Deposit formation and erosion of refractory in Al induction-channel furnaces

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The refractory material in Al melting is strongly attacked by the Al melt. The lifetime of refractory in those furnace is only 9-12 months. In case of Fe melting, the erosion of refractory does not occur and deposits have not fractal-like structure. The temperature of Al melt varies in range 750-850 C that is about by 100 C higher than melting temperature of Al. The power of the inductor of typical furnaces is above 1000 kW. The typical diameter of the channel is 10-20 cm. Channel-crossection has an eliptic shape in order to improve penetrabilty of melt through the channel. relatively short lifetime in Al melting is caused by deposition in points A, B and erosion at the other points. Thus, the elliptic cross-section of channel became more excentric during explatation. induction-channel furnace of Al melting

 
 
 
The calculations and experimental measurements showed that two vortices forms in channel cross-section with the average velocity 1 m/s. At the same time, the velocity along the channel axis is only 0.1 m/s. However, the fluctuations of melt circulation occurs, especially at the intersection region of both vortices: A, B in previous figure. Thus, the thickness of laminar layer at points A, B is much smaller than in other points along perimeter. It sugests that simulataneous deposition and erosion occurs due to variation of the laminar layer. fluctuations in channel cross-section

 
 
 
Most important doping elements in Al melt with main properties are shown in this table (see, e.g., http://www.webelements.com). The most typical doping element is Mg especially for making mechanically hard materials with light weight.
  density [kg/m3] melt. temp. [C]
Al  2702  660.37 
Cr  7190  1857 
Cu  8920  1083.4 
Fe  7870  1535 
Mg  1740  648.8
Mn  7200  1244 
Ni  8900  1453 
Pb  11340  327.502 
Si  2330  1410 
Ti  4500  1660
Zn  7130  419.58 
Zr  6506  1852 

 
 
 
The doping fractions for typical materials can found, e.g., in . The percentage of doping elements for mechanically very hard Al material is given in this table. We will investigate Al with following doping elements: AlCuPb, AlMgSiPb, AlMg (easy removable deposits), AlSiNi, AlMgMn, AlMn (hardly removable deposits).

 
 
 
The phase diagrams of aluminium with dopping elements are necessary to know how much of the doping element dissolves into alumium melt at given temperature. Thsee diagrams can be obtained from SGTE Phase diagram collection. An example of Al-Mn phase diagram is shown. In general, the higher the melting temperature of the doping element lower the solvability in melt. The N2 gas and also soda are added to increase the solvability. But as the fraction of them is small, they are disregarded in our calculations. binary phase diagram of Al-Mn

 
 
 
The constitution of refractory material Didolit 20-T (made in DIDIER-WERKE_AG) on oxide basis is shown in this table. It consists mostly from mullite 3Al2O3-2SiO2 and tridymite SiO2. The increase in Al2O3 percentage would improve the chemical resistance of refractory material but make the equipment more expensive. Moreover, silica has the advantage of strong expansion effect for getting high material density after heating and for closing cracks after reheating.
  wt %
SiO2 53.8
Al2O3 25.6
BaO 6.0
K2O 2.0
CaO 1.4
TiO2 1.0
Fe2O3 0.7
Na2O 0.5
P2O5 0.5
MgO 0.4
MnO <0.1
S Other 8.0

 
 
 
The table above shows how much enthalpy in gram-calories releases in reaction of a given element with half a mole of O2 gas. Higher absolute value of enthalpy makes a given oxide more chemically stable. Mg and Al are placed in the top of table. It means that they can disposses the oxygen atom from more active oxides in refractory such as Fe2O3, P2O5 bypassing the energy barrier of the reaction.

Bulletin 542, U.S. Bureau of Mines, 1954.

Ca(b) + O2(g) -> CaO(c) -151.730
Mg(l) + O2(g) -> MgO(c) -145.810
2/3 Al(l) + O2(g) -> 1/3 Al2O3(corundum) -135.983
Ba(l) + O2(g) -> BaO(c) -135.900
Ti(a) + O2(g) -> TiO2(c) -114.180
Si(c) + O2(g) -> SiO2(b-quartz)  -104.960
2 Na(l) + O2(g) -> Na2O(c) -100.150
Mn(b) + O2(g) -> MnO(c) -91.900
2 K(l) + O2(g) -> K2O(c) -87.380 (below 776 C)
2/5 P4(g) + O2(g) -> 1/10 P4O10(g) -72.233
2/3 Fe(a) + O2(g) -> 1/3 Fe2O3(b) -66.667 (below 760 C)
Ni(b) + O2(g) -> NiO(c) -57.460
Pb(l) + O2(g) -> PbO(yellow) -53.02
2 Cu(c) + O2(g) -> Cu2O(c) -40.550

 
 
 
Therefore, Al and Mg attacks the refractory material. The simulations can be made using the methods of chemical kinetics. Low sintering temperature ~ 1000 C and the composition of refractory material suggests that the refractory is build up from SiO2 grains that are bound by calcium-alumina cement material. But the cement contains such weak oxides as Fe2O3, P4O10 and K2O. The chemical erosion destroys the cement the SiO2 grains become weakly connected with other ceramics material at the surface melt-refractory. The intense flow melt produces a final stress and grains are able to liberate. High porosity of ceramic (~ 20 %) significantly increases the effective area of surface attacked by chemical erosion of refractory. chemical erosion of refractory

 
 
 
Al and Mg can get the oxygen also from scrap and waste material added in the process of melting and also from surounding atmosphere. Al2O3 and MgO dissolves into Al melt in extremely low quantities (~ 10-6 %). Therefore, together with erosion of refractory material deposition of those oxides is possible. The rate of deposition is proportional to the concnetration of oxides in the bulk of melt and it is modelled assuming that laminar layer forms directly at the refractory surface. The difference in deposition in various channel areas arises from difference in thicknesses of laminar layer that decreases significantly in regions of strong turbulence. The figure shows hypothetical reaction of Al with O2 that is present in atmosphere. hypothetical intermediate stages in reaction of Al with O2

 
 
 
sample Nr. 1 2 3
a -SiO2     40 %
BaAl2Si2O8     40 %
Al6Si2O13     15 %
a -Al2O3 3 % 2 % 5 %
MgAl2O4 30 % 57 %  
NaAlO2 25 % 10 %  
MgO 20 % 5 %  
Al(OH)3 10 % 10 %  
Na2CO3H2O 7 % 7 %  
Other 5 % 9 %  
photo of deposit from induction channel furnace
The figure above shows characteristic piece of ceramics coated with deposits. The lighter layer is ceramics, while the dark one is the deposit. The surface of the deposit is coated with metallic aluminium left in furnace. One can see that the deposit surface is rough with characteristic length-scale - 2 cm. These wavy structures cannot be explained by microscopic theory, but they arise due to instability of fluxes and other factors. The chemical composition of sample is shown in table. The first sample is taken directly from upper part of deposit, the second one - from layer of deposit close to ceramics, while the last sample corresponds to ceramics. Elements Na and H are present in deposits because not only N2 but also soda are added in melting process in order to increase the solubility of doping elements. Someties stone formation in Al melting is possible.

 
 
 
The phase diagram of MgO-Al2O3 shows a peak at spinel composition. Therefore, deposition of spinel composition must be significant. As can be seen from the table above, the formation of spinel forms relatively slowly in comparision of deposition itself. Therefore, seperate deposition of Al2O3 and MgO could be simulated.

Taylor, Ch.R. (1985) Electrical furnace and steelmaking, Iron & Steel Society

binary phase diagram of MgO-Al2O3

 
 
 
The erosion takes place also when Mg is not present in melt. One of the simplest models describing simultaneous erosion and deposition is shown here. It is assumed that Al2O molecule is very active and responsible for chemical rections with refractory. The melt redistribution between Al2O, Al2O2 and Al2O3 occurs inside the bulk of melt. When deposition rate is high enough, Al2O molecules could not penetrate inside pores and erosion in this area is impossible. simultaneous erosion-deposition mechanismsimultaneous erosion-deposition mechanismsimultaneous erosion-deposition mechanism

References

A. Jakovics, B. Nacke, I. Madzhulis, V. Frishfelds, "Influence of melt Flow and Temperature on Erosion of Refractory and Deposit Formation in Aluminium Melting Furnaces", 4th International Conference MHD at dawn of 3rd Millenium, Giens, France, September 18-22, 2000
 
 
Page compiled by Vilnis Frishfelds. Last update: 03.08.00.