Pyrometallurgy of iron is still the most important pyrometallurgical process economically.

Transcription

1 1

2 Pyrometallurgy of iron is still the most important pyrometallurgical process economically. Prehistorically, iron was prepared by simply heating it with charcoal in a fired clay pot. Coke is coal that has been heated to drive off the volatile components. 2

3 The iron ore, limestone, and coke are added to the top of the furnace. Coke is coal that has been heated in an inert atmosphere to drive off volatile components (~ 80 90% C). Coke is the fuel, producing heat in the lower part of the furnace. Is also the source of the reducing gases CO & H 2. Limestone (CaCO 3 ) serves as the source of CaO which reacts with silicates & other impurities in the ore to form slag. Coke is used to satisfy these requirements Produces the heat Produces the reducing gas needed for the smelting of iron 3

4 Pig iron - High impure iron, which is very brittle. Pig iron contains ~4% C and significant levels of S & P Steel - Iron, containing controlled amounts of C ( wt%), and other alloying elements (but virtually no O,N,P,S) Blast furnace slag used for cement production and road construction. 4

5 5

6 6

7 Most rocks are composed of silica (SiO 2 ) and silicates (SiO 3 2- ) and are almost always present in the ore. These compounds don t melt at the furnace temperature and would eventually clog it up. An important chemical method to remove these is by use of a flux which combines with the silica & silicates to produce a slag. Slag is used in road making, and can also be combined with cement. The heat of the furnace decomposes the limestone to give calcium oxide (e.g. of a calcination reaction). CaO (a basic oxide) reacts with silicon dioxide to give calcium silicate. 7

8 95 meters high and 14 meters diameter Air is blown into the bottom of the furnace, and combusts with the coke to raise the furnace temp up to 2000oC : Additionally, significant amount of reducing gas is required increasing cost. 8

9 Coke reacts with oxygen to form CO (the reducing agent): CO is also produced by the reaction of water vapor in the air with C: Since this reaction is endothermic, if the blast furnace gets too hot, water vapor is added to cool it down without interrupting the chemistry. Also around 250 C iron oxides are reduced by CO(g) and H 2 (g): Molten iron is produced lower down the furnace and removed at the bottom. If iron is going to be made into steel it is poured directly into a basic oxygen furnace. Also called indirect reduction Reactions occur at the upper part of the furnace (T<1000 o C). Reactions produced CO 2. CO 2 cannot exist in the presence of C at high temp as it reacts with the carbon of the coke reproducing CO CO 2 + C = 2CO 9

10 In this case the reducer is coke carbon: FeO + C = Fe + CO CO is one of the products => direct reduction Also called direct reduction Reactions occur at the lower furnace where the blast air is introduced 10

11 200 mph = 322 kmh The overall reaction in the BF may be indicated as: Iron oxide ore + Gangue + Reducing agent (C) + Flux + Air = Pig iron (l) + Slag (l) + Gases (N 2, CO, CO 2 ) Primary reduction reactions are endothermic 11

12 95 meters high and 14 meters diameter The reactor vessel Stack: upper portion of the reactor: In the stack, hematite or magnetite is reduced to sponge iron (DRI-direct reduced iron) by the action of carbon monoxide. Bosh: lower portion of the reactor: This is the part of the furnace where the contents melt. This is where the slag is formed. Hearth: lower bottom of the reactor: Molten pig iron and molten slag segregate from each other and are tapped off separately. Slag is lower density than iron, and so it floats on the molten metal. 12

13 As charge descends toward the hearth, it undergoes a number of consecutive changes and transformations. Different processes occur at different levels of the furnace This is due to changes in temperature, composition, and pressure of gases as they ascend from zone to zone The nearer to the hearth, the higher the temperature and pressure of gases and the greater the content of CO Notes: 1. Reactions 7, 8 - SiO2, Si, and CaS form the slag. 2. Reactions 9, 10 - some of the P and Mn go into pig iron, some goes into the slag. 13

14 Hence evaporation of moisture from the charge starts as soon as it enters the furnace 14

15 The highest decomposition point is limestone at o C. Iron is reduced by successive separation of oxygen from the oxides Oxygen of the iron is taken away by the reducers, namely CO, C, H 2.. The principal reducer is CO CO 2 is evolved in all three reactions => indirect reactions Reactions possible in the upper zones of the BF (T < 1000 o C) 15

16 CO 2 cannot exist in the presence of C at high temp as it reacts with the carbon of the coke reproducing CO CO 2 + C = 2CO In the lower part of the furnace, coke lumps are immersed in molten slag which causes an intimate contact of the coke carbon with the slag oxides CO is one of the products => direct reduction Indirect reduction liberates heat while direct reduction absorbs heat Hence furnace should be operated in a manner minimizing the progress of the direct reduction 15

17 Coke lumps are the only solid material reaching the tuyere level At about 1m from the tuyere, there is no more oxygen available for carbon reaction Hydrocarbons can also be used as a fuel, and these react similarly to produce carbon monoxide and hydrogen. CH 4(g) + 0.5O 2(g) CO (g) + 2H 2(g) 16

18 Silicon is reduced in the blast furnace from silica contained in the slag by the solid carbon of the coke: Reaction proceeds at high temperatures (> 1050 o C) with the absorption of heat. Such temperatures only possible at the lower part of the furnace The greater the Si content in the iron, the higher the temperature in the furnace and the greater the consumption of coke per tonne of iron produced Manganese properties similar to iron. Reduction proceeds as follows: MnO is difficult to reduce hence this reaction goes on in the lower part of the furnace. Refer to Ellingham diagram Here it is reduced directly by the solid carbon of the coke: This requires much coke as with silicon 17

19 18

20 Pure iron melts at 1527 o C. If C is dissolved in it, melting point is lowered Iron carburization in the blast furnace is due to CO: 3Fe + 2CO = Fe 3 C + CO 2 The resulting Fe 3 C dissolves in iron and sharply decreases its melting 19

21 point 19

22 Slag is formed by the oxides of Si, Al, Mn, Ca, Mg, and Fe, which have failed to be reduced in the BF These oxides are hard to remove and are not done in the BF About half of the Mn is reduced and passes to iron while the unreduced oxides remain in the slag The limestone descends to the bottom of the blast furnace where it is thermally decomposed: CaCO 3(s) CaO (s) + CO 2(g) The basic calcium oxide produced in this reaction then goes on to react with acidic impurities in the iron: FeS (s) + CaO (s) + C (s) CaS (s) + FeO (s) + CO (s) CaO (s) + SiO 2(g) CaSiO 3(s) 20

23 Sulphur gets into the furnace with charged ore coke contains sulphur Both reactions absorbs great amount of heat and therefore more coke is required 21

24 Temperature of the iron, slag and top gas will rise and Si content of the hot metal will increase SiO 2 + 2C = Si + 2CO (Endothermic reaction) and occurs at high temperature. To lower the Si content in the hot 22

25 metal, reduce the coke rate to restore the heat balance. Production will increase but amount of C burned to CO would decrease resulting in a decrease in indirect reductions What would be the effect of reducing coke content has on indirect reductions as a result on increased hot-blast temperature? 22

26 The difficulties experienced by raising the hot blast temperature without increasing the hearth temperature (which causes Si content in hot metal) can be overcome by the injection of steam Steam reacting with coke is a strong endothermic reaction and it is only possible if the blast is given additional pre-heat 23

27 Natural gas or oil may be injected. These burn at the tuyeres to form a mixture of CO and H 2 By addition of hydrocarbon to the BF => less coke needed 24

28 25

29 Impurity levels must be lowered by oxidation 26