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Outline the chemical reactions involved in the lime-soda method of softening water.
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Soda lime is a process used in water treatment to remove hardness from water. This process is now obsolete but was very useful for the treatment of large volumes of hard water. Addition of lime Ca(OH)2 and soda (Na2CO3) to the hard water precipitates calcium as the carbonate and magnesium as its hydroxide. The amounts of the two chemicals required are easily calculated from the analysis of the water and stoichiometry of the reactions. The lime-soda uses lime, Ca(OH)2 and soda ash, Na2CO3, to precipitate hardness from solution.

Carbon dioxide and carbonate hardness (Calcium and Magnesium bicarbonate) are complexed by lime. In this process Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)2) and soda ash (Na2CO3).

Following are the reactions that take place in this process:

As slacked lime is added to water, it will react with any carbon dioxide present as follows:

Ca(OH)2+CO2⟶CaCO3↓+H2O⋯⋯①

The lime will react with carbonate hardness as follows:

Ca(OH)2+Ca(HCO3 )2  ⟶ 2CaCO3↓+ 2H2 O ⋯⋯ ②

Ca(OH)2+Mg(HCO3)2⟶MgCO3+CaCO3↓+2H2O⋯⋯③

The product magnesium carbonate in equation 3 is soluble. To remove it, more lime is added:

Ca(OH)2+MgCO3⟶CaCO3↓+Mg(OH)2↓⋯⋯④

Also, magnesium non-carbonate hardness, such as magnesium sulfate, is removed:

Ca(OH)2+MgSO4⟶CaSO4+Mg(OH)2↓ ⋯⋯ ⑤

Lime addition removes only magnesium hardness and calcium carbonate hardness.

In equation 5, magnesium is precipitated, however, an equivalent amount of calcium is added. The water now contains the original calcium non-carbonate hardness and the calcium non-carbonate hardness produced in equation 5. Soda ash is added to remove calcium non-carbonate hardness:

Na2CO3+CaSO4⟶Na2SO4+CaCO3↓⋯⋯⑥

To precipitate CaCO3 requires a pH of about 9.5 and to precipitate Mg(OH)2 requires a pH of about 10.8, therefore, an excess lime of about 1.25 mg/l is required to raise the pH.

The amount of lime required:

lime=carbon dioxide+carbonate hardness+magnesium ion+1.25

All in (mg/l)

 

The amount of soda ash required:

soda ash (mg/l) =non‑carbonate hardness (mg/l)

After softening, the water will have high pH and contain the excess lime and the magnesium hydroxide and the calcium carbonate that did not precipitate. Re carbonation (adding carbon dioxide) is used to stabilize the water. The excess lime and magnesium hydroxide are stabilized by adding carbon dioxide, which also reduces pH from 10.8 to 9.5 as the following:

CO2+Ca(OH)2⟶MgCO3↓+H2O

CO2+Mg(OH)2⟶MgCO3+H2O

 

Further re carbonation, will bring the pH to about 8.5 and stablize the calcium carbonate as the following:

CO2+CaCO3+H2 O⟶Ca(HCO3 )2

It is not possible to remove all of the hardness from water. In actual practice, about 50 to 80 mg/l will remain as a residual hardness.

 

Limitation of Soda Lime Process :

Lime soda softening cannot produce a water at completely free of hardness because of the solubility (little) of CaCO3 and Mg(OH)2. Thus the minimum calcium hardness can be achieved is about 30 mg/L as CaCO3, and the magnesium hardness is about 10 mg/L as CaCO3. We normally tolerate a final total hardness on the order of 75 to 120 mg/L as CaCO3, but the magnesium content should not exceed 40 mg/L as CaCO3 (because a greater hardness of magnesium forms scales on heat exchange elements).

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"Softening of water" means removal of calcium, magnesium, iron salts and similar other metallic ions, which would form insoluble metallic soaps. One of the important industrial methods employed for softening of water is "Lime-Soda process".

Lime-Soda process :

By this process, soluble calcium and magnesium salts are rendered insoluble by adding calculated amount of lime [(CaOH)2] and soda [Na2CO3]. The insoluble precipitates of CaCO3 and Mg(OH)2 are removed by filtration. By this method, both temporary and permanent hardness are removed.

For the removal of permanent hardness due to Ca-salts, lime is not necessary, but it is necessary for Mg salts. Extra addition of Ca(OH)2 causes hardness. So calculated quantities of lime and soda are to be added after the determination of actual hardness. In the actual process the water is thoroughly mixed with the chemicals and allowed to react for sufficient time. Activated charcoal is added as activator, alum etc .are added as coagulants. To avoid after-precipitation of CaCO3, sludge of the previous operation is added, which supplies the nuclei for the precipitation.

Reactions with lime Ca(OH)2 :-

Temporary hardness substances:

                                       Ca(HCO3)2+Ca(OH)2-->2CaCO3↓+2H2O
                                       Mg(HCO3)2+2Ca(OH)2-->Mg(OH)2↓+2CaCO3↓+2H2O

Permanent hardness substances :

                                       MgCl2+Ca(OH)2-->Mg(OH)2↓+CaCl2
                                       MgSO4+Ca(OH)2-->Mg(OH)2↓+CaSO4
                                       FeSO4+Ca(OH)2-->Fe(OH)2+CaSO4
                                 4Fe(OH)2+O2+2H2O-->4Fe(OH)3↓
                                 Al2(SO4)3+3Ca(OH)2-->2Al(OH)3↓+3CaSO4

Coagulants :

                                     NaAlO2+2H2O-->Al(OH)3↓+NaOH
                                     1 NaOH=1/2Ca(OH)2

Carbon Dioxide :

                                   CO2+Ca(OH)2-->CaCO3↓+H2O

Acids :

                                   2HCl+Ca(OH)2-->CaCl2+2H2O
                                   H2SO4+Ca(OH)2-->CaSO4+2H2O

Bicarbonates of sodium or potassium :

                                2NaHCO3+Ca(OH)2-->CaCO3↓+Na2CO3+2H2O
                                2KHCO3+Ca(OH)2-->CaCO3↓+K2CO3+2H2O

Reactions with soda(Na2CO3) :

                                CaCl2+Na2CO3-->CaCO3+2NaCl
                                CaSO4+Na2CO3-->CaCO3+Na2SO4

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