The Transport Characteristics of 238U, 232Th, 226Ra, and 40K in the Production Cycle of Phosphate Rock

Abstract

Background: Phosphate rock and its by-product are widely used in various industries to produce phosphoric acid, gypsum, gypsum board, and fertilizer. Owing to its high level of natural radioactive nuclides (e.g., 238 U and 226 Ra), the radiological safety of workers who work with phosphate rock should be systematically managed. In this study, 238 U, 232 Th, 226 Ra, and 40 K levels were measured to analyze the transport characteristics of these radionuclides in the production cycle of phosphate rock. Materials and Methods: Energy dispersive X-ray fluorescence and gamma spectrometry were used to determine the activity of 238 U, 232 Th, 226 Ra, and 40 K. To evaluate the extent of secular disequilibrium, the analytical results were compared using statistical methods. Finally, the distribution of radioactivity across different stages of the phosphate rock production cycle was evaluated. Results and Discussion: The concentration ratios of 226 Ra and 238 U in phosphate rock were close to 1.0, while those found in gypsum and fertilizer were extremely different, reflecting disequilibrium after the chemical reaction process. The nuclide with the highest activity level in the production cycle of phosphate rock was 40 K, and the median 40 K activity was 8.972 Bq•g-1 and 1.496 Bq•g-1 , respectively. For the 238 U series, the activity of 238 U and 226 Ra was greatest in phosphate rock, and the distribution of activity values clearly showed the transport characteristics of the radionuclides, both for the byproducts of the decay sequences and for their final products. Conclusion: Although the activity of 40 K in k-related fertilizer was relatively high, it made a relatively low contribution to the total radiological effect. However, the activity levels of 226 Ra and 238 U in phosphate rock were found to be relatively high, near the upper end of the acceptable limits. Therefore, it is necessary to systematically manage the radiological safety of workers engaged in phosphate rock processing.

Refinement of waste phosphogypsum from Prahovo, Serbia: characterization and assessment of application in civil engineering

“…The results obtained are presented in Table 4. Similar phosphogypsum activity levels for the radionuclides 238 U, 226 Ra, 232 Th and 40 K have also been reported in other studies (Sahu et al , 2014; Jung et al , 2017). The activity concentration values for natural radionuclides before and after the sealing of the samples’ containers were within experimental uncertainty of the measurements, except in the case of 238 U and 40 K (Table 4).…”

“…Using the available literature data and general knowledge of the phosphoric acid production process, the most significant impurities identified in raw phosphogypsum were detrital quartz (by volume), RaSO 4 (by radioactivity) and ferric oxyhydroxides affecting the colour ( e.g. Pérez-López et al , 2016; Jung et al , 2017). As all of these components are essentially insoluble in water, in contrast to calcium sulfate, the recrystallization procedure was employed to improve gypsum purity, radioactivity level and colour.…”

“…Through the manufacturing process, ~80% of 226 Ra, 14% of 238 U and 30% of 232 Th tend to be enriched in phosphogypsum, and the rest remains in phosphoric acid (Jung et al , 2017). This is due to the extremely low solubility product of RaSO 4 (3.66 × 10 –11 ).…”