The process of extracting metals from their ores and refining them for use is known as metallurgy. In other words, the process of obtaining a metal from its ores is called metallurgy of the metal.
Common terms used in discussing metallurgical operations :
1. Charge : The mixture of materials fed to a furnace for obtaining the metal is called charge.
2. Dressing of the ore : The removal of impurities associated with ore is called dressing or enrichment of the ore.
3. Calcination : The process of heating a metal-rich ore to a high temperature to convert the metal into its oxide, either in absence or insufficient supply of air is called calcination.
4. Roasting : The process of heating a finely ground ore to a high temperature in excess of air is called roasting. Roasting converts the metal present in the ore to its oxide.
5. Flux : A flux is a substance that is mixed with the fumance charge (calcined or roasted ore and coke) to remove the infusible impurities present in the ore.
6. Slag : Flux combines with the infusible impurities to convert them into a fusible substance called slag. Being light, slag floats over the molten metal and is removed from there.
Impurities present in metal oxides may be acidic or basic. For acidic impurities, such as SiO2 or P2O5, a basic flux (e.g., CaO) is added to the charge. If basic impurities such as MnO are present, silica is added to the charge.
|SiO2 +||CaO →||CaSiO3|
|P2O5 +||3CaO →||Ca3(PO4)2|
|MnO +||SiO2 →||MnSiO3|
7. Gangue or matrix : The ore mined from the earth’s crust contains some unwanted substances or impurities, such as sand, rocky or clayey materials. These substances are called gangue or matrix. The gangue has to be removed before the process of extraction of metals starts.
8. Smelting: The process of obtaining the metal by reducing its oxide ore with coke is known as smelting.
Principles of metallurgy
The extraction of a metal from its ore depends upon the reactivity of the metals.
1. Metals at the top of the activity series (K, Na, Ca, Mg, etc.) are highly reactive. They do not occur in the free state. They are extracted by the electrolysis of the molten ore.
2. Metals in the middle of the activity series (Zn, Fe, Pb, etc.) are moderately reactive. These are obtained by roasting and calcination of their sulphide or carbonate ore.
3. Metals at the bottom of the activity series (Au, Ag, Pt, Cu) being the least reactive are found in the free state. Copper and silver also occur as their sulphide or oxide ores. These are obtained by the process of roasting.