Spheroidal alkali metal polyphosphates of low apparent density, and process for making them

A process for making hollow spheroidal alkali metal polyphosphates in a spray tower which comprises:

• forcing an aqueous solution of an alkali metal orthophosphate and an inflating agent selected from the group consisting of urea, a low-valent phosphorus compound, an alkali metal carbonate and an alkali metal bicarbonate under a pressure of 6 to 40 bars into a fluidization zone arranged inside the spray tower and flow-connected to a plurality of single medium spray nozzles, injecting a finely dispersed permanent gas selected from the group consisting of compressed air, carbon dioxide, nitrogen and mixtures thereof into the fluidization zone, maintaining the finely dispersed permanent gas under a pressure at least 20% higher than that selected for the solution, and spraying the solution containing the finely dispersed permanent gas via the plurality of single medium nozzles through a flame zone produced inside the spray tower with the resultant formation of alkali metal polyphosphates having a bulk density of 200 to 320 g/l and an alkali metal polyphosphate content of at least 97.9%.