MINERAL ACID LEACHING OF SCANDIUM FROM BAUXITE RESIDUE National Technical University of Athens School of Chemical Engineering MINERAL ACID LEACHING OF SCANDIUM FROM BAUXITE RESIDUE Maria OCHSENKUEHN-PETROPOULOU, Theopisti LYMPEROPOULOU, Lamprini-Areti TSAKANIKA, Klaus-Michael OCHSENKUEHN, Konstantinos HATZILYBERIS, Paraskevas GEORGIOU, Chrysanthos STERGIOPOULOS and Fotios TSOPELAS 2nd International BAUXITE RESIDUE VALORISATION AND BEST PRACTICES CONFERENCE, 7-10 May 2018 Athens, 9 May 2018

Bauxite Residue Bauxite Residue or Red mud: A by-product of alumina production after Bayer process (~40-50 % Fe2O3) Global production: 150 Mt/yr Accumulated amount: 3 billion t Annual production in Greece : 750.000 tons (AoG) Characteristics: Highly alkaline (pH>11) Very fine grained Rich in elements of techno-economical interest (V, Zr, Nb, REEs) Enriched in REEs by a factor of 2 respectively to the original bauxite Scandium comprises 98% of the total value of REEs in Greek BR Full exploitation of BR in AoG can produce about 1 kt/yr of REE oxides

Concentration [g/t] of Sc and REEs in Greek red mud batches (1993-2016) BR 1993 BR 2001 BR 2007 BR 2012 BR 2014 BR 2016 Mean Sc 127.9 107.0 130.0 110.0 114.7 98.0 114.6± 12.4 Total REEs 986.1 868.0 1010.5 1040.3 729.7 - 926.9± 128.2 Results obtained by the Lab of Inorganic & Analytical Chemistry, School of Chemical Engineering NTUA Sc main uses(critical metal for future) Al-Sc alloys (high tech products [0.5-0.8%Sc] for aerospace, defense technology, automobiles, athletic equipment) SOFCs (as electrolyte, improving oxygen-ion conductivity)

Ga, Nb, Zr, REEs and radioactive elements Th, U in BR (g/t BR 2014)   Sc Ga Y Zr Nb La Ce Pr Nd Sm AVERAGE 114.7 28.6 92.3 484.8 94.0 87.1 305.7 20.4 74.8 15.5 STDEV 6.8 3.4 7.1 63.6 14.6 10.3 27.7 2.1 7.6 1.6   Eu Gd Tb Dy Ho Er Tm Yb Lu Th U AVERAGE 3.3 15.4 2.4 3.2 9.9 1.6 11.0 83.8 10.2 STDEV 0.3 1.4 0.2 1.0 0.7 0.1 0.9 4.8

Recovery of REEs from Red mud Lab scale – Pilot plan Bayer process Bauxite Alumina Bauxite Residue Mineral Acid Leaching HCl, HNO3, aqua regia, H2SO4 Sc separation (Ion exchange) Individual REEs (Selective extraction/ RP-HPLC)

% leaching recovery

Economics of Sc production Main cost categories throughout the stages of Sc2O3 production from BR: Stage Cost category Investment Cost (annual basis1) Reagents Cost Other Operating Cost2 Acid Leaching + ++ Ion Exchange Solvent Extraction +++ 1 total investment cost distributed within a lifetime of 25 years 2 labor, operating, maintenance cost & overheads Sc Production Cost Breakdown per processing stage

Economics of leaching process Most critical factor in operation: Reagents cost Purchase cost ratio HNO3/H2SO4 at least 2:1 Leaching agent: HNO3 HNO3 cost ≈ 350 $/kg Sc2O3 (16-17% of total production unit cost of Sc2O3) Leaching agent: H2SO4 H2SO4 cost ≈ 140-150 $/kg Sc2O3 Using H2SO4 compresses leaching cost about 2.3 times

Effect of leaching agent on Th, U, Ga, Zr and Nb recovery

Leaching agents Acid High Sc recovery Low Fe recovery Environmental impact Aqua regia HCl HNO3 H2SO4

Optimization of leaching process with sulfuric acid Investigated variables Leaching time Liquid to solid ratio (pulp density) Acid molarity Final pH Temperature Pressure Multi stage leaching (re-leaching with fresh acid) Leachate recycling (reusing of PLS on fresh BR)

Effect of leaching time L/S=1:20 (ambient conditions) Rapid reaction 5-60 min

Effect of L/S ratio Critical on Sc concentration No significant effect on Sc recovery

Effect of H2SO4 Molarity Higher molarities (>3M): no significant effect on Sc recovery, but enhance Fe dissolution economical and environmental impact no gel formation even at lower L/S ratios

Effect of final pH Optimum pH range < 0 - 0.3 L/S=5,10,20 &1-3M H2SO4, 60 min, ambient conditions

Effect of temperature and pressure Normal leaching conditions: L/S=10 (10%), 2M H2SO4, t=60min Extreme conditions: L/S=3.3 (30%), 2M H2SO4, t=420min, T=85oC Sc = 17ppm but Fe>30000 ppm and Ti>3400 ppm

Concentration of main elements Fe Ti Si Gel formation mg/L 1:5 (20%) 2M (25 °C, 1 atm,1h) 1900 2300 >1375 yes 1:10 (10%) 3M (25 °C, 1 atm, 1h) 1040 1430 1143 after 2-3days 1:2.5 (40%) 4M (25 °C, 1 atm, 24h) 7688 6550 35 no 1:5 (20%) 4M (25 °C, 1 atm, 24h) 5900 3260 50 1:10 (10%) 4M autoclave, 24h 32500 3400 25 1:10 (10%) 4M (85oC, 4h) 31840 3370 88 High pulp density leads to gel formation. The problem is solved by working with higher molarities.

Leaching time t=60min, Ambient conditions Multi stage effect (re-leaching with fresh acid) Leaching time t=60min, Ambient conditions

L/S= 7.5 - 10 (13 – 10 %), 2M H2SO4, pH ~0, t=60min Leachate recycling (reusing of PLS on fresh BR) Leaching conditions: L/S= 7.5 - 10 (13 – 10 %), 2M H2SO4, pH ~0, t=60min Ambient conditions

One way ANOVA (three categorical – two dependent variables) Statistical evaluation One way ANOVA (three categorical – two dependent variables) One way ANOVA Parameter Contribution to Sc % recovery Contribution to Sc concentration Acid molarity 6.23 % 6.71 % L/S ratio 22.24 % 70.39 % Leaching time 70.05 % 20.23 %

Statistical evaluation Contour plot of C100 vs molarity, ratio Contour plot of C100 vs time, ratio

Model for the theoretical prediction based on diffusion theory Symbols ηL/Sc : the efficiency of the leaching process regarding Sc, in mg of Sc in leachate /mg of Sc in leached sample of BR, (CSc)BR : the concentration of Sc in BR, expressed, in mg of Sc/kg of dry BR, (L/S)reactant ≈ (L/S)feed : the Liquid to Solids ratio , in L of acid/kg of dry BR, cs : the concentration (w/v) of the saturated solution in contact with the particles, in mg of Sc/L of acid, KL=k’/b, : the overall mass transfer coefficient from particles to the Liquid phase, where: k’ : the diffusion coefficient, in dm2/min b : the effective thickness of the liquid film surrounding the particles, in dm, S(=Atot/Md) : the specific surface area of BR particles participating in the leaching process, where: Atot : the total area of the solid-liquid interface, in dm2, Md : the total mass of dry BR particles, in kg, and τ : the mixing residence time, in min.

Molarity: 1 M H2SO4 Statistical evaluation Model

Development of flow-sheet for selective Sc leaching from BR The conceptual design for a continuous work L=Leaching reactor. P= Unit for Purification of leachate from solids. M= Mixing unit. BR= Bauxite Residue streams with moisture content. Subscripts: i = input (alkaline solids stream), o = output (acidic solids stream). A= Acid streams (sulfuric acid solutions). Subscripts: f= leaching feed, p= product (PLS), n= new (fresh solution), r= recycling PLS, d= dense acid (93-98% w/w). W= Water stream

Conclusions (1/2) Leaching time up to 60min. Fast process. Leaching time statistically significant for Sc recovery. Low acid molarities (≈1M) are environmentally and economically favorable but result to gel formation due to Si compounds, especially for low L/S ratios. Higher molarities (≈2-3M) increase Sc recovery, allow to work with lower L/S (or, higher pulp density) at the expense of selectivity and economical impact. L/S (pulp density) crucially increases Sc concentration. High pulp density though, result to handling difficulty due to gel formation. Final pH values are associated to acid molarity. Adjusting final pH value close to zero enhances Sc recovery.

Conclusions (2/2) Temperature and pressure effect seems insignificant for Sc recovery when using low acid molarity . High temperatures combined with high molarities (>3M) improve Sc recovery , while selectivity is reduced (higher Fe concentrations). Multi stage leaching improves Sc recovery . Iron recovery remains low. PLS recycling with pH adjustment almost triples Sc concentration but affects the selectivity. Combination of high molarity, high pulp density, elevated temperature, prolonged time and recycling lead to high Sc concentration in the PLS up to 30 ppm, but enhance iron concentration due to its almost total dissolution. Most promising scenario for selective leaching under ambient conditions resulted till now in: 1-2M H2SO4,L/S=10, t=60 min, PLS recycling 1.5 times combined with pH adjustment.

Acknowledgments The research leading to these results has been performed within the SCALE “Production of Scandium compounds and Scandium Aluminium alloys from European metallurgical by-products” project and received funding from the European Community's Horizon 2020 Program (H2020/2014-2020), under Grant Agreement No. 730105. The company II-VI is also gratefully acknowledged for the fruitful collaboration in a previous joint project.

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