CONCENTRATION OF THE EXTRACTION FLUID FROM SULFURIC-ACID TREATMENT OF PHOSPHOGYPSUM BY MEMBRANE DISTILLATION

An attempt was made to apply membrane distillation for the concentration of the sulfuric acid solution obtained after extraction of apatite phosphogypsum, in order to recover lanthane compounds. The process was carried out step-wise, till the acid concentration of the solution reached 40%. Under such conditions the lantane compounds precipitate after cooling of the solution. The temperature of the feed and the cold solution was kept at 333 K and 293 K respectively. The distillation flux decreased from 215 dm3/m2-d to 80 dm3/M2-d with the increase of acid concentration in the feed from 16% to 44%. The acid rejection coefficient was close to 100%, in all experiments performed. The most effective process was that performed in a flow apparatus, where the membrane sides were reversed and the distillate condensation was carried directly into a stream of cooling water. The content of lantane compounds in the precipitate obtained from the concentrated solution amounted to 25%. The experiments were performed with sulfuric acid solutions and a model solution consisting of 12% sulfuric acid saturated with CaSO4.