Method for preliminary treatment of molten iron

A method for preliminary treatment of molten iron by conducting desiliconization and dephosphorization of molten iron with a converter container, the method comprising:

• charging untreated molten iron tapped from a blast furnace into the converter container to conduct a first desiliconization;
• then retaining the molten iron and a part of a slag after the first desiliconization in the converter container to conduct intermediate slag removal;
• subsequently adding a lime-based flux solvent to the molten iron and slag retained in the converter container after the first desiliconization while blowing oxygen to conduct a first dephosphorization of the molten iron, not less than 30 mass % of a slag having a slag basicity of not less than 1.2 after the first dephosphorization is retained in the converter container,
• then charging more untreated molten iron into the converter container containing the slag after the first dephosphorization to conduct a second desiliconization, and adjusting a basicity of the slag during the second desiliconization such that the basicity is in the range of 0.8 to 1.5, silicon concentration of molten iron at the end of the second desiliconization is not more than 0.2 mass %, a basicity of the slag after the second desiliconization is in the range of 0.5 to 1.5 and a temperature of the molten iron is in the range of 1240° C. to 1400° C.;
• thereafter conducting a second intermediate slag removal of discharging not less than 40 mass % of the slag after the second desiliconization from the converter container;
• and
• subsequently conducting a second dephosphorization in the converter container, wherein
• the basicity of the slag during the second desiliconization is obtained by dividing a numerical value obtained by adding an amount of CaO contained in the slag after the first dephosphorization with a CaO amount generated, with a numerical value obtained by adding a SiO2 amount in a residual dephosphorized slag with a SiO2 amount generated,
• the CaO amount generated is calculated by multiplying (i) a CaO content in an auxiliary material, (ii) a retention time of the auxiliary material in a furnace, and (iii) a dissolution rate of the auxiliary material in the furnace, and
• the SiO2 amount generated is calculated by multiplying (i) an oxygen feeding rate in blowing, (ii) the molecular weight of SiO2 divided by the atomic weight of Si and further divided by a theoretical Si amount burnt by oxygen of 1 Nm3, (iii) an oxygen efficiency for desiliconization in blowing, and (iv) a total amount of molten iron and scrap.