Hot Metal

Hot Metal

Hot metal (HM) is the output of a blast furnace (BF). It is liquid iron which is produced by the reduction of descending ore burden (iron ore lump, sinter, and pellet) by the ascending reducing gases. HM gets collected in the hearth of the BF. From the hearth, the HM is tapped from the taphole of the BF after an interval of time. Normally in large BFs, HM tapping rates of 7 ton/min and liquid tapping velocities of 5 m/sec, in tap holes of 70 mm diameter and 3.5 m long, are typically encountered. The tapping rate of HM is strongly influenced by the taphole condition and taphole length. Generally the temperature of tapped HM varies in the range of 1420 deg C to 1480 deg C. The tapped HM is handled in the two stages namely (i) handling of the HM in the cast house i.e. from taphole to the hot metal ladles (open top or torpedo), and (ii) transport of HM ladles to the point of HM consumption.

Presently most of the HM is consumed within integrated steel plants for steel making. The HM is transferred to the steel melting shop for making of steel. The HM which is not sent for steel making is cast into pig iron in pig casting machine for use in steel making later as cold charge or is sold to foundries or to mini steel plants having induction furnaces as merchant pig iron. HM can also be granulated by a process which is known as ‘Granshot’ process. Presently the Granshot plants for the production of GPI are working at six places namely (i) Uddeholm, Sweden, (ii) SSAB Lulea, Sweden, (iii) Voest Alpine, Donawitz, (iv) Saldanha steel, South Africa, (v) SSAB Oxelosund, Sweden, and (vi) Essar Steel, India.

HM contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulphur (S), trace metals and some gases besides iron (Fe) which is the main constituent of HM. P and S are considered as impurities in the HM. HM contains around 3.5 % to 4.5 % of C with S content less than 0.05 % and P content can be up to 0.12 %. There are two main qualities of HM normally being produced in the BF. These are basic grade iron and the foundry grade iron.

Basic grade of HM has less than 1.0 % of Si, and lower than 1 % of Mn. This type of HM is mainly used for steel making. There are several grades specified in various standards based on Si and Mn content of the HM. Foundry grade of HM is mostly being used in iron foundries as pig iron 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 HM. 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 HM depends on its Si, Mn and P contents and can be found by using the following equation.

C = 4.6 % – 0.27*(% Si) – 0.32*(% P) + 0.03*(%Mn)

The density D of HM in grams per cubic centimetres (g/cc) at 1450 deg C can be found using the following equation.

D = 7.16 – [0.1*(% Si) + 0.07*(% C)]

At the time of tapping, the HM is normally has a density in the range of 6.8 g/cc to 7 g/cc.

The concentration of gases in the HM generally ranges 0.009 % to 0.03 %. Gases in HM are usually oxygen (O2) and nitrogen (N2) and to a lesser extent hydrogen (H2). The non-metallic inclusions in the HM are generally in the range of 0.03 % to 0.23 %. The non-metallic inclusions are usually in the form of sulphides and oxides.

The dynamic viscosity of HM can vary in the range of 0.002 pascal second to 0.010 pascal second depending on the tapping temperature. The electrical resistivity of HM having a composition 3.7 % of C, 1.5 % of Si, and 0.6 % of Mn at 1400 deg C is equal to 0.000148 ohm metre. The surface tension of HM having a composition 4.12 % of C, 1.7 % of Si, 0.62 % of Mn, 0.08 % of S, and 0.11 % of P at 1300 deg C is equal to 1.08 Newton per metre.

The composition of HM especially the impurities such as S, P, and trace elements depends on the quality of the burden materials consisting of ore, coke and fluxes as well as quality of coal used for the injection.

Si content in HM

Si is the main element which decides whether the HM is of basic grade or foundry grade. A low Si content in HM while lowering of the refining costs during steel making also reduces BF energy consumption since the Si transfer reactions are endothermic. For every 0.1 % increase in HM Si, an extra around 25 mega calories per ton of HM (Mcal/tHM) is consumed, equivalent to a 3 kg/tHM to 4 kg/tHM increase in the reductant rate. Si in the HM originates from silica (SiO2) in coal, coke and the ore burden.

A material balance carried out in one of the BF with PCI found that the iron-bearing materials (sinter, iron ore and pellets) contributed the highest amounts of Si (80 % relative contribution), followed by coke (12 %) and then coal (8 %). Most of the Si ends up in the slag (94 %) with 4 % in the HM and 2 % in the dust carried by the BF top gas. Transfer of Si into the HM and slag takes place in the lower part of the BF mainly through gaseous silicon monoxide (SiO). The SiO2 is partially reduced by the C present in the raw materials to either gaseous SiO or solid silicon carbide (SiC). The SiC is further oxidized to SiO by reaction with CO. C in the HM then reduces the SiO to Si. Gaseous silicon mono sulphide (SiS) can also play a role in the transfer of Si. Certain studies have shown that the SiO generation rate from coal char is greater than that from coke which, in turn, is greater than that from iron ore slag.

The chemistry of HM principally depends on the extent of the slag-metal-gas reactions taking place and the partition of Si between these three phases. Reactions in the BF hearth between the HM and slag determine the final percentage of Si in the tapped HM. As the liquid metal droplets trickle through the slag layer, part of the Si already picked up by the Fe reacts with oxides in the slag resulting in Si removal from the HM. The Si content of the HM can be controlled by a number of factors (Fig 1) as given below.

  • Utilization of coke and coal which have a low content of SiO2.
  • Lowering the raceway adiabatic flame temperature (RAFT) to reduce the production (gasification rate) of gaseous SiO, though this can decrease the HM temperature.
  • Controlling the cohesive zone height. A low cohesive zone height can help decrease the temperature at the tuyere level, diminishing the SiO generation rate and hence Si content of the HM.
  • Controlling the composition of the slag in the high temperature zone. Acidic slags generate SiO, whereas slags with a high basicity (low SiO2 activity) and high FeO absorb SiO, oxidizing it to SiO2. Injecting Fe oxides or fluxes into the tuyeres increases tuyere slag basicity and hence lowers Si content of HM. However, if the basicity is too high, slag viscosity increases and the SiO absorption rate then decreases. Lower temperatures promote the silicate capacity of the slags. Operating with a low cohesive zone produces slags with a higher FeO content that absorb the SiO. De-siliconization of the HM in the packed coke bed and hearth regions can be enhanced by suitable slag chemistry. Iron oxide (FeO) and manganese oxide (MnO) in the slag can oxidize Si at the metal-slag interface to SiO2, transferring Si to the slag. Increasing the availability of O2 at the metal-slag interface also enhances metal de-siliconization.

Fig 1 Factors controlling silicon content in hot metal

The interplay of the many mechanisms affecting transfer of Si to the HM and the different BF operating conditions can explain why some operators report lower Si metal contents with PCI, whilst others found higher Si levels.

S content in HM

A low S content of HM is desired to avoid expensive desulphurization before steelmaking. Additionally, S in the HM retards C dissolution from coke and coal char and hence the consumption of char. Most of the S in the HM originates in the coke and coal. The principal mechanism for transferring S to the HM is through sulphur di-oxide (SO2) emitted from the coke and coal ash. C in the HM reduces SO2 to S. Gaseous SiS, formed by the reaction of calcium sulphide (CaS) in the coke and coal ash with gaseous SiO also transfers S (and Si) to the HM.

The S content of HM can be controlled by the following actions.

  • Utilization of coke and coal with low S content. This also lowers the consumption of fluxes and additives added to improve the slag S uptake.
  • Adjusting the conditions of BF to manipulate the partition of S between the gas, HM and slag phases. Of course, control of the S content can only be considered in connection with other requirements of the BF process.

Gas phase desulphurization of the HM (around the raceway) becomes important when S concentration is higher than 0.1 % for high C HM. The possible reactions are as follows

H2 + S (metal) = H2S

C (metal) + 2S (metal) = CS2

CO + S (metal) = COS

Since the reaction rate of the first reaction, which produces gaseous hydrogen sulphide (H2S), is higher than those of the other two reactions, an increase in the partial pressure of H2 enhances the gas desulphurization. Therefore gas phase desulphurization plays a larger role with higher injection rates of coal since the amount of H2 in the furnace increases.

S is transferred to the slag as the Fe droplets flow down through the coke bed. Oxides in the slag react with S in the HM to form sulphides. The transfer is promoted by high slag basicity, high temperatures, high slag reduction degree, and low O2 potential. Fluxes can be injected to increase slag basicity. Unfortunately, it is difficult to remove S and unwanted alkalis simultaneously as alkali removal requires an acidic slag. The lower the FeO in the slag, the higher the amounts of S retained, since FeO in slag promotes S transfer to the HM. Most of the desulphurization occurs as the HM droplets pass through the liquid slag layer. Hence the thicker the slag layer the more effective is the desulphurization.

Trace metals

Non-ferrous metals also originate in coke, coal, and ore burden. Coals with P content below 0.08 % are normally desired. Thermodynamics and metallurgy within the BF concentrate the trace metals into the different output streams. The more volatile elements, such as cadmium (Cd) and mercury (Hg), leave through the top BF gas and are removed in the gas cleaning plant (GCP), and hence these are not found in the HM. The less volatile ones, such as zinc (Zn) and copper (Cu), partition between the HM and slag.

The majority of the Zn from all the input sources dissolves into the HM because of the overpressure in the BF process, with around 70 % leaving in the HM and slag. Lead (Pb) has a lower evaporation temperature than Zn, and can accumulate in the BF, lowering productivity. It is principally emitted in the top BF gas (absorbed on the dust particles), where it is removed in the GCP. Its transfer into HM is considered to be of minor importance.