SODIUM TRIPOLYPHOSPHATE 

In many tripolyphosphate, the representative and most importantly, sodium

tripolyphosphate (abbreviated as STPP), it is the phosphate industry in the bulk of

industrial products, mainly for synthetic detergent. Sodium tripolyphosphate present the

world's annual production capacity of 3.58 million t, production and sales of about 3

million t. Although the environmental problems, some developed countries, stagnant

production of sodium tripolyphosphate or decreased significantly, but due to rise in

developing countries is still in development stage, therefore, the world's total production

capacity of sodium tripolyphosphate is still 2% per yeargrowth rate of 3%. China's

production of sodium tripolyphosphate increased rapidly in recent years. Quality close to

or reached the international advanced level. 

The nature of sodium tripolyphosphate 

Sodium tripolyphosphate as a white powder, apparent density (also called packing

density) is 0.30 ~ 1.10g/cm3. Anhydrous sodium tripolyphosphate as the two different

configurations, bringing some of the physical properties of different, but the same

chemical properties. , P3O105-is a good complexing agent, with calcium, magnesium,

iron and other metal ions to form soluble complexes. For example, STPP and Ca2 +

complexation reaction to form Na3 [CaP3O10], these soluble complexes with high

stability, see Table 4-8. 100gSTPP Ca2 + complex is about 10.87g. 

  

(1) solubility as shown in Figure 4-10, sodium tripolyphosphate in water solubility is a

function of temperature. At room temperature, sodium tripolyphosphate hexahydrate

solubility of 13gNa5P3O10/100g solution.STPP-Ⅰ and STPP-Ⅱ hexahydrate more soluble

in water than in the range of 0 ~ 100 ℃ are saturated crystalline hexahydrate, and

STPP-Ⅰ than STPP-Ⅱ has a faster hydration, which is As STPP-Ⅰ the configuration of four-

coordinate the presence of sodium ions, showing a stronger affinity for water, therefore,

STPP solubility has its particularity. The STPP-Ⅰ and STPP-Ⅱ in the given conditions, the

maximum available without salt and water solubility and dissolution rate of hydrate

formation of six two factors determine the rate. When STPP-Ⅰ and STPP-Ⅱ soluble in

water. Start solubility (referred to as instantaneous solubility) are high, over time, are

reduced to six hydrate equilibrium solubility. As STPP-Ⅰ type quickly dissolved in water

and formation of hexahydrate, the solubility rapidly from STPP-Ⅰ type hexahydrate

solubility decreased solubility, supersaturation of the hydrate will Na5P3O10 • 6H2O

crystals precipitated from the solution, then the formation of insoluble druse or clumps,

which gives synthetic detergent slurry preparation operations difficult. The STPP-Ⅱ type

no such agglomeration, so the production of synthetic detergents usually STPP-Ⅱ type

more sodium tripolyphosphate. Should be noted that the factors affecting the solubility of

anhydrous STPP are many. 1% Na5P3O10 aqueous solution pH = 9.7, for preparation

aspects of reasons, usually industrial sodium tripolyphosphate, pH value of 9.5 ~ 10.1. 

Anhydrous STPP with moisture absorption, easy to absorb water vapor six hydrate

formation. The hygroscopic substance is STPP STPP in the wet air hydration. Air relative

humidity of hydration for the STPP has significant influence, relative humidity, hydration

rate of speed; the contrary, the hydration rate is slower. 

(2) hydrolytic stability of the sodium tripolyphosphate in water or damp air, occurs

hydration generated hexahydrate, which is a metastable hexahydrate, further hydrolysis

of pyrophosphate and orthophosphate form.

Factors affecting hydrolysis: ① solution pH, pH value decreases as the hydrolysis rate

increases. Table 4-6; ② temperature, temperature rise, hydrolysis speed, see Table 4-6

and Table 4-7. 

  

Should be emphasized that at room temperature sodium tripolyphosphate with a high

hydrolytic stability. For example, in pH = 9 ~ 10, when the concentration of 0.1 ~

10gSTPP/100mL day, degree of hydrolysis is only 0.01% ~ 0.02%. 

(3) complexation of water-soluble sodium tripolyphosphate is a good linear

polyphosphate 

  As STPP with Ca2 +, Mg2 +, Fe2 +, Fe3 +, Cu2 + and other complex, therefore, STPP

can be used as hard water softeners, the quality improver in food processing and H2O2

stabilizer. 

In addition, STPP have emulsifying, plastic melting and so on. 

The structure of sodium tripolyphosphate 

Formula of sodium tripolyphosphate Na5P3O10, are linear

polyphosphate.Tripolyphosphate anion P3O105-can be seen as composed of three PO4

tetrahedra condensation: 

  

There anhydrous sodium tripolyphosphate hexahydrate and two of them there are the

two anhydrous different configurations: STPP-Ⅰ type (high temperature) and STPP-Ⅱ-type

(low temperature). Heated to temperatures above 417 ℃, STPP-Ⅱ type can easily turn to

STPP-Ⅰ type, but the STPP-Ⅰ type to STPP-Ⅱ type is difficult and extremely

slow. Therefore, at room temperature sodium tripolyphosphate anhydrous form of the two

can be considered stable and coexistence. Sodium tripolyphosphate for industrial

products is often the STPP-Ⅰ type and STPP-Ⅱ type of mixture, as determined by the ratio

between the conditions of the production process. 

STPP-Ⅰ and STPP-Ⅱ type structure of the main difference is that cations (Na +) the

coordination of different. In the STPP-Ⅱ type in all of the sodium ions were carried out six

oxygen octahedral, in the STPP-Ⅰ type in the number of sodium ions is only around four

oxygen to form a tetrahedral coordination, shown in Figure 4-9. Cation because of the

two structures - the formation of anion electrostatic interaction between the three-

dimensional network structure, but also in the STPP-Ⅱ presents a unique arrangement of

a similar sheet. STPP-Ⅰ and STPP-Ⅱ type can be X-ray, IR spectroscopy and Raman

spectroscopy to distinguish between them. 

  

Preparation of sodium tripolyphosphate principle 

Sodium tripolyphosphate by the hot phosphoric acid or phosphoric acid with alkali (soda

ash, caustic soda) in and and dehydration polycondensation. Mainly in its preparation

and response and condensation, this condensation reaction principle and focus on the

problem. 

(1) phosphoric acid and phosphoric acid and soda ash in the reaction of sodium salt

mixture obtained, the reaction is: 

  

      Such as hot phosphoric acid, because of its purity and concentration are both high,

without purification, and liquid which can be polymerized directly. 

      If using WPA, because it contains SO42-, H2SiF6, Fe2O3 • 4H3PO4, Al2O3 •

4H3PO4, CaH4 (PO4) 2, etc., and occurs in the following reaction:

  

      WPA in and, if the extraction of phosphoric acid with soda ash in and to pH = 4.2 ~

5.0, then precipitate the most fully sodium fluoride. So most of the fluoride phosphate

sodium fluoride precipitation. Recycling sodium fluoride used in the production of various

fluoride compounds. Separated again in the first and will help reduce the fluorine content

in solution and the preparation of high-quality STPP. And in the process, the metal

sesquioxide phosphate precipitation, as "caustic sludge" removed. 

The SO42-ion can be added to BaCO3 removal: 

  

The use of a solvent (butanol, etc.) and extraction with the method of removal of

phosphoric acid can be better in the calcium, magnesium, aluminum and fluoride. 

In the phosphoric acid process, the control and the degree of quality standards is a

necessary condition for STPP. Is called in and the degree of phosphorylation were in and

level. For the production of STPP, it means the use of soda ash and phosphoric acid to

obtain the system needed to meet the composition STPP sodium salt mixture. That

Na2HPO4: NaH2PO4 molar ratio of 2:1. Because 

  

Therefore, the neutralization degree can be expressed as: 

  

In phosphate, the commonly used Na2O/P2O5 phosphate molar ratio to represent the

composition of which is expressed in and the corresponding degree. For STPP,

Na2O/P2O5 = 5 / 3 = 1.67. That the significance of these two methods are the same,

both in practical application of factory production. 

When the extent of control 66.67% (or Na2O/P2O5 = 1.67), the polymerization reaction

products were Na5P3O10 and H2O. Then Na5P3O10 and P2O5 content of the theory of

values should be 100% and 57.88%. 

And the degree of greater than 66.67% (or Na2O/P2O5 = 1.67), the polymerization

reaction products were Na5P3O10 and H2O. Then Na5P3O10 and theoretical values of

P2O5 content should be 100% and 57.88%. 

And the degree of greater than 66.67% (or Na2O/P2O5> 1.67), excessive condensation

to pyrophosphate Na2HPO4 will: 

  

So that the final product in the pyrophosphate content of STPP increased, resulting in

decreased product P2O5 less than 57.88%. 

In and is less than 66.67% (or Na2O/P2O5 <1.67). Excessive dehydration condensation

occurs NaH2PO4 generate partial sodium salt: 

  

P2O5 content of lead products increased more than 57.88%. Particularly insoluble

metaphosphate, the product will increase the water content of insoluble matter. 

It can be clearly seen and the degree of control in the meaning and importance. Shown in

Figure 4-11 from the phase diagram of polyphosphate can be well illustrated. STPP to

stabilize and improve the quality, must be strictly controlled and the degree of accuracy. 

  

     (2) condensation reaction mechanism of the sodium salt is qualified mixture

dehydrated at 350 ~ 402 ℃ polycondensation, can be obtained sodium

tripolyphosphate. 

  

The course of polycondensation, generally considered the temperature 180 ~ 290 ℃

polycondensation first time orthophosphate into pyrophosphate: 

  

When the temperature rose to 290 ~ 310 ℃ when the condensation into pyrophosphate,

sodium tripolyphosphate, polycondensation was carried out to enable faster and more

complete, the reaction temperature should be controlled at 350 ~ 400 ℃. 

  

Some people think that burning orthophosphate, Na2HPO4 and NaH2PO4 first into

pyrophosphate and metaphosphate: 

  

At 185 ~ 220 ℃ in order to generate maximum speed of the intermediate compound, and

then pyrophosphate and metaphosphate mutual reaction of sodium tripolyphosphate, and

290 ~ 310 ℃ when the fastest response. 

  

The main factors that affect the condensation reaction are: 

a. sodium phosphate ratio as mentioned above, precise control and the degree to

Na2O/P2O5 = 1.67, is qualified STPP Preparation of the necessary conditions are also

important factors affect the condensation reaction. 

b. temperature experiments show that the higher the temperature, the time required to

complete polycondensation shorter. For example, 225 ℃ 2 hours, 250 ℃ 50 minutes

when, 300 ℃ 20 minutes; and the higher the temperature, the higher the content of the

product in the STPP, 225 ℃ when the STPP content 36%, 250 ℃ 48% when 300 ℃

was 84.5%.According to known condensation reaction mechanism, when the

temperature rose to 290 ~ 310 ℃, the focus on the rapid condensation of phosphate into

the STPP, in the actual production, in order to Na2HPO4 and NaH2PO4 quickly and

completely into STPP, the reaction temperature is usually controlled at 400 ± 20 ℃. 

c. Catalyst in the refining and in the medium with 0.5% ~ 1% NH4NO3 as a catalyst to

accelerate the formation of STPP to condensation reaction at lower temperatures, see

Table 4-9. Also conducive to the formation of STPP-Ⅱ, reduce the content of STPP-Ⅰ. 

  

In the polymerization of STPP is sodium salt can be used as a catalyst for the reaction of

the compounds are mainly H2O, nitrate, urea and ammonia salts and organic salts,

usually NH4NO3. According to the literature, add the appropriate amount of soluble

potassium salt (as KNO3), can improve the content of STPP products to reduce the

formation of insoluble metaphosphate, for detergent production. 

As STPP-Ⅰ type hydration moisture quickly, easy to agglomerate, from the practical view,

STPP-Ⅱ type in the production of synthetic detergents is more valuable, so products

STPP-Ⅰ type content should not be too high.China Sodium Tripolyphosphate STPP-Ⅰ type

of internal control target levels of 5% to 20%, in order to get high content of STPP-Ⅱ type

sodium tripolyphosphate, in industrial production to take the following measures: 

a. adding an appropriate amount of nitrate as a catalyst, lowering the temperature of

condensation reaction, stability STPP-Ⅱ type, make the products to increase brightness. 

b. sodium salt in anhydrous condensation is to maintain adequate steam pressure, help

STPP-Ⅰ to Ⅱ type of transformation. U.S. chemical company Monsanto is to maintain a

certain amount of steam pressure to produce STPP-Ⅱ High content of sodium

tripolyphosphate, provided to P & G company. 

c. the positive sodium salt (Na2O: P2O5 = 5:3), by adding a small amount of STPP

crystals can improve the STPP-Ⅱ type yield, and STPP-Ⅰ and six hydrates than STPP-Ⅰ

type more effective , the amount of about 2%. 

d. control of polycondensation reaction temperature in the 400 ± 20 ℃.Because STPP-Ⅰ

is a high temperature, 450 ~ 622 ℃ in the range of stability, the higher the temperature,

the more favorable STPP-Ⅰ type. 

Methods of production of sodium tripolyphosphate 

Current world production of sodium tripolyphosphate basically two process routes: the

use of phosphoric acid and sodium carbonate in hot and called hot phosphoric acid

process; wet phosphoric acid and soda ash in and called the wet process phosphoric

acid process. As a high energy thermal phosphoric acid, combined with phosphoric acid

purification technology improving, so production of phosphoric acid STPP developed

rapidly in recent 10 years. 

In the production of sodium tripolyphosphate, sodium salt, if the first is obtained by

anhydrous sodium salt, and then condensation into the STPP, called two-step method; if

directly from the salt solution is prepared sodium finished STPP, is called a

step method. Therefore, STPP thermal phosphoric acid production process has one step

and two-step, one step and two-step method WPA. 

(1) Thermal Process Phosphoric Acid STPP with hot phosphoric acid production STPP,

the main production process including the preparation of phosphoric acid, phosphoric

acid medium and, condensation drying sodium salt, and recovery and tail gas

emissions. Process shown in Figure 4-12. 

  

Hot phosphoric acid by the oxidation of hydration reaction of yellow

phosphorus. Phosphoric acid, both at home and abroad too intermittent method, that is

crude and adjusted (fine tuning) is intermittent. In and should pay attention to the

following: 

a in the and operation: You can post the first acid base, or first base after the acid. But

Either way, the feed rate to be even appropriate, because the neutralization reaction will

generate a lot of CO2 gas and water vapor, should prevent the flash pan pulp, pay

attention to safety. 

b adding alkali ways: First, add in the preparation of a good lye. System is generally in

the carbon steel tank with water soluble base, heated to 40 ~ 50 ℃, stir, put a certain

amount of soda ash, alkali was obtained in 50% of soda ash solution, and placed in the

slot and used for phosphoric acid.Second, in and of solid sodium carbonate. After

measurement of phosphate from the high slot and into the stainless steel tank, starting

mixer, adding the solid sodium carbonate in the ongoing and H3PO4. As the light of

domestic soda ash is alkaline, relative density of 0.56 ~ 0.74, so the feed must be

uniform to avoid localized phenomenon occurred in alkali, precipitated solid package of

soda ash Na2HPO4, hinder its further reaction. 

c good control and the degree of: feeding stopped, boiled for 30 minutes, in order to

complete reaction in and control the reaction end point pH 6.5 ~ 7.0. And the liquid slurry

in the relative density of 1.50 ~ 1.60, then add the appropriate amount (0.5% ~ 1%)

NH4NO3, obtained sodium salt is qualified and to prepare polymer solution used. 

And after the mixture is in the sodium phosphate salt solution (with dry salt is about 40%

to 50%) with a pump to the high slot, and then released by the compressed air tank by

the high pressure spray through the nozzle into the spray drying tower for drying , heat

furnace gas drying tower by the heavy oil burning furnace or other heat source supply,

the inlet temperature of 600 ℃ ~ 700 ℃, and the material and flow, dried powder of

sodium salt, most of the discharged from the bottom of the column, a small furnace gas

to be exported away, through the cyclone dust collection, gas stove sent emptying after

washing. 

From the spray drying tower and the unloading end of cyclone recovery is sodium salt,

delivered by a bucket elevator hopper, then into the rotary furnace for calcining

polymerization polymerization, the use of gas from the heavy oil burning furnace or by

other heat supply, gas temperature is 600 ~ 700 ℃, material polymerization temperature

of 350 ~ 450 ℃, residence time in oven for 30 to 60 minutes. Polymerization of sodium

tripolyphosphate, by cooling shall be finished, sent packing. 

(2) production of phosphoric acid with sodium tripolyphosphate production of phosphoric

acid STPP,, the wet process phosphoric acid main preparation and purification,

neutralization, concentration, aggregation, and recovery and tail gas emissions. 

WPA must defluorination, desulfurization purification. Because impurities in phosphoric

acid not only finished in the STPP, decreased, but the process of dehydration

condensation play against catalytic. WPA in the chemical purification, the first removal of

phosphoric acid with soda ash fluoride, control solution of pH 4.2 to 5.0, making it

Na2SiF6 precipitation.Defluorination separation (or separation) of phosphoric acid

desulfurization with BaCO3, according to the content of phosphate in the SO42-inputs

determine the amount of BaCO3 and maintain desulfurization temperature of 60 ~ 80 ℃,

reaction time of 15 minutes to maintain so that SO42-content of less than 0.15 %. When

necessary, with Na2S (or P2S5) for arsenic removal and the removal of heavy metal

ions. 

WPA and operations not only in the first acid, alkali, or alkali after the first acid, and can

simultaneously adding acid. Soda with a pre-prepared solution (concentration 35%) in the

heating and stirring conditions, the ongoing, pH value controlled at 6.5. At this point

phosphoric acid Ca2 +, Mg2 +, Al3 +, Fe3 +, etc. out of the formation of phosphate

precipitation, as residue, filter press separation. Soda residue can be comprehensive

utilization, with alkali (caustic soda) dissolved Preparation Na3PO4, or add NH4NO3,

potassium salt, etc. made of NPK fertilizer. Filtrate were adjusted to meet the production

requirements of them and the degree.After the adjustment pressure filtration and sodium

salt solution (relative density of about 1.20) into a single effect, double effect, or three-

way vacuum evaporator, concentrated and vacuum of 53.3 ~ 60kPa, the relative density

of the slurry was concentrated to about 1.50, by adding an appropriate amount of

NH4NO3, and then in the aggregate and liquid sent to the drying section in. 

    Aggregation may be one step drying may also be a two-step method, depending on

the specific circumstances of each plant. 

The use of sodium tripolyphosphate and quality indicators 

Mainly for the synthesis of sodium tripolyphosphate detergent, water softener, drilling

mud and mineral flotation of dispersing agent, a catalyst for organic synthesis, as well as

for medicine and food additives. As STPP application is very broad, all producing

countries to develop a quality index, the country STPP product specifications and

indicators shown in Table 4-10.