Pig Iron is a high Fe (iron), low residual metallic material which is used for the production of high quality iron and steel products in a wide variety of furnaces engaged in steelmaking and foundries. The material is not a scrap substitute but rather as a source of clean iron units which can be used to supplement and enhance the scrap charge. Pig iron is an intermediate product and the first product of ironmaking produced during the smelting of iron ore. It is produced when liquid iron (hot metal) is cast in the pig moulds.
Pig iron is basically an alloy of iron (Fe), carbon (C), silicon (Si), and manganese (Mn). It also contains some percentage of sulphur (S) and phosphorus (P) as impurity elements, along with some titanium (Ti) and other trace elements. The C content of pig iron is high, typically in the range of 3.5 % to 4.5%. With its defined and closely controlled specification and the absence of metallic impurities, pig iron is a reliable and consistent charge material for both the electric steelmaking as well as the ferrous castings production. The various grades of pig iron have specific chemical properties designed for their end use. The pig iron also contains valuable alloying elements and reduces the energy consumption of a melt.
The term ‘pig iron’ can be traced back to the method of casting blast furnace iron into moulds arranged in sand beds such that they could be fed from a common runner. The term has arisen from the old method of casting liquid iron from a blast furnace into moulds arranged in sand beds (Fig 1). The traditional shape of the moulds used for these ingots was a branching structure formed in sand. It had many individual ingots at right angles to a central channel or runner. Such a configuration is similar in appearance to a litter of piglets suckling on a sow. Name of these ingots was given from the word ‘piglets’. When the liquid metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner runner (the sow). Since pig iron was intended for remelting, the uneven size of the ingots and inclusion of small amounts of sand were insignificant when compared with its ease of casting and of handling.
Fig 1 Sand moulds for casting of pig iron
Presently sand moulds are no more used for the casting of liquid iron into pig iron. These days pig iron is cast in pig casting machines. A schematic view of a two strand pig casting machine is shown in Fig 2.
Fig 2 Schematic view of casting pig iron in a pig casting machine
Pig moulds have different sizes and shapes. Pig iron is supplied in a variety of sizes and weights, ranging from 3 kg upto more than 50 kg. Large pieces have normally notches. There are normally one or two notches. The depth of the notches is to be such so as to make the pig iron easily breakable. A few varieties of pig irons showing different shapes are shown in Fig 3.
Fig 3 Size and shape of pig iron
The dimensional drawings of a 45 kg pig iron having two notches and a 22.5 kg pig iron having a single notch are shown in Fig 4.
Fig 4 Typical drawings of pig iron
Properties of pig iron
Pig iron is hard, wear resistant, fairly fusible and very brittle. It is not useful directly as a material except for limited applications. It has a low melting point as compared to steel. Typically its hardness varies between 250 BHN to 450 BHN, tensile strength between 25 kN to 50 kN and % elongation in the range of around 0 % and 0.5 %. The lower melting point of pig iron combined with higher density of the charge due to it, results into lower energy requirements and faster melting time when compared to that of an all steel charge make-up in a melting furnace. The physical characteristics of the pig iron are given in Tab 1.
|Tab 1 Physical characteristics of pig iron
|A variety of shapes
|Weight of single piece
|3.3 to 3.6
|Angle of repose
Pig iron contains C, Si, Mn, P, S, trace metals and some gases besides iron (Fe) which is the main constituent of the pig iron. Fe content in the iron is around 94 %. P and S are considered as impurities in the pig iron. Pig iron contains around 3.5 % to 4.5 % of C with S content less than 0.05 % and P content can be upto 0.12 %.There are two main qualities of pig iron which is normally produced. These are basic grade pig iron and the foundry grade pig iron. Besides there are special types of high purity pig irons.
Basic grade of pig iron has less than 1.0 % of Si, and lower than 1 % of Mn. This type of pig iron is mainly used for steel making. There are several grades specified in various standards based on Si and Mn content of the pig iron.
Foundry grade of pig iron is mostly being used in iron foundries for remelting and casting into cast iron products. It contains higher amount of Si. Various standards specify composition limits for Si and Mn for different grades of this type of pig iron. Si content in foundry grade is much higher and usually is in the range of 1.5 % to 3.5 %. It can be as high as 4.25 %.
The approximate content of C in the pig iron depends on its Si, Mn and P contents and can be found by using the equation C = 4.6 % – 0.27 x (% Si) – 0.32 x (% P) + 0.03 x (% Mn). The range of chemical composition of different types of pig iron is given in Tab 2.
|Tab 2 Composition of different types of pig irons
|Pig iron grade
Uses of pig iron
Traditionally pig iron was worked into wrought iron in forging shops. Later this iron was used in puddling furnaces. In these processes, pig iron is melted and a strong current of air is directed over it while it is being stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. An intermediate product of puddling is known as refined pig iron or refined iron. Pig iron is also melted in the cupolas and the electric furnaces for the production of cast iron castings in the foundries. More recently pig iron is used in making steel in induction furnace and electric arc furnace. It also replaces scrap in the basic oxygen furnace (BOF) during the period of scrap shortage.
Pig iron can also be used to produce gray iron. This is achieved by remelting pig iron, often along with substantial quantities of steel and scrap iron, removing undesirable contaminants, adding alloys and adjusting of the C content.
Some pig iron grades are suitable for producing ductile irons. These are high purity pig irons and depending on the grade of ductile iron being produced, these pig irons can be low in the elements like Si, Mn, S, and P. These types of pig irons are useful to dilute all elements in a ductile iron charge (except C) which can be harmful to the ductile iron process.
Nickel pig iron
Nickel (Ni) pig iron is a ferro-nickel pig iron containing 3 % to 13 % of Ni. It is a low grade ferro-nickel invented in China as a cheaper alternative to pure Ni for the production of stainless steel. The alternative was developed as a response to high price of pure Ni. It contains much less Ni than the conventional ferro-nickel ((25 % to 40 % Ni) and has higher concentrations S and P.
Nickel pig iron is presently being used to produce 200 series stainless steels, with limited application to the 300 series. There is considerable upside potential for nickel pig iron to be used more widely in the production of 300 series stainless steels with improvements in technology.
The production process of nickel pig iron utilizes laterite Ni ores instead of pure Ni. The low grade Ni ore, containing around 0.8 % to 2 % Ni, is mixed with coking coal, gravel, and sand as an aggregate. This mixture is heated in either in a small blast furnace (around 300 cum or less), or in an electric arc furnace depending on the desired grade. Impurities are then removed through smelting and sintering processes and the resulting nickel pig iron contains 3 % to 13 % of Ni. Ni pig iron equals ‘dirty nickel’, because the production process is not environmentally friendly, the carbon dioxide emissions being particularly high.