Solutocapillary Effects of Sulphur in CaO-Al2O3 Slags, in Marangoni Instability and Convection Onset, Related to the Desulphurization of low Carbon, Aluminium Killed Steels
Abstract
Based on the “one layer model” applied to desulphurization slag layer with boundary conditions, imposed due to the sulphur transfer through the interface steel-slag during desulphurization process, the threshold conditions of instability for Marangoni convection in the desulphurization slag at 1873 K are evaluated in the CaO-Al2O3 system at ratios CaO/Al2O3 = 1.5 and 1.0, for sulphur concentration lower than the solubility limit and in conditions of fully liquid slag. Threshold critical values ∆σc = (51−79.62)10-6 N/m, for the surface tension in slag and interfacial tension slag-steel, closed to the value ∆σcrit = 30.748·10-6 N/m obtained in steel and Δ(S)crit = (20−44.73)10-4 mass%, for the local fluctuations of sulphur concentration , were found for onset instability and Marangoni convection in slag, at the minimal value of the critical dimensionless solutal number Masc = 80. There are small differences between numerical values of the quantities for threshold conditions obtained on the base of the surface tension of the slag and on the base of the interfacial tension steel-slag. The instability with onset of the turbulent convection is predicted to take place starting from values ∆σ > 312.36−501.61)·10-6 N/m, corresponding of Δ(S) = (126−228.82)·10-4 mass%. An enhancement coefficient of the mass transfer of sulphur at interface on the slag side is established in the conditions of the onset Marangoni flow and convection. The solutal parameter Δ(S) is the most sensitive in contributing to instability conditions and in producing mixing effects in slag in a thickness layer δeff at interface, by energy dissipation of Marangoni flow and helps to make important differences between conditions in slags C/A = 1.5 and C/A = 1.
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