Radiological implications of NORM in the production of rare-earth compounds

Abstract

The composite rare earth chloride produced from monazite contain low levels of natural radionuclides and is being used as the input material for the production of individual rare earth compounds which have wide applications. Gross alpha, beta and 228 Ra activity concentrations in composite rare earth chloride and individual rare earth compounds such as oxides of Ce, Nd, Pr, Sm, Gd etc are presented. The significant radionuclide of environmental concern is identified as 228 Ra and the activity level varied between 0.1 and 7.8Bq.g −1 in different compounds. Sporadic 228 Ra levels up to 16Bq.g −1 was observed in Lanthanum oxide. The external gamma exposure rates and airborne activity due to thorium and thoron progeny in the process locations are studied. The activity levels in liquid effluent and potential exposure scenarios are indicated. Radiation protection and management of wastes generated by industries that process minerals and raw materials containing naturally occurring radioactive materials (NORM) are of concern in the context of long term environmental implications of waste disposal. Chemical compounds of rare-earth (RE) elements (La to Lu) have wide applications in daily life. They are commercially exploited from minerals such as monazite, bastnasite, xenotime, allanite etc. Most of these natural ores and minerals contain low or significant levels of thorium and uranium series nuclides. The thorium content varies from 0.1% to 10% and uranium content varies from very low percentage to 0.8% depending on the mineral and region of occurrence (1). Monazite (Ce,La,Th)PO4, a phosphate mineral containing nearly 9% thorium as ThO2 is the starting material for the production of rare earth compounds in India. Caustic digestion followed by selective extraction in HCl is the method used to separate composite rare-earths chloride. Limited studies on the radiation protection aspects in rare-earths industry are reported (2, 3). A variety of diversified RE compounds that have wide applications in daily life and advanced scientific research are produced from the composite RE chloride by different chemical processes such as precipi- tation, solvent extraction and ion-exchange methods. The present studies are carried out in a rare-earths producing plant typically convert composite RE chloride to different compounds such as RE fluoride, oxide, carbonate and other individual rare earths compounds with emphasis on radiological aspects.