Experiments have been made to study the crystallization process of free lime and magnesia from the molten LD-converter slag saturated with magnesia.
Many types of free lime exist depending on the cooling conditions and the slag basicity. In rapidly cooled slags with a basicity higher than 4, two types of free lime exist, i.e. primary lime solid solution (L_pri) and lime solid solution (L_per) as residuum which results from a peritectoid reaction between L_pri and the liquid slag. On the other hand, in slowly cooled slag, three types of free lime exist.
1) CaO rich phase (L_wus) which contains a lot of minute precipitates of white spots of wustite.
2) Lime (L_C3S) as a mixture of 2CaO·SiO₂ and lime solid solution which are formed by the decomposition of 3CaO·SiO₂.
3) Lime (L_res) which exists in the form of a string along boundaries of 2CaO·Fe₂O₃ crystals.
Free magnesia forms a solid solution with FeO or/and MnO in slag and changes into magnesio wustite (MW).
The mechanism of the formation of such various phases is discussed.