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Mod-01 Lec-05 Lecture-05-General Methods of Metal Extraction — Transcript

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0:00[Music]
0:20Well friends,
0:22I now come to
0:25module three of this lecture series.
0:30In module two, we ended up by discussing
0:35mineral beneficiation techniques
0:38which are used to prepare or
0:42into a product
0:45which will be acceptable to an
0:47extraction proc extraction process
0:50industry.
0:52I had mentioned that you cannot simply
0:54mine an ore and start working on it to
0:58extract metals. The ore has to be
1:01dressed properly. It has to be sized.
1:05Various minerals will have to be
1:07separated.
1:09The gang materials have to be rejected.
1:12And for doing these there are many many
1:15unit operations.
1:20Having done that, we now have
1:23a dressed ore from which we will have to
1:28have a process for extracting metal
1:31either as an elemental form or as a
1:34compound maybe more than one metal. And
1:37in this module we will discuss the
1:41principle of extraction
1:44of metals or compounds and the various
1:48refining techniques.
1:50The learning objective should be to
1:53understand why some metals exist in
1:55nature as stable complex minerals and
1:58some others as less compound or even in
2:01free state.
2:04Why do I mention this?
2:07Because unless we understand the nature
2:09of minerals
2:11which depend on the nature of
2:14reactivities of elements, we really
2:17cannot design an extraction process.
2:20We would also have to understand some
2:23thermodynamic and kinetic principles of
2:25heterogeneous reactions. Most reactions
2:28in extractive metal are heterogeneous
2:30means they involve more than one phase.
2:35There can be two phases. Sometimes there
2:36are three phases solid, liquid and gas.
2:40And then having done that we'll go into
2:43the principles of pyometry and
2:45hydromeology.
2:47I will discuss electromeology in the
2:49next module after several lectures. Why?
2:52I'll discuss it later. I will explain uh
2:56when I get there.
2:59Now before we start
3:03we need to know
3:08why not all metals are similar
3:12in reactivities.
3:14First of all you know that we make a
3:16distinction between non-metals
3:20and metals. All these are nonmetals. On
3:24the right hand side here all nonmetals.
3:28These are all metals
3:30almost 55 56 of them. There are many
3:33more because under lanthanum and actinia
3:38actinium series there are many many
3:39metals. So all these are
3:43metals excepting hydrogen which is has
3:45been put there.
3:49Now what is a metal?
3:52I have earlier mentioned to you that
3:55generally we say by the word metal
4:00something which has a very typical
4:01lusture
4:04it has good electronic electronic
4:06conductivity has mechanical properties
4:10like strength hardness malleability
4:12ductility etc.
4:15And a metal is a base. By the word base,
4:18I mean a metal
4:21reacts with acid.
4:25But not all metals. Those which are more
4:29metallic will react more strongly. There
4:32are some metals which have other
4:34properties but have no reactivity
4:37towards acids. So the metallic nature in
4:41terms of these properties which are kind
4:44of vague vary. There are also exceptions
4:49like arsenic
4:51and bismouth.
4:53They are metals
4:55but they are very brittle. So they don't
4:57have mechanical strength. On the other
5:01hand nonmetals like iodine and graphite
5:04they have lusture. You've seen graphite
5:07they shine
5:10and graphite also has a very high um
5:14conductivity but there's not but none of
5:17them is a metal and then you know
5:20diamond diamond is a nonmetal but it is
5:24the hardest substance now so the
5:27classification between metals and
5:28nonmetals is not so clearcut it's bit
5:32vague but basically we understand what
5:35what is a metal? What is a non-metal?
5:39But then not all metals are equally
5:43metallic.
5:45In the periodic table,
5:48what we see is that the metallic nature
5:53increases
5:54as we move from right to left
5:59and as we move from top to bottom.
6:05The metals in this area are much more
6:08metallic than the metals in this area.
6:12Like they will be much more reactive
6:16towards acids than the metals in these
6:19areas.
6:21And because they their reactivities
6:25differ,
6:27they form different kinds of compounds.
6:29the the metals in this area. The kind of
6:31compounds they will form will be
6:34different from the metals in this area,
6:37the compounds that will formed by metals
6:39in this area. Actually, you know that
6:42metals like gold, they don't form any
6:44compounds at all because they are so
6:45stable. Their reactivities are so small.
6:49Peings like lead its reactivity is
6:52small. It does it forms very weak uh
6:55compounds. So again
6:59metallic nature is not the same in all
7:02metals and their reactivities are not
7:05the same in all metals.
7:08Why I am saying it? Because the
7:12fundamental aim of an extraction process
7:15is to break a mineral,
7:18separate the metal from the non-metal
7:23with which it is generally
7:26in a combined state. And what are the
7:28nonmetals? It will be mostly oxygen,
7:31sulfur or halogen. So most minerals are
7:37compounds of metals
7:40with oxygen which means they're oxides
7:45or with halogen helides or sulfur and of
7:51course there are combinations of oxides.
7:55So there can be silicates, aluminum
7:56silicates etc etc. So the whole idea
8:00would be to break the compound and take
8:03the metal out. Now there are some
8:07compounds which are very weak compounds
8:09like cineabar
8:12mercury sulfide. All one has to do is to
8:15just heat it and mercury liquid will
8:18flow out because HG will get separated
8:21from sulfur. It's very easy. It does not
8:24need any reducing agent nothing. Only by
8:27heating we can break it.
8:29There are some which are very stable
8:31compounds like aluminum oxide L23.
8:34It's very difficult to break it. What we
8:37have to do is to dissolve it in some
8:40solvent
8:42and then electrolyze it at high
8:45temperature. So not only aluminina is
8:49electrolyed in a solvent by application
8:52of electricity, it also has to be done
8:55at a very high temperature because the
8:58solvent in this case would be cryolyte
9:00which is a a hallide
9:04and that that would need high
9:06temperatures to melt to start
9:09something like lead
9:12which which is which can be very easily
9:14converted to oxide. side very easily
9:17reduced by carbon because carbon will
9:20take out the oxygen and lead will be
9:22left behind. So these are the techniques
9:25of breaking
9:28the compound to liberate the metal and
9:30all extraction processes in a sense are
9:35processes of liberating
9:38the metal from its state of association
9:42with nonmetals like oxygen, sulfur
9:47and halogen.
9:52Again let me repeat that there are some
9:56minerals
9:58which are compounds which are very
9:59stable.
10:01There are some minerals which are very
10:04weak and very easily decomposed
10:07and that depends on the reactivities of
10:10these metals. Now how do we define the
10:14reactivities?
10:17A very common way of defining reactivity
10:22of a metal is in terms of electrode
10:27potential
10:29which you must have studied uh in course
10:33in thermodynamics.
10:37Electro potentials
10:39are
10:42the potential that is created
10:45between metal and metal ions in an
10:48aquous media and it is quoted with
10:52reference to the electro potential
10:54between hydrogen hydrogen ions.
10:57Now the hydrogen
11:00electro potential at 25°
11:04is defined as zero.
11:11It is at zero. There are metals
11:15whose electro potential is below
11:17hydrogen
11:19means these potentials are negative
11:23and there are good number whose electro
11:25potentials
11:27are positive and higher than hydrogen.
11:31What this means is is that these are
11:34more active than hydrogen.
11:39So if we have hydrogen ions in an acid
11:43so dilute H2SO4
11:46these
11:47elements will replace
11:51hydrogen liberate hydrogen and dissolve
11:53themselves. That is why zinc
11:56if you put zinc in an acid
12:00it will dissolve and hydrogen will be
12:02liberated. This is true with all of
12:04them. So all these are electropositive
12:09in the sense they are positive potential
12:13as compared to hydrogen. Whereas
12:18these elements my metals which have
12:22negative hydrogen potential will not
12:25liberate hydrogen from acids. You you
12:28have heard you have learned this in your
12:30thermodynamics course. So this electrode
12:33potential
12:34indicates
12:36relative reactivities of metals.
12:40Now I mentioned that an element placed
12:45higher compared to hydrogen will
12:47liberate hydrogen from an acid.
12:51Similar principle operate in the case of
12:55elements
12:56other than hydrogen. Like
13:01the ele the metal calcium
13:04should be able to liberate
13:07the metals placed below from their
13:11compounds
13:12and form a more stable compounds and
13:14liberate that metal.
13:17This will be the basis of metallothermic
13:19reaction to which I'll come later on.
13:23So what I'm trying to say is when you
13:25have a series like this which indicates
13:28relative reactivities
13:31we can
13:34say that the metals placed higher above
13:37being more reactive will replace those
13:40below that from their compound all or
13:43replace hydrogen from aquos media and
13:46dissolve themselves. Now there are other
13:49kinds of series I'm talking about aquas
13:50solutions. What about say such a series
13:54in helides?
13:56It is possible to have electro
13:59potentials of metal metal ions in molten
14:02chlorides with reference to chlorine
14:05chlorine
14:07electro potential
14:09and we'll have a series
14:12but it is may not be identical to this.
14:17the the very active reactive metals will
14:21still be placed high above in that but
14:24their relative the order may not be the
14:27same.
14:29The another thing that we uh criteria
14:32that we use to understand reactivities
14:34is free energies of formation
14:38of oxides, chlorides and other
14:40compounds.
14:43Metals which are placed high in the
14:47electro potential series who which are
14:50very act reactive they form very stable
14:55compounds and stable compounds will be
14:58characterized by
15:01more negative values for free energies
15:04of formations of their compounds. So you
15:07see for calcium oxide
15:10it's -254
15:13kilo calories per mole unit whereas for
15:17those which are placed below calcium the
15:20values are less negative means compounds
15:25of calcium would be more stable
15:27compounds of sodium would be less
15:30stable. Now again the relative the
15:34ordering of metals according to this
15:36criterion
15:38may not be the same as ordering here but
15:41generally those which are placed higher
15:43here will be placed higher here as well.
15:47Look at for example the stability of
15:49chlorides. You find here that this metal
15:54is placed higher in the chloride series
15:58whereas calcium is placed higher in the
16:01oxide series.
16:04The other criterion is electro
16:06negativity. I will not discuss that now
16:09because perhaps you will study it in in
16:12your thermodynamics course. But what you
16:14should understand from a series like
16:16this is that there is a very clearcut
16:20way of understanding
16:22which metals are more active as compared
16:28to the other metals in terms of values
16:31of electro potentials
16:34in ecosolutions as compared to hydrogen
16:37electrode
16:39or electro potentials in
16:43hallide melts in terms of the
16:45corresponding halogen electrode
16:49and also in terms of the free energies
16:52of formation of various compounds.
16:55Mostly we code values of oxide and
16:57chloride. There are some other ways of
17:00also um assigning relative reactivity
17:05but we need not discuss that.
17:10Now we have understood
17:13that not all metals
17:17are similar in terms of reactivity. And
17:20if they are not similar, what will
17:22happen? They'll obviously form different
17:26kinds of compounds.
17:30I have listed some characteristics here
17:34that we have seen metals with electro
17:38potential values
17:40range the the more reactive ones in this
17:44range one group then another group then
17:49another group and at the bottom are
17:51those where you see we got negative
17:54electro potentials and there we have
17:58some precious metals or absolutely inert
18:00metals like gold also.
18:03Now the most reactive metals are
18:06characterized by lower values of electro
18:09negativity,
18:14higher electro potentials
18:16and the free energies formations of
18:19their compounds like oxides and helides
18:21are more negative.
18:24The order according to these criteria as
18:27I mentioned again are not identical.
18:30But their modes of occurrence in nature
18:33will depend on their reactivities.
18:48We can make some general statements such
18:51as the following.
18:55Highly reactive metals
19:03which are characterized by factors such
19:05as high electro potentials and high
19:07negative free energies of formation
19:11of compounds occur in nature as simple
19:14compounds very often. Thus the alkali or
19:18alkaline earth metals are mostly found
19:20as chlorides, carbonates or sulfates.
19:25Simple oxides of these metals are
19:27however rare as they will react with
19:29other acidic oxides such as silica. So
19:32they will end up forming minerals such
19:34as aluminous silicates and silicates.
19:40And then we come to those which occupy
19:43intermediate position. We come lower
19:46down.
19:48There we find the metals principally
19:50found as silicates.
19:52Some of those which are placed higher up
19:54in the intermediate position may form
19:56simple oxides as well. Those occupying a
20:00lower position may be found abundantly
20:03also as sulfides and sulfides become
20:06more important from the point of view of
20:08winning of for metals placed in the
20:10lower half of the series
20:13and metals occupy very low positions
20:15show an increasing tendency to occur
20:18either as selenides
20:20arenides or antimonides or they may be
20:24available in the native state.
20:27Now all the metals that appear below
20:29lead in the first column in the table I
20:31had shown may be found in free state
20:36and this becomes the most state of
20:38common state of occurrence for silver
20:41and all metals placed below it.
20:45Now
20:50we I have given a table here to indicate
20:53this uh in little more detail.
20:57If you look at the first group which is
21:00the lithium
21:03led by lithium and electro potential
21:06value is 3.01 to 2.38.
21:11These metals forms chlorides carbonates
21:14sulfides they're easily oxidized in air.
21:18They gives very stable oxide reacts
21:20spontaneously with water and forms ionic
21:23compounds.
21:26If you come to the the metals below that
21:30values of electric potential 2.1 to 1.44
21:35they form mainly oxides silicates or
21:38complex oxides oxides on heating.
21:42They will not oxide in left in air. If
21:44they are heated they will form oxides
21:47and they will give stable oxides.
21:49They're attacked by steam. They will not
21:52be attacked by water. They will attack
21:54by steam.
21:56and they will form mainly ionic
21:58compounds again. Then you come to again
22:02another group lower down in electro
22:05potential values.
22:08These will form simple or complex oxides
22:11or sulfides. They will oxidize
22:15on strong heating
22:18and forms moderately stable oxides.
22:21oxides of less reactive metals are
22:25easily reduced
22:27and they're attacked by steam.
22:32Then we come to this group which are
22:35again
22:38less reactive with values of electro
22:41potential from 335 to minus.799
22:45they forms as I mentioned earlier
22:49selenides and arsenides in addition to
22:52sulfites and complex oxides forms
22:55relatively unstable oxides some of which
23:00decompose
23:01at high temperature like mercury
23:03sulfide. They are not attacked by steam
23:06in many cases show complex bonding in
23:10compounds. Lastly, we have the least
23:14reactive group of metals.
23:17They may occur as sulfides, telures or
23:20they may occur in free state. They'll
23:23give very less stable oxides,
23:27some unstable even at room temperature
23:29such as oxides of lead and gold. They
23:34are not attacked by steam or ordinary
23:36acides
23:39and they show complex bonding in salts.
23:42So you see the the way they exist in
23:45nature, the kind of compounds they form,
23:49the count of minerals they form, they
23:52all depend really on their relative
23:55reactivities and the and we measure
23:59reactivities in terms of electric
24:00potentials or free energies of
24:02formations of compounds.
24:05So the obviously we need to have
24:08different kinds of processes for
24:10extraction of metals from different kind
24:12of metals.
24:14Those which are form very weak compounds
24:17they'll be very easy to break and there
24:20it will be very easy to liberate the
24:21metal but there will be some where we'll
24:24have to have very special processes for
24:27liberating the material metal because
24:29it's very strongly bound with the
24:31nonmetal.
24:35Now
24:38the history of
24:43extractive metalology actually starts
24:45with pyomelogology because we have
24:47discussed how the ancients produce
24:50metals. It was always with the
24:52application of fire. Unless we leave
24:56aside what they found in the native
24:58state, the iron from the falling
25:01meteorites or gold that was found here
25:04and there. But when they started
25:07producing copper or bronze or zinc and
25:11then iron, it always was with
25:14application of fire. And many processes
25:18were developed by trial and error. And
25:22many we understood much later the
25:26science of it. We understood much later
25:29by analyzing what was being done and why
25:33it was being done. But our ancients did
25:36not go into science. They found somehow
25:39that this is how we should work to
25:42produce this metal or that metal. Now
25:46pyomeatology
25:48definitely have has some distinct
25:51advantages.
25:54First of all, compounds become less
25:57stable at high temperatures
26:00because temperatures
26:03create vibrations amongst atoms and
26:06molecules. So that that is how you make
26:09things unstable.
26:12And when you have made them unstable,
26:15you can use various techniques. You can
26:17apply electrical potential or you can
26:19bring in a reducing agent or whatever to
26:22take out the metal and separate the
26:24rest.
26:27There's another advantage of using high
26:28temperatures. At high temperatures,
26:31reaction rates are accelerated.
26:35You know many reactions are
26:40highly temperature sensitive.
26:43Give you an ordinary example. If sugar
26:45is not dissolving in in water, if you
26:48heat the water, the dissolution will be
26:50speeded up. Sugar will dissolve much
26:53faster.
26:54So the process of process of dissolution
26:57of sugar is temperature sensitive.
27:00We say such processes as thermally
27:04activated processes.
27:06And in pyometal energy, we use heat
27:11to accelerate processes by use of higher
27:16temperatures.
27:18Then there's a third advantage. When you
27:21go to higher temperatures,
27:24thermodynamically many reactions become
27:27feasible which were not feasible at
27:29lower temperatures. Means no matter how
27:32much of heat you give, you may not you
27:36cannot do that at lower temperatures.
27:38Even mercury sulfide cineabar Ags which
27:42is a very weak compound which
27:45dissociates from heating, it needs a
27:48minimum temperature. It will not
27:50dissociate at a at room temperature. It
27:54will dissociate when you go to couple of
27:55hundred degrees, 600, 700°. So I I don't
28:00know the exact figure. Then only it will
28:02decompose. Which means decomposition
28:06is made possible thermodynamically
28:11only at a particular
28:14temperature.
28:16Take simple example of say decomposition
28:19of calcium carbonate.
28:21Calcium carbonate on heating decomposes
28:25to calcium oxide and CO2.
28:28Now in theory
28:31as you keep increasing the temperature
28:33the partial pressure of CO2 increases.
28:38So it it's not that it suddenly
28:40decomposes. Like you should know water
28:43if you have water at room temperature it
28:46has a vapor pressure but we say boiling
28:49point of water is 100°
28:52when the atmospheric pressure is one
28:55because at 100° it's partial pressure
28:58becomes one atmosphere. Similarly,
29:00calcium carbonate
29:02will have some PCCO2 at lower
29:05temperatures, but it will really
29:07decompose with one atmospheric pressure
29:09only when you go to about 910° or so.
29:12You exceed 900°.
29:15So mercury sulfide
29:18at 100 200° may have a small
29:21vapor pressure of mercury but to really
29:25decompose it and get the metal in large
29:27quantities you have to go to a certain
29:29temperature. Similarly when we talk
29:31about reduction by carbon
29:34or reduction or some some process
29:36everywhere thermodynamics
29:39dictates the minimum temp temperature
29:42you need and many reactions become uh
29:45possible only when you
29:53when you go to a minimum temperature.
29:57The fourth advantage of pyometlogy is
30:00that at higher temperatures many phases
30:05become molten. Metal may will melt. Then
30:09the gang materials
30:12which are fluxed to make what we call
30:14slag will melt. So there's a clear
30:17separation between metal and slag.
30:21Consider for example steel making.
30:25In the blast furnace we charge iron or
30:28we charge coke. We also charge fluxes
30:34that is limestone
30:36quartz. So we form to start with calcium
30:42silicate phase. Many other gang
30:44materials go into that and then we form
30:48a phase called slag which dissolves a
30:53lot of impurities from iron so that we
30:56will finally get a metal which is
30:59relatively pure and the slag face takes
31:02out uh the gang materials. So we have a
31:06greater ease of separation of metal and
31:09slag at high temperature. These are the
31:11advantages of pyomelogy.
31:16Now in pyomelogy
31:19there are three terms which are very
31:22used very frequently and you should know
31:24what they mean.
31:27These are unit processes.
31:31Previously we have talked about unit
31:33operations where the nature physical and
31:38chemical nature of the mineral is not
31:40changed. By unit process we mean an
31:43operation where we change the physical
31:46and chemical nature of a process. For
31:48example,
31:50by calcination, we mean
31:53heating to decompose minerals
31:58to eliminate
32:00products such as CO2 or even water.
32:05So if you have calcium carbonate,
32:09we heat it to around 900°, it decomposes
32:12to produce calcium oxide and CO2 goes
32:16out. So we have produced calcium oxide
32:20from calcium carbonate by calcination.
32:24So that is the meaning of the word
32:25calcination.
32:27Do not think that if we have water
32:31molecules associated with a compound
32:35a mineral they will all go out at 100°.
32:39No. There are many minerals which have
32:44water molecules in them. Not all of it
32:46will go at 100 degrees.
32:49Actually, some may require several
32:51hundred degrees
32:54temperature rise to release that water
32:57molecule and that would also be under uh
33:00in a calcination process body completely
33:03can to eliminate all water.
33:06Now the calcination temperature of
33:10various carbonates like this. You have
33:14magnesium carbonate decomposes at 4 417,
33:21manganese carbonate 377,
33:24iron carbonate 400,
33:27hydrates like Al23 with XH2 or decompose
33:31at lower temperatures less than 700 for
33:34and so the kills have to be operated
33:36like that. So this is calcination.
33:43Then there is the word called roasting.
33:46The word roasting means
33:49heating the ore below its fusion points.
33:54You have to very careful not to fuse it.
33:57It's still solid but you heat it
34:01near below the fusing point
34:04so as to change its chemical nature.
34:08And a very common thing will be to heat
34:10the sulfides to convert the sulfide into
34:14oxide. Lead sulfide roasted to form lead
34:18oxide. Copper sulfide roasted to form
34:22copper sulfide
34:24etc etc.
34:26There can be other kinds of
34:30roasting also and some of the the first
34:33one I mentioned is oxidizing roasting
34:36where the idea is to produce an oxide.
34:39By the word volatileizing roasting, I
34:42mean that you allow a some part of that
34:48mineral to get separated through
34:51volatilization.
34:55And by chloridizing roasting we mean
34:59that we heat it below the fusion point
35:02using a chloridizing agent like
35:04hydrochloric acid or chlorine to produce
35:08either a liquid chloride
35:11or a gaseous chloride will take out some
35:15element from the aggregate of uh other
35:19elements
35:20they'll get separated out. So suppose
35:22you have several uh metal compounds. One
35:28is able to be chloridized. One forms
35:32chloride easily. So by reacting with the
35:34chloridizing agent we can separate it
35:36out as a liquid or as a gas. That we
35:40call chloridizing roasting.
35:44There can be other kinds of roasting.
35:47For example,
35:50reduction roasting.
35:52We may have
35:55a an or which has in it
36:00Fe23 as well as many other oxides.
36:03And if we reduce it partially and
36:08produce Fe304 in place of F23 then we
36:12would be able to separate it out by
36:15magnetic separation. This will be called
36:18reduction roasting.
36:20We can also reduce something to a lower
36:24valance state which will be more
36:26amendable to leeching by separation. So
36:30reduction roasting is when during
36:32heating you are bringing in a reducing
36:35atmosphere to go to a lower valance
36:38state of one particular metal. There are
36:41also other kinds of uh roasting
36:44reactions and we need not discuss all
36:46that now.
37:03There are some very interesting
37:09operations in pyometry
37:12and let me mention
37:15some examples.
37:18I had mentioned that there are some
37:20oxides and some some compounds which are
37:24very weak like here I have mentioned
37:27mercury sulfide
37:30I say ma it can be solid or liquid
37:35actually a is the the non-metallic part
37:40and simply by heating we can dissociate
37:43that and during dissociation we
37:47can may produce metal in the gas phase
37:51or we may produce the metal in the solid
37:54or liquid phase.
37:57And if
37:59in the right hand side if we have
38:02something in the gas phase then you know
38:06by application of vacuum we should be
38:09able to
38:11encourage the reaction to go to the
38:13right side.
38:16Now when we talk about reduction
38:18processes,
38:20if we have the metal as a compound solid
38:23or liquid, if we have a reducing agent
38:27solid or liquid,
38:30then we may we can produce the metal
38:33either as a gas
38:37or solid or liquid. But suppose the
38:41other the reducing agent produces a gas
38:47then in either case the reaction becomes
38:50sensitive to vacuum. We can apply vacuum
38:53and help
38:55uh to go the reaction forward. So many
38:59pyometric operations make use of vacuum.
39:04Of course it will be true for
39:05sublimation or distillation also. Like
39:07if you have a metal we want to uh purify
39:10it and if we heat it it forms a gas
39:15this is not 00 it should be metal in
39:18solution.
39:22All all these cases vacuum will help.
39:25Now in these things R is a reducing
39:27agent M is a metal being extracted or
39:30refined. A is the acid radical which
39:33could be sulfur oxygen etc. And in all
39:36cases equilibrium
39:39is shifted to derived by application of
39:42vacuum. There can be a a reverse thing
39:46also like consider reduction of
39:50zirconium dioxide by calcium.
39:55This is can be achieved by gaseous
39:59calcium reducing solid zirconia to
40:03produce zirconium solid and two calcium
40:06oxide solid. In this case, it is
40:10pressure which is going to drive this
40:12reaction to the right because you have a
40:14gas phase on the left hand side. So
40:16there will be examples like this in
40:18pyomelogy
40:20where we will see that we can use
40:24vacuum of pressure to make things happen
40:28at appropriate temperatures.
40:34Now
40:36in pyomelogy
40:38there is extensive application of
40:40thermodynamics and kinetics and there
40:43are many books which discuss this.
40:48I
40:50can suggest the two books where I'm a
40:53co-author. They are very simple books.
40:56One is called chemical and metological
40:58thermodynamics by
41:00KK Prasad and myself and also KP Abraham
41:05and another which is which is quite old
41:08now actually principles of extractive
41:10metalology by
41:13Aos and myself that was published many
41:17years ago 1999 but it's still very
41:20useful
41:24you'll have to read these things now one
41:26of The most fundamental concepts,
41:30thermodynamic concepts
41:33that finds extensive application
41:36in understanding metalological
41:40reduction processes
41:42is what we call alingram diagrams.
41:49This will be discussed extensively in
41:52your thermodynamics course and I kind of
41:55assume
41:57you have studied it already.
42:01I will very briefly say what it what
42:04does the diagram shows and what we learn
42:07from the diagram.
42:09Elingum diagrams
42:12actually plot
42:15free energies of formation
42:18of oxides
42:20as against temperature.
42:23Free energies of formation of oxides
42:28versus temperature.
42:30Now they are plotted
42:33always by writing the reaction with one
42:38oxygen molecule
42:41like we write 2 Ti plus O2 is equal to 2
42:45TIO2.
42:47So it is giving free energies of
42:49formations of 2 TiO2
42:53not 1 TO2.
42:55If you take it venadedium, it plots free
43:00energies of formation when the reaction
43:02is
43:04this. It's not one
43:07V25. The idea is if we write always a
43:11reaction in terms of 102
43:15then we can
43:17add these figures to find what will
43:20happen when a more reactive metal
43:24replaces another.
43:27The more reactive replaces
43:30a less reactive metal from its oxide.
43:32All you have to do is to substract one
43:35value of free energy of formation from
43:37the other. I will explain that later on.
43:40But please take note of this that all
43:42these lines
43:44represent values of free energies of
43:46formation of the compound when the
43:50reaction is written in terms of
43:54O2. So reaction is TI plus O2 TIO2. Here
44:00it will be
44:04four clay.
44:08No say four. No. What do I go? Lithium
44:13TiO2 Si + O2 SiO2
44:184 Cu + O2 2 Cu2O.
44:22So all these oxides are represented
44:26their free energies of formation
44:28are represented by these lines.
44:34So reaction in general
44:39for
44:40these lines
44:42can be written as
44:512xm
44:53by y
44:55+2
44:57g = 2x y mx X O Y
45:05all these lines are written in terms of
45:08one oxygen molecule.
45:10Here of course M is metal
45:16and these lines also represent
45:21that
45:23metal
45:25and oxide
45:28are in their standard states
45:37in their pure states.
45:39and oxygen
45:42will react with metal in the standard
45:44state to produce the oxide in the
45:48standard state.
45:50And the equations
45:53for these lines
45:56obviously would be delta G not T delta H
46:00knot minus T delta S not
46:07now I'm not writing delta G not T
46:10because after all
46:12or T here because delta G not values and
46:16delta S not H not values generally do
46:20not change with temperature.
46:22I would not explain why they do not
46:24change with temperature. They more or
46:25less remain independent of temperature.
46:30This is is the equation of every line.
46:35So we have delta G not delta H knot is
46:38the intercept. Delta S not is the slope.
46:46This explains why the lines are
46:51more or less parallel
46:54because every reaction is so written
46:57that you see there is entropy
47:02on the left hand side in the gas phase
47:05which is disappearing as the oxide is
47:08forming. Entropy change is positive and
47:12therefore all of them are positive
47:14lines. Delta S not is always positive
47:18and with higher and higher temperature
47:20this term becomes more positive. So
47:23delta G not becomes less and less
47:26negative. That's how we get all these
47:28parallel lines.
47:30I hope you understand that why these
47:33lines are sloping upwards
47:38and why most of them are nearly
47:42parallel.
47:43The slope of each line is
47:48referring to the entropy change for
47:50disappearance of the oxygen. Now look at
47:53this line here. This equation is for C
47:57plus O2 plus CO2.
48:01In this reaction, there is no entropy
48:04change because you have a gas phase on
48:06the left hand side,
48:08the same amount of
48:11gas on the right hand side, the gram
48:14moles, same volumes. And therefore since
48:17same number of gram moles are on the
48:20left hand side same number of ground
48:21gram moles on the right hand side
48:23there's no entropy change therefore
48:25delta s not is zero and therefore this
48:28line is horizontal
48:32this there's no entropy change in this
48:35now
48:36all these lines are parallel because you
48:39have written them in terms of only O2
48:42and that's why if we didn't write in
48:45terms of O2 if we represented these
48:47lines writing in terms of
48:52one molecule of the oxide then things
48:55will get complicated. So you have
48:57understood that these lines represent
49:00variation of free energies of formation
49:04with variation in temperature.
49:07The slope giving
49:10the positive slope is because of delta s
49:13not which is because of disappearance of
49:17oxygen
49:19the entropy represent oxygen on the left
49:21hand side
49:23there's one very interesting line which
49:26goes downwards
49:28and this this reaction that is
49:322 C plus O2 giving you 2 CO
49:37This is the only one line here which is
49:39sloping downwards. Why it is sloping
49:42downwards? Because you have one molecule
49:45of gas on the left hand side O2.
49:50We have two molecules of gas on the
49:52right hand side. So you are creating lot
49:56more gas out of this reaction. So the
49:59entropy here is increasing on the right
50:02hand side. So now this gives you a
50:04negative slope. That's why when the
50:07these lines have a positive slope, this
50:10has a negative slope. Now this of course
50:13uh is a very very important observation
50:17and absolutely the most important thing
50:20in process metal and I'll come to that
50:23in a minute.
50:26You will find there are some lines which
50:28suddenly go up mean change their slope
50:33come here then change their slope. This
50:36is explained
50:38by
50:40a heat of
50:43melting
50:44of the metal or the oxide
50:49because if there is a melting of the
50:53metal or melting of the oxide that is
50:56being formed then the slope will change
50:59because this value will change the
51:01intercept value will change. So try you
51:04must understand in some cases it should
51:06go up and in some case it should go
51:09down. If the oxide was to melt the slope
51:11will go this way.
51:14So now we have understood the eling
51:18diagrams.
51:20The importance of this line we must
51:22understand. Now this line shows that as
51:26the temperature increases
51:28carbon monoxide becomes increasingly
51:31stable because free energies of
51:34formation of carbon
51:37monoxide becomes less more and more
51:40negative.
51:43So if we reduce an oxide by carbon
51:49monoxide
51:51then chances are that somewhere or other
51:56it will get reduced and where it will
51:58get reduced we find from the
52:01intersection. Consider this reaction
52:05a particle which is very clear. 2 TIO
52:092 Ti plus O2 + 2 2 TIO.
52:14The free energies
52:19of formations intersect here which means
52:22beyond this temperature
52:25CO becomes relatively more stable as
52:28compared to
52:30TiO2 and therefore TiO2 get reduced by
52:34carbon to form CO.
52:37So as this line which is carbon forming
52:42carbon monoxide moves downwards
52:44intersects all of them. So in theory we
52:48can reduce any oxide
52:52by carbon to carbon monoxide and carbon
52:55can take out that oxygen from that
52:58oxide. I will continue with this uh in
53:02my next lecture because this is the
53:04heart of reduction by carbon and this we
53:08have to understand very very thoroughly.
53:10So in this lecture we have just started
53:14with the relative reactivities of
53:16metals. I'm trying to understand why not
53:19all metals form same kind of compounds
53:22of
53:23free energies of formation
53:26or in terms of electro potentials and
53:28depending on their relative order in
53:29that series they form different kinds of
53:32compounds some more stable some less
53:34stable and then you have started with
53:36principles of pyomeatology
53:38saying what are the advantages of
53:40pyometlogy at high temperatures some
53:43reactions become feasible all reactions
53:45are accelerated
53:47phases are become molten so that there
53:50is a separation very easily etc etc and
53:53then I have come to reduction by carbon
53:56which has been the backbone of
53:59extractive metalology I'll continue with
54:02this in the next lecture thank you
54:10[Music]

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