LIME-SODA PROCESS
Basic principle:
• Lime [Ca(OH)2] and soda [Na2CO3] are the reagents used to precipitate the dissolved salts of Ca+2 and Mg+2 as CaCO3 and Mg(OH)2.
• The precipitated CaCO3 and Mg(OH)2 are filtered off.
• Lime reacts with temporary hardness, CO2, acids, bicarbonates and alums.
• Lime cannot remove the calcium permanent hardness which should be removed by soda.
• The precipitation reactions with lime and soda are very slow.
• Only calculated amounts of lime and soda are to be added. Excess amount of lime & soda causes boiler troubles like caustic embrittlement.
• The above mentioned formulae are based on the chemical reactions involved in the process as mentioned below.
Treatment with lime:
1. Removal of temporary hardness of Ca+2 Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O
2. Removal of Temporary hardness of Mg+2 Mg(HCO3)2 + 2Ca(OH)2 Mg(OH)2 + 2CaCO3 + 2H2O
3. Removal of Permanent hardness of Mg+2 MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2 MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4 Mg(NO3)2 + Ca(OH)2 Mg(OH)2 + Ca(NO3)2
4. Removal of CO2 CO2 + Ca(OH)2 CaCO3 + H2O
5. Removal of acids 2HCl + Ca(OH)2 CaCl2 + 2H2O H2SO4 + Ca(OH)2 CaSO4 + 2H2O
6. Removal of bicarbonates of Na+ and K+ 2NaHCO3 + Ca(OH)2 CaCO3 + Na2CO3 + 2H2O 2KHCO3 + Ca(OH)2 2CaCO3 + K2CO3 + 2H2O
7. Removal of alums
FeSO4 + Ca(OH)2 CaSO4 + Fe(OH)2 Al2(SO4)3 + 3Ca(OH)2 3CaSO4 + 2Al(OH)3 NaAlO2 + 2H2O Al(OH)3 + NaOH 2NaOH + CaCl2 Ca(OH)2 + 2NaCl
Treatment with soda:
Removal of Permanent hardness of Ca +2 CaCl2 + Na2CO3 CaCO3 + 2NaCl CaSO4 + Na2CO3 CaCO3 + Na2SO4 Ca(NO3)2 + Na2CO3 CaCO3 + 2NaNO3
Conclusions:
1. Half mole of lime is required for the removal of one mole of HCl and bicarbonates of Na+ and K+ each.
2. One mole of lime is required for the removal of one mole of temporary hardness of Ca+2, permanent hardness of Mg+2, CO2, FeSO4, H2SO4 each
3. Two moles of lime is required for the removal of one mole of temporary hardness of Mg+2 .
4. Three moles of lime is required for the removal of one mole of Al2(SO4)3.
5. NaOH produced by NaAlO2 reacts with hardness causing salts and produces lime. Hence its hardness must be subtracted from the lime requirement.
6. During the treatment of lime to remove the permanent hardness of Mg+2, acids and alums the permanent hardness of Ca+2 is generated in water. Hence they require both lime and soda treatment.
IPUNOTES 