Prayon Phosphoric Acid

History

Prayon has been selling its know-how, which includes equipment as well as process technology, since 1950's and now between 50% and 60% of the world's phosphoric acid is produced in plants using Prayon Technology.

More than 120 plants have been built, ranging in capacity from 25 to 1500 t/d P₂O₅. The most widely adopted Prayon process is its dihydrate process undergoing a number of modifications over the years, resulting in today's fourth-generation process - the Prayon Mark IV Dihydrate Process.

KEMWorks Inc. is proud to be a licensee for the full range of Prayon technology, including phosphoric acid production and fluorine recovery.

Features

The Prayon Mark IV process offers several special features, which ensure a highly efficient design:

• The patented low level flash cooler and high circulation rate improves filtration and reduces scaling by sodium and potassium fluorosilicates and gypsum
• A multi-compartmented design that gives flexibility and allows the plant to be optimized for any rock
• Mark IV reaction and agitation system provides high P₂O₅ recovery and low power consumption.

The Process

Reaction

The Prayon system consists of a single concrete tank divided into a number of compartments depending on the size of the plant. This modular design avoids the scale-up problems of many of the large single tank reactors. It also allows the process to operate with a defined sulfate gradient - a feature absent in single tank reactors.

Agitation

• One agitator is placed in each of the attack tank compartments. The agitators have three sets of impellers.
• The top impeller extends above the slurry surface and serves three functions:
• To break any foam generated, reducing or eliminating the need for defoamer
• To splash slurry above the surface to dilute the sulfuric acid, and to disperse the rock slurry as it enters the attack tank
• To improve the removal of heat through the ventilation system.

The middle impeller is a 45° pitched blade, that produces a high amount of shear. This promotes the incorporation of reactants into the slurry mass.

The bottom impeller blades are propeller types to ensure a clean tank floor.

Low Level Flash Cooler

The Low Level Flash Cooler (LLFC) allows a high volume of slurry to be pumped with a low delta T. This minimizes scaling and reduces spontaneous nucleation of gypsum crystals. Few crystals mean larger crystals and a higher filtration rate.

Filtration

A tilting-pan filter or belt filter are used depending on the size of the plant and layout considerations.

For phosphoric acid plants requiring a smaller filter or a multiple small units PRAYON offers the Gaudfrin-Phosfil linear belt filter commercialized by PROFILE. 

PRAYON's experience in phosphoric acid production linked with the Filtration expertise of PROFILE has been incorporated into the Gaudfrin belt filter design to provide a machine totally compatible with all the requirements for phosphoric acid separation from the Gypsum or Hemihydrate slurry with minimum solids content and dilution of the acid with minimum scaling and maximum on-line time.

Features that characterize this filter are:

• Pre-separation vacuum box with excellent access;
• Single self aligning wear strip;
• Unique belt support system maintaining lateral horizontality.

For filter sizes above about 70 m2 the first choice would be a tilting pan filter.

The PRAYON tilting Pan Filter was developed "in-house" at Prayon at the end of the 1940's and has been developed progressively over the past 50 years to its present highly developed form.

From small beginnings the filter is now produced in sizes up to an active surface of 250 m² with 32 filtration cells or pans. Over two-thirds of the phosphoric acid produced in the world uses this type of filter.

Even the biggest filters to have a cycle time close to 2 minutes while smaller filters can have cycle times of just over one minute. This coupled with the excellent hydraulic properties of the new cell design means that at the same time high productivity and high washing efficiency can be attained. The latest design of cloth-fixing incorporated into the "fast-drain" design enables cloth changes to be made in less than 10 minutes per pan.

PRAYON now offers the TDI filter, that has a tangential cake discharge. This filter offers:

• a very compact design (ratio of filtering surface to base area);
• with fewer mechanical parts compared with other tilting pan filters;
• savings on energy consumption and maintenance costs.

Evaporation

Acid is concentrated in a forced-circulation evaporator operating under partial vacuum. The vacuum system is designed so that no vacuum pump or steam ejectors are required.

Fluorine Recovery

Fluorine may be recovered from the evaporators to produce fluorosilicic acid (FSA) at up to 25% H2SiF6. The Prayon process enables FSA to be recovered at very high recovery and with as low as 100 ppm P2O5 contamination.