GuruIntroduction
Water is necessary for life, but not all water is the same. Hard water, with a high mineral content, can cause problems in homes and businesses. Different methods can soften hard water and make it more user-friendly, benefiting our daily lives.
Various method for treatment of hard water or softening of water
- By External Treatment
- By Internal Treatment
1.External Treatment
➤Lime Soda Process
It is the most important method of chemical water softening. In lime soda process all the soluble hardness causing impurities are chemically converted into insoluble precipitates, which may be removed by settling and filtration.
In this method calculated quantity of lime [`C_aOH_2`] and soda (`Na_2Co_3`) are added into water.The precipitated formed are finely divided, so they do not settle easily and cannot be filtered easily.Therefore we add a small amount of coagulants.
Since natural waters generally contain a large proportion of temporary hardness, it is often convenient and economical to remove temporary hardness by lime treatment. Lime is rather cheap and it removes temporary hardness efficiently without introducing any soluble salts in the water.Magnesium hydroxide produced in the above reactions precipitates as an insoluble sludge.
The reaction of soda with the permanent calcium hardness produces insoluble `Caco_3`. Addition of a coagulant such as sodium aluminate or alum helps in accelerating the coagulation of the carbonate sludge, which is subsequently removed by filtration. Water softened by this process contains appreciable concentrations of soluble salts, such as sodium sulphate, and cannot be used in high-pressure boiler installations.
(B)Internal treatment
Internal treatment consists of adding chemicals directly to the water in the boilers for removing dangerous scale forming salts which were not completely removed in the external treatment for water softening. This is mainly used as a corrective treatment to remove the slight residual hardness and also sometimes to remove the corrosive tendencies in water.
This treatment is not usually applied to raw waters, except for small boilers, but it is usually practised in larger power stations. In modern heavy-duty high-pressure boilers, water of zero hardness is required, since even an egg-shell thickness of scale may be extremely detrimental.
1.Carbonate conditioning
In this, method we add sodium carbonate to boiler water, so that salt like CaSO4 etc are converted into calcium carbonate and can be removed
`Caso_4+Na_2co_3rightarrow Na_2so_4+Caco_3`
For a salt to be precipitate, sufficient amount of the ions forming the salt must be present, so that the product of their ionic concentration is high from their solubility product. Thus, for a salt like `Caco_3` to be precipitated, the product of the concentration of
`Ca^{+2}` and `Co_3^-` must high from the solubility product of `Caco_3`.
The above principles are used in the carbonate conditioning when sodium carbonate solution is added to boiler water, the concentration of `Co_3^{-2}` ion increased and when it becomes greater than the solubility product of `So_4^-` (i.e. greater than K' × [`So_4^-`], only `Caco_3` get precipitated and `CaSO_4` remains in solution. Thus the deposition of scale forming `CaSO_4` is prevented
`Na_2co_3+Caso_4rightarrow Caco_3+Na_2so_4`
Carbonate conditioning is used only for low pressure boiler. In high pressure boiler the excess `Na_2co_3` might be converted into NaOH due to hydrolysis as follows
`Na_2co_3+2H_2 or 2Naoh+H_2Co_3`
2.Phosphate conditioning
Phosphate conditioning is applicable to high pressure boilers. In this method an excess of soluble phosphate is added to boiler water. It react with Ca and Mg salts and form soft sludge of Ca and Mg phosphate which can be removed by blow down process.
`3Mcl_2+2Na_3Po_4rightarrow M_3{left(Po_4right)}_2+6Nacl`
The three sodium orthophosphates viz., `Na_3Po_4`, `Na_2HPo_4` and `NaH_2Po_4` have been used for phosphate conditioning.Sodium pyrophosphate (`Na_4P_2O_7`) and sodium metaphosphate (`NaPO_3`) are also used for the same. The typical reactions of the various phosphates with the hardness represented as `CaCO_3`, may be summarized as follows:
The quality of the feed water decide the choice of a particular phosphate to be used. For instance,if the feed water tends to produce an acidic condition in the boiler, the alkaline `Na_3Po_4` should be choosen. This treatment could be supplemented with NaOH if the required alkalinity could not be maintained with `Na_3Po_4` alone.
If the feed water produces almost the right alkalinity desired in the boiler, it is preferable to use `Na_2HPo_4` which is practically neutral. If the boiler water becomes too alkaline, the acidic `Na_2HPo_4` would be selected.
temperatures to orthophosphate.
`NaPo_3+H_2o=NaH_2Po_4`
`Na_4P_2o_7+H_2o=2Na_2HPo_4`
Thus, their behavior within the boiler is identical with that of orthophosphates mentioned above. However, `NaPo_3` solutions are practically neutral, whereas `NaH_2Po_4` solutions are acidic.Hence the former would be preferred if the use of `NaH_2Po_4` causes feedline corrosion.
The use of internal treatment combined with suitable blow down to remove sludge has contributed largely to the operation of the modern high-pressure steam boilers without the formation of hard scales. However, precaution should be taken to inspect them at least once in six months and remove the scale and sludge accumulations.
3.Colloidal conditioning
4.Calgon conditioning
Another approach for preventing scale formation is to convert the scale forming salts into highly soluble complexes which are not easily precipitated under the boiler conditions. In order to achieve this, sodium hexameta phosphate (Na PO3)6 or Na2 [Na4P6O18] (its trade name is calgon) is generally employed. This substance interacts with the residual calcium ions forming highly soluble calcium hexametaphosphate and thus prevents the precipitation of scale forming salts.
Conclusion
Selecting the appropriate water softening technique is influenced by water hardness levels, budget limitations, and individual preferences. Being knowledgeable about different treatment choices helps people make educated decisions about enhancing water quality. Whether choosing ion exchange or investigating innovations like salt-free conditioners, the objective remains consistent - to experience the advantages of soft water and safeguard one's home and appliances from the detrimental impacts of hard water minerals.