On sulphur behaviour in steelmaking with sponge iron in an electric arc furnace

The sulphur behaviour in steelmaking by melting of charges containing from 38.7 to 95.4 wt.% sponge iron in the metallic input in a 70 t UHP electric arc furnace is investigated. The effect of both slag composition and temperature on the sulphide capacity of the slag and on the sulphur distribution ratio between slag and metal is studied. The kinetics of desulphurization are also treated. Regression analysis shows that the logarithm of sulphide capacity varies linearly with total lime equivalent, theoretical optical basicity and logarithm of slag basicity. Equations relating the sulphide capacity to these parameters as well as to the temperature are given. Based on these equations, functional relationships for the calculation of sulphur distribution between slag and metal are derived. Metallurgical reactions for metal desulphurization by slag and equilibrium relationships are given from which the slag-metal sulphur partition ratio can be calculated. In all cases, the results of the calculations are in line with the experimental data. The rate of desulphurization is expressed by an equation based on the boundary layer diffusion theory. Under the conditions of the present investigation, the activation energy of desulphurization is found to be 39 kJ/mol and the mass transfer coefficient of sulphur at 1600°C is equal to 0.014 cm/s.