4.3 Extraction of Crude Metal from Concentrated Ore

  Ores and Metallurgy

1. Calcination: It is a process of heating the concentrated ore strongly in a limited supply of air or in the absence of air. This process brings about the following changes:

• The carbonate ore gets decomposed to form the oxide of the metal e.g., $$\begin{equation} \begin{aligned} FeC{O_3}\left( {siderite} \right)\mathop \to \limits^\Delta FeO + C{O_2} \\ PbC{O_3}\left( {ceurssite} \right)\mathop \to \limits^\Delta PbO + C{O_2} \\ CaC{O_3}\left( {calcite\ ore\ or\ limestone} \right)\mathop \to \limits^\Delta CaO + C{O_2} \\ ZnC{O_3}\left( {calamine} \right)\mathop \to \limits^\Delta ZnO + C{O_2} \\ CaC{O_3}.MgC{O_3}\left( s \right)\mathop \to \limits^\Delta CaO\left( s \right) + MgO\left( s \right) + 2C{O_2}\left( g \right) \\\end{aligned} \end{equation} $$

• Water of crystallisation present in the hydrated oxide ore gets lost as moisture, e.g., $$\begin{equation} \begin{aligned} 2F{e_2}{O_3}.3{H_2}O\left( {limonite} \right)\mathop \to \limits^\Delta 2F{e_2}{O_3}\left( s \right) + 3{H_2}O\left( g \right) \uparrow \\ A{l_2}{O_3}.2{H_2}O\left( {bauxite} \right)\mathop \to \limits^\Delta A{l_2}{O_3}\left( s \right) + 2{H_2}O\left( g \right) \uparrow \\\end{aligned} \end{equation} $$

• Organic matter, if present in the ore, gets expelled and the ore becomes porous. Volatile impurities are removed.
  

• Roasting at moderate temperature: Some portion of the sulphide ores like galena ($PbS$), Zinc Blende ($ZnS$) is converted into metallic oxide. If the temperature is fairy low (about ${500^ \circ }C$) and the concentration of $SO_2$ in the gaseous environment is more, sulphate may be produced that are stable, and high temperature is needed to decompose them. $$\begin{equation} \begin{aligned} 2PbS + 3{O_2}\mathop \to \limits^\Delta 2PbO + S{O_2} \\ PbS + 2{O_2}\mathop \to \limits^\Delta PbS{O_4} \\ ZnS + 2{O_2}\mathop \to \limits^\Delta ZnS{O_4} \\ 2ZnS + 2{O_2}\mathop \to \limits^\Delta 2ZnO + 2S{O_2} \\\end{aligned} \end{equation} $$ Note: Sometimes roasting may not bring about complete oxidation. $$2CuFe{S_2}\left( {copper\ pyrite} \right) + 4{O_2} \to C{u_2}S + 2FeO + 3S{O_2}$$

• Roasting at high temperature: The sulphide ores of some of the metals like $Cu$, $Pb$, $Hg$, $Sb$ etc., when heated strongly in the free supply of air or $O_2$ are reduced directly to the metal rather than to the metallic oxides, e.g., $$\begin{equation} \begin{aligned} C{u_2}S\left( {copperglance} \right) + {O_2} \to 2Cu + S{O_2} \\ PbS\left( {galena} \right) + {O_2} \to Pb + S{O_2} \\\end{aligned} \end{equation} $$ The reduction of the oxide ore directly into metal by heating it in air or $O_2$ is called self-reduction or auto-reduction, air reduction. The $SO_2$ produced is utilised for manufacturing of $H_2SO_4$.

• It removes easily oxidisable volatile impurities like arsenic (as $As_2O_3$), sulphur (as $SO_2$), phosphorus (as $P_4O_10$) and antimony (as $Sb_2O_3$). $$\begin{equation} \begin{aligned} 4M + 3{O_2} \to 2{M_2}{O_3} \uparrow \quad \left( {M = As,Sb} \right) \\ S + {O_2} \to S{O_2} \uparrow \\ {P_4} + 4{O_2} \to {P_4}{O_{10}} \uparrow \\\end{aligned} \end{equation} $$ These oxides are volatile and hence escape as gases through the chimney.

• When the concentrated tin stone ore $SnO_2$ (ore of $Sn$) is heated strongly in a free supply of air (roasting), the impurities of $CuS$ and $FeS$ present in the ore are converted into $CuSO_4$ and $FeSO_4$ respectively. $$\begin{equation} \begin{aligned} CuS + 2{O_2}\mathop \to \limits^\Delta CuS{O_4} \\ FeS + 2{O_2}\mathop \to \limits^\Delta FeS{O_4} \\\end{aligned} \end{equation} $$

Both calcination and roasting are generally carried out in a reverberatory furnance. In case of roasting, the air holes are kept open while they are partially or completely closed during calcinations.

3. Smelting: It is a process by which a metal is obtained, either as the element or as a simple compound, from its ore by heating beyond the melting point, ordinarily in the presence of oxidizing agents, such as air, or reducing agents, such as coke. In many extraction processes, an oxide is added deliberately to combine with other impurities and form a stable molten phase immiscible with molten metal called a slag. The principle of slag formation is:

Properties of SLAG:

• It is a fusible mass.

• It has low melting point.

• It is lighter than and immiscible with the molten metal.

It is due to these impurities that the slag floats as a separate layer on the molten metal and can thus be easily separated from the metal. The layer of the slag on the molten metal prevents the metal from being oxidised.

Flux- The ores after concentration contain some earthy matter called gangue which on heating combines with this earthy matter to form an easily fusible material called slag. Such a substance is known as flux. Types of Flux are:

• Acidic Flux: It is an acidic oxide (oxide of a non-metal) like $SiO_2$, $P_2O_5$, $B_2O_3$ (from borax). It is used to remove the basic impurity like $CaO$, $FeO$, $MgO$ etc. The acidic flux combines with the basic impurity and forms a slag. $$FeO\left( {{\text{Impurity}}} \right) + Si{O_2}\left( {{\text{Flux}}} \right) \to FeSi{O_3}\left( {{\text{Slag }}\left( {{\text{Ferrous Silicate}}} \right)} \right)$$

• Basic Flux: It is a basic oxide (i.e., oxide of a metal) like $CaO$ (obtained from limestone $CaCO_3$), $MgO$ (from magnesite, $MgCO_3$), haematite ($Fe_2O_3$) etc. $$CaC{O_3}\left( {{\text{Impurity}}} \right) + Si{O_2}\left( {{\text{Flux}}} \right) \to CaSi{O_3}\left( {{\text{Slag }}\left( {{\text{Calcium Silicate}}} \right)} \right) + C{O_2}$$

(b) Reduction of the oxide to metal: It can be carried out by following methods:

1. Chemical Reduction Method:

• Reduction by carbon or smelting: The process of extraction of a metal by reducing its oxide ore with carbon (coal, coke, charcoal, etc.) is called smelting. $$\begin{equation} \begin{aligned} ZnO + C\mathop \to \limits^{{{1200}^ \circ }C} Zn + CO \uparrow \left( {{\text{Extraction of Zinc}}} \right) \\ 2F{e_2}{O_3} + 3C \to 2Fe\left( {{\text{spongy iron}}} \right) + 3C{O_2} \\ Sn{O_2} + 2C\left( {{\text{anthracite}}} \right)\mathop \to \limits^{{{1800}^ \circ }C} Sn + 2CO\left( {{\text{Extraction of Tin}}} \right) \\ MgO + C\mathop \to \limits^{{\text{Electric Furnance}}} Mg + CO \\\end{aligned} \end{equation} $$ Released carbon monoxide also brings the reduction $$\begin{equation} \begin{aligned} F{e_2}{O_3} + 3CO \to 2Fe + 3C{O_2} \\ ZnO + CO \to Zn + C{O_2} \\\end{aligned} \end{equation} $$ Smelting is carried out in blast furnace at high temperature. So that metal is produced in liquid state and sometimes in vapour state, e.g. $Zn$.

• Reduction by Hydrogen: Hydrogen can also reduce certain metal oxides to metals. $$M{O_3} + 3{H_2} \to M + 3{H_2}O\left( {M = MO\;{\text{or }}W} \right)$$

• Aluminium reduction method (Aluminothermy or Gold-Schmidt thermite process): This is the process of reducing certain metal oxides which cannot be reduced by carbon like $TiO_2$, $Cr_2O_3$ and $Mn_3O_4$ etc., by using aluminium powder as reducing agent. $$\begin{equation} \begin{aligned} C{r_2}{O_3} + 2Al \to 2Cr + A{l_2}{O_3} + Heat \\ 3M{n_3}{O_4} + 8Al \to 4A{l_2}{O_3} + 9Mn + Heat \\\end{aligned} \end{equation} $$ A large amount of heat is evolved in this process so that metal is obtained in liquid state.

• Reduction by other metals: Some other electro-positive metals can also be used as reducing agents like sodium, magnesium, etc. $$\begin{equation} \begin{aligned} B{r_2}{O_3} + 3Mg \to 3MgO + 2B \\ Ti{O_2} + 4Na \to Ti + 2N{a_2}O \\\end{aligned} \end{equation} $$ Metal halides can also be reduced to metal (Kroll’s process). $$\begin{equation} \begin{aligned} TiC{l_4} + 2Mg\mathop \to \limits^\Delta Ti + 2MgC{l_2} \\ VC{l_4} + 2Mg\mathop \to \limits^\Delta V + 2MgC{l_2} \\\end{aligned} \end{equation} $$

2. Self Reduction Method: