UNDERSTANDING PHOSPHATE CHEMISTRY

Coordinated Phosphate Theory

• Dissociation of phosphoric acidH3PO4=H+ + H2PO4

- K1=7.5x10-3 (1)H2PO4

- = H+ + HPO42- K2=6.2x10-8 (2)

HPO42- = H+ + PO4

3- K3=4.8 x 10-13 (3)• Consider the reaction

PO43- + H2O = OH- + HPO4

2-

K3= 4.8 x 10-1 = [H+] [ PO43-]/[HPO4

2-]pH – 12.318 = log (PO43-/HPO4

2-)• Degree of hydrolysis of Na3PO4 to Na2HPO4

pH % complete< 9.0 100

9.0 10010.0 10011.0 9512.0 6713.0 1714.0 2

Consider the reactionHPO42- + H2O = OH- + H2PO4-

pH- 7.218 = log[HPO42-/H2PO4-]Degree of hydrolysis of dibasic

phosphate to monobasic phosphatepH % Complete

< 5.0 1005.0 1006.0 94 7.0 61 8.0 149.0 1

10.0 0.1>10.0 0.0

Understanding Phosphate Chemistry I

• Sodium phosphates are pH buffers.

• Disodium phophates arrests caustic soda.Na2HPO4 + NaOH = Na3PO4 + H2O

• Sodium to phosphate molar ratio (Na/PO4)

• Disodium phosphate is present if coordinate of PO4& pH lies within the control boundary.

12.03:1Na3PO4(TSP)

8.82:1Na2HPO4(DSP)

4.81:1NaH2PO4 (MSP)

pH of % solution

Na/PO4Orthophosphates

Understanding phosphate chemistry I…..

• Caustic is tied up in another form so that it cannot concentratewithin the control boundary.

• Na:PO4 is used to describe mixtures of phosphates

MIXTURE Na:PO4

TSP + DSP 2:1 to 3:165% TSP + 35% DSP 2.65 : 1

• Equations to show boiler water chemical balanceH3PO4 + H2O = HPO4

2- + 2H+ + H2ONaH2PO4 + H2O = HPO4

2- + H+ + Na+ + H2ONa2HPO4 + H2O = HPO4

2- + 2Na+ + H2ONa3PO4 + H2O = HPO4

2- + Na+ + OH-

NaOH + H2O = OH- + Na+ + H2O

Understanding phosphate chemistry II

• The products of ionization of sodium phosphates are a function of pH

• CONTROLLING CAUSTICpH increases with increasing Na:PO4 ratios (at equal PO

concentrations).Solution pH and PO4 conc identify the form of phosphate

When the PO4-pH coordinate falls on 3:1When the PO4-pH coordinate falls on 2:1When the PO4-pH coordinate falls between 3:1 to 2:1

When the PO4-pH coordinate falls above 3:1

Na3PO4Na2HPO4Mixture of TSP & DSPTSP & caustic

Na: PO4 ratioExistence

Understanding phosphate chemistry II ….

• The farther the PO4-pH coordinate is kept below the line 3:1, the greater the caustic absorbing capacity.

• What happens when free caustic is produced in the boiler?

Na2HPO4 + H2O = (2Na+ + HPO42-) + H2O

NaOH + H2O = (Na+ + OH-) + H2O(2Na+ + HPO4

2-) + (Na+ + OH-) = 3Na+ + HPO42- + OH-

3Na+ + HPO42- + OH- = Na3PO4 + H2O

Understanding phosphate chemistry III

• What happens when excess caustic is present?50 Na2HPO4 + 50Na3PO4 + NaOH = 49Na2HPO4 + 51Na3PO4 + H2O

(No excess caustic)50 Na2HPO4 + 50 Na3PO4 + 51 NaOH = 100 Na3PO4 + NaOH + 50 H2O

(Free caustic is present)• It is desirable to keep the Na: PO4 ratio between 2.1 to 2.8

during coordinate PO4-pH treatment.• Na:PO4 ratio is to be determined only from the measurement of

pH & PO4 in boiler water & not by the measurement of Na & PO4• The control charts are based on Na:PO4 of pur sodium

phosphate solution.• Other sources of Na in boiler water

1. Condenser leaks 2. Sodium sulphite in industrial boilers3. Other impurities.

Understanding phosphate chemistry IV• As the boiler pressures & temperatures increased with a

subsequent reduction in the design margin, caustic corrosion problems began to reappear.

• Tri Sodium phosphate has retrograde solubility.EFFECT OF SODIUM PHOSPHATE PRECIPITATION

Decreases Na:PO4 of solution

Greater than solution

< (2.13:1)

Solution Na:PO4 remains same

Equal or greater than solution

= (2.13:1)

Decreases Na:PO4 of solution

Greater than solution

Between (2.13:1) to (2.85:1)

Solution Na:PO4 remains the same

Equal to solution= (2.85 : 1)

Increases Na:PO4 of solution

Less than in solution

> ( 2.85 : 1)EffectSoild phase Na:PO4Solution Na:PO4

Variation of pH Vs PO4 with different ammonia levels in boiler

water

Phosphate hide-out in high pressure boilers

• Phosphate hide-out occurs in dirty boilers.• PO4 conc decreases as the pressure is increased & PO4 conc

increases as the pressure is decreased.• MECHANISMS

1. Formation of Na/PO4 solids in occluded zones , such as under porous deposits or crevices.2. Reaction of PO4

3- and Na+ forming iron oxides and their subsequent inclusion within the oxide layer.Fe3O4 + 3Na3PO4 +H2 + 2H2O = 3NaFePO4 + 6 NaOH

• Mechanism No:1 operates in a deposit laden & dirty boiler.• Mechanism No:2 operates in units with magnetite during their

formation such as immediately following an acid cleaning in which old magnetite layer is removed.

Phosphate hide-out in high pressure boilers… Contd

• Degree of phosphate hide-out during load changes is an indicator of unit cleanliness.

• A reduction in phosphate hide-out over a period of time is an indicator of the thickening or “maturing” of the magnetite layerand signal a need for chemical cleaning.

• Within the range of congruent phosphate control, phosphate hide-out does not upset the ratio of Na:PO4 in solution.

Congruent Control• It was found that the caustic attack continued to occur even when

Na/PO4 mole ratio = 3.0• Presence of caustic due to precipitation of di-sodium phosphate at

high temperature• Congruent composition occurs at mole ratios of Na/PO4

2.85 at 300 deg C [ 86 kg/cm2 (g)]2.65 at 365 deg C [ 197 kg/cm2(g)]

• Congruent composition: Composition of liquid & composition of solid phases are same.

• Congruent control offers very little protection against- silica volatalization - scale formation – very susceptible

to unexpected ingress of Na.• PARAMETERS LIMITS

Phosphate mg/L 2 – 4Sodium hydroxide mg/L NILpH 8.5- 9.3Mole ratio Na:PO4 2.6 – 1DSP mg/L 4 - 15

Neutral water treatment• In Russia, the NWT chemistry was implemented by use of

gaseous O2.• In Germany by using H2O2• Oxygen conc in feed water should be kept between 200 to 400

ppbREQUIREMENTS

• For NWT, feed water cation conductivity to be maintained between 0.15 to 0.30 micromho/cm

• pH of feed water between 6.5 to 7.5• Organics in condensate and DM water to be less than 0.37 ppm

and 1.0 ppm respectively.• No condenser leaks should be allowed.• Copper alloy tubes should not be used in the feed water

heaters and in the condensers.