Physical and chemical pre-treatment of mining residues
WP1 achievement led to the selection and delivery of two tailings: one coming from New Kankberg (Sweden, received late December 2015) from a mine currently exploited by BOLIDEN and a second one coming from a legacy tailings near Covas (Portugal, sample received in February 2016). The pre-treatment of New Kankberg tailings was studied first. Investigation (mineral processing trials) requiring to gather mineralogical and mineral liberation information, New Kankberg samples were characterized using QEMSCAN analysis to reveal the liberation potential of phosphates. The investigation aimed to identify the occurrence of phosphates (monazite and apatite) and their liberation potential.
Based on this characterization, mineral processing experiments were carried out. The objective was to evaluate the performance of several mineral processing techniques to concentrate REE from tailings. Samples of different size were collected to perform flotation tests, gravity separation tests (Wilfley table + Multi-gravity separation tests) and magnetic separation tests.
Results obtained showed a very good recovery of phosphates (70%) and a concentration factor of 10 with regards to received tailings. REE recovery reached 50% with a concentration factor of 9. These results highlight the efficiency of flotation to recover REE from New Kankberg tailings.
Magnetic separation tests
Monazite which the main REE-bearing mineral is paramagnetic whereas apatite does not exhibit any magnetic characteristics. Thus, after beneficiating phosphates through flotation, it is possible to recover specifically monazite through magnetic separation. Batch tests where carried out using a Boxmag high intensity magnetic separation batch cell. The effect of the magnetic field intensity on monazite recovery was studied (0 to 18 000 G). Each test was carried out on several grams (< 100 grams). Very good concentration results were obtained on flotation concentrates i) phosphates recovery = 25%, concentration factor = 1.4, ii) REE recovery = 75%, concentration factor = 3.
Gravity separation tests
Due to the size distribution of tailings particles, Wifley table density separation is less efficient to recover phosphates than flotation (even if monazite is a very dense mineral). Heavy fine particles (monazite) behave similarly to the one of bigger and lighter particles. Phosphates recovery did not exceed 15% (with a concentration factor of 8) and REE recovery did not exceed 18% (with a concentration factor of 7).
Multi Gravity separator (MGS)
Results obtained are intermediate between flotation and Wilfley shaking table. Phosphates recovery reached 32%, REE recovery reached 43% with a concentration factor of 6. Because of the size distribution of monazite (D80 = 10µm), ultrafine particles of heavy monazite are lost in light particles stream.
Following the achievement of batch tests, a flotation pilot was settled in BRGM piloting facility in May 2016) and operated during two days in June 2016 on 1 ton of tailings to recover a REE concentrate. 16 kg of REE concentrate was recovered from this pilot operation. This piloting activity validated mineral processing pre-treatment on New Kankberg tailings. This concentrate will then be used to study leaching steps (CEA) and REE recovery routes from pregnant leach solutions (Chalmers & KIT).
The second tailings studied in ENVIREE, comes from Portugal (Covas legacy mine). Pre-treatment activities achieved on Covas tailings include i) crushing and regrinding of the tailings to 100 µm, ii) chemical analysis by size class of material, ii) magnetic separation. Mineral processing tests are still ongoing. To date, pre-treatment carried out on this tailings are less efficient than the ones carried out on New Kankberg tailings. This result is mainly due to the repartition of REE in material. In Covas tailings REE are not borne by a specific mineral phase that could be enriched thanks to physical and/or chemical pre-treatment techniques. REE seem dispersed in the matrix and thus are difficult to concentrate. Magnetic separation technique allowed concentrating REE by a factor 4 but the most interesting seems to concern tungsten (ferberite FeWO4 and scheelite CaWO4) with a concentration factor of 100. Next tests will include flotation (flotation of sulphides (arsenopyrite + chalcopyrite) then flotation of oxides (calcite, scheelite, ferberite and hematite)) followed by magnetic separation. These tests will be achieved at the end of October 2016.