Influence of melt Flow and Temperature on ERosion of Refractory and Deposit Formation in Aluminium Melting Furnaces

A. Jakovics* , I. Madzhulis, V. Frishfelds

University of Latvia, Zellu 8, Riga LV-1002, Latvia

B. Nacke

Institute for Electroheat, University of Hannover,
Wilhelm Busch Str. 4, Hannover D-30167, Germany

The deposition and erosion mechanism in induction-channel furnace for Al melting in alumino-silicate refractory is considered. The possibility of simultaneous erosion and deposition in the same cross-section of the channel is shown. The chemical reaction model causing the erosion of refractory is proposed. The erosion process is described by chemically active aluminium oxides while the deposition is caused by chemically stable aluminium oxides. The variations of erosion and deposition in the same cross-section of the channel are explained by variation of the thickness of laminar sub-layer along the perimeter.

Keywords: Al melting, erosion-deposition, induction-channel furnace
  1. Introduction
  2. The erosion of refractory brings the main exploitation difficulties with high-power induction-channel furnaces of aluminium melting. However, the analysis of experimental data shows that intense deposition is also possible. The last effect causes much more difficulties in steel melting furnaces [4]. Practical observations in industrial furnaces show that erosion and deposition can take place at different areas of channel furnace simultaneously [1], so that the eccentricity of ellipse-like cross-section increases. Therefore, it is necessary to find which effects should be included to describe both erosion and deposition. The physical model and calculations of fluid dynamic would allow to find the optimal geometry of channels increasing, consequently, the exploitation lifetime of the equipment. Empirical studies show that inappropriate regime of exploitation of the furnace (especially at the beginning of exploitation) is the usual factor causing intense erosion and deposition. The presence of doping elements (e.g., Mg) and addition of waste material with Al2O3 rust significantly increases the deposition rate. In order to escape from erosion, the refractory surface is sometimes coated with fine material that closes larger pores of ceramics. The lifetime of aluminium melting channel furnaces typically varies from some weeks to several months due to erosion and deposition.

    Figure 1. Photo of the deposit coated ceramics piece logged from channel furnace of aluminium melting

    Figure 1 shows characteristic piece of ceramics (light) coated with deposits (dark). The surface of the deposit is sheeted with thin metallic aluminium layer left in the furnace. One can see that the deposit surface has a wavy-like structure with characteristic length-scale - 2 cm. Such a structure cannot be explained by microscopic theory, but it arises due to instability of fluxes and other factors. The chemical composition of the sample at four different positions is shown in Table 1. The first position corresponds to the upper side of deposit, the second one - a little bit deeper, next one - the deposit layer close to ceramics, while the last one - ceramics. The oxides with high melting point Al2O3, MgO and especially spinel structure Al2MgO4 are the dominating constituents in the deposits. Elements Na and H are present since not only nitrogen but also soda is added in melting process in order to increase the solubility of doping elements. We will neglect the large variety of doping elements except Mg for a moment. Refractory material is build up with high silica fraction, because silica has the advantage of strong expansion effect for getting high material density after heating and for closing cracks after reheating [2]. However, chemical resistance of silica is lower than of alumina.


    Table 1. Approximate mass fraction of different phases in samples of one aluminium-melting furnace

    sample Nr.





    a -SiO2


    40 %



    40 %



    15 %

    a -Al2O3

    3 %


    2 %

    5 %


    30 %

    30 %

    57 %



    25 %

    40 %

    10 %



    20 %

    10 %

    5 %



    10 %

    10 %

    10 %


    Na2CO3× H2O

    7 %

    7 %

    7 %



    5 %

    3 %

    9 %

  3. Mechanism of Chemical Erosion and Deposition
  4. Al and Mg atoms are chemically very active and can easily take part in chemical reactions with refractory. The refractory contains such oxides as Fe2O3, P2O5 which are holding the oxygen atom less tightly than Al or Mg oxides. Consider, e.g., following reactions of a given element with half a mole of O2 and amount of released enthalpy (kcal/mol) at 800 ° C [3]:

    Mg(l)+ ½ O2(g)® MgO(c) D H = - 145.810,

    2/3 Al(l)+ ½ O2(g)® 1/3 Al2O3(corundum) D H = - 135.983,

    ½ Si(c)+ ½ O2(g)® ½ SiO2(b -quartz) D H = - 104.960,

    ¾ Fe(b )+ ½ O2(g)® ¼ Fe3O4(b ) D H = - 65.748.

    These reactions show that reactions of Al and Mg with less stable oxides of refractory are possible. These reactions involve, e.g., MgO and Al2O3 in condensed phase. Of course, the amount of released enthalpy is less for separate Al2O3 and MgO molecules in melt. Al2O3 cannot be formed without intermediate stages, because the collision of several molecules is unlike. Moreover, Al2O3 formation at once cannot describe simultaneous erosion and deposition, because Al2O3 would deposit at the same point it was created. Intermediate reaction products are necessary for modelling of the processes in the channel.

    If MgO is present in melt then spinel Al2MgO4 formation is possible in deposits since corresponding substance is more stable, because the peak is present in phase diagram of Al2O3-MgO at spinel composition [2]. Table 1 suggests that Mg fraction is approximately constant in different layers of deposit. Hence, Al2MgO4 forms relatively slowly from already deposited Al2O3 and MgO. The Mg percentage in melt varies from 0.1% to 4.5%, but erosion occurs in praxis also for pure 99.5 % aluminium melt. Therefore, we will concentrate in the paper exclusively in the reaction of Al with refractory.

    Consider, that other aluminium oxides Al2O, Al2O2 act as intermediate products, since they are less stable as Al2O3. Therefore, we will assume the following simple irreversible reaction scheme (see Figure 3):

    reaction at refractory surface: Al2O + refract. ® Al2O2 +…

    reaction in bulk of Al melt: 2 Al2O « Al2O2 + 2 Al

    reaction in bulk of Al melt: Al2O + Al2O2 « Al2O3 + 2 Al

    deposition of Al2O3: Al2O3(in melt) « Al2O3(corundum)

    Other possible intermediate stages will be ignored. Thus, the active Al2O molecules are considered to be responsible for the erosion of refractory. The reaction rates in bulk between oxides Al2O, Al2O2, Al2O3 are low because they are reacting via collisions, and probability of such collisions is proportional to the product of respective densities. The formation of Al2O occurs due to entropy factor. Al2O2 will be considered here as inert in respect to refractory, despite it is more active than Al2O3. Nor Al2O, nor Al2O2 form solid phase at 800 ° C and so they do not make deposits. Moreover, Al2O, Al2O2 certainly cannot attack very inert Al2O3 (corundum) deposits. The refractory material is not homogeneous, but forms a porous structure. Therefore, the Al2O attacks more chemically unstable parts into pores such as cement material that joining the grains. Consequently, the grains are able to liberate after sufficient amount of cement has been eroded. Liberated grains contain the major part in easily removable deposits not shown in photo Figure 1. The erosion-deposition mechanism is shown schematically in Figure 3. If the deposition rate is high enough, corundum layer delays or even blocks the access for Al2O molecules in pores and erosion becomes impossible.

  5. Erosion and deposition model
  6. Let us consider the aluminium melt with small fraction of Al2O, Al2O2 and Al2O3. The temperature is almost constant in channel cross-section that follows from characteristic turbulent velocities (about 1 m/s) and small channel diameter (10 - 20 cm). Calculations of melt dynamics in electromagnetic field showed that two vortices are characteristic in channel cross-section (see Figure 2). The intersection of both vortices possesses a high turbulence with characteristic pulsation. The experiments with channel melting furnaces show a simultaneous erosion at points B, D and deposition at - A, C. That can be described by variation of thickness of laminar layer at refractory. High turbulence at points A, C significantly diminishes the thickness of laminar layer in comparison with points B, D.

    Figure 2. Cross-section of channel

    Figure 3. Erosion-deposition mechanism

    Consider that total number of oxygen atoms in the melt are N. One part them N1 builds molecules Al2O, other N2 - Al2O2 and rest N-N1-N2 - Al2O3. Let us denote binding energies as e 1>0, e 2>0, e 3>0 of molecules Al2O, Al2O2, Al2O3, respectively. Free energy in mean field approximation is

    , (1)

    where N0 is the total number of accessible sites for single, double or triple oxygen atoms in melt. The total number of accessible sites N0 for oxygen atoms equals to the half number of Al atoms in melt NAl: . Let us denote relative concentrations of Al2O as , Al2O2 - , Al2O3 - . The total concentration of oxygen is . The minimum of free energy (1) at large enough Al2O3 concentration yields:

    . (1a)

    There is still a significant number of active Al2O and Al2O2 molecules because of entropy factor. Intense turbulence makes the distribution of oxides practically homogeneous in the bulk of channel cross-section. Variations occur only near refractory surface due to laminar layer. The concentrations of components c1, c2, c3 vary according to diffusion equations

    , (2)

    where x axis is perpendicular to refractory surface, D1, D2 and D3 are diffusion coefficients for each component, respectively. Basing on diffusion controlled assumption, the boundary concentration of Al2O3 at flat corundum deposit is equal to equilibrium concentration of Al2O3 in melt

    , (3)

    where the parameters Acor = 0.017, Ecor = 0.85 eV are asymptotic values at 800 ° C of Al-O phase diagram. The concentration of Al2O at refractory surface is zero in diffusion controlled approach. The laminar sub-layer at points A and C in Figure 2 is the narrowest due to high turbulence. The laminar sub-layer at points B and D is much thicker as the fluid flow here is closer to laminar.

    The thickness of laminar layer d for turbulent flow and smooth refractory surface can be estimated as [4]

    , (4)

    where k is kinematic viscosity coefficient, u* is dynamic velocity that could be expressed by velocity pulsation, vi are the velocity components. k is about 10-6 m2/s for aluminium melt. Hydrodynamic simulations at points A, C (see Figure 2) yielded . Therefore, the thickness of laminar layer is d » 0.1 mm.

    First consider the simplest case when the thickness of laminar layer is constant along the perimeter of channel cross-section (see Figure 2) and erosion of refractory is the only source of oxygen atoms. One-dimensional planar diffusion model can be used, as the thickness of laminar layer is much lower than average diameter of cross-section. The minimum of free energy in bulk (2) and conservation law of oxygen gives the boundary conditions at x = 0 (boundary of laminar layer). The stationary solution can be expressed by constant fluxes of respective oxygen atoms:

    . (5)

    The boundary co-ordinate of refractory shifts during the melting process, i.e., is positive for deposition and negative for erosion. The sign of this velocity is a function of the ratio between Al2O and Al2O3 fluxes towards the refractory depending on the structure of inhomogeneous refractory material and erosion-deposition nature. The ratio between these Al2O and Al2O3 fluxes for the case with constant thickness of laminar layer d equals to 1.

    The diffusion coefficient inside a pore D1 is lower than the diffusion coefficient in laminar layer D1. Therefore, the flux j1 is


    where d is approximately equal to the characteristic size of grains (see Figure 3) at refractory surface.

    Figure 4. Model with two different thicknesses of laminar layer

    Now consider the case when the thicknesses of laminar layer is not constant along the channel perimeter in Figure 2. Let us assume that there are two characteristic thicknesses of laminar layer d A, d B and the sizes of both regions are equal as shown in Figure 4. The flux ratios in both regions are

    , , (5a)

    If laminar layer in B is thicker at , one gets that the erosion dominates in B but deposition in A. The intensive deposition in region A reduces the diffusion of Al2O in pores. Due to stronger deposition, diffusion in pores is slowed down or cancels at all in this region, i.e., . Then follows that stationary flux ratio in B at is

    . (6)

    This ratio can be much higher than 1. Henceforth, the strong erosion in B occurs and, consequently, increasing of the eccentricity of the elliptic cross-section. One can conclude that the intensity of erosion-deposition process increases during exploitation of furnace. The resultant Al2O3 deposition flux towards the refractory at point A and c1 << 3 c3 » c, c3 » ceq is

    . (7)

    We can conclude that simultaneous erosion and deposition is describable by variation of the thickness of laminar layer. In order to decrease the rate of erosion, it necessary to improve the smoothness of the surface and increase the resistance of refractory to erosion active molecules. Making the thickness of laminar layer more homogeneous following from fluid dynamics calculations can essentially reduce the erosion rate. The quantitative analysis about the chemical erosion will be performed by finding more data about oxidation reactions. For this reason chemical composition of more samples of various furnaces for aluminium melting should be performed.

  7. Conclusions
  8. The deposition-erosion model is made which shows the mechanism of simultaneous erosion and deposition in the same channel cross-section of Al melting furnace.

    The necessary multistage oxidation scheme is constructed which includes aluminium oxides acting as intermediate products. The different behaviour of refractory surface bases on the different chemical properties of aluminium oxides and varying thickness of laminar sub-layer.

  9. References

  1. Drewek R. Verschleißmechanismen in Induktions- Rinnenöfen für Gußeisen und Aluminium, VDI Verlag, 1995.
  2. Taylor Ch.R. Electrical furnace and steelmaking, Iron & Steel Society, 1985.
  3. Bulletin 542, U.S. Bureau of Mines, 1954.
  4. Bethers U, Jakovics A, Jekabsons N, Madzhulis I, Nacke B. The theoretical investigation of the conditions of the build-up formation in the induction channel furnaces. Magnitnaya Gidrodinamika 1994, 30; 247-258.