5.0 Metallurgy of some important metals

• From Galena by Carbon Reduction: The sulphide ore is roasted/smelted to give oxide in reverberatory furnace in access of air at high temperature. $$2PbS + 3{O_2} \to 2PbO + 2S{O_2}$$ The oxide can then be easily reduced to metallic lead using coke or $CO$ in blast furnace. $$PbO + C \to Pb + CO$$

• From Galena by Self Reduction: The sulphide ore is roasted in reverberatory furnace in a limited supply of air at moderate temperature so that $PbS$ is partially oxidized. $$\begin{equation} \begin{aligned} PbS + 2{O_2} \to PbS{O_4} \\ PbS + 3{O_2} \to 2PbO + 2S{O_2} \\\end{aligned} \end{equation} $$ Supply of air is cutoff and the temperature is increased to melt the charge in the reverberatory furnace and limestone is added. $$\begin{equation} \begin{aligned} PbS + 2PbO \to 3Pb + S{O_2} \\ PbS + PbS{O_4} \to 2Pb + 2S{O_2} \\ Si{O_2} + CaO\left( {flux} \right) \to CaSi{O_3}\left( {Slag} \right) \\ PbSi{O_3} + CaO \to PbO + CaSi{O_3}\left( {Slag} \right) \\\end{aligned} \end{equation} $$ $CaO$ prevents the formation of $PbSiO_3$.
  
  Purification: By Electrolytic Refining.

• Crushing and Grinding
  
  
  
  

• Electrolytic Reduction Method: (Hall- Herault’s method)
  
  To lower fusion point of alumina, Cryolite ($Na_3AlF_6) is added.
  
  Use of Cryolite:
  
  1. To lower the melting point.
  2. Ions obtain increase conductivity.
  3. Improves electrical conductivity of cell as $Al_2O_3$ is a poor conductor. $$\begin{equation} \begin{aligned} N{a_3}Al{F_6}\mathop \to \limits^{900^\circ C} 3NaF + Al{F_3} \\ Al{F_3}\mathop \Leftrightarrow \limits^{electrolysis} A{l^{3 + }} + 3{F^ - } \\\end{aligned} \end{equation} $$
  At Cathode: $A{l^{3 + }} + 3{e^ - } \to Al$
  At Anode: $2{F^ - } \to {F_2} + 2{e^ - }$
  
  Coke Powder is added to prevent corrosion and decrease loss of heat in radiation form. $$\begin{equation} \begin{aligned} 3{F_2} + A{l_2}{O_3} \to 2Al{F_3} + \frac{3}{2}{O_2} \uparrow \\ C + {O_2} \to C{O_2} \uparrow \\ 2C + {O_2} \to 2CO \uparrow \\\end{aligned} \end{equation} $$
  
  

• Electrolytic Refining Method: (Hoop’s Method)
  
  $BaF_2$ (which is costlier than $CaF_2$) is used. It contains neutral impurities.
  At Cathode: $Al \to A{l^{3 + }} + 3{e^ - }$
  At Anode: $A{l^{3 + }} + 3{e^ - } \to Al$

Naturally, the resultant reaction will take place when the right hand side in the above equation is negative. In $\Delta {G_ \circ }$ vs $T$ plot representing reaction ($1$), the plot goes upward and that representing the change $C \to CO\left( {C,CO} \right)$ goes downward.

At temperatures above $1073\ K$ (approx.), the $C,CO$ line comes below the $Fe,FeO$ line $\left[ {\Delta {G_{\left[ {C,CO} \right]}} < \Delta {G_{\left[ {Fe,FeO} \right]}}} \right]$. So in this range, coke will be reducing the $FeO$ and will itself be oxidised to $CO$.

In a similar way the reduction of $Fe_3O_4$ and $Fe_2O_3$ at relatively lower temperatures by $CO$ can be explained on the basis of lower lying points of intersection of their curves with the $CO,CO_2$ curve in Fig. $6$.

Fig. 6: Gibbs energy ($\Delta {G_ \circ }$ vs $T$ plots (schematic) for formation of some oxides (Ellingham diagram)

In the Blast furnace, reduction of iron oxides takes place in different temperature ranges. Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about $2200\ K$ in the lower portion itself. The burning of coke therefore supplies most of the heat required in the process. The $CO$ and heat moves to upper part of the furnace. In upper part, the temperature is lower and the iron oxides ($Fe_2O_3$ and $Fe_3O_4$) coming from the top are reduced in steps to $FeO$. Thus, the reduction reactions taking place in the lower temperature range and in the higher temperature range, depend on the points of corresponding intersections in the $\Delta {G_ \circ }$ vs $T$ plots. These reactions can be summarized as follows:

At $500-800\ K$ (lower temperature range in the blast furnace):

$$\begin{equation} \begin{aligned} 3F{e_2}{O_3} + CO \to 2F{e_3}{O_4} + C{O_2} \\ F{e_3}{O_4} + 4CO \to 3Fe + 4C{O_2} \\ F{e_2}{O_3} + CO \to 2FeO + C{O_2} \\\end{aligned} \end{equation} $$

At $900-1500\ K$ (higher temperature range in the blast furnace):

$$\begin{equation} \begin{aligned} C + C{O_2} \to 2CO \\ FeO + CO \to Fe + C{O_2} \\\end{aligned} \end{equation} $$

Limestone is also decomposed to $CaO$ which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron. The iron obtained from Blast furnace contains about $4\%$ carbon and many impurities in smaller amount (e.g., $S$, $P$, $Si$, $Mn$). This is known as pig iron and cast into variety of shapes. Cast iron is different from pig iron and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about $3\%$) and is extremely hard and brittle. Further Reductions Wrought iron or malleable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reverberatory furnace lined with haematite. This haematite oxidises carbon to carbon monoxide: $$F{e_2}{O_3} + 3C \to 2Fe + 3CO$$

Limestone is added as a flux and sulphur, silicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers.