Induction Furnace Refractory Lining with Silica Ramming Mass
Induction Furnace Refractory Lining with Silica Ramming Mass
Induction furnaces are used for melting cast iron, mild steel and various alloy steels in foundries and making of steel in mini steel plants using sponge iron. Lining is the important part of induction furnace. Furnace performance is directly related to the performance of its lining. Well laid and stabilized lining results in smooth working of furnace, optimum output, and good control of the metallurgical reactions. The lining practice best suited to a particular furnace depends upon the capacity and design of the furnace, operation practice adopted during making of a heat, and furnace output. For successful and consistent performance of the lining, the important aspects are (i) use of proper grade and quality of the lining material, (ii) careful and systematic lining practice, and (iii) consistency in working conditions. Fig 1 shows the installed refractory lining of a coreless induction furnace,
Fig 1 Installed refractory lining of a coreless induction furnace
The characteristics of the lining material needed for consistent lining life include (i) thermal characteristics which means that it has to withstand the stresses developed by thermal cycles during the furnace operation, (ii) chemically inert to metal being melted, (iii) structural strength under operating conditions, (iv) high erosion resistance, (v) ease of installation, (vi) reparability of the lining, (vii) ease of dismantling, and (viii) economics. As such, it is very difficult to judge the suitability of a particular lining under various conditions like operating temperature, metal being melted, the type of formed slag, and furnace capacity. Chemical inertness to the liquid metal can be achieved by using acid and neutral lining for the acidic slag and neutral or basic lining for the basic slags.
Normally, the selection of refractory for the furnace lining is based on the type of slag generated during melting. There are three types of ramming masses namely (i) acidic, (ii) basic, and (iii) neutral. If the slag contains high amount of acidic components then a silica (SiO2) lining is used. For slags with a high basicity index, magnesite (MgO) linings are the choice. Neutral refractory has become the new trend for lining in induction furnace. The ramming refractory mass used for neutral lining in the induction furnace consists of a mixture of alumina (Al2O3) and sintered MgO blended according to a certain granulometry. Such blends are tested prior to their use for estimating their corrosion resistance and mechanical behaviour.
The most commonly used lining material for induction furnace is acidic lining. It normally consists of high purity silica ramming mass. Silica ramming mass is used for melting carbon steels with carbon content higher than 0.1 %. It is produced by calcining, crushing and grading of the white crystalline quartz containing 99.90 % of SiO2. The purity of quartzite is important since the impurities present produces unpredictable and higher amount of liquid phase at high temperature thereby lowering chemical and mechanical resistance of the lining. Due to calcining of quartz, SiO2 is in stabilized condition and its spalling tendency is removed and it neither expands nor contracts. High purity calcined silica yields more lining life. It also results in considerable uniformity in physical properties.
Silica is the most commonly used lining material for induction furnace since it has the following advantages.
- There is a dense sintered layer at the face which is in contact with liquid metal. The tightness of the sintered ensures no infiltration of liquid metal in the lining.
- Since the thermal conductivity is lower than other refractory materials there are lower thermal losses when compared with the other refractory materials.
- There is good resistance to temperature change.
- Being a low cost material, the cost of lining of the furnace is low.
- Because of dry preparation of the mass, the heating and sintering time needed is short.
- The specific cost of refractory per ton of steel made is low.
Silica ramming mass can safely be used up to an operating temperature of 1600 deg C. Since it expands very little, it is superior to both alumina and magnesia refractories to resist thermal shocks. Though silica lining has good endurance against thermal shock, it has poor resistance against steelmaking slags. Temperature control is very necessary for a satisfactory lining life.
Typical chemical composition of silica ramming mass consists of SiO2 – 98.9 % minimum, Al2O3 – 0.6 % maximum, Fe2O3 – 0.2 % maximum, and CaO – 0.1 % maximum. The typical physical properties of the silica ramming mass are at Tab 1.
|Tab 1 Typical properties of silica ramming mass
|Free energy at 1450 deg C
|Average Thermal conductivity between 0 deg C and 1200 deg C
The more compact lining results in greater strength and life. The compactness (packing density) depends upon granulometry of the ramming mass. It is to be such that it forms the least open space between particles. The typical granulometry of commercial silica ramming mass consists of + 5 mm – nil, 4mm to 5 mm – 3 % maximum, 3 mm to 4 mm – around 8 %, 2 mm to 3 mm – around 17 %, 1 mm to 2 mm – around 12 %, 0.2 mm to 1 mm – around 27 %, 0.06 mm to 0.20 mm – around 15 %, and less than 0.06 mm – around 18 %.
Particular attention is needed to be paid towards the proportion of fines which is to be within a certain tolerance. It has adverse influence on the service behaviour of lining if present in more amount than required as it is the finest particles which takes part in the sintering reaction.
The binding agent is added to the ramming mass so that the refractory lining of crucible must sinter during heating up and develop strength before liquid metal is charged. Boric acid is mixed as binder. The boric oxide reacts with silica particles to produce a low melting point glassy phase which fills the interstitial holes between the quartz grains. The addition of right quantity of boric acid is very important for optimum life of lining. The quantity depends upon (i) temperature of liquid metal bath, (ii) chemical composition of quartzite mass, and (iii) thickness of crucible wall. The relationship of percentage of boric acid and bath temperature is shown in Fig 2.
Fig 2 Relationship of percentage of boric acid and bath temperature
The process of ramming
The ramming process has the following steps.
Mixing of the ramming mass – First the required quantity of ramming mass is worked out. The quantity of the ramming mass needed depends upon the furnace design. Then the quantity of boric acid needed is calculated. The ramming mass is preheated in tray made of sheet to a temperature of around 100 deg C in summer and around 140 deg C in winter. The mass is preheated in batch of 50 kgs for the removal of the traces of moisture. The mass is then transferred to cooling trays and is cooled down to 50 deg C. Boric acid is sieved through 0.20 mm screen and the calculated quantity is weighed and added to the mass. The mass is mixed thoroughly by hand. The mixed batch is checked for ensuring uniform mixing of boric acid. This is done by hand picking a small amount of mass to make thick water slurry with the help of distilled water. A pH paper is dipped in the slurry. The presence of boric acid shows a pH value less than 7. This is repeated by picking 3 samples from each batch from different location in tray.
Lining of the coil – The water cooled copper coil is coated with refractory mortar and dried well before start of lining. Then thick asbestos sheets are wrapped around, the coil lining.
Ramming of the crucible – The furnace bottom is rammed by using flat head tools. The bottom is rammed in several layers of different thicknesses. The layer at the bottom is thickest and is around 60 mm. The subsequent layers are 20 mm thick. The ramming of the alternate layers is done with spiked and flat head tools. The bottom is built 10 mm above the required height and the extra mass is scrapped uniformly. The level is then checked.
The metallic former duly cleaned from outside is then placed in a position which is concentric to the coils. It is held in position with the help of the wooden spacers. A heavy weight is kept inside the former to resist its coming up during further ramming. The angular space between the asbestos sheet and the former is rammed in 50 mm to 60 mm layers using spiked and flat head tools from the top. Use of blunt or worn tools can result in poor compaction. The ramming is continued till 100 mm gap from the top. Then thin layer of sodium silicate solution is applied over top of rammed silica layer in the crucible before ramming of the topping mix. Dilute sodium silicate solution is added to the topping silica ramming mass. The spout is formed by the same topping mix. Pneumatic rammers/electric vibrators are normally used in large furnaces for crucible formation.
Sintering of the furnace crucible – for inductive sintering of the mains frequency furnace, the furnace is filled upto the coil upper edge with proper centering, while heavy scrap is used for medium frequency furnaces. The power supply is regulated through switching on the lowest transformer tap and the power is kept switching on and off at intervals of few minutes so that the rise of the temperature of 100 deg C/hour is achieved for furnaces up to 6 tons capacity and 50 deg C/hour for big furnaces with thick lining. This rise in temperature is monitored up to about 800 deg C. After the temperature of 800 deg C is reached, the power is raised at a rate of around 150 deg C/hour. The heating is continued up to the melting of sintering charge. For measurement of temperatures Chromel -Alumel thermo-couples are used. As the charge slowly melts, further charging of the furnace is done for producing full furnace heat. The temperature is maintained low during entire melting through constant addition. As soon as the furnace is filled with liquid metal the power is increased in order to reach the sinter temperature. The final metal temperature is to be raised to around 30 deg C to 50 deg C above the normal operating temperature and held at this temperature for an hour to stabilize the temperature of the refractory lining and also sufficient thickness of refractory gets fused to withstand the physical shock of crucible. The furnace is not to be put out of service or cooled to less than 1000 deg C during the initial period of the working of the furnace.
Lining wear and the causes of wear
The lining life of induction furnace lined with silica ramming mass depends upon the lining practice and operating practice of the furnace besides quality of the silica ramming mass. It is quite common to get inconsistent lining life of the furnace. There are cases when sudden failure of lining takes place. The main factors which affects the lining life of the induction furnace are (i) incorrect granulometry of the ramming mass, (ii) non-uniform distribution of the binding agent, (iii) superheating of the metal bath in the furnace, (iv) penetration of metal, (v) minimum slag free metal resulting in minimum erosion at slag line, (vi) loss of refractory powder, and (vii) topping/lining interface cracking
For the proper failure analysis in case of pre-mature failure of the refractory lining, it is important that proper records about output, working temperature, and other parameters are maintained. These records not only help in finding the cause of failure but also help in the continuous performance of the lining life.
Repair of the lining
During operation of the furnace, the furnace lining is subjected to various kinds of thermal stresses, mechanical loading and metal lining reactions. As a result lining wear takes place. The following are a few methods for the repair of the lining. The repair depends upon the nature of wear. The different kinds of wear of the refractory lining are (i) cracking, (ii) localized wear, (iii) erosion of bottom, (iv) erosion of side walls, and (v) slag line erosion.
Quite often small hair line cracks are seen on the lining surface after cooling. On cooling the lining contracts and when it cannot withstand contraction-stresses it develops small cracks and thereby release stresses. However cracks of this nature usually close when the furnace is heated up. It is not desirable to charge borings or fine metallic particles which can enter the cracks and prevent them from self-closing.
Localized lining wear consists of small localized broken furnace lining or localized worn-out portion of the furnace lining. This localized wear can be easily patched by using air setting refractory which can be trowelled with pressure. The exposed surface of patch is left to dry by allowing moisture to escape.
Erosion of bottom can be repaired by pouring lining material on the eroded area and ramming with flat hammer for minor wear at the bottom.
In case of erosion of side walls, the worn section of the induction furnace can be repaired by using dry monolithic lining behind a part former. The former is to be slightly less than original diameter of the lining.
Because of slag-line erosion grooves are formed at the slag level in the induction furnace. These grooves can be repaired either in empty furnace by putting silica ramming mass similar as described in localized wear. The repair can also be done when the furnace is working. For such repair the metal level is kept below the area to be repaired. Slag is removed and loose monolithic mass is added to the furnace through the movement of bath the mass is drawn to the side walls and adheres to the wall.