Chemistry > Ores and Metallurgy > 4.0 Steps of Metallurgy
Ores and Metallurgy
1.0 Basic Definitions
2.0 Classification of Ores
3.0 Metallurgy
4.0 Steps of Metallurgy
4.1 Crushing or Grinding
4.2 Concentration of Ores
4.3 Extraction of Crude Metal from Concentrated Ore
4.4 Purification or Refining of Metals
5.0 Metallurgy of some important metals
4.4 Purification or Refining of Metals
4.2 Concentration of Ores
4.3 Extraction of Crude Metal from Concentrated Ore
4.4 Purification or Refining of Metals
A metal extracted by any method is usually contaminated with some impurity. For obtaining metal of high purity, several techniques are used depending upon the differences in properties of the metal and the impurity.
(a) Physical Methods:
Physical Methods | Description | |
1- | Distillation | It is used to purify those metals which themselves are volatile and the impurities in them are nonvolatile or vice-versa. Some low boiling metal like Zinc ($Zn$), Mercury ($Hg$) and Cadmium ($Cd$) are purified by this method. Impure Metal is evaporated to obtain the pure metal as distillate. |
2- | Liquation | It is used for the purification of the metal, which itself is readily fusible But the impurities are infusible. Low melting metal like Tin ($Sn$), Lead ($Pb$), Mercury ($Hg$) can be made to flow on a sloping surface and thus separated from higher melting impurities. |
3- | Electrolysis | Impure metal is made to act as anode and a strip of same metal in pure form is used as cathode and put in a suitable electrolytic bath containing soluble salt of same metal. The more basic metal remains in the solution and the less basic ones goes to the anode mud. Ex- Copper, Zinc. |
4- | Zone Refining | Metals of high purity are obtained. Silicon, germanium, boron, gallium, Indium are purified (which are used in semiconductors). It is based on the fact that impurities are more soluble in the melt than in the pure metal. |
5- | Vapour phase refining | Metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal. Two requirements are : 1. Metal should form a volatile compound with an unavailable reagent. 2. The volatile compound should be easily decomposable, so that recovery is easy. VAN ARKEL METHOD: It is used to purify those metals of which metal iodide are volatile and stable. $Ti$, $Zr$, $Hf$, $Th$, $U$ and $B$ are purified by this method. $$M + {I_2}\mathop \to \limits^\Delta M{I_4}\left( {{\text{volatile and stable}}} \right)\mathop \to \limits^{hightemperature} M\left( {{\text{Pure}}} \right) + {I_2}$$ MOND’S PROCESS: This method is used to purify impure Nickel. $$Ni\left( {{\text{impure}}} \right) + 4CO\mathop \to \limits^{50^\circ C} Ni{(CO)_4}\left( {{\text{volatile}}} \right)\mathop \to \limits^{250^\circ C} Ni\left( {{\text{pure}}} \right) + 4CO \uparrow $$ |
(b) Chemical Methods:
1. Oxidative Refining: The method is used when the impurities present in the metal have a great affinity for oxygen and are more readily oxidized than the metal. Then these oxides are removed as follows :
- These oxides may form a scum on the surface of the metal, which can easily be removed by skimming.
- If oxides are volatile, they escape from the mouth of the furnance.
- The oxides may form a slag with the lining of the inside surface of the furnace and thus be removed. In the formation of the slag, the lining acts as a flux.
It is used for refining metals like $Pb$, $Ag$, $Cu$, $Fe$ etc.
Bessemerisation (Purification of Iron):
The iron obtained from blast furnace is a brittle material called cast iron or pig iron. It contains about $4\%$ elemental carbon and smaller amounts of other impurities such as elemental $Si$, $P$, $S$ and $Mn$ that are formed from their compounds in the reducing atmosphere of the furnace. Oxidative Refining is the most important method for purifying the iron and converting it to the steel.
Molten iron from the blast furnace is exposed to a jet of pure $O_2$ gas for about $20$ minutes in a furnace that is lined with basic oxide such as $CaO$. The impurities in the iron are oxidized and the acidic oxides that form react with basic $CaO$ to yield a molten slag that can be poured off. In this process $10\%-15\%$ iron gets oxidized to $Fe_2O_3$ and it goes to waste. Reactions:
$$\begin{equation} \begin{aligned} 1.{\text{ }}2Mn + {O_2}\mathop \to \limits^\Delta 2MnO\left( {Basic{\text{ }}Oxide} \right) \\ {\text{ }}Si + {O_2}\mathop \to \limits^\Delta Si{O_2}\left( {Acidic\,Oxide} \right) \\ {\text{ }}MnO + Si{O_2}\mathop \to \limits^\Delta MnSi{O_3}\left( {Slag} \right){\text{ }} \\\end{aligned} \end{equation} $$
$$\begin{equation} \begin{aligned} 2.{\text{ }}{P_2} + 5{O_2}\mathop \to \limits^\Delta {P_4}{O_{10}} \\ {\text{ }}6CaO\left( {Basic{\text{ }}Oxide} \right) + {P_4}{O_{10}}\left( {Acidic{\text{ }}Oxide} \right)\mathop \to \limits^\Delta 2C{a_3}{(P{O_4})_2}\left( {Slag} \right) \\\end{aligned} \end{equation} $$
Cupellation (Removal of Lead):
It is used to purify those metals having other metals as impurity that can be removed in the form of their volatile oxides. $$Pb\left( {{\text{Argentiferrous lead}}} \right) + Ag + {O_2}\left( {{\text{Air}}} \right)\mathop \to \limits^\Delta PbO \uparrow + Ag \downarrow $$
2. Parting Process: Crude gold obtained by MacArthur-Forrest cyanide and chlorination process contains $Ag$, $Cu$, $Zn$ and $Pb$ as impurity. $Zn$ and $Pb$ are removed by cupellation process. $Cu$ and $Ag$ are removed by parting process.
- Parting with sulfuric acid or nitric acid.
- Parting with $Cl_2$
3. Poling Process: It is employed to purify those metals which have impurity of their own oxide. It is used for the purification of copper and tin.
- Purification of impure copper: Impure copper is remelted in a reverberatory furnace lined with $SiO_2$ and a blast of $O_2$ blows into the furnace. $O_2$ oxidises $S$, $Sb$ and $As$ to their respective oxides which being volatile are removed. $Fe$ is oxidized to $FeO$ which forms a slag of $FeSiO_3$ with $SiO_2$ lining of the furnace. Molten copper left behind contains $CuO$ as impurity which reacts with Hydrocarbon/ Marsh gas Methane ($CH_4$) to give pure Copper Metal which is about $99.5\%$ pure and is called tough pitch copper. $$\begin{equation} \begin{aligned} {\text{Green Wood}} \to {\text{Hydrocarbons}} \to C{H_4} \\ 4CuO + C{H_4} \to 4Cu\left( {{\text{pure metal}}} \right) + C{O_2} + 2{H_2}O \\\end{aligned} \end{equation} $$
- Purification of impure tin: Impure tin metal contains the impurities of $Cu$, $Fe$, $W$ and $SnO_2$. The impurity of $SnO_2$ is due to the incomplete reduction of tin stone ore ($SnO_2$) during smelting. $$\begin{equation} \begin{aligned} {\text{Green Wood}} \to {\text{Hydrocarbons}} \to C{H_4} \\ 2Sn{O_2} + C{H_4} \to 2Sn + C{O_2} + 2{H_2}O \\\end{aligned} \end{equation} $$
