Continuous process for manufacturing freely flowing solid acidic P/K fertilizer

1. A continuous process for manufacturing a solid, free-flowing acidic phosphorus/potassium (P/K) fertilizer, comprising steps of

i) providing agriculturally acceptable phosphoric acid (PA) and partially neutralizing it with a neutralizer selected from monopotassium phosphate (MKP) and potassium hydroxide (KOH), thereby providing an aqueous mixture which provides a pH of less than 3.0 when diluted ten times in water;

ii) drying said aqueous mixture of step i) in a vacuum dryer ensuring a reaction temperature between 70 and 120° C. and a pressure of at most 500 mbar whereby obtaining a solid material comprising less than 5 wt % water;

iii) cooling down said solid material of step ii) to ambient temperature; and

iv) mixing said cooled solid material with a solid magnesium source in an amount of from 0.1 to 3 wt %, based on said cooled solid material;

thereby obtaining the free flowing solid fertilizer without tendency to cake, comprising an acidic phosphate of formula KH₅(PO₄)₂ and water in an amount of 5 wt % or less.

2. A process according to claim 1 , wherein said step of drying comprises contacting a liquid stream comprising a phosphate of formula KH₅(PO₄)₂ with a solid surface comprising the same phosphate.

3. A process according to claim 2 , wherein said step of drying comprises continually combining a liquid stream comprising a phosphate of formula KH₅(PO₄)₂ with a partially dried solid comprising the same phosphate.

4. A process according to claim 3 , comprising dripping said liquid stream onto said partially dried solid.

5. A continuous process according to claim 1 , further comprising the step of preparing a concentrated solution of PA, and a liquid phase comprising a neutralizer selected from MKP and KOH.

6. A continuous process according to claim 5 , comprising reacting said PA and said neutralizer in a reactor kept at a temperature of from 70 to 120° C.

7. A continuous process according to claim 1 , wherein said step of drying is performed in a drying unit kept at a temperature of from 70 to 120° C., at a pressure of at most 500 mbar.

8. A process according to claim 5 , wherein said step of reacting said PA with said neutralizer is separated in time and space from said step of drying.

9. A process according to claim 1 , wherein said neutralizing comprises reacting MKP with PA in a weight ratio of from 1.32 to 1.85.

10. A process according to claim 1 , wherein said neutralizing comprises reacting MKP with PA in a weight ratio of from 1.43 to 1.85.

11. A process according to claim 1 , wherein said neutralizing comprises reacting KOH with PA in a weight ratio of from 0.28 to 0.33.

12. A process according to claim 1 , wherein said neutralizing comprises reacting KOH with PA in a weight ratio of from 0.29 to 0.33.

13. A process according to claim 1 , wherein said amount of water is 1 wt % or less.

14. A process according to claim 1 , wherein said amount of water is between 0. 1 and 0.6 wt %.

15. A process according to claim 1 , wherein said magnesium source is selected from magnesium oxide, magnesium hydroxide, and magnesium carbonate.

16. A process according to claim 1 , wherein said reactor and said vacuum dryer are integrated in an apparatus through which the products of the reaction between said PA and said neutralizer continuously move, thereby continually converting the reactants into freely flowing solid acidic potassium phosphate.

17. A process according to claim 16 , wherein said apparatus comprises reactor unit, dryer unit, cooling unit, transporting means for moving said products between the units, and controlling means for checking the product composition and humidity and for regulating the velocity of product movement.

18. A particulate, freely-flowing acidic P/K fertilizer prepared from PA, and MKP or KOH, comprising particles coated with magnesium phosphate.

19. A particulate, freely-flowing fertilizer according to claim 18 , comprising acidic potassium phosphate of the formula KH₅(PO₄)₂ in an amount of at least 85 wt % and water in an amount of at most 5 wt %.

20. A particulate, freely-flowing fertilizer according to claim 19 , exhibiting a low tendency to caking and being stable during storage.

21. A particulate, freely-flowing fertilizer according to claim 18 , consisting of at least 85 wt % of potassium phosphate of the formula KH₅(PO₄)₂, water in an amount of less than 5 wt %, and magnesium in an amount of at most 2 wt %.