Efficient zinc extraction with novel phenyl phosphate PTOPP in sulfuric acid system

Abstract

Traditional acidic phosphate extractants (D2EHPA and HEH/EHP) exhibit poor extraction performance under high-acid conditions and require saponification pretreatment to maintain the pH stability of the aqueous phase during the solvent extraction. However, saponification pretreatment produces a large amount of saline wastewater, resulting in high treatment costs and difficulty of the industrial waste salts utilization. Thus, we synthesized an efficient p-tert-octyl phenyl phosphate (PTOPP) extractant suitable for a high-acid system based on phenyl substitution and carbon-chain design. The structure and content of the synthesized products were confirmed to be [(CH3)3CCH2C(CH3)2C6H4O]2P(O)OH (diphenyl phosphate, P10B4, 88.26%) and (CH3)3CCH2C(CH3)2C6H4OP(O)(OH)2 (monophenyl phosphate, DHP10B4, 10.68%) via HRMS, LCMS, and NMR characterization combined with potentiometric titration. After three-stage countercurrent extraction and two-stage countercurrent stripping, the zinc extraction efficiency of PTOPP reached 85%, and the zinc concentration of aqueous phase increased from 20 g/L to 66.53 g/L with a 3.3 enrichment multiple. In comparison, the enrichment multiples of D2EHPA and HEH/EHP under the same conditions were only 1.5 and 1.1, respectively. The zinc extraction mechanism with PTOPP was determined by the slope method and structural characterization of the extracted compound (PTOPP-Zn). During the zinc extraction process, the P-OH and P=O groups in PTOPP interacted with Zn2+ to form Zn–O bonds through proton exchange and coordination reactions, and the molecular formula of the extracted compound was ZnA2·HA. The novel PTOPP significantly improves the zinc extraction performance and achieves a remarkable enrichment effect without saponification pretreatment, thereby avoiding the consumption of NaOH and the discharge of Na2SO4 wastewater. It has promising applications in the fields of low-grade zinc mineral resource recovery and heavy-metal wastewater disposal.