Method and crystallizing tank and arrangement thereof for crystallizing calcium nitrate from the nitro-phosphate process

A method for crystallizing calcium nitrate from an aqueous calcium nitrate composition comprising from 6 to 12 weight % nitric acid, from 11 to 17 weight % phosphoric acid, and from 36 to 49 weight % dissolved calcium nitrate, comprising steps of:

• a) filling at least one vertical crystallizing tank through an inlet with the aqueous calcium nitrate composition, the crystallizing tank comprising:
• a vertical cylindrical section defined by a top end and a bottom end, each end having a circular cross section;
• a first inlet for introducing the aqueous calcium nitrate composition into the cylindrical section;
• a first outlet for draining a slurry of crystallized calcium nitrate at the bottom end;
• a second inlet for introducing an aqueous washing composition;
• concentric banks of cooling coils running alongside each other and parallel to an axis running through centers of the circular cross section of the top end and the bottom end of the cylindrical section;
• an agitator equipped with paddles fixed to its rotation axis which runs through centers of two circular bases of the cylindrical section, driven by an agitator motor and
• a temperature measurement device for measuring a temperature in the crystallizing tank;
• b) circulating a cooling fluid through the banks of cooling coils, with an initial temperature ranging from −40° C. to −5° C. and
• c) rotating the agitator such that a minimum heat transfer is achieved;
• characterized in that the crystallizing tank comprises three banks of cooling coils and the agitator is rotated at such speed that a minimum average heat transfer of 400 W/m2·K is achieved on the bank of cooling coils most distant from the agitator.