Prayon’s first industrial DA-HF plant

PROJECT PROFILE

Prayon’s first industrial DA-HF plant

The DA-HF phosphoric acid process has been successfully implemented at Grupa Azoty’s Police fertilizer production plant in Poland, as part of a revamp of the site’s existing DH plant. Sébastien Havelange and Alexandre Wavreille of Prayon Technologies outline the performance improvements achieved by this first-of-its-kind plant.

The world faces ever growing environmental challenges, making the development of new industrial technologies increasingly essential.

Today, most of the world’s phosphoric acid is produced via the DH (Di-Hydrate) process route. For some years now, Prayon has been developing an improved phosphoric acid production process known as DA-HF (Dihydrate Attack-Hemihydrate Filtration). These are also the initials of Dorina Fati and Antoine Hoxha, the two Prayon researchers who developed the process.

DA-HF and its underlying assumptions have been thoroughly tested at pilot-scale at Prayon’s Engis site in Belgium. Recently, the DA-HF process has also been demonstrated at full-scale commercially with the installation of an industrial plant at Grupa Azoty’s Police site in Poland.

Compared to the standard DH process, Prayon’s DA-HF process has a number of distinct advantages:

• The weak product acid contains higher P2O5 levels of up to 34 percent.
• This allows merchant grade acid (MGA, 54% P2O5 ) to be produced using smaller concentration units.
• A higher process efficiency with P2O5 recovery above 97 percent.
•  Less water is consumed during washing of the calcium sulphate cake.
• Compared to the DH process, the hemi-hydrate (HH) calcium sulphate by-product obtained is potentially more recoverable and suitable for use in cement plants. This is due to its lower P2O5 content and its self-drying behaviour when it naturally rehydrates to DH.

Fig. 1: The calcium sulphate crystallisation system in phosphoric acid production

Fig. 2: DA-HF process description

As already stated, the first plant in the world to run this process is currently operating in Poland for Grupa Azoty, Police. This plant was engineered by Desmet Ballestra – who also helped commission the plant in collaboration with a team from Prayon Technologies in 2019. Some results from this first-of-its-kind plant are now available and are presented in this article.

DA-HF process description

The DA-HF process is based on the phase diagram for calcium sulphate (Figure 1). This shows how different forms of calcium sulphate crystallise, according to temperature and global acidity (including sulphate content).

DA-HF is a double-crystallisation process (Figure 2). The first attack of the rock with sulphuric acid results in the crystallisation of DH in a reaction tank at low sulphate levels. DH is then converted into HH at higher sulphate levels (3-3.5% SO3 ) in a conversion tank. The conversion is then followed by filtration to recover the high P2O5 content acid. This two-step double-crystallisation process, with different sulphate contents, is beneficial as it minimises both unreacted and co-crystallised P2O5 losses.

Unlike the very high efficiency CPP (Central Prayon Process) operated by Prayon in Engis, Belgium, only one single filtration stage is required in DA-HF. This makes DA-HF simpler and easier to operate compared to CPP.

With the conversion operating at 3-3.5 percent SO3 , the sulphate content in the product acid is higher than with the DH process. However, this can be easily reduced, if necessary, by installing a desulphation unit.

First industrial results

The DA-HF process has been successfully implemented in Grupa Azoty’s plant in Police, Poland, as part of a revamp of the site’s existing DH plant. To simulate Grupa Azoty’s production conditions, successful tests were completed at Prayon’s R&D centre prior to the full-scale implementation.

The revamping project for Grupa Azoty at the Police site mainly involved the installation of a conversion tank and the modification of the existing tilting pan filter and gypsum evacuation to enable it to handle hemihydrate filtration cake. The revamp increased nominal plant capacity from 420 tonnes of P2O5 per day to 500 t/d. The revamp was designed to provide production flexibility – as it allows the plant to still operate in standard DH mode, if necessary, with the conversion tank operating as an extra standard digestion tank. Nominal production capacity is also increased in DH mode due to this extra tank.

Fig. 3: P2O5 losses in discharged cake for the Grupa Azoty phosphoric acid plant: DH (right) versus DA-HF (left) modes

After completing performance tests and the first technological start-up run, it has now been possible to collect statistically representative results for the new DA-HF plant (Figure 3).

Figure 3 shows the distribution of P2O5 losses (within the discharged cake) for Grupa Azoty’s plant running in DA-HF and DH modes after revamping. Analysis of these results shows that, after applying a Student’s t-test to the data, there is a significant difference between losses in DA-HF and DH modes with, promisingly, significantly lower P2O5 losses for the plant in DA-HF mode. The results therefore conclusively demonstrate that plant efficiency is significantly higher when operating in DA-HF mode.

Initial results also show that considerably more cadmium is removed from the phosphoric acid end-product in DA-HF mode, compared to DH mode, due to its concentration in solids.

Conclusion

This article briefly describes the DA-HF process and its key advantages, compared to the standard DH process that is most widely operated around the world currently. We also show the performance improvements achieved following the conversion of an existing DH phosphoric acid plant into a DA-HF plant by Prayon Technologies, as part of a revamp project for Grupa Azoty. This first-of-its-kind DA-HF plant increased:

• Plant P2O5 capacity from 420 t/d to 500 t/d
• Its global efficiency – with total P2O5 content in the discharged cake decreasing from 2.5 percent to 1.2 percent
• The P2O5 content in the product acid to 32-33 percent
• The quality of the calcium sulphate by-product.

The first results for this industrial plant demonstrate that the limited investment costs and modifications needed as part a DH plant revamp will rapidly be compensated by the multiple advantages DA-HF can offer.

About the authors

Sébastien Havelange is a process engineer and Alexandre Wavreille a senior process engineer at Prayon Technologies.

References

1. Leruth, H., 2019. New process for improved phosphoric acid production: DA-HF process-first industrial plant Symphos 2019: The 5th International Symposium on Innovation and Technology in the Phosphate Industry, Benguerir, Morocco, October 2019.