The freezing acidolysis solution of the nitric acid-phosphate fertilizer process has a high calcium content

which makes it difficult to produce fine phosphate and high water-soluble phosphate fertilizer products. Here

based on the potential crystallization principle of calcium sulfate in NH

4

NO

3

-H

3

PO

4

-H

2

O

the deep decalcification (

i.e.

calcium removal) technology to achieve

α

-high-strength gypsum originated from freezing acidolysis-solutions has been firstly proposed and investigated. Typically

calcium can be removed from the factory-provided freezing acidolysis-solution by neutralizing it with ammonia

followed by the addition of ammonium sulfate solution. As a result

the formation of calcium sulfate in the reaction system undergoes the nucleation and growth of CaSO

4

·2H

2

O (DH)

as well as its dissolution and crystallization into short columnar

α

-CaSO

4

·0.5H

2

O (

α

-HH). Remarkably

with the molar ratio of SO

4

2-

/Ca

2+

at 1.8

the degree of neutralization (NH

3

/HNO

3

molar ratio) at 1.7

the reaction temperature of 94 ℃

and the reaction time of 300 min

the decalcification rate can reach 86.89%

of which the high-strength

α

-CaSO

4

·0.5H

2

O (

α

-HH) will be obtained. Noteworthy

the deep decalcification product meets the standards for the production of fine phosphates

and highly water-soluble phosphate fertilizers. Consequently

the 2 h flexural strength of

α

-HH is 5.3 MPa and the dry compressive strength is 36.8 MPa

which is up to the standard of commercial

α

-HH.