Ores and minerals: Commonly occurring ores and minerals of iron, copper, tin, lead, magnesium, aluminium, zinc and silver.
Extractive metallurgy: Chemical principles and reactions only (industrial details excluded); Carbon reduction method (iron and tin); Self reduction method (copper and lead); Electrolytic reduction method (magnesium and aluminium); Cyanide process (silver and gold).
Main Topics in the TMH Book
IRON AND TIN
COPPER AND LEAD
MAGNESIUM AND ALUMINIUM
Ores and minerals of iron
Haematite is the principal ore.
Ores and minerals of Tin
Ores and minerals of Copper
Cuprite or ruby copper
Minerals of Lead
Minerals of Magnesium
Minerals of Alumium
Feldspar, Mica, Kaolinite
Alunite or Alumstone
Aluminates of Magensium, Iron and Manganese
Minerals of Silver
Minerals of Zinc
(from X book by Viraf Dalal)
Minerals of Gold
Mainly native gold
Extracting iron from iron ore using a Blast Furnace
The common ores of iron are both iron oxides, and these can be reduced to iron by heating them with carbon in the form of coke. Coke is produced by heating coal in the absence of air.
Coke is cheap and provides both the reducing agent for the reaction and also the heat source - as you will see below.
The most commonly used iron ores are haematite (US: hematite), Fe2O3, and magnetite, Fe3O4.
The heat source
The air blown into the bottom of the furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important not to waste any.
The coke (essentially impure carbon) burns in the blast of hot air to form carbon dioxide - a strongly exothermic reaction. This reaction is the main source of heat in the furnace.
The reduction of the ore
At the high temperature at the bottom of the furnace, carbon dioxide reacts with carbon to produce carbon monoxide.
It is the carbon monoxide which is the main reducing agent in the furnace.
In the hotter parts of the furnace, the carbon itself also acts as a reducing agent. Notice that at these temperatures, the other product of the reaction is carbon monoxide, not carbon dioxide.
The temperature of the furnace is hot enough to melt the iron which trickles down to the bottom where it can be tapped off.
The function of the limestone
Iron ore isn't pure iron oxide - it also contains an assortment of rocky material. This wouldn't melt at the temperature of the furnace, and would eventually clog it up. The limestone is added to convert this into slag which melts and runs to the bottom.
The heat of the furnace decomposes the limestone to give calcium oxide.
This is an endothermic reaction, absorbing heat from the furnace. It is therefore important not to add too much limestone because it would otherwise cool the furnace.
Calcium oxide is a basic oxide and reacts with acidic oxides such as silicon dioxide present in the rock. Calcium oxide reacts with silicon dioxide to give calcium silicate.
The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron. It can be tapped off from time to time as slag.
Slag is used in road making and as "slag cement" - a final ground slag which can be used in cement, often mixed with Portland cement.
The molten iron from the bottom of the furnace can be used as cast iron.
Cast iron is very runny when it is molten and doesn't shrink much when it solidifies. It is therefore ideal for making castings - hence its name. However, it is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle. If you hit it hard, it tends to shatter rather than bend or dent.
Cast iron is used for things like manhole covers, guttering and drainpipes, cylinder blocks in car engines, Aga-type cookers, and very expensive and very heavy cookware.
Extraction of Tin
The ore is tin stone that contains 10% of the metal as SnO2.
SnO2 + 2C = Sn + 2CO
The moltenmetal is collected from the bottom of the blast furnace.
The metal may be purified elctrolytically
Extraction of Copper and Lead
Extraction of Copper
Extracting copper from its ores
The method used to extract copper from its ores depends on the nature of the ore. Sulphide ores such as chalcopyrite are converted to copper by a different method from silicate, carbonate or sulphate ores.
Getting copper from chalcopyrite, CuFeS2
Chalcopyrite (also known as copper pyrites) and similar sulphide ores are the commonest ores of copper. The ores typically contain low percentages of copper and have to be concentrated by, for example, froth flotation before refining.
The concentrated ore is heated strongly with silicon dioxide (silica) and air or oxygen in a furnace or series of furnaces.
• The copper(II) ions in the chalcopyrite are reduced to copper(I) sulphide (which is reduced further to copper metal in the final stage).
• The iron in the chalcopyrite ends up converted into an iron(II) silicate slag which is removed.
• Most of the sulphur in the chalcopyrite turns into sulphur dioxide gas. This is used to make sulphuric acid via the Contact Process.
The copper(I) sulphide produced is converted to copper with a final blast of air.
The end product of this is called blister copper - a porous brittle form of copper, about 98 - 99.5% pure.
The redox processes in this reaction
look at the second reaction.
Examine the oxidation states of everything.
• In the copper(I) sulphide, the copper is +1 and the sulphur -2.
• The oxidation states of the elements oxygen (in the gas) and copper (in the metal) are 0.
• In sulphur dioxide, the oxygen has an oxidation state of -2 and the sulphur +4.
That means that both the copper and the oxygen have been reduced (decrease in oxidation state). The sulphur has been oxidised (increase in oxidation state).
The reducing agent is therefore the sulphide ion in the copper(I) sulphide.
The other reaction
In the CuFeS2, the copper and iron are both in oxidation state +2, the oxidation state of the silicon remains unchanged at +4.
use this information to work out what has been oxidised and what reduced in this case!
You should find that copper has been reduced from +2 to +1; oxygen (in the gas) has been reduced from 0 to -2 (oxygen in the SiO2 is unchanged); and three of the four sulphurs on the left-hand side have been oxidised from -2 to +4 (the other is unchanged).
Once again, the sulphide ions are acting as the reducing agent.
Purification of copper
When copper is made from sulphide ores by the first method above, it is impure. The blister copper is first treated to remove any remaining sulphur (trapped as bubbles of sulphur dioxide in the copper - hence "blister copper") and then cast into anodes for refining using electrolysis.
The purification uses an electrolyte of copper(II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.
At the cathode, copper(II) ions are deposited as copper.
At the anode, copper goes into solution as copper(II) ions.
For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same.
All that happens is that there is a transfer of copper from the anode to the cathode. The cathode gets bigger as more and more pure copper is deposited; the anode gradually disappears.
What happens to the impurities?
Any metal in the impure anode which is below copper in the electrochemical series (reactivity series) doesn't go into solution as ions. It stays as a metal and falls to the bottom of the cell as an "anode sludge" together with any unreactive material left over from the ore. The anode sludge will contain valuable metals such as silver and gold.
Metals above copper in the electrochemical series (like zinc) will form ions at the anode and go into solution. However, they won't get discharged at the cathode provided their concentration doesn't get too high.
The concentration of ions like zinc will increase with time, and the concentration of the copper(II) ions in the solution will fall. For every zinc ion going into solution there will obviously be one fewer copper ion formed.
The copper(II) sulphate solution has to be continuously purified to make up for this.
Extraction of Lead
Ores and their preparation
The most important lead ore is galena (lead sulphide). Other important ores such as cerrusite (lead carbonate) and anglesite (lead sulphate) may be regarded as weathered products of galena and they are usually found nearer to the surface.
Lead and zinc ores often occur together and for most extraction methods they have to be separated. The most common technique is selective froth flotation. The ore is first processed to a fine suspension in water by grinding in ball or rod mills - preferably to a particle size of less than 0.25 mm. The dilution of this suspension (or pulp) can vary from 5 to 40% solids by weight. Air is then bubbled through this pulp contained in a cell or tank and due to the previous addition of various chemicals and proper agitation, the required mineral particles become attached to the air bubbles and are carried to the surface to form a stable mineralized froth which is skimmed off. The unwanted or gangue particles are unaffected and remain in the pulp.
The chemicals added include frothing agents to produce the stable froth and collecting or promoting agents to give the desired mineral the right kind of surface - for example non wetting - for collection. Modifying agents are also added, notably depressants, which prevent collection of certain minerals, and activators which remove the effects of depressants. Thus, for example, with lead-zinc sulphide ores, zinc sulphate, sodium cyanide or sodium sulphite can be used to depress the zinc sulphide, while the lead sulphide is floated off to form one concentrate. The zinc sulphide is then activated by copper sulphate and floated off as a second concentrate. The froth is broken down by water sprays and the resulting mineral suspension is dewatered by appropriate filtration equipment.
The first stage in smelting is to remove most of the sulphur from the concentrate. This is achieved by roasting in a Dwight Lloyd Sintering Machine in which a layer of a mixture of concentrate, flux and some returned sinter fines is moistened and spread on the continuous grate of the sinter machine and ignited. Combustion is rapidly propagated by a current of air blown upwards through the ore mixture by wind boxes.
The sulphur in the mixture provides the fuel for the exothermic reactions which take place; the returned sinter fines are added to dilute the fuel content to prevent overheating. As the charge on the continuous grate moves forward, the sulphide is largely converted to oxide and the fine powders are agglomerated into lumps, which are broken up as they leave the machine, to a size convenient for use in a blast furnace - the next stage in the process. The sinter plant gases are routed to gas cleaning equipment for recovery of fume and the removal of sulphur-containing gases to form sulphuric acid.
The graded sinter is mixed with coke and flux such as limestone, and fed into the top of the blast furnace, where it is smelted using an air blast (sometimes preheated) introduced near the bottom, The chemical processes taking place in the furnace result in the production of lead bullion (lead containing only metallic impurities, usually including gold and silver, hence the use of the name bullion) which is tapped off from the bottom of the furnace and either cast into ingots or collected molten in ladies for transfer to the refining process.
An almost identical process, sintering followed by reduction in a blast furnace, is an integral part of the more complex Imperial Smelting Process for the simultaneous production of zinc and lead. In this blast furnace, a mixed lead/zinc sinter is added and the lead bullion is tapped conventionally from the bottom of the furnace but the metallic zinc is distilled off as a vapour and captured in a shower of molten lead. This is allowed to cool and zinc can be floated off, while the lead is recirculated to the collector.
These traditional two stage processes offer a large number of opportunities for hazardous dust and fume to be released. This necessitates the use of extensive exhaust ventilation and results in large volumes of lead-laden exhaust gases which must be cleaned before they can be discharged to atmosphere. The collected dusts are returned to the smelting process.
Direct smelting processes
The environmental problems and inefficient use of energy associated with the sinter/blast furnace and Imperial Smelting Furnace processes have provided the incentive for much research into more economical and pollution-free processes for the production of lead. Most of this research has been aimed at devising processes in which lead is converted directly from the sulphide to the metal without the need to produce lead oxide in an initial step and then reduce it to the metal in a separate operation. As a result a number of such direct smelting processes are now in existence, though at varying stages of development.
Direct smelting processes offer several significant advantages over conventional methods. The first and most obvious of these is that sintering is no longer necessary and a major environmental problem, i.e. the creation of dust, is avoided. Moreover, the heat evolved during oxidising the ore (sintering in the two-step processes) is no longer wasted but is put to direct effect in the smelting operation, thus providing a considerable fuel saving. The volumes of gas requiring filtering are much reduced, and at the same time the sulphur dioxide concentration of the off-gases is greater and therefore more suitable for sulphuric acid manufacture.
The major difficulty in all direct smelting processes lies in obtaining both a lead bullion with an acceptably low sulphur content and a slag with a sufficiently low lead content for it to be safely and economically discarded. In several cases further treatment of either the crude bullion or the slag (or both) is required in a separate operation. There are several direct smelting processes which come close to meeting the desired criteria - the Russian Kivcet, the QSL (QueneauSchuhmann-Lurgi), the lsasmelt and Outokumpu processes are examples.
The use of these newer processes is likely to increase but at the current time, the relative importance of the different smelting methods in terms of metal produced is as follows:
* Conventional blast furnace 80%
* Imperial Smelting process 10%
* Direct processes 10%
For extraction of lead - extra information
Eletrolytic Reduction method
• dolomite (CaMg(CO3)2),
magnesite (MgCO3) and Carnallite (KMgCl3.6H2O).
Electrolysis of magnesium
• Dolomite and seawater is precipitated
as insoluble magnesium hydroxide
Mg(OH)2 which is subsequently treated
with HCl to give MgCl2.
• MgCl2 is fed into electrolysis cell to
produce Mg metal at cathode and Cl2
Extracting aluminium from bauxite
The ore is first converted into pure aluminium oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminium compound. The aluminium oxide has too high a melting point to electrolyse on its own.
Purifiying the aluminium oxide - the Bayer Process
Reaction with sodium hydroxide solution
Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.
High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. The higher the temperature, the higher the pressure needed.
With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate.
The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. Some of the silicon dioxide reacts, but goes on to form a sodium aluminosilicate which precipitates out.
All of these solids are separated from the sodium tetrahydroxoaluminate solution by filtration. They form a "red mud" which is just stored in huge lagoons.
Precipitation of aluminium hydroxide
The sodium tetrahydroxoaluminate solution is cooled, and "seeded" with some previously produced aluminium hydroxide. This provides something for the new aluminium hydroxide to precipitate around.
Formation of pure aluminium oxide
Aluminium oxide (sometimes known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C.
Conversion of the aluminium oxide into aluminium by electrolysis
The aluminium oxide is electrolysed in solution in molten cryolite, Na3AlF6. Cryolite is another aluminium ore, but is rare and expensive, and most is now made chemically.
The electrolysis cell
Although the carbon lining of the cell is labelled as the cathode, the effective cathode is mainly the molten aluminium that forms on the bottom of the cell.
Molten aluminium is syphoned out of the cell from time to time, and new aluminium oxide added at the top.
The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.
The electrode reactions
Aluminium is released at the cathode. Aluminium ions are reduced by gaining 3 electrons.
Oxygen is produced initially at the anode.
However, at the temperature of the cell, the carbon anodes burn in this oxygen to give carbon dioxide and carbon monoxide.
Continual replacement of the anodes is a major expense.
The cyanide process was developed (1887) by J. S. MacArthur
It is now the most important and widely used process for extracting gold from ores.
The ore is first finely ground and concentrated by flotation.
To remove certain impurities, it may be roasted.
It is then mixed with a dilute solution of sodium cyanide (or potassium or calcium cyanide) while air is bubbled through it.
Soluble aurocyanide complex ion, Au(CN)-2^-1 is formed .
Silver, usually present as an impurity, also forms a similar soluble ion.
The solution is separated from the ore by methods such as filtration, and the gold is precipitated by adding powdered zinc.
The precipitate usually contains silver, which is also precipitated, and unreacted zinc.
The precipitate is further refined, e.g., by smelting to remove the zinc and by treating with nitric acid to dissolve the silver.
Cyanide Process - Silver
The application of the MacArthur-Forrest process of gold extraction by solutions of
potassium cyanide has been adapted to the extraction of silver.
Cyaniding for Silver.--The recovery from finely divided ore containing the silver as sulphide and chloride amounts
to 80 to 95 per cent.,
(Note.--Slimes includes all material that will pass a 2oo-mesh sieve. Most of it will pass a 3oo-mesh)
The stronger cyanide solutions employed make the greatest possible economic reduction in bulk necessary to reduce
Hand picking to remove rich material is followed by an elaborate crushing and concentrating campaign. Crushers,stamps, jigs, and Wilfley or other concentrating tables are used to remove the poorest portions of the ore. At every stage
slimes are recovered by settling in Dorr thickeners or other devices as they are partly produced from the soft and rich material.
All slimes pass to the special plant for separate treatment.
In cases where it is possible the ore is graded and the rich and poor portions concentrated separately, all the slimes being mixed.
The preliminary concentration may separate 60 to 70 per cent. The metallics of the rich mineral are recovered from the rich material and from concentrates of both grades after prolonged fine grinding --22 to 3o hours---in tube or other mills.
When much silver is present calcium hypochlorite and caustic soda are added as oxidising agents in the mill. The grinding flattens the silver and makes it flakey.
It is separated in classifiers, and after roasting, fluxed, melted, and refined.
The pulp after removal of the metallics is settled, and washed free from chlorides prior to cyaniding.
Elaborate plant of every description for crushing, concentrating, washing, and filtering is employed.
At Cobalt plant in Canada the final concentrate may be not more than 2 per cent. of the original ore treated. The slimes may total 6o per cent.
Treatment of Concentrates.--After removing metallicS the pulp is settled, washed free from chlorides, and dewatered by filtration. It is transferred to tanks and treated with 0.5 per cent. cyanide solution, the solution being precipitated after filtration with sodium sulphide as above and filter-pressed.
Treatment of Slimes.--They are allowed to settle and further dewatered by vacuum filtration till only 25 per cent. of water remains.
The mud--pulp--is transferred to large tanks, 30 feet by 10 feet, capable of treating 80 to 85 tons,provided with agitating appliances for cyanide treatment. About 2 tons of solution per ton of slime is employed. This solution contains about 0.25 per cent. of cyanide, and the ore is agitated from 48 to 72 hours.
The liquor is drawn off at the end of the treatment, filtered, and clarified.
Sodium sulphide is added to precipitate the silver and regenerate the cyanide, in tanks fitted with agitators similar to the treatment tanks. The sulphide is recovered by filtration through filter presses, the solution going back to storage tanks.
Treatment of Silver Sulphide Precipitates.--
The cyaniding of concentrate and slimes give silver sulphide as precipitate.
The sulphide collected in the' filter presses is thoroughly agitated in a tank with caustic soda solution. This mixture is pumped continuously through a revolving cylinder containing aluminium in massive form such as ingots, the silver sulphide is reduced by nascent hydrogen produced by the action of caustic soda on aluminium, sodium sulphide being formed.
The reduction may occupy from 14 to 20 hours, and
circulation is continued till tests show that reduction is complete.
3Ag2S + 2A1 + 12NaHO = 6Ag + 2Al(NaO)3 + 6H2O + 3Na2S
2A1 + 6NaHO = 2Al(NaO)3 + 6H
6H + 3Ag2S = 3H2S + 6Ag
3H2S + 6NaHO = 3Na2S + 6H20
The silver pulp is filtered through a filter press and the solution of sulphide sent back for re-use.
After thorough washing to remove traces of soluble sulphide the precipitate, together with that removed as flake silver, is melted and refined.
The recovery of silver by cyaniding processes varies with (a) the form of occurrence; (5) fineness of milling; (c) strength of solutions; (d) efficiency of agitation and filtration. In some ores it reaches 95 per cent., but may fall as low
as 80 per cent.
The elaborate concentration methods followed are necessary to reduce the cyanide consumption, which is high.
JEE Question 2007 Paper I
Extraction of zinc from zinc blende is achieved by
(A) electrolytic reduction
(B) roasting followed by reduction with carbon
(C) roasting followed by reduction with another metal
(D) roasting followed by self-reduction
Roasting followed by reduction with carbon.