1.1 Introduction to Ferro Alloys

1.1 Introduction to Ferro Alloys

 

Ferroalloys (and master alloys in general) have been developed to improve the properties of steels and alloys by introducing specific alloying elements in desirable quantities in the most feasible technical and economic way. Ferroalloys are namely alloys of one or more alloying elements with iron, employed to add chemical elements into molten metal. Not a single steel grade is produced without ferroalloys (Wood and Owen, 2005).

Ferroalloys production is an important part of the manufacturing chain between the mining and steel and alloys metallurgy: the main task of the ferroalloys industry is the primary recovery (reduction) of needed metals from natural minerals. As ores also include non-metallic minerals (gangue), they have to be dressed (beneficiated, enriched) by one or several successive methods (gravitational, magnetic, electric, and flotation separation, or in some cases by chemical means) to produce useful mineral concentrates in which the leading content of the metal is much higher in comparison with the original ore (Gasik and Lyakishev, 2005).

This allows the production of higher-grade ferroalloys with a higher content of leading elements and a lower content of impurity elements (usually phosphorus, sulfur, and non-ferrous metals), and it significantly reduces specific energy consumption and production costs.

There are several reasons why ferroalloys are used to add necessary elements. The alloying element might be difficult to obtain in pure form and it makes less sense to purify it (also from iron) if the purpose is to add it back to iron-based steel melt. It just may not be stable in a condensed form at steelmaking temperatures. The alloying element alone might have too much affinity to oxygen or nitrogen, which would lead to its premature oxidation before it would be utilized. Eventually, the costs of 1 kg of alloying element in its ferroalloy form are many times lower than the costs for its pure form.

Ferroalloys are usually classified in two groups: bulk (major) ferroalloys (produced in large quantities) and Noble (minor) ferroalloys (produced in smaller quantities, but of a high importance). Bulk ferroalloys are used in steelmaking and steel or iron foundries exclusively, whereas the use of special ferroalloys is far more varied. About 85% to 90% of all ferroalloys are used in steelmaking; the remaining ferroalloys are used for non-ferrous alloys (e.g., those that are nickel or titanium based) and by the chemicals industry (Gasik and Lyakishev, 2005).

Historically, the ferroalloys production technology used in the 19th century was developed for blast furnaces (high-carbon ferromanganese, low-grade ferrosilicon), as at those times it was the main route for cast iron processing. However, in a blast furnace it is not possible to produce ferroalloys with elements that have a higher affinity for oxygen or with low carbon content. This led to the development of ferroalloys to be manufactured (smelted) in electric furnaces at the beginning of the 20th century. Today, almost all ferroalloys are produced in submerged arc furnaces, where raw materials (ores), reductants (coke, silicon-based ferroalloys, aluminum), iron additions (iron ore or steel scrap), and fluxes (lime, magnesia, dolomite, limestone, fluorspar, etc.) are loaded and smelted, followed by the tapping of slag and metal.

With the first experience of ferroalloys smelting, it became evident that more research was needed to improve our understanding of the thermodynamics of liquid melts; ferroalloy and slag formation (Holappa and Xiao, 2004); the kinetics of reduction and phase transformation; the structure, properties, and stability of solid ferroalloys, mineral raw materials, and reducing agents applied; and the electric and thermal properties of all charge components. Later, the issues of better energy and resource utilization, lower emissions requirements (Tanaka, 2008), and increasing demands to improve the purity and costs of ferroalloys set new challenges for improving the quality and price competitiveness of ferroalloys and their production processes.

It is now well understood that the main technical and economic performance indicators of ferroalloys production and application (i.e., their competitiveness) could be substantially improved with systematic fundamental and applied research, allowing for the development and implementation of better energy-saving smelting processes, the creation of more sophisticated electric furnace equipment with automated control systems, and enhancements to process management.