Removal of radioactivity from zircon

Details Removal of radioactivity from zircon Removal of radioactivity from zircon

1993-12-14

1995-12-26

Mai, Ngoclan

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Radioactive metal

423 20, 423 80, 423 84, 423 3, C01G 4300

Patent

active

054785383

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a treatment for partial removal of radioactive components from concentrates of zircon.
In a particular embodiment the present invention provides a process for the removal of all or part of the radionuclides contained in zircon concentrates. In a general aspect the process of the invention comprises three basic steps, namely: contained in the thermally treated zircon: and zircon.
Additional steps may be employed as will be described below.
All commercially available zircons contain radioactivity, in the form of uranium-238 and thorium-232 radionuclides in the zircon lattice, and their respective radioactive progeny elements. The significance of the progeny elements formed by radioactive decay of the parent radionuclides is that for each radioactive decomposition of a parent there will ultimately follow a chain of decomposition until stable elements, subject to no further decomposition, are formed. In the case of thorium-232 effectively nine further decompositions follow the initial decomposition while thirteen further decompositions follow the initial decompositions of uranium-238. Commercially available zircons typically are of sufficient age from formation in original host rocks to allow establishment of "secular equilibrium", at which all radioactive progeny elements express the same rate of decomposition as the parent radionuclides. Under such circumstances the rate of radionuclide decomposition, i.e. the radioactivity of the zircon, is the sum of: the rate of decompositions of each parent multiplied by the number of effective decompositions in the decomposition chain of the parent. That is, the decay chain acts as a multiplier for the radioactivity of the parents.
A large proportion of commercially produced zircon concentrates enters ceramics products in the form of glazes and pacifiers. As a prior treatment for the production of frits or for direct use in glazes and porcelain, zircon is milled in order to improve its incorporation into the various ceramics applications. Such milling, to produce at least a portion of ultra fine (sub-micron) material in the size distribution, results in dust arising either in the dry milling of the zircon or in drying of the wet milled zircon and in subsequent handling of the milled product.
The use of zircon having the typical low level of radioactivity associated with commercially available zircons can be shown to pose potential health risks. Inhalation of fine zircon dusts results in retention of particles within the lungs of those exposed to dust laden air. Partial dissolution of radionuclides from these particles in body fluids can result in distribution of alpha radiation to areas within the body where radiological damage can occur.
Table 1 provides an analysis of the rate of radioactive decomposition (1 decomposition per second is known as a Becquerel, Bq) associated with radioactive elements present within a range of commercially available zircons. Also included in Table 1 is an assessment of the level of 0.1 micron dust in the breathing environment which would result in exposure to alpha radiation at greater than the limits for exposure recommended by the International Council for Radiological Protection (ICRP) of 1 mSv (milli Sieverts) per annum for members of the public and 5 mSV per annum for general workers. In most workplaces handling of milled products will result in areas of plant having dust levels in excess of 1.0 mg m.sup.-3.
It is clear from Table 1 that milling of zircon and handling of milled and micronised zircon represents a significant potential health risk to workers for virtually all commercially available zircons.


TABLE 1 __________________________________________________________________________ Analysis and Behaviour in Milling of Commercially Available Zircons A B C D __________________________________________________________________________ ZrO.sub.2 + HfO2 65.9 66.1 66.3 66.4 SiO.sub.2 31.4 32.3 33.0 32.3 Fe.sub.2 O.sub.3 0.080 0.040 0.052 0.021 Al.sub.2 O.sub.3 0.12 0.08 0.29 0.20 U.sub.3 O

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