction Furnace

Initial cost is high

There should be no error in charge in terms of composition because there is practically very less available for analysis.

Thermal efficiency is less (upto 60%) than core type due to energy loss in motor generator set or spark gap convertor.

Applications of Coreless type / High frequency Induction Furnace

Used for melting general, special, alloy and high quality steels in small quantity.

Core type / Low Frequency Induction Furnace

It is shown in Fig.

Core type induction furnace

It works as an ordinary transformer.

The primary coil has many turns which is wound on a laminated steel core, but the secondary has only one turn which is a channel of molten metal.

The furnace uses an A.C. supply of 60 cycles per second (Hz).

Secondary current (high current and low voltage) produced in molten metal around core generates heat due to electrical resistance to flow of secondary current.

This channel of molten metal is connected to the main metal container which holds metal charge. So the metal in the channel gets heated, it circulates through the metal in the container and melting process goes on.

Metal is poured into ladle when it reaches the required pouring temperature

Advantages of Core type / Low Frequency Induction Furnace

It is the most efficient induction furnace.

Thermal efficiency is about 80%

Rapid and clean melting.

Uniform molten metal due to magnetic stirring.

Accurate control of temperature and composition.

Economical operation.

Limitations of Core type / Low Frequency Induction Furnace

Furnace can not be started with solid metal charge. The channels have to be filled with molten metal produced by any other furnace.

If by mistake metal in channel gets soildifield, can be remitted by heat created in secondary coil.

Normally used to melt only one type of metal/alloy continuously because by this the liquid metal in channel will always be available.

Applications of Core type / Low Frequency Induction Furnace

Generally used for melting non-ferrous metal and alloys continuously for a longtime.

Generally used as a holding furnace for permanent mould and die casting because of ability to control temperature easily.

Normally capacity is upto 5 tons.

Arc Furnace

Direct Arc Furnace

The interior of the furnace is preheated before placing metal charge in the furnace.

Preheating is done by alternatively striking and breaking the arc between the vertical electrodes and the used electrode pieces kept on the hearth.

After preheating is done, the electrode pieces placed on the hearth are removed.

Few spare roofs should be available at all times because of their shorter life time.

Charging may be done from charging door which is also used for removing slag or from furnace roof which is lifted for charging.

Transformer rating – 800 kVA to 40000 kVA

A 50 ton direct arc furnace require current of 2500 amps and voltage of 250 volts.

Electrode guides placed on roof are water cooled to dissipate damaging heat.

For removing slags before pouring molten metal, the furnace is tilted backward to pour off slag from charging door.

The furnace is tilted forward for pouring molten metal into ladle.

After each heat, hearth, side walls and roof are repaired using suitable refractory material.

Indirect Arc Furnace

It has capacity from few kgs to 2 tons.

Generally used for less capacity than direct arc type.

In this type electric arc is produced between two graphite electrodes. So charge is not a part of arc.

It is called rocking furnace because, it rocks back and forth during melting. So the metal comes in contact with the heated refractory lining for melting.

Charge melts due to

– heat from arc,

– heat from hot refractory walls, due to conduction when furnace rocks and molten metal rolls over it.

Indirect arc furnace is used for melting

– cast-iron

– steel

– copper and its alloys

Gets lower temperature and has lower efficiency than arc type.

Construction of Indirect Arc Furnace

An indirect arc furnace has a barrel type shell made out of steel plates having refractory lining inside.

There are three openings, two for two graphite electrodes and third one is for charging metal charge into furnace.

Pouring tap is built up with charging door.

These rollers are driven by rocking unit to rock the furnace back and forth for melting.

Working of Rocking Unit

 During rocking of furnace, the liquid metal flows over the heated refractory lining and absorbs heat.

 It also results in proper mixing of molten metal.

 Rocking

– increases speed of melting.

– stress the molten metal.

– avoids overheating of refractory lining, thus its life time increased.

The angle of rocking furnace is adjusted in such a manner that liquid metal level remains below the pouring tap.

Process of Operation of Indirect Arc Furnace

Pig iron is charged first.

Scrap is poured over pig iron.

When electric power is supplied, graphite electrodes are brought nearer to each other till the current jumps and electric arc is produced between them.

The heat is generated due to arc which melts charge. After some metal has been melted, rocking unit is activated.

Rocking helps in better exchange of heat between refractory lining, molten metal and solid metal.

Advantages of Indirect Arc Furnace

Metal charge is not a part of electrical circuit.

Low cost scrap metal can be used.

Operation and control are simple.