Prozesstechnische Anpassungen in der Scheelitaufbereitung der Aufbereitungsanlage der Wolfram Bergbau- und Hütten AG bei veränderten Rohgutzusammensetzungen

Since 1975, Wolfram Mining and Hütten AG has been mining scheelite from the Felbertal deposit near Mittersill in the Austrian Alps. The deposit is very inhomogeneous and comprises orebodies of mineralogical composition. The processing involves crushing, sensor-assisted sorting (separation of coarse rejects), grinding and subsequent scheelite flotation. To avoid over-grinding of the scheelite, flotation is integrated into the grinding circuit (keyword: grinding circuit flotation; the separation of oversize particles only takes place after flotation). Since the beginning of mining activity, due to their talc contend, some orebodies have caused occasional processing issues. The talc is naturally hydrophobic and therefore reports strongly to the final concentrate, leading to significant dilution. The grade of the concentrates drops sharply. In 2021, the problem with talc aggravated sharply due to strongly increasing talc contend in the flotation feed. With depth, a talc-rich zone originally located along the side of the main ore body moved into its center. The previous approach of leaving the talc rich portion behind became unfeasible due to related ore losses. As a first countermeasure, extensive tests were carried out with talc depressants. Occasional positive results in the laboratory could not be replicated in the plant. On the contrary, all plant test involving talc depressants led to deteriorating performance with respect to both, concentrate grade and recovery. The theoretically best solution to remove unwanted talc from the scheelite flotation is by a separate upstream talc flotation stage, however, this was tested without success in 2004. To avoid dragging residual reagents from the scheelite flotation into the talc flotation, it was necessary to break up the circuite. This required the installation of another ball mill to separately grind the coarse portion of the tailings from the scheelite rougher flotation in order to feed this directly to the main scheelite flotation. In the talc flotation stage, it was not possible to achieve a stable foam blanket. A sufficient amount of talc froth could only be achieved with frother dosages, that led to froth overflow in the subsequent scheelite flotation. In January 2023, the talc problem escalated to the extend that some of the scheelite concentrates could only be processed in the refinery with considerable additional efforts. There was an urgent need for action. Despite the good-feel expectation that residual reagents from the scheelite flotation lead to notable scheelite losses in talc flotation, following laboratory tests, the tailings from the scheelite cleaner stages was added only ahead of the the third cell of the scheelite rougher flotation and the first two cells of the original scheelite rouger flotation were now operated as a talc flotation stage. Beside the possible carryover of residual reagents, the disadvantage of this circuit is a shorter residual time in the scheelite rougher flotation. However, experience in the plant shows that the scheelite losses in the talc stage are negligible and the reduced flotation volume in the scheelite stage can be compensated by higher dosage of frother. The integration of the talc flotation stage enables the company to process talc rich ores to on-spec scheelite concentrates.