Lime-soda process
It is used for feed water treatment for low and medium pressure boilers. Lime and soda convert the soluble impurities, into insoluble salts, which get precipitated and after settling they are filtered off. Lime is generally 10% excess is added to water.Its advantages are Temporary hardness is removed by the following reactions:
Ca(HCO3)2+ Ca(OH)2 2CaCO3+ 2H2
Mg (HCO3)2 + 2Ca(OH)2 2CaCO3+ Mg(OH)2+ 2H2O
CaCO3&Mg(OH)2
are precipitated.
The removal of temporary hardness by lime does not introduce any soluble salt in water and reduce the total dissolve solids.Lime also removes all permanent magnesium –hardness
MgSO4/MgCl2+ Ca(OH)2 Mg(OH)2(s)+ CaSO4/ CaCl2
An equivalent amount of permanent calcium hardness is introduced, which has to be removed by soda treatment. Here, there is no softening. Lime also removes free acids, CO2and H2S Soda Treatment: It removes the remaining calcium permanent hardness.
CaSO4/CaCl2+ Na2CO3 CaCO3(s) + Na2SO4/2NaCl
Lime-soda process
(1) Batch process
(2) Continuous process
(1)batch process:
In this process chemicals are added in batches in water taken in tanks, agitated for necessary time and the precipitate allowed to settle and then filtered. Fresh water is again taken into the tank and the operation is repeated.
(2)Continuous process
Water together with lime and soda solutions enters a smaller tank continuously at a predetermined rate.The chemicals and water to be treated enter a bigger tank or chamber (see the figure). The second chamber is tall structure having a larger cross section at the top than at the bottom. As chemical reactions settle down,because towards the upper portion of chamber the velocity in less due to larger cross section,clear water is removed continuously from the top and sludge from the bottom. A part of sludge is recycled which helps in reuse of excess chemicals left behind in the sludge and also helps in precipitation by providing nuclei.
Cold lime soda process
When the chemicals are added to water at atmospheric temperature, the process is known as cold lime soda process.At this temperature precipitate is finely divided and do not settle easily, nor can it be easily filtered. Therefore it is necessary to add coagulants like alum, aluminium sulfonate, sodium aluminate etc.
Coagulants get hydrolyzedand form flocculent and gelations precipitated which entraps the finely divided particles and settle down readily or are easily filtered off. Ferrous sulfate and sodium aluminate are costlier than alum. Ferrous sulfate helps to remove oxygen and sodium aluminate helps the removal of silica and also oil.
Hot lime soda process
When this treatment is given at a higher temperature the process is known as hot lime soda process. Water is heated by waste steam or hot gases and calculated amount of chemicals is added as in cold process.
Advantages
As reactions proceed, quickly at higher temperature, the reactions are completed in about 15 min., whereas in the cold process, several hours are needed. This greatly increases the capacity of the plant.
Precipitate and sludge formed settle rapidly at higher temperature; hence coagulants are not needed.
Filtration becomes easier because of the bigger size, of the particles and the reduced viscosity of solution at the higher temperature; hence the capacity of the filter is also increased.
Residual hardness, i.e. hardness of softened water needs careful consideration. Solubility of calcium salts is very slightly higher at low temperature. More than this, the time taken for the completion of the reaction at a lower temperature is greater. In the hot process the reaction time is less with the result that the hot process gives softer water. In some places, water is highly alkaline, and contains considerable amounts of sodium bicarbonates. On adding lime, calcium carbonate precipitates forming sodium carbonate.
2NaHCO3+ Ca(OH)2 CaCO3+ Na2CO3+ 2H2O
In such a case, soda may not be heated for the removal of permanent hardness. As a matter of fact, cases are known where gypsum has to be added to remove soda so produced.
Na2CO3+ CaSO4 Na2SO4+ CaCO3(s)
