Q. Describe Lime-soda method for softening of hard water. Discuss the advantages of the process and the chemistry involved in it.
OR
Describe the continuous lime soda process of softening of hard water. Compare between continuous cold and hot Lime-soda process.
Ans.
Continuous Lime-soda process: The basic principle in this method is to convert all the soluble hardness causing constituents into insoluble precipitates by appropriate chemical reactions. In this process, a suspension of calcium hydroxide (lime) and sodium carbonate (soda) is added to water in calculated quantities bases on the concentration of impurities.
Reactions involved in treatment:
(a) Ca (HCO3)2 + Ca (OH)2 = 2CaCO3 +2H2O
(b) Mg (HCO3)2 + 2Ca (OH)2 = Mg (OH)2 + 2CaCO3 + 2H2O
MgCO3 + Ca (OH)2 = Mg (OH)2 +CaCO3
(c) Mg+2 + Ca (OH)2 = Mg (OH)2 + CaCO3
(d) Ca+2 + Na2CO3 = CaCO3 + 2Na+
(e) CO2 + Ca (OH)2 = CaCO3 + 2Na+
(f) 2H+ + Ca (OH)2 = Ca+2 + 2H2O
(g) Ca+2+ + Na2CO3 = CaCO3 + 2Na+
(h) 2HCO-3 + Ca (OH)2 = = CaCO3 + 2H2O + CO2-3
(i) Fe2+ + Ca (OH)2 = Ca+2 + Fe (OH)2
(j) Al2 (SO4)3 + 3Ca (OH)2 = 3CaSO4 + 2Al (OH)3
(k) 3CaSO4 + 3Na2CO3 = 3Na2SO4 + 3CaCO3
Since 100 parts of CaCO3 are equivalent to 74 parts of Ca (OH2), the lime required for removing hardness is
Lime required = 74/100 [Temperature Ca+2 + 2* Temperature Mg+2 + Permanent Mg+2 + Fe+2 + Al+3 + CO2 + H+ (HCl or H2SO4) + HCO-Na3+AlO2]
Soda required = 106/100[Permanent (Ca+2 + Mg+2 + Al3+ + Fe2+ + H+-HCO-3]
Advantages of Lime-soda process
1. Fewer amounts of coagulants are required.
2. The pH of soft water is higher which prevents corrosion of the distribution pipes.
3. Amount of impurities such as iron and manganese are also decreased.
4. Water produced after treatment is of alkaline nature. Amount of micro organism are reduced.
5. It is very economical method.
Disadvantages of Lime-Soda process:
1. This process cannot be used for domestic purpose because it is difficult to find amount of lime-soda required.
2. It is not suitable for laundry industry, where calcium carbonate may end up on the clothing.
3. Skilled and careful supervision is required for efficient and economical operation.
4. It creates the problem of disposal of sludge.
5. The hardness of water is not completely removed.
Comparison between cold and hot lime-soda process:
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S. No
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Cold lime-soda process
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Hot lime-soda process
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1.
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Reactions are slow
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Reactions are fast and complete
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2.
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Filtration is slow
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Filtration is fast
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3.
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Less efficient
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Very efficient
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4.
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Softening capacity is low
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Softening capacity is high
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5.
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Use of coagulant is necessary
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No coagulants are required
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6.
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Requirement of lime consumption is as per stochiometry
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Less quantity of lime is required as temporary hardness is removed by boiling.
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7.
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Steam is not used
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Steam is used, which increases fuel consumption.
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8.
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Residual hardness of soft water produced is 50-60 ppm.
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Residual hardness of soft water produced is 15-30 ppm.
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