Ferro-Silicon


Ferro-Silicon

Ferro-silicon (Fe-Si) is a metallic ferro-alloy having iron (Fe) and silicon (Si) as its main elements. In commercial terminology It is defined as a ferro-alloy containing 4 % or more of Fe, more than 8 % but not more than 96 % of Si, 3 % or less phosphorus (P), 30 % or less of manganese (Mn), less than 3 % of magnesium (Mg), and 10 % or less any other element. However, the regular grades of the ferro-alloy normally contain Si in the range of 15 % to 90 %. The usual Si contents in the Fe-Si available in the market are 15 %, 45 %, 65 %, 75 %, and 90 %. The remainder is Fe and minor elements. The minor elements, such as aluminum (Al), calcium (Ca), carbon (C), manganese (Mn), phosphorus (P), and sulphur (S) are present in small percentages in Fe-Si.

Commercially, Fe-Si is differentiated by its grade and size. Fe-Si grades are defined by the percentages of Si and minor elements contained in the product. The principal characteristic is the percentage of Si contained in the ferro-alloy and the grades are referred to primarily by reference to that percentage. Hence 75 % Fe-Si contains around 75 % of Si in it. Fe-Si grades are further defined by the percentages of minor elements present in the product. ‘Regular grade 75 % Fe-Si’ denote that the product containing the indicated percentages of Si and recognized maximum percentages of minor elements. Other grades of Fe-Si differ from regular grades by having more restrictive limits on the content of elements such as Al, titanium (Ti), and/or Ca in the ferro-alloy. Fe-Si is also produced in a grade that contains controlled amounts of minor elements for the purpose of adding them to steel or foundry iron using Fe-Si as the carrier. Such Fe-Si products are sometimes called ‘inoculants’.

Commercial grade of Fe-Si with Si content of 15 % is generally produced in the blast furnace (BF) lined with acidic fire bricks. Fe-Si with higher Si content is normally produced in SAF. Fe-Si is produced industrially by carbo-thermic reduction of silicon dioxide (SiO2) with C. It is produced by smelting Fe containing materials and Si containing materials. Fe is in the form of iron ore, iron or steel scrap, or mill scale and Si is normally in the form of quartzite lumps. These are combined with carbonaceous material such as coal or petroleum coke and a bulking agent such as wood chips. Quartzite is the source for Si in this carbo-thermic process. The smelting of Fe-Si is a continuous process carried out in the electric submerged arc furnace (SAF) with the self-baking electrodes.

All grades of Fe-Si are produced using essentially the same process, but certain additional steps are required to produce higher?purity grades of Fe-Si. Such grades are produced using raw materials containing lower amounts of impurities. In addition, refinement of the liquid Fe-Si to remove unwanted impurities and the addition of special alloying elements occur in the ladles. This further processing to produce higher purity Fe-Si is known as ladle metallurgy. Specialty grade 15 % Fe-Si for dense medium application is typically produced by remelting 75 % Fe-Si with steel scrap in an electric arc furnace and casting into a high?pressure water spray.

Fe-Si is usually produced in four grades. These are (i) standard grade, (ii) low Al grade, (iii) low C grade, and (iv) high purity grade having low content of Ti. The standard grade of Fe- Si contains Al up to 2 % while the low Al grade has Al content of 0.5 % maximum. Fe-Si contains a high proportion of Fe silicides.

Fe-Si with 15 % Si is not used for metallurgical purposes in the production of steel or cast iron. Specialty grade 15 % Fe-Si  is combined with water to create a dense medium for gravity (sink/float) separation of minerals, aggregates, and metals.

Fe-Si is added usually along with ferro-manganese (Fe-Mn) as ladle addition during steelmaking. Both Si and Mn play an important role in the manufacturing of steel as deoxidizing, desulphurizing, and alloying agents. Si is the primary and more powerful deoxidizer than Mn.

Modern steel making practice aims to keep the furnace availability as high as possible and hence the heat is tapped in the ‘open’ condition with relying mostly on ladle deoxidation. Fe- Si, Fe-Mn, and Si-Mn are used as the primary deoxidizers. The amount of Si added depends on tapping temperature, oxygen (O2) content of liquid steel, and the residual Si level needed in the cast steel.

Preliminary furnace deoxidation with Fe- Si is to be done with care since Si has the power to reduce P2O5 (phosphorus penta oxide) in the slag. If slag phosphorus (P) content is high and the slag is not flushed, it may remain insufficiently basic after the Fe-Si addition. These conditions can lead to P reversion in the heat, especially if the slag is hot, and the bath analysis exceeds about 0.10 % Si and 0.40 % Mn at C levels of over 0.15 %.

Fe-Si is mainly used during steelmaking and in foundries for the production of C steels, stainless steels as a deoxidizing agent and for the alloying of steel and cast iron. It is used as a reducing agent, particularly in the production of stainless steel. As a reducing agent, Si reacts with chromium (Cr) oxides to form Si oxides, returning Cr to the liquid steel, and thus increasing the overall Cr recovery of the process.

Fe-Si is used as the source of Si for alloying purposes in the production of certain steel alloys, particularly Si electrical steel, which may contain 3 % or more of Si. Fe-Si is used by iron foundries as the source of Si needed for alloying purposes in iron castings. During the production of cast iron, it is also used for inoculation of the iron to accelerate graphitization. In arc welding Fe-Si can be found in some electrode coatings.

Almost all Fe-Si consumed in the steel and cast iron industry contains Si ranging from 65 % to 90 %. Fe-Si is used primarily in sized lump form. Size is important because it affects the performance of the Fe-Si in its designated use. Large lumps are generally used in primary steelmaking furnaces because they penetrate the layer of slag on top of the liquid steel more readily. Smaller lumps are more commonly used for alloying purposes to insure rapid dissolution in liquid steel. Fines are less desirable than lumps because it is more difficult to recover the Si content in them.

Properties of Fe-Si

Fe-Si is a non-hazardous solid with an appearance of silvery grey. In fact, the colour of Fe-Si can vary between silvery grey and dark grey. It has a metallic surface. The shape of Fe-Si powder particles can be spherical or irregular. The material has no odour but can be dangerous when inhaled. CAS number of Fe-Si alloy is 8049-17-0.

Fe-Si may slowly produce hydrogen (H2) when it comes in contact with water. The dust of Fe-Si is soluble in water with the solubility of 0.015 milli grams Si per litre at pH of 5.8-5-9 and temperature of 20 deg C. Toxic and flammable gases are produced when this alloy reacts with moisture, bases and acids.

Fe-Si is non-inflammable solid with a lowest explosive limit of +/- 60 milli grams per cubic meter. The auto ignition temperature of Fe-Si is higher than 400 deg C. However Fe-Si particles suspended in air can cause dust explosion under certain conditions. Hence spreading of dust needs to be prohibited. It is also important not to have any open flames near Fe-Si powder. While fire-fighting, the containers of Fe-Si are to be cooled with running water even after the flames are out.

The Fe-Si storage is to be provided with adequate ventilation and facilities for cleansing. Accumulation of dust is to be avoided.

The density of Fe-Si depends on the content of the Si in it and it goes on reducing as the Si content increases. The densities of some grades of Fe-Si are (i) around 5.15 grams per cubic centimeter (g/cc) with 45 % Si, (ii) 3.5 g/cc with 75 % Si, and (iii) 2.4 g/cc with 90 % Si. The densities of some of the grades of Fe-Si along with the solidus and liquidus temperatures are given in Tab 1.

Tab 1 Physical properties of Fe-Si
Sl.No. Si content Solidus point Liquidus point Density
% Deg C Deg C g/cc
1 0 1538 1538 7.87
2 20 1200 1212 6.76
3 35 1203 1410 5.65
4 50 1212 1220 5.1
5 60 1207 1230 4.27
6 80 1207 1360 3.44
7 100 1414 1414 2.33

Boiling point of Si is 2355 deg C. Fe-Si is known to possess good resistance to abrasion and good resistance to corrosion. It has high magnetism, which allows its magnetic recovery.

Fe -Si is usually available in lump or granular form. Coarser lumps are used for better slag penetration during deoxidation while finer sizes ensure rapid dissolution when used as an alloying addition. Fe-Si is fairly friable and excessive handling generates unwanted fines.

Iron-silicon phase diagram

The phase diagram of the Fe-Si binary system is at Fig 1. Iron-silicon phase diagram shows the phases which are to be expected at equilibrium for different combinations of Si content and temperature.

Fig 1 Phase diagram of the Fe-Si binary system

 Application

The following are the common application areas of Fe-Si.

  • It is a source of Si to reduce metals from their oxides and to deoxidize steel and other ferrous alloys. This prevents the loss of C from the liquid steel (so called ‘blocking the heat’).
  • It is used in the manufacture of other ferroalloys.
  • It is used in the production of Si, corrosion-resistant and high-temperature resistant ferrous Si alloys, and Si steel for electro motors and transformer cores.
  • It is used in the manufacture of cast iron. It is also used for inoculation of the iron to accelerate graphitization.
  • It is used for the production of pre-alloys like magnesium Fe-Si (FeSiMg), used for modification of melted malleable iron. FeSiMg is instrumental in the formation of nodules, which give ductile iron its flexible property. Unlike gray cast iron, which forms graphite flakes, ductile iron contains graphite nodules, or pores, which make cracking more difficult. FeSiMg contains 3 % to 42 % of magnesium and small amounts of rare earth metals.
  • It is used as an additive to cast irons for controlling the initial content of Si.
  • It is used in steel melting, casting, mineral processing and melting rod industry.
  • It is used in the Pidgeon process to make magnesium from dolomite.
  • It is used for heavy media separation and atomization.
  • It is used in casting, melting and related metallurgy industry.
  • It is used for the production of semiconductor pure Si in electric industry and Si copper in chemical industry.