Steel MakingProcess & Practice in BOFOleh Maulud Hidayat

Charging 1. Feeding system2. Scrap Charging methodes 3. HBI as coolant

BOF & Feeding System

Scrap charging methode to BOFThe HBI is loaded first and placed at the rear of the scrap scoop. The rest of the scrap charge such as heavy melt, alloys, mill scrap etc. is then loaded into the scoop. This procedure is essential to avoid contact between the HBI and the retained slag on the furnace bottom when charging

Benefits of such charging practiceThis charging practice prevents HBI skulling over the furnace bottom and allows a normal slag washing practice to be performed and followed by either retained slag or non-retained slag. The practice of non-retained slag requires the operator to drain the slag out of the furnace after the slag washing is completed. Draining the old slag is required under the following conditions: a) Highly oxidised slag; b) Turn down chemistry is low in C/Mn; c) A reblow occurred; d) High hot metal Si/Ti in the following charge.BHP has experience of charging up to 10% HBI in the 280 tonne BOF at Port Kembla Steelworks using the scrap scoop system.

Charging of HBI as a Coolant

Most BOF shops prefer to use clean low residual scrap as a coolant. A typical chemistry specification for coolant scrap used in flat products shops is shown below.Typical Coolant Scrap Chemistry Specification ElementMax. Wt. %C0.3,Mn1.0,Si1.0,P0.04,S0.04,Cu0.004,Ni0.004, Sn0.004,Mo0.004As can be seen, the residual requirements are very low and could be only met by premium quality scrap or virgin iron units. Since BoodarieTM Iron is low in residual elements, it satisfies these stringent requirements.

Charging Practice HBI for coolantCharging Practice When HBI is Used as an In-Blow Coolant When BoodarieTM Iron is used as an in-blow coolant, the charging process can start prior to hot metal charging and continue during oxygen blowing. The balance of scrap in the scoop must be adjusted to allow for the HBI charge. Lance height and quantity of oxygen also must be adjusted for HBI. The HBI chemistry and quantity used must be taken into consideration for flux additions. BHP's Port Kembla Steelworks has experience in charging HBI up to 24 tonnes into the 280 tonne BOF vessel. In BHP practice part of the HBI is added prior to start blow (maximum 4 tonnes/batch)and the rest is batched in during the blow (maximum 2 tonnes/batch). Increasing the batch size could cause excessive flaring. No HBI is dumped when decarburisation is at its peak.

Charging Practice When HBI is Used as an End-Point Coolant When HBI is used as an end-point coolant, the charging process can start immediately after the sublance in-blow sample is taken and the heat balance computation has been performed using the dynamic model. It was found during trials at Port Kembla that to avoid flaring the batch size should be limited to a maximum of 2 tonnes/batch.

Using HBI as coolantUsing HBI as a Coolant In BOF shops where productivity is limited by scrap scoop capacity, it is possible to add a large quantity of HBI into the furnace via the overhead bin system. This may be done during charging and blowing. Other shops use metallics as an in-blow coolant where they are dribble-fed during oxygen blowing. Iron ore pellets or limestone are often added after completion of the main oxygen blow to adjust hot metal temperature (end-point coolant), since time does not permit an exact weight of scrap to be charged to compensate for the heat generated in the blow. Compared to conventional coolants (such as iron ore pellets, reclaimed scrap metallics, and limestone), HBI will provide a better gain in yield and productivity for the same amount of cooling. As shown in the table below limestone and iron ore have much greater cooling effects compared to HBI.Coolant Practical Cooling Effect Relative to Scrap Scrap 1.0, Iron Ore 2.0-3.0, Limestone 3.0-4.0, HBI 1.2The cooling effect of HBI is higher than scrap due to the endorthermic reaction that occurs when iron oxide is reduced during melting

RefiningOxidation of few elements

Refining effect of oxygen blowing

Fundamental Reactions :Carbon Oxidation (1) Fundamental Reactions - Carbon OxidationCarbon oxidation, or decarburisation, is the most important reaction during oxygen steelmaking. Hot metal has a carbon content typically in excess of 4.0% and this is reduced to less than 0.1%. Oxygen is supplied via a top lance, bottom tuyeres or a combination of both. There are three distinct periods of carbon oxidation: The first period, characterised by a low rate of decarburisation and dominated by Si oxidation. This lasts only a few minutes (initial blow). At the end of this period, Si in the steel is completely oxidised and largely reports to the slag.

Fundamental Reactions :Carbon Oxidation (2) Fundamental Reactions - Carbon OxidationThe second period is characterised by a steadily increasing decarburisation rate. This is the dominant period. In this period, the decarburisation rate is limited by the rate at which oxygen can be supplied, which in turn governs BOF productivity. This period is also characterised by a high rate of CO evolution that helps remove dissolved gases such as nitrogen and hydrogen, and also promotes thermal and chemical homogenisation of the bath. During this period some reduction of FeO in the slag occurs due to post combustion reactions in the slag bulk. The third and the final period is characterised by a low rate of decarburisation and begins when bath carbon content falls to approximately 0.3%. In this period, the decarburisation rate is governed by the mass transfer rate of carbon from the bath to the reaction interface. Most of the oxygen supplied will react with iron to form iron oxide that will lead to yield loss. As the carbon level in the bath is reduced, the slag FeO level will continue to rise. End-point carbon and temperature control are the main tasks of this period.

Fundamental Reactions :Silicon Oxidation Fundamental Reactions - Silicon Oxidation In BOF steelmaking, the silicon brought in by hot metal and scrap gets completely oxidised during the initial blowing period as shown in the chart below. The silicon oxidation reaction is strongly exothermic and helps raise hot metal temperature very quickly early in the heat. The product of silicon oxidation is silica and this forms a large part of the slag make. Silica is acidic and is very aggressive towards the basic dolomitic refractories. Therefore, early formation of a lime rich liquid slag is very important to minimise silica attack on furnace refractories. Slag volume is largely dependent on the silica formed as a reaction product or brought in as gangue in the feed material. The larger the slag volume, the higher the yield loss and also the degree of slopping, therefore it is desirable to limit the amount of silica formation. On the other hand, a larger slag bulk helps dephosphorisation. Therefore, each operation needs to optimise the slag formation. In BOF operation where high percentage HBI usage is planned, it is important to consider the slag volume fraction resulting from the gangue.

Fundamental Reactions :Dephosphorization (1)Fundamental Reactions - Dephosphorisation As shown in the chart below phosphorous oxidation begins at the start of the blow. When the slag FeO content is reduced during the main decarburisation period some phosphorus reversion occurs. The phosphorus level drops towards the end of the blow when the slag FeO rises again due to low bath carbon.Change in the bath chemistry during the blow The oxidation reaction is as shown below. This reaction is exothermic. 2[P]+5(FeO)= (P2O5) + 5[Fe] It is important to have an oxidising slag (high FeO) for this reaction to occur and to promote formation of P2O5. P2O5 is a very unstable oxide and in the presence of lime helps form a more stable phosphate: 3(CaO) + (P2O5)= (3CaO.P2O5) Combining the two reactions, net reaction for dephosphorisation is: 2[P]+5(FeO) + 3(CaO) = (3CaO.P2O5) + 5[Fe]

Fundamental Reactions :Dephosphorization (2)In scrap and hot metal, phosphorus is present in the elemental form and hence needs to be oxidised prior to joining the slag phase as explained above. In BoodarieTM Iron, phosphorus is present in the oxide form (P2O5). Therefore, when the briquette melts, the phosphorus content readily becomes part of the slag make and is easily removed.If fluxes are added to the BOF so as to maintain a constant slag basicity, the phosphorus level in the steel will actually be lower when high phosphorus HBI replaces scrap in the charge. This is because the increased slag bulk, at constant phosphorus partition, allows more phosphorus to be contained in the slag. If the fluxes are kept fixed, then the phosphorus level will only increase by a negligible amount. In practice the lack of slag-metal equilibrium and the fact that the phosphorus in the HBI is in the oxidised form will mean that this predicted higher phosphorus level may not be attained.The results obtained from adding 10t of HBI#1 to a 280t BOF at BHP's Port Kembla Steelworks are shown in Figure below. The distribution in steel phosphorus was unchanged from normal zero HBI feed operation. Similar results have been obtained at POSCO.

Fundamental Reactions :DesulfurizationFundamental Reactions - Desulphurisation As shown in the chart below, the degree of sulphur removal is quite small in the BOF process. This is because the sulphur partition ratio, (S)/[S] is very low (4-8) in a BOF due to highly oxidising conditions. In a BOF, some sulphur in the charge (about 10-20%) directly reacts with oxygen to form gaseous SO2. The remainder enters into solution and in the presence of lime rich slag will react according to the following equation: [S] + (CaO)+ [Fe] = (CaS) + (FeO)

Change in the bath chemistry during the blow Not much sulphur can be captured by the slag phase since FeO levels are too high for any significant amount of partitioning to occur. When producing low sulphur steels it is important not to charge large pieces of high sulphur scrap since the bulk of the sulphur will report to the steel. In BoodarieTM Iron sulphur is typically less than 0.018%. That is much lower than in most commercial scrap. Therefore, BoodarieTM Iron can be used advantageously in the production of low sulphur steels.

Operation

Oxygen Blowing Control and Furnace Slops

Sloping in BOF

Oxygen Blowing Control and Furnace Slops

Oxygen Blowing Control and Furnace Slops Liquid foaming slag acts as a medium to hold the metal droplets ejected from the jet cavity formed by the oxygen lance. Foaming slag captures these droplets and prevents them from depositing on the lance or on the waste heat recovery system and protects these elements from damage. Also foaming slag prevents corrosion of the BOF refractory lining from corrosion wear by free oxygen. Therefore, liquid foaming slag is essential for the BOF process.However excessive foaming leading to spillage or overflow of slag from the BOF, commonly known as slopping, is a problem. Furnace slopping in BOF operations leads to prolonged processing times and accelerated furnace lining wear. Extreme slopping can cause building roof emissions and infringe environmental license conditions. Therefore, every effort is made to minimise slopping. The reason for slopping is the excessive expansion of slag volume (foaming) due to formation of a large number of tiny gas bubbles in the slag. In BOF operation, an increase in slopping is largely associated with an increase in hot metal Si and Ti levels. With HBI in the charge, the slag volume is increased due to the presence of gangue material and associated increase in fluxing (to adjust basicity). Therefore, low gangue HBI such as BoodarieTM Iron are better suited for BOF operations intending to use a high percentage of HBI in the charge. Operators have found that slopping is reduced when HBI is used to replace high gangue coolants, such as metallics reclaimed from scrap. Slopping potential is greatly reduced when HBI is used to replace coolants such as limestone and iron ore.

Charging & Emision BOF

BOF Roof Emissions BOF roof emissions typically occur during either hot metal charging (charging emission) or oxygen blowing (flaring emission). Selection of an excessively large batch size could lead to HBI fusing into a large lump at the start of the heat and then remelting later during the blow, when the bath temperature is increasing rapidly. This releases a large volume of CO gas due to the oxidation reaction. Therefore, it is important to determine the correct HBI batch size to minimise flaring.

BOF Roof Emissions BOF roof emissions typically occur during either hot metal charging (charging emission) or oxygen blowing (flaring emission). Selection of an excessively large batch size could lead to HBI fusing into a large lump at the start of the heat and then remelting later during the blow, when the bath temperature is increasing rapidly. This releases a large volume of CO gas due to the oxidation reaction. Therefore, it is important to determine the correct HBI batch size to minimise flaring.

Heat Balance and End-Point Temperature Control

Heat Balance and End-Point Temperature Control Slag Design and Flux Addition Practice

Heat Balance and End-Point Temperature Control In BOF operation, it is important to understand the cooling effect of each and every charge material in order to achieve the correct tap temperature. BOF operators use both static and dynamic models to accurately predict the bath temperature. The dynamic model predicts the end-point temperature based on the actual measurement of the in-blow sample.BHP's Port Kembla Steelworks conducted trials to understand the cooling effect of HBI with up to 6% HBI in the charge. These trials have shown that the cooling effect of HBI was about 7C per tonne of HBI and was not significantly different from steel scrap. These trials showed that the end-point control was not affected when the HBI charge is less than 10 tonne per heat (less than 3% of the charge). The temperature control deteriorated when the HBI charge was increased to greater than 10 tonnes (greater than 3% of the charge). This finding could be due to excessive heat loss due to flaring emissions observed when HBI in the charge was greater than 3%. For higher HBI levels in the charge some adjustments to dynamic models are necessary.