TO CONCENTRATE SOLUTIONS SUCH AS PHOSPHORIC ACID, DROPLETS THEREOF ARE ENTRAINED IN A HIGH VELOCITY ASCENDING CURRENT OF HOT GAS SUCH AS FLUE GAS, AIR, ETC., OPTIONALLY CONTAINING A REACTANT GAS SUCH AS AMMONIA. THE ENTRAINED DROPLETS ARE PASSED TO A DECELERATING ZONE WHERE THE VELOCITY PROFILE OF THE GAS IS SUCH THAT CONTINUOUS CIRCULATION OF DROPLETS IS POSSIBLE, WITH DROPLETS PROXIMATE THE AXIS OF SAID ZONE BEING ENTRAINED UPWARDLY AND DROPLETS PROXIMATE THE SIDE OF SAID ZONE FALLING DOWNWARDLY WHERE THEY CAN BE RE-ENTRAINED BY ENTERING HOT GAS. IN THIS MANNER IT IS POSSIBLE TO RECIRCULATE THE DROPLETS AND CONTROL THE EXTENT OF CONCENTRATION WITHIN A REASONABLY DIMENSIONED APPARATUS AND WITHOUT ANY DIFFICULT SEPARATION PROBLEM BECAUSE OF EXCESSIVELY SMALL DROPS.