Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 6M

Year: May 2013

In this method hard water is treated with calculated amount of slaked lime and soda ash in reaction tanks, so as to convert hardness producing chemicals into insoluble compounds which are then removed by settling and filtration. If this process is carried out at room temperature it is called Cold lime soda process and if carried out at higher temperature is known as hot lime soda process.

Treatment with lime:

1. Removal of temporary hardness of $Ca^{+2}$

   $Ca(HCO_3)_2 + Ca(OH)_2 → 2CaCO_3 + 2H_2O$

2. Removal of Temporary hardness of $Mg^{+2}$

   $Mg(HCO_3)_2 + 2Ca(OH)_2 → Mg(OH)_2 + 2CaCO_3 + 2H_2O$

3. Removal of Permanent hardness of $Mg^{+2}$

   $MgCl_2 + Ca(OH)_2 → Mg(OH)_2 + CaCl_2 \\ MgSO_4 + Ca(OH)_2 → Mg(OH)_2 + CaSO_4 \\ Mg(NO_3)_2 + Ca(OH)_2 → Mg(OH)_2 + Ca(NO_3)_2 $

4. Removal of $CO_2$

   $CO_2 + Ca(OH)_2 → CaCO_3 + H_2O$

5. Removal of acids

   $2HCl + Ca(OH)_2 → CaCl_2 + 2H_2O \\ H_2SO_4 + Ca(OH)_2 → CaSO_4 + 2H_2O$

6. Removal of bicarbonates of $Na^+ \ \ and \ \ K^+$

   $2NaHCO_3 + Ca(OH)_2 → CaCO_3 + Na_2CO_3 + 2H_2O \\ 2KHCO_3 + Ca(OH)_2 → 2CaCO_3 + K_2CO_3 + 2H_2O $

7. Removal of alums

   $FeSO_4 + Ca(OH)_2 → CaSO_4 + Fe(OH)_2 \\ Al_2(SO_4)_3 + 3Ca(OH)_2 → 3CaSO_4 + 2Al(OH)_3 \\ NaAlO_2 + 2H_2O . → Al(OH)_3 + NaOH \\ 2NaOH + CaCl_2 → Ca(OH)_2 + 2NaCl$

   Treatment with soda:

   Removal of Permanent hardness of $Ca^{+2}$

   $CaCl_2 + Na_2CO_3 → CaCO_3 + 2NaCl \\ CaSO_4 + Na_2CO_3 → CaCO_3 + Na_2SO_4 \\ Ca(NO_3)_2 + Na_2CO_3 → CaCO_3 + 2NaNO3$