Blast furnace – Quality of Raw Materials and Hot Metal Quality

Steel production from coal and iron ore is still the world's main method of manufacturing high-quality steels for the most demanding applications in terms of working properties, notably in the transport, energy and packaging sectors. It is based on the blast furnace and the oxygen furnace, and accounts for nearly 70% of the world's steel production. The electric furnace scrap recycling process, whose product portfolio is gradually moving upmarket in terms of quality, continues to develop in the Western world, not least because of its significantly lower CO ₂ impact than that of the blast furnace-converter process. An alternative route for producing steel from ore, via gas pre-reduction in a shaft furnace, followed by smelting of the pre-reduced material in an electric furnace, continues to develop, but at much lower production levels than the other two routes.

Good control and consistency of cast iron quality (chemical composition, temperature) are major assets of the blast furnace-converter chain. Indeed, the reducing conditions of the blast furnace crucible, and the possibility of transferring certain undesirable impurities from the cast iron to the slag, or of sharing elements between the cast iron and the slag, offer the means to regulate cast iron quality and optimize the interface between the blast furnace and the steel mill. Cast iron quality criteria relate to both its chemical composition and its consistency.

The quality of cast iron and slag depends first and foremost on the quality of the raw materials fed into the furnace, i.e. iron-bearing materials and coke. This dependence has become all the more critical as the quality of ores and coking coals tends to deteriorate, requiring a high degree of flexibility with regard to raw materials. In addition, controlling the behavior of recirculating elements (alkalis, zinc) contained in raw materials at low levels, but which can cause serious running disorders, is essential for consistent cast iron quality.

The characteristics of the hot wind (temperature, pressure, oxygen content, speed) play a major role in the distribution of gas and energy inputs in the tuyere zone. Blast furnace productivity and energy consumption depend to a large extent on the operating point setting of this zone.

The various aspects contributing to cast iron quality are discussed in this article:

• properties and quality of ferrous materials and solid fuels charged to the blast furnace,

• characteristics and quality of coals injected into nozzles,

• properties of liquid cast iron and slag,

• mechanisms...