Boiler feed water, specially that for use in high pressure steam boilers, should be as soft as possible to avoid formation of boiler scale and corrosion of boiler tubes.
As a rule water for industrial use must be soft. Modern methods to soften hard water are the following:
(1) Lime Soda treatment
(2) Phosphate conditioning
(3) Base-exchange or Permute process, and
(4) De ionization of water.
This procedure removes both permanent and temporary hardness. Hard water is treated with slaked lime and soda ash when the calcium and:magnesium are precipitated as CaCO3 and Mg(0H)2
Typical reactions are:
Ca(HCO3)2 + Ca(OH)2 2 CaCO3 +2H20
Mg(HCO3)+Ca(OH)2 = CaCO3 + MgCO3 + 2H20
Then, since the MgCO3 is fairly soluble (100 ppm), it reacts with excess of lime, precipitating Mg(OH)2: MgCO3+ Ca(OH)2 Mg(OH)2 + CaCO3
For non-carbonate hardness, i.e., hardness due to the presence of calcium and magnesium salts, the chemical reactions are:
MgC12 + Na2CO3+Ca(OH)2 = Mg(OH)24-CaCO34-2NaCl
CaSO4+Na2CO3 = CaCO3 + Na2S04
The precipitates are removed by filtration, small amount of A11(SO4)3 or NaA102% are usually added to help the precipitate formation. The requirement of lime and soda ash is calculated by the analysis of raw water.
It is evident that, for carbonate hardness, one mole of calcium bicarbonate requires one mole of lime; while for each mole of magnesium bicarbonate two moles of lime are required. For non carbonate hardness also, the magnesium salt requires more chemicals (one mole each of soda ash and lime, while the calcium salts need only one mole of soda ash).
Coagulation and precipitation are greatly facilitated by heating. The hot lime-soda process, (by using hot water), is invariably used for boiler feed water. The effluent from the hot process is reasonably soft and has a hardness of about 20-25 ppm only. The effluent get a further treatment with phosphate in order to soften it completely for use as a boiler feed water. For feeding high pressure steam boilers, the effluent from the hot lime soda process is demineralised by ion exchange resins.
e.g., mono- and di-sodiun phosphates art added to boiler water to precipitate the calcium ions as easily removable calcium phosphate sludge, instead of a hard crust or scale.
3CaSO4-1-2Na8PO4 = Ca3(PO4),±3Na2SO4
Tri sodium phosphate (T.S.P) has a distinct alkaline reaction, and precipitates magnesium ions as the hydroxide which is also a readily removable sludge. Sodium bexa-meta phosphate or Calgon is also used for water softening, but instead of precipitating the ions causing hardness, namely Ca, Mg, Fe ions. etc., It forms complexes with them and thereby prevents the formation of scale or insoluble soaps, e.g. Naar Na4(P03)61+ CaSO4 = Na1[Na2Ca(P03)6] +NaiSO4 (Calgon) The addition of 1-2 ppm of Calgon to water for softening operations prevents an after-deposit of CaCO3 scale (which adversely affects the heat transfer and fluid flow) in pipes. This is known as the threshold treatment of water, and is important in water pipes in heat exchangers and also in laundering.
This is the modern and most effective method of removing both temporary and permanent hardness of water. The Permeate is the trade name for artificially prepared sodium aluminum silicate allied to the natural mineral zeolites. The perrnutit precipitated as a gel by mixing solutions of sodium silicate automate. The base exchange material permutit may br formulated as -Ng as where Ze is the zeolites radical, often formulated as Al2H4S18012. In the permutit process, the hard water is allowed to percolate immense a bed of grounds of permutite, when the calcium ant magnesium salts in the water react with the prrmutlt forming insoluble calcium and magnesium aluminum silicates which area retained in this filter bed. The issuing water, free from Ca and Mg, is soft.
Na2Ze Ca(HCO3)1 CaZe 2NaHCO3 (PPO Na2Ze MgSO4 MgZe Na2SO4, (PPO The ca++ and Mg++ ions in hard water are thus exchanged for an equivalent amount of sodium loos in the permutit and the hard water gets softened. Permutit process furnishes water of practically zero hardness. After use for some time, when the parmutit gets exhausted and loses its activity, it is regenerated by treatment with a 10 per cent solution of common salt. The NaCl displaces the calcium and magnesium from the exhausted permutit and replaces these by sodium, so that the bed is regenerated and ready for use again. CaZe 2NaCI CaC12 Na2Ze Very hard water is first given a lime treatment for partial softening, and then followed up with the perrnutit treatment.
Demineralisation and deionisation of water Hydrogen Cation Exchangers:
The methods generally used for softening water scarcely alter its mineral content, since sodium ions are added in amounts equivalent to the calcium or magnesium ions removed. But by using new types of ion-exchange materials, it is possible to obtain demineralised or deionized water without recourse to distillation. The hydrogen cation exchangers are either synthetic organic resins, e.g. sulphonated polystyrene resins, sulphonated phenol-formaldehyde resins or such materials as sulphonated coal,lignite or wood e.g. Zeo-Carb-H. These exchangers contain the carbonate groups which can exchange H+ ions for metal cation (such as Ca++, Mg4-4″ or Na4- ions) in solution. Such organic ion.exchange materials are employed to treat water in a wide pH range (inorganic silicate exchangers, like permutits, tend to disintegrate in low pH waters).
The untreated raw water is allowed to percolate through a bed of Hydrogen-cation exchanger,
e.g., Zeo-Carb-H, represented by HR, when all the cations are removed according to the following reactions:
Ca(HCO3)2+2HR = CaR2-1-2H20+2CO2
A similar reaction occurs with magnesium or sodium bicarbonates. The CO2 formed in the reaction may be removed by air wind degassing apparatus. Sulfates and chlorides of metals react as follows
CaSO4 + 2HR = CaR2+ H2SO4
NaCl + HR = NaR + HCl
The effluent water, therefore, contains the acids of the anions or the intake water, namely H2CO3, HCl, H2S04, etc.
The exchange bed is regenerated periodically by treatment with 0.2 molar sulfuric acid solution.
As the water obtained thus contains acid, it is not suitable for most purposes and therefore the effluent from the hydrogen cation exchanger is passed through a bed of anion exchange synthetic resin, when acids are removed by anion exchange ‘factions. Anion-exchange resin is a basic resin and is synthesized from high molecular weight amines (R4NC1) and alkali hydroxides. This resin (e.g., Amber lite IRA-400) behaves in a manner analogous to NaOH and may be symbolized R4NOH.
Anion exchange reaction may be represented as follows.
R4NOH + HCl = R4NCl + H2O
The anion exchange base is regenerated by treatment with 0.2 molar NaOH solution.
The effluent from the anion exchanger is demineralised or deionized water which is pure and completely free from all electrolytes.
The removal of all the cation and anion of the dissolved electrolytic impurities in water is known as demineralisation or de ionization of water. The only other process for the removal of all the ions in water is distillation. The deionized water is soft and is conveyed in aluminum and Polyvinyl chloride (PVC) pipes.
The anion exchange resin does not remove silica which may ht an undesirable impurity in boiler feed water as it forms a hard scale. For its removal, sodium fluoride is added to the intake water, when HE is produced in the hydrogen cation exchange unit. The HF in turn reacts with the silica, forming H2SiFe as the end-product which is readily removed by the basic resin.