Radioactive distribution in rare earth production
There are two sources of radioactivity in rare earth production. On the one hand, rare earth elements themselves have a few radioactive isotopes that are less abundant in nature. On the other hand, it is a natural radionuclide such as uranium , thorium and radium associated with rare earth minerals. The natural radioisotope of rare earth elements has a low specific radioactivity, so the rare earth element itself is not treated as a radioactive element. Rare earth minerals associated uranium, thorium and radium and other natural radionuclides in the rare earth production is the main source of radioactivity, and the intermediate product has been distributed in the rare earth and rare earth alloy products.
The natural uranium, strontium content and specific radioactivity of some rare earth minerals, intermediate products and rare earth alloy products are listed in Table 1, Table 2 and Table 3. It can be seen from the table that the alpha specific radioactivity of the Baotou mixed rare earth ore concentrate is on the nationally controlled 7.4×10 4 Bq/kg control line. When the production capacity is large, the daily operation amount may exceed the national control standard. Bastnaesite, monazite and specific radioactivity fergusonite ore concentrate were higher than the national minimum control standards. The rare earth intermediate alloy product has higher specific radioactivity, and it needs to be strengthened for storage and transportation. The radioactivity specific strength of most other products is lower than the national sanitary standard limit.
Table 1 Contents of uranium and thorium in several rare earth concentrates in China and their specific radioactivity
Rare earth concentrate ore type | REO/% | ThO2/% | U3O8/% | Total specific radioactivity intensity / (Bq / kg) |
Mixed mine Bastnasite Monazite mine Brown sugar mine | 24.43～40.26 50 42.7~60.3 22.02~30.66 | 0.111～0.246 0.36 4.3 to 7.18 1.48~4.38 | - 0.0051 0.22~0.88 2.12~2.14 | 5.37×10 4 to 7.77×10 4 1.2×10 5 0.37～3.7×10 6 0.37～3.7×10 6 |
Table 2 Natural strontium content and specific radioactivity in some intermediate products produced by rare earth mixed ore
Intermediate product name | REO/% | ThO2/% | Total specific radioactivity intensity / (Bq / kg) |
Double salt Mixed rare earth oxide Ceria | 42~45 45~48 | 0.056~0.22 ≤0.03 ≤0.03 | 3.26×10 4 to 7.8×10 4 0.41×10 4 to 1.11×10 4 0.44×10 3 |
Table 3 Natural rare earth content and its specific radioactivity in raw materials and products of rare earth intermediate alloys
Raw material, product name | Rare earth rich slag | Rare-earth iron-silicon alloy | Rare earth magnesium alloy | Calcium rare earth alloy |
Rare earth content (REO) /% Natural strontium content /% Total specific radioactivity intensity / (×104Bq/kg) | ≥8 0.056～0.059 2.22~3.48 | 23~34 0.10~0.20 3.92~7.77 | 6 to 20 0.05~0.12 2.22~6.22 | 22.76 0.32 0.74~1.48 |
Taking Baotou Mine as an example, the distribution of radioactive elements in the production process is: when alkali treatment of Baotou Mine, 96.25% of the strontium enters the slag, and less than 3.75% of the strontium enters the chlorinated rare earth product, and the rest enters the wastewater. When the concentrated sulfuric acid is used for the roasting treatment, 90% of the niobium enters the water leaching residue, and the rest is transferred to the rare earth chloride and the waste water. When the rare earth alloy is smelted in an electric furnace, almost 50% of the bismuth entering the alloy and the residue enters the dust [1] .
From the radiation level of each workplace in the production of rare earths, except for the slightly higher radioactivity level of uranium and plutonium recovery processes, the radioactive water in most workplaces is lower than the national allowable standard, and individual posts (such as pre-treatment processes) are slightly higher. National standards, and as the production process progresses and the uranium and uranium content of the treated materials decreases, the radioactivity level of the operating environment will decrease significantly until it is equivalent to the normal environmental level. However, since there is no clear conclusion about the long-term hazard caused by the long-term exposure of low-dose long-term exposure to the current level of knowledge, it is not possible to despise radioactive substances for human health in low-radiation posts. Harmful.
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