Radioactivity in slag

Slag is one of the main by-products of almost all metallurgical processes. It is a non-metallic material which is produced together with the metallic products of these processes. This product is largely employed in road building waterway stabilization, agriculture and in many other sectors. The using of these alternative materials helps to minimize the extractive quarrying of primary aggregates thereby protecting more of natural resources and landscape. Air-cooling of metallurgical slag produces a crystalline structured mass which, after crushing and screening, provides an eminently suitable material for use as a construction aggregate in bound or unbound form, like any natural rock. Blast-furnace and steel slag have certain advantages compared to natural stone: the high raw density coupled with the high abrasion resistance, the rough surface and the cubic shaped grain. Steel slag that is used for hydraulic structures has to ensure - like natural aggregates - environmental compatibility, sufficient volume stability and frost resistance as well as a high compressive strength. Nowadays, there is a tendency to obtain new building materials having good isolation properties and low density by using cheap and practically inexhaustible solid waste products like furnace slag, fly coal ash and phosphogypsum, without combustion. The Romanian furnace slag can be used alone or mixed with fly ash to obtain some binder materials with mechanical resistance comparable to the Portland cement. From the furnace slag and fly coal ash, in the presence of sulphatic activating additives like gypsum, phosphogypsum and/or lime, one can obtain the ceramic blocks that can substitute the usual bricks. The concentration levels of U, Th and K radioactive elements in slag are correlated with the limestone and dolomite used as fluxes in the iron metallurgy. Possible concentration of radioactivity in iron and steel process involves the necessity of radiometric measurements to identify and quantify the natural and/or artificial radioisotopes in iron and steel raw materials, intermediate and finite products. Gamma-ray spectrometry is a powerful non-destructive analytical tool for the qualitative and quantitative determination of the gamma emitters. The increased requirements for knowledge of toxic emissions, along with the increasing use of new sources of raw materials (new deposits, industrial by products, wastes) are making it necessary to carefully analyze the distribution of impurity elements between the main products and by-products of ferrous metallurgical processes. Trace elements may present an environmental hazard in the vicinity of industrial activities related to ironmaking and steelmaking, with implications in the environmental pollution – contamination of soils and waters in the vicinity of a steel plant. The situation in the region of an industrial centre should be seriously considered both from the standpoint of ecology and of the existence of a technogenic deposit of elements. Instrumental Neutron Activation Analysis (INAA) is a suitable multi-elemental analytical technique for the non-destructive investigation of solid samples requiring minimum preparation, having high sensitivity and good accuracy.

The aim of this study was to investigate, by low background gamma-ray spectrometry, the radioactivity level of slag from Arcelor-Mittal Iron and Steel Works of Galati, as well as by Instrumental Neutron Activation (INAA) the major, minor and trace element content of metallurgical slag, as a completion to earlier INAA application on various metallurgical samples

RADIOACTIVITY ANALYSIS

The low-background high resolution gamma-ray spectrometry technique was applied at GamaSpec laboratory of “Horia Hulubei” National Institute of Physics and Nuclear Engineering (IFIN-HH) in Magurele to determine the natural radioactivity of a steel slag material resulted from the metallurgical industry at Galati. The spectrometric chain was equipped with a HPGe (EG&G Ortec) detector of 2.0 keV resolution at 1332 keV of 60Co, and 30% detection efficiency relative to 3”x 3” NaI (Tl) standard, coupled to a PC based multichannel analyzer (Maestro 32 MCA Ortec). GAMMAW software program was used for the spectra processing. The dried and homogenized metallurgical slag sample was measured in 72-mm diameter and 40-mm height plastic beaker placed on the detector end cap, after 21 days of keeping sealed in the measuring box, for a counting time of 24.14 h. The system was calibrated for the detection efficiency using relatively high activity 152Eu, 137Cs, and 241Am water equivalent volume standard sources, as well as IAEA reference materials of similar density and volume with those of measured sample (IAEA135 sediment, IAEA Soil-6, IAEA S-18 uranium ore), placed into identical beakers (the same measuring geometry). For a low-level background counting of samples, shielding of the detector to reduce ambient background radiation is an essential request. For environmental application covering the energy range from zero to 2000 keV, 10 cm of lead is sufficient. This thickness of low background virgin lead for bulk shield together with a 0.5 mm cadmium and 1.5 mm copper graded liner is needed. Similarly, in the GamaSpec laboratory of IFIN-HH, a lead shield of 10 cm thickness, coated with Sn and Cu foils of 1 mm, and 1.5 mm thickness, respectively, were used. The integral background rate for the gamma-ray energy range of 20 - 2760 keV and 24-h counting time was of 1.76 cps, a value comparable to the accepted value of 1 cps for well-shielded spectrometers in the energy range of 50-1500 keV [21]. The natural background gamma-ray spectrum is mainly due to the gamma emitting radionuclides of the uranium-radium (238U-226Ra) and thorium (232Th) radioactive series, as well as 40K radionuclide. The aleatory temporal variation of the radon’s activity in the natural background of the laboratory requires an alternately counting of the sample and natural background, especially for low level radioactivity samples. To determine radium radioactivity, the samples are sealed and measured after 3-4 weeks in order to establish the radioactive equilibrium between 226Ra and its gaseous radioactive descendant 222Rn (radon). 214Pb and 214Bi radon’s decay progenies are being measured by gamma-ray spectrometry. U, Th and K concentrations can be determined by measuring uranium (238U), thorium (232Th) and potassium (40K) radioactivity in the sample.