Extraction of rare earth elements from mine tailings

Transcription

1 Extraction of rare earth elements from mine tailings Sebastiaan Peelman Delft University of Technology Department of Material Science and Engineering Metals Production, Refining and recycling 1

2 Contents Introduction Characterization of the mine tailings Leaching Residue analysis Proposed process route Conclusions 2

3 Introduction Critical raw materials defined by the EU 3

4 Exploration of the tailings have revealed: LKAB mine tailings ppm REE 4-8% P (apatite) M tons of tailings currently in the dam largest REE deposit in Europe 4

5 Apatite flotation The tailings were first treated by Pålsson et al. at LTU Sweden The REE and P content of the tailings were upgraded using flotation Particle sizes after upgrading range from µm 5

6 Characterization of the upgraded tailings Main minerals: (Fluor)Apatite Calcite Dolomite Trace minerals: Silica Magnetite REE phosphate Likely monazite 6

7 Traditional rare earth extraction Primary REE ores are usually treated via either: Alkaline pressure leaching Sulfuric acid roasting Both processes are not economical to apply to this low grade REE source Does nor account for the P content Methods for P recovery were researched for REE recovery 7

8 Leaching H 3 PO 4 is the main P product produced from apatite Traditionally H 2 SO 4 leaching is used: Ca 5 (PO 4 ) 3 F + 5H 2 SO 4 5CaSO 4 + 3H 3 PO 4 + HF CaSO 4 is detrimental to REE recovery HCl and HNO 3 as alternative: Ca 5 (PO 4 ) 3 F + 10HCl 5CaCl 2 + 3H 3 PO 4 + HF Ca 5 (PO 4 ) 3 F + 10HNO 3 5Ca(NO 3 ) 2 + 3H 3 PO 4 + HF 8

9 HCl leaching: tailings behavior Ca minerals highly reactive non excess acid required REE mineral resistant to dissolution large excess of acid required 9

10 HCl leaching: REE behavior Heavy REE react differently from light REE REE are chemically very similar Indicates the presence of 2 REE minerals 10

11 Identifying the second REE mineral A REE mineral other than monazite appears present REE can incorporated within the apatite lattice through substitution REE should co-dissolve with the apatite Dissolving apatite also produces H 3 PO 4 H 3 PO 4 dissociates into H 2 PO 4-, HPO 4 2- and PO 4 3- depending on the ph 11

12 Identifying the second REE mineral Phosphate anions can react with dissolved REE to form insoluble precipitates A ph low enough (0-1) will inhibit the formation of these anions Only at high acid concentration (excess) can any dissolved REE remain in solution Apatite is likely the second REE mineral and a source of heavy REE REE 3+ + PO 4 3 REEPO 4 K =

13 HNO 3 leaching HNO 3 leaching was compared to HCl leaching Results HNO 3 are very similar to HCl increased acid concentration has little effect Monazite is not dissolving in HCl or HNO 3 Supports that apatite is the second REE mineral 13

14 Residue analysis: SEM/EDS REE particles maintain the same approximate size before and after leaching and show no sign of partial dissolution 14

15 Remaining minerals: Quartz Hematite Residue analysis: XRD Magnesio silicates monazite 15

16 Ca(NO 3 ) 2 precipitation Ca recovery possible by cooling solution Low L/S (2-3) leads to more effective recovery 50-75% of Ca was removed No REE losses observed in the precipitates 16

17 Proposed process flowsheet 17

18 Conclusions Total REE recovery not achievable under present conditions Two REE minerals are present in the upgraded tailings: apatite and monazite Apatite is rich in heavy REE and monazite is rich in light REE Apatite readily dissolves in acidic solutions, but a low end ph is required for the dissolved REE to remain in the solution Monazite does not dissolve, even in heated and concentrated HNO 3 18

19 Conclusions Phosphorous is recovered as H 3 PO 4 together with the heavy REE Calcium can be partially removed and valorized as Ca(NO 3 ) 2,when HNO 3 is used as leachant, by chilling the solution The leach residue can be treated as a low grade monazite concentrate and be processed by alkaline treatment or roasting treatment as a source of light REE 19

20 Thank you for your attention 20