Ferroalloys are alloys of iron with a high percentage of one or more of other elements. Ferroalloys industry is very closely related to iron and steel industry since ferroalloys are used in steel making, alloying of steels and in iron or steel foundries. In the production of steel, ferro alloys are used for deoxidation of steels as well as for introduction of the alloying elements in the steel. Ferroalloys impart distinctive qualities to steel and cast iron. Depending upon the process of steel making and the product quality envisaged, the requirement of ferroalloys varies widely.
Ferroalloys are usually classified into two groups namely (i) Bulk ferroalloys and (ii) Noble or special ferroalloys. More than 85 % of ferroalloys produced are used in the steel industry.
Bulk ferroalloys consist of principal alloys namely ferro manganese (Fe-Mn), silico manganese (Si-Mn), ferro chrome/charge chrome (Fe-Cr) and ferro silicon (Fe-Si). These are shown in Fig 1.
Fig 1 Bulk ferroalloys
- Ferro manganese – Fe – Mn is a ferroalloy with high content of manganese (Mn). It is produced by heating a mixture of the oxides of MnO2 and Fe203 with carbon usually as coal and coke, in either a blast furnace or a submerged arc furnace. The oxides undergo carbo thermal reduction to produce Fe- Mn. It is produced as three types of products namely (i) standard high carbon (C) Fe-Mn, (ii) medium carbon Fe-Mn and (iii) low carbon Fe- Mn. High carbon Fe – Mn has manganese in the range of 72 % to 82 %, C in the range of 6 % to 8 % and silicon (Si) in the range of around 1.5 %. Medium carbon Fe- Mn has manganese in the range of 74 % to 82 %, C in the range of 1 % to 3 % and silicon (Si) in the range of around 1.5 %. Low carbon Fe- Mn has manganese in the range of 80 % to 85 %, C in the range of 0.1 % to 0.7 % and silicon (Si) in the range of 1 % to 2 %.
- Silico manganese – Si – Mn is a ferroalloy with high content of manganese and silicon and is produced by heating a mixture of oxides of MnO2, SiO2 and Fe2O3 with carbon in a furnace. These oxides undergo a thermal decomposition reaction. The standard grade contains manganese in the range of 62 -68 %, Si in the range of 12 % to 18 % and carbon in the range of around 2.0 %. The low carbon grade of Si- Mn has a carbon level of 0.1 % maximum. For steel melting shop operators Si – Mn is more preferred ferroalloy for deoxidation.
- Ferro chrome/charge chrome – Ferrochrome (Fe-Cr) is an alloy of chromium and iron containing chromium and iron containing between 50 % and 70 % of chromium. Ferro-chrome with chrome content below 56 % is known as ‘charge chrome’. Carbon content classifies the Fe- Cr into high carbon (6 % to 8 %), medium carbon (3 % to 4 %) and low carbon (0.1 % to 0.5 %). The chromium content in Fe – Cr varies in the range of 60 % to 70 %. Ferrochrome is produced by electric arc melting of chromite, an iron magnesium chromium oxide and the most important chromium ore. Ferrochrome production is essentially a carbothermic reduction reaction taking place at high temperatures. Chromite ore is reduced by coal and coke to form the iron-chromium alloy. Charge – chrome is produced from a chrome containing ore with lower chrome content. Alternatively, high carbon Fe-Cr produced from higher grade ore, is more commonly used in specialist applications such as engineering steels where a high Cr to Fe ratio and minimum levels of other elements such as sulfur, phosphorus and titanium are important. Low carbon Fe – Cr is used during steel production to correct chrome percentages, without causing undesirable variations in the carbon or trace element percentages. It is also a low cost alternative to metallic chrome for uses in super alloys and other special melting applications.
- Ferro silicon – Ferro silicon (Fe-Si) is a ferroalloy of iron and silicon. Fe – Si contains 65 % to 90 % of silicon and minor amounts of iron, aluminum and carbon. Fe – Si is usually produced in four grades. These are standard grade, low aluminum grade, low carbon grade and high purity grade which is low in titanium. It is produced by reduction of quartzite (SiO2) with coke in presence of iron ore. Ferro silicon is made in the submerged arc furnace.
Noble ferroalloys are the vital inputs for the production special and alloy steels. These ferroalloys are of high value and consumed in low volumes. They are namely ferro molybdenum (Fe-Mo), ferro vanadium (Fe-V), ferro tungsten (Fe-W), ferro titanium (Fe-Ti) and ferro niobium (Fe – Nb). Noble ferro alloys are mostly manufactured through Alumino-thermic process. These are used in the production of steel as de-oxidant and alloying agent. Their quantum of consumption varies widely based on steel making process and grade of steel.
- Ferro molybdenum – It is an important noble ferroalloy with a molybdenum content of 60% to 70 %. Molybdenite concentrates are roasted to form molybdic oxide. This oxide is mixed with iron oxide and aluminum and is reduced in an alumino-thermic reaction to produce ferro molybdenum.
- Ferro vanadium – It contains 35 % to 80 % vanadium. The production is carried out in an electric-arc furnace. Scrap iron is first melted, and a mixture of V2O5, aluminum, and a flux such as calcium fluoride or calcium oxide is added. In the ensuing reaction, the aluminum metal is converted to alumina, forming a slag, and the V2O5 is reduced to ferro vanadium.
- Ferro tungsten – Ferro tungsten contains 75 % to 85 % tungsten. The raw materials for ferro-tungsten production are rich ore or ore concentrates of wolframite or scheelite. The tungsten trioxide in these ores are reduced either carbothermically in electric arc furnaces or metallothermically by silicon and/or aluminum. Ferro tungsten has a steel grey appearance and a fine-grained structure consisting of FeW and Fe2W.
- Ferro titanium – Ferro titanium is produced in two grades containing titanium in the range of 35 % to 35 % and 65 % to 75 %. Ferro titanium is manufactured from various raw materials such as titanium scrap, ilemenite sand, rutile and titanium sponge. Fe- T is produced by mixing titanium bearing raw materials and iron scrap and melting them together in an induction furnace.
- Ferro niobium – Ferro niobium contains niobium in the range of 60 % to 70 %. Basic raw material for producing ferro niobium is pyrochlore ore. From this ore niobium penta oxide (Nb2O5) is produced. Nb2O5 is mixed with iron oxide and aluminum and is then reduced by alumino-thermic reaction to produce ferro niobium.