The process of extraction of moderately reactive metals involves the following steps:
Step I :
Calcination: It is the process in which a carbonate ore is heated strongly in the absence of air to convert it into metal oxide.
When calamine ore (zinc carbonate) is heated strongly in the absence of air calamine ore decomposes to form zinc oxide and carbon dioxide.
This whole process is called calcination.
Roasting: It is the process in which a sulphide ore is strongly heated in the presence of air to convert it into metal oxide.
When zinc blende ore (zinc sulphide) is strongly heated in air (roasted), it forms zinc oxide and sulphur dioxide:
Step II: Reducing agents like carbon, aluminium, sodium or calcium are used to convert metal oxides obtained by calcination or roasting of ores. The reducing agent used depends on the chemical reactivity of the metal to be extracted.
Example: Reduction of zinc oxide using carbon. Zinc metal is extracted by the reduction of its oxide with carbon (or coke). Hence, on heating of zinc oxide with carbon results in the production of zinc metal.
EXTRACTION OF LESS REACTIVE METALS
The less reactive metals which are quite low in the activity series are extracted by the reduction of their oxides by heating.
Extraction of Mercury: Heating of the sulphide ore in air results in the extraction of mercury.
Mercury metal is produced from the sulphide ore called cinnabar, HgS, which is actually mercury sulphide. Thereafter, the following steps take place:
The concentrated mercury sulphide ore (cinnabar ore) is roasted in air when mercury oxide is formed.
When this mercury oxide is heated to about , it decomposes (gets reduced) to form mercury metal:
REFINING OF METALS
The process of purifying impure metals is called refining of metals.
The most important and most widely used method for refining impure metals is electrolytic refining.
For the refining of an impure metal by electrolysis:
A thick block of the impure metal is made anode, i.e. it is connected to the positive terminal of the battery).
A thin strip of the pure metal is made cathode, i.e. it is connected to the negative terminal of the battery).
A water soluble salt of the metal to be refined is taken as electrolyte.
Example: Electrolytic Refining of Copper: Acidified copper sulphate solution acts as an electrolyte. Here, a thick block of impure copper metal is made anode and a thin strip of pure copper metal is made cathode. Thus, pure copper metal is produced on the cathode.