The compounds of various metals found in nature as ores are mixed with impurities like sand and rock. The various processes involved in the extraction of metals from their ores and their subsequent refining are known as metallurgy. An overview of various processes involved during metallurgy is given below.

Concentration of ore

Unwanted rocks, sand and grit from the mineral ore are called gangue or matrix. These have to be removed so that the mineral ore is concentrated with higher percentage of metal. Ores are mined from deep within the earth's crust in the form of rocks. The minerals are embedded in these rocks. The rocks are first crushed into smaller pieces by crushers. Then they are ground to powder by ball mill and other processes so that powdered ore is obtained. Depending on the type of ore, hydraulic washing, froth floatation process, magnetic separation and chemical separation techniques are applied for concentrating an ore.

Hydraulic washing :

As the name suggests, hydraulic washing process is done by washing the ores with streams of water. If an ore is heavier or denser than the gangue, then the gangue particles are washed way with the stream. The heavier or denser ore particles remain behind and can be collected. Hydraulic washing is done for ores that have tin or lead, as they are found to be heavier than the gangue.  

ii) Froth-floating process :

This process is used for sulphide ores. Oils can wet sulphides. Oil floats on water. Sulphide ores are first ground to powder and water is added. Then pine oil is added and the emulsion is agitated by passing compressed air. Oil and froth float on the surface along with the sulphide ore. The gangue particles being insoluble in oil remain at the bottom of the water tank. The froth is removed and allowed to settle down. This is called the froth-floating process. This process is used for sulphide ores of Cu, Pb and Zn.  

iii) Magnetic separation:

After grinding, the streams of material are put through magnetic fields to separate the nickel-iron metal granules from the silicate grains. Repeated cycling through the magnetic field gives highly pure bags of free nickel iron metal. One of several alternative ways is to drop a stream of material onto magnetic drums, as shown in the figure below. This method also shows an impact grinder discussed in the next paragraph. The silicates and weakly magnetic material deflect off the drum whereas the magnetic granules and material holding magnetic grains stick to the magnetic drum until the scrape off point.

An optional additional piece of equipment is an "impact grinder" or "centrifugal grinder" whereby a very rapidly spinning wheel accelerates the material down its spokes and flings it against an impact block. Any silicate impurities still attached to the free metal are shattered off. It's feasible to have drum speeds sufficient to flatten the metal granules by impact. A centrifugal grinder may be used after mechanical grinding and sieving, and before further magnetic separation. In fact, most of the shattered silicate will be small particles which could be sieved out.

Magnetic beneficiation can be used not only for separating pure nickel-iron metal granules, but also for minerals which have weak magnetic properties. This is done at Earth mines. In space, where gravity is lower and more sensitive processes are possible, magnetic beneficiation can play a significantly greater role.

Sometimes, a reverse situation may occur: the ore is non magnetic and the gangue is magnetic. In this case also magnetic separation may be used for concentration of the non-magnetic ore.

iv) Chemical separation:

Different chemical affinity between the ore and the gangue is utilized for concentrating the ore. One example shown below will make this process clear.

1. Separation of Al from bauxite ore: This process is known as Bayer's process. Bauxite ore is reacted with hot NaOH. A water soluble sodium aluminate is formed. The gangue is insoluble in NaOH as well as water. The sodium aluminate formed is removed by adding water and filtering the solution.

 

2. The solution containing sodium aluminate is treated with HCl. A precipitate of aluminium hydroxide is obtained.

3. The precipitate is separated and dried, then heated. Pure aluminium oxide is obtained. The oxide can be reduced to obtain pure Al.

Conversion of ore into metal oxide

Ores concentrated by any of the above processes are still in their original oxide, carbonate, sulphide or halide forms. To obtain pure metals from these chemicals, it is better convert the metal-compounds into oxides. Oxides of metals can be reduced easily and pure metal can be obtained. To convert the concentrated ores into metal oxides, two processes are used. These processes are calcination (heating in inadequate quantity of air) and roasting the ore (heating in adequate quantity of air).

i) Calcination :

Carbonate ores are heated in absence of air. The absence of air and heat converts the CO₃ into CO₂ and O. The O remains with the metal as metal-oxide. Heating also expels any water content in the ore. In case these are any volatile impurities or gases trapped in the ore, they are also removed by heating.

Example of how calamine ore or zinc carbonate is converted to ZnO by calcination is shown below.

Calcination can be done for ores containing carbonates.

ii) Roasting :

Sulphide ores are roasted or heated in plenty of air. The sulphide S changes to sulphur dioxide. The metal reacts with oxygen in the air to become a metal-oxide. Heating removes gaseous and other volatile impurities.

Example below shows how zinc blende ore (zinc sulphide) is converted to ZnO by roasting.

Other types of ores, namely metal-oxide ores and metal-chloride ores remain unaffected by calcination and roasting processes. On treating with heat, impurities and water are removed from these ores.

Reduction of metal-oxide

Reduction process is used for converting metal-oxides into metal. Metal-chloride can also be reduced directly. The reduction reaction chosen depends on the chemical reaction or reactivity of metals. Generally reduction by heat, chemical reduction or electrolytic reduction processes are utilized.

i) Reduction by heat:

Metals that are unreactive, like Hg, can be reduced from their ores by heating them. Mercury ore cinnabar is actually mercury sulphide. This can be heated at 300°C so that S is removed as SO₂ and HgO is obtained. Hg is a very unreactive metal. HgO dissociates into Hg and oxygen soon. The reaction is shown below.

It is interesting to note that for the cinnabar ore; roasting and reduction processes go on one after another.

ii) Chemical reduction :

Various reducing agents are used for different metal-oxides to obtain free metals. Carbon, Al, Na, Ca are some reducing agents that are put in use.

1) Reduction by carbon:

Oxides of Zn, Fe, Ni, Sn, Pb are reduced by heating them with carbon. Metal-oxide is mixed with coke, a source of carbon, and heated in a furnace. Carbon reacts with oxygen and free metal is obtained. Example below shows how Zn is obtained from ZnO on reduction with coke.

Reduction by carbon cannot be done for more reactive metals like Mn, Al, Cr, etc. Cu-oxide can be reduced by coke, but Ca-oxide cannot be reduced by coke.

2) Reduction by Al :

This process is called as the Thermite process. Al is more reactive than carbon. Some metal-oxides that cannot be reduced by coke are reduced by Al. Al itself attracts oxygen from the metal-oxide and becomes aluminium oxide, and this frees the metal. Mn and Cr metals oxides are extracted and reduced by Al. Example below shows what happens when manganese dioxide is heated with aluminium powder.

3) Reduction by electrolysis:

Highly reactive metal-oxides and metal-chlorides are not easy to be reduced by chemical reactions. Metals such as Na, K, Mn, Ca have to be freed from their ores by electrolytic processes. These metals are so reactive that they themselves are powerful reducing agents. Molten metal-oxides or chlorides form the electrolyte in the electrolytic cell. The cathode of the cell provides the electrons needed for the metal to free itself from the metal-oxide or metal-chloride bonds.

Al₂O₃ is reduced at the cathode of an electrolytic cell as shown below. Al₂O₃ is melted and forms the electrolyte. Free Al⁺⁺ ions are attracted to the negatively charged cathode. Al⁺⁺ is reduced by supply of electrons at the cathode.

Chlorides of metals like Na, Mg are melted to form electrolytes. The reactions are shown below.

Chlorine gas is liberated at the anode. The electrolysis has been done with molten metal ores and not aqueous solutions because these metals are highly reactive and will react with water to give hydroxides and not pure metals. In the electrolysis, metals atoms get deposited on the cathode electrodes which then have to carefully removed and stored.

Refining of metals

Free metals obtained by various reduction processes may have several impurities which may be other metals. For example while obtaining Na, K is a common metal that may be separated along with Na. Thus K is an impurity here. These impurities have to be removed by processes known as refining. Refining is nothing but purification of metals. There are various methods of refining; some of which we will discuss below.

i) Liquation method :

In liquation method, metals with low melting points are refined. Metals like Sn, Pb, Bi have low melting points compared to the impurities in the metal. A block of impure metal is placed on a sloping furnace. The temperature of the furnace is maintained a bit above the melting point of the metal to be refined. The metal melts and flows off the slope; it is then collected and cooled.

ii) Distillation method :

Zn, Cd and Hg form vapours easily. They can be heated and distilled out from their impurities. Impure metal block is heated at a temperature so that the metal atoms start to evaporate. The temperature is then held constant. The vapours are condensed and separated out in a container called receiver. Non-volatile impurities are left behind in the distillation chamber.

iii) Oxidation method :

Sometimes impurities are able to get oxidized more easily than the metal itself. In this case oxidative method is used. For example if impurities are S, C, Si or P, they can get oxidized more easily than the metal itself. For example in case of pig iron Fe, these non-metals are present as impurities. When air is passed over hot molten pig iron, these non-metals get oxidized to CO₂, SO₂, P₂O₅ and can be removed easily.

iv) Electrolytic refining :

In this method electrolysis is used to refine metals. Metals like Cu, Zn, Sn, Pb, Ag, Au are refined by electrolysis method. In an electrolytic cell, a block of impure metal is made into the anode, a thin strip of pure metal is made into a cathode, and an electrolyte is made out of a suitable metal-salt of the metal to be refined. When an electric current is passed through the cell, ions from the anode enter the electrolyte. The same number of metal ions from the electrolyte gets deposited on the cathode. This is a preferential deposition. Impurities remain in the electrolyte. Some of the impurities may be deposited below the anode. As an example we will study electrolytic refining of copper.

Electrolytic refining of copper :

In an electrolytic tank, acidified copper sulphate (CuSO₄ + dil H₂O₄) solution forms the electrolyte. A block of impure copper is made into an anode by connecting the positive terminal of a power supply (battery). A thin strip of highly pure copper metal is the cathode of the cell. The negative terminal of the power supply is connected to it.

A small electric current is passed through the cell. Atoms from the anode enter the electrolyte. The copper from the anode gets converted into copper sulphide. An equal number of copper atoms from the solution get deposited on the cathode. This is to keep the concentration of the solution constant. Impurities from the anode block either remain in solution or collect below the anode, as they are unable to displace copper form the sulphate solution. The impurities remain insoluble in the electrolyte and they are called anode mud.

Copper sulphate solution contains ions of Cu⁺⁺ and SO₄^--. The following reactions take place at the anode and cathode when an electric current is passed.

Pure copper is scraped or removed from the cathode. Anode becomes thinner as the electrolysis process proceeds. Some important metals like gold and silver are present in the anode mud. These can be recovered separately.

High purity metals

Besides the processes for refining metals discussed above, sometimes, some more processes have to be employed to obtain really high purity metals. Highly pure metals, with impurities less 0.1% are needed in semiconductor industries, atomic energy application, space applications, etc. We will discuss here two methods, namely, Van Arkel method and zone refining method used for getting ultra pure Ti and Ge respectively.

1) Van Arkel method :

In this process metals are converted into other metal-compounds like metal-iodides. Metal-iodides are decomposed easily and highly pure metals are then obtained. For example in a pure block of Ti, Fe can be an impurity. This block is heated in iodine at a temperature of 250°C. Ti converts into TiI4 but the impurities do not. TiI4 is passed over hot tungsten filament so that TiI4 is decomposed and ultra pure Ti is obtained. The reaction is shown below.

 

2)Â Zone refining :

This method applies the fact when a metal crystallizes on cooling; impurities are automatically expelled as they do not form part of the crystal. Ge metal is refined by zone refining. Impure germanium is made into a rod.

A heater in a circular geometry heats the rod from all direction. The heater can be moved over the length of the rod. First a piece of ultra pure Ge (seed) is attached to the rod on the left hand side. The heater is placed over it. The Ge seed and the impure Ge rod melt. The heater is moved to the right slowly. The molten seed and the Ge from the rod re-crystallize as the temperature drops. The ultra pure Ge crystal grows in size. The impurities from the Ge rod are expelled from the re-crystallized Ge. As the heater is moved to the right, the ultra-pure Ge moves in the left direction and the impurities move in the opposite direction. Other metals refined by this process are Si and Ga.

Reference:
www.tutorvista.com
home.att.net
www.permanent.com
www.britannica.com
www.nitt.edu
en.wikipedia.org
www.efunda.com

Editorial Team, Mindfiesta