Sodium tripolyphosphate is often used in food, as a water retention agent, quality improver, pH regulator, metal chelator. The appearance is white powder, soluble in water.\
Sodium tripolyphosphate with different generation temperature has a high temperature type (Ⅰ type) and low temperature type (Ⅱ type), the difference between the two bond length and bond Angle different, Ⅰ and Ⅱ hydration products are generated after six hydrate Na5P3O10 · 6 h2o, under the same conditions, Ⅰ type rapid hydration heat is high, soluble in water are prone to agglomerate phenomenon, this is because the structure of the type Ⅰ four ligand of sodium ions, four ligand has a strong affinity for water, whereas Ⅱ type very slow speed in the water is generated six water content.
Sodium tripolyphosphate is fairly stable at room temperature, and will slowly hydrolyze in humid air to eventually produce sodium orthophosphate. The reaction is as follows :Na5P3O10+2H2O→2Na2HPO4+NaH2PO4.
The ability to chelate metal ions
Na5P3O10+Ca→Na3(Cap3O10)+2Na is formed by the complexation of sodium tripolyphosphate with metal ions such as Ca, Mg and Fe soluble in water.
The complexing capacity of sodium tripolyphosphate is generally expressed by the calcium value, that is, the gram number of calcium ions that can be complexed by 100g phosphate, and the theoretical value is 13.4.
The sodium tripolyphosphate aqueous solution is weakly alkaline (the pH value of 1% aqueous solution is about 9.7), it is in the pH range of 4.3~14 water), the formation of suspension (similar to emulsion) effect, that is, the dispersion effect. Sodium tripolyphosphate can also make liquid, solid particles better soluble in liquid (such as water) media, so that the appearance of the solution is completely transparent, as if the real solution, this is solubilization. Due to the above unique properties, sodium tripolyphosphate has become an important and ideal raw material in laundry powder.
The production process
Sodium tripolyphosphate by phosphoric acid through sodium soda neutralization into sodium orthophosphate, and then by condensation. Phosphoric acid can be produced either by wet or by heat. Wet method is the phosphate ore and inorganic acid (usually with sulfuric acid, but also available hydrochloric acid) reaction, after extraction, refining to get phosphoric acid. Thermal method is in the electric furnace will be phosphate ore and coke and silica roasting together, phosphate ore reduction into phosphorus, and then oxidation, hydration, phosphoric acid.
The process of preparing sodium tripolyphosphate from orthophosphate is simple, the process is short and the control operation is convenient. There are two main approaches.
(1) Spray drying - converter condensation polymerization two-stage method: the spray dryer is a cylinder, the upper part is connected with a horizontal burner with a gas nozzle. The gas with a temperature of 400~500℃ is passed into the upper part of the spray drying tower through the burner, and the orthophosphate solution is sprayed into the spray with a high pressure (4.2MPa) pump at the same time. Dry products containing less than 5% water are constantly removed from the lower part of the dryer. The orthophosphate mixture is dehydrated and condensed into sodium tripolyphosphate in a rotary furnace. The characteristics of this method are that the product does not need to be crushed, the drying efficiency of the spray dryer is high, the drying material is very uniform, and the stratification phenomenon will not occur. The disadvantage is that the drying and condensation polymerization are carried out in two stages, and the dust loss is large when the dehydration is directly heated.
(2) Dry-dehydration method: spray slurry into the rotary furnace, the furnace with a nozzle into the hot gas or combustion of oil