LIME SODA PROCESS- Principle:: Calculated quantity of lime & soda are added to hard water to convert soluble impurities into insoluble one which are easily removed by filtration. Reactions are as follows.

i) Removal of temporary hardness-

Ca$(HCO_{3})_{2}$+Ca$(OH)_{2}\rightarrow 2CaCO_{3} \downarrow +2H_{2}O$

Mg$(HCO_{3})_{2}$+2Ca$(OH)^{\rightarrow}_{2} Mg(OH)_{2}\downarrow+2CaCO_{3}\downarrow+2H_{2}O$

ii)Removal of permanent hardness-

$MgCl_{2}+Ca(OH)_{2}\rightarrow Mg(OH)_{2}\downarrow +CaCl_{2}$

$CaCl_{2}+Na_{2}CO_{3}$ $\rightarrow CaCO_{3}\downarrow+2NaCl$

$MgSO_{4}+Ca(OH)_{2}$ $\rightarrow Mg(OH)_{2}\downarrow+CaSO_{4}$

$CaSO_{4}+Na_{2}CO_{3}$ $\rightarrow CaCO_{3}\downarrow+Na_{2}SO_{4}$

$MgCO_{3}+Ca(OH)_{2}$ $\rightarrow Mg(OH)_{2}\downarrow+CaCO_{3}$

iii)Neutralization free acids-

2HCl +Ca$(OH)_{2}$ $\rightarrow CaCl_{2}+2H_{2}O$

$CaCl_{2}+Na_{2}CO_{3}\rightarrow CaCO_{3}\downarrow +2NaCl$

$H_{2}SO_{4}+Ca(OH)_{2} \rightarrow CaSO_{4}+2H_{2}O$

$CaSO_{4}+Na_{3}CO_{3}\rightarrow CaCO_{3}\downarrow +Na_{2}SO_{4}$

$2HNO_{3}+Ca(OH)_{2}\rightarrow Mg(NO_{3})_{2}\downarrow +2H_{2}O$

$CaNO_{3}+Na_{2}CO_{3}\rightarrow CaCO_{3}\downarrow +2NaNO_{3}$

iv) Removal of salts-

$FeSO_{4}+Ca(OH)_{2}\rightarrow Fe(OH)_{2}+CaSO_{4}$

$CaSO_{4}+Na_{2}CO_{3}\rightarrow CaCO_{3}\downarrow +Na_{2}SO_{4}$

$2Fe(OH)_{2}+1/2 \ O_{2}+H_{2}O\rightarrow2Fe(OH)_{3}\downarrow$

$Al_{2}(SO_{4})_{3}+3Ca(OH)_{2}\rightarrow 2Al(OH)_{3}+3CaSO_{4}$

$CaSO_{4}+Na_{2}CO_{3} \rightarrow CaCO_{3}\downarrow+Na_{2}SO_{4}$

$AlCl_{3}+3Ca(OH)_{2} \rightarrow 2Al(OH)_{3}+3CaCl_{2}$

v) Removal of gases-

$CO_{2}+Ca(OH)_{2}\rightarrow CaCO_{3}\downarrow +2H_{2}O$

$H_{2}s_Ca(OH)_{2}\rightarrow CaS\downarrow +2H_{2}O$

2.vi) Removal of bicarbonate-

$2NaHCO_{3}+Ca(OH)_{2}\rightarrow CaCO_{3}\downarrow+2H_{2}O+Na_{2}CO_{3}$

$2HCO_{3}+Ca(OH)_{2}\rightarrow CaCO_{3}\downarrow+2H_{2}O+CO_{3}$

3. vii) Reaction with coagulant-

$NaAlO_{2}+2H_{2}O\rightarrow NaOH+Al(OH)_{3}$