HIGH-TEMPERATURE METHODS FOR PROCESSING MONAZITE .1. REACTION WITH CALCIUM-CHLORIDE AND CALCIUM-CARBONATE

Rare earth element compounds are usually extracted from monazite by decomposing the mineral with sodium hydroxide and then separating the rare earth elements first from phosphate and then from the impurity thorium. This paper presents a new method for the decomposition of monazite which simplifies the subsequent separation of the rare earth elements from thorium. In the new method, monazite is decomposed by reacting it with calcium chloride and calcium carbonate in a both reducing and sulphidizing environment at temperatures between 1250 and 1460 K. The products from this reaction are rare earth element oxysulphides and oxychlorides, a thorium-rich oxide solid solution and a calcium chlorophosphate (chlorapatite). An analogous reaction occurs if calcium fluoride is used instead of calcium chloride. The rare earth elements are removed from the decomposed monazite by leaching with 3% hydrochloric acid. The leach solution is readily filtered from the leach residue which contains both the gangue minerals and the thorium-rich oxide. The thorium-rich oxide is resistant to acid attack and so is suitable for disposal by burial. In comparison with the sodium hydroxide process, more rapid decomposition of the monazite is achieved without the need for grinding, and separation of the thorium waste from the solution containing the rare earth elements is made easier. However, the method does not offer improved rare earth element/thorium separation. In comparison with previously reported high temperature decomposition methods, there is an improvement in the rare earth element/thorium separation because the decomposition is carried out in a reducing and sulphidizing atmosphere. © 1990.