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Difficulties encountered during extraction of alkali metals
Alkali metals, can not be extracted from their ores by the usual methods of extraction of metals because of the following difficulties:
(i) Alkali metals are strong reducing agents and hence can not be extracted by reduction of their oxides or chlorides.
(ii) Alkali metals being highly electropositive can not be displaced from the aqueous solutions of their salts by other metals.
(iii) Alkali metals can not be isolated by electrolysis of the aqueous solution of their salts since hydrogen is liberated at the cathode instead of the alkali metal because the discharge potentials of alkali metals are much higher than that of the hydrogen. However, by using mercury as cathode, the alkali metals can be deposited at the cathode but the alkali metals so deposited readily combines with mercury to form an amalgam from which its recovery is very difficult.
Therefore in view of above difficulties, only successful method is the electrolysis of their molten (fused) salts usually chlorides.
Extraction of Lithium
Minerals of Lithium
(i) Spodumene, LiAl(SiO3)2 – 6% Lithium
(ii) Triphylite (Li, Na)2PO4 (Fe, Mn)3(PO4)2 – 4% Lithium
(iii) Petalite LiAl(Si2O5)4 – 2.7 – 3.7% Lithium
(iv) Lepidolite (Li, Na, K)2(SiO3)3(FOH)2 – 1.5% Lithium
(v) Amblygonite LiAl(PO4)F
It involves the following steps:
1. Preparation of Lithium chloride
The minerals are first of all converted into lithium chloride by any one of the following methods:
(i) Acid treatment method
The finely powdered silicate ore is first heated to about 1373 K to make it more friable and then with H2SO4 at 523 K. The Li2SO4H2O thus formed is cooled, leached with water and then filtered to remove silica (SiO2). The filtrate thus obtained is treated with a calculated amount of Na2CO3 to precipitate aluminium and iron as carbonates which are filtered off. Excess of Na2CO3 is then added to the filtrate to precipitate Li2CO3. This is filtered and dissolved in HCl to obtain LiCl which is purified by extraction with alcohol.
(ii) Fusion method
The powdered silicate mineral is fused with CaCO3 and the fused mass is extracted with HCl and filtered. The filterate contains chlorides Li, Al, Ca, Na and K whereas silicon is removed as insoluble residue. The filterate is evaporated to dryness and the residue is extracted with pyridine in which only LiCl dissolves. Pyridine is distilled off while LiCl is left behind. The method discussed above may be summed up in the following flow-sheet.
Alt text: fusion method to prepare lithium chloride
2. Electrolysis of Lithium chloride
A mixture of dry lithium chloride (55%) and potassium chloride (45%) is fused and electrolysed in an electrolytic cell shown in the figure.
Alt text: electrolysis of Lithium chloride
Potassium chloride is added to increase the conductivity of lithium chloride and to lower the fusion temperature. The cell is operated at a temperature of about 723 K and voltage of 8-9 volts is applied.
As a result of electrolysis, the following reactions take place:
LiCI++ ↑ Li+ + CI-
At cathode: Li+ + e- → Li
At anode : 2CI- - 2e- → CI2
Chlorine gas, a valuable by product liberated at the anode leaves the cell through the exit while molten lithium rises to the surface of the fused electrolytes and collects in the cast iron enclosure surrounding the cathode. The metal thus obtained is 99% pure and is preserved by keeping it wrapped in paraffin wax. It may be noted here that lithium being the lightest metal known (density = 0.534 g ) can not be stored in kerosene oil since it floats on the surface.