For the ever-growing demand of gallium (Ga) resources in electronic applications, the recovery of Ga from the mining residues or waste has received considerable interest. The present paper is focused on the selective recovery of Ga from the sulfuric acid leach liquor of coal fly ash (CFA) by stepwise separation using acidic extractant. The results show that Ga(III) can be effectively separated by controlling the equilibrium pH values and meanwhile employing appropriate extractant based on the different ionic potential and electronic configuration of metal ions. At equilibrium pH ≤ 0.8, the impurities Ti(IV) and Fe(III) in the liquor can be preferably removed by a two-stage cross current extraction with 1 mol/L P507. Then Ga(III) can be selectively separated against Al(III) using 0.5 mol/L Cyanex 272 at equilibrium pH of 2.4–2.6. The separation factors of Fe/Ga, Fe/Al and Ga/Al can reach 145, 133 and 40, respectively. The extraction mechanism of Ga(III) as well as the Ga(III) complexes with Cyanex 272 were investigated using slope analysis and spectroscopic methods including FT-IR, NMR and ESI-MS, indicating that the extraction of Ga(III) with Cyanex 272 follows an ion exchange mechanism. The Ga(III) complexes with Cyanex 272 (H 2 L 2 ) contains three coordination compounds, GaL 3 , Ga(HL)L 3 and Ga(HL) 3 L 3 . Both the P-O and P = O groups of Cyanex 272 play the roles in the coordination reaction of Ga(III). The study provides a feasible way for the selective recovery of Ga from coal fly ash.
Stepwise separation of multi-species with a single type of chemical additives (such as surfactants) is an auspicious but challenging issue in many engineering processes as one surfactant usually lacks the ability to interact with different components. One promising strategy is to develop an environment-responsive surfactant, which possesses various functional groups to anchor target constituents under different environmental conditions. In this work, we have prepared a pH-responsive surfactant (2-(benzylthio)-acetohydroxamic acid, BTHA) possessing two pH-switchable groups (i.e., thioether and hydroxamate) that can selectively interact with different target substances, thus enabling stepwise and efficient separation of a three-component mineral system. The single in-house synthesized BTHA is successfully applied to the sequential separation of pyrite, diaspore, and kaolinite from high-sulfur bauxite by simply tuning the solution pH. Specifically, under the acidic condition (pH 6.0), the hydroxamate group of BTHA is deactivated, thus only the thioether groups can directionally interact with pyrite and enable the selective flotation recovery of pyrite to 85.9%. When the solution pH is subsequently increased to 8.0, the active hydroxamate group of BTHA can tightly chelate with diaspore and result in its efficient separation from kaolinite. The bifunctional surfactant exhibits strong and selective interactions with target substrates via pH modulation, allowing the adsorption selectivity and efficient stepwise separation, which provides an innovative paradigm for the development of novel surfactants with diverse applications in the mineral, metallurgical, chemical, and renewables recycling industry.
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