Optimized photometric technique for rare-earth elements determination in phosphate ore

Abstract

The Socialist Republic of Vietnam is 21st in the list of the world’s countries for phosphate ore reserves. The largest apatite ore deposit Lao Cai with the stretch of over 100 km and the total area of 45.56 km2 situated along the right bank of the Hong River has been in operation since 1940. According to the prospecting data, the Lao Cai ore reserves exceed 30 mln tons. As the mining operations keep expanding and the environmental regulations grow stricter, it is necessary to monitor the concentration of rare-earth metals in the apatite ore. Inductively coupled plasma mass spectrometry (ICP–MS), a method that enables to directly analyze mineral raw materials and sites for rare-earth elements, cannot be consistently utilized by industrial sector. An alternative spectrophotometric technique, which helps determine the total amount of rare-earth elements in analyzed objects, is time- and labour-consuming. This paper describes the results of a trial that was conducted for an optimized spectrophotometric technique. The advantage of the optimized technique is that it skips the stage when a sample gets decomposed with a mixture of hydrofluoric, nitric and sulphuric acids, which saves time and labour and improves the work safety. The results obtained from both conventional and optimized spectrophotometric techniques have good convergence: the discrepancy between them is 1.68%, which does not exceed the applicable error. The authors performed parallel analysis of two samples of ground Lao Cai apatite ore for rare-earth elements determination. Thus, ICP-MS, as well as conventional and optimized spectrophotometric techniques were applied. The authors determined what caused the errors in the case of the latter and how such errors can be minimized.