Metal Production- Ironmaking Flashcards
What factors must be considered as they will effect the viability of the process?
Thermodynamics
Kinetics
Ease of separation
Refractory wear
Environmental aspects
What are the environmental aspects to consider?
Removal of gaseous emissions
Waste removal (some are byproducts not waste)
Health and safety legislation
What is ironmaking?
Reduction of iron oxide with a reducing agent and heat. A complex process involving chemical reactors with heat and mass transfer
Four main ingredients for ironmaking (very general)
Ore
Flux
Reducing agent
Oxygen
What is the ore?
Contains the iron in the form of iron oxides, Fe2O3 and Fe3O4. Iron oxide content varies with source
What is the flux and what is it for?
CaCO3/CaO primarily added to remove the waste material from the molten metal to the slag. Sintered together with the iron ore before use.
What is the reducing agent?
Most common is carbon in the form of coal. However coal not suitable for modern blast furnace use so coke is used in coke ovens. Coke is coal with the volatiles burned away
What does oxygen do?
Added to the furnace to provide heat and also to combine with the carbon in the coke to form CO which is the actual reducing agent
What does sintering produce?
Strong but porous particles of feed ore and flux with a large surface area to volume ratio.
How is sintering done (basic)?
The finely divided mixture of iron ore, flux and return singer is packed together, ignited and air is sucked through the singer bed. This gas pulse is heated and passes through the singer bed causing reactions to occur. Done in a sinter plant
Why sinter?
To get efficient reduction, need a high surface area to volume ratio. If finely divided particles were put into the blast furnace, the high pressures would pack them together and constrict the flow of gases. Sintering gives us a product that is strong enough and porous enough for efficient reduction of the ore
How does sintering work (more detail)?
As hot gas pulse passes through sinter bed, the temperature of the surrounding solid increases. Reaction of lime, iron oxide and silica occurs to form mixed oxides called CaFeSiO4 at contact points. When temperature high enough, these oxides melt. When gas pulse has passes, the temperature falls and the oxides solidify and act as glue binding the particles together. Gas pulse takes about 15mins to pass through a standard sized sinter bed.
Ratios of ingredients for sintering
42% iron ore and limestone
42% return sinter
5% coke
2% flue dust
9% water
Ore and limestone composition ratios for sintering and what form (and size) is it in?
Typical ore composition 45% Fe2O3, 45% Fe3O4, 10% SiO2.
In particulate form about 12mm diameter.
Limestone added to give CaO/SiO2 ratio of 1
What is return sinter?
Previously processed material unsuitable for blast furnace
Products of sinter bed, ratios
25% volatiles and dust.
15% warm return sinter (too brittle for blast furnace use and is returned to sinter bed).
25% cold return sinter (further screening removes any more material less than about 25mm, again returned to sinter bed).
35% usable material greater than 25mm diameter goes to blast furnace (porosity about 50%)
Other benefits of sintering
Elimination of sulphur (very important later).
Removes CO2, SO2, H2O.
Heats up blast furnace feedstock.
Some reduction of iron ore takes place.
Savings in metallurgical coke.
Increase in efficiency.
Arrangement of blast furnace
Bottom 5-6m is hearth (12m wide). Next 3-5m is Bosch (diagonal wider). Top 17m is stack (diagonal thinner). Sinter and coke in from top at RT. Waste gas out at top at 300C. Air at 1200C enters low in Bosch region through tuyeres (inside is tuyere zone). Liquid iron and slag at 1500C exits bottom (side of hearth). 12000tpd of iron.
Why does the furnace diameter change?
As the solid passes down the stack it heats up and expands so diameter of stack increases. Melting occurs in Bosch region. This leads to reduction in volume so the furnace diameter decreases in Bosch region.
Basics of operation of blast furnace
Cold solid goes in at top, hot air fed in at bottom, molten iron and slag leave at bottom, warm air exits at top. Heat from the hot air is being transferred to the solid as you go down the furnace.
The three zones of heat transfer in the blast furnace
Top zone until half way down the stack.
Middle zone (3-4m) or thermal reserve zone.
Bottom zone
Top zone of blast furnace temperatures and reactions)
Solid and gas temperatures converge at about 1000C (about half way down stack). The C is unreactive at T less than 1000C so in upper heat exchanger coke doesn’t react. Get small amount of reduction of ore by any CO present through these reactions:
3Fe2O3+CO->2Fe3O4-CO2
Fe3O4+CO->3FeO+CO2
Fe2O3+CO->2FeO+CO2
As C unreactive CO content in top half of furnace is low so no Fe formed.
Bottom zone of blast furnace (temperatures, reactions, movement of heat)
Temperatures of solid and gas diverge (1400C and 2000C). Temperatures high enough so O2 reacts with C to form CO and heat (exothermic).
1/O2+C->CO
Above this get FeO+C->Fe+CO
Heat produced is what raises Tg to 2000C. Hot gas rich in CO rises up furnace and contacts falling solid. Heat transferred to falling solid so Ts rises and the important reactions can start to occur.
Mid zone of blast furnace (temperatures, equilibrium)
Solid and gas temperatures are equal. Varies with the burden but usually between 900 and 1050C. Zones somewhere between 1 and 4m high. In this thermal reserve zone where CO comes into contact with the FeO there is chemical and thermal equilibrium.
FeO+CO->Fe+CO2
Heat entering this zone from the hot gas is balanced by heat requirements of the solution loss reaction. Therefore no change in temperature.
