Dolomite application in steel making
Dolomite and quick lime are secondary materials used for ferrous metallurgy as fluxes. However, they have a huge role to affect the temperature of the metallurgical process, product quality, separation process of liquid steel and slag, and life cycle of equipment. In steel making, we can use limestone or dolostone. However, in a production, limestone and dolostone will be decomposed and liberate a large amount of CO2 into the atmosphere. This matter cause kiln heat loss. Thence, we often use burnt lime and dolomite to reduce production expense.
1. Diagram of steelmaking technology
2. The role of lime and dolomite in the metallurgical process
2.1 Limestone and dolomite are used as flux and scorifier
The main composition of slag is metal oxides (including base oxide, acid oxide and amphoteric oxide).Oxides has very high melting temperatures and lower weight than liquid steel. During the steel smelting process, infusing lime (CaO) and dolomite (CaO.MgO) could reduce steel melting temperature and liquid steel viscosity. Thence, slag easily separates from liquid steel and floats to the kiln surface.
In the base kiln, at the first stage of steel smelting, one part of SiO2 will create links with FeO and form fayalite compounds:
SiO2 + 2FeO = Fe2SiO4
Then, depending on the solubility of CaO and MgO in fayalite, iron oxide will be splitted:
Fe2SiO4 + 2CaO = Ca2SiO4 + 2FeO
Phosphorus is harmful element in steel because it reduces the ductility of steel, causing brittleness, especially when carbon content in steel is high. High phosphoric steel is required only in some cases at steels surface with high smoothness, or brittle shavings chip making convenience steel production process.
As quick lime and burnt dolomite have high content of CaO and MgO, P2O5 react with CaO similarly with MgO as below:
P2O5 + 3CaO = 3CaO.P2O5 Hoặc P2O5 + 4CaO = 4CaO.P2O5
As 3CaO.P2O5 and 4CaO.P2O5 are complexes which could not be decomposed in high temperature, the above reactions happen for phosphorus oxidation. The general reaction as follows:
2[P] + 5FeO + 3CaO = 3CaO.P2O5 + 5[Fe] + Q
Sulfur is a harmful impurity in steel as it cause hot shortness. Therefore, the higher quality steel, the lower sulfur content required. Clean steel has to be desulfurized strictly.
Sulfur is soluble into Fe. The higher temperature, the greater solubility. Desulfurization reaction is similar as CaO and MgO:
FeS + CaO = CaS + FeO – Q
2.4 Create proper alkalinity to protect kiln wall
In alkaline kiln, in steel-making slag components (both oxidation slag and reverted slag), MgO always has a detected value. In non-MgO steel-making coordination, MgO in slag is MgO which melts from kiln wall. Thence, we need to infuse dolomite for scarifying in order to keep required balance of MgO in dolomite, which will also reduces kiln wall melting.
3. Dolomite used for metallurgy