What method can be used to obtain industrial grade sodium tripolyphosphate?

Chemical precipitation method is to add some specific chemicals to the wet process phosphoric acid, so that it forms a precipitation with the impurities in the acid, and is separated by filtration to remove the impurities. The main advantages of this method are: simple process, less investment, low production cost, the disadvantage is that the purification effect is poor, and it will introduce other ions, which will bring trouble to the subsequent deep purification. Chemical precipitation method is one of the common methods for defluorination of wet phosphoric acid or removal of various harmful heavy metals, and the representative methods are neutralization precipitation method and sulfide precipitation method. Here are a few examples:

(1) Fluorine and silicon removal: fluorine in wet phosphoric acid, usually in the form of SiF6, industrial production often adds sodium salt (such as NaCl) or potassium salt, and forms insoluble Na2SiF6 or K2SiF6 precipitation, which can be filtered and separated to remove fluorine and silicon.

(2) Removal of sulfate: add calcium salt (when the requirements are not high) or barium salt (when the requirements are high), and form insoluble CaSO4 or BaSO4 precipitation, which can be removed by filtration and separation.

(3) Removal of iron, aluminum, magnesium: Adding alkaline substances (such as Na2CO3, NH3, etc.) and wet phosphoric acid for partial neutralization (PH = 2.5 ~ 3.0) to form a series of insoluble iron, aluminum, magnesium phosphate complex compounds, which can be removed by filtration and separation.

(4) Removal of arsenic, lead, cadmium: add sulfide (such as Na2S, P2S5, etc.), and form insoluble arsenic sulfide or sulfide heavy metal compound precipitation, can be removed by filtration separation. In the early years, when China produced industrial sodium tripolyphosphate products, the above-mentioned chemical precipitation method was used to purify wet phosphoric acid. That is to use soda ash and wet process phosphoric acid for "two-stage neutralization", the initial phase of neutralization to form the above ①②③ precipitation, by pressing filtration to eliminate impurities, the clear liquid is continued to neutralize with alkali to prepare sodium tripolhosphate products.

Production practice shows that the content of sodium tripolyphosphate (Na5P3O10) in the prepared products is usually only 85 ~ 92%, which only meets the second grade of the national standard "Industrial sodium tripolyphosphate" GB9983-2004, and it is difficult to make a grade, but also can not reach the index of excellent grade. In addition, the recovery rate of P2O5 in phosphoric acid is only 75-80%. Therefore, after the 1990s, China has used thermal phosphoric acid to produce industrial grade sodium tripolyphosphate products.

The chemical precipitation purification method of "two-stage neutralization" is also used to produce feed grade calcium hydrogen phosphate by wet phosphoric acid in China. The calcium hydrogen phosphate obtained in the neutralization process in the early stage can only be used as fertilizer because it contains more impurities and does not meet the relevant standards. Feed grade calcium hydrogen phosphate was obtained only after neutralization. The ratio of the two grades of products (in terms of P2O5) is 0.5/0.5 to 0.3/0.7. Based on the same principle, using the above-mentioned "two-stage neutralization" chemical precipitation purification method, wet process phosphoric acid can also be used as raw material to produce fertilizer grade and industrial grade ammonium phosphate two products at the same time. However, there have been no reports of pilot tests or industrial production.