Metal Production- Oxygen Steelmaking Flashcards
Describe the resultant hot metal from the blast furnace
Contains about 4-5% C. Essentially a cast iron
What needs to happen to the hot metal from the blast furnace to produce steel?
The % C needs to be reduced to 0.05%.
Retain Fe, reduce amount of C, Si, Mn.
Control (eliminate) as far as possible S and P.
Definitions of low, medium and high carbon steel
Low: less than 0.04% C
Medium between 0.05 and 0.18% C
High: above 0.4% C
What is steelmaking?
Essentially an oxidation process to remove all the oxidisable elements from the hot metal and scrap to the furnace slag as the steel is decarburised with oxygen blowing
Reactions of C, Si, P, Mn and Fe in steelmaking
C+1/2O2->CO
Si+O2->SiO2
P+5/4O2->1/2P2O5
Mn+1/2O2->MnO
Fe+O2->FeO
All the Si, Mn and P oxides are fluxed with lime (CaO) to form molten slag.
What do the oxidation reactions of C, P, Si and Mn do?
They are exothermic so provide the necessary heat to melt scrap. Means don’t need to add more heat externally but means there is less control of temperature.
Why are there high reaction rates in steelmaking?
The slag forms a foam containing small metal droplets with a huge surface area to mass ratio
Why are highly basic slags used?
To reduce refractory wear and promote dephosphorisation
Features of the basic oxygen steelmaking vessel
Looks like a massive thick flask. Made of an outer steel shell and thick internal refractory lining. In bottom is molten metal. Water cooled lance comes down through top to blow oxygen onto the steel. At top is a water cooled fume collection hood with one exit for fluxes and coolants and another for converter fumes to cleaning plant. Quite high up vessel is horizontal tap hole. Whole vessel can be pivoted for pouring.
Steps in the BOS process
- Charge scrap (maybe 20%)
- Charge hot metal
- Add fluxes and oxygen blowing for 10-15mins
- Sampling
- Tapping steel (below slag) into new container and add alloy additions.
- Slag off into slag pot (pour out of opposite side to taphole
Control of the delivery of oxygen
Oxygen blow controlled by a sophisticated model depending on steel grade and amount of scrap etc. Dynamic control allows continuous adjustment of O2 delivery. Use a secondary (or sub-) lance which can sample and provide feedback and audiometry and waste gas analysis. This reduces analysis time but increases cost
Advantages of scrap and limit of use
Recycling and process benefits. Hot metal is expensive.
Amount limited in oxygen steelmaking in theory to less than 50% and in reality to about 20%. Otherwise not enough heat generated to melt the scrap
Describe the top blowing processes and potential problem
Oxygen blown down through lance onto metal at Mach 2. Bare metal is exposed. Fe droplets are dragged into slag producing an emulsion with a huge surface area to mass ratio. Slag is highly oxidising and gives good reaction rates with dissolved C, P, Si, Mn etc. Can get excess foam produced by the CO gas which can then flow over the sides. Need correct balance between too much and too little foam.
Factors effecting foam levels
- Rate of CO evolution (need to avoid excessive foaming).
- Energy of oxygen jet.
- Foam stability:
i) metal/slag interfacial tension
ii) presence of absorption film (surfactants like sulfur)
iii) slag viscosity (viscous film drains slowly)
Describe the bottom oxygen blowing processes and how it has changed
Various names like Q-BOP, OBM. Essentially an evolution of the Bessemer process which used air. Gases are blown in from the bottom of the vessel. Silicon oxidation causes temperatures in tuyeres zone to rise to 1900C. Used to to use CH4 decomposition to cool down. But this is still insufficient so was replaced by inert gases like Ar. Also CH4 can lead to high H contents in the steel.
