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Desulfurization Process, dephosphorization and Secondary Refining

The search for efficiency in the desulfurization process (reduction in the sulfur content) of pig iron led to new procedures and equipment innovations, which increased significantly the final quality of steel and reduced the consumption of power, in addition to turning toxic waste into inert material. The current desulfurization processes use high-reactivity desulfurization agent, consisting basically of calcium oxide, calcium carbide, metallic magnesium and other elements which react with the sulfur found in the metal, by turning it into sulfide and converting it into slag.

Since the reducing environment is an essential factor for the desulfurization reaction, the processes require, in addition to balanced compounds, a controlled, continuous, homogenous addition of the agents to the metal. For that purpose, an inert gas (Nitrogen) is used to carry and supply the desulfurization agent. TECNOSULFUR desulfurization operations include all from the simple process of adding the agent into the pig iron ladle, to the more sophisticated process of deep injection, through the immersion of a refractory rotary spear in torpedo cars or ladles.

The dephosphorization processes (reduction in the phosphorus content) of pig iron consist in oxidizing the phosphorus and combining it with calcium and other components, which are injected into the hot metal deeply, resulting in compounds which are transferred to the slag and, thus, removed from the metal.

Complementary desulfurization treatments, final adjustments of the chemical composition and control of inclusions are carried out in the secondary refining of steel. Equipment allowing for the electric (ladle furnace) or chemical (CAS-OB, AHF or IRUT) heating, or simply bubbling inert gas is typically used. The chemical composition is adjusted by adding alloys through silos or wires. Ancillary injection equipment can be used to employ micropulverized desulfurization agents.

Since the molten pig iron is the main raw material to manufacture steel, which has been requiring more and more resistance and applicability, the demand for desulfurization and dephosphorization processes is increasing, because high contents of phosphorus, sulfur and other nonmetallic inclusions in the steel lead to more hot shortness and less resistance to impacts, among other losses.


PERFORMANCE OF TECNOSULFUR PRODUCTS

TECNOSULFUR products are employed throughout the process of transformation of pig iron into molten steel. The process begins by reducing the content of sulfur found in the pig iron, at the Desulfurization Station. Afterwards, the dephosphorization agent is applied to the pig iron at the Dephosphorization Station by way of initial refining, for the purpose of reducing the phosphorus content. From the Dephosphorization Station, the pig iron goes to the converters or electric furnaces of the steel mill, where it is transformed into steel. From the converters or electric furnaces, the steel goes to the ladle furnace, to the bubbling station or to chemical heating, where the synthetic slag is applied as second refining and/or the micropulverized desulfurization agent is injected. The next step is the vacuum degassing, where steel is refined for the third time, through the use of other synthetic slag specifications. After this treatment, the steel goes to the rolling/drawing chain.