Natural radioactivity concentration in materials used in the manufacture of refractory products

Refractory materials are use in a wide range of industrial applications. Raw materials to produce refractory products include alumina, prepared starting from bauxite, and zirconia, prepared starting from zircon sand. These two raw materials can contain significant natural radioactivity concentrations. Therefore refractory materials are regarded, also in the framwork of the European and national legislation, as possible sources of significant exposure to ionizing radiation. This paper presents the results obtained from a representative survey on Italian factories producing refractory materials: the determination of radioactivity concentrations of 40K and of the members of the natural decay series has been performed on samples deriving from different refractory factories. The radioactivity concentrations were measured with a HPGe gamma-spectrometer connected to a multi-channel analyser. Materials analysed were graphite, silicon carbide, cement, chamotte, zircon sand, zircon silicate, bauxite, corundum and calcined alumina. The samples were crushed, dried, weighed, and sealed in a Marinelli beaker. From the results of gamma-spectrometry, the materials can be divided into three homogeneous groups characterized by similar features. In the first group, including graphite, silicon carbide, cement, chamotte and calcined alumina, the natural radioactivity concentrations are very low, of the order of a few hundred Bq per kg at most. The materials belonging to the second group, including zircon sand and zircon silicate, have shown activity concentrations of 238U and its decay products ranging from 2000 to 3000 Bq per kg and activity concentrations of 232Th and its decay products ranging from 500 to 600 Bq per kg. Furthermore, after having done the self-absorption corrections due to the low radiation energies of 234Th and 210Pb, the decay chains of 238U and 232Th appear in secular equilibrium. Bauxite and corundum, belonging to the last group, are characterized by similar 238U and 232Th concentrations ranging from 100 to 500 Bq per kg. These materials show a marked disequilibrium between 210Pb and the other members of 238U decay chain. In many cases mineral processing can modify the secular equilibrium among the nuclides of the natural radioactive chains. To prepare bauxite for processing, mined ore is crushed and ground in ball mills to a finely divided state. Most bauxite as mined has a low moisture content. If the moisture content is high, then the bauxite must be dried in rotary kilns before it is sent to storage. Substantially corundum is produced from the mineral bauxite by very high temperature calcination. The thermal volatilisation of 210Pb and 210Po probably occurs during the drying and/or heating operations. On the contrary, zircon sand are prepared by grinding and sieving processes so that the secular equilibrium among the nuclides of the natural radioactive chains is undisturbed. The acquired information about natural radioactivity concentrations and secular equilibrium states, in a second time, will be used by the authors to assess the radiological and radioecological implications of the manufacture of refractory materials.