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So, in the last lecture, I have told you something about the alternative charge material in the
electric arc furnace. There I have said three different types of charge materials are there
in the practice. One is the hot metal, another is the directly reduced iron or sponge iron
or h b i, and third, I have said about the iron carbide. I have also mentioned, that
proportion of hot metal is not unlimited. Because, electric arc furnace traditionally,
was developed for melting unit. If we charge hot metal, since hot metal contains varying
amount of impurities, starting from carbon, silicon, manganese, phosphorus.
So, depending on the proportion of hot metal in the mix, one has to carry out refining;
that is, one has to carry out oxidation, one has to supply oxygen and hence the refining
time may increase with the proportion of increase in the hot metal and hence tap-to-tap time
may increase with the more proportion of hot metal in the feed. So, taking in to all these
aspects under consideration, a 30 percent hot metal is considered to be an optimum amount
for maintaining the reasonable tap to tap time, comparable with the 100 percent describe
practice.
Increasing amount of hot metal, though it can be used, but it will increase tap to tap
time and increase in tap to tap time will be associated with all disadvantages, like
increased refractory wear, increased electrode consumption, then increase electrical power
consumption and so on and so forth. Now, the, another charge material is the directly reduced
iron and that is what I will be telling you something about directly reduce iron today.
In fact, this directly reduced iron is produced by reduction of iron ore, either with coal
or with gaseous reducing agent or with gaseous reducing agent. So, the temperature in the
production of DRI rarely exceeds 1000 to 1100 degree Celsius. That means, we are not melting
any component of the iron ore, simply the oxygen content of iron oxide is getting removed.
So, whatever impurities will be in the iron ore, the same will be transferred into the
DRI. So, essentially DRI or h b i, both are the
same thing, DRI it contains, it contains free iron plus oxygen combined with iron, plus
carbon which is left, plus gangue minerals and these gangue minerals could be S i O 2,
could be A l 2 O 3, could be T i O 2 etcetera. That means, whatever present in the iron ore
is transferred into the DRI , because, no melting is being done. The reduction is carried
out in solid state. Therefore, it is important to characterize DRI. Characterize means - what
is the quality of DRI that is being used in the electric arc furnace as a substitute for
scrap? So, the quality of DRI, quality of DRI, first
important parameter is percent metallization, percent metallization and this percent metallization,
that is equal to free iron, free iron upon total iron into 100. That is the one important
characteristic of DRI before it is considered to be used as a substitute of a scrap in electric
arc furnace. The important thing to know for an operator or for the person who will be
using DRI is - what is the percentage metallization? Because any amount of oxygen that is left
with iron in DRI, it will be removed in the electric arc furnace. For example, if we have
say 90 percent metallization, then 90 percent is the free iron and 10 percent iron is in
the form of FeO or Fe 2 O 3 or Fe 3 O 4, whatever the form may be. So, it is in that reference,
the percentage metallization is an important component of DRI.
Second important thing is that, how oxygen is present. That is, either oxygen is present
as FeO or Fe 2 O 3 or Fe 3 O 4. Now, this is also important, that is, in which form
oxygen is present in combining with the iron, FeO, Fe 2 O 3 or Fe 3 O 4. Because, any oxygen
which is with the gangue mineral, it cannot be removed and it is not removed. That point
should be clear. What removal of oxygen we are talking about is a removal of oxygen from
Fe 2 O 3. So, if we say 90 percent metallization, that means, 90 percent is the free iron and
rest iron is either in the form of Fe O or in the form of Fe 2 O 3 or in the form of
Fe 3 O 4. Now, it is important to know in which form iron is, one and because, ultimately
when this DRI will be charged in the electric arc furnace, we will like to reduce oxygen
and get iron back as a yield. So, if the iron is present in Fe 2 O 3 or Fe 3 O 4, then higher
amount of energy is required, but if it is present in the form of Fe O again, higher
heat load is required because Fe O plus C is an endothermic reaction.
Now, let us calculate, say, what is the amount of oxygen that is present in DRI. So, let
us assume that, oxygen is present in the form of FeO, oxygen in DRI is present in form of
FeO. So, we can calculate now, a percentage FeO that will be equal to percent iron total
minus percent iron metallic into 72 by 56. Therefore, oxygen in percent, that will be
equal to 0.285 into percent FeO, because 16 kg oxygen is present in 72 kg of FeO, from
that we can get oxygen present is equal to this. Now, this is the oxygen present in the
form of FeO. Now, this oxygen removal also requires carbon. So, therefore, carbon in
DRI is important, because ultimately, this FeO will react with carbon and we will get
Fe plus CO and this reaction requires carbon. So, if we look at this reaction, then what
you see is that, 16 kg oxygen it requires 12 kg carbon. So, 1 kg oxygen requires 0.75
kg carbon. So, what I wanted to say from here is that, if carbon content in DRI is maintained
at 0.75 ratio to the oxygen of FeO, then carbon will react stoichiometrically with FeO during
electric arc furnace operation. That means what? If it reacts stoichiometrically, good,
it will reduce FeO and you will get the iron. It is a endothermic reaction, no heat will
be produced - that is one case. Now, if carbon content in DRI is more than the stoichiometric
amount, that means, if carbon content in DRI is greater than stoichiometric amount, amount,
what does that mean? There will be extra carbon left in DRI.
So, to remove extra carbon, I have to inject oxygen, and C plus O is equal to C O. So,
it will generate a large amount of heat; so, that is what is important. So, what is important
is that carbon, content in DRI that can be taken care of during the production of DRI,
it should be greater than the stoichiometric amount of carbon required to reduce FeO to
iron. So, if the carbon content in DRI is greater than the stoichiometric amount, then
extra oxygen has to be injected and on account of extra oxygen injection, the reaction between
carbon and oxygen will be producing carbon monoxide. So, here two advantages: one this
is slightly exothermic reaction, it will produce the amount of heat, and second, the carbon
monoxide bubble which has been formed, they will also help in foaming of the slag.
So, this particular carbon content in DRI is greater than the stoichiometric amount,
will need extra amount of oxygen to be supplied and that helps in two ways, one way is you
produce heat through C plus O is equal to C O reaction, second the C O bubble, it induces
foaming in the slag. So, these are the advantages of having carbon content in DRI greater than
stoichiometric amount. So, what this discussion leads to, that quality of DRI, there are certain
important thing. First important is that metallization and second important thing is the carbon content
in DRI. Now, remember at this point of time, when I am talking of the oxygen content of
DRI, then I only mean, that oxygen which is present with iron. Because, no oxygen which
is present in the gangue mineral is removed in the process of manufacture of DRI, though
this should be very clear.
So, now, let us see the effect of metallization. So, take a case for example, consider 100
kg iron ore, say 56 percent iron. So, it has around 80 percent Fe 2 O 3 and 20 percent
gangue. 80 kg Fe 2 O 3 and 20 kg gangue. So, I carry out now, metallization. I produce
now, DRI from this particular iron ore. Then I calculate some kg iron. Then kg iron which
is combined with FeO and kg gangue. So, if the metallization, for example, is 80 percent,
then 45 kg is the iron, 11 kg iron is present as FeO and gangue will of course, will be
20 kg. If I go for 85 percent metallization, then
I have 48 kg iron, 8 kg Fe to FeO and well of course, gangue will remain the same. Similarly,
90 percent metallization and 95 percent metallization, here kg free iron will be 50 and kg free iron
will be 53 kg, here it is 6, here it is 3, here it is 20, here it is 20. Because, whatever
amount of gangue is there, it will transfer to DRI. So, in the amount of gangue nothing
will change. So, what is important to see from here? If I increase degree of metallization,
what it is doing? It is increasing the free iron that is being charged with the DRI. That
is, first it will do increase in free iron. It is good, because, it will contribute to
the productivity directly. Second, it is decreasing the iron which is combined as FeO. That is
also good, because, less amount of energy would be required to reduce FeO by carbon
to Feplus C O. As a result, what will happen, less heat will be required during processing,
less heat will be required, productivity will be more. So, if I want to illustrate schematically,
if I take here productivity and plot it against percentage metallization, I go from 80 percent
to some where as 95 percent, then productivity will increase as the degree of metallization
increases, 80 percent to 95 percent. In order to produce a high percent metallization, the
investment cost is also important, because, it takes sufficient time and sufficient thermal
energy to produce highly metalize Wproduct, that is a very high quality product. when
you, besides that point, from our argument side, if we increase the metallization in
our DRI, then our productivity is increased. Now, let us see, the another effect, for example,
for a constant degree of metallization, if we increase the proportion of a sponge iron,
that is I take 90 percent scrap, 10 percent sponge iron, 70 percent scrap, 30 percent
sponge iron, 60 percent scrap, 40 percent sponge iron, in that, I vary the proportion,
then let me see what effects it does on the various constituents.
So, what I am doing now, I am taking now, 100 kg sponge iron or DRI of 90 percent metallization.
Now, just a background calculation, you require around 132 kg of iron ore of 80 percent Fe
2 O 3 and 20 percent gangue. So, if you want to calculate again, what is the effect of
changing the proportion of DRI in the feed, on free iron Fe to FeO and the gangue, then
you have to perform the calculation by considering how much amount of iron ore is required, how
much amount of iron it has and how much amount of gangue it has. I have done this calculation
I am presenting here. So, I am taking a case, say scrap, then I
take sponge iron, then I take free iron, then I take iron which is present as FeO and the
gangue content. So, if I take now, 90 percent scrap, 10 percent sponge iron, 10 kg, free
iron will be 6.6 kg, 0.8 kg and 2.6 kg. If I take now, 70 percent scrap, 30 percent sponge
iron, I have 19.8 kg free iron, 2.4 kg as iron is FeO and 7.8 kg in the gangue. Now,
if I take 60 percent scrap, then 40 percent sponge iron, 26.4 kg is the free iron, 3.2
kg as the iron as FeO and 10.4 kg gangue. If I take now, 50 percent scrap and 50 percent
sponge iron, 33 kg will be free iron, 4 kg will be Fe FeO and 13 kg present as a gangue.
What this calculation indicates? Increase in proportion of sponge iron, talking of the
same productivity, I have taken the same productivity. So, increasing proportion of a sponge iron,
what is doing, first free iron is increasing. This is good. No problem. Second effect, iron
which is present as FeO is also increasing. It is not good. Why it is not good, because
FeO plus C is an endothermic reaction. So, more amount of heat you require, to reduce
FeO to iron, because you do not want to lose FeO in the slag. You have an option, you can
lose FeO also in the slag, but you do not want to do it, because it contains iron.
So, Fe as FeO is also increasing as you see. Fe FeO, from 0.8 kg it goes to 4 kg. So, third
thing is that, gangue content is also increasing. Gangue content is increasing in the mix that
you have prepared; that means, if you take 50 percent scrap and 50 percent sponge iron,
13 kg is the gangue. If you take 90 percent scrap, 10 percent sponge iron where 2.6 kg
is the gangue material. Gangue content is increasing means what? More slag volume. More
slag volume means what? More heat load, because you have to supply extra amount of energy.
So, what is the consequence of it? The consequence of it, one, increase in the proportion of
a sponge iron in the feed, it is increasing Fe which is combined with FeO as well as the
gangue, so, therefore, the tap to tap time will be influenced. For a small proportion
of sponge iron, say 10 percent or let us say between 20 and 30 percent, the tap to tap
time may decrease, because, that extra load is not that much. But beyond that, the load
will be too much and tap to tap time will also begin to increase, instead of decreasing.
It appears, as if there is an optimum percentage of sponge iron there, tap to tap time should
also be decreasing. Increasing tap to tap time, by increasing the proportion of sponge
iron is not very good, because if the tap to tap time increases, electrode consumption
will also increase, refractory wear will also increase and productive will also be affected.
So, these are the, some of the calculation that I thought I will illustrate.
Now, the next thing is that, the charging methods. So, let us see now, the charging
methods. Say one can have batch charging; that means, the DRI is charged in batches.
So, batch charging is rather good for small capacity furnaces, for small capacity furnaces, say less than 5 tons
capacity and this batch charging is also preferred up to 25 to 30 percent DRI. Any increasing
amount, one has to see another method. So, another method is continuous charging. Continuous
charging is done for furnaces of 10 tons capacity or more than 10 tons, or more than 10 tons
capacity and this continuous charging has certain advantages. So, the advantages of
continuous charging, you can think of, we have the roof of the electric arc furnace,
where three electrodes are inserted. Still there is space on the roof and that
space can be used for charging sponge iron continuously as and when it is required in
the electric arc furnace. So, you can think of the advantages, because now, you do not
need to open the roof for charging, as you, yes, you have to do for batch process of charging.
So, the first advantage is less power off time, because now, you do not need to switch
off the power while charging, as in case of batch charging. Advantages are there, less
power off time. Second, heat losses resulting from delays are eliminated. It is obvious,
also is it not? Because, what you are doing? You are not removing the roof as you did for
batch charging. So, whatever the heat losses, which are there during charging, it is not
there. The delay is also not there, because the process is going on and charging is also
going on. So, that way heat losses resulting from delays are eliminated.
Third, the consequence of this is to lower electrical losses. Fourth, overlap of charging
and refining. What does it mean; that means, both charging and refining can be carried
out simultaneously. Because, the process is on, charging is continuous. So, charging and
refining are overlapping in each other, that is good. Fifth, say, reaction between carbon
and FeO of DRI is taking place during the charging of sponge iron. So, as a result this
reaction produces heat. This reaction produces strong carbon boil during charging. Now, remember,
this reaction between C and FeO, it takes place during charging and the C O which results,
it creates a strong bath agitation in the bath.
So, this carbon boil, it improves heat transfer and slag metal mixing. These are the certain
advantages associated with continuous charging. And now, it is obvious also, now anything
which is continuous, all the delays which are associated with the batch, they are eliminated
and hence the advantages are obvious. Now, say little bit about the Indian condition.
So, under Indian condition, the power consumption is around 600 kilo watt hour per ton with
heavy melting scrap. So, if 40 to 50 percent of scrap is substituted by sponge iron, then
power consumption will go up. Say, I have roughly a figure of 10 percent. Now, this
is mainly because, you have introduced extra gangue content, you require to remove oxygen
of FeO and because of this, the power consumption also increases.
Now, in fact, electric furnace can be charged with 100 percent sponge iron also. Now, this
is only possible for ultra-high power furnaces. So, this is about the assessment of DRI as
a feed for electric arc furnace. Now, third is use of Iron Carbide, which is Fe 3 C. Now,
this Fe 3 C, it contains 6 percent carbon. The melting point of Fe 3 C is 1837 degree
Celsius. It is a very high melting point. So, it can also be used as a alternative charge
material, but certain precautions are to be exercised during charging. First of all, say
Fe 3 C, is charged below the slag layer. Why, because then, it can dissolve.
So, with this Fe 3 C, it dissolves in slag, it dissolves and carbon reacts with oxygen.
It will give two benefits, one saving in power and another benefit is because carbon monoxide
is evolving through the charging of Fe 3 C. So, it will also contribute to foamy slag
practice. So, this is about the alternative charge materials. In the future, the proportion
of sponge iron, hot metal or Fe 3 C will increase and its increase would depend upon what is
the quality of scrap available and what is the quality of steel that is desired. So,
that is all about the alternative charge material.
Now, the next topic that I want to tell you, is about stainless steel making. Now, some
of the manifestation of a stainless steel is that, the stainless steel denoted by S
S. It contains 10 to 30 percent Chromium. Minimum 10 percent chromium to maintain corrosion
resistance. Second, stainless steel also contains varying amounts of
alloying elements like nickel, molybdenum, vanadium, titanium etcetera. Now, these alloying
elements are added in order to get some special property. Now, certain types of stainless
steel, one, austenitic and this austenitic steel composition, a rough idea I am giving
of composition, 18 percent chromium, 8 percent nickel, carbon may vary from 0.03 to 0.15
percent. So, remember, the austenitic steel is characterized
by low carbon and high chromium. That is important for the lecture which is coming now. Another
grade is ferritic. Now, ferritic grade has 12 to 30 percent chromium, 0.08 to 0.12 percent
carbon. Again low carbon, high chromium. Third grade is Martencitic. They have approximately
12.7 percent chromium. The carbon content, it may go from 0.15 to 0.25, but certain grades
have carbon in between 0.6 to 0.95. Another variation is duplex steels. Now, in the duplex,
((when)) chromium is around 25 percent. Then we have a still another variety - the precipitation
hardening steels, and they have say 18 to 20 chromium, plus copper, titanium, aluminium,
plus 8 to 10 percent nickel. They are highly specialized steel and used for certain specific
purposes.
Now, say, about the production of stainless steel, electric arc furnace is used for production
of stainless steel through melting stainless steel scrap. Hardly refining was required,
because stainless steel scrap was available of the composition which is required for the
product, so, melted and stainless steel of that particular rate is there. But again,
over the years, what was happened, shortage of high quality stainless steel scrap
has required to find out alternative charge material, so that, the quality of the steel
remains as it is required. So, again the problem of having the shortage of high quality stainless
steel scrap. So, this require alternative charge material. Now, what are these alternative
charge material for production of a stainless steel? One, carbon steel scrap. Then, of course,
you have a stainless steel scrap, then, in order to meet the chromium content, you have
to charge Ferrochrome. Now, there are, three grades of ferrochrome
are available. One grade is low carbon ferrochrome and this low carbon ferrochrome contains approximately
0.1 percent carbon. Then we have medium carbon ferrochrome and this grade contains around 2 percent carbon.
Then, third grade, we have high carbon ferrochrome and this may have approximately 7 percent
carbon. What is the problem in this? The problem is very simple. You are charging carbon steel
scrap, stainless scrap, you mix and calculate the amount of chromium is far less. Then you
have to add the charge material which contains the chromium; that means, ferrochrome is to
be produced. You can argue, sir, let us use low carbon ferrochrome, fine.
The production of low carbon ferrochrome is very expensive as compared to production of
high carbon ferrochrome. If you rate the ferrolite, production of ferrolite, you will recall from
there, that production of low carbon ferrochrome is very expensive as compared to production
of high carbon ferrochrome. As a technologist, as a producer of a stainless steel from electric
arc furnace, you will love or you will like to use medium or high carbon ferrochrome if
the technology exists. Otherwise, you have no choice except, except to use low carbon
ferrochrome and to see what is the cost of the stainless steel, that is coming out of
the furnace, because the cost of production of ferrochrome will also be added up in the
cost of the stainless steel. So, this is the main issue, whether we go
for low carbon or high carbon or whatever type of ferrochrome. The another alloying
element is a primary nickel, you also required to it, because certain grades which require
nickel, you have to add nickel. So, what is become important now, for production of stainless
steel is that, refining became essential to produce stainless steel in electric arc furnace.
As you have noted, the different grades of stainless steel require carbon very low. So,
if you use high carbon or if you use carbon steel scrap, then carbon content of the bath
is very high. So, you have to decrease the carbon; that
means, you have to refine. So, the refining became essential and the refining of a stainless
steel essentially consists of carbon and chromium and to obtain the, both carbon and chromium
up to the specification. What is the sign says? The sign says if you have a melt which
contains iron, carbon and chromium and if you supply oxygen, for removal of carbon,
if you see the Ellingham diagram, you will note that, the oxidation of carbon begins
only at high temperature.
So, unless you have a very high temperature, the carbon oxidation will not proceed. So,
let us see, now, what is the underlying thermodynamics of decarburization of chromium melt. So, thermodynamics
of decarburization of chromium melt. What we are doing now, we are trying to arrive
the conditions, under which carbon oxidation will proceed or chromium oxidation will proceed.
So, chromium oxidizes to Cr 2 O 3or Cr 3 O 4. Now, let us take it, it oxidizes to Cr
3 O 4 and we write down the reaction, say Cr 3 O 4 plus 4 C that is equal to 4 C O,
the gaseous phase plus 3 chromium. Now, if you write down the equilibrium constant
k, that will be equal to h 3 C O h c and activity of Cr 3 O 4. Remember, again the liquid steel
we have, Henry’s, follows Henry’s law. So, accordingly we put, for example, certain
assumption, let us take activity of Cr 3 O 4 is equal to 1, h chromium that is equal
to f Cr into weight percent chromium and h C that is equal to h C into weight percent
carbon. So, if we replace h 3 C r h c r, an activity of C r 3 O 4 by 1, then we get k
that is equal to f Cr W chromium 3 p C O to the power 4 and f C into weight percent carbon
to the power 4. Here also f C to the power 4 and activity h Cr 3 O 4 is equal to 1. Now,
say f C and f Cr, they are activity coefficient of carbon and chromium in the melt.
Now, this is a very complicated relationship. Now, say Hilty and Kaveney have proposed the
following equation. Log weight percent chromium in the metal upon weight percent carbon in
the metal, that is equal to minus 1 3 8 double 0 upon T plus 8.76 minus 0.925 log of p C
O.
Now, this equation is valid, when there is no nickel. T is to be substituted in degree
Kelvin. Now, in presence of nickel, the following equation is to be used. Log weight percent
chromium melt upon weight percent carbon in melt, that is equal to minus 13800 T plus
4.21 weight percent nickel plus 8.76 minus 0.925 log of p C O.
Now, why it is though? So, nickel increases the activity coefficient of carbon
in the melt. Now, what we can do now, we utilize these two equations and we can calculate the
ratio of percentage chromium upon percentage carbon in the bath as a function of temperature,
as well as, as a function of C O. To illustrate what will happen if we increase the temperature
or we decrease partial pressure of carbon monoxide and this we will be taking in the
next lecture.