chemistry(water tech-2-external treatment)

7/30/2019 chemistry(water tech-2-external treatment)

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EXTERNAL TREATMENT (SOFTENING METHODS)WATER

Water used for industrial purposessufficiently pure.The process of removing hardness producingsubstances from water is called softening of water.

1. Lime-Soda process: In this method soluble Ca & Mg salts in waterare converted in to insoluble compounds, byadding calculated amount of Lime and soda.CaCO3 and Mg(OH)2, so precipitated, arefiltered off.


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Cold lime soda process: In this process calculated amount of chemicals(lime and soda) are mixed with water at RTAt RT the precipitates formed are finely divided,will not be settle down easily.

So, it is essential to add small amount of coagulants (Alum, Sodium aluminate, Aluminiumsulphate).These coagulants help in the fine precipitation.

Sodium aluminate also helps the removal of silica and oil.This cold L-S process provides water-containing50-60ppm.


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NaAlO2

+ H2O NaOH + Al(OH)

3

Al2 (SO4) 3+Ca(HCO3) 2 Al(OH) 3+ CaSO 4+ CO 2 HOT LIME SODA PROSESS: It involves treating water with softening chemicals

at a temperature of 80-150oCThe reactions proceed fasterSoftening capacity increasedPrecipitates and sludge formed settle down rapidlyNo coagulants are neededMuch of the dissolved gases driven out of the waterFiltration of softened water is fast


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Advantages of Lime Soda process: Very economical.Lesser amount of coagulants is needed.This process increases the pH of water andprotects from corrosion.Mineral quantity is reduced.

Disadvantages of Lime Soda process: Careful operation and skilled supervision isrequired.Disposal of large amount of sludge is difficult.Removes hardness up to 15ppm.


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2.ZEOLITE (PERMUTIT) PROCESS: Zeolite is hydrated sodium alumino silicate.It is also called as Permutits.

It capable of exchanging its sodium ionsStructure of sodium Zeolite isNa2O.Al2O3.xSiO2.yH2O.

Where x = 2-10, y = 2-6


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Zeolites are two types

1.Natural Zeolites: These arenon-porous.

Ex.: Na2O.Al2O3.4SiO2.2H2OSynthetic Zeolites: These are porous

and possess gel structure.These have higher exchange capacityPrepared by heating together chinaclay, faldspar and soda.


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Process: Na 2Ze + Ca(HCO 3 ) 2 NaHCO 3 + CaZe Regeneration:

After some time the zeolite is converted in to Ca andMg Zeolites and it gets exhausted.At this stage the supply of this hard water is stoppedand the exhausted Zeolite is reclaimed by treatingwith 10% NaCl

CaZe + NaCl CaCl 2 + Na 2ZeLimitations:

If the water is turbid, the suspended matter is to beremoved.Large quantity of iron and manganese should beremoved; otherwise they form Mn and Fe Zeolites.if mineral acids are present, they may destroy theZeolite bed.


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Advantages:It removes the hardness up to 10ppm.The equipment used is very compact.No impurities are precipitated, so there is no formationof sludge or scale.

It requires less time.It requires less skill.

Disadvantages:

Treated water contains more sodium.It removes only positive ions.High turbidity water cannot be treated .


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Ion Exchange Process: It is also called de-ionization or de-mineralisation.Ion exchange resins are in soluble, long chain,cross-linked polymers with a micro porous

structure.Resins containing acid groups (COOH, SO3H) arecapable of exchanging their H+ ions with othercations.Resins containing acid groups (OH-, NH2-) arecapable of exchanging their OH- ions with otheranions.Cation exchange resinAnion exchange resin


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Demineralization of Brackish Water The process of removing common salt from thewater is known as desalination ordemineralization.The water containing dissolved salts with apeculiar salty taste is called Brackish Water.

Brackish water is demineralized by two methods:


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Electro dialysis: In this method ions are pulled out of the salt water by

passing direct currentIn this process electrodes and plastic membrane pair

are used.

When direct current is passed through saline water,Sodium ions start moving towards negative pole

The chloride ions start moving towards the positive polethrough the membrane.

During this process the conc. of NaCl decreases in thecentral component while it increases in two sidecomponents

Desalinated water is removed from the centralcomponent from time to time


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For efficient separation ions-selectivemembranes are employed.Ex. RCOO-, RSO3- and R4N+Cl- Electro dialysis cell consists of a large

number of paired sets of plastic membranes. Advantages: Most compact unitMost economical

If electricity is easily available, it is bestsuited


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Reverse Osmosis:

When two solutions of unequal concentrationsare separated by a *SPM, flow of solvent takesplace from dilute to concentrated sides, due toosmosis.If a hydrostatic pressure in excess of osmotic

pressure is applied on the concentrated side thesolvent flow reverses.The membrane filtration is also called as Super-filtration or Hyper-filtration.In this method 15-40 Kg/Cm2 is applied.


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Advantages:It removes ionic, non-ionic, colloidal substancesetc.Cost is on the replacement of SPM (life time2years)Converts sea water into drinking water andobtaining water for very high pressure boilers.

chemistry(water tech-2-external treatment)

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