Phosphoric Acid Purification

JSC NIUIF is the leading organization in the Russian Federation in the process development of WPA purification from sulphates, suspensions and fluorine impurities. We offer a wide range of turnkey solutions and specialized research in this area.

Concentrated WPA, obtained by sulphuric acid decomposition of phosphate raw materials, usually contains 49-54% P2O5, 2.0-4.0% SO3, 2.5-4.0% suspensions and 0.4-0.6% fluorine in the liquid phase. The specified content of impurities in WPA is unacceptable for many subsequent processing.

To remove excess sulphates from WPA, we offer a desulphurization process based on the addition of phosphate raw materials under special conditions with calcium sulphate precipitation. The reactor for WPA desulphurization is designed taking into account the physicochemical regularities of phosphate raw materials dissolution in sulphuric-phosphoric acid solutions, allows to maximize the intensification of the process and ensure the production of easily purified suspensions. Suspensions are separated in thin-layer or radial sump tanks to obtain desulphurized purified WPA with 50-54% P2O5, 0.3-0.5% SO3 and 0.2-0.4% suspensions content. This WPA is commercial, and also serves as an intermediate product for manufacture of liquid complex fertilizers (LCF), purified phosphoric acid (PPA), feed monocalcium phosphate (MCP) and sodium tripolyphosphate (STPP). WPA purification in the production of phosphate fertilizers is carried out to stabilize the operation of vacuum evaporation units for acid concentration, improve fertilizer quality and obtain new exclusive MAP grades that have no analogues on the world market. A similar process is also used for desulphurization of non-concentrated WPA of various processes.

The increase in the importance of WPA purification process determines both the involvement of more impure natural phosphate raw materials in the processing, which makes it difficult to obtain fertilizer grades established at the market, and the use of WPA as a feedstock for the production of feed, technical and food phosphates. At the same time, concentrated WPA has become a commercial product that is transported over long distances and in large volumes.

JSC NIUIF expertise is based on many years of experience in operating plants for desulphurization and purification of WPA using thin-layer sump tanks (domestic and foreign manufacture) of various designs and dimensions, as well as a set of physicochemical laboratory studies of the purification processes of concentrated and non-concentrated WPA of dihydrate (DH) and hemihydrate (HH) processes, in which purification rates, the type and optimal flocculant consumption rate are determined, sediment compositions are studied. On the basis of experimental data, process flowsheets for the production of concentrated MAP and DAP grades are proposed. The experience gained made it possible to refine the methodology for calculating the required surface of a thin-layer sump tank using experimental data on WPA purification rates.

Operational experience allowed JSC NIUIF to develop various options for stabilizing WPA quality for both DH and HH processes by setting a purification unit for a weak WPA in a filtration unit closed cycle or concentrated WPA using various equipment (both thin-layer sump tanks with different plate projection areas, and belt vacuum filters). NIUIF has developed purification technology of weak WPA before its concentration in MAP production, according to which purification is carried out in a filtration unit closed cycle using highly-efficient thin-layer sump tanks. In this case, the sump tank is installed in cascade to the existing filtration unit tanks and product weak WPA storage facilities. The latter makes the gravity flow of production acid and condensed WPA possible.

In general, the use of thin-layer sump tanks for WPA purification from suspensions, depending on the quality of the parent acid, production capacity, type of flocculant and the sump tank design, allows to provide a residual suspension content of 0.05-0.5% in the purified WPA and a product yield of up to 80% of the parent WPA.

Expansion of application field for concentrated desulphurized purified WPA is gained by fluorine removal. The defluorination of concentrated WPA for feed phosphates and liquid fertilizer production is carried out with the preliminary introduction of inert additives (diatomite, infusorial flour, etc.) in vacuum (steam heating) or catalytic systems (heating with off-gasses) from 50-54% P2O5 and 0.35- 0.40% F up to 60-65% P2O5 and residual fluorine content not more than 0.1-0.2%.