The extraction and recovery of rare earth elements, aluminium, titanium, and iron from South African coal fly ash

Abstract

Coal fly ash (CFA) is a waste material with the potential for high-value and high-volume utilisation in wastewater treatment and alumina production. The sinter-H2SO4 leach process can extract over 80 wt.% of aluminium, which can be commercially processed to generate smelter-grade Al2O3. However, due to the high costs involved in the pre-treatment and downstream purification steps, the process has proved neither feasible nor sustainable. This study reports on a holistic approach to adding value in alumina processing by extracting aluminium (Al), rare earth elements (REEs), titanium (Ti), and iron (Fe) as saleable products. The overall aim is to produce a rare-earth concentrate, smelter-grade alumina, high-purity titanium dioxide, and iron-based coagulants. The potential application of the generated secondary solid residue as an adsorbent for wastewater treatment is also evaluated. In the first experimental stage, CFA was characterised for its composition. XRD analysis revealed an amorphous component with crystalline mullite, quartz, and magnetite/haematite. SEM-EDS and TIMA analyses showed that Al-Si-rich grains are the predominant phase, with discrete REE-bearing grains (phosphates and silicates) and Fe-oxide (magnetite/haematite) grains. Traces of REEs, Ti, Ca, Si, and Fe were also found in the Al-Si-rich grains. Discrete Fe-oxide grains were further recovered using dry magnetic separation to reduce downstream purification costs. The results showed that up to 65.9% Fe-oxides could be recovered at 1.05 T (magnetic field strength). However, the magnetic fraction (CFA-MF) also contained mullite and quartz impurities. The non-magnetic fraction (non-MF) was further processed for the extraction of the selected REEs (Sc, Y, La, Ce, and Nd), Al, Fe, and Ti. In the preliminary leaching tests, the effects of leaching parameters were investigated using direct H2SO4 leach and sinter-H2SO4 leach processes. The former resulted in poor metal extraction due to the phase composition of CFA, while the latter showed a much higher extraction efficiency. In the sinter-H2SO4 leach process, a maximum of 72% Al, 63% Fe, and 40% Ti were extracted in 6 M H2SO4 by leaching at 70°C in a 1:4 S/L ratio for 10 hours. Under the same experimental conditions, less than 20% of the selected REEs were extracted, except for Sc which had 35% extraction. Further investigation was conducted using the sinter-HCl leach process, and the extraction efficiency was significantly improved, with over 50% REEs recovered in 4 M HCl by leaching for 6 hours at 70°C. The lower extraction iii efficiency of REEs in the sinter-H2SO4 leach process was ascribed to calcium sulphate precipitation and the complex phase composition. Nonetheless, the sinter-H2SO4 leach process was optimised using a Design of Experiments (DOE) for alumina production. Consequently, a CFA leach liquor and a solid residue (sintered CFA residue) were generated for further processing. The CFA leach liquor was concentrated with Al(III), Fe(II/III), and Ti(IV). It was processed by integrating solvent extraction (SX), crystallisation, and oxidation-precipitation processes. All Fe(III) in solution was first reduced to Fe(II), and Ti(IV) was selectively extracted using Primene JM-T/Shellsol D70, followed by stripping in NH4OH and calcination to produce TiO2. The synthesised TiO2 showed lower purity (95.03 wt.%) and BET surface area (10.05 m2/g) for application as a photocatalyst in wastewater treatment. However, it was classified as a type II pigment grade. Aluminium in the raffinate was directly crystallised and calcined to produce smelter-grade Al2O3. The remaining Fe(II)-rich solution was processed at pH 5.0, using an oxidation-precipitation process. Due to the gelatinous Fe(III) precipitate, which was difficult to filter, the recovered CFA-MF was recycled as seeding material to improve the precipitation process. Seeding significantly improved the filtration process for all concentration levels (Cs = 0.5, 1.0, and 2.0). The seeded Fe(III) precipitate at Cs = 1.0 was further used to produce polyferric sulphate (PFS) coagulants by dissolution in dilute sulphuric acid. The generated CFA-PFS was mainly concentrated with Fe(III) and contained minor amounts of Al(III), Mg(II), Mn(II), and Ca(II). The synthesised CFA-PFS coagulant was tested on brewery wastewater and found compatible with commercial PFS. Throughout the purification processes, REEs which were dissolved in the sinter-H2SO4 leach process, mostly remained in solution. The sintered CFA solid residue (CFA-SR) was characterised as a silica and calcium component with significant concentrations of REEs. Phase composition analysis revealed an amorphous material with a Si-Ca-S-rich phase and a Si-rich phase. Traces of Al, Fe, Ti, and REEs were mostly distributed in the Si-Ca-S-rich phase. To minimise the formation of silica gel, the dry digestion-water leach process was carried out in a sulphates and chloride system for REE extraction. The obtained results indicated that the solubility of calcium sulphates and the complex phase composition might be limiting factors for efficient REE extraction. Nevertheless, an REE concentrate was generated with low purity. As a result, the REE- concentrated solution was set as the final product. Overall, combining the sinter-H2SO4 leach iv process with the dry digestion-water leach process was considered a promising alternative for concentrating and selectively extracting REEs from CFA. A CFA secondary solid residue (CFA-SSR) was generated after the two-step leaching process for disposal. The material was characterised for potential recycling in wastewater treatment as an adsorbent. Characterisation showed an amorphous and crystalline component with mesopores measuring 11.7 nm. The average particle size was 44.76 μm, while the SBET was 29.236 m2/g (surface area). Batch tests showed that CFA-SSR has the capacity to adsorb heavy metals (i.e. Ni, Zn, and Fe) in acid mine drainage (AMD). The overall findings indicated that the adsorption capacity could be enhanced by functionalising CFA-SSR to improve selectivity and surface properties. A process flowsheet was designed for the proposed CFA processing using magnetic separation, a two-step leaching process, and an integrated purification process. The economic evaluation showed that more than one value-added recovery could be an alternative to generate revenue and offset some of the operational costs in alumina processing from CFA.