INTERACTION OF MONAZITE AND AMMONIUM BIFLUORIDE

Abstract

Up-to-date technologies providing scientific and technological progress are based on the unique properties of rare-earth metals and their compounds. In recent years, attempts have been made to revive rare-earth metals production in Russia, mainly using the sources from which rare-earth elements can be extracted simultaneously (apatites, phosphorites and products of their processing). Another source of rare-earth elements, which is a ready concentrate, is monazite concentrate stored in the warehouse of the state government institution of the Sverdlovsk region «UralMonatsit» in Krasnoufimsk. The known industrial technologies of its processing (alkaline and sulfuric acid) have a number of drawbacks, in particular, the incompleteness of rare-earth elements extraction and the complexity of separating thorium from rare-earth elements. Therefore, there is the relevance of the work on improving the processing technologies of monazite raw materials with extraction of rare-earth elements concentrate and its refinement from impurities, including purification from thorium. To decompose the difficult-to-open crystalline lattice of monazite effectively, its fluorination with ammonium hydrofluoride as a relatively environmentally friendly and regenerable reagent is proposed. The maim aim of the research is to study the process and determine the features of interaction of monazite concentrate with ammonium bifluoride to produce a product suitable for further sulfuric acid processing. The object of the research is a sample of monazite concentrate in the storage of the SGI SR UralMonatsit. Research methods. Atomic emission spectroscopy with inductively coupled plasma, X-ray fluorescence analysis, neutron activation analysis, X-ray phase analysis, scanning electron microscopy with microanalysis, method of combined thermogravimetric/differential-thermogravimetric/differential scanning calorimetric analysis. Results. The paper considers the first stage of the proposed fluorammonium-sulfuric acid processing of monazite concentrates, i.e. the hydrofluorination stage. In the interaction of monazite concentrate with ammonium bifluoride, the impurities contained in the concentrate are the first to enter into a reaction with the formation of fluorides and fluoromammonium compounds; after that the rare-earth metals phosphates enter into reaction. The resulting phosphoric acid reacts with ammonium bifluoride to form ammonium hydrophosphates and a slight phosphorus emission into the gaseous phase. This is confirmed by the results of studies carried out in the temperature range of 170–250 °С: 49,2–83,3 % of phosphorus remains in the solid phase during fluorination, 12,0–32,8 % of phosphorus changes into the liquid phase, 11,2–28,1 % of phosphorus changes into gaseous phase. With a further increase in temperature up to 500 °C, there are no signs of reaction proceeding on the obtained TGA and DSC curves. This allows us to say that complete distillation of phosphorus from the fluorination products in the studied temperature range is impossible. According to the material balance, 10,4-38,4 % of silicon from its content in the initial concentrate transforms into the gas phase in the form of SiF4, 55,3–75,9 % – into the leaching solution in the form of soluble (NH4)2SiF6, which can be decomposed to gaseous SiF4 with further acid processing. On the basis of the results obtained, optimal conditions for fluorination of the monazite concentrate with ammonium hydrofluoride are chosen: temperature is 210 °C; ammonium bifluoride content is 80 % of stoichiometry; the duration of the process is 1,5 hours.

Keywords

• monazite
• monazite concentrate
• ammonium hydrogen difluoride
• ammonium bifluoride
• rare-earth elements
• thorium
• silicon
• phosphorus