Extraction of metal from concentrated ore - class-XII

$2Cu _2S+3O _2\longrightarrow 2Cu _2O+2SO _2$

Carbon reduction occurs in smelting from oxide ore. $PbO+C\longrightarrow Pb+CO$

The sulphide ores of certain less electropositive metals like those of Hg, Pb, Cu, etc. when heated in air bring about the conversion of a part of the sulphide ore to oxide or sulphate which then reacts with the remaining sulphide to give the matal and $SO _2$. No external reducing agent is used in this process.
$2Cu _2S+3O _2\rightarrow 2Cu _2O+3SO _2$
$2Cu _2O+Cu _2S\rightarrow 6Cu+ SO _2$

 The three steps involved in extraction are:
1) Concentration of the ore to remove impurities.
2) Reduction of the ore to get the metal.
3) Purification of the ore.

Refining and magnetic separation are methods of extraction of metals .Enrichment of ore is also known as concentration of ore. It means purification of ore.

Impurities are removed prior to the extraction of metal. The process used for removal of gangue from the ore are based on the differences between physical and chemical properties of the gangue.

During roasting, the sulphide ores are converted into oxides by heating strongly in excess of air.
$ZnS + 3O _2 \rightarrow 2ZnO + 2SO _2$

Reduction is the process of removal of oxygen for extraction of metals from their oxide ores. The common reducing agents used for reduction of metal oxides are: Carbon monoxide,  Carbon, Hydrogen.

During electrolysis, electrons are being added directly to the metal ions at the cathode (the negative electrode). The downside (particularly in the aluminum case) is the cost of the electricity. An advantage is that it can produce very pure metals.

After the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures  to convert ore into raw metals, while hydrometallurgy employs aqueous treatment for the same purpose.

Metals more reactive than carbon are usually extracted by electrolysis. Electrolysis is the process of breaking down a compound using electrical energy and is needed to extract  the most reactive metals. 

The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. Electro-refining is used to dissolve an impure metallic anode and produce a high purity cathode. 

In many cases, it is possible to separate the metal compound from unwanted rocky material by physical means. A common example of this involves froth flotation. In this method, the unwanted object binds with air and comes on the surface as bubbles.

In electrolytic refining, impure metal is of positive charged anode and pure metal is of negative charged cathode.

Heating of ore in the presence of oxygen is called roasting. It is a Pyrochemical Process. 

Dressing of ore can be carried out by first handpicking followed by crushing and grinding and then pulverization.

Formula of Limonite is $F _eO(OH)\cdot nH _2O$. It is an oxide ore.

Smelting is the process where heat is applied to ore to extract the base metal, using a reducing agent. So, it is used to carry out reduction of metal oxides to metal by heating with coke.

Cinnabar ($HgS$), when heated with a current of $O _2$ gets reduced to $Hg$ without the need of any additional reducing agent. The process involved is known as self reduction

Oxides of highly reactive metals cannot be reduced by $CO$. Hence $CaO$ and $Al _2O _3$ are not reduced by $CO$ and both $ZnO $ and $Fe _2O _3$ are reduced by $CO$

The alkali halides crystallize in the NaCl structure also known as rock-salt structure (rock salt is the mineral form of sodium chloride); exceptions being CsCl, CsBr, and CsI, which crystallize in the CsCl structure. In the NaCl structure (face-centered cubic lattice), each cation is surrounding by six anions, and vice versa (coordination number of 6). In the CsCl structure, each cation is surrounded by eight anions and vice versa (coordination number of 8).

These are the methods for extraction of metals from oxide ores:
Carbon reduction, involving reduction with CO or C
Self reduction
Thermite reduction, which employs Al
Electrolytic reduction.

The advantage(s) of using carbon to reduced a number of oxides and other compounds are
 (i) easy availability of coke.
 (ii) low cost of carbon.
 (iii) exothermic nature of reaction which requires less energy to keep the furnace hot.

During extraction of metals from oxide ores, the reduction of metal ore to crude metal can be carried out by using several methods such as pyrometallurgy, hydrometallurgy and electrometallurgy. Various reducing agents such as carbon, hydrogen, aluminum or other metals can be used. CO also reduces metal ores to crude metals.

Aluminothermy used for the spot welding of large iron structures is based upon two facts 
(i) As compared to iron, aluminum has greater affinity for oxygen. hence, aluminum acts are reducing agent.
(ii) Reaction between iron oxide and aluminum is exothermic. High temperature is involved in the reaction.
Aluminum reduces iron oxide to iron. 

Oxides of various metal oxides are converted into metals on heating but CuO (copper oxide) does not not converted into metal on heating but by reaction with Cu2S.

The methods chiefly used for the extraction of lead and tin from their ores are respectively self reduction and carbon reduction. Self reduction for lead is as follows.
$2PbS+3O _2 \rightarrow 2PbO+2SO _2 \uparrow$
$PbS+2O _2 \rightarrow PbSO _4$
$2PbO + PbS \rightarrow 3Pb+SO _2 \uparrow$
$PbSO _4+PbS \rightarrow 2Pb+2SO _2 \uparrow$
Carbon reduction for tin is as follows.
$SnO _2+2C \rightarrow Sn+2CO \uparrow$

$ AgNO _3 \xrightarrow{\Delta} Ag + \frac {1}{2} O _2  +NO _2 $

When $\Delta$G is negative, a process or chemical reaction proceeds spontaneously in the forward direction and similar condition is required for a reduction process also. 

Cinnabar ore is relatively concentrated, it can be processed directly without any intermediate steps to remove waste material and without requirement of a reducing agent. 

During extraction of metals from oxide ores, the reduction of metal ore to crude metal can be carried out by using several methods such as pyrometallurgy, hydrometallurgy and electrometallurgy. Various reducing agents such as carbon, hydrogen, aluminum or other metals can be used. CO also reduces metal ores to crude metals.

$H _2$ is not widely used as the reducing agent in metallurgic process because metals react with $H _2$ at elevated temperature forming hydrides. When $H _2$ and $O _2$ present in the air are combined it will be explosive. Reducing power of $H _2$ does not increase with temperature, which is essential in metallurgy.

Self-reduction process:  This process is also called auto reduction process or air reduction process. 

For the removal of base metals the impure bullion is charged to a softening furnace whereby oxidation these elements form drosses or slags and, with some lead, are skimmed from the bath. This is the softening process. 

Aqua regia is three parts conc. $ HCI$ and one part conc. $HNO _3$.

Self reduction process of Pb is

Reduction is loss of oxygens.
In all the above reactions the metal oxides are reduced to metals by the elimination of oxygens,so all are reduction ractions.
Hence options A,B,C & D are correct.

As sodium  (alkali metal) is more reactive than Al so it cannot be extracted from $Na _2O$ by using Al as a reducing agent.

Metals can be recovered from their ores by following chemical methods:

The heat is used up in changing the state overcoming the forces of attraction.

The compounds of reactive metals from which metals extracted and purified are called ores of metals. Metals can be extracted from ore by reduction and electrolysis.

It is easier to obtain a metal from its oxide, as compared to its sulphides and carbonates. Therefore, prior to reduction, the metal sulphides and carbonates must be converted to metal oxides.

Ores contain large amount of impurities, which is called as gangue. The process of removing of impurities by physical means of methods like hand picking, crushing or pulverizing and grinding) is called dressing of ore.

Reduction is the process of removal of oxygen for extraction of metals from their oxide ores.

Electrolysis is the process of decomposition of a substance by passing electricity through it in a molten or dissolved state. 

Different processes applied in order to increase the percentage of metal in the ore is called concentration of ore.

Ores found in the form of sulphide and carbonates are first converted to their oxides by the process of roasting and calcination. Heating of sulphide ores in the presence of excess air to convert them into oxides is known as roasting.

It is easy to obtain metals from their oxides. So, ores found in the form of sulphide and carbonates are first converted to their oxides by the process of roasting and calcination.

Mercury has a low boiling point. Therefore it can be purified by the process of distillation because then mercury will evaporate and collected in pure form and impurities will remain back. 

During electrolytic refining of Alumina, the molten electrolyte used contains pure alumina $(Al _2O _3)$. The electrolyte consists of alumina dissolved in fused Cryolite (Na$ _3$AlF$ _6$) and Fluorspar(CaF$ _2$).

During calcination, carbonate ores are changed into oxides by heating strongly in limited air.
$ZnCO _3 \rightarrow ZnO + CO _2$

Metal oxide of the low reactivity can be reduced by carbon in the simple extraction process. For example; lead oxide is reduced by carbon and it extract lead from its ore.

The walls of a furnace should not react with metals or ore. So retractory metals are used as they are chemically inert.

Auto-reduction is a process in which sulphide ores of less electropositive metals like Hg, Pb, Cu etc., are heated in air so as to convert part of the ore into oxide or sulphate which then reacts with the remaining of sulphide ore in absence of air to give the metal and sulphur dioxide.

Carbon monoxide gas from coke is responsible for flame at the mouth of Bessemer converter, as it burns to form carbon dioxide.

Magnesium is a very reactive metal. At the high temperatures required for carbon reduction, magnesium reacts with carbon to form magnesium carbide (MgC2), making the reduction process ineffective.

When chromite $(FeCr _2O _4)$ is reduced with carbon in an electric-arc furnace, ferrochrome (Fe and Cr) is obtained.

Carbon reduction process is used to reduce less electropositive metals.
$Mg, Al$ have high electropositivity. Hence, cannot be extracted from oxides through carbon reduction process.

 Aluminium being more electropositive than carbon, hence it is not reduced by carbon. Other metal oxides can be reduced by carbon.

The correct reactions are as follows-

$2PbS+3O _2\rightarrow 2PbO+2SO _2$
$PbS+2PbO\rightarrow 3Pb+SO _2$ (self reduction)
$PbO+C\rightarrow Pb+CO$
$PbO+CO\rightarrow Pb+CO _2$ (carbon reduction)

The given roasting and auto-reduction reactions are classic steps in the extraction of lead (Pb) from galena (PbS). Lead sulfide reacts with oxygen to form lead sulfate and lead oxide, which then react further with remaining sulfide to yield molten lead and sulfur dioxide.

Generally carbon is more reactive than $Zn$ and $Cu$. Hence, $ZnO$ and $CuO$ can be reduced by carbon leading to the formation of corresponding metals,  but carbon is less reactive than $Ca$ and $Al$, so $CaO$ and $Al _2O _3$ can not be reduced by carbon .