THE BEHAVIOUR OF RARE EARTH ELEMENTS AND STRONTIUM DURING THE SULPHURIC ACID PROCESSING OF THE KHIBINY APATITE CONCENTRATE

The features of the behaviour of rare earth elements (REE) and strontium in hemihydrate and dihydrate processes of sulphuric acid processing of Khibiny apatite concentrate have been studied on the basis of investigation of the residues of aqueous leaching of phosphogypsum obtained under production conditions. It has been shown that the increased degree of REE ingress into phosphogypsum of the hemihydrate process, compared to phosphogypsum of the dihydrate process, is determined by the technological features of these processes: higher temperature and REE concentration in the liquid phase of the pulp of the hemihydrate process, compared to the liquid phase of the pulp of the dihydrate process. In contrast to existing ideas, it is theoretically justified and experimentally confirmed that a significant part of REE in phosphogypsum of the hemihydrate process is present in the form of rhabdophane. Rhabdophane was not found in phosphogypsum from the dihydrate process, and it is most likely that REE in phosphogypsum from the dihydrate process are isomorphically cocrystallised with gypsum. It has been established that strontium occurs partially in the form of celestine in phosphogypsum of both processes. It is shown for the first time that celestine, present in phosphogypsum of the dihydrate process, contains isomorphically cocrystallised REE. It has been suggested that isomorphically cocrystallised REE may be also present in celestine from phosphogypsum of the hemihydrate process. The influence of REE forms present in phosphogypsum on the possibility of its sulphuric acid leaching is discussed. The results obtained in the work are of high practical importance because sulphuric acid leaching is widely recommended for organising the industrial extraction of REE from phosphogypsum obtained during sulphuric acid processing of the Khibiny apatite concentrate.