Refractories for the Cast Iron Melting

Abstract

Refractory is a very important component in economically successful melting of cast iron. Refractory is used to line the furnace or any other vessel used for melting or holding of the molten metal. This chapter has discussed the different type of furnaces used for the melting of cast iron, the special features of those furnaces and the operational parameters of those furnaces with special emphasis on the coreless induction furnace, which is most commonly used. It has dealt with the installation practices of the refractory lining and also has discussed the mode of failure of the refractory lining and the precautions to be taken during installation and during use.

1. Introduction

Cast iron is an important engineering material and has numerous applications in Civil Engineering, Architecture, Agriculture, etc. Manufacturing of Cast iron products is comprised of first melting and then casting process, normally followed in Cast iron foundries. Cast iron contains about 2–4% C and 1–2% Si and the melting takes place at a temperature of 1550C. The furnaces used for melting the Cast iron need to be lined internally with Refractory material which can withstand that temperature continuously in presence of molten metal and slag to protect the integrity of the furnace structure. Different type of furnaces can be used for melting the Cast iron and the quality of the Refractory material used depends upon the furnace type. Although the Refractory cost per ton of processed cast iron is not high and significant but the refractory plays a very vital role towards the economic viability of the project.

The Refractory lining influences the furnace operational efficiency and the productivity. Higher is the refractory life, lower is the stoppage time and higher is furnace availability and productivity. Refractory quality influences quality of the castings also, because of the inclusion, coming from refractory. The incorporation of inclusion in the casting impairs its mechanical property and appearance.

We shall discuss here about the different types of furnaces used for the cast iron melting, different types of the Refractories used for different kind of furnaces, installation of refractories and the cause of failure of Refractories.

2. Furnaces used for melting cast iron

Three types of furnaces are mostly used for the Cast iron melting in the foundries. These are:-

1. Cupola Furnace

2. Induction Furnace

3. Arc furnace

The refractories are designed to withstand the operational conditions and the environment inside the furnace and therefore it is essential to know the furnace and operational details connected to refractory selection.

2.1 Cupola furnace

The typical Cupola furnace cross sections are shown in Figure 1 [1].

Figure 1.

Cupola furnace.

The cupola is a shaft type cylindrical structure as shown in Figure 1. The principle of operation is similar to Blast furnace. The pig iron and scraps, lime stones and coke are charged from the top and air is blown from the bottom when the oxidation of Carbon generates heat which melts the iron and it is collected at the bottom of the furnace from where it is tapped out. The hot air moves upward through the bed exchanging its heat with the downward moving cold burden and preheating it. Two major reactions take place inside the cupola. The first one is the oxidation of carbon in the coke C + O2 = CO2, which is highly exothermic reaction and increases the temperature inside and melts the iron. The molten iron picks up the carbon according to the reaction 3Fe+2CO = Fe3C + CO2 which is an endothermic reaction. Temperature inside the Cupola can reach up to 1600C and temperature control is not easy and precise in the Cupola furnace. Cupolas are used mainly where the large volume of Cast iron is melted.

2.2 Arc furnace

The typical arrangement of Arc furnace is shown in Figure 2 [1]. This furnace is a cylindrical Refractory lined steel shell fitted with three Carbon electrodes inserted through the Refractory lined roof of the furnace. The scrap and the pig iron and the fluxes are charged through the door inside the furnace and the power is made on. The arc between the charge and the electrodes produces arcs which generates high temperature and melts the charge.

Figure 2.

Arc furnace.

2.3 Induction furnace

This is the most widely used furnace in the foundry for melting the cast iron because of its easy and precise control of temperature and melt chemistry. The principle of induction furnace is same as a transformer. The electric current is passed through a water cooled coil and when electrically conductive solid metal kept inside the coil, it gets heated up because of induction. Two different types of induction furnaces are there.

2.3.1 Coreless induction furnace

The coreless induction furnace basically consists of the cylindrical refractory crucible surrounded by the induction coil supported by the transformer yokes (Figure 3). The energy is transmitted in such induction furnaces by passing an electric current through the coil which creates a magnetic field. Voltages are induced in the feed material due to this magnetic field, whereby eddy currents are created due to the conductivity of the metal. The induced current heats the charged material in accordance with Joule’s law and it melts after a certain heating time.

Figure 3.

Vertical section of coreless induction furnace.

Any current I, AC or DC, passing through an electrically conducting material causes a voltage drop V resulting in energy conversion to heat. Heat generated in the process is defined by V.I = R.I², where R is the electrical resistance of the current path. The resistance of the current path is inversely proportional to the cross-section area in which the current is flowing.

Coreless induction furnace normally has capacity up to 50 Ton. It can run by the main frequency of 50 Hz or by frequencies 200-1000 Hz called medium frequency furnace. Medium frequency furnaces are mainly used in industries because they have got certain advantages over the main frequency furnaces e.g.,

1. It requires less energy to melt the same quantity of metal.

2. It does not require to maintain any metal heel inside the furnace.

3. The furnace can be emptied and therefore no chance of mixing in case of different metals are produced in the same furnace.

4. No necessity to use the starter block to start cold heat.

2.3.2 Channel induction furnace

Channel induction furnaces are mostly used for holding the molten metal or for superheating the molten metal. It is used along with the Cupola to hold molten metal. The arrangement of the furnace is shown in Figure 4.

Figure 4.

Channel induction furnace.

Power consumption – Theoretically, melting one ton of Cast iron at 1500C should consume 396 Kwh of energy, but in actual practice it takes about 500 Kwh of energy because many types of energy losses takes place which is shown in Figure 5 [2].

Figure 5.

Energy balance in the coreless induction furnace.

2.4 Comparison of the furnaces

All the different types of furnaces discussed have some special features, advantages and disadvantages. Selection of the right kind of furnace depends upon the grades of metal to be produced, required melting capacity, availability and cost of raw materials and consumables, cost of electricity and coke, capital investment, operational cost, environmental restrictions, available space etc. Amongst all these different type of furnace used for Cast Iron melting, most used furnaces are Cupola and Coreless Induction furnace. The comparison of the special features of Cupola and Coreless Induction furnace is shown in Table 1 [3, 4].

Features	Cupola	Induction furnace
Energy consumption/ton of metal (KWh)	825–890	490–520
Efficiency%	44–47	75–79
Energy Consumption/ton of metal (Auxiliary Equipment) (KWh)	20–70	6–10
Throughput (Ton/h)	5–100	0.1–50
Temperature variation (K)	20–50	5
Chemistry control
Delta C%	0.5–0.7	0.1
Delta Si%	0.5–1.2	0.1
Slag quantity (Kg/ton)	40–100	10–30
Dust quantity (Kg/ton)	5–15	0.1–0.3
Metal losses (%)	0.5–1.5	0.1–0.3
Mode of operation	Continuous	Intermittent
Charge quality	Can handle highly oxidized and low quality scrap	Require high quality scrap
Maintenance cost	Higher	Lower
Capital investment	25% higher	25% lower
Refractory cost	Lower (1.8USD/t)	Higher(3.1USD/t)
Flexibility	Limited	Very high
Refractory lining	Complicated	Easy
Temperature homogeneity	Good	Very good
Chemistry adjustment	Good	Very good

Table 1.

Comparison of features of cupola and induction furnace.

3. Refractory

Materials are inert inorganic solid materials which can withstand high temperature in contact with solid, liquid and gases to retain its integrity and mechanical strength. These are basically Oxides, Nitrides, Carbides and Borides of Aluminum, Silicon, Alkaline earth metals and transition metals. Selection criteria of refractory for a high temperature process depend mainly upon the following.

1. Operational temperature

2. Chemical constituents, it will be in contact with and their state of occurrence at that temperature (solid, liquid or gas)

3. Batch process or continuous process

4. Mechanical abuse by the solid charge, degree of turbulence of liquid or gas velocity

5. Thickness of the refractory lining and shell temperature permitted as per design

6. Furnace size and geometry.

7. Economic considerations

Refractory lining has primarily two main functions

1. To withstand the hostile environment inside the furnace to protect of the furnace steel structure.

2. To reduce the outward flow of energy from furnace inside to conserve energy.

3.1 Cupola

It is to be borne in mind that there is lot of difference between a modern Cupola and the Cupola of earlier days. In earlier days Cupola used to be run for a day and next day it was used to be put down for Refractory maintenance and slag cleaning. Very low quality patching material comprised of sand, burnt brick bats and clay mixture were used for patching purpose. But to-day the modern Cupola is designed for continuous running and therefore high quality Refractories are used to line inside. Cupola is a vertical shaft furnace in which different operational condition exists across its height (zone) and different quality of refractories are used at different zone of the furnace (Figure 6a) [5]. The thermal profile across different zone is shown in Figure 6b [6].

Figure 6.

a. Different zones of the cupola furnace. b. the temperature profile inside cupola.

Charging is done from the top and the uppermost zone experiences heavy mechanical abuse and abrasion by the falling charge material like scrap, limestone and coke and descending burden.

Preheating and calcining zone are heated by the upward moving gas and the temperature there is low, below 1000C (Figure 6b). High density and low porosity 60–70% Al2O3 refractory bricks or dense low cement Castable can be used here for lining.

In the melting and well zone high quality Al2O3-SiC-C refractory either in brick form or as low cement self- flow Castable is used. Corundum based Castable or bricks can also be used here.

The tuyere is lined with high strength Al2O3-SiC-C or Corundum based low cement Castable. The refractories are degraded mostly by the chemical corrosion by the fluid slag generated in the melting process. The primary condition for the corrosion is the wetting of the refractory surface by the molten slag. The wettability of a solid surface by a liquid happens when the contact angle is low. The addition of Carbon to the refractory increases the contact angle of slag to refractory surface to make it non-wetting and hence more corrosion resistant [7]. The thickness of the lining depends upon the diameter of the Cupola and the intended shell temperature. To maintain low shell temperature and lower refractory thickness the use of insulation refractory is inevitable, otherwise the water cooling of the shell is required.

The use of water cooling leads to energy loss and increases the fuel cost.

Chemical and physical properties of refractory castables used for different zone of the cupola furnace are shown in Table 2.

Area of use	Al2O3%	SiO2%	SiC+C%	CaO%	Bulk density (gm/cc)	Maximum Service temp (°C)
Upper stack/ Cal.zone	60–70	25–35	—	2–4	2.2–2.5	1600–1700
Melting zone	10–40	5–6	45–80	0.5–1.5	2.65–2.70	1700
Well /Hearth	45–75	5–6	20–45	0.5–1.0	2.75–3.00	1700
Taphole area	65–70	4–5	20–22	1–3	2.90–3.00	1700
Iron Runner	60–75	5–6	18–20	1–2	2.9–3.00	1700

Table 2.

Typical properties of Castables suitable for different zone of cupola.

3.2 Coreless induction furnace

The Refractory issue is much more critical in case of Coreless induction furnace. In Coreless induction furnace the refractory lining separates the molten metal from the electrical copper coil behind (Figure 3) through which water is circulated to keep it cool.

The refractory lining thickness is to be optimized taking into consideration of the following:-

1. The higher is the thickness better it is to prevent heat loss through the lining.

2. Higher is the lining thickness lesser is the electrical efficiency of heating (Figure 7) [3].

3. Safety is a very important issue to be looked into during the operation of induction furnace. If a crack forms in the lining during the operation of the furnace, the molten metal may penetrate inside and strikes the coil which may lead to severe explosion.

Figure 7.

Effect of refractory lining thickness on electrical efficiency.

Thinner the lining more is the chance of metal penetration. On the other hand, higher is the refractory lining thickness lesser will be the heat loss through the lining but more will be the loss in electrical energy input given, to heat the charge inside the furnace.

Decreasing refractory thickness improves the coil efficiency but at the same time admits higher thermal losses through the thinner crucible wall. However, since coil losses exceed the thermal losses across the crucible wall nearly by the factor of 10, coil losses play the dominant role here. Taking above all points into consideration high safety margin is eliminated by use of advanced crucible monitoring equipment and the lining thickness is optimized.

The refractory lining thickness commonly maintained is from 75 to 125 mm based on the furnace capacity. The concept of refractory lining design for the induction furnace is quite different from conventional refractory lining.

The main attention is given to arrest crack formation and crack propagation, across the lining thickness, which may pose the danger of penetration of molten metal to strike the coil. During the operation of the furnace the refractory lining always experiences temperature fluctuation and related thermal shock, because the furnace does not run continuously at same temperature. The thermal shock generates crack in the lining, therefore, lining by brick or ramming masses of conventional type are not suitable because in sintered refractory brick, in castable or in a chemically bonded ramming mass, if a crack forms, it propagates very fast.

One of the measures taken to handle this problem is to use the dry powdery mass for lining. The material is designed such that during the use only one third of the lining thickness at the working face will get sintered hard which will withstand the chemical and mechanical abuse of molten metal and charging scrap. The next one third of the thickness will be in semi-sintered stage and the rest one third of the thickness at the back, in touch with inductor coil, will be in loose form (Figure 8) [8]. Under such condition if a crack forms at the working face, it cannot propagate up to the coil and will be arrested in between. The diagram also shows the temperature profile of the lining in case of Quartzite lining (Silica Ramming Mass) which is mostly used in case of Cast iron melting.

Figure 8.

The thermal profile across the refractory lining thickness in coreless induction furnace (b is total thickness).

The requirements of a ramming mass suitable for the lining of a coreless induction furnace are

1. It should have a softening point minimum 150C above the operating temperature

2. It should sinter in working layer to have a strength to withstand the pressure of molten metal.

3. Should have a poor sinter-ability not to allow sinter at back

4. Should be chemically inert at the operating temperature in contact with the molten metal and slag.

5. Should have permanent expansion at a temperature 1000-1200C

6. Should have a low thermal conductivity

7. Should be economic

3.2.1 Silica ramming mass

Meets all the criteria mentioned before, to make it a most commonly used lining material for the Coreless induction furnace for melting Cast iron. It is made out of high purity Quartzite or Quartz containing minimum 98.5% SiO2. 0.2–1.5% Boron containing compounds like Boric acid or Boron oxide is used as the sintering aid and mixed with the ramming mass during supply. The percentage of these additives depends upon the operational temperature of the furnace. The grain size distribution in the ramming mass is of utmost importance. The grain size distribution determines its packing density on compaction. Higher is the packing density better will be the performance.

Quartz the main mineral phase in this ramming mass can exist in different crystalline phases and undergoes polymorphic changes at different temperature as shown in Table 3.

Change in crystalline phase	Transformation Temperature (°C)	Associated volume change%
β to α- Quartz	573	+0.8–1.3
β Quartz to β- Tridymite	870	+14.4
β Quartz to β-Cristobalite	1250	+17.4

Table 3.

Polymorphic changes in silica relevant to function of silica ramming Mass.

Due to this polymorphic transformation and associated volume change from Cristobalite to Tridymite the Refractory lining at the middle layer remains tight and does not allow any crack to proceed further and stops any liquid metal penetration. Chemical and physical properties of Silica Ramming mass used for the lining of Induction furnaces are shown in Table 4.

SiO2%	Al2O3%	Fe2O3%	Others%	Bulk density (gm/cc)	Max service temp C
99.2	0.5	0.1	0.2	2.1	1650
98.8	0.7	0.07	0.40	2.1	1700

Table 4.

Properties of typical silica ramming Mass.

3.2.1.1 Installation

Installation is a very important part towards the efficient running of the furnace. The campaign life of the lining and the electrical efficiency depends upon the quality of the installation. Best quality refractory material will not produce the desired performance unless the installation is sound. Figure 9 [3] shows how the electrical efficiency is related to the packing density. The aim of good installation is to get maximum and uniform packing density. The process of packing the Refractory mass between the coil and the central former can be done both manually as well as through mechanization. For smaller furnaces below 5 ton capacity manual ramming may be done but for larger furnaces mechanical ramming method must be adopted to ensure best and uniform packing.

Figure 9.

Relation between the packing density and the energy consumption (MFT-GE/10000/8000KW/250 Hz).

It is necessary to check, before installation that the ramming mass does not contain any moisture and is perfectly dry. It is very safe to heat the ramming mass before installation to ensure the removal of any moisture. In actual installation process the ramming mass poured on the bottom is first rammed to compact. The ramming material should be poured in such a way that every time sufficient material is poured to get a compacted height of 50 mm and gradually the desired height is build up. The material must not be poured from a height which may segregate the coarse and fine particles of the mass. In case of pouring from a height, a long funnel must be used to ensure no segregation. After the bottom is ready its level is checked and then a mild steel former of the shape, shown in Figure 10a is placed on the packed bottom and the annular space between the coil and the former outer wall is packed with the Ramming mass. Figure 10b shows some of the tools being used for the compaction of the refractory ramming mass.

Figure 10.

a. Steel former. b. Ramming tools.

Ramming is done dry and therefore is difficult to compact and the tools are also of different design than those used for wet ramming.

Before ramming the inside wall of the coil is plastered with mixture of fine Alumina powder and high Alumina cement mixed with water. The presence of coarse grains may damage the Copper coil. The coating thickness will be 3–5 mm. This provides an extra layer of protection over the coil and also forms a separation layer between the coil and the ramming mass which makes the removal of the used up lining easier after the campaign life of the ramming mass is over.

There is a practice to put some insulation layer like Asbestos sheet over the coil to thermally insulate the coil, but this is not recommended as a correct practice. It consumes both money and time and reduces the lining life by helping sintering front to move towards the coil. This is against the philosophy of the lining design of the coreless induction furnace and moreover Asbestos creates health hazard.

After completion of lining, the scrap is charged inside and the power is put on. The steel former used for the lining is allowed to melt in the process of sintering the lining. There are methods in which the steel former can be taken out after the ramming is over and before the power is made on. This process is much more economical because the same former can be used number of times and it saves the cost of the former. For removing the former before melting, special binder is added in the ramming mass and the former is heated up to 400C by gas burner inside when the ramming mass, in contact with it, forms a hard layer and enables the pull out of the former which has a taper design to facilitate the removal.

The first heat is very important because it is to be done with care to sinter and stabilize the lining and next heat onwards it can be run in normal routine way. It is also recommended to use clean and good quality scrap in first few heats. The former used for the lining is allowed to melt in service and it actually holds the dry material till it sinters and acquires strength to stand on its own.

After a new lining is constructed, its first heat up procedure is very important. A special heat up schedule is followed which helps in stabilization of the Refractory lining. This is called sintering cycle and should be followed as per the instruction of the supplier of the lining material because it depends upon the grain size distribution, raw material character, quality and quantity of the sintering aid used and also upon the furnace capacity. The most common sintering aid is Boric acid or Boron Oxide used for Silica Ramming mass. Table 5 shows the typical heat up schedule used for a certain ramming product.

Furnace size (Ton)	Boron oxide (B2O3)	Boric acid (H3BO3)
1–3	180 C/h	120 C/h
4–15	150 C/h	100 C/h
+15	100 C/h	60 C/h

Table 5.

Heat up schedule of the coreless induction furnace.

After rising, the temperature is hold at a certain temperature, called sintering temperature, which depends upon the quantity of additive used. Table 6 shows the relation between the kind of sintering aid used, its percentage and the sintering temperature.

% Additive	B2O3	H3BO3
1.0	1530C	—
0.8%	1580C	—
0.6%	1630C	—
0.4%	1680C	—
2%	—	1530C
1.6%	—	1580C
1.2	—	1630C
0.8%	—	1680C

Table 6.

Typical relation between additive% and sintering temperature.

3.2.1.2 Mechanism of refractory degradation

Refractory lining life depends not upon quality of the refractory alone but more upon the other parameters e.g., furnace size, quality of installation and the deviations from SOP etc. Same Refractory performs differently in different cast iron melting units.

Following are the major causes of refractory degradation process takes place in induction furnace.

1. Chemical corrosion and erosion

2. Crack formation

3. Erosion

4. Superheating

5. Build up

3.2.1.2.1 Chemical corrosion and erosion

It causes the gradual loss of the lining thickness due to the chemical reaction of the refractory material with the charge material or alloying elements in the metal. The presence of carbon and other oxides like Mn, Mg, and Al present in the melt reacts with SiO2 and reduces it following the reaction as below.

$$\text{2X}+\text{SiO2}=\text{2XO}+\mathrm{Si}$$E1

The content of the said impurities must be low to avoid the chemical corrosion. Carbon in Cast iron also attacks SiO2 at a temperature above 1450C to 1480C when the boiling process sets in. The FeO present in the slag reacts with SiO2 to form low melting compound Fayalite (2FeO.SiO2) having melting point 1180C and corrodes SiO2 Refractory lining. Slag also contains Manganese silicate (MnO.SiO2) with 1250°C as its melting temperature. Both of these compounds occur in the slag in proportion 10 to 30% and 2 to 10% respectively. Input of rusty scrap makes the situation worse.

The content of the said impurities must be low to avoid the chemical corrosion. Carbon in Cast iron also attacks SiO2 at a temperature above 1450C to 1480C when the boiling process sets in.

During the charging of the scrap in molten metal or during the CO boiling process, molten metal is splashed on the refractory lining and later gets oxidized and forms FeO and then to Fayalite.

When the metal is hold in the furnace for longer time metal gets oxidized to FeO and causes the erosion in the lower part of the crucible. During holding, the temperature of the molten metal should be kept as low as possible to retard the oxidation process and generation of FeO. In melting of nodular iron, SiO2-Al2O3-MgO eutectic is formed at 1365C [9] and the lining gets eroded because if it’s higher tapping temperature.

If the scrap, used as feed to furnace, contains Zn, then it vaporizes beyond 900 C and permeates through the lining material and condenses on the inductor coil. This deposited Zn layer may cause arc formation in the inductor coil and damages it [8]. To avoid this problem the initial three charges must be free from any Zn and the lining is allowed to sinter and get dense to retard the permeation of Zn vapor. The same is applicable during the charges after cold heat because the time is to be allowed to heal up the cooling crack through which the vapor permeates easily. A high temperature gradient from hot to cold face of the lining is also recommended to allow the condensation zone away from the coil. The coil coating material with high thermal conductivity is recommended.

The CO gas also permeates through the lining and gets converted to C and CO2 as per the Boudouard reaction

$$\text{2CO}=\text{CO2}+\mathrm{C}$$E2

This Carbon gets deposited on the coil. Beside this, Sulfur vapor generated from the MgS in recycled nodular iron also penetrates through the lining and reacts with the Oxygen and the moisture in the lining to form Sulfuric acid as per the reaction

$${\mathrm{S}}_2+{\text{3O}}_2+{\text{2H}}_2\mathrm{O}={\text{2H}}_2{\mathrm{SO}}_4$$E3

Sulfuric acid attacks the Copper coil to form Copper Sulfate and damages the coil.

3.2.1.2.2 Crack formation

in the lining is inevitable because of thermo-mechanical stresses developed in heating and cooling of the lining and due to volumetric changes during the polymorphic transitions of Quartz. But formation of deep crack is dangerous which may allow the passage of molten metal through it to strike the coil. Formation of small cracks is not a concern rather formation of smaller cracks absorbs the stress and does not allow the formation of bigger cracks. Cracks can be of different type and the reason of their formation is shown in Figure 11 [10].

Figure 11.

Different types of lining cracks and their causes.

The lamination is formed due to separation of two layers during compaction of the lining and care must be taken to avoid such layer formation. The vertical cracks are formed during over sintering also and care must be taken to reduce the amount of sintering agent in such case. Crack also formed if the density is not uniform throughout the lining. Segregation of the material can also cause such non uniformity. To avoid segregation the refractory material must not be poured from much height or to pour the material through a long funnel during lining.

3.2.1.2.3 Erosion

Erosion is more of a physical process and aggravates chemical corrosion by exposing the fresh surface available for chemical reaction and corrosion. Erosion is connected to the extent of turbulence of the bath of molten metal. Higher the turbulence more is the erosion.

The characteristic of induction melting is that the bath is in constant movement, which is called inductive stirring. The amount of stirring is determined by the size of the furnace, the power put into the metal, the frequency of the electromagnetic field and the type/amount of metal in the furnace.

When a furnace is operated at a frequency lower than ideal, the result may be a violent stirring action that may produce inclusions of slag and refractory particles. Metal loss may be excessive due to excess surface area of the melt and oxidation of volatiles.

In many cases the refractory lining life is reduced because of using too low of a frequency to produce strong stirring. On the other hand, if too high a frequency is selected for the size of the furnace, there may be a complete lack of stirring, uneven heating throughout the charge, excessive side-wall temperatures and difficulty in attaining homogeneous melts.

The degree of agitation in molten metal can be indicated by Stirring Index, which is defined as [11].

$$\mathrm{SI}=\frac{6000\sqrt{\mathrm{KW}.\frac{\mathrm{D}}{\mathrm{SG}.\mathrm{f}.\mathrm{\rho }}}}{A}$$E4

Where, SI- Stirring Index, KW = Power of Furnace in KWh, D = Melt diameter, SG = Sp. Gravity of metal, ρ = Metal resistivity, f = Frequency, A = Cross sectional area of the melt (πD²/4). Relation between furnace size and frequency is shown in Figure 12 [12].

Figure 12.

Relation between the furnace size and the ideal frequency of operation.

Due to erosion when the lining gets thinner, the furnace draws more power and the melting rate becomes faster and this is an indicator of lining erosion. Data in Table 7 [2] illustrates this effect in a 3 ton capacity furnace of 700 KW rating.

Campaign	Power input (KW)	Energy consumption (KWh/t)
New Lining	615	656
Lining after 1 week	655	622
Lining after 3 week	750	598

Table 7.

Effect of refractory lining age on energy consumption.

3.2.1.2.4 Buildup

When the slag makes contact with the refractory lining of a furnace wall (or other areas of the holding vessel) that is colder than the melting point of the slag, the slag is cooled below its freezing point and adheres to the refractory furnace wall or inductor channel. The source of these build up material are the oxides from the oxidation of the metal or contaminants charged into the furnace e.g. molding sands. Buildup normally occurs in the areas where the flow or the turbulence is minimum. Some of the major mineral forms found in the buildup, are shown in Table 8 [13].

The buildup gradually reduces the working volume of the furnace and forced to take shutdown for the new lining. The remedy is to use the better quality of scrap with lesser contaminants. Sometimes the use of flux reduces the build up by reacting with it to reduce the melting point so it goes into the slag.

3.2.1.2.5 Superheating

The generation of localized heat, which leads to high temperature at some spots, is very detrimental for the refractory lining and may cause lining failure. The major reasons for the localized superheating of the lining are shown in Figure 13 [10].

Figure 13.

Factors that cause superheating of molten cast iron.

During the scrap charging in the furnace, it may so happen that some scraps remain at hanging position at the top while the liquid metal is formed below and an air gap forms in between the liquid metal at the bottom and the charge at the top in hanging position and is called Bridging. This is a very dangerous situation for the refractory because the liquid metal below will be superheated and will have high stirring effect due to high power density and lesser quantity of molten metal. The metal temperature can shoot up above the melting point of refractory and erosion will be high because of strong agitation of molten metal. Under such condition the refractory lining can give way and molten metal can penetrate through the lining to strike the water cooled coil causing severe explosion (Table 8).

Formula	Mineral	Melting point (°C)
FeO	Wustite	1379
Fe2O3	Hematite	1625
2MnO.SiO2	Galaxite	1850
2FeO.SiO2	Fayalite	1216
MgO.SiO2	Forsterite	1888
2(Fe, Mg)OSiO2	Olivine	1798
CaS	Oldhamite	2522.5
CaO.MgO.SiO2	Diopside	1389.7
CaO.Al2O3.SiO2	Anorthite	1555.6
MgO.Al2O3.5SiO2	Cordierite	1576

Table 8.

Some minerals, found in the buildup material in cast iron melting.

Once the bridging of scrap happens the power must be switched off immediately. The scrap sizes are very important to control the bridging and the charges must be of different sizes.

The trapped metal pieces inside the refractory lining can also cause the local superheating of the refractory lining. The penetrated metal fin inside the lining can also cause the superheating of the lining.

3.3 Transport of molten metal

For transportation of the molten cast iron in foundries Ladles can be used and these ladles can also be lined with Silica Ramming mass in similar way as it is being done for induction furnace. The advantage of Silica Ramming mass is its low cost and lower drop in metal temperature because of its low thermal conductivity. Smaller foundry ladles also use sol-gel castable lining which is amenable for fast drying and heat up.

In case of cupola also, the liquid metal can be transported through ladle lined by Silica Ramming mass which is most economical. In case of bigger ladle Alumina—Silicon Carbide bricks can be used which gives better campaign life but it is having much higher thermal conductivity and need insulation at the back to prevent the heat loss.

4. Concluding remarks

The driving force acting on industries, in general, today are related to economy, environment and safety and health issues and that brings the changes in current practices. The cast iron industry is not an exception to that.

For example, so far Boron compounds are being used as the sintering aid to Silica Ramming mass, but Boron compounds are found to have detrimental effects on human health. Trials are on the way to develop Boron free Silica Ramming masses and initial trial results are very encouraging.

New methods for melting, like Electron beam melting, Microwave melting, Solar furnaces [13] are under trial and Refractory requirements will be changed along with the changed furnace type in future.