Integrated approach for the processing of a complex tungsten Skarn ore (Tabuaço, Portugal)

Abstract

The Tabuaço tungsten project, located in Northern Portugal, consists of two skarn layers, namely “Main” and “Lower” skarns, which display significantly different mineralogical and geochemical features. Both skarns contain fine-grained disseminated scheelite, but the Lower-skarn gangue is dominated by silicates whereas the Main-skarn gangue contains calcium-bearing minerals including fluorite, apatite, and vesuvianite, in close association with scheelite. Preliminary feasibility studies showed that direct separation of calcium-bearing minerals by flotation with fatty acids, which are environmentally benign, was unsuccessful due to their similar surface properties. Several routes for each skarn type were proposed with a strong focus on the Main skarn that displays the most complex mineralogical associations. Enhanced gravity separation (Falcon concentrator), high intensity magnetic separation, and flotation with fatty acids were investigated for the Main Skarn level with the objective of producing a marketable scheelite concentrate. Flotation was thoroughly investigated in terms of depressants, producing optimally a concentrate assaying 9.2%WO3 at 87.9%WO3 recovery from a ~1.1%WO3 feed, by means of a 1:1 ratio of sodium carbonate and sodium silicate. Fluorite is the most problematic mineral in flotation with fatty acids and, consequently, new fatty-acids-based collector formulations have been developed to improve the separation contrast between scheelite and fluorite, producing a concentrate assaying 14.1%WO3 with 77.1%WO3 recovery for the fully-optimised conditions. Furthermore, the performance of two types of Falcon bowls (SB and UF) were investigated and optimised for gangue minerals rejection. The Falcon concentrator was used as a desliming and pre-concentrating apparatus: one stage of Falcon SB allowed rejection of 84% of the total amount of fluorite, more than 95% of the slimes, and 85 wt% of the mass. Consequently, it improved significantly the performance of the fully-optimised scheelite flotation as the final concentrate, after one Falcon SB stage and 4 flotation stages, assayed 62.9%WO3 with 59.4%WO3 recovery, which constituted a marketable scheelite concentrate. Both the Falcon SB parameters and the number of fully-optimised flotation stages can be optimised to maximise either the WO3 grade or the WO3 recovery, regarding the product specifications. Finally, the high intensity magnetic separation allowed rejection of around 45 wt% of the total yield prior to the milling stage with only 6.0%WO3 losses, which would decrease significantly the energy consumed during the milling stage.