The kinetics of lime dissolution containing different amounts of magnesium oxide (4.3–7.6%) in steelmaking slags has been studied in a non-stationary diffusion, at temperatures ranging from 1 300–1 600°C,
directly in the hot process. The decrease in solubility of lime with the increasing content of magnesium
oxide is due to the formation of high temperature solid chemical compounds (Ca2SiO4, (Mg,Fe)2SiO4) and
solutions based on MgO like ((Mg,Fe) O) in the volume of lime samples. Formed slightly soluble phases
hinder the process of CaO dissolution and slow down the rate of dissolution of lime samples. Based on
differential equations of mass balance of the dissolving substance and taking into account its transition
into the melt phase, and accumulation in the consistently dissolving layers of solid, values of mass transfer coefficients and diffusion coefficients were calculated. Mass transfer coefficients, despite a decrease
in the concentration gradient, increase in the process of slag saturation with lime. Activation energy has
been calculated under conditions of non-stationary linear and spatial semi-infinite diffusion. Based on the
dependency of the logarithm of diffusion coefficients on the inverse temperature derived value of the
activation energy of diffusion is 230.2±14.0 kJ/mol.