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We are now starting module number 6, which will be about extraction of metals from sulphides
and we will start with copper metallurgy. I have already said one or two things, as
an introduction during my last lecture, but let me repeat what I have said.
In this module 6, our learning objectives will be following that is regarding the complex
nature of sulphides and sulphide metallurgy. Then, we will take on “Pyrometallurgical
Extraction Process”, for copper, zinc and lead. These are three very common metals and
often called as base metals. We will touch nickel also because nickel is found as sulphides
and as well as oxides. Then, “Hydrometallurgical Extraction Processes”,
for copper and zinc, not to for lead, then we will talk about the concept of Process
Fuel Equivalent, which gives us a way of comparing energy requirements.
Now, these are the standard methods of producing metal from sulphides. One is straightforward;
it is ‘Thermal Decomposition’. This is applicable in the case of mercury sulphide:
Hg S, which can be simply heated and it decomposes to give mercury metal and sulphur. Most common
and logical will be to take the sulphide, roast it to make an oxide, and then reduce
by carbon, like the way you treat an oxide. The third one is what we apply in the case
of copper, and also in the case of nickel, will see when nickel is in the form of sulphide.
In this case, the idea is not to roast copper sulphide to copper oxide and then reduce by
carbon. It is a very different technique. Copper sulphides are often associated with
iron sulphide, and if you try to roast them, iron gets oxidizing preference with respect
to copper and iron will start forming iron oxides. We ensure that not all iron gets oxidized
and we leave oxidation of iron incomplete. The iron oxide that forms F e O is slagged
out. We are left with copper sulphide and some residual iron sulphide. These two form
what is known as a matte; matte is a mixture of sulphides from there we get metal by very
interesting oxidation technique where simply by oxidizing it we make produce copper. How
we will do that we will see later. The other will be flash smelting, where roasting, smelting,
are combined and speeded up. The process is speeded up. We can also have metallothermic
reduction of sulphide; there can be hydrometallurgical processing of sulphide.
A sulphide can be chlorinated, change into a chloride, from which the metal can be extracted
by electrolysis or by a metallothermic reduction. Finally, there is also a process, in some
cases matte, which is a mixture of sulphides can be refined to produce pure metal by an
electrolytic process. The commercial process is for nickel only.
Let us start with copper. Now I mentioned that there is a beneficiation technique called
floatation, which makes it possible to concentrate low grade copper ores to produce a concentrate
that will have much higher concentrations of copper. And flotation, as I mentioned,
is a technique where we have very fine ores; ores is ground very fine. Then we make a froth,
we have that fine suspended, in a medium where there is a froth, and when the bubbles come
out, the sulphide particles attach themselves to the bubbles; they float to the surface.
The gangue minerals like silica and other oxides which do not attach themselves to the
bubbles; they stay behind. So, from the top we can scheme out a layer which will be very
rich in the sulphide particles.
So, starting with an ore which may contain only one to two percent copper, one can after
grinding and flotation, one can produce a concentrate which will have 13 to 35 percent
copper, depending on circumstances. And the gangue and the other sulphides which have
not been floated will be taken out. There is a way of doing differential flotation
also where we float one sulphide, not the other sulphide, with another state float the
other sulphide. So, here we are doing a differential flotation, only to get the copper sulphide
mineral floating out of other gangue and other sulphides and there are other sulphides are
not floated. So, the conditions are so maintained. So, we will start now with a concentrate,
which contains sulphide from15 to 35 percent copper. This is the starting material.
Now, there are several ways which are similar, but they evolved over time. The conventional
or traditional route is this. The concentrate goes for reverberatory or electric furnace
smelting, or it can go from hearth or fluid bed roasting, calcinations and then go for
a reverberatory or electric furnace smelting. Now, in the smelting process, I have mentioned
little while ago is the aim is not to produce an oxide, but to have controlled oxidation,
so that we produce a matte. Matte is a mixture of copper sulphide and iron sulphide, which
means that copper sulphide which was in the concentrate, is left practically untouched.
Only the iron sulphide part is oxidized that too incompletely, so that only it goes to
the Fe O state. That Fe O is slagged out, and the rest of the iron sulphide and copper
sulphide, they form a sulphide solution called matte, which can have 35 to 60 percent copper,
the rest will be iron as iron sulphide. So, all the oxygen has been eliminated from
the system, and the slag that comes out, which has most of the iron, slagged out Fe O with
silica, limestone, quartz etcetera, in a slag. It will also take out a small amount of copper;
obviously, that we cannot ensure. But, basically oxygen is eliminated, from the system. But
what we get is a matte, a mixture of iron sulphide and copper sulphide. Then this goes
through a process called converting. Converting, a process of oxidation, where as we oxidize
iron sulphide and copper sulphide matte, a stage comes where suddenly copper comes out
of the system, and there is some slag also. The slag will be put back to the reverberatory
furnace because it may contain some iron. The copper that has come out goes for refining.
Now, a newer route is that which will eliminate reverberatory furnace smelting. We would have
and these steps everything is combined into one step called flash smelting, means everything
is smelted, calcined and flashed, very fast. It produces a matte, it goes straight for
converting, there are even newer processes, where everything is combined; all steps are
combined into one continuous smelting process, which can produce either matte for conversion
or it can straight away produce the copper, called blister copper. What it is? I will
tell you later. And the slag that contains some copper will go for slag cleaning, for
copper recovery. After we have produced impure copper called blister copper, which is 98.5
percent copper. It will go for refining. In the refining process we will produce cathode
copper; 99.99 percent copper. But, we will also get whole lot of valuable byproducts
in slime. All the precious metals like silver, gold, platinum, palladium, etcetera.
I will go into the details of the process now and look at these steps, one by one. Now,
to understand what happens when we roast the sulphide concentration, the concentrate in
a reverberatory furnace, we should note that during roasting sulphides are first oxidized.
Iron sulphides are first oxidized in preference to copper sulphides. Now, we do not want iron
sulphides to form Fe 2 O 3. Because, if Fe 2 O 3 forms and then this cannot be slagged
very easily. It does not dissolve in the slag that you are creating by adding limestone,
quartz etcetera. And the gangue, there is a slag phase where Fe 2 O 3 does not dissolve
very well, but Fe O does. So, the iron sulphides will be oxidized only
up to the Fe O stage, not beyond. Sometimes they say little bit of Fe 3 O 4 is ok, because
that helps in protecting the refractory, is goes and gets absorbed in the refractory surface.
But let us ignore that. We have iron sulphide and copper sulphide in a starting material.
We will ensure that one does not go beyond Fe O, which means we will have to leave much
of Fe S not oxidized and that is accepted. So, the Fe O that forms will go into the slag
face. No copper oxide is formed, because we have not even completed oxidation of iron
sulphide. Copper sulphide cannot be oxidized and yes we have oxidized the whole of iron
sulphide. We are not oxidizing whole of iron sulphide. Only part of that is oxidized, only
up to Fe O stage, which is taken out iron sulphide and copper sulphide left behind will
form matte. No copper oxide is formed. So, the slag will
not have copper oxide. We have mentioned only a very minor amount of copper oxide, which
will go into the slag and we are deliberately leaving Fe S behind to ensure that Fe 2 O
4 and Fe 2 O 3 are not formed. This is the basic technique. Now, during smelting like
this subsequently, if there is subsequent if there is some Fe 2 O 3 left, if supposing
by somehow Fe 2 O 3 forms, then in the subsequent step, the residual Fe S that we have will
reduce Fe 2 O 3 and Fe 3 O 4 to low Fe O state and that will be slagged off.
So, we want that Fe S to play this role also. We must have some Fe S that subsequent if
there are higher oxides; they will be reducing to the Fe O stage for slagging. This stage,
smelting is coming after or now. So, our aim is to produce only a mixture of copper and
iron sulphides. Fe O is removed in slag. Then we will go for what is known as converting.
Converting is where we start oxidizing the matte. First that iron sulphide is oxidized
to produce Fe O. That is removed in a converter and then copper sulphide starts getting oxidized.
Once it reaches the Cu 2 O and when it forms Cu 2 O then there is a very interesting reaction
between Cu 2 S and Cu 2 O that forms copper straight away. The reaction is written as
Cu 2 S plus 2 Cu 2 O to produce copper and S O 2.
So, what is it we had? Starting with a sulphide concentrate, we have roasted it to ensure
that we are left with incomplete roasting of iron sulphide, we will produce a matte,
and then we will have a smelting process. In the smelting process residual Fe S will
ensure that Fe 2 O 3, Fe 3 O 4 forms Fe O and that is slagged off. Then we have left
up with the matte, which will go for the converting step, where in a converter oxygen will be
blown to oxidize iron sulphide to Fe O stage and it is slagged off. And once copper starts
getting oxidized, once it form Cu 2 O, these two will reacts and it will produce copper
straight away. Now, all the reactions that are taking place are exothermic. So, in theory
you are not supplying any energy, but in practice that is not so. Because when you have a reverberatory
furnace or in electric furnace where you do the smelting operation, obviously there will
be lot of energy losses. So, energy will have to be supplied. But, the converting step where
you have a molten matte and oxygen is being injected, it is a highly exothermic process,
no external heat supply is required.
So, what are the reactions? We first have a roasting reaction. See the copper sulphide
is mineral, is Cu Fe S 2. It forms; it tends to form Cu O and Fe 2 O 3. It tends to form
sulphates. It can decompose to give Cu 2 S and Fe S also. It can give Fe 2 O 3. It can
give Cu O. Cu S O 4 and Fe S O 4 and this compound (reaction is not balanced). Like
the roasting of sulphides, all kinds of phases can come about.
Our aim will be to control the partial pressure of oxygen, partial pressure of S O 2 in such
a manner that we will essentially have Cu 2 S, Fe S and Fe O and that Fe O during the
smelting reactions, which will take place at 1250 degrees with fluxes. It will produce
two layers; a slag layer which will remove the iron part and we will have a matte; metallic
sulphides mixture of iron sulphide and copper sulphide.
Then during smelting in a smelter will continue with oxidation. Many reactions can take place.
If any Cu O is there, it will convert itself to Cu 2 S forms Fe O. It will slagged off.
If sulphate is there, it will form Cu 2 S, it will be slagged off. Everything is molten
and many reactions take place.
And then finally, by smelting, we will have iron sulphide and copper sulphide. Now, we
have to remove iron, and sulphur, and other impurities from their matte and we will do
that by slagging. We will continue in the converter. We blow oxygen to oxidize Fe S
to Fe O stage, then Fe O will form react with Si O 2 to form fayalite slag. It will go into
the slag phase. The slag will contain 1 to 5 percent calcium oxide, magnesium oxide.
Iron will be mostly 40 to 50 percent. There will be some slag losses will be there; it
would have silicon. So, in the converter where the molten matte
is being oxidized by blowing oxygen, these reactions take place. How we blow oxygen?
We will see that at one time attempts were made to blow oxygen from their bottom; like
it was done in the Bessemer converter, the air was blown. But, in copper converter things,
oxygen comes from the side. Why it is from the side, I will discuss little later. And
once we start doing that we automatically will produce blister copper. Blister copper
means because once that copper is formed and then solidified and it is broken, it has a
structure like blisters, there is lots of a holds. So, that is why it is called blister
copper. And the reactions are that in the matte there
is copper sulphide. It reacts with the oxygen coming in produces Cu 2 O. Gradually Cu 2
S starts converting into Cu 2 O. At one stage, once you have twice as much Cu 2 O as Cu 2
S, and then these two react very quickly with each other to produce copper liquid and S
O 2 gas. The overall reaction is Cu 2 S, being oxidized by oxygen to produce copper and S
O 2. So, the entire process lot of S O 2 will be coming out. The whole thing is exothermic;
all the reactions. The only energy required is to make sure it compensates for the heat
losses that go from the high temperature reactors. You do not need any heat supply during the
converting reactions because they are very highly exothermic. Like today’s steel converters
where in pig iron the oxygen is injected into pig iron to produce steel, you do not need
any external heat supply; the same thing is also in the copper converter. But, in the
smelting stage, which is done in a reverberatory furnace or electric arc furnace. Although
you have exothermic reaction, you will need heat supply.
Now, this is these are the two kinds of reverberatory furnaces used for smelting. This is the conventional
reverberatory furnace, which uses liquid fuel or gaseous fuel. You have air and oxygen coming
in. There is fuel coming from this side. The converter slag will be taken out this way,
matte will come out this way, and outgoing gases will go out this way. Now, there are
many complications of this design and we are not going into that. But, you see the flame
is thrown from this side, and this is the feeding drag converter, and things are happening
inside a long converter. There are also submerged arc electric furnaces,
where energy is not supplied by oxidation of a fuel, but by electrodes that stick into
the molten charge. So, this is a cutaway view of submerged arc electric furnace, and this
is a cutaway view of submerged electric arc furnace and this is a cutaway view of a reverberatory
furnace. Now, let me read out what is written here. Conventionally a smelting operation
is carried out in a reverberatory furnace filled with either coal fired with either
coal or oil. A typical reverberatory furnace is shown.
Smelting has also being carried out in electric furnaces; that is also shown. And electric
furnace is more advantageous than a reverberatory furnace, if the highly electric power is available
freely. If we have the highly electric power supplying, then you will go for a submerged
arc electric furnaces. And it is also done inexpensively because
the generation of large volume of combustion gases is avoided. When you are using coal
or liquid fuel then, as it is lot of S O 2 is coming out from the system, but you would
also generate C O, C O 2. The amount of gases, combustion gases coming out to be much larger.
In the arc furnace, you are only getting S O 2. You are not getting C O, C O 2 that will
come from combustion of oil or coal. So, that will be an advantage of electric arc furnace;
however, the electric arc furnaces can only the operated where electricity is available.
Unfortunately, an electric arc furnace consumes a large amount of energy, when fossil fuel
is burnt especially to generate electricity. Because, it is a roundabout way of getting
energy, if you are using a fossil fuel into the furnace, either furnace oil, or coal,
and you are generating heat directly, the process of heat generation will be much better.
But, when if you are getting electricity that has been generated in the thermal power plant
again by combustion of fossil fuels then, you have a two step thing that you are generating
at some point from there it is giving electricity, which electricity is being consumed here.
So, if this electricity for the submerged arc is coming from a thermal power plant then
you are not generating C O, C O 2 here, but you are generating C O, C O 2 elsewhere where
the thermal power plant is located. If it is hydro electricity this problem is not there,
otherwise you have to remember that you have only displaced the problem of combustion gases,
in this case, if thermal power plant is giving with the electricity.
But then in this process, in the plant, you are generating less amount of combustion gases,
because you are only generating what is coming from the reactions which do not include a
combustion reaction. Now, there are both of these have now given away to flash smelting
and continuous smelting operations, which I will discuss little later. Now, in the early
days of copper concentrate smelting the average capacity of the reverberatory furnace was
only 100 tons per day, but at present it has gone to almost ten times that much, 1000 tons
per day. Now, when your reactor becomes bigger then
relatively energy losses would be lower. So, it will be give more energy efficient. When
reactors are smaller, imagine ten small reactors with ten times as much as surface area, as
compare to a something with capacity ten times larger. This will have lesser surface area
and surface to volume ratio. Therefore, heat losses will be less.
So, the tendency is to make the reverberatory furnace as big as possible. But, then trying
to operate with the bigger furnace also has operational problems. There will be other
kinds of problem. So, the industry has to decide what it can do. So, this is the step
where you produce from the roasted material, a matte. A matte, as I said, is a mixture
of sulphide. This matte will go for converting and the purpose of converting is to remove
iron. Part of that iron, you have removed earlier, by making Fe O, and removing partly,
but again lot of that iron that is left there as Fe S, will be removed in the converting
step. Sulphur and other impurities from matte will also be removed. For this the molten
matte is produced as a result of smelting. The molten matte produced from the smelting
step is charged into a side blown converter which is a cylindrical vessel; I will show
you a picture of that. With a capacity of 100 to 120 tons per of matte; it is not a
continuous process, because you will charge the matte, it is like steel making, you will
produce blister copper, pour it out and then take fresh amount of matte and again do converting.
So, at a time you will produce 100 to 120 tons of matte. A typical vessel will be 4
meters in diameter, 9 meters in length, so it is a big cylinder, and its line with a
layer of chrome-magnesite refractory about 40 centimeter thick.
Now, next figure shows you a typical side blown converter. This is the steel cylindrical
vessel. There are pneumatic punchers, so that it can be rotated if necessary. This is the
hood from which… This is necessary because you will have to pour out blister copper.
So, that thing has to rotate. Tuyeres pipes, through which, the oxygen will blown in. There
is intake of cerealicious flux because you have to create a slag. This is flux gun, how
the gun it will be thrown into that etcetera. Gases will come out of here. All the details
are there. This is more important. See, this is the converter.
I am looking at here from the sectional point of view. It is charged. This is the hood,
at every stage; whatever the gases are coming will go out to this hood. Then this is straightened
up, put straight under the hood. We start blowing oxygen from this side. And after it
is blown, we can tilt it the other way; pour out the product which is blister copper. So,
charge, blow, tilt the other way, and fully take out the blister copper, this is the converter
operation. Now, in the converter, the atmosphere is highly
oxidizing, compared with the neutral or mildly oxidizing atmosphere during smelting. Air
or oxygen enriched here up to a maximum limit of 32 volume percent of oxygen in the blast
is injected into the molten matte through tuyeres. Each tuyeres is about five centimeter
in diameter and there are about forty tuyeres in a converter. The total volume of gas flowing
through these tuyeres is about 600 meter cube per minute. The products of the converter
are slag and blister copper. So, you have three steps. You have roasting;
converting, smelting and then converting. Now, let us come to what is happening during
converting. So, during converting you have starting with a mixture of sulphides; copper
sulphide and iron sulphide. Then iron sulphide is converted to Fe O. It is taken out as a
slag. You are left now with a metal, very often it is called a white metal, because
it looks white, it is copper sulphide. Now it is copper sulphide turn to start getting
oxidized.
You gradually begin to build up Cu 2 O and then you create two layers. What are these
layers and that you come to know if you look at the phase diagram here, let me go through
this first. The reality volumes are the two layers and can be determined by the lever
rule. When the sulphur level eventually drops to 1.2 percent, only the metallic copper phase
remains and at this stage, care to be exercise to ensure the metal is not over oxidized to
Cu 2 O. Now, as I said, there is a stage comes when
Cu 2 S reacts with 2 Cu 2 O to produce copper and you are left it copper and copper sulphide.
And in that case, you will get two layers. Air is coming through this layer and this
is the phase diagram of copper and copper sulphide. You see there is a complete separation
here. This is the melting point of copper; this is the melting point of copper sulphide.
Now, if the converting operation is taking place here, you are going from copper sulphide
towards this direction. You go from a to b to c to d and there is a clear separation
between here and here. There is a copper layer with very small amounts of sulphur and you
have copper sulphide here. This is the two layers you are getting. And we are injecting
here continuously into the copper sulphide layer, copper is at the bottom. The completion
of blow can be examining the fracture of the sample. The blistery appearance of this sample lends to
the name blister copper to this product. In industrial practice, the blister produced
contains 0.02 to 0.05 percent sulphur; along with 0.2 to 0.5 percent dissolved oxygen.
So, the blister copper, we are producing here, you will find that there is always a bit of
sulphur and it will also contain some oxygen, because after all you are injecting oxygen.
Now, attempts made in the early days to produce blister copper in the bottom blown converter
used in this steel industry ended in a failure. Now, of course, the there is top blown converters,
but in Bessemer converters had oxygen coming from the bottom. So, same thing was tried
in the case of copper. It ended up in a failure because after point
b is reached, and once you have reached this point b, you are coming from copper sulphide
part to the copper part. Once you reach this point, means after you have removed certain
amount copper sulphide and produce certain amount of copper, the relative amounts you
will know from the lever rule. A layer of copper rich liquid will be formed at the bottom.
So, once you start producing copper, you have gone this way, a copper rich liquid will be
formed at the bottom in contact with the tuyers. And there will be very little heat generation
due to Cu 2 S oxidation in this layer. There is no more Cu 2 S left. Cu 2 S oxidation is
generating heat. Once it produced copper and the copper would sink and cover the tuyers
and because there is no heat being generated you have a problem there.
There will be very little heat generation due to Cu 2 S oxidation, though this heat
is generated due to oxidation of copper. But, there is not much difference in the heat generated
per mole of oxygen for copper oxidation and that for Cu 2 S oxidation. The efficiency
of copper oxidation is much lower. Consequently, the temperature drops rapidly in the tuyers
region of the Bessemer converter. This leads to the clogging of the tuyers with solid and
stoppage of the conversion of the matte to the metal.
So, if we are trying to inject from the bottom, after sometime the process will have to be
discontinue, because as copper sulphide begins to produce copper metal. That metal will come
down to the bottom, and then it will get frozen, and then it will stop will interfere with
the incoming oxygen. So, the tuyers will be clogged with the solid and the conversion
of the metal matte to metal will stop. So, then the industry have to come up with
the invention that have a side blown converter and the conversion of white metal to blister
copper in a converter becomes possible. So, from the side oxygen will be blown into the
Cu 2 S layer. Copper goes on forming; it goes to the bottom as almost pure copper and copper.
So, this layer gradually becomes thinner and thinner and this layer becomes thicker and
thicker. When it is all over, this entire converter will be turned and the blister copper
will be taken out. This is how the converter operates.
Now, again I am repeating what I have said earlier that,” the oxidation taking place
during roasting and that resulting from the leakage of air into the furnace during smelting
determine the extent of oxidation of the iron sulphide in the charge to the slag”. During
roasting it is an oxidation process. There is some leakage of air also into the furnace
during smelting. All these tend to oxidize iron sulphide to iron oxide and it goes into
the slag. Generally, the object is to produce a matte. There should be any that contains
35 to 45 percent copper, 20 to 22 percent sulphur and 25 to 35 percent iron in the matte.
This iron is as sulphide; that copper also is a sulphide.
This not only minimizes the loss of copper to the slag, but also provides a matte; again
there should be a need, with a sufficient quantity of iron sulphide for use in the next
stage. Now, in the next stage where there is converting, the iron sulphide oxidation
will provide all the heat required to ensure an autogenously converting operation. You
need to have certain amount of iron sulphide because that will oxidize and begin to give
you lot of heat so that you do not need any extra heat in the converter. Now, the relationship
between the percentage copper in the slag and that in the matte is given in the figure.
Lot of experiments has been done in the industry to find out how to cut down copper losses
in the slag. What has been found is something you should expect from common sense that if
you have a very rich matte with a lot more copper then copper losses in the slag perhaps
will also be more. That is what it says. The relationship between the percentage copper
in the slag and that in the matte is shown in the figure for various reverberatory furnace
operations around the world. So, wherever we are producing matte in the
smelting step; data have been taken and you have been found that percentage copper in
the reverberatory furnace made the higher it is; the higher will be the copper losses
in the slag. But you know it is never more than one percent. The copper losses are not
that alarming.
Now, once we have produced a blister copper, we have produced an impure copper. That copper will now have
to be refined and refining of copper is a very elaborate process. In the sense that
it will give whole lot of products, let us see how. One is first of all there is some
sulphur. The sulphur can be removed from the liquid copper by slow oxidation. If we have
the copper pool, the blister copper in the molten state and it is slowly oxidized then
all the sulphur will be removed as S O 2. Some copper might get oxidized also, and in
the process some oxygen may go into the copper. So, to eliminate that oxygen now, the industry
years ago, have found a very simple process, which is still applied in industry. There
is a pool of copper which has to be refined; it has impurities, it had sulphur. That sulphur
is removed by keeping it through a slow oxidation that can be done in a reverberatory furnace.
Just keep the door open and the air leakage itself to gradually remove all the sulphur.
But, the air also then goes into copper. We want to remove that and that is done by a
process called Poling. Poling is stirring the bath with green branches
taken from trees. It is a very crude process, but very effective process, and industry even
today apply the technique. When green branches are taken from trees and those branches are
use to stir the bath, those branches generate reducing gases like methane, and the methane
will reduce the oxygen. People have tried in modern industry to remove that oxygen by
hydrogen injection, but that is too elaborate and too complicated, hence industry still
uses poling and it is done by stirring green branch trees.
This is done in 400 ton capacity reverberatory furnace where 12 to 16 hours door kept open
to mild air blast. This is the technique. Now, during this slow oxidation many other
things are also removed such as sulphur, iron, selenium and zinc. Solid oxides being skimmed
off and poling is done at the end. So, initially we allow lot of oxidation of the bath for
many hours and oxides keep on floating to the surface and they are skimmed off. We allow
all the impurities mentioned to get oxidized and get removed. When we are sure that all
those have been gone, we know there is lot of oxygen has gone in now, so it will be poled
using green branches. So, poling is done at the end. Though the method is crude it is
still the most common method. Now, finally, after we have removed all these
impurities by slow oxidation we get a copper which is 99.7 percent pure. Now this copper
has to be refined further. And it will be done by electrolytic refining, which is done
in concrete or wooden tanks of dimensions are roughly given, using 250 to 320 kg copper
anode, a thick anode, which is that impure copper in a copper sulphate solution, 35 gram
per liter in sulphuric acid. This in this is not nine.
It should be zero with some additives like glue and alcohol. Why glue and alcohol, because,
we get a nice cathode deposits. Now, the scheme is simple. The impure copper will be made
into a thick anode and there is a pure copper sheet cathode. We have the electrolyte with
copper ions in that. As electro refining starts copper will dissolve from anode and it will
get deposited on the cathode. Using glue and other additives is to make sure that the deposit
is very coherent. So, this gradually becomes thinner and this gradually becomes thicker.
Now, what happens when copper is coming from here to here, what happens with the impurities
that are beginning, this. Now there two things can happen. There are some metals here which
enter the solution because they can dissolve in the solution. Like, if you have impurities
like iron, cobalt, nickel, selenium, tellurium, we have sulphuric acid electrolyte, they will
go into the solution. But, there are in this impure anode are precious metals. As copper
leaves this anode while these impurities go into the solution, the precious metals simply
drop below the anode to form sludge. Below the anode there will be some insolubles
which you call the sludge and that contain all the precious metals. So, all the precious
metals that were in the original concentrate, they will come all through, because there
will always been in copper sulphide and it will end up in the copper finally. And only
here it will get separated, because other impurities can dissolve, copper will go through
electrolytic refining to the other electrode. And the sludge that comes, the approximate
composition is this, which are grams per ton. Silver, gold, lead, arsenic, tellurium, nickel
etcetera, depending on the amount of sludge, considerable amount of this precious metals
are collected and there price actually can account for the cost of electro refining and
many other things.
Now, you got sludge. There, from the sludge again we have to recover all the precious
metals one by one. And for that there is again an elaborate flow sheet. Let me look at that
flow sheet. We use the word, ‘sludge’ and also sometimes the word, ‘slimes’.
This is the anode sludge or slimes that collect below the anode. With dilute sulphuric acid
leaching, any residual copper that may be in the sludge can be taken into solution.
Then there will be the residue, the residue will be treated with slightly stronger H 2
S O 4. And we will say that we will do what is called,
‘sulphation leaching’ in the solution. There will be copper and copper cementation
can give you tellurium, means by adding copper it may get tellurium. The residue by roasting
will give selenium oxide, if you go for selenium recovery. There will be a residue, which will
go for lead smelting to get lead. You get a metal and it will go through some other
process to get another metal; some other step and finally, we can recover silver grains
etcetera and the slime will have gold, platinum, palladium. These are processing steps of chemistry
that have been developed over many years. So, the sludge that has all these valuable
impurities, it can be treated by chemical steps to recover them. And I need not go into
the details of this. So, this is the basis of copper metallurgy.
But, now there have been developments and remember once we have talked about flash roasting,
flash smelting, etcetera. When roasting is done in an earth roaster, where there are
rotating platforms and the calcine is going from one bed to another bed to another bed,
so that things are exposed to the oxidizing atmosphere. People found that maximum an oxidation
takes place during flight, not when they are sitting on a rotating platform; it is only
when they are discharge from one to the other; maximum takes place. So, people argued why
have these platforms, why not let the practical drop from the top straight down to the bottom
and during flight they get oxidized the way you want to do that, in a furnace.
This is the basis of newer process of flash smelting, means smelting in a flash and that
combines roasting and smelting in one unit. There are different kinds of flash roasters.
Here is a cutaway view of Inco flash roaster. You see from here chalcopyrite concentrate
is coming here; oxygen is creating an oxidizing environment. So, during the process of this
flights, all the reaction that you want, would takes place. You produce a matte here and
you produce a slag here. Now, of course, this means you need to control the oxidizing potential
very accurately. So, from this side oxygen comes in, because from the end you have putting
oxygen, the oxidation potential will be higher here, because you know as you go towards from
matte generation of matte, you need higher and higher oxidation potential.
So, initially what is happening is the sulphides are getting roasted then by the time it is
coming higher oxidation potential it forms a matte, and matte comes here, slag comes
here. Similar process here is cutaway view of how to come to flash smelting furnace,
I think it is there in Ghatshila. Here concentrate is coming from here. There is a concentrate
burner. This is the reaction shaft and from here it is the off-gases are coming in the
process of flight and the fuel air is all here, you produce like a matte. So, you have
expedited the process of roasting or smelting. They have combined in one step in a flash
roaster. We will find that there is a newer process where we will have one more step,
the conversion of matte to blister copper is also combined in one step. Those are the
ultimate, but the more you combine the steps, the more difficult it becomes to control things.
You need higher technology. I will come to that in the next lecture.
Thank you very much.