by using preheated reactants an aqueous suspension of finely ground calcium carbonate can be caused to react very rapidly with phosphoric acid with violent formation of gas and foam. According to the invention such violent reaction between hot reactants is utilized in a very special manner, whereby there takes place an intensive mixing of the reactants without the aid of power-consuming mechanical mixers and where the reaction mixture shortly afterwards is discharged or expelled to the air. Most of the reaction takes place while the reaction mixture is suspended in the air in the form of separate drops of foam. It is during this stage that the sticky consistency normally occurs, and which has been described above as critical with regard to mixability, requirements of power, etc. When the brief period of suspension is over the reaction mixture has the form of a moist, non-sticky grain material