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Superphosphate of lime is also known as calcium superphosphate . This is one of the few water soluble phosphates. A mixture of calcium dihydrogen phosphate [Ca(H2P04)2] and gypsum (CaS04. 2H20) is known as superphosphate of lime.
Superphosphate is made by treating well powdered phosphate rock (calcium phosphate) with calculated quantities of commercial concentrated sulphuric acid (chamber acids) "
Ca3(P04)2 + 2H2S04+ 4H20 -» Ca(H2P04)2 + 2(CaS04. 2H20) + heat.
(The composition of the super phosphate may change and need not be always represented by this formula only).
Industrial method :- The plant used for the preparation of superphosphate of lime is shown in fig 1.5.
The phosphate rock is ground to a fine powder. It is charged into a cast iron mixer. A calculated quantity of conc. H2S04 (Chamber acid) is added. The reaction mixture is stirred with blades present in the mixer. When the reaction had started it is dumped into one of the two dens, Dj and D2, through either the valve V, or the valve V2. The reaction is allowed to take place for 24-36 hours in the dens. In this period temperature rises to about 373 -383 K. [The carbonate and the fluoride impurities in the phosphate rock react with H2S04 and liberate C02 and HF gases. They escape through the outlet at the top]. The final product is a hard mass (due to the presence of gypsum). This mass is crushed to fine powder and sold as superphosphate of lime.
Uses :- Superphosphate of lime is a good phosphatic fertilizer. TheCaS04initisan insoluble waste product and is of no value to the plants. To avoid this waste product CaS04, superphosphate is changed into "triple phosphate" which completely dissolves in water !!!!!