Metal Production- Ironmaking

Question 1

What factors must be considered as they will effect the viability of the process?

Answer 1

Thermodynamics
Kinetics
Ease of separation
Refractory wear
Environmental aspects

Question 2

What are the environmental aspects to consider?

Answer 2

Removal of gaseous emissions
Waste removal (some are byproducts not waste)
Health and safety legislation

Question 3

What is ironmaking?

Answer 3

Reduction of iron oxide with a reducing agent and heat. A complex process involving chemical reactors with heat and mass transfer

Question 4

Four main ingredients for ironmaking (very general)

Answer 4

Ore
Flux
Reducing agent
Oxygen

Question 5

What is the ore?

Answer 5

Contains the iron in the form of iron oxides, Fe2O3 and Fe3O4. Iron oxide content varies with source

Question 6

What is the flux and what is it for?

Answer 6

CaCO3/CaO primarily added to remove the waste material from the molten metal to the slag. Sintered together with the iron ore before use.

Question 7

What is the reducing agent?

Answer 7

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

Question 8

What does oxygen do?

Answer 8

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

Question 9

What does sintering produce?

Answer 9

Strong but porous particles of feed ore and flux with a large surface area to volume ratio.

Question 10

How is sintering done (basic)?

Answer 10

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

Question 11

Why sinter?

Answer 11

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

Question 12

How does sintering work (more detail)?

Answer 12

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.

Question 13

Ratios of ingredients for sintering

Answer 13

42% iron ore and limestone
42% return sinter
5% coke
2% flue dust
9% water

Question 14

Ore and limestone composition ratios for sintering and what form (and size) is it in?

Answer 14

Typical ore composition 45% Fe2O3, 45% Fe3O4, 10% SiO2.
In particulate form about 12mm diameter.
Limestone added to give CaO/SiO2 ratio of 1

Question 15

What is return sinter?

Answer 15

Previously processed material unsuitable for blast furnace

Question 16

Products of sinter bed, ratios

Answer 16

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%)

Question 17

Other benefits of sintering

Answer 17

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.

Question 18

Arrangement of blast furnace

Answer 18

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.

Question 19

Why does the furnace diameter change?

Answer 19

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.

Question 20

Basics of operation of blast furnace

Answer 20

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.

Question 21

The three zones of heat transfer in the blast furnace

Answer 21

Top zone until half way down the stack.
Middle zone (3-4m) or thermal reserve zone.
Bottom zone

Question 22

Top zone of blast furnace temperatures and reactions)

Answer 22

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.

Question 23

Bottom zone of blast furnace (temperatures, reactions, movement of heat)

Answer 23

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.

Question 24

Mid zone of blast furnace (temperatures, equilibrium)

Answer 24

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.

Question 25

What is the main reduction reaction and what does the CO2 formed do?

Answer 25

FeO+CO->Fe+CO2
Mainly occurs in thermal reserve zone.
But below the thermal reserve zone, the CO2 produced by this reaction comes into contact with unreacted C in the feedstock and so:
CO2+C->2CO
This is the Boudouard reaction

Question 26

Reduction reaction if hydrogen was used

Answer 26

FeO+H2->Fe+H2O

Question 27

Temperature depth profiles for gas and solid temperature down the blast furnace

Answer 27

Temperature across depth going down. Solid curves down from low T (increases in T) to 1000C in top zone. Stays at 1000C down thermal reserve (mid) zone. Curves down exponential decay (increase in T) to max T (below 2000C, maybe 1400C) at bottom of bottom zone.
Gas has same shape but starts at higher T (below 1000) and less steep exponential decay so ends at 2000C.