Metal Production- Oxygen Steelmaking continued

Question 1

Where can refining reactions occur

Answer 1

Simultaneously or in a sequential fashion.
Within the oxygen jet
Under the jet
In the slag metal emulsion
In the slag foam
At the slag iron interface
On the refractory walls

Question 2

What are the kinetics of reaction influenced by?

Answer 2

Conditions of slag formation during the oxygen blow

Question 3

Desiliconisation- when it occurs, reactions, info

Answer 3

First element to be removed to the slag.
Si+2O->SiO2
Si+2FeO->SiO2+2Fe
SiO2 has very high free energy of formation so provides a lot of heat.
Si removal first and P removal does not start until all the Si is oxidised

Question 4

Decarburisation- when, reactions, info

Answer 4

Once Si removed, C removal can occur. Occurs by different mechanisms depending on the process (LD or Q-BOP).
LD: C+(FeO)sl -> Fe+CO
Occurs by emulsification of Fe droplets in the slag foam. Kinetics can be fast because if large surface area of the droplets.
Q-BOP: C+1/2O2 -> CO
Direct oxidation by oxygen in the tuyere region (less dependent on foam)

Question 5

Rate of decarburisation vs time and the regions on the graph

Answer 5

Starts at origin. Linear up is region A. Then horizontal for a while is region B then diagonal back down the x axis.
Region A is SiO2 formation and slag build up.
Region Zn is metal emulsion formed in slag droplets. Retention time about 5mins. Bigger the droplet the more carbon it contains

Question 6

% of element vs amount of oxygen blown (time) graph for C, S, P, Mn, Si

Answer 6

C starts just below 5% and flat for a very little then curve down to linear decrease. S stays pretty flat throughout maybe decreasing a bit. P fluctuates down then up then down. Mn stays fairly flat with bit of fluctuation. Si starts lowest and decreases exponentially quickly to zero.

Question 7

Temperature of liquid metal and slag and how they progress for BOS

Answer 7

T of liquid metal increases from 1250-1450C at beginning to 1600-1680 at end of blow. T of slag can be as much as 300C higher than metal in first few minutes of blow. At end of oxygen blow the difference is only about 50C

Question 8

Formula for slag basicity

Answer 8

%CaO/%SiO2 (optical basicity)

Question 9

Slag temperature compared to steel temperature in Q-BOP

Answer 9

Slag T lower than steel T

Question 10

What type of compounds do we want to minimise that form in the slag?

Answer 10

Fluorides like CaF. They are dangerous as can lead to HF formation

Question 11

Removal of manganese- reactions and when they take place

Answer 11

1) Mn+O->MnO (direct)
2) Mn+FeO->MnO+Fe (indirect)
First reaction takes place at beginning of blow when man concentration is high and metal temperature low. Major part of refining occurs via reaction 2.

Question 12

Conditions that favour phosphorous removal

Answer 12

In general high basicity and low slag temperature favour P removal. Fluxing agents are added to decrease slag viscosity and if lime is injected the reaction rates can improve. Slag formation is also key to P removal and seems to be favoured if the steel has low Si content

Question 13

Reaction for removal of phosphorous

Answer 13

2P+5FeO+3CaO->3CaO.P2O5+5Fe
Oxidising potential of FeO needed to oxidise P.
CaO needed to tie-up P2O5 and prevent P reversion.

Question 14

How does LP or Q-BOP influence P reversion?

Answer 14

LD slag has comparatively low basicity due to SiO2 in slag and so is possibility of P reversion (back from slag to metal).
In Q-BOP we have a very basic slag and there is less chance of reversion.

Question 15

Problem with removing sulfur

Answer 15

Ideal conditions for S removal are reducing conditions which is the opposite of the conditions in BOS steelmaking. So S removal occurs in torpedo ladle or more usually in secondary steelmaking.

Question 16

When can some desulfurisation happen?

Answer 16

In conjunction with de-C and de-P.
Direct oxidation: S+O2->SO2
Slag metal interface: S+O2–>S2- +O

Question 17

Reactions for de-S prior to BOS

Answer 17

CaC2+S=CaS(sl)+2C
Mg(g)+S=MgS(sl)
Na2O+S=Na2S+O
CaO+S=CaS+O

Question 18

Problems with H and N in steel

Answer 18

They have potentially negative effects on the properties

Question 19

How are N2 and H2 removed from the converter and how do H and N contents vary during oxygen blow?

Answer 19

Removed by CO.
In the ladle, H content is around 5-10ppm
Higher H content for Q-BOP than LD and increases during oxygen blow.
Initial N contents between 12-18ppm.
At tapping N content has increased to 25ppm

Question 20

Problems with not getting the process right first time

Answer 20

Has implications further downstream.
Time delay
May affect another parameter
Increase in iron oxidation and so reduction in yield
Increase in refractory wear

Question 21

What happens during tapping?

Answer 21

Steel is tapped (turn-down) into a ladle and then taken off for secondary steelmaking. Most of the slag is also tapped off but some remains in the BOS vessel

Question 22

What is done with the remaining slag in the BOS vessel?

Answer 22

Slag washing: the vessel is rocked from side to side to coat the refractory lining with slag.
Slag splashing: a jet of nitrogen blows the slag around the vessel causing it to adhere to the vessel walls.
This increases lifetime of refractory lining to over 60,000 heats.

Question 23

Compare LD with Q-BOP for de-Si

Answer 23

De-Si: LD has very fast process and highly exothermic.

Question 24

Compare LD with Q-BOP for de-C

Answer 24

LD: indirect oxidation via metal emulsion.
C+FeO=Fe+CO. Si to SiO2 slag formation slower than in Q-BOP. O in metal higher, FeO in slag higher. Steel needs more degassing.
Q-BOP: direct oxidation via tuyeres.
2C+O2=2CO. Initial slag formation period shorter than LD. O in metal lower, FeO in slag lower

Question 25

Compare LD with Q-BOP for CaO

Answer 25

LD: lime added from top. Is more porous and promotes more rapid dissolution.
Q-BOP: lime injected. More rapid slag formation than in LD process.

Question 26

Compare LD with Q-BOP for de-Mn

Answer 26

LD: Mn+FeO(sl)=MnO(sl)+Fe
%Mn increases then falls because CaO dissolution is slow and so slag formation takes time.
Q-BOP: constant %Mn which then decreases because rapid formation of (CaO+FeO) slag

Question 27

Compare LD with Q-BOP for de-P

Answer 27

LD: 2P+FeO+3CaO=3CaO.P2O5+Fe.
Does not occur until all Si is oxidised. Slow slag formation so lower basicity tends towards P reversion. Higher P content in metal.
Q-BOP: slag formation faster and more basic slag formed so there is less tendency towards P reversion.