External exposure levels and fluoride content in the urine of workers at aluminium electrolysis

Introduction. In the context of large-scale modernization and intensification of aluminium production, it becomes relevant to assess the actual fluoride loads and the main factors that determine the occupational exposure of workers to fluorides for health risk analysis. Materials and methods. Monitoring studies of the content of hydrofluoride and aerosols of fluoride salts in the air of electrolysis shops were carried out with traditional and modernized technologies for producing aluminium and fluorine in urine in 108 workers of the primary professions and 35 people in the control group. Determination of the concentration of fluorides was carried out by photometric methods, and fluorine in urine - by ionometric methods. Results. The total concentration of fluoride compounds in the air of the modernized shops was 1.4-2.1 times lower than the values in traditional shops. The fluoride content in the urine of workers in these workshops ranged from 0.8 to 4.7 mg/l, 1.1-6.7 times higher than the level of the control group and 1.1-2.3 times the limit value of the biological exposure index. The highest external and internal fluoride loads and associated health risks are found in workers who maintain electrolyzers and anodes. A closer, noticeable (r = 0.644) correlation was found between the content of fluoride in urine and the levels of gaseous hydrofluoride in the air, indicating its predominant effect on workers compared to other determinants (total concentration of fluorides, seniority and age). Conclusion. The studies’ results indicate a high additional informative value and reliability of biomonitoring of fluoride in urine, which, combined with analysis of air pollution with fluorides, provides a completely objective assessment of the risk of exposure to workers. The presence of high levels of hydrofluoride in the air of electrolysis shops creates a higher occupational health risk for workers.

Proving the Availability of Ni-Fe Anode for Electro-Reduction of Solid V2O3 in Molten Fluoride Salts

The availability of casting Ni-Fe alloy as inert anode for direct electro-reduction of V2O3 in molten Na3AlF6-K3AlF6-AlF3 was investigated. The electrochemical oxidation behavior of anode as well as microstructural evolutions of formed oxide scale were systematically studied. The electrochemical characterization and reaction mechanism of cathode oxide were also investigated to evaluate the influence of alloy anode on cathodic reduction process. The in situ formed three-layered oxide scale is compact and coherent, which is composed of an outermost Fe2O3+FeAl2O4 skin layer, a Fe2O3 middle layer and a FeAl2O4 inner layer. The skin layer has a continuous, smooth structure and shows electrochemical activity. The Fe2O3 layer with compact structure prevents inward diffusion of electrolyte and outward migration of metal cations. The innermost FeAl2O4 layer shows a loose structure and functions as buffer layer to improve the peeling resistance of oxide scale. With the continuous extension of polarization time, the inner FeAl2O4 layer is slowly oxidized and becomes thinner, simultaneously, the dense Fe2O3 layer becomes thicker. Ultimately, metal vanadium product with fine rod-like particles can be obtained and the oxygen content in the metal vanadium is below 0.3 mass% within electrolyzing time of 2 h. The corresponding current efficiency is around 63%.

Revealing specific features of structure formation in composites based on nanopowders of synthesized zirconium dioxide

Peculiarities of formation of microstructure in composites based on chemically synthesized zirconium nanopowders obtained by the method of decomposition from fluoride salts were considered. Hydrofluoric acid, concentrated nitric acid, aqueous ammonia solution, metallic zirconium, and polyvinyl alcohol were used. It was established that the reduction of porosity in nanopowders in the sintering process is the main problem in the formation of high-density materials. Analysis of various initial nanopowders, their morphology, and features of sintering by the method of hot pressing with direct transmission of electric current was made. Peculiarities of obtaining the composites based on them with the addition of Al2O3 nanopowders applying the electric sintering method were considered. It was shown that the increase in the content of alumina nano additives leads to an increase in strength and crack resistance of the samples due to simultaneous inhibition of abnormal grain growth and formation of a finer structure with a high content of tetragonal phase. The influence of sintering modes on the formation of the microstructure of zirconium nanopowders has been studied for different contents of alumina additives. Electric current promotes the surface activity of nanopowders and its variable value promotes partial fragmentation of agglomerated grains thus affecting the composite structure. Physical-mechanical properties of the obtained samples, optimal compositions of mixtures, and possibilities of improving some parameters were determined. It was found that nanopowders of zirconium dioxide obtained by the method of decomposition from fluoride salts are quite suitable for the production of composite materials with high physical and mechanical properties. They can compete with imported analogs and enable obtaining of crack resistance of 7.8 MPa·m1/2 and strength of 820 MPa.

In Vitro Enamel Remineralization Efficacy of Calcium Silicate-Sodium Phosphate-Fluoride Salts versus NovaMin Bioactive Glass, Following Tooth Whitening

Objectives This study aimed to evaluate the effect of in-office bleaching on the enamel surface and the efficacy of calcium silicate-sodium phosphate-fluoride salt (CS) and NovaMin bioactive glass (NM) dentifrice in remineralizing bleached enamel. Materials and Methods Forty extracted premolars were sectioned mesio-distally, and the facial and lingual enamel were flattened and polished. The samples were equally divided into nonbleached and bleached with 38% hydrogen peroxide (HP). Each group was further divided according to the remineralization protocol (n = 10); no remineralization treatment (nontreated), CS, or NM, applied for 3 minutes two times/day for 7 days, or CS combined with NR-5 boosting serum (CS+NR-5) applied for 3 minutes once/day for 3 days. The average Knoop hardness number (KHN) and surface roughness (utilizing atomic force microscopy) were measured. Surface topography/elemental analysis was analyzed by using scanning electron microscopy/energy dispersive X-ray analysis. All the tests were performed at baseline, after bleaching, and following each remineralization protocol. Data were statistically analyzed by two-way analysis of variance and Bonferroni post hoc multiple comparison tests (α = 0.05). Results HP significantly reduced KHN and increased roughness (p < 0.05). All remineralization materials increased the hardness and reduced the surface roughness after bleaching except NM, which demonstrated significantly increased roughness (p < 0.05). Ca/P ratio decreased after bleaching (p < 0.05), and following treatment, CS and CS+NR-5 exhibited higher remineralization capacity in comparison to NM (p < 0.05). Conclusion Although none of the material tested was able to reverse the negative effect of high-concentration in-office HP on enamel completely, the remineralization efficacy of CS and CS+NR-5 was superior to that of NM.