Selection of Proper Refractory Materials for Energy Saving in Aluminium Melting and Holding Furnaces

Refractory lining of an aluminum melt ing and holding furnace are engineered with a primary design objective to keep the furnace condition stable throughout the life of the fu rnace. In consideration of energy saving, ideally all the heat added to the furnace should be used to heat the load or stock but in practice, a lot of heat is lost in several ways. Energy input to metal output depends on factors apart from energy losses through the furnace wall i.e . through flue gas, moisture in fuel, hydrogen in fuel, opening of furnace door. These heat loses can be differentiate as, wall losses at steady state operating condition and heat storage loss during transient condition. Practically apart from flue gas loss, majority of the heat loss took place during the transient condition. Heat loss through refractory wall during steady state condition depends on thermal conductivity, resistance against thermo chemical attack from aggressive liquid alumin ium and its alloy and resistance against mechanical wear. High porosity, low thermal conductive materials due to lower strength and lower resistance towards chemical attack reduce life of the furnace and in contrast high density refractory materials needs multilayer backup to save potential energy loss through the refractory wall. This paper discusses the proper selection criteria and best suitable solution of refractory materials for aluminium Melt ing & Holding fu rnace which can contribute potential energy saving.


Introduction
There are many factors which limit the uses of refractory such as service environ ment, service temperature, mechanical degradation and the ability to install or repair of the refractory materials in a cost effective manner. These limitat ions are related to the energy efficiency of the processes, as degradation of refractory reduces the thickness of walls, cause heat loss through the walls and increases exponentially. Th is condition requires cooling of the furnaces for maintenance and again reheating for further uses which causes huge loss of energy and production time. Refractories for alu minu m melting and holding furnaces must withstand mechanical abuse fro m charging, fro m thermal shock due to cyclic heating and from co mplex forces on the refractories when molten metal penetrates their surface. Penetration can destroy the furnace.
In metal melt ing furnaces, wear of the refractory lining is not uniform in nature; it is severe in specific areas where most corrosive condition exists. In melting furnaces the most severe condition occurs at the metal lines where solid refractory co mes in contact with liquid metal and gaseous environment above the liquid metals.
In case of aluminu m melting and holding furnaces corundum deposit as surface agglomerates, leads to spalling of refractory wall due to alu mina surface concretion and porosity increases in the refractory structure for internal corundum growth.
Below the metal lines the degraded refractory reduces the thickness of the wall and floor. Th is condition reduces the thermal efficiency of the fu rnaces leading to high heat loss and can lead to failu re of thermal balance of the furnaces.

Energy Losses in Aluminum Melting Furnaces
The heat loses can be differentiate as, wall losses at steady state operating condition and heat storage loss during transient condition. Practically apart fro m flue gas loss, M elting and Holding Furnaces majority of the heat loss took place during the transient condition due to the opening loses and depends on type of furnace and operating condition wh ich is sometimes unavoidable due to process requirement. The majority of the heat loss i.e. around 40% of heat input is flue gas loss and only an estimated 10% of available heat is lost through the refractory wall during steady state operating conditions. [1] To reduce the energy loss through refractory wall in steady state condition refractory wall should have low thermal conductivity, resistance against thermo-chemical attack fro m alu minum and its alloying elements and resistance against mechanical abuses.
The potential for energy savings through the refractory lin ing in alu minu m furnaces will now be d iscussed, and in particular, how refractory materials can contribute significantly to overall energy savings.

Refractory Lining in Transient Phase
The furnace refractory lin ing is often exposed to thermal shock during skimming, cleaning, flu xing and charging. The severity of these forces increases sharply with increasing furnace size and varies widely with operation practice. Mechanical shock and abrasion are severe where large quantities of cold scrap are direct ly charged with solid materials through the main door or through doors on one side wall or are charged into a well containing molten alu minu m. All this practice leads to massive energy losses and as a consequence, stresses generates in the lining which can cause damage of the refractory lining and eventually a reduction of the furnace refractory life. When a furnace needs to shut down, cooling and shrinking of the lining create a gap between the hot face and back-up insulation, which could cause a lin ing failure during restart. Most of the lin ing shrinkage is caused by the contraction of the alu minu m in cracks of the lin ing as alu minu m has shrinkage greater than 6% [2]. In the lin ing apart from hot metal contact area sills, door jambs and furnace door are the most vulnerable area to thermal shock. The energy efficient high porosity light weight insulating refractory materials cannot be used in the hot face lining due to the lower chemical and mechanical resistance, less energy efficient, high thermal conductive, dense refractory materials must have to be used which requires a mult i layer refractory lin ing.
Degradation of refractory lin ing in the refractory hot face leads to the freeze plane changes towards the insulating material which ult imately causes aluminu m infilt rations into the porous insulating lining through the hot face layer cracks.
To maintain stable furnace conditions and to save energy, it is essential to install hot face refractory materials wh ich are resistant to contacts with the liquid metal and atmosphere. The refractory lining of the fu rnace under transient conditions should have excellent thermal shock propert ies i.e volume stable, high thermal conductivity, low thermal expansion and also to maximize energy efficiency material should have low heat capacity in order to reduce storage heat loss.

Corrosion and Metal Penetration
The main p roblem associated with alu minu m melting and holding furnaces is the corrosiveness of molten alu minum and its alloying elements. The corrosion of alumino-silicate refractories by molten alu minu m, generally leads to the formation of an alu mina deposit on the refractory. The presence of alkalies and under a reducing at mosphere increases the corrosion also metal contamination in mo lten alu min iu m confinement depends on the characteristics of the refractory i.e their chemical and mineralogical co mposition, types of binder, and permeability. The susceptibility of the refractory to the corrosion in the furnaces depends on these characteristics. Such corrosion promotes the formation of inclusion in the mo lten metal, wh ich can also originate fro m its direct o xidation at the metal line. Increase in permeab ility favors the corrosion by molten metal and permits the gas migrat ion like o xygen and water vapors.
The products of oxidation of mo lten alumin iu m alloy are a function of the alloy co mposition. At operating condition of alu min iu m melting furnace, the presence of Mg favours the formation of spinel (MgAl 2 O 4 ). At less than 3 wt %, Al 2 O 3 is the most stable o xide in these conditions, while at more than 18 % it is MgO. [3] These o xides may result fro m the action of o xygen gas.
Also the alkali content (less than 2%) in the refractory may be present as amo rphous phases such as sodium silicate or crystalline compound such as beta alumina (NaA l 11 O 17 ). These phases are highly prone to reduction by alu minum which releases the alkali to mo lten alu minum.
6NaA l 11 O 17 + 2Al  6Na + 34 Al 2 O 3 (1) The presence of AlF 3 in the refractory as non-wetting agent may also release the alkali in to the mo lten metal. The reaction is as follo ws [4] 6NaA l 11 O 17 + 2Al  6Na + 34 Al 2 O 3 (2) NaF + A l  3Na + AlF 3 (3) Because of refractory corrosion and wear, the temperature profile of a furnace lin ing changes depending on the operating conditions and furnace design, which needs a close watch because the initial calcu lated thermal profile changes . The thermal profile of the lining changes with corundum build-up on the lining, alu minu m infiltration in pores and cracks, flu x salt infiltration of the lining, reduction of the wall thickness due to mechanical wear.

Mechanism of Corundum B uil d-Up
Along the operation of furnaces, the corrosion of refractories by molten alu minum is generally acco mpanied by the formation of a surface deposit of corundum on the lin ing, especially at the metal line, where the effect of atmosphere and alkaline vapors is at maximu m. Microscopic observations of the corundum layer shows that the corundum layer looks like a co mposite material with fine grains of corundum surrounded by an interconnected metallic network [5]. The metallic alu miniu m network provides the channels to supply fresh aluminu m to the reaction interface during the corrosion process. The deposited corundum layer is difficu lt to remove during furnace cleaning due to its high adherence to the lining.
There are t wo specific mechanis ms of corundum formation based on location within the furnace.

Internal Corundum Growth
Internal corundum fo rmation occurs due to reduction process of refractory co mponents by aluminu m metal and alloying elements. Belo w the liquid metal line refractory surface is the main area for internal corundum g rowth. The reaction takes place in the pore system of the refractory material and leads to a decomposition of the matrix. When the furnaces lined with alu mino-silicate refractories, alu min iu m gradually penetrate into the refractory and mo lten alu minu m reduces the refractory o xides such as silica, thereby forming corundum.
The reactions can be described as: [6] 3 SiO 2 (refractory) + 4 Al (metal) = 2 A l 2 O 3 (corundum) + 3 Si (4) 3 Mg (alloying metal) + 4 Al 2 O 3 (refractory) = 3 MgAl 2 O 4 (spinel) +2 Al (5) The conversion to corundum is dependent on the temperature and process is faster with increase in temperature. According to Brondyke[7], the corrosion reaction increases the volume of penetrated product. This expansion creates cracks within the refractory making it prone to further penetration. However, calculat ions based on the reaction in equation-4 indicate a 23% reduction in volume. However there is a change in volume based on the reduction of silica by mo lten alu minu m. Such changes can cause stresses within the refractory matrix. The amount and speed of such penetration and reaction determines the life of a refractory lin ing. Below the metal line, corundum formation may also occur as a result of alu minum o xidation by gaseous oxygen present in the infiltrated porosity. This phenomenon has been referred to as "internal corundum growth".

External Corundum Gro wth
The second mechanism of corundum format ion is referred to as "external corundum growth. External corundum growth is a function of o xygen part ial pressure in the furnace in the presence of aluminu m and alloying elements like silicon and magnesiu m. At the liquid metal line, solid refractory, liquid alu minu m and at mospheric oxygen are all in contact at one point and known as the triple point. The corundum build-up occurs at the metal line, of the furnace at the interface of the atmosphere, alu minu m bath, and the refractory lin ing. Availability of o xygen leads to a more severe corundum formation and the damage is extended to some height above the metal line. This zone of excessive corrosion is commonly known as the "bellyband area". Molten aluminu m penetrates into the refractory, by capillary act ion and o xid izes by reaction with at mospheric o xygen leading to the formation of large corundum mushrooms that adhere to the refractory lin ing. The effect ive furnace volu me shrinks due to this process and reduces the energy input to metal output ratio. The alloying elements like Mg speed up the corrosion process and can reduce the refractory oxides mo re aggressively than aluminum.
Mg + Si0 2 (s) -------> MgO + Si (6) The adhered corundum layer is initially soft and may be removed easily but in later stages the product is dense, hard and difficult to remove during cleaning and can cause mechanical damage to the lining during removal. So it is evident that corundum build up is one of the major factors impacting energy consumption because corundum growth reduces the furnace capacity in the hearth area also changes the thermal conductivity of the wall also its reduces the furnace life due to differential thermal behavior. Corundum gro wth requires intense cleaning of the lin ing with cleaning flu xes and mechanical tools which leads to wear on the hot face lining subsequently heat loss in the furnace. Flu x Salt penetrates in to the refractory linings whenever the temperature of the furnace is above the melting point of the flu x. The flu x infiltrat ion increases the thermal conductivity of the wall and reduces the hot strengths due to glass phase penetration. So metimes the exothermic cleaning flu xes are added besides the use of cover flu xes, to loosen and disperse corundum build up on the wall in order to M elting and Holding Furnaces maintain the furnace capacity. This requires process shutdown to apply the cleaning flu x on the hot walls of a drained furnace which u ltimately cost to the energy and production loss.
Corundum formation with alu min iu m infiltration leads to pressure build up due to crystallization in the pores and cracks and eventually spalling of the refractory material. [8] Thus presence of corundum layer in an alu minosilicate refractory reduces the mechanical strength of the refractory lin ing and makes it most vulnerable to mechanical failu re during thermal cycling wh ich ultimately lead to a furnace lin ing failure. Since the corundum has a high thermal conductivity than the refractory, the thermal efficiency of the holding furnace is directly proportional to the volume of dense corundum layer. Thus energy is lost through the refractory lin ing .To maintain the temperature o f the furnace additional heating will be required which accelerate the corrosion process, since corrosion increases with increasing furnace temperature. Thus thee processes have a more negative impact to energy costs.

Refractory Selection Considerations
Fro m the above discussion it is understood the refractory materials for melting & hold ing furnace should have volume stable, accuracy in shape & dimension, good mechanical strength and resistance to abrasion, oxidation and reduction. Practically no refractory materials co mbine all this properties in the best possible way. Generally there are two main practical ways to avoid ing corrosion of refractory i.e. Addition of additive in refractory material during its manufacturing and protective coating on refractory wall.
Alumina silicate refractories having less silica content or higher alu mina silica ratio normally exh ib it a superior resistance to aluminiu m attack. Fo r alu minum metal to attack a refractory lin ing, t wo conditions must be fulfilled. First, the refractory material must be reducib le by molten alu minum metal and second, the two reacting species must be in contact with each other.
The fact is that refractory materials such as silica will be reduced by molten alu minum alloy to fo rm corundu m. In order to minimize contact between molten alu minu m and the refractory, "anti-wetting" additives have been added to refractory mixes. The primary role of such additives is to reduce the wettability of the refractory by mo lten alu minu m. This will min imize the contact between the two species and result in improved corrosion resistance. Alu minum fluoride, calciu m fluoride and bariu m sulfate have been generally used as anti-wetting additives.
In conventional cement bonded monolithic materials, additives like BaSO 4 , CaF 2 o r A lF 2 can temporarily p revent the refractory lining fro m penetration by alu minum metal and alloying agents. However, these additives possess limited temperature stability and tend to react with certain flu xes even at low temperatures thereby losing their effectiveness.
The use of a non-wetting addit ive is not always a sufficient solution to improve the corrosion resistance of refractories against molten alu minu m. Once in contact with liquid alu minu m, the coarse refractory aggregates, which do not benefit fro m the non-wetting additives in the mat rix, may be corroded. In some cases, the corrosion of aggregates promotes corrosion in the surrounding matrix, even in the presence of a non-wetting agent.
In general phosphate bonded materials show excellent resistance to alu min iu m metal penetration and corundum growth. The phosphate bond generally provides a lower modulus of elasticity co mpared to more brittle conventional cement and ceramic bonded materials. The flexib le bonding mechanis m results in higher impact resistance. Phosphate bonded bricks with low alkali content possess high hot strengths at elevated temperatures wh ich can significantly increase their performance against mechanical abuse and chemical attack. These combined properties are very useful in the area where the mechanical and chemical abuse is more. The characteristics are important in hot wall applications of side well charging furnaces where arch and metal line areas are exposed to mechanical wear in co mbination with chemical attack.
During manufacturing Phosphate bonded bricks are either fired above 1000°C or heat treated between 150°C and 800°C. In lo w temperatures fired materials addition of non-wetting additives can be done which is otherwise decompose at higher temperature above 1000°C. [9] A lso Lower firing temperatures creates smaller pore sizes and improved non-wetting properties, which ult imately leads to higher penetration resistance against alkali. But at lower firing temperatures below 600°C modulus of elasticity is less due to incomplete conversion of phosphate phases.
Both bricks and monolithic materials are used in fu rnaces and both have advantages over each other. Since the bricks are pre-fired materials, the structural properties are defined before use. 85 % phosphate bonded bricks are the most volume stable and have excellent thermal shock resistance. For this reason in severe applications areas such as impact zones of floors, ramps, and lower side walls, phosphate bonded bricks are still a p referred solution. But in bricks lin ing the jo ints of a b rick lining is the weakest link metal infiltrat ion, mineral transformat ions and corundum growth in the joints are frequent problems leading to uncontrolled e xpansions and bursting of bricks. According to Schacht the modulus of elasticity in the mo rtar joints is significantly softer than the brick modulus of elasticity, so on thermo-mechanical perspective the mortar jo int has a deep influence on the total structural behavior of the lining system [10].
In the case of monolithic lining, it shrinks during furnace operation because it does not receive uniform thermal treatment throughout the lin ing thickness. In some cases shrinkage can compensate the thermal expansion but result a significantly different thermo-mechanical behavior compared to brick lin ings. In larger monolithic applicat ions, some crack init iation and development is obvious due to the stresses along the thermal grad ient in the lin ing fro m the shrinkage or expansion. But the advantage of monolith ic materials is the absence of larger jo ints in the structure which makes the lining less vulnerable to metal penetration and can be used by every installation method including casting, gunning, patching and ramming. Ho wever, monolith ic materials at operating temperature are not in equilibriu m with regard to volu me stability. Therefore monolith ic materials are more susceptible to chemical attack in the environment of alu minum furnaces.
Phosphate bonded monolithic Refractories, used in the alu minu m industry, are based on liquid phosphate bonded two component binders, dry phosphate salt based systems mixed with water, or plastic Refractories containing a mono-alu min iu m phosphate binder. The weakness of all these materials is their relatively lo wer hot strengths and lower high temperature wear resistance compared to phosphate bonded bricks. The problems is that lower hot properties of acidic monolithic phosphate bonded materials due to relat ively h igh liquid content necessary for proper placement. Phosphate binder systems are acidic and therefore dispersants, the water content of phosphate bonded materials are more. As a result, such linings have relatively higher porosity compared to phosphate bonded bricks.
It is also possible to make refectories resistant to the mo lten metal by applying protective coatings by filling the open porosity. It also makes the surface ho mogeneous and thus minimizing the abrasion and slag erosion. But the coating has a limited durability and needs to be re-impregnated periodically.

Summary
The furnace life and energy efficiency of a furnace depends on the proper selection of refractory materials and its behavior in the furnace environment. The properties and quality of the refractory determine the extent of heat loss during steady state condition and storage heat loss during transient condition. The stoppage of furnace operation caused by refractory failure due to corrosion and mechanical wear leads to major impact on energy saving. The reduction in downtime, due to refractory failure, increases the energy saving and it can be achieved by using phosphate bonded refractory materials. A mult i-layer lin ing with optimized performance of layers with the service environ ment and proper installation improves the energy efficiency of a furnace.