Interactions of dolomitic lime with iron silicate melts.

An investigation has been made of the dissolution of dolomitic lime in iron silicate (fayalite) melts at temperatures corresponding to those obtained during the early stages of an LD blow (1300°C). The melts were contained in iron crucibles and held in a furnace under an argon atmosphere. Cylinders of dolomitic lime, prepared from dolomitic limestone, were preheated to the temperature of the melt and immersed for times ranging from 15 to 540 seconds. The reacted cylinders were withdrawn, and quenched under an argon jet for microscopic examination and scanning electron microscope analysis. Equivalent experiments were undertaken with calcitic lime cylinders prepared from limestone. A rotating bob-fixed crucible viscometer technique has been established, and viscosities of synthetic slags within the system CaO-FeO-SiO2-MgO, were measured. Cone fusion studies were used to determine the melting behaviour of the slag system. Dolomitic lime and iron silicate (fayalite) melt reacted to form magnesiowustite and dicalcium silicate, and a globular wustite and forsterite-fayalite olivine series was established at the melt-cylinder interface region. The dissolution of dolomitic lime was controlled by the transport of magnesia into the melt, by diffusion, from the forsterite-fayalite zone.. Corresponding iron silicate-calcitic lime reactions prodiiced a liquid cal-ciowftstite phase contained within a continuous envelope of dicalcium silicate, surrounding the lime sample, which assumed a granular morphology when a 5 mass % MgO addition was made to the melt. Magnesia levels up to 7.5 mass % MgO raised the viscosity of fayalite by 250 mPa.s at 1300°C. Magnesia increased the liquidus temperatureand melting range of synthetic CaO-FeO-SiO2 melts at all levels of addition. A modified film theory approach has been used to calculate the experimental mass transfer coefficients for CaO and MgO.The relative rates of calcitic and dolomitic lime solution have been assessed and the results evaluated in relation to the industrial situation. It has been shown that dolomitic lime does not represent a replacement for calcitic lime. However, it is essential for reducing refractory wear rates and enhancing the formation of a basic early formed slag, when used in conjunction with calcitic lime, provided it is added at the commencement of the blow.