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Company Background

Project Summary

Background on K-OBM

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Company Background

New Zealand Steel (a BHP Steel company) has become one of the world leaders in production of steel from ironsands.

Steel making, using the black ironsands of the West Coast of New Zealand’s North Island, has been present on the banks of the Waiuku Estuary for over 30 years.  But it was in 1769 that these sands were first noted by Captain James Cook who called the area “the Desert Coast”.  Those who ventured into the area were intrigued by the fact that the sand appeared magnetic and questioned whether the material could have commercial value as raw materials for a local iron and steel industry.  Nearly a hundred years later in 1850, pioneer experiments to smelt iron from the ironsands were carried out, but it was not until 1965 that the first steel was produced by “New Zealand Steel”.  The association with Broken Hill Proprietary Company Limited (BHP), one of the world leaders in best practice steel making, began in 1989.  Today, New Zealand Steel operates as the world’s only producer of steel from ironsand.

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Project Summary

(Kombiniert Oxygen Bottom blowing Maxhűtte)

The K-OBM is a steel-making vessel with a steel shell lined with refractory bricks.  The vessel is used to convert batches (heats) of hot metal (impure iron) and steel scrap into steel by blowing oxygen through the bath.  The oxidation of elements in the hot metal and scrap increases the temperature of the metal while changing its composition to that of refined steel.

The end point temperature of each heat is sampled by submerging a disposable thermocouple in the steel.

The purpose of the project is to improve the prediction of the end point steel temperature, by predicting the energy lost from the molten bath to the K-OBM vessel during each heat. This prediction will need to encompass the thermal history of the vessel between successive heats and sequences, and the lining wear through a vessel lifetime. An additional outcome may be a better prediction of the heating time required to bring a new vessel to the operating temperature.

Data: 

• The initial temperature of the hot metal is about 1350ºC, and the scrap is about 20ºC.  The end point steel temperature is in the range 1680 - 1730ºC.
• The weight of hot metal and scrap is about 81 tonnes.
• Approximately 3 - 4 tonnes of lime (CaO) is added to each heat to form a protective slag.
• Each heat cycle (i.e. charge, oxygen blow, turndown, possible re-blow, and tap) takes about 40 minutes.
• The vessel sits idle for 5 to 15 minutes between heats.
• A set of consecutive heats is called a sequence, A sequence can be from 1 to 20 heats.  The vessel is empty for between 45 minutes and 10 hours between sequences.
• When empty, the vessel can have different conditions, e.g. mouth cover on or off, and different cooling gas flows through the oxygen injection ports.
• Refractory lining thickness is measured with a laser scanner about every 74 heats.  When new, the refractory lining is between 550mm and 900mm thick.  When worn out, it is between 50mm and 400mm thick.  A lining lasts about 1000 heats.  (This will be a key factor).
• Brick hot face temperatures can be measured with a pyrometer between sequences (not routinely measured).
• A mass of slag may be held in the vessel between sequences.
• The dimensions of the vessel, and the thermal properties of the refractory bricks are available.

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Background on the K-OBM

(Kombiniert Oxygen Bottom blowing Maxhűtte)

The KOBM is a refractory lined vessel in which hot metal from the Vanadium Recovery Unit (VRU), and occasionally directly from the melters, is refined.  A computer model is used to optimise the composition and temperature of the steel.  Compositional changes involve the removal of carbon, silicon, manganese, phosphorus, titanium, sulphur and vanadium from the metal bath.  The oxidation of elements and dissolution of sulphur into lime are the major mechanisms used to facilitate this process.  The final steel temperature is determined by regulating exothermic reactions in the hot metal and by cold scrap metal addition.

The vessel, which can rotate through 360º, has an opening at the top (the mouth) and an upper side tap hole.  There is a capacity of up to 75 tonnes of hot (T ≈ 1350˚ C) metal that is loaded (charged) through the mouth by overhead crane.  In addition to this hot metal, 5 to 7 tonnes of scrap is added to increase the yield.

Oxygen is blown through the molten metal bath by five bottom-mounted tubes, called tuyeres  (f ≈ 5500 m ³ /hr) and a single top lance (f ≈ 9500 m ³ /hr).  This combination of top and bottom oxygen flow is where the K-OBM derives its name.

When an iron ladle leaves the VRU the chemical composition and temperature of the hot metal is measured.  From this hot metal information the K-OBM computer model can optimise refinement conditions, e.g. total oxygen required.  The oxygen blown through the metal oxidises C to CO and some Fe to FeO.  Elements in the hot metal, scrap and other flux and fuel additions are oxidised and melted to form SiO₂, MnO, P₂O₅, TiO₂, Al₂O₃ and V₂O₃.  These materials have a lower density than the molten metal and therefore separate out to form a layer on the metal surface termed slag.  Lime is added to produce a slag chemistry that facilitates hot metal dephosphorization and desulphurization.  The compositional changes of the major elements involved in the refinement process are presented in the following table: 

Table: Weight percentages of the hot metal and steel.

To refine one KOBM charge and tap the steel takes approximately 25 and 5 minutes respectively.  Therefore one refinement cycle is about 30 minutes.  Once a refinement cycle is completed the KOBM is rotated on its side and steel is tapped out through the side tap hole.  As the steel is tapped alloys are added to take advantage of the large mixing energy available.  The amount and type of alloys added depends on the steel composition desired.  The tapped steel flows into a refractory lined vessel termed a ladle.  This sits on a ladle car and is transported to the Ladle Treatment Station (LTS) where further alloy additions are made to finely adjust the steel composition before casting.

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