Técnicas de processamento mineral em escórias provenientes de beneficiamento metalúrgico da columbita/tantalita

Abstract

The production and consumption of electricity are linked to the development of a nation. But it is necessary to diversify the energy matrix so as not to be dependent on seasonality, price fluctuations, and lack of inputs, among others. In addition, the search for sustainable energy sources is essential, with nuclear energy being an example. Several studies are being carried out to recover uranium from uranium mining residues, phosphate rock, mineral coal and even seawater, which can help in the sustainability of the nuclear matrix. In this line, the main objective of this study is to evaluate mineral processing techniques for the recovery of uranium oxide (U3O8) and thorium oxide (ThO2) retained in niobium and tantalum ferroalloy slag from the metallurgical processing of columbite/tantalite. This residue/tailing is produced by NORM facilities and its destination is storage in initial deposits, but with no forecast of future use. The methodology used was the collection of slag samples, and their preparation through grinding, crushing, homogenization and quartering. In the sequence were: analyzed the contents of the head sample; determined the density of the slag; physical tests were carried out (granule size separation, gravimetric separation, electrostatic separation and magnetic separation); the 160 sulfuric acid leaching tests (with process parameters pH, time, percentage of solids in the pulp and slag granulometry); determined the metallurgical recovery; determination of the mass and cost of H2SO4 needed to treat 1 ton of pulp; determined the cost of storing the slag and the cost of licensing, building and operating a medium and low radiation radioactive material deposit. All products generated by the assays were analyzed by X-ray fluorescence. The head sample had a content of 1.78% of U3O8 and 3.66% of ThO2. The slag density was 3.56 g.cm-3 – a value that agrees, within the limits of uncertainty, with the density of the gang that makes up the slag (Al2O3 and SiO2). The granulometric separation showed that the levels of uranium and thorium oxides were distributed along with the granulometric ranges. The gravimetric separation resulted in three products: fine, concentrated and mixed. Most of the slag mass (64.5%) was mixed. The electrical separation resulted in three products: conductive, non-conductive and mixed. Most of the slag mass showed conductive behaviour (63.5%) in contrast to what was expected since the oxides of higher composition (Al2O3 and SiO2) are dielectric. The magnetic separation resulted in three products: magnetic, non-magnetic and mixed. There was a certain proportion between the slag mass distribution between magnetic (40%) and non-magnetic (48%). This result is contrary to what was expected, since aluminium and silicon oxides, major components of slag, are dielectric. Thus, the slag should have a higher proportion of non-magnetics. It was found that the most relevant parameters for leaching were pH and time. Thus, the highest recoveries were observed at t > 8h and pH < 1.5. The granulometry, in contrast to what was expected, had little influence on this study. The maximum recovery was 83% for uranium oxide and 81.6% for thorium oxide – in a t = 9 h and pH = 1 (Assay 70). The optimal process conditions, that is, recovery above 80% for uranium, above 70% for thorium and lower cost for treating one ton of pulp were observed in Assay 66: pH = 1.5; t = 9 h; granulometry of 200 μm; 40% solids; (81 +- 4.0) % U3O8 recovery; ThO2 recovery of (73 +- 1.0) %; cost of H2SO4 from USD 10.01 to USD 15.51 per ton of pulp. The cost of planning, construction and licensing for a volume of 80,000 tons of slag was estimated at 126.6 million dollars, taking as a reference a Spanish deposit for the storage of low and medium radiation tailings. The cost of transferring the slag to CNEN, if possible and without taking into account transport costs, was estimated at 19.7 million dollars. The result of the head sample, 1.78% of U3O8, is superior to that of the mines that produced the most uranium in 2021 – except for Cigar Lake. The proximity of the slag density to that of the gangue that composes it was decisive in the results of the gravimetric separation, where the highest product was the mixed ones. The results of the physical separations showed that there was no significant variation of uranium and thorium oxides between the products. Thus, they were not efficient to obtain materials with a higher concentration of U3O8 or ThO2, requiring leaching. The leaching tests proved to be efficient since recoveries above 80% were obtained for the two oxides. Metallurgical recovery, taking Assay 66, maybe feasible since the cost of sulfuric acid is within the range of a traditional uranium mine and the current options, building deposits or transferring the slag, are estimated to be in the dozens million dollars, do not generate revenue for NORM facilities and they are not sustainable, as they leave the management of waste/tailings to the next generations.