Dissolution rate and diffusivity of lime in steelmaking slag and development of fluoride-free fluxes

摘要

A rotating disk technique was used to determine the dissolution rate anddiffusivity of CaO and MgO in slags. The dissolution rate was deduced from themeasured changes in concentration of oxides in slag with respect to reactiontime. The experimental set- up was initially tested with dissolution of magnesia inthe CaO – 55 wt% Al2O3 slag at 1430 ºC and a measured rate of 2.7 ´10 -5g/cm2.s was obtained. The dissolution rate was increased by slag chemistry andranged from 6.5´10-5 to 2.1´10-4 g/cm2.s.The dissolution rate of CaO was measured in CaO – 42 wt% Al2O3 – 8% SiO2based slag. The measured dissolution rates were found to be strongly dependenton the slag chemistry and temperature and ranged from 5.03´10 -5 to 3.3´10 -4g/cm2.s.The dissolution rates were strongly dependent on the rotation speed and resultsindicate mass transfer in the slag phase to be rate- limiting step.The diffusivity of MgO / CaO was calculated from the dissolution rate andsolubility data, using known mass transfer correlations. The diffusivity of MgOin the calcium aluminate slag at 1430 ºC was found to be about 1.1´10-5 cm2/s.Additions of 5 and 10 wt% Fe2O3 increased the diffusivity by a factor ~ 1.5 to 3,respectively. However, with introduction of (CaF2 5 wt% + Fe2O3 5 wt%) and(CaF2 5 wt% + Fe2O3 10 wt%) in the slag, the diffusivity increased considerablyby a factor of about 29 and 11, respectively.The diffusivity of CaO in calcium aluminosilicate was measured to be in theorder of 10-6 to 10-5 over a temperature range of 1430 – 1600 ºC. CaF2 increasedthe diffusivity by a factor of 3 to 5 while MnOx and FeOx, ilmenite and TiO 2increased the diffusivity substantially and SiO2 had an opposite effect. Themeasured diffusivities are in accord with published data on comparable systemsand are discussed with reference to Eyring theory.It was concluded that MnOx, FeOx and ilmenite in the slag increase thedissolution rate and diffusivity of lime, showing comparable results with respectto CaF2.